3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 An named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<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.
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
456 binmode() should be called after open() but before any I/O is done on
459 On some systems binmode() is necessary when you're not working with a
460 text file. For the sake of portability it is a good idea to always use
461 it when appropriate, and to never use it when it isn't appropriate.
463 In other words: Regardless of platform, use binmode() on binary
464 files, and do not use binmode() on text files.
466 The C<open> pragma can be used to establish default disciplines.
469 The operating system, device drivers, C libraries, and Perl run-time
470 system all work together to let the programmer treat a single
471 character (C<\n>) as the line terminator, irrespective of the external
472 representation. On many operating systems, the native text file
473 representation matches the internal representation, but on some
474 platforms the external representation of C<\n> is made up of more than
477 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
478 character to end each line in the external representation of text (even
479 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
480 on Unix and most VMS files). Consequently binmode() has no effect on
481 these operating systems. In other systems like OS/2, DOS and the various
482 flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
483 what's stored in text files are the two characters C<\cM\cJ>. That means
484 that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
485 disk will be converted to C<\n> on input, and any C<\n> in your program
486 will be converted back to C<\cM\cJ> on output. This is what you want for
487 text files, but it can be disastrous for binary files.
489 Another consequence of using binmode() (on some systems) is that
490 special end-of-file markers will be seen as part of the data stream.
491 For systems from the Microsoft family this means that if your binary
492 data contains C<\cZ>, the I/O subsystem will regard it as the end of
493 the file, unless you use binmode().
495 binmode() is not only important for readline() and print() operations,
496 but also when using read(), seek(), sysread(), syswrite() and tell()
497 (see L<perlport> for more details). See the C<$/> and C<$\> variables
498 in L<perlvar> for how to manually set your input and output
499 line-termination sequences.
501 =item bless REF,CLASSNAME
505 This function tells the thingy referenced by REF that it is now an object
506 in the CLASSNAME package. If CLASSNAME is omitted, the current package
507 is used. Because a C<bless> is often the last thing in a constructor,
508 it returns the reference for convenience. Always use the two-argument
509 version if the function doing the blessing might be inherited by a
510 derived class. See L<perltoot> and L<perlobj> for more about the blessing
511 (and blessings) of objects.
513 Consider always blessing objects in CLASSNAMEs that are mixed case.
514 Namespaces with all lowercase names are considered reserved for
515 Perl pragmata. Builtin types have all uppercase names, so to prevent
516 confusion, you may wish to avoid such package names as well. Make sure
517 that CLASSNAME is a true value.
519 See L<perlmod/"Perl Modules">.
525 Returns the context of the current subroutine call. In scalar context,
526 returns the caller's package name if there is a caller, that is, if
527 we're in a subroutine or C<eval> or C<require>, and the undefined value
528 otherwise. In list context, returns
530 ($package, $filename, $line) = caller;
532 With EXPR, it returns some extra information that the debugger uses to
533 print a stack trace. The value of EXPR indicates how many call frames
534 to go back before the current one.
536 ($package, $filename, $line, $subroutine, $hasargs,
537 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
539 Here $subroutine may be C<(eval)> if the frame is not a subroutine
540 call, but an C<eval>. In such a case additional elements $evaltext and
541 C<$is_require> are set: C<$is_require> is true if the frame is created by a
542 C<require> or C<use> statement, $evaltext contains the text of the
543 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
544 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
545 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
546 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
547 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
548 was compiled with. The C<$hints> and C<$bitmask> values are subject to
549 change between versions of Perl, and are not meant for external use.
551 Furthermore, when called from within the DB package, caller returns more
552 detailed information: it sets the list variable C<@DB::args> to be the
553 arguments with which the subroutine was invoked.
555 Be aware that the optimizer might have optimized call frames away before
556 C<caller> had a chance to get the information. That means that C<caller(N)>
557 might not return information about the call frame you expect it do, for
558 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
559 previous time C<caller> was called.
563 Changes the working directory to EXPR, if possible. If EXPR is omitted,
564 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
565 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
566 set, C<chdir> does nothing. It returns true upon success, false
567 otherwise. See the example under C<die>.
571 Changes the permissions of a list of files. The first element of the
572 list must be the numerical mode, which should probably be an octal
573 number, and which definitely should I<not> a string of octal digits:
574 C<0644> is okay, C<'0644'> is not. Returns the number of files
575 successfully changed. See also L</oct>, if all you have is a string.
577 $cnt = chmod 0755, 'foo', 'bar';
578 chmod 0755, @executables;
579 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
581 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
582 $mode = 0644; chmod $mode, 'foo'; # this is best
584 You can also import the symbolic C<S_I*> constants from the Fcntl
589 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
590 # This is identical to the chmod 0755 of the above example.
598 This safer version of L</chop> removes any trailing string
599 that corresponds to the current value of C<$/> (also known as
600 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
601 number of characters removed from all its arguments. It's often used to
602 remove the newline from the end of an input record when you're worried
603 that the final record may be missing its newline. When in paragraph
604 mode (C<$/ = "">), it removes all trailing newlines from the string.
605 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
606 a reference to an integer or the like, see L<perlvar>) chomp() won't
608 If VARIABLE is omitted, it chomps C<$_>. Example:
611 chomp; # avoid \n on last field
616 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
618 You can actually chomp anything that's an lvalue, including an assignment:
621 chomp($answer = <STDIN>);
623 If you chomp a list, each element is chomped, and the total number of
624 characters removed is returned.
632 Chops off the last character of a string and returns the character
633 chopped. It is much more efficient than C<s/.$//s> because it neither
634 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
635 If VARIABLE is a hash, it chops the hash's values, but not its keys.
637 You can actually chop anything that's an lvalue, including an assignment.
639 If you chop a list, each element is chopped. Only the value of the
640 last C<chop> is returned.
642 Note that C<chop> returns the last character. To return all but the last
643 character, use C<substr($string, 0, -1)>.
647 Changes the owner (and group) of a list of files. The first two
648 elements of the list must be the I<numeric> uid and gid, in that
649 order. A value of -1 in either position is interpreted by most
650 systems to leave that value unchanged. Returns the number of files
651 successfully changed.
653 $cnt = chown $uid, $gid, 'foo', 'bar';
654 chown $uid, $gid, @filenames;
656 Here's an example that looks up nonnumeric uids in the passwd file:
659 chomp($user = <STDIN>);
661 chomp($pattern = <STDIN>);
663 ($login,$pass,$uid,$gid) = getpwnam($user)
664 or die "$user not in passwd file";
666 @ary = glob($pattern); # expand filenames
667 chown $uid, $gid, @ary;
669 On most systems, you are not allowed to change the ownership of the
670 file unless you're the superuser, although you should be able to change
671 the group to any of your secondary groups. On insecure systems, these
672 restrictions may be relaxed, but this is not a portable assumption.
673 On POSIX systems, you can detect this condition this way:
675 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
676 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
682 Returns the character represented by that NUMBER in the character set.
683 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
684 chr(0x263a) is a Unicode smiley face. Note that characters from
685 127 to 255 (inclusive) are not encoded in Unicode for backward
686 compatibility reasons.
688 For the reverse, use L</ord>.
689 See L<utf8> for more about Unicode.
691 If NUMBER is omitted, uses C<$_>.
693 =item chroot FILENAME
697 This function works like the system call by the same name: it makes the
698 named directory the new root directory for all further pathnames that
699 begin with a C</> by your process and all its children. (It doesn't
700 change your current working directory, which is unaffected.) For security
701 reasons, this call is restricted to the superuser. If FILENAME is
702 omitted, does a C<chroot> to C<$_>.
704 =item close FILEHANDLE
708 Closes the file or pipe associated with the file handle, returning true
709 only if stdio successfully flushes buffers and closes the system file
710 descriptor. Closes the currently selected filehandle if the argument
713 You don't have to close FILEHANDLE if you are immediately going to do
714 another C<open> on it, because C<open> will close it for you. (See
715 C<open>.) However, an explicit C<close> on an input file resets the line
716 counter (C<$.>), while the implicit close done by C<open> does not.
718 If the file handle came from a piped open C<close> will additionally
719 return false if one of the other system calls involved fails or if the
720 program exits with non-zero status. (If the only problem was that the
721 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
722 also waits for the process executing on the pipe to complete, in case you
723 want to look at the output of the pipe afterwards, and
724 implicitly puts the exit status value of that command into C<$?>.
726 Prematurely closing the read end of a pipe (i.e. before the process
727 writing to it at the other end has closed it) will result in a
728 SIGPIPE being delivered to the writer. If the other end can't
729 handle that, be sure to read all the data before closing the pipe.
733 open(OUTPUT, '|sort >foo') # pipe to sort
734 or die "Can't start sort: $!";
735 #... # print stuff to output
736 close OUTPUT # wait for sort to finish
737 or warn $! ? "Error closing sort pipe: $!"
738 : "Exit status $? from sort";
739 open(INPUT, 'foo') # get sort's results
740 or die "Can't open 'foo' for input: $!";
742 FILEHANDLE may be an expression whose value can be used as an indirect
743 filehandle, usually the real filehandle name.
745 =item closedir DIRHANDLE
747 Closes a directory opened by C<opendir> and returns the success of that
750 DIRHANDLE may be an expression whose value can be used as an indirect
751 dirhandle, usually the real dirhandle name.
753 =item connect SOCKET,NAME
755 Attempts to connect to a remote socket, just as the connect system call
756 does. Returns true if it succeeded, false otherwise. NAME should be a
757 packed address of the appropriate type for the socket. See the examples in
758 L<perlipc/"Sockets: Client/Server Communication">.
762 Actually a flow control statement rather than a function. If there is a
763 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
764 C<foreach>), it is always executed just before the conditional is about to
765 be evaluated again, just like the third part of a C<for> loop in C. Thus
766 it can be used to increment a loop variable, even when the loop has been
767 continued via the C<next> statement (which is similar to the C C<continue>
770 C<last>, C<next>, or C<redo> may appear within a C<continue>
771 block. C<last> and C<redo> will behave as if they had been executed within
772 the main block. So will C<next>, but since it will execute a C<continue>
773 block, it may be more entertaining.
776 ### redo always comes here
779 ### next always comes here
781 # then back the top to re-check EXPR
783 ### last always comes here
785 Omitting the C<continue> section is semantically equivalent to using an
786 empty one, logically enough. In that case, C<next> goes directly back
787 to check the condition at the top of the loop.
793 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
794 takes cosine of C<$_>.
796 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
797 function, or use this relation:
799 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
801 =item crypt PLAINTEXT,SALT
803 Encrypts a string exactly like the crypt(3) function in the C library
804 (assuming that you actually have a version there that has not been
805 extirpated as a potential munition). This can prove useful for checking
806 the password file for lousy passwords, amongst other things. Only the
807 guys wearing white hats should do this.
809 Note that C<crypt> is intended to be a one-way function, much like
810 breaking eggs to make an omelette. There is no (known) corresponding
811 decrypt function (in other words, the crypt() is a one-way hash
812 function). As a result, this function isn't all that useful for
813 cryptography. (For that, see your nearby CPAN mirror.)
815 When verifying an existing encrypted string you should use the
816 encrypted text as the salt (like C<crypt($plain, $crypted) eq
817 $crypted>). This allows your code to work with the standard C<crypt>
818 and with more exotic implementations. In other words, do not assume
819 anything about the returned string itself, or how many bytes in
820 the encrypted string matter.
822 Traditionally the result is a string of 13 bytes: two first bytes of
823 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
824 the first eight bytes of the encrypted string mattered, but
825 alternative hashing schemes (like MD5), higher level security schemes
826 (like C2), and implementations on non-UNIX platforms may produce
829 When choosing a new salt create a random two character string whose
830 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
831 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
833 Here's an example that makes sure that whoever runs this program knows
836 $pwd = (getpwuid($<))[1];
840 chomp($word = <STDIN>);
844 if (crypt($word, $pwd) ne $pwd) {
850 Of course, typing in your own password to whoever asks you
853 The L<crypt> function is unsuitable for encrypting large quantities
854 of data, not least of all because you can't get the information
855 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
856 on your favorite CPAN mirror for a slew of potentially useful
859 If using crypt() on an Unicode string (which potentially has
860 characters with codepoints above 255), Perl tries to make sense of
861 the situation by using only the low eight bits of the characters when
866 [This function has been largely superseded by the C<untie> function.]
868 Breaks the binding between a DBM file and a hash.
870 =item dbmopen HASH,DBNAME,MASK
872 [This function has been largely superseded by the C<tie> function.]
874 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
875 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
876 argument is I<not> a filehandle, even though it looks like one). DBNAME
877 is the name of the database (without the F<.dir> or F<.pag> extension if
878 any). If the database does not exist, it is created with protection
879 specified by MASK (as modified by the C<umask>). If your system supports
880 only the older DBM functions, you may perform only one C<dbmopen> in your
881 program. In older versions of Perl, if your system had neither DBM nor
882 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
885 If you don't have write access to the DBM file, you can only read hash
886 variables, not set them. If you want to test whether you can write,
887 either use file tests or try setting a dummy hash entry inside an C<eval>,
888 which will trap the error.
890 Note that functions such as C<keys> and C<values> may return huge lists
891 when used on large DBM files. You may prefer to use the C<each>
892 function to iterate over large DBM files. Example:
894 # print out history file offsets
895 dbmopen(%HIST,'/usr/lib/news/history',0666);
896 while (($key,$val) = each %HIST) {
897 print $key, ' = ', unpack('L',$val), "\n";
901 See also L<AnyDBM_File> for a more general description of the pros and
902 cons of the various dbm approaches, as well as L<DB_File> for a particularly
905 You can control which DBM library you use by loading that library
906 before you call dbmopen():
909 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
910 or die "Can't open netscape history file: $!";
916 Returns a Boolean value telling whether EXPR has a value other than
917 the undefined value C<undef>. If EXPR is not present, C<$_> will be
920 Many operations return C<undef> to indicate failure, end of file,
921 system error, uninitialized variable, and other exceptional
922 conditions. This function allows you to distinguish C<undef> from
923 other values. (A simple Boolean test will not distinguish among
924 C<undef>, zero, the empty string, and C<"0">, which are all equally
925 false.) Note that since C<undef> is a valid scalar, its presence
926 doesn't I<necessarily> indicate an exceptional condition: C<pop>
927 returns C<undef> when its argument is an empty array, I<or> when the
928 element to return happens to be C<undef>.
930 You may also use C<defined(&func)> to check whether subroutine C<&func>
931 has ever been defined. The return value is unaffected by any forward
932 declarations of C<&foo>. Note that a subroutine which is not defined
933 may still be callable: its package may have an C<AUTOLOAD> method that
934 makes it spring into existence the first time that it is called -- see
937 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
938 used to report whether memory for that aggregate has ever been
939 allocated. This behavior may disappear in future versions of Perl.
940 You should instead use a simple test for size:
942 if (@an_array) { print "has array elements\n" }
943 if (%a_hash) { print "has hash members\n" }
945 When used on a hash element, it tells you whether the value is defined,
946 not whether the key exists in the hash. Use L</exists> for the latter
951 print if defined $switch{'D'};
952 print "$val\n" while defined($val = pop(@ary));
953 die "Can't readlink $sym: $!"
954 unless defined($value = readlink $sym);
955 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
956 $debugging = 0 unless defined $debugging;
958 Note: Many folks tend to overuse C<defined>, and then are surprised to
959 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
960 defined values. For example, if you say
964 The pattern match succeeds, and C<$1> is defined, despite the fact that it
965 matched "nothing". But it didn't really match nothing--rather, it
966 matched something that happened to be zero characters long. This is all
967 very above-board and honest. When a function returns an undefined value,
968 it's an admission that it couldn't give you an honest answer. So you
969 should use C<defined> only when you're questioning the integrity of what
970 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
973 See also L</undef>, L</exists>, L</ref>.
977 Given an expression that specifies a hash element, array element, hash slice,
978 or array slice, deletes the specified element(s) from the hash or array.
979 In the case of an array, if the array elements happen to be at the end,
980 the size of the array will shrink to the highest element that tests
981 true for exists() (or 0 if no such element exists).
983 Returns each element so deleted or the undefined value if there was no such
984 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
985 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
986 from a C<tie>d hash or array may not necessarily return anything.
988 Deleting an array element effectively returns that position of the array
989 to its initial, uninitialized state. Subsequently testing for the same
990 element with exists() will return false. Note that deleting array
991 elements in the middle of an array will not shift the index of the ones
992 after them down--use splice() for that. See L</exists>.
994 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
996 foreach $key (keys %HASH) {
1000 foreach $index (0 .. $#ARRAY) {
1001 delete $ARRAY[$index];
1006 delete @HASH{keys %HASH};
1008 delete @ARRAY[0 .. $#ARRAY];
1010 But both of these are slower than just assigning the empty list
1011 or undefining %HASH or @ARRAY:
1013 %HASH = (); # completely empty %HASH
1014 undef %HASH; # forget %HASH ever existed
1016 @ARRAY = (); # completely empty @ARRAY
1017 undef @ARRAY; # forget @ARRAY ever existed
1019 Note that the EXPR can be arbitrarily complicated as long as the final
1020 operation is a hash element, array element, hash slice, or array slice
1023 delete $ref->[$x][$y]{$key};
1024 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1026 delete $ref->[$x][$y][$index];
1027 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1031 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1032 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1033 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1034 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1035 an C<eval(),> the error message is stuffed into C<$@> and the
1036 C<eval> is terminated with the undefined value. This makes
1037 C<die> the way to raise an exception.
1039 Equivalent examples:
1041 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1042 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1044 If the value of EXPR does not end in a newline, the current script line
1045 number and input line number (if any) are also printed, and a newline
1046 is supplied. Note that the "input line number" (also known as "chunk")
1047 is subject to whatever notion of "line" happens to be currently in
1048 effect, and is also available as the special variable C<$.>.
1049 See L<perlvar/"$/"> and L<perlvar/"$.">.
1051 Hint: sometimes appending C<", stopped"> to your message
1052 will cause it to make better sense when the string C<"at foo line 123"> is
1053 appended. Suppose you are running script "canasta".
1055 die "/etc/games is no good";
1056 die "/etc/games is no good, stopped";
1058 produce, respectively
1060 /etc/games is no good at canasta line 123.
1061 /etc/games is no good, stopped at canasta line 123.
1063 See also exit(), warn(), and the Carp module.
1065 If LIST is empty and C<$@> already contains a value (typically from a
1066 previous eval) that value is reused after appending C<"\t...propagated">.
1067 This is useful for propagating exceptions:
1070 die unless $@ =~ /Expected exception/;
1072 If C<$@> is empty then the string C<"Died"> is used.
1074 die() can also be called with a reference argument. If this happens to be
1075 trapped within an eval(), $@ contains the reference. This behavior permits
1076 a more elaborate exception handling implementation using objects that
1077 maintain arbitrary state about the nature of the exception. Such a scheme
1078 is sometimes preferable to matching particular string values of $@ using
1079 regular expressions. Here's an example:
1081 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1083 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1084 # handle Some::Module::Exception
1087 # handle all other possible exceptions
1091 Because perl will stringify uncaught exception messages before displaying
1092 them, you may want to overload stringification operations on such custom
1093 exception objects. See L<overload> for details about that.
1095 You can arrange for a callback to be run just before the C<die>
1096 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1097 handler will be called with the error text and can change the error
1098 message, if it sees fit, by calling C<die> again. See
1099 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1100 L<"eval BLOCK"> for some examples. Although this feature was meant
1101 to be run only right before your program was to exit, this is not
1102 currently the case--the C<$SIG{__DIE__}> hook is currently called
1103 even inside eval()ed blocks/strings! If one wants the hook to do
1104 nothing in such situations, put
1108 as the first line of the handler (see L<perlvar/$^S>). Because
1109 this promotes strange action at a distance, this counterintuitive
1110 behavior may be fixed in a future release.
1114 Not really a function. Returns the value of the last command in the
1115 sequence of commands indicated by BLOCK. When modified by a loop
1116 modifier, executes the BLOCK once before testing the loop condition.
1117 (On other statements the loop modifiers test the conditional first.)
1119 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1120 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1121 See L<perlsyn> for alternative strategies.
1123 =item do SUBROUTINE(LIST)
1125 A deprecated form of subroutine call. See L<perlsub>.
1129 Uses the value of EXPR as a filename and executes the contents of the
1130 file as a Perl script. Its primary use is to include subroutines
1131 from a Perl subroutine library.
1139 except that it's more efficient and concise, keeps track of the current
1140 filename for error messages, searches the @INC libraries, and updates
1141 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1142 variables. It also differs in that code evaluated with C<do FILENAME>
1143 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1144 same, however, in that it does reparse the file every time you call it,
1145 so you probably don't want to do this inside a loop.
