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<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, 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<our>, C<prototype>,
204 C<qx>, 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 regard 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 arbitrary 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 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1436 output before the exec, but this may not be supported on some platforms
1437 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1438 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1439 open handles in order to avoid lost output.
1441 Note that C<exec> will not call your C<END> blocks, nor will it call
1442 any C<DESTROY> methods in your objects.
1446 Given an expression that specifies a hash element or array element,
1447 returns true if the specified element in the hash or array has ever
1448 been initialized, even if the corresponding value is undefined. The
1449 element is not autovivified if it doesn't exist.
1451 print "Exists\n" if exists $hash{$key};
1452 print "Defined\n" if defined $hash{$key};
1453 print "True\n" if $hash{$key};
1455 print "Exists\n" if exists $array[$index];
1456 print "Defined\n" if defined $array[$index];
1457 print "True\n" if $array[$index];
1459 A hash or array element can be true only if it's defined, and defined if
1460 it exists, but the reverse doesn't necessarily hold true.
1462 Given an expression that specifies the name of a subroutine,
1463 returns true if the specified subroutine has ever been declared, even
1464 if it is undefined. Mentioning a subroutine name for exists or defined
1465 does not count as declaring it.
1467 print "Exists\n" if exists &subroutine;
1468 print "Defined\n" if defined &subroutine;
1470 Note that the EXPR can be arbitrarily complicated as long as the final
1471 operation is a hash or array key lookup or subroutine name:
1473 if (exists $ref->{A}->{B}->{$key}) { }
1474 if (exists $hash{A}{B}{$key}) { }
1476 if (exists $ref->{A}->{B}->[$ix]) { }
1477 if (exists $hash{A}{B}[$ix]) { }
1479 if (exists &{$ref->{A}{B}{$key}}) { }
1481 Although the deepest nested array or hash will not spring into existence
1482 just because its existence was tested, any intervening ones will.
1483 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1484 into existence due to the existence test for the $key element above.
1485 This happens anywhere the arrow operator is used, including even:
1488 if (exists $ref->{"Some key"}) { }
1489 print $ref; # prints HASH(0x80d3d5c)
1491 This surprising autovivification in what does not at first--or even
1492 second--glance appear to be an lvalue context may be fixed in a future
1495 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1496 on how exists() acts when used on a pseudo-hash.
1498 Use of a subroutine call, rather than a subroutine name, as an argument
1499 to exists() is an error.
1502 exists &sub(); # Error
1506 Evaluates EXPR and exits immediately with that value. Example:
1509 exit 0 if $ans =~ /^[Xx]/;
1511 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1512 universally recognized values for EXPR are C<0> for success and C<1>
1513 for error; other values are subject to interpretation depending on the
1514 environment in which the Perl program is running. For example, exiting
1515 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1516 the mailer to return the item undelivered, but that's not true everywhere.
1518 Don't use C<exit> to abort a subroutine if there's any chance that
1519 someone might want to trap whatever error happened. Use C<die> instead,
1520 which can be trapped by an C<eval>.
1522 The exit() function does not always exit immediately. It calls any
1523 defined C<END> routines first, but these C<END> routines may not
1524 themselves abort the exit. Likewise any object destructors that need to
1525 be called are called before the real exit. If this is a problem, you
1526 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1527 See L<perlmod> for details.
1533 Returns I<e> (the natural logarithm base) to the power of EXPR.
1534 If EXPR is omitted, gives C<exp($_)>.
1536 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1538 Implements the fcntl(2) function. You'll probably have to say
1542 first to get the correct constant definitions. Argument processing and
1543 value return works just like C<ioctl> below.
1547 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1548 or die "can't fcntl F_GETFL: $!";
1550 You don't have to check for C<defined> on the return from C<fnctl>.
1551 Like C<ioctl>, it maps a C<0> return from the system call into
1552 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1553 in numeric context. It is also exempt from the normal B<-w> warnings
1554 on improper numeric conversions.
1556 Note that C<fcntl> will produce a fatal error if used on a machine that
1557 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1558 manpage to learn what functions are available on your system.
1560 =item fileno FILEHANDLE
1562 Returns the file descriptor for a filehandle, or undefined if the
1563 filehandle is not open. This is mainly useful for constructing
1564 bitmaps for C<select> and low-level POSIX tty-handling operations.
1565 If FILEHANDLE is an expression, the value is taken as an indirect
1566 filehandle, generally its name.
1568 You can use this to find out whether two handles refer to the
1569 same underlying descriptor:
1571 if (fileno(THIS) == fileno(THAT)) {
1572 print "THIS and THAT are dups\n";
1575 =item flock FILEHANDLE,OPERATION
1577 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1578 for success, false on failure. Produces a fatal error if used on a
1579 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1580 C<flock> is Perl's portable file locking interface, although it locks
1581 only entire files, not records.
1583 Two potentially non-obvious but traditional C<flock> semantics are
1584 that it waits indefinitely until the lock is granted, and that its locks
1585 B<merely advisory>. Such discretionary locks are more flexible, but offer
1586 fewer guarantees. This means that files locked with C<flock> may be
1587 modified by programs that do not also use C<flock>. See L<perlport>,
1588 your port's specific documentation, or your system-specific local manpages
1589 for details. It's best to assume traditional behavior if you're writing
1590 portable programs. (But if you're not, you should as always feel perfectly
1591 free to write for your own system's idiosyncrasies (sometimes called
1592 "features"). Slavish adherence to portability concerns shouldn't get
1593 in the way of your getting your job done.)
1595 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1596 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1597 you can use the symbolic names if you import them from the Fcntl module,
1598 either individually, or as a group using the ':flock' tag. LOCK_SH
1599 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1600 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1601 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1602 waiting for the lock (check the return status to see if you got it).
1604 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1605 before locking or unlocking it.
1607 Note that the emulation built with lockf(3) doesn't provide shared
1608 locks, and it requires that FILEHANDLE be open with write intent. These
1609 are the semantics that lockf(3) implements. Most if not all systems
1610 implement lockf(3) in terms of fcntl(2) locking, though, so the
1611 differing semantics shouldn't bite too many people.
1613 Note also that some versions of C<flock> cannot lock things over the
1614 network; you would need to use the more system-specific C<fcntl> for
1615 that. If you like you can force Perl to ignore your system's flock(2)
1616 function, and so provide its own fcntl(2)-based emulation, by passing
1617 the switch C<-Ud_flock> to the F<Configure> program when you configure
1620 Here's a mailbox appender for BSD systems.
1622 use Fcntl ':flock'; # import LOCK_* constants
1625 flock(MBOX,LOCK_EX);
1626 # and, in case someone appended
1627 # while we were waiting...
1632 flock(MBOX,LOCK_UN);
1635 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1636 or die "Can't open mailbox: $!";
1639 print MBOX $msg,"\n\n";
1642 On systems that support a real flock(), locks are inherited across fork()
1643 calls, whereas those that must resort to the more capricious fcntl()
1644 function lose the locks, making it harder to write servers.
1646 See also L<DB_File> for other flock() examples.
1650 Does a fork(2) system call to create a new process running the
1651 same program at the same point. It returns the child pid to the
1652 parent process, C<0> to the child process, or C<undef> if the fork is
1653 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1654 are shared, while everything else is copied. On most systems supporting
1655 fork(), great care has gone into making it extremely efficient (for
1656 example, using copy-on-write technology on data pages), making it the
1657 dominant paradigm for multitasking over the last few decades.
1659 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1660 output before forking the child process, but this may not be supported
1661 on some platforms (see L<perlport>). To be safe, you may need to set
1662 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1663 C<IO::Handle> on any open handles in order to avoid duplicate output.
1665 If you C<fork> without ever waiting on your children, you will
1666 accumulate zombies. On some systems, you can avoid this by setting
1667 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1668 forking and reaping moribund children.
1670 Note that if your forked child inherits system file descriptors like
1671 STDIN and STDOUT that are actually connected by a pipe or socket, even
1672 if you exit, then the remote server (such as, say, a CGI script or a
1673 backgrounded job launched from a remote shell) won't think you're done.
1674 You should reopen those to F</dev/null> if it's any issue.
1678 Declare a picture format for use by the C<write> function. For
1682 Test: @<<<<<<<< @||||| @>>>>>
1683 $str, $%, '$' . int($num)
1687 $num = $cost/$quantity;
1691 See L<perlform> for many details and examples.
1693 =item formline PICTURE,LIST
1695 This is an internal function used by C<format>s, though you may call it,
1696 too. It formats (see L<perlform>) a list of values according to the
1697 contents of PICTURE, placing the output into the format output
1698 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1699 Eventually, when a C<write> is done, the contents of
1700 C<$^A> are written to some filehandle, but you could also read C<$^A>
1701 yourself and then set C<$^A> back to C<"">. Note that a format typically
1702 does one C<formline> per line of form, but the C<formline> function itself
1703 doesn't care how many newlines are embedded in the PICTURE. This means
1704 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1705 You may therefore need to use multiple formlines to implement a single
1706 record format, just like the format compiler.
1708 Be careful if you put double quotes around the picture, because an C<@>
1709 character may be taken to mean the beginning of an array name.
1710 C<formline> always returns true. See L<perlform> for other examples.
1712 =item getc FILEHANDLE
1716 Returns the next character from the input file attached to FILEHANDLE,
1717 or the undefined value at end of file, or if there was an error.
1718 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1719 efficient. However, it cannot be used by itself to fetch single
1720 characters without waiting for the user to hit enter. For that, try
1721 something more like:
1724 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1727 system "stty", '-icanon', 'eol', "\001";
1733 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1736 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1740 Determination of whether $BSD_STYLE should be set
1741 is left as an exercise to the reader.
1743 The C<POSIX::getattr> function can do this more portably on
1744 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1745 module from your nearest CPAN site; details on CPAN can be found on
1750 Implements the C library function of the same name, which on most
1751 systems returns the current login from F</etc/utmp>, if any. If null,
1754 $login = getlogin || getpwuid($<) || "Kilroy";
1756 Do not consider C<getlogin> for authentication: it is not as
1757 secure as C<getpwuid>.
1759 =item getpeername SOCKET
1761 Returns the packed sockaddr address of other end of the SOCKET connection.
1764 $hersockaddr = getpeername(SOCK);
1765 ($port, $iaddr) = sockaddr_in($hersockaddr);
1766 $herhostname = gethostbyaddr($iaddr, AF_INET);
1767 $herstraddr = inet_ntoa($iaddr);
1771 Returns the current process group for the specified PID. Use
1772 a PID of C<0> to get the current process group for the
1773 current process. Will raise an exception if used on a machine that
1774 doesn't implement getpgrp(2). If PID is omitted, returns process
1775 group of current process. Note that the POSIX version of C<getpgrp>
1776 does not accept a PID argument, so only C<PID==0> is truly portable.
1780 Returns the process id of the parent process.
1782 =item getpriority WHICH,WHO
1784 Returns the current priority for a process, a process group, or a user.
1785 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1786 machine that doesn't implement getpriority(2).
1792 =item gethostbyname NAME
1794 =item getnetbyname NAME
1796 =item getprotobyname NAME
1802 =item getservbyname NAME,PROTO
1804 =item gethostbyaddr ADDR,ADDRTYPE
1806 =item getnetbyaddr ADDR,ADDRTYPE
1808 =item getprotobynumber NUMBER
1810 =item getservbyport PORT,PROTO
1828 =item sethostent STAYOPEN
1830 =item setnetent STAYOPEN
1832 =item setprotoent STAYOPEN
1834 =item setservent STAYOPEN
1848 These routines perform the same functions as their counterparts in the
1849 system library. In list context, the return values from the
1850 various get routines are as follows:
1852 ($name,$passwd,$uid,$gid,
1853 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1854 ($name,$passwd,$gid,$members) = getgr*
1855 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1856 ($name,$aliases,$addrtype,$net) = getnet*
1857 ($name,$aliases,$proto) = getproto*
1858 ($name,$aliases,$port,$proto) = getserv*
1860 (If the entry doesn't exist you get a null list.)
1862 The exact meaning of the $gcos field varies but it usually contains
1863 the real name of the user (as opposed to the login name) and other
1864 information pertaining to the user. Beware, however, that in many
1865 system users are able to change this information and therefore it
1866 cannot be trusted and therefore the $gcos is tainted (see
1867 L<perlsec>). The $passwd and $shell, user's encrypted password and
1868 login shell, are also tainted, because of the same reason.
1870 In scalar context, you get the name, unless the function was a
1871 lookup by name, in which case you get the other thing, whatever it is.
1872 (If the entry doesn't exist you get the undefined value.) For example:
1874 $uid = getpwnam($name);
1875 $name = getpwuid($num);
1877 $gid = getgrnam($name);
1878 $name = getgrgid($num;
1882 In I<getpw*()> the fields $quota, $comment, and $expire are special
1883 cases in the sense that in many systems they are unsupported. If the
1884 $quota is unsupported, it is an empty scalar. If it is supported, it
1885 usually encodes the disk quota. If the $comment field is unsupported,
1886 it is an empty scalar. If it is supported it usually encodes some
1887 administrative comment about the user. In some systems the $quota
1888 field may be $change or $age, fields that have to do with password
1889 aging. In some systems the $comment field may be $class. The $expire
1890 field, if present, encodes the expiration period of the account or the
1891 password. For the availability and the exact meaning of these fields
1892 in your system, please consult your getpwnam(3) documentation and your
1893 F<pwd.h> file. You can also find out from within Perl what your
1894 $quota and $comment fields mean and whether you have the $expire field
1895 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1896 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1897 files are only supported if your vendor has implemented them in the
1898 intuitive fashion that calling the regular C library routines gets the
1899 shadow versions if you're running under privilege or if there exists
1900 the shadow(3) functions as found in System V ( this includes Solaris
1901 and Linux.) Those systems which implement a proprietary shadow password
1902 facility are unlikely to be supported.
1904 The $members value returned by I<getgr*()> is a space separated list of
1905 the login names of the members of the group.
1907 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1908 C, it will be returned to you via C<$?> if the function call fails. The
1909 C<@addrs> value returned by a successful call is a list of the raw
1910 addresses returned by the corresponding system library call. In the
1911 Internet domain, each address is four bytes long and you can unpack it
1912 by saying something like:
1914 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1916 The Socket library makes this slightly easier:
1919 $iaddr = inet_aton("127.1"); # or whatever address
1920 $name = gethostbyaddr($iaddr, AF_INET);
1922 # or going the other way
1923 $straddr = inet_ntoa($iaddr);
1925 If you get tired of remembering which element of the return list
1926 contains which return value, by-name interfaces are provided
1927 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1928 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1929 and C<User::grent>. These override the normal built-ins, supplying
1930 versions that return objects with the appropriate names
1931 for each field. For example:
1935 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1937 Even though it looks like they're the same method calls (uid),
1938 they aren't, because a C<File::stat> object is different from
1939 a C<User::pwent> object.
1941 =item getsockname SOCKET
1943 Returns the packed sockaddr address of this end of the SOCKET connection,
1944 in case you don't know the address because you have several different
1945 IPs that the connection might have come in on.
1948 $mysockaddr = getsockname(SOCK);
1949 ($port, $myaddr) = sockaddr_in($mysockaddr);
1950 printf "Connect to %s [%s]\n",
1951 scalar gethostbyaddr($myaddr, AF_INET),
1954 =item getsockopt SOCKET,LEVEL,OPTNAME
1956 Returns the socket option requested, or undef if there is an error.
1962 Returns the value of EXPR with filename expansions such as the
1963 standard Unix shell F</bin/csh> would do. This is the internal function
1964 implementing the C<< <*.c> >> operator, but you can use it directly.
1965 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1966 discussed in more detail in L<perlop/"I/O Operators">.
1968 Beginning with v5.6.0, this operator is implemented using the standard
1969 C<File::Glob> extension. See L<File::Glob> for details.
1973 Converts a time as returned by the time function to a 8-element list
1974 with the time localized for the standard Greenwich time zone.
