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.
608 Changes the working directory to EXPR, if possible. If EXPR is omitted,
609 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
610 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
611 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
612 neither is set, C<chdir> does nothing. It returns true upon success,
613 false otherwise. See the example under C<die>.
617 Changes the permissions of a list of files. The first element of the
618 list must be the numerical mode, which should probably be an octal
619 number, and which definitely should I<not> be a string of octal digits:
620 C<0644> is okay, C<'0644'> is not. Returns the number of files
621 successfully changed. See also L</oct>, if all you have is a string.
623 $cnt = chmod 0755, 'foo', 'bar';
624 chmod 0755, @executables;
625 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
627 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
628 $mode = 0644; chmod $mode, 'foo'; # this is best
630 You can also import the symbolic C<S_I*> constants from the Fcntl
635 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
636 # This is identical to the chmod 0755 of the above example.
644 This safer version of L</chop> removes any trailing string
645 that corresponds to the current value of C<$/> (also known as
646 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
647 number of characters removed from all its arguments. It's often used to
648 remove the newline from the end of an input record when you're worried
649 that the final record may be missing its newline. When in paragraph
650 mode (C<$/ = "">), it removes all trailing newlines from the string.
651 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
652 a reference to an integer or the like, see L<perlvar>) chomp() won't
654 If VARIABLE is omitted, it chomps C<$_>. Example:
657 chomp; # avoid \n on last field
662 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
664 You can actually chomp anything that's an lvalue, including an assignment:
667 chomp($answer = <STDIN>);
669 If you chomp a list, each element is chomped, and the total number of
670 characters removed is returned.
672 If the C<encoding> pragma is in scope then the lengths returned are
673 calculated from the length of C<$/> in Unicode characters, which is not
674 always the same as the length of C<$/> in the native encoding.
676 Note that parentheses are necessary when you're chomping anything
677 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
678 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
679 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
680 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
689 Chops off the last character of a string and returns the character
690 chopped. It is much more efficient than C<s/.$//s> because it neither
691 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
692 If VARIABLE is a hash, it chops the hash's values, but not its keys.
694 You can actually chop anything that's an lvalue, including an assignment.
696 If you chop a list, each element is chopped. Only the value of the
697 last C<chop> is returned.
699 Note that C<chop> returns the last character. To return all but the last
700 character, use C<substr($string, 0, -1)>.
706 Changes the owner (and group) of a list of files. The first two
707 elements of the list must be the I<numeric> uid and gid, in that
708 order. A value of -1 in either position is interpreted by most
709 systems to leave that value unchanged. Returns the number of files
710 successfully changed.
712 $cnt = chown $uid, $gid, 'foo', 'bar';
713 chown $uid, $gid, @filenames;
715 Here's an example that looks up nonnumeric uids in the passwd file:
718 chomp($user = <STDIN>);
720 chomp($pattern = <STDIN>);
722 ($login,$pass,$uid,$gid) = getpwnam($user)
723 or die "$user not in passwd file";
725 @ary = glob($pattern); # expand filenames
726 chown $uid, $gid, @ary;
728 On most systems, you are not allowed to change the ownership of the
729 file unless you're the superuser, although you should be able to change
730 the group to any of your secondary groups. On insecure systems, these
731 restrictions may be relaxed, but this is not a portable assumption.
732 On POSIX systems, you can detect this condition this way:
734 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
735 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
741 Returns the character represented by that NUMBER in the character set.
742 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
743 chr(0x263a) is a Unicode smiley face. Note that characters from 128
744 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
745 backward compatibility reasons (but see L<encoding>).
747 Negative values give the Unicode replacement character (chr(0xfffd)),
748 except under the L</bytes> pragma, where low eight bits of the value
749 (truncated to an integer) are used.
751 If NUMBER is omitted, uses C<$_>.
753 For the reverse, use L</ord>.
755 Note that under the C<bytes> pragma the NUMBER is masked to
758 See L<perlunicode> and L<encoding> for more about Unicode.
760 =item chroot FILENAME
764 This function works like the system call by the same name: it makes the
765 named directory the new root directory for all further pathnames that
766 begin with a C</> by your process and all its children. (It doesn't
767 change your current working directory, which is unaffected.) For security
768 reasons, this call is restricted to the superuser. If FILENAME is
769 omitted, does a C<chroot> to C<$_>.
