3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 An named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
204 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239 C<utime>, C<wait>, C<waitpid>
241 For more information about the portability of these functions, see
242 L<perlport> and other available platform-specific documentation.
244 =head2 Alphabetical Listing of Perl Functions
248 =item I<-X> FILEHANDLE
254 A file test, where X is one of the letters listed below. This unary
255 operator takes one argument, either a filename or a filehandle, and
256 tests the associated file to see if something is true about it. If the
257 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
258 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
259 the undefined value if the file doesn't exist. Despite the funny
260 names, precedence is the same as any other named unary operator, and
261 the argument may be parenthesized like any other unary operator. The
262 operator may be any of:
263 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
264 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
277 -z File has zero size.
278 -s File has nonzero size (returns size).
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
304 next unless -f $_; # ignore specials
308 The interpretation of the file permission operators C<-r>, C<-R>,
309 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310 of the file and the uids and gids of the user. There may be other
311 reasons you can't actually read, write, or execute the file. Such
312 reasons may be for example network filesystem access controls, ACLs
313 (access control lists), read-only filesystems, and unrecognized
316 Also note that, for the superuser on the local filesystems, the C<-r>,
317 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
318 if any execute bit is set in the mode. Scripts run by the superuser
319 may thus need to do a stat() to determine the actual mode of the file,
320 or temporarily set their effective uid to something else.
322 If you are using ACLs, there is a pragma called C<filetest> that may
323 produce more accurate results than the bare stat() mode bits.
324 When under the C<use filetest 'access'> the above-mentioned filetests
325 will test whether the permission can (not) be granted using the
326 access() family of system calls. Also note that the C<-x> and C<-X> may
327 under this pragma return true even if there are no execute permission
328 bits set (nor any extra execute permission ACLs). This strangeness is
329 due to the underlying system calls' definitions. Read the
330 documentation for the C<filetest> pragma for more information.
332 Note that C<-s/a/b/> does not do a negated substitution. Saying
333 C<-exp($foo)> still works as expected, however--only single letters
334 following a minus are interpreted as file tests.
336 The C<-T> and C<-B> switches work as follows. The first block or so of the
337 file is examined for odd characters such as strange control codes or
338 characters with the high bit set. If too many strange characters (E<gt>30%)
339 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
340 containing null in the first block is considered a binary file. If C<-T>
341 or C<-B> is used on a filehandle, the current stdio buffer is examined
342 rather than the first block. Both C<-T> and C<-B> return true on a null
343 file, or a file at EOF when testing a filehandle. Because you have to
344 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
345 against the file first, as in C<next unless -f $file && -T $file>.
347 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
348 the special filehandle consisting of a solitary underline, then the stat
349 structure of the previous file test (or stat operator) is used, saving
350 a system call. (This doesn't work with C<-t>, and you need to remember
351 that lstat() and C<-l> will leave values in the stat structure for the
352 symbolic link, not the real file.) Example:
354 print "Can do.\n" if -r $a || -w _ || -x _;
357 print "Readable\n" if -r _;
358 print "Writable\n" if -w _;
359 print "Executable\n" if -x _;
360 print "Setuid\n" if -u _;
361 print "Setgid\n" if -g _;
362 print "Sticky\n" if -k _;
363 print "Text\n" if -T _;
364 print "Binary\n" if -B _;
370 Returns the absolute value of its argument.
371 If VALUE is omitted, uses C<$_>.
373 =item accept NEWSOCKET,GENERICSOCKET
375 Accepts an incoming socket connect, just as the accept(2) system call
376 does. Returns the packed address if it succeeded, false otherwise.
377 See the example in L<perlipc/"Sockets: Client/Server Communication">.
383 Arranges to have a SIGALRM delivered to this process after the
384 specified number of seconds have elapsed. If SECONDS is not specified,
385 the value stored in C<$_> is used. (On some machines,
386 unfortunately, the elapsed time may be up to one second less than you
387 specified because of how seconds are counted.) Only one timer may be
388 counting at once. Each call disables the previous timer, and an
389 argument of C<0> may be supplied to cancel the previous timer without
390 starting a new one. The returned value is the amount of time remaining
391 on the previous timer.
393 For delays of finer granularity than one second, you may use Perl's
394 four-argument version of select() leaving the first three arguments
395 undefined, or you might be able to use the C<syscall> interface to
396 access setitimer(2) if your system supports it. The Time::HiRes module
397 from CPAN may also prove useful.
399 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
400 (C<sleep> may be internally implemented in your system with C<alarm>)
402 If you want to use C<alarm> to time out a system call you need to use an
403 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
404 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
405 restart system calls on some systems. Using C<eval>/C<die> always works,
406 modulo the caveats given in L<perlipc/"Signals">.
409 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
411 $nread = sysread SOCKET, $buffer, $size;
415 die unless $@ eq "alarm\n"; # propagate unexpected errors
424 Returns the arctangent of Y/X in the range -PI to PI.
426 For the tangent operation, you may use the C<Math::Trig::tan>
427 function, or use the familiar relation:
429 sub tan { sin($_[0]) / cos($_[0]) }
431 =item bind SOCKET,NAME
433 Binds a network address to a socket, just as the bind system call
434 does. Returns true if it succeeded, false otherwise. NAME should be a
435 packed address of the appropriate type for the socket. See the examples in
436 L<perlipc/"Sockets: Client/Server Communication">.
438 =item binmode FILEHANDLE
440 Arranges for FILEHANDLE to be read or written in "binary" mode on
441 systems whose run-time libraries force the programmer to guess
442 between binary and text files. If FILEHANDLE is an expression, the
443 value is taken as the name of the filehandle. binmode() should be
444 called after the C<open> but before any I/O is done on the filehandle.
445 The only way to reset binary mode on a filehandle is to reopen the
448 The operating system, device drivers, C libraries, and Perl run-time
449 system all conspire to let the programmer conveniently treat a
450 simple, one-byte C<\n> as the line terminator, irrespective of its
451 external representation. On Unix and its brethren, the native file
452 representation exactly matches the internal representation, making
453 everyone's lives unbelievably simpler. Consequently, L<binmode>
454 has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
455 to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and
456 C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
457 C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because
458 it's only one byte, and the internal and external representations
461 In legacy systems like MS-DOS and its embellishments, your program
462 sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
463 that's not what's physically stored on disk. What's worse, these
464 systems refuse to help you with this; it's up to you to remember
465 what to do. And you mustn't go applying binmode() with wild abandon,
466 either, because if your system does care about binmode(), then using
467 it when you shouldn't is just as perilous as failing to use it when
470 That means that on any version of Microsoft WinXX that you might
471 care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
472 to be converted to C<\n> when read into your program, and causes
473 any C<\n> in your program to be converted back to C<\cM\cJ> on
474 output to disk. This sad discrepancy leads to no end of
475 problems in not just the readline operator, but also when using
476 seek(), tell(), and read() calls. See L<perlport> for other painful
477 details. See the C<$/> and C<$\> variables in L<perlvar> for how
478 to manually set your input and output line-termination sequences.
480 =item bless REF,CLASSNAME
484 This function tells the thingy referenced by REF that it is now an object
485 in the CLASSNAME package. If CLASSNAME is omitted, the current package
486 is used. Because a C<bless> is often the last thing in a constructor,
487 it returns the reference for convenience. Always use the two-argument
488 version if the function doing the blessing might be inherited by a
489 derived class. See L<perltoot> and L<perlobj> for more about the blessing
490 (and blessings) of objects.
492 Consider always blessing objects in CLASSNAMEs that are mixed case.
493 Namespaces with all lowercase names are considered reserved for
494 Perl pragmata. Builtin types have all uppercase names, so to prevent
495 confusion, you may wish to avoid such package names as well. Make sure
496 that CLASSNAME is a true value.
498 See L<perlmod/"Perl Modules">.
504 Returns the context of the current subroutine call. In scalar context,
505 returns the caller's package name if there is a caller, that is, if
506 we're in a subroutine or C<eval> or C<require>, and the undefined value
507 otherwise. In list context, returns
509 ($package, $filename, $line) = caller;
511 With EXPR, it returns some extra information that the debugger uses to
512 print a stack trace. The value of EXPR indicates how many call frames
513 to go back before the current one.
515 ($package, $filename, $line, $subroutine, $hasargs,
516 $wantarray, $evaltext, $is_require, $hints) = caller($i);
518 Here $subroutine may be C<(eval)> if the frame is not a subroutine
519 call, but an C<eval>. In such a case additional elements $evaltext and
520 C<$is_require> are set: C<$is_require> is true if the frame is created by a
521 C<require> or C<use> statement, $evaltext contains the text of the
522 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
523 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
524 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
525 frame. C<$hints> contains pragmatic hints that the caller was
526 compiled with. The C<$hints> value is subject to change between versions
527 of Perl, and is not meant for external use.
529 Furthermore, when called from within the DB package, caller returns more
530 detailed information: it sets the list variable C<@DB::args> to be the
531 arguments with which the subroutine was invoked.
533 Be aware that the optimizer might have optimized call frames away before
534 C<caller> had a chance to get the information. That means that C<caller(N)>
535 might not return information about the call frame you expect it do, for
536 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
537 previous time C<caller> was called.
541 Changes the working directory to EXPR, if possible. If EXPR is omitted,
542 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
543 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
544 set, C<chdir> does nothing. It returns true upon success, false
545 otherwise. See the example under C<die>.
549 Changes the permissions of a list of files. The first element of the
550 list must be the numerical mode, which should probably be an octal
551 number, and which definitely should I<not> a string of octal digits:
552 C<0644> is okay, C<'0644'> is not. Returns the number of files
553 successfully changed. See also L</oct>, if all you have is a string.
555 $cnt = chmod 0755, 'foo', 'bar';
556 chmod 0755, @executables;
557 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
559 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
560 $mode = 0644; chmod $mode, 'foo'; # this is best
562 You can also import the symbolic C<S_I*> constants from the Fcntl
567 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
568 # This is identical to the chmod 0755 of the above example.
576 This safer version of L</chop> removes any trailing string
577 that corresponds to the current value of C<$/> (also known as
578 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
579 number of characters removed from all its arguments. It's often used to
580 remove the newline from the end of an input record when you're worried
581 that the final record may be missing its newline. When in paragraph
582 mode (C<$/ = "">), it removes all trailing newlines from the string.
583 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
584 a reference to an integer or the like, see L<perlvar>) chomp() won't
586 If VARIABLE is omitted, it chomps C<$_>. Example:
589 chomp; # avoid \n on last field
594 You can actually chomp anything that's an lvalue, including an assignment:
597 chomp($answer = <STDIN>);
599 If you chomp a list, each element is chomped, and the total number of
600 characters removed is returned.
608 Chops off the last character of a string and returns the character
609 chopped. It's used primarily to remove the newline from the end of an
610 input record, but is much more efficient than C<s/\n//> because it neither
611 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
615 chop; # avoid \n on last field
620 You can actually chop anything that's an lvalue, including an assignment:
623 chop($answer = <STDIN>);
625 If you chop a list, each element is chopped. Only the value of the
626 last C<chop> is returned.
628 Note that C<chop> returns the last character. To return all but the last
629 character, use C<substr($string, 0, -1)>.
633 Changes the owner (and group) of a list of files. The first two
634 elements of the list must be the I<numeric> uid and gid, in that
635 order. A value of -1 in either position is interpreted by most
636 systems to leave that value unchanged. Returns the number of files
637 successfully changed.
639 $cnt = chown $uid, $gid, 'foo', 'bar';
640 chown $uid, $gid, @filenames;
642 Here's an example that looks up nonnumeric uids in the passwd file:
645 chomp($user = <STDIN>);
647 chomp($pattern = <STDIN>);
649 ($login,$pass,$uid,$gid) = getpwnam($user)
650 or die "$user not in passwd file";
652 @ary = glob($pattern); # expand filenames
653 chown $uid, $gid, @ary;
655 On most systems, you are not allowed to change the ownership of the
656 file unless you're the superuser, although you should be able to change
657 the group to any of your secondary groups. On insecure systems, these
658 restrictions may be relaxed, but this is not a portable assumption.
659 On POSIX systems, you can detect this condition this way:
661 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
662 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
668 Returns the character represented by that NUMBER in the character set.
669 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
670 chr(0x263a) is a Unicode smiley face (but only within the scope of
671 a C<use utf8>). For the reverse, use L</ord>.
672 See L<utf8> for more about Unicode.
674 If NUMBER is omitted, uses C<$_>.
676 =item chroot FILENAME
680 This function works like the system call by the same name: it makes the
681 named directory the new root directory for all further pathnames that
682 begin with a C</> by your process and all its children. (It doesn't
683 change your current working directory, which is unaffected.) For security
684 reasons, this call is restricted to the superuser. If FILENAME is
685 omitted, does a C<chroot> to C<$_>.
687 =item close FILEHANDLE
691 Closes the file or pipe associated with the file handle, returning true
692 only if stdio successfully flushes buffers and closes the system file
693 descriptor. Closes the currently selected filehandle if the argument
696 You don't have to close FILEHANDLE if you are immediately going to do
697 another C<open> on it, because C<open> will close it for you. (See
698 C<open>.) However, an explicit C<close> on an input file resets the line
699 counter (C<$.>), while the implicit close done by C<open> does not.
701 If the file handle came from a piped open C<close> will additionally
702 return false if one of the other system calls involved fails or if the
703 program exits with non-zero status. (If the only problem was that the
704 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
705 also waits for the process executing on the pipe to complete, in case you
706 want to look at the output of the pipe afterwards, and
707 implicitly puts the exit status value of that command into C<$?>.
709 Prematurely closing the read end of a pipe (i.e. before the process
710 writing to it at the other end has closed it) will result in a
711 SIGPIPE being delivered to the writer. If the other end can't
712 handle that, be sure to read all the data before closing the pipe.
716 open(OUTPUT, '|sort >foo') # pipe to sort
717 or die "Can't start sort: $!";
718 #... # print stuff to output
719 close OUTPUT # wait for sort to finish
720 or warn $! ? "Error closing sort pipe: $!"
721 : "Exit status $? from sort";
722 open(INPUT, 'foo') # get sort's results
723 or die "Can't open 'foo' for input: $!";
725 FILEHANDLE may be an expression whose value can be used as an indirect
726 filehandle, usually the real filehandle name.
728 =item closedir DIRHANDLE
730 Closes a directory opened by C<opendir> and returns the success of that
733 DIRHANDLE may be an expression whose value can be used as an indirect
734 dirhandle, usually the real dirhandle name.
736 =item connect SOCKET,NAME
738 Attempts to connect to a remote socket, just as the connect system call
739 does. Returns true if it succeeded, false otherwise. NAME should be a
740 packed address of the appropriate type for the socket. See the examples in
741 L<perlipc/"Sockets: Client/Server Communication">.
745 Actually a flow control statement rather than a function. If there is a
746 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
747 C<foreach>), it is always executed just before the conditional is about to
748 be evaluated again, just like the third part of a C<for> loop in C. Thus
749 it can be used to increment a loop variable, even when the loop has been
750 continued via the C<next> statement (which is similar to the C C<continue>
753 C<last>, C<next>, or C<redo> may appear within a C<continue>
754 block. C<last> and C<redo> will behave as if they had been executed within
755 the main block. So will C<next>, but since it will execute a C<continue>
756 block, it may be more entertaining.
759 ### redo always comes here
762 ### next always comes here
764 # then back the top to re-check EXPR
766 ### last always comes here
768 Omitting the C<continue> section is semantically equivalent to using an
769 empty one, logically enough. In that case, C<next> goes directly back
770 to check the condition at the top of the loop.
774 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
775 takes cosine of C<$_>.
777 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
778 function, or use this relation:
780 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
782 =item crypt PLAINTEXT,SALT
784 Encrypts a string exactly like the crypt(3) function in the C library
785 (assuming that you actually have a version there that has not been
786 extirpated as a potential munition). This can prove useful for checking
787 the password file for lousy passwords, amongst other things. Only the
788 guys wearing white hats should do this.
790 Note that C<crypt> is intended to be a one-way function, much like breaking
791 eggs to make an omelette. There is no (known) corresponding decrypt
792 function. As a result, this function isn't all that useful for
793 cryptography. (For that, see your nearby CPAN mirror.)
795 When verifying an existing encrypted string you should use the encrypted
796 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
797 allows your code to work with the standard C<crypt> and with more
798 exotic implementations. When choosing a new salt create a random two
799 character string whose characters come from the set C<[./0-9A-Za-z]>
800 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
802 Here's an example that makes sure that whoever runs this program knows
805 $pwd = (getpwuid($<))[1];
809 chomp($word = <STDIN>);
813 if (crypt($word, $pwd) ne $pwd) {
819 Of course, typing in your own password to whoever asks you
822 The L<crypt> function is unsuitable for encrypting large quantities
823 of data, not least of all because you can't get the information
824 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
825 on your favorite CPAN mirror for a slew of potentially useful
830 [This function has been largely superseded by the C<untie> function.]
832 Breaks the binding between a DBM file and a hash.
834 =item dbmopen HASH,DBNAME,MASK
836 [This function has been largely superseded by the C<tie> function.]
838 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
839 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
840 argument is I<not> a filehandle, even though it looks like one). DBNAME
841 is the name of the database (without the F<.dir> or F<.pag> extension if
842 any). If the database does not exist, it is created with protection
843 specified by MASK (as modified by the C<umask>). If your system supports
844 only the older DBM functions, you may perform only one C<dbmopen> in your
845 program. In older versions of Perl, if your system had neither DBM nor
846 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
849 If you don't have write access to the DBM file, you can only read hash
850 variables, not set them. If you want to test whether you can write,
851 either use file tests or try setting a dummy hash entry inside an C<eval>,
852 which will trap the error.
854 Note that functions such as C<keys> and C<values> may return huge lists
855 when used on large DBM files. You may prefer to use the C<each>
856 function to iterate over large DBM files. Example:
858 # print out history file offsets
859 dbmopen(%HIST,'/usr/lib/news/history',0666);
860 while (($key,$val) = each %HIST) {
861 print $key, ' = ', unpack('L',$val), "\n";
865 See also L<AnyDBM_File> for a more general description of the pros and
866 cons of the various dbm approaches, as well as L<DB_File> for a particularly
869 You can control which DBM library you use by loading that library
870 before you call dbmopen():
873 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
874 or die "Can't open netscape history file: $!";
880 Returns a Boolean value telling whether EXPR has a value other than
881 the undefined value C<undef>. If EXPR is not present, C<$_> will be
884 Many operations return C<undef> to indicate failure, end of file,
885 system error, uninitialized variable, and other exceptional
886 conditions. This function allows you to distinguish C<undef> from
887 other values. (A simple Boolean test will not distinguish among
888 C<undef>, zero, the empty string, and C<"0">, which are all equally
889 false.) Note that since C<undef> is a valid scalar, its presence
890 doesn't I<necessarily> indicate an exceptional condition: C<pop>
891 returns C<undef> when its argument is an empty array, I<or> when the
892 element to return happens to be C<undef>.
894 You may also use C<defined(&func)> to check whether subroutine C<&func>
895 has ever been defined. The return value is unaffected by any forward
896 declarations of C<&foo>.
898 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
899 used to report whether memory for that aggregate has ever been
900 allocated. This behavior may disappear in future versions of Perl.
901 You should instead use a simple test for size:
903 if (@an_array) { print "has array elements\n" }
904 if (%a_hash) { print "has hash members\n" }
906 When used on a hash element, it tells you whether the value is defined,
907 not whether the key exists in the hash. Use L</exists> for the latter
912 print if defined $switch{'D'};
913 print "$val\n" while defined($val = pop(@ary));
914 die "Can't readlink $sym: $!"
