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 an C<eval BLOCK> statement,
543 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
544 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
545 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
546 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
547 was compiled with. The C<$hints> and C<$bitmask> values are subject to
548 change between versions of Perl, and are not meant for external use.
550 Furthermore, when called from within the DB package, caller returns more
551 detailed information: it sets the list variable C<@DB::args> to be the
552 arguments with which the subroutine was invoked.
554 Be aware that the optimizer might have optimized call frames away before
555 C<caller> had a chance to get the information. That means that C<caller(N)>
556 might not return information about the call frame you expect it do, for
557 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
558 previous time C<caller> was called.
562 Changes the working directory to EXPR, if possible. If EXPR is omitted,
563 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
564 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
565 set, C<chdir> does nothing. It returns true upon success, false
566 otherwise. See the example under C<die>.
570 Changes the permissions of a list of files. The first element of the
571 list must be the numerical mode, which should probably be an octal
572 number, and which definitely should I<not> a string of octal digits:
573 C<0644> is okay, C<'0644'> is not. Returns the number of files
574 successfully changed. See also L</oct>, if all you have is a string.
576 $cnt = chmod 0755, 'foo', 'bar';
577 chmod 0755, @executables;
578 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
580 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
581 $mode = 0644; chmod $mode, 'foo'; # this is best
583 You can also import the symbolic C<S_I*> constants from the Fcntl
588 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
589 # This is identical to the chmod 0755 of the above example.
597 This safer version of L</chop> removes any trailing string
598 that corresponds to the current value of C<$/> (also known as
599 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
600 number of characters removed from all its arguments. It's often used to
601 remove the newline from the end of an input record when you're worried
602 that the final record may be missing its newline. When in paragraph
603 mode (C<$/ = "">), it removes all trailing newlines from the string.
604 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
605 a reference to an integer or the like, see L<perlvar>) chomp() won't
607 If VARIABLE is omitted, it chomps C<$_>. Example:
610 chomp; # avoid \n on last field
615 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
617 You can actually chomp anything that's an lvalue, including an assignment:
620 chomp($answer = <STDIN>);
622 If you chomp a list, each element is chomped, and the total number of
623 characters removed is returned.
631 Chops off the last character of a string and returns the character
632 chopped. It's used primarily to remove the newline from the end of an
633 input record, but is much more efficient than C<s/\n//> because it neither
634 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
638 chop; # avoid \n on last field
643 If VARIABLE is a hash, it chops the hash's values, but not its keys.
645 You can actually chop anything that's an lvalue, including an assignment:
648 chop($answer = <STDIN>);
650 If you chop a list, each element is chopped. Only the value of the
651 last C<chop> is returned.
653 Note that C<chop> returns the last character. To return all but the last
654 character, use C<substr($string, 0, -1)>.
658 Changes the owner (and group) of a list of files. The first two
659 elements of the list must be the I<numeric> uid and gid, in that
660 order. A value of -1 in either position is interpreted by most
661 systems to leave that value unchanged. Returns the number of files
662 successfully changed.
664 $cnt = chown $uid, $gid, 'foo', 'bar';
665 chown $uid, $gid, @filenames;
667 Here's an example that looks up nonnumeric uids in the passwd file:
670 chomp($user = <STDIN>);
672 chomp($pattern = <STDIN>);
674 ($login,$pass,$uid,$gid) = getpwnam($user)
675 or die "$user not in passwd file";
677 @ary = glob($pattern); # expand filenames
678 chown $uid, $gid, @ary;
680 On most systems, you are not allowed to change the ownership of the
681 file unless you're the superuser, although you should be able to change
682 the group to any of your secondary groups. On insecure systems, these
683 restrictions may be relaxed, but this is not a portable assumption.
684 On POSIX systems, you can detect this condition this way:
686 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
687 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
693 Returns the character represented by that NUMBER in the character set.
694 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
695 chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>,
696 characters higher than 127 are encoded in Unicode; if you don't want
697 this, temporarily C<use bytes> or use C<pack("C*",...)>
699 For the reverse, use L</ord>.
700 See L<utf8> for more about Unicode.
702 If NUMBER is omitted, uses C<$_>.
704 =item chroot FILENAME
708 This function works like the system call by the same name: it makes the
709 named directory the new root directory for all further pathnames that
710 begin with a C</> by your process and all its children. (It doesn't
711 change your current working directory, which is unaffected.) For security
712 reasons, this call is restricted to the superuser. If FILENAME is
713 omitted, does a C<chroot> to C<$_>.
715 =item close FILEHANDLE
719 Closes the file or pipe associated with the file handle, returning true
720 only if stdio successfully flushes buffers and closes the system file
721 descriptor. Closes the currently selected filehandle if the argument
724 You don't have to close FILEHANDLE if you are immediately going to do
725 another C<open> on it, because C<open> will close it for you. (See
726 C<open>.) However, an explicit C<close> on an input file resets the line
727 counter (C<$.>), while the implicit close done by C<open> does not.
729 If the file handle came from a piped open C<close> will additionally
730 return false if one of the other system calls involved fails or if the
731 program exits with non-zero status. (If the only problem was that the
732 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
733 also waits for the process executing on the pipe to complete, in case you
734 want to look at the output of the pipe afterwards, and
735 implicitly puts the exit status value of that command into C<$?>.
737 Prematurely closing the read end of a pipe (i.e. before the process
738 writing to it at the other end has closed it) will result in a
739 SIGPIPE being delivered to the writer. If the other end can't
740 handle that, be sure to read all the data before closing the pipe.
744 open(OUTPUT, '|sort >foo') # pipe to sort
745 or die "Can't start sort: $!";
746 #... # print stuff to output
747 close OUTPUT # wait for sort to finish
748 or warn $! ? "Error closing sort pipe: $!"
749 : "Exit status $? from sort";
750 open(INPUT, 'foo') # get sort's results
751 or die "Can't open 'foo' for input: $!";
753 FILEHANDLE may be an expression whose value can be used as an indirect
754 filehandle, usually the real filehandle name.
756 =item closedir DIRHANDLE
758 Closes a directory opened by C<opendir> and returns the success of that
761 DIRHANDLE may be an expression whose value can be used as an indirect
762 dirhandle, usually the real dirhandle name.
764 =item connect SOCKET,NAME
766 Attempts to connect to a remote socket, just as the connect system call
767 does. Returns true if it succeeded, false otherwise. NAME should be a
768 packed address of the appropriate type for the socket. See the examples in
769 L<perlipc/"Sockets: Client/Server Communication">.
773 Actually a flow control statement rather than a function. If there is a
774 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
775 C<foreach>), it is always executed just before the conditional is about to
776 be evaluated again, just like the third part of a C<for> loop in C. Thus
777 it can be used to increment a loop variable, even when the loop has been
778 continued via the C<next> statement (which is similar to the C C<continue>
781 C<last>, C<next>, or C<redo> may appear within a C<continue>
782 block. C<last> and C<redo> will behave as if they had been executed within
783 the main block. So will C<next>, but since it will execute a C<continue>
784 block, it may be more entertaining.
787 ### redo always comes here
790 ### next always comes here
792 # then back the top to re-check EXPR
794 ### last always comes here
796 Omitting the C<continue> section is semantically equivalent to using an
797 empty one, logically enough. In that case, C<next> goes directly back
798 to check the condition at the top of the loop.
804 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
805 takes cosine of C<$_>.
807 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
808 function, or use this relation:
810 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
812 =item crypt PLAINTEXT,SALT
814 Encrypts a string exactly like the crypt(3) function in the C library
815 (assuming that you actually have a version there that has not been
816 extirpated as a potential munition). This can prove useful for checking
817 the password file for lousy passwords, amongst other things. Only the
818 guys wearing white hats should do this.
820 Note that C<crypt> is intended to be a one-way function, much like breaking
821 eggs to make an omelette. There is no (known) corresponding decrypt
822 function. As a result, this function isn't all that useful for
823 cryptography. (For that, see your nearby CPAN mirror.)
825 When verifying an existing encrypted string you should use the encrypted
826 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
827 allows your code to work with the standard C<crypt> and with more
828 exotic implementations. When choosing a new salt create a random two
829 character string whose characters come from the set C<[./0-9A-Za-z]>
830 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
832 Here's an example that makes sure that whoever runs this program knows
835 $pwd = (getpwuid($<))[1];
839 chomp($word = <STDIN>);
843 if (crypt($word, $pwd) ne $pwd) {
849 Of course, typing in your own password to whoever asks you
852 The L<crypt> function is unsuitable for encrypting large quantities
853 of data, not least of all because you can't get the information
854 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
855 on your favorite CPAN mirror for a slew of potentially useful
860 [This function has been largely superseded by the C<untie> function.]
862 Breaks the binding between a DBM file and a hash.
864 =item dbmopen HASH,DBNAME,MASK
866 [This function has been largely superseded by the C<tie> function.]
868 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
869 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
870 argument is I<not> a filehandle, even though it looks like one). DBNAME
871 is the name of the database (without the F<.dir> or F<.pag> extension if
872 any). If the database does not exist, it is created with protection
873 specified by MASK (as modified by the C<umask>). If your system supports
874 only the older DBM functions, you may perform only one C<dbmopen> in your
875 program. In older versions of Perl, if your system had neither DBM nor
876 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
879 If you don't have write access to the DBM file, you can only read hash
880 variables, not set them. If you want to test whether you can write,
881 either use file tests or try setting a dummy hash entry inside an C<eval>,
882 which will trap the error.
884 Note that functions such as C<keys> and C<values> may return huge lists
885 when used on large DBM files. You may prefer to use the C<each>
886 function to iterate over large DBM files. Example:
888 # print out history file offsets
889 dbmopen(%HIST,'/usr/lib/news/history',0666);
890 while (($key,$val) = each %HIST) {
891 print $key, ' = ', unpack('L',$val), "\n";
895 See also L<AnyDBM_File> for a more general description of the pros and
896 cons of the various dbm approaches, as well as L<DB_File> for a particularly
899 You can control which DBM library you use by loading that library
900 before you call dbmopen():
903 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
904 or die "Can't open netscape history file: $!";
910 Returns a Boolean value telling whether EXPR has a value other than
911 the undefined value C<undef>. If EXPR is not present, C<$_> will be
914 Many operations return C<undef> to indicate failure, end of file,
915 system error, uninitialized variable, and other exceptional
916 conditions. This function allows you to distinguish C<undef> from
917 other values. (A simple Boolean test will not distinguish among
918 C<undef>, zero, the empty string, and C<"0">, which are all equally
919 false.) Note that since C<undef> is a valid scalar, its presence
920 doesn't I<necessarily> indicate an exceptional condition: C<pop>
921 returns C<undef> when its argument is an empty array, I<or> when the
922 element to return happens to be C<undef>.
924 You may also use C<defined(&func)> to check whether subroutine C<&func>
925 has ever been defined. The return value is unaffected by any forward
926 declarations of C<&foo>.
928 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
929 used to report whether memory for that aggregate has ever been
930 allocated. This behavior may disappear in future versions of Perl.
931 You should instead use a simple test for size:
933 if (@an_array) { print "has array elements\n" }
934 if (%a_hash) { print "has hash members\n" }
936 When used on a hash element, it tells you whether the value is defined,
937 not whether the key exists in the hash. Use L</exists> for the latter
942 print if defined $switch{'D'};
943 print "$val\n" while defined($val = pop(@ary));
944 die "Can't readlink $sym: $!"
945 unless defined($value = readlink $sym);
946 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
947 $debugging = 0 unless defined $debugging;
949 Note: Many folks tend to overuse C<defined>, and then are surprised to
950 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
951 defined values. For example, if you say
955 The pattern match succeeds, and C<$1> is defined, despite the fact that it
956 matched "nothing". But it didn't really match nothing--rather, it
957 matched something that happened to be zero characters long. This is all
958 very above-board and honest. When a function returns an undefined value,
959 it's an admission that it couldn't give you an honest answer. So you
960 should use C<defined> only when you're questioning the integrity of what
961 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
964 See also L</undef>, L</exists>, L</ref>.
968 Given an expression that specifies a hash element, array element, hash slice,
969 or array slice, deletes the specified element(s) from the hash or array.
970 In the case of an array, if the array elements happen to be at the end,
971 the size of the array will shrink to the highest element that tests
972 true for exists() (or 0 if no such element exists).
974 Returns each element so deleted or the undefined value if there was no such
975 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
976 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
977 from a C<tie>d hash or array may not necessarily return anything.
979 Deleting an array element effectively returns that position of the array
980 to its initial, uninitialized state. Subsequently testing for the same
981 element with exists() will return false. Note that deleting array
982 elements in the middle of an array will not shift the index of the ones
983 after them down--use splice() for that. See L</exists>.
985 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
987 foreach $key (keys %HASH) {
991 foreach $index (0 .. $#ARRAY) {
992 delete $ARRAY[$index];
997 delete @HASH{keys %HASH};
999 delete @ARRAY[0 .. $#ARRAY];
1001 But both of these are slower than just assigning the empty list
1002 or undefining %HASH or @ARRAY:
1004 %HASH = (); # completely empty %HASH
1005 undef %HASH; # forget %HASH ever existed
1007 @ARRAY = (); # completely empty @ARRAY
1008 undef @ARRAY; # forget @ARRAY ever existed
1010 Note that the EXPR can be arbitrarily complicated as long as the final
1011 operation is a hash element, array element, hash slice, or array slice
1014 delete $ref->[$x][$y]{$key};
1015 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1017 delete $ref->[$x][$y][$index];
1018 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1022 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1023 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1024 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1025 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1026 an C<eval(),> the error message is stuffed into C<$@> and the
1027 C<eval> is terminated with the undefined value. This makes
1028 C<die> the way to raise an exception.
1030 Equivalent examples:
1032 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1033 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1035 If the value of EXPR does not end in a newline, the current script line
1036 number and input line number (if any) are also printed, and a newline
1037 is supplied. Note that the "input line number" (also known as "chunk")
1038 is subject to whatever notion of "line" happens to be currently in
1039 effect, and is also available as the special variable C<$.>.
1040 See L<perlvar/"$/"> and L<perlvar/"$.">.
1042 Hint: sometimes appending C<", stopped"> to your message
1043 will cause it to make better sense when the string C<"at foo line 123"> is
1044 appended. Suppose you are running script "canasta".
1046 die "/etc/games is no good";
1047 die "/etc/games is no good, stopped";
1049 produce, respectively
1051 /etc/games is no good at canasta line 123.
1052 /etc/games is no good, stopped at canasta line 123.
1054 See also exit(), warn(), and the Carp module.
1056 If LIST is empty and C<$@> already contains a value (typically from a
1057 previous eval) that value is reused after appending C<"\t...propagated">.
1058 This is useful for propagating exceptions:
1061 die unless $@ =~ /Expected exception/;
1063 If C<$@> is empty then the string C<"Died"> is used.
1065 die() can also be called with a reference argument. If this happens to be
1066 trapped within an eval(), $@ contains the reference. This behavior permits
1067 a more elaborate exception handling implementation using objects that
1068 maintain arbitrary state about the nature of the exception. Such a scheme
1069 is sometimes preferable to matching particular string values of $@ using
1070 regular expressions. Here's an example:
1072 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1074 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1075 # handle Some::Module::Exception
1078 # handle all other possible exceptions
1082 Because perl will stringify uncaught exception messages before displaying
1083 them, you may want to overload stringification operations on such custom
1084 exception objects. See L<overload> for details about that.
1086 You can arrange for a callback to be run just before the C<die>
1087 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1088 handler will be called with the error text and can change the error
1089 message, if it sees fit, by calling C<die> again. See
1090 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1091 L<"eval BLOCK"> for some examples. Although this feature was meant
1092 to be run only right before your program was to exit, this is not
1093 currently the case--the C<$SIG{__DIE__}> hook is currently called
1094 even inside eval()ed blocks/strings! If one wants the hook to do
1095 nothing in such situations, put
1099 as the first line of the handler (see L<perlvar/$^S>). Because
1100 this promotes strange action at a distance, this counterintuitive
1101 behavior may be fixed in a future release.
1105 Not really a function. Returns the value of the last command in the
1106 sequence of commands indicated by BLOCK. When modified by a loop
1107 modifier, executes the BLOCK once before testing the loop condition.
1108 (On other statements the loop modifiers test the conditional first.)
1110 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1111 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1112 See L<perlsyn> for alternative strategies.
1114 =item do SUBROUTINE(LIST)
1116 A deprecated form of subroutine call. See L<perlsub>.
1120 Uses the value of EXPR as a filename and executes the contents of the
1121 file as a Perl script. Its primary use is to include subroutines
1122 from a Perl subroutine library.
1128 scalar eval `cat stat.pl`;
1130 except that it's more efficient and concise, keeps track of the current
1131 filename for error messages, searches the @INC libraries, and updates
1132 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1133 variables. It also differs in that code evaluated with C<do FILENAME>
1134 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1135 same, however, in that it does reparse the file every time you call it,
1136 so you probably don't want to do this inside a loop.
1138 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1139 error. If C<do> can read the file but cannot compile it, it
1140 returns undef and sets an error message in C<$@>. If the file is
1141 successfully compiled, C<do> returns the value of the last expression
1144 Note that inclusion of library modules is better done with the
1145 C<use> and C<require> operators, which also do automatic error checking
1146 and raise an exception if there's a problem.
1148 You might like to use C<do> to read in a program configuration
1149 file. Manual error checking can be done this way:
1151 # read in config files: system first, then user
1152 for $file ("/share/prog/defaults.rc",
1153 "$ENV{HOME}/.someprogrc")
1155 unless ($return = do $file) {
1156 warn "couldn't parse $file: $@" if $@;
1157 warn "couldn't do $file: $!" unless defined $return;
1158 warn "couldn't run $file" unless $return;
1166 This function causes an immediate core dump. See also the B<-u>
1167 command-line switch in L<perlrun>, which does the same thing.
1168 Primarily this is so that you can use the B<undump> program (not
1169 supplied) to turn your core dump into an executable binary after
1170 having initialized all your variables at the beginning of the
1171 program. When the new binary is executed it will begin by executing
1172 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1173 Think of it as a goto with an intervening core dump and reincarnation.
1174 If C<LABEL> is omitted, restarts the program from the top.
1176 B<WARNING>: Any files opened at the time of the dump will I<not>
1177 be open any more when the program is reincarnated, with possible
1178 resulting confusion on the part of Perl.
1180 This function is now largely obsolete, partly because it's very
1181 hard to convert a core file into an executable, and because the
1182 real compiler backends for generating portable bytecode and compilable
1183 C code have superseded it.
1185 If you're looking to use L<dump> to speed up your program, consider
1186 generating bytecode or native C code as described in L<perlcc>. If
1187 you're just trying to accelerate a CGI script, consider using the
1188 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1189 You might also consider autoloading or selfloading, which at least
1190 make your program I<appear> to run faster.
1194 When called in list context, returns a 2-element list consisting of the
1195 key and value for the next element of a hash, so that you can iterate over
1196 it. When called in scalar context, returns only the key for the next
1197 element in the hash.
1199 Entries are returned in an apparently random order. The actual random
1200 order is subject to change in future versions of perl, but it is guaranteed
1201 to be in the same order as either the C<keys> or C<values> function
1202 would produce on the same (unmodified) hash.