1975 Typically used as follows:
1978 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1981 All list elements are numeric, and come straight out of the C `struct
1982 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1983 specified time. $mday is the day of the month, and $mon is the month
1984 itself, in the range C<0..11> with 0 indicating January and 11
1985 indicating December. $year is the number of years since 1900. That
1986 is, $year is C<123> in year 2023. $wday is the day of the week, with
1987 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
1988 the year, in the range C<0..364> (or C<0..365> in leap years.)
1990 Note that the $year element is I<not> simply the last two digits of
1991 the year. If you assume it is, then you create non-Y2K-compliant
1992 programs--and you wouldn't want to do that, would you?
1994 The proper way to get a complete 4-digit year is simply:
1998 And to get the last two digits of the year (e.g., '01' in 2001) do:
2000 $year = sprintf("%02d", $year % 100);
2002 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2004 In scalar context, C<gmtime()> returns the ctime(3) value:
2006 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2008 Also see the C<timegm> function provided by the C<Time::Local> module,
2009 and the strftime(3) function available via the POSIX module.
2011 This scalar value is B<not> locale dependent (see L<perllocale>), but
2012 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2013 strftime(3) and mktime(3) functions available via the POSIX module. To
2014 get somewhat similar but locale dependent date strings, set up your
2015 locale environment variables appropriately (please see L<perllocale>)
2016 and try for example:
2018 use POSIX qw(strftime);
2019 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2021 Note that the C<%a> and C<%b> escapes, which represent the short forms
2022 of the day of the week and the month of the year, may not necessarily
2023 be three characters wide in all locales.
2031 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2032 execution there. It may not be used to go into any construct that
2033 requires initialization, such as a subroutine or a C<foreach> loop. It
2034 also can't be used to go into a construct that is optimized away,
2035 or to get out of a block or subroutine given to C<sort>.
2036 It can be used to go almost anywhere else within the dynamic scope,
2037 including out of subroutines, but it's usually better to use some other
2038 construct such as C<last> or C<die>. The author of Perl has never felt the
2039 need to use this form of C<goto> (in Perl, that is--C is another matter).
2041 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2042 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2043 necessarily recommended if you're optimizing for maintainability:
2045 goto ("FOO", "BAR", "GLARCH")[$i];
2047 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2048 In fact, it isn't a goto in the normal sense at all, and doesn't have
2049 the stigma associated with other gotos. Instead, it
2050 substitutes a call to the named subroutine for the currently running
2051 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2052 another subroutine and then pretend that the other subroutine had been
2053 called in the first place (except that any modifications to C<@_>
2054 in the current subroutine are propagated to the other subroutine.)
2055 After the C<goto>, not even C<caller> will be able to tell that this
2056 routine was called first.
2058 NAME needn't be the name of a subroutine; it can be a scalar variable
2059 containing a code reference, or a block which evaluates to a code
2062 =item grep BLOCK LIST
2064 =item grep EXPR,LIST
2066 This is similar in spirit to, but not the same as, grep(1) and its
2067 relatives. In particular, it is not limited to using regular expressions.
2069 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2070 C<$_> to each element) and returns the list value consisting of those
2071 elements for which the expression evaluated to true. In scalar
2072 context, returns the number of times the expression was true.
2074 @foo = grep(!/^#/, @bar); # weed out comments
2078 @foo = grep {!/^#/} @bar; # weed out comments
2080 Note that, because C<$_> is a reference into the list value, it can
2081 be used to modify the elements of the array. While this is useful and
2082 supported, it can cause bizarre results if the LIST is not a named array.
2083 Similarly, grep returns aliases into the original list, much as a for
2084 loop's index variable aliases the list elements. That is, modifying an
2085 element of a list returned by grep (for example, in a C<foreach>, C<map>
2086 or another C<grep>) actually modifies the element in the original list.
2087 This is usually something to be avoided when writing clear code.
2089 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2095 Interprets EXPR as a hex string and returns the corresponding value.
2096 (To convert strings that might start with either 0, 0x, or 0b, see
2097 L</oct>.) If EXPR is omitted, uses C<$_>.
2099 print hex '0xAf'; # prints '175'
2100 print hex 'aF'; # same
2102 Hex strings may only represent integers. Strings that would cause
2103 integer overflow trigger a warning.
2107 There is no builtin C<import> function. It is just an ordinary
2108 method (subroutine) defined (or inherited) by modules that wish to export
2109 names to another module. The C<use> function calls the C<import> method
2110 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2112 =item index STR,SUBSTR,POSITION
2114 =item index STR,SUBSTR
2116 The index function searches for one string within another, but without
2117 the wildcard-like behavior of a full regular-expression pattern match.
2118 It returns the position of the first occurrence of SUBSTR in STR at
2119 or after POSITION. If POSITION is omitted, starts searching from the
2120 beginning of the string. The return value is based at C<0> (or whatever
2121 you've set the C<$[> variable to--but don't do that). If the substring
2122 is not found, returns one less than the base, ordinarily C<-1>.
2128 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2129 You should not use this function for rounding: one because it truncates
2130 towards C<0>, and two because machine representations of floating point
2131 numbers can sometimes produce counterintuitive results. For example,
2132 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2133 because it's really more like -268.99999999999994315658 instead. Usually,
2134 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2135 functions will serve you better than will int().
2137 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2139 Implements the ioctl(2) function. You'll probably first have to say
2141 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2143 to get the correct function definitions. If F<ioctl.ph> doesn't
2144 exist or doesn't have the correct definitions you'll have to roll your
2145 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2146 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2147 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2148 written depending on the FUNCTION--a pointer to the string value of SCALAR
2149 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2150 has no string value but does have a numeric value, that value will be
2151 passed rather than a pointer to the string value. To guarantee this to be
2152 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2153 functions may be needed to manipulate the values of structures used by
2156 The return value of C<ioctl> (and C<fcntl>) is as follows:
2158 if OS returns: then Perl returns:
2160 0 string "0 but true"
2161 anything else that number
2163 Thus Perl returns true on success and false on failure, yet you can
2164 still easily determine the actual value returned by the operating
2167 $retval = ioctl(...) || -1;
2168 printf "System returned %d\n", $retval;
2170 The special string "C<0> but true" is exempt from B<-w> complaints
2171 about improper numeric conversions.
2173 Here's an example of setting a filehandle named C<REMOTE> to be
2174 non-blocking at the system level. You'll have to negotiate C<$|>
2175 on your own, though.
2177 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2179 $flags = fcntl(REMOTE, F_GETFL, 0)
2180 or die "Can't get flags for the socket: $!\n";
2182 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2183 or die "Can't set flags for the socket: $!\n";
2185 =item join EXPR,LIST
2187 Joins the separate strings of LIST into a single string with fields
2188 separated by the value of EXPR, and returns that new string. Example:
2190 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2192 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2193 first argument. Compare L</split>.
2197 Returns a list consisting of all the keys of the named hash. (In
2198 scalar context, returns the number of keys.) The keys are returned in
2199 an apparently random order. The actual random order is subject to
2200 change in future versions of perl, but it is guaranteed to be the same
2201 order as either the C<values> or C<each> function produces (given
2202 that the hash has not been modified). As a side effect, it resets
2205 Here is yet another way to print your environment:
2208 @values = values %ENV;
2210 print pop(@keys), '=', pop(@values), "\n";
2213 or how about sorted by key:
2215 foreach $key (sort(keys %ENV)) {
2216 print $key, '=', $ENV{$key}, "\n";
2219 To sort a hash by value, you'll need to use a C<sort> function.
2220 Here's a descending numeric sort of a hash by its values:
2222 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2223 printf "%4d %s\n", $hash{$key}, $key;
2226 As an lvalue C<keys> allows you to increase the number of hash buckets
2227 allocated for the given hash. This can gain you a measure of efficiency if
2228 you know the hash is going to get big. (This is similar to pre-extending
2229 an array by assigning a larger number to $#array.) If you say
2233 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2234 in fact, since it rounds up to the next power of two. These
2235 buckets will be retained even if you do C<%hash = ()>, use C<undef
2236 %hash> if you want to free the storage while C<%hash> is still in scope.
2237 You can't shrink the number of buckets allocated for the hash using
2238 C<keys> in this way (but you needn't worry about doing this by accident,
2239 as trying has no effect).
2241 See also C<each>, C<values> and C<sort>.
2243 =item kill SIGNAL, LIST
2245 Sends a signal to a list of processes. Returns the number of
2246 processes successfully signaled (which is not necessarily the
2247 same as the number actually killed).
2249 $cnt = kill 1, $child1, $child2;
2252 If SIGNAL is zero, no signal is sent to the process. This is a
2253 useful way to check that the process is alive and hasn't changed
2254 its UID. See L<perlport> for notes on the portability of this
2257 Unlike in the shell, if SIGNAL is negative, it kills
2258 process groups instead of processes. (On System V, a negative I<PROCESS>
2259 number will also kill process groups, but that's not portable.) That
2260 means you usually want to use positive not negative signals. You may also
2261 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2267 The C<last> command is like the C<break> statement in C (as used in
2268 loops); it immediately exits the loop in question. If the LABEL is
2269 omitted, the command refers to the innermost enclosing loop. The
2270 C<continue> block, if any, is not executed:
2272 LINE: while (<STDIN>) {
2273 last LINE if /^$/; # exit when done with header
2277 C<last> cannot be used to exit a block which returns a value such as
2278 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2279 a grep() or map() operation.
2281 Note that a block by itself is semantically identical to a loop
2282 that executes once. Thus C<last> can be used to effect an early
2283 exit out of such a block.
2285 See also L</continue> for an illustration of how C<last>, C<next>, and
2292 Returns an lowercased version of EXPR. This is the internal function
2293 implementing the C<\L> escape in double-quoted strings.
2294 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2297 If EXPR is omitted, uses C<$_>.
2303 Returns the value of EXPR with the first character lowercased. This is
2304 the internal function implementing the C<\l> escape in double-quoted strings.
2305 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2307 If EXPR is omitted, uses C<$_>.
2313 Returns the length in characters of the value of EXPR. If EXPR is
2314 omitted, returns length of C<$_>. Note that this cannot be used on
2315 an entire array or hash to find out how many elements these have.
2316 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2318 =item link OLDFILE,NEWFILE
2320 Creates a new filename linked to the old filename. Returns true for
2321 success, false otherwise.
2323 =item listen SOCKET,QUEUESIZE
2325 Does the same thing that the listen system call does. Returns true if
2326 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2330 You really probably want to be using C<my> instead, because C<local> isn't
2331 what most people think of as "local". See L<perlsub/"Private Variables
2332 via my()"> for details.
2334 A local modifies the listed variables to be local to the enclosing
2335 block, file, or eval. If more than one value is listed, the list must
2336 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2337 for details, including issues with tied arrays and hashes.
2339 =item localtime EXPR
2341 Converts a time as returned by the time function to a 9-element list
2342 with the time analyzed for the local time zone. Typically used as
2346 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2349 All list elements are numeric, and come straight out of the C `struct
2350 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2351 specified time. $mday is the day of the month, and $mon is the month
2352 itself, in the range C<0..11> with 0 indicating January and 11
2353 indicating December. $year is the number of years since 1900. That
2354 is, $year is C<123> in year 2023. $wday is the day of the week, with
2355 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2356 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2357 is true if the specified time occurs during daylight savings time,
2360 Note that the $year element is I<not> simply the last two digits of
2361 the year. If you assume it is, then you create non-Y2K-compliant
2362 programs--and you wouldn't want to do that, would you?
2364 The proper way to get a complete 4-digit year is simply:
2368 And to get the last two digits of the year (e.g., '01' in 2001) do:
2370 $year = sprintf("%02d", $year % 100);
2372 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2374 In scalar context, C<localtime()> returns the ctime(3) value:
2376 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2378 This scalar value is B<not> locale dependent, see L<perllocale>, but
2379 instead a Perl builtin. Also see the C<Time::Local> module
2380 (to convert the second, minutes, hours, ... back to seconds since the
2381 stroke of midnight the 1st of January 1970, the value returned by
2382 time()), and the strftime(3) and mktime(3) functions available via the
2383 POSIX module. To get somewhat similar but locale dependent date
2384 strings, set up your locale environment variables appropriately
2385 (please see L<perllocale>) and try for example:
2387 use POSIX qw(strftime);
2388 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2390 Note that the C<%a> and C<%b>, the short forms of the day of the week
2391 and the month of the year, may not necessarily be three characters wide.
2397 This function places an advisory lock on a variable, subroutine,
2398 or referenced object contained in I<THING> until the lock goes out
2399 of scope. This is a built-in function only if your version of Perl
2400 was built with threading enabled, and if you've said C<use Threads>.
2401 Otherwise a user-defined function by this name will be called. See
2408 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2409 returns log of C<$_>. To get the log of another base, use basic algebra:
2410 The base-N log of a number is equal to the natural log of that number
2411 divided by the natural log of N. For example:
2415 return log($n)/log(10);
2418 See also L</exp> for the inverse operation.
2420 =item lstat FILEHANDLE
2426 Does the same thing as the C<stat> function (including setting the
2427 special C<_> filehandle) but stats a symbolic link instead of the file
2428 the symbolic link points to. If symbolic links are unimplemented on
2429 your system, a normal C<stat> is done.
2431 If EXPR is omitted, stats C<$_>.
2435 The match operator. See L<perlop>.
2437 =item map BLOCK LIST
2441 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2442 C<$_> to each element) and returns the list value composed of the
2443 results of each such evaluation. In scalar context, returns the
2444 total number of elements so generated. Evaluates BLOCK or EXPR in
2445 list context, so each element of LIST may produce zero, one, or
2446 more elements in the returned value.
2448 @chars = map(chr, @nums);
2450 translates a list of numbers to the corresponding characters. And
2452 %hash = map { getkey($_) => $_ } @array;
2454 is just a funny way to write
2457 foreach $_ (@array) {
2458 $hash{getkey($_)} = $_;
2461 Note that, because C<$_> is a reference into the list value, it can
2462 be used to modify the elements of the array. While this is useful and
2463 supported, it can cause bizarre results if the LIST is not a named array.
2464 Using a regular C<foreach> loop for this purpose would be clearer in
2465 most cases. See also L</grep> for an array composed of those items of
2466 the original list for which the BLOCK or EXPR evaluates to true.
2468 =item mkdir FILENAME,MASK
2470 =item mkdir FILENAME
2472 Creates the directory specified by FILENAME, with permissions
2473 specified by MASK (as modified by C<umask>). If it succeeds it
2474 returns true, otherwise it returns false and sets C<$!> (errno).
2475 If omitted, MASK defaults to 0777.
2477 In general, it is better to create directories with permissive MASK,
2478 and let the user modify that with their C<umask>, than it is to supply
2479 a restrictive MASK and give the user no way to be more permissive.
2480 The exceptions to this rule are when the file or directory should be
2481 kept private (mail files, for instance). The perlfunc(1) entry on
2482 C<umask> discusses the choice of MASK in more detail.
2484 =item msgctl ID,CMD,ARG
2486 Calls the System V IPC function msgctl(2). You'll probably have to say
2490 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2491 then ARG must be a variable which will hold the returned C<msqid_ds>
2492 structure. Returns like C<ioctl>: the undefined value for error,
2493 C<"0 but true"> for zero, or the actual return value otherwise. See also
2494 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2496 =item msgget KEY,FLAGS
2498 Calls the System V IPC function msgget(2). Returns the message queue
2499 id, or the undefined value if there is an error. See also C<IPC::SysV>
2500 and C<IPC::Msg> documentation.
2502 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2504 Calls the System V IPC function msgrcv to receive a message from
2505 message queue ID into variable VAR with a maximum message size of
2506 SIZE. Note that when a message is received, the message type as a
2507 native long integer will be the first thing in VAR, followed by the
2508 actual message. This packing may be opened with C<unpack("l! a*")>.
