3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 An named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
204 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239 C<utime>, C<wait>, C<waitpid>
241 For more information about the portability of these functions, see
242 L<perlport> and other available platform-specific documentation.
244 =head2 Alphabetical Listing of Perl Functions
248 =item I<-X> FILEHANDLE
254 A file test, where X is one of the letters listed below. This unary
255 operator takes one argument, either a filename or a filehandle, and
256 tests the associated file to see if something is true about it. If the
257 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
258 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
259 the undefined value if the file doesn't exist. Despite the funny
260 names, precedence is the same as any other named unary operator, and
261 the argument may be parenthesized like any other unary operator. The
262 operator may be any of:
263 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
264 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
277 -z File has zero size.
278 -s File has nonzero size (returns size).
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
304 next unless -f $_; # ignore specials
308 The interpretation of the file permission operators C<-r>, C<-R>,
309 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310 of the file and the uids and gids of the user. There may be other
311 reasons you can't actually read, write, or execute the file. Such
312 reasons may be for example network filesystem access controls, ACLs
313 (access control lists), read-only filesystems, and unrecognized
316 Also note that, for the superuser on the local filesystems, the C<-r>,
317 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
318 if any execute bit is set in the mode. Scripts run by the superuser
319 may thus need to do a stat() to determine the actual mode of the file,
320 or temporarily set their effective uid to something else.
322 If you are using ACLs, there is a pragma called C<filetest> that may
323 produce more accurate results than the bare stat() mode bits.
324 When under the C<use filetest 'access'> the above-mentioned filetests
325 will test whether the permission can (not) be granted using the
326 access() family of system calls. Also note that the C<-x> and C<-X> may
327 under this pragma return true even if there are no execute permission
328 bits set (nor any extra execute permission ACLs). This strangeness is
329 due to the underlying system calls' definitions. Read the
330 documentation for the C<filetest> pragma for more information.
332 Note that C<-s/a/b/> does not do a negated substitution. Saying
333 C<-exp($foo)> still works as expected, however--only single letters
334 following a minus are interpreted as file tests.
336 The C<-T> and C<-B> switches work as follows. The first block or so of the
337 file is examined for odd characters such as strange control codes or
338 characters with the high bit set. If too many strange characters (E<gt>30%)
339 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
340 containing null in the first block is considered a binary file. If C<-T>
341 or C<-B> is used on a filehandle, the current stdio buffer is examined
342 rather than the first block. Both C<-T> and C<-B> return true on a null
343 file, or a file at EOF when testing a filehandle. Because you have to
344 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
345 against the file first, as in C<next unless -f $file && -T $file>.
347 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
348 the special filehandle consisting of a solitary underline, then the stat
349 structure of the previous file test (or stat operator) is used, saving
350 a system call. (This doesn't work with C<-t>, and you need to remember
351 that lstat() and C<-l> will leave values in the stat structure for the
352 symbolic link, not the real file.) Example:
354 print "Can do.\n" if -r $a || -w _ || -x _;
357 print "Readable\n" if -r _;
358 print "Writable\n" if -w _;
359 print "Executable\n" if -x _;
360 print "Setuid\n" if -u _;
361 print "Setgid\n" if -g _;
362 print "Sticky\n" if -k _;
363 print "Text\n" if -T _;
364 print "Binary\n" if -B _;
370 Returns the absolute value of its argument.
371 If VALUE is omitted, uses C<$_>.
373 =item accept NEWSOCKET,GENERICSOCKET
375 Accepts an incoming socket connect, just as the accept(2) system call
376 does. Returns the packed address if it succeeded, false otherwise.
377 See the example in L<perlipc/"Sockets: Client/Server Communication">.
379 On systems that support a close-on-exec flag on files, the flag will
380 be set for the newly opened file descriptor, as determined by the
381 value of $^F. See L<perlvar/$^F>.
387 Arranges to have a SIGALRM delivered to this process after the
388 specified number of seconds have elapsed. If SECONDS is not specified,
389 the value stored in C<$_> is used. (On some machines,
390 unfortunately, the elapsed time may be up to one second less than you
391 specified because of how seconds are counted.) Only one timer may be
392 counting at once. Each call disables the previous timer, and an
393 argument of C<0> may be supplied to cancel the previous timer without
394 starting a new one. The returned value is the amount of time remaining
395 on the previous timer.
397 For delays of finer granularity than one second, you may use Perl's
398 four-argument version of select() leaving the first three arguments
399 undefined, or you might be able to use the C<syscall> interface to
400 access setitimer(2) if your system supports it. The Time::HiRes module
401 from CPAN may also prove useful.
403 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
404 (C<sleep> may be internally implemented in your system with C<alarm>)
406 If you want to use C<alarm> to time out a system call you need to use an
407 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
408 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
409 restart system calls on some systems. Using C<eval>/C<die> always works,
410 modulo the caveats given in L<perlipc/"Signals">.
413 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
415 $nread = sysread SOCKET, $buffer, $size;
419 die unless $@ eq "alarm\n"; # propagate unexpected errors
428 Returns the arctangent of Y/X in the range -PI to PI.
430 For the tangent operation, you may use the C<Math::Trig::tan>
431 function, or use the familiar relation:
433 sub tan { sin($_[0]) / cos($_[0]) }
435 =item bind SOCKET,NAME
437 Binds a network address to a socket, just as the bind system call
438 does. Returns true if it succeeded, false otherwise. NAME should be a
439 packed address of the appropriate type for the socket. See the examples in
440 L<perlipc/"Sockets: Client/Server Communication">.
442 =item binmode FILEHANDLE
444 Arranges for FILEHANDLE to be read or written in "binary" mode on
445 systems whose run-time libraries force the programmer to guess
446 between binary and text files. If FILEHANDLE is an expression, the
447 value is taken as the name of the filehandle. binmode() should be
448 called after the C<open> but before any I/O is done on the filehandle.
449 The only way to reset binary mode on a filehandle is to reopen the
452 The operating system, device drivers, C libraries, and Perl run-time
453 system all conspire to let the programmer conveniently treat a
454 simple, one-byte C<\n> as the line terminator, irrespective of its
455 external representation. On Unix and its brethren, the native file
456 representation exactly matches the internal representation, making
457 everyone's lives unbelievably simpler. Consequently, L<binmode>
458 has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
459 to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and
460 C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
461 C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because
462 it's only one byte, and the internal and external representations
465 In legacy systems like MS-DOS and its embellishments, your program
466 sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
467 that's not what's physically stored on disk. What's worse, these
468 systems refuse to help you with this; it's up to you to remember
469 what to do. And you mustn't go applying binmode() with wild abandon,
470 either, because if your system does care about binmode(), then using
471 it when you shouldn't is just as perilous as failing to use it when
474 That means that on any version of Microsoft WinXX that you might
475 care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
476 to be converted to C<\n> when read into your program, and causes
477 any C<\n> in your program to be converted back to C<\cM\cJ> on
478 output to disk. This sad discrepancy leads to no end of
479 problems in not just the readline operator, but also when using
480 seek(), tell(), and read() calls. See L<perlport> for other painful
481 details. See the C<$/> and C<$\> variables in L<perlvar> for how
482 to manually set your input and output line-termination sequences.
484 =item bless REF,CLASSNAME
488 This function tells the thingy referenced by REF that it is now an object
489 in the CLASSNAME package. If CLASSNAME is omitted, the current package
490 is used. Because a C<bless> is often the last thing in a constructor,
491 it returns the reference for convenience. Always use the two-argument
492 version if the function doing the blessing might be inherited by a
493 derived class. See L<perltoot> and L<perlobj> for more about the blessing
494 (and blessings) of objects.
496 Consider always blessing objects in CLASSNAMEs that are mixed case.
497 Namespaces with all lowercase names are considered reserved for
498 Perl pragmata. Builtin types have all uppercase names, so to prevent
499 confusion, you may wish to avoid such package names as well. Make sure
500 that CLASSNAME is a true value.
502 See L<perlmod/"Perl Modules">.
508 Returns the context of the current subroutine call. In scalar context,
509 returns the caller's package name if there is a caller, that is, if
510 we're in a subroutine or C<eval> or C<require>, and the undefined value
511 otherwise. In list context, returns
513 ($package, $filename, $line) = caller;
515 With EXPR, it returns some extra information that the debugger uses to
516 print a stack trace. The value of EXPR indicates how many call frames
517 to go back before the current one.
519 ($package, $filename, $line, $subroutine, $hasargs,
520 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
522 Here $subroutine may be C<(eval)> if the frame is not a subroutine
523 call, but an C<eval>. In such a case additional elements $evaltext and
524 C<$is_require> are set: C<$is_require> is true if the frame is created by a
525 C<require> or C<use> statement, $evaltext contains the text of the
526 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
527 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
528 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
529 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
530 was compiled with. The C<$hints> and C<$bitmask> values are subject to
531 change between versions of Perl, and are not meant for external use.
533 Furthermore, when called from within the DB package, caller returns more
534 detailed information: it sets the list variable C<@DB::args> to be the
535 arguments with which the subroutine was invoked.
537 Be aware that the optimizer might have optimized call frames away before
538 C<caller> had a chance to get the information. That means that C<caller(N)>
539 might not return information about the call frame you expect it do, for
540 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
541 previous time C<caller> was called.
545 Changes the working directory to EXPR, if possible. If EXPR is omitted,
546 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
547 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
548 set, C<chdir> does nothing. It returns true upon success, false
549 otherwise. See the example under C<die>.
553 Changes the permissions of a list of files. The first element of the
554 list must be the numerical mode, which should probably be an octal
555 number, and which definitely should I<not> a string of octal digits:
556 C<0644> is okay, C<'0644'> is not. Returns the number of files
557 successfully changed. See also L</oct>, if all you have is a string.
559 $cnt = chmod 0755, 'foo', 'bar';
560 chmod 0755, @executables;
561 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
563 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
564 $mode = 0644; chmod $mode, 'foo'; # this is best
566 You can also import the symbolic C<S_I*> constants from the Fcntl
571 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
572 # This is identical to the chmod 0755 of the above example.
580 This safer version of L</chop> removes any trailing string
581 that corresponds to the current value of C<$/> (also known as
582 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
583 number of characters removed from all its arguments. It's often used to
584 remove the newline from the end of an input record when you're worried
585 that the final record may be missing its newline. When in paragraph
586 mode (C<$/ = "">), it removes all trailing newlines from the string.
587 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
588 a reference to an integer or the like, see L<perlvar>) chomp() won't
590 If VARIABLE is omitted, it chomps C<$_>. Example:
593 chomp; # avoid \n on last field
598 You can actually chomp anything that's an lvalue, including an assignment:
601 chomp($answer = <STDIN>);
603 If you chomp a list, each element is chomped, and the total number of
604 characters removed is returned.
612 Chops off the last character of a string and returns the character
613 chopped. It's used primarily to remove the newline from the end of an
614 input record, but is much more efficient than C<s/\n//> because it neither
615 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
619 chop; # avoid \n on last field
624 You can actually chop anything that's an lvalue, including an assignment:
627 chop($answer = <STDIN>);
629 If you chop a list, each element is chopped. Only the value of the
630 last C<chop> is returned.
632 Note that C<chop> returns the last character. To return all but the last
633 character, use C<substr($string, 0, -1)>.
637 Changes the owner (and group) of a list of files. The first two
638 elements of the list must be the I<numeric> uid and gid, in that
639 order. A value of -1 in either position is interpreted by most
640 systems to leave that value unchanged. Returns the number of files
641 successfully changed.
643 $cnt = chown $uid, $gid, 'foo', 'bar';
644 chown $uid, $gid, @filenames;
646 Here's an example that looks up nonnumeric uids in the passwd file:
649 chomp($user = <STDIN>);
651 chomp($pattern = <STDIN>);
653 ($login,$pass,$uid,$gid) = getpwnam($user)
654 or die "$user not in passwd file";
656 @ary = glob($pattern); # expand filenames
657 chown $uid, $gid, @ary;
659 On most systems, you are not allowed to change the ownership of the
660 file unless you're the superuser, although you should be able to change
661 the group to any of your secondary groups. On insecure systems, these
662 restrictions may be relaxed, but this is not a portable assumption.
663 On POSIX systems, you can detect this condition this way:
665 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
666 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
672 Returns the character represented by that NUMBER in the character set.
673 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
674 chr(0x263a) is a Unicode smiley face (but only within the scope of
675 a C<use utf8>). For the reverse, use L</ord>.
676 See L<utf8> for more about Unicode.
678 If NUMBER is omitted, uses C<$_>.
680 =item chroot FILENAME
684 This function works like the system call by the same name: it makes the
685 named directory the new root directory for all further pathnames that
686 begin with a C</> by your process and all its children. (It doesn't
687 change your current working directory, which is unaffected.) For security
688 reasons, this call is restricted to the superuser. If FILENAME is
689 omitted, does a C<chroot> to C<$_>.
691 =item close FILEHANDLE
695 Closes the file or pipe associated with the file handle, returning true
696 only if stdio successfully flushes buffers and closes the system file
697 descriptor. Closes the currently selected filehandle if the argument
700 You don't have to close FILEHANDLE if you are immediately going to do
701 another C<open> on it, because C<open> will close it for you. (See
702 C<open>.) However, an explicit C<close> on an input file resets the line
703 counter (C<$.>), while the implicit close done by C<open> does not.
705 If the file handle came from a piped open C<close> will additionally
706 return false if one of the other system calls involved fails or if the
707 program exits with non-zero status. (If the only problem was that the
708 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
709 also waits for the process executing on the pipe to complete, in case you
710 want to look at the output of the pipe afterwards, and
711 implicitly puts the exit status value of that command into C<$?>.
713 Prematurely closing the read end of a pipe (i.e. before the process
714 writing to it at the other end has closed it) will result in a
715 SIGPIPE being delivered to the writer. If the other end can't
716 handle that, be sure to read all the data before closing the pipe.
720 open(OUTPUT, '|sort >foo') # pipe to sort
721 or die "Can't start sort: $!";
722 #... # print stuff to output
723 close OUTPUT # wait for sort to finish
724 or warn $! ? "Error closing sort pipe: $!"
725 : "Exit status $? from sort";
726 open(INPUT, 'foo') # get sort's results
727 or die "Can't open 'foo' for input: $!";
729 FILEHANDLE may be an expression whose value can be used as an indirect
730 filehandle, usually the real filehandle name.
732 =item closedir DIRHANDLE
734 Closes a directory opened by C<opendir> and returns the success of that
737 DIRHANDLE may be an expression whose value can be used as an indirect
738 dirhandle, usually the real dirhandle name.
740 =item connect SOCKET,NAME
742 Attempts to connect to a remote socket, just as the connect system call
743 does. Returns true if it succeeded, false otherwise. NAME should be a
744 packed address of the appropriate type for the socket. See the examples in
745 L<perlipc/"Sockets: Client/Server Communication">.
749 Actually a flow control statement rather than a function. If there is a
750 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
751 C<foreach>), it is always executed just before the conditional is about to
752 be evaluated again, just like the third part of a C<for> loop in C. Thus
753 it can be used to increment a loop variable, even when the loop has been
754 continued via the C<next> statement (which is similar to the C C<continue>
757 C<last>, C<next>, or C<redo> may appear within a C<continue>
758 block. C<last> and C<redo> will behave as if they had been executed within
759 the main block. So will C<next>, but since it will execute a C<continue>
760 block, it may be more entertaining.
763 ### redo always comes here
766 ### next always comes here
768 # then back the top to re-check EXPR
770 ### last always comes here
772 Omitting the C<continue> section is semantically equivalent to using an
773 empty one, logically enough. In that case, C<next> goes directly back
774 to check the condition at the top of the loop.
778 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
779 takes cosine of C<$_>.
781 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
782 function, or use this relation:
784 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
786 =item crypt PLAINTEXT,SALT
788 Encrypts a string exactly like the crypt(3) function in the C library
789 (assuming that you actually have a version there that has not been
790 extirpated as a potential munition). This can prove useful for checking
791 the password file for lousy passwords, amongst other things. Only the
792 guys wearing white hats should do this.
794 Note that C<crypt> is intended to be a one-way function, much like breaking
795 eggs to make an omelette. There is no (known) corresponding decrypt
796 function. As a result, this function isn't all that useful for
797 cryptography. (For that, see your nearby CPAN mirror.)
799 When verifying an existing encrypted string you should use the encrypted
800 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
801 allows your code to work with the standard C<crypt> and with more
802 exotic implementations. When choosing a new salt create a random two
803 character string whose characters come from the set C<[./0-9A-Za-z]>
804 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
806 Here's an example that makes sure that whoever runs this program knows
809 $pwd = (getpwuid($<))[1];
813 chomp($word = <STDIN>);
817 if (crypt($word, $pwd) ne $pwd) {
823 Of course, typing in your own password to whoever asks you
826 The L<crypt> function is unsuitable for encrypting large quantities
827 of data, not least of all because you can't get the information
828 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
829 on your favorite CPAN mirror for a slew of potentially useful
834 [This function has been largely superseded by the C<untie> function.]
836 Breaks the binding between a DBM file and a hash.
838 =item dbmopen HASH,DBNAME,MASK
840 [This function has been largely superseded by the C<tie> function.]
842 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
843 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
844 argument is I<not> a filehandle, even though it looks like one). DBNAME
845 is the name of the database (without the F<.dir> or F<.pag> extension if
846 any). If the database does not exist, it is created with protection
847 specified by MASK (as modified by the C<umask>). If your system supports
848 only the older DBM functions, you may perform only one C<dbmopen> in your
849 program. In older versions of Perl, if your system had neither DBM nor
850 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
853 If you don't have write access to the DBM file, you can only read hash
854 variables, not set them. If you want to test whether you can write,
855 either use file tests or try setting a dummy hash entry inside an C<eval>,
856 which will trap the error.
858 Note that functions such as C<keys> and C<values> may return huge lists
859 when used on large DBM files. You may prefer to use the C<each>
860 function to iterate over large DBM files. Example:
862 # print out history file offsets
863 dbmopen(%HIST,'/usr/lib/news/history',0666);
864 while (($key,$val) = each %HIST) {
865 print $key, ' = ', unpack('L',$val), "\n";
869 See also L<AnyDBM_File> for a more general description of the pros and
870 cons of the various dbm approaches, as well as L<DB_File> for a particularly
873 You can control which DBM library you use by loading that library
874 before you call dbmopen():
877 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
878 or die "Can't open netscape history file: $!";
884 Returns a Boolean value telling whether EXPR has a value other than
885 the undefined value C<undef>. If EXPR is not present, C<$_> will be
888 Many operations return C<undef> to indicate failure, end of file,
889 system error, uninitialized variable, and other exceptional
890 conditions. This function allows you to distinguish C<undef> from
891 other values. (A simple Boolean test will not distinguish among
892 C<undef>, zero, the empty string, and C<"0">, which are all equally
893 false.) Note that since C<undef> is a valid scalar, its presence
894 doesn't I<necessarily> indicate an exceptional condition: C<pop>
895 returns C<undef> when its argument is an empty array, I<or> when the
896 element to return happens to be C<undef>.
898 You may also use C<defined(&func)> to check whether subroutine C<&func>
899 has ever been defined. The return value is unaffected by any forward
900 declarations of C<&foo>.
902 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
903 used to report whether memory for that aggregate has ever been
904 allocated. This behavior may disappear in future versions of Perl.
905 You should instead use a simple test for size:
907 if (@an_array) { print "has array elements\n" }
908 if (%a_hash) { print "has hash members\n" }
910 When used on a hash element, it tells you whether the value is defined,
911 not whether the key exists in the hash. Use L</exists> for the latter
916 print if defined $switch{'D'};
917 print "$val\n" while defined($val = pop(@ary));
918 die "Can't readlink $sym: $!"
