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<$_> is used. (On some machines,
386 unfortunately, the elapsed time may be up to one second less than you
387 specified because of how seconds are counted.) Only one timer may be
388 counting at once. Each call disables the previous timer, and an
389 argument of C<0> may be supplied to cancel the previous timer without
390 starting a new one. The returned value is the amount of time remaining
391 on the previous timer.
393 For delays of finer granularity than one second, you may use Perl's
394 four-argument version of select() leaving the first three arguments
395 undefined, or you might be able to use the C<syscall> interface to
396 access setitimer(2) if your system supports it. The Time::HiRes module
397 from CPAN may also prove useful.
399 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
400 (C<sleep> may be internally implemented in your system with C<alarm>)
402 If you want to use C<alarm> to time out a system call you need to use an
403 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
404 fail with C<$!> 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, $hasargs,
516 $wantarray, $evaltext, $is_require, $hints) = caller($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 $evaltext is undefined. (Note also that
524 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
525 frame. C<$hints> contains pragmatic hints that the caller was
526 compiled with. It currently only reflects the hint corresponding to
529 Furthermore, when called from within the DB package, caller returns more
530 detailed information: it sets the list variable C<@DB::args> to be the
531 arguments with which the subroutine was invoked.
533 Be aware that the optimizer might have optimized call frames away before
534 C<caller> had a chance to get the information. That means that C<caller(N)>
535 might not return information about the call frame you expect it do, for
536 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
537 previous time C<caller> was called.
541 Changes the working directory to EXPR, if possible. If EXPR is omitted,
542 changes to the user's home directory. Returns true upon success,
543 false otherwise. See the example under C<die>.
547 Changes the permissions of a list of files. The first element of the
548 list must be the numerical mode, which should probably be an octal
549 number, and which definitely should I<not> a string of octal digits:
550 C<0644> is okay, C<'0644'> is not. Returns the number of files
551 successfully changed. See also L</oct>, if all you have is a string.
553 $cnt = chmod 0755, 'foo', 'bar';
554 chmod 0755, @executables;
555 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
557 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
558 $mode = 0644; chmod $mode, 'foo'; # this is best
566 This safer version of L</chop> removes any trailing string
567 that corresponds to the current value of C<$/> (also known as
568 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
569 number of characters removed from all its arguments. It's often used to
570 remove the newline from the end of an input record when you're worried
571 that the final record may be missing its newline. When in paragraph
572 mode (C<$/ = "">), it removes all trailing newlines from the string.
573 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
574 a reference to an integer or the like, see L<perlvar>) chomp() won't
576 If VARIABLE is omitted, it chomps C<$_>. Example:
579 chomp; # avoid \n on last field
584 You can actually chomp anything that's an lvalue, including an assignment:
587 chomp($answer = <STDIN>);
589 If you chomp a list, each element is chomped, and the total number of
590 characters removed is returned.
598 Chops off the last character of a string and returns the character
599 chopped. It's used primarily to remove the newline from the end of an
600 input record, but is much more efficient than C<s/\n//> because it neither
601 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
605 chop; # avoid \n on last field
610 You can actually chop anything that's an lvalue, including an assignment:
613 chop($answer = <STDIN>);
615 If you chop a list, each element is chopped. Only the value of the
616 last C<chop> is returned.
618 Note that C<chop> returns the last character. To return all but the last
619 character, use C<substr($string, 0, -1)>.
623 Changes the owner (and group) of a list of files. The first two
624 elements of the list must be the I<numeric> uid and gid, in that
625 order. A value of -1 in either position is interpreted by most
626 systems to leave that value unchanged. Returns the number of files
627 successfully changed.
629 $cnt = chown $uid, $gid, 'foo', 'bar';
630 chown $uid, $gid, @filenames;
632 Here's an example that looks up nonnumeric uids in the passwd file:
635 chomp($user = <STDIN>);
637 chomp($pattern = <STDIN>);
639 ($login,$pass,$uid,$gid) = getpwnam($user)
640 or die "$user not in passwd file";
642 @ary = glob($pattern); # expand filenames
643 chown $uid, $gid, @ary;
645 On most systems, you are not allowed to change the ownership of the
646 file unless you're the superuser, although you should be able to change
647 the group to any of your secondary groups. On insecure systems, these
648 restrictions may be relaxed, but this is not a portable assumption.
649 On POSIX systems, you can detect this condition this way:
651 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
652 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
658 Returns the character represented by that NUMBER in the character set.
659 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
660 chr(0x263a) is a Unicode smiley face (but only within the scope of
661 a C<use utf8>). For the reverse, use L</ord>.
662 See L<utf8> for more about Unicode.
664 If NUMBER is omitted, uses C<$_>.
666 =item chroot FILENAME
670 This function works like the system call by the same name: it makes the
671 named directory the new root directory for all further pathnames that
672 begin with a C</> by your process and all its children. (It doesn't
673 change your current working directory, which is unaffected.) For security
674 reasons, this call is restricted to the superuser. If FILENAME is
675 omitted, does a C<chroot> to C<$_>.
677 =item close FILEHANDLE
681 Closes the file or pipe associated with the file handle, returning true
682 only if stdio successfully flushes buffers and closes the system file
683 descriptor. Closes the currently selected filehandle if the argument
686 You don't have to close FILEHANDLE if you are immediately going to do
687 another C<open> on it, because C<open> will close it for you. (See
688 C<open>.) However, an explicit C<close> on an input file resets the line
689 counter (C<$.>), while the implicit close done by C<open> does not.
691 If the file handle came from a piped open C<close> will additionally
692 return false if one of the other system calls involved fails or if the
693 program exits with non-zero status. (If the only problem was that the
694 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
695 also waits for the process executing on the pipe to complete, in case you
696 want to look at the output of the pipe afterwards, and
697 implicitly puts the exit status value of that command into C<$?>.
699 Prematurely closing the read end of a pipe (i.e. before the process
700 writing to it at the other end has closed it) will result in a
701 SIGPIPE being delivered to the writer. If the other end can't
702 handle that, be sure to read all the data before closing the pipe.
706 open(OUTPUT, '|sort >foo') # pipe to sort
707 or die "Can't start sort: $!";
708 #... # print stuff to output
709 close OUTPUT # wait for sort to finish
710 or warn $! ? "Error closing sort pipe: $!"
711 : "Exit status $? from sort";
712 open(INPUT, 'foo') # get sort's results
713 or die "Can't open 'foo' for input: $!";
715 FILEHANDLE may be an expression whose value can be used as an indirect
716 filehandle, usually the real filehandle name.
718 =item closedir DIRHANDLE
720 Closes a directory opened by C<opendir> and returns the success of that
723 DIRHANDLE may be an expression whose value can be used as an indirect
724 dirhandle, usually the real dirhandle name.
726 =item connect SOCKET,NAME
728 Attempts to connect to a remote socket, just as the connect system call
729 does. Returns true if it succeeded, false otherwise. NAME should be a
730 packed address of the appropriate type for the socket. See the examples in
731 L<perlipc/"Sockets: Client/Server Communication">.
735 Actually a flow control statement rather than a function. If there is a
736 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
737 C<foreach>), it is always executed just before the conditional is about to
738 be evaluated again, just like the third part of a C<for> loop in C. Thus
739 it can be used to increment a loop variable, even when the loop has been
740 continued via the C<next> statement (which is similar to the C C<continue>
743 C<last>, C<next>, or C<redo> may appear within a C<continue>
744 block. C<last> and C<redo> will behave as if they had been executed within
745 the main block. So will C<next>, but since it will execute a C<continue>
746 block, it may be more entertaining.
749 ### redo always comes here
752 ### next always comes here
754 # then back the top to re-check EXPR
756 ### last always comes here
758 Omitting the C<continue> section is semantically equivalent to using an
759 empty one, logically enough. In that case, C<next> goes directly back
760 to check the condition at the top of the loop.
764 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
765 takes cosine of C<$_>.
767 For the inverse cosine operation, you may use the C<POSIX::acos()>
768 function, or use this relation:
770 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
772 =item crypt PLAINTEXT,SALT
774 Encrypts a string exactly like the crypt(3) function in the C library
775 (assuming that you actually have a version there that has not been
776 extirpated as a potential munition). This can prove useful for checking
777 the password file for lousy passwords, amongst other things. Only the
778 guys wearing white hats should do this.
780 Note that C<crypt> is intended to be a one-way function, much like breaking
781 eggs to make an omelette. There is no (known) corresponding decrypt
782 function. As a result, this function isn't all that useful for
783 cryptography. (For that, see your nearby CPAN mirror.)
785 When verifying an existing encrypted string you should use the encrypted
786 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
787 allows your code to work with the standard C<crypt> and with more
788 exotic implementations. When choosing a new salt create a random two
789 character string whose characters come from the set C<[./0-9A-Za-z]>
790 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
792 Here's an example that makes sure that whoever runs this program knows
795 $pwd = (getpwuid($<))[1];
799 chomp($word = <STDIN>);
803 if (crypt($word, $pwd) ne $pwd) {
809 Of course, typing in your own password to whoever asks you
812 The L<crypt> function is unsuitable for encrypting large quantities
813 of data, not least of all because you can't get the information
814 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
815 on your favorite CPAN mirror for a slew of potentially useful
820 [This function has been largely superseded by the C<untie> function.]
822 Breaks the binding between a DBM file and a hash.
824 =item dbmopen HASH,DBNAME,MASK
826 [This function has been largely superseded by the C<tie> function.]
828 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
829 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
830 argument is I<not> a filehandle, even though it looks like one). DBNAME
831 is the name of the database (without the F<.dir> or F<.pag> extension if
832 any). If the database does not exist, it is created with protection
833 specified by MASK (as modified by the C<umask>). If your system supports
834 only the older DBM functions, you may perform only one C<dbmopen> in your
835 program. In older versions of Perl, if your system had neither DBM nor
836 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
839 If you don't have write access to the DBM file, you can only read hash
840 variables, not set them. If you want to test whether you can write,
841 either use file tests or try setting a dummy hash entry inside an C<eval>,
842 which will trap the error.
844 Note that functions such as C<keys> and C<values> may return huge lists
845 when used on large DBM files. You may prefer to use the C<each>
846 function to iterate over large DBM files. Example:
848 # print out history file offsets
849 dbmopen(%HIST,'/usr/lib/news/history',0666);
850 while (($key,$val) = each %HIST) {
851 print $key, ' = ', unpack('L',$val), "\n";
855 See also L<AnyDBM_File> for a more general description of the pros and
856 cons of the various dbm approaches, as well as L<DB_File> for a particularly
859 You can control which DBM library you use by loading that library
860 before you call dbmopen():
863 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
864 or die "Can't open netscape history file: $!";
870 Returns a Boolean value telling whether EXPR has a value other than
871 the undefined value C<undef>. If EXPR is not present, C<$_> will be
874 Many operations return C<undef> to indicate failure, end of file,
875 system error, uninitialized variable, and other exceptional
876 conditions. This function allows you to distinguish C<undef> from
877 other values. (A simple Boolean test will not distinguish among
878 C<undef>, zero, the empty string, and C<"0">, which are all equally
879 false.) Note that since C<undef> is a valid scalar, its presence
880 doesn't I<necessarily> indicate an exceptional condition: C<pop>
881 returns C<undef> when its argument is an empty array, I<or> when the
882 element to return happens to be C<undef>.
884 You may also use C<defined(&func)> to check whether subroutine C<&func>
885 has ever been defined. The return value is unaffected by any forward
886 declarations of C<&foo>.
888 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
889 used to report whether memory for that aggregate has ever been
890 allocated. This behavior may disappear in future versions of Perl.
891 You should instead use a simple test for size:
893 if (@an_array) { print "has array elements\n" }
894 if (%a_hash) { print "has hash members\n" }
896 When used on a hash element, it tells you whether the value is defined,
897 not whether the key exists in the hash. Use L</exists> for the latter
902 print if defined $switch{'D'};
903 print "$val\n" while defined($val = pop(@ary));
904 die "Can't readlink $sym: $!"
905 unless defined($value = readlink $sym);
906 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
907 $debugging = 0 unless defined $debugging;
909 Note: Many folks tend to overuse C<defined>, and then are surprised to
910 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
911 defined values. For example, if you say
915 The pattern match succeeds, and C<$1> is defined, despite the fact that it
916 matched "nothing". But it didn't really match nothing--rather, it
917 matched something that happened to be zero characters long. This is all
918 very above-board and honest. When a function returns an undefined value,
919 it's an admission that it couldn't give you an honest answer. So you
920 should use C<defined> only when you're questioning the integrity of what
921 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
924 See also L</undef>, L</exists>, L</ref>.
928 Given an expression that specifies a hash element, array element, hash slice,
929 or array slice, deletes the specified element(s) from the hash or array.
930 If the array elements happen to be at the end of the array, the size
931 of the array will shrink by that number of elements.
933 Returns each element so deleted or the undefined value if there was no such
934 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
935 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
936 from a C<tie>d hash or array may not necessarily return anything.
938 Deleting an array element effectively returns that position of the array
939 to its initial, uninitialized state. Subsequently testing for the same
940 element with exists() will return false. See L</exists>.
942 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
944 foreach $key (keys %HASH) {
948 foreach $index (0 .. $#ARRAY) {
949 delete $ARRAY[$index];
954 delete @HASH{keys %HASH};
956 delete @ARRAY{0 .. $#ARRAY};
958 But both of these are slower than just assigning the empty list
959 or undefining %HASH or @ARRAY:
961 %HASH = (); # completely empty %HASH
962 undef %HASH; # forget %HASH ever existed
964 @ARRAY = (); # completely empty @ARRAY
965 undef @ARRAY; # forget @ARRAY ever existed
967 Note that the EXPR can be arbitrarily complicated as long as the final
968 operation is a hash element, array element, hash slice, or array slice
971 delete $ref->[$x][$y]{$key};
972 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
974 delete $ref->[$x][$y][$index];
975 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
979 Outside an C<eval>, prints the value of LIST to C<STDERR> and
980 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
981 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
982 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
983 an C<eval(),> the error message is stuffed into C<$@> and the
984 C<eval> is terminated with the undefined value. This makes
985 C<die> the way to raise an exception.
989 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
990 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
992 If the value of EXPR does not end in a newline, the current script line
993 number and input line number (if any) are also printed, and a newline
994 is supplied. Note that the "input line number" (also known as "chunk")
995 is subject to whatever notion of "line" happens to be currently in
996 effect, and is also available as the special variable C<$.>.
997 See L<perlvar/"$/"> and L<perlvar/"$.">.
999 Hint: sometimes appending C<", stopped"> to your message
1000 will cause it to make better sense when the string C<"at foo line 123"> is
1001 appended. Suppose you are running script "canasta".
1003 die "/etc/games is no good";
1004 die "/etc/games is no good, stopped";
1006 produce, respectively
1008 /etc/games is no good at canasta line 123.
1009 /etc/games is no good, stopped at canasta line 123.
1011 See also exit(), warn(), and the Carp module.
1013 If LIST is empty and C<$@> already contains a value (typically from a
1014 previous eval) that value is reused after appending C<"\t...propagated">.
1015 This is useful for propagating exceptions:
1018 die unless $@ =~ /Expected exception/;
1020 If C<$@> is empty then the string C<"Died"> is used.
1022 die() can also be called with a reference argument. If this happens to be
1023 trapped within an eval(), $@ contains the reference. This behavior permits
1024 a more elaborate exception handling implementation using objects that
1025 maintain arbitary state about the nature of the exception. Such a scheme
1026 is sometimes preferable to matching particular string values of $@ using
1027 regular expressions. Here's an example:
1029 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1031 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1032 # handle Some::Module::Exception
1035 # handle all other possible exceptions
1039 Because perl will stringify uncaught exception messages before displaying
1040 them, you may want to overload stringification operations on such custom
1041 exception objects. See L<overload> for details about that.
1043 You can arrange for a callback to be run just before the C<die>
1044 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1045 handler will be called with the error text and can change the error
1046 message, if it sees fit, by calling C<die> again. See
1047 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1048 L<"eval BLOCK"> for some examples. Although this feature was meant
1049 to be run only right before your program was to exit, this is not
1050 currently the case--the C<$SIG{__DIE__}> hook is currently called
1051 even inside eval()ed blocks/strings! If one wants the hook to do
1052 nothing in such situations, put
1056 as the first line of the handler (see L<perlvar/$^S>). Because
1057 this promotes strange action at a distance, this counterintuitive
1058 behavior may be fixed in a future release.
1062 Not really a function. Returns the value of the last command in the
1063 sequence of commands indicated by BLOCK. When modified by a loop
1064 modifier, executes the BLOCK once before testing the loop condition.
1065 (On other statements the loop modifiers test the conditional first.)
1067 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1068 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1069 See L<perlsyn> for alternative strategies.
1071 =item do SUBROUTINE(LIST)
1073 A deprecated form of subroutine call. See L<perlsub>.
1077 Uses the value of EXPR as a filename and executes the contents of the
1078 file as a Perl script. Its primary use is to include subroutines
1079 from a Perl subroutine library.
1085 scalar eval `cat stat.pl`;
1087 except that it's more efficient and concise, keeps track of the current
1088 filename for error messages, searches the @INC libraries, and updates
1089 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1090 variables. It also differs in that code evaluated with C<do FILENAME>
1091 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1092 same, however, in that it does reparse the file every time you call it,
1093 so you probably don't want to do this inside a loop.
1095 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1096 error. If C<do> can read the file but cannot compile it, it
1097 returns undef and sets an error message in C<$@>. If the file is
1098 successfully compiled, C<do> returns the value of the last expression
1101 Note that inclusion of library modules is better done with the
1102 C<use> and C<require> operators, which also do automatic error checking
1103 and raise an exception if there's a problem.
1105 You might like to use C<do> to read in a program configuration
1106 file. Manual error checking can be done this way:
1108 # read in config files: system first, then user
1109 for $file ("/share/prog/defaults.rc",
1110 "$ENV{HOME}/.someprogrc")
1112 unless ($return = do $file) {
1113 warn "couldn't parse $file: $@" if $@;
1114 warn "couldn't do $file: $!" unless defined $return;
1115 warn "couldn't run $file" unless $return;
1123 This function causes an immediate core dump. See also the B<-u>
1124 command-line switch in L<perlrun>, which does the same thing.
1125 Primarily this is so that you can use the B<undump> program (not
1126 supplied) to turn your core dump into an executable binary after
1127 having initialized all your variables at the beginning of the
1128 program. When the new binary is executed it will begin by executing
1129 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1130 Think of it as a goto with an intervening core dump and reincarnation.
