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 where the run-time libraries distinguish between binary and
446 text files. If FILEHANDLE is an expression, the value is taken as the
447 name of the filehandle. binmode() should be called after open() but
448 before any I/O is done on the filehandle. The only way to reset
449 binary mode on a filehandle is to reopen the file.
451 On many systems binmode() has no effect, and on some systems it is
452 necessary when you're not working with a text file. For the sake of
453 portability it is a good idea to always use it when appropriate, and
454 to never use it when it isn't appropriate.
456 In other words: Regardless of platform, use binmode() on binary
457 files, and do not use binmode() on text files.
459 The operating system, device drivers, C libraries, and Perl run-time
460 system all work together to let the programmer treat a single
461 character (C<\n>) as the line terminator, irrespective of the external
462 representation. On many operating systems, the native text file
463 representation matches the internal representation, but on some
464 platforms the external representation of C<\n> is made up of more than
467 Mac OS and all variants of Unix use a single character to end each line
468 in the external representation of text (even though that single
469 character is not necessarily the same across these platforms).
470 Consequently binmode() has no effect on these operating systems. In
471 other systems like VMS, MS-DOS and the various flavors of MS-Windows
472 your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
473 files are the two characters C<\cM\cJ>. That means that, if you don't
474 use binmode() on these systems, C<\cM\cJ> sequences on disk will be
475 converted to C<\n> on input, and any C<\n> in your program will be
476 converted back to C<\cM\cJ> on output. This is what you want for text
477 files, but it can be disastrous for binary files.
479 Another consequence of using binmode() (on some systems) is that
480 special end-of-file markers will be seen as part of the data stream.
481 For systems from the Microsoft family this means that if your binary
482 data contains C<\cZ>, the I/O subsystem will ragard it as the end of
483 the file, unless you use binmode().
485 binmode() is not only important for readline() and print() operations,
486 but also when using read(), seek(), sysread(), syswrite() and tell()
487 (see L<perlport> for more details). See the C<$/> and C<$\> variables
488 in L<perlvar> for how to manually set your input and output
489 line-termination sequences.
491 =item bless REF,CLASSNAME
495 This function tells the thingy referenced by REF that it is now an object
496 in the CLASSNAME package. If CLASSNAME is omitted, the current package
497 is used. Because a C<bless> is often the last thing in a constructor,
498 it returns the reference for convenience. Always use the two-argument
499 version if the function doing the blessing might be inherited by a
500 derived class. See L<perltoot> and L<perlobj> for more about the blessing
501 (and blessings) of objects.
503 Consider always blessing objects in CLASSNAMEs that are mixed case.
504 Namespaces with all lowercase names are considered reserved for
505 Perl pragmata. Builtin types have all uppercase names, so to prevent
506 confusion, you may wish to avoid such package names as well. Make sure
507 that CLASSNAME is a true value.
509 See L<perlmod/"Perl Modules">.
515 Returns the context of the current subroutine call. In scalar context,
516 returns the caller's package name if there is a caller, that is, if
517 we're in a subroutine or C<eval> or C<require>, and the undefined value
518 otherwise. In list context, returns
520 ($package, $filename, $line) = caller;
522 With EXPR, it returns some extra information that the debugger uses to
523 print a stack trace. The value of EXPR indicates how many call frames
524 to go back before the current one.
526 ($package, $filename, $line, $subroutine, $hasargs,
527 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
529 Here $subroutine may be C<(eval)> if the frame is not a subroutine
530 call, but an C<eval>. In such a case additional elements $evaltext and
531 C<$is_require> are set: C<$is_require> is true if the frame is created by a
532 C<require> or C<use> statement, $evaltext contains the text of the
533 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
534 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
535 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
536 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
537 was compiled with. The C<$hints> and C<$bitmask> values are subject to
538 change between versions of Perl, and are not meant for external use.
540 Furthermore, when called from within the DB package, caller returns more
541 detailed information: it sets the list variable C<@DB::args> to be the
542 arguments with which the subroutine was invoked.
544 Be aware that the optimizer might have optimized call frames away before
545 C<caller> had a chance to get the information. That means that C<caller(N)>
546 might not return information about the call frame you expect it do, for
547 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
548 previous time C<caller> was called.
552 Changes the working directory to EXPR, if possible. If EXPR is omitted,
553 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
554 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
555 set, C<chdir> does nothing. It returns true upon success, false
556 otherwise. See the example under C<die>.
560 Changes the permissions of a list of files. The first element of the
561 list must be the numerical mode, which should probably be an octal
562 number, and which definitely should I<not> a string of octal digits:
563 C<0644> is okay, C<'0644'> is not. Returns the number of files
564 successfully changed. See also L</oct>, if all you have is a string.
566 $cnt = chmod 0755, 'foo', 'bar';
567 chmod 0755, @executables;
568 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
570 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
571 $mode = 0644; chmod $mode, 'foo'; # this is best
573 You can also import the symbolic C<S_I*> constants from the Fcntl
578 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
579 # This is identical to the chmod 0755 of the above example.
587 This safer version of L</chop> removes any trailing string
588 that corresponds to the current value of C<$/> (also known as
589 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
590 number of characters removed from all its arguments. It's often used to
591 remove the newline from the end of an input record when you're worried
592 that the final record may be missing its newline. When in paragraph
593 mode (C<$/ = "">), it removes all trailing newlines from the string.
594 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
595 a reference to an integer or the like, see L<perlvar>) chomp() won't
597 If VARIABLE is omitted, it chomps C<$_>. Example:
600 chomp; # avoid \n on last field
605 You can actually chomp anything that's an lvalue, including an assignment:
608 chomp($answer = <STDIN>);
610 If you chomp a list, each element is chomped, and the total number of
611 characters removed is returned.
619 Chops off the last character of a string and returns the character
620 chopped. It's used primarily to remove the newline from the end of an
621 input record, but is much more efficient than C<s/\n//> because it neither
622 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
626 chop; # avoid \n on last field
631 You can actually chop anything that's an lvalue, including an assignment:
634 chop($answer = <STDIN>);
636 If you chop a list, each element is chopped. Only the value of the
637 last C<chop> is returned.
639 Note that C<chop> returns the last character. To return all but the last
640 character, use C<substr($string, 0, -1)>.
644 Changes the owner (and group) of a list of files. The first two
645 elements of the list must be the I<numeric> uid and gid, in that
646 order. A value of -1 in either position is interpreted by most
647 systems to leave that value unchanged. Returns the number of files
648 successfully changed.
650 $cnt = chown $uid, $gid, 'foo', 'bar';
651 chown $uid, $gid, @filenames;
653 Here's an example that looks up nonnumeric uids in the passwd file:
656 chomp($user = <STDIN>);
658 chomp($pattern = <STDIN>);
660 ($login,$pass,$uid,$gid) = getpwnam($user)
661 or die "$user not in passwd file";
663 @ary = glob($pattern); # expand filenames
664 chown $uid, $gid, @ary;
666 On most systems, you are not allowed to change the ownership of the
667 file unless you're the superuser, although you should be able to change
668 the group to any of your secondary groups. On insecure systems, these
669 restrictions may be relaxed, but this is not a portable assumption.
670 On POSIX systems, you can detect this condition this way:
672 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
673 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
679 Returns the character represented by that NUMBER in the character set.
680 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
681 chr(0x263a) is a Unicode smiley face (but only within the scope of
682 a C<use utf8>). For the reverse, use L</ord>.
683 See L<utf8> for more about Unicode.
685 If NUMBER is omitted, uses C<$_>.
687 =item chroot FILENAME
691 This function works like the system call by the same name: it makes the
692 named directory the new root directory for all further pathnames that
693 begin with a C</> by your process and all its children. (It doesn't
694 change your current working directory, which is unaffected.) For security
695 reasons, this call is restricted to the superuser. If FILENAME is
696 omitted, does a C<chroot> to C<$_>.
698 =item close FILEHANDLE
702 Closes the file or pipe associated with the file handle, returning true
703 only if stdio successfully flushes buffers and closes the system file
704 descriptor. Closes the currently selected filehandle if the argument
707 You don't have to close FILEHANDLE if you are immediately going to do
708 another C<open> on it, because C<open> will close it for you. (See
709 C<open>.) However, an explicit C<close> on an input file resets the line
710 counter (C<$.>), while the implicit close done by C<open> does not.
712 If the file handle came from a piped open C<close> will additionally
713 return false if one of the other system calls involved fails or if the
714 program exits with non-zero status. (If the only problem was that the
715 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
716 also waits for the process executing on the pipe to complete, in case you
717 want to look at the output of the pipe afterwards, and
718 implicitly puts the exit status value of that command into C<$?>.
720 Prematurely closing the read end of a pipe (i.e. before the process
721 writing to it at the other end has closed it) will result in a
722 SIGPIPE being delivered to the writer. If the other end can't
723 handle that, be sure to read all the data before closing the pipe.
727 open(OUTPUT, '|sort >foo') # pipe to sort
728 or die "Can't start sort: $!";
729 #... # print stuff to output
730 close OUTPUT # wait for sort to finish
731 or warn $! ? "Error closing sort pipe: $!"
732 : "Exit status $? from sort";
733 open(INPUT, 'foo') # get sort's results
734 or die "Can't open 'foo' for input: $!";
736 FILEHANDLE may be an expression whose value can be used as an indirect
737 filehandle, usually the real filehandle name.
739 =item closedir DIRHANDLE
741 Closes a directory opened by C<opendir> and returns the success of that
744 DIRHANDLE may be an expression whose value can be used as an indirect
745 dirhandle, usually the real dirhandle name.
747 =item connect SOCKET,NAME
749 Attempts to connect to a remote socket, just as the connect system call
750 does. Returns true if it succeeded, false otherwise. NAME should be a
751 packed address of the appropriate type for the socket. See the examples in
752 L<perlipc/"Sockets: Client/Server Communication">.
756 Actually a flow control statement rather than a function. If there is a
757 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
758 C<foreach>), it is always executed just before the conditional is about to
759 be evaluated again, just like the third part of a C<for> loop in C. Thus
760 it can be used to increment a loop variable, even when the loop has been
761 continued via the C<next> statement (which is similar to the C C<continue>
764 C<last>, C<next>, or C<redo> may appear within a C<continue>
765 block. C<last> and C<redo> will behave as if they had been executed within
766 the main block. So will C<next>, but since it will execute a C<continue>
767 block, it may be more entertaining.
770 ### redo always comes here
773 ### next always comes here
775 # then back the top to re-check EXPR
777 ### last always comes here
779 Omitting the C<continue> section is semantically equivalent to using an
780 empty one, logically enough. In that case, C<next> goes directly back
781 to check the condition at the top of the loop.
785 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
786 takes cosine of C<$_>.
788 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
789 function, or use this relation:
791 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
793 =item crypt PLAINTEXT,SALT
795 Encrypts a string exactly like the crypt(3) function in the C library
796 (assuming that you actually have a version there that has not been
797 extirpated as a potential munition). This can prove useful for checking
798 the password file for lousy passwords, amongst other things. Only the
799 guys wearing white hats should do this.
801 Note that C<crypt> is intended to be a one-way function, much like breaking
802 eggs to make an omelette. There is no (known) corresponding decrypt
803 function. As a result, this function isn't all that useful for
804 cryptography. (For that, see your nearby CPAN mirror.)
806 When verifying an existing encrypted string you should use the encrypted
807 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
808 allows your code to work with the standard C<crypt> and with more
809 exotic implementations. When choosing a new salt create a random two
810 character string whose characters come from the set C<[./0-9A-Za-z]>
811 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
813 Here's an example that makes sure that whoever runs this program knows
816 $pwd = (getpwuid($<))[1];
820 chomp($word = <STDIN>);
824 if (crypt($word, $pwd) ne $pwd) {
830 Of course, typing in your own password to whoever asks you
833 The L<crypt> function is unsuitable for encrypting large quantities
834 of data, not least of all because you can't get the information
835 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
836 on your favorite CPAN mirror for a slew of potentially useful
841 [This function has been largely superseded by the C<untie> function.]
843 Breaks the binding between a DBM file and a hash.
845 =item dbmopen HASH,DBNAME,MASK
847 [This function has been largely superseded by the C<tie> function.]
849 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
850 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
851 argument is I<not> a filehandle, even though it looks like one). DBNAME
852 is the name of the database (without the F<.dir> or F<.pag> extension if
853 any). If the database does not exist, it is created with protection
854 specified by MASK (as modified by the C<umask>). If your system supports
855 only the older DBM functions, you may perform only one C<dbmopen> in your
856 program. In older versions of Perl, if your system had neither DBM nor
857 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
860 If you don't have write access to the DBM file, you can only read hash
861 variables, not set them. If you want to test whether you can write,
862 either use file tests or try setting a dummy hash entry inside an C<eval>,
863 which will trap the error.
865 Note that functions such as C<keys> and C<values> may return huge lists
866 when used on large DBM files. You may prefer to use the C<each>
867 function to iterate over large DBM files. Example:
869 # print out history file offsets
870 dbmopen(%HIST,'/usr/lib/news/history',0666);
871 while (($key,$val) = each %HIST) {
872 print $key, ' = ', unpack('L',$val), "\n";
876 See also L<AnyDBM_File> for a more general description of the pros and
877 cons of the various dbm approaches, as well as L<DB_File> for a particularly
880 You can control which DBM library you use by loading that library
881 before you call dbmopen():
884 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
885 or die "Can't open netscape history file: $!";
891 Returns a Boolean value telling whether EXPR has a value other than
892 the undefined value C<undef>. If EXPR is not present, C<$_> will be
895 Many operations return C<undef> to indicate failure, end of file,
896 system error, uninitialized variable, and other exceptional
897 conditions. This function allows you to distinguish C<undef> from
898 other values. (A simple Boolean test will not distinguish among
899 C<undef>, zero, the empty string, and C<"0">, which are all equally
900 false.) Note that since C<undef> is a valid scalar, its presence
901 doesn't I<necessarily> indicate an exceptional condition: C<pop>
902 returns C<undef> when its argument is an empty array, I<or> when the
903 element to return happens to be C<undef>.
905 You may also use C<defined(&func)> to check whether subroutine C<&func>
906 has ever been defined. The return value is unaffected by any forward
907 declarations of C<&foo>.
909 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
910 used to report whether memory for that aggregate has ever been
911 allocated. This behavior may disappear in future versions of Perl.
912 You should instead use a simple test for size:
914 if (@an_array) { print "has array elements\n" }
915 if (%a_hash) { print "has hash members\n" }
917 When used on a hash element, it tells you whether the value is defined,
918 not whether the key exists in the hash. Use L</exists> for the latter
923 print if defined $switch{'D'};
924 print "$val\n" while defined($val = pop(@ary));
925 die "Can't readlink $sym: $!"
926 unless defined($value = readlink $sym);
927 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
928 $debugging = 0 unless defined $debugging;
930 Note: Many folks tend to overuse C<defined>, and then are surprised to
931 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
932 defined values. For example, if you say
936 The pattern match succeeds, and C<$1> is defined, despite the fact that it
937 matched "nothing". But it didn't really match nothing--rather, it
938 matched something that happened to be zero characters long. This is all
939 very above-board and honest. When a function returns an undefined value,
940 it's an admission that it couldn't give you an honest answer. So you
941 should use C<defined> only when you're questioning the integrity of what
942 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
945 See also L</undef>, L</exists>, L</ref>.
949 Given an expression that specifies a hash element, array element, hash slice,
950 or array slice, deletes the specified element(s) from the hash or array.
951 In the case of an array, if the array elements happen to be at the end,
952 the size of the array will shrink to the highest element that tests
953 true for exists() (or 0 if no such element exists).
955 Returns each element so deleted or the undefined value if there was no such
956 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
957 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
958 from a C<tie>d hash or array may not necessarily return anything.
960 Deleting an array element effectively returns that position of the array
961 to its initial, uninitialized state. Subsequently testing for the same
962 element with exists() will return false. Note that deleting array
963 elements in the middle of an array will not shift the index of the ones
964 after them down--use splice() for that. See L</exists>.
966 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
968 foreach $key (keys %HASH) {
972 foreach $index (0 .. $#ARRAY) {
973 delete $ARRAY[$index];
978 delete @HASH{keys %HASH};
980 delete @ARRAY[0 .. $#ARRAY];
982 But both of these are slower than just assigning the empty list
983 or undefining %HASH or @ARRAY:
985 %HASH = (); # completely empty %HASH
986 undef %HASH; # forget %HASH ever existed
988 @ARRAY = (); # completely empty @ARRAY
989 undef @ARRAY; # forget @ARRAY ever existed
991 Note that the EXPR can be arbitrarily complicated as long as the final
992 operation is a hash element, array element, hash slice, or array slice
995 delete $ref->[$x][$y]{$key};
996 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
998 delete $ref->[$x][$y][$index];
999 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1003 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1004 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1005 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
1006 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
1007 an C<eval(),> the error message is stuffed into C<$@> and the
1008 C<eval> is terminated with the undefined value. This makes
1009 C<die> the way to raise an exception.
1011 Equivalent examples:
1013 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1014 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1016 If the value of EXPR does not end in a newline, the current script line
1017 number and input line number (if any) are also printed, and a newline
1018 is supplied. Note that the "input line number" (also known as "chunk")
1019 is subject to whatever notion of "line" happens to be currently in
1020 effect, and is also available as the special variable C<$.>.
1021 See L<perlvar/"$/"> and L<perlvar/"$.">.
1023 Hint: sometimes appending C<", stopped"> to your message
1024 will cause it to make better sense when the string C<"at foo line 123"> is
1025 appended. Suppose you are running script "canasta".
1027 die "/etc/games is no good";
1028 die "/etc/games is no good, stopped";
1030 produce, respectively
1032 /etc/games is no good at canasta line 123.
1033 /etc/games is no good, stopped at canasta line 123.
1035 See also exit(), warn(), and the Carp module.
1037 If LIST is empty and C<$@> already contains a value (typically from a
1038 previous eval) that value is reused after appending C<"\t...propagated">.
1039 This is useful for propagating exceptions:
1042 die unless $@ =~ /Expected exception/;
1044 If C<$@> is empty then the string C<"Died"> is used.
1046 die() can also be called with a reference argument. If this happens to be
1047 trapped within an eval(), $@ contains the reference. This behavior permits
1048 a more elaborate exception handling implementation using objects that
1049 maintain arbitary state about the nature of the exception. Such a scheme
1050 is sometimes preferable to matching particular string values of $@ using
1051 regular expressions. Here's an example:
1053 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1055 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1056 # handle Some::Module::Exception
1059 # handle all other possible exceptions
1063 Because perl will stringify uncaught exception messages before displaying
1064 them, you may want to overload stringification operations on such custom
1065 exception objects. See L<overload> for details about that.
1067 You can arrange for a callback to be run just before the C<die>
1068 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1069 handler will be called with the error text and can change the error
1070 message, if it sees fit, by calling C<die> again. See
1071 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1072 L<"eval BLOCK"> for some examples. Although this feature was meant
1073 to be run only right before your program was to exit, this is not
1074 currently the case--the C<$SIG{__DIE__}> hook is currently called
1075 even inside eval()ed blocks/strings! If one wants the hook to do
1076 nothing in such situations, put
1080 as the first line of the handler (see L<perlvar/$^S>). Because
1081 this promotes strange action at a distance, this counterintuitive
1082 behavior may be fixed in a future release.