771 =item close FILEHANDLE
775 Closes the file or pipe associated with the file handle, returning
776 true only if IO buffers are successfully flushed and closes the system
777 file descriptor. Closes the currently selected filehandle if the
780 You don't have to close FILEHANDLE if you are immediately going to do
781 another C<open> on it, because C<open> will close it for you. (See
782 C<open>.) However, an explicit C<close> on an input file resets the line
783 counter (C<$.>), while the implicit close done by C<open> does not.
785 If the file handle came from a piped open, C<close> will additionally
786 return false if one of the other system calls involved fails, or if the
787 program exits with non-zero status. (If the only problem was that the
788 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
789 also waits for the process executing on the pipe to complete, in case you
790 want to look at the output of the pipe afterwards, and
791 implicitly puts the exit status value of that command into C<$?> and
792 C<${^CHILD_ERROR_NATIVE}>.
794 Prematurely closing the read end of a pipe (i.e. before the process
795 writing to it at the other end has closed it) will result in a
796 SIGPIPE being delivered to the writer. If the other end can't
797 handle that, be sure to read all the data before closing the pipe.
801 open(OUTPUT, '|sort >foo') # pipe to sort
802 or die "Can't start sort: $!";
803 #... # print stuff to output
804 close OUTPUT # wait for sort to finish
805 or warn $! ? "Error closing sort pipe: $!"
806 : "Exit status $? from sort";
807 open(INPUT, 'foo') # get sort's results
808 or die "Can't open 'foo' for input: $!";
810 FILEHANDLE may be an expression whose value can be used as an indirect
811 filehandle, usually the real filehandle name.
813 =item closedir DIRHANDLE
815 Closes a directory opened by C<opendir> and returns the success of that
818 =item connect SOCKET,NAME
820 Attempts to connect to a remote socket, just as the connect system call
821 does. Returns true if it succeeded, false otherwise. NAME should be a
822 packed address of the appropriate type for the socket. See the examples in
823 L<perlipc/"Sockets: Client/Server Communication">.
827 Actually a flow control statement rather than a function. If there is a
828 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
829 C<foreach>), it is always executed just before the conditional is about to
830 be evaluated again, just like the third part of a C<for> loop in C. Thus
831 it can be used to increment a loop variable, even when the loop has been
832 continued via the C<next> statement (which is similar to the C C<continue>
835 C<last>, C<next>, or C<redo> may appear within a C<continue>
836 block. C<last> and C<redo> will behave as if they had been executed within
837 the main block. So will C<next>, but since it will execute a C<continue>
838 block, it may be more entertaining.
841 ### redo always comes here
844 ### next always comes here
846 # then back the top to re-check EXPR
848 ### last always comes here
850 Omitting the C<continue> section is semantically equivalent to using an
851 empty one, logically enough. In that case, C<next> goes directly back
852 to check the condition at the top of the loop.
858 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
859 takes cosine of C<$_>.
861 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
862 function, or use this relation:
864 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
866 =item crypt PLAINTEXT,SALT
868 Encrypts a string exactly like the crypt(3) function in the C library
869 (assuming that you actually have a version there that has not been
870 extirpated as a potential munition). This can prove useful for checking
871 the password file for lousy passwords, amongst other things. Only the
872 guys wearing white hats should do this.
874 Note that L<crypt|/crypt> is intended to be a one-way function, much like
875 breaking eggs to make an omelette. There is no (known) corresponding
876 decrypt function (in other words, the crypt() is a one-way hash
877 function). As a result, this function isn't all that useful for
878 cryptography. (For that, see your nearby CPAN mirror.)
880 When verifying an existing encrypted string you should use the
881 encrypted text as the salt (like C<crypt($plain, $crypted) eq
882 $crypted>). This allows your code to work with the standard L<crypt|/crypt>
883 and with more exotic implementations. In other words, do not assume
884 anything about the returned string itself, or how many bytes in
885 the encrypted string matter.