919 unless defined($value = readlink $sym);
920 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
921 $debugging = 0 unless defined $debugging;
923 Note: Many folks tend to overuse C<defined>, and then are surprised to
924 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
925 defined values. For example, if you say
929 The pattern match succeeds, and C<$1> is defined, despite the fact that it
930 matched "nothing". But it didn't really match nothing--rather, it
931 matched something that happened to be zero characters long. This is all
932 very above-board and honest. When a function returns an undefined value,
933 it's an admission that it couldn't give you an honest answer. So you
934 should use C<defined> only when you're questioning the integrity of what
935 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
938 See also L</undef>, L</exists>, L</ref>.
942 Given an expression that specifies a hash element, array element, hash slice,
943 or array slice, deletes the specified element(s) from the hash or array.
944 In the case of an array, if the array elements happen to be at the end,
945 the size of the array will shrink to the highest element that tests
946 true for exists() (or 0 if no such element exists).
948 Returns each element so deleted or the undefined value if there was no such
949 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
950 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
951 from a C<tie>d hash or array may not necessarily return anything.
953 Deleting an array element effectively returns that position of the array
954 to its initial, uninitialized state. Subsequently testing for the same
955 element with exists() will return false. Note that deleting array
956 elements in the middle of an array will not shift the index of the ones
957 after them down--use splice() for that. See L</exists>.
959 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
961 foreach $key (keys %HASH) {
965 foreach $index (0 .. $#ARRAY) {
966 delete $ARRAY[$index];
971 delete @HASH{keys %HASH};
973 delete @ARRAY[0 .. $#ARRAY];
975 But both of these are slower than just assigning the empty list
976 or undefining %HASH or @ARRAY:
978 %HASH = (); # completely empty %HASH
979 undef %HASH; # forget %HASH ever existed
981 @ARRAY = (); # completely empty @ARRAY
982 undef @ARRAY; # forget @ARRAY ever existed
984 Note that the EXPR can be arbitrarily complicated as long as the final
985 operation is a hash element, array element, hash slice, or array slice
988 delete $ref->[$x][$y]{$key};
989 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
991 delete $ref->[$x][$y][$index];
992 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
996 Outside an C<eval>, prints the value of LIST to C<STDERR> and
997 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
998 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
999 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
1000 an C<eval(),> the error message is stuffed into C<$@> and the
1001 C<eval> is terminated with the undefined value. This makes
1002 C<die> the way to raise an exception.
1004 Equivalent examples:
1006 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1007 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1009 If the value of EXPR does not end in a newline, the current script line
1010 number and input line number (if any) are also printed, and a newline
1011 is supplied. Note that the "input line number" (also known as "chunk")
1012 is subject to whatever notion of "line" happens to be currently in
1013 effect, and is also available as the special variable C<$.>.
1014 See L<perlvar/"$/"> and L<perlvar/"$.">.
1016 Hint: sometimes appending C<", stopped"> to your message
1017 will cause it to make better sense when the string C<"at foo line 123"> is
1018 appended. Suppose you are running script "canasta".
1020 die "/etc/games is no good";
1021 die "/etc/games is no good, stopped";
1023 produce, respectively
1025 /etc/games is no good at canasta line 123.
1026 /etc/games is no good, stopped at canasta line 123.
1028 See also exit(), warn(), and the Carp module.
1030 If LIST is empty and C<$@> already contains a value (typically from a
1031 previous eval) that value is reused after appending C<"\t...propagated">.
1032 This is useful for propagating exceptions:
1035 die unless $@ =~ /Expected exception/;
1037 If C<$@> is empty then the string C<"Died"> is used.
1039 die() can also be called with a reference argument. If this happens to be
1040 trapped within an eval(), $@ contains the reference. This behavior permits
1041 a more elaborate exception handling implementation using objects that
1042 maintain arbitary state about the nature of the exception. Such a scheme
1043 is sometimes preferable to matching particular string values of $@ using
1044 regular expressions. Here's an example:
1046 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1048 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1049 # handle Some::Module::Exception
1052 # handle all other possible exceptions
1056 Because perl will stringify uncaught exception messages before displaying
1057 them, you may want to overload stringification operations on such custom
1058 exception objects. See L<overload> for details about that.
1060 You can arrange for a callback to be run just before the C<die>
1061 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1062 handler will be called with the error text and can change the error
1063 message, if it sees fit, by calling C<die> again. See
1064 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1065 L<"eval BLOCK"> for some examples. Although this feature was meant
1066 to be run only right before your program was to exit, this is not
1067 currently the case--the C<$SIG{__DIE__}> hook is currently called
1068 even inside eval()ed blocks/strings! If one wants the hook to do
1069 nothing in such situations, put
1073 as the first line of the handler (see L<perlvar/$^S>). Because
1074 this promotes strange action at a distance, this counterintuitive
1075 behavior may be fixed in a future release.
1079 Not really a function. Returns the value of the last command in the
1080 sequence of commands indicated by BLOCK. When modified by a loop
1081 modifier, executes the BLOCK once before testing the loop condition.
1082 (On other statements the loop modifiers test the conditional first.)
1084 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1085 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1086 See L<perlsyn> for alternative strategies.
1088 =item do SUBROUTINE(LIST)
1090 A deprecated form of subroutine call. See L<perlsub>.
1094 Uses the value of EXPR as a filename and executes the contents of the
1095 file as a Perl script. Its primary use is to include subroutines
1096 from a Perl subroutine library.
1102 scalar eval `cat stat.pl`;
1104 except that it's more efficient and concise, keeps track of the current
1105 filename for error messages, searches the @INC libraries, and updates
1106 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1107 variables. It also differs in that code evaluated with C<do FILENAME>
1108 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1109 same, however, in that it does reparse the file every time you call it,
1110 so you probably don't want to do this inside a loop.
1112 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1113 error. If C<do> can read the file but cannot compile it, it
1114 returns undef and sets an error message in C<$@>. If the file is
1115 successfully compiled, C<do> returns the value of the last expression
1118 Note that inclusion of library modules is better done with the
1119 C<use> and C<require> operators, which also do automatic error checking
1120 and raise an exception if there's a problem.
1122 You might like to use C<do> to read in a program configuration
1123 file. Manual error checking can be done this way:
1125 # read in config files: system first, then user
1126 for $file ("/share/prog/defaults.rc",
1127 "$ENV{HOME}/.someprogrc")
1129 unless ($return = do $file) {
1130 warn "couldn't parse $file: $@" if $@;
1131 warn "couldn't do $file: $!" unless defined $return;
1132 warn "couldn't run $file" unless $return;
1140 This function causes an immediate core dump. See also the B<-u>
1141 command-line switch in L<perlrun>, which does the same thing.
1142 Primarily this is so that you can use the B<undump> program (not
1143 supplied) to turn your core dump into an executable binary after
1144 having initialized all your variables at the beginning of the
1145 program. When the new binary is executed it will begin by executing
1146 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1147 Think of it as a goto with an intervening core dump and reincarnation.
1148 If C<LABEL> is omitted, restarts the program from the top.
1150 B<WARNING>: Any files opened at the time of the dump will I<not>
1151 be open any more when the program is reincarnated, with possible
1152 resulting confusion on the part of Perl.
1154 This function is now largely obsolete, partly because it's very
1155 hard to convert a core file into an executable, and because the
1156 real compiler backends for generating portable bytecode and compilable
1157 C code have superseded it.
1159 If you're looking to use L<dump> to speed up your program, consider
1160 generating bytecode or native C code as described in L<perlcc>. If
1161 you're just trying to accelerate a CGI script, consider using the
1162 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1163 You might also consider autoloading or selfloading, which at least
1164 make your program I<appear> to run faster.
1168 When called in list context, returns a 2-element list consisting of the
1169 key and value for the next element of a hash, so that you can iterate over
1170 it. When called in scalar context, returns the key for only the "next"
1171 element in the hash.
1173 Entries are returned in an apparently random order. The actual random
1174 order is subject to change in future versions of perl, but it is guaranteed
1175 to be in the same order as either the C<keys> or C<values> function
1176 would produce on the same (unmodified) hash.
1178 When the hash is entirely read, a null array is returned in list context
1179 (which when assigned produces a false (C<0>) value), and C<undef> in
1180 scalar context. The next call to C<each> after that will start iterating
1181 again. There is a single iterator for each hash, shared by all C<each>,
1182 C<keys>, and C<values> function calls in the program; it can be reset by
1183 reading all the elements from the hash, or by evaluating C<keys HASH> or
1184 C<values HASH>. If you add or delete elements of a hash while you're
1185 iterating over it, you may get entries skipped or duplicated, so don't.
1187 The following prints out your environment like the printenv(1) program,
1188 only in a different order:
1190 while (($key,$value) = each %ENV) {
1191 print "$key=$value\n";
1194 See also C<keys>, C<values> and C<sort>.
1196 =item eof FILEHANDLE
1202 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1203 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1204 gives the real filehandle. (Note that this function actually
1205 reads a character and then C<ungetc>s it, so isn't very useful in an
1206 interactive context.) Do not read from a terminal file (or call
1207 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1208 as terminals may lose the end-of-file condition if you do.
1210 An C<eof> without an argument uses the last file read. Using C<eof()>
1211 with empty parentheses is very different. It refers to the pseudo file
1212 formed from the files listed on the command line and accessed via the
1213 C<E<lt>E<gt>> operator. Since C<E<lt>E<gt>> isn't explicitly opened,
1214 as a normal filehandle is, an C<eof()> before C<E<lt>E<gt>> has been
1215 used will cause C<@ARGV> to be examined to determine if input is
1218 In a C<while (E<lt>E<gt>)> loop, C<eof> or C<eof(ARGV)> can be used to
1219 detect the end of each file, C<eof()> will only detect the end of the
1220 last file. Examples:
1222 # reset line numbering on each input file
1224 next if /^\s*#/; # skip comments
1227 close ARGV if eof; # Not eof()!
1230 # insert dashes just before last line of last file
1232 if (eof()) { # check for end of current file
1233 print "--------------\n";
1234 close(ARGV); # close or last; is needed if we
1235 # are reading from the terminal
1240 Practical hint: you almost never need to use C<eof> in Perl, because the
1241 input operators typically return C<undef> when they run out of data, or if
1248 In the first form, the return value of EXPR is parsed and executed as if it
1249 were a little Perl program. The value of the expression (which is itself
1250 determined within scalar context) is first parsed, and if there weren't any
1251 errors, executed in the context of the current Perl program, so that any
1252 variable settings or subroutine and format definitions remain afterwards.
1253 Note that the value is parsed every time the eval executes. If EXPR is
1254 omitted, evaluates C<$_>. This form is typically used to delay parsing
1255 and subsequent execution of the text of EXPR until run time.
1257 In the second form, the code within the BLOCK is parsed only once--at the
1258 same time the code surrounding the eval itself was parsed--and executed
1259 within the context of the current Perl program. This form is typically
1260 used to trap exceptions more efficiently than the first (see below), while
1261 also providing the benefit of checking the code within BLOCK at compile
1264 The final semicolon, if any, may be omitted from the value of EXPR or within
1267 In both forms, the value returned is the value of the last expression
1268 evaluated inside the mini-program; a return statement may be also used, just
1269 as with subroutines. The expression providing the return value is evaluated
1270 in void, scalar, or list context, depending on the context of the eval itself.
1271 See L</wantarray> for more on how the evaluation context can be determined.
1273 If there is a syntax error or runtime error, or a C<die> statement is
1274 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1275 error message. If there was no error, C<$@> is guaranteed to be a null
1276 string. Beware that using C<eval> neither silences perl from printing
1277 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1278 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1279 L</warn> and L<perlvar>.
1281 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1282 determining whether a particular feature (such as C<socket> or C<symlink>)
1283 is implemented. It is also Perl's exception trapping mechanism, where
1284 the die operator is used to raise exceptions.
1286 If the code to be executed doesn't vary, you may use the eval-BLOCK
1287 form to trap run-time errors without incurring the penalty of
1288 recompiling each time. The error, if any, is still returned in C<$@>.
1291 # make divide-by-zero nonfatal
1292 eval { $answer = $a / $b; }; warn $@ if $@;
1294 # same thing, but less efficient
1295 eval '$answer = $a / $b'; warn $@ if $@;
1297 # a compile-time error
1298 eval { $answer = }; # WRONG
1301 eval '$answer ='; # sets $@
1303 Due to the current arguably broken state of C<__DIE__> hooks, when using
1304 the C<eval{}> form as an exception trap in libraries, you may wish not
1305 to trigger any C<__DIE__> hooks that user code may have installed.
1306 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1307 as shown in this example:
1309 # a very private exception trap for divide-by-zero
1310 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1313 This is especially significant, given that C<__DIE__> hooks can call
1314 C<die> again, which has the effect of changing their error messages:
1316 # __DIE__ hooks may modify error messages
1318 local $SIG{'__DIE__'} =
1319 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1320 eval { die "foo lives here" };
1321 print $@ if $@; # prints "bar lives here"
1324 Because this promotes action at a distance, this counterintuitive behavior
1325 may be fixed in a future release.
1327 With an C<eval>, you should be especially careful to remember what's
1328 being looked at when:
1334 eval { $x }; # CASE 4
1336 eval "\$$x++"; # CASE 5
1339 Cases 1 and 2 above behave identically: they run the code contained in
1340 the variable $x. (Although case 2 has misleading double quotes making
1341 the reader wonder what else might be happening (nothing is).) Cases 3
1342 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1343 does nothing but return the value of $x. (Case 4 is preferred for
1344 purely visual reasons, but it also has the advantage of compiling at
1345 compile-time instead of at run-time.) Case 5 is a place where
1346 normally you I<would> like to use double quotes, except that in this
1347 particular situation, you can just use symbolic references instead, as
1350 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1351 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1355 =item exec PROGRAM LIST
1357 The C<exec> function executes a system command I<and never returns>--
1358 use C<system> instead of C<exec> if you want it to return. It fails and
1359 returns false only if the command does not exist I<and> it is executed
1360 directly instead of via your system's command shell (see below).
1362 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1363 warns you if there is a following statement which isn't C<die>, C<warn>,
1364 or C<exit> (if C<-w> is set - but you always do that). If you
1365 I<really> want to follow an C<exec> with some other statement, you
1366 can use one of these styles to avoid the warning:
1368 exec ('foo') or print STDERR "couldn't exec foo: $!";
1369 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1371 If there is more than one argument in LIST, or if LIST is an array
1372 with more than one value, calls execvp(3) with the arguments in LIST.
1373 If there is only one scalar argument or an array with one element in it,
1374 the argument is checked for shell metacharacters, and if there are any,
1375 the entire argument is passed to the system's command shell for parsing
1376 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1377 If there are no shell metacharacters in the argument, it is split into
1378 words and passed directly to C<execvp>, which is more efficient.
1381 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1382 exec "sort $outfile | uniq";
1384 If you don't really want to execute the first argument, but want to lie
1385 to the program you are executing about its own name, you can specify
1386 the program you actually want to run as an "indirect object" (without a
1387 comma) in front of the LIST. (This always forces interpretation of the
1388 LIST as a multivalued list, even if there is only a single scalar in
1391 $shell = '/bin/csh';
1392 exec $shell '-sh'; # pretend it's a login shell
1396 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1398 When the arguments get executed via the system shell, results will
1399 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1402 Using an indirect object with C<exec> or C<system> is also more
1403 secure. This usage (which also works fine with system()) forces
1404 interpretation of the arguments as a multivalued list, even if the
1405 list had just one argument. That way you're safe from the shell
1406 expanding wildcards or splitting up words with whitespace in them.
1408 @args = ( "echo surprise" );
1410 exec @args; # subject to shell escapes
1412 exec { $args[0] } @args; # safe even with one-arg list
1414 The first version, the one without the indirect object, ran the I<echo>
1415 program, passing it C<"surprise"> an argument. The second version
1416 didn't--it tried to run a program literally called I<"echo surprise">,
1417 didn't find it, and set C<$?> to a non-zero value indicating failure.
1419 Note that C<exec> will not call your C<END> blocks, nor will it call
1420 any C<DESTROY> methods in your objects.
1424 Given an expression that specifies a hash element or array element,
1425 returns true if the specified element in the hash or array has ever
1426 been initialized, even if the corresponding value is undefined. The
1427 element is not autovivified if it doesn't exist.
1429 print "Exists\n" if exists $hash{$key};
1430 print "Defined\n" if defined $hash{$key};
1431 print "True\n" if $hash{$key};
1433 print "Exists\n" if exists $array[$index];
1434 print "Defined\n" if defined $array[$index];
1435 print "True\n" if $array[$index];
1437 A hash or array element can be true only if it's defined, and defined if
1438 it exists, but the reverse doesn't necessarily hold true.
1440 Given an expression that specifies the name of a subroutine,
1441 returns true if the specified subroutine has ever been declared, even
1442 if it is undefined. Mentioning a subroutine name for exists or defined
1443 does not count as declaring it.
1445 print "Exists\n" if exists &subroutine;
1446 print "Defined\n" if defined &subroutine;
1448 Note that the EXPR can be arbitrarily complicated as long as the final
1449 operation is a hash or array key lookup or subroutine name:
1451 if (exists $ref->{A}->{B}->{$key}) { }
1452 if (exists $hash{A}{B}{$key}) { }
1454 if (exists $ref->{A}->{B}->[$ix]) { }
1455 if (exists $hash{A}{B}[$ix]) { }
1457 if (exists &{$ref->{A}{B}{$key}}) { }
1459 Although the deepest nested array or hash will not spring into existence
1460 just because its existence was tested, any intervening ones will.
1461 Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring
1462 into existence due to the existence test for the $key element above.
1463 This happens anywhere the arrow operator is used, including even:
1466 if (exists $ref->{"Some key"}) { }
1467 print $ref; # prints HASH(0x80d3d5c)
1469 This surprising autovivification in what does not at first--or even
1470 second--glance appear to be an lvalue context may be fixed in a future
1473 See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
1474 used on a pseudo-hash.
1476 Use of a subroutine call, rather than a subroutine name, as an argument
1477 to exists() is an error.
1480 exists &sub(); # Error
1484 Evaluates EXPR and exits immediately with that value. Example:
1487 exit 0 if $ans =~ /^[Xx]/;
1489 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1490 universally recognized values for EXPR are C<0> for success and C<1>
1491 for error; other values are subject to interpretation depending on the
1492 environment in which the Perl program is running. For example, exiting
1493 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1494 the mailer to return the item undelivered, but that's not true everywhere.
1496 Don't use C<exit> to abort a subroutine if there's any chance that
1497 someone might want to trap whatever error happened. Use C<die> instead,
1498 which can be trapped by an C<eval>.
1500 The exit() function does not always exit immediately. It calls any
1501 defined C<END> routines first, but these C<END> routines may not
1502 themselves abort the exit. Likewise any object destructors that need to
1503 be called are called before the real exit. If this is a problem, you
1504 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1505 See L<perlmod> for details.
1511 Returns I<e> (the natural logarithm base) to the power of EXPR.
1512 If EXPR is omitted, gives C<exp($_)>.
1514 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1516 Implements the fcntl(2) function. You'll probably have to say
1520 first to get the correct constant definitions. Argument processing and
1521 value return works just like C<ioctl> below.
1525 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1526 or die "can't fcntl F_GETFL: $!";
1528 You don't have to check for C<defined> on the return from C<fnctl>.
1529 Like C<ioctl>, it maps a C<0> return from the system call into
1530 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1531 in numeric context. It is also exempt from the normal B<-w> warnings
1532 on improper numeric conversions.
1534 Note that C<fcntl> will produce a fatal error if used on a machine that
1535 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1536 manpage to learn what functions are available on your system.
1538 =item fileno FILEHANDLE
1540 Returns the file descriptor for a filehandle, or undefined if the
1541 filehandle is not open. This is mainly useful for constructing
1542 bitmaps for C<select> and low-level POSIX tty-handling operations.
1543 If FILEHANDLE is an expression, the value is taken as an indirect
1544 filehandle, generally its name.