1131 If C<LABEL> is omitted, restarts the program from the top.
1133 B<WARNING>: Any files opened at the time of the dump will I<not>
1134 be open any more when the program is reincarnated, with possible
1135 resulting confusion on the part of Perl.
1137 This function is now largely obsolete, partly because it's very
1138 hard to convert a core file into an executable, and because the
1139 real compiler backends for generating portable bytecode and compilable
1140 C code have superseded it.
1142 If you're looking to use L<dump> to speed up your program, consider
1143 generating bytecode or native C code as described in L<perlcc>. If
1144 you're just trying to accelerate a CGI script, consider using the
1145 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1146 You might also consider autoloading or selfloading, which at least
1147 make your program I<appear> to run faster.
1151 When called in list context, returns a 2-element list consisting of the
1152 key and value for the next element of a hash, so that you can iterate over
1153 it. When called in scalar context, returns the key for only the "next"
1154 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1155 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1158 Entries are returned in an apparently random order. The actual random
1159 order is subject to change in future versions of perl, but it is guaranteed
1160 to be in the same order as either the C<keys> or C<values> function
1161 would produce on the same (unmodified) hash.
1163 When the hash is entirely read, a null array is returned in list context
1164 (which when assigned produces a false (C<0>) value), and C<undef> in
1165 scalar context. The next call to C<each> after that will start iterating
1166 again. There is a single iterator for each hash, shared by all C<each>,
1167 C<keys>, and C<values> function calls in the program; it can be reset by
1168 reading all the elements from the hash, or by evaluating C<keys HASH> or
1169 C<values HASH>. If you add or delete elements of a hash while you're
1170 iterating over it, you may get entries skipped or duplicated, so don't.
1172 The following prints out your environment like the printenv(1) program,
1173 only in a different order:
1175 while (($key,$value) = each %ENV) {
1176 print "$key=$value\n";
1179 See also C<keys>, C<values> and C<sort>.
1181 =item eof FILEHANDLE
1187 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1188 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1189 gives the real filehandle. (Note that this function actually
1190 reads a character and then C<ungetc>s it, so isn't very useful in an
1191 interactive context.) Do not read from a terminal file (or call
1192 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1193 as terminals may lose the end-of-file condition if you do.
1195 An C<eof> without an argument uses the last file read. Using C<eof()>
1196 with empty parentheses is very different. It refers to the pseudo file
1197 formed from the files listed on the command line and accessed via the
1198 C<E<lt>E<gt>> operator. Since C<E<lt>E<gt>> isn't explicitly opened,
1199 as a normal filehandle is, an C<eof()> before C<E<lt>E<gt>> has been
1200 used will cause C<@ARGV> to be examined to determine if input is
1203 In a C<while (E<lt>E<gt>)> loop, C<eof> or C<eof(ARGV)> can be used to
1204 detect the end of each file, C<eof()> will only detect the end of the
1205 last file. Examples:
1207 # reset line numbering on each input file
1209 next if /^\s*#/; # skip comments
1212 close ARGV if eof; # Not eof()!
1215 # insert dashes just before last line of last file
1217 if (eof()) { # check for end of current file
1218 print "--------------\n";
1219 close(ARGV); # close or last; is needed if we
1220 # are reading from the terminal
1225 Practical hint: you almost never need to use C<eof> in Perl, because the
1226 input operators typically return C<undef> when they run out of data, or if
1233 In the first form, the return value of EXPR is parsed and executed as if it
1234 were a little Perl program. The value of the expression (which is itself
1235 determined within scalar context) is first parsed, and if there weren't any
1236 errors, executed in the context of the current Perl program, so that any
1237 variable settings or subroutine and format definitions remain afterwards.
1238 Note that the value is parsed every time the eval executes. If EXPR is
1239 omitted, evaluates C<$_>. This form is typically used to delay parsing
1240 and subsequent execution of the text of EXPR until run time.
1242 In the second form, the code within the BLOCK is parsed only once--at the
1243 same time the code surrounding the eval itself was parsed--and executed
1244 within the context of the current Perl program. This form is typically
1245 used to trap exceptions more efficiently than the first (see below), while
1246 also providing the benefit of checking the code within BLOCK at compile
1249 The final semicolon, if any, may be omitted from the value of EXPR or within
1252 In both forms, the value returned is the value of the last expression
1253 evaluated inside the mini-program; a return statement may be also used, just
1254 as with subroutines. The expression providing the return value is evaluated
1255 in void, scalar, or list context, depending on the context of the eval itself.
1256 See L</wantarray> for more on how the evaluation context can be determined.
1258 If there is a syntax error or runtime error, or a C<die> statement is
1259 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1260 error message. If there was no error, C<$@> is guaranteed to be a null
1261 string. Beware that using C<eval> neither silences perl from printing
1262 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1263 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1264 L</warn> and L<perlvar>.
1266 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1267 determining whether a particular feature (such as C<socket> or C<symlink>)
1268 is implemented. It is also Perl's exception trapping mechanism, where
1269 the die operator is used to raise exceptions.
1271 If the code to be executed doesn't vary, you may use the eval-BLOCK
1272 form to trap run-time errors without incurring the penalty of
1273 recompiling each time. The error, if any, is still returned in C<$@>.
1276 # make divide-by-zero nonfatal
1277 eval { $answer = $a / $b; }; warn $@ if $@;
1279 # same thing, but less efficient
1280 eval '$answer = $a / $b'; warn $@ if $@;
1282 # a compile-time error
1283 eval { $answer = }; # WRONG
1286 eval '$answer ='; # sets $@
1288 Due to the current arguably broken state of C<__DIE__> hooks, when using
1289 the C<eval{}> form as an exception trap in libraries, you may wish not
1290 to trigger any C<__DIE__> hooks that user code may have installed.
1291 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1292 as shown in this example:
1294 # a very private exception trap for divide-by-zero
1295 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1298 This is especially significant, given that C<__DIE__> hooks can call
1299 C<die> again, which has the effect of changing their error messages:
1301 # __DIE__ hooks may modify error messages
1303 local $SIG{'__DIE__'} =
1304 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1305 eval { die "foo lives here" };
1306 print $@ if $@; # prints "bar lives here"
1309 Because this promotes action at a distance, this counterintuitive behavior
1310 may be fixed in a future release.
1312 With an C<eval>, you should be especially careful to remember what's
1313 being looked at when:
1319 eval { $x }; # CASE 4
1321 eval "\$$x++"; # CASE 5
1324 Cases 1 and 2 above behave identically: they run the code contained in
1325 the variable $x. (Although case 2 has misleading double quotes making
1326 the reader wonder what else might be happening (nothing is).) Cases 3
1327 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1328 does nothing but return the value of $x. (Case 4 is preferred for
1329 purely visual reasons, but it also has the advantage of compiling at
1330 compile-time instead of at run-time.) Case 5 is a place where
1331 normally you I<would> like to use double quotes, except that in this
1332 particular situation, you can just use symbolic references instead, as
1335 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1336 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1340 =item exec PROGRAM LIST
1342 The C<exec> function executes a system command I<and never returns>--
1343 use C<system> instead of C<exec> if you want it to return. It fails and
1344 returns false only if the command does not exist I<and> it is executed
1345 directly instead of via your system's command shell (see below).
1347 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1348 warns you if there is a following statement which isn't C<die>, C<warn>,
1349 or C<exit> (if C<-w> is set - but you always do that). If you
1350 I<really> want to follow an C<exec> with some other statement, you
1351 can use one of these styles to avoid the warning:
1353 exec ('foo') or print STDERR "couldn't exec foo: $!";
1354 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1356 If there is more than one argument in LIST, or if LIST is an array
1357 with more than one value, calls execvp(3) with the arguments in LIST.
1358 If there is only one scalar argument or an array with one element in it,
1359 the argument is checked for shell metacharacters, and if there are any,
1360 the entire argument is passed to the system's command shell for parsing
1361 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1362 If there are no shell metacharacters in the argument, it is split into
1363 words and passed directly to C<execvp>, which is more efficient.
1366 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1367 exec "sort $outfile | uniq";
1369 If you don't really want to execute the first argument, but want to lie
1370 to the program you are executing about its own name, you can specify
1371 the program you actually want to run as an "indirect object" (without a
1372 comma) in front of the LIST. (This always forces interpretation of the
1373 LIST as a multivalued list, even if there is only a single scalar in
1376 $shell = '/bin/csh';
1377 exec $shell '-sh'; # pretend it's a login shell
1381 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1383 When the arguments get executed via the system shell, results will
1384 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1387 Using an indirect object with C<exec> or C<system> is also more
1388 secure. This usage (which also works fine with system()) forces
1389 interpretation of the arguments as a multivalued list, even if the
1390 list had just one argument. That way you're safe from the shell
1391 expanding wildcards or splitting up words with whitespace in them.
1393 @args = ( "echo surprise" );
1395 exec @args; # subject to shell escapes
1397 exec { $args[0] } @args; # safe even with one-arg list
1399 The first version, the one without the indirect object, ran the I<echo>
1400 program, passing it C<"surprise"> an argument. The second version
1401 didn't--it tried to run a program literally called I<"echo surprise">,
1402 didn't find it, and set C<$?> to a non-zero value indicating failure.
1404 Note that C<exec> will not call your C<END> blocks, nor will it call
1405 any C<DESTROY> methods in your objects.
1409 Given an expression that specifies a hash element or array element,
1410 returns true if the specified element in the hash or array has ever
1411 been initialized, even if the corresponding value is undefined. The
1412 element is not autovivified if it doesn't exist.
1414 print "Exists\n" if exists $hash{$key};
1415 print "Defined\n" if defined $hash{$key};
1416 print "True\n" if $hash{$key};
1418 print "Exists\n" if exists $array[$index];
1419 print "Defined\n" if defined $array[$index];
1420 print "True\n" if $array[$index];
1422 A hash or array element can be true only if it's defined, and defined if
1423 it exists, but the reverse doesn't necessarily hold true.
1425 Note that the EXPR can be arbitrarily complicated as long as the final
1426 operation is a hash or array key lookup:
1428 if (exists $ref->{A}->{B}->{$key}) { }
1429 if (exists $hash{A}{B}{$key}) { }
1431 if (exists $ref->{A}->{B}->[$ix]) { }
1432 if (exists $hash{A}{B}[$ix]) { }
1434 Although the deepest nested array or hash will not spring into existence
1435 just because its existence was tested, any intervening ones will.
1436 Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring
1437 into existence due to the existence test for the $key element above.
1438 This happens anywhere the arrow operator is used, including even:
1441 if (exists $ref->{"Some key"}) { }
1442 print $ref; # prints HASH(0x80d3d5c)
1444 This surprising autovivification in what does not at first--or even
1445 second--glance appear to be an lvalue context may be fixed in a future
1448 See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
1449 used on a pseudo-hash.
1453 Evaluates EXPR and exits immediately with that value. Example:
1456 exit 0 if $ans =~ /^[Xx]/;
1458 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1459 universally recognized values for EXPR are C<0> for success and C<1>
1460 for error; other values are subject to interpretation depending on the
1461 environment in which the Perl program is running. For example, exiting
1462 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1463 the mailer to return the item undelivered, but that's not true everywhere.
1465 Don't use C<exit> to abort a subroutine if there's any chance that
1466 someone might want to trap whatever error happened. Use C<die> instead,
1467 which can be trapped by an C<eval>.
1469 The exit() function does not always exit immediately. It calls any
1470 defined C<END> routines first, but these C<END> routines may not
1471 themselves abort the exit. Likewise any object destructors that need to
1472 be called are called before the real exit. If this is a problem, you
1473 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1474 See L<perlmod> for details.
1480 Returns I<e> (the natural logarithm base) to the power of EXPR.
1481 If EXPR is omitted, gives C<exp($_)>.
1483 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1485 Implements the fcntl(2) function. You'll probably have to say
1489 first to get the correct constant definitions. Argument processing and
1490 value return works just like C<ioctl> below.
1494 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1495 or die "can't fcntl F_GETFL: $!";
1497 You don't have to check for C<defined> on the return from C<fnctl>.
1498 Like C<ioctl>, it maps a C<0> return from the system call into
1499 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1500 in numeric context. It is also exempt from the normal B<-w> warnings
1501 on improper numeric conversions.
1503 Note that C<fcntl> will produce a fatal error if used on a machine that
1504 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1505 manpage to learn what functions are available on your system.
1507 =item fileno FILEHANDLE
1509 Returns the file descriptor for a filehandle, or undefined if the
1510 filehandle is not open. This is mainly useful for constructing
1511 bitmaps for C<select> and low-level POSIX tty-handling operations.
1512 If FILEHANDLE is an expression, the value is taken as an indirect
1513 filehandle, generally its name.
1515 You can use this to find out whether two handles refer to the
1516 same underlying descriptor:
1518 if (fileno(THIS) == fileno(THAT)) {
1519 print "THIS and THAT are dups\n";
1522 =item flock FILEHANDLE,OPERATION
1524 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1525 for success, false on failure. Produces a fatal error if used on a
1526 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1527 C<flock> is Perl's portable file locking interface, although it locks
1528 only entire files, not records.
1530 Two potentially non-obvious but traditional C<flock> semantics are
1531 that it waits indefinitely until the lock is granted, and that its locks
1532 B<merely advisory>. Such discretionary locks are more flexible, but offer
1533 fewer guarantees. This means that files locked with C<flock> may be
1534 modified by programs that do not also use C<flock>. See L<perlport>,
1535 your port's specific documentation, or your system-specific local manpages
1536 for details. It's best to assume traditional behavior if you're writing
1537 portable programs. (But if you're not, you should as always feel perfectly
1538 free to write for your own system's idiosyncrasies (sometimes called
1539 "features"). Slavish adherence to portability concerns shouldn't get
1540 in the way of your getting your job done.)
1542 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1543 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1544 you can use the symbolic names if import them from the Fcntl module,
1545 either individually, or as a group using the ':flock' tag. LOCK_SH
1546 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1547 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1548 LOCK_EX then C<flock> will return immediately rather than blocking
1549 waiting for the lock (check the return status to see if you got it).
1551 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1552 before locking or unlocking it.
1554 Note that the emulation built with lockf(3) doesn't provide shared
1555 locks, and it requires that FILEHANDLE be open with write intent. These
1556 are the semantics that lockf(3) implements. Most if not all systems
1557 implement lockf(3) in terms of fcntl(2) locking, though, so the
1558 differing semantics shouldn't bite too many people.
1560 Note also that some versions of C<flock> cannot lock things over the
1561 network; you would need to use the more system-specific C<fcntl> for
1562 that. If you like you can force Perl to ignore your system's flock(2)
1563 function, and so provide its own fcntl(2)-based emulation, by passing
1564 the switch C<-Ud_flock> to the F<Configure> program when you configure
1567 Here's a mailbox appender for BSD systems.
1569 use Fcntl ':flock'; # import LOCK_* constants
1572 flock(MBOX,LOCK_EX);
1573 # and, in case someone appended
1574 # while we were waiting...
1579 flock(MBOX,LOCK_UN);
1582 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1583 or die "Can't open mailbox: $!";
1586 print MBOX $msg,"\n\n";
1589 On systems that support a real flock(), locks are inherited across fork()
1590 calls, whereas those that must resort to the more capricious fcntl()
1591 function lose the locks, making it harder to write servers.
1593 See also L<DB_File> for other flock() examples.
1597 Does a fork(2) system call to create a new process running the
1598 same program at the same point. It returns the child pid to the
1599 parent process, C<0> to the child process, or C<undef> if the fork is
1600 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1601 are shared, while everything else is copied. On most systems supporting
1602 fork(), great care has gone into making it extremely efficient (for
1603 example, using copy-on-write technology on data pages), making it the
1604 dominant paradigm for multitasking over the last few decades.
1606 All files opened for output are flushed before forking the child process.
1608 If you C<fork> without ever waiting on your children, you will
1609 accumulate zombies. On some systems, you can avoid this by setting
1610 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1611 forking and reaping moribund children.
1613 Note that if your forked child inherits system file descriptors like
1614 STDIN and STDOUT that are actually connected by a pipe or socket, even
1615 if you exit, then the remote server (such as, say, a CGI script or a
1616 backgrounded job launched from a remote shell) won't think you're done.
1617 You should reopen those to F</dev/null> if it's any issue.
1621 Declare a picture format for use by the C<write> function. For
1625 Test: @<<<<<<<< @||||| @>>>>>
1626 $str, $%, '$' . int($num)
1630 $num = $cost/$quantity;
1634 See L<perlform> for many details and examples.
1636 =item formline PICTURE,LIST
1638 This is an internal function used by C<format>s, though you may call it,
1639 too. It formats (see L<perlform>) a list of values according to the
1640 contents of PICTURE, placing the output into the format output
1641 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1642 Eventually, when a C<write> is done, the contents of
1643 C<$^A> are written to some filehandle, but you could also read C<$^A>
1644 yourself and then set C<$^A> back to C<"">. Note that a format typically
1645 does one C<formline> per line of form, but the C<formline> function itself
1646 doesn't care how many newlines are embedded in the PICTURE. This means
1647 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1648 You may therefore need to use multiple formlines to implement a single
1649 record format, just like the format compiler.
1651 Be careful if you put double quotes around the picture, because an C<@>
1652 character may be taken to mean the beginning of an array name.
1653 C<formline> always returns true. See L<perlform> for other examples.
1655 =item getc FILEHANDLE
1659 Returns the next character from the input file attached to FILEHANDLE,
1660 or the undefined value at end of file, or if there was an error.
1661 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1662 efficient. However, it cannot be used by itself to fetch single
1663 characters without waiting for the user to hit enter. For that, try
1664 something more like:
1667 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1670 system "stty", '-icanon', 'eol', "\001";
1676 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1679 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1683 Determination of whether $BSD_STYLE should be set
1684 is left as an exercise to the reader.
1686 The C<POSIX::getattr> function can do this more portably on
1687 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1688 module from your nearest CPAN site; details on CPAN can be found on
1693 Implements the C library function of the same name, which on most
1694 systems returns the current login from F</etc/utmp>, if any. If null,
1697 $login = getlogin || getpwuid($<) || "Kilroy";
1699 Do not consider C<getlogin> for authentication: it is not as
1700 secure as C<getpwuid>.