1204 When the hash is entirely read, a null array is returned in list context
1205 (which when assigned produces a false (C<0>) value), and C<undef> in
1206 scalar context. The next call to C<each> after that will start iterating
1207 again. There is a single iterator for each hash, shared by all C<each>,
1208 C<keys>, and C<values> function calls in the program; it can be reset by
1209 reading all the elements from the hash, or by evaluating C<keys HASH> or
1210 C<values HASH>. If you add or delete elements of a hash while you're
1211 iterating over it, you may get entries skipped or duplicated, so
1212 don't. Exception: It is always safe to delete the item most recently
1213 returned by C<each()>, which means that the following code will work:
1215 while (($key, $value) = each %hash) {
1217 delete $hash{$key}; # This is safe
1220 The following prints out your environment like the printenv(1) program,
1221 only in a different order:
1223 while (($key,$value) = each %ENV) {
1224 print "$key=$value\n";
1227 See also C<keys>, C<values> and C<sort>.
1229 =item eof FILEHANDLE
1235 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1236 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1237 gives the real filehandle. (Note that this function actually
1238 reads a character and then C<ungetc>s it, so isn't very useful in an
1239 interactive context.) Do not read from a terminal file (or call
1240 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1241 as terminals may lose the end-of-file condition if you do.
1243 An C<eof> without an argument uses the last file read. Using C<eof()>
1244 with empty parentheses is very different. It refers to the pseudo file
1245 formed from the files listed on the command line and accessed via the
1246 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1247 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1248 used will cause C<@ARGV> to be examined to determine if input is
1251 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1252 detect the end of each file, C<eof()> will only detect the end of the
1253 last file. Examples:
1255 # reset line numbering on each input file
1257 next if /^\s*#/; # skip comments
1260 close ARGV if eof; # Not eof()!
1263 # insert dashes just before last line of last file
1265 if (eof()) { # check for end of current file
1266 print "--------------\n";
1267 close(ARGV); # close or last; is needed if we
1268 # are reading from the terminal
1273 Practical hint: you almost never need to use C<eof> in Perl, because the
1274 input operators typically return C<undef> when they run out of data, or if
1281 In the first form, the return value of EXPR is parsed and executed as if it
1282 were a little Perl program. The value of the expression (which is itself
1283 determined within scalar context) is first parsed, and if there weren't any
1284 errors, executed in the lexical context of the current Perl program, so
1285 that any variable settings or subroutine and format definitions remain
1286 afterwards. Note that the value is parsed every time the eval executes.
1287 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1288 delay parsing and subsequent execution of the text of EXPR until run time.
1290 In the second form, the code within the BLOCK is parsed only once--at the
1291 same time the code surrounding the eval itself was parsed--and executed
1292 within the context of the current Perl program. This form is typically
1293 used to trap exceptions more efficiently than the first (see below), while
1294 also providing the benefit of checking the code within BLOCK at compile
1297 The final semicolon, if any, may be omitted from the value of EXPR or within
1300 In both forms, the value returned is the value of the last expression
1301 evaluated inside the mini-program; a return statement may be also used, just
1302 as with subroutines. The expression providing the return value is evaluated
1303 in void, scalar, or list context, depending on the context of the eval itself.
1304 See L</wantarray> for more on how the evaluation context can be determined.
1306 If there is a syntax error or runtime error, or a C<die> statement is
1307 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1308 error message. If there was no error, C<$@> is guaranteed to be a null
1309 string. Beware that using C<eval> neither silences perl from printing
1310 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1311 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1312 L</warn> and L<perlvar>.
1314 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1315 determining whether a particular feature (such as C<socket> or C<symlink>)
1316 is implemented. It is also Perl's exception trapping mechanism, where
1317 the die operator is used to raise exceptions.
1319 If the code to be executed doesn't vary, you may use the eval-BLOCK
1320 form to trap run-time errors without incurring the penalty of
1321 recompiling each time. The error, if any, is still returned in C<$@>.
1324 # make divide-by-zero nonfatal
1325 eval { $answer = $a / $b; }; warn $@ if $@;
1327 # same thing, but less efficient
1328 eval '$answer = $a / $b'; warn $@ if $@;
1330 # a compile-time error
1331 eval { $answer = }; # WRONG
1334 eval '$answer ='; # sets $@
1336 Due to the current arguably broken state of C<__DIE__> hooks, when using
1337 the C<eval{}> form as an exception trap in libraries, you may wish not
1338 to trigger any C<__DIE__> hooks that user code may have installed.
1339 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1340 as shown in this example:
1342 # a very private exception trap for divide-by-zero
1343 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1346 This is especially significant, given that C<__DIE__> hooks can call
1347 C<die> again, which has the effect of changing their error messages:
1349 # __DIE__ hooks may modify error messages
1351 local $SIG{'__DIE__'} =
1352 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1353 eval { die "foo lives here" };
1354 print $@ if $@; # prints "bar lives here"
1357 Because this promotes action at a distance, this counterintuitive behavior
1358 may be fixed in a future release.
1360 With an C<eval>, you should be especially careful to remember what's
1361 being looked at when:
1367 eval { $x }; # CASE 4
1369 eval "\$$x++"; # CASE 5
1372 Cases 1 and 2 above behave identically: they run the code contained in
1373 the variable $x. (Although case 2 has misleading double quotes making
1374 the reader wonder what else might be happening (nothing is).) Cases 3
1375 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1376 does nothing but return the value of $x. (Case 4 is preferred for
1377 purely visual reasons, but it also has the advantage of compiling at
1378 compile-time instead of at run-time.) Case 5 is a place where
1379 normally you I<would> like to use double quotes, except that in this
1380 particular situation, you can just use symbolic references instead, as
1383 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1384 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1388 =item exec PROGRAM LIST
1390 The C<exec> function executes a system command I<and never returns>--
1391 use C<system> instead of C<exec> if you want it to return. It fails and
1392 returns false only if the command does not exist I<and> it is executed
1393 directly instead of via your system's command shell (see below).
1395 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1396 warns you if there is a following statement which isn't C<die>, C<warn>,
1397 or C<exit> (if C<-w> is set - but you always do that). If you
1398 I<really> want to follow an C<exec> with some other statement, you
1399 can use one of these styles to avoid the warning:
1401 exec ('foo') or print STDERR "couldn't exec foo: $!";
1402 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1404 If there is more than one argument in LIST, or if LIST is an array
1405 with more than one value, calls execvp(3) with the arguments in LIST.
1406 If there is only one scalar argument or an array with one element in it,
1407 the argument is checked for shell metacharacters, and if there are any,
1408 the entire argument is passed to the system's command shell for parsing
1409 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1410 If there are no shell metacharacters in the argument, it is split into
1411 words and passed directly to C<execvp>, which is more efficient.
1414 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1415 exec "sort $outfile | uniq";
1417 If you don't really want to execute the first argument, but want to lie
1418 to the program you are executing about its own name, you can specify
1419 the program you actually want to run as an "indirect object" (without a
1420 comma) in front of the LIST. (This always forces interpretation of the
1421 LIST as a multivalued list, even if there is only a single scalar in
1424 $shell = '/bin/csh';
1425 exec $shell '-sh'; # pretend it's a login shell
1429 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1431 When the arguments get executed via the system shell, results will
1432 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1435 Using an indirect object with C<exec> or C<system> is also more
1436 secure. This usage (which also works fine with system()) forces
1437 interpretation of the arguments as a multivalued list, even if the
1438 list had just one argument. That way you're safe from the shell
1439 expanding wildcards or splitting up words with whitespace in them.
1441 @args = ( "echo surprise" );
1443 exec @args; # subject to shell escapes
1445 exec { $args[0] } @args; # safe even with one-arg list
1447 The first version, the one without the indirect object, ran the I<echo>
1448 program, passing it C<"surprise"> an argument. The second version
1449 didn't--it tried to run a program literally called I<"echo surprise">,
1450 didn't find it, and set C<$?> to a non-zero value indicating failure.
1452 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1453 output before the exec, but this may not be supported on some platforms
1454 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1455 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1456 open handles in order to avoid lost output.
1458 Note that C<exec> will not call your C<END> blocks, nor will it call
1459 any C<DESTROY> methods in your objects.
1463 Given an expression that specifies a hash element or array element,
1464 returns true if the specified element in the hash or array has ever
1465 been initialized, even if the corresponding value is undefined. The
1466 element is not autovivified if it doesn't exist.
1468 print "Exists\n" if exists $hash{$key};
1469 print "Defined\n" if defined $hash{$key};
1470 print "True\n" if $hash{$key};
1472 print "Exists\n" if exists $array[$index];
1473 print "Defined\n" if defined $array[$index];
1474 print "True\n" if $array[$index];
1476 A hash or array element can be true only if it's defined, and defined if
1477 it exists, but the reverse doesn't necessarily hold true.
1479 Given an expression that specifies the name of a subroutine,
1480 returns true if the specified subroutine has ever been declared, even
1481 if it is undefined. Mentioning a subroutine name for exists or defined
1482 does not count as declaring it.
1484 print "Exists\n" if exists &subroutine;
1485 print "Defined\n" if defined &subroutine;
1487 Note that the EXPR can be arbitrarily complicated as long as the final
1488 operation is a hash or array key lookup or subroutine name:
1490 if (exists $ref->{A}->{B}->{$key}) { }
1491 if (exists $hash{A}{B}{$key}) { }
1493 if (exists $ref->{A}->{B}->[$ix]) { }
1494 if (exists $hash{A}{B}[$ix]) { }
1496 if (exists &{$ref->{A}{B}{$key}}) { }
1498 Although the deepest nested array or hash will not spring into existence
1499 just because its existence was tested, any intervening ones will.
1500 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1501 into existence due to the existence test for the $key element above.
1502 This happens anywhere the arrow operator is used, including even:
1505 if (exists $ref->{"Some key"}) { }
1506 print $ref; # prints HASH(0x80d3d5c)
1508 This surprising autovivification in what does not at first--or even
1509 second--glance appear to be an lvalue context may be fixed in a future
1512 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1513 on how exists() acts when used on a pseudo-hash.
1515 Use of a subroutine call, rather than a subroutine name, as an argument
1516 to exists() is an error.
1519 exists &sub(); # Error
1523 Evaluates EXPR and exits immediately with that value. Example:
1526 exit 0 if $ans =~ /^[Xx]/;
1528 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1529 universally recognized values for EXPR are C<0> for success and C<1>
1530 for error; other values are subject to interpretation depending on the
1531 environment in which the Perl program is running. For example, exiting
1532 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1533 the mailer to return the item undelivered, but that's not true everywhere.
1535 Don't use C<exit> to abort a subroutine if there's any chance that
1536 someone might want to trap whatever error happened. Use C<die> instead,
1537 which can be trapped by an C<eval>.
1539 The exit() function does not always exit immediately. It calls any
1540 defined C<END> routines first, but these C<END> routines may not
1541 themselves abort the exit. Likewise any object destructors that need to
1542 be called are called before the real exit. If this is a problem, you
1543 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1544 See L<perlmod> for details.
1550 Returns I<e> (the natural logarithm base) to the power of EXPR.
1551 If EXPR is omitted, gives C<exp($_)>.
1553 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1555 Implements the fcntl(2) function. You'll probably have to say
1559 first to get the correct constant definitions. Argument processing and
1560 value return works just like C<ioctl> below.
1564 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1565 or die "can't fcntl F_GETFL: $!";
1567 You don't have to check for C<defined> on the return from C<fnctl>.
1568 Like C<ioctl>, it maps a C<0> return from the system call into
1569 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1570 in numeric context. It is also exempt from the normal B<-w> warnings
1571 on improper numeric conversions.
1573 Note that C<fcntl> will produce a fatal error if used on a machine that
1574 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1575 manpage to learn what functions are available on your system.
1577 =item fileno FILEHANDLE
1579 Returns the file descriptor for a filehandle, or undefined if the
1580 filehandle is not open. This is mainly useful for constructing
1581 bitmaps for C<select> and low-level POSIX tty-handling operations.
1582 If FILEHANDLE is an expression, the value is taken as an indirect
1583 filehandle, generally its name.
1585 You can use this to find out whether two handles refer to the
1586 same underlying descriptor:
1588 if (fileno(THIS) == fileno(THAT)) {
1589 print "THIS and THAT are dups\n";
1592 =item flock FILEHANDLE,OPERATION
1594 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1595 for success, false on failure. Produces a fatal error if used on a
1596 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1597 C<flock> is Perl's portable file locking interface, although it locks
1598 only entire files, not records.
1600 Two potentially non-obvious but traditional C<flock> semantics are
1601 that it waits indefinitely until the lock is granted, and that its locks
1602 B<merely advisory>. Such discretionary locks are more flexible, but offer
1603 fewer guarantees. This means that files locked with C<flock> may be
1604 modified by programs that do not also use C<flock>. See L<perlport>,
1605 your port's specific documentation, or your system-specific local manpages
1606 for details. It's best to assume traditional behavior if you're writing
1607 portable programs. (But if you're not, you should as always feel perfectly
1608 free to write for your own system's idiosyncrasies (sometimes called
1609 "features"). Slavish adherence to portability concerns shouldn't get
1610 in the way of your getting your job done.)
1612 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1613 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1614 you can use the symbolic names if you import them from the Fcntl module,
1615 either individually, or as a group using the ':flock' tag. LOCK_SH
1616 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1617 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1618 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1619 waiting for the lock (check the return status to see if you got it).
1621 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1622 before locking or unlocking it.
1624 Note that the emulation built with lockf(3) doesn't provide shared
1625 locks, and it requires that FILEHANDLE be open with write intent. These
1626 are the semantics that lockf(3) implements. Most if not all systems
1627 implement lockf(3) in terms of fcntl(2) locking, though, so the
1628 differing semantics shouldn't bite too many people.
1630 Note also that some versions of C<flock> cannot lock things over the
1631 network; you would need to use the more system-specific C<fcntl> for
1632 that. If you like you can force Perl to ignore your system's flock(2)
1633 function, and so provide its own fcntl(2)-based emulation, by passing
1634 the switch C<-Ud_flock> to the F<Configure> program when you configure
1637 Here's a mailbox appender for BSD systems.
1639 use Fcntl ':flock'; # import LOCK_* constants
1642 flock(MBOX,LOCK_EX);
1643 # and, in case someone appended
1644 # while we were waiting...
1649 flock(MBOX,LOCK_UN);
1652 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1653 or die "Can't open mailbox: $!";
1656 print MBOX $msg,"\n\n";
1659 On systems that support a real flock(), locks are inherited across fork()
1660 calls, whereas those that must resort to the more capricious fcntl()
1661 function lose the locks, making it harder to write servers.
1663 See also L<DB_File> for other flock() examples.
1667 Does a fork(2) system call to create a new process running the
1668 same program at the same point. It returns the child pid to the
1669 parent process, C<0> to the child process, or C<undef> if the fork is
1670 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1671 are shared, while everything else is copied. On most systems supporting
1672 fork(), great care has gone into making it extremely efficient (for
1673 example, using copy-on-write technology on data pages), making it the
1674 dominant paradigm for multitasking over the last few decades.
1676 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1677 output before forking the child process, but this may not be supported
1678 on some platforms (see L<perlport>). To be safe, you may need to set
1679 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1680 C<IO::Handle> on any open handles in order to avoid duplicate output.
1682 If you C<fork> without ever waiting on your children, you will
1683 accumulate zombies. On some systems, you can avoid this by setting
1684 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1685 forking and reaping moribund children.
1687 Note that if your forked child inherits system file descriptors like
1688 STDIN and STDOUT that are actually connected by a pipe or socket, even
1689 if you exit, then the remote server (such as, say, a CGI script or a
1690 backgrounded job launched from a remote shell) won't think you're done.
1691 You should reopen those to F</dev/null> if it's any issue.
1695 Declare a picture format for use by the C<write> function. For
1699 Test: @<<<<<<<< @||||| @>>>>>
1700 $str, $%, '$' . int($num)
1704 $num = $cost/$quantity;
1708 See L<perlform> for many details and examples.
1710 =item formline PICTURE,LIST
1712 This is an internal function used by C<format>s, though you may call it,
1713 too. It formats (see L<perlform>) a list of values according to the
1714 contents of PICTURE, placing the output into the format output
1715 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1716 Eventually, when a C<write> is done, the contents of
1717 C<$^A> are written to some filehandle, but you could also read C<$^A>
1718 yourself and then set C<$^A> back to C<"">. Note that a format typically
1719 does one C<formline> per line of form, but the C<formline> function itself
1720 doesn't care how many newlines are embedded in the PICTURE. This means
1721 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1722 You may therefore need to use multiple formlines to implement a single
1723 record format, just like the format compiler.
1725 Be careful if you put double quotes around the picture, because an C<@>
1726 character may be taken to mean the beginning of an array name.
1727 C<formline> always returns true. See L<perlform> for other examples.
1729 =item getc FILEHANDLE
1733 Returns the next character from the input file attached to FILEHANDLE,
1734 or the undefined value at end of file, or if there was an error.
1735 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1736 efficient. However, it cannot be used by itself to fetch single
1737 characters without waiting for the user to hit enter. For that, try
1738 something more like:
1741 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1744 system "stty", '-icanon', 'eol', "\001";
1750 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1753 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1757 Determination of whether $BSD_STYLE should be set
1758 is left as an exercise to the reader.
1760 The C<POSIX::getattr> function can do this more portably on
1761 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1762 module from your nearest CPAN site; details on CPAN can be found on
1767 Implements the C library function of the same name, which on most
1768 systems returns the current login from F</etc/utmp>, if any. If null,
1771 $login = getlogin || getpwuid($<) || "Kilroy";
1773 Do not consider C<getlogin> for authentication: it is not as
1774 secure as C<getpwuid>.
1776 =item getpeername SOCKET
1778 Returns the packed sockaddr address of other end of the SOCKET connection.
1781 $hersockaddr = getpeername(SOCK);
1782 ($port, $iaddr) = sockaddr_in($hersockaddr);
1783 $herhostname = gethostbyaddr($iaddr, AF_INET);
1784 $herstraddr = inet_ntoa($iaddr);
1788 Returns the current process group for the specified PID. Use
1789 a PID of C<0> to get the current process group for the
1790 current process. Will raise an exception if used on a machine that
1791 doesn't implement getpgrp(2). If PID is omitted, returns process
1792 group of current process. Note that the POSIX version of C<getpgrp>
1793 does not accept a PID argument, so only C<PID==0> is truly portable.
1797 Returns the process id of the parent process.
1799 =item getpriority WHICH,WHO
1801 Returns the current priority for a process, a process group, or a user.
1802 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1803 machine that doesn't implement getpriority(2).
1809 =item gethostbyname NAME
1811 =item getnetbyname NAME
1813 =item getprotobyname NAME
1819 =item getservbyname NAME,PROTO
1821 =item gethostbyaddr ADDR,ADDRTYPE
1823 =item getnetbyaddr ADDR,ADDRTYPE
1825 =item getprotobynumber NUMBER
1827 =item getservbyport PORT,PROTO
1845 =item sethostent STAYOPEN
1847 =item setnetent STAYOPEN
1849 =item setprotoent STAYOPEN
1851 =item setservent STAYOPEN
1865 These routines perform the same functions as their counterparts in the
1866 system library. In list context, the return values from the
1867 various get routines are as follows:
1869 ($name,$passwd,$uid,$gid,
1870 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1871 ($name,$passwd,$gid,$members) = getgr*
1872 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1873 ($name,$aliases,$addrtype,$net) = getnet*
1874 ($name,$aliases,$proto) = getproto*
1875 ($name,$aliases,$port,$proto) = getserv*
1877 (If the entry doesn't exist you get a null list.)