1147 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1148 error. If C<do> can read the file but cannot compile it, it
1149 returns undef and sets an error message in C<$@>. If the file is
1150 successfully compiled, C<do> returns the value of the last expression
1153 Note that inclusion of library modules is better done with the
1154 C<use> and C<require> operators, which also do automatic error checking
1155 and raise an exception if there's a problem.
1157 You might like to use C<do> to read in a program configuration
1158 file. Manual error checking can be done this way:
1160 # read in config files: system first, then user
1161 for $file ("/share/prog/defaults.rc",
1162 "$ENV{HOME}/.someprogrc")
1164 unless ($return = do $file) {
1165 warn "couldn't parse $file: $@" if $@;
1166 warn "couldn't do $file: $!" unless defined $return;
1167 warn "couldn't run $file" unless $return;
1175 This function causes an immediate core dump. See also the B<-u>
1176 command-line switch in L<perlrun>, which does the same thing.
1177 Primarily this is so that you can use the B<undump> program (not
1178 supplied) to turn your core dump into an executable binary after
1179 having initialized all your variables at the beginning of the
1180 program. When the new binary is executed it will begin by executing
1181 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1182 Think of it as a goto with an intervening core dump and reincarnation.
1183 If C<LABEL> is omitted, restarts the program from the top.
1185 B<WARNING>: Any files opened at the time of the dump will I<not>
1186 be open any more when the program is reincarnated, with possible
1187 resulting confusion on the part of Perl.
1189 This function is now largely obsolete, partly because it's very
1190 hard to convert a core file into an executable, and because the
1191 real compiler backends for generating portable bytecode and compilable
1192 C code have superseded it.
1194 If you're looking to use L<dump> to speed up your program, consider
1195 generating bytecode or native C code as described in L<perlcc>. If
1196 you're just trying to accelerate a CGI script, consider using the
1197 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1198 You might also consider autoloading or selfloading, which at least
1199 make your program I<appear> to run faster.
1203 When called in list context, returns a 2-element list consisting of the
1204 key and value for the next element of a hash, so that you can iterate over
1205 it. When called in scalar context, returns only the key for the next
1206 element in the hash.
1208 Entries are returned in an apparently random order. The actual random
1209 order is subject to change in future versions of perl, but it is guaranteed
1210 to be in the same order as either the C<keys> or C<values> function
1211 would produce on the same (unmodified) hash.
1213 When the hash is entirely read, a null array is returned in list context
1214 (which when assigned produces a false (C<0>) value), and C<undef> in
1215 scalar context. The next call to C<each> after that will start iterating
1216 again. There is a single iterator for each hash, shared by all C<each>,
1217 C<keys>, and C<values> function calls in the program; it can be reset by
1218 reading all the elements from the hash, or by evaluating C<keys HASH> or
1219 C<values HASH>. If you add or delete elements of a hash while you're
1220 iterating over it, you may get entries skipped or duplicated, so
1221 don't. Exception: It is always safe to delete the item most recently
1222 returned by C<each()>, which means that the following code will work:
1224 while (($key, $value) = each %hash) {
1226 delete $hash{$key}; # This is safe
1229 The following prints out your environment like the printenv(1) program,
1230 only in a different order:
1232 while (($key,$value) = each %ENV) {
1233 print "$key=$value\n";
1236 See also C<keys>, C<values> and C<sort>.
1238 =item eof FILEHANDLE
1244 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1245 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1246 gives the real filehandle. (Note that this function actually
1247 reads a character and then C<ungetc>s it, so isn't very useful in an
1248 interactive context.) Do not read from a terminal file (or call
1249 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1250 as terminals may lose the end-of-file condition if you do.
1252 An C<eof> without an argument uses the last file read. Using C<eof()>
1253 with empty parentheses is very different. It refers to the pseudo file
1254 formed from the files listed on the command line and accessed via the
1255 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1256 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1257 used will cause C<@ARGV> to be examined to determine if input is
1260 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1261 detect the end of each file, C<eof()> will only detect the end of the
1262 last file. Examples:
1264 # reset line numbering on each input file
1266 next if /^\s*#/; # skip comments
1269 close ARGV if eof; # Not eof()!
1272 # insert dashes just before last line of last file
1274 if (eof()) { # check for end of current file
1275 print "--------------\n";
1276 close(ARGV); # close or last; is needed if we
1277 # are reading from the terminal
1282 Practical hint: you almost never need to use C<eof> in Perl, because the
1283 input operators typically return C<undef> when they run out of data, or if
1290 In the first form, the return value of EXPR is parsed and executed as if it
1291 were a little Perl program. The value of the expression (which is itself
1292 determined within scalar context) is first parsed, and if there weren't any
1293 errors, executed in the lexical context of the current Perl program, so
1294 that any variable settings or subroutine and format definitions remain
1295 afterwards. Note that the value is parsed every time the eval executes.
1296 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1297 delay parsing and subsequent execution of the text of EXPR until run time.
1299 In the second form, the code within the BLOCK is parsed only once--at the
1300 same time the code surrounding the eval itself was parsed--and executed
1301 within the context of the current Perl program. This form is typically
1302 used to trap exceptions more efficiently than the first (see below), while
1303 also providing the benefit of checking the code within BLOCK at compile
1306 The final semicolon, if any, may be omitted from the value of EXPR or within
1309 In both forms, the value returned is the value of the last expression
1310 evaluated inside the mini-program; a return statement may be also used, just
1311 as with subroutines. The expression providing the return value is evaluated
1312 in void, scalar, or list context, depending on the context of the eval itself.
1313 See L</wantarray> for more on how the evaluation context can be determined.
1315 If there is a syntax error or runtime error, or a C<die> statement is
1316 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1317 error message. If there was no error, C<$@> is guaranteed to be a null
1318 string. Beware that using C<eval> neither silences perl from printing
1319 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1320 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1321 L</warn> and L<perlvar>.
1323 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1324 determining whether a particular feature (such as C<socket> or C<symlink>)
1325 is implemented. It is also Perl's exception trapping mechanism, where
1326 the die operator is used to raise exceptions.
1328 If the code to be executed doesn't vary, you may use the eval-BLOCK
1329 form to trap run-time errors without incurring the penalty of
1330 recompiling each time. The error, if any, is still returned in C<$@>.
1333 # make divide-by-zero nonfatal
1334 eval { $answer = $a / $b; }; warn $@ if $@;
1336 # same thing, but less efficient
1337 eval '$answer = $a / $b'; warn $@ if $@;
1339 # a compile-time error
1340 eval { $answer = }; # WRONG
1343 eval '$answer ='; # sets $@
1345 Due to the current arguably broken state of C<__DIE__> hooks, when using
1346 the C<eval{}> form as an exception trap in libraries, you may wish not
1347 to trigger any C<__DIE__> hooks that user code may have installed.
1348 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1349 as shown in this example:
1351 # a very private exception trap for divide-by-zero
1352 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1355 This is especially significant, given that C<__DIE__> hooks can call
1356 C<die> again, which has the effect of changing their error messages:
1358 # __DIE__ hooks may modify error messages
1360 local $SIG{'__DIE__'} =
1361 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1362 eval { die "foo lives here" };
1363 print $@ if $@; # prints "bar lives here"
1366 Because this promotes action at a distance, this counterintuitive behavior
1367 may be fixed in a future release.
1369 With an C<eval>, you should be especially careful to remember what's
1370 being looked at when:
1376 eval { $x }; # CASE 4
1378 eval "\$$x++"; # CASE 5
1381 Cases 1 and 2 above behave identically: they run the code contained in
1382 the variable $x. (Although case 2 has misleading double quotes making
1383 the reader wonder what else might be happening (nothing is).) Cases 3
1384 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1385 does nothing but return the value of $x. (Case 4 is preferred for
1386 purely visual reasons, but it also has the advantage of compiling at
1387 compile-time instead of at run-time.) Case 5 is a place where
1388 normally you I<would> like to use double quotes, except that in this
1389 particular situation, you can just use symbolic references instead, as
1392 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1393 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1397 =item exec PROGRAM LIST
1399 The C<exec> function executes a system command I<and never returns>--
1400 use C<system> instead of C<exec> if you want it to return. It fails and
1401 returns false only if the command does not exist I<and> it is executed
1402 directly instead of via your system's command shell (see below).
1404 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1405 warns you if there is a following statement which isn't C<die>, C<warn>,
1406 or C<exit> (if C<-w> is set - but you always do that). If you
1407 I<really> want to follow an C<exec> with some other statement, you
1408 can use one of these styles to avoid the warning:
1410 exec ('foo') or print STDERR "couldn't exec foo: $!";
1411 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1413 If there is more than one argument in LIST, or if LIST is an array
1414 with more than one value, calls execvp(3) with the arguments in LIST.
1415 If there is only one scalar argument or an array with one element in it,
1416 the argument is checked for shell metacharacters, and if there are any,
1417 the entire argument is passed to the system's command shell for parsing
1418 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1419 If there are no shell metacharacters in the argument, it is split into
1420 words and passed directly to C<execvp>, which is more efficient.
1423 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1424 exec "sort $outfile | uniq";
1426 If you don't really want to execute the first argument, but want to lie
1427 to the program you are executing about its own name, you can specify
1428 the program you actually want to run as an "indirect object" (without a
1429 comma) in front of the LIST. (This always forces interpretation of the
1430 LIST as a multivalued list, even if there is only a single scalar in
1433 $shell = '/bin/csh';
1434 exec $shell '-sh'; # pretend it's a login shell
1438 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1440 When the arguments get executed via the system shell, results will
1441 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1444 Using an indirect object with C<exec> or C<system> is also more
1445 secure. This usage (which also works fine with system()) forces
1446 interpretation of the arguments as a multivalued list, even if the
1447 list had just one argument. That way you're safe from the shell
1448 expanding wildcards or splitting up words with whitespace in them.
1450 @args = ( "echo surprise" );
1452 exec @args; # subject to shell escapes
1454 exec { $args[0] } @args; # safe even with one-arg list
1456 The first version, the one without the indirect object, ran the I<echo>
1457 program, passing it C<"surprise"> an argument. The second version
1458 didn't--it tried to run a program literally called I<"echo surprise">,
1459 didn't find it, and set C<$?> to a non-zero value indicating failure.
1461 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1462 output before the exec, but this may not be supported on some platforms
1463 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1464 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1465 open handles in order to avoid lost output.
1467 Note that C<exec> will not call your C<END> blocks, nor will it call
1468 any C<DESTROY> methods in your objects.
1472 Given an expression that specifies a hash element or array element,
1473 returns true if the specified element in the hash or array has ever
1474 been initialized, even if the corresponding value is undefined. The
1475 element is not autovivified if it doesn't exist.
1477 print "Exists\n" if exists $hash{$key};
1478 print "Defined\n" if defined $hash{$key};
1479 print "True\n" if $hash{$key};
1481 print "Exists\n" if exists $array[$index];
1482 print "Defined\n" if defined $array[$index];
1483 print "True\n" if $array[$index];
1485 A hash or array element can be true only if it's defined, and defined if
1486 it exists, but the reverse doesn't necessarily hold true.
1488 Given an expression that specifies the name of a subroutine,
1489 returns true if the specified subroutine has ever been declared, even
1490 if it is undefined. Mentioning a subroutine name for exists or defined
1491 does not count as declaring it. Note that a subroutine which does not
1492 exist may still be callable: its package may have an C<AUTOLOAD>
1493 method that makes it spring into existence the first time that it is
1494 called -- see L<perlsub>.
1496 print "Exists\n" if exists &subroutine;
1497 print "Defined\n" if defined &subroutine;
1499 Note that the EXPR can be arbitrarily complicated as long as the final
1500 operation is a hash or array key lookup or subroutine name:
1502 if (exists $ref->{A}->{B}->{$key}) { }
1503 if (exists $hash{A}{B}{$key}) { }
1505 if (exists $ref->{A}->{B}->[$ix]) { }
1506 if (exists $hash{A}{B}[$ix]) { }
1508 if (exists &{$ref->{A}{B}{$key}}) { }
1510 Although the deepest nested array or hash will not spring into existence
1511 just because its existence was tested, any intervening ones will.
1512 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1513 into existence due to the existence test for the $key element above.
1514 This happens anywhere the arrow operator is used, including even:
1517 if (exists $ref->{"Some key"}) { }
1518 print $ref; # prints HASH(0x80d3d5c)
1520 This surprising autovivification in what does not at first--or even
1521 second--glance appear to be an lvalue context may be fixed in a future
1524 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1525 on how exists() acts when used on a pseudo-hash.
1527 Use of a subroutine call, rather than a subroutine name, as an argument
1528 to exists() is an error.
1531 exists &sub(); # Error
1535 Evaluates EXPR and exits immediately with that value. Example:
1538 exit 0 if $ans =~ /^[Xx]/;
1540 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1541 universally recognized values for EXPR are C<0> for success and C<1>
1542 for error; other values are subject to interpretation depending on the
1543 environment in which the Perl program is running. For example, exiting
1544 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1545 the mailer to return the item undelivered, but that's not true everywhere.
1547 Don't use C<exit> to abort a subroutine if there's any chance that
1548 someone might want to trap whatever error happened. Use C<die> instead,
1549 which can be trapped by an C<eval>.
1551 The exit() function does not always exit immediately. It calls any
1552 defined C<END> routines first, but these C<END> routines may not
1553 themselves abort the exit. Likewise any object destructors that need to
1554 be called are called before the real exit. If this is a problem, you
1555 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1556 See L<perlmod> for details.
1562 Returns I<e> (the natural logarithm base) to the power of EXPR.
1563 If EXPR is omitted, gives C<exp($_)>.
1565 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1567 Implements the fcntl(2) function. You'll probably have to say
1571 first to get the correct constant definitions. Argument processing and
1572 value return works just like C<ioctl> below.
1576 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1577 or die "can't fcntl F_GETFL: $!";
1579 You don't have to check for C<defined> on the return from C<fnctl>.
1580 Like C<ioctl>, it maps a C<0> return from the system call into
1581 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1582 in numeric context. It is also exempt from the normal B<-w> warnings
1583 on improper numeric conversions.
1585 Note that C<fcntl> will produce a fatal error if used on a machine that
1586 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1587 manpage to learn what functions are available on your system.
1589 =item fileno FILEHANDLE
1591 Returns the file descriptor for a filehandle, or undefined if the
1592 filehandle is not open. This is mainly useful for constructing
1593 bitmaps for C<select> and low-level POSIX tty-handling operations.
1594 If FILEHANDLE is an expression, the value is taken as an indirect
1595 filehandle, generally its name.
1597 You can use this to find out whether two handles refer to the
1598 same underlying descriptor:
1600 if (fileno(THIS) == fileno(THAT)) {
1601 print "THIS and THAT are dups\n";
1604 (Filehandles connected to memory objects via new features of C<open> may
1605 return undefined even though they are open.)
1608 =item flock FILEHANDLE,OPERATION
1610 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1611 for success, false on failure. Produces a fatal error if used on a
1612 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1613 C<flock> is Perl's portable file locking interface, although it locks
1614 only entire files, not records.
1616 Two potentially non-obvious but traditional C<flock> semantics are
1617 that it waits indefinitely until the lock is granted, and that its locks
1618 B<merely advisory>. Such discretionary locks are more flexible, but offer
1619 fewer guarantees. This means that files locked with C<flock> may be
1620 modified by programs that do not also use C<flock>. See L<perlport>,
1621 your port's specific documentation, or your system-specific local manpages
1622 for details. It's best to assume traditional behavior if you're writing
1623 portable programs. (But if you're not, you should as always feel perfectly
1624 free to write for your own system's idiosyncrasies (sometimes called
1625 "features"). Slavish adherence to portability concerns shouldn't get
1626 in the way of your getting your job done.)
1628 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1629 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1630 you can use the symbolic names if you import them from the Fcntl module,
1631 either individually, or as a group using the ':flock' tag. LOCK_SH
1632 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1633 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1634 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1635 waiting for the lock (check the return status to see if you got it).
1637 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1638 before locking or unlocking it.
1640 Note that the emulation built with lockf(3) doesn't provide shared
1641 locks, and it requires that FILEHANDLE be open with write intent. These
1642 are the semantics that lockf(3) implements. Most if not all systems
1643 implement lockf(3) in terms of fcntl(2) locking, though, so the
1644 differing semantics shouldn't bite too many people.
1646 Note also that some versions of C<flock> cannot lock things over the
1647 network; you would need to use the more system-specific C<fcntl> for
1648 that. If you like you can force Perl to ignore your system's flock(2)
1649 function, and so provide its own fcntl(2)-based emulation, by passing
1650 the switch C<-Ud_flock> to the F<Configure> program when you configure
1653 Here's a mailbox appender for BSD systems.
1655 use Fcntl ':flock'; # import LOCK_* constants
1658 flock(MBOX,LOCK_EX);
1659 # and, in case someone appended
1660 # while we were waiting...
1665 flock(MBOX,LOCK_UN);
1668 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1669 or die "Can't open mailbox: $!";
1672 print MBOX $msg,"\n\n";
1675 On systems that support a real flock(), locks are inherited across fork()
1676 calls, whereas those that must resort to the more capricious fcntl()
1677 function lose the locks, making it harder to write servers.
1679 See also L<DB_File> for other flock() examples.
1683 Does a fork(2) system call to create a new process running the
1684 same program at the same point. It returns the child pid to the
1685 parent process, C<0> to the child process, or C<undef> if the fork is
1686 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1687 are shared, while everything else is copied. On most systems supporting
1688 fork(), great care has gone into making it extremely efficient (for
1689 example, using copy-on-write technology on data pages), making it the
1690 dominant paradigm for multitasking over the last few decades.
1692 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1693 output before forking the child process, but this may not be supported
1694 on some platforms (see L<perlport>). To be safe, you may need to set
1695 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1696 C<IO::Handle> on any open handles in order to avoid duplicate output.
1698 If you C<fork> without ever waiting on your children, you will
1699 accumulate zombies. On some systems, you can avoid this by setting
1700 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1701 forking and reaping moribund children.
1703 Note that if your forked child inherits system file descriptors like
1704 STDIN and STDOUT that are actually connected by a pipe or socket, even
1705 if you exit, then the remote server (such as, say, a CGI script or a
1706 backgrounded job launched from a remote shell) won't think you're done.
1707 You should reopen those to F</dev/null> if it's any issue.
1711 Declare a picture format for use by the C<write> function. For
1715 Test: @<<<<<<<< @||||| @>>>>>
1716 $str, $%, '$' . int($num)
1720 $num = $cost/$quantity;
1724 See L<perlform> for many details and examples.
1726 =item formline PICTURE,LIST
1728 This is an internal function used by C<format>s, though you may call it,
1729 too. It formats (see L<perlform>) a list of values according to the
1730 contents of PICTURE, placing the output into the format output
1731 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1732 Eventually, when a C<write> is done, the contents of
1733 C<$^A> are written to some filehandle, but you could also read C<$^A>
1734 yourself and then set C<$^A> back to C<"">. Note that a format typically
1735 does one C<formline> per line of form, but the C<formline> function itself
1736 doesn't care how many newlines are embedded in the PICTURE. This means
1737 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1738 You may therefore need to use multiple formlines to implement a single
1739 record format, just like the format compiler.
1741 Be careful if you put double quotes around the picture, because an C<@>
1742 character may be taken to mean the beginning of an array name.
1743 C<formline> always returns true. See L<perlform> for other examples.
1745 =item getc FILEHANDLE
1749 Returns the next character from the input file attached to FILEHANDLE,
1750 or the undefined value at end of file, or if there was an error.
1751 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1752 efficient. However, it cannot be used by itself to fetch single
1753 characters without waiting for the user to hit enter. For that, try
1754 something more like:
1757 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1760 system "stty", '-icanon', 'eol', "\001";
1766 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1769 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1773 Determination of whether $BSD_STYLE should be set
1774 is left as an exercise to the reader.
1776 The C<POSIX::getattr> function can do this more portably on
1777 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1778 module from your nearest CPAN site; details on CPAN can be found on
1783 Implements the C library function of the same name, which on most
1784 systems returns the current login from F</etc/utmp>, if any. If null,
1787 $login = getlogin || getpwuid($<) || "Kilroy";
1789 Do not consider C<getlogin> for authentication: it is not as
1790 secure as C<getpwuid>.
1792 =item getpeername SOCKET
1794 Returns the packed sockaddr address of other end of the SOCKET connection.
1797 $hersockaddr = getpeername(SOCK);
1798 ($port, $iaddr) = sockaddr_in($hersockaddr);
1799 $herhostname = gethostbyaddr($iaddr, AF_INET);
1800 $herstraddr = inet_ntoa($iaddr);
1804 Returns the current process group for the specified PID. Use
1805 a PID of C<0> to get the current process group for the
1806 current process. Will raise an exception if used on a machine that
1807 doesn't implement getpgrp(2). If PID is omitted, returns process
1808 group of current process. Note that the POSIX version of C<getpgrp>
1809 does not accept a PID argument, so only C<PID==0> is truly portable.