1702 =item getpeername SOCKET
1704 Returns the packed sockaddr address of other end of the SOCKET connection.
1707 $hersockaddr = getpeername(SOCK);
1708 ($port, $iaddr) = sockaddr_in($hersockaddr);
1709 $herhostname = gethostbyaddr($iaddr, AF_INET);
1710 $herstraddr = inet_ntoa($iaddr);
1714 Returns the current process group for the specified PID. Use
1715 a PID of C<0> to get the current process group for the
1716 current process. Will raise an exception if used on a machine that
1717 doesn't implement getpgrp(2). If PID is omitted, returns process
1718 group of current process. Note that the POSIX version of C<getpgrp>
1719 does not accept a PID argument, so only C<PID==0> is truly portable.
1723 Returns the process id of the parent process.
1725 =item getpriority WHICH,WHO
1727 Returns the current priority for a process, a process group, or a user.
1728 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1729 machine that doesn't implement getpriority(2).
1735 =item gethostbyname NAME
1737 =item getnetbyname NAME
1739 =item getprotobyname NAME
1745 =item getservbyname NAME,PROTO
1747 =item gethostbyaddr ADDR,ADDRTYPE
1749 =item getnetbyaddr ADDR,ADDRTYPE
1751 =item getprotobynumber NUMBER
1753 =item getservbyport PORT,PROTO
1771 =item sethostent STAYOPEN
1773 =item setnetent STAYOPEN
1775 =item setprotoent STAYOPEN
1777 =item setservent STAYOPEN
1791 These routines perform the same functions as their counterparts in the
1792 system library. In list context, the return values from the
1793 various get routines are as follows:
1795 ($name,$passwd,$uid,$gid,
1796 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1797 ($name,$passwd,$gid,$members) = getgr*
1798 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1799 ($name,$aliases,$addrtype,$net) = getnet*
1800 ($name,$aliases,$proto) = getproto*
1801 ($name,$aliases,$port,$proto) = getserv*
1803 (If the entry doesn't exist you get a null list.)
1805 In scalar context, you get the name, unless the function was a
1806 lookup by name, in which case you get the other thing, whatever it is.
1807 (If the entry doesn't exist you get the undefined value.) For example:
1809 $uid = getpwnam($name);
1810 $name = getpwuid($num);
1812 $gid = getgrnam($name);
1813 $name = getgrgid($num;
1817 In I<getpw*()> the fields $quota, $comment, and $expire are
1818 special cases in the sense that in many systems they are unsupported.
1819 If the $quota is unsupported, it is an empty scalar. If it is
1820 supported, it usually encodes the disk quota. If the $comment
1821 field is unsupported, it is an empty scalar. If it is supported it
1822 usually encodes some administrative comment about the user. In some
1823 systems the $quota field may be $change or $age, fields that have
1824 to do with password aging. In some systems the $comment field may
1825 be $class. The $expire field, if present, encodes the expiration
1826 period of the account or the password. For the availability and the
1827 exact meaning of these fields in your system, please consult your
1828 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1829 out from within Perl what your $quota and $comment fields mean
1830 and whether you have the $expire field by using the C<Config> module
1831 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1832 and C<d_pwexpire>. Shadow password files are only supported if your
1833 vendor has implemented them in the intuitive fashion that calling the
1834 regular C library routines gets the shadow versions if you're running
1835 under privilege. Those that incorrectly implement a separate library
1836 call are not supported.
1838 The $members value returned by I<getgr*()> is a space separated list of
1839 the login names of the members of the group.
1841 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1842 C, it will be returned to you via C<$?> if the function call fails. The
1843 C<@addrs> value returned by a successful call is a list of the raw
1844 addresses returned by the corresponding system library call. In the
1845 Internet domain, each address is four bytes long and you can unpack it
1846 by saying something like:
1848 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1850 The Socket library makes this slightly easier:
1853 $iaddr = inet_aton("127.1"); # or whatever address
1854 $name = gethostbyaddr($iaddr, AF_INET);
1856 # or going the other way
1857 $straddr = inet_ntoa($iaddr);
1859 If you get tired of remembering which element of the return list
1860 contains which return value, by-name interfaces are provided
1861 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1862 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1863 and C<User::grent>. These override the normal built-ins, supplying
1864 versions that return objects with the appropriate names
1865 for each field. For example:
1869 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1871 Even though it looks like they're the same method calls (uid),
1872 they aren't, because a C<File::stat> object is different from
1873 a C<User::pwent> object.
1875 =item getsockname SOCKET
1877 Returns the packed sockaddr address of this end of the SOCKET connection,
1878 in case you don't know the address because you have several different
1879 IPs that the connection might have come in on.
1882 $mysockaddr = getsockname(SOCK);
1883 ($port, $myaddr) = sockaddr_in($mysockaddr);
1884 printf "Connect to %s [%s]\n",
1885 scalar gethostbyaddr($myaddr, AF_INET),
1888 =item getsockopt SOCKET,LEVEL,OPTNAME
1890 Returns the socket option requested, or undef if there is an error.
1896 Returns the value of EXPR with filename expansions such as the
1897 standard Unix shell F</bin/csh> would do. This is the internal function
1898 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1899 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1900 discussed in more detail in L<perlop/"I/O Operators">.
1904 Converts a time as returned by the time function to a 9-element list
1905 with the time localized for the standard Greenwich time zone.
1906 Typically used as follows:
1909 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1912 All list elements are numeric, and come straight out of a struct tm.
1913 In particular this means that $mon has the range C<0..11> and $wday
1914 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1915 number of years since 1900, that is, $year is C<123> in year 2023,
1916 I<not> simply the last two digits of the year. If you assume it is,
1917 then you create non-Y2K-compliant programs--and you wouldn't want to do
1920 The proper way to get a complete 4-digit year is simply:
1924 And to get the last two digits of the year (e.g., '01' in 2001) do:
1926 $year = sprintf("%02d", $year % 100);
1928 If EXPR is omitted, does C<gmtime(time())>.
1930 In scalar context, returns the ctime(3) value:
1932 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1934 Also see the C<timegm> function provided by the C<Time::Local> module,
1935 and the strftime(3) function available via the POSIX module.
1937 This scalar value is B<not> locale dependent (see L<perllocale>), but
1938 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1939 strftime(3) and mktime(3) functions available via the POSIX module. To
1940 get somewhat similar but locale dependent date strings, set up your
1941 locale environment variables appropriately (please see L<perllocale>)
1942 and try for example:
1944 use POSIX qw(strftime);
1945 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1947 Note that the C<%a> and C<%b> escapes, which represent the short forms
1948 of the day of the week and the month of the year, may not necessarily
1949 be three characters wide in all locales.
1957 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1958 execution there. It may not be used to go into any construct that
1959 requires initialization, such as a subroutine or a C<foreach> loop. It
1960 also can't be used to go into a construct that is optimized away,
1961 or to get out of a block or subroutine given to C<sort>.
1962 It can be used to go almost anywhere else within the dynamic scope,
1963 including out of subroutines, but it's usually better to use some other
1964 construct such as C<last> or C<die>. The author of Perl has never felt the
1965 need to use this form of C<goto> (in Perl, that is--C is another matter).
1967 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1968 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1969 necessarily recommended if you're optimizing for maintainability:
1971 goto ("FOO", "BAR", "GLARCH")[$i];
1973 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
1974 In fact, it isn't a goto in the normal sense at all, and doesn't have
1975 the stigma associated with other gotos. Instead, it
1976 substitutes a call to the named subroutine for the currently running
1977 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
1978 another subroutine and then pretend that the other subroutine had been
1979 called in the first place (except that any modifications to C<@_>
1980 in the current subroutine are propagated to the other subroutine.)
1981 After the C<goto>, not even C<caller> will be able to tell that this
1982 routine was called first.
1984 NAME needn't be the name of a subroutine; it can be a scalar variable
1985 containing a code reference, or a block which evaluates to a code
1988 =item grep BLOCK LIST
1990 =item grep EXPR,LIST
1992 This is similar in spirit to, but not the same as, grep(1) and its
1993 relatives. In particular, it is not limited to using regular expressions.
1995 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1996 C<$_> to each element) and returns the list value consisting of those
1997 elements for which the expression evaluated to true. In scalar
1998 context, returns the number of times the expression was true.
2000 @foo = grep(!/^#/, @bar); # weed out comments
2004 @foo = grep {!/^#/} @bar; # weed out comments
2006 Note that, because C<$_> is a reference into the list value, it can
2007 be used to modify the elements of the array. While this is useful and
2008 supported, it can cause bizarre results if the LIST is not a named array.
2009 Similarly, grep returns aliases into the original list, much as a for
2010 loop's index variable aliases the list elements. That is, modifying an
2011 element of a list returned by grep (for example, in a C<foreach>, C<map>
2012 or another C<grep>) actually modifies the element in the original list.
2013 This is usually something to be avoided when writing clear code.
2015 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2021 Interprets EXPR as a hex string and returns the corresponding value.
2022 (To convert strings that might start with either 0, 0x, or 0b, see
2023 L</oct>.) If EXPR is omitted, uses C<$_>.
2025 print hex '0xAf'; # prints '175'
2026 print hex 'aF'; # same
2028 Hex strings may only represent integers. Strings that would cause
2029 integer overflow trigger a warning.
2033 There is no builtin C<import> function. It is just an ordinary
2034 method (subroutine) defined (or inherited) by modules that wish to export
2035 names to another module. The C<use> function calls the C<import> method
2036 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2038 =item index STR,SUBSTR,POSITION
2040 =item index STR,SUBSTR
2042 The index function searches for one string within another, but without
2043 the wildcard-like behavior of a full regular-expression pattern match.
2044 It returns the position of the first occurrence of SUBSTR in STR at
2045 or after POSITION. If POSITION is omitted, starts searching from the
2046 beginning of the string. The return value is based at C<0> (or whatever
2047 you've set the C<$[> variable to--but don't do that). If the substring
2048 is not found, returns one less than the base, ordinarily C<-1>.
2054 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2055 You should not use this function for rounding: one because it truncates
2056 towards C<0>, and two because machine representations of floating point
2057 numbers can sometimes produce counterintuitive results. For example,
2058 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2059 because it's really more like -268.99999999999994315658 instead. Usually,
2060 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2061 functions will serve you better than will int().
2063 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2065 Implements the ioctl(2) function. You'll probably first have to say
2067 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2069 to get the correct function definitions. If F<ioctl.ph> doesn't
2070 exist or doesn't have the correct definitions you'll have to roll your
2071 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2072 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2073 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2074 written depending on the FUNCTION--a pointer to the string value of SCALAR
2075 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2076 has no string value but does have a numeric value, that value will be
2077 passed rather than a pointer to the string value. To guarantee this to be
2078 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2079 functions may be needed to manipulate the values of structures used by
2082 The return value of C<ioctl> (and C<fcntl>) is as follows:
2084 if OS returns: then Perl returns:
2086 0 string "0 but true"
2087 anything else that number
2089 Thus Perl returns true on success and false on failure, yet you can
2090 still easily determine the actual value returned by the operating
2093 $retval = ioctl(...) || -1;
2094 printf "System returned %d\n", $retval;
2096 The special string "C<0> but true" is exempt from B<-w> complaints
2097 about improper numeric conversions.
2099 Here's an example of setting a filehandle named C<REMOTE> to be
2100 non-blocking at the system level. You'll have to negotiate C<$|>
2101 on your own, though.
2103 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2105 $flags = fcntl(REMOTE, F_GETFL, 0)
2106 or die "Can't get flags for the socket: $!\n";
2108 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2109 or die "Can't set flags for the socket: $!\n";
2111 =item join EXPR,LIST
2113 Joins the separate strings of LIST into a single string with fields
2114 separated by the value of EXPR, and returns that new string. Example:
2116 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2118 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2119 first argument. Compare L</split>.
2123 Returns a list consisting of all the keys of the named hash. (In
2124 scalar context, returns the number of keys.) The keys are returned in
2125 an apparently random order. The actual random order is subject to
2126 change in future versions of perl, but it is guaranteed to be the same
2127 order as either the C<values> or C<each> function produces (given
2128 that the hash has not been modified). As a side effect, it resets
2131 Here is yet another way to print your environment:
2134 @values = values %ENV;
2136 print pop(@keys), '=', pop(@values), "\n";
2139 or how about sorted by key:
2141 foreach $key (sort(keys %ENV)) {
2142 print $key, '=', $ENV{$key}, "\n";
2145 To sort a hash by value, you'll need to use a C<sort> function.
2146 Here's a descending numeric sort of a hash by its values:
2148 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2149 printf "%4d %s\n", $hash{$key}, $key;
2152 As an lvalue C<keys> allows you to increase the number of hash buckets
2153 allocated for the given hash. This can gain you a measure of efficiency if
2154 you know the hash is going to get big. (This is similar to pre-extending
2155 an array by assigning a larger number to $#array.) If you say
2159 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2160 in fact, since it rounds up to the next power of two. These
2161 buckets will be retained even if you do C<%hash = ()>, use C<undef
2162 %hash> if you want to free the storage while C<%hash> is still in scope.
2163 You can't shrink the number of buckets allocated for the hash using
2164 C<keys> in this way (but you needn't worry about doing this by accident,
2165 as trying has no effect).
2167 See also C<each>, C<values> and C<sort>.
2169 =item kill SIGNAL, LIST
2171 Sends a signal to a list of processes. Returns the number of
2172 processes successfully signaled (which is not necessarily the
2173 same as the number actually killed).
2175 $cnt = kill 1, $child1, $child2;
2178 If SIGNAL is zero, no signal is sent to the process. This is a
2179 useful way to check that the process is alive and hasn't changed
2180 its UID. See L<perlport> for notes on the portability of this
2183 Unlike in the shell, if SIGNAL is negative, it kills
2184 process groups instead of processes. (On System V, a negative I<PROCESS>
2185 number will also kill process groups, but that's not portable.) That
2186 means you usually want to use positive not negative signals. You may also
2187 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2193 The C<last> command is like the C<break> statement in C (as used in
2194 loops); it immediately exits the loop in question. If the LABEL is
2195 omitted, the command refers to the innermost enclosing loop. The
2196 C<continue> block, if any, is not executed:
2198 LINE: while (<STDIN>) {
2199 last LINE if /^$/; # exit when done with header
2203 C<last> cannot be used to exit a block which returns a value such as
2204 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2205 a grep() or map() operation.
2207 Note that a block by itself is semantically identical to a loop
2208 that executes once. Thus C<last> can be used to effect an early
2209 exit out of such a block.
2211 See also L</continue> for an illustration of how C<last>, C<next>, and
2218 Returns an lowercased version of EXPR. This is the internal function
2219 implementing the C<\L> escape in double-quoted strings.
2220 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2223 If EXPR is omitted, uses C<$_>.
2229 Returns the value of EXPR with the first character lowercased. This is
2230 the internal function implementing the C<\l> escape in double-quoted strings.
2231 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2233 If EXPR is omitted, uses C<$_>.
2239 Returns the length in characters of the value of EXPR. If EXPR is
2240 omitted, returns length of C<$_>. Note that this cannot be used on
2241 an entire array or hash to find out how many elements these have.
2242 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2244 =item link OLDFILE,NEWFILE
2246 Creates a new filename linked to the old filename. Returns true for
2247 success, false otherwise.
2249 =item listen SOCKET,QUEUESIZE
2251 Does the same thing that the listen system call does. Returns true if
2252 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2256 You really probably want to be using C<my> instead, because C<local> isn't
2257 what most people think of as "local". See L<perlsub/"Private Variables
2258 via my()"> for details.
2260 A local modifies the listed variables to be local to the enclosing
2261 block, file, or eval. If more than one value is listed, the list must
2262 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2263 for details, including issues with tied arrays and hashes.
2265 =item localtime EXPR
2267 Converts a time as returned by the time function to a 9-element list
2268 with the time analyzed for the local time zone. Typically used as
2272 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2275 All list elements are numeric, and come straight out of a struct tm.
2276 In particular this means that $mon has the range C<0..11> and $wday
2277 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2278 number of years since 1900, that is, $year is C<123> in year 2023,
2279 and I<not> simply the last two digits of the year. If you assume it is,
2280 then you create non-Y2K-compliant programs--and you wouldn't want to do
2283 The proper way to get a complete 4-digit year is simply:
2287 And to get the last two digits of the year (e.g., '01' in 2001) do:
2289 $year = sprintf("%02d", $year % 100);
2291 If EXPR is omitted, uses the current time (C<localtime(time)>).
2293 In scalar context, returns the ctime(3) value:
2295 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2297 This scalar value is B<not> locale dependent, see L<perllocale>, but
2298 instead a Perl builtin. Also see the C<Time::Local> module
2299 (to convert the second, minutes, hours, ... back to seconds since the
2300 stroke of midnight the 1st of January 1970, the value returned by
2301 time()), and the strftime(3) and mktime(3) function available via the
2302 POSIX module. To get somewhat similar but locale dependent date
2303 strings, set up your locale environment variables appropriately
2304 (please see L<perllocale>) and try for example:
2306 use POSIX qw(strftime);
2307 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2309 Note that the C<%a> and C<%b>, the short forms of the day of the week
2310 and the month of the year, may not necessarily be three characters wide.
2316 This function places an advisory lock on a variable, subroutine,
2317 or referenced object contained in I<THING> until the lock goes out
2318 of scope. This is a built-in function only if your version of Perl
2319 was built with threading enabled, and if you've said C<use Threads>.
2320 Otherwise a user-defined function by this name will be called. See
2327 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2328 returns log of C<$_>. To get the log of another base, use basic algebra:
2329 The base-N log of a number is equal to the natural log of that number
2330 divided by the natural log of N. For example:
2334 return log($n)/log(10);
2337 See also L</exp> for the inverse operation.
2339 =item lstat FILEHANDLE
2345 Does the same thing as the C<stat> function (including setting the
2346 special C<_> filehandle) but stats a symbolic link instead of the file
2347 the symbolic link points to. If symbolic links are unimplemented on
2348 your system, a normal C<stat> is done.
2350 If EXPR is omitted, stats C<$_>.
2354 The match operator. See L<perlop>.
2356 =item map BLOCK LIST
2360 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2361 C<$_> to each element) and returns the list value composed of the
2362 results of each such evaluation. In scalar context, returns the
2363 total number of elements so generated. Evaluates BLOCK or EXPR in
2364 list context, so each element of LIST may produce zero, one, or
2365 more elements in the returned value.
