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 (>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, DISCIPLINE
444 =item binmode FILEHANDLE
446 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
447 on systems where the run-time libraries distinguish between binary and
448 text files. If FILEHANDLE is an expression, the value is taken as the
449 name of the filehandle. DISCIPLINE can be either of C<":raw"> for
450 binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
451 omitted, it defaults to C<":raw">.
453 binmode() should be called after open() but before any I/O is done on
456 On many systems binmode() currently has no effect, but in future, it
457 will be extended to support user-defined input and output disciplines.
458 On some systems binmode() is necessary when you're not working with a
459 text file. For the sake of portability it is a good idea to always use
460 it when appropriate, and to never use it when it isn't appropriate.
462 In other words: Regardless of platform, use binmode() on binary
463 files, and do not use binmode() on text files.
465 The C<open> pragma can be used to establish default disciplines.
468 The operating system, device drivers, C libraries, and Perl run-time
469 system all work together to let the programmer treat a single
470 character (C<\n>) as the line terminator, irrespective of the external
471 representation. On many operating systems, the native text file
472 representation matches the internal representation, but on some
473 platforms the external representation of C<\n> is made up of more than
476 Mac OS and all variants of Unix use a single character to end each line
477 in the external representation of text (even though that single
478 character is not necessarily the same across these platforms).
479 Consequently binmode() has no effect on these operating systems. In
480 other systems like VMS, MS-DOS and the various flavors of MS-Windows
481 your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
482 files are the two characters C<\cM\cJ>. That means that, if you don't
483 use binmode() on these systems, C<\cM\cJ> sequences on disk will be
484 converted to C<\n> on input, and any C<\n> in your program will be
485 converted back to C<\cM\cJ> on output. This is what you want for text
486 files, but it can be disastrous for binary files.
488 Another consequence of using binmode() (on some systems) is that
489 special end-of-file markers will be seen as part of the data stream.
490 For systems from the Microsoft family this means that if your binary
491 data contains C<\cZ>, the I/O subsystem will ragard it as the end of
492 the file, unless you use binmode().
494 binmode() is not only important for readline() and print() operations,
495 but also when using read(), seek(), sysread(), syswrite() and tell()
496 (see L<perlport> for more details). See the C<$/> and C<$\> variables
497 in L<perlvar> for how to manually set your input and output
498 line-termination sequences.
500 =item bless REF,CLASSNAME
504 This function tells the thingy referenced by REF that it is now an object
505 in the CLASSNAME package. If CLASSNAME is omitted, the current package
506 is used. Because a C<bless> is often the last thing in a constructor,
507 it returns the reference for convenience. Always use the two-argument
508 version if the function doing the blessing might be inherited by a
509 derived class. See L<perltoot> and L<perlobj> for more about the blessing
510 (and blessings) of objects.
512 Consider always blessing objects in CLASSNAMEs that are mixed case.
513 Namespaces with all lowercase names are considered reserved for
514 Perl pragmata. Builtin types have all uppercase names, so to prevent
515 confusion, you may wish to avoid such package names as well. Make sure
516 that CLASSNAME is a true value.
518 See L<perlmod/"Perl Modules">.
524 Returns the context of the current subroutine call. In scalar context,
525 returns the caller's package name if there is a caller, that is, if
526 we're in a subroutine or C<eval> or C<require>, and the undefined value
527 otherwise. In list context, returns
529 ($package, $filename, $line) = caller;
531 With EXPR, it returns some extra information that the debugger uses to
532 print a stack trace. The value of EXPR indicates how many call frames
533 to go back before the current one.
535 ($package, $filename, $line, $subroutine, $hasargs,
536 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
538 Here $subroutine may be C<(eval)> if the frame is not a subroutine
539 call, but an C<eval>. In such a case additional elements $evaltext and
540 C<$is_require> are set: C<$is_require> is true if the frame is created by a
541 C<require> or C<use> statement, $evaltext contains the text of the
542 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
543 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
544 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
545 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
546 was compiled with. The C<$hints> and C<$bitmask> values are subject to
547 change between versions of Perl, and are not meant for external use.
549 Furthermore, when called from within the DB package, caller returns more
550 detailed information: it sets the list variable C<@DB::args> to be the
551 arguments with which the subroutine was invoked.
553 Be aware that the optimizer might have optimized call frames away before
554 C<caller> had a chance to get the information. That means that C<caller(N)>
555 might not return information about the call frame you expect it do, for
556 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
557 previous time C<caller> was called.
561 Changes the working directory to EXPR, if possible. If EXPR is omitted,
562 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
563 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
564 set, C<chdir> does nothing. It returns true upon success, false
565 otherwise. See the example under C<die>.
569 Changes the permissions of a list of files. The first element of the
570 list must be the numerical mode, which should probably be an octal
571 number, and which definitely should I<not> a string of octal digits:
572 C<0644> is okay, C<'0644'> is not. Returns the number of files
573 successfully changed. See also L</oct>, if all you have is a string.
575 $cnt = chmod 0755, 'foo', 'bar';
576 chmod 0755, @executables;
577 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
579 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
580 $mode = 0644; chmod $mode, 'foo'; # this is best
582 You can also import the symbolic C<S_I*> constants from the Fcntl
587 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
588 # This is identical to the chmod 0755 of the above example.
596 This safer version of L</chop> removes any trailing string
597 that corresponds to the current value of C<$/> (also known as
598 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
599 number of characters removed from all its arguments. It's often used to
600 remove the newline from the end of an input record when you're worried
601 that the final record may be missing its newline. When in paragraph
602 mode (C<$/ = "">), it removes all trailing newlines from the string.
603 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
604 a reference to an integer or the like, see L<perlvar>) chomp() won't
606 If VARIABLE is omitted, it chomps C<$_>. Example:
609 chomp; # avoid \n on last field
614 You can actually chomp anything that's an lvalue, including an assignment:
617 chomp($answer = <STDIN>);
619 If you chomp a list, each element is chomped, and the total number of
620 characters removed is returned.
628 Chops off the last character of a string and returns the character
629 chopped. It's used primarily to remove the newline from the end of an
630 input record, but is much more efficient than C<s/\n//> because it neither
631 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
635 chop; # avoid \n on last field
640 You can actually chop anything that's an lvalue, including an assignment:
643 chop($answer = <STDIN>);
645 If you chop a list, each element is chopped. Only the value of the
646 last C<chop> is returned.
648 Note that C<chop> returns the last character. To return all but the last
649 character, use C<substr($string, 0, -1)>.
653 Changes the owner (and group) of a list of files. The first two
654 elements of the list must be the I<numeric> uid and gid, in that
655 order. A value of -1 in either position is interpreted by most
656 systems to leave that value unchanged. Returns the number of files
657 successfully changed.
659 $cnt = chown $uid, $gid, 'foo', 'bar';
660 chown $uid, $gid, @filenames;
662 Here's an example that looks up nonnumeric uids in the passwd file:
665 chomp($user = <STDIN>);
667 chomp($pattern = <STDIN>);
669 ($login,$pass,$uid,$gid) = getpwnam($user)
670 or die "$user not in passwd file";
672 @ary = glob($pattern); # expand filenames
673 chown $uid, $gid, @ary;
675 On most systems, you are not allowed to change the ownership of the
676 file unless you're the superuser, although you should be able to change
677 the group to any of your secondary groups. On insecure systems, these
678 restrictions may be relaxed, but this is not a portable assumption.
679 On POSIX systems, you can detect this condition this way:
681 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
682 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
688 Returns the character represented by that NUMBER in the character set.
689 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
690 chr(0x263a) is a Unicode smiley face (but only within the scope of
691 a C<use utf8>). For the reverse, use L</ord>.
692 See L<utf8> for more about Unicode.
694 If NUMBER is omitted, uses C<$_>.
696 =item chroot FILENAME
700 This function works like the system call by the same name: it makes the
701 named directory the new root directory for all further pathnames that
702 begin with a C</> by your process and all its children. (It doesn't
703 change your current working directory, which is unaffected.) For security
704 reasons, this call is restricted to the superuser. If FILENAME is
705 omitted, does a C<chroot> to C<$_>.
707 =item close FILEHANDLE
711 Closes the file or pipe associated with the file handle, returning true
712 only if stdio successfully flushes buffers and closes the system file
713 descriptor. Closes the currently selected filehandle if the argument
716 You don't have to close FILEHANDLE if you are immediately going to do
717 another C<open> on it, because C<open> will close it for you. (See
718 C<open>.) However, an explicit C<close> on an input file resets the line
719 counter (C<$.>), while the implicit close done by C<open> does not.
721 If the file handle came from a piped open C<close> will additionally
722 return false if one of the other system calls involved fails or if the
723 program exits with non-zero status. (If the only problem was that the
724 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
725 also waits for the process executing on the pipe to complete, in case you
726 want to look at the output of the pipe afterwards, and
727 implicitly puts the exit status value of that command into C<$?>.
729 Prematurely closing the read end of a pipe (i.e. before the process
730 writing to it at the other end has closed it) will result in a
731 SIGPIPE being delivered to the writer. If the other end can't
732 handle that, be sure to read all the data before closing the pipe.
736 open(OUTPUT, '|sort >foo') # pipe to sort
737 or die "Can't start sort: $!";
738 #... # print stuff to output
739 close OUTPUT # wait for sort to finish
740 or warn $! ? "Error closing sort pipe: $!"
741 : "Exit status $? from sort";
742 open(INPUT, 'foo') # get sort's results
743 or die "Can't open 'foo' for input: $!";
745 FILEHANDLE may be an expression whose value can be used as an indirect
746 filehandle, usually the real filehandle name.
748 =item closedir DIRHANDLE
750 Closes a directory opened by C<opendir> and returns the success of that
753 DIRHANDLE may be an expression whose value can be used as an indirect
754 dirhandle, usually the real dirhandle name.
756 =item connect SOCKET,NAME
758 Attempts to connect to a remote socket, just as the connect system call
759 does. Returns true if it succeeded, false otherwise. NAME should be a
760 packed address of the appropriate type for the socket. See the examples in
761 L<perlipc/"Sockets: Client/Server Communication">.
765 Actually a flow control statement rather than a function. If there is a
766 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
767 C<foreach>), it is always executed just before the conditional is about to
768 be evaluated again, just like the third part of a C<for> loop in C. Thus
769 it can be used to increment a loop variable, even when the loop has been
770 continued via the C<next> statement (which is similar to the C C<continue>
773 C<last>, C<next>, or C<redo> may appear within a C<continue>
774 block. C<last> and C<redo> will behave as if they had been executed within
775 the main block. So will C<next>, but since it will execute a C<continue>
776 block, it may be more entertaining.
779 ### redo always comes here
782 ### next always comes here
784 # then back the top to re-check EXPR
786 ### last always comes here
788 Omitting the C<continue> section is semantically equivalent to using an
789 empty one, logically enough. In that case, C<next> goes directly back
790 to check the condition at the top of the loop.
794 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
795 takes cosine of C<$_>.
797 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
798 function, or use this relation:
800 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
802 =item crypt PLAINTEXT,SALT
804 Encrypts a string exactly like the crypt(3) function in the C library
805 (assuming that you actually have a version there that has not been
806 extirpated as a potential munition). This can prove useful for checking
807 the password file for lousy passwords, amongst other things. Only the
808 guys wearing white hats should do this.
810 Note that C<crypt> is intended to be a one-way function, much like breaking
811 eggs to make an omelette. There is no (known) corresponding decrypt
812 function. As a result, this function isn't all that useful for
813 cryptography. (For that, see your nearby CPAN mirror.)
815 When verifying an existing encrypted string you should use the encrypted
816 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
817 allows your code to work with the standard C<crypt> and with more
818 exotic implementations. When choosing a new salt create a random two
819 character string whose characters come from the set C<[./0-9A-Za-z]>
820 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
822 Here's an example that makes sure that whoever runs this program knows
825 $pwd = (getpwuid($<))[1];
829 chomp($word = <STDIN>);
833 if (crypt($word, $pwd) ne $pwd) {
839 Of course, typing in your own password to whoever asks you
842 The L<crypt> function is unsuitable for encrypting large quantities
843 of data, not least of all because you can't get the information
844 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
845 on your favorite CPAN mirror for a slew of potentially useful
850 [This function has been largely superseded by the C<untie> function.]
852 Breaks the binding between a DBM file and a hash.
854 =item dbmopen HASH,DBNAME,MASK
856 [This function has been largely superseded by the C<tie> function.]
858 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
859 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
860 argument is I<not> a filehandle, even though it looks like one). DBNAME
861 is the name of the database (without the F<.dir> or F<.pag> extension if
862 any). If the database does not exist, it is created with protection
863 specified by MASK (as modified by the C<umask>). If your system supports
864 only the older DBM functions, you may perform only one C<dbmopen> in your
865 program. In older versions of Perl, if your system had neither DBM nor
866 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
869 If you don't have write access to the DBM file, you can only read hash
870 variables, not set them. If you want to test whether you can write,
871 either use file tests or try setting a dummy hash entry inside an C<eval>,
872 which will trap the error.
874 Note that functions such as C<keys> and C<values> may return huge lists
875 when used on large DBM files. You may prefer to use the C<each>
876 function to iterate over large DBM files. Example:
878 # print out history file offsets
879 dbmopen(%HIST,'/usr/lib/news/history',0666);
880 while (($key,$val) = each %HIST) {
881 print $key, ' = ', unpack('L',$val), "\n";
885 See also L<AnyDBM_File> for a more general description of the pros and
886 cons of the various dbm approaches, as well as L<DB_File> for a particularly
889 You can control which DBM library you use by loading that library
890 before you call dbmopen():
893 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
894 or die "Can't open netscape history file: $!";
900 Returns a Boolean value telling whether EXPR has a value other than
901 the undefined value C<undef>. If EXPR is not present, C<$_> will be
904 Many operations return C<undef> to indicate failure, end of file,
905 system error, uninitialized variable, and other exceptional
906 conditions. This function allows you to distinguish C<undef> from
907 other values. (A simple Boolean test will not distinguish among
908 C<undef>, zero, the empty string, and C<"0">, which are all equally
909 false.) Note that since C<undef> is a valid scalar, its presence
910 doesn't I<necessarily> indicate an exceptional condition: C<pop>
911 returns C<undef> when its argument is an empty array, I<or> when the
912 element to return happens to be C<undef>.
914 You may also use C<defined(&func)> to check whether subroutine C<&func>
915 has ever been defined. The return value is unaffected by any forward
916 declarations of C<&foo>.
918 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
919 used to report whether memory for that aggregate has ever been
920 allocated. This behavior may disappear in future versions of Perl.
921 You should instead use a simple test for size:
923 if (@an_array) { print "has array elements\n" }
924 if (%a_hash) { print "has hash members\n" }
926 When used on a hash element, it tells you whether the value is defined,
927 not whether the key exists in the hash. Use L</exists> for the latter
932 print if defined $switch{'D'};
933 print "$val\n" while defined($val = pop(@ary));
934 die "Can't readlink $sym: $!"
935 unless defined($value = readlink $sym);
936 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
937 $debugging = 0 unless defined $debugging;
939 Note: Many folks tend to overuse C<defined>, and then are surprised to
940 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
941 defined values. For example, if you say
945 The pattern match succeeds, and C<$1> is defined, despite the fact that it
946 matched "nothing". But it didn't really match nothing--rather, it
947 matched something that happened to be zero characters long. This is all
948 very above-board and honest. When a function returns an undefined value,
949 it's an admission that it couldn't give you an honest answer. So you
950 should use C<defined> only when you're questioning the integrity of what
951 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
954 See also L</undef>, L</exists>, L</ref>.
958 Given an expression that specifies a hash element, array element, hash slice,
959 or array slice, deletes the specified element(s) from the hash or array.
960 In the case of an array, if the array elements happen to be at the end,
961 the size of the array will shrink to the highest element that tests
962 true for exists() (or 0 if no such element exists).
964 Returns each element so deleted or the undefined value if there was no such
965 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
966 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
967 from a C<tie>d hash or array may not necessarily return anything.
969 Deleting an array element effectively returns that position of the array
970 to its initial, uninitialized state. Subsequently testing for the same
971 element with exists() will return false. Note that deleting array
972 elements in the middle of an array will not shift the index of the ones
973 after them down--use splice() for that. See L</exists>.
975 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
977 foreach $key (keys %HASH) {
981 foreach $index (0 .. $#ARRAY) {
982 delete $ARRAY[$index];
987 delete @HASH{keys %HASH};
989 delete @ARRAY[0 .. $#ARRAY];
991 But both of these are slower than just assigning the empty list
992 or undefining %HASH or @ARRAY:
994 %HASH = (); # completely empty %HASH
995 undef %HASH; # forget %HASH ever existed
997 @ARRAY = (); # completely empty @ARRAY
998 undef @ARRAY; # forget @ARRAY ever existed
1000 Note that the EXPR can be arbitrarily complicated as long as the final
1001 operation is a hash element, array element, hash slice, or array slice
1004 delete $ref->[$x][$y]{$key};
1005 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1007 delete $ref->[$x][$y][$index];
1008 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1012 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1013 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1014 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1015 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1016 an C<eval(),> the error message is stuffed into C<$@> and the
1017 C<eval> is terminated with the undefined value. This makes
1018 C<die> the way to raise an exception.
1020 Equivalent examples:
1022 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1023 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1025 If the value of EXPR does not end in a newline, the current script line
1026 number and input line number (if any) are also printed, and a newline
1027 is supplied. Note that the "input line number" (also known as "chunk")
1028 is subject to whatever notion of "line" happens to be currently in
1029 effect, and is also available as the special variable C<$.>.
1030 See L<perlvar/"$/"> and L<perlvar/"$.">.
1032 Hint: sometimes appending C<", stopped"> to your message
1033 will cause it to make better sense when the string C<"at foo line 123"> is
1034 appended. Suppose you are running script "canasta".
1036 die "/etc/games is no good";
1037 die "/etc/games is no good, stopped";
1039 produce, respectively
1041 /etc/games is no good at canasta line 123.
1042 /etc/games is no good, stopped at canasta line 123.
1044 See also exit(), warn(), and the Carp module.
1046 If LIST is empty and C<$@> already contains a value (typically from a
1047 previous eval) that value is reused after appending C<"\t...propagated">.
1048 This is useful for propagating exceptions:
1051 die unless $@ =~ /Expected exception/;
1053 If C<$@> is empty then the string C<"Died"> is used.
1055 die() can also be called with a reference argument. If this happens to be
1056 trapped within an eval(), $@ contains the reference. This behavior permits
1057 a more elaborate exception handling implementation using objects that
1058 maintain arbitary state about the nature of the exception. Such a scheme
1059 is sometimes preferable to matching particular string values of $@ using
1060 regular expressions. Here's an example:
1062 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1064 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1065 # handle Some::Module::Exception
1068 # handle all other possible exceptions
1072 Because perl will stringify uncaught exception messages before displaying
1073 them, you may want to overload stringification operations on such custom
1074 exception objects. See L<overload> for details about that.
1076 You can arrange for a callback to be run just before the C<die>
1077 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1078 handler will be called with the error text and can change the error
1079 message, if it sees fit, by calling C<die> again. See
1080 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1081 L<"eval BLOCK"> for some examples. Although this feature was meant
1082 to be run only right before your program was to exit, this is not
1083 currently the case--the C<$SIG{__DIE__}> hook is currently called
1084 even inside eval()ed blocks/strings! If one wants the hook to do
1085 nothing in such situations, put
1089 as the first line of the handler (see L<perlvar/$^S>). Because
1090 this promotes strange action at a distance, this counterintuitive
1091 behavior may be fixed in a future release.