915 unless defined($value = readlink $sym);
916 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
917 $debugging = 0 unless defined $debugging;
919 Note: Many folks tend to overuse C<defined>, and then are surprised to
920 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
921 defined values. For example, if you say
925 The pattern match succeeds, and C<$1> is defined, despite the fact that it
926 matched "nothing". But it didn't really match nothing--rather, it
927 matched something that happened to be zero characters long. This is all
928 very above-board and honest. When a function returns an undefined value,
929 it's an admission that it couldn't give you an honest answer. So you
930 should use C<defined> only when you're questioning the integrity of what
931 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
934 See also L</undef>, L</exists>, L</ref>.
938 Given an expression that specifies a hash element, array element, hash slice,
939 or array slice, deletes the specified element(s) from the hash or array.
940 In the case of an array, if the array elements happen to be at the end,
941 the size of the array will shrink to the highest element that tests
942 true for exists() (or 0 if no such element exists).
944 Returns each element so deleted or the undefined value if there was no such
945 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
946 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
947 from a C<tie>d hash or array may not necessarily return anything.
949 Deleting an array element effectively returns that position of the array
950 to its initial, uninitialized state. Subsequently testing for the same
951 element with exists() will return false. Note that deleting array
952 elements in the middle of an array will not shift the index of the ones
953 after them down--use splice() for that. See L</exists>.
955 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
957 foreach $key (keys %HASH) {
961 foreach $index (0 .. $#ARRAY) {
962 delete $ARRAY[$index];
967 delete @HASH{keys %HASH};
969 delete @ARRAY[0 .. $#ARRAY];
971 But both of these are slower than just assigning the empty list
972 or undefining %HASH or @ARRAY:
974 %HASH = (); # completely empty %HASH
975 undef %HASH; # forget %HASH ever existed
977 @ARRAY = (); # completely empty @ARRAY
978 undef @ARRAY; # forget @ARRAY ever existed
980 Note that the EXPR can be arbitrarily complicated as long as the final
981 operation is a hash element, array element, hash slice, or array slice
984 delete $ref->[$x][$y]{$key};
985 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
987 delete $ref->[$x][$y][$index];
988 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
992 Outside an C<eval>, prints the value of LIST to C<STDERR> and
993 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
994 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
995 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
996 an C<eval(),> the error message is stuffed into C<$@> and the
997 C<eval> is terminated with the undefined value. This makes
998 C<die> the way to raise an exception.
1000 Equivalent examples:
1002 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1003 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1005 If the value of EXPR does not end in a newline, the current script line
1006 number and input line number (if any) are also printed, and a newline
1007 is supplied. Note that the "input line number" (also known as "chunk")
1008 is subject to whatever notion of "line" happens to be currently in
1009 effect, and is also available as the special variable C<$.>.
1010 See L<perlvar/"$/"> and L<perlvar/"$.">.
1012 Hint: sometimes appending C<", stopped"> to your message
1013 will cause it to make better sense when the string C<"at foo line 123"> is
1014 appended. Suppose you are running script "canasta".
1016 die "/etc/games is no good";
1017 die "/etc/games is no good, stopped";
1019 produce, respectively
1021 /etc/games is no good at canasta line 123.
1022 /etc/games is no good, stopped at canasta line 123.
1024 See also exit(), warn(), and the Carp module.
1026 If LIST is empty and C<$@> already contains a value (typically from a
1027 previous eval) that value is reused after appending C<"\t...propagated">.
1028 This is useful for propagating exceptions:
1031 die unless $@ =~ /Expected exception/;
1033 If C<$@> is empty then the string C<"Died"> is used.
1035 die() can also be called with a reference argument. If this happens to be
1036 trapped within an eval(), $@ contains the reference. This behavior permits
1037 a more elaborate exception handling implementation using objects that
1038 maintain arbitary state about the nature of the exception. Such a scheme
1039 is sometimes preferable to matching particular string values of $@ using
1040 regular expressions. Here's an example:
1042 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1044 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1045 # handle Some::Module::Exception
1048 # handle all other possible exceptions
1052 Because perl will stringify uncaught exception messages before displaying
1053 them, you may want to overload stringification operations on such custom
1054 exception objects. See L<overload> for details about that.
1056 You can arrange for a callback to be run just before the C<die>
1057 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1058 handler will be called with the error text and can change the error
1059 message, if it sees fit, by calling C<die> again. See
1060 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1061 L<"eval BLOCK"> for some examples. Although this feature was meant
1062 to be run only right before your program was to exit, this is not
1063 currently the case--the C<$SIG{__DIE__}> hook is currently called
1064 even inside eval()ed blocks/strings! If one wants the hook to do
1065 nothing in such situations, put
1069 as the first line of the handler (see L<perlvar/$^S>). Because
1070 this promotes strange action at a distance, this counterintuitive
1071 behavior may be fixed in a future release.
1075 Not really a function. Returns the value of the last command in the
1076 sequence of commands indicated by BLOCK. When modified by a loop
1077 modifier, executes the BLOCK once before testing the loop condition.
1078 (On other statements the loop modifiers test the conditional first.)
1080 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1081 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1082 See L<perlsyn> for alternative strategies.
1084 =item do SUBROUTINE(LIST)
1086 A deprecated form of subroutine call. See L<perlsub>.
1090 Uses the value of EXPR as a filename and executes the contents of the
1091 file as a Perl script. Its primary use is to include subroutines
1092 from a Perl subroutine library.
1098 scalar eval `cat stat.pl`;
1100 except that it's more efficient and concise, keeps track of the current
1101 filename for error messages, searches the @INC libraries, and updates
1102 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1103 variables. It also differs in that code evaluated with C<do FILENAME>
1104 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1105 same, however, in that it does reparse the file every time you call it,
1106 so you probably don't want to do this inside a loop.
1108 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1109 error. If C<do> can read the file but cannot compile it, it
1110 returns undef and sets an error message in C<$@>. If the file is
1111 successfully compiled, C<do> returns the value of the last expression
1114 Note that inclusion of library modules is better done with the
1115 C<use> and C<require> operators, which also do automatic error checking
1116 and raise an exception if there's a problem.
1118 You might like to use C<do> to read in a program configuration
1119 file. Manual error checking can be done this way:
1121 # read in config files: system first, then user
1122 for $file ("/share/prog/defaults.rc",
1123 "$ENV{HOME}/.someprogrc")
1125 unless ($return = do $file) {
1126 warn "couldn't parse $file: $@" if $@;
1127 warn "couldn't do $file: $!" unless defined $return;
1128 warn "couldn't run $file" unless $return;
1136 This function causes an immediate core dump. See also the B<-u>
1137 command-line switch in L<perlrun>, which does the same thing.
1138 Primarily this is so that you can use the B<undump> program (not
1139 supplied) to turn your core dump into an executable binary after
1140 having initialized all your variables at the beginning of the
1141 program. When the new binary is executed it will begin by executing
1142 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1143 Think of it as a goto with an intervening core dump and reincarnation.
1144 If C<LABEL> is omitted, restarts the program from the top.
1146 B<WARNING>: Any files opened at the time of the dump will I<not>
1147 be open any more when the program is reincarnated, with possible
1148 resulting confusion on the part of Perl.
1150 This function is now largely obsolete, partly because it's very
1151 hard to convert a core file into an executable, and because the
1152 real compiler backends for generating portable bytecode and compilable
1153 C code have superseded it.
1155 If you're looking to use L<dump> to speed up your program, consider
1156 generating bytecode or native C code as described in L<perlcc>. If
1157 you're just trying to accelerate a CGI script, consider using the
1158 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1159 You might also consider autoloading or selfloading, which at least
1160 make your program I<appear> to run faster.
1164 When called in list context, returns a 2-element list consisting of the
1165 key and value for the next element of a hash, so that you can iterate over
1166 it. When called in scalar context, returns the key for only the "next"
1167 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1168 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1171 Entries are returned in an apparently random order. The actual random
1172 order is subject to change in future versions of perl, but it is guaranteed
1173 to be in the same order as either the C<keys> or C<values> function
1174 would produce on the same (unmodified) hash.
1176 When the hash is entirely read, a null array is returned in list context
1177 (which when assigned produces a false (C<0>) value), and C<undef> in
1178 scalar context. The next call to C<each> after that will start iterating
1179 again. There is a single iterator for each hash, shared by all C<each>,
1180 C<keys>, and C<values> function calls in the program; it can be reset by
1181 reading all the elements from the hash, or by evaluating C<keys HASH> or
1182 C<values HASH>. If you add or delete elements of a hash while you're
1183 iterating over it, you may get entries skipped or duplicated, so don't.
1185 The following prints out your environment like the printenv(1) program,
1186 only in a different order:
1188 while (($key,$value) = each %ENV) {
1189 print "$key=$value\n";
1192 See also C<keys>, C<values> and C<sort>.
1194 =item eof FILEHANDLE
1200 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1201 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1202 gives the real filehandle. (Note that this function actually
1203 reads a character and then C<ungetc>s it, so isn't very useful in an
1204 interactive context.) Do not read from a terminal file (or call
1205 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1206 as terminals may lose the end-of-file condition if you do.
1208 An C<eof> without an argument uses the last file read. Using C<eof()>
1209 with empty parentheses is very different. It refers to the pseudo file
1210 formed from the files listed on the command line and accessed via the
1211 C<E<lt>E<gt>> operator. Since C<E<lt>E<gt>> isn't explicitly opened,
1212 as a normal filehandle is, an C<eof()> before C<E<lt>E<gt>> has been
1213 used will cause C<@ARGV> to be examined to determine if input is
1216 In a C<while (E<lt>E<gt>)> loop, C<eof> or C<eof(ARGV)> can be used to
1217 detect the end of each file, C<eof()> will only detect the end of the
1218 last file. Examples:
1220 # reset line numbering on each input file
1222 next if /^\s*#/; # skip comments
1225 close ARGV if eof; # Not eof()!
1228 # insert dashes just before last line of last file
1230 if (eof()) { # check for end of current file
1231 print "--------------\n";
1232 close(ARGV); # close or last; is needed if we
1233 # are reading from the terminal
1238 Practical hint: you almost never need to use C<eof> in Perl, because the
1239 input operators typically return C<undef> when they run out of data, or if
1246 In the first form, the return value of EXPR is parsed and executed as if it
1247 were a little Perl program. The value of the expression (which is itself
1248 determined within scalar context) is first parsed, and if there weren't any
1249 errors, executed in the context of the current Perl program, so that any
1250 variable settings or subroutine and format definitions remain afterwards.
1251 Note that the value is parsed every time the eval executes. If EXPR is
1252 omitted, evaluates C<$_>. This form is typically used to delay parsing
1253 and subsequent execution of the text of EXPR until run time.
1255 In the second form, the code within the BLOCK is parsed only once--at the
1256 same time the code surrounding the eval itself was parsed--and executed
1257 within the context of the current Perl program. This form is typically
1258 used to trap exceptions more efficiently than the first (see below), while
1259 also providing the benefit of checking the code within BLOCK at compile
1262 The final semicolon, if any, may be omitted from the value of EXPR or within
1265 In both forms, the value returned is the value of the last expression
1266 evaluated inside the mini-program; a return statement may be also used, just
1267 as with subroutines. The expression providing the return value is evaluated
1268 in void, scalar, or list context, depending on the context of the eval itself.
1269 See L</wantarray> for more on how the evaluation context can be determined.
1271 If there is a syntax error or runtime error, or a C<die> statement is
1272 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1273 error message. If there was no error, C<$@> is guaranteed to be a null
1274 string. Beware that using C<eval> neither silences perl from printing
1275 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1276 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1277 L</warn> and L<perlvar>.
1279 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1280 determining whether a particular feature (such as C<socket> or C<symlink>)
1281 is implemented. It is also Perl's exception trapping mechanism, where
1282 the die operator is used to raise exceptions.
1284 If the code to be executed doesn't vary, you may use the eval-BLOCK
1285 form to trap run-time errors without incurring the penalty of
1286 recompiling each time. The error, if any, is still returned in C<$@>.
1289 # make divide-by-zero nonfatal
1290 eval { $answer = $a / $b; }; warn $@ if $@;
1292 # same thing, but less efficient
1293 eval '$answer = $a / $b'; warn $@ if $@;
1295 # a compile-time error
1296 eval { $answer = }; # WRONG
1299 eval '$answer ='; # sets $@
1301 Due to the current arguably broken state of C<__DIE__> hooks, when using
1302 the C<eval{}> form as an exception trap in libraries, you may wish not
1303 to trigger any C<__DIE__> hooks that user code may have installed.
1304 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1305 as shown in this example:
1307 # a very private exception trap for divide-by-zero
1308 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1311 This is especially significant, given that C<__DIE__> hooks can call
1312 C<die> again, which has the effect of changing their error messages:
1314 # __DIE__ hooks may modify error messages
1316 local $SIG{'__DIE__'} =
1317 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1318 eval { die "foo lives here" };
1319 print $@ if $@; # prints "bar lives here"
1322 Because this promotes action at a distance, this counterintuitive behavior
1323 may be fixed in a future release.
1325 With an C<eval>, you should be especially careful to remember what's
1326 being looked at when:
1332 eval { $x }; # CASE 4
1334 eval "\$$x++"; # CASE 5
1337 Cases 1 and 2 above behave identically: they run the code contained in
1338 the variable $x. (Although case 2 has misleading double quotes making
1339 the reader wonder what else might be happening (nothing is).) Cases 3
1340 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1341 does nothing but return the value of $x. (Case 4 is preferred for
1342 purely visual reasons, but it also has the advantage of compiling at
1343 compile-time instead of at run-time.) Case 5 is a place where
1344 normally you I<would> like to use double quotes, except that in this
1345 particular situation, you can just use symbolic references instead, as
1348 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1349 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1353 =item exec PROGRAM LIST
1355 The C<exec> function executes a system command I<and never returns>--
1356 use C<system> instead of C<exec> if you want it to return. It fails and
1357 returns false only if the command does not exist I<and> it is executed
1358 directly instead of via your system's command shell (see below).
1360 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1361 warns you if there is a following statement which isn't C<die>, C<warn>,
1362 or C<exit> (if C<-w> is set - but you always do that). If you
1363 I<really> want to follow an C<exec> with some other statement, you
1364 can use one of these styles to avoid the warning:
1366 exec ('foo') or print STDERR "couldn't exec foo: $!";
1367 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1369 If there is more than one argument in LIST, or if LIST is an array
1370 with more than one value, calls execvp(3) with the arguments in LIST.
1371 If there is only one scalar argument or an array with one element in it,
1372 the argument is checked for shell metacharacters, and if there are any,
1373 the entire argument is passed to the system's command shell for parsing
1374 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1375 If there are no shell metacharacters in the argument, it is split into
1376 words and passed directly to C<execvp>, which is more efficient.
1379 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1380 exec "sort $outfile | uniq";
1382 If you don't really want to execute the first argument, but want to lie
1383 to the program you are executing about its own name, you can specify
1384 the program you actually want to run as an "indirect object" (without a
1385 comma) in front of the LIST. (This always forces interpretation of the
1386 LIST as a multivalued list, even if there is only a single scalar in
1389 $shell = '/bin/csh';
1390 exec $shell '-sh'; # pretend it's a login shell
1394 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1396 When the arguments get executed via the system shell, results will
1397 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1400 Using an indirect object with C<exec> or C<system> is also more
1401 secure. This usage (which also works fine with system()) forces
1402 interpretation of the arguments as a multivalued list, even if the
1403 list had just one argument. That way you're safe from the shell
1404 expanding wildcards or splitting up words with whitespace in them.
1406 @args = ( "echo surprise" );
1408 exec @args; # subject to shell escapes
1410 exec { $args[0] } @args; # safe even with one-arg list
1412 The first version, the one without the indirect object, ran the I<echo>
1413 program, passing it C<"surprise"> an argument. The second version
1414 didn't--it tried to run a program literally called I<"echo surprise">,
1415 didn't find it, and set C<$?> to a non-zero value indicating failure.
1417 Note that C<exec> will not call your C<END> blocks, nor will it call
1418 any C<DESTROY> methods in your objects.
1422 Given an expression that specifies a hash element or array element,
1423 returns true if the specified element in the hash or array has ever
1424 been initialized, even if the corresponding value is undefined. The
1425 element is not autovivified if it doesn't exist.
1427 print "Exists\n" if exists $hash{$key};
1428 print "Defined\n" if defined $hash{$key};
1429 print "True\n" if $hash{$key};
1431 print "Exists\n" if exists $array[$index];
1432 print "Defined\n" if defined $array[$index];
1433 print "True\n" if $array[$index];
1435 A hash or array element can be true only if it's defined, and defined if
1436 it exists, but the reverse doesn't necessarily hold true.
1438 Given an expression that specifies the name of a subroutine,
1439 returns true if the specified subroutine has ever been declared, even
1440 if it is undefined. Mentioning a subroutine name for exists or defined
1441 does not count as declaring it.
1443 print "Exists\n" if exists &subroutine;
1444 print "Defined\n" if defined &subroutine;
1446 Note that the EXPR can be arbitrarily complicated as long as the final
1447 operation is a hash or array key lookup or subroutine name:
1449 if (exists $ref->{A}->{B}->{$key}) { }
1450 if (exists $hash{A}{B}{$key}) { }
1452 if (exists $ref->{A}->{B}->[$ix]) { }
1453 if (exists $hash{A}{B}[$ix]) { }
1455 if (exists &{$ref->{A}{B}{$key}}) { }
1457 Although the deepest nested array or hash will not spring into existence
1458 just because its existence was tested, any intervening ones will.
1459 Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring
1460 into existence due to the existence test for the $key element above.
1461 This happens anywhere the arrow operator is used, including even:
1464 if (exists $ref->{"Some key"}) { }
1465 print $ref; # prints HASH(0x80d3d5c)
1467 This surprising autovivification in what does not at first--or even
1468 second--glance appear to be an lvalue context may be fixed in a future
1471 See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
1472 used on a pseudo-hash.
1474 Use of a subroutine call, rather than a subroutine name, as an argument
1475 to exists() is an error.
1478 exists &sub(); # Error
1482 Evaluates EXPR and exits immediately with that value. Example:
1485 exit 0 if $ans =~ /^[Xx]/;
1487 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1488 universally recognized values for EXPR are C<0> for success and C<1>
1489 for error; other values are subject to interpretation depending on the
1490 environment in which the Perl program is running. For example, exiting
1491 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1492 the mailer to return the item undelivered, but that's not true everywhere.
1494 Don't use C<exit> to abort a subroutine if there's any chance that
1495 someone might want to trap whatever error happened. Use C<die> instead,
1496 which can be trapped by an C<eval>.
1498 The exit() function does not always exit immediately. It calls any
1499 defined C<END> routines first, but these C<END> routines may not
1500 themselves abort the exit. Likewise any object destructors that need to
1501 be called are called before the real exit. If this is a problem, you
1502 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1503 See L<perlmod> for details.
1509 Returns I<e> (the natural logarithm base) to the power of EXPR.
1510 If EXPR is omitted, gives C<exp($_)>.
1512 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1514 Implements the fcntl(2) function. You'll probably have to say
1518 first to get the correct constant definitions. Argument processing and
1519 value return works just like C<ioctl> below.
1523 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1524 or die "can't fcntl F_GETFL: $!";
1526 You don't have to check for C<defined> on the return from C<fnctl>.
1527 Like C<ioctl>, it maps a C<0> return from the system call into
1528 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1529 in numeric context. It is also exempt from the normal B<-w> warnings
1530 on improper numeric conversions.
1532 Note that C<fcntl> will produce a fatal error if used on a machine that
1533 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1534 manpage to learn what functions are available on your system.
1536 =item fileno FILEHANDLE
1538 Returns the file descriptor for a filehandle, or undefined if the
1539 filehandle is not open. This is mainly useful for constructing
1540 bitmaps for C<select> and low-level POSIX tty-handling operations.