1813 Returns the process id of the parent process.
1815 =item getpriority WHICH,WHO
1817 Returns the current priority for a process, a process group, or a user.
1818 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1819 machine that doesn't implement getpriority(2).
1825 =item gethostbyname NAME
1827 =item getnetbyname NAME
1829 =item getprotobyname NAME
1835 =item getservbyname NAME,PROTO
1837 =item gethostbyaddr ADDR,ADDRTYPE
1839 =item getnetbyaddr ADDR,ADDRTYPE
1841 =item getprotobynumber NUMBER
1843 =item getservbyport PORT,PROTO
1861 =item sethostent STAYOPEN
1863 =item setnetent STAYOPEN
1865 =item setprotoent STAYOPEN
1867 =item setservent STAYOPEN
1881 These routines perform the same functions as their counterparts in the
1882 system library. In list context, the return values from the
1883 various get routines are as follows:
1885 ($name,$passwd,$uid,$gid,
1886 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1887 ($name,$passwd,$gid,$members) = getgr*
1888 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1889 ($name,$aliases,$addrtype,$net) = getnet*
1890 ($name,$aliases,$proto) = getproto*
1891 ($name,$aliases,$port,$proto) = getserv*
1893 (If the entry doesn't exist you get a null list.)
1895 The exact meaning of the $gcos field varies but it usually contains
1896 the real name of the user (as opposed to the login name) and other
1897 information pertaining to the user. Beware, however, that in many
1898 system users are able to change this information and therefore it
1899 cannot be trusted and therefore the $gcos is tainted (see
1900 L<perlsec>). The $passwd and $shell, user's encrypted password and
1901 login shell, are also tainted, because of the same reason.
1903 In scalar context, you get the name, unless the function was a
1904 lookup by name, in which case you get the other thing, whatever it is.
1905 (If the entry doesn't exist you get the undefined value.) For example:
1907 $uid = getpwnam($name);
1908 $name = getpwuid($num);
1910 $gid = getgrnam($name);
1911 $name = getgrgid($num;
1915 In I<getpw*()> the fields $quota, $comment, and $expire are special
1916 cases in the sense that in many systems they are unsupported. If the
1917 $quota is unsupported, it is an empty scalar. If it is supported, it
1918 usually encodes the disk quota. If the $comment field is unsupported,
1919 it is an empty scalar. If it is supported it usually encodes some
1920 administrative comment about the user. In some systems the $quota
1921 field may be $change or $age, fields that have to do with password
1922 aging. In some systems the $comment field may be $class. The $expire
1923 field, if present, encodes the expiration period of the account or the
1924 password. For the availability and the exact meaning of these fields
1925 in your system, please consult your getpwnam(3) documentation and your
1926 F<pwd.h> file. You can also find out from within Perl what your
1927 $quota and $comment fields mean and whether you have the $expire field
1928 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1929 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1930 files are only supported if your vendor has implemented them in the
1931 intuitive fashion that calling the regular C library routines gets the
1932 shadow versions if you're running under privilege or if there exists
1933 the shadow(3) functions as found in System V ( this includes Solaris
1934 and Linux.) Those systems which implement a proprietary shadow password
1935 facility are unlikely to be supported.
1937 The $members value returned by I<getgr*()> is a space separated list of
1938 the login names of the members of the group.
1940 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1941 C, it will be returned to you via C<$?> if the function call fails. The
1942 C<@addrs> value returned by a successful call is a list of the raw
1943 addresses returned by the corresponding system library call. In the
1944 Internet domain, each address is four bytes long and you can unpack it
1945 by saying something like:
1947 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1949 The Socket library makes this slightly easier:
1952 $iaddr = inet_aton("127.1"); # or whatever address
1953 $name = gethostbyaddr($iaddr, AF_INET);
1955 # or going the other way
1956 $straddr = inet_ntoa($iaddr);
1958 If you get tired of remembering which element of the return list
1959 contains which return value, by-name interfaces are provided
1960 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1961 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1962 and C<User::grent>. These override the normal built-ins, supplying
1963 versions that return objects with the appropriate names
1964 for each field. For example:
1968 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1970 Even though it looks like they're the same method calls (uid),
1971 they aren't, because a C<File::stat> object is different from
1972 a C<User::pwent> object.
1974 =item getsockname SOCKET
1976 Returns the packed sockaddr address of this end of the SOCKET connection,
1977 in case you don't know the address because you have several different
1978 IPs that the connection might have come in on.
1981 $mysockaddr = getsockname(SOCK);
1982 ($port, $myaddr) = sockaddr_in($mysockaddr);
1983 printf "Connect to %s [%s]\n",
1984 scalar gethostbyaddr($myaddr, AF_INET),
1987 =item getsockopt SOCKET,LEVEL,OPTNAME
1989 Returns the socket option requested, or undef if there is an error.
1995 Returns the value of EXPR with filename expansions such as the
1996 standard Unix shell F</bin/csh> would do. This is the internal function
1997 implementing the C<< <*.c> >> operator, but you can use it directly.
1998 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1999 discussed in more detail in L<perlop/"I/O Operators">.
2001 Beginning with v5.6.0, this operator is implemented using the standard
2002 C<File::Glob> extension. See L<File::Glob> for details.
2006 Converts a time as returned by the time function to a 8-element list
2007 with the time localized for the standard Greenwich time zone.
2008 Typically used as follows:
2011 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2014 All list elements are numeric, and come straight out of the C `struct
2015 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2016 specified time. $mday is the day of the month, and $mon is the month
2017 itself, in the range C<0..11> with 0 indicating January and 11
2018 indicating December. $year is the number of years since 1900. That
2019 is, $year is C<123> in year 2023. $wday is the day of the week, with
2020 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2021 the year, in the range C<0..364> (or C<0..365> in leap years.)
2023 Note that the $year element is I<not> simply the last two digits of
2024 the year. If you assume it is, then you create non-Y2K-compliant
2025 programs--and you wouldn't want to do that, would you?
2027 The proper way to get a complete 4-digit year is simply:
2031 And to get the last two digits of the year (e.g., '01' in 2001) do:
2033 $year = sprintf("%02d", $year % 100);
2035 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2037 In scalar context, C<gmtime()> returns the ctime(3) value:
2039 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2041 Also see the C<timegm> function provided by the C<Time::Local> module,
2042 and the strftime(3) function available via the POSIX module.
2044 This scalar value is B<not> locale dependent (see L<perllocale>), but
2045 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2046 strftime(3) and mktime(3) functions available via the POSIX module. To
2047 get somewhat similar but locale dependent date strings, set up your
2048 locale environment variables appropriately (please see L<perllocale>)
2049 and try for example:
2051 use POSIX qw(strftime);
2052 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2054 Note that the C<%a> and C<%b> escapes, which represent the short forms
2055 of the day of the week and the month of the year, may not necessarily
2056 be three characters wide in all locales.
2064 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2065 execution there. It may not be used to go into any construct that
2066 requires initialization, such as a subroutine or a C<foreach> loop. It
2067 also can't be used to go into a construct that is optimized away,
2068 or to get out of a block or subroutine given to C<sort>.
2069 It can be used to go almost anywhere else within the dynamic scope,
2070 including out of subroutines, but it's usually better to use some other
2071 construct such as C<last> or C<die>. The author of Perl has never felt the
2072 need to use this form of C<goto> (in Perl, that is--C is another matter).
2074 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2075 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2076 necessarily recommended if you're optimizing for maintainability:
2078 goto ("FOO", "BAR", "GLARCH")[$i];
2080 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2081 In fact, it isn't a goto in the normal sense at all, and doesn't have
2082 the stigma associated with other gotos. Instead, it
2083 substitutes a call to the named subroutine for the currently running
2084 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2085 another subroutine and then pretend that the other subroutine had been
2086 called in the first place (except that any modifications to C<@_>
2087 in the current subroutine are propagated to the other subroutine.)
2088 After the C<goto>, not even C<caller> will be able to tell that this
2089 routine was called first.
2091 NAME needn't be the name of a subroutine; it can be a scalar variable
2092 containing a code reference, or a block which evaluates to a code
2095 =item grep BLOCK LIST
2097 =item grep EXPR,LIST
2099 This is similar in spirit to, but not the same as, grep(1) and its
2100 relatives. In particular, it is not limited to using regular expressions.
2102 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2103 C<$_> to each element) and returns the list value consisting of those
2104 elements for which the expression evaluated to true. In scalar
2105 context, returns the number of times the expression was true.
2107 @foo = grep(!/^#/, @bar); # weed out comments
2111 @foo = grep {!/^#/} @bar; # weed out comments
2113 Note that C<$_> is an alias to the list value, so it can be used to
2114 modify the elements of the LIST. While this is useful and supported,
2115 it can cause bizarre results if the elements of LIST are not variables.
2116 Similarly, grep returns aliases into the original list, much as a for
2117 loop's index variable aliases the list elements. That is, modifying an
2118 element of a list returned by grep (for example, in a C<foreach>, C<map>
2119 or another C<grep>) actually modifies the element in the original list.
2120 This is usually something to be avoided when writing clear code.
2122 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2128 Interprets EXPR as a hex string and returns the corresponding value.
2129 (To convert strings that might start with either 0, 0x, or 0b, see
2130 L</oct>.) If EXPR is omitted, uses C<$_>.
2132 print hex '0xAf'; # prints '175'
2133 print hex 'aF'; # same
2135 Hex strings may only represent integers. Strings that would cause
2136 integer overflow trigger a warning. Leading whitespace is not stripped,
2141 There is no builtin C<import> function. It is just an ordinary
2142 method (subroutine) defined (or inherited) by modules that wish to export
2143 names to another module. The C<use> function calls the C<import> method
2144 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2146 =item index STR,SUBSTR,POSITION
2148 =item index STR,SUBSTR
2150 The index function searches for one string within another, but without
2151 the wildcard-like behavior of a full regular-expression pattern match.
2152 It returns the position of the first occurrence of SUBSTR in STR at
2153 or after POSITION. If POSITION is omitted, starts searching from the
2154 beginning of the string. The return value is based at C<0> (or whatever
2155 you've set the C<$[> variable to--but don't do that). If the substring
2156 is not found, returns one less than the base, ordinarily C<-1>.
2162 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2163 You should not use this function for rounding: one because it truncates
2164 towards C<0>, and two because machine representations of floating point
2165 numbers can sometimes produce counterintuitive results. For example,
2166 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2167 because it's really more like -268.99999999999994315658 instead. Usually,
2168 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2169 functions will serve you better than will int().
2171 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2173 Implements the ioctl(2) function. You'll probably first have to say
2175 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2177 to get the correct function definitions. If F<ioctl.ph> doesn't
2178 exist or doesn't have the correct definitions you'll have to roll your
2179 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2180 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2181 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2182 written depending on the FUNCTION--a pointer to the string value of SCALAR
2183 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2184 has no string value but does have a numeric value, that value will be
2185 passed rather than a pointer to the string value. To guarantee this to be
2186 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2187 functions may be needed to manipulate the values of structures used by
2190 The return value of C<ioctl> (and C<fcntl>) is as follows:
2192 if OS returns: then Perl returns:
2194 0 string "0 but true"
2195 anything else that number
2197 Thus Perl returns true on success and false on failure, yet you can
2198 still easily determine the actual value returned by the operating
2201 $retval = ioctl(...) || -1;
2202 printf "System returned %d\n", $retval;
2204 The special string "C<0> but true" is exempt from B<-w> complaints
2205 about improper numeric conversions.
2207 Here's an example of setting a filehandle named C<REMOTE> to be
2208 non-blocking at the system level. You'll have to negotiate C<$|>
2209 on your own, though.
2211 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2213 $flags = fcntl(REMOTE, F_GETFL, 0)
2214 or die "Can't get flags for the socket: $!\n";
2216 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2217 or die "Can't set flags for the socket: $!\n";
2219 =item join EXPR,LIST
2221 Joins the separate strings of LIST into a single string with fields
2222 separated by the value of EXPR, and returns that new string. Example:
2224 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2226 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2227 first argument. Compare L</split>.
2231 Returns a list consisting of all the keys of the named hash. (In
2232 scalar context, returns the number of keys.) The keys are returned in
2233 an apparently random order. The actual random order is subject to
2234 change in future versions of perl, but it is guaranteed to be the same
2235 order as either the C<values> or C<each> function produces (given
2236 that the hash has not been modified). As a side effect, it resets
2239 Here is yet another way to print your environment:
2242 @values = values %ENV;
2244 print pop(@keys), '=', pop(@values), "\n";
2247 or how about sorted by key:
2249 foreach $key (sort(keys %ENV)) {
2250 print $key, '=', $ENV{$key}, "\n";
2253 The returned values are copies of the original keys in the hash, so
2254 modifying them will not affect the original hash. Compare L</values>.
2256 To sort a hash by value, you'll need to use a C<sort> function.
2257 Here's a descending numeric sort of a hash by its values:
2259 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2260 printf "%4d %s\n", $hash{$key}, $key;
2263 As an lvalue C<keys> allows you to increase the number of hash buckets
2264 allocated for the given hash. This can gain you a measure of efficiency if
2265 you know the hash is going to get big. (This is similar to pre-extending
2266 an array by assigning a larger number to $#array.) If you say
2270 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2271 in fact, since it rounds up to the next power of two. These
2272 buckets will be retained even if you do C<%hash = ()>, use C<undef
2273 %hash> if you want to free the storage while C<%hash> is still in scope.
2274 You can't shrink the number of buckets allocated for the hash using
2275 C<keys> in this way (but you needn't worry about doing this by accident,
2276 as trying has no effect).
2278 See also C<each>, C<values> and C<sort>.
2280 =item kill SIGNAL, LIST
2282 Sends a signal to a list of processes. Returns the number of
2283 processes successfully signaled (which is not necessarily the
2284 same as the number actually killed).
2286 $cnt = kill 1, $child1, $child2;
2289 If SIGNAL is zero, no signal is sent to the process. This is a
2290 useful way to check that the process is alive and hasn't changed
2291 its UID. See L<perlport> for notes on the portability of this
2294 Unlike in the shell, if SIGNAL is negative, it kills
2295 process groups instead of processes. (On System V, a negative I<PROCESS>
2296 number will also kill process groups, but that's not portable.) That
2297 means you usually want to use positive not negative signals. You may also
2298 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2304 The C<last> command is like the C<break> statement in C (as used in
2305 loops); it immediately exits the loop in question. If the LABEL is
2306 omitted, the command refers to the innermost enclosing loop. The
2307 C<continue> block, if any, is not executed:
2309 LINE: while (<STDIN>) {
2310 last LINE if /^$/; # exit when done with header
2314 C<last> cannot be used to exit a block which returns a value such as
2315 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2316 a grep() or map() operation.
2318 Note that a block by itself is semantically identical to a loop
2319 that executes once. Thus C<last> can be used to effect an early
2320 exit out of such a block.
2322 See also L</continue> for an illustration of how C<last>, C<next>, and
2329 Returns an lowercased version of EXPR. This is the internal function
2330 implementing the C<\L> escape in double-quoted strings. Respects
2331 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2334 If EXPR is omitted, uses C<$_>.
2340 Returns the value of EXPR with the first character lowercased. This
2341 is the internal function implementing the C<\l> escape in
2342 double-quoted strings. Respects current LC_CTYPE locale if C<use
2343 locale> in force. See L<perllocale> and L<perlunicode>.
2345 If EXPR is omitted, uses C<$_>.
2351 Returns the length in characters of the value of EXPR. If EXPR is
2352 omitted, returns length of C<$_>. Note that this cannot be used on
2353 an entire array or hash to find out how many elements these have.
2354 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2356 =item link OLDFILE,NEWFILE
2358 Creates a new filename linked to the old filename. Returns true for
2359 success, false otherwise.
2361 =item listen SOCKET,QUEUESIZE
2363 Does the same thing that the listen system call does. Returns true if
2364 it succeeded, false otherwise. See the example in
2365 L<perlipc/"Sockets: Client/Server Communication">.
2369 You really probably want to be using C<my> instead, because C<local> isn't
2370 what most people think of as "local". See
2371 L<perlsub/"Private Variables via my()"> for details.
2373 A local modifies the listed variables to be local to the enclosing
2374 block, file, or eval. If more than one value is listed, the list must
2375 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2376 for details, including issues with tied arrays and hashes.
2378 =item localtime EXPR
2380 Converts a time as returned by the time function to a 9-element list
2381 with the time analyzed for the local time zone. Typically used as
2385 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2388 All list elements are numeric, and come straight out of the C `struct
2389 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2390 specified time. $mday is the day of the month, and $mon is the month
2391 itself, in the range C<0..11> with 0 indicating January and 11
2392 indicating December. $year is the number of years since 1900. That
2393 is, $year is C<123> in year 2023. $wday is the day of the week, with
2394 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2395 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2396 is true if the specified time occurs during daylight savings time,
2399 Note that the $year element is I<not> simply the last two digits of
2400 the year. If you assume it is, then you create non-Y2K-compliant
2401 programs--and you wouldn't want to do that, would you?
2403 The proper way to get a complete 4-digit year is simply:
2407 And to get the last two digits of the year (e.g., '01' in 2001) do:
2409 $year = sprintf("%02d", $year % 100);
2411 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2413 In scalar context, C<localtime()> returns the ctime(3) value:
2415 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2417 This scalar value is B<not> locale dependent, see L<perllocale>, but
2418 instead a Perl builtin. Also see the C<Time::Local> module
2419 (to convert the second, minutes, hours, ... back to seconds since the
2420 stroke of midnight the 1st of January 1970, the value returned by
2421 time()), and the strftime(3) and mktime(3) functions available via the
2422 POSIX module. To get somewhat similar but locale dependent date
2423 strings, set up your locale environment variables appropriately
2424 (please see L<perllocale>) and try for example:
2426 use POSIX qw(strftime);
2427 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2429 Note that the C<%a> and C<%b>, the short forms of the day of the week
2430 and the month of the year, may not necessarily be three characters wide.
2436 This function places an advisory lock on a variable, subroutine,
2437 or referenced object contained in I<THING> until the lock goes out
2438 of scope. This is a built-in function only if your version of Perl
2439 was built with threading enabled, and if you've said C<use Threads>.
2440 Otherwise a user-defined function by this name will be called. See
2447 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2448 returns log of C<$_>. To get the log of another base, use basic algebra:
2449 The base-N log of a number is equal to the natural log of that number
2450 divided by the natural log of N. For example:
2454 return log($n)/log(10);
2457 See also L</exp> for the inverse operation.
2463 Does the same thing as the C<stat> function (including setting the
2464 special C<_> filehandle) but stats a symbolic link instead of the file
2465 the symbolic link points to. If symbolic links are unimplemented on
2466 your system, a normal C<stat> is done.
2468 If EXPR is omitted, stats C<$_>.
2472 The match operator. See L<perlop>.
2474 =item map BLOCK LIST
2478 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2479 C<$_> to each element) and returns the list value composed of the
2480 results of each such evaluation. In scalar context, returns the
2481 total number of elements so generated. Evaluates BLOCK or EXPR in
2482 list context, so each element of LIST may produce zero, one, or
2483 more elements in the returned value.
2485 @chars = map(chr, @nums);
2487 translates a list of numbers to the corresponding characters. And
2489 %hash = map { getkey($_) => $_ } @array;
2491 is just a funny way to write
2494 foreach $_ (@array) {
2495 $hash{getkey($_)} = $_;
2498 Note that C<$_> is an alias to the list value, so it can be used to
2499 modify the elements of the LIST. While this is useful and supported,
2500 it can cause bizarre results if the elements of LIST are not variables.
2501 Using a regular C<foreach> loop for this purpose would be clearer in
2502 most cases. See also L</grep> for an array composed of those items of
2503 the original list for which the BLOCK or EXPR evaluates to true.
2505 C<{> starts both hash references and blocks, so C<map { ...> could be either
2506 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2507 ahead for the closing C<}> it has to take a guess at which its dealing with
2508 based what it finds just after the C<{>. Usually it gets it right, but if it
2509 doesn't it won't realize something is wrong until it gets to the C<}> and
2510 encounters the missing (or unexpected) comma. The syntax error will be
2511 reported close to the C<}> but you'll need to change something near the C<{>
2512 such as using a unary C<+> to give perl some help:
2514 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2515 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2516 %hash = map { ("\L$_", 1) } @array # this also works
2517 %hash = map { lc($_), 1 } @array # as does this.
2518 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2520 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2522 or to force an anon hash constructor use C<+{>
2524 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2526 and you get list of anonymous hashes each with only 1 entry.
2528 =item mkdir FILENAME,MASK
2530 =item mkdir FILENAME
2532 Creates the directory specified by FILENAME, with permissions
2533 specified by MASK (as modified by C<umask>). If it succeeds it
2534 returns true, otherwise it returns false and sets C<$!> (errno).