887 Traditionally the result is a string of 13 bytes: two first bytes of
888 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
889 the first eight bytes of the encrypted string mattered, but
890 alternative hashing schemes (like MD5), higher level security schemes
891 (like C2), and implementations on non-UNIX platforms may produce
894 When choosing a new salt create a random two character string whose
895 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
896 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
897 characters is just a recommendation; the characters allowed in
898 the salt depend solely on your system's crypt library, and Perl can't
899 restrict what salts C<crypt()> accepts.
901 Here's an example that makes sure that whoever runs this program knows
904 $pwd = (getpwuid($<))[1];
908 chomp($word = <STDIN>);
912 if (crypt($word, $pwd) ne $pwd) {
918 Of course, typing in your own password to whoever asks you
921 The L<crypt|/crypt> function is unsuitable for encrypting large quantities
922 of data, not least of all because you can't get the information
923 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
924 on your favorite CPAN mirror for a slew of potentially useful
927 If using crypt() on a Unicode string (which I<potentially> has
928 characters with codepoints above 255), Perl tries to make sense
929 of the situation by trying to downgrade (a copy of the string)
930 the string back to an eight-bit byte string before calling crypt()
931 (on that copy). If that works, good. If not, crypt() dies with
932 C<Wide character in crypt>.
936 [This function has been largely superseded by the C<untie> function.]
938 Breaks the binding between a DBM file and a hash.
940 =item dbmopen HASH,DBNAME,MASK
942 [This function has been largely superseded by the C<tie> function.]
944 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
945 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
946 argument is I<not> a filehandle, even though it looks like one). DBNAME
947 is the name of the database (without the F<.dir> or F<.pag> extension if
948 any). If the database does not exist, it is created with protection
949 specified by MASK (as modified by the C<umask>). If your system supports
950 only the older DBM functions, you may perform only one C<dbmopen> in your
951 program. In older versions of Perl, if your system had neither DBM nor
952 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
955 If you don't have write access to the DBM file, you can only read hash
956 variables, not set them. If you want to test whether you can write,
957 either use file tests or try setting a dummy hash entry inside an C<eval>,
958 which will trap the error.
960 Note that functions such as C<keys> and C<values> may return huge lists
961 when used on large DBM files. You may prefer to use the C<each>
962 function to iterate over large DBM files. Example:
964 # print out history file offsets
965 dbmopen(%HIST,'/usr/lib/news/history',0666);
966 while (($key,$val) = each %HIST) {
967 print $key, ' = ', unpack('L',$val), "\n";
971 See also L<AnyDBM_File> for a more general description of the pros and
972 cons of the various dbm approaches, as well as L<DB_File> for a particularly
975 You can control which DBM library you use by loading that library
976 before you call dbmopen():
979 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
980 or die "Can't open netscape history file: $!";
986 Returns a Boolean value telling whether EXPR has a value other than
987 the undefined value C<undef>. If EXPR is not present, C<$_> will be
990 Many operations return C<undef> to indicate failure, end of file,
991 system error, uninitialized variable, and other exceptional
992 conditions. This function allows you to distinguish C<undef> from
993 other values. (A simple Boolean test will not distinguish among
994 C<undef>, zero, the empty string, and C<"0">, which are all equally
995 false.) Note that since C<undef> is a valid scalar, its presence
996 doesn't I<necessarily> indicate an exceptional condition: C<pop>
997 returns C<undef> when its argument is an empty array, I<or> when the
998 element to return happens to be C<undef>.
1000 You may also use C<defined(&func)> to check whether subroutine C<&func>
1001 has ever been defined. The return value is unaffected by any forward
1002 declarations of C<&func>. Note that a subroutine which is not defined
1003 may still be callable: its package may have an C<AUTOLOAD> method that
1004 makes it spring into existence the first time that it is called -- see
1007 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1008 used to report whether memory for that aggregate has ever been
1009 allocated. This behavior may disappear in future versions of Perl.
1010 You should instead use a simple test for size:
1012 if (@an_array) { print "has array elements\n" }
1013 if (%a_hash) { print "has hash members\n" }
1015 When used on a hash element, it tells you whether the value is defined,
1016 not whether the key exists in the hash. Use L</exists> for the latter
1021 print if defined $switch{'D'};
1022 print "$val\n" while defined($val = pop(@ary));
1023 die "Can't readlink $sym: $!"