1546 You can use this to find out whether two handles refer to the
1547 same underlying descriptor:
1549 if (fileno(THIS) == fileno(THAT)) {
1550 print "THIS and THAT are dups\n";
1553 =item flock FILEHANDLE,OPERATION
1555 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1556 for success, false on failure. Produces a fatal error if used on a
1557 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1558 C<flock> is Perl's portable file locking interface, although it locks
1559 only entire files, not records.
1561 Two potentially non-obvious but traditional C<flock> semantics are
1562 that it waits indefinitely until the lock is granted, and that its locks
1563 B<merely advisory>. Such discretionary locks are more flexible, but offer
1564 fewer guarantees. This means that files locked with C<flock> may be
1565 modified by programs that do not also use C<flock>. See L<perlport>,
1566 your port's specific documentation, or your system-specific local manpages
1567 for details. It's best to assume traditional behavior if you're writing
1568 portable programs. (But if you're not, you should as always feel perfectly
1569 free to write for your own system's idiosyncrasies (sometimes called
1570 "features"). Slavish adherence to portability concerns shouldn't get
1571 in the way of your getting your job done.)
1573 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1574 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1575 you can use the symbolic names if you import them from the Fcntl module,
1576 either individually, or as a group using the ':flock' tag. LOCK_SH
1577 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1578 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1579 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1580 waiting for the lock (check the return status to see if you got it).
1582 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1583 before locking or unlocking it.
1585 Note that the emulation built with lockf(3) doesn't provide shared
1586 locks, and it requires that FILEHANDLE be open with write intent. These
1587 are the semantics that lockf(3) implements. Most if not all systems
1588 implement lockf(3) in terms of fcntl(2) locking, though, so the
1589 differing semantics shouldn't bite too many people.
1591 Note also that some versions of C<flock> cannot lock things over the
1592 network; you would need to use the more system-specific C<fcntl> for
1593 that. If you like you can force Perl to ignore your system's flock(2)
1594 function, and so provide its own fcntl(2)-based emulation, by passing
1595 the switch C<-Ud_flock> to the F<Configure> program when you configure
1598 Here's a mailbox appender for BSD systems.
1600 use Fcntl ':flock'; # import LOCK_* constants
1603 flock(MBOX,LOCK_EX);
1604 # and, in case someone appended
1605 # while we were waiting...
1610 flock(MBOX,LOCK_UN);
1613 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1614 or die "Can't open mailbox: $!";
1617 print MBOX $msg,"\n\n";
1620 On systems that support a real flock(), locks are inherited across fork()
1621 calls, whereas those that must resort to the more capricious fcntl()
1622 function lose the locks, making it harder to write servers.
1624 See also L<DB_File> for other flock() examples.
1628 Does a fork(2) system call to create a new process running the
1629 same program at the same point. It returns the child pid to the
1630 parent process, C<0> to the child process, or C<undef> if the fork is
1631 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1632 are shared, while everything else is copied. On most systems supporting
1633 fork(), great care has gone into making it extremely efficient (for
1634 example, using copy-on-write technology on data pages), making it the
1635 dominant paradigm for multitasking over the last few decades.
1637 All files opened for output are flushed before forking the child process.
1639 If you C<fork> without ever waiting on your children, you will
1640 accumulate zombies. On some systems, you can avoid this by setting
1641 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1642 forking and reaping moribund children.
1644 Note that if your forked child inherits system file descriptors like
1645 STDIN and STDOUT that are actually connected by a pipe or socket, even
1646 if you exit, then the remote server (such as, say, a CGI script or a
1647 backgrounded job launched from a remote shell) won't think you're done.
1648 You should reopen those to F</dev/null> if it's any issue.
1652 Declare a picture format for use by the C<write> function. For
1656 Test: @<<<<<<<< @||||| @>>>>>
1657 $str, $%, '$' . int($num)
1661 $num = $cost/$quantity;
1665 See L<perlform> for many details and examples.
1667 =item formline PICTURE,LIST
1669 This is an internal function used by C<format>s, though you may call it,
1670 too. It formats (see L<perlform>) a list of values according to the
1671 contents of PICTURE, placing the output into the format output
1672 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1673 Eventually, when a C<write> is done, the contents of
1674 C<$^A> are written to some filehandle, but you could also read C<$^A>
1675 yourself and then set C<$^A> back to C<"">. Note that a format typically
1676 does one C<formline> per line of form, but the C<formline> function itself
1677 doesn't care how many newlines are embedded in the PICTURE. This means
1678 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1679 You may therefore need to use multiple formlines to implement a single
1680 record format, just like the format compiler.
1682 Be careful if you put double quotes around the picture, because an C<@>
1683 character may be taken to mean the beginning of an array name.
1684 C<formline> always returns true. See L<perlform> for other examples.
1686 =item getc FILEHANDLE
1690 Returns the next character from the input file attached to FILEHANDLE,
1691 or the undefined value at end of file, or if there was an error.
1692 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1693 efficient. However, it cannot be used by itself to fetch single
1694 characters without waiting for the user to hit enter. For that, try
1695 something more like:
1698 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1701 system "stty", '-icanon', 'eol', "\001";
1707 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1710 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1714 Determination of whether $BSD_STYLE should be set
1715 is left as an exercise to the reader.
1717 The C<POSIX::getattr> function can do this more portably on
1718 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1719 module from your nearest CPAN site; details on CPAN can be found on
1724 Implements the C library function of the same name, which on most
1725 systems returns the current login from F</etc/utmp>, if any. If null,
1728 $login = getlogin || getpwuid($<) || "Kilroy";
1730 Do not consider C<getlogin> for authentication: it is not as
1731 secure as C<getpwuid>.
1733 =item getpeername SOCKET
1735 Returns the packed sockaddr address of other end of the SOCKET connection.
1738 $hersockaddr = getpeername(SOCK);
1739 ($port, $iaddr) = sockaddr_in($hersockaddr);
1740 $herhostname = gethostbyaddr($iaddr, AF_INET);
1741 $herstraddr = inet_ntoa($iaddr);
1745 Returns the current process group for the specified PID. Use
1746 a PID of C<0> to get the current process group for the
1747 current process. Will raise an exception if used on a machine that
1748 doesn't implement getpgrp(2). If PID is omitted, returns process
1749 group of current process. Note that the POSIX version of C<getpgrp>
1750 does not accept a PID argument, so only C<PID==0> is truly portable.
1754 Returns the process id of the parent process.
1756 =item getpriority WHICH,WHO
1758 Returns the current priority for a process, a process group, or a user.
1759 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1760 machine that doesn't implement getpriority(2).
1766 =item gethostbyname NAME
1768 =item getnetbyname NAME
1770 =item getprotobyname NAME
1776 =item getservbyname NAME,PROTO
1778 =item gethostbyaddr ADDR,ADDRTYPE
1780 =item getnetbyaddr ADDR,ADDRTYPE
1782 =item getprotobynumber NUMBER
1784 =item getservbyport PORT,PROTO
1802 =item sethostent STAYOPEN
1804 =item setnetent STAYOPEN
1806 =item setprotoent STAYOPEN
1808 =item setservent STAYOPEN
1822 These routines perform the same functions as their counterparts in the
1823 system library. In list context, the return values from the
1824 various get routines are as follows:
1826 ($name,$passwd,$uid,$gid,
1827 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1828 ($name,$passwd,$gid,$members) = getgr*
1829 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1830 ($name,$aliases,$addrtype,$net) = getnet*
1831 ($name,$aliases,$proto) = getproto*
1832 ($name,$aliases,$port,$proto) = getserv*
1834 (If the entry doesn't exist you get a null list.)
1836 In scalar context, you get the name, unless the function was a
1837 lookup by name, in which case you get the other thing, whatever it is.
1838 (If the entry doesn't exist you get the undefined value.) For example:
1840 $uid = getpwnam($name);
1841 $name = getpwuid($num);
1843 $gid = getgrnam($name);
1844 $name = getgrgid($num;
1848 In I<getpw*()> the fields $quota, $comment, and $expire are
1849 special cases in the sense that in many systems they are unsupported.
1850 If the $quota is unsupported, it is an empty scalar. If it is
1851 supported, it usually encodes the disk quota. If the $comment
1852 field is unsupported, it is an empty scalar. If it is supported it
1853 usually encodes some administrative comment about the user. In some
1854 systems the $quota field may be $change or $age, fields that have
1855 to do with password aging. In some systems the $comment field may
1856 be $class. The $expire field, if present, encodes the expiration
1857 period of the account or the password. For the availability and the
1858 exact meaning of these fields in your system, please consult your
1859 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1860 out from within Perl what your $quota and $comment fields mean
1861 and whether you have the $expire field by using the C<Config> module
1862 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1863 and C<d_pwexpire>. Shadow password files are only supported if your
1864 vendor has implemented them in the intuitive fashion that calling the
1865 regular C library routines gets the shadow versions if you're running
1866 under privilege. Those that incorrectly implement a separate library
1867 call are not supported.
1869 The $members value returned by I<getgr*()> is a space separated list of
1870 the login names of the members of the group.
1872 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1873 C, it will be returned to you via C<$?> if the function call fails. The
1874 C<@addrs> value returned by a successful call is a list of the raw
1875 addresses returned by the corresponding system library call. In the
1876 Internet domain, each address is four bytes long and you can unpack it
1877 by saying something like:
1879 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1881 The Socket library makes this slightly easier:
1884 $iaddr = inet_aton("127.1"); # or whatever address
1885 $name = gethostbyaddr($iaddr, AF_INET);
1887 # or going the other way
1888 $straddr = inet_ntoa($iaddr);
1890 If you get tired of remembering which element of the return list
1891 contains which return value, by-name interfaces are provided
1892 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1893 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1894 and C<User::grent>. These override the normal built-ins, supplying
1895 versions that return objects with the appropriate names
1896 for each field. For example:
1900 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1902 Even though it looks like they're the same method calls (uid),
1903 they aren't, because a C<File::stat> object is different from
1904 a C<User::pwent> object.
1906 =item getsockname SOCKET
1908 Returns the packed sockaddr address of this end of the SOCKET connection,
1909 in case you don't know the address because you have several different
1910 IPs that the connection might have come in on.
1913 $mysockaddr = getsockname(SOCK);
1914 ($port, $myaddr) = sockaddr_in($mysockaddr);
1915 printf "Connect to %s [%s]\n",
1916 scalar gethostbyaddr($myaddr, AF_INET),
1919 =item getsockopt SOCKET,LEVEL,OPTNAME
1921 Returns the socket option requested, or undef if there is an error.
1927 Returns the value of EXPR with filename expansions such as the
1928 standard Unix shell F</bin/csh> would do. This is the internal function
1929 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1930 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1931 discussed in more detail in L<perlop/"I/O Operators">.
1935 Converts a time as returned by the time function to a 9-element list
1936 with the time localized for the standard Greenwich time zone.
1937 Typically used as follows:
1940 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1943 All list elements are numeric, and come straight out of a struct tm.
1944 In particular this means that $mon has the range C<0..11> and $wday
1945 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1946 number of years since 1900, that is, $year is C<123> in year 2023,
1947 I<not> simply the last two digits of the year. If you assume it is,
1948 then you create non-Y2K-compliant programs--and you wouldn't want to do
1951 The proper way to get a complete 4-digit year is simply:
1955 And to get the last two digits of the year (e.g., '01' in 2001) do:
1957 $year = sprintf("%02d", $year % 100);
1959 If EXPR is omitted, does C<gmtime(time())>.
1961 In scalar context, returns the ctime(3) value:
1963 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1965 Also see the C<timegm> function provided by the C<Time::Local> module,
1966 and the strftime(3) function available via the POSIX module.
1968 This scalar value is B<not> locale dependent (see L<perllocale>), but
1969 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1970 strftime(3) and mktime(3) functions available via the POSIX module. To
1971 get somewhat similar but locale dependent date strings, set up your
1972 locale environment variables appropriately (please see L<perllocale>)
1973 and try for example:
1975 use POSIX qw(strftime);
1976 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1978 Note that the C<%a> and C<%b> escapes, which represent the short forms
1979 of the day of the week and the month of the year, may not necessarily
1980 be three characters wide in all locales.
1988 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1989 execution there. It may not be used to go into any construct that
1990 requires initialization, such as a subroutine or a C<foreach> loop. It
1991 also can't be used to go into a construct that is optimized away,
1992 or to get out of a block or subroutine given to C<sort>.
1993 It can be used to go almost anywhere else within the dynamic scope,
1994 including out of subroutines, but it's usually better to use some other
1995 construct such as C<last> or C<die>. The author of Perl has never felt the
1996 need to use this form of C<goto> (in Perl, that is--C is another matter).
1998 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1999 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2000 necessarily recommended if you're optimizing for maintainability:
2002 goto ("FOO", "BAR", "GLARCH")[$i];
2004 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2005 In fact, it isn't a goto in the normal sense at all, and doesn't have
2006 the stigma associated with other gotos. Instead, it
2007 substitutes a call to the named subroutine for the currently running
2008 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2009 another subroutine and then pretend that the other subroutine had been
2010 called in the first place (except that any modifications to C<@_>
2011 in the current subroutine are propagated to the other subroutine.)
2012 After the C<goto>, not even C<caller> will be able to tell that this
2013 routine was called first.
2015 NAME needn't be the name of a subroutine; it can be a scalar variable
2016 containing a code reference, or a block which evaluates to a code
2019 =item grep BLOCK LIST
2021 =item grep EXPR,LIST
2023 This is similar in spirit to, but not the same as, grep(1) and its
2024 relatives. In particular, it is not limited to using regular expressions.
2026 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2027 C<$_> to each element) and returns the list value consisting of those
2028 elements for which the expression evaluated to true. In scalar
2029 context, returns the number of times the expression was true.
2031 @foo = grep(!/^#/, @bar); # weed out comments
2035 @foo = grep {!/^#/} @bar; # weed out comments
2037 Note that, because C<$_> is a reference into the list value, it can
2038 be used to modify the elements of the array. While this is useful and
2039 supported, it can cause bizarre results if the LIST is not a named array.
2040 Similarly, grep returns aliases into the original list, much as a for
2041 loop's index variable aliases the list elements. That is, modifying an
2042 element of a list returned by grep (for example, in a C<foreach>, C<map>
2043 or another C<grep>) actually modifies the element in the original list.
2044 This is usually something to be avoided when writing clear code.
2046 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2052 Interprets EXPR as a hex string and returns the corresponding value.
2053 (To convert strings that might start with either 0, 0x, or 0b, see
2054 L</oct>.) If EXPR is omitted, uses C<$_>.
2056 print hex '0xAf'; # prints '175'
2057 print hex 'aF'; # same
2059 Hex strings may only represent integers. Strings that would cause
2060 integer overflow trigger a warning.
2064 There is no builtin C<import> function. It is just an ordinary
2065 method (subroutine) defined (or inherited) by modules that wish to export
2066 names to another module. The C<use> function calls the C<import> method
2067 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2069 =item index STR,SUBSTR,POSITION
2071 =item index STR,SUBSTR
2073 The index function searches for one string within another, but without
2074 the wildcard-like behavior of a full regular-expression pattern match.
2075 It returns the position of the first occurrence of SUBSTR in STR at
2076 or after POSITION. If POSITION is omitted, starts searching from the
2077 beginning of the string. The return value is based at C<0> (or whatever
2078 you've set the C<$[> variable to--but don't do that). If the substring
2079 is not found, returns one less than the base, ordinarily C<-1>.
2085 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2086 You should not use this function for rounding: one because it truncates
2087 towards C<0>, and two because machine representations of floating point
2088 numbers can sometimes produce counterintuitive results. For example,
2089 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2090 because it's really more like -268.99999999999994315658 instead. Usually,
2091 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2092 functions will serve you better than will int().
2094 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2096 Implements the ioctl(2) function. You'll probably first have to say
2098 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2100 to get the correct function definitions. If F<ioctl.ph> doesn't
2101 exist or doesn't have the correct definitions you'll have to roll your
2102 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2103 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2104 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2105 written depending on the FUNCTION--a pointer to the string value of SCALAR
2106 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2107 has no string value but does have a numeric value, that value will be
2108 passed rather than a pointer to the string value. To guarantee this to be
2109 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2110 functions may be needed to manipulate the values of structures used by
2113 The return value of C<ioctl> (and C<fcntl>) is as follows:
2115 if OS returns: then Perl returns:
2117 0 string "0 but true"
2118 anything else that number
2120 Thus Perl returns true on success and false on failure, yet you can
2121 still easily determine the actual value returned by the operating
2124 $retval = ioctl(...) || -1;
2125 printf "System returned %d\n", $retval;
2127 The special string "C<0> but true" is exempt from B<-w> complaints
2128 about improper numeric conversions.
2130 Here's an example of setting a filehandle named C<REMOTE> to be
2131 non-blocking at the system level. You'll have to negotiate C<$|>
2132 on your own, though.
2134 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2136 $flags = fcntl(REMOTE, F_GETFL, 0)
2137 or die "Can't get flags for the socket: $!\n";
2139 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2140 or die "Can't set flags for the socket: $!\n";
2142 =item join EXPR,LIST
2144 Joins the separate strings of LIST into a single string with fields
2145 separated by the value of EXPR, and returns that new string. Example:
2147 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2149 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2150 first argument. Compare L</split>.
2154 Returns a list consisting of all the keys of the named hash. (In
2155 scalar context, returns the number of keys.) The keys are returned in
2156 an apparently random order. The actual random order is subject to
2157 change in future versions of perl, but it is guaranteed to be the same
2158 order as either the C<values> or C<each> function produces (given
2159 that the hash has not been modified). As a side effect, it resets
2162 Here is yet another way to print your environment:
2165 @values = values %ENV;
2167 print pop(@keys), '=', pop(@values), "\n";
2170 or how about sorted by key:
2172 foreach $key (sort(keys %ENV)) {
2173 print $key, '=', $ENV{$key}, "\n";
2176 To sort a hash by value, you'll need to use a C<sort> function.
2177 Here's a descending numeric sort of a hash by its values:
2179 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2180 printf "%4d %s\n", $hash{$key}, $key;
2183 As an lvalue C<keys> allows you to increase the number of hash buckets
2184 allocated for the given hash. This can gain you a measure of efficiency if
2185 you know the hash is going to get big. (This is similar to pre-extending
2186 an array by assigning a larger number to $#array.) If you say
2190 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2191 in fact, since it rounds up to the next power of two. These
2192 buckets will be retained even if you do C<%hash = ()>, use C<undef
2193 %hash> if you want to free the storage while C<%hash> is still in scope.
2194 You can't shrink the number of buckets allocated for the hash using
2195 C<keys> in this way (but you needn't worry about doing this by accident,
2196 as trying has no effect).
2198 See also C<each>, C<values> and C<sort>.
2200 =item kill SIGNAL, LIST
2202 Sends a signal to a list of processes. Returns the number of
2203 processes successfully signaled (which is not necessarily the
2204 same as the number actually killed).
2206 $cnt = kill 1, $child1, $child2;
2209 If SIGNAL is zero, no signal is sent to the process. This is a
2210 useful way to check that the process is alive and hasn't changed
2211 its UID. See L<perlport> for notes on the portability of this
2214 Unlike in the shell, if SIGNAL is negative, it kills
2215 process groups instead of processes. (On System V, a negative I<PROCESS>
2216 number will also kill process groups, but that's not portable.) That
2217 means you usually want to use positive not negative signals. You may also
2218 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2224 The C<last> command is like the C<break> statement in C (as used in
2225 loops); it immediately exits the loop in question. If the LABEL is
2226 omitted, the command refers to the innermost enclosing loop. The
2227 C<continue> block, if any, is not executed:
2229 LINE: while (<STDIN>) {
2230 last LINE if /^$/; # exit when done with header
2234 C<last> cannot be used to exit a block which returns a value such as
2235 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2236 a grep() or map() operation.
2238 Note that a block by itself is semantically identical to a loop
2239 that executes once. Thus C<last> can be used to effect an early
2240 exit out of such a block.
2242 See also L</continue> for an illustration of how C<last>, C<next>, and
2249 Returns an lowercased version of EXPR. This is the internal function
2250 implementing the C<\L> escape in double-quoted strings.
