3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 A named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140 C<umask>, C<unlink>, C<utime>
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>,
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
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Script start time minus file modification time, in days.
298 -A Same for access time.
299 -C Same for inode change time (Unix, may differ for other platforms)
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 IO 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.) (Also, if the stat buffer was filled by
354 a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
357 print "Can do.\n" if -r $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
369 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
370 test operators, in a way that C<-f -w -x $file> is equivalent to
371 C<-x $file && -w _ && -f _>. (This is only syntax fancy : if you use
372 the return value of C<-f $file> as an argument to another filetest
373 operator, no special magic will happen.)
379 Returns the absolute value of its argument.
380 If VALUE is omitted, uses C<$_>.
382 =item accept NEWSOCKET,GENERICSOCKET
384 Accepts an incoming socket connect, just as the accept(2) system call
385 does. Returns the packed address if it succeeded, false otherwise.
386 See the example in L<perlipc/"Sockets: Client/Server Communication">.
388 On systems that support a close-on-exec flag on files, the flag will
389 be set for the newly opened file descriptor, as determined by the
390 value of $^F. See L<perlvar/$^F>.
396 Arranges to have a SIGALRM delivered to this process after the
397 specified number of wallclock seconds have elapsed. If SECONDS is not
398 specified, the value stored in C<$_> is used. (On some machines,
399 unfortunately, the elapsed time may be up to one second less or more
400 than you specified because of how seconds are counted, and process
401 scheduling may delay the delivery of the signal even further.)
403 Only one timer may be counting at once. Each call disables the
404 previous timer, and an argument of C<0> may be supplied to cancel the
405 previous timer without starting a new one. The returned value is the
406 amount of time remaining on the previous timer.
408 For delays of finer granularity than one second, you may use Perl's
409 four-argument version of select() leaving the first three arguments
410 undefined, or you might be able to use the C<syscall> interface to
411 access setitimer(2) if your system supports it. The Time::HiRes
412 module (from CPAN, and starting from Perl 5.8 part of the standard
413 distribution) may also prove useful.
415 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
416 (C<sleep> may be internally implemented in your system with C<alarm>)
418 If you want to use C<alarm> to time out a system call you need to use an
419 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
420 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
421 restart system calls on some systems. Using C<eval>/C<die> always works,
422 modulo the caveats given in L<perlipc/"Signals">.
425 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
427 $nread = sysread SOCKET, $buffer, $size;
431 die unless $@ eq "alarm\n"; # propagate unexpected errors
438 For more information see L<perlipc>.
442 Returns the arctangent of Y/X in the range -PI to PI.
444 For the tangent operation, you may use the C<Math::Trig::tan>
445 function, or use the familiar relation:
447 sub tan { sin($_[0]) / cos($_[0]) }
449 =item bind SOCKET,NAME
451 Binds a network address to a socket, just as the bind system call
452 does. Returns true if it succeeded, false otherwise. NAME should be a
453 packed address of the appropriate type for the socket. See the examples in
454 L<perlipc/"Sockets: Client/Server Communication">.
456 =item binmode FILEHANDLE, LAYER
458 =item binmode FILEHANDLE
460 Arranges for FILEHANDLE to be read or written in "binary" or "text"
461 mode on systems where the run-time libraries distinguish between
462 binary and text files. If FILEHANDLE is an expression, the value is
463 taken as the name of the filehandle. Returns true on success,
464 otherwise it returns C<undef> and sets C<$!> (errno).
466 On some systems (in general, DOS and Windows-based systems) binmode()
467 is necessary when you're not working with a text file. For the sake
468 of portability it is a good idea to always use it when appropriate,
469 and to never use it when it isn't appropriate. Also, people can
470 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
472 In other words: regardless of platform, use binmode() on binary data,
473 like for example images.
475 If LAYER is present it is a single string, but may contain multiple
476 directives. The directives alter the behaviour of the file handle.
477 When LAYER is present using binmode on text file makes sense.
479 If LAYER is omitted or specified as C<:raw> the filehandle is made
480 suitable for passing binary data. This includes turning off possible CRLF
481 translation and marking it as bytes (as opposed to Unicode characters).
482 Note that, despite what may be implied in I<"Programming Perl"> (the
483 Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
484 -- other layers which would affect binary nature of the stream are
485 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
486 PERLIO environment variable.
488 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
489 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
490 establish default I/O layers. See L<open>.
492 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
493 in "Programming Perl, 3rd Edition". However, since the publishing of this
494 book, by many known as "Camel III", the consensus of the naming of this
495 functionality has moved from "discipline" to "layer". All documentation
496 of this version of Perl therefore refers to "layers" rather than to
497 "disciplines". Now back to the regularly scheduled documentation...>
499 To mark FILEHANDLE as UTF-8, use C<:utf8>.
501 In general, binmode() should be called after open() but before any I/O
502 is done on the filehandle. Calling binmode() will normally flush any
503 pending buffered output data (and perhaps pending input data) on the
504 handle. An exception to this is the C<:encoding> layer that
505 changes the default character encoding of the handle, see L<open>.
506 The C<:encoding> layer sometimes needs to be called in
507 mid-stream, and it doesn't flush the stream. The C<:encoding>
508 also implicitly pushes on top of itself the C<:utf8> layer because
509 internally Perl will operate on UTF-8 encoded Unicode characters.
511 The operating system, device drivers, C libraries, and Perl run-time
512 system all work together to let the programmer treat a single
513 character (C<\n>) as the line terminator, irrespective of the external
514 representation. On many operating systems, the native text file
515 representation matches the internal representation, but on some
516 platforms the external representation of C<\n> is made up of more than
519 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
520 character to end each line in the external representation of text (even
521 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
522 on Unix and most VMS files). In other systems like OS/2, DOS and the
523 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
524 but what's stored in text files are the two characters C<\cM\cJ>. That
525 means that, if you don't use binmode() on these systems, C<\cM\cJ>
526 sequences on disk will be converted to C<\n> on input, and any C<\n> in
527 your program will be converted back to C<\cM\cJ> on output. This is what
528 you want for text files, but it can be disastrous for binary files.
530 Another consequence of using binmode() (on some systems) is that
531 special end-of-file markers will be seen as part of the data stream.
532 For systems from the Microsoft family this means that if your binary
533 data contains C<\cZ>, the I/O subsystem will regard it as the end of
534 the file, unless you use binmode().
536 binmode() is not only important for readline() and print() operations,
537 but also when using read(), seek(), sysread(), syswrite() and tell()
538 (see L<perlport> for more details). See the C<$/> and C<$\> variables
539 in L<perlvar> for how to manually set your input and output
540 line-termination sequences.
542 =item bless REF,CLASSNAME
546 This function tells the thingy referenced by REF that it is now an object
547 in the CLASSNAME package. If CLASSNAME is omitted, the current package
548 is used. Because a C<bless> is often the last thing in a constructor,
549 it returns the reference for convenience. Always use the two-argument
550 version if the function doing the blessing might be inherited by a
551 derived class. See L<perltoot> and L<perlobj> for more about the blessing
552 (and blessings) of objects.
554 Consider always blessing objects in CLASSNAMEs that are mixed case.
555 Namespaces with all lowercase names are considered reserved for
556 Perl pragmata. Builtin types have all uppercase names, so to prevent
557 confusion, you may wish to avoid such package names as well. Make sure
558 that CLASSNAME is a true value.
560 See L<perlmod/"Perl Modules">.
566 Returns the context of the current subroutine call. In scalar context,
567 returns the caller's package name if there is a caller, that is, if
568 we're in a subroutine or C<eval> or C<require>, and the undefined value
569 otherwise. In list context, returns
571 ($package, $filename, $line) = caller;
573 With EXPR, it returns some extra information that the debugger uses to
574 print a stack trace. The value of EXPR indicates how many call frames
575 to go back before the current one.
577 ($package, $filename, $line, $subroutine, $hasargs,
578 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
580 Here $subroutine may be C<(eval)> if the frame is not a subroutine
581 call, but an C<eval>. In such a case additional elements $evaltext and
582 C<$is_require> are set: C<$is_require> is true if the frame is created by a
583 C<require> or C<use> statement, $evaltext contains the text of the
584 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
585 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
586 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
587 frame.) $subroutine may also be C<(unknown)> if this particular
588 subroutine happens to have been deleted from the symbol table.
589 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
590 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
591 compiled with. The C<$hints> and C<$bitmask> values are subject to change
592 between versions of Perl, and are not meant for external use.
594 Furthermore, when called from within the DB package, caller returns more
595 detailed information: it sets the list variable C<@DB::args> to be the
596 arguments with which the subroutine was invoked.
598 Be aware that the optimizer might have optimized call frames away before
599 C<caller> had a chance to get the information. That means that C<caller(N)>
600 might not return information about the call frame you expect it do, for
601 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
602 previous time C<caller> was called.
606 Changes the working directory to EXPR, if possible. If EXPR is omitted,
607 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
608 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
609 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
610 neither is set, C<chdir> does nothing. It returns true upon success,
611 false otherwise. See the example under C<die>.
615 Changes the permissions of a list of files. The first element of the
616 list must be the numerical mode, which should probably be an octal
617 number, and which definitely should I<not> be a string of octal digits:
618 C<0644> is okay, C<'0644'> is not. Returns the number of files
619 successfully changed. See also L</oct>, if all you have is a string.
621 $cnt = chmod 0755, 'foo', 'bar';
622 chmod 0755, @executables;
623 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
625 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
626 $mode = 0644; chmod $mode, 'foo'; # this is best
628 You can also import the symbolic C<S_I*> constants from the Fcntl
633 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
634 # This is identical to the chmod 0755 of the above example.
642 This safer version of L</chop> removes any trailing string
643 that corresponds to the current value of C<$/> (also known as
644 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
645 number of characters removed from all its arguments. It's often used to
646 remove the newline from the end of an input record when you're worried
647 that the final record may be missing its newline. When in paragraph
648 mode (C<$/ = "">), it removes all trailing newlines from the string.
649 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
650 a reference to an integer or the like, see L<perlvar>) chomp() won't
652 If VARIABLE is omitted, it chomps C<$_>. Example:
655 chomp; # avoid \n on last field
660 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
662 You can actually chomp anything that's an lvalue, including an assignment:
665 chomp($answer = <STDIN>);
667 If you chomp a list, each element is chomped, and the total number of
668 characters removed is returned.
670 If the C<encoding> pragma is in scope then the lengths returned are
671 calculated from the length of C<$/> in Unicode characters, which is not
672 always the same as the length of C<$/> in the native encoding.
674 Note that parentheses are necessary when you're chomping anything
675 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
676 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
677 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
678 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
687 Chops off the last character of a string and returns the character
688 chopped. It is much more efficient than C<s/.$//s> because it neither
689 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
690 If VARIABLE is a hash, it chops the hash's values, but not its keys.
692 You can actually chop anything that's an lvalue, including an assignment.
694 If you chop a list, each element is chopped. Only the value of the
695 last C<chop> is returned.
697 Note that C<chop> returns the last character. To return all but the last
698 character, use C<substr($string, 0, -1)>.
704 Changes the owner (and group) of a list of files. The first two
705 elements of the list must be the I<numeric> uid and gid, in that
706 order. A value of -1 in either position is interpreted by most
707 systems to leave that value unchanged. Returns the number of files
708 successfully changed.
710 $cnt = chown $uid, $gid, 'foo', 'bar';
711 chown $uid, $gid, @filenames;
713 Here's an example that looks up nonnumeric uids in the passwd file:
716 chomp($user = <STDIN>);
718 chomp($pattern = <STDIN>);
720 ($login,$pass,$uid,$gid) = getpwnam($user)
721 or die "$user not in passwd file";
723 @ary = glob($pattern); # expand filenames
724 chown $uid, $gid, @ary;
726 On most systems, you are not allowed to change the ownership of the
727 file unless you're the superuser, although you should be able to change
728 the group to any of your secondary groups. On insecure systems, these
729 restrictions may be relaxed, but this is not a portable assumption.
730 On POSIX systems, you can detect this condition this way:
732 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
733 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
739 Returns the character represented by that NUMBER in the character set.
740 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
741 chr(0x263a) is a Unicode smiley face. Note that characters from 128
742 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
743 backward compatibility reasons (but see L<encoding>).
745 If NUMBER is omitted, uses C<$_>.
747 For the reverse, use L</ord>.
749 Note that under the C<bytes> pragma the NUMBER is masked to
752 See L<perlunicode> and L<encoding> for more about Unicode.
754 =item chroot FILENAME
758 This function works like the system call by the same name: it makes the
759 named directory the new root directory for all further pathnames that
760 begin with a C</> by your process and all its children. (It doesn't
761 change your current working directory, which is unaffected.) For security
762 reasons, this call is restricted to the superuser. If FILENAME is
763 omitted, does a C<chroot> to C<$_>.
765 =item close FILEHANDLE
769 Closes the file or pipe associated with the file handle, returning
770 true only if IO buffers are successfully flushed and closes the system
771 file descriptor. Closes the currently selected filehandle if the
774 You don't have to close FILEHANDLE if you are immediately going to do
775 another C<open> on it, because C<open> will close it for you. (See
776 C<open>.) However, an explicit C<close> on an input file resets the line
777 counter (C<$.>), while the implicit close done by C<open> does not.
779 If the file handle came from a piped open, C<close> will additionally
780 return false if one of the other system calls involved fails, or if the
781 program exits with non-zero status. (If the only problem was that the
782 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
783 also waits for the process executing on the pipe to complete, in case you
784 want to look at the output of the pipe afterwards, and
785 implicitly puts the exit status value of that command into C<$?>.
787 Prematurely closing the read end of a pipe (i.e. before the process
788 writing to it at the other end has closed it) will result in a
789 SIGPIPE being delivered to the writer. If the other end can't
790 handle that, be sure to read all the data before closing the pipe.
794 open(OUTPUT, '|sort >foo') # pipe to sort
795 or die "Can't start sort: $!";
796 #... # print stuff to output
797 close OUTPUT # wait for sort to finish
798 or warn $! ? "Error closing sort pipe: $!"
799 : "Exit status $? from sort";
800 open(INPUT, 'foo') # get sort's results
801 or die "Can't open 'foo' for input: $!";
803 FILEHANDLE may be an expression whose value can be used as an indirect
804 filehandle, usually the real filehandle name.
806 =item closedir DIRHANDLE
808 Closes a directory opened by C<opendir> and returns the success of that
811 =item connect SOCKET,NAME
813 Attempts to connect to a remote socket, just as the connect system call
814 does. Returns true if it succeeded, false otherwise. NAME should be a
815 packed address of the appropriate type for the socket. See the examples in
816 L<perlipc/"Sockets: Client/Server Communication">.
820 Actually a flow control statement rather than a function. If there is a
821 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
822 C<foreach>), it is always executed just before the conditional is about to
823 be evaluated again, just like the third part of a C<for> loop in C. Thus
824 it can be used to increment a loop variable, even when the loop has been
825 continued via the C<next> statement (which is similar to the C C<continue>
828 C<last>, C<next>, or C<redo> may appear within a C<continue>
829 block. C<last> and C<redo> will behave as if they had been executed within
830 the main block. So will C<next>, but since it will execute a C<continue>
831 block, it may be more entertaining.
834 ### redo always comes here
837 ### next always comes here
839 # then back the top to re-check EXPR
841 ### last always comes here
843 Omitting the C<continue> section is semantically equivalent to using an
844 empty one, logically enough. In that case, C<next> goes directly back
845 to check the condition at the top of the loop.
851 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
852 takes cosine of C<$_>.
854 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
855 function, or use this relation:
857 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
859 =item crypt PLAINTEXT,SALT
861 Encrypts a string exactly like the crypt(3) function in the C library
862 (assuming that you actually have a version there that has not been
863 extirpated as a potential munition). This can prove useful for checking
864 the password file for lousy passwords, amongst other things. Only the
865 guys wearing white hats should do this.
867 Note that L<crypt|/crypt> is intended to be a one-way function, much like
868 breaking eggs to make an omelette. There is no (known) corresponding
869 decrypt function (in other words, the crypt() is a one-way hash
870 function). As a result, this function isn't all that useful for
871 cryptography. (For that, see your nearby CPAN mirror.)
873 When verifying an existing encrypted string you should use the
874 encrypted text as the salt (like C<crypt($plain, $crypted) eq
875 $crypted>). This allows your code to work with the standard L<crypt|/crypt>
876 and with more exotic implementations. In other words, do not assume
877 anything about the returned string itself, or how many bytes in
878 the encrypted string matter.
880 Traditionally the result is a string of 13 bytes: two first bytes of
881 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
882 the first eight bytes of the encrypted string mattered, but
883 alternative hashing schemes (like MD5), higher level security schemes
884 (like C2), and implementations on non-UNIX platforms may produce
887 When choosing a new salt create a random two character string whose
888 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
889 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
890 characters is just a recommendation; the characters allowed in
891 the salt depend solely on your system's crypt library, and Perl can't
892 restrict what salts C<crypt()> accepts.
894 Here's an example that makes sure that whoever runs this program knows
897 $pwd = (getpwuid($<))[1];
901 chomp($word = <STDIN>);
905 if (crypt($word, $pwd) ne $pwd) {
911 Of course, typing in your own password to whoever asks you
914 The L<crypt|/crypt> function is unsuitable for encrypting large quantities
915 of data, not least of all because you can't get the information
916 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
917 on your favorite CPAN mirror for a slew of potentially useful
920 If using crypt() on a Unicode string (which I<potentially> has
921 characters with codepoints above 255), Perl tries to make sense
922 of the situation by trying to downgrade (a copy of the string)
923 the string back to an eight-bit byte string before calling crypt()
924 (on that copy). If that works, good. If not, crypt() dies with
925 C<Wide character in crypt>.
929 [This function has been largely superseded by the C<untie> function.]
931 Breaks the binding between a DBM file and a hash.
933 =item dbmopen HASH,DBNAME,MASK
935 [This function has been largely superseded by the C<tie> function.]
937 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
938 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
939 argument is I<not> a filehandle, even though it looks like one). DBNAME
940 is the name of the database (without the F<.dir> or F<.pag> extension if
941 any). If the database does not exist, it is created with protection
942 specified by MASK (as modified by the C<umask>). If your system supports
943 only the older DBM functions, you may perform only one C<dbmopen> in your
944 program. In older versions of Perl, if your system had neither DBM nor
945 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
948 If you don't have write access to the DBM file, you can only read hash
949 variables, not set them. If you want to test whether you can write,
950 either use file tests or try setting a dummy hash entry inside an C<eval>,
951 which will trap the error.
953 Note that functions such as C<keys> and C<values> may return huge lists
954 when used on large DBM files. You may prefer to use the C<each>
955 function to iterate over large DBM files. Example:
957 # print out history file offsets
958 dbmopen(%HIST,'/usr/lib/news/history',0666);
959 while (($key,$val) = each %HIST) {
960 print $key, ' = ', unpack('L',$val), "\n";
964 See also L<AnyDBM_File> for a more general description of the pros and
965 cons of the various dbm approaches, as well as L<DB_File> for a particularly
968 You can control which DBM library you use by loading that library
969 before you call dbmopen():
972 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
973 or die "Can't open netscape history file: $!";
979 Returns a Boolean value telling whether EXPR has a value other than
980 the undefined value C<undef>. If EXPR is not present, C<$_> will be
983 Many operations return C<undef> to indicate failure, end of file,
984 system error, uninitialized variable, and other exceptional
985 conditions. This function allows you to distinguish C<undef> from
986 other values. (A simple Boolean test will not distinguish among
987 C<undef>, zero, the empty string, and C<"0">, which are all equally
988 false.) Note that since C<undef> is a valid scalar, its presence
989 doesn't I<necessarily> indicate an exceptional condition: C<pop>
990 returns C<undef> when its argument is an empty array, I<or> when the
991 element to return happens to be C<undef>.
993 You may also use C<defined(&func)> to check whether subroutine C<&func>
994 has ever been defined. The return value is unaffected by any forward
995 declarations of C<&func>. Note that a subroutine which is not defined
996 may still be callable: its package may have an C<AUTOLOAD> method that
997 makes it spring into existence the first time that it is called -- see
1000 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1001 used to report whether memory for that aggregate has ever been
1002 allocated. This behavior may disappear in future versions of Perl.
1003 You should instead use a simple test for size:
1005 if (@an_array) { print "has array elements\n" }
1006 if (%a_hash) { print "has hash members\n" }
1008 When used on a hash element, it tells you whether the value is defined,
1009 not whether the key exists in the hash. Use L</exists> for the latter
1014 print if defined $switch{'D'};
1015 print "$val\n" while defined($val = pop(@ary));
1016 die "Can't readlink $sym: $!"
1017 unless defined($value = readlink $sym);
1018 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1019 $debugging = 0 unless defined $debugging;
1021 Note: Many folks tend to overuse C<defined>, and then are surprised to
1022 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1023 defined values. For example, if you say
1027 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1028 matched "nothing". But it didn't really match nothing--rather, it
1029 matched something that happened to be zero characters long. This is all
1030 very above-board and honest. When a function returns an undefined value,
1031 it's an admission that it couldn't give you an honest answer. So you
1032 should use C<defined> only when you're questioning the integrity of what
1033 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1036 See also L</undef>, L</exists>, L</ref>.
1040 Given an expression that specifies a hash element, array element, hash slice,
1041 or array slice, deletes the specified element(s) from the hash or array.
1042 In the case of an array, if the array elements happen to be at the end,
1043 the size of the array will shrink to the highest element that tests
1044 true for exists() (or 0 if no such element exists).
1046 Returns a list with the same number of elements as the number of elements
1047 for which deletion was attempted. Each element of that list consists of
1048 either the value of the element deleted, or the undefined value. In scalar
1049 context, this means that you get the value of the last element deleted (or
1050 the undefined value if that element did not exist).
1052 %hash = (foo => 11, bar => 22, baz => 33);
1053 $scalar = delete $hash{foo}; # $scalar is 11
1054 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1055 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1057 Deleting from C<%ENV> modifies the environment. Deleting from
1058 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1059 from a C<tie>d hash or array may not necessarily return anything.
1061 Deleting an array element effectively returns that position of the array
1062 to its initial, uninitialized state. Subsequently testing for the same
1063 element with exists() will return false. Note that deleting array
1064 elements in the middle of an array will not shift the index of the ones
1065 after them down--use splice() for that. See L</exists>.
1067 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1069 foreach $key (keys %HASH) {
1073 foreach $index (0 .. $#ARRAY) {
1074 delete $ARRAY[$index];
1079 delete @HASH{keys %HASH};
1081 delete @ARRAY[0 .. $#ARRAY];
1083 But both of these are slower than just assigning the empty list
1084 or undefining %HASH or @ARRAY:
1086 %HASH = (); # completely empty %HASH
1087 undef %HASH; # forget %HASH ever existed
1089 @ARRAY = (); # completely empty @ARRAY
1090 undef @ARRAY; # forget @ARRAY ever existed
1092 Note that the EXPR can be arbitrarily complicated as long as the final
1093 operation is a hash element, array element, hash slice, or array slice
1096 delete $ref->[$x][$y]{$key};
1097 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1099 delete $ref->[$x][$y][$index];
1100 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1104 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1105 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1106 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1107 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1108 an C<eval(),> the error message is stuffed into C<$@> and the
1109 C<eval> is terminated with the undefined value. This makes
1110 C<die> the way to raise an exception.
1112 Equivalent examples:
1114 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1115 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1117 If the last element of LIST does not end in a newline, the current
1118 script line number and input line number (if any) are also printed,
1119 and a newline is supplied. Note that the "input line number" (also
1120 known as "chunk") is subject to whatever notion of "line" happens to
1121 be currently in effect, and is also available as the special variable
1122 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1124 Hint: sometimes appending C<", stopped"> to your message will cause it
1125 to make better sense when the string C<"at foo line 123"> is appended.
1126 Suppose you are running script "canasta".
1128 die "/etc/games is no good";
1129 die "/etc/games is no good, stopped";
1131 produce, respectively
1133 /etc/games is no good at canasta line 123.
1134 /etc/games is no good, stopped at canasta line 123.
1136 See also exit(), warn(), and the Carp module.
1138 If LIST is empty and C<$@> already contains a value (typically from a
1139 previous eval) that value is reused after appending C<"\t...propagated">.
