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 a 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 If VARIABLE is omitted, it chomps C<$_>. Example:
552 chomp; # avoid \n on last field
557 You can actually chomp anything that's an lvalue, including an assignment:
560 chomp($answer = <STDIN>);
562 If you chomp a list, each element is chomped, and the total number of
563 characters removed is returned.
571 Chops off the last character of a string and returns the character
572 chopped. It's used primarily to remove the newline from the end of an
573 input record, but is much more efficient than C<s/\n//> because it neither
574 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
578 chop; # avoid \n on last field
583 You can actually chop anything that's an lvalue, including an assignment:
586 chop($answer = <STDIN>);
588 If you chop a list, each element is chopped. Only the value of the
589 last C<chop()> is returned.
591 Note that C<chop()> returns the last character. To return all but the last
592 character, use C<substr($string, 0, -1)>.
596 Changes the owner (and group) of a list of files. The first two
597 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
598 Returns the number of files successfully changed.
600 $cnt = chown $uid, $gid, 'foo', 'bar';
601 chown $uid, $gid, @filenames;
603 Here's an example that looks up nonnumeric uids in the passwd file:
606 chop($user = <STDIN>);
608 chop($pattern = <STDIN>);
610 ($login,$pass,$uid,$gid) = getpwnam($user)
611 or die "$user not in passwd file";
613 @ary = glob($pattern); # expand filenames
614 chown $uid, $gid, @ary;
616 On most systems, you are not allowed to change the ownership of the
617 file unless you're the superuser, although you should be able to change
618 the group to any of your secondary groups. On insecure systems, these
619 restrictions may be relaxed, but this is not a portable assumption.
625 Returns the character represented by that NUMBER in the character set.
626 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
627 chr(0x263a) is a Unicode smiley face (but only within the scope of
628 a C<use utf8>). For the reverse, use L</ord>.
629 See L<utf8> for more about Unicode.
631 If NUMBER is omitted, uses C<$_>.
633 =item chroot FILENAME
637 This function works like the system call by the same name: it makes the
638 named directory the new root directory for all further pathnames that
639 begin with a C<"/"> by your process and all its children. (It doesn't
640 change your current working directory, which is unaffected.) For security
641 reasons, this call is restricted to the superuser. If FILENAME is
642 omitted, does a C<chroot()> to C<$_>.
644 =item close FILEHANDLE
648 Closes the file or pipe associated with the file handle, returning TRUE
649 only if stdio successfully flushes buffers and closes the system file
650 descriptor. Closes the currently selected filehandle if the argument
653 You don't have to close FILEHANDLE if you are immediately going to do
654 another C<open()> on it, because C<open()> will close it for you. (See
655 C<open()>.) However, an explicit C<close()> on an input file resets the line
656 counter (C<$.>), while the implicit close done by C<open()> does not.
658 If the file handle came from a piped open C<close()> will additionally
659 return FALSE if one of the other system calls involved fails or if the
660 program exits with non-zero status. (If the only problem was that the
661 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
662 also waits for the process executing on the pipe to complete, in case you
663 want to look at the output of the pipe afterwards, and
664 implicitly puts the exit status value of that command into C<$?>.
668 open(OUTPUT, '|sort >foo') # pipe to sort
669 or die "Can't start sort: $!";
670 #... # print stuff to output
671 close OUTPUT # wait for sort to finish
672 or warn $! ? "Error closing sort pipe: $!"
673 : "Exit status $? from sort";
674 open(INPUT, 'foo') # get sort's results
675 or die "Can't open 'foo' for input: $!";
677 FILEHANDLE may be an expression whose value can be used as an indirect
678 filehandle, usually the real filehandle name.
680 =item closedir DIRHANDLE
682 Closes a directory opened by C<opendir()> and returns the success of that
685 DIRHANDLE may be an expression whose value can be used as an indirect
686 dirhandle, usually the real dirhandle name.
688 =item connect SOCKET,NAME
690 Attempts to connect to a remote socket, just as the connect system call
691 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
692 packed address of the appropriate type for the socket. See the examples in
693 L<perlipc/"Sockets: Client/Server Communication">.
697 Actually a flow control statement rather than a function. If there is a
698 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
699 C<foreach>), it is always executed just before the conditional is about to
700 be evaluated again, just like the third part of a C<for> loop in C. Thus
701 it can be used to increment a loop variable, even when the loop has been
702 continued via the C<next> statement (which is similar to the C C<continue>
705 C<last>, C<next>, or C<redo> may appear within a C<continue>
706 block. C<last> and C<redo> will behave as if they had been executed within
707 the main block. So will C<next>, but since it will execute a C<continue>
708 block, it may be more entertaining.
711 ### redo always comes here
714 ### next always comes here
716 # then back the top to re-check EXPR
718 ### last always comes here
720 Omitting the C<continue> section is semantically equivalent to using an
721 empty one, logically enough. In that case, C<next> goes directly back
722 to check the condition at the top of the loop.
726 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
727 takes cosine of C<$_>.
729 For the inverse cosine operation, you may use the C<POSIX::acos()>
730 function, or use this relation:
732 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
734 =item crypt PLAINTEXT,SALT
736 Encrypts a string exactly like the crypt(3) function in the C library
737 (assuming that you actually have a version there that has not been
738 extirpated as a potential munition). This can prove useful for checking
739 the password file for lousy passwords, amongst other things. Only the
740 guys wearing white hats should do this.
742 Note that C<crypt()> is intended to be a one-way function, much like breaking
743 eggs to make an omelette. There is no (known) corresponding decrypt
744 function. As a result, this function isn't all that useful for
745 cryptography. (For that, see your nearby CPAN mirror.)
747 When verifying an existing encrypted string you should use the encrypted
748 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
749 allows your code to work with the standard C<crypt()> and with more
750 exotic implementations. When choosing a new salt create a random two
751 character string whose characters come from the set C<[./0-9A-Za-z]>
752 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
754 Here's an example that makes sure that whoever runs this program knows
757 $pwd = (getpwuid($<))[1];
761 chomp($word = <STDIN>);
765 if (crypt($word, $pwd) ne $pwd) {
771 Of course, typing in your own password to whoever asks you
776 [This function has been largely superseded by the C<untie()> function.]
778 Breaks the binding between a DBM file and a hash.
780 =item dbmopen HASH,DBNAME,MODE
782 [This function has been largely superseded by the C<tie()> function.]
784 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
785 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
786 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
787 is the name of the database (without the F<.dir> or F<.pag> extension if
788 any). If the database does not exist, it is created with protection
789 specified by MODE (as modified by the C<umask()>). If your system supports
790 only the older DBM functions, you may perform only one C<dbmopen()> in your
791 program. In older versions of Perl, if your system had neither DBM nor
792 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
795 If you don't have write access to the DBM file, you can only read hash
796 variables, not set them. If you want to test whether you can write,
797 either use file tests or try setting a dummy hash entry inside an C<eval()>,
798 which will trap the error.
800 Note that functions such as C<keys()> and C<values()> may return huge lists
801 when used on large DBM files. You may prefer to use the C<each()>
802 function to iterate over large DBM files. Example:
804 # print out history file offsets
805 dbmopen(%HIST,'/usr/lib/news/history',0666);
806 while (($key,$val) = each %HIST) {
807 print $key, ' = ', unpack('L',$val), "\n";
811 See also L<AnyDBM_File> for a more general description of the pros and
812 cons of the various dbm approaches, as well as L<DB_File> for a particularly
815 You can control which DBM library you use by loading that library
816 before you call dbmopen():
819 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
820 or die "Can't open netscape history file: $!";
826 Returns a Boolean value telling whether EXPR has a value other than
827 the undefined value C<undef>. If EXPR is not present, C<$_> will be
830 Many operations return C<undef> to indicate failure, end of file,
831 system error, uninitialized variable, and other exceptional
832 conditions. This function allows you to distinguish C<undef> from
833 other values. (A simple Boolean test will not distinguish among
834 C<undef>, zero, the empty string, and C<"0">, which are all equally
835 false.) Note that since C<undef> is a valid scalar, its presence
836 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
837 returns C<undef> when its argument is an empty array, I<or> when the
838 element to return happens to be C<undef>.
840 You may also use C<defined()> to check whether a subroutine exists, by
841 saying C<defined &func> without parentheses. On the other hand, use
842 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
843 produce intuitive results, and should probably be avoided.
845 When used on a hash element, it tells you whether the value is defined,
846 not whether the key exists in the hash. Use L</exists> for the latter
851 print if defined $switch{'D'};
852 print "$val\n" while defined($val = pop(@ary));
853 die "Can't readlink $sym: $!"
854 unless defined($value = readlink $sym);
855 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
856 $debugging = 0 unless defined $debugging;
858 Note: Many folks tend to overuse C<defined()>, and then are surprised to
859 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
860 defined values. For example, if you say
864 The pattern match succeeds, and C<$1> is defined, despite the fact that it
865 matched "nothing". But it didn't really match nothing--rather, it
866 matched something that happened to be zero characters long. This is all
867 very above-board and honest. When a function returns an undefined value,
868 it's an admission that it couldn't give you an honest answer. So you
869 should use C<defined()> only when you're questioning the integrity of what
870 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
873 Currently, using C<defined()> on an entire array or hash reports whether
874 memory for that aggregate has ever been allocated. So an array you set
875 to the empty list appears undefined initially, and one that once was full
876 and that you then set to the empty list still appears defined. You
877 should instead use a simple test for size:
879 if (@an_array) { print "has array elements\n" }
880 if (%a_hash) { print "has hash members\n" }
882 Using C<undef()> on these, however, does clear their memory and then report
883 them as not defined anymore, but you shouldn't do that unless you don't
884 plan to use them again, because it saves time when you load them up
885 again to have memory already ready to be filled. The normal way to
886 free up space used by an aggregate is to assign the empty list.
888 This counterintuitive behavior of C<defined()> on aggregates may be
889 changed, fixed, or broken in a future release of Perl.
891 See also L</undef>, L</exists>, L</ref>.
895 Deletes the specified key(s) and their associated values from a hash.
896 For each key, returns the deleted value associated with that key, or
897 the undefined value if there was no such key. Deleting from C<$ENV{}>
898 modifies the environment. Deleting from a hash tied to a DBM file
899 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
900 doesn't necessarily return anything.)
902 The following deletes all the values of a hash:
904 foreach $key (keys %HASH) {
910 delete @HASH{keys %HASH}
912 But both of these are slower than just assigning the empty list
915 %hash = (); # completely empty %hash
916 undef %hash; # forget %hash every existed
918 Note that the EXPR can be arbitrarily complicated as long as the final
919 operation is a hash element lookup or hash slice:
921 delete $ref->[$x][$y]{$key};
922 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
926 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
927 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
928 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
929 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
930 C<$@> and the C<eval()> is terminated with the undefined value. This makes
931 C<die()> the way to raise an exception.
935 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
936 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
938 If the value of EXPR does not end in a newline, the current script line
939 number and input line number (if any) are also printed, and a newline
940 is supplied. Note that the "input line number" (also known as "chunk")
941 is subject to whatever notion of "line" happens to be currently in
942 effect, and is also available as the special variable C<$.>.
943 See L<perlvar/"$/"> and L<perlvar/"$.">.
945 Hint: sometimes appending C<", stopped"> to your message
946 will cause it to make better sense when the string C<"at foo line 123"> is
947 appended. Suppose you are running script "canasta".
949 die "/etc/games is no good";
950 die "/etc/games is no good, stopped";
952 produce, respectively
954 /etc/games is no good at canasta line 123.
955 /etc/games is no good, stopped at canasta line 123.
957 See also exit(), warn(), and the Carp module.
959 If LIST is empty and C<$@> already contains a value (typically from a
960 previous eval) that value is reused after appending C<"\t...propagated">.
961 This is useful for propagating exceptions:
964 die unless $@ =~ /Expected exception/;
966 If C<$@> is empty then the string C<"Died"> is used.
968 die() can also be called with a reference argument. If this happens to be
969 trapped within an eval(), $@ contains the reference. This behavior permits
970 a more elaborate exception handling implementation using objects that
971 maintain arbitary state about the nature of the exception. Such a scheme
972 is sometimes preferable to matching particular string values of $@ using
973 regular expressions. Here's an example:
975 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
977 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
978 # handle Some::Module::Exception
981 # handle all other possible exceptions
985 Since perl will stringify uncaught exception messages before displaying
986 them, you may want to overload stringification operations on such custom
987 exception objects. See L<overload> for details about that.
989 You can arrange for a callback to be run just before the C<die()> does
990 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
991 will be called with the error text and can change the error message, if
992 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
993 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
995 Note that the C<$SIG{__DIE__}> hook is currently called even inside
996 eval()ed blocks/strings! If one wants the hook to do nothing in such
1001 as the first line of the handler (see L<perlvar/$^S>). Because this
1002 promotes action at a distance, this counterintuitive behavior may be fixed
1003 in a future release.
1007 Not really a function. Returns the value of the last command in the
1008 sequence of commands indicated by BLOCK. When modified by a loop
1009 modifier, executes the BLOCK once before testing the loop condition.
1010 (On other statements the loop modifiers test the conditional first.)
1012 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1013 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1014 See L<perlsyn> for alternative strategies.
1016 =item do SUBROUTINE(LIST)
1018 A deprecated form of subroutine call. See L<perlsub>.
1022 Uses the value of EXPR as a filename and executes the contents of the
1023 file as a Perl script. Its primary use is to include subroutines
1024 from a Perl subroutine library.
1030 scalar eval `cat stat.pl`;
1032 except that it's more efficient and concise, keeps track of the current
1033 filename for error messages, searches the @INC libraries, and updates
1034 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1035 variables. It also differs in that code evaluated with C<do FILENAME>
1036 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1037 same, however, in that it does reparse the file every time you call it,
1038 so you probably don't want to do this inside a loop.
1040 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1041 error. If C<do> can read the file but cannot compile it, it
1042 returns undef and sets an error message in C<$@>. If the file is
1043 successfully compiled, C<do> returns the value of the last expression
1046 Note that inclusion of library modules is better done with the
1047 C<use()> and C<require()> operators, which also do automatic error checking
1048 and raise an exception if there's a problem.
1050 You might like to use C<do> to read in a program configuration
1051 file. Manual error checking can be done this way:
1053 # read in config files: system first, then user
1054 for $file ("/share/prog/defaults.rc",
1055 "$ENV{HOME}/.someprogrc")
1057 unless ($return = do $file) {
1058 warn "couldn't parse $file: $@" if $@;
1059 warn "couldn't do $file: $!" unless defined $return;
1060 warn "couldn't run $file" unless $return;
1068 This causes an immediate core dump. Primarily this is so that you can
1069 use the B<undump> program to turn your core dump into an executable binary
1070 after having initialized all your variables at the beginning of the
1071 program. When the new binary is executed it will begin by executing a
1072 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
1073 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
1074 is omitted, restarts the program from the top. WARNING: Any files
1075 opened at the time of the dump will NOT be open any more when the
1076 program is reincarnated, with possible resulting confusion on the part
1077 of Perl. See also B<-u> option in L<perlrun>.
1082 require 'getopt.pl';
1094 dump QUICKSTART if $ARGV[0] eq '-d';
1099 This operator is largely obsolete, partly because it's very hard to
1100 convert a core file into an executable, and because the real perl-to-C
1101 compiler has superseded it.
1105 When called in list context, returns a 2-element list consisting of the
1106 key and value for the next element of a hash, so that you can iterate over
1107 it. When called in scalar context, returns the key for only the "next"
1108 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1109 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1112 Entries are returned in an apparently random order. The actual random
1113 order is subject to change in future versions of perl, but it is guaranteed
1114 to be in the same order as either the C<keys()> or C<values()> function
1115 would produce on the same (unmodified) hash.