1879 The exact meaning of the $gcos field varies but it usually contains
1880 the real name of the user (as opposed to the login name) and other
1881 information pertaining to the user. Beware, however, that in many
1882 system users are able to change this information and therefore it
1883 cannot be trusted and therefore the $gcos is tainted (see
1884 L<perlsec>). The $passwd and $shell, user's encrypted password and
1885 login shell, are also tainted, because of the same reason.
1887 In scalar context, you get the name, unless the function was a
1888 lookup by name, in which case you get the other thing, whatever it is.
1889 (If the entry doesn't exist you get the undefined value.) For example:
1891 $uid = getpwnam($name);
1892 $name = getpwuid($num);
1894 $gid = getgrnam($name);
1895 $name = getgrgid($num;
1899 In I<getpw*()> the fields $quota, $comment, and $expire are special
1900 cases in the sense that in many systems they are unsupported. If the
1901 $quota is unsupported, it is an empty scalar. If it is supported, it
1902 usually encodes the disk quota. If the $comment field is unsupported,
1903 it is an empty scalar. If it is supported it usually encodes some
1904 administrative comment about the user. In some systems the $quota
1905 field may be $change or $age, fields that have to do with password
1906 aging. In some systems the $comment field may be $class. The $expire
1907 field, if present, encodes the expiration period of the account or the
1908 password. For the availability and the exact meaning of these fields
1909 in your system, please consult your getpwnam(3) documentation and your
1910 F<pwd.h> file. You can also find out from within Perl what your
1911 $quota and $comment fields mean and whether you have the $expire field
1912 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1913 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1914 files are only supported if your vendor has implemented them in the
1915 intuitive fashion that calling the regular C library routines gets the
1916 shadow versions if you're running under privilege or if there exists
1917 the shadow(3) functions as found in System V ( this includes Solaris
1918 and Linux.) Those systems which implement a proprietary shadow password
1919 facility are unlikely to be supported.
1921 The $members value returned by I<getgr*()> is a space separated list of
1922 the login names of the members of the group.
1924 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1925 C, it will be returned to you via C<$?> if the function call fails. The
1926 C<@addrs> value returned by a successful call is a list of the raw
1927 addresses returned by the corresponding system library call. In the
1928 Internet domain, each address is four bytes long and you can unpack it
1929 by saying something like:
1931 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1933 The Socket library makes this slightly easier:
1936 $iaddr = inet_aton("127.1"); # or whatever address
1937 $name = gethostbyaddr($iaddr, AF_INET);
1939 # or going the other way
1940 $straddr = inet_ntoa($iaddr);
1942 If you get tired of remembering which element of the return list
1943 contains which return value, by-name interfaces are provided
1944 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1945 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1946 and C<User::grent>. These override the normal built-ins, supplying
1947 versions that return objects with the appropriate names
1948 for each field. For example:
1952 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1954 Even though it looks like they're the same method calls (uid),
1955 they aren't, because a C<File::stat> object is different from
1956 a C<User::pwent> object.
1958 =item getsockname SOCKET
1960 Returns the packed sockaddr address of this end of the SOCKET connection,
1961 in case you don't know the address because you have several different
1962 IPs that the connection might have come in on.
1965 $mysockaddr = getsockname(SOCK);
1966 ($port, $myaddr) = sockaddr_in($mysockaddr);
1967 printf "Connect to %s [%s]\n",
1968 scalar gethostbyaddr($myaddr, AF_INET),
1971 =item getsockopt SOCKET,LEVEL,OPTNAME
1973 Returns the socket option requested, or undef if there is an error.
1979 Returns the value of EXPR with filename expansions such as the
1980 standard Unix shell F</bin/csh> would do. This is the internal function
1981 implementing the C<< <*.c> >> operator, but you can use it directly.
1982 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1983 discussed in more detail in L<perlop/"I/O Operators">.
1985 Beginning with v5.6.0, this operator is implemented using the standard
1986 C<File::Glob> extension. See L<File::Glob> for details.
1990 Converts a time as returned by the time function to a 8-element list
1991 with the time localized for the standard Greenwich time zone.
1992 Typically used as follows:
1995 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1998 All list elements are numeric, and come straight out of the C `struct
1999 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2000 specified time. $mday is the day of the month, and $mon is the month
2001 itself, in the range C<0..11> with 0 indicating January and 11
2002 indicating December. $year is the number of years since 1900. That
2003 is, $year is C<123> in year 2023. $wday is the day of the week, with
2004 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2005 the year, in the range C<0..364> (or C<0..365> in leap years.)
2007 Note that the $year element is I<not> simply the last two digits of
2008 the year. If you assume it is, then you create non-Y2K-compliant
2009 programs--and you wouldn't want to do that, would you?
2011 The proper way to get a complete 4-digit year is simply:
2015 And to get the last two digits of the year (e.g., '01' in 2001) do:
2017 $year = sprintf("%02d", $year % 100);
2019 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2021 In scalar context, C<gmtime()> returns the ctime(3) value:
2023 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2025 Also see the C<timegm> function provided by the C<Time::Local> module,
2026 and the strftime(3) function available via the POSIX module.
2028 This scalar value is B<not> locale dependent (see L<perllocale>), but
2029 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2030 strftime(3) and mktime(3) functions available via the POSIX module. To
2031 get somewhat similar but locale dependent date strings, set up your
2032 locale environment variables appropriately (please see L<perllocale>)
2033 and try for example:
2035 use POSIX qw(strftime);
2036 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2038 Note that the C<%a> and C<%b> escapes, which represent the short forms
2039 of the day of the week and the month of the year, may not necessarily
2040 be three characters wide in all locales.
2048 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2049 execution there. It may not be used to go into any construct that
2050 requires initialization, such as a subroutine or a C<foreach> loop. It
2051 also can't be used to go into a construct that is optimized away,
2052 or to get out of a block or subroutine given to C<sort>.
2053 It can be used to go almost anywhere else within the dynamic scope,
2054 including out of subroutines, but it's usually better to use some other
2055 construct such as C<last> or C<die>. The author of Perl has never felt the
2056 need to use this form of C<goto> (in Perl, that is--C is another matter).
2058 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2059 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2060 necessarily recommended if you're optimizing for maintainability:
2062 goto ("FOO", "BAR", "GLARCH")[$i];
2064 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2065 In fact, it isn't a goto in the normal sense at all, and doesn't have
2066 the stigma associated with other gotos. Instead, it
2067 substitutes a call to the named subroutine for the currently running
2068 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2069 another subroutine and then pretend that the other subroutine had been
2070 called in the first place (except that any modifications to C<@_>
2071 in the current subroutine are propagated to the other subroutine.)
2072 After the C<goto>, not even C<caller> will be able to tell that this
2073 routine was called first.
2075 NAME needn't be the name of a subroutine; it can be a scalar variable
2076 containing a code reference, or a block which evaluates to a code
2079 =item grep BLOCK LIST
2081 =item grep EXPR,LIST
2083 This is similar in spirit to, but not the same as, grep(1) and its
2084 relatives. In particular, it is not limited to using regular expressions.
2086 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2087 C<$_> to each element) and returns the list value consisting of those
2088 elements for which the expression evaluated to true. In scalar
2089 context, returns the number of times the expression was true.
2091 @foo = grep(!/^#/, @bar); # weed out comments
2095 @foo = grep {!/^#/} @bar; # weed out comments
2097 Note that C<$_> is an alias to the list value, so it can be used to
2098 modify the elements of the LIST. While this is useful and supported,
2099 it can cause bizarre results if the elements of LIST are not variables.
2100 Similarly, grep returns aliases into the original list, much as a for
2101 loop's index variable aliases the list elements. That is, modifying an
2102 element of a list returned by grep (for example, in a C<foreach>, C<map>
2103 or another C<grep>) actually modifies the element in the original list.
2104 This is usually something to be avoided when writing clear code.
2106 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2112 Interprets EXPR as a hex string and returns the corresponding value.
2113 (To convert strings that might start with either 0, 0x, or 0b, see
2114 L</oct>.) If EXPR is omitted, uses C<$_>.
2116 print hex '0xAf'; # prints '175'
2117 print hex 'aF'; # same
2119 Hex strings may only represent integers. Strings that would cause
2120 integer overflow trigger a warning.
2124 There is no builtin C<import> function. It is just an ordinary
2125 method (subroutine) defined (or inherited) by modules that wish to export
2126 names to another module. The C<use> function calls the C<import> method
2127 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2129 =item index STR,SUBSTR,POSITION
2131 =item index STR,SUBSTR
2133 The index function searches for one string within another, but without
2134 the wildcard-like behavior of a full regular-expression pattern match.
2135 It returns the position of the first occurrence of SUBSTR in STR at
2136 or after POSITION. If POSITION is omitted, starts searching from the
2137 beginning of the string. The return value is based at C<0> (or whatever
2138 you've set the C<$[> variable to--but don't do that). If the substring
2139 is not found, returns one less than the base, ordinarily C<-1>.
2145 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2146 You should not use this function for rounding: one because it truncates
2147 towards C<0>, and two because machine representations of floating point
2148 numbers can sometimes produce counterintuitive results. For example,
2149 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2150 because it's really more like -268.99999999999994315658 instead. Usually,
2151 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2152 functions will serve you better than will int().
2154 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2156 Implements the ioctl(2) function. You'll probably first have to say
2158 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2160 to get the correct function definitions. If F<ioctl.ph> doesn't
2161 exist or doesn't have the correct definitions you'll have to roll your
2162 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2163 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2164 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2165 written depending on the FUNCTION--a pointer to the string value of SCALAR
2166 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2167 has no string value but does have a numeric value, that value will be
2168 passed rather than a pointer to the string value. To guarantee this to be
2169 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2170 functions may be needed to manipulate the values of structures used by
2173 The return value of C<ioctl> (and C<fcntl>) is as follows:
2175 if OS returns: then Perl returns:
2177 0 string "0 but true"
2178 anything else that number
2180 Thus Perl returns true on success and false on failure, yet you can
2181 still easily determine the actual value returned by the operating
2184 $retval = ioctl(...) || -1;
2185 printf "System returned %d\n", $retval;
2187 The special string "C<0> but true" is exempt from B<-w> complaints
2188 about improper numeric conversions.
2190 Here's an example of setting a filehandle named C<REMOTE> to be
2191 non-blocking at the system level. You'll have to negotiate C<$|>
2192 on your own, though.
2194 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2196 $flags = fcntl(REMOTE, F_GETFL, 0)
2197 or die "Can't get flags for the socket: $!\n";
2199 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2200 or die "Can't set flags for the socket: $!\n";
2202 =item join EXPR,LIST
2204 Joins the separate strings of LIST into a single string with fields
2205 separated by the value of EXPR, and returns that new string. Example:
2207 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2209 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2210 first argument. Compare L</split>.
2214 Returns a list consisting of all the keys of the named hash. (In
2215 scalar context, returns the number of keys.) The keys are returned in
2216 an apparently random order. The actual random order is subject to
2217 change in future versions of perl, but it is guaranteed to be the same
2218 order as either the C<values> or C<each> function produces (given
2219 that the hash has not been modified). As a side effect, it resets
2222 Here is yet another way to print your environment:
2225 @values = values %ENV;
2227 print pop(@keys), '=', pop(@values), "\n";
2230 or how about sorted by key:
2232 foreach $key (sort(keys %ENV)) {
2233 print $key, '=', $ENV{$key}, "\n";
2236 The returned values are copies of the original keys in the hash, so
2237 modifying them will not affect the original hash. Compare L</values>.
2239 To sort a hash by value, you'll need to use a C<sort> function.
2240 Here's a descending numeric sort of a hash by its values:
2242 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2243 printf "%4d %s\n", $hash{$key}, $key;
2246 As an lvalue C<keys> allows you to increase the number of hash buckets
2247 allocated for the given hash. This can gain you a measure of efficiency if
2248 you know the hash is going to get big. (This is similar to pre-extending
2249 an array by assigning a larger number to $#array.) If you say
2253 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2254 in fact, since it rounds up to the next power of two. These
2255 buckets will be retained even if you do C<%hash = ()>, use C<undef
2256 %hash> if you want to free the storage while C<%hash> is still in scope.
2257 You can't shrink the number of buckets allocated for the hash using
2258 C<keys> in this way (but you needn't worry about doing this by accident,
2259 as trying has no effect).
2261 See also C<each>, C<values> and C<sort>.
2263 =item kill SIGNAL, LIST
2265 Sends a signal to a list of processes. Returns the number of
2266 processes successfully signaled (which is not necessarily the
2267 same as the number actually killed).
2269 $cnt = kill 1, $child1, $child2;
2272 If SIGNAL is zero, no signal is sent to the process. This is a
2273 useful way to check that the process is alive and hasn't changed
2274 its UID. See L<perlport> for notes on the portability of this
2277 Unlike in the shell, if SIGNAL is negative, it kills
2278 process groups instead of processes. (On System V, a negative I<PROCESS>
2279 number will also kill process groups, but that's not portable.) That
2280 means you usually want to use positive not negative signals. You may also
2281 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2287 The C<last> command is like the C<break> statement in C (as used in
2288 loops); it immediately exits the loop in question. If the LABEL is
2289 omitted, the command refers to the innermost enclosing loop. The
2290 C<continue> block, if any, is not executed:
2292 LINE: while (<STDIN>) {
2293 last LINE if /^$/; # exit when done with header
2297 C<last> cannot be used to exit a block which returns a value such as
2298 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2299 a grep() or map() operation.
2301 Note that a block by itself is semantically identical to a loop
2302 that executes once. Thus C<last> can be used to effect an early
2303 exit out of such a block.
2305 See also L</continue> for an illustration of how C<last>, C<next>, and
2312 Returns an lowercased version of EXPR. This is the internal function
2313 implementing the C<\L> escape in double-quoted strings.
2314 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2317 If EXPR is omitted, uses C<$_>.
2323 Returns the value of EXPR with the first character lowercased. This is
2324 the internal function implementing the C<\l> escape in double-quoted strings.
2325 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2327 If EXPR is omitted, uses C<$_>.
2333 Returns the length in characters of the value of EXPR. If EXPR is
2334 omitted, returns length of C<$_>. Note that this cannot be used on
2335 an entire array or hash to find out how many elements these have.
2336 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2338 =item link OLDFILE,NEWFILE
2340 Creates a new filename linked to the old filename. Returns true for
2341 success, false otherwise.
2343 =item listen SOCKET,QUEUESIZE
2345 Does the same thing that the listen system call does. Returns true if
2346 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2350 You really probably want to be using C<my> instead, because C<local> isn't
2351 what most people think of as "local". See
2352 L<perlsub/"Private Variables via my()"> for details.
2354 A local modifies the listed variables to be local to the enclosing
2355 block, file, or eval. If more than one value is listed, the list must
2356 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2357 for details, including issues with tied arrays and hashes.
2359 =item localtime EXPR
2361 Converts a time as returned by the time function to a 9-element list
2362 with the time analyzed for the local time zone. Typically used as
2366 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2369 All list elements are numeric, and come straight out of the C `struct
2370 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2371 specified time. $mday is the day of the month, and $mon is the month
2372 itself, in the range C<0..11> with 0 indicating January and 11
2373 indicating December. $year is the number of years since 1900. That
2374 is, $year is C<123> in year 2023. $wday is the day of the week, with
2375 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2376 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2377 is true if the specified time occurs during daylight savings time,
2380 Note that the $year element is I<not> simply the last two digits of
2381 the year. If you assume it is, then you create non-Y2K-compliant
2382 programs--and you wouldn't want to do that, would you?
2384 The proper way to get a complete 4-digit year is simply:
2388 And to get the last two digits of the year (e.g., '01' in 2001) do:
2390 $year = sprintf("%02d", $year % 100);
2392 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2394 In scalar context, C<localtime()> returns the ctime(3) value:
2396 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2398 This scalar value is B<not> locale dependent, see L<perllocale>, but
2399 instead a Perl builtin. Also see the C<Time::Local> module
2400 (to convert the second, minutes, hours, ... back to seconds since the
2401 stroke of midnight the 1st of January 1970, the value returned by
2402 time()), and the strftime(3) and mktime(3) functions available via the
2403 POSIX module. To get somewhat similar but locale dependent date
2404 strings, set up your locale environment variables appropriately
2405 (please see L<perllocale>) and try for example:
2407 use POSIX qw(strftime);
2408 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2410 Note that the C<%a> and C<%b>, the short forms of the day of the week
2411 and the month of the year, may not necessarily be three characters wide.
2417 This function places an advisory lock on a variable, subroutine,
2418 or referenced object contained in I<THING> until the lock goes out
2419 of scope. This is a built-in function only if your version of Perl
2420 was built with threading enabled, and if you've said C<use Threads>.
2421 Otherwise a user-defined function by this name will be called. See
2428 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2429 returns log of C<$_>. To get the log of another base, use basic algebra:
2430 The base-N log of a number is equal to the natural log of that number
2431 divided by the natural log of N. For example:
2435 return log($n)/log(10);
2438 See also L</exp> for the inverse operation.
2444 Does the same thing as the C<stat> function (including setting the
2445 special C<_> filehandle) but stats a symbolic link instead of the file
2446 the symbolic link points to. If symbolic links are unimplemented on
2447 your system, a normal C<stat> is done.
2449 If EXPR is omitted, stats C<$_>.
2453 The match operator. See L<perlop>.
2455 =item map BLOCK LIST
2459 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2460 C<$_> to each element) and returns the list value composed of the
2461 results of each such evaluation. In scalar context, returns the
2462 total number of elements so generated. Evaluates BLOCK or EXPR in
2463 list context, so each element of LIST may produce zero, one, or
2464 more elements in the returned value.
2466 @chars = map(chr, @nums);
2468 translates a list of numbers to the corresponding characters. And
2470 %hash = map { getkey($_) => $_ } @array;
2472 is just a funny way to write
2475 foreach $_ (@array) {
2476 $hash{getkey($_)} = $_;
2479 Note that C<$_> is an alias to the list value, so it can be used to
2480 modify the elements of the LIST. While this is useful and supported,
2481 it can cause bizarre results if the elements of LIST are not variables.
2482 Using a regular C<foreach> loop for this purpose would be clearer in
2483 most cases. See also L</grep> for an array composed of those items of
2484 the original list for which the BLOCK or EXPR evaluates to true.
2486 =item mkdir FILENAME,MASK
2488 =item mkdir FILENAME
2490 Creates the directory specified by FILENAME, with permissions
2491 specified by MASK (as modified by C<umask>). If it succeeds it
2492 returns true, otherwise it returns false and sets C<$!> (errno).
2493 If omitted, MASK defaults to 0777.
2495 In general, it is better to create directories with permissive MASK,
2496 and let the user modify that with their C<umask>, than it is to supply
2497 a restrictive MASK and give the user no way to be more permissive.