2251 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2254 If EXPR is omitted, uses C<$_>.
2260 Returns the value of EXPR with the first character lowercased. This is
2261 the internal function implementing the C<\l> escape in double-quoted strings.
2262 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2264 If EXPR is omitted, uses C<$_>.
2270 Returns the length in characters of the value of EXPR. If EXPR is
2271 omitted, returns length of C<$_>. Note that this cannot be used on
2272 an entire array or hash to find out how many elements these have.
2273 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2275 =item link OLDFILE,NEWFILE
2277 Creates a new filename linked to the old filename. Returns true for
2278 success, false otherwise.
2280 =item listen SOCKET,QUEUESIZE
2282 Does the same thing that the listen system call does. Returns true if
2283 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2287 You really probably want to be using C<my> instead, because C<local> isn't
2288 what most people think of as "local". See L<perlsub/"Private Variables
2289 via my()"> for details.
2291 A local modifies the listed variables to be local to the enclosing
2292 block, file, or eval. If more than one value is listed, the list must
2293 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2294 for details, including issues with tied arrays and hashes.
2296 =item localtime EXPR
2298 Converts a time as returned by the time function to a 9-element list
2299 with the time analyzed for the local time zone. Typically used as
2303 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2306 All list elements are numeric, and come straight out of a struct tm.
2307 In particular this means that $mon has the range C<0..11> and $wday
2308 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2309 number of years since 1900, that is, $year is C<123> in year 2023,
2310 and I<not> simply the last two digits of the year. If you assume it is,
2311 then you create non-Y2K-compliant programs--and you wouldn't want to do
2314 The proper way to get a complete 4-digit year is simply:
2318 And to get the last two digits of the year (e.g., '01' in 2001) do:
2320 $year = sprintf("%02d", $year % 100);
2322 If EXPR is omitted, uses the current time (C<localtime(time)>).
2324 In scalar context, returns the ctime(3) value:
2326 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2328 This scalar value is B<not> locale dependent, see L<perllocale>, but
2329 instead a Perl builtin. Also see the C<Time::Local> module
2330 (to convert the second, minutes, hours, ... back to seconds since the
2331 stroke of midnight the 1st of January 1970, the value returned by
2332 time()), and the strftime(3) and mktime(3) functions available via the
2333 POSIX module. To get somewhat similar but locale dependent date
2334 strings, set up your locale environment variables appropriately
2335 (please see L<perllocale>) and try for example:
2337 use POSIX qw(strftime);
2338 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2340 Note that the C<%a> and C<%b>, the short forms of the day of the week
2341 and the month of the year, may not necessarily be three characters wide.
2347 This function places an advisory lock on a variable, subroutine,
2348 or referenced object contained in I<THING> until the lock goes out
2349 of scope. This is a built-in function only if your version of Perl
2350 was built with threading enabled, and if you've said C<use Threads>.
2351 Otherwise a user-defined function by this name will be called. See
2358 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2359 returns log of C<$_>. To get the log of another base, use basic algebra:
2360 The base-N log of a number is equal to the natural log of that number
2361 divided by the natural log of N. For example:
2365 return log($n)/log(10);
2368 See also L</exp> for the inverse operation.
2370 =item lstat FILEHANDLE
2376 Does the same thing as the C<stat> function (including setting the
2377 special C<_> filehandle) but stats a symbolic link instead of the file
2378 the symbolic link points to. If symbolic links are unimplemented on
2379 your system, a normal C<stat> is done.
2381 If EXPR is omitted, stats C<$_>.
2385 The match operator. See L<perlop>.
2387 =item map BLOCK LIST
2391 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2392 C<$_> to each element) and returns the list value composed of the
2393 results of each such evaluation. In scalar context, returns the
2394 total number of elements so generated. Evaluates BLOCK or EXPR in
2395 list context, so each element of LIST may produce zero, one, or
2396 more elements in the returned value.
2398 @chars = map(chr, @nums);
2400 translates a list of numbers to the corresponding characters. And
2402 %hash = map { getkey($_) => $_ } @array;
2404 is just a funny way to write
2407 foreach $_ (@array) {
2408 $hash{getkey($_)} = $_;
2411 Note that, because C<$_> is a reference into the list value, it can
2412 be used to modify the elements of the array. While this is useful and
2413 supported, it can cause bizarre results if the LIST is not a named array.
2414 Using a regular C<foreach> loop for this purpose would be clearer in
2415 most cases. See also L</grep> for an array composed of those items of
2416 the original list for which the BLOCK or EXPR evaluates to true.
2418 =item mkdir FILENAME,MASK
2420 =item mkdir FILENAME
2422 Creates the directory specified by FILENAME, with permissions
2423 specified by MASK (as modified by C<umask>). If it succeeds it
2424 returns true, otherwise it returns false and sets C<$!> (errno).
2425 If omitted, MASK defaults to 0777.
2427 In general, it is better to create directories with permissive MASK,
2428 and let the user modify that with their C<umask>, than it is to supply
2429 a restrictive MASK and give the user no way to be more permissive.
2430 The exceptions to this rule are when the file or directory should be
2431 kept private (mail files, for instance). The perlfunc(1) entry on
2432 C<umask> discusses the choice of MASK in more detail.
2434 =item msgctl ID,CMD,ARG
2436 Calls the System V IPC function msgctl(2). You'll probably have to say
2440 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2441 then ARG must be a variable which will hold the returned C<msqid_ds>
2442 structure. Returns like C<ioctl>: the undefined value for error,
2443 C<"0 but true"> for zero, or the actual return value otherwise. See also
2444 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2446 =item msgget KEY,FLAGS
2448 Calls the System V IPC function msgget(2). Returns the message queue
2449 id, or the undefined value if there is an error. See also C<IPC::SysV>
2450 and C<IPC::Msg> documentation.
2452 =item msgsnd ID,MSG,FLAGS
2454 Calls the System V IPC function msgsnd to send the message MSG to the
2455 message queue ID. MSG must begin with the long integer message type,
2456 which may be created with C<pack("l", $type)>. Returns true if
2457 successful, or false if there is an error. See also C<IPC::SysV>
2458 and C<IPC::SysV::Msg> documentation.
2460 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2462 Calls the System V IPC function msgrcv to receive a message from
2463 message queue ID into variable VAR with a maximum message size of
2464 SIZE. Note that if a message is received, the message type will be
2465 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2466 size of the message type. Returns true if successful, or false if
2467 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2471 =item my EXPR : ATTRIBUTES
2473 A C<my> declares the listed variables to be local (lexically) to the
2474 enclosing block, file, or C<eval>. If
2475 more than one value is listed, the list must be placed in parentheses. See
2476 L<perlsub/"Private Variables via my()"> for details.
2482 The C<next> command is like the C<continue> statement in C; it starts
2483 the next iteration of the loop:
2485 LINE: while (<STDIN>) {
2486 next LINE if /^#/; # discard comments
2490 Note that if there were a C<continue> block on the above, it would get
2491 executed even on discarded lines. If the LABEL is omitted, the command
2492 refers to the innermost enclosing loop.
2494 C<next> cannot be used to exit a block which returns a value such as
2495 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2496 a grep() or map() operation.
2498 Note that a block by itself is semantically identical to a loop
2499 that executes once. Thus C<next> will exit such a block early.
2501 See also L</continue> for an illustration of how C<last>, C<next>, and
2504 =item no Module LIST
2506 See the L</use> function, which C<no> is the opposite of.
2512 Interprets EXPR as an octal string and returns the corresponding
2513 value. (If EXPR happens to start off with C<0x>, interprets it as a
2514 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2515 binary string.) The following will handle decimal, binary, octal, and
2516 hex in the standard Perl or C notation:
2518 $val = oct($val) if $val =~ /^0/;
2520 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2521 in octal), use sprintf() or printf():
2523 $perms = (stat("filename"))[2] & 07777;
2524 $oct_perms = sprintf "%lo", $perms;
2526 The oct() function is commonly used when a string such as C<644> needs
2527 to be converted into a file mode, for example. (Although perl will
2528 automatically convert strings into numbers as needed, this automatic
2529 conversion assumes base 10.)
2531 =item open FILEHANDLE,MODE,EXPR
2533 =item open FILEHANDLE,EXPR
2535 =item open FILEHANDLE
2537 Opens the file whose filename is given by EXPR, and associates it with
2538 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2539 name of the real filehandle wanted. If EXPR is omitted, the scalar
2540 variable of the same name as the FILEHANDLE contains the filename.
2541 (Note that lexical variables--those declared with C<my>--will not work
2542 for this purpose; so if you're using C<my>, specify EXPR in your call
2543 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2546 If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2547 If MODE is C<'E<gt>'>, the file is truncated and opened for
2548 output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
2549 the file is opened for appending, again being created if necessary.
2550 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2551 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2552 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2553 file first. You can't usually use either read-write mode for updating
2554 textfiles, since they have variable length records. See the B<-i>
2555 switch in L<perlrun> for a better approach. The file is created with
2556 permissions of C<0666> modified by the process' C<umask> value.
2558 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2559 C<'w+'>, C<'a'>, and C<'a+'>.
2561 In the 2-arguments (and 1-argument) form of the call the mode and
2562 filename should be concatenated (in this order), possibly separated by
2563 spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2565 If the filename begins with C<'|'>, the filename is interpreted as a
2566 command to which output is to be piped, and if the filename ends with a
2567 C<'|'>, the filename is interpreted as a command which pipes output to
2568 us. See L<perlipc/"Using open() for IPC">
2569 for more examples of this. (You are not allowed to C<open> to a command
2570 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2571 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2573 If MODE is C<'|-'>, the filename is interpreted as a
2574 command to which output is to be piped, and if MODE is
2575 C<'-|'>, the filename is interpreted as a command which pipes output to
2576 us. In the 2-arguments (and 1-argument) form one should replace dash
2577 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2578 for more examples of this. (You are not allowed to C<open> to a command
2579 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2580 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2582 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2583 and opening C<'E<gt>-'> opens STDOUT.
2586 nonzero upon success, the undefined value otherwise. If the C<open>
2587 involved a pipe, the return value happens to be the pid of the
2590 If you're unfortunate enough to be running Perl on a system that
2591 distinguishes between text files and binary files (modern operating
2592 systems don't care), then you should check out L</binmode> for tips for
2593 dealing with this. The key distinction between systems that need C<binmode>
2594 and those that don't is their text file formats. Systems like Unix, MacOS, and
2595 Plan9, which delimit lines with a single character, and which encode that
2596 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2598 When opening a file, it's usually a bad idea to continue normal execution
2599 if the request failed, so C<open> is frequently used in connection with
2600 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2601 where you want to make a nicely formatted error message (but there are
2602 modules that can help with that problem)) you should always check
2603 the return value from opening a file. The infrequent exception is when
2604 working with an unopened filehandle is actually what you want to do.
2609 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2610 while (<ARTICLE>) {...
2612 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2613 # if the open fails, output is discarded
2615 open(DBASE, '+<', 'dbase.mine') # open for update
2616 or die "Can't open 'dbase.mine' for update: $!";
2618 open(DBASE, '+<dbase.mine') # ditto
2619 or die "Can't open 'dbase.mine' for update: $!";
2621 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2622 or die "Can't start caesar: $!";
2624 open(ARTICLE, "caesar <$article |") # ditto
2625 or die "Can't start caesar: $!";
2627 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2628 or die "Can't start sort: $!";
2630 # process argument list of files along with any includes
2632 foreach $file (@ARGV) {
2633 process($file, 'fh00');
2637 my($filename, $input) = @_;
2638 $input++; # this is a string increment
2639 unless (open($input, $filename)) {
2640 print STDERR "Can't open $filename: $!\n";
2645 while (<$input>) { # note use of indirection
2646 if (/^#include "(.*)"/) {
2647 process($1, $input);
2654 You may also, in the Bourne shell tradition, specify an EXPR beginning
2655 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2656 name of a filehandle (or file descriptor, if numeric) to be
2657 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2658 C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
2659 mode you specify should match the mode of the original filehandle.
2660 (Duping a filehandle does not take into account any existing contents of
2661 stdio buffers.) Duping file handles is not yet supported for 3-argument
2664 Here is a script that saves, redirects, and restores STDOUT and
2668 open(OLDOUT, ">&STDOUT");
2669 open(OLDERR, ">&STDERR");
2671 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2672 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2674 select(STDERR); $| = 1; # make unbuffered
2675 select(STDOUT); $| = 1; # make unbuffered
2677 print STDOUT "stdout 1\n"; # this works for
2678 print STDERR "stderr 1\n"; # subprocesses too
2683 open(STDOUT, ">&OLDOUT");
2684 open(STDERR, ">&OLDERR");
2686 print STDOUT "stdout 2\n";
2687 print STDERR "stderr 2\n";
2689 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2690 equivalent of C's C<fdopen> of that file descriptor; this is more
2691 parsimonious of file descriptors. For example:
2693 open(FILEHANDLE, "<&=$fd")
2695 Note that this feature depends on the fdopen() C library function.
2696 On many UNIX systems, fdopen() is known to fail when file descriptors
2697 exceed a certain value, typically 255. If you need more file
2698 descriptors than that, consider rebuilding Perl to use the C<sfio>
2701 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2702 with 2-arguments (or 1-argument) form of open(), then
2703 there is an implicit fork done, and the return value of open is the pid
2704 of the child within the parent process, and C<0> within the child
2705 process. (Use C<defined($pid)> to determine whether the open was successful.)
2706 The filehandle behaves normally for the parent, but i/o to that
2707 filehandle is piped from/to the STDOUT/STDIN of the child process.
2708 In the child process the filehandle isn't opened--i/o happens from/to
2709 the new STDOUT or STDIN. Typically this is used like the normal
2710 piped open when you want to exercise more control over just how the
2711 pipe command gets executed, such as when you are running setuid, and
2712 don't want to have to scan shell commands for metacharacters.
2713 The following triples are more or less equivalent:
2715 open(FOO, "|tr '[a-z]' '[A-Z]'");
2716 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2717 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2719 open(FOO, "cat -n '$file'|");
2720 open(FOO, '-|', "cat -n '$file'");
2721 open(FOO, '-|') || exec 'cat', '-n', $file;
2723 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2725 NOTE: On any operation that may do a fork, all files opened for output
2726 are flushed before the fork is attempted. On systems that support a
2727 close-on-exec flag on files, the flag will be set for the newly opened
2728 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2730 Closing any piped filehandle causes the parent process to wait for the
2731 child to finish, and returns the status value in C<$?>.
2733 The filename passed to 2-argument (or 1-argument) form of open()
2734 will have leading and trailing
2735 whitespace deleted, and the normal redirection characters
2736 honored. This property, known as "magic open",
2737 can often be used to good effect. A user could specify a filename of
2738 F<"rsh cat file |">, or you could change certain filenames as needed:
2740 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2741 open(FH, $filename) or die "Can't open $filename: $!";
2743 Use 3-argument form to open a file with arbitrary weird characters in it,
2745 open(FOO, '<', $file);
2747 otherwise it's necessary to protect any leading and trailing whitespace:
2749 $file =~ s#^(\s)#./$1#;
2750 open(FOO, "< $file\0");
2752 (this may not work on some bizzare filesystems). One should
2753 conscientiously choose between the the I<magic> and 3-arguments form
2758 will allow the user to specify an argument of the form C<"rsh cat file |">,
2759 but will not work on a filename which happens to have a trailing space, while
2761 open IN, '<', $ARGV[0];
2763 will have exactly the opposite restrictions.
2765 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2766 should use the C<sysopen> function, which involves no such magic (but
2767 may use subtly different filemodes than Perl open(), which is mapped
2768 to C fopen()). This is
2769 another way to protect your filenames from interpretation. For example:
2772 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2773 or die "sysopen $path: $!";
2774 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2775 print HANDLE "stuff $$\n");
2777 print "File contains: ", <HANDLE>;
2779 Using the constructor from the C<IO::Handle> package (or one of its
2780 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2781 filehandles that have the scope of whatever variables hold references to
2782 them, and automatically close whenever and however you leave that scope:
2786 sub read_myfile_munged {
2788 my $handle = new IO::File;
2789 open($handle, "myfile") or die "myfile: $!";
2791 or return (); # Automatically closed here.
2792 mung $first or die "mung failed"; # Or here.
2793 return $first, <$handle> if $ALL; # Or here.
2797 See L</seek> for some details about mixing reading and writing.
2799 =item opendir DIRHANDLE,EXPR
2801 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2802 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2803 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2809 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2810 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2811 See L<utf8> for more about Unicode.
2815 An C<our> declares the listed variables to be valid globals within
2816 the enclosing block, file, or C<eval>. That is, it has the same
2817 scoping rules as a "my" declaration, but does not create a local
2818 variable. If more than one value is listed, the list must be placed
2819 in parentheses. The C<our> declaration has no semantic effect unless
2820 "use strict vars" is in effect, in which case it lets you use the
2821 declared global variable without qualifying it with a package name.
2822 (But only within the lexical scope of the C<our> declaration. In this
2823 it differs from "use vars", which is package scoped.)
2825 An C<our> declaration declares a global variable that will be visible
2826 across its entire lexical scope, even across package boundaries. The
2827 package in which the variable is entered is determined at the point
2828 of the declaration, not at the point of use. This means the following
2832 our $bar; # declares $Foo::bar for rest of lexical scope
2836 print $bar; # prints 20
2838 Multiple C<our> declarations in the same lexical scope are allowed
2839 if they are in different packages. If they happened to be in the same
2840 package, Perl will emit warnings if you have asked for them.
2844 our $bar; # declares $Foo::bar for rest of lexical scope
2848 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2849 print $bar; # prints 30
2851 our $bar; # emits warning
2853 =item pack TEMPLATE,LIST
2855 Takes a LIST of values and converts it into a string using the rules
2856 given by the TEMPLATE. The resulting string is the concatenation of
2857 the converted values. Typically, each converted value looks
2858 like its machine-level representation. For example, on 32-bit machines
2859 a converted integer may be represented by a sequence of 4 bytes.
2862 sequence of characters that give the order and type of values, as
2865 a A string with arbitrary binary data, will be null padded.
2866 A An ascii string, will be space padded.
2867 Z A null terminated (asciz) string, will be null padded.
2869 b A bit string (ascending bit order inside each byte, like vec()).
2870 B A bit string (descending bit order inside each byte).
2871 h A hex string (low nybble first).
2872 H A hex string (high nybble first).
2874 c A signed char value.
2875 C An unsigned char value. Only does bytes. See U for Unicode.
2877 s A signed short value.
2878 S An unsigned short value.
2879 (This 'short' is _exactly_ 16 bits, which may differ from
2880 what a local C compiler calls 'short'. If you want
2881 native-length shorts, use the '!' suffix.)
2883 i A signed integer value.
2884 I An unsigned integer value.
2885 (This 'integer' is _at_least_ 32 bits wide. Its exact
2886 size depends on what a local C compiler calls 'int',
2887 and may even be larger than the 'long' described in
2890 l A signed long value.
2891 L An unsigned long value.
2892 (This 'long' is _exactly_ 32 bits, which may differ from
2893 what a local C compiler calls 'long'. If you want
2894 native-length longs, use the '!' suffix.)
2896 n An unsigned short in "network" (big-endian) order.
2897 N An unsigned long in "network" (big-endian) order.
2898 v An unsigned short in "VAX" (little-endian) order.
2899 V An unsigned long in "VAX" (little-endian) order.
2900 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2901 _exactly_ 32 bits, respectively.)
2903 q A signed quad (64-bit) value.
2904 Q An unsigned quad value.
2905 (Quads are available only if your system supports 64-bit
2906 integer values _and_ if Perl has been compiled to support those.
2907 Causes a fatal error otherwise.)
2909 f A single-precision float in the native format.
2910 d A double-precision float in the native format.
2912 p A pointer to a null-terminated string.
2913 P A pointer to a structure (fixed-length string).
2915 u A uuencoded string.