2509 Taints the variable. Returns true if successful, or false if there is
2510 an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2512 =item msgsnd ID,MSG,FLAGS
2514 Calls the System V IPC function msgsnd to send the message MSG to the
2515 message queue ID. MSG must begin with the native long integer message
2516 type, and be followed by the length of the actual message, and finally
2517 the message itself. This kind of packing can be achieved with
2518 C<pack("l! a*", $type, $message)>. Returns true if successful,
2519 or false if there is an error. See also C<IPC::SysV>
2520 and C<IPC::SysV::Msg> documentation.
2524 =item my EXPR : ATTRIBUTES
2526 A C<my> declares the listed variables to be local (lexically) to the
2527 enclosing block, file, or C<eval>. If
2528 more than one value is listed, the list must be placed in parentheses. See
2529 L<perlsub/"Private Variables via my()"> for details.
2535 The C<next> command is like the C<continue> statement in C; it starts
2536 the next iteration of the loop:
2538 LINE: while (<STDIN>) {
2539 next LINE if /^#/; # discard comments
2543 Note that if there were a C<continue> block on the above, it would get
2544 executed even on discarded lines. If the LABEL is omitted, the command
2545 refers to the innermost enclosing loop.
2547 C<next> cannot be used to exit a block which returns a value such as
2548 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2549 a grep() or map() operation.
2551 Note that a block by itself is semantically identical to a loop
2552 that executes once. Thus C<next> will exit such a block early.
2554 See also L</continue> for an illustration of how C<last>, C<next>, and
2557 =item no Module LIST
2559 See the L</use> function, which C<no> is the opposite of.
2565 Interprets EXPR as an octal string and returns the corresponding
2566 value. (If EXPR happens to start off with C<0x>, interprets it as a
2567 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2568 binary string.) The following will handle decimal, binary, octal, and
2569 hex in the standard Perl or C notation:
2571 $val = oct($val) if $val =~ /^0/;
2573 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2574 in octal), use sprintf() or printf():
2576 $perms = (stat("filename"))[2] & 07777;
2577 $oct_perms = sprintf "%lo", $perms;
2579 The oct() function is commonly used when a string such as C<644> needs
2580 to be converted into a file mode, for example. (Although perl will
2581 automatically convert strings into numbers as needed, this automatic
2582 conversion assumes base 10.)
2584 =item open FILEHANDLE,MODE,LIST
2586 =item open FILEHANDLE,EXPR
2588 =item open FILEHANDLE
2590 Opens the file whose filename is given by EXPR, and associates it with
2591 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2592 name of the real filehandle wanted. (This is considered a symbolic
2593 reference, so C<use strict 'refs'> should I<not> be in effect.)
2595 If EXPR is omitted, the scalar
2596 variable of the same name as the FILEHANDLE contains the filename.
2597 (Note that lexical variables--those declared with C<my>--will not work
2598 for this purpose; so if you're using C<my>, specify EXPR in your call
2599 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2602 If MODE is C<< '<' >> or nothing, the file is opened for input.
2603 If MODE is C<< '>' >>, the file is truncated and opened for
2604 output, being created if necessary. If MODE is C<<< '>>' >>>,
2605 the file is opened for appending, again being created if necessary.
2606 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2607 you want both read and write access to the file; thus C<< '+<' >> is almost
2608 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2609 file first. You can't usually use either read-write mode for updating
2610 textfiles, since they have variable length records. See the B<-i>
2611 switch in L<perlrun> for a better approach. The file is created with
2612 permissions of C<0666> modified by the process' C<umask> value.
2614 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2615 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2617 In the 2-arguments (and 1-argument) form of the call the mode and
2618 filename should be concatenated (in this order), possibly separated by
2619 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2621 If the filename begins with C<'|'>, the filename is interpreted as a
2622 command to which output is to be piped, and if the filename ends with a
2623 C<'|'>, the filename is interpreted as a command which pipes output to
2624 us. See L<perlipc/"Using open() for IPC">
2625 for more examples of this. (You are not allowed to C<open> to a command
2626 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2627 and L<perlipc/"Bidirectional Communication with Another Process">
2630 If MODE is C<'|-'>, the filename is interpreted as a
2631 command to which output is to be piped, and if MODE is
2632 C<'-|'>, the filename is interpreted as a command which pipes output to
2633 us. In the 2-arguments (and 1-argument) form one should replace dash
2634 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2635 for more examples of this. (You are not allowed to C<open> to a command
2636 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2637 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2639 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2640 and opening C<< '>-' >> opens STDOUT.
2643 nonzero upon success, the undefined value otherwise. If the C<open>
2644 involved a pipe, the return value happens to be the pid of the
2647 If you're unfortunate enough to be running Perl on a system that
2648 distinguishes between text files and binary files (modern operating
2649 systems don't care), then you should check out L</binmode> for tips for
2650 dealing with this. The key distinction between systems that need C<binmode>
2651 and those that don't is their text file formats. Systems like Unix, MacOS, and
2652 Plan9, which delimit lines with a single character, and which encode that
2653 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2655 When opening a file, it's usually a bad idea to continue normal execution
2656 if the request failed, so C<open> is frequently used in connection with
2657 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2658 where you want to make a nicely formatted error message (but there are
2659 modules that can help with that problem)) you should always check
2660 the return value from opening a file. The infrequent exception is when
2661 working with an unopened filehandle is actually what you want to do.
2666 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2667 while (<ARTICLE>) {...
2669 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2670 # if the open fails, output is discarded
2672 open(DBASE, '+<', 'dbase.mine') # open for update
2673 or die "Can't open 'dbase.mine' for update: $!";
2675 open(DBASE, '+<dbase.mine') # ditto
2676 or die "Can't open 'dbase.mine' for update: $!";
2678 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2679 or die "Can't start caesar: $!";
2681 open(ARTICLE, "caesar <$article |") # ditto
2682 or die "Can't start caesar: $!";
2684 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2685 or die "Can't start sort: $!";
2687 # process argument list of files along with any includes
2689 foreach $file (@ARGV) {
2690 process($file, 'fh00');
2694 my($filename, $input) = @_;
2695 $input++; # this is a string increment
2696 unless (open($input, $filename)) {
2697 print STDERR "Can't open $filename: $!\n";
2702 while (<$input>) { # note use of indirection
2703 if (/^#include "(.*)"/) {
2704 process($1, $input);
2711 You may also, in the Bourne shell tradition, specify an EXPR beginning
2712 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2713 name of a filehandle (or file descriptor, if numeric) to be
2714 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2715 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2716 mode you specify should match the mode of the original filehandle.
2717 (Duping a filehandle does not take into account any existing contents of
2718 stdio buffers.) Duping file handles is not yet supported for 3-argument
2721 Here is a script that saves, redirects, and restores STDOUT and
2725 open(OLDOUT, ">&STDOUT");
2726 open(OLDERR, ">&STDERR");
2728 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2729 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2731 select(STDERR); $| = 1; # make unbuffered
2732 select(STDOUT); $| = 1; # make unbuffered
2734 print STDOUT "stdout 1\n"; # this works for
2735 print STDERR "stderr 1\n"; # subprocesses too
2740 open(STDOUT, ">&OLDOUT");
2741 open(STDERR, ">&OLDERR");
2743 print STDOUT "stdout 2\n";
2744 print STDERR "stderr 2\n";
2746 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2747 equivalent of C's C<fdopen> of that file descriptor; this is more
2748 parsimonious of file descriptors. For example:
2750 open(FILEHANDLE, "<&=$fd")
2752 Note that this feature depends on the fdopen() C library function.
2753 On many UNIX systems, fdopen() is known to fail when file descriptors
2754 exceed a certain value, typically 255. If you need more file
2755 descriptors than that, consider rebuilding Perl to use the C<sfio>
2758 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2759 with 2-arguments (or 1-argument) form of open(), then
2760 there is an implicit fork done, and the return value of open is the pid
2761 of the child within the parent process, and C<0> within the child
2762 process. (Use C<defined($pid)> to determine whether the open was successful.)
2763 The filehandle behaves normally for the parent, but i/o to that
2764 filehandle is piped from/to the STDOUT/STDIN of the child process.
2765 In the child process the filehandle isn't opened--i/o happens from/to
2766 the new STDOUT or STDIN. Typically this is used like the normal
2767 piped open when you want to exercise more control over just how the
2768 pipe command gets executed, such as when you are running setuid, and
2769 don't want to have to scan shell commands for metacharacters.
2770 The following triples are more or less equivalent:
2772 open(FOO, "|tr '[a-z]' '[A-Z]'");
2773 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2774 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2776 open(FOO, "cat -n '$file'|");
2777 open(FOO, '-|', "cat -n '$file'");
2778 open(FOO, '-|') || exec 'cat', '-n', $file;
2780 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2782 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2783 output before any operation that may do a fork, but this may not be
2784 supported on some platforms (see L<perlport>). To be safe, you may need
2785 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2786 of C<IO::Handle> on any open handles.
2788 On systems that support a
2789 close-on-exec flag on files, the flag will be set for the newly opened
2790 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2792 Closing any piped filehandle causes the parent process to wait for the
2793 child to finish, and returns the status value in C<$?>.
2795 The filename passed to 2-argument (or 1-argument) form of open()
2796 will have leading and trailing
2797 whitespace deleted, and the normal redirection characters
2798 honored. This property, known as "magic open",
2799 can often be used to good effect. A user could specify a filename of
2800 F<"rsh cat file |">, or you could change certain filenames as needed:
2802 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2803 open(FH, $filename) or die "Can't open $filename: $!";
2805 Use 3-argument form to open a file with arbitrary weird characters in it,
2807 open(FOO, '<', $file);
2809 otherwise it's necessary to protect any leading and trailing whitespace:
2811 $file =~ s#^(\s)#./$1#;
2812 open(FOO, "< $file\0");
2814 (this may not work on some bizzare filesystems). One should
2815 conscientiously choose between the I<magic> and 3-arguments form
2820 will allow the user to specify an argument of the form C<"rsh cat file |">,
2821 but will not work on a filename which happens to have a trailing space, while
2823 open IN, '<', $ARGV[0];
2825 will have exactly the opposite restrictions.
2827 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2828 should use the C<sysopen> function, which involves no such magic (but
2829 may use subtly different filemodes than Perl open(), which is mapped
2830 to C fopen()). This is
2831 another way to protect your filenames from interpretation. For example:
2834 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2835 or die "sysopen $path: $!";
2836 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2837 print HANDLE "stuff $$\n");
2839 print "File contains: ", <HANDLE>;
2841 Using the constructor from the C<IO::Handle> package (or one of its
2842 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2843 filehandles that have the scope of whatever variables hold references to
2844 them, and automatically close whenever and however you leave that scope:
2848 sub read_myfile_munged {
2850 my $handle = new IO::File;
2851 open($handle, "myfile") or die "myfile: $!";
2853 or return (); # Automatically closed here.
2854 mung $first or die "mung failed"; # Or here.
2855 return $first, <$handle> if $ALL; # Or here.
2859 See L</seek> for some details about mixing reading and writing.
2861 =item opendir DIRHANDLE,EXPR
2863 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2864 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2865 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2871 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2872 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2873 See L<utf8> for more about Unicode.
2877 An C<our> declares the listed variables to be valid globals within
2878 the enclosing block, file, or C<eval>. That is, it has the same
2879 scoping rules as a "my" declaration, but does not create a local
2880 variable. If more than one value is listed, the list must be placed
2881 in parentheses. The C<our> declaration has no semantic effect unless
2882 "use strict vars" is in effect, in which case it lets you use the
2883 declared global variable without qualifying it with a package name.
2884 (But only within the lexical scope of the C<our> declaration. In this
2885 it differs from "use vars", which is package scoped.)
2887 An C<our> declaration declares a global variable that will be visible
2888 across its entire lexical scope, even across package boundaries. The
2889 package in which the variable is entered is determined at the point
2890 of the declaration, not at the point of use. This means the following
2894 our $bar; # declares $Foo::bar for rest of lexical scope
2898 print $bar; # prints 20
2900 Multiple C<our> declarations in the same lexical scope are allowed
2901 if they are in different packages. If they happened to be in the same
2902 package, Perl will emit warnings if you have asked for them.
2906 our $bar; # declares $Foo::bar for rest of lexical scope
2910 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2911 print $bar; # prints 30
2913 our $bar; # emits warning
2915 =item pack TEMPLATE,LIST
2917 Takes a LIST of values and converts it into a string using the rules
2918 given by the TEMPLATE. The resulting string is the concatenation of
2919 the converted values. Typically, each converted value looks
2920 like its machine-level representation. For example, on 32-bit machines
2921 a converted integer may be represented by a sequence of 4 bytes.
2924 sequence of characters that give the order and type of values, as
2927 a A string with arbitrary binary data, will be null padded.
2928 A An ASCII string, will be space padded.
2929 Z A null terminated (asciz) string, will be null padded.
2931 b A bit string (ascending bit order inside each byte, like vec()).
2932 B A bit string (descending bit order inside each byte).
2933 h A hex string (low nybble first).
2934 H A hex string (high nybble first).
2936 c A signed char value.
2937 C An unsigned char value. Only does bytes. See U for Unicode.
2939 s A signed short value.
2940 S An unsigned short value.
2941 (This 'short' is _exactly_ 16 bits, which may differ from
2942 what a local C compiler calls 'short'. If you want
2943 native-length shorts, use the '!' suffix.)
2945 i A signed integer value.
2946 I An unsigned integer value.
2947 (This 'integer' is _at_least_ 32 bits wide. Its exact
2948 size depends on what a local C compiler calls 'int',
2949 and may even be larger than the 'long' described in
2952 l A signed long value.
2953 L An unsigned long value.
2954 (This 'long' is _exactly_ 32 bits, which may differ from
2955 what a local C compiler calls 'long'. If you want
2956 native-length longs, use the '!' suffix.)
2958 n An unsigned short in "network" (big-endian) order.
2959 N An unsigned long in "network" (big-endian) order.
2960 v An unsigned short in "VAX" (little-endian) order.
2961 V An unsigned long in "VAX" (little-endian) order.
2962 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2963 _exactly_ 32 bits, respectively.)
2965 q A signed quad (64-bit) value.
2966 Q An unsigned quad value.
2967 (Quads are available only if your system supports 64-bit
2968 integer values _and_ if Perl has been compiled to support those.
2969 Causes a fatal error otherwise.)
2971 f A single-precision float in the native format.
2972 d A double-precision float in the native format.
2974 p A pointer to a null-terminated string.
2975 P A pointer to a structure (fixed-length string).
2977 u A uuencoded string.
2978 U A Unicode character number. Encodes to UTF-8 internally.
2979 Works even if C<use utf8> is not in effect.
2981 w A BER compressed integer. Its bytes represent an unsigned
2982 integer in base 128, most significant digit first, with as
2983 few digits as possible. Bit eight (the high bit) is set
2984 on each byte except the last.
2988 @ Null fill to absolute position.
2990 The following rules apply:
2996 Each letter may optionally be followed by a number giving a repeat
2997 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2998 C<H>, and C<P> the pack function will gobble up that many values from
2999 the LIST. A C<*> for the repeat count means to use however many items are
3000 left, except for C<@>, C<x>, C<X>, where it is equivalent
3001 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3004 When used with C<Z>, C<*> results in the addition of a trailing null
3005 byte (so the packed result will be one longer than the byte C<length>
3008 The repeat count for C<u> is interpreted as the maximal number of bytes
3009 to encode per line of output, with 0 and 1 replaced by 45.
3013 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3014 string of length count, padding with nulls or spaces as necessary. When
3015 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3016 after the first null, and C<a> returns data verbatim. When packing,
3017 C<a>, and C<Z> are equivalent.
3019 If the value-to-pack is too long, it is truncated. If too long and an
3020 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3021 by a null byte. Thus C<Z> always packs a trailing null byte under
3026 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3027 Each byte of the input field of pack() generates 1 bit of the result.