1095 Not really a function. Returns the value of the last command in the
1096 sequence of commands indicated by BLOCK. When modified by a loop
1097 modifier, executes the BLOCK once before testing the loop condition.
1098 (On other statements the loop modifiers test the conditional first.)
1100 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1101 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1102 See L<perlsyn> for alternative strategies.
1104 =item do SUBROUTINE(LIST)
1106 A deprecated form of subroutine call. See L<perlsub>.
1110 Uses the value of EXPR as a filename and executes the contents of the
1111 file as a Perl script. Its primary use is to include subroutines
1112 from a Perl subroutine library.
1118 scalar eval `cat stat.pl`;
1120 except that it's more efficient and concise, keeps track of the current
1121 filename for error messages, searches the @INC libraries, and updates
1122 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1123 variables. It also differs in that code evaluated with C<do FILENAME>
1124 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1125 same, however, in that it does reparse the file every time you call it,
1126 so you probably don't want to do this inside a loop.
1128 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1129 error. If C<do> can read the file but cannot compile it, it
1130 returns undef and sets an error message in C<$@>. If the file is
1131 successfully compiled, C<do> returns the value of the last expression
1134 Note that inclusion of library modules is better done with the
1135 C<use> and C<require> operators, which also do automatic error checking
1136 and raise an exception if there's a problem.
1138 You might like to use C<do> to read in a program configuration
1139 file. Manual error checking can be done this way:
1141 # read in config files: system first, then user
1142 for $file ("/share/prog/defaults.rc",
1143 "$ENV{HOME}/.someprogrc")
1145 unless ($return = do $file) {
1146 warn "couldn't parse $file: $@" if $@;
1147 warn "couldn't do $file: $!" unless defined $return;
1148 warn "couldn't run $file" unless $return;
1156 This function causes an immediate core dump. See also the B<-u>
1157 command-line switch in L<perlrun>, which does the same thing.
1158 Primarily this is so that you can use the B<undump> program (not
1159 supplied) to turn your core dump into an executable binary after
1160 having initialized all your variables at the beginning of the
1161 program. When the new binary is executed it will begin by executing
1162 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1163 Think of it as a goto with an intervening core dump and reincarnation.
1164 If C<LABEL> is omitted, restarts the program from the top.
1166 B<WARNING>: Any files opened at the time of the dump will I<not>
1167 be open any more when the program is reincarnated, with possible
1168 resulting confusion on the part of Perl.
1170 This function is now largely obsolete, partly because it's very
1171 hard to convert a core file into an executable, and because the
1172 real compiler backends for generating portable bytecode and compilable
1173 C code have superseded it.
1175 If you're looking to use L<dump> to speed up your program, consider
1176 generating bytecode or native C code as described in L<perlcc>. If
1177 you're just trying to accelerate a CGI script, consider using the
1178 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1179 You might also consider autoloading or selfloading, which at least
1180 make your program I<appear> to run faster.
1184 When called in list context, returns a 2-element list consisting of the
1185 key and value for the next element of a hash, so that you can iterate over
1186 it. When called in scalar context, returns the key for only the "next"
1187 element in the hash.
1189 Entries are returned in an apparently random order. The actual random
1190 order is subject to change in future versions of perl, but it is guaranteed
1191 to be in the same order as either the C<keys> or C<values> function
1192 would produce on the same (unmodified) hash.
1194 When the hash is entirely read, a null array is returned in list context
1195 (which when assigned produces a false (C<0>) value), and C<undef> in
1196 scalar context. The next call to C<each> after that will start iterating
1197 again. There is a single iterator for each hash, shared by all C<each>,
1198 C<keys>, and C<values> function calls in the program; it can be reset by
1199 reading all the elements from the hash, or by evaluating C<keys HASH> or
1200 C<values HASH>. If you add or delete elements of a hash while you're
1201 iterating over it, you may get entries skipped or duplicated, so don't.
1203 The following prints out your environment like the printenv(1) program,
1204 only in a different order:
1206 while (($key,$value) = each %ENV) {
1207 print "$key=$value\n";
1210 See also C<keys>, C<values> and C<sort>.
1212 =item eof FILEHANDLE
1218 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1219 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1220 gives the real filehandle. (Note that this function actually
1221 reads a character and then C<ungetc>s it, so isn't very useful in an
1222 interactive context.) Do not read from a terminal file (or call
1223 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1224 as terminals may lose the end-of-file condition if you do.
1226 An C<eof> without an argument uses the last file read. Using C<eof()>
1227 with empty parentheses is very different. It refers to the pseudo file
1228 formed from the files listed on the command line and accessed via the
1229 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1230 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1231 used will cause C<@ARGV> to be examined to determine if input is
1234 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1235 detect the end of each file, C<eof()> will only detect the end of the
1236 last file. Examples:
1238 # reset line numbering on each input file
1240 next if /^\s*#/; # skip comments
1243 close ARGV if eof; # Not eof()!
1246 # insert dashes just before last line of last file
1248 if (eof()) { # check for end of current file
1249 print "--------------\n";
1250 close(ARGV); # close or last; is needed if we
1251 # are reading from the terminal
1256 Practical hint: you almost never need to use C<eof> in Perl, because the
1257 input operators typically return C<undef> when they run out of data, or if
1264 In the first form, the return value of EXPR is parsed and executed as if it
1265 were a little Perl program. The value of the expression (which is itself
1266 determined within scalar context) is first parsed, and if there weren't any
1267 errors, executed in the context of the current Perl program, so that any
1268 variable settings or subroutine and format definitions remain afterwards.
1269 Note that the value is parsed every time the eval executes. If EXPR is
1270 omitted, evaluates C<$_>. This form is typically used to delay parsing
1271 and subsequent execution of the text of EXPR until run time.
1273 In the second form, the code within the BLOCK is parsed only once--at the
1274 same time the code surrounding the eval itself was parsed--and executed
1275 within the context of the current Perl program. This form is typically
1276 used to trap exceptions more efficiently than the first (see below), while
1277 also providing the benefit of checking the code within BLOCK at compile
1280 The final semicolon, if any, may be omitted from the value of EXPR or within
1283 In both forms, the value returned is the value of the last expression
1284 evaluated inside the mini-program; a return statement may be also used, just
1285 as with subroutines. The expression providing the return value is evaluated
1286 in void, scalar, or list context, depending on the context of the eval itself.
1287 See L</wantarray> for more on how the evaluation context can be determined.
1289 If there is a syntax error or runtime error, or a C<die> statement is
1290 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1291 error message. If there was no error, C<$@> is guaranteed to be a null
1292 string. Beware that using C<eval> neither silences perl from printing
1293 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1294 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1295 L</warn> and L<perlvar>.
1297 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1298 determining whether a particular feature (such as C<socket> or C<symlink>)
1299 is implemented. It is also Perl's exception trapping mechanism, where
1300 the die operator is used to raise exceptions.
1302 If the code to be executed doesn't vary, you may use the eval-BLOCK
1303 form to trap run-time errors without incurring the penalty of
1304 recompiling each time. The error, if any, is still returned in C<$@>.
1307 # make divide-by-zero nonfatal
1308 eval { $answer = $a / $b; }; warn $@ if $@;
1310 # same thing, but less efficient
1311 eval '$answer = $a / $b'; warn $@ if $@;
1313 # a compile-time error
1314 eval { $answer = }; # WRONG
1317 eval '$answer ='; # sets $@
1319 Due to the current arguably broken state of C<__DIE__> hooks, when using
1320 the C<eval{}> form as an exception trap in libraries, you may wish not
1321 to trigger any C<__DIE__> hooks that user code may have installed.
1322 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1323 as shown in this example:
1325 # a very private exception trap for divide-by-zero
1326 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1329 This is especially significant, given that C<__DIE__> hooks can call
1330 C<die> again, which has the effect of changing their error messages:
1332 # __DIE__ hooks may modify error messages
1334 local $SIG{'__DIE__'} =
1335 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1336 eval { die "foo lives here" };
1337 print $@ if $@; # prints "bar lives here"
1340 Because this promotes action at a distance, this counterintuitive behavior
1341 may be fixed in a future release.
1343 With an C<eval>, you should be especially careful to remember what's
1344 being looked at when:
1350 eval { $x }; # CASE 4
1352 eval "\$$x++"; # CASE 5
1355 Cases 1 and 2 above behave identically: they run the code contained in
1356 the variable $x. (Although case 2 has misleading double quotes making
1357 the reader wonder what else might be happening (nothing is).) Cases 3
1358 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1359 does nothing but return the value of $x. (Case 4 is preferred for
1360 purely visual reasons, but it also has the advantage of compiling at
1361 compile-time instead of at run-time.) Case 5 is a place where
1362 normally you I<would> like to use double quotes, except that in this
1363 particular situation, you can just use symbolic references instead, as
1366 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1367 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1371 =item exec PROGRAM LIST
1373 The C<exec> function executes a system command I<and never returns>--
1374 use C<system> instead of C<exec> if you want it to return. It fails and
1375 returns false only if the command does not exist I<and> it is executed
1376 directly instead of via your system's command shell (see below).
1378 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1379 warns you if there is a following statement which isn't C<die>, C<warn>,
1380 or C<exit> (if C<-w> is set - but you always do that). If you
1381 I<really> want to follow an C<exec> with some other statement, you
1382 can use one of these styles to avoid the warning:
1384 exec ('foo') or print STDERR "couldn't exec foo: $!";
1385 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1387 If there is more than one argument in LIST, or if LIST is an array
1388 with more than one value, calls execvp(3) with the arguments in LIST.
1389 If there is only one scalar argument or an array with one element in it,
1390 the argument is checked for shell metacharacters, and if there are any,
1391 the entire argument is passed to the system's command shell for parsing
1392 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1393 If there are no shell metacharacters in the argument, it is split into
1394 words and passed directly to C<execvp>, which is more efficient.
1397 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1398 exec "sort $outfile | uniq";
1400 If you don't really want to execute the first argument, but want to lie
1401 to the program you are executing about its own name, you can specify
1402 the program you actually want to run as an "indirect object" (without a
1403 comma) in front of the LIST. (This always forces interpretation of the
1404 LIST as a multivalued list, even if there is only a single scalar in
1407 $shell = '/bin/csh';
1408 exec $shell '-sh'; # pretend it's a login shell
1412 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1414 When the arguments get executed via the system shell, results will
1415 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1418 Using an indirect object with C<exec> or C<system> is also more
1419 secure. This usage (which also works fine with system()) forces
1420 interpretation of the arguments as a multivalued list, even if the
1421 list had just one argument. That way you're safe from the shell
1422 expanding wildcards or splitting up words with whitespace in them.
1424 @args = ( "echo surprise" );
1426 exec @args; # subject to shell escapes
1428 exec { $args[0] } @args; # safe even with one-arg list
1430 The first version, the one without the indirect object, ran the I<echo>
1431 program, passing it C<"surprise"> an argument. The second version
1432 didn't--it tried to run a program literally called I<"echo surprise">,
1433 didn't find it, and set C<$?> to a non-zero value indicating failure.
1435 Note that C<exec> will not call your C<END> blocks, nor will it call
1436 any C<DESTROY> methods in your objects.
1440 Given an expression that specifies a hash element or array element,
1441 returns true if the specified element in the hash or array has ever
1442 been initialized, even if the corresponding value is undefined. The
1443 element is not autovivified if it doesn't exist.
1445 print "Exists\n" if exists $hash{$key};
1446 print "Defined\n" if defined $hash{$key};
1447 print "True\n" if $hash{$key};
1449 print "Exists\n" if exists $array[$index];
1450 print "Defined\n" if defined $array[$index];
1451 print "True\n" if $array[$index];
1453 A hash or array element can be true only if it's defined, and defined if
1454 it exists, but the reverse doesn't necessarily hold true.
1456 Given an expression that specifies the name of a subroutine,
1457 returns true if the specified subroutine has ever been declared, even
1458 if it is undefined. Mentioning a subroutine name for exists or defined
1459 does not count as declaring it.
1461 print "Exists\n" if exists &subroutine;
1462 print "Defined\n" if defined &subroutine;
1464 Note that the EXPR can be arbitrarily complicated as long as the final
1465 operation is a hash or array key lookup or subroutine name:
1467 if (exists $ref->{A}->{B}->{$key}) { }
1468 if (exists $hash{A}{B}{$key}) { }
1470 if (exists $ref->{A}->{B}->[$ix]) { }
1471 if (exists $hash{A}{B}[$ix]) { }
1473 if (exists &{$ref->{A}{B}{$key}}) { }
1475 Although the deepest nested array or hash will not spring into existence
1476 just because its existence was tested, any intervening ones will.
1477 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1478 into existence due to the existence test for the $key element above.
1479 This happens anywhere the arrow operator is used, including even:
1482 if (exists $ref->{"Some key"}) { }
1483 print $ref; # prints HASH(0x80d3d5c)
1485 This surprising autovivification in what does not at first--or even
1486 second--glance appear to be an lvalue context may be fixed in a future
1489 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1490 on how exists() acts when used on a pseudo-hash.
1492 Use of a subroutine call, rather than a subroutine name, as an argument
1493 to exists() is an error.
1496 exists &sub(); # Error
1500 Evaluates EXPR and exits immediately with that value. Example:
1503 exit 0 if $ans =~ /^[Xx]/;
1505 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1506 universally recognized values for EXPR are C<0> for success and C<1>
1507 for error; other values are subject to interpretation depending on the
1508 environment in which the Perl program is running. For example, exiting
1509 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1510 the mailer to return the item undelivered, but that's not true everywhere.
1512 Don't use C<exit> to abort a subroutine if there's any chance that
1513 someone might want to trap whatever error happened. Use C<die> instead,
1514 which can be trapped by an C<eval>.
1516 The exit() function does not always exit immediately. It calls any
1517 defined C<END> routines first, but these C<END> routines may not
1518 themselves abort the exit. Likewise any object destructors that need to
1519 be called are called before the real exit. If this is a problem, you
1520 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1521 See L<perlmod> for details.
1527 Returns I<e> (the natural logarithm base) to the power of EXPR.
1528 If EXPR is omitted, gives C<exp($_)>.
1530 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1532 Implements the fcntl(2) function. You'll probably have to say
1536 first to get the correct constant definitions. Argument processing and
1537 value return works just like C<ioctl> below.
1541 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1542 or die "can't fcntl F_GETFL: $!";
1544 You don't have to check for C<defined> on the return from C<fnctl>.
1545 Like C<ioctl>, it maps a C<0> return from the system call into
1546 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1547 in numeric context. It is also exempt from the normal B<-w> warnings
1548 on improper numeric conversions.
1550 Note that C<fcntl> will produce a fatal error if used on a machine that
1551 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1552 manpage to learn what functions are available on your system.
1554 =item fileno FILEHANDLE
1556 Returns the file descriptor for a filehandle, or undefined if the
1557 filehandle is not open. This is mainly useful for constructing
1558 bitmaps for C<select> and low-level POSIX tty-handling operations.
1559 If FILEHANDLE is an expression, the value is taken as an indirect
1560 filehandle, generally its name.
1562 You can use this to find out whether two handles refer to the
1563 same underlying descriptor:
1565 if (fileno(THIS) == fileno(THAT)) {
1566 print "THIS and THAT are dups\n";
1569 =item flock FILEHANDLE,OPERATION
1571 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1572 for success, false on failure. Produces a fatal error if used on a
1573 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1574 C<flock> is Perl's portable file locking interface, although it locks
1575 only entire files, not records.
1577 Two potentially non-obvious but traditional C<flock> semantics are
1578 that it waits indefinitely until the lock is granted, and that its locks
1579 B<merely advisory>. Such discretionary locks are more flexible, but offer
1580 fewer guarantees. This means that files locked with C<flock> may be
1581 modified by programs that do not also use C<flock>. See L<perlport>,
1582 your port's specific documentation, or your system-specific local manpages
1583 for details. It's best to assume traditional behavior if you're writing
1584 portable programs. (But if you're not, you should as always feel perfectly
1585 free to write for your own system's idiosyncrasies (sometimes called
1586 "features"). Slavish adherence to portability concerns shouldn't get
1587 in the way of your getting your job done.)
1589 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1590 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1591 you can use the symbolic names if you import them from the Fcntl module,
1592 either individually, or as a group using the ':flock' tag. LOCK_SH
1593 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1594 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1595 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1596 waiting for the lock (check the return status to see if you got it).
1598 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1599 before locking or unlocking it.
1601 Note that the emulation built with lockf(3) doesn't provide shared
1602 locks, and it requires that FILEHANDLE be open with write intent. These
1603 are the semantics that lockf(3) implements. Most if not all systems
1604 implement lockf(3) in terms of fcntl(2) locking, though, so the
1605 differing semantics shouldn't bite too many people.
1607 Note also that some versions of C<flock> cannot lock things over the
1608 network; you would need to use the more system-specific C<fcntl> for
1609 that. If you like you can force Perl to ignore your system's flock(2)
1610 function, and so provide its own fcntl(2)-based emulation, by passing
1611 the switch C<-Ud_flock> to the F<Configure> program when you configure
1614 Here's a mailbox appender for BSD systems.
1616 use Fcntl ':flock'; # import LOCK_* constants
1619 flock(MBOX,LOCK_EX);
1620 # and, in case someone appended
1621 # while we were waiting...
1626 flock(MBOX,LOCK_UN);
1629 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1630 or die "Can't open mailbox: $!";
1633 print MBOX $msg,"\n\n";
1636 On systems that support a real flock(), locks are inherited across fork()
1637 calls, whereas those that must resort to the more capricious fcntl()
1638 function lose the locks, making it harder to write servers.
1640 See also L<DB_File> for other flock() examples.
1644 Does a fork(2) system call to create a new process running the
1645 same program at the same point. It returns the child pid to the
1646 parent process, C<0> to the child process, or C<undef> if the fork is
1647 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1648 are shared, while everything else is copied. On most systems supporting
1649 fork(), great care has gone into making it extremely efficient (for
1650 example, using copy-on-write technology on data pages), making it the
1651 dominant paradigm for multitasking over the last few decades.
1653 All files opened for output are flushed before forking the child process.
1655 If you C<fork> without ever waiting on your children, you will
1656 accumulate zombies. On some systems, you can avoid this by setting
1657 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1658 forking and reaping moribund children.
1660 Note that if your forked child inherits system file descriptors like
1661 STDIN and STDOUT that are actually connected by a pipe or socket, even
1662 if you exit, then the remote server (such as, say, a CGI script or a
1663 backgrounded job launched from a remote shell) won't think you're done.
1664 You should reopen those to F</dev/null> if it's any issue.
1668 Declare a picture format for use by the C<write> function. For
1672 Test: @<<<<<<<< @||||| @>>>>>
1673 $str, $%, '$' . int($num)
1677 $num = $cost/$quantity;
1681 See L<perlform> for many details and examples.
1683 =item formline PICTURE,LIST
1685 This is an internal function used by C<format>s, though you may call it,
1686 too. It formats (see L<perlform>) a list of values according to the
1687 contents of PICTURE, placing the output into the format output
1688 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1689 Eventually, when a C<write> is done, the contents of
1690 C<$^A> are written to some filehandle, but you could also read C<$^A>
1691 yourself and then set C<$^A> back to C<"">. Note that a format typically
1692 does one C<formline> per line of form, but the C<formline> function itself
1693 doesn't care how many newlines are embedded in the PICTURE. This means
1694 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1695 You may therefore need to use multiple formlines to implement a single
1696 record format, just like the format compiler.
