3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 An named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
204 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239 C<utime>, C<wait>, C<waitpid>
241 For more information about the portability of these functions, see
242 L<perlport> and other available platform-specific documentation.
244 =head2 Alphabetical Listing of Perl Functions
248 =item I<-X> FILEHANDLE
254 A file test, where X is one of the letters listed below. This unary
255 operator takes one argument, either a filename or a filehandle, and
256 tests the associated file to see if something is true about it. If the
257 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
258 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
259 the undefined value if the file doesn't exist. Despite the funny
260 names, precedence is the same as any other named unary operator, and
261 the argument may be parenthesized like any other unary operator. The
262 operator may be any of:
263 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
264 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
277 -z File has zero size.
278 -s File has nonzero size (returns size).
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
304 next unless -f $_; # ignore specials
308 The interpretation of the file permission operators C<-r>, C<-R>,
309 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310 of the file and the uids and gids of the user. There may be other
311 reasons you can't actually read, write, or execute the file. Such
312 reasons may be for example network filesystem access controls, ACLs
313 (access control lists), read-only filesystems, and unrecognized
316 Also note that, for the superuser on the local filesystems, the C<-r>,
317 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
318 if any execute bit is set in the mode. Scripts run by the superuser
319 may thus need to do a stat() to determine the actual mode of the file,
320 or temporarily set their effective uid to something else.
322 If you are using ACLs, there is a pragma called C<filetest> that may
323 produce more accurate results than the bare stat() mode bits.
324 When under the C<use filetest 'access'> the above-mentioned filetests
325 will test whether the permission can (not) be granted using the
326 access() family of system calls. Also note that the C<-x> and C<-X> may
327 under this pragma return true even if there are no execute permission
328 bits set (nor any extra execute permission ACLs). This strangeness is
329 due to the underlying system calls' definitions. Read the
330 documentation for the C<filetest> pragma for more information.
332 Note that C<-s/a/b/> does not do a negated substitution. Saying
333 C<-exp($foo)> still works as expected, however--only single letters
334 following a minus are interpreted as file tests.
336 The C<-T> and C<-B> switches work as follows. The first block or so of the
337 file is examined for odd characters such as strange control codes or
338 characters with the high bit set. If too many strange characters (E<gt>30%)
339 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
340 containing null in the first block is considered a binary file. If C<-T>
341 or C<-B> is used on a filehandle, the current stdio buffer is examined
342 rather than the first block. Both C<-T> and C<-B> return true on a null
343 file, or a file at EOF when testing a filehandle. Because you have to
344 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
345 against the file first, as in C<next unless -f $file && -T $file>.
347 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
348 the special filehandle consisting of a solitary underline, then the stat
349 structure of the previous file test (or stat operator) is used, saving
350 a system call. (This doesn't work with C<-t>, and you need to remember
351 that lstat() and C<-l> will leave values in the stat structure for the
352 symbolic link, not the real file.) Example:
354 print "Can do.\n" if -r $a || -w _ || -x _;
357 print "Readable\n" if -r _;
358 print "Writable\n" if -w _;
359 print "Executable\n" if -x _;
360 print "Setuid\n" if -u _;
361 print "Setgid\n" if -g _;
362 print "Sticky\n" if -k _;
363 print "Text\n" if -T _;
364 print "Binary\n" if -B _;
370 Returns the absolute value of its argument.
371 If VALUE is omitted, uses C<$_>.
373 =item accept NEWSOCKET,GENERICSOCKET
375 Accepts an incoming socket connect, just as the accept(2) system call
376 does. Returns the packed address if it succeeded, false otherwise.
377 See the example in L<perlipc/"Sockets: Client/Server Communication">.
383 Arranges to have a SIGALRM delivered to this process after the
384 specified number of seconds have elapsed. If SECONDS is not specified,
385 the value stored in C<$_> is used. (On some machines,
386 unfortunately, the elapsed time may be up to one second less than you
387 specified because of how seconds are counted.) Only one timer may be
388 counting at once. Each call disables the previous timer, and an
389 argument of C<0> may be supplied to cancel the previous timer without
390 starting a new one. The returned value is the amount of time remaining
391 on the previous timer.
393 For delays of finer granularity than one second, you may use Perl's
394 four-argument version of select() leaving the first three arguments
395 undefined, or you might be able to use the C<syscall> interface to
396 access setitimer(2) if your system supports it. The Time::HiRes module
397 from CPAN may also prove useful.
399 It is usually a mistake to intermix C<alarm>
402 If you want to use C<alarm> to time out a system call you need to use an
403 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
404 fail with C<$!> 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 FILEHANDLE to be read or written in "binary" mode on
441 systems whose run-time libraries force the programmer to guess
442 between binary and text files. If FILEHANDLE is an expression, the
443 value is taken as the name of the filehandle. binmode() should be
444 called after the C<open> but before any I/O is done on the filehandle.
445 The only way to reset binary mode on a filehandle is to reopen the
448 The operating system, device drivers, C libraries, and Perl run-time
449 system all conspire to let the programmer conveniently treat a
450 simple, one-byte C<\n> as the line terminator, irrespective of its
451 external representation. On Unix and its brethren, the native file
452 representation exactly matches the internal representation, making
453 everyone's lives unbelievably simpler. Consequently, L<binmode>
454 has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
455 to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and
456 C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
457 C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because
458 it's only one byte, and the internal and external representations
461 In legacy systems like MS-DOS and its embellishments, your program
462 sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
463 that's not what's physically stored on disk. What's worse, these
464 systems refuse to help you with this; it's up to you to remember
465 what to do. And you mustn't go applying binmode() with wild abandon,
466 either, because if your system does care about binmode(), then using
467 it when you shouldn't is just as perilous as failing to use it when
470 That means that on any version of Microsoft WinXX that you might
471 care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
472 to be converted to C<\n> when read into your program, and causes
473 any C<\n> in your program to be converted back to C<\cM\cJ> on
474 output to disk. This sad discrepancy leads to no end of
475 problems in not just the readline operator, but also when using
476 seek(), tell(), and read() calls. See L<perlport> for other painful
477 details. See the C<$/> and C<$\> variables in L<perlvar> for how
478 to manually set your input and output line-termination sequences.
480 =item bless REF,CLASSNAME
484 This function tells the thingy referenced by REF that it is now an object
485 in the CLASSNAME package. If CLASSNAME is omitted, the current package
486 is used. Because a C<bless> is often the last thing in a constructor,
487 it returns the reference for convenience. Always use the two-argument
488 version if the function doing the blessing might be inherited by a
489 derived class. See L<perltoot> and L<perlobj> for more about the blessing
490 (and blessings) of objects.
492 Consider always blessing objects in CLASSNAMEs that are mixed case.
493 Namespaces with all lowercase names are considered reserved for
494 Perl pragmata. Builtin types have all uppercase names, so to prevent
495 confusion, you may wish to avoid such package names as well. Make sure
496 that CLASSNAME is a true value.
498 See L<perlmod/"Perl Modules">.
504 Returns the context of the current subroutine call. In scalar context,
505 returns the caller's package name if there is a caller, that is, if
506 we're in a subroutine or C<eval> or C<require>, and the undefined value
507 otherwise. In list context, returns
509 ($package, $filename, $line) = caller;
511 With EXPR, it returns some extra information that the debugger uses to
512 print a stack trace. The value of EXPR indicates how many call frames
513 to go back before the current one.
515 ($package, $filename, $line, $subroutine,
516 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
518 Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
519 call, but an C<eval>. In such a case additional elements $evaltext and
520 C<$is_require> are set: C<$is_require> is true if the frame is created by a
521 C<require> or C<use> statement, $evaltext contains the text of the
522 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
523 $filename is C<"(eval)">, but $evaltext is undefined. (Note also that
524 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
527 Furthermore, when called from within the DB package, caller returns more
528 detailed information: it sets the list variable C<@DB::args> to be the
529 arguments with which the subroutine was invoked.
531 Be aware that the optimizer might have optimized call frames away before
532 C<caller> had a chance to get the information. That means that C<caller(N)>
533 might not return information about the call frame you expect it do, for
534 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
535 previous time C<caller> was called.
539 Changes the working directory to EXPR, if possible. If EXPR is omitted,
540 changes to the user's home directory. Returns true upon success,
541 false otherwise. See the example under C<die>.
545 Changes the permissions of a list of files. The first element of the
546 list must be the numerical mode, which should probably be an octal
547 number, and which definitely should I<not> a string of octal digits:
548 C<0644> is okay, C<'0644'> is not. Returns the number of files
549 successfully changed. See also L</oct>, if all you have is a string.
551 $cnt = chmod 0755, 'foo', 'bar';
552 chmod 0755, @executables;
553 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
555 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
556 $mode = 0644; chmod $mode, 'foo'; # this is best
564 This safer version of L</chop> removes any trailing string
565 that corresponds to the current value of C<$/> (also known as
566 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
567 number of characters removed from all its arguments. It's often used to
568 remove the newline from the end of an input record when you're worried
569 that the final record may be missing its newline. When in paragraph
570 mode (C<$/ = "">), it removes all trailing newlines from the string.
571 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
572 a reference to an integer or the like, see L<perlvar>) chomp() won't
574 If VARIABLE is omitted, it chomps C<$_>. Example:
577 chomp; # avoid \n on last field
582 You can actually chomp anything that's an lvalue, including an assignment:
585 chomp($answer = <STDIN>);
587 If you chomp a list, each element is chomped, and the total number of
588 characters removed is returned.
596 Chops off the last character of a string and returns the character
597 chopped. It's used primarily to remove the newline from the end of an
598 input record, but is much more efficient than C<s/\n//> because it neither
599 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
603 chop; # avoid \n on last field
608 You can actually chop anything that's an lvalue, including an assignment:
611 chop($answer = <STDIN>);
613 If you chop a list, each element is chopped. Only the value of the
614 last C<chop> is returned.
616 Note that C<chop> returns the last character. To return all but the last
617 character, use C<substr($string, 0, -1)>.
621 Changes the owner (and group) of a list of files. The first two
622 elements of the list must be the I<numeric> uid and gid, in that
623 order. A value of -1 in either position is interpreted by most
624 systems to leave that value unchanged. Returns the number of files
625 successfully changed.
627 $cnt = chown $uid, $gid, 'foo', 'bar';
628 chown $uid, $gid, @filenames;
630 Here's an example that looks up nonnumeric uids in the passwd file:
633 chomp($user = <STDIN>);
635 chomp($pattern = <STDIN>);
637 ($login,$pass,$uid,$gid) = getpwnam($user)
638 or die "$user not in passwd file";
640 @ary = glob($pattern); # expand filenames
641 chown $uid, $gid, @ary;
643 On most systems, you are not allowed to change the ownership of the
644 file unless you're the superuser, although you should be able to change
645 the group to any of your secondary groups. On insecure systems, these
646 restrictions may be relaxed, but this is not a portable assumption.
647 On POSIX systems, you can detect this condition this way:
649 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
650 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
656 Returns the character represented by that NUMBER in the character set.
657 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
658 chr(0x263a) is a Unicode smiley face (but only within the scope of
659 a C<use utf8>). For the reverse, use L</ord>.
660 See L<utf8> for more about Unicode.
662 If NUMBER is omitted, uses C<$_>.
664 =item chroot FILENAME
668 This function works like the system call by the same name: it makes the
669 named directory the new root directory for all further pathnames that
670 begin with a C<"/"> by your process and all its children. (It doesn't
671 change your current working directory, which is unaffected.) For security
672 reasons, this call is restricted to the superuser. If FILENAME is
673 omitted, does a C<chroot> to C<$_>.
675 =item close FILEHANDLE
679 Closes the file or pipe associated with the file handle, returning true
680 only if stdio successfully flushes buffers and closes the system file
681 descriptor. Closes the currently selected filehandle if the argument
684 You don't have to close FILEHANDLE if you are immediately going to do
685 another C<open> on it, because C<open> will close it for you. (See
686 C<open>.) However, an explicit C<close> on an input file resets the line
687 counter (C<$.>), while the implicit close done by C<open> does not.
689 If the file handle came from a piped open C<close> will additionally
690 return false if one of the other system calls involved fails or if the
691 program exits with non-zero status. (If the only problem was that the
692 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
693 also waits for the process executing on the pipe to complete, in case you
694 want to look at the output of the pipe afterwards, and
695 implicitly puts the exit status value of that command into C<$?>.
697 Prematurely closing the read end of a pipe (i.e. before the process
698 writing to it at the other end has closed it) will result in a
699 SIGPIPE being delivered to the writer. If the other end can't
700 handle that, be sure to read all the data before closing the pipe.
704 open(OUTPUT, '|sort >foo') # pipe to sort
705 or die "Can't start sort: $!";
706 #... # print stuff to output
707 close OUTPUT # wait for sort to finish
708 or warn $! ? "Error closing sort pipe: $!"
709 : "Exit status $? from sort";
710 open(INPUT, 'foo') # get sort's results
711 or die "Can't open 'foo' for input: $!";
713 FILEHANDLE may be an expression whose value can be used as an indirect
714 filehandle, usually the real filehandle name.
716 =item closedir DIRHANDLE
718 Closes a directory opened by C<opendir> and returns the success of that
721 DIRHANDLE may be an expression whose value can be used as an indirect
722 dirhandle, usually the real dirhandle name.
724 =item connect SOCKET,NAME
726 Attempts to connect to a remote socket, just as the connect system call
727 does. Returns true if it succeeded, false otherwise. NAME should be a
728 packed address of the appropriate type for the socket. See the examples in
729 L<perlipc/"Sockets: Client/Server Communication">.
733 Actually a flow control statement rather than a function. If there is a
734 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
735 C<foreach>), it is always executed just before the conditional is about to
736 be evaluated again, just like the third part of a C<for> loop in C. Thus
737 it can be used to increment a loop variable, even when the loop has been
738 continued via the C<next> statement (which is similar to the C C<continue>
741 C<last>, C<next>, or C<redo> may appear within a C<continue>
742 block. C<last> and C<redo> will behave as if they had been executed within
743 the main block. So will C<next>, but since it will execute a C<continue>
744 block, it may be more entertaining.
747 ### redo always comes here
750 ### next always comes here
752 # then back the top to re-check EXPR
754 ### last always comes here
756 Omitting the C<continue> section is semantically equivalent to using an
757 empty one, logically enough. In that case, C<next> goes directly back
758 to check the condition at the top of the loop.
762 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
763 takes cosine of C<$_>.
765 For the inverse cosine operation, you may use the C<POSIX::acos()>
766 function, or use this relation:
768 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
770 =item crypt PLAINTEXT,SALT
772 Encrypts a string exactly like the crypt(3) function in the C library
773 (assuming that you actually have a version there that has not been
774 extirpated as a potential munition). This can prove useful for checking
775 the password file for lousy passwords, amongst other things. Only the
776 guys wearing white hats should do this.
778 Note that C<crypt> is intended to be a one-way function, much like breaking
779 eggs to make an omelette. There is no (known) corresponding decrypt
780 function. As a result, this function isn't all that useful for
781 cryptography. (For that, see your nearby CPAN mirror.)
783 When verifying an existing encrypted string you should use the encrypted
784 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
785 allows your code to work with the standard C<crypt> and with more
786 exotic implementations. When choosing a new salt create a random two
787 character string whose characters come from the set C<[./0-9A-Za-z]>
788 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
790 Here's an example that makes sure that whoever runs this program knows
793 $pwd = (getpwuid($<))[1];
797 chomp($word = <STDIN>);
801 if (crypt($word, $pwd) ne $pwd) {
807 Of course, typing in your own password to whoever asks you
810 The L<crypt> function is unsuitable for encrypting large quantities
811 of data, not least of all because you can't get the information
812 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
813 on your favorite CPAN mirror for a slew of potentially useful
818 [This function has been largely superseded by the C<untie> function.]
820 Breaks the binding between a DBM file and a hash.
822 =item dbmopen HASH,DBNAME,MASK
824 [This function has been largely superseded by the C<tie> function.]
826 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
827 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
828 argument is I<not> a filehandle, even though it looks like one). DBNAME
829 is the name of the database (without the F<.dir> or F<.pag> extension if
830 any). If the database does not exist, it is created with protection
831 specified by MASK (as modified by the C<umask>). If your system supports
832 only the older DBM functions, you may perform only one C<dbmopen> in your
833 program. In older versions of Perl, if your system had neither DBM nor
834 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
837 If you don't have write access to the DBM file, you can only read hash
838 variables, not set them. If you want to test whether you can write,
839 either use file tests or try setting a dummy hash entry inside an C<eval>,
840 which will trap the error.
842 Note that functions such as C<keys> and C<values> may return huge lists
843 when used on large DBM files. You may prefer to use the C<each>
844 function to iterate over large DBM files. Example:
846 # print out history file offsets
847 dbmopen(%HIST,'/usr/lib/news/history',0666);
848 while (($key,$val) = each %HIST) {
849 print $key, ' = ', unpack('L',$val), "\n";
853 See also L<AnyDBM_File> for a more general description of the pros and
854 cons of the various dbm approaches, as well as L<DB_File> for a particularly
857 You can control which DBM library you use by loading that library
858 before you call dbmopen():
861 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
862 or die "Can't open netscape history file: $!";
868 Returns a Boolean value telling whether EXPR has a value other than
869 the undefined value C<undef>. If EXPR is not present, C<$_> will be
872 Many operations return C<undef> to indicate failure, end of file,
873 system error, uninitialized variable, and other exceptional
874 conditions. This function allows you to distinguish C<undef> from
875 other values. (A simple Boolean test will not distinguish among
876 C<undef>, zero, the empty string, and C<"0">, which are all equally
877 false.) Note that since C<undef> is a valid scalar, its presence
878 doesn't I<necessarily> indicate an exceptional condition: C<pop>
879 returns C<undef> when its argument is an empty array, I<or> when the
880 element to return happens to be C<undef>.