2535 If omitted, MASK defaults to 0777.
2537 In general, it is better to create directories with permissive MASK,
2538 and let the user modify that with their C<umask>, than it is to supply
2539 a restrictive MASK and give the user no way to be more permissive.
2540 The exceptions to this rule are when the file or directory should be
2541 kept private (mail files, for instance). The perlfunc(1) entry on
2542 C<umask> discusses the choice of MASK in more detail.
2544 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2545 number of trailing slashes. Some operating and filesystems do not get
2546 this right, so Perl automatically removes all trailing slashes to keep
2549 =item msgctl ID,CMD,ARG
2551 Calls the System V IPC function msgctl(2). You'll probably have to say
2555 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2556 then ARG must be a variable which will hold the returned C<msqid_ds>
2557 structure. Returns like C<ioctl>: the undefined value for error,
2558 C<"0 but true"> for zero, or the actual return value otherwise. See also
2559 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2561 =item msgget KEY,FLAGS
2563 Calls the System V IPC function msgget(2). Returns the message queue
2564 id, or the undefined value if there is an error. See also
2565 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2567 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2569 Calls the System V IPC function msgrcv to receive a message from
2570 message queue ID into variable VAR with a maximum message size of
2571 SIZE. Note that when a message is received, the message type as a
2572 native long integer will be the first thing in VAR, followed by the
2573 actual message. This packing may be opened with C<unpack("l! a*")>.
2574 Taints the variable. Returns true if successful, or false if there is
2575 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2576 C<IPC::SysV::Msg> documentation.
2578 =item msgsnd ID,MSG,FLAGS
2580 Calls the System V IPC function msgsnd to send the message MSG to the
2581 message queue ID. MSG must begin with the native long integer message
2582 type, and be followed by the length of the actual message, and finally
2583 the message itself. This kind of packing can be achieved with
2584 C<pack("l! a*", $type, $message)>. Returns true if successful,
2585 or false if there is an error. See also C<IPC::SysV>
2586 and C<IPC::SysV::Msg> documentation.
2590 =item my EXPR : ATTRIBUTES
2592 A C<my> declares the listed variables to be local (lexically) to the
2593 enclosing block, file, or C<eval>. If
2594 more than one value is listed, the list must be placed in parentheses. See
2595 L<perlsub/"Private Variables via my()"> for details.
2601 The C<next> command is like the C<continue> statement in C; it starts
2602 the next iteration of the loop:
2604 LINE: while (<STDIN>) {
2605 next LINE if /^#/; # discard comments
2609 Note that if there were a C<continue> block on the above, it would get
2610 executed even on discarded lines. If the LABEL is omitted, the command
2611 refers to the innermost enclosing loop.
2613 C<next> cannot be used to exit a block which returns a value such as
2614 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2615 a grep() or map() operation.
2617 Note that a block by itself is semantically identical to a loop
2618 that executes once. Thus C<next> will exit such a block early.
2620 See also L</continue> for an illustration of how C<last>, C<next>, and
2623 =item no Module LIST
2625 See the L</use> function, which C<no> is the opposite of.
2631 Interprets EXPR as an octal string and returns the corresponding
2632 value. (If EXPR happens to start off with C<0x>, interprets it as a
2633 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2634 binary string. Leading whitespace is ignored in all three cases.)
2635 The following will handle decimal, binary, octal, and hex in the standard
2638 $val = oct($val) if $val =~ /^0/;
2640 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2641 in octal), use sprintf() or printf():
2643 $perms = (stat("filename"))[2] & 07777;
2644 $oct_perms = sprintf "%lo", $perms;
2646 The oct() function is commonly used when a string such as C<644> needs
2647 to be converted into a file mode, for example. (Although perl will
2648 automatically convert strings into numbers as needed, this automatic
2649 conversion assumes base 10.)
2651 =item open FILEHANDLE,EXPR
2653 =item open FILEHANDLE,MODE,EXPR
2655 =item open FILEHANDLE,MODE,EXPR,LIST
2657 =item open FILEHANDLE
2659 Opens the file whose filename is given by EXPR, and associates it with
2660 FILEHANDLE. If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2661 assigned a reference to a new anonymous filehandle, otherwise if FILEHANDLE is an expression,
2662 its value is used as the name of the real filehandle wanted. (This is considered a symbolic
2663 reference, so C<use strict 'refs'> should I<not> be in effect.)
2665 If EXPR is omitted, the scalar
2666 variable of the same name as the FILEHANDLE contains the filename.
2667 (Note that lexical variables--those declared with C<my>--will not work
2668 for this purpose; so if you're using C<my>, specify EXPR in your call
2669 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2672 If three or more arguments are specified then the mode of opening and the file name
2673 are separate. If MODE is C<< '<' >> or nothing, the file is opened for input.
2674 If MODE is C<< '>' >>, the file is truncated and opened for
2675 output, being created if necessary. If MODE is C<<< '>>' >>>,
2676 the file is opened for appending, again being created if necessary.
2677 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2678 you want both read and write access to the file; thus C<< '+<' >> is almost
2679 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2680 file first. You can't usually use either read-write mode for updating
2681 textfiles, since they have variable length records. See the B<-i>
2682 switch in L<perlrun> for a better approach. The file is created with
2683 permissions of C<0666> modified by the process' C<umask> value.
2685 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2686 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2688 In the 2-arguments (and 1-argument) form of the call the mode and
2689 filename should be concatenated (in this order), possibly separated by
2690 spaces. It is possible to omit the mode in these forms if the mode is
2693 If the filename begins with C<'|'>, the filename is interpreted as a
2694 command to which output is to be piped, and if the filename ends with a
2695 C<'|'>, the filename is interpreted as a command which pipes output to
2696 us. See L<perlipc/"Using open() for IPC">
2697 for more examples of this. (You are not allowed to C<open> to a command
2698 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2699 and L<perlipc/"Bidirectional Communication with Another Process">
2702 For three or more arguments if MODE is C<'|-'>, the filename is interpreted as a
2703 command to which output is to be piped, and if MODE is
2704 C<'-|'>, the filename is interpreted as a command which pipes output to
2705 us. In the 2-arguments (and 1-argument) form one should replace dash
2706 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2707 for more examples of this. (You are not allowed to C<open> to a command
2708 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2709 and L<perlipc/"Bidirectional Communication"> for alternatives.) In 3+ arg form of
2710 pipe opens then if LIST is specified (extra arguments after the command name) then
2711 LIST becomes arguments to the command invoked if the platform supports it.
2712 The meaning of C<open> with more than three arguments for non-pipe modes
2713 is not yet specified. Experimental "layers" may give extra LIST arguments meaning.
2715 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2716 and opening C<< '>-' >> opens STDOUT.
2719 nonzero upon success, the undefined value otherwise. If the C<open>
2720 involved a pipe, the return value happens to be the pid of the
2723 If you're unfortunate enough to be running Perl on a system that
2724 distinguishes between text files and binary files (modern operating
2725 systems don't care), then you should check out L</binmode> for tips for
2726 dealing with this. The key distinction between systems that need C<binmode>
2727 and those that don't is their text file formats. Systems like Unix, MacOS, and
2728 Plan9, which delimit lines with a single character, and which encode that
2729 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2731 In the three argument form MODE may also contain a list of IO "layers" (see L<open> and
2732 L<PerlIO> for more details) to be applied to the handle. This can be used to achieve the
2733 effect of C<binmode> as well as more complex behaviours.
2735 When opening a file, it's usually a bad idea to continue normal execution
2736 if the request failed, so C<open> is frequently used in connection with
2737 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2738 where you want to make a nicely formatted error message (but there are
2739 modules that can help with that problem)) you should always check
2740 the return value from opening a file. The infrequent exception is when
2741 working with an unopened filehandle is actually what you want to do.
2743 As a special case the 3 arg form with a read/write mode and the third argument
2746 open(TMP, "+>", undef) or die ...
2748 opens a filehandle to an anonymous temporary file.
2754 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2755 while (<ARTICLE>) {...
2757 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2758 # if the open fails, output is discarded
2760 open(DBASE, '+<', 'dbase.mine') # open for update
2761 or die "Can't open 'dbase.mine' for update: $!";
2763 open(DBASE, '+<dbase.mine') # ditto
2764 or die "Can't open 'dbase.mine' for update: $!";
2766 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2767 or die "Can't start caesar: $!";
2769 open(ARTICLE, "caesar <$article |") # ditto
2770 or die "Can't start caesar: $!";
2772 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2773 or die "Can't start sort: $!";
2775 # process argument list of files along with any includes
2777 foreach $file (@ARGV) {
2778 process($file, 'fh00');
2782 my($filename, $input) = @_;
2783 $input++; # this is a string increment
2784 unless (open($input, $filename)) {
2785 print STDERR "Can't open $filename: $!\n";
2790 while (<$input>) { # note use of indirection
2791 if (/^#include "(.*)"/) {
2792 process($1, $input);
2799 You may also, in the Bourne shell tradition, specify an EXPR beginning
2800 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2801 name of a filehandle (or file descriptor, if numeric) to be
2802 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2803 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2804 mode you specify should match the mode of the original filehandle.
2805 (Duping a filehandle does not take into account any existing contents of
2806 stdio buffers.) If you use the 3 arg form then you can pass either a number,
2807 the name of a filehandle or the normal "reference to a glob".
2809 Here is a script that saves, redirects, and restores STDOUT and
2813 open(my $oldout, ">&", \*STDOUT);
2814 open(OLDERR, ">&STDERR");
2816 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2817 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2819 select(STDERR); $| = 1; # make unbuffered
2820 select(STDOUT); $| = 1; # make unbuffered
2822 print STDOUT "stdout 1\n"; # this works for
2823 print STDERR "stderr 1\n"; # subprocesses too
2828 open(STDOUT, ">&OLDOUT");
2829 open(STDERR, ">&OLDERR");
2831 print STDOUT "stdout 2\n";
2832 print STDERR "stderr 2\n";
2834 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2835 do an equivalent of C's C<fdopen> of that file descriptor; this is
2836 more parsimonious of file descriptors. For example:
2838 open(FILEHANDLE, "<&=$fd")
2842 open(FILEHANDLE, "<&=", $fd)
2844 Note that if Perl is using the standard C libraries' fdopen() then on
2845 many UNIX systems, fdopen() is known to fail when file descriptors
2846 exceed a certain value, typically 255. If you need more file
2847 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2849 You can see whether Perl has been compiled with PerlIO or not by
2850 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2851 is C<define>, you have PerlIO, otherwise you don't.
2853 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2854 with 2-arguments (or 1-argument) form of open(), then
2855 there is an implicit fork done, and the return value of open is the pid
2856 of the child within the parent process, and C<0> within the child
2857 process. (Use C<defined($pid)> to determine whether the open was successful.)
2858 The filehandle behaves normally for the parent, but i/o to that
2859 filehandle is piped from/to the STDOUT/STDIN of the child process.
2860 In the child process the filehandle isn't opened--i/o happens from/to
2861 the new STDOUT or STDIN. Typically this is used like the normal
2862 piped open when you want to exercise more control over just how the
2863 pipe command gets executed, such as when you are running setuid, and
2864 don't want to have to scan shell commands for metacharacters.
2865 The following triples are more or less equivalent:
2867 open(FOO, "|tr '[a-z]' '[A-Z]'");
2868 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2869 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2870 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2872 open(FOO, "cat -n '$file'|");
2873 open(FOO, '-|', "cat -n '$file'");
2874 open(FOO, '-|') || exec 'cat', '-n', $file;
2875 open(FOO, '-|', "cat", '-n', $file);
2877 The last example in each block shows the pipe as "list form", which is
2878 not yet supported on all platforms.
2880 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2882 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2883 output before any operation that may do a fork, but this may not be
2884 supported on some platforms (see L<perlport>). To be safe, you may need
2885 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2886 of C<IO::Handle> on any open handles.
2888 On systems that support a
2889 close-on-exec flag on files, the flag will be set for the newly opened
2890 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2892 Closing any piped filehandle causes the parent process to wait for the
2893 child to finish, and returns the status value in C<$?>.
2895 The filename passed to 2-argument (or 1-argument) form of open()
2896 will have leading and trailing
2897 whitespace deleted, and the normal redirection characters
2898 honored. This property, known as "magic open",
2899 can often be used to good effect. A user could specify a filename of
2900 F<"rsh cat file |">, or you could change certain filenames as needed:
2902 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2903 open(FH, $filename) or die "Can't open $filename: $!";
2905 Use 3-argument form to open a file with arbitrary weird characters in it,
2907 open(FOO, '<', $file);
2909 otherwise it's necessary to protect any leading and trailing whitespace:
2911 $file =~ s#^(\s)#./$1#;
2912 open(FOO, "< $file\0");
2914 (this may not work on some bizarre filesystems). One should
2915 conscientiously choose between the I<magic> and 3-arguments form
2920 will allow the user to specify an argument of the form C<"rsh cat file |">,
2921 but will not work on a filename which happens to have a trailing space, while
2923 open IN, '<', $ARGV[0];
2925 will have exactly the opposite restrictions.
2927 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2928 should use the C<sysopen> function, which involves no such magic (but
2929 may use subtly different filemodes than Perl open(), which is mapped
2930 to C fopen()). This is
2931 another way to protect your filenames from interpretation. For example:
2934 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2935 or die "sysopen $path: $!";
2936 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2937 print HANDLE "stuff $$\n";
2939 print "File contains: ", <HANDLE>;
2941 Using the constructor from the C<IO::Handle> package (or one of its
2942 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2943 filehandles that have the scope of whatever variables hold references to
2944 them, and automatically close whenever and however you leave that scope:
2948 sub read_myfile_munged {
2950 my $handle = new IO::File;
2951 open($handle, "myfile") or die "myfile: $!";
2953 or return (); # Automatically closed here.
2954 mung $first or die "mung failed"; # Or here.
2955 return $first, <$handle> if $ALL; # Or here.
2959 See L</seek> for some details about mixing reading and writing.
2961 =item opendir DIRHANDLE,EXPR
2963 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2964 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2965 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2971 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2972 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2973 See L<utf8> for more about Unicode.
2977 =item our EXPR : ATTRIBUTES
2979 An C<our> declares the listed variables to be valid globals within
2980 the enclosing block, file, or C<eval>. That is, it has the same
2981 scoping rules as a "my" declaration, but does not create a local
2982 variable. If more than one value is listed, the list must be placed
2983 in parentheses. The C<our> declaration has no semantic effect unless
2984 "use strict vars" is in effect, in which case it lets you use the
2985 declared global variable without qualifying it with a package name.
2986 (But only within the lexical scope of the C<our> declaration. In this
2987 it differs from "use vars", which is package scoped.)
2989 An C<our> declaration declares a global variable that will be visible
2990 across its entire lexical scope, even across package boundaries. The
2991 package in which the variable is entered is determined at the point
2992 of the declaration, not at the point of use. This means the following
2996 our $bar; # declares $Foo::bar for rest of lexical scope
3000 print $bar; # prints 20
3002 Multiple C<our> declarations in the same lexical scope are allowed
3003 if they are in different packages. If they happened to be in the same
3004 package, Perl will emit warnings if you have asked for them.
3008 our $bar; # declares $Foo::bar for rest of lexical scope
3012 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3013 print $bar; # prints 30
3015 our $bar; # emits warning
3017 An C<our> declaration may also have a list of attributes associated
3018 with it. B<WARNING>: This is an experimental feature that may be
3019 changed or removed in future releases of Perl. It should not be
3022 The only currently recognized attribute is C<unique> which indicates
3023 that a single copy of the global is to be used by all interpreters
3024 should the program happen to be running in a multi-interpreter
3025 environment. (The default behaviour would be for each interpreter to
3026 have its own copy of the global.) In such an environment, this
3027 attribute also has the effect of making the global readonly.
3030 our @EXPORT : unique = qw(foo);
3031 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3032 our $VERSION : unique = "1.00";
3034 Multi-interpreter environments can come to being either through the
3035 fork() emulation on Windows platforms, or by embedding perl in a
3036 multi-threaded application. The C<unique> attribute does nothing in
3037 all other environments.
3039 =item pack TEMPLATE,LIST
3041 Takes a LIST of values and converts it into a string using the rules
3042 given by the TEMPLATE. The resulting string is the concatenation of
3043 the converted values. Typically, each converted value looks
3044 like its machine-level representation. For example, on 32-bit machines
3045 a converted integer may be represented by a sequence of 4 bytes.
3048 sequence of characters that give the order and type of values, as
3051 a A string with arbitrary binary data, will be null padded.
3052 A An ASCII string, will be space padded.
3053 Z A null terminated (asciz) string, will be null padded.
3055 b A bit string (ascending bit order inside each byte, like vec()).
3056 B A bit string (descending bit order inside each byte).
3057 h A hex string (low nybble first).
3058 H A hex string (high nybble first).
3060 c A signed char value.
3061 C An unsigned char value. Only does bytes. See U for Unicode.
3063 s A signed short value.
3064 S An unsigned short value.
3065 (This 'short' is _exactly_ 16 bits, which may differ from
3066 what a local C compiler calls 'short'. If you want
3067 native-length shorts, use the '!' suffix.)
3069 i A signed integer value.
3070 I An unsigned integer value.
3071 (This 'integer' is _at_least_ 32 bits wide. Its exact
3072 size depends on what a local C compiler calls 'int',
3073 and may even be larger than the 'long' described in
3076 l A signed long value.
3077 L An unsigned long value.
3078 (This 'long' is _exactly_ 32 bits, which may differ from
3079 what a local C compiler calls 'long'. If you want
3080 native-length longs, use the '!' suffix.)
3082 n An unsigned short in "network" (big-endian) order.
3083 N An unsigned long in "network" (big-endian) order.
3084 v An unsigned short in "VAX" (little-endian) order.
3085 V An unsigned long in "VAX" (little-endian) order.
3086 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3087 _exactly_ 32 bits, respectively.)
3089 q A signed quad (64-bit) value.
3090 Q An unsigned quad value.
3091 (Quads are available only if your system supports 64-bit
3092 integer values _and_ if Perl has been compiled to support those.
3093 Causes a fatal error otherwise.)
3095 f A single-precision float in the native format.
3096 d A double-precision float in the native format.
3098 p A pointer to a null-terminated string.
3099 P A pointer to a structure (fixed-length string).
3101 u A uuencoded string.
3102 U A Unicode character number. Encodes to UTF-8 internally
3103 (or UTF-EBCDIC in EBCDIC platforms).
3105 w A BER compressed integer. Its bytes represent an unsigned
3106 integer in base 128, most significant digit first, with as
3107 few digits as possible. Bit eight (the high bit) is set
3108 on each byte except the last.
3112 @ Null fill to absolute position.
3114 The following rules apply:
3120 Each letter may optionally be followed by a number giving a repeat
3121 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3122 C<H>, and C<P> the pack function will gobble up that many values from
3123 the LIST. A C<*> for the repeat count means to use however many items are
3124 left, except for C<@>, C<x>, C<X>, where it is equivalent
3125 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3128 When used with C<Z>, C<*> results in the addition of a trailing null
3129 byte (so the packed result will be one longer than the byte C<length>
3132 The repeat count for C<u> is interpreted as the maximal number of bytes
3133 to encode per line of output, with 0 and 1 replaced by 45.
3137 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3138 string of length count, padding with nulls or spaces as necessary. When
3139 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3140 after the first null, and C<a> returns data verbatim. When packing,
3141 C<a>, and C<Z> are equivalent.
3143 If the value-to-pack is too long, it is truncated. If too long and an
3144 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3145 by a null byte. Thus C<Z> always packs a trailing null byte under
3150 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3151 Each byte of the input field of pack() generates 1 bit of the result.
3152 Each result bit is based on the least-significant bit of the corresponding
3153 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3154 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3156 Starting from the beginning of the input string of pack(), each 8-tuple
3157 of bytes is converted to 1 byte of output. With format C<b>
3158 the first byte of the 8-tuple determines the least-significant bit of a
3159 byte, and with format C<B> it determines the most-significant bit of
3162 If the length of the input string is not exactly divisible by 8, the
3163 remainder is packed as if the input string were padded by null bytes
3164 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3166 If the input string of pack() is longer than needed, extra bytes are ignored.
3167 A C<*> for the repeat count of pack() means to use all the bytes of
3168 the input field. On unpack()ing the bits are converted to a string
3169 of C<"0">s and C<"1">s.
3173 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3174 representable as hexadecimal digits, 0-9a-f) long.
3176 Each byte of the input field of pack() generates 4 bits of the result.
3177 For non-alphabetical bytes the result is based on the 4 least-significant
3178 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3179 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3180 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3181 is compatible with the usual hexadecimal digits, so that C<"a"> and
3182 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3183 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3185 Starting from the beginning of the input string of pack(), each pair
3186 of bytes is converted to 1 byte of output. With format C<h> the
3187 first byte of the pair determines the least-significant nybble of the
3188 output byte, and with format C<H> it determines the most-significant
3191 If the length of the input string is not even, it behaves as if padded
3192 by a null byte at the end. Similarly, during unpack()ing the "extra"
3193 nybbles are ignored.