3028 Each result bit is based on the least-significant bit of the corresponding
3029 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3030 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3032 Starting from the beginning of the input string of pack(), each 8-tuple
3033 of bytes is converted to 1 byte of output. With format C<b>
3034 the first byte of the 8-tuple determines the least-significant bit of a
3035 byte, and with format C<B> it determines the most-significant bit of
3038 If the length of the input string is not exactly divisible by 8, the
3039 remainder is packed as if the input string were padded by null bytes
3040 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3042 If the input string of pack() is longer than needed, extra bytes are ignored.
3043 A C<*> for the repeat count of pack() means to use all the bytes of
3044 the input field. On unpack()ing the bits are converted to a string
3045 of C<"0">s and C<"1">s.
3049 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3050 representable as hexadecimal digits, 0-9a-f) long.
3052 Each byte of the input field of pack() generates 4 bits of the result.
3053 For non-alphabetical bytes the result is based on the 4 least-significant
3054 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3055 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3056 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3057 is compatible with the usual hexadecimal digits, so that C<"a"> and
3058 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3059 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3061 Starting from the beginning of the input string of pack(), each pair
3062 of bytes is converted to 1 byte of output. With format C<h> the
3063 first byte of the pair determines the least-significant nybble of the
3064 output byte, and with format C<H> it determines the most-significant
3067 If the length of the input string is not even, it behaves as if padded
3068 by a null byte at the end. Similarly, during unpack()ing the "extra"
3069 nybbles are ignored.
3071 If the input string of pack() is longer than needed, extra bytes are ignored.
3072 A C<*> for the repeat count of pack() means to use all the bytes of
3073 the input field. On unpack()ing the bits are converted to a string
3074 of hexadecimal digits.
3078 The C<p> type packs a pointer to a null-terminated string. You are
3079 responsible for ensuring the string is not a temporary value (which can
3080 potentially get deallocated before you get around to using the packed result).
3081 The C<P> type packs a pointer to a structure of the size indicated by the
3082 length. A NULL pointer is created if the corresponding value for C<p> or
3083 C<P> is C<undef>, similarly for unpack().
3087 The C</> template character allows packing and unpacking of strings where
3088 the packed structure contains a byte count followed by the string itself.
3089 You write I<length-item>C</>I<string-item>.
3091 The I<length-item> can be any C<pack> template letter,
3092 and describes how the length value is packed.
3093 The ones likely to be of most use are integer-packing ones like
3094 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3095 and C<N> (for Sun XDR).
3097 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3098 For C<unpack> the length of the string is obtained from the I<length-item>,
3099 but if you put in the '*' it will be ignored.
3101 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3102 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3103 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3105 The I<length-item> is not returned explicitly from C<unpack>.
3107 Adding a count to the I<length-item> letter is unlikely to do anything
3108 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3109 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3110 which Perl does not regard as legal in numeric strings.
3114 The integer types C<s>, C<S>, C<l>, and C<L> may be
3115 immediately followed by a C<!> suffix to signify native shorts or
3116 longs--as you can see from above for example a bare C<l> does mean
3117 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3118 may be larger. This is an issue mainly in 64-bit platforms. You can
3119 see whether using C<!> makes any difference by
3121 print length(pack("s")), " ", length(pack("s!")), "\n";
3122 print length(pack("l")), " ", length(pack("l!")), "\n";
3124 C<i!> and C<I!> also work but only because of completeness;
3125 they are identical to C<i> and C<I>.
3127 The actual sizes (in bytes) of native shorts, ints, longs, and long
3128 longs on the platform where Perl was built are also available via
3132 print $Config{shortsize}, "\n";
3133 print $Config{intsize}, "\n";
3134 print $Config{longsize}, "\n";
3135 print $Config{longlongsize}, "\n";
3137 (The C<$Config{longlongsize}> will be undefine if your system does
3138 not support long longs.)
3142 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3143 are inherently non-portable between processors and operating systems
3144 because they obey the native byteorder and endianness. For example a
3145 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3146 (arranged in and handled by the CPU registers) into bytes as
3148 0x12 0x34 0x56 0x78 # little-endian
3149 0x78 0x56 0x34 0x12 # big-endian
3151 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3152 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3153 Power, and Cray are big-endian. MIPS can be either: Digital used it
3154 in little-endian mode; SGI uses it in big-endian mode.
3156 The names `big-endian' and `little-endian' are comic references to
3157 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3158 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3159 the egg-eating habits of the Lilliputians.
3161 Some systems may have even weirder byte orders such as
3166 You can see your system's preference with
3168 print join(" ", map { sprintf "%#02x", $_ }
3169 unpack("C*",pack("L",0x12345678))), "\n";
3171 The byteorder on the platform where Perl was built is also available
3175 print $Config{byteorder}, "\n";
3177 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3178 and C<'87654321'> are big-endian.
3180 If you want portable packed integers use the formats C<n>, C<N>,
3181 C<v>, and C<V>, their byte endianness and size is known.
3182 See also L<perlport>.
3186 Real numbers (floats and doubles) are in the native machine format only;
3187 due to the multiplicity of floating formats around, and the lack of a
3188 standard "network" representation, no facility for interchange has been
3189 made. This means that packed floating point data written on one machine
3190 may not be readable on another - even if both use IEEE floating point
3191 arithmetic (as the endian-ness of the memory representation is not part
3192 of the IEEE spec). See also L<perlport>.
3194 Note that Perl uses doubles internally for all numeric calculation, and
3195 converting from double into float and thence back to double again will
3196 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3201 You must yourself do any alignment or padding by inserting for example
3202 enough C<'x'>es while packing. There is no way to pack() and unpack()
3203 could know where the bytes are going to or coming from. Therefore
3204 C<pack> (and C<unpack>) handle their output and input as flat
3209 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3213 If TEMPLATE requires more arguments to pack() than actually given, pack()
3214 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3215 to pack() than actually given, extra arguments are ignored.
3221 $foo = pack("CCCC",65,66,67,68);
3223 $foo = pack("C4",65,66,67,68);
3225 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3226 # same thing with Unicode circled letters
3228 $foo = pack("ccxxcc",65,66,67,68);
3231 # note: the above examples featuring "C" and "c" are true
3232 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3233 # and UTF-8. In EBCDIC the first example would be
3234 # $foo = pack("CCCC",193,194,195,196);
3236 $foo = pack("s2",1,2);
3237 # "\1\0\2\0" on little-endian
3238 # "\0\1\0\2" on big-endian
3240 $foo = pack("a4","abcd","x","y","z");
3243 $foo = pack("aaaa","abcd","x","y","z");
3246 $foo = pack("a14","abcdefg");
3247 # "abcdefg\0\0\0\0\0\0\0"
3249 $foo = pack("i9pl", gmtime);
3250 # a real struct tm (on my system anyway)
3252 $utmp_template = "Z8 Z8 Z16 L";
3253 $utmp = pack($utmp_template, @utmp1);
3254 # a struct utmp (BSDish)
3256 @utmp2 = unpack($utmp_template, $utmp);
3257 # "@utmp1" eq "@utmp2"
3260 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3263 $foo = pack('sx2l', 12, 34);
3264 # short 12, two zero bytes padding, long 34
3265 $bar = pack('s@4l', 12, 34);
3266 # short 12, zero fill to position 4, long 34
3269 The same template may generally also be used in unpack().
3273 =item package NAMESPACE
3275 Declares the compilation unit as being in the given namespace. The scope
3276 of the package declaration is from the declaration itself through the end
3277 of the enclosing block, file, or eval (the same as the C<my> operator).
3278 All further unqualified dynamic identifiers will be in this namespace.
3279 A package statement affects only dynamic variables--including those
3280 you've used C<local> on--but I<not> lexical variables, which are created
3281 with C<my>. Typically it would be the first declaration in a file to
3282 be included by the C<require> or C<use> operator. You can switch into a
3283 package in more than one place; it merely influences which symbol table
3284 is used by the compiler for the rest of that block. You can refer to
3285 variables and filehandles in other packages by prefixing the identifier
3286 with the package name and a double colon: C<$Package::Variable>.
3287 If the package name is null, the C<main> package as assumed. That is,
3288 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3289 still seen in older code).
3291 If NAMESPACE is omitted, then there is no current package, and all
3292 identifiers must be fully qualified or lexicals. This is stricter
3293 than C<use strict>, since it also extends to function names.
3295 See L<perlmod/"Packages"> for more information about packages, modules,
3296 and classes. See L<perlsub> for other scoping issues.
3298 =item pipe READHANDLE,WRITEHANDLE
3300 Opens a pair of connected pipes like the corresponding system call.
3301 Note that if you set up a loop of piped processes, deadlock can occur
3302 unless you are very careful. In addition, note that Perl's pipes use
3303 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3304 after each command, depending on the application.
3306 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3307 for examples of such things.
3309 On systems that support a close-on-exec flag on files, the flag will be set
3310 for the newly opened file descriptors as determined by the value of $^F.
3317 Pops and returns the last value of the array, shortening the array by
3318 one element. Has an effect similar to
3322 If there are no elements in the array, returns the undefined value
3323 (although this may happen at other times as well). If ARRAY is
3324 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3325 array in subroutines, just like C<shift>.
3331 Returns the offset of where the last C<m//g> search left off for the variable
3332 is in question (C<$_> is used when the variable is not specified). May be
3333 modified to change that offset. Such modification will also influence
3334 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3337 =item print FILEHANDLE LIST
3343 Prints a string or a list of strings. Returns true if successful.
3344 FILEHANDLE may be a scalar variable name, in which case the variable
3345 contains the name of or a reference to the filehandle, thus introducing
3346 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3347 the next token is a term, it may be misinterpreted as an operator
3348 unless you interpose a C<+> or put parentheses around the arguments.)
3349 If FILEHANDLE is omitted, prints by default to standard output (or
3350 to the last selected output channel--see L</select>). If LIST is
3351 also omitted, prints C<$_> to the currently selected output channel.
3352 To set the default output channel to something other than STDOUT
3353 use the select operation. The current value of C<$,> (if any) is
3354 printed between each LIST item. The current value of C<$\> (if
3355 any) is printed after the entire LIST has been printed. Because
3356 print takes a LIST, anything in the LIST is evaluated in list
3357 context, and any subroutine that you call will have one or more of
3358 its expressions evaluated in list context. Also be careful not to
3359 follow the print keyword with a left parenthesis unless you want
3360 the corresponding right parenthesis to terminate the arguments to
3361 the print--interpose a C<+> or put parentheses around all the
3364 Note that if you're storing FILEHANDLES in an array or other expression,
3365 you will have to use a block returning its value instead:
3367 print { $files[$i] } "stuff\n";
3368 print { $OK ? STDOUT : STDERR } "stuff\n";
3370 =item printf FILEHANDLE FORMAT, LIST
3372 =item printf FORMAT, LIST
3374 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3375 (the output record separator) is not appended. The first argument
3376 of the list will be interpreted as the C<printf> format. If C<use locale> is
3377 in effect, the character used for the decimal point in formatted real numbers
3378 is affected by the LC_NUMERIC locale. See L<perllocale>.
3380 Don't fall into the trap of using a C<printf> when a simple
3381 C<print> would do. The C<print> is more efficient and less
3384 =item prototype FUNCTION
3386 Returns the prototype of a function as a string (or C<undef> if the
3387 function has no prototype). FUNCTION is a reference to, or the name of,
3388 the function whose prototype you want to retrieve.
3390 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3391 name for Perl builtin. If the builtin is not I<overridable> (such as
3392 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3393 C<system>) returns C<undef> because the builtin does not really behave
3394 like a Perl function. Otherwise, the string describing the equivalent
3395 prototype is returned.
3397 =item push ARRAY,LIST
3399 Treats ARRAY as a stack, and pushes the values of LIST
3400 onto the end of ARRAY. The length of ARRAY increases by the length of
3401 LIST. Has the same effect as
3404 $ARRAY[++$#ARRAY] = $value;
3407 but is more efficient. Returns the new number of elements in the array.
3419 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3421 =item quotemeta EXPR
3425 Returns the value of EXPR with all non-alphanumeric
3426 characters backslashed. (That is, all characters not matching
3427 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3428 returned string, regardless of any locale settings.)
3429 This is the internal function implementing
3430 the C<\Q> escape in double-quoted strings.
3432 If EXPR is omitted, uses C<$_>.
3438 Returns a random fractional number greater than or equal to C<0> and less
3439 than the value of EXPR. (EXPR should be positive.) If EXPR is
3440 omitted, the value C<1> is used. Automatically calls C<srand> unless
3441 C<srand> has already been called. See also C<srand>.
3443 (Note: If your rand function consistently returns numbers that are too
3444 large or too small, then your version of Perl was probably compiled
3445 with the wrong number of RANDBITS.)
3447 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3449 =item read FILEHANDLE,SCALAR,LENGTH
3451 Attempts to read LENGTH bytes of data into variable SCALAR from the
3452 specified FILEHANDLE. Returns the number of bytes actually read,
3453 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3454 or shrunk to the length actually read. An OFFSET may be specified to
3455 place the read data at some other place than the beginning of the
3456 string. This call is actually implemented in terms of stdio's fread(3)
3457 call. To get a true read(2) system call, see C<sysread>.
3459 =item readdir DIRHANDLE
3461 Returns the next directory entry for a directory opened by C<opendir>.
3462 If used in list context, returns all the rest of the entries in the
3463 directory. If there are no more entries, returns an undefined value in
3464 scalar context or a null list in list context.
3466 If you're planning to filetest the return values out of a C<readdir>, you'd
3467 better prepend the directory in question. Otherwise, because we didn't
3468 C<chdir> there, it would have been testing the wrong file.
3470 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3471 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3476 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3477 context, each call reads and returns the next line, until end-of-file is
3478 reached, whereupon the subsequent call returns undef. In list context,
3479 reads until end-of-file is reached and returns a list of lines. Note that
3480 the notion of "line" used here is however you may have defined it
3481 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3483 When C<$/> is set to C<undef>, when readline() is in scalar
3484 context (i.e. file slurp mode), and when an empty file is read, it
3485 returns C<''> the first time, followed by C<undef> subsequently.
3487 This is the internal function implementing the C<< <EXPR> >>
3488 operator, but you can use it directly. The C<< <EXPR> >>
3489 operator is discussed in more detail in L<perlop/"I/O Operators">.
3492 $line = readline(*STDIN); # same thing
3498 Returns the value of a symbolic link, if symbolic links are
3499 implemented. If not, gives a fatal error. If there is some system
3500 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3501 omitted, uses C<$_>.
3505 EXPR is executed as a system command.
3506 The collected standard output of the command is returned.
3507 In scalar context, it comes back as a single (potentially
3508 multi-line) string. In list context, returns a list of lines
3509 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3510 This is the internal function implementing the C<qx/EXPR/>
3511 operator, but you can use it directly. The C<qx/EXPR/>
3512 operator is discussed in more detail in L<perlop/"I/O Operators">.
3514 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3516 Receives a message on a socket. Attempts to receive LENGTH bytes of
3517 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3518 will be grown or shrunk to the length actually read. Takes the same
3519 flags as the system call of the same name. Returns the address of the
3520 sender if SOCKET's protocol supports this; returns an empty string
3521 otherwise. If there's an error, returns the undefined value. This call
3522 is actually implemented in terms of recvfrom(2) system call. See
3523 L<perlipc/"UDP: Message Passing"> for examples.
3529 The C<redo> command restarts the loop block without evaluating the
3530 conditional again. The C<continue> block, if any, is not executed. If
3531 the LABEL is omitted, the command refers to the innermost enclosing
3532 loop. This command is normally used by programs that want to lie to
3533 themselves about what was just input:
3535 # a simpleminded Pascal comment stripper
3536 # (warning: assumes no { or } in strings)
3537 LINE: while (<STDIN>) {
3538 while (s|({.*}.*){.*}|$1 |) {}
3543 if (/}/) { # end of comment?