1541 If FILEHANDLE is an expression, the value is taken as an indirect
1542 filehandle, generally its name.
1544 You can use this to find out whether two handles refer to the
1545 same underlying descriptor:
1547 if (fileno(THIS) == fileno(THAT)) {
1548 print "THIS and THAT are dups\n";
1551 =item flock FILEHANDLE,OPERATION
1553 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1554 for success, false on failure. Produces a fatal error if used on a
1555 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1556 C<flock> is Perl's portable file locking interface, although it locks
1557 only entire files, not records.
1559 Two potentially non-obvious but traditional C<flock> semantics are
1560 that it waits indefinitely until the lock is granted, and that its locks
1561 B<merely advisory>. Such discretionary locks are more flexible, but offer
1562 fewer guarantees. This means that files locked with C<flock> may be
1563 modified by programs that do not also use C<flock>. See L<perlport>,
1564 your port's specific documentation, or your system-specific local manpages
1565 for details. It's best to assume traditional behavior if you're writing
1566 portable programs. (But if you're not, you should as always feel perfectly
1567 free to write for your own system's idiosyncrasies (sometimes called
1568 "features"). Slavish adherence to portability concerns shouldn't get
1569 in the way of your getting your job done.)
1571 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1572 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1573 you can use the symbolic names if you import them from the Fcntl module,
1574 either individually, or as a group using the ':flock' tag. LOCK_SH
1575 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1576 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1577 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1578 waiting for the lock (check the return status to see if you got it).
1580 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1581 before locking or unlocking it.
1583 Note that the emulation built with lockf(3) doesn't provide shared
1584 locks, and it requires that FILEHANDLE be open with write intent. These
1585 are the semantics that lockf(3) implements. Most if not all systems
1586 implement lockf(3) in terms of fcntl(2) locking, though, so the
1587 differing semantics shouldn't bite too many people.
1589 Note also that some versions of C<flock> cannot lock things over the
1590 network; you would need to use the more system-specific C<fcntl> for
1591 that. If you like you can force Perl to ignore your system's flock(2)
1592 function, and so provide its own fcntl(2)-based emulation, by passing
1593 the switch C<-Ud_flock> to the F<Configure> program when you configure
1596 Here's a mailbox appender for BSD systems.
1598 use Fcntl ':flock'; # import LOCK_* constants
1601 flock(MBOX,LOCK_EX);
1602 # and, in case someone appended
1603 # while we were waiting...
1608 flock(MBOX,LOCK_UN);
1611 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1612 or die "Can't open mailbox: $!";
1615 print MBOX $msg,"\n\n";
1618 On systems that support a real flock(), locks are inherited across fork()
1619 calls, whereas those that must resort to the more capricious fcntl()
1620 function lose the locks, making it harder to write servers.
1622 See also L<DB_File> for other flock() examples.
1626 Does a fork(2) system call to create a new process running the
1627 same program at the same point. It returns the child pid to the
1628 parent process, C<0> to the child process, or C<undef> if the fork is
1629 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1630 are shared, while everything else is copied. On most systems supporting
1631 fork(), great care has gone into making it extremely efficient (for
1632 example, using copy-on-write technology on data pages), making it the
1633 dominant paradigm for multitasking over the last few decades.
1635 All files opened for output are flushed before forking the child process.
1637 If you C<fork> without ever waiting on your children, you will
1638 accumulate zombies. On some systems, you can avoid this by setting
1639 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1640 forking and reaping moribund children.
1642 Note that if your forked child inherits system file descriptors like
1643 STDIN and STDOUT that are actually connected by a pipe or socket, even
1644 if you exit, then the remote server (such as, say, a CGI script or a
1645 backgrounded job launched from a remote shell) won't think you're done.
1646 You should reopen those to F</dev/null> if it's any issue.
1650 Declare a picture format for use by the C<write> function. For
1654 Test: @<<<<<<<< @||||| @>>>>>
1655 $str, $%, '$' . int($num)
1659 $num = $cost/$quantity;
1663 See L<perlform> for many details and examples.
1665 =item formline PICTURE,LIST
1667 This is an internal function used by C<format>s, though you may call it,
1668 too. It formats (see L<perlform>) a list of values according to the
1669 contents of PICTURE, placing the output into the format output
1670 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1671 Eventually, when a C<write> is done, the contents of
1672 C<$^A> are written to some filehandle, but you could also read C<$^A>
1673 yourself and then set C<$^A> back to C<"">. Note that a format typically
1674 does one C<formline> per line of form, but the C<formline> function itself
1675 doesn't care how many newlines are embedded in the PICTURE. This means
1676 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1677 You may therefore need to use multiple formlines to implement a single
1678 record format, just like the format compiler.
1680 Be careful if you put double quotes around the picture, because an C<@>
1681 character may be taken to mean the beginning of an array name.
1682 C<formline> always returns true. See L<perlform> for other examples.
1684 =item getc FILEHANDLE
1688 Returns the next character from the input file attached to FILEHANDLE,
1689 or the undefined value at end of file, or if there was an error.
1690 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1691 efficient. However, it cannot be used by itself to fetch single
1692 characters without waiting for the user to hit enter. For that, try
1693 something more like:
1696 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1699 system "stty", '-icanon', 'eol', "\001";
1705 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1708 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1712 Determination of whether $BSD_STYLE should be set
1713 is left as an exercise to the reader.
1715 The C<POSIX::getattr> function can do this more portably on
1716 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1717 module from your nearest CPAN site; details on CPAN can be found on
1722 Implements the C library function of the same name, which on most
1723 systems returns the current login from F</etc/utmp>, if any. If null,
1726 $login = getlogin || getpwuid($<) || "Kilroy";
1728 Do not consider C<getlogin> for authentication: it is not as
1729 secure as C<getpwuid>.
1731 =item getpeername SOCKET
1733 Returns the packed sockaddr address of other end of the SOCKET connection.
1736 $hersockaddr = getpeername(SOCK);
1737 ($port, $iaddr) = sockaddr_in($hersockaddr);
1738 $herhostname = gethostbyaddr($iaddr, AF_INET);
1739 $herstraddr = inet_ntoa($iaddr);
1743 Returns the current process group for the specified PID. Use
1744 a PID of C<0> to get the current process group for the
1745 current process. Will raise an exception if used on a machine that
1746 doesn't implement getpgrp(2). If PID is omitted, returns process
1747 group of current process. Note that the POSIX version of C<getpgrp>
1748 does not accept a PID argument, so only C<PID==0> is truly portable.
1752 Returns the process id of the parent process.
1754 =item getpriority WHICH,WHO
1756 Returns the current priority for a process, a process group, or a user.
1757 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1758 machine that doesn't implement getpriority(2).
1764 =item gethostbyname NAME
1766 =item getnetbyname NAME
1768 =item getprotobyname NAME
1774 =item getservbyname NAME,PROTO
1776 =item gethostbyaddr ADDR,ADDRTYPE
1778 =item getnetbyaddr ADDR,ADDRTYPE
1780 =item getprotobynumber NUMBER
1782 =item getservbyport PORT,PROTO
1800 =item sethostent STAYOPEN
1802 =item setnetent STAYOPEN
1804 =item setprotoent STAYOPEN
1806 =item setservent STAYOPEN
1820 These routines perform the same functions as their counterparts in the
1821 system library. In list context, the return values from the
1822 various get routines are as follows:
1824 ($name,$passwd,$uid,$gid,
1825 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1826 ($name,$passwd,$gid,$members) = getgr*
1827 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1828 ($name,$aliases,$addrtype,$net) = getnet*
1829 ($name,$aliases,$proto) = getproto*
1830 ($name,$aliases,$port,$proto) = getserv*
1832 (If the entry doesn't exist you get a null list.)
1834 In scalar context, you get the name, unless the function was a
1835 lookup by name, in which case you get the other thing, whatever it is.
1836 (If the entry doesn't exist you get the undefined value.) For example:
1838 $uid = getpwnam($name);
1839 $name = getpwuid($num);
1841 $gid = getgrnam($name);
1842 $name = getgrgid($num;
1846 In I<getpw*()> the fields $quota, $comment, and $expire are
1847 special cases in the sense that in many systems they are unsupported.
1848 If the $quota is unsupported, it is an empty scalar. If it is
1849 supported, it usually encodes the disk quota. If the $comment
1850 field is unsupported, it is an empty scalar. If it is supported it
1851 usually encodes some administrative comment about the user. In some
1852 systems the $quota field may be $change or $age, fields that have
1853 to do with password aging. In some systems the $comment field may
1854 be $class. The $expire field, if present, encodes the expiration
1855 period of the account or the password. For the availability and the
1856 exact meaning of these fields in your system, please consult your
1857 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1858 out from within Perl what your $quota and $comment fields mean
1859 and whether you have the $expire field by using the C<Config> module
1860 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1861 and C<d_pwexpire>. Shadow password files are only supported if your
1862 vendor has implemented them in the intuitive fashion that calling the
1863 regular C library routines gets the shadow versions if you're running
1864 under privilege. Those that incorrectly implement a separate library
1865 call are not supported.
1867 The $members value returned by I<getgr*()> is a space separated list of
1868 the login names of the members of the group.
1870 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1871 C, it will be returned to you via C<$?> if the function call fails. The
1872 C<@addrs> value returned by a successful call is a list of the raw
1873 addresses returned by the corresponding system library call. In the
1874 Internet domain, each address is four bytes long and you can unpack it
1875 by saying something like:
1877 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1879 The Socket library makes this slightly easier:
1882 $iaddr = inet_aton("127.1"); # or whatever address
1883 $name = gethostbyaddr($iaddr, AF_INET);
1885 # or going the other way
1886 $straddr = inet_ntoa($iaddr);
1888 If you get tired of remembering which element of the return list
1889 contains which return value, by-name interfaces are provided
1890 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1891 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1892 and C<User::grent>. These override the normal built-ins, supplying
1893 versions that return objects with the appropriate names
1894 for each field. For example:
1898 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1900 Even though it looks like they're the same method calls (uid),
1901 they aren't, because a C<File::stat> object is different from
1902 a C<User::pwent> object.
1904 =item getsockname SOCKET
1906 Returns the packed sockaddr address of this end of the SOCKET connection,
1907 in case you don't know the address because you have several different
1908 IPs that the connection might have come in on.
1911 $mysockaddr = getsockname(SOCK);
1912 ($port, $myaddr) = sockaddr_in($mysockaddr);
1913 printf "Connect to %s [%s]\n",
1914 scalar gethostbyaddr($myaddr, AF_INET),
1917 =item getsockopt SOCKET,LEVEL,OPTNAME
1919 Returns the socket option requested, or undef if there is an error.
1925 Returns the value of EXPR with filename expansions such as the
1926 standard Unix shell F</bin/csh> would do. This is the internal function
1927 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1928 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1929 discussed in more detail in L<perlop/"I/O Operators">.
1933 Converts a time as returned by the time function to a 9-element list
1934 with the time localized for the standard Greenwich time zone.
1935 Typically used as follows:
1938 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1941 All list elements are numeric, and come straight out of a struct tm.
1942 In particular this means that $mon has the range C<0..11> and $wday
1943 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1944 number of years since 1900, that is, $year is C<123> in year 2023,
1945 I<not> simply the last two digits of the year. If you assume it is,
1946 then you create non-Y2K-compliant programs--and you wouldn't want to do
1949 The proper way to get a complete 4-digit year is simply:
1953 And to get the last two digits of the year (e.g., '01' in 2001) do:
1955 $year = sprintf("%02d", $year % 100);
1957 If EXPR is omitted, does C<gmtime(time())>.
1959 In scalar context, returns the ctime(3) value:
1961 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1963 Also see the C<timegm> function provided by the C<Time::Local> module,
1964 and the strftime(3) function available via the POSIX module.
1966 This scalar value is B<not> locale dependent (see L<perllocale>), but
1967 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1968 strftime(3) and mktime(3) functions available via the POSIX module. To
1969 get somewhat similar but locale dependent date strings, set up your
1970 locale environment variables appropriately (please see L<perllocale>)
1971 and try for example:
1973 use POSIX qw(strftime);
1974 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1976 Note that the C<%a> and C<%b> escapes, which represent the short forms
1977 of the day of the week and the month of the year, may not necessarily
1978 be three characters wide in all locales.
1986 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1987 execution there. It may not be used to go into any construct that
1988 requires initialization, such as a subroutine or a C<foreach> loop. It
1989 also can't be used to go into a construct that is optimized away,
1990 or to get out of a block or subroutine given to C<sort>.
1991 It can be used to go almost anywhere else within the dynamic scope,
1992 including out of subroutines, but it's usually better to use some other
1993 construct such as C<last> or C<die>. The author of Perl has never felt the
1994 need to use this form of C<goto> (in Perl, that is--C is another matter).
1996 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1997 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1998 necessarily recommended if you're optimizing for maintainability:
2000 goto ("FOO", "BAR", "GLARCH")[$i];
2002 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2003 In fact, it isn't a goto in the normal sense at all, and doesn't have
2004 the stigma associated with other gotos. Instead, it
2005 substitutes a call to the named subroutine for the currently running
2006 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2007 another subroutine and then pretend that the other subroutine had been
2008 called in the first place (except that any modifications to C<@_>
2009 in the current subroutine are propagated to the other subroutine.)
2010 After the C<goto>, not even C<caller> will be able to tell that this
2011 routine was called first.
2013 NAME needn't be the name of a subroutine; it can be a scalar variable
2014 containing a code reference, or a block which evaluates to a code
2017 =item grep BLOCK LIST
2019 =item grep EXPR,LIST
2021 This is similar in spirit to, but not the same as, grep(1) and its
2022 relatives. In particular, it is not limited to using regular expressions.
2024 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2025 C<$_> to each element) and returns the list value consisting of those
2026 elements for which the expression evaluated to true. In scalar
2027 context, returns the number of times the expression was true.
2029 @foo = grep(!/^#/, @bar); # weed out comments
2033 @foo = grep {!/^#/} @bar; # weed out comments
2035 Note that, because C<$_> is a reference into the list value, it can
2036 be used to modify the elements of the array. While this is useful and
2037 supported, it can cause bizarre results if the LIST is not a named array.
2038 Similarly, grep returns aliases into the original list, much as a for
2039 loop's index variable aliases the list elements. That is, modifying an
2040 element of a list returned by grep (for example, in a C<foreach>, C<map>
2041 or another C<grep>) actually modifies the element in the original list.
2042 This is usually something to be avoided when writing clear code.
2044 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2050 Interprets EXPR as a hex string and returns the corresponding value.
2051 (To convert strings that might start with either 0, 0x, or 0b, see
2052 L</oct>.) If EXPR is omitted, uses C<$_>.
2054 print hex '0xAf'; # prints '175'
2055 print hex 'aF'; # same
2057 Hex strings may only represent integers. Strings that would cause
2058 integer overflow trigger a warning.
2062 There is no builtin C<import> function. It is just an ordinary
2063 method (subroutine) defined (or inherited) by modules that wish to export
2064 names to another module. The C<use> function calls the C<import> method
2065 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2067 =item index STR,SUBSTR,POSITION
2069 =item index STR,SUBSTR
2071 The index function searches for one string within another, but without
2072 the wildcard-like behavior of a full regular-expression pattern match.
2073 It returns the position of the first occurrence of SUBSTR in STR at
2074 or after POSITION. If POSITION is omitted, starts searching from the
2075 beginning of the string. The return value is based at C<0> (or whatever
2076 you've set the C<$[> variable to--but don't do that). If the substring
2077 is not found, returns one less than the base, ordinarily C<-1>.
2083 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2084 You should not use this function for rounding: one because it truncates
2085 towards C<0>, and two because machine representations of floating point
2086 numbers can sometimes produce counterintuitive results. For example,
2087 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2088 because it's really more like -268.99999999999994315658 instead. Usually,
2089 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2090 functions will serve you better than will int().
2092 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2094 Implements the ioctl(2) function. You'll probably first have to say
2096 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2098 to get the correct function definitions. If F<ioctl.ph> doesn't
2099 exist or doesn't have the correct definitions you'll have to roll your
2100 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2101 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2102 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2103 written depending on the FUNCTION--a pointer to the string value of SCALAR
2104 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2105 has no string value but does have a numeric value, that value will be
2106 passed rather than a pointer to the string value. To guarantee this to be
2107 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2108 functions may be needed to manipulate the values of structures used by
2111 The return value of C<ioctl> (and C<fcntl>) is as follows:
2113 if OS returns: then Perl returns:
2115 0 string "0 but true"
2116 anything else that number
2118 Thus Perl returns true on success and false on failure, yet you can
2119 still easily determine the actual value returned by the operating
2122 $retval = ioctl(...) || -1;
2123 printf "System returned %d\n", $retval;
2125 The special string "C<0> but true" is exempt from B<-w> complaints
2126 about improper numeric conversions.
2128 Here's an example of setting a filehandle named C<REMOTE> to be
2129 non-blocking at the system level. You'll have to negotiate C<$|>
2130 on your own, though.
2132 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2134 $flags = fcntl(REMOTE, F_GETFL, 0)
2135 or die "Can't get flags for the socket: $!\n";
2137 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2138 or die "Can't set flags for the socket: $!\n";
2140 =item join EXPR,LIST
2142 Joins the separate strings of LIST into a single string with fields
2143 separated by the value of EXPR, and returns that new string. Example:
2145 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2147 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2148 first argument. Compare L</split>.
2152 Returns a list consisting of all the keys of the named hash. (In
2153 scalar context, returns the number of keys.) The keys are returned in
2154 an apparently random order. The actual random order is subject to
2155 change in future versions of perl, but it is guaranteed to be the same
2156 order as either the C<values> or C<each> function produces (given
2157 that the hash has not been modified). As a side effect, it resets
2160 Here is yet another way to print your environment:
2163 @values = values %ENV;
2165 print pop(@keys), '=', pop(@values), "\n";
2168 or how about sorted by key:
2170 foreach $key (sort(keys %ENV)) {
2171 print $key, '=', $ENV{$key}, "\n";
2174 To sort a hash by value, you'll need to use a C<sort> function.
2175 Here's a descending numeric sort of a hash by its values:
2177 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2178 printf "%4d %s\n", $hash{$key}, $key;
2181 As an lvalue C<keys> allows you to increase the number of hash buckets
2182 allocated for the given hash. This can gain you a measure of efficiency if
2183 you know the hash is going to get big. (This is similar to pre-extending
2184 an array by assigning a larger number to $#array.) If you say
2188 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2189 in fact, since it rounds up to the next power of two. These
2190 buckets will be retained even if you do C<%hash = ()>, use C<undef
2191 %hash> if you want to free the storage while C<%hash> is still in scope.
2192 You can't shrink the number of buckets allocated for the hash using
2193 C<keys> in this way (but you needn't worry about doing this by accident,
2194 as trying has no effect).
2196 See also C<each>, C<values> and C<sort>.
2198 =item kill SIGNAL, LIST
2200 Sends a signal to a list of processes. Returns the number of
2201 processes successfully signaled (which is not necessarily the
2202 same as the number actually killed).
2204 $cnt = kill 1, $child1, $child2;
2207 If SIGNAL is zero, no signal is sent to the process. This is a
2208 useful way to check that the process is alive and hasn't changed
2209 its UID. See L<perlport> for notes on the portability of this
2212 Unlike in the shell, if SIGNAL is negative, it kills
2213 process groups instead of processes. (On System V, a negative I<PROCESS>
2214 number will also kill process groups, but that's not portable.) That
2215 means you usually want to use positive not negative signals. You may also
2216 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2222 The C<last> command is like the C<break> statement in C (as used in
2223 loops); it immediately exits the loop in question. If the LABEL is
2224 omitted, the command refers to the innermost enclosing loop. The
2225 C<continue> block, if any, is not executed:
2227 LINE: while (<STDIN>) {
2228 last LINE if /^$/; # exit when done with header
2232 C<last> cannot be used to exit a block which returns a value such as
2233 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2234 a grep() or map() operation.
