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">.
379 On systems that support a close-on-exec flag on files, the flag will
380 be set for the newly opened file descriptor, as determined by the
381 value of $^F. See L<perlvar/$^F>.
387 Arranges to have a SIGALRM delivered to this process after the
388 specified number of seconds have elapsed. If SECONDS is not specified,
389 the value stored in C<$_> is used. (On some machines,
390 unfortunately, the elapsed time may be up to one second less than you
391 specified because of how seconds are counted.) Only one timer may be
392 counting at once. Each call disables the previous timer, and an
393 argument of C<0> may be supplied to cancel the previous timer without
394 starting a new one. The returned value is the amount of time remaining
395 on the previous timer.
397 For delays of finer granularity than one second, you may use Perl's
398 four-argument version of select() leaving the first three arguments
399 undefined, or you might be able to use the C<syscall> interface to
400 access setitimer(2) if your system supports it. The Time::HiRes module
401 from CPAN may also prove useful.
403 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
404 (C<sleep> may be internally implemented in your system with C<alarm>)
406 If you want to use C<alarm> to time out a system call you need to use an
407 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
408 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
409 restart system calls on some systems. Using C<eval>/C<die> always works,
410 modulo the caveats given in L<perlipc/"Signals">.
413 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
415 $nread = sysread SOCKET, $buffer, $size;
419 die unless $@ eq "alarm\n"; # propagate unexpected errors
428 Returns the arctangent of Y/X in the range -PI to PI.
430 For the tangent operation, you may use the C<Math::Trig::tan>
431 function, or use the familiar relation:
433 sub tan { sin($_[0]) / cos($_[0]) }
435 =item bind SOCKET,NAME
437 Binds a network address to a socket, just as the bind system call
438 does. Returns true if it succeeded, false otherwise. NAME should be a
439 packed address of the appropriate type for the socket. See the examples in
440 L<perlipc/"Sockets: Client/Server Communication">.
442 =item binmode FILEHANDLE
444 Arranges for FILEHANDLE to be read or written in "binary" mode on
445 systems whose run-time libraries force the programmer to guess
446 between binary and text files. If FILEHANDLE is an expression, the
447 value is taken as the name of the filehandle. binmode() should be
448 called after the C<open> but before any I/O is done on the filehandle.
449 The only way to reset binary mode on a filehandle is to reopen the
452 The operating system, device drivers, C libraries, and Perl run-time
453 system all conspire to let the programmer conveniently treat a
454 simple, one-byte C<\n> as the line terminator, irrespective of its
455 external representation. On Unix and its brethren, the native file
456 representation exactly matches the internal representation, making
457 everyone's lives unbelievably simpler. Consequently, L<binmode>
458 has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
459 to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and
460 C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
461 C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because
462 it's only one byte, and the internal and external representations
465 In legacy systems like MS-DOS and its embellishments, your program
466 sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
467 that's not what's physically stored on disk. What's worse, these
468 systems refuse to help you with this; it's up to you to remember
469 what to do. And you mustn't go applying binmode() with wild abandon,
470 either, because if your system does care about binmode(), then using
471 it when you shouldn't is just as perilous as failing to use it when
474 That means that on any version of Microsoft WinXX that you might
475 care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
476 to be converted to C<\n> when read into your program, and causes
477 any C<\n> in your program to be converted back to C<\cM\cJ> on
478 output to disk. This sad discrepancy leads to no end of
479 problems in not just the readline operator, but also when using
480 seek(), tell(), and read() calls. See L<perlport> for other painful
481 details. See the C<$/> and C<$\> variables in L<perlvar> for how
482 to manually set your input and output line-termination sequences.
484 =item bless REF,CLASSNAME
488 This function tells the thingy referenced by REF that it is now an object
489 in the CLASSNAME package. If CLASSNAME is omitted, the current package
490 is used. Because a C<bless> is often the last thing in a constructor,
491 it returns the reference for convenience. Always use the two-argument
492 version if the function doing the blessing might be inherited by a
493 derived class. See L<perltoot> and L<perlobj> for more about the blessing
494 (and blessings) of objects.
496 Consider always blessing objects in CLASSNAMEs that are mixed case.
497 Namespaces with all lowercase names are considered reserved for
498 Perl pragmata. Builtin types have all uppercase names, so to prevent
499 confusion, you may wish to avoid such package names as well. Make sure
500 that CLASSNAME is a true value.
502 See L<perlmod/"Perl Modules">.
508 Returns the context of the current subroutine call. In scalar context,
509 returns the caller's package name if there is a caller, that is, if
510 we're in a subroutine or C<eval> or C<require>, and the undefined value
511 otherwise. In list context, returns
513 ($package, $filename, $line) = caller;
515 With EXPR, it returns some extra information that the debugger uses to
516 print a stack trace. The value of EXPR indicates how many call frames
517 to go back before the current one.
519 ($package, $filename, $line, $subroutine, $hasargs,
520 $wantarray, $evaltext, $is_require, $hints) = caller($i);
522 Here $subroutine may be C<(eval)> if the frame is not a subroutine
523 call, but an C<eval>. In such a case additional elements $evaltext and
524 C<$is_require> are set: C<$is_require> is true if the frame is created by a
525 C<require> or C<use> statement, $evaltext contains the text of the
526 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
527 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
528 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
529 frame. C<$hints> contains pragmatic hints that the caller was
530 compiled with. The C<$hints> value is subject to change between versions
531 of Perl, and is not meant for external use.
533 Furthermore, when called from within the DB package, caller returns more
534 detailed information: it sets the list variable C<@DB::args> to be the
535 arguments with which the subroutine was invoked.
537 Be aware that the optimizer might have optimized call frames away before
538 C<caller> had a chance to get the information. That means that C<caller(N)>
539 might not return information about the call frame you expect it do, for
540 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
541 previous time C<caller> was called.
545 Changes the working directory to EXPR, if possible. If EXPR is omitted,
546 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
547 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
548 set, C<chdir> does nothing. It returns true upon success, false
549 otherwise. See the example under C<die>.
553 Changes the permissions of a list of files. The first element of the
554 list must be the numerical mode, which should probably be an octal
555 number, and which definitely should I<not> a string of octal digits:
556 C<0644> is okay, C<'0644'> is not. Returns the number of files
557 successfully changed. See also L</oct>, if all you have is a string.
559 $cnt = chmod 0755, 'foo', 'bar';
560 chmod 0755, @executables;
561 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
563 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
564 $mode = 0644; chmod $mode, 'foo'; # this is best
566 You can also import the symbolic C<S_I*> constants from the Fcntl
571 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
572 # This is identical to the chmod 0755 of the above example.
580 This safer version of L</chop> removes any trailing string
581 that corresponds to the current value of C<$/> (also known as
582 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
583 number of characters removed from all its arguments. It's often used to
584 remove the newline from the end of an input record when you're worried
585 that the final record may be missing its newline. When in paragraph
586 mode (C<$/ = "">), it removes all trailing newlines from the string.
587 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
588 a reference to an integer or the like, see L<perlvar>) chomp() won't
590 If VARIABLE is omitted, it chomps C<$_>. Example:
593 chomp; # avoid \n on last field
598 You can actually chomp anything that's an lvalue, including an assignment:
601 chomp($answer = <STDIN>);
603 If you chomp a list, each element is chomped, and the total number of
604 characters removed is returned.
612 Chops off the last character of a string and returns the character
613 chopped. It's used primarily to remove the newline from the end of an
614 input record, but is much more efficient than C<s/\n//> because it neither
615 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
619 chop; # avoid \n on last field
624 You can actually chop anything that's an lvalue, including an assignment:
627 chop($answer = <STDIN>);
629 If you chop a list, each element is chopped. Only the value of the
630 last C<chop> is returned.
632 Note that C<chop> returns the last character. To return all but the last
633 character, use C<substr($string, 0, -1)>.
637 Changes the owner (and group) of a list of files. The first two
638 elements of the list must be the I<numeric> uid and gid, in that
639 order. A value of -1 in either position is interpreted by most
640 systems to leave that value unchanged. Returns the number of files
641 successfully changed.
643 $cnt = chown $uid, $gid, 'foo', 'bar';
644 chown $uid, $gid, @filenames;
646 Here's an example that looks up nonnumeric uids in the passwd file:
649 chomp($user = <STDIN>);
651 chomp($pattern = <STDIN>);
653 ($login,$pass,$uid,$gid) = getpwnam($user)
654 or die "$user not in passwd file";
656 @ary = glob($pattern); # expand filenames
657 chown $uid, $gid, @ary;
659 On most systems, you are not allowed to change the ownership of the
660 file unless you're the superuser, although you should be able to change
661 the group to any of your secondary groups. On insecure systems, these
662 restrictions may be relaxed, but this is not a portable assumption.
663 On POSIX systems, you can detect this condition this way:
665 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
666 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
672 Returns the character represented by that NUMBER in the character set.
673 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
674 chr(0x263a) is a Unicode smiley face (but only within the scope of
675 a C<use utf8>). For the reverse, use L</ord>.
676 See L<utf8> for more about Unicode.
678 If NUMBER is omitted, uses C<$_>.
680 =item chroot FILENAME
684 This function works like the system call by the same name: it makes the
685 named directory the new root directory for all further pathnames that
686 begin with a C</> by your process and all its children. (It doesn't
687 change your current working directory, which is unaffected.) For security
688 reasons, this call is restricted to the superuser. If FILENAME is
689 omitted, does a C<chroot> to C<$_>.
691 =item close FILEHANDLE
695 Closes the file or pipe associated with the file handle, returning true
696 only if stdio successfully flushes buffers and closes the system file
697 descriptor. Closes the currently selected filehandle if the argument
700 You don't have to close FILEHANDLE if you are immediately going to do
701 another C<open> on it, because C<open> will close it for you. (See
702 C<open>.) However, an explicit C<close> on an input file resets the line
703 counter (C<$.>), while the implicit close done by C<open> does not.
705 If the file handle came from a piped open C<close> will additionally
706 return false if one of the other system calls involved fails or if the
707 program exits with non-zero status. (If the only problem was that the
708 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
709 also waits for the process executing on the pipe to complete, in case you
710 want to look at the output of the pipe afterwards, and
711 implicitly puts the exit status value of that command into C<$?>.
713 Prematurely closing the read end of a pipe (i.e. before the process
714 writing to it at the other end has closed it) will result in a
715 SIGPIPE being delivered to the writer. If the other end can't
716 handle that, be sure to read all the data before closing the pipe.
720 open(OUTPUT, '|sort >foo') # pipe to sort
721 or die "Can't start sort: $!";
722 #... # print stuff to output
723 close OUTPUT # wait for sort to finish
724 or warn $! ? "Error closing sort pipe: $!"
725 : "Exit status $? from sort";
726 open(INPUT, 'foo') # get sort's results
727 or die "Can't open 'foo' for input: $!";
729 FILEHANDLE may be an expression whose value can be used as an indirect
730 filehandle, usually the real filehandle name.
732 =item closedir DIRHANDLE
734 Closes a directory opened by C<opendir> and returns the success of that
737 DIRHANDLE may be an expression whose value can be used as an indirect
738 dirhandle, usually the real dirhandle name.
740 =item connect SOCKET,NAME
742 Attempts to connect to a remote socket, just as the connect system call
743 does. Returns true if it succeeded, false otherwise. NAME should be a
744 packed address of the appropriate type for the socket. See the examples in
745 L<perlipc/"Sockets: Client/Server Communication">.
749 Actually a flow control statement rather than a function. If there is a
750 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
751 C<foreach>), it is always executed just before the conditional is about to
752 be evaluated again, just like the third part of a C<for> loop in C. Thus
753 it can be used to increment a loop variable, even when the loop has been
754 continued via the C<next> statement (which is similar to the C C<continue>
757 C<last>, C<next>, or C<redo> may appear within a C<continue>
758 block. C<last> and C<redo> will behave as if they had been executed within
759 the main block. So will C<next>, but since it will execute a C<continue>
760 block, it may be more entertaining.
763 ### redo always comes here
766 ### next always comes here
768 # then back the top to re-check EXPR
770 ### last always comes here
772 Omitting the C<continue> section is semantically equivalent to using an
773 empty one, logically enough. In that case, C<next> goes directly back
774 to check the condition at the top of the loop.
778 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
779 takes cosine of C<$_>.
781 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
782 function, or use this relation:
784 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
786 =item crypt PLAINTEXT,SALT
788 Encrypts a string exactly like the crypt(3) function in the C library
789 (assuming that you actually have a version there that has not been
790 extirpated as a potential munition). This can prove useful for checking
791 the password file for lousy passwords, amongst other things. Only the
792 guys wearing white hats should do this.
794 Note that C<crypt> is intended to be a one-way function, much like breaking
795 eggs to make an omelette. There is no (known) corresponding decrypt
796 function. As a result, this function isn't all that useful for
797 cryptography. (For that, see your nearby CPAN mirror.)
799 When verifying an existing encrypted string you should use the encrypted
800 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
801 allows your code to work with the standard C<crypt> and with more
802 exotic implementations. When choosing a new salt create a random two
803 character string whose characters come from the set C<[./0-9A-Za-z]>
804 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
806 Here's an example that makes sure that whoever runs this program knows
809 $pwd = (getpwuid($<))[1];
813 chomp($word = <STDIN>);
817 if (crypt($word, $pwd) ne $pwd) {
823 Of course, typing in your own password to whoever asks you
826 The L<crypt> function is unsuitable for encrypting large quantities
827 of data, not least of all because you can't get the information
828 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
829 on your favorite CPAN mirror for a slew of potentially useful
834 [This function has been largely superseded by the C<untie> function.]
836 Breaks the binding between a DBM file and a hash.
838 =item dbmopen HASH,DBNAME,MASK
840 [This function has been largely superseded by the C<tie> function.]
842 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
843 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
844 argument is I<not> a filehandle, even though it looks like one). DBNAME
845 is the name of the database (without the F<.dir> or F<.pag> extension if
846 any). If the database does not exist, it is created with protection
847 specified by MASK (as modified by the C<umask>). If your system supports
848 only the older DBM functions, you may perform only one C<dbmopen> in your
849 program. In older versions of Perl, if your system had neither DBM nor
850 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
853 If you don't have write access to the DBM file, you can only read hash
854 variables, not set them. If you want to test whether you can write,
855 either use file tests or try setting a dummy hash entry inside an C<eval>,
856 which will trap the error.
858 Note that functions such as C<keys> and C<values> may return huge lists
859 when used on large DBM files. You may prefer to use the C<each>
860 function to iterate over large DBM files. Example:
862 # print out history file offsets
863 dbmopen(%HIST,'/usr/lib/news/history',0666);
864 while (($key,$val) = each %HIST) {
865 print $key, ' = ', unpack('L',$val), "\n";
869 See also L<AnyDBM_File> for a more general description of the pros and
870 cons of the various dbm approaches, as well as L<DB_File> for a particularly
873 You can control which DBM library you use by loading that library
874 before you call dbmopen():
877 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
878 or die "Can't open netscape history file: $!";
884 Returns a Boolean value telling whether EXPR has a value other than
885 the undefined value C<undef>. If EXPR is not present, C<$_> will be
888 Many operations return C<undef> to indicate failure, end of file,
889 system error, uninitialized variable, and other exceptional
890 conditions. This function allows you to distinguish C<undef> from
891 other values. (A simple Boolean test will not distinguish among
892 C<undef>, zero, the empty string, and C<"0">, which are all equally
893 false.) Note that since C<undef> is a valid scalar, its presence
894 doesn't I<necessarily> indicate an exceptional condition: C<pop>
895 returns C<undef> when its argument is an empty array, I<or> when the
896 element to return happens to be C<undef>.
898 You may also use C<defined(&func)> to check whether subroutine C<&func>
899 has ever been defined. The return value is unaffected by any forward
900 declarations of C<&foo>.
902 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
903 used to report whether memory for that aggregate has ever been
904 allocated. This behavior may disappear in future versions of Perl.
905 You should instead use a simple test for size:
907 if (@an_array) { print "has array elements\n" }
908 if (%a_hash) { print "has hash members\n" }
910 When used on a hash element, it tells you whether the value is defined,
911 not whether the key exists in the hash. Use L</exists> for the latter
916 print if defined $switch{'D'};
917 print "$val\n" while defined($val = pop(@ary));
918 die "Can't readlink $sym: $!"
919 unless defined($value = readlink $sym);
920 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
921 $debugging = 0 unless defined $debugging;
923 Note: Many folks tend to overuse C<defined>, and then are surprised to
924 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
925 defined values. For example, if you say
929 The pattern match succeeds, and C<$1> is defined, despite the fact that it
930 matched "nothing". But it didn't really match nothing--rather, it
931 matched something that happened to be zero characters long. This is all
932 very above-board and honest. When a function returns an undefined value,
933 it's an admission that it couldn't give you an honest answer. So you
934 should use C<defined> only when you're questioning the integrity of what
935 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
938 See also L</undef>, L</exists>, L</ref>.
942 Given an expression that specifies a hash element, array element, hash slice,
943 or array slice, deletes the specified element(s) from the hash or array.
944 In the case of an array, if the array elements happen to be at the end,
945 the size of the array will shrink to the highest element that tests
946 true for exists() (or 0 if no such element exists).
948 Returns each element so deleted or the undefined value if there was no such
949 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
950 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
951 from a C<tie>d hash or array may not necessarily return anything.
953 Deleting an array element effectively returns that position of the array
954 to its initial, uninitialized state. Subsequently testing for the same
955 element with exists() will return false. Note that deleting array
956 elements in the middle of an array will not shift the index of the ones
957 after them down--use splice() for that. See L</exists>.
959 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
961 foreach $key (keys %HASH) {
965 foreach $index (0 .. $#ARRAY) {
966 delete $ARRAY[$index];
971 delete @HASH{keys %HASH};
973 delete @ARRAY[0 .. $#ARRAY];
975 But both of these are slower than just assigning the empty list
976 or undefining %HASH or @ARRAY:
978 %HASH = (); # completely empty %HASH
979 undef %HASH; # forget %HASH ever existed
981 @ARRAY = (); # completely empty @ARRAY
982 undef @ARRAY; # forget @ARRAY ever existed
984 Note that the EXPR can be arbitrarily complicated as long as the final
985 operation is a hash element, array element, hash slice, or array slice
988 delete $ref->[$x][$y]{$key};
989 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
991 delete $ref->[$x][$y][$index];
992 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
996 Outside an C<eval>, prints the value of LIST to C<STDERR> and
997 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
998 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
999 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
1000 an C<eval(),> the error message is stuffed into C<$@> and the
1001 C<eval> is terminated with the undefined value. This makes
1002 C<die> the way to raise an exception.
1004 Equivalent examples:
1006 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1007 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1009 If the value of EXPR does not end in a newline, the current script line
1010 number and input line number (if any) are also printed, and a newline
1011 is supplied. Note that the "input line number" (also known as "chunk")
1012 is subject to whatever notion of "line" happens to be currently in
1013 effect, and is also available as the special variable C<$.>.
1014 See L<perlvar/"$/"> and L<perlvar/"$.">.
1016 Hint: sometimes appending C<", stopped"> to your message
1017 will cause it to make better sense when the string C<"at foo line 123"> is
1018 appended. Suppose you are running script "canasta".
1020 die "/etc/games is no good";
1021 die "/etc/games is no good, stopped";
1023 produce, respectively
1025 /etc/games is no good at canasta line 123.