3195 If the input string of pack() is longer than needed, extra bytes are ignored.
3196 A C<*> for the repeat count of pack() means to use all the bytes of
3197 the input field. On unpack()ing the bits are converted to a string
3198 of hexadecimal digits.
3202 The C<p> type packs a pointer to a null-terminated string. You are
3203 responsible for ensuring the string is not a temporary value (which can
3204 potentially get deallocated before you get around to using the packed result).
3205 The C<P> type packs a pointer to a structure of the size indicated by the
3206 length. A NULL pointer is created if the corresponding value for C<p> or
3207 C<P> is C<undef>, similarly for unpack().
3211 The C</> template character allows packing and unpacking of strings where
3212 the packed structure contains a byte count followed by the string itself.
3213 You write I<length-item>C</>I<string-item>.
3215 The I<length-item> can be any C<pack> template letter,
3216 and describes how the length value is packed.
3217 The ones likely to be of most use are integer-packing ones like
3218 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3219 and C<N> (for Sun XDR).
3221 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3222 For C<unpack> the length of the string is obtained from the I<length-item>,
3223 but if you put in the '*' it will be ignored.
3225 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3226 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3227 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3229 The I<length-item> is not returned explicitly from C<unpack>.
3231 Adding a count to the I<length-item> letter is unlikely to do anything
3232 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3233 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3234 which Perl does not regard as legal in numeric strings.
3238 The integer types C<s>, C<S>, C<l>, and C<L> may be
3239 immediately followed by a C<!> suffix to signify native shorts or
3240 longs--as you can see from above for example a bare C<l> does mean
3241 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3242 may be larger. This is an issue mainly in 64-bit platforms. You can
3243 see whether using C<!> makes any difference by
3245 print length(pack("s")), " ", length(pack("s!")), "\n";
3246 print length(pack("l")), " ", length(pack("l!")), "\n";
3248 C<i!> and C<I!> also work but only because of completeness;
3249 they are identical to C<i> and C<I>.
3251 The actual sizes (in bytes) of native shorts, ints, longs, and long
3252 longs on the platform where Perl was built are also available via
3256 print $Config{shortsize}, "\n";
3257 print $Config{intsize}, "\n";
3258 print $Config{longsize}, "\n";
3259 print $Config{longlongsize}, "\n";
3261 (The C<$Config{longlongsize}> will be undefine if your system does
3262 not support long longs.)
3266 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3267 are inherently non-portable between processors and operating systems
3268 because they obey the native byteorder and endianness. For example a
3269 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3270 (arranged in and handled by the CPU registers) into bytes as
3272 0x12 0x34 0x56 0x78 # big-endian
3273 0x78 0x56 0x34 0x12 # little-endian
3275 Basically, the Intel and VAX CPUs are little-endian, while everybody
3276 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3277 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3278 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3281 The names `big-endian' and `little-endian' are comic references to
3282 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3283 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3284 the egg-eating habits of the Lilliputians.
3286 Some systems may have even weirder byte orders such as
3291 You can see your system's preference with
3293 print join(" ", map { sprintf "%#02x", $_ }
3294 unpack("C*",pack("L",0x12345678))), "\n";
3296 The byteorder on the platform where Perl was built is also available
3300 print $Config{byteorder}, "\n";
3302 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3303 and C<'87654321'> are big-endian.
3305 If you want portable packed integers use the formats C<n>, C<N>,
3306 C<v>, and C<V>, their byte endianness and size are known.
3307 See also L<perlport>.
3311 Real numbers (floats and doubles) are in the native machine format only;
3312 due to the multiplicity of floating formats around, and the lack of a
3313 standard "network" representation, no facility for interchange has been
3314 made. This means that packed floating point data written on one machine
3315 may not be readable on another - even if both use IEEE floating point
3316 arithmetic (as the endian-ness of the memory representation is not part
3317 of the IEEE spec). See also L<perlport>.
3319 Note that Perl uses doubles internally for all numeric calculation, and
3320 converting from double into float and thence back to double again will
3321 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3326 If the pattern begins with a C<U>, the resulting string will be treated
3327 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3328 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3329 characters. If you don't want this to happen, you can begin your pattern
3330 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3331 string, and then follow this with a C<U*> somewhere in your pattern.
3335 You must yourself do any alignment or padding by inserting for example
3336 enough C<'x'>es while packing. There is no way to pack() and unpack()
3337 could know where the bytes are going to or coming from. Therefore
3338 C<pack> (and C<unpack>) handle their output and input as flat
3343 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3347 If TEMPLATE requires more arguments to pack() than actually given, pack()
3348 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3349 to pack() than actually given, extra arguments are ignored.
3355 $foo = pack("CCCC",65,66,67,68);
3357 $foo = pack("C4",65,66,67,68);
3359 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3360 # same thing with Unicode circled letters
3362 $foo = pack("ccxxcc",65,66,67,68);
3365 # note: the above examples featuring "C" and "c" are true
3366 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3367 # and UTF-8. In EBCDIC the first example would be
3368 # $foo = pack("CCCC",193,194,195,196);
3370 $foo = pack("s2",1,2);
3371 # "\1\0\2\0" on little-endian
3372 # "\0\1\0\2" on big-endian
3374 $foo = pack("a4","abcd","x","y","z");
3377 $foo = pack("aaaa","abcd","x","y","z");
3380 $foo = pack("a14","abcdefg");
3381 # "abcdefg\0\0\0\0\0\0\0"
3383 $foo = pack("i9pl", gmtime);
3384 # a real struct tm (on my system anyway)
3386 $utmp_template = "Z8 Z8 Z16 L";
3387 $utmp = pack($utmp_template, @utmp1);
3388 # a struct utmp (BSDish)
3390 @utmp2 = unpack($utmp_template, $utmp);
3391 # "@utmp1" eq "@utmp2"
3394 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3397 $foo = pack('sx2l', 12, 34);
3398 # short 12, two zero bytes padding, long 34
3399 $bar = pack('s@4l', 12, 34);
3400 # short 12, zero fill to position 4, long 34
3403 The same template may generally also be used in unpack().
3405 =item package NAMESPACE
3409 Declares the compilation unit as being in the given namespace. The scope
3410 of the package declaration is from the declaration itself through the end
3411 of the enclosing block, file, or eval (the same as the C<my> operator).
3412 All further unqualified dynamic identifiers will be in this namespace.
3413 A package statement affects only dynamic variables--including those
3414 you've used C<local> on--but I<not> lexical variables, which are created
3415 with C<my>. Typically it would be the first declaration in a file to
3416 be included by the C<require> or C<use> operator. You can switch into a
3417 package in more than one place; it merely influences which symbol table
3418 is used by the compiler for the rest of that block. You can refer to
3419 variables and filehandles in other packages by prefixing the identifier
3420 with the package name and a double colon: C<$Package::Variable>.
3421 If the package name is null, the C<main> package as assumed. That is,
3422 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3423 still seen in older code).
3425 If NAMESPACE is omitted, then there is no current package, and all
3426 identifiers must be fully qualified or lexicals. However, you are
3427 strongly advised not to make use of this feature. Its use can cause
3428 unexpected behaviour, even crashing some versions of Perl. It is
3429 deprecated, and will be removed from a future release.
3431 See L<perlmod/"Packages"> for more information about packages, modules,
3432 and classes. See L<perlsub> for other scoping issues.
3434 =item pipe READHANDLE,WRITEHANDLE
3436 Opens a pair of connected pipes like the corresponding system call.
3437 Note that if you set up a loop of piped processes, deadlock can occur
3438 unless you are very careful. In addition, note that Perl's pipes use
3439 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3440 after each command, depending on the application.
3442 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3443 for examples of such things.
3445 On systems that support a close-on-exec flag on files, the flag will be set
3446 for the newly opened file descriptors as determined by the value of $^F.
3453 Pops and returns the last value of the array, shortening the array by
3454 one element. Has an effect similar to
3458 If there are no elements in the array, returns the undefined value
3459 (although this may happen at other times as well). If ARRAY is
3460 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3461 array in subroutines, just like C<shift>.
3467 Returns the offset of where the last C<m//g> search left off for the variable
3468 in question (C<$_> is used when the variable is not specified). May be
3469 modified to change that offset. Such modification will also influence
3470 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3473 =item print FILEHANDLE LIST
3479 Prints a string or a list of strings. Returns true if successful.
3480 FILEHANDLE may be a scalar variable name, in which case the variable
3481 contains the name of or a reference to the filehandle, thus introducing
3482 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3483 the next token is a term, it may be misinterpreted as an operator
3484 unless you interpose a C<+> or put parentheses around the arguments.)
3485 If FILEHANDLE is omitted, prints by default to standard output (or
3486 to the last selected output channel--see L</select>). If LIST is
3487 also omitted, prints C<$_> to the currently selected output channel.
3488 To set the default output channel to something other than STDOUT
3489 use the select operation. The current value of C<$,> (if any) is
3490 printed between each LIST item. The current value of C<$\> (if
3491 any) is printed after the entire LIST has been printed. Because
3492 print takes a LIST, anything in the LIST is evaluated in list
3493 context, and any subroutine that you call will have one or more of
3494 its expressions evaluated in list context. Also be careful not to
3495 follow the print keyword with a left parenthesis unless you want
3496 the corresponding right parenthesis to terminate the arguments to
3497 the print--interpose a C<+> or put parentheses around all the
3500 Note that if you're storing FILEHANDLES in an array or other expression,
3501 you will have to use a block returning its value instead:
3503 print { $files[$i] } "stuff\n";
3504 print { $OK ? STDOUT : STDERR } "stuff\n";
3506 =item printf FILEHANDLE FORMAT, LIST
3508 =item printf FORMAT, LIST
3510 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3511 (the output record separator) is not appended. The first argument
3512 of the list will be interpreted as the C<printf> format. See C<sprintf>
3513 for an explanation of the format argument. If C<use locale> is in effect,
3514 the character used for the decimal point in formatted real numbers is
3515 affected by the LC_NUMERIC locale. See L<perllocale>.
3517 Don't fall into the trap of using a C<printf> when a simple
3518 C<print> would do. The C<print> is more efficient and less
3521 =item prototype FUNCTION
3523 Returns the prototype of a function as a string (or C<undef> if the
3524 function has no prototype). FUNCTION is a reference to, or the name of,
3525 the function whose prototype you want to retrieve.
3527 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3528 name for Perl builtin. If the builtin is not I<overridable> (such as
3529 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3530 C<system>) returns C<undef> because the builtin does not really behave
3531 like a Perl function. Otherwise, the string describing the equivalent
3532 prototype is returned.
3534 =item push ARRAY,LIST
3536 Treats ARRAY as a stack, and pushes the values of LIST
3537 onto the end of ARRAY. The length of ARRAY increases by the length of
3538 LIST. Has the same effect as
3541 $ARRAY[++$#ARRAY] = $value;
3544 but is more efficient. Returns the new number of elements in the array.
3556 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3558 =item quotemeta EXPR
3562 Returns the value of EXPR with all non-"word"
3563 characters backslashed. (That is, all characters not matching
3564 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3565 returned string, regardless of any locale settings.)
3566 This is the internal function implementing
3567 the C<\Q> escape in double-quoted strings.
3569 If EXPR is omitted, uses C<$_>.
3575 Returns a random fractional number greater than or equal to C<0> and less
3576 than the value of EXPR. (EXPR should be positive.) If EXPR is
3577 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3578 unless C<srand> has already been called. See also C<srand>.
3580 Apply C<int()> to the value returned by C<rand()> if you want random
3581 integers instead of random fractional numbers. For example,
3585 returns a random integer between C<0> and C<9>, inclusive.
3587 (Note: If your rand function consistently returns numbers that are too
3588 large or too small, then your version of Perl was probably compiled
3589 with the wrong number of RANDBITS.)
3591 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3593 =item read FILEHANDLE,SCALAR,LENGTH
3595 Attempts to read LENGTH bytes of data into variable SCALAR from the
3596 specified FILEHANDLE. Returns the number of bytes actually read, C<0>
3597 at end of file, or undef if there was an error. SCALAR will be grown
3598 or shrunk to the length actually read. If SCALAR needs growing, the
3599 new bytes will be zero bytes. An OFFSET may be specified to place
3600 the read data into some other place in SCALAR than the beginning.
3601 The call is actually implemented in terms of stdio's fread(3) call.
3602 To get a true read(2) system call, see C<sysread>.
3604 =item readdir DIRHANDLE
3606 Returns the next directory entry for a directory opened by C<opendir>.
3607 If used in list context, returns all the rest of the entries in the
3608 directory. If there are no more entries, returns an undefined value in
3609 scalar context or a null list in list context.
3611 If you're planning to filetest the return values out of a C<readdir>, you'd
3612 better prepend the directory in question. Otherwise, because we didn't
3613 C<chdir> there, it would have been testing the wrong file.
3615 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3616 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3621 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3622 context, each call reads and returns the next line, until end-of-file is
3623 reached, whereupon the subsequent call returns undef. In list context,
3624 reads until end-of-file is reached and returns a list of lines. Note that
3625 the notion of "line" used here is however you may have defined it
3626 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3628 When C<$/> is set to C<undef>, when readline() is in scalar
3629 context (i.e. file slurp mode), and when an empty file is read, it
3630 returns C<''> the first time, followed by C<undef> subsequently.
3632 This is the internal function implementing the C<< <EXPR> >>
3633 operator, but you can use it directly. The C<< <EXPR> >>
3634 operator is discussed in more detail in L<perlop/"I/O Operators">.
3637 $line = readline(*STDIN); # same thing
3643 Returns the value of a symbolic link, if symbolic links are
3644 implemented. If not, gives a fatal error. If there is some system
3645 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3646 omitted, uses C<$_>.
3650 EXPR is executed as a system command.
3651 The collected standard output of the command is returned.
3652 In scalar context, it comes back as a single (potentially
3653 multi-line) string. In list context, returns a list of lines
3654 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3655 This is the internal function implementing the C<qx/EXPR/>
3656 operator, but you can use it directly. The C<qx/EXPR/>
3657 operator is discussed in more detail in L<perlop/"I/O Operators">.
3659 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3661 Receives a message on a socket. Attempts to receive LENGTH bytes of
3662 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3663 will be grown or shrunk to the length actually read. Takes the same
3664 flags as the system call of the same name. Returns the address of the
3665 sender if SOCKET's protocol supports this; returns an empty string
3666 otherwise. If there's an error, returns the undefined value. This call
3667 is actually implemented in terms of recvfrom(2) system call. See
3668 L<perlipc/"UDP: Message Passing"> for examples.
3674 The C<redo> command restarts the loop block without evaluating the
3675 conditional again. The C<continue> block, if any, is not executed. If
3676 the LABEL is omitted, the command refers to the innermost enclosing
3677 loop. This command is normally used by programs that want to lie to
3678 themselves about what was just input:
3680 # a simpleminded Pascal comment stripper
3681 # (warning: assumes no { or } in strings)
3682 LINE: while (<STDIN>) {
3683 while (s|({.*}.*){.*}|$1 |) {}
3688 if (/}/) { # end of comment?
3697 C<redo> cannot be used to retry a block which returns a value such as
3698 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3699 a grep() or map() operation.
3701 Note that a block by itself is semantically identical to a loop
3702 that executes once. Thus C<redo> inside such a block will effectively
3703 turn it into a looping construct.
3705 See also L</continue> for an illustration of how C<last>, C<next>, and
3712 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3713 is not specified, C<$_> will be used. The value returned depends on the
3714 type of thing the reference is a reference to.
3715 Builtin types include:
3725 If the referenced object has been blessed into a package, then that package
3726 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3728 if (ref($r) eq "HASH") {
3729 print "r is a reference to a hash.\n";
3732 print "r is not a reference at all.\n";
3734 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3735 print "r is a reference to something that isa hash.\n";
3738 See also L<perlref>.
3740 =item rename OLDNAME,NEWNAME
3742 Changes the name of a file; an existing file NEWNAME will be
3743 clobbered. Returns true for success, false otherwise.
3745 Behavior of this function varies wildly depending on your system
3746 implementation. For example, it will usually not work across file system
3747 boundaries, even though the system I<mv> command sometimes compensates
3748 for this. Other restrictions include whether it works on directories,
3749 open files, or pre-existing files. Check L<perlport> and either the
3750 rename(2) manpage or equivalent system documentation for details.
3752 =item require VERSION
3758 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3761 If a VERSION is specified as a literal of the form v5.6.1,
3762 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3763 at least as recent as that version, at run time. (For compatibility
3764 with older versions of Perl, a numeric argument will also be interpreted
3765 as VERSION.) Compare with L</use>, which can do a similar check at
3768 require v5.6.1; # run time version check
3769 require 5.6.1; # ditto
3770 require 5.005_03; # float version allowed for compatibility
3772 Otherwise, demands that a library file be included if it hasn't already
3773 been included. The file is included via the do-FILE mechanism, which is
3774 essentially just a variety of C<eval>. Has semantics similar to the following
3779 return 1 if $INC{$filename};
3780 my($realfilename,$result);
3782 foreach $prefix (@INC) {
3783 $realfilename = "$prefix/$filename";
3784 if (-f $realfilename) {
3785 $INC{$filename} = $realfilename;
3786 $result = do $realfilename;
3790 die "Can't find $filename in \@INC";
3792 delete $INC{$filename} if $@ || !$result;
3794 die "$filename did not return true value" unless $result;
3798 Note that the file will not be included twice under the same specified
3799 name. The file must return true as the last statement to indicate
3800 successful execution of any initialization code, so it's customary to
3801 end such a file with C<1;> unless you're sure it'll return true
3802 otherwise. But it's better just to put the C<1;>, in case you add more
3805 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3806 replaces "F<::>" with "F</>" in the filename for you,
3807 to make it easy to load standard modules. This form of loading of
3808 modules does not risk altering your namespace.
3810 In other words, if you try this:
3812 require Foo::Bar; # a splendid bareword
3814 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3815 directories specified in the C<@INC> array.
3817 But if you try this:
3819 $class = 'Foo::Bar';
3820 require $class; # $class is not a bareword
3822 require "Foo::Bar"; # not a bareword because of the ""
3824 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3825 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3827 eval "require $class";
3829 You can also insert hooks into the import facility, by putting directly
3830 Perl code into the @INC array. There are three forms of hooks: subroutine
3831 references, array references and blessed objects.
3833 Subroutine references are the simplest case. When the inclusion system
3834 walks through @INC and encounters a subroutine, this subroutine gets
3835 called with two parameters, the first being a reference to itself, and the
3836 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
3837 subroutine should return C<undef> or a filehandle, from which the file to
3838 include will be read. If C<undef> is returned, C<require> will look at
3839 the remaining elements of @INC.
3841 If the hook is an array reference, its first element must be a subroutine
3842 reference. This subroutine is called as above, but the first parameter is
3843 the array reference. This enables to pass indirectly some arguments to
3846 In other words, you can write:
3848 push @INC, \&my_sub;
3850 my ($coderef, $filename) = @_; # $coderef is \&my_sub
3856 push @INC, [ \&my_sub, $x, $y, ... ];
3858 my ($arrayref, $filename) = @_;
3859 # Retrieve $x, $y, ...
3860 my @parameters = @$arrayref[1..$#$arrayref];
3864 If the hook is an object, it must provide an INC method, that will be
3865 called as above, the first parameter being the object itself. (Note that
3866 you must fully qualify the sub's name, as it is always forced into package
3867 C<main>.) Here is a typical code layout:
3873 my ($self, $filename) = @_;
3877 # In the main program
3878 push @INC, new Foo(...);
3880 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3886 Generally used in a C<continue> block at the end of a loop to clear
3887 variables and reset C<??> searches so that they work again. The
3888 expression is interpreted as a list of single characters (hyphens
3889 allowed for ranges). All variables and arrays beginning with one of
3890 those letters are reset to their pristine state. If the expression is
3891 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3892 only variables or searches in the current package. Always returns
3895 reset 'X'; # reset all X variables
3896 reset 'a-z'; # reset lower case variables
3897 reset; # just reset ?one-time? searches
3899 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3900 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3901 variables--lexical variables are unaffected, but they clean themselves
3902 up on scope exit anyway, so you'll probably want to use them instead.
3909 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3910 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3911 context, depending on how the return value will be used, and the context
3912 may vary from one execution to the next (see C<wantarray>). If no EXPR
3913 is given, returns an empty list in list context, the undefined value in
3914 scalar context, and (of course) nothing at all in a void context.
3916 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3917 or do FILE will automatically return the value of the last expression
3922 In list context, returns a list value consisting of the elements
3923 of LIST in the opposite order. In scalar context, concatenates the
3924 elements of LIST and returns a string value with all characters
3925 in the opposite order.