2916 U A Unicode character number. Encodes to UTF-8 internally.
2917 Works even if C<use utf8> is not in effect.
2919 w A BER compressed integer. Its bytes represent an unsigned
2920 integer in base 128, most significant digit first, with as
2921 few digits as possible. Bit eight (the high bit) is set
2922 on each byte except the last.
2926 @ Null fill to absolute position.
2928 The following rules apply:
2934 Each letter may optionally be followed by a number giving a repeat
2935 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2936 C<H>, and C<P> the pack function will gobble up that many values from
2937 the LIST. A C<*> for the repeat count means to use however many items are
2938 left, except for C<@>, C<x>, C<X>, where it is equivalent
2939 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2942 When used with C<Z>, C<*> results in the addition of a trailing null
2943 byte (so the packed result will be one longer than the byte C<length>
2946 The repeat count for C<u> is interpreted as the maximal number of bytes
2947 to encode per line of output, with 0 and 1 replaced by 45.
2951 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
2952 string of length count, padding with nulls or spaces as necessary. When
2953 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2954 after the first null, and C<a> returns data verbatim. When packing,
2955 C<a>, and C<Z> are equivalent.
2957 If the value-to-pack is too long, it is truncated. If too long and an
2958 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2959 by a null byte. Thus C<Z> always packs a trailing null byte under
2964 Likewise, the C<b> and C<B> fields pack a string that many bits long.
2965 Each byte of the input field of pack() generates 1 bit of the result.
2966 Each result bit is based on the least-significant bit of the corresponding
2967 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2968 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2970 Starting from the beginning of the input string of pack(), each 8-tuple
2971 of bytes is converted to 1 byte of output. With format C<b>
2972 the first byte of the 8-tuple determines the least-significant bit of a
2973 byte, and with format C<B> it determines the most-significant bit of
2976 If the length of the input string is not exactly divisible by 8, the
2977 remainder is packed as if the input string were padded by null bytes
2978 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2980 If the input string of pack() is longer than needed, extra bytes are ignored.
2981 A C<*> for the repeat count of pack() means to use all the bytes of
2982 the input field. On unpack()ing the bits are converted to a string
2983 of C<"0">s and C<"1">s.
2987 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
2988 representable as hexadecimal digits, 0-9a-f) long.
2990 Each byte of the input field of pack() generates 4 bits of the result.
2991 For non-alphabetical bytes the result is based on the 4 least-significant
2992 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
2993 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
2994 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
2995 is compatible with the usual hexadecimal digits, so that C<"a"> and
2996 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
2997 C<"g".."z"> and C<"G".."Z"> is not well-defined.
2999 Starting from the beginning of the input string of pack(), each pair
3000 of bytes is converted to 1 byte of output. With format C<h> the
3001 first byte of the pair determines the least-significant nybble of the
3002 output byte, and with format C<H> it determines the most-significant
3005 If the length of the input string is not even, it behaves as if padded
3006 by a null byte at the end. Similarly, during unpack()ing the "extra"
3007 nybbles are ignored.
3009 If the input string of pack() is longer than needed, extra bytes are ignored.
3010 A C<*> for the repeat count of pack() means to use all the bytes of
3011 the input field. On unpack()ing the bits are converted to a string
3012 of hexadecimal digits.
3016 The C<p> type packs a pointer to a null-terminated string. You are
3017 responsible for ensuring the string is not a temporary value (which can
3018 potentially get deallocated before you get around to using the packed result).
3019 The C<P> type packs a pointer to a structure of the size indicated by the
3020 length. A NULL pointer is created if the corresponding value for C<p> or
3021 C<P> is C<undef>, similarly for unpack().
3025 The C</> template character allows packing and unpacking of strings where
3026 the packed structure contains a byte count followed by the string itself.
3027 You write I<length-item>C</>I<string-item>.
3029 The I<length-item> can be any C<pack> template letter,
3030 and describes how the length value is packed.
3031 The ones likely to be of most use are integer-packing ones like
3032 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3033 and C<N> (for Sun XDR).
3035 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3036 For C<unpack> the length of the string is obtained from the I<length-item>,
3037 but if you put in the '*' it will be ignored.
3039 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3040 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3041 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3043 The I<length-item> is not returned explicitly from C<unpack>.
3045 Adding a count to the I<length-item> letter is unlikely to do anything
3046 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3047 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3048 which Perl does not regard as legal in numeric strings.
3052 The integer types C<s>, C<S>, C<l>, and C<L> may be
3053 immediately followed by a C<!> suffix to signify native shorts or
3054 longs--as you can see from above for example a bare C<l> does mean
3055 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3056 may be larger. This is an issue mainly in 64-bit platforms. You can
3057 see whether using C<!> makes any difference by
3059 print length(pack("s")), " ", length(pack("s!")), "\n";
3060 print length(pack("l")), " ", length(pack("l!")), "\n";
3062 C<i!> and C<I!> also work but only because of completeness;
3063 they are identical to C<i> and C<I>.
3065 The actual sizes (in bytes) of native shorts, ints, longs, and long
3066 longs on the platform where Perl was built are also available via
3070 print $Config{shortsize}, "\n";
3071 print $Config{intsize}, "\n";
3072 print $Config{longsize}, "\n";
3073 print $Config{longlongsize}, "\n";
3075 (The C<$Config{longlongsize}> will be undefine if your system does
3076 not support long longs.)
3080 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3081 are inherently non-portable between processors and operating systems
3082 because they obey the native byteorder and endianness. For example a
3083 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3084 (arranged in and handled by the CPU registers) into bytes as
3086 0x12 0x34 0x56 0x78 # little-endian
3087 0x78 0x56 0x34 0x12 # big-endian
3089 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3090 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3091 Power, and Cray are big-endian. MIPS can be either: Digital used it
3092 in little-endian mode; SGI uses it in big-endian mode.
3094 The names `big-endian' and `little-endian' are comic references to
3095 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3096 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3097 the egg-eating habits of the Lilliputians.
3099 Some systems may have even weirder byte orders such as
3104 You can see your system's preference with
3106 print join(" ", map { sprintf "%#02x", $_ }
3107 unpack("C*",pack("L",0x12345678))), "\n";
3109 The byteorder on the platform where Perl was built is also available
3113 print $Config{byteorder}, "\n";
3115 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3116 and C<'87654321'> are big-endian.
3118 If you want portable packed integers use the formats C<n>, C<N>,
3119 C<v>, and C<V>, their byte endianness and size is known.
3120 See also L<perlport>.
3124 Real numbers (floats and doubles) are in the native machine format only;
3125 due to the multiplicity of floating formats around, and the lack of a
3126 standard "network" representation, no facility for interchange has been
3127 made. This means that packed floating point data written on one machine
3128 may not be readable on another - even if both use IEEE floating point
3129 arithmetic (as the endian-ness of the memory representation is not part
3130 of the IEEE spec). See also L<perlport>.
3132 Note that Perl uses doubles internally for all numeric calculation, and
3133 converting from double into float and thence back to double again will
3134 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3139 You must yourself do any alignment or padding by inserting for example
3140 enough C<'x'>es while packing. There is no way to pack() and unpack()
3141 could know where the bytes are going to or coming from. Therefore
3142 C<pack> (and C<unpack>) handle their output and input as flat
3147 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3151 If TEMPLATE requires more arguments to pack() than actually given, pack()
3152 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3153 to pack() than actually given, extra arguments are ignored.
3159 $foo = pack("CCCC",65,66,67,68);
3161 $foo = pack("C4",65,66,67,68);
3163 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3164 # same thing with Unicode circled letters
3166 $foo = pack("ccxxcc",65,66,67,68);
3169 # note: the above examples featuring "C" and "c" are true
3170 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3171 # and UTF-8. In EBCDIC the first example would be
3172 # $foo = pack("CCCC",193,194,195,196);
3174 $foo = pack("s2",1,2);
3175 # "\1\0\2\0" on little-endian
3176 # "\0\1\0\2" on big-endian
3178 $foo = pack("a4","abcd","x","y","z");
3181 $foo = pack("aaaa","abcd","x","y","z");
3184 $foo = pack("a14","abcdefg");
3185 # "abcdefg\0\0\0\0\0\0\0"
3187 $foo = pack("i9pl", gmtime);
3188 # a real struct tm (on my system anyway)
3190 $utmp_template = "Z8 Z8 Z16 L";
3191 $utmp = pack($utmp_template, @utmp1);
3192 # a struct utmp (BSDish)
3194 @utmp2 = unpack($utmp_template, $utmp);
3195 # "@utmp1" eq "@utmp2"
3198 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3201 $foo = pack('sx2l', 12, 34);
3202 # short 12, two zero bytes padding, long 34
3203 $bar = pack('s@4l', 12, 34);
3204 # short 12, zero fill to position 4, long 34
3207 The same template may generally also be used in unpack().
3211 =item package NAMESPACE
3213 Declares the compilation unit as being in the given namespace. The scope
3214 of the package declaration is from the declaration itself through the end
3215 of the enclosing block, file, or eval (the same as the C<my> operator).
3216 All further unqualified dynamic identifiers will be in this namespace.
3217 A package statement affects only dynamic variables--including those
3218 you've used C<local> on--but I<not> lexical variables, which are created
3219 with C<my>. Typically it would be the first declaration in a file to
3220 be included by the C<require> or C<use> operator. You can switch into a
3221 package in more than one place; it merely influences which symbol table
3222 is used by the compiler for the rest of that block. You can refer to
3223 variables and filehandles in other packages by prefixing the identifier
3224 with the package name and a double colon: C<$Package::Variable>.
3225 If the package name is null, the C<main> package as assumed. That is,
3226 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3227 still seen in older code).
3229 If NAMESPACE is omitted, then there is no current package, and all
3230 identifiers must be fully qualified or lexicals. This is stricter
3231 than C<use strict>, since it also extends to function names.
3233 See L<perlmod/"Packages"> for more information about packages, modules,
3234 and classes. See L<perlsub> for other scoping issues.
3236 =item pipe READHANDLE,WRITEHANDLE
3238 Opens a pair of connected pipes like the corresponding system call.
3239 Note that if you set up a loop of piped processes, deadlock can occur
3240 unless you are very careful. In addition, note that Perl's pipes use
3241 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3242 after each command, depending on the application.
3244 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3245 for examples of such things.
3247 On systems that support a close-on-exec flag on files, the flag will be set
3248 for the newly opened file descriptors as determined by the value of $^F.
3255 Pops and returns the last value of the array, shortening the array by
3256 one element. Has an effect similar to
3260 If there are no elements in the array, returns the undefined value
3261 (although this may happen at other times as well). If ARRAY is
3262 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3263 array in subroutines, just like C<shift>.
3269 Returns the offset of where the last C<m//g> search left off for the variable
3270 is in question (C<$_> is used when the variable is not specified). May be
3271 modified to change that offset. Such modification will also influence
3272 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3275 =item print FILEHANDLE LIST
3281 Prints a string or a list of strings. Returns true if successful.
3282 FILEHANDLE may be a scalar variable name, in which case the variable
3283 contains the name of or a reference to the filehandle, thus introducing
3284 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3285 the next token is a term, it may be misinterpreted as an operator
3286 unless you interpose a C<+> or put parentheses around the arguments.)
3287 If FILEHANDLE is omitted, prints by default to standard output (or
3288 to the last selected output channel--see L</select>). If LIST is
3289 also omitted, prints C<$_> to the currently selected output channel.
3290 To set the default output channel to something other than STDOUT
3291 use the select operation. The current value of C<$,> (if any) is
3292 printed between each LIST item. The current value of C<$\> (if
3293 any) is printed after the entire LIST has been printed. Because
3294 print takes a LIST, anything in the LIST is evaluated in list
3295 context, and any subroutine that you call will have one or more of
3296 its expressions evaluated in list context. Also be careful not to
3297 follow the print keyword with a left parenthesis unless you want
3298 the corresponding right parenthesis to terminate the arguments to
3299 the print--interpose a C<+> or put parentheses around all the
3302 Note that if you're storing FILEHANDLES in an array or other expression,
3303 you will have to use a block returning its value instead:
3305 print { $files[$i] } "stuff\n";
3306 print { $OK ? STDOUT : STDERR } "stuff\n";
3308 =item printf FILEHANDLE FORMAT, LIST
3310 =item printf FORMAT, LIST
3312 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3313 (the output record separator) is not appended. The first argument
3314 of the list will be interpreted as the C<printf> format. If C<use locale> is
3315 in effect, the character used for the decimal point in formatted real numbers
3316 is affected by the LC_NUMERIC locale. See L<perllocale>.
3318 Don't fall into the trap of using a C<printf> when a simple
3319 C<print> would do. The C<print> is more efficient and less
3322 =item prototype FUNCTION
3324 Returns the prototype of a function as a string (or C<undef> if the
3325 function has no prototype). FUNCTION is a reference to, or the name of,
3326 the function whose prototype you want to retrieve.
3328 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3329 name for Perl builtin. If the builtin is not I<overridable> (such as
3330 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3331 C<system>) returns C<undef> because the builtin does not really behave
3332 like a Perl function. Otherwise, the string describing the equivalent
3333 prototype is returned.
3335 =item push ARRAY,LIST
3337 Treats ARRAY as a stack, and pushes the values of LIST
3338 onto the end of ARRAY. The length of ARRAY increases by the length of
3339 LIST. Has the same effect as
3342 $ARRAY[++$#ARRAY] = $value;
3345 but is more efficient. Returns the new number of elements in the array.
3357 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3359 =item quotemeta EXPR
3363 Returns the value of EXPR with all non-alphanumeric
3364 characters backslashed. (That is, all characters not matching
3365 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3366 returned string, regardless of any locale settings.)
3367 This is the internal function implementing
3368 the C<\Q> escape in double-quoted strings.
3370 If EXPR is omitted, uses C<$_>.
3376 Returns a random fractional number greater than or equal to C<0> and less
3377 than the value of EXPR. (EXPR should be positive.) If EXPR is
3378 omitted, the value C<1> is used. Automatically calls C<srand> unless
3379 C<srand> has already been called. See also C<srand>.
3381 (Note: If your rand function consistently returns numbers that are too
3382 large or too small, then your version of Perl was probably compiled
3383 with the wrong number of RANDBITS.)
3385 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3387 =item read FILEHANDLE,SCALAR,LENGTH
3389 Attempts to read LENGTH bytes of data into variable SCALAR from the
3390 specified FILEHANDLE. Returns the number of bytes actually read,
3391 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3392 or shrunk to the length actually read. An OFFSET may be specified to
3393 place the read data at some other place than the beginning of the
3394 string. This call is actually implemented in terms of stdio's fread(3)
3395 call. To get a true read(2) system call, see C<sysread>.
3397 =item readdir DIRHANDLE
3399 Returns the next directory entry for a directory opened by C<opendir>.
3400 If used in list context, returns all the rest of the entries in the
3401 directory. If there are no more entries, returns an undefined value in
3402 scalar context or a null list in list context.
3404 If you're planning to filetest the return values out of a C<readdir>, you'd
3405 better prepend the directory in question. Otherwise, because we didn't
3406 C<chdir> there, it would have been testing the wrong file.
3408 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3409 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3414 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3415 context, each call reads and returns the next line, until end-of-file is
3416 reached, whereupon the subsequent call returns undef. In list context,
3417 reads until end-of-file is reached and returns a list of lines. Note that
3418 the notion of "line" used here is however you may have defined it
3419 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3421 When C<$/> is set to C<undef>, when readline() is in scalar
3422 context (i.e. file slurp mode), and when an empty file is read, it
3423 returns C<''> the first time, followed by C<undef> subsequently.
3425 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3426 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3427 operator is discussed in more detail in L<perlop/"I/O Operators">.
3430 $line = readline(*STDIN); # same thing
3436 Returns the value of a symbolic link, if symbolic links are
3437 implemented. If not, gives a fatal error. If there is some system
3438 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3439 omitted, uses C<$_>.
3443 EXPR is executed as a system command.
3444 The collected standard output of the command is returned.
3445 In scalar context, it comes back as a single (potentially
3446 multi-line) string. In list context, returns a list of lines
3447 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3448 This is the internal function implementing the C<qx/EXPR/>
3449 operator, but you can use it directly. The C<qx/EXPR/>
3450 operator is discussed in more detail in L<perlop/"I/O Operators">.
3452 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3454 Receives a message on a socket. Attempts to receive LENGTH bytes of
3455 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3456 will be grown or shrunk to the length actually read. Takes the same
3457 flags as the system call of the same name. Returns the address of the
3458 sender if SOCKET's protocol supports this; returns an empty string
3459 otherwise. If there's an error, returns the undefined value. This call
3460 is actually implemented in terms of recvfrom(2) system call. See
3461 L<perlipc/"UDP: Message Passing"> for examples.
3467 The C<redo> command restarts the loop block without evaluating the
3468 conditional again. The C<continue> block, if any, is not executed. If
3469 the LABEL is omitted, the command refers to the innermost enclosing
3470 loop. This command is normally used by programs that want to lie to
3471 themselves about what was just input:
3473 # a simpleminded Pascal comment stripper
3474 # (warning: assumes no { or } in strings)
3475 LINE: while (<STDIN>) {
3476 while (s|({.*}.*){.*}|$1 |) {}
3481 if (/}/) { # end of comment?
3490 C<redo> cannot be used to retry a block which returns a value such as
3491 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3492 a grep() or map() operation.
3494 Note that a block by itself is semantically identical to a loop
3495 that executes once. Thus C<redo> inside such a block will effectively
3496 turn it into a looping construct.
3498 See also L</continue> for an illustration of how C<last>, C<next>, and
3505 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3506 is not specified, C<$_> will be used. The value returned depends on the
3507 type of thing the reference is a reference to.
3508 Builtin types include:
3518 If the referenced object has been blessed into a package, then that package
3519 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3521 if (ref($r) eq "HASH") {
3522 print "r is a reference to a hash.\n";
3525 print "r is not a reference at all.\n";
3527 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3528 print "r is a reference to something that isa hash.\n";
3531 See also L<perlref>.
3533 =item rename OLDNAME,NEWNAME
3535 Changes the name of a file; an existing file NEWNAME will be
3536 clobbered. Returns true for success, false otherwise.
3538 Behavior of this function varies wildly depending on your system
3539 implementation. For example, it will usually not work across file system
3540 boundaries, even though the system I<mv> command sometimes compensates
3541 for this. Other restrictions include whether it works on directories,
3542 open files, or pre-existing files. Check L<perlport> and either the
3543 rename(2) manpage or equivalent system documentation for details.
3545 =item require VERSION
3551 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3554 If a VERSION is specified as a literal of the form v5.6.0,
3555 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3556 at least as recent as that version, at run time. (For compatibility
3557 with older versions of Perl, a numeric argument will also be interpreted
3558 as VERSION.) Compare with L</use>, which can do a similar check at
3561 require v5.6.0; # run time version check
3562 require 5.005_03; # same, number still supported for compatibility
3564 Otherwise, demands that a library file be included if it hasn't already
3565 been included. The file is included via the do-FILE mechanism, which is
3566 essentially just a variety of C<eval>. Has semantics similar to the following
3571 return 1 if $INC{$filename};
3572 my($realfilename,$result);
3574 foreach $prefix (@INC) {
3575 $realfilename = "$prefix/$filename";
3576 if (-f $realfilename) {
3577 $INC{$filename} = $realfilename;
3578 $result = do $realfilename;
3582 die "Can't find $filename in \@INC";
3584 delete $INC{$filename} if $@ || !$result;
3586 die "$filename did not return true value" unless $result;
3590 Note that the file will not be included twice under the same specified
3591 name. The file must return true as the last statement to indicate
3592 successful execution of any initialization code, so it's customary to
3593 end such a file with C<1;> unless you're sure it'll return true
3594 otherwise. But it's better just to put the C<1;>, in case you add more
3597 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3598 replaces "F<::>" with "F</>" in the filename for you,
3599 to make it easy to load standard modules. This form of loading of
3600 modules does not risk altering your namespace.
