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 A 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<sockatmark>, 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<sockatmark>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
249 =item I<-X> FILEHANDLE
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 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>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Age of file in days when script started.
298 -A Same for access time.
299 -C Same for inode change time.
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current stdio buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) Example:
355 print "Can do.\n" if -r $a || -w _ || -x _;
358 print "Readable\n" if -r _;
359 print "Writable\n" if -w _;
360 print "Executable\n" if -x _;
361 print "Setuid\n" if -u _;
362 print "Setgid\n" if -g _;
363 print "Sticky\n" if -k _;
364 print "Text\n" if -T _;
365 print "Binary\n" if -B _;
371 Returns the absolute value of its argument.
372 If VALUE is omitted, uses C<$_>.
374 =item accept NEWSOCKET,GENERICSOCKET
376 Accepts an incoming socket connect, just as the accept(2) system call
377 does. Returns the packed address if it succeeded, false otherwise.
378 See the example in L<perlipc/"Sockets: Client/Server Communication">.
380 On systems that support a close-on-exec flag on files, the flag will
381 be set for the newly opened file descriptor, as determined by the
382 value of $^F. See L<perlvar/$^F>.
388 Arranges to have a SIGALRM delivered to this process after the
389 specified number of seconds have elapsed. If SECONDS is not specified,
390 the value stored in C<$_> is used. (On some machines,
391 unfortunately, the elapsed time may be up to one second less than you
392 specified because of how seconds are counted.) Only one timer may be
393 counting at once. Each call disables the previous timer, and an
394 argument of C<0> may be supplied to cancel the previous timer without
395 starting a new one. The returned value is the amount of time remaining
396 on the previous timer.
398 For delays of finer granularity than one second, you may use Perl's
399 four-argument version of select() leaving the first three arguments
400 undefined, or you might be able to use the C<syscall> interface to
401 access setitimer(2) if your system supports it. The Time::HiRes
402 module (from CPAN, and starting from Perl 5.8 part of the standard
403 distribution) may also prove useful.
405 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
406 (C<sleep> may be internally implemented in your system with C<alarm>)
408 If you want to use C<alarm> to time out a system call you need to use an
409 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
410 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
411 restart system calls on some systems. Using C<eval>/C<die> always works,
412 modulo the caveats given in L<perlipc/"Signals">.
415 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
417 $nread = sysread SOCKET, $buffer, $size;
421 die unless $@ eq "alarm\n"; # propagate unexpected errors
430 Returns the arctangent of Y/X in the range -PI to PI.
432 For the tangent operation, you may use the C<Math::Trig::tan>
433 function, or use the familiar relation:
435 sub tan { sin($_[0]) / cos($_[0]) }
437 =item bind SOCKET,NAME
439 Binds a network address to a socket, just as the bind system call
440 does. Returns true if it succeeded, false otherwise. NAME should be a
441 packed address of the appropriate type for the socket. See the examples in
442 L<perlipc/"Sockets: Client/Server Communication">.
444 =item binmode FILEHANDLE, DISCIPLINE
446 =item binmode FILEHANDLE
448 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
449 on systems where the run-time libraries distinguish between binary and
450 text files. If FILEHANDLE is an expression, the value is taken as the
451 name of the filehandle. DISCIPLINE can be either of C<:raw> for
452 binary mode or C<:crlf> for "text" mode. If the DISCIPLINE is
453 omitted, it defaults to C<:raw>. Returns true on success, C<undef> on
454 failure. The C<:raw> are C<:clrf>, and any other directives of the
455 form C<:...>, are called I/O I<disciplines>.
457 The C<open> pragma can be used to establish default I/O disciplines.
460 In general, binmode() should be called after open() but before any I/O
461 is done on the filehandle. Calling binmode() will flush any possibly
462 pending buffered input or output data on the handle. The only
463 exception to this is the C<:encoding> discipline that changes
464 the default character encoding of the handle, see L<open>.
465 The C<:encoding> discipline sometimes needs to be called in
466 mid-stream, and it doesn't flush the stream.
468 On some systems binmode() is necessary when you're not working with a
469 text file. For the sake of portability it is a good idea to always use
470 it when appropriate, and to never use it when it isn't appropriate.
472 In other words: Regardless of platform, use binmode() on binary
473 files, and do not use binmode() on text files.
475 The operating system, device drivers, C libraries, and Perl run-time
476 system all work together to let the programmer treat a single
477 character (C<\n>) as the line terminator, irrespective of the external
478 representation. On many operating systems, the native text file
479 representation matches the internal representation, but on some
480 platforms the external representation of C<\n> is made up of more than
483 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
484 character to end each line in the external representation of text (even
485 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
486 on Unix and most VMS files). Consequently binmode() has no effect on
487 these operating systems. In other systems like OS/2, DOS and the various
488 flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
489 what's stored in text files are the two characters C<\cM\cJ>. That means
490 that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
491 disk will be converted to C<\n> on input, and any C<\n> in your program
492 will be converted back to C<\cM\cJ> on output. This is what you want for
493 text files, but it can be disastrous for binary files.
495 Another consequence of using binmode() (on some systems) is that
496 special end-of-file markers will be seen as part of the data stream.
497 For systems from the Microsoft family this means that if your binary
498 data contains C<\cZ>, the I/O subsystem will regard it as the end of
499 the file, unless you use binmode().
501 binmode() is not only important for readline() and print() operations,
502 but also when using read(), seek(), sysread(), syswrite() and tell()
503 (see L<perlport> for more details). See the C<$/> and C<$\> variables
504 in L<perlvar> for how to manually set your input and output
505 line-termination sequences.
507 =item bless REF,CLASSNAME
511 This function tells the thingy referenced by REF that it is now an object
512 in the CLASSNAME package. If CLASSNAME is omitted, the current package
513 is used. Because a C<bless> is often the last thing in a constructor,
514 it returns the reference for convenience. Always use the two-argument
515 version if the function doing the blessing might be inherited by a
516 derived class. See L<perltoot> and L<perlobj> for more about the blessing
517 (and blessings) of objects.
519 Consider always blessing objects in CLASSNAMEs that are mixed case.
520 Namespaces with all lowercase names are considered reserved for
521 Perl pragmata. Builtin types have all uppercase names, so to prevent
522 confusion, you may wish to avoid such package names as well. Make sure
523 that CLASSNAME is a true value.
525 See L<perlmod/"Perl Modules">.
531 Returns the context of the current subroutine call. In scalar context,
532 returns the caller's package name if there is a caller, that is, if
533 we're in a subroutine or C<eval> or C<require>, and the undefined value
534 otherwise. In list context, returns
536 ($package, $filename, $line) = caller;
538 With EXPR, it returns some extra information that the debugger uses to
539 print a stack trace. The value of EXPR indicates how many call frames
540 to go back before the current one.
542 ($package, $filename, $line, $subroutine, $hasargs,
543 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
545 Here $subroutine may be C<(eval)> if the frame is not a subroutine
546 call, but an C<eval>. In such a case additional elements $evaltext and
547 C<$is_require> are set: C<$is_require> is true if the frame is created by a
548 C<require> or C<use> statement, $evaltext contains the text of the
549 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
550 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
551 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
552 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
553 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
554 was compiled with. The C<$hints> and C<$bitmask> values are subject to
555 change between versions of Perl, and are not meant for external use.
557 Furthermore, when called from within the DB package, caller returns more
558 detailed information: it sets the list variable C<@DB::args> to be the
559 arguments with which the subroutine was invoked.
561 Be aware that the optimizer might have optimized call frames away before
562 C<caller> had a chance to get the information. That means that C<caller(N)>
563 might not return information about the call frame you expect it do, for
564 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
565 previous time C<caller> was called.
569 Changes the working directory to EXPR, if possible. If EXPR is omitted,
570 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
571 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
572 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
573 neither is set, C<chdir> does nothing. It returns true upon success,
574 false otherwise. See the example under C<die>.
578 Changes the permissions of a list of files. The first element of the
579 list must be the numerical mode, which should probably be an octal
580 number, and which definitely should I<not> a string of octal digits:
581 C<0644> is okay, C<'0644'> is not. Returns the number of files
582 successfully changed. See also L</oct>, if all you have is a string.
584 $cnt = chmod 0755, 'foo', 'bar';
585 chmod 0755, @executables;
586 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
588 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
589 $mode = 0644; chmod $mode, 'foo'; # this is best
591 You can also import the symbolic C<S_I*> constants from the Fcntl
596 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
597 # This is identical to the chmod 0755 of the above example.
605 This safer version of L</chop> removes any trailing string
606 that corresponds to the current value of C<$/> (also known as
607 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
608 number of characters removed from all its arguments. It's often used to
609 remove the newline from the end of an input record when you're worried
610 that the final record may be missing its newline. When in paragraph
611 mode (C<$/ = "">), it removes all trailing newlines from the string.
612 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
613 a reference to an integer or the like, see L<perlvar>) chomp() won't
615 If VARIABLE is omitted, it chomps C<$_>. Example:
618 chomp; # avoid \n on last field
623 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
625 You can actually chomp anything that's an lvalue, including an assignment:
628 chomp($answer = <STDIN>);
630 If you chomp a list, each element is chomped, and the total number of
631 characters removed is returned.
639 Chops off the last character of a string and returns the character
640 chopped. It is much more efficient than C<s/.$//s> because it neither
641 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
642 If VARIABLE is a hash, it chops the hash's values, but not its keys.
644 You can actually chop anything that's an lvalue, including an assignment.
646 If you chop a list, each element is chopped. Only the value of the
647 last C<chop> is returned.
649 Note that C<chop> returns the last character. To return all but the last
650 character, use C<substr($string, 0, -1)>.
654 Changes the owner (and group) of a list of files. The first two
655 elements of the list must be the I<numeric> uid and gid, in that
656 order. A value of -1 in either position is interpreted by most
657 systems to leave that value unchanged. Returns the number of files
658 successfully changed.
660 $cnt = chown $uid, $gid, 'foo', 'bar';
661 chown $uid, $gid, @filenames;
663 Here's an example that looks up nonnumeric uids in the passwd file:
666 chomp($user = <STDIN>);
668 chomp($pattern = <STDIN>);
670 ($login,$pass,$uid,$gid) = getpwnam($user)
671 or die "$user not in passwd file";
673 @ary = glob($pattern); # expand filenames
674 chown $uid, $gid, @ary;
676 On most systems, you are not allowed to change the ownership of the
677 file unless you're the superuser, although you should be able to change
678 the group to any of your secondary groups. On insecure systems, these
679 restrictions may be relaxed, but this is not a portable assumption.
680 On POSIX systems, you can detect this condition this way:
682 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
683 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
689 Returns the character represented by that NUMBER in the character set.
690 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
691 chr(0x263a) is a Unicode smiley face. Note that characters from 127
692 to 255 (inclusive) are by default not encoded in Unicode for backward
693 compatibility reasons (but see L<encoding>).
695 For the reverse, use L</ord>.
696 See L<perlunicode> and L<encoding> for more about Unicode.
698 If NUMBER is omitted, uses C<$_>.
700 =item chroot FILENAME
704 This function works like the system call by the same name: it makes the
705 named directory the new root directory for all further pathnames that
706 begin with a C</> by your process and all its children. (It doesn't
707 change your current working directory, which is unaffected.) For security
708 reasons, this call is restricted to the superuser. If FILENAME is
709 omitted, does a C<chroot> to C<$_>.
711 =item close FILEHANDLE
715 Closes the file or pipe associated with the file handle, returning true
716 only if stdio successfully flushes buffers and closes the system file
717 descriptor. Closes the currently selected filehandle if the argument
720 You don't have to close FILEHANDLE if you are immediately going to do
721 another C<open> on it, because C<open> will close it for you. (See
722 C<open>.) However, an explicit C<close> on an input file resets the line
723 counter (C<$.>), while the implicit close done by C<open> does not.
725 If the file handle came from a piped open C<close> will additionally
726 return false if one of the other system calls involved fails or if the
727 program exits with non-zero status. (If the only problem was that the
728 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
729 also waits for the process executing on the pipe to complete, in case you
730 want to look at the output of the pipe afterwards, and
731 implicitly puts the exit status value of that command into C<$?>.
733 Prematurely closing the read end of a pipe (i.e. before the process
734 writing to it at the other end has closed it) will result in a
735 SIGPIPE being delivered to the writer. If the other end can't
736 handle that, be sure to read all the data before closing the pipe.
740 open(OUTPUT, '|sort >foo') # pipe to sort
741 or die "Can't start sort: $!";
742 #... # print stuff to output
743 close OUTPUT # wait for sort to finish
744 or warn $! ? "Error closing sort pipe: $!"
745 : "Exit status $? from sort";
746 open(INPUT, 'foo') # get sort's results
747 or die "Can't open 'foo' for input: $!";
749 FILEHANDLE may be an expression whose value can be used as an indirect
750 filehandle, usually the real filehandle name.
752 =item closedir DIRHANDLE
754 Closes a directory opened by C<opendir> and returns the success of that
757 DIRHANDLE may be an expression whose value can be used as an indirect
758 dirhandle, usually the real dirhandle name.
760 =item connect SOCKET,NAME
762 Attempts to connect to a remote socket, just as the connect system call
763 does. Returns true if it succeeded, false otherwise. NAME should be a
764 packed address of the appropriate type for the socket. See the examples in
765 L<perlipc/"Sockets: Client/Server Communication">.
769 Actually a flow control statement rather than a function. If there is a
770 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
771 C<foreach>), it is always executed just before the conditional is about to
772 be evaluated again, just like the third part of a C<for> loop in C. Thus
773 it can be used to increment a loop variable, even when the loop has been
774 continued via the C<next> statement (which is similar to the C C<continue>
777 C<last>, C<next>, or C<redo> may appear within a C<continue>
778 block. C<last> and C<redo> will behave as if they had been executed within
779 the main block. So will C<next>, but since it will execute a C<continue>
780 block, it may be more entertaining.
783 ### redo always comes here
786 ### next always comes here
788 # then back the top to re-check EXPR
790 ### last always comes here
792 Omitting the C<continue> section is semantically equivalent to using an
793 empty one, logically enough. In that case, C<next> goes directly back
794 to check the condition at the top of the loop.
800 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
801 takes cosine of C<$_>.
803 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
804 function, or use this relation:
806 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
808 =item crypt PLAINTEXT,SALT
810 Encrypts a string exactly like the crypt(3) function in the C library
811 (assuming that you actually have a version there that has not been
812 extirpated as a potential munition). This can prove useful for checking
813 the password file for lousy passwords, amongst other things. Only the
814 guys wearing white hats should do this.
816 Note that C<crypt> is intended to be a one-way function, much like
817 breaking eggs to make an omelette. There is no (known) corresponding
818 decrypt function (in other words, the crypt() is a one-way hash
819 function). As a result, this function isn't all that useful for
820 cryptography. (For that, see your nearby CPAN mirror.)
822 When verifying an existing encrypted string you should use the
823 encrypted text as the salt (like C<crypt($plain, $crypted) eq
824 $crypted>). This allows your code to work with the standard C<crypt>
825 and with more exotic implementations. In other words, do not assume
826 anything about the returned string itself, or how many bytes in
827 the encrypted string matter.
829 Traditionally the result is a string of 13 bytes: two first bytes of
830 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
831 the first eight bytes of the encrypted string mattered, but
832 alternative hashing schemes (like MD5), higher level security schemes
833 (like C2), and implementations on non-UNIX platforms may produce
836 When choosing a new salt create a random two character string whose
837 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
838 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
840 Here's an example that makes sure that whoever runs this program knows
843 $pwd = (getpwuid($<))[1];
847 chomp($word = <STDIN>);
851 if (crypt($word, $pwd) ne $pwd) {
857 Of course, typing in your own password to whoever asks you
860 The L<crypt> function is unsuitable for encrypting large quantities
861 of data, not least of all because you can't get the information
862 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
863 on your favorite CPAN mirror for a slew of potentially useful
866 If using crypt() on a Unicode string (which potentially has
867 characters with codepoints above 255), Perl tries to make sense of
868 the situation by using only the low eight bits of the characters when
873 [This function has been largely superseded by the C<untie> function.]
875 Breaks the binding between a DBM file and a hash.
877 =item dbmopen HASH,DBNAME,MASK
879 [This function has been largely superseded by the C<tie> function.]
881 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
882 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
883 argument is I<not> a filehandle, even though it looks like one). DBNAME
884 is the name of the database (without the F<.dir> or F<.pag> extension if
885 any). If the database does not exist, it is created with protection
886 specified by MASK (as modified by the C<umask>). If your system supports
887 only the older DBM functions, you may perform only one C<dbmopen> in your
888 program. In older versions of Perl, if your system had neither DBM nor
889 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
892 If you don't have write access to the DBM file, you can only read hash
893 variables, not set them. If you want to test whether you can write,
894 either use file tests or try setting a dummy hash entry inside an C<eval>,
895 which will trap the error.
897 Note that functions such as C<keys> and C<values> may return huge lists
898 when used on large DBM files. You may prefer to use the C<each>
899 function to iterate over large DBM files. Example:
901 # print out history file offsets
902 dbmopen(%HIST,'/usr/lib/news/history',0666);
903 while (($key,$val) = each %HIST) {
904 print $key, ' = ', unpack('L',$val), "\n";
908 See also L<AnyDBM_File> for a more general description of the pros and
909 cons of the various dbm approaches, as well as L<DB_File> for a particularly
912 You can control which DBM library you use by loading that library
913 before you call dbmopen():
916 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
917 or die "Can't open netscape history file: $!";
923 Returns a Boolean value telling whether EXPR has a value other than
924 the undefined value C<undef>. If EXPR is not present, C<$_> will be
927 Many operations return C<undef> to indicate failure, end of file,
928 system error, uninitialized variable, and other exceptional
929 conditions. This function allows you to distinguish C<undef> from
930 other values. (A simple Boolean test will not distinguish among
931 C<undef>, zero, the empty string, and C<"0">, which are all equally
932 false.) Note that since C<undef> is a valid scalar, its presence
933 doesn't I<necessarily> indicate an exceptional condition: C<pop>
934 returns C<undef> when its argument is an empty array, I<or> when the
935 element to return happens to be C<undef>.
937 You may also use C<defined(&func)> to check whether subroutine C<&func>
938 has ever been defined. The return value is unaffected by any forward
939 declarations of C<&foo>. Note that a subroutine which is not defined
940 may still be callable: its package may have an C<AUTOLOAD> method that
941 makes it spring into existence the first time that it is called -- see
944 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
945 used to report whether memory for that aggregate has ever been
946 allocated. This behavior may disappear in future versions of Perl.
947 You should instead use a simple test for size:
949 if (@an_array) { print "has array elements\n" }
950 if (%a_hash) { print "has hash members\n" }
952 When used on a hash element, it tells you whether the value is defined,
953 not whether the key exists in the hash. Use L</exists> for the latter
958 print if defined $switch{'D'};
959 print "$val\n" while defined($val = pop(@ary));
960 die "Can't readlink $sym: $!"
961 unless defined($value = readlink $sym);
962 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
963 $debugging = 0 unless defined $debugging;
965 Note: Many folks tend to overuse C<defined>, and then are surprised to
966 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
967 defined values. For example, if you say
971 The pattern match succeeds, and C<$1> is defined, despite the fact that it
972 matched "nothing". But it didn't really match nothing--rather, it
973 matched something that happened to be zero characters long. This is all
974 very above-board and honest. When a function returns an undefined value,
975 it's an admission that it couldn't give you an honest answer. So you
976 should use C<defined> only when you're questioning the integrity of what
977 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
980 See also L</undef>, L</exists>, L</ref>.
984 Given an expression that specifies a hash element, array element, hash slice,
985 or array slice, deletes the specified element(s) from the hash or array.
986 In the case of an array, if the array elements happen to be at the end,
987 the size of the array will shrink to the highest element that tests
988 true for exists() (or 0 if no such element exists).
990 Returns each element so deleted or the undefined value if there was no such
991 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
992 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
993 from a C<tie>d hash or array may not necessarily return anything.
995 Deleting an array element effectively returns that position of the array
996 to its initial, uninitialized state. Subsequently testing for the same
997 element with exists() will return false. Note that deleting array
998 elements in the middle of an array will not shift the index of the ones
999 after them down--use splice() for that. See L</exists>.
1001 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1003 foreach $key (keys %HASH) {
1007 foreach $index (0 .. $#ARRAY) {
1008 delete $ARRAY[$index];
1013 delete @HASH{keys %HASH};
1015 delete @ARRAY[0 .. $#ARRAY];
1017 But both of these are slower than just assigning the empty list
1018 or undefining %HASH or @ARRAY:
1020 %HASH = (); # completely empty %HASH
1021 undef %HASH; # forget %HASH ever existed
1023 @ARRAY = (); # completely empty @ARRAY
1024 undef @ARRAY; # forget @ARRAY ever existed
1026 Note that the EXPR can be arbitrarily complicated as long as the final
1027 operation is a hash element, array element, hash slice, or array slice
1030 delete $ref->[$x][$y]{$key};
1031 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1033 delete $ref->[$x][$y][$index];
1034 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1038 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1039 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1040 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1041 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1042 an C<eval(),> the error message is stuffed into C<$@> and the
1043 C<eval> is terminated with the undefined value. This makes
1044 C<die> the way to raise an exception.
1046 Equivalent examples:
1048 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1049 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1051 If the value of EXPR does not end in a newline, the current script line
1052 number and input line number (if any) are also printed, and a newline
1053 is supplied. Note that the "input line number" (also known as "chunk")
1054 is subject to whatever notion of "line" happens to be currently in
1055 effect, and is also available as the special variable C<$.>.