1026 /etc/games is no good, stopped at canasta line 123.
1028 See also exit(), warn(), and the Carp module.
1030 If LIST is empty and C<$@> already contains a value (typically from a
1031 previous eval) that value is reused after appending C<"\t...propagated">.
1032 This is useful for propagating exceptions:
1035 die unless $@ =~ /Expected exception/;
1037 If C<$@> is empty then the string C<"Died"> is used.
1039 die() can also be called with a reference argument. If this happens to be
1040 trapped within an eval(), $@ contains the reference. This behavior permits
1041 a more elaborate exception handling implementation using objects that
1042 maintain arbitary state about the nature of the exception. Such a scheme
1043 is sometimes preferable to matching particular string values of $@ using
1044 regular expressions. Here's an example:
1046 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1048 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1049 # handle Some::Module::Exception
1052 # handle all other possible exceptions
1056 Because perl will stringify uncaught exception messages before displaying
1057 them, you may want to overload stringification operations on such custom
1058 exception objects. See L<overload> for details about that.
1060 You can arrange for a callback to be run just before the C<die>
1061 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1062 handler will be called with the error text and can change the error
1063 message, if it sees fit, by calling C<die> again. See
1064 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1065 L<"eval BLOCK"> for some examples. Although this feature was meant
1066 to be run only right before your program was to exit, this is not
1067 currently the case--the C<$SIG{__DIE__}> hook is currently called
1068 even inside eval()ed blocks/strings! If one wants the hook to do
1069 nothing in such situations, put
1073 as the first line of the handler (see L<perlvar/$^S>). Because
1074 this promotes strange action at a distance, this counterintuitive
1075 behavior may be fixed in a future release.
1079 Not really a function. Returns the value of the last command in the
1080 sequence of commands indicated by BLOCK. When modified by a loop
1081 modifier, executes the BLOCK once before testing the loop condition.
1082 (On other statements the loop modifiers test the conditional first.)
1084 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1085 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1086 See L<perlsyn> for alternative strategies.
1088 =item do SUBROUTINE(LIST)
1090 A deprecated form of subroutine call. See L<perlsub>.
1094 Uses the value of EXPR as a filename and executes the contents of the
1095 file as a Perl script. Its primary use is to include subroutines
1096 from a Perl subroutine library.
1102 scalar eval `cat stat.pl`;
1104 except that it's more efficient and concise, keeps track of the current
1105 filename for error messages, searches the @INC libraries, and updates
1106 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1107 variables. It also differs in that code evaluated with C<do FILENAME>
1108 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1109 same, however, in that it does reparse the file every time you call it,
1110 so you probably don't want to do this inside a loop.
1112 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1113 error. If C<do> can read the file but cannot compile it, it
1114 returns undef and sets an error message in C<$@>. If the file is
1115 successfully compiled, C<do> returns the value of the last expression
1118 Note that inclusion of library modules is better done with the
1119 C<use> and C<require> operators, which also do automatic error checking
1120 and raise an exception if there's a problem.
1122 You might like to use C<do> to read in a program configuration
1123 file. Manual error checking can be done this way:
1125 # read in config files: system first, then user
1126 for $file ("/share/prog/defaults.rc",
1127 "$ENV{HOME}/.someprogrc")
1129 unless ($return = do $file) {
1130 warn "couldn't parse $file: $@" if $@;
1131 warn "couldn't do $file: $!" unless defined $return;
1132 warn "couldn't run $file" unless $return;
1140 This function causes an immediate core dump. See also the B<-u>
1141 command-line switch in L<perlrun>, which does the same thing.
1142 Primarily this is so that you can use the B<undump> program (not
1143 supplied) to turn your core dump into an executable binary after
1144 having initialized all your variables at the beginning of the
1145 program. When the new binary is executed it will begin by executing
1146 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1147 Think of it as a goto with an intervening core dump and reincarnation.
1148 If C<LABEL> is omitted, restarts the program from the top.
1150 B<WARNING>: Any files opened at the time of the dump will I<not>
1151 be open any more when the program is reincarnated, with possible
1152 resulting confusion on the part of Perl.
1154 This function is now largely obsolete, partly because it's very
1155 hard to convert a core file into an executable, and because the
1156 real compiler backends for generating portable bytecode and compilable
1157 C code have superseded it.
1159 If you're looking to use L<dump> to speed up your program, consider
1160 generating bytecode or native C code as described in L<perlcc>. If
1161 you're just trying to accelerate a CGI script, consider using the
1162 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1163 You might also consider autoloading or selfloading, which at least
1164 make your program I<appear> to run faster.
1168 When called in list context, returns a 2-element list consisting of the
1169 key and value for the next element of a hash, so that you can iterate over
1170 it. When called in scalar context, returns the key for only the "next"
1171 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1172 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1175 Entries are returned in an apparently random order. The actual random
1176 order is subject to change in future versions of perl, but it is guaranteed
1177 to be in the same order as either the C<keys> or C<values> function
1178 would produce on the same (unmodified) hash.
1180 When the hash is entirely read, a null array is returned in list context
1181 (which when assigned produces a false (C<0>) value), and C<undef> in
1182 scalar context. The next call to C<each> after that will start iterating
1183 again. There is a single iterator for each hash, shared by all C<each>,
1184 C<keys>, and C<values> function calls in the program; it can be reset by
1185 reading all the elements from the hash, or by evaluating C<keys HASH> or
1186 C<values HASH>. If you add or delete elements of a hash while you're
1187 iterating over it, you may get entries skipped or duplicated, so don't.
1189 The following prints out your environment like the printenv(1) program,
1190 only in a different order:
1192 while (($key,$value) = each %ENV) {
1193 print "$key=$value\n";
1196 See also C<keys>, C<values> and C<sort>.
1198 =item eof FILEHANDLE
1204 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1205 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1206 gives the real filehandle. (Note that this function actually
1207 reads a character and then C<ungetc>s it, so isn't very useful in an
1208 interactive context.) Do not read from a terminal file (or call
1209 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1210 as terminals may lose the end-of-file condition if you do.
1212 An C<eof> without an argument uses the last file read. Using C<eof()>
1213 with empty parentheses is very different. It refers to the pseudo file
1214 formed from the files listed on the command line and accessed via the
1215 C<E<lt>E<gt>> operator. Since C<E<lt>E<gt>> isn't explicitly opened,
1216 as a normal filehandle is, an C<eof()> before C<E<lt>E<gt>> has been
1217 used will cause C<@ARGV> to be examined to determine if input is
1220 In a C<while (E<lt>E<gt>)> loop, C<eof> or C<eof(ARGV)> can be used to
1221 detect the end of each file, C<eof()> will only detect the end of the
1222 last file. Examples:
1224 # reset line numbering on each input file
1226 next if /^\s*#/; # skip comments
1229 close ARGV if eof; # Not eof()!
1232 # insert dashes just before last line of last file
1234 if (eof()) { # check for end of current file
1235 print "--------------\n";
1236 close(ARGV); # close or last; is needed if we
1237 # are reading from the terminal
1242 Practical hint: you almost never need to use C<eof> in Perl, because the
1243 input operators typically return C<undef> when they run out of data, or if
1250 In the first form, the return value of EXPR is parsed and executed as if it
1251 were a little Perl program. The value of the expression (which is itself
1252 determined within scalar context) is first parsed, and if there weren't any
1253 errors, executed in the context of the current Perl program, so that any
1254 variable settings or subroutine and format definitions remain afterwards.
1255 Note that the value is parsed every time the eval executes. If EXPR is
1256 omitted, evaluates C<$_>. This form is typically used to delay parsing
1257 and subsequent execution of the text of EXPR until run time.
1259 In the second form, the code within the BLOCK is parsed only once--at the
1260 same time the code surrounding the eval itself was parsed--and executed
1261 within the context of the current Perl program. This form is typically
1262 used to trap exceptions more efficiently than the first (see below), while
1263 also providing the benefit of checking the code within BLOCK at compile
1266 The final semicolon, if any, may be omitted from the value of EXPR or within
1269 In both forms, the value returned is the value of the last expression
1270 evaluated inside the mini-program; a return statement may be also used, just
1271 as with subroutines. The expression providing the return value is evaluated
1272 in void, scalar, or list context, depending on the context of the eval itself.
1273 See L</wantarray> for more on how the evaluation context can be determined.
1275 If there is a syntax error or runtime error, or a C<die> statement is
1276 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1277 error message. If there was no error, C<$@> is guaranteed to be a null
1278 string. Beware that using C<eval> neither silences perl from printing
1279 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1280 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1281 L</warn> and L<perlvar>.
1283 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1284 determining whether a particular feature (such as C<socket> or C<symlink>)
1285 is implemented. It is also Perl's exception trapping mechanism, where
1286 the die operator is used to raise exceptions.
1288 If the code to be executed doesn't vary, you may use the eval-BLOCK
1289 form to trap run-time errors without incurring the penalty of
1290 recompiling each time. The error, if any, is still returned in C<$@>.
1293 # make divide-by-zero nonfatal
1294 eval { $answer = $a / $b; }; warn $@ if $@;
1296 # same thing, but less efficient
1297 eval '$answer = $a / $b'; warn $@ if $@;
1299 # a compile-time error
1300 eval { $answer = }; # WRONG
1303 eval '$answer ='; # sets $@
1305 Due to the current arguably broken state of C<__DIE__> hooks, when using
1306 the C<eval{}> form as an exception trap in libraries, you may wish not
1307 to trigger any C<__DIE__> hooks that user code may have installed.
1308 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1309 as shown in this example:
1311 # a very private exception trap for divide-by-zero
1312 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1315 This is especially significant, given that C<__DIE__> hooks can call
1316 C<die> again, which has the effect of changing their error messages:
1318 # __DIE__ hooks may modify error messages
1320 local $SIG{'__DIE__'} =
1321 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1322 eval { die "foo lives here" };
1323 print $@ if $@; # prints "bar lives here"
1326 Because this promotes action at a distance, this counterintuitive behavior
1327 may be fixed in a future release.
1329 With an C<eval>, you should be especially careful to remember what's
1330 being looked at when:
1336 eval { $x }; # CASE 4
1338 eval "\$$x++"; # CASE 5
1341 Cases 1 and 2 above behave identically: they run the code contained in
1342 the variable $x. (Although case 2 has misleading double quotes making
1343 the reader wonder what else might be happening (nothing is).) Cases 3
1344 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1345 does nothing but return the value of $x. (Case 4 is preferred for
1346 purely visual reasons, but it also has the advantage of compiling at
1347 compile-time instead of at run-time.) Case 5 is a place where
1348 normally you I<would> like to use double quotes, except that in this
1349 particular situation, you can just use symbolic references instead, as
1352 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1353 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1357 =item exec PROGRAM LIST
1359 The C<exec> function executes a system command I<and never returns>--
1360 use C<system> instead of C<exec> if you want it to return. It fails and
1361 returns false only if the command does not exist I<and> it is executed
1362 directly instead of via your system's command shell (see below).
1364 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1365 warns you if there is a following statement which isn't C<die>, C<warn>,
1366 or C<exit> (if C<-w> is set - but you always do that). If you
1367 I<really> want to follow an C<exec> with some other statement, you
1368 can use one of these styles to avoid the warning:
1370 exec ('foo') or print STDERR "couldn't exec foo: $!";
1371 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1373 If there is more than one argument in LIST, or if LIST is an array
1374 with more than one value, calls execvp(3) with the arguments in LIST.
1375 If there is only one scalar argument or an array with one element in it,
1376 the argument is checked for shell metacharacters, and if there are any,
1377 the entire argument is passed to the system's command shell for parsing
1378 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1379 If there are no shell metacharacters in the argument, it is split into
1380 words and passed directly to C<execvp>, which is more efficient.
1383 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1384 exec "sort $outfile | uniq";
1386 If you don't really want to execute the first argument, but want to lie
1387 to the program you are executing about its own name, you can specify
1388 the program you actually want to run as an "indirect object" (without a
1389 comma) in front of the LIST. (This always forces interpretation of the
1390 LIST as a multivalued list, even if there is only a single scalar in
1393 $shell = '/bin/csh';
1394 exec $shell '-sh'; # pretend it's a login shell
1398 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1400 When the arguments get executed via the system shell, results will
1401 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1404 Using an indirect object with C<exec> or C<system> is also more
1405 secure. This usage (which also works fine with system()) forces
1406 interpretation of the arguments as a multivalued list, even if the
1407 list had just one argument. That way you're safe from the shell
1408 expanding wildcards or splitting up words with whitespace in them.
1410 @args = ( "echo surprise" );
1412 exec @args; # subject to shell escapes
1414 exec { $args[0] } @args; # safe even with one-arg list
1416 The first version, the one without the indirect object, ran the I<echo>
1417 program, passing it C<"surprise"> an argument. The second version
1418 didn't--it tried to run a program literally called I<"echo surprise">,
1419 didn't find it, and set C<$?> to a non-zero value indicating failure.
1421 Note that C<exec> will not call your C<END> blocks, nor will it call
1422 any C<DESTROY> methods in your objects.
1426 Given an expression that specifies a hash element or array element,
1427 returns true if the specified element in the hash or array has ever
1428 been initialized, even if the corresponding value is undefined. The
1429 element is not autovivified if it doesn't exist.
1431 print "Exists\n" if exists $hash{$key};
1432 print "Defined\n" if defined $hash{$key};
1433 print "True\n" if $hash{$key};
1435 print "Exists\n" if exists $array[$index];
1436 print "Defined\n" if defined $array[$index];
1437 print "True\n" if $array[$index];
1439 A hash or array element can be true only if it's defined, and defined if
1440 it exists, but the reverse doesn't necessarily hold true.
1442 Given an expression that specifies the name of a subroutine,
1443 returns true if the specified subroutine has ever been declared, even
1444 if it is undefined. Mentioning a subroutine name for exists or defined
1445 does not count as declaring it.
1447 print "Exists\n" if exists &subroutine;
1448 print "Defined\n" if defined &subroutine;
1450 Note that the EXPR can be arbitrarily complicated as long as the final
1451 operation is a hash or array key lookup or subroutine name:
1453 if (exists $ref->{A}->{B}->{$key}) { }
1454 if (exists $hash{A}{B}{$key}) { }
1456 if (exists $ref->{A}->{B}->[$ix]) { }
1457 if (exists $hash{A}{B}[$ix]) { }
1459 if (exists &{$ref->{A}{B}{$key}}) { }
1461 Although the deepest nested array or hash will not spring into existence
1462 just because its existence was tested, any intervening ones will.
1463 Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring
1464 into existence due to the existence test for the $key element above.
1465 This happens anywhere the arrow operator is used, including even:
1468 if (exists $ref->{"Some key"}) { }
1469 print $ref; # prints HASH(0x80d3d5c)
1471 This surprising autovivification in what does not at first--or even
1472 second--glance appear to be an lvalue context may be fixed in a future
1475 See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
1476 used on a pseudo-hash.
1478 Use of a subroutine call, rather than a subroutine name, as an argument
1479 to exists() is an error.
1482 exists &sub(); # Error
1486 Evaluates EXPR and exits immediately with that value. Example:
1489 exit 0 if $ans =~ /^[Xx]/;
1491 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1492 universally recognized values for EXPR are C<0> for success and C<1>
1493 for error; other values are subject to interpretation depending on the
1494 environment in which the Perl program is running. For example, exiting
1495 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1496 the mailer to return the item undelivered, but that's not true everywhere.
1498 Don't use C<exit> to abort a subroutine if there's any chance that
1499 someone might want to trap whatever error happened. Use C<die> instead,
1500 which can be trapped by an C<eval>.
1502 The exit() function does not always exit immediately. It calls any
1503 defined C<END> routines first, but these C<END> routines may not
1504 themselves abort the exit. Likewise any object destructors that need to
1505 be called are called before the real exit. If this is a problem, you
1506 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1507 See L<perlmod> for details.
1513 Returns I<e> (the natural logarithm base) to the power of EXPR.
1514 If EXPR is omitted, gives C<exp($_)>.
1516 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1518 Implements the fcntl(2) function. You'll probably have to say
1522 first to get the correct constant definitions. Argument processing and
1523 value return works just like C<ioctl> below.
1527 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1528 or die "can't fcntl F_GETFL: $!";
1530 You don't have to check for C<defined> on the return from C<fnctl>.
1531 Like C<ioctl>, it maps a C<0> return from the system call into
1532 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1533 in numeric context. It is also exempt from the normal B<-w> warnings
1534 on improper numeric conversions.
1536 Note that C<fcntl> will produce a fatal error if used on a machine that
1537 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1538 manpage to learn what functions are available on your system.
1540 =item fileno FILEHANDLE
1542 Returns the file descriptor for a filehandle, or undefined if the
1543 filehandle is not open. This is mainly useful for constructing
1544 bitmaps for C<select> and low-level POSIX tty-handling operations.
1545 If FILEHANDLE is an expression, the value is taken as an indirect
1546 filehandle, generally its name.
1548 You can use this to find out whether two handles refer to the
1549 same underlying descriptor:
1551 if (fileno(THIS) == fileno(THAT)) {
1552 print "THIS and THAT are dups\n";
1555 =item flock FILEHANDLE,OPERATION
1557 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1558 for success, false on failure. Produces a fatal error if used on a
1559 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1560 C<flock> is Perl's portable file locking interface, although it locks
1561 only entire files, not records.
1563 Two potentially non-obvious but traditional C<flock> semantics are
1564 that it waits indefinitely until the lock is granted, and that its locks
1565 B<merely advisory>. Such discretionary locks are more flexible, but offer
1566 fewer guarantees. This means that files locked with C<flock> may be
1567 modified by programs that do not also use C<flock>. See L<perlport>,
1568 your port's specific documentation, or your system-specific local manpages
1569 for details. It's best to assume traditional behavior if you're writing
1570 portable programs. (But if you're not, you should as always feel perfectly
1571 free to write for your own system's idiosyncrasies (sometimes called
1572 "features"). Slavish adherence to portability concerns shouldn't get
1573 in the way of your getting your job done.)
1575 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1576 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1577 you can use the symbolic names if you import them from the Fcntl module,
1578 either individually, or as a group using the ':flock' tag. LOCK_SH
1579 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1580 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1581 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1582 waiting for the lock (check the return status to see if you got it).
1584 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1585 before locking or unlocking it.
1587 Note that the emulation built with lockf(3) doesn't provide shared
1588 locks, and it requires that FILEHANDLE be open with write intent. These
1589 are the semantics that lockf(3) implements. Most if not all systems
1590 implement lockf(3) in terms of fcntl(2) locking, though, so the
1591 differing semantics shouldn't bite too many people.
1593 Note also that some versions of C<flock> cannot lock things over the
1594 network; you would need to use the more system-specific C<fcntl> for
1595 that. If you like you can force Perl to ignore your system's flock(2)
1596 function, and so provide its own fcntl(2)-based emulation, by passing
1597 the switch C<-Ud_flock> to the F<Configure> program when you configure
1600 Here's a mailbox appender for BSD systems.
1602 use Fcntl ':flock'; # import LOCK_* constants
1605 flock(MBOX,LOCK_EX);
1606 # and, in case someone appended
1607 # while we were waiting...
1612 flock(MBOX,LOCK_UN);
1615 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1616 or die "Can't open mailbox: $!";
1619 print MBOX $msg,"\n\n";
1622 On systems that support a real flock(), locks are inherited across fork()
1623 calls, whereas those that must resort to the more capricious fcntl()
1624 function lose the locks, making it harder to write servers.