3927 print reverse <>; # line tac, last line first
3929 undef $/; # for efficiency of <>
3930 print scalar reverse <>; # character tac, last line tsrif
3932 This operator is also handy for inverting a hash, although there are some
3933 caveats. If a value is duplicated in the original hash, only one of those
3934 can be represented as a key in the inverted hash. Also, this has to
3935 unwind one hash and build a whole new one, which may take some time
3936 on a large hash, such as from a DBM file.
3938 %by_name = reverse %by_address; # Invert the hash
3940 =item rewinddir DIRHANDLE
3942 Sets the current position to the beginning of the directory for the
3943 C<readdir> routine on DIRHANDLE.
3945 =item rindex STR,SUBSTR,POSITION
3947 =item rindex STR,SUBSTR
3949 Works just like index() except that it returns the position of the LAST
3950 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3951 last occurrence at or before that position.
3953 =item rmdir FILENAME
3957 Deletes the directory specified by FILENAME if that directory is empty. If it
3958 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3959 FILENAME is omitted, uses C<$_>.
3963 The substitution operator. See L<perlop>.
3967 Forces EXPR to be interpreted in scalar context and returns the value
3970 @counts = ( scalar @a, scalar @b, scalar @c );
3972 There is no equivalent operator to force an expression to
3973 be interpolated in list context because in practice, this is never
3974 needed. If you really wanted to do so, however, you could use
3975 the construction C<@{[ (some expression) ]}>, but usually a simple
3976 C<(some expression)> suffices.
3978 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3979 parenthesized list, this behaves as a scalar comma expression, evaluating
3980 all but the last element in void context and returning the final element
3981 evaluated in scalar context. This is seldom what you want.
3983 The following single statement:
3985 print uc(scalar(&foo,$bar)),$baz;
3987 is the moral equivalent of these two:
3990 print(uc($bar),$baz);
3992 See L<perlop> for more details on unary operators and the comma operator.
3994 =item seek FILEHANDLE,POSITION,WHENCE
3996 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3997 FILEHANDLE may be an expression whose value gives the name of the
3998 filehandle. The values for WHENCE are C<0> to set the new position to
3999 POSITION, C<1> to set it to the current position plus POSITION, and
4000 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
4001 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
4002 (start of the file, current position, end of the file) from the Fcntl
4003 module. Returns C<1> upon success, C<0> otherwise.
4005 If you want to position file for C<sysread> or C<syswrite>, don't use
4006 C<seek>--buffering makes its effect on the file's system position
4007 unpredictable and non-portable. Use C<sysseek> instead.
4009 Due to the rules and rigors of ANSI C, on some systems you have to do a
4010 seek whenever you switch between reading and writing. Amongst other
4011 things, this may have the effect of calling stdio's clearerr(3).
4012 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4016 This is also useful for applications emulating C<tail -f>. Once you hit
4017 EOF on your read, and then sleep for a while, you might have to stick in a
4018 seek() to reset things. The C<seek> doesn't change the current position,
4019 but it I<does> clear the end-of-file condition on the handle, so that the
4020 next C<< <FILE> >> makes Perl try again to read something. We hope.
4022 If that doesn't work (some stdios are particularly cantankerous), then
4023 you may need something more like this:
4026 for ($curpos = tell(FILE); $_ = <FILE>;
4027 $curpos = tell(FILE)) {
4028 # search for some stuff and put it into files
4030 sleep($for_a_while);
4031 seek(FILE, $curpos, 0);
4034 =item seekdir DIRHANDLE,POS
4036 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4037 must be a value returned by C<telldir>. Has the same caveats about
4038 possible directory compaction as the corresponding system library
4041 =item select FILEHANDLE
4045 Returns the currently selected filehandle. Sets the current default
4046 filehandle for output, if FILEHANDLE is supplied. This has two
4047 effects: first, a C<write> or a C<print> without a filehandle will
4048 default to this FILEHANDLE. Second, references to variables related to
4049 output will refer to this output channel. For example, if you have to
4050 set the top of form format for more than one output channel, you might
4058 FILEHANDLE may be an expression whose value gives the name of the
4059 actual filehandle. Thus:
4061 $oldfh = select(STDERR); $| = 1; select($oldfh);
4063 Some programmers may prefer to think of filehandles as objects with
4064 methods, preferring to write the last example as:
4067 STDERR->autoflush(1);
4069 =item select RBITS,WBITS,EBITS,TIMEOUT
4071 This calls the select(2) system call with the bit masks specified, which
4072 can be constructed using C<fileno> and C<vec>, along these lines:
4074 $rin = $win = $ein = '';
4075 vec($rin,fileno(STDIN),1) = 1;
4076 vec($win,fileno(STDOUT),1) = 1;
4079 If you want to select on many filehandles you might wish to write a
4083 my(@fhlist) = split(' ',$_[0]);
4086 vec($bits,fileno($_),1) = 1;
4090 $rin = fhbits('STDIN TTY SOCK');
4094 ($nfound,$timeleft) =
4095 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4097 or to block until something becomes ready just do this
4099 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4101 Most systems do not bother to return anything useful in $timeleft, so
4102 calling select() in scalar context just returns $nfound.
4104 Any of the bit masks can also be undef. The timeout, if specified, is
4105 in seconds, which may be fractional. Note: not all implementations are
4106 capable of returning the$timeleft. If not, they always return
4107 $timeleft equal to the supplied $timeout.
4109 You can effect a sleep of 250 milliseconds this way:
4111 select(undef, undef, undef, 0.25);
4113 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4114 or <FH>) with C<select>, except as permitted by POSIX, and even
4115 then only on POSIX systems. You have to use C<sysread> instead.
4117 =item semctl ID,SEMNUM,CMD,ARG
4119 Calls the System V IPC function C<semctl>. You'll probably have to say
4123 first to get the correct constant definitions. If CMD is IPC_STAT or
4124 GETALL, then ARG must be a variable which will hold the returned
4125 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4126 the undefined value for error, "C<0 but true>" for zero, or the actual
4127 return value otherwise. The ARG must consist of a vector of native
4128 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4129 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4132 =item semget KEY,NSEMS,FLAGS
4134 Calls the System V IPC function semget. Returns the semaphore id, or
4135 the undefined value if there is an error. See also
4136 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4139 =item semop KEY,OPSTRING
4141 Calls the System V IPC function semop to perform semaphore operations
4142 such as signalling and waiting. OPSTRING must be a packed array of
4143 semop structures. Each semop structure can be generated with
4144 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4145 operations is implied by the length of OPSTRING. Returns true if
4146 successful, or false if there is an error. As an example, the
4147 following code waits on semaphore $semnum of semaphore id $semid:
4149 $semop = pack("s!3", $semnum, -1, 0);
4150 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4152 To signal the semaphore, replace C<-1> with C<1>. See also
4153 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4156 =item send SOCKET,MSG,FLAGS,TO
4158 =item send SOCKET,MSG,FLAGS
4160 Sends a message on a socket. Takes the same flags as the system call
4161 of the same name. On unconnected sockets you must specify a
4162 destination to send TO, in which case it does a C C<sendto>. Returns
4163 the number of characters sent, or the undefined value if there is an
4164 error. The C system call sendmsg(2) is currently unimplemented.
4165 See L<perlipc/"UDP: Message Passing"> for examples.
4167 =item setpgrp PID,PGRP
4169 Sets the current process group for the specified PID, C<0> for the current
4170 process. Will produce a fatal error if used on a machine that doesn't
4171 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4172 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4173 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4176 =item setpriority WHICH,WHO,PRIORITY
4178 Sets the current priority for a process, a process group, or a user.
4179 (See setpriority(2).) Will produce a fatal error if used on a machine
4180 that doesn't implement setpriority(2).
4182 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4184 Sets the socket option requested. Returns undefined if there is an
4185 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4192 Shifts the first value of the array off and returns it, shortening the
4193 array by 1 and moving everything down. If there are no elements in the
4194 array, returns the undefined value. If ARRAY is omitted, shifts the
4195 C<@_> array within the lexical scope of subroutines and formats, and the
4196 C<@ARGV> array at file scopes or within the lexical scopes established by
4197 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4200 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4201 same thing to the left end of an array that C<pop> and C<push> do to the
4204 =item shmctl ID,CMD,ARG
4206 Calls the System V IPC function shmctl. You'll probably have to say
4210 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4211 then ARG must be a variable which will hold the returned C<shmid_ds>
4212 structure. Returns like ioctl: the undefined value for error, "C<0> but
4213 true" for zero, or the actual return value otherwise.
4214 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4216 =item shmget KEY,SIZE,FLAGS
4218 Calls the System V IPC function shmget. Returns the shared memory
4219 segment id, or the undefined value if there is an error.
4220 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4222 =item shmread ID,VAR,POS,SIZE
4224 =item shmwrite ID,STRING,POS,SIZE
4226 Reads or writes the System V shared memory segment ID starting at
4227 position POS for size SIZE by attaching to it, copying in/out, and
4228 detaching from it. When reading, VAR must be a variable that will
4229 hold the data read. When writing, if STRING is too long, only SIZE
4230 bytes are used; if STRING is too short, nulls are written to fill out
4231 SIZE bytes. Return true if successful, or false if there is an error.
4232 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4233 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4235 =item shutdown SOCKET,HOW
4237 Shuts down a socket connection in the manner indicated by HOW, which
4238 has the same interpretation as in the system call of the same name.
4240 shutdown(SOCKET, 0); # I/we have stopped reading data
4241 shutdown(SOCKET, 1); # I/we have stopped writing data
4242 shutdown(SOCKET, 2); # I/we have stopped using this socket
4244 This is useful with sockets when you want to tell the other
4245 side you're done writing but not done reading, or vice versa.
4246 It's also a more insistent form of close because it also
4247 disables the file descriptor in any forked copies in other
4254 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4255 returns sine of C<$_>.
4257 For the inverse sine operation, you may use the C<Math::Trig::asin>
4258 function, or use this relation:
4260 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4266 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4267 May be interrupted if the process receives a signal such as C<SIGALRM>.
4268 Returns the number of seconds actually slept. You probably cannot
4269 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4272 On some older systems, it may sleep up to a full second less than what
4273 you requested, depending on how it counts seconds. Most modern systems
4274 always sleep the full amount. They may appear to sleep longer than that,
4275 however, because your process might not be scheduled right away in a
4276 busy multitasking system.
4278 For delays of finer granularity than one second, you may use Perl's
4279 C<syscall> interface to access setitimer(2) if your system supports
4280 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4281 and starting from Perl 5.8 part of the standard distribution) may also
4284 See also the POSIX module's C<pause> function.
4286 =item sockatmark SOCKET
4288 Returns true if the socket is positioned at the out-of-band mark
4289 (also known as the urgent data mark), false otherwise. Use right
4290 after reading from the socket.
4292 Not available directly, one has to import the function from
4293 the IO::Socket extension
4295 use IO::Socket 'sockatmark';
4297 Even this doesn't guarantee that sockatmark() really is available,
4298 though, because sockatmark() is a relatively recent addition to
4299 the family of socket functions. If it is unavailable, attempt to
4302 IO::Socket::atmark not implemented on this architecture ...
4304 See also L<IO::Socket>.
4306 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4308 Opens a socket of the specified kind and attaches it to filehandle
4309 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4310 the system call of the same name. You should C<use Socket> first
4311 to get the proper definitions imported. See the examples in
4312 L<perlipc/"Sockets: Client/Server Communication">.
4314 On systems that support a close-on-exec flag on files, the flag will
4315 be set for the newly opened file descriptor, as determined by the
4316 value of $^F. See L<perlvar/$^F>.
4318 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4320 Creates an unnamed pair of sockets in the specified domain, of the
4321 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4322 for the system call of the same name. If unimplemented, yields a fatal
4323 error. Returns true if successful.
4325 On systems that support a close-on-exec flag on files, the flag will
4326 be set for the newly opened file descriptors, as determined by the value
4327 of $^F. See L<perlvar/$^F>.
4329 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4330 to C<pipe(Rdr, Wtr)> is essentially:
4333 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4334 shutdown(Rdr, 1); # no more writing for reader
4335 shutdown(Wtr, 0); # no more reading for writer
4337 See L<perlipc> for an example of socketpair use.
4339 =item sort SUBNAME LIST
4341 =item sort BLOCK LIST
4345 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4346 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4347 specified, it gives the name of a subroutine that returns an integer
4348 less than, equal to, or greater than C<0>, depending on how the elements
4349 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4350 operators are extremely useful in such routines.) SUBNAME may be a
4351 scalar variable name (unsubscripted), in which case the value provides
4352 the name of (or a reference to) the actual subroutine to use. In place
4353 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4356 If the subroutine's prototype is C<($$)>, the elements to be compared
4357 are passed by reference in C<@_>, as for a normal subroutine. This is
4358 slower than unprototyped subroutines, where the elements to be
4359 compared are passed into the subroutine
4360 as the package global variables $a and $b (see example below). Note that
4361 in the latter case, it is usually counter-productive to declare $a and
4364 In either case, the subroutine may not be recursive. The values to be
4365 compared are always passed by reference, so don't modify them.
4367 You also cannot exit out of the sort block or subroutine using any of the
4368 loop control operators described in L<perlsyn> or with C<goto>.
4370 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4371 current collation locale. See L<perllocale>.
4373 Perl does B<not> guarantee that sort is stable. (A I<stable> sort
4374 preserves the input order of elements that compare equal.) 5.7 and
4375 5.8 happen to use a stable mergesort, but 5.6 and earlier used quicksort,
4376 which is not stable. Do not assume that future perls will continue to
4382 @articles = sort @files;
4384 # same thing, but with explicit sort routine
4385 @articles = sort {$a cmp $b} @files;
4387 # now case-insensitively
4388 @articles = sort {uc($a) cmp uc($b)} @files;
4390 # same thing in reversed order
4391 @articles = sort {$b cmp $a} @files;
4393 # sort numerically ascending
4394 @articles = sort {$a <=> $b} @files;
4396 # sort numerically descending
4397 @articles = sort {$b <=> $a} @files;
4399 # this sorts the %age hash by value instead of key
4400 # using an in-line function
4401 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4403 # sort using explicit subroutine name
4405 $age{$a} <=> $age{$b}; # presuming numeric
4407 @sortedclass = sort byage @class;
4409 sub backwards { $b cmp $a }
4410 @harry = qw(dog cat x Cain Abel);
4411 @george = qw(gone chased yz Punished Axed);
4413 # prints AbelCaincatdogx
4414 print sort backwards @harry;
4415 # prints xdogcatCainAbel
4416 print sort @george, 'to', @harry;
4417 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4419 # inefficiently sort by descending numeric compare using
4420 # the first integer after the first = sign, or the
4421 # whole record case-insensitively otherwise
4424 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4429 # same thing, but much more efficiently;
4430 # we'll build auxiliary indices instead
4434 push @nums, /=(\d+)/;
4439 $nums[$b] <=> $nums[$a]
4441 $caps[$a] cmp $caps[$b]
4445 # same thing, but without any temps
4446 @new = map { $_->[0] }
4447 sort { $b->[1] <=> $a->[1]
4450 } map { [$_, /=(\d+)/, uc($_)] } @old;
4452 # using a prototype allows you to use any comparison subroutine
4453 # as a sort subroutine (including other package's subroutines)
4455 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4458 @new = sort other::backwards @old;
4460 If you're using strict, you I<must not> declare $a
4461 and $b as lexicals. They are package globals. That means
4462 if you're in the C<main> package and type
4464 @articles = sort {$b <=> $a} @files;
4466 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4467 but if you're in the C<FooPack> package, it's the same as typing
4469 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4471 The comparison function is required to behave. If it returns
4472 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4473 sometimes saying the opposite, for example) the results are not
4476 =item splice ARRAY,OFFSET,LENGTH,LIST
4478 =item splice ARRAY,OFFSET,LENGTH
4480 =item splice ARRAY,OFFSET
4484 Removes the elements designated by OFFSET and LENGTH from an array, and
4485 replaces them with the elements of LIST, if any. In list context,
4486 returns the elements removed from the array. In scalar context,
4487 returns the last element removed, or C<undef> if no elements are
4488 removed. The array grows or shrinks as necessary.
4489 If OFFSET is negative then it starts that far from the end of the array.
4490 If LENGTH is omitted, removes everything from OFFSET onward.
4491 If LENGTH is negative, leaves that many elements off the end of the array.
4492 If both OFFSET and LENGTH are omitted, removes everything.
4494 The following equivalences hold (assuming C<$[ == 0>):
4496 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4497 pop(@a) splice(@a,-1)
4498 shift(@a) splice(@a,0,1)
4499 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4500 $a[$x] = $y splice(@a,$x,1,$y)
4502 Example, assuming array lengths are passed before arrays:
4504 sub aeq { # compare two list values
4505 my(@a) = splice(@_,0,shift);
4506 my(@b) = splice(@_,0,shift);
4507 return 0 unless @a == @b; # same len?
4509 return 0 if pop(@a) ne pop(@b);
4513 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4515 =item split /PATTERN/,EXPR,LIMIT
4517 =item split /PATTERN/,EXPR
4519 =item split /PATTERN/
4523 Splits a string into a list of strings and returns that list. By default,
4524 empty leading fields are preserved, and empty trailing ones are deleted.
4526 In scalar context, returns the number of fields found and splits into
4527 the C<@_> array. Use of split in scalar context is deprecated, however,
4528 because it clobbers your subroutine arguments.
4530 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4531 splits on whitespace (after skipping any leading whitespace). Anything
4532 matching PATTERN is taken to be a delimiter separating the fields. (Note
4533 that the delimiter may be longer than one character.)
4535 If LIMIT is specified and positive, it represents the maximum number
4536 of fields the EXPR will be split into, though the actual number of
4537 fields returned depends on the number of times PATTERN matches within
4538 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4539 stripped (which potential users of C<pop> would do well to remember).
4540 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4541 had been specified. Note that splitting an EXPR that evaluates to the
4542 empty string always returns the empty list, regardless of the LIMIT
4545 A pattern matching the null string (not to be confused with
4546 a null pattern C<//>, which is just one member of the set of patterns
4547 matching a null string) will split the value of EXPR into separate
4548 characters at each point it matches that way. For example:
4550 print join(':', split(/ */, 'hi there'));
4552 produces the output 'h:i:t:h:e:r:e'.
4554 Using the empty pattern C<//> specifically matches the null string, and is
4555 not be confused with the use of C<//> to mean "the last successful pattern
4558 Empty leading (or trailing) fields are produced when there positive width
4559 matches at the beginning (or end) of the string; a zero-width match at the
4560 beginning (or end) of the string does not produce an empty field. For
4563 print join(':', split(/(?=\w)/, 'hi there!'));
4565 produces the output 'h:i :t:h:e:r:e!'.
4567 The LIMIT parameter can be used to split a line partially
4569 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4571 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4572 one larger than the number of variables in the list, to avoid
4573 unnecessary work. For the list above LIMIT would have been 4 by
4574 default. In time critical applications it behooves you not to split
4575 into more fields than you really need.
4577 If the PATTERN contains parentheses, additional list elements are
4578 created from each matching substring in the delimiter.
4580 split(/([,-])/, "1-10,20", 3);
4582 produces the list value
4584 (1, '-', 10, ',', 20)
4586 If you had the entire header of a normal Unix email message in $header,
4587 you could split it up into fields and their values this way:
4589 $header =~ s/\n\s+/ /g; # fix continuation lines
4590 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4592 The pattern C</PATTERN/> may be replaced with an expression to specify
4593 patterns that vary at runtime. (To do runtime compilation only once,
4594 use C</$variable/o>.)
4596 As a special case, specifying a PATTERN of space (C<' '>) will split on
4597 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4598 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4599 will give you as many null initial fields as there are leading spaces.
4600 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4601 whitespace produces a null first field. A C<split> with no arguments
4602 really does a C<split(' ', $_)> internally.
4604 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4609 open(PASSWD, '/etc/passwd');
4612 ($login, $passwd, $uid, $gid,
4613 $gcos, $home, $shell) = split(/:/);
4617 As with regular pattern matching, any capturing parentheses that are not
4618 matched in a C<split()> will be set to C<undef> when returned:
4620 @fields = split /(A)|B/, "1A2B3";
4621 # @fields is (1, 'A', 2, undef, 3)
4623 =item sprintf FORMAT, LIST
4625 Returns a string formatted by the usual C<printf> conventions of the C
4626 library function C<sprintf>. See below for more details
4627 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4628 the general principles.
4632 # Format number with up to 8 leading zeroes
4633 $result = sprintf("%08d", $number);
4635 # Round number to 3 digits after decimal point
4636 $rounded = sprintf("%.3f", $number);
4638 Perl does its own C<sprintf> formatting--it emulates the C
4639 function C<sprintf>, but it doesn't use it (except for floating-point
4640 numbers, and even then only the standard modifiers are allowed). As a
4641 result, any non-standard extensions in your local C<sprintf> are not
4642 available from Perl.