1117 When the hash is entirely read, a null array is returned in list context
1118 (which when assigned produces a FALSE (C<0>) value), and C<undef> in
1119 scalar context. The next call to C<each()> after that will start iterating
1120 again. There is a single iterator for each hash, shared by all C<each()>,
1121 C<keys()>, and C<values()> function calls in the program; it can be reset by
1122 reading all the elements from the hash, or by evaluating C<keys HASH> or
1123 C<values HASH>. If you add or delete elements of a hash while you're
1124 iterating over it, you may get entries skipped or duplicated, so don't.
1126 The following prints out your environment like the printenv(1) program,
1127 only in a different order:
1129 while (($key,$value) = each %ENV) {
1130 print "$key=$value\n";
1133 See also C<keys()>, C<values()> and C<sort()>.
1135 =item eof FILEHANDLE
1141 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1142 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1143 gives the real filehandle. (Note that this function actually
1144 reads a character and then C<ungetc()>s it, so isn't very useful in an
1145 interactive context.) Do not read from a terminal file (or call
1146 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1147 as terminals may lose the end-of-file condition if you do.
1149 An C<eof> without an argument uses the last file read as argument.
1150 Using C<eof()> with empty parentheses is very different. It indicates
1151 the pseudo file formed of the files listed on the command line, i.e.,
1152 C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)> loop to
1153 detect the end of only the last file. Use C<eof(ARGV)> or eof without the
1154 parentheses to test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1156 # reset line numbering on each input file
1158 next if /^\s*#/; # skip comments
1161 close ARGV if eof; # Not eof()!
1164 # insert dashes just before last line of last file
1166 if (eof()) { # check for end of current file
1167 print "--------------\n";
1168 close(ARGV); # close or last; is needed if we
1169 # are reading from the terminal
1174 Practical hint: you almost never need to use C<eof> in Perl, because the
1175 input operators return false values when they run out of data, or if there
1182 In the first form, the return value of EXPR is parsed and executed as if it
1183 were a little Perl program. The value of the expression (which is itself
1184 determined within scalar context) is first parsed, and if there weren't any
1185 errors, executed in the context of the current Perl program, so that any
1186 variable settings or subroutine and format definitions remain afterwards.
1187 Note that the value is parsed every time the eval executes. If EXPR is
1188 omitted, evaluates C<$_>. This form is typically used to delay parsing
1189 and subsequent execution of the text of EXPR until run time.
1191 In the second form, the code within the BLOCK is parsed only once--at the
1192 same time the code surrounding the eval itself was parsed--and executed
1193 within the context of the current Perl program. This form is typically
1194 used to trap exceptions more efficiently than the first (see below), while
1195 also providing the benefit of checking the code within BLOCK at compile
1198 The final semicolon, if any, may be omitted from the value of EXPR or within
1201 In both forms, the value returned is the value of the last expression
1202 evaluated inside the mini-program; a return statement may be also used, just
1203 as with subroutines. The expression providing the return value is evaluated
1204 in void, scalar, or list context, depending on the context of the eval itself.
1205 See L</wantarray> for more on how the evaluation context can be determined.
1207 If there is a syntax error or runtime error, or a C<die()> statement is
1208 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1209 error message. If there was no error, C<$@> is guaranteed to be a null
1210 string. Beware that using C<eval()> neither silences perl from printing
1211 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1212 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1213 L</warn> and L<perlvar>.
1215 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1216 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1217 is implemented. It is also Perl's exception trapping mechanism, where
1218 the die operator is used to raise exceptions.
1220 If the code to be executed doesn't vary, you may use the eval-BLOCK
1221 form to trap run-time errors without incurring the penalty of
1222 recompiling each time. The error, if any, is still returned in C<$@>.
1225 # make divide-by-zero nonfatal
1226 eval { $answer = $a / $b; }; warn $@ if $@;
1228 # same thing, but less efficient
1229 eval '$answer = $a / $b'; warn $@ if $@;
1231 # a compile-time error
1232 eval { $answer = }; # WRONG
1235 eval '$answer ='; # sets $@
1237 Due to the current arguably broken state of C<__DIE__> hooks, when using
1238 the C<eval{}> form as an exception trap in libraries, you may wish not
1239 to trigger any C<__DIE__> hooks that user code may have installed.
1240 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1241 as shown in this example:
1243 # a very private exception trap for divide-by-zero
1244 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1247 This is especially significant, given that C<__DIE__> hooks can call
1248 C<die()> again, which has the effect of changing their error messages:
1250 # __DIE__ hooks may modify error messages
1252 local $SIG{'__DIE__'} =
1253 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1254 eval { die "foo lives here" };
1255 print $@ if $@; # prints "bar lives here"
1258 Because this promotes action at a distance, this counterintuive behavior
1259 may be fixed in a future release.
1261 With an C<eval()>, you should be especially careful to remember what's
1262 being looked at when:
1268 eval { $x }; # CASE 4
1270 eval "\$$x++"; # CASE 5
1273 Cases 1 and 2 above behave identically: they run the code contained in
1274 the variable C<$x>. (Although case 2 has misleading double quotes making
1275 the reader wonder what else might be happening (nothing is).) Cases 3
1276 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1277 does nothing but return the value of C<$x>. (Case 4 is preferred for
1278 purely visual reasons, but it also has the advantage of compiling at
1279 compile-time instead of at run-time.) Case 5 is a place where
1280 normally you I<WOULD> like to use double quotes, except that in this
1281 particular situation, you can just use symbolic references instead, as
1284 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1285 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1289 =item exec PROGRAM LIST
1291 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1292 use C<system()> instead of C<exec()> if you want it to return. It fails and
1293 returns FALSE only if the command does not exist I<and> it is executed
1294 directly instead of via your system's command shell (see below).
1296 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1297 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1298 or C<exit()> (if C<-w> is set - but you always do that). If you
1299 I<really> want to follow an C<exec()> with some other statement, you
1300 can use one of these styles to avoid the warning:
1302 exec ('foo') or print STDERR "couldn't exec foo: $!";
1303 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1305 If there is more than one argument in LIST, or if LIST is an array
1306 with more than one value, calls execvp(3) with the arguments in LIST.
1307 If there is only one scalar argument or an array with one element in it,
1308 the argument is checked for shell metacharacters, and if there are any,
1309 the entire argument is passed to the system's command shell for parsing
1310 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1311 If there are no shell metacharacters in the argument, it is split into
1312 words and passed directly to C<execvp()>, which is more efficient. Note:
1313 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1314 set C<$|> to avoid lost output. Examples:
1316 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1317 exec "sort $outfile | uniq";
1319 If you don't really want to execute the first argument, but want to lie
1320 to the program you are executing about its own name, you can specify
1321 the program you actually want to run as an "indirect object" (without a
1322 comma) in front of the LIST. (This always forces interpretation of the
1323 LIST as a multivalued list, even if there is only a single scalar in
1326 $shell = '/bin/csh';
1327 exec $shell '-sh'; # pretend it's a login shell
1331 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1333 When the arguments get executed via the system shell, results will
1334 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1337 Using an indirect object with C<exec()> or C<system()> is also more secure.
1338 This usage forces interpretation of the arguments as a multivalued list,
1339 even if the list had just one argument. That way you're safe from the
1340 shell expanding wildcards or splitting up words with whitespace in them.
1342 @args = ( "echo surprise" );
1344 exec @args; # subject to shell escapes
1346 exec { $args[0] } @args; # safe even with one-arg list
1348 The first version, the one without the indirect object, ran the I<echo>
1349 program, passing it C<"surprise"> an argument. The second version
1350 didn't--it tried to run a program literally called I<"echo surprise">,
1351 didn't find it, and set C<$?> to a non-zero value indicating failure.
1353 Note that C<exec()> will not call your C<END> blocks, nor will it call
1354 any C<DESTROY> methods in your objects.
1358 Returns TRUE if the specified hash key exists in its hash array, even
1359 if the corresponding value is undefined.
1361 print "Exists\n" if exists $array{$key};
1362 print "Defined\n" if defined $array{$key};
1363 print "True\n" if $array{$key};
1365 A hash element can be TRUE only if it's defined, and defined if
1366 it exists, but the reverse doesn't necessarily hold true.
1368 Note that the EXPR can be arbitrarily complicated as long as the final
1369 operation is a hash key lookup:
1371 if (exists $ref->{A}->{B}->{$key}) { }
1372 if (exists $hash{A}{B}{$key}) { }
1374 Although the last element will not spring into existence just because
1375 its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1376 and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
1377 existence test for a $key element. This happens anywhere the arrow
1378 operator is used, including even
1381 if (exists $ref->{"Some key"}) { }
1382 print $ref; # prints HASH(0x80d3d5c)
1384 This surprising autovivification in what does not at first--or even
1385 second--glance appear to be an lvalue context may be fixed in a future
1390 Evaluates EXPR and exits immediately with that value. Example:
1393 exit 0 if $ans =~ /^[Xx]/;
1395 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1396 universally recognized values for EXPR are C<0> for success and C<1>
1397 for error; other values are subject to interpretation depending on the
1398 environment in which the Perl program is running. For example, exiting
1399 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1400 the mailer to return the item undelivered, but that's not true everywhere.
1402 Don't use C<exit()> to abort a subroutine if there's any chance that
1403 someone might want to trap whatever error happened. Use C<die()> instead,
1404 which can be trapped by an C<eval()>.
1406 The exit() function does not always exit immediately. It calls any
1407 defined C<END> routines first, but these C<END> routines may not
1408 themselves abort the exit. Likewise any object destructors that need to
1409 be called are called before the real exit. If this is a problem, you
1410 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1411 See L<perlsub> for details.
1417 Returns I<e> (the natural logarithm base) to the power of EXPR.
1418 If EXPR is omitted, gives C<exp($_)>.
1420 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1422 Implements the fcntl(2) function. You'll probably have to say
1426 first to get the correct constant definitions. Argument processing and
1427 value return works just like C<ioctl()> below.
1431 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1432 or die "can't fcntl F_GETFL: $!";
1434 You don't have to check for C<defined()> on the return from C<fnctl()>.
1435 Like C<ioctl()>, it maps a C<0> return from the system call into "C<0>
1436 but true" in Perl. This string is true in boolean context and C<0>
1437 in numeric context. It is also exempt from the normal B<-w> warnings
1438 on improper numeric conversions.
1440 Note that C<fcntl()> will produce a fatal error if used on a machine that
1441 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1442 manpage to learn what functions are available on your system.
1444 =item fileno FILEHANDLE
1446 Returns the file descriptor for a filehandle, or undefined if the
1447 filehandle is not open. This is mainly useful for constructing
1448 bitmaps for C<select()> and low-level POSIX tty-handling operations.
1449 If FILEHANDLE is an expression, the value is taken as an indirect
1450 filehandle, generally its name.
1452 You can use this to find out whether two handles refer to the
1453 same underlying descriptor:
1455 if (fileno(THIS) == fileno(THAT)) {
1456 print "THIS and THAT are dups\n";
1459 =item flock FILEHANDLE,OPERATION
1461 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE
1462 for success, FALSE on failure. Produces a fatal error if used on a
1463 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1464 C<flock()> is Perl's portable file locking interface, although it locks
1465 only entire files, not records.
1467 Two potentially non-obvious but traditional C<flock> semantics are
1468 that it waits indefinitely until the lock is granted, and that its locks
1469 B<merely advisory>. Such discretionary locks are more flexible, but offer
1470 fewer guarantees. This means that files locked with C<flock()> may be
1471 modified by programs that do not also use C<flock()>. See L<perlport>,
1472 your port's specific documentation, or your system-specific local manpages
1473 for details. It's best to assume traditional behavior if you're writing
1474 portable programs. (But if you're not, you should as always feel perfectly
1475 free to write for your own system's idiosyncrasies (sometimes called
1476 "features"). Slavish adherence to portability concerns shouldn't get
1477 in the way of your getting your job done.)
1479 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1480 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1481 you can use the symbolic names if import them from the Fcntl module,
1482 either individually, or as a group using the ':flock' tag. LOCK_SH
1483 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1484 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1485 LOCK_EX then C<flock()> will return immediately rather than blocking
1486 waiting for the lock (check the return status to see if you got it).
1488 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1489 before locking or unlocking it.
1491 Note that the emulation built with lockf(3) doesn't provide shared
1492 locks, and it requires that FILEHANDLE be open with write intent. These
1493 are the semantics that lockf(3) implements. Most if not all systems
1494 implement lockf(3) in terms of fcntl(2) locking, though, so the
1495 differing semantics shouldn't bite too many people.
1497 Note also that some versions of C<flock()> cannot lock things over the
1498 network; you would need to use the more system-specific C<fcntl()> for
1499 that. If you like you can force Perl to ignore your system's flock(2)
1500 function, and so provide its own fcntl(2)-based emulation, by passing
1501 the switch C<-Ud_flock> to the F<Configure> program when you configure
1504 Here's a mailbox appender for BSD systems.
1506 use Fcntl ':flock'; # import LOCK_* constants
1509 flock(MBOX,LOCK_EX);
1510 # and, in case someone appended
1511 # while we were waiting...
1516 flock(MBOX,LOCK_UN);
1519 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1520 or die "Can't open mailbox: $!";
1523 print MBOX $msg,"\n\n";
1526 On systems that support a real flock(), locks are inherited across fork()
1527 calls, whereas those that must resort to the more capricious fcntl()
1528 function lose the locks, making it harder to write servers.
1530 See also L<DB_File> for other flock() examples.
1534 Does a fork(2) system call to create a new process running the
1535 same program at the same point. It returns the child pid to the
1536 parent process, C<0> to the child process, or C<undef> if the fork is
1537 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1538 are shared, while everything else is copied. On most systems supporting
1539 fork(), great care has gone into making it extremely efficient (for
1540 example, using copy-on-write technology on data pages), making it the
1541 dominant paradigm for multitasking over the last few decades.
1543 Note: unflushed buffers remain unflushed in both processes, which means
1544 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1545 method of C<IO::Handle> to avoid duplicate output.
1547 If you C<fork()> without ever waiting on your children, you will
1548 accumulate zombies. On some systems, you can avoid this by setting
1549 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1550 forking and reaping moribund children.
1552 Note that if your forked child inherits system file descriptors like
1553 STDIN and STDOUT that are actually connected by a pipe or socket, even
1554 if you exit, then the remote server (such as, say, a CGI script or a
1555 backgrounded job launced from a remote shell) won't think you're done.
1556 You should reopen those to F</dev/null> if it's any issue.
1560 Declare a picture format for use by the C<write()> function. For
1564 Test: @<<<<<<<< @||||| @>>>>>
1565 $str, $%, '$' . int($num)
1569 $num = $cost/$quantity;
1573 See L<perlform> for many details and examples.
1575 =item formline PICTURE,LIST
1577 This is an internal function used by C<format>s, though you may call it,
1578 too. It formats (see L<perlform>) a list of values according to the
1579 contents of PICTURE, placing the output into the format output
1580 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1581 Eventually, when a C<write()> is done, the contents of
1582 C<$^A> are written to some filehandle, but you could also read C<$^A>
1583 yourself and then set C<$^A> back to C<"">. Note that a format typically
1584 does one C<formline()> per line of form, but the C<formline()> function itself
1585 doesn't care how many newlines are embedded in the PICTURE. This means
1586 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1587 You may therefore need to use multiple formlines to implement a single
1588 record format, just like the format compiler.
1590 Be careful if you put double quotes around the picture, because an "C<@>"
1591 character may be taken to mean the beginning of an array name.
1592 C<formline()> always returns TRUE. See L<perlform> for other examples.
1594 =item getc FILEHANDLE
1598 Returns the next character from the input file attached to FILEHANDLE,
1599 or the undefined value at end of file, or if there was an error.
1600 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1601 efficient. However, it cannot be used by itself to fetch single
1602 characters without waiting for the user to hit enter. For that, try
1603 something more like:
1606 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1609 system "stty", '-icanon', 'eol', "\001";
1615 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1618 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1622 Determination of whether $BSD_STYLE should be set
1623 is left as an exercise to the reader.
1625 The C<POSIX::getattr()> function can do this more portably on
1626 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1627 module from your nearest CPAN site; details on CPAN can be found on
1632 Implements the C library function of the same name, which on most
1633 systems returns the current login from F</etc/utmp>, if any. If null,
1636 $login = getlogin || getpwuid($<) || "Kilroy";
1638 Do not consider C<getlogin()> for authentication: it is not as
1639 secure as C<getpwuid()>.