1626 See also L<DB_File> for other flock() examples.
1630 Does a fork(2) system call to create a new process running the
1631 same program at the same point. It returns the child pid to the
1632 parent process, C<0> to the child process, or C<undef> if the fork is
1633 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1634 are shared, while everything else is copied. On most systems supporting
1635 fork(), great care has gone into making it extremely efficient (for
1636 example, using copy-on-write technology on data pages), making it the
1637 dominant paradigm for multitasking over the last few decades.
1639 All files opened for output are flushed before forking the child process.
1641 If you C<fork> without ever waiting on your children, you will
1642 accumulate zombies. On some systems, you can avoid this by setting
1643 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1644 forking and reaping moribund children.
1646 Note that if your forked child inherits system file descriptors like
1647 STDIN and STDOUT that are actually connected by a pipe or socket, even
1648 if you exit, then the remote server (such as, say, a CGI script or a
1649 backgrounded job launched from a remote shell) won't think you're done.
1650 You should reopen those to F</dev/null> if it's any issue.
1654 Declare a picture format for use by the C<write> function. For
1658 Test: @<<<<<<<< @||||| @>>>>>
1659 $str, $%, '$' . int($num)
1663 $num = $cost/$quantity;
1667 See L<perlform> for many details and examples.
1669 =item formline PICTURE,LIST
1671 This is an internal function used by C<format>s, though you may call it,
1672 too. It formats (see L<perlform>) a list of values according to the
1673 contents of PICTURE, placing the output into the format output
1674 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1675 Eventually, when a C<write> is done, the contents of
1676 C<$^A> are written to some filehandle, but you could also read C<$^A>
1677 yourself and then set C<$^A> back to C<"">. Note that a format typically
1678 does one C<formline> per line of form, but the C<formline> function itself
1679 doesn't care how many newlines are embedded in the PICTURE. This means
1680 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1681 You may therefore need to use multiple formlines to implement a single
1682 record format, just like the format compiler.
1684 Be careful if you put double quotes around the picture, because an C<@>
1685 character may be taken to mean the beginning of an array name.
1686 C<formline> always returns true. See L<perlform> for other examples.
1688 =item getc FILEHANDLE
1692 Returns the next character from the input file attached to FILEHANDLE,
1693 or the undefined value at end of file, or if there was an error.
1694 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1695 efficient. However, it cannot be used by itself to fetch single
1696 characters without waiting for the user to hit enter. For that, try
1697 something more like:
1700 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1703 system "stty", '-icanon', 'eol', "\001";
1709 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1712 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1716 Determination of whether $BSD_STYLE should be set
1717 is left as an exercise to the reader.
1719 The C<POSIX::getattr> function can do this more portably on
1720 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1721 module from your nearest CPAN site; details on CPAN can be found on
1726 Implements the C library function of the same name, which on most
1727 systems returns the current login from F</etc/utmp>, if any. If null,
1730 $login = getlogin || getpwuid($<) || "Kilroy";
1732 Do not consider C<getlogin> for authentication: it is not as
1733 secure as C<getpwuid>.
1735 =item getpeername SOCKET
1737 Returns the packed sockaddr address of other end of the SOCKET connection.
1740 $hersockaddr = getpeername(SOCK);
1741 ($port, $iaddr) = sockaddr_in($hersockaddr);
1742 $herhostname = gethostbyaddr($iaddr, AF_INET);
1743 $herstraddr = inet_ntoa($iaddr);
1747 Returns the current process group for the specified PID. Use
1748 a PID of C<0> to get the current process group for the
1749 current process. Will raise an exception if used on a machine that
1750 doesn't implement getpgrp(2). If PID is omitted, returns process
1751 group of current process. Note that the POSIX version of C<getpgrp>
1752 does not accept a PID argument, so only C<PID==0> is truly portable.
1756 Returns the process id of the parent process.
1758 =item getpriority WHICH,WHO
1760 Returns the current priority for a process, a process group, or a user.
1761 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1762 machine that doesn't implement getpriority(2).
1768 =item gethostbyname NAME
1770 =item getnetbyname NAME
1772 =item getprotobyname NAME
1778 =item getservbyname NAME,PROTO
1780 =item gethostbyaddr ADDR,ADDRTYPE
1782 =item getnetbyaddr ADDR,ADDRTYPE
1784 =item getprotobynumber NUMBER
1786 =item getservbyport PORT,PROTO
1804 =item sethostent STAYOPEN
1806 =item setnetent STAYOPEN
1808 =item setprotoent STAYOPEN
1810 =item setservent STAYOPEN
1824 These routines perform the same functions as their counterparts in the
1825 system library. In list context, the return values from the
1826 various get routines are as follows:
1828 ($name,$passwd,$uid,$gid,
1829 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1830 ($name,$passwd,$gid,$members) = getgr*
1831 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1832 ($name,$aliases,$addrtype,$net) = getnet*
1833 ($name,$aliases,$proto) = getproto*
1834 ($name,$aliases,$port,$proto) = getserv*
1836 (If the entry doesn't exist you get a null list.)
1838 In scalar context, you get the name, unless the function was a
1839 lookup by name, in which case you get the other thing, whatever it is.
1840 (If the entry doesn't exist you get the undefined value.) For example:
1842 $uid = getpwnam($name);
1843 $name = getpwuid($num);
1845 $gid = getgrnam($name);
1846 $name = getgrgid($num;
1850 In I<getpw*()> the fields $quota, $comment, and $expire are
1851 special cases in the sense that in many systems they are unsupported.
1852 If the $quota is unsupported, it is an empty scalar. If it is
1853 supported, it usually encodes the disk quota. If the $comment
1854 field is unsupported, it is an empty scalar. If it is supported it
1855 usually encodes some administrative comment about the user. In some
1856 systems the $quota field may be $change or $age, fields that have
1857 to do with password aging. In some systems the $comment field may
1858 be $class. The $expire field, if present, encodes the expiration
1859 period of the account or the password. For the availability and the
1860 exact meaning of these fields in your system, please consult your
1861 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1862 out from within Perl what your $quota and $comment fields mean
1863 and whether you have the $expire field by using the C<Config> module
1864 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1865 and C<d_pwexpire>. Shadow password files are only supported if your
1866 vendor has implemented them in the intuitive fashion that calling the
1867 regular C library routines gets the shadow versions if you're running
1868 under privilege. Those that incorrectly implement a separate library
1869 call are not supported.
1871 The $members value returned by I<getgr*()> is a space separated list of
1872 the login names of the members of the group.
1874 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1875 C, it will be returned to you via C<$?> if the function call fails. The
1876 C<@addrs> value returned by a successful call is a list of the raw
1877 addresses returned by the corresponding system library call. In the
1878 Internet domain, each address is four bytes long and you can unpack it
1879 by saying something like:
1881 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1883 The Socket library makes this slightly easier:
1886 $iaddr = inet_aton("127.1"); # or whatever address
1887 $name = gethostbyaddr($iaddr, AF_INET);
1889 # or going the other way
1890 $straddr = inet_ntoa($iaddr);
1892 If you get tired of remembering which element of the return list
1893 contains which return value, by-name interfaces are provided
1894 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1895 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1896 and C<User::grent>. These override the normal built-ins, supplying
1897 versions that return objects with the appropriate names
1898 for each field. For example:
1902 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1904 Even though it looks like they're the same method calls (uid),
1905 they aren't, because a C<File::stat> object is different from
1906 a C<User::pwent> object.
1908 =item getsockname SOCKET
1910 Returns the packed sockaddr address of this end of the SOCKET connection,
1911 in case you don't know the address because you have several different
1912 IPs that the connection might have come in on.
1915 $mysockaddr = getsockname(SOCK);
1916 ($port, $myaddr) = sockaddr_in($mysockaddr);
1917 printf "Connect to %s [%s]\n",
1918 scalar gethostbyaddr($myaddr, AF_INET),
1921 =item getsockopt SOCKET,LEVEL,OPTNAME
1923 Returns the socket option requested, or undef if there is an error.
1929 Returns the value of EXPR with filename expansions such as the
1930 standard Unix shell F</bin/csh> would do. This is the internal function
1931 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1932 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1933 discussed in more detail in L<perlop/"I/O Operators">.
1937 Converts a time as returned by the time function to a 9-element list
1938 with the time localized for the standard Greenwich time zone.
1939 Typically used as follows:
1942 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1945 All list elements are numeric, and come straight out of a struct tm.
1946 In particular this means that $mon has the range C<0..11> and $wday
1947 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1948 number of years since 1900, that is, $year is C<123> in year 2023,
1949 I<not> simply the last two digits of the year. If you assume it is,
1950 then you create non-Y2K-compliant programs--and you wouldn't want to do
1953 The proper way to get a complete 4-digit year is simply:
1957 And to get the last two digits of the year (e.g., '01' in 2001) do:
1959 $year = sprintf("%02d", $year % 100);
1961 If EXPR is omitted, does C<gmtime(time())>.
1963 In scalar context, returns the ctime(3) value:
1965 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1967 Also see the C<timegm> function provided by the C<Time::Local> module,
1968 and the strftime(3) function available via the POSIX module.
1970 This scalar value is B<not> locale dependent (see L<perllocale>), but
1971 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1972 strftime(3) and mktime(3) functions available via the POSIX module. To
1973 get somewhat similar but locale dependent date strings, set up your
1974 locale environment variables appropriately (please see L<perllocale>)
1975 and try for example:
1977 use POSIX qw(strftime);
1978 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1980 Note that the C<%a> and C<%b> escapes, which represent the short forms
1981 of the day of the week and the month of the year, may not necessarily
1982 be three characters wide in all locales.
1990 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1991 execution there. It may not be used to go into any construct that
1992 requires initialization, such as a subroutine or a C<foreach> loop. It
1993 also can't be used to go into a construct that is optimized away,
1994 or to get out of a block or subroutine given to C<sort>.
1995 It can be used to go almost anywhere else within the dynamic scope,
1996 including out of subroutines, but it's usually better to use some other
1997 construct such as C<last> or C<die>. The author of Perl has never felt the
1998 need to use this form of C<goto> (in Perl, that is--C is another matter).
2000 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2001 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2002 necessarily recommended if you're optimizing for maintainability:
2004 goto ("FOO", "BAR", "GLARCH")[$i];
2006 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2007 In fact, it isn't a goto in the normal sense at all, and doesn't have
2008 the stigma associated with other gotos. Instead, it
2009 substitutes a call to the named subroutine for the currently running
2010 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2011 another subroutine and then pretend that the other subroutine had been
2012 called in the first place (except that any modifications to C<@_>
2013 in the current subroutine are propagated to the other subroutine.)
2014 After the C<goto>, not even C<caller> will be able to tell that this
2015 routine was called first.
2017 NAME needn't be the name of a subroutine; it can be a scalar variable
2018 containing a code reference, or a block which evaluates to a code
2021 =item grep BLOCK LIST
2023 =item grep EXPR,LIST
2025 This is similar in spirit to, but not the same as, grep(1) and its
2026 relatives. In particular, it is not limited to using regular expressions.
2028 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2029 C<$_> to each element) and returns the list value consisting of those
2030 elements for which the expression evaluated to true. In scalar
2031 context, returns the number of times the expression was true.
2033 @foo = grep(!/^#/, @bar); # weed out comments
2037 @foo = grep {!/^#/} @bar; # weed out comments
2039 Note that, because C<$_> is a reference into the list value, it can
2040 be used to modify the elements of the array. While this is useful and
2041 supported, it can cause bizarre results if the LIST is not a named array.
2042 Similarly, grep returns aliases into the original list, much as a for
2043 loop's index variable aliases the list elements. That is, modifying an
2044 element of a list returned by grep (for example, in a C<foreach>, C<map>
2045 or another C<grep>) actually modifies the element in the original list.
2046 This is usually something to be avoided when writing clear code.
2048 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2054 Interprets EXPR as a hex string and returns the corresponding value.
2055 (To convert strings that might start with either 0, 0x, or 0b, see
2056 L</oct>.) If EXPR is omitted, uses C<$_>.
2058 print hex '0xAf'; # prints '175'
2059 print hex 'aF'; # same
2061 Hex strings may only represent integers. Strings that would cause
2062 integer overflow trigger a warning.
2066 There is no builtin C<import> function. It is just an ordinary
2067 method (subroutine) defined (or inherited) by modules that wish to export
2068 names to another module. The C<use> function calls the C<import> method
2069 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2071 =item index STR,SUBSTR,POSITION
2073 =item index STR,SUBSTR
2075 The index function searches for one string within another, but without
2076 the wildcard-like behavior of a full regular-expression pattern match.
2077 It returns the position of the first occurrence of SUBSTR in STR at
2078 or after POSITION. If POSITION is omitted, starts searching from the
2079 beginning of the string. The return value is based at C<0> (or whatever
2080 you've set the C<$[> variable to--but don't do that). If the substring
2081 is not found, returns one less than the base, ordinarily C<-1>.
2087 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2088 You should not use this function for rounding: one because it truncates
2089 towards C<0>, and two because machine representations of floating point
2090 numbers can sometimes produce counterintuitive results. For example,
2091 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2092 because it's really more like -268.99999999999994315658 instead. Usually,
2093 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2094 functions will serve you better than will int().
2096 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2098 Implements the ioctl(2) function. You'll probably first have to say
2100 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2102 to get the correct function definitions. If F<ioctl.ph> doesn't
2103 exist or doesn't have the correct definitions you'll have to roll your
2104 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2105 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2106 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2107 written depending on the FUNCTION--a pointer to the string value of SCALAR
2108 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2109 has no string value but does have a numeric value, that value will be
2110 passed rather than a pointer to the string value. To guarantee this to be
2111 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2112 functions may be needed to manipulate the values of structures used by
2115 The return value of C<ioctl> (and C<fcntl>) is as follows:
2117 if OS returns: then Perl returns:
2119 0 string "0 but true"
2120 anything else that number
2122 Thus Perl returns true on success and false on failure, yet you can
2123 still easily determine the actual value returned by the operating
2126 $retval = ioctl(...) || -1;
2127 printf "System returned %d\n", $retval;
2129 The special string "C<0> but true" is exempt from B<-w> complaints
2130 about improper numeric conversions.
2132 Here's an example of setting a filehandle named C<REMOTE> to be
2133 non-blocking at the system level. You'll have to negotiate C<$|>
2134 on your own, though.
2136 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2138 $flags = fcntl(REMOTE, F_GETFL, 0)
2139 or die "Can't get flags for the socket: $!\n";
2141 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2142 or die "Can't set flags for the socket: $!\n";
2144 =item join EXPR,LIST
2146 Joins the separate strings of LIST into a single string with fields
2147 separated by the value of EXPR, and returns that new string. Example:
2149 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2151 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2152 first argument. Compare L</split>.
2156 Returns a list consisting of all the keys of the named hash. (In
2157 scalar context, returns the number of keys.) The keys are returned in
2158 an apparently random order. The actual random order is subject to
2159 change in future versions of perl, but it is guaranteed to be the same
2160 order as either the C<values> or C<each> function produces (given
2161 that the hash has not been modified). As a side effect, it resets
2164 Here is yet another way to print your environment:
2167 @values = values %ENV;
2169 print pop(@keys), '=', pop(@values), "\n";
2172 or how about sorted by key:
2174 foreach $key (sort(keys %ENV)) {
2175 print $key, '=', $ENV{$key}, "\n";
2178 To sort a hash by value, you'll need to use a C<sort> function.
2179 Here's a descending numeric sort of a hash by its values:
2181 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2182 printf "%4d %s\n", $hash{$key}, $key;
2185 As an lvalue C<keys> allows you to increase the number of hash buckets
2186 allocated for the given hash. This can gain you a measure of efficiency if
2187 you know the hash is going to get big. (This is similar to pre-extending
2188 an array by assigning a larger number to $#array.) If you say
2192 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2193 in fact, since it rounds up to the next power of two. These
2194 buckets will be retained even if you do C<%hash = ()>, use C<undef
2195 %hash> if you want to free the storage while C<%hash> is still in scope.
2196 You can't shrink the number of buckets allocated for the hash using
2197 C<keys> in this way (but you needn't worry about doing this by accident,
2198 as trying has no effect).
2200 See also C<each>, C<values> and C<sort>.
2202 =item kill SIGNAL, LIST
2204 Sends a signal to a list of processes. Returns the number of
2205 processes successfully signaled (which is not necessarily the
2206 same as the number actually killed).
2208 $cnt = kill 1, $child1, $child2;
2211 If SIGNAL is zero, no signal is sent to the process. This is a
2212 useful way to check that the process is alive and hasn't changed
2213 its UID. See L<perlport> for notes on the portability of this
2216 Unlike in the shell, if SIGNAL is negative, it kills
2217 process groups instead of processes. (On System V, a negative I<PROCESS>
2218 number will also kill process groups, but that's not portable.) That
2219 means you usually want to use positive not negative signals. You may also
2220 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2226 The C<last> command is like the C<break> statement in C (as used in
2227 loops); it immediately exits the loop in question. If the LABEL is
2228 omitted, the command refers to the innermost enclosing loop. The
2229 C<continue> block, if any, is not executed:
2231 LINE: while (<STDIN>) {
2232 last LINE if /^$/; # exit when done with header
2236 C<last> cannot be used to exit a block which returns a value such as
2237 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2238 a grep() or map() operation.
2240 Note that a block by itself is semantically identical to a loop
2241 that executes once. Thus C<last> can be used to effect an early
2242 exit out of such a block.
2244 See also L</continue> for an illustration of how C<last>, C<next>, and
2251 Returns an lowercased version of EXPR. This is the internal function
2252 implementing the C<\L> escape in double-quoted strings.
2253 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2256 If EXPR is omitted, uses C<$_>.
2262 Returns the value of EXPR with the first character lowercased. This is
2263 the internal function implementing the C<\l> escape in double-quoted strings.
2264 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2266 If EXPR is omitted, uses C<$_>.
2272 Returns the length in characters of the value of EXPR. If EXPR is
2273 omitted, returns length of C<$_>. Note that this cannot be used on
2274 an entire array or hash to find out how many elements these have.
2275 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2277 =item link OLDFILE,NEWFILE
2279 Creates a new filename linked to the old filename. Returns true for
2280 success, false otherwise.
2282 =item listen SOCKET,QUEUESIZE
2284 Does the same thing that the listen system call does. Returns true if
2285 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2289 You really probably want to be using C<my> instead, because C<local> isn't
2290 what most people think of as "local". See L<perlsub/"Private Variables
2291 via my()"> for details.
2293 A local modifies the listed variables to be local to the enclosing
2294 block, file, or eval. If more than one value is listed, the list must
2295 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2296 for details, including issues with tied arrays and hashes.
2298 =item localtime EXPR
2300 Converts a time as returned by the time function to a 9-element list
2301 with the time analyzed for the local time zone. Typically used as
2305 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2308 All list elements are numeric, and come straight out of a struct tm.