1698 Be careful if you put double quotes around the picture, because an C<@>
1699 character may be taken to mean the beginning of an array name.
1700 C<formline> always returns true. See L<perlform> for other examples.
1702 =item getc FILEHANDLE
1706 Returns the next character from the input file attached to FILEHANDLE,
1707 or the undefined value at end of file, or if there was an error.
1708 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1709 efficient. However, it cannot be used by itself to fetch single
1710 characters without waiting for the user to hit enter. For that, try
1711 something more like:
1714 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1717 system "stty", '-icanon', 'eol', "\001";
1723 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1726 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1730 Determination of whether $BSD_STYLE should be set
1731 is left as an exercise to the reader.
1733 The C<POSIX::getattr> function can do this more portably on
1734 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1735 module from your nearest CPAN site; details on CPAN can be found on
1740 Implements the C library function of the same name, which on most
1741 systems returns the current login from F</etc/utmp>, if any. If null,
1744 $login = getlogin || getpwuid($<) || "Kilroy";
1746 Do not consider C<getlogin> for authentication: it is not as
1747 secure as C<getpwuid>.
1749 =item getpeername SOCKET
1751 Returns the packed sockaddr address of other end of the SOCKET connection.
1754 $hersockaddr = getpeername(SOCK);
1755 ($port, $iaddr) = sockaddr_in($hersockaddr);
1756 $herhostname = gethostbyaddr($iaddr, AF_INET);
1757 $herstraddr = inet_ntoa($iaddr);
1761 Returns the current process group for the specified PID. Use
1762 a PID of C<0> to get the current process group for the
1763 current process. Will raise an exception if used on a machine that
1764 doesn't implement getpgrp(2). If PID is omitted, returns process
1765 group of current process. Note that the POSIX version of C<getpgrp>
1766 does not accept a PID argument, so only C<PID==0> is truly portable.
1770 Returns the process id of the parent process.
1772 =item getpriority WHICH,WHO
1774 Returns the current priority for a process, a process group, or a user.
1775 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1776 machine that doesn't implement getpriority(2).
1782 =item gethostbyname NAME
1784 =item getnetbyname NAME
1786 =item getprotobyname NAME
1792 =item getservbyname NAME,PROTO
1794 =item gethostbyaddr ADDR,ADDRTYPE
1796 =item getnetbyaddr ADDR,ADDRTYPE
1798 =item getprotobynumber NUMBER
1800 =item getservbyport PORT,PROTO
1818 =item sethostent STAYOPEN
1820 =item setnetent STAYOPEN
1822 =item setprotoent STAYOPEN
1824 =item setservent STAYOPEN
1838 These routines perform the same functions as their counterparts in the
1839 system library. In list context, the return values from the
1840 various get routines are as follows:
1842 ($name,$passwd,$uid,$gid,
1843 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1844 ($name,$passwd,$gid,$members) = getgr*
1845 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1846 ($name,$aliases,$addrtype,$net) = getnet*
1847 ($name,$aliases,$proto) = getproto*
1848 ($name,$aliases,$port,$proto) = getserv*
1850 (If the entry doesn't exist you get a null list.)
1852 In scalar context, you get the name, unless the function was a
1853 lookup by name, in which case you get the other thing, whatever it is.
1854 (If the entry doesn't exist you get the undefined value.) For example:
1856 $uid = getpwnam($name);
1857 $name = getpwuid($num);
1859 $gid = getgrnam($name);
1860 $name = getgrgid($num;
1864 In I<getpw*()> the fields $quota, $comment, and $expire are
1865 special cases in the sense that in many systems they are unsupported.
1866 If the $quota is unsupported, it is an empty scalar. If it is
1867 supported, it usually encodes the disk quota. If the $comment
1868 field is unsupported, it is an empty scalar. If it is supported it
1869 usually encodes some administrative comment about the user. In some
1870 systems the $quota field may be $change or $age, fields that have
1871 to do with password aging. In some systems the $comment field may
1872 be $class. The $expire field, if present, encodes the expiration
1873 period of the account or the password. For the availability and the
1874 exact meaning of these fields in your system, please consult your
1875 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1876 out from within Perl what your $quota and $comment fields mean
1877 and whether you have the $expire field by using the C<Config> module
1878 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1879 and C<d_pwexpire>. Shadow password files are only supported if your
1880 vendor has implemented them in the intuitive fashion that calling the
1881 regular C library routines gets the shadow versions if you're running
1882 under privilege. Those that incorrectly implement a separate library
1883 call are not supported.
1885 The $members value returned by I<getgr*()> is a space separated list of
1886 the login names of the members of the group.
1888 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1889 C, it will be returned to you via C<$?> if the function call fails. The
1890 C<@addrs> value returned by a successful call is a list of the raw
1891 addresses returned by the corresponding system library call. In the
1892 Internet domain, each address is four bytes long and you can unpack it
1893 by saying something like:
1895 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1897 The Socket library makes this slightly easier:
1900 $iaddr = inet_aton("127.1"); # or whatever address
1901 $name = gethostbyaddr($iaddr, AF_INET);
1903 # or going the other way
1904 $straddr = inet_ntoa($iaddr);
1906 If you get tired of remembering which element of the return list
1907 contains which return value, by-name interfaces are provided
1908 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1909 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1910 and C<User::grent>. These override the normal built-ins, supplying
1911 versions that return objects with the appropriate names
1912 for each field. For example:
1916 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1918 Even though it looks like they're the same method calls (uid),
1919 they aren't, because a C<File::stat> object is different from
1920 a C<User::pwent> object.
1922 =item getsockname SOCKET
1924 Returns the packed sockaddr address of this end of the SOCKET connection,
1925 in case you don't know the address because you have several different
1926 IPs that the connection might have come in on.
1929 $mysockaddr = getsockname(SOCK);
1930 ($port, $myaddr) = sockaddr_in($mysockaddr);
1931 printf "Connect to %s [%s]\n",
1932 scalar gethostbyaddr($myaddr, AF_INET),
1935 =item getsockopt SOCKET,LEVEL,OPTNAME
1937 Returns the socket option requested, or undef if there is an error.
1943 Returns the value of EXPR with filename expansions such as the
1944 standard Unix shell F</bin/csh> would do. This is the internal function
1945 implementing the C<< <*.c> >> operator, but you can use it directly.
1946 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1947 discussed in more detail in L<perlop/"I/O Operators">.
1949 Beginning with v5.6.0, this operator is implemented using the standard
1950 C<File::Glob> extension. See L<File::Glob> for details.
1954 Converts a time as returned by the time function to a 8-element list
1955 with the time localized for the standard Greenwich time zone.
1956 Typically used as follows:
1959 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1962 All list elements are numeric, and come straight out of the C `struct
1963 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1964 specified time. $mday is the day of the month, and $mon is the month
1965 itself, in the range C<0..11> with 0 indicating January and 11
1966 indicating December. $year is the number of years since 1900. That
1967 is, $year is C<123> in year 2023. $wday is the day of the week, with
1968 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
1969 the year, in the range C<1..365> (or C<1..366> in leap years.)
1971 Note that the $year element is I<not> simply the last two digits of
1972 the year. If you assume it is, then you create non-Y2K-compliant
1973 programs--and you wouldn't want to do that, would you?
1975 The proper way to get a complete 4-digit year is simply:
1979 And to get the last two digits of the year (e.g., '01' in 2001) do:
1981 $year = sprintf("%02d", $year % 100);
1983 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
1985 In scalar context, C<gmtime()> returns the ctime(3) value:
1987 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1989 Also see the C<timegm> function provided by the C<Time::Local> module,
1990 and the strftime(3) function available via the POSIX module.
1992 This scalar value is B<not> locale dependent (see L<perllocale>), but
1993 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1994 strftime(3) and mktime(3) functions available via the POSIX module. To
1995 get somewhat similar but locale dependent date strings, set up your
1996 locale environment variables appropriately (please see L<perllocale>)
1997 and try for example:
1999 use POSIX qw(strftime);
2000 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2002 Note that the C<%a> and C<%b> escapes, which represent the short forms
2003 of the day of the week and the month of the year, may not necessarily
2004 be three characters wide in all locales.
2012 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2013 execution there. It may not be used to go into any construct that
2014 requires initialization, such as a subroutine or a C<foreach> loop. It
2015 also can't be used to go into a construct that is optimized away,
2016 or to get out of a block or subroutine given to C<sort>.
2017 It can be used to go almost anywhere else within the dynamic scope,
2018 including out of subroutines, but it's usually better to use some other
2019 construct such as C<last> or C<die>. The author of Perl has never felt the
2020 need to use this form of C<goto> (in Perl, that is--C is another matter).
2022 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2023 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2024 necessarily recommended if you're optimizing for maintainability:
2026 goto ("FOO", "BAR", "GLARCH")[$i];
2028 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2029 In fact, it isn't a goto in the normal sense at all, and doesn't have
2030 the stigma associated with other gotos. Instead, it
2031 substitutes a call to the named subroutine for the currently running
2032 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2033 another subroutine and then pretend that the other subroutine had been
2034 called in the first place (except that any modifications to C<@_>
2035 in the current subroutine are propagated to the other subroutine.)
2036 After the C<goto>, not even C<caller> will be able to tell that this
2037 routine was called first.
2039 NAME needn't be the name of a subroutine; it can be a scalar variable
2040 containing a code reference, or a block which evaluates to a code
2043 =item grep BLOCK LIST
2045 =item grep EXPR,LIST
2047 This is similar in spirit to, but not the same as, grep(1) and its
2048 relatives. In particular, it is not limited to using regular expressions.
2050 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2051 C<$_> to each element) and returns the list value consisting of those
2052 elements for which the expression evaluated to true. In scalar
2053 context, returns the number of times the expression was true.
2055 @foo = grep(!/^#/, @bar); # weed out comments
2059 @foo = grep {!/^#/} @bar; # weed out comments
2061 Note that, because C<$_> is a reference into the list value, it can
2062 be used to modify the elements of the array. While this is useful and
2063 supported, it can cause bizarre results if the LIST is not a named array.
2064 Similarly, grep returns aliases into the original list, much as a for
2065 loop's index variable aliases the list elements. That is, modifying an
2066 element of a list returned by grep (for example, in a C<foreach>, C<map>
2067 or another C<grep>) actually modifies the element in the original list.
2068 This is usually something to be avoided when writing clear code.
2070 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2076 Interprets EXPR as a hex string and returns the corresponding value.
2077 (To convert strings that might start with either 0, 0x, or 0b, see
2078 L</oct>.) If EXPR is omitted, uses C<$_>.
2080 print hex '0xAf'; # prints '175'
2081 print hex 'aF'; # same
2083 Hex strings may only represent integers. Strings that would cause
2084 integer overflow trigger a warning.
2088 There is no builtin C<import> function. It is just an ordinary
2089 method (subroutine) defined (or inherited) by modules that wish to export
2090 names to another module. The C<use> function calls the C<import> method
2091 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2093 =item index STR,SUBSTR,POSITION
2095 =item index STR,SUBSTR
2097 The index function searches for one string within another, but without
2098 the wildcard-like behavior of a full regular-expression pattern match.
2099 It returns the position of the first occurrence of SUBSTR in STR at
2100 or after POSITION. If POSITION is omitted, starts searching from the
2101 beginning of the string. The return value is based at C<0> (or whatever
2102 you've set the C<$[> variable to--but don't do that). If the substring
2103 is not found, returns one less than the base, ordinarily C<-1>.
2109 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2110 You should not use this function for rounding: one because it truncates
2111 towards C<0>, and two because machine representations of floating point
2112 numbers can sometimes produce counterintuitive results. For example,
2113 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2114 because it's really more like -268.99999999999994315658 instead. Usually,
2115 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2116 functions will serve you better than will int().
2118 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2120 Implements the ioctl(2) function. You'll probably first have to say
2122 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2124 to get the correct function definitions. If F<ioctl.ph> doesn't
2125 exist or doesn't have the correct definitions you'll have to roll your
2126 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2127 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2128 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2129 written depending on the FUNCTION--a pointer to the string value of SCALAR
2130 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2131 has no string value but does have a numeric value, that value will be
2132 passed rather than a pointer to the string value. To guarantee this to be
2133 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2134 functions may be needed to manipulate the values of structures used by
2137 The return value of C<ioctl> (and C<fcntl>) is as follows:
2139 if OS returns: then Perl returns:
2141 0 string "0 but true"
2142 anything else that number
2144 Thus Perl returns true on success and false on failure, yet you can
2145 still easily determine the actual value returned by the operating
2148 $retval = ioctl(...) || -1;
2149 printf "System returned %d\n", $retval;
2151 The special string "C<0> but true" is exempt from B<-w> complaints
2152 about improper numeric conversions.
2154 Here's an example of setting a filehandle named C<REMOTE> to be
2155 non-blocking at the system level. You'll have to negotiate C<$|>
2156 on your own, though.
2158 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2160 $flags = fcntl(REMOTE, F_GETFL, 0)
2161 or die "Can't get flags for the socket: $!\n";
2163 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2164 or die "Can't set flags for the socket: $!\n";
2166 =item join EXPR,LIST
2168 Joins the separate strings of LIST into a single string with fields
2169 separated by the value of EXPR, and returns that new string. Example:
2171 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2173 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2174 first argument. Compare L</split>.
2178 Returns a list consisting of all the keys of the named hash. (In
2179 scalar context, returns the number of keys.) The keys are returned in
2180 an apparently random order. The actual random order is subject to
2181 change in future versions of perl, but it is guaranteed to be the same
2182 order as either the C<values> or C<each> function produces (given
2183 that the hash has not been modified). As a side effect, it resets
2186 Here is yet another way to print your environment:
2189 @values = values %ENV;
2191 print pop(@keys), '=', pop(@values), "\n";
2194 or how about sorted by key:
2196 foreach $key (sort(keys %ENV)) {
2197 print $key, '=', $ENV{$key}, "\n";
2200 To sort a hash by value, you'll need to use a C<sort> function.
2201 Here's a descending numeric sort of a hash by its values:
2203 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2204 printf "%4d %s\n", $hash{$key}, $key;
2207 As an lvalue C<keys> allows you to increase the number of hash buckets
2208 allocated for the given hash. This can gain you a measure of efficiency if
2209 you know the hash is going to get big. (This is similar to pre-extending
2210 an array by assigning a larger number to $#array.) If you say
2214 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2215 in fact, since it rounds up to the next power of two. These
2216 buckets will be retained even if you do C<%hash = ()>, use C<undef
2217 %hash> if you want to free the storage while C<%hash> is still in scope.
2218 You can't shrink the number of buckets allocated for the hash using
2219 C<keys> in this way (but you needn't worry about doing this by accident,
2220 as trying has no effect).
2222 See also C<each>, C<values> and C<sort>.
2224 =item kill SIGNAL, LIST
2226 Sends a signal to a list of processes. Returns the number of
2227 processes successfully signaled (which is not necessarily the
2228 same as the number actually killed).
2230 $cnt = kill 1, $child1, $child2;
2233 If SIGNAL is zero, no signal is sent to the process. This is a
2234 useful way to check that the process is alive and hasn't changed
2235 its UID. See L<perlport> for notes on the portability of this
2238 Unlike in the shell, if SIGNAL is negative, it kills
2239 process groups instead of processes. (On System V, a negative I<PROCESS>
2240 number will also kill process groups, but that's not portable.) That
2241 means you usually want to use positive not negative signals. You may also
2242 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2248 The C<last> command is like the C<break> statement in C (as used in
2249 loops); it immediately exits the loop in question. If the LABEL is
2250 omitted, the command refers to the innermost enclosing loop. The
2251 C<continue> block, if any, is not executed:
2253 LINE: while (<STDIN>) {
2254 last LINE if /^$/; # exit when done with header
2258 C<last> cannot be used to exit a block which returns a value such as
2259 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2260 a grep() or map() operation.
2262 Note that a block by itself is semantically identical to a loop
2263 that executes once. Thus C<last> can be used to effect an early
2264 exit out of such a block.
2266 See also L</continue> for an illustration of how C<last>, C<next>, and
2273 Returns an lowercased version of EXPR. This is the internal function
2274 implementing the C<\L> escape in double-quoted strings.
2275 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2278 If EXPR is omitted, uses C<$_>.
2284 Returns the value of EXPR with the first character lowercased. This is
2285 the internal function implementing the C<\l> escape in double-quoted strings.
2286 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2288 If EXPR is omitted, uses C<$_>.
2294 Returns the length in characters of the value of EXPR. If EXPR is
2295 omitted, returns length of C<$_>. Note that this cannot be used on
2296 an entire array or hash to find out how many elements these have.
2297 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2299 =item link OLDFILE,NEWFILE
2301 Creates a new filename linked to the old filename. Returns true for
2302 success, false otherwise.
2304 =item listen SOCKET,QUEUESIZE
2306 Does the same thing that the listen system call does. Returns true if
2307 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2311 You really probably want to be using C<my> instead, because C<local> isn't
2312 what most people think of as "local". See L<perlsub/"Private Variables
2313 via my()"> for details.
2315 A local modifies the listed variables to be local to the enclosing
2316 block, file, or eval. If more than one value is listed, the list must
2317 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2318 for details, including issues with tied arrays and hashes.
2320 =item localtime EXPR
2322 Converts a time as returned by the time function to a 9-element list
2323 with the time analyzed for the local time zone. Typically used as
2327 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2330 All list elements are numeric, and come straight out of the C `struct
2331 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2332 specified time. $mday is the day of the month, and $mon is the month
2333 itself, in the range C<0..11> with 0 indicating January and 11
2334 indicating December. $year is the number of years since 1900. That
2335 is, $year is C<123> in year 2023. $wday is the day of the week, with
2336 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2337 the year, in the range C<1..365> (or C<1..366> in leap years.) $isdst
2338 is true if the specified time occurs during daylight savings time,
2341 Note that the $year element is I<not> simply the last two digits of
2342 the year. If you assume it is, then you create non-Y2K-compliant
2343 programs--and you wouldn't want to do that, would you?
2345 The proper way to get a complete 4-digit year is simply:
2349 And to get the last two digits of the year (e.g., '01' in 2001) do:
2351 $year = sprintf("%02d", $year % 100);
2353 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2355 In scalar context, C<localtime()> returns the ctime(3) value:
2357 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2359 This scalar value is B<not> locale dependent, see L<perllocale>, but
2360 instead a Perl builtin. Also see the C<Time::Local> module
2361 (to convert the second, minutes, hours, ... back to seconds since the
2362 stroke of midnight the 1st of January 1970, the value returned by
2363 time()), and the strftime(3) and mktime(3) functions available via the
2364 POSIX module. To get somewhat similar but locale dependent date
2365 strings, set up your locale environment variables appropriately
2366 (please see L<perllocale>) and try for example:
2368 use POSIX qw(strftime);
2369 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2371 Note that the C<%a> and C<%b>, the short forms of the day of the week
2372 and the month of the year, may not necessarily be three characters wide.
2378 This function places an advisory lock on a variable, subroutine,
2379 or referenced object contained in I<THING> until the lock goes out
2380 of scope. This is a built-in function only if your version of Perl
2381 was built with threading enabled, and if you've said C<use Threads>.