2236 Note that a block by itself is semantically identical to a loop
2237 that executes once. Thus C<last> can be used to effect an early
2238 exit out of such a block.
2240 See also L</continue> for an illustration of how C<last>, C<next>, and
2247 Returns an lowercased version of EXPR. This is the internal function
2248 implementing the C<\L> escape in double-quoted strings.
2249 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2252 If EXPR is omitted, uses C<$_>.
2258 Returns the value of EXPR with the first character lowercased. This is
2259 the internal function implementing the C<\l> escape in double-quoted strings.
2260 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2262 If EXPR is omitted, uses C<$_>.
2268 Returns the length in characters of the value of EXPR. If EXPR is
2269 omitted, returns length of C<$_>. Note that this cannot be used on
2270 an entire array or hash to find out how many elements these have.
2271 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2273 =item link OLDFILE,NEWFILE
2275 Creates a new filename linked to the old filename. Returns true for
2276 success, false otherwise.
2278 =item listen SOCKET,QUEUESIZE
2280 Does the same thing that the listen system call does. Returns true if
2281 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2285 You really probably want to be using C<my> instead, because C<local> isn't
2286 what most people think of as "local". See L<perlsub/"Private Variables
2287 via my()"> for details.
2289 A local modifies the listed variables to be local to the enclosing
2290 block, file, or eval. If more than one value is listed, the list must
2291 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2292 for details, including issues with tied arrays and hashes.
2294 =item localtime EXPR
2296 Converts a time as returned by the time function to a 9-element list
2297 with the time analyzed for the local time zone. Typically used as
2301 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2304 All list elements are numeric, and come straight out of a struct tm.
2305 In particular this means that $mon has the range C<0..11> and $wday
2306 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2307 number of years since 1900, that is, $year is C<123> in year 2023,
2308 and I<not> simply the last two digits of the year. If you assume it is,
2309 then you create non-Y2K-compliant programs--and you wouldn't want to do
2312 The proper way to get a complete 4-digit year is simply:
2316 And to get the last two digits of the year (e.g., '01' in 2001) do:
2318 $year = sprintf("%02d", $year % 100);
2320 If EXPR is omitted, uses the current time (C<localtime(time)>).
2322 In scalar context, returns the ctime(3) value:
2324 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2326 This scalar value is B<not> locale dependent, see L<perllocale>, but
2327 instead a Perl builtin. Also see the C<Time::Local> module
2328 (to convert the second, minutes, hours, ... back to seconds since the
2329 stroke of midnight the 1st of January 1970, the value returned by
2330 time()), and the strftime(3) and mktime(3) functions available via the
2331 POSIX module. To get somewhat similar but locale dependent date
2332 strings, set up your locale environment variables appropriately
2333 (please see L<perllocale>) and try for example:
2335 use POSIX qw(strftime);
2336 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2338 Note that the C<%a> and C<%b>, the short forms of the day of the week
2339 and the month of the year, may not necessarily be three characters wide.
2345 This function places an advisory lock on a variable, subroutine,
2346 or referenced object contained in I<THING> until the lock goes out
2347 of scope. This is a built-in function only if your version of Perl
2348 was built with threading enabled, and if you've said C<use Threads>.
2349 Otherwise a user-defined function by this name will be called. See
2356 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2357 returns log of C<$_>. To get the log of another base, use basic algebra:
2358 The base-N log of a number is equal to the natural log of that number
2359 divided by the natural log of N. For example:
2363 return log($n)/log(10);
2366 See also L</exp> for the inverse operation.
2368 =item lstat FILEHANDLE
2374 Does the same thing as the C<stat> function (including setting the
2375 special C<_> filehandle) but stats a symbolic link instead of the file
2376 the symbolic link points to. If symbolic links are unimplemented on
2377 your system, a normal C<stat> is done.
2379 If EXPR is omitted, stats C<$_>.
2383 The match operator. See L<perlop>.
2385 =item map BLOCK LIST
2389 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2390 C<$_> to each element) and returns the list value composed of the
2391 results of each such evaluation. In scalar context, returns the
2392 total number of elements so generated. Evaluates BLOCK or EXPR in
2393 list context, so each element of LIST may produce zero, one, or
2394 more elements in the returned value.
2396 @chars = map(chr, @nums);
2398 translates a list of numbers to the corresponding characters. And
2400 %hash = map { getkey($_) => $_ } @array;
2402 is just a funny way to write
2405 foreach $_ (@array) {
2406 $hash{getkey($_)} = $_;
2409 Note that, because C<$_> is a reference into the list value, it can
2410 be used to modify the elements of the array. While this is useful and
2411 supported, it can cause bizarre results if the LIST is not a named array.
2412 Using a regular C<foreach> loop for this purpose would be clearer in
2413 most cases. See also L</grep> for an array composed of those items of
2414 the original list for which the BLOCK or EXPR evaluates to true.
2416 =item mkdir FILENAME,MASK
2418 Creates the directory specified by FILENAME, with permissions
2419 specified by MASK (as modified by C<umask>). If it succeeds it
2420 returns true, otherwise it returns false and sets C<$!> (errno).
2422 In general, it is better to create directories with permissive MASK,
2423 and let the user modify that with their C<umask>, than it is to supply
2424 a restrictive MASK and give the user no way to be more permissive.
2425 The exceptions to this rule are when the file or directory should be
2426 kept private (mail files, for instance). The perlfunc(1) entry on
2427 C<umask> discusses the choice of MASK in more detail.
2429 =item msgctl ID,CMD,ARG
2431 Calls the System V IPC function msgctl(2). You'll probably have to say
2435 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2436 then ARG must be a variable which will hold the returned C<msqid_ds>
2437 structure. Returns like C<ioctl>: the undefined value for error,
2438 C<"0 but true"> for zero, or the actual return value otherwise. See also
2439 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2441 =item msgget KEY,FLAGS
2443 Calls the System V IPC function msgget(2). Returns the message queue
2444 id, or the undefined value if there is an error. See also C<IPC::SysV>
2445 and C<IPC::Msg> documentation.
2447 =item msgsnd ID,MSG,FLAGS
2449 Calls the System V IPC function msgsnd to send the message MSG to the
2450 message queue ID. MSG must begin with the long integer message type,
2451 which may be created with C<pack("l", $type)>. Returns true if
2452 successful, or false if there is an error. See also C<IPC::SysV>
2453 and C<IPC::SysV::Msg> documentation.
2455 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2457 Calls the System V IPC function msgrcv to receive a message from
2458 message queue ID into variable VAR with a maximum message size of
2459 SIZE. Note that if a message is received, the message type will be
2460 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2461 size of the message type. Returns true if successful, or false if
2462 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2466 =item my EXPR : ATTRIBUTES
2468 A C<my> declares the listed variables to be local (lexically) to the
2469 enclosing block, file, or C<eval>. If
2470 more than one value is listed, the list must be placed in parentheses. See
2471 L<perlsub/"Private Variables via my()"> for details.
2477 The C<next> command is like the C<continue> statement in C; it starts
2478 the next iteration of the loop:
2480 LINE: while (<STDIN>) {
2481 next LINE if /^#/; # discard comments
2485 Note that if there were a C<continue> block on the above, it would get
2486 executed even on discarded lines. If the LABEL is omitted, the command
2487 refers to the innermost enclosing loop.
2489 C<next> cannot be used to exit a block which returns a value such as
2490 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2491 a grep() or map() operation.
2493 Note that a block by itself is semantically identical to a loop
2494 that executes once. Thus C<next> will exit such a block early.
2496 See also L</continue> for an illustration of how C<last>, C<next>, and
2499 =item no Module LIST
2501 See the L</use> function, which C<no> is the opposite of.
2507 Interprets EXPR as an octal string and returns the corresponding
2508 value. (If EXPR happens to start off with C<0x>, interprets it as a
2509 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2510 binary string.) The following will handle decimal, binary, octal, and
2511 hex in the standard Perl or C notation:
2513 $val = oct($val) if $val =~ /^0/;
2515 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2516 in octal), use sprintf() or printf():
2518 $perms = (stat("filename"))[2] & 07777;
2519 $oct_perms = sprintf "%lo", $perms;
2521 The oct() function is commonly used when a string such as C<644> needs
2522 to be converted into a file mode, for example. (Although perl will
2523 automatically convert strings into numbers as needed, this automatic
2524 conversion assumes base 10.)
2526 =item open FILEHANDLE,MODE,EXPR
2528 =item open FILEHANDLE,EXPR
2530 =item open FILEHANDLE
2532 Opens the file whose filename is given by EXPR, and associates it with
2533 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2534 name of the real filehandle wanted. If EXPR is omitted, the scalar
2535 variable of the same name as the FILEHANDLE contains the filename.
2536 (Note that lexical variables--those declared with C<my>--will not work
2537 for this purpose; so if you're using C<my>, specify EXPR in your call
2538 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2541 If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2542 If MODE is C<'E<gt>'>, the file is truncated and opened for
2543 output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
2544 the file is opened for appending, again being created if necessary.
2545 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2546 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2547 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2548 file first. You can't usually use either read-write mode for updating
2549 textfiles, since they have variable length records. See the B<-i>
2550 switch in L<perlrun> for a better approach. The file is created with
2551 permissions of C<0666> modified by the process' C<umask> value.
2553 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2554 C<'w+'>, C<'a'>, and C<'a+'>.
2556 In the 2-arguments (and 1-argument) form of the call the mode and
2557 filename should be concatenated (in this order), possibly separated by
2558 spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2560 If the filename begins with C<'|'>, the filename is interpreted as a
2561 command to which output is to be piped, and if the filename ends with a
2562 C<'|'>, the filename is interpreted as a command which pipes output to
2563 us. See L<perlipc/"Using open() for IPC">
2564 for more examples of this. (You are not allowed to C<open> to a command
2565 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2566 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2568 If MODE is C<'|-'>, the filename is interpreted as a
2569 command to which output is to be piped, and if MODE is
2570 C<'-|'>, the filename is interpreted as a command which pipes output to
2571 us. In the 2-arguments (and 1-argument) form one should replace dash
2572 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2573 for more examples of this. (You are not allowed to C<open> to a command
2574 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2575 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2577 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2578 and opening C<'E<gt>-'> opens STDOUT.
2581 nonzero upon success, the undefined value otherwise. If the C<open>
2582 involved a pipe, the return value happens to be the pid of the
2585 If you're unfortunate enough to be running Perl on a system that
2586 distinguishes between text files and binary files (modern operating
2587 systems don't care), then you should check out L</binmode> for tips for
2588 dealing with this. The key distinction between systems that need C<binmode>
2589 and those that don't is their text file formats. Systems like Unix, MacOS, and
2590 Plan9, which delimit lines with a single character, and which encode that
2591 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2593 When opening a file, it's usually a bad idea to continue normal execution
2594 if the request failed, so C<open> is frequently used in connection with
2595 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2596 where you want to make a nicely formatted error message (but there are
2597 modules that can help with that problem)) you should always check
2598 the return value from opening a file. The infrequent exception is when
2599 working with an unopened filehandle is actually what you want to do.
2604 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2605 while (<ARTICLE>) {...
2607 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2608 # if the open fails, output is discarded
2610 open(DBASE, '+<', 'dbase.mine') # open for update
2611 or die "Can't open 'dbase.mine' for update: $!";
2613 open(DBASE, '+<dbase.mine') # ditto
2614 or die "Can't open 'dbase.mine' for update: $!";
2616 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2617 or die "Can't start caesar: $!";
2619 open(ARTICLE, "caesar <$article |") # ditto
2620 or die "Can't start caesar: $!";
2622 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2623 or die "Can't start sort: $!";
2625 # process argument list of files along with any includes
2627 foreach $file (@ARGV) {
2628 process($file, 'fh00');
2632 my($filename, $input) = @_;
2633 $input++; # this is a string increment
2634 unless (open($input, $filename)) {
2635 print STDERR "Can't open $filename: $!\n";
2640 while (<$input>) { # note use of indirection
2641 if (/^#include "(.*)"/) {
2642 process($1, $input);
2649 You may also, in the Bourne shell tradition, specify an EXPR beginning
2650 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2651 name of a filehandle (or file descriptor, if numeric) to be
2652 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2653 C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
2654 mode you specify should match the mode of the original filehandle.
2655 (Duping a filehandle does not take into account any existing contents of
2656 stdio buffers.) Duping file handles is not yet supported for 3-argument
2659 Here is a script that saves, redirects, and restores STDOUT and
2663 open(OLDOUT, ">&STDOUT");
2664 open(OLDERR, ">&STDERR");
2666 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2667 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2669 select(STDERR); $| = 1; # make unbuffered
2670 select(STDOUT); $| = 1; # make unbuffered
2672 print STDOUT "stdout 1\n"; # this works for
2673 print STDERR "stderr 1\n"; # subprocesses too
2678 open(STDOUT, ">&OLDOUT");
2679 open(STDERR, ">&OLDERR");
2681 print STDOUT "stdout 2\n";
2682 print STDERR "stderr 2\n";
2684 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2685 equivalent of C's C<fdopen> of that file descriptor; this is more
2686 parsimonious of file descriptors. For example:
2688 open(FILEHANDLE, "<&=$fd")
2690 Note that this feature depends on the fdopen() C library function.
2691 On many UNIX systems, fdopen() is known to fail when file descriptors
2692 exceed a certain value, typically 255. If you need more file
2693 descriptors than that, consider rebuilding Perl to use the C<sfio>
2696 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2697 with 2-arguments (or 1-argument) form of open(), then
2698 there is an implicit fork done, and the return value of open is the pid
2699 of the child within the parent process, and C<0> within the child
2700 process. (Use C<defined($pid)> to determine whether the open was successful.)
2701 The filehandle behaves normally for the parent, but i/o to that
2702 filehandle is piped from/to the STDOUT/STDIN of the child process.
2703 In the child process the filehandle isn't opened--i/o happens from/to
2704 the new STDOUT or STDIN. Typically this is used like the normal
2705 piped open when you want to exercise more control over just how the
2706 pipe command gets executed, such as when you are running setuid, and
2707 don't want to have to scan shell commands for metacharacters.
2708 The following triples are more or less equivalent:
2710 open(FOO, "|tr '[a-z]' '[A-Z]'");
2711 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2712 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2714 open(FOO, "cat -n '$file'|");
2715 open(FOO, '-|', "cat -n '$file'");
2716 open(FOO, '-|') || exec 'cat', '-n', $file;
2718 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2720 NOTE: On any operation that may do a fork, all files opened for output
2721 are flushed before the fork is attempted. On systems that support a
2722 close-on-exec flag on files, the flag will be set for the newly opened
2723 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2725 Closing any piped filehandle causes the parent process to wait for the
2726 child to finish, and returns the status value in C<$?>.
2728 The filename passed to 2-argument (or 1-argument) form of open()
2729 will have leading and trailing
2730 whitespace deleted, and the normal redirection characters
2731 honored. This property, known as "magic open",
2732 can often be used to good effect. A user could specify a filename of
2733 F<"rsh cat file |">, or you could change certain filenames as needed:
2735 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2736 open(FH, $filename) or die "Can't open $filename: $!";
2738 Use 3-argument form to open a file with arbitrary weird characters in it,
2740 open(FOO, '<', $file);
2742 otherwise it's necessary to protect any leading and trailing whitespace:
2744 $file =~ s#^(\s)#./$1#;
2745 open(FOO, "< $file\0");
2747 (this may not work on some bizzare filesystems). One should
2748 conscientiously choose between the the I<magic> and 3-arguments form
2753 will allow the user to specify an argument of the form C<"rsh cat file |">,
2754 but will not work on a filename which happens to have a trailing space, while
2756 open IN, '<', $ARGV[0];
2758 will have exactly the opposite restrictions.
2760 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2761 should use the C<sysopen> function, which involves no such magic (but
2762 may use subtly different filemodes than Perl open(), which is mapped
2763 to C fopen()). This is
2764 another way to protect your filenames from interpretation. For example:
2767 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2768 or die "sysopen $path: $!";
2769 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2770 print HANDLE "stuff $$\n");
2772 print "File contains: ", <HANDLE>;
2774 Using the constructor from the C<IO::Handle> package (or one of its
2775 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2776 filehandles that have the scope of whatever variables hold references to
2777 them, and automatically close whenever and however you leave that scope:
2781 sub read_myfile_munged {
2783 my $handle = new IO::File;
2784 open($handle, "myfile") or die "myfile: $!";
2786 or return (); # Automatically closed here.
2787 mung $first or die "mung failed"; # Or here.
2788 return $first, <$handle> if $ALL; # Or here.
2792 See L</seek> for some details about mixing reading and writing.
2794 =item opendir DIRHANDLE,EXPR
2796 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2797 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2798 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2804 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2805 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2806 See L<utf8> for more about Unicode.
2810 An C<our> declares the listed variables to be valid globals within
2811 the enclosing block, file, or C<eval>. That is, it has the same
2812 scoping rules as a "my" declaration, but does not create a local
2813 variable. If more than one value is listed, the list must be placed
2814 in parentheses. The C<our> declaration has no semantic effect unless
2815 "use strict vars" is in effect, in which case it lets you use the
2816 declared global variable without qualifying it with a package name.
2817 (But only within the lexical scope of the C<our> declaration. In this
2818 it differs from "use vars", which is package scoped.)
2820 An C<our> declaration declares a global variable that will be visible
2821 across its entire lexical scope, even across package boundaries. The
2822 package in which the variable is entered is determined at the point
2823 of the declaration, not at the point of use. This means the following
2827 our $bar; # declares $Foo::bar for rest of lexical scope
2831 print $bar; # prints 20
2833 Multiple C<our> declarations in the same lexical scope are allowed
2834 if they are in different packages. If they happened to be in the same
2835 package, Perl will emit warnings if you have asked for them.
2839 our $bar; # declares $Foo::bar for rest of lexical scope
2843 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2844 print $bar; # prints 30
2846 our $bar; # emits warning
2848 =item pack TEMPLATE,LIST
2850 Takes a LIST of values and converts it into a string using the rules
2851 given by the TEMPLATE. The resulting string is the concatenation of
2852 the converted values. Typically, each converted value looks
2853 like its machine-level representation. For example, on 32-bit machines
2854 a converted integer may be represented by a sequence of 4 bytes.
2857 sequence of characters that give the order and type of values, as
2860 a A string with arbitrary binary data, will be null padded.
2861 A An ascii string, will be space padded.
2862 Z A null terminated (asciz) string, will be null padded.
2864 b A bit string (ascending bit order inside each byte, like vec()).
2865 B A bit string (descending bit order inside each byte).
2866 h A hex string (low nybble first).
2867 H A hex string (high nybble first).
2869 c A signed char value.
2870 C An unsigned char value. Only does bytes. See U for Unicode.
2872 s A signed short value.
2873 S An unsigned short value.
2874 (This 'short' is _exactly_ 16 bits, which may differ from
2875 what a local C compiler calls 'short'. If you want
2876 native-length shorts, use the '!' suffix.)
2878 i A signed integer value.
2879 I An unsigned integer value.
2880 (This 'integer' is _at_least_ 32 bits wide. Its exact
2881 size depends on what a local C compiler calls 'int',
2882 and may even be larger than the 'long' described in
2885 l A signed long value.
2886 L An unsigned long value.
2887 (This 'long' is _exactly_ 32 bits, which may differ from
2888 what a local C compiler calls 'long'. If you want
2889 native-length longs, use the '!' suffix.)
2891 n An unsigned short in "network" (big-endian) order.
2892 N An unsigned long in "network" (big-endian) order.
2893 v An unsigned short in "VAX" (little-endian) order.