1056 See L<perlvar/"$/"> and L<perlvar/"$.">.
1058 Hint: sometimes appending C<", stopped"> to your message
1059 will cause it to make better sense when the string C<"at foo line 123"> is
1060 appended. Suppose you are running script "canasta".
1062 die "/etc/games is no good";
1063 die "/etc/games is no good, stopped";
1065 produce, respectively
1067 /etc/games is no good at canasta line 123.
1068 /etc/games is no good, stopped at canasta line 123.
1070 See also exit(), warn(), and the Carp module.
1072 If LIST is empty and C<$@> already contains a value (typically from a
1073 previous eval) that value is reused after appending C<"\t...propagated">.
1074 This is useful for propagating exceptions:
1077 die unless $@ =~ /Expected exception/;
1079 If C<$@> is empty then the string C<"Died"> is used.
1081 die() can also be called with a reference argument. If this happens to be
1082 trapped within an eval(), $@ contains the reference. This behavior permits
1083 a more elaborate exception handling implementation using objects that
1084 maintain arbitrary state about the nature of the exception. Such a scheme
1085 is sometimes preferable to matching particular string values of $@ using
1086 regular expressions. Here's an example:
1088 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1090 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1091 # handle Some::Module::Exception
1094 # handle all other possible exceptions
1098 Because perl will stringify uncaught exception messages before displaying
1099 them, you may want to overload stringification operations on such custom
1100 exception objects. See L<overload> for details about that.
1102 You can arrange for a callback to be run just before the C<die>
1103 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1104 handler will be called with the error text and can change the error
1105 message, if it sees fit, by calling C<die> again. See
1106 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1107 L<"eval BLOCK"> for some examples. Although this feature was meant
1108 to be run only right before your program was to exit, this is not
1109 currently the case--the C<$SIG{__DIE__}> hook is currently called
1110 even inside eval()ed blocks/strings! If one wants the hook to do
1111 nothing in such situations, put
1115 as the first line of the handler (see L<perlvar/$^S>). Because
1116 this promotes strange action at a distance, this counterintuitive
1117 behavior may be fixed in a future release.
1121 Not really a function. Returns the value of the last command in the
1122 sequence of commands indicated by BLOCK. When modified by a loop
1123 modifier, executes the BLOCK once before testing the loop condition.
1124 (On other statements the loop modifiers test the conditional first.)
1126 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1127 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1128 See L<perlsyn> for alternative strategies.
1130 =item do SUBROUTINE(LIST)
1132 A deprecated form of subroutine call. See L<perlsub>.
1136 Uses the value of EXPR as a filename and executes the contents of the
1137 file as a Perl script. Its primary use is to include subroutines
1138 from a Perl subroutine library.
1146 except that it's more efficient and concise, keeps track of the current
1147 filename for error messages, searches the @INC libraries, and updates
1148 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1149 variables. It also differs in that code evaluated with C<do FILENAME>
1150 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1151 same, however, in that it does reparse the file every time you call it,
1152 so you probably don't want to do this inside a loop.
1154 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1155 error. If C<do> can read the file but cannot compile it, it
1156 returns undef and sets an error message in C<$@>. If the file is
1157 successfully compiled, C<do> returns the value of the last expression
1160 Note that inclusion of library modules is better done with the
1161 C<use> and C<require> operators, which also do automatic error checking
1162 and raise an exception if there's a problem.
1164 You might like to use C<do> to read in a program configuration
1165 file. Manual error checking can be done this way:
1167 # read in config files: system first, then user
1168 for $file ("/share/prog/defaults.rc",
1169 "$ENV{HOME}/.someprogrc")
1171 unless ($return = do $file) {
1172 warn "couldn't parse $file: $@" if $@;
1173 warn "couldn't do $file: $!" unless defined $return;
1174 warn "couldn't run $file" unless $return;
1182 This function causes an immediate core dump. See also the B<-u>
1183 command-line switch in L<perlrun>, which does the same thing.
1184 Primarily this is so that you can use the B<undump> program (not
1185 supplied) to turn your core dump into an executable binary after
1186 having initialized all your variables at the beginning of the
1187 program. When the new binary is executed it will begin by executing
1188 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1189 Think of it as a goto with an intervening core dump and reincarnation.
1190 If C<LABEL> is omitted, restarts the program from the top.
1192 B<WARNING>: Any files opened at the time of the dump will I<not>
1193 be open any more when the program is reincarnated, with possible
1194 resulting confusion on the part of Perl.
1196 This function is now largely obsolete, partly because it's very
1197 hard to convert a core file into an executable, and because the
1198 real compiler backends for generating portable bytecode and compilable
1199 C code have superseded it.
1201 If you're looking to use L<dump> to speed up your program, consider
1202 generating bytecode or native C code as described in L<perlcc>. If
1203 you're just trying to accelerate a CGI script, consider using the
1204 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1205 You might also consider autoloading or selfloading, which at least
1206 make your program I<appear> to run faster.
1210 When called in list context, returns a 2-element list consisting of the
1211 key and value for the next element of a hash, so that you can iterate over
1212 it. When called in scalar context, returns only the key for the next
1213 element in the hash.
1215 Entries are returned in an apparently random order. The actual random
1216 order is subject to change in future versions of perl, but it is guaranteed
1217 to be in the same order as either the C<keys> or C<values> function
1218 would produce on the same (unmodified) hash.
1220 When the hash is entirely read, a null array is returned in list context
1221 (which when assigned produces a false (C<0>) value), and C<undef> in
1222 scalar context. The next call to C<each> after that will start iterating
1223 again. There is a single iterator for each hash, shared by all C<each>,
1224 C<keys>, and C<values> function calls in the program; it can be reset by
1225 reading all the elements from the hash, or by evaluating C<keys HASH> or
1226 C<values HASH>. If you add or delete elements of a hash while you're
1227 iterating over it, you may get entries skipped or duplicated, so
1228 don't. Exception: It is always safe to delete the item most recently
1229 returned by C<each()>, which means that the following code will work:
1231 while (($key, $value) = each %hash) {
1233 delete $hash{$key}; # This is safe
1236 The following prints out your environment like the printenv(1) program,
1237 only in a different order:
1239 while (($key,$value) = each %ENV) {
1240 print "$key=$value\n";
1243 See also C<keys>, C<values> and C<sort>.
1245 =item eof FILEHANDLE
1251 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1252 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1253 gives the real filehandle. (Note that this function actually
1254 reads a character and then C<ungetc>s it, so isn't very useful in an
1255 interactive context.) Do not read from a terminal file (or call
1256 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1257 as terminals may lose the end-of-file condition if you do.
1259 An C<eof> without an argument uses the last file read. Using C<eof()>
1260 with empty parentheses is very different. It refers to the pseudo file
1261 formed from the files listed on the command line and accessed via the
1262 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1263 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1264 used will cause C<@ARGV> to be examined to determine if input is
1267 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1268 detect the end of each file, C<eof()> will only detect the end of the
1269 last file. Examples:
1271 # reset line numbering on each input file
1273 next if /^\s*#/; # skip comments
1276 close ARGV if eof; # Not eof()!
1279 # insert dashes just before last line of last file
1281 if (eof()) { # check for end of current file
1282 print "--------------\n";
1283 close(ARGV); # close or last; is needed if we
1284 # are reading from the terminal
1289 Practical hint: you almost never need to use C<eof> in Perl, because the
1290 input operators typically return C<undef> when they run out of data, or if
1297 In the first form, the return value of EXPR is parsed and executed as if it
1298 were a little Perl program. The value of the expression (which is itself
1299 determined within scalar context) is first parsed, and if there weren't any
1300 errors, executed in the lexical context of the current Perl program, so
1301 that any variable settings or subroutine and format definitions remain
1302 afterwards. Note that the value is parsed every time the eval executes.
1303 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1304 delay parsing and subsequent execution of the text of EXPR until run time.
1306 In the second form, the code within the BLOCK is parsed only once--at the
1307 same time the code surrounding the eval itself was parsed--and executed
1308 within the context of the current Perl program. This form is typically
1309 used to trap exceptions more efficiently than the first (see below), while
1310 also providing the benefit of checking the code within BLOCK at compile
1313 The final semicolon, if any, may be omitted from the value of EXPR or within
1316 In both forms, the value returned is the value of the last expression
1317 evaluated inside the mini-program; a return statement may be also used, just
1318 as with subroutines. The expression providing the return value is evaluated
1319 in void, scalar, or list context, depending on the context of the eval itself.
1320 See L</wantarray> for more on how the evaluation context can be determined.
1322 If there is a syntax error or runtime error, or a C<die> statement is
1323 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1324 error message. If there was no error, C<$@> is guaranteed to be a null
1325 string. Beware that using C<eval> neither silences perl from printing
1326 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1327 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1328 L</warn> and L<perlvar>.
1330 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1331 determining whether a particular feature (such as C<socket> or C<symlink>)
1332 is implemented. It is also Perl's exception trapping mechanism, where
1333 the die operator is used to raise exceptions.
1335 If the code to be executed doesn't vary, you may use the eval-BLOCK
1336 form to trap run-time errors without incurring the penalty of
1337 recompiling each time. The error, if any, is still returned in C<$@>.
1340 # make divide-by-zero nonfatal
1341 eval { $answer = $a / $b; }; warn $@ if $@;
1343 # same thing, but less efficient
1344 eval '$answer = $a / $b'; warn $@ if $@;
1346 # a compile-time error
1347 eval { $answer = }; # WRONG
1350 eval '$answer ='; # sets $@
1352 Due to the current arguably broken state of C<__DIE__> hooks, when using
1353 the C<eval{}> form as an exception trap in libraries, you may wish not
1354 to trigger any C<__DIE__> hooks that user code may have installed.
1355 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1356 as shown in this example:
1358 # a very private exception trap for divide-by-zero
1359 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1362 This is especially significant, given that C<__DIE__> hooks can call
1363 C<die> again, which has the effect of changing their error messages:
1365 # __DIE__ hooks may modify error messages
1367 local $SIG{'__DIE__'} =
1368 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1369 eval { die "foo lives here" };
1370 print $@ if $@; # prints "bar lives here"
1373 Because this promotes action at a distance, this counterintuitive behavior
1374 may be fixed in a future release.
1376 With an C<eval>, you should be especially careful to remember what's
1377 being looked at when:
1383 eval { $x }; # CASE 4
1385 eval "\$$x++"; # CASE 5
1388 Cases 1 and 2 above behave identically: they run the code contained in
1389 the variable $x. (Although case 2 has misleading double quotes making
1390 the reader wonder what else might be happening (nothing is).) Cases 3
1391 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1392 does nothing but return the value of $x. (Case 4 is preferred for
1393 purely visual reasons, but it also has the advantage of compiling at
1394 compile-time instead of at run-time.) Case 5 is a place where
1395 normally you I<would> like to use double quotes, except that in this
1396 particular situation, you can just use symbolic references instead, as
1399 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1400 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1404 =item exec PROGRAM LIST
1406 The C<exec> function executes a system command I<and never returns>--
1407 use C<system> instead of C<exec> if you want it to return. It fails and
1408 returns false only if the command does not exist I<and> it is executed
1409 directly instead of via your system's command shell (see below).
1411 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1412 warns you if there is a following statement which isn't C<die>, C<warn>,
1413 or C<exit> (if C<-w> is set - but you always do that). If you
1414 I<really> want to follow an C<exec> with some other statement, you
1415 can use one of these styles to avoid the warning:
1417 exec ('foo') or print STDERR "couldn't exec foo: $!";
1418 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1420 If there is more than one argument in LIST, or if LIST is an array
1421 with more than one value, calls execvp(3) with the arguments in LIST.
1422 If there is only one scalar argument or an array with one element in it,
1423 the argument is checked for shell metacharacters, and if there are any,
1424 the entire argument is passed to the system's command shell for parsing
1425 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1426 If there are no shell metacharacters in the argument, it is split into
1427 words and passed directly to C<execvp>, which is more efficient.
1430 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1431 exec "sort $outfile | uniq";
1433 If you don't really want to execute the first argument, but want to lie
1434 to the program you are executing about its own name, you can specify
1435 the program you actually want to run as an "indirect object" (without a
1436 comma) in front of the LIST. (This always forces interpretation of the
1437 LIST as a multivalued list, even if there is only a single scalar in
1440 $shell = '/bin/csh';
1441 exec $shell '-sh'; # pretend it's a login shell
1445 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1447 When the arguments get executed via the system shell, results will
1448 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1451 Using an indirect object with C<exec> or C<system> is also more
1452 secure. This usage (which also works fine with system()) forces
1453 interpretation of the arguments as a multivalued list, even if the
1454 list had just one argument. That way you're safe from the shell
1455 expanding wildcards or splitting up words with whitespace in them.
1457 @args = ( "echo surprise" );
1459 exec @args; # subject to shell escapes
1461 exec { $args[0] } @args; # safe even with one-arg list
1463 The first version, the one without the indirect object, ran the I<echo>
1464 program, passing it C<"surprise"> an argument. The second version
1465 didn't--it tried to run a program literally called I<"echo surprise">,
1466 didn't find it, and set C<$?> to a non-zero value indicating failure.
1468 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1469 output before the exec, but this may not be supported on some platforms
1470 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1471 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1472 open handles in order to avoid lost output.
1474 Note that C<exec> will not call your C<END> blocks, nor will it call
1475 any C<DESTROY> methods in your objects.
1479 Given an expression that specifies a hash element or array element,
1480 returns true if the specified element in the hash or array has ever
1481 been initialized, even if the corresponding value is undefined. The
1482 element is not autovivified if it doesn't exist.
1484 print "Exists\n" if exists $hash{$key};
1485 print "Defined\n" if defined $hash{$key};
1486 print "True\n" if $hash{$key};
1488 print "Exists\n" if exists $array[$index];
1489 print "Defined\n" if defined $array[$index];
1490 print "True\n" if $array[$index];
1492 A hash or array element can be true only if it's defined, and defined if
1493 it exists, but the reverse doesn't necessarily hold true.
1495 Given an expression that specifies the name of a subroutine,
1496 returns true if the specified subroutine has ever been declared, even
1497 if it is undefined. Mentioning a subroutine name for exists or defined
1498 does not count as declaring it. Note that a subroutine which does not
1499 exist may still be callable: its package may have an C<AUTOLOAD>
1500 method that makes it spring into existence the first time that it is
1501 called -- see L<perlsub>.
1503 print "Exists\n" if exists &subroutine;
1504 print "Defined\n" if defined &subroutine;
1506 Note that the EXPR can be arbitrarily complicated as long as the final
1507 operation is a hash or array key lookup or subroutine name:
1509 if (exists $ref->{A}->{B}->{$key}) { }
1510 if (exists $hash{A}{B}{$key}) { }
1512 if (exists $ref->{A}->{B}->[$ix]) { }
1513 if (exists $hash{A}{B}[$ix]) { }
1515 if (exists &{$ref->{A}{B}{$key}}) { }
1517 Although the deepest nested array or hash will not spring into existence
1518 just because its existence was tested, any intervening ones will.
1519 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1520 into existence due to the existence test for the $key element above.
1521 This happens anywhere the arrow operator is used, including even:
1524 if (exists $ref->{"Some key"}) { }
1525 print $ref; # prints HASH(0x80d3d5c)
1527 This surprising autovivification in what does not at first--or even
1528 second--glance appear to be an lvalue context may be fixed in a future
1531 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1532 on how exists() acts when used on a pseudo-hash.
1534 Use of a subroutine call, rather than a subroutine name, as an argument
1535 to exists() is an error.
1538 exists &sub(); # Error
1542 Evaluates EXPR and exits immediately with that value. Example:
1545 exit 0 if $ans =~ /^[Xx]/;
1547 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1548 universally recognized values for EXPR are C<0> for success and C<1>
1549 for error; other values are subject to interpretation depending on the
1550 environment in which the Perl program is running. For example, exiting
1551 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1552 the mailer to return the item undelivered, but that's not true everywhere.
1554 Don't use C<exit> to abort a subroutine if there's any chance that
1555 someone might want to trap whatever error happened. Use C<die> instead,
1556 which can be trapped by an C<eval>.
1558 The exit() function does not always exit immediately. It calls any
1559 defined C<END> routines first, but these C<END> routines may not
1560 themselves abort the exit. Likewise any object destructors that need to
1561 be called are called before the real exit. If this is a problem, you
1562 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1563 See L<perlmod> for details.
1569 Returns I<e> (the natural logarithm base) to the power of EXPR.
1570 If EXPR is omitted, gives C<exp($_)>.
1572 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1574 Implements the fcntl(2) function. You'll probably have to say
1578 first to get the correct constant definitions. Argument processing and
1579 value return works just like C<ioctl> below.
1583 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1584 or die "can't fcntl F_GETFL: $!";
1586 You don't have to check for C<defined> on the return from C<fnctl>.
1587 Like C<ioctl>, it maps a C<0> return from the system call into
1588 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1589 in numeric context. It is also exempt from the normal B<-w> warnings
1590 on improper numeric conversions.
1592 Note that C<fcntl> will produce a fatal error if used on a machine that
1593 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1594 manpage to learn what functions are available on your system.
1596 =item fileno FILEHANDLE
1598 Returns the file descriptor for a filehandle, or undefined if the
1599 filehandle is not open. This is mainly useful for constructing
1600 bitmaps for C<select> and low-level POSIX tty-handling operations.
1601 If FILEHANDLE is an expression, the value is taken as an indirect
1602 filehandle, generally its name.
1604 You can use this to find out whether two handles refer to the
1605 same underlying descriptor:
1607 if (fileno(THIS) == fileno(THAT)) {
1608 print "THIS and THAT are dups\n";
1611 (Filehandles connected to memory objects via new features of C<open> may
1612 return undefined even though they are open.)
1615 =item flock FILEHANDLE,OPERATION
1617 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1618 for success, false on failure. Produces a fatal error if used on a
1619 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1620 C<flock> is Perl's portable file locking interface, although it locks
1621 only entire files, not records.
1623 Two potentially non-obvious but traditional C<flock> semantics are
1624 that it waits indefinitely until the lock is granted, and that its locks
1625 B<merely advisory>. Such discretionary locks are more flexible, but offer
1626 fewer guarantees. This means that files locked with C<flock> may be
1627 modified by programs that do not also use C<flock>. See L<perlport>,
1628 your port's specific documentation, or your system-specific local manpages
1629 for details. It's best to assume traditional behavior if you're writing
1630 portable programs. (But if you're not, you should as always feel perfectly
1631 free to write for your own system's idiosyncrasies (sometimes called
1632 "features"). Slavish adherence to portability concerns shouldn't get
1633 in the way of your getting your job done.)
1635 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1636 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1637 you can use the symbolic names if you import them from the Fcntl module,
1638 either individually, or as a group using the ':flock' tag. LOCK_SH
1639 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1640 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1641 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1642 waiting for the lock (check the return status to see if you got it).
1644 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1645 before locking or unlocking it.
1647 Note that the emulation built with lockf(3) doesn't provide shared
1648 locks, and it requires that FILEHANDLE be open with write intent. These
1649 are the semantics that lockf(3) implements. Most if not all systems
1650 implement lockf(3) in terms of fcntl(2) locking, though, so the
1651 differing semantics shouldn't bite too many people.
1653 Note also that some versions of C<flock> cannot lock things over the
1654 network; you would need to use the more system-specific C<fcntl> for
1655 that. If you like you can force Perl to ignore your system's flock(2)
1656 function, and so provide its own fcntl(2)-based emulation, by passing
1657 the switch C<-Ud_flock> to the F<Configure> program when you configure
1660 Here's a mailbox appender for BSD systems.
1662 use Fcntl ':flock'; # import LOCK_* constants
1665 flock(MBOX,LOCK_EX);
1666 # and, in case someone appended
1667 # while we were waiting...
1672 flock(MBOX,LOCK_UN);
1675 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1676 or die "Can't open mailbox: $!";
1679 print MBOX $msg,"\n\n";
1682 On systems that support a real flock(), locks are inherited across fork()
1683 calls, whereas those that must resort to the more capricious fcntl()
1684 function lose the locks, making it harder to write servers.
1686 See also L<DB_File> for other flock() examples.
1690 Does a fork(2) system call to create a new process running the
1691 same program at the same point. It returns the child pid to the
1692 parent process, C<0> to the child process, or C<undef> if the fork is
1693 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1694 are shared, while everything else is copied. On most systems supporting
1695 fork(), great care has gone into making it extremely efficient (for
1696 example, using copy-on-write technology on data pages), making it the
1697 dominant paradigm for multitasking over the last few decades.
1699 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1700 output before forking the child process, but this may not be supported
1701 on some platforms (see L<perlport>). To be safe, you may need to set
1702 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1703 C<IO::Handle> on any open handles in order to avoid duplicate output.
1705 If you C<fork> without ever waiting on your children, you will
1706 accumulate zombies. On some systems, you can avoid this by setting
1707 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1708 forking and reaping moribund children.
1710 Note that if your forked child inherits system file descriptors like
1711 STDIN and STDOUT that are actually connected by a pipe or socket, even
1712 if you exit, then the remote server (such as, say, a CGI script or a
1713 backgrounded job launched from a remote shell) won't think you're done.