3552 C<redo> cannot be used to retry a block which returns a value such as
3553 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3554 a grep() or map() operation.
3556 Note that a block by itself is semantically identical to a loop
3557 that executes once. Thus C<redo> inside such a block will effectively
3558 turn it into a looping construct.
3560 See also L</continue> for an illustration of how C<last>, C<next>, and
3567 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3568 is not specified, C<$_> will be used. The value returned depends on the
3569 type of thing the reference is a reference to.
3570 Builtin types include:
3580 If the referenced object has been blessed into a package, then that package
3581 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3583 if (ref($r) eq "HASH") {
3584 print "r is a reference to a hash.\n";
3587 print "r is not a reference at all.\n";
3589 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3590 print "r is a reference to something that isa hash.\n";
3593 See also L<perlref>.
3595 =item rename OLDNAME,NEWNAME
3597 Changes the name of a file; an existing file NEWNAME will be
3598 clobbered. Returns true for success, false otherwise.
3600 Behavior of this function varies wildly depending on your system
3601 implementation. For example, it will usually not work across file system
3602 boundaries, even though the system I<mv> command sometimes compensates
3603 for this. Other restrictions include whether it works on directories,
3604 open files, or pre-existing files. Check L<perlport> and either the
3605 rename(2) manpage or equivalent system documentation for details.
3607 =item require VERSION
3613 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3616 If a VERSION is specified as a literal of the form v5.6.1,
3617 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3618 at least as recent as that version, at run time. (For compatibility
3619 with older versions of Perl, a numeric argument will also be interpreted
3620 as VERSION.) Compare with L</use>, which can do a similar check at
3623 require v5.6.1; # run time version check
3624 require 5.6.1; # ditto
3625 require 5.005_03; # float version allowed for compatibility
3627 Otherwise, demands that a library file be included if it hasn't already
3628 been included. The file is included via the do-FILE mechanism, which is
3629 essentially just a variety of C<eval>. Has semantics similar to the following
3634 return 1 if $INC{$filename};
3635 my($realfilename,$result);
3637 foreach $prefix (@INC) {
3638 $realfilename = "$prefix/$filename";
3639 if (-f $realfilename) {
3640 $INC{$filename} = $realfilename;
3641 $result = do $realfilename;
3645 die "Can't find $filename in \@INC";
3647 delete $INC{$filename} if $@ || !$result;
3649 die "$filename did not return true value" unless $result;
3653 Note that the file will not be included twice under the same specified
3654 name. The file must return true as the last statement to indicate
3655 successful execution of any initialization code, so it's customary to
3656 end such a file with C<1;> unless you're sure it'll return true
3657 otherwise. But it's better just to put the C<1;>, in case you add more
3660 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3661 replaces "F<::>" with "F</>" in the filename for you,
3662 to make it easy to load standard modules. This form of loading of
3663 modules does not risk altering your namespace.
3665 In other words, if you try this:
3667 require Foo::Bar; # a splendid bareword
3669 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3670 directories specified in the C<@INC> array.
3672 But if you try this:
3674 $class = 'Foo::Bar';
3675 require $class; # $class is not a bareword
3677 require "Foo::Bar"; # not a bareword because of the ""
3679 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3680 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3682 eval "require $class";
3684 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3690 Generally used in a C<continue> block at the end of a loop to clear
3691 variables and reset C<??> searches so that they work again. The
3692 expression is interpreted as a list of single characters (hyphens
3693 allowed for ranges). All variables and arrays beginning with one of
3694 those letters are reset to their pristine state. If the expression is
3695 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3696 only variables or searches in the current package. Always returns
3699 reset 'X'; # reset all X variables
3700 reset 'a-z'; # reset lower case variables
3701 reset; # just reset ?one-time? searches
3703 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3704 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3705 variables--lexical variables are unaffected, but they clean themselves
3706 up on scope exit anyway, so you'll probably want to use them instead.
3713 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3714 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3715 context, depending on how the return value will be used, and the context
3716 may vary from one execution to the next (see C<wantarray>). If no EXPR
3717 is given, returns an empty list in list context, the undefined value in
3718 scalar context, and (of course) nothing at all in a void context.
3720 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3721 or do FILE will automatically return the value of the last expression
3726 In list context, returns a list value consisting of the elements
3727 of LIST in the opposite order. In scalar context, concatenates the
3728 elements of LIST and returns a string value with all characters
3729 in the opposite order.
3731 print reverse <>; # line tac, last line first
3733 undef $/; # for efficiency of <>
3734 print scalar reverse <>; # character tac, last line tsrif
3736 This operator is also handy for inverting a hash, although there are some
3737 caveats. If a value is duplicated in the original hash, only one of those
3738 can be represented as a key in the inverted hash. Also, this has to
3739 unwind one hash and build a whole new one, which may take some time
3740 on a large hash, such as from a DBM file.
3742 %by_name = reverse %by_address; # Invert the hash
3744 =item rewinddir DIRHANDLE
3746 Sets the current position to the beginning of the directory for the
3747 C<readdir> routine on DIRHANDLE.
3749 =item rindex STR,SUBSTR,POSITION
3751 =item rindex STR,SUBSTR
3753 Works just like index() except that it returns the position of the LAST
3754 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3755 last occurrence at or before that position.
3757 =item rmdir FILENAME
3761 Deletes the directory specified by FILENAME if that directory is empty. If it
3762 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3763 FILENAME is omitted, uses C<$_>.
3767 The substitution operator. See L<perlop>.
3771 Forces EXPR to be interpreted in scalar context and returns the value
3774 @counts = ( scalar @a, scalar @b, scalar @c );
3776 There is no equivalent operator to force an expression to
3777 be interpolated in list context because in practice, this is never
3778 needed. If you really wanted to do so, however, you could use
3779 the construction C<@{[ (some expression) ]}>, but usually a simple
3780 C<(some expression)> suffices.
3782 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3783 parenthesized list, this behaves as a scalar comma expression, evaluating
3784 all but the last element in void context and returning the final element
3785 evaluated in scalar context. This is seldom what you want.
3787 The following single statement:
3789 print uc(scalar(&foo,$bar)),$baz;
3791 is the moral equivalent of these two:
3794 print(uc($bar),$baz);
3796 See L<perlop> for more details on unary operators and the comma operator.
3798 =item seek FILEHANDLE,POSITION,WHENCE
3800 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3801 FILEHANDLE may be an expression whose value gives the name of the
3802 filehandle. The values for WHENCE are C<0> to set the new position to
3803 POSITION, C<1> to set it to the current position plus POSITION, and
3804 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3805 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3806 (start of the file, current position, end of the file) from the Fcntl
3807 module. Returns C<1> upon success, C<0> otherwise.
3809 If you want to position file for C<sysread> or C<syswrite>, don't use
3810 C<seek>--buffering makes its effect on the file's system position
3811 unpredictable and non-portable. Use C<sysseek> instead.
3813 Due to the rules and rigors of ANSI C, on some systems you have to do a
3814 seek whenever you switch between reading and writing. Amongst other
3815 things, this may have the effect of calling stdio's clearerr(3).
3816 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3820 This is also useful for applications emulating C<tail -f>. Once you hit
3821 EOF on your read, and then sleep for a while, you might have to stick in a
3822 seek() to reset things. The C<seek> doesn't change the current position,
3823 but it I<does> clear the end-of-file condition on the handle, so that the
3824 next C<< <FILE> >> makes Perl try again to read something. We hope.
3826 If that doesn't work (some stdios are particularly cantankerous), then
3827 you may need something more like this:
3830 for ($curpos = tell(FILE); $_ = <FILE>;
3831 $curpos = tell(FILE)) {
3832 # search for some stuff and put it into files
3834 sleep($for_a_while);
3835 seek(FILE, $curpos, 0);
3838 =item seekdir DIRHANDLE,POS
3840 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3841 must be a value returned by C<telldir>. Has the same caveats about
3842 possible directory compaction as the corresponding system library
3845 =item select FILEHANDLE
3849 Returns the currently selected filehandle. Sets the current default
3850 filehandle for output, if FILEHANDLE is supplied. This has two
3851 effects: first, a C<write> or a C<print> without a filehandle will
3852 default to this FILEHANDLE. Second, references to variables related to
3853 output will refer to this output channel. For example, if you have to
3854 set the top of form format for more than one output channel, you might
3862 FILEHANDLE may be an expression whose value gives the name of the
3863 actual filehandle. Thus:
3865 $oldfh = select(STDERR); $| = 1; select($oldfh);
3867 Some programmers may prefer to think of filehandles as objects with
3868 methods, preferring to write the last example as:
3871 STDERR->autoflush(1);
3873 =item select RBITS,WBITS,EBITS,TIMEOUT
3875 This calls the select(2) system call with the bit masks specified, which
3876 can be constructed using C<fileno> and C<vec>, along these lines:
3878 $rin = $win = $ein = '';
3879 vec($rin,fileno(STDIN),1) = 1;
3880 vec($win,fileno(STDOUT),1) = 1;
3883 If you want to select on many filehandles you might wish to write a
3887 my(@fhlist) = split(' ',$_[0]);
3890 vec($bits,fileno($_),1) = 1;
3894 $rin = fhbits('STDIN TTY SOCK');
3898 ($nfound,$timeleft) =
3899 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3901 or to block until something becomes ready just do this
3903 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3905 Most systems do not bother to return anything useful in $timeleft, so
3906 calling select() in scalar context just returns $nfound.
3908 Any of the bit masks can also be undef. The timeout, if specified, is
3909 in seconds, which may be fractional. Note: not all implementations are
3910 capable of returning the$timeleft. If not, they always return
3911 $timeleft equal to the supplied $timeout.
3913 You can effect a sleep of 250 milliseconds this way:
3915 select(undef, undef, undef, 0.25);
3917 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3918 or <FH>) with C<select>, except as permitted by POSIX, and even
3919 then only on POSIX systems. You have to use C<sysread> instead.
3921 =item semctl ID,SEMNUM,CMD,ARG
3923 Calls the System V IPC function C<semctl>. You'll probably have to say
3927 first to get the correct constant definitions. If CMD is IPC_STAT or
3928 GETALL, then ARG must be a variable which will hold the returned
3929 semid_ds structure or semaphore value array. Returns like C<ioctl>:
3930 the undefined value for error, "C<0 but true>" for zero, or the actual
3931 return value otherwise. The ARG must consist of a vector of native
3932 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
3933 See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3935 =item semget KEY,NSEMS,FLAGS
3937 Calls the System V IPC function semget. Returns the semaphore id, or
3938 the undefined value if there is an error. See also C<IPC::SysV> and
3939 C<IPC::SysV::Semaphore> documentation.
3941 =item semop KEY,OPSTRING
3943 Calls the System V IPC function semop to perform semaphore operations
3944 such as signaling and waiting. OPSTRING must be a packed array of
3945 semop structures. Each semop structure can be generated with
3946 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3947 operations is implied by the length of OPSTRING. Returns true if
3948 successful, or false if there is an error. As an example, the
3949 following code waits on semaphore $semnum of semaphore id $semid:
3951 $semop = pack("sss", $semnum, -1, 0);
3952 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3954 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3955 and C<IPC::SysV::Semaphore> documentation.
3957 =item send SOCKET,MSG,FLAGS,TO
3959 =item send SOCKET,MSG,FLAGS
3961 Sends a message on a socket. Takes the same flags as the system call
3962 of the same name. On unconnected sockets you must specify a
3963 destination to send TO, in which case it does a C C<sendto>. Returns
3964 the number of characters sent, or the undefined value if there is an
3965 error. The C system call sendmsg(2) is currently unimplemented.
3966 See L<perlipc/"UDP: Message Passing"> for examples.
3968 =item setpgrp PID,PGRP
3970 Sets the current process group for the specified PID, C<0> for the current
3971 process. Will produce a fatal error if used on a machine that doesn't
3972 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3973 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3974 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3977 =item setpriority WHICH,WHO,PRIORITY
3979 Sets the current priority for a process, a process group, or a user.
3980 (See setpriority(2).) Will produce a fatal error if used on a machine
3981 that doesn't implement setpriority(2).
3983 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3985 Sets the socket option requested. Returns undefined if there is an
3986 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3993 Shifts the first value of the array off and returns it, shortening the
3994 array by 1 and moving everything down. If there are no elements in the
3995 array, returns the undefined value. If ARRAY is omitted, shifts the
3996 C<@_> array within the lexical scope of subroutines and formats, and the
3997 C<@ARGV> array at file scopes or within the lexical scopes established by
3998 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4001 See also C<unshift>, C<push>, and C<pop>. C<shift()> and C<unshift> do the
4002 same thing to the left end of an array that C<pop> and C<push> do to the
4005 =item shmctl ID,CMD,ARG
4007 Calls the System V IPC function shmctl. You'll probably have to say
4011 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4012 then ARG must be a variable which will hold the returned C<shmid_ds>
4013 structure. Returns like ioctl: the undefined value for error, "C<0> but
4014 true" for zero, or the actual return value otherwise.
4015 See also C<IPC::SysV> documentation.
4017 =item shmget KEY,SIZE,FLAGS
4019 Calls the System V IPC function shmget. Returns the shared memory
4020 segment id, or the undefined value if there is an error.
4021 See also C<IPC::SysV> documentation.
4023 =item shmread ID,VAR,POS,SIZE
4025 =item shmwrite ID,STRING,POS,SIZE
4027 Reads or writes the System V shared memory segment ID starting at
4028 position POS for size SIZE by attaching to it, copying in/out, and
4029 detaching from it. When reading, VAR must be a variable that will
4030 hold the data read. When writing, if STRING is too long, only SIZE
4031 bytes are used; if STRING is too short, nulls are written to fill out
4032 SIZE bytes. Return true if successful, or false if there is an error.
4033 shmread() taints the variable. See also C<IPC::SysV> documentation and
4034 the C<IPC::Shareable> module from CPAN.
4036 =item shutdown SOCKET,HOW
4038 Shuts down a socket connection in the manner indicated by HOW, which
4039 has the same interpretation as in the system call of the same name.
4041 shutdown(SOCKET, 0); # I/we have stopped reading data
4042 shutdown(SOCKET, 1); # I/we have stopped writing data
4043 shutdown(SOCKET, 2); # I/we have stopped using this socket
4045 This is useful with sockets when you want to tell the other
4046 side you're done writing but not done reading, or vice versa.
4047 It's also a more insistent form of close because it also
4048 disables the file descriptor in any forked copies in other
4055 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4056 returns sine of C<$_>.
4058 For the inverse sine operation, you may use the C<Math::Trig::asin>
4059 function, or use this relation:
4061 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4067 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4068 May be interrupted if the process receives a signal such as C<SIGALRM>.
4069 Returns the number of seconds actually slept. You probably cannot
4070 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4073 On some older systems, it may sleep up to a full second less than what
4074 you requested, depending on how it counts seconds. Most modern systems
4075 always sleep the full amount. They may appear to sleep longer than that,
4076 however, because your process might not be scheduled right away in a
4077 busy multitasking system.
4079 For delays of finer granularity than one second, you may use Perl's
4080 C<syscall> interface to access setitimer(2) if your system supports
4081 it, or else see L</select> above. The Time::HiRes module from CPAN
4084 See also the POSIX module's C<sigpause> function.
4086 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4088 Opens a socket of the specified kind and attaches it to filehandle
4089 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4090 the system call of the same name. You should C<use Socket> first
4091 to get the proper definitions imported. See the examples in
4092 L<perlipc/"Sockets: Client/Server Communication">.
4094 On systems that support a close-on-exec flag on files, the flag will
4095 be set for the newly opened file descriptor, as determined by the
4096 value of $^F. See L<perlvar/$^F>.
4098 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4100 Creates an unnamed pair of sockets in the specified domain, of the
4101 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4102 for the system call of the same name. If unimplemented, yields a fatal
4103 error. Returns true if successful.