1140 This is useful for propagating exceptions:
1143 die unless $@ =~ /Expected exception/;
1145 If LIST is empty and C<$@> contains an object reference that has a
1146 C<PROPAGATE> method, that method will be called with additional file
1147 and line number parameters. The return value replaces the value in
1148 C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1151 If C<$@> is empty then the string C<"Died"> is used.
1153 die() can also be called with a reference argument. If this happens to be
1154 trapped within an eval(), $@ contains the reference. This behavior permits
1155 a more elaborate exception handling implementation using objects that
1156 maintain arbitrary state about the nature of the exception. Such a scheme
1157 is sometimes preferable to matching particular string values of $@ using
1158 regular expressions. Here's an example:
1160 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1162 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1163 # handle Some::Module::Exception
1166 # handle all other possible exceptions
1170 Because perl will stringify uncaught exception messages before displaying
1171 them, you may want to overload stringification operations on such custom
1172 exception objects. See L<overload> for details about that.
1174 You can arrange for a callback to be run just before the C<die>
1175 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1176 handler will be called with the error text and can change the error
1177 message, if it sees fit, by calling C<die> again. See
1178 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1179 L<"eval BLOCK"> for some examples. Although this feature was meant
1180 to be run only right before your program was to exit, this is not
1181 currently the case--the C<$SIG{__DIE__}> hook is currently called
1182 even inside eval()ed blocks/strings! If one wants the hook to do
1183 nothing in such situations, put
1187 as the first line of the handler (see L<perlvar/$^S>). Because
1188 this promotes strange action at a distance, this counterintuitive
1189 behavior may be fixed in a future release.
1193 Not really a function. Returns the value of the last command in the
1194 sequence of commands indicated by BLOCK. When modified by a loop
1195 modifier, executes the BLOCK once before testing the loop condition.
1196 (On other statements the loop modifiers test the conditional first.)
1198 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1199 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1200 See L<perlsyn> for alternative strategies.
1202 =item do SUBROUTINE(LIST)
1204 A deprecated form of subroutine call. See L<perlsub>.
1208 Uses the value of EXPR as a filename and executes the contents of the
1209 file as a Perl script.
1217 except that it's more efficient and concise, keeps track of the current
1218 filename for error messages, searches the @INC directories, and updates
1219 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1220 variables. It also differs in that code evaluated with C<do FILENAME>
1221 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1222 same, however, in that it does reparse the file every time you call it,
1223 so you probably don't want to do this inside a loop.
1225 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1226 error. If C<do> can read the file but cannot compile it, it
1227 returns undef and sets an error message in C<$@>. If the file is
1228 successfully compiled, C<do> returns the value of the last expression
1231 Note that inclusion of library modules is better done with the
1232 C<use> and C<require> operators, which also do automatic error checking
1233 and raise an exception if there's a problem.
1235 You might like to use C<do> to read in a program configuration
1236 file. Manual error checking can be done this way:
1238 # read in config files: system first, then user
1239 for $file ("/share/prog/defaults.rc",
1240 "$ENV{HOME}/.someprogrc")
1242 unless ($return = do $file) {
1243 warn "couldn't parse $file: $@" if $@;
1244 warn "couldn't do $file: $!" unless defined $return;
1245 warn "couldn't run $file" unless $return;
1253 This function causes an immediate core dump. See also the B<-u>
1254 command-line switch in L<perlrun>, which does the same thing.
1255 Primarily this is so that you can use the B<undump> program (not
1256 supplied) to turn your core dump into an executable binary after
1257 having initialized all your variables at the beginning of the
1258 program. When the new binary is executed it will begin by executing
1259 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1260 Think of it as a goto with an intervening core dump and reincarnation.
1261 If C<LABEL> is omitted, restarts the program from the top.
1263 B<WARNING>: Any files opened at the time of the dump will I<not>
1264 be open any more when the program is reincarnated, with possible
1265 resulting confusion on the part of Perl.
1267 This function is now largely obsolete, partly because it's very
1268 hard to convert a core file into an executable, and because the
1269 real compiler backends for generating portable bytecode and compilable
1270 C code have superseded it. That's why you should now invoke it as
1271 C<CORE::dump()>, if you don't want to be warned against a possible
1274 If you're looking to use L<dump> to speed up your program, consider
1275 generating bytecode or native C code as described in L<perlcc>. If
1276 you're just trying to accelerate a CGI script, consider using the
1277 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1278 You might also consider autoloading or selfloading, which at least
1279 make your program I<appear> to run faster.
1283 When called in list context, returns a 2-element list consisting of the
1284 key and value for the next element of a hash, so that you can iterate over
1285 it. When called in scalar context, returns only the key for the next
1286 element in the hash.
1288 Entries are returned in an apparently random order. The actual random
1289 order is subject to change in future versions of perl, but it is
1290 guaranteed to be in the same order as either the C<keys> or C<values>
1291 function would produce on the same (unmodified) hash. Since Perl
1292 5.8.1 the ordering is different even between different runs of Perl
1293 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1295 When the hash is entirely read, a null array is returned in list context
1296 (which when assigned produces a false (C<0>) value), and C<undef> in
1297 scalar context. The next call to C<each> after that will start iterating
1298 again. There is a single iterator for each hash, shared by all C<each>,
1299 C<keys>, and C<values> function calls in the program; it can be reset by
1300 reading all the elements from the hash, or by evaluating C<keys HASH> or
1301 C<values HASH>. If you add or delete elements of a hash while you're
1302 iterating over it, you may get entries skipped or duplicated, so
1303 don't. Exception: It is always safe to delete the item most recently
1304 returned by C<each()>, which means that the following code will work:
1306 while (($key, $value) = each %hash) {
1308 delete $hash{$key}; # This is safe
1311 The following prints out your environment like the printenv(1) program,
1312 only in a different order:
1314 while (($key,$value) = each %ENV) {
1315 print "$key=$value\n";
1318 See also C<keys>, C<values> and C<sort>.
1320 =item eof FILEHANDLE
1326 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1327 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1328 gives the real filehandle. (Note that this function actually
1329 reads a character and then C<ungetc>s it, so isn't very useful in an
1330 interactive context.) Do not read from a terminal file (or call
1331 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1332 as terminals may lose the end-of-file condition if you do.
1334 An C<eof> without an argument uses the last file read. Using C<eof()>
1335 with empty parentheses is very different. It refers to the pseudo file
1336 formed from the files listed on the command line and accessed via the
1337 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1338 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1339 used will cause C<@ARGV> to be examined to determine if input is
1340 available. Similarly, an C<eof()> after C<< <> >> has returned
1341 end-of-file will assume you are processing another C<@ARGV> list,
1342 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1343 see L<perlop/"I/O Operators">.
1345 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1346 detect the end of each file, C<eof()> will only detect the end of the
1347 last file. Examples:
1349 # reset line numbering on each input file
1351 next if /^\s*#/; # skip comments
1354 close ARGV if eof; # Not eof()!
1357 # insert dashes just before last line of last file
1359 if (eof()) { # check for end of last file
1360 print "--------------\n";
1363 last if eof(); # needed if we're reading from a terminal
1366 Practical hint: you almost never need to use C<eof> in Perl, because the
1367 input operators typically return C<undef> when they run out of data, or if
1374 In the first form, the return value of EXPR is parsed and executed as if it
1375 were a little Perl program. The value of the expression (which is itself
1376 determined within scalar context) is first parsed, and if there weren't any
1377 errors, executed in the lexical context of the current Perl program, so
1378 that any variable settings or subroutine and format definitions remain
1379 afterwards. Note that the value is parsed every time the eval executes.
1380 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1381 delay parsing and subsequent execution of the text of EXPR until run time.
1383 In the second form, the code within the BLOCK is parsed only once--at the
1384 same time the code surrounding the eval itself was parsed--and executed
1385 within the context of the current Perl program. This form is typically
1386 used to trap exceptions more efficiently than the first (see below), while
1387 also providing the benefit of checking the code within BLOCK at compile
1390 The final semicolon, if any, may be omitted from the value of EXPR or within
1393 In both forms, the value returned is the value of the last expression
1394 evaluated inside the mini-program; a return statement may be also used, just
1395 as with subroutines. The expression providing the return value is evaluated
1396 in void, scalar, or list context, depending on the context of the eval itself.
1397 See L</wantarray> for more on how the evaluation context can be determined.
1399 If there is a syntax error or runtime error, or a C<die> statement is
1400 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1401 error message. If there was no error, C<$@> is guaranteed to be a null
1402 string. Beware that using C<eval> neither silences perl from printing
1403 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1404 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1405 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1406 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1408 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1409 determining whether a particular feature (such as C<socket> or C<symlink>)
1410 is implemented. It is also Perl's exception trapping mechanism, where
1411 the die operator is used to raise exceptions.
1413 If the code to be executed doesn't vary, you may use the eval-BLOCK
1414 form to trap run-time errors without incurring the penalty of
1415 recompiling each time. The error, if any, is still returned in C<$@>.
1418 # make divide-by-zero nonfatal
1419 eval { $answer = $a / $b; }; warn $@ if $@;
1421 # same thing, but less efficient
1422 eval '$answer = $a / $b'; warn $@ if $@;
1424 # a compile-time error
1425 eval { $answer = }; # WRONG
1428 eval '$answer ='; # sets $@
1430 Due to the current arguably broken state of C<__DIE__> hooks, when using
1431 the C<eval{}> form as an exception trap in libraries, you may wish not
1432 to trigger any C<__DIE__> hooks that user code may have installed.
1433 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1434 as shown in this example:
1436 # a very private exception trap for divide-by-zero
1437 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1440 This is especially significant, given that C<__DIE__> hooks can call
1441 C<die> again, which has the effect of changing their error messages:
1443 # __DIE__ hooks may modify error messages
1445 local $SIG{'__DIE__'} =
1446 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1447 eval { die "foo lives here" };
1448 print $@ if $@; # prints "bar lives here"
1451 Because this promotes action at a distance, this counterintuitive behavior
1452 may be fixed in a future release.
1454 With an C<eval>, you should be especially careful to remember what's
1455 being looked at when:
1461 eval { $x }; # CASE 4
1463 eval "\$$x++"; # CASE 5
1466 Cases 1 and 2 above behave identically: they run the code contained in
1467 the variable $x. (Although case 2 has misleading double quotes making
1468 the reader wonder what else might be happening (nothing is).) Cases 3
1469 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1470 does nothing but return the value of $x. (Case 4 is preferred for
1471 purely visual reasons, but it also has the advantage of compiling at
1472 compile-time instead of at run-time.) Case 5 is a place where
1473 normally you I<would> like to use double quotes, except that in this
1474 particular situation, you can just use symbolic references instead, as
1477 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1478 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1480 Note that as a very special case, an C<eval ''> executed within the C<DB>
1481 package doesn't see the usual surrounding lexical scope, but rather the
1482 scope of the first non-DB piece of code that called it. You don't normally
1483 need to worry about this unless you are writing a Perl debugger.
1487 =item exec PROGRAM LIST
1489 The C<exec> function executes a system command I<and never returns>--
1490 use C<system> instead of C<exec> if you want it to return. It fails and
1491 returns false only if the command does not exist I<and> it is executed
1492 directly instead of via your system's command shell (see below).
1494 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1495 warns you if there is a following statement which isn't C<die>, C<warn>,
1496 or C<exit> (if C<-w> is set - but you always do that). If you
1497 I<really> want to follow an C<exec> with some other statement, you
1498 can use one of these styles to avoid the warning:
1500 exec ('foo') or print STDERR "couldn't exec foo: $!";
1501 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1503 If there is more than one argument in LIST, or if LIST is an array
1504 with more than one value, calls execvp(3) with the arguments in LIST.
1505 If there is only one scalar argument or an array with one element in it,
1506 the argument is checked for shell metacharacters, and if there are any,
1507 the entire argument is passed to the system's command shell for parsing
1508 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1509 If there are no shell metacharacters in the argument, it is split into
1510 words and passed directly to C<execvp>, which is more efficient.
1513 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1514 exec "sort $outfile | uniq";
1516 If you don't really want to execute the first argument, but want to lie
1517 to the program you are executing about its own name, you can specify
1518 the program you actually want to run as an "indirect object" (without a
1519 comma) in front of the LIST. (This always forces interpretation of the
1520 LIST as a multivalued list, even if there is only a single scalar in
1523 $shell = '/bin/csh';
1524 exec $shell '-sh'; # pretend it's a login shell
1528 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1530 When the arguments get executed via the system shell, results will
1531 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1534 Using an indirect object with C<exec> or C<system> is also more
1535 secure. This usage (which also works fine with system()) forces
1536 interpretation of the arguments as a multivalued list, even if the
1537 list had just one argument. That way you're safe from the shell
1538 expanding wildcards or splitting up words with whitespace in them.
1540 @args = ( "echo surprise" );
1542 exec @args; # subject to shell escapes
1544 exec { $args[0] } @args; # safe even with one-arg list
1546 The first version, the one without the indirect object, ran the I<echo>
1547 program, passing it C<"surprise"> an argument. The second version
1548 didn't--it tried to run a program literally called I<"echo surprise">,
1549 didn't find it, and set C<$?> to a non-zero value indicating failure.
1551 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1552 output before the exec, but this may not be supported on some platforms
1553 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1554 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1555 open handles in order to avoid lost output.
1557 Note that C<exec> will not call your C<END> blocks, nor will it call
1558 any C<DESTROY> methods in your objects.
1562 Given an expression that specifies a hash element or array element,
1563 returns true if the specified element in the hash or array has ever
1564 been initialized, even if the corresponding value is undefined. The
1565 element is not autovivified if it doesn't exist.
1567 print "Exists\n" if exists $hash{$key};
1568 print "Defined\n" if defined $hash{$key};
1569 print "True\n" if $hash{$key};
1571 print "Exists\n" if exists $array[$index];
1572 print "Defined\n" if defined $array[$index];
1573 print "True\n" if $array[$index];
1575 A hash or array element can be true only if it's defined, and defined if
1576 it exists, but the reverse doesn't necessarily hold true.
1578 Given an expression that specifies the name of a subroutine,
1579 returns true if the specified subroutine has ever been declared, even
1580 if it is undefined. Mentioning a subroutine name for exists or defined
1581 does not count as declaring it. Note that a subroutine which does not
1582 exist may still be callable: its package may have an C<AUTOLOAD>
1583 method that makes it spring into existence the first time that it is
1584 called -- see L<perlsub>.
1586 print "Exists\n" if exists &subroutine;
1587 print "Defined\n" if defined &subroutine;
1589 Note that the EXPR can be arbitrarily complicated as long as the final
1590 operation is a hash or array key lookup or subroutine name:
1592 if (exists $ref->{A}->{B}->{$key}) { }
1593 if (exists $hash{A}{B}{$key}) { }
1595 if (exists $ref->{A}->{B}->[$ix]) { }
1596 if (exists $hash{A}{B}[$ix]) { }
1598 if (exists &{$ref->{A}{B}{$key}}) { }
1600 Although the deepest nested array or hash will not spring into existence
1601 just because its existence was tested, any intervening ones will.
1602 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1603 into existence due to the existence test for the $key element above.
1604 This happens anywhere the arrow operator is used, including even:
1607 if (exists $ref->{"Some key"}) { }
1608 print $ref; # prints HASH(0x80d3d5c)
1610 This surprising autovivification in what does not at first--or even
1611 second--glance appear to be an lvalue context may be fixed in a future
1614 Use of a subroutine call, rather than a subroutine name, as an argument
1615 to exists() is an error.
1618 exists &sub(); # Error
1622 Evaluates EXPR and exits immediately with that value. Example:
1625 exit 0 if $ans =~ /^[Xx]/;
1627 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1628 universally recognized values for EXPR are C<0> for success and C<1>
1629 for error; other values are subject to interpretation depending on the
1630 environment in which the Perl program is running. For example, exiting
1631 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1632 the mailer to return the item undelivered, but that's not true everywhere.
1634 Don't use C<exit> to abort a subroutine if there's any chance that
1635 someone might want to trap whatever error happened. Use C<die> instead,
1636 which can be trapped by an C<eval>.
1638 The exit() function does not always exit immediately. It calls any
1639 defined C<END> routines first, but these C<END> routines may not
1640 themselves abort the exit. Likewise any object destructors that need to
1641 be called are called before the real exit. If this is a problem, you
1642 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1643 See L<perlmod> for details.
1649 Returns I<e> (the natural logarithm base) to the power of EXPR.
1650 If EXPR is omitted, gives C<exp($_)>.
1652 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1654 Implements the fcntl(2) function. You'll probably have to say
1658 first to get the correct constant definitions. Argument processing and
1659 value return works just like C<ioctl> below.
1663 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1664 or die "can't fcntl F_GETFL: $!";
1666 You don't have to check for C<defined> on the return from C<fcntl>.
1667 Like C<ioctl>, it maps a C<0> return from the system call into
1668 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1669 in numeric context. It is also exempt from the normal B<-w> warnings
1670 on improper numeric conversions.
1672 Note that C<fcntl> will produce a fatal error if used on a machine that
1673 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1674 manpage to learn what functions are available on your system.
1676 Here's an example of setting a filehandle named C<REMOTE> to be
1677 non-blocking at the system level. You'll have to negotiate C<$|>
1678 on your own, though.
1680 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1682 $flags = fcntl(REMOTE, F_GETFL, 0)
1683 or die "Can't get flags for the socket: $!\n";
1685 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1686 or die "Can't set flags for the socket: $!\n";
1688 =item fileno FILEHANDLE
1690 Returns the file descriptor for a filehandle, or undefined if the
1691 filehandle is not open. This is mainly useful for constructing
1692 bitmaps for C<select> and low-level POSIX tty-handling operations.
1693 If FILEHANDLE is an expression, the value is taken as an indirect
1694 filehandle, generally its name.
1696 You can use this to find out whether two handles refer to the
1697 same underlying descriptor:
1699 if (fileno(THIS) == fileno(THAT)) {
1700 print "THIS and THAT are dups\n";
1703 (Filehandles connected to memory objects via new features of C<open> may
1704 return undefined even though they are open.)
1707 =item flock FILEHANDLE,OPERATION
1709 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1710 for success, false on failure. Produces a fatal error if used on a
1711 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1712 C<flock> is Perl's portable file locking interface, although it locks
1713 only entire files, not records.
1715 Two potentially non-obvious but traditional C<flock> semantics are
1716 that it waits indefinitely until the lock is granted, and that its locks
1717 B<merely advisory>. Such discretionary locks are more flexible, but offer
1718 fewer guarantees. This means that files locked with C<flock> may be
1719 modified by programs that do not also use C<flock>. See L<perlport>,
1720 your port's specific documentation, or your system-specific local manpages
1721 for details. It's best to assume traditional behavior if you're writing
1722 portable programs. (But if you're not, you should as always feel perfectly
1723 free to write for your own system's idiosyncrasies (sometimes called
1724 "features"). Slavish adherence to portability concerns shouldn't get
1725 in the way of your getting your job done.)
1727 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1728 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1729 you can use the symbolic names if you import them from the Fcntl module,
1730 either individually, or as a group using the ':flock' tag. LOCK_SH
1731 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1732 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1733 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1734 waiting for the lock (check the return status to see if you got it).
1736 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1737 before locking or unlocking it.
1739 Note that the emulation built with lockf(3) doesn't provide shared
1740 locks, and it requires that FILEHANDLE be open with write intent. These
1741 are the semantics that lockf(3) implements. Most if not all systems
1742 implement lockf(3) in terms of fcntl(2) locking, though, so the
1743 differing semantics shouldn't bite too many people.
1745 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1746 be open with read intent to use LOCK_SH and requires that it be open
1747 with write intent to use LOCK_EX.
1749 Note also that some versions of C<flock> cannot lock things over the
1750 network; you would need to use the more system-specific C<fcntl> for
1751 that. If you like you can force Perl to ignore your system's flock(2)
1752 function, and so provide its own fcntl(2)-based emulation, by passing
1753 the switch C<-Ud_flock> to the F<Configure> program when you configure
1756 Here's a mailbox appender for BSD systems.
1758 use Fcntl ':flock'; # import LOCK_* constants
1761 flock(MBOX,LOCK_EX);
1762 # and, in case someone appended
1763 # while we were waiting...
1768 flock(MBOX,LOCK_UN);
1771 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1772 or die "Can't open mailbox: $!";
1775 print MBOX $msg,"\n\n";
1778 On systems that support a real flock(), locks are inherited across fork()
1779 calls, whereas those that must resort to the more capricious fcntl()
1780 function lose the locks, making it harder to write servers.
1782 See also L<DB_File> for other flock() examples.
1786 Does a fork(2) system call to create a new process running the
1787 same program at the same point. It returns the child pid to the
1788 parent process, C<0> to the child process, or C<undef> if the fork is
1789 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1790 are shared, while everything else is copied. On most systems supporting
1791 fork(), great care has gone into making it extremely efficient (for
1792 example, using copy-on-write technology on data pages), making it the
1793 dominant paradigm for multitasking over the last few decades.
1795 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1796 output before forking the child process, but this may not be supported
1797 on some platforms (see L<perlport>). To be safe, you may need to set
1798 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1799 C<IO::Handle> on any open handles in order to avoid duplicate output.
1801 If you C<fork> without ever waiting on your children, you will
1802 accumulate zombies. On some systems, you can avoid this by setting
1803 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1804 forking and reaping moribund children.
1806 Note that if your forked child inherits system file descriptors like
1807 STDIN and STDOUT that are actually connected by a pipe or socket, even
1808 if you exit, then the remote server (such as, say, a CGI script or a
1809 backgrounded job launched from a remote shell) won't think you're done.
1810 You should reopen those to F</dev/null> if it's any issue.
1814 Declare a picture format for use by the C<write> function. For
1818 Test: @<<<<<<<< @||||| @>>>>>
1819 $str, $%, '$' . int($num)
1823 $num = $cost/$quantity;
1827 See L<perlform> for many details and examples.
1829 =item formline PICTURE,LIST
1831 This is an internal function used by C<format>s, though you may call it,
1832 too. It formats (see L<perlform>) a list of values according to the
1833 contents of PICTURE, placing the output into the format output
1834 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1835 Eventually, when a C<write> is done, the contents of
1836 C<$^A> are written to some filehandle, but you could also read C<$^A>
1837 yourself and then set C<$^A> back to C<"">. Note that a format typically
1838 does one C<formline> per line of form, but the C<formline> function itself
1839 doesn't care how many newlines are embedded in the PICTURE. This means
1840 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1841 You may therefore need to use multiple formlines to implement a single
1842 record format, just like the format compiler.
1844 Be careful if you put double quotes around the picture, because an C<@>
1845 character may be taken to mean the beginning of an array name.
1846 C<formline> always returns true. See L<perlform> for other examples.
1848 =item getc FILEHANDLE
1852 Returns the next character from the input file attached to FILEHANDLE,
1853 or the undefined value at end of file, or if there was an error (in
1854 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1855 STDIN. This is not particularly efficient. However, it cannot be
1856 used by itself to fetch single characters without waiting for the user
1857 to hit enter. For that, try something more like:
1860 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1863 system "stty", '-icanon', 'eol', "\001";
1869 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1872 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1876 Determination of whether $BSD_STYLE should be set
1877 is left as an exercise to the reader.
1879 The C<POSIX::getattr> function can do this more portably on
1880 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1881 module from your nearest CPAN site; details on CPAN can be found on
1886 Implements the C library function of the same name, which on most
1887 systems returns the current login from F</etc/utmp>, if any. If null,
1890 $login = getlogin || getpwuid($<) || "Kilroy";
1892 Do not consider C<getlogin> for authentication: it is not as
1893 secure as C<getpwuid>.
1895 =item getpeername SOCKET
1897 Returns the packed sockaddr address of other end of the SOCKET connection.
1900 $hersockaddr = getpeername(SOCK);
1901 ($port, $iaddr) = sockaddr_in($hersockaddr);
1902 $herhostname = gethostbyaddr($iaddr, AF_INET);
1903 $herstraddr = inet_ntoa($iaddr);
1907 Returns the current process group for the specified PID. Use
1908 a PID of C<0> to get the current process group for the
1909 current process. Will raise an exception if used on a machine that
1910 doesn't implement getpgrp(2). If PID is omitted, returns process
1911 group of current process. Note that the POSIX version of C<getpgrp>
1912 does not accept a PID argument, so only C<PID==0> is truly portable.
1916 Returns the process id of the parent process.
1918 Note for Linux users: on Linux, the C functions C<getpid()> and
1919 C<getppid()> return different values from different threads. In order to
1920 be portable, this behavior is not reflected by the perl-level function
1921 C<getppid()>, that returns a consistent value across threads. If you want
1922 to call the underlying C<getppid()>, you may use the CPAN module
1925 =item getpriority WHICH,WHO
1927 Returns the current priority for a process, a process group, or a user.
1928 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1929 machine that doesn't implement getpriority(2).