1714 You should reopen those to F</dev/null> if it's any issue.
1718 Declare a picture format for use by the C<write> function. For
1722 Test: @<<<<<<<< @||||| @>>>>>
1723 $str, $%, '$' . int($num)
1727 $num = $cost/$quantity;
1731 See L<perlform> for many details and examples.
1733 =item formline PICTURE,LIST
1735 This is an internal function used by C<format>s, though you may call it,
1736 too. It formats (see L<perlform>) a list of values according to the
1737 contents of PICTURE, placing the output into the format output
1738 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1739 Eventually, when a C<write> is done, the contents of
1740 C<$^A> are written to some filehandle, but you could also read C<$^A>
1741 yourself and then set C<$^A> back to C<"">. Note that a format typically
1742 does one C<formline> per line of form, but the C<formline> function itself
1743 doesn't care how many newlines are embedded in the PICTURE. This means
1744 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1745 You may therefore need to use multiple formlines to implement a single
1746 record format, just like the format compiler.
1748 Be careful if you put double quotes around the picture, because an C<@>
1749 character may be taken to mean the beginning of an array name.
1750 C<formline> always returns true. See L<perlform> for other examples.
1752 =item getc FILEHANDLE
1756 Returns the next character from the input file attached to FILEHANDLE,
1757 or the undefined value at end of file, or if there was an error.
1758 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1759 efficient. However, it cannot be used by itself to fetch single
1760 characters without waiting for the user to hit enter. For that, try
1761 something more like:
1764 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1767 system "stty", '-icanon', 'eol', "\001";
1773 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1776 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1780 Determination of whether $BSD_STYLE should be set
1781 is left as an exercise to the reader.
1783 The C<POSIX::getattr> function can do this more portably on
1784 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1785 module from your nearest CPAN site; details on CPAN can be found on
1790 Implements the C library function of the same name, which on most
1791 systems returns the current login from F</etc/utmp>, if any. If null,
1794 $login = getlogin || getpwuid($<) || "Kilroy";
1796 Do not consider C<getlogin> for authentication: it is not as
1797 secure as C<getpwuid>.
1799 =item getpeername SOCKET
1801 Returns the packed sockaddr address of other end of the SOCKET connection.
1804 $hersockaddr = getpeername(SOCK);
1805 ($port, $iaddr) = sockaddr_in($hersockaddr);
1806 $herhostname = gethostbyaddr($iaddr, AF_INET);
1807 $herstraddr = inet_ntoa($iaddr);
1811 Returns the current process group for the specified PID. Use
1812 a PID of C<0> to get the current process group for the
1813 current process. Will raise an exception if used on a machine that
1814 doesn't implement getpgrp(2). If PID is omitted, returns process
1815 group of current process. Note that the POSIX version of C<getpgrp>
1816 does not accept a PID argument, so only C<PID==0> is truly portable.
1820 Returns the process id of the parent process.
1822 =item getpriority WHICH,WHO
1824 Returns the current priority for a process, a process group, or a user.
1825 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1826 machine that doesn't implement getpriority(2).
1832 =item gethostbyname NAME
1834 =item getnetbyname NAME
1836 =item getprotobyname NAME
1842 =item getservbyname NAME,PROTO
1844 =item gethostbyaddr ADDR,ADDRTYPE
1846 =item getnetbyaddr ADDR,ADDRTYPE
1848 =item getprotobynumber NUMBER
1850 =item getservbyport PORT,PROTO
1868 =item sethostent STAYOPEN
1870 =item setnetent STAYOPEN
1872 =item setprotoent STAYOPEN
1874 =item setservent STAYOPEN
1888 These routines perform the same functions as their counterparts in the
1889 system library. In list context, the return values from the
1890 various get routines are as follows:
1892 ($name,$passwd,$uid,$gid,
1893 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1894 ($name,$passwd,$gid,$members) = getgr*
1895 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1896 ($name,$aliases,$addrtype,$net) = getnet*
1897 ($name,$aliases,$proto) = getproto*
1898 ($name,$aliases,$port,$proto) = getserv*
1900 (If the entry doesn't exist you get a null list.)
1902 The exact meaning of the $gcos field varies but it usually contains
1903 the real name of the user (as opposed to the login name) and other
1904 information pertaining to the user. Beware, however, that in many
1905 system users are able to change this information and therefore it
1906 cannot be trusted and therefore the $gcos is tainted (see
1907 L<perlsec>). The $passwd and $shell, user's encrypted password and
1908 login shell, are also tainted, because of the same reason.
1910 In scalar context, you get the name, unless the function was a
1911 lookup by name, in which case you get the other thing, whatever it is.
1912 (If the entry doesn't exist you get the undefined value.) For example:
1914 $uid = getpwnam($name);
1915 $name = getpwuid($num);
1917 $gid = getgrnam($name);
1918 $name = getgrgid($num;
1922 In I<getpw*()> the fields $quota, $comment, and $expire are special
1923 cases in the sense that in many systems they are unsupported. If the
1924 $quota is unsupported, it is an empty scalar. If it is supported, it
1925 usually encodes the disk quota. If the $comment field is unsupported,
1926 it is an empty scalar. If it is supported it usually encodes some
1927 administrative comment about the user. In some systems the $quota
1928 field may be $change or $age, fields that have to do with password
1929 aging. In some systems the $comment field may be $class. The $expire
1930 field, if present, encodes the expiration period of the account or the
1931 password. For the availability and the exact meaning of these fields
1932 in your system, please consult your getpwnam(3) documentation and your
1933 F<pwd.h> file. You can also find out from within Perl what your
1934 $quota and $comment fields mean and whether you have the $expire field
1935 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1936 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1937 files are only supported if your vendor has implemented them in the
1938 intuitive fashion that calling the regular C library routines gets the
1939 shadow versions if you're running under privilege or if there exists
1940 the shadow(3) functions as found in System V ( this includes Solaris
1941 and Linux.) Those systems which implement a proprietary shadow password
1942 facility are unlikely to be supported.
1944 The $members value returned by I<getgr*()> is a space separated list of
1945 the login names of the members of the group.
1947 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1948 C, it will be returned to you via C<$?> if the function call fails. The
1949 C<@addrs> value returned by a successful call is a list of the raw
1950 addresses returned by the corresponding system library call. In the
1951 Internet domain, each address is four bytes long and you can unpack it
1952 by saying something like:
1954 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1956 The Socket library makes this slightly easier:
1959 $iaddr = inet_aton("127.1"); # or whatever address
1960 $name = gethostbyaddr($iaddr, AF_INET);
1962 # or going the other way
1963 $straddr = inet_ntoa($iaddr);
1965 If you get tired of remembering which element of the return list
1966 contains which return value, by-name interfaces are provided
1967 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1968 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1969 and C<User::grent>. These override the normal built-ins, supplying
1970 versions that return objects with the appropriate names
1971 for each field. For example:
1975 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1977 Even though it looks like they're the same method calls (uid),
1978 they aren't, because a C<File::stat> object is different from
1979 a C<User::pwent> object.
1981 =item getsockname SOCKET
1983 Returns the packed sockaddr address of this end of the SOCKET connection,
1984 in case you don't know the address because you have several different
1985 IPs that the connection might have come in on.
1988 $mysockaddr = getsockname(SOCK);
1989 ($port, $myaddr) = sockaddr_in($mysockaddr);
1990 printf "Connect to %s [%s]\n",
1991 scalar gethostbyaddr($myaddr, AF_INET),
1994 =item getsockopt SOCKET,LEVEL,OPTNAME
1996 Returns the socket option requested, or undef if there is an error.
2002 Returns the value of EXPR with filename expansions such as the
2003 standard Unix shell F</bin/csh> would do. This is the internal function
2004 implementing the C<< <*.c> >> operator, but you can use it directly.
2005 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2006 discussed in more detail in L<perlop/"I/O Operators">.
2008 Beginning with v5.6.0, this operator is implemented using the standard
2009 C<File::Glob> extension. See L<File::Glob> for details.
2013 Converts a time as returned by the time function to an 8-element list
2014 with the time localized for the standard Greenwich time zone.
2015 Typically used as follows:
2018 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2021 All list elements are numeric, and come straight out of the C `struct
2022 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2023 specified time. $mday is the day of the month, and $mon is the month
2024 itself, in the range C<0..11> with 0 indicating January and 11
2025 indicating December. $year is the number of years since 1900. That
2026 is, $year is C<123> in year 2023. $wday is the day of the week, with
2027 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2028 the year, in the range C<0..364> (or C<0..365> in leap years.)
2030 Note that the $year element is I<not> simply the last two digits of
2031 the year. If you assume it is, then you create non-Y2K-compliant
2032 programs--and you wouldn't want to do that, would you?
2034 The proper way to get a complete 4-digit year is simply:
2038 And to get the last two digits of the year (e.g., '01' in 2001) do:
2040 $year = sprintf("%02d", $year % 100);
2042 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2044 In scalar context, C<gmtime()> returns the ctime(3) value:
2046 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2048 Also see the C<timegm> function provided by the C<Time::Local> module,
2049 and the strftime(3) function available via the POSIX module.
2051 This scalar value is B<not> locale dependent (see L<perllocale>), but
2052 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2053 strftime(3) and mktime(3) functions available via the POSIX module. To
2054 get somewhat similar but locale dependent date strings, set up your
2055 locale environment variables appropriately (please see L<perllocale>)
2056 and try for example:
2058 use POSIX qw(strftime);
2059 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2061 Note that the C<%a> and C<%b> escapes, which represent the short forms
2062 of the day of the week and the month of the year, may not necessarily
2063 be three characters wide in all locales.
2071 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2072 execution there. It may not be used to go into any construct that
2073 requires initialization, such as a subroutine or a C<foreach> loop. It
2074 also can't be used to go into a construct that is optimized away,
2075 or to get out of a block or subroutine given to C<sort>.
2076 It can be used to go almost anywhere else within the dynamic scope,
2077 including out of subroutines, but it's usually better to use some other
2078 construct such as C<last> or C<die>. The author of Perl has never felt the
2079 need to use this form of C<goto> (in Perl, that is--C is another matter).
2081 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2082 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2083 necessarily recommended if you're optimizing for maintainability:
2085 goto ("FOO", "BAR", "GLARCH")[$i];
2087 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2088 In fact, it isn't a goto in the normal sense at all, and doesn't have
2089 the stigma associated with other gotos. Instead, it
2090 substitutes a call to the named subroutine for the currently running
2091 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2092 another subroutine and then pretend that the other subroutine had been
2093 called in the first place (except that any modifications to C<@_>
2094 in the current subroutine are propagated to the other subroutine.)
2095 After the C<goto>, not even C<caller> will be able to tell that this
2096 routine was called first.
2098 NAME needn't be the name of a subroutine; it can be a scalar variable
2099 containing a code reference, or a block which evaluates to a code
2102 =item grep BLOCK LIST
2104 =item grep EXPR,LIST
2106 This is similar in spirit to, but not the same as, grep(1) and its
2107 relatives. In particular, it is not limited to using regular expressions.
2109 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2110 C<$_> to each element) and returns the list value consisting of those
2111 elements for which the expression evaluated to true. In scalar
2112 context, returns the number of times the expression was true.
2114 @foo = grep(!/^#/, @bar); # weed out comments
2118 @foo = grep {!/^#/} @bar; # weed out comments
2120 Note that C<$_> is an alias to the list value, so it can be used to
2121 modify the elements of the LIST. While this is useful and supported,
2122 it can cause bizarre results if the elements of LIST are not variables.
2123 Similarly, grep returns aliases into the original list, much as a for
2124 loop's index variable aliases the list elements. That is, modifying an
2125 element of a list returned by grep (for example, in a C<foreach>, C<map>
2126 or another C<grep>) actually modifies the element in the original list.
2127 This is usually something to be avoided when writing clear code.
2129 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2135 Interprets EXPR as a hex string and returns the corresponding value.
2136 (To convert strings that might start with either 0, 0x, or 0b, see
2137 L</oct>.) If EXPR is omitted, uses C<$_>.
2139 print hex '0xAf'; # prints '175'
2140 print hex 'aF'; # same
2142 Hex strings may only represent integers. Strings that would cause
2143 integer overflow trigger a warning. Leading whitespace is not stripped,
2148 There is no builtin C<import> function. It is just an ordinary
2149 method (subroutine) defined (or inherited) by modules that wish to export
2150 names to another module. The C<use> function calls the C<import> method
2151 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2153 =item index STR,SUBSTR,POSITION
2155 =item index STR,SUBSTR
2157 The index function searches for one string within another, but without
2158 the wildcard-like behavior of a full regular-expression pattern match.
2159 It returns the position of the first occurrence of SUBSTR in STR at
2160 or after POSITION. If POSITION is omitted, starts searching from the
2161 beginning of the string. The return value is based at C<0> (or whatever
2162 you've set the C<$[> variable to--but don't do that). If the substring
2163 is not found, returns one less than the base, ordinarily C<-1>.
2169 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2170 You should not use this function for rounding: one because it truncates
2171 towards C<0>, and two because machine representations of floating point
2172 numbers can sometimes produce counterintuitive results. For example,
2173 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2174 because it's really more like -268.99999999999994315658 instead. Usually,
2175 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2176 functions will serve you better than will int().
2178 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2180 Implements the ioctl(2) function. You'll probably first have to say
2182 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2184 to get the correct function definitions. If F<ioctl.ph> doesn't
2185 exist or doesn't have the correct definitions you'll have to roll your
2186 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2187 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2188 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2189 written depending on the FUNCTION--a pointer to the string value of SCALAR
2190 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2191 has no string value but does have a numeric value, that value will be
2192 passed rather than a pointer to the string value. To guarantee this to be
2193 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2194 functions may be needed to manipulate the values of structures used by
2197 The return value of C<ioctl> (and C<fcntl>) is as follows:
2199 if OS returns: then Perl returns:
2201 0 string "0 but true"
2202 anything else that number
2204 Thus Perl returns true on success and false on failure, yet you can
2205 still easily determine the actual value returned by the operating
2208 $retval = ioctl(...) || -1;
2209 printf "System returned %d\n", $retval;
2211 The special string "C<0> but true" is exempt from B<-w> complaints
2212 about improper numeric conversions.
2214 Here's an example of setting a filehandle named C<REMOTE> to be
2215 non-blocking at the system level. You'll have to negotiate C<$|>
2216 on your own, though.
2218 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2220 $flags = fcntl(REMOTE, F_GETFL, 0)
2221 or die "Can't get flags for the socket: $!\n";
2223 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2224 or die "Can't set flags for the socket: $!\n";
2226 =item join EXPR,LIST
2228 Joins the separate strings of LIST into a single string with fields
2229 separated by the value of EXPR, and returns that new string. Example:
2231 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2233 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2234 first argument. Compare L</split>.
2238 Returns a list consisting of all the keys of the named hash. (In
2239 scalar context, returns the number of keys.) The keys are returned in
2240 an apparently random order. The actual random order is subject to
2241 change in future versions of perl, but it is guaranteed to be the same
2242 order as either the C<values> or C<each> function produces (given
2243 that the hash has not been modified). As a side effect, it resets
2246 Here is yet another way to print your environment:
2249 @values = values %ENV;
2251 print pop(@keys), '=', pop(@values), "\n";
2254 or how about sorted by key:
2256 foreach $key (sort(keys %ENV)) {
2257 print $key, '=', $ENV{$key}, "\n";
2260 The returned values are copies of the original keys in the hash, so
2261 modifying them will not affect the original hash. Compare L</values>.
2263 To sort a hash by value, you'll need to use a C<sort> function.
2264 Here's a descending numeric sort of a hash by its values:
2266 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2267 printf "%4d %s\n", $hash{$key}, $key;
2270 As an lvalue C<keys> allows you to increase the number of hash buckets
2271 allocated for the given hash. This can gain you a measure of efficiency if
2272 you know the hash is going to get big. (This is similar to pre-extending
2273 an array by assigning a larger number to $#array.) If you say
2277 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2278 in fact, since it rounds up to the next power of two. These
2279 buckets will be retained even if you do C<%hash = ()>, use C<undef
2280 %hash> if you want to free the storage while C<%hash> is still in scope.
2281 You can't shrink the number of buckets allocated for the hash using
2282 C<keys> in this way (but you needn't worry about doing this by accident,
2283 as trying has no effect).
2285 See also C<each>, C<values> and C<sort>.
2287 =item kill SIGNAL, LIST
2289 Sends a signal to a list of processes. Returns the number of
2290 processes successfully signaled (which is not necessarily the
2291 same as the number actually killed).
2293 $cnt = kill 1, $child1, $child2;
2296 If SIGNAL is zero, no signal is sent to the process. This is a
2297 useful way to check that the process is alive and hasn't changed
2298 its UID. See L<perlport> for notes on the portability of this
2301 Unlike in the shell, if SIGNAL is negative, it kills
2302 process groups instead of processes. (On System V, a negative I<PROCESS>
2303 number will also kill process groups, but that's not portable.) That
2304 means you usually want to use positive not negative signals. You may also
2305 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2311 The C<last> command is like the C<break> statement in C (as used in
2312 loops); it immediately exits the loop in question. If the LABEL is
2313 omitted, the command refers to the innermost enclosing loop. The
2314 C<continue> block, if any, is not executed:
2316 LINE: while (<STDIN>) {
2317 last LINE if /^$/; # exit when done with header
2321 C<last> cannot be used to exit a block which returns a value such as
2322 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2323 a grep() or map() operation.
2325 Note that a block by itself is semantically identical to a loop
2326 that executes once. Thus C<last> can be used to effect an early
2327 exit out of such a block.
2329 See also L</continue> for an illustration of how C<last>, C<next>, and
2336 Returns a lowercased version of EXPR. This is the internal function
2337 implementing the C<\L> escape in double-quoted strings. Respects
2338 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2339 and L<perlunicode> for more details about locale and Unicode support.
2341 If EXPR is omitted, uses C<$_>.
2347 Returns the value of EXPR with the first character lowercased. This
2348 is the internal function implementing the C<\l> escape in
2349 double-quoted strings. Respects current LC_CTYPE locale if C<use
2350 locale> in force. See L<perllocale> and L<perlunicode> for more
2351 details about locale and Unicode support.
2353 If EXPR is omitted, uses C<$_>.
2359 Returns the length in characters of the value of EXPR. If EXPR is
2360 omitted, returns length of C<$_>. Note that this cannot be used on
2361 an entire array or hash to find out how many elements these have.
2362 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2364 =item link OLDFILE,NEWFILE
2366 Creates a new filename linked to the old filename. Returns true for
2367 success, false otherwise.
2369 =item listen SOCKET,QUEUESIZE
2371 Does the same thing that the listen system call does. Returns true if
2372 it succeeded, false otherwise. See the example in
2373 L<perlipc/"Sockets: Client/Server Communication">.
2377 You really probably want to be using C<my> instead, because C<local> isn't
2378 what most people think of as "local". See
2379 L<perlsub/"Private Variables via my()"> for details.
2381 A local modifies the listed variables to be local to the enclosing
2382 block, file, or eval. If more than one value is listed, the list must
2383 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2384 for details, including issues with tied arrays and hashes.
2386 =item localtime EXPR
2388 Converts a time as returned by the time function to a 9-element list
2389 with the time analyzed for the local time zone. Typically used as
2393 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2396 All list elements are numeric, and come straight out of the C `struct
2397 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2398 specified time. $mday is the day of the month, and $mon is the month
2399 itself, in the range C<0..11> with 0 indicating January and 11
2400 indicating December. $year is the number of years since 1900. That
2401 is, $year is C<123> in year 2023. $wday is the day of the week, with
2402 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2403 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2404 is true if the specified time occurs during daylight savings time,
2407 Note that the $year element is I<not> simply the last two digits of
2408 the year. If you assume it is, then you create non-Y2K-compliant
2409 programs--and you wouldn't want to do that, would you?
2411 The proper way to get a complete 4-digit year is simply:
2415 And to get the last two digits of the year (e.g., '01' in 2001) do:
2417 $year = sprintf("%02d", $year % 100);
2419 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2421 In scalar context, C<localtime()> returns the ctime(3) value:
2423 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2425 This scalar value is B<not> locale dependent, see L<perllocale>, but
2426 instead a Perl builtin. Also see the C<Time::Local> module
2427 (to convert the second, minutes, hours, ... back to seconds since the
2428 stroke of midnight the 1st of January 1970, the value returned by
2429 time()), and the strftime(3) and mktime(3) functions available via the
2430 POSIX module. To get somewhat similar but locale dependent date
2431 strings, set up your locale environment variables appropriately
2432 (please see L<perllocale>) and try for example:
2434 use POSIX qw(strftime);
2435 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2437 Note that the C<%a> and C<%b>, the short forms of the day of the week
2438 and the month of the year, may not necessarily be three characters wide.
2444 This function places an advisory lock on a variable, subroutine,
2445 or referenced object contained in I<THING> until the lock goes out
2446 of scope. This is a built-in function only if your version of Perl
2447 was built with threading enabled, and if you've said C<use Thread>.
2448 Otherwise a user-defined function by this name will be called.
2455 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2456 returns log of C<$_>. To get the log of another base, use basic algebra:
2457 The base-N log of a number is equal to the natural log of that number
2458 divided by the natural log of N. For example:
2462 return log($n)/log(10);
2465 See also L</exp> for the inverse operation.
2471 Does the same thing as the C<stat> function (including setting the
2472 special C<_> filehandle) but stats a symbolic link instead of the file
2473 the symbolic link points to. If symbolic links are unimplemented on
2474 your system, a normal C<stat> is done.