4105 On systems that support a close-on-exec flag on files, the flag will
4106 be set for the newly opened file descriptors, as determined by the value
4107 of $^F. See L<perlvar/$^F>.
4109 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4110 to C<pipe(Rdr, Wtr)> is essentially:
4113 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4114 shutdown(Rdr, 1); # no more writing for reader
4115 shutdown(Wtr, 0); # no more reading for writer
4117 See L<perlipc> for an example of socketpair use.
4119 =item sort SUBNAME LIST
4121 =item sort BLOCK LIST
4125 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4126 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4127 specified, it gives the name of a subroutine that returns an integer
4128 less than, equal to, or greater than C<0>, depending on how the elements
4129 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4130 operators are extremely useful in such routines.) SUBNAME may be a
4131 scalar variable name (unsubscripted), in which case the value provides
4132 the name of (or a reference to) the actual subroutine to use. In place
4133 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4136 If the subroutine's prototype is C<($$)>, the elements to be compared
4137 are passed by reference in C<@_>, as for a normal subroutine. This is
4138 slower than unprototyped subroutines, where the elements to be
4139 compared are passed into the subroutine
4140 as the package global variables $a and $b (see example below). Note that
4141 in the latter case, it is usually counter-productive to declare $a and
4144 In either case, the subroutine may not be recursive. The values to be
4145 compared are always passed by reference, so don't modify them.
4147 You also cannot exit out of the sort block or subroutine using any of the
4148 loop control operators described in L<perlsyn> or with C<goto>.
4150 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4151 current collation locale. See L<perllocale>.
4156 @articles = sort @files;
4158 # same thing, but with explicit sort routine
4159 @articles = sort {$a cmp $b} @files;
4161 # now case-insensitively
4162 @articles = sort {uc($a) cmp uc($b)} @files;
4164 # same thing in reversed order
4165 @articles = sort {$b cmp $a} @files;
4167 # sort numerically ascending
4168 @articles = sort {$a <=> $b} @files;
4170 # sort numerically descending
4171 @articles = sort {$b <=> $a} @files;
4173 # this sorts the %age hash by value instead of key
4174 # using an in-line function
4175 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4177 # sort using explicit subroutine name
4179 $age{$a} <=> $age{$b}; # presuming numeric
4181 @sortedclass = sort byage @class;
4183 sub backwards { $b cmp $a }
4184 @harry = qw(dog cat x Cain Abel);
4185 @george = qw(gone chased yz Punished Axed);
4187 # prints AbelCaincatdogx
4188 print sort backwards @harry;
4189 # prints xdogcatCainAbel
4190 print sort @george, 'to', @harry;
4191 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4193 # inefficiently sort by descending numeric compare using
4194 # the first integer after the first = sign, or the
4195 # whole record case-insensitively otherwise
4198 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4203 # same thing, but much more efficiently;
4204 # we'll build auxiliary indices instead
4208 push @nums, /=(\d+)/;
4213 $nums[$b] <=> $nums[$a]
4215 $caps[$a] cmp $caps[$b]
4219 # same thing, but without any temps
4220 @new = map { $_->[0] }
4221 sort { $b->[1] <=> $a->[1]
4224 } map { [$_, /=(\d+)/, uc($_)] } @old;
4226 # using a prototype allows you to use any comparison subroutine
4227 # as a sort subroutine (including other package's subroutines)
4229 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4232 @new = sort other::backwards @old;
4234 If you're using strict, you I<must not> declare $a
4235 and $b as lexicals. They are package globals. That means
4236 if you're in the C<main> package, it's
4238 @articles = sort {$main::b <=> $main::a} @files;
4242 @articles = sort {$::b <=> $::a} @files;
4244 but if you're in the C<FooPack> package, it's
4246 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4248 The comparison function is required to behave. If it returns
4249 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4250 sometimes saying the opposite, for example) the results are not
4253 =item splice ARRAY,OFFSET,LENGTH,LIST
4255 =item splice ARRAY,OFFSET,LENGTH
4257 =item splice ARRAY,OFFSET
4261 Removes the elements designated by OFFSET and LENGTH from an array, and
4262 replaces them with the elements of LIST, if any. In list context,
4263 returns the elements removed from the array. In scalar context,
4264 returns the last element removed, or C<undef> if no elements are
4265 removed. The array grows or shrinks as necessary.
4266 If OFFSET is negative then it starts that far from the end of the array.
4267 If LENGTH is omitted, removes everything from OFFSET onward.
4268 If LENGTH is negative, leaves that many elements off the end of the array.
4269 If both OFFSET and LENGTH are omitted, removes everything.
4271 The following equivalences hold (assuming C<$[ == 0>):
4273 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4274 pop(@a) splice(@a,-1)
4275 shift(@a) splice(@a,0,1)
4276 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4277 $a[$x] = $y splice(@a,$x,1,$y)
4279 Example, assuming array lengths are passed before arrays:
4281 sub aeq { # compare two list values
4282 my(@a) = splice(@_,0,shift);
4283 my(@b) = splice(@_,0,shift);
4284 return 0 unless @a == @b; # same len?
4286 return 0 if pop(@a) ne pop(@b);
4290 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4292 =item split /PATTERN/,EXPR,LIMIT
4294 =item split /PATTERN/,EXPR
4296 =item split /PATTERN/
4300 Splits a string into a list of strings and returns that list. By default,
4301 empty leading fields are preserved, and empty trailing ones are deleted.
4303 If not in list context, returns the number of fields found and splits into
4304 the C<@_> array. (In list context, you can force the split into C<@_> by
4305 using C<??> as the pattern delimiters, but it still returns the list
4306 value.) The use of implicit split to C<@_> is deprecated, however, because
4307 it clobbers your subroutine arguments.
4309 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4310 splits on whitespace (after skipping any leading whitespace). Anything
4311 matching PATTERN is taken to be a delimiter separating the fields. (Note
4312 that the delimiter may be longer than one character.)
4314 If LIMIT is specified and positive, splits into no more than that
4315 many fields (though it may split into fewer). If LIMIT is unspecified
4316 or zero, trailing null fields are stripped (which potential users
4317 of C<pop> would do well to remember). If LIMIT is negative, it is
4318 treated as if an arbitrarily large LIMIT had been specified.
4320 A pattern matching the null string (not to be confused with
4321 a null pattern C<//>, which is just one member of the set of patterns
4322 matching a null string) will split the value of EXPR into separate
4323 characters at each point it matches that way. For example:
4325 print join(':', split(/ */, 'hi there'));
4327 produces the output 'h:i:t:h:e:r:e'.
4329 The LIMIT parameter can be used to split a line partially
4331 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4333 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4334 one larger than the number of variables in the list, to avoid
4335 unnecessary work. For the list above LIMIT would have been 4 by
4336 default. In time critical applications it behooves you not to split
4337 into more fields than you really need.
4339 If the PATTERN contains parentheses, additional list elements are
4340 created from each matching substring in the delimiter.
4342 split(/([,-])/, "1-10,20", 3);
4344 produces the list value
4346 (1, '-', 10, ',', 20)
4348 If you had the entire header of a normal Unix email message in $header,
4349 you could split it up into fields and their values this way:
4351 $header =~ s/\n\s+/ /g; # fix continuation lines
4352 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4354 The pattern C</PATTERN/> may be replaced with an expression to specify
4355 patterns that vary at runtime. (To do runtime compilation only once,
4356 use C</$variable/o>.)
4358 As a special case, specifying a PATTERN of space (C<' '>) will split on
4359 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4360 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4361 will give you as many null initial fields as there are leading spaces.
4362 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4363 whitespace produces a null first field. A C<split> with no arguments
4364 really does a C<split(' ', $_)> internally.
4368 open(PASSWD, '/etc/passwd');
4370 ($login, $passwd, $uid, $gid,
4371 $gcos, $home, $shell) = split(/:/);
4375 (Note that $shell above will still have a newline on it. See L</chop>,
4376 L</chomp>, and L</join>.)
4378 =item sprintf FORMAT, LIST
4380 Returns a string formatted by the usual C<printf> conventions of the
4381 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4382 on your system for an explanation of the general principles.
4384 Perl does its own C<sprintf> formatting--it emulates the C
4385 function C<sprintf>, but it doesn't use it (except for floating-point
4386 numbers, and even then only the standard modifiers are allowed). As a
4387 result, any non-standard extensions in your local C<sprintf> are not
4388 available from Perl.
4390 Perl's C<sprintf> permits the following universally-known conversions:
4393 %c a character with the given number
4395 %d a signed integer, in decimal
4396 %u an unsigned integer, in decimal
4397 %o an unsigned integer, in octal
4398 %x an unsigned integer, in hexadecimal
4399 %e a floating-point number, in scientific notation
4400 %f a floating-point number, in fixed decimal notation
4401 %g a floating-point number, in %e or %f notation
4403 In addition, Perl permits the following widely-supported conversions:
4405 %X like %x, but using upper-case letters
4406 %E like %e, but using an upper-case "E"
4407 %G like %g, but with an upper-case "E" (if applicable)
4408 %b an unsigned integer, in binary
4409 %p a pointer (outputs the Perl value's address in hexadecimal)
4410 %n special: *stores* the number of characters output so far
4411 into the next variable in the parameter list
4413 Finally, for backward (and we do mean "backward") compatibility, Perl
4414 permits these unnecessary but widely-supported conversions:
4417 %D a synonym for %ld
4418 %U a synonym for %lu
4419 %O a synonym for %lo
4422 Perl permits the following universally-known flags between the C<%>
4423 and the conversion letter:
4425 space prefix positive number with a space
4426 + prefix positive number with a plus sign
4427 - left-justify within the field
4428 0 use zeros, not spaces, to right-justify
4429 # prefix non-zero octal with "0", non-zero hex with "0x"
4430 number minimum field width
4431 .number "precision": digits after decimal point for
4432 floating-point, max length for string, minimum length
4434 l interpret integer as C type "long" or "unsigned long"
4435 h interpret integer as C type "short" or "unsigned short"
4436 If no flags, interpret integer as C type "int" or "unsigned"
4438 There are also two Perl-specific flags:
4440 V interpret integer as Perl's standard integer type
4441 v interpret string as a vector of integers, output as
4442 numbers separated either by dots, or by an arbitrary
4443 string received from the argument list when the flag
4446 Where a number would appear in the flags, an asterisk (C<*>) may be
4447 used instead, in which case Perl uses the next item in the parameter
4448 list as the given number (that is, as the field width or precision).
4449 If a field width obtained through C<*> is negative, it has the same
4450 effect as the C<-> flag: left-justification.
4452 The C<v> flag is useful for displaying ordinal values of characters
4453 in arbitrary strings:
4455 printf "version is v%vd\n", $^V; # Perl's version
4456 printf "address is %*vX\n", ":", $addr; # IPv6 address
4457 printf "bits are %*vb\n", " ", $bits; # random bitstring
4459 If C<use locale> is in effect, the character used for the decimal
4460 point in formatted real numbers is affected by the LC_NUMERIC locale.
4463 If Perl understands "quads" (64-bit integers) (this requires
4464 either that the platform natively support quads or that Perl
4465 be specifically compiled to support quads), the characters
4469 print quads, and they may optionally be preceded by
4477 You can find out whether your Perl supports quads via L<Config>:
4480 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4483 If Perl understands "long doubles" (this requires that the platform
4484 support long doubles), the flags
4488 may optionally be preceded by
4496 You can find out whether your Perl supports long doubles via L<Config>:
4499 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4505 Return the square root of EXPR. If EXPR is omitted, returns square
4506 root of C<$_>. Only works on non-negative operands, unless you've
4507 loaded the standard Math::Complex module.
4510 print sqrt(-2); # prints 1.4142135623731i
4516 Sets the random number seed for the C<rand> operator. If EXPR is
4517 omitted, uses a semi-random value supplied by the kernel (if it supports
4518 the F</dev/urandom> device) or based on the current time and process
4519 ID, among other things. In versions of Perl prior to 5.004 the default
4520 seed was just the current C<time>. This isn't a particularly good seed,
4521 so many old programs supply their own seed value (often C<time ^ $$> or
4522 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4524 In fact, it's usually not necessary to call C<srand> at all, because if
4525 it is not called explicitly, it is called implicitly at the first use of
4526 the C<rand> operator. However, this was not the case in version of Perl
4527 before 5.004, so if your script will run under older Perl versions, it
4528 should call C<srand>.
4530 Note that you need something much more random than the default seed for
4531 cryptographic purposes. Checksumming the compressed output of one or more
4532 rapidly changing operating system status programs is the usual method. For
4535 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4537 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4540 Do I<not> call C<srand> multiple times in your program unless you know
4541 exactly what you're doing and why you're doing it. The point of the
4542 function is to "seed" the C<rand> function so that C<rand> can produce
4543 a different sequence each time you run your program. Just do it once at the
4544 top of your program, or you I<won't> get random numbers out of C<rand>!
4546 Frequently called programs (like CGI scripts) that simply use
4550 for a seed can fall prey to the mathematical property that
4554 one-third of the time. So don't do that.
4556 =item stat FILEHANDLE
4562 Returns a 13-element list giving the status info for a file, either
4563 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4564 it stats C<$_>. Returns a null list if the stat fails. Typically used
4567 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4568 $atime,$mtime,$ctime,$blksize,$blocks)
4571 Not all fields are supported on all filesystem types. Here are the
4572 meaning of the fields:
4574 0 dev device number of filesystem
4576 2 mode file mode (type and permissions)
4577 3 nlink number of (hard) links to the file
4578 4 uid numeric user ID of file's owner
4579 5 gid numeric group ID of file's owner
4580 6 rdev the device identifier (special files only)
4581 7 size total size of file, in bytes
4582 8 atime last access time in seconds since the epoch
4583 9 mtime last modify time in seconds since the epoch
4584 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4585 11 blksize preferred block size for file system I/O
4586 12 blocks actual number of blocks allocated
4588 (The epoch was at 00:00 January 1, 1970 GMT.)
4590 If stat is passed the special filehandle consisting of an underline, no
4591 stat is done, but the current contents of the stat structure from the
4592 last stat or filetest are returned. Example:
4594 if (-x $file && (($d) = stat(_)) && $d < 0) {
4595 print "$file is executable NFS file\n";
4598 (This works on machines only for which the device number is negative
4601 Because the mode contains both the file type and its permissions, you
4602 should mask off the file type portion and (s)printf using a C<"%o">
4603 if you want to see the real permissions.
4605 $mode = (stat($filename))[2];
4606 printf "Permissions are %04o\n", $mode & 07777;
4608 In scalar context, C<stat> returns a boolean value indicating success
4609 or failure, and, if successful, sets the information associated with
4610 the special filehandle C<_>.
4612 The File::stat module provides a convenient, by-name access mechanism:
4615 $sb = stat($filename);
4616 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4617 $filename, $sb->size, $sb->mode & 07777,
4618 scalar localtime $sb->mtime;
4620 You can import symbolic mode constants (C<S_IF*>) and functions
4621 (C<S_IS*>) from the Fcntl module:
4625 $mode = (stat($filename))[2];
4627 $user_rwx = ($mode & S_IRWXU) >> 6;
4628 $group_read = ($mode & S_IRGRP) >> 3;
4629 $other_execute = $mode & S_IXOTH;
4631 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4633 $is_setuid = $mode & S_ISUID;
4634 $is_setgid = S_ISDIR($mode);
4636 You could write the last two using the C<-u> and C<-d> operators.
4637 The commonly available S_IF* constants are
4639 # Permissions: read, write, execute, for user, group, others.
4641 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4642 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4643 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4645 # Setuid/Setgid/Stickiness.
4647 S_ISUID S_ISGID S_ISVTX S_ISTXT
4649 # File types. Not necessarily all are available on your system.