882 You may also use C<defined> to check whether a subroutine exists, by
883 saying C<defined &func> without parentheses. On the other hand, use
884 of C<defined> upon aggregates (hashes and arrays) is not guaranteed to
885 produce intuitive results, and should probably be avoided.
887 When used on a hash element, it tells you whether the value is defined,
888 not whether the key exists in the hash. Use L</exists> for the latter
893 print if defined $switch{'D'};
894 print "$val\n" while defined($val = pop(@ary));
895 die "Can't readlink $sym: $!"
896 unless defined($value = readlink $sym);
897 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
898 $debugging = 0 unless defined $debugging;
900 Note: Many folks tend to overuse C<defined>, and then are surprised to
901 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
902 defined values. For example, if you say
906 The pattern match succeeds, and C<$1> is defined, despite the fact that it
907 matched "nothing". But it didn't really match nothing--rather, it
908 matched something that happened to be zero characters long. This is all
909 very above-board and honest. When a function returns an undefined value,
910 it's an admission that it couldn't give you an honest answer. So you
911 should use C<defined> only when you're questioning the integrity of what
912 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
915 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
916 used to report whether memory for that aggregate has ever been
917 allocated. This behavior may disappear in future versions of Perl.
918 You should instead use a simple test for size:
920 if (@an_array) { print "has array elements\n" }
921 if (%a_hash) { print "has hash members\n" }
923 See also L</undef>, L</exists>, L</ref>.
927 Deletes the specified key(s) and their associated values from a hash.
928 For each key, returns the deleted value associated with that key, or
929 the undefined value if there was no such key. Deleting from C<$ENV{}>
930 modifies the environment. Deleting from a hash tied to a DBM file
931 deletes the entry from the DBM file. (But deleting from a C<tie>d hash
932 doesn't necessarily return anything.)
934 The following deletes all the values of a hash:
936 foreach $key (keys %HASH) {
942 delete @HASH{keys %HASH}
944 But both of these are slower than just assigning the empty list
947 %hash = (); # completely empty %hash
948 undef %hash; # forget %hash every existed
950 Note that the EXPR can be arbitrarily complicated as long as the final
951 operation is a hash element lookup or hash slice:
953 delete $ref->[$x][$y]{$key};
954 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
958 Outside an C<eval>, prints the value of LIST to C<STDERR> and
959 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
960 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
961 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
962 an C<eval(),> the error message is stuffed into C<$@> and the
963 C<eval> is terminated with the undefined value. This makes
964 C<die> the way to raise an exception.
968 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
969 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
971 If the value of EXPR does not end in a newline, the current script line
972 number and input line number (if any) are also printed, and a newline
973 is supplied. Note that the "input line number" (also known as "chunk")
974 is subject to whatever notion of "line" happens to be currently in
975 effect, and is also available as the special variable C<$.>.
976 See L<perlvar/"$/"> and L<perlvar/"$.">.
978 Hint: sometimes appending C<", stopped"> to your message
979 will cause it to make better sense when the string C<"at foo line 123"> is
980 appended. Suppose you are running script "canasta".
982 die "/etc/games is no good";
983 die "/etc/games is no good, stopped";
985 produce, respectively
987 /etc/games is no good at canasta line 123.
988 /etc/games is no good, stopped at canasta line 123.
990 See also exit(), warn(), and the Carp module.
992 If LIST is empty and C<$@> already contains a value (typically from a
993 previous eval) that value is reused after appending C<"\t...propagated">.
994 This is useful for propagating exceptions:
997 die unless $@ =~ /Expected exception/;
999 If C<$@> is empty then the string C<"Died"> is used.
1001 die() can also be called with a reference argument. If this happens to be
1002 trapped within an eval(), $@ contains the reference. This behavior permits
1003 a more elaborate exception handling implementation using objects that
1004 maintain arbitary state about the nature of the exception. Such a scheme
1005 is sometimes preferable to matching particular string values of $@ using
1006 regular expressions. Here's an example:
1008 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1010 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1011 # handle Some::Module::Exception
1014 # handle all other possible exceptions
1018 Because perl will stringify uncaught exception messages before displaying
1019 them, you may want to overload stringification operations on such custom
1020 exception objects. See L<overload> for details about that.
1022 You can arrange for a callback to be run just before the C<die>
1023 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1024 handler will be called with the error text and can change the error
1025 message, if it sees fit, by calling C<die> again. See
1026 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1027 L<"eval BLOCK"> for some examples. Although this feature was meant
1028 to be run only right before your program was to exit, this is not
1029 currently the case--the C<$SIG{__DIE__}> hook is currently called
1030 even inside eval()ed blocks/strings! If one wants the hook to do
1031 nothing in such situations, put
1035 as the first line of the handler (see L<perlvar/$^S>). Because
1036 this promotes strange action at a distance, this counterintuitive
1037 behavior may be fixed in a future release.
1041 Not really a function. Returns the value of the last command in the
1042 sequence of commands indicated by BLOCK. When modified by a loop
1043 modifier, executes the BLOCK once before testing the loop condition.
1044 (On other statements the loop modifiers test the conditional first.)
1046 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1047 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1048 See L<perlsyn> for alternative strategies.
1050 =item do SUBROUTINE(LIST)
1052 A deprecated form of subroutine call. See L<perlsub>.
1056 Uses the value of EXPR as a filename and executes the contents of the
1057 file as a Perl script. Its primary use is to include subroutines
1058 from a Perl subroutine library.
1064 scalar eval `cat stat.pl`;
1066 except that it's more efficient and concise, keeps track of the current
1067 filename for error messages, searches the @INC libraries, and updates
1068 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1069 variables. It also differs in that code evaluated with C<do FILENAME>
1070 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1071 same, however, in that it does reparse the file every time you call it,
1072 so you probably don't want to do this inside a loop.
1074 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1075 error. If C<do> can read the file but cannot compile it, it
1076 returns undef and sets an error message in C<$@>. If the file is
1077 successfully compiled, C<do> returns the value of the last expression
1080 Note that inclusion of library modules is better done with the
1081 C<use> and C<require> operators, which also do automatic error checking
1082 and raise an exception if there's a problem.
1084 You might like to use C<do> to read in a program configuration
1085 file. Manual error checking can be done this way:
1087 # read in config files: system first, then user
1088 for $file ("/share/prog/defaults.rc",
1089 "$ENV{HOME}/.someprogrc")
1091 unless ($return = do $file) {
1092 warn "couldn't parse $file: $@" if $@;
1093 warn "couldn't do $file: $!" unless defined $return;
1094 warn "couldn't run $file" unless $return;
1102 This function causes an immediate core dump. See also the B<-u>
1103 command-line switch in L<perlrun>, which does the same thing.
1104 Primarily this is so that you can use the B<undump> program (not
1105 supplied) to turn your core dump into an executable binary after
1106 having initialized all your variables at the beginning of the
1107 program. When the new binary is executed it will begin by executing
1108 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1109 Think of it as a goto with an intervening core dump and reincarnation.
1110 If C<LABEL> is omitted, restarts the program from the top.
1112 B<WARNING>: Any files opened at the time of the dump will I<not>
1113 be open any more when the program is reincarnated, with possible
1114 resulting confusion on the part of Perl.
1116 This function is now largely obsolete, partly because it's very
1117 hard to convert a core file into an executable, and because the
1118 real compiler backends for generating portable bytecode and compilable
1119 C code have superseded it.
1121 If you're looking to use L<dump> to speed up your program, consider
1122 generating bytecode or native C code as described in L<perlcc>. If
1123 you're just trying to accelerate a CGI script, consider using the
1124 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1125 You might also consider autoloading or selfloading, which at least
1126 make your program I<appear> to run faster.
1130 When called in list context, returns a 2-element list consisting of the
1131 key and value for the next element of a hash, so that you can iterate over
1132 it. When called in scalar context, returns the key for only the "next"
1133 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1134 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1137 Entries are returned in an apparently random order. The actual random
1138 order is subject to change in future versions of perl, but it is guaranteed
1139 to be in the same order as either the C<keys> or C<values> function
1140 would produce on the same (unmodified) hash.
1142 When the hash is entirely read, a null array is returned in list context
1143 (which when assigned produces a false (C<0>) value), and C<undef> in
1144 scalar context. The next call to C<each> after that will start iterating
1145 again. There is a single iterator for each hash, shared by all C<each>,
1146 C<keys>, and C<values> function calls in the program; it can be reset by
1147 reading all the elements from the hash, or by evaluating C<keys HASH> or
1148 C<values HASH>. If you add or delete elements of a hash while you're
1149 iterating over it, you may get entries skipped or duplicated, so don't.
1151 The following prints out your environment like the printenv(1) program,
1152 only in a different order:
1154 while (($key,$value) = each %ENV) {
1155 print "$key=$value\n";
1158 See also C<keys>, C<values> and C<sort>.
1160 =item eof FILEHANDLE
1166 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1167 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1168 gives the real filehandle. (Note that this function actually
1169 reads a character and then C<ungetc>s it, so isn't very useful in an
1170 interactive context.) Do not read from a terminal file (or call
1171 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1172 as terminals may lose the end-of-file condition if you do.
1174 An C<eof> without an argument uses the last file read as argument.
1175 Using C<eof()> with empty parentheses is very different. It indicates
1176 the pseudo file formed of the files listed on the command line,
1177 i.e., C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)>
1178 loop to detect the end of only the last file. Use C<eof(ARGV)> or
1179 C<eof> without the parentheses to test I<each> file in a while
1180 (E<lt>E<gt>) loop. Examples:
1182 # reset line numbering on each input file
1184 next if /^\s*#/; # skip comments
1187 close ARGV if eof; # Not eof()!
1190 # insert dashes just before last line of last file
1192 if (eof()) { # check for end of current file
1193 print "--------------\n";
1194 close(ARGV); # close or last; is needed if we
1195 # are reading from the terminal
1200 Practical hint: you almost never need to use C<eof> in Perl, because the
1201 input operators return false values when they run out of data, or if there
1208 In the first form, the return value of EXPR is parsed and executed as if it
1209 were a little Perl program. The value of the expression (which is itself
1210 determined within scalar context) is first parsed, and if there weren't any
1211 errors, executed in the context of the current Perl program, so that any
1212 variable settings or subroutine and format definitions remain afterwards.
1213 Note that the value is parsed every time the eval executes. If EXPR is
1214 omitted, evaluates C<$_>. This form is typically used to delay parsing
1215 and subsequent execution of the text of EXPR until run time.
1217 In the second form, the code within the BLOCK is parsed only once--at the
1218 same time the code surrounding the eval itself was parsed--and executed
1219 within the context of the current Perl program. This form is typically
1220 used to trap exceptions more efficiently than the first (see below), while
1221 also providing the benefit of checking the code within BLOCK at compile
1224 The final semicolon, if any, may be omitted from the value of EXPR or within
1227 In both forms, the value returned is the value of the last expression
1228 evaluated inside the mini-program; a return statement may be also used, just
1229 as with subroutines. The expression providing the return value is evaluated
1230 in void, scalar, or list context, depending on the context of the eval itself.
1231 See L</wantarray> for more on how the evaluation context can be determined.
1233 If there is a syntax error or runtime error, or a C<die> statement is
1234 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1235 error message. If there was no error, C<$@> is guaranteed to be a null
1236 string. Beware that using C<eval> neither silences perl from printing
1237 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1238 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1239 L</warn> and L<perlvar>.
1241 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1242 determining whether a particular feature (such as C<socket> or C<symlink>)
1243 is implemented. It is also Perl's exception trapping mechanism, where
1244 the die operator is used to raise exceptions.
1246 If the code to be executed doesn't vary, you may use the eval-BLOCK
1247 form to trap run-time errors without incurring the penalty of
1248 recompiling each time. The error, if any, is still returned in C<$@>.
1251 # make divide-by-zero nonfatal
1252 eval { $answer = $a / $b; }; warn $@ if $@;
1254 # same thing, but less efficient
1255 eval '$answer = $a / $b'; warn $@ if $@;
1257 # a compile-time error
1258 eval { $answer = }; # WRONG
1261 eval '$answer ='; # sets $@
1263 Due to the current arguably broken state of C<__DIE__> hooks, when using
1264 the C<eval{}> form as an exception trap in libraries, you may wish not
1265 to trigger any C<__DIE__> hooks that user code may have installed.
1266 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1267 as shown in this example:
1269 # a very private exception trap for divide-by-zero
1270 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1273 This is especially significant, given that C<__DIE__> hooks can call
1274 C<die> again, which has the effect of changing their error messages:
1276 # __DIE__ hooks may modify error messages
1278 local $SIG{'__DIE__'} =
1279 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1280 eval { die "foo lives here" };
1281 print $@ if $@; # prints "bar lives here"
1284 Because this promotes action at a distance, this counterintuitive behavior
1285 may be fixed in a future release.
1287 With an C<eval>, you should be especially careful to remember what's
1288 being looked at when:
1294 eval { $x }; # CASE 4
1296 eval "\$$x++"; # CASE 5
1299 Cases 1 and 2 above behave identically: they run the code contained in
1300 the variable $x. (Although case 2 has misleading double quotes making
1301 the reader wonder what else might be happening (nothing is).) Cases 3
1302 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1303 does nothing but return the value of $x. (Case 4 is preferred for
1304 purely visual reasons, but it also has the advantage of compiling at
1305 compile-time instead of at run-time.) Case 5 is a place where
1306 normally you I<would> like to use double quotes, except that in this
1307 particular situation, you can just use symbolic references instead, as
1310 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1311 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1315 =item exec PROGRAM LIST
1317 The C<exec> function executes a system command I<and never returns>--
1318 use C<system> instead of C<exec> if you want it to return. It fails and
1319 returns false only if the command does not exist I<and> it is executed
1320 directly instead of via your system's command shell (see below).
1322 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1323 warns you if there is a following statement which isn't C<die>, C<warn>,
1324 or C<exit> (if C<-w> is set - but you always do that). If you
1325 I<really> want to follow an C<exec> with some other statement, you
1326 can use one of these styles to avoid the warning:
1328 exec ('foo') or print STDERR "couldn't exec foo: $!";
1329 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1331 If there is more than one argument in LIST, or if LIST is an array
1332 with more than one value, calls execvp(3) with the arguments in LIST.
1333 If there is only one scalar argument or an array with one element in it,
1334 the argument is checked for shell metacharacters, and if there are any,
1335 the entire argument is passed to the system's command shell for parsing
1336 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1337 If there are no shell metacharacters in the argument, it is split into
1338 words and passed directly to C<execvp>, which is more efficient.
1341 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1342 exec "sort $outfile | uniq";
1344 If you don't really want to execute the first argument, but want to lie
1345 to the program you are executing about its own name, you can specify
1346 the program you actually want to run as an "indirect object" (without a
1347 comma) in front of the LIST. (This always forces interpretation of the
1348 LIST as a multivalued list, even if there is only a single scalar in
1351 $shell = '/bin/csh';
1352 exec $shell '-sh'; # pretend it's a login shell
1356 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1358 When the arguments get executed via the system shell, results will
1359 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1362 Using an indirect object with C<exec> or C<system> is also more
1363 secure. This usage (which also works fine with system()) forces
1364 interpretation of the arguments as a multivalued list, even if the
1365 list had just one argument. That way you're safe from the shell
1366 expanding wildcards or splitting up words with whitespace in them.
1368 @args = ( "echo surprise" );
1370 exec @args; # subject to shell escapes
1372 exec { $args[0] } @args; # safe even with one-arg list
1374 The first version, the one without the indirect object, ran the I<echo>
1375 program, passing it C<"surprise"> an argument. The second version
1376 didn't--it tried to run a program literally called I<"echo surprise">,
1377 didn't find it, and set C<$?> to a non-zero value indicating failure.
1379 Note that C<exec> will not call your C<END> blocks, nor will it call
1380 any C<DESTROY> methods in your objects.
1384 Returns true if the specified hash key exists in its hash, even
1385 if the corresponding value is undefined.
1387 print "Exists\n" if exists $array{$key};
1388 print "Defined\n" if defined $array{$key};
1389 print "True\n" if $array{$key};
1391 A hash element can be true only if it's defined, and defined if
1392 it exists, but the reverse doesn't necessarily hold true.
1394 Note that the EXPR can be arbitrarily complicated as long as the final
1395 operation is a hash key lookup:
1397 if (exists $ref->{A}->{B}->{$key}) { }
1398 if (exists $hash{A}{B}{$key}) { }
1400 Although the last element will not spring into existence just because
1401 its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1402 and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
1403 existence test for a $key element. This happens anywhere the arrow
1404 operator is used, including even
1407 if (exists $ref->{"Some key"}) { }
1408 print $ref; # prints HASH(0x80d3d5c)
1410 This surprising autovivification in what does not at first--or even
1411 second--glance appear to be an lvalue context may be fixed in a future
1416 Evaluates EXPR and exits immediately with that value. Example:
1419 exit 0 if $ans =~ /^[Xx]/;
1421 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1422 universally recognized values for EXPR are C<0> for success and C<1>
1423 for error; other values are subject to interpretation depending on the
1424 environment in which the Perl program is running. For example, exiting
1425 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1426 the mailer to return the item undelivered, but that's not true everywhere.