1024 unless defined($value = readlink $sym);
1025 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1026 $debugging = 0 unless defined $debugging;
1028 Note: Many folks tend to overuse C<defined>, and then are surprised to
1029 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1030 defined values. For example, if you say
1034 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1035 matched "nothing". But it didn't really match nothing--rather, it
1036 matched something that happened to be zero characters long. This is all
1037 very above-board and honest. When a function returns an undefined value,
1038 it's an admission that it couldn't give you an honest answer. So you
1039 should use C<defined> only when you're questioning the integrity of what
1040 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1043 See also L</undef>, L</exists>, L</ref>.
1047 Given an expression that specifies a hash element, array element, hash slice,
1048 or array slice, deletes the specified element(s) from the hash or array.
1049 In the case of an array, if the array elements happen to be at the end,
1050 the size of the array will shrink to the highest element that tests
1051 true for exists() (or 0 if no such element exists).
1053 Returns a list with the same number of elements as the number of elements
1054 for which deletion was attempted. Each element of that list consists of
1055 either the value of the element deleted, or the undefined value. In scalar
1056 context, this means that you get the value of the last element deleted (or
1057 the undefined value if that element did not exist).
1059 %hash = (foo => 11, bar => 22, baz => 33);
1060 $scalar = delete $hash{foo}; # $scalar is 11
1061 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1062 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1064 Deleting from C<%ENV> modifies the environment. Deleting from
1065 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1066 from a C<tie>d hash or array may not necessarily return anything.
1068 Deleting an array element effectively returns that position of the array
1069 to its initial, uninitialized state. Subsequently testing for the same
1070 element with exists() will return false. Note that deleting array
1071 elements in the middle of an array will not shift the index of the ones
1072 after them down--use splice() for that. See L</exists>.
1074 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1076 foreach $key (keys %HASH) {
1080 foreach $index (0 .. $#ARRAY) {
1081 delete $ARRAY[$index];
1086 delete @HASH{keys %HASH};
1088 delete @ARRAY[0 .. $#ARRAY];
1090 But both of these are slower than just assigning the empty list
1091 or undefining %HASH or @ARRAY:
1093 %HASH = (); # completely empty %HASH
1094 undef %HASH; # forget %HASH ever existed
1096 @ARRAY = (); # completely empty @ARRAY
1097 undef @ARRAY; # forget @ARRAY ever existed
1099 Note that the EXPR can be arbitrarily complicated as long as the final
1100 operation is a hash element, array element, hash slice, or array slice
1103 delete $ref->[$x][$y]{$key};
1104 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1106 delete $ref->[$x][$y][$index];
1107 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1111 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1112 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1113 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1114 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1115 an C<eval(),> the error message is stuffed into C<$@> and the
1116 C<eval> is terminated with the undefined value. This makes
1117 C<die> the way to raise an exception.
1119 Equivalent examples:
1121 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1122 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1124 If the last element of LIST does not end in a newline, the current
1125 script line number and input line number (if any) are also printed,
1126 and a newline is supplied. Note that the "input line number" (also
1127 known as "chunk") is subject to whatever notion of "line" happens to
1128 be currently in effect, and is also available as the special variable
1129 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1131 Hint: sometimes appending C<", stopped"> to your message will cause it
1132 to make better sense when the string C<"at foo line 123"> is appended.
1133 Suppose you are running script "canasta".
1135 die "/etc/games is no good";
1136 die "/etc/games is no good, stopped";
1138 produce, respectively
1140 /etc/games is no good at canasta line 123.
1141 /etc/games is no good, stopped at canasta line 123.
1143 See also exit(), warn(), and the Carp module.
1145 If LIST is empty and C<$@> already contains a value (typically from a
1146 previous eval) that value is reused after appending C<"\t...propagated">.