1935 =item gethostbyname NAME
1937 =item getnetbyname NAME
1939 =item getprotobyname NAME
1945 =item getservbyname NAME,PROTO
1947 =item gethostbyaddr ADDR,ADDRTYPE
1949 =item getnetbyaddr ADDR,ADDRTYPE
1951 =item getprotobynumber NUMBER
1953 =item getservbyport PORT,PROTO
1971 =item sethostent STAYOPEN
1973 =item setnetent STAYOPEN
1975 =item setprotoent STAYOPEN
1977 =item setservent STAYOPEN
1991 These routines perform the same functions as their counterparts in the
1992 system library. In list context, the return values from the
1993 various get routines are as follows:
1995 ($name,$passwd,$uid,$gid,
1996 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1997 ($name,$passwd,$gid,$members) = getgr*
1998 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1999 ($name,$aliases,$addrtype,$net) = getnet*
2000 ($name,$aliases,$proto) = getproto*
2001 ($name,$aliases,$port,$proto) = getserv*
2003 (If the entry doesn't exist you get a null list.)
2005 The exact meaning of the $gcos field varies but it usually contains
2006 the real name of the user (as opposed to the login name) and other
2007 information pertaining to the user. Beware, however, that in many
2008 system users are able to change this information and therefore it
2009 cannot be trusted and therefore the $gcos is tainted (see
2010 L<perlsec>). The $passwd and $shell, user's encrypted password and
2011 login shell, are also tainted, because of the same reason.
2013 In scalar context, you get the name, unless the function was a
2014 lookup by name, in which case you get the other thing, whatever it is.
2015 (If the entry doesn't exist you get the undefined value.) For example:
2017 $uid = getpwnam($name);
2018 $name = getpwuid($num);
2020 $gid = getgrnam($name);
2021 $name = getgrgid($num);
2025 In I<getpw*()> the fields $quota, $comment, and $expire are special
2026 cases in the sense that in many systems they are unsupported. If the
2027 $quota is unsupported, it is an empty scalar. If it is supported, it
2028 usually encodes the disk quota. If the $comment field is unsupported,
2029 it is an empty scalar. If it is supported it usually encodes some
2030 administrative comment about the user. In some systems the $quota
2031 field may be $change or $age, fields that have to do with password
2032 aging. In some systems the $comment field may be $class. The $expire
2033 field, if present, encodes the expiration period of the account or the
2034 password. For the availability and the exact meaning of these fields
2035 in your system, please consult your getpwnam(3) documentation and your
2036 F<pwd.h> file. You can also find out from within Perl what your
2037 $quota and $comment fields mean and whether you have the $expire field
2038 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2039 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2040 files are only supported if your vendor has implemented them in the
2041 intuitive fashion that calling the regular C library routines gets the
2042 shadow versions if you're running under privilege or if there exists
2043 the shadow(3) functions as found in System V ( this includes Solaris
2044 and Linux.) Those systems which implement a proprietary shadow password
2045 facility are unlikely to be supported.
2047 The $members value returned by I<getgr*()> is a space separated list of
2048 the login names of the members of the group.
2050 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2051 C, it will be returned to you via C<$?> if the function call fails. The
2052 C<@addrs> value returned by a successful call is a list of the raw
2053 addresses returned by the corresponding system library call. In the
2054 Internet domain, each address is four bytes long and you can unpack it
2055 by saying something like:
2057 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2059 The Socket library makes this slightly easier:
2062 $iaddr = inet_aton("127.1"); # or whatever address
2063 $name = gethostbyaddr($iaddr, AF_INET);
2065 # or going the other way
2066 $straddr = inet_ntoa($iaddr);
2068 If you get tired of remembering which element of the return list
2069 contains which return value, by-name interfaces are provided
2070 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2071 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2072 and C<User::grent>. These override the normal built-ins, supplying
2073 versions that return objects with the appropriate names
2074 for each field. For example:
2078 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2080 Even though it looks like they're the same method calls (uid),
2081 they aren't, because a C<File::stat> object is different from
2082 a C<User::pwent> object.
2084 =item getsockname SOCKET
2086 Returns the packed sockaddr address of this end of the SOCKET connection,
2087 in case you don't know the address because you have several different
2088 IPs that the connection might have come in on.
2091 $mysockaddr = getsockname(SOCK);
2092 ($port, $myaddr) = sockaddr_in($mysockaddr);
2093 printf "Connect to %s [%s]\n",
2094 scalar gethostbyaddr($myaddr, AF_INET),
2097 =item getsockopt SOCKET,LEVEL,OPTNAME
2099 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2100 Options may exist at multiple protocol levels depending on the socket
2101 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2102 C<Socket> module) will exist. To query options at another level the
2103 protocol number of the appropriate protocol controlling the option
2104 should be supplied. For example, to indicate that an option is to be
2105 interpreted by the TCP protocol, LEVEL should be set to the protocol
2106 number of TCP, which you can get using getprotobyname.
2108 The call returns a packed string representing the requested socket option,
2109 or C<undef> if there is an error (the error reason will be in $!). What
2110 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2111 your system documentation for details. A very common case however is that
2112 the option is an integer, in which case the result will be an packed
2113 integer which you can decode using unpack with the C<i> (or C<I>) format.
2115 An example testing if Nagle's algorithm is turned on on a socket:
2117 use Socket qw(:all);
2119 defined(my $tcp = getprotobyname("tcp"))
2120 or die "Could not determine the protocol number for tcp";
2121 # my $tcp = IPPROTO_TCP; # Alternative
2122 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2123 or die "Could not query TCP_NODELAY socket option: $!";
2124 my $nodelay = unpack("I", $packed);
2125 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2132 In list context, returns a (possibly empty) list of filename expansions on
2133 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2134 scalar context, glob iterates through such filename expansions, returning
2135 undef when the list is exhausted. This is the internal function
2136 implementing the C<< <*.c> >> operator, but you can use it directly. If
2137 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2138 more detail in L<perlop/"I/O Operators">.
2140 Beginning with v5.6.0, this operator is implemented using the standard
2141 C<File::Glob> extension. See L<File::Glob> for details.
2145 Converts a time as returned by the time function to an 8-element list
2146 with the time localized for the standard Greenwich time zone.
2147 Typically used as follows:
2150 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2153 All list elements are numeric, and come straight out of the C `struct
2154 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2155 specified time. $mday is the day of the month, and $mon is the month
2156 itself, in the range C<0..11> with 0 indicating January and 11
2157 indicating December. $year is the number of years since 1900. That
2158 is, $year is C<123> in year 2023. $wday is the day of the week, with
2159 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2160 the year, in the range C<0..364> (or C<0..365> in leap years.)
2162 Note that the $year element is I<not> simply the last two digits of
2163 the year. If you assume it is, then you create non-Y2K-compliant
2164 programs--and you wouldn't want to do that, would you?
2166 The proper way to get a complete 4-digit year is simply:
2170 And to get the last two digits of the year (e.g., '01' in 2001) do:
2172 $year = sprintf("%02d", $year % 100);
2174 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2176 In scalar context, C<gmtime()> returns the ctime(3) value:
2178 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2180 If you need local time instead of GMT use the L</localtime> builtin.
2181 See also the C<timegm> function provided by the C<Time::Local> module,
2182 and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
2184 This scalar value is B<not> locale dependent (see L<perllocale>), but is
2185 instead a Perl builtin. To get somewhat similar but locale dependent date
2186 strings, see the example in L</localtime>.
2194 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2195 execution there. It may not be used to go into any construct that
2196 requires initialization, such as a subroutine or a C<foreach> loop. It
2197 also can't be used to go into a construct that is optimized away,
2198 or to get out of a block or subroutine given to C<sort>.
2199 It can be used to go almost anywhere else within the dynamic scope,
2200 including out of subroutines, but it's usually better to use some other
2201 construct such as C<last> or C<die>. The author of Perl has never felt the
2202 need to use this form of C<goto> (in Perl, that is--C is another matter).
2203 (The difference being that C does not offer named loops combined with
2204 loop control. Perl does, and this replaces most structured uses of C<goto>
2205 in other languages.)
2207 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2208 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2209 necessarily recommended if you're optimizing for maintainability:
2211 goto ("FOO", "BAR", "GLARCH")[$i];
2213 The C<goto-&NAME> form is quite different from the other forms of
2214 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2215 doesn't have the stigma associated with other gotos. Instead, it
2216 exits the current subroutine (losing any changes set by local()) and
2217 immediately calls in its place the named subroutine using the current
2218 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2219 load another subroutine and then pretend that the other subroutine had
2220 been called in the first place (except that any modifications to C<@_>
2221 in the current subroutine are propagated to the other subroutine.)
2222 After the C<goto>, not even C<caller> will be able to tell that this
2223 routine was called first.
2225 NAME needn't be the name of a subroutine; it can be a scalar variable
2226 containing a code reference, or a block which evaluates to a code
2229 =item grep BLOCK LIST
2231 =item grep EXPR,LIST
2233 This is similar in spirit to, but not the same as, grep(1) and its
2234 relatives. In particular, it is not limited to using regular expressions.
2236 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2237 C<$_> to each element) and returns the list value consisting of those
2238 elements for which the expression evaluated to true. In scalar
2239 context, returns the number of times the expression was true.
2241 @foo = grep(!/^#/, @bar); # weed out comments
2245 @foo = grep {!/^#/} @bar; # weed out comments
2247 Note that C<$_> is an alias to the list value, so it can be used to
2248 modify the elements of the LIST. While this is useful and supported,
2249 it can cause bizarre results if the elements of LIST are not variables.
2250 Similarly, grep returns aliases into the original list, much as a for
2251 loop's index variable aliases the list elements. That is, modifying an
2252 element of a list returned by grep (for example, in a C<foreach>, C<map>
2253 or another C<grep>) actually modifies the element in the original list.
2254 This is usually something to be avoided when writing clear code.
2256 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2257 been declared with C<my $_>) then, in addition the be locally aliased to
2258 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2259 can't be seen from the outside, avoiding any potential side-effects.
2261 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2267 Interprets EXPR as a hex string and returns the corresponding value.
2268 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2269 L</oct>.) If EXPR is omitted, uses C<$_>.
2271 print hex '0xAf'; # prints '175'
2272 print hex 'aF'; # same
2274 Hex strings may only represent integers. Strings that would cause
2275 integer overflow trigger a warning. Leading whitespace is not stripped,
2276 unlike oct(). To present something as hex, look into L</printf>,
2277 L</sprintf>, or L</unpack>.
2281 There is no builtin C<import> function. It is just an ordinary
2282 method (subroutine) defined (or inherited) by modules that wish to export
2283 names to another module. The C<use> function calls the C<import> method
2284 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2286 =item index STR,SUBSTR,POSITION
2288 =item index STR,SUBSTR
2290 The index function searches for one string within another, but without
2291 the wildcard-like behavior of a full regular-expression pattern match.
2292 It returns the position of the first occurrence of SUBSTR in STR at
2293 or after POSITION. If POSITION is omitted, starts searching from the
2294 beginning of the string. The return value is based at C<0> (or whatever
2295 you've set the C<$[> variable to--but don't do that). If the substring
2296 is not found, returns one less than the base, ordinarily C<-1>.
2302 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2303 You should not use this function for rounding: one because it truncates
2304 towards C<0>, and two because machine representations of floating point
2305 numbers can sometimes produce counterintuitive results. For example,
2306 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2307 because it's really more like -268.99999999999994315658 instead. Usually,
2308 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2309 functions will serve you better than will int().
2311 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2313 Implements the ioctl(2) function. You'll probably first have to say
2315 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2317 to get the correct function definitions. If F<ioctl.ph> doesn't
2318 exist or doesn't have the correct definitions you'll have to roll your
2319 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2320 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2321 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2322 written depending on the FUNCTION--a pointer to the string value of SCALAR
2323 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2324 has no string value but does have a numeric value, that value will be
2325 passed rather than a pointer to the string value. To guarantee this to be
2326 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2327 functions may be needed to manipulate the values of structures used by
2330 The return value of C<ioctl> (and C<fcntl>) is as follows:
2332 if OS returns: then Perl returns:
2334 0 string "0 but true"
2335 anything else that number
2337 Thus Perl returns true on success and false on failure, yet you can
2338 still easily determine the actual value returned by the operating
2341 $retval = ioctl(...) || -1;
2342 printf "System returned %d\n", $retval;
2344 The special string C<"0 but true"> is exempt from B<-w> complaints
2345 about improper numeric conversions.
2347 =item join EXPR,LIST
2349 Joins the separate strings of LIST into a single string with fields
2350 separated by the value of EXPR, and returns that new string. Example:
2352 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2354 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2355 first argument. Compare L</split>.
2359 Returns a list consisting of all the keys of the named hash.
2360 (In scalar context, returns the number of keys.)
2362 The keys are returned in an apparently random order. The actual
2363 random order is subject to change in future versions of perl, but it
2364 is guaranteed to be the same order as either the C<values> or C<each>
2365 function produces (given that the hash has not been modified). Since
2366 Perl 5.8.1 the ordering is different even between different runs of
2367 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2370 As a side effect, calling keys() resets the HASH's internal iterator,
2371 see L</each>. (In particular, calling keys() in void context resets
2372 the iterator with no other overhead.)
2374 Here is yet another way to print your environment:
2377 @values = values %ENV;
2379 print pop(@keys), '=', pop(@values), "\n";
2382 or how about sorted by key:
2384 foreach $key (sort(keys %ENV)) {
2385 print $key, '=', $ENV{$key}, "\n";
2388 The returned values are copies of the original keys in the hash, so
2389 modifying them will not affect the original hash. Compare L</values>.
2391 To sort a hash by value, you'll need to use a C<sort> function.
2392 Here's a descending numeric sort of a hash by its values:
2394 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2395 printf "%4d %s\n", $hash{$key}, $key;
2398 As an lvalue C<keys> allows you to increase the number of hash buckets
2399 allocated for the given hash. This can gain you a measure of efficiency if
2400 you know the hash is going to get big. (This is similar to pre-extending
2401 an array by assigning a larger number to $#array.) If you say
2405 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2406 in fact, since it rounds up to the next power of two. These
2407 buckets will be retained even if you do C<%hash = ()>, use C<undef
2408 %hash> if you want to free the storage while C<%hash> is still in scope.
2409 You can't shrink the number of buckets allocated for the hash using
2410 C<keys> in this way (but you needn't worry about doing this by accident,
2411 as trying has no effect).
2413 See also C<each>, C<values> and C<sort>.
2415 =item kill SIGNAL, LIST
2417 Sends a signal to a list of processes. Returns the number of
2418 processes successfully signaled (which is not necessarily the
2419 same as the number actually killed).
2421 $cnt = kill 1, $child1, $child2;
2424 If SIGNAL is zero, no signal is sent to the process. This is a
2425 useful way to check that a child process is alive and hasn't changed
2426 its UID. See L<perlport> for notes on the portability of this
2429 Unlike in the shell, if SIGNAL is negative, it kills
2430 process groups instead of processes. (On System V, a negative I<PROCESS>
2431 number will also kill process groups, but that's not portable.) That
2432 means you usually want to use positive not negative signals. You may also
2433 use a signal name in quotes.
2435 See L<perlipc/"Signals"> for more details.
2441 The C<last> command is like the C<break> statement in C (as used in
2442 loops); it immediately exits the loop in question. If the LABEL is
2443 omitted, the command refers to the innermost enclosing loop. The
2444 C<continue> block, if any, is not executed:
2446 LINE: while (<STDIN>) {
2447 last LINE if /^$/; # exit when done with header
2451 C<last> cannot be used to exit a block which returns a value such as
2452 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2453 a grep() or map() operation.
2455 Note that a block by itself is semantically identical to a loop
2456 that executes once. Thus C<last> can be used to effect an early
2457 exit out of such a block.
2459 See also L</continue> for an illustration of how C<last>, C<next>, and
2466 Returns a lowercased version of EXPR. This is the internal function
2467 implementing the C<\L> escape in double-quoted strings. Respects
2468 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2469 and L<perlunicode> for more details about locale and Unicode support.
2471 If EXPR is omitted, uses C<$_>.
2477 Returns the value of EXPR with the first character lowercased. This
2478 is the internal function implementing the C<\l> escape in
2479 double-quoted strings. Respects current LC_CTYPE locale if C<use
2480 locale> in force. See L<perllocale> and L<perlunicode> for more
2481 details about locale and Unicode support.
2483 If EXPR is omitted, uses C<$_>.
2489 Returns the length in I<characters> of the value of EXPR. If EXPR is
2490 omitted, returns length of C<$_>. Note that this cannot be used on
2491 an entire array or hash to find out how many elements these have.
2492 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2494 Note the I<characters>: if the EXPR is in Unicode, you will get the
2495 number of characters, not the number of bytes. To get the length
2496 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2498 =item link OLDFILE,NEWFILE
2500 Creates a new filename linked to the old filename. Returns true for
2501 success, false otherwise.
2503 =item listen SOCKET,QUEUESIZE
2505 Does the same thing that the listen system call does. Returns true if
2506 it succeeded, false otherwise. See the example in
2507 L<perlipc/"Sockets: Client/Server Communication">.
2511 You really probably want to be using C<my> instead, because C<local> isn't
2512 what most people think of as "local". See
2513 L<perlsub/"Private Variables via my()"> for details.
2515 A local modifies the listed variables to be local to the enclosing
2516 block, file, or eval. If more than one value is listed, the list must
2517 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2518 for details, including issues with tied arrays and hashes.
2520 =item localtime EXPR
2524 Converts a time as returned by the time function to a 9-element list
2525 with the time analyzed for the local time zone. Typically used as
2529 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2532 All list elements are numeric, and come straight out of the C `struct
2533 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2534 of the specified time.
2536 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2537 the range C<0..11> with 0 indicating January and 11 indicating December.
2538 This makes it easy to get a month name from a list:
2540 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2541 print "$abbr[$mon] $mday";
2542 # $mon=9, $mday=18 gives "Oct 18"
2544 C<$year> is the number of years since 1900, not just the last two digits
2545 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2546 to get a complete 4-digit year is simply:
2550 To get the last two digits of the year (e.g., '01' in 2001) do:
2552 $year = sprintf("%02d", $year % 100);
2554 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2555 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2556 (or C<0..365> in leap years.)
2558 C<$isdst> is true if the specified time occurs during Daylight Saving
2559 Time, false otherwise.
2561 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2563 In scalar context, C<localtime()> returns the ctime(3) value:
2565 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2567 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2568 instead of local time use the L</gmtime> builtin. See also the
2569 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2570 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2571 and mktime(3) functions.
2573 To get somewhat similar but locale dependent date strings, set up your
2574 locale environment variables appropriately (please see L<perllocale>) and
2577 use POSIX qw(strftime);
2578 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2579 # or for GMT formatted appropriately for your locale:
2580 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2582 Note that the C<%a> and C<%b>, the short forms of the day of the week
2583 and the month of the year, may not necessarily be three characters wide.
2587 This function places an advisory lock on a shared variable, or referenced
2588 object contained in I<THING> until the lock goes out of scope.
2590 lock() is a "weak keyword" : this means that if you've defined a function
2591 by this name (before any calls to it), that function will be called
2592 instead. (However, if you've said C<use threads>, lock() is always a
2593 keyword.) See L<threads>.
2599 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2600 returns log of C<$_>. To get the log of another base, use basic algebra:
2601 The base-N log of a number is equal to the natural log of that number
2602 divided by the natural log of N. For example:
2606 return log($n)/log(10);
2609 See also L</exp> for the inverse operation.
2615 Does the same thing as the C<stat> function (including setting the
2616 special C<_> filehandle) but stats a symbolic link instead of the file
2617 the symbolic link points to. If symbolic links are unimplemented on
2618 your system, a normal C<stat> is done. For much more detailed
2619 information, please see the documentation for C<stat>.
2621 If EXPR is omitted, stats C<$_>.
2625 The match operator. See L<perlop>.
2627 =item map BLOCK LIST
2631 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2632 C<$_> to each element) and returns the list value composed of the
2633 results of each such evaluation. In scalar context, returns the
2634 total number of elements so generated. Evaluates BLOCK or EXPR in
2635 list context, so each element of LIST may produce zero, one, or
2636 more elements in the returned value.
2638 @chars = map(chr, @nums);
2640 translates a list of numbers to the corresponding characters. And
2642 %hash = map { getkey($_) => $_ } @array;
2644 is just a funny way to write
2647 foreach $_ (@array) {
2648 $hash{getkey($_)} = $_;
2651 Note that C<$_> is an alias to the list value, so it can be used to
2652 modify the elements of the LIST. While this is useful and supported,
2653 it can cause bizarre results if the elements of LIST are not variables.
2654 Using a regular C<foreach> loop for this purpose would be clearer in
2655 most cases. See also L</grep> for an array composed of those items of
2656 the original list for which the BLOCK or EXPR evaluates to true.
2658 If C<$_> is lexical in the scope where the C<map> appears (because it has
2659 been declared with C<my $_>) then, in addition the be locally aliased to
2660 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2661 can't be seen from the outside, avoiding any potential side-effects.
2663 C<{> starts both hash references and blocks, so C<map { ...> could be either
2664 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2665 ahead for the closing C<}> it has to take a guess at which its dealing with
2666 based what it finds just after the C<{>. Usually it gets it right, but if it
2667 doesn't it won't realize something is wrong until it gets to the C<}> and
2668 encounters the missing (or unexpected) comma. The syntax error will be
2669 reported close to the C<}> but you'll need to change something near the C<{>
2670 such as using a unary C<+> to give perl some help:
2672 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2673 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2674 %hash = map { ("\L$_", 1) } @array # this also works
2675 %hash = map { lc($_), 1 } @array # as does this.
2676 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2678 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2680 or to force an anon hash constructor use C<+{>
2682 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2684 and you get list of anonymous hashes each with only 1 entry.
2686 =item mkdir FILENAME,MASK
2688 =item mkdir FILENAME
2690 Creates the directory specified by FILENAME, with permissions
2691 specified by MASK (as modified by C<umask>). If it succeeds it
2692 returns true, otherwise it returns false and sets C<$!> (errno).
2693 If omitted, MASK defaults to 0777.
2695 In general, it is better to create directories with permissive MASK,
2696 and let the user modify that with their C<umask>, than it is to supply
2697 a restrictive MASK and give the user no way to be more permissive.
2698 The exceptions to this rule are when the file or directory should be
2699 kept private (mail files, for instance). The perlfunc(1) entry on
2700 C<umask> discusses the choice of MASK in more detail.
2702 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2703 number of trailing slashes. Some operating and filesystems do not get
2704 this right, so Perl automatically removes all trailing slashes to keep
2707 =item msgctl ID,CMD,ARG
2709 Calls the System V IPC function msgctl(2). You'll probably have to say
2713 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2714 then ARG must be a variable which will hold the returned C<msqid_ds>
2715 structure. Returns like C<ioctl>: the undefined value for error,
2716 C<"0 but true"> for zero, or the actual return value otherwise. See also
2717 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2719 =item msgget KEY,FLAGS
2721 Calls the System V IPC function msgget(2). Returns the message queue
2722 id, or the undefined value if there is an error. See also
2723 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2725 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2727 Calls the System V IPC function msgrcv to receive a message from
2728 message queue ID into variable VAR with a maximum message size of
2729 SIZE. Note that when a message is received, the message type as a
2730 native long integer will be the first thing in VAR, followed by the
2731 actual message. This packing may be opened with C<unpack("l! a*")>.
2732 Taints the variable. Returns true if successful, or false if there is
2733 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2734 C<IPC::SysV::Msg> documentation.
2736 =item msgsnd ID,MSG,FLAGS
2738 Calls the System V IPC function msgsnd to send the message MSG to the
2739 message queue ID. MSG must begin with the native long integer message
2740 type, and be followed by the length of the actual message, and finally
2741 the message itself. This kind of packing can be achieved with
2742 C<pack("l! a*", $type, $message)>. Returns true if successful,
2743 or false if there is an error. See also C<IPC::SysV>
2744 and C<IPC::SysV::Msg> documentation.
2750 =item my EXPR : ATTRS
2752 =item my TYPE EXPR : ATTRS
2754 A C<my> declares the listed variables to be local (lexically) to the
2755 enclosing block, file, or C<eval>. If more than one value is listed,
2756 the list must be placed in parentheses.
2758 The exact semantics and interface of TYPE and ATTRS are still
2759 evolving. TYPE is currently bound to the use of C<fields> pragma,
2760 and attributes are handled using the C<attributes> pragma, or starting
2761 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2762 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2763 L<attributes>, and L<Attribute::Handlers>.