1428 Don't use C<exit> to abort a subroutine if there's any chance that
1429 someone might want to trap whatever error happened. Use C<die> instead,
1430 which can be trapped by an C<eval>.
1432 The exit() function does not always exit immediately. It calls any
1433 defined C<END> routines first, but these C<END> routines may not
1434 themselves abort the exit. Likewise any object destructors that need to
1435 be called are called before the real exit. If this is a problem, you
1436 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1437 See L<perlsub> for details.
1443 Returns I<e> (the natural logarithm base) to the power of EXPR.
1444 If EXPR is omitted, gives C<exp($_)>.
1446 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1448 Implements the fcntl(2) function. You'll probably have to say
1452 first to get the correct constant definitions. Argument processing and
1453 value return works just like C<ioctl> below.
1457 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1458 or die "can't fcntl F_GETFL: $!";
1460 You don't have to check for C<defined> on the return from C<fnctl>.
1461 Like C<ioctl>, it maps a C<0> return from the system call into C<"0
1462 but true"> in Perl. This string is true in boolean context and C<0>
1463 in numeric context. It is also exempt from the normal B<-w> warnings
1464 on improper numeric conversions.
1466 Note that C<fcntl> will produce a fatal error if used on a machine that
1467 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1468 manpage to learn what functions are available on your system.
1470 =item fileno FILEHANDLE
1472 Returns the file descriptor for a filehandle, or undefined if the
1473 filehandle is not open. This is mainly useful for constructing
1474 bitmaps for C<select> and low-level POSIX tty-handling operations.
1475 If FILEHANDLE is an expression, the value is taken as an indirect
1476 filehandle, generally its name.
1478 You can use this to find out whether two handles refer to the
1479 same underlying descriptor:
1481 if (fileno(THIS) == fileno(THAT)) {
1482 print "THIS and THAT are dups\n";
1485 =item flock FILEHANDLE,OPERATION
1487 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1488 for success, false on failure. Produces a fatal error if used on a
1489 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1490 C<flock> is Perl's portable file locking interface, although it locks
1491 only entire files, not records.
1493 Two potentially non-obvious but traditional C<flock> semantics are
1494 that it waits indefinitely until the lock is granted, and that its locks
1495 B<merely advisory>. Such discretionary locks are more flexible, but offer
1496 fewer guarantees. This means that files locked with C<flock> may be
1497 modified by programs that do not also use C<flock>. See L<perlport>,
1498 your port's specific documentation, or your system-specific local manpages
1499 for details. It's best to assume traditional behavior if you're writing
1500 portable programs. (But if you're not, you should as always feel perfectly
1501 free to write for your own system's idiosyncrasies (sometimes called
1502 "features"). Slavish adherence to portability concerns shouldn't get
1503 in the way of your getting your job done.)
1505 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1506 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1507 you can use the symbolic names if import them from the Fcntl module,
1508 either individually, or as a group using the ':flock' tag. LOCK_SH
1509 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1510 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1511 LOCK_EX then C<flock> will return immediately rather than blocking
1512 waiting for the lock (check the return status to see if you got it).
1514 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1515 before locking or unlocking it.
1517 Note that the emulation built with lockf(3) doesn't provide shared
1518 locks, and it requires that FILEHANDLE be open with write intent. These
1519 are the semantics that lockf(3) implements. Most if not all systems
1520 implement lockf(3) in terms of fcntl(2) locking, though, so the
1521 differing semantics shouldn't bite too many people.
1523 Note also that some versions of C<flock> cannot lock things over the
1524 network; you would need to use the more system-specific C<fcntl> for
1525 that. If you like you can force Perl to ignore your system's flock(2)
1526 function, and so provide its own fcntl(2)-based emulation, by passing
1527 the switch C<-Ud_flock> to the F<Configure> program when you configure
1530 Here's a mailbox appender for BSD systems.
1532 use Fcntl ':flock'; # import LOCK_* constants
1535 flock(MBOX,LOCK_EX);
1536 # and, in case someone appended
1537 # while we were waiting...
1542 flock(MBOX,LOCK_UN);
1545 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1546 or die "Can't open mailbox: $!";
1549 print MBOX $msg,"\n\n";
1552 On systems that support a real flock(), locks are inherited across fork()
1553 calls, whereas those that must resort to the more capricious fcntl()
1554 function lose the locks, making it harder to write servers.
1556 See also L<DB_File> for other flock() examples.
1560 Does a fork(2) system call to create a new process running the
1561 same program at the same point. It returns the child pid to the
1562 parent process, C<0> to the child process, or C<undef> if the fork is
1563 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1564 are shared, while everything else is copied. On most systems supporting
1565 fork(), great care has gone into making it extremely efficient (for
1566 example, using copy-on-write technology on data pages), making it the
1567 dominant paradigm for multitasking over the last few decades.
1569 All files opened for output are flushed before forking the child process.
1571 If you C<fork> without ever waiting on your children, you will
1572 accumulate zombies. On some systems, you can avoid this by setting
1573 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1574 forking and reaping moribund children.
1576 Note that if your forked child inherits system file descriptors like
1577 STDIN and STDOUT that are actually connected by a pipe or socket, even
1578 if you exit, then the remote server (such as, say, a CGI script or a
1579 backgrounded job launched from a remote shell) won't think you're done.
1580 You should reopen those to F</dev/null> if it's any issue.
1584 Declare a picture format for use by the C<write> function. For
1588 Test: @<<<<<<<< @||||| @>>>>>
1589 $str, $%, '$' . int($num)
1593 $num = $cost/$quantity;
1597 See L<perlform> for many details and examples.
1599 =item formline PICTURE,LIST
1601 This is an internal function used by C<format>s, though you may call it,
1602 too. It formats (see L<perlform>) a list of values according to the
1603 contents of PICTURE, placing the output into the format output
1604 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1605 Eventually, when a C<write> is done, the contents of
1606 C<$^A> are written to some filehandle, but you could also read C<$^A>
1607 yourself and then set C<$^A> back to C<"">. Note that a format typically
1608 does one C<formline> per line of form, but the C<formline> function itself
1609 doesn't care how many newlines are embedded in the PICTURE. This means
1610 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1611 You may therefore need to use multiple formlines to implement a single
1612 record format, just like the format compiler.
1614 Be careful if you put double quotes around the picture, because an C<@>
1615 character may be taken to mean the beginning of an array name.
1616 C<formline> always returns true. See L<perlform> for other examples.
1618 =item getc FILEHANDLE
1622 Returns the next character from the input file attached to FILEHANDLE,
1623 or the undefined value at end of file, or if there was an error.
1624 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1625 efficient. However, it cannot be used by itself to fetch single
1626 characters without waiting for the user to hit enter. For that, try
1627 something more like:
1630 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1633 system "stty", '-icanon', 'eol', "\001";
1639 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1642 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1646 Determination of whether $BSD_STYLE should be set
1647 is left as an exercise to the reader.
1649 The C<POSIX::getattr> function can do this more portably on
1650 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1651 module from your nearest CPAN site; details on CPAN can be found on
1656 Implements the C library function of the same name, which on most
1657 systems returns the current login from F</etc/utmp>, if any. If null,
1660 $login = getlogin || getpwuid($<) || "Kilroy";
1662 Do not consider C<getlogin> for authentication: it is not as
1663 secure as C<getpwuid>.
1665 =item getpeername SOCKET
1667 Returns the packed sockaddr address of other end of the SOCKET connection.
1670 $hersockaddr = getpeername(SOCK);
1671 ($port, $iaddr) = sockaddr_in($hersockaddr);
1672 $herhostname = gethostbyaddr($iaddr, AF_INET);
1673 $herstraddr = inet_ntoa($iaddr);
1677 Returns the current process group for the specified PID. Use
1678 a PID of C<0> to get the current process group for the
1679 current process. Will raise an exception if used on a machine that
1680 doesn't implement getpgrp(2). If PID is omitted, returns process
1681 group of current process. Note that the POSIX version of C<getpgrp>
1682 does not accept a PID argument, so only C<PID==0> is truly portable.
1686 Returns the process id of the parent process.
1688 =item getpriority WHICH,WHO
1690 Returns the current priority for a process, a process group, or a user.
1691 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1692 machine that doesn't implement getpriority(2).
1698 =item gethostbyname NAME
1700 =item getnetbyname NAME
1702 =item getprotobyname NAME
1708 =item getservbyname NAME,PROTO
1710 =item gethostbyaddr ADDR,ADDRTYPE
1712 =item getnetbyaddr ADDR,ADDRTYPE
1714 =item getprotobynumber NUMBER
1716 =item getservbyport PORT,PROTO
1734 =item sethostent STAYOPEN
1736 =item setnetent STAYOPEN
1738 =item setprotoent STAYOPEN
1740 =item setservent STAYOPEN
1754 These routines perform the same functions as their counterparts in the
1755 system library. In list context, the return values from the
1756 various get routines are as follows:
1758 ($name,$passwd,$uid,$gid,
1759 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1760 ($name,$passwd,$gid,$members) = getgr*
1761 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1762 ($name,$aliases,$addrtype,$net) = getnet*
1763 ($name,$aliases,$proto) = getproto*
1764 ($name,$aliases,$port,$proto) = getserv*
1766 (If the entry doesn't exist you get a null list.)
1768 In scalar context, you get the name, unless the function was a
1769 lookup by name, in which case you get the other thing, whatever it is.
1770 (If the entry doesn't exist you get the undefined value.) For example:
1772 $uid = getpwnam($name);
1773 $name = getpwuid($num);
1775 $gid = getgrnam($name);
1776 $name = getgrgid($num;
1780 In I<getpw*()> the fields $quota, $comment, and $expire are
1781 special cases in the sense that in many systems they are unsupported.
1782 If the $quota is unsupported, it is an empty scalar. If it is
1783 supported, it usually encodes the disk quota. If the $comment
1784 field is unsupported, it is an empty scalar. If it is supported it
1785 usually encodes some administrative comment about the user. In some
1786 systems the $quota field may be $change or $age, fields that have
1787 to do with password aging. In some systems the $comment field may
1788 be $class. The $expire field, if present, encodes the expiration
1789 period of the account or the password. For the availability and the
1790 exact meaning of these fields in your system, please consult your
1791 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1792 out from within Perl what your $quota and $comment fields mean
1793 and whether you have the $expire field by using the C<Config> module
1794 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1795 and C<d_pwexpire>. Shadow password files are only supported if your
1796 vendor has implemented them in the intuitive fashion that calling the
1797 regular C library routines gets the shadow versions if you're running
1798 under privilege. Those that incorrectly implement a separate library
1799 call are not supported.
1801 The $members value returned by I<getgr*()> is a space separated list of
1802 the login names of the members of the group.
1804 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1805 C, it will be returned to you via C<$?> if the function call fails. The
1806 C<@addrs> value returned by a successful call is a list of the raw
1807 addresses returned by the corresponding system library call. In the
1808 Internet domain, each address is four bytes long and you can unpack it
1809 by saying something like:
1811 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1813 The Socket library makes this slightly easier:
1816 $iaddr = inet_aton("127.1"); # or whatever address
1817 $name = gethostbyaddr($iaddr, AF_INET);
1819 # or going the other way
1820 $straddr = inet_ntoa($iaddr);
1822 If you get tired of remembering which element of the return list
1823 contains which return value, by-name interfaces are provided
1824 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1825 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1826 and C<User::grent>. These override the normal built-ins, supplying
1827 versions that return objects with the appropriate names
1828 for each field. For example:
1832 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1834 Even though it looks like they're the same method calls (uid),
1835 they aren't, because a C<File::stat> object is different from
1836 a C<User::pwent> object.
1838 =item getsockname SOCKET
1840 Returns the packed sockaddr address of this end of the SOCKET connection,
1841 in case you don't know the address because you have several different
1842 IPs that the connection might have come in on.
1845 $mysockaddr = getsockname(SOCK);
1846 ($port, $myaddr) = sockaddr_in($mysockaddr);
1847 printf "Connect to %s [%s]\n",
1848 scalar gethostbyaddr($myaddr, AF_INET),
1851 =item getsockopt SOCKET,LEVEL,OPTNAME
1853 Returns the socket option requested, or undef if there is an error.
1859 Returns the value of EXPR with filename expansions such as the
1860 standard Unix shell F</bin/csh> would do. This is the internal function
1861 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1862 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1863 discussed in more detail in L<perlop/"I/O Operators">.
1867 Converts a time as returned by the time function to a 9-element list
1868 with the time localized for the standard Greenwich time zone.
1869 Typically used as follows:
1872 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1875 All list elements are numeric, and come straight out of a struct tm.
1876 In particular this means that $mon has the range C<0..11> and $wday
1877 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1878 number of years since 1900, that is, $year is C<123> in year 2023,
1879 I<not> simply the last two digits of the year. If you assume it is,
1880 then you create non-Y2K-compliant programs--and you wouldn't want to do
1883 If EXPR is omitted, does C<gmtime(time())>.
1885 In scalar context, returns the ctime(3) value:
1887 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1889 Also see the C<timegm> function provided by the C<Time::Local> module,
1890 and the strftime(3) function available via the POSIX module.
1892 This scalar value is B<not> locale dependent (see L<perllocale>), but
1893 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1894 strftime(3) and mktime(3) functions available via the POSIX module. To
1895 get somewhat similar but locale dependent date strings, set up your
1896 locale environment variables appropriately (please see L<perllocale>)
1897 and try for example:
1899 use POSIX qw(strftime);
1900 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1902 Note that the C<%a> and C<%b> escapes, which represent the short forms
1903 of the day of the week and the month of the year, may not necessarily
1904 be three characters wide in all locales.
1912 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1913 execution there. It may not be used to go into any construct that
1914 requires initialization, such as a subroutine or a C<foreach> loop. It
1915 also can't be used to go into a construct that is optimized away,
1916 or to get out of a block or subroutine given to C<sort>.
1917 It can be used to go almost anywhere else within the dynamic scope,
1918 including out of subroutines, but it's usually better to use some other
1919 construct such as C<last> or C<die>. The author of Perl has never felt the
1920 need to use this form of C<goto> (in Perl, that is--C is another matter).
1922 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1923 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1924 necessarily recommended if you're optimizing for maintainability:
1926 goto ("FOO", "BAR", "GLARCH")[$i];
1928 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1929 named subroutine for the currently running subroutine. This is used by
1930 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1931 pretend that the other subroutine had been called in the first place
1932 (except that any modifications to C<@_> in the current subroutine are
1933 propagated to the other subroutine.) After the C<goto>, not even C<caller>
1934 will be able to tell that this routine was called first.
1936 =item grep BLOCK LIST
1938 =item grep EXPR,LIST
1940 This is similar in spirit to, but not the same as, grep(1) and its
1941 relatives. In particular, it is not limited to using regular expressions.
1943 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1944 C<$_> to each element) and returns the list value consisting of those
1945 elements for which the expression evaluated to true. In scalar
1946 context, returns the number of times the expression was true.
1948 @foo = grep(!/^#/, @bar); # weed out comments
1952 @foo = grep {!/^#/} @bar; # weed out comments
1954 Note that, because C<$_> is a reference into the list value, it can
1955 be used to modify the elements of the array. While this is useful and
1956 supported, it can cause bizarre results if the LIST is not a named array.
1957 Similarly, grep returns aliases into the original list, much as a for
1958 loop's index variable aliases the list elements. That is, modifying an
1959 element of a list returned by grep (for example, in a C<foreach>, C<map>
1960 or another C<grep>) actually modifies the element in the original list.
1961 This is usually something to be avoided when writing clear code.
1963 See also L</map> for a list composed of the results of the BLOCK or EXPR.
1969 Interprets EXPR as a hex string and returns the corresponding value.
1970 (To convert strings that might start with either 0, 0x, or 0b, see
1971 L</oct>.) If EXPR is omitted, uses C<$_>.
1973 print hex '0xAf'; # prints '175'
1974 print hex 'aF'; # same
1976 Hex strings may only represent integers. Strings that would cause
1977 integer overflow trigger a mandatory error message.
1981 There is no builtin C<import> function. It is just an ordinary
1982 method (subroutine) defined (or inherited) by modules that wish to export
1983 names to another module. The C<use> function calls the C<import> method
1984 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1986 =item index STR,SUBSTR,POSITION
1988 =item index STR,SUBSTR
1990 The index function searches for one string within another, but without
1991 the wildcard-like behavior of a full regular-expression pattern match.
1992 It returns the position of the first occurrence of SUBSTR in STR at
1993 or after POSITION. If POSITION is omitted, starts searching from the
1994 beginning of the string. The return value is based at C<0> (or whatever
1995 you've set the C<$[> variable to--but don't do that). If the substring
1996 is not found, returns one less than the base, ordinarily C<-1>.
2002 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2003 You should not use this function for rounding: one because it truncates
2004 towards C<0>, and two because machine representations of floating point
2005 numbers can sometimes produce counterintuitive results. For example,
2006 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2007 because it's really more like -268.99999999999994315658 instead. Usually,
2008 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2009 functions will serve you better than will int().