2476 If EXPR is omitted, stats C<$_>.
2480 The match operator. See L<perlop>.
2482 =item map BLOCK LIST
2486 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2487 C<$_> to each element) and returns the list value composed of the
2488 results of each such evaluation. In scalar context, returns the
2489 total number of elements so generated. Evaluates BLOCK or EXPR in
2490 list context, so each element of LIST may produce zero, one, or
2491 more elements in the returned value.
2493 @chars = map(chr, @nums);
2495 translates a list of numbers to the corresponding characters. And
2497 %hash = map { getkey($_) => $_ } @array;
2499 is just a funny way to write
2502 foreach $_ (@array) {
2503 $hash{getkey($_)} = $_;
2506 Note that C<$_> is an alias to the list value, so it can be used to
2507 modify the elements of the LIST. While this is useful and supported,
2508 it can cause bizarre results if the elements of LIST are not variables.
2509 Using a regular C<foreach> loop for this purpose would be clearer in
2510 most cases. See also L</grep> for an array composed of those items of
2511 the original list for which the BLOCK or EXPR evaluates to true.
2513 C<{> starts both hash references and blocks, so C<map { ...> could be either
2514 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2515 ahead for the closing C<}> it has to take a guess at which its dealing with
2516 based what it finds just after the C<{>. Usually it gets it right, but if it
2517 doesn't it won't realize something is wrong until it gets to the C<}> and
2518 encounters the missing (or unexpected) comma. The syntax error will be
2519 reported close to the C<}> but you'll need to change something near the C<{>
2520 such as using a unary C<+> to give perl some help:
2522 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2523 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2524 %hash = map { ("\L$_", 1) } @array # this also works
2525 %hash = map { lc($_), 1 } @array # as does this.
2526 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2528 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2530 or to force an anon hash constructor use C<+{>
2532 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2534 and you get list of anonymous hashes each with only 1 entry.
2536 =item mkdir FILENAME,MASK
2538 =item mkdir FILENAME
2540 Creates the directory specified by FILENAME, with permissions
2541 specified by MASK (as modified by C<umask>). If it succeeds it
2542 returns true, otherwise it returns false and sets C<$!> (errno).
2543 If omitted, MASK defaults to 0777.
2545 In general, it is better to create directories with permissive MASK,
2546 and let the user modify that with their C<umask>, than it is to supply
2547 a restrictive MASK and give the user no way to be more permissive.
2548 The exceptions to this rule are when the file or directory should be
2549 kept private (mail files, for instance). The perlfunc(1) entry on
2550 C<umask> discusses the choice of MASK in more detail.
2552 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2553 number of trailing slashes. Some operating and filesystems do not get
2554 this right, so Perl automatically removes all trailing slashes to keep
2557 =item msgctl ID,CMD,ARG
2559 Calls the System V IPC function msgctl(2). You'll probably have to say
2563 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2564 then ARG must be a variable which will hold the returned C<msqid_ds>
2565 structure. Returns like C<ioctl>: the undefined value for error,
2566 C<"0 but true"> for zero, or the actual return value otherwise. See also
2567 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2569 =item msgget KEY,FLAGS
2571 Calls the System V IPC function msgget(2). Returns the message queue
2572 id, or the undefined value if there is an error. See also
2573 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2575 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2577 Calls the System V IPC function msgrcv to receive a message from
2578 message queue ID into variable VAR with a maximum message size of
2579 SIZE. Note that when a message is received, the message type as a
2580 native long integer will be the first thing in VAR, followed by the
2581 actual message. This packing may be opened with C<unpack("l! a*")>.
2582 Taints the variable. Returns true if successful, or false if there is
2583 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2584 C<IPC::SysV::Msg> documentation.
2586 =item msgsnd ID,MSG,FLAGS
2588 Calls the System V IPC function msgsnd to send the message MSG to the
2589 message queue ID. MSG must begin with the native long integer message
2590 type, and be followed by the length of the actual message, and finally
2591 the message itself. This kind of packing can be achieved with
2592 C<pack("l! a*", $type, $message)>. Returns true if successful,
2593 or false if there is an error. See also C<IPC::SysV>
2594 and C<IPC::SysV::Msg> documentation.
2598 =item my EXPR : ATTRIBUTES
2600 A C<my> declares the listed variables to be local (lexically) to the
2601 enclosing block, file, or C<eval>. If
2602 more than one value is listed, the list must be placed in parentheses. See
2603 L<perlsub/"Private Variables via my()"> for details.
2609 The C<next> command is like the C<continue> statement in C; it starts
2610 the next iteration of the loop:
2612 LINE: while (<STDIN>) {
2613 next LINE if /^#/; # discard comments
2617 Note that if there were a C<continue> block on the above, it would get
2618 executed even on discarded lines. If the LABEL is omitted, the command
2619 refers to the innermost enclosing loop.
2621 C<next> cannot be used to exit a block which returns a value such as
2622 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2623 a grep() or map() operation.
2625 Note that a block by itself is semantically identical to a loop
2626 that executes once. Thus C<next> will exit such a block early.
2628 See also L</continue> for an illustration of how C<last>, C<next>, and
2631 =item no Module LIST
2633 See the L</use> function, which C<no> is the opposite of.
2639 Interprets EXPR as an octal string and returns the corresponding
2640 value. (If EXPR happens to start off with C<0x>, interprets it as a
2641 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2642 binary string. Leading whitespace is ignored in all three cases.)
2643 The following will handle decimal, binary, octal, and hex in the standard
2646 $val = oct($val) if $val =~ /^0/;
2648 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2649 in octal), use sprintf() or printf():
2651 $perms = (stat("filename"))[2] & 07777;
2652 $oct_perms = sprintf "%lo", $perms;
2654 The oct() function is commonly used when a string such as C<644> needs
2655 to be converted into a file mode, for example. (Although perl will
2656 automatically convert strings into numbers as needed, this automatic
2657 conversion assumes base 10.)
2659 =item open FILEHANDLE,EXPR
2661 =item open FILEHANDLE,MODE,EXPR
2663 =item open FILEHANDLE,MODE,EXPR,LIST
2665 =item open FILEHANDLE
2667 Opens the file whose filename is given by EXPR, and associates it with
2670 (The following is a comprehensive reference to open(): for a gentler
2671 introduction you may consider L<perlopentut>.)
2673 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2674 assigned a reference to a new anonymous filehandle, otherwise if
2675 FILEHANDLE is an expression, its value is used as the name of the real
2676 filehandle wanted. (This is considered a symbolic reference, so C<use
2677 strict 'refs'> should I<not> be in effect.)
2679 If EXPR is omitted, the scalar variable of the same name as the
2680 FILEHANDLE contains the filename. (Note that lexical variables--those
2681 declared with C<my>--will not work for this purpose; so if you're
2682 using C<my>, specify EXPR in your call to open.)
2684 If three or more arguments are specified then the mode of opening and
2685 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2686 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2687 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2688 the file is opened for appending, again being created if necessary.
2690 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2691 indicate that you want both read and write access to the file; thus
2692 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2693 '+>' >> mode would clobber the file first. You can't usually use
2694 either read-write mode for updating textfiles, since they have
2695 variable length records. See the B<-i> switch in L<perlrun> for a
2696 better approach. The file is created with permissions of C<0666>
2697 modified by the process' C<umask> value.
2699 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2700 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2702 In the 2-arguments (and 1-argument) form of the call the mode and
2703 filename should be concatenated (in this order), possibly separated by
2704 spaces. It is possible to omit the mode in these forms if the mode is
2707 If the filename begins with C<'|'>, the filename is interpreted as a
2708 command to which output is to be piped, and if the filename ends with a
2709 C<'|'>, the filename is interpreted as a command which pipes output to
2710 us. See L<perlipc/"Using open() for IPC">
2711 for more examples of this. (You are not allowed to C<open> to a command
2712 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2713 and L<perlipc/"Bidirectional Communication with Another Process">
2716 For three or more arguments if MODE is C<'|-'>, the filename is
2717 interpreted as a command to which output is to be piped, and if MODE
2718 is C<'-|'>, the filename is interpreted as a command which pipes
2719 output to us. In the 2-arguments (and 1-argument) form one should
2720 replace dash (C<'-'>) with the command.
2721 See L<perlipc/"Using open() for IPC"> for more examples of this.
2722 (You are not allowed to C<open> to a command that pipes both in I<and>
2723 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2724 L<perlipc/"Bidirectional Communication"> for alternatives.)
2726 In the three-or-more argument form of pipe opens, if LIST is specified
2727 (extra arguments after the command name) then LIST becomes arguments
2728 to the command invoked if the platform supports it. The meaning of
2729 C<open> with more than three arguments for non-pipe modes is not yet
2730 specified. Experimental "layers" may give extra LIST arguments
2733 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2734 and opening C<< '>-' >> opens STDOUT.
2736 You may use the three-argument form of open to specify
2737 I<I/O disciplines> that affect how the input and output
2738 are processed: see L</binmode> and L<open>.
2740 Open returns nonzero upon success, the undefined value otherwise. If
2741 the C<open> involved a pipe, the return value happens to be the pid of
2744 If you're running Perl on a system that distinguishes between text
2745 files and binary files, then you should check out L</binmode> for tips
2746 for dealing with this. The key distinction between systems that need
2747 C<binmode> and those that don't is their text file formats. Systems
2748 like Unix, MacOS, and Plan9, which delimit lines with a single
2749 character, and which encode that character in C as C<"\n">, do not
2750 need C<binmode>. The rest need it.
2752 In the three argument form MODE may also contain a list of IO "layers"
2753 (see L<open> and L<PerlIO> for more details) to be applied to the
2754 handle. This can be used to achieve the effect of C<binmode> as well
2755 as more complex behaviours.
2757 When opening a file, it's usually a bad idea to continue normal execution
2758 if the request failed, so C<open> is frequently used in connection with
2759 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2760 where you want to make a nicely formatted error message (but there are
2761 modules that can help with that problem)) you should always check
2762 the return value from opening a file. The infrequent exception is when
2763 working with an unopened filehandle is actually what you want to do.
2765 As a special case the 3 arg form with a read/write mode and the third
2766 argument being C<undef>:
2768 open(TMP, "+>", undef) or die ...
2770 opens a filehandle to an anonymous temporary file.
2775 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2776 while (<ARTICLE>) {...
2778 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2779 # if the open fails, output is discarded
2781 open(DBASE, '+<', 'dbase.mine') # open for update
2782 or die "Can't open 'dbase.mine' for update: $!";
2784 open(DBASE, '+<dbase.mine') # ditto
2785 or die "Can't open 'dbase.mine' for update: $!";
2787 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2788 or die "Can't start caesar: $!";
2790 open(ARTICLE, "caesar <$article |") # ditto
2791 or die "Can't start caesar: $!";
2793 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2794 or die "Can't start sort: $!";
2796 # process argument list of files along with any includes
2798 foreach $file (@ARGV) {
2799 process($file, 'fh00');
2803 my($filename, $input) = @_;
2804 $input++; # this is a string increment
2805 unless (open($input, $filename)) {
2806 print STDERR "Can't open $filename: $!\n";
2811 while (<$input>) { # note use of indirection
2812 if (/^#include "(.*)"/) {
2813 process($1, $input);
2820 You may also, in the Bourne shell tradition, specify an EXPR beginning
2821 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2822 name of a filehandle (or file descriptor, if numeric) to be
2823 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2824 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2825 mode you specify should match the mode of the original filehandle.
2826 (Duping a filehandle does not take into account any existing contents of
2827 stdio buffers.) If you use the 3 arg form then you can pass either a number,
2828 the name of a filehandle or the normal "reference to a glob".
2830 Here is a script that saves, redirects, and restores STDOUT and
2834 open(my $oldout, ">&", \*STDOUT);
2835 open(OLDERR, ">&STDERR");
2837 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2838 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2840 select(STDERR); $| = 1; # make unbuffered
2841 select(STDOUT); $| = 1; # make unbuffered
2843 print STDOUT "stdout 1\n"; # this works for
2844 print STDERR "stderr 1\n"; # subprocesses too
2849 open(STDOUT, ">&OLDOUT");
2850 open(STDERR, ">&OLDERR");
2852 print STDOUT "stdout 2\n";
2853 print STDERR "stderr 2\n";
2855 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2856 do an equivalent of C's C<fdopen> of that file descriptor; this is
2857 more parsimonious of file descriptors. For example:
2859 open(FILEHANDLE, "<&=$fd")
2863 open(FILEHANDLE, "<&=", $fd)
2865 Note that if Perl is using the standard C libraries' fdopen() then on
2866 many UNIX systems, fdopen() is known to fail when file descriptors
2867 exceed a certain value, typically 255. If you need more file
2868 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2870 You can see whether Perl has been compiled with PerlIO or not by
2871 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2872 is C<define>, you have PerlIO, otherwise you don't.
2874 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2875 with 2-arguments (or 1-argument) form of open(), then
2876 there is an implicit fork done, and the return value of open is the pid
2877 of the child within the parent process, and C<0> within the child
2878 process. (Use C<defined($pid)> to determine whether the open was successful.)
2879 The filehandle behaves normally for the parent, but i/o to that
2880 filehandle is piped from/to the STDOUT/STDIN of the child process.
2881 In the child process the filehandle isn't opened--i/o happens from/to
2882 the new STDOUT or STDIN. Typically this is used like the normal
2883 piped open when you want to exercise more control over just how the
2884 pipe command gets executed, such as when you are running setuid, and
2885 don't want to have to scan shell commands for metacharacters.
2886 The following triples are more or less equivalent:
2888 open(FOO, "|tr '[a-z]' '[A-Z]'");
2889 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2890 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2891 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2893 open(FOO, "cat -n '$file'|");
2894 open(FOO, '-|', "cat -n '$file'");
2895 open(FOO, '-|') || exec 'cat', '-n', $file;
2896 open(FOO, '-|', "cat", '-n', $file);
2898 The last example in each block shows the pipe as "list form", which is
2899 not yet supported on all platforms.
2901 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2903 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2904 output before any operation that may do a fork, but this may not be
2905 supported on some platforms (see L<perlport>). To be safe, you may need
2906 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2907 of C<IO::Handle> on any open handles.
2909 On systems that support a close-on-exec flag on files, the flag will
2910 be set for the newly opened file descriptor as determined by the value
2911 of $^F. See L<perlvar/$^F>.
2913 Closing any piped filehandle causes the parent process to wait for the
2914 child to finish, and returns the status value in C<$?>.
2916 The filename passed to 2-argument (or 1-argument) form of open() will
2917 have leading and trailing whitespace deleted, and the normal
2918 redirection characters honored. This property, known as "magic open",
2919 can often be used to good effect. A user could specify a filename of
2920 F<"rsh cat file |">, or you could change certain filenames as needed:
2922 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2923 open(FH, $filename) or die "Can't open $filename: $!";
2925 Use 3-argument form to open a file with arbitrary weird characters in it,
2927 open(FOO, '<', $file);
2929 otherwise it's necessary to protect any leading and trailing whitespace:
2931 $file =~ s#^(\s)#./$1#;
2932 open(FOO, "< $file\0");
2934 (this may not work on some bizarre filesystems). One should
2935 conscientiously choose between the I<magic> and 3-arguments form
2940 will allow the user to specify an argument of the form C<"rsh cat file |">,
2941 but will not work on a filename which happens to have a trailing space, while
2943 open IN, '<', $ARGV[0];
2945 will have exactly the opposite restrictions.
2947 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2948 should use the C<sysopen> function, which involves no such magic (but
2949 may use subtly different filemodes than Perl open(), which is mapped
2950 to C fopen()). This is
2951 another way to protect your filenames from interpretation. For example:
2954 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2955 or die "sysopen $path: $!";
2956 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2957 print HANDLE "stuff $$\n";
2959 print "File contains: ", <HANDLE>;
2961 Using the constructor from the C<IO::Handle> package (or one of its
2962 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2963 filehandles that have the scope of whatever variables hold references to
2964 them, and automatically close whenever and however you leave that scope:
2968 sub read_myfile_munged {
2970 my $handle = new IO::File;
2971 open($handle, "myfile") or die "myfile: $!";
2973 or return (); # Automatically closed here.
2974 mung $first or die "mung failed"; # Or here.
2975 return $first, <$handle> if $ALL; # Or here.
2979 See L</seek> for some details about mixing reading and writing.
2981 =item opendir DIRHANDLE,EXPR
2983 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2984 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2985 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2991 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
2992 or Unicode) value of the first character of EXPR. If EXPR is omitted,
2995 For the reverse, see L</chr>.
2996 See L<perlunicode> and L<encoding> for more about Unicode.
3000 =item our EXPR : ATTRIBUTES
3002 An C<our> declares the listed variables to be valid globals within
3003 the enclosing block, file, or C<eval>. That is, it has the same
3004 scoping rules as a "my" declaration, but does not create a local
3005 variable. If more than one value is listed, the list must be placed
3006 in parentheses. The C<our> declaration has no semantic effect unless
3007 "use strict vars" is in effect, in which case it lets you use the
3008 declared global variable without qualifying it with a package name.
3009 (But only within the lexical scope of the C<our> declaration. In this
3010 it differs from "use vars", which is package scoped.)
3012 An C<our> declaration declares a global variable that will be visible
3013 across its entire lexical scope, even across package boundaries. The
3014 package in which the variable is entered is determined at the point
3015 of the declaration, not at the point of use. This means the following
3019 our $bar; # declares $Foo::bar for rest of lexical scope
3023 print $bar; # prints 20
3025 Multiple C<our> declarations in the same lexical scope are allowed
3026 if they are in different packages. If they happened to be in the same
3027 package, Perl will emit warnings if you have asked for them.
3031 our $bar; # declares $Foo::bar for rest of lexical scope
3035 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3036 print $bar; # prints 30
3038 our $bar; # emits warning
3040 An C<our> declaration may also have a list of attributes associated
3041 with it. B<WARNING>: This is an experimental feature that may be
3042 changed or removed in future releases of Perl. It should not be
3045 The only currently recognized attribute is C<unique> which indicates
3046 that a single copy of the global is to be used by all interpreters
3047 should the program happen to be running in a multi-interpreter
3048 environment. (The default behaviour would be for each interpreter to
3049 have its own copy of the global.) In such an environment, this
3050 attribute also has the effect of making the global readonly.
3053 our @EXPORT : unique = qw(foo);
3054 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3055 our $VERSION : unique = "1.00";
3057 Multi-interpreter environments can come to being either through the
3058 fork() emulation on Windows platforms, or by embedding perl in a
3059 multi-threaded application. The C<unique> attribute does nothing in
3060 all other environments.
3062 =item pack TEMPLATE,LIST
3064 Takes a LIST of values and converts it into a string using the rules
3065 given by the TEMPLATE. The resulting string is the concatenation of
3066 the converted values. Typically, each converted value looks
3067 like its machine-level representation. For example, on 32-bit machines
3068 a converted integer may be represented by a sequence of 4 bytes.
3071 sequence of characters that give the order and type of values, as
3074 a A string with arbitrary binary data, will be null padded.
3075 A A text (ASCII) string, will be space padded.
3076 Z A null terminated (ASCIZ) string, will be null padded.
3078 b A bit string (ascending bit order inside each byte, like vec()).
3079 B A bit string (descending bit order inside each byte).
3080 h A hex string (low nybble first).
3081 H A hex string (high nybble first).
3083 c A signed char value.
3084 C An unsigned char value. Only does bytes. See U for Unicode.
3086 s A signed short value.
3087 S An unsigned short value.
3088 (This 'short' is _exactly_ 16 bits, which may differ from
3089 what a local C compiler calls 'short'. If you want
3090 native-length shorts, use the '!' suffix.)
3092 i A signed integer value.
3093 I An unsigned integer value.
3094 (This 'integer' is _at_least_ 32 bits wide. Its exact
3095 size depends on what a local C compiler calls 'int',
3096 and may even be larger than the 'long' described in
3099 l A signed long value.
3100 L An unsigned long value.
3101 (This 'long' is _exactly_ 32 bits, which may differ from
3102 what a local C compiler calls 'long'. If you want
3103 native-length longs, use the '!' suffix.)
3105 n An unsigned short in "network" (big-endian) order.
3106 N An unsigned long in "network" (big-endian) order.
3107 v An unsigned short in "VAX" (little-endian) order.
3108 V An unsigned long in "VAX" (little-endian) order.
3109 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3110 _exactly_ 32 bits, respectively.)
3112 q A signed quad (64-bit) value.
3113 Q An unsigned quad value.
3114 (Quads are available only if your system supports 64-bit
3115 integer values _and_ if Perl has been compiled to support those.
3116 Causes a fatal error otherwise.)
3118 f A single-precision float in the native format.
3119 d A double-precision float in the native format.
3121 p A pointer to a null-terminated string.
3122 P A pointer to a structure (fixed-length string).
3124 u A uuencoded string.
3125 U A Unicode character number. Encodes to UTF-8 internally
3126 (or UTF-EBCDIC in EBCDIC platforms).
3128 w A BER compressed integer. Its bytes represent an unsigned
3129 integer in base 128, most significant digit first, with as
3130 few digits as possible. Bit eight (the high bit) is set
3131 on each byte except the last.
3135 @ Null fill to absolute position.
3137 The following rules apply:
3143 Each letter may optionally be followed by a number giving a repeat
3144 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3145 C<H>, and C<P> the pack function will gobble up that many values from
3146 the LIST. A C<*> for the repeat count means to use however many items are
3147 left, except for C<@>, C<x>, C<X>, where it is equivalent
3148 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3151 When used with C<Z>, C<*> results in the addition of a trailing null
3152 byte (so the packed result will be one longer than the byte C<length>
3155 The repeat count for C<u> is interpreted as the maximal number of bytes
3156 to encode per line of output, with 0 and 1 replaced by 45.