2769 The C<next> command is like the C<continue> statement in C; it starts
2770 the next iteration of the loop:
2772 LINE: while (<STDIN>) {
2773 next LINE if /^#/; # discard comments
2777 Note that if there were a C<continue> block on the above, it would get
2778 executed even on discarded lines. If the LABEL is omitted, the command
2779 refers to the innermost enclosing loop.
2781 C<next> cannot be used to exit a block which returns a value such as
2782 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2783 a grep() or map() operation.
2785 Note that a block by itself is semantically identical to a loop
2786 that executes once. Thus C<next> will exit such a block early.
2788 See also L</continue> for an illustration of how C<last>, C<next>, and
2791 =item no Module VERSION LIST
2793 =item no Module VERSION
2795 =item no Module LIST
2799 See the C<use> function, of which C<no> is the opposite.
2805 Interprets EXPR as an octal string and returns the corresponding
2806 value. (If EXPR happens to start off with C<0x>, interprets it as a
2807 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2808 binary string. Leading whitespace is ignored in all three cases.)
2809 The following will handle decimal, binary, octal, and hex in the standard
2812 $val = oct($val) if $val =~ /^0/;
2814 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2815 in octal), use sprintf() or printf():
2817 $perms = (stat("filename"))[2] & 07777;
2818 $oct_perms = sprintf "%lo", $perms;
2820 The oct() function is commonly used when a string such as C<644> needs
2821 to be converted into a file mode, for example. (Although perl will
2822 automatically convert strings into numbers as needed, this automatic
2823 conversion assumes base 10.)
2825 =item open FILEHANDLE,EXPR
2827 =item open FILEHANDLE,MODE,EXPR
2829 =item open FILEHANDLE,MODE,EXPR,LIST
2831 =item open FILEHANDLE,MODE,REFERENCE
2833 =item open FILEHANDLE
2835 Opens the file whose filename is given by EXPR, and associates it with
2838 (The following is a comprehensive reference to open(): for a gentler
2839 introduction you may consider L<perlopentut>.)
2841 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2842 the variable is assigned a reference to a new anonymous filehandle,
2843 otherwise if FILEHANDLE is an expression, its value is used as the name of
2844 the real filehandle wanted. (This is considered a symbolic reference, so
2845 C<use strict 'refs'> should I<not> be in effect.)
2847 If EXPR is omitted, the scalar variable of the same name as the
2848 FILEHANDLE contains the filename. (Note that lexical variables--those
2849 declared with C<my>--will not work for this purpose; so if you're
2850 using C<my>, specify EXPR in your call to open.)
2852 If three or more arguments are specified then the mode of opening and
2853 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2854 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2855 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2856 the file is opened for appending, again being created if necessary.
2858 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2859 indicate that you want both read and write access to the file; thus
2860 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2861 '+>' >> mode would clobber the file first. You can't usually use
2862 either read-write mode for updating textfiles, since they have
2863 variable length records. See the B<-i> switch in L<perlrun> for a
2864 better approach. The file is created with permissions of C<0666>
2865 modified by the process' C<umask> value.
2867 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2868 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2870 In the 2-arguments (and 1-argument) form of the call the mode and
2871 filename should be concatenated (in this order), possibly separated by
2872 spaces. It is possible to omit the mode in these forms if the mode is
2875 If the filename begins with C<'|'>, the filename is interpreted as a
2876 command to which output is to be piped, and if the filename ends with a
2877 C<'|'>, the filename is interpreted as a command which pipes output to
2878 us. See L<perlipc/"Using open() for IPC">
2879 for more examples of this. (You are not allowed to C<open> to a command
2880 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2881 and L<perlipc/"Bidirectional Communication with Another Process">
2884 For three or more arguments if MODE is C<'|-'>, the filename is
2885 interpreted as a command to which output is to be piped, and if MODE
2886 is C<'-|'>, the filename is interpreted as a command which pipes
2887 output to us. In the 2-arguments (and 1-argument) form one should
2888 replace dash (C<'-'>) with the command.
2889 See L<perlipc/"Using open() for IPC"> for more examples of this.
2890 (You are not allowed to C<open> to a command that pipes both in I<and>
2891 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2892 L<perlipc/"Bidirectional Communication"> for alternatives.)
2894 In the three-or-more argument form of pipe opens, if LIST is specified
2895 (extra arguments after the command name) then LIST becomes arguments
2896 to the command invoked if the platform supports it. The meaning of
2897 C<open> with more than three arguments for non-pipe modes is not yet
2898 specified. Experimental "layers" may give extra LIST arguments
2901 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2902 and opening C<< '>-' >> opens STDOUT.
2904 You may use the three-argument form of open to specify IO "layers"
2905 (sometimes also referred to as "disciplines") to be applied to the handle
2906 that affect how the input and output are processed (see L<open> and
2907 L<PerlIO> for more details). For example
2909 open(FH, "<:utf8", "file")
2911 will open the UTF-8 encoded file containing Unicode characters,
2912 see L<perluniintro>. (Note that if layers are specified in the
2913 three-arg form then default layers set by the C<open> pragma are
2916 Open returns nonzero upon success, the undefined value otherwise. If
2917 the C<open> involved a pipe, the return value happens to be the pid of
2920 If you're running Perl on a system that distinguishes between text
2921 files and binary files, then you should check out L</binmode> for tips
2922 for dealing with this. The key distinction between systems that need
2923 C<binmode> and those that don't is their text file formats. Systems
2924 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2925 character, and which encode that character in C as C<"\n">, do not
2926 need C<binmode>. The rest need it.
2928 When opening a file, it's usually a bad idea to continue normal execution
2929 if the request failed, so C<open> is frequently used in connection with
2930 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2931 where you want to make a nicely formatted error message (but there are
2932 modules that can help with that problem)) you should always check
2933 the return value from opening a file. The infrequent exception is when
2934 working with an unopened filehandle is actually what you want to do.
2936 As a special case the 3 arg form with a read/write mode and the third
2937 argument being C<undef>:
2939 open(TMP, "+>", undef) or die ...
2941 opens a filehandle to an anonymous temporary file. Also using "+<"
2942 works for symmetry, but you really should consider writing something
2943 to the temporary file first. You will need to seek() to do the
2946 Since v5.8.0, perl has built using PerlIO by default. Unless you've
2947 changed this (ie Configure -Uuseperlio), you can open file handles to
2948 "in memory" files held in Perl scalars via:
2950 open($fh, '>', \$variable) || ..
2952 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2953 file, you have to close it first:
2956 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2961 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2962 while (<ARTICLE>) {...
2964 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2965 # if the open fails, output is discarded
2967 open(DBASE, '+<', 'dbase.mine') # open for update
2968 or die "Can't open 'dbase.mine' for update: $!";
2970 open(DBASE, '+<dbase.mine') # ditto
2971 or die "Can't open 'dbase.mine' for update: $!";
2973 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2974 or die "Can't start caesar: $!";
2976 open(ARTICLE, "caesar <$article |") # ditto
2977 or die "Can't start caesar: $!";
2979 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
2980 or die "Can't start sort: $!";
2983 open(MEMORY,'>', \$var)
2984 or die "Can't open memory file: $!";
2985 print MEMORY "foo!\n"; # output will end up in $var
2987 # process argument list of files along with any includes
2989 foreach $file (@ARGV) {
2990 process($file, 'fh00');
2994 my($filename, $input) = @_;
2995 $input++; # this is a string increment
2996 unless (open($input, $filename)) {
2997 print STDERR "Can't open $filename: $!\n";
3002 while (<$input>) { # note use of indirection
3003 if (/^#include "(.*)"/) {
3004 process($1, $input);
3011 See L<perliol> for detailed info on PerlIO.
3013 You may also, in the Bourne shell tradition, specify an EXPR beginning
3014 with C<< '>&' >>, in which case the rest of the string is interpreted
3015 as the name of a filehandle (or file descriptor, if numeric) to be
3016 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3017 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3018 The mode you specify should match the mode of the original filehandle.
3019 (Duping a filehandle does not take into account any existing contents
3020 of IO buffers.) If you use the 3 arg form then you can pass either a
3021 number, the name of a filehandle or the normal "reference to a glob".
3023 Here is a script that saves, redirects, and restores C<STDOUT> and
3024 C<STDERR> using various methods:
3027 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3028 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3030 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3031 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3033 select STDERR; $| = 1; # make unbuffered
3034 select STDOUT; $| = 1; # make unbuffered
3036 print STDOUT "stdout 1\n"; # this works for
3037 print STDERR "stderr 1\n"; # subprocesses too
3039 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3040 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3042 print STDOUT "stdout 2\n";
3043 print STDERR "stderr 2\n";
3045 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3046 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3047 that file descriptor (and not call L<dup(2)>); this is more
3048 parsimonious of file descriptors. For example:
3050 # open for input, reusing the fileno of $fd
3051 open(FILEHANDLE, "<&=$fd")
3055 open(FILEHANDLE, "<&=", $fd)
3059 # open for append, using the fileno of OLDFH
3060 open(FH, ">>&=", OLDFH)
3064 open(FH, ">>&=OLDFH")
3066 Being parsimonious on filehandles is also useful (besides being
3067 parsimonious) for example when something is dependent on file
3068 descriptors, like for example locking using flock(). If you do just
3069 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3070 descriptor as B, and therefore flock(A) will not flock(B), and vice
3071 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3072 the same file descriptor.
3074 Note that if you are using Perls older than 5.8.0, Perl will be using
3075 the standard C libraries' fdopen() to implement the "=" functionality.
3076 On many UNIX systems fdopen() fails when file descriptors exceed a
3077 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3078 most often the default.
3080 You can see whether Perl has been compiled with PerlIO or not by
3081 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3082 is C<define>, you have PerlIO, otherwise you don't.
3084 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3085 with 2-arguments (or 1-argument) form of open(), then
3086 there is an implicit fork done, and the return value of open is the pid
3087 of the child within the parent process, and C<0> within the child
3088 process. (Use C<defined($pid)> to determine whether the open was successful.)
3089 The filehandle behaves normally for the parent, but i/o to that
3090 filehandle is piped from/to the STDOUT/STDIN of the child process.
3091 In the child process the filehandle isn't opened--i/o happens from/to
3092 the new STDOUT or STDIN. Typically this is used like the normal
3093 piped open when you want to exercise more control over just how the
3094 pipe command gets executed, such as when you are running setuid, and
3095 don't want to have to scan shell commands for metacharacters.
3096 The following triples are more or less equivalent:
3098 open(FOO, "|tr '[a-z]' '[A-Z]'");
3099 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3100 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3101 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3103 open(FOO, "cat -n '$file'|");
3104 open(FOO, '-|', "cat -n '$file'");
3105 open(FOO, '-|') || exec 'cat', '-n', $file;
3106 open(FOO, '-|', "cat", '-n', $file);
3108 The last example in each block shows the pipe as "list form", which is
3109 not yet supported on all platforms. A good rule of thumb is that if
3110 your platform has true C<fork()> (in other words, if your platform is
3111 UNIX) you can use the list form.
3113 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3115 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3116 output before any operation that may do a fork, but this may not be
3117 supported on some platforms (see L<perlport>). To be safe, you may need
3118 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3119 of C<IO::Handle> on any open handles.
3121 On systems that support a close-on-exec flag on files, the flag will
3122 be set for the newly opened file descriptor as determined by the value
3123 of $^F. See L<perlvar/$^F>.
3125 Closing any piped filehandle causes the parent process to wait for the
3126 child to finish, and returns the status value in C<$?>.
3128 The filename passed to 2-argument (or 1-argument) form of open() will
3129 have leading and trailing whitespace deleted, and the normal
3130 redirection characters honored. This property, known as "magic open",
3131 can often be used to good effect. A user could specify a filename of
3132 F<"rsh cat file |">, or you could change certain filenames as needed:
3134 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3135 open(FH, $filename) or die "Can't open $filename: $!";
3137 Use 3-argument form to open a file with arbitrary weird characters in it,
3139 open(FOO, '<', $file);
3141 otherwise it's necessary to protect any leading and trailing whitespace:
3143 $file =~ s#^(\s)#./$1#;
3144 open(FOO, "< $file\0");
3146 (this may not work on some bizarre filesystems). One should
3147 conscientiously choose between the I<magic> and 3-arguments form
3152 will allow the user to specify an argument of the form C<"rsh cat file |">,
3153 but will not work on a filename which happens to have a trailing space, while
3155 open IN, '<', $ARGV[0];
3157 will have exactly the opposite restrictions.
3159 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3160 should use the C<sysopen> function, which involves no such magic (but
3161 may use subtly different filemodes than Perl open(), which is mapped
3162 to C fopen()). This is
3163 another way to protect your filenames from interpretation. For example:
3166 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3167 or die "sysopen $path: $!";
3168 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3169 print HANDLE "stuff $$\n";
3171 print "File contains: ", <HANDLE>;
3173 Using the constructor from the C<IO::Handle> package (or one of its
3174 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3175 filehandles that have the scope of whatever variables hold references to
3176 them, and automatically close whenever and however you leave that scope:
3180 sub read_myfile_munged {
3182 my $handle = new IO::File;
3183 open($handle, "myfile") or die "myfile: $!";
3185 or return (); # Automatically closed here.
3186 mung $first or die "mung failed"; # Or here.
3187 return $first, <$handle> if $ALL; # Or here.
3191 See L</seek> for some details about mixing reading and writing.
3193 =item opendir DIRHANDLE,EXPR
3195 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3196 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3197 DIRHANDLE may be an expression whose value can be used as an indirect
3198 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3199 scalar variable (or array or hash element), the variable is assigned a
3200 reference to a new anonymous dirhandle.
3201 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3207 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3208 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3211 For the reverse, see L</chr>.
3212 See L<perlunicode> and L<encoding> for more about Unicode.
3218 =item our EXPR : ATTRS
3220 =item our TYPE EXPR : ATTRS
3222 An C<our> declares the listed variables to be valid globals within
3223 the enclosing block, file, or C<eval>. That is, it has the same
3224 scoping rules as a "my" declaration, but does not create a local
3225 variable. If more than one value is listed, the list must be placed
3226 in parentheses. The C<our> declaration has no semantic effect unless
3227 "use strict vars" is in effect, in which case it lets you use the
3228 declared global variable without qualifying it with a package name.
3229 (But only within the lexical scope of the C<our> declaration. In this
3230 it differs from "use vars", which is package scoped.)
3232 An C<our> declaration declares a global variable that will be visible
3233 across its entire lexical scope, even across package boundaries. The
3234 package in which the variable is entered is determined at the point
3235 of the declaration, not at the point of use. This means the following
3239 our $bar; # declares $Foo::bar for rest of lexical scope
3243 print $bar; # prints 20
3245 Multiple C<our> declarations in the same lexical scope are allowed
3246 if they are in different packages. If they happened to be in the same
3247 package, Perl will emit warnings if you have asked for them.
3251 our $bar; # declares $Foo::bar for rest of lexical scope
3255 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3256 print $bar; # prints 30
3258 our $bar; # emits warning
3260 An C<our> declaration may also have a list of attributes associated
3263 The exact semantics and interface of TYPE and ATTRS are still
3264 evolving. TYPE is currently bound to the use of C<fields> pragma,
3265 and attributes are handled using the C<attributes> pragma, or starting
3266 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3267 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3268 L<attributes>, and L<Attribute::Handlers>.
3270 The only currently recognized C<our()> attribute is C<unique> which
3271 indicates that a single copy of the global is to be used by all
3272 interpreters should the program happen to be running in a
3273 multi-interpreter environment. (The default behaviour would be for
3274 each interpreter to have its own copy of the global.) Examples:
3276 our @EXPORT : unique = qw(foo);
3277 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3278 our $VERSION : unique = "1.00";
3280 Note that this attribute also has the effect of making the global
3281 readonly when the first new interpreter is cloned (for example,
3282 when the first new thread is created).
3284 Multi-interpreter environments can come to being either through the
3285 fork() emulation on Windows platforms, or by embedding perl in a
3286 multi-threaded application. The C<unique> attribute does nothing in
3287 all other environments.
3289 Warning: the current implementation of this attribute operates on the
3290 typeglob associated with the variable; this means that C<our $x : unique>
3291 also has the effect of C<our @x : unique; our %x : unique>. This may be
3294 =item pack TEMPLATE,LIST
3296 Takes a LIST of values and converts it into a string using the rules
3297 given by the TEMPLATE. The resulting string is the concatenation of
3298 the converted values. Typically, each converted value looks
3299 like its machine-level representation. For example, on 32-bit machines
3300 an integer may be represented by a sequence of 4 bytes which will be
3301 converted to a sequence of 4 characters.
3303 The TEMPLATE is a sequence of characters that give the order and type
3304 of values, as follows:
3306 a A string with arbitrary binary data, will be null padded.
3307 A A text (ASCII) string, will be space padded.
3308 Z A null terminated (ASCIZ) string, will be null padded.
3310 b A bit string (ascending bit order inside each byte, like vec()).
3311 B A bit string (descending bit order inside each byte).
3312 h A hex string (low nybble first).
3313 H A hex string (high nybble first).
3315 c A signed char (8-bit) value.
3316 C An unsigned C char (octet) even under Unicode. Should normally not
3317 be used. See U and W instead.
3318 W An unsigned char value (can be greater than 255).
3320 s A signed short (16-bit) value.
3321 S An unsigned short value.
3323 l A signed long (32-bit) value.
3324 L An unsigned long value.
3326 q A signed quad (64-bit) value.
3327 Q An unsigned quad value.
3328 (Quads are available only if your system supports 64-bit
3329 integer values _and_ if Perl has been compiled to support those.
3330 Causes a fatal error otherwise.)
3332 i A signed integer value.
3333 I A unsigned integer value.
3334 (This 'integer' is _at_least_ 32 bits wide. Its exact
3335 size depends on what a local C compiler calls 'int'.)
3337 n An unsigned short (16-bit) in "network" (big-endian) order.
3338 N An unsigned long (32-bit) in "network" (big-endian) order.
3339 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3340 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3342 j A Perl internal signed integer value (IV).
3343 J A Perl internal unsigned integer value (UV).
3345 f A single-precision float in the native format.
3346 d A double-precision float in the native format.
3348 F A Perl internal floating point value (NV) in the native format
3349 D A long double-precision float in the native format.
3350 (Long doubles are available only if your system supports long
3351 double values _and_ if Perl has been compiled to support those.
3352 Causes a fatal error otherwise.)
3354 p A pointer to a null-terminated string.
3355 P A pointer to a structure (fixed-length string).
3357 u A uuencoded string.
3358 U A Unicode character number. Encodes to UTF-8 internally
3359 (or UTF-EBCDIC in EBCDIC platforms).
3361 w A BER compressed integer. Its bytes represent an unsigned
3362 integer in base 128, most significant digit first, with as
3363 few digits as possible. Bit eight (the high bit) is set
3364 on each byte except the last.
3368 @ Null fill or truncate to absolute position, counted from the
3369 start of the innermost ()-group.
3370 . Null fill or truncate to absolute position specified by value.
3371 ( Start of a ()-group.
3373 Some letters in the TEMPLATE may optionally be followed by one or
3374 more of these modifiers (the second column lists the letters for
3375 which the modifier is valid):
3377 ! sSlLiI Forces native (short, long, int) sizes instead
3378 of fixed (16-/32-bit) sizes.
3380 xX Make x and X act as alignment commands.
3382 nNvV Treat integers as signed instead of unsigned.
3384 @. Specify position as byte offset in the internal
3385 representation of the packed string. Efficient but
3388 > sSiIlLqQ Force big-endian byte-order on the type.
3389 jJfFdDpP (The "big end" touches the construct.)
3391 < sSiIlLqQ Force little-endian byte-order on the type.
3392 jJfFdDpP (The "little end" touches the construct.)
3394 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3395 in which case they force a certain byte-order on all components of
3396 that group, including subgroups.
3398 The following rules apply:
3404 Each letter may optionally be followed by a number giving a repeat
3405 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3406 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3407 that many values from the LIST. A C<*> for the repeat count means to
3408 use however many items are left, except for C<@>, C<x>, C<X>, where it
3409 is equivalent to C<0>, for <.> where it means relative to string start
3410 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3411 A numeric repeat count may optionally be enclosed in brackets, as in
3412 C<pack 'C[80]', @arr>.
3414 One can replace the numeric repeat count by a template enclosed in brackets;
3415 then the packed length of this template in bytes is used as a count.
3416 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3417 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3418 If the template in brackets contains alignment commands (such as C<x![d]>),
3419 its packed length is calculated as if the start of the template has the maximal
3422 When used with C<Z>, C<*> results in the addition of a trailing null
3423 byte (so the packed result will be one longer than the byte C<length>
3426 When used with C<@>, the repeat count represents an offset from the start
3427 of the innermost () group.
3429 When used with C<.>, the repeat count is used to determine the starting
3430 position from where the value offset is calculated. If the repeat count
3431 is 0, it's relative to the current position. If the repeat count is C<*>,
3432 the offset is relative to the start of the packed string. And if its an
3433 integer C<n> the offset is relative to the start of the n-th innermost
3434 () group (or the start of the string if C<n> is bigger then the group
3437 The repeat count for C<u> is interpreted as the maximal number of bytes
3438 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3439 count should not be more than 65.
3443 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3444 string of length count, padding with nulls or spaces as necessary. When
3445 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3446 after the first null, and C<a> returns data verbatim.
3448 If the value-to-pack is too long, it is truncated. If too long and an
3449 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3450 by a null byte. Thus C<Z> always packs a trailing null (except when the
3455 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3456 Each character of the input field of pack() generates 1 bit of the result.
3457 Each result bit is based on the least-significant bit of the corresponding
3458 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3459 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3461 Starting from the beginning of the input string of pack(), each 8-tuple
3462 of characters is converted to 1 character of output. With format C<b>
3463 the first character of the 8-tuple determines the least-significant bit of a
3464 character, and with format C<B> it determines the most-significant bit of
3467 If the length of the input string is not exactly divisible by 8, the
3468 remainder is packed as if the input string were padded by null characters
3469 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3471 If the input string of pack() is longer than needed, extra characters are
3472 ignored. A C<*> for the repeat count of pack() means to use all the
3473 characters of the input field. On unpack()ing the bits are converted to a
3474 string of C<"0">s and C<"1">s.
3478 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3479 representable as hexadecimal digits, 0-9a-f) long.
3481 Each character of the input field of pack() generates 4 bits of the result.
3482 For non-alphabetical characters the result is based on the 4 least-significant
3483 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3484 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3485 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3486 is compatible with the usual hexadecimal digits, so that C<"a"> and
3487 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3488 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3490 Starting from the beginning of the input string of pack(), each pair
3491 of characters is converted to 1 character of output. With format C<h> the
3492 first character of the pair determines the least-significant nybble of the
3493 output character, and with format C<H> it determines the most-significant
3496 If the length of the input string is not even, it behaves as if padded
3497 by a null character at the end. Similarly, during unpack()ing the "extra"
3498 nybbles are ignored.
3500 If the input string of pack() is longer than needed, extra characters are
3502 A C<*> for the repeat count of pack() means to use all the characters of
3503 the input field. On unpack()ing the nybbles are converted to a string
3504 of hexadecimal digits.
3508 The C<p> type packs a pointer to a null-terminated string. You are
3509 responsible for ensuring the string is not a temporary value (which can
3510 potentially get deallocated before you get around to using the packed result).
3511 The C<P> type packs a pointer to a structure of the size indicated by the
3512 length. A NULL pointer is created if the corresponding value for C<p> or
3513 C<P> is C<undef>, similarly for unpack().
3515 If your system has a strange pointer size (i.e. a pointer is neither as
3516 big as an int nor as big as a long), it may not be possible to pack or
3517 unpack pointers in big- or little-endian byte order. Attempting to do
3518 so will result in a fatal error.
3522 The C</> template character allows packing and unpacking of a sequence of
3523 items where the packed structure contains a packed item count followed by
3524 the packed items themselves.
3525 You write I<length-item>C</>I<sequence-item>.
3527 The I<length-item> can be any C<pack> template letter, and describes
3528 how the length value is packed. The ones likely to be of most use are
3529 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3530 SNMP) and C<N> (for Sun XDR).
3532 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3533 the minimum of that and the number of available items is used as argument
3534 for the I<length-item>. If it has no repeat count or uses a '*', the number
3535 of available items is used. For C<unpack> the repeat count is always obtained
3536 by decoding the packed item count, and the I<sequence-item> must not have a
3539 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3540 the I<length-item> is a string length, not a number of strings. If there is
3541 an explicit repeat count for pack, the packed string will be adjusted to that
3544 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3545 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond', 'J')
3546 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3547 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3549 The I<length-item> is not returned explicitly from C<unpack>.