1086 Not really a function. Returns the value of the last command in the
1087 sequence of commands indicated by BLOCK. When modified by a loop
1088 modifier, executes the BLOCK once before testing the loop condition.
1089 (On other statements the loop modifiers test the conditional first.)
1091 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1092 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1093 See L<perlsyn> for alternative strategies.
1095 =item do SUBROUTINE(LIST)
1097 A deprecated form of subroutine call. See L<perlsub>.
1101 Uses the value of EXPR as a filename and executes the contents of the
1102 file as a Perl script. Its primary use is to include subroutines
1103 from a Perl subroutine library.
1109 scalar eval `cat stat.pl`;
1111 except that it's more efficient and concise, keeps track of the current
1112 filename for error messages, searches the @INC libraries, and updates
1113 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1114 variables. It also differs in that code evaluated with C<do FILENAME>
1115 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1116 same, however, in that it does reparse the file every time you call it,
1117 so you probably don't want to do this inside a loop.
1119 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1120 error. If C<do> can read the file but cannot compile it, it
1121 returns undef and sets an error message in C<$@>. If the file is
1122 successfully compiled, C<do> returns the value of the last expression
1125 Note that inclusion of library modules is better done with the
1126 C<use> and C<require> operators, which also do automatic error checking
1127 and raise an exception if there's a problem.
1129 You might like to use C<do> to read in a program configuration
1130 file. Manual error checking can be done this way:
1132 # read in config files: system first, then user
1133 for $file ("/share/prog/defaults.rc",
1134 "$ENV{HOME}/.someprogrc")
1136 unless ($return = do $file) {
1137 warn "couldn't parse $file: $@" if $@;
1138 warn "couldn't do $file: $!" unless defined $return;
1139 warn "couldn't run $file" unless $return;
1147 This function causes an immediate core dump. See also the B<-u>
1148 command-line switch in L<perlrun>, which does the same thing.
1149 Primarily this is so that you can use the B<undump> program (not
1150 supplied) to turn your core dump into an executable binary after
1151 having initialized all your variables at the beginning of the
1152 program. When the new binary is executed it will begin by executing
1153 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1154 Think of it as a goto with an intervening core dump and reincarnation.
1155 If C<LABEL> is omitted, restarts the program from the top.
1157 B<WARNING>: Any files opened at the time of the dump will I<not>
1158 be open any more when the program is reincarnated, with possible
1159 resulting confusion on the part of Perl.
1161 This function is now largely obsolete, partly because it's very
1162 hard to convert a core file into an executable, and because the
1163 real compiler backends for generating portable bytecode and compilable
1164 C code have superseded it.
1166 If you're looking to use L<dump> to speed up your program, consider
1167 generating bytecode or native C code as described in L<perlcc>. If
1168 you're just trying to accelerate a CGI script, consider using the
1169 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1170 You might also consider autoloading or selfloading, which at least
1171 make your program I<appear> to run faster.
1175 When called in list context, returns a 2-element list consisting of the
1176 key and value for the next element of a hash, so that you can iterate over
1177 it. When called in scalar context, returns the key for only the "next"
1178 element in the hash.
1180 Entries are returned in an apparently random order. The actual random
1181 order is subject to change in future versions of perl, but it is guaranteed
1182 to be in the same order as either the C<keys> or C<values> function
1183 would produce on the same (unmodified) hash.
1185 When the hash is entirely read, a null array is returned in list context
1186 (which when assigned produces a false (C<0>) value), and C<undef> in
1187 scalar context. The next call to C<each> after that will start iterating
1188 again. There is a single iterator for each hash, shared by all C<each>,
1189 C<keys>, and C<values> function calls in the program; it can be reset by
1190 reading all the elements from the hash, or by evaluating C<keys HASH> or
1191 C<values HASH>. If you add or delete elements of a hash while you're
1192 iterating over it, you may get entries skipped or duplicated, so don't.
1194 The following prints out your environment like the printenv(1) program,
1195 only in a different order:
1197 while (($key,$value) = each %ENV) {
1198 print "$key=$value\n";
1201 See also C<keys>, C<values> and C<sort>.
1203 =item eof FILEHANDLE
1209 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1210 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1211 gives the real filehandle. (Note that this function actually
1212 reads a character and then C<ungetc>s it, so isn't very useful in an
1213 interactive context.) Do not read from a terminal file (or call
1214 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1215 as terminals may lose the end-of-file condition if you do.
1217 An C<eof> without an argument uses the last file read. Using C<eof()>
1218 with empty parentheses is very different. It refers to the pseudo file
1219 formed from the files listed on the command line and accessed via the
1220 C<E<lt>E<gt>> operator. Since C<E<lt>E<gt>> isn't explicitly opened,
1221 as a normal filehandle is, an C<eof()> before C<E<lt>E<gt>> has been
1222 used will cause C<@ARGV> to be examined to determine if input is
1225 In a C<while (E<lt>E<gt>)> loop, C<eof> or C<eof(ARGV)> can be used to
1226 detect the end of each file, C<eof()> will only detect the end of the
1227 last file. Examples:
1229 # reset line numbering on each input file
1231 next if /^\s*#/; # skip comments
1234 close ARGV if eof; # Not eof()!
1237 # insert dashes just before last line of last file
1239 if (eof()) { # check for end of current file
1240 print "--------------\n";
1241 close(ARGV); # close or last; is needed if we
1242 # are reading from the terminal
1247 Practical hint: you almost never need to use C<eof> in Perl, because the
1248 input operators typically return C<undef> when they run out of data, or if
1255 In the first form, the return value of EXPR is parsed and executed as if it
1256 were a little Perl program. The value of the expression (which is itself
1257 determined within scalar context) is first parsed, and if there weren't any
1258 errors, executed in the context of the current Perl program, so that any
1259 variable settings or subroutine and format definitions remain afterwards.
1260 Note that the value is parsed every time the eval executes. If EXPR is
1261 omitted, evaluates C<$_>. This form is typically used to delay parsing
1262 and subsequent execution of the text of EXPR until run time.
1264 In the second form, the code within the BLOCK is parsed only once--at the
1265 same time the code surrounding the eval itself was parsed--and executed
1266 within the context of the current Perl program. This form is typically
1267 used to trap exceptions more efficiently than the first (see below), while
1268 also providing the benefit of checking the code within BLOCK at compile
1271 The final semicolon, if any, may be omitted from the value of EXPR or within
1274 In both forms, the value returned is the value of the last expression
1275 evaluated inside the mini-program; a return statement may be also used, just
1276 as with subroutines. The expression providing the return value is evaluated
1277 in void, scalar, or list context, depending on the context of the eval itself.
1278 See L</wantarray> for more on how the evaluation context can be determined.
1280 If there is a syntax error or runtime error, or a C<die> statement is
1281 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1282 error message. If there was no error, C<$@> is guaranteed to be a null
1283 string. Beware that using C<eval> neither silences perl from printing
1284 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1285 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1286 L</warn> and L<perlvar>.
1288 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1289 determining whether a particular feature (such as C<socket> or C<symlink>)
1290 is implemented. It is also Perl's exception trapping mechanism, where
1291 the die operator is used to raise exceptions.
1293 If the code to be executed doesn't vary, you may use the eval-BLOCK
1294 form to trap run-time errors without incurring the penalty of
1295 recompiling each time. The error, if any, is still returned in C<$@>.
1298 # make divide-by-zero nonfatal
1299 eval { $answer = $a / $b; }; warn $@ if $@;
1301 # same thing, but less efficient
1302 eval '$answer = $a / $b'; warn $@ if $@;
1304 # a compile-time error
1305 eval { $answer = }; # WRONG
1308 eval '$answer ='; # sets $@
1310 Due to the current arguably broken state of C<__DIE__> hooks, when using
1311 the C<eval{}> form as an exception trap in libraries, you may wish not
1312 to trigger any C<__DIE__> hooks that user code may have installed.
1313 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1314 as shown in this example:
1316 # a very private exception trap for divide-by-zero
1317 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1320 This is especially significant, given that C<__DIE__> hooks can call
1321 C<die> again, which has the effect of changing their error messages:
1323 # __DIE__ hooks may modify error messages
1325 local $SIG{'__DIE__'} =
1326 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1327 eval { die "foo lives here" };
1328 print $@ if $@; # prints "bar lives here"
1331 Because this promotes action at a distance, this counterintuitive behavior
1332 may be fixed in a future release.
1334 With an C<eval>, you should be especially careful to remember what's
1335 being looked at when:
1341 eval { $x }; # CASE 4
1343 eval "\$$x++"; # CASE 5
1346 Cases 1 and 2 above behave identically: they run the code contained in
1347 the variable $x. (Although case 2 has misleading double quotes making
1348 the reader wonder what else might be happening (nothing is).) Cases 3
1349 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1350 does nothing but return the value of $x. (Case 4 is preferred for
1351 purely visual reasons, but it also has the advantage of compiling at
1352 compile-time instead of at run-time.) Case 5 is a place where
1353 normally you I<would> like to use double quotes, except that in this
1354 particular situation, you can just use symbolic references instead, as
1357 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1358 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1362 =item exec PROGRAM LIST
1364 The C<exec> function executes a system command I<and never returns>--
1365 use C<system> instead of C<exec> if you want it to return. It fails and
1366 returns false only if the command does not exist I<and> it is executed
1367 directly instead of via your system's command shell (see below).
1369 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1370 warns you if there is a following statement which isn't C<die>, C<warn>,
1371 or C<exit> (if C<-w> is set - but you always do that). If you
1372 I<really> want to follow an C<exec> with some other statement, you
1373 can use one of these styles to avoid the warning:
1375 exec ('foo') or print STDERR "couldn't exec foo: $!";
1376 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1378 If there is more than one argument in LIST, or if LIST is an array
1379 with more than one value, calls execvp(3) with the arguments in LIST.
1380 If there is only one scalar argument or an array with one element in it,
1381 the argument is checked for shell metacharacters, and if there are any,
1382 the entire argument is passed to the system's command shell for parsing
1383 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1384 If there are no shell metacharacters in the argument, it is split into
1385 words and passed directly to C<execvp>, which is more efficient.
1388 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1389 exec "sort $outfile | uniq";
1391 If you don't really want to execute the first argument, but want to lie
1392 to the program you are executing about its own name, you can specify
1393 the program you actually want to run as an "indirect object" (without a
1394 comma) in front of the LIST. (This always forces interpretation of the
1395 LIST as a multivalued list, even if there is only a single scalar in
1398 $shell = '/bin/csh';
1399 exec $shell '-sh'; # pretend it's a login shell
1403 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1405 When the arguments get executed via the system shell, results will
1406 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1409 Using an indirect object with C<exec> or C<system> is also more
1410 secure. This usage (which also works fine with system()) forces
1411 interpretation of the arguments as a multivalued list, even if the
1412 list had just one argument. That way you're safe from the shell
1413 expanding wildcards or splitting up words with whitespace in them.
1415 @args = ( "echo surprise" );
1417 exec @args; # subject to shell escapes
1419 exec { $args[0] } @args; # safe even with one-arg list
1421 The first version, the one without the indirect object, ran the I<echo>
1422 program, passing it C<"surprise"> an argument. The second version
1423 didn't--it tried to run a program literally called I<"echo surprise">,
1424 didn't find it, and set C<$?> to a non-zero value indicating failure.
1426 Note that C<exec> will not call your C<END> blocks, nor will it call
1427 any C<DESTROY> methods in your objects.
1431 Given an expression that specifies a hash element or array element,
1432 returns true if the specified element in the hash or array has ever
1433 been initialized, even if the corresponding value is undefined. The
1434 element is not autovivified if it doesn't exist.
1436 print "Exists\n" if exists $hash{$key};
1437 print "Defined\n" if defined $hash{$key};
1438 print "True\n" if $hash{$key};
1440 print "Exists\n" if exists $array[$index];
1441 print "Defined\n" if defined $array[$index];
1442 print "True\n" if $array[$index];
1444 A hash or array element can be true only if it's defined, and defined if
1445 it exists, but the reverse doesn't necessarily hold true.
1447 Given an expression that specifies the name of a subroutine,
1448 returns true if the specified subroutine has ever been declared, even
1449 if it is undefined. Mentioning a subroutine name for exists or defined
1450 does not count as declaring it.
1452 print "Exists\n" if exists &subroutine;
1453 print "Defined\n" if defined &subroutine;
1455 Note that the EXPR can be arbitrarily complicated as long as the final
1456 operation is a hash or array key lookup or subroutine name:
1458 if (exists $ref->{A}->{B}->{$key}) { }
1459 if (exists $hash{A}{B}{$key}) { }
1461 if (exists $ref->{A}->{B}->[$ix]) { }
1462 if (exists $hash{A}{B}[$ix]) { }
1464 if (exists &{$ref->{A}{B}{$key}}) { }
1466 Although the deepest nested array or hash will not spring into existence
1467 just because its existence was tested, any intervening ones will.
1468 Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring
1469 into existence due to the existence test for the $key element above.
1470 This happens anywhere the arrow operator is used, including even:
1473 if (exists $ref->{"Some key"}) { }
1474 print $ref; # prints HASH(0x80d3d5c)
1476 This surprising autovivification in what does not at first--or even
1477 second--glance appear to be an lvalue context may be fixed in a future
1480 See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
1481 used on a pseudo-hash.
1483 Use of a subroutine call, rather than a subroutine name, as an argument
1484 to exists() is an error.
1487 exists &sub(); # Error
1491 Evaluates EXPR and exits immediately with that value. Example:
1494 exit 0 if $ans =~ /^[Xx]/;
1496 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1497 universally recognized values for EXPR are C<0> for success and C<1>
1498 for error; other values are subject to interpretation depending on the
1499 environment in which the Perl program is running. For example, exiting
1500 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1501 the mailer to return the item undelivered, but that's not true everywhere.
1503 Don't use C<exit> to abort a subroutine if there's any chance that
1504 someone might want to trap whatever error happened. Use C<die> instead,
1505 which can be trapped by an C<eval>.
1507 The exit() function does not always exit immediately. It calls any
1508 defined C<END> routines first, but these C<END> routines may not
1509 themselves abort the exit. Likewise any object destructors that need to
1510 be called are called before the real exit. If this is a problem, you
1511 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1512 See L<perlmod> for details.
1518 Returns I<e> (the natural logarithm base) to the power of EXPR.
1519 If EXPR is omitted, gives C<exp($_)>.
1521 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1523 Implements the fcntl(2) function. You'll probably have to say
1527 first to get the correct constant definitions. Argument processing and
1528 value return works just like C<ioctl> below.
1532 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1533 or die "can't fcntl F_GETFL: $!";
1535 You don't have to check for C<defined> on the return from C<fnctl>.
1536 Like C<ioctl>, it maps a C<0> return from the system call into
1537 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1538 in numeric context. It is also exempt from the normal B<-w> warnings
1539 on improper numeric conversions.
1541 Note that C<fcntl> will produce a fatal error if used on a machine that
1542 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1543 manpage to learn what functions are available on your system.
1545 =item fileno FILEHANDLE
1547 Returns the file descriptor for a filehandle, or undefined if the
1548 filehandle is not open. This is mainly useful for constructing
1549 bitmaps for C<select> and low-level POSIX tty-handling operations.
1550 If FILEHANDLE is an expression, the value is taken as an indirect
1551 filehandle, generally its name.
1553 You can use this to find out whether two handles refer to the
1554 same underlying descriptor:
1556 if (fileno(THIS) == fileno(THAT)) {
1557 print "THIS and THAT are dups\n";
1560 =item flock FILEHANDLE,OPERATION
1562 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1563 for success, false on failure. Produces a fatal error if used on a
1564 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1565 C<flock> is Perl's portable file locking interface, although it locks
1566 only entire files, not records.
1568 Two potentially non-obvious but traditional C<flock> semantics are
1569 that it waits indefinitely until the lock is granted, and that its locks
1570 B<merely advisory>. Such discretionary locks are more flexible, but offer
1571 fewer guarantees. This means that files locked with C<flock> may be
1572 modified by programs that do not also use C<flock>. See L<perlport>,
1573 your port's specific documentation, or your system-specific local manpages
1574 for details. It's best to assume traditional behavior if you're writing
1575 portable programs. (But if you're not, you should as always feel perfectly
1576 free to write for your own system's idiosyncrasies (sometimes called
1577 "features"). Slavish adherence to portability concerns shouldn't get
1578 in the way of your getting your job done.)
1580 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1581 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1582 you can use the symbolic names if you import them from the Fcntl module,
1583 either individually, or as a group using the ':flock' tag. LOCK_SH
1584 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1585 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1586 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1587 waiting for the lock (check the return status to see if you got it).
1589 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1590 before locking or unlocking it.
1592 Note that the emulation built with lockf(3) doesn't provide shared
1593 locks, and it requires that FILEHANDLE be open with write intent. These
1594 are the semantics that lockf(3) implements. Most if not all systems
1595 implement lockf(3) in terms of fcntl(2) locking, though, so the
1596 differing semantics shouldn't bite too many people.
1598 Note also that some versions of C<flock> cannot lock things over the
1599 network; you would need to use the more system-specific C<fcntl> for
1600 that. If you like you can force Perl to ignore your system's flock(2)
1601 function, and so provide its own fcntl(2)-based emulation, by passing
1602 the switch C<-Ud_flock> to the F<Configure> program when you configure
1605 Here's a mailbox appender for BSD systems.
1607 use Fcntl ':flock'; # import LOCK_* constants
1610 flock(MBOX,LOCK_EX);
1611 # and, in case someone appended
1612 # while we were waiting...
1617 flock(MBOX,LOCK_UN);
1620 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1621 or die "Can't open mailbox: $!";
1624 print MBOX $msg,"\n\n";
1627 On systems that support a real flock(), locks are inherited across fork()
1628 calls, whereas those that must resort to the more capricious fcntl()
1629 function lose the locks, making it harder to write servers.
1631 See also L<DB_File> for other flock() examples.
1635 Does a fork(2) system call to create a new process running the
1636 same program at the same point. It returns the child pid to the
1637 parent process, C<0> to the child process, or C<undef> if the fork is
1638 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1639 are shared, while everything else is copied. On most systems supporting
1640 fork(), great care has gone into making it extremely efficient (for
1641 example, using copy-on-write technology on data pages), making it the
1642 dominant paradigm for multitasking over the last few decades.
1644 All files opened for output are flushed before forking the child process.
1646 If you C<fork> without ever waiting on your children, you will
1647 accumulate zombies. On some systems, you can avoid this by setting
1648 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1649 forking and reaping moribund children.
1651 Note that if your forked child inherits system file descriptors like
1652 STDIN and STDOUT that are actually connected by a pipe or socket, even
1653 if you exit, then the remote server (such as, say, a CGI script or a
1654 backgrounded job launched from a remote shell) won't think you're done.