2367 @chars = map(chr, @nums);
2369 translates a list of numbers to the corresponding characters. And
2371 %hash = map { getkey($_) => $_ } @array;
2373 is just a funny way to write
2376 foreach $_ (@array) {
2377 $hash{getkey($_)} = $_;
2380 Note that, because C<$_> is a reference into the list value, it can
2381 be used to modify the elements of the array. While this is useful and
2382 supported, it can cause bizarre results if the LIST is not a named array.
2383 Using a regular C<foreach> loop for this purpose would be clearer in
2384 most cases. See also L</grep> for an array composed of those items of
2385 the original list for which the BLOCK or EXPR evaluates to true.
2387 =item mkdir FILENAME,MASK
2389 Creates the directory specified by FILENAME, with permissions
2390 specified by MASK (as modified by C<umask>). If it succeeds it
2391 returns true, otherwise it returns false and sets C<$!> (errno).
2393 In general, it is better to create directories with permissive MASK,
2394 and let the user modify that with their C<umask>, than it is to supply
2395 a restrictive MASK and give the user no way to be more permissive.
2396 The exceptions to this rule are when the file or directory should be
2397 kept private (mail files, for instance). The perlfunc(1) entry on
2398 C<umask> discusses the choice of MASK in more detail.
2400 =item msgctl ID,CMD,ARG
2402 Calls the System V IPC function msgctl(2). You'll probably have to say
2406 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2407 then ARG must be a variable which will hold the returned C<msqid_ds>
2408 structure. Returns like C<ioctl>: the undefined value for error,
2409 C<"0 but true"> for zero, or the actual return value otherwise. See also
2410 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2412 =item msgget KEY,FLAGS
2414 Calls the System V IPC function msgget(2). Returns the message queue
2415 id, or the undefined value if there is an error. See also C<IPC::SysV>
2416 and C<IPC::Msg> documentation.
2418 =item msgsnd ID,MSG,FLAGS
2420 Calls the System V IPC function msgsnd to send the message MSG to the
2421 message queue ID. MSG must begin with the long integer message type,
2422 which may be created with C<pack("l", $type)>. Returns true if
2423 successful, or false if there is an error. See also C<IPC::SysV>
2424 and C<IPC::SysV::Msg> documentation.
2426 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2428 Calls the System V IPC function msgrcv to receive a message from
2429 message queue ID into variable VAR with a maximum message size of
2430 SIZE. Note that if a message is received, the message type will be
2431 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2432 size of the message type. Returns true if successful, or false if
2433 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2437 =item my EXPR : ATTRIBUTES
2439 A C<my> declares the listed variables to be local (lexically) to the
2440 enclosing block, file, or C<eval>. If
2441 more than one value is listed, the list must be placed in parentheses. See
2442 L<perlsub/"Private Variables via my()"> for details.
2448 The C<next> command is like the C<continue> statement in C; it starts
2449 the next iteration of the loop:
2451 LINE: while (<STDIN>) {
2452 next LINE if /^#/; # discard comments
2456 Note that if there were a C<continue> block on the above, it would get
2457 executed even on discarded lines. If the LABEL is omitted, the command
2458 refers to the innermost enclosing loop.
2460 C<next> cannot be used to exit a block which returns a value such as
2461 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2462 a grep() or map() operation.
2464 Note that a block by itself is semantically identical to a loop
2465 that executes once. Thus C<next> will exit such a block early.
2467 See also L</continue> for an illustration of how C<last>, C<next>, and
2470 =item no Module LIST
2472 See the L</use> function, which C<no> is the opposite of.
2478 Interprets EXPR as an octal string and returns the corresponding
2479 value. (If EXPR happens to start off with C<0x>, interprets it as a
2480 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2481 binary string.) The following will handle decimal, binary, octal, and
2482 hex in the standard Perl or C notation:
2484 $val = oct($val) if $val =~ /^0/;
2486 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2487 in octal), use sprintf() or printf():
2489 $perms = (stat("filename"))[2] & 07777;
2490 $oct_perms = sprintf "%lo", $perms;
2492 The oct() function is commonly used when a string such as C<644> needs
2493 to be converted into a file mode, for example. (Although perl will
2494 automatically convert strings into numbers as needed, this automatic
2495 conversion assumes base 10.)
2497 =item open FILEHANDLE,MODE,EXPR
2499 =item open FILEHANDLE,EXPR
2501 =item open FILEHANDLE
2503 Opens the file whose filename is given by EXPR, and associates it with
2504 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2505 name of the real filehandle wanted. If EXPR is omitted, the scalar
2506 variable of the same name as the FILEHANDLE contains the filename.
2507 (Note that lexical variables--those declared with C<my>--will not work
2508 for this purpose; so if you're using C<my>, specify EXPR in your call
2509 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2512 If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2513 If MODE is C<'E<gt>'>, the file is truncated and opened for
2514 output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
2515 the file is opened for appending, again being created if necessary.
2516 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2517 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2518 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2519 file first. You can't usually use either read-write mode for updating
2520 textfiles, since they have variable length records. See the B<-i>
2521 switch in L<perlrun> for a better approach. The file is created with
2522 permissions of C<0666> modified by the process' C<umask> value.
2524 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2525 C<'w+'>, C<'a'>, and C<'a+'>.
2527 In the 2-arguments (and 1-argument) form of the call the mode and
2528 filename should be concatenated (in this order), possibly separated by
2529 spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2531 If the filename begins with C<'|'>, the filename is interpreted as a
2532 command to which output is to be piped, and if the filename ends with a
2533 C<'|'>, the filename is interpreted as a command which pipes output to
2534 us. See L<perlipc/"Using open() for IPC">
2535 for more examples of this. (You are not allowed to C<open> to a command
2536 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2537 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2539 If MODE is C<'|-'>, the filename is interpreted as a
2540 command to which output is to be piped, and if MODE is
2541 C<'-|'>, the filename is interpreted as a command which pipes output to
2542 us. In the 2-arguments (and 1-argument) form one should replace dash
2543 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2544 for more examples of this. (You are not allowed to C<open> to a command
2545 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2546 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2548 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2549 and opening C<'E<gt>-'> opens STDOUT.
2552 nonzero upon success, the undefined value otherwise. If the C<open>
2553 involved a pipe, the return value happens to be the pid of the
2556 If you're unfortunate enough to be running Perl on a system that
2557 distinguishes between text files and binary files (modern operating
2558 systems don't care), then you should check out L</binmode> for tips for
2559 dealing with this. The key distinction between systems that need C<binmode>
2560 and those that don't is their text file formats. Systems like Unix, MacOS, and
2561 Plan9, which delimit lines with a single character, and which encode that
2562 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2564 When opening a file, it's usually a bad idea to continue normal execution
2565 if the request failed, so C<open> is frequently used in connection with
2566 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2567 where you want to make a nicely formatted error message (but there are
2568 modules that can help with that problem)) you should always check
2569 the return value from opening a file. The infrequent exception is when
2570 working with an unopened filehandle is actually what you want to do.
2575 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2576 while (<ARTICLE>) {...
2578 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2579 # if the open fails, output is discarded
2581 open(DBASE, '+<', 'dbase.mine') # open for update
2582 or die "Can't open 'dbase.mine' for update: $!";
2584 open(DBASE, '+<dbase.mine') # ditto
2585 or die "Can't open 'dbase.mine' for update: $!";
2587 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2588 or die "Can't start caesar: $!";
2590 open(ARTICLE, "caesar <$article |") # ditto
2591 or die "Can't start caesar: $!";
2593 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2594 or die "Can't start sort: $!";
2596 # process argument list of files along with any includes
2598 foreach $file (@ARGV) {
2599 process($file, 'fh00');
2603 my($filename, $input) = @_;
2604 $input++; # this is a string increment
2605 unless (open($input, $filename)) {
2606 print STDERR "Can't open $filename: $!\n";
2611 while (<$input>) { # note use of indirection
2612 if (/^#include "(.*)"/) {
2613 process($1, $input);
2620 You may also, in the Bourne shell tradition, specify an EXPR beginning
2621 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2622 name of a filehandle (or file descriptor, if numeric) to be
2623 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2624 C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
2625 mode you specify should match the mode of the original filehandle.
2626 (Duping a filehandle does not take into account any existing contents of
2627 stdio buffers.) Duping file handles is not yet supported for 3-argument
2630 Here is a script that saves, redirects, and restores STDOUT and
2634 open(OLDOUT, ">&STDOUT");
2635 open(OLDERR, ">&STDERR");
2637 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2638 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2640 select(STDERR); $| = 1; # make unbuffered
2641 select(STDOUT); $| = 1; # make unbuffered
2643 print STDOUT "stdout 1\n"; # this works for
2644 print STDERR "stderr 1\n"; # subprocesses too
2649 open(STDOUT, ">&OLDOUT");
2650 open(STDERR, ">&OLDERR");
2652 print STDOUT "stdout 2\n";
2653 print STDERR "stderr 2\n";
2655 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2656 equivalent of C's C<fdopen> of that file descriptor; this is more
2657 parsimonious of file descriptors. For example:
2659 open(FILEHANDLE, "<&=$fd")
2661 Note that this feature depends on the fdopen() C library function.
2662 On many UNIX systems, fdopen() is known to fail when file descriptors
2663 exceed a certain value, typically 255. If you need more file
2664 descriptors than that, consider rebuilding Perl to use the C<sfio>
2667 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2668 with 2-arguments (or 1-argument) form of open(), then
2669 there is an implicit fork done, and the return value of open is the pid
2670 of the child within the parent process, and C<0> within the child
2671 process. (Use C<defined($pid)> to determine whether the open was successful.)
2672 The filehandle behaves normally for the parent, but i/o to that
2673 filehandle is piped from/to the STDOUT/STDIN of the child process.
2674 In the child process the filehandle isn't opened--i/o happens from/to
2675 the new STDOUT or STDIN. Typically this is used like the normal
2676 piped open when you want to exercise more control over just how the
2677 pipe command gets executed, such as when you are running setuid, and
2678 don't want to have to scan shell commands for metacharacters.
2679 The following triples are more or less equivalent:
2681 open(FOO, "|tr '[a-z]' '[A-Z]'");
2682 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2683 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2685 open(FOO, "cat -n '$file'|");
2686 open(FOO, '-|', "cat -n '$file'");
2687 open(FOO, '-|') || exec 'cat', '-n', $file;
2689 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2691 NOTE: On any operation that may do a fork, all files opened for output
2692 are flushed before the fork is attempted. On systems that support a
2693 close-on-exec flag on files, the flag will be set for the newly opened
2694 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2696 Closing any piped filehandle causes the parent process to wait for the
2697 child to finish, and returns the status value in C<$?>.
2699 The filename passed to 2-argument (or 1-argument) form of open()
2700 will have leading and trailing
2701 whitespace deleted, and the normal redirection characters
2702 honored. This property, known as "magic open",
2703 can often be used to good effect. A user could specify a filename of
2704 F<"rsh cat file |">, or you could change certain filenames as needed:
2706 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2707 open(FH, $filename) or die "Can't open $filename: $!";
2709 Use 3-argument form to open a file with arbitrary weird characters in it,
2711 open(FOO, '<', $file);
2713 otherwise it's necessary to protect any leading and trailing whitespace:
2715 $file =~ s#^(\s)#./$1#;
2716 open(FOO, "< $file\0");
2718 (this may not work on some bizzare filesystems). One should
2719 conscientiously choose between the the I<magic> and 3-arguments form
2724 will allow the user to specify an argument of the form C<"rsh cat file |">,
2725 but will not work on a filename which happens to have a trailing space, while
2727 open IN, '<', $ARGV[0];
2729 will have exactly the opposite restrictions.
2731 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2732 should use the C<sysopen> function, which involves no such magic (but
2733 may use subtly different filemodes than Perl open(), which is mapped
2734 to C fopen()). This is
2735 another way to protect your filenames from interpretation. For example:
2738 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2739 or die "sysopen $path: $!";
2740 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2741 print HANDLE "stuff $$\n");
2743 print "File contains: ", <HANDLE>;
2745 Using the constructor from the C<IO::Handle> package (or one of its
2746 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2747 filehandles that have the scope of whatever variables hold references to
2748 them, and automatically close whenever and however you leave that scope:
2752 sub read_myfile_munged {
2754 my $handle = new IO::File;
2755 open($handle, "myfile") or die "myfile: $!";
2757 or return (); # Automatically closed here.
2758 mung $first or die "mung failed"; # Or here.
2759 return $first, <$handle> if $ALL; # Or here.
2763 See L</seek> for some details about mixing reading and writing.
2765 =item opendir DIRHANDLE,EXPR
2767 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2768 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2769 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2775 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2776 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2777 See L<utf8> for more about Unicode.
2781 An C<our> declares the listed variables to be valid globals within
2782 the enclosing block, file, or C<eval>. That is, it has the same
2783 scoping rules as a "my" declaration, but does not create a local
2784 variable. If more than one value is listed, the list must be placed
2785 in parentheses. The C<our> declaration has no semantic effect unless
2786 "use strict vars" is in effect, in which case it lets you use the
2787 declared global variable without qualifying it with a package name.
2788 (But only within the lexical scope of the C<our> declaration. In this
2789 it differs from "use vars", which is package scoped.)
2791 =item pack TEMPLATE,LIST
2793 Takes a LIST of values and converts it into a string using the rules
2794 given by the TEMPLATE. The resulting string is the concatenation of
2795 the converted values. Typically, each converted value looks
2796 like its machine-level representation. For example, on 32-bit machines
2797 a converted integer may be represented by a sequence of 4 bytes.
2800 sequence of characters that give the order and type of values, as
2803 a A string with arbitrary binary data, will be null padded.
2804 A An ascii string, will be space padded.
2805 Z A null terminated (asciz) string, will be null padded.
2807 b A bit string (ascending bit order inside each byte, like vec()).
2808 B A bit string (descending bit order inside each byte).
2809 h A hex string (low nybble first).
2810 H A hex string (high nybble first).
2812 c A signed char value.
2813 C An unsigned char value. Only does bytes. See U for Unicode.
2815 s A signed short value.
2816 S An unsigned short value.
2817 (This 'short' is _exactly_ 16 bits, which may differ from
2818 what a local C compiler calls 'short'. If you want
2819 native-length shorts, use the '!' suffix.)
2821 i A signed integer value.
2822 I An unsigned integer value.
2823 (This 'integer' is _at_least_ 32 bits wide. Its exact
2824 size depends on what a local C compiler calls 'int',
2825 and may even be larger than the 'long' described in
2828 l A signed long value.
2829 L An unsigned long value.
2830 (This 'long' is _exactly_ 32 bits, which may differ from
2831 what a local C compiler calls 'long'. If you want
2832 native-length longs, use the '!' suffix.)
2834 n An unsigned short in "network" (big-endian) order.
2835 N An unsigned long in "network" (big-endian) order.
2836 v An unsigned short in "VAX" (little-endian) order.
2837 V An unsigned long in "VAX" (little-endian) order.
2838 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2839 _exactly_ 32 bits, respectively.)
2841 q A signed quad (64-bit) value.
2842 Q An unsigned quad value.
2843 (Quads are available only if your system supports 64-bit
2844 integer values _and_ if Perl has been compiled to support those.
2845 Causes a fatal error otherwise.)
2847 f A single-precision float in the native format.
2848 d A double-precision float in the native format.
2850 p A pointer to a null-terminated string.
2851 P A pointer to a structure (fixed-length string).
2853 u A uuencoded string.
2854 U A Unicode character number. Encodes to UTF-8 internally.
2855 Works even if C<use utf8> is not in effect.
2857 w A BER compressed integer. Its bytes represent an unsigned
2858 integer in base 128, most significant digit first, with as
2859 few digits as possible. Bit eight (the high bit) is set
2860 on each byte except the last.
2864 @ Null fill to absolute position.
2866 The following rules apply:
2872 Each letter may optionally be followed by a number giving a repeat
2873 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2874 C<H>, and C<P> the pack function will gobble up that many values from
2875 the LIST. A C<*> for the repeat count means to use however many items are
2876 left, except for C<@>, C<x>, C<X>, where it is equivalent
2877 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2880 When used with C<Z>, C<*> results in the addition of a trailing null
2881 byte (so the packed result will be one longer than the byte C<length>
2884 The repeat count for C<u> is interpreted as the maximal number of bytes
2885 to encode per line of output, with 0 and 1 replaced by 45.
2889 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
2890 string of length count, padding with nulls or spaces as necessary. When
2891 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2892 after the first null, and C<a> returns data verbatim. When packing,
2893 C<a>, and C<Z> are equivalent.
2895 If the value-to-pack is too long, it is truncated. If too long and an
2896 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2897 by a null byte. Thus C<Z> always packs a trailing null byte under
2902 Likewise, the C<b> and C<B> fields pack a string that many bits long.
2903 Each byte of the input field of pack() generates 1 bit of the result.
2904 Each result bit is based on the least-significant bit of the corresponding
2905 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2906 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2908 Starting from the beginning of the input string of pack(), each 8-tuple
2909 of bytes is converted to 1 byte of output. With format C<b>
2910 the first byte of the 8-tuple determines the least-significant bit of a
2911 byte, and with format C<B> it determines the most-significant bit of
2914 If the length of the input string is not exactly divisible by 8, the
2915 remainder is packed as if the input string were padded by null bytes
2916 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2918 If the input string of pack() is longer than needed, extra bytes are ignored.
2919 A C<*> for the repeat count of pack() means to use all the bytes of
2920 the input field. On unpack()ing the bits are converted to a string
2921 of C<"0">s and C<"1">s.
2925 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
2926 representable as hexadecimal digits, 0-9a-f) long.
2928 Each byte of the input field of pack() generates 4 bits of the result.
2929 For non-alphabetical bytes the result is based on the 4 least-significant
2930 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
2931 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
2932 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
2933 is compatible with the usual hexadecimal digits, so that C<"a"> and
2934 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
2935 C<"g".."z"> and C<"G".."Z"> is not well-defined.
2937 Starting from the beginning of the input string of pack(), each pair
2938 of bytes is converted to 1 byte of output. With format C<h> the
2939 first byte of the pair determines the least-significant nybble of the
2940 output byte, and with format C<H> it determines the most-significant
2943 If the length of the input string is not even, it behaves as if padded
2944 by a null byte at the end. Similarly, during unpack()ing the "extra"
2945 nybbles are ignored.
2947 If the input string of pack() is longer than needed, extra bytes are ignored.
2948 A C<*> for the repeat count of pack() means to use all the bytes of
2949 the input field. On unpack()ing the bits are converted to a string
2950 of hexadecimal digits.