3551 Adding a count to the I<length-item> letter is unlikely to do anything
3552 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3553 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3554 which Perl does not regard as legal in numeric strings.
3558 The integer types C<s>, C<S>, C<l>, and C<L> may be
3559 followed by a C<!> modifier to signify native shorts or
3560 longs--as you can see from above for example a bare C<l> does mean
3561 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3562 may be larger. This is an issue mainly in 64-bit platforms. You can
3563 see whether using C<!> makes any difference by
3565 print length(pack("s")), " ", length(pack("s!")), "\n";
3566 print length(pack("l")), " ", length(pack("l!")), "\n";
3568 C<i!> and C<I!> also work but only because of completeness;
3569 they are identical to C<i> and C<I>.
3571 The actual sizes (in bytes) of native shorts, ints, longs, and long
3572 longs on the platform where Perl was built are also available via
3576 print $Config{shortsize}, "\n";
3577 print $Config{intsize}, "\n";
3578 print $Config{longsize}, "\n";
3579 print $Config{longlongsize}, "\n";
3581 (The C<$Config{longlongsize}> will be undefined if your system does
3582 not support long longs.)
3586 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3587 are inherently non-portable between processors and operating systems
3588 because they obey the native byteorder and endianness. For example a
3589 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3590 (arranged in and handled by the CPU registers) into bytes as
3592 0x12 0x34 0x56 0x78 # big-endian
3593 0x78 0x56 0x34 0x12 # little-endian
3595 Basically, the Intel and VAX CPUs are little-endian, while everybody
3596 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3597 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3598 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3601 The names `big-endian' and `little-endian' are comic references to
3602 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3603 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3604 the egg-eating habits of the Lilliputians.
3606 Some systems may have even weirder byte orders such as
3611 You can see your system's preference with
3613 print join(" ", map { sprintf "%#02x", $_ }
3614 unpack("W*",pack("L",0x12345678))), "\n";
3616 The byteorder on the platform where Perl was built is also available
3620 print $Config{byteorder}, "\n";
3622 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3623 and C<'87654321'> are big-endian.
3625 If you want portable packed integers you can either use the formats
3626 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3627 modifiers. These modifiers are only available as of perl 5.9.2.
3628 See also L<perlport>.
3632 All integer and floating point formats as well as C<p> and C<P> and
3633 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3634 to force big- or little- endian byte-order, respectively.
3635 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3636 signed integers, 64-bit integers and floating point values. However,
3637 there are some things to keep in mind.
3639 Exchanging signed integers between different platforms only works
3640 if all platforms store them in the same format. Most platforms store
3641 signed integers in two's complement, so usually this is not an issue.
3643 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3644 formats on big- or little-endian machines. Otherwise, attempting to
3645 do so will result in a fatal error.
3647 Forcing big- or little-endian byte-order on floating point values for
3648 data exchange can only work if all platforms are using the same
3649 binary representation (e.g. IEEE floating point format). Even if all
3650 platforms are using IEEE, there may be subtle differences. Being able
3651 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3652 but also very dangerous if you don't know exactly what you're doing.
3653 It is definetely not a general way to portably store floating point
3656 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3657 all types inside the group that accept the byte-order modifiers,
3658 including all subgroups. It will silently be ignored for all other
3659 types. You are not allowed to override the byte-order within a group
3660 that already has a byte-order modifier suffix.
3664 Real numbers (floats and doubles) are in the native machine format only;
3665 due to the multiplicity of floating formats around, and the lack of a
3666 standard "network" representation, no facility for interchange has been
3667 made. This means that packed floating point data written on one machine
3668 may not be readable on another - even if both use IEEE floating point
3669 arithmetic (as the endian-ness of the memory representation is not part
3670 of the IEEE spec). See also L<perlport>.
3672 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3673 modifiers to force big- or little-endian byte-order on floating point values.
3675 Note that Perl uses doubles (or long doubles, if configured) internally for
3676 all numeric calculation, and converting from double into float and thence back
3677 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3678 will not in general equal $foo).
3682 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3683 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3684 where the packed string is processed in its UTF-8-encoded Unicode form on
3685 a byte by byte basis. Character mode is the default unless the format string
3686 starts with an C<U>. You can switch mode at any moment with an explicit
3687 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3688 or until the end of the ()-group in which it was entered.
3692 You must yourself do any alignment or padding by inserting for example
3693 enough C<'x'>es while packing. There is no way to pack() and unpack()
3694 could know where the characters are going to or coming from. Therefore
3695 C<pack> (and C<unpack>) handle their output and input as flat
3696 sequences of characters.
3700 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3701 take a repeat count, both as postfix, and for unpack() also via the C</>
3702 template character. Within each repetition of a group, positioning with
3703 C<@> starts again at 0. Therefore, the result of
3705 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3707 is the string "\0a\0\0bc".
3711 C<x> and C<X> accept C<!> modifier. In this case they act as
3712 alignment commands: they jump forward/back to the closest position
3713 aligned at a multiple of C<count> characters. For example, to pack() or
3714 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3715 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3716 aligned on the double's size.
3718 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3719 both result in no-ops.
3723 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3724 will represent signed 16-/32-bit integers in big-/little-endian order.
3725 This is only portable if all platforms sharing the packed data use the
3726 same binary representation for signed integers (e.g. all platforms are
3727 using two's complement representation).
3731 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3732 White space may be used to separate pack codes from each other, but
3733 modifiers and a repeat count must follow immediately.
3737 If TEMPLATE requires more arguments to pack() than actually given, pack()
3738 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3739 to pack() than actually given, extra arguments are ignored.
3745 $foo = pack("WWWW",65,66,67,68);
3747 $foo = pack("W4",65,66,67,68);
3749 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3750 # same thing with Unicode circled letters.
3751 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3752 # same thing with Unicode circled letters. You don't get the UTF-8
3753 # bytes because the U at the start of the format caused a switch to
3754 # U0-mode, so the UTF-8 bytes get joined into characters
3755 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3756 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3757 # This is the UTF-8 encoding of the string in the previous example
3759 $foo = pack("ccxxcc",65,66,67,68);
3762 # note: the above examples featuring "W" and "c" are true
3763 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3764 # and UTF-8. In EBCDIC the first example would be
3765 # $foo = pack("WWWW",193,194,195,196);
3767 $foo = pack("s2",1,2);
3768 # "\1\0\2\0" on little-endian
3769 # "\0\1\0\2" on big-endian
3771 $foo = pack("a4","abcd","x","y","z");
3774 $foo = pack("aaaa","abcd","x","y","z");
3777 $foo = pack("a14","abcdefg");
3778 # "abcdefg\0\0\0\0\0\0\0"
3780 $foo = pack("i9pl", gmtime);
3781 # a real struct tm (on my system anyway)
3783 $utmp_template = "Z8 Z8 Z16 L";
3784 $utmp = pack($utmp_template, @utmp1);
3785 # a struct utmp (BSDish)
3787 @utmp2 = unpack($utmp_template, $utmp);
3788 # "@utmp1" eq "@utmp2"
3791 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3794 $foo = pack('sx2l', 12, 34);
3795 # short 12, two zero bytes padding, long 34
3796 $bar = pack('s@4l', 12, 34);
3797 # short 12, zero fill to position 4, long 34
3799 $baz = pack('s.l', 12, 4, 34);
3800 # short 12, zero fill to position 4, long 34
3802 $foo = pack('nN', 42, 4711);
3803 # pack big-endian 16- and 32-bit unsigned integers
3804 $foo = pack('S>L>', 42, 4711);
3806 $foo = pack('s<l<', -42, 4711);
3807 # pack little-endian 16- and 32-bit signed integers
3808 $foo = pack('(sl)<', -42, 4711);
3811 The same template may generally also be used in unpack().
3813 =item package NAMESPACE
3817 Declares the compilation unit as being in the given namespace. The scope
3818 of the package declaration is from the declaration itself through the end
3819 of the enclosing block, file, or eval (the same as the C<my> operator).
3820 All further unqualified dynamic identifiers will be in this namespace.
3821 A package statement affects only dynamic variables--including those
3822 you've used C<local> on--but I<not> lexical variables, which are created
3823 with C<my>. Typically it would be the first declaration in a file to
3824 be included by the C<require> or C<use> operator. You can switch into a
3825 package in more than one place; it merely influences which symbol table
3826 is used by the compiler for the rest of that block. You can refer to
3827 variables and filehandles in other packages by prefixing the identifier
3828 with the package name and a double colon: C<$Package::Variable>.
3829 If the package name is null, the C<main> package as assumed. That is,
3830 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3831 still seen in older code).
3833 If NAMESPACE is omitted, then there is no current package, and all
3834 identifiers must be fully qualified or lexicals. However, you are
3835 strongly advised not to make use of this feature. Its use can cause
3836 unexpected behaviour, even crashing some versions of Perl. It is
3837 deprecated, and will be removed from a future release.
3839 See L<perlmod/"Packages"> for more information about packages, modules,
3840 and classes. See L<perlsub> for other scoping issues.
3842 =item pipe READHANDLE,WRITEHANDLE
3844 Opens a pair of connected pipes like the corresponding system call.
3845 Note that if you set up a loop of piped processes, deadlock can occur
3846 unless you are very careful. In addition, note that Perl's pipes use
3847 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3848 after each command, depending on the application.
3850 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3851 for examples of such things.
3853 On systems that support a close-on-exec flag on files, the flag will be set
3854 for the newly opened file descriptors as determined by the value of $^F.
3861 Pops and returns the last value of the array, shortening the array by
3862 one element. Has an effect similar to
3866 If there are no elements in the array, returns the undefined value
3867 (although this may happen at other times as well). If ARRAY is
3868 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3869 array in subroutines, just like C<shift>.
3875 Returns the offset of where the last C<m//g> search left off for the variable
3876 in question (C<$_> is used when the variable is not specified). Note that
3877 0 is a valid match offset, while C<undef> indicates that the search position
3878 is reset (usually due to match failure, but can also be because no match has
3879 yet been performed on the scalar). C<pos> directly accesses the location used
3880 by the regexp engine to store the offset, so assigning to C<pos> will change
3881 that offset, and so will also influence the C<\G> zero-width assertion in
3882 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
3883 the return from C<pos> won't change either in this case. See L<perlre> and
3886 =item print FILEHANDLE LIST
3892 Prints a string or a list of strings. Returns true if successful.
3893 FILEHANDLE may be a scalar variable name, in which case the variable
3894 contains the name of or a reference to the filehandle, thus introducing
3895 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3896 the next token is a term, it may be misinterpreted as an operator
3897 unless you interpose a C<+> or put parentheses around the arguments.)
3898 If FILEHANDLE is omitted, prints by default to standard output (or
3899 to the last selected output channel--see L</select>). If LIST is
3900 also omitted, prints C<$_> to the currently selected output channel.
3901 To set the default output channel to something other than STDOUT
3902 use the select operation. The current value of C<$,> (if any) is
3903 printed between each LIST item. The current value of C<$\> (if
3904 any) is printed after the entire LIST has been printed. Because
3905 print takes a LIST, anything in the LIST is evaluated in list
3906 context, and any subroutine that you call will have one or more of
3907 its expressions evaluated in list context. Also be careful not to
3908 follow the print keyword with a left parenthesis unless you want
3909 the corresponding right parenthesis to terminate the arguments to
3910 the print--interpose a C<+> or put parentheses around all the
3913 Note that if you're storing FILEHANDLES in an array or other expression,
3914 you will have to use a block returning its value instead:
3916 print { $files[$i] } "stuff\n";
3917 print { $OK ? STDOUT : STDERR } "stuff\n";
3919 =item printf FILEHANDLE FORMAT, LIST
3921 =item printf FORMAT, LIST
3923 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3924 (the output record separator) is not appended. The first argument
3925 of the list will be interpreted as the C<printf> format. See C<sprintf>
3926 for an explanation of the format argument. If C<use locale> is in effect,
3927 the character used for the decimal point in formatted real numbers is
3928 affected by the LC_NUMERIC locale. See L<perllocale>.
3930 Don't fall into the trap of using a C<printf> when a simple
3931 C<print> would do. The C<print> is more efficient and less
3934 =item prototype FUNCTION
3936 Returns the prototype of a function as a string (or C<undef> if the
3937 function has no prototype). FUNCTION is a reference to, or the name of,
3938 the function whose prototype you want to retrieve.
3940 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3941 name for Perl builtin. If the builtin is not I<overridable> (such as
3942 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3943 C<system>) returns C<undef> because the builtin does not really behave
3944 like a Perl function. Otherwise, the string describing the equivalent
3945 prototype is returned.
3947 =item push ARRAY,LIST
3949 Treats ARRAY as a stack, and pushes the values of LIST
3950 onto the end of ARRAY. The length of ARRAY increases by the length of
3951 LIST. Has the same effect as
3954 $ARRAY[++$#ARRAY] = $value;
3957 but is more efficient. Returns the new number of elements in the array.
3969 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3971 =item quotemeta EXPR
3975 Returns the value of EXPR with all non-"word"
3976 characters backslashed. (That is, all characters not matching
3977 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3978 returned string, regardless of any locale settings.)
3979 This is the internal function implementing
3980 the C<\Q> escape in double-quoted strings.
3982 If EXPR is omitted, uses C<$_>.
3988 Returns a random fractional number greater than or equal to C<0> and less
3989 than the value of EXPR. (EXPR should be positive.) If EXPR is
3990 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3991 also special-cased as C<1> - this has not been documented before perl 5.8.0
3992 and is subject to change in future versions of perl. Automatically calls
3993 C<srand> unless C<srand> has already been called. See also C<srand>.
3995 Apply C<int()> to the value returned by C<rand()> if you want random
3996 integers instead of random fractional numbers. For example,
4000 returns a random integer between C<0> and C<9>, inclusive.
4002 (Note: If your rand function consistently returns numbers that are too
4003 large or too small, then your version of Perl was probably compiled
4004 with the wrong number of RANDBITS.)
4006 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4008 =item read FILEHANDLE,SCALAR,LENGTH
4010 Attempts to read LENGTH I<characters> of data into variable SCALAR
4011 from the specified FILEHANDLE. Returns the number of characters
4012 actually read, C<0> at end of file, or undef if there was an error (in
4013 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4014 so that the last character actually read is the last character of the
4015 scalar after the read.
4017 An OFFSET may be specified to place the read data at some place in the
4018 string other than the beginning. A negative OFFSET specifies
4019 placement at that many characters counting backwards from the end of
4020 the string. A positive OFFSET greater than the length of SCALAR
4021 results in the string being padded to the required size with C<"\0">
4022 bytes before the result of the read is appended.
4024 The call is actually implemented in terms of either Perl's or system's
4025 fread() call. To get a true read(2) system call, see C<sysread>.
4027 Note the I<characters>: depending on the status of the filehandle,
4028 either (8-bit) bytes or characters are read. By default all
4029 filehandles operate on bytes, but for example if the filehandle has
4030 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4031 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4032 characters, not bytes. Similarly for the C<:encoding> pragma:
4033 in that case pretty much any characters can be read.
4035 =item readdir DIRHANDLE
4037 Returns the next directory entry for a directory opened by C<opendir>.
4038 If used in list context, returns all the rest of the entries in the
4039 directory. If there are no more entries, returns an undefined value in
4040 scalar context or a null list in list context.
4042 If you're planning to filetest the return values out of a C<readdir>, you'd
4043 better prepend the directory in question. Otherwise, because we didn't
4044 C<chdir> there, it would have been testing the wrong file.
4046 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4047 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4052 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4053 context, each call reads and returns the next line, until end-of-file is
4054 reached, whereupon the subsequent call returns undef. In list context,
4055 reads until end-of-file is reached and returns a list of lines. Note that
4056 the notion of "line" used here is however you may have defined it
4057 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4059 When C<$/> is set to C<undef>, when readline() is in scalar
4060 context (i.e. file slurp mode), and when an empty file is read, it
4061 returns C<''> the first time, followed by C<undef> subsequently.
4063 This is the internal function implementing the C<< <EXPR> >>
4064 operator, but you can use it directly. The C<< <EXPR> >>
4065 operator is discussed in more detail in L<perlop/"I/O Operators">.
4068 $line = readline(*STDIN); # same thing
4070 If readline encounters an operating system error, C<$!> will be set with the
4071 corresponding error message. It can be helpful to check C<$!> when you are
4072 reading from filehandles you don't trust, such as a tty or a socket. The
4073 following example uses the operator form of C<readline>, and takes the necessary
4074 steps to ensure that C<readline> was successful.
4078 unless (defined( $line = <> )) {
4089 Returns the value of a symbolic link, if symbolic links are
4090 implemented. If not, gives a fatal error. If there is some system
4091 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4092 omitted, uses C<$_>.
4096 EXPR is executed as a system command.
4097 The collected standard output of the command is returned.
4098 In scalar context, it comes back as a single (potentially
4099 multi-line) string. In list context, returns a list of lines
4100 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4101 This is the internal function implementing the C<qx/EXPR/>
4102 operator, but you can use it directly. The C<qx/EXPR/>
4103 operator is discussed in more detail in L<perlop/"I/O Operators">.
4105 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4107 Receives a message on a socket. Attempts to receive LENGTH characters
4108 of data into variable SCALAR from the specified SOCKET filehandle.
4109 SCALAR will be grown or shrunk to the length actually read. Takes the
4110 same flags as the system call of the same name. Returns the address
4111 of the sender if SOCKET's protocol supports this; returns an empty
4112 string otherwise. If there's an error, returns the undefined value.
4113 This call is actually implemented in terms of recvfrom(2) system call.
4114 See L<perlipc/"UDP: Message Passing"> for examples.
4116 Note the I<characters>: depending on the status of the socket, either
4117 (8-bit) bytes or characters are received. By default all sockets
4118 operate on bytes, but for example if the socket has been changed using
4119 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4120 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4121 characters, not bytes. Similarly for the C<:encoding> pragma:
4122 in that case pretty much any characters can be read.
4128 The C<redo> command restarts the loop block without evaluating the
4129 conditional again. The C<continue> block, if any, is not executed. If
4130 the LABEL is omitted, the command refers to the innermost enclosing
4131 loop. This command is normally used by programs that want to lie to
4132 themselves about what was just input:
4134 # a simpleminded Pascal comment stripper
4135 # (warning: assumes no { or } in strings)
4136 LINE: while (<STDIN>) {
4137 while (s|({.*}.*){.*}|$1 |) {}
4142 if (/}/) { # end of comment?
4151 C<redo> cannot be used to retry a block which returns a value such as
4152 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4153 a grep() or map() operation.
4155 Note that a block by itself is semantically identical to a loop
4156 that executes once. Thus C<redo> inside such a block will effectively
4157 turn it into a looping construct.
4159 See also L</continue> for an illustration of how C<last>, C<next>, and
4166 Returns a non-empty string if EXPR is a reference, the empty
4167 string otherwise. If EXPR
4168 is not specified, C<$_> will be used. The value returned depends on the
4169 type of thing the reference is a reference to.
4170 Builtin types include:
4180 If the referenced object has been blessed into a package, then that package
4181 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4183 if (ref($r) eq "HASH") {
4184 print "r is a reference to a hash.\n";
4187 print "r is not a reference at all.\n";
4189 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
4190 print "r is a reference to something that isa hash.\n";
4193 See also L<perlref>.
4195 =item rename OLDNAME,NEWNAME
4197 Changes the name of a file; an existing file NEWNAME will be
4198 clobbered. Returns true for success, false otherwise.
4200 Behavior of this function varies wildly depending on your system
4201 implementation. For example, it will usually not work across file system
4202 boundaries, even though the system I<mv> command sometimes compensates
4203 for this. Other restrictions include whether it works on directories,
4204 open files, or pre-existing files. Check L<perlport> and either the
4205 rename(2) manpage or equivalent system documentation for details.
4207 =item require VERSION
4213 Demands a version of Perl specified by VERSION, or demands some semantics
4214 specified by EXPR or by C<$_> if EXPR is not supplied.
4216 VERSION may be either a numeric argument such as 5.006, which will be
4217 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4218 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4219 VERSION is greater than the version of the current Perl interpreter.
4220 Compare with L</use>, which can do a similar check at compile time.
4222 Specifying VERSION as a literal of the form v5.6.1 should generally be
4223 avoided, because it leads to misleading error messages under earlier
4224 versions of Perl which do not support this syntax. The equivalent numeric
4225 version should be used instead.
4227 require v5.6.1; # run time version check
4228 require 5.6.1; # ditto
4229 require 5.006_001; # ditto; preferred for backwards compatibility
4231 Otherwise, demands that a library file be included if it hasn't already
4232 been included. The file is included via the do-FILE mechanism, which is
4233 essentially just a variety of C<eval>. Has semantics similar to the
4234 following subroutine:
4237 my ($filename) = @_;
4238 if (exists $INC{$filename}) {
4239 return 1 if $INC{$filename};
4240 die "Compilation failed in require";
4242 my ($realfilename,$result);
4244 foreach $prefix (@INC) {
4245 $realfilename = "$prefix/$filename";
4246 if (-f $realfilename) {
4247 $INC{$filename} = $realfilename;
4248 $result = do $realfilename;
4252 die "Can't find $filename in \@INC";
4255 $INC{$filename} = undef;
4257 } elsif (!$result) {
4258 delete $INC{$filename};
4259 die "$filename did not return true value";
4265 Note that the file will not be included twice under the same specified
4268 The file must return true as the last statement to indicate
4269 successful execution of any initialization code, so it's customary to
4270 end such a file with C<1;> unless you're sure it'll return true
4271 otherwise. But it's better just to put the C<1;>, in case you add more
4274 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4275 replaces "F<::>" with "F</>" in the filename for you,
4276 to make it easy to load standard modules. This form of loading of
4277 modules does not risk altering your namespace.
4279 In other words, if you try this:
4281 require Foo::Bar; # a splendid bareword
4283 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4284 directories specified in the C<@INC> array.
4286 But if you try this:
4288 $class = 'Foo::Bar';
4289 require $class; # $class is not a bareword
4291 require "Foo::Bar"; # not a bareword because of the ""
4293 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4294 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4296 eval "require $class";
4298 Now that you understand how C<require> looks for files in the case of
4299 a bareword argument, there is a little extra functionality going on
4300 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4301 it will first look for a filename with a "F<.pmc>" extension. A file
4302 with this extension is assumed to be Perl bytecode generated by
4303 L<B::Bytecode|B::Bytecode>. If this file is found, and it's modification
4304 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4305 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4307 You can also insert hooks into the import facility, by putting directly
4308 Perl code into the @INC array. There are three forms of hooks: subroutine
4309 references, array references and blessed objects.
4311 Subroutine references are the simplest case. When the inclusion system
4312 walks through @INC and encounters a subroutine, this subroutine gets
4313 called with two parameters, the first being a reference to itself, and the
4314 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4315 subroutine should return C<undef> or a filehandle, from which the file to
4316 include will be read. If C<undef> is returned, C<require> will look at
4317 the remaining elements of @INC.
4319 If the hook is an array reference, its first element must be a subroutine
4320 reference. This subroutine is called as above, but the first parameter is
4321 the array reference. This enables to pass indirectly some arguments to
4324 In other words, you can write:
4326 push @INC, \&my_sub;
4328 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4334 push @INC, [ \&my_sub, $x, $y, ... ];
4336 my ($arrayref, $filename) = @_;
4337 # Retrieve $x, $y, ...
4338 my @parameters = @$arrayref[1..$#$arrayref];
4342 If the hook is an object, it must provide an INC method, that will be
4343 called as above, the first parameter being the object itself. (Note that
4344 you must fully qualify the sub's name, as it is always forced into package
4345 C<main>.) Here is a typical code layout:
4351 my ($self, $filename) = @_;
4355 # In the main program
4356 push @INC, new Foo(...);
4358 Note that these hooks are also permitted to set the %INC entry
4359 corresponding to the files they have loaded. See L<perlvar/%INC>.
4361 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4367 Generally used in a C<continue> block at the end of a loop to clear
4368 variables and reset C<??> searches so that they work again. The
4369 expression is interpreted as a list of single characters (hyphens
4370 allowed for ranges). All variables and arrays beginning with one of
4371 those letters are reset to their pristine state. If the expression is
4372 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4373 only variables or searches in the current package. Always returns
4376 reset 'X'; # reset all X variables
4377 reset 'a-z'; # reset lower case variables
4378 reset; # just reset ?one-time? searches
4380 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4381 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4382 variables--lexical variables are unaffected, but they clean themselves
4383 up on scope exit anyway, so you'll probably want to use them instead.