2498 The exceptions to this rule are when the file or directory should be
2499 kept private (mail files, for instance). The perlfunc(1) entry on
2500 C<umask> discusses the choice of MASK in more detail.
2502 =item msgctl ID,CMD,ARG
2504 Calls the System V IPC function msgctl(2). You'll probably have to say
2508 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2509 then ARG must be a variable which will hold the returned C<msqid_ds>
2510 structure. Returns like C<ioctl>: the undefined value for error,
2511 C<"0 but true"> for zero, or the actual return value otherwise. See also
2512 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2514 =item msgget KEY,FLAGS
2516 Calls the System V IPC function msgget(2). Returns the message queue
2517 id, or the undefined value if there is an error. See also
2518 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2520 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2522 Calls the System V IPC function msgrcv to receive a message from
2523 message queue ID into variable VAR with a maximum message size of
2524 SIZE. Note that when a message is received, the message type as a
2525 native long integer will be the first thing in VAR, followed by the
2526 actual message. This packing may be opened with C<unpack("l! a*")>.
2527 Taints the variable. Returns true if successful, or false if there is
2528 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2529 C<IPC::SysV::Msg> documentation.
2531 =item msgsnd ID,MSG,FLAGS
2533 Calls the System V IPC function msgsnd to send the message MSG to the
2534 message queue ID. MSG must begin with the native long integer message
2535 type, and be followed by the length of the actual message, and finally
2536 the message itself. This kind of packing can be achieved with
2537 C<pack("l! a*", $type, $message)>. Returns true if successful,
2538 or false if there is an error. See also C<IPC::SysV>
2539 and C<IPC::SysV::Msg> documentation.
2543 =item my EXPR : ATTRIBUTES
2545 A C<my> declares the listed variables to be local (lexically) to the
2546 enclosing block, file, or C<eval>. If
2547 more than one value is listed, the list must be placed in parentheses. See
2548 L<perlsub/"Private Variables via my()"> for details.
2554 The C<next> command is like the C<continue> statement in C; it starts
2555 the next iteration of the loop:
2557 LINE: while (<STDIN>) {
2558 next LINE if /^#/; # discard comments
2562 Note that if there were a C<continue> block on the above, it would get
2563 executed even on discarded lines. If the LABEL is omitted, the command
2564 refers to the innermost enclosing loop.
2566 C<next> cannot be used to exit a block which returns a value such as
2567 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2568 a grep() or map() operation.
2570 Note that a block by itself is semantically identical to a loop
2571 that executes once. Thus C<next> will exit such a block early.
2573 See also L</continue> for an illustration of how C<last>, C<next>, and
2576 =item no Module LIST
2578 See the L</use> function, which C<no> is the opposite of.
2584 Interprets EXPR as an octal string and returns the corresponding
2585 value. (If EXPR happens to start off with C<0x>, interprets it as a
2586 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2587 binary string.) The following will handle decimal, binary, octal, and
2588 hex in the standard Perl or C notation:
2590 $val = oct($val) if $val =~ /^0/;
2592 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2593 in octal), use sprintf() or printf():
2595 $perms = (stat("filename"))[2] & 07777;
2596 $oct_perms = sprintf "%lo", $perms;
2598 The oct() function is commonly used when a string such as C<644> needs
2599 to be converted into a file mode, for example. (Although perl will
2600 automatically convert strings into numbers as needed, this automatic
2601 conversion assumes base 10.)
2603 =item open FILEHANDLE,MODE,LIST
2605 =item open FILEHANDLE,EXPR
2607 =item open FILEHANDLE
2609 Opens the file whose filename is given by EXPR, and associates it with
2610 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2611 name of the real filehandle wanted. (This is considered a symbolic
2612 reference, so C<use strict 'refs'> should I<not> be in effect.)
2614 If EXPR is omitted, the scalar
2615 variable of the same name as the FILEHANDLE contains the filename.
2616 (Note that lexical variables--those declared with C<my>--will not work
2617 for this purpose; so if you're using C<my>, specify EXPR in your call
2618 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2621 If MODE is C<< '<' >> or nothing, the file is opened for input.
2622 If MODE is C<< '>' >>, the file is truncated and opened for
2623 output, being created if necessary. If MODE is C<<< '>>' >>>,
2624 the file is opened for appending, again being created if necessary.
2625 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2626 you want both read and write access to the file; thus C<< '+<' >> is almost
2627 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2628 file first. You can't usually use either read-write mode for updating
2629 textfiles, since they have variable length records. See the B<-i>
2630 switch in L<perlrun> for a better approach. The file is created with
2631 permissions of C<0666> modified by the process' C<umask> value.
2633 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2634 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2636 In the 2-arguments (and 1-argument) form of the call the mode and
2637 filename should be concatenated (in this order), possibly separated by
2638 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2640 If the filename begins with C<'|'>, the filename is interpreted as a
2641 command to which output is to be piped, and if the filename ends with a
2642 C<'|'>, the filename is interpreted as a command which pipes output to
2643 us. See L<perlipc/"Using open() for IPC">
2644 for more examples of this. (You are not allowed to C<open> to a command
2645 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2646 and L<perlipc/"Bidirectional Communication with Another Process">
2649 If MODE is C<'|-'>, the filename is interpreted as a
2650 command to which output is to be piped, and if MODE is
2651 C<'-|'>, the filename is interpreted as a command which pipes output to
2652 us. In the 2-arguments (and 1-argument) form one should replace dash
2653 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2654 for more examples of this. (You are not allowed to C<open> to a command
2655 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2656 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2658 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2659 and opening C<< '>-' >> opens STDOUT.
2662 nonzero upon success, the undefined value otherwise. If the C<open>
2663 involved a pipe, the return value happens to be the pid of the
2666 If you're unfortunate enough to be running Perl on a system that
2667 distinguishes between text files and binary files (modern operating
2668 systems don't care), then you should check out L</binmode> for tips for
2669 dealing with this. The key distinction between systems that need C<binmode>
2670 and those that don't is their text file formats. Systems like Unix, MacOS, and
2671 Plan9, which delimit lines with a single character, and which encode that
2672 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2674 When opening a file, it's usually a bad idea to continue normal execution
2675 if the request failed, so C<open> is frequently used in connection with
2676 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2677 where you want to make a nicely formatted error message (but there are
2678 modules that can help with that problem)) you should always check
2679 the return value from opening a file. The infrequent exception is when
2680 working with an unopened filehandle is actually what you want to do.
2685 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2686 while (<ARTICLE>) {...
2688 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2689 # if the open fails, output is discarded
2691 open(DBASE, '+<', 'dbase.mine') # open for update
2692 or die "Can't open 'dbase.mine' for update: $!";
2694 open(DBASE, '+<dbase.mine') # ditto
2695 or die "Can't open 'dbase.mine' for update: $!";
2697 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2698 or die "Can't start caesar: $!";
2700 open(ARTICLE, "caesar <$article |") # ditto
2701 or die "Can't start caesar: $!";
2703 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2704 or die "Can't start sort: $!";
2706 # process argument list of files along with any includes
2708 foreach $file (@ARGV) {
2709 process($file, 'fh00');
2713 my($filename, $input) = @_;
2714 $input++; # this is a string increment
2715 unless (open($input, $filename)) {
2716 print STDERR "Can't open $filename: $!\n";
2721 while (<$input>) { # note use of indirection
2722 if (/^#include "(.*)"/) {
2723 process($1, $input);
2730 You may also, in the Bourne shell tradition, specify an EXPR beginning
2731 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2732 name of a filehandle (or file descriptor, if numeric) to be
2733 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2734 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2735 mode you specify should match the mode of the original filehandle.
2736 (Duping a filehandle does not take into account any existing contents of
2737 stdio buffers.) Duping file handles is not yet supported for 3-argument
2740 Here is a script that saves, redirects, and restores STDOUT and
2744 open(OLDOUT, ">&STDOUT");
2745 open(OLDERR, ">&STDERR");
2747 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2748 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2750 select(STDERR); $| = 1; # make unbuffered
2751 select(STDOUT); $| = 1; # make unbuffered
2753 print STDOUT "stdout 1\n"; # this works for
2754 print STDERR "stderr 1\n"; # subprocesses too
2759 open(STDOUT, ">&OLDOUT");
2760 open(STDERR, ">&OLDERR");
2762 print STDOUT "stdout 2\n";
2763 print STDERR "stderr 2\n";
2765 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2766 equivalent of C's C<fdopen> of that file descriptor; this is more
2767 parsimonious of file descriptors. For example:
2769 open(FILEHANDLE, "<&=$fd")
2771 Note that this feature depends on the fdopen() C library function.
2772 On many UNIX systems, fdopen() is known to fail when file descriptors
2773 exceed a certain value, typically 255. If you need more file
2774 descriptors than that, consider rebuilding Perl to use the C<sfio>
2777 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2778 with 2-arguments (or 1-argument) form of open(), then
2779 there is an implicit fork done, and the return value of open is the pid
2780 of the child within the parent process, and C<0> within the child
2781 process. (Use C<defined($pid)> to determine whether the open was successful.)
2782 The filehandle behaves normally for the parent, but i/o to that
2783 filehandle is piped from/to the STDOUT/STDIN of the child process.
2784 In the child process the filehandle isn't opened--i/o happens from/to
2785 the new STDOUT or STDIN. Typically this is used like the normal
2786 piped open when you want to exercise more control over just how the
2787 pipe command gets executed, such as when you are running setuid, and
2788 don't want to have to scan shell commands for metacharacters.
2789 The following triples are more or less equivalent:
2791 open(FOO, "|tr '[a-z]' '[A-Z]'");
2792 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2793 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2795 open(FOO, "cat -n '$file'|");
2796 open(FOO, '-|', "cat -n '$file'");
2797 open(FOO, '-|') || exec 'cat', '-n', $file;
2799 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2801 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2802 output before any operation that may do a fork, but this may not be
2803 supported on some platforms (see L<perlport>). To be safe, you may need
2804 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2805 of C<IO::Handle> on any open handles.
2807 On systems that support a
2808 close-on-exec flag on files, the flag will be set for the newly opened
2809 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2811 Closing any piped filehandle causes the parent process to wait for the
2812 child to finish, and returns the status value in C<$?>.
2814 The filename passed to 2-argument (or 1-argument) form of open()
2815 will have leading and trailing
2816 whitespace deleted, and the normal redirection characters
2817 honored. This property, known as "magic open",
2818 can often be used to good effect. A user could specify a filename of
2819 F<"rsh cat file |">, or you could change certain filenames as needed:
2821 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2822 open(FH, $filename) or die "Can't open $filename: $!";
2824 Use 3-argument form to open a file with arbitrary weird characters in it,
2826 open(FOO, '<', $file);
2828 otherwise it's necessary to protect any leading and trailing whitespace:
2830 $file =~ s#^(\s)#./$1#;
2831 open(FOO, "< $file\0");
2833 (this may not work on some bizarre filesystems). One should
2834 conscientiously choose between the I<magic> and 3-arguments form
2839 will allow the user to specify an argument of the form C<"rsh cat file |">,
2840 but will not work on a filename which happens to have a trailing space, while
2842 open IN, '<', $ARGV[0];
2844 will have exactly the opposite restrictions.
2846 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2847 should use the C<sysopen> function, which involves no such magic (but
2848 may use subtly different filemodes than Perl open(), which is mapped
2849 to C fopen()). This is
2850 another way to protect your filenames from interpretation. For example:
2853 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2854 or die "sysopen $path: $!";
2855 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2856 print HANDLE "stuff $$\n");
2858 print "File contains: ", <HANDLE>;
2860 Using the constructor from the C<IO::Handle> package (or one of its
2861 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2862 filehandles that have the scope of whatever variables hold references to
2863 them, and automatically close whenever and however you leave that scope:
2867 sub read_myfile_munged {
2869 my $handle = new IO::File;
2870 open($handle, "myfile") or die "myfile: $!";
2872 or return (); # Automatically closed here.
2873 mung $first or die "mung failed"; # Or here.
2874 return $first, <$handle> if $ALL; # Or here.
2878 See L</seek> for some details about mixing reading and writing.
2880 =item opendir DIRHANDLE,EXPR
2882 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2883 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2884 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2890 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2891 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2892 See L<utf8> for more about Unicode.
2896 An C<our> declares the listed variables to be valid globals within
2897 the enclosing block, file, or C<eval>. That is, it has the same
2898 scoping rules as a "my" declaration, but does not create a local
2899 variable. If more than one value is listed, the list must be placed
2900 in parentheses. The C<our> declaration has no semantic effect unless
2901 "use strict vars" is in effect, in which case it lets you use the
2902 declared global variable without qualifying it with a package name.
2903 (But only within the lexical scope of the C<our> declaration. In this
2904 it differs from "use vars", which is package scoped.)
2906 An C<our> declaration declares a global variable that will be visible
2907 across its entire lexical scope, even across package boundaries. The
2908 package in which the variable is entered is determined at the point
2909 of the declaration, not at the point of use. This means the following
2913 our $bar; # declares $Foo::bar for rest of lexical scope
2917 print $bar; # prints 20
2919 Multiple C<our> declarations in the same lexical scope are allowed
2920 if they are in different packages. If they happened to be in the same
2921 package, Perl will emit warnings if you have asked for them.
2925 our $bar; # declares $Foo::bar for rest of lexical scope
2929 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2930 print $bar; # prints 30
2932 our $bar; # emits warning
2934 =item pack TEMPLATE,LIST
2936 Takes a LIST of values and converts it into a string using the rules
2937 given by the TEMPLATE. The resulting string is the concatenation of
2938 the converted values. Typically, each converted value looks
2939 like its machine-level representation. For example, on 32-bit machines
2940 a converted integer may be represented by a sequence of 4 bytes.
2943 sequence of characters that give the order and type of values, as
2946 a A string with arbitrary binary data, will be null padded.
2947 A An ASCII string, will be space padded.
2948 Z A null terminated (asciz) string, will be null padded.
2950 b A bit string (ascending bit order inside each byte, like vec()).
2951 B A bit string (descending bit order inside each byte).
2952 h A hex string (low nybble first).
2953 H A hex string (high nybble first).
2955 c A signed char value.
2956 C An unsigned char value. Only does bytes. See U for Unicode.
2958 s A signed short value.
2959 S An unsigned short value.
2960 (This 'short' is _exactly_ 16 bits, which may differ from
2961 what a local C compiler calls 'short'. If you want
2962 native-length shorts, use the '!' suffix.)
2964 i A signed integer value.
2965 I An unsigned integer value.
2966 (This 'integer' is _at_least_ 32 bits wide. Its exact
2967 size depends on what a local C compiler calls 'int',
2968 and may even be larger than the 'long' described in
2971 l A signed long value.
2972 L An unsigned long value.
2973 (This 'long' is _exactly_ 32 bits, which may differ from
2974 what a local C compiler calls 'long'. If you want
2975 native-length longs, use the '!' suffix.)
2977 n An unsigned short in "network" (big-endian) order.
2978 N An unsigned long in "network" (big-endian) order.
2979 v An unsigned short in "VAX" (little-endian) order.
2980 V An unsigned long in "VAX" (little-endian) order.
2981 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2982 _exactly_ 32 bits, respectively.)
2984 q A signed quad (64-bit) value.
2985 Q An unsigned quad value.
2986 (Quads are available only if your system supports 64-bit
2987 integer values _and_ if Perl has been compiled to support those.
2988 Causes a fatal error otherwise.)
2990 f A single-precision float in the native format.
2991 d A double-precision float in the native format.
2993 p A pointer to a null-terminated string.
2994 P A pointer to a structure (fixed-length string).
2996 u A uuencoded string.
2997 U A Unicode character number. Encodes to UTF-8 internally.
2998 Works even if C<use utf8> is not in effect.
3000 w A BER compressed integer. Its bytes represent an unsigned
3001 integer in base 128, most significant digit first, with as
3002 few digits as possible. Bit eight (the high bit) is set
3003 on each byte except the last.
3007 @ Null fill to absolute position.
3009 The following rules apply:
3015 Each letter may optionally be followed by a number giving a repeat
3016 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3017 C<H>, and C<P> the pack function will gobble up that many values from
3018 the LIST. A C<*> for the repeat count means to use however many items are
3019 left, except for C<@>, C<x>, C<X>, where it is equivalent
3020 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3023 When used with C<Z>, C<*> results in the addition of a trailing null
3024 byte (so the packed result will be one longer than the byte C<length>
3027 The repeat count for C<u> is interpreted as the maximal number of bytes
3028 to encode per line of output, with 0 and 1 replaced by 45.
3032 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3033 string of length count, padding with nulls or spaces as necessary. When
3034 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3035 after the first null, and C<a> returns data verbatim. When packing,
3036 C<a>, and C<Z> are equivalent.
3038 If the value-to-pack is too long, it is truncated. If too long and an
3039 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3040 by a null byte. Thus C<Z> always packs a trailing null byte under
3045 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3046 Each byte of the input field of pack() generates 1 bit of the result.
3047 Each result bit is based on the least-significant bit of the corresponding
3048 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3049 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3051 Starting from the beginning of the input string of pack(), each 8-tuple
3052 of bytes is converted to 1 byte of output. With format C<b>
3053 the first byte of the 8-tuple determines the least-significant bit of a
3054 byte, and with format C<B> it determines the most-significant bit of
3057 If the length of the input string is not exactly divisible by 8, the
3058 remainder is packed as if the input string were padded by null bytes
3059 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3061 If the input string of pack() is longer than needed, extra bytes are ignored.
3062 A C<*> for the repeat count of pack() means to use all the bytes of
3063 the input field. On unpack()ing the bits are converted to a string
3064 of C<"0">s and C<"1">s.
3068 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3069 representable as hexadecimal digits, 0-9a-f) long.
3071 Each byte of the input field of pack() generates 4 bits of the result.
3072 For non-alphabetical bytes the result is based on the 4 least-significant
3073 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3074 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3075 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3076 is compatible with the usual hexadecimal digits, so that C<"a"> and
3077 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3078 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3080 Starting from the beginning of the input string of pack(), each pair
3081 of bytes is converted to 1 byte of output. With format C<h> the
3082 first byte of the pair determines the least-significant nybble of the
3083 output byte, and with format C<H> it determines the most-significant
3086 If the length of the input string is not even, it behaves as if padded
3087 by a null byte at the end. Similarly, during unpack()ing the "extra"
3088 nybbles are ignored.
3090 If the input string of pack() is longer than needed, extra bytes are ignored.
3091 A C<*> for the repeat count of pack() means to use all the bytes of
3092 the input field. On unpack()ing the bits are converted to a string
3093 of hexadecimal digits.
3097 The C<p> type packs a pointer to a null-terminated string. You are
3098 responsible for ensuring the string is not a temporary value (which can
3099 potentially get deallocated before you get around to using the packed result).
3100 The C<P> type packs a pointer to a structure of the size indicated by the
3101 length. A NULL pointer is created if the corresponding value for C<p> or
3102 C<P> is C<undef>, similarly for unpack().
3106 The C</> template character allows packing and unpacking of strings where
3107 the packed structure contains a byte count followed by the string itself.
3108 You write I<length-item>C</>I<string-item>.
3110 The I<length-item> can be any C<pack> template letter,
3111 and describes how the length value is packed.
3112 The ones likely to be of most use are integer-packing ones like
3113 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3114 and C<N> (for Sun XDR).
3116 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3117 For C<unpack> the length of the string is obtained from the I<length-item>,
3118 but if you put in the '*' it will be ignored.
3120 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3121 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3122 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3124 The I<length-item> is not returned explicitly from C<unpack>.
3126 Adding a count to the I<length-item> letter is unlikely to do anything
3127 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3128 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3129 which Perl does not regard as legal in numeric strings.