2954 The C<p> type packs a pointer to a null-terminated string. You are
2955 responsible for ensuring the string is not a temporary value (which can
2956 potentially get deallocated before you get around to using the packed result).
2957 The C<P> type packs a pointer to a structure of the size indicated by the
2958 length. A NULL pointer is created if the corresponding value for C<p> or
2959 C<P> is C<undef>, similarly for unpack().
2963 The C</> template character allows packing and unpacking of strings where
2964 the packed structure contains a byte count followed by the string itself.
2965 You write I<length-item>C</>I<string-item>.
2967 The I<length-item> can be any C<pack> template letter,
2968 and describes how the length value is packed.
2969 The ones likely to be of most use are integer-packing ones like
2970 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
2971 and C<N> (for Sun XDR).
2973 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
2974 For C<unpack> the length of the string is obtained from the I<length-item>,
2975 but if you put in the '*' it will be ignored.
2977 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
2978 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
2979 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
2981 The I<length-item> is not returned explicitly from C<unpack>.
2983 Adding a count to the I<length-item> letter is unlikely to do anything
2984 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
2985 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
2986 which Perl does not regard as legal in numeric strings.
2990 The integer types C<s>, C<S>, C<l>, and C<L> may be
2991 immediately followed by a C<!> suffix to signify native shorts or
2992 longs--as you can see from above for example a bare C<l> does mean
2993 exactly 32 bits, the native C<long> (as seen by the local C compiler)
2994 may be larger. This is an issue mainly in 64-bit platforms. You can
2995 see whether using C<!> makes any difference by
2997 print length(pack("s")), " ", length(pack("s!")), "\n";
2998 print length(pack("l")), " ", length(pack("l!")), "\n";
3000 C<i!> and C<I!> also work but only because of completeness;
3001 they are identical to C<i> and C<I>.
3003 The actual sizes (in bytes) of native shorts, ints, longs, and long
3004 longs on the platform where Perl was built are also available via
3008 print $Config{shortsize}, "\n";
3009 print $Config{intsize}, "\n";
3010 print $Config{longsize}, "\n";
3011 print $Config{longlongsize}, "\n";
3013 (The C<$Config{longlongsize}> will be undefine if your system does
3014 not support long longs.)
3018 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3019 are inherently non-portable between processors and operating systems
3020 because they obey the native byteorder and endianness. For example a
3021 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3022 (arranged in and handled by the CPU registers) into bytes as
3024 0x12 0x34 0x56 0x78 # little-endian
3025 0x78 0x56 0x34 0x12 # big-endian
3027 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3028 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3029 Power, and Cray are big-endian. MIPS can be either: Digital used it
3030 in little-endian mode; SGI uses it in big-endian mode.
3032 The names `big-endian' and `little-endian' are comic references to
3033 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3034 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3035 the egg-eating habits of the Lilliputians.
3037 Some systems may have even weirder byte orders such as
3042 You can see your system's preference with
3044 print join(" ", map { sprintf "%#02x", $_ }
3045 unpack("C*",pack("L",0x12345678))), "\n";
3047 The byteorder on the platform where Perl was built is also available
3051 print $Config{byteorder}, "\n";
3053 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3054 and C<'87654321'> are big-endian.
3056 If you want portable packed integers use the formats C<n>, C<N>,
3057 C<v>, and C<V>, their byte endianness and size is known.
3058 See also L<perlport>.
3062 Real numbers (floats and doubles) are in the native machine format only;
3063 due to the multiplicity of floating formats around, and the lack of a
3064 standard "network" representation, no facility for interchange has been
3065 made. This means that packed floating point data written on one machine
3066 may not be readable on another - even if both use IEEE floating point
3067 arithmetic (as the endian-ness of the memory representation is not part
3068 of the IEEE spec). See also L<perlport>.
3070 Note that Perl uses doubles internally for all numeric calculation, and
3071 converting from double into float and thence back to double again will
3072 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3077 You must yourself do any alignment or padding by inserting for example
3078 enough C<'x'>es while packing. There is no way to pack() and unpack()
3079 could know where the bytes are going to or coming from. Therefore
3080 C<pack> (and C<unpack>) handle their output and input as flat
3085 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3089 If TEMPLATE requires more arguments to pack() than actually given, pack()
3090 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3091 to pack() than actually given, extra arguments are ignored.
3097 $foo = pack("CCCC",65,66,67,68);
3099 $foo = pack("C4",65,66,67,68);
3101 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3102 # same thing with Unicode circled letters
3104 $foo = pack("ccxxcc",65,66,67,68);
3107 # note: the above examples featuring "C" and "c" are true
3108 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3109 # and UTF-8. In EBCDIC the first example would be
3110 # $foo = pack("CCCC",193,194,195,196);
3112 $foo = pack("s2",1,2);
3113 # "\1\0\2\0" on little-endian
3114 # "\0\1\0\2" on big-endian
3116 $foo = pack("a4","abcd","x","y","z");
3119 $foo = pack("aaaa","abcd","x","y","z");
3122 $foo = pack("a14","abcdefg");
3123 # "abcdefg\0\0\0\0\0\0\0"
3125 $foo = pack("i9pl", gmtime);
3126 # a real struct tm (on my system anyway)
3128 $utmp_template = "Z8 Z8 Z16 L";
3129 $utmp = pack($utmp_template, @utmp1);
3130 # a struct utmp (BSDish)
3132 @utmp2 = unpack($utmp_template, $utmp);
3133 # "@utmp1" eq "@utmp2"
3136 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3139 $foo = pack('sx2l', 12, 34);
3140 # short 12, two zero bytes padding, long 34
3141 $bar = pack('s@4l', 12, 34);
3142 # short 12, zero fill to position 4, long 34
3145 The same template may generally also be used in unpack().
3149 =item package NAMESPACE
3151 Declares the compilation unit as being in the given namespace. The scope
3152 of the package declaration is from the declaration itself through the end
3153 of the enclosing block, file, or eval (the same as the C<my> operator).
3154 All further unqualified dynamic identifiers will be in this namespace.
3155 A package statement affects only dynamic variables--including those
3156 you've used C<local> on--but I<not> lexical variables, which are created
3157 with C<my>. Typically it would be the first declaration in a file to
3158 be included by the C<require> or C<use> operator. You can switch into a
3159 package in more than one place; it merely influences which symbol table
3160 is used by the compiler for the rest of that block. You can refer to
3161 variables and filehandles in other packages by prefixing the identifier
3162 with the package name and a double colon: C<$Package::Variable>.
3163 If the package name is null, the C<main> package as assumed. That is,
3164 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3165 still seen in older code).
3167 If NAMESPACE is omitted, then there is no current package, and all
3168 identifiers must be fully qualified or lexicals. This is stricter
3169 than C<use strict>, since it also extends to function names.
3171 See L<perlmod/"Packages"> for more information about packages, modules,
3172 and classes. See L<perlsub> for other scoping issues.
3174 =item pipe READHANDLE,WRITEHANDLE
3176 Opens a pair of connected pipes like the corresponding system call.
3177 Note that if you set up a loop of piped processes, deadlock can occur
3178 unless you are very careful. In addition, note that Perl's pipes use
3179 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3180 after each command, depending on the application.
3182 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3183 for examples of such things.
3185 On systems that support a close-on-exec flag on files, the flag will be set
3186 for the newly opened file descriptors as determined by the value of $^F.
3193 Pops and returns the last value of the array, shortening the array by
3194 one element. Has an effect similar to
3198 If there are no elements in the array, returns the undefined value
3199 (although this may happen at other times as well). If ARRAY is
3200 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3201 array in subroutines, just like C<shift>.
3207 Returns the offset of where the last C<m//g> search left off for the variable
3208 is in question (C<$_> is used when the variable is not specified). May be
3209 modified to change that offset. Such modification will also influence
3210 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3213 =item print FILEHANDLE LIST
3219 Prints a string or a list of strings. Returns true if successful.
3220 FILEHANDLE may be a scalar variable name, in which case the variable
3221 contains the name of or a reference to the filehandle, thus introducing
3222 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3223 the next token is a term, it may be misinterpreted as an operator
3224 unless you interpose a C<+> or put parentheses around the arguments.)
3225 If FILEHANDLE is omitted, prints by default to standard output (or
3226 to the last selected output channel--see L</select>). If LIST is
3227 also omitted, prints C<$_> to the currently selected output channel.
3228 To set the default output channel to something other than STDOUT
3229 use the select operation. The current value of C<$,> (if any) is
3230 printed between each LIST item. The current value of C<$\> (if
3231 any) is printed after the entire LIST has been printed. Because
3232 print takes a LIST, anything in the LIST is evaluated in list
3233 context, and any subroutine that you call will have one or more of
3234 its expressions evaluated in list context. Also be careful not to
3235 follow the print keyword with a left parenthesis unless you want
3236 the corresponding right parenthesis to terminate the arguments to
3237 the print--interpose a C<+> or put parentheses around all the
3240 Note that if you're storing FILEHANDLES in an array or other expression,
3241 you will have to use a block returning its value instead:
3243 print { $files[$i] } "stuff\n";
3244 print { $OK ? STDOUT : STDERR } "stuff\n";
3246 =item printf FILEHANDLE FORMAT, LIST
3248 =item printf FORMAT, LIST
3250 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3251 (the output record separator) is not appended. The first argument
3252 of the list will be interpreted as the C<printf> format. If C<use locale> is
3253 in effect, the character used for the decimal point in formatted real numbers
3254 is affected by the LC_NUMERIC locale. See L<perllocale>.
3256 Don't fall into the trap of using a C<printf> when a simple
3257 C<print> would do. The C<print> is more efficient and less
3260 =item prototype FUNCTION
3262 Returns the prototype of a function as a string (or C<undef> if the
3263 function has no prototype). FUNCTION is a reference to, or the name of,
3264 the function whose prototype you want to retrieve.
3266 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3267 name for Perl builtin. If the builtin is not I<overridable> (such as
3268 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3269 C<system>) returns C<undef> because the builtin does not really behave
3270 like a Perl function. Otherwise, the string describing the equivalent
3271 prototype is returned.
3273 =item push ARRAY,LIST
3275 Treats ARRAY as a stack, and pushes the values of LIST
3276 onto the end of ARRAY. The length of ARRAY increases by the length of
3277 LIST. Has the same effect as
3280 $ARRAY[++$#ARRAY] = $value;
3283 but is more efficient. Returns the new number of elements in the array.
3295 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3297 =item quotemeta EXPR
3301 Returns the value of EXPR with all non-alphanumeric
3302 characters backslashed. (That is, all characters not matching
3303 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3304 returned string, regardless of any locale settings.)
3305 This is the internal function implementing
3306 the C<\Q> escape in double-quoted strings.
3308 If EXPR is omitted, uses C<$_>.
3314 Returns a random fractional number greater than or equal to C<0> and less
3315 than the value of EXPR. (EXPR should be positive.) If EXPR is
3316 omitted, the value C<1> is used. Automatically calls C<srand> unless
3317 C<srand> has already been called. See also C<srand>.
3319 (Note: If your rand function consistently returns numbers that are too
3320 large or too small, then your version of Perl was probably compiled
3321 with the wrong number of RANDBITS.)
3323 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3325 =item read FILEHANDLE,SCALAR,LENGTH
3327 Attempts to read LENGTH bytes of data into variable SCALAR from the
3328 specified FILEHANDLE. Returns the number of bytes actually read,
3329 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3330 or shrunk to the length actually read. An OFFSET may be specified to
3331 place the read data at some other place than the beginning of the
3332 string. This call is actually implemented in terms of stdio's fread(3)
3333 call. To get a true read(2) system call, see C<sysread>.
3335 =item readdir DIRHANDLE
3337 Returns the next directory entry for a directory opened by C<opendir>.
3338 If used in list context, returns all the rest of the entries in the
3339 directory. If there are no more entries, returns an undefined value in
3340 scalar context or a null list in list context.
3342 If you're planning to filetest the return values out of a C<readdir>, you'd
3343 better prepend the directory in question. Otherwise, because we didn't
3344 C<chdir> there, it would have been testing the wrong file.
3346 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3347 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3352 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3353 context, each call reads and returns the next line, until end-of-file is
3354 reached, whereupon the subsequent call returns undef. In list context,
3355 reads until end-of-file is reached and returns a list of lines. Note that
3356 the notion of "line" used here is however you may have defined it
3357 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3359 When C<$/> is set to C<undef>, when readline() is in scalar
3360 context (i.e. file slurp mode), and when an empty file is read, it
3361 returns C<''> the first time, followed by C<undef> subsequently.
3363 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3364 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3365 operator is discussed in more detail in L<perlop/"I/O Operators">.
3368 $line = readline(*STDIN); # same thing
3374 Returns the value of a symbolic link, if symbolic links are
3375 implemented. If not, gives a fatal error. If there is some system
3376 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3377 omitted, uses C<$_>.
3381 EXPR is executed as a system command.
3382 The collected standard output of the command is returned.
3383 In scalar context, it comes back as a single (potentially
3384 multi-line) string. In list context, returns a list of lines
3385 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3386 This is the internal function implementing the C<qx/EXPR/>
3387 operator, but you can use it directly. The C<qx/EXPR/>
3388 operator is discussed in more detail in L<perlop/"I/O Operators">.
3390 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3392 Receives a message on a socket. Attempts to receive LENGTH bytes of
3393 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3394 will be grown or shrunk to the length actually read. Takes the same
3395 flags as the system call of the same name. Returns the address of the
3396 sender if SOCKET's protocol supports this; returns an empty string
3397 otherwise. If there's an error, returns the undefined value. This call
3398 is actually implemented in terms of recvfrom(2) system call. See
3399 L<perlipc/"UDP: Message Passing"> for examples.
3405 The C<redo> command restarts the loop block without evaluating the
3406 conditional again. The C<continue> block, if any, is not executed. If
3407 the LABEL is omitted, the command refers to the innermost enclosing
3408 loop. This command is normally used by programs that want to lie to
3409 themselves about what was just input:
3411 # a simpleminded Pascal comment stripper
3412 # (warning: assumes no { or } in strings)
3413 LINE: while (<STDIN>) {
3414 while (s|({.*}.*){.*}|$1 |) {}
3419 if (/}/) { # end of comment?
3428 C<redo> cannot be used to retry a block which returns a value such as
3429 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3430 a grep() or map() operation.
3432 Note that a block by itself is semantically identical to a loop
3433 that executes once. Thus C<redo> inside such a block will effectively
3434 turn it into a looping construct.
3436 See also L</continue> for an illustration of how C<last>, C<next>, and
3443 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3444 is not specified, C<$_> will be used. The value returned depends on the
3445 type of thing the reference is a reference to.
3446 Builtin types include:
3456 If the referenced object has been blessed into a package, then that package
3457 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3459 if (ref($r) eq "HASH") {
3460 print "r is a reference to a hash.\n";
3463 print "r is not a reference at all.\n";
3465 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3466 print "r is a reference to something that isa hash.\n";
3469 See also L<perlref>.
3471 =item rename OLDNAME,NEWNAME
3473 Changes the name of a file; an existing file NEWNAME will be
3474 clobbered. Returns true for success, false otherwise.
3476 Behavior of this function varies wildly depending on your system
3477 implementation. For example, it will usually not work across file system
3478 boundaries, even though the system I<mv> command sometimes compensates
3479 for this. Other restrictions include whether it works on directories,
3480 open files, or pre-existing files. Check L<perlport> and either the
3481 rename(2) manpage or equivalent system documentation for details.
3487 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3488 supplied. If EXPR is numeric, demands that the current version of Perl
3489 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3491 Otherwise, demands that a library file be included if it hasn't already
3492 been included. The file is included via the do-FILE mechanism, which is
3493 essentially just a variety of C<eval>. Has semantics similar to the following
3498 return 1 if $INC{$filename};
3499 my($realfilename,$result);
3501 foreach $prefix (@INC) {
3502 $realfilename = "$prefix/$filename";
3503 if (-f $realfilename) {
3504 $INC{$filename} = $realfilename;
3505 $result = do $realfilename;
3509 die "Can't find $filename in \@INC";
3511 delete $INC{$filename} if $@ || !$result;
3513 die "$filename did not return true value" unless $result;
3517 Note that the file will not be included twice under the same specified
3518 name. The file must return true as the last statement to indicate
3519 successful execution of any initialization code, so it's customary to
3520 end such a file with C<1;> unless you're sure it'll return true
3521 otherwise. But it's better just to put the C<1;>, in case you add more
3524 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3525 replaces "F<::>" with "F</>" in the filename for you,
3526 to make it easy to load standard modules. This form of loading of
3527 modules does not risk altering your namespace.
3529 In other words, if you try this:
3531 require Foo::Bar; # a splendid bareword
3533 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3534 directories specified in the C<@INC> array.
3536 But if you try this:
3538 $class = 'Foo::Bar';
3539 require $class; # $class is not a bareword
3541 require "Foo::Bar"; # not a bareword because of the ""
3543 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3544 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3546 eval "require $class";
3548 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3554 Generally used in a C<continue> block at the end of a loop to clear
3555 variables and reset C<??> searches so that they work again. The
3556 expression is interpreted as a list of single characters (hyphens
3557 allowed for ranges). All variables and arrays beginning with one of
3558 those letters are reset to their pristine state. If the expression is
3559 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3560 only variables or searches in the current package. Always returns
3563 reset 'X'; # reset all X variables
3564 reset 'a-z'; # reset lower case variables
3565 reset; # just reset ?one-time? searches
3567 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3568 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3569 variables--lexical variables are unaffected, but they clean themselves
3570 up on scope exit anyway, so you'll probably want to use them instead.
3577 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3578 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3579 context, depending on how the return value will be used, and the context
3580 may vary from one execution to the next (see C<wantarray>). If no EXPR
3581 is given, returns an empty list in list context, the undefined value in
3582 scalar context, and (of course) nothing at all in a void context.
3584 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3585 or do FILE will automatically return the value of the last expression
3590 In list context, returns a list value consisting of the elements
3591 of LIST in the opposite order. In scalar context, concatenates the
3592 elements of LIST and returns a string value with all characters
3593 in the opposite order.
3595 print reverse <>; # line tac, last line first
3597 undef $/; # for efficiency of <>
3598 print scalar reverse <>; # character tac, last line tsrif
3600 This operator is also handy for inverting a hash, although there are some
3601 caveats. If a value is duplicated in the original hash, only one of those
3602 can be represented as a key in the inverted hash. Also, this has to
3603 unwind one hash and build a whole new one, which may take some time
3604 on a large hash, such as from a DBM file.