2011 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2013 Implements the ioctl(2) function. You'll probably first have to say
2015 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2017 to get the correct function definitions. If F<ioctl.ph> doesn't
2018 exist or doesn't have the correct definitions you'll have to roll your
2019 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2020 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2021 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2022 written depending on the FUNCTION--a pointer to the string value of SCALAR
2023 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2024 has no string value but does have a numeric value, that value will be
2025 passed rather than a pointer to the string value. To guarantee this to be
2026 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2027 functions may be needed to manipulate the values of structures used by
2030 The return value of C<ioctl> (and C<fcntl>) is as follows:
2032 if OS returns: then Perl returns:
2034 0 string "0 but true"
2035 anything else that number
2037 Thus Perl returns true on success and false on failure, yet you can
2038 still easily determine the actual value returned by the operating
2041 $retval = ioctl(...) || -1;
2042 printf "System returned %d\n", $retval;
2044 The special string "C<0> but true" is exempt from B<-w> complaints
2045 about improper numeric conversions.
2047 Here's an example of setting a filehandle named C<REMOTE> to be
2048 non-blocking at the system level. You'll have to negotiate C<$|>
2049 on your own, though.
2051 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2053 $flags = fcntl(REMOTE, F_GETFL, 0)
2054 or die "Can't get flags for the socket: $!\n";
2056 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2057 or die "Can't set flags for the socket: $!\n";
2059 =item join EXPR,LIST
2061 Joins the separate strings of LIST into a single string with fields
2062 separated by the value of EXPR, and returns that new string. Example:
2064 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2070 Returns a list consisting of all the keys of the named hash. (In
2071 scalar context, returns the number of keys.) The keys are returned in
2072 an apparently random order. The actual random order is subject to
2073 change in future versions of perl, but it is guaranteed to be the same
2074 order as either the C<values> or C<each> function produces (given
2075 that the hash has not been modified). As a side effect, it resets
2078 Here is yet another way to print your environment:
2081 @values = values %ENV;
2083 print pop(@keys), '=', pop(@values), "\n";
2086 or how about sorted by key:
2088 foreach $key (sort(keys %ENV)) {
2089 print $key, '=', $ENV{$key}, "\n";
2092 To sort a hash by value, you'll need to use a C<sort> function.
2093 Here's a descending numeric sort of a hash by its values:
2095 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2096 printf "%4d %s\n", $hash{$key}, $key;
2099 As an lvalue C<keys> allows you to increase the number of hash buckets
2100 allocated for the given hash. This can gain you a measure of efficiency if
2101 you know the hash is going to get big. (This is similar to pre-extending
2102 an array by assigning a larger number to $#array.) If you say
2106 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2107 in fact, since it rounds up to the next power of two. These
2108 buckets will be retained even if you do C<%hash = ()>, use C<undef
2109 %hash> if you want to free the storage while C<%hash> is still in scope.
2110 You can't shrink the number of buckets allocated for the hash using
2111 C<keys> in this way (but you needn't worry about doing this by accident,
2112 as trying has no effect).
2114 See also C<each>, C<values> and C<sort>.
2118 Sends a signal to a list of processes. The first element of
2119 the list must be the signal to send. Returns the number of
2120 processes successfully signaled.
2122 $cnt = kill 1, $child1, $child2;
2125 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2126 process groups instead of processes. (On System V, a negative I<PROCESS>
2127 number will also kill process groups, but that's not portable.) That
2128 means you usually want to use positive not negative signals. You may also
2129 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2135 The C<last> command is like the C<break> statement in C (as used in
2136 loops); it immediately exits the loop in question. If the LABEL is
2137 omitted, the command refers to the innermost enclosing loop. The
2138 C<continue> block, if any, is not executed:
2140 LINE: while (<STDIN>) {
2141 last LINE if /^$/; # exit when done with header
2145 C<last> cannot be used to exit a block which returns a value such as
2146 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2147 a grep() or map() operation.
2149 See also L</continue> for an illustration of how C<last>, C<next>, and
2156 Returns an lowercased version of EXPR. This is the internal function
2157 implementing the C<\L> escape in double-quoted strings.
2158 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2161 If EXPR is omitted, uses C<$_>.
2167 Returns the value of EXPR with the first character lowercased. This is
2168 the internal function implementing the C<\l> escape in double-quoted strings.
2169 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2171 If EXPR is omitted, uses C<$_>.
2177 Returns the length in characters of the value of EXPR. If EXPR is
2178 omitted, returns length of C<$_>. Note that this cannot be used on
2179 an entire array or hash to find out how many elements these have.
2180 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2182 =item link OLDFILE,NEWFILE
2184 Creates a new filename linked to the old filename. Returns true for
2185 success, false otherwise.
2187 =item listen SOCKET,QUEUESIZE
2189 Does the same thing that the listen system call does. Returns true if
2190 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2194 You really probably want to be using C<my> instead, because C<local> isn't
2195 what most people think of as "local". See L<perlsub/"Private Variables
2196 via my()"> for details.
2198 A local modifies the listed variables to be local to the enclosing
2199 block, file, or eval. If more than one value is listed, the list must
2200 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2201 for details, including issues with tied arrays and hashes.
2203 =item localtime EXPR
2205 Converts a time as returned by the time function to a 9-element list
2206 with the time analyzed for the local time zone. Typically used as
2210 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2213 All list elements are numeric, and come straight out of a struct tm.
2214 In particular this means that $mon has the range C<0..11> and $wday
2215 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2216 number of years since 1900, that is, $year is C<123> in year 2023,
2217 and I<not> simply the last two digits of the year. If you assume it is,
2218 then you create non-Y2K-compliant programs--and you wouldn't want to do
2221 If EXPR is omitted, uses the current time (C<localtime(time)>).
2223 In scalar context, returns the ctime(3) value:
2225 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2227 This scalar value is B<not> locale dependent, see L<perllocale>, but
2228 instead a Perl builtin. Also see the C<Time::Local> module, and the
2229 strftime(3) and mktime(3) function available via the POSIX module. To
2230 get somewhat similar but locale dependent date strings, set up your
2231 locale environment variables appropriately (please see L<perllocale>)
2232 and try for example:
2234 use POSIX qw(strftime);
2235 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2237 Note that the C<%a> and C<%b>, the short forms of the day of the week
2238 and the month of the year, may not necessarily be three characters wide.
2244 This function places an advisory lock on a variable, subroutine,
2245 or referenced object contained in I<THING> until the lock goes out
2246 of scope. This is a built-in function only if your version of Perl
2247 was built with threading enabled, and if you've said C<use Threads>.
2248 Otherwise a user-defined function by this name will be called. See
2255 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2256 returns log of C<$_>. To get the log of another base, use basic algebra:
2257 The base-N log of a number is equal to the natural log of that number
2258 divided by the natural log of N. For example:
2262 return log($n)/log(10);
2265 See also L</exp> for the inverse operation.
2267 =item lstat FILEHANDLE
2273 Does the same thing as the C<stat> function (including setting the
2274 special C<_> filehandle) but stats a symbolic link instead of the file
2275 the symbolic link points to. If symbolic links are unimplemented on
2276 your system, a normal C<stat> is done.
2278 If EXPR is omitted, stats C<$_>.
2282 The match operator. See L<perlop>.
2284 =item map BLOCK LIST
2288 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2289 C<$_> to each element) and returns the list value composed of the
2290 results of each such evaluation. In scalar context, returns the
2291 total number of elements so generated. Evaluates BLOCK or EXPR in
2292 list context, so each element of LIST may produce zero, one, or
2293 more elements in the returned value.
2295 @chars = map(chr, @nums);
2297 translates a list of numbers to the corresponding characters. And
2299 %hash = map { getkey($_) => $_ } @array;
2301 is just a funny way to write
2304 foreach $_ (@array) {
2305 $hash{getkey($_)} = $_;
2308 Note that, because C<$_> is a reference into the list value, it can
2309 be used to modify the elements of the array. While this is useful and
2310 supported, it can cause bizarre results if the LIST is not a named array.
2311 Using a regular C<foreach> loop for this purpose would be clearer in
2312 most cases. See also L</grep> for an array composed of those items of
2313 the original list for which the BLOCK or EXPR evaluates to true.
2315 =item mkdir FILENAME,MASK
2317 Creates the directory specified by FILENAME, with permissions
2318 specified by MASK (as modified by C<umask>). If it succeeds it
2319 returns true, otherwise it returns false and sets C<$!> (errno).
2321 In general, it is better to create directories with permissive MASK,
2322 and let the user modify that with their C<umask>, than it is to supply
2323 a restrictive MASK and give the user no way to be more permissive.
2324 The exceptions to this rule are when the file or directory should be
2325 kept private (mail files, for instance). The perlfunc(1) entry on
2326 C<umask> discusses the choice of MASK in more detail.
2328 =item msgctl ID,CMD,ARG
2330 Calls the System V IPC function msgctl(2). You'll probably have to say
2334 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2335 then ARG must be a variable which will hold the returned C<msqid_ds>
2336 structure. Returns like C<ioctl>: the undefined value for error, C<"0 but
2337 true"> for zero, or the actual return value otherwise. See also
2338 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2340 =item msgget KEY,FLAGS
2342 Calls the System V IPC function msgget(2). Returns the message queue
2343 id, or the undefined value if there is an error. See also C<IPC::SysV>
2344 and C<IPC::Msg> documentation.
2346 =item msgsnd ID,MSG,FLAGS
2348 Calls the System V IPC function msgsnd to send the message MSG to the
2349 message queue ID. MSG must begin with the long integer message type,
2350 which may be created with C<pack("l", $type)>. Returns true if
2351 successful, or false if there is an error. See also C<IPC::SysV>
2352 and C<IPC::SysV::Msg> documentation.
2354 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2356 Calls the System V IPC function msgrcv to receive a message from
2357 message queue ID into variable VAR with a maximum message size of
2358 SIZE. Note that if a message is received, the message type will be
2359 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2360 size of the message type. Returns true if successful, or false if
2361 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2365 A C<my> declares the listed variables to be local (lexically) to the
2366 enclosing block, file, or C<eval>. If
2367 more than one value is listed, the list must be placed in parentheses. See
2368 L<perlsub/"Private Variables via my()"> for details.
2374 The C<next> command is like the C<continue> statement in C; it starts
2375 the next iteration of the loop:
2377 LINE: while (<STDIN>) {
2378 next LINE if /^#/; # discard comments
2382 Note that if there were a C<continue> block on the above, it would get
2383 executed even on discarded lines. If the LABEL is omitted, the command
2384 refers to the innermost enclosing loop.
2386 C<next> cannot be used to exit a block which returns a value such as
2387 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2388 a grep() or map() operation.
2390 See also L</continue> for an illustration of how C<last>, C<next>, and
2393 =item no Module LIST
2395 See the L</use> function, which C<no> is the opposite of.
2401 Interprets EXPR as an octal string and returns the corresponding
2402 value. (If EXPR happens to start off with C<0x>, interprets it as a
2403 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2404 binary string.) The following will handle decimal, binary, octal, and
2405 hex in the standard Perl or C notation:
2407 $val = oct($val) if $val =~ /^0/;
2409 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2410 in octal), use sprintf() or printf():
2412 $perms = (stat("filename"))[2] & 07777;
2413 $oct_perms = sprintf "%lo", $perms;
2415 The oct() function is commonly used when a string such as C<644> needs
2416 to be converted into a file mode, for example. (Although perl will
2417 automatically convert strings into numbers as needed, this automatic
2418 conversion assumes base 10.)
2420 =item open FILEHANDLE,EXPR
2422 =item open FILEHANDLE
2424 Opens the file whose filename is given by EXPR, and associates it with
2425 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2426 name of the real filehandle wanted. If EXPR is omitted, the scalar
2427 variable of the same name as the FILEHANDLE contains the filename.
2428 (Note that lexical variables--those declared with C<my>--will not work
2429 for this purpose; so if you're using C<my>, specify EXPR in your call
2430 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2433 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2434 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2435 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2436 the file is opened for appending, again being created if necessary.
2437 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2438 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2439 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2440 file first. You can't usually use either read-write mode for updating
2441 textfiles, since they have variable length records. See the B<-i>
2442 switch in L<perlrun> for a better approach. The file is created with
2443 permissions of C<0666> modified by the process' C<umask> value.
2445 The prefix and the filename may be separated with spaces.
2446 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2447 C<'w+'>, C<'a'>, and C<'a+'>.
2449 If the filename begins with C<'|'>, the filename is interpreted as a
2450 command to which output is to be piped, and if the filename ends with a
2451 C<'|'>, the filename is interpreted as a command which pipes output to
2452 us. See L<perlipc/"Using open() for IPC">
2453 for more examples of this. (You are not allowed to C<open> to a command
2454 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2455 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2457 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2458 nonzero upon success, the undefined value otherwise. If the C<open>
2459 involved a pipe, the return value happens to be the pid of the
2462 If you're unfortunate enough to be running Perl on a system that
2463 distinguishes between text files and binary files (modern operating
2464 systems don't care), then you should check out L</binmode> for tips for
2465 dealing with this. The key distinction between systems that need C<binmode>
2466 and those that don't is their text file formats. Systems like Unix, MacOS, and
2467 Plan9, which delimit lines with a single character, and which encode that
2468 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2470 When opening a file, it's usually a bad idea to continue normal execution
2471 if the request failed, so C<open> is frequently used in connection with
2472 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2473 where you want to make a nicely formatted error message (but there are
2474 modules that can help with that problem)) you should always check
2475 the return value from opening a file. The infrequent exception is when
2476 working with an unopened filehandle is actually what you want to do.
2481 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2482 while (<ARTICLE>) {...
2484 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2485 # if the open fails, output is discarded
2487 open(DBASE, '+<dbase.mine') # open for update
2488 or die "Can't open 'dbase.mine' for update: $!";
2490 open(ARTICLE, "caesar <$article |") # decrypt article
2491 or die "Can't start caesar: $!";
2493 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2494 or die "Can't start sort: $!";
2496 # process argument list of files along with any includes
2498 foreach $file (@ARGV) {
2499 process($file, 'fh00');
2503 my($filename, $input) = @_;
2504 $input++; # this is a string increment
2505 unless (open($input, $filename)) {
2506 print STDERR "Can't open $filename: $!\n";
2511 while (<$input>) { # note use of indirection
2512 if (/^#include "(.*)"/) {
2513 process($1, $input);
2520 You may also, in the Bourne shell tradition, specify an EXPR beginning
2521 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2522 name of a filehandle (or file descriptor, if numeric) to be
2523 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2524 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2525 mode you specify should match the mode of the original filehandle.
2526 (Duping a filehandle does not take into account any existing contents of
2528 Here is a script that saves, redirects, and restores STDOUT and
2532 open(OLDOUT, ">&STDOUT");
2533 open(OLDERR, ">&STDERR");
2535 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2536 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2538 select(STDERR); $| = 1; # make unbuffered
2539 select(STDOUT); $| = 1; # make unbuffered
2541 print STDOUT "stdout 1\n"; # this works for
2542 print STDERR "stderr 1\n"; # subprocesses too
2547 open(STDOUT, ">&OLDOUT");
2548 open(STDERR, ">&OLDERR");
2550 print STDOUT "stdout 2\n";
2551 print STDERR "stderr 2\n";
2553 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2554 equivalent of C's C<fdopen> of that file descriptor; this is more
2555 parsimonious of file descriptors. For example:
2557 open(FILEHANDLE, "<&=$fd")
2559 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2560 there is an implicit fork done, and the return value of open is the pid
2561 of the child within the parent process, and C<0> within the child
2562 process. (Use C<defined($pid)> to determine whether the open was successful.)
2563 The filehandle behaves normally for the parent, but i/o to that
2564 filehandle is piped from/to the STDOUT/STDIN of the child process.
2565 In the child process the filehandle isn't opened--i/o happens from/to
2566 the new STDOUT or STDIN. Typically this is used like the normal
2567 piped open when you want to exercise more control over just how the
2568 pipe command gets executed, such as when you are running setuid, and
2569 don't want to have to scan shell commands for metacharacters.
2570 The following pairs are more or less equivalent:
2572 open(FOO, "|tr '[a-z]' '[A-Z]'");
2573 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2575 open(FOO, "cat -n '$file'|");
2576 open(FOO, "-|") || exec 'cat', '-n', $file;
2578 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2580 NOTE: On any operation that may do a fork, all files opened for output
2581 are flushed before the fork is attempted. On systems that support a
2582 close-on-exec flag on files, the flag will be set for the newly opened
2583 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2585 Closing any piped filehandle causes the parent process to wait for the
2586 child to finish, and returns the status value in C<$?>.
2588 The filename passed to open will have leading and trailing
2589 whitespace deleted, and the normal redirection characters
2590 honored. This property, known as "magic open",
2591 can often be used to good effect. A user could specify a filename of
2592 F<"rsh cat file |">, or you could change certain filenames as needed:
2594 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2595 open(FH, $filename) or die "Can't open $filename: $!";
2597 However, to open a file with arbitrary weird characters in it, it's
2598 necessary to protect any leading and trailing whitespace:
2600 $file =~ s#^(\s)#./$1#;
2601 open(FOO, "< $file\0");
2603 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2604 should use the C<sysopen> function, which involves no such magic. This is
2605 another way to protect your filenames from interpretation. For example:
2608 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2609 or die "sysopen $path: $!";
2610 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2611 print HANDLE "stuff $$\n");
2613 print "File contains: ", <HANDLE>;
2615 Using the constructor from the C<IO::Handle> package (or one of its
2616 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2617 filehandles that have the scope of whatever variables hold references to
2618 them, and automatically close whenever and however you leave that scope:
2622 sub read_myfile_munged {
2624 my $handle = new IO::File;
2625 open($handle, "myfile") or die "myfile: $!";
2627 or return (); # Automatically closed here.