4644 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4645 pass it an array as your first argument. The array is given scalar context,
4646 and instead of using the 0th element of the array as the format, Perl will
4647 use the count of elements in the array as the format, which is almost never
4650 Perl's C<sprintf> permits the following universally-known conversions:
4653 %c a character with the given number
4655 %d a signed integer, in decimal
4656 %u an unsigned integer, in decimal
4657 %o an unsigned integer, in octal
4658 %x an unsigned integer, in hexadecimal
4659 %e a floating-point number, in scientific notation
4660 %f a floating-point number, in fixed decimal notation
4661 %g a floating-point number, in %e or %f notation
4663 In addition, Perl permits the following widely-supported conversions:
4665 %X like %x, but using upper-case letters
4666 %E like %e, but using an upper-case "E"
4667 %G like %g, but with an upper-case "E" (if applicable)
4668 %b an unsigned integer, in binary
4669 %p a pointer (outputs the Perl value's address in hexadecimal)
4670 %n special: *stores* the number of characters output so far
4671 into the next variable in the parameter list
4673 Finally, for backward (and we do mean "backward") compatibility, Perl
4674 permits these unnecessary but widely-supported conversions:
4677 %D a synonym for %ld
4678 %U a synonym for %lu
4679 %O a synonym for %lo
4682 Note that the number of exponent digits in the scientific notation by
4683 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4684 exponent less than 100 is system-dependent: it may be three or less
4685 (zero-padded as necessary). In other words, 1.23 times ten to the
4686 99th may be either "1.23e99" or "1.23e099".
4688 Perl permits the following universally-known flags between the C<%>
4689 and the conversion letter:
4691 space prefix positive number with a space
4692 + prefix positive number with a plus sign
4693 - left-justify within the field
4694 0 use zeros, not spaces, to right-justify
4695 # prefix non-zero octal with "0", non-zero hex with "0x"
4696 number minimum field width
4697 .number "precision": digits after decimal point for
4698 floating-point, max length for string, minimum length
4700 l interpret integer as C type "long" or "unsigned long"
4701 h interpret integer as C type "short" or "unsigned short"
4702 If no flags, interpret integer as C type "int" or "unsigned"
4704 Perl supports parameter ordering, in other words, fetching the
4705 parameters in some explicitly specified "random" ordering as opposed
4706 to the default implicit sequential ordering. The syntax is, instead
4707 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4708 where the I<digits> is the wanted index, from one upwards. For example:
4710 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4711 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4713 Note that using the reordering syntax does not interfere with the usual
4714 implicit sequential fetching of the parameters:
4716 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4717 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4718 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4719 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4720 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4722 There are also two Perl-specific flags:
4724 V interpret integer as Perl's standard integer type
4725 v interpret string as a vector of integers, output as
4726 numbers separated either by dots, or by an arbitrary
4727 string received from the argument list when the flag
4730 Where a number would appear in the flags, an asterisk (C<*>) may be
4731 used instead, in which case Perl uses the next item in the parameter
4732 list as the given number (that is, as the field width or precision).
4733 If a field width obtained through C<*> is negative, it has the same
4734 effect as the C<-> flag: left-justification.
4736 The C<v> flag is useful for displaying ordinal values of characters
4737 in arbitrary strings:
4739 printf "version is v%vd\n", $^V; # Perl's version
4740 printf "address is %*vX\n", ":", $addr; # IPv6 address
4741 printf "bits are %*vb\n", " ", $bits; # random bitstring
4743 If C<use locale> is in effect, the character used for the decimal
4744 point in formatted real numbers is affected by the LC_NUMERIC locale.
4747 If Perl understands "quads" (64-bit integers) (this requires
4748 either that the platform natively support quads or that Perl
4749 be specifically compiled to support quads), the characters
4753 print quads, and they may optionally be preceded by
4761 You can find out whether your Perl supports quads via L<Config>:
4764 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4767 If Perl understands "long doubles" (this requires that the platform
4768 support long doubles), the flags
4772 may optionally be preceded by
4780 You can find out whether your Perl supports long doubles via L<Config>:
4783 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4789 Return the square root of EXPR. If EXPR is omitted, returns square
4790 root of C<$_>. Only works on non-negative operands, unless you've
4791 loaded the standard Math::Complex module.
4794 print sqrt(-2); # prints 1.4142135623731i
4800 Sets the random number seed for the C<rand> operator.
4802 The point of the function is to "seed" the C<rand> function so that
4803 C<rand> can produce a different sequence each time you run your
4806 If srand() is not called explicitly, it is called implicitly at the
4807 first use of the C<rand> operator. However, this was not the case in
4808 versions of Perl before 5.004, so if your script will run under older
4809 Perl versions, it should call C<srand>.
4811 Most programs won't even call srand() at all, except those that
4812 need a cryptographically-strong starting point rather than the
4813 generally acceptable default, which is based on time of day,
4814 process ID, and memory allocation, or the F</dev/urandom> device,
4817 You can call srand($seed) with the same $seed to reproduce the
4818 I<same> sequence from rand(), but this is usually reserved for
4819 generating predictable results for testing or debugging.
4820 Otherwise, don't call srand() more than once in your program.
4822 Do B<not> call srand() (i.e. without an argument) more than once in
4823 a script. The internal state of the random number generator should
4824 contain more entropy than can be provided by any seed, so calling
4825 srand() again actually I<loses> randomness.
4827 Most implementations of C<srand> take an integer and will silently
4828 truncate decimal numbers. This means C<srand(42)> will usually
4829 produce the same results as C<srand(42.1)>. To be safe, always pass
4830 C<srand> an integer.
4832 In versions of Perl prior to 5.004 the default seed was just the
4833 current C<time>. This isn't a particularly good seed, so many old
4834 programs supply their own seed value (often C<time ^ $$> or C<time ^
4835 ($$ + ($$ << 15))>), but that isn't necessary any more.
4837 Note that you need something much more random than the default seed for
4838 cryptographic purposes. Checksumming the compressed output of one or more
4839 rapidly changing operating system status programs is the usual method. For
4842 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4844 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4847 Frequently called programs (like CGI scripts) that simply use
4851 for a seed can fall prey to the mathematical property that
4855 one-third of the time. So don't do that.
4857 =item stat FILEHANDLE
4863 Returns a 13-element list giving the status info for a file, either
4864 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4865 it stats C<$_>. Returns a null list if the stat fails. Typically used
4868 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4869 $atime,$mtime,$ctime,$blksize,$blocks)
4872 Not all fields are supported on all filesystem types. Here are the
4873 meaning of the fields:
4875 0 dev device number of filesystem
4877 2 mode file mode (type and permissions)
4878 3 nlink number of (hard) links to the file
4879 4 uid numeric user ID of file's owner
4880 5 gid numeric group ID of file's owner
4881 6 rdev the device identifier (special files only)
4882 7 size total size of file, in bytes
4883 8 atime last access time in seconds since the epoch
4884 9 mtime last modify time in seconds since the epoch
4885 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4886 11 blksize preferred block size for file system I/O
4887 12 blocks actual number of blocks allocated
4889 (The epoch was at 00:00 January 1, 1970 GMT.)
4891 If stat is passed the special filehandle consisting of an underline, no
4892 stat is done, but the current contents of the stat structure from the
4893 last stat or filetest are returned. Example:
4895 if (-x $file && (($d) = stat(_)) && $d < 0) {
4896 print "$file is executable NFS file\n";
4899 (This works on machines only for which the device number is negative
4902 Because the mode contains both the file type and its permissions, you
4903 should mask off the file type portion and (s)printf using a C<"%o">
4904 if you want to see the real permissions.
4906 $mode = (stat($filename))[2];
4907 printf "Permissions are %04o\n", $mode & 07777;
4909 In scalar context, C<stat> returns a boolean value indicating success
4910 or failure, and, if successful, sets the information associated with
4911 the special filehandle C<_>.
4913 The File::stat module provides a convenient, by-name access mechanism:
4916 $sb = stat($filename);
4917 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4918 $filename, $sb->size, $sb->mode & 07777,
4919 scalar localtime $sb->mtime;
4921 You can import symbolic mode constants (C<S_IF*>) and functions
4922 (C<S_IS*>) from the Fcntl module:
4926 $mode = (stat($filename))[2];
4928 $user_rwx = ($mode & S_IRWXU) >> 6;
4929 $group_read = ($mode & S_IRGRP) >> 3;
4930 $other_execute = $mode & S_IXOTH;
4932 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4934 $is_setuid = $mode & S_ISUID;
4935 $is_setgid = S_ISDIR($mode);
4937 You could write the last two using the C<-u> and C<-d> operators.
4938 The commonly available S_IF* constants are
4940 # Permissions: read, write, execute, for user, group, others.
4942 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4943 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4944 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4946 # Setuid/Setgid/Stickiness.
4948 S_ISUID S_ISGID S_ISVTX S_ISTXT
4950 # File types. Not necessarily all are available on your system.
4952 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4954 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4956 S_IREAD S_IWRITE S_IEXEC
4958 and the S_IF* functions are
4960 S_IFMODE($mode) the part of $mode containing the permission bits
4961 and the setuid/setgid/sticky bits
4963 S_IFMT($mode) the part of $mode containing the file type
4964 which can be bit-anded with e.g. S_IFREG
4965 or with the following functions
4967 # The operators -f, -d, -l, -b, -c, -p, and -s.
4969 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4970 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4972 # No direct -X operator counterpart, but for the first one
4973 # the -g operator is often equivalent. The ENFMT stands for
4974 # record flocking enforcement, a platform-dependent feature.
4976 S_ISENFMT($mode) S_ISWHT($mode)
4978 See your native chmod(2) and stat(2) documentation for more details
4979 about the S_* constants.
4985 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4986 doing many pattern matches on the string before it is next modified.
4987 This may or may not save time, depending on the nature and number of
4988 patterns you are searching on, and on the distribution of character
4989 frequencies in the string to be searched--you probably want to compare
4990 run times with and without it to see which runs faster. Those loops
4991 which scan for many short constant strings (including the constant
4992 parts of more complex patterns) will benefit most. You may have only
4993 one C<study> active at a time--if you study a different scalar the first
4994 is "unstudied". (The way C<study> works is this: a linked list of every
4995 character in the string to be searched is made, so we know, for
4996 example, where all the C<'k'> characters are. From each search string,
4997 the rarest character is selected, based on some static frequency tables
4998 constructed from some C programs and English text. Only those places
4999 that contain this "rarest" character are examined.)
5001 For example, here is a loop that inserts index producing entries
5002 before any line containing a certain pattern:
5006 print ".IX foo\n" if /\bfoo\b/;
5007 print ".IX bar\n" if /\bbar\b/;
5008 print ".IX blurfl\n" if /\bblurfl\b/;
5013 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5014 will be looked at, because C<f> is rarer than C<o>. In general, this is
5015 a big win except in pathological cases. The only question is whether
5016 it saves you more time than it took to build the linked list in the
5019 Note that if you have to look for strings that you don't know till
5020 runtime, you can build an entire loop as a string and C<eval> that to
5021 avoid recompiling all your patterns all the time. Together with
5022 undefining C<$/> to input entire files as one record, this can be very
5023 fast, often faster than specialized programs like fgrep(1). The following
5024 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5025 out the names of those files that contain a match:
5027 $search = 'while (<>) { study;';
5028 foreach $word (@words) {
5029 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5034 eval $search; # this screams
5035 $/ = "\n"; # put back to normal input delimiter
5036 foreach $file (sort keys(%seen)) {
5044 =item sub NAME BLOCK
5046 This is subroutine definition, not a real function I<per se>. With just a
5047 NAME (and possibly prototypes or attributes), it's just a forward declaration.
5048 Without a NAME, it's an anonymous function declaration, and does actually
5049 return a value: the CODE ref of the closure you just created. See L<perlsub>
5050 and L<perlref> for details.
5052 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5054 =item substr EXPR,OFFSET,LENGTH
5056 =item substr EXPR,OFFSET
5058 Extracts a substring out of EXPR and returns it. First character is at
5059 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5060 If OFFSET is negative (or more precisely, less than C<$[>), starts
5061 that far from the end of the string. If LENGTH is omitted, returns
5062 everything to the end of the string. If LENGTH is negative, leaves that
5063 many characters off the end of the string.
5065 You can use the substr() function as an lvalue, in which case EXPR
5066 must itself be an lvalue. If you assign something shorter than LENGTH,
5067 the string will shrink, and if you assign something longer than LENGTH,
5068 the string will grow to accommodate it. To keep the string the same
5069 length you may need to pad or chop your value using C<sprintf>.
5071 If OFFSET and LENGTH specify a substring that is partly outside the
5072 string, only the part within the string is returned. If the substring
5073 is beyond either end of the string, substr() returns the undefined
5074 value and produces a warning. When used as an lvalue, specifying a
5075 substring that is entirely outside the string is a fatal error.
5076 Here's an example showing the behavior for boundary cases:
5079 substr($name, 4) = 'dy'; # $name is now 'freddy'
5080 my $null = substr $name, 6, 2; # returns '' (no warning)
5081 my $oops = substr $name, 7; # returns undef, with warning
5082 substr($name, 7) = 'gap'; # fatal error
5084 An alternative to using substr() as an lvalue is to specify the
5085 replacement string as the 4th argument. This allows you to replace
5086 parts of the EXPR and return what was there before in one operation,
5087 just as you can with splice().
5089 =item symlink OLDFILE,NEWFILE
5091 Creates a new filename symbolically linked to the old filename.
5092 Returns C<1> for success, C<0> otherwise. On systems that don't support
5093 symbolic links, produces a fatal error at run time. To check for that,
5096 $symlink_exists = eval { symlink("",""); 1 };
5100 Calls the system call specified as the first element of the list,
5101 passing the remaining elements as arguments to the system call. If
5102 unimplemented, produces a fatal error. The arguments are interpreted
5103 as follows: if a given argument is numeric, the argument is passed as
5104 an int. If not, the pointer to the string value is passed. You are
5105 responsible to make sure a string is pre-extended long enough to
5106 receive any result that might be written into a string. You can't use a
5107 string literal (or other read-only string) as an argument to C<syscall>
5108 because Perl has to assume that any string pointer might be written
5110 integer arguments are not literals and have never been interpreted in a
5111 numeric context, you may need to add C<0> to them to force them to look
5112 like numbers. This emulates the C<syswrite> function (or vice versa):
5114 require 'syscall.ph'; # may need to run h2ph
5116 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5118 Note that Perl supports passing of up to only 14 arguments to your system call,
5119 which in practice should usually suffice.
5121 Syscall returns whatever value returned by the system call it calls.
5122 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5123 Note that some system calls can legitimately return C<-1>. The proper
5124 way to handle such calls is to assign C<$!=0;> before the call and
5125 check the value of C<$!> if syscall returns C<-1>.
5127 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5128 number of the read end of the pipe it creates. There is no way
5129 to retrieve the file number of the other end. You can avoid this
5130 problem by using C<pipe> instead.
5132 =item sysopen FILEHANDLE,FILENAME,MODE
5134 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5136 Opens the file whose filename is given by FILENAME, and associates it
5137 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5138 the name of the real filehandle wanted. This function calls the
5139 underlying operating system's C<open> function with the parameters
5140 FILENAME, MODE, PERMS.
5142 The possible values and flag bits of the MODE parameter are
5143 system-dependent; they are available via the standard module C<Fcntl>.
5144 See the documentation of your operating system's C<open> to see which
5145 values and flag bits are available. You may combine several flags
5146 using the C<|>-operator.
5148 Some of the most common values are C<O_RDONLY> for opening the file in
5149 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5150 and C<O_RDWR> for opening the file in read-write mode, and.
5152 For historical reasons, some values work on almost every system
5153 supported by perl: zero means read-only, one means write-only, and two
5154 means read/write. We know that these values do I<not> work under
5155 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5156 use them in new code.
5158 If the file named by FILENAME does not exist and the C<open> call creates
5159 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5160 PERMS specifies the permissions of the newly created file. If you omit
5161 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5162 These permission values need to be in octal, and are modified by your
5163 process's current C<umask>.
5165 In many systems the C<O_EXCL> flag is available for opening files in
5166 exclusive mode. This is B<not> locking: exclusiveness means here that
5167 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5170 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5172 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5173 that takes away the user's option to have a more permissive umask.
5174 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5177 Note that C<sysopen> depends on the fdopen() C library function.
5178 On many UNIX systems, fdopen() is known to fail when file descriptors
5179 exceed a certain value, typically 255. If you need more file
5180 descriptors than that, consider rebuilding Perl to use the C<sfio>
5181 library, or perhaps using the POSIX::open() function.
5183 See L<perlopentut> for a kinder, gentler explanation of opening files.
5185 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5187 =item sysread FILEHANDLE,SCALAR,LENGTH
5189 Attempts to read LENGTH bytes of data into variable SCALAR from the
5190 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
5191 so mixing this with other kinds of reads, C<print>, C<write>,
5192 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
5193 usually buffers data. Returns the number of bytes actually read, C<0>
5194 at end of file, or undef if there was an error. SCALAR will be grown or
5195 shrunk so that the last byte actually read is the last byte of the
5196 scalar after the read.
5198 An OFFSET may be specified to place the read data at some place in the
5199 string other than the beginning. A negative OFFSET specifies
5200 placement at that many bytes counting backwards from the end of the
5201 string. A positive OFFSET greater than the length of SCALAR results
5202 in the string being padded to the required size with C<"\0"> bytes before
5203 the result of the read is appended.
5205 There is no syseof() function, which is ok, since eof() doesn't work
5206 very well on device files (like ttys) anyway. Use sysread() and check
5207 for a return value for 0 to decide whether you're done.
5209 =item sysseek FILEHANDLE,POSITION,WHENCE
5211 Sets FILEHANDLE's system position using the system call lseek(2). It
5212 bypasses stdio, so mixing this with reads (other than C<sysread>),
5213 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
5214 FILEHANDLE may be an expression whose value gives the name of the
5215 filehandle. The values for WHENCE are C<0> to set the new position to
5216 POSITION, C<1> to set the it to the current position plus POSITION,
5217 and C<2> to set it to EOF plus POSITION (typically negative). For
5218 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
5219 C<SEEK_END> (start of the file, current position, end of the file)
5220 from the Fcntl module.
5222 Returns the new position, or the undefined value on failure. A position
5223 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5224 true on success and false on failure, yet you can still easily determine
5229 =item system PROGRAM LIST
5231 Does exactly the same thing as C<exec LIST>, except that a fork is
5232 done first, and the parent process waits for the child process to
5233 complete. Note that argument processing varies depending on the
5234 number of arguments. If there is more than one argument in LIST,
5235 or if LIST is an array with more than one value, starts the program
5236 given by the first element of the list with arguments given by the
5237 rest of the list. If there is only one scalar argument, the argument
5238 is checked for shell metacharacters, and if there are any, the
5239 entire argument is passed to the system's command shell for parsing
5240 (this is C</bin/sh -c> on Unix platforms, but varies on other
5241 platforms). If there are no shell metacharacters in the argument,
5242 it is split into words and passed directly to C<execvp>, which is
5245 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5246 output before any operation that may do a fork, but this may not be
5247 supported on some platforms (see L<perlport>). To be safe, you may need
5248 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5249 of C<IO::Handle> on any open handles.
5251 The return value is the exit status of the program as
5252 returned by the C<wait> call. To get the actual exit value divide by
5253 256. See also L</exec>. This is I<not> what you want to use to capture
5254 the output from a command, for that you should use merely backticks or
5255 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5256 indicates a failure to start the program (inspect $! for the reason).
5258 Like C<exec>, C<system> allows you to lie to a program about its name if
5259 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5261 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
5262 program they're running doesn't actually interrupt your program.
5264 @args = ("command", "arg1", "arg2");
5266 or die "system @args failed: $?"
5268 You can check all the failure possibilities by inspecting
5271 $exit_value = $? >> 8;
5272 $signal_num = $? & 127;
5273 $dumped_core = $? & 128;
5275 When the arguments get executed via the system shell, results
5276 and return codes will be subject to its quirks and capabilities.
5277 See L<perlop/"`STRING`"> and L</exec> for details.
5279 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5281 =item syswrite FILEHANDLE,SCALAR,LENGTH
5283 =item syswrite FILEHANDLE,SCALAR
5285 Attempts to write LENGTH bytes of data from variable SCALAR to the
5286 specified FILEHANDLE, using the system call write(2). If LENGTH
5287 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5288 this with reads (other than C<sysread())>, C<print>, C<write>,
5289 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5290 usually buffers data. Returns the number of bytes actually written,
5291 or C<undef> if there was an error. If the LENGTH is greater than
5292 the available data in the SCALAR after the OFFSET, only as much
5293 data as is available will be written.
5295 An OFFSET may be specified to write the data from some part of the
5296 string other than the beginning. A negative OFFSET specifies writing
5297 that many bytes counting backwards from the end of the string. In the
5298 case the SCALAR is empty you can use OFFSET but only zero offset.