3133 The integer types C<s>, C<S>, C<l>, and C<L> may be
3134 immediately followed by a C<!> suffix to signify native shorts or
3135 longs--as you can see from above for example a bare C<l> does mean
3136 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3137 may be larger. This is an issue mainly in 64-bit platforms. You can
3138 see whether using C<!> makes any difference by
3140 print length(pack("s")), " ", length(pack("s!")), "\n";
3141 print length(pack("l")), " ", length(pack("l!")), "\n";
3143 C<i!> and C<I!> also work but only because of completeness;
3144 they are identical to C<i> and C<I>.
3146 The actual sizes (in bytes) of native shorts, ints, longs, and long
3147 longs on the platform where Perl was built are also available via
3151 print $Config{shortsize}, "\n";
3152 print $Config{intsize}, "\n";
3153 print $Config{longsize}, "\n";
3154 print $Config{longlongsize}, "\n";
3156 (The C<$Config{longlongsize}> will be undefine if your system does
3157 not support long longs.)
3161 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3162 are inherently non-portable between processors and operating systems
3163 because they obey the native byteorder and endianness. For example a
3164 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3165 (arranged in and handled by the CPU registers) into bytes as
3167 0x12 0x34 0x56 0x78 # big-endian
3168 0x78 0x56 0x34 0x12 # little-endian
3170 Basically, the Intel and VAX CPUs are little-endian, while everybody
3171 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3172 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3173 used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode.
3175 The names `big-endian' and `little-endian' are comic references to
3176 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3177 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3178 the egg-eating habits of the Lilliputians.
3180 Some systems may have even weirder byte orders such as
3185 You can see your system's preference with
3187 print join(" ", map { sprintf "%#02x", $_ }
3188 unpack("C*",pack("L",0x12345678))), "\n";
3190 The byteorder on the platform where Perl was built is also available
3194 print $Config{byteorder}, "\n";
3196 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3197 and C<'87654321'> are big-endian.
3199 If you want portable packed integers use the formats C<n>, C<N>,
3200 C<v>, and C<V>, their byte endianness and size is known.
3201 See also L<perlport>.
3205 Real numbers (floats and doubles) are in the native machine format only;
3206 due to the multiplicity of floating formats around, and the lack of a
3207 standard "network" representation, no facility for interchange has been
3208 made. This means that packed floating point data written on one machine
3209 may not be readable on another - even if both use IEEE floating point
3210 arithmetic (as the endian-ness of the memory representation is not part
3211 of the IEEE spec). See also L<perlport>.
3213 Note that Perl uses doubles internally for all numeric calculation, and
3214 converting from double into float and thence back to double again will
3215 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3220 If the pattern begins with a C<U>, the resulting string will be treated
3221 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3222 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3223 characters. If you don't want this to happen, you can begin your pattern
3224 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3225 string, and then follow this with a C<U*> somewhere in your pattern.
3229 You must yourself do any alignment or padding by inserting for example
3230 enough C<'x'>es while packing. There is no way to pack() and unpack()
3231 could know where the bytes are going to or coming from. Therefore
3232 C<pack> (and C<unpack>) handle their output and input as flat
3237 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3241 If TEMPLATE requires more arguments to pack() than actually given, pack()
3242 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3243 to pack() than actually given, extra arguments are ignored.
3249 $foo = pack("CCCC",65,66,67,68);
3251 $foo = pack("C4",65,66,67,68);
3253 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3254 # same thing with Unicode circled letters
3256 $foo = pack("ccxxcc",65,66,67,68);
3259 # note: the above examples featuring "C" and "c" are true
3260 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3261 # and UTF-8. In EBCDIC the first example would be
3262 # $foo = pack("CCCC",193,194,195,196);
3264 $foo = pack("s2",1,2);
3265 # "\1\0\2\0" on little-endian
3266 # "\0\1\0\2" on big-endian
3268 $foo = pack("a4","abcd","x","y","z");
3271 $foo = pack("aaaa","abcd","x","y","z");
3274 $foo = pack("a14","abcdefg");
3275 # "abcdefg\0\0\0\0\0\0\0"
3277 $foo = pack("i9pl", gmtime);
3278 # a real struct tm (on my system anyway)
3280 $utmp_template = "Z8 Z8 Z16 L";
3281 $utmp = pack($utmp_template, @utmp1);
3282 # a struct utmp (BSDish)
3284 @utmp2 = unpack($utmp_template, $utmp);
3285 # "@utmp1" eq "@utmp2"
3288 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3291 $foo = pack('sx2l', 12, 34);
3292 # short 12, two zero bytes padding, long 34
3293 $bar = pack('s@4l', 12, 34);
3294 # short 12, zero fill to position 4, long 34
3297 The same template may generally also be used in unpack().
3299 =item package NAMESPACE
3303 Declares the compilation unit as being in the given namespace. The scope
3304 of the package declaration is from the declaration itself through the end
3305 of the enclosing block, file, or eval (the same as the C<my> operator).
3306 All further unqualified dynamic identifiers will be in this namespace.
3307 A package statement affects only dynamic variables--including those
3308 you've used C<local> on--but I<not> lexical variables, which are created
3309 with C<my>. Typically it would be the first declaration in a file to
3310 be included by the C<require> or C<use> operator. You can switch into a
3311 package in more than one place; it merely influences which symbol table
3312 is used by the compiler for the rest of that block. You can refer to
3313 variables and filehandles in other packages by prefixing the identifier
3314 with the package name and a double colon: C<$Package::Variable>.
3315 If the package name is null, the C<main> package as assumed. That is,
3316 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3317 still seen in older code).
3319 If NAMESPACE is omitted, then there is no current package, and all
3320 identifiers must be fully qualified or lexicals. This is stricter
3321 than C<use strict>, since it also extends to function names.
3323 See L<perlmod/"Packages"> for more information about packages, modules,
3324 and classes. See L<perlsub> for other scoping issues.
3326 =item pipe READHANDLE,WRITEHANDLE
3328 Opens a pair of connected pipes like the corresponding system call.
3329 Note that if you set up a loop of piped processes, deadlock can occur
3330 unless you are very careful. In addition, note that Perl's pipes use
3331 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3332 after each command, depending on the application.
3334 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3335 for examples of such things.
3337 On systems that support a close-on-exec flag on files, the flag will be set
3338 for the newly opened file descriptors as determined by the value of $^F.
3345 Pops and returns the last value of the array, shortening the array by
3346 one element. Has an effect similar to
3350 If there are no elements in the array, returns the undefined value
3351 (although this may happen at other times as well). If ARRAY is
3352 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3353 array in subroutines, just like C<shift>.
3359 Returns the offset of where the last C<m//g> search left off for the variable
3360 in question (C<$_> is used when the variable is not specified). May be
3361 modified to change that offset. Such modification will also influence
3362 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3365 =item print FILEHANDLE LIST
3371 Prints a string or a list of strings. Returns true if successful.
3372 FILEHANDLE may be a scalar variable name, in which case the variable
3373 contains the name of or a reference to the filehandle, thus introducing
3374 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3375 the next token is a term, it may be misinterpreted as an operator
3376 unless you interpose a C<+> or put parentheses around the arguments.)
3377 If FILEHANDLE is omitted, prints by default to standard output (or
3378 to the last selected output channel--see L</select>). If LIST is
3379 also omitted, prints C<$_> to the currently selected output channel.
3380 To set the default output channel to something other than STDOUT
3381 use the select operation. The current value of C<$,> (if any) is
3382 printed between each LIST item. The current value of C<$\> (if
3383 any) is printed after the entire LIST has been printed. Because
3384 print takes a LIST, anything in the LIST is evaluated in list
3385 context, and any subroutine that you call will have one or more of
3386 its expressions evaluated in list context. Also be careful not to
3387 follow the print keyword with a left parenthesis unless you want
3388 the corresponding right parenthesis to terminate the arguments to
3389 the print--interpose a C<+> or put parentheses around all the
3392 Note that if you're storing FILEHANDLES in an array or other expression,
3393 you will have to use a block returning its value instead:
3395 print { $files[$i] } "stuff\n";
3396 print { $OK ? STDOUT : STDERR } "stuff\n";
3398 =item printf FILEHANDLE FORMAT, LIST
3400 =item printf FORMAT, LIST
3402 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3403 (the output record separator) is not appended. The first argument
3404 of the list will be interpreted as the C<printf> format. If C<use locale> is
3405 in effect, the character used for the decimal point in formatted real numbers
3406 is affected by the LC_NUMERIC locale. See L<perllocale>.
3408 Don't fall into the trap of using a C<printf> when a simple
3409 C<print> would do. The C<print> is more efficient and less
3412 =item prototype FUNCTION
3414 Returns the prototype of a function as a string (or C<undef> if the
3415 function has no prototype). FUNCTION is a reference to, or the name of,
3416 the function whose prototype you want to retrieve.
3418 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3419 name for Perl builtin. If the builtin is not I<overridable> (such as
3420 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3421 C<system>) returns C<undef> because the builtin does not really behave
3422 like a Perl function. Otherwise, the string describing the equivalent
3423 prototype is returned.
3425 =item push ARRAY,LIST
3427 Treats ARRAY as a stack, and pushes the values of LIST
3428 onto the end of ARRAY. The length of ARRAY increases by the length of
3429 LIST. Has the same effect as
3432 $ARRAY[++$#ARRAY] = $value;
3435 but is more efficient. Returns the new number of elements in the array.
3447 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3449 =item quotemeta EXPR
3453 Returns the value of EXPR with all non-"word"
3454 characters backslashed. (That is, all characters not matching
3455 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3456 returned string, regardless of any locale settings.)
3457 This is the internal function implementing
3458 the C<\Q> escape in double-quoted strings.
3460 If EXPR is omitted, uses C<$_>.
3466 Returns a random fractional number greater than or equal to C<0> and less
3467 than the value of EXPR. (EXPR should be positive.) If EXPR is
3468 omitted, the value C<1> is used. Automatically calls C<srand> unless
3469 C<srand> has already been called. See also C<srand>.
3471 (Note: If your rand function consistently returns numbers that are too
3472 large or too small, then your version of Perl was probably compiled
3473 with the wrong number of RANDBITS.)
3475 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3477 =item read FILEHANDLE,SCALAR,LENGTH
3479 Attempts to read LENGTH bytes of data into variable SCALAR from the
3480 specified FILEHANDLE. Returns the number of bytes actually read,
3481 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3482 or shrunk to the length actually read. An OFFSET may be specified to
3483 place the read data at some other place than the beginning of the
3484 string. This call is actually implemented in terms of stdio's fread(3)
3485 call. To get a true read(2) system call, see C<sysread>.
3487 =item readdir DIRHANDLE
3489 Returns the next directory entry for a directory opened by C<opendir>.
3490 If used in list context, returns all the rest of the entries in the
3491 directory. If there are no more entries, returns an undefined value in
3492 scalar context or a null list in list context.
3494 If you're planning to filetest the return values out of a C<readdir>, you'd
3495 better prepend the directory in question. Otherwise, because we didn't
3496 C<chdir> there, it would have been testing the wrong file.
3498 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3499 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3504 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3505 context, each call reads and returns the next line, until end-of-file is
3506 reached, whereupon the subsequent call returns undef. In list context,
3507 reads until end-of-file is reached and returns a list of lines. Note that
3508 the notion of "line" used here is however you may have defined it
3509 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3511 When C<$/> is set to C<undef>, when readline() is in scalar
3512 context (i.e. file slurp mode), and when an empty file is read, it
3513 returns C<''> the first time, followed by C<undef> subsequently.
3515 This is the internal function implementing the C<< <EXPR> >>
3516 operator, but you can use it directly. The C<< <EXPR> >>
3517 operator is discussed in more detail in L<perlop/"I/O Operators">.
3520 $line = readline(*STDIN); # same thing
3526 Returns the value of a symbolic link, if symbolic links are
3527 implemented. If not, gives a fatal error. If there is some system
3528 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3529 omitted, uses C<$_>.
3533 EXPR is executed as a system command.
3534 The collected standard output of the command is returned.
3535 In scalar context, it comes back as a single (potentially
3536 multi-line) string. In list context, returns a list of lines
3537 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3538 This is the internal function implementing the C<qx/EXPR/>
3539 operator, but you can use it directly. The C<qx/EXPR/>
3540 operator is discussed in more detail in L<perlop/"I/O Operators">.
3542 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3544 Receives a message on a socket. Attempts to receive LENGTH bytes of
3545 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3546 will be grown or shrunk to the length actually read. Takes the same
3547 flags as the system call of the same name. Returns the address of the
3548 sender if SOCKET's protocol supports this; returns an empty string
3549 otherwise. If there's an error, returns the undefined value. This call
3550 is actually implemented in terms of recvfrom(2) system call. See
3551 L<perlipc/"UDP: Message Passing"> for examples.
3557 The C<redo> command restarts the loop block without evaluating the
3558 conditional again. The C<continue> block, if any, is not executed. If
3559 the LABEL is omitted, the command refers to the innermost enclosing
3560 loop. This command is normally used by programs that want to lie to
3561 themselves about what was just input:
3563 # a simpleminded Pascal comment stripper
3564 # (warning: assumes no { or } in strings)
3565 LINE: while (<STDIN>) {
3566 while (s|({.*}.*){.*}|$1 |) {}
3571 if (/}/) { # end of comment?
3580 C<redo> cannot be used to retry a block which returns a value such as
3581 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3582 a grep() or map() operation.
3584 Note that a block by itself is semantically identical to a loop
3585 that executes once. Thus C<redo> inside such a block will effectively
3586 turn it into a looping construct.
3588 See also L</continue> for an illustration of how C<last>, C<next>, and
3595 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3596 is not specified, C<$_> will be used. The value returned depends on the
3597 type of thing the reference is a reference to.
3598 Builtin types include:
3608 If the referenced object has been blessed into a package, then that package
3609 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3611 if (ref($r) eq "HASH") {
3612 print "r is a reference to a hash.\n";
3615 print "r is not a reference at all.\n";
3617 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3618 print "r is a reference to something that isa hash.\n";
3621 See also L<perlref>.
3623 =item rename OLDNAME,NEWNAME
3625 Changes the name of a file; an existing file NEWNAME will be
3626 clobbered. Returns true for success, false otherwise.
3628 Behavior of this function varies wildly depending on your system
3629 implementation. For example, it will usually not work across file system
3630 boundaries, even though the system I<mv> command sometimes compensates
3631 for this. Other restrictions include whether it works on directories,
3632 open files, or pre-existing files. Check L<perlport> and either the
3633 rename(2) manpage or equivalent system documentation for details.
3635 =item require VERSION
3641 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3644 If a VERSION is specified as a literal of the form v5.6.1,
3645 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3646 at least as recent as that version, at run time. (For compatibility
3647 with older versions of Perl, a numeric argument will also be interpreted
3648 as VERSION.) Compare with L</use>, which can do a similar check at
3651 require v5.6.1; # run time version check
3652 require 5.6.1; # ditto
3653 require 5.005_03; # float version allowed for compatibility
3655 Otherwise, demands that a library file be included if it hasn't already
3656 been included. The file is included via the do-FILE mechanism, which is
3657 essentially just a variety of C<eval>. Has semantics similar to the following
3662 return 1 if $INC{$filename};
3663 my($realfilename,$result);
3665 foreach $prefix (@INC) {
3666 $realfilename = "$prefix/$filename";
3667 if (-f $realfilename) {
3668 $INC{$filename} = $realfilename;
3669 $result = do $realfilename;
3673 die "Can't find $filename in \@INC";
3675 delete $INC{$filename} if $@ || !$result;
3677 die "$filename did not return true value" unless $result;
3681 Note that the file will not be included twice under the same specified
3682 name. The file must return true as the last statement to indicate
3683 successful execution of any initialization code, so it's customary to
3684 end such a file with C<1;> unless you're sure it'll return true
3685 otherwise. But it's better just to put the C<1;>, in case you add more
3688 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3689 replaces "F<::>" with "F</>" in the filename for you,
3690 to make it easy to load standard modules. This form of loading of
3691 modules does not risk altering your namespace.
3693 In other words, if you try this:
3695 require Foo::Bar; # a splendid bareword
3697 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3698 directories specified in the C<@INC> array.
3700 But if you try this:
3702 $class = 'Foo::Bar';
3703 require $class; # $class is not a bareword
3705 require "Foo::Bar"; # not a bareword because of the ""
3707 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3708 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3710 eval "require $class";
3712 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3718 Generally used in a C<continue> block at the end of a loop to clear
3719 variables and reset C<??> searches so that they work again. The
3720 expression is interpreted as a list of single characters (hyphens
3721 allowed for ranges). All variables and arrays beginning with one of
3722 those letters are reset to their pristine state. If the expression is
3723 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3724 only variables or searches in the current package. Always returns
3727 reset 'X'; # reset all X variables
3728 reset 'a-z'; # reset lower case variables
3729 reset; # just reset ?one-time? searches
3731 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3732 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3733 variables--lexical variables are unaffected, but they clean themselves
3734 up on scope exit anyway, so you'll probably want to use them instead.
3741 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3742 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3743 context, depending on how the return value will be used, and the context
3744 may vary from one execution to the next (see C<wantarray>). If no EXPR
3745 is given, returns an empty list in list context, the undefined value in
3746 scalar context, and (of course) nothing at all in a void context.
3748 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3749 or do FILE will automatically return the value of the last expression
3754 In list context, returns a list value consisting of the elements
3755 of LIST in the opposite order. In scalar context, concatenates the
3756 elements of LIST and returns a string value with all characters
3757 in the opposite order.
3759 print reverse <>; # line tac, last line first
3761 undef $/; # for efficiency of <>
3762 print scalar reverse <>; # character tac, last line tsrif
3764 This operator is also handy for inverting a hash, although there are some
3765 caveats. If a value is duplicated in the original hash, only one of those
3766 can be represented as a key in the inverted hash. Also, this has to
3767 unwind one hash and build a whole new one, which may take some time
3768 on a large hash, such as from a DBM file.
3770 %by_name = reverse %by_address; # Invert the hash
3772 =item rewinddir DIRHANDLE
3774 Sets the current position to the beginning of the directory for the
3775 C<readdir> routine on DIRHANDLE.
3777 =item rindex STR,SUBSTR,POSITION
3779 =item rindex STR,SUBSTR
3781 Works just like index() except that it returns the position of the LAST
3782 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3783 last occurrence at or before that position.
3785 =item rmdir FILENAME
3789 Deletes the directory specified by FILENAME if that directory is empty. If it
3790 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3791 FILENAME is omitted, uses C<$_>.
3795 The substitution operator. See L<perlop>.
3799 Forces EXPR to be interpreted in scalar context and returns the value
3802 @counts = ( scalar @a, scalar @b, scalar @c );
3804 There is no equivalent operator to force an expression to
3805 be interpolated in list context because in practice, this is never
3806 needed. If you really wanted to do so, however, you could use
3807 the construction C<@{[ (some expression) ]}>, but usually a simple
3808 C<(some expression)> suffices.
3810 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3811 parenthesized list, this behaves as a scalar comma expression, evaluating
3812 all but the last element in void context and returning the final element
3813 evaluated in scalar context. This is seldom what you want.
3815 The following single statement:
3817 print uc(scalar(&foo,$bar)),$baz;
3819 is the moral equivalent of these two:
3822 print(uc($bar),$baz);
3824 See L<perlop> for more details on unary operators and the comma operator.
3826 =item seek FILEHANDLE,POSITION,WHENCE
3828 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3829 FILEHANDLE may be an expression whose value gives the name of the
3830 filehandle. The values for WHENCE are C<0> to set the new position to
3831 POSITION, C<1> to set it to the current position plus POSITION, and
3832 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3833 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3834 (start of the file, current position, end of the file) from the Fcntl
3835 module. Returns C<1> upon success, C<0> otherwise.