4390 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4391 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4392 context, depending on how the return value will be used, and the context
4393 may vary from one execution to the next (see C<wantarray>). If no EXPR
4394 is given, returns an empty list in list context, the undefined value in
4395 scalar context, and (of course) nothing at all in a void context.
4397 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4398 or do FILE will automatically return the value of the last expression
4403 In list context, returns a list value consisting of the elements
4404 of LIST in the opposite order. In scalar context, concatenates the
4405 elements of LIST and returns a string value with all characters
4406 in the opposite order.
4408 print reverse <>; # line tac, last line first
4410 undef $/; # for efficiency of <>
4411 print scalar reverse <>; # character tac, last line tsrif
4413 Used without arguments in scalar context, reverse() reverses C<$_>.
4415 This operator is also handy for inverting a hash, although there are some
4416 caveats. If a value is duplicated in the original hash, only one of those
4417 can be represented as a key in the inverted hash. Also, this has to
4418 unwind one hash and build a whole new one, which may take some time
4419 on a large hash, such as from a DBM file.
4421 %by_name = reverse %by_address; # Invert the hash
4423 =item rewinddir DIRHANDLE
4425 Sets the current position to the beginning of the directory for the
4426 C<readdir> routine on DIRHANDLE.
4428 =item rindex STR,SUBSTR,POSITION
4430 =item rindex STR,SUBSTR
4432 Works just like index() except that it returns the position of the LAST
4433 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4434 last occurrence at or before that position.
4436 =item rmdir FILENAME
4440 Deletes the directory specified by FILENAME if that directory is
4441 empty. If it succeeds it returns true, otherwise it returns false and
4442 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4446 The substitution operator. See L<perlop>.
4450 Forces EXPR to be interpreted in scalar context and returns the value
4453 @counts = ( scalar @a, scalar @b, scalar @c );
4455 There is no equivalent operator to force an expression to
4456 be interpolated in list context because in practice, this is never
4457 needed. If you really wanted to do so, however, you could use
4458 the construction C<@{[ (some expression) ]}>, but usually a simple
4459 C<(some expression)> suffices.
4461 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4462 parenthesized list, this behaves as a scalar comma expression, evaluating
4463 all but the last element in void context and returning the final element
4464 evaluated in scalar context. This is seldom what you want.
4466 The following single statement:
4468 print uc(scalar(&foo,$bar)),$baz;
4470 is the moral equivalent of these two:
4473 print(uc($bar),$baz);
4475 See L<perlop> for more details on unary operators and the comma operator.
4477 =item seek FILEHANDLE,POSITION,WHENCE
4479 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4480 FILEHANDLE may be an expression whose value gives the name of the
4481 filehandle. The values for WHENCE are C<0> to set the new position
4482 I<in bytes> to POSITION, C<1> to set it to the current position plus
4483 POSITION, and C<2> to set it to EOF plus POSITION (typically
4484 negative). For WHENCE you may use the constants C<SEEK_SET>,
4485 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4486 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4489 Note the I<in bytes>: even if the filehandle has been set to
4490 operate on characters (for example by using the C<:utf8> open
4491 layer), tell() will return byte offsets, not character offsets
4492 (because implementing that would render seek() and tell() rather slow).
4494 If you want to position file for C<sysread> or C<syswrite>, don't use
4495 C<seek>--buffering makes its effect on the file's system position
4496 unpredictable and non-portable. Use C<sysseek> instead.
4498 Due to the rules and rigors of ANSI C, on some systems you have to do a
4499 seek whenever you switch between reading and writing. Amongst other
4500 things, this may have the effect of calling stdio's clearerr(3).
4501 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4505 This is also useful for applications emulating C<tail -f>. Once you hit
4506 EOF on your read, and then sleep for a while, you might have to stick in a
4507 seek() to reset things. The C<seek> doesn't change the current position,
4508 but it I<does> clear the end-of-file condition on the handle, so that the
4509 next C<< <FILE> >> makes Perl try again to read something. We hope.
4511 If that doesn't work (some IO implementations are particularly
4512 cantankerous), then you may need something more like this:
4515 for ($curpos = tell(FILE); $_ = <FILE>;
4516 $curpos = tell(FILE)) {
4517 # search for some stuff and put it into files
4519 sleep($for_a_while);
4520 seek(FILE, $curpos, 0);
4523 =item seekdir DIRHANDLE,POS
4525 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4526 must be a value returned by C<telldir>. Has the same caveats about
4527 possible directory compaction as the corresponding system library
4530 =item select FILEHANDLE
4534 Returns the currently selected filehandle. Sets the current default
4535 filehandle for output, if FILEHANDLE is supplied. This has two
4536 effects: first, a C<write> or a C<print> without a filehandle will
4537 default to this FILEHANDLE. Second, references to variables related to
4538 output will refer to this output channel. For example, if you have to
4539 set the top of form format for more than one output channel, you might
4547 FILEHANDLE may be an expression whose value gives the name of the
4548 actual filehandle. Thus:
4550 $oldfh = select(STDERR); $| = 1; select($oldfh);
4552 Some programmers may prefer to think of filehandles as objects with
4553 methods, preferring to write the last example as:
4556 STDERR->autoflush(1);
4558 =item select RBITS,WBITS,EBITS,TIMEOUT
4560 This calls the select(2) system call with the bit masks specified, which
4561 can be constructed using C<fileno> and C<vec>, along these lines:
4563 $rin = $win = $ein = '';
4564 vec($rin,fileno(STDIN),1) = 1;
4565 vec($win,fileno(STDOUT),1) = 1;
4568 If you want to select on many filehandles you might wish to write a
4572 my(@fhlist) = split(' ',$_[0]);
4575 vec($bits,fileno($_),1) = 1;
4579 $rin = fhbits('STDIN TTY SOCK');
4583 ($nfound,$timeleft) =
4584 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4586 or to block until something becomes ready just do this
4588 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4590 Most systems do not bother to return anything useful in $timeleft, so
4591 calling select() in scalar context just returns $nfound.
4593 Any of the bit masks can also be undef. The timeout, if specified, is
4594 in seconds, which may be fractional. Note: not all implementations are
4595 capable of returning the $timeleft. If not, they always return
4596 $timeleft equal to the supplied $timeout.
4598 You can effect a sleep of 250 milliseconds this way:
4600 select(undef, undef, undef, 0.25);
4602 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4603 is implementation-dependent. See also L<perlport> for notes on the
4604 portability of C<select>.
4606 On error, C<select> returns C<undef> and sets C<$!>.
4608 Note: on some Unixes, the select(2) system call may report a socket file
4609 descriptor as "ready for reading", when actually no data is available,
4610 thus a subsequent read blocks. It can be avoided using always the
4611 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4614 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4615 or <FH>) with C<select>, except as permitted by POSIX, and even
4616 then only on POSIX systems. You have to use C<sysread> instead.
4618 =item semctl ID,SEMNUM,CMD,ARG
4620 Calls the System V IPC function C<semctl>. You'll probably have to say
4624 first to get the correct constant definitions. If CMD is IPC_STAT or
4625 GETALL, then ARG must be a variable which will hold the returned
4626 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4627 the undefined value for error, "C<0 but true>" for zero, or the actual
4628 return value otherwise. The ARG must consist of a vector of native
4629 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4630 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4633 =item semget KEY,NSEMS,FLAGS
4635 Calls the System V IPC function semget. Returns the semaphore id, or
4636 the undefined value if there is an error. See also
4637 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4640 =item semop KEY,OPSTRING
4642 Calls the System V IPC function semop to perform semaphore operations
4643 such as signalling and waiting. OPSTRING must be a packed array of
4644 semop structures. Each semop structure can be generated with
4645 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4646 operations is implied by the length of OPSTRING. Returns true if
4647 successful, or false if there is an error. As an example, the
4648 following code waits on semaphore $semnum of semaphore id $semid:
4650 $semop = pack("s!3", $semnum, -1, 0);
4651 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4653 To signal the semaphore, replace C<-1> with C<1>. See also
4654 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4657 =item send SOCKET,MSG,FLAGS,TO
4659 =item send SOCKET,MSG,FLAGS
4661 Sends a message on a socket. Attempts to send the scalar MSG to the
4662 SOCKET filehandle. Takes the same flags as the system call of the
4663 same name. On unconnected sockets you must specify a destination to
4664 send TO, in which case it does a C C<sendto>. Returns the number of
4665 characters sent, or the undefined value if there is an error. The C
4666 system call sendmsg(2) is currently unimplemented. See
4667 L<perlipc/"UDP: Message Passing"> for examples.
4669 Note the I<characters>: depending on the status of the socket, either
4670 (8-bit) bytes or characters are sent. By default all sockets operate
4671 on bytes, but for example if the socket has been changed using
4672 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4673 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4674 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4675 in that case pretty much any characters can be sent.
4677 =item setpgrp PID,PGRP
4679 Sets the current process group for the specified PID, C<0> for the current
4680 process. Will produce a fatal error if used on a machine that doesn't
4681 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4682 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4683 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4686 =item setpriority WHICH,WHO,PRIORITY
4688 Sets the current priority for a process, a process group, or a user.
4689 (See setpriority(2).) Will produce a fatal error if used on a machine
4690 that doesn't implement setpriority(2).
4692 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4694 Sets the socket option requested. Returns undefined if there is an
4695 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4702 Shifts the first value of the array off and returns it, shortening the
4703 array by 1 and moving everything down. If there are no elements in the
4704 array, returns the undefined value. If ARRAY is omitted, shifts the
4705 C<@_> array within the lexical scope of subroutines and formats, and the
4706 C<@ARGV> array at file scopes or within the lexical scopes established by
4707 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4710 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4711 same thing to the left end of an array that C<pop> and C<push> do to the
4714 =item shmctl ID,CMD,ARG
4716 Calls the System V IPC function shmctl. You'll probably have to say
4720 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4721 then ARG must be a variable which will hold the returned C<shmid_ds>
4722 structure. Returns like ioctl: the undefined value for error, "C<0> but
4723 true" for zero, or the actual return value otherwise.
4724 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4726 =item shmget KEY,SIZE,FLAGS
4728 Calls the System V IPC function shmget. Returns the shared memory
4729 segment id, or the undefined value if there is an error.
4730 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4732 =item shmread ID,VAR,POS,SIZE
4734 =item shmwrite ID,STRING,POS,SIZE
4736 Reads or writes the System V shared memory segment ID starting at
4737 position POS for size SIZE by attaching to it, copying in/out, and
4738 detaching from it. When reading, VAR must be a variable that will
4739 hold the data read. When writing, if STRING is too long, only SIZE
4740 bytes are used; if STRING is too short, nulls are written to fill out
4741 SIZE bytes. Return true if successful, or false if there is an error.
4742 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4743 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4745 =item shutdown SOCKET,HOW
4747 Shuts down a socket connection in the manner indicated by HOW, which
4748 has the same interpretation as in the system call of the same name.
4750 shutdown(SOCKET, 0); # I/we have stopped reading data
4751 shutdown(SOCKET, 1); # I/we have stopped writing data
4752 shutdown(SOCKET, 2); # I/we have stopped using this socket
4754 This is useful with sockets when you want to tell the other
4755 side you're done writing but not done reading, or vice versa.
4756 It's also a more insistent form of close because it also
4757 disables the file descriptor in any forked copies in other
4764 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4765 returns sine of C<$_>.
4767 For the inverse sine operation, you may use the C<Math::Trig::asin>
4768 function, or use this relation:
4770 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4776 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4777 May be interrupted if the process receives a signal such as C<SIGALRM>.
4778 Returns the number of seconds actually slept. You probably cannot
4779 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4782 On some older systems, it may sleep up to a full second less than what
4783 you requested, depending on how it counts seconds. Most modern systems
4784 always sleep the full amount. They may appear to sleep longer than that,
4785 however, because your process might not be scheduled right away in a
4786 busy multitasking system.
4788 For delays of finer granularity than one second, you may use Perl's
4789 C<syscall> interface to access setitimer(2) if your system supports
4790 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4791 and starting from Perl 5.8 part of the standard distribution) may also
4794 See also the POSIX module's C<pause> function.
4796 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4798 Opens a socket of the specified kind and attaches it to filehandle
4799 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4800 the system call of the same name. You should C<use Socket> first
4801 to get the proper definitions imported. See the examples in
4802 L<perlipc/"Sockets: Client/Server Communication">.
4804 On systems that support a close-on-exec flag on files, the flag will
4805 be set for the newly opened file descriptor, as determined by the
4806 value of $^F. See L<perlvar/$^F>.
4808 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4810 Creates an unnamed pair of sockets in the specified domain, of the
4811 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4812 for the system call of the same name. If unimplemented, yields a fatal
4813 error. Returns true if successful.
4815 On systems that support a close-on-exec flag on files, the flag will
4816 be set for the newly opened file descriptors, as determined by the value
4817 of $^F. See L<perlvar/$^F>.
4819 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4820 to C<pipe(Rdr, Wtr)> is essentially:
4823 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4824 shutdown(Rdr, 1); # no more writing for reader
4825 shutdown(Wtr, 0); # no more reading for writer
4827 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4828 emulate socketpair using IP sockets to localhost if your system implements
4829 sockets but not socketpair.
4831 =item sort SUBNAME LIST
4833 =item sort BLOCK LIST
4837 In list context, this sorts the LIST and returns the sorted list value.
4838 In scalar context, the behaviour of C<sort()> is undefined.
4840 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4841 order. If SUBNAME is specified, it gives the name of a subroutine
4842 that returns an integer less than, equal to, or greater than C<0>,
4843 depending on how the elements of the list are to be ordered. (The C<<
4844 <=> >> and C<cmp> operators are extremely useful in such routines.)
4845 SUBNAME may be a scalar variable name (unsubscripted), in which case
4846 the value provides the name of (or a reference to) the actual
4847 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4848 an anonymous, in-line sort subroutine.
4850 If the subroutine's prototype is C<($$)>, the elements to be compared
4851 are passed by reference in C<@_>, as for a normal subroutine. This is
4852 slower than unprototyped subroutines, where the elements to be
4853 compared are passed into the subroutine
4854 as the package global variables $a and $b (see example below). Note that
4855 in the latter case, it is usually counter-productive to declare $a and
4858 In either case, the subroutine may not be recursive. The values to be
4859 compared are always passed by reference, so don't modify them.
4861 You also cannot exit out of the sort block or subroutine using any of the
4862 loop control operators described in L<perlsyn> or with C<goto>.
4864 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4865 current collation locale. See L<perllocale>.
4867 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4868 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4869 preserves the input order of elements that compare equal. Although
4870 quicksort's run time is O(NlogN) when averaged over all arrays of
4871 length N, the time can be O(N**2), I<quadratic> behavior, for some
4872 inputs.) In 5.7, the quicksort implementation was replaced with
4873 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4874 But benchmarks indicated that for some inputs, on some platforms,
4875 the original quicksort was faster. 5.8 has a sort pragma for
4876 limited control of the sort. Its rather blunt control of the
4877 underlying algorithm may not persist into future perls, but the
4878 ability to characterize the input or output in implementation
4879 independent ways quite probably will. See L<sort>.
4884 @articles = sort @files;
4886 # same thing, but with explicit sort routine
4887 @articles = sort {$a cmp $b} @files;
4889 # now case-insensitively
4890 @articles = sort {uc($a) cmp uc($b)} @files;
4892 # same thing in reversed order
4893 @articles = sort {$b cmp $a} @files;
4895 # sort numerically ascending
4896 @articles = sort {$a <=> $b} @files;
4898 # sort numerically descending
4899 @articles = sort {$b <=> $a} @files;
4901 # this sorts the %age hash by value instead of key
4902 # using an in-line function
4903 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4905 # sort using explicit subroutine name
4907 $age{$a} <=> $age{$b}; # presuming numeric
4909 @sortedclass = sort byage @class;
4911 sub backwards { $b cmp $a }
4912 @harry = qw(dog cat x Cain Abel);
4913 @george = qw(gone chased yz Punished Axed);
4915 # prints AbelCaincatdogx
4916 print sort backwards @harry;
4917 # prints xdogcatCainAbel
4918 print sort @george, 'to', @harry;
4919 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4921 # inefficiently sort by descending numeric compare using
4922 # the first integer after the first = sign, or the
4923 # whole record case-insensitively otherwise
4926 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4931 # same thing, but much more efficiently;
4932 # we'll build auxiliary indices instead
4936 push @nums, /=(\d+)/;
4941 $nums[$b] <=> $nums[$a]
4943 $caps[$a] cmp $caps[$b]
4947 # same thing, but without any temps
4948 @new = map { $_->[0] }
4949 sort { $b->[1] <=> $a->[1]
4952 } map { [$_, /=(\d+)/, uc($_)] } @old;
4954 # using a prototype allows you to use any comparison subroutine
4955 # as a sort subroutine (including other package's subroutines)
4957 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4960 @new = sort other::backwards @old;
4962 # guarantee stability, regardless of algorithm
4964 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4966 # force use of mergesort (not portable outside Perl 5.8)
4967 use sort '_mergesort'; # note discouraging _
4968 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4970 If you're using strict, you I<must not> declare $a
4971 and $b as lexicals. They are package globals. That means
4972 if you're in the C<main> package and type
4974 @articles = sort {$b <=> $a} @files;
4976 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4977 but if you're in the C<FooPack> package, it's the same as typing
4979 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4981 The comparison function is required to behave. If it returns
4982 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4983 sometimes saying the opposite, for example) the results are not
4986 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
4987 (not-a-number), and because C<sort> will trigger a fatal error unless the
4988 result of a comparison is defined, when sorting with a comparison function
4989 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
4990 The following example takes advantage of the fact that C<NaN != NaN> to
4991 eliminate any C<NaN>s from the input.
4993 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
4995 =item splice ARRAY,OFFSET,LENGTH,LIST
4997 =item splice ARRAY,OFFSET,LENGTH
4999 =item splice ARRAY,OFFSET
5003 Removes the elements designated by OFFSET and LENGTH from an array, and
5004 replaces them with the elements of LIST, if any. In list context,
5005 returns the elements removed from the array. In scalar context,
5006 returns the last element removed, or C<undef> if no elements are
5007 removed. The array grows or shrinks as necessary.
5008 If OFFSET is negative then it starts that far from the end of the array.
5009 If LENGTH is omitted, removes everything from OFFSET onward.
5010 If LENGTH is negative, removes the elements from OFFSET onward
5011 except for -LENGTH elements at the end of the array.
5012 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5013 past the end of the array, perl issues a warning, and splices at the
5016 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5018 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5019 pop(@a) splice(@a,-1)
5020 shift(@a) splice(@a,0,1)
5021 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5022 $a[$i] = $y splice(@a,$i,1,$y)
5024 Example, assuming array lengths are passed before arrays:
5026 sub aeq { # compare two list values
5027 my(@a) = splice(@_,0,shift);
5028 my(@b) = splice(@_,0,shift);
5029 return 0 unless @a == @b; # same len?
5031 return 0 if pop(@a) ne pop(@b);
5035 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5037 =item split /PATTERN/,EXPR,LIMIT
5039 =item split /PATTERN/,EXPR
5041 =item split /PATTERN/
5045 Splits the string EXPR into a list of strings and returns that list. By
5046 default, empty leading fields are preserved, and empty trailing ones are
5047 deleted. (If all fields are empty, they are considered to be trailing.)
5049 In scalar context, returns the number of fields found and splits into
5050 the C<@_> array. Use of split in scalar context is deprecated, however,
5051 because it clobbers your subroutine arguments.
5053 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5054 splits on whitespace (after skipping any leading whitespace). Anything
5055 matching PATTERN is taken to be a delimiter separating the fields. (Note
5056 that the delimiter may be longer than one character.)
5058 If LIMIT is specified and positive, it represents the maximum number
5059 of fields the EXPR will be split into, though the actual number of
5060 fields returned depends on the number of times PATTERN matches within
5061 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5062 stripped (which potential users of C<pop> would do well to remember).
5063 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5064 had been specified. Note that splitting an EXPR that evaluates to the
5065 empty string always returns the empty list, regardless of the LIMIT
5068 A pattern matching the null string (not to be confused with
5069 a null pattern C<//>, which is just one member of the set of patterns
5070 matching a null string) will split the value of EXPR into separate
5071 characters at each point it matches that way. For example:
5073 print join(':', split(/ */, 'hi there'));
5075 produces the output 'h:i:t:h:e:r:e'.
5077 Using the empty pattern C<//> specifically matches the null string, and is
5078 not be confused with the use of C<//> to mean "the last successful pattern
5081 Empty leading (or trailing) fields are produced when there are positive width
5082 matches at the beginning (or end) of the string; a zero-width match at the
5083 beginning (or end) of the string does not produce an empty field. For
5086 print join(':', split(/(?=\w)/, 'hi there!'));
5088 produces the output 'h:i :t:h:e:r:e!'.
5090 The LIMIT parameter can be used to split a line partially
5092 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5094 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5095 a LIMIT one larger than the number of variables in the list, to avoid
5096 unnecessary work. For the list above LIMIT would have been 4 by
5097 default. In time critical applications it behooves you not to split
5098 into more fields than you really need.
5100 If the PATTERN contains parentheses, additional list elements are
5101 created from each matching substring in the delimiter.
5103 split(/([,-])/, "1-10,20", 3);
5105 produces the list value
5107 (1, '-', 10, ',', 20)
5109 If you had the entire header of a normal Unix email message in $header,
5110 you could split it up into fields and their values this way:
5112 $header =~ s/\n\s+/ /g; # fix continuation lines
5113 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5115 The pattern C</PATTERN/> may be replaced with an expression to specify
5116 patterns that vary at runtime. (To do runtime compilation only once,
5117 use C</$variable/o>.)
5119 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5120 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5121 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5122 will give you as many null initial fields as there are leading spaces.
5123 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5124 whitespace produces a null first field. A C<split> with no arguments
5125 really does a S<C<split(' ', $_)>> internally.
5127 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5132 open(PASSWD, '/etc/passwd');
5135 ($login, $passwd, $uid, $gid,
5136 $gcos, $home, $shell) = split(/:/);
5140 As with regular pattern matching, any capturing parentheses that are not
5141 matched in a C<split()> will be set to C<undef> when returned:
5143 @fields = split /(A)|B/, "1A2B3";
5144 # @fields is (1, 'A', 2, undef, 3)
5146 =item sprintf FORMAT, LIST
5148 Returns a string formatted by the usual C<printf> conventions of the C
5149 library function C<sprintf>. See below for more details
5150 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5151 the general principles.
5155 # Format number with up to 8 leading zeroes
5156 $result = sprintf("%08d", $number);
5158 # Round number to 3 digits after decimal point
5159 $rounded = sprintf("%.3f", $number);
5161 Perl does its own C<sprintf> formatting--it emulates the C
5162 function C<sprintf>, but it doesn't use it (except for floating-point
5163 numbers, and even then only the standard modifiers are allowed). As a
5164 result, any non-standard extensions in your local C<sprintf> are not
5165 available from Perl.
5167 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5168 pass it an array as your first argument. The array is given scalar context,
5169 and instead of using the 0th element of the array as the format, Perl will
5170 use the count of elements in the array as the format, which is almost never
5173 Perl's C<sprintf> permits the following universally-known conversions:
5176 %c a character with the given number
5178 %d a signed integer, in decimal
5179 %u an unsigned integer, in decimal
5180 %o an unsigned integer, in octal
5181 %x an unsigned integer, in hexadecimal
5182 %e a floating-point number, in scientific notation
5183 %f a floating-point number, in fixed decimal notation
5184 %g a floating-point number, in %e or %f notation
5186 In addition, Perl permits the following widely-supported conversions:
5188 %X like %x, but using upper-case letters
5189 %E like %e, but using an upper-case "E"
5190 %G like %g, but with an upper-case "E" (if applicable)
5191 %b an unsigned integer, in binary
5192 %p a pointer (outputs the Perl value's address in hexadecimal)
5193 %n special: *stores* the number of characters output so far
5194 into the next variable in the parameter list
5196 Finally, for backward (and we do mean "backward") compatibility, Perl
5197 permits these unnecessary but widely-supported conversions:
5200 %D a synonym for %ld
5201 %U a synonym for %lu
5202 %O a synonym for %lo
5205 Note that the number of exponent digits in the scientific notation produced
5206 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5207 exponent less than 100 is system-dependent: it may be three or less
5208 (zero-padded as necessary). In other words, 1.23 times ten to the
5209 99th may be either "1.23e99" or "1.23e099".