2382 Otherwise a user-defined function by this name will be called. See
2389 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2390 returns log of C<$_>. To get the log of another base, use basic algebra:
2391 The base-N log of a number is equal to the natural log of that number
2392 divided by the natural log of N. For example:
2396 return log($n)/log(10);
2399 See also L</exp> for the inverse operation.
2401 =item lstat FILEHANDLE
2407 Does the same thing as the C<stat> function (including setting the
2408 special C<_> filehandle) but stats a symbolic link instead of the file
2409 the symbolic link points to. If symbolic links are unimplemented on
2410 your system, a normal C<stat> is done.
2412 If EXPR is omitted, stats C<$_>.
2416 The match operator. See L<perlop>.
2418 =item map BLOCK LIST
2422 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2423 C<$_> to each element) and returns the list value composed of the
2424 results of each such evaluation. In scalar context, returns the
2425 total number of elements so generated. Evaluates BLOCK or EXPR in
2426 list context, so each element of LIST may produce zero, one, or
2427 more elements in the returned value.
2429 @chars = map(chr, @nums);
2431 translates a list of numbers to the corresponding characters. And
2433 %hash = map { getkey($_) => $_ } @array;
2435 is just a funny way to write
2438 foreach $_ (@array) {
2439 $hash{getkey($_)} = $_;
2442 Note that, because C<$_> is a reference into the list value, it can
2443 be used to modify the elements of the array. While this is useful and
2444 supported, it can cause bizarre results if the LIST is not a named array.
2445 Using a regular C<foreach> loop for this purpose would be clearer in
2446 most cases. See also L</grep> for an array composed of those items of
2447 the original list for which the BLOCK or EXPR evaluates to true.
2449 =item mkdir FILENAME,MASK
2451 =item mkdir FILENAME
2453 Creates the directory specified by FILENAME, with permissions
2454 specified by MASK (as modified by C<umask>). If it succeeds it
2455 returns true, otherwise it returns false and sets C<$!> (errno).
2456 If omitted, MASK defaults to 0777.
2458 In general, it is better to create directories with permissive MASK,
2459 and let the user modify that with their C<umask>, than it is to supply
2460 a restrictive MASK and give the user no way to be more permissive.
2461 The exceptions to this rule are when the file or directory should be
2462 kept private (mail files, for instance). The perlfunc(1) entry on
2463 C<umask> discusses the choice of MASK in more detail.
2465 =item msgctl ID,CMD,ARG
2467 Calls the System V IPC function msgctl(2). You'll probably have to say
2471 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2472 then ARG must be a variable which will hold the returned C<msqid_ds>
2473 structure. Returns like C<ioctl>: the undefined value for error,
2474 C<"0 but true"> for zero, or the actual return value otherwise. See also
2475 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2477 =item msgget KEY,FLAGS
2479 Calls the System V IPC function msgget(2). Returns the message queue
2480 id, or the undefined value if there is an error. See also C<IPC::SysV>
2481 and C<IPC::Msg> documentation.
2483 =item msgsnd ID,MSG,FLAGS
2485 Calls the System V IPC function msgsnd to send the message MSG to the
2486 message queue ID. MSG must begin with the native long integer message
2487 type, which may be created with C<pack("l!", $type)>. Returns true if
2488 successful, or false if there is an error. See also C<IPC::SysV> and
2489 C<IPC::SysV::Msg> documentation.
2491 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2493 Calls the System V IPC function msgrcv to receive a message from
2494 message queue ID into variable VAR with a maximum message size of
2495 SIZE. Note that if a message is received, the message type will be
2496 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2497 size of the message type. Returns true if successful, or false if
2498 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2502 =item my EXPR : ATTRIBUTES
2504 A C<my> declares the listed variables to be local (lexically) to the
2505 enclosing block, file, or C<eval>. If
2506 more than one value is listed, the list must be placed in parentheses. See
2507 L<perlsub/"Private Variables via my()"> for details.
2513 The C<next> command is like the C<continue> statement in C; it starts
2514 the next iteration of the loop:
2516 LINE: while (<STDIN>) {
2517 next LINE if /^#/; # discard comments
2521 Note that if there were a C<continue> block on the above, it would get
2522 executed even on discarded lines. If the LABEL is omitted, the command
2523 refers to the innermost enclosing loop.
2525 C<next> cannot be used to exit a block which returns a value such as
2526 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2527 a grep() or map() operation.
2529 Note that a block by itself is semantically identical to a loop
2530 that executes once. Thus C<next> will exit such a block early.
2532 See also L</continue> for an illustration of how C<last>, C<next>, and
2535 =item no Module LIST
2537 See the L</use> function, which C<no> is the opposite of.
2543 Interprets EXPR as an octal string and returns the corresponding
2544 value. (If EXPR happens to start off with C<0x>, interprets it as a
2545 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2546 binary string.) The following will handle decimal, binary, octal, and
2547 hex in the standard Perl or C notation:
2549 $val = oct($val) if $val =~ /^0/;
2551 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2552 in octal), use sprintf() or printf():
2554 $perms = (stat("filename"))[2] & 07777;
2555 $oct_perms = sprintf "%lo", $perms;
2557 The oct() function is commonly used when a string such as C<644> needs
2558 to be converted into a file mode, for example. (Although perl will
2559 automatically convert strings into numbers as needed, this automatic
2560 conversion assumes base 10.)
2562 =item open FILEHANDLE,MODE,LIST
2564 =item open FILEHANDLE,EXPR
2566 =item open FILEHANDLE
2568 Opens the file whose filename is given by EXPR, and associates it with
2569 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2570 name of the real filehandle wanted. If EXPR is omitted, the scalar
2571 variable of the same name as the FILEHANDLE contains the filename.
2572 (Note that lexical variables--those declared with C<my>--will not work
2573 for this purpose; so if you're using C<my>, specify EXPR in your call
2574 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2577 If MODE is C<< '<' >> or nothing, the file is opened for input.
2578 If MODE is C<< '>' >>, the file is truncated and opened for
2579 output, being created if necessary. If MODE is C<<< '>>' >>>,
2580 the file is opened for appending, again being created if necessary.
2581 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2582 you want both read and write access to the file; thus C<< '+<' >> is almost
2583 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2584 file first. You can't usually use either read-write mode for updating
2585 textfiles, since they have variable length records. See the B<-i>
2586 switch in L<perlrun> for a better approach. The file is created with
2587 permissions of C<0666> modified by the process' C<umask> value.
2589 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2590 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2592 In the 2-arguments (and 1-argument) form of the call the mode and
2593 filename should be concatenated (in this order), possibly separated by
2594 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2596 If the filename begins with C<'|'>, the filename is interpreted as a
2597 command to which output is to be piped, and if the filename ends with a
2598 C<'|'>, the filename is interpreted as a command which pipes output to
2599 us. See L<perlipc/"Using open() for IPC">
2600 for more examples of this. (You are not allowed to C<open> to a command
2601 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2602 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2604 If MODE is C<'|-'>, the filename is interpreted as a
2605 command to which output is to be piped, and if MODE is
2606 C<'-|'>, the filename is interpreted as a command which pipes output to
2607 us. In the 2-arguments (and 1-argument) form one should replace dash
2608 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2609 for more examples of this. (You are not allowed to C<open> to a command
2610 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2611 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2613 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2614 and opening C<< '>-' >> opens STDOUT.
2617 nonzero upon success, the undefined value otherwise. If the C<open>
2618 involved a pipe, the return value happens to be the pid of the
2621 If you're unfortunate enough to be running Perl on a system that
2622 distinguishes between text files and binary files (modern operating
2623 systems don't care), then you should check out L</binmode> for tips for
2624 dealing with this. The key distinction between systems that need C<binmode>
2625 and those that don't is their text file formats. Systems like Unix, MacOS, and
2626 Plan9, which delimit lines with a single character, and which encode that
2627 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2629 When opening a file, it's usually a bad idea to continue normal execution
2630 if the request failed, so C<open> is frequently used in connection with
2631 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2632 where you want to make a nicely formatted error message (but there are
2633 modules that can help with that problem)) you should always check
2634 the return value from opening a file. The infrequent exception is when
2635 working with an unopened filehandle is actually what you want to do.
2640 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2641 while (<ARTICLE>) {...
2643 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2644 # if the open fails, output is discarded
2646 open(DBASE, '+<', 'dbase.mine') # open for update
2647 or die "Can't open 'dbase.mine' for update: $!";
2649 open(DBASE, '+<dbase.mine') # ditto
2650 or die "Can't open 'dbase.mine' for update: $!";
2652 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2653 or die "Can't start caesar: $!";
2655 open(ARTICLE, "caesar <$article |") # ditto
2656 or die "Can't start caesar: $!";
2658 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2659 or die "Can't start sort: $!";
2661 # process argument list of files along with any includes
2663 foreach $file (@ARGV) {
2664 process($file, 'fh00');
2668 my($filename, $input) = @_;
2669 $input++; # this is a string increment
2670 unless (open($input, $filename)) {
2671 print STDERR "Can't open $filename: $!\n";
2676 while (<$input>) { # note use of indirection
2677 if (/^#include "(.*)"/) {
2678 process($1, $input);
2685 You may also, in the Bourne shell tradition, specify an EXPR beginning
2686 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2687 name of a filehandle (or file descriptor, if numeric) to be
2688 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2689 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2690 mode you specify should match the mode of the original filehandle.
2691 (Duping a filehandle does not take into account any existing contents of
2692 stdio buffers.) Duping file handles is not yet supported for 3-argument
2695 Here is a script that saves, redirects, and restores STDOUT and
2699 open(OLDOUT, ">&STDOUT");
2700 open(OLDERR, ">&STDERR");
2702 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2703 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2705 select(STDERR); $| = 1; # make unbuffered
2706 select(STDOUT); $| = 1; # make unbuffered
2708 print STDOUT "stdout 1\n"; # this works for
2709 print STDERR "stderr 1\n"; # subprocesses too
2714 open(STDOUT, ">&OLDOUT");
2715 open(STDERR, ">&OLDERR");
2717 print STDOUT "stdout 2\n";
2718 print STDERR "stderr 2\n";
2720 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2721 equivalent of C's C<fdopen> of that file descriptor; this is more
2722 parsimonious of file descriptors. For example:
2724 open(FILEHANDLE, "<&=$fd")
2726 Note that this feature depends on the fdopen() C library function.
2727 On many UNIX systems, fdopen() is known to fail when file descriptors
2728 exceed a certain value, typically 255. If you need more file
2729 descriptors than that, consider rebuilding Perl to use the C<sfio>
2732 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2733 with 2-arguments (or 1-argument) form of open(), then
2734 there is an implicit fork done, and the return value of open is the pid
2735 of the child within the parent process, and C<0> within the child
2736 process. (Use C<defined($pid)> to determine whether the open was successful.)
2737 The filehandle behaves normally for the parent, but i/o to that
2738 filehandle is piped from/to the STDOUT/STDIN of the child process.
2739 In the child process the filehandle isn't opened--i/o happens from/to
2740 the new STDOUT or STDIN. Typically this is used like the normal
2741 piped open when you want to exercise more control over just how the
2742 pipe command gets executed, such as when you are running setuid, and
2743 don't want to have to scan shell commands for metacharacters.
2744 The following triples are more or less equivalent:
2746 open(FOO, "|tr '[a-z]' '[A-Z]'");
2747 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2748 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2750 open(FOO, "cat -n '$file'|");
2751 open(FOO, '-|', "cat -n '$file'");
2752 open(FOO, '-|') || exec 'cat', '-n', $file;
2754 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2756 NOTE: On any operation that may do a fork, all files opened for output
2757 are flushed before the fork is attempted. On systems that support a
2758 close-on-exec flag on files, the flag will be set for the newly opened
2759 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2761 Closing any piped filehandle causes the parent process to wait for the
2762 child to finish, and returns the status value in C<$?>.
2764 The filename passed to 2-argument (or 1-argument) form of open()
2765 will have leading and trailing
2766 whitespace deleted, and the normal redirection characters
2767 honored. This property, known as "magic open",
2768 can often be used to good effect. A user could specify a filename of
2769 F<"rsh cat file |">, or you could change certain filenames as needed:
2771 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2772 open(FH, $filename) or die "Can't open $filename: $!";
2774 Use 3-argument form to open a file with arbitrary weird characters in it,
2776 open(FOO, '<', $file);
2778 otherwise it's necessary to protect any leading and trailing whitespace:
2780 $file =~ s#^(\s)#./$1#;
2781 open(FOO, "< $file\0");
2783 (this may not work on some bizzare filesystems). One should
2784 conscientiously choose between the the I<magic> and 3-arguments form
2789 will allow the user to specify an argument of the form C<"rsh cat file |">,
2790 but will not work on a filename which happens to have a trailing space, while
2792 open IN, '<', $ARGV[0];
2794 will have exactly the opposite restrictions.
2796 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2797 should use the C<sysopen> function, which involves no such magic (but
2798 may use subtly different filemodes than Perl open(), which is mapped
2799 to C fopen()). This is
2800 another way to protect your filenames from interpretation. For example:
2803 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2804 or die "sysopen $path: $!";
2805 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2806 print HANDLE "stuff $$\n");
2808 print "File contains: ", <HANDLE>;
2810 Using the constructor from the C<IO::Handle> package (or one of its
2811 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2812 filehandles that have the scope of whatever variables hold references to
2813 them, and automatically close whenever and however you leave that scope:
2817 sub read_myfile_munged {
2819 my $handle = new IO::File;
2820 open($handle, "myfile") or die "myfile: $!";
2822 or return (); # Automatically closed here.
2823 mung $first or die "mung failed"; # Or here.
2824 return $first, <$handle> if $ALL; # Or here.
2828 See L</seek> for some details about mixing reading and writing.
2830 =item opendir DIRHANDLE,EXPR
2832 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2833 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2834 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2840 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2841 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2842 See L<utf8> for more about Unicode.
2846 An C<our> declares the listed variables to be valid globals within
2847 the enclosing block, file, or C<eval>. That is, it has the same
2848 scoping rules as a "my" declaration, but does not create a local
2849 variable. If more than one value is listed, the list must be placed
2850 in parentheses. The C<our> declaration has no semantic effect unless
2851 "use strict vars" is in effect, in which case it lets you use the
2852 declared global variable without qualifying it with a package name.
2853 (But only within the lexical scope of the C<our> declaration. In this
2854 it differs from "use vars", which is package scoped.)
2856 An C<our> declaration declares a global variable that will be visible
2857 across its entire lexical scope, even across package boundaries. The
2858 package in which the variable is entered is determined at the point
2859 of the declaration, not at the point of use. This means the following
2863 our $bar; # declares $Foo::bar for rest of lexical scope
2867 print $bar; # prints 20
2869 Multiple C<our> declarations in the same lexical scope are allowed
2870 if they are in different packages. If they happened to be in the same
2871 package, Perl will emit warnings if you have asked for them.
2875 our $bar; # declares $Foo::bar for rest of lexical scope
2879 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2880 print $bar; # prints 30
2882 our $bar; # emits warning
2884 =item pack TEMPLATE,LIST
2886 Takes a LIST of values and converts it into a string using the rules
2887 given by the TEMPLATE. The resulting string is the concatenation of
2888 the converted values. Typically, each converted value looks
2889 like its machine-level representation. For example, on 32-bit machines
2890 a converted integer may be represented by a sequence of 4 bytes.
2893 sequence of characters that give the order and type of values, as
2896 a A string with arbitrary binary data, will be null padded.
2897 A An ascii string, will be space padded.
2898 Z A null terminated (asciz) string, will be null padded.
2900 b A bit string (ascending bit order inside each byte, like vec()).
2901 B A bit string (descending bit order inside each byte).
2902 h A hex string (low nybble first).
2903 H A hex string (high nybble first).
2905 c A signed char value.
2906 C An unsigned char value. Only does bytes. See U for Unicode.
2908 s A signed short value.
2909 S An unsigned short value.
2910 (This 'short' is _exactly_ 16 bits, which may differ from
2911 what a local C compiler calls 'short'. If you want
2912 native-length shorts, use the '!' suffix.)
2914 i A signed integer value.
2915 I An unsigned integer value.
2916 (This 'integer' is _at_least_ 32 bits wide. Its exact
2917 size depends on what a local C compiler calls 'int',
2918 and may even be larger than the 'long' described in
2921 l A signed long value.
2922 L An unsigned long value.
2923 (This 'long' is _exactly_ 32 bits, which may differ from
2924 what a local C compiler calls 'long'. If you want
2925 native-length longs, use the '!' suffix.)
2927 n An unsigned short in "network" (big-endian) order.
2928 N An unsigned long in "network" (big-endian) order.
2929 v An unsigned short in "VAX" (little-endian) order.
2930 V An unsigned long in "VAX" (little-endian) order.
2931 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2932 _exactly_ 32 bits, respectively.)
2934 q A signed quad (64-bit) value.
2935 Q An unsigned quad value.
2936 (Quads are available only if your system supports 64-bit
2937 integer values _and_ if Perl has been compiled to support those.
2938 Causes a fatal error otherwise.)
2940 f A single-precision float in the native format.
2941 d A double-precision float in the native format.
2943 p A pointer to a null-terminated string.
2944 P A pointer to a structure (fixed-length string).
2946 u A uuencoded string.
2947 U A Unicode character number. Encodes to UTF-8 internally.
2948 Works even if C<use utf8> is not in effect.
2950 w A BER compressed integer. Its bytes represent an unsigned
2951 integer in base 128, most significant digit first, with as
2952 few digits as possible. Bit eight (the high bit) is set
2953 on each byte except the last.
2957 @ Null fill to absolute position.
2959 The following rules apply:
2965 Each letter may optionally be followed by a number giving a repeat
2966 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2967 C<H>, and C<P> the pack function will gobble up that many values from
2968 the LIST. A C<*> for the repeat count means to use however many items are
2969 left, except for C<@>, C<x>, C<X>, where it is equivalent
2970 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2973 When used with C<Z>, C<*> results in the addition of a trailing null
2974 byte (so the packed result will be one longer than the byte C<length>
2977 The repeat count for C<u> is interpreted as the maximal number of bytes
2978 to encode per line of output, with 0 and 1 replaced by 45.
2982 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
2983 string of length count, padding with nulls or spaces as necessary. When
2984 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2985 after the first null, and C<a> returns data verbatim. When packing,
2986 C<a>, and C<Z> are equivalent.
2988 If the value-to-pack is too long, it is truncated. If too long and an
2989 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2990 by a null byte. Thus C<Z> always packs a trailing null byte under
2995 Likewise, the C<b> and C<B> fields pack a string that many bits long.
2996 Each byte of the input field of pack() generates 1 bit of the result.
2997 Each result bit is based on the least-significant bit of the corresponding
2998 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2999 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3001 Starting from the beginning of the input string of pack(), each 8-tuple
3002 of bytes is converted to 1 byte of output. With format C<b>
3003 the first byte of the 8-tuple determines the least-significant bit of a
3004 byte, and with format C<B> it determines the most-significant bit of
3007 If the length of the input string is not exactly divisible by 8, the
3008 remainder is packed as if the input string were padded by null bytes
3009 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3011 If the input string of pack() is longer than needed, extra bytes are ignored.
3012 A C<*> for the repeat count of pack() means to use all the bytes of
3013 the input field. On unpack()ing the bits are converted to a string
3014 of C<"0">s and C<"1">s.
3018 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3019 representable as hexadecimal digits, 0-9a-f) long.