3837 If you want to position file for C<sysread> or C<syswrite>, don't use
3838 C<seek>--buffering makes its effect on the file's system position
3839 unpredictable and non-portable. Use C<sysseek> instead.
3841 Due to the rules and rigors of ANSI C, on some systems you have to do a
3842 seek whenever you switch between reading and writing. Amongst other
3843 things, this may have the effect of calling stdio's clearerr(3).
3844 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3848 This is also useful for applications emulating C<tail -f>. Once you hit
3849 EOF on your read, and then sleep for a while, you might have to stick in a
3850 seek() to reset things. The C<seek> doesn't change the current position,
3851 but it I<does> clear the end-of-file condition on the handle, so that the
3852 next C<< <FILE> >> makes Perl try again to read something. We hope.
3854 If that doesn't work (some stdios are particularly cantankerous), then
3855 you may need something more like this:
3858 for ($curpos = tell(FILE); $_ = <FILE>;
3859 $curpos = tell(FILE)) {
3860 # search for some stuff and put it into files
3862 sleep($for_a_while);
3863 seek(FILE, $curpos, 0);
3866 =item seekdir DIRHANDLE,POS
3868 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3869 must be a value returned by C<telldir>. Has the same caveats about
3870 possible directory compaction as the corresponding system library
3873 =item select FILEHANDLE
3877 Returns the currently selected filehandle. Sets the current default
3878 filehandle for output, if FILEHANDLE is supplied. This has two
3879 effects: first, a C<write> or a C<print> without a filehandle will
3880 default to this FILEHANDLE. Second, references to variables related to
3881 output will refer to this output channel. For example, if you have to
3882 set the top of form format for more than one output channel, you might
3890 FILEHANDLE may be an expression whose value gives the name of the
3891 actual filehandle. Thus:
3893 $oldfh = select(STDERR); $| = 1; select($oldfh);
3895 Some programmers may prefer to think of filehandles as objects with
3896 methods, preferring to write the last example as:
3899 STDERR->autoflush(1);
3901 =item select RBITS,WBITS,EBITS,TIMEOUT
3903 This calls the select(2) system call with the bit masks specified, which
3904 can be constructed using C<fileno> and C<vec>, along these lines:
3906 $rin = $win = $ein = '';
3907 vec($rin,fileno(STDIN),1) = 1;
3908 vec($win,fileno(STDOUT),1) = 1;
3911 If you want to select on many filehandles you might wish to write a
3915 my(@fhlist) = split(' ',$_[0]);
3918 vec($bits,fileno($_),1) = 1;
3922 $rin = fhbits('STDIN TTY SOCK');
3926 ($nfound,$timeleft) =
3927 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3929 or to block until something becomes ready just do this
3931 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3933 Most systems do not bother to return anything useful in $timeleft, so
3934 calling select() in scalar context just returns $nfound.
3936 Any of the bit masks can also be undef. The timeout, if specified, is
3937 in seconds, which may be fractional. Note: not all implementations are
3938 capable of returning the$timeleft. If not, they always return
3939 $timeleft equal to the supplied $timeout.
3941 You can effect a sleep of 250 milliseconds this way:
3943 select(undef, undef, undef, 0.25);
3945 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3946 or <FH>) with C<select>, except as permitted by POSIX, and even
3947 then only on POSIX systems. You have to use C<sysread> instead.
3949 =item semctl ID,SEMNUM,CMD,ARG
3951 Calls the System V IPC function C<semctl>. You'll probably have to say
3955 first to get the correct constant definitions. If CMD is IPC_STAT or
3956 GETALL, then ARG must be a variable which will hold the returned
3957 semid_ds structure or semaphore value array. Returns like C<ioctl>:
3958 the undefined value for error, "C<0 but true>" for zero, or the actual
3959 return value otherwise. The ARG must consist of a vector of native
3960 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
3961 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
3964 =item semget KEY,NSEMS,FLAGS
3966 Calls the System V IPC function semget. Returns the semaphore id, or
3967 the undefined value if there is an error. See also
3968 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
3971 =item semop KEY,OPSTRING
3973 Calls the System V IPC function semop to perform semaphore operations
3974 such as signaling and waiting. OPSTRING must be a packed array of
3975 semop structures. Each semop structure can be generated with
3976 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3977 operations is implied by the length of OPSTRING. Returns true if
3978 successful, or false if there is an error. As an example, the
3979 following code waits on semaphore $semnum of semaphore id $semid:
3981 $semop = pack("sss", $semnum, -1, 0);
3982 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3984 To signal the semaphore, replace C<-1> with C<1>. See also
3985 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
3988 =item send SOCKET,MSG,FLAGS,TO
3990 =item send SOCKET,MSG,FLAGS
3992 Sends a message on a socket. Takes the same flags as the system call
3993 of the same name. On unconnected sockets you must specify a
3994 destination to send TO, in which case it does a C C<sendto>. Returns
3995 the number of characters sent, or the undefined value if there is an
3996 error. The C system call sendmsg(2) is currently unimplemented.
3997 See L<perlipc/"UDP: Message Passing"> for examples.
3999 =item setpgrp PID,PGRP
4001 Sets the current process group for the specified PID, C<0> for the current
4002 process. Will produce a fatal error if used on a machine that doesn't
4003 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4004 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4005 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4008 =item setpriority WHICH,WHO,PRIORITY
4010 Sets the current priority for a process, a process group, or a user.
4011 (See setpriority(2).) Will produce a fatal error if used on a machine
4012 that doesn't implement setpriority(2).
4014 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4016 Sets the socket option requested. Returns undefined if there is an
4017 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4024 Shifts the first value of the array off and returns it, shortening the
4025 array by 1 and moving everything down. If there are no elements in the
4026 array, returns the undefined value. If ARRAY is omitted, shifts the
4027 C<@_> array within the lexical scope of subroutines and formats, and the
4028 C<@ARGV> array at file scopes or within the lexical scopes established by
4029 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4032 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4033 same thing to the left end of an array that C<pop> and C<push> do to the
4036 =item shmctl ID,CMD,ARG
4038 Calls the System V IPC function shmctl. You'll probably have to say
4042 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4043 then ARG must be a variable which will hold the returned C<shmid_ds>
4044 structure. Returns like ioctl: the undefined value for error, "C<0> but
4045 true" for zero, or the actual return value otherwise.
4046 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4048 =item shmget KEY,SIZE,FLAGS
4050 Calls the System V IPC function shmget. Returns the shared memory
4051 segment id, or the undefined value if there is an error.
4052 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4054 =item shmread ID,VAR,POS,SIZE
4056 =item shmwrite ID,STRING,POS,SIZE
4058 Reads or writes the System V shared memory segment ID starting at
4059 position POS for size SIZE by attaching to it, copying in/out, and
4060 detaching from it. When reading, VAR must be a variable that will
4061 hold the data read. When writing, if STRING is too long, only SIZE
4062 bytes are used; if STRING is too short, nulls are written to fill out
4063 SIZE bytes. Return true if successful, or false if there is an error.
4064 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4065 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4067 =item shutdown SOCKET,HOW
4069 Shuts down a socket connection in the manner indicated by HOW, which
4070 has the same interpretation as in the system call of the same name.
4072 shutdown(SOCKET, 0); # I/we have stopped reading data
4073 shutdown(SOCKET, 1); # I/we have stopped writing data
4074 shutdown(SOCKET, 2); # I/we have stopped using this socket
4076 This is useful with sockets when you want to tell the other
4077 side you're done writing but not done reading, or vice versa.
4078 It's also a more insistent form of close because it also
4079 disables the file descriptor in any forked copies in other
4086 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4087 returns sine of C<$_>.
4089 For the inverse sine operation, you may use the C<Math::Trig::asin>
4090 function, or use this relation:
4092 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4098 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4099 May be interrupted if the process receives a signal such as C<SIGALRM>.
4100 Returns the number of seconds actually slept. You probably cannot
4101 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4104 On some older systems, it may sleep up to a full second less than what
4105 you requested, depending on how it counts seconds. Most modern systems
4106 always sleep the full amount. They may appear to sleep longer than that,
4107 however, because your process might not be scheduled right away in a
4108 busy multitasking system.
4110 For delays of finer granularity than one second, you may use Perl's
4111 C<syscall> interface to access setitimer(2) if your system supports
4112 it, or else see L</select> above. The Time::HiRes module from CPAN
4115 See also the POSIX module's C<pause> function.
4117 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4119 Opens a socket of the specified kind and attaches it to filehandle
4120 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4121 the system call of the same name. You should C<use Socket> first
4122 to get the proper definitions imported. See the examples in
4123 L<perlipc/"Sockets: Client/Server Communication">.
4125 On systems that support a close-on-exec flag on files, the flag will
4126 be set for the newly opened file descriptor, as determined by the
4127 value of $^F. See L<perlvar/$^F>.
4129 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4131 Creates an unnamed pair of sockets in the specified domain, of the
4132 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4133 for the system call of the same name. If unimplemented, yields a fatal
4134 error. Returns true if successful.
4136 On systems that support a close-on-exec flag on files, the flag will
4137 be set for the newly opened file descriptors, as determined by the value
4138 of $^F. See L<perlvar/$^F>.
4140 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4141 to C<pipe(Rdr, Wtr)> is essentially:
4144 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4145 shutdown(Rdr, 1); # no more writing for reader
4146 shutdown(Wtr, 0); # no more reading for writer
4148 See L<perlipc> for an example of socketpair use.
4150 =item sort SUBNAME LIST
4152 =item sort BLOCK LIST
4156 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4157 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4158 specified, it gives the name of a subroutine that returns an integer
4159 less than, equal to, or greater than C<0>, depending on how the elements
4160 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4161 operators are extremely useful in such routines.) SUBNAME may be a
4162 scalar variable name (unsubscripted), in which case the value provides
4163 the name of (or a reference to) the actual subroutine to use. In place
4164 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4167 If the subroutine's prototype is C<($$)>, the elements to be compared
4168 are passed by reference in C<@_>, as for a normal subroutine. This is
4169 slower than unprototyped subroutines, where the elements to be
4170 compared are passed into the subroutine
4171 as the package global variables $a and $b (see example below). Note that
4172 in the latter case, it is usually counter-productive to declare $a and
4175 In either case, the subroutine may not be recursive. The values to be
4176 compared are always passed by reference, so don't modify them.
4178 You also cannot exit out of the sort block or subroutine using any of the
4179 loop control operators described in L<perlsyn> or with C<goto>.
4181 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4182 current collation locale. See L<perllocale>.
4187 @articles = sort @files;
4189 # same thing, but with explicit sort routine
4190 @articles = sort {$a cmp $b} @files;
4192 # now case-insensitively
4193 @articles = sort {uc($a) cmp uc($b)} @files;
4195 # same thing in reversed order
4196 @articles = sort {$b cmp $a} @files;
4198 # sort numerically ascending
4199 @articles = sort {$a <=> $b} @files;
4201 # sort numerically descending
4202 @articles = sort {$b <=> $a} @files;
4204 # this sorts the %age hash by value instead of key
4205 # using an in-line function
4206 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4208 # sort using explicit subroutine name
4210 $age{$a} <=> $age{$b}; # presuming numeric
4212 @sortedclass = sort byage @class;
4214 sub backwards { $b cmp $a }
4215 @harry = qw(dog cat x Cain Abel);
4216 @george = qw(gone chased yz Punished Axed);
4218 # prints AbelCaincatdogx
4219 print sort backwards @harry;
4220 # prints xdogcatCainAbel
4221 print sort @george, 'to', @harry;
4222 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4224 # inefficiently sort by descending numeric compare using
4225 # the first integer after the first = sign, or the
4226 # whole record case-insensitively otherwise
4229 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4234 # same thing, but much more efficiently;
4235 # we'll build auxiliary indices instead
4239 push @nums, /=(\d+)/;
4244 $nums[$b] <=> $nums[$a]
4246 $caps[$a] cmp $caps[$b]
4250 # same thing, but without any temps
4251 @new = map { $_->[0] }
4252 sort { $b->[1] <=> $a->[1]
4255 } map { [$_, /=(\d+)/, uc($_)] } @old;
4257 # using a prototype allows you to use any comparison subroutine
4258 # as a sort subroutine (including other package's subroutines)
4260 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4263 @new = sort other::backwards @old;
4265 If you're using strict, you I<must not> declare $a
4266 and $b as lexicals. They are package globals. That means
4267 if you're in the C<main> package and type
4269 @articles = sort {$b <=> $a} @files;
4271 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4272 but if you're in the C<FooPack> package, it's the same as typing
4274 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4276 The comparison function is required to behave. If it returns
4277 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4278 sometimes saying the opposite, for example) the results are not
4281 =item splice ARRAY,OFFSET,LENGTH,LIST
4283 =item splice ARRAY,OFFSET,LENGTH
4285 =item splice ARRAY,OFFSET
4289 Removes the elements designated by OFFSET and LENGTH from an array, and
4290 replaces them with the elements of LIST, if any. In list context,
4291 returns the elements removed from the array. In scalar context,
4292 returns the last element removed, or C<undef> if no elements are
4293 removed. The array grows or shrinks as necessary.
4294 If OFFSET is negative then it starts that far from the end of the array.
4295 If LENGTH is omitted, removes everything from OFFSET onward.
4296 If LENGTH is negative, leaves that many elements off the end of the array.
4297 If both OFFSET and LENGTH are omitted, removes everything.
4299 The following equivalences hold (assuming C<$[ == 0>):
4301 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4302 pop(@a) splice(@a,-1)
4303 shift(@a) splice(@a,0,1)
4304 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4305 $a[$x] = $y splice(@a,$x,1,$y)
4307 Example, assuming array lengths are passed before arrays:
4309 sub aeq { # compare two list values
4310 my(@a) = splice(@_,0,shift);
4311 my(@b) = splice(@_,0,shift);
4312 return 0 unless @a == @b; # same len?
4314 return 0 if pop(@a) ne pop(@b);
4318 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4320 =item split /PATTERN/,EXPR,LIMIT
4322 =item split /PATTERN/,EXPR
4324 =item split /PATTERN/
4328 Splits a string into a list of strings and returns that list. By default,
4329 empty leading fields are preserved, and empty trailing ones are deleted.
4331 In scalar context, returns the number of fields found and splits into
4332 the C<@_> array. Use of split in scalar context is deprecated, however,
4333 because it clobbers your subroutine arguments.
4335 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4336 splits on whitespace (after skipping any leading whitespace). Anything
4337 matching PATTERN is taken to be a delimiter separating the fields. (Note
4338 that the delimiter may be longer than one character.)
4340 If LIMIT is specified and positive, splits into no more than that
4341 many fields (though it may split into fewer). If LIMIT is unspecified
4342 or zero, trailing null fields are stripped (which potential users
4343 of C<pop> would do well to remember). If LIMIT is negative, it is
4344 treated as if an arbitrarily large LIMIT had been specified.
4346 A pattern matching the null string (not to be confused with
4347 a null pattern C<//>, which is just one member of the set of patterns
4348 matching a null string) will split the value of EXPR into separate
4349 characters at each point it matches that way. For example:
4351 print join(':', split(/ */, 'hi there'));
4353 produces the output 'h:i:t:h:e:r:e'.
4355 The LIMIT parameter can be used to split a line partially
4357 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4359 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4360 one larger than the number of variables in the list, to avoid
4361 unnecessary work. For the list above LIMIT would have been 4 by
4362 default. In time critical applications it behooves you not to split
4363 into more fields than you really need.
4365 If the PATTERN contains parentheses, additional list elements are
4366 created from each matching substring in the delimiter.
4368 split(/([,-])/, "1-10,20", 3);
4370 produces the list value
4372 (1, '-', 10, ',', 20)
4374 If you had the entire header of a normal Unix email message in $header,
4375 you could split it up into fields and their values this way:
4377 $header =~ s/\n\s+/ /g; # fix continuation lines
4378 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4380 The pattern C</PATTERN/> may be replaced with an expression to specify
4381 patterns that vary at runtime. (To do runtime compilation only once,
4382 use C</$variable/o>.)
4384 As a special case, specifying a PATTERN of space (C<' '>) will split on
4385 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4386 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4387 will give you as many null initial fields as there are leading spaces.
4388 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4389 whitespace produces a null first field. A C<split> with no arguments
4390 really does a C<split(' ', $_)> internally.
4392 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4397 open(PASSWD, '/etc/passwd');
4399 ($login, $passwd, $uid, $gid,
4400 $gcos, $home, $shell) = split(/:/);
4404 (Note that $shell above will still have a newline on it. See L</chop>,
4405 L</chomp>, and L</join>.)
4407 =item sprintf FORMAT, LIST
4409 Returns a string formatted by the usual C<printf> conventions of the C
4410 library function C<sprintf>. See below for more details
4411 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4412 the general principles.
4416 # Format number with up to 8 leading zeroes
4417 $result = sprintf("%08d", $number);
4419 # Round number to 3 digits after decimal point
4420 $rounded = sprintf("%.3f", $number);
4422 Perl does its own C<sprintf> formatting--it emulates the C
4423 function C<sprintf>, but it doesn't use it (except for floating-point
4424 numbers, and even then only the standard modifiers are allowed). As a
4425 result, any non-standard extensions in your local C<sprintf> are not
4426 available from Perl.
4428 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4429 pass it an array as your first argument. The array is given scalar context,
4430 and instead of using the 0th element of the array as the format, Perl will
4431 use the count of elements in the array as the format, which is almost never
4434 Perl's C<sprintf> permits the following universally-known conversions:
4437 %c a character with the given number
4439 %d a signed integer, in decimal
4440 %u an unsigned integer, in decimal
4441 %o an unsigned integer, in octal
4442 %x an unsigned integer, in hexadecimal
4443 %e a floating-point number, in scientific notation
4444 %f a floating-point number, in fixed decimal notation
4445 %g a floating-point number, in %e or %f notation
4447 In addition, Perl permits the following widely-supported conversions:
4449 %X like %x, but using upper-case letters
4450 %E like %e, but using an upper-case "E"
4451 %G like %g, but with an upper-case "E" (if applicable)
4452 %b an unsigned integer, in binary
4453 %p a pointer (outputs the Perl value's address in hexadecimal)
4454 %n special: *stores* the number of characters output so far
4455 into the next variable in the parameter list
4457 Finally, for backward (and we do mean "backward") compatibility, Perl
4458 permits these unnecessary but widely-supported conversions:
4461 %D a synonym for %ld
4462 %U a synonym for %lu
4463 %O a synonym for %lo
4466 Note that the number of exponent digits in the scientific notation by
4467 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4468 exponent less than 100 is system-dependent: it may be three or less
4469 (zero-padded as necessary). In other words, 1.23 times ten to the
4470 99th may be either "1.23e99" or "1.23e099".