2309 In particular this means that $mon has the range C<0..11> and $wday
2310 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2311 number of years since 1900, that is, $year is C<123> in year 2023,
2312 and I<not> simply the last two digits of the year. If you assume it is,
2313 then you create non-Y2K-compliant programs--and you wouldn't want to do
2316 The proper way to get a complete 4-digit year is simply:
2320 And to get the last two digits of the year (e.g., '01' in 2001) do:
2322 $year = sprintf("%02d", $year % 100);
2324 If EXPR is omitted, uses the current time (C<localtime(time)>).
2326 In scalar context, returns the ctime(3) value:
2328 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2330 This scalar value is B<not> locale dependent, see L<perllocale>, but
2331 instead a Perl builtin. Also see the C<Time::Local> module
2332 (to convert the second, minutes, hours, ... back to seconds since the
2333 stroke of midnight the 1st of January 1970, the value returned by
2334 time()), and the strftime(3) and mktime(3) functions available via the
2335 POSIX module. To get somewhat similar but locale dependent date
2336 strings, set up your locale environment variables appropriately
2337 (please see L<perllocale>) and try for example:
2339 use POSIX qw(strftime);
2340 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2342 Note that the C<%a> and C<%b>, the short forms of the day of the week
2343 and the month of the year, may not necessarily be three characters wide.
2349 This function places an advisory lock on a variable, subroutine,
2350 or referenced object contained in I<THING> until the lock goes out
2351 of scope. This is a built-in function only if your version of Perl
2352 was built with threading enabled, and if you've said C<use Threads>.
2353 Otherwise a user-defined function by this name will be called. See
2360 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2361 returns log of C<$_>. To get the log of another base, use basic algebra:
2362 The base-N log of a number is equal to the natural log of that number
2363 divided by the natural log of N. For example:
2367 return log($n)/log(10);
2370 See also L</exp> for the inverse operation.
2372 =item lstat FILEHANDLE
2378 Does the same thing as the C<stat> function (including setting the
2379 special C<_> filehandle) but stats a symbolic link instead of the file
2380 the symbolic link points to. If symbolic links are unimplemented on
2381 your system, a normal C<stat> is done.
2383 If EXPR is omitted, stats C<$_>.
2387 The match operator. See L<perlop>.
2389 =item map BLOCK LIST
2393 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2394 C<$_> to each element) and returns the list value composed of the
2395 results of each such evaluation. In scalar context, returns the
2396 total number of elements so generated. Evaluates BLOCK or EXPR in
2397 list context, so each element of LIST may produce zero, one, or
2398 more elements in the returned value.
2400 @chars = map(chr, @nums);
2402 translates a list of numbers to the corresponding characters. And
2404 %hash = map { getkey($_) => $_ } @array;
2406 is just a funny way to write
2409 foreach $_ (@array) {
2410 $hash{getkey($_)} = $_;
2413 Note that, because C<$_> is a reference into the list value, it can
2414 be used to modify the elements of the array. While this is useful and
2415 supported, it can cause bizarre results if the LIST is not a named array.
2416 Using a regular C<foreach> loop for this purpose would be clearer in
2417 most cases. See also L</grep> for an array composed of those items of
2418 the original list for which the BLOCK or EXPR evaluates to true.
2420 =item mkdir FILENAME,MASK
2422 Creates the directory specified by FILENAME, with permissions
2423 specified by MASK (as modified by C<umask>). If it succeeds it
2424 returns true, otherwise it returns false and sets C<$!> (errno).
2426 In general, it is better to create directories with permissive MASK,
2427 and let the user modify that with their C<umask>, than it is to supply
2428 a restrictive MASK and give the user no way to be more permissive.
2429 The exceptions to this rule are when the file or directory should be
2430 kept private (mail files, for instance). The perlfunc(1) entry on
2431 C<umask> discusses the choice of MASK in more detail.
2433 =item msgctl ID,CMD,ARG
2435 Calls the System V IPC function msgctl(2). You'll probably have to say
2439 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2440 then ARG must be a variable which will hold the returned C<msqid_ds>
2441 structure. Returns like C<ioctl>: the undefined value for error,
2442 C<"0 but true"> for zero, or the actual return value otherwise. See also
2443 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2445 =item msgget KEY,FLAGS
2447 Calls the System V IPC function msgget(2). Returns the message queue
2448 id, or the undefined value if there is an error. See also C<IPC::SysV>
2449 and C<IPC::Msg> documentation.
2451 =item msgsnd ID,MSG,FLAGS
2453 Calls the System V IPC function msgsnd to send the message MSG to the
2454 message queue ID. MSG must begin with the long integer message type,
2455 which may be created with C<pack("l", $type)>. Returns true if
2456 successful, or false if there is an error. See also C<IPC::SysV>
2457 and C<IPC::SysV::Msg> documentation.
2459 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2461 Calls the System V IPC function msgrcv to receive a message from
2462 message queue ID into variable VAR with a maximum message size of
2463 SIZE. Note that if a message is received, the message type will be
2464 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2465 size of the message type. Returns true if successful, or false if
2466 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2470 =item my EXPR : ATTRIBUTES
2472 A C<my> declares the listed variables to be local (lexically) to the
2473 enclosing block, file, or C<eval>. If
2474 more than one value is listed, the list must be placed in parentheses. See
2475 L<perlsub/"Private Variables via my()"> for details.
2481 The C<next> command is like the C<continue> statement in C; it starts
2482 the next iteration of the loop:
2484 LINE: while (<STDIN>) {
2485 next LINE if /^#/; # discard comments
2489 Note that if there were a C<continue> block on the above, it would get
2490 executed even on discarded lines. If the LABEL is omitted, the command
2491 refers to the innermost enclosing loop.
2493 C<next> cannot be used to exit a block which returns a value such as
2494 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2495 a grep() or map() operation.
2497 Note that a block by itself is semantically identical to a loop
2498 that executes once. Thus C<next> will exit such a block early.
2500 See also L</continue> for an illustration of how C<last>, C<next>, and
2503 =item no Module LIST
2505 See the L</use> function, which C<no> is the opposite of.
2511 Interprets EXPR as an octal string and returns the corresponding
2512 value. (If EXPR happens to start off with C<0x>, interprets it as a
2513 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2514 binary string.) The following will handle decimal, binary, octal, and
2515 hex in the standard Perl or C notation:
2517 $val = oct($val) if $val =~ /^0/;
2519 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2520 in octal), use sprintf() or printf():
2522 $perms = (stat("filename"))[2] & 07777;
2523 $oct_perms = sprintf "%lo", $perms;
2525 The oct() function is commonly used when a string such as C<644> needs
2526 to be converted into a file mode, for example. (Although perl will
2527 automatically convert strings into numbers as needed, this automatic
2528 conversion assumes base 10.)
2530 =item open FILEHANDLE,MODE,EXPR
2532 =item open FILEHANDLE,EXPR
2534 =item open FILEHANDLE
2536 Opens the file whose filename is given by EXPR, and associates it with
2537 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2538 name of the real filehandle wanted. If EXPR is omitted, the scalar
2539 variable of the same name as the FILEHANDLE contains the filename.
2540 (Note that lexical variables--those declared with C<my>--will not work
2541 for this purpose; so if you're using C<my>, specify EXPR in your call
2542 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2545 If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2546 If MODE is C<'E<gt>'>, the file is truncated and opened for
2547 output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
2548 the file is opened for appending, again being created if necessary.
2549 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2550 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2551 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2552 file first. You can't usually use either read-write mode for updating
2553 textfiles, since they have variable length records. See the B<-i>
2554 switch in L<perlrun> for a better approach. The file is created with
2555 permissions of C<0666> modified by the process' C<umask> value.
2557 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2558 C<'w+'>, C<'a'>, and C<'a+'>.
2560 In the 2-arguments (and 1-argument) form of the call the mode and
2561 filename should be concatenated (in this order), possibly separated by
2562 spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2564 If the filename begins with C<'|'>, the filename is interpreted as a
2565 command to which output is to be piped, and if the filename ends with a
2566 C<'|'>, the filename is interpreted as a command which pipes output to
2567 us. See L<perlipc/"Using open() for IPC">
2568 for more examples of this. (You are not allowed to C<open> to a command
2569 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2570 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2572 If MODE is C<'|-'>, the filename is interpreted as a
2573 command to which output is to be piped, and if MODE is
2574 C<'-|'>, the filename is interpreted as a command which pipes output to
2575 us. In the 2-arguments (and 1-argument) form one should replace dash
2576 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2577 for more examples of this. (You are not allowed to C<open> to a command
2578 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2579 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2581 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2582 and opening C<'E<gt>-'> opens STDOUT.
2585 nonzero upon success, the undefined value otherwise. If the C<open>
2586 involved a pipe, the return value happens to be the pid of the
2589 If you're unfortunate enough to be running Perl on a system that
2590 distinguishes between text files and binary files (modern operating
2591 systems don't care), then you should check out L</binmode> for tips for
2592 dealing with this. The key distinction between systems that need C<binmode>
2593 and those that don't is their text file formats. Systems like Unix, MacOS, and
2594 Plan9, which delimit lines with a single character, and which encode that
2595 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2597 When opening a file, it's usually a bad idea to continue normal execution
2598 if the request failed, so C<open> is frequently used in connection with
2599 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2600 where you want to make a nicely formatted error message (but there are
2601 modules that can help with that problem)) you should always check
2602 the return value from opening a file. The infrequent exception is when
2603 working with an unopened filehandle is actually what you want to do.
2608 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2609 while (<ARTICLE>) {...
2611 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2612 # if the open fails, output is discarded
2614 open(DBASE, '+<', 'dbase.mine') # open for update
2615 or die "Can't open 'dbase.mine' for update: $!";
2617 open(DBASE, '+<dbase.mine') # ditto
2618 or die "Can't open 'dbase.mine' for update: $!";
2620 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2621 or die "Can't start caesar: $!";
2623 open(ARTICLE, "caesar <$article |") # ditto
2624 or die "Can't start caesar: $!";
2626 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2627 or die "Can't start sort: $!";
2629 # process argument list of files along with any includes
2631 foreach $file (@ARGV) {
2632 process($file, 'fh00');
2636 my($filename, $input) = @_;
2637 $input++; # this is a string increment
2638 unless (open($input, $filename)) {
2639 print STDERR "Can't open $filename: $!\n";
2644 while (<$input>) { # note use of indirection
2645 if (/^#include "(.*)"/) {
2646 process($1, $input);
2653 You may also, in the Bourne shell tradition, specify an EXPR beginning
2654 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2655 name of a filehandle (or file descriptor, if numeric) to be
2656 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2657 C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
2658 mode you specify should match the mode of the original filehandle.
2659 (Duping a filehandle does not take into account any existing contents of
2660 stdio buffers.) Duping file handles is not yet supported for 3-argument
2663 Here is a script that saves, redirects, and restores STDOUT and
2667 open(OLDOUT, ">&STDOUT");
2668 open(OLDERR, ">&STDERR");
2670 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2671 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2673 select(STDERR); $| = 1; # make unbuffered
2674 select(STDOUT); $| = 1; # make unbuffered
2676 print STDOUT "stdout 1\n"; # this works for
2677 print STDERR "stderr 1\n"; # subprocesses too
2682 open(STDOUT, ">&OLDOUT");
2683 open(STDERR, ">&OLDERR");
2685 print STDOUT "stdout 2\n";
2686 print STDERR "stderr 2\n";
2688 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2689 equivalent of C's C<fdopen> of that file descriptor; this is more
2690 parsimonious of file descriptors. For example:
2692 open(FILEHANDLE, "<&=$fd")
2694 Note that this feature depends on the fdopen() C library function.
2695 On many UNIX systems, fdopen() is known to fail when file descriptors
2696 exceed a certain value, typically 255. If you need more file
2697 descriptors than that, consider rebuilding Perl to use the C<sfio>
2700 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2701 with 2-arguments (or 1-argument) form of open(), then
2702 there is an implicit fork done, and the return value of open is the pid
2703 of the child within the parent process, and C<0> within the child
2704 process. (Use C<defined($pid)> to determine whether the open was successful.)
2705 The filehandle behaves normally for the parent, but i/o to that
2706 filehandle is piped from/to the STDOUT/STDIN of the child process.
2707 In the child process the filehandle isn't opened--i/o happens from/to
2708 the new STDOUT or STDIN. Typically this is used like the normal
2709 piped open when you want to exercise more control over just how the
2710 pipe command gets executed, such as when you are running setuid, and
2711 don't want to have to scan shell commands for metacharacters.
2712 The following triples are more or less equivalent:
2714 open(FOO, "|tr '[a-z]' '[A-Z]'");
2715 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2716 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2718 open(FOO, "cat -n '$file'|");
2719 open(FOO, '-|', "cat -n '$file'");
2720 open(FOO, '-|') || exec 'cat', '-n', $file;
2722 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2724 NOTE: On any operation that may do a fork, all files opened for output
2725 are flushed before the fork is attempted. On systems that support a
2726 close-on-exec flag on files, the flag will be set for the newly opened
2727 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2729 Closing any piped filehandle causes the parent process to wait for the
2730 child to finish, and returns the status value in C<$?>.
2732 The filename passed to 2-argument (or 1-argument) form of open()
2733 will have leading and trailing
2734 whitespace deleted, and the normal redirection characters
2735 honored. This property, known as "magic open",
2736 can often be used to good effect. A user could specify a filename of
2737 F<"rsh cat file |">, or you could change certain filenames as needed:
2739 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2740 open(FH, $filename) or die "Can't open $filename: $!";
2742 Use 3-argument form to open a file with arbitrary weird characters in it,
2744 open(FOO, '<', $file);
2746 otherwise it's necessary to protect any leading and trailing whitespace:
2748 $file =~ s#^(\s)#./$1#;
2749 open(FOO, "< $file\0");
2751 (this may not work on some bizzare filesystems). One should
2752 conscientiously choose between the the I<magic> and 3-arguments form
2757 will allow the user to specify an argument of the form C<"rsh cat file |">,
2758 but will not work on a filename which happens to have a trailing space, while
2760 open IN, '<', $ARGV[0];
2762 will have exactly the opposite restrictions.
2764 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2765 should use the C<sysopen> function, which involves no such magic (but
2766 may use subtly different filemodes than Perl open(), which is mapped
2767 to C fopen()). This is
2768 another way to protect your filenames from interpretation. For example:
2771 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2772 or die "sysopen $path: $!";
2773 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2774 print HANDLE "stuff $$\n");
2776 print "File contains: ", <HANDLE>;
2778 Using the constructor from the C<IO::Handle> package (or one of its
2779 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2780 filehandles that have the scope of whatever variables hold references to
2781 them, and automatically close whenever and however you leave that scope:
2785 sub read_myfile_munged {
2787 my $handle = new IO::File;
2788 open($handle, "myfile") or die "myfile: $!";
2790 or return (); # Automatically closed here.
2791 mung $first or die "mung failed"; # Or here.
2792 return $first, <$handle> if $ALL; # Or here.
2796 See L</seek> for some details about mixing reading and writing.
2798 =item opendir DIRHANDLE,EXPR
2800 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2801 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2802 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2808 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2809 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2810 See L<utf8> for more about Unicode.
2814 An C<our> declares the listed variables to be valid globals within
2815 the enclosing block, file, or C<eval>. That is, it has the same
2816 scoping rules as a "my" declaration, but does not create a local
2817 variable. If more than one value is listed, the list must be placed
2818 in parentheses. The C<our> declaration has no semantic effect unless
2819 "use strict vars" is in effect, in which case it lets you use the
2820 declared global variable without qualifying it with a package name.
2821 (But only within the lexical scope of the C<our> declaration. In this
2822 it differs from "use vars", which is package scoped.)
2824 An C<our> declaration declares a global variable that will be visible
2825 across its entire lexical scope, even across package boundaries. The
2826 package in which the variable is entered is determined at the point
2827 of the declaration, not at the point of use. This means the following
2831 our $bar; # declares $Foo::bar for rest of lexical scope
2835 print $bar; # prints 20
2837 Multiple C<our> declarations in the same lexical scope are allowed
2838 if they are in different packages. If they happened to be in the same
2839 package, Perl will emit warnings if you have asked for them.
2843 our $bar; # declares $Foo::bar for rest of lexical scope
2847 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2848 print $bar; # prints 30
2850 our $bar; # emits warning
2852 =item pack TEMPLATE,LIST
2854 Takes a LIST of values and converts it into a string using the rules
2855 given by the TEMPLATE. The resulting string is the concatenation of
2856 the converted values. Typically, each converted value looks
2857 like its machine-level representation. For example, on 32-bit machines
2858 a converted integer may be represented by a sequence of 4 bytes.
2861 sequence of characters that give the order and type of values, as
2864 a A string with arbitrary binary data, will be null padded.
2865 A An ascii string, will be space padded.
2866 Z A null terminated (asciz) string, will be null padded.
2868 b A bit string (ascending bit order inside each byte, like vec()).
2869 B A bit string (descending bit order inside each byte).
2870 h A hex string (low nybble first).
2871 H A hex string (high nybble first).
2873 c A signed char value.
2874 C An unsigned char value. Only does bytes. See U for Unicode.
2876 s A signed short value.
2877 S An unsigned short value.
2878 (This 'short' is _exactly_ 16 bits, which may differ from
2879 what a local C compiler calls 'short'. If you want
2880 native-length shorts, use the '!' suffix.)
2882 i A signed integer value.
2883 I An unsigned integer value.
2884 (This 'integer' is _at_least_ 32 bits wide. Its exact
2885 size depends on what a local C compiler calls 'int',
2886 and may even be larger than the 'long' described in
2889 l A signed long value.
2890 L An unsigned long value.
2891 (This 'long' is _exactly_ 32 bits, which may differ from
2892 what a local C compiler calls 'long'. If you want
2893 native-length longs, use the '!' suffix.)
2895 n An unsigned short in "network" (big-endian) order.
2896 N An unsigned long in "network" (big-endian) order.
2897 v An unsigned short in "VAX" (little-endian) order.
2898 V An unsigned long in "VAX" (little-endian) order.
2899 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2900 _exactly_ 32 bits, respectively.)
2902 q A signed quad (64-bit) value.
2903 Q An unsigned quad value.
2904 (Quads are available only if your system supports 64-bit
2905 integer values _and_ if Perl has been compiled to support those.
2906 Causes a fatal error otherwise.)
2908 f A single-precision float in the native format.
2909 d A double-precision float in the native format.
2911 p A pointer to a null-terminated string.
2912 P A pointer to a structure (fixed-length string).
2914 u A uuencoded string.
2915 U A Unicode character number. Encodes to UTF-8 internally.
2916 Works even if C<use utf8> is not in effect.
2918 w A BER compressed integer. Its bytes represent an unsigned
2919 integer in base 128, most significant digit first, with as
2920 few digits as possible. Bit eight (the high bit) is set
2921 on each byte except the last.
2925 @ Null fill to absolute position.
2927 The following rules apply:
2933 Each letter may optionally be followed by a number giving a repeat
2934 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2935 C<H>, and C<P> the pack function will gobble up that many values from
2936 the LIST. A C<*> for the repeat count means to use however many items are
2937 left, except for C<@>, C<x>, C<X>, where it is equivalent
2938 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2941 When used with C<Z>, C<*> results in the addition of a trailing null
2942 byte (so the packed result will be one longer than the byte C<length>
2945 The repeat count for C<u> is interpreted as the maximal number of bytes
2946 to encode per line of output, with 0 and 1 replaced by 45.