3160 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3161 string of length count, padding with nulls or spaces as necessary. When
3162 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3163 after the first null, and C<a> returns data verbatim. When packing,
3164 C<a>, and C<Z> are equivalent.
3166 If the value-to-pack is too long, it is truncated. If too long and an
3167 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3168 by a null byte. Thus C<Z> always packs a trailing null byte under
3173 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3174 Each byte of the input field of pack() generates 1 bit of the result.
3175 Each result bit is based on the least-significant bit of the corresponding
3176 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3177 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3179 Starting from the beginning of the input string of pack(), each 8-tuple
3180 of bytes is converted to 1 byte of output. With format C<b>
3181 the first byte of the 8-tuple determines the least-significant bit of a
3182 byte, and with format C<B> it determines the most-significant bit of
3185 If the length of the input string is not exactly divisible by 8, the
3186 remainder is packed as if the input string were padded by null bytes
3187 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3189 If the input string of pack() is longer than needed, extra bytes are ignored.
3190 A C<*> for the repeat count of pack() means to use all the bytes of
3191 the input field. On unpack()ing the bits are converted to a string
3192 of C<"0">s and C<"1">s.
3196 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3197 representable as hexadecimal digits, 0-9a-f) long.
3199 Each byte of the input field of pack() generates 4 bits of the result.
3200 For non-alphabetical bytes the result is based on the 4 least-significant
3201 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3202 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3203 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3204 is compatible with the usual hexadecimal digits, so that C<"a"> and
3205 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3206 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3208 Starting from the beginning of the input string of pack(), each pair
3209 of bytes is converted to 1 byte of output. With format C<h> the
3210 first byte of the pair determines the least-significant nybble of the
3211 output byte, and with format C<H> it determines the most-significant
3214 If the length of the input string is not even, it behaves as if padded
3215 by a null byte at the end. Similarly, during unpack()ing the "extra"
3216 nybbles are ignored.
3218 If the input string of pack() is longer than needed, extra bytes are ignored.
3219 A C<*> for the repeat count of pack() means to use all the bytes of
3220 the input field. On unpack()ing the bits are converted to a string
3221 of hexadecimal digits.
3225 The C<p> type packs a pointer to a null-terminated string. You are
3226 responsible for ensuring the string is not a temporary value (which can
3227 potentially get deallocated before you get around to using the packed result).
3228 The C<P> type packs a pointer to a structure of the size indicated by the
3229 length. A NULL pointer is created if the corresponding value for C<p> or
3230 C<P> is C<undef>, similarly for unpack().
3234 The C</> template character allows packing and unpacking of strings where
3235 the packed structure contains a byte count followed by the string itself.
3236 You write I<length-item>C</>I<string-item>.
3238 The I<length-item> can be any C<pack> template letter,
3239 and describes how the length value is packed.
3240 The ones likely to be of most use are integer-packing ones like
3241 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3242 and C<N> (for Sun XDR).
3244 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3245 For C<unpack> the length of the string is obtained from the I<length-item>,
3246 but if you put in the '*' it will be ignored.
3248 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3249 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3250 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3252 The I<length-item> is not returned explicitly from C<unpack>.
3254 Adding a count to the I<length-item> letter is unlikely to do anything
3255 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3256 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3257 which Perl does not regard as legal in numeric strings.
3261 The integer types C<s>, C<S>, C<l>, and C<L> may be
3262 immediately followed by a C<!> suffix to signify native shorts or
3263 longs--as you can see from above for example a bare C<l> does mean
3264 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3265 may be larger. This is an issue mainly in 64-bit platforms. You can
3266 see whether using C<!> makes any difference by
3268 print length(pack("s")), " ", length(pack("s!")), "\n";
3269 print length(pack("l")), " ", length(pack("l!")), "\n";
3271 C<i!> and C<I!> also work but only because of completeness;
3272 they are identical to C<i> and C<I>.
3274 The actual sizes (in bytes) of native shorts, ints, longs, and long
3275 longs on the platform where Perl was built are also available via
3279 print $Config{shortsize}, "\n";
3280 print $Config{intsize}, "\n";
3281 print $Config{longsize}, "\n";
3282 print $Config{longlongsize}, "\n";
3284 (The C<$Config{longlongsize}> will be undefine if your system does
3285 not support long longs.)
3289 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3290 are inherently non-portable between processors and operating systems
3291 because they obey the native byteorder and endianness. For example a
3292 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3293 (arranged in and handled by the CPU registers) into bytes as
3295 0x12 0x34 0x56 0x78 # big-endian
3296 0x78 0x56 0x34 0x12 # little-endian
3298 Basically, the Intel and VAX CPUs are little-endian, while everybody
3299 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3300 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3301 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3304 The names `big-endian' and `little-endian' are comic references to
3305 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3306 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3307 the egg-eating habits of the Lilliputians.
3309 Some systems may have even weirder byte orders such as
3314 You can see your system's preference with
3316 print join(" ", map { sprintf "%#02x", $_ }
3317 unpack("C*",pack("L",0x12345678))), "\n";
3319 The byteorder on the platform where Perl was built is also available
3323 print $Config{byteorder}, "\n";
3325 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3326 and C<'87654321'> are big-endian.
3328 If you want portable packed integers use the formats C<n>, C<N>,
3329 C<v>, and C<V>, their byte endianness and size are known.
3330 See also L<perlport>.
3334 Real numbers (floats and doubles) are in the native machine format only;
3335 due to the multiplicity of floating formats around, and the lack of a
3336 standard "network" representation, no facility for interchange has been
3337 made. This means that packed floating point data written on one machine
3338 may not be readable on another - even if both use IEEE floating point
3339 arithmetic (as the endian-ness of the memory representation is not part
3340 of the IEEE spec). See also L<perlport>.
3342 Note that Perl uses doubles internally for all numeric calculation, and
3343 converting from double into float and thence back to double again will
3344 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3349 If the pattern begins with a C<U>, the resulting string will be treated
3350 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3351 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3352 characters. If you don't want this to happen, you can begin your pattern
3353 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3354 string, and then follow this with a C<U*> somewhere in your pattern.
3358 You must yourself do any alignment or padding by inserting for example
3359 enough C<'x'>es while packing. There is no way to pack() and unpack()
3360 could know where the bytes are going to or coming from. Therefore
3361 C<pack> (and C<unpack>) handle their output and input as flat
3366 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3370 If TEMPLATE requires more arguments to pack() than actually given, pack()
3371 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3372 to pack() than actually given, extra arguments are ignored.
3378 $foo = pack("CCCC",65,66,67,68);
3380 $foo = pack("C4",65,66,67,68);
3382 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3383 # same thing with Unicode circled letters
3385 $foo = pack("ccxxcc",65,66,67,68);
3388 # note: the above examples featuring "C" and "c" are true
3389 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3390 # and UTF-8. In EBCDIC the first example would be
3391 # $foo = pack("CCCC",193,194,195,196);
3393 $foo = pack("s2",1,2);
3394 # "\1\0\2\0" on little-endian
3395 # "\0\1\0\2" on big-endian
3397 $foo = pack("a4","abcd","x","y","z");
3400 $foo = pack("aaaa","abcd","x","y","z");
3403 $foo = pack("a14","abcdefg");
3404 # "abcdefg\0\0\0\0\0\0\0"
3406 $foo = pack("i9pl", gmtime);
3407 # a real struct tm (on my system anyway)
3409 $utmp_template = "Z8 Z8 Z16 L";
3410 $utmp = pack($utmp_template, @utmp1);
3411 # a struct utmp (BSDish)
3413 @utmp2 = unpack($utmp_template, $utmp);
3414 # "@utmp1" eq "@utmp2"
3417 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3420 $foo = pack('sx2l', 12, 34);
3421 # short 12, two zero bytes padding, long 34
3422 $bar = pack('s@4l', 12, 34);
3423 # short 12, zero fill to position 4, long 34
3426 The same template may generally also be used in unpack().
3428 =item package NAMESPACE
3432 Declares the compilation unit as being in the given namespace. The scope
3433 of the package declaration is from the declaration itself through the end
3434 of the enclosing block, file, or eval (the same as the C<my> operator).
3435 All further unqualified dynamic identifiers will be in this namespace.
3436 A package statement affects only dynamic variables--including those
3437 you've used C<local> on--but I<not> lexical variables, which are created
3438 with C<my>. Typically it would be the first declaration in a file to
3439 be included by the C<require> or C<use> operator. You can switch into a
3440 package in more than one place; it merely influences which symbol table
3441 is used by the compiler for the rest of that block. You can refer to
3442 variables and filehandles in other packages by prefixing the identifier
3443 with the package name and a double colon: C<$Package::Variable>.
3444 If the package name is null, the C<main> package as assumed. That is,
3445 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3446 still seen in older code).
3448 If NAMESPACE is omitted, then there is no current package, and all
3449 identifiers must be fully qualified or lexicals. However, you are
3450 strongly advised not to make use of this feature. Its use can cause
3451 unexpected behaviour, even crashing some versions of Perl. It is
3452 deprecated, and will be removed from a future release.
3454 See L<perlmod/"Packages"> for more information about packages, modules,
3455 and classes. See L<perlsub> for other scoping issues.
3457 =item pipe READHANDLE,WRITEHANDLE
3459 Opens a pair of connected pipes like the corresponding system call.
3460 Note that if you set up a loop of piped processes, deadlock can occur
3461 unless you are very careful. In addition, note that Perl's pipes use
3462 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3463 after each command, depending on the application.
3465 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3466 for examples of such things.
3468 On systems that support a close-on-exec flag on files, the flag will be set
3469 for the newly opened file descriptors as determined by the value of $^F.
3476 Pops and returns the last value of the array, shortening the array by
3477 one element. Has an effect similar to
3481 If there are no elements in the array, returns the undefined value
3482 (although this may happen at other times as well). If ARRAY is
3483 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3484 array in subroutines, just like C<shift>.
3490 Returns the offset of where the last C<m//g> search left off for the variable
3491 in question (C<$_> is used when the variable is not specified). May be
3492 modified to change that offset. Such modification will also influence
3493 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3496 =item print FILEHANDLE LIST
3502 Prints a string or a list of strings. Returns true if successful.
3503 FILEHANDLE may be a scalar variable name, in which case the variable
3504 contains the name of or a reference to the filehandle, thus introducing
3505 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3506 the next token is a term, it may be misinterpreted as an operator
3507 unless you interpose a C<+> or put parentheses around the arguments.)
3508 If FILEHANDLE is omitted, prints by default to standard output (or
3509 to the last selected output channel--see L</select>). If LIST is
3510 also omitted, prints C<$_> to the currently selected output channel.
3511 To set the default output channel to something other than STDOUT
3512 use the select operation. The current value of C<$,> (if any) is
3513 printed between each LIST item. The current value of C<$\> (if
3514 any) is printed after the entire LIST has been printed. Because
3515 print takes a LIST, anything in the LIST is evaluated in list
3516 context, and any subroutine that you call will have one or more of
3517 its expressions evaluated in list context. Also be careful not to
3518 follow the print keyword with a left parenthesis unless you want
3519 the corresponding right parenthesis to terminate the arguments to
3520 the print--interpose a C<+> or put parentheses around all the
3523 Note that if you're storing FILEHANDLES in an array or other expression,
3524 you will have to use a block returning its value instead:
3526 print { $files[$i] } "stuff\n";
3527 print { $OK ? STDOUT : STDERR } "stuff\n";
3529 =item printf FILEHANDLE FORMAT, LIST
3531 =item printf FORMAT, LIST
3533 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3534 (the output record separator) is not appended. The first argument
3535 of the list will be interpreted as the C<printf> format. See C<sprintf>
3536 for an explanation of the format argument. If C<use locale> is in effect,
3537 the character used for the decimal point in formatted real numbers is
3538 affected by the LC_NUMERIC locale. See L<perllocale>.
3540 Don't fall into the trap of using a C<printf> when a simple
3541 C<print> would do. The C<print> is more efficient and less
3544 =item prototype FUNCTION
3546 Returns the prototype of a function as a string (or C<undef> if the
3547 function has no prototype). FUNCTION is a reference to, or the name of,
3548 the function whose prototype you want to retrieve.
3550 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3551 name for Perl builtin. If the builtin is not I<overridable> (such as
3552 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3553 C<system>) returns C<undef> because the builtin does not really behave
3554 like a Perl function. Otherwise, the string describing the equivalent
3555 prototype is returned.
3557 =item push ARRAY,LIST
3559 Treats ARRAY as a stack, and pushes the values of LIST
3560 onto the end of ARRAY. The length of ARRAY increases by the length of
3561 LIST. Has the same effect as
3564 $ARRAY[++$#ARRAY] = $value;
3567 but is more efficient. Returns the new number of elements in the array.
3579 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3581 =item quotemeta EXPR
3585 Returns the value of EXPR with all non-"word"
3586 characters backslashed. (That is, all characters not matching
3587 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3588 returned string, regardless of any locale settings.)
3589 This is the internal function implementing
3590 the C<\Q> escape in double-quoted strings.
3592 If EXPR is omitted, uses C<$_>.
3598 Returns a random fractional number greater than or equal to C<0> and less
3599 than the value of EXPR. (EXPR should be positive.) If EXPR is
3600 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3601 unless C<srand> has already been called. See also C<srand>.
3603 Apply C<int()> to the value returned by C<rand()> if you want random
3604 integers instead of random fractional numbers. For example,
3608 returns a random integer between C<0> and C<9>, inclusive.
3610 (Note: If your rand function consistently returns numbers that are too
3611 large or too small, then your version of Perl was probably compiled
3612 with the wrong number of RANDBITS.)
3614 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3616 =item read FILEHANDLE,SCALAR,LENGTH
3618 Attempts to read LENGTH bytes of data into variable SCALAR from the
3619 specified FILEHANDLE. Returns the number of bytes actually read, C<0>
3620 at end of file, or undef if there was an error. SCALAR will be grown
3621 or shrunk to the length actually read. If SCALAR needs growing, the
3622 new bytes will be zero bytes. An OFFSET may be specified to place
3623 the read data into some other place in SCALAR than the beginning.
3624 The call is actually implemented in terms of stdio's fread(3) call.
3625 To get a true read(2) system call, see C<sysread>.
3627 =item readdir DIRHANDLE
3629 Returns the next directory entry for a directory opened by C<opendir>.
3630 If used in list context, returns all the rest of the entries in the
3631 directory. If there are no more entries, returns an undefined value in
3632 scalar context or a null list in list context.
3634 If you're planning to filetest the return values out of a C<readdir>, you'd
3635 better prepend the directory in question. Otherwise, because we didn't
3636 C<chdir> there, it would have been testing the wrong file.
3638 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3639 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3644 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3645 context, each call reads and returns the next line, until end-of-file is
3646 reached, whereupon the subsequent call returns undef. In list context,
3647 reads until end-of-file is reached and returns a list of lines. Note that
3648 the notion of "line" used here is however you may have defined it
3649 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3651 When C<$/> is set to C<undef>, when readline() is in scalar
3652 context (i.e. file slurp mode), and when an empty file is read, it
3653 returns C<''> the first time, followed by C<undef> subsequently.
3655 This is the internal function implementing the C<< <EXPR> >>
3656 operator, but you can use it directly. The C<< <EXPR> >>
3657 operator is discussed in more detail in L<perlop/"I/O Operators">.
3660 $line = readline(*STDIN); # same thing
3666 Returns the value of a symbolic link, if symbolic links are
3667 implemented. If not, gives a fatal error. If there is some system
3668 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3669 omitted, uses C<$_>.
3673 EXPR is executed as a system command.
3674 The collected standard output of the command is returned.
3675 In scalar context, it comes back as a single (potentially
3676 multi-line) string. In list context, returns a list of lines
3677 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3678 This is the internal function implementing the C<qx/EXPR/>
3679 operator, but you can use it directly. The C<qx/EXPR/>
3680 operator is discussed in more detail in L<perlop/"I/O Operators">.
3682 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3684 Receives a message on a socket. Attempts to receive LENGTH bytes of
3685 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3686 will be grown or shrunk to the length actually read. Takes the same
3687 flags as the system call of the same name. Returns the address of the
3688 sender if SOCKET's protocol supports this; returns an empty string
3689 otherwise. If there's an error, returns the undefined value. This call
3690 is actually implemented in terms of recvfrom(2) system call. See
3691 L<perlipc/"UDP: Message Passing"> for examples.
3697 The C<redo> command restarts the loop block without evaluating the
3698 conditional again. The C<continue> block, if any, is not executed. If
3699 the LABEL is omitted, the command refers to the innermost enclosing
3700 loop. This command is normally used by programs that want to lie to
3701 themselves about what was just input:
3703 # a simpleminded Pascal comment stripper
3704 # (warning: assumes no { or } in strings)
3705 LINE: while (<STDIN>) {
3706 while (s|({.*}.*){.*}|$1 |) {}
3711 if (/}/) { # end of comment?
3720 C<redo> cannot be used to retry a block which returns a value such as
3721 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3722 a grep() or map() operation.
3724 Note that a block by itself is semantically identical to a loop
3725 that executes once. Thus C<redo> inside such a block will effectively
3726 turn it into a looping construct.
3728 See also L</continue> for an illustration of how C<last>, C<next>, and
3735 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3736 is not specified, C<$_> will be used. The value returned depends on the
3737 type of thing the reference is a reference to.
3738 Builtin types include:
3748 If the referenced object has been blessed into a package, then that package
3749 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3751 if (ref($r) eq "HASH") {
3752 print "r is a reference to a hash.\n";
3755 print "r is not a reference at all.\n";
3757 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3758 print "r is a reference to something that isa hash.\n";
3761 See also L<perlref>.
3763 =item rename OLDNAME,NEWNAME
3765 Changes the name of a file; an existing file NEWNAME will be
3766 clobbered. Returns true for success, false otherwise.
3768 Behavior of this function varies wildly depending on your system
3769 implementation. For example, it will usually not work across file system
3770 boundaries, even though the system I<mv> command sometimes compensates
3771 for this. Other restrictions include whether it works on directories,
3772 open files, or pre-existing files. Check L<perlport> and either the
3773 rename(2) manpage or equivalent system documentation for details.
3775 =item require VERSION
3781 Demands a version of Perl specified by VERSION, or demands some semantics
3782 specified by EXPR or by C<$_> if EXPR is not supplied.
3784 VERSION may be either a numeric argument such as 5.006, which will be
3785 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3786 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3787 VERSION is greater than the version of the current Perl interpreter.
3788 Compare with L</use>, which can do a similar check at compile time.
3790 Specifying VERSION as a literal of the form v5.6.1 should generally be
3791 avoided, because it leads to misleading error messages under earlier
3792 versions of Perl which do not support this syntax. The equivalent numeric
3793 version should be used instead.
3795 require v5.6.1; # run time version check
3796 require 5.6.1; # ditto
3797 require 5.006_001; # ditto; preferred for backwards compatibility
3799 Otherwise, demands that a library file be included if it hasn't already
3800 been included. The file is included via the do-FILE mechanism, which is
3801 essentially just a variety of C<eval>. Has semantics similar to the following
3806 return 1 if $INC{$filename};
3807 my($realfilename,$result);
3809 foreach $prefix (@INC) {
3810 $realfilename = "$prefix/$filename";
3811 if (-f $realfilename) {
3812 $INC{$filename} = $realfilename;
3813 $result = do $realfilename;
3817 die "Can't find $filename in \@INC";
3819 delete $INC{$filename} if $@ || !$result;
3821 die "$filename did not return true value" unless $result;
3825 Note that the file will not be included twice under the same specified
3826 name. The file must return true as the last statement to indicate
3827 successful execution of any initialization code, so it's customary to
3828 end such a file with C<1;> unless you're sure it'll return true
3829 otherwise. But it's better just to put the C<1;>, in case you add more
3832 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3833 replaces "F<::>" with "F</>" in the filename for you,
3834 to make it easy to load standard modules. This form of loading of
3835 modules does not risk altering your namespace.
3837 In other words, if you try this:
3839 require Foo::Bar; # a splendid bareword
3841 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3842 directories specified in the C<@INC> array.
3844 But if you try this:
3846 $class = 'Foo::Bar';
3847 require $class; # $class is not a bareword
3849 require "Foo::Bar"; # not a bareword because of the ""
3851 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3852 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3854 eval "require $class";
3856 You can also insert hooks into the import facility, by putting directly
3857 Perl code into the @INC array. There are three forms of hooks: subroutine
3858 references, array references and blessed objects.
3860 Subroutine references are the simplest case. When the inclusion system
3861 walks through @INC and encounters a subroutine, this subroutine gets
3862 called with two parameters, the first being a reference to itself, and the
3863 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
3864 subroutine should return C<undef> or a filehandle, from which the file to
3865 include will be read. If C<undef> is returned, C<require> will look at
3866 the remaining elements of @INC.
3868 If the hook is an array reference, its first element must be a subroutine
3869 reference. This subroutine is called as above, but the first parameter is
3870 the array reference. This enables to pass indirectly some arguments to
3873 In other words, you can write:
3875 push @INC, \&my_sub;
3877 my ($coderef, $filename) = @_; # $coderef is \&my_sub
3883 push @INC, [ \&my_sub, $x, $y, ... ];
3885 my ($arrayref, $filename) = @_;
3886 # Retrieve $x, $y, ...