3021 Each byte of the input field of pack() generates 4 bits of the result.
3022 For non-alphabetical bytes the result is based on the 4 least-significant
3023 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3024 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3025 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3026 is compatible with the usual hexadecimal digits, so that C<"a"> and
3027 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3028 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3030 Starting from the beginning of the input string of pack(), each pair
3031 of bytes is converted to 1 byte of output. With format C<h> the
3032 first byte of the pair determines the least-significant nybble of the
3033 output byte, and with format C<H> it determines the most-significant
3036 If the length of the input string is not even, it behaves as if padded
3037 by a null byte at the end. Similarly, during unpack()ing the "extra"
3038 nybbles are ignored.
3040 If the input string of pack() is longer than needed, extra bytes are ignored.
3041 A C<*> for the repeat count of pack() means to use all the bytes of
3042 the input field. On unpack()ing the bits are converted to a string
3043 of hexadecimal digits.
3047 The C<p> type packs a pointer to a null-terminated string. You are
3048 responsible for ensuring the string is not a temporary value (which can
3049 potentially get deallocated before you get around to using the packed result).
3050 The C<P> type packs a pointer to a structure of the size indicated by the
3051 length. A NULL pointer is created if the corresponding value for C<p> or
3052 C<P> is C<undef>, similarly for unpack().
3056 The C</> template character allows packing and unpacking of strings where
3057 the packed structure contains a byte count followed by the string itself.
3058 You write I<length-item>C</>I<string-item>.
3060 The I<length-item> can be any C<pack> template letter,
3061 and describes how the length value is packed.
3062 The ones likely to be of most use are integer-packing ones like
3063 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3064 and C<N> (for Sun XDR).
3066 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3067 For C<unpack> the length of the string is obtained from the I<length-item>,
3068 but if you put in the '*' it will be ignored.
3070 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3071 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3072 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3074 The I<length-item> is not returned explicitly from C<unpack>.
3076 Adding a count to the I<length-item> letter is unlikely to do anything
3077 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3078 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3079 which Perl does not regard as legal in numeric strings.
3083 The integer types C<s>, C<S>, C<l>, and C<L> may be
3084 immediately followed by a C<!> suffix to signify native shorts or
3085 longs--as you can see from above for example a bare C<l> does mean
3086 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3087 may be larger. This is an issue mainly in 64-bit platforms. You can
3088 see whether using C<!> makes any difference by
3090 print length(pack("s")), " ", length(pack("s!")), "\n";
3091 print length(pack("l")), " ", length(pack("l!")), "\n";
3093 C<i!> and C<I!> also work but only because of completeness;
3094 they are identical to C<i> and C<I>.
3096 The actual sizes (in bytes) of native shorts, ints, longs, and long
3097 longs on the platform where Perl was built are also available via
3101 print $Config{shortsize}, "\n";
3102 print $Config{intsize}, "\n";
3103 print $Config{longsize}, "\n";
3104 print $Config{longlongsize}, "\n";
3106 (The C<$Config{longlongsize}> will be undefine if your system does
3107 not support long longs.)
3111 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3112 are inherently non-portable between processors and operating systems
3113 because they obey the native byteorder and endianness. For example a
3114 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3115 (arranged in and handled by the CPU registers) into bytes as
3117 0x12 0x34 0x56 0x78 # little-endian
3118 0x78 0x56 0x34 0x12 # big-endian
3120 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3121 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3122 Power, and Cray are big-endian. MIPS can be either: Digital used it
3123 in little-endian mode; SGI uses it in big-endian mode.
3125 The names `big-endian' and `little-endian' are comic references to
3126 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3127 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3128 the egg-eating habits of the Lilliputians.
3130 Some systems may have even weirder byte orders such as
3135 You can see your system's preference with
3137 print join(" ", map { sprintf "%#02x", $_ }
3138 unpack("C*",pack("L",0x12345678))), "\n";
3140 The byteorder on the platform where Perl was built is also available
3144 print $Config{byteorder}, "\n";
3146 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3147 and C<'87654321'> are big-endian.
3149 If you want portable packed integers use the formats C<n>, C<N>,
3150 C<v>, and C<V>, their byte endianness and size is known.
3151 See also L<perlport>.
3155 Real numbers (floats and doubles) are in the native machine format only;
3156 due to the multiplicity of floating formats around, and the lack of a
3157 standard "network" representation, no facility for interchange has been
3158 made. This means that packed floating point data written on one machine
3159 may not be readable on another - even if both use IEEE floating point
3160 arithmetic (as the endian-ness of the memory representation is not part
3161 of the IEEE spec). See also L<perlport>.
3163 Note that Perl uses doubles internally for all numeric calculation, and
3164 converting from double into float and thence back to double again will
3165 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3170 You must yourself do any alignment or padding by inserting for example
3171 enough C<'x'>es while packing. There is no way to pack() and unpack()
3172 could know where the bytes are going to or coming from. Therefore
3173 C<pack> (and C<unpack>) handle their output and input as flat
3178 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3182 If TEMPLATE requires more arguments to pack() than actually given, pack()
3183 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3184 to pack() than actually given, extra arguments are ignored.
3190 $foo = pack("CCCC",65,66,67,68);
3192 $foo = pack("C4",65,66,67,68);
3194 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3195 # same thing with Unicode circled letters
3197 $foo = pack("ccxxcc",65,66,67,68);
3200 # note: the above examples featuring "C" and "c" are true
3201 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3202 # and UTF-8. In EBCDIC the first example would be
3203 # $foo = pack("CCCC",193,194,195,196);
3205 $foo = pack("s2",1,2);
3206 # "\1\0\2\0" on little-endian
3207 # "\0\1\0\2" on big-endian
3209 $foo = pack("a4","abcd","x","y","z");
3212 $foo = pack("aaaa","abcd","x","y","z");
3215 $foo = pack("a14","abcdefg");
3216 # "abcdefg\0\0\0\0\0\0\0"
3218 $foo = pack("i9pl", gmtime);
3219 # a real struct tm (on my system anyway)
3221 $utmp_template = "Z8 Z8 Z16 L";
3222 $utmp = pack($utmp_template, @utmp1);
3223 # a struct utmp (BSDish)
3225 @utmp2 = unpack($utmp_template, $utmp);
3226 # "@utmp1" eq "@utmp2"
3229 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3232 $foo = pack('sx2l', 12, 34);
3233 # short 12, two zero bytes padding, long 34
3234 $bar = pack('s@4l', 12, 34);
3235 # short 12, zero fill to position 4, long 34
3238 The same template may generally also be used in unpack().
3242 =item package NAMESPACE
3244 Declares the compilation unit as being in the given namespace. The scope
3245 of the package declaration is from the declaration itself through the end
3246 of the enclosing block, file, or eval (the same as the C<my> operator).
3247 All further unqualified dynamic identifiers will be in this namespace.
3248 A package statement affects only dynamic variables--including those
3249 you've used C<local> on--but I<not> lexical variables, which are created
3250 with C<my>. Typically it would be the first declaration in a file to
3251 be included by the C<require> or C<use> operator. You can switch into a
3252 package in more than one place; it merely influences which symbol table
3253 is used by the compiler for the rest of that block. You can refer to
3254 variables and filehandles in other packages by prefixing the identifier
3255 with the package name and a double colon: C<$Package::Variable>.
3256 If the package name is null, the C<main> package as assumed. That is,
3257 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3258 still seen in older code).
3260 If NAMESPACE is omitted, then there is no current package, and all
3261 identifiers must be fully qualified or lexicals. This is stricter
3262 than C<use strict>, since it also extends to function names.
3264 See L<perlmod/"Packages"> for more information about packages, modules,
3265 and classes. See L<perlsub> for other scoping issues.
3267 =item pipe READHANDLE,WRITEHANDLE
3269 Opens a pair of connected pipes like the corresponding system call.
3270 Note that if you set up a loop of piped processes, deadlock can occur
3271 unless you are very careful. In addition, note that Perl's pipes use
3272 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3273 after each command, depending on the application.
3275 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3276 for examples of such things.
3278 On systems that support a close-on-exec flag on files, the flag will be set
3279 for the newly opened file descriptors as determined by the value of $^F.
3286 Pops and returns the last value of the array, shortening the array by
3287 one element. Has an effect similar to
3291 If there are no elements in the array, returns the undefined value
3292 (although this may happen at other times as well). If ARRAY is
3293 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3294 array in subroutines, just like C<shift>.
3300 Returns the offset of where the last C<m//g> search left off for the variable
3301 is in question (C<$_> is used when the variable is not specified). May be
3302 modified to change that offset. Such modification will also influence
3303 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3306 =item print FILEHANDLE LIST
3312 Prints a string or a list of strings. Returns true if successful.
3313 FILEHANDLE may be a scalar variable name, in which case the variable
3314 contains the name of or a reference to the filehandle, thus introducing
3315 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3316 the next token is a term, it may be misinterpreted as an operator
3317 unless you interpose a C<+> or put parentheses around the arguments.)
3318 If FILEHANDLE is omitted, prints by default to standard output (or
3319 to the last selected output channel--see L</select>). If LIST is
3320 also omitted, prints C<$_> to the currently selected output channel.
3321 To set the default output channel to something other than STDOUT
3322 use the select operation. The current value of C<$,> (if any) is
3323 printed between each LIST item. The current value of C<$\> (if
3324 any) is printed after the entire LIST has been printed. Because
3325 print takes a LIST, anything in the LIST is evaluated in list
3326 context, and any subroutine that you call will have one or more of
3327 its expressions evaluated in list context. Also be careful not to
3328 follow the print keyword with a left parenthesis unless you want
3329 the corresponding right parenthesis to terminate the arguments to
3330 the print--interpose a C<+> or put parentheses around all the
3333 Note that if you're storing FILEHANDLES in an array or other expression,
3334 you will have to use a block returning its value instead:
3336 print { $files[$i] } "stuff\n";
3337 print { $OK ? STDOUT : STDERR } "stuff\n";
3339 =item printf FILEHANDLE FORMAT, LIST
3341 =item printf FORMAT, LIST
3343 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3344 (the output record separator) is not appended. The first argument
3345 of the list will be interpreted as the C<printf> format. If C<use locale> is
3346 in effect, the character used for the decimal point in formatted real numbers
3347 is affected by the LC_NUMERIC locale. See L<perllocale>.
3349 Don't fall into the trap of using a C<printf> when a simple
3350 C<print> would do. The C<print> is more efficient and less
3353 =item prototype FUNCTION
3355 Returns the prototype of a function as a string (or C<undef> if the
3356 function has no prototype). FUNCTION is a reference to, or the name of,
3357 the function whose prototype you want to retrieve.
3359 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3360 name for Perl builtin. If the builtin is not I<overridable> (such as
3361 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3362 C<system>) returns C<undef> because the builtin does not really behave
3363 like a Perl function. Otherwise, the string describing the equivalent
3364 prototype is returned.
3366 =item push ARRAY,LIST
3368 Treats ARRAY as a stack, and pushes the values of LIST
3369 onto the end of ARRAY. The length of ARRAY increases by the length of
3370 LIST. Has the same effect as
3373 $ARRAY[++$#ARRAY] = $value;
3376 but is more efficient. Returns the new number of elements in the array.
3388 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3390 =item quotemeta EXPR
3394 Returns the value of EXPR with all non-alphanumeric
3395 characters backslashed. (That is, all characters not matching
3396 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3397 returned string, regardless of any locale settings.)
3398 This is the internal function implementing
3399 the C<\Q> escape in double-quoted strings.
3401 If EXPR is omitted, uses C<$_>.
3407 Returns a random fractional number greater than or equal to C<0> and less
3408 than the value of EXPR. (EXPR should be positive.) If EXPR is
3409 omitted, the value C<1> is used. Automatically calls C<srand> unless
3410 C<srand> has already been called. See also C<srand>.
3412 (Note: If your rand function consistently returns numbers that are too
3413 large or too small, then your version of Perl was probably compiled
3414 with the wrong number of RANDBITS.)
3416 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3418 =item read FILEHANDLE,SCALAR,LENGTH
3420 Attempts to read LENGTH bytes of data into variable SCALAR from the
3421 specified FILEHANDLE. Returns the number of bytes actually read,
3422 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3423 or shrunk to the length actually read. An OFFSET may be specified to
3424 place the read data at some other place than the beginning of the
3425 string. This call is actually implemented in terms of stdio's fread(3)
3426 call. To get a true read(2) system call, see C<sysread>.
3428 =item readdir DIRHANDLE
3430 Returns the next directory entry for a directory opened by C<opendir>.
3431 If used in list context, returns all the rest of the entries in the
3432 directory. If there are no more entries, returns an undefined value in
3433 scalar context or a null list in list context.
3435 If you're planning to filetest the return values out of a C<readdir>, you'd
3436 better prepend the directory in question. Otherwise, because we didn't
3437 C<chdir> there, it would have been testing the wrong file.
3439 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3440 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3445 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3446 context, each call reads and returns the next line, until end-of-file is
3447 reached, whereupon the subsequent call returns undef. In list context,
3448 reads until end-of-file is reached and returns a list of lines. Note that
3449 the notion of "line" used here is however you may have defined it
3450 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3452 When C<$/> is set to C<undef>, when readline() is in scalar
3453 context (i.e. file slurp mode), and when an empty file is read, it
3454 returns C<''> the first time, followed by C<undef> subsequently.
3456 This is the internal function implementing the C<< <EXPR> >>
3457 operator, but you can use it directly. The C<< <EXPR> >>
3458 operator is discussed in more detail in L<perlop/"I/O Operators">.
3461 $line = readline(*STDIN); # same thing
3467 Returns the value of a symbolic link, if symbolic links are
3468 implemented. If not, gives a fatal error. If there is some system
3469 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3470 omitted, uses C<$_>.
3474 EXPR is executed as a system command.
3475 The collected standard output of the command is returned.
3476 In scalar context, it comes back as a single (potentially
3477 multi-line) string. In list context, returns a list of lines
3478 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3479 This is the internal function implementing the C<qx/EXPR/>
3480 operator, but you can use it directly. The C<qx/EXPR/>
3481 operator is discussed in more detail in L<perlop/"I/O Operators">.
3483 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3485 Receives a message on a socket. Attempts to receive LENGTH bytes of
3486 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3487 will be grown or shrunk to the length actually read. Takes the same
3488 flags as the system call of the same name. Returns the address of the
3489 sender if SOCKET's protocol supports this; returns an empty string
3490 otherwise. If there's an error, returns the undefined value. This call
3491 is actually implemented in terms of recvfrom(2) system call. See
3492 L<perlipc/"UDP: Message Passing"> for examples.
3498 The C<redo> command restarts the loop block without evaluating the
3499 conditional again. The C<continue> block, if any, is not executed. If
3500 the LABEL is omitted, the command refers to the innermost enclosing
3501 loop. This command is normally used by programs that want to lie to
3502 themselves about what was just input:
3504 # a simpleminded Pascal comment stripper
3505 # (warning: assumes no { or } in strings)
3506 LINE: while (<STDIN>) {
3507 while (s|({.*}.*){.*}|$1 |) {}
3512 if (/}/) { # end of comment?
3521 C<redo> cannot be used to retry a block which returns a value such as
3522 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3523 a grep() or map() operation.
3525 Note that a block by itself is semantically identical to a loop
3526 that executes once. Thus C<redo> inside such a block will effectively
3527 turn it into a looping construct.
3529 See also L</continue> for an illustration of how C<last>, C<next>, and
3536 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3537 is not specified, C<$_> will be used. The value returned depends on the
3538 type of thing the reference is a reference to.
3539 Builtin types include:
3549 If the referenced object has been blessed into a package, then that package
3550 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3552 if (ref($r) eq "HASH") {
3553 print "r is a reference to a hash.\n";
3556 print "r is not a reference at all.\n";
3558 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3559 print "r is a reference to something that isa hash.\n";
3562 See also L<perlref>.
3564 =item rename OLDNAME,NEWNAME
3566 Changes the name of a file; an existing file NEWNAME will be
3567 clobbered. Returns true for success, false otherwise.
3569 Behavior of this function varies wildly depending on your system
3570 implementation. For example, it will usually not work across file system
3571 boundaries, even though the system I<mv> command sometimes compensates
3572 for this. Other restrictions include whether it works on directories,
3573 open files, or pre-existing files. Check L<perlport> and either the
3574 rename(2) manpage or equivalent system documentation for details.
3576 =item require VERSION
3582 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3585 If a VERSION is specified as a literal of the form v5.6.1,
3586 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3587 at least as recent as that version, at run time. (For compatibility
3588 with older versions of Perl, a numeric argument will also be interpreted
3589 as VERSION.) Compare with L</use>, which can do a similar check at
3592 require v5.6.1; # run time version check
3593 require 5.6.1; # ditto
3594 require 5.005_03; # float version allowed for compatibility
3596 Otherwise, demands that a library file be included if it hasn't already
3597 been included. The file is included via the do-FILE mechanism, which is
3598 essentially just a variety of C<eval>. Has semantics similar to the following
3603 return 1 if $INC{$filename};
3604 my($realfilename,$result);
3606 foreach $prefix (@INC) {
3607 $realfilename = "$prefix/$filename";
3608 if (-f $realfilename) {
3609 $INC{$filename} = $realfilename;
3610 $result = do $realfilename;
3614 die "Can't find $filename in \@INC";
3616 delete $INC{$filename} if $@ || !$result;
3618 die "$filename did not return true value" unless $result;
3622 Note that the file will not be included twice under the same specified
3623 name. The file must return true as the last statement to indicate
3624 successful execution of any initialization code, so it's customary to
3625 end such a file with C<1;> unless you're sure it'll return true
3626 otherwise. But it's better just to put the C<1;>, in case you add more
3629 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3630 replaces "F<::>" with "F</>" in the filename for you,
3631 to make it easy to load standard modules. This form of loading of
3632 modules does not risk altering your namespace.
3634 In other words, if you try this:
3636 require Foo::Bar; # a splendid bareword
3638 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3639 directories specified in the C<@INC> array.
3641 But if you try this:
3643 $class = 'Foo::Bar';
3644 require $class; # $class is not a bareword
3646 require "Foo::Bar"; # not a bareword because of the ""
3648 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3649 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3651 eval "require $class";
3653 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3659 Generally used in a C<continue> block at the end of a loop to clear
3660 variables and reset C<??> searches so that they work again. The
3661 expression is interpreted as a list of single characters (hyphens
3662 allowed for ranges). All variables and arrays beginning with one of
3663 those letters are reset to their pristine state. If the expression is
3664 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3665 only variables or searches in the current package. Always returns
3668 reset 'X'; # reset all X variables
3669 reset 'a-z'; # reset lower case variables
3670 reset; # just reset ?one-time? searches
3672 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3673 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3674 variables--lexical variables are unaffected, but they clean themselves
3675 up on scope exit anyway, so you'll probably want to use them instead.
3682 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3683 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3684 context, depending on how the return value will be used, and the context
3685 may vary from one execution to the next (see C<wantarray>). If no EXPR
3686 is given, returns an empty list in list context, the undefined value in
3687 scalar context, and (of course) nothing at all in a void context.
3689 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3690 or do FILE will automatically return the value of the last expression
3695 In list context, returns a list value consisting of the elements
3696 of LIST in the opposite order. In scalar context, concatenates the
3697 elements of LIST and returns a string value with all characters
3698 in the opposite order.
3700 print reverse <>; # line tac, last line first
3702 undef $/; # for efficiency of <>
3703 print scalar reverse <>; # character tac, last line tsrif
3705 This operator is also handy for inverting a hash, although there are some
3706 caveats. If a value is duplicated in the original hash, only one of those
3707 can be represented as a key in the inverted hash. Also, this has to
3708 unwind one hash and build a whole new one, which may take some time
3709 on a large hash, such as from a DBM file.