1147 This is useful for propagating exceptions:
1150 die unless $@ =~ /Expected exception/;
1152 If LIST is empty and C<$@> contains an object reference that has a
1153 C<PROPAGATE> method, that method will be called with additional file
1154 and line number parameters. The return value replaces the value in
1155 C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1158 If C<$@> is empty then the string C<"Died"> is used.
1160 die() can also be called with a reference argument. If this happens to be
1161 trapped within an eval(), $@ contains the reference. This behavior permits
1162 a more elaborate exception handling implementation using objects that
1163 maintain arbitrary state about the nature of the exception. Such a scheme
1164 is sometimes preferable to matching particular string values of $@ using
1165 regular expressions. Here's an example:
1167 use Scalar::Util 'blessed';
1169 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1171 if (blessed($@) && $@->isa("Some::Module::Exception")) {
1172 # handle Some::Module::Exception
1175 # handle all other possible exceptions
1179 Because perl will stringify uncaught exception messages before displaying
1180 them, you may want to overload stringification operations on such custom
1181 exception objects. See L<overload> for details about that.
1183 You can arrange for a callback to be run just before the C<die>
1184 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1185 handler will be called with the error text and can change the error
1186 message, if it sees fit, by calling C<die> again. See
1187 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1188 L<"eval BLOCK"> for some examples. Although this feature was meant
1189 to be run only right before your program was to exit, this is not
1190 currently the case--the C<$SIG{__DIE__}> hook is currently called
1191 even inside eval()ed blocks/strings! If one wants the hook to do
1192 nothing in such situations, put
1196 as the first line of the handler (see L<perlvar/$^S>). Because
1197 this promotes strange action at a distance, this counterintuitive
1198 behavior may be fixed in a future release.
1202 Not really a function. Returns the value of the last command in the
1203 sequence of commands indicated by BLOCK. When modified by a loop
1204 modifier, executes the BLOCK once before testing the loop condition.
1205 (On other statements the loop modifiers test the conditional first.)
1207 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1208 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1209 See L<perlsyn> for alternative strategies.
1211 =item do SUBROUTINE(LIST)
1213 A deprecated form of subroutine call. See L<perlsub>.
1217 Uses the value of EXPR as a filename and executes the contents of the
1218 file as a Perl script.
1226 except that it's more efficient and concise, keeps track of the current
1227 filename for error messages, searches the @INC directories, and updates
1228 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1229 variables. It also differs in that code evaluated with C<do FILENAME>
1230 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1231 same, however, in that it does reparse the file every time you call it,
1232 so you probably don't want to do this inside a loop.
1234 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1235 error. If C<do> can read the file but cannot compile it, it
1236 returns undef and sets an error message in C<$@>. If the file is
1237 successfully compiled, C<do> returns the value of the last expression
1240 Note that inclusion of library modules is better done with the
1241 C<use> and C<require> operators, which also do automatic error checking
1242 and raise an exception if there's a problem.
1244 You might like to use C<do> to read in a program configuration
1245 file. Manual error checking can be done this way:
1247 # read in config files: system first, then user
1248 for $file ("/share/prog/defaults.rc",
1249 "$ENV{HOME}/.someprogrc")
1251 unless ($return = do $file) {
1252 warn "couldn't parse $file: $@" if $@;
1253 warn "couldn't do $file: $!" unless defined $return;
1254 warn "couldn't run $file" unless $return;
1262 This function causes an immediate core dump. See also the B<-u>
1263 command-line switch in L<perlrun>, which does the same thing.
1264 Primarily this is so that you can use the B<undump> program (not
1265 supplied) to turn your core dump into an executable binary after
1266 having initialized all your variables at the beginning of the
1267 program. When the new binary is executed it will begin by executing
1268 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1269 Think of it as a goto with an intervening core dump and reincarnation.
1270 If C<LABEL> is omitted, restarts the program from the top.
1272 B<WARNING>: Any files opened at the time of the dump will I<not>
1273 be open any more when the program is reincarnated, with possible
1274 resulting confusion on the part of Perl.
1276 This function is now largely obsolete, partly because it's very
1277 hard to convert a core file into an executable, and because the
1278 real compiler backends for generating portable bytecode and compilable
1279 C code have superseded it. That's why you should now invoke it as
1280 C<CORE::dump()>, if you don't want to be warned against a possible
1283 If you're looking to use L<dump> to speed up your program, consider
1284 generating bytecode or native C code as described in L<perlcc>. If
1285 you're just trying to accelerate a CGI script, consider using the
1286 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1287 You might also consider autoloading or selfloading, which at least
1288 make your program I<appear> to run faster.