5211 Between the C<%> and the format letter, you may specify a number of
5212 additional attributes controlling the interpretation of the format.
5213 In order, these are:
5217 =item format parameter index
5219 An explicit format parameter index, such as C<2$>. By default sprintf
5220 will format the next unused argument in the list, but this allows you
5221 to take the arguments out of order. Eg:
5223 printf '%2$d %1$d', 12, 34; # prints "34 12"
5224 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5229 space prefix positive number with a space
5230 + prefix positive number with a plus sign
5231 - left-justify within the field
5232 0 use zeros, not spaces, to right-justify
5233 # prefix non-zero octal with "0", non-zero hex with "0x",
5234 non-zero binary with "0b"
5238 printf '<% d>', 12; # prints "< 12>"
5239 printf '<%+d>', 12; # prints "<+12>"
5240 printf '<%6s>', 12; # prints "< 12>"
5241 printf '<%-6s>', 12; # prints "<12 >"
5242 printf '<%06s>', 12; # prints "<000012>"
5243 printf '<%#x>', 12; # prints "<0xc>"
5247 The vector flag C<v>, optionally specifying the join string to use.
5248 This flag tells perl to interpret the supplied string as a vector
5249 of integers, one for each character in the string, separated by
5250 a given string (a dot C<.> by default). This can be useful for
5251 displaying ordinal values of characters in arbitrary strings:
5253 printf "version is v%vd\n", $^V; # Perl's version
5255 Put an asterisk C<*> before the C<v> to override the string to
5256 use to separate the numbers:
5258 printf "address is %*vX\n", ":", $addr; # IPv6 address
5259 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5261 You can also explicitly specify the argument number to use for
5262 the join string using eg C<*2$v>:
5264 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5266 =item (minimum) width
5268 Arguments are usually formatted to be only as wide as required to
5269 display the given value. You can override the width by putting
5270 a number here, or get the width from the next argument (with C<*>)
5271 or from a specified argument (with eg C<*2$>):
5273 printf '<%s>', "a"; # prints "<a>"
5274 printf '<%6s>', "a"; # prints "< a>"
5275 printf '<%*s>', 6, "a"; # prints "< a>"
5276 printf '<%*2$s>', "a", 6; # prints "< a>"
5277 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5279 If a field width obtained through C<*> is negative, it has the same
5280 effect as the C<-> flag: left-justification.
5282 =item precision, or maximum width
5284 You can specify a precision (for numeric conversions) or a maximum
5285 width (for string conversions) by specifying a C<.> followed by a number.
5286 For floating point formats, with the exception of 'g' and 'G', this specifies
5287 the number of decimal places to show (the default being 6), eg:
5289 # these examples are subject to system-specific variation
5290 printf '<%f>', 1; # prints "<1.000000>"
5291 printf '<%.1f>', 1; # prints "<1.0>"
5292 printf '<%.0f>', 1; # prints "<1>"
5293 printf '<%e>', 10; # prints "<1.000000e+01>"
5294 printf '<%.1e>', 10; # prints "<1.0e+01>"
5296 For 'g' and 'G', this specifies the maximum number of digits to show,
5297 including prior to the decimal point as well as after it, eg:
5299 # these examples are subject to system-specific variation
5300 printf '<%g>', 1; # prints "<1>"
5301 printf '<%.10g>', 1; # prints "<1>"
5302 printf '<%g>', 100; # prints "<100>"
5303 printf '<%.1g>', 100; # prints "<1e+02>"
5304 printf '<%.2g>', 100.01; # prints "<1e+02>"
5305 printf '<%.5g>', 100.01; # prints "<100.01>"
5306 printf '<%.4g>', 100.01; # prints "<100>"
5308 For integer conversions, specifying a precision implies that the
5309 output of the number itself should be zero-padded to this width:
5311 printf '<%.6x>', 1; # prints "<000001>"
5312 printf '<%#.6x>', 1; # prints "<0x000001>"
5313 printf '<%-10.6x>', 1; # prints "<000001 >"
5315 For string conversions, specifying a precision truncates the string
5316 to fit in the specified width:
5318 printf '<%.5s>', "truncated"; # prints "<trunc>"
5319 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5321 You can also get the precision from the next argument using C<.*>:
5323 printf '<%.6x>', 1; # prints "<000001>"
5324 printf '<%.*x>', 6, 1; # prints "<000001>"
5326 You cannot currently get the precision from a specified number,
5327 but it is intended that this will be possible in the future using
5330 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5334 For numeric conversions, you can specify the size to interpret the
5335 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5336 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5337 whatever the default integer size is on your platform (usually 32 or 64
5338 bits), but you can override this to use instead one of the standard C types,
5339 as supported by the compiler used to build Perl:
5341 l interpret integer as C type "long" or "unsigned long"
5342 h interpret integer as C type "short" or "unsigned short"
5343 q, L or ll interpret integer as C type "long long", "unsigned long long".
5344 or "quads" (typically 64-bit integers)
5346 The last will produce errors if Perl does not understand "quads" in your
5347 installation. (This requires that either the platform natively supports quads
5348 or Perl was specifically compiled to support quads.) You can find out
5349 whether your Perl supports quads via L<Config>:
5352 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5355 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5356 to be the default floating point size on your platform (double or long double),
5357 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5358 platform supports them. You can find out whether your Perl supports long
5359 doubles via L<Config>:
5362 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5364 You can find out whether Perl considers 'long double' to be the default
5365 floating point size to use on your platform via L<Config>:
5368 ($Config{uselongdouble} eq 'define') &&
5369 print "long doubles by default\n";
5371 It can also be the case that long doubles and doubles are the same thing:
5374 ($Config{doublesize} == $Config{longdblsize}) &&
5375 print "doubles are long doubles\n";
5377 The size specifier C<V> has no effect for Perl code, but it is supported
5378 for compatibility with XS code; it means 'use the standard size for
5379 a Perl integer (or floating-point number)', which is already the
5380 default for Perl code.
5382 =item order of arguments
5384 Normally, sprintf takes the next unused argument as the value to
5385 format for each format specification. If the format specification
5386 uses C<*> to require additional arguments, these are consumed from
5387 the argument list in the order in which they appear in the format
5388 specification I<before> the value to format. Where an argument is
5389 specified using an explicit index, this does not affect the normal
5390 order for the arguments (even when the explicitly specified index
5391 would have been the next argument in any case).
5395 printf '<%*.*s>', $a, $b, $c;
5397 would use C<$a> for the width, C<$b> for the precision and C<$c>
5398 as the value to format, while:
5400 print '<%*1$.*s>', $a, $b;
5402 would use C<$a> for the width and the precision, and C<$b> as the
5405 Here are some more examples - beware that when using an explicit
5406 index, the C<$> may need to be escaped:
5408 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5409 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5410 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5411 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5415 If C<use locale> is in effect, the character used for the decimal
5416 point in formatted real numbers is affected by the LC_NUMERIC locale.
5423 Return the square root of EXPR. If EXPR is omitted, returns square
5424 root of C<$_>. Only works on non-negative operands, unless you've
5425 loaded the standard Math::Complex module.
5428 print sqrt(-2); # prints 1.4142135623731i
5434 Sets the random number seed for the C<rand> operator.
5436 The point of the function is to "seed" the C<rand> function so that
5437 C<rand> can produce a different sequence each time you run your
5440 If srand() is not called explicitly, it is called implicitly at the
5441 first use of the C<rand> operator. However, this was not the case in
5442 versions of Perl before 5.004, so if your script will run under older
5443 Perl versions, it should call C<srand>.
5445 Most programs won't even call srand() at all, except those that
5446 need a cryptographically-strong starting point rather than the
5447 generally acceptable default, which is based on time of day,
5448 process ID, and memory allocation, or the F</dev/urandom> device,
5451 You can call srand($seed) with the same $seed to reproduce the
5452 I<same> sequence from rand(), but this is usually reserved for
5453 generating predictable results for testing or debugging.
5454 Otherwise, don't call srand() more than once in your program.
5456 Do B<not> call srand() (i.e. without an argument) more than once in
5457 a script. The internal state of the random number generator should
5458 contain more entropy than can be provided by any seed, so calling
5459 srand() again actually I<loses> randomness.
5461 Most implementations of C<srand> take an integer and will silently
5462 truncate decimal numbers. This means C<srand(42)> will usually
5463 produce the same results as C<srand(42.1)>. To be safe, always pass
5464 C<srand> an integer.
5466 In versions of Perl prior to 5.004 the default seed was just the
5467 current C<time>. This isn't a particularly good seed, so many old
5468 programs supply their own seed value (often C<time ^ $$> or C<time ^
5469 ($$ + ($$ << 15))>), but that isn't necessary any more.
5471 Note that you need something much more random than the default seed for
5472 cryptographic purposes. Checksumming the compressed output of one or more
5473 rapidly changing operating system status programs is the usual method. For
5476 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5478 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5481 Frequently called programs (like CGI scripts) that simply use
5485 for a seed can fall prey to the mathematical property that
5489 one-third of the time. So don't do that.
5491 =item stat FILEHANDLE
5497 Returns a 13-element list giving the status info for a file, either
5498 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5499 it stats C<$_>. Returns a null list if the stat fails. Typically used
5502 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5503 $atime,$mtime,$ctime,$blksize,$blocks)
5506 Not all fields are supported on all filesystem types. Here are the
5507 meanings of the fields:
5509 0 dev device number of filesystem
5511 2 mode file mode (type and permissions)
5512 3 nlink number of (hard) links to the file
5513 4 uid numeric user ID of file's owner
5514 5 gid numeric group ID of file's owner
5515 6 rdev the device identifier (special files only)
5516 7 size total size of file, in bytes
5517 8 atime last access time in seconds since the epoch
5518 9 mtime last modify time in seconds since the epoch
5519 10 ctime inode change time in seconds since the epoch (*)
5520 11 blksize preferred block size for file system I/O
5521 12 blocks actual number of blocks allocated
5523 (The epoch was at 00:00 January 1, 1970 GMT.)
5525 (*) Not all fields are supported on all filesystem types. Notably, the
5526 ctime field is non-portable. In particular, you cannot expect it to be a
5527 "creation time", see L<perlport/"Files and Filesystems"> for details.
5529 If C<stat> is passed the special filehandle consisting of an underline, no
5530 stat is done, but the current contents of the stat structure from the
5531 last C<stat>, C<lstat>, or filetest are returned. Example:
5533 if (-x $file && (($d) = stat(_)) && $d < 0) {
5534 print "$file is executable NFS file\n";
5537 (This works on machines only for which the device number is negative
5540 Because the mode contains both the file type and its permissions, you
5541 should mask off the file type portion and (s)printf using a C<"%o">
5542 if you want to see the real permissions.
5544 $mode = (stat($filename))[2];
5545 printf "Permissions are %04o\n", $mode & 07777;
5547 In scalar context, C<stat> returns a boolean value indicating success
5548 or failure, and, if successful, sets the information associated with
5549 the special filehandle C<_>.
5551 The File::stat module provides a convenient, by-name access mechanism:
5554 $sb = stat($filename);
5555 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5556 $filename, $sb->size, $sb->mode & 07777,
5557 scalar localtime $sb->mtime;
5559 You can import symbolic mode constants (C<S_IF*>) and functions
5560 (C<S_IS*>) from the Fcntl module:
5564 $mode = (stat($filename))[2];
5566 $user_rwx = ($mode & S_IRWXU) >> 6;
5567 $group_read = ($mode & S_IRGRP) >> 3;
5568 $other_execute = $mode & S_IXOTH;
5570 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5572 $is_setuid = $mode & S_ISUID;
5573 $is_setgid = S_ISDIR($mode);
5575 You could write the last two using the C<-u> and C<-d> operators.
5576 The commonly available C<S_IF*> constants are
5578 # Permissions: read, write, execute, for user, group, others.
5580 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5581 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5582 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5584 # Setuid/Setgid/Stickiness/SaveText.
5585 # Note that the exact meaning of these is system dependent.
5587 S_ISUID S_ISGID S_ISVTX S_ISTXT
5589 # File types. Not necessarily all are available on your system.
5591 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5593 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5595 S_IREAD S_IWRITE S_IEXEC
5597 and the C<S_IF*> functions are
5599 S_IMODE($mode) the part of $mode containing the permission bits
5600 and the setuid/setgid/sticky bits
5602 S_IFMT($mode) the part of $mode containing the file type
5603 which can be bit-anded with e.g. S_IFREG
5604 or with the following functions
5606 # The operators -f, -d, -l, -b, -c, -p, and -S.
5608 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5609 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5611 # No direct -X operator counterpart, but for the first one
5612 # the -g operator is often equivalent. The ENFMT stands for
5613 # record flocking enforcement, a platform-dependent feature.
5615 S_ISENFMT($mode) S_ISWHT($mode)
5617 See your native chmod(2) and stat(2) documentation for more details
5618 about the C<S_*> constants. To get status info for a symbolic link
5619 instead of the target file behind the link, use the C<lstat> function.
5625 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5626 doing many pattern matches on the string before it is next modified.
5627 This may or may not save time, depending on the nature and number of
5628 patterns you are searching on, and on the distribution of character
5629 frequencies in the string to be searched--you probably want to compare
5630 run times with and without it to see which runs faster. Those loops
5631 which scan for many short constant strings (including the constant
5632 parts of more complex patterns) will benefit most. You may have only
5633 one C<study> active at a time--if you study a different scalar the first
5634 is "unstudied". (The way C<study> works is this: a linked list of every
5635 character in the string to be searched is made, so we know, for
5636 example, where all the C<'k'> characters are. From each search string,
5637 the rarest character is selected, based on some static frequency tables
5638 constructed from some C programs and English text. Only those places
5639 that contain this "rarest" character are examined.)
5641 For example, here is a loop that inserts index producing entries
5642 before any line containing a certain pattern:
5646 print ".IX foo\n" if /\bfoo\b/;
5647 print ".IX bar\n" if /\bbar\b/;
5648 print ".IX blurfl\n" if /\bblurfl\b/;
5653 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5654 will be looked at, because C<f> is rarer than C<o>. In general, this is
5655 a big win except in pathological cases. The only question is whether
5656 it saves you more time than it took to build the linked list in the
5659 Note that if you have to look for strings that you don't know till
5660 runtime, you can build an entire loop as a string and C<eval> that to
5661 avoid recompiling all your patterns all the time. Together with
5662 undefining C<$/> to input entire files as one record, this can be very
5663 fast, often faster than specialized programs like fgrep(1). The following
5664 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5665 out the names of those files that contain a match:
5667 $search = 'while (<>) { study;';
5668 foreach $word (@words) {
5669 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5674 eval $search; # this screams
5675 $/ = "\n"; # put back to normal input delimiter
5676 foreach $file (sort keys(%seen)) {
5680 =item sub NAME BLOCK
5682 =item sub NAME (PROTO) BLOCK
5684 =item sub NAME : ATTRS BLOCK
5686 =item sub NAME (PROTO) : ATTRS BLOCK
5688 This is subroutine definition, not a real function I<per se>.
5689 Without a BLOCK it's just a forward declaration. Without a NAME,
5690 it's an anonymous function declaration, and does actually return
5691 a value: the CODE ref of the closure you just created.
5693 See L<perlsub> and L<perlref> for details about subroutines and
5694 references, and L<attributes> and L<Attribute::Handlers> for more
5695 information about attributes.
5697 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5699 =item substr EXPR,OFFSET,LENGTH
5701 =item substr EXPR,OFFSET
5703 Extracts a substring out of EXPR and returns it. First character is at
5704 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5705 If OFFSET is negative (or more precisely, less than C<$[>), starts
5706 that far from the end of the string. If LENGTH is omitted, returns
5707 everything to the end of the string. If LENGTH is negative, leaves that
5708 many characters off the end of the string.
5710 You can use the substr() function as an lvalue, in which case EXPR
5711 must itself be an lvalue. If you assign something shorter than LENGTH,
5712 the string will shrink, and if you assign something longer than LENGTH,
5713 the string will grow to accommodate it. To keep the string the same
5714 length you may need to pad or chop your value using C<sprintf>.
5716 If OFFSET and LENGTH specify a substring that is partly outside the
5717 string, only the part within the string is returned. If the substring
5718 is beyond either end of the string, substr() returns the undefined
5719 value and produces a warning. When used as an lvalue, specifying a
5720 substring that is entirely outside the string is a fatal error.
5721 Here's an example showing the behavior for boundary cases:
5724 substr($name, 4) = 'dy'; # $name is now 'freddy'
5725 my $null = substr $name, 6, 2; # returns '' (no warning)
5726 my $oops = substr $name, 7; # returns undef, with warning
5727 substr($name, 7) = 'gap'; # fatal error
5729 An alternative to using substr() as an lvalue is to specify the
5730 replacement string as the 4th argument. This allows you to replace
5731 parts of the EXPR and return what was there before in one operation,
5732 just as you can with splice().
5734 Note that the lvalue returned by by the 3-arg version of substr() acts as
5735 a 'magic bullet'; each time it is assigned to, it remembers which part
5736 of the original string is being modified; for example:
5739 for (substr($x,1,2)) {
5740 $_ = 'a'; print $x,"\n"; # prints 1a4
5741 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
5743 $_ = 'pq'; print $x,"\n"; # prints 5pq9
5747 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
5750 =item symlink OLDFILE,NEWFILE
5752 Creates a new filename symbolically linked to the old filename.
5753 Returns C<1> for success, C<0> otherwise. On systems that don't support
5754 symbolic links, produces a fatal error at run time. To check for that,
5757 $symlink_exists = eval { symlink("",""); 1 };
5759 =item syscall NUMBER, LIST
5761 Calls the system call specified as the first element of the list,
5762 passing the remaining elements as arguments to the system call. If
5763 unimplemented, produces a fatal error. The arguments are interpreted
5764 as follows: if a given argument is numeric, the argument is passed as
5765 an int. If not, the pointer to the string value is passed. You are
5766 responsible to make sure a string is pre-extended long enough to
5767 receive any result that might be written into a string. You can't use a
5768 string literal (or other read-only string) as an argument to C<syscall>
5769 because Perl has to assume that any string pointer might be written
5771 integer arguments are not literals and have never been interpreted in a
5772 numeric context, you may need to add C<0> to them to force them to look
5773 like numbers. This emulates the C<syswrite> function (or vice versa):
5775 require 'syscall.ph'; # may need to run h2ph
5777 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5779 Note that Perl supports passing of up to only 14 arguments to your system call,
5780 which in practice should usually suffice.
5782 Syscall returns whatever value returned by the system call it calls.
5783 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5784 Note that some system calls can legitimately return C<-1>. The proper
5785 way to handle such calls is to assign C<$!=0;> before the call and
5786 check the value of C<$!> if syscall returns C<-1>.
5788 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5789 number of the read end of the pipe it creates. There is no way
5790 to retrieve the file number of the other end. You can avoid this
5791 problem by using C<pipe> instead.
5793 =item sysopen FILEHANDLE,FILENAME,MODE
5795 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5797 Opens the file whose filename is given by FILENAME, and associates it
5798 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5799 the name of the real filehandle wanted. This function calls the
5800 underlying operating system's C<open> function with the parameters
5801 FILENAME, MODE, PERMS.
5803 The possible values and flag bits of the MODE parameter are
5804 system-dependent; they are available via the standard module C<Fcntl>.
5805 See the documentation of your operating system's C<open> to see which
5806 values and flag bits are available. You may combine several flags
5807 using the C<|>-operator.
5809 Some of the most common values are C<O_RDONLY> for opening the file in
5810 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5811 and C<O_RDWR> for opening the file in read-write mode.
5813 For historical reasons, some values work on almost every system
5814 supported by perl: zero means read-only, one means write-only, and two
5815 means read/write. We know that these values do I<not> work under
5816 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5817 use them in new code.
5819 If the file named by FILENAME does not exist and the C<open> call creates
5820 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5821 PERMS specifies the permissions of the newly created file. If you omit
5822 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5823 These permission values need to be in octal, and are modified by your
5824 process's current C<umask>.
5826 In many systems the C<O_EXCL> flag is available for opening files in
5827 exclusive mode. This is B<not> locking: exclusiveness means here that
5828 if the file already exists, sysopen() fails. C<O_EXCL> may not work
5829 on network filesystems, and has no effect unless the C<O_CREAT> flag
5830 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
5831 being opened if it is a symbolic link. It does not protect against
5832 symbolic links in the file's path.
5834 Sometimes you may want to truncate an already-existing file. This
5835 can be done using the C<O_TRUNC> flag. The behavior of
5836 C<O_TRUNC> with C<O_RDONLY> is undefined.
5838 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5839 that takes away the user's option to have a more permissive umask.
5840 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5843 Note that C<sysopen> depends on the fdopen() C library function.
5844 On many UNIX systems, fdopen() is known to fail when file descriptors
5845 exceed a certain value, typically 255. If you need more file
5846 descriptors than that, consider rebuilding Perl to use the C<sfio>
5847 library, or perhaps using the POSIX::open() function.
5849 See L<perlopentut> for a kinder, gentler explanation of opening files.
5851 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5853 =item sysread FILEHANDLE,SCALAR,LENGTH
5855 Attempts to read LENGTH bytes of data into variable SCALAR from the
5856 specified FILEHANDLE, using the system call read(2). It bypasses
5857 buffered IO, so mixing this with other kinds of reads, C<print>,
5858 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5859 perlio or stdio layers usually buffers data. Returns the number of
5860 bytes actually read, C<0> at end of file, or undef if there was an
5861 error (in the latter case C<$!> is also set). SCALAR will be grown or
5862 shrunk so that the last byte actually read is the last byte of the
5863 scalar after the read.
5865 An OFFSET may be specified to place the read data at some place in the
5866 string other than the beginning. A negative OFFSET specifies
5867 placement at that many characters counting backwards from the end of
5868 the string. A positive OFFSET greater than the length of SCALAR
5869 results in the string being padded to the required size with C<"\0">
5870 bytes before the result of the read is appended.
5872 There is no syseof() function, which is ok, since eof() doesn't work
5873 very well on device files (like ttys) anyway. Use sysread() and check
5874 for a return value for 0 to decide whether you're done.
5876 Note that if the filehandle has been marked as C<:utf8> Unicode
5877 characters are read instead of bytes (the LENGTH, OFFSET, and the
5878 return value of sysread() are in Unicode characters).
5879 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5880 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5882 =item sysseek FILEHANDLE,POSITION,WHENCE
5884 Sets FILEHANDLE's system position in bytes using the system call
5885 lseek(2). FILEHANDLE may be an expression whose value gives the name
5886 of the filehandle. The values for WHENCE are C<0> to set the new
5887 position to POSITION, C<1> to set the it to the current position plus
5888 POSITION, and C<2> to set it to EOF plus POSITION (typically
5891 Note the I<in bytes>: even if the filehandle has been set to operate
5892 on characters (for example by using the C<:utf8> I/O layer), tell()
5893 will return byte offsets, not character offsets (because implementing
5894 that would render sysseek() very slow).
5896 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5897 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
5898 C<seek>, C<tell>, or C<eof> may cause confusion.
5900 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5901 and C<SEEK_END> (start of the file, current position, end of the file)
5902 from the Fcntl module. Use of the constants is also more portable
5903 than relying on 0, 1, and 2. For example to define a "systell" function:
5905 use Fcntl 'SEEK_CUR';
5906 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5908 Returns the new position, or the undefined value on failure. A position
5909 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5910 true on success and false on failure, yet you can still easily determine
5915 =item system PROGRAM LIST
5917 Does exactly the same thing as C<exec LIST>, except that a fork is
5918 done first, and the parent process waits for the child process to
5919 complete. Note that argument processing varies depending on the
5920 number of arguments. If there is more than one argument in LIST,
5921 or if LIST is an array with more than one value, starts the program
5922 given by the first element of the list with arguments given by the
5923 rest of the list. If there is only one scalar argument, the argument
5924 is checked for shell metacharacters, and if there are any, the
5925 entire argument is passed to the system's command shell for parsing
5926 (this is C</bin/sh -c> on Unix platforms, but varies on other
5927 platforms). If there are no shell metacharacters in the argument,
5928 it is split into words and passed directly to C<execvp>, which is
5931 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5932 output before any operation that may do a fork, but this may not be
5933 supported on some platforms (see L<perlport>). To be safe, you may need
5934 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5935 of C<IO::Handle> on any open handles.
5937 The return value is the exit status of the program as returned by the
5938 C<wait> call. To get the actual exit value shift right by eight (see below).
5939 See also L</exec>. This is I<not> what you want to use to capture
5940 the output from a command, for that you should use merely backticks or
5941 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5942 indicates a failure to start the program (inspect $! for the reason).