1655 You should reopen those to F</dev/null> if it's any issue.
1659 Declare a picture format for use by the C<write> function. For
1663 Test: @<<<<<<<< @||||| @>>>>>
1664 $str, $%, '$' . int($num)
1668 $num = $cost/$quantity;
1672 See L<perlform> for many details and examples.
1674 =item formline PICTURE,LIST
1676 This is an internal function used by C<format>s, though you may call it,
1677 too. It formats (see L<perlform>) a list of values according to the
1678 contents of PICTURE, placing the output into the format output
1679 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1680 Eventually, when a C<write> is done, the contents of
1681 C<$^A> are written to some filehandle, but you could also read C<$^A>
1682 yourself and then set C<$^A> back to C<"">. Note that a format typically
1683 does one C<formline> per line of form, but the C<formline> function itself
1684 doesn't care how many newlines are embedded in the PICTURE. This means
1685 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1686 You may therefore need to use multiple formlines to implement a single
1687 record format, just like the format compiler.
1689 Be careful if you put double quotes around the picture, because an C<@>
1690 character may be taken to mean the beginning of an array name.
1691 C<formline> always returns true. See L<perlform> for other examples.
1693 =item getc FILEHANDLE
1697 Returns the next character from the input file attached to FILEHANDLE,
1698 or the undefined value at end of file, or if there was an error.
1699 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1700 efficient. However, it cannot be used by itself to fetch single
1701 characters without waiting for the user to hit enter. For that, try
1702 something more like:
1705 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1708 system "stty", '-icanon', 'eol', "\001";
1714 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1717 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1721 Determination of whether $BSD_STYLE should be set
1722 is left as an exercise to the reader.
1724 The C<POSIX::getattr> function can do this more portably on
1725 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1726 module from your nearest CPAN site; details on CPAN can be found on
1731 Implements the C library function of the same name, which on most
1732 systems returns the current login from F</etc/utmp>, if any. If null,
1735 $login = getlogin || getpwuid($<) || "Kilroy";
1737 Do not consider C<getlogin> for authentication: it is not as
1738 secure as C<getpwuid>.
1740 =item getpeername SOCKET
1742 Returns the packed sockaddr address of other end of the SOCKET connection.
1745 $hersockaddr = getpeername(SOCK);
1746 ($port, $iaddr) = sockaddr_in($hersockaddr);
1747 $herhostname = gethostbyaddr($iaddr, AF_INET);
1748 $herstraddr = inet_ntoa($iaddr);
1752 Returns the current process group for the specified PID. Use
1753 a PID of C<0> to get the current process group for the
1754 current process. Will raise an exception if used on a machine that
1755 doesn't implement getpgrp(2). If PID is omitted, returns process
1756 group of current process. Note that the POSIX version of C<getpgrp>
1757 does not accept a PID argument, so only C<PID==0> is truly portable.
1761 Returns the process id of the parent process.
1763 =item getpriority WHICH,WHO
1765 Returns the current priority for a process, a process group, or a user.
1766 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1767 machine that doesn't implement getpriority(2).
1773 =item gethostbyname NAME
1775 =item getnetbyname NAME
1777 =item getprotobyname NAME
1783 =item getservbyname NAME,PROTO
1785 =item gethostbyaddr ADDR,ADDRTYPE
1787 =item getnetbyaddr ADDR,ADDRTYPE
1789 =item getprotobynumber NUMBER
1791 =item getservbyport PORT,PROTO
1809 =item sethostent STAYOPEN
1811 =item setnetent STAYOPEN
1813 =item setprotoent STAYOPEN
1815 =item setservent STAYOPEN
1829 These routines perform the same functions as their counterparts in the
1830 system library. In list context, the return values from the
1831 various get routines are as follows:
1833 ($name,$passwd,$uid,$gid,
1834 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1835 ($name,$passwd,$gid,$members) = getgr*
1836 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1837 ($name,$aliases,$addrtype,$net) = getnet*
1838 ($name,$aliases,$proto) = getproto*
1839 ($name,$aliases,$port,$proto) = getserv*
1841 (If the entry doesn't exist you get a null list.)
1843 In scalar context, you get the name, unless the function was a
1844 lookup by name, in which case you get the other thing, whatever it is.
1845 (If the entry doesn't exist you get the undefined value.) For example:
1847 $uid = getpwnam($name);
1848 $name = getpwuid($num);
1850 $gid = getgrnam($name);
1851 $name = getgrgid($num;
1855 In I<getpw*()> the fields $quota, $comment, and $expire are
1856 special cases in the sense that in many systems they are unsupported.
1857 If the $quota is unsupported, it is an empty scalar. If it is
1858 supported, it usually encodes the disk quota. If the $comment
1859 field is unsupported, it is an empty scalar. If it is supported it
1860 usually encodes some administrative comment about the user. In some
1861 systems the $quota field may be $change or $age, fields that have
1862 to do with password aging. In some systems the $comment field may
1863 be $class. The $expire field, if present, encodes the expiration
1864 period of the account or the password. For the availability and the
1865 exact meaning of these fields in your system, please consult your
1866 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1867 out from within Perl what your $quota and $comment fields mean
1868 and whether you have the $expire field by using the C<Config> module
1869 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1870 and C<d_pwexpire>. Shadow password files are only supported if your
1871 vendor has implemented them in the intuitive fashion that calling the
1872 regular C library routines gets the shadow versions if you're running
1873 under privilege. Those that incorrectly implement a separate library
1874 call are not supported.
1876 The $members value returned by I<getgr*()> is a space separated list of
1877 the login names of the members of the group.
1879 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1880 C, it will be returned to you via C<$?> if the function call fails. The
1881 C<@addrs> value returned by a successful call is a list of the raw
1882 addresses returned by the corresponding system library call. In the
1883 Internet domain, each address is four bytes long and you can unpack it
1884 by saying something like:
1886 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1888 The Socket library makes this slightly easier:
1891 $iaddr = inet_aton("127.1"); # or whatever address
1892 $name = gethostbyaddr($iaddr, AF_INET);
1894 # or going the other way
1895 $straddr = inet_ntoa($iaddr);
1897 If you get tired of remembering which element of the return list
1898 contains which return value, by-name interfaces are provided
1899 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1900 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1901 and C<User::grent>. These override the normal built-ins, supplying
1902 versions that return objects with the appropriate names
1903 for each field. For example:
1907 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1909 Even though it looks like they're the same method calls (uid),
1910 they aren't, because a C<File::stat> object is different from
1911 a C<User::pwent> object.
1913 =item getsockname SOCKET
1915 Returns the packed sockaddr address of this end of the SOCKET connection,
1916 in case you don't know the address because you have several different
1917 IPs that the connection might have come in on.
1920 $mysockaddr = getsockname(SOCK);
1921 ($port, $myaddr) = sockaddr_in($mysockaddr);
1922 printf "Connect to %s [%s]\n",
1923 scalar gethostbyaddr($myaddr, AF_INET),
1926 =item getsockopt SOCKET,LEVEL,OPTNAME
1928 Returns the socket option requested, or undef if there is an error.
1934 Returns the value of EXPR with filename expansions such as the
1935 standard Unix shell F</bin/csh> would do. This is the internal function
1936 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1937 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1938 discussed in more detail in L<perlop/"I/O Operators">.
1942 Converts a time as returned by the time function to a 9-element list
1943 with the time localized for the standard Greenwich time zone.
1944 Typically used as follows:
1947 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1950 All list elements are numeric, and come straight out of a struct tm.
1951 In particular this means that $mon has the range C<0..11> and $wday
1952 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1953 number of years since 1900, that is, $year is C<123> in year 2023,
1954 I<not> simply the last two digits of the year. If you assume it is,
1955 then you create non-Y2K-compliant programs--and you wouldn't want to do
1958 The proper way to get a complete 4-digit year is simply:
1962 And to get the last two digits of the year (e.g., '01' in 2001) do:
1964 $year = sprintf("%02d", $year % 100);
1966 If EXPR is omitted, does C<gmtime(time())>.
1968 In scalar context, returns the ctime(3) value:
1970 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1972 Also see the C<timegm> function provided by the C<Time::Local> module,
1973 and the strftime(3) function available via the POSIX module.
1975 This scalar value is B<not> locale dependent (see L<perllocale>), but
1976 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1977 strftime(3) and mktime(3) functions available via the POSIX module. To
1978 get somewhat similar but locale dependent date strings, set up your
1979 locale environment variables appropriately (please see L<perllocale>)
1980 and try for example:
1982 use POSIX qw(strftime);
1983 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1985 Note that the C<%a> and C<%b> escapes, which represent the short forms
1986 of the day of the week and the month of the year, may not necessarily
1987 be three characters wide in all locales.
1995 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1996 execution there. It may not be used to go into any construct that
1997 requires initialization, such as a subroutine or a C<foreach> loop. It
1998 also can't be used to go into a construct that is optimized away,
1999 or to get out of a block or subroutine given to C<sort>.
2000 It can be used to go almost anywhere else within the dynamic scope,
2001 including out of subroutines, but it's usually better to use some other
2002 construct such as C<last> or C<die>. The author of Perl has never felt the
2003 need to use this form of C<goto> (in Perl, that is--C is another matter).
2005 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2006 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2007 necessarily recommended if you're optimizing for maintainability:
2009 goto ("FOO", "BAR", "GLARCH")[$i];
2011 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2012 In fact, it isn't a goto in the normal sense at all, and doesn't have
2013 the stigma associated with other gotos. Instead, it
2014 substitutes a call to the named subroutine for the currently running
2015 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2016 another subroutine and then pretend that the other subroutine had been
2017 called in the first place (except that any modifications to C<@_>
2018 in the current subroutine are propagated to the other subroutine.)
2019 After the C<goto>, not even C<caller> will be able to tell that this
2020 routine was called first.
2022 NAME needn't be the name of a subroutine; it can be a scalar variable
2023 containing a code reference, or a block which evaluates to a code
2026 =item grep BLOCK LIST
2028 =item grep EXPR,LIST
2030 This is similar in spirit to, but not the same as, grep(1) and its
2031 relatives. In particular, it is not limited to using regular expressions.
2033 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2034 C<$_> to each element) and returns the list value consisting of those
2035 elements for which the expression evaluated to true. In scalar
2036 context, returns the number of times the expression was true.
2038 @foo = grep(!/^#/, @bar); # weed out comments
2042 @foo = grep {!/^#/} @bar; # weed out comments
2044 Note that, because C<$_> is a reference into the list value, it can
2045 be used to modify the elements of the array. While this is useful and
2046 supported, it can cause bizarre results if the LIST is not a named array.
2047 Similarly, grep returns aliases into the original list, much as a for
2048 loop's index variable aliases the list elements. That is, modifying an
2049 element of a list returned by grep (for example, in a C<foreach>, C<map>
2050 or another C<grep>) actually modifies the element in the original list.
2051 This is usually something to be avoided when writing clear code.
2053 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2059 Interprets EXPR as a hex string and returns the corresponding value.
2060 (To convert strings that might start with either 0, 0x, or 0b, see
2061 L</oct>.) If EXPR is omitted, uses C<$_>.
2063 print hex '0xAf'; # prints '175'
2064 print hex 'aF'; # same
2066 Hex strings may only represent integers. Strings that would cause
2067 integer overflow trigger a warning.
2071 There is no builtin C<import> function. It is just an ordinary
2072 method (subroutine) defined (or inherited) by modules that wish to export
2073 names to another module. The C<use> function calls the C<import> method
2074 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2076 =item index STR,SUBSTR,POSITION
2078 =item index STR,SUBSTR
2080 The index function searches for one string within another, but without
2081 the wildcard-like behavior of a full regular-expression pattern match.
2082 It returns the position of the first occurrence of SUBSTR in STR at
2083 or after POSITION. If POSITION is omitted, starts searching from the
2084 beginning of the string. The return value is based at C<0> (or whatever
2085 you've set the C<$[> variable to--but don't do that). If the substring
2086 is not found, returns one less than the base, ordinarily C<-1>.
2092 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2093 You should not use this function for rounding: one because it truncates
2094 towards C<0>, and two because machine representations of floating point
2095 numbers can sometimes produce counterintuitive results. For example,
2096 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2097 because it's really more like -268.99999999999994315658 instead. Usually,
2098 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2099 functions will serve you better than will int().
2101 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2103 Implements the ioctl(2) function. You'll probably first have to say
2105 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2107 to get the correct function definitions. If F<ioctl.ph> doesn't
2108 exist or doesn't have the correct definitions you'll have to roll your
2109 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2110 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2111 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2112 written depending on the FUNCTION--a pointer to the string value of SCALAR
2113 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2114 has no string value but does have a numeric value, that value will be
2115 passed rather than a pointer to the string value. To guarantee this to be
2116 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2117 functions may be needed to manipulate the values of structures used by
2120 The return value of C<ioctl> (and C<fcntl>) is as follows:
2122 if OS returns: then Perl returns:
2124 0 string "0 but true"
2125 anything else that number
2127 Thus Perl returns true on success and false on failure, yet you can
2128 still easily determine the actual value returned by the operating
2131 $retval = ioctl(...) || -1;
2132 printf "System returned %d\n", $retval;
2134 The special string "C<0> but true" is exempt from B<-w> complaints
2135 about improper numeric conversions.
2137 Here's an example of setting a filehandle named C<REMOTE> to be
2138 non-blocking at the system level. You'll have to negotiate C<$|>
2139 on your own, though.
2141 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2143 $flags = fcntl(REMOTE, F_GETFL, 0)
2144 or die "Can't get flags for the socket: $!\n";
2146 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2147 or die "Can't set flags for the socket: $!\n";
2149 =item join EXPR,LIST
2151 Joins the separate strings of LIST into a single string with fields
2152 separated by the value of EXPR, and returns that new string. Example:
2154 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2156 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2157 first argument. Compare L</split>.
2161 Returns a list consisting of all the keys of the named hash. (In
2162 scalar context, returns the number of keys.) The keys are returned in
2163 an apparently random order. The actual random order is subject to
2164 change in future versions of perl, but it is guaranteed to be the same
2165 order as either the C<values> or C<each> function produces (given
2166 that the hash has not been modified). As a side effect, it resets
2169 Here is yet another way to print your environment:
2172 @values = values %ENV;
2174 print pop(@keys), '=', pop(@values), "\n";
2177 or how about sorted by key:
2179 foreach $key (sort(keys %ENV)) {
2180 print $key, '=', $ENV{$key}, "\n";
2183 To sort a hash by value, you'll need to use a C<sort> function.
2184 Here's a descending numeric sort of a hash by its values:
2186 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2187 printf "%4d %s\n", $hash{$key}, $key;
2190 As an lvalue C<keys> allows you to increase the number of hash buckets
2191 allocated for the given hash. This can gain you a measure of efficiency if
2192 you know the hash is going to get big. (This is similar to pre-extending
2193 an array by assigning a larger number to $#array.) If you say
2197 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2198 in fact, since it rounds up to the next power of two. These
2199 buckets will be retained even if you do C<%hash = ()>, use C<undef
2200 %hash> if you want to free the storage while C<%hash> is still in scope.
2201 You can't shrink the number of buckets allocated for the hash using
2202 C<keys> in this way (but you needn't worry about doing this by accident,
2203 as trying has no effect).
2205 See also C<each>, C<values> and C<sort>.
2207 =item kill SIGNAL, LIST
2209 Sends a signal to a list of processes. Returns the number of
2210 processes successfully signaled (which is not necessarily the
2211 same as the number actually killed).
2213 $cnt = kill 1, $child1, $child2;
2216 If SIGNAL is zero, no signal is sent to the process. This is a
2217 useful way to check that the process is alive and hasn't changed
2218 its UID. See L<perlport> for notes on the portability of this
2221 Unlike in the shell, if SIGNAL is negative, it kills
2222 process groups instead of processes. (On System V, a negative I<PROCESS>
2223 number will also kill process groups, but that's not portable.) That
2224 means you usually want to use positive not negative signals. You may also
2225 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2231 The C<last> command is like the C<break> statement in C (as used in
2232 loops); it immediately exits the loop in question. If the LABEL is
2233 omitted, the command refers to the innermost enclosing loop. The
2234 C<continue> block, if any, is not executed:
2236 LINE: while (<STDIN>) {
2237 last LINE if /^$/; # exit when done with header
2241 C<last> cannot be used to exit a block which returns a value such as
2242 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2243 a grep() or map() operation.
2245 Note that a block by itself is semantically identical to a loop
2246 that executes once. Thus C<last> can be used to effect an early
2247 exit out of such a block.
2249 See also L</continue> for an illustration of how C<last>, C<next>, and
2256 Returns an lowercased version of EXPR. This is the internal function
2257 implementing the C<\L> escape in double-quoted strings.
2258 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2261 If EXPR is omitted, uses C<$_>.
2267 Returns the value of EXPR with the first character lowercased. This is
2268 the internal function implementing the C<\l> escape in double-quoted strings.
2269 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2271 If EXPR is omitted, uses C<$_>.
2277 Returns the length in characters of the value of EXPR. If EXPR is
2278 omitted, returns length of C<$_>. Note that this cannot be used on
2279 an entire array or hash to find out how many elements these have.
2280 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2282 =item link OLDFILE,NEWFILE
2284 Creates a new filename linked to the old filename. Returns true for
2285 success, false otherwise.
2287 =item listen SOCKET,QUEUESIZE
2289 Does the same thing that the listen system call does. Returns true if
2290 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2294 You really probably want to be using C<my> instead, because C<local> isn't
2295 what most people think of as "local". See L<perlsub/"Private Variables
2296 via my()"> for details.
2298 A local modifies the listed variables to be local to the enclosing
2299 block, file, or eval. If more than one value is listed, the list must
2300 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2301 for details, including issues with tied arrays and hashes.
2303 =item localtime EXPR
2305 Converts a time as returned by the time function to a 9-element list
2306 with the time analyzed for the local time zone. Typically used as
2310 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2313 All list elements are numeric, and come straight out of a struct tm.
2314 In particular this means that $mon has the range C<0..11> and $wday
2315 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2316 number of years since 1900, that is, $year is C<123> in year 2023,
2317 and I<not> simply the last two digits of the year. If you assume it is,
2318 then you create non-Y2K-compliant programs--and you wouldn't want to do
2321 The proper way to get a complete 4-digit year is simply:
2325 And to get the last two digits of the year (e.g., '01' in 2001) do:
2327 $year = sprintf("%02d", $year % 100);
2329 If EXPR is omitted, uses the current time (C<localtime(time)>).
2331 In scalar context, returns the ctime(3) value:
2333 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2335 This scalar value is B<not> locale dependent, see L<perllocale>, but
2336 instead a Perl builtin. Also see the C<Time::Local> module
2337 (to convert the second, minutes, hours, ... back to seconds since the
2338 stroke of midnight the 1st of January 1970, the value returned by
2339 time()), and the strftime(3) and mktime(3) functions available via the
2340 POSIX module. To get somewhat similar but locale dependent date
2341 strings, set up your locale environment variables appropriately
2342 (please see L<perllocale>) and try for example:
2344 use POSIX qw(strftime);
2345 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2347 Note that the C<%a> and C<%b>, the short forms of the day of the week
2348 and the month of the year, may not necessarily be three characters wide.
2354 This function places an advisory lock on a variable, subroutine,
2355 or referenced object contained in I<THING> until the lock goes out
2356 of scope. This is a built-in function only if your version of Perl
2357 was built with threading enabled, and if you've said C<use Threads>.
2358 Otherwise a user-defined function by this name will be called. See
2365 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2366 returns log of C<$_>. To get the log of another base, use basic algebra:
2367 The base-N log of a number is equal to the natural log of that number
2368 divided by the natural log of N. For example:
2372 return log($n)/log(10);
2375 See also L</exp> for the inverse operation.
2377 =item lstat FILEHANDLE
2383 Does the same thing as the C<stat> function (including setting the
2384 special C<_> filehandle) but stats a symbolic link instead of the file
2385 the symbolic link points to. If symbolic links are unimplemented on
2386 your system, a normal C<stat> is done.