1292 When called in list context, returns a 2-element list consisting of the
1293 key and value for the next element of a hash, so that you can iterate over
1294 it. When called in scalar context, returns only the key for the next
1295 element in the hash.
1297 Entries are returned in an apparently random order. The actual random
1298 order is subject to change in future versions of perl, but it is
1299 guaranteed to be in the same order as either the C<keys> or C<values>
1300 function would produce on the same (unmodified) hash. Since Perl
1301 5.8.1 the ordering is different even between different runs of Perl
1302 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1304 When the hash is entirely read, a null array is returned in list context
1305 (which when assigned produces a false (C<0>) value), and C<undef> in
1306 scalar context. The next call to C<each> after that will start iterating
1307 again. There is a single iterator for each hash, shared by all C<each>,
1308 C<keys>, and C<values> function calls in the program; it can be reset by
1309 reading all the elements from the hash, or by evaluating C<keys HASH> or
1310 C<values HASH>. If you add or delete elements of a hash while you're
1311 iterating over it, you may get entries skipped or duplicated, so
1312 don't. Exception: It is always safe to delete the item most recently
1313 returned by C<each()>, which means that the following code will work:
1315 while (($key, $value) = each %hash) {
1317 delete $hash{$key}; # This is safe
1320 The following prints out your environment like the printenv(1) program,
1321 only in a different order:
1323 while (($key,$value) = each %ENV) {
1324 print "$key=$value\n";
1327 See also C<keys>, C<values> and C<sort>.
1329 =item eof FILEHANDLE
1335 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1336 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1337 gives the real filehandle. (Note that this function actually
1338 reads a character and then C<ungetc>s it, so isn't very useful in an
1339 interactive context.) Do not read from a terminal file (or call
1340 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1341 as terminals may lose the end-of-file condition if you do.
1343 An C<eof> without an argument uses the last file read. Using C<eof()>
1344 with empty parentheses is very different. It refers to the pseudo file
1345 formed from the files listed on the command line and accessed via the
1346 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1347 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1348 used will cause C<@ARGV> to be examined to determine if input is
1349 available. Similarly, an C<eof()> after C<< <> >> has returned
1350 end-of-file will assume you are processing another C<@ARGV> list,
1351 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1352 see L<perlop/"I/O Operators">.
1354 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1355 detect the end of each file, C<eof()> will only detect the end of the
1356 last file. Examples:
1358 # reset line numbering on each input file
1360 next if /^\s*#/; # skip comments
1363 close ARGV if eof; # Not eof()!
1366 # insert dashes just before last line of last file
1368 if (eof()) { # check for end of last file
1369 print "--------------\n";
1372 last if eof(); # needed if we're reading from a terminal
1375 Practical hint: you almost never need to use C<eof> in Perl, because the
1376 input operators typically return C<undef> when they run out of data, or if
1385 In the first form, the return value of EXPR is parsed and executed as if it
1386 were a little Perl program. The value of the expression (which is itself
1387 determined within scalar context) is first parsed, and if there weren't any
1388 errors, executed in the lexical context of the current Perl program, so
1389 that any variable settings or subroutine and format definitions remain
1390 afterwards. Note that the value is parsed every time the eval executes.
1391 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1392 delay parsing and subsequent execution of the text of EXPR until run time.
1394 In the second form, the code within the BLOCK is parsed only once--at the
1395 same time the code surrounding the eval itself was parsed--and executed
1396 within the context of the current Perl program. This form is typically
1397 used to trap exceptions more efficiently than the first (see below), while
1398 also providing the benefit of checking the code within BLOCK at compile
1401 The final semicolon, if any, may be omitted from the value of EXPR or within
1404 In both forms, the value returned is the value of the last expression
1405 evaluated inside the mini-program; a return statement may be also used, just
1406 as with subroutines. The expression providing the return value is evaluated
1407 in void, scalar, or list context, depending on the context of the eval itself.
1408 See L</wantarray> for more on how the evaluation context can be determined.
1410 If there is a syntax error or runtime error, or a C<die> statement is
1411 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1412 error message. If there was no error, C<$@> is guaranteed to be a null
1413 string. Beware that using C<eval> neither silences perl from printing
1414 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1415 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1416 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1417 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1419 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1420 determining whether a particular feature (such as C<socket> or C<symlink>)
1421 is implemented. It is also Perl's exception trapping mechanism, where
1422 the die operator is used to raise exceptions.