1641 =item getpeername SOCKET
1643 Returns the packed sockaddr address of other end of the SOCKET connection.
1646 $hersockaddr = getpeername(SOCK);
1647 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1648 $herhostname = gethostbyaddr($iaddr, AF_INET);
1649 $herstraddr = inet_ntoa($iaddr);
1653 Returns the current process group for the specified PID. Use
1654 a PID of C<0> to get the current process group for the
1655 current process. Will raise an exception if used on a machine that
1656 doesn't implement getpgrp(2). If PID is omitted, returns process
1657 group of current process. Note that the POSIX version of C<getpgrp()>
1658 does not accept a PID argument, so only C<PID==0> is truly portable.
1662 Returns the process id of the parent process.
1664 =item getpriority WHICH,WHO
1666 Returns the current priority for a process, a process group, or a user.
1667 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1668 machine that doesn't implement getpriority(2).
1674 =item gethostbyname NAME
1676 =item getnetbyname NAME
1678 =item getprotobyname NAME
1684 =item getservbyname NAME,PROTO
1686 =item gethostbyaddr ADDR,ADDRTYPE
1688 =item getnetbyaddr ADDR,ADDRTYPE
1690 =item getprotobynumber NUMBER
1692 =item getservbyport PORT,PROTO
1710 =item sethostent STAYOPEN
1712 =item setnetent STAYOPEN
1714 =item setprotoent STAYOPEN
1716 =item setservent STAYOPEN
1730 These routines perform the same functions as their counterparts in the
1731 system library. In list context, the return values from the
1732 various get routines are as follows:
1734 ($name,$passwd,$uid,$gid,
1735 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1736 ($name,$passwd,$gid,$members) = getgr*
1737 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1738 ($name,$aliases,$addrtype,$net) = getnet*
1739 ($name,$aliases,$proto) = getproto*
1740 ($name,$aliases,$port,$proto) = getserv*
1742 (If the entry doesn't exist you get a null list.)
1744 In scalar context, you get the name, unless the function was a
1745 lookup by name, in which case you get the other thing, whatever it is.
1746 (If the entry doesn't exist you get the undefined value.) For example:
1748 $uid = getpwnam($name);
1749 $name = getpwuid($num);
1751 $gid = getgrnam($name);
1752 $name = getgrgid($num;
1756 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are
1757 special cases in the sense that in many systems they are unsupported.
1758 If the C<$quota> is unsupported, it is an empty scalar. If it is
1759 supported, it usually encodes the disk quota. If the C<$comment>
1760 field is unsupported, it is an empty scalar. If it is supported it
1761 usually encodes some administrative comment about the user. In some
1762 systems the $quota field may be C<$change> or C<$age>, fields that have
1763 to do with password aging. In some systems the C<$comment> field may
1764 be C<$class>. The C<$expire> field, if present, encodes the expiration
1765 period of the account or the password. For the availability and the
1766 exact meaning of these fields in your system, please consult your
1767 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1768 out from within Perl what your C<$quota> and C<$comment> fields mean
1769 and whether you have the C<$expire> field by using the C<Config> module
1770 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1771 and C<d_pwexpire>. Shadow password files are only supported if your
1772 vendor has implemented them in the intuitive fashion that calling the
1773 regular C library routines gets the shadow versions if you're running
1774 under privilege. Those that incorrectly implement a separate library
1775 call are not supported.
1777 The C<$members> value returned by I<getgr*()> is a space separated list of
1778 the login names of the members of the group.
1780 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1781 C, it will be returned to you via C<$?> if the function call fails. The
1782 C<@addrs> value returned by a successful call is a list of the raw
1783 addresses returned by the corresponding system library call. In the
1784 Internet domain, each address is four bytes long and you can unpack it
1785 by saying something like:
1787 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1789 The Socket library makes this slightly easier:
1792 $iaddr = inet_aton("127.1"); # or whatever address
1793 $name = gethostbyaddr($iaddr, AF_INET);
1795 # or going the other way
1796 $straddr = inet_ntoa($iaddr");
1798 If you get tired of remembering which element of the return list contains
1799 which return value, by-name interfaces are also provided in modules:
1800 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1801 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1802 normal built-in, replacing them with versions that return objects with
1803 the appropriate names for each field. For example:
1807 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1809 Even though it looks like they're the same method calls (uid),
1810 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1812 =item getsockname SOCKET
1814 Returns the packed sockaddr address of this end of the SOCKET connection.
1817 $mysockaddr = getsockname(SOCK);
1818 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1820 =item getsockopt SOCKET,LEVEL,OPTNAME
1822 Returns the socket option requested, or undef if there is an error.
1828 Returns the value of EXPR with filename expansions such as the
1829 standard Unix shell F</bin/csh> would do. This is the internal function
1830 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1831 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1832 discussed in more detail in L<perlop/"I/O Operators">.
1836 Converts a time as returned by the time function to a 9-element array
1837 with the time localized for the standard Greenwich time zone.
1838 Typically used as follows:
1841 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1844 All array elements are numeric, and come straight out of a struct tm.
1845 In particular this means that C<$mon> has the range C<0..11> and C<$wday>
1846 has the range C<0..6> with sunday as day C<0>. Also, C<$year> is the
1847 number of years since 1900, that is, C<$year> is C<123> in year 2023,
1848 I<not> simply the last two digits of the year. If you assume it is,
1849 then you create non-Y2K-compliant programs--and you wouldn't want to do
1852 If EXPR is omitted, does C<gmtime(time())>.
1854 In scalar context, returns the ctime(3) value:
1856 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1858 Also see the C<timegm()> function provided by the C<Time::Local> module,
1859 and the strftime(3) function available via the POSIX module.
1861 This scalar value is B<not> locale dependent (see L<perllocale>), but
1862 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1863 strftime(3) and mktime(3) functions available via the POSIX module. To
1864 get somewhat similar but locale dependent date strings, set up your
1865 locale environment variables appropriately (please see L<perllocale>)
1866 and try for example:
1868 use POSIX qw(strftime);
1869 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1871 Note that the C<%a> and C<%b> escapes, which represent the short forms
1872 of the day of the week and the month of the year, may not necessarily
1873 be three characters wide in all locales.
1881 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1882 execution there. It may not be used to go into any construct that
1883 requires initialization, such as a subroutine or a C<foreach> loop. It
1884 also can't be used to go into a construct that is optimized away,
1885 or to get out of a block or subroutine given to C<sort()>.
1886 It can be used to go almost anywhere else within the dynamic scope,
1887 including out of subroutines, but it's usually better to use some other
1888 construct such as C<last> or C<die()>. The author of Perl has never felt the
1889 need to use this form of C<goto> (in Perl, that is--C is another matter).
1891 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1892 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1893 necessarily recommended if you're optimizing for maintainability:
1895 goto ("FOO", "BAR", "GLARCH")[$i];
1897 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1898 named subroutine for the currently running subroutine. This is used by
1899 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1900 pretend that the other subroutine had been called in the first place
1901 (except that any modifications to C<@_> in the current subroutine are
1902 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1903 will be able to tell that this routine was called first.
1905 =item grep BLOCK LIST
1907 =item grep EXPR,LIST
1909 This is similar in spirit to, but not the same as, grep(1) and its
1910 relatives. In particular, it is not limited to using regular expressions.
1912 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1913 C<$_> to each element) and returns the list value consisting of those
1914 elements for which the expression evaluated to TRUE. In scalar
1915 context, returns the number of times the expression was TRUE.
1917 @foo = grep(!/^#/, @bar); # weed out comments
1921 @foo = grep {!/^#/} @bar; # weed out comments
1923 Note that, because C<$_> is a reference into the list value, it can
1924 be used to modify the elements of the array. While this is useful and
1925 supported, it can cause bizarre results if the LIST is not a named array.
1926 Similarly, grep returns aliases into the original list, much as a for
1927 loop's index variable aliases the list elements. That is, modifying an
1928 element of a list returned by grep (for example, in a C<foreach>, C<map()>
1929 or another C<grep()>) actually modifies the element in the original list.
1930 This is usually something to be avoided when writing clear code.
1932 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1938 Interprets EXPR as a hex string and returns the corresponding value.
1939 (To convert strings that might start with either 0, 0x, or 0b, see
1940 L</oct>.) If EXPR is omitted, uses C<$_>.
1942 print hex '0xAf'; # prints '175'
1943 print hex 'aF'; # same
1947 There is no builtin C<import()> function. It is just an ordinary
1948 method (subroutine) defined (or inherited) by modules that wish to export
1949 names to another module. The C<use()> function calls the C<import()> method
1950 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1952 =item index STR,SUBSTR,POSITION
1954 =item index STR,SUBSTR
1956 The index function searches for one string within another, but without
1957 the wildcard-like behavior of a full regular-expression pattern match.
1958 It returns the position of the first occurrence of SUBSTR in STR at
1959 or after POSITION. If POSITION is omitted, starts searching from the
1960 beginning of the string. The return value is based at C<0> (or whatever
1961 you've set the C<$[> variable to--but don't do that). If the substring
1962 is not found, returns one less than the base, ordinarily C<-1>.
1968 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1969 You should not use this function for rounding: one because it truncates
1970 towards C<0>, and two because machine representations of floating point
1971 numbers can sometimes produce counterintuitive results. For example,
1972 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
1973 because it's really more like -268.99999999999994315658 instead. Usually,
1974 the C<sprintf()>, C<printf()>, or the C<POSIX::floor> and C<POSIX::ceil>
1975 functions will serve you better than will int().
1977 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1979 Implements the ioctl(2) function. You'll probably first have to say
1981 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1983 to get the correct function definitions. If F<ioctl.ph> doesn't
1984 exist or doesn't have the correct definitions you'll have to roll your
1985 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1986 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1987 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1988 written depending on the FUNCTION--a pointer to the string value of SCALAR
1989 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1990 has no string value but does have a numeric value, that value will be
1991 passed rather than a pointer to the string value. To guarantee this to be
1992 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1993 functions are useful for manipulating the values of structures used by
1994 C<ioctl()>. The following example sets the erase character to DEL.
1998 die "NO TIOCGETP" if $@ || !$getp;
1999 $sgttyb_t = "ccccs"; # 4 chars and a short
2000 if (ioctl(STDIN,$getp,$sgttyb)) {
2001 @ary = unpack($sgttyb_t,$sgttyb);
2003 $sgttyb = pack($sgttyb_t,@ary);
2004 ioctl(STDIN,&TIOCSETP,$sgttyb)
2005 || die "Can't ioctl: $!";
2008 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
2010 if OS returns: then Perl returns:
2012 0 string "0 but true"
2013 anything else that number
2015 Thus Perl returns TRUE on success and FALSE on failure, yet you can
2016 still easily determine the actual value returned by the operating
2019 $retval = ioctl(...) || -1;
2020 printf "System returned %d\n", $retval;
2022 The special string "C<0> but true" is exempt from B<-w> complaints
2023 about improper numeric conversions.
2025 =item join EXPR,LIST
2027 Joins the separate strings of LIST into a single string with fields
2028 separated by the value of EXPR, and returns that new string. Example:
2030 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2036 Returns a list consisting of all the keys of the named hash. (In a
2037 scalar context, returns the number of keys.) The keys are returned in
2038 an apparently random order. The actual random order is subject to
2039 change in future versions of perl, but it is guaranteed to be the same
2040 order as either the C<values()> or C<each()> function produces (given
2041 that the hash has not been modified). As a side effect, it resets
2044 Here is yet another way to print your environment:
2047 @values = values %ENV;
2048 while ($#keys >= 0) {
2049 print pop(@keys), '=', pop(@values), "\n";
2052 or how about sorted by key:
2054 foreach $key (sort(keys %ENV)) {
2055 print $key, '=', $ENV{$key}, "\n";
2058 To sort a hash by value, you'll need to use a C<sort()> function.
2059 Here's a descending numeric sort of a hash by its values:
2061 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2062 printf "%4d %s\n", $hash{$key}, $key;
2065 As an lvalue C<keys()> allows you to increase the number of hash buckets
2066 allocated for the given hash. This can gain you a measure of efficiency if
2067 you know the hash is going to get big. (This is similar to pre-extending
2068 an array by assigning a larger number to $#array.) If you say
2072 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2073 in fact, since it rounds up to the next power of two. These
2074 buckets will be retained even if you do C<%hash = ()>, use C<undef
2075 %hash> if you want to free the storage while C<%hash> is still in scope.
2076 You can't shrink the number of buckets allocated for the hash using
2077 C<keys()> in this way (but you needn't worry about doing this by accident,
2078 as trying has no effect).
2080 See also C<each()>, C<values()> and C<sort()>.
2084 Sends a signal to a list of processes. The first element of
2085 the list must be the signal to send. Returns the number of
2086 processes successfully signaled.
2088 $cnt = kill 1, $child1, $child2;
2091 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2092 process groups instead of processes. (On System V, a negative I<PROCESS>
2093 number will also kill process groups, but that's not portable.) That
2094 means you usually want to use positive not negative signals. You may also
2095 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2101 The C<last> command is like the C<break> statement in C (as used in
2102 loops); it immediately exits the loop in question. If the LABEL is
2103 omitted, the command refers to the innermost enclosing loop. The
2104 C<continue> block, if any, is not executed:
2106 LINE: while (<STDIN>) {
2107 last LINE if /^$/; # exit when done with header
2111 C<last> cannot be used to exit a block which returns a value such as
2112 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2113 a grep() or map() operation.
2115 See also L</continue> for an illustration of how C<last>, C<next>, and
2122 Returns an lowercased version of EXPR. This is the internal function
2123 implementing the C<\L> escape in double-quoted strings.
2124 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2126 If EXPR is omitted, uses C<$_>.
2132 Returns the value of EXPR with the first character lowercased. This is
2133 the internal function implementing the C<\l> escape in double-quoted strings.
2134 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2136 If EXPR is omitted, uses C<$_>.
2142 Returns the length in characters of the value of EXPR. If EXPR is
2143 omitted, returns length of C<$_>. Note that this cannot be used on
2144 an entire array or hash to find out how many elements these have.
2145 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2147 =item link OLDFILE,NEWFILE
2149 Creates a new filename linked to the old filename. Returns TRUE for
2150 success, FALSE otherwise.
2152 =item listen SOCKET,QUEUESIZE
2154 Does the same thing that the listen system call does. Returns TRUE if
2155 it succeeded, FALSE otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2159 You really probably want to be using C<my()> instead, because C<local()> isn't
2160 what most people think of as "local". See L<perlsub/"Private Variables
2161 via my()"> for details.
2163 A local modifies the listed variables to be local to the enclosing
2164 block, file, or eval. If more than one value is listed, the list must
2165 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2166 for details, including issues with tied arrays and hashes.
2168 =item localtime EXPR
2170 Converts a time as returned by the time function to a 9-element array
2171 with the time analyzed for the local time zone. Typically used as
2175 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2178 All array elements are numeric, and come straight out of a struct tm.
2179 In particular this means that C<$mon> has the range C<0..11> and C<$wday>
2180 has the range C<0..6> with sunday as day C<0>. Also, C<$year> is the
2181 number of years since 1900, that is, C<$year> is C<123> in year 2023,
2182 and I<not> simply the last two digits of the year. If you assume it is,
2183 then you create non-Y2K-compliant programs--and you wouldn't want to do
2186 If EXPR is omitted, uses the current time (C<localtime(time)>).
2188 In scalar context, returns the ctime(3) value:
2190 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2192 This scalar value is B<not> locale dependent, see L<perllocale>, but
2193 instead a Perl builtin. Also see the C<Time::Local> module, and the
2194 strftime(3) and mktime(3) function available via the POSIX module. To
2195 get somewhat similar but locale dependent date strings, set up your
2196 locale environment variables appropriately (please see L<perllocale>)
2197 and try for example:
2199 use POSIX qw(strftime);
2200 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2202 Note that the C<%a> and C<%b>, the short forms of the day of the week
2203 and the month of the year, may not necessarily be three characters wide.