2894 V An unsigned long in "VAX" (little-endian) order.
2895 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2896 _exactly_ 32 bits, respectively.)
2898 q A signed quad (64-bit) value.
2899 Q An unsigned quad value.
2900 (Quads are available only if your system supports 64-bit
2901 integer values _and_ if Perl has been compiled to support those.
2902 Causes a fatal error otherwise.)
2904 f A single-precision float in the native format.
2905 d A double-precision float in the native format.
2907 p A pointer to a null-terminated string.
2908 P A pointer to a structure (fixed-length string).
2910 u A uuencoded string.
2911 U A Unicode character number. Encodes to UTF-8 internally.
2912 Works even if C<use utf8> is not in effect.
2914 w A BER compressed integer. Its bytes represent an unsigned
2915 integer in base 128, most significant digit first, with as
2916 few digits as possible. Bit eight (the high bit) is set
2917 on each byte except the last.
2921 @ Null fill to absolute position.
2923 The following rules apply:
2929 Each letter may optionally be followed by a number giving a repeat
2930 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2931 C<H>, and C<P> the pack function will gobble up that many values from
2932 the LIST. A C<*> for the repeat count means to use however many items are
2933 left, except for C<@>, C<x>, C<X>, where it is equivalent
2934 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2937 When used with C<Z>, C<*> results in the addition of a trailing null
2938 byte (so the packed result will be one longer than the byte C<length>
2941 The repeat count for C<u> is interpreted as the maximal number of bytes
2942 to encode per line of output, with 0 and 1 replaced by 45.
2946 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
2947 string of length count, padding with nulls or spaces as necessary. When
2948 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2949 after the first null, and C<a> returns data verbatim. When packing,
2950 C<a>, and C<Z> are equivalent.
2952 If the value-to-pack is too long, it is truncated. If too long and an
2953 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2954 by a null byte. Thus C<Z> always packs a trailing null byte under
2959 Likewise, the C<b> and C<B> fields pack a string that many bits long.
2960 Each byte of the input field of pack() generates 1 bit of the result.
2961 Each result bit is based on the least-significant bit of the corresponding
2962 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2963 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2965 Starting from the beginning of the input string of pack(), each 8-tuple
2966 of bytes is converted to 1 byte of output. With format C<b>
2967 the first byte of the 8-tuple determines the least-significant bit of a
2968 byte, and with format C<B> it determines the most-significant bit of
2971 If the length of the input string is not exactly divisible by 8, the
2972 remainder is packed as if the input string were padded by null bytes
2973 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2975 If the input string of pack() is longer than needed, extra bytes are ignored.
2976 A C<*> for the repeat count of pack() means to use all the bytes of
2977 the input field. On unpack()ing the bits are converted to a string
2978 of C<"0">s and C<"1">s.
2982 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
2983 representable as hexadecimal digits, 0-9a-f) long.
2985 Each byte of the input field of pack() generates 4 bits of the result.
2986 For non-alphabetical bytes the result is based on the 4 least-significant
2987 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
2988 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
2989 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
2990 is compatible with the usual hexadecimal digits, so that C<"a"> and
2991 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
2992 C<"g".."z"> and C<"G".."Z"> is not well-defined.
2994 Starting from the beginning of the input string of pack(), each pair
2995 of bytes is converted to 1 byte of output. With format C<h> the
2996 first byte of the pair determines the least-significant nybble of the
2997 output byte, and with format C<H> it determines the most-significant
3000 If the length of the input string is not even, it behaves as if padded
3001 by a null byte at the end. Similarly, during unpack()ing the "extra"
3002 nybbles are ignored.
3004 If the input string of pack() is longer than needed, extra bytes are ignored.
3005 A C<*> for the repeat count of pack() means to use all the bytes of
3006 the input field. On unpack()ing the bits are converted to a string
3007 of hexadecimal digits.
3011 The C<p> type packs a pointer to a null-terminated string. You are
3012 responsible for ensuring the string is not a temporary value (which can
3013 potentially get deallocated before you get around to using the packed result).
3014 The C<P> type packs a pointer to a structure of the size indicated by the
3015 length. A NULL pointer is created if the corresponding value for C<p> or
3016 C<P> is C<undef>, similarly for unpack().
3020 The C</> template character allows packing and unpacking of strings where
3021 the packed structure contains a byte count followed by the string itself.
3022 You write I<length-item>C</>I<string-item>.
3024 The I<length-item> can be any C<pack> template letter,
3025 and describes how the length value is packed.
3026 The ones likely to be of most use are integer-packing ones like
3027 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3028 and C<N> (for Sun XDR).
3030 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3031 For C<unpack> the length of the string is obtained from the I<length-item>,
3032 but if you put in the '*' it will be ignored.
3034 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3035 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3036 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3038 The I<length-item> is not returned explicitly from C<unpack>.
3040 Adding a count to the I<length-item> letter is unlikely to do anything
3041 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3042 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3043 which Perl does not regard as legal in numeric strings.
3047 The integer types C<s>, C<S>, C<l>, and C<L> may be
3048 immediately followed by a C<!> suffix to signify native shorts or
3049 longs--as you can see from above for example a bare C<l> does mean
3050 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3051 may be larger. This is an issue mainly in 64-bit platforms. You can
3052 see whether using C<!> makes any difference by
3054 print length(pack("s")), " ", length(pack("s!")), "\n";
3055 print length(pack("l")), " ", length(pack("l!")), "\n";
3057 C<i!> and C<I!> also work but only because of completeness;
3058 they are identical to C<i> and C<I>.
3060 The actual sizes (in bytes) of native shorts, ints, longs, and long
3061 longs on the platform where Perl was built are also available via
3065 print $Config{shortsize}, "\n";
3066 print $Config{intsize}, "\n";
3067 print $Config{longsize}, "\n";
3068 print $Config{longlongsize}, "\n";
3070 (The C<$Config{longlongsize}> will be undefine if your system does
3071 not support long longs.)
3075 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3076 are inherently non-portable between processors and operating systems
3077 because they obey the native byteorder and endianness. For example a
3078 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3079 (arranged in and handled by the CPU registers) into bytes as
3081 0x12 0x34 0x56 0x78 # little-endian
3082 0x78 0x56 0x34 0x12 # big-endian
3084 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3085 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3086 Power, and Cray are big-endian. MIPS can be either: Digital used it
3087 in little-endian mode; SGI uses it in big-endian mode.
3089 The names `big-endian' and `little-endian' are comic references to
3090 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3091 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3092 the egg-eating habits of the Lilliputians.
3094 Some systems may have even weirder byte orders such as
3099 You can see your system's preference with
3101 print join(" ", map { sprintf "%#02x", $_ }
3102 unpack("C*",pack("L",0x12345678))), "\n";
3104 The byteorder on the platform where Perl was built is also available
3108 print $Config{byteorder}, "\n";
3110 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3111 and C<'87654321'> are big-endian.
3113 If you want portable packed integers use the formats C<n>, C<N>,
3114 C<v>, and C<V>, their byte endianness and size is known.
3115 See also L<perlport>.
3119 Real numbers (floats and doubles) are in the native machine format only;
3120 due to the multiplicity of floating formats around, and the lack of a
3121 standard "network" representation, no facility for interchange has been
3122 made. This means that packed floating point data written on one machine
3123 may not be readable on another - even if both use IEEE floating point
3124 arithmetic (as the endian-ness of the memory representation is not part
3125 of the IEEE spec). See also L<perlport>.
3127 Note that Perl uses doubles internally for all numeric calculation, and
3128 converting from double into float and thence back to double again will
3129 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3134 You must yourself do any alignment or padding by inserting for example
3135 enough C<'x'>es while packing. There is no way to pack() and unpack()
3136 could know where the bytes are going to or coming from. Therefore
3137 C<pack> (and C<unpack>) handle their output and input as flat
3142 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3146 If TEMPLATE requires more arguments to pack() than actually given, pack()
3147 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3148 to pack() than actually given, extra arguments are ignored.
3154 $foo = pack("CCCC",65,66,67,68);
3156 $foo = pack("C4",65,66,67,68);
3158 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3159 # same thing with Unicode circled letters
3161 $foo = pack("ccxxcc",65,66,67,68);
3164 # note: the above examples featuring "C" and "c" are true
3165 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3166 # and UTF-8. In EBCDIC the first example would be
3167 # $foo = pack("CCCC",193,194,195,196);
3169 $foo = pack("s2",1,2);
3170 # "\1\0\2\0" on little-endian
3171 # "\0\1\0\2" on big-endian
3173 $foo = pack("a4","abcd","x","y","z");
3176 $foo = pack("aaaa","abcd","x","y","z");
3179 $foo = pack("a14","abcdefg");
3180 # "abcdefg\0\0\0\0\0\0\0"
3182 $foo = pack("i9pl", gmtime);
3183 # a real struct tm (on my system anyway)
3185 $utmp_template = "Z8 Z8 Z16 L";
3186 $utmp = pack($utmp_template, @utmp1);
3187 # a struct utmp (BSDish)
3189 @utmp2 = unpack($utmp_template, $utmp);
3190 # "@utmp1" eq "@utmp2"
3193 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3196 $foo = pack('sx2l', 12, 34);
3197 # short 12, two zero bytes padding, long 34
3198 $bar = pack('s@4l', 12, 34);
3199 # short 12, zero fill to position 4, long 34
3202 The same template may generally also be used in unpack().
3206 =item package NAMESPACE
3208 Declares the compilation unit as being in the given namespace. The scope
3209 of the package declaration is from the declaration itself through the end
3210 of the enclosing block, file, or eval (the same as the C<my> operator).
3211 All further unqualified dynamic identifiers will be in this namespace.
3212 A package statement affects only dynamic variables--including those
3213 you've used C<local> on--but I<not> lexical variables, which are created
3214 with C<my>. Typically it would be the first declaration in a file to
3215 be included by the C<require> or C<use> operator. You can switch into a
3216 package in more than one place; it merely influences which symbol table
3217 is used by the compiler for the rest of that block. You can refer to
3218 variables and filehandles in other packages by prefixing the identifier
3219 with the package name and a double colon: C<$Package::Variable>.
3220 If the package name is null, the C<main> package as assumed. That is,
3221 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3222 still seen in older code).
3224 If NAMESPACE is omitted, then there is no current package, and all
3225 identifiers must be fully qualified or lexicals. This is stricter
3226 than C<use strict>, since it also extends to function names.
3228 See L<perlmod/"Packages"> for more information about packages, modules,
3229 and classes. See L<perlsub> for other scoping issues.
3231 =item pipe READHANDLE,WRITEHANDLE
3233 Opens a pair of connected pipes like the corresponding system call.
3234 Note that if you set up a loop of piped processes, deadlock can occur
3235 unless you are very careful. In addition, note that Perl's pipes use
3236 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3237 after each command, depending on the application.
3239 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3240 for examples of such things.
3242 On systems that support a close-on-exec flag on files, the flag will be set
3243 for the newly opened file descriptors as determined by the value of $^F.
3250 Pops and returns the last value of the array, shortening the array by
3251 one element. Has an effect similar to
3255 If there are no elements in the array, returns the undefined value
3256 (although this may happen at other times as well). If ARRAY is
3257 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3258 array in subroutines, just like C<shift>.
3264 Returns the offset of where the last C<m//g> search left off for the variable
3265 is in question (C<$_> is used when the variable is not specified). May be
3266 modified to change that offset. Such modification will also influence
3267 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3270 =item print FILEHANDLE LIST
3276 Prints a string or a list of strings. Returns true if successful.
3277 FILEHANDLE may be a scalar variable name, in which case the variable
3278 contains the name of or a reference to the filehandle, thus introducing
3279 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3280 the next token is a term, it may be misinterpreted as an operator
3281 unless you interpose a C<+> or put parentheses around the arguments.)
3282 If FILEHANDLE is omitted, prints by default to standard output (or
3283 to the last selected output channel--see L</select>). If LIST is
3284 also omitted, prints C<$_> to the currently selected output channel.
3285 To set the default output channel to something other than STDOUT
3286 use the select operation. The current value of C<$,> (if any) is
3287 printed between each LIST item. The current value of C<$\> (if
3288 any) is printed after the entire LIST has been printed. Because
3289 print takes a LIST, anything in the LIST is evaluated in list
3290 context, and any subroutine that you call will have one or more of
3291 its expressions evaluated in list context. Also be careful not to
3292 follow the print keyword with a left parenthesis unless you want
3293 the corresponding right parenthesis to terminate the arguments to
3294 the print--interpose a C<+> or put parentheses around all the
3297 Note that if you're storing FILEHANDLES in an array or other expression,
3298 you will have to use a block returning its value instead:
3300 print { $files[$i] } "stuff\n";
3301 print { $OK ? STDOUT : STDERR } "stuff\n";
3303 =item printf FILEHANDLE FORMAT, LIST
3305 =item printf FORMAT, LIST
3307 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3308 (the output record separator) is not appended. The first argument
3309 of the list will be interpreted as the C<printf> format. If C<use locale> is
3310 in effect, the character used for the decimal point in formatted real numbers
3311 is affected by the LC_NUMERIC locale. See L<perllocale>.
3313 Don't fall into the trap of using a C<printf> when a simple
3314 C<print> would do. The C<print> is more efficient and less
3317 =item prototype FUNCTION
3319 Returns the prototype of a function as a string (or C<undef> if the
3320 function has no prototype). FUNCTION is a reference to, or the name of,
3321 the function whose prototype you want to retrieve.
3323 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3324 name for Perl builtin. If the builtin is not I<overridable> (such as
3325 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3326 C<system>) returns C<undef> because the builtin does not really behave
3327 like a Perl function. Otherwise, the string describing the equivalent
3328 prototype is returned.
3330 =item push ARRAY,LIST
3332 Treats ARRAY as a stack, and pushes the values of LIST
3333 onto the end of ARRAY. The length of ARRAY increases by the length of
3334 LIST. Has the same effect as
3337 $ARRAY[++$#ARRAY] = $value;
3340 but is more efficient. Returns the new number of elements in the array.
3352 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3354 =item quotemeta EXPR
3358 Returns the value of EXPR with all non-alphanumeric
3359 characters backslashed. (That is, all characters not matching
3360 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3361 returned string, regardless of any locale settings.)
3362 This is the internal function implementing
3363 the C<\Q> escape in double-quoted strings.
3365 If EXPR is omitted, uses C<$_>.
3371 Returns a random fractional number greater than or equal to C<0> and less
3372 than the value of EXPR. (EXPR should be positive.) If EXPR is
3373 omitted, the value C<1> is used. Automatically calls C<srand> unless
3374 C<srand> has already been called. See also C<srand>.
3376 (Note: If your rand function consistently returns numbers that are too
3377 large or too small, then your version of Perl was probably compiled
3378 with the wrong number of RANDBITS.)
3380 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3382 =item read FILEHANDLE,SCALAR,LENGTH
3384 Attempts to read LENGTH bytes of data into variable SCALAR from the
3385 specified FILEHANDLE. Returns the number of bytes actually read,
3386 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3387 or shrunk to the length actually read. An OFFSET may be specified to
3388 place the read data at some other place than the beginning of the
3389 string. This call is actually implemented in terms of stdio's fread(3)
3390 call. To get a true read(2) system call, see C<sysread>.
3392 =item readdir DIRHANDLE
3394 Returns the next directory entry for a directory opened by C<opendir>.
3395 If used in list context, returns all the rest of the entries in the
3396 directory. If there are no more entries, returns an undefined value in
3397 scalar context or a null list in list context.
3399 If you're planning to filetest the return values out of a C<readdir>, you'd
3400 better prepend the directory in question. Otherwise, because we didn't
3401 C<chdir> there, it would have been testing the wrong file.
3403 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3404 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3409 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3410 context, each call reads and returns the next line, until end-of-file is
3411 reached, whereupon the subsequent call returns undef. In list context,
3412 reads until end-of-file is reached and returns a list of lines. Note that
3413 the notion of "line" used here is however you may have defined it
3414 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3416 When C<$/> is set to C<undef>, when readline() is in scalar
3417 context (i.e. file slurp mode), and when an empty file is read, it
3418 returns C<''> the first time, followed by C<undef> subsequently.
3420 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3421 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3422 operator is discussed in more detail in L<perlop/"I/O Operators">.
3425 $line = readline(*STDIN); # same thing
3431 Returns the value of a symbolic link, if symbolic links are
3432 implemented. If not, gives a fatal error. If there is some system
3433 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3434 omitted, uses C<$_>.
3438 EXPR is executed as a system command.
3439 The collected standard output of the command is returned.
3440 In scalar context, it comes back as a single (potentially
3441 multi-line) string. In list context, returns a list of lines
3442 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3443 This is the internal function implementing the C<qx/EXPR/>
3444 operator, but you can use it directly. The C<qx/EXPR/>
3445 operator is discussed in more detail in L<perlop/"I/O Operators">.
3447 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3449 Receives a message on a socket. Attempts to receive LENGTH bytes of
3450 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3451 will be grown or shrunk to the length actually read. Takes the same
3452 flags as the system call of the same name. Returns the address of the
3453 sender if SOCKET's protocol supports this; returns an empty string
3454 otherwise. If there's an error, returns the undefined value. This call
3455 is actually implemented in terms of recvfrom(2) system call. See
3456 L<perlipc/"UDP: Message Passing"> for examples.
3462 The C<redo> command restarts the loop block without evaluating the
3463 conditional again. The C<continue> block, if any, is not executed. If
3464 the LABEL is omitted, the command refers to the innermost enclosing
3465 loop. This command is normally used by programs that want to lie to
3466 themselves about what was just input:
3468 # a simpleminded Pascal comment stripper
3469 # (warning: assumes no { or } in strings)
3470 LINE: while (<STDIN>) {
3471 while (s|({.*}.*){.*}|$1 |) {}
3476 if (/}/) { # end of comment?
3485 C<redo> cannot be used to retry a block which returns a value such as
3486 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3487 a grep() or map() operation.
3489 Note that a block by itself is semantically identical to a loop
3490 that executes once. Thus C<redo> inside such a block will effectively
3491 turn it into a looping construct.
3493 See also L</continue> for an illustration of how C<last>, C<next>, and
3500 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3501 is not specified, C<$_> will be used. The value returned depends on the
3502 type of thing the reference is a reference to.
3503 Builtin types include:
3513 If the referenced object has been blessed into a package, then that package
3514 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3516 if (ref($r) eq "HASH") {
3517 print "r is a reference to a hash.\n";
3520 print "r is not a reference at all.\n";
3522 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3523 print "r is a reference to something that isa hash.\n";
3526 See also L<perlref>.
3528 =item rename OLDNAME,NEWNAME
3530 Changes the name of a file; an existing file NEWNAME will be
3531 clobbered. Returns true for success, false otherwise.
3533 Behavior of this function varies wildly depending on your system
3534 implementation. For example, it will usually not work across file system
3535 boundaries, even though the system I<mv> command sometimes compensates
3536 for this. Other restrictions include whether it works on directories,
3537 open files, or pre-existing files. Check L<perlport> and either the
3538 rename(2) manpage or equivalent system documentation for details.
3540 =item require VERSION
3546 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3547 supplied. If a version number or tuple is specified, or if EXPR is
3548 numeric, demands that the current version of Perl
3549 (C<$^V> or C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3551 Otherwise, demands that a library file be included if it hasn't already
3552 been included. The file is included via the do-FILE mechanism, which is
3553 essentially just a variety of C<eval>. Has semantics similar to the following
3558 return 1 if $INC{$filename};
3559 my($realfilename,$result);
3561 foreach $prefix (@INC) {
3562 $realfilename = "$prefix/$filename";
3563 if (-f $realfilename) {
3564 $INC{$filename} = $realfilename;
3565 $result = do $realfilename;
3569 die "Can't find $filename in \@INC";
3571 delete $INC{$filename} if $@ || !$result;
3573 die "$filename did not return true value" unless $result;
3577 Note that the file will not be included twice under the same specified
3578 name. The file must return true as the last statement to indicate
3579 successful execution of any initialization code, so it's customary to
3580 end such a file with C<1;> unless you're sure it'll return true
3581 otherwise. But it's better just to put the C<1;>, in case you add more
3584 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3585 replaces "F<::>" with "F</>" in the filename for you,
3586 to make it easy to load standard modules. This form of loading of
3587 modules does not risk altering your namespace.