2950 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
2951 string of length count, padding with nulls or spaces as necessary. When
2952 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2953 after the first null, and C<a> returns data verbatim. When packing,
2954 C<a>, and C<Z> are equivalent.
2956 If the value-to-pack is too long, it is truncated. If too long and an
2957 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2958 by a null byte. Thus C<Z> always packs a trailing null byte under
2963 Likewise, the C<b> and C<B> fields pack a string that many bits long.
2964 Each byte of the input field of pack() generates 1 bit of the result.
2965 Each result bit is based on the least-significant bit of the corresponding
2966 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2967 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2969 Starting from the beginning of the input string of pack(), each 8-tuple
2970 of bytes is converted to 1 byte of output. With format C<b>
2971 the first byte of the 8-tuple determines the least-significant bit of a
2972 byte, and with format C<B> it determines the most-significant bit of
2975 If the length of the input string is not exactly divisible by 8, the
2976 remainder is packed as if the input string were padded by null bytes
2977 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2979 If the input string of pack() is longer than needed, extra bytes are ignored.
2980 A C<*> for the repeat count of pack() means to use all the bytes of
2981 the input field. On unpack()ing the bits are converted to a string
2982 of C<"0">s and C<"1">s.
2986 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
2987 representable as hexadecimal digits, 0-9a-f) long.
2989 Each byte of the input field of pack() generates 4 bits of the result.
2990 For non-alphabetical bytes the result is based on the 4 least-significant
2991 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
2992 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
2993 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
2994 is compatible with the usual hexadecimal digits, so that C<"a"> and
2995 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
2996 C<"g".."z"> and C<"G".."Z"> is not well-defined.
2998 Starting from the beginning of the input string of pack(), each pair
2999 of bytes is converted to 1 byte of output. With format C<h> the
3000 first byte of the pair determines the least-significant nybble of the
3001 output byte, and with format C<H> it determines the most-significant
3004 If the length of the input string is not even, it behaves as if padded
3005 by a null byte at the end. Similarly, during unpack()ing the "extra"
3006 nybbles are ignored.
3008 If the input string of pack() is longer than needed, extra bytes are ignored.
3009 A C<*> for the repeat count of pack() means to use all the bytes of
3010 the input field. On unpack()ing the bits are converted to a string
3011 of hexadecimal digits.
3015 The C<p> type packs a pointer to a null-terminated string. You are
3016 responsible for ensuring the string is not a temporary value (which can
3017 potentially get deallocated before you get around to using the packed result).
3018 The C<P> type packs a pointer to a structure of the size indicated by the
3019 length. A NULL pointer is created if the corresponding value for C<p> or
3020 C<P> is C<undef>, similarly for unpack().
3024 The C</> template character allows packing and unpacking of strings where
3025 the packed structure contains a byte count followed by the string itself.
3026 You write I<length-item>C</>I<string-item>.
3028 The I<length-item> can be any C<pack> template letter,
3029 and describes how the length value is packed.
3030 The ones likely to be of most use are integer-packing ones like
3031 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3032 and C<N> (for Sun XDR).
3034 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3035 For C<unpack> the length of the string is obtained from the I<length-item>,
3036 but if you put in the '*' it will be ignored.
3038 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3039 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3040 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3042 The I<length-item> is not returned explicitly from C<unpack>.
3044 Adding a count to the I<length-item> letter is unlikely to do anything
3045 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3046 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3047 which Perl does not regard as legal in numeric strings.
3051 The integer types C<s>, C<S>, C<l>, and C<L> may be
3052 immediately followed by a C<!> suffix to signify native shorts or
3053 longs--as you can see from above for example a bare C<l> does mean
3054 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3055 may be larger. This is an issue mainly in 64-bit platforms. You can
3056 see whether using C<!> makes any difference by
3058 print length(pack("s")), " ", length(pack("s!")), "\n";
3059 print length(pack("l")), " ", length(pack("l!")), "\n";
3061 C<i!> and C<I!> also work but only because of completeness;
3062 they are identical to C<i> and C<I>.
3064 The actual sizes (in bytes) of native shorts, ints, longs, and long
3065 longs on the platform where Perl was built are also available via
3069 print $Config{shortsize}, "\n";
3070 print $Config{intsize}, "\n";
3071 print $Config{longsize}, "\n";
3072 print $Config{longlongsize}, "\n";
3074 (The C<$Config{longlongsize}> will be undefine if your system does
3075 not support long longs.)
3079 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3080 are inherently non-portable between processors and operating systems
3081 because they obey the native byteorder and endianness. For example a
3082 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3083 (arranged in and handled by the CPU registers) into bytes as
3085 0x12 0x34 0x56 0x78 # little-endian
3086 0x78 0x56 0x34 0x12 # big-endian
3088 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3089 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3090 Power, and Cray are big-endian. MIPS can be either: Digital used it
3091 in little-endian mode; SGI uses it in big-endian mode.
3093 The names `big-endian' and `little-endian' are comic references to
3094 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3095 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3096 the egg-eating habits of the Lilliputians.
3098 Some systems may have even weirder byte orders such as
3103 You can see your system's preference with
3105 print join(" ", map { sprintf "%#02x", $_ }
3106 unpack("C*",pack("L",0x12345678))), "\n";
3108 The byteorder on the platform where Perl was built is also available
3112 print $Config{byteorder}, "\n";
3114 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3115 and C<'87654321'> are big-endian.
3117 If you want portable packed integers use the formats C<n>, C<N>,
3118 C<v>, and C<V>, their byte endianness and size is known.
3119 See also L<perlport>.
3123 Real numbers (floats and doubles) are in the native machine format only;
3124 due to the multiplicity of floating formats around, and the lack of a
3125 standard "network" representation, no facility for interchange has been
3126 made. This means that packed floating point data written on one machine
3127 may not be readable on another - even if both use IEEE floating point
3128 arithmetic (as the endian-ness of the memory representation is not part
3129 of the IEEE spec). See also L<perlport>.
3131 Note that Perl uses doubles internally for all numeric calculation, and
3132 converting from double into float and thence back to double again will
3133 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3138 You must yourself do any alignment or padding by inserting for example
3139 enough C<'x'>es while packing. There is no way to pack() and unpack()
3140 could know where the bytes are going to or coming from. Therefore
3141 C<pack> (and C<unpack>) handle their output and input as flat
3146 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3150 If TEMPLATE requires more arguments to pack() than actually given, pack()
3151 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3152 to pack() than actually given, extra arguments are ignored.
3158 $foo = pack("CCCC",65,66,67,68);
3160 $foo = pack("C4",65,66,67,68);
3162 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3163 # same thing with Unicode circled letters
3165 $foo = pack("ccxxcc",65,66,67,68);
3168 # note: the above examples featuring "C" and "c" are true
3169 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3170 # and UTF-8. In EBCDIC the first example would be
3171 # $foo = pack("CCCC",193,194,195,196);
3173 $foo = pack("s2",1,2);
3174 # "\1\0\2\0" on little-endian
3175 # "\0\1\0\2" on big-endian
3177 $foo = pack("a4","abcd","x","y","z");
3180 $foo = pack("aaaa","abcd","x","y","z");
3183 $foo = pack("a14","abcdefg");
3184 # "abcdefg\0\0\0\0\0\0\0"
3186 $foo = pack("i9pl", gmtime);
3187 # a real struct tm (on my system anyway)
3189 $utmp_template = "Z8 Z8 Z16 L";
3190 $utmp = pack($utmp_template, @utmp1);
3191 # a struct utmp (BSDish)
3193 @utmp2 = unpack($utmp_template, $utmp);
3194 # "@utmp1" eq "@utmp2"
3197 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3200 $foo = pack('sx2l', 12, 34);
3201 # short 12, two zero bytes padding, long 34
3202 $bar = pack('s@4l', 12, 34);
3203 # short 12, zero fill to position 4, long 34
3206 The same template may generally also be used in unpack().
3210 =item package NAMESPACE
3212 Declares the compilation unit as being in the given namespace. The scope
3213 of the package declaration is from the declaration itself through the end
3214 of the enclosing block, file, or eval (the same as the C<my> operator).
3215 All further unqualified dynamic identifiers will be in this namespace.
3216 A package statement affects only dynamic variables--including those
3217 you've used C<local> on--but I<not> lexical variables, which are created
3218 with C<my>. Typically it would be the first declaration in a file to
3219 be included by the C<require> or C<use> operator. You can switch into a
3220 package in more than one place; it merely influences which symbol table
3221 is used by the compiler for the rest of that block. You can refer to
3222 variables and filehandles in other packages by prefixing the identifier
3223 with the package name and a double colon: C<$Package::Variable>.
3224 If the package name is null, the C<main> package as assumed. That is,
3225 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3226 still seen in older code).
3228 If NAMESPACE is omitted, then there is no current package, and all
3229 identifiers must be fully qualified or lexicals. This is stricter
3230 than C<use strict>, since it also extends to function names.
3232 See L<perlmod/"Packages"> for more information about packages, modules,
3233 and classes. See L<perlsub> for other scoping issues.
3235 =item pipe READHANDLE,WRITEHANDLE
3237 Opens a pair of connected pipes like the corresponding system call.
3238 Note that if you set up a loop of piped processes, deadlock can occur
3239 unless you are very careful. In addition, note that Perl's pipes use
3240 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3241 after each command, depending on the application.
3243 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3244 for examples of such things.
3246 On systems that support a close-on-exec flag on files, the flag will be set
3247 for the newly opened file descriptors as determined by the value of $^F.
3254 Pops and returns the last value of the array, shortening the array by
3255 one element. Has an effect similar to
3259 If there are no elements in the array, returns the undefined value
3260 (although this may happen at other times as well). If ARRAY is
3261 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3262 array in subroutines, just like C<shift>.
3268 Returns the offset of where the last C<m//g> search left off for the variable
3269 is in question (C<$_> is used when the variable is not specified). May be
3270 modified to change that offset. Such modification will also influence
3271 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3274 =item print FILEHANDLE LIST
3280 Prints a string or a list of strings. Returns true if successful.
3281 FILEHANDLE may be a scalar variable name, in which case the variable
3282 contains the name of or a reference to the filehandle, thus introducing
3283 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3284 the next token is a term, it may be misinterpreted as an operator
3285 unless you interpose a C<+> or put parentheses around the arguments.)
3286 If FILEHANDLE is omitted, prints by default to standard output (or
3287 to the last selected output channel--see L</select>). If LIST is
3288 also omitted, prints C<$_> to the currently selected output channel.
3289 To set the default output channel to something other than STDOUT
3290 use the select operation. The current value of C<$,> (if any) is
3291 printed between each LIST item. The current value of C<$\> (if
3292 any) is printed after the entire LIST has been printed. Because
3293 print takes a LIST, anything in the LIST is evaluated in list
3294 context, and any subroutine that you call will have one or more of
3295 its expressions evaluated in list context. Also be careful not to
3296 follow the print keyword with a left parenthesis unless you want
3297 the corresponding right parenthesis to terminate the arguments to
3298 the print--interpose a C<+> or put parentheses around all the
3301 Note that if you're storing FILEHANDLES in an array or other expression,
3302 you will have to use a block returning its value instead:
3304 print { $files[$i] } "stuff\n";
3305 print { $OK ? STDOUT : STDERR } "stuff\n";
3307 =item printf FILEHANDLE FORMAT, LIST
3309 =item printf FORMAT, LIST
3311 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3312 (the output record separator) is not appended. The first argument
3313 of the list will be interpreted as the C<printf> format. If C<use locale> is
3314 in effect, the character used for the decimal point in formatted real numbers
3315 is affected by the LC_NUMERIC locale. See L<perllocale>.
3317 Don't fall into the trap of using a C<printf> when a simple
3318 C<print> would do. The C<print> is more efficient and less
3321 =item prototype FUNCTION
3323 Returns the prototype of a function as a string (or C<undef> if the
3324 function has no prototype). FUNCTION is a reference to, or the name of,
3325 the function whose prototype you want to retrieve.
3327 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3328 name for Perl builtin. If the builtin is not I<overridable> (such as
3329 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3330 C<system>) returns C<undef> because the builtin does not really behave
3331 like a Perl function. Otherwise, the string describing the equivalent
3332 prototype is returned.
3334 =item push ARRAY,LIST
3336 Treats ARRAY as a stack, and pushes the values of LIST
3337 onto the end of ARRAY. The length of ARRAY increases by the length of
3338 LIST. Has the same effect as
3341 $ARRAY[++$#ARRAY] = $value;
3344 but is more efficient. Returns the new number of elements in the array.
3356 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3358 =item quotemeta EXPR
3362 Returns the value of EXPR with all non-alphanumeric
3363 characters backslashed. (That is, all characters not matching
3364 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3365 returned string, regardless of any locale settings.)
3366 This is the internal function implementing
3367 the C<\Q> escape in double-quoted strings.
3369 If EXPR is omitted, uses C<$_>.
3375 Returns a random fractional number greater than or equal to C<0> and less
3376 than the value of EXPR. (EXPR should be positive.) If EXPR is
3377 omitted, the value C<1> is used. Automatically calls C<srand> unless
3378 C<srand> has already been called. See also C<srand>.
3380 (Note: If your rand function consistently returns numbers that are too
3381 large or too small, then your version of Perl was probably compiled
3382 with the wrong number of RANDBITS.)
3384 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3386 =item read FILEHANDLE,SCALAR,LENGTH
3388 Attempts to read LENGTH bytes of data into variable SCALAR from the
3389 specified FILEHANDLE. Returns the number of bytes actually read,
3390 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3391 or shrunk to the length actually read. An OFFSET may be specified to
3392 place the read data at some other place than the beginning of the
3393 string. This call is actually implemented in terms of stdio's fread(3)
3394 call. To get a true read(2) system call, see C<sysread>.
3396 =item readdir DIRHANDLE
3398 Returns the next directory entry for a directory opened by C<opendir>.
3399 If used in list context, returns all the rest of the entries in the
3400 directory. If there are no more entries, returns an undefined value in
3401 scalar context or a null list in list context.
3403 If you're planning to filetest the return values out of a C<readdir>, you'd
3404 better prepend the directory in question. Otherwise, because we didn't
3405 C<chdir> there, it would have been testing the wrong file.
3407 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3408 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3413 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3414 context, each call reads and returns the next line, until end-of-file is
3415 reached, whereupon the subsequent call returns undef. In list context,
3416 reads until end-of-file is reached and returns a list of lines. Note that
3417 the notion of "line" used here is however you may have defined it
3418 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3420 When C<$/> is set to C<undef>, when readline() is in scalar
3421 context (i.e. file slurp mode), and when an empty file is read, it
3422 returns C<''> the first time, followed by C<undef> subsequently.
3424 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3425 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3426 operator is discussed in more detail in L<perlop/"I/O Operators">.
3429 $line = readline(*STDIN); # same thing
3435 Returns the value of a symbolic link, if symbolic links are
3436 implemented. If not, gives a fatal error. If there is some system
3437 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3438 omitted, uses C<$_>.
3442 EXPR is executed as a system command.
3443 The collected standard output of the command is returned.
3444 In scalar context, it comes back as a single (potentially
3445 multi-line) string. In list context, returns a list of lines
3446 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3447 This is the internal function implementing the C<qx/EXPR/>
3448 operator, but you can use it directly. The C<qx/EXPR/>
3449 operator is discussed in more detail in L<perlop/"I/O Operators">.
3451 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3453 Receives a message on a socket. Attempts to receive LENGTH bytes of
3454 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3455 will be grown or shrunk to the length actually read. Takes the same
3456 flags as the system call of the same name. Returns the address of the
3457 sender if SOCKET's protocol supports this; returns an empty string
3458 otherwise. If there's an error, returns the undefined value. This call
3459 is actually implemented in terms of recvfrom(2) system call. See
3460 L<perlipc/"UDP: Message Passing"> for examples.
3466 The C<redo> command restarts the loop block without evaluating the
3467 conditional again. The C<continue> block, if any, is not executed. If
3468 the LABEL is omitted, the command refers to the innermost enclosing
3469 loop. This command is normally used by programs that want to lie to
3470 themselves about what was just input:
3472 # a simpleminded Pascal comment stripper
3473 # (warning: assumes no { or } in strings)
3474 LINE: while (<STDIN>) {
3475 while (s|({.*}.*){.*}|$1 |) {}
3480 if (/}/) { # end of comment?
3489 C<redo> cannot be used to retry a block which returns a value such as
3490 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3491 a grep() or map() operation.
3493 Note that a block by itself is semantically identical to a loop
3494 that executes once. Thus C<redo> inside such a block will effectively
3495 turn it into a looping construct.
3497 See also L</continue> for an illustration of how C<last>, C<next>, and
3504 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3505 is not specified, C<$_> will be used. The value returned depends on the
3506 type of thing the reference is a reference to.
3507 Builtin types include:
3517 If the referenced object has been blessed into a package, then that package
3518 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3520 if (ref($r) eq "HASH") {
3521 print "r is a reference to a hash.\n";
3524 print "r is not a reference at all.\n";
3526 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3527 print "r is a reference to something that isa hash.\n";
3530 See also L<perlref>.
3532 =item rename OLDNAME,NEWNAME
3534 Changes the name of a file; an existing file NEWNAME will be
3535 clobbered. Returns true for success, false otherwise.
3537 Behavior of this function varies wildly depending on your system
3538 implementation. For example, it will usually not work across file system
3539 boundaries, even though the system I<mv> command sometimes compensates
3540 for this. Other restrictions include whether it works on directories,
3541 open files, or pre-existing files. Check L<perlport> and either the
3542 rename(2) manpage or equivalent system documentation for details.
3544 =item require VERSION
3550 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3551 supplied. If a version number or tuple is specified, or if EXPR is
3552 numeric, demands that the current version of Perl
3553 (C<$^V> or C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3555 Otherwise, demands that a library file be included if it hasn't already
3556 been included. The file is included via the do-FILE mechanism, which is
3557 essentially just a variety of C<eval>. Has semantics similar to the following
3562 return 1 if $INC{$filename};
3563 my($realfilename,$result);
3565 foreach $prefix (@INC) {
3566 $realfilename = "$prefix/$filename";
3567 if (-f $realfilename) {
3568 $INC{$filename} = $realfilename;
3569 $result = do $realfilename;
3573 die "Can't find $filename in \@INC";
3575 delete $INC{$filename} if $@ || !$result;
3577 die "$filename did not return true value" unless $result;
3581 Note that the file will not be included twice under the same specified
3582 name. The file must return true as the last statement to indicate
3583 successful execution of any initialization code, so it's customary to
3584 end such a file with C<1;> unless you're sure it'll return true
3585 otherwise. But it's better just to put the C<1;>, in case you add more
3588 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3589 replaces "F<::>" with "F</>" in the filename for you,
3590 to make it easy to load standard modules. This form of loading of
3591 modules does not risk altering your namespace.
3593 In other words, if you try this:
3595 require Foo::Bar; # a splendid bareword
3597 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3598 directories specified in the C<@INC> array.
3600 But if you try this:
3602 $class = 'Foo::Bar';
3603 require $class; # $class is not a bareword
3605 require "Foo::Bar"; # not a bareword because of the ""
3607 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3608 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3610 eval "require $class";
3612 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3618 Generally used in a C<continue> block at the end of a loop to clear
3619 variables and reset C<??> searches so that they work again. The
3620 expression is interpreted as a list of single characters (hyphens
3621 allowed for ranges). All variables and arrays beginning with one of
3622 those letters are reset to their pristine state. If the expression is
3623 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3624 only variables or searches in the current package. Always returns
3627 reset 'X'; # reset all X variables
3628 reset 'a-z'; # reset lower case variables
3629 reset; # just reset ?one-time? searches
3631 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3632 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3633 variables--lexical variables are unaffected, but they clean themselves
3634 up on scope exit anyway, so you'll probably want to use them instead.