1424 If the code to be executed doesn't vary, you may use the eval-BLOCK
1425 form to trap run-time errors without incurring the penalty of
1426 recompiling each time. The error, if any, is still returned in C<$@>.
1429 # make divide-by-zero nonfatal
1430 eval { $answer = $a / $b; }; warn $@ if $@;
1432 # same thing, but less efficient
1433 eval '$answer = $a / $b'; warn $@ if $@;
1435 # a compile-time error
1436 eval { $answer = }; # WRONG
1439 eval '$answer ='; # sets $@
1441 Due to the current arguably broken state of C<__DIE__> hooks, when using
1442 the C<eval{}> form as an exception trap in libraries, you may wish not
1443 to trigger any C<__DIE__> hooks that user code may have installed.
1444 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1445 as shown in this example:
1447 # a very private exception trap for divide-by-zero
1448 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1451 This is especially significant, given that C<__DIE__> hooks can call
1452 C<die> again, which has the effect of changing their error messages:
1454 # __DIE__ hooks may modify error messages
1456 local $SIG{'__DIE__'} =
1457 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1458 eval { die "foo lives here" };
1459 print $@ if $@; # prints "bar lives here"
1462 Because this promotes action at a distance, this counterintuitive behavior
1463 may be fixed in a future release.
1465 With an C<eval>, you should be especially careful to remember what's
1466 being looked at when:
1472 eval { $x }; # CASE 4
1474 eval "\$$x++"; # CASE 5
1477 Cases 1 and 2 above behave identically: they run the code contained in
1478 the variable $x. (Although case 2 has misleading double quotes making
1479 the reader wonder what else might be happening (nothing is).) Cases 3
1480 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1481 does nothing but return the value of $x. (Case 4 is preferred for
1482 purely visual reasons, but it also has the advantage of compiling at
1483 compile-time instead of at run-time.) Case 5 is a place where
1484 normally you I<would> like to use double quotes, except that in this
1485 particular situation, you can just use symbolic references instead, as
1488 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1489 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1491 Note that as a very special case, an C<eval ''> executed within the C<DB>
1492 package doesn't see the usual surrounding lexical scope, but rather the
1493 scope of the first non-DB piece of code that called it. You don't normally
1494 need to worry about this unless you are writing a Perl debugger.
1498 =item exec PROGRAM LIST
1500 The C<exec> function executes a system command I<and never returns>--
1501 use C<system> instead of C<exec> if you want it to return. It fails and
1502 returns false only if the command does not exist I<and> it is executed
1503 directly instead of via your system's command shell (see below).
1505 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1506 warns you if there is a following statement which isn't C<die>, C<warn>,
1507 or C<exit> (if C<-w> is set - but you always do that). If you
1508 I<really> want to follow an C<exec> with some other statement, you
1509 can use one of these styles to avoid the warning:
1511 exec ('foo') or print STDERR "couldn't exec foo: $!";
1512 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1514 If there is more than one argument in LIST, or if LIST is an array
1515 with more than one value, calls execvp(3) with the arguments in LIST.
1516 If there is only one scalar argument or an array with one element in it,
1517 the argument is checked for shell metacharacters, and if there are any,
1518 the entire argument is passed to the system's command shell for parsing
1519 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1520 If there are no shell metacharacters in the argument, it is split into
1521 words and passed directly to C<execvp>, which is more efficient.
1524 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1525 exec "sort $outfile | uniq";
1527 If you don't really want to execute the first argument, but want to lie
1528 to the program you are executing about its own name, you can specify
1529 the program you actually want to run as an "indirect object" (without a
1530 comma) in front of the LIST. (This always forces interpretation of the
1531 LIST as a multivalued list, even if there is only a single scalar in
1534 $shell = '/bin/csh';
1535 exec $shell '-sh'; # pretend it's a login shell
1539 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1541 When the arguments get executed via the system shell, results will
1542 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1545 Using an indirect object with C<exec> or C<system> is also more
1546 secure. This usage (which also works fine with system()) forces
1547 interpretation of the arguments as a multivalued list, even if the
1548 list had just one argument. That way you're safe from the shell
1549 expanding wildcards or splitting up words with whitespace in them.