3589 In other words, if you try this:
3591 require Foo::Bar; # a splendid bareword
3593 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3594 directories specified in the C<@INC> array.
3596 But if you try this:
3598 $class = 'Foo::Bar';
3599 require $class; # $class is not a bareword
3601 require "Foo::Bar"; # not a bareword because of the ""
3603 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3604 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3606 eval "require $class";
3608 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3614 Generally used in a C<continue> block at the end of a loop to clear
3615 variables and reset C<??> searches so that they work again. The
3616 expression is interpreted as a list of single characters (hyphens
3617 allowed for ranges). All variables and arrays beginning with one of
3618 those letters are reset to their pristine state. If the expression is
3619 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3620 only variables or searches in the current package. Always returns
3623 reset 'X'; # reset all X variables
3624 reset 'a-z'; # reset lower case variables
3625 reset; # just reset ?one-time? searches
3627 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3628 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3629 variables--lexical variables are unaffected, but they clean themselves
3630 up on scope exit anyway, so you'll probably want to use them instead.
3637 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3638 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3639 context, depending on how the return value will be used, and the context
3640 may vary from one execution to the next (see C<wantarray>). If no EXPR
3641 is given, returns an empty list in list context, the undefined value in
3642 scalar context, and (of course) nothing at all in a void context.
3644 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3645 or do FILE will automatically return the value of the last expression
3650 In list context, returns a list value consisting of the elements
3651 of LIST in the opposite order. In scalar context, concatenates the
3652 elements of LIST and returns a string value with all characters
3653 in the opposite order.
3655 print reverse <>; # line tac, last line first
3657 undef $/; # for efficiency of <>
3658 print scalar reverse <>; # character tac, last line tsrif
3660 This operator is also handy for inverting a hash, although there are some
3661 caveats. If a value is duplicated in the original hash, only one of those
3662 can be represented as a key in the inverted hash. Also, this has to
3663 unwind one hash and build a whole new one, which may take some time
3664 on a large hash, such as from a DBM file.
3666 %by_name = reverse %by_address; # Invert the hash
3668 =item rewinddir DIRHANDLE
3670 Sets the current position to the beginning of the directory for the
3671 C<readdir> routine on DIRHANDLE.
3673 =item rindex STR,SUBSTR,POSITION
3675 =item rindex STR,SUBSTR
3677 Works just like index() except that it returns the position of the LAST
3678 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3679 last occurrence at or before that position.
3681 =item rmdir FILENAME
3685 Deletes the directory specified by FILENAME if that directory is empty. If it
3686 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3687 FILENAME is omitted, uses C<$_>.
3691 The substitution operator. See L<perlop>.
3695 Forces EXPR to be interpreted in scalar context and returns the value
3698 @counts = ( scalar @a, scalar @b, scalar @c );
3700 There is no equivalent operator to force an expression to
3701 be interpolated in list context because in practice, this is never
3702 needed. If you really wanted to do so, however, you could use
3703 the construction C<@{[ (some expression) ]}>, but usually a simple
3704 C<(some expression)> suffices.
3706 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3707 parenthesized list, this behaves as a scalar comma expression, evaluating
3708 all but the last element in void context and returning the final element
3709 evaluated in scalar context. This is seldom what you want.
3711 The following single statement:
3713 print uc(scalar(&foo,$bar)),$baz;
3715 is the moral equivalent of these two:
3718 print(uc($bar),$baz);
3720 See L<perlop> for more details on unary operators and the comma operator.
3722 =item seek FILEHANDLE,POSITION,WHENCE
3724 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3725 FILEHANDLE may be an expression whose value gives the name of the
3726 filehandle. The values for WHENCE are C<0> to set the new position to
3727 POSITION, C<1> to set it to the current position plus POSITION, and
3728 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3729 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3730 (start of the file, current position, end of the file) from the Fcntl
3731 module. Returns C<1> upon success, C<0> otherwise.
3733 If you want to position file for C<sysread> or C<syswrite>, don't use
3734 C<seek>--buffering makes its effect on the file's system position
3735 unpredictable and non-portable. Use C<sysseek> instead.
3737 Due to the rules and rigors of ANSI C, on some systems you have to do a
3738 seek whenever you switch between reading and writing. Amongst other
3739 things, this may have the effect of calling stdio's clearerr(3).
3740 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3744 This is also useful for applications emulating C<tail -f>. Once you hit
3745 EOF on your read, and then sleep for a while, you might have to stick in a
3746 seek() to reset things. The C<seek> doesn't change the current position,
3747 but it I<does> clear the end-of-file condition on the handle, so that the
3748 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3750 If that doesn't work (some stdios are particularly cantankerous), then
3751 you may need something more like this:
3754 for ($curpos = tell(FILE); $_ = <FILE>;
3755 $curpos = tell(FILE)) {
3756 # search for some stuff and put it into files
3758 sleep($for_a_while);
3759 seek(FILE, $curpos, 0);
3762 =item seekdir DIRHANDLE,POS
3764 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3765 must be a value returned by C<telldir>. Has the same caveats about
3766 possible directory compaction as the corresponding system library
3769 =item select FILEHANDLE
3773 Returns the currently selected filehandle. Sets the current default
3774 filehandle for output, if FILEHANDLE is supplied. This has two
3775 effects: first, a C<write> or a C<print> without a filehandle will
3776 default to this FILEHANDLE. Second, references to variables related to
3777 output will refer to this output channel. For example, if you have to
3778 set the top of form format for more than one output channel, you might
3786 FILEHANDLE may be an expression whose value gives the name of the
3787 actual filehandle. Thus:
3789 $oldfh = select(STDERR); $| = 1; select($oldfh);
3791 Some programmers may prefer to think of filehandles as objects with
3792 methods, preferring to write the last example as:
3795 STDERR->autoflush(1);
3797 =item select RBITS,WBITS,EBITS,TIMEOUT
3799 This calls the select(2) system call with the bit masks specified, which
3800 can be constructed using C<fileno> and C<vec>, along these lines:
3802 $rin = $win = $ein = '';
3803 vec($rin,fileno(STDIN),1) = 1;
3804 vec($win,fileno(STDOUT),1) = 1;
3807 If you want to select on many filehandles you might wish to write a
3811 my(@fhlist) = split(' ',$_[0]);
3814 vec($bits,fileno($_),1) = 1;
3818 $rin = fhbits('STDIN TTY SOCK');
3822 ($nfound,$timeleft) =
3823 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3825 or to block until something becomes ready just do this
3827 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3829 Most systems do not bother to return anything useful in $timeleft, so
3830 calling select() in scalar context just returns $nfound.
3832 Any of the bit masks can also be undef. The timeout, if specified, is
3833 in seconds, which may be fractional. Note: not all implementations are
3834 capable of returning the$timeleft. If not, they always return
3835 $timeleft equal to the supplied $timeout.
3837 You can effect a sleep of 250 milliseconds this way:
3839 select(undef, undef, undef, 0.25);
3841 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3842 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3843 then only on POSIX systems. You have to use C<sysread> instead.
3845 =item semctl ID,SEMNUM,CMD,ARG
3847 Calls the System V IPC function C<semctl>. You'll probably have to say
3851 first to get the correct constant definitions. If CMD is IPC_STAT or
3852 GETALL, then ARG must be a variable which will hold the returned
3853 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3854 undefined value for error, "C<0 but true>" for zero, or the actual return
3855 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3857 =item semget KEY,NSEMS,FLAGS
3859 Calls the System V IPC function semget. Returns the semaphore id, or
3860 the undefined value if there is an error. See also C<IPC::SysV> and
3861 C<IPC::SysV::Semaphore> documentation.
3863 =item semop KEY,OPSTRING
3865 Calls the System V IPC function semop to perform semaphore operations
3866 such as signaling and waiting. OPSTRING must be a packed array of
3867 semop structures. Each semop structure can be generated with
3868 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3869 operations is implied by the length of OPSTRING. Returns true if
3870 successful, or false if there is an error. As an example, the
3871 following code waits on semaphore $semnum of semaphore id $semid:
3873 $semop = pack("sss", $semnum, -1, 0);
3874 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3876 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3877 and C<IPC::SysV::Semaphore> documentation.
3879 =item send SOCKET,MSG,FLAGS,TO
3881 =item send SOCKET,MSG,FLAGS
3883 Sends a message on a socket. Takes the same flags as the system call
3884 of the same name. On unconnected sockets you must specify a
3885 destination to send TO, in which case it does a C C<sendto>. Returns
3886 the number of characters sent, or the undefined value if there is an
3887 error. The C system call sendmsg(2) is currently unimplemented.
3888 See L<perlipc/"UDP: Message Passing"> for examples.
3890 =item setpgrp PID,PGRP
3892 Sets the current process group for the specified PID, C<0> for the current
3893 process. Will produce a fatal error if used on a machine that doesn't
3894 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3895 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3896 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3899 =item setpriority WHICH,WHO,PRIORITY
3901 Sets the current priority for a process, a process group, or a user.
3902 (See setpriority(2).) Will produce a fatal error if used on a machine
3903 that doesn't implement setpriority(2).
3905 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3907 Sets the socket option requested. Returns undefined if there is an
3908 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3915 Shifts the first value of the array off and returns it, shortening the
3916 array by 1 and moving everything down. If there are no elements in the
3917 array, returns the undefined value. If ARRAY is omitted, shifts the
3918 C<@_> array within the lexical scope of subroutines and formats, and the
3919 C<@ARGV> array at file scopes or within the lexical scopes established by
3920 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
3923 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3924 same thing to the left end of an array that C<pop> and C<push> do to the
3927 =item shmctl ID,CMD,ARG
3929 Calls the System V IPC function shmctl. You'll probably have to say
3933 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3934 then ARG must be a variable which will hold the returned C<shmid_ds>
3935 structure. Returns like ioctl: the undefined value for error, "C<0> but
3936 true" for zero, or the actual return value otherwise.
3937 See also C<IPC::SysV> documentation.
3939 =item shmget KEY,SIZE,FLAGS
3941 Calls the System V IPC function shmget. Returns the shared memory
3942 segment id, or the undefined value if there is an error.
3943 See also C<IPC::SysV> documentation.
3945 =item shmread ID,VAR,POS,SIZE
3947 =item shmwrite ID,STRING,POS,SIZE
3949 Reads or writes the System V shared memory segment ID starting at
3950 position POS for size SIZE by attaching to it, copying in/out, and
3951 detaching from it. When reading, VAR must be a variable that will
3952 hold the data read. When writing, if STRING is too long, only SIZE
3953 bytes are used; if STRING is too short, nulls are written to fill out
3954 SIZE bytes. Return true if successful, or false if there is an error.
3955 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3958 =item shutdown SOCKET,HOW
3960 Shuts down a socket connection in the manner indicated by HOW, which
3961 has the same interpretation as in the system call of the same name.
3963 shutdown(SOCKET, 0); # I/we have stopped reading data
3964 shutdown(SOCKET, 1); # I/we have stopped writing data
3965 shutdown(SOCKET, 2); # I/we have stopped using this socket
3967 This is useful with sockets when you want to tell the other
3968 side you're done writing but not done reading, or vice versa.
3969 It's also a more insistent form of close because it also
3970 disables the file descriptor in any forked copies in other
3977 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3978 returns sine of C<$_>.
3980 For the inverse sine operation, you may use the C<Math::Trig::asin>
3981 function, or use this relation:
3983 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3989 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3990 May be interrupted if the process receives a signal such as C<SIGALRM>.
3991 Returns the number of seconds actually slept. You probably cannot
3992 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
3995 On some older systems, it may sleep up to a full second less than what
3996 you requested, depending on how it counts seconds. Most modern systems
3997 always sleep the full amount. They may appear to sleep longer than that,
3998 however, because your process might not be scheduled right away in a
3999 busy multitasking system.
4001 For delays of finer granularity than one second, you may use Perl's
4002 C<syscall> interface to access setitimer(2) if your system supports
4003 it, or else see L</select> above. The Time::HiRes module from CPAN
4006 See also the POSIX module's C<sigpause> function.
4008 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4010 Opens a socket of the specified kind and attaches it to filehandle
4011 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4012 the system call of the same name. You should C<use Socket> first
4013 to get the proper definitions imported. See the examples in
4014 L<perlipc/"Sockets: Client/Server Communication">.
4016 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4018 Creates an unnamed pair of sockets in the specified domain, of the
4019 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4020 for the system call of the same name. If unimplemented, yields a fatal
4021 error. Returns true if successful.
4023 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4024 to C<pipe(Rdr, Wtr)> is essentially:
4027 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4028 shutdown(Rdr, 1); # no more writing for reader
4029 shutdown(Wtr, 0); # no more reading for writer
4031 See L<perlipc> for an example of socketpair use.
4033 =item sort SUBNAME LIST
4035 =item sort BLOCK LIST
4039 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4040 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4041 specified, it gives the name of a subroutine that returns an integer
4042 less than, equal to, or greater than C<0>, depending on how the elements
4043 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
4044 operators are extremely useful in such routines.) SUBNAME may be a
4045 scalar variable name (unsubscripted), in which case the value provides
4046 the name of (or a reference to) the actual subroutine to use. In place
4047 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4050 If the subroutine's prototype is C<($$)>, the elements to be compared
4051 are passed by reference in C<@_>, as for a normal subroutine. If not,
4052 the normal calling code for subroutines is bypassed in the interests of
4053 efficiency, and the elements to be compared are passed into the subroutine
4054 as the package global variables $a and $b (see example below). Note that
4055 in the latter case, it is usually counter-productive to declare $a and
4058 In either case, the subroutine may not be recursive. The values to be
4059 compared are always passed by reference, so don't modify them.
4061 You also cannot exit out of the sort block or subroutine using any of the
4062 loop control operators described in L<perlsyn> or with C<goto>.
4064 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4065 current collation locale. See L<perllocale>.
4070 @articles = sort @files;
4072 # same thing, but with explicit sort routine
4073 @articles = sort {$a cmp $b} @files;
4075 # now case-insensitively
4076 @articles = sort {uc($a) cmp uc($b)} @files;
4078 # same thing in reversed order
4079 @articles = sort {$b cmp $a} @files;
4081 # sort numerically ascending
4082 @articles = sort {$a <=> $b} @files;
4084 # sort numerically descending
4085 @articles = sort {$b <=> $a} @files;
4087 # this sorts the %age hash by value instead of key
4088 # using an in-line function
4089 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4091 # sort using explicit subroutine name
4093 $age{$a} <=> $age{$b}; # presuming numeric
4095 @sortedclass = sort byage @class;
4097 sub backwards { $b cmp $a }
4098 @harry = qw(dog cat x Cain Abel);
4099 @george = qw(gone chased yz Punished Axed);
4101 # prints AbelCaincatdogx
4102 print sort backwards @harry;
4103 # prints xdogcatCainAbel
4104 print sort @george, 'to', @harry;
4105 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4107 # inefficiently sort by descending numeric compare using
4108 # the first integer after the first = sign, or the
4109 # whole record case-insensitively otherwise
4112 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4117 # same thing, but much more efficiently;
4118 # we'll build auxiliary indices instead
4122 push @nums, /=(\d+)/;
4127 $nums[$b] <=> $nums[$a]
4129 $caps[$a] cmp $caps[$b]
4133 # same thing, but without any temps
4134 @new = map { $_->[0] }
4135 sort { $b->[1] <=> $a->[1]
4138 } map { [$_, /=(\d+)/, uc($_)] } @old;
4140 # using a prototype allows you to use any comparison subroutine
4141 # as a sort subroutine (including other package's subroutines)
4143 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4146 @new = sort other::backwards @old;
4148 If you're using strict, you I<must not> declare $a
4149 and $b as lexicals. They are package globals. That means
4150 if you're in the C<main> package, it's
4152 @articles = sort {$main::b <=> $main::a} @files;
4156 @articles = sort {$::b <=> $::a} @files;
4158 but if you're in the C<FooPack> package, it's
4160 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4162 The comparison function is required to behave. If it returns
4163 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4164 sometimes saying the opposite, for example) the results are not
4167 =item splice ARRAY,OFFSET,LENGTH,LIST
4169 =item splice ARRAY,OFFSET,LENGTH
4171 =item splice ARRAY,OFFSET
4173 Removes the elements designated by OFFSET and LENGTH from an array, and
4174 replaces them with the elements of LIST, if any. In list context,
4175 returns the elements removed from the array. In scalar context,
4176 returns the last element removed, or C<undef> if no elements are
4177 removed. The array grows or shrinks as necessary.
4178 If OFFSET is negative then it starts that far from the end of the array.
4179 If LENGTH is omitted, removes everything from OFFSET onward.
4180 If LENGTH is negative, leave that many elements off the end of the array.
4181 The following equivalences hold (assuming C<$[ == 0>):
4183 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4184 pop(@a) splice(@a,-1)
4185 shift(@a) splice(@a,0,1)
4186 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4187 $a[$x] = $y splice(@a,$x,1,$y)
4189 Example, assuming array lengths are passed before arrays:
4191 sub aeq { # compare two list values
4192 my(@a) = splice(@_,0,shift);
4193 my(@b) = splice(@_,0,shift);
4194 return 0 unless @a == @b; # same len?
4196 return 0 if pop(@a) ne pop(@b);
4200 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4202 =item split /PATTERN/,EXPR,LIMIT
4204 =item split /PATTERN/,EXPR
4206 =item split /PATTERN/
4210 Splits a string into a list of strings and returns that list. By default,
4211 empty leading fields are preserved, and empty trailing ones are deleted.
4213 If not in list context, returns the number of fields found and splits into
4214 the C<@_> array. (In list context, you can force the split into C<@_> by
4215 using C<??> as the pattern delimiters, but it still returns the list
4216 value.) The use of implicit split to C<@_> is deprecated, however, because
4217 it clobbers your subroutine arguments.
4219 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4220 splits on whitespace (after skipping any leading whitespace). Anything
4221 matching PATTERN is taken to be a delimiter separating the fields. (Note
4222 that the delimiter may be longer than one character.)
4224 If LIMIT is specified and positive, splits into no more than that
4225 many fields (though it may split into fewer). If LIMIT is unspecified
4226 or zero, trailing null fields are stripped (which potential users
4227 of C<pop> would do well to remember). If LIMIT is negative, it is
4228 treated as if an arbitrarily large LIMIT had been specified.
4230 A pattern matching the null string (not to be confused with
4231 a null pattern C<//>, which is just one member of the set of patterns
4232 matching a null string) will split the value of EXPR into separate
4233 characters at each point it matches that way. For example:
4235 print join(':', split(/ */, 'hi there'));
4237 produces the output 'h:i:t:h:e:r:e'.
4239 The LIMIT parameter can be used to split a line partially
4241 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4243 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4244 one larger than the number of variables in the list, to avoid
4245 unnecessary work. For the list above LIMIT would have been 4 by
4246 default. In time critical applications it behooves you not to split
4247 into more fields than you really need.
4249 If the PATTERN contains parentheses, additional list elements are
4250 created from each matching substring in the delimiter.
4252 split(/([,-])/, "1-10,20", 3);
4254 produces the list value
4256 (1, '-', 10, ',', 20)
4258 If you had the entire header of a normal Unix email message in $header,
4259 you could split it up into fields and their values this way:
4261 $header =~ s/\n\s+/ /g; # fix continuation lines
4262 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4264 The pattern C</PATTERN/> may be replaced with an expression to specify
4265 patterns that vary at runtime. (To do runtime compilation only once,
4266 use C</$variable/o>.)