3641 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3642 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3643 context, depending on how the return value will be used, and the context
3644 may vary from one execution to the next (see C<wantarray>). If no EXPR
3645 is given, returns an empty list in list context, the undefined value in
3646 scalar context, and (of course) nothing at all in a void context.
3648 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3649 or do FILE will automatically return the value of the last expression
3654 In list context, returns a list value consisting of the elements
3655 of LIST in the opposite order. In scalar context, concatenates the
3656 elements of LIST and returns a string value with all characters
3657 in the opposite order.
3659 print reverse <>; # line tac, last line first
3661 undef $/; # for efficiency of <>
3662 print scalar reverse <>; # character tac, last line tsrif
3664 This operator is also handy for inverting a hash, although there are some
3665 caveats. If a value is duplicated in the original hash, only one of those
3666 can be represented as a key in the inverted hash. Also, this has to
3667 unwind one hash and build a whole new one, which may take some time
3668 on a large hash, such as from a DBM file.
3670 %by_name = reverse %by_address; # Invert the hash
3672 =item rewinddir DIRHANDLE
3674 Sets the current position to the beginning of the directory for the
3675 C<readdir> routine on DIRHANDLE.
3677 =item rindex STR,SUBSTR,POSITION
3679 =item rindex STR,SUBSTR
3681 Works just like index() except that it returns the position of the LAST
3682 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3683 last occurrence at or before that position.
3685 =item rmdir FILENAME
3689 Deletes the directory specified by FILENAME if that directory is empty. If it
3690 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3691 FILENAME is omitted, uses C<$_>.
3695 The substitution operator. See L<perlop>.
3699 Forces EXPR to be interpreted in scalar context and returns the value
3702 @counts = ( scalar @a, scalar @b, scalar @c );
3704 There is no equivalent operator to force an expression to
3705 be interpolated in list context because in practice, this is never
3706 needed. If you really wanted to do so, however, you could use
3707 the construction C<@{[ (some expression) ]}>, but usually a simple
3708 C<(some expression)> suffices.
3710 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3711 parenthesized list, this behaves as a scalar comma expression, evaluating
3712 all but the last element in void context and returning the final element
3713 evaluated in scalar context. This is seldom what you want.
3715 The following single statement:
3717 print uc(scalar(&foo,$bar)),$baz;
3719 is the moral equivalent of these two:
3722 print(uc($bar),$baz);
3724 See L<perlop> for more details on unary operators and the comma operator.
3726 =item seek FILEHANDLE,POSITION,WHENCE
3728 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3729 FILEHANDLE may be an expression whose value gives the name of the
3730 filehandle. The values for WHENCE are C<0> to set the new position to
3731 POSITION, C<1> to set it to the current position plus POSITION, and
3732 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3733 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3734 (start of the file, current position, end of the file) from the Fcntl
3735 module. Returns C<1> upon success, C<0> otherwise.
3737 If you want to position file for C<sysread> or C<syswrite>, don't use
3738 C<seek>--buffering makes its effect on the file's system position
3739 unpredictable and non-portable. Use C<sysseek> instead.
3741 Due to the rules and rigors of ANSI C, on some systems you have to do a
3742 seek whenever you switch between reading and writing. Amongst other
3743 things, this may have the effect of calling stdio's clearerr(3).
3744 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3748 This is also useful for applications emulating C<tail -f>. Once you hit
3749 EOF on your read, and then sleep for a while, you might have to stick in a
3750 seek() to reset things. The C<seek> doesn't change the current position,
3751 but it I<does> clear the end-of-file condition on the handle, so that the
3752 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3754 If that doesn't work (some stdios are particularly cantankerous), then
3755 you may need something more like this:
3758 for ($curpos = tell(FILE); $_ = <FILE>;
3759 $curpos = tell(FILE)) {
3760 # search for some stuff and put it into files
3762 sleep($for_a_while);
3763 seek(FILE, $curpos, 0);
3766 =item seekdir DIRHANDLE,POS
3768 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3769 must be a value returned by C<telldir>. Has the same caveats about
3770 possible directory compaction as the corresponding system library
3773 =item select FILEHANDLE
3777 Returns the currently selected filehandle. Sets the current default
3778 filehandle for output, if FILEHANDLE is supplied. This has two
3779 effects: first, a C<write> or a C<print> without a filehandle will
3780 default to this FILEHANDLE. Second, references to variables related to
3781 output will refer to this output channel. For example, if you have to
3782 set the top of form format for more than one output channel, you might
3790 FILEHANDLE may be an expression whose value gives the name of the
3791 actual filehandle. Thus:
3793 $oldfh = select(STDERR); $| = 1; select($oldfh);
3795 Some programmers may prefer to think of filehandles as objects with
3796 methods, preferring to write the last example as:
3799 STDERR->autoflush(1);
3801 =item select RBITS,WBITS,EBITS,TIMEOUT
3803 This calls the select(2) system call with the bit masks specified, which
3804 can be constructed using C<fileno> and C<vec>, along these lines:
3806 $rin = $win = $ein = '';
3807 vec($rin,fileno(STDIN),1) = 1;
3808 vec($win,fileno(STDOUT),1) = 1;
3811 If you want to select on many filehandles you might wish to write a
3815 my(@fhlist) = split(' ',$_[0]);
3818 vec($bits,fileno($_),1) = 1;
3822 $rin = fhbits('STDIN TTY SOCK');
3826 ($nfound,$timeleft) =
3827 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3829 or to block until something becomes ready just do this
3831 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3833 Most systems do not bother to return anything useful in $timeleft, so
3834 calling select() in scalar context just returns $nfound.
3836 Any of the bit masks can also be undef. The timeout, if specified, is
3837 in seconds, which may be fractional. Note: not all implementations are
3838 capable of returning the$timeleft. If not, they always return
3839 $timeleft equal to the supplied $timeout.
3841 You can effect a sleep of 250 milliseconds this way:
3843 select(undef, undef, undef, 0.25);
3845 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3846 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3847 then only on POSIX systems. You have to use C<sysread> instead.
3849 =item semctl ID,SEMNUM,CMD,ARG
3851 Calls the System V IPC function C<semctl>. You'll probably have to say
3855 first to get the correct constant definitions. If CMD is IPC_STAT or
3856 GETALL, then ARG must be a variable which will hold the returned
3857 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3858 undefined value for error, "C<0 but true>" for zero, or the actual return
3859 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3861 =item semget KEY,NSEMS,FLAGS
3863 Calls the System V IPC function semget. Returns the semaphore id, or
3864 the undefined value if there is an error. See also C<IPC::SysV> and
3865 C<IPC::SysV::Semaphore> documentation.
3867 =item semop KEY,OPSTRING
3869 Calls the System V IPC function semop to perform semaphore operations
3870 such as signaling and waiting. OPSTRING must be a packed array of
3871 semop structures. Each semop structure can be generated with
3872 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3873 operations is implied by the length of OPSTRING. Returns true if
3874 successful, or false if there is an error. As an example, the
3875 following code waits on semaphore $semnum of semaphore id $semid:
3877 $semop = pack("sss", $semnum, -1, 0);
3878 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3880 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3881 and C<IPC::SysV::Semaphore> documentation.
3883 =item send SOCKET,MSG,FLAGS,TO
3885 =item send SOCKET,MSG,FLAGS
3887 Sends a message on a socket. Takes the same flags as the system call
3888 of the same name. On unconnected sockets you must specify a
3889 destination to send TO, in which case it does a C C<sendto>. Returns
3890 the number of characters sent, or the undefined value if there is an
3891 error. The C system call sendmsg(2) is currently unimplemented.
3892 See L<perlipc/"UDP: Message Passing"> for examples.
3894 =item setpgrp PID,PGRP
3896 Sets the current process group for the specified PID, C<0> for the current
3897 process. Will produce a fatal error if used on a machine that doesn't
3898 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3899 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3900 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3903 =item setpriority WHICH,WHO,PRIORITY
3905 Sets the current priority for a process, a process group, or a user.
3906 (See setpriority(2).) Will produce a fatal error if used on a machine
3907 that doesn't implement setpriority(2).
3909 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3911 Sets the socket option requested. Returns undefined if there is an
3912 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3919 Shifts the first value of the array off and returns it, shortening the
3920 array by 1 and moving everything down. If there are no elements in the
3921 array, returns the undefined value. If ARRAY is omitted, shifts the
3922 C<@_> array within the lexical scope of subroutines and formats, and the
3923 C<@ARGV> array at file scopes or within the lexical scopes established by
3924 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
3927 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3928 same thing to the left end of an array that C<pop> and C<push> do to the
3931 =item shmctl ID,CMD,ARG
3933 Calls the System V IPC function shmctl. You'll probably have to say
3937 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3938 then ARG must be a variable which will hold the returned C<shmid_ds>
3939 structure. Returns like ioctl: the undefined value for error, "C<0> but
3940 true" for zero, or the actual return value otherwise.
3941 See also C<IPC::SysV> documentation.
3943 =item shmget KEY,SIZE,FLAGS
3945 Calls the System V IPC function shmget. Returns the shared memory
3946 segment id, or the undefined value if there is an error.
3947 See also C<IPC::SysV> documentation.
3949 =item shmread ID,VAR,POS,SIZE
3951 =item shmwrite ID,STRING,POS,SIZE
3953 Reads or writes the System V shared memory segment ID starting at
3954 position POS for size SIZE by attaching to it, copying in/out, and
3955 detaching from it. When reading, VAR must be a variable that will
3956 hold the data read. When writing, if STRING is too long, only SIZE
3957 bytes are used; if STRING is too short, nulls are written to fill out
3958 SIZE bytes. Return true if successful, or false if there is an error.
3959 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3962 =item shutdown SOCKET,HOW
3964 Shuts down a socket connection in the manner indicated by HOW, which
3965 has the same interpretation as in the system call of the same name.
3967 shutdown(SOCKET, 0); # I/we have stopped reading data
3968 shutdown(SOCKET, 1); # I/we have stopped writing data
3969 shutdown(SOCKET, 2); # I/we have stopped using this socket
3971 This is useful with sockets when you want to tell the other
3972 side you're done writing but not done reading, or vice versa.
3973 It's also a more insistent form of close because it also
3974 disables the file descriptor in any forked copies in other
3981 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3982 returns sine of C<$_>.
3984 For the inverse sine operation, you may use the C<Math::Trig::asin>
3985 function, or use this relation:
3987 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3993 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3994 May be interrupted if the process receives a signal such as C<SIGALRM>.
3995 Returns the number of seconds actually slept. You probably cannot
3996 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
3999 On some older systems, it may sleep up to a full second less than what
4000 you requested, depending on how it counts seconds. Most modern systems
4001 always sleep the full amount. They may appear to sleep longer than that,
4002 however, because your process might not be scheduled right away in a
4003 busy multitasking system.
4005 For delays of finer granularity than one second, you may use Perl's
4006 C<syscall> interface to access setitimer(2) if your system supports
4007 it, or else see L</select> above. The Time::HiRes module from CPAN
4010 See also the POSIX module's C<sigpause> function.
4012 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4014 Opens a socket of the specified kind and attaches it to filehandle
4015 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4016 the system call of the same name. You should C<use Socket> first
4017 to get the proper definitions imported. See the examples in
4018 L<perlipc/"Sockets: Client/Server Communication">.
4020 On systems that support a close-on-exec flag on files, the flag will
4021 be set for the newly opened file descriptor, as determined by the
4022 value of $^F. See L<perlvar/$^F>.
4024 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4026 Creates an unnamed pair of sockets in the specified domain, of the
4027 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4028 for the system call of the same name. If unimplemented, yields a fatal
4029 error. Returns true if successful.
4031 On systems that support a close-on-exec flag on files, the flag will
4032 be set for the newly opened file descriptors, as determined by the value
4033 of $^F. See L<perlvar/$^F>.
4035 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4036 to C<pipe(Rdr, Wtr)> is essentially:
4039 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4040 shutdown(Rdr, 1); # no more writing for reader
4041 shutdown(Wtr, 0); # no more reading for writer
4043 See L<perlipc> for an example of socketpair use.
4045 =item sort SUBNAME LIST
4047 =item sort BLOCK LIST
4051 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4052 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4053 specified, it gives the name of a subroutine that returns an integer
4054 less than, equal to, or greater than C<0>, depending on how the elements
4055 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
4056 operators are extremely useful in such routines.) SUBNAME may be a
4057 scalar variable name (unsubscripted), in which case the value provides
4058 the name of (or a reference to) the actual subroutine to use. In place
4059 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4062 If the subroutine's prototype is C<($$)>, the elements to be compared
4063 are passed by reference in C<@_>, as for a normal subroutine. If not,
4064 the normal calling code for subroutines is bypassed in the interests of
4065 efficiency, and the elements to be compared are passed into the subroutine
4066 as the package global variables $a and $b (see example below). Note that
4067 in the latter case, it is usually counter-productive to declare $a and
4070 In either case, the subroutine may not be recursive. The values to be
4071 compared are always passed by reference, so don't modify them.
4073 You also cannot exit out of the sort block or subroutine using any of the
4074 loop control operators described in L<perlsyn> or with C<goto>.
4076 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4077 current collation locale. See L<perllocale>.
4082 @articles = sort @files;
4084 # same thing, but with explicit sort routine
4085 @articles = sort {$a cmp $b} @files;
4087 # now case-insensitively
4088 @articles = sort {uc($a) cmp uc($b)} @files;
4090 # same thing in reversed order
4091 @articles = sort {$b cmp $a} @files;
4093 # sort numerically ascending
4094 @articles = sort {$a <=> $b} @files;
4096 # sort numerically descending
4097 @articles = sort {$b <=> $a} @files;
4099 # this sorts the %age hash by value instead of key
4100 # using an in-line function
4101 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4103 # sort using explicit subroutine name
4105 $age{$a} <=> $age{$b}; # presuming numeric
4107 @sortedclass = sort byage @class;
4109 sub backwards { $b cmp $a }
4110 @harry = qw(dog cat x Cain Abel);
4111 @george = qw(gone chased yz Punished Axed);
4113 # prints AbelCaincatdogx
4114 print sort backwards @harry;
4115 # prints xdogcatCainAbel
4116 print sort @george, 'to', @harry;
4117 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4119 # inefficiently sort by descending numeric compare using
4120 # the first integer after the first = sign, or the
4121 # whole record case-insensitively otherwise
4124 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4129 # same thing, but much more efficiently;
4130 # we'll build auxiliary indices instead
4134 push @nums, /=(\d+)/;
4139 $nums[$b] <=> $nums[$a]
4141 $caps[$a] cmp $caps[$b]
4145 # same thing, but without any temps
4146 @new = map { $_->[0] }
4147 sort { $b->[1] <=> $a->[1]
4150 } map { [$_, /=(\d+)/, uc($_)] } @old;
4152 # using a prototype allows you to use any comparison subroutine
4153 # as a sort subroutine (including other package's subroutines)
4155 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4158 @new = sort other::backwards @old;
4160 If you're using strict, you I<must not> declare $a
4161 and $b as lexicals. They are package globals. That means
4162 if you're in the C<main> package, it's
4164 @articles = sort {$main::b <=> $main::a} @files;
4168 @articles = sort {$::b <=> $::a} @files;
4170 but if you're in the C<FooPack> package, it's
4172 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4174 The comparison function is required to behave. If it returns
4175 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4176 sometimes saying the opposite, for example) the results are not
4179 =item splice ARRAY,OFFSET,LENGTH,LIST
4181 =item splice ARRAY,OFFSET,LENGTH
4183 =item splice ARRAY,OFFSET
4187 Removes the elements designated by OFFSET and LENGTH from an array, and
4188 replaces them with the elements of LIST, if any. In list context,
4189 returns the elements removed from the array. In scalar context,
4190 returns the last element removed, or C<undef> if no elements are
4191 removed. The array grows or shrinks as necessary.
4192 If OFFSET is negative then it starts that far from the end of the array.
4193 If LENGTH is omitted, removes everything from OFFSET onward.
4194 If LENGTH is negative, leaves that many elements off the end of the array.
4195 If both OFFSET and LENGTH are omitted, removes everything.
4197 The following equivalences hold (assuming C<$[ == 0>):
4199 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4200 pop(@a) splice(@a,-1)
4201 shift(@a) splice(@a,0,1)
4202 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4203 $a[$x] = $y splice(@a,$x,1,$y)
4205 Example, assuming array lengths are passed before arrays:
4207 sub aeq { # compare two list values
4208 my(@a) = splice(@_,0,shift);
4209 my(@b) = splice(@_,0,shift);
4210 return 0 unless @a == @b; # same len?
4212 return 0 if pop(@a) ne pop(@b);
4216 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4218 =item split /PATTERN/,EXPR,LIMIT
4220 =item split /PATTERN/,EXPR
4222 =item split /PATTERN/
4226 Splits a string into a list of strings and returns that list. By default,
4227 empty leading fields are preserved, and empty trailing ones are deleted.
4229 If not in list context, returns the number of fields found and splits into
4230 the C<@_> array. (In list context, you can force the split into C<@_> by
4231 using C<??> as the pattern delimiters, but it still returns the list
4232 value.) The use of implicit split to C<@_> is deprecated, however, because
4233 it clobbers your subroutine arguments.
4235 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4236 splits on whitespace (after skipping any leading whitespace). Anything
4237 matching PATTERN is taken to be a delimiter separating the fields. (Note
4238 that the delimiter may be longer than one character.)
4240 If LIMIT is specified and positive, splits into no more than that
4241 many fields (though it may split into fewer). If LIMIT is unspecified
4242 or zero, trailing null fields are stripped (which potential users
4243 of C<pop> would do well to remember). If LIMIT is negative, it is
4244 treated as if an arbitrarily large LIMIT had been specified.
4246 A pattern matching the null string (not to be confused with
4247 a null pattern C<//>, which is just one member of the set of patterns
4248 matching a null string) will split the value of EXPR into separate
4249 characters at each point it matches that way. For example:
4251 print join(':', split(/ */, 'hi there'));
4253 produces the output 'h:i:t:h:e:r:e'.
4255 The LIMIT parameter can be used to split a line partially
4257 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4259 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4260 one larger than the number of variables in the list, to avoid
4261 unnecessary work. For the list above LIMIT would have been 4 by
4262 default. In time critical applications it behooves you not to split
4263 into more fields than you really need.
4265 If the PATTERN contains parentheses, additional list elements are
4266 created from each matching substring in the delimiter.
4268 split(/([,-])/, "1-10,20", 3);
4270 produces the list value
4272 (1, '-', 10, ',', 20)
4274 If you had the entire header of a normal Unix email message in $header,
4275 you could split it up into fields and their values this way:
4277 $header =~ s/\n\s+/ /g; # fix continuation lines
4278 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4280 The pattern C</PATTERN/> may be replaced with an expression to specify
4281 patterns that vary at runtime. (To do runtime compilation only once,
4282 use C</$variable/o>.)
4284 As a special case, specifying a PATTERN of space (C<' '>) will split on
4285 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4286 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4287 will give you as many null initial fields as there are leading spaces.
4288 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4289 whitespace produces a null first field. A C<split> with no arguments
4290 really does a C<split(' ', $_)> internally.