5944 Like C<exec>, C<system> allows you to lie to a program about its name if
5945 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5947 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
5948 C<system>, if you expect your program to terminate on receipt of these
5949 signals you will need to arrange to do so yourself based on the return
5952 @args = ("command", "arg1", "arg2");
5954 or die "system @args failed: $?"
5956 You can check all the failure possibilities by inspecting
5960 print "failed to execute: $!\n";
5963 printf "child died with signal %d, %s coredump\n",
5964 ($? & 127), ($? & 128) ? 'with' : 'without';
5967 printf "child exited with value %d\n", $? >> 8;
5970 or more portably by using the W*() calls of the POSIX extension;
5971 see L<perlport> for more information.
5973 When the arguments get executed via the system shell, results
5974 and return codes will be subject to its quirks and capabilities.
5975 See L<perlop/"`STRING`"> and L</exec> for details.
5977 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5979 =item syswrite FILEHANDLE,SCALAR,LENGTH
5981 =item syswrite FILEHANDLE,SCALAR
5983 Attempts to write LENGTH bytes of data from variable SCALAR to the
5984 specified FILEHANDLE, using the system call write(2). If LENGTH is
5985 not specified, writes whole SCALAR. It bypasses buffered IO, so
5986 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5987 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
5988 stdio layers usually buffers data. Returns the number of bytes
5989 actually written, or C<undef> if there was an error (in this case the
5990 errno variable C<$!> is also set). If the LENGTH is greater than the
5991 available data in the SCALAR after the OFFSET, only as much data as is
5992 available will be written.
5994 An OFFSET may be specified to write the data from some part of the
5995 string other than the beginning. A negative OFFSET specifies writing
5996 that many characters counting backwards from the end of the string.
5997 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5999 Note that if the filehandle has been marked as C<:utf8>, Unicode
6000 characters are written instead of bytes (the LENGTH, OFFSET, and the
6001 return value of syswrite() are in UTF-8 encoded Unicode characters).
6002 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6003 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6005 =item tell FILEHANDLE
6009 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6010 error. FILEHANDLE may be an expression whose value gives the name of
6011 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6014 Note the I<in bytes>: even if the filehandle has been set to
6015 operate on characters (for example by using the C<:utf8> open
6016 layer), tell() will return byte offsets, not character offsets
6017 (because that would render seek() and tell() rather slow).
6019 The return value of tell() for the standard streams like the STDIN
6020 depends on the operating system: it may return -1 or something else.
6021 tell() on pipes, fifos, and sockets usually returns -1.
6023 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6025 Do not use tell() (or other buffered I/O operations) on a file handle
6026 that has been manipulated by sysread(), syswrite() or sysseek().
6027 Those functions ignore the buffering, while tell() does not.
6029 =item telldir DIRHANDLE
6031 Returns the current position of the C<readdir> routines on DIRHANDLE.
6032 Value may be given to C<seekdir> to access a particular location in a
6033 directory. Has the same caveats about possible directory compaction as
6034 the corresponding system library routine.
6036 =item tie VARIABLE,CLASSNAME,LIST
6038 This function binds a variable to a package class that will provide the
6039 implementation for the variable. VARIABLE is the name of the variable
6040 to be enchanted. CLASSNAME is the name of a class implementing objects
6041 of correct type. Any additional arguments are passed to the C<new>
6042 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6043 or C<TIEHASH>). Typically these are arguments such as might be passed
6044 to the C<dbm_open()> function of C. The object returned by the C<new>
6045 method is also returned by the C<tie> function, which would be useful
6046 if you want to access other methods in CLASSNAME.
6048 Note that functions such as C<keys> and C<values> may return huge lists
6049 when used on large objects, like DBM files. You may prefer to use the
6050 C<each> function to iterate over such. Example:
6052 # print out history file offsets
6054 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6055 while (($key,$val) = each %HIST) {
6056 print $key, ' = ', unpack('L',$val), "\n";
6060 A class implementing a hash should have the following methods:
6062 TIEHASH classname, LIST
6064 STORE this, key, value
6069 NEXTKEY this, lastkey
6074 A class implementing an ordinary array should have the following methods:
6076 TIEARRAY classname, LIST
6078 STORE this, key, value
6080 STORESIZE this, count
6086 SPLICE this, offset, length, LIST
6091 A class implementing a file handle should have the following methods:
6093 TIEHANDLE classname, LIST
6094 READ this, scalar, length, offset
6097 WRITE this, scalar, length, offset
6099 PRINTF this, format, LIST
6103 SEEK this, position, whence
6105 OPEN this, mode, LIST
6110 A class implementing a scalar should have the following methods:
6112 TIESCALAR classname, LIST
6118 Not all methods indicated above need be implemented. See L<perltie>,
6119 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6121 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6122 for you--you need to do that explicitly yourself. See L<DB_File>
6123 or the F<Config> module for interesting C<tie> implementations.
6125 For further details see L<perltie>, L<"tied VARIABLE">.
6129 Returns a reference to the object underlying VARIABLE (the same value
6130 that was originally returned by the C<tie> call that bound the variable
6131 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6136 Returns the number of non-leap seconds since whatever time the system
6137 considers to be the epoch, suitable for feeding to C<gmtime> and
6138 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6139 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6140 1904 in the current local time zone for its epoch.
6142 For measuring time in better granularity than one second,
6143 you may use either the Time::HiRes module (from CPAN, and starting from
6144 Perl 5.8 part of the standard distribution), or if you have
6145 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6146 See L<perlfaq8> for details.
6150 Returns a four-element list giving the user and system times, in
6151 seconds, for this process and the children of this process.
6153 ($user,$system,$cuser,$csystem) = times;
6155 In scalar context, C<times> returns C<$user>.
6159 The transliteration operator. Same as C<y///>. See L<perlop>.
6161 =item truncate FILEHANDLE,LENGTH
6163 =item truncate EXPR,LENGTH
6165 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6166 specified length. Produces a fatal error if truncate isn't implemented
6167 on your system. Returns true if successful, the undefined value
6170 The behavior is undefined if LENGTH is greater than the length of the
6177 Returns an uppercased version of EXPR. This is the internal function
6178 implementing the C<\U> escape in double-quoted strings. Respects
6179 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6180 and L<perlunicode> for more details about locale and Unicode support.
6181 It does not attempt to do titlecase mapping on initial letters. See
6182 C<ucfirst> for that.
6184 If EXPR is omitted, uses C<$_>.
6190 Returns the value of EXPR with the first character in uppercase
6191 (titlecase in Unicode). This is the internal function implementing
6192 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6193 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6194 for more details about locale and Unicode support.
6196 If EXPR is omitted, uses C<$_>.
6202 Sets the umask for the process to EXPR and returns the previous value.
6203 If EXPR is omitted, merely returns the current umask.
6205 The Unix permission C<rwxr-x---> is represented as three sets of three
6206 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6207 and isn't one of the digits). The C<umask> value is such a number
6208 representing disabled permissions bits. The permission (or "mode")
6209 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6210 even if you tell C<sysopen> to create a file with permissions C<0777>,
6211 if your umask is C<0022> then the file will actually be created with
6212 permissions C<0755>. If your C<umask> were C<0027> (group can't
6213 write; others can't read, write, or execute), then passing
6214 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6217 Here's some advice: supply a creation mode of C<0666> for regular
6218 files (in C<sysopen>) and one of C<0777> for directories (in
6219 C<mkdir>) and executable files. This gives users the freedom of
6220 choice: if they want protected files, they might choose process umasks
6221 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6222 Programs should rarely if ever make policy decisions better left to
6223 the user. The exception to this is when writing files that should be
6224 kept private: mail files, web browser cookies, I<.rhosts> files, and
6227 If umask(2) is not implemented on your system and you are trying to
6228 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6229 fatal error at run time. If umask(2) is not implemented and you are
6230 not trying to restrict access for yourself, returns C<undef>.
6232 Remember that a umask is a number, usually given in octal; it is I<not> a
6233 string of octal digits. See also L</oct>, if all you have is a string.
6239 Undefines the value of EXPR, which must be an lvalue. Use only on a
6240 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6241 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6242 will probably not do what you expect on most predefined variables or
6243 DBM list values, so don't do that; see L<delete>.) Always returns the
6244 undefined value. You can omit the EXPR, in which case nothing is
6245 undefined, but you still get an undefined value that you could, for
6246 instance, return from a subroutine, assign to a variable or pass as a
6247 parameter. Examples:
6250 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6254 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6255 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6256 select undef, undef, undef, 0.25;
6257 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6259 Note that this is a unary operator, not a list operator.
6265 Deletes a list of files. Returns the number of files successfully
6268 $cnt = unlink 'a', 'b', 'c';
6272 Note: C<unlink> will not delete directories unless you are superuser and
6273 the B<-U> flag is supplied to Perl. Even if these conditions are
6274 met, be warned that unlinking a directory can inflict damage on your
6275 filesystem. Use C<rmdir> instead.
6277 If LIST is omitted, uses C<$_>.
6279 =item unpack TEMPLATE,EXPR
6281 =item unpack TEMPLATE
6283 C<unpack> does the reverse of C<pack>: it takes a string
6284 and expands it out into a list of values.
6285 (In scalar context, it returns merely the first value produced.)
6287 If EXPR is omitted, unpacks the C<$_> string.
6289 The string is broken into chunks described by the TEMPLATE. Each chunk
6290 is converted separately to a value. Typically, either the string is a result
6291 of C<pack>, or the characters of the string represent a C structure of some
6294 The TEMPLATE has the same format as in the C<pack> function.
6295 Here's a subroutine that does substring:
6298 my($what,$where,$howmuch) = @_;
6299 unpack("x$where a$howmuch", $what);
6304 sub ordinal { unpack("W",$_[0]); } # same as ord()
6306 In addition to fields allowed in pack(), you may prefix a field with
6307 a %<number> to indicate that
6308 you want a <number>-bit checksum of the items instead of the items
6309 themselves. Default is a 16-bit checksum. Checksum is calculated by
6310 summing numeric values of expanded values (for string fields the sum of
6311 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6313 For example, the following
6314 computes the same number as the System V sum program:
6318 unpack("%32W*",<>) % 65535;
6321 The following efficiently counts the number of set bits in a bit vector:
6323 $setbits = unpack("%32b*", $selectmask);
6325 The C<p> and C<P> formats should be used with care. Since Perl
6326 has no way of checking whether the value passed to C<unpack()>
6327 corresponds to a valid memory location, passing a pointer value that's
6328 not known to be valid is likely to have disastrous consequences.
6330 If there are more pack codes or if the repeat count of a field or a group
6331 is larger than what the remainder of the input string allows, the result
6332 is not well defined: in some cases, the repeat count is decreased, or
6333 C<unpack()> will produce null strings or zeroes, or terminate with an
6334 error. If the input string is longer than one described by the TEMPLATE,
6335 the rest is ignored.
6337 See L</pack> for more examples and notes.
6339 =item untie VARIABLE
6341 Breaks the binding between a variable and a package. (See C<tie>.)
6342 Has no effect if the variable is not tied.
6344 =item unshift ARRAY,LIST
6346 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6347 depending on how you look at it. Prepends list to the front of the
6348 array, and returns the new number of elements in the array.
6350 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6352 Note the LIST is prepended whole, not one element at a time, so the
6353 prepended elements stay in the same order. Use C<reverse> to do the
6356 =item use Module VERSION LIST
6358 =item use Module VERSION
6360 =item use Module LIST
6366 Imports some semantics into the current package from the named module,
6367 generally by aliasing certain subroutine or variable names into your
6368 package. It is exactly equivalent to
6370 BEGIN { require Module; import Module LIST; }
6372 except that Module I<must> be a bareword.
6374 VERSION may be either a numeric argument such as 5.006, which will be
6375 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6376 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6377 greater than the version of the current Perl interpreter; Perl will not
6378 attempt to parse the rest of the file. Compare with L</require>, which can
6379 do a similar check at run time.
6381 Specifying VERSION as a literal of the form v5.6.1 should generally be
6382 avoided, because it leads to misleading error messages under earlier
6383 versions of Perl which do not support this syntax. The equivalent numeric
6384 version should be used instead.
6386 use v5.6.1; # compile time version check
6388 use 5.006_001; # ditto; preferred for backwards compatibility
6390 This is often useful if you need to check the current Perl version before
6391 C<use>ing library modules that have changed in incompatible ways from
6392 older versions of Perl. (We try not to do this more than we have to.)
6394 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6395 C<require> makes sure the module is loaded into memory if it hasn't been
6396 yet. The C<import> is not a builtin--it's just an ordinary static method
6397 call into the C<Module> package to tell the module to import the list of
6398 features back into the current package. The module can implement its
6399 C<import> method any way it likes, though most modules just choose to
6400 derive their C<import> method via inheritance from the C<Exporter> class that
6401 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6402 method can be found then the call is skipped, even if there is an AUTOLOAD
6405 If you do not want to call the package's C<import> method (for instance,
6406 to stop your namespace from being altered), explicitly supply the empty list:
6410 That is exactly equivalent to
6412 BEGIN { require Module }
6414 If the VERSION argument is present between Module and LIST, then the
6415 C<use> will call the VERSION method in class Module with the given
6416 version as an argument. The default VERSION method, inherited from
6417 the UNIVERSAL class, croaks if the given version is larger than the
6418 value of the variable C<$Module::VERSION>.
6420 Again, there is a distinction between omitting LIST (C<import> called
6421 with no arguments) and an explicit empty LIST C<()> (C<import> not
6422 called). Note that there is no comma after VERSION!
6424 Because this is a wide-open interface, pragmas (compiler directives)
6425 are also implemented this way. Currently implemented pragmas are:
6430 use sigtrap qw(SEGV BUS);
6431 use strict qw(subs vars refs);
6432 use subs qw(afunc blurfl);
6433 use warnings qw(all);
6434 use sort qw(stable _quicksort _mergesort);
6436 Some of these pseudo-modules import semantics into the current
6437 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6438 which import symbols into the current package (which are effective
6439 through the end of the file).
6441 There's a corresponding C<no> command that unimports meanings imported
6442 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6443 It behaves exactly as C<import> does with respect to VERSION, an
6444 omitted LIST, empty LIST, or no unimport method being found.
6450 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6451 for the C<-M> and C<-m> command-line options to perl that give C<use>
6452 functionality from the command-line.
6456 Changes the access and modification times on each file of a list of
6457 files. The first two elements of the list must be the NUMERICAL access
6458 and modification times, in that order. Returns the number of files
6459 successfully changed. The inode change time of each file is set
6460 to the current time. For example, this code has the same effect as the
6461 Unix touch(1) command when the files I<already exist> and belong to
6462 the user running the program:
6465 $atime = $mtime = time;
6466 utime $atime, $mtime, @ARGV;
6468 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6469 the utime(2) function in the C library will be called with a null second
6470 argument. On most systems, this will set the file's access and
6471 modification times to the current time (i.e. equivalent to the example
6472 above) and will even work on other users' files where you have write
6475 utime undef, undef, @ARGV;
6477 Under NFS this will use the time of the NFS server, not the time of
6478 the local machine. If there is a time synchronization problem, the
6479 NFS server and local machine will have different times. The Unix
6480 touch(1) command will in fact normally use this form instead of the
6481 one shown in the first example.
6483 Note that only passing one of the first two elements as C<undef> will
6484 be equivalent of passing it as 0 and will not have the same effect as
6485 described when they are both C<undef>. This case will also trigger an
6486 uninitialized warning.
6490 Returns a list consisting of all the values of the named hash.
6491 (In a scalar context, returns the number of values.)
6493 The values are returned in an apparently random order. The actual
6494 random order is subject to change in future versions of perl, but it
6495 is guaranteed to be the same order as either the C<keys> or C<each>
6496 function would produce on the same (unmodified) hash. Since Perl
6497 5.8.1 the ordering is different even between different runs of Perl
6498 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6500 As a side effect, calling values() resets the HASH's internal iterator,
6501 see L</each>. (In particular, calling values() in void context resets
6502 the iterator with no other overhead.)
6504 Note that the values are not copied, which means modifying them will
6505 modify the contents of the hash:
6507 for (values %hash) { s/foo/bar/g } # modifies %hash values
6508 for (@hash{keys %hash}) { s/foo/bar/g } # same
6510 See also C<keys>, C<each>, and C<sort>.
6512 =item vec EXPR,OFFSET,BITS
6514 Treats the string in EXPR as a bit vector made up of elements of
6515 width BITS, and returns the value of the element specified by OFFSET
6516 as an unsigned integer. BITS therefore specifies the number of bits
6517 that are reserved for each element in the bit vector. This must
6518 be a power of two from 1 to 32 (or 64, if your platform supports
6521 If BITS is 8, "elements" coincide with bytes of the input string.
6523 If BITS is 16 or more, bytes of the input string are grouped into chunks
6524 of size BITS/8, and each group is converted to a number as with
6525 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6526 for BITS==64). See L<"pack"> for details.
6528 If bits is 4 or less, the string is broken into bytes, then the bits
6529 of each byte are broken into 8/BITS groups. Bits of a byte are
6530 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6531 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6532 breaking the single input byte C<chr(0x36)> into two groups gives a list
6533 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6535 C<vec> may also be assigned to, in which case parentheses are needed
6536 to give the expression the correct precedence as in
6538 vec($image, $max_x * $x + $y, 8) = 3;
6540 If the selected element is outside the string, the value 0 is returned.
6541 If an element off the end of the string is written to, Perl will first
6542 extend the string with sufficiently many zero bytes. It is an error
6543 to try to write off the beginning of the string (i.e. negative OFFSET).
6545 The string should not contain any character with the value > 255 (which
6546 can only happen if you're using UTF-8 encoding). If it does, it will be
6547 treated as something which is not UTF-8 encoded. When the C<vec> was
6548 assigned to, other parts of your program will also no longer consider the
6549 string to be UTF-8 encoded. In other words, if you do have such characters
6550 in your string, vec() will operate on the actual byte string, and not the
6551 conceptual character string.
6553 Strings created with C<vec> can also be manipulated with the logical
6554 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6555 vector operation is desired when both operands are strings.
6556 See L<perlop/"Bitwise String Operators">.
6558 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6559 The comments show the string after each step. Note that this code works
6560 in the same way on big-endian or little-endian machines.
6563 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6565 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6566 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6568 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6569 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6570 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6571 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6572 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6573 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6575 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6576 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6577 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6580 To transform a bit vector into a string or list of 0's and 1's, use these:
6582 $bits = unpack("b*", $vector);
6583 @bits = split(//, unpack("b*", $vector));
6585 If you know the exact length in bits, it can be used in place of the C<*>.
6587 Here is an example to illustrate how the bits actually fall in place:
6593 unpack("V",$_) 01234567890123456789012345678901
6594 ------------------------------------------------------------------
6599 for ($shift=0; $shift < $width; ++$shift) {
6600 for ($off=0; $off < 32/$width; ++$off) {
6601 $str = pack("B*", "0"x32);
6602 $bits = (1<<$shift);
6603 vec($str, $off, $width) = $bits;
6604 $res = unpack("b*",$str);
6605 $val = unpack("V", $str);
6612 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6613 $off, $width, $bits, $val, $res
6617 Regardless of the machine architecture on which it is run, the above
6618 example should print the following table:
6621 unpack("V",$_) 01234567890123456789012345678901
6622 ------------------------------------------------------------------
6623 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6624 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6625 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6626 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6627 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6628 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6629 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6630 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6631 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6632 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6633 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6634 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6635 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6636 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6637 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6638 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6639 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6640 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6641 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6642 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6643 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6644 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6645 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6646 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6647 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6648 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6649 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6650 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6651 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6652 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6653 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6654 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6655 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6656 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6657 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6658 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6659 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6660 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6661 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6662 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6663 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6664 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6665 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6666 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6667 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6668 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6669 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6670 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6671 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6672 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6673 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6674 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6675 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6676 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6677 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6678 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6679 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6680 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6681 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6682 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6683 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6684 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6685 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6686 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6687 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6688 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6689 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6690 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6691 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6692 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6693 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6694 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6695 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6696 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6697 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6698 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6699 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6700 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6701 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6702 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6703 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6704 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6705 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6706 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6707 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6708 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6709 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6710 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6711 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6712 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6713 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6714 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6715 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6716 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6717 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6718 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6719 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6720 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6721 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6722 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6723 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6724 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6725 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6726 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6727 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6728 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6729 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6730 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6731 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6732 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6733 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6734 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6735 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6736 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6737 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6738 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6739 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6740 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6741 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6742 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6743 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6744 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6745 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6746 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6747 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6748 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6749 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6750 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6754 Behaves like the wait(2) system call on your system: it waits for a child
6755 process to terminate and returns the pid of the deceased process, or
6756 C<-1> if there are no child processes. The status is returned in C<$?>.
6757 Note that a return value of C<-1> could mean that child processes are
6758 being automatically reaped, as described in L<perlipc>.
6760 =item waitpid PID,FLAGS
6762 Waits for a particular child process to terminate and returns the pid of
6763 the deceased process, or C<-1> if there is no such child process. On some
6764 systems, a value of 0 indicates that there are processes still running.
6765 The status is returned in C<$?>. If you say
6767 use POSIX ":sys_wait_h";
6770 $kid = waitpid(-1, WNOHANG);
6773 then you can do a non-blocking wait for all pending zombie processes.
6774 Non-blocking wait is available on machines supporting either the
6775 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6776 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6777 system call by remembering the status values of processes that have
6778 exited but have not been harvested by the Perl script yet.)
6780 Note that on some systems, a return value of C<-1> could mean that child
6781 processes are being automatically reaped. See L<perlipc> for details,
6782 and for other examples.
6786 Returns true if the context of the currently executing subroutine or
6787 C<eval> is looking for a list value. Returns false if the context is
6788 looking for a scalar. Returns the undefined value if the context is
6789 looking for no value (void context).
6791 return unless defined wantarray; # don't bother doing more
6792 my @a = complex_calculation();
6793 return wantarray ? @a : "@a";
6795 C<wantarray()>'s result is unspecified in the top level of a file,
6796 in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
6799 This function should have been named wantlist() instead.
6803 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6806 If LIST is empty and C<$@> already contains a value (typically from a
6807 previous eval) that value is used after appending C<"\t...caught">
6808 to C<$@>. This is useful for staying almost, but not entirely similar to
6811 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6813 No message is printed if there is a C<$SIG{__WARN__}> handler
6814 installed. It is the handler's responsibility to deal with the message
6815 as it sees fit (like, for instance, converting it into a C<die>). Most
6816 handlers must therefore make arrangements to actually display the
6817 warnings that they are not prepared to deal with, by calling C<warn>
6818 again in the handler. Note that this is quite safe and will not
6819 produce an endless loop, since C<__WARN__> hooks are not called from
6822 You will find this behavior is slightly different from that of
6823 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6824 instead call C<die> again to change it).
6826 Using a C<__WARN__> handler provides a powerful way to silence all
6827 warnings (even the so-called mandatory ones). An example:
6829 # wipe out *all* compile-time warnings
6830 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6832 my $foo = 20; # no warning about duplicate my $foo,
6833 # but hey, you asked for it!
6834 # no compile-time or run-time warnings before here
6837 # run-time warnings enabled after here
6838 warn "\$foo is alive and $foo!"; # does show up
6840 See L<perlvar> for details on setting C<%SIG> entries, and for more
6841 examples. See the Carp module for other kinds of warnings using its
6842 carp() and cluck() functions.
6844 =item write FILEHANDLE
6850 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6851 using the format associated with that file. By default the format for
6852 a file is the one having the same name as the filehandle, but the
6853 format for the current output channel (see the C<select> function) may be set
6854 explicitly by assigning the name of the format to the C<$~> variable.
6856 Top of form processing is handled automatically: if there is
6857 insufficient room on the current page for the formatted record, the
6858 page is advanced by writing a form feed, a special top-of-page format
6859 is used to format the new page header, and then the record is written.
6860 By default the top-of-page format is the name of the filehandle with
6861 "_TOP" appended, but it may be dynamically set to the format of your
6862 choice by assigning the name to the C<$^> variable while the filehandle is
6863 selected. The number of lines remaining on the current page is in
6864 variable C<$->, which can be set to C<0> to force a new page.
6866 If FILEHANDLE is unspecified, output goes to the current default output
6867 channel, which starts out as STDOUT but may be changed by the
6868 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6869 is evaluated and the resulting string is used to look up the name of
6870 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6872 Note that write is I<not> the opposite of C<read>. Unfortunately.
6876 The transliteration operator. Same as C<tr///>. See L<perlop>.