2209 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2210 returns log of C<$_>. To get the log of another base, use basic algebra:
2211 The base-N log of a number is is equal to the natural log of that number
2212 divided by the natural log of N. For example:
2216 return log($n)/log(10);
2219 See also L</exp> for the inverse operation.
2221 =item lstat FILEHANDLE
2227 Does the same thing as the C<stat()> function (including setting the
2228 special C<_> filehandle) but stats a symbolic link instead of the file
2229 the symbolic link points to. If symbolic links are unimplemented on
2230 your system, a normal C<stat()> is done.
2232 If EXPR is omitted, stats C<$_>.
2236 The match operator. See L<perlop>.
2238 =item map BLOCK LIST
2242 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2243 element) and returns the list value composed of the results of each such
2244 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2245 may produce zero, one, or more elements in the returned value.
2247 In scalar context, returns the total number of elements so generated.
2249 @chars = map(chr, @nums);
2251 translates a list of numbers to the corresponding characters. And
2253 %hash = map { getkey($_) => $_ } @array;
2255 is just a funny way to write
2258 foreach $_ (@array) {
2259 $hash{getkey($_)} = $_;
2262 Note that, because C<$_> is a reference into the list value, it can
2263 be used to modify the elements of the array. While this is useful and
2264 supported, it can cause bizarre results if the LIST is not a named array.
2265 Using a regular C<foreach> loop for this purpose would be clearer in
2266 most cases. See also L</grep> for an array composed of those items of
2267 the original list for which the BLOCK or EXPR evaluates to true.
2269 =item mkdir FILENAME,MODE
2271 Creates the directory specified by FILENAME, with permissions
2272 specified by MODE (as modified by C<umask>). If it succeeds it
2273 returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2275 In general, it is better to create directories with permissive MODEs,
2276 and let the user modify that with their C<umask>, than it is to supply
2277 a restrictive MODE and give the user no way to be more permissive.
2278 The exceptions to this rule are when the file or directory should be
2279 kept private (mail files, for instance). The perlfunc(1) entry on
2280 C<umask> discusses the choice of MODE in more detail.
2282 =item msgctl ID,CMD,ARG
2284 Calls the System V IPC function msgctl(2). You'll probably have to say
2288 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2289 then ARG must be a variable which will hold the returned C<msqid_ds>
2290 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2291 true" for zero, or the actual return value otherwise. See also
2292 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2294 =item msgget KEY,FLAGS
2296 Calls the System V IPC function msgget(2). Returns the message queue
2297 id, or the undefined value if there is an error. See also C<IPC::SysV>
2298 and C<IPC::SysV::Msg> documentation.
2300 =item msgsnd ID,MSG,FLAGS
2302 Calls the System V IPC function msgsnd to send the message MSG to the
2303 message queue ID. MSG must begin with the long integer message type,
2304 which may be created with C<pack("l", $type)>. Returns TRUE if
2305 successful, or FALSE if there is an error. See also C<IPC::SysV>
2306 and C<IPC::SysV::Msg> documentation.
2308 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2310 Calls the System V IPC function msgrcv to receive a message from
2311 message queue ID into variable VAR with a maximum message size of
2312 SIZE. Note that if a message is received, the message type will be
2313 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2314 size of the message type. Returns TRUE if successful, or FALSE if
2315 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2319 A C<my()> declares the listed variables to be local (lexically) to the
2320 enclosing block, file, or C<eval()>. If
2321 more than one value is listed, the list must be placed in parentheses. See
2322 L<perlsub/"Private Variables via my()"> for details.
2328 The C<next> command is like the C<continue> statement in C; it starts
2329 the next iteration of the loop:
2331 LINE: while (<STDIN>) {
2332 next LINE if /^#/; # discard comments
2336 Note that if there were a C<continue> block on the above, it would get
2337 executed even on discarded lines. If the LABEL is omitted, the command
2338 refers to the innermost enclosing loop.
2340 C<next> cannot be used to exit a block which returns a value such as
2341 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2342 a grep() or map() operation.
2344 See also L</continue> for an illustration of how C<last>, C<next>, and
2347 =item no Module LIST
2349 See the L</use> function, which C<no> is the opposite of.
2355 Interprets EXPR as an octal string and returns the corresponding
2356 value. (If EXPR happens to start off with C<0x>, interprets it as a
2357 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2358 binary string.) The following will handle decimal, binary, octal, and
2359 hex in the standard Perl or C notation:
2361 $val = oct($val) if $val =~ /^0/;
2363 If EXPR is omitted, uses C<$_>. This function is commonly used when
2364 a string such as C<644> needs to be converted into a file mode, for
2365 example. (Although perl will automatically convert strings into
2366 numbers as needed, this automatic conversion assumes base 10.)
2368 =item open FILEHANDLE,EXPR
2370 =item open FILEHANDLE
2372 Opens the file whose filename is given by EXPR, and associates it with
2373 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2374 name of the real filehandle wanted. If EXPR is omitted, the scalar
2375 variable of the same name as the FILEHANDLE contains the filename.
2376 (Note that lexical variables--those declared with C<my()>--will not work
2377 for this purpose; so if you're using C<my()>, specify EXPR in your call
2378 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2381 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2382 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2383 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2384 the file is opened for appending, again being created if necessary.
2385 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2386 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2387 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2388 file first. You can't usually use either read-write mode for updating
2389 textfiles, since they have variable length records. See the B<-i>
2390 switch in L<perlrun> for a better approach. The file is created with
2391 permissions of C<0666> modified by the process' C<umask> value.
2393 The prefix and the filename may be separated with spaces.
2394 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2395 C<'w+'>, C<'a'>, and C<'a+'>.
2397 If the filename begins with C<'|'>, the filename is interpreted as a
2398 command to which output is to be piped, and if the filename ends with a
2399 C<'|'>, the filename is interpreted as a command which pipes output to
2400 us. See L<perlipc/"Using open() for IPC">
2401 for more examples of this. (You are not allowed to C<open()> to a command
2402 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2403 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2405 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2406 nonzero upon success, the undefined value otherwise. If the C<open()>
2407 involved a pipe, the return value happens to be the pid of the
2410 If you're unfortunate enough to be running Perl on a system that
2411 distinguishes between text files and binary files (modern operating
2412 systems don't care), then you should check out L</binmode> for tips for
2413 dealing with this. The key distinction between systems that need C<binmode()>
2414 and those that don't is their text file formats. Systems like Unix, MacOS, and
2415 Plan9, which delimit lines with a single character, and which encode that
2416 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2418 When opening a file, it's usually a bad idea to continue normal execution
2419 if the request failed, so C<open()> is frequently used in connection with
2420 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2421 where you want to make a nicely formatted error message (but there are
2422 modules that can help with that problem)) you should always check
2423 the return value from opening a file. The infrequent exception is when
2424 working with an unopened filehandle is actually what you want to do.
2429 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2430 while (<ARTICLE>) {...
2432 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2433 # if the open fails, output is discarded
2435 open(DBASE, '+<dbase.mine') # open for update
2436 or die "Can't open 'dbase.mine' for update: $!";
2438 open(ARTICLE, "caesar <$article |") # decrypt article
2439 or die "Can't start caesar: $!";
2441 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2442 or die "Can't start sort: $!";
2444 # process argument list of files along with any includes
2446 foreach $file (@ARGV) {
2447 process($file, 'fh00');
2451 my($filename, $input) = @_;
2452 $input++; # this is a string increment
2453 unless (open($input, $filename)) {
2454 print STDERR "Can't open $filename: $!\n";
2459 while (<$input>) { # note use of indirection
2460 if (/^#include "(.*)"/) {
2461 process($1, $input);
2468 You may also, in the Bourne shell tradition, specify an EXPR beginning
2469 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2470 name of a filehandle (or file descriptor, if numeric) to be
2471 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2472 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2473 mode you specify should match the mode of the original filehandle.
2474 (Duping a filehandle does not take into account any existing contents of
2476 Here is a script that saves, redirects, and restores STDOUT and
2480 open(OLDOUT, ">&STDOUT");
2481 open(OLDERR, ">&STDERR");
2483 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2484 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2486 select(STDERR); $| = 1; # make unbuffered
2487 select(STDOUT); $| = 1; # make unbuffered
2489 print STDOUT "stdout 1\n"; # this works for
2490 print STDERR "stderr 1\n"; # subprocesses too
2495 open(STDOUT, ">&OLDOUT");
2496 open(STDERR, ">&OLDERR");
2498 print STDOUT "stdout 2\n";
2499 print STDERR "stderr 2\n";
2501 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2502 equivalent of C's C<fdopen()> of that file descriptor; this is more
2503 parsimonious of file descriptors. For example:
2505 open(FILEHANDLE, "<&=$fd")
2507 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2508 there is an implicit fork done, and the return value of open is the pid
2509 of the child within the parent process, and C<0> within the child
2510 process. (Use C<defined($pid)> to determine whether the open was successful.)
2511 The filehandle behaves normally for the parent, but i/o to that
2512 filehandle is piped from/to the STDOUT/STDIN of the child process.
2513 In the child process the filehandle isn't opened--i/o happens from/to
2514 the new STDOUT or STDIN. Typically this is used like the normal
2515 piped open when you want to exercise more control over just how the
2516 pipe command gets executed, such as when you are running setuid, and
2517 don't want to have to scan shell commands for metacharacters.
2518 The following pairs are more or less equivalent:
2520 open(FOO, "|tr '[a-z]' '[A-Z]'");
2521 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2523 open(FOO, "cat -n '$file'|");
2524 open(FOO, "-|") || exec 'cat', '-n', $file;
2526 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2528 NOTE: On any operation that may do a fork, any unflushed buffers remain
2529 unflushed in both processes, which means you may need to set C<$|> to
2530 avoid duplicate output. On systems that support a close-on-exec flag on
2531 files, the flag will be set for the newly opened file descriptor as
2532 determined by the value of $^F. See L<perlvar/$^F>.
2534 Closing any piped filehandle causes the parent process to wait for the
2535 child to finish, and returns the status value in C<$?>.
2537 The filename passed to open will have leading and trailing
2538 whitespace deleted, and the normal redirection characters
2539 honored. This property, known as "magic open",
2540 can often be used to good effect. A user could specify a filename of
2541 F<"rsh cat file |">, or you could change certain filenames as needed:
2543 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2544 open(FH, $filename) or die "Can't open $filename: $!";
2546 However, to open a file with arbitrary weird characters in it, it's
2547 necessary to protect any leading and trailing whitespace:
2549 $file =~ s#^(\s)#./$1#;
2550 open(FOO, "< $file\0");
2552 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2553 should use the C<sysopen()> function, which involves no such magic. This is
2554 another way to protect your filenames from interpretation. For example:
2557 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2558 or die "sysopen $path: $!";
2559 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2560 print HANDLE "stuff $$\n");
2562 print "File contains: ", <HANDLE>;
2564 Using the constructor from the C<IO::Handle> package (or one of its
2565 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2566 filehandles that have the scope of whatever variables hold references to
2567 them, and automatically close whenever and however you leave that scope:
2571 sub read_myfile_munged {
2573 my $handle = new IO::File;
2574 open($handle, "myfile") or die "myfile: $!";
2576 or return (); # Automatically closed here.
2577 mung $first or die "mung failed"; # Or here.
2578 return $first, <$handle> if $ALL; # Or here.
2582 See L</seek> for some details about mixing reading and writing.
2584 =item opendir DIRHANDLE,EXPR
2586 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2587 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2588 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2594 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2595 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2596 See L<utf8> for more about Unicode.
2598 =item pack TEMPLATE,LIST
2600 Takes an array or list of values and packs it into a binary structure,
2601 returning the string containing the structure. The TEMPLATE is a
2602 sequence of characters that give the order and type of values, as
2605 a A string with arbitrary binary data, will be null padded.
2606 A An ascii string, will be space padded.
2607 Z A null terminated (asciz) string, will be null padded.
2609 b A bit string (ascending bit order, like vec()).
2610 B A bit string (descending bit order).
2611 h A hex string (low nybble first).
2612 H A hex string (high nybble first).
2614 c A signed char value.
2615 C An unsigned char value. Only does bytes. See U for Unicode.
2617 s A signed short value.
2618 S An unsigned short value.
2619 (This 'short' is _exactly_ 16 bits, which may differ from
2620 what a local C compiler calls 'short'.)
2622 i A signed integer value.
2623 I An unsigned integer value.
2624 (This 'integer' is _at least_ 32 bits wide. Its exact
2625 size depends on what a local C compiler calls 'int',
2626 and may even be larger than the 'long' described in
2629 l A signed long value.
2630 L An unsigned long value.
2631 (This 'long' is _exactly_ 32 bits, which may differ from
2632 what a local C compiler calls 'long'.)
2634 n A short in "network" (big-endian) order.
2635 N A long in "network" (big-endian) order.
2636 v A short in "VAX" (little-endian) order.
2637 V A long in "VAX" (little-endian) order.
2638 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2639 _exactly_ 32 bits, respectively.)
2641 q A signed quad (64-bit) value.
2642 Q An unsigned quad value.
2643 (Available only if your system supports 64-bit integer values
2644 _and_ if Perl has been compiled to support those.
2645 Causes a fatal error otherwise.)
2647 f A single-precision float in the native format.
2648 d A double-precision float in the native format.
2650 p A pointer to a null-terminated string.
2651 P A pointer to a structure (fixed-length string).
2653 u A uuencoded string.
2654 U A Unicode character number. Encodes to UTF-8 internally.
2655 Works even if C<use utf8> is not in effect.
2657 w A BER compressed integer. Its bytes represent an unsigned
2658 integer in base 128, most significant digit first, with as
2659 few digits as possible. Bit eight (the high bit) is set
2660 on each byte except the last.
2664 @ Null fill to absolute position.
2666 The following rules apply:
2672 Each letter may optionally be followed by a number giving a repeat
2673 count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2674 C<"H">, and C<"P"> the pack function will gobble up that many values from
2675 the LIST. A C<*> for the repeat count means to use however many items are
2680 The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
2681 string of length count, padding with nulls or spaces as necessary. When
2682 unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2683 after the first null, and C<"a"> returns data verbatim.
2687 Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2691 The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2695 The C<"p"> type packs a pointer to a null-terminated string. You are
2696 responsible for ensuring the string is not a temporary value (which can
2697 potentially get deallocated before you get around to using the packed result).
2698 The C<"P"> type packs a pointer to a structure of the size indicated by the
2699 length. A NULL pointer is created if the corresponding value for C<"p"> or
2704 The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
2705 immediately followed by a C<"!"> to signify native shorts or longs--as
2706 you can see from above for example a bare C<"l"> does mean exactly 32
2707 bits, the native C<long> (as seen by the local C compiler) may be
2708 larger. This is an issue mainly in 64-bit platforms. You can see
2709 whether using C<"!"> makes any difference by
2711 print length(pack("s")), " ", length(pack("s!")), "\n";
2712 print length(pack("l")), " ", length(pack("l!")), "\n";
2714 C<"i!"> and C<"I!"> also work but only because of completeness;
2715 they are identical to C<"i"> and C<"I">.
2717 The actual sizes (in bytes) of native shorts, ints, and longs on
2718 the platform where Perl was built are also available via L<Config>:
2721 print $Config{shortsize}, "\n";
2722 print $Config{intsize}, "\n";
2723 print $Config{longsize}, "\n";
2727 The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
2728 are inherently non-portable between processors and operating systems
2729 because they obey the native byteorder and endianness. For example a
2730 4-byte integer 0x87654321 (2271560481 decimal) be ordered natively
2731 (arranged in and handled by the CPU registers) into bytes as
2733 0x12 0x34 0x56 0x78 # little-endian
2734 0x78 0x56 0x34 0x12 # big-endian
2736 Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
2737 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
2738 Power, and Cray are big-endian. MIPS can be either: Digital used it
2739 in little-endian mode, SGI uses it in big-endian mode.