4651 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4653 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4655 S_IREAD S_IWRITE S_IEXEC
4657 and the S_IF* functions are
4659 S_IFMODE($mode) the part of $mode containing the permission bits
4660 and the setuid/setgid/sticky bits
4662 S_IFMT($mode) the part of $mode containing the file type
4663 which can be bit-anded with e.g. S_IFREG
4664 or with the following functions
4666 # The operators -f, -d, -l, -b, -c, -p, and -s.
4668 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4669 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4671 # No direct -X operator counterpart, but for the first one
4672 # the -g operator is often equivalent. The ENFMT stands for
4673 # record flocking enforcement, a platform-dependent feature.
4675 S_ISENFMT($mode) S_ISWHT($mode)
4677 See your native chmod(2) and stat(2) documentation for more details
4678 about the S_* constants.
4684 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4685 doing many pattern matches on the string before it is next modified.
4686 This may or may not save time, depending on the nature and number of
4687 patterns you are searching on, and on the distribution of character
4688 frequencies in the string to be searched--you probably want to compare
4689 run times with and without it to see which runs faster. Those loops
4690 which scan for many short constant strings (including the constant
4691 parts of more complex patterns) will benefit most. You may have only
4692 one C<study> active at a time--if you study a different scalar the first
4693 is "unstudied". (The way C<study> works is this: a linked list of every
4694 character in the string to be searched is made, so we know, for
4695 example, where all the C<'k'> characters are. From each search string,
4696 the rarest character is selected, based on some static frequency tables
4697 constructed from some C programs and English text. Only those places
4698 that contain this "rarest" character are examined.)
4700 For example, here is a loop that inserts index producing entries
4701 before any line containing a certain pattern:
4705 print ".IX foo\n" if /\bfoo\b/;
4706 print ".IX bar\n" if /\bbar\b/;
4707 print ".IX blurfl\n" if /\bblurfl\b/;
4712 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4713 will be looked at, because C<f> is rarer than C<o>. In general, this is
4714 a big win except in pathological cases. The only question is whether
4715 it saves you more time than it took to build the linked list in the
4718 Note that if you have to look for strings that you don't know till
4719 runtime, you can build an entire loop as a string and C<eval> that to
4720 avoid recompiling all your patterns all the time. Together with
4721 undefining C<$/> to input entire files as one record, this can be very
4722 fast, often faster than specialized programs like fgrep(1). The following
4723 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4724 out the names of those files that contain a match:
4726 $search = 'while (<>) { study;';
4727 foreach $word (@words) {
4728 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4733 eval $search; # this screams
4734 $/ = "\n"; # put back to normal input delimiter
4735 foreach $file (sort keys(%seen)) {
4743 =item sub NAME BLOCK
4745 This is subroutine definition, not a real function I<per se>. With just a
4746 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4747 Without a NAME, it's an anonymous function declaration, and does actually
4748 return a value: the CODE ref of the closure you just created. See L<perlsub>
4749 and L<perlref> for details.
4751 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4753 =item substr EXPR,OFFSET,LENGTH
4755 =item substr EXPR,OFFSET
4757 Extracts a substring out of EXPR and returns it. First character is at
4758 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4759 If OFFSET is negative (or more precisely, less than C<$[>), starts
4760 that far from the end of the string. If LENGTH is omitted, returns
4761 everything to the end of the string. If LENGTH is negative, leaves that
4762 many characters off the end of the string.
4764 You can use the substr() function as an lvalue, in which case EXPR
4765 must itself be an lvalue. If you assign something shorter than LENGTH,
4766 the string will shrink, and if you assign something longer than LENGTH,
4767 the string will grow to accommodate it. To keep the string the same
4768 length you may need to pad or chop your value using C<sprintf>.
4770 If OFFSET and LENGTH specify a substring that is partly outside the
4771 string, only the part within the string is returned. If the substring
4772 is beyond either end of the string, substr() returns the undefined
4773 value and produces a warning. When used as an lvalue, specifying a
4774 substring that is entirely outside the string is a fatal error.
4775 Here's an example showing the behavior for boundary cases:
4778 substr($name, 4) = 'dy'; # $name is now 'freddy'
4779 my $null = substr $name, 6, 2; # returns '' (no warning)
4780 my $oops = substr $name, 7; # returns undef, with warning
4781 substr($name, 7) = 'gap'; # fatal error
4783 An alternative to using substr() as an lvalue is to specify the
4784 replacement string as the 4th argument. This allows you to replace
4785 parts of the EXPR and return what was there before in one operation,
4786 just as you can with splice().
4788 =item symlink OLDFILE,NEWFILE
4790 Creates a new filename symbolically linked to the old filename.
4791 Returns C<1> for success, C<0> otherwise. On systems that don't support
4792 symbolic links, produces a fatal error at run time. To check for that,
4795 $symlink_exists = eval { symlink("",""); 1 };
4799 Calls the system call specified as the first element of the list,
4800 passing the remaining elements as arguments to the system call. If
4801 unimplemented, produces a fatal error. The arguments are interpreted
4802 as follows: if a given argument is numeric, the argument is passed as
4803 an int. If not, the pointer to the string value is passed. You are
4804 responsible to make sure a string is pre-extended long enough to
4805 receive any result that might be written into a string. You can't use a
4806 string literal (or other read-only string) as an argument to C<syscall>
4807 because Perl has to assume that any string pointer might be written
4809 integer arguments are not literals and have never been interpreted in a
4810 numeric context, you may need to add C<0> to them to force them to look
4811 like numbers. This emulates the C<syswrite> function (or vice versa):
4813 require 'syscall.ph'; # may need to run h2ph
4815 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4817 Note that Perl supports passing of up to only 14 arguments to your system call,
4818 which in practice should usually suffice.
4820 Syscall returns whatever value returned by the system call it calls.
4821 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4822 Note that some system calls can legitimately return C<-1>. The proper
4823 way to handle such calls is to assign C<$!=0;> before the call and
4824 check the value of C<$!> if syscall returns C<-1>.
4826 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4827 number of the read end of the pipe it creates. There is no way
4828 to retrieve the file number of the other end. You can avoid this
4829 problem by using C<pipe> instead.
4831 =item sysopen FILEHANDLE,FILENAME,MODE
4833 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4835 Opens the file whose filename is given by FILENAME, and associates it
4836 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4837 the name of the real filehandle wanted. This function calls the
4838 underlying operating system's C<open> function with the parameters
4839 FILENAME, MODE, PERMS.
4841 The possible values and flag bits of the MODE parameter are
4842 system-dependent; they are available via the standard module C<Fcntl>.
4843 See the documentation of your operating system's C<open> to see which
4844 values and flag bits are available. You may combine several flags
4845 using the C<|>-operator.
4847 Some of the most common values are C<O_RDONLY> for opening the file in
4848 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4849 and C<O_RDWR> for opening the file in read-write mode, and.
4851 For historical reasons, some values work on almost every system
4852 supported by perl: zero means read-only, one means write-only, and two
4853 means read/write. We know that these values do I<not> work under
4854 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4855 use them in new code.
4857 If the file named by FILENAME does not exist and the C<open> call creates
4858 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4859 PERMS specifies the permissions of the newly created file. If you omit
4860 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4861 These permission values need to be in octal, and are modified by your
4862 process's current C<umask>.
4864 In many systems the C<O_EXCL> flag is available for opening files in
4865 exclusive mode. This is B<not> locking: exclusiveness means here that
4866 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4869 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4871 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4872 that takes away the user's option to have a more permissive umask.
4873 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4876 Note that C<sysopen> depends on the fdopen() C library function.
4877 On many UNIX systems, fdopen() is known to fail when file descriptors
4878 exceed a certain value, typically 255. If you need more file
4879 descriptors than that, consider rebuilding Perl to use the C<sfio>
4880 library, or perhaps using the POSIX::open() function.
4882 See L<perlopentut> for a kinder, gentler explanation of opening files.
4884 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4886 =item sysread FILEHANDLE,SCALAR,LENGTH
4888 Attempts to read LENGTH bytes of data into variable SCALAR from the
4889 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4890 so mixing this with other kinds of reads, C<print>, C<write>,
4891 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4892 usually buffers data. Returns the number of bytes actually read, C<0>
4893 at end of file, or undef if there was an error. SCALAR will be grown or
4894 shrunk so that the last byte actually read is the last byte of the
4895 scalar after the read.
4897 An OFFSET may be specified to place the read data at some place in the
4898 string other than the beginning. A negative OFFSET specifies
4899 placement at that many bytes counting backwards from the end of the
4900 string. A positive OFFSET greater than the length of SCALAR results
4901 in the string being padded to the required size with C<"\0"> bytes before
4902 the result of the read is appended.
4904 There is no syseof() function, which is ok, since eof() doesn't work
4905 very well on device files (like ttys) anyway. Use sysread() and check
4906 for a return value for 0 to decide whether you're done.
4908 =item sysseek FILEHANDLE,POSITION,WHENCE
4910 Sets FILEHANDLE's system position using the system call lseek(2). It
4911 bypasses stdio, so mixing this with reads (other than C<sysread>),
4912 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4913 FILEHANDLE may be an expression whose value gives the name of the
4914 filehandle. The values for WHENCE are C<0> to set the new position to
4915 POSITION, C<1> to set the it to the current position plus POSITION,
4916 and C<2> to set it to EOF plus POSITION (typically negative). For
4917 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4918 C<SEEK_END> (start of the file, current position, end of the file)
4919 from the Fcntl module.
4921 Returns the new position, or the undefined value on failure. A position
4922 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4923 true on success and false on failure, yet you can still easily determine
4928 =item system PROGRAM LIST
4930 Does exactly the same thing as C<exec LIST>, except that a fork is
4931 done first, and the parent process waits for the child process to
4932 complete. Note that argument processing varies depending on the
4933 number of arguments. If there is more than one argument in LIST,
4934 or if LIST is an array with more than one value, starts the program
4935 given by the first element of the list with arguments given by the
4936 rest of the list. If there is only one scalar argument, the argument
4937 is checked for shell metacharacters, and if there are any, the
4938 entire argument is passed to the system's command shell for parsing
4939 (this is C</bin/sh -c> on Unix platforms, but varies on other
4940 platforms). If there are no shell metacharacters in the argument,
4941 it is split into words and passed directly to C<execvp>, which is
4944 Beginning with v5.6.0, Perl will attempt to flush all files opened for
4945 output before any operation that may do a fork, but this may not be
4946 supported on some platforms (see L<perlport>). To be safe, you may need
4947 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4948 of C<IO::Handle> on any open handles.
4950 The return value is the exit status of the program as
4951 returned by the C<wait> call. To get the actual exit value divide by
4952 256. See also L</exec>. This is I<not> what you want to use to capture
4953 the output from a command, for that you should use merely backticks or
4954 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4955 indicates a failure to start the program (inspect $! for the reason).
4957 Like C<exec>, C<system> allows you to lie to a program about its name if
4958 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4960 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4961 program they're running doesn't actually interrupt your program.
4963 @args = ("command", "arg1", "arg2");
4965 or die "system @args failed: $?"
4967 You can check all the failure possibilities by inspecting
4970 $exit_value = $? >> 8;
4971 $signal_num = $? & 127;
4972 $dumped_core = $? & 128;
4974 When the arguments get executed via the system shell, results
4975 and return codes will be subject to its quirks and capabilities.
4976 See L<perlop/"`STRING`"> and L</exec> for details.
4978 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4980 =item syswrite FILEHANDLE,SCALAR,LENGTH
4982 =item syswrite FILEHANDLE,SCALAR
4984 Attempts to write LENGTH bytes of data from variable SCALAR to the
4985 specified FILEHANDLE, using the system call write(2). If LENGTH
4986 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4987 this with reads (other than C<sysread())>, C<print>, C<write>,
4988 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4989 usually buffers data. Returns the number of bytes actually written,
4990 or C<undef> if there was an error. If the LENGTH is greater than
4991 the available data in the SCALAR after the OFFSET, only as much
4992 data as is available will be written.
4994 An OFFSET may be specified to write the data from some part of the
4995 string other than the beginning. A negative OFFSET specifies writing
4996 that many bytes counting backwards from the end of the string. In the
4997 case the SCALAR is empty you can use OFFSET but only zero offset.
4999 =item tell FILEHANDLE
5003 Returns the current position for FILEHANDLE. FILEHANDLE may be an
5004 expression whose value gives the name of the actual filehandle. If
5005 FILEHANDLE is omitted, assumes the file last read.
5007 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5009 =item telldir DIRHANDLE
5011 Returns the current position of the C<readdir> routines on DIRHANDLE.
5012 Value may be given to C<seekdir> to access a particular location in a
5013 directory. Has the same caveats about possible directory compaction as
5014 the corresponding system library routine.
5016 =item tie VARIABLE,CLASSNAME,LIST
5018 This function binds a variable to a package class that will provide the
5019 implementation for the variable. VARIABLE is the name of the variable
5020 to be enchanted. CLASSNAME is the name of a class implementing objects
5021 of correct type. Any additional arguments are passed to the C<new>
5022 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5023 or C<TIEHASH>). Typically these are arguments such as might be passed
5024 to the C<dbm_open()> function of C. The object returned by the C<new>
5025 method is also returned by the C<tie> function, which would be useful
5026 if you want to access other methods in CLASSNAME.
5028 Note that functions such as C<keys> and C<values> may return huge lists
5029 when used on large objects, like DBM files. You may prefer to use the
5030 C<each> function to iterate over such. Example:
5032 # print out history file offsets
5034 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5035 while (($key,$val) = each %HIST) {
5036 print $key, ' = ', unpack('L',$val), "\n";
5040 A class implementing a hash should have the following methods:
5042 TIEHASH classname, LIST
5044 STORE this, key, value
5049 NEXTKEY this, lastkey
5052 A class implementing an ordinary array should have the following methods:
5054 TIEARRAY classname, LIST
5056 STORE this, key, value
5058 STORESIZE this, count
5064 SPLICE this, offset, length, LIST
5068 A class implementing a file handle should have the following methods:
5070 TIEHANDLE classname, LIST
5071 READ this, scalar, length, offset
5074 WRITE this, scalar, length, offset
5076 PRINTF this, format, LIST
5080 A class implementing a scalar should have the following methods:
5082 TIESCALAR classname, LIST
5087 Not all methods indicated above need be implemented. See L<perltie>,
5088 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5090 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5091 for you--you need to do that explicitly yourself. See L<DB_File>
5092 or the F<Config> module for interesting C<tie> implementations.
5094 For further details see L<perltie>, L<"tied VARIABLE">.
5098 Returns a reference to the object underlying VARIABLE (the same value
5099 that was originally returned by the C<tie> call that bound the variable
5100 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5105 Returns the number of non-leap seconds since whatever time the system
5106 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5107 and 00:00:00 UTC, January 1, 1970 for most other systems).
5108 Suitable for feeding to C<gmtime> and C<localtime>.
5110 For measuring time in better granularity than one second,
5111 you may use either the Time::HiRes module from CPAN, or
5112 if you have gettimeofday(2), you may be able to use the
5113 C<syscall> interface of Perl, see L<perlfaq8> for details.
5117 Returns a four-element list giving the user and system times, in
5118 seconds, for this process and the children of this process.
5120 ($user,$system,$cuser,$csystem) = times;
5124 The transliteration operator. Same as C<y///>. See L<perlop>.
5126 =item truncate FILEHANDLE,LENGTH
5128 =item truncate EXPR,LENGTH
5130 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5131 specified length. Produces a fatal error if truncate isn't implemented
5132 on your system. Returns true if successful, the undefined value
5139 Returns an uppercased version of EXPR. This is the internal function
5140 implementing the C<\U> escape in double-quoted strings.
5141 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5142 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5143 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5145 If EXPR is omitted, uses C<$_>.
5151 Returns the value of EXPR with the first character
5152 in uppercase (titlecase in Unicode). This is
5153 the internal function implementing the C<\u> escape in double-quoted strings.
5154 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5157 If EXPR is omitted, uses C<$_>.