2628 mung $first or die "mung failed"; # Or here.
2629 return $first, <$handle> if $ALL; # Or here.
2633 See L</seek> for some details about mixing reading and writing.
2635 =item opendir DIRHANDLE,EXPR
2637 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2638 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2639 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2645 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2646 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2647 See L<utf8> for more about Unicode.
2649 =item pack TEMPLATE,LIST
2651 Takes a list of values and packs it into a binary structure,
2652 returning the string containing the structure. The TEMPLATE is a
2653 sequence of characters that give the order and type of values, as
2656 a A string with arbitrary binary data, will be null padded.
2657 A An ascii string, will be space padded.
2658 Z A null terminated (asciz) string, will be null padded.
2660 b A bit string (ascending bit order, like vec()).
2661 B A bit string (descending bit order).
2662 h A hex string (low nybble first).
2663 H A hex string (high nybble first).
2665 c A signed char value.
2666 C An unsigned char value. Only does bytes. See U for Unicode.
2668 s A signed short value.
2669 S An unsigned short value.
2670 (This 'short' is _exactly_ 16 bits, which may differ from
2671 what a local C compiler calls 'short'.)
2673 i A signed integer value.
2674 I An unsigned integer value.
2675 (This 'integer' is _at_least_ 32 bits wide. Its exact
2676 size depends on what a local C compiler calls 'int',
2677 and may even be larger than the 'long' described in
2680 l A signed long value.
2681 L An unsigned long value.
2682 (This 'long' is _exactly_ 32 bits, which may differ from
2683 what a local C compiler calls 'long'.)
2685 n A short in "network" (big-endian) order.
2686 N A long in "network" (big-endian) order.
2687 v A short in "VAX" (little-endian) order.
2688 V A long in "VAX" (little-endian) order.
2689 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2690 _exactly_ 32 bits, respectively.)
2692 q A signed quad (64-bit) value.
2693 Q An unsigned quad value.
2694 (Available only if your system supports 64-bit integer values
2695 _and_ if Perl has been compiled to support those.
2696 Causes a fatal error otherwise.)
2698 f A single-precision float in the native format.
2699 d A double-precision float in the native format.
2701 p A pointer to a null-terminated string.
2702 P A pointer to a structure (fixed-length string).
2704 u A uuencoded string.
2705 U A Unicode character number. Encodes to UTF-8 internally.
2706 Works even if C<use utf8> is not in effect.
2708 w A BER compressed integer. Its bytes represent an unsigned
2709 integer in base 128, most significant digit first, with as
2710 few digits as possible. Bit eight (the high bit) is set
2711 on each byte except the last.
2715 @ Null fill to absolute position.
2717 The following rules apply:
2723 Each letter may optionally be followed by a number giving a repeat
2724 count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2725 C<"H">, and C<"P"> the pack function will gobble up that many values from
2726 the LIST. A C<*> for the repeat count means to use however many items are
2731 The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
2732 string of length count, padding with nulls or spaces as necessary. When
2733 unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2734 after the first null, and C<"a"> returns data verbatim.
2738 Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2742 The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2746 The C<"p"> type packs a pointer to a null-terminated string. You are
2747 responsible for ensuring the string is not a temporary value (which can
2748 potentially get deallocated before you get around to using the packed result).
2749 The C<"P"> type packs a pointer to a structure of the size indicated by the
2750 length. A NULL pointer is created if the corresponding value for C<"p"> or
2755 The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
2756 immediately followed by a C<"!"> to signify native shorts or longs--as
2757 you can see from above for example a bare C<"l"> does mean exactly 32
2758 bits, the native C<long> (as seen by the local C compiler) may be
2759 larger. This is an issue mainly in 64-bit platforms. You can see
2760 whether using C<"!"> makes any difference by
2762 print length(pack("s")), " ", length(pack("s!")), "\n";
2763 print length(pack("l")), " ", length(pack("l!")), "\n";
2765 C<"i!"> and C<"I!"> also work but only because of completeness;
2766 they are identical to C<"i"> and C<"I">.
2768 The actual sizes (in bytes) of native shorts, ints, and longs on
2769 the platform where Perl was built are also available via L<Config>:
2771 The actual sizes (in bytes) of native shorts, ints, longs, and long
2772 longs on the platform where Perl was built are also available via
2776 print $Config{shortsize}, "\n";
2777 print $Config{intsize}, "\n";
2778 print $Config{longsize}, "\n";
2779 print $Config{longlongsize}, "\n";
2783 The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
2784 are inherently non-portable between processors and operating systems
2785 because they obey the native byteorder and endianness. For example a
2786 4-byte integer 0x87654321 (2271560481 decimal) be ordered natively
2787 (arranged in and handled by the CPU registers) into bytes as
2789 0x12 0x34 0x56 0x78 # little-endian
2790 0x78 0x56 0x34 0x12 # big-endian
2792 Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
2793 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
2794 Power, and Cray are big-endian. MIPS can be either: Digital used it
2795 in little-endian mode; SGI uses it in big-endian mode.
2797 The names `big-endian' and `little-endian' are comic references to
2798 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
2799 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
2800 the egg-eating habits of the Lilliputians.
2802 Some systems may even have weird byte orders such as
2807 You can see your system's preference with
2809 print join(" ", map { sprintf "%#02x", $_ }
2810 unpack("C*",pack("L",0x12345678))), "\n";
2812 The byteorder on the platform where Perl was built is also available
2816 print $Config{byteorder}, "\n";
2818 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
2819 and C<'87654321'> are big-endian.
2821 If you want portable packed integers use the formats C<"n">, C<"N">,
2822 C<"v">, and C<"V">, their byte endianness and size is known.
2826 Real numbers (floats and doubles) are in the native machine format only;
2827 due to the multiplicity of floating formats around, and the lack of a
2828 standard "network" representation, no facility for interchange has been
2829 made. This means that packed floating point data written on one machine
2830 may not be readable on another - even if both use IEEE floating point
2831 arithmetic (as the endian-ness of the memory representation is not part
2834 Note that Perl uses doubles internally for all numeric calculation, and
2835 converting from double into float and thence back to double again will
2836 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2843 $foo = pack("CCCC",65,66,67,68);
2845 $foo = pack("C4",65,66,67,68);
2847 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2848 # same thing with Unicode circled letters
2850 $foo = pack("ccxxcc",65,66,67,68);
2853 $foo = pack("s2",1,2);
2854 # "\1\0\2\0" on little-endian
2855 # "\0\1\0\2" on big-endian
2857 $foo = pack("a4","abcd","x","y","z");
2860 $foo = pack("aaaa","abcd","x","y","z");
2863 $foo = pack("a14","abcdefg");
2864 # "abcdefg\0\0\0\0\0\0\0"
2866 $foo = pack("i9pl", gmtime);
2867 # a real struct tm (on my system anyway)
2869 $utmp_template = "Z8 Z8 Z16 L";
2870 $utmp = pack($utmp_template, @utmp1);
2871 # a struct utmp (BSDish)
2873 @utmp2 = unpack($utmp_template, $utmp);
2874 # "@utmp1" eq "@utmp2"
2877 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2880 The same template may generally also be used in unpack().
2884 =item package NAMESPACE
2886 Declares the compilation unit as being in the given namespace. The scope
2887 of the package declaration is from the declaration itself through the end
2888 of the enclosing block, file, or eval (the same as the C<my> operator).
2889 All further unqualified dynamic identifiers will be in this namespace.
2890 A package statement affects only dynamic variables--including those
2891 you've used C<local> on--but I<not> lexical variables, which are created
2892 with C<my>. Typically it would be the first declaration in a file to
2893 be included by the C<require> or C<use> operator. You can switch into a
2894 package in more than one place; it merely influences which symbol table
2895 is used by the compiler for the rest of that block. You can refer to
2896 variables and filehandles in other packages by prefixing the identifier
2897 with the package name and a double colon: C<$Package::Variable>.
2898 If the package name is null, the C<main> package as assumed. That is,
2899 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
2900 still seen in older code).
2902 If NAMESPACE is omitted, then there is no current package, and all
2903 identifiers must be fully qualified or lexicals. This is stricter
2904 than C<use strict>, since it also extends to function names.
2906 See L<perlmod/"Packages"> for more information about packages, modules,
2907 and classes. See L<perlsub> for other scoping issues.
2909 =item pipe READHANDLE,WRITEHANDLE
2911 Opens a pair of connected pipes like the corresponding system call.
2912 Note that if you set up a loop of piped processes, deadlock can occur
2913 unless you are very careful. In addition, note that Perl's pipes use
2914 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2915 after each command, depending on the application.
2917 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2918 for examples of such things.
2920 On systems that support a close-on-exec flag on files, the flag will be set
2921 for the newly opened file descriptors as determined by the value of $^F.
2928 Pops and returns the last value of the array, shortening the array by
2929 one element. Has an effect similar to
2933 If there are no elements in the array, returns the undefined value
2934 (although this may happen at other times as well). If ARRAY is
2935 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
2936 array in subroutines, just like C<shift>.
2942 Returns the offset of where the last C<m//g> search left off for the variable
2943 is in question (C<$_> is used when the variable is not specified). May be
2944 modified to change that offset. Such modification will also influence
2945 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2948 =item print FILEHANDLE LIST
2954 Prints a string or a list of strings. Returns true if successful.
2955 FILEHANDLE may be a scalar variable name, in which case the variable
2956 contains the name of or a reference to the filehandle, thus introducing
2957 one level of indirection. (NOTE: If FILEHANDLE is a variable and
2958 the next token is a term, it may be misinterpreted as an operator
2959 unless you interpose a C<+> or put parentheses around the arguments.)
2960 If FILEHANDLE is omitted, prints by default to standard output (or
2961 to the last selected output channel--see L</select>). If LIST is
2962 also omitted, prints C<$_> to the currently selected output channel.
2963 To set the default output channel to something other than STDOUT
2964 use the select operation. The current value of C<$,> (if any) is
2965 printed between each LIST item. The current value of C<$\> (if
2966 any) is printed after the entire LIST has been printed. Because
2967 print takes a LIST, anything in the LIST is evaluated in list
2968 context, and any subroutine that you call will have one or more of
2969 its expressions evaluated in list context. Also be careful not to
2970 follow the print keyword with a left parenthesis unless you want
2971 the corresponding right parenthesis to terminate the arguments to
2972 the print--interpose a C<+> or put parentheses around all the
2975 Note that if you're storing FILEHANDLES in an array or other expression,
2976 you will have to use a block returning its value instead:
2978 print { $files[$i] } "stuff\n";
2979 print { $OK ? STDOUT : STDERR } "stuff\n";
2981 =item printf FILEHANDLE FORMAT, LIST
2983 =item printf FORMAT, LIST
2985 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2986 (the output record separator) is not appended. The first argument
2987 of the list will be interpreted as the C<printf> format. If C<use locale> is
2988 in effect, the character used for the decimal point in formatted real numbers
2989 is affected by the LC_NUMERIC locale. See L<perllocale>.
2991 Don't fall into the trap of using a C<printf> when a simple
2992 C<print> would do. The C<print> is more efficient and less
2995 =item prototype FUNCTION
2997 Returns the prototype of a function as a string (or C<undef> if the
2998 function has no prototype). FUNCTION is a reference to, or the name of,
2999 the function whose prototype you want to retrieve.
3001 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3002 name for Perl builtin. If the builtin is not I<overridable> (such as
3003 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3004 C<system>) returns C<undef> because the builtin does not really behave
3005 like a Perl function. Otherwise, the string describing the equivalent
3006 prototype is returned.
3008 =item push ARRAY,LIST
3010 Treats ARRAY as a stack, and pushes the values of LIST
3011 onto the end of ARRAY. The length of ARRAY increases by the length of
3012 LIST. Has the same effect as
3015 $ARRAY[++$#ARRAY] = $value;
3018 but is more efficient. Returns the new number of elements in the array.
3030 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3032 =item quotemeta EXPR
3036 Returns the value of EXPR with all non-alphanumeric
3037 characters backslashed. (That is, all characters not matching
3038 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3039 returned string, regardless of any locale settings.)
3040 This is the internal function implementing
3041 the C<\Q> escape in double-quoted strings.
3043 If EXPR is omitted, uses C<$_>.
3049 Returns a random fractional number greater than or equal to C<0> and less
3050 than the value of EXPR. (EXPR should be positive.) If EXPR is
3051 omitted, the value C<1> is used. Automatically calls C<srand> unless
3052 C<srand> has already been called. See also C<srand>.
3054 (Note: If your rand function consistently returns numbers that are too
3055 large or too small, then your version of Perl was probably compiled
3056 with the wrong number of RANDBITS.)
3058 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3060 =item read FILEHANDLE,SCALAR,LENGTH
3062 Attempts to read LENGTH bytes of data into variable SCALAR from the
3063 specified FILEHANDLE. Returns the number of bytes actually read,
3064 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3065 or shrunk to the length actually read. An OFFSET may be specified to
3066 place the read data at some other place than the beginning of the
3067 string. This call is actually implemented in terms of stdio's fread(3)
3068 call. To get a true read(2) system call, see C<sysread>.
3070 =item readdir DIRHANDLE
3072 Returns the next directory entry for a directory opened by C<opendir>.
3073 If used in list context, returns all the rest of the entries in the
3074 directory. If there are no more entries, returns an undefined value in
3075 scalar context or a null list in list context.
3077 If you're planning to filetest the return values out of a C<readdir>, you'd
3078 better prepend the directory in question. Otherwise, because we didn't
3079 C<chdir> there, it would have been testing the wrong file.
3081 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3082 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3087 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3088 context, each call reads and returns the next line, until end-of-file is
3089 reached, whereupon the subsequent call returns undef. In list context,
3090 reads until end-of-file is reached and returns a list of lines. Note that
3091 the notion of "line" used here is however you may have defined it
3092 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3094 When C<$/> is set to C<undef>, when readline() is in scalar
3095 context (i.e. file slurp mode), and when an empty file is read, it
3096 returns C<''> the first time, followed by C<undef> subsequently.
3098 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3099 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3100 operator is discussed in more detail in L<perlop/"I/O Operators">.
3103 $line = readline(*STDIN); # same thing
3109 Returns the value of a symbolic link, if symbolic links are
3110 implemented. If not, gives a fatal error. If there is some system
3111 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3112 omitted, uses C<$_>.
3116 EXPR is executed as a system command.
3117 The collected standard output of the command is returned.
3118 In scalar context, it comes back as a single (potentially
3119 multi-line) string. In list context, returns a list of lines
3120 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3121 This is the internal function implementing the C<qx/EXPR/>
3122 operator, but you can use it directly. The C<qx/EXPR/>
3123 operator is discussed in more detail in L<perlop/"I/O Operators">.
3125 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3127 Receives a message on a socket. Attempts to receive LENGTH bytes of
3128 data into variable SCALAR from the specified SOCKET filehandle.
3129 Actually does a C C<recvfrom>, so that it can return the address of the
3130 sender. Returns the undefined value if there's an error. SCALAR will
3131 be grown or shrunk to the length actually read. Takes the same flags
3132 as the system call of the same name.
3133 See L<perlipc/"UDP: Message Passing"> for examples.
3139 The C<redo> command restarts the loop block without evaluating the
3140 conditional again. The C<continue> block, if any, is not executed. If
3141 the LABEL is omitted, the command refers to the innermost enclosing
3142 loop. This command is normally used by programs that want to lie to
3143 themselves about what was just input:
3145 # a simpleminded Pascal comment stripper
3146 # (warning: assumes no { or } in strings)
3147 LINE: while (<STDIN>) {
3148 while (s|({.*}.*){.*}|$1 |) {}
3153 if (/}/) { # end of comment?
3162 C<redo> cannot be used to retry a block which returns a value such as
3163 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3164 a grep() or map() operation.
3166 See also L</continue> for an illustration of how C<last>, C<next>, and
3173 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3174 is not specified, C<$_> will be used. The value returned depends on the
3175 type of thing the reference is a reference to.
3176 Builtin types include:
3186 If the referenced object has been blessed into a package, then that package
3187 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3189 if (ref($r) eq "HASH") {
3190 print "r is a reference to a hash.\n";
3193 print "r is not a reference at all.\n";
3195 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3196 print "r is a reference to something that isa hash.\n";
3199 See also L<perlref>.
3201 =item rename OLDNAME,NEWNAME
3203 Changes the name of a file; an existing file NEWNAME will be
3204 clobbered. Returns true for success, false otherwise.
3206 Behavior of this function varies wildly depending on your system
3207 implementation. For example, it will usually not work across file system
3208 boundaries, even though the system I<mv> command sometimes compensates
3209 for this. Other restrictions include whether it works on directories,
3210 open files, or pre-existing files. Check L<perlport> and either the
3211 rename(2) manpage or equivalent system documentation for details.