2741 The names `big-endian' and `little-endian' are joking references to
2742 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
2743 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
2744 the egg-eating habits of the lilliputs.
2746 Some systems may even have weird byte orders such as
2751 You can see your system's preference with
2753 print join(" ", map { sprintf "%#02x", $_ }
2754 unpack("C*",pack("L",0x12345678))), "\n";
2756 The byteorder on the platform where Perl was built is also available
2760 print $Config{byteorder}, "\n";
2762 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
2763 and C<'87654321'> are big-endian.
2765 If you want portable packed integers use the formats C<"n">, C<"N">,
2766 C<"v">, and C<"V">, their byte endianness and size is known.
2770 Real numbers (floats and doubles) are in the native machine format only;
2771 due to the multiplicity of floating formats around, and the lack of a
2772 standard "network" representation, no facility for interchange has been
2773 made. This means that packed floating point data written on one machine
2774 may not be readable on another - even if both use IEEE floating point
2775 arithmetic (as the endian-ness of the memory representation is not part
2778 Note that Perl uses doubles internally for all numeric calculation, and
2779 converting from double into float and thence back to double again will
2780 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2787 $foo = pack("CCCC",65,66,67,68);
2789 $foo = pack("C4",65,66,67,68);
2791 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2792 # same thing with Unicode circled letters
2794 $foo = pack("ccxxcc",65,66,67,68);
2797 $foo = pack("s2",1,2);
2798 # "\1\0\2\0" on little-endian
2799 # "\0\1\0\2" on big-endian
2801 $foo = pack("a4","abcd","x","y","z");
2804 $foo = pack("aaaa","abcd","x","y","z");
2807 $foo = pack("a14","abcdefg");
2808 # "abcdefg\0\0\0\0\0\0\0"
2810 $foo = pack("i9pl", gmtime);
2811 # a real struct tm (on my system anyway)
2813 $utmp_template = "Z8 Z8 Z16 L";
2814 $utmp = pack($utmp_template, @utmp1);
2815 # a struct utmp (BSDish)
2817 @utmp2 = unpack($utmp_template, $utmp);
2818 # "@utmp1" eq "@utmp2"
2821 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2824 The same template may generally also be used in unpack().
2828 =item package NAMESPACE
2830 Declares the compilation unit as being in the given namespace. The scope
2831 of the package declaration is from the declaration itself through the end
2832 of the enclosing block, file, or eval (the same as the C<my()> operator).
2833 All further unqualified dynamic identifiers will be in this namespace.
2834 A package statement affects only dynamic variables--including those
2835 you've used C<local()> on--but I<not> lexical variables, which are created
2836 with C<my()>. Typically it would be the first declaration in a file to
2837 be included by the C<require> or C<use> operator. You can switch into a
2838 package in more than one place; it merely influences which symbol table
2839 is used by the compiler for the rest of that block. You can refer to
2840 variables and filehandles in other packages by prefixing the identifier
2841 with the package name and a double colon: C<$Package::Variable>.
2842 If the package name is null, the C<main> package as assumed. That is,
2843 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
2844 still seen in older code).
2846 If NAMESPACE is omitted, then there is no current package, and all
2847 identifiers must be fully qualified or lexicals. This is stricter
2848 than C<use strict>, since it also extends to function names.
2850 See L<perlmod/"Packages"> for more information about packages, modules,
2851 and classes. See L<perlsub> for other scoping issues.
2853 =item pipe READHANDLE,WRITEHANDLE
2855 Opens a pair of connected pipes like the corresponding system call.
2856 Note that if you set up a loop of piped processes, deadlock can occur
2857 unless you are very careful. In addition, note that Perl's pipes use
2858 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2859 after each command, depending on the application.
2861 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2862 for examples of such things.
2864 On systems that support a close-on-exec flag on files, the flag will be set
2865 for the newly opened file descriptors as determined by the value of $^F.
2872 Pops and returns the last value of the array, shortening the array by
2873 one element. Has a similar effect to
2875 $tmp = $ARRAY[$#ARRAY--];
2877 If there are no elements in the array, returns the undefined value.
2878 If ARRAY is omitted, pops the C<@ARGV> array in the main program, and
2879 the C<@_> array in subroutines, just like C<shift()>.
2885 Returns the offset of where the last C<m//g> search left off for the variable
2886 is in question (C<$_> is used when the variable is not specified). May be
2887 modified to change that offset. Such modification will also influence
2888 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2891 =item print FILEHANDLE LIST
2897 Prints a string or a comma-separated list of strings. Returns TRUE
2898 if successful. FILEHANDLE may be a scalar variable name, in which case
2899 the variable contains the name of or a reference to the filehandle, thus
2900 introducing one level of indirection. (NOTE: If FILEHANDLE is a variable
2901 and the next token is a term, it may be misinterpreted as an operator
2902 unless you interpose a C<+> or put parentheses around the arguments.)
2903 If FILEHANDLE is omitted, prints by default to standard output (or to the
2904 last selected output channel--see L</select>). If LIST is also omitted,
2905 prints C<$_> to the currently selected output channel. To set the default
2906 output channel to something other than STDOUT use the select operation.
2907 Note that, because print takes a LIST, anything in the LIST is evaluated
2908 in list context, and any subroutine that you call will have one or
2909 more of its expressions evaluated in list context. Also be careful
2910 not to follow the print keyword with a left parenthesis unless you want
2911 the corresponding right parenthesis to terminate the arguments to the
2912 print--interpose a C<+> or put parentheses around all the arguments.
2914 Note that if you're storing FILEHANDLES in an array or other expression,
2915 you will have to use a block returning its value instead:
2917 print { $files[$i] } "stuff\n";
2918 print { $OK ? STDOUT : STDERR } "stuff\n";
2920 =item printf FILEHANDLE FORMAT, LIST
2922 =item printf FORMAT, LIST
2924 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2925 (the output record separator) is not appended. The first argument
2926 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2927 in effect, the character used for the decimal point in formatted real numbers
2928 is affected by the LC_NUMERIC locale. See L<perllocale>.
2930 Don't fall into the trap of using a C<printf()> when a simple
2931 C<print()> would do. The C<print()> is more efficient and less
2934 =item prototype FUNCTION
2936 Returns the prototype of a function as a string (or C<undef> if the
2937 function has no prototype). FUNCTION is a reference to, or the name of,
2938 the function whose prototype you want to retrieve.
2940 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
2941 name for Perl builtin. If the builtin is not I<overridable> (such as
2942 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2943 C<system()>) returns C<undef> because the builtin does not really behave
2944 like a Perl function. Otherwise, the string describing the equivalent
2945 prototype is returned.
2947 =item push ARRAY,LIST
2949 Treats ARRAY as a stack, and pushes the values of LIST
2950 onto the end of ARRAY. The length of ARRAY increases by the length of
2951 LIST. Has the same effect as
2954 $ARRAY[++$#ARRAY] = $value;
2957 but is more efficient. Returns the new number of elements in the array.
2969 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
2971 =item quotemeta EXPR
2975 Returns the value of EXPR with all non-alphanumeric
2976 characters backslashed. (That is, all characters not matching
2977 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2978 returned string, regardless of any locale settings.)
2979 This is the internal function implementing
2980 the C<\Q> escape in double-quoted strings.
2982 If EXPR is omitted, uses C<$_>.
2988 Returns a random fractional number greater than or equal to C<0> and less
2989 than the value of EXPR. (EXPR should be positive.) If EXPR is
2990 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2991 C<srand()> has already been called. See also C<srand()>.
2993 (Note: If your rand function consistently returns numbers that are too
2994 large or too small, then your version of Perl was probably compiled
2995 with the wrong number of RANDBITS.)
2997 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2999 =item read FILEHANDLE,SCALAR,LENGTH
3001 Attempts to read LENGTH bytes of data into variable SCALAR from the
3002 specified FILEHANDLE. Returns the number of bytes actually read,
3003 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3004 or shrunk to the length actually read. An OFFSET may be specified to
3005 place the read data at some other place than the beginning of the
3006 string. This call is actually implemented in terms of stdio's fread(3)
3007 call. To get a true read(2) system call, see C<sysread()>.
3009 =item readdir DIRHANDLE
3011 Returns the next directory entry for a directory opened by C<opendir()>.
3012 If used in list context, returns all the rest of the entries in the
3013 directory. If there are no more entries, returns an undefined value in
3014 scalar context or a null list in list context.
3016 If you're planning to filetest the return values out of a C<readdir()>, you'd
3017 better prepend the directory in question. Otherwise, because we didn't
3018 C<chdir()> there, it would have been testing the wrong file.
3020 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3021 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3026 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3027 context, each call reads and returns the next line, until end-of-file is
3028 reached, whereupon the subsequent call returns undef. In list context,
3029 reads until end-of-file is reached and returns a list of lines. Note that
3030 the notion of "line" used here is however you may have defined it
3031 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3033 When C<$/> is set to C<undef>, when readline() is in scalar
3034 context (i.e. file slurp mode), and when an empty file is read, it
3035 returns C<''> the first time, followed by C<undef> subsequently.
3037 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3038 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3039 operator is discussed in more detail in L<perlop/"I/O Operators">.
3042 $line = readline(*STDIN); # same thing
3048 Returns the value of a symbolic link, if symbolic links are
3049 implemented. If not, gives a fatal error. If there is some system
3050 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3051 omitted, uses C<$_>.
3055 EXPR is executed as a system command.
3056 The collected standard output of the command is returned.
3057 In scalar context, it comes back as a single (potentially
3058 multi-line) string. In list context, returns a list of lines
3059 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3060 This is the internal function implementing the C<qx/EXPR/>
3061 operator, but you can use it directly. The C<qx/EXPR/>
3062 operator is discussed in more detail in L<perlop/"I/O Operators">.
3064 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3066 Receives a message on a socket. Attempts to receive LENGTH bytes of
3067 data into variable SCALAR from the specified SOCKET filehandle.
3068 Actually does a C C<recvfrom()>, so that it can return the address of the
3069 sender. Returns the undefined value if there's an error. SCALAR will
3070 be grown or shrunk to the length actually read. Takes the same flags
3071 as the system call of the same name.
3072 See L<perlipc/"UDP: Message Passing"> for examples.
3078 The C<redo> command restarts the loop block without evaluating the
3079 conditional again. The C<continue> block, if any, is not executed. If
3080 the LABEL is omitted, the command refers to the innermost enclosing
3081 loop. This command is normally used by programs that want to lie to
3082 themselves about what was just input:
3084 # a simpleminded Pascal comment stripper
3085 # (warning: assumes no { or } in strings)
3086 LINE: while (<STDIN>) {
3087 while (s|({.*}.*){.*}|$1 |) {}
3092 if (/}/) { # end of comment?
3101 C<redo> cannot be used to retry a block which returns a value such as
3102 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3103 a grep() or map() operation.
3105 See also L</continue> for an illustration of how C<last>, C<next>, and
3112 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
3113 is not specified, C<$_> will be used. The value returned depends on the
3114 type of thing the reference is a reference to.
3115 Builtin types include:
3124 If the referenced object has been blessed into a package, then that package
3125 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
3127 if (ref($r) eq "HASH") {
3128 print "r is a reference to a hash.\n";
3131 print "r is not a reference at all.\n";
3133 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3134 print "r is a reference to something that isa hash.\n";
3137 See also L<perlref>.
3139 =item rename OLDNAME,NEWNAME
3141 Changes the name of a file. Returns C<1> for success, C<0> otherwise.
3142 Behavior of this function varies wildly depending on your system
3143 implementation. For example, it will usually not work across file system
3144 boundaries, even though the system I<mv> command sometimes compensates
3145 for this. Other restrictions include whether it works on directories,
3146 open files, or pre-existing files. Check L<perlport> and either the
3147 rename(2) manpage or equivalent system documentation for details.
3153 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3154 supplied. If EXPR is numeric, demands that the current version of Perl
3155 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3157 Otherwise, demands that a library file be included if it hasn't already
3158 been included. The file is included via the do-FILE mechanism, which is
3159 essentially just a variety of C<eval()>. Has semantics similar to the following
3164 return 1 if $INC{$filename};
3165 my($realfilename,$result);
3167 foreach $prefix (@INC) {
3168 $realfilename = "$prefix/$filename";
3169 if (-f $realfilename) {
3170 $result = do $realfilename;
3174 die "Can't find $filename in \@INC";
3177 die "$filename did not return true value" unless $result;
3178 $INC{$filename} = $realfilename;
3182 Note that the file will not be included twice under the same specified
3183 name. The file must return TRUE as the last statement to indicate
3184 successful execution of any initialization code, so it's customary to
3185 end such a file with "C<1;>" unless you're sure it'll return TRUE
3186 otherwise. But it's better just to put the "C<1;>", in case you add more
3189 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3190 replaces "F<::>" with "F</>" in the filename for you,
3191 to make it easy to load standard modules. This form of loading of
3192 modules does not risk altering your namespace.
3194 In other words, if you try this:
3196 require Foo::Bar; # a splendid bareword
3198 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3199 directories specified in the C<@INC> array.
3201 But if you try this:
3203 $class = 'Foo::Bar';
3204 require $class; # $class is not a bareword
3206 require "Foo::Bar"; # not a bareword because of the ""
3208 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3209 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3211 eval "require $class";
3213 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3219 Generally used in a C<continue> block at the end of a loop to clear
3220 variables and reset C<??> searches so that they work again. The
3221 expression is interpreted as a list of single characters (hyphens
3222 allowed for ranges). All variables and arrays beginning with one of
3223 those letters are reset to their pristine state. If the expression is
3224 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3225 only variables or searches in the current package. Always returns
3228 reset 'X'; # reset all X variables
3229 reset 'a-z'; # reset lower case variables
3230 reset; # just reset ?one-time? searches
3232 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3233 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3234 variables--lexical variables are unaffected, but they clean themselves
3235 up on scope exit anyway, so you'll probably want to use them instead.
3242 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
3243 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3244 context, depending on how the return value will be used, and the context
3245 may vary from one execution to the next (see C<wantarray()>). If no EXPR
3246 is given, returns an empty list in list context, the undefined value in
3247 scalar context, and (of course) nothing at all in a void context.
3249 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3250 or do FILE will automatically return the value of the last expression
3255 In list context, returns a list value consisting of the elements
3256 of LIST in the opposite order. In scalar context, concatenates the
3257 elements of LIST and returns a string value with all characters
3258 in the opposite order.
3260 print reverse <>; # line tac, last line first
3262 undef $/; # for efficiency of <>
3263 print scalar reverse <>; # character tac, last line tsrif
3265 This operator is also handy for inverting a hash, although there are some
3266 caveats. If a value is duplicated in the original hash, only one of those
3267 can be represented as a key in the inverted hash. Also, this has to
3268 unwind one hash and build a whole new one, which may take some time
3269 on a large hash, such as from a DBM file.
3271 %by_name = reverse %by_address; # Invert the hash
3273 =item rewinddir DIRHANDLE
3275 Sets the current position to the beginning of the directory for the
3276 C<readdir()> routine on DIRHANDLE.
3278 =item rindex STR,SUBSTR,POSITION
3280 =item rindex STR,SUBSTR
3282 Works just like index() except that it returns the position of the LAST
3283 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3284 last occurrence at or before that position.
3286 =item rmdir FILENAME
3290 Deletes the directory specified by FILENAME if that directory is empty. If it
3291 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
3292 FILENAME is omitted, uses C<$_>.
3296 The substitution operator. See L<perlop>.
3300 Forces EXPR to be interpreted in scalar context and returns the value
3303 @counts = ( scalar @a, scalar @b, scalar @c );
3305 There is no equivalent operator to force an expression to
3306 be interpolated in list context because in practice, this is never
3307 needed. If you really wanted to do so, however, you could use
3308 the construction C<@{[ (some expression) ]}>, but usually a simple
3309 C<(some expression)> suffices.
3311 Wince C<scalar> is unary operator, if you accidentally use for EXPR a
3312 parenthesized list, this behaves as a scalar comma expression, evaluating
3313 all but the last element in void context and returning the final element
3314 evaluated in scalar context. This is seldom what you want.