3887 my @parameters = @$arrayref[1..$#$arrayref];
3891 If the hook is an object, it must provide an INC method, that will be
3892 called as above, the first parameter being the object itself. (Note that
3893 you must fully qualify the sub's name, as it is always forced into package
3894 C<main>.) Here is a typical code layout:
3900 my ($self, $filename) = @_;
3904 # In the main program
3905 push @INC, new Foo(...);
3907 Note that these hooks are also permitted to set the %INC entry
3908 corresponding to the files they have loaded. See L<perlvar/%INC>.
3910 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3916 Generally used in a C<continue> block at the end of a loop to clear
3917 variables and reset C<??> searches so that they work again. The
3918 expression is interpreted as a list of single characters (hyphens
3919 allowed for ranges). All variables and arrays beginning with one of
3920 those letters are reset to their pristine state. If the expression is
3921 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3922 only variables or searches in the current package. Always returns
3925 reset 'X'; # reset all X variables
3926 reset 'a-z'; # reset lower case variables
3927 reset; # just reset ?one-time? searches
3929 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3930 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3931 variables--lexical variables are unaffected, but they clean themselves
3932 up on scope exit anyway, so you'll probably want to use them instead.
3939 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3940 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3941 context, depending on how the return value will be used, and the context
3942 may vary from one execution to the next (see C<wantarray>). If no EXPR
3943 is given, returns an empty list in list context, the undefined value in
3944 scalar context, and (of course) nothing at all in a void context.
3946 (Note that in the absence of an explicit C<return>, a subroutine, eval,
3947 or do FILE will automatically return the value of the last expression
3952 In list context, returns a list value consisting of the elements
3953 of LIST in the opposite order. In scalar context, concatenates the
3954 elements of LIST and returns a string value with all characters
3955 in the opposite order.
3957 print reverse <>; # line tac, last line first
3959 undef $/; # for efficiency of <>
3960 print scalar reverse <>; # character tac, last line tsrif
3962 This operator is also handy for inverting a hash, although there are some
3963 caveats. If a value is duplicated in the original hash, only one of those
3964 can be represented as a key in the inverted hash. Also, this has to
3965 unwind one hash and build a whole new one, which may take some time
3966 on a large hash, such as from a DBM file.
3968 %by_name = reverse %by_address; # Invert the hash
3970 =item rewinddir DIRHANDLE
3972 Sets the current position to the beginning of the directory for the
3973 C<readdir> routine on DIRHANDLE.
3975 =item rindex STR,SUBSTR,POSITION
3977 =item rindex STR,SUBSTR
3979 Works just like index() except that it returns the position of the LAST
3980 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3981 last occurrence at or before that position.
3983 =item rmdir FILENAME
3987 Deletes the directory specified by FILENAME if that directory is empty. If it
3988 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3989 FILENAME is omitted, uses C<$_>.
3993 The substitution operator. See L<perlop>.
3997 Forces EXPR to be interpreted in scalar context and returns the value
4000 @counts = ( scalar @a, scalar @b, scalar @c );
4002 There is no equivalent operator to force an expression to
4003 be interpolated in list context because in practice, this is never
4004 needed. If you really wanted to do so, however, you could use
4005 the construction C<@{[ (some expression) ]}>, but usually a simple
4006 C<(some expression)> suffices.
4008 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4009 parenthesized list, this behaves as a scalar comma expression, evaluating
4010 all but the last element in void context and returning the final element
4011 evaluated in scalar context. This is seldom what you want.
4013 The following single statement:
4015 print uc(scalar(&foo,$bar)),$baz;
4017 is the moral equivalent of these two:
4020 print(uc($bar),$baz);
4022 See L<perlop> for more details on unary operators and the comma operator.
4024 =item seek FILEHANDLE,POSITION,WHENCE
4026 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4027 FILEHANDLE may be an expression whose value gives the name of the
4028 filehandle. The values for WHENCE are C<0> to set the new position to
4029 POSITION, C<1> to set it to the current position plus POSITION, and
4030 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
4031 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
4032 (start of the file, current position, end of the file) from the Fcntl
4033 module. Returns C<1> upon success, C<0> otherwise.
4035 If you want to position file for C<sysread> or C<syswrite>, don't use
4036 C<seek>--buffering makes its effect on the file's system position
4037 unpredictable and non-portable. Use C<sysseek> instead.
4039 Due to the rules and rigors of ANSI C, on some systems you have to do a
4040 seek whenever you switch between reading and writing. Amongst other
4041 things, this may have the effect of calling stdio's clearerr(3).
4042 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4046 This is also useful for applications emulating C<tail -f>. Once you hit
4047 EOF on your read, and then sleep for a while, you might have to stick in a
4048 seek() to reset things. The C<seek> doesn't change the current position,
4049 but it I<does> clear the end-of-file condition on the handle, so that the
4050 next C<< <FILE> >> makes Perl try again to read something. We hope.
4052 If that doesn't work (some stdios are particularly cantankerous), then
4053 you may need something more like this:
4056 for ($curpos = tell(FILE); $_ = <FILE>;
4057 $curpos = tell(FILE)) {
4058 # search for some stuff and put it into files
4060 sleep($for_a_while);
4061 seek(FILE, $curpos, 0);
4064 =item seekdir DIRHANDLE,POS
4066 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4067 must be a value returned by C<telldir>. Has the same caveats about
4068 possible directory compaction as the corresponding system library
4071 =item select FILEHANDLE
4075 Returns the currently selected filehandle. Sets the current default
4076 filehandle for output, if FILEHANDLE is supplied. This has two
4077 effects: first, a C<write> or a C<print> without a filehandle will
4078 default to this FILEHANDLE. Second, references to variables related to
4079 output will refer to this output channel. For example, if you have to
4080 set the top of form format for more than one output channel, you might
4088 FILEHANDLE may be an expression whose value gives the name of the
4089 actual filehandle. Thus:
4091 $oldfh = select(STDERR); $| = 1; select($oldfh);
4093 Some programmers may prefer to think of filehandles as objects with
4094 methods, preferring to write the last example as:
4097 STDERR->autoflush(1);
4099 =item select RBITS,WBITS,EBITS,TIMEOUT
4101 This calls the select(2) system call with the bit masks specified, which
4102 can be constructed using C<fileno> and C<vec>, along these lines:
4104 $rin = $win = $ein = '';
4105 vec($rin,fileno(STDIN),1) = 1;
4106 vec($win,fileno(STDOUT),1) = 1;
4109 If you want to select on many filehandles you might wish to write a
4113 my(@fhlist) = split(' ',$_[0]);
4116 vec($bits,fileno($_),1) = 1;
4120 $rin = fhbits('STDIN TTY SOCK');
4124 ($nfound,$timeleft) =
4125 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4127 or to block until something becomes ready just do this
4129 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4131 Most systems do not bother to return anything useful in $timeleft, so
4132 calling select() in scalar context just returns $nfound.
4134 Any of the bit masks can also be undef. The timeout, if specified, is
4135 in seconds, which may be fractional. Note: not all implementations are
4136 capable of returning the $timeleft. If not, they always return
4137 $timeleft equal to the supplied $timeout.
4139 You can effect a sleep of 250 milliseconds this way:
4141 select(undef, undef, undef, 0.25);
4143 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4144 or <FH>) with C<select>, except as permitted by POSIX, and even
4145 then only on POSIX systems. You have to use C<sysread> instead.
4147 =item semctl ID,SEMNUM,CMD,ARG
4149 Calls the System V IPC function C<semctl>. You'll probably have to say
4153 first to get the correct constant definitions. If CMD is IPC_STAT or
4154 GETALL, then ARG must be a variable which will hold the returned
4155 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4156 the undefined value for error, "C<0 but true>" for zero, or the actual
4157 return value otherwise. The ARG must consist of a vector of native
4158 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4159 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4162 =item semget KEY,NSEMS,FLAGS
4164 Calls the System V IPC function semget. Returns the semaphore id, or
4165 the undefined value if there is an error. See also
4166 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4169 =item semop KEY,OPSTRING
4171 Calls the System V IPC function semop to perform semaphore operations
4172 such as signalling and waiting. OPSTRING must be a packed array of
4173 semop structures. Each semop structure can be generated with
4174 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4175 operations is implied by the length of OPSTRING. Returns true if
4176 successful, or false if there is an error. As an example, the
4177 following code waits on semaphore $semnum of semaphore id $semid:
4179 $semop = pack("s!3", $semnum, -1, 0);
4180 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4182 To signal the semaphore, replace C<-1> with C<1>. See also
4183 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4186 =item send SOCKET,MSG,FLAGS,TO
4188 =item send SOCKET,MSG,FLAGS
4190 Sends a message on a socket. Takes the same flags as the system call
4191 of the same name. On unconnected sockets you must specify a
4192 destination to send TO, in which case it does a C C<sendto>. Returns
4193 the number of characters sent, or the undefined value if there is an
4194 error. The C system call sendmsg(2) is currently unimplemented.
4195 See L<perlipc/"UDP: Message Passing"> for examples.
4197 =item setpgrp PID,PGRP
4199 Sets the current process group for the specified PID, C<0> for the current
4200 process. Will produce a fatal error if used on a machine that doesn't
4201 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4202 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4203 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4206 =item setpriority WHICH,WHO,PRIORITY
4208 Sets the current priority for a process, a process group, or a user.
4209 (See setpriority(2).) Will produce a fatal error if used on a machine
4210 that doesn't implement setpriority(2).
4212 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4214 Sets the socket option requested. Returns undefined if there is an
4215 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4222 Shifts the first value of the array off and returns it, shortening the
4223 array by 1 and moving everything down. If there are no elements in the
4224 array, returns the undefined value. If ARRAY is omitted, shifts the
4225 C<@_> array within the lexical scope of subroutines and formats, and the
4226 C<@ARGV> array at file scopes or within the lexical scopes established by
4227 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4230 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4231 same thing to the left end of an array that C<pop> and C<push> do to the
4234 =item shmctl ID,CMD,ARG
4236 Calls the System V IPC function shmctl. You'll probably have to say
4240 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4241 then ARG must be a variable which will hold the returned C<shmid_ds>
4242 structure. Returns like ioctl: the undefined value for error, "C<0> but
4243 true" for zero, or the actual return value otherwise.
4244 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4246 =item shmget KEY,SIZE,FLAGS
4248 Calls the System V IPC function shmget. Returns the shared memory
4249 segment id, or the undefined value if there is an error.
4250 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4252 =item shmread ID,VAR,POS,SIZE
4254 =item shmwrite ID,STRING,POS,SIZE
4256 Reads or writes the System V shared memory segment ID starting at
4257 position POS for size SIZE by attaching to it, copying in/out, and
4258 detaching from it. When reading, VAR must be a variable that will
4259 hold the data read. When writing, if STRING is too long, only SIZE
4260 bytes are used; if STRING is too short, nulls are written to fill out
4261 SIZE bytes. Return true if successful, or false if there is an error.
4262 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4263 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4265 =item shutdown SOCKET,HOW
4267 Shuts down a socket connection in the manner indicated by HOW, which
4268 has the same interpretation as in the system call of the same name.
4270 shutdown(SOCKET, 0); # I/we have stopped reading data
4271 shutdown(SOCKET, 1); # I/we have stopped writing data
4272 shutdown(SOCKET, 2); # I/we have stopped using this socket
4274 This is useful with sockets when you want to tell the other
4275 side you're done writing but not done reading, or vice versa.
4276 It's also a more insistent form of close because it also
4277 disables the file descriptor in any forked copies in other
4284 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4285 returns sine of C<$_>.
4287 For the inverse sine operation, you may use the C<Math::Trig::asin>
4288 function, or use this relation:
4290 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4296 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4297 May be interrupted if the process receives a signal such as C<SIGALRM>.
4298 Returns the number of seconds actually slept. You probably cannot
4299 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4302 On some older systems, it may sleep up to a full second less than what
4303 you requested, depending on how it counts seconds. Most modern systems
4304 always sleep the full amount. They may appear to sleep longer than that,
4305 however, because your process might not be scheduled right away in a
4306 busy multitasking system.
4308 For delays of finer granularity than one second, you may use Perl's
4309 C<syscall> interface to access setitimer(2) if your system supports
4310 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4311 and starting from Perl 5.8 part of the standard distribution) may also
4314 See also the POSIX module's C<pause> function.
4316 =item sockatmark SOCKET
4318 Returns true if the socket is positioned at the out-of-band mark
4319 (also known as the urgent data mark), false otherwise. Use right
4320 after reading from the socket.
4322 Not available directly, one has to import the function from
4323 the IO::Socket extension
4325 use IO::Socket 'sockatmark';
4327 Even this doesn't guarantee that sockatmark() really is available,
4328 though, because sockatmark() is a relatively recent addition to
4329 the family of socket functions. If it is unavailable, attempt to
4332 IO::Socket::atmark not implemented on this architecture ...
4334 See also L<IO::Socket>.
4336 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4338 Opens a socket of the specified kind and attaches it to filehandle
4339 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4340 the system call of the same name. You should C<use Socket> first
4341 to get the proper definitions imported. See the examples in
4342 L<perlipc/"Sockets: Client/Server Communication">.
4344 On systems that support a close-on-exec flag on files, the flag will
4345 be set for the newly opened file descriptor, as determined by the
4346 value of $^F. See L<perlvar/$^F>.
4348 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4350 Creates an unnamed pair of sockets in the specified domain, of the
4351 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4352 for the system call of the same name. If unimplemented, yields a fatal
4353 error. Returns true if successful.
4355 On systems that support a close-on-exec flag on files, the flag will
4356 be set for the newly opened file descriptors, as determined by the value
4357 of $^F. See L<perlvar/$^F>.
4359 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4360 to C<pipe(Rdr, Wtr)> is essentially:
4363 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4364 shutdown(Rdr, 1); # no more writing for reader
4365 shutdown(Wtr, 0); # no more reading for writer
4367 See L<perlipc> for an example of socketpair use.
4369 =item sort SUBNAME LIST
4371 =item sort BLOCK LIST
4375 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4376 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4377 specified, it gives the name of a subroutine that returns an integer
4378 less than, equal to, or greater than C<0>, depending on how the elements
4379 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4380 operators are extremely useful in such routines.) SUBNAME may be a
4381 scalar variable name (unsubscripted), in which case the value provides
4382 the name of (or a reference to) the actual subroutine to use. In place
4383 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4386 If the subroutine's prototype is C<($$)>, the elements to be compared
4387 are passed by reference in C<@_>, as for a normal subroutine. This is
4388 slower than unprototyped subroutines, where the elements to be
4389 compared are passed into the subroutine
4390 as the package global variables $a and $b (see example below). Note that
4391 in the latter case, it is usually counter-productive to declare $a and
4394 In either case, the subroutine may not be recursive. The values to be
4395 compared are always passed by reference, so don't modify them.
4397 You also cannot exit out of the sort block or subroutine using any of the
4398 loop control operators described in L<perlsyn> or with C<goto>.
4400 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4401 current collation locale. See L<perllocale>.
4403 Perl does B<not> guarantee that sort is stable. (A I<stable> sort
4404 preserves the input order of elements that compare equal.) 5.7 and
4405 5.8 happen to use a stable mergesort, but 5.6 and earlier used quicksort,
4406 which is not stable. Do not assume that future perls will continue to
4412 @articles = sort @files;
4414 # same thing, but with explicit sort routine
4415 @articles = sort {$a cmp $b} @files;
4417 # now case-insensitively
4418 @articles = sort {uc($a) cmp uc($b)} @files;
4420 # same thing in reversed order
4421 @articles = sort {$b cmp $a} @files;
4423 # sort numerically ascending
4424 @articles = sort {$a <=> $b} @files;
4426 # sort numerically descending
4427 @articles = sort {$b <=> $a} @files;
4429 # this sorts the %age hash by value instead of key
4430 # using an in-line function
4431 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4433 # sort using explicit subroutine name
4435 $age{$a} <=> $age{$b}; # presuming numeric
4437 @sortedclass = sort byage @class;
4439 sub backwards { $b cmp $a }
4440 @harry = qw(dog cat x Cain Abel);
4441 @george = qw(gone chased yz Punished Axed);
4443 # prints AbelCaincatdogx
4444 print sort backwards @harry;
4445 # prints xdogcatCainAbel
4446 print sort @george, 'to', @harry;
4447 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4449 # inefficiently sort by descending numeric compare using
4450 # the first integer after the first = sign, or the
4451 # whole record case-insensitively otherwise
4454 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4459 # same thing, but much more efficiently;
4460 # we'll build auxiliary indices instead
4464 push @nums, /=(\d+)/;
4469 $nums[$b] <=> $nums[$a]
4471 $caps[$a] cmp $caps[$b]
4475 # same thing, but without any temps
4476 @new = map { $_->[0] }
4477 sort { $b->[1] <=> $a->[1]
4480 } map { [$_, /=(\d+)/, uc($_)] } @old;
4482 # using a prototype allows you to use any comparison subroutine
4483 # as a sort subroutine (including other package's subroutines)
4485 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4488 @new = sort other::backwards @old;
4490 If you're using strict, you I<must not> declare $a
4491 and $b as lexicals. They are package globals. That means
4492 if you're in the C<main> package and type
4494 @articles = sort {$b <=> $a} @files;
4496 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4497 but if you're in the C<FooPack> package, it's the same as typing
4499 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4501 The comparison function is required to behave. If it returns
4502 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4503 sometimes saying the opposite, for example) the results are not
4506 =item splice ARRAY,OFFSET,LENGTH,LIST
4508 =item splice ARRAY,OFFSET,LENGTH
4510 =item splice ARRAY,OFFSET
4514 Removes the elements designated by OFFSET and LENGTH from an array, and
4515 replaces them with the elements of LIST, if any. In list context,
4516 returns the elements removed from the array. In scalar context,
4517 returns the last element removed, or C<undef> if no elements are
4518 removed. The array grows or shrinks as necessary.
4519 If OFFSET is negative then it starts that far from the end of the array.
4520 If LENGTH is omitted, removes everything from OFFSET onward.
4521 If LENGTH is negative, leaves that many elements off the end of the array.
4522 If both OFFSET and LENGTH are omitted, removes everything.
4524 The following equivalences hold (assuming C<$[ == 0>):
4526 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4527 pop(@a) splice(@a,-1)
4528 shift(@a) splice(@a,0,1)
4529 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4530 $a[$x] = $y splice(@a,$x,1,$y)
4532 Example, assuming array lengths are passed before arrays:
4534 sub aeq { # compare two list values
4535 my(@a) = splice(@_,0,shift);
4536 my(@b) = splice(@_,0,shift);
4537 return 0 unless @a == @b; # same len?
4539 return 0 if pop(@a) ne pop(@b);
4543 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4545 =item split /PATTERN/,EXPR,LIMIT
4547 =item split /PATTERN/,EXPR
4549 =item split /PATTERN/
4553 Splits a string into a list of strings and returns that list. By default,
4554 empty leading fields are preserved, and empty trailing ones are deleted.
4556 In scalar context, returns the number of fields found and splits into
4557 the C<@_> array. Use of split in scalar context is deprecated, however,
4558 because it clobbers your subroutine arguments.
4560 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4561 splits on whitespace (after skipping any leading whitespace). Anything
4562 matching PATTERN is taken to be a delimiter separating the fields. (Note
4563 that the delimiter may be longer than one character.)
4565 If LIMIT is specified and positive, it represents the maximum number
4566 of fields the EXPR will be split into, though the actual number of
4567 fields returned depends on the number of times PATTERN matches within
4568 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4569 stripped (which potential users of C<pop> would do well to remember).
4570 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4571 had been specified. Note that splitting an EXPR that evaluates to the
4572 empty string always returns the empty list, regardless of the LIMIT
4575 A pattern matching the null string (not to be confused with
4576 a null pattern C<//>, which is just one member of the set of patterns
4577 matching a null string) will split the value of EXPR into separate
4578 characters at each point it matches that way. For example:
4580 print join(':', split(/ */, 'hi there'));
4582 produces the output 'h:i:t:h:e:r:e'.
4584 Using the empty pattern C<//> specifically matches the null string, and is
4585 not be confused with the use of C<//> to mean "the last successful pattern
4588 Empty leading (or trailing) fields are produced when there positive width
4589 matches at the beginning (or end) of the string; a zero-width match at the
4590 beginning (or end) of the string does not produce an empty field. For
4593 print join(':', split(/(?=\w)/, 'hi there!'));
4595 produces the output 'h:i :t:h:e:r:e!'.
4597 The LIMIT parameter can be used to split a line partially
4599 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4601 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4602 one larger than the number of variables in the list, to avoid
4603 unnecessary work. For the list above LIMIT would have been 4 by
4604 default. In time critical applications it behooves you not to split
4605 into more fields than you really need.
4607 If the PATTERN contains parentheses, additional list elements are
4608 created from each matching substring in the delimiter.
4610 split(/([,-])/, "1-10,20", 3);
4612 produces the list value
4614 (1, '-', 10, ',', 20)
4616 If you had the entire header of a normal Unix email message in $header,
4617 you could split it up into fields and their values this way:
4619 $header =~ s/\n\s+/ /g; # fix continuation lines
4620 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4622 The pattern C</PATTERN/> may be replaced with an expression to specify
4623 patterns that vary at runtime. (To do runtime compilation only once,
4624 use C</$variable/o>.)
4626 As a special case, specifying a PATTERN of space (C<' '>) will split on
4627 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4628 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4629 will give you as many null initial fields as there are leading spaces.
4630 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4631 whitespace produces a null first field. A C<split> with no arguments
4632 really does a C<split(' ', $_)> internally.