2388 If EXPR is omitted, stats C<$_>.
2392 The match operator. See L<perlop>.
2394 =item map BLOCK LIST
2398 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2399 C<$_> to each element) and returns the list value composed of the
2400 results of each such evaluation. In scalar context, returns the
2401 total number of elements so generated. Evaluates BLOCK or EXPR in
2402 list context, so each element of LIST may produce zero, one, or
2403 more elements in the returned value.
2405 @chars = map(chr, @nums);
2407 translates a list of numbers to the corresponding characters. And
2409 %hash = map { getkey($_) => $_ } @array;
2411 is just a funny way to write
2414 foreach $_ (@array) {
2415 $hash{getkey($_)} = $_;
2418 Note that, because C<$_> is a reference into the list value, it can
2419 be used to modify the elements of the array. While this is useful and
2420 supported, it can cause bizarre results if the LIST is not a named array.
2421 Using a regular C<foreach> loop for this purpose would be clearer in
2422 most cases. See also L</grep> for an array composed of those items of
2423 the original list for which the BLOCK or EXPR evaluates to true.
2425 =item mkdir FILENAME,MASK
2427 =item mkdir FILENAME
2429 Creates the directory specified by FILENAME, with permissions
2430 specified by MASK (as modified by C<umask>). If it succeeds it
2431 returns true, otherwise it returns false and sets C<$!> (errno).
2432 If omitted, MASK defaults to 0777.
2434 In general, it is better to create directories with permissive MASK,
2435 and let the user modify that with their C<umask>, than it is to supply
2436 a restrictive MASK and give the user no way to be more permissive.
2437 The exceptions to this rule are when the file or directory should be
2438 kept private (mail files, for instance). The perlfunc(1) entry on
2439 C<umask> discusses the choice of MASK in more detail.
2441 =item msgctl ID,CMD,ARG
2443 Calls the System V IPC function msgctl(2). You'll probably have to say
2447 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2448 then ARG must be a variable which will hold the returned C<msqid_ds>
2449 structure. Returns like C<ioctl>: the undefined value for error,
2450 C<"0 but true"> for zero, or the actual return value otherwise. See also
2451 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2453 =item msgget KEY,FLAGS
2455 Calls the System V IPC function msgget(2). Returns the message queue
2456 id, or the undefined value if there is an error. See also C<IPC::SysV>
2457 and C<IPC::Msg> documentation.
2459 =item msgsnd ID,MSG,FLAGS
2461 Calls the System V IPC function msgsnd to send the message MSG to the
2462 message queue ID. MSG must begin with the long integer message type,
2463 which may be created with C<pack("l", $type)>. Returns true if
2464 successful, or false if there is an error. See also C<IPC::SysV>
2465 and C<IPC::SysV::Msg> documentation.
2467 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2469 Calls the System V IPC function msgrcv to receive a message from
2470 message queue ID into variable VAR with a maximum message size of
2471 SIZE. Note that if a message is received, the message type will be
2472 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2473 size of the message type. Returns true if successful, or false if
2474 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2478 =item my EXPR : ATTRIBUTES
2480 A C<my> declares the listed variables to be local (lexically) to the
2481 enclosing block, file, or C<eval>. If
2482 more than one value is listed, the list must be placed in parentheses. See
2483 L<perlsub/"Private Variables via my()"> for details.
2489 The C<next> command is like the C<continue> statement in C; it starts
2490 the next iteration of the loop:
2492 LINE: while (<STDIN>) {
2493 next LINE if /^#/; # discard comments
2497 Note that if there were a C<continue> block on the above, it would get
2498 executed even on discarded lines. If the LABEL is omitted, the command
2499 refers to the innermost enclosing loop.
2501 C<next> cannot be used to exit a block which returns a value such as
2502 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2503 a grep() or map() operation.
2505 Note that a block by itself is semantically identical to a loop
2506 that executes once. Thus C<next> will exit such a block early.
2508 See also L</continue> for an illustration of how C<last>, C<next>, and
2511 =item no Module LIST
2513 See the L</use> function, which C<no> is the opposite of.
2519 Interprets EXPR as an octal string and returns the corresponding
2520 value. (If EXPR happens to start off with C<0x>, interprets it as a
2521 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2522 binary string.) The following will handle decimal, binary, octal, and
2523 hex in the standard Perl or C notation:
2525 $val = oct($val) if $val =~ /^0/;
2527 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2528 in octal), use sprintf() or printf():
2530 $perms = (stat("filename"))[2] & 07777;
2531 $oct_perms = sprintf "%lo", $perms;
2533 The oct() function is commonly used when a string such as C<644> needs
2534 to be converted into a file mode, for example. (Although perl will
2535 automatically convert strings into numbers as needed, this automatic
2536 conversion assumes base 10.)
2538 =item open FILEHANDLE,MODE,EXPR
2540 =item open FILEHANDLE,EXPR
2542 =item open FILEHANDLE
2544 Opens the file whose filename is given by EXPR, and associates it with
2545 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2546 name of the real filehandle wanted. If EXPR is omitted, the scalar
2547 variable of the same name as the FILEHANDLE contains the filename.
2548 (Note that lexical variables--those declared with C<my>--will not work
2549 for this purpose; so if you're using C<my>, specify EXPR in your call
2550 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2553 If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2554 If MODE is C<'E<gt>'>, the file is truncated and opened for
2555 output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
2556 the file is opened for appending, again being created if necessary.
2557 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2558 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2559 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2560 file first. You can't usually use either read-write mode for updating
2561 textfiles, since they have variable length records. See the B<-i>
2562 switch in L<perlrun> for a better approach. The file is created with
2563 permissions of C<0666> modified by the process' C<umask> value.
2565 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2566 C<'w+'>, C<'a'>, and C<'a+'>.
2568 In the 2-arguments (and 1-argument) form of the call the mode and
2569 filename should be concatenated (in this order), possibly separated by
2570 spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2572 If the filename begins with C<'|'>, the filename is interpreted as a
2573 command to which output is to be piped, and if the filename ends with a
2574 C<'|'>, the filename is interpreted as a command which pipes output to
2575 us. See L<perlipc/"Using open() for IPC">
2576 for more examples of this. (You are not allowed to C<open> to a command
2577 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2578 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2580 If MODE is C<'|-'>, the filename is interpreted as a
2581 command to which output is to be piped, and if MODE is
2582 C<'-|'>, the filename is interpreted as a command which pipes output to
2583 us. In the 2-arguments (and 1-argument) form one should replace dash
2584 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2585 for more examples of this. (You are not allowed to C<open> to a command
2586 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2587 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2589 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2590 and opening C<'E<gt>-'> opens STDOUT.
2593 nonzero upon success, the undefined value otherwise. If the C<open>
2594 involved a pipe, the return value happens to be the pid of the
2597 If you're unfortunate enough to be running Perl on a system that
2598 distinguishes between text files and binary files (modern operating
2599 systems don't care), then you should check out L</binmode> for tips for
2600 dealing with this. The key distinction between systems that need C<binmode>
2601 and those that don't is their text file formats. Systems like Unix, MacOS, and
2602 Plan9, which delimit lines with a single character, and which encode that
2603 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2605 When opening a file, it's usually a bad idea to continue normal execution
2606 if the request failed, so C<open> is frequently used in connection with
2607 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2608 where you want to make a nicely formatted error message (but there are
2609 modules that can help with that problem)) you should always check
2610 the return value from opening a file. The infrequent exception is when
2611 working with an unopened filehandle is actually what you want to do.
2616 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2617 while (<ARTICLE>) {...
2619 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2620 # if the open fails, output is discarded
2622 open(DBASE, '+<', 'dbase.mine') # open for update
2623 or die "Can't open 'dbase.mine' for update: $!";
2625 open(DBASE, '+<dbase.mine') # ditto
2626 or die "Can't open 'dbase.mine' for update: $!";
2628 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2629 or die "Can't start caesar: $!";
2631 open(ARTICLE, "caesar <$article |") # ditto
2632 or die "Can't start caesar: $!";
2634 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2635 or die "Can't start sort: $!";
2637 # process argument list of files along with any includes
2639 foreach $file (@ARGV) {
2640 process($file, 'fh00');
2644 my($filename, $input) = @_;
2645 $input++; # this is a string increment
2646 unless (open($input, $filename)) {
2647 print STDERR "Can't open $filename: $!\n";
2652 while (<$input>) { # note use of indirection
2653 if (/^#include "(.*)"/) {
2654 process($1, $input);
2661 You may also, in the Bourne shell tradition, specify an EXPR beginning
2662 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2663 name of a filehandle (or file descriptor, if numeric) to be
2664 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2665 C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
2666 mode you specify should match the mode of the original filehandle.
2667 (Duping a filehandle does not take into account any existing contents of
2668 stdio buffers.) Duping file handles is not yet supported for 3-argument
2671 Here is a script that saves, redirects, and restores STDOUT and
2675 open(OLDOUT, ">&STDOUT");
2676 open(OLDERR, ">&STDERR");
2678 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2679 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2681 select(STDERR); $| = 1; # make unbuffered
2682 select(STDOUT); $| = 1; # make unbuffered
2684 print STDOUT "stdout 1\n"; # this works for
2685 print STDERR "stderr 1\n"; # subprocesses too
2690 open(STDOUT, ">&OLDOUT");
2691 open(STDERR, ">&OLDERR");
2693 print STDOUT "stdout 2\n";
2694 print STDERR "stderr 2\n";
2696 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2697 equivalent of C's C<fdopen> of that file descriptor; this is more
2698 parsimonious of file descriptors. For example:
2700 open(FILEHANDLE, "<&=$fd")
2702 Note that this feature depends on the fdopen() C library function.
2703 On many UNIX systems, fdopen() is known to fail when file descriptors
2704 exceed a certain value, typically 255. If you need more file
2705 descriptors than that, consider rebuilding Perl to use the C<sfio>
2708 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2709 with 2-arguments (or 1-argument) form of open(), then
2710 there is an implicit fork done, and the return value of open is the pid
2711 of the child within the parent process, and C<0> within the child
2712 process. (Use C<defined($pid)> to determine whether the open was successful.)
2713 The filehandle behaves normally for the parent, but i/o to that
2714 filehandle is piped from/to the STDOUT/STDIN of the child process.
2715 In the child process the filehandle isn't opened--i/o happens from/to
2716 the new STDOUT or STDIN. Typically this is used like the normal
2717 piped open when you want to exercise more control over just how the
2718 pipe command gets executed, such as when you are running setuid, and
2719 don't want to have to scan shell commands for metacharacters.
2720 The following triples are more or less equivalent:
2722 open(FOO, "|tr '[a-z]' '[A-Z]'");
2723 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2724 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2726 open(FOO, "cat -n '$file'|");
2727 open(FOO, '-|', "cat -n '$file'");
2728 open(FOO, '-|') || exec 'cat', '-n', $file;
2730 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2732 NOTE: On any operation that may do a fork, all files opened for output
2733 are flushed before the fork is attempted. On systems that support a
2734 close-on-exec flag on files, the flag will be set for the newly opened
2735 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2737 Closing any piped filehandle causes the parent process to wait for the
2738 child to finish, and returns the status value in C<$?>.
2740 The filename passed to 2-argument (or 1-argument) form of open()
2741 will have leading and trailing
2742 whitespace deleted, and the normal redirection characters
2743 honored. This property, known as "magic open",
2744 can often be used to good effect. A user could specify a filename of
2745 F<"rsh cat file |">, or you could change certain filenames as needed:
2747 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2748 open(FH, $filename) or die "Can't open $filename: $!";
2750 Use 3-argument form to open a file with arbitrary weird characters in it,
2752 open(FOO, '<', $file);
2754 otherwise it's necessary to protect any leading and trailing whitespace:
2756 $file =~ s#^(\s)#./$1#;
2757 open(FOO, "< $file\0");
2759 (this may not work on some bizzare filesystems). One should
2760 conscientiously choose between the the I<magic> and 3-arguments form
2765 will allow the user to specify an argument of the form C<"rsh cat file |">,
2766 but will not work on a filename which happens to have a trailing space, while
2768 open IN, '<', $ARGV[0];
2770 will have exactly the opposite restrictions.
2772 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2773 should use the C<sysopen> function, which involves no such magic (but
2774 may use subtly different filemodes than Perl open(), which is mapped
2775 to C fopen()). This is
2776 another way to protect your filenames from interpretation. For example:
2779 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2780 or die "sysopen $path: $!";
2781 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2782 print HANDLE "stuff $$\n");
2784 print "File contains: ", <HANDLE>;
2786 Using the constructor from the C<IO::Handle> package (or one of its
2787 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2788 filehandles that have the scope of whatever variables hold references to
2789 them, and automatically close whenever and however you leave that scope:
2793 sub read_myfile_munged {
2795 my $handle = new IO::File;
2796 open($handle, "myfile") or die "myfile: $!";
2798 or return (); # Automatically closed here.
2799 mung $first or die "mung failed"; # Or here.
2800 return $first, <$handle> if $ALL; # Or here.
2804 See L</seek> for some details about mixing reading and writing.
2806 =item opendir DIRHANDLE,EXPR
2808 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2809 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2810 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2816 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2817 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2818 See L<utf8> for more about Unicode.
2822 An C<our> declares the listed variables to be valid globals within
2823 the enclosing block, file, or C<eval>. That is, it has the same
2824 scoping rules as a "my" declaration, but does not create a local
2825 variable. If more than one value is listed, the list must be placed
2826 in parentheses. The C<our> declaration has no semantic effect unless
2827 "use strict vars" is in effect, in which case it lets you use the
2828 declared global variable without qualifying it with a package name.
2829 (But only within the lexical scope of the C<our> declaration. In this
2830 it differs from "use vars", which is package scoped.)
2832 An C<our> declaration declares a global variable that will be visible
2833 across its entire lexical scope, even across package boundaries. The
2834 package in which the variable is entered is determined at the point
2835 of the declaration, not at the point of use. This means the following
2839 our $bar; # declares $Foo::bar for rest of lexical scope
2843 print $bar; # prints 20
2845 Multiple C<our> declarations in the same lexical scope are allowed
2846 if they are in different packages. If they happened to be in the same
2847 package, Perl will emit warnings if you have asked for them.
2851 our $bar; # declares $Foo::bar for rest of lexical scope
2855 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2856 print $bar; # prints 30
2858 our $bar; # emits warning
2860 =item pack TEMPLATE,LIST
2862 Takes a LIST of values and converts it into a string using the rules
2863 given by the TEMPLATE. The resulting string is the concatenation of
2864 the converted values. Typically, each converted value looks
2865 like its machine-level representation. For example, on 32-bit machines
2866 a converted integer may be represented by a sequence of 4 bytes.
2869 sequence of characters that give the order and type of values, as
2872 a A string with arbitrary binary data, will be null padded.
2873 A An ascii string, will be space padded.
2874 Z A null terminated (asciz) string, will be null padded.
2876 b A bit string (ascending bit order inside each byte, like vec()).
2877 B A bit string (descending bit order inside each byte).
2878 h A hex string (low nybble first).
2879 H A hex string (high nybble first).
2881 c A signed char value.
2882 C An unsigned char value. Only does bytes. See U for Unicode.
2884 s A signed short value.
2885 S An unsigned short value.
2886 (This 'short' is _exactly_ 16 bits, which may differ from
2887 what a local C compiler calls 'short'. If you want
2888 native-length shorts, use the '!' suffix.)
2890 i A signed integer value.
2891 I An unsigned integer value.
2892 (This 'integer' is _at_least_ 32 bits wide. Its exact
2893 size depends on what a local C compiler calls 'int',
2894 and may even be larger than the 'long' described in
2897 l A signed long value.
2898 L An unsigned long value.
2899 (This 'long' is _exactly_ 32 bits, which may differ from
2900 what a local C compiler calls 'long'. If you want
2901 native-length longs, use the '!' suffix.)
2903 n An unsigned short in "network" (big-endian) order.
2904 N An unsigned long in "network" (big-endian) order.
2905 v An unsigned short in "VAX" (little-endian) order.
2906 V An unsigned long in "VAX" (little-endian) order.
2907 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2908 _exactly_ 32 bits, respectively.)
2910 q A signed quad (64-bit) value.
2911 Q An unsigned quad value.
2912 (Quads are available only if your system supports 64-bit
2913 integer values _and_ if Perl has been compiled to support those.
2914 Causes a fatal error otherwise.)
2916 f A single-precision float in the native format.
2917 d A double-precision float in the native format.
2919 p A pointer to a null-terminated string.
2920 P A pointer to a structure (fixed-length string).
2922 u A uuencoded string.
2923 U A Unicode character number. Encodes to UTF-8 internally.
2924 Works even if C<use utf8> is not in effect.
2926 w A BER compressed integer. Its bytes represent an unsigned
2927 integer in base 128, most significant digit first, with as
2928 few digits as possible. Bit eight (the high bit) is set
2929 on each byte except the last.
2933 @ Null fill to absolute position.
2935 The following rules apply:
2941 Each letter may optionally be followed by a number giving a repeat
2942 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2943 C<H>, and C<P> the pack function will gobble up that many values from
2944 the LIST. A C<*> for the repeat count means to use however many items are
2945 left, except for C<@>, C<x>, C<X>, where it is equivalent
2946 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2949 When used with C<Z>, C<*> results in the addition of a trailing null
2950 byte (so the packed result will be one longer than the byte C<length>
2953 The repeat count for C<u> is interpreted as the maximal number of bytes
2954 to encode per line of output, with 0 and 1 replaced by 45.
2958 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
2959 string of length count, padding with nulls or spaces as necessary. When
2960 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2961 after the first null, and C<a> returns data verbatim. When packing,
2962 C<a>, and C<Z> are equivalent.
2964 If the value-to-pack is too long, it is truncated. If too long and an
2965 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2966 by a null byte. Thus C<Z> always packs a trailing null byte under
2971 Likewise, the C<b> and C<B> fields pack a string that many bits long.
2972 Each byte of the input field of pack() generates 1 bit of the result.
2973 Each result bit is based on the least-significant bit of the corresponding
2974 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2975 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2977 Starting from the beginning of the input string of pack(), each 8-tuple
2978 of bytes is converted to 1 byte of output. With format C<b>
2979 the first byte of the 8-tuple determines the least-significant bit of a
2980 byte, and with format C<B> it determines the most-significant bit of
2983 If the length of the input string is not exactly divisible by 8, the
2984 remainder is packed as if the input string were padded by null bytes
2985 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2987 If the input string of pack() is longer than needed, extra bytes are ignored.
2988 A C<*> for the repeat count of pack() means to use all the bytes of
2989 the input field. On unpack()ing the bits are converted to a string
2990 of C<"0">s and C<"1">s.
2994 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
2995 representable as hexadecimal digits, 0-9a-f) long.