4294 open(PASSWD, '/etc/passwd');
4296 ($login, $passwd, $uid, $gid,
4297 $gcos, $home, $shell) = split(/:/);
4301 (Note that $shell above will still have a newline on it. See L</chop>,
4302 L</chomp>, and L</join>.)
4304 =item sprintf FORMAT, LIST
4306 Returns a string formatted by the usual C<printf> conventions of the
4307 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4308 on your system for an explanation of the general principles.
4310 Perl does its own C<sprintf> formatting--it emulates the C
4311 function C<sprintf>, but it doesn't use it (except for floating-point
4312 numbers, and even then only the standard modifiers are allowed). As a
4313 result, any non-standard extensions in your local C<sprintf> are not
4314 available from Perl.
4316 Perl's C<sprintf> permits the following universally-known conversions:
4319 %c a character with the given number
4321 %d a signed integer, in decimal
4322 %u an unsigned integer, in decimal
4323 %o an unsigned integer, in octal
4324 %x an unsigned integer, in hexadecimal
4325 %e a floating-point number, in scientific notation
4326 %f a floating-point number, in fixed decimal notation
4327 %g a floating-point number, in %e or %f notation
4329 In addition, Perl permits the following widely-supported conversions:
4331 %X like %x, but using upper-case letters
4332 %E like %e, but using an upper-case "E"
4333 %G like %g, but with an upper-case "E" (if applicable)
4334 %b an unsigned integer, in binary
4335 %p a pointer (outputs the Perl value's address in hexadecimal)
4336 %n special: *stores* the number of characters output so far
4337 into the next variable in the parameter list
4339 And the following Perl-specific conversion:
4341 %v a string, output as a tuple of integers ("Perl" is 80.101.114.108)
4343 Finally, for backward (and we do mean "backward") compatibility, Perl
4344 permits these unnecessary but widely-supported conversions:
4347 %D a synonym for %ld
4348 %U a synonym for %lu
4349 %O a synonym for %lo
4352 Perl permits the following universally-known flags between the C<%>
4353 and the conversion letter:
4355 space prefix positive number with a space
4356 + prefix positive number with a plus sign
4357 - left-justify within the field
4358 0 use zeros, not spaces, to right-justify
4359 # prefix non-zero octal with "0", non-zero hex with "0x"
4360 number minimum field width
4361 .number "precision": digits after decimal point for
4362 floating-point, max length for string, minimum length
4364 l interpret integer as C type "long" or "unsigned long"
4365 h interpret integer as C type "short" or "unsigned short"
4366 If no flags, interpret integer as C type "int" or "unsigned"
4368 There is also one Perl-specific flag:
4370 V interpret integer as Perl's standard integer type
4372 Where a number would appear in the flags, an asterisk (C<*>) may be
4373 used instead, in which case Perl uses the next item in the parameter
4374 list as the given number (that is, as the field width or precision).
4375 If a field width obtained through C<*> is negative, it has the same
4376 effect as the C<-> flag: left-justification.
4378 If C<use locale> is in effect, the character used for the decimal
4379 point in formatted real numbers is affected by the LC_NUMERIC locale.
4382 If Perl understands "quads" (64-bit integers) (this requires
4383 either that the platform natively support quads or that Perl
4384 be specifically compiled to support quads), the characters
4388 print quads, and they may optionally be preceded by
4396 You can find out whether your Perl supports quads via L<Config>:
4399 ($Config{use64bits} eq 'define' || $Config{longsize} == 8) &&
4402 If Perl understands "long doubles" (this requires that the platform
4403 support long doubles), the flags
4407 may optionally be preceded by
4415 You can find out whether your Perl supports long doubles via L<Config>:
4418 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4424 Return the square root of EXPR. If EXPR is omitted, returns square
4425 root of C<$_>. Only works on non-negative operands, unless you've
4426 loaded the standard Math::Complex module.
4429 print sqrt(-2); # prints 1.4142135623731i
4435 Sets the random number seed for the C<rand> operator. If EXPR is
4436 omitted, uses a semi-random value supplied by the kernel (if it supports
4437 the F</dev/urandom> device) or based on the current time and process
4438 ID, among other things. In versions of Perl prior to 5.004 the default
4439 seed was just the current C<time>. This isn't a particularly good seed,
4440 so many old programs supply their own seed value (often C<time ^ $$> or
4441 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4443 In fact, it's usually not necessary to call C<srand> at all, because if
4444 it is not called explicitly, it is called implicitly at the first use of
4445 the C<rand> operator. However, this was not the case in version of Perl
4446 before 5.004, so if your script will run under older Perl versions, it
4447 should call C<srand>.
4449 Note that you need something much more random than the default seed for
4450 cryptographic purposes. Checksumming the compressed output of one or more
4451 rapidly changing operating system status programs is the usual method. For
4454 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4456 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4459 Do I<not> call C<srand> multiple times in your program unless you know
4460 exactly what you're doing and why you're doing it. The point of the
4461 function is to "seed" the C<rand> function so that C<rand> can produce
4462 a different sequence each time you run your program. Just do it once at the
4463 top of your program, or you I<won't> get random numbers out of C<rand>!
4465 Frequently called programs (like CGI scripts) that simply use
4469 for a seed can fall prey to the mathematical property that
4473 one-third of the time. So don't do that.
4475 =item stat FILEHANDLE
4481 Returns a 13-element list giving the status info for a file, either
4482 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4483 it stats C<$_>. Returns a null list if the stat fails. Typically used
4486 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4487 $atime,$mtime,$ctime,$blksize,$blocks)
4490 Not all fields are supported on all filesystem types. Here are the
4491 meaning of the fields:
4493 0 dev device number of filesystem
4495 2 mode file mode (type and permissions)
4496 3 nlink number of (hard) links to the file
4497 4 uid numeric user ID of file's owner
4498 5 gid numeric group ID of file's owner
4499 6 rdev the device identifier (special files only)
4500 7 size total size of file, in bytes
4501 8 atime last access time in seconds since the epoch
4502 9 mtime last modify time in seconds since the epoch
4503 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4504 11 blksize preferred block size for file system I/O
4505 12 blocks actual number of blocks allocated
4507 (The epoch was at 00:00 January 1, 1970 GMT.)
4509 If stat is passed the special filehandle consisting of an underline, no
4510 stat is done, but the current contents of the stat structure from the
4511 last stat or filetest are returned. Example:
4513 if (-x $file && (($d) = stat(_)) && $d < 0) {
4514 print "$file is executable NFS file\n";
4517 (This works on machines only for which the device number is negative
4520 Because the mode contains both the file type and its permissions, you
4521 should mask off the file type portion and (s)printf using a C<"%o">
4522 if you want to see the real permissions.
4524 $mode = (stat($filename))[2];
4525 printf "Permissions are %04o\n", $mode & 07777;
4527 In scalar context, C<stat> returns a boolean value indicating success
4528 or failure, and, if successful, sets the information associated with
4529 the special filehandle C<_>.
4531 The File::stat module provides a convenient, by-name access mechanism:
4534 $sb = stat($filename);
4535 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4536 $filename, $sb->size, $sb->mode & 07777,
4537 scalar localtime $sb->mtime;
4539 You can import symbolic mode constants (C<S_IF*>) and functions
4540 (C<S_IS*>) from the Fcntl module:
4544 $mode = (stat($filename))[2];
4546 $user_rwx = ($mode & S_IRWXU) >> 6;
4547 $group_read = ($mode & S_IRGRP) >> 3;
4548 $other_execute = $mode & S_IXOTH;
4550 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4552 $is_setuid = $mode & S_ISUID;
4553 $is_setgid = S_ISDIR($mode);
4555 You could write the last two using the C<-u> and C<-d> operators.
4556 The commonly available S_IF* constants are
4558 # Permissions: read, write, execute, for user, group, others.
4560 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4561 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4562 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4564 # Setuid/Setgid/Stickiness.
4566 S_ISUID S_ISGID S_ISVTX S_ISTXT
4568 # File types. Not necessarily all are available on your system.
4570 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4572 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4574 S_IREAD S_IWRITE S_IEXEC
4576 and the S_IF* functions are
4578 S_IFMODE($mode) the part of $mode containg the permission bits
4579 and the setuid/setgid/sticky bits
4581 S_IFMT($mode) the part of $mode containing the file type
4582 which can be bit-anded with e.g. S_IFREG
4583 or with the following functions
4585 # The operators -f, -d, -l, -b, -c, -p, and -s.
4587 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4588 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4590 # No direct -X operator counterpart, but for the first one
4591 # the -g operator is often equivalent. The ENFMT stands for
4592 # record flocking enforcement, a platform-dependent feature.
4594 S_ISENFMT($mode) S_ISWHT($mode)
4596 See your native chmod(2) and stat(2) documentation for more details
4597 about the S_* constants.
4603 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4604 doing many pattern matches on the string before it is next modified.
4605 This may or may not save time, depending on the nature and number of
4606 patterns you are searching on, and on the distribution of character
4607 frequencies in the string to be searched--you probably want to compare
4608 run times with and without it to see which runs faster. Those loops
4609 which scan for many short constant strings (including the constant
4610 parts of more complex patterns) will benefit most. You may have only
4611 one C<study> active at a time--if you study a different scalar the first
4612 is "unstudied". (The way C<study> works is this: a linked list of every
4613 character in the string to be searched is made, so we know, for
4614 example, where all the C<'k'> characters are. From each search string,
4615 the rarest character is selected, based on some static frequency tables
4616 constructed from some C programs and English text. Only those places
4617 that contain this "rarest" character are examined.)
4619 For example, here is a loop that inserts index producing entries
4620 before any line containing a certain pattern:
4624 print ".IX foo\n" if /\bfoo\b/;
4625 print ".IX bar\n" if /\bbar\b/;
4626 print ".IX blurfl\n" if /\bblurfl\b/;
4631 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4632 will be looked at, because C<f> is rarer than C<o>. In general, this is
4633 a big win except in pathological cases. The only question is whether
4634 it saves you more time than it took to build the linked list in the
4637 Note that if you have to look for strings that you don't know till
4638 runtime, you can build an entire loop as a string and C<eval> that to
4639 avoid recompiling all your patterns all the time. Together with
4640 undefining C<$/> to input entire files as one record, this can be very
4641 fast, often faster than specialized programs like fgrep(1). The following
4642 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4643 out the names of those files that contain a match:
4645 $search = 'while (<>) { study;';
4646 foreach $word (@words) {
4647 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4652 eval $search; # this screams
4653 $/ = "\n"; # put back to normal input delimiter
4654 foreach $file (sort keys(%seen)) {
4662 =item sub NAME BLOCK
4664 This is subroutine definition, not a real function I<per se>. With just a
4665 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4666 Without a NAME, it's an anonymous function declaration, and does actually
4667 return a value: the CODE ref of the closure you just created. See L<perlsub>
4668 and L<perlref> for details.
4670 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4672 =item substr EXPR,OFFSET,LENGTH
4674 =item substr EXPR,OFFSET
4676 Extracts a substring out of EXPR and returns it. First character is at
4677 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4678 If OFFSET is negative (or more precisely, less than C<$[>), starts
4679 that far from the end of the string. If LENGTH is omitted, returns
4680 everything to the end of the string. If LENGTH is negative, leaves that
4681 many characters off the end of the string.
4683 You can use the substr() function as an lvalue, in which case EXPR
4684 must itself be an lvalue. If you assign something shorter than LENGTH,
4685 the string will shrink, and if you assign something longer than LENGTH,
4686 the string will grow to accommodate it. To keep the string the same
4687 length you may need to pad or chop your value using C<sprintf>.
4689 If OFFSET and LENGTH specify a substring that is partly outside the
4690 string, only the part within the string is returned. If the substring
4691 is beyond either end of the string, substr() returns the undefined
4692 value and produces a warning. When used as an lvalue, specifying a
4693 substring that is entirely outside the string is a fatal error.
4694 Here's an example showing the behavior for boundary cases:
4697 substr($name, 4) = 'dy'; # $name is now 'freddy'
4698 my $null = substr $name, 6, 2; # returns '' (no warning)
4699 my $oops = substr $name, 7; # returns undef, with warning
4700 substr($name, 7) = 'gap'; # fatal error
4702 An alternative to using substr() as an lvalue is to specify the
4703 replacement string as the 4th argument. This allows you to replace
4704 parts of the EXPR and return what was there before in one operation,
4705 just as you can with splice().
4707 =item symlink OLDFILE,NEWFILE
4709 Creates a new filename symbolically linked to the old filename.
4710 Returns C<1> for success, C<0> otherwise. On systems that don't support
4711 symbolic links, produces a fatal error at run time. To check for that,
4714 $symlink_exists = eval { symlink("",""); 1 };
4718 Calls the system call specified as the first element of the list,
4719 passing the remaining elements as arguments to the system call. If
4720 unimplemented, produces a fatal error. The arguments are interpreted
4721 as follows: if a given argument is numeric, the argument is passed as
4722 an int. If not, the pointer to the string value is passed. You are
4723 responsible to make sure a string is pre-extended long enough to
4724 receive any result that might be written into a string. You can't use a
4725 string literal (or other read-only string) as an argument to C<syscall>
4726 because Perl has to assume that any string pointer might be written
4728 integer arguments are not literals and have never been interpreted in a
4729 numeric context, you may need to add C<0> to them to force them to look
4730 like numbers. This emulates the C<syswrite> function (or vice versa):
4732 require 'syscall.ph'; # may need to run h2ph
4734 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4736 Note that Perl supports passing of up to only 14 arguments to your system call,
4737 which in practice should usually suffice.
4739 Syscall returns whatever value returned by the system call it calls.
4740 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4741 Note that some system calls can legitimately return C<-1>. The proper
4742 way to handle such calls is to assign C<$!=0;> before the call and
4743 check the value of C<$!> if syscall returns C<-1>.
4745 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4746 number of the read end of the pipe it creates. There is no way
4747 to retrieve the file number of the other end. You can avoid this
4748 problem by using C<pipe> instead.
4750 =item sysopen FILEHANDLE,FILENAME,MODE
4752 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4754 Opens the file whose filename is given by FILENAME, and associates it
4755 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4756 the name of the real filehandle wanted. This function calls the
4757 underlying operating system's C<open> function with the parameters
4758 FILENAME, MODE, PERMS.
4760 The possible values and flag bits of the MODE parameter are
4761 system-dependent; they are available via the standard module C<Fcntl>.
4762 See the documentation of your operating system's C<open> to see which
4763 values and flag bits are available. You may combine several flags
4764 using the C<|>-operator.
4766 Some of the most common values are C<O_RDONLY> for opening the file in
4767 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4768 and C<O_RDWR> for opening the file in read-write mode, and.
4770 For historical reasons, some values work on almost every system
4771 supported by perl: zero means read-only, one means write-only, and two
4772 means read/write. We know that these values do I<not> work under
4773 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4774 use them in new code.
4776 If the file named by FILENAME does not exist and the C<open> call creates
4777 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4778 PERMS specifies the permissions of the newly created file. If you omit
4779 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4780 These permission values need to be in octal, and are modified by your
4781 process's current C<umask>.
4783 In many systems the C<O_EXCL> flag is available for opening files in
4784 exclusive mode. This is B<not> locking: exclusiveness means here that
4785 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4788 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4790 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4791 that takes away the user's option to have a more permissive umask.
4792 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4795 Note that C<sysopen> depends on the fdopen() C library function.
4796 On many UNIX systems, fdopen() is known to fail when file descriptors
4797 exceed a certain value, typically 255. If you need more file
4798 descriptors than that, consider rebuilding Perl to use the C<sfio>
4799 library, or perhaps using the POSIX::open() function.
4801 See L<perlopentut> for a kinder, gentler explanation of opening files.
4803 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4805 =item sysread FILEHANDLE,SCALAR,LENGTH
4807 Attempts to read LENGTH bytes of data into variable SCALAR from the
4808 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4809 so mixing this with other kinds of reads, C<print>, C<write>,
4810 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4811 usually buffers data. Returns the number of bytes actually read, C<0>
4812 at end of file, or undef if there was an error. SCALAR will be grown or
4813 shrunk so that the last byte actually read is the last byte of the
4814 scalar after the read.
4816 An OFFSET may be specified to place the read data at some place in the
4817 string other than the beginning. A negative OFFSET specifies
4818 placement at that many bytes counting backwards from the end of the
4819 string. A positive OFFSET greater than the length of SCALAR results
4820 in the string being padded to the required size with C<"\0"> bytes before
4821 the result of the read is appended.
4823 There is no syseof() function, which is ok, since eof() doesn't work
4824 very well on device files (like ttys) anyway. Use sysread() and check
4825 for a return value for 0 to decide whether you're done.
4827 =item sysseek FILEHANDLE,POSITION,WHENCE
4829 Sets FILEHANDLE's system position using the system call lseek(2). It
4830 bypasses stdio, so mixing this with reads (other than C<sysread>),
4831 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4832 FILEHANDLE may be an expression whose value gives the name of the
4833 filehandle. The values for WHENCE are C<0> to set the new position to
4834 POSITION, C<1> to set the it to the current position plus POSITION,
4835 and C<2> to set it to EOF plus POSITION (typically negative). For
4836 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4837 C<SEEK_END> (start of the file, current position, end of the file)
4838 from the Fcntl module.
4840 Returns the new position, or the undefined value on failure. A position
4841 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4842 true on success and false on failure, yet you can still easily determine
4847 =item system PROGRAM LIST
4849 Does exactly the same thing as C<exec LIST>, except that a fork is
4850 done first, and the parent process waits for the child process to
4851 complete. Note that argument processing varies depending on the
4852 number of arguments. If there is more than one argument in LIST,
4853 or if LIST is an array with more than one value, starts the program
4854 given by the first element of the list with arguments given by the
4855 rest of the list. If there is only one scalar argument, the argument
4856 is checked for shell metacharacters, and if there are any, the
4857 entire argument is passed to the system's command shell for parsing
4858 (this is C</bin/sh -c> on Unix platforms, but varies on other
4859 platforms). If there are no shell metacharacters in the argument,
4860 it is split into words and passed directly to C<execvp>, which is
4863 All files opened for output are flushed before attempting the exec().
4865 The return value is the exit status of the program as
4866 returned by the C<wait> call. To get the actual exit value divide by
4867 256. See also L</exec>. This is I<not> what you want to use to capture
4868 the output from a command, for that you should use merely backticks or
4869 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4870 indicates a failure to start the program (inspect $! for the reason).
4872 Like C<exec>, C<system> allows you to lie to a program about its name if
4873 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4875 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4876 program they're running doesn't actually interrupt your program.
4878 @args = ("command", "arg1", "arg2");
4880 or die "system @args failed: $?"
4882 You can check all the failure possibilities by inspecting
4885 $exit_value = $? >> 8;
4886 $signal_num = $? & 127;
4887 $dumped_core = $? & 128;
4889 When the arguments get executed via the system shell, results
4890 and return codes will be subject to its quirks and capabilities.
4891 See L<perlop/"`STRING`"> and L</exec> for details.
4893 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4895 =item syswrite FILEHANDLE,SCALAR,LENGTH
4897 =item syswrite FILEHANDLE,SCALAR
4899 Attempts to write LENGTH bytes of data from variable SCALAR to the
4900 specified FILEHANDLE, using the system call write(2). If LENGTH
4901 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4902 this with reads (other than C<sysread())>, C<print>, C<write>,
4903 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4904 usually buffers data. Returns the number of bytes actually written,
4905 or C<undef> if there was an error. If the LENGTH is greater than
4906 the available data in the SCALAR after the OFFSET, only as much
4907 data as is available will be written.