4634 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4639 open(PASSWD, '/etc/passwd');
4642 ($login, $passwd, $uid, $gid,
4643 $gcos, $home, $shell) = split(/:/);
4647 As with regular pattern matching, any capturing parentheses that are not
4648 matched in a C<split()> will be set to C<undef> when returned:
4650 @fields = split /(A)|B/, "1A2B3";
4651 # @fields is (1, 'A', 2, undef, 3)
4653 =item sprintf FORMAT, LIST
4655 Returns a string formatted by the usual C<printf> conventions of the C
4656 library function C<sprintf>. See below for more details
4657 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4658 the general principles.
4662 # Format number with up to 8 leading zeroes
4663 $result = sprintf("%08d", $number);
4665 # Round number to 3 digits after decimal point
4666 $rounded = sprintf("%.3f", $number);
4668 Perl does its own C<sprintf> formatting--it emulates the C
4669 function C<sprintf>, but it doesn't use it (except for floating-point
4670 numbers, and even then only the standard modifiers are allowed). As a
4671 result, any non-standard extensions in your local C<sprintf> are not
4672 available from Perl.
4674 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4675 pass it an array as your first argument. The array is given scalar context,
4676 and instead of using the 0th element of the array as the format, Perl will
4677 use the count of elements in the array as the format, which is almost never
4680 Perl's C<sprintf> permits the following universally-known conversions:
4683 %c a character with the given number
4685 %d a signed integer, in decimal
4686 %u an unsigned integer, in decimal
4687 %o an unsigned integer, in octal
4688 %x an unsigned integer, in hexadecimal
4689 %e a floating-point number, in scientific notation
4690 %f a floating-point number, in fixed decimal notation
4691 %g a floating-point number, in %e or %f notation
4693 In addition, Perl permits the following widely-supported conversions:
4695 %X like %x, but using upper-case letters
4696 %E like %e, but using an upper-case "E"
4697 %G like %g, but with an upper-case "E" (if applicable)
4698 %b an unsigned integer, in binary
4699 %p a pointer (outputs the Perl value's address in hexadecimal)
4700 %n special: *stores* the number of characters output so far
4701 into the next variable in the parameter list
4703 Finally, for backward (and we do mean "backward") compatibility, Perl
4704 permits these unnecessary but widely-supported conversions:
4707 %D a synonym for %ld
4708 %U a synonym for %lu
4709 %O a synonym for %lo
4712 Note that the number of exponent digits in the scientific notation by
4713 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4714 exponent less than 100 is system-dependent: it may be three or less
4715 (zero-padded as necessary). In other words, 1.23 times ten to the
4716 99th may be either "1.23e99" or "1.23e099".
4718 Perl permits the following universally-known flags between the C<%>
4719 and the conversion letter:
4721 space prefix positive number with a space
4722 + prefix positive number with a plus sign
4723 - left-justify within the field
4724 0 use zeros, not spaces, to right-justify
4725 # prefix non-zero octal with "0", non-zero hex with "0x"
4726 number minimum field width
4727 .number "precision": digits after decimal point for
4728 floating-point, max length for string, minimum length
4730 l interpret integer as C type "long" or "unsigned long"
4731 h interpret integer as C type "short" or "unsigned short"
4732 If no flags, interpret integer as C type "int" or "unsigned"
4734 Perl supports parameter ordering, in other words, fetching the
4735 parameters in some explicitly specified "random" ordering as opposed
4736 to the default implicit sequential ordering. The syntax is, instead
4737 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4738 where the I<digits> is the wanted index, from one upwards. For example:
4740 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4741 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4743 Note that using the reordering syntax does not interfere with the usual
4744 implicit sequential fetching of the parameters:
4746 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4747 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4748 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4749 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4750 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4752 There are also two Perl-specific flags:
4754 V interpret integer as Perl's standard integer type
4755 v interpret string as a vector of integers, output as
4756 numbers separated either by dots, or by an arbitrary
4757 string received from the argument list when the flag
4760 Where a number would appear in the flags, an asterisk (C<*>) may be
4761 used instead, in which case Perl uses the next item in the parameter
4762 list as the given number (that is, as the field width or precision).
4763 If a field width obtained through C<*> is negative, it has the same
4764 effect as the C<-> flag: left-justification.
4766 The C<v> flag is useful for displaying ordinal values of characters
4767 in arbitrary strings:
4769 printf "version is v%vd\n", $^V; # Perl's version
4770 printf "address is %*vX\n", ":", $addr; # IPv6 address
4771 printf "bits are %*vb\n", " ", $bits; # random bitstring
4773 If C<use locale> is in effect, the character used for the decimal
4774 point in formatted real numbers is affected by the LC_NUMERIC locale.
4777 If Perl understands "quads" (64-bit integers) (this requires
4778 either that the platform natively support quads or that Perl
4779 be specifically compiled to support quads), the characters
4783 print quads, and they may optionally be preceded by
4791 You can find out whether your Perl supports quads via L<Config>:
4794 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4797 If Perl understands "long doubles" (this requires that the platform
4798 support long doubles), the flags
4802 may optionally be preceded by
4810 You can find out whether your Perl supports long doubles via L<Config>:
4813 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4819 Return the square root of EXPR. If EXPR is omitted, returns square
4820 root of C<$_>. Only works on non-negative operands, unless you've
4821 loaded the standard Math::Complex module.
4824 print sqrt(-2); # prints 1.4142135623731i
4830 Sets the random number seed for the C<rand> operator.
4832 The point of the function is to "seed" the C<rand> function so that
4833 C<rand> can produce a different sequence each time you run your
4836 If srand() is not called explicitly, it is called implicitly at the
4837 first use of the C<rand> operator. However, this was not the case in
4838 versions of Perl before 5.004, so if your script will run under older
4839 Perl versions, it should call C<srand>.
4841 Most programs won't even call srand() at all, except those that
4842 need a cryptographically-strong starting point rather than the
4843 generally acceptable default, which is based on time of day,
4844 process ID, and memory allocation, or the F</dev/urandom> device,
4847 You can call srand($seed) with the same $seed to reproduce the
4848 I<same> sequence from rand(), but this is usually reserved for
4849 generating predictable results for testing or debugging.
4850 Otherwise, don't call srand() more than once in your program.
4852 Do B<not> call srand() (i.e. without an argument) more than once in
4853 a script. The internal state of the random number generator should
4854 contain more entropy than can be provided by any seed, so calling
4855 srand() again actually I<loses> randomness.
4857 Most implementations of C<srand> take an integer and will silently
4858 truncate decimal numbers. This means C<srand(42)> will usually
4859 produce the same results as C<srand(42.1)>. To be safe, always pass
4860 C<srand> an integer.
4862 In versions of Perl prior to 5.004 the default seed was just the
4863 current C<time>. This isn't a particularly good seed, so many old
4864 programs supply their own seed value (often C<time ^ $$> or C<time ^
4865 ($$ + ($$ << 15))>), but that isn't necessary any more.
4867 Note that you need something much more random than the default seed for
4868 cryptographic purposes. Checksumming the compressed output of one or more
4869 rapidly changing operating system status programs is the usual method. For
4872 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4874 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4877 Frequently called programs (like CGI scripts) that simply use
4881 for a seed can fall prey to the mathematical property that
4885 one-third of the time. So don't do that.
4887 =item stat FILEHANDLE
4893 Returns a 13-element list giving the status info for a file, either
4894 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4895 it stats C<$_>. Returns a null list if the stat fails. Typically used
4898 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4899 $atime,$mtime,$ctime,$blksize,$blocks)
4902 Not all fields are supported on all filesystem types. Here are the
4903 meaning of the fields:
4905 0 dev device number of filesystem
4907 2 mode file mode (type and permissions)
4908 3 nlink number of (hard) links to the file
4909 4 uid numeric user ID of file's owner
4910 5 gid numeric group ID of file's owner
4911 6 rdev the device identifier (special files only)
4912 7 size total size of file, in bytes
4913 8 atime last access time in seconds since the epoch
4914 9 mtime last modify time in seconds since the epoch
4915 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4916 11 blksize preferred block size for file system I/O
4917 12 blocks actual number of blocks allocated
4919 (The epoch was at 00:00 January 1, 1970 GMT.)
4921 If stat is passed the special filehandle consisting of an underline, no
4922 stat is done, but the current contents of the stat structure from the
4923 last stat or filetest are returned. Example:
4925 if (-x $file && (($d) = stat(_)) && $d < 0) {
4926 print "$file is executable NFS file\n";
4929 (This works on machines only for which the device number is negative
4932 Because the mode contains both the file type and its permissions, you
4933 should mask off the file type portion and (s)printf using a C<"%o">
4934 if you want to see the real permissions.
4936 $mode = (stat($filename))[2];
4937 printf "Permissions are %04o\n", $mode & 07777;
4939 In scalar context, C<stat> returns a boolean value indicating success
4940 or failure, and, if successful, sets the information associated with
4941 the special filehandle C<_>.
4943 The File::stat module provides a convenient, by-name access mechanism:
4946 $sb = stat($filename);
4947 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4948 $filename, $sb->size, $sb->mode & 07777,
4949 scalar localtime $sb->mtime;
4951 You can import symbolic mode constants (C<S_IF*>) and functions
4952 (C<S_IS*>) from the Fcntl module:
4956 $mode = (stat($filename))[2];
4958 $user_rwx = ($mode & S_IRWXU) >> 6;
4959 $group_read = ($mode & S_IRGRP) >> 3;
4960 $other_execute = $mode & S_IXOTH;
4962 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4964 $is_setuid = $mode & S_ISUID;
4965 $is_setgid = S_ISDIR($mode);
4967 You could write the last two using the C<-u> and C<-d> operators.
4968 The commonly available S_IF* constants are
4970 # Permissions: read, write, execute, for user, group, others.
4972 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4973 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4974 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4976 # Setuid/Setgid/Stickiness.
4978 S_ISUID S_ISGID S_ISVTX S_ISTXT
4980 # File types. Not necessarily all are available on your system.
4982 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4984 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4986 S_IREAD S_IWRITE S_IEXEC
4988 and the S_IF* functions are
4990 S_IFMODE($mode) the part of $mode containing the permission bits
4991 and the setuid/setgid/sticky bits
4993 S_IFMT($mode) the part of $mode containing the file type
4994 which can be bit-anded with e.g. S_IFREG
4995 or with the following functions
4997 # The operators -f, -d, -l, -b, -c, -p, and -s.
4999 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5000 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5002 # No direct -X operator counterpart, but for the first one
5003 # the -g operator is often equivalent. The ENFMT stands for
5004 # record flocking enforcement, a platform-dependent feature.
5006 S_ISENFMT($mode) S_ISWHT($mode)
5008 See your native chmod(2) and stat(2) documentation for more details
5009 about the S_* constants.
5015 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5016 doing many pattern matches on the string before it is next modified.
5017 This may or may not save time, depending on the nature and number of
5018 patterns you are searching on, and on the distribution of character
5019 frequencies in the string to be searched--you probably want to compare
5020 run times with and without it to see which runs faster. Those loops
5021 which scan for many short constant strings (including the constant
5022 parts of more complex patterns) will benefit most. You may have only
5023 one C<study> active at a time--if you study a different scalar the first
5024 is "unstudied". (The way C<study> works is this: a linked list of every
5025 character in the string to be searched is made, so we know, for
5026 example, where all the C<'k'> characters are. From each search string,
5027 the rarest character is selected, based on some static frequency tables
5028 constructed from some C programs and English text. Only those places
5029 that contain this "rarest" character are examined.)
5031 For example, here is a loop that inserts index producing entries
5032 before any line containing a certain pattern:
5036 print ".IX foo\n" if /\bfoo\b/;
5037 print ".IX bar\n" if /\bbar\b/;
5038 print ".IX blurfl\n" if /\bblurfl\b/;
5043 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5044 will be looked at, because C<f> is rarer than C<o>. In general, this is
5045 a big win except in pathological cases. The only question is whether
5046 it saves you more time than it took to build the linked list in the
5049 Note that if you have to look for strings that you don't know till
5050 runtime, you can build an entire loop as a string and C<eval> that to
5051 avoid recompiling all your patterns all the time. Together with
5052 undefining C<$/> to input entire files as one record, this can be very
5053 fast, often faster than specialized programs like fgrep(1). The following
5054 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5055 out the names of those files that contain a match:
5057 $search = 'while (<>) { study;';
5058 foreach $word (@words) {
5059 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5064 eval $search; # this screams
5065 $/ = "\n"; # put back to normal input delimiter
5066 foreach $file (sort keys(%seen)) {
5074 =item sub NAME BLOCK
5076 This is subroutine definition, not a real function I<per se>. With just a
5077 NAME (and possibly prototypes or attributes), it's just a forward declaration.
5078 Without a NAME, it's an anonymous function declaration, and does actually
5079 return a value: the CODE ref of the closure you just created. See L<perlsub>
5080 and L<perlref> for details.
5082 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5084 =item substr EXPR,OFFSET,LENGTH
5086 =item substr EXPR,OFFSET
5088 Extracts a substring out of EXPR and returns it. First character is at
5089 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5090 If OFFSET is negative (or more precisely, less than C<$[>), starts
5091 that far from the end of the string. If LENGTH is omitted, returns
5092 everything to the end of the string. If LENGTH is negative, leaves that
5093 many characters off the end of the string.
5095 You can use the substr() function as an lvalue, in which case EXPR
5096 must itself be an lvalue. If you assign something shorter than LENGTH,
5097 the string will shrink, and if you assign something longer than LENGTH,
5098 the string will grow to accommodate it. To keep the string the same
5099 length you may need to pad or chop your value using C<sprintf>.
5101 If OFFSET and LENGTH specify a substring that is partly outside the
5102 string, only the part within the string is returned. If the substring
5103 is beyond either end of the string, substr() returns the undefined
5104 value and produces a warning. When used as an lvalue, specifying a
5105 substring that is entirely outside the string is a fatal error.
5106 Here's an example showing the behavior for boundary cases:
5109 substr($name, 4) = 'dy'; # $name is now 'freddy'
5110 my $null = substr $name, 6, 2; # returns '' (no warning)
5111 my $oops = substr $name, 7; # returns undef, with warning
5112 substr($name, 7) = 'gap'; # fatal error
5114 An alternative to using substr() as an lvalue is to specify the
5115 replacement string as the 4th argument. This allows you to replace
5116 parts of the EXPR and return what was there before in one operation,
5117 just as you can with splice().
5119 =item symlink OLDFILE,NEWFILE
5121 Creates a new filename symbolically linked to the old filename.
5122 Returns C<1> for success, C<0> otherwise. On systems that don't support
5123 symbolic links, produces a fatal error at run time. To check for that,
5126 $symlink_exists = eval { symlink("",""); 1 };
5130 Calls the system call specified as the first element of the list,
5131 passing the remaining elements as arguments to the system call. If
5132 unimplemented, produces a fatal error. The arguments are interpreted
5133 as follows: if a given argument is numeric, the argument is passed as
5134 an int. If not, the pointer to the string value is passed. You are
5135 responsible to make sure a string is pre-extended long enough to
5136 receive any result that might be written into a string. You can't use a
5137 string literal (or other read-only string) as an argument to C<syscall>
5138 because Perl has to assume that any string pointer might be written
5140 integer arguments are not literals and have never been interpreted in a
5141 numeric context, you may need to add C<0> to them to force them to look
5142 like numbers. This emulates the C<syswrite> function (or vice versa):
5144 require 'syscall.ph'; # may need to run h2ph
5146 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5148 Note that Perl supports passing of up to only 14 arguments to your system call,
5149 which in practice should usually suffice.
5151 Syscall returns whatever value returned by the system call it calls.
5152 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5153 Note that some system calls can legitimately return C<-1>. The proper
5154 way to handle such calls is to assign C<$!=0;> before the call and
5155 check the value of C<$!> if syscall returns C<-1>.
5157 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5158 number of the read end of the pipe it creates. There is no way
5159 to retrieve the file number of the other end. You can avoid this
5160 problem by using C<pipe> instead.
5162 =item sysopen FILEHANDLE,FILENAME,MODE
5164 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5166 Opens the file whose filename is given by FILENAME, and associates it
5167 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5168 the name of the real filehandle wanted. This function calls the
5169 underlying operating system's C<open> function with the parameters
5170 FILENAME, MODE, PERMS.
5172 The possible values and flag bits of the MODE parameter are
5173 system-dependent; they are available via the standard module C<Fcntl>.
5174 See the documentation of your operating system's C<open> to see which
5175 values and flag bits are available. You may combine several flags
5176 using the C<|>-operator.
5178 Some of the most common values are C<O_RDONLY> for opening the file in
5179 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5180 and C<O_RDWR> for opening the file in read-write mode, and.
5182 For historical reasons, some values work on almost every system
5183 supported by perl: zero means read-only, one means write-only, and two
5184 means read/write. We know that these values do I<not> work under
5185 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5186 use them in new code.
5188 If the file named by FILENAME does not exist and the C<open> call creates
5189 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5190 PERMS specifies the permissions of the newly created file. If you omit
5191 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5192 These permission values need to be in octal, and are modified by your
5193 process's current C<umask>.
5195 In many systems the C<O_EXCL> flag is available for opening files in
5196 exclusive mode. This is B<not> locking: exclusiveness means here that
5197 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5200 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5202 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5203 that takes away the user's option to have a more permissive umask.
5204 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5207 Note that C<sysopen> depends on the fdopen() C library function.
5208 On many UNIX systems, fdopen() is known to fail when file descriptors
5209 exceed a certain value, typically 255. If you need more file
5210 descriptors than that, consider rebuilding Perl to use the C<sfio>
5211 library, or perhaps using the POSIX::open() function.
5213 See L<perlopentut> for a kinder, gentler explanation of opening files.
5215 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5217 =item sysread FILEHANDLE,SCALAR,LENGTH
5219 Attempts to read LENGTH bytes of data into variable SCALAR from the
5220 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
5221 so mixing this with other kinds of reads, C<print>, C<write>,
5222 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
5223 usually buffers data. Returns the number of bytes actually read, C<0>
5224 at end of file, or undef if there was an error. SCALAR will be grown or
5225 shrunk so that the last byte actually read is the last byte of the
5226 scalar after the read.
5228 An OFFSET may be specified to place the read data at some place in the
5229 string other than the beginning. A negative OFFSET specifies
5230 placement at that many bytes counting backwards from the end of the
5231 string. A positive OFFSET greater than the length of SCALAR results
5232 in the string being padded to the required size with C<"\0"> bytes before
5233 the result of the read is appended.
5235 There is no syseof() function, which is ok, since eof() doesn't work
5236 very well on device files (like ttys) anyway. Use sysread() and check
5237 for a return value for 0 to decide whether you're done.
5239 =item sysseek FILEHANDLE,POSITION,WHENCE
5241 Sets FILEHANDLE's system position using the system call lseek(2). It
5242 bypasses stdio, so mixing this with reads (other than C<sysread>),
5243 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
5244 FILEHANDLE may be an expression whose value gives the name of the
5245 filehandle. The values for WHENCE are C<0> to set the new position to
5246 POSITION, C<1> to set the it to the current position plus POSITION,
5247 and C<2> to set it to EOF plus POSITION (typically negative). For
5248 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
5249 C<SEEK_END> (start of the file, current position, end of the file)
5250 from the Fcntl module.
5252 Returns the new position, or the undefined value on failure. A position
5253 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5254 true on success and false on failure, yet you can still easily determine
5259 =item system PROGRAM LIST
5261 Does exactly the same thing as C<exec LIST>, except that a fork is
5262 done first, and the parent process waits for the child process to
5263 complete. Note that argument processing varies depending on the
5264 number of arguments. If there is more than one argument in LIST,
5265 or if LIST is an array with more than one value, starts the program
5266 given by the first element of the list with arguments given by the
5267 rest of the list. If there is only one scalar argument, the argument
5268 is checked for shell metacharacters, and if there are any, the
5269 entire argument is passed to the system's command shell for parsing
5270 (this is C</bin/sh -c> on Unix platforms, but varies on other
5271 platforms). If there are no shell metacharacters in the argument,
5272 it is split into words and passed directly to C<execvp>, which is
5275 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5276 output before any operation that may do a fork, but this may not be
5277 supported on some platforms (see L<perlport>). To be safe, you may need
5278 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5279 of C<IO::Handle> on any open handles.
5281 The return value is the exit status of the program as
5282 returned by the C<wait> call. To get the actual exit value divide by
5283 256. See also L</exec>. This is I<not> what you want to use to capture
5284 the output from a command, for that you should use merely backticks or
5285 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5286 indicates a failure to start the program (inspect $! for the reason).
5288 Like C<exec>, C<system> allows you to lie to a program about its name if
5289 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5291 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
5292 program they're running doesn't actually interrupt your program.
5294 @args = ("command", "arg1", "arg2");
5296 or die "system @args failed: $?"
5298 You can check all the failure possibilities by inspecting
5301 $exit_value = $? >> 8;
5302 $signal_num = $? & 127;
5303 $dumped_core = $? & 128;
5305 When the arguments get executed via the system shell, results
5306 and return codes will be subject to its quirks and capabilities.
5307 See L<perlop/"`STRING`"> and L</exec> for details.
5309 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5311 =item syswrite FILEHANDLE,SCALAR,LENGTH
5313 =item syswrite FILEHANDLE,SCALAR
5315 Attempts to write LENGTH bytes of data from variable SCALAR to the
5316 specified FILEHANDLE, using the system call write(2). If LENGTH
5317 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5318 this with reads (other than C<sysread())>, C<print>, C<write>,
5319 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5320 usually buffers data. Returns the number of bytes actually written,
5321 or C<undef> if there was an error. If the LENGTH is greater than
5322 the available data in the SCALAR after the OFFSET, only as much
5323 data as is available will be written.