3217 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3218 supplied. If EXPR is numeric, demands that the current version of Perl
3219 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3221 Otherwise, demands that a library file be included if it hasn't already
3222 been included. The file is included via the do-FILE mechanism, which is
3223 essentially just a variety of C<eval>. Has semantics similar to the following
3228 return 1 if $INC{$filename};
3229 my($realfilename,$result);
3231 foreach $prefix (@INC) {
3232 $realfilename = "$prefix/$filename";
3233 if (-f $realfilename) {
3234 $result = do $realfilename;
3238 die "Can't find $filename in \@INC";
3241 die "$filename did not return true value" unless $result;
3242 $INC{$filename} = $realfilename;
3246 Note that the file will not be included twice under the same specified
3247 name. The file must return true as the last statement to indicate
3248 successful execution of any initialization code, so it's customary to
3249 end such a file with C<1;> unless you're sure it'll return true
3250 otherwise. But it's better just to put the C<1;>, in case you add more
3253 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3254 replaces "F<::>" with "F</>" in the filename for you,
3255 to make it easy to load standard modules. This form of loading of
3256 modules does not risk altering your namespace.
3258 In other words, if you try this:
3260 require Foo::Bar; # a splendid bareword
3262 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3263 directories specified in the C<@INC> array.
3265 But if you try this:
3267 $class = 'Foo::Bar';
3268 require $class; # $class is not a bareword
3270 require "Foo::Bar"; # not a bareword because of the ""
3272 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3273 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3275 eval "require $class";
3277 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3283 Generally used in a C<continue> block at the end of a loop to clear
3284 variables and reset C<??> searches so that they work again. The
3285 expression is interpreted as a list of single characters (hyphens
3286 allowed for ranges). All variables and arrays beginning with one of
3287 those letters are reset to their pristine state. If the expression is
3288 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3289 only variables or searches in the current package. Always returns
3292 reset 'X'; # reset all X variables
3293 reset 'a-z'; # reset lower case variables
3294 reset; # just reset ?one-time? searches
3296 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3297 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3298 variables--lexical variables are unaffected, but they clean themselves
3299 up on scope exit anyway, so you'll probably want to use them instead.
3306 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3307 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3308 context, depending on how the return value will be used, and the context
3309 may vary from one execution to the next (see C<wantarray>). If no EXPR
3310 is given, returns an empty list in list context, the undefined value in
3311 scalar context, and (of course) nothing at all in a void context.
3313 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3314 or do FILE will automatically return the value of the last expression
3319 In list context, returns a list value consisting of the elements
3320 of LIST in the opposite order. In scalar context, concatenates the
3321 elements of LIST and returns a string value with all characters
3322 in the opposite order.
3324 print reverse <>; # line tac, last line first
3326 undef $/; # for efficiency of <>
3327 print scalar reverse <>; # character tac, last line tsrif
3329 This operator is also handy for inverting a hash, although there are some
3330 caveats. If a value is duplicated in the original hash, only one of those
3331 can be represented as a key in the inverted hash. Also, this has to
3332 unwind one hash and build a whole new one, which may take some time
3333 on a large hash, such as from a DBM file.
3335 %by_name = reverse %by_address; # Invert the hash
3337 =item rewinddir DIRHANDLE
3339 Sets the current position to the beginning of the directory for the
3340 C<readdir> routine on DIRHANDLE.
3342 =item rindex STR,SUBSTR,POSITION
3344 =item rindex STR,SUBSTR
3346 Works just like index() except that it returns the position of the LAST
3347 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3348 last occurrence at or before that position.
3350 =item rmdir FILENAME
3354 Deletes the directory specified by FILENAME if that directory is empty. If it
3355 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3356 FILENAME is omitted, uses C<$_>.
3360 The substitution operator. See L<perlop>.
3364 Forces EXPR to be interpreted in scalar context and returns the value
3367 @counts = ( scalar @a, scalar @b, scalar @c );
3369 There is no equivalent operator to force an expression to
3370 be interpolated in list context because in practice, this is never
3371 needed. If you really wanted to do so, however, you could use
3372 the construction C<@{[ (some expression) ]}>, but usually a simple
3373 C<(some expression)> suffices.
3375 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3376 parenthesized list, this behaves as a scalar comma expression, evaluating
3377 all but the last element in void context and returning the final element
3378 evaluated in scalar context. This is seldom what you want.
3380 The following single statement:
3382 print uc(scalar(&foo,$bar)),$baz;
3384 is the moral equivalent of these two:
3387 print(uc($bar),$baz);
3389 See L<perlop> for more details on unary operators and the comma operator.
3391 =item seek FILEHANDLE,POSITION,WHENCE
3393 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3394 FILEHANDLE may be an expression whose value gives the name of the
3395 filehandle. The values for WHENCE are C<0> to set the new position to
3396 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3397 set it to EOF plus POSITION (typically negative). For WHENCE you may
3398 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3399 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3401 If you want to position file for C<sysread> or C<syswrite>, don't use
3402 C<seek>--buffering makes its effect on the file's system position
3403 unpredictable and non-portable. Use C<sysseek> instead.
3405 Due to the rules and rigors of ANSI C, on some systems you have to do a
3406 seek whenever you switch between reading and writing. Amongst other
3407 things, this may have the effect of calling stdio's clearerr(3).
3408 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3412 This is also useful for applications emulating C<tail -f>. Once you hit
3413 EOF on your read, and then sleep for a while, you might have to stick in a
3414 seek() to reset things. The C<seek> doesn't change the current position,
3415 but it I<does> clear the end-of-file condition on the handle, so that the
3416 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3418 If that doesn't work (some stdios are particularly cantankerous), then
3419 you may need something more like this:
3422 for ($curpos = tell(FILE); $_ = <FILE>;
3423 $curpos = tell(FILE)) {
3424 # search for some stuff and put it into files
3426 sleep($for_a_while);
3427 seek(FILE, $curpos, 0);
3430 =item seekdir DIRHANDLE,POS
3432 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3433 must be a value returned by C<telldir>. Has the same caveats about
3434 possible directory compaction as the corresponding system library
3437 =item select FILEHANDLE
3441 Returns the currently selected filehandle. Sets the current default
3442 filehandle for output, if FILEHANDLE is supplied. This has two
3443 effects: first, a C<write> or a C<print> without a filehandle will
3444 default to this FILEHANDLE. Second, references to variables related to
3445 output will refer to this output channel. For example, if you have to
3446 set the top of form format for more than one output channel, you might
3454 FILEHANDLE may be an expression whose value gives the name of the
3455 actual filehandle. Thus:
3457 $oldfh = select(STDERR); $| = 1; select($oldfh);
3459 Some programmers may prefer to think of filehandles as objects with
3460 methods, preferring to write the last example as:
3463 STDERR->autoflush(1);
3465 =item select RBITS,WBITS,EBITS,TIMEOUT
3467 This calls the select(2) system call with the bit masks specified, which
3468 can be constructed using C<fileno> and C<vec>, along these lines:
3470 $rin = $win = $ein = '';
3471 vec($rin,fileno(STDIN),1) = 1;
3472 vec($win,fileno(STDOUT),1) = 1;
3475 If you want to select on many filehandles you might wish to write a
3479 my(@fhlist) = split(' ',$_[0]);
3482 vec($bits,fileno($_),1) = 1;
3486 $rin = fhbits('STDIN TTY SOCK');
3490 ($nfound,$timeleft) =
3491 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3493 or to block until something becomes ready just do this
3495 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3497 Most systems do not bother to return anything useful in $timeleft, so
3498 calling select() in scalar context just returns $nfound.
3500 Any of the bit masks can also be undef. The timeout, if specified, is
3501 in seconds, which may be fractional. Note: not all implementations are
3502 capable of returning the$timeleft. If not, they always return
3503 $timeleft equal to the supplied $timeout.
3505 You can effect a sleep of 250 milliseconds this way:
3507 select(undef, undef, undef, 0.25);
3509 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3510 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3511 then only on POSIX systems. You have to use C<sysread> instead.
3513 =item semctl ID,SEMNUM,CMD,ARG
3515 Calls the System V IPC function C<semctl>. You'll probably have to say
3519 first to get the correct constant definitions. If CMD is IPC_STAT or
3520 GETALL, then ARG must be a variable which will hold the returned
3521 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3522 undefined value for error, "C<0 but true>" for zero, or the actual return
3523 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3525 =item semget KEY,NSEMS,FLAGS
3527 Calls the System V IPC function semget. Returns the semaphore id, or
3528 the undefined value if there is an error. See also C<IPC::SysV> and
3529 C<IPC::SysV::Semaphore> documentation.
3531 =item semop KEY,OPSTRING
3533 Calls the System V IPC function semop to perform semaphore operations
3534 such as signaling and waiting. OPSTRING must be a packed array of
3535 semop structures. Each semop structure can be generated with
3536 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3537 operations is implied by the length of OPSTRING. Returns true if
3538 successful, or false if there is an error. As an example, the
3539 following code waits on semaphore $semnum of semaphore id $semid:
3541 $semop = pack("sss", $semnum, -1, 0);
3542 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3544 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3545 and C<IPC::SysV::Semaphore> documentation.
3547 =item send SOCKET,MSG,FLAGS,TO
3549 =item send SOCKET,MSG,FLAGS
3551 Sends a message on a socket. Takes the same flags as the system call
3552 of the same name. On unconnected sockets you must specify a
3553 destination to send TO, in which case it does a C C<sendto>. Returns
3554 the number of characters sent, or the undefined value if there is an
3555 error. The C system call sendmsg(2) is currently unimplemented.
3556 See L<perlipc/"UDP: Message Passing"> for examples.
3558 =item setpgrp PID,PGRP
3560 Sets the current process group for the specified PID, C<0> for the current
3561 process. Will produce a fatal error if used on a machine that doesn't
3562 implement setpgrp(2). If the arguments are omitted, it defaults to
3563 C<0,0>. Note that the POSIX version of C<setpgrp> does not accept any
3564 arguments, so only C<setpgrp(0,0)> is portable. See also C<POSIX::setsid()>.
3566 =item setpriority WHICH,WHO,PRIORITY
3568 Sets the current priority for a process, a process group, or a user.
3569 (See setpriority(2).) Will produce a fatal error if used on a machine
3570 that doesn't implement setpriority(2).
3572 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3574 Sets the socket option requested. Returns undefined if there is an
3575 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3582 Shifts the first value of the array off and returns it, shortening the
3583 array by 1 and moving everything down. If there are no elements in the
3584 array, returns the undefined value. If ARRAY is omitted, shifts the
3585 C<@_> array within the lexical scope of subroutines and formats, and the
3586 C<@ARGV> array at file scopes or within the lexical scopes established by
3587 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3588 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3589 same thing to the left end of an array that C<pop> and C<push> do to the
3592 =item shmctl ID,CMD,ARG
3594 Calls the System V IPC function shmctl. You'll probably have to say
3598 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3599 then ARG must be a variable which will hold the returned C<shmid_ds>
3600 structure. Returns like ioctl: the undefined value for error, "C<0> but
3601 true" for zero, or the actual return value otherwise.
3602 See also C<IPC::SysV> documentation.
3604 =item shmget KEY,SIZE,FLAGS
3606 Calls the System V IPC function shmget. Returns the shared memory
3607 segment id, or the undefined value if there is an error.
3608 See also C<IPC::SysV> documentation.
3610 =item shmread ID,VAR,POS,SIZE
3612 =item shmwrite ID,STRING,POS,SIZE
3614 Reads or writes the System V shared memory segment ID starting at
3615 position POS for size SIZE by attaching to it, copying in/out, and
3616 detaching from it. When reading, VAR must be a variable that will
3617 hold the data read. When writing, if STRING is too long, only SIZE
3618 bytes are used; if STRING is too short, nulls are written to fill out
3619 SIZE bytes. Return true if successful, or false if there is an error.
3620 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3623 =item shutdown SOCKET,HOW
3625 Shuts down a socket connection in the manner indicated by HOW, which
3626 has the same interpretation as in the system call of the same name.
3628 shutdown(SOCKET, 0); # I/we have stopped reading data
3629 shutdown(SOCKET, 1); # I/we have stopped writing data
3630 shutdown(SOCKET, 2); # I/we have stopped using this socket
3632 This is useful with sockets when you want to tell the other
3633 side you're done writing but not done reading, or vice versa.
3634 It's also a more insistent form of close because it also
3635 disables the file descriptor in any forked copies in other
3642 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3643 returns sine of C<$_>.
3645 For the inverse sine operation, you may use the C<POSIX::asin>
3646 function, or use this relation:
3648 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3654 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3655 May be interrupted if the process receives a signal such as C<SIGALRM>.
3656 Returns the number of seconds actually slept. You probably cannot
3657 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
3660 On some older systems, it may sleep up to a full second less than what
3661 you requested, depending on how it counts seconds. Most modern systems
3662 always sleep the full amount. They may appear to sleep longer than that,
3663 however, because your process might not be scheduled right away in a
3664 busy multitasking system.
3666 For delays of finer granularity than one second, you may use Perl's
3667 C<syscall> interface to access setitimer(2) if your system supports it,
3668 or else see L</select> above.
3670 See also the POSIX module's C<sigpause> function.
3672 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3674 Opens a socket of the specified kind and attaches it to filehandle
3675 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
3676 the system call of the same name. You should C<use Socket> first
3677 to get the proper definitions imported. See the examples in
3678 L<perlipc/"Sockets: Client/Server Communication">.
3680 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3682 Creates an unnamed pair of sockets in the specified domain, of the
3683 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3684 for the system call of the same name. If unimplemented, yields a fatal
3685 error. Returns true if successful.
3687 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
3688 to C<pipe(Rdr, Wtr)> is essentially:
3691 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3692 shutdown(Rdr, 1); # no more writing for reader
3693 shutdown(Wtr, 0); # no more reading for writer
3695 See L<perlipc> for an example of socketpair use.
3697 =item sort SUBNAME LIST
3699 =item sort BLOCK LIST
3703 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3704 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
3705 specified, it gives the name of a subroutine that returns an integer
3706 less than, equal to, or greater than C<0>, depending on how the elements
3707 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3708 operators are extremely useful in such routines.) SUBNAME may be a
3709 scalar variable name (unsubscripted), in which case the value provides
3710 the name of (or a reference to) the actual subroutine to use. In place
3711 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3714 In the interests of efficiency the normal calling code for subroutines is
3715 bypassed, with the following effects: the subroutine may not be a
3716 recursive subroutine, and the two elements to be compared are passed into
3717 the subroutine not via C<@_> but as the package global variables $a and
3718 $b (see example below). They are passed by reference, so don't
3719 modify $a and $b. And don't try to declare them as lexicals either.
3721 You also cannot exit out of the sort block or subroutine using any of the
3722 loop control operators described in L<perlsyn> or with C<goto>.
3724 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3725 current collation locale. See L<perllocale>.
3730 @articles = sort @files;
3732 # same thing, but with explicit sort routine
3733 @articles = sort {$a cmp $b} @files;
3735 # now case-insensitively
3736 @articles = sort {uc($a) cmp uc($b)} @files;
3738 # same thing in reversed order
3739 @articles = sort {$b cmp $a} @files;
3741 # sort numerically ascending
3742 @articles = sort {$a <=> $b} @files;
3744 # sort numerically descending
3745 @articles = sort {$b <=> $a} @files;
3747 # this sorts the %age hash by value instead of key
3748 # using an in-line function
3749 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3751 # sort using explicit subroutine name
3753 $age{$a} <=> $age{$b}; # presuming numeric
3755 @sortedclass = sort byage @class;
3757 sub backwards { $b cmp $a }
3758 @harry = qw(dog cat x Cain Abel);
3759 @george = qw(gone chased yz Punished Axed);
3761 # prints AbelCaincatdogx
3762 print sort backwards @harry;
3763 # prints xdogcatCainAbel
3764 print sort @george, 'to', @harry;
3765 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3767 # inefficiently sort by descending numeric compare using
3768 # the first integer after the first = sign, or the
3769 # whole record case-insensitively otherwise
3772 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3777 # same thing, but much more efficiently;
3778 # we'll build auxiliary indices instead
3782 push @nums, /=(\d+)/;
3787 $nums[$b] <=> $nums[$a]
3789 $caps[$a] cmp $caps[$b]
3793 # same thing, but without any temps
3794 @new = map { $_->[0] }
3795 sort { $b->[1] <=> $a->[1]
3798 } map { [$_, /=(\d+)/, uc($_)] } @old;
3800 If you're using strict, you I<must not> declare $a
3801 and $b as lexicals. They are package globals. That means
3802 if you're in the C<main> package, it's
3804 @articles = sort {$main::b <=> $main::a} @files;
3808 @articles = sort {$::b <=> $::a} @files;
3810 but if you're in the C<FooPack> package, it's
3812 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3814 The comparison function is required to behave. If it returns
3815 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3816 sometimes saying the opposite, for example) the results are not
3819 =item splice ARRAY,OFFSET,LENGTH,LIST
3821 =item splice ARRAY,OFFSET,LENGTH
3823 =item splice ARRAY,OFFSET
3825 Removes the elements designated by OFFSET and LENGTH from an array, and
3826 replaces them with the elements of LIST, if any. In list context,
3827 returns the elements removed from the array. In scalar context,
3828 returns the last element removed, or C<undef> if no elements are
3829 removed. The array grows or shrinks as necessary.
3830 If OFFSET is negative then it starts that far from the end of the array.
3831 If LENGTH is omitted, removes everything from OFFSET onward.
3832 If LENGTH is negative, leave that many elements off the end of the array.
3833 The following equivalences hold (assuming C<$[ == 0>):
3835 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3836 pop(@a) splice(@a,-1)
3837 shift(@a) splice(@a,0,1)
3838 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3839 $a[$x] = $y splice(@a,$x,1,$y)
3841 Example, assuming array lengths are passed before arrays:
3843 sub aeq { # compare two list values
3844 my(@a) = splice(@_,0,shift);
3845 my(@b) = splice(@_,0,shift);
3846 return 0 unless @a == @b; # same len?