3316 The following single statement:
3318 print uc(scalar(&foo,$bar)),$baz;
3320 is the moral equivalent of these two:
3323 print(uc($bar),$baz);
3325 See L<perlop> for more details on unary operators and the comma operator.
3327 =item seek FILEHANDLE,POSITION,WHENCE
3329 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
3330 FILEHANDLE may be an expression whose value gives the name of the
3331 filehandle. The values for WHENCE are C<0> to set the new position to
3332 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3333 set it to EOF plus POSITION (typically negative). For WHENCE you may
3334 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3335 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3337 If you want to position file for C<sysread()> or C<syswrite()>, don't use
3338 C<seek()> -- buffering makes its effect on the file's system position
3339 unpredictable and non-portable. Use C<sysseek()> instead.
3341 Due to the rules and rigors of ANSI C, on some systems you have to do a
3342 seek whenever you switch between reading and writing. Amongst other
3343 things, this may have the effect of calling stdio's clearerr(3).
3344 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3348 This is also useful for applications emulating C<tail -f>. Once you hit
3349 EOF on your read, and then sleep for a while, you might have to stick in a
3350 seek() to reset things. The C<seek()> doesn't change the current position,
3351 but it I<does> clear the end-of-file condition on the handle, so that the
3352 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3354 If that doesn't work (some stdios are particularly cantankerous), then
3355 you may need something more like this:
3358 for ($curpos = tell(FILE); $_ = <FILE>;
3359 $curpos = tell(FILE)) {
3360 # search for some stuff and put it into files
3362 sleep($for_a_while);
3363 seek(FILE, $curpos, 0);
3366 =item seekdir DIRHANDLE,POS
3368 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3369 must be a value returned by C<telldir()>. Has the same caveats about
3370 possible directory compaction as the corresponding system library
3373 =item select FILEHANDLE
3377 Returns the currently selected filehandle. Sets the current default
3378 filehandle for output, if FILEHANDLE is supplied. This has two
3379 effects: first, a C<write()> or a C<print()> without a filehandle will
3380 default to this FILEHANDLE. Second, references to variables related to
3381 output will refer to this output channel. For example, if you have to
3382 set the top of form format for more than one output channel, you might
3390 FILEHANDLE may be an expression whose value gives the name of the
3391 actual filehandle. Thus:
3393 $oldfh = select(STDERR); $| = 1; select($oldfh);
3395 Some programmers may prefer to think of filehandles as objects with
3396 methods, preferring to write the last example as:
3399 STDERR->autoflush(1);
3401 =item select RBITS,WBITS,EBITS,TIMEOUT
3403 This calls the select(2) system call with the bit masks specified, which
3404 can be constructed using C<fileno()> and C<vec()>, along these lines:
3406 $rin = $win = $ein = '';
3407 vec($rin,fileno(STDIN),1) = 1;
3408 vec($win,fileno(STDOUT),1) = 1;
3411 If you want to select on many filehandles you might wish to write a
3415 my(@fhlist) = split(' ',$_[0]);
3418 vec($bits,fileno($_),1) = 1;
3422 $rin = fhbits('STDIN TTY SOCK');
3426 ($nfound,$timeleft) =
3427 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3429 or to block until something becomes ready just do this
3431 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3433 Most systems do not bother to return anything useful in C<$timeleft>, so
3434 calling select() in scalar context just returns C<$nfound>.
3436 Any of the bit masks can also be undef. The timeout, if specified, is
3437 in seconds, which may be fractional. Note: not all implementations are
3438 capable of returning theC<$timeleft>. If not, they always return
3439 C<$timeleft> equal to the supplied C<$timeout>.
3441 You can effect a sleep of 250 milliseconds this way:
3443 select(undef, undef, undef, 0.25);
3445 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3446 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3447 then only on POSIX systems. You have to use C<sysread()> instead.
3449 =item semctl ID,SEMNUM,CMD,ARG
3451 Calls the System V IPC function C<semctl()>. You'll probably have to say
3455 first to get the correct constant definitions. If CMD is IPC_STAT or
3456 GETALL, then ARG must be a variable which will hold the returned
3457 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3458 undefined value for error, "C<0> but true" for zero, or the actual return
3459 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3461 =item semget KEY,NSEMS,FLAGS
3463 Calls the System V IPC function semget. Returns the semaphore id, or
3464 the undefined value if there is an error. See also C<IPC::SysV> and
3465 C<IPC::SysV::Semaphore> documentation.
3467 =item semop KEY,OPSTRING
3469 Calls the System V IPC function semop to perform semaphore operations
3470 such as signaling and waiting. OPSTRING must be a packed array of
3471 semop structures. Each semop structure can be generated with
3472 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3473 operations is implied by the length of OPSTRING. Returns TRUE if
3474 successful, or FALSE if there is an error. As an example, the
3475 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3477 $semop = pack("sss", $semnum, -1, 0);
3478 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3480 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3481 and C<IPC::SysV::Semaphore> documentation.
3483 =item send SOCKET,MSG,FLAGS,TO
3485 =item send SOCKET,MSG,FLAGS
3487 Sends a message on a socket. Takes the same flags as the system call
3488 of the same name. On unconnected sockets you must specify a
3489 destination to send TO, in which case it does a C C<sendto()>. Returns
3490 the number of characters sent, or the undefined value if there is an
3491 error. The C system call sendmsg(2) is currently unimplemented.
3492 See L<perlipc/"UDP: Message Passing"> for examples.
3494 =item setpgrp PID,PGRP
3496 Sets the current process group for the specified PID, C<0> for the current
3497 process. Will produce a fatal error if used on a machine that doesn't
3498 implement setpgrp(2). If the arguments are omitted, it defaults to
3499 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3500 arguments, so only C<setpgrp(0,0)> is portable. See also C<POSIX::setsid()>.
3502 =item setpriority WHICH,WHO,PRIORITY
3504 Sets the current priority for a process, a process group, or a user.
3505 (See setpriority(2).) Will produce a fatal error if used on a machine
3506 that doesn't implement setpriority(2).
3508 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3510 Sets the socket option requested. Returns undefined if there is an
3511 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3518 Shifts the first value of the array off and returns it, shortening the
3519 array by 1 and moving everything down. If there are no elements in the
3520 array, returns the undefined value. If ARRAY is omitted, shifts the
3521 C<@_> array within the lexical scope of subroutines and formats, and the
3522 C<@ARGV> array at file scopes or within the lexical scopes established by
3523 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3524 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3525 same thing to the left end of an array that C<pop()> and C<push()> do to the
3528 =item shmctl ID,CMD,ARG
3530 Calls the System V IPC function shmctl. You'll probably have to say
3534 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3535 then ARG must be a variable which will hold the returned C<shmid_ds>
3536 structure. Returns like ioctl: the undefined value for error, "C<0> but
3537 true" for zero, or the actual return value otherwise.
3538 See also C<IPC::SysV> documentation.
3540 =item shmget KEY,SIZE,FLAGS
3542 Calls the System V IPC function shmget. Returns the shared memory
3543 segment id, or the undefined value if there is an error.
3544 See also C<IPC::SysV> documentation.
3546 =item shmread ID,VAR,POS,SIZE
3548 =item shmwrite ID,STRING,POS,SIZE
3550 Reads or writes the System V shared memory segment ID starting at
3551 position POS for size SIZE by attaching to it, copying in/out, and
3552 detaching from it. When reading, VAR must be a variable that will
3553 hold the data read. When writing, if STRING is too long, only SIZE
3554 bytes are used; if STRING is too short, nulls are written to fill out
3555 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3556 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3559 =item shutdown SOCKET,HOW
3561 Shuts down a socket connection in the manner indicated by HOW, which
3562 has the same interpretation as in the system call of the same name.
3564 shutdown(SOCKET, 0); # I/we have stopped reading data
3565 shutdown(SOCKET, 1); # I/we have stopped writing data
3566 shutdown(SOCKET, 2); # I/we have stopped using this socket
3568 This is useful with sockets when you want to tell the other
3569 side you're done writing but not done reading, or vice versa.
3570 It's also a more insistent form of close because it also
3571 disables the filedescriptor in any forked copies in other
3578 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3579 returns sine of C<$_>.
3581 For the inverse sine operation, you may use the C<POSIX::asin()>
3582 function, or use this relation:
3584 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3590 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3591 May be interrupted if the process receives a signal such as C<SIGALRM>.
3592 Returns the number of seconds actually slept. You probably cannot
3593 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3596 On some older systems, it may sleep up to a full second less than what
3597 you requested, depending on how it counts seconds. Most modern systems
3598 always sleep the full amount. They may appear to sleep longer than that,
3599 however, because your process might not be scheduled right away in a
3600 busy multitasking system.
3602 For delays of finer granularity than one second, you may use Perl's
3603 C<syscall()> interface to access setitimer(2) if your system supports it,
3604 or else see L</select> above.
3606 See also the POSIX module's C<sigpause()> function.
3608 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3610 Opens a socket of the specified kind and attaches it to filehandle
3611 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3612 system call of the same name. You should "C<use Socket;>" first to get
3613 the proper definitions imported. See the examples in L<perlipc/"Sockets: Client/Server Communication">.
3615 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3617 Creates an unnamed pair of sockets in the specified domain, of the
3618 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3619 for the system call of the same name. If unimplemented, yields a fatal
3620 error. Returns TRUE if successful.
3622 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3623 to C<pipe(Rdr, Wtr)> is essentially:
3626 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3627 shutdown(Rdr, 1); # no more writing for reader
3628 shutdown(Wtr, 0); # no more reading for writer
3630 See L<perlipc> for an example of socketpair use.
3632 =item sort SUBNAME LIST
3634 =item sort BLOCK LIST
3638 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3639 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3640 specified, it gives the name of a subroutine that returns an integer
3641 less than, equal to, or greater than C<0>, depending on how the elements
3642 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3643 operators are extremely useful in such routines.) SUBNAME may be a
3644 scalar variable name (unsubscripted), in which case the value provides
3645 the name of (or a reference to) the actual subroutine to use. In place
3646 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3649 In the interests of efficiency the normal calling code for subroutines is
3650 bypassed, with the following effects: the subroutine may not be a
3651 recursive subroutine, and the two elements to be compared are passed into
3652 the subroutine not via C<@_> but as the package global variables C<$a> and
3653 C<$b> (see example below). They are passed by reference, so don't
3654 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3656 You also cannot exit out of the sort block or subroutine using any of the
3657 loop control operators described in L<perlsyn> or with C<goto()>.
3659 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3660 current collation locale. See L<perllocale>.
3665 @articles = sort @files;
3667 # same thing, but with explicit sort routine
3668 @articles = sort {$a cmp $b} @files;
3670 # now case-insensitively
3671 @articles = sort {uc($a) cmp uc($b)} @files;
3673 # same thing in reversed order
3674 @articles = sort {$b cmp $a} @files;
3676 # sort numerically ascending
3677 @articles = sort {$a <=> $b} @files;
3679 # sort numerically descending
3680 @articles = sort {$b <=> $a} @files;
3682 # sort using explicit subroutine name
3684 $age{$a} <=> $age{$b}; # presuming numeric
3686 @sortedclass = sort byage @class;
3688 # this sorts the %age hash by value instead of key
3689 # using an in-line function
3690 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3692 sub backwards { $b cmp $a; }
3693 @harry = ('dog','cat','x','Cain','Abel');
3694 @george = ('gone','chased','yz','Punished','Axed');
3696 # prints AbelCaincatdogx
3697 print sort backwards @harry;
3698 # prints xdogcatCainAbel
3699 print sort @george, 'to', @harry;
3700 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3702 # inefficiently sort by descending numeric compare using
3703 # the first integer after the first = sign, or the
3704 # whole record case-insensitively otherwise
3707 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3712 # same thing, but much more efficiently;
3713 # we'll build auxiliary indices instead
3717 push @nums, /=(\d+)/;
3722 $nums[$b] <=> $nums[$a]
3724 $caps[$a] cmp $caps[$b]
3728 # same thing using a Schwartzian Transform (no temps)
3729 @new = map { $_->[0] }
3730 sort { $b->[1] <=> $a->[1]
3733 } map { [$_, /=(\d+)/, uc($_)] } @old;
3735 If you're using strict, you I<MUST NOT> declare C<$a>
3736 and C<$b> as lexicals. They are package globals. That means
3737 if you're in the C<main> package, it's
3739 @articles = sort {$main::b <=> $main::a} @files;
3743 @articles = sort {$::b <=> $::a} @files;
3745 but if you're in the C<FooPack> package, it's
3747 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3749 The comparison function is required to behave. If it returns
3750 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3751 sometimes saying the opposite, for example) the results are not
3754 =item splice ARRAY,OFFSET,LENGTH,LIST
3756 =item splice ARRAY,OFFSET,LENGTH
3758 =item splice ARRAY,OFFSET
3760 Removes the elements designated by OFFSET and LENGTH from an array, and
3761 replaces them with the elements of LIST, if any. In list context,
3762 returns the elements removed from the array. In scalar context,
3763 returns the last element removed, or C<undef> if no elements are
3764 removed. The array grows or shrinks as necessary.
3765 If OFFSET is negative then it start that far from the end of the array.
3766 If LENGTH is omitted, removes everything from OFFSET onward.
3767 If LENGTH is negative, leave that many elements off the end of the array.
3768 The following equivalences hold (assuming C<$[ == 0>):
3770 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3771 pop(@a) splice(@a,-1)
3772 shift(@a) splice(@a,0,1)
3773 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3774 $a[$x] = $y splice(@a,$x,1,$y)
3776 Example, assuming array lengths are passed before arrays:
3778 sub aeq { # compare two list values
3779 my(@a) = splice(@_,0,shift);
3780 my(@b) = splice(@_,0,shift);
3781 return 0 unless @a == @b; # same len?
3783 return 0 if pop(@a) ne pop(@b);
3787 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3789 =item split /PATTERN/,EXPR,LIMIT
3791 =item split /PATTERN/,EXPR
3793 =item split /PATTERN/
3797 Splits a string into an array of strings, and returns it. By default,
3798 empty leading fields are preserved, and empty trailing ones are deleted.
3800 If not in list context, returns the number of fields found and splits into
3801 the C<@_> array. (In list context, you can force the split into C<@_> by
3802 using C<??> as the pattern delimiters, but it still returns the list
3803 value.) The use of implicit split to C<@_> is deprecated, however, because
3804 it clobbers your subroutine arguments.
3806 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3807 splits on whitespace (after skipping any leading whitespace). Anything
3808 matching PATTERN is taken to be a delimiter separating the fields. (Note
3809 that the delimiter may be longer than one character.)
3811 If LIMIT is specified and positive, splits into no more than that
3812 many fields (though it may split into fewer). If LIMIT is unspecified
3813 or zero, trailing null fields are stripped (which potential users
3814 of C<pop()> would do well to remember). If LIMIT is negative, it is
3815 treated as if an arbitrarily large LIMIT had been specified.
3817 A pattern matching the null string (not to be confused with
3818 a null pattern C<//>, which is just one member of the set of patterns
3819 matching a null string) will split the value of EXPR into separate
3820 characters at each point it matches that way. For example:
3822 print join(':', split(/ */, 'hi there'));
3824 produces the output 'h:i:t:h:e:r:e'.
3826 The LIMIT parameter can be used to split a line partially
3828 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3830 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3831 one larger than the number of variables in the list, to avoid
3832 unnecessary work. For the list above LIMIT would have been 4 by
3833 default. In time critical applications it behooves you not to split
3834 into more fields than you really need.
3836 If the PATTERN contains parentheses, additional array elements are
3837 created from each matching substring in the delimiter.
3839 split(/([,-])/, "1-10,20", 3);
3841 produces the list value
3843 (1, '-', 10, ',', 20)
3845 If you had the entire header of a normal Unix email message in C<$header>,
3846 you could split it up into fields and their values this way:
3848 $header =~ s/\n\s+/ /g; # fix continuation lines
3849 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3851 The pattern C</PATTERN/> may be replaced with an expression to specify
3852 patterns that vary at runtime. (To do runtime compilation only once,
3853 use C</$variable/o>.)