4268 As a special case, specifying a PATTERN of space (C<' '>) will split on
4269 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4270 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4271 will give you as many null initial fields as there are leading spaces.
4272 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4273 whitespace produces a null first field. A C<split> with no arguments
4274 really does a C<split(' ', $_)> internally.
4278 open(PASSWD, '/etc/passwd');
4280 ($login, $passwd, $uid, $gid,
4281 $gcos, $home, $shell) = split(/:/);
4285 (Note that $shell above will still have a newline on it. See L</chop>,
4286 L</chomp>, and L</join>.)
4288 =item sprintf FORMAT, LIST
4290 Returns a string formatted by the usual C<printf> conventions of the
4291 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4292 on your system for an explanation of the general principles.
4294 Perl does its own C<sprintf> formatting--it emulates the C
4295 function C<sprintf>, but it doesn't use it (except for floating-point
4296 numbers, and even then only the standard modifiers are allowed). As a
4297 result, any non-standard extensions in your local C<sprintf> are not
4298 available from Perl.
4300 Perl's C<sprintf> permits the following universally-known conversions:
4303 %c a character with the given number
4305 %d a signed integer, in decimal
4306 %u an unsigned integer, in decimal
4307 %o an unsigned integer, in octal
4308 %x an unsigned integer, in hexadecimal
4309 %e a floating-point number, in scientific notation
4310 %f a floating-point number, in fixed decimal notation
4311 %g a floating-point number, in %e or %f notation
4313 In addition, Perl permits the following widely-supported conversions:
4315 %X like %x, but using upper-case letters
4316 %E like %e, but using an upper-case "E"
4317 %G like %g, but with an upper-case "E" (if applicable)
4318 %b an unsigned integer, in binary
4319 %p a pointer (outputs the Perl value's address in hexadecimal)
4320 %n special: *stores* the number of characters output so far
4321 into the next variable in the parameter list
4323 And the following Perl-specific conversion:
4325 %v a string, output as a tuple of integers ("Perl" is 80.101.114.108)
4327 Finally, for backward (and we do mean "backward") compatibility, Perl
4328 permits these unnecessary but widely-supported conversions:
4331 %D a synonym for %ld
4332 %U a synonym for %lu
4333 %O a synonym for %lo
4336 Perl permits the following universally-known flags between the C<%>
4337 and the conversion letter:
4339 space prefix positive number with a space
4340 + prefix positive number with a plus sign
4341 - left-justify within the field
4342 0 use zeros, not spaces, to right-justify
4343 # prefix non-zero octal with "0", non-zero hex with "0x"
4344 number minimum field width
4345 .number "precision": digits after decimal point for
4346 floating-point, max length for string, minimum length
4348 l interpret integer as C type "long" or "unsigned long"
4349 h interpret integer as C type "short" or "unsigned short"
4350 If no flags, interpret integer as C type "int" or "unsigned"
4352 There is also one Perl-specific flag:
4354 V interpret integer as Perl's standard integer type
4356 Where a number would appear in the flags, an asterisk (C<*>) may be
4357 used instead, in which case Perl uses the next item in the parameter
4358 list as the given number (that is, as the field width or precision).
4359 If a field width obtained through C<*> is negative, it has the same
4360 effect as the C<-> flag: left-justification.
4362 If C<use locale> is in effect, the character used for the decimal
4363 point in formatted real numbers is affected by the LC_NUMERIC locale.
4366 If Perl understands "quads" (64-bit integers) (this requires
4367 either that the platform natively supports quads or that Perl
4368 has been specifically compiled to support quads), the characters
4372 print quads, and they may optionally be preceded by
4380 You can find out whether your Perl supports quads via L<Config>:
4383 ($Config{use64bits} eq 'define' || $Config{longsize} == 8) &&
4386 If Perl understands "long doubles" (this requires that the platform
4387 supports long doubles), the flags
4391 may optionally be preceded by
4399 You can find out whether your Perl supports long doubles via L<Config>:
4402 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4408 Return the square root of EXPR. If EXPR is omitted, returns square
4409 root of C<$_>. Only works on non-negative operands, unless you've
4410 loaded the standard Math::Complex module.
4413 print sqrt(-2); # prints 1.4142135623731i
4419 Sets the random number seed for the C<rand> operator. If EXPR is
4420 omitted, uses a semi-random value supplied by the kernel (if it supports
4421 the F</dev/urandom> device) or based on the current time and process
4422 ID, among other things. In versions of Perl prior to 5.004 the default
4423 seed was just the current C<time>. This isn't a particularly good seed,
4424 so many old programs supply their own seed value (often C<time ^ $$> or
4425 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4427 In fact, it's usually not necessary to call C<srand> at all, because if
4428 it is not called explicitly, it is called implicitly at the first use of
4429 the C<rand> operator. However, this was not the case in version of Perl
4430 before 5.004, so if your script will run under older Perl versions, it
4431 should call C<srand>.
4433 Note that you need something much more random than the default seed for
4434 cryptographic purposes. Checksumming the compressed output of one or more
4435 rapidly changing operating system status programs is the usual method. For
4438 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4440 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4443 Do I<not> call C<srand> multiple times in your program unless you know
4444 exactly what you're doing and why you're doing it. The point of the
4445 function is to "seed" the C<rand> function so that C<rand> can produce
4446 a different sequence each time you run your program. Just do it once at the
4447 top of your program, or you I<won't> get random numbers out of C<rand>!
4449 Frequently called programs (like CGI scripts) that simply use
4453 for a seed can fall prey to the mathematical property that
4457 one-third of the time. So don't do that.
4459 =item stat FILEHANDLE
4465 Returns a 13-element list giving the status info for a file, either
4466 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4467 it stats C<$_>. Returns a null list if the stat fails. Typically used
4470 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4471 $atime,$mtime,$ctime,$blksize,$blocks)
4474 Not all fields are supported on all filesystem types. Here are the
4475 meaning of the fields:
4477 0 dev device number of filesystem
4479 2 mode file mode (type and permissions)
4480 3 nlink number of (hard) links to the file
4481 4 uid numeric user ID of file's owner
4482 5 gid numeric group ID of file's owner
4483 6 rdev the device identifier (special files only)
4484 7 size total size of file, in bytes
4485 8 atime last access time in seconds since the epoch
4486 9 mtime last modify time in seconds since the epoch
4487 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4488 11 blksize preferred block size for file system I/O
4489 12 blocks actual number of blocks allocated
4491 (The epoch was at 00:00 January 1, 1970 GMT.)
4493 If stat is passed the special filehandle consisting of an underline, no
4494 stat is done, but the current contents of the stat structure from the
4495 last stat or filetest are returned. Example:
4497 if (-x $file && (($d) = stat(_)) && $d < 0) {
4498 print "$file is executable NFS file\n";
4501 (This works on machines only for which the device number is negative
4504 Because the mode contains both the file type and its permissions, you
4505 should mask off the file type portion and (s)printf using a C<"%o">
4506 if you want to see the real permissions.
4508 $mode = (stat($filename))[2];
4509 printf "Permissions are %04o\n", $mode & 07777;
4511 In scalar context, C<stat> returns a boolean value indicating success
4512 or failure, and, if successful, sets the information associated with
4513 the special filehandle C<_>.
4515 The File::stat module provides a convenient, by-name access mechanism:
4518 $sb = stat($filename);
4519 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4520 $filename, $sb->size, $sb->mode & 07777,
4521 scalar localtime $sb->mtime;
4523 You can import symbolic mode constants (C<S_IF*>) and functions
4524 (C<S_IS*>) from the Fcntl module:
4528 $mode = (stat($filename))[2];
4530 $user_rwx = ($mode & S_IRWXU) >> 6;
4531 $group_read = ($mode & S_IRGRP) >> 3;
4532 $other_execute = $mode & S_IXOTH;
4534 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4536 $is_setuid = $mode & S_ISUID;
4537 $is_setgid = S_ISDIR($mode);
4539 You could write the last two using the C<-u> and C<-d> operators.
4540 The commonly available S_IF* constants are
4542 # Permissions: read, write, execute, for user, group, others.
4544 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4545 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4546 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4548 # Setuid/Setgid/Stickiness.
4550 S_ISUID S_ISGID S_ISVTX S_ISTXT
4552 # File types. Not necessarily all are available on your system.
4554 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4556 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4558 S_IREAD S_IWRITE S_IEXEC
4560 and the S_IF* functions are
4562 S_IFMODE($mode) the part of $mode containg the permission bits
4563 and the setuid/setgid/sticky bits
4565 S_IFMT($mode) the part of $mode containing the file type
4566 which can be bit-anded with e.g. S_IFREG
4567 or with the following functions
4569 # The operators -f, -d, -l, -b, -c, -p, and -s.
4571 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4572 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4574 # No direct -X operator counterpart, but for the first one
4575 # the -g operator is often equivalent. The ENFMT stands for
4576 # record flocking enforcement, a platform-dependent feature.
4578 S_ISENFMT($mode) S_ISWHT($mode)
4580 See your native chmod(2) and stat(2) documentation for more details
4581 about the S_* constants.
4587 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4588 doing many pattern matches on the string before it is next modified.
4589 This may or may not save time, depending on the nature and number of
4590 patterns you are searching on, and on the distribution of character
4591 frequencies in the string to be searched--you probably want to compare
4592 run times with and without it to see which runs faster. Those loops
4593 which scan for many short constant strings (including the constant
4594 parts of more complex patterns) will benefit most. You may have only
4595 one C<study> active at a time--if you study a different scalar the first
4596 is "unstudied". (The way C<study> works is this: a linked list of every
4597 character in the string to be searched is made, so we know, for
4598 example, where all the C<'k'> characters are. From each search string,
4599 the rarest character is selected, based on some static frequency tables
4600 constructed from some C programs and English text. Only those places
4601 that contain this "rarest" character are examined.)
4603 For example, here is a loop that inserts index producing entries
4604 before any line containing a certain pattern:
4608 print ".IX foo\n" if /\bfoo\b/;
4609 print ".IX bar\n" if /\bbar\b/;
4610 print ".IX blurfl\n" if /\bblurfl\b/;
4615 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4616 will be looked at, because C<f> is rarer than C<o>. In general, this is
4617 a big win except in pathological cases. The only question is whether
4618 it saves you more time than it took to build the linked list in the
4621 Note that if you have to look for strings that you don't know till
4622 runtime, you can build an entire loop as a string and C<eval> that to
4623 avoid recompiling all your patterns all the time. Together with
4624 undefining C<$/> to input entire files as one record, this can be very
4625 fast, often faster than specialized programs like fgrep(1). The following
4626 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4627 out the names of those files that contain a match:
4629 $search = 'while (<>) { study;';
4630 foreach $word (@words) {
4631 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4636 eval $search; # this screams
4637 $/ = "\n"; # put back to normal input delimiter
4638 foreach $file (sort keys(%seen)) {
4646 =item sub NAME BLOCK
4648 This is subroutine definition, not a real function I<per se>. With just a
4649 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4650 Without a NAME, it's an anonymous function declaration, and does actually
4651 return a value: the CODE ref of the closure you just created. See L<perlsub>
4652 and L<perlref> for details.
4654 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4656 =item substr EXPR,OFFSET,LENGTH
4658 =item substr EXPR,OFFSET
4660 Extracts a substring out of EXPR and returns it. First character is at
4661 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4662 If OFFSET is negative (or more precisely, less than C<$[>), starts
4663 that far from the end of the string. If LENGTH is omitted, returns
4664 everything to the end of the string. If LENGTH is negative, leaves that
4665 many characters off the end of the string.
4667 You can use the substr() function as an lvalue, in which case EXPR
4668 must itself be an lvalue. If you assign something shorter than LENGTH,
4669 the string will shrink, and if you assign something longer than LENGTH,
4670 the string will grow to accommodate it. To keep the string the same
4671 length you may need to pad or chop your value using C<sprintf>.
4673 If OFFSET and LENGTH specify a substring that is partly outside the
4674 string, only the part within the string is returned. If the substring
4675 is beyond either end of the string, substr() returns the undefined
4676 value and produces a warning. When used as an lvalue, specifying a
4677 substring that is entirely outside the string is a fatal error.
4678 Here's an example showing the behavior for boundary cases:
4681 substr($name, 4) = 'dy'; # $name is now 'freddy'
4682 my $null = substr $name, 6, 2; # returns '' (no warning)
4683 my $oops = substr $name, 7; # returns undef, with warning
4684 substr($name, 7) = 'gap'; # fatal error
4686 An alternative to using substr() as an lvalue is to specify the
4687 replacement string as the 4th argument. This allows you to replace
4688 parts of the EXPR and return what was there before in one operation,
4689 just as you can with splice().
4691 =item symlink OLDFILE,NEWFILE
4693 Creates a new filename symbolically linked to the old filename.
4694 Returns C<1> for success, C<0> otherwise. On systems that don't support
4695 symbolic links, produces a fatal error at run time. To check for that,
4698 $symlink_exists = eval { symlink("",""); 1 };
4702 Calls the system call specified as the first element of the list,
4703 passing the remaining elements as arguments to the system call. If
4704 unimplemented, produces a fatal error. The arguments are interpreted
4705 as follows: if a given argument is numeric, the argument is passed as
4706 an int. If not, the pointer to the string value is passed. You are
4707 responsible to make sure a string is pre-extended long enough to
4708 receive any result that might be written into a string. You can't use a
4709 string literal (or other read-only string) as an argument to C<syscall>
4710 because Perl has to assume that any string pointer might be written
4712 integer arguments are not literals and have never been interpreted in a
4713 numeric context, you may need to add C<0> to them to force them to look
4714 like numbers. This emulates the C<syswrite> function (or vice versa):
4716 require 'syscall.ph'; # may need to run h2ph
4718 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4720 Note that Perl supports passing of up to only 14 arguments to your system call,
4721 which in practice should usually suffice.
4723 Syscall returns whatever value returned by the system call it calls.
4724 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4725 Note that some system calls can legitimately return C<-1>. The proper
4726 way to handle such calls is to assign C<$!=0;> before the call and
4727 check the value of C<$!> if syscall returns C<-1>.
4729 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4730 number of the read end of the pipe it creates. There is no way
4731 to retrieve the file number of the other end. You can avoid this
4732 problem by using C<pipe> instead.
4734 =item sysopen FILEHANDLE,FILENAME,MODE
4736 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4738 Opens the file whose filename is given by FILENAME, and associates it
4739 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4740 the name of the real filehandle wanted. This function calls the
4741 underlying operating system's C<open> function with the parameters
4742 FILENAME, MODE, PERMS.
4744 The possible values and flag bits of the MODE parameter are
4745 system-dependent; they are available via the standard module C<Fcntl>.
4746 See the documentation of your operating system's C<open> to see which
4747 values and flag bits are available. You may combine several flags
4748 using the C<|>-operator.
4750 Some of the most common values are C<O_RDONLY> for opening the file in
4751 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4752 and C<O_RDWR> for opening the file in read-write mode, and.
4754 For historical reasons, some values work on almost every system
4755 supported by perl: zero means read-only, one means write-only, and two
4756 means read/write. We know that these values do I<not> work under
4757 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4758 use them in new code.
4760 If the file named by FILENAME does not exist and the C<open> call creates
4761 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4762 PERMS specifies the permissions of the newly created file. If you omit
4763 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4764 These permission values need to be in octal, and are modified by your
4765 process's current C<umask>.
4767 In many systems the C<O_EXCL> flag is available for opening files in
4768 exclusive mode. This is B<not> locking: exclusiveness means here that
4769 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4772 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4774 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4775 that takes away the user's option to have a more permissive umask.
4776 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4779 Note that C<sysopen> depends on the fdopen() C library function.
4780 On many UNIX systems, fdopen() is known to fail when file descriptors
4781 exceed a certain value, typically 255. If you need more file
4782 descriptors than that, consider rebuilding Perl to use the C<sfio>
4783 library, or perhaps using the POSIX::open() function.
4785 See L<perlopentut> for a kinder, gentler explanation of opening files.
4787 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4789 =item sysread FILEHANDLE,SCALAR,LENGTH
4791 Attempts to read LENGTH bytes of data into variable SCALAR from the
4792 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4793 so mixing this with other kinds of reads, C<print>, C<write>,
4794 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4795 usually buffers data. Returns the number of bytes actually read, C<0>
4796 at end of file, or undef if there was an error. SCALAR will be grown or
4797 shrunk so that the last byte actually read is the last byte of the
4798 scalar after the read.
4800 An OFFSET may be specified to place the read data at some place in the
4801 string other than the beginning. A negative OFFSET specifies
4802 placement at that many bytes counting backwards from the end of the
4803 string. A positive OFFSET greater than the length of SCALAR results
4804 in the string being padded to the required size with C<"\0"> bytes before
4805 the result of the read is appended.
4807 There is no syseof() function, which is ok, since eof() doesn't work
4808 very well on device files (like ttys) anyway. Use sysread() and check
4809 for a return value for 0 to decide whether you're done.
4811 =item sysseek FILEHANDLE,POSITION,WHENCE
4813 Sets FILEHANDLE's system position using the system call lseek(2). It
4814 bypasses stdio, so mixing this with reads (other than C<sysread>),
4815 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4816 FILEHANDLE may be an expression whose value gives the name of the
4817 filehandle. The values for WHENCE are C<0> to set the new position to
4818 POSITION, C<1> to set the it to the current position plus POSITION,
4819 and C<2> to set it to EOF plus POSITION (typically negative). For
4820 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4821 C<SEEK_END> (start of the file, current position, end of the file)
4822 from the Fcntl module.
4824 Returns the new position, or the undefined value on failure. A position
4825 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4826 true on success and false on failure, yet you can still easily determine
4831 =item system PROGRAM LIST
4833 Does exactly the same thing as C<exec LIST>, except that a fork is
4834 done first, and the parent process waits for the child process to
4835 complete. Note that argument processing varies depending on the
4836 number of arguments. If there is more than one argument in LIST,
4837 or if LIST is an array with more than one value, starts the program
4838 given by the first element of the list with arguments given by the
4839 rest of the list. If there is only one scalar argument, the argument
4840 is checked for shell metacharacters, and if there are any, the
4841 entire argument is passed to the system's command shell for parsing
4842 (this is C</bin/sh -c> on Unix platforms, but varies on other
4843 platforms). If there are no shell metacharacters in the argument,
4844 it is split into words and passed directly to C<execvp>, which is
4847 All files opened for output are flushed before attempting the exec().
4849 The return value is the exit status of the program as
4850 returned by the C<wait> call. To get the actual exit value divide by
4851 256. See also L</exec>. This is I<not> what you want to use to capture
4852 the output from a command, for that you should use merely backticks or
4853 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4854 indicates a failure to start the program (inspect $! for the reason).
4856 Like C<exec>, C<system> allows you to lie to a program about its name if
4857 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4859 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4860 program they're running doesn't actually interrupt your program.
4862 @args = ("command", "arg1", "arg2");
4864 or die "system @args failed: $?"
4866 You can check all the failure possibilities by inspecting
4869 $exit_value = $? >> 8;
4870 $signal_num = $? & 127;
4871 $dumped_core = $? & 128;
4873 When the arguments get executed via the system shell, results
4874 and return codes will be subject to its quirks and capabilities.
4875 See L<perlop/"`STRING`"> and L</exec> for details.
4877 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4879 =item syswrite FILEHANDLE,SCALAR,LENGTH
4881 =item syswrite FILEHANDLE,SCALAR
4883 Attempts to write LENGTH bytes of data from variable SCALAR to the
4884 specified FILEHANDLE, using the system call write(2). If LENGTH
4885 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4886 this with reads (other than C<sysread())>, C<print>, C<write>,
4887 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4888 usually buffers data. Returns the number of bytes actually written,
4889 or C<undef> if there was an error. If the LENGTH is greater than
4890 the available data in the SCALAR after the OFFSET, only as much
4891 data as is available will be written.