3606 %by_name = reverse %by_address; # Invert the hash
3608 =item rewinddir DIRHANDLE
3610 Sets the current position to the beginning of the directory for the
3611 C<readdir> routine on DIRHANDLE.
3613 =item rindex STR,SUBSTR,POSITION
3615 =item rindex STR,SUBSTR
3617 Works just like index() except that it returns the position of the LAST
3618 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3619 last occurrence at or before that position.
3621 =item rmdir FILENAME
3625 Deletes the directory specified by FILENAME if that directory is empty. If it
3626 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3627 FILENAME is omitted, uses C<$_>.
3631 The substitution operator. See L<perlop>.
3635 Forces EXPR to be interpreted in scalar context and returns the value
3638 @counts = ( scalar @a, scalar @b, scalar @c );
3640 There is no equivalent operator to force an expression to
3641 be interpolated in list context because in practice, this is never
3642 needed. If you really wanted to do so, however, you could use
3643 the construction C<@{[ (some expression) ]}>, but usually a simple
3644 C<(some expression)> suffices.
3646 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3647 parenthesized list, this behaves as a scalar comma expression, evaluating
3648 all but the last element in void context and returning the final element
3649 evaluated in scalar context. This is seldom what you want.
3651 The following single statement:
3653 print uc(scalar(&foo,$bar)),$baz;
3655 is the moral equivalent of these two:
3658 print(uc($bar),$baz);
3660 See L<perlop> for more details on unary operators and the comma operator.
3662 =item seek FILEHANDLE,POSITION,WHENCE
3664 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3665 FILEHANDLE may be an expression whose value gives the name of the
3666 filehandle. The values for WHENCE are C<0> to set the new position to
3667 POSITION, C<1> to set it to the current position plus POSITION, and
3668 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3669 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3670 (start of the file, current position, end of the file) from any of the
3671 modules Fcntl, C<IO::Seekable>, or POSIX. Returns C<1> upon success,
3674 If you want to position file for C<sysread> or C<syswrite>, don't use
3675 C<seek>--buffering makes its effect on the file's system position
3676 unpredictable and non-portable. Use C<sysseek> instead.
3678 Due to the rules and rigors of ANSI C, on some systems you have to do a
3679 seek whenever you switch between reading and writing. Amongst other
3680 things, this may have the effect of calling stdio's clearerr(3).
3681 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3685 This is also useful for applications emulating C<tail -f>. Once you hit
3686 EOF on your read, and then sleep for a while, you might have to stick in a
3687 seek() to reset things. The C<seek> doesn't change the current position,
3688 but it I<does> clear the end-of-file condition on the handle, so that the
3689 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3691 If that doesn't work (some stdios are particularly cantankerous), then
3692 you may need something more like this:
3695 for ($curpos = tell(FILE); $_ = <FILE>;
3696 $curpos = tell(FILE)) {
3697 # search for some stuff and put it into files
3699 sleep($for_a_while);
3700 seek(FILE, $curpos, 0);
3703 =item seekdir DIRHANDLE,POS
3705 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3706 must be a value returned by C<telldir>. Has the same caveats about
3707 possible directory compaction as the corresponding system library
3710 =item select FILEHANDLE
3714 Returns the currently selected filehandle. Sets the current default
3715 filehandle for output, if FILEHANDLE is supplied. This has two
3716 effects: first, a C<write> or a C<print> without a filehandle will
3717 default to this FILEHANDLE. Second, references to variables related to
3718 output will refer to this output channel. For example, if you have to
3719 set the top of form format for more than one output channel, you might
3727 FILEHANDLE may be an expression whose value gives the name of the
3728 actual filehandle. Thus:
3730 $oldfh = select(STDERR); $| = 1; select($oldfh);
3732 Some programmers may prefer to think of filehandles as objects with
3733 methods, preferring to write the last example as:
3736 STDERR->autoflush(1);
3738 =item select RBITS,WBITS,EBITS,TIMEOUT
3740 This calls the select(2) system call with the bit masks specified, which
3741 can be constructed using C<fileno> and C<vec>, along these lines:
3743 $rin = $win = $ein = '';
3744 vec($rin,fileno(STDIN),1) = 1;
3745 vec($win,fileno(STDOUT),1) = 1;
3748 If you want to select on many filehandles you might wish to write a
3752 my(@fhlist) = split(' ',$_[0]);
3755 vec($bits,fileno($_),1) = 1;
3759 $rin = fhbits('STDIN TTY SOCK');
3763 ($nfound,$timeleft) =
3764 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3766 or to block until something becomes ready just do this
3768 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3770 Most systems do not bother to return anything useful in $timeleft, so
3771 calling select() in scalar context just returns $nfound.
3773 Any of the bit masks can also be undef. The timeout, if specified, is
3774 in seconds, which may be fractional. Note: not all implementations are
3775 capable of returning the$timeleft. If not, they always return
3776 $timeleft equal to the supplied $timeout.
3778 You can effect a sleep of 250 milliseconds this way:
3780 select(undef, undef, undef, 0.25);
3782 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3783 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3784 then only on POSIX systems. You have to use C<sysread> instead.
3786 =item semctl ID,SEMNUM,CMD,ARG
3788 Calls the System V IPC function C<semctl>. You'll probably have to say
3792 first to get the correct constant definitions. If CMD is IPC_STAT or
3793 GETALL, then ARG must be a variable which will hold the returned
3794 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3795 undefined value for error, "C<0 but true>" for zero, or the actual return
3796 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3798 =item semget KEY,NSEMS,FLAGS
3800 Calls the System V IPC function semget. Returns the semaphore id, or
3801 the undefined value if there is an error. See also C<IPC::SysV> and
3802 C<IPC::SysV::Semaphore> documentation.
3804 =item semop KEY,OPSTRING
3806 Calls the System V IPC function semop to perform semaphore operations
3807 such as signaling and waiting. OPSTRING must be a packed array of
3808 semop structures. Each semop structure can be generated with
3809 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3810 operations is implied by the length of OPSTRING. Returns true if
3811 successful, or false if there is an error. As an example, the
3812 following code waits on semaphore $semnum of semaphore id $semid:
3814 $semop = pack("sss", $semnum, -1, 0);
3815 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3817 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3818 and C<IPC::SysV::Semaphore> documentation.
3820 =item send SOCKET,MSG,FLAGS,TO
3822 =item send SOCKET,MSG,FLAGS
3824 Sends a message on a socket. Takes the same flags as the system call
3825 of the same name. On unconnected sockets you must specify a
3826 destination to send TO, in which case it does a C C<sendto>. Returns
3827 the number of characters sent, or the undefined value if there is an
3828 error. The C system call sendmsg(2) is currently unimplemented.
3829 See L<perlipc/"UDP: Message Passing"> for examples.
3831 =item setpgrp PID,PGRP
3833 Sets the current process group for the specified PID, C<0> for the current
3834 process. Will produce a fatal error if used on a machine that doesn't
3835 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3836 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3837 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3840 =item setpriority WHICH,WHO,PRIORITY
3842 Sets the current priority for a process, a process group, or a user.
3843 (See setpriority(2).) Will produce a fatal error if used on a machine
3844 that doesn't implement setpriority(2).
3846 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3848 Sets the socket option requested. Returns undefined if there is an
3849 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3856 Shifts the first value of the array off and returns it, shortening the
3857 array by 1 and moving everything down. If there are no elements in the
3858 array, returns the undefined value. If ARRAY is omitted, shifts the
3859 C<@_> array within the lexical scope of subroutines and formats, and the
3860 C<@ARGV> array at file scopes or within the lexical scopes established by
3861 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<STOP {}>, and C<END {}>
3864 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3865 same thing to the left end of an array that C<pop> and C<push> do to the
3868 =item shmctl ID,CMD,ARG
3870 Calls the System V IPC function shmctl. You'll probably have to say
3874 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3875 then ARG must be a variable which will hold the returned C<shmid_ds>
3876 structure. Returns like ioctl: the undefined value for error, "C<0> but
3877 true" for zero, or the actual return value otherwise.
3878 See also C<IPC::SysV> documentation.
3880 =item shmget KEY,SIZE,FLAGS
3882 Calls the System V IPC function shmget. Returns the shared memory
3883 segment id, or the undefined value if there is an error.
3884 See also C<IPC::SysV> documentation.
3886 =item shmread ID,VAR,POS,SIZE
3888 =item shmwrite ID,STRING,POS,SIZE
3890 Reads or writes the System V shared memory segment ID starting at
3891 position POS for size SIZE by attaching to it, copying in/out, and
3892 detaching from it. When reading, VAR must be a variable that will
3893 hold the data read. When writing, if STRING is too long, only SIZE
3894 bytes are used; if STRING is too short, nulls are written to fill out
3895 SIZE bytes. Return true if successful, or false if there is an error.
3896 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3899 =item shutdown SOCKET,HOW
3901 Shuts down a socket connection in the manner indicated by HOW, which
3902 has the same interpretation as in the system call of the same name.
3904 shutdown(SOCKET, 0); # I/we have stopped reading data
3905 shutdown(SOCKET, 1); # I/we have stopped writing data
3906 shutdown(SOCKET, 2); # I/we have stopped using this socket
3908 This is useful with sockets when you want to tell the other
3909 side you're done writing but not done reading, or vice versa.
3910 It's also a more insistent form of close because it also
3911 disables the file descriptor in any forked copies in other
3918 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3919 returns sine of C<$_>.
3921 For the inverse sine operation, you may use the C<POSIX::asin>
3922 function, or use this relation:
3924 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3930 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3931 May be interrupted if the process receives a signal such as C<SIGALRM>.
3932 Returns the number of seconds actually slept. You probably cannot
3933 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
3936 On some older systems, it may sleep up to a full second less than what
3937 you requested, depending on how it counts seconds. Most modern systems
3938 always sleep the full amount. They may appear to sleep longer than that,
3939 however, because your process might not be scheduled right away in a
3940 busy multitasking system.
3942 For delays of finer granularity than one second, you may use Perl's
3943 C<syscall> interface to access setitimer(2) if your system supports
3944 it, or else see L</select> above. The Time::HiRes module from CPAN
3947 See also the POSIX module's C<sigpause> function.
3949 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3951 Opens a socket of the specified kind and attaches it to filehandle
3952 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
3953 the system call of the same name. You should C<use Socket> first
3954 to get the proper definitions imported. See the examples in
3955 L<perlipc/"Sockets: Client/Server Communication">.
3957 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3959 Creates an unnamed pair of sockets in the specified domain, of the
3960 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3961 for the system call of the same name. If unimplemented, yields a fatal
3962 error. Returns true if successful.
3964 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
3965 to C<pipe(Rdr, Wtr)> is essentially:
3968 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3969 shutdown(Rdr, 1); # no more writing for reader
3970 shutdown(Wtr, 0); # no more reading for writer
3972 See L<perlipc> for an example of socketpair use.
3974 =item sort SUBNAME LIST
3976 =item sort BLOCK LIST
3980 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3981 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
3982 specified, it gives the name of a subroutine that returns an integer
3983 less than, equal to, or greater than C<0>, depending on how the elements
3984 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3985 operators are extremely useful in such routines.) SUBNAME may be a
3986 scalar variable name (unsubscripted), in which case the value provides
3987 the name of (or a reference to) the actual subroutine to use. In place
3988 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3991 If the subroutine's prototype is C<($$)>, the elements to be compared
3992 are passed by reference in C<@_>, as for a normal subroutine. If not,
3993 the normal calling code for subroutines is bypassed in the interests of
3994 efficiency, and the elements to be compared are passed into the subroutine
3995 as the package global variables $a and $b (see example below). Note that
3996 in the latter case, it is usually counter-productive to declare $a and
3999 In either case, the subroutine may not be recursive. The values to be
4000 compared are always passed by reference, so don't modify them.
4002 You also cannot exit out of the sort block or subroutine using any of the
4003 loop control operators described in L<perlsyn> or with C<goto>.
4005 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4006 current collation locale. See L<perllocale>.
4011 @articles = sort @files;
4013 # same thing, but with explicit sort routine
4014 @articles = sort {$a cmp $b} @files;
4016 # now case-insensitively
4017 @articles = sort {uc($a) cmp uc($b)} @files;
4019 # same thing in reversed order
4020 @articles = sort {$b cmp $a} @files;
4022 # sort numerically ascending
4023 @articles = sort {$a <=> $b} @files;
4025 # sort numerically descending
4026 @articles = sort {$b <=> $a} @files;
4028 # this sorts the %age hash by value instead of key
4029 # using an in-line function
4030 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4032 # sort using explicit subroutine name
4034 $age{$a} <=> $age{$b}; # presuming numeric
4036 @sortedclass = sort byage @class;
4038 sub backwards { $b cmp $a }
4039 @harry = qw(dog cat x Cain Abel);
4040 @george = qw(gone chased yz Punished Axed);
4042 # prints AbelCaincatdogx
4043 print sort backwards @harry;
4044 # prints xdogcatCainAbel
4045 print sort @george, 'to', @harry;
4046 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4048 # inefficiently sort by descending numeric compare using
4049 # the first integer after the first = sign, or the
4050 # whole record case-insensitively otherwise
4053 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4058 # same thing, but much more efficiently;
4059 # we'll build auxiliary indices instead
4063 push @nums, /=(\d+)/;
4068 $nums[$b] <=> $nums[$a]
4070 $caps[$a] cmp $caps[$b]
4074 # same thing, but without any temps
4075 @new = map { $_->[0] }
4076 sort { $b->[1] <=> $a->[1]
4079 } map { [$_, /=(\d+)/, uc($_)] } @old;
4081 # using a prototype allows you to use any comparison subroutine
4082 # as a sort subroutine (including other package's subroutines)
4084 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4087 @new = sort other::backwards @old;
4089 If you're using strict, you I<must not> declare $a
4090 and $b as lexicals. They are package globals. That means
4091 if you're in the C<main> package, it's
4093 @articles = sort {$main::b <=> $main::a} @files;
4097 @articles = sort {$::b <=> $::a} @files;
4099 but if you're in the C<FooPack> package, it's
4101 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4103 The comparison function is required to behave. If it returns
4104 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4105 sometimes saying the opposite, for example) the results are not
4108 =item splice ARRAY,OFFSET,LENGTH,LIST
4110 =item splice ARRAY,OFFSET,LENGTH
4112 =item splice ARRAY,OFFSET
4114 Removes the elements designated by OFFSET and LENGTH from an array, and
4115 replaces them with the elements of LIST, if any. In list context,
4116 returns the elements removed from the array. In scalar context,
4117 returns the last element removed, or C<undef> if no elements are
4118 removed. The array grows or shrinks as necessary.
4119 If OFFSET is negative then it starts that far from the end of the array.
4120 If LENGTH is omitted, removes everything from OFFSET onward.
4121 If LENGTH is negative, leave that many elements off the end of the array.
4122 The following equivalences hold (assuming C<$[ == 0>):
4124 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4125 pop(@a) splice(@a,-1)
4126 shift(@a) splice(@a,0,1)
4127 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4128 $a[$x] = $y splice(@a,$x,1,$y)
4130 Example, assuming array lengths are passed before arrays:
4132 sub aeq { # compare two list values
4133 my(@a) = splice(@_,0,shift);
4134 my(@b) = splice(@_,0,shift);
4135 return 0 unless @a == @b; # same len?
4137 return 0 if pop(@a) ne pop(@b);
4141 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4143 =item split /PATTERN/,EXPR,LIMIT
4145 =item split /PATTERN/,EXPR
4147 =item split /PATTERN/
4151 Splits a string into a list of strings and returns that list. By default,
4152 empty leading fields are preserved, and empty trailing ones are deleted.
4154 If not in list context, returns the number of fields found and splits into
4155 the C<@_> array. (In list context, you can force the split into C<@_> by
4156 using C<??> as the pattern delimiters, but it still returns the list
4157 value.) The use of implicit split to C<@_> is deprecated, however, because
4158 it clobbers your subroutine arguments.
4160 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4161 splits on whitespace (after skipping any leading whitespace). Anything
4162 matching PATTERN is taken to be a delimiter separating the fields. (Note
4163 that the delimiter may be longer than one character.)
4165 If LIMIT is specified and positive, splits into no more than that
4166 many fields (though it may split into fewer). If LIMIT is unspecified
4167 or zero, trailing null fields are stripped (which potential users
4168 of C<pop> would do well to remember). If LIMIT is negative, it is
4169 treated as if an arbitrarily large LIMIT had been specified.
4171 A pattern matching the null string (not to be confused with
4172 a null pattern C<//>, which is just one member of the set of patterns
4173 matching a null string) will split the value of EXPR into separate
4174 characters at each point it matches that way. For example:
4176 print join(':', split(/ */, 'hi there'));
4178 produces the output 'h:i:t:h:e:r:e'.
4180 The LIMIT parameter can be used to split a line partially
4182 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4184 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4185 one larger than the number of variables in the list, to avoid
4186 unnecessary work. For the list above LIMIT would have been 4 by
4187 default. In time critical applications it behooves you not to split
4188 into more fields than you really need.
4190 If the PATTERN contains parentheses, additional list elements are
4191 created from each matching substring in the delimiter.
4193 split(/([,-])/, "1-10,20", 3);
4195 produces the list value
4197 (1, '-', 10, ',', 20)
4199 If you had the entire header of a normal Unix email message in $header,
4200 you could split it up into fields and their values this way:
4202 $header =~ s/\n\s+/ /g; # fix continuation lines
4203 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4205 The pattern C</PATTERN/> may be replaced with an expression to specify
4206 patterns that vary at runtime. (To do runtime compilation only once,
4207 use C</$variable/o>.)
4209 As a special case, specifying a PATTERN of space (C<' '>) will split on
4210 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4211 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4212 will give you as many null initial fields as there are leading spaces.
4213 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4214 whitespace produces a null first field. A C<split> with no arguments
4215 really does a C<split(' ', $_)> internally.
4219 open(PASSWD, '/etc/passwd');
4221 ($login, $passwd, $uid, $gid,
4222 $gcos, $home, $shell) = split(/:/);
4226 (Note that $shell above will still have a newline on it. See L</chop>,
4227 L</chomp>, and L</join>.)
4229 =item sprintf FORMAT, LIST
4231 Returns a string formatted by the usual C<printf> conventions of the
4232 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4233 on your system for an explanation of the general principles.