3855 As a special case, specifying a PATTERN of space (C<' '>) will split on
3856 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3857 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3858 will give you as many null initial fields as there are leading spaces.
3859 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3860 whitespace produces a null first field. A C<split()> with no arguments
3861 really does a C<split(' ', $_)> internally.
3865 open(PASSWD, '/etc/passwd');
3867 ($login, $passwd, $uid, $gid,
3868 $gcos, $home, $shell) = split(/:/);
3872 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3873 L</chomp>, and L</join>.)
3875 =item sprintf FORMAT, LIST
3877 Returns a string formatted by the usual C<printf()> conventions of the
3878 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3879 on your system for an explanation of the general principles.
3881 Perl does its own C<sprintf()> formatting -- it emulates the C
3882 function C<sprintf()>, but it doesn't use it (except for floating-point
3883 numbers, and even then only the standard modifiers are allowed). As a
3884 result, any non-standard extensions in your local C<sprintf()> are not
3885 available from Perl.
3887 Perl's C<sprintf()> permits the following universally-known conversions:
3890 %c a character with the given number
3892 %d a signed integer, in decimal
3893 %u an unsigned integer, in decimal
3894 %o an unsigned integer, in octal
3895 %x an unsigned integer, in hexadecimal
3896 %e a floating-point number, in scientific notation
3897 %f a floating-point number, in fixed decimal notation
3898 %g a floating-point number, in %e or %f notation
3900 In addition, Perl permits the following widely-supported conversions:
3902 %X like %x, but using upper-case letters
3903 %E like %e, but using an upper-case "E"
3904 %G like %g, but with an upper-case "E" (if applicable)
3905 %b an unsigned integer, in binary
3906 %p a pointer (outputs the Perl value's address in hexadecimal)
3907 %n special: *stores* the number of characters output so far
3908 into the next variable in the parameter list
3910 Finally, for backward (and we do mean "backward") compatibility, Perl
3911 permits these unnecessary but widely-supported conversions:
3914 %D a synonym for %ld
3915 %U a synonym for %lu
3916 %O a synonym for %lo
3919 Perl permits the following universally-known flags between the C<%>
3920 and the conversion letter:
3922 space prefix positive number with a space
3923 + prefix positive number with a plus sign
3924 - left-justify within the field
3925 0 use zeros, not spaces, to right-justify
3926 # prefix non-zero octal with "0", non-zero hex with "0x"
3927 number minimum field width
3928 .number "precision": digits after decimal point for
3929 floating-point, max length for string, minimum length
3931 l interpret integer as C type "long" or "unsigned long"
3932 h interpret integer as C type "short" or "unsigned short"
3934 There is also one Perl-specific flag:
3936 V interpret integer as Perl's standard integer type
3938 Where a number would appear in the flags, an asterisk ("C<*>") may be
3939 used instead, in which case Perl uses the next item in the parameter
3940 list as the given number (that is, as the field width or precision).
3941 If a field width obtained through "C<*>" is negative, it has the same
3942 effect as the "C<->" flag: left-justification.
3944 If C<use locale> is in effect, the character used for the decimal
3945 point in formatted real numbers is affected by the LC_NUMERIC locale.
3952 Return the square root of EXPR. If EXPR is omitted, returns square
3953 root of C<$_>. Only works on non-negative operands, unless you've
3954 loaded the standard Math::Complex module.
3957 print sqrt(-2); # prints 1.4142135623731i
3963 Sets the random number seed for the C<rand()> operator. If EXPR is
3964 omitted, uses a semi-random value supplied by the kernel (if it supports
3965 the F</dev/urandom> device) or based on the current time and process
3966 ID, among other things. In versions of Perl prior to 5.004 the default
3967 seed was just the current C<time()>. This isn't a particularly good seed,
3968 so many old programs supply their own seed value (often C<time ^ $$> or
3969 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3971 In fact, it's usually not necessary to call C<srand()> at all, because if
3972 it is not called explicitly, it is called implicitly at the first use of
3973 the C<rand()> operator. However, this was not the case in version of Perl
3974 before 5.004, so if your script will run under older Perl versions, it
3975 should call C<srand()>.
3977 Note that you need something much more random than the default seed for
3978 cryptographic purposes. Checksumming the compressed output of one or more
3979 rapidly changing operating system status programs is the usual method. For
3982 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3984 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3987 Do I<not> call C<srand()> multiple times in your program unless you know
3988 exactly what you're doing and why you're doing it. The point of the
3989 function is to "seed" the C<rand()> function so that C<rand()> can produce
3990 a different sequence each time you run your program. Just do it once at the
3991 top of your program, or you I<won't> get random numbers out of C<rand()>!
3993 Frequently called programs (like CGI scripts) that simply use
3997 for a seed can fall prey to the mathematical property that
4001 one-third of the time. So don't do that.
4003 =item stat FILEHANDLE
4009 Returns a 13-element list giving the status info for a file, either
4010 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4011 it stats C<$_>. Returns a null list if the stat fails. Typically used
4014 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4015 $atime,$mtime,$ctime,$blksize,$blocks)
4018 Not all fields are supported on all filesystem types. Here are the
4019 meaning of the fields:
4021 0 dev device number of filesystem
4023 2 mode file mode (type and permissions)
4024 3 nlink number of (hard) links to the file
4025 4 uid numeric user ID of file's owner
4026 5 gid numeric group ID of file's owner
4027 6 rdev the device identifier (special files only)
4028 7 size total size of file, in bytes
4029 8 atime last access time since the epoch
4030 9 mtime last modify time since the epoch
4031 10 ctime inode change time (NOT creation time!) since the epoch
4032 11 blksize preferred block size for file system I/O
4033 12 blocks actual number of blocks allocated
4035 (The epoch was at 00:00 January 1, 1970 GMT.)
4037 If stat is passed the special filehandle consisting of an underline, no
4038 stat is done, but the current contents of the stat structure from the
4039 last stat or filetest are returned. Example:
4041 if (-x $file && (($d) = stat(_)) && $d < 0) {
4042 print "$file is executable NFS file\n";
4045 (This works on machines only for which the device number is negative under NFS.)
4047 Because the mode contains both the file type and its permissions, you
4048 should mask off the file type portion and (s)printf using a C<"%o">
4049 if you want to see the real permissions.
4051 $mode = (stat($filename))[2];
4052 printf "Permissions are %04o\n", $mode & 07777;
4055 In scalar context, C<stat()> returns a boolean value indicating success
4056 or failure, and, if successful, sets the information associated with
4057 the special filehandle C<_>.
4059 The File::stat module provides a convenient, by-name access mechanism:
4062 $sb = stat($filename);
4063 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4064 $filename, $sb->size, $sb->mode & 07777,
4065 scalar localtime $sb->mtime;
4071 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4072 doing many pattern matches on the string before it is next modified.
4073 This may or may not save time, depending on the nature and number of
4074 patterns you are searching on, and on the distribution of character
4075 frequencies in the string to be searched -- you probably want to compare
4076 run times with and without it to see which runs faster. Those loops
4077 which scan for many short constant strings (including the constant
4078 parts of more complex patterns) will benefit most. You may have only
4079 one C<study()> active at a time -- if you study a different scalar the first
4080 is "unstudied". (The way C<study()> works is this: a linked list of every
4081 character in the string to be searched is made, so we know, for
4082 example, where all the C<'k'> characters are. From each search string,
4083 the rarest character is selected, based on some static frequency tables
4084 constructed from some C programs and English text. Only those places
4085 that contain this "rarest" character are examined.)
4087 For example, here is a loop that inserts index producing entries
4088 before any line containing a certain pattern:
4092 print ".IX foo\n" if /\bfoo\b/;
4093 print ".IX bar\n" if /\bbar\b/;
4094 print ".IX blurfl\n" if /\bblurfl\b/;
4099 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
4100 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
4101 a big win except in pathological cases. The only question is whether
4102 it saves you more time than it took to build the linked list in the
4105 Note that if you have to look for strings that you don't know till
4106 runtime, you can build an entire loop as a string and C<eval()> that to
4107 avoid recompiling all your patterns all the time. Together with
4108 undefining C<$/> to input entire files as one record, this can be very
4109 fast, often faster than specialized programs like fgrep(1). The following
4110 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4111 out the names of those files that contain a match:
4113 $search = 'while (<>) { study;';
4114 foreach $word (@words) {
4115 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4120 eval $search; # this screams
4121 $/ = "\n"; # put back to normal input delimiter
4122 foreach $file (sort keys(%seen)) {
4130 =item sub NAME BLOCK
4132 This is subroutine definition, not a real function I<per se>. With just a
4133 NAME (and possibly prototypes), it's just a forward declaration. Without
4134 a NAME, it's an anonymous function declaration, and does actually return a
4135 value: the CODE ref of the closure you just created. See L<perlsub> and
4136 L<perlref> for details.
4138 =item substr EXPR,OFFSET,LEN,REPLACEMENT
4140 =item substr EXPR,OFFSET,LEN
4142 =item substr EXPR,OFFSET
4144 Extracts a substring out of EXPR and returns it. First character is at
4145 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4146 If OFFSET is negative (or more precisely, less than C<$[>), starts
4147 that far from the end of the string. If LEN is omitted, returns
4148 everything to the end of the string. If LEN is negative, leaves that
4149 many characters off the end of the string.
4151 If you specify a substring that is partly outside the string, the part
4152 within the string is returned. If the substring is totally outside
4153 the string a warning is produced.
4155 You can use the substr() function as an lvalue, in which case EXPR
4156 must itself be an lvalue. If you assign something shorter than LEN,
4157 the string will shrink, and if you assign something longer than LEN,
4158 the string will grow to accommodate it. To keep the string the same
4159 length you may need to pad or chop your value using C<sprintf()>.
4161 An alternative to using substr() as an lvalue is to specify the
4162 replacement string as the 4th argument. This allows you to replace
4163 parts of the EXPR and return what was there before in one operation,
4164 just as you can with splice().
4166 =item symlink OLDFILE,NEWFILE
4168 Creates a new filename symbolically linked to the old filename.
4169 Returns C<1> for success, C<0> otherwise. On systems that don't support
4170 symbolic links, produces a fatal error at run time. To check for that,
4173 $symlink_exists = eval { symlink("",""); 1 };
4177 Calls the system call specified as the first element of the list,
4178 passing the remaining elements as arguments to the system call. If
4179 unimplemented, produces a fatal error. The arguments are interpreted
4180 as follows: if a given argument is numeric, the argument is passed as
4181 an int. If not, the pointer to the string value is passed. You are
4182 responsible to make sure a string is pre-extended long enough to
4183 receive any result that might be written into a string. You can't use a
4184 string literal (or other read-only string) as an argument to C<syscall()>
4185 because Perl has to assume that any string pointer might be written
4187 integer arguments are not literals and have never been interpreted in a
4188 numeric context, you may need to add C<0> to them to force them to look
4189 like numbers. This emulates the C<syswrite()> function (or vice versa):
4191 require 'syscall.ph'; # may need to run h2ph
4193 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4195 Note that Perl supports passing of up to only 14 arguments to your system call,
4196 which in practice should usually suffice.
4198 Syscall returns whatever value returned by the system call it calls.
4199 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
4200 Note that some system calls can legitimately return C<-1>. The proper
4201 way to handle such calls is to assign C<$!=0;> before the call and
4202 check the value of C<$!> if syscall returns C<-1>.
4204 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4205 number of the read end of the pipe it creates. There is no way
4206 to retrieve the file number of the other end. You can avoid this
4207 problem by using C<pipe()> instead.
4209 =item sysopen FILEHANDLE,FILENAME,MODE
4211 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4213 Opens the file whose filename is given by FILENAME, and associates it
4214 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4215 the name of the real filehandle wanted. This function calls the
4216 underlying operating system's C<open()> function with the parameters
4217 FILENAME, MODE, PERMS.
4219 The possible values and flag bits of the MODE parameter are
4220 system-dependent; they are available via the standard module C<Fcntl>.
4221 For historical reasons, some values work on almost every system
4222 supported by perl: zero means read-only, one means write-only, and two
4223 means read/write. We know that these values do I<not> work under
4224 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4225 use them in new code.
4227 If the file named by FILENAME does not exist and the C<open()> call creates
4228 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4229 PERMS specifies the permissions of the newly created file. If you omit
4230 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
4231 These permission values need to be in octal, and are modified by your
4232 process's current C<umask>.
4234 You should seldom if ever use C<0644> as argument to C<sysopen()>, because
4235 that takes away the user's option to have a more permissive umask.
4236 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4239 See L<perlopentut> for a kinder, gentler explanation of opening files.
4241 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4243 =item sysread FILEHANDLE,SCALAR,LENGTH
4245 Attempts to read LENGTH bytes of data into variable SCALAR from the
4246 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4247 so mixing this with other kinds of reads, C<print()>, C<write()>,
4248 C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
4249 usually buffers data. Returns the number of bytes actually read, C<0>
4250 at end of file, or undef if there was an error. SCALAR will be grown or
4251 shrunk so that the last byte actually read is the last byte of the
4252 scalar after the read.
4254 An OFFSET may be specified to place the read data at some place in the
4255 string other than the beginning. A negative OFFSET specifies
4256 placement at that many bytes counting backwards from the end of the
4257 string. A positive OFFSET greater than the length of SCALAR results
4258 in the string being padded to the required size with C<"\0"> bytes before
4259 the result of the read is appended.
4261 There is no syseof() function, which is ok, since eof() doesn't work
4262 very well on device files (like ttys) anyway. Use sysread() and check
4263 ofr a return value for 0 to decide whether you're done.
4265 =item sysseek FILEHANDLE,POSITION,WHENCE
4267 Sets FILEHANDLE's system position using the system call lseek(2). It
4268 bypasses stdio, so mixing this with reads (other than C<sysread()>),
4269 C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
4270 confusion. FILEHANDLE may be an expression whose value gives the name
4271 of the filehandle. The values for WHENCE are C<0> to set the new
4272 position to POSITION, C<1> to set the it to the current position plus
4273 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4274 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4275 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4277 Returns the new position, or the undefined value on failure. A position
4278 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
4279 TRUE on success and FALSE on failure, yet you can still easily determine
4284 =item system PROGRAM LIST
4286 Does exactly the same thing as "C<exec LIST>", except that a fork is done
4287 first, and the parent process waits for the child process to complete.
4288 Note that argument processing varies depending on the number of
4289 arguments. If there is more than one argument in LIST, or if LIST is
4290 an array with more than one value, starts the program given by the
4291 first element of the list with arguments given by the rest of the list.
4292 If there is only one scalar argument, the argument is
4293 checked for shell metacharacters, and if there are any, the entire
4294 argument is passed to the system's command shell for parsing (this is
4295 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
4296 there are no shell metacharacters in the argument, it is split into
4297 words and passed directly to C<execvp()>, which is more efficient.
4299 The return value is the exit status of the program as
4300 returned by the C<wait()> call. To get the actual exit value divide by
4301 256. See also L</exec>. This is I<NOT> what you want to use to capture
4302 the output from a command, for that you should use merely backticks or
4303 C<qx//>, as described in L<perlop/"`STRING`">.
4305 Like C<exec()>, C<system()> allows you to lie to a program about its name if
4306 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
4308 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4309 program they're running doesn't actually interrupt your program.
4311 @args = ("command", "arg1", "arg2");
4313 or die "system @args failed: $?"
4315 You can check all the failure possibilities by inspecting
4318 $exit_value = $? >> 8;
4319 $signal_num = $? & 127;
4320 $dumped_core = $? & 128;
4322 When the arguments get executed via the system shell, results
4323 and return codes will be subject to its quirks and capabilities.