3711 %by_name = reverse %by_address; # Invert the hash
3713 =item rewinddir DIRHANDLE
3715 Sets the current position to the beginning of the directory for the
3716 C<readdir> routine on DIRHANDLE.
3718 =item rindex STR,SUBSTR,POSITION
3720 =item rindex STR,SUBSTR
3722 Works just like index() except that it returns the position of the LAST
3723 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3724 last occurrence at or before that position.
3726 =item rmdir FILENAME
3730 Deletes the directory specified by FILENAME if that directory is empty. If it
3731 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3732 FILENAME is omitted, uses C<$_>.
3736 The substitution operator. See L<perlop>.
3740 Forces EXPR to be interpreted in scalar context and returns the value
3743 @counts = ( scalar @a, scalar @b, scalar @c );
3745 There is no equivalent operator to force an expression to
3746 be interpolated in list context because in practice, this is never
3747 needed. If you really wanted to do so, however, you could use
3748 the construction C<@{[ (some expression) ]}>, but usually a simple
3749 C<(some expression)> suffices.
3751 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3752 parenthesized list, this behaves as a scalar comma expression, evaluating
3753 all but the last element in void context and returning the final element
3754 evaluated in scalar context. This is seldom what you want.
3756 The following single statement:
3758 print uc(scalar(&foo,$bar)),$baz;
3760 is the moral equivalent of these two:
3763 print(uc($bar),$baz);
3765 See L<perlop> for more details on unary operators and the comma operator.
3767 =item seek FILEHANDLE,POSITION,WHENCE
3769 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3770 FILEHANDLE may be an expression whose value gives the name of the
3771 filehandle. The values for WHENCE are C<0> to set the new position to
3772 POSITION, C<1> to set it to the current position plus POSITION, and
3773 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3774 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3775 (start of the file, current position, end of the file) from the Fcntl
3776 module. Returns C<1> upon success, C<0> otherwise.
3778 If you want to position file for C<sysread> or C<syswrite>, don't use
3779 C<seek>--buffering makes its effect on the file's system position
3780 unpredictable and non-portable. Use C<sysseek> instead.
3782 Due to the rules and rigors of ANSI C, on some systems you have to do a
3783 seek whenever you switch between reading and writing. Amongst other
3784 things, this may have the effect of calling stdio's clearerr(3).
3785 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3789 This is also useful for applications emulating C<tail -f>. Once you hit
3790 EOF on your read, and then sleep for a while, you might have to stick in a
3791 seek() to reset things. The C<seek> doesn't change the current position,
3792 but it I<does> clear the end-of-file condition on the handle, so that the
3793 next C<< <FILE> >> makes Perl try again to read something. We hope.
3795 If that doesn't work (some stdios are particularly cantankerous), then
3796 you may need something more like this:
3799 for ($curpos = tell(FILE); $_ = <FILE>;
3800 $curpos = tell(FILE)) {
3801 # search for some stuff and put it into files
3803 sleep($for_a_while);
3804 seek(FILE, $curpos, 0);
3807 =item seekdir DIRHANDLE,POS
3809 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3810 must be a value returned by C<telldir>. Has the same caveats about
3811 possible directory compaction as the corresponding system library
3814 =item select FILEHANDLE
3818 Returns the currently selected filehandle. Sets the current default
3819 filehandle for output, if FILEHANDLE is supplied. This has two
3820 effects: first, a C<write> or a C<print> without a filehandle will
3821 default to this FILEHANDLE. Second, references to variables related to
3822 output will refer to this output channel. For example, if you have to
3823 set the top of form format for more than one output channel, you might
3831 FILEHANDLE may be an expression whose value gives the name of the
3832 actual filehandle. Thus:
3834 $oldfh = select(STDERR); $| = 1; select($oldfh);
3836 Some programmers may prefer to think of filehandles as objects with
3837 methods, preferring to write the last example as:
3840 STDERR->autoflush(1);
3842 =item select RBITS,WBITS,EBITS,TIMEOUT
3844 This calls the select(2) system call with the bit masks specified, which
3845 can be constructed using C<fileno> and C<vec>, along these lines:
3847 $rin = $win = $ein = '';
3848 vec($rin,fileno(STDIN),1) = 1;
3849 vec($win,fileno(STDOUT),1) = 1;
3852 If you want to select on many filehandles you might wish to write a
3856 my(@fhlist) = split(' ',$_[0]);
3859 vec($bits,fileno($_),1) = 1;
3863 $rin = fhbits('STDIN TTY SOCK');
3867 ($nfound,$timeleft) =
3868 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3870 or to block until something becomes ready just do this
3872 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3874 Most systems do not bother to return anything useful in $timeleft, so
3875 calling select() in scalar context just returns $nfound.
3877 Any of the bit masks can also be undef. The timeout, if specified, is
3878 in seconds, which may be fractional. Note: not all implementations are
3879 capable of returning the$timeleft. If not, they always return
3880 $timeleft equal to the supplied $timeout.
3882 You can effect a sleep of 250 milliseconds this way:
3884 select(undef, undef, undef, 0.25);
3886 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3887 or <FH>) with C<select>, except as permitted by POSIX, and even
3888 then only on POSIX systems. You have to use C<sysread> instead.
3890 =item semctl ID,SEMNUM,CMD,ARG
3892 Calls the System V IPC function C<semctl>. You'll probably have to say
3896 first to get the correct constant definitions. If CMD is IPC_STAT or
3897 GETALL, then ARG must be a variable which will hold the returned
3898 semid_ds structure or semaphore value array. Returns like C<ioctl>:
3899 the undefined value for error, "C<0 but true>" for zero, or the actual
3900 return value otherwise. The ARG must consist of a vector of native
3901 short integers, which may may be created with C<pack("s!",(0)x$nsem)>.
3902 See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3904 =item semget KEY,NSEMS,FLAGS
3906 Calls the System V IPC function semget. Returns the semaphore id, or
3907 the undefined value if there is an error. See also C<IPC::SysV> and
3908 C<IPC::SysV::Semaphore> documentation.
3910 =item semop KEY,OPSTRING
3912 Calls the System V IPC function semop to perform semaphore operations
3913 such as signaling and waiting. OPSTRING must be a packed array of
3914 semop structures. Each semop structure can be generated with
3915 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3916 operations is implied by the length of OPSTRING. Returns true if
3917 successful, or false if there is an error. As an example, the
3918 following code waits on semaphore $semnum of semaphore id $semid:
3920 $semop = pack("sss", $semnum, -1, 0);
3921 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3923 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3924 and C<IPC::SysV::Semaphore> documentation.
3926 =item send SOCKET,MSG,FLAGS,TO
3928 =item send SOCKET,MSG,FLAGS
3930 Sends a message on a socket. Takes the same flags as the system call
3931 of the same name. On unconnected sockets you must specify a
3932 destination to send TO, in which case it does a C C<sendto>. Returns
3933 the number of characters sent, or the undefined value if there is an
3934 error. The C system call sendmsg(2) is currently unimplemented.
3935 See L<perlipc/"UDP: Message Passing"> for examples.
3937 =item setpgrp PID,PGRP
3939 Sets the current process group for the specified PID, C<0> for the current
3940 process. Will produce a fatal error if used on a machine that doesn't
3941 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3942 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3943 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3946 =item setpriority WHICH,WHO,PRIORITY
3948 Sets the current priority for a process, a process group, or a user.
3949 (See setpriority(2).) Will produce a fatal error if used on a machine
3950 that doesn't implement setpriority(2).
3952 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3954 Sets the socket option requested. Returns undefined if there is an
3955 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3962 Shifts the first value of the array off and returns it, shortening the
3963 array by 1 and moving everything down. If there are no elements in the
3964 array, returns the undefined value. If ARRAY is omitted, shifts the
3965 C<@_> array within the lexical scope of subroutines and formats, and the
3966 C<@ARGV> array at file scopes or within the lexical scopes established by
3967 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
3970 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3971 same thing to the left end of an array that C<pop> and C<push> do to the
3974 =item shmctl ID,CMD,ARG
3976 Calls the System V IPC function shmctl. You'll probably have to say
3980 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3981 then ARG must be a variable which will hold the returned C<shmid_ds>
3982 structure. Returns like ioctl: the undefined value for error, "C<0> but
3983 true" for zero, or the actual return value otherwise.
3984 See also C<IPC::SysV> documentation.
3986 =item shmget KEY,SIZE,FLAGS
3988 Calls the System V IPC function shmget. Returns the shared memory
3989 segment id, or the undefined value if there is an error.
3990 See also C<IPC::SysV> documentation.
3992 =item shmread ID,VAR,POS,SIZE
3994 =item shmwrite ID,STRING,POS,SIZE
3996 Reads or writes the System V shared memory segment ID starting at
3997 position POS for size SIZE by attaching to it, copying in/out, and
3998 detaching from it. When reading, VAR must be a variable that will
3999 hold the data read. When writing, if STRING is too long, only SIZE
4000 bytes are used; if STRING is too short, nulls are written to fill out
4001 SIZE bytes. Return true if successful, or false if there is an error.
4002 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
4005 =item shutdown SOCKET,HOW
4007 Shuts down a socket connection in the manner indicated by HOW, which
4008 has the same interpretation as in the system call of the same name.
4010 shutdown(SOCKET, 0); # I/we have stopped reading data
4011 shutdown(SOCKET, 1); # I/we have stopped writing data
4012 shutdown(SOCKET, 2); # I/we have stopped using this socket
4014 This is useful with sockets when you want to tell the other
4015 side you're done writing but not done reading, or vice versa.
4016 It's also a more insistent form of close because it also
4017 disables the file descriptor in any forked copies in other
4024 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4025 returns sine of C<$_>.
4027 For the inverse sine operation, you may use the C<Math::Trig::asin>
4028 function, or use this relation:
4030 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4036 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4037 May be interrupted if the process receives a signal such as C<SIGALRM>.
4038 Returns the number of seconds actually slept. You probably cannot
4039 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4042 On some older systems, it may sleep up to a full second less than what
4043 you requested, depending on how it counts seconds. Most modern systems
4044 always sleep the full amount. They may appear to sleep longer than that,
4045 however, because your process might not be scheduled right away in a
4046 busy multitasking system.
4048 For delays of finer granularity than one second, you may use Perl's
4049 C<syscall> interface to access setitimer(2) if your system supports
4050 it, or else see L</select> above. The Time::HiRes module from CPAN
4053 See also the POSIX module's C<sigpause> function.
4055 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4057 Opens a socket of the specified kind and attaches it to filehandle
4058 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4059 the system call of the same name. You should C<use Socket> first
4060 to get the proper definitions imported. See the examples in
4061 L<perlipc/"Sockets: Client/Server Communication">.
4063 On systems that support a close-on-exec flag on files, the flag will
4064 be set for the newly opened file descriptor, as determined by the
4065 value of $^F. See L<perlvar/$^F>.
4067 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4069 Creates an unnamed pair of sockets in the specified domain, of the
4070 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4071 for the system call of the same name. If unimplemented, yields a fatal
4072 error. Returns true if successful.
4074 On systems that support a close-on-exec flag on files, the flag will
4075 be set for the newly opened file descriptors, as determined by the value
4076 of $^F. See L<perlvar/$^F>.
4078 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4079 to C<pipe(Rdr, Wtr)> is essentially:
4082 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4083 shutdown(Rdr, 1); # no more writing for reader
4084 shutdown(Wtr, 0); # no more reading for writer
4086 See L<perlipc> for an example of socketpair use.
4088 =item sort SUBNAME LIST
4090 =item sort BLOCK LIST
4094 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4095 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4096 specified, it gives the name of a subroutine that returns an integer
4097 less than, equal to, or greater than C<0>, depending on how the elements
4098 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4099 operators are extremely useful in such routines.) SUBNAME may be a
4100 scalar variable name (unsubscripted), in which case the value provides
4101 the name of (or a reference to) the actual subroutine to use. In place
4102 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4105 If the subroutine's prototype is C<($$)>, the elements to be compared
4106 are passed by reference in C<@_>, as for a normal subroutine. If not,
4107 the normal calling code for subroutines is bypassed in the interests of
4108 efficiency, and the elements to be compared are passed into the subroutine
4109 as the package global variables $a and $b (see example below). Note that
4110 in the latter case, it is usually counter-productive to declare $a and
4113 In either case, the subroutine may not be recursive. The values to be
4114 compared are always passed by reference, so don't modify them.
4116 You also cannot exit out of the sort block or subroutine using any of the
4117 loop control operators described in L<perlsyn> or with C<goto>.
4119 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4120 current collation locale. See L<perllocale>.
4125 @articles = sort @files;
4127 # same thing, but with explicit sort routine
4128 @articles = sort {$a cmp $b} @files;
4130 # now case-insensitively
4131 @articles = sort {uc($a) cmp uc($b)} @files;
4133 # same thing in reversed order
4134 @articles = sort {$b cmp $a} @files;
4136 # sort numerically ascending
4137 @articles = sort {$a <=> $b} @files;
4139 # sort numerically descending
4140 @articles = sort {$b <=> $a} @files;
4142 # this sorts the %age hash by value instead of key
4143 # using an in-line function
4144 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4146 # sort using explicit subroutine name
4148 $age{$a} <=> $age{$b}; # presuming numeric
4150 @sortedclass = sort byage @class;
4152 sub backwards { $b cmp $a }
4153 @harry = qw(dog cat x Cain Abel);
4154 @george = qw(gone chased yz Punished Axed);
4156 # prints AbelCaincatdogx
4157 print sort backwards @harry;
4158 # prints xdogcatCainAbel
4159 print sort @george, 'to', @harry;
4160 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4162 # inefficiently sort by descending numeric compare using
4163 # the first integer after the first = sign, or the
4164 # whole record case-insensitively otherwise
4167 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4172 # same thing, but much more efficiently;
4173 # we'll build auxiliary indices instead
4177 push @nums, /=(\d+)/;
4182 $nums[$b] <=> $nums[$a]
4184 $caps[$a] cmp $caps[$b]
4188 # same thing, but without any temps
4189 @new = map { $_->[0] }
4190 sort { $b->[1] <=> $a->[1]
4193 } map { [$_, /=(\d+)/, uc($_)] } @old;
4195 # using a prototype allows you to use any comparison subroutine
4196 # as a sort subroutine (including other package's subroutines)
4198 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4201 @new = sort other::backwards @old;
4203 If you're using strict, you I<must not> declare $a
4204 and $b as lexicals. They are package globals. That means
4205 if you're in the C<main> package, it's
4207 @articles = sort {$main::b <=> $main::a} @files;
4211 @articles = sort {$::b <=> $::a} @files;
4213 but if you're in the C<FooPack> package, it's
4215 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4217 The comparison function is required to behave. If it returns
4218 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4219 sometimes saying the opposite, for example) the results are not
4222 =item splice ARRAY,OFFSET,LENGTH,LIST
4224 =item splice ARRAY,OFFSET,LENGTH
4226 =item splice ARRAY,OFFSET
4230 Removes the elements designated by OFFSET and LENGTH from an array, and
4231 replaces them with the elements of LIST, if any. In list context,
4232 returns the elements removed from the array. In scalar context,
4233 returns the last element removed, or C<undef> if no elements are
4234 removed. The array grows or shrinks as necessary.
4235 If OFFSET is negative then it starts that far from the end of the array.
4236 If LENGTH is omitted, removes everything from OFFSET onward.
4237 If LENGTH is negative, leaves that many elements off the end of the array.
4238 If both OFFSET and LENGTH are omitted, removes everything.
4240 The following equivalences hold (assuming C<$[ == 0>):
4242 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4243 pop(@a) splice(@a,-1)
4244 shift(@a) splice(@a,0,1)
4245 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4246 $a[$x] = $y splice(@a,$x,1,$y)
4248 Example, assuming array lengths are passed before arrays:
4250 sub aeq { # compare two list values
4251 my(@a) = splice(@_,0,shift);
4252 my(@b) = splice(@_,0,shift);
4253 return 0 unless @a == @b; # same len?
4255 return 0 if pop(@a) ne pop(@b);
4259 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4261 =item split /PATTERN/,EXPR,LIMIT
4263 =item split /PATTERN/,EXPR
4265 =item split /PATTERN/
4269 Splits a string into a list of strings and returns that list. By default,
4270 empty leading fields are preserved, and empty trailing ones are deleted.
4272 If not in list context, returns the number of fields found and splits into
4273 the C<@_> array. (In list context, you can force the split into C<@_> by
4274 using C<??> as the pattern delimiters, but it still returns the list
4275 value.) The use of implicit split to C<@_> is deprecated, however, because
4276 it clobbers your subroutine arguments.
4278 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4279 splits on whitespace (after skipping any leading whitespace). Anything
4280 matching PATTERN is taken to be a delimiter separating the fields. (Note
4281 that the delimiter may be longer than one character.)
4283 If LIMIT is specified and positive, splits into no more than that
4284 many fields (though it may split into fewer). If LIMIT is unspecified
4285 or zero, trailing null fields are stripped (which potential users
4286 of C<pop> would do well to remember). If LIMIT is negative, it is
4287 treated as if an arbitrarily large LIMIT had been specified.
4289 A pattern matching the null string (not to be confused with
4290 a null pattern C<//>, which is just one member of the set of patterns
4291 matching a null string) will split the value of EXPR into separate
4292 characters at each point it matches that way. For example:
4294 print join(':', split(/ */, 'hi there'));
4296 produces the output 'h:i:t:h:e:r:e'.
4298 The LIMIT parameter can be used to split a line partially
4300 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4302 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4303 one larger than the number of variables in the list, to avoid
4304 unnecessary work. For the list above LIMIT would have been 4 by
4305 default. In time critical applications it behooves you not to split
4306 into more fields than you really need.
4308 If the PATTERN contains parentheses, additional list elements are
4309 created from each matching substring in the delimiter.
4311 split(/([,-])/, "1-10,20", 3);
4313 produces the list value
4315 (1, '-', 10, ',', 20)
4317 If you had the entire header of a normal Unix email message in $header,
4318 you could split it up into fields and their values this way:
4320 $header =~ s/\n\s+/ /g; # fix continuation lines
4321 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4323 The pattern C</PATTERN/> may be replaced with an expression to specify
4324 patterns that vary at runtime. (To do runtime compilation only once,
4325 use C</$variable/o>.)
4327 As a special case, specifying a PATTERN of space (C<' '>) will split on
4328 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4329 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4330 will give you as many null initial fields as there are leading spaces.
4331 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4332 whitespace produces a null first field. A C<split> with no arguments
4333 really does a C<split(' ', $_)> internally.
4337 open(PASSWD, '/etc/passwd');
4339 ($login, $passwd, $uid, $gid,
4340 $gcos, $home, $shell) = split(/:/);
4344 (Note that $shell above will still have a newline on it. See L</chop>,
4345 L</chomp>, and L</join>.)
4347 =item sprintf FORMAT, LIST
4349 Returns a string formatted by the usual C<printf> conventions of the
4350 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4351 on your system for an explanation of the general principles.
4353 Perl does its own C<sprintf> formatting--it emulates the C
4354 function C<sprintf>, but it doesn't use it (except for floating-point
4355 numbers, and even then only the standard modifiers are allowed). As a
4356 result, any non-standard extensions in your local C<sprintf> are not
4357 available from Perl.