4472 Perl permits the following universally-known flags between the C<%>
4473 and the conversion letter:
4475 space prefix positive number with a space
4476 + prefix positive number with a plus sign
4477 - left-justify within the field
4478 0 use zeros, not spaces, to right-justify
4479 # prefix non-zero octal with "0", non-zero hex with "0x"
4480 number minimum field width
4481 .number "precision": digits after decimal point for
4482 floating-point, max length for string, minimum length
4484 l interpret integer as C type "long" or "unsigned long"
4485 h interpret integer as C type "short" or "unsigned short"
4486 If no flags, interpret integer as C type "int" or "unsigned"
4488 Perl supports parameter ordering, in other words, fetching the
4489 parameters in some explicitly specified "random" ordering as opposed
4490 to the default implicit sequential ordering. The syntax is, instead
4491 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4492 where the I<digits> is the wanted index, from one upwards. For example:
4494 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4495 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4497 Note that using the reordering syntax does not interfere with the usual
4498 implicit sequential fetching of the parameters:
4500 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4501 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4502 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4503 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4504 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4506 There are also two Perl-specific flags:
4508 V interpret integer as Perl's standard integer type
4509 v interpret string as a vector of integers, output as
4510 numbers separated either by dots, or by an arbitrary
4511 string received from the argument list when the flag
4514 Where a number would appear in the flags, an asterisk (C<*>) may be
4515 used instead, in which case Perl uses the next item in the parameter
4516 list as the given number (that is, as the field width or precision).
4517 If a field width obtained through C<*> is negative, it has the same
4518 effect as the C<-> flag: left-justification.
4520 The C<v> flag is useful for displaying ordinal values of characters
4521 in arbitrary strings:
4523 printf "version is v%vd\n", $^V; # Perl's version
4524 printf "address is %*vX\n", ":", $addr; # IPv6 address
4525 printf "bits are %*vb\n", " ", $bits; # random bitstring
4527 If C<use locale> is in effect, the character used for the decimal
4528 point in formatted real numbers is affected by the LC_NUMERIC locale.
4531 If Perl understands "quads" (64-bit integers) (this requires
4532 either that the platform natively support quads or that Perl
4533 be specifically compiled to support quads), the characters
4537 print quads, and they may optionally be preceded by
4545 You can find out whether your Perl supports quads via L<Config>:
4548 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4551 If Perl understands "long doubles" (this requires that the platform
4552 support long doubles), the flags
4556 may optionally be preceded by
4564 You can find out whether your Perl supports long doubles via L<Config>:
4567 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4573 Return the square root of EXPR. If EXPR is omitted, returns square
4574 root of C<$_>. Only works on non-negative operands, unless you've
4575 loaded the standard Math::Complex module.
4578 print sqrt(-2); # prints 1.4142135623731i
4584 Sets the random number seed for the C<rand> operator. If EXPR is
4585 omitted, uses a semi-random value supplied by the kernel (if it supports
4586 the F</dev/urandom> device) or based on the current time and process
4587 ID, among other things. In versions of Perl prior to 5.004 the default
4588 seed was just the current C<time>. This isn't a particularly good seed,
4589 so many old programs supply their own seed value (often C<time ^ $$> or
4590 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4592 In fact, it's usually not necessary to call C<srand> at all, because if
4593 it is not called explicitly, it is called implicitly at the first use of
4594 the C<rand> operator. However, this was not the case in version of Perl
4595 before 5.004, so if your script will run under older Perl versions, it
4596 should call C<srand>.
4598 Note that you need something much more random than the default seed for
4599 cryptographic purposes. Checksumming the compressed output of one or more
4600 rapidly changing operating system status programs is the usual method. For
4603 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4605 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4608 Do I<not> call C<srand> multiple times in your program unless you know
4609 exactly what you're doing and why you're doing it. The point of the
4610 function is to "seed" the C<rand> function so that C<rand> can produce
4611 a different sequence each time you run your program. Just do it once at the
4612 top of your program, or you I<won't> get random numbers out of C<rand>!
4614 Frequently called programs (like CGI scripts) that simply use
4618 for a seed can fall prey to the mathematical property that
4622 one-third of the time. So don't do that.
4624 =item stat FILEHANDLE
4630 Returns a 13-element list giving the status info for a file, either
4631 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4632 it stats C<$_>. Returns a null list if the stat fails. Typically used
4635 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4636 $atime,$mtime,$ctime,$blksize,$blocks)
4639 Not all fields are supported on all filesystem types. Here are the
4640 meaning of the fields:
4642 0 dev device number of filesystem
4644 2 mode file mode (type and permissions)
4645 3 nlink number of (hard) links to the file
4646 4 uid numeric user ID of file's owner
4647 5 gid numeric group ID of file's owner
4648 6 rdev the device identifier (special files only)
4649 7 size total size of file, in bytes
4650 8 atime last access time in seconds since the epoch
4651 9 mtime last modify time in seconds since the epoch
4652 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4653 11 blksize preferred block size for file system I/O
4654 12 blocks actual number of blocks allocated
4656 (The epoch was at 00:00 January 1, 1970 GMT.)
4658 If stat is passed the special filehandle consisting of an underline, no
4659 stat is done, but the current contents of the stat structure from the
4660 last stat or filetest are returned. Example:
4662 if (-x $file && (($d) = stat(_)) && $d < 0) {
4663 print "$file is executable NFS file\n";
4666 (This works on machines only for which the device number is negative
4669 Because the mode contains both the file type and its permissions, you
4670 should mask off the file type portion and (s)printf using a C<"%o">
4671 if you want to see the real permissions.
4673 $mode = (stat($filename))[2];
4674 printf "Permissions are %04o\n", $mode & 07777;
4676 In scalar context, C<stat> returns a boolean value indicating success
4677 or failure, and, if successful, sets the information associated with
4678 the special filehandle C<_>.
4680 The File::stat module provides a convenient, by-name access mechanism:
4683 $sb = stat($filename);
4684 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4685 $filename, $sb->size, $sb->mode & 07777,
4686 scalar localtime $sb->mtime;
4688 You can import symbolic mode constants (C<S_IF*>) and functions
4689 (C<S_IS*>) from the Fcntl module:
4693 $mode = (stat($filename))[2];
4695 $user_rwx = ($mode & S_IRWXU) >> 6;
4696 $group_read = ($mode & S_IRGRP) >> 3;
4697 $other_execute = $mode & S_IXOTH;
4699 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4701 $is_setuid = $mode & S_ISUID;
4702 $is_setgid = S_ISDIR($mode);
4704 You could write the last two using the C<-u> and C<-d> operators.
4705 The commonly available S_IF* constants are
4707 # Permissions: read, write, execute, for user, group, others.
4709 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4710 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4711 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4713 # Setuid/Setgid/Stickiness.
4715 S_ISUID S_ISGID S_ISVTX S_ISTXT
4717 # File types. Not necessarily all are available on your system.
4719 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4721 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4723 S_IREAD S_IWRITE S_IEXEC
4725 and the S_IF* functions are
4727 S_IFMODE($mode) the part of $mode containing the permission bits
4728 and the setuid/setgid/sticky bits
4730 S_IFMT($mode) the part of $mode containing the file type
4731 which can be bit-anded with e.g. S_IFREG
4732 or with the following functions
4734 # The operators -f, -d, -l, -b, -c, -p, and -s.
4736 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4737 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4739 # No direct -X operator counterpart, but for the first one
4740 # the -g operator is often equivalent. The ENFMT stands for
4741 # record flocking enforcement, a platform-dependent feature.
4743 S_ISENFMT($mode) S_ISWHT($mode)
4745 See your native chmod(2) and stat(2) documentation for more details
4746 about the S_* constants.
4752 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4753 doing many pattern matches on the string before it is next modified.
4754 This may or may not save time, depending on the nature and number of
4755 patterns you are searching on, and on the distribution of character
4756 frequencies in the string to be searched--you probably want to compare
4757 run times with and without it to see which runs faster. Those loops
4758 which scan for many short constant strings (including the constant
4759 parts of more complex patterns) will benefit most. You may have only
4760 one C<study> active at a time--if you study a different scalar the first
4761 is "unstudied". (The way C<study> works is this: a linked list of every
4762 character in the string to be searched is made, so we know, for
4763 example, where all the C<'k'> characters are. From each search string,
4764 the rarest character is selected, based on some static frequency tables
4765 constructed from some C programs and English text. Only those places
4766 that contain this "rarest" character are examined.)
4768 For example, here is a loop that inserts index producing entries
4769 before any line containing a certain pattern:
4773 print ".IX foo\n" if /\bfoo\b/;
4774 print ".IX bar\n" if /\bbar\b/;
4775 print ".IX blurfl\n" if /\bblurfl\b/;
4780 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4781 will be looked at, because C<f> is rarer than C<o>. In general, this is
4782 a big win except in pathological cases. The only question is whether
4783 it saves you more time than it took to build the linked list in the
4786 Note that if you have to look for strings that you don't know till
4787 runtime, you can build an entire loop as a string and C<eval> that to
4788 avoid recompiling all your patterns all the time. Together with
4789 undefining C<$/> to input entire files as one record, this can be very
4790 fast, often faster than specialized programs like fgrep(1). The following
4791 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4792 out the names of those files that contain a match:
4794 $search = 'while (<>) { study;';
4795 foreach $word (@words) {
4796 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4801 eval $search; # this screams
4802 $/ = "\n"; # put back to normal input delimiter
4803 foreach $file (sort keys(%seen)) {
4811 =item sub NAME BLOCK
4813 This is subroutine definition, not a real function I<per se>. With just a
4814 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4815 Without a NAME, it's an anonymous function declaration, and does actually
4816 return a value: the CODE ref of the closure you just created. See L<perlsub>
4817 and L<perlref> for details.
4819 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4821 =item substr EXPR,OFFSET,LENGTH
4823 =item substr EXPR,OFFSET
4825 Extracts a substring out of EXPR and returns it. First character is at
4826 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4827 If OFFSET is negative (or more precisely, less than C<$[>), starts
4828 that far from the end of the string. If LENGTH is omitted, returns
4829 everything to the end of the string. If LENGTH is negative, leaves that
4830 many characters off the end of the string.
4832 You can use the substr() function as an lvalue, in which case EXPR
4833 must itself be an lvalue. If you assign something shorter than LENGTH,
4834 the string will shrink, and if you assign something longer than LENGTH,
4835 the string will grow to accommodate it. To keep the string the same
4836 length you may need to pad or chop your value using C<sprintf>.
4838 If OFFSET and LENGTH specify a substring that is partly outside the
4839 string, only the part within the string is returned. If the substring
4840 is beyond either end of the string, substr() returns the undefined
4841 value and produces a warning. When used as an lvalue, specifying a
4842 substring that is entirely outside the string is a fatal error.
4843 Here's an example showing the behavior for boundary cases:
4846 substr($name, 4) = 'dy'; # $name is now 'freddy'
4847 my $null = substr $name, 6, 2; # returns '' (no warning)
4848 my $oops = substr $name, 7; # returns undef, with warning
4849 substr($name, 7) = 'gap'; # fatal error
4851 An alternative to using substr() as an lvalue is to specify the
4852 replacement string as the 4th argument. This allows you to replace
4853 parts of the EXPR and return what was there before in one operation,
4854 just as you can with splice().
4856 =item symlink OLDFILE,NEWFILE
4858 Creates a new filename symbolically linked to the old filename.
4859 Returns C<1> for success, C<0> otherwise. On systems that don't support
4860 symbolic links, produces a fatal error at run time. To check for that,
4863 $symlink_exists = eval { symlink("",""); 1 };
4867 Calls the system call specified as the first element of the list,
4868 passing the remaining elements as arguments to the system call. If
4869 unimplemented, produces a fatal error. The arguments are interpreted
4870 as follows: if a given argument is numeric, the argument is passed as
4871 an int. If not, the pointer to the string value is passed. You are
4872 responsible to make sure a string is pre-extended long enough to
4873 receive any result that might be written into a string. You can't use a
4874 string literal (or other read-only string) as an argument to C<syscall>
4875 because Perl has to assume that any string pointer might be written
4877 integer arguments are not literals and have never been interpreted in a
4878 numeric context, you may need to add C<0> to them to force them to look
4879 like numbers. This emulates the C<syswrite> function (or vice versa):
4881 require 'syscall.ph'; # may need to run h2ph
4883 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4885 Note that Perl supports passing of up to only 14 arguments to your system call,
4886 which in practice should usually suffice.
4888 Syscall returns whatever value returned by the system call it calls.
4889 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4890 Note that some system calls can legitimately return C<-1>. The proper
4891 way to handle such calls is to assign C<$!=0;> before the call and
4892 check the value of C<$!> if syscall returns C<-1>.
4894 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4895 number of the read end of the pipe it creates. There is no way
4896 to retrieve the file number of the other end. You can avoid this
4897 problem by using C<pipe> instead.
4899 =item sysopen FILEHANDLE,FILENAME,MODE
4901 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4903 Opens the file whose filename is given by FILENAME, and associates it
4904 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4905 the name of the real filehandle wanted. This function calls the
4906 underlying operating system's C<open> function with the parameters
4907 FILENAME, MODE, PERMS.
4909 The possible values and flag bits of the MODE parameter are
4910 system-dependent; they are available via the standard module C<Fcntl>.
4911 See the documentation of your operating system's C<open> to see which
4912 values and flag bits are available. You may combine several flags
4913 using the C<|>-operator.
4915 Some of the most common values are C<O_RDONLY> for opening the file in
4916 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4917 and C<O_RDWR> for opening the file in read-write mode, and.
4919 For historical reasons, some values work on almost every system
4920 supported by perl: zero means read-only, one means write-only, and two
4921 means read/write. We know that these values do I<not> work under
4922 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4923 use them in new code.
4925 If the file named by FILENAME does not exist and the C<open> call creates
4926 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4927 PERMS specifies the permissions of the newly created file. If you omit
4928 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4929 These permission values need to be in octal, and are modified by your
4930 process's current C<umask>.
4932 In many systems the C<O_EXCL> flag is available for opening files in
4933 exclusive mode. This is B<not> locking: exclusiveness means here that
4934 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4937 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4939 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4940 that takes away the user's option to have a more permissive umask.
4941 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4944 Note that C<sysopen> depends on the fdopen() C library function.
4945 On many UNIX systems, fdopen() is known to fail when file descriptors
4946 exceed a certain value, typically 255. If you need more file
4947 descriptors than that, consider rebuilding Perl to use the C<sfio>
4948 library, or perhaps using the POSIX::open() function.
4950 See L<perlopentut> for a kinder, gentler explanation of opening files.
4952 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4954 =item sysread FILEHANDLE,SCALAR,LENGTH
4956 Attempts to read LENGTH bytes of data into variable SCALAR from the
4957 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4958 so mixing this with other kinds of reads, C<print>, C<write>,
4959 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4960 usually buffers data. Returns the number of bytes actually read, C<0>
4961 at end of file, or undef if there was an error. SCALAR will be grown or
4962 shrunk so that the last byte actually read is the last byte of the
4963 scalar after the read.
4965 An OFFSET may be specified to place the read data at some place in the
4966 string other than the beginning. A negative OFFSET specifies
4967 placement at that many bytes counting backwards from the end of the
4968 string. A positive OFFSET greater than the length of SCALAR results
4969 in the string being padded to the required size with C<"\0"> bytes before
4970 the result of the read is appended.
4972 There is no syseof() function, which is ok, since eof() doesn't work
4973 very well on device files (like ttys) anyway. Use sysread() and check
4974 for a return value for 0 to decide whether you're done.
4976 =item sysseek FILEHANDLE,POSITION,WHENCE
4978 Sets FILEHANDLE's system position using the system call lseek(2). It
4979 bypasses stdio, so mixing this with reads (other than C<sysread>),
4980 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4981 FILEHANDLE may be an expression whose value gives the name of the
4982 filehandle. The values for WHENCE are C<0> to set the new position to
4983 POSITION, C<1> to set the it to the current position plus POSITION,
4984 and C<2> to set it to EOF plus POSITION (typically negative). For
4985 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4986 C<SEEK_END> (start of the file, current position, end of the file)
4987 from the Fcntl module.
4989 Returns the new position, or the undefined value on failure. A position
4990 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4991 true on success and false on failure, yet you can still easily determine
4996 =item system PROGRAM LIST
4998 Does exactly the same thing as C<exec LIST>, except that a fork is
4999 done first, and the parent process waits for the child process to
5000 complete. Note that argument processing varies depending on the
5001 number of arguments. If there is more than one argument in LIST,
5002 or if LIST is an array with more than one value, starts the program
5003 given by the first element of the list with arguments given by the
5004 rest of the list. If there is only one scalar argument, the argument
5005 is checked for shell metacharacters, and if there are any, the
5006 entire argument is passed to the system's command shell for parsing
5007 (this is C</bin/sh -c> on Unix platforms, but varies on other
5008 platforms). If there are no shell metacharacters in the argument,
5009 it is split into words and passed directly to C<execvp>, which is
5012 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5013 output before any operation that may do a fork, but this may not be
5014 supported on some platforms (see L<perlport>). To be safe, you may need
5015 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5016 of C<IO::Handle> on any open handles.
5018 The return value is the exit status of the program as
5019 returned by the C<wait> call. To get the actual exit value divide by
5020 256. See also L</exec>. This is I<not> what you want to use to capture
5021 the output from a command, for that you should use merely backticks or
5022 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5023 indicates a failure to start the program (inspect $! for the reason).
5025 Like C<exec>, C<system> allows you to lie to a program about its name if
5026 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5028 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
5029 program they're running doesn't actually interrupt your program.
5031 @args = ("command", "arg1", "arg2");
5033 or die "system @args failed: $?"
5035 You can check all the failure possibilities by inspecting
5038 $exit_value = $? >> 8;
5039 $signal_num = $? & 127;
5040 $dumped_core = $? & 128;
5042 When the arguments get executed via the system shell, results
5043 and return codes will be subject to its quirks and capabilities.
5044 See L<perlop/"`STRING`"> and L</exec> for details.
5046 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5048 =item syswrite FILEHANDLE,SCALAR,LENGTH
5050 =item syswrite FILEHANDLE,SCALAR
5052 Attempts to write LENGTH bytes of data from variable SCALAR to the
5053 specified FILEHANDLE, using the system call write(2). If LENGTH
5054 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5055 this with reads (other than C<sysread())>, C<print>, C<write>,
5056 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5057 usually buffers data. Returns the number of bytes actually written,
5058 or C<undef> if there was an error. If the LENGTH is greater than
5059 the available data in the SCALAR after the OFFSET, only as much
5060 data as is available will be written.
5062 An OFFSET may be specified to write the data from some part of the
5063 string other than the beginning. A negative OFFSET specifies writing
5064 that many bytes counting backwards from the end of the string. In the
5065 case the SCALAR is empty you can use OFFSET but only zero offset.
5067 =item tell FILEHANDLE
5071 Returns the current position for FILEHANDLE. FILEHANDLE may be an
5072 expression whose value gives the name of the actual filehandle. If
5073 FILEHANDLE is omitted, assumes the file last read.
5075 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5077 =item telldir DIRHANDLE
5079 Returns the current position of the C<readdir> routines on DIRHANDLE.
5080 Value may be given to C<seekdir> to access a particular location in a
5081 directory. Has the same caveats about possible directory compaction as
5082 the corresponding system library routine.
5084 =item tie VARIABLE,CLASSNAME,LIST
5086 This function binds a variable to a package class that will provide the
5087 implementation for the variable. VARIABLE is the name of the variable
5088 to be enchanted. CLASSNAME is the name of a class implementing objects
5089 of correct type. Any additional arguments are passed to the C<new>
5090 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5091 or C<TIEHASH>). Typically these are arguments such as might be passed
5092 to the C<dbm_open()> function of C. The object returned by the C<new>
5093 method is also returned by the C<tie> function, which would be useful
5094 if you want to access other methods in CLASSNAME.