5163 Sets the umask for the process to EXPR and returns the previous value.
5164 If EXPR is omitted, merely returns the current umask.
5166 The Unix permission C<rwxr-x---> is represented as three sets of three
5167 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5168 and isn't one of the digits). The C<umask> value is such a number
5169 representing disabled permissions bits. The permission (or "mode")
5170 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5171 even if you tell C<sysopen> to create a file with permissions C<0777>,
5172 if your umask is C<0022> then the file will actually be created with
5173 permissions C<0755>. If your C<umask> were C<0027> (group can't
5174 write; others can't read, write, or execute), then passing
5175 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5178 Here's some advice: supply a creation mode of C<0666> for regular
5179 files (in C<sysopen>) and one of C<0777> for directories (in
5180 C<mkdir>) and executable files. This gives users the freedom of
5181 choice: if they want protected files, they might choose process umasks
5182 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5183 Programs should rarely if ever make policy decisions better left to
5184 the user. The exception to this is when writing files that should be
5185 kept private: mail files, web browser cookies, I<.rhosts> files, and
5188 If umask(2) is not implemented on your system and you are trying to
5189 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5190 fatal error at run time. If umask(2) is not implemented and you are
5191 not trying to restrict access for yourself, returns C<undef>.
5193 Remember that a umask is a number, usually given in octal; it is I<not> a
5194 string of octal digits. See also L</oct>, if all you have is a string.
5200 Undefines the value of EXPR, which must be an lvalue. Use only on a
5201 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5202 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5203 will probably not do what you expect on most predefined variables or
5204 DBM list values, so don't do that; see L<delete>.) Always returns the
5205 undefined value. You can omit the EXPR, in which case nothing is
5206 undefined, but you still get an undefined value that you could, for
5207 instance, return from a subroutine, assign to a variable or pass as a
5208 parameter. Examples:
5211 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5215 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5216 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5217 select undef, undef, undef, 0.25;
5218 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5220 Note that this is a unary operator, not a list operator.
5226 Deletes a list of files. Returns the number of files successfully
5229 $cnt = unlink 'a', 'b', 'c';
5233 Note: C<unlink> will not delete directories unless you are superuser and
5234 the B<-U> flag is supplied to Perl. Even if these conditions are
5235 met, be warned that unlinking a directory can inflict damage on your
5236 filesystem. Use C<rmdir> instead.
5238 If LIST is omitted, uses C<$_>.
5240 =item unpack TEMPLATE,EXPR
5242 C<unpack> does the reverse of C<pack>: it takes a string
5243 and expands it out into a list of values.
5244 (In scalar context, it returns merely the first value produced.)
5246 The string is broken into chunks described by the TEMPLATE. Each chunk
5247 is converted separately to a value. Typically, either the string is a result
5248 of C<pack>, or the bytes of the string represent a C structure of some
5251 The TEMPLATE has the same format as in the C<pack> function.
5252 Here's a subroutine that does substring:
5255 my($what,$where,$howmuch) = @_;
5256 unpack("x$where a$howmuch", $what);
5261 sub ordinal { unpack("c",$_[0]); } # same as ord()
5263 In addition to fields allowed in pack(), you may prefix a field with
5264 a %<number> to indicate that
5265 you want a <number>-bit checksum of the items instead of the items
5266 themselves. Default is a 16-bit checksum. Checksum is calculated by
5267 summing numeric values of expanded values (for string fields the sum of
5268 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5270 For example, the following
5271 computes the same number as the System V sum program:
5275 unpack("%32C*",<>) % 65535;
5278 The following efficiently counts the number of set bits in a bit vector:
5280 $setbits = unpack("%32b*", $selectmask);
5282 The C<p> and C<P> formats should be used with care. Since Perl
5283 has no way of checking whether the value passed to C<unpack()>
5284 corresponds to a valid memory location, passing a pointer value that's
5285 not known to be valid is likely to have disastrous consequences.
5287 If the repeat count of a field is larger than what the remainder of
5288 the input string allows, repeat count is decreased. If the input string
5289 is longer than one described by the TEMPLATE, the rest is ignored.
5291 See L</pack> for more examples and notes.
5293 =item untie VARIABLE
5295 Breaks the binding between a variable and a package. (See C<tie>.)
5297 =item unshift ARRAY,LIST
5299 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5300 depending on how you look at it. Prepends list to the front of the
5301 array, and returns the new number of elements in the array.
5303 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5305 Note the LIST is prepended whole, not one element at a time, so the
5306 prepended elements stay in the same order. Use C<reverse> to do the
5309 =item use Module VERSION LIST
5311 =item use Module VERSION
5313 =item use Module LIST
5319 Imports some semantics into the current package from the named module,
5320 generally by aliasing certain subroutine or variable names into your
5321 package. It is exactly equivalent to
5323 BEGIN { require Module; import Module LIST; }
5325 except that Module I<must> be a bareword.
5327 VERSION, which can be specified as a literal of the form v5.6.1, demands
5328 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5329 as recent as that version. (For compatibility with older versions of Perl,
5330 a numeric literal will also be interpreted as VERSION.) If the version
5331 of the running Perl interpreter is less than VERSION, then an error
5332 message is printed and Perl exits immediately without attempting to
5333 parse the rest of the file. Compare with L</require>, which can do a
5334 similar check at run time.
5336 use v5.6.1; # compile time version check
5338 use 5.005_03; # float version allowed for compatibility
5340 This is often useful if you need to check the current Perl version before
5341 C<use>ing library modules that have changed in incompatible ways from
5342 older versions of Perl. (We try not to do this more than we have to.)
5344 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5345 C<require> makes sure the module is loaded into memory if it hasn't been
5346 yet. The C<import> is not a builtin--it's just an ordinary static method
5347 call into the C<Module> package to tell the module to import the list of
5348 features back into the current package. The module can implement its
5349 C<import> method any way it likes, though most modules just choose to
5350 derive their C<import> method via inheritance from the C<Exporter> class that
5351 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5352 method can be found then the call is skipped.
5354 If you don't want your namespace altered, explicitly supply an empty list:
5358 That is exactly equivalent to
5360 BEGIN { require Module }
5362 If the VERSION argument is present between Module and LIST, then the
5363 C<use> will call the VERSION method in class Module with the given
5364 version as an argument. The default VERSION method, inherited from
5365 the UNIVERSAL class, croaks if the given version is larger than the
5366 value of the variable C<$Module::VERSION>.
5368 Again, there is a distinction between omitting LIST (C<import> called
5369 with no arguments) and an explicit empty LIST C<()> (C<import> not
5370 called). Note that there is no comma after VERSION!
5372 Because this is a wide-open interface, pragmas (compiler directives)
5373 are also implemented this way. Currently implemented pragmas are:
5377 use sigtrap qw(SEGV BUS);
5378 use strict qw(subs vars refs);
5379 use subs qw(afunc blurfl);
5380 use warnings qw(all);
5382 Some of these pseudo-modules import semantics into the current
5383 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5384 which import symbols into the current package (which are effective
5385 through the end of the file).
5387 There's a corresponding C<no> command that unimports meanings imported
5388 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5394 If no C<unimport> method can be found the call fails with a fatal error.
5396 See L<perlmod> for a list of standard modules and pragmas.
5400 Changes the access and modification times on each file of a list of
5401 files. The first two elements of the list must be the NUMERICAL access
5402 and modification times, in that order. Returns the number of files
5403 successfully changed. The inode change time of each file is set
5404 to the current time. This code has the same effect as the C<touch>
5405 command if the files already exist:
5409 utime $now, $now, @ARGV;
5413 Returns a list consisting of all the values of the named hash. (In a
5414 scalar context, returns the number of values.) The values are
5415 returned in an apparently random order. The actual random order is
5416 subject to change in future versions of perl, but it is guaranteed to
5417 be the same order as either the C<keys> or C<each> function would
5418 produce on the same (unmodified) hash.
5420 Note that you cannot modify the values of a hash this way, because the
5421 returned list is just a copy. You need to use a hash slice for that,
5422 since it's lvaluable in a way that values() is not.
5424 for (values %hash) { s/foo/bar/g } # FAILS!
5425 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5427 As a side effect, calling values() resets the HASH's internal iterator.
5428 See also C<keys>, C<each>, and C<sort>.
5430 =item vec EXPR,OFFSET,BITS
5432 Treats the string in EXPR as a bit vector made up of elements of
5433 width BITS, and returns the value of the element specified by OFFSET
5434 as an unsigned integer. BITS therefore specifies the number of bits
5435 that are reserved for each element in the bit vector. This must
5436 be a power of two from 1 to 32 (or 64, if your platform supports
5439 If BITS is 8, "elements" coincide with bytes of the input string.
5441 If BITS is 16 or more, bytes of the input string are grouped into chunks
5442 of size BITS/8, and each group is converted to a number as with
5443 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5444 for BITS==64). See L<"pack"> for details.
5446 If bits is 4 or less, the string is broken into bytes, then the bits
5447 of each byte are broken into 8/BITS groups. Bits of a byte are
5448 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5449 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5450 breaking the single input byte C<chr(0x36)> into two groups gives a list
5451 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5453 C<vec> may also be assigned to, in which case parentheses are needed
5454 to give the expression the correct precedence as in
5456 vec($image, $max_x * $x + $y, 8) = 3;
5458 If the selected element is off the end of the string, the value 0 is
5459 returned. If an element off the end of the string is written to,
5460 Perl will first extend the string with sufficiently many zero bytes.
5462 Strings created with C<vec> can also be manipulated with the logical
5463 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5464 vector operation is desired when both operands are strings.
5465 See L<perlop/"Bitwise String Operators">.
5467 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5468 The comments show the string after each step. Note that this code works
5469 in the same way on big-endian or little-endian machines.
5472 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5474 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5475 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5477 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5478 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5479 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5480 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5481 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5482 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5484 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5485 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5486 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5489 To transform a bit vector into a string or list of 0's and 1's, use these:
5491 $bits = unpack("b*", $vector);
5492 @bits = split(//, unpack("b*", $vector));
5494 If you know the exact length in bits, it can be used in place of the C<*>.
5496 Here is an example to illustrate how the bits actually fall in place:
5502 unpack("V",$_) 01234567890123456789012345678901
5503 ------------------------------------------------------------------
5508 for ($shift=0; $shift < $width; ++$shift) {
5509 for ($off=0; $off < 32/$width; ++$off) {
5510 $str = pack("B*", "0"x32);
5511 $bits = (1<<$shift);
5512 vec($str, $off, $width) = $bits;
5513 $res = unpack("b*",$str);
5514 $val = unpack("V", $str);
5521 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5522 $off, $width, $bits, $val, $res
5526 Regardless of the machine architecture on which it is run, the above
5527 example should print the following table:
5530 unpack("V",$_) 01234567890123456789012345678901
5531 ------------------------------------------------------------------
5532 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5533 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5534 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5535 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5536 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5537 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5538 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5539 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5540 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5541 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5542 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5543 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5544 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5545 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5546 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5547 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5548 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5549 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5550 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5551 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5552 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5553 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5554 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5555 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5556 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5557 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5558 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5559 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5560 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5561 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5562 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5563 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5564 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5565 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5566 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5567 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5568 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5569 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5570 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5571 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5572 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5573 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5574 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5575 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5576 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5577 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5578 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5579 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5580 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5581 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5582 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5583 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5584 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5585 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5586 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5587 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5588 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5589 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5590 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5591 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5592 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5593 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5594 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5595 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5596 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5597 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5598 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5599 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5600 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5601 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5602 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5603 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5604 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5605 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5606 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5607 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5608 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5609 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5610 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5611 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5612 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5613 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5614 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5615 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5616 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5617 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5618 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5619 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5620 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5621 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5622 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5623 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5624 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5625 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5626 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5627 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5628 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5629 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5630 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5631 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5632 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5633 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5634 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5635 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5636 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5637 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5638 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5639 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5640 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5641 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5642 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5643 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5644 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5645 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5646 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5647 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5648 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5649 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5650 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5651 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5652 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5653 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5654 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5655 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5656 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5657 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5658 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5659 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5663 Behaves like the wait(2) system call on your system: it waits for a child
5664 process to terminate and returns the pid of the deceased process, or
5665 C<-1> if there are no child processes. The status is returned in C<$?>.
5666 Note that a return value of C<-1> could mean that child processes are
5667 being automatically reaped, as described in L<perlipc>.
5669 =item waitpid PID,FLAGS
5671 Waits for a particular child process to terminate and returns the pid of
5672 the deceased process, or C<-1> if there is no such child process. On some
5673 systems, a value of 0 indicates that there are processes still running.
5674 The status is returned in C<$?>. If you say
5676 use POSIX ":sys_wait_h";
5679 $kid = waitpid(-1,&WNOHANG);
5682 then you can do a non-blocking wait for all pending zombie processes.
5683 Non-blocking wait is available on machines supporting either the
5684 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5685 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5686 system call by remembering the status values of processes that have
5687 exited but have not been harvested by the Perl script yet.)
5689 Note that on some systems, a return value of C<-1> could mean that child
5690 processes are being automatically reaped. See L<perlipc> for details,
5691 and for other examples.
5695 Returns true if the context of the currently executing subroutine is
5696 looking for a list value. Returns false if the context is looking
5697 for a scalar. Returns the undefined value if the context is looking
5698 for no value (void context).
5700 return unless defined wantarray; # don't bother doing more
5701 my @a = complex_calculation();
5702 return wantarray ? @a : "@a";
5704 This function should have been named wantlist() instead.
5708 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5711 If LIST is empty and C<$@> already contains a value (typically from a
5712 previous eval) that value is used after appending C<"\t...caught">
5713 to C<$@>. This is useful for staying almost, but not entirely similar to
5716 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5718 No message is printed if there is a C<$SIG{__WARN__}> handler
5719 installed. It is the handler's responsibility to deal with the message
5720 as it sees fit (like, for instance, converting it into a C<die>). Most
5721 handlers must therefore make arrangements to actually display the
5722 warnings that they are not prepared to deal with, by calling C<warn>
5723 again in the handler. Note that this is quite safe and will not
5724 produce an endless loop, since C<__WARN__> hooks are not called from
5727 You will find this behavior is slightly different from that of
5728 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5729 instead call C<die> again to change it).
5731 Using a C<__WARN__> handler provides a powerful way to silence all
5732 warnings (even the so-called mandatory ones). An example:
5734 # wipe out *all* compile-time warnings
5735 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5737 my $foo = 20; # no warning about duplicate my $foo,
5738 # but hey, you asked for it!
5739 # no compile-time or run-time warnings before here
5742 # run-time warnings enabled after here
5743 warn "\$foo is alive and $foo!"; # does show up
5745 See L<perlvar> for details on setting C<%SIG> entries, and for more
5746 examples. See the Carp module for other kinds of warnings using its
5747 carp() and cluck() functions.
5749 =item write FILEHANDLE
5755 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5756 using the format associated with that file. By default the format for
5757 a file is the one having the same name as the filehandle, but the
5758 format for the current output channel (see the C<select> function) may be set
5759 explicitly by assigning the name of the format to the C<$~> variable.
5761 Top of form processing is handled automatically: if there is
5762 insufficient room on the current page for the formatted record, the
5763 page is advanced by writing a form feed, a special top-of-page format
5764 is used to format the new page header, and then the record is written.
5765 By default the top-of-page format is the name of the filehandle with
5766 "_TOP" appended, but it may be dynamically set to the format of your
5767 choice by assigning the name to the C<$^> variable while the filehandle is
5768 selected. The number of lines remaining on the current page is in
5769 variable C<$->, which can be set to C<0> to force a new page.
5771 If FILEHANDLE is unspecified, output goes to the current default output
5772 channel, which starts out as STDOUT but may be changed by the
5773 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5774 is evaluated and the resulting string is used to look up the name of
5775 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5777 Note that write is I<not> the opposite of C<read>. Unfortunately.
5781 The transliteration operator. Same as C<tr///>. See L<perlop>.