4359 Perl's C<sprintf> permits the following universally-known conversions:
4362 %c a character with the given number
4364 %d a signed integer, in decimal
4365 %u an unsigned integer, in decimal
4366 %o an unsigned integer, in octal
4367 %x an unsigned integer, in hexadecimal
4368 %e a floating-point number, in scientific notation
4369 %f a floating-point number, in fixed decimal notation
4370 %g a floating-point number, in %e or %f notation
4372 In addition, Perl permits the following widely-supported conversions:
4374 %X like %x, but using upper-case letters
4375 %E like %e, but using an upper-case "E"
4376 %G like %g, but with an upper-case "E" (if applicable)
4377 %b an unsigned integer, in binary
4378 %p a pointer (outputs the Perl value's address in hexadecimal)
4379 %n special: *stores* the number of characters output so far
4380 into the next variable in the parameter list
4382 Finally, for backward (and we do mean "backward") compatibility, Perl
4383 permits these unnecessary but widely-supported conversions:
4386 %D a synonym for %ld
4387 %U a synonym for %lu
4388 %O a synonym for %lo
4391 Perl permits the following universally-known flags between the C<%>
4392 and the conversion letter:
4394 space prefix positive number with a space
4395 + prefix positive number with a plus sign
4396 - left-justify within the field
4397 0 use zeros, not spaces, to right-justify
4398 # prefix non-zero octal with "0", non-zero hex with "0x"
4399 number minimum field width
4400 .number "precision": digits after decimal point for
4401 floating-point, max length for string, minimum length
4403 l interpret integer as C type "long" or "unsigned long"
4404 h interpret integer as C type "short" or "unsigned short"
4405 If no flags, interpret integer as C type "int" or "unsigned"
4407 There are also two Perl-specific flags:
4409 V interpret integer as Perl's standard integer type
4410 v interpret string as a vector of integers, output as
4411 numbers separated either by dots, or by an arbitrary
4412 string received from the argument list when the flag
4415 Where a number would appear in the flags, an asterisk (C<*>) may be
4416 used instead, in which case Perl uses the next item in the parameter
4417 list as the given number (that is, as the field width or precision).
4418 If a field width obtained through C<*> is negative, it has the same
4419 effect as the C<-> flag: left-justification.
4421 The C<v> flag is useful for displaying ordinal values of characters
4422 in arbitrary strings:
4424 printf "version is v%vd\n", $^V; # Perl's version
4425 printf "address is %*vX\n", ":", $addr; # IPv6 address
4426 printf "bits are %*vb\n", " ", $bits; # random bitstring
4428 If C<use locale> is in effect, the character used for the decimal
4429 point in formatted real numbers is affected by the LC_NUMERIC locale.
4432 If Perl understands "quads" (64-bit integers) (this requires
4433 either that the platform natively support quads or that Perl
4434 be specifically compiled to support quads), the characters
4438 print quads, and they may optionally be preceded by
4446 You can find out whether your Perl supports quads via L<Config>:
4449 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4452 If Perl understands "long doubles" (this requires that the platform
4453 support long doubles), the flags
4457 may optionally be preceded by
4465 You can find out whether your Perl supports long doubles via L<Config>:
4468 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4474 Return the square root of EXPR. If EXPR is omitted, returns square
4475 root of C<$_>. Only works on non-negative operands, unless you've
4476 loaded the standard Math::Complex module.
4479 print sqrt(-2); # prints 1.4142135623731i
4485 Sets the random number seed for the C<rand> operator. If EXPR is
4486 omitted, uses a semi-random value supplied by the kernel (if it supports
4487 the F</dev/urandom> device) or based on the current time and process
4488 ID, among other things. In versions of Perl prior to 5.004 the default
4489 seed was just the current C<time>. This isn't a particularly good seed,
4490 so many old programs supply their own seed value (often C<time ^ $$> or
4491 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4493 In fact, it's usually not necessary to call C<srand> at all, because if
4494 it is not called explicitly, it is called implicitly at the first use of
4495 the C<rand> operator. However, this was not the case in version of Perl
4496 before 5.004, so if your script will run under older Perl versions, it
4497 should call C<srand>.
4499 Note that you need something much more random than the default seed for
4500 cryptographic purposes. Checksumming the compressed output of one or more
4501 rapidly changing operating system status programs is the usual method. For
4504 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4506 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4509 Do I<not> call C<srand> multiple times in your program unless you know
4510 exactly what you're doing and why you're doing it. The point of the
4511 function is to "seed" the C<rand> function so that C<rand> can produce
4512 a different sequence each time you run your program. Just do it once at the
4513 top of your program, or you I<won't> get random numbers out of C<rand>!
4515 Frequently called programs (like CGI scripts) that simply use
4519 for a seed can fall prey to the mathematical property that
4523 one-third of the time. So don't do that.
4525 =item stat FILEHANDLE
4531 Returns a 13-element list giving the status info for a file, either
4532 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4533 it stats C<$_>. Returns a null list if the stat fails. Typically used
4536 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4537 $atime,$mtime,$ctime,$blksize,$blocks)
4540 Not all fields are supported on all filesystem types. Here are the
4541 meaning of the fields:
4543 0 dev device number of filesystem
4545 2 mode file mode (type and permissions)
4546 3 nlink number of (hard) links to the file
4547 4 uid numeric user ID of file's owner
4548 5 gid numeric group ID of file's owner
4549 6 rdev the device identifier (special files only)
4550 7 size total size of file, in bytes
4551 8 atime last access time in seconds since the epoch
4552 9 mtime last modify time in seconds since the epoch
4553 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4554 11 blksize preferred block size for file system I/O
4555 12 blocks actual number of blocks allocated
4557 (The epoch was at 00:00 January 1, 1970 GMT.)
4559 If stat is passed the special filehandle consisting of an underline, no
4560 stat is done, but the current contents of the stat structure from the
4561 last stat or filetest are returned. Example:
4563 if (-x $file && (($d) = stat(_)) && $d < 0) {
4564 print "$file is executable NFS file\n";
4567 (This works on machines only for which the device number is negative
4570 Because the mode contains both the file type and its permissions, you
4571 should mask off the file type portion and (s)printf using a C<"%o">
4572 if you want to see the real permissions.
4574 $mode = (stat($filename))[2];
4575 printf "Permissions are %04o\n", $mode & 07777;
4577 In scalar context, C<stat> returns a boolean value indicating success
4578 or failure, and, if successful, sets the information associated with
4579 the special filehandle C<_>.
4581 The File::stat module provides a convenient, by-name access mechanism:
4584 $sb = stat($filename);
4585 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4586 $filename, $sb->size, $sb->mode & 07777,
4587 scalar localtime $sb->mtime;
4589 You can import symbolic mode constants (C<S_IF*>) and functions
4590 (C<S_IS*>) from the Fcntl module:
4594 $mode = (stat($filename))[2];
4596 $user_rwx = ($mode & S_IRWXU) >> 6;
4597 $group_read = ($mode & S_IRGRP) >> 3;
4598 $other_execute = $mode & S_IXOTH;
4600 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4602 $is_setuid = $mode & S_ISUID;
4603 $is_setgid = S_ISDIR($mode);
4605 You could write the last two using the C<-u> and C<-d> operators.
4606 The commonly available S_IF* constants are
4608 # Permissions: read, write, execute, for user, group, others.
4610 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4611 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4612 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4614 # Setuid/Setgid/Stickiness.
4616 S_ISUID S_ISGID S_ISVTX S_ISTXT
4618 # File types. Not necessarily all are available on your system.
4620 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4622 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4624 S_IREAD S_IWRITE S_IEXEC
4626 and the S_IF* functions are
4628 S_IFMODE($mode) the part of $mode containg the permission bits
4629 and the setuid/setgid/sticky bits
4631 S_IFMT($mode) the part of $mode containing the file type
4632 which can be bit-anded with e.g. S_IFREG
4633 or with the following functions
4635 # The operators -f, -d, -l, -b, -c, -p, and -s.
4637 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4638 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4640 # No direct -X operator counterpart, but for the first one
4641 # the -g operator is often equivalent. The ENFMT stands for
4642 # record flocking enforcement, a platform-dependent feature.
4644 S_ISENFMT($mode) S_ISWHT($mode)
4646 See your native chmod(2) and stat(2) documentation for more details
4647 about the S_* constants.
4653 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4654 doing many pattern matches on the string before it is next modified.
4655 This may or may not save time, depending on the nature and number of
4656 patterns you are searching on, and on the distribution of character
4657 frequencies in the string to be searched--you probably want to compare
4658 run times with and without it to see which runs faster. Those loops
4659 which scan for many short constant strings (including the constant
4660 parts of more complex patterns) will benefit most. You may have only
4661 one C<study> active at a time--if you study a different scalar the first
4662 is "unstudied". (The way C<study> works is this: a linked list of every
4663 character in the string to be searched is made, so we know, for
4664 example, where all the C<'k'> characters are. From each search string,
4665 the rarest character is selected, based on some static frequency tables
4666 constructed from some C programs and English text. Only those places
4667 that contain this "rarest" character are examined.)
4669 For example, here is a loop that inserts index producing entries
4670 before any line containing a certain pattern:
4674 print ".IX foo\n" if /\bfoo\b/;
4675 print ".IX bar\n" if /\bbar\b/;
4676 print ".IX blurfl\n" if /\bblurfl\b/;
4681 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4682 will be looked at, because C<f> is rarer than C<o>. In general, this is
4683 a big win except in pathological cases. The only question is whether
4684 it saves you more time than it took to build the linked list in the
4687 Note that if you have to look for strings that you don't know till
4688 runtime, you can build an entire loop as a string and C<eval> that to
4689 avoid recompiling all your patterns all the time. Together with
4690 undefining C<$/> to input entire files as one record, this can be very
4691 fast, often faster than specialized programs like fgrep(1). The following
4692 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4693 out the names of those files that contain a match:
4695 $search = 'while (<>) { study;';
4696 foreach $word (@words) {
4697 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4702 eval $search; # this screams
4703 $/ = "\n"; # put back to normal input delimiter
4704 foreach $file (sort keys(%seen)) {
4712 =item sub NAME BLOCK
4714 This is subroutine definition, not a real function I<per se>. With just a
4715 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4716 Without a NAME, it's an anonymous function declaration, and does actually
4717 return a value: the CODE ref of the closure you just created. See L<perlsub>
4718 and L<perlref> for details.
4720 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4722 =item substr EXPR,OFFSET,LENGTH
4724 =item substr EXPR,OFFSET
4726 Extracts a substring out of EXPR and returns it. First character is at
4727 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4728 If OFFSET is negative (or more precisely, less than C<$[>), starts
4729 that far from the end of the string. If LENGTH is omitted, returns
4730 everything to the end of the string. If LENGTH is negative, leaves that
4731 many characters off the end of the string.
4733 You can use the substr() function as an lvalue, in which case EXPR
4734 must itself be an lvalue. If you assign something shorter than LENGTH,
4735 the string will shrink, and if you assign something longer than LENGTH,
4736 the string will grow to accommodate it. To keep the string the same
4737 length you may need to pad or chop your value using C<sprintf>.
4739 If OFFSET and LENGTH specify a substring that is partly outside the
4740 string, only the part within the string is returned. If the substring
4741 is beyond either end of the string, substr() returns the undefined
4742 value and produces a warning. When used as an lvalue, specifying a
4743 substring that is entirely outside the string is a fatal error.
4744 Here's an example showing the behavior for boundary cases:
4747 substr($name, 4) = 'dy'; # $name is now 'freddy'
4748 my $null = substr $name, 6, 2; # returns '' (no warning)
4749 my $oops = substr $name, 7; # returns undef, with warning
4750 substr($name, 7) = 'gap'; # fatal error
4752 An alternative to using substr() as an lvalue is to specify the
4753 replacement string as the 4th argument. This allows you to replace
4754 parts of the EXPR and return what was there before in one operation,
4755 just as you can with splice().
4757 =item symlink OLDFILE,NEWFILE
4759 Creates a new filename symbolically linked to the old filename.
4760 Returns C<1> for success, C<0> otherwise. On systems that don't support
4761 symbolic links, produces a fatal error at run time. To check for that,
4764 $symlink_exists = eval { symlink("",""); 1 };
4768 Calls the system call specified as the first element of the list,
4769 passing the remaining elements as arguments to the system call. If
4770 unimplemented, produces a fatal error. The arguments are interpreted
4771 as follows: if a given argument is numeric, the argument is passed as
4772 an int. If not, the pointer to the string value is passed. You are
4773 responsible to make sure a string is pre-extended long enough to
4774 receive any result that might be written into a string. You can't use a
4775 string literal (or other read-only string) as an argument to C<syscall>
4776 because Perl has to assume that any string pointer might be written
4778 integer arguments are not literals and have never been interpreted in a
4779 numeric context, you may need to add C<0> to them to force them to look
4780 like numbers. This emulates the C<syswrite> function (or vice versa):
4782 require 'syscall.ph'; # may need to run h2ph
4784 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4786 Note that Perl supports passing of up to only 14 arguments to your system call,
4787 which in practice should usually suffice.
4789 Syscall returns whatever value returned by the system call it calls.
4790 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4791 Note that some system calls can legitimately return C<-1>. The proper
4792 way to handle such calls is to assign C<$!=0;> before the call and
4793 check the value of C<$!> if syscall returns C<-1>.
4795 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4796 number of the read end of the pipe it creates. There is no way
4797 to retrieve the file number of the other end. You can avoid this
4798 problem by using C<pipe> instead.
4800 =item sysopen FILEHANDLE,FILENAME,MODE
4802 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4804 Opens the file whose filename is given by FILENAME, and associates it
4805 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4806 the name of the real filehandle wanted. This function calls the
4807 underlying operating system's C<open> function with the parameters
4808 FILENAME, MODE, PERMS.
4810 The possible values and flag bits of the MODE parameter are
4811 system-dependent; they are available via the standard module C<Fcntl>.
4812 See the documentation of your operating system's C<open> to see which
4813 values and flag bits are available. You may combine several flags
4814 using the C<|>-operator.
4816 Some of the most common values are C<O_RDONLY> for opening the file in
4817 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4818 and C<O_RDWR> for opening the file in read-write mode, and.
4820 For historical reasons, some values work on almost every system
4821 supported by perl: zero means read-only, one means write-only, and two
4822 means read/write. We know that these values do I<not> work under
4823 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4824 use them in new code.
4826 If the file named by FILENAME does not exist and the C<open> call creates
4827 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4828 PERMS specifies the permissions of the newly created file. If you omit
4829 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4830 These permission values need to be in octal, and are modified by your
4831 process's current C<umask>.
4833 In many systems the C<O_EXCL> flag is available for opening files in
4834 exclusive mode. This is B<not> locking: exclusiveness means here that
4835 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4838 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4840 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4841 that takes away the user's option to have a more permissive umask.
4842 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4845 Note that C<sysopen> depends on the fdopen() C library function.
4846 On many UNIX systems, fdopen() is known to fail when file descriptors
4847 exceed a certain value, typically 255. If you need more file
4848 descriptors than that, consider rebuilding Perl to use the C<sfio>
4849 library, or perhaps using the POSIX::open() function.
4851 See L<perlopentut> for a kinder, gentler explanation of opening files.
4853 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4855 =item sysread FILEHANDLE,SCALAR,LENGTH
4857 Attempts to read LENGTH bytes of data into variable SCALAR from the
4858 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4859 so mixing this with other kinds of reads, C<print>, C<write>,
4860 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4861 usually buffers data. Returns the number of bytes actually read, C<0>
4862 at end of file, or undef if there was an error. SCALAR will be grown or
4863 shrunk so that the last byte actually read is the last byte of the
4864 scalar after the read.
4866 An OFFSET may be specified to place the read data at some place in the
4867 string other than the beginning. A negative OFFSET specifies
4868 placement at that many bytes counting backwards from the end of the
4869 string. A positive OFFSET greater than the length of SCALAR results
4870 in the string being padded to the required size with C<"\0"> bytes before
4871 the result of the read is appended.
4873 There is no syseof() function, which is ok, since eof() doesn't work
4874 very well on device files (like ttys) anyway. Use sysread() and check
4875 for a return value for 0 to decide whether you're done.
4877 =item sysseek FILEHANDLE,POSITION,WHENCE
4879 Sets FILEHANDLE's system position using the system call lseek(2). It
4880 bypasses stdio, so mixing this with reads (other than C<sysread>),
4881 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4882 FILEHANDLE may be an expression whose value gives the name of the
4883 filehandle. The values for WHENCE are C<0> to set the new position to
4884 POSITION, C<1> to set the it to the current position plus POSITION,
4885 and C<2> to set it to EOF plus POSITION (typically negative). For
4886 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4887 C<SEEK_END> (start of the file, current position, end of the file)
4888 from the Fcntl module.
4890 Returns the new position, or the undefined value on failure. A position
4891 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4892 true on success and false on failure, yet you can still easily determine
4897 =item system PROGRAM LIST
4899 Does exactly the same thing as C<exec LIST>, except that a fork is
4900 done first, and the parent process waits for the child process to
4901 complete. Note that argument processing varies depending on the
4902 number of arguments. If there is more than one argument in LIST,
4903 or if LIST is an array with more than one value, starts the program
4904 given by the first element of the list with arguments given by the
4905 rest of the list. If there is only one scalar argument, the argument
4906 is checked for shell metacharacters, and if there are any, the
4907 entire argument is passed to the system's command shell for parsing
4908 (this is C</bin/sh -c> on Unix platforms, but varies on other
4909 platforms). If there are no shell metacharacters in the argument,
4910 it is split into words and passed directly to C<execvp>, which is
4913 All files opened for output are flushed before attempting the exec().
4915 The return value is the exit status of the program as
4916 returned by the C<wait> call. To get the actual exit value divide by
4917 256. See also L</exec>. This is I<not> what you want to use to capture
4918 the output from a command, for that you should use merely backticks or
4919 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4920 indicates a failure to start the program (inspect $! for the reason).
4922 Like C<exec>, C<system> allows you to lie to a program about its name if
4923 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4925 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4926 program they're running doesn't actually interrupt your program.
4928 @args = ("command", "arg1", "arg2");
4930 or die "system @args failed: $?"
4932 You can check all the failure possibilities by inspecting
4935 $exit_value = $? >> 8;
4936 $signal_num = $? & 127;
4937 $dumped_core = $? & 128;
4939 When the arguments get executed via the system shell, results
4940 and return codes will be subject to its quirks and capabilities.
4941 See L<perlop/"`STRING`"> and L</exec> for details.
4943 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4945 =item syswrite FILEHANDLE,SCALAR,LENGTH
4947 =item syswrite FILEHANDLE,SCALAR
4949 Attempts to write LENGTH bytes of data from variable SCALAR to the
4950 specified FILEHANDLE, using the system call write(2). If LENGTH
4951 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4952 this with reads (other than C<sysread())>, C<print>, C<write>,
4953 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4954 usually buffers data. Returns the number of bytes actually written,
4955 or C<undef> if there was an error. If the LENGTH is greater than
4956 the available data in the SCALAR after the OFFSET, only as much
4957 data as is available will be written.
4959 An OFFSET may be specified to write the data from some part of the
4960 string other than the beginning. A negative OFFSET specifies writing
4961 that many bytes counting backwards from the end of the string. In the
4962 case the SCALAR is empty you can use OFFSET but only zero offset.