4235 Perl does its own C<sprintf> formatting--it emulates the C
4236 function C<sprintf>, but it doesn't use it (except for floating-point
4237 numbers, and even then only the standard modifiers are allowed). As a
4238 result, any non-standard extensions in your local C<sprintf> are not
4239 available from Perl.
4241 Perl's C<sprintf> permits the following universally-known conversions:
4244 %c a character with the given number
4246 %d a signed integer, in decimal
4247 %u an unsigned integer, in decimal
4248 %o an unsigned integer, in octal
4249 %x an unsigned integer, in hexadecimal
4250 %e a floating-point number, in scientific notation
4251 %f a floating-point number, in fixed decimal notation
4252 %g a floating-point number, in %e or %f notation
4254 In addition, Perl permits the following widely-supported conversions:
4256 %X like %x, but using upper-case letters
4257 %E like %e, but using an upper-case "E"
4258 %G like %g, but with an upper-case "E" (if applicable)
4259 %b an unsigned integer, in binary
4260 %p a pointer (outputs the Perl value's address in hexadecimal)
4261 %n special: *stores* the number of characters output so far
4262 into the next variable in the parameter list
4264 Finally, for backward (and we do mean "backward") compatibility, Perl
4265 permits these unnecessary but widely-supported conversions:
4268 %D a synonym for %ld
4269 %U a synonym for %lu
4270 %O a synonym for %lo
4273 Perl permits the following universally-known flags between the C<%>
4274 and the conversion letter:
4276 space prefix positive number with a space
4277 + prefix positive number with a plus sign
4278 - left-justify within the field
4279 0 use zeros, not spaces, to right-justify
4280 # prefix non-zero octal with "0", non-zero hex with "0x"
4281 number minimum field width
4282 .number "precision": digits after decimal point for
4283 floating-point, max length for string, minimum length
4285 l interpret integer as C type "long" or "unsigned long"
4286 h interpret integer as C type "short" or "unsigned short"
4287 If no flags, interpret integer as C type "int" or "unsigned"
4289 There is also one Perl-specific flag:
4291 V interpret integer as Perl's standard integer type
4293 Where a number would appear in the flags, an asterisk (C<*>) may be
4294 used instead, in which case Perl uses the next item in the parameter
4295 list as the given number (that is, as the field width or precision).
4296 If a field width obtained through C<*> is negative, it has the same
4297 effect as the C<-> flag: left-justification.
4299 If C<use locale> is in effect, the character used for the decimal
4300 point in formatted real numbers is affected by the LC_NUMERIC locale.
4303 If Perl understands "quads" (64-bit integers) (this requires
4304 either that the platform natively supports quads or that Perl
4305 has been specifically compiled to support quads), the characters
4309 print quads, and they may optionally be preceded by
4317 You can find out whether your Perl supports quads via L<Config>:
4320 ($Config{use64bits} eq 'define' || $Config{longsize} == 8) &&
4323 If Perl understands "long doubles" (this requires that the platform
4324 supports long doubles), the flags
4328 may optionally be preceded by
4336 You can find out whether your Perl supports long doubles via L<Config>:
4339 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4345 Return the square root of EXPR. If EXPR is omitted, returns square
4346 root of C<$_>. Only works on non-negative operands, unless you've
4347 loaded the standard Math::Complex module.
4350 print sqrt(-2); # prints 1.4142135623731i
4356 Sets the random number seed for the C<rand> operator. If EXPR is
4357 omitted, uses a semi-random value supplied by the kernel (if it supports
4358 the F</dev/urandom> device) or based on the current time and process
4359 ID, among other things. In versions of Perl prior to 5.004 the default
4360 seed was just the current C<time>. This isn't a particularly good seed,
4361 so many old programs supply their own seed value (often C<time ^ $$> or
4362 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4364 In fact, it's usually not necessary to call C<srand> at all, because if
4365 it is not called explicitly, it is called implicitly at the first use of
4366 the C<rand> operator. However, this was not the case in version of Perl
4367 before 5.004, so if your script will run under older Perl versions, it
4368 should call C<srand>.
4370 Note that you need something much more random than the default seed for
4371 cryptographic purposes. Checksumming the compressed output of one or more
4372 rapidly changing operating system status programs is the usual method. For
4375 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4377 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4380 Do I<not> call C<srand> multiple times in your program unless you know
4381 exactly what you're doing and why you're doing it. The point of the
4382 function is to "seed" the C<rand> function so that C<rand> can produce
4383 a different sequence each time you run your program. Just do it once at the
4384 top of your program, or you I<won't> get random numbers out of C<rand>!
4386 Frequently called programs (like CGI scripts) that simply use
4390 for a seed can fall prey to the mathematical property that
4394 one-third of the time. So don't do that.
4396 =item stat FILEHANDLE
4402 Returns a 13-element list giving the status info for a file, either
4403 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4404 it stats C<$_>. Returns a null list if the stat fails. Typically used
4407 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4408 $atime,$mtime,$ctime,$blksize,$blocks)
4411 Not all fields are supported on all filesystem types. Here are the
4412 meaning of the fields:
4414 0 dev device number of filesystem
4416 2 mode file mode (type and permissions)
4417 3 nlink number of (hard) links to the file
4418 4 uid numeric user ID of file's owner
4419 5 gid numeric group ID of file's owner
4420 6 rdev the device identifier (special files only)
4421 7 size total size of file, in bytes
4422 8 atime last access time in seconds since the epoch
4423 9 mtime last modify time in seconds since the epoch
4424 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4425 11 blksize preferred block size for file system I/O
4426 12 blocks actual number of blocks allocated
4428 (The epoch was at 00:00 January 1, 1970 GMT.)
4430 If stat is passed the special filehandle consisting of an underline, no
4431 stat is done, but the current contents of the stat structure from the
4432 last stat or filetest are returned. Example:
4434 if (-x $file && (($d) = stat(_)) && $d < 0) {
4435 print "$file is executable NFS file\n";
4438 (This works on machines only for which the device number is negative under NFS.)
4440 Because the mode contains both the file type and its permissions, you
4441 should mask off the file type portion and (s)printf using a C<"%o">
4442 if you want to see the real permissions.
4444 $mode = (stat($filename))[2];
4445 printf "Permissions are %04o\n", $mode & 07777;
4447 In scalar context, C<stat> returns a boolean value indicating success
4448 or failure, and, if successful, sets the information associated with
4449 the special filehandle C<_>.
4451 The File::stat module provides a convenient, by-name access mechanism:
4454 $sb = stat($filename);
4455 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4456 $filename, $sb->size, $sb->mode & 07777,
4457 scalar localtime $sb->mtime;
4463 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4464 doing many pattern matches on the string before it is next modified.
4465 This may or may not save time, depending on the nature and number of
4466 patterns you are searching on, and on the distribution of character
4467 frequencies in the string to be searched--you probably want to compare
4468 run times with and without it to see which runs faster. Those loops
4469 which scan for many short constant strings (including the constant
4470 parts of more complex patterns) will benefit most. You may have only
4471 one C<study> active at a time--if you study a different scalar the first
4472 is "unstudied". (The way C<study> works is this: a linked list of every
4473 character in the string to be searched is made, so we know, for
4474 example, where all the C<'k'> characters are. From each search string,
4475 the rarest character is selected, based on some static frequency tables
4476 constructed from some C programs and English text. Only those places
4477 that contain this "rarest" character are examined.)
4479 For example, here is a loop that inserts index producing entries
4480 before any line containing a certain pattern:
4484 print ".IX foo\n" if /\bfoo\b/;
4485 print ".IX bar\n" if /\bbar\b/;
4486 print ".IX blurfl\n" if /\bblurfl\b/;
4491 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4492 will be looked at, because C<f> is rarer than C<o>. In general, this is
4493 a big win except in pathological cases. The only question is whether
4494 it saves you more time than it took to build the linked list in the
4497 Note that if you have to look for strings that you don't know till
4498 runtime, you can build an entire loop as a string and C<eval> that to
4499 avoid recompiling all your patterns all the time. Together with
4500 undefining C<$/> to input entire files as one record, this can be very
4501 fast, often faster than specialized programs like fgrep(1). The following
4502 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4503 out the names of those files that contain a match:
4505 $search = 'while (<>) { study;';
4506 foreach $word (@words) {
4507 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4512 eval $search; # this screams
4513 $/ = "\n"; # put back to normal input delimiter
4514 foreach $file (sort keys(%seen)) {
4522 =item sub NAME BLOCK
4524 This is subroutine definition, not a real function I<per se>. With just a
4525 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4526 Without a NAME, it's an anonymous function declaration, and does actually
4527 return a value: the CODE ref of the closure you just created. See L<perlsub>
4528 and L<perlref> for details.
4530 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4532 =item substr EXPR,OFFSET,LENGTH
4534 =item substr EXPR,OFFSET
4536 Extracts a substring out of EXPR and returns it. First character is at
4537 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4538 If OFFSET is negative (or more precisely, less than C<$[>), starts
4539 that far from the end of the string. If LENGTH is omitted, returns
4540 everything to the end of the string. If LENGTH is negative, leaves that
4541 many characters off the end of the string.
4543 You can use the substr() function as an lvalue, in which case EXPR
4544 must itself be an lvalue. If you assign something shorter than LENGTH,
4545 the string will shrink, and if you assign something longer than LENGTH,
4546 the string will grow to accommodate it. To keep the string the same
4547 length you may need to pad or chop your value using C<sprintf>.
4549 If OFFSET and LENGTH specify a substring that is partly outside the
4550 string, only the part within the string is returned. If the substring
4551 is beyond either end of the string, substr() returns the undefined
4552 value and produces a warning. When used as an lvalue, specifying a
4553 substring that is entirely outside the string is a fatal error.
4554 Here's an example showing the behavior for boundary cases:
4557 substr($name, 4) = 'dy'; # $name is now 'freddy'
4558 my $null = substr $name, 6, 2; # returns '' (no warning)
4559 my $oops = substr $name, 7; # returns undef, with warning
4560 substr($name, 7) = 'gap'; # fatal error
4562 An alternative to using substr() as an lvalue is to specify the
4563 replacement string as the 4th argument. This allows you to replace
4564 parts of the EXPR and return what was there before in one operation,
4565 just as you can with splice().
4567 =item symlink OLDFILE,NEWFILE
4569 Creates a new filename symbolically linked to the old filename.
4570 Returns C<1> for success, C<0> otherwise. On systems that don't support
4571 symbolic links, produces a fatal error at run time. To check for that,
4574 $symlink_exists = eval { symlink("",""); 1 };
4578 Calls the system call specified as the first element of the list,
4579 passing the remaining elements as arguments to the system call. If
4580 unimplemented, produces a fatal error. The arguments are interpreted
4581 as follows: if a given argument is numeric, the argument is passed as
4582 an int. If not, the pointer to the string value is passed. You are
4583 responsible to make sure a string is pre-extended long enough to
4584 receive any result that might be written into a string. You can't use a
4585 string literal (or other read-only string) as an argument to C<syscall>
4586 because Perl has to assume that any string pointer might be written
4588 integer arguments are not literals and have never been interpreted in a
4589 numeric context, you may need to add C<0> to them to force them to look
4590 like numbers. This emulates the C<syswrite> function (or vice versa):
4592 require 'syscall.ph'; # may need to run h2ph
4594 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4596 Note that Perl supports passing of up to only 14 arguments to your system call,
4597 which in practice should usually suffice.
4599 Syscall returns whatever value returned by the system call it calls.
4600 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4601 Note that some system calls can legitimately return C<-1>. The proper
4602 way to handle such calls is to assign C<$!=0;> before the call and
4603 check the value of C<$!> if syscall returns C<-1>.
4605 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4606 number of the read end of the pipe it creates. There is no way
4607 to retrieve the file number of the other end. You can avoid this
4608 problem by using C<pipe> instead.
4610 =item sysopen FILEHANDLE,FILENAME,MODE
4612 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4614 Opens the file whose filename is given by FILENAME, and associates it
4615 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4616 the name of the real filehandle wanted. This function calls the
4617 underlying operating system's C<open> function with the parameters
4618 FILENAME, MODE, PERMS.
4620 The possible values and flag bits of the MODE parameter are
4621 system-dependent; they are available via the standard module C<Fcntl>.
4622 See the documentation of your operating system's C<open> to see which
4623 values and flag bits are available. You may combine several flags
4624 using the C<|>-operator.
4626 Some of the most common values are C<O_RDONLY> for opening the file in
4627 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4628 and C<O_RDWR> for opening the file in read-write mode, and.
4630 For historical reasons, some values work on almost every system
4631 supported by perl: zero means read-only, one means write-only, and two
4632 means read/write. We know that these values do I<not> work under
4633 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4634 use them in new code.
4636 If the file named by FILENAME does not exist and the C<open> call creates
4637 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4638 PERMS specifies the permissions of the newly created file. If you omit
4639 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4640 These permission values need to be in octal, and are modified by your
4641 process's current C<umask>.
4643 In many systems the C<O_EXCL> flag is available for opening files in
4644 exclusive mode. This is B<not> locking: exclusiveness means here that
4645 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4648 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4650 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4651 that takes away the user's option to have a more permissive umask.
4652 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4655 Note that C<sysopen> depends on the fdopen() C library function.
4656 On many UNIX systems, fdopen() is known to fail when file descriptors
4657 exceed a certain value, typically 255. If you need more file
4658 descriptors than that, consider rebuilding Perl to use the C<sfio>
4659 library, or perhaps using the POSIX::open() function.
4661 See L<perlopentut> for a kinder, gentler explanation of opening files.
4663 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4665 =item sysread FILEHANDLE,SCALAR,LENGTH
4667 Attempts to read LENGTH bytes of data into variable SCALAR from the
4668 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4669 so mixing this with other kinds of reads, C<print>, C<write>,
4670 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4671 usually buffers data. Returns the number of bytes actually read, C<0>
4672 at end of file, or undef if there was an error. SCALAR will be grown or
4673 shrunk so that the last byte actually read is the last byte of the
4674 scalar after the read.
4676 An OFFSET may be specified to place the read data at some place in the
4677 string other than the beginning. A negative OFFSET specifies
4678 placement at that many bytes counting backwards from the end of the
4679 string. A positive OFFSET greater than the length of SCALAR results
4680 in the string being padded to the required size with C<"\0"> bytes before
4681 the result of the read is appended.
4683 There is no syseof() function, which is ok, since eof() doesn't work
4684 very well on device files (like ttys) anyway. Use sysread() and check
4685 for a return value for 0 to decide whether you're done.
4687 =item sysseek FILEHANDLE,POSITION,WHENCE
4689 Sets FILEHANDLE's system position using the system call lseek(2). It
4690 bypasses stdio, so mixing this with reads (other than C<sysread>),
4691 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4692 FILEHANDLE may be an expression whose value gives the name of the
4693 filehandle. The values for WHENCE are C<0> to set the new position to
4694 POSITION, C<1> to set the it to the current position plus POSITION,
4695 and C<2> to set it to EOF plus POSITION (typically negative). For
4696 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4697 C<SEEK_END> (start of the file, current position, end of the file)
4698 from any of the modules Fcntl, C<IO::Seekable>, or POSIX.
4700 Returns the new position, or the undefined value on failure. A position
4701 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4702 true on success and false on failure, yet you can still easily determine
4707 =item system PROGRAM LIST
4709 Does exactly the same thing as C<exec LIST>, except that a fork is
4710 done first, and the parent process waits for the child process to
4711 complete. Note that argument processing varies depending on the
4712 number of arguments. If there is more than one argument in LIST,
4713 or if LIST is an array with more than one value, starts the program
4714 given by the first element of the list with arguments given by the
4715 rest of the list. If there is only one scalar argument, the argument
4716 is checked for shell metacharacters, and if there are any, the
4717 entire argument is passed to the system's command shell for parsing
4718 (this is C</bin/sh -c> on Unix platforms, but varies on other
4719 platforms). If there are no shell metacharacters in the argument,
4720 it is split into words and passed directly to C<execvp>, which is
4723 All files opened for output are flushed before attempting the exec().
4725 The return value is the exit status of the program as
4726 returned by the C<wait> call. To get the actual exit value divide by
4727 256. See also L</exec>. This is I<not> what you want to use to capture
4728 the output from a command, for that you should use merely backticks or
4729 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4730 indicates a failure to start the program (inspect $! for the reason).
4732 Like C<exec>, C<system> allows you to lie to a program about its name if
4733 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4735 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4736 program they're running doesn't actually interrupt your program.
4738 @args = ("command", "arg1", "arg2");
4740 or die "system @args failed: $?"
4742 You can check all the failure possibilities by inspecting
4745 $exit_value = $? >> 8;
4746 $signal_num = $? & 127;
4747 $dumped_core = $? & 128;
4749 When the arguments get executed via the system shell, results
4750 and return codes will be subject to its quirks and capabilities.
4751 See L<perlop/"`STRING`"> and L</exec> for details.
4753 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4755 =item syswrite FILEHANDLE,SCALAR,LENGTH
4757 =item syswrite FILEHANDLE,SCALAR
4759 Attempts to write LENGTH bytes of data from variable SCALAR to the
4760 specified FILEHANDLE, using the system call write(2). If LENGTH
4761 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4762 this with reads (other than C<sysread())>, C<print>, C<write>,
4763 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4764 usually buffers data. Returns the number of bytes actually written,
4765 or C<undef> if there was an error. If the LENGTH is greater than
4766 the available data in the SCALAR after the OFFSET, only as much
4767 data as is available will be written.
4769 An OFFSET may be specified to write the data from some part of the
4770 string other than the beginning. A negative OFFSET specifies writing
4771 that many bytes counting backwards from the end of the string. In the
4772 case the SCALAR is empty you can use OFFSET but only zero offset.
4774 =item tell FILEHANDLE
4778 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4779 expression whose value gives the name of the actual filehandle. If
4780 FILEHANDLE is omitted, assumes the file last read.
4782 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4784 =item telldir DIRHANDLE
4786 Returns the current position of the C<readdir> routines on DIRHANDLE.
4787 Value may be given to C<seekdir> to access a particular location in a
4788 directory. Has the same caveats about possible directory compaction as
4789 the corresponding system library routine.