2997 Each byte of the input field of pack() generates 4 bits of the result.
2998 For non-alphabetical bytes the result is based on the 4 least-significant
2999 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3000 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3001 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3002 is compatible with the usual hexadecimal digits, so that C<"a"> and
3003 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3004 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3006 Starting from the beginning of the input string of pack(), each pair
3007 of bytes is converted to 1 byte of output. With format C<h> the
3008 first byte of the pair determines the least-significant nybble of the
3009 output byte, and with format C<H> it determines the most-significant
3012 If the length of the input string is not even, it behaves as if padded
3013 by a null byte at the end. Similarly, during unpack()ing the "extra"
3014 nybbles are ignored.
3016 If the input string of pack() is longer than needed, extra bytes are ignored.
3017 A C<*> for the repeat count of pack() means to use all the bytes of
3018 the input field. On unpack()ing the bits are converted to a string
3019 of hexadecimal digits.
3023 The C<p> type packs a pointer to a null-terminated string. You are
3024 responsible for ensuring the string is not a temporary value (which can
3025 potentially get deallocated before you get around to using the packed result).
3026 The C<P> type packs a pointer to a structure of the size indicated by the
3027 length. A NULL pointer is created if the corresponding value for C<p> or
3028 C<P> is C<undef>, similarly for unpack().
3032 The C</> template character allows packing and unpacking of strings where
3033 the packed structure contains a byte count followed by the string itself.
3034 You write I<length-item>C</>I<string-item>.
3036 The I<length-item> can be any C<pack> template letter,
3037 and describes how the length value is packed.
3038 The ones likely to be of most use are integer-packing ones like
3039 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3040 and C<N> (for Sun XDR).
3042 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3043 For C<unpack> the length of the string is obtained from the I<length-item>,
3044 but if you put in the '*' it will be ignored.
3046 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3047 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3048 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3050 The I<length-item> is not returned explicitly from C<unpack>.
3052 Adding a count to the I<length-item> letter is unlikely to do anything
3053 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3054 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3055 which Perl does not regard as legal in numeric strings.
3059 The integer types C<s>, C<S>, C<l>, and C<L> may be
3060 immediately followed by a C<!> suffix to signify native shorts or
3061 longs--as you can see from above for example a bare C<l> does mean
3062 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3063 may be larger. This is an issue mainly in 64-bit platforms. You can
3064 see whether using C<!> makes any difference by
3066 print length(pack("s")), " ", length(pack("s!")), "\n";
3067 print length(pack("l")), " ", length(pack("l!")), "\n";
3069 C<i!> and C<I!> also work but only because of completeness;
3070 they are identical to C<i> and C<I>.
3072 The actual sizes (in bytes) of native shorts, ints, longs, and long
3073 longs on the platform where Perl was built are also available via
3077 print $Config{shortsize}, "\n";
3078 print $Config{intsize}, "\n";
3079 print $Config{longsize}, "\n";
3080 print $Config{longlongsize}, "\n";
3082 (The C<$Config{longlongsize}> will be undefine if your system does
3083 not support long longs.)
3087 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3088 are inherently non-portable between processors and operating systems
3089 because they obey the native byteorder and endianness. For example a
3090 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3091 (arranged in and handled by the CPU registers) into bytes as
3093 0x12 0x34 0x56 0x78 # little-endian
3094 0x78 0x56 0x34 0x12 # big-endian
3096 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3097 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3098 Power, and Cray are big-endian. MIPS can be either: Digital used it
3099 in little-endian mode; SGI uses it in big-endian mode.
3101 The names `big-endian' and `little-endian' are comic references to
3102 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3103 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3104 the egg-eating habits of the Lilliputians.
3106 Some systems may have even weirder byte orders such as
3111 You can see your system's preference with
3113 print join(" ", map { sprintf "%#02x", $_ }
3114 unpack("C*",pack("L",0x12345678))), "\n";
3116 The byteorder on the platform where Perl was built is also available
3120 print $Config{byteorder}, "\n";
3122 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3123 and C<'87654321'> are big-endian.
3125 If you want portable packed integers use the formats C<n>, C<N>,
3126 C<v>, and C<V>, their byte endianness and size is known.
3127 See also L<perlport>.
3131 Real numbers (floats and doubles) are in the native machine format only;
3132 due to the multiplicity of floating formats around, and the lack of a
3133 standard "network" representation, no facility for interchange has been
3134 made. This means that packed floating point data written on one machine
3135 may not be readable on another - even if both use IEEE floating point
3136 arithmetic (as the endian-ness of the memory representation is not part
3137 of the IEEE spec). See also L<perlport>.
3139 Note that Perl uses doubles internally for all numeric calculation, and
3140 converting from double into float and thence back to double again will
3141 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3146 You must yourself do any alignment or padding by inserting for example
3147 enough C<'x'>es while packing. There is no way to pack() and unpack()
3148 could know where the bytes are going to or coming from. Therefore
3149 C<pack> (and C<unpack>) handle their output and input as flat
3154 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3158 If TEMPLATE requires more arguments to pack() than actually given, pack()
3159 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3160 to pack() than actually given, extra arguments are ignored.
3166 $foo = pack("CCCC",65,66,67,68);
3168 $foo = pack("C4",65,66,67,68);
3170 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3171 # same thing with Unicode circled letters
3173 $foo = pack("ccxxcc",65,66,67,68);
3176 # note: the above examples featuring "C" and "c" are true
3177 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3178 # and UTF-8. In EBCDIC the first example would be
3179 # $foo = pack("CCCC",193,194,195,196);
3181 $foo = pack("s2",1,2);
3182 # "\1\0\2\0" on little-endian
3183 # "\0\1\0\2" on big-endian
3185 $foo = pack("a4","abcd","x","y","z");
3188 $foo = pack("aaaa","abcd","x","y","z");
3191 $foo = pack("a14","abcdefg");
3192 # "abcdefg\0\0\0\0\0\0\0"
3194 $foo = pack("i9pl", gmtime);
3195 # a real struct tm (on my system anyway)
3197 $utmp_template = "Z8 Z8 Z16 L";
3198 $utmp = pack($utmp_template, @utmp1);
3199 # a struct utmp (BSDish)
3201 @utmp2 = unpack($utmp_template, $utmp);
3202 # "@utmp1" eq "@utmp2"
3205 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3208 $foo = pack('sx2l', 12, 34);
3209 # short 12, two zero bytes padding, long 34
3210 $bar = pack('s@4l', 12, 34);
3211 # short 12, zero fill to position 4, long 34
3214 The same template may generally also be used in unpack().
3218 =item package NAMESPACE
3220 Declares the compilation unit as being in the given namespace. The scope
3221 of the package declaration is from the declaration itself through the end
3222 of the enclosing block, file, or eval (the same as the C<my> operator).
3223 All further unqualified dynamic identifiers will be in this namespace.
3224 A package statement affects only dynamic variables--including those
3225 you've used C<local> on--but I<not> lexical variables, which are created
3226 with C<my>. Typically it would be the first declaration in a file to
3227 be included by the C<require> or C<use> operator. You can switch into a
3228 package in more than one place; it merely influences which symbol table
3229 is used by the compiler for the rest of that block. You can refer to
3230 variables and filehandles in other packages by prefixing the identifier
3231 with the package name and a double colon: C<$Package::Variable>.
3232 If the package name is null, the C<main> package as assumed. That is,
3233 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3234 still seen in older code).
3236 If NAMESPACE is omitted, then there is no current package, and all
3237 identifiers must be fully qualified or lexicals. This is stricter
3238 than C<use strict>, since it also extends to function names.
3240 See L<perlmod/"Packages"> for more information about packages, modules,
3241 and classes. See L<perlsub> for other scoping issues.
3243 =item pipe READHANDLE,WRITEHANDLE
3245 Opens a pair of connected pipes like the corresponding system call.
3246 Note that if you set up a loop of piped processes, deadlock can occur
3247 unless you are very careful. In addition, note that Perl's pipes use
3248 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3249 after each command, depending on the application.
3251 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3252 for examples of such things.
3254 On systems that support a close-on-exec flag on files, the flag will be set
3255 for the newly opened file descriptors as determined by the value of $^F.
3262 Pops and returns the last value of the array, shortening the array by
3263 one element. Has an effect similar to
3267 If there are no elements in the array, returns the undefined value
3268 (although this may happen at other times as well). If ARRAY is
3269 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3270 array in subroutines, just like C<shift>.
3276 Returns the offset of where the last C<m//g> search left off for the variable
3277 is in question (C<$_> is used when the variable is not specified). May be
3278 modified to change that offset. Such modification will also influence
3279 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3282 =item print FILEHANDLE LIST
3288 Prints a string or a list of strings. Returns true if successful.
3289 FILEHANDLE may be a scalar variable name, in which case the variable
3290 contains the name of or a reference to the filehandle, thus introducing
3291 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3292 the next token is a term, it may be misinterpreted as an operator
3293 unless you interpose a C<+> or put parentheses around the arguments.)
3294 If FILEHANDLE is omitted, prints by default to standard output (or
3295 to the last selected output channel--see L</select>). If LIST is
3296 also omitted, prints C<$_> to the currently selected output channel.
3297 To set the default output channel to something other than STDOUT
3298 use the select operation. The current value of C<$,> (if any) is
3299 printed between each LIST item. The current value of C<$\> (if
3300 any) is printed after the entire LIST has been printed. Because
3301 print takes a LIST, anything in the LIST is evaluated in list
3302 context, and any subroutine that you call will have one or more of
3303 its expressions evaluated in list context. Also be careful not to
3304 follow the print keyword with a left parenthesis unless you want
3305 the corresponding right parenthesis to terminate the arguments to
3306 the print--interpose a C<+> or put parentheses around all the
3309 Note that if you're storing FILEHANDLES in an array or other expression,
3310 you will have to use a block returning its value instead:
3312 print { $files[$i] } "stuff\n";
3313 print { $OK ? STDOUT : STDERR } "stuff\n";
3315 =item printf FILEHANDLE FORMAT, LIST
3317 =item printf FORMAT, LIST
3319 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3320 (the output record separator) is not appended. The first argument
3321 of the list will be interpreted as the C<printf> format. If C<use locale> is
3322 in effect, the character used for the decimal point in formatted real numbers
3323 is affected by the LC_NUMERIC locale. See L<perllocale>.
3325 Don't fall into the trap of using a C<printf> when a simple
3326 C<print> would do. The C<print> is more efficient and less
3329 =item prototype FUNCTION
3331 Returns the prototype of a function as a string (or C<undef> if the
3332 function has no prototype). FUNCTION is a reference to, or the name of,
3333 the function whose prototype you want to retrieve.
3335 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3336 name for Perl builtin. If the builtin is not I<overridable> (such as
3337 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3338 C<system>) returns C<undef> because the builtin does not really behave
3339 like a Perl function. Otherwise, the string describing the equivalent
3340 prototype is returned.
3342 =item push ARRAY,LIST
3344 Treats ARRAY as a stack, and pushes the values of LIST
3345 onto the end of ARRAY. The length of ARRAY increases by the length of
3346 LIST. Has the same effect as
3349 $ARRAY[++$#ARRAY] = $value;
3352 but is more efficient. Returns the new number of elements in the array.
3364 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3366 =item quotemeta EXPR
3370 Returns the value of EXPR with all non-alphanumeric
3371 characters backslashed. (That is, all characters not matching
3372 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3373 returned string, regardless of any locale settings.)
3374 This is the internal function implementing
3375 the C<\Q> escape in double-quoted strings.
3377 If EXPR is omitted, uses C<$_>.
3383 Returns a random fractional number greater than or equal to C<0> and less
3384 than the value of EXPR. (EXPR should be positive.) If EXPR is
3385 omitted, the value C<1> is used. Automatically calls C<srand> unless
3386 C<srand> has already been called. See also C<srand>.
3388 (Note: If your rand function consistently returns numbers that are too
3389 large or too small, then your version of Perl was probably compiled
3390 with the wrong number of RANDBITS.)
3392 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3394 =item read FILEHANDLE,SCALAR,LENGTH
3396 Attempts to read LENGTH bytes of data into variable SCALAR from the
3397 specified FILEHANDLE. Returns the number of bytes actually read,
3398 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3399 or shrunk to the length actually read. An OFFSET may be specified to
3400 place the read data at some other place than the beginning of the
3401 string. This call is actually implemented in terms of stdio's fread(3)
3402 call. To get a true read(2) system call, see C<sysread>.
3404 =item readdir DIRHANDLE
3406 Returns the next directory entry for a directory opened by C<opendir>.
3407 If used in list context, returns all the rest of the entries in the
3408 directory. If there are no more entries, returns an undefined value in
3409 scalar context or a null list in list context.
3411 If you're planning to filetest the return values out of a C<readdir>, you'd
3412 better prepend the directory in question. Otherwise, because we didn't
3413 C<chdir> there, it would have been testing the wrong file.
3415 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3416 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3421 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3422 context, each call reads and returns the next line, until end-of-file is
3423 reached, whereupon the subsequent call returns undef. In list context,
3424 reads until end-of-file is reached and returns a list of lines. Note that
3425 the notion of "line" used here is however you may have defined it
3426 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3428 When C<$/> is set to C<undef>, when readline() is in scalar
3429 context (i.e. file slurp mode), and when an empty file is read, it
3430 returns C<''> the first time, followed by C<undef> subsequently.
3432 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3433 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3434 operator is discussed in more detail in L<perlop/"I/O Operators">.
3437 $line = readline(*STDIN); # same thing
3443 Returns the value of a symbolic link, if symbolic links are
3444 implemented. If not, gives a fatal error. If there is some system
3445 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3446 omitted, uses C<$_>.
3450 EXPR is executed as a system command.
3451 The collected standard output of the command is returned.
3452 In scalar context, it comes back as a single (potentially
3453 multi-line) string. In list context, returns a list of lines
3454 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3455 This is the internal function implementing the C<qx/EXPR/>
3456 operator, but you can use it directly. The C<qx/EXPR/>
3457 operator is discussed in more detail in L<perlop/"I/O Operators">.
3459 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3461 Receives a message on a socket. Attempts to receive LENGTH bytes of
3462 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3463 will be grown or shrunk to the length actually read. Takes the same
3464 flags as the system call of the same name. Returns the address of the
3465 sender if SOCKET's protocol supports this; returns an empty string
3466 otherwise. If there's an error, returns the undefined value. This call
3467 is actually implemented in terms of recvfrom(2) system call. See
3468 L<perlipc/"UDP: Message Passing"> for examples.
3474 The C<redo> command restarts the loop block without evaluating the
3475 conditional again. The C<continue> block, if any, is not executed. If
3476 the LABEL is omitted, the command refers to the innermost enclosing
3477 loop. This command is normally used by programs that want to lie to
3478 themselves about what was just input:
3480 # a simpleminded Pascal comment stripper
3481 # (warning: assumes no { or } in strings)
3482 LINE: while (<STDIN>) {
3483 while (s|({.*}.*){.*}|$1 |) {}
3488 if (/}/) { # end of comment?
3497 C<redo> cannot be used to retry a block which returns a value such as
3498 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3499 a grep() or map() operation.
3501 Note that a block by itself is semantically identical to a loop
3502 that executes once. Thus C<redo> inside such a block will effectively
3503 turn it into a looping construct.
3505 See also L</continue> for an illustration of how C<last>, C<next>, and
3512 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3513 is not specified, C<$_> will be used. The value returned depends on the
3514 type of thing the reference is a reference to.
3515 Builtin types include:
3525 If the referenced object has been blessed into a package, then that package
3526 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3528 if (ref($r) eq "HASH") {
3529 print "r is a reference to a hash.\n";
3532 print "r is not a reference at all.\n";
3534 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3535 print "r is a reference to something that isa hash.\n";
3538 See also L<perlref>.
3540 =item rename OLDNAME,NEWNAME
3542 Changes the name of a file; an existing file NEWNAME will be
3543 clobbered. Returns true for success, false otherwise.
3545 Behavior of this function varies wildly depending on your system
3546 implementation. For example, it will usually not work across file system
3547 boundaries, even though the system I<mv> command sometimes compensates
3548 for this. Other restrictions include whether it works on directories,
3549 open files, or pre-existing files. Check L<perlport> and either the
3550 rename(2) manpage or equivalent system documentation for details.
3552 =item require VERSION
3558 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3561 If a VERSION is specified as a literal of the form v5.6.1,
3562 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3563 at least as recent as that version, at run time. (For compatibility
3564 with older versions of Perl, a numeric argument will also be interpreted
3565 as VERSION.) Compare with L</use>, which can do a similar check at
3568 require v5.6.1; # run time version check
3569 require 5.6.1; # ditto
3570 require 5.005_03; # float version allowed for compatibility
3572 Otherwise, demands that a library file be included if it hasn't already
3573 been included. The file is included via the do-FILE mechanism, which is
3574 essentially just a variety of C<eval>. Has semantics similar to the following
3579 return 1 if $INC{$filename};
3580 my($realfilename,$result);
3582 foreach $prefix (@INC) {
3583 $realfilename = "$prefix/$filename";
3584 if (-f $realfilename) {
3585 $INC{$filename} = $realfilename;
3586 $result = do $realfilename;
3590 die "Can't find $filename in \@INC";
3592 delete $INC{$filename} if $@ || !$result;
3594 die "$filename did not return true value" unless $result;
3598 Note that the file will not be included twice under the same specified
3599 name. The file must return true as the last statement to indicate
3600 successful execution of any initialization code, so it's customary to
3601 end such a file with C<1;> unless you're sure it'll return true
3602 otherwise. But it's better just to put the C<1;>, in case you add more
3605 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3606 replaces "F<::>" with "F</>" in the filename for you,
3607 to make it easy to load standard modules. This form of loading of
3608 modules does not risk altering your namespace.
3610 In other words, if you try this:
3612 require Foo::Bar; # a splendid bareword
3614 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3615 directories specified in the C<@INC> array.
3617 But if you try this:
3619 $class = 'Foo::Bar';
3620 require $class; # $class is not a bareword
3622 require "Foo::Bar"; # not a bareword because of the ""
3624 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3625 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3627 eval "require $class";
3629 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3635 Generally used in a C<continue> block at the end of a loop to clear
3636 variables and reset C<??> searches so that they work again. The
3637 expression is interpreted as a list of single characters (hyphens
3638 allowed for ranges). All variables and arrays beginning with one of
3639 those letters are reset to their pristine state. If the expression is
3640 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3641 only variables or searches in the current package. Always returns
3644 reset 'X'; # reset all X variables
3645 reset 'a-z'; # reset lower case variables
3646 reset; # just reset ?one-time? searches
3648 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3649 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3650 variables--lexical variables are unaffected, but they clean themselves
3651 up on scope exit anyway, so you'll probably want to use them instead.
3658 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3659 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3660 context, depending on how the return value will be used, and the context
3661 may vary from one execution to the next (see C<wantarray>). If no EXPR
3662 is given, returns an empty list in list context, the undefined value in
3663 scalar context, and (of course) nothing at all in a void context.
3665 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3666 or do FILE will automatically return the value of the last expression
3671 In list context, returns a list value consisting of the elements
3672 of LIST in the opposite order. In scalar context, concatenates the
3673 elements of LIST and returns a string value with all characters
3674 in the opposite order.
3676 print reverse <>; # line tac, last line first
3678 undef $/; # for efficiency of <>
3679 print scalar reverse <>; # character tac, last line tsrif
3681 This operator is also handy for inverting a hash, although there are some
3682 caveats. If a value is duplicated in the original hash, only one of those
3683 can be represented as a key in the inverted hash. Also, this has to
3684 unwind one hash and build a whole new one, which may take some time
3685 on a large hash, such as from a DBM file.