4964 =item tell FILEHANDLE
4968 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4969 expression whose value gives the name of the actual filehandle. If
4970 FILEHANDLE is omitted, assumes the file last read.
4972 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4974 =item telldir DIRHANDLE
4976 Returns the current position of the C<readdir> routines on DIRHANDLE.
4977 Value may be given to C<seekdir> to access a particular location in a
4978 directory. Has the same caveats about possible directory compaction as
4979 the corresponding system library routine.
4981 =item tie VARIABLE,CLASSNAME,LIST
4983 This function binds a variable to a package class that will provide the
4984 implementation for the variable. VARIABLE is the name of the variable
4985 to be enchanted. CLASSNAME is the name of a class implementing objects
4986 of correct type. Any additional arguments are passed to the C<new>
4987 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4988 or C<TIEHASH>). Typically these are arguments such as might be passed
4989 to the C<dbm_open()> function of C. The object returned by the C<new>
4990 method is also returned by the C<tie> function, which would be useful
4991 if you want to access other methods in CLASSNAME.
4993 Note that functions such as C<keys> and C<values> may return huge lists
4994 when used on large objects, like DBM files. You may prefer to use the
4995 C<each> function to iterate over such. Example:
4997 # print out history file offsets
4999 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5000 while (($key,$val) = each %HIST) {
5001 print $key, ' = ', unpack('L',$val), "\n";
5005 A class implementing a hash should have the following methods:
5007 TIEHASH classname, LIST
5009 STORE this, key, value
5014 NEXTKEY this, lastkey
5017 A class implementing an ordinary array should have the following methods:
5019 TIEARRAY classname, LIST
5021 STORE this, key, value
5023 STORESIZE this, count
5029 SPLICE this, offset, length, LIST
5033 A class implementing a file handle should have the following methods:
5035 TIEHANDLE classname, LIST
5036 READ this, scalar, length, offset
5039 WRITE this, scalar, length, offset
5041 PRINTF this, format, LIST
5045 A class implementing a scalar should have the following methods:
5047 TIESCALAR classname, LIST
5052 Not all methods indicated above need be implemented. See L<perltie>,
5053 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5055 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5056 for you--you need to do that explicitly yourself. See L<DB_File>
5057 or the F<Config> module for interesting C<tie> implementations.
5059 For further details see L<perltie>, L<"tied VARIABLE">.
5063 Returns a reference to the object underlying VARIABLE (the same value
5064 that was originally returned by the C<tie> call that bound the variable
5065 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5070 Returns the number of non-leap seconds since whatever time the system
5071 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5072 and 00:00:00 UTC, January 1, 1970 for most other systems).
5073 Suitable for feeding to C<gmtime> and C<localtime>.
5075 For measuring time in better granularity than one second,
5076 you may use either the Time::HiRes module from CPAN, or
5077 if you have gettimeofday(2), you may be able to use the
5078 C<syscall> interface of Perl, see L<perlfaq8> for details.
5082 Returns a four-element list giving the user and system times, in
5083 seconds, for this process and the children of this process.
5085 ($user,$system,$cuser,$csystem) = times;
5089 The transliteration operator. Same as C<y///>. See L<perlop>.
5091 =item truncate FILEHANDLE,LENGTH
5093 =item truncate EXPR,LENGTH
5095 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5096 specified length. Produces a fatal error if truncate isn't implemented
5097 on your system. Returns true if successful, the undefined value
5104 Returns an uppercased version of EXPR. This is the internal function
5105 implementing the C<\U> escape in double-quoted strings.
5106 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5107 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5108 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5110 If EXPR is omitted, uses C<$_>.
5116 Returns the value of EXPR with the first character
5117 in uppercase (titlecase in Unicode). This is
5118 the internal function implementing the C<\u> escape in double-quoted strings.
5119 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5122 If EXPR is omitted, uses C<$_>.
5128 Sets the umask for the process to EXPR and returns the previous value.
5129 If EXPR is omitted, merely returns the current umask.
5131 The Unix permission C<rwxr-x---> is represented as three sets of three
5132 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5133 and isn't one of the digits). The C<umask> value is such a number
5134 representing disabled permissions bits. The permission (or "mode")
5135 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5136 even if you tell C<sysopen> to create a file with permissions C<0777>,
5137 if your umask is C<0022> then the file will actually be created with
5138 permissions C<0755>. If your C<umask> were C<0027> (group can't
5139 write; others can't read, write, or execute), then passing
5140 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5143 Here's some advice: supply a creation mode of C<0666> for regular
5144 files (in C<sysopen>) and one of C<0777> for directories (in
5145 C<mkdir>) and executable files. This gives users the freedom of
5146 choice: if they want protected files, they might choose process umasks
5147 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5148 Programs should rarely if ever make policy decisions better left to
5149 the user. The exception to this is when writing files that should be
5150 kept private: mail files, web browser cookies, I<.rhosts> files, and
5153 If umask(2) is not implemented on your system and you are trying to
5154 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5155 fatal error at run time. If umask(2) is not implemented and you are
5156 not trying to restrict access for yourself, returns C<undef>.
5158 Remember that a umask is a number, usually given in octal; it is I<not> a
5159 string of octal digits. See also L</oct>, if all you have is a string.
5165 Undefines the value of EXPR, which must be an lvalue. Use only on a
5166 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5167 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5168 will probably not do what you expect on most predefined variables or
5169 DBM list values, so don't do that; see L<delete>.) Always returns the
5170 undefined value. You can omit the EXPR, in which case nothing is
5171 undefined, but you still get an undefined value that you could, for
5172 instance, return from a subroutine, assign to a variable or pass as a
5173 parameter. Examples:
5176 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5180 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5181 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5182 select undef, undef, undef, 0.25;
5183 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5185 Note that this is a unary operator, not a list operator.
5191 Deletes a list of files. Returns the number of files successfully
5194 $cnt = unlink 'a', 'b', 'c';
5198 Note: C<unlink> will not delete directories unless you are superuser and
5199 the B<-U> flag is supplied to Perl. Even if these conditions are
5200 met, be warned that unlinking a directory can inflict damage on your
5201 filesystem. Use C<rmdir> instead.
5203 If LIST is omitted, uses C<$_>.
5205 =item unpack TEMPLATE,EXPR
5207 C<unpack> does the reverse of C<pack>: it takes a string
5208 and expands it out into a list of values.
5209 (In scalar context, it returns merely the first value produced.)
5211 The string is broken into chunks described by the TEMPLATE. Each chunk
5212 is converted separately to a value. Typically, either the string is a result
5213 of C<pack>, or the bytes of the string represent a C structure of some
5216 The TEMPLATE has the same format as in the C<pack> function.
5217 Here's a subroutine that does substring:
5220 my($what,$where,$howmuch) = @_;
5221 unpack("x$where a$howmuch", $what);
5226 sub ordinal { unpack("c",$_[0]); } # same as ord()
5228 In addition to fields allowed in pack(), you may prefix a field with
5229 a %<number> to indicate that
5230 you want a <number>-bit checksum of the items instead of the items
5231 themselves. Default is a 16-bit checksum. Checksum is calculated by
5232 summing numeric values of expanded values (for string fields the sum of
5233 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5235 For example, the following
5236 computes the same number as the System V sum program:
5240 unpack("%32C*",<>) % 65535;
5243 The following efficiently counts the number of set bits in a bit vector:
5245 $setbits = unpack("%32b*", $selectmask);
5247 The C<p> and C<P> formats should be used with care. Since Perl
5248 has no way of checking whether the value passed to C<unpack()>
5249 corresponds to a valid memory location, passing a pointer value that's
5250 not known to be valid is likely to have disastrous consequences.
5252 If the repeat count of a field is larger than what the remainder of
5253 the input string allows, repeat count is decreased. If the input string
5254 is longer than one described by the TEMPLATE, the rest is ignored.
5256 See L</pack> for more examples and notes.
5258 =item untie VARIABLE
5260 Breaks the binding between a variable and a package. (See C<tie>.)
5262 =item unshift ARRAY,LIST
5264 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5265 depending on how you look at it. Prepends list to the front of the
5266 array, and returns the new number of elements in the array.
5268 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5270 Note the LIST is prepended whole, not one element at a time, so the
5271 prepended elements stay in the same order. Use C<reverse> to do the
5274 =item use Module VERSION LIST
5276 =item use Module VERSION
5278 =item use Module LIST
5284 Imports some semantics into the current package from the named module,
5285 generally by aliasing certain subroutine or variable names into your
5286 package. It is exactly equivalent to
5288 BEGIN { require Module; import Module LIST; }
5290 except that Module I<must> be a bareword.
5292 VERSION, which can be specified as a literal of the form v5.6.1, demands
5293 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5294 as recent as that version. (For compatibility with older versions of Perl,
5295 a numeric literal will also be interpreted as VERSION.) If the version
5296 of the running Perl interpreter is less than VERSION, then an error
5297 message is printed and Perl exits immediately without attempting to
5298 parse the rest of the file. Compare with L</require>, which can do a
5299 similar check at run time.
5301 use v5.6.1; # compile time version check
5303 use 5.005_03; # float version allowed for compatibility
5305 This is often useful if you need to check the current Perl version before
5306 C<use>ing library modules that have changed in incompatible ways from
5307 older versions of Perl. (We try not to do this more than we have to.)
5309 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5310 C<require> makes sure the module is loaded into memory if it hasn't been
5311 yet. The C<import> is not a builtin--it's just an ordinary static method
5312 call into the C<Module> package to tell the module to import the list of
5313 features back into the current package. The module can implement its
5314 C<import> method any way it likes, though most modules just choose to
5315 derive their C<import> method via inheritance from the C<Exporter> class that
5316 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5317 method can be found then the call is skipped.
5319 If you don't want your namespace altered, explicitly supply an empty list:
5323 That is exactly equivalent to
5325 BEGIN { require Module }
5327 If the VERSION argument is present between Module and LIST, then the
5328 C<use> will call the VERSION method in class Module with the given
5329 version as an argument. The default VERSION method, inherited from
5330 the UNIVERSAL class, croaks if the given version is larger than the
5331 value of the variable C<$Module::VERSION>.
5333 Again, there is a distinction between omitting LIST (C<import> called
5334 with no arguments) and an explicit empty LIST C<()> (C<import> not
5335 called). Note that there is no comma after VERSION!
5337 Because this is a wide-open interface, pragmas (compiler directives)
5338 are also implemented this way. Currently implemented pragmas are:
5342 use sigtrap qw(SEGV BUS);
5343 use strict qw(subs vars refs);
5344 use subs qw(afunc blurfl);
5345 use warnings qw(all);
5347 Some of these pseudo-modules import semantics into the current
5348 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5349 which import symbols into the current package (which are effective
5350 through the end of the file).
5352 There's a corresponding C<no> command that unimports meanings imported
5353 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5359 If no C<unimport> method can be found the call fails with a fatal error.
5361 See L<perlmod> for a list of standard modules and pragmas.
5365 Changes the access and modification times on each file of a list of
5366 files. The first two elements of the list must be the NUMERICAL access
5367 and modification times, in that order. Returns the number of files
5368 successfully changed. The inode change time of each file is set
5369 to the current time. This code has the same effect as the C<touch>
5370 command if the files already exist:
5374 utime $now, $now, @ARGV;
5378 Returns a list consisting of all the values of the named hash. (In a
5379 scalar context, returns the number of values.) The values are
5380 returned in an apparently random order. The actual random order is
5381 subject to change in future versions of perl, but it is guaranteed to
5382 be the same order as either the C<keys> or C<each> function would
5383 produce on the same (unmodified) hash.
5385 Note that you cannot modify the values of a hash this way, because the
5386 returned list is just a copy. You need to use a hash slice for that,
5387 since it's lvaluable in a way that values() is not.
5389 for (values %hash) { s/foo/bar/g } # FAILS!
5390 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5392 As a side effect, calling values() resets the HASH's internal iterator.
5393 See also C<keys>, C<each>, and C<sort>.
5395 =item vec EXPR,OFFSET,BITS
5397 Treats the string in EXPR as a bit vector made up of elements of
5398 width BITS, and returns the value of the element specified by OFFSET
5399 as an unsigned integer. BITS therefore specifies the number of bits
5400 that are reserved for each element in the bit vector. This must
5401 be a power of two from 1 to 32 (or 64, if your platform supports
5404 If BITS is 8, "elements" coincide with bytes of the input string.
5406 If BITS is 16 or more, bytes of the input string are grouped into chunks
5407 of size BITS/8, and each group is converted to a number as with
5408 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5409 for BITS==64). See L<"pack"> for details.
5411 If bits is 4 or less, the string is broken into bytes, then the bits
5412 of each byte are broken into 8/BITS groups. Bits of a byte are
5413 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5414 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5415 breaking the single input byte C<chr(0x36)> into two groups gives a list
5416 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5418 C<vec> may also be assigned to, in which case parentheses are needed
5419 to give the expression the correct precedence as in
5421 vec($image, $max_x * $x + $y, 8) = 3;
5423 If the selected element is off the end of the string, the value 0 is
5424 returned. If an element off the end of the string is written to,
5425 Perl will first extend the string with sufficiently many zero bytes.
5427 Strings created with C<vec> can also be manipulated with the logical
5428 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5429 vector operation is desired when both operands are strings.
5430 See L<perlop/"Bitwise String Operators">.
5432 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5433 The comments show the string after each step. Note that this code works
5434 in the same way on big-endian or little-endian machines.
5437 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5439 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5440 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5442 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5443 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5444 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5445 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5446 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5447 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5449 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5450 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5451 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5454 To transform a bit vector into a string or list of 0's and 1's, use these:
5456 $bits = unpack("b*", $vector);
5457 @bits = split(//, unpack("b*", $vector));
5459 If you know the exact length in bits, it can be used in place of the C<*>.
5461 Here is an example to illustrate how the bits actually fall in place:
5467 unpack("V",$_) 01234567890123456789012345678901
5468 ------------------------------------------------------------------
5473 for ($shift=0; $shift < $width; ++$shift) {
5474 for ($off=0; $off < 32/$width; ++$off) {
5475 $str = pack("B*", "0"x32);
5476 $bits = (1<<$shift);
5477 vec($str, $off, $width) = $bits;
5478 $res = unpack("b*",$str);
5479 $val = unpack("V", $str);
5486 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5487 $off, $width, $bits, $val, $res
5491 Regardless of the machine architecture on which it is run, the above
5492 example should print the following table:
5495 unpack("V",$_) 01234567890123456789012345678901
5496 ------------------------------------------------------------------
5497 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5498 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5499 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5500 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5501 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5502 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5503 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5504 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5505 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5506 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5507 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5508 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5509 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5510 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5511 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5512 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5513 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5514 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5515 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5516 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5517 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5518 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5519 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5520 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5521 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5522 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5523 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5524 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5525 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5526 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5527 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5528 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5529 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5530 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5531 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5532 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5533 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5534 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5535 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5536 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5537 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5538 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5539 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5540 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5541 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5542 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5543 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5544 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5545 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5546 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5547 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5548 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5549 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5550 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5551 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5552 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5553 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5554 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5555 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5556 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5557 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5558 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5559 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5560 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5561 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5562 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5563 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5564 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5565 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5566 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5567 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5568 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5569 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5570 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5571 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5572 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5573 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5574 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5575 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5576 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5577 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5578 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5579 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5580 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5581 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5582 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5583 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5584 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5585 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5586 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5587 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5588 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5589 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5590 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5591 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5592 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5593 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5594 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5595 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5596 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5597 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5598 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5599 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5600 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5601 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5602 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5603 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5604 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5605 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5606 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5607 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5608 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5609 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5610 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5611 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5612 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5613 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5614 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5615 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5616 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5617 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5618 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5619 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5620 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5621 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5622 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5623 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5624 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5628 Behaves like the wait(2) system call on your system: it waits for a child
5629 process to terminate and returns the pid of the deceased process, or
5630 C<-1> if there are no child processes. The status is returned in C<$?>.
5631 Note that a return value of C<-1> could mean that child processes are
5632 being automatically reaped, as described in L<perlipc>.
5634 =item waitpid PID,FLAGS
5636 Waits for a particular child process to terminate and returns the pid of
5637 the deceased process, or C<-1> if there is no such child process. On some
5638 systems, a value of 0 indicates that there are processes still running.
5639 The status is returned in C<$?>. If you say
5641 use POSIX ":sys_wait_h";
5644 $kid = waitpid(-1,&WNOHANG);
5647 then you can do a non-blocking wait for all pending zombie processes.
5648 Non-blocking wait is available on machines supporting either the
5649 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5650 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5651 system call by remembering the status values of processes that have
5652 exited but have not been harvested by the Perl script yet.)
5654 Note that on some systems, a return value of C<-1> could mean that child
5655 processes are being automatically reaped. See L<perlipc> for details,
5656 and for other examples.
5660 Returns true if the context of the currently executing subroutine is
5661 looking for a list value. Returns false if the context is looking
5662 for a scalar. Returns the undefined value if the context is looking
5663 for no value (void context).
5665 return unless defined wantarray; # don't bother doing more
5666 my @a = complex_calculation();
5667 return wantarray ? @a : "@a";
5669 This function should have been named wantlist() instead.
5673 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5676 If LIST is empty and C<$@> already contains a value (typically from a
5677 previous eval) that value is used after appending C<"\t...caught">
5678 to C<$@>. This is useful for staying almost, but not entirely similar to
5681 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5683 No message is printed if there is a C<$SIG{__WARN__}> handler
5684 installed. It is the handler's responsibility to deal with the message
5685 as it sees fit (like, for instance, converting it into a C<die>). Most
5686 handlers must therefore make arrangements to actually display the
5687 warnings that they are not prepared to deal with, by calling C<warn>
5688 again in the handler. Note that this is quite safe and will not
5689 produce an endless loop, since C<__WARN__> hooks are not called from
5692 You will find this behavior is slightly different from that of
5693 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5694 instead call C<die> again to change it).
5696 Using a C<__WARN__> handler provides a powerful way to silence all
5697 warnings (even the so-called mandatory ones). An example:
5699 # wipe out *all* compile-time warnings
5700 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5702 my $foo = 20; # no warning about duplicate my $foo,
5703 # but hey, you asked for it!
5704 # no compile-time or run-time warnings before here
5707 # run-time warnings enabled after here
5708 warn "\$foo is alive and $foo!"; # does show up
5710 See L<perlvar> for details on setting C<%SIG> entries, and for more
5711 examples. See the Carp module for other kinds of warnings using its
5712 carp() and cluck() functions.
5714 =item write FILEHANDLE
5720 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5721 using the format associated with that file. By default the format for
5722 a file is the one having the same name as the filehandle, but the
5723 format for the current output channel (see the C<select> function) may be set
5724 explicitly by assigning the name of the format to the C<$~> variable.
5726 Top of form processing is handled automatically: if there is
5727 insufficient room on the current page for the formatted record, the
5728 page is advanced by writing a form feed, a special top-of-page format
5729 is used to format the new page header, and then the record is written.
5730 By default the top-of-page format is the name of the filehandle with
5731 "_TOP" appended, but it may be dynamically set to the format of your
5732 choice by assigning the name to the C<$^> variable while the filehandle is
5733 selected. The number of lines remaining on the current page is in
5734 variable C<$->, which can be set to C<0> to force a new page.
5736 If FILEHANDLE is unspecified, output goes to the current default output
5737 channel, which starts out as STDOUT but may be changed by the
5738 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5739 is evaluated and the resulting string is used to look up the name of
5740 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5742 Note that write is I<not> the opposite of C<read>. Unfortunately.
5746 The transliteration operator. Same as C<tr///>. See L<perlop>.