3602 In other words, if you try this:
3604 require Foo::Bar; # a splendid bareword
3606 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3607 directories specified in the C<@INC> array.
3609 But if you try this:
3611 $class = 'Foo::Bar';
3612 require $class; # $class is not a bareword
3614 require "Foo::Bar"; # not a bareword because of the ""
3616 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3617 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3619 eval "require $class";
3621 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3627 Generally used in a C<continue> block at the end of a loop to clear
3628 variables and reset C<??> searches so that they work again. The
3629 expression is interpreted as a list of single characters (hyphens
3630 allowed for ranges). All variables and arrays beginning with one of
3631 those letters are reset to their pristine state. If the expression is
3632 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3633 only variables or searches in the current package. Always returns
3636 reset 'X'; # reset all X variables
3637 reset 'a-z'; # reset lower case variables
3638 reset; # just reset ?one-time? searches
3640 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3641 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3642 variables--lexical variables are unaffected, but they clean themselves
3643 up on scope exit anyway, so you'll probably want to use them instead.
3650 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3651 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3652 context, depending on how the return value will be used, and the context
3653 may vary from one execution to the next (see C<wantarray>). If no EXPR
3654 is given, returns an empty list in list context, the undefined value in
3655 scalar context, and (of course) nothing at all in a void context.
3657 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3658 or do FILE will automatically return the value of the last expression
3663 In list context, returns a list value consisting of the elements
3664 of LIST in the opposite order. In scalar context, concatenates the
3665 elements of LIST and returns a string value with all characters
3666 in the opposite order.
3668 print reverse <>; # line tac, last line first
3670 undef $/; # for efficiency of <>
3671 print scalar reverse <>; # character tac, last line tsrif
3673 This operator is also handy for inverting a hash, although there are some
3674 caveats. If a value is duplicated in the original hash, only one of those
3675 can be represented as a key in the inverted hash. Also, this has to
3676 unwind one hash and build a whole new one, which may take some time
3677 on a large hash, such as from a DBM file.
3679 %by_name = reverse %by_address; # Invert the hash
3681 =item rewinddir DIRHANDLE
3683 Sets the current position to the beginning of the directory for the
3684 C<readdir> routine on DIRHANDLE.
3686 =item rindex STR,SUBSTR,POSITION
3688 =item rindex STR,SUBSTR
3690 Works just like index() except that it returns the position of the LAST
3691 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3692 last occurrence at or before that position.
3694 =item rmdir FILENAME
3698 Deletes the directory specified by FILENAME if that directory is empty. If it
3699 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3700 FILENAME is omitted, uses C<$_>.
3704 The substitution operator. See L<perlop>.
3708 Forces EXPR to be interpreted in scalar context and returns the value
3711 @counts = ( scalar @a, scalar @b, scalar @c );
3713 There is no equivalent operator to force an expression to
3714 be interpolated in list context because in practice, this is never
3715 needed. If you really wanted to do so, however, you could use
3716 the construction C<@{[ (some expression) ]}>, but usually a simple
3717 C<(some expression)> suffices.
3719 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3720 parenthesized list, this behaves as a scalar comma expression, evaluating
3721 all but the last element in void context and returning the final element
3722 evaluated in scalar context. This is seldom what you want.
3724 The following single statement:
3726 print uc(scalar(&foo,$bar)),$baz;
3728 is the moral equivalent of these two:
3731 print(uc($bar),$baz);
3733 See L<perlop> for more details on unary operators and the comma operator.
3735 =item seek FILEHANDLE,POSITION,WHENCE
3737 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3738 FILEHANDLE may be an expression whose value gives the name of the
3739 filehandle. The values for WHENCE are C<0> to set the new position to
3740 POSITION, C<1> to set it to the current position plus POSITION, and
3741 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3742 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3743 (start of the file, current position, end of the file) from the Fcntl
3744 module. Returns C<1> upon success, C<0> otherwise.
3746 If you want to position file for C<sysread> or C<syswrite>, don't use
3747 C<seek>--buffering makes its effect on the file's system position
3748 unpredictable and non-portable. Use C<sysseek> instead.
3750 Due to the rules and rigors of ANSI C, on some systems you have to do a
3751 seek whenever you switch between reading and writing. Amongst other
3752 things, this may have the effect of calling stdio's clearerr(3).
3753 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3757 This is also useful for applications emulating C<tail -f>. Once you hit
3758 EOF on your read, and then sleep for a while, you might have to stick in a
3759 seek() to reset things. The C<seek> doesn't change the current position,
3760 but it I<does> clear the end-of-file condition on the handle, so that the
3761 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3763 If that doesn't work (some stdios are particularly cantankerous), then
3764 you may need something more like this:
3767 for ($curpos = tell(FILE); $_ = <FILE>;
3768 $curpos = tell(FILE)) {
3769 # search for some stuff and put it into files
3771 sleep($for_a_while);
3772 seek(FILE, $curpos, 0);
3775 =item seekdir DIRHANDLE,POS
3777 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3778 must be a value returned by C<telldir>. Has the same caveats about
3779 possible directory compaction as the corresponding system library
3782 =item select FILEHANDLE
3786 Returns the currently selected filehandle. Sets the current default
3787 filehandle for output, if FILEHANDLE is supplied. This has two
3788 effects: first, a C<write> or a C<print> without a filehandle will
3789 default to this FILEHANDLE. Second, references to variables related to
3790 output will refer to this output channel. For example, if you have to
3791 set the top of form format for more than one output channel, you might
3799 FILEHANDLE may be an expression whose value gives the name of the
3800 actual filehandle. Thus:
3802 $oldfh = select(STDERR); $| = 1; select($oldfh);
3804 Some programmers may prefer to think of filehandles as objects with
3805 methods, preferring to write the last example as:
3808 STDERR->autoflush(1);
3810 =item select RBITS,WBITS,EBITS,TIMEOUT
3812 This calls the select(2) system call with the bit masks specified, which
3813 can be constructed using C<fileno> and C<vec>, along these lines:
3815 $rin = $win = $ein = '';
3816 vec($rin,fileno(STDIN),1) = 1;
3817 vec($win,fileno(STDOUT),1) = 1;
3820 If you want to select on many filehandles you might wish to write a
3824 my(@fhlist) = split(' ',$_[0]);
3827 vec($bits,fileno($_),1) = 1;
3831 $rin = fhbits('STDIN TTY SOCK');
3835 ($nfound,$timeleft) =
3836 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3838 or to block until something becomes ready just do this
3840 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3842 Most systems do not bother to return anything useful in $timeleft, so
3843 calling select() in scalar context just returns $nfound.
3845 Any of the bit masks can also be undef. The timeout, if specified, is
3846 in seconds, which may be fractional. Note: not all implementations are
3847 capable of returning the$timeleft. If not, they always return
3848 $timeleft equal to the supplied $timeout.
3850 You can effect a sleep of 250 milliseconds this way:
3852 select(undef, undef, undef, 0.25);
3854 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3855 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3856 then only on POSIX systems. You have to use C<sysread> instead.
3858 =item semctl ID,SEMNUM,CMD,ARG
3860 Calls the System V IPC function C<semctl>. You'll probably have to say
3864 first to get the correct constant definitions. If CMD is IPC_STAT or
3865 GETALL, then ARG must be a variable which will hold the returned
3866 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3867 undefined value for error, "C<0 but true>" for zero, or the actual return
3868 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3870 =item semget KEY,NSEMS,FLAGS
3872 Calls the System V IPC function semget. Returns the semaphore id, or
3873 the undefined value if there is an error. See also C<IPC::SysV> and
3874 C<IPC::SysV::Semaphore> documentation.
3876 =item semop KEY,OPSTRING
3878 Calls the System V IPC function semop to perform semaphore operations
3879 such as signaling and waiting. OPSTRING must be a packed array of
3880 semop structures. Each semop structure can be generated with
3881 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3882 operations is implied by the length of OPSTRING. Returns true if
3883 successful, or false if there is an error. As an example, the
3884 following code waits on semaphore $semnum of semaphore id $semid:
3886 $semop = pack("sss", $semnum, -1, 0);
3887 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3889 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3890 and C<IPC::SysV::Semaphore> documentation.
3892 =item send SOCKET,MSG,FLAGS,TO
3894 =item send SOCKET,MSG,FLAGS
3896 Sends a message on a socket. Takes the same flags as the system call
3897 of the same name. On unconnected sockets you must specify a
3898 destination to send TO, in which case it does a C C<sendto>. Returns
3899 the number of characters sent, or the undefined value if there is an
3900 error. The C system call sendmsg(2) is currently unimplemented.
3901 See L<perlipc/"UDP: Message Passing"> for examples.
3903 =item setpgrp PID,PGRP
3905 Sets the current process group for the specified PID, C<0> for the current
3906 process. Will produce a fatal error if used on a machine that doesn't
3907 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3908 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3909 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3912 =item setpriority WHICH,WHO,PRIORITY
3914 Sets the current priority for a process, a process group, or a user.
3915 (See setpriority(2).) Will produce a fatal error if used on a machine
3916 that doesn't implement setpriority(2).
3918 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3920 Sets the socket option requested. Returns undefined if there is an
3921 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3928 Shifts the first value of the array off and returns it, shortening the
3929 array by 1 and moving everything down. If there are no elements in the
3930 array, returns the undefined value. If ARRAY is omitted, shifts the
3931 C<@_> array within the lexical scope of subroutines and formats, and the
3932 C<@ARGV> array at file scopes or within the lexical scopes established by
3933 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
3936 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3937 same thing to the left end of an array that C<pop> and C<push> do to the
3940 =item shmctl ID,CMD,ARG
3942 Calls the System V IPC function shmctl. You'll probably have to say
3946 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3947 then ARG must be a variable which will hold the returned C<shmid_ds>
3948 structure. Returns like ioctl: the undefined value for error, "C<0> but
3949 true" for zero, or the actual return value otherwise.
3950 See also C<IPC::SysV> documentation.
3952 =item shmget KEY,SIZE,FLAGS
3954 Calls the System V IPC function shmget. Returns the shared memory
3955 segment id, or the undefined value if there is an error.
3956 See also C<IPC::SysV> documentation.
3958 =item shmread ID,VAR,POS,SIZE
3960 =item shmwrite ID,STRING,POS,SIZE
3962 Reads or writes the System V shared memory segment ID starting at
3963 position POS for size SIZE by attaching to it, copying in/out, and
3964 detaching from it. When reading, VAR must be a variable that will
3965 hold the data read. When writing, if STRING is too long, only SIZE
3966 bytes are used; if STRING is too short, nulls are written to fill out
3967 SIZE bytes. Return true if successful, or false if there is an error.
3968 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3971 =item shutdown SOCKET,HOW
3973 Shuts down a socket connection in the manner indicated by HOW, which
3974 has the same interpretation as in the system call of the same name.
3976 shutdown(SOCKET, 0); # I/we have stopped reading data
3977 shutdown(SOCKET, 1); # I/we have stopped writing data
3978 shutdown(SOCKET, 2); # I/we have stopped using this socket
3980 This is useful with sockets when you want to tell the other
3981 side you're done writing but not done reading, or vice versa.
3982 It's also a more insistent form of close because it also
3983 disables the file descriptor in any forked copies in other
3990 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3991 returns sine of C<$_>.
3993 For the inverse sine operation, you may use the C<Math::Trig::asin>
3994 function, or use this relation:
3996 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4002 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4003 May be interrupted if the process receives a signal such as C<SIGALRM>.
4004 Returns the number of seconds actually slept. You probably cannot
4005 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4008 On some older systems, it may sleep up to a full second less than what
4009 you requested, depending on how it counts seconds. Most modern systems
4010 always sleep the full amount. They may appear to sleep longer than that,
4011 however, because your process might not be scheduled right away in a
4012 busy multitasking system.
4014 For delays of finer granularity than one second, you may use Perl's
4015 C<syscall> interface to access setitimer(2) if your system supports
4016 it, or else see L</select> above. The Time::HiRes module from CPAN
4019 See also the POSIX module's C<sigpause> function.
4021 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4023 Opens a socket of the specified kind and attaches it to filehandle
4024 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4025 the system call of the same name. You should C<use Socket> first
4026 to get the proper definitions imported. See the examples in
4027 L<perlipc/"Sockets: Client/Server Communication">.
4029 On systems that support a close-on-exec flag on files, the flag will
4030 be set for the newly opened file descriptor, as determined by the
4031 value of $^F. See L<perlvar/$^F>.
4033 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4035 Creates an unnamed pair of sockets in the specified domain, of the
4036 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4037 for the system call of the same name. If unimplemented, yields a fatal
4038 error. Returns true if successful.
4040 On systems that support a close-on-exec flag on files, the flag will
4041 be set for the newly opened file descriptors, as determined by the value
4042 of $^F. See L<perlvar/$^F>.
4044 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4045 to C<pipe(Rdr, Wtr)> is essentially:
4048 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4049 shutdown(Rdr, 1); # no more writing for reader
4050 shutdown(Wtr, 0); # no more reading for writer
4052 See L<perlipc> for an example of socketpair use.
4054 =item sort SUBNAME LIST
4056 =item sort BLOCK LIST
4060 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4061 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4062 specified, it gives the name of a subroutine that returns an integer
4063 less than, equal to, or greater than C<0>, depending on how the elements
4064 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
4065 operators are extremely useful in such routines.) SUBNAME may be a
4066 scalar variable name (unsubscripted), in which case the value provides
4067 the name of (or a reference to) the actual subroutine to use. In place
4068 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4071 If the subroutine's prototype is C<($$)>, the elements to be compared
4072 are passed by reference in C<@_>, as for a normal subroutine. If not,
4073 the normal calling code for subroutines is bypassed in the interests of
4074 efficiency, and the elements to be compared are passed into the subroutine
4075 as the package global variables $a and $b (see example below). Note that
4076 in the latter case, it is usually counter-productive to declare $a and
4079 In either case, the subroutine may not be recursive. The values to be
4080 compared are always passed by reference, so don't modify them.
4082 You also cannot exit out of the sort block or subroutine using any of the
4083 loop control operators described in L<perlsyn> or with C<goto>.
4085 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4086 current collation locale. See L<perllocale>.
4091 @articles = sort @files;
4093 # same thing, but with explicit sort routine
4094 @articles = sort {$a cmp $b} @files;
4096 # now case-insensitively
4097 @articles = sort {uc($a) cmp uc($b)} @files;
4099 # same thing in reversed order
4100 @articles = sort {$b cmp $a} @files;
4102 # sort numerically ascending
4103 @articles = sort {$a <=> $b} @files;
4105 # sort numerically descending
4106 @articles = sort {$b <=> $a} @files;
4108 # this sorts the %age hash by value instead of key
4109 # using an in-line function
4110 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4112 # sort using explicit subroutine name
4114 $age{$a} <=> $age{$b}; # presuming numeric
4116 @sortedclass = sort byage @class;
4118 sub backwards { $b cmp $a }
4119 @harry = qw(dog cat x Cain Abel);
4120 @george = qw(gone chased yz Punished Axed);
4122 # prints AbelCaincatdogx
4123 print sort backwards @harry;
4124 # prints xdogcatCainAbel
4125 print sort @george, 'to', @harry;
4126 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4128 # inefficiently sort by descending numeric compare using
4129 # the first integer after the first = sign, or the
4130 # whole record case-insensitively otherwise
4133 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4138 # same thing, but much more efficiently;
4139 # we'll build auxiliary indices instead
4143 push @nums, /=(\d+)/;
4148 $nums[$b] <=> $nums[$a]
4150 $caps[$a] cmp $caps[$b]
4154 # same thing, but without any temps
4155 @new = map { $_->[0] }
4156 sort { $b->[1] <=> $a->[1]
4159 } map { [$_, /=(\d+)/, uc($_)] } @old;
4161 # using a prototype allows you to use any comparison subroutine
4162 # as a sort subroutine (including other package's subroutines)
4164 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4167 @new = sort other::backwards @old;
4169 If you're using strict, you I<must not> declare $a
4170 and $b as lexicals. They are package globals. That means
4171 if you're in the C<main> package, it's
4173 @articles = sort {$main::b <=> $main::a} @files;
4177 @articles = sort {$::b <=> $::a} @files;
4179 but if you're in the C<FooPack> package, it's
4181 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4183 The comparison function is required to behave. If it returns
4184 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4185 sometimes saying the opposite, for example) the results are not
4188 =item splice ARRAY,OFFSET,LENGTH,LIST
4190 =item splice ARRAY,OFFSET,LENGTH
4192 =item splice ARRAY,OFFSET
4196 Removes the elements designated by OFFSET and LENGTH from an array, and
4197 replaces them with the elements of LIST, if any. In list context,
4198 returns the elements removed from the array. In scalar context,
4199 returns the last element removed, or C<undef> if no elements are
4200 removed. The array grows or shrinks as necessary.
4201 If OFFSET is negative then it starts that far from the end of the array.
4202 If LENGTH is omitted, removes everything from OFFSET onward.
4203 If LENGTH is negative, leaves that many elements off the end of the array.
4204 If both OFFSET and LENGTH are omitted, removes everything.
4206 The following equivalences hold (assuming C<$[ == 0>):
4208 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4209 pop(@a) splice(@a,-1)
4210 shift(@a) splice(@a,0,1)
4211 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4212 $a[$x] = $y splice(@a,$x,1,$y)
4214 Example, assuming array lengths are passed before arrays:
4216 sub aeq { # compare two list values
4217 my(@a) = splice(@_,0,shift);
4218 my(@b) = splice(@_,0,shift);
4219 return 0 unless @a == @b; # same len?
4221 return 0 if pop(@a) ne pop(@b);
4225 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4227 =item split /PATTERN/,EXPR,LIMIT
4229 =item split /PATTERN/,EXPR
4231 =item split /PATTERN/
4235 Splits a string into a list of strings and returns that list. By default,
4236 empty leading fields are preserved, and empty trailing ones are deleted.
4238 If not in list context, returns the number of fields found and splits into
4239 the C<@_> array. (In list context, you can force the split into C<@_> by
4240 using C<??> as the pattern delimiters, but it still returns the list
4241 value.) The use of implicit split to C<@_> is deprecated, however, because
4242 it clobbers your subroutine arguments.
4244 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4245 splits on whitespace (after skipping any leading whitespace). Anything
4246 matching PATTERN is taken to be a delimiter separating the fields. (Note
4247 that the delimiter may be longer than one character.)
4249 If LIMIT is specified and positive, splits into no more than that
4250 many fields (though it may split into fewer). If LIMIT is unspecified
4251 or zero, trailing null fields are stripped (which potential users
4252 of C<pop> would do well to remember). If LIMIT is negative, it is
4253 treated as if an arbitrarily large LIMIT had been specified.
4255 A pattern matching the null string (not to be confused with
4256 a null pattern C<//>, which is just one member of the set of patterns
4257 matching a null string) will split the value of EXPR into separate
4258 characters at each point it matches that way. For example:
4260 print join(':', split(/ */, 'hi there'));
4262 produces the output 'h:i:t:h:e:r:e'.
4264 The LIMIT parameter can be used to split a line partially
4266 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4268 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4269 one larger than the number of variables in the list, to avoid
4270 unnecessary work. For the list above LIMIT would have been 4 by
4271 default. In time critical applications it behooves you not to split
4272 into more fields than you really need.
4274 If the PATTERN contains parentheses, additional list elements are
4275 created from each matching substring in the delimiter.
4277 split(/([,-])/, "1-10,20", 3);
4279 produces the list value
4281 (1, '-', 10, ',', 20)
4283 If you had the entire header of a normal Unix email message in $header,
4284 you could split it up into fields and their values this way:
4286 $header =~ s/\n\s+/ /g; # fix continuation lines
4287 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4289 The pattern C</PATTERN/> may be replaced with an expression to specify
4290 patterns that vary at runtime. (To do runtime compilation only once,
4291 use C</$variable/o>.)
4293 As a special case, specifying a PATTERN of space (C<' '>) will split on
4294 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4295 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4296 will give you as many null initial fields as there are leading spaces.