3687 %by_name = reverse %by_address; # Invert the hash
3689 =item rewinddir DIRHANDLE
3691 Sets the current position to the beginning of the directory for the
3692 C<readdir> routine on DIRHANDLE.
3694 =item rindex STR,SUBSTR,POSITION
3696 =item rindex STR,SUBSTR
3698 Works just like index() except that it returns the position of the LAST
3699 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3700 last occurrence at or before that position.
3702 =item rmdir FILENAME
3706 Deletes the directory specified by FILENAME if that directory is empty. If it
3707 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3708 FILENAME is omitted, uses C<$_>.
3712 The substitution operator. See L<perlop>.
3716 Forces EXPR to be interpreted in scalar context and returns the value
3719 @counts = ( scalar @a, scalar @b, scalar @c );
3721 There is no equivalent operator to force an expression to
3722 be interpolated in list context because in practice, this is never
3723 needed. If you really wanted to do so, however, you could use
3724 the construction C<@{[ (some expression) ]}>, but usually a simple
3725 C<(some expression)> suffices.
3727 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3728 parenthesized list, this behaves as a scalar comma expression, evaluating
3729 all but the last element in void context and returning the final element
3730 evaluated in scalar context. This is seldom what you want.
3732 The following single statement:
3734 print uc(scalar(&foo,$bar)),$baz;
3736 is the moral equivalent of these two:
3739 print(uc($bar),$baz);
3741 See L<perlop> for more details on unary operators and the comma operator.
3743 =item seek FILEHANDLE,POSITION,WHENCE
3745 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3746 FILEHANDLE may be an expression whose value gives the name of the
3747 filehandle. The values for WHENCE are C<0> to set the new position to
3748 POSITION, C<1> to set it to the current position plus POSITION, and
3749 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3750 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3751 (start of the file, current position, end of the file) from the Fcntl
3752 module. Returns C<1> upon success, C<0> otherwise.
3754 If you want to position file for C<sysread> or C<syswrite>, don't use
3755 C<seek>--buffering makes its effect on the file's system position
3756 unpredictable and non-portable. Use C<sysseek> instead.
3758 Due to the rules and rigors of ANSI C, on some systems you have to do a
3759 seek whenever you switch between reading and writing. Amongst other
3760 things, this may have the effect of calling stdio's clearerr(3).
3761 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3765 This is also useful for applications emulating C<tail -f>. Once you hit
3766 EOF on your read, and then sleep for a while, you might have to stick in a
3767 seek() to reset things. The C<seek> doesn't change the current position,
3768 but it I<does> clear the end-of-file condition on the handle, so that the
3769 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3771 If that doesn't work (some stdios are particularly cantankerous), then
3772 you may need something more like this:
3775 for ($curpos = tell(FILE); $_ = <FILE>;
3776 $curpos = tell(FILE)) {
3777 # search for some stuff and put it into files
3779 sleep($for_a_while);
3780 seek(FILE, $curpos, 0);
3783 =item seekdir DIRHANDLE,POS
3785 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3786 must be a value returned by C<telldir>. Has the same caveats about
3787 possible directory compaction as the corresponding system library
3790 =item select FILEHANDLE
3794 Returns the currently selected filehandle. Sets the current default
3795 filehandle for output, if FILEHANDLE is supplied. This has two
3796 effects: first, a C<write> or a C<print> without a filehandle will
3797 default to this FILEHANDLE. Second, references to variables related to
3798 output will refer to this output channel. For example, if you have to
3799 set the top of form format for more than one output channel, you might
3807 FILEHANDLE may be an expression whose value gives the name of the
3808 actual filehandle. Thus:
3810 $oldfh = select(STDERR); $| = 1; select($oldfh);
3812 Some programmers may prefer to think of filehandles as objects with
3813 methods, preferring to write the last example as:
3816 STDERR->autoflush(1);
3818 =item select RBITS,WBITS,EBITS,TIMEOUT
3820 This calls the select(2) system call with the bit masks specified, which
3821 can be constructed using C<fileno> and C<vec>, along these lines:
3823 $rin = $win = $ein = '';
3824 vec($rin,fileno(STDIN),1) = 1;
3825 vec($win,fileno(STDOUT),1) = 1;
3828 If you want to select on many filehandles you might wish to write a
3832 my(@fhlist) = split(' ',$_[0]);
3835 vec($bits,fileno($_),1) = 1;
3839 $rin = fhbits('STDIN TTY SOCK');
3843 ($nfound,$timeleft) =
3844 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3846 or to block until something becomes ready just do this
3848 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3850 Most systems do not bother to return anything useful in $timeleft, so
3851 calling select() in scalar context just returns $nfound.
3853 Any of the bit masks can also be undef. The timeout, if specified, is
3854 in seconds, which may be fractional. Note: not all implementations are
3855 capable of returning the$timeleft. If not, they always return
3856 $timeleft equal to the supplied $timeout.
3858 You can effect a sleep of 250 milliseconds this way:
3860 select(undef, undef, undef, 0.25);
3862 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3863 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3864 then only on POSIX systems. You have to use C<sysread> instead.
3866 =item semctl ID,SEMNUM,CMD,ARG
3868 Calls the System V IPC function C<semctl>. You'll probably have to say
3872 first to get the correct constant definitions. If CMD is IPC_STAT or
3873 GETALL, then ARG must be a variable which will hold the returned
3874 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3875 undefined value for error, "C<0 but true>" for zero, or the actual return
3876 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3878 =item semget KEY,NSEMS,FLAGS
3880 Calls the System V IPC function semget. Returns the semaphore id, or
3881 the undefined value if there is an error. See also C<IPC::SysV> and
3882 C<IPC::SysV::Semaphore> documentation.
3884 =item semop KEY,OPSTRING
3886 Calls the System V IPC function semop to perform semaphore operations
3887 such as signaling and waiting. OPSTRING must be a packed array of
3888 semop structures. Each semop structure can be generated with
3889 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3890 operations is implied by the length of OPSTRING. Returns true if
3891 successful, or false if there is an error. As an example, the
3892 following code waits on semaphore $semnum of semaphore id $semid:
3894 $semop = pack("sss", $semnum, -1, 0);
3895 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3897 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3898 and C<IPC::SysV::Semaphore> documentation.
3900 =item send SOCKET,MSG,FLAGS,TO
3902 =item send SOCKET,MSG,FLAGS
3904 Sends a message on a socket. Takes the same flags as the system call
3905 of the same name. On unconnected sockets you must specify a
3906 destination to send TO, in which case it does a C C<sendto>. Returns
3907 the number of characters sent, or the undefined value if there is an
3908 error. The C system call sendmsg(2) is currently unimplemented.
3909 See L<perlipc/"UDP: Message Passing"> for examples.
3911 =item setpgrp PID,PGRP
3913 Sets the current process group for the specified PID, C<0> for the current
3914 process. Will produce a fatal error if used on a machine that doesn't
3915 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3916 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3917 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3920 =item setpriority WHICH,WHO,PRIORITY
3922 Sets the current priority for a process, a process group, or a user.
3923 (See setpriority(2).) Will produce a fatal error if used on a machine
3924 that doesn't implement setpriority(2).
3926 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3928 Sets the socket option requested. Returns undefined if there is an
3929 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3936 Shifts the first value of the array off and returns it, shortening the
3937 array by 1 and moving everything down. If there are no elements in the
3938 array, returns the undefined value. If ARRAY is omitted, shifts the
3939 C<@_> array within the lexical scope of subroutines and formats, and the
3940 C<@ARGV> array at file scopes or within the lexical scopes established by
3941 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
3944 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3945 same thing to the left end of an array that C<pop> and C<push> do to the
3948 =item shmctl ID,CMD,ARG
3950 Calls the System V IPC function shmctl. You'll probably have to say
3954 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3955 then ARG must be a variable which will hold the returned C<shmid_ds>
3956 structure. Returns like ioctl: the undefined value for error, "C<0> but
3957 true" for zero, or the actual return value otherwise.
3958 See also C<IPC::SysV> documentation.
3960 =item shmget KEY,SIZE,FLAGS
3962 Calls the System V IPC function shmget. Returns the shared memory
3963 segment id, or the undefined value if there is an error.
3964 See also C<IPC::SysV> documentation.
3966 =item shmread ID,VAR,POS,SIZE
3968 =item shmwrite ID,STRING,POS,SIZE
3970 Reads or writes the System V shared memory segment ID starting at
3971 position POS for size SIZE by attaching to it, copying in/out, and
3972 detaching from it. When reading, VAR must be a variable that will
3973 hold the data read. When writing, if STRING is too long, only SIZE
3974 bytes are used; if STRING is too short, nulls are written to fill out
3975 SIZE bytes. Return true if successful, or false if there is an error.
3976 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3979 =item shutdown SOCKET,HOW
3981 Shuts down a socket connection in the manner indicated by HOW, which
3982 has the same interpretation as in the system call of the same name.
3984 shutdown(SOCKET, 0); # I/we have stopped reading data
3985 shutdown(SOCKET, 1); # I/we have stopped writing data
3986 shutdown(SOCKET, 2); # I/we have stopped using this socket
3988 This is useful with sockets when you want to tell the other
3989 side you're done writing but not done reading, or vice versa.
3990 It's also a more insistent form of close because it also
3991 disables the file descriptor in any forked copies in other
3998 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3999 returns sine of C<$_>.
4001 For the inverse sine operation, you may use the C<Math::Trig::asin>
4002 function, or use this relation:
4004 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4010 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4011 May be interrupted if the process receives a signal such as C<SIGALRM>.
4012 Returns the number of seconds actually slept. You probably cannot
4013 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4016 On some older systems, it may sleep up to a full second less than what
4017 you requested, depending on how it counts seconds. Most modern systems
4018 always sleep the full amount. They may appear to sleep longer than that,
4019 however, because your process might not be scheduled right away in a
4020 busy multitasking system.
4022 For delays of finer granularity than one second, you may use Perl's
4023 C<syscall> interface to access setitimer(2) if your system supports
4024 it, or else see L</select> above. The Time::HiRes module from CPAN
4027 See also the POSIX module's C<sigpause> function.
4029 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4031 Opens a socket of the specified kind and attaches it to filehandle
4032 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4033 the system call of the same name. You should C<use Socket> first
4034 to get the proper definitions imported. See the examples in
4035 L<perlipc/"Sockets: Client/Server Communication">.
4037 On systems that support a close-on-exec flag on files, the flag will
4038 be set for the newly opened file descriptor, as determined by the
4039 value of $^F. See L<perlvar/$^F>.
4041 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4043 Creates an unnamed pair of sockets in the specified domain, of the
4044 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4045 for the system call of the same name. If unimplemented, yields a fatal
4046 error. Returns true if successful.
4048 On systems that support a close-on-exec flag on files, the flag will
4049 be set for the newly opened file descriptors, as determined by the value
4050 of $^F. See L<perlvar/$^F>.
4052 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4053 to C<pipe(Rdr, Wtr)> is essentially:
4056 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4057 shutdown(Rdr, 1); # no more writing for reader
4058 shutdown(Wtr, 0); # no more reading for writer
4060 See L<perlipc> for an example of socketpair use.
4062 =item sort SUBNAME LIST
4064 =item sort BLOCK LIST
4068 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4069 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4070 specified, it gives the name of a subroutine that returns an integer
4071 less than, equal to, or greater than C<0>, depending on how the elements
4072 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
4073 operators are extremely useful in such routines.) SUBNAME may be a
4074 scalar variable name (unsubscripted), in which case the value provides
4075 the name of (or a reference to) the actual subroutine to use. In place
4076 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4079 If the subroutine's prototype is C<($$)>, the elements to be compared
4080 are passed by reference in C<@_>, as for a normal subroutine. If not,
4081 the normal calling code for subroutines is bypassed in the interests of
4082 efficiency, and the elements to be compared are passed into the subroutine
4083 as the package global variables $a and $b (see example below). Note that
4084 in the latter case, it is usually counter-productive to declare $a and
4087 In either case, the subroutine may not be recursive. The values to be
4088 compared are always passed by reference, so don't modify them.
4090 You also cannot exit out of the sort block or subroutine using any of the
4091 loop control operators described in L<perlsyn> or with C<goto>.
4093 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4094 current collation locale. See L<perllocale>.
4099 @articles = sort @files;
4101 # same thing, but with explicit sort routine
4102 @articles = sort {$a cmp $b} @files;
4104 # now case-insensitively
4105 @articles = sort {uc($a) cmp uc($b)} @files;
4107 # same thing in reversed order
4108 @articles = sort {$b cmp $a} @files;
4110 # sort numerically ascending
4111 @articles = sort {$a <=> $b} @files;
4113 # sort numerically descending
4114 @articles = sort {$b <=> $a} @files;
4116 # this sorts the %age hash by value instead of key
4117 # using an in-line function
4118 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4120 # sort using explicit subroutine name
4122 $age{$a} <=> $age{$b}; # presuming numeric
4124 @sortedclass = sort byage @class;
4126 sub backwards { $b cmp $a }
4127 @harry = qw(dog cat x Cain Abel);
4128 @george = qw(gone chased yz Punished Axed);
4130 # prints AbelCaincatdogx
4131 print sort backwards @harry;
4132 # prints xdogcatCainAbel
4133 print sort @george, 'to', @harry;
4134 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4136 # inefficiently sort by descending numeric compare using
4137 # the first integer after the first = sign, or the
4138 # whole record case-insensitively otherwise
4141 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4146 # same thing, but much more efficiently;
4147 # we'll build auxiliary indices instead
4151 push @nums, /=(\d+)/;
4156 $nums[$b] <=> $nums[$a]
4158 $caps[$a] cmp $caps[$b]
4162 # same thing, but without any temps
4163 @new = map { $_->[0] }
4164 sort { $b->[1] <=> $a->[1]
4167 } map { [$_, /=(\d+)/, uc($_)] } @old;
4169 # using a prototype allows you to use any comparison subroutine
4170 # as a sort subroutine (including other package's subroutines)
4172 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4175 @new = sort other::backwards @old;
4177 If you're using strict, you I<must not> declare $a
4178 and $b as lexicals. They are package globals. That means
4179 if you're in the C<main> package, it's
4181 @articles = sort {$main::b <=> $main::a} @files;
4185 @articles = sort {$::b <=> $::a} @files;
4187 but if you're in the C<FooPack> package, it's
4189 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4191 The comparison function is required to behave. If it returns
4192 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4193 sometimes saying the opposite, for example) the results are not
4196 =item splice ARRAY,OFFSET,LENGTH,LIST
4198 =item splice ARRAY,OFFSET,LENGTH
4200 =item splice ARRAY,OFFSET
4204 Removes the elements designated by OFFSET and LENGTH from an array, and
4205 replaces them with the elements of LIST, if any. In list context,
4206 returns the elements removed from the array. In scalar context,
4207 returns the last element removed, or C<undef> if no elements are
4208 removed. The array grows or shrinks as necessary.
4209 If OFFSET is negative then it starts that far from the end of the array.
4210 If LENGTH is omitted, removes everything from OFFSET onward.
4211 If LENGTH is negative, leaves that many elements off the end of the array.
4212 If both OFFSET and LENGTH are omitted, removes everything.
4214 The following equivalences hold (assuming C<$[ == 0>):
4216 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4217 pop(@a) splice(@a,-1)
4218 shift(@a) splice(@a,0,1)
4219 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4220 $a[$x] = $y splice(@a,$x,1,$y)
4222 Example, assuming array lengths are passed before arrays:
4224 sub aeq { # compare two list values
4225 my(@a) = splice(@_,0,shift);
4226 my(@b) = splice(@_,0,shift);
4227 return 0 unless @a == @b; # same len?
4229 return 0 if pop(@a) ne pop(@b);
4233 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4235 =item split /PATTERN/,EXPR,LIMIT
4237 =item split /PATTERN/,EXPR
4239 =item split /PATTERN/
4243 Splits a string into a list of strings and returns that list. By default,
4244 empty leading fields are preserved, and empty trailing ones are deleted.
4246 If not in list context, returns the number of fields found and splits into
4247 the C<@_> array. (In list context, you can force the split into C<@_> by
4248 using C<??> as the pattern delimiters, but it still returns the list
4249 value.) The use of implicit split to C<@_> is deprecated, however, because
4250 it clobbers your subroutine arguments.
4252 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4253 splits on whitespace (after skipping any leading whitespace). Anything
4254 matching PATTERN is taken to be a delimiter separating the fields. (Note
4255 that the delimiter may be longer than one character.)
4257 If LIMIT is specified and positive, splits into no more than that
4258 many fields (though it may split into fewer). If LIMIT is unspecified
4259 or zero, trailing null fields are stripped (which potential users
4260 of C<pop> would do well to remember). If LIMIT is negative, it is
4261 treated as if an arbitrarily large LIMIT had been specified.
4263 A pattern matching the null string (not to be confused with
4264 a null pattern C<//>, which is just one member of the set of patterns
4265 matching a null string) will split the value of EXPR into separate
4266 characters at each point it matches that way. For example:
4268 print join(':', split(/ */, 'hi there'));
4270 produces the output 'h:i:t:h:e:r:e'.
4272 The LIMIT parameter can be used to split a line partially
4274 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4276 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4277 one larger than the number of variables in the list, to avoid
4278 unnecessary work. For the list above LIMIT would have been 4 by
4279 default. In time critical applications it behooves you not to split
4280 into more fields than you really need.
4282 If the PATTERN contains parentheses, additional list elements are
4283 created from each matching substring in the delimiter.
4285 split(/([,-])/, "1-10,20", 3);
4287 produces the list value
4289 (1, '-', 10, ',', 20)
4291 If you had the entire header of a normal Unix email message in $header,
4292 you could split it up into fields and their values this way:
4294 $header =~ s/\n\s+/ /g; # fix continuation lines
4295 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4297 The pattern C</PATTERN/> may be replaced with an expression to specify
4298 patterns that vary at runtime. (To do runtime compilation only once,
4299 use C</$variable/o>.)
4301 As a special case, specifying a PATTERN of space (C<' '>) will split on
4302 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4303 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4304 will give you as many null initial fields as there are leading spaces.
4305 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4306 whitespace produces a null first field. A C<split> with no arguments
4307 really does a C<split(' ', $_)> internally.
4311 open(PASSWD, '/etc/passwd');
4313 ($login, $passwd, $uid, $gid,
4314 $gcos, $home, $shell) = split(/:/);
4318 (Note that $shell above will still have a newline on it. See L</chop>,
4319 L</chomp>, and L</join>.)
4321 =item sprintf FORMAT, LIST
4323 Returns a string formatted by the usual C<printf> conventions of the
4324 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4325 on your system for an explanation of the general principles.
4327 Perl does its own C<sprintf> formatting--it emulates the C
4328 function C<sprintf>, but it doesn't use it (except for floating-point
4329 numbers, and even then only the standard modifiers are allowed). As a
4330 result, any non-standard extensions in your local C<sprintf> are not
4331 available from Perl.