5300 =item tell FILEHANDLE
5304 Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE
5305 may be an expression whose value gives the name of the actual filehandle.
5306 If FILEHANDLE is omitted, assumes the file last read.
5308 The return value of tell() for the standard streams like the STDIN
5309 depends on the operating system: it may return -1 or something else.
5310 tell() on pipes, fifos, and sockets usually returns -1.
5312 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5314 =item telldir DIRHANDLE
5316 Returns the current position of the C<readdir> routines on DIRHANDLE.
5317 Value may be given to C<seekdir> to access a particular location in a
5318 directory. Has the same caveats about possible directory compaction as
5319 the corresponding system library routine.
5321 =item tie VARIABLE,CLASSNAME,LIST
5323 This function binds a variable to a package class that will provide the
5324 implementation for the variable. VARIABLE is the name of the variable
5325 to be enchanted. CLASSNAME is the name of a class implementing objects
5326 of correct type. Any additional arguments are passed to the C<new>
5327 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5328 or C<TIEHASH>). Typically these are arguments such as might be passed
5329 to the C<dbm_open()> function of C. The object returned by the C<new>
5330 method is also returned by the C<tie> function, which would be useful
5331 if you want to access other methods in CLASSNAME.
5333 Note that functions such as C<keys> and C<values> may return huge lists
5334 when used on large objects, like DBM files. You may prefer to use the
5335 C<each> function to iterate over such. Example:
5337 # print out history file offsets
5339 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5340 while (($key,$val) = each %HIST) {
5341 print $key, ' = ', unpack('L',$val), "\n";
5345 A class implementing a hash should have the following methods:
5347 TIEHASH classname, LIST
5349 STORE this, key, value
5354 NEXTKEY this, lastkey
5358 A class implementing an ordinary array should have the following methods:
5360 TIEARRAY classname, LIST
5362 STORE this, key, value
5364 STORESIZE this, count
5370 SPLICE this, offset, length, LIST
5375 A class implementing a file handle should have the following methods:
5377 TIEHANDLE classname, LIST
5378 READ this, scalar, length, offset
5381 WRITE this, scalar, length, offset
5383 PRINTF this, format, LIST
5387 SEEK this, position, whence
5389 OPEN this, mode, LIST
5394 A class implementing a scalar should have the following methods:
5396 TIESCALAR classname, LIST
5402 Not all methods indicated above need be implemented. See L<perltie>,
5403 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5405 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5406 for you--you need to do that explicitly yourself. See L<DB_File>
5407 or the F<Config> module for interesting C<tie> implementations.
5409 For further details see L<perltie>, L<"tied VARIABLE">.
5413 Returns a reference to the object underlying VARIABLE (the same value
5414 that was originally returned by the C<tie> call that bound the variable
5415 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5420 Returns the number of non-leap seconds since whatever time the system
5421 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5422 and 00:00:00 UTC, January 1, 1970 for most other systems).
5423 Suitable for feeding to C<gmtime> and C<localtime>.
5425 For measuring time in better granularity than one second,
5426 you may use either the Time::HiRes module from CPAN, or
5427 if you have gettimeofday(2), you may be able to use the
5428 C<syscall> interface of Perl, see L<perlfaq8> for details.
5432 Returns a four-element list giving the user and system times, in
5433 seconds, for this process and the children of this process.
5435 ($user,$system,$cuser,$csystem) = times;
5437 In scalar context, C<times> returns C<$user>.
5441 The transliteration operator. Same as C<y///>. See L<perlop>.
5443 =item truncate FILEHANDLE,LENGTH
5445 =item truncate EXPR,LENGTH
5447 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5448 specified length. Produces a fatal error if truncate isn't implemented
5449 on your system. Returns true if successful, the undefined value
5456 Returns an uppercased version of EXPR. This is the internal function
5457 implementing the C<\U> escape in double-quoted strings. Respects
5458 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5459 and L<perlunicode>. It does not attempt to do titlecase mapping on
5460 initial letters. See C<ucfirst> for that.
5462 If EXPR is omitted, uses C<$_>.
5468 Returns the value of EXPR with the first character in uppercase
5469 (titlecase in Unicode). This is the internal function implementing
5470 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5471 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>.
5473 If EXPR is omitted, uses C<$_>.
5479 Sets the umask for the process to EXPR and returns the previous value.
5480 If EXPR is omitted, merely returns the current umask.
5482 The Unix permission C<rwxr-x---> is represented as three sets of three
5483 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5484 and isn't one of the digits). The C<umask> value is such a number
5485 representing disabled permissions bits. The permission (or "mode")
5486 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5487 even if you tell C<sysopen> to create a file with permissions C<0777>,
5488 if your umask is C<0022> then the file will actually be created with
5489 permissions C<0755>. If your C<umask> were C<0027> (group can't
5490 write; others can't read, write, or execute), then passing
5491 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5494 Here's some advice: supply a creation mode of C<0666> for regular
5495 files (in C<sysopen>) and one of C<0777> for directories (in
5496 C<mkdir>) and executable files. This gives users the freedom of
5497 choice: if they want protected files, they might choose process umasks
5498 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5499 Programs should rarely if ever make policy decisions better left to
5500 the user. The exception to this is when writing files that should be
5501 kept private: mail files, web browser cookies, I<.rhosts> files, and
5504 If umask(2) is not implemented on your system and you are trying to
5505 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5506 fatal error at run time. If umask(2) is not implemented and you are
5507 not trying to restrict access for yourself, returns C<undef>.
5509 Remember that a umask is a number, usually given in octal; it is I<not> a
5510 string of octal digits. See also L</oct>, if all you have is a string.
5516 Undefines the value of EXPR, which must be an lvalue. Use only on a
5517 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5518 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5519 will probably not do what you expect on most predefined variables or
5520 DBM list values, so don't do that; see L<delete>.) Always returns the
5521 undefined value. You can omit the EXPR, in which case nothing is
5522 undefined, but you still get an undefined value that you could, for
5523 instance, return from a subroutine, assign to a variable or pass as a
5524 parameter. Examples:
5527 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5531 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5532 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5533 select undef, undef, undef, 0.25;
5534 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5536 Note that this is a unary operator, not a list operator.
5542 Deletes a list of files. Returns the number of files successfully
5545 $cnt = unlink 'a', 'b', 'c';
5549 Note: C<unlink> will not delete directories unless you are superuser and
5550 the B<-U> flag is supplied to Perl. Even if these conditions are
5551 met, be warned that unlinking a directory can inflict damage on your
5552 filesystem. Use C<rmdir> instead.
5554 If LIST is omitted, uses C<$_>.
5556 =item unpack TEMPLATE,EXPR
5558 C<unpack> does the reverse of C<pack>: it takes a string
5559 and expands it out into a list of values.
5560 (In scalar context, it returns merely the first value produced.)
5562 The string is broken into chunks described by the TEMPLATE. Each chunk
5563 is converted separately to a value. Typically, either the string is a result
5564 of C<pack>, or the bytes of the string represent a C structure of some
5567 The TEMPLATE has the same format as in the C<pack> function.
5568 Here's a subroutine that does substring:
5571 my($what,$where,$howmuch) = @_;
5572 unpack("x$where a$howmuch", $what);
5577 sub ordinal { unpack("c",$_[0]); } # same as ord()
5579 In addition to fields allowed in pack(), you may prefix a field with
5580 a %<number> to indicate that
5581 you want a <number>-bit checksum of the items instead of the items
5582 themselves. Default is a 16-bit checksum. Checksum is calculated by
5583 summing numeric values of expanded values (for string fields the sum of
5584 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5586 For example, the following
5587 computes the same number as the System V sum program:
5591 unpack("%32C*",<>) % 65535;
5594 The following efficiently counts the number of set bits in a bit vector:
5596 $setbits = unpack("%32b*", $selectmask);
5598 The C<p> and C<P> formats should be used with care. Since Perl
5599 has no way of checking whether the value passed to C<unpack()>
5600 corresponds to a valid memory location, passing a pointer value that's
5601 not known to be valid is likely to have disastrous consequences.
5603 If the repeat count of a field is larger than what the remainder of
5604 the input string allows, repeat count is decreased. If the input string
5605 is longer than one described by the TEMPLATE, the rest is ignored.
5607 See L</pack> for more examples and notes.
5609 =item untie VARIABLE
5611 Breaks the binding between a variable and a package. (See C<tie>.)
5613 =item unshift ARRAY,LIST
5615 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5616 depending on how you look at it. Prepends list to the front of the
5617 array, and returns the new number of elements in the array.
5619 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5621 Note the LIST is prepended whole, not one element at a time, so the
5622 prepended elements stay in the same order. Use C<reverse> to do the
5625 =item use Module VERSION LIST
5627 =item use Module VERSION
5629 =item use Module LIST
5635 Imports some semantics into the current package from the named module,
5636 generally by aliasing certain subroutine or variable names into your
5637 package. It is exactly equivalent to
5639 BEGIN { require Module; import Module LIST; }
5641 except that Module I<must> be a bareword.
5643 VERSION, which can be specified as a literal of the form v5.6.1, demands
5644 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5645 as recent as that version. (For compatibility with older versions of Perl,
5646 a numeric literal will also be interpreted as VERSION.) If the version
5647 of the running Perl interpreter is less than VERSION, then an error
5648 message is printed and Perl exits immediately without attempting to
5649 parse the rest of the file. Compare with L</require>, which can do a
5650 similar check at run time.
5652 use v5.6.1; # compile time version check
5654 use 5.005_03; # float version allowed for compatibility
5656 This is often useful if you need to check the current Perl version before
5657 C<use>ing library modules that have changed in incompatible ways from
5658 older versions of Perl. (We try not to do this more than we have to.)
5660 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5661 C<require> makes sure the module is loaded into memory if it hasn't been
5662 yet. The C<import> is not a builtin--it's just an ordinary static method
5663 call into the C<Module> package to tell the module to import the list of
5664 features back into the current package. The module can implement its
5665 C<import> method any way it likes, though most modules just choose to
5666 derive their C<import> method via inheritance from the C<Exporter> class that
5667 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5668 method can be found then the call is skipped.
5670 If you do not want to call the package's C<import> method (for instance,
5671 to stop your namespace from being altered), explicitly supply the empty list:
5675 That is exactly equivalent to
5677 BEGIN { require Module }
5679 If the VERSION argument is present between Module and LIST, then the
5680 C<use> will call the VERSION method in class Module with the given
5681 version as an argument. The default VERSION method, inherited from
5682 the UNIVERSAL class, croaks if the given version is larger than the
5683 value of the variable C<$Module::VERSION>.
5685 Again, there is a distinction between omitting LIST (C<import> called
5686 with no arguments) and an explicit empty LIST C<()> (C<import> not
5687 called). Note that there is no comma after VERSION!
5689 Because this is a wide-open interface, pragmas (compiler directives)
5690 are also implemented this way. Currently implemented pragmas are:
5695 use sigtrap qw(SEGV BUS);
5696 use strict qw(subs vars refs);
5697 use subs qw(afunc blurfl);
5698 use warnings qw(all);
5700 Some of these pseudo-modules import semantics into the current
5701 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5702 which import symbols into the current package (which are effective
5703 through the end of the file).
5705 There's a corresponding C<no> command that unimports meanings imported
5706 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5712 If no C<unimport> method can be found the call fails with a fatal error.
5714 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5715 for the C<-M> and C<-m> command-line options to perl that give C<use>
5716 functionality from the command-line.
5720 Changes the access and modification times on each file of a list of
5721 files. The first two elements of the list must be the NUMERICAL access
5722 and modification times, in that order. Returns the number of files
5723 successfully changed. The inode change time of each file is set
5724 to the current time. This code has the same effect as the C<touch>
5725 command if the files already exist:
5729 utime $now, $now, @ARGV;
5731 If the first two elements of the list are C<undef>, then the utime(2)
5732 function in the C library will be called with a null second argument.
5733 On most systems, this will set the file's access and modification
5734 times to the current time. (i.e. equivalent to the example above.)
5736 utime undef, undef, @ARGV;
5740 Returns a list consisting of all the values of the named hash. (In a
5741 scalar context, returns the number of values.) The values are
5742 returned in an apparently random order. The actual random order is
5743 subject to change in future versions of perl, but it is guaranteed to
5744 be the same order as either the C<keys> or C<each> function would
5745 produce on the same (unmodified) hash.
5747 Note that the values are not copied, which means modifying them will
5748 modify the contents of the hash:
5750 for (values %hash) { s/foo/bar/g } # modifies %hash values
5751 for (@hash{keys %hash}) { s/foo/bar/g } # same
5753 As a side effect, calling values() resets the HASH's internal iterator.
5754 See also C<keys>, C<each>, and C<sort>.
5756 =item vec EXPR,OFFSET,BITS
5758 Treats the string in EXPR as a bit vector made up of elements of
5759 width BITS, and returns the value of the element specified by OFFSET
5760 as an unsigned integer. BITS therefore specifies the number of bits
5761 that are reserved for each element in the bit vector. This must
5762 be a power of two from 1 to 32 (or 64, if your platform supports
5765 If BITS is 8, "elements" coincide with bytes of the input string.
5767 If BITS is 16 or more, bytes of the input string are grouped into chunks
5768 of size BITS/8, and each group is converted to a number as with
5769 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5770 for BITS==64). See L<"pack"> for details.
5772 If bits is 4 or less, the string is broken into bytes, then the bits
5773 of each byte are broken into 8/BITS groups. Bits of a byte are
5774 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5775 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5776 breaking the single input byte C<chr(0x36)> into two groups gives a list
5777 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5779 C<vec> may also be assigned to, in which case parentheses are needed
5780 to give the expression the correct precedence as in
5782 vec($image, $max_x * $x + $y, 8) = 3;
5784 If the selected element is outside the string, the value 0 is returned.
5785 If an element off the end of the string is written to, Perl will first
5786 extend the string with sufficiently many zero bytes. It is an error
5787 to try to write off the beginning of the string (i.e. negative OFFSET).
5789 The string should not contain any character with the value > 255 (which
5790 can only happen if you're using UTF8 encoding). If it does, it will be
5791 treated as something which is not UTF8 encoded. When the C<vec> was
5792 assigned to, other parts of your program will also no longer consider the
5793 string to be UTF8 encoded. In other words, if you do have such characters
5794 in your string, vec() will operate on the actual byte string, and not the
5795 conceptual character string.
5797 Strings created with C<vec> can also be manipulated with the logical
5798 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5799 vector operation is desired when both operands are strings.
5800 See L<perlop/"Bitwise String Operators">.
5802 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5803 The comments show the string after each step. Note that this code works
5804 in the same way on big-endian or little-endian machines.
5807 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5809 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5810 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5812 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5813 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5814 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5815 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5816 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5817 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5819 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5820 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5821 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5824 To transform a bit vector into a string or list of 0's and 1's, use these:
5826 $bits = unpack("b*", $vector);
5827 @bits = split(//, unpack("b*", $vector));
5829 If you know the exact length in bits, it can be used in place of the C<*>.
5831 Here is an example to illustrate how the bits actually fall in place:
5837 unpack("V",$_) 01234567890123456789012345678901
5838 ------------------------------------------------------------------
5843 for ($shift=0; $shift < $width; ++$shift) {
5844 for ($off=0; $off < 32/$width; ++$off) {
5845 $str = pack("B*", "0"x32);
5846 $bits = (1<<$shift);
5847 vec($str, $off, $width) = $bits;
5848 $res = unpack("b*",$str);
5849 $val = unpack("V", $str);
5856 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5857 $off, $width, $bits, $val, $res
5861 Regardless of the machine architecture on which it is run, the above
5862 example should print the following table:
5865 unpack("V",$_) 01234567890123456789012345678901
5866 ------------------------------------------------------------------
5867 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5868 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5869 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5870 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5871 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5872 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5873 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5874 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5875 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5876 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5877 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5878 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5879 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5880 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5881 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5882 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5883 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5884 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5885 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5886 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5887 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5888 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5889 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5890 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5891 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5892 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5893 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5894 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5895 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5896 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5897 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5898 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5899 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5900 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5901 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5902 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5903 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5904 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5905 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5906 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5907 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5908 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5909 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5910 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5911 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5912 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5913 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5914 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5915 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5916 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5917 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5918 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5919 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5920 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5921 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5922 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5923 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5924 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5925 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5926 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5927 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5928 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5929 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5930 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5931 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5932 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5933 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5934 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5935 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5936 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5937 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5938 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5939 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5940 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5941 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5942 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5943 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5944 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5945 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5946 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5947 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5948 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5949 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5950 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5951 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5952 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5953 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5954 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5955 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5956 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5957 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5958 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5959 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5960 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5961 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5962 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5963 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5964 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5965 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5966 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5967 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5968 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5969 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5970 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5971 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5972 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5973 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5974 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5975 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5976 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5977 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5978 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5979 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5980 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5981 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5982 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5983 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5984 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5985 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5986 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5987 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5988 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5989 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5990 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5991 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5992 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5993 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5994 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5998 Behaves like the wait(2) system call on your system: it waits for a child
5999 process to terminate and returns the pid of the deceased process, or
6000 C<-1> if there are no child processes. The status is returned in C<$?>.
6001 Note that a return value of C<-1> could mean that child processes are
6002 being automatically reaped, as described in L<perlipc>.
6004 =item waitpid PID,FLAGS
6006 Waits for a particular child process to terminate and returns the pid of
6007 the deceased process, or C<-1> if there is no such child process. On some
6008 systems, a value of 0 indicates that there are processes still running.
6009 The status is returned in C<$?>. If you say
6011 use POSIX ":sys_wait_h";
6014 $kid = waitpid(-1, WNOHANG);
6017 then you can do a non-blocking wait for all pending zombie processes.
6018 Non-blocking wait is available on machines supporting either the
6019 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6020 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6021 system call by remembering the status values of processes that have
6022 exited but have not been harvested by the Perl script yet.)
6024 Note that on some systems, a return value of C<-1> could mean that child
6025 processes are being automatically reaped. See L<perlipc> for details,
6026 and for other examples.
6030 Returns true if the context of the currently executing subroutine is
6031 looking for a list value. Returns false if the context is looking
6032 for a scalar. Returns the undefined value if the context is looking
6033 for no value (void context).
6035 return unless defined wantarray; # don't bother doing more
6036 my @a = complex_calculation();
6037 return wantarray ? @a : "@a";
6039 This function should have been named wantlist() instead.
6043 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6046 If LIST is empty and C<$@> already contains a value (typically from a
6047 previous eval) that value is used after appending C<"\t...caught">
6048 to C<$@>. This is useful for staying almost, but not entirely similar to
6051 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6053 No message is printed if there is a C<$SIG{__WARN__}> handler
6054 installed. It is the handler's responsibility to deal with the message
6055 as it sees fit (like, for instance, converting it into a C<die>). Most
6056 handlers must therefore make arrangements to actually display the
6057 warnings that they are not prepared to deal with, by calling C<warn>
6058 again in the handler. Note that this is quite safe and will not
6059 produce an endless loop, since C<__WARN__> hooks are not called from
6062 You will find this behavior is slightly different from that of
6063 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6064 instead call C<die> again to change it).
6066 Using a C<__WARN__> handler provides a powerful way to silence all
6067 warnings (even the so-called mandatory ones). An example:
6069 # wipe out *all* compile-time warnings
6070 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6072 my $foo = 20; # no warning about duplicate my $foo,
6073 # but hey, you asked for it!
6074 # no compile-time or run-time warnings before here
6077 # run-time warnings enabled after here
6078 warn "\$foo is alive and $foo!"; # does show up
6080 See L<perlvar> for details on setting C<%SIG> entries, and for more
6081 examples. See the Carp module for other kinds of warnings using its
6082 carp() and cluck() functions.
6084 =item write FILEHANDLE
6090 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6091 using the format associated with that file. By default the format for
6092 a file is the one having the same name as the filehandle, but the
6093 format for the current output channel (see the C<select> function) may be set
6094 explicitly by assigning the name of the format to the C<$~> variable.
6096 Top of form processing is handled automatically: if there is
6097 insufficient room on the current page for the formatted record, the
6098 page is advanced by writing a form feed, a special top-of-page format
6099 is used to format the new page header, and then the record is written.
6100 By default the top-of-page format is the name of the filehandle with
6101 "_TOP" appended, but it may be dynamically set to the format of your
6102 choice by assigning the name to the C<$^> variable while the filehandle is
6103 selected. The number of lines remaining on the current page is in
6104 variable C<$->, which can be set to C<0> to force a new page.
6106 If FILEHANDLE is unspecified, output goes to the current default output
6107 channel, which starts out as STDOUT but may be changed by the
6108 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6109 is evaluated and the resulting string is used to look up the name of
6110 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6112 Note that write is I<not> the opposite of C<read>. Unfortunately.
6116 The transliteration operator. Same as C<tr///>. See L<perlop>.