4893 An OFFSET may be specified to write the data from some part of the
4894 string other than the beginning. A negative OFFSET specifies writing
4895 that many bytes counting backwards from the end of the string. In the
4896 case the SCALAR is empty you can use OFFSET but only zero offset.
4898 =item tell FILEHANDLE
4902 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4903 expression whose value gives the name of the actual filehandle. If
4904 FILEHANDLE is omitted, assumes the file last read.
4906 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4908 =item telldir DIRHANDLE
4910 Returns the current position of the C<readdir> routines on DIRHANDLE.
4911 Value may be given to C<seekdir> to access a particular location in a
4912 directory. Has the same caveats about possible directory compaction as
4913 the corresponding system library routine.
4915 =item tie VARIABLE,CLASSNAME,LIST
4917 This function binds a variable to a package class that will provide the
4918 implementation for the variable. VARIABLE is the name of the variable
4919 to be enchanted. CLASSNAME is the name of a class implementing objects
4920 of correct type. Any additional arguments are passed to the C<new>
4921 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4922 or C<TIEHASH>). Typically these are arguments such as might be passed
4923 to the C<dbm_open()> function of C. The object returned by the C<new>
4924 method is also returned by the C<tie> function, which would be useful
4925 if you want to access other methods in CLASSNAME.
4927 Note that functions such as C<keys> and C<values> may return huge lists
4928 when used on large objects, like DBM files. You may prefer to use the
4929 C<each> function to iterate over such. Example:
4931 # print out history file offsets
4933 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4934 while (($key,$val) = each %HIST) {
4935 print $key, ' = ', unpack('L',$val), "\n";
4939 A class implementing a hash should have the following methods:
4941 TIEHASH classname, LIST
4943 STORE this, key, value
4948 NEXTKEY this, lastkey
4951 A class implementing an ordinary array should have the following methods:
4953 TIEARRAY classname, LIST
4955 STORE this, key, value
4957 STORESIZE this, count
4963 SPLICE this, offset, length, LIST
4967 A class implementing a file handle should have the following methods:
4969 TIEHANDLE classname, LIST
4970 READ this, scalar, length, offset
4973 WRITE this, scalar, length, offset
4975 PRINTF this, format, LIST
4979 A class implementing a scalar should have the following methods:
4981 TIESCALAR classname, LIST
4986 Not all methods indicated above need be implemented. See L<perltie>,
4987 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
4989 Unlike C<dbmopen>, the C<tie> function will not use or require a module
4990 for you--you need to do that explicitly yourself. See L<DB_File>
4991 or the F<Config> module for interesting C<tie> implementations.
4993 For further details see L<perltie>, L<"tied VARIABLE">.
4997 Returns a reference to the object underlying VARIABLE (the same value
4998 that was originally returned by the C<tie> call that bound the variable
4999 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5004 Returns the number of non-leap seconds since whatever time the system
5005 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5006 and 00:00:00 UTC, January 1, 1970 for most other systems).
5007 Suitable for feeding to C<gmtime> and C<localtime>.
5009 For measuring time in better granularity than one second,
5010 you may use either the Time::HiRes module from CPAN, or
5011 if you have gettimeofday(2), you may be able to use the
5012 C<syscall> interface of Perl, see L<perlfaq8> for details.
5016 Returns a four-element list giving the user and system times, in
5017 seconds, for this process and the children of this process.
5019 ($user,$system,$cuser,$csystem) = times;
5023 The transliteration operator. Same as C<y///>. See L<perlop>.
5025 =item truncate FILEHANDLE,LENGTH
5027 =item truncate EXPR,LENGTH
5029 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5030 specified length. Produces a fatal error if truncate isn't implemented
5031 on your system. Returns true if successful, the undefined value
5038 Returns an uppercased version of EXPR. This is the internal function
5039 implementing the C<\U> escape in double-quoted strings.
5040 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5041 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5042 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5044 If EXPR is omitted, uses C<$_>.
5050 Returns the value of EXPR with the first character
5051 in uppercase (titlecase in Unicode). This is
5052 the internal function implementing the C<\u> escape in double-quoted strings.
5053 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5056 If EXPR is omitted, uses C<$_>.
5062 Sets the umask for the process to EXPR and returns the previous value.
5063 If EXPR is omitted, merely returns the current umask.
5065 The Unix permission C<rwxr-x---> is represented as three sets of three
5066 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5067 and isn't one of the digits). The C<umask> value is such a number
5068 representing disabled permissions bits. The permission (or "mode")
5069 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5070 even if you tell C<sysopen> to create a file with permissions C<0777>,
5071 if your umask is C<0022> then the file will actually be created with
5072 permissions C<0755>. If your C<umask> were C<0027> (group can't
5073 write; others can't read, write, or execute), then passing
5074 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5077 Here's some advice: supply a creation mode of C<0666> for regular
5078 files (in C<sysopen>) and one of C<0777> for directories (in
5079 C<mkdir>) and executable files. This gives users the freedom of
5080 choice: if they want protected files, they might choose process umasks
5081 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5082 Programs should rarely if ever make policy decisions better left to
5083 the user. The exception to this is when writing files that should be
5084 kept private: mail files, web browser cookies, I<.rhosts> files, and
5087 If umask(2) is not implemented on your system and you are trying to
5088 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5089 fatal error at run time. If umask(2) is not implemented and you are
5090 not trying to restrict access for yourself, returns C<undef>.
5092 Remember that a umask is a number, usually given in octal; it is I<not> a
5093 string of octal digits. See also L</oct>, if all you have is a string.
5099 Undefines the value of EXPR, which must be an lvalue. Use only on a
5100 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5101 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5102 will probably not do what you expect on most predefined variables or
5103 DBM list values, so don't do that; see L<delete>.) Always returns the
5104 undefined value. You can omit the EXPR, in which case nothing is
5105 undefined, but you still get an undefined value that you could, for
5106 instance, return from a subroutine, assign to a variable or pass as a
5107 parameter. Examples:
5110 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5114 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5115 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5116 select undef, undef, undef, 0.25;
5117 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5119 Note that this is a unary operator, not a list operator.
5125 Deletes a list of files. Returns the number of files successfully
5128 $cnt = unlink 'a', 'b', 'c';
5132 Note: C<unlink> will not delete directories unless you are superuser and
5133 the B<-U> flag is supplied to Perl. Even if these conditions are
5134 met, be warned that unlinking a directory can inflict damage on your
5135 filesystem. Use C<rmdir> instead.
5137 If LIST is omitted, uses C<$_>.
5139 =item unpack TEMPLATE,EXPR
5141 C<unpack> does the reverse of C<pack>: it takes a string
5142 and expands it out into a list of values.
5143 (In scalar context, it returns merely the first value produced.)
5145 The string is broken into chunks described by the TEMPLATE. Each chunk
5146 is converted separately to a value. Typically, either the string is a result
5147 of C<pack>, or the bytes of the string represent a C structure of some
5150 The TEMPLATE has the same format as in the C<pack> function.
5151 Here's a subroutine that does substring:
5154 my($what,$where,$howmuch) = @_;
5155 unpack("x$where a$howmuch", $what);
5160 sub ordinal { unpack("c",$_[0]); } # same as ord()
5162 In addition to fields allowed in pack(), you may prefix a field with
5163 a %E<lt>numberE<gt> to indicate that
5164 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
5165 themselves. Default is a 16-bit checksum. Checksum is calculated by
5166 summing numeric values of expanded values (for string fields the sum of
5167 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5169 For example, the following
5170 computes the same number as the System V sum program:
5174 unpack("%32C*",<>) % 65535;
5177 The following efficiently counts the number of set bits in a bit vector:
5179 $setbits = unpack("%32b*", $selectmask);
5181 The C<p> and C<P> formats should be used with care. Since Perl
5182 has no way of checking whether the value passed to C<unpack()>
5183 corresponds to a valid memory location, passing a pointer value that's
5184 not known to be valid is likely to have disastrous consequences.
5186 If the repeat count of a field is larger than what the remainder of
5187 the input string allows, repeat count is decreased. If the input string
5188 is longer than one described by the TEMPLATE, the rest is ignored.
5190 See L</pack> for more examples and notes.
5192 =item untie VARIABLE
5194 Breaks the binding between a variable and a package. (See C<tie>.)
5196 =item unshift ARRAY,LIST
5198 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5199 depending on how you look at it. Prepends list to the front of the
5200 array, and returns the new number of elements in the array.
5202 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5204 Note the LIST is prepended whole, not one element at a time, so the
5205 prepended elements stay in the same order. Use C<reverse> to do the
5208 =item use Module LIST
5212 =item use Module VERSION LIST
5216 Imports some semantics into the current package from the named module,
5217 generally by aliasing certain subroutine or variable names into your
5218 package. It is exactly equivalent to
5220 BEGIN { require Module; import Module LIST; }
5222 except that Module I<must> be a bareword.
5224 If the first argument to C<use> is a number or a version tuple, it is
5225 treated as a version instead of a module name. If the version
5226 of the Perl interpreter is less than VERSION, then an error message
5227 is printed and Perl exits immediately.
5229 use 5.005_03; # version number
5230 use v5.6.0; # version tuple
5232 This is often useful if you need to check the current Perl version before
5233 C<use>ing library modules that have changed in incompatible ways from
5234 older versions of Perl. (We try not to do this more than we have to.)
5236 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5237 C<require> makes sure the module is loaded into memory if it hasn't been
5238 yet. The C<import> is not a builtin--it's just an ordinary static method
5239 call into the C<Module> package to tell the module to import the list of
5240 features back into the current package. The module can implement its
5241 C<import> method any way it likes, though most modules just choose to
5242 derive their C<import> method via inheritance from the C<Exporter> class that
5243 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5244 method can be found then the call is skipped.
5246 If you don't want your namespace altered, explicitly supply an empty list:
5250 That is exactly equivalent to
5252 BEGIN { require Module }
5254 If the VERSION argument is present between Module and LIST, then the
5255 C<use> will call the VERSION method in class Module with the given
5256 version as an argument. The default VERSION method, inherited from
5257 the Universal class, croaks if the given version is larger than the
5258 value of the variable C<$Module::VERSION>. (Note that there is not a
5259 comma after VERSION!)
5261 Because this is a wide-open interface, pragmas (compiler directives)
5262 are also implemented this way. Currently implemented pragmas are:
5266 use sigtrap qw(SEGV BUS);
5267 use strict qw(subs vars refs);
5268 use subs qw(afunc blurfl);
5269 use warnings qw(all);
5271 Some of these pseudo-modules import semantics into the current
5272 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5273 which import symbols into the current package (which are effective
5274 through the end of the file).
5276 There's a corresponding C<no> command that unimports meanings imported
5277 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5283 If no C<unimport> method can be found the call fails with a fatal error.
5285 See L<perlmod> for a list of standard modules and pragmas.
5289 Changes the access and modification times on each file of a list of
5290 files. The first two elements of the list must be the NUMERICAL access
5291 and modification times, in that order. Returns the number of files
5292 successfully changed. The inode change time of each file is set
5293 to the current time. This code has the same effect as the C<touch>
5294 command if the files already exist:
5298 utime $now, $now, @ARGV;
5302 Returns a list consisting of all the values of the named hash. (In a
5303 scalar context, returns the number of values.) The values are
5304 returned in an apparently random order. The actual random order is
5305 subject to change in future versions of perl, but it is guaranteed to
5306 be the same order as either the C<keys> or C<each> function would
5307 produce on the same (unmodified) hash.
5309 Note that you cannot modify the values of a hash this way, because the
5310 returned list is just a copy. You need to use a hash slice for that,
5311 since it's lvaluable in a way that values() is not.
5313 for (values %hash) { s/foo/bar/g } # FAILS!
5314 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5316 As a side effect, calling values() resets the HASH's internal iterator.
5317 See also C<keys>, C<each>, and C<sort>.
5319 =item vec EXPR,OFFSET,BITS
5321 Treats the string in EXPR as a bit vector made up of elements of
5322 width BITS, and returns the value of the element specified by OFFSET
5323 as an unsigned integer. BITS therefore specifies the number of bits
5324 that are reserved for each element in the bit vector. This must
5325 be a power of two from 1 to 32 (or 64, if your platform supports
5328 If BITS is 8, "elements" coincide with bytes of the input string.
5330 If BITS is 16 or more, bytes of the input string are grouped into chunks
5331 of size BITS/8, and each group is converted to a number as with
5332 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5333 for BITS==64). See L<"pack"> for details.
5335 If bits is 4 or less, the string is broken into bytes, then the bits
5336 of each byte are broken into 8/BITS groups. Bits of a byte are
5337 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5338 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5339 breaking the single input byte C<chr(0x36)> into two groups gives a list
5340 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5342 C<vec> may also be assigned to, in which case parentheses are needed
5343 to give the expression the correct precedence as in
5345 vec($image, $max_x * $x + $y, 8) = 3;
5347 If the selected element is off the end of the string, the value 0 is
5348 returned. If an element off the end of the string is written to,
5349 Perl will first extend the string with sufficiently many zero bytes.
5351 Strings created with C<vec> can also be manipulated with the logical
5352 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5353 vector operation is desired when both operands are strings.
5354 See L<perlop/"Bitwise String Operators">.
5356 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5357 The comments show the string after each step. Note that this code works
5358 in the same way on big-endian or little-endian machines.
5361 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5363 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5364 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5366 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5367 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5368 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5369 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5370 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5371 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5373 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5374 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5375 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5378 To transform a bit vector into a string or list of 0's and 1's, use these:
5380 $bits = unpack("b*", $vector);
5381 @bits = split(//, unpack("b*", $vector));
5383 If you know the exact length in bits, it can be used in place of the C<*>.
5385 Here is an example to illustrate how the bits actually fall in place:
5391 unpack("V",$_) 01234567890123456789012345678901
5392 ------------------------------------------------------------------
5397 for ($shift=0; $shift < $width; ++$shift) {
5398 for ($off=0; $off < 32/$width; ++$off) {
5399 $str = pack("B*", "0"x32);
5400 $bits = (1<<$shift);
5401 vec($str, $off, $width) = $bits;
5402 $res = unpack("b*",$str);
5403 $val = unpack("V", $str);
5410 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5411 $off, $width, $bits, $val, $res
5415 Regardless of the machine architecture on which it is run, the above
5416 example should print the following table:
5419 unpack("V",$_) 01234567890123456789012345678901
5420 ------------------------------------------------------------------
5421 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5422 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5423 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5424 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5425 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5426 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5427 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5428 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5429 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5430 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5431 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5432 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5433 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5434 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5435 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5436 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5437 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5438 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5439 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5440 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5441 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5442 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5443 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5444 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5445 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5446 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5447 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5448 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5449 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5450 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5451 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5452 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5453 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5454 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5455 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5456 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5457 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5458 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5459 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5460 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5461 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5462 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5463 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5464 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5465 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5466 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5467 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5468 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5469 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5470 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5471 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5472 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5473 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5474 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5475 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5476 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5477 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5478 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5479 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5480 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5481 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5482 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5483 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5484 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5485 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5486 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5487 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5488 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5489 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5490 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5491 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5492 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5493 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5494 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5495 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5496 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5497 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5498 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5499 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5500 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5501 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5502 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5503 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5504 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5505 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5506 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5507 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5508 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5509 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5510 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5511 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5512 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5513 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5514 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5515 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5516 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5517 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5518 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5519 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5520 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5521 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5522 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5523 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5524 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5525 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5526 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5527 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5528 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5529 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5530 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5531 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5532 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5533 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5534 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5535 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5536 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5537 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5538 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5539 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5540 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5541 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5542 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5543 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5544 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5545 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5546 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5547 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5548 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5552 Behaves like the wait(2) system call on your system: it waits for a child
5553 process to terminate and returns the pid of the deceased process, or
5554 C<-1> if there are no child processes. The status is returned in C<$?>.
5555 Note that a return value of C<-1> could mean that child processes are
5556 being automatically reaped, as described in L<perlipc>.
5558 =item waitpid PID,FLAGS
5560 Waits for a particular child process to terminate and returns the pid of
5561 the deceased process, or C<-1> if there is no such child process. On some
5562 systems, a value of 0 indicates that there are processes still running.
5563 The status is returned in C<$?>. If you say
5565 use POSIX ":sys_wait_h";
5568 $kid = waitpid(-1,&WNOHANG);
5571 then you can do a non-blocking wait for all pending zombie processes.
5572 Non-blocking wait is available on machines supporting either the
5573 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5574 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5575 system call by remembering the status values of processes that have
5576 exited but have not been harvested by the Perl script yet.)
5578 Note that on some systems, a return value of C<-1> could mean that child
5579 processes are being automatically reaped. See L<perlipc> for details,
5580 and for other examples.
5584 Returns true if the context of the currently executing subroutine is
5585 looking for a list value. Returns false if the context is looking
5586 for a scalar. Returns the undefined value if the context is looking
5587 for no value (void context).
5589 return unless defined wantarray; # don't bother doing more
5590 my @a = complex_calculation();
5591 return wantarray ? @a : "@a";
5593 This function should have been named wantlist() instead.
5597 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5600 If LIST is empty and C<$@> already contains a value (typically from a
5601 previous eval) that value is used after appending C<"\t...caught">
5602 to C<$@>. This is useful for staying almost, but not entirely similar to
5605 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5607 No message is printed if there is a C<$SIG{__WARN__}> handler
5608 installed. It is the handler's responsibility to deal with the message
5609 as it sees fit (like, for instance, converting it into a C<die>). Most
5610 handlers must therefore make arrangements to actually display the
5611 warnings that they are not prepared to deal with, by calling C<warn>
5612 again in the handler. Note that this is quite safe and will not
5613 produce an endless loop, since C<__WARN__> hooks are not called from
5616 You will find this behavior is slightly different from that of
5617 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5618 instead call C<die> again to change it).
5620 Using a C<__WARN__> handler provides a powerful way to silence all
5621 warnings (even the so-called mandatory ones). An example:
5623 # wipe out *all* compile-time warnings
5624 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5626 my $foo = 20; # no warning about duplicate my $foo,
5627 # but hey, you asked for it!
5628 # no compile-time or run-time warnings before here
5631 # run-time warnings enabled after here
5632 warn "\$foo is alive and $foo!"; # does show up
5634 See L<perlvar> for details on setting C<%SIG> entries, and for more
5635 examples. See the Carp module for other kinds of warnings using its
5636 carp() and cluck() functions.
5638 =item write FILEHANDLE
5644 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5645 using the format associated with that file. By default the format for
5646 a file is the one having the same name as the filehandle, but the
5647 format for the current output channel (see the C<select> function) may be set
5648 explicitly by assigning the name of the format to the C<$~> variable.
5650 Top of form processing is handled automatically: if there is
5651 insufficient room on the current page for the formatted record, the
5652 page is advanced by writing a form feed, a special top-of-page format
5653 is used to format the new page header, and then the record is written.
5654 By default the top-of-page format is the name of the filehandle with
5655 "_TOP" appended, but it may be dynamically set to the format of your
5656 choice by assigning the name to the C<$^> variable while the filehandle is
5657 selected. The number of lines remaining on the current page is in
5658 variable C<$->, which can be set to C<0> to force a new page.
5660 If FILEHANDLE is unspecified, output goes to the current default output
5661 channel, which starts out as STDOUT but may be changed by the
5662 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5663 is evaluated and the resulting string is used to look up the name of
5664 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5666 Note that write is I<not> the opposite of C<read>. Unfortunately.
5670 The transliteration operator. Same as C<tr///>. See L<perlop>.