4333 Perl's C<sprintf> permits the following universally-known conversions:
4336 %c a character with the given number
4338 %d a signed integer, in decimal
4339 %u an unsigned integer, in decimal
4340 %o an unsigned integer, in octal
4341 %x an unsigned integer, in hexadecimal
4342 %e a floating-point number, in scientific notation
4343 %f a floating-point number, in fixed decimal notation
4344 %g a floating-point number, in %e or %f notation
4346 In addition, Perl permits the following widely-supported conversions:
4348 %X like %x, but using upper-case letters
4349 %E like %e, but using an upper-case "E"
4350 %G like %g, but with an upper-case "E" (if applicable)
4351 %b an unsigned integer, in binary
4352 %p a pointer (outputs the Perl value's address in hexadecimal)
4353 %n special: *stores* the number of characters output so far
4354 into the next variable in the parameter list
4356 Finally, for backward (and we do mean "backward") compatibility, Perl
4357 permits these unnecessary but widely-supported conversions:
4360 %D a synonym for %ld
4361 %U a synonym for %lu
4362 %O a synonym for %lo
4365 Perl permits the following universally-known flags between the C<%>
4366 and the conversion letter:
4368 space prefix positive number with a space
4369 + prefix positive number with a plus sign
4370 - left-justify within the field
4371 0 use zeros, not spaces, to right-justify
4372 # prefix non-zero octal with "0", non-zero hex with "0x"
4373 number minimum field width
4374 .number "precision": digits after decimal point for
4375 floating-point, max length for string, minimum length
4377 l interpret integer as C type "long" or "unsigned long"
4378 h interpret integer as C type "short" or "unsigned short"
4379 If no flags, interpret integer as C type "int" or "unsigned"
4381 There are also two Perl-specific flags:
4383 V interpret integer as Perl's standard integer type
4384 v interpret string as a vector of integers, output as
4385 numbers separated either by dots, or by an arbitrary
4386 string received from the argument list when the flag
4389 Where a number would appear in the flags, an asterisk (C<*>) may be
4390 used instead, in which case Perl uses the next item in the parameter
4391 list as the given number (that is, as the field width or precision).
4392 If a field width obtained through C<*> is negative, it has the same
4393 effect as the C<-> flag: left-justification.
4395 The C<v> flag is useful for displaying ordinal values of characters
4396 in arbitrary strings:
4398 printf "version is v%vd\n", $^V; # Perl's version
4399 printf "address is %*vX\n", ":", $addr; # IPv6 address
4400 printf "bits are %*vb\n", " ", $bits; # random bitstring
4402 If C<use locale> is in effect, the character used for the decimal
4403 point in formatted real numbers is affected by the LC_NUMERIC locale.
4406 If Perl understands "quads" (64-bit integers) (this requires
4407 either that the platform natively support quads or that Perl
4408 be specifically compiled to support quads), the characters
4412 print quads, and they may optionally be preceded by
4420 You can find out whether your Perl supports quads via L<Config>:
4423 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4426 If Perl understands "long doubles" (this requires that the platform
4427 support long doubles), the flags
4431 may optionally be preceded by
4439 You can find out whether your Perl supports long doubles via L<Config>:
4442 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4448 Return the square root of EXPR. If EXPR is omitted, returns square
4449 root of C<$_>. Only works on non-negative operands, unless you've
4450 loaded the standard Math::Complex module.
4453 print sqrt(-2); # prints 1.4142135623731i
4459 Sets the random number seed for the C<rand> operator. If EXPR is
4460 omitted, uses a semi-random value supplied by the kernel (if it supports
4461 the F</dev/urandom> device) or based on the current time and process
4462 ID, among other things. In versions of Perl prior to 5.004 the default
4463 seed was just the current C<time>. This isn't a particularly good seed,
4464 so many old programs supply their own seed value (often C<time ^ $$> or
4465 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4467 In fact, it's usually not necessary to call C<srand> at all, because if
4468 it is not called explicitly, it is called implicitly at the first use of
4469 the C<rand> operator. However, this was not the case in version of Perl
4470 before 5.004, so if your script will run under older Perl versions, it
4471 should call C<srand>.
4473 Note that you need something much more random than the default seed for
4474 cryptographic purposes. Checksumming the compressed output of one or more
4475 rapidly changing operating system status programs is the usual method. For
4478 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4480 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4483 Do I<not> call C<srand> multiple times in your program unless you know
4484 exactly what you're doing and why you're doing it. The point of the
4485 function is to "seed" the C<rand> function so that C<rand> can produce
4486 a different sequence each time you run your program. Just do it once at the
4487 top of your program, or you I<won't> get random numbers out of C<rand>!
4489 Frequently called programs (like CGI scripts) that simply use
4493 for a seed can fall prey to the mathematical property that
4497 one-third of the time. So don't do that.
4499 =item stat FILEHANDLE
4505 Returns a 13-element list giving the status info for a file, either
4506 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4507 it stats C<$_>. Returns a null list if the stat fails. Typically used
4510 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4511 $atime,$mtime,$ctime,$blksize,$blocks)
4514 Not all fields are supported on all filesystem types. Here are the
4515 meaning of the fields:
4517 0 dev device number of filesystem
4519 2 mode file mode (type and permissions)
4520 3 nlink number of (hard) links to the file
4521 4 uid numeric user ID of file's owner
4522 5 gid numeric group ID of file's owner
4523 6 rdev the device identifier (special files only)
4524 7 size total size of file, in bytes
4525 8 atime last access time in seconds since the epoch
4526 9 mtime last modify time in seconds since the epoch
4527 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4528 11 blksize preferred block size for file system I/O
4529 12 blocks actual number of blocks allocated
4531 (The epoch was at 00:00 January 1, 1970 GMT.)
4533 If stat is passed the special filehandle consisting of an underline, no
4534 stat is done, but the current contents of the stat structure from the
4535 last stat or filetest are returned. Example:
4537 if (-x $file && (($d) = stat(_)) && $d < 0) {
4538 print "$file is executable NFS file\n";
4541 (This works on machines only for which the device number is negative
4544 Because the mode contains both the file type and its permissions, you
4545 should mask off the file type portion and (s)printf using a C<"%o">
4546 if you want to see the real permissions.
4548 $mode = (stat($filename))[2];
4549 printf "Permissions are %04o\n", $mode & 07777;
4551 In scalar context, C<stat> returns a boolean value indicating success
4552 or failure, and, if successful, sets the information associated with
4553 the special filehandle C<_>.
4555 The File::stat module provides a convenient, by-name access mechanism:
4558 $sb = stat($filename);
4559 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4560 $filename, $sb->size, $sb->mode & 07777,
4561 scalar localtime $sb->mtime;
4563 You can import symbolic mode constants (C<S_IF*>) and functions
4564 (C<S_IS*>) from the Fcntl module:
4568 $mode = (stat($filename))[2];
4570 $user_rwx = ($mode & S_IRWXU) >> 6;
4571 $group_read = ($mode & S_IRGRP) >> 3;
4572 $other_execute = $mode & S_IXOTH;
4574 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4576 $is_setuid = $mode & S_ISUID;
4577 $is_setgid = S_ISDIR($mode);
4579 You could write the last two using the C<-u> and C<-d> operators.
4580 The commonly available S_IF* constants are
4582 # Permissions: read, write, execute, for user, group, others.
4584 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4585 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4586 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4588 # Setuid/Setgid/Stickiness.
4590 S_ISUID S_ISGID S_ISVTX S_ISTXT
4592 # File types. Not necessarily all are available on your system.
4594 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4596 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4598 S_IREAD S_IWRITE S_IEXEC
4600 and the S_IF* functions are
4602 S_IFMODE($mode) the part of $mode containg the permission bits
4603 and the setuid/setgid/sticky bits
4605 S_IFMT($mode) the part of $mode containing the file type
4606 which can be bit-anded with e.g. S_IFREG
4607 or with the following functions
4609 # The operators -f, -d, -l, -b, -c, -p, and -s.
4611 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4612 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4614 # No direct -X operator counterpart, but for the first one
4615 # the -g operator is often equivalent. The ENFMT stands for
4616 # record flocking enforcement, a platform-dependent feature.
4618 S_ISENFMT($mode) S_ISWHT($mode)
4620 See your native chmod(2) and stat(2) documentation for more details
4621 about the S_* constants.
4627 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4628 doing many pattern matches on the string before it is next modified.
4629 This may or may not save time, depending on the nature and number of
4630 patterns you are searching on, and on the distribution of character
4631 frequencies in the string to be searched--you probably want to compare
4632 run times with and without it to see which runs faster. Those loops
4633 which scan for many short constant strings (including the constant
4634 parts of more complex patterns) will benefit most. You may have only
4635 one C<study> active at a time--if you study a different scalar the first
4636 is "unstudied". (The way C<study> works is this: a linked list of every
4637 character in the string to be searched is made, so we know, for
4638 example, where all the C<'k'> characters are. From each search string,
4639 the rarest character is selected, based on some static frequency tables
4640 constructed from some C programs and English text. Only those places
4641 that contain this "rarest" character are examined.)
4643 For example, here is a loop that inserts index producing entries
4644 before any line containing a certain pattern:
4648 print ".IX foo\n" if /\bfoo\b/;
4649 print ".IX bar\n" if /\bbar\b/;
4650 print ".IX blurfl\n" if /\bblurfl\b/;
4655 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4656 will be looked at, because C<f> is rarer than C<o>. In general, this is
4657 a big win except in pathological cases. The only question is whether
4658 it saves you more time than it took to build the linked list in the
4661 Note that if you have to look for strings that you don't know till
4662 runtime, you can build an entire loop as a string and C<eval> that to
4663 avoid recompiling all your patterns all the time. Together with
4664 undefining C<$/> to input entire files as one record, this can be very
4665 fast, often faster than specialized programs like fgrep(1). The following
4666 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4667 out the names of those files that contain a match:
4669 $search = 'while (<>) { study;';
4670 foreach $word (@words) {
4671 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4676 eval $search; # this screams
4677 $/ = "\n"; # put back to normal input delimiter
4678 foreach $file (sort keys(%seen)) {
4686 =item sub NAME BLOCK
4688 This is subroutine definition, not a real function I<per se>. With just a
4689 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4690 Without a NAME, it's an anonymous function declaration, and does actually
4691 return a value: the CODE ref of the closure you just created. See L<perlsub>
4692 and L<perlref> for details.
4694 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4696 =item substr EXPR,OFFSET,LENGTH
4698 =item substr EXPR,OFFSET
4700 Extracts a substring out of EXPR and returns it. First character is at
4701 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4702 If OFFSET is negative (or more precisely, less than C<$[>), starts
4703 that far from the end of the string. If LENGTH is omitted, returns
4704 everything to the end of the string. If LENGTH is negative, leaves that
4705 many characters off the end of the string.
4707 You can use the substr() function as an lvalue, in which case EXPR
4708 must itself be an lvalue. If you assign something shorter than LENGTH,
4709 the string will shrink, and if you assign something longer than LENGTH,
4710 the string will grow to accommodate it. To keep the string the same
4711 length you may need to pad or chop your value using C<sprintf>.
4713 If OFFSET and LENGTH specify a substring that is partly outside the
4714 string, only the part within the string is returned. If the substring
4715 is beyond either end of the string, substr() returns the undefined
4716 value and produces a warning. When used as an lvalue, specifying a
4717 substring that is entirely outside the string is a fatal error.
4718 Here's an example showing the behavior for boundary cases:
4721 substr($name, 4) = 'dy'; # $name is now 'freddy'
4722 my $null = substr $name, 6, 2; # returns '' (no warning)
4723 my $oops = substr $name, 7; # returns undef, with warning
4724 substr($name, 7) = 'gap'; # fatal error
4726 An alternative to using substr() as an lvalue is to specify the
4727 replacement string as the 4th argument. This allows you to replace
4728 parts of the EXPR and return what was there before in one operation,
4729 just as you can with splice().
4731 =item symlink OLDFILE,NEWFILE
4733 Creates a new filename symbolically linked to the old filename.
4734 Returns C<1> for success, C<0> otherwise. On systems that don't support
4735 symbolic links, produces a fatal error at run time. To check for that,
4738 $symlink_exists = eval { symlink("",""); 1 };
4742 Calls the system call specified as the first element of the list,
4743 passing the remaining elements as arguments to the system call. If
4744 unimplemented, produces a fatal error. The arguments are interpreted
4745 as follows: if a given argument is numeric, the argument is passed as
4746 an int. If not, the pointer to the string value is passed. You are
4747 responsible to make sure a string is pre-extended long enough to
4748 receive any result that might be written into a string. You can't use a
4749 string literal (or other read-only string) as an argument to C<syscall>
4750 because Perl has to assume that any string pointer might be written
4752 integer arguments are not literals and have never been interpreted in a
4753 numeric context, you may need to add C<0> to them to force them to look
4754 like numbers. This emulates the C<syswrite> function (or vice versa):
4756 require 'syscall.ph'; # may need to run h2ph
4758 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4760 Note that Perl supports passing of up to only 14 arguments to your system call,
4761 which in practice should usually suffice.
4763 Syscall returns whatever value returned by the system call it calls.
4764 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4765 Note that some system calls can legitimately return C<-1>. The proper
4766 way to handle such calls is to assign C<$!=0;> before the call and
4767 check the value of C<$!> if syscall returns C<-1>.
4769 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4770 number of the read end of the pipe it creates. There is no way
4771 to retrieve the file number of the other end. You can avoid this
4772 problem by using C<pipe> instead.
4774 =item sysopen FILEHANDLE,FILENAME,MODE
4776 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4778 Opens the file whose filename is given by FILENAME, and associates it
4779 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4780 the name of the real filehandle wanted. This function calls the
4781 underlying operating system's C<open> function with the parameters
4782 FILENAME, MODE, PERMS.
4784 The possible values and flag bits of the MODE parameter are
4785 system-dependent; they are available via the standard module C<Fcntl>.
4786 See the documentation of your operating system's C<open> to see which
4787 values and flag bits are available. You may combine several flags
4788 using the C<|>-operator.
4790 Some of the most common values are C<O_RDONLY> for opening the file in
4791 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4792 and C<O_RDWR> for opening the file in read-write mode, and.
4794 For historical reasons, some values work on almost every system
4795 supported by perl: zero means read-only, one means write-only, and two
4796 means read/write. We know that these values do I<not> work under
4797 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4798 use them in new code.
4800 If the file named by FILENAME does not exist and the C<open> call creates
4801 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4802 PERMS specifies the permissions of the newly created file. If you omit
4803 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4804 These permission values need to be in octal, and are modified by your
4805 process's current C<umask>.
4807 In many systems the C<O_EXCL> flag is available for opening files in
4808 exclusive mode. This is B<not> locking: exclusiveness means here that
4809 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4812 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4814 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4815 that takes away the user's option to have a more permissive umask.
4816 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4819 Note that C<sysopen> depends on the fdopen() C library function.
4820 On many UNIX systems, fdopen() is known to fail when file descriptors
4821 exceed a certain value, typically 255. If you need more file
4822 descriptors than that, consider rebuilding Perl to use the C<sfio>
4823 library, or perhaps using the POSIX::open() function.
4825 See L<perlopentut> for a kinder, gentler explanation of opening files.
4827 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4829 =item sysread FILEHANDLE,SCALAR,LENGTH
4831 Attempts to read LENGTH bytes of data into variable SCALAR from the
4832 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4833 so mixing this with other kinds of reads, C<print>, C<write>,
4834 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4835 usually buffers data. Returns the number of bytes actually read, C<0>
4836 at end of file, or undef if there was an error. SCALAR will be grown or
4837 shrunk so that the last byte actually read is the last byte of the
4838 scalar after the read.
4840 An OFFSET may be specified to place the read data at some place in the
4841 string other than the beginning. A negative OFFSET specifies
4842 placement at that many bytes counting backwards from the end of the
4843 string. A positive OFFSET greater than the length of SCALAR results
4844 in the string being padded to the required size with C<"\0"> bytes before
4845 the result of the read is appended.
4847 There is no syseof() function, which is ok, since eof() doesn't work
4848 very well on device files (like ttys) anyway. Use sysread() and check
4849 for a return value for 0 to decide whether you're done.
4851 =item sysseek FILEHANDLE,POSITION,WHENCE
4853 Sets FILEHANDLE's system position using the system call lseek(2). It
4854 bypasses stdio, so mixing this with reads (other than C<sysread>),
4855 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4856 FILEHANDLE may be an expression whose value gives the name of the
4857 filehandle. The values for WHENCE are C<0> to set the new position to
4858 POSITION, C<1> to set the it to the current position plus POSITION,
4859 and C<2> to set it to EOF plus POSITION (typically negative). For
4860 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4861 C<SEEK_END> (start of the file, current position, end of the file)
4862 from the Fcntl module.
4864 Returns the new position, or the undefined value on failure. A position
4865 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4866 true on success and false on failure, yet you can still easily determine
4871 =item system PROGRAM LIST
4873 Does exactly the same thing as C<exec LIST>, except that a fork is
4874 done first, and the parent process waits for the child process to
4875 complete. Note that argument processing varies depending on the
4876 number of arguments. If there is more than one argument in LIST,
4877 or if LIST is an array with more than one value, starts the program
4878 given by the first element of the list with arguments given by the
4879 rest of the list. If there is only one scalar argument, the argument
4880 is checked for shell metacharacters, and if there are any, the
4881 entire argument is passed to the system's command shell for parsing
4882 (this is C</bin/sh -c> on Unix platforms, but varies on other
4883 platforms). If there are no shell metacharacters in the argument,
4884 it is split into words and passed directly to C<execvp>, which is
4887 All files opened for output are flushed before attempting the exec().
4889 The return value is the exit status of the program as
4890 returned by the C<wait> call. To get the actual exit value divide by
4891 256. See also L</exec>. This is I<not> what you want to use to capture
4892 the output from a command, for that you should use merely backticks or
4893 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4894 indicates a failure to start the program (inspect $! for the reason).
4896 Like C<exec>, C<system> allows you to lie to a program about its name if
4897 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4899 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4900 program they're running doesn't actually interrupt your program.
4902 @args = ("command", "arg1", "arg2");
4904 or die "system @args failed: $?"
4906 You can check all the failure possibilities by inspecting
4909 $exit_value = $? >> 8;
4910 $signal_num = $? & 127;
4911 $dumped_core = $? & 128;
4913 When the arguments get executed via the system shell, results
4914 and return codes will be subject to its quirks and capabilities.
4915 See L<perlop/"`STRING`"> and L</exec> for details.
4917 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4919 =item syswrite FILEHANDLE,SCALAR,LENGTH
4921 =item syswrite FILEHANDLE,SCALAR
4923 Attempts to write LENGTH bytes of data from variable SCALAR to the
4924 specified FILEHANDLE, using the system call write(2). If LENGTH
4925 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4926 this with reads (other than C<sysread())>, C<print>, C<write>,
4927 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4928 usually buffers data. Returns the number of bytes actually written,
4929 or C<undef> if there was an error. If the LENGTH is greater than
4930 the available data in the SCALAR after the OFFSET, only as much
4931 data as is available will be written.
4933 An OFFSET may be specified to write the data from some part of the
4934 string other than the beginning. A negative OFFSET specifies writing
4935 that many bytes counting backwards from the end of the string. In the
4936 case the SCALAR is empty you can use OFFSET but only zero offset.
4938 =item tell FILEHANDLE
4942 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4943 expression whose value gives the name of the actual filehandle. If
4944 FILEHANDLE is omitted, assumes the file last read.
4946 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4948 =item telldir DIRHANDLE
4950 Returns the current position of the C<readdir> routines on DIRHANDLE.
4951 Value may be given to C<seekdir> to access a particular location in a
4952 directory. Has the same caveats about possible directory compaction as
4953 the corresponding system library routine.