4297 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4298 whitespace produces a null first field. A C<split> with no arguments
4299 really does a C<split(' ', $_)> internally.
4303 open(PASSWD, '/etc/passwd');
4305 ($login, $passwd, $uid, $gid,
4306 $gcos, $home, $shell) = split(/:/);
4310 (Note that $shell above will still have a newline on it. See L</chop>,
4311 L</chomp>, and L</join>.)
4313 =item sprintf FORMAT, LIST
4315 Returns a string formatted by the usual C<printf> conventions of the
4316 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4317 on your system for an explanation of the general principles.
4319 Perl does its own C<sprintf> formatting--it emulates the C
4320 function C<sprintf>, but it doesn't use it (except for floating-point
4321 numbers, and even then only the standard modifiers are allowed). As a
4322 result, any non-standard extensions in your local C<sprintf> are not
4323 available from Perl.
4325 Perl's C<sprintf> permits the following universally-known conversions:
4328 %c a character with the given number
4330 %d a signed integer, in decimal
4331 %u an unsigned integer, in decimal
4332 %o an unsigned integer, in octal
4333 %x an unsigned integer, in hexadecimal
4334 %e a floating-point number, in scientific notation
4335 %f a floating-point number, in fixed decimal notation
4336 %g a floating-point number, in %e or %f notation
4338 In addition, Perl permits the following widely-supported conversions:
4340 %X like %x, but using upper-case letters
4341 %E like %e, but using an upper-case "E"
4342 %G like %g, but with an upper-case "E" (if applicable)
4343 %b an unsigned integer, in binary
4344 %p a pointer (outputs the Perl value's address in hexadecimal)
4345 %n special: *stores* the number of characters output so far
4346 into the next variable in the parameter list
4348 Finally, for backward (and we do mean "backward") compatibility, Perl
4349 permits these unnecessary but widely-supported conversions:
4352 %D a synonym for %ld
4353 %U a synonym for %lu
4354 %O a synonym for %lo
4357 Perl permits the following universally-known flags between the C<%>
4358 and the conversion letter:
4360 space prefix positive number with a space
4361 + prefix positive number with a plus sign
4362 - left-justify within the field
4363 0 use zeros, not spaces, to right-justify
4364 # prefix non-zero octal with "0", non-zero hex with "0x"
4365 number minimum field width
4366 .number "precision": digits after decimal point for
4367 floating-point, max length for string, minimum length
4369 l interpret integer as C type "long" or "unsigned long"
4370 h interpret integer as C type "short" or "unsigned short"
4371 If no flags, interpret integer as C type "int" or "unsigned"
4373 There are also two Perl-specific flags:
4375 V interpret integer as Perl's standard integer type
4376 v interpret string as a vector of integers, output as
4377 numbers separated either by dots, or by an arbitrary
4378 string received from the argument list when the flag
4381 Where a number would appear in the flags, an asterisk (C<*>) may be
4382 used instead, in which case Perl uses the next item in the parameter
4383 list as the given number (that is, as the field width or precision).
4384 If a field width obtained through C<*> is negative, it has the same
4385 effect as the C<-> flag: left-justification.
4387 The C<v> flag is useful for displaying ordinal values of characters
4388 in arbitrary strings:
4390 printf "version is v%vd\n", $^V; # Perl's version
4391 printf "address is %*vX\n", ":", $addr; # IPv6 address
4392 printf "bits are %*vb\n", "", $bits; # random bitstring
4394 If C<use locale> is in effect, the character used for the decimal
4395 point in formatted real numbers is affected by the LC_NUMERIC locale.
4398 If Perl understands "quads" (64-bit integers) (this requires
4399 either that the platform natively support quads or that Perl
4400 be specifically compiled to support quads), the characters
4404 print quads, and they may optionally be preceded by
4412 You can find out whether your Perl supports quads via L<Config>:
4415 ($Config{use64bits} eq 'define' || $Config{longsize} == 8) &&
4418 If Perl understands "long doubles" (this requires that the platform
4419 support long doubles), the flags
4423 may optionally be preceded by
4431 You can find out whether your Perl supports long doubles via L<Config>:
4434 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4440 Return the square root of EXPR. If EXPR is omitted, returns square
4441 root of C<$_>. Only works on non-negative operands, unless you've
4442 loaded the standard Math::Complex module.
4445 print sqrt(-2); # prints 1.4142135623731i
4451 Sets the random number seed for the C<rand> operator. If EXPR is
4452 omitted, uses a semi-random value supplied by the kernel (if it supports
4453 the F</dev/urandom> device) or based on the current time and process
4454 ID, among other things. In versions of Perl prior to 5.004 the default
4455 seed was just the current C<time>. This isn't a particularly good seed,
4456 so many old programs supply their own seed value (often C<time ^ $$> or
4457 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4459 In fact, it's usually not necessary to call C<srand> at all, because if
4460 it is not called explicitly, it is called implicitly at the first use of
4461 the C<rand> operator. However, this was not the case in version of Perl
4462 before 5.004, so if your script will run under older Perl versions, it
4463 should call C<srand>.
4465 Note that you need something much more random than the default seed for
4466 cryptographic purposes. Checksumming the compressed output of one or more
4467 rapidly changing operating system status programs is the usual method. For
4470 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4472 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4475 Do I<not> call C<srand> multiple times in your program unless you know
4476 exactly what you're doing and why you're doing it. The point of the
4477 function is to "seed" the C<rand> function so that C<rand> can produce
4478 a different sequence each time you run your program. Just do it once at the
4479 top of your program, or you I<won't> get random numbers out of C<rand>!
4481 Frequently called programs (like CGI scripts) that simply use
4485 for a seed can fall prey to the mathematical property that
4489 one-third of the time. So don't do that.
4491 =item stat FILEHANDLE
4497 Returns a 13-element list giving the status info for a file, either
4498 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4499 it stats C<$_>. Returns a null list if the stat fails. Typically used
4502 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4503 $atime,$mtime,$ctime,$blksize,$blocks)
4506 Not all fields are supported on all filesystem types. Here are the
4507 meaning of the fields:
4509 0 dev device number of filesystem
4511 2 mode file mode (type and permissions)
4512 3 nlink number of (hard) links to the file
4513 4 uid numeric user ID of file's owner
4514 5 gid numeric group ID of file's owner
4515 6 rdev the device identifier (special files only)
4516 7 size total size of file, in bytes
4517 8 atime last access time in seconds since the epoch
4518 9 mtime last modify time in seconds since the epoch
4519 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4520 11 blksize preferred block size for file system I/O
4521 12 blocks actual number of blocks allocated
4523 (The epoch was at 00:00 January 1, 1970 GMT.)
4525 If stat is passed the special filehandle consisting of an underline, no
4526 stat is done, but the current contents of the stat structure from the
4527 last stat or filetest are returned. Example:
4529 if (-x $file && (($d) = stat(_)) && $d < 0) {
4530 print "$file is executable NFS file\n";
4533 (This works on machines only for which the device number is negative
4536 Because the mode contains both the file type and its permissions, you
4537 should mask off the file type portion and (s)printf using a C<"%o">
4538 if you want to see the real permissions.
4540 $mode = (stat($filename))[2];
4541 printf "Permissions are %04o\n", $mode & 07777;
4543 In scalar context, C<stat> returns a boolean value indicating success
4544 or failure, and, if successful, sets the information associated with
4545 the special filehandle C<_>.
4547 The File::stat module provides a convenient, by-name access mechanism:
4550 $sb = stat($filename);
4551 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4552 $filename, $sb->size, $sb->mode & 07777,
4553 scalar localtime $sb->mtime;
4555 You can import symbolic mode constants (C<S_IF*>) and functions
4556 (C<S_IS*>) from the Fcntl module:
4560 $mode = (stat($filename))[2];
4562 $user_rwx = ($mode & S_IRWXU) >> 6;
4563 $group_read = ($mode & S_IRGRP) >> 3;
4564 $other_execute = $mode & S_IXOTH;
4566 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4568 $is_setuid = $mode & S_ISUID;
4569 $is_setgid = S_ISDIR($mode);
4571 You could write the last two using the C<-u> and C<-d> operators.
4572 The commonly available S_IF* constants are
4574 # Permissions: read, write, execute, for user, group, others.
4576 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4577 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4578 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4580 # Setuid/Setgid/Stickiness.
4582 S_ISUID S_ISGID S_ISVTX S_ISTXT
4584 # File types. Not necessarily all are available on your system.
4586 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4588 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4590 S_IREAD S_IWRITE S_IEXEC
4592 and the S_IF* functions are
4594 S_IFMODE($mode) the part of $mode containg the permission bits
4595 and the setuid/setgid/sticky bits
4597 S_IFMT($mode) the part of $mode containing the file type
4598 which can be bit-anded with e.g. S_IFREG
4599 or with the following functions
4601 # The operators -f, -d, -l, -b, -c, -p, and -s.
4603 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4604 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4606 # No direct -X operator counterpart, but for the first one
4607 # the -g operator is often equivalent. The ENFMT stands for
4608 # record flocking enforcement, a platform-dependent feature.
4610 S_ISENFMT($mode) S_ISWHT($mode)
4612 See your native chmod(2) and stat(2) documentation for more details
4613 about the S_* constants.
4619 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4620 doing many pattern matches on the string before it is next modified.
4621 This may or may not save time, depending on the nature and number of
4622 patterns you are searching on, and on the distribution of character
4623 frequencies in the string to be searched--you probably want to compare
4624 run times with and without it to see which runs faster. Those loops
4625 which scan for many short constant strings (including the constant
4626 parts of more complex patterns) will benefit most. You may have only
4627 one C<study> active at a time--if you study a different scalar the first
4628 is "unstudied". (The way C<study> works is this: a linked list of every
4629 character in the string to be searched is made, so we know, for
4630 example, where all the C<'k'> characters are. From each search string,
4631 the rarest character is selected, based on some static frequency tables
4632 constructed from some C programs and English text. Only those places
4633 that contain this "rarest" character are examined.)
4635 For example, here is a loop that inserts index producing entries
4636 before any line containing a certain pattern:
4640 print ".IX foo\n" if /\bfoo\b/;
4641 print ".IX bar\n" if /\bbar\b/;
4642 print ".IX blurfl\n" if /\bblurfl\b/;
4647 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4648 will be looked at, because C<f> is rarer than C<o>. In general, this is
4649 a big win except in pathological cases. The only question is whether
4650 it saves you more time than it took to build the linked list in the
4653 Note that if you have to look for strings that you don't know till
4654 runtime, you can build an entire loop as a string and C<eval> that to
4655 avoid recompiling all your patterns all the time. Together with
4656 undefining C<$/> to input entire files as one record, this can be very
4657 fast, often faster than specialized programs like fgrep(1). The following
4658 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4659 out the names of those files that contain a match:
4661 $search = 'while (<>) { study;';
4662 foreach $word (@words) {
4663 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4668 eval $search; # this screams
4669 $/ = "\n"; # put back to normal input delimiter
4670 foreach $file (sort keys(%seen)) {
4678 =item sub NAME BLOCK
4680 This is subroutine definition, not a real function I<per se>. With just a
4681 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4682 Without a NAME, it's an anonymous function declaration, and does actually
4683 return a value: the CODE ref of the closure you just created. See L<perlsub>
4684 and L<perlref> for details.
4686 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4688 =item substr EXPR,OFFSET,LENGTH
4690 =item substr EXPR,OFFSET
4692 Extracts a substring out of EXPR and returns it. First character is at
4693 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4694 If OFFSET is negative (or more precisely, less than C<$[>), starts
4695 that far from the end of the string. If LENGTH is omitted, returns
4696 everything to the end of the string. If LENGTH is negative, leaves that
4697 many characters off the end of the string.
4699 You can use the substr() function as an lvalue, in which case EXPR
4700 must itself be an lvalue. If you assign something shorter than LENGTH,
4701 the string will shrink, and if you assign something longer than LENGTH,
4702 the string will grow to accommodate it. To keep the string the same
4703 length you may need to pad or chop your value using C<sprintf>.
4705 If OFFSET and LENGTH specify a substring that is partly outside the
4706 string, only the part within the string is returned. If the substring
4707 is beyond either end of the string, substr() returns the undefined
4708 value and produces a warning. When used as an lvalue, specifying a
4709 substring that is entirely outside the string is a fatal error.
4710 Here's an example showing the behavior for boundary cases:
4713 substr($name, 4) = 'dy'; # $name is now 'freddy'
4714 my $null = substr $name, 6, 2; # returns '' (no warning)
4715 my $oops = substr $name, 7; # returns undef, with warning
4716 substr($name, 7) = 'gap'; # fatal error
4718 An alternative to using substr() as an lvalue is to specify the
4719 replacement string as the 4th argument. This allows you to replace
4720 parts of the EXPR and return what was there before in one operation,
4721 just as you can with splice().
4723 =item symlink OLDFILE,NEWFILE
4725 Creates a new filename symbolically linked to the old filename.
4726 Returns C<1> for success, C<0> otherwise. On systems that don't support
4727 symbolic links, produces a fatal error at run time. To check for that,
4730 $symlink_exists = eval { symlink("",""); 1 };
4734 Calls the system call specified as the first element of the list,
4735 passing the remaining elements as arguments to the system call. If
4736 unimplemented, produces a fatal error. The arguments are interpreted
4737 as follows: if a given argument is numeric, the argument is passed as
4738 an int. If not, the pointer to the string value is passed. You are
4739 responsible to make sure a string is pre-extended long enough to
4740 receive any result that might be written into a string. You can't use a
4741 string literal (or other read-only string) as an argument to C<syscall>
4742 because Perl has to assume that any string pointer might be written
4744 integer arguments are not literals and have never been interpreted in a
4745 numeric context, you may need to add C<0> to them to force them to look
4746 like numbers. This emulates the C<syswrite> function (or vice versa):
4748 require 'syscall.ph'; # may need to run h2ph
4750 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4752 Note that Perl supports passing of up to only 14 arguments to your system call,
4753 which in practice should usually suffice.
4755 Syscall returns whatever value returned by the system call it calls.
4756 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4757 Note that some system calls can legitimately return C<-1>. The proper
4758 way to handle such calls is to assign C<$!=0;> before the call and
4759 check the value of C<$!> if syscall returns C<-1>.
4761 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4762 number of the read end of the pipe it creates. There is no way
4763 to retrieve the file number of the other end. You can avoid this
4764 problem by using C<pipe> instead.
4766 =item sysopen FILEHANDLE,FILENAME,MODE
4768 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4770 Opens the file whose filename is given by FILENAME, and associates it
4771 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4772 the name of the real filehandle wanted. This function calls the
4773 underlying operating system's C<open> function with the parameters
4774 FILENAME, MODE, PERMS.
4776 The possible values and flag bits of the MODE parameter are
4777 system-dependent; they are available via the standard module C<Fcntl>.
4778 See the documentation of your operating system's C<open> to see which
4779 values and flag bits are available. You may combine several flags
4780 using the C<|>-operator.
4782 Some of the most common values are C<O_RDONLY> for opening the file in
4783 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4784 and C<O_RDWR> for opening the file in read-write mode, and.
4786 For historical reasons, some values work on almost every system
4787 supported by perl: zero means read-only, one means write-only, and two
4788 means read/write. We know that these values do I<not> work under
4789 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4790 use them in new code.
4792 If the file named by FILENAME does not exist and the C<open> call creates
4793 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4794 PERMS specifies the permissions of the newly created file. If you omit
4795 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4796 These permission values need to be in octal, and are modified by your
4797 process's current C<umask>.
4799 In many systems the C<O_EXCL> flag is available for opening files in
4800 exclusive mode. This is B<not> locking: exclusiveness means here that
4801 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4804 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4806 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4807 that takes away the user's option to have a more permissive umask.
4808 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4811 Note that C<sysopen> depends on the fdopen() C library function.
4812 On many UNIX systems, fdopen() is known to fail when file descriptors
4813 exceed a certain value, typically 255. If you need more file
4814 descriptors than that, consider rebuilding Perl to use the C<sfio>
4815 library, or perhaps using the POSIX::open() function.
4817 See L<perlopentut> for a kinder, gentler explanation of opening files.
4819 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4821 =item sysread FILEHANDLE,SCALAR,LENGTH
4823 Attempts to read LENGTH bytes of data into variable SCALAR from the
4824 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4825 so mixing this with other kinds of reads, C<print>, C<write>,
4826 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4827 usually buffers data. Returns the number of bytes actually read, C<0>
4828 at end of file, or undef if there was an error. SCALAR will be grown or
4829 shrunk so that the last byte actually read is the last byte of the
4830 scalar after the read.
4832 An OFFSET may be specified to place the read data at some place in the
4833 string other than the beginning. A negative OFFSET specifies
4834 placement at that many bytes counting backwards from the end of the
4835 string. A positive OFFSET greater than the length of SCALAR results
4836 in the string being padded to the required size with C<"\0"> bytes before
4837 the result of the read is appended.
4839 There is no syseof() function, which is ok, since eof() doesn't work
4840 very well on device files (like ttys) anyway. Use sysread() and check
4841 for a return value for 0 to decide whether you're done.
4843 =item sysseek FILEHANDLE,POSITION,WHENCE
4845 Sets FILEHANDLE's system position using the system call lseek(2). It
4846 bypasses stdio, so mixing this with reads (other than C<sysread>),
4847 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4848 FILEHANDLE may be an expression whose value gives the name of the
4849 filehandle. The values for WHENCE are C<0> to set the new position to
4850 POSITION, C<1> to set the it to the current position plus POSITION,
4851 and C<2> to set it to EOF plus POSITION (typically negative). For
4852 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4853 C<SEEK_END> (start of the file, current position, end of the file)
4854 from the Fcntl module.
4856 Returns the new position, or the undefined value on failure. A position
4857 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4858 true on success and false on failure, yet you can still easily determine
4863 =item system PROGRAM LIST
4865 Does exactly the same thing as C<exec LIST>, except that a fork is
4866 done first, and the parent process waits for the child process to
4867 complete. Note that argument processing varies depending on the
4868 number of arguments. If there is more than one argument in LIST,
4869 or if LIST is an array with more than one value, starts the program
4870 given by the first element of the list with arguments given by the
4871 rest of the list. If there is only one scalar argument, the argument
4872 is checked for shell metacharacters, and if there are any, the
4873 entire argument is passed to the system's command shell for parsing
4874 (this is C</bin/sh -c> on Unix platforms, but varies on other
4875 platforms). If there are no shell metacharacters in the argument,
4876 it is split into words and passed directly to C<execvp>, which is
4879 All files opened for output are flushed before attempting the exec().
4881 The return value is the exit status of the program as
4882 returned by the C<wait> call. To get the actual exit value divide by
4883 256. See also L</exec>. This is I<not> what you want to use to capture
4884 the output from a command, for that you should use merely backticks or
4885 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4886 indicates a failure to start the program (inspect $! for the reason).
4888 Like C<exec>, C<system> allows you to lie to a program about its name if
4889 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4891 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4892 program they're running doesn't actually interrupt your program.
4894 @args = ("command", "arg1", "arg2");
4896 or die "system @args failed: $?"
4898 You can check all the failure possibilities by inspecting
4901 $exit_value = $? >> 8;
4902 $signal_num = $? & 127;
4903 $dumped_core = $? & 128;
4905 When the arguments get executed via the system shell, results
4906 and return codes will be subject to its quirks and capabilities.
4907 See L<perlop/"`STRING`"> and L</exec> for details.
4909 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4911 =item syswrite FILEHANDLE,SCALAR,LENGTH
4913 =item syswrite FILEHANDLE,SCALAR
4915 Attempts to write LENGTH bytes of data from variable SCALAR to the
4916 specified FILEHANDLE, using the system call write(2). If LENGTH
4917 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4918 this with reads (other than C<sysread())>, C<print>, C<write>,
4919 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4920 usually buffers data. Returns the number of bytes actually written,
4921 or C<undef> if there was an error. If the LENGTH is greater than
4922 the available data in the SCALAR after the OFFSET, only as much
4923 data as is available will be written.