5096 Note that functions such as C<keys> and C<values> may return huge lists
5097 when used on large objects, like DBM files. You may prefer to use the
5098 C<each> function to iterate over such. Example:
5100 # print out history file offsets
5102 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5103 while (($key,$val) = each %HIST) {
5104 print $key, ' = ', unpack('L',$val), "\n";
5108 A class implementing a hash should have the following methods:
5110 TIEHASH classname, LIST
5112 STORE this, key, value
5117 NEXTKEY this, lastkey
5121 A class implementing an ordinary array should have the following methods:
5123 TIEARRAY classname, LIST
5125 STORE this, key, value
5127 STORESIZE this, count
5133 SPLICE this, offset, length, LIST
5138 A class implementing a file handle should have the following methods:
5140 TIEHANDLE classname, LIST
5141 READ this, scalar, length, offset
5144 WRITE this, scalar, length, offset
5146 PRINTF this, format, LIST
5150 SEEK this, position, whence
5152 OPEN this, mode, LIST
5157 A class implementing a scalar should have the following methods:
5159 TIESCALAR classname, LIST
5165 Not all methods indicated above need be implemented. See L<perltie>,
5166 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5168 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5169 for you--you need to do that explicitly yourself. See L<DB_File>
5170 or the F<Config> module for interesting C<tie> implementations.
5172 For further details see L<perltie>, L<"tied VARIABLE">.
5176 Returns a reference to the object underlying VARIABLE (the same value
5177 that was originally returned by the C<tie> call that bound the variable
5178 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5183 Returns the number of non-leap seconds since whatever time the system
5184 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5185 and 00:00:00 UTC, January 1, 1970 for most other systems).
5186 Suitable for feeding to C<gmtime> and C<localtime>.
5188 For measuring time in better granularity than one second,
5189 you may use either the Time::HiRes module from CPAN, or
5190 if you have gettimeofday(2), you may be able to use the
5191 C<syscall> interface of Perl, see L<perlfaq8> for details.
5195 Returns a four-element list giving the user and system times, in
5196 seconds, for this process and the children of this process.
5198 ($user,$system,$cuser,$csystem) = times;
5202 The transliteration operator. Same as C<y///>. See L<perlop>.
5204 =item truncate FILEHANDLE,LENGTH
5206 =item truncate EXPR,LENGTH
5208 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5209 specified length. Produces a fatal error if truncate isn't implemented
5210 on your system. Returns true if successful, the undefined value
5217 Returns an uppercased version of EXPR. This is the internal function
5218 implementing the C<\U> escape in double-quoted strings.
5219 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5220 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5221 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5223 If EXPR is omitted, uses C<$_>.
5229 Returns the value of EXPR with the first character
5230 in uppercase (titlecase in Unicode). This is
5231 the internal function implementing the C<\u> escape in double-quoted strings.
5232 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5235 If EXPR is omitted, uses C<$_>.
5241 Sets the umask for the process to EXPR and returns the previous value.
5242 If EXPR is omitted, merely returns the current umask.
5244 The Unix permission C<rwxr-x---> is represented as three sets of three
5245 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5246 and isn't one of the digits). The C<umask> value is such a number
5247 representing disabled permissions bits. The permission (or "mode")
5248 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5249 even if you tell C<sysopen> to create a file with permissions C<0777>,
5250 if your umask is C<0022> then the file will actually be created with
5251 permissions C<0755>. If your C<umask> were C<0027> (group can't
5252 write; others can't read, write, or execute), then passing
5253 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5256 Here's some advice: supply a creation mode of C<0666> for regular
5257 files (in C<sysopen>) and one of C<0777> for directories (in
5258 C<mkdir>) and executable files. This gives users the freedom of
5259 choice: if they want protected files, they might choose process umasks
5260 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5261 Programs should rarely if ever make policy decisions better left to
5262 the user. The exception to this is when writing files that should be
5263 kept private: mail files, web browser cookies, I<.rhosts> files, and
5266 If umask(2) is not implemented on your system and you are trying to
5267 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5268 fatal error at run time. If umask(2) is not implemented and you are
5269 not trying to restrict access for yourself, returns C<undef>.
5271 Remember that a umask is a number, usually given in octal; it is I<not> a
5272 string of octal digits. See also L</oct>, if all you have is a string.
5278 Undefines the value of EXPR, which must be an lvalue. Use only on a
5279 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5280 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5281 will probably not do what you expect on most predefined variables or
5282 DBM list values, so don't do that; see L<delete>.) Always returns the
5283 undefined value. You can omit the EXPR, in which case nothing is
5284 undefined, but you still get an undefined value that you could, for
5285 instance, return from a subroutine, assign to a variable or pass as a
5286 parameter. Examples:
5289 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5293 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5294 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5295 select undef, undef, undef, 0.25;
5296 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5298 Note that this is a unary operator, not a list operator.
5304 Deletes a list of files. Returns the number of files successfully
5307 $cnt = unlink 'a', 'b', 'c';
5311 Note: C<unlink> will not delete directories unless you are superuser and
5312 the B<-U> flag is supplied to Perl. Even if these conditions are
5313 met, be warned that unlinking a directory can inflict damage on your
5314 filesystem. Use C<rmdir> instead.
5316 If LIST is omitted, uses C<$_>.
5318 =item unpack TEMPLATE,EXPR
5320 C<unpack> does the reverse of C<pack>: it takes a string
5321 and expands it out into a list of values.
5322 (In scalar context, it returns merely the first value produced.)
5324 The string is broken into chunks described by the TEMPLATE. Each chunk
5325 is converted separately to a value. Typically, either the string is a result
5326 of C<pack>, or the bytes of the string represent a C structure of some
5329 The TEMPLATE has the same format as in the C<pack> function.
5330 Here's a subroutine that does substring:
5333 my($what,$where,$howmuch) = @_;
5334 unpack("x$where a$howmuch", $what);
5339 sub ordinal { unpack("c",$_[0]); } # same as ord()
5341 In addition to fields allowed in pack(), you may prefix a field with
5342 a %<number> to indicate that
5343 you want a <number>-bit checksum of the items instead of the items
5344 themselves. Default is a 16-bit checksum. Checksum is calculated by
5345 summing numeric values of expanded values (for string fields the sum of
5346 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5348 For example, the following
5349 computes the same number as the System V sum program:
5353 unpack("%32C*",<>) % 65535;
5356 The following efficiently counts the number of set bits in a bit vector:
5358 $setbits = unpack("%32b*", $selectmask);
5360 The C<p> and C<P> formats should be used with care. Since Perl
5361 has no way of checking whether the value passed to C<unpack()>
5362 corresponds to a valid memory location, passing a pointer value that's
5363 not known to be valid is likely to have disastrous consequences.
5365 If the repeat count of a field is larger than what the remainder of
5366 the input string allows, repeat count is decreased. If the input string
5367 is longer than one described by the TEMPLATE, the rest is ignored.
5369 See L</pack> for more examples and notes.
5371 =item untie VARIABLE
5373 Breaks the binding between a variable and a package. (See C<tie>.)
5375 =item unshift ARRAY,LIST
5377 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5378 depending on how you look at it. Prepends list to the front of the
5379 array, and returns the new number of elements in the array.
5381 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5383 Note the LIST is prepended whole, not one element at a time, so the
5384 prepended elements stay in the same order. Use C<reverse> to do the
5387 =item use Module VERSION LIST
5389 =item use Module VERSION
5391 =item use Module LIST
5397 Imports some semantics into the current package from the named module,
5398 generally by aliasing certain subroutine or variable names into your
5399 package. It is exactly equivalent to
5401 BEGIN { require Module; import Module LIST; }
5403 except that Module I<must> be a bareword.
5405 VERSION, which can be specified as a literal of the form v5.6.1, demands
5406 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5407 as recent as that version. (For compatibility with older versions of Perl,
5408 a numeric literal will also be interpreted as VERSION.) If the version
5409 of the running Perl interpreter is less than VERSION, then an error
5410 message is printed and Perl exits immediately without attempting to
5411 parse the rest of the file. Compare with L</require>, which can do a
5412 similar check at run time.
5414 use v5.6.1; # compile time version check
5416 use 5.005_03; # float version allowed for compatibility
5418 This is often useful if you need to check the current Perl version before
5419 C<use>ing library modules that have changed in incompatible ways from
5420 older versions of Perl. (We try not to do this more than we have to.)
5422 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5423 C<require> makes sure the module is loaded into memory if it hasn't been
5424 yet. The C<import> is not a builtin--it's just an ordinary static method
5425 call into the C<Module> package to tell the module to import the list of
5426 features back into the current package. The module can implement its
5427 C<import> method any way it likes, though most modules just choose to
5428 derive their C<import> method via inheritance from the C<Exporter> class that
5429 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5430 method can be found then the call is skipped.
5432 If you do not want to call the package's C<import> method (for instance,
5433 to stop your namespace from being altered), explicitly supply the empty list:
5437 That is exactly equivalent to
5439 BEGIN { require Module }
5441 If the VERSION argument is present between Module and LIST, then the
5442 C<use> will call the VERSION method in class Module with the given
5443 version as an argument. The default VERSION method, inherited from
5444 the UNIVERSAL class, croaks if the given version is larger than the
5445 value of the variable C<$Module::VERSION>.
5447 Again, there is a distinction between omitting LIST (C<import> called
5448 with no arguments) and an explicit empty LIST C<()> (C<import> not
5449 called). Note that there is no comma after VERSION!
5451 Because this is a wide-open interface, pragmas (compiler directives)
5452 are also implemented this way. Currently implemented pragmas are:
5457 use sigtrap qw(SEGV BUS);
5458 use strict qw(subs vars refs);
5459 use subs qw(afunc blurfl);
5460 use warnings qw(all);
5462 Some of these pseudo-modules import semantics into the current
5463 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5464 which import symbols into the current package (which are effective
5465 through the end of the file).
5467 There's a corresponding C<no> command that unimports meanings imported
5468 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5474 If no C<unimport> method can be found the call fails with a fatal error.
5476 See L<perlmod> for a list of standard modules and pragmas. See L<perlrun>
5477 for the C<-M> and C<-m> command-line options to perl that give C<use>
5478 functionality from the command-line.
5482 Changes the access and modification times on each file of a list of
5483 files. The first two elements of the list must be the NUMERICAL access
5484 and modification times, in that order. Returns the number of files
5485 successfully changed. The inode change time of each file is set
5486 to the current time. This code has the same effect as the C<touch>
5487 command if the files already exist:
5491 utime $now, $now, @ARGV;
5495 Returns a list consisting of all the values of the named hash. (In a
5496 scalar context, returns the number of values.) The values are
5497 returned in an apparently random order. The actual random order is
5498 subject to change in future versions of perl, but it is guaranteed to
5499 be the same order as either the C<keys> or C<each> function would
5500 produce on the same (unmodified) hash.
5502 Note that the values are not copied, which means modifying them will
5503 modify the contents of the hash:
5505 for (values %hash) { s/foo/bar/g } # modifies %hash values
5506 for (@hash{keys %hash}) { s/foo/bar/g } # same
5508 As a side effect, calling values() resets the HASH's internal iterator.
5509 See also C<keys>, C<each>, and C<sort>.
5511 =item vec EXPR,OFFSET,BITS
5513 Treats the string in EXPR as a bit vector made up of elements of
5514 width BITS, and returns the value of the element specified by OFFSET
5515 as an unsigned integer. BITS therefore specifies the number of bits
5516 that are reserved for each element in the bit vector. This must
5517 be a power of two from 1 to 32 (or 64, if your platform supports
5520 If BITS is 8, "elements" coincide with bytes of the input string.
5522 If BITS is 16 or more, bytes of the input string are grouped into chunks
5523 of size BITS/8, and each group is converted to a number as with
5524 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5525 for BITS==64). See L<"pack"> for details.
5527 If bits is 4 or less, the string is broken into bytes, then the bits
5528 of each byte are broken into 8/BITS groups. Bits of a byte are
5529 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5530 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5531 breaking the single input byte C<chr(0x36)> into two groups gives a list
5532 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5534 C<vec> may also be assigned to, in which case parentheses are needed
5535 to give the expression the correct precedence as in
5537 vec($image, $max_x * $x + $y, 8) = 3;
5539 If the selected element is outside the string, the value 0 is returned.
5540 If an element off the end of the string is written to, Perl will first
5541 extend the string with sufficiently many zero bytes. It is an error
5542 to try to write off the beginning of the string (i.e. negative OFFSET).
5544 The string should not contain any character with the value > 255 (which
5545 can only happen if you're using UTF8 encoding). If it does, it will be
5546 treated as something which is not UTF8 encoded. When the C<vec> was
5547 assigned to, other parts of your program will also no longer consider the
5548 string to be UTF8 encoded. In other words, if you do have such characters
5549 in your string, vec() will operate on the actual byte string, and not the
5550 conceptual character string.
5552 Strings created with C<vec> can also be manipulated with the logical
5553 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5554 vector operation is desired when both operands are strings.
5555 See L<perlop/"Bitwise String Operators">.
5557 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5558 The comments show the string after each step. Note that this code works
5559 in the same way on big-endian or little-endian machines.
5562 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5564 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5565 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5567 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5568 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5569 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5570 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5571 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5572 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5574 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5575 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5576 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5579 To transform a bit vector into a string or list of 0's and 1's, use these:
5581 $bits = unpack("b*", $vector);
5582 @bits = split(//, unpack("b*", $vector));
5584 If you know the exact length in bits, it can be used in place of the C<*>.
5586 Here is an example to illustrate how the bits actually fall in place:
5592 unpack("V",$_) 01234567890123456789012345678901
5593 ------------------------------------------------------------------
5598 for ($shift=0; $shift < $width; ++$shift) {
5599 for ($off=0; $off < 32/$width; ++$off) {
5600 $str = pack("B*", "0"x32);
5601 $bits = (1<<$shift);
5602 vec($str, $off, $width) = $bits;
5603 $res = unpack("b*",$str);
5604 $val = unpack("V", $str);
5611 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5612 $off, $width, $bits, $val, $res
5616 Regardless of the machine architecture on which it is run, the above
5617 example should print the following table:
5620 unpack("V",$_) 01234567890123456789012345678901
5621 ------------------------------------------------------------------
5622 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5623 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5624 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5625 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5626 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5627 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5628 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5629 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5630 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5631 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5632 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5633 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5634 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5635 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5636 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5637 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5638 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5639 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5640 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5641 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5642 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5643 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5644 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5645 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5646 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5647 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5648 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5649 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5650 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5651 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5652 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5653 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5654 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5655 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5656 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5657 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5658 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5659 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5660 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5661 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5662 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5663 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5664 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5665 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5666 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5667 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5668 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5669 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5670 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5671 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5672 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5673 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5674 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5675 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5676 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5677 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5678 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5679 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5680 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5681 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5682 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5683 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5684 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5685 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5686 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5687 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5688 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5689 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5690 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5691 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5692 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5693 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5694 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5695 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5696 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5697 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5698 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5699 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5700 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5701 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5702 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5703 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5704 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5705 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5706 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5707 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5708 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5709 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5710 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5711 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5712 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5713 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5714 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5715 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5716 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5717 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5718 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5719 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5720 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5721 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5722 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5723 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5724 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5725 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5726 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5727 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5728 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5729 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5730 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5731 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5732 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5733 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5734 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5735 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5736 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5737 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5738 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5739 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5740 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5741 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5742 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5743 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5744 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5745 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5746 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5747 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5748 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5749 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5753 Behaves like the wait(2) system call on your system: it waits for a child
5754 process to terminate and returns the pid of the deceased process, or
5755 C<-1> if there are no child processes. The status is returned in C<$?>.
5756 Note that a return value of C<-1> could mean that child processes are
5757 being automatically reaped, as described in L<perlipc>.
5759 =item waitpid PID,FLAGS
5761 Waits for a particular child process to terminate and returns the pid of
5762 the deceased process, or C<-1> if there is no such child process. On some
5763 systems, a value of 0 indicates that there are processes still running.
5764 The status is returned in C<$?>. If you say
5766 use POSIX ":sys_wait_h";
5769 $kid = waitpid(-1,&WNOHANG);
5772 then you can do a non-blocking wait for all pending zombie processes.
5773 Non-blocking wait is available on machines supporting either the
5774 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5775 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5776 system call by remembering the status values of processes that have
5777 exited but have not been harvested by the Perl script yet.)
5779 Note that on some systems, a return value of C<-1> could mean that child
5780 processes are being automatically reaped. See L<perlipc> for details,
5781 and for other examples.
5785 Returns true if the context of the currently executing subroutine is
5786 looking for a list value. Returns false if the context is looking
5787 for a scalar. Returns the undefined value if the context is looking
5788 for no value (void context).
5790 return unless defined wantarray; # don't bother doing more
5791 my @a = complex_calculation();
5792 return wantarray ? @a : "@a";
5794 This function should have been named wantlist() instead.
5798 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5801 If LIST is empty and C<$@> already contains a value (typically from a
5802 previous eval) that value is used after appending C<"\t...caught">
5803 to C<$@>. This is useful for staying almost, but not entirely similar to
5806 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5808 No message is printed if there is a C<$SIG{__WARN__}> handler
5809 installed. It is the handler's responsibility to deal with the message
5810 as it sees fit (like, for instance, converting it into a C<die>). Most
5811 handlers must therefore make arrangements to actually display the
5812 warnings that they are not prepared to deal with, by calling C<warn>
5813 again in the handler. Note that this is quite safe and will not
5814 produce an endless loop, since C<__WARN__> hooks are not called from
5817 You will find this behavior is slightly different from that of
5818 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5819 instead call C<die> again to change it).
5821 Using a C<__WARN__> handler provides a powerful way to silence all
5822 warnings (even the so-called mandatory ones). An example:
5824 # wipe out *all* compile-time warnings
5825 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5827 my $foo = 20; # no warning about duplicate my $foo,
5828 # but hey, you asked for it!
5829 # no compile-time or run-time warnings before here
5832 # run-time warnings enabled after here
5833 warn "\$foo is alive and $foo!"; # does show up
5835 See L<perlvar> for details on setting C<%SIG> entries, and for more
5836 examples. See the Carp module for other kinds of warnings using its
5837 carp() and cluck() functions.
5839 =item write FILEHANDLE
5845 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5846 using the format associated with that file. By default the format for
5847 a file is the one having the same name as the filehandle, but the
5848 format for the current output channel (see the C<select> function) may be set
5849 explicitly by assigning the name of the format to the C<$~> variable.
5851 Top of form processing is handled automatically: if there is
5852 insufficient room on the current page for the formatted record, the
5853 page is advanced by writing a form feed, a special top-of-page format
5854 is used to format the new page header, and then the record is written.
5855 By default the top-of-page format is the name of the filehandle with
5856 "_TOP" appended, but it may be dynamically set to the format of your
5857 choice by assigning the name to the C<$^> variable while the filehandle is
5858 selected. The number of lines remaining on the current page is in
5859 variable C<$->, which can be set to C<0> to force a new page.
5861 If FILEHANDLE is unspecified, output goes to the current default output
5862 channel, which starts out as STDOUT but may be changed by the
5863 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5864 is evaluated and the resulting string is used to look up the name of
5865 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5867 Note that write is I<not> the opposite of C<read>. Unfortunately.
5871 The transliteration operator. Same as C<tr///>. See L<perlop>.