4955 =item tie VARIABLE,CLASSNAME,LIST
4957 This function binds a variable to a package class that will provide the
4958 implementation for the variable. VARIABLE is the name of the variable
4959 to be enchanted. CLASSNAME is the name of a class implementing objects
4960 of correct type. Any additional arguments are passed to the C<new>
4961 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4962 or C<TIEHASH>). Typically these are arguments such as might be passed
4963 to the C<dbm_open()> function of C. The object returned by the C<new>
4964 method is also returned by the C<tie> function, which would be useful
4965 if you want to access other methods in CLASSNAME.
4967 Note that functions such as C<keys> and C<values> may return huge lists
4968 when used on large objects, like DBM files. You may prefer to use the
4969 C<each> function to iterate over such. Example:
4971 # print out history file offsets
4973 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4974 while (($key,$val) = each %HIST) {
4975 print $key, ' = ', unpack('L',$val), "\n";
4979 A class implementing a hash should have the following methods:
4981 TIEHASH classname, LIST
4983 STORE this, key, value
4988 NEXTKEY this, lastkey
4991 A class implementing an ordinary array should have the following methods:
4993 TIEARRAY classname, LIST
4995 STORE this, key, value
4997 STORESIZE this, count
5003 SPLICE this, offset, length, LIST
5007 A class implementing a file handle should have the following methods:
5009 TIEHANDLE classname, LIST
5010 READ this, scalar, length, offset
5013 WRITE this, scalar, length, offset
5015 PRINTF this, format, LIST
5019 A class implementing a scalar should have the following methods:
5021 TIESCALAR classname, LIST
5026 Not all methods indicated above need be implemented. See L<perltie>,
5027 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5029 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5030 for you--you need to do that explicitly yourself. See L<DB_File>
5031 or the F<Config> module for interesting C<tie> implementations.
5033 For further details see L<perltie>, L<"tied VARIABLE">.
5037 Returns a reference to the object underlying VARIABLE (the same value
5038 that was originally returned by the C<tie> call that bound the variable
5039 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5044 Returns the number of non-leap seconds since whatever time the system
5045 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5046 and 00:00:00 UTC, January 1, 1970 for most other systems).
5047 Suitable for feeding to C<gmtime> and C<localtime>.
5049 For measuring time in better granularity than one second,
5050 you may use either the Time::HiRes module from CPAN, or
5051 if you have gettimeofday(2), you may be able to use the
5052 C<syscall> interface of Perl, see L<perlfaq8> for details.
5056 Returns a four-element list giving the user and system times, in
5057 seconds, for this process and the children of this process.
5059 ($user,$system,$cuser,$csystem) = times;
5063 The transliteration operator. Same as C<y///>. See L<perlop>.
5065 =item truncate FILEHANDLE,LENGTH
5067 =item truncate EXPR,LENGTH
5069 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5070 specified length. Produces a fatal error if truncate isn't implemented
5071 on your system. Returns true if successful, the undefined value
5078 Returns an uppercased version of EXPR. This is the internal function
5079 implementing the C<\U> escape in double-quoted strings.
5080 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5081 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5082 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5084 If EXPR is omitted, uses C<$_>.
5090 Returns the value of EXPR with the first character
5091 in uppercase (titlecase in Unicode). This is
5092 the internal function implementing the C<\u> escape in double-quoted strings.
5093 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5096 If EXPR is omitted, uses C<$_>.
5102 Sets the umask for the process to EXPR and returns the previous value.
5103 If EXPR is omitted, merely returns the current umask.
5105 The Unix permission C<rwxr-x---> is represented as three sets of three
5106 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5107 and isn't one of the digits). The C<umask> value is such a number
5108 representing disabled permissions bits. The permission (or "mode")
5109 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5110 even if you tell C<sysopen> to create a file with permissions C<0777>,
5111 if your umask is C<0022> then the file will actually be created with
5112 permissions C<0755>. If your C<umask> were C<0027> (group can't
5113 write; others can't read, write, or execute), then passing
5114 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5117 Here's some advice: supply a creation mode of C<0666> for regular
5118 files (in C<sysopen>) and one of C<0777> for directories (in
5119 C<mkdir>) and executable files. This gives users the freedom of
5120 choice: if they want protected files, they might choose process umasks
5121 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5122 Programs should rarely if ever make policy decisions better left to
5123 the user. The exception to this is when writing files that should be
5124 kept private: mail files, web browser cookies, I<.rhosts> files, and
5127 If umask(2) is not implemented on your system and you are trying to
5128 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5129 fatal error at run time. If umask(2) is not implemented and you are
5130 not trying to restrict access for yourself, returns C<undef>.
5132 Remember that a umask is a number, usually given in octal; it is I<not> a
5133 string of octal digits. See also L</oct>, if all you have is a string.
5139 Undefines the value of EXPR, which must be an lvalue. Use only on a
5140 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5141 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5142 will probably not do what you expect on most predefined variables or
5143 DBM list values, so don't do that; see L<delete>.) Always returns the
5144 undefined value. You can omit the EXPR, in which case nothing is
5145 undefined, but you still get an undefined value that you could, for
5146 instance, return from a subroutine, assign to a variable or pass as a
5147 parameter. Examples:
5150 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5154 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5155 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5156 select undef, undef, undef, 0.25;
5157 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5159 Note that this is a unary operator, not a list operator.
5165 Deletes a list of files. Returns the number of files successfully
5168 $cnt = unlink 'a', 'b', 'c';
5172 Note: C<unlink> will not delete directories unless you are superuser and
5173 the B<-U> flag is supplied to Perl. Even if these conditions are
5174 met, be warned that unlinking a directory can inflict damage on your
5175 filesystem. Use C<rmdir> instead.
5177 If LIST is omitted, uses C<$_>.
5179 =item unpack TEMPLATE,EXPR
5181 C<unpack> does the reverse of C<pack>: it takes a string
5182 and expands it out into a list of values.
5183 (In scalar context, it returns merely the first value produced.)
5185 The string is broken into chunks described by the TEMPLATE. Each chunk
5186 is converted separately to a value. Typically, either the string is a result
5187 of C<pack>, or the bytes of the string represent a C structure of some
5190 The TEMPLATE has the same format as in the C<pack> function.
5191 Here's a subroutine that does substring:
5194 my($what,$where,$howmuch) = @_;
5195 unpack("x$where a$howmuch", $what);
5200 sub ordinal { unpack("c",$_[0]); } # same as ord()
5202 In addition to fields allowed in pack(), you may prefix a field with
5203 a %E<lt>numberE<gt> to indicate that
5204 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
5205 themselves. Default is a 16-bit checksum. Checksum is calculated by
5206 summing numeric values of expanded values (for string fields the sum of
5207 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5209 For example, the following
5210 computes the same number as the System V sum program:
5214 unpack("%32C*",<>) % 65535;
5217 The following efficiently counts the number of set bits in a bit vector:
5219 $setbits = unpack("%32b*", $selectmask);
5221 The C<p> and C<P> formats should be used with care. Since Perl
5222 has no way of checking whether the value passed to C<unpack()>
5223 corresponds to a valid memory location, passing a pointer value that's
5224 not known to be valid is likely to have disastrous consequences.
5226 If the repeat count of a field is larger than what the remainder of
5227 the input string allows, repeat count is decreased. If the input string
5228 is longer than one described by the TEMPLATE, the rest is ignored.
5230 See L</pack> for more examples and notes.
5232 =item untie VARIABLE
5234 Breaks the binding between a variable and a package. (See C<tie>.)
5236 =item unshift ARRAY,LIST
5238 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5239 depending on how you look at it. Prepends list to the front of the
5240 array, and returns the new number of elements in the array.
5242 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5244 Note the LIST is prepended whole, not one element at a time, so the
5245 prepended elements stay in the same order. Use C<reverse> to do the
5248 =item use Module VERSION LIST
5250 =item use Module VERSION
5252 =item use Module LIST
5258 Imports some semantics into the current package from the named module,
5259 generally by aliasing certain subroutine or variable names into your
5260 package. It is exactly equivalent to
5262 BEGIN { require Module; import Module LIST; }
5264 except that Module I<must> be a bareword.
5266 VERSION, which can be specified as a literal of the form v5.6.1, demands
5267 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5268 as recent as that version. (For compatibility with older versions of Perl,
5269 a numeric literal will also be interpreted as VERSION.) If the version
5270 of the running Perl interpreter is less than VERSION, then an error
5271 message is printed and Perl exits immediately without attempting to
5272 parse the rest of the file. Compare with L</require>, which can do a
5273 similar check at run time.
5275 use v5.6.1; # compile time version check
5277 use 5.005_03; # float version allowed for compatibility
5279 This is often useful if you need to check the current Perl version before
5280 C<use>ing library modules that have changed in incompatible ways from
5281 older versions of Perl. (We try not to do this more than we have to.)
5283 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5284 C<require> makes sure the module is loaded into memory if it hasn't been
5285 yet. The C<import> is not a builtin--it's just an ordinary static method
5286 call into the C<Module> package to tell the module to import the list of
5287 features back into the current package. The module can implement its
5288 C<import> method any way it likes, though most modules just choose to
5289 derive their C<import> method via inheritance from the C<Exporter> class that
5290 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5291 method can be found then the call is skipped.
5293 If you don't want your namespace altered, explicitly supply an empty list:
5297 That is exactly equivalent to
5299 BEGIN { require Module }
5301 If the VERSION argument is present between Module and LIST, then the
5302 C<use> will call the VERSION method in class Module with the given
5303 version as an argument. The default VERSION method, inherited from
5304 the UNIVERSAL class, croaks if the given version is larger than the
5305 value of the variable C<$Module::VERSION>.
5307 Again, there is a distinction between omitting LIST (C<import> called
5308 with no arguments) and an explicit empty LIST C<()> (C<import> not
5309 called). Note that there is no comma after VERSION!
5311 Because this is a wide-open interface, pragmas (compiler directives)
5312 are also implemented this way. Currently implemented pragmas are:
5316 use sigtrap qw(SEGV BUS);
5317 use strict qw(subs vars refs);
5318 use subs qw(afunc blurfl);
5319 use warnings qw(all);
5321 Some of these pseudo-modules import semantics into the current
5322 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5323 which import symbols into the current package (which are effective
5324 through the end of the file).
5326 There's a corresponding C<no> command that unimports meanings imported
5327 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5333 If no C<unimport> method can be found the call fails with a fatal error.
5335 See L<perlmod> for a list of standard modules and pragmas.
5339 Changes the access and modification times on each file of a list of
5340 files. The first two elements of the list must be the NUMERICAL access
5341 and modification times, in that order. Returns the number of files
5342 successfully changed. The inode change time of each file is set
5343 to the current time. This code has the same effect as the C<touch>
5344 command if the files already exist:
5348 utime $now, $now, @ARGV;
5352 Returns a list consisting of all the values of the named hash. (In a
5353 scalar context, returns the number of values.) The values are
5354 returned in an apparently random order. The actual random order is
5355 subject to change in future versions of perl, but it is guaranteed to
5356 be the same order as either the C<keys> or C<each> function would
5357 produce on the same (unmodified) hash.
5359 Note that you cannot modify the values of a hash this way, because the
5360 returned list is just a copy. You need to use a hash slice for that,
5361 since it's lvaluable in a way that values() is not.
5363 for (values %hash) { s/foo/bar/g } # FAILS!
5364 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5366 As a side effect, calling values() resets the HASH's internal iterator.
5367 See also C<keys>, C<each>, and C<sort>.
5369 =item vec EXPR,OFFSET,BITS
5371 Treats the string in EXPR as a bit vector made up of elements of
5372 width BITS, and returns the value of the element specified by OFFSET
5373 as an unsigned integer. BITS therefore specifies the number of bits
5374 that are reserved for each element in the bit vector. This must
5375 be a power of two from 1 to 32 (or 64, if your platform supports
5378 If BITS is 8, "elements" coincide with bytes of the input string.
5380 If BITS is 16 or more, bytes of the input string are grouped into chunks
5381 of size BITS/8, and each group is converted to a number as with
5382 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5383 for BITS==64). See L<"pack"> for details.
5385 If bits is 4 or less, the string is broken into bytes, then the bits
5386 of each byte are broken into 8/BITS groups. Bits of a byte are
5387 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5388 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5389 breaking the single input byte C<chr(0x36)> into two groups gives a list
5390 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5392 C<vec> may also be assigned to, in which case parentheses are needed
5393 to give the expression the correct precedence as in
5395 vec($image, $max_x * $x + $y, 8) = 3;
5397 If the selected element is off the end of the string, the value 0 is
5398 returned. If an element off the end of the string is written to,
5399 Perl will first extend the string with sufficiently many zero bytes.
5401 Strings created with C<vec> can also be manipulated with the logical
5402 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5403 vector operation is desired when both operands are strings.
5404 See L<perlop/"Bitwise String Operators">.
5406 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5407 The comments show the string after each step. Note that this code works
5408 in the same way on big-endian or little-endian machines.
5411 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5413 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5414 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5416 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5417 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5418 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5419 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5420 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5421 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5423 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5424 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5425 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5428 To transform a bit vector into a string or list of 0's and 1's, use these:
5430 $bits = unpack("b*", $vector);
5431 @bits = split(//, unpack("b*", $vector));
5433 If you know the exact length in bits, it can be used in place of the C<*>.
5435 Here is an example to illustrate how the bits actually fall in place:
5441 unpack("V",$_) 01234567890123456789012345678901
5442 ------------------------------------------------------------------
5447 for ($shift=0; $shift < $width; ++$shift) {
5448 for ($off=0; $off < 32/$width; ++$off) {
5449 $str = pack("B*", "0"x32);
5450 $bits = (1<<$shift);
5451 vec($str, $off, $width) = $bits;
5452 $res = unpack("b*",$str);
5453 $val = unpack("V", $str);
5460 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5461 $off, $width, $bits, $val, $res
5465 Regardless of the machine architecture on which it is run, the above
5466 example should print the following table:
5469 unpack("V",$_) 01234567890123456789012345678901
5470 ------------------------------------------------------------------
5471 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5472 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5473 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5474 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5475 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5476 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5477 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5478 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5479 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5480 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5481 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5482 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5483 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5484 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5485 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5486 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5487 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5488 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5489 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5490 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5491 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5492 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5493 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5494 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5495 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5496 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5497 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5498 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5499 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5500 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5501 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5502 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5503 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5504 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5505 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5506 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5507 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5508 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5509 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5510 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5511 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5512 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5513 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5514 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5515 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5516 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5517 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5518 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5519 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5520 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5521 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5522 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5523 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5524 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5525 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5526 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5527 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5528 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5529 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5530 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5531 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5532 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5533 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5534 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5535 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5536 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5537 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5538 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5539 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5540 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5541 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5542 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5543 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5544 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5545 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5546 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5547 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5548 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5549 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5550 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5551 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5552 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5553 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5554 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5555 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5556 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5557 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5558 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5559 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5560 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5561 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5562 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5563 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5564 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5565 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5566 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5567 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5568 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5569 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5570 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5571 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5572 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5573 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5574 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5575 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5576 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5577 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5578 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5579 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5580 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5581 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5582 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5583 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5584 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5585 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5586 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5587 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5588 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5589 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5590 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5591 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5592 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5593 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5594 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5595 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5596 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5597 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5598 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5602 Behaves like the wait(2) system call on your system: it waits for a child
5603 process to terminate and returns the pid of the deceased process, or
5604 C<-1> if there are no child processes. The status is returned in C<$?>.
5605 Note that a return value of C<-1> could mean that child processes are
5606 being automatically reaped, as described in L<perlipc>.
5608 =item waitpid PID,FLAGS
5610 Waits for a particular child process to terminate and returns the pid of
5611 the deceased process, or C<-1> if there is no such child process. On some
5612 systems, a value of 0 indicates that there are processes still running.
5613 The status is returned in C<$?>. If you say
5615 use POSIX ":sys_wait_h";
5618 $kid = waitpid(-1,&WNOHANG);
5621 then you can do a non-blocking wait for all pending zombie processes.
5622 Non-blocking wait is available on machines supporting either the
5623 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5624 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5625 system call by remembering the status values of processes that have
5626 exited but have not been harvested by the Perl script yet.)
5628 Note that on some systems, a return value of C<-1> could mean that child
5629 processes are being automatically reaped. See L<perlipc> for details,
5630 and for other examples.
5634 Returns true if the context of the currently executing subroutine is
5635 looking for a list value. Returns false if the context is looking
5636 for a scalar. Returns the undefined value if the context is looking
5637 for no value (void context).
5639 return unless defined wantarray; # don't bother doing more
5640 my @a = complex_calculation();
5641 return wantarray ? @a : "@a";
5643 This function should have been named wantlist() instead.
5647 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5650 If LIST is empty and C<$@> already contains a value (typically from a
5651 previous eval) that value is used after appending C<"\t...caught">
5652 to C<$@>. This is useful for staying almost, but not entirely similar to
5655 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5657 No message is printed if there is a C<$SIG{__WARN__}> handler
5658 installed. It is the handler's responsibility to deal with the message
5659 as it sees fit (like, for instance, converting it into a C<die>). Most
5660 handlers must therefore make arrangements to actually display the
5661 warnings that they are not prepared to deal with, by calling C<warn>
5662 again in the handler. Note that this is quite safe and will not
5663 produce an endless loop, since C<__WARN__> hooks are not called from
5666 You will find this behavior is slightly different from that of
5667 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5668 instead call C<die> again to change it).
5670 Using a C<__WARN__> handler provides a powerful way to silence all
5671 warnings (even the so-called mandatory ones). An example:
5673 # wipe out *all* compile-time warnings
5674 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5676 my $foo = 20; # no warning about duplicate my $foo,
5677 # but hey, you asked for it!
5678 # no compile-time or run-time warnings before here
5681 # run-time warnings enabled after here
5682 warn "\$foo is alive and $foo!"; # does show up
5684 See L<perlvar> for details on setting C<%SIG> entries, and for more
5685 examples. See the Carp module for other kinds of warnings using its
5686 carp() and cluck() functions.
5688 =item write FILEHANDLE
5694 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5695 using the format associated with that file. By default the format for
5696 a file is the one having the same name as the filehandle, but the
5697 format for the current output channel (see the C<select> function) may be set
5698 explicitly by assigning the name of the format to the C<$~> variable.
5700 Top of form processing is handled automatically: if there is
5701 insufficient room on the current page for the formatted record, the
5702 page is advanced by writing a form feed, a special top-of-page format
5703 is used to format the new page header, and then the record is written.
5704 By default the top-of-page format is the name of the filehandle with
5705 "_TOP" appended, but it may be dynamically set to the format of your
5706 choice by assigning the name to the C<$^> variable while the filehandle is
5707 selected. The number of lines remaining on the current page is in
5708 variable C<$->, which can be set to C<0> to force a new page.
5710 If FILEHANDLE is unspecified, output goes to the current default output
5711 channel, which starts out as STDOUT but may be changed by the
5712 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5713 is evaluated and the resulting string is used to look up the name of
5714 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5716 Note that write is I<not> the opposite of C<read>. Unfortunately.
5720 The transliteration operator. Same as C<tr///>. See L<perlop>.