3848 return 0 if pop(@a) ne pop(@b);
3852 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3854 =item split /PATTERN/,EXPR,LIMIT
3856 =item split /PATTERN/,EXPR
3858 =item split /PATTERN/
3862 Splits a string into a list of strings and returns that list. By default,
3863 empty leading fields are preserved, and empty trailing ones are deleted.
3865 If not in list context, returns the number of fields found and splits into
3866 the C<@_> array. (In list context, you can force the split into C<@_> by
3867 using C<??> as the pattern delimiters, but it still returns the list
3868 value.) The use of implicit split to C<@_> is deprecated, however, because
3869 it clobbers your subroutine arguments.
3871 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3872 splits on whitespace (after skipping any leading whitespace). Anything
3873 matching PATTERN is taken to be a delimiter separating the fields. (Note
3874 that the delimiter may be longer than one character.)
3876 If LIMIT is specified and positive, splits into no more than that
3877 many fields (though it may split into fewer). If LIMIT is unspecified
3878 or zero, trailing null fields are stripped (which potential users
3879 of C<pop> would do well to remember). If LIMIT is negative, it is
3880 treated as if an arbitrarily large LIMIT had been specified.
3882 A pattern matching the null string (not to be confused with
3883 a null pattern C<//>, which is just one member of the set of patterns
3884 matching a null string) will split the value of EXPR into separate
3885 characters at each point it matches that way. For example:
3887 print join(':', split(/ */, 'hi there'));
3889 produces the output 'h:i:t:h:e:r:e'.
3891 The LIMIT parameter can be used to split a line partially
3893 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3895 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3896 one larger than the number of variables in the list, to avoid
3897 unnecessary work. For the list above LIMIT would have been 4 by
3898 default. In time critical applications it behooves you not to split
3899 into more fields than you really need.
3901 If the PATTERN contains parentheses, additional list elements are
3902 created from each matching substring in the delimiter.
3904 split(/([,-])/, "1-10,20", 3);
3906 produces the list value
3908 (1, '-', 10, ',', 20)
3910 If you had the entire header of a normal Unix email message in $header,
3911 you could split it up into fields and their values this way:
3913 $header =~ s/\n\s+/ /g; # fix continuation lines
3914 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3916 The pattern C</PATTERN/> may be replaced with an expression to specify
3917 patterns that vary at runtime. (To do runtime compilation only once,
3918 use C</$variable/o>.)
3920 As a special case, specifying a PATTERN of space (C<' '>) will split on
3921 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
3922 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3923 will give you as many null initial fields as there are leading spaces.
3924 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
3925 whitespace produces a null first field. A C<split> with no arguments
3926 really does a C<split(' ', $_)> internally.
3930 open(PASSWD, '/etc/passwd');
3932 ($login, $passwd, $uid, $gid,
3933 $gcos, $home, $shell) = split(/:/);
3937 (Note that $shell above will still have a newline on it. See L</chop>,
3938 L</chomp>, and L</join>.)
3940 =item sprintf FORMAT, LIST
3942 Returns a string formatted by the usual C<printf> conventions of the
3943 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
3944 on your system for an explanation of the general principles.
3946 Perl does its own C<sprintf> formatting--it emulates the C
3947 function C<sprintf>, but it doesn't use it (except for floating-point
3948 numbers, and even then only the standard modifiers are allowed). As a
3949 result, any non-standard extensions in your local C<sprintf> are not
3950 available from Perl.
3952 Perl's C<sprintf> permits the following universally-known conversions:
3955 %c a character with the given number
3957 %d a signed integer, in decimal
3958 %u an unsigned integer, in decimal
3959 %o an unsigned integer, in octal
3960 %x an unsigned integer, in hexadecimal
3961 %e a floating-point number, in scientific notation
3962 %f a floating-point number, in fixed decimal notation
3963 %g a floating-point number, in %e or %f notation
3965 In addition, Perl permits the following widely-supported conversions:
3967 %X like %x, but using upper-case letters
3968 %E like %e, but using an upper-case "E"
3969 %G like %g, but with an upper-case "E" (if applicable)
3970 %b an unsigned integer, in binary
3971 %p a pointer (outputs the Perl value's address in hexadecimal)
3972 %n special: *stores* the number of characters output so far
3973 into the next variable in the parameter list
3975 Finally, for backward (and we do mean "backward") compatibility, Perl
3976 permits these unnecessary but widely-supported conversions:
3979 %D a synonym for %ld
3980 %U a synonym for %lu
3981 %O a synonym for %lo
3984 Perl permits the following universally-known flags between the C<%>
3985 and the conversion letter:
3987 space prefix positive number with a space
3988 + prefix positive number with a plus sign
3989 - left-justify within the field
3990 0 use zeros, not spaces, to right-justify
3991 # prefix non-zero octal with "0", non-zero hex with "0x"
3992 number minimum field width
3993 .number "precision": digits after decimal point for
3994 floating-point, max length for string, minimum length
3996 l interpret integer as C type "long" or "unsigned long"
3997 h interpret integer as C type "short" or "unsigned short"
3999 There is also one Perl-specific flag:
4001 V interpret integer as Perl's standard integer type
4003 Where a number would appear in the flags, an asterisk (C<*>) may be
4004 used instead, in which case Perl uses the next item in the parameter
4005 list as the given number (that is, as the field width or precision).
4006 If a field width obtained through C<*> is negative, it has the same
4007 effect as the C<-> flag: left-justification.
4009 If C<use locale> is in effect, the character used for the decimal
4010 point in formatted real numbers is affected by the LC_NUMERIC locale.
4017 Return the square root of EXPR. If EXPR is omitted, returns square
4018 root of C<$_>. Only works on non-negative operands, unless you've
4019 loaded the standard Math::Complex module.
4022 print sqrt(-2); # prints 1.4142135623731i
4028 Sets the random number seed for the C<rand> operator. If EXPR is
4029 omitted, uses a semi-random value supplied by the kernel (if it supports
4030 the F</dev/urandom> device) or based on the current time and process
4031 ID, among other things. In versions of Perl prior to 5.004 the default
4032 seed was just the current C<time>. This isn't a particularly good seed,
4033 so many old programs supply their own seed value (often C<time ^ $$> or
4034 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4036 In fact, it's usually not necessary to call C<srand> at all, because if
4037 it is not called explicitly, it is called implicitly at the first use of
4038 the C<rand> operator. However, this was not the case in version of Perl
4039 before 5.004, so if your script will run under older Perl versions, it
4040 should call C<srand>.
4042 Note that you need something much more random than the default seed for
4043 cryptographic purposes. Checksumming the compressed output of one or more
4044 rapidly changing operating system status programs is the usual method. For
4047 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4049 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4052 Do I<not> call C<srand> multiple times in your program unless you know
4053 exactly what you're doing and why you're doing it. The point of the
4054 function is to "seed" the C<rand> function so that C<rand> can produce
4055 a different sequence each time you run your program. Just do it once at the
4056 top of your program, or you I<won't> get random numbers out of C<rand>!
4058 Frequently called programs (like CGI scripts) that simply use
4062 for a seed can fall prey to the mathematical property that
4066 one-third of the time. So don't do that.
4068 =item stat FILEHANDLE
4074 Returns a 13-element list giving the status info for a file, either
4075 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4076 it stats C<$_>. Returns a null list if the stat fails. Typically used
4079 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4080 $atime,$mtime,$ctime,$blksize,$blocks)
4083 Not all fields are supported on all filesystem types. Here are the
4084 meaning of the fields:
4086 0 dev device number of filesystem
4088 2 mode file mode (type and permissions)
4089 3 nlink number of (hard) links to the file
4090 4 uid numeric user ID of file's owner
4091 5 gid numeric group ID of file's owner
4092 6 rdev the device identifier (special files only)
4093 7 size total size of file, in bytes
4094 8 atime last access time since the epoch
4095 9 mtime last modify time since the epoch
4096 10 ctime inode change time (NOT creation time!) since the epoch
4097 11 blksize preferred block size for file system I/O
4098 12 blocks actual number of blocks allocated
4100 (The epoch was at 00:00 January 1, 1970 GMT.)
4102 If stat is passed the special filehandle consisting of an underline, no
4103 stat is done, but the current contents of the stat structure from the
4104 last stat or filetest are returned. Example:
4106 if (-x $file && (($d) = stat(_)) && $d < 0) {
4107 print "$file is executable NFS file\n";
4110 (This works on machines only for which the device number is negative under NFS.)
4112 Because the mode contains both the file type and its permissions, you
4113 should mask off the file type portion and (s)printf using a C<"%o">
4114 if you want to see the real permissions.
4116 $mode = (stat($filename))[2];
4117 printf "Permissions are %04o\n", $mode & 07777;
4119 In scalar context, C<stat> returns a boolean value indicating success
4120 or failure, and, if successful, sets the information associated with
4121 the special filehandle C<_>.
4123 The File::stat module provides a convenient, by-name access mechanism:
4126 $sb = stat($filename);
4127 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4128 $filename, $sb->size, $sb->mode & 07777,
4129 scalar localtime $sb->mtime;
4135 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4136 doing many pattern matches on the string before it is next modified.
4137 This may or may not save time, depending on the nature and number of
4138 patterns you are searching on, and on the distribution of character
4139 frequencies in the string to be searched--you probably want to compare
4140 run times with and without it to see which runs faster. Those loops
4141 which scan for many short constant strings (including the constant
4142 parts of more complex patterns) will benefit most. You may have only
4143 one C<study> active at a time--if you study a different scalar the first
4144 is "unstudied". (The way C<study> works is this: a linked list of every
4145 character in the string to be searched is made, so we know, for
4146 example, where all the C<'k'> characters are. From each search string,
4147 the rarest character is selected, based on some static frequency tables
4148 constructed from some C programs and English text. Only those places
4149 that contain this "rarest" character are examined.)
4151 For example, here is a loop that inserts index producing entries
4152 before any line containing a certain pattern:
4156 print ".IX foo\n" if /\bfoo\b/;
4157 print ".IX bar\n" if /\bbar\b/;
4158 print ".IX blurfl\n" if /\bblurfl\b/;
4163 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
4164 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
4165 a big win except in pathological cases. The only question is whether
4166 it saves you more time than it took to build the linked list in the
4169 Note that if you have to look for strings that you don't know till
4170 runtime, you can build an entire loop as a string and C<eval> that to
4171 avoid recompiling all your patterns all the time. Together with
4172 undefining C<$/> to input entire files as one record, this can be very
4173 fast, often faster than specialized programs like fgrep(1). The following
4174 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4175 out the names of those files that contain a match:
4177 $search = 'while (<>) { study;';
4178 foreach $word (@words) {
4179 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4184 eval $search; # this screams
4185 $/ = "\n"; # put back to normal input delimiter
4186 foreach $file (sort keys(%seen)) {
4194 =item sub NAME BLOCK
4196 This is subroutine definition, not a real function I<per se>. With just a
4197 NAME (and possibly prototypes), it's just a forward declaration. Without
4198 a NAME, it's an anonymous function declaration, and does actually return a
4199 value: the CODE ref of the closure you just created. See L<perlsub> and
4200 L<perlref> for details.
4202 =item substr EXPR,OFFSET,LEN,REPLACEMENT
4204 =item substr EXPR,OFFSET,LEN
4206 =item substr EXPR,OFFSET
4208 Extracts a substring out of EXPR and returns it. First character is at
4209 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4210 If OFFSET is negative (or more precisely, less than C<$[>), starts
4211 that far from the end of the string. If LEN is omitted, returns
4212 everything to the end of the string. If LEN is negative, leaves that
4213 many characters off the end of the string.
4215 If you specify a substring that is partly outside the string, the part
4216 within the string is returned. If the substring is totally outside
4217 the string a warning is produced.
4219 You can use the substr() function as an lvalue, in which case EXPR
4220 must itself be an lvalue. If you assign something shorter than LEN,
4221 the string will shrink, and if you assign something longer than LEN,
4222 the string will grow to accommodate it. To keep the string the same
4223 length you may need to pad or chop your value using C<sprintf>.
4225 An alternative to using substr() as an lvalue is to specify the
4226 replacement string as the 4th argument. This allows you to replace
4227 parts of the EXPR and return what was there before in one operation,
4228 just as you can with splice().
4230 =item symlink OLDFILE,NEWFILE
4232 Creates a new filename symbolically linked to the old filename.
4233 Returns C<1> for success, C<0> otherwise. On systems that don't support
4234 symbolic links, produces a fatal error at run time. To check for that,
4237 $symlink_exists = eval { symlink("",""); 1 };
4241 Calls the system call specified as the first element of the list,
4242 passing the remaining elements as arguments to the system call. If
4243 unimplemented, produces a fatal error. The arguments are interpreted
4244 as follows: if a given argument is numeric, the argument is passed as
4245 an int. If not, the pointer to the string value is passed. You are
4246 responsible to make sure a string is pre-extended long enough to
4247 receive any result that might be written into a string. You can't use a
4248 string literal (or other read-only string) as an argument to C<syscall>
4249 because Perl has to assume that any string pointer might be written
4251 integer arguments are not literals and have never been interpreted in a
4252 numeric context, you may need to add C<0> to them to force them to look
4253 like numbers. This emulates the C<syswrite> function (or vice versa):
4255 require 'syscall.ph'; # may need to run h2ph
4257 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4259 Note that Perl supports passing of up to only 14 arguments to your system call,
4260 which in practice should usually suffice.
4262 Syscall returns whatever value returned by the system call it calls.
4263 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4264 Note that some system calls can legitimately return C<-1>. The proper
4265 way to handle such calls is to assign C<$!=0;> before the call and
4266 check the value of C<$!> if syscall returns C<-1>.
4268 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4269 number of the read end of the pipe it creates. There is no way
4270 to retrieve the file number of the other end. You can avoid this
4271 problem by using C<pipe> instead.
4273 =item sysopen FILEHANDLE,FILENAME,MODE
4275 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4277 Opens the file whose filename is given by FILENAME, and associates it
4278 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4279 the name of the real filehandle wanted. This function calls the
4280 underlying operating system's C<open> function with the parameters
4281 FILENAME, MODE, PERMS.
4283 The possible values and flag bits of the MODE parameter are
4284 system-dependent; they are available via the standard module C<Fcntl>.
4285 For historical reasons, some values work on almost every system
4286 supported by perl: zero means read-only, one means write-only, and two
4287 means read/write. We know that these values do I<not> work under
4288 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4289 use them in new code.
4291 If the file named by FILENAME does not exist and the C<open> call creates
4292 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4293 PERMS specifies the permissions of the newly created file. If you omit
4294 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4295 These permission values need to be in octal, and are modified by your
4296 process's current C<umask>.
4298 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4299 that takes away the user's option to have a more permissive umask.
4300 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4303 See L<perlopentut> for a kinder, gentler explanation of opening files.
4305 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4307 =item sysread FILEHANDLE,SCALAR,LENGTH
4309 Attempts to read LENGTH bytes of data into variable SCALAR from the
4310 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4311 so mixing this with other kinds of reads, C<print>, C<write>,
4312 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4313 usually buffers data. Returns the number of bytes actually read, C<0>
4314 at end of file, or undef if there was an error. SCALAR will be grown or
4315 shrunk so that the last byte actually read is the last byte of the
4316 scalar after the read.
4318 An OFFSET may be specified to place the read data at some place in the
4319 string other than the beginning. A negative OFFSET specifies
4320 placement at that many bytes counting backwards from the end of the
4321 string. A positive OFFSET greater than the length of SCALAR results
4322 in the string being padded to the required size with C<"\0"> bytes before
4323 the result of the read is appended.
4325 There is no syseof() function, which is ok, since eof() doesn't work
4326 very well on device files (like ttys) anyway. Use sysread() and check
4327 for a return value for 0 to decide whether you're done.
4329 =item sysseek FILEHANDLE,POSITION,WHENCE
4331 Sets FILEHANDLE's system position using the system call lseek(2). It
4332 bypasses stdio, so mixing this with reads (other than C<sysread>),
4333 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause
4334 confusion. FILEHANDLE may be an expression whose value gives the name
4335 of the filehandle. The values for WHENCE are C<0> to set the new
4336 position to POSITION, C<1> to set the it to the current position plus
4337 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4338 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4339 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4341 Returns the new position, or the undefined value on failure. A position
4342 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4343 true on success and false on failure, yet you can still easily determine
4348 =item system PROGRAM LIST
4350 Does exactly the same thing as C<exec LIST>, except that a fork is
4351 done first, and the parent process waits for the child process to
4352 complete. Note that argument processing varies depending on the
4353 number of arguments. If there is more than one argument in LIST,
4354 or if LIST is an array with more than one value, starts the program
4355 given by the first element of the list with arguments given by the
4356 rest of the list. If there is only one scalar argument, the argument
4357 is checked for shell metacharacters, and if there are any, the
4358 entire argument is passed to the system's command shell for parsing
4359 (this is C</bin/sh -c> on Unix platforms, but varies on other
4360 platforms). If there are no shell metacharacters in the argument,
4361 it is split into words and passed directly to C<execvp>, which is
4364 All files opened for output are flushed before attempting the exec().
4366 The return value is the exit status of the program as
4367 returned by the C<wait> call. To get the actual exit value divide by
4368 256. See also L</exec>. This is I<not> what you want to use to capture
4369 the output from a command, for that you should use merely backticks or
4370 C<qx//>, as described in L<perlop/"`STRING`">.
4372 Like C<exec>, C<system> allows you to lie to a program about its name if
4373 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4375 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4376 program they're running doesn't actually interrupt your program.