5325 An OFFSET may be specified to write the data from some part of the
5326 string other than the beginning. A negative OFFSET specifies writing
5327 that many bytes counting backwards from the end of the string. In the
5328 case the SCALAR is empty you can use OFFSET but only zero offset.
5330 =item tell FILEHANDLE
5334 Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE
5335 may be an expression whose value gives the name of the actual filehandle.
5336 If FILEHANDLE is omitted, assumes the file last read.
5338 The return value of tell() for the standard streams like the STDIN
5339 depends on the operating system: it may return -1 or something else.
5340 tell() on pipes, fifos, and sockets usually returns -1.
5342 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5344 =item telldir DIRHANDLE
5346 Returns the current position of the C<readdir> routines on DIRHANDLE.
5347 Value may be given to C<seekdir> to access a particular location in a
5348 directory. Has the same caveats about possible directory compaction as
5349 the corresponding system library routine.
5351 =item tie VARIABLE,CLASSNAME,LIST
5353 This function binds a variable to a package class that will provide the
5354 implementation for the variable. VARIABLE is the name of the variable
5355 to be enchanted. CLASSNAME is the name of a class implementing objects
5356 of correct type. Any additional arguments are passed to the C<new>
5357 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5358 or C<TIEHASH>). Typically these are arguments such as might be passed
5359 to the C<dbm_open()> function of C. The object returned by the C<new>
5360 method is also returned by the C<tie> function, which would be useful
5361 if you want to access other methods in CLASSNAME.
5363 Note that functions such as C<keys> and C<values> may return huge lists
5364 when used on large objects, like DBM files. You may prefer to use the
5365 C<each> function to iterate over such. Example:
5367 # print out history file offsets
5369 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5370 while (($key,$val) = each %HIST) {
5371 print $key, ' = ', unpack('L',$val), "\n";
5375 A class implementing a hash should have the following methods:
5377 TIEHASH classname, LIST
5379 STORE this, key, value
5384 NEXTKEY this, lastkey
5388 A class implementing an ordinary array should have the following methods:
5390 TIEARRAY classname, LIST
5392 STORE this, key, value
5394 STORESIZE this, count
5400 SPLICE this, offset, length, LIST
5405 A class implementing a file handle should have the following methods:
5407 TIEHANDLE classname, LIST
5408 READ this, scalar, length, offset
5411 WRITE this, scalar, length, offset
5413 PRINTF this, format, LIST
5417 SEEK this, position, whence
5419 OPEN this, mode, LIST
5424 A class implementing a scalar should have the following methods:
5426 TIESCALAR classname, LIST
5432 Not all methods indicated above need be implemented. See L<perltie>,
5433 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5435 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5436 for you--you need to do that explicitly yourself. See L<DB_File>
5437 or the F<Config> module for interesting C<tie> implementations.
5439 For further details see L<perltie>, L<"tied VARIABLE">.
5443 Returns a reference to the object underlying VARIABLE (the same value
5444 that was originally returned by the C<tie> call that bound the variable
5445 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5450 Returns the number of non-leap seconds since whatever time the system
5451 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5452 and 00:00:00 UTC, January 1, 1970 for most other systems).
5453 Suitable for feeding to C<gmtime> and C<localtime>.
5455 For measuring time in better granularity than one second,
5456 you may use either the Time::HiRes module from CPAN, or
5457 if you have gettimeofday(2), you may be able to use the
5458 C<syscall> interface of Perl, see L<perlfaq8> for details.
5462 Returns a four-element list giving the user and system times, in
5463 seconds, for this process and the children of this process.
5465 ($user,$system,$cuser,$csystem) = times;
5467 In scalar context, C<times> returns C<$user>.
5471 The transliteration operator. Same as C<y///>. See L<perlop>.
5473 =item truncate FILEHANDLE,LENGTH
5475 =item truncate EXPR,LENGTH
5477 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5478 specified length. Produces a fatal error if truncate isn't implemented
5479 on your system. Returns true if successful, the undefined value
5486 Returns an uppercased version of EXPR. This is the internal function
5487 implementing the C<\U> escape in double-quoted strings. Respects
5488 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5489 and L<perlunicode> for more details about locale and Unicode support.
5490 It does not attempt to do titlecase mapping on initial letters. See
5491 C<ucfirst> for that.
5493 If EXPR is omitted, uses C<$_>.
5499 Returns the value of EXPR with the first character in uppercase
5500 (titlecase in Unicode). This is the internal function implementing
5501 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5502 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5503 for more details about locale and Unicode support.
5505 If EXPR is omitted, uses C<$_>.
5511 Sets the umask for the process to EXPR and returns the previous value.
5512 If EXPR is omitted, merely returns the current umask.
5514 The Unix permission C<rwxr-x---> is represented as three sets of three
5515 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5516 and isn't one of the digits). The C<umask> value is such a number
5517 representing disabled permissions bits. The permission (or "mode")
5518 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5519 even if you tell C<sysopen> to create a file with permissions C<0777>,
5520 if your umask is C<0022> then the file will actually be created with
5521 permissions C<0755>. If your C<umask> were C<0027> (group can't
5522 write; others can't read, write, or execute), then passing
5523 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5526 Here's some advice: supply a creation mode of C<0666> for regular
5527 files (in C<sysopen>) and one of C<0777> for directories (in
5528 C<mkdir>) and executable files. This gives users the freedom of
5529 choice: if they want protected files, they might choose process umasks
5530 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5531 Programs should rarely if ever make policy decisions better left to
5532 the user. The exception to this is when writing files that should be
5533 kept private: mail files, web browser cookies, I<.rhosts> files, and
5536 If umask(2) is not implemented on your system and you are trying to
5537 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5538 fatal error at run time. If umask(2) is not implemented and you are
5539 not trying to restrict access for yourself, returns C<undef>.
5541 Remember that a umask is a number, usually given in octal; it is I<not> a
5542 string of octal digits. See also L</oct>, if all you have is a string.
5548 Undefines the value of EXPR, which must be an lvalue. Use only on a
5549 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5550 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5551 will probably not do what you expect on most predefined variables or
5552 DBM list values, so don't do that; see L<delete>.) Always returns the
5553 undefined value. You can omit the EXPR, in which case nothing is
5554 undefined, but you still get an undefined value that you could, for
5555 instance, return from a subroutine, assign to a variable or pass as a
5556 parameter. Examples:
5559 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5563 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5564 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5565 select undef, undef, undef, 0.25;
5566 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5568 Note that this is a unary operator, not a list operator.
5574 Deletes a list of files. Returns the number of files successfully
5577 $cnt = unlink 'a', 'b', 'c';
5581 Note: C<unlink> will not delete directories unless you are superuser and
5582 the B<-U> flag is supplied to Perl. Even if these conditions are
5583 met, be warned that unlinking a directory can inflict damage on your
5584 filesystem. Use C<rmdir> instead.
5586 If LIST is omitted, uses C<$_>.
5588 =item unpack TEMPLATE,EXPR
5590 C<unpack> does the reverse of C<pack>: it takes a string
5591 and expands it out into a list of values.
5592 (In scalar context, it returns merely the first value produced.)
5594 The string is broken into chunks described by the TEMPLATE. Each chunk
5595 is converted separately to a value. Typically, either the string is a result
5596 of C<pack>, or the bytes of the string represent a C structure of some
5599 The TEMPLATE has the same format as in the C<pack> function.
5600 Here's a subroutine that does substring:
5603 my($what,$where,$howmuch) = @_;
5604 unpack("x$where a$howmuch", $what);
5609 sub ordinal { unpack("c",$_[0]); } # same as ord()
5611 In addition to fields allowed in pack(), you may prefix a field with
5612 a %<number> to indicate that
5613 you want a <number>-bit checksum of the items instead of the items
5614 themselves. Default is a 16-bit checksum. Checksum is calculated by
5615 summing numeric values of expanded values (for string fields the sum of
5616 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5618 For example, the following
5619 computes the same number as the System V sum program:
5623 unpack("%32C*",<>) % 65535;
5626 The following efficiently counts the number of set bits in a bit vector:
5628 $setbits = unpack("%32b*", $selectmask);
5630 The C<p> and C<P> formats should be used with care. Since Perl
5631 has no way of checking whether the value passed to C<unpack()>
5632 corresponds to a valid memory location, passing a pointer value that's
5633 not known to be valid is likely to have disastrous consequences.
5635 If the repeat count of a field is larger than what the remainder of
5636 the input string allows, repeat count is decreased. If the input string
5637 is longer than one described by the TEMPLATE, the rest is ignored.
5639 See L</pack> for more examples and notes.
5641 =item untie VARIABLE
5643 Breaks the binding between a variable and a package. (See C<tie>.)
5645 =item unshift ARRAY,LIST
5647 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5648 depending on how you look at it. Prepends list to the front of the
5649 array, and returns the new number of elements in the array.
5651 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5653 Note the LIST is prepended whole, not one element at a time, so the
5654 prepended elements stay in the same order. Use C<reverse> to do the
5657 =item use Module VERSION LIST
5659 =item use Module VERSION
5661 =item use Module LIST
5667 Imports some semantics into the current package from the named module,
5668 generally by aliasing certain subroutine or variable names into your
5669 package. It is exactly equivalent to
5671 BEGIN { require Module; import Module LIST; }
5673 except that Module I<must> be a bareword.
5675 VERSION may be either a numeric argument such as 5.006, which will be
5676 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5677 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5678 greater than the version of the current Perl interpreter; Perl will not
5679 attempt to parse the rest of the file. Compare with L</require>, which can
5680 do a similar check at run time.
5682 Specifying VERSION as a literal of the form v5.6.1 should generally be
5683 avoided, because it leads to misleading error messages under earlier
5684 versions of Perl which do not support this syntax. The equivalent numeric
5685 version should be used instead.
5687 use v5.6.1; # compile time version check
5689 use 5.006_001; # ditto; preferred for backwards compatibility
5691 This is often useful if you need to check the current Perl version before
5692 C<use>ing library modules that have changed in incompatible ways from
5693 older versions of Perl. (We try not to do this more than we have to.)
5695 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5696 C<require> makes sure the module is loaded into memory if it hasn't been
5697 yet. The C<import> is not a builtin--it's just an ordinary static method
5698 call into the C<Module> package to tell the module to import the list of
5699 features back into the current package. The module can implement its
5700 C<import> method any way it likes, though most modules just choose to
5701 derive their C<import> method via inheritance from the C<Exporter> class that
5702 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5703 method can be found then the call is skipped.
5705 If you do not want to call the package's C<import> method (for instance,
5706 to stop your namespace from being altered), explicitly supply the empty list:
5710 That is exactly equivalent to
5712 BEGIN { require Module }
5714 If the VERSION argument is present between Module and LIST, then the
5715 C<use> will call the VERSION method in class Module with the given
5716 version as an argument. The default VERSION method, inherited from
5717 the UNIVERSAL class, croaks if the given version is larger than the
5718 value of the variable C<$Module::VERSION>.
5720 Again, there is a distinction between omitting LIST (C<import> called
5721 with no arguments) and an explicit empty LIST C<()> (C<import> not
5722 called). Note that there is no comma after VERSION!
5724 Because this is a wide-open interface, pragmas (compiler directives)
5725 are also implemented this way. Currently implemented pragmas are:
5730 use sigtrap qw(SEGV BUS);
5731 use strict qw(subs vars refs);
5732 use subs qw(afunc blurfl);
5733 use warnings qw(all);
5735 Some of these pseudo-modules import semantics into the current
5736 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5737 which import symbols into the current package (which are effective
5738 through the end of the file).
5740 There's a corresponding C<no> command that unimports meanings imported
5741 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5747 If no C<unimport> method can be found the call fails with a fatal error.
5749 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5750 for the C<-M> and C<-m> command-line options to perl that give C<use>
5751 functionality from the command-line.
5755 Changes the access and modification times on each file of a list of
5756 files. The first two elements of the list must be the NUMERICAL access
5757 and modification times, in that order. Returns the number of files
5758 successfully changed. The inode change time of each file is set
5759 to the current time. This code has the same effect as the C<touch>
5760 command if the files already exist:
5764 utime $now, $now, @ARGV;
5766 If the first two elements of the list are C<undef>, then the utime(2)
5767 function in the C library will be called with a null second argument.
5768 On most systems, this will set the file's access and modification
5769 times to the current time. (i.e. equivalent to the example above.)
5771 utime undef, undef, @ARGV;
5775 Returns a list consisting of all the values of the named hash. (In a
5776 scalar context, returns the number of values.) The values are
5777 returned in an apparently random order. The actual random order is
5778 subject to change in future versions of perl, but it is guaranteed to
5779 be the same order as either the C<keys> or C<each> function would
5780 produce on the same (unmodified) hash.
5782 Note that the values are not copied, which means modifying them will
5783 modify the contents of the hash:
5785 for (values %hash) { s/foo/bar/g } # modifies %hash values
5786 for (@hash{keys %hash}) { s/foo/bar/g } # same
5788 As a side effect, calling values() resets the HASH's internal iterator.
5789 See also C<keys>, C<each>, and C<sort>.
5791 =item vec EXPR,OFFSET,BITS
5793 Treats the string in EXPR as a bit vector made up of elements of
5794 width BITS, and returns the value of the element specified by OFFSET
5795 as an unsigned integer. BITS therefore specifies the number of bits
5796 that are reserved for each element in the bit vector. This must
5797 be a power of two from 1 to 32 (or 64, if your platform supports
5800 If BITS is 8, "elements" coincide with bytes of the input string.
5802 If BITS is 16 or more, bytes of the input string are grouped into chunks
5803 of size BITS/8, and each group is converted to a number as with
5804 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5805 for BITS==64). See L<"pack"> for details.
5807 If bits is 4 or less, the string is broken into bytes, then the bits
5808 of each byte are broken into 8/BITS groups. Bits of a byte are
5809 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5810 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5811 breaking the single input byte C<chr(0x36)> into two groups gives a list
5812 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5814 C<vec> may also be assigned to, in which case parentheses are needed
5815 to give the expression the correct precedence as in
5817 vec($image, $max_x * $x + $y, 8) = 3;
5819 If the selected element is outside the string, the value 0 is returned.
5820 If an element off the end of the string is written to, Perl will first
5821 extend the string with sufficiently many zero bytes. It is an error
5822 to try to write off the beginning of the string (i.e. negative OFFSET).
5824 The string should not contain any character with the value > 255 (which
5825 can only happen if you're using UTF8 encoding). If it does, it will be
5826 treated as something which is not UTF8 encoded. When the C<vec> was
5827 assigned to, other parts of your program will also no longer consider the
5828 string to be UTF8 encoded. In other words, if you do have such characters
5829 in your string, vec() will operate on the actual byte string, and not the
5830 conceptual character string.
5832 Strings created with C<vec> can also be manipulated with the logical
5833 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5834 vector operation is desired when both operands are strings.
5835 See L<perlop/"Bitwise String Operators">.
5837 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5838 The comments show the string after each step. Note that this code works
5839 in the same way on big-endian or little-endian machines.
5842 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5844 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5845 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5847 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5848 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5849 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5850 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5851 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5852 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5854 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5855 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5856 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5859 To transform a bit vector into a string or list of 0's and 1's, use these:
5861 $bits = unpack("b*", $vector);
5862 @bits = split(//, unpack("b*", $vector));
5864 If you know the exact length in bits, it can be used in place of the C<*>.
5866 Here is an example to illustrate how the bits actually fall in place:
5872 unpack("V",$_) 01234567890123456789012345678901
5873 ------------------------------------------------------------------
5878 for ($shift=0; $shift < $width; ++$shift) {
5879 for ($off=0; $off < 32/$width; ++$off) {
5880 $str = pack("B*", "0"x32);
5881 $bits = (1<<$shift);
5882 vec($str, $off, $width) = $bits;
5883 $res = unpack("b*",$str);
5884 $val = unpack("V", $str);
5891 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5892 $off, $width, $bits, $val, $res
5896 Regardless of the machine architecture on which it is run, the above
5897 example should print the following table:
5900 unpack("V",$_) 01234567890123456789012345678901
5901 ------------------------------------------------------------------
5902 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5903 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5904 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5905 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5906 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5907 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5908 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5909 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5910 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5911 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5912 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5913 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5914 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5915 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5916 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5917 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5918 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5919 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5920 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5921 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5922 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5923 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5924 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5925 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5926 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5927 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5928 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5929 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5930 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5931 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5932 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5933 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5934 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5935 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5936 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5937 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5938 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5939 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5940 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5941 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5942 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5943 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5944 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5945 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5946 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5947 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5948 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5949 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5950 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5951 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5952 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5953 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5954 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5955 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5956 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5957 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5958 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5959 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5960 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5961 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5962 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5963 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5964 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5965 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5966 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5967 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5968 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5969 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5970 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5971 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5972 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5973 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5974 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5975 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5976 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5977 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5978 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5979 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5980 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5981 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5982 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5983 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5984 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5985 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5986 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5987 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5988 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5989 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5990 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5991 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5992 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5993 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5994 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5995 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5996 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5997 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5998 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5999 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6000 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6001 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6002 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6003 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6004 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6005 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6006 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6007 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6008 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6009 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6010 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6011 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6012 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6013 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6014 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6015 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6016 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6017 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6018 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6019 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6020 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6021 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6022 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6023 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6024 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6025 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6026 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6027 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6028 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6029 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6033 Behaves like the wait(2) system call on your system: it waits for a child
6034 process to terminate and returns the pid of the deceased process, or
6035 C<-1> if there are no child processes. The status is returned in C<$?>.
6036 Note that a return value of C<-1> could mean that child processes are
6037 being automatically reaped, as described in L<perlipc>.
6039 =item waitpid PID,FLAGS
6041 Waits for a particular child process to terminate and returns the pid of
6042 the deceased process, or C<-1> if there is no such child process. On some
6043 systems, a value of 0 indicates that there are processes still running.
6044 The status is returned in C<$?>. If you say
6046 use POSIX ":sys_wait_h";
6049 $kid = waitpid(-1, WNOHANG);
6052 then you can do a non-blocking wait for all pending zombie processes.
6053 Non-blocking wait is available on machines supporting either the
6054 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6055 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6056 system call by remembering the status values of processes that have
6057 exited but have not been harvested by the Perl script yet.)
6059 Note that on some systems, a return value of C<-1> could mean that child
6060 processes are being automatically reaped. See L<perlipc> for details,
6061 and for other examples.
6065 Returns true if the context of the currently executing subroutine is
6066 looking for a list value. Returns false if the context is looking
6067 for a scalar. Returns the undefined value if the context is looking
6068 for no value (void context).
6070 return unless defined wantarray; # don't bother doing more
6071 my @a = complex_calculation();
6072 return wantarray ? @a : "@a";
6074 This function should have been named wantlist() instead.
6078 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6081 If LIST is empty and C<$@> already contains a value (typically from a
6082 previous eval) that value is used after appending C<"\t...caught">
6083 to C<$@>. This is useful for staying almost, but not entirely similar to
6086 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6088 No message is printed if there is a C<$SIG{__WARN__}> handler
6089 installed. It is the handler's responsibility to deal with the message
6090 as it sees fit (like, for instance, converting it into a C<die>). Most
6091 handlers must therefore make arrangements to actually display the
6092 warnings that they are not prepared to deal with, by calling C<warn>
6093 again in the handler. Note that this is quite safe and will not
6094 produce an endless loop, since C<__WARN__> hooks are not called from
6097 You will find this behavior is slightly different from that of
6098 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6099 instead call C<die> again to change it).
6101 Using a C<__WARN__> handler provides a powerful way to silence all
6102 warnings (even the so-called mandatory ones). An example:
6104 # wipe out *all* compile-time warnings
6105 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6107 my $foo = 20; # no warning about duplicate my $foo,
6108 # but hey, you asked for it!
6109 # no compile-time or run-time warnings before here
6112 # run-time warnings enabled after here
6113 warn "\$foo is alive and $foo!"; # does show up
6115 See L<perlvar> for details on setting C<%SIG> entries, and for more
6116 examples. See the Carp module for other kinds of warnings using its
6117 carp() and cluck() functions.
6119 =item write FILEHANDLE
6125 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6126 using the format associated with that file. By default the format for
6127 a file is the one having the same name as the filehandle, but the
6128 format for the current output channel (see the C<select> function) may be set
6129 explicitly by assigning the name of the format to the C<$~> variable.
6131 Top of form processing is handled automatically: if there is
6132 insufficient room on the current page for the formatted record, the
6133 page is advanced by writing a form feed, a special top-of-page format
6134 is used to format the new page header, and then the record is written.
6135 By default the top-of-page format is the name of the filehandle with
6136 "_TOP" appended, but it may be dynamically set to the format of your
6137 choice by assigning the name to the C<$^> variable while the filehandle is
6138 selected. The number of lines remaining on the current page is in
6139 variable C<$->, which can be set to C<0> to force a new page.
6141 If FILEHANDLE is unspecified, output goes to the current default output
6142 channel, which starts out as STDOUT but may be changed by the
6143 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6144 is evaluated and the resulting string is used to look up the name of
6145 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6147 Note that write is I<not> the opposite of C<read>. Unfortunately.
6151 The transliteration operator. Same as C<tr///>. See L<perlop>.