4791 =item tie VARIABLE,CLASSNAME,LIST
4793 This function binds a variable to a package class that will provide the
4794 implementation for the variable. VARIABLE is the name of the variable
4795 to be enchanted. CLASSNAME is the name of a class implementing objects
4796 of correct type. Any additional arguments are passed to the C<new>
4797 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4798 or C<TIEHASH>). Typically these are arguments such as might be passed
4799 to the C<dbm_open()> function of C. The object returned by the C<new>
4800 method is also returned by the C<tie> function, which would be useful
4801 if you want to access other methods in CLASSNAME.
4803 Note that functions such as C<keys> and C<values> may return huge lists
4804 when used on large objects, like DBM files. You may prefer to use the
4805 C<each> function to iterate over such. Example:
4807 # print out history file offsets
4809 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4810 while (($key,$val) = each %HIST) {
4811 print $key, ' = ', unpack('L',$val), "\n";
4815 A class implementing a hash should have the following methods:
4817 TIEHASH classname, LIST
4819 STORE this, key, value
4824 NEXTKEY this, lastkey
4827 A class implementing an ordinary array should have the following methods:
4829 TIEARRAY classname, LIST
4831 STORE this, key, value
4833 STORESIZE this, count
4839 SPLICE this, offset, length, LIST
4843 A class implementing a file handle should have the following methods:
4845 TIEHANDLE classname, LIST
4846 READ this, scalar, length, offset
4849 WRITE this, scalar, length, offset
4851 PRINTF this, format, LIST
4855 A class implementing a scalar should have the following methods:
4857 TIESCALAR classname, LIST
4862 Not all methods indicated above need be implemented. See L<perltie>,
4863 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
4865 Unlike C<dbmopen>, the C<tie> function will not use or require a module
4866 for you--you need to do that explicitly yourself. See L<DB_File>
4867 or the F<Config> module for interesting C<tie> implementations.
4869 For further details see L<perltie>, L<"tied VARIABLE">.
4873 Returns a reference to the object underlying VARIABLE (the same value
4874 that was originally returned by the C<tie> call that bound the variable
4875 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4880 Returns the number of non-leap seconds since whatever time the system
4881 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4882 and 00:00:00 UTC, January 1, 1970 for most other systems).
4883 Suitable for feeding to C<gmtime> and C<localtime>.
4885 For measuring time in better granularity than one second,
4886 you may use either the Time::HiRes module from CPAN, or
4887 if you have gettimeofday(2), you may be able to use the
4888 C<syscall> interface of Perl, see L<perlfaq8> for details.
4892 Returns a four-element list giving the user and system times, in
4893 seconds, for this process and the children of this process.
4895 ($user,$system,$cuser,$csystem) = times;
4899 The transliteration operator. Same as C<y///>. See L<perlop>.
4901 =item truncate FILEHANDLE,LENGTH
4903 =item truncate EXPR,LENGTH
4905 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4906 specified length. Produces a fatal error if truncate isn't implemented
4907 on your system. Returns true if successful, the undefined value
4914 Returns an uppercased version of EXPR. This is the internal function
4915 implementing the C<\U> escape in double-quoted strings.
4916 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4917 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4918 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
4920 If EXPR is omitted, uses C<$_>.
4926 Returns the value of EXPR with the first character
4927 in uppercase (titlecase in Unicode). This is
4928 the internal function implementing the C<\u> escape in double-quoted strings.
4929 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
4932 If EXPR is omitted, uses C<$_>.
4938 Sets the umask for the process to EXPR and returns the previous value.
4939 If EXPR is omitted, merely returns the current umask.
4941 The Unix permission C<rwxr-x---> is represented as three sets of three
4942 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4943 and isn't one of the digits). The C<umask> value is such a number
4944 representing disabled permissions bits. The permission (or "mode")
4945 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4946 even if you tell C<sysopen> to create a file with permissions C<0777>,
4947 if your umask is C<0022> then the file will actually be created with
4948 permissions C<0755>. If your C<umask> were C<0027> (group can't
4949 write; others can't read, write, or execute), then passing
4950 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
4953 Here's some advice: supply a creation mode of C<0666> for regular
4954 files (in C<sysopen>) and one of C<0777> for directories (in
4955 C<mkdir>) and executable files. This gives users the freedom of
4956 choice: if they want protected files, they might choose process umasks
4957 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4958 Programs should rarely if ever make policy decisions better left to
4959 the user. The exception to this is when writing files that should be
4960 kept private: mail files, web browser cookies, I<.rhosts> files, and
4963 If umask(2) is not implemented on your system and you are trying to
4964 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4965 fatal error at run time. If umask(2) is not implemented and you are
4966 not trying to restrict access for yourself, returns C<undef>.
4968 Remember that a umask is a number, usually given in octal; it is I<not> a
4969 string of octal digits. See also L</oct>, if all you have is a string.
4975 Undefines the value of EXPR, which must be an lvalue. Use only on a
4976 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
4977 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
4978 will probably not do what you expect on most predefined variables or
4979 DBM list values, so don't do that; see L<delete>.) Always returns the
4980 undefined value. You can omit the EXPR, in which case nothing is
4981 undefined, but you still get an undefined value that you could, for
4982 instance, return from a subroutine, assign to a variable or pass as a
4983 parameter. Examples:
4986 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4990 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4991 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4992 select undef, undef, undef, 0.25;
4993 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4995 Note that this is a unary operator, not a list operator.
5001 Deletes a list of files. Returns the number of files successfully
5004 $cnt = unlink 'a', 'b', 'c';
5008 Note: C<unlink> will not delete directories unless you are superuser and
5009 the B<-U> flag is supplied to Perl. Even if these conditions are
5010 met, be warned that unlinking a directory can inflict damage on your
5011 filesystem. Use C<rmdir> instead.
5013 If LIST is omitted, uses C<$_>.
5015 =item unpack TEMPLATE,EXPR
5017 C<unpack> does the reverse of C<pack>: it takes a string
5018 and expands it out into a list of values.
5019 (In scalar context, it returns merely the first value produced.)
5021 The string is broken into chunks described by the TEMPLATE. Each chunk
5022 is converted separately to a value. Typically, either the string is a result
5023 of C<pack>, or the bytes of the string represent a C structure of some
5026 The TEMPLATE has the same format as in the C<pack> function.
5027 Here's a subroutine that does substring:
5030 my($what,$where,$howmuch) = @_;
5031 unpack("x$where a$howmuch", $what);
5036 sub ordinal { unpack("c",$_[0]); } # same as ord()
5038 In addition to fields allowed in pack(), you may prefix a field with
5039 a %E<lt>numberE<gt> to indicate that
5040 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
5041 themselves. Default is a 16-bit checksum. Checksum is calculated by
5042 summing numeric values of expanded values (for string fields the sum of
5043 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5045 For example, the following
5046 computes the same number as the System V sum program:
5050 unpack("%32C*",<>) % 65535;
5053 The following efficiently counts the number of set bits in a bit vector:
5055 $setbits = unpack("%32b*", $selectmask);
5057 The C<p> and C<P> formats should be used with care. Since Perl
5058 has no way of checking whether the value passed to C<unpack()>
5059 corresponds to a valid memory location, passing a pointer value that's
5060 not known to be valid is likely to have disastrous consequences.
5062 If the repeat count of a field is larger than what the remainder of
5063 the input string allows, repeat count is decreased. If the input string
5064 is longer than one described by the TEMPLATE, the rest is ignored.
5066 See L</pack> for more examples and notes.
5068 =item untie VARIABLE
5070 Breaks the binding between a variable and a package. (See C<tie>.)
5072 =item unshift ARRAY,LIST
5074 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5075 depending on how you look at it. Prepends list to the front of the
5076 array, and returns the new number of elements in the array.
5078 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5080 Note the LIST is prepended whole, not one element at a time, so the
5081 prepended elements stay in the same order. Use C<reverse> to do the
5084 =item use Module LIST
5088 =item use Module VERSION LIST
5092 Imports some semantics into the current package from the named module,
5093 generally by aliasing certain subroutine or variable names into your
5094 package. It is exactly equivalent to
5096 BEGIN { require Module; import Module LIST; }
5098 except that Module I<must> be a bareword.
5100 If the first argument to C<use> is a number, it is treated as a version
5101 number instead of a module name. If the version of the Perl interpreter
5102 is less than VERSION, then an error message is printed and Perl exits
5103 immediately. This is often useful if you need to check the current
5104 Perl version before C<use>ing library modules that have changed in
5105 incompatible ways from older versions of Perl. (We try not to do
5106 this more than we have to.)
5108 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5109 C<require> makes sure the module is loaded into memory if it hasn't been
5110 yet. The C<import> is not a builtin--it's just an ordinary static method
5111 call into the C<Module> package to tell the module to import the list of
5112 features back into the current package. The module can implement its
5113 C<import> method any way it likes, though most modules just choose to
5114 derive their C<import> method via inheritance from the C<Exporter> class that
5115 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5116 method can be found then the error is currently silently ignored. This
5117 may change to a fatal error in a future version.
5119 If you don't want your namespace altered, explicitly supply an empty list:
5123 That is exactly equivalent to
5125 BEGIN { require Module }
5127 If the VERSION argument is present between Module and LIST, then the
5128 C<use> will call the VERSION method in class Module with the given
5129 version as an argument. The default VERSION method, inherited from
5130 the Universal class, croaks if the given version is larger than the
5131 value of the variable C<$Module::VERSION>. (Note that there is not a
5132 comma after VERSION!)
5134 Because this is a wide-open interface, pragmas (compiler directives)
5135 are also implemented this way. Currently implemented pragmas are:
5139 use sigtrap qw(SEGV BUS);
5140 use strict qw(subs vars refs);
5141 use subs qw(afunc blurfl);
5142 use warnings qw(all);
5144 Some of these pseudo-modules import semantics into the current
5145 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5146 which import symbols into the current package (which are effective
5147 through the end of the file).
5149 There's a corresponding C<no> command that unimports meanings imported
5150 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5156 If no C<unimport> method can be found the call fails with a fatal error.
5158 See L<perlmod> for a list of standard modules and pragmas.
5162 Changes the access and modification times on each file of a list of
5163 files. The first two elements of the list must be the NUMERICAL access
5164 and modification times, in that order. Returns the number of files
5165 successfully changed. The inode change time of each file is set
5166 to the current time. This code has the same effect as the C<touch>
5167 command if the files already exist:
5171 utime $now, $now, @ARGV;
5175 Returns a list consisting of all the values of the named hash. (In a
5176 scalar context, returns the number of values.) The values are
5177 returned in an apparently random order. The actual random order is
5178 subject to change in future versions of perl, but it is guaranteed to
5179 be the same order as either the C<keys> or C<each> function would
5180 produce on the same (unmodified) hash.
5182 Note that you cannot modify the values of a hash this way, because the
5183 returned list is just a copy. You need to use a hash slice for that,
5184 since it's lvaluable in a way that values() is not.
5186 for (values %hash) { s/foo/bar/g } # FAILS!
5187 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5189 As a side effect, calling values() resets the HASH's internal iterator.
5190 See also C<keys>, C<each>, and C<sort>.
5192 =item vec EXPR,OFFSET,BITS
5194 Treats the string in EXPR as a bit vector made up of elements of
5195 width BITS, and returns the value of the element specified by OFFSET
5196 as an unsigned integer. BITS therefore specifies the number of bits
5197 that are reserved for each element in the bit vector. This must
5198 be a power of two from 1 to 32 (or 64, if your platform supports
5201 If BITS is 8, "elements" coincide with bytes of the input string.
5203 If BITS is 16 or more, bytes of the input string are grouped into chunks
5204 of size BITS/8, and each group is converted to a number as with
5205 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5206 for BITS==64). See L<"pack"> for details.
5208 If bits is 4 or less, the string is broken into bytes, then the bits
5209 of each byte are broken into 8/BITS groups. Bits of a byte are
5210 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5211 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5212 breaking the single input byte C<chr(0x36)> into two groups gives a list
5213 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5215 C<vec> may also be assigned to, in which case parentheses are needed
5216 to give the expression the correct precedence as in
5218 vec($image, $max_x * $x + $y, 8) = 3;
5220 Vectors created with C<vec> can also be manipulated with the logical
5221 operators C<|>, C<&>, and C<^>, which will assume a bit vector
5222 operation is desired when both operands are strings.
5223 See L<perlop/"Bitwise String Operators">.
5225 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5226 The comments show the string after each step. Note that this code works
5227 in the same way on big-endian or little-endian machines.
5230 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5232 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5233 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5235 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5236 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5237 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5238 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5239 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5240 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5242 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5243 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5244 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5247 To transform a bit vector into a string or list of 0's and 1's, use these:
5249 $bits = unpack("b*", $vector);
5250 @bits = split(//, unpack("b*", $vector));
5252 If you know the exact length in bits, it can be used in place of the C<*>.
5254 Here is an example to illustrate how the bits actually fall in place:
5260 unpack("V",$_) 01234567890123456789012345678901
5261 ------------------------------------------------------------------
5266 for ($shift=0; $shift < $width; ++$shift) {
5267 for ($off=0; $off < 32/$width; ++$off) {
5268 $str = pack("B*", "0"x32);
5269 $bits = (1<<$shift);
5270 vec($str, $off, $width) = $bits;
5271 $res = unpack("b*",$str);
5272 $val = unpack("V", $str);
5279 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5280 $off, $width, $bits, $val, $res
5284 Regardless of the machine architecture on which it is run, the above
5285 example should print the following table:
5288 unpack("V",$_) 01234567890123456789012345678901
5289 ------------------------------------------------------------------
5290 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5291 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5292 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5293 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5294 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5295 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5296 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5297 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5298 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5299 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5300 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5301 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5302 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5303 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5304 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5305 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5306 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5307 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5308 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5309 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5310 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5311 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5312 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5313 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5314 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5315 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5316 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5317 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5318 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5319 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5320 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5321 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5322 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5323 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5324 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5325 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5326 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5327 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5328 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5329 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5330 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5331 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5332 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5333 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5334 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5335 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5336 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5337 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5338 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5339 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5340 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5341 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5342 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5343 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5344 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5345 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5346 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5347 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5348 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5349 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5350 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5351 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5352 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5353 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5354 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5355 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5356 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5357 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5358 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5359 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5360 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5361 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5362 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5363 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5364 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5365 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5366 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5367 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5368 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5369 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5370 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5371 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5372 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5373 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5374 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5375 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5376 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5377 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5378 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5379 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5380 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5381 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5382 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5383 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5384 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5385 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5386 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5387 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5388 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5389 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5390 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5391 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5392 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5393 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5394 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5395 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5396 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5397 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5398 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5399 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5400 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5401 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5402 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5403 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5404 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5405 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5406 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5407 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5408 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5409 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5410 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5411 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5412 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5413 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5414 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5415 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5416 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5417 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5421 Behaves like the wait(2) system call on your system: it waits for a child
5422 process to terminate and returns the pid of the deceased process, or
5423 C<-1> if there are no child processes. The status is returned in C<$?>.
5424 Note that a return value of C<-1> could mean that child processes are
5425 being automatically reaped, as described in L<perlipc>.
5427 =item waitpid PID,FLAGS
5429 Waits for a particular child process to terminate and returns the pid of
5430 the deceased process, or C<-1> if there is no such child process. On some
5431 systems, a value of 0 indicates that there are processes still running.
5432 The status is returned in C<$?>. If you say
5434 use POSIX ":sys_wait_h";
5437 $kid = waitpid(-1,&WNOHANG);
5440 then you can do a non-blocking wait for all pending zombie processes.
5441 Non-blocking wait is available on machines supporting either the
5442 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5443 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5444 system call by remembering the status values of processes that have
5445 exited but have not been harvested by the Perl script yet.)
5447 Note that on some systems, a return value of C<-1> could mean that child
5448 processes are being automatically reaped. See L<perlipc> for details,
5449 and for other examples.
5453 Returns true if the context of the currently executing subroutine is
5454 looking for a list value. Returns false if the context is looking
5455 for a scalar. Returns the undefined value if the context is looking
5456 for no value (void context).
5458 return unless defined wantarray; # don't bother doing more
5459 my @a = complex_calculation();
5460 return wantarray ? @a : "@a";
5462 This function should have been named wantlist() instead.
5466 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5469 If LIST is empty and C<$@> already contains a value (typically from a
5470 previous eval) that value is used after appending C<"\t...caught">
5471 to C<$@>. This is useful for staying almost, but not entirely similar to
5474 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5476 No message is printed if there is a C<$SIG{__WARN__}> handler
5477 installed. It is the handler's responsibility to deal with the message
5478 as it sees fit (like, for instance, converting it into a C<die>). Most
5479 handlers must therefore make arrangements to actually display the
5480 warnings that they are not prepared to deal with, by calling C<warn>
5481 again in the handler. Note that this is quite safe and will not
5482 produce an endless loop, since C<__WARN__> hooks are not called from
5485 You will find this behavior is slightly different from that of
5486 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5487 instead call C<die> again to change it).
5489 Using a C<__WARN__> handler provides a powerful way to silence all
5490 warnings (even the so-called mandatory ones). An example:
5492 # wipe out *all* compile-time warnings
5493 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5495 my $foo = 20; # no warning about duplicate my $foo,
5496 # but hey, you asked for it!
5497 # no compile-time or run-time warnings before here
5500 # run-time warnings enabled after here
5501 warn "\$foo is alive and $foo!"; # does show up
5503 See L<perlvar> for details on setting C<%SIG> entries, and for more
5504 examples. See the Carp module for other kinds of warnings using its
5505 carp() and cluck() functions.
5507 =item write FILEHANDLE
5513 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5514 using the format associated with that file. By default the format for
5515 a file is the one having the same name as the filehandle, but the
5516 format for the current output channel (see the C<select> function) may be set
5517 explicitly by assigning the name of the format to the C<$~> variable.
5519 Top of form processing is handled automatically: if there is
5520 insufficient room on the current page for the formatted record, the
5521 page is advanced by writing a form feed, a special top-of-page format
5522 is used to format the new page header, and then the record is written.
5523 By default the top-of-page format is the name of the filehandle with
5524 "_TOP" appended, but it may be dynamically set to the format of your
5525 choice by assigning the name to the C<$^> variable while the filehandle is
5526 selected. The number of lines remaining on the current page is in
5527 variable C<$->, which can be set to C<0> to force a new page.
5529 If FILEHANDLE is unspecified, output goes to the current default output
5530 channel, which starts out as STDOUT but may be changed by the
5531 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5532 is evaluated and the resulting string is used to look up the name of
5533 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5535 Note that write is I<not> the opposite of C<read>. Unfortunately.
5539 The transliteration operator. Same as C<tr///>. See L<perlop>.