4909 An OFFSET may be specified to write the data from some part of the
4910 string other than the beginning. A negative OFFSET specifies writing
4911 that many bytes counting backwards from the end of the string. In the
4912 case the SCALAR is empty you can use OFFSET but only zero offset.
4914 =item tell FILEHANDLE
4918 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4919 expression whose value gives the name of the actual filehandle. If
4920 FILEHANDLE is omitted, assumes the file last read.
4922 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4924 =item telldir DIRHANDLE
4926 Returns the current position of the C<readdir> routines on DIRHANDLE.
4927 Value may be given to C<seekdir> to access a particular location in a
4928 directory. Has the same caveats about possible directory compaction as
4929 the corresponding system library routine.
4931 =item tie VARIABLE,CLASSNAME,LIST
4933 This function binds a variable to a package class that will provide the
4934 implementation for the variable. VARIABLE is the name of the variable
4935 to be enchanted. CLASSNAME is the name of a class implementing objects
4936 of correct type. Any additional arguments are passed to the C<new>
4937 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4938 or C<TIEHASH>). Typically these are arguments such as might be passed
4939 to the C<dbm_open()> function of C. The object returned by the C<new>
4940 method is also returned by the C<tie> function, which would be useful
4941 if you want to access other methods in CLASSNAME.
4943 Note that functions such as C<keys> and C<values> may return huge lists
4944 when used on large objects, like DBM files. You may prefer to use the
4945 C<each> function to iterate over such. Example:
4947 # print out history file offsets
4949 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4950 while (($key,$val) = each %HIST) {
4951 print $key, ' = ', unpack('L',$val), "\n";
4955 A class implementing a hash should have the following methods:
4957 TIEHASH classname, LIST
4959 STORE this, key, value
4964 NEXTKEY this, lastkey
4967 A class implementing an ordinary array should have the following methods:
4969 TIEARRAY classname, LIST
4971 STORE this, key, value
4973 STORESIZE this, count
4979 SPLICE this, offset, length, LIST
4983 A class implementing a file handle should have the following methods:
4985 TIEHANDLE classname, LIST
4986 READ this, scalar, length, offset
4989 WRITE this, scalar, length, offset
4991 PRINTF this, format, LIST
4995 A class implementing a scalar should have the following methods:
4997 TIESCALAR classname, LIST
5002 Not all methods indicated above need be implemented. See L<perltie>,
5003 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5005 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5006 for you--you need to do that explicitly yourself. See L<DB_File>
5007 or the F<Config> module for interesting C<tie> implementations.
5009 For further details see L<perltie>, L<"tied VARIABLE">.
5013 Returns a reference to the object underlying VARIABLE (the same value
5014 that was originally returned by the C<tie> call that bound the variable
5015 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5020 Returns the number of non-leap seconds since whatever time the system
5021 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5022 and 00:00:00 UTC, January 1, 1970 for most other systems).
5023 Suitable for feeding to C<gmtime> and C<localtime>.
5025 For measuring time in better granularity than one second,
5026 you may use either the Time::HiRes module from CPAN, or
5027 if you have gettimeofday(2), you may be able to use the
5028 C<syscall> interface of Perl, see L<perlfaq8> for details.
5032 Returns a four-element list giving the user and system times, in
5033 seconds, for this process and the children of this process.
5035 ($user,$system,$cuser,$csystem) = times;
5039 The transliteration operator. Same as C<y///>. See L<perlop>.
5041 =item truncate FILEHANDLE,LENGTH
5043 =item truncate EXPR,LENGTH
5045 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5046 specified length. Produces a fatal error if truncate isn't implemented
5047 on your system. Returns true if successful, the undefined value
5054 Returns an uppercased version of EXPR. This is the internal function
5055 implementing the C<\U> escape in double-quoted strings.
5056 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5057 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5058 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5060 If EXPR is omitted, uses C<$_>.
5066 Returns the value of EXPR with the first character
5067 in uppercase (titlecase in Unicode). This is
5068 the internal function implementing the C<\u> escape in double-quoted strings.
5069 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5072 If EXPR is omitted, uses C<$_>.
5078 Sets the umask for the process to EXPR and returns the previous value.
5079 If EXPR is omitted, merely returns the current umask.
5081 The Unix permission C<rwxr-x---> is represented as three sets of three
5082 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5083 and isn't one of the digits). The C<umask> value is such a number
5084 representing disabled permissions bits. The permission (or "mode")
5085 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5086 even if you tell C<sysopen> to create a file with permissions C<0777>,
5087 if your umask is C<0022> then the file will actually be created with
5088 permissions C<0755>. If your C<umask> were C<0027> (group can't
5089 write; others can't read, write, or execute), then passing
5090 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5093 Here's some advice: supply a creation mode of C<0666> for regular
5094 files (in C<sysopen>) and one of C<0777> for directories (in
5095 C<mkdir>) and executable files. This gives users the freedom of
5096 choice: if they want protected files, they might choose process umasks
5097 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5098 Programs should rarely if ever make policy decisions better left to
5099 the user. The exception to this is when writing files that should be
5100 kept private: mail files, web browser cookies, I<.rhosts> files, and
5103 If umask(2) is not implemented on your system and you are trying to
5104 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5105 fatal error at run time. If umask(2) is not implemented and you are
5106 not trying to restrict access for yourself, returns C<undef>.
5108 Remember that a umask is a number, usually given in octal; it is I<not> a
5109 string of octal digits. See also L</oct>, if all you have is a string.
5115 Undefines the value of EXPR, which must be an lvalue. Use only on a
5116 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5117 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5118 will probably not do what you expect on most predefined variables or
5119 DBM list values, so don't do that; see L<delete>.) Always returns the
5120 undefined value. You can omit the EXPR, in which case nothing is
5121 undefined, but you still get an undefined value that you could, for
5122 instance, return from a subroutine, assign to a variable or pass as a
5123 parameter. Examples:
5126 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5130 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5131 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5132 select undef, undef, undef, 0.25;
5133 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5135 Note that this is a unary operator, not a list operator.
5141 Deletes a list of files. Returns the number of files successfully
5144 $cnt = unlink 'a', 'b', 'c';
5148 Note: C<unlink> will not delete directories unless you are superuser and
5149 the B<-U> flag is supplied to Perl. Even if these conditions are
5150 met, be warned that unlinking a directory can inflict damage on your
5151 filesystem. Use C<rmdir> instead.
5153 If LIST is omitted, uses C<$_>.
5155 =item unpack TEMPLATE,EXPR
5157 C<unpack> does the reverse of C<pack>: it takes a string
5158 and expands it out into a list of values.
5159 (In scalar context, it returns merely the first value produced.)
5161 The string is broken into chunks described by the TEMPLATE. Each chunk
5162 is converted separately to a value. Typically, either the string is a result
5163 of C<pack>, or the bytes of the string represent a C structure of some
5166 The TEMPLATE has the same format as in the C<pack> function.
5167 Here's a subroutine that does substring:
5170 my($what,$where,$howmuch) = @_;
5171 unpack("x$where a$howmuch", $what);
5176 sub ordinal { unpack("c",$_[0]); } # same as ord()
5178 In addition to fields allowed in pack(), you may prefix a field with
5179 a %E<lt>numberE<gt> to indicate that
5180 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
5181 themselves. Default is a 16-bit checksum. Checksum is calculated by
5182 summing numeric values of expanded values (for string fields the sum of
5183 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5185 For example, the following
5186 computes the same number as the System V sum program:
5190 unpack("%32C*",<>) % 65535;
5193 The following efficiently counts the number of set bits in a bit vector:
5195 $setbits = unpack("%32b*", $selectmask);
5197 The C<p> and C<P> formats should be used with care. Since Perl
5198 has no way of checking whether the value passed to C<unpack()>
5199 corresponds to a valid memory location, passing a pointer value that's
5200 not known to be valid is likely to have disastrous consequences.
5202 If the repeat count of a field is larger than what the remainder of
5203 the input string allows, repeat count is decreased. If the input string
5204 is longer than one described by the TEMPLATE, the rest is ignored.
5206 See L</pack> for more examples and notes.
5208 =item untie VARIABLE
5210 Breaks the binding between a variable and a package. (See C<tie>.)
5212 =item unshift ARRAY,LIST
5214 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5215 depending on how you look at it. Prepends list to the front of the
5216 array, and returns the new number of elements in the array.
5218 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5220 Note the LIST is prepended whole, not one element at a time, so the
5221 prepended elements stay in the same order. Use C<reverse> to do the
5224 =item use Module LIST
5228 =item use Module VERSION LIST
5232 Imports some semantics into the current package from the named module,
5233 generally by aliasing certain subroutine or variable names into your
5234 package. It is exactly equivalent to
5236 BEGIN { require Module; import Module LIST; }
5238 except that Module I<must> be a bareword.
5240 If the first argument to C<use> is a number or a version tuple, it is
5241 treated as a version instead of a module name. If the version
5242 of the Perl interpreter is less than VERSION, then an error message
5243 is printed and Perl exits immediately.
5245 use 5.005_03; # version number
5246 use v5.6.0; # version tuple
5248 This is often useful if you need to check the current Perl version before
5249 C<use>ing library modules that have changed in incompatible ways from
5250 older versions of Perl. (We try not to do this more than we have to.)
5252 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5253 C<require> makes sure the module is loaded into memory if it hasn't been
5254 yet. The C<import> is not a builtin--it's just an ordinary static method
5255 call into the C<Module> package to tell the module to import the list of
5256 features back into the current package. The module can implement its
5257 C<import> method any way it likes, though most modules just choose to
5258 derive their C<import> method via inheritance from the C<Exporter> class that
5259 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5260 method can be found then the call is skipped.
5262 If you don't want your namespace altered, explicitly supply an empty list:
5266 That is exactly equivalent to
5268 BEGIN { require Module }
5270 If the VERSION argument is present between Module and LIST, then the
5271 C<use> will call the VERSION method in class Module with the given
5272 version as an argument. The default VERSION method, inherited from
5273 the Universal class, croaks if the given version is larger than the
5274 value of the variable C<$Module::VERSION>. (Note that there is not a
5275 comma after VERSION!)
5277 Because this is a wide-open interface, pragmas (compiler directives)
5278 are also implemented this way. Currently implemented pragmas are:
5282 use sigtrap qw(SEGV BUS);
5283 use strict qw(subs vars refs);
5284 use subs qw(afunc blurfl);
5285 use warnings qw(all);
5287 Some of these pseudo-modules import semantics into the current
5288 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5289 which import symbols into the current package (which are effective
5290 through the end of the file).
5292 There's a corresponding C<no> command that unimports meanings imported
5293 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5299 If no C<unimport> method can be found the call fails with a fatal error.
5301 See L<perlmod> for a list of standard modules and pragmas.
5305 Changes the access and modification times on each file of a list of
5306 files. The first two elements of the list must be the NUMERICAL access
5307 and modification times, in that order. Returns the number of files
5308 successfully changed. The inode change time of each file is set
5309 to the current time. This code has the same effect as the C<touch>
5310 command if the files already exist:
5314 utime $now, $now, @ARGV;
5318 Returns a list consisting of all the values of the named hash. (In a
5319 scalar context, returns the number of values.) The values are
5320 returned in an apparently random order. The actual random order is
5321 subject to change in future versions of perl, but it is guaranteed to
5322 be the same order as either the C<keys> or C<each> function would
5323 produce on the same (unmodified) hash.
5325 Note that you cannot modify the values of a hash this way, because the
5326 returned list is just a copy. You need to use a hash slice for that,
5327 since it's lvaluable in a way that values() is not.
5329 for (values %hash) { s/foo/bar/g } # FAILS!
5330 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5332 As a side effect, calling values() resets the HASH's internal iterator.
5333 See also C<keys>, C<each>, and C<sort>.
5335 =item vec EXPR,OFFSET,BITS
5337 Treats the string in EXPR as a bit vector made up of elements of
5338 width BITS, and returns the value of the element specified by OFFSET
5339 as an unsigned integer. BITS therefore specifies the number of bits
5340 that are reserved for each element in the bit vector. This must
5341 be a power of two from 1 to 32 (or 64, if your platform supports
5344 If BITS is 8, "elements" coincide with bytes of the input string.
5346 If BITS is 16 or more, bytes of the input string are grouped into chunks
5347 of size BITS/8, and each group is converted to a number as with
5348 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5349 for BITS==64). See L<"pack"> for details.
5351 If bits is 4 or less, the string is broken into bytes, then the bits
5352 of each byte are broken into 8/BITS groups. Bits of a byte are
5353 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5354 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5355 breaking the single input byte C<chr(0x36)> into two groups gives a list
5356 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5358 C<vec> may also be assigned to, in which case parentheses are needed
5359 to give the expression the correct precedence as in
5361 vec($image, $max_x * $x + $y, 8) = 3;
5363 If the selected element is off the end of the string, the value 0 is
5364 returned. If an element off the end of the string is written to,
5365 Perl will first extend the string with sufficiently many zero bytes.
5367 Strings created with C<vec> can also be manipulated with the logical
5368 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5369 vector operation is desired when both operands are strings.
5370 See L<perlop/"Bitwise String Operators">.
5372 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5373 The comments show the string after each step. Note that this code works
5374 in the same way on big-endian or little-endian machines.
5377 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5379 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5380 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5382 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5383 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5384 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5385 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5386 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5387 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5389 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5390 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5391 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5394 To transform a bit vector into a string or list of 0's and 1's, use these:
5396 $bits = unpack("b*", $vector);
5397 @bits = split(//, unpack("b*", $vector));
5399 If you know the exact length in bits, it can be used in place of the C<*>.
5401 Here is an example to illustrate how the bits actually fall in place:
5407 unpack("V",$_) 01234567890123456789012345678901
5408 ------------------------------------------------------------------
5413 for ($shift=0; $shift < $width; ++$shift) {
5414 for ($off=0; $off < 32/$width; ++$off) {
5415 $str = pack("B*", "0"x32);
5416 $bits = (1<<$shift);
5417 vec($str, $off, $width) = $bits;
5418 $res = unpack("b*",$str);
5419 $val = unpack("V", $str);
5426 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5427 $off, $width, $bits, $val, $res
5431 Regardless of the machine architecture on which it is run, the above
5432 example should print the following table:
5435 unpack("V",$_) 01234567890123456789012345678901
5436 ------------------------------------------------------------------
5437 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5438 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5439 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5440 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5441 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5442 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5443 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5444 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5445 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5446 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5447 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5448 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5449 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5450 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5451 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5452 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5453 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5454 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5455 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5456 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5457 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5458 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5459 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5460 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5461 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5462 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5463 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5464 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5465 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5466 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5467 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5468 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5469 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5470 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5471 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5472 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5473 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5474 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5475 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5476 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5477 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5478 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5479 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5480 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5481 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5482 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5483 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5484 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5485 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5486 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5487 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5488 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5489 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5490 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5491 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5492 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5493 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5494 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5495 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5496 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5497 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5498 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5499 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5500 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5501 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5502 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5503 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5504 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5505 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5506 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5507 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5508 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5509 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5510 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5511 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5512 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5513 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5514 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5515 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5516 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5517 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5518 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5519 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5520 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5521 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5522 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5523 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5524 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5525 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5526 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5527 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5528 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5529 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5530 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5531 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5532 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5533 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5534 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5535 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5536 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5537 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5538 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5539 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5540 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5541 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5542 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5543 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5544 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5545 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5546 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5547 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5548 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5549 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5550 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5551 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5552 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5553 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5554 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5555 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5556 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5557 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5558 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5559 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5560 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5561 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5562 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5563 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5564 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5568 Behaves like the wait(2) system call on your system: it waits for a child
5569 process to terminate and returns the pid of the deceased process, or
5570 C<-1> if there are no child processes. The status is returned in C<$?>.
5571 Note that a return value of C<-1> could mean that child processes are
5572 being automatically reaped, as described in L<perlipc>.
5574 =item waitpid PID,FLAGS
5576 Waits for a particular child process to terminate and returns the pid of
5577 the deceased process, or C<-1> if there is no such child process. On some
5578 systems, a value of 0 indicates that there are processes still running.
5579 The status is returned in C<$?>. If you say
5581 use POSIX ":sys_wait_h";
5584 $kid = waitpid(-1,&WNOHANG);
5587 then you can do a non-blocking wait for all pending zombie processes.
5588 Non-blocking wait is available on machines supporting either the
5589 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5590 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5591 system call by remembering the status values of processes that have
5592 exited but have not been harvested by the Perl script yet.)
5594 Note that on some systems, a return value of C<-1> could mean that child
5595 processes are being automatically reaped. See L<perlipc> for details,
5596 and for other examples.
5600 Returns true if the context of the currently executing subroutine is
5601 looking for a list value. Returns false if the context is looking
5602 for a scalar. Returns the undefined value if the context is looking
5603 for no value (void context).
5605 return unless defined wantarray; # don't bother doing more
5606 my @a = complex_calculation();
5607 return wantarray ? @a : "@a";
5609 This function should have been named wantlist() instead.
5613 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5616 If LIST is empty and C<$@> already contains a value (typically from a
5617 previous eval) that value is used after appending C<"\t...caught">
5618 to C<$@>. This is useful for staying almost, but not entirely similar to
5621 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5623 No message is printed if there is a C<$SIG{__WARN__}> handler
5624 installed. It is the handler's responsibility to deal with the message
5625 as it sees fit (like, for instance, converting it into a C<die>). Most
5626 handlers must therefore make arrangements to actually display the
5627 warnings that they are not prepared to deal with, by calling C<warn>
5628 again in the handler. Note that this is quite safe and will not
5629 produce an endless loop, since C<__WARN__> hooks are not called from
5632 You will find this behavior is slightly different from that of
5633 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5634 instead call C<die> again to change it).
5636 Using a C<__WARN__> handler provides a powerful way to silence all
5637 warnings (even the so-called mandatory ones). An example:
5639 # wipe out *all* compile-time warnings
5640 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5642 my $foo = 20; # no warning about duplicate my $foo,
5643 # but hey, you asked for it!
5644 # no compile-time or run-time warnings before here
5647 # run-time warnings enabled after here
5648 warn "\$foo is alive and $foo!"; # does show up
5650 See L<perlvar> for details on setting C<%SIG> entries, and for more
5651 examples. See the Carp module for other kinds of warnings using its
5652 carp() and cluck() functions.
5654 =item write FILEHANDLE
5660 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5661 using the format associated with that file. By default the format for
5662 a file is the one having the same name as the filehandle, but the
5663 format for the current output channel (see the C<select> function) may be set
5664 explicitly by assigning the name of the format to the C<$~> variable.
5666 Top of form processing is handled automatically: if there is
5667 insufficient room on the current page for the formatted record, the
5668 page is advanced by writing a form feed, a special top-of-page format
5669 is used to format the new page header, and then the record is written.
5670 By default the top-of-page format is the name of the filehandle with
5671 "_TOP" appended, but it may be dynamically set to the format of your
5672 choice by assigning the name to the C<$^> variable while the filehandle is
5673 selected. The number of lines remaining on the current page is in
5674 variable C<$->, which can be set to C<0> to force a new page.
5676 If FILEHANDLE is unspecified, output goes to the current default output
5677 channel, which starts out as STDOUT but may be changed by the
5678 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5679 is evaluated and the resulting string is used to look up the name of
5680 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5682 Note that write is I<not> the opposite of C<read>. Unfortunately.
5686 The transliteration operator. Same as C<tr///>. See L<perlop>.