4324 See L<perlop/"`STRING`"> and L</exec> for details.
4326 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4328 =item syswrite FILEHANDLE,SCALAR,LENGTH
4330 =item syswrite FILEHANDLE,SCALAR
4332 Attempts to write LENGTH bytes of data from variable SCALAR to the
4333 specified FILEHANDLE, using the system call write(2). If LENGTH is
4334 not specified, writes whole SCALAR. It bypasses
4335 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
4336 C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
4337 because stdio usually buffers data. Returns the number of bytes
4338 actually written, or C<undef> if there was an error. If the LENGTH is
4339 greater than the available data in the SCALAR after the OFFSET, only as
4340 much data as is available will be written.
4342 An OFFSET may be specified to write the data from some part of the
4343 string other than the beginning. A negative OFFSET specifies writing
4344 that many bytes counting backwards from the end of the string. In the
4345 case the SCALAR is empty you can use OFFSET but only zero offset.
4347 =item tell FILEHANDLE
4351 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4352 expression whose value gives the name of the actual filehandle. If
4353 FILEHANDLE is omitted, assumes the file last read.
4355 There is no C<systell()> function. Use C<sysseek(FH, 0, 1)> for that.
4357 =item telldir DIRHANDLE
4359 Returns the current position of the C<readdir()> routines on DIRHANDLE.
4360 Value may be given to C<seekdir()> to access a particular location in a
4361 directory. Has the same caveats about possible directory compaction as
4362 the corresponding system library routine.
4364 =item tie VARIABLE,CLASSNAME,LIST
4366 This function binds a variable to a package class that will provide the
4367 implementation for the variable. VARIABLE is the name of the variable
4368 to be enchanted. CLASSNAME is the name of a class implementing objects
4369 of correct type. Any additional arguments are passed to the "C<new()>"
4370 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4371 or C<TIEHASH>). Typically these are arguments such as might be passed
4372 to the C<dbm_open()> function of C. The object returned by the "C<new()>"
4373 method is also returned by the C<tie()> function, which would be useful
4374 if you want to access other methods in CLASSNAME.
4376 Note that functions such as C<keys()> and C<values()> may return huge lists
4377 when used on large objects, like DBM files. You may prefer to use the
4378 C<each()> function to iterate over such. Example:
4380 # print out history file offsets
4382 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4383 while (($key,$val) = each %HIST) {
4384 print $key, ' = ', unpack('L',$val), "\n";
4388 A class implementing a hash should have the following methods:
4390 TIEHASH classname, LIST
4392 STORE this, key, value
4397 NEXTKEY this, lastkey
4400 A class implementing an ordinary array should have the following methods:
4402 TIEARRAY classname, LIST
4404 STORE this, key, value
4406 STORESIZE this, count
4412 SPLICE this, offset, length, LIST
4416 A class implementing a file handle should have the following methods:
4418 TIEHANDLE classname, LIST
4419 READ this, scalar, length, offset
4422 WRITE this, scalar, length, offset
4424 PRINTF this, format, LIST
4428 A class implementing a scalar should have the following methods:
4430 TIESCALAR classname, LIST
4435 Not all methods indicated above need be implemented. See L<perltie>,
4436 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
4438 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4439 for you--you need to do that explicitly yourself. See L<DB_File>
4440 or the F<Config> module for interesting C<tie()> implementations.
4442 For further details see L<perltie>, L<"tied VARIABLE">.
4446 Returns a reference to the object underlying VARIABLE (the same value
4447 that was originally returned by the C<tie()> call that bound the variable
4448 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4453 Returns the number of non-leap seconds since whatever time the system
4454 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4455 and 00:00:00 UTC, January 1, 1970 for most other systems).
4456 Suitable for feeding to C<gmtime()> and C<localtime()>.
4460 Returns a four-element list giving the user and system times, in
4461 seconds, for this process and the children of this process.
4463 ($user,$system,$cuser,$csystem) = times;
4467 The transliteration operator. Same as C<y///>. See L<perlop>.
4469 =item truncate FILEHANDLE,LENGTH
4471 =item truncate EXPR,LENGTH
4473 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4474 specified length. Produces a fatal error if truncate isn't implemented
4475 on your system. Returns TRUE if successful, the undefined value
4482 Returns an uppercased version of EXPR. This is the internal function
4483 implementing the C<\U> escape in double-quoted strings.
4484 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4485 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4486 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4488 If EXPR is omitted, uses C<$_>.
4494 Returns the value of EXPR with the first character
4495 in uppercase (titlecase in Unicode). This is
4496 the internal function implementing the C<\u> escape in double-quoted strings.
4497 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
4500 If EXPR is omitted, uses C<$_>.
4506 Sets the umask for the process to EXPR and returns the previous value.
4507 If EXPR is omitted, merely returns the current umask.
4509 The Unix permission C<rwxr-x---> is represented as three sets of three
4510 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4511 and isn't one of the digits). The C<umask> value is such a number
4512 representing disabled permissions bits. The permission (or "mode")
4513 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4514 even if you tell C<sysopen> to create a file with permissions C<0777>,
4515 if your umask is C<0022> then the file will actually be created with
4516 permissions C<0755>. If your C<umask> were C<0027> (group can't
4517 write; others can't read, write, or execute), then passing
4518 C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
4521 Here's some advice: supply a creation mode of C<0666> for regular
4522 files (in C<sysopen()>) and one of C<0777> for directories (in
4523 C<mkdir()>) and executable files. This gives users the freedom of
4524 choice: if they want protected files, they might choose process umasks
4525 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4526 Programs should rarely if ever make policy decisions better left to
4527 the user. The exception to this is when writing files that should be
4528 kept private: mail files, web browser cookies, I<.rhosts> files, and
4531 If umask(2) is not implemented on your system and you are trying to
4532 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4533 fatal error at run time. If umask(2) is not implemented and you are
4534 not trying to restrict access for yourself, returns C<undef>.
4536 Remember that a umask is a number, usually given in octal; it is I<not> a
4537 string of octal digits. See also L</oct>, if all you have is a string.
4543 Undefines the value of EXPR, which must be an lvalue. Use only on a
4544 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4545 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4546 will probably not do what you expect on most predefined variables or
4547 DBM list values, so don't do that; see L<delete>.) Always returns the
4548 undefined value. You can omit the EXPR, in which case nothing is
4549 undefined, but you still get an undefined value that you could, for
4550 instance, return from a subroutine, assign to a variable or pass as a
4551 parameter. Examples:
4554 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4558 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4559 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4560 select undef, undef, undef, 0.25;
4561 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4563 Note that this is a unary operator, not a list operator.
4569 Deletes a list of files. Returns the number of files successfully
4572 $cnt = unlink 'a', 'b', 'c';
4576 Note: C<unlink()> will not delete directories unless you are superuser and
4577 the B<-U> flag is supplied to Perl. Even if these conditions are
4578 met, be warned that unlinking a directory can inflict damage on your
4579 filesystem. Use C<rmdir()> instead.
4581 If LIST is omitted, uses C<$_>.
4583 =item unpack TEMPLATE,EXPR
4585 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4586 structure and expands it out into a list value, returning the array
4587 value. (In scalar context, it returns merely the first value
4588 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4589 Here's a subroutine that does substring:
4592 my($what,$where,$howmuch) = @_;
4593 unpack("x$where a$howmuch", $what);
4598 sub ordinal { unpack("c",$_[0]); } # same as ord()
4600 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4601 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4602 themselves. Default is a 16-bit checksum. For example, the following
4603 computes the same number as the System V sum program:
4606 $checksum += unpack("%32C*", $_);
4610 The following efficiently counts the number of set bits in a bit vector:
4612 $setbits = unpack("%32b*", $selectmask);
4614 See L</pack> for more examples.
4616 =item untie VARIABLE
4618 Breaks the binding between a variable and a package. (See C<tie()>.)
4620 =item unshift ARRAY,LIST
4622 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4623 depending on how you look at it. Prepends list to the front of the
4624 array, and returns the new number of elements in the array.
4626 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4628 Note the LIST is prepended whole, not one element at a time, so the
4629 prepended elements stay in the same order. Use C<reverse()> to do the
4632 =item use Module LIST
4636 =item use Module VERSION LIST
4640 Imports some semantics into the current package from the named module,
4641 generally by aliasing certain subroutine or variable names into your
4642 package. It is exactly equivalent to
4644 BEGIN { require Module; import Module LIST; }
4646 except that Module I<must> be a bareword.
4648 If the first argument to C<use> is a number, it is treated as a version
4649 number instead of a module name. If the version of the Perl interpreter
4650 is less than VERSION, then an error message is printed and Perl exits
4651 immediately. This is often useful if you need to check the current
4652 Perl version before C<use>ing library modules that have changed in
4653 incompatible ways from older versions of Perl. (We try not to do
4654 this more than we have to.)
4656 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4657 C<require> makes sure the module is loaded into memory if it hasn't been
4658 yet. The C<import()> is not a builtin--it's just an ordinary static method
4659 call into the "C<Module>" package to tell the module to import the list of
4660 features back into the current package. The module can implement its
4661 C<import()> method any way it likes, though most modules just choose to
4662 derive their C<import()> method via inheritance from the C<Exporter> class that
4663 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4664 method can be found then the error is currently silently ignored. This
4665 may change to a fatal error in a future version.
4667 If you don't want your namespace altered, explicitly supply an empty list:
4671 That is exactly equivalent to
4673 BEGIN { require Module }
4675 If the VERSION argument is present between Module and LIST, then the
4676 C<use> will call the VERSION method in class Module with the given
4677 version as an argument. The default VERSION method, inherited from
4678 the Universal class, croaks if the given version is larger than the
4679 value of the variable C<$Module::VERSION>. (Note that there is not a
4680 comma after VERSION!)
4682 Because this is a wide-open interface, pragmas (compiler directives)
4683 are also implemented this way. Currently implemented pragmas are:
4687 use sigtrap qw(SEGV BUS);
4688 use strict qw(subs vars refs);
4689 use subs qw(afunc blurfl);
4691 Some of these these pseudo-modules import semantics into the current
4692 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4693 which import symbols into the current package (which are effective
4694 through the end of the file).
4696 There's a corresponding "C<no>" command that unimports meanings imported
4697 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4702 If no C<unimport()> method can be found the call fails with a fatal error.
4704 See L<perlmod> for a list of standard modules and pragmas.
4708 Changes the access and modification times on each file of a list of
4709 files. The first two elements of the list must be the NUMERICAL access
4710 and modification times, in that order. Returns the number of files
4711 successfully changed. The inode modification time of each file is set
4712 to the current time. This code has the same effect as the "C<touch>"
4713 command if the files already exist:
4717 utime $now, $now, @ARGV;
4721 Returns a list consisting of all the values of the named hash. (In a
4722 scalar context, returns the number of values.) The values are
4723 returned in an apparently random order. The actual random order is
4724 subject to change in future versions of perl, but it is guaranteed to
4725 be the same order as either the C<keys()> or C<each()> function would
4726 produce on the same (unmodified) hash.
4728 Note that you cannot modify the values of a hash this way, because the
4729 returned list is just a copy. You need to use a hash slice for that,
4730 since it's lvaluable in a way that values() is not.
4732 for (values %hash) { s/foo/bar/g } # FAILS!
4733 for (@hash{keys %hash}) { s/foo/bar/g } # ok
4735 As a side effect, calling values() resets the HASH's internal iterator.
4736 See also C<keys()>, C<each()>, and C<sort()>.
4738 =item vec EXPR,OFFSET,BITS
4740 Treats the string in EXPR as a vector of unsigned integers, and
4741 returns the value of the bit field specified by OFFSET. BITS specifies
4742 the number of bits that are reserved for each entry in the bit
4743 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4744 assigned to, in which case parentheses are needed to give the expression
4745 the correct precedence as in
4747 vec($image, $max_x * $x + $y, 8) = 3;
4749 Vectors created with C<vec()> can also be manipulated with the logical
4750 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4751 desired when both operands are strings. See L<perlop/"Bitwise String Operators">.
4753 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4754 The comments show the string after each step. Note that this code works
4755 in the same way on big-endian or little-endian machines.
4758 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4759 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4760 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4761 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4762 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4763 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4764 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4766 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4767 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4768 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4771 To transform a bit vector into a string or array of 0's and 1's, use these:
4773 $bits = unpack("b*", $vector);
4774 @bits = split(//, unpack("b*", $vector));
4776 If you know the exact length in bits, it can be used in place of the C<*>.
4780 Behaves like the wait(2) system call on your system: it waits for a child
4781 process to terminate and returns the pid of the deceased process, or
4782 C<-1> if there are no child processes. The status is rketurned in C<$?>.
4783 Note that a return value of C<-1> could mean that child processes are
4784 being automatically reaped, as described in L<perlipc>.
4786 =item waitpid PID,FLAGS
4788 Waits for a particular child process to terminate and returns the pid of
4789 the deceased process, or C<-1> if there is no such child process. On some
4790 systems, a value of 0 indicates that there are processes still running.
4791 The status is returned in C<$?>. If you say
4793 use POSIX ":sys_wait_h";
4796 $kid = waitpid(-1,&WNOHANG);
4799 then you can do a non-blocking wait for all pending zombie processes.
4800 Non-blocking wait is available on machines supporting either the
4801 waitpid(2) or wait4(2) system calls. However, waiting for a particular
4802 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
4803 system call by remembering the status values of processes that have
4804 exited but have not been harvested by the Perl script yet.)
4806 Note that on some systems, a return value of C<-1> could mean that child
4807 processes are being automatically reaped. See L<perlipc> for details,
4808 and for other examples.
4812 Returns TRUE if the context of the currently executing subroutine is
4813 looking for a list value. Returns FALSE if the context is looking
4814 for a scalar. Returns the undefined value if the context is looking
4815 for no value (void context).
4817 return unless defined wantarray; # don't bother doing more
4818 my @a = complex_calculation();
4819 return wantarray ? @a : "@a";
4823 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4826 If LIST is empty and C<$@> already contains a value (typically from a
4827 previous eval) that value is used after appending C<"\t...caught">
4828 to C<$@>. This is useful for staying almost, but not entirely similar to
4831 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4833 No message is printed if there is a C<$SIG{__WARN__}> handler
4834 installed. It is the handler's responsibility to deal with the message
4835 as it sees fit (like, for instance, converting it into a C<die()>). Most
4836 handlers must therefore make arrangements to actually display the
4837 warnings that they are not prepared to deal with, by calling C<warn()>
4838 again in the handler. Note that this is quite safe and will not
4839 produce an endless loop, since C<__WARN__> hooks are not called from
4842 You will find this behavior is slightly different from that of
4843 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4844 instead call C<die()> again to change it).
4846 Using a C<__WARN__> handler provides a powerful way to silence all
4847 warnings (even the so-called mandatory ones). An example:
4849 # wipe out *all* compile-time warnings
4850 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4852 my $foo = 20; # no warning about duplicate my $foo,
4853 # but hey, you asked for it!
4854 # no compile-time or run-time warnings before here
4857 # run-time warnings enabled after here
4858 warn "\$foo is alive and $foo!"; # does show up
4860 See L<perlvar> for details on setting C<%SIG> entries, and for more
4861 examples. See the Carp module for other kinds of warnings using its
4862 carp() and cluck() functions.
4864 =item write FILEHANDLE
4870 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4871 using the format associated with that file. By default the format for
4872 a file is the one having the same name as the filehandle, but the
4873 format for the current output channel (see the C<select()> function) may be set
4874 explicitly by assigning the name of the format to the C<$~> variable.
4876 Top of form processing is handled automatically: if there is
4877 insufficient room on the current page for the formatted record, the
4878 page is advanced by writing a form feed, a special top-of-page format
4879 is used to format the new page header, and then the record is written.
4880 By default the top-of-page format is the name of the filehandle with
4881 "_TOP" appended, but it may be dynamically set to the format of your
4882 choice by assigning the name to the C<$^> variable while the filehandle is
4883 selected. The number of lines remaining on the current page is in
4884 variable C<$->, which can be set to C<0> to force a new page.
4886 If FILEHANDLE is unspecified, output goes to the current default output
4887 channel, which starts out as STDOUT but may be changed by the
4888 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4889 is evaluated and the resulting string is used to look up the name of
4890 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4892 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4896 The transliteration operator. Same as C<tr///>. See L<perlop>.