4378 @args = ("command", "arg1", "arg2");
4380 or die "system @args failed: $?"
4382 You can check all the failure possibilities by inspecting
4385 $exit_value = $? >> 8;
4386 $signal_num = $? & 127;
4387 $dumped_core = $? & 128;
4389 When the arguments get executed via the system shell, results
4390 and return codes will be subject to its quirks and capabilities.
4391 See L<perlop/"`STRING`"> and L</exec> for details.
4393 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4395 =item syswrite FILEHANDLE,SCALAR,LENGTH
4397 =item syswrite FILEHANDLE,SCALAR
4399 Attempts to write LENGTH bytes of data from variable SCALAR to the
4400 specified FILEHANDLE, using the system call write(2). If LENGTH
4401 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4402 this with reads (other than C<sysread())>, C<print>, C<write>,
4403 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4404 usually buffers data. Returns the number of bytes actually written,
4405 or C<undef> if there was an error. If the LENGTH is greater than
4406 the available data in the SCALAR after the OFFSET, only as much
4407 data as is available will be written.
4409 An OFFSET may be specified to write the data from some part of the
4410 string other than the beginning. A negative OFFSET specifies writing
4411 that many bytes counting backwards from the end of the string. In the
4412 case the SCALAR is empty you can use OFFSET but only zero offset.
4414 =item tell FILEHANDLE
4418 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4419 expression whose value gives the name of the actual filehandle. If
4420 FILEHANDLE is omitted, assumes the file last read.
4422 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4424 =item telldir DIRHANDLE
4426 Returns the current position of the C<readdir> routines on DIRHANDLE.
4427 Value may be given to C<seekdir> to access a particular location in a
4428 directory. Has the same caveats about possible directory compaction as
4429 the corresponding system library routine.
4431 =item tie VARIABLE,CLASSNAME,LIST
4433 This function binds a variable to a package class that will provide the
4434 implementation for the variable. VARIABLE is the name of the variable
4435 to be enchanted. CLASSNAME is the name of a class implementing objects
4436 of correct type. Any additional arguments are passed to the C<new>
4437 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4438 or C<TIEHASH>). Typically these are arguments such as might be passed
4439 to the C<dbm_open()> function of C. The object returned by the C<new>
4440 method is also returned by the C<tie> function, which would be useful
4441 if you want to access other methods in CLASSNAME.
4443 Note that functions such as C<keys> and C<values> may return huge lists
4444 when used on large objects, like DBM files. You may prefer to use the
4445 C<each> function to iterate over such. Example:
4447 # print out history file offsets
4449 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4450 while (($key,$val) = each %HIST) {
4451 print $key, ' = ', unpack('L',$val), "\n";
4455 A class implementing a hash should have the following methods:
4457 TIEHASH classname, LIST
4459 STORE this, key, value
4464 NEXTKEY this, lastkey
4467 A class implementing an ordinary array should have the following methods:
4469 TIEARRAY classname, LIST
4471 STORE this, key, value
4473 STORESIZE this, count
4479 SPLICE this, offset, length, LIST
4483 A class implementing a file handle should have the following methods:
4485 TIEHANDLE classname, LIST
4486 READ this, scalar, length, offset
4489 WRITE this, scalar, length, offset
4491 PRINTF this, format, LIST
4495 A class implementing a scalar should have the following methods:
4497 TIESCALAR classname, LIST
4502 Not all methods indicated above need be implemented. See L<perltie>,
4503 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
4505 Unlike C<dbmopen>, the C<tie> function will not use or require a module
4506 for you--you need to do that explicitly yourself. See L<DB_File>
4507 or the F<Config> module for interesting C<tie> implementations.
4509 For further details see L<perltie>, L<"tied VARIABLE">.
4513 Returns a reference to the object underlying VARIABLE (the same value
4514 that was originally returned by the C<tie> call that bound the variable
4515 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4520 Returns the number of non-leap seconds since whatever time the system
4521 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4522 and 00:00:00 UTC, January 1, 1970 for most other systems).
4523 Suitable for feeding to C<gmtime> and C<localtime>.
4527 Returns a four-element list giving the user and system times, in
4528 seconds, for this process and the children of this process.
4530 ($user,$system,$cuser,$csystem) = times;
4534 The transliteration operator. Same as C<y///>. See L<perlop>.
4536 =item truncate FILEHANDLE,LENGTH
4538 =item truncate EXPR,LENGTH
4540 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4541 specified length. Produces a fatal error if truncate isn't implemented
4542 on your system. Returns true if successful, the undefined value
4549 Returns an uppercased version of EXPR. This is the internal function
4550 implementing the C<\U> escape in double-quoted strings.
4551 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4552 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4553 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
4555 If EXPR is omitted, uses C<$_>.
4561 Returns the value of EXPR with the first character
4562 in uppercase (titlecase in Unicode). This is
4563 the internal function implementing the C<\u> escape in double-quoted strings.
4564 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
4567 If EXPR is omitted, uses C<$_>.
4573 Sets the umask for the process to EXPR and returns the previous value.
4574 If EXPR is omitted, merely returns the current umask.
4576 The Unix permission C<rwxr-x---> is represented as three sets of three
4577 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4578 and isn't one of the digits). The C<umask> value is such a number
4579 representing disabled permissions bits. The permission (or "mode")
4580 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4581 even if you tell C<sysopen> to create a file with permissions C<0777>,
4582 if your umask is C<0022> then the file will actually be created with
4583 permissions C<0755>. If your C<umask> were C<0027> (group can't
4584 write; others can't read, write, or execute), then passing
4585 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
4588 Here's some advice: supply a creation mode of C<0666> for regular
4589 files (in C<sysopen>) and one of C<0777> for directories (in
4590 C<mkdir>) and executable files. This gives users the freedom of
4591 choice: if they want protected files, they might choose process umasks
4592 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4593 Programs should rarely if ever make policy decisions better left to
4594 the user. The exception to this is when writing files that should be
4595 kept private: mail files, web browser cookies, I<.rhosts> files, and
4598 If umask(2) is not implemented on your system and you are trying to
4599 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4600 fatal error at run time. If umask(2) is not implemented and you are
4601 not trying to restrict access for yourself, returns C<undef>.
4603 Remember that a umask is a number, usually given in octal; it is I<not> a
4604 string of octal digits. See also L</oct>, if all you have is a string.
4610 Undefines the value of EXPR, which must be an lvalue. Use only on a
4611 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
4612 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
4613 will probably not do what you expect on most predefined variables or
4614 DBM list values, so don't do that; see L<delete>.) Always returns the
4615 undefined value. You can omit the EXPR, in which case nothing is
4616 undefined, but you still get an undefined value that you could, for
4617 instance, return from a subroutine, assign to a variable or pass as a
4618 parameter. Examples:
4621 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4625 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4626 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4627 select undef, undef, undef, 0.25;
4628 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4630 Note that this is a unary operator, not a list operator.
4636 Deletes a list of files. Returns the number of files successfully
4639 $cnt = unlink 'a', 'b', 'c';
4643 Note: C<unlink> will not delete directories unless you are superuser and
4644 the B<-U> flag is supplied to Perl. Even if these conditions are
4645 met, be warned that unlinking a directory can inflict damage on your
4646 filesystem. Use C<rmdir> instead.
4648 If LIST is omitted, uses C<$_>.
4650 =item unpack TEMPLATE,EXPR
4652 C<unpack> does the reverse of C<pack>: it takes a string
4653 representing a structure and expands it out into a list of values.
4654 (In scalar context, it returns merely the first value produced.)
4655 The TEMPLATE has the same format as in the C<pack> function.
4656 Here's a subroutine that does substring:
4659 my($what,$where,$howmuch) = @_;
4660 unpack("x$where a$howmuch", $what);
4665 sub ordinal { unpack("c",$_[0]); } # same as ord()
4667 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4668 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4669 themselves. Default is a 16-bit checksum. For example, the following
4670 computes the same number as the System V sum program:
4674 unpack("%32C*",<>) % 65535;
4677 The following efficiently counts the number of set bits in a bit vector:
4679 $setbits = unpack("%32b*", $selectmask);
4681 The C<"p"> and C<"P"> formats should be used with care. Since Perl
4682 has no way of checking whether the value passed to C<unpack()>
4683 corresponds to a valid memory location, passing a pointer value that's
4684 not known to be valid is likely to have disastrous consequences.
4686 See L</pack> for more examples.
4688 =item untie VARIABLE
4690 Breaks the binding between a variable and a package. (See C<tie>.)
4692 =item unshift ARRAY,LIST
4694 Does the opposite of a C<shift>. Or the opposite of a C<push>,
4695 depending on how you look at it. Prepends list to the front of the
4696 array, and returns the new number of elements in the array.
4698 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4700 Note the LIST is prepended whole, not one element at a time, so the
4701 prepended elements stay in the same order. Use C<reverse> to do the
4704 =item use Module LIST
4708 =item use Module VERSION LIST
4712 Imports some semantics into the current package from the named module,
4713 generally by aliasing certain subroutine or variable names into your
4714 package. It is exactly equivalent to
4716 BEGIN { require Module; import Module LIST; }
4718 except that Module I<must> be a bareword.
4720 If the first argument to C<use> is a number, it is treated as a version
4721 number instead of a module name. If the version of the Perl interpreter
4722 is less than VERSION, then an error message is printed and Perl exits
4723 immediately. This is often useful if you need to check the current
4724 Perl version before C<use>ing library modules that have changed in
4725 incompatible ways from older versions of Perl. (We try not to do
4726 this more than we have to.)
4728 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
4729 C<require> makes sure the module is loaded into memory if it hasn't been
4730 yet. The C<import> is not a builtin--it's just an ordinary static method
4731 call into the C<Module> package to tell the module to import the list of
4732 features back into the current package. The module can implement its
4733 C<import> method any way it likes, though most modules just choose to
4734 derive their C<import> method via inheritance from the C<Exporter> class that
4735 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
4736 method can be found then the error is currently silently ignored. This
4737 may change to a fatal error in a future version.
4739 If you don't want your namespace altered, explicitly supply an empty list:
4743 That is exactly equivalent to
4745 BEGIN { require Module }
4747 If the VERSION argument is present between Module and LIST, then the
4748 C<use> will call the VERSION method in class Module with the given
4749 version as an argument. The default VERSION method, inherited from
4750 the Universal class, croaks if the given version is larger than the
4751 value of the variable C<$Module::VERSION>. (Note that there is not a
4752 comma after VERSION!)
4754 Because this is a wide-open interface, pragmas (compiler directives)
4755 are also implemented this way. Currently implemented pragmas are:
4759 use sigtrap qw(SEGV BUS);
4760 use strict qw(subs vars refs);
4761 use subs qw(afunc blurfl);
4763 Some of these pseudo-modules import semantics into the current
4764 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4765 which import symbols into the current package (which are effective
4766 through the end of the file).
4768 There's a corresponding C<no> command that unimports meanings imported
4769 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
4774 If no C<unimport> method can be found the call fails with a fatal error.
4776 See L<perlmod> for a list of standard modules and pragmas.
4780 Changes the access and modification times on each file of a list of
4781 files. The first two elements of the list must be the NUMERICAL access
4782 and modification times, in that order. Returns the number of files
4783 successfully changed. The inode modification time of each file is set
4784 to the current time. This code has the same effect as the C<touch>
4785 command if the files already exist:
4789 utime $now, $now, @ARGV;
4793 Returns a list consisting of all the values of the named hash. (In a
4794 scalar context, returns the number of values.) The values are
4795 returned in an apparently random order. The actual random order is
4796 subject to change in future versions of perl, but it is guaranteed to
4797 be the same order as either the C<keys> or C<each> function would
4798 produce on the same (unmodified) hash.
4800 Note that you cannot modify the values of a hash this way, because the
4801 returned list is just a copy. You need to use a hash slice for that,
4802 since it's lvaluable in a way that values() is not.
4804 for (values %hash) { s/foo/bar/g } # FAILS!
4805 for (@hash{keys %hash}) { s/foo/bar/g } # ok
4807 As a side effect, calling values() resets the HASH's internal iterator.
4808 See also C<keys>, C<each>, and C<sort>.
4810 =item vec EXPR,OFFSET,BITS
4812 Treats the string in EXPR as a vector of unsigned integers, and
4813 returns the value of the bit field specified by OFFSET. BITS specifies
4814 the number of bits that are reserved for each entry in the bit
4815 vector. This must be a power of two from 1 to 32. C<vec> may also be
4816 assigned to, in which case parentheses are needed to give the expression
4817 the correct precedence as in
4819 vec($image, $max_x * $x + $y, 8) = 3;
4821 Vectors created with C<vec> can also be manipulated with the logical
4822 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4823 desired when both operands are strings. See L<perlop/"Bitwise String Operators">.
4825 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4826 The comments show the string after each step. Note that this code works
4827 in the same way on big-endian or little-endian machines.
4830 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4831 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4832 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4833 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4834 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4835 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4836 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4838 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4839 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4840 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4843 To transform a bit vector into a string or list of 0's and 1's, use these:
4845 $bits = unpack("b*", $vector);
4846 @bits = split(//, unpack("b*", $vector));
4848 If you know the exact length in bits, it can be used in place of the C<*>.
4852 Behaves like the wait(2) system call on your system: it waits for a child
4853 process to terminate and returns the pid of the deceased process, or
4854 C<-1> if there are no child processes. The status is returned in C<$?>.
4855 Note that a return value of C<-1> could mean that child processes are
4856 being automatically reaped, as described in L<perlipc>.
4858 =item waitpid PID,FLAGS
4860 Waits for a particular child process to terminate and returns the pid of
4861 the deceased process, or C<-1> if there is no such child process. On some
4862 systems, a value of 0 indicates that there are processes still running.
4863 The status is returned in C<$?>. If you say
4865 use POSIX ":sys_wait_h";
4868 $kid = waitpid(-1,&WNOHANG);
4871 then you can do a non-blocking wait for all pending zombie processes.
4872 Non-blocking wait is available on machines supporting either the
4873 waitpid(2) or wait4(2) system calls. However, waiting for a particular
4874 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
4875 system call by remembering the status values of processes that have
4876 exited but have not been harvested by the Perl script yet.)
4878 Note that on some systems, a return value of C<-1> could mean that child
4879 processes are being automatically reaped. See L<perlipc> for details,
4880 and for other examples.
4884 Returns true if the context of the currently executing subroutine is
4885 looking for a list value. Returns false if the context is looking
4886 for a scalar. Returns the undefined value if the context is looking
4887 for no value (void context).
4889 return unless defined wantarray; # don't bother doing more
4890 my @a = complex_calculation();
4891 return wantarray ? @a : "@a";
4893 This function should have been named wantlist() instead.
4897 Produces a message on STDERR just like C<die>, but doesn't exit or throw
4900 If LIST is empty and C<$@> already contains a value (typically from a
4901 previous eval) that value is used after appending C<"\t...caught">
4902 to C<$@>. This is useful for staying almost, but not entirely similar to
4905 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4907 No message is printed if there is a C<$SIG{__WARN__}> handler
4908 installed. It is the handler's responsibility to deal with the message
4909 as it sees fit (like, for instance, converting it into a C<die>). Most
4910 handlers must therefore make arrangements to actually display the
4911 warnings that they are not prepared to deal with, by calling C<warn>
4912 again in the handler. Note that this is quite safe and will not
4913 produce an endless loop, since C<__WARN__> hooks are not called from
4916 You will find this behavior is slightly different from that of
4917 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4918 instead call C<die> again to change it).
4920 Using a C<__WARN__> handler provides a powerful way to silence all
4921 warnings (even the so-called mandatory ones). An example:
4923 # wipe out *all* compile-time warnings
4924 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4926 my $foo = 20; # no warning about duplicate my $foo,
4927 # but hey, you asked for it!
4928 # no compile-time or run-time warnings before here
4931 # run-time warnings enabled after here
4932 warn "\$foo is alive and $foo!"; # does show up
4934 See L<perlvar> for details on setting C<%SIG> entries, and for more
4935 examples. See the Carp module for other kinds of warnings using its
4936 carp() and cluck() functions.
4938 =item write FILEHANDLE
4944 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4945 using the format associated with that file. By default the format for
4946 a file is the one having the same name as the filehandle, but the
4947 format for the current output channel (see the C<select> function) may be set
4948 explicitly by assigning the name of the format to the C<$~> variable.
4950 Top of form processing is handled automatically: if there is
4951 insufficient room on the current page for the formatted record, the
4952 page is advanced by writing a form feed, a special top-of-page format
4953 is used to format the new page header, and then the record is written.
4954 By default the top-of-page format is the name of the filehandle with
4955 "_TOP" appended, but it may be dynamically set to the format of your
4956 choice by assigning the name to the C<$^> variable while the filehandle is
4957 selected. The number of lines remaining on the current page is in
4958 variable C<$->, which can be set to C<0> to force a new page.
4960 If FILEHANDLE is unspecified, output goes to the current default output
4961 channel, which starts out as STDOUT but may be changed by the
4962 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
4963 is evaluated and the resulting string is used to look up the name of
4964 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4966 Note that write is I<not> the opposite of C<read>. Unfortunately.
4970 The transliteration operator. Same as C<tr///>. See L<perlop>.