4925 An OFFSET may be specified to write the data from some part of the
4926 string other than the beginning. A negative OFFSET specifies writing
4927 that many bytes counting backwards from the end of the string. In the
4928 case the SCALAR is empty you can use OFFSET but only zero offset.
4930 =item tell FILEHANDLE
4934 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4935 expression whose value gives the name of the actual filehandle. If
4936 FILEHANDLE is omitted, assumes the file last read.
4938 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4940 =item telldir DIRHANDLE
4942 Returns the current position of the C<readdir> routines on DIRHANDLE.
4943 Value may be given to C<seekdir> to access a particular location in a
4944 directory. Has the same caveats about possible directory compaction as
4945 the corresponding system library routine.
4947 =item tie VARIABLE,CLASSNAME,LIST
4949 This function binds a variable to a package class that will provide the
4950 implementation for the variable. VARIABLE is the name of the variable
4951 to be enchanted. CLASSNAME is the name of a class implementing objects
4952 of correct type. Any additional arguments are passed to the C<new>
4953 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4954 or C<TIEHASH>). Typically these are arguments such as might be passed
4955 to the C<dbm_open()> function of C. The object returned by the C<new>
4956 method is also returned by the C<tie> function, which would be useful
4957 if you want to access other methods in CLASSNAME.
4959 Note that functions such as C<keys> and C<values> may return huge lists
4960 when used on large objects, like DBM files. You may prefer to use the
4961 C<each> function to iterate over such. Example:
4963 # print out history file offsets
4965 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4966 while (($key,$val) = each %HIST) {
4967 print $key, ' = ', unpack('L',$val), "\n";
4971 A class implementing a hash should have the following methods:
4973 TIEHASH classname, LIST
4975 STORE this, key, value
4980 NEXTKEY this, lastkey
4983 A class implementing an ordinary array should have the following methods:
4985 TIEARRAY classname, LIST
4987 STORE this, key, value
4989 STORESIZE this, count
4995 SPLICE this, offset, length, LIST
4999 A class implementing a file handle should have the following methods:
5001 TIEHANDLE classname, LIST
5002 READ this, scalar, length, offset
5005 WRITE this, scalar, length, offset
5007 PRINTF this, format, LIST
5011 A class implementing a scalar should have the following methods:
5013 TIESCALAR classname, LIST
5018 Not all methods indicated above need be implemented. See L<perltie>,
5019 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5021 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5022 for you--you need to do that explicitly yourself. See L<DB_File>
5023 or the F<Config> module for interesting C<tie> implementations.
5025 For further details see L<perltie>, L<"tied VARIABLE">.
5029 Returns a reference to the object underlying VARIABLE (the same value
5030 that was originally returned by the C<tie> call that bound the variable
5031 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5036 Returns the number of non-leap seconds since whatever time the system
5037 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5038 and 00:00:00 UTC, January 1, 1970 for most other systems).
5039 Suitable for feeding to C<gmtime> and C<localtime>.
5041 For measuring time in better granularity than one second,
5042 you may use either the Time::HiRes module from CPAN, or
5043 if you have gettimeofday(2), you may be able to use the
5044 C<syscall> interface of Perl, see L<perlfaq8> for details.
5048 Returns a four-element list giving the user and system times, in
5049 seconds, for this process and the children of this process.
5051 ($user,$system,$cuser,$csystem) = times;
5055 The transliteration operator. Same as C<y///>. See L<perlop>.
5057 =item truncate FILEHANDLE,LENGTH
5059 =item truncate EXPR,LENGTH
5061 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5062 specified length. Produces a fatal error if truncate isn't implemented
5063 on your system. Returns true if successful, the undefined value
5070 Returns an uppercased version of EXPR. This is the internal function
5071 implementing the C<\U> escape in double-quoted strings.
5072 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5073 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5074 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5076 If EXPR is omitted, uses C<$_>.
5082 Returns the value of EXPR with the first character
5083 in uppercase (titlecase in Unicode). This is
5084 the internal function implementing the C<\u> escape in double-quoted strings.
5085 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5088 If EXPR is omitted, uses C<$_>.
5094 Sets the umask for the process to EXPR and returns the previous value.
5095 If EXPR is omitted, merely returns the current umask.
5097 The Unix permission C<rwxr-x---> is represented as three sets of three
5098 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5099 and isn't one of the digits). The C<umask> value is such a number
5100 representing disabled permissions bits. The permission (or "mode")
5101 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5102 even if you tell C<sysopen> to create a file with permissions C<0777>,
5103 if your umask is C<0022> then the file will actually be created with
5104 permissions C<0755>. If your C<umask> were C<0027> (group can't
5105 write; others can't read, write, or execute), then passing
5106 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5109 Here's some advice: supply a creation mode of C<0666> for regular
5110 files (in C<sysopen>) and one of C<0777> for directories (in
5111 C<mkdir>) and executable files. This gives users the freedom of
5112 choice: if they want protected files, they might choose process umasks
5113 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5114 Programs should rarely if ever make policy decisions better left to
5115 the user. The exception to this is when writing files that should be
5116 kept private: mail files, web browser cookies, I<.rhosts> files, and
5119 If umask(2) is not implemented on your system and you are trying to
5120 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5121 fatal error at run time. If umask(2) is not implemented and you are
5122 not trying to restrict access for yourself, returns C<undef>.
5124 Remember that a umask is a number, usually given in octal; it is I<not> a
5125 string of octal digits. See also L</oct>, if all you have is a string.
5131 Undefines the value of EXPR, which must be an lvalue. Use only on a
5132 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5133 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5134 will probably not do what you expect on most predefined variables or
5135 DBM list values, so don't do that; see L<delete>.) Always returns the
5136 undefined value. You can omit the EXPR, in which case nothing is
5137 undefined, but you still get an undefined value that you could, for
5138 instance, return from a subroutine, assign to a variable or pass as a
5139 parameter. Examples:
5142 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5146 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5147 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5148 select undef, undef, undef, 0.25;
5149 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5151 Note that this is a unary operator, not a list operator.
5157 Deletes a list of files. Returns the number of files successfully
5160 $cnt = unlink 'a', 'b', 'c';
5164 Note: C<unlink> will not delete directories unless you are superuser and
5165 the B<-U> flag is supplied to Perl. Even if these conditions are
5166 met, be warned that unlinking a directory can inflict damage on your
5167 filesystem. Use C<rmdir> instead.
5169 If LIST is omitted, uses C<$_>.
5171 =item unpack TEMPLATE,EXPR
5173 C<unpack> does the reverse of C<pack>: it takes a string
5174 and expands it out into a list of values.
5175 (In scalar context, it returns merely the first value produced.)
5177 The string is broken into chunks described by the TEMPLATE. Each chunk
5178 is converted separately to a value. Typically, either the string is a result
5179 of C<pack>, or the bytes of the string represent a C structure of some
5182 The TEMPLATE has the same format as in the C<pack> function.
5183 Here's a subroutine that does substring:
5186 my($what,$where,$howmuch) = @_;
5187 unpack("x$where a$howmuch", $what);
5192 sub ordinal { unpack("c",$_[0]); } # same as ord()
5194 In addition to fields allowed in pack(), you may prefix a field with
5195 a %E<lt>numberE<gt> to indicate that
5196 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
5197 themselves. Default is a 16-bit checksum. Checksum is calculated by
5198 summing numeric values of expanded values (for string fields the sum of
5199 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5201 For example, the following
5202 computes the same number as the System V sum program:
5206 unpack("%32C*",<>) % 65535;
5209 The following efficiently counts the number of set bits in a bit vector:
5211 $setbits = unpack("%32b*", $selectmask);
5213 The C<p> and C<P> formats should be used with care. Since Perl
5214 has no way of checking whether the value passed to C<unpack()>
5215 corresponds to a valid memory location, passing a pointer value that's
5216 not known to be valid is likely to have disastrous consequences.
5218 If the repeat count of a field is larger than what the remainder of
5219 the input string allows, repeat count is decreased. If the input string
5220 is longer than one described by the TEMPLATE, the rest is ignored.
5222 See L</pack> for more examples and notes.
5224 =item untie VARIABLE
5226 Breaks the binding between a variable and a package. (See C<tie>.)
5228 =item unshift ARRAY,LIST
5230 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5231 depending on how you look at it. Prepends list to the front of the
5232 array, and returns the new number of elements in the array.
5234 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5236 Note the LIST is prepended whole, not one element at a time, so the
5237 prepended elements stay in the same order. Use C<reverse> to do the
5240 =item use Module VERSION LIST
5242 =item use Module VERSION
5244 =item use Module LIST
5250 Imports some semantics into the current package from the named module,
5251 generally by aliasing certain subroutine or variable names into your
5252 package. It is exactly equivalent to
5254 BEGIN { require Module; import Module LIST; }
5256 except that Module I<must> be a bareword.
5258 VERSION, which can be specified as a literal of the form v5.6.0, demands
5259 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5260 as recent as that version. (For compatibility with older versions of Perl,
5261 a numeric literal will also be interpreted as VERSION.) If the version
5262 of the running Perl interpreter is less than VERSION, then an error
5263 message is printed and Perl exits immediately without attempting to
5264 parse the rest of the file. Compare with L</require>, which can do a
5265 similar check at run time.
5267 use v5.6.0; # compile time version check
5268 use 5.005_03; # same, number still supported for compatibility
5270 This is often useful if you need to check the current Perl version before
5271 C<use>ing library modules that have changed in incompatible ways from
5272 older versions of Perl. (We try not to do this more than we have to.)
5274 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5275 C<require> makes sure the module is loaded into memory if it hasn't been
5276 yet. The C<import> is not a builtin--it's just an ordinary static method
5277 call into the C<Module> package to tell the module to import the list of
5278 features back into the current package. The module can implement its
5279 C<import> method any way it likes, though most modules just choose to
5280 derive their C<import> method via inheritance from the C<Exporter> class that
5281 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5282 method can be found then the call is skipped.
5284 If you don't want your namespace altered, explicitly supply an empty list:
5288 That is exactly equivalent to
5290 BEGIN { require Module }
5292 If the VERSION argument is present between Module and LIST, then the
5293 C<use> will call the VERSION method in class Module with the given
5294 version as an argument. The default VERSION method, inherited from
5295 the UNIVERSAL class, croaks if the given version is larger than the
5296 value of the variable C<$Module::VERSION>.
5298 Again, there is a distinction between omitting LIST (C<import> called
5299 with no arguments) and an explicit empty LIST C<()> (C<import> not
5300 called). Note that there is no comma after VERSION!
5302 Because this is a wide-open interface, pragmas (compiler directives)
5303 are also implemented this way. Currently implemented pragmas are:
5307 use sigtrap qw(SEGV BUS);
5308 use strict qw(subs vars refs);
5309 use subs qw(afunc blurfl);
5310 use warnings qw(all);
5312 Some of these pseudo-modules import semantics into the current
5313 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5314 which import symbols into the current package (which are effective
5315 through the end of the file).
5317 There's a corresponding C<no> command that unimports meanings imported
5318 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5324 If no C<unimport> method can be found the call fails with a fatal error.
5326 See L<perlmod> for a list of standard modules and pragmas.
5330 Changes the access and modification times on each file of a list of
5331 files. The first two elements of the list must be the NUMERICAL access
5332 and modification times, in that order. Returns the number of files
5333 successfully changed. The inode change time of each file is set
5334 to the current time. This code has the same effect as the C<touch>
5335 command if the files already exist:
5339 utime $now, $now, @ARGV;
5343 Returns a list consisting of all the values of the named hash. (In a
5344 scalar context, returns the number of values.) The values are
5345 returned in an apparently random order. The actual random order is
5346 subject to change in future versions of perl, but it is guaranteed to
5347 be the same order as either the C<keys> or C<each> function would
5348 produce on the same (unmodified) hash.
5350 Note that you cannot modify the values of a hash this way, because the
5351 returned list is just a copy. You need to use a hash slice for that,
5352 since it's lvaluable in a way that values() is not.
5354 for (values %hash) { s/foo/bar/g } # FAILS!
5355 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5357 As a side effect, calling values() resets the HASH's internal iterator.
5358 See also C<keys>, C<each>, and C<sort>.
5360 =item vec EXPR,OFFSET,BITS
5362 Treats the string in EXPR as a bit vector made up of elements of
5363 width BITS, and returns the value of the element specified by OFFSET
5364 as an unsigned integer. BITS therefore specifies the number of bits
5365 that are reserved for each element in the bit vector. This must
5366 be a power of two from 1 to 32 (or 64, if your platform supports
5369 If BITS is 8, "elements" coincide with bytes of the input string.
5371 If BITS is 16 or more, bytes of the input string are grouped into chunks
5372 of size BITS/8, and each group is converted to a number as with
5373 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5374 for BITS==64). See L<"pack"> for details.
5376 If bits is 4 or less, the string is broken into bytes, then the bits
5377 of each byte are broken into 8/BITS groups. Bits of a byte are
5378 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5379 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5380 breaking the single input byte C<chr(0x36)> into two groups gives a list
5381 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5383 C<vec> may also be assigned to, in which case parentheses are needed
5384 to give the expression the correct precedence as in
5386 vec($image, $max_x * $x + $y, 8) = 3;
5388 If the selected element is off the end of the string, the value 0 is
5389 returned. If an element off the end of the string is written to,
5390 Perl will first extend the string with sufficiently many zero bytes.
5392 Strings created with C<vec> can also be manipulated with the logical
5393 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5394 vector operation is desired when both operands are strings.
5395 See L<perlop/"Bitwise String Operators">.
5397 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5398 The comments show the string after each step. Note that this code works
5399 in the same way on big-endian or little-endian machines.
5402 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5404 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5405 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5407 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5408 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5409 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5410 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5411 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5412 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5414 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5415 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5416 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5419 To transform a bit vector into a string or list of 0's and 1's, use these:
5421 $bits = unpack("b*", $vector);
5422 @bits = split(//, unpack("b*", $vector));
5424 If you know the exact length in bits, it can be used in place of the C<*>.
5426 Here is an example to illustrate how the bits actually fall in place:
5432 unpack("V",$_) 01234567890123456789012345678901
5433 ------------------------------------------------------------------
5438 for ($shift=0; $shift < $width; ++$shift) {
5439 for ($off=0; $off < 32/$width; ++$off) {
5440 $str = pack("B*", "0"x32);
5441 $bits = (1<<$shift);
5442 vec($str, $off, $width) = $bits;
5443 $res = unpack("b*",$str);
5444 $val = unpack("V", $str);
5451 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5452 $off, $width, $bits, $val, $res
5456 Regardless of the machine architecture on which it is run, the above
5457 example should print the following table:
5460 unpack("V",$_) 01234567890123456789012345678901
5461 ------------------------------------------------------------------
5462 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5463 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5464 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5465 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5466 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5467 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5468 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5469 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5470 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5471 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5472 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5473 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5474 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5475 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5476 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5477 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5478 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5479 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5480 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5481 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5482 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5483 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5484 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5485 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5486 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5487 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5488 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5489 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5490 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5491 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5492 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5493 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5494 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5495 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5496 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5497 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5498 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5499 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5500 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5501 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5502 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5503 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5504 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5505 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5506 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5507 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5508 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5509 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5510 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5511 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5512 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5513 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5514 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5515 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5516 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5517 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5518 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5519 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5520 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5521 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5522 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5523 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5524 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5525 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5526 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5527 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5528 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5529 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5530 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5531 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5532 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5533 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5534 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5535 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5536 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5537 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5538 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5539 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5540 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5541 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5542 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5543 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5544 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5545 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5546 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5547 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5548 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5549 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5550 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5551 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5552 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5553 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5554 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5555 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5556 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5557 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5558 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5559 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5560 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5561 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5562 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5563 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5564 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5565 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5566 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5567 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5568 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5569 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5570 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5571 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5572 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5573 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5574 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5575 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5576 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5577 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5578 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5579 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5580 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5581 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5582 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5583 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5584 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5585 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5586 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5587 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5588 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5589 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5593 Behaves like the wait(2) system call on your system: it waits for a child
5594 process to terminate and returns the pid of the deceased process, or
5595 C<-1> if there are no child processes. The status is returned in C<$?>.
5596 Note that a return value of C<-1> could mean that child processes are
5597 being automatically reaped, as described in L<perlipc>.
5599 =item waitpid PID,FLAGS
5601 Waits for a particular child process to terminate and returns the pid of
5602 the deceased process, or C<-1> if there is no such child process. On some
5603 systems, a value of 0 indicates that there are processes still running.
5604 The status is returned in C<$?>. If you say
5606 use POSIX ":sys_wait_h";
5609 $kid = waitpid(-1,&WNOHANG);
5612 then you can do a non-blocking wait for all pending zombie processes.
5613 Non-blocking wait is available on machines supporting either the
5614 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5615 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5616 system call by remembering the status values of processes that have
5617 exited but have not been harvested by the Perl script yet.)
5619 Note that on some systems, a return value of C<-1> could mean that child
5620 processes are being automatically reaped. See L<perlipc> for details,
5621 and for other examples.
5625 Returns true if the context of the currently executing subroutine is
5626 looking for a list value. Returns false if the context is looking
5627 for a scalar. Returns the undefined value if the context is looking
5628 for no value (void context).
5630 return unless defined wantarray; # don't bother doing more
5631 my @a = complex_calculation();
5632 return wantarray ? @a : "@a";
5634 This function should have been named wantlist() instead.
5638 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5641 If LIST is empty and C<$@> already contains a value (typically from a
5642 previous eval) that value is used after appending C<"\t...caught">
5643 to C<$@>. This is useful for staying almost, but not entirely similar to
5646 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5648 No message is printed if there is a C<$SIG{__WARN__}> handler
5649 installed. It is the handler's responsibility to deal with the message
5650 as it sees fit (like, for instance, converting it into a C<die>). Most
5651 handlers must therefore make arrangements to actually display the
5652 warnings that they are not prepared to deal with, by calling C<warn>
5653 again in the handler. Note that this is quite safe and will not
5654 produce an endless loop, since C<__WARN__> hooks are not called from
5657 You will find this behavior is slightly different from that of
5658 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5659 instead call C<die> again to change it).
5661 Using a C<__WARN__> handler provides a powerful way to silence all
5662 warnings (even the so-called mandatory ones). An example:
5664 # wipe out *all* compile-time warnings
5665 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5667 my $foo = 20; # no warning about duplicate my $foo,
5668 # but hey, you asked for it!
5669 # no compile-time or run-time warnings before here
5672 # run-time warnings enabled after here
5673 warn "\$foo is alive and $foo!"; # does show up
5675 See L<perlvar> for details on setting C<%SIG> entries, and for more
5676 examples. See the Carp module for other kinds of warnings using its
5677 carp() and cluck() functions.
5679 =item write FILEHANDLE
5685 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5686 using the format associated with that file. By default the format for
5687 a file is the one having the same name as the filehandle, but the
5688 format for the current output channel (see the C<select> function) may be set
5689 explicitly by assigning the name of the format to the C<$~> variable.
5691 Top of form processing is handled automatically: if there is
5692 insufficient room on the current page for the formatted record, the
5693 page is advanced by writing a form feed, a special top-of-page format
5694 is used to format the new page header, and then the record is written.
5695 By default the top-of-page format is the name of the filehandle with
5696 "_TOP" appended, but it may be dynamically set to the format of your
5697 choice by assigning the name to the C<$^> variable while the filehandle is
5698 selected. The number of lines remaining on the current page is in
5699 variable C<$->, which can be set to C<0> to force a new page.
5701 If FILEHANDLE is unspecified, output goes to the current default output
5702 channel, which starts out as STDOUT but may be changed by the
5703 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5704 is evaluated and the resulting string is used to look up the name of
5705 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5707 Note that write is I<not> the opposite of C<read>. Unfortunately.
5711 The transliteration operator. Same as C<tr///>. See L<perlop>.