1551 @args = ( "echo surprise" );
1553 exec @args; # subject to shell escapes
1555 exec { $args[0] } @args; # safe even with one-arg list
1557 The first version, the one without the indirect object, ran the I<echo>
1558 program, passing it C<"surprise"> an argument. The second version
1559 didn't--it tried to run a program literally called I<"echo surprise">,
1560 didn't find it, and set C<$?> to a non-zero value indicating failure.
1562 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1563 output before the exec, but this may not be supported on some platforms
1564 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1565 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1566 open handles in order to avoid lost output.
1568 Note that C<exec> will not call your C<END> blocks, nor will it call
1569 any C<DESTROY> methods in your objects.
1573 Given an expression that specifies a hash element or array element,
1574 returns true if the specified element in the hash or array has ever
1575 been initialized, even if the corresponding value is undefined. The
1576 element is not autovivified if it doesn't exist.
1578 print "Exists\n" if exists $hash{$key};
1579 print "Defined\n" if defined $hash{$key};
1580 print "True\n" if $hash{$key};
1582 print "Exists\n" if exists $array[$index];
1583 print "Defined\n" if defined $array[$index];
1584 print "True\n" if $array[$index];
1586 A hash or array element can be true only if it's defined, and defined if
1587 it exists, but the reverse doesn't necessarily hold true.
1589 Given an expression that specifies the name of a subroutine,
1590 returns true if the specified subroutine has ever been declared, even
1591 if it is undefined. Mentioning a subroutine name for exists or defined
1592 does not count as declaring it. Note that a subroutine which does not
1593 exist may still be callable: its package may have an C<AUTOLOAD>
1594 method that makes it spring into existence the first time that it is
1595 called -- see L<perlsub>.
1597 print "Exists\n" if exists &subroutine;
1598 print "Defined\n" if defined &subroutine;
1600 Note that the EXPR can be arbitrarily complicated as long as the final
1601 operation is a hash or array key lookup or subroutine name:
1603 if (exists $ref->{A}->{B}->{$key}) { }
1604 if (exists $hash{A}{B}{$key}) { }
1606 if (exists $ref->{A}->{B}->[$ix]) { }
1607 if (exists $hash{A}{B}[$ix]) { }
1609 if (exists &{$ref->{A}{B}{$key}}) { }
1611 Although the deepest nested array or hash will not spring into existence
1612 just because its existence was tested, any intervening ones will.
1613 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1614 into existence due to the existence test for the $key element above.
1615 This happens anywhere the arrow operator is used, including even:
1618 if (exists $ref->{"Some key"}) { }
1619 print $ref; # prints HASH(0x80d3d5c)
1621 This surprising autovivification in what does not at first--or even
1622 second--glance appear to be an lvalue context may be fixed in a future
1625 Use of a subroutine call, rather than a subroutine name, as an argument
1626 to exists() is an error.
1629 exists &sub(); # Error
1635 Evaluates EXPR and exits immediately with that value. Example:
1638 exit 0 if $ans =~ /^[Xx]/;
1640 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1641 universally recognized values for EXPR are C<0> for success and C<1>
1642 for error; other values are subject to interpretation depending on the
1643 environment in which the Perl program is running. For example, exiting
1644 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1645 the mailer to return the item undelivered, but that's not true everywhere.
1647 Don't use C<exit> to abort a subroutine if there's any chance that
1648 someone might want to trap whatever error happened. Use C<die> instead,
1649 which can be trapped by an C<eval>.
1651 The exit() function does not always exit immediately. It calls any
1652 defined C<END> routines first, but these C<END> routines may not
1653 themselves abort the exit. Likewise any object destructors that need to
1654 be called are called before the real exit. If this is a problem, you
1655 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1656 See L<perlmod> for details.
1662 Returns I<e> (the natural logarithm base) to the power of EXPR.
1663 If EXPR is omitted, gives C<exp($_)>.
1665 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1667 Implements the fcntl(2) function. You'll probably have to say
1671 first to get the correct constant definitions. Argument processing and
1672 value return works just like C<ioctl> below.
1676 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1677 or die "can't fcntl F_GETFL: $!";
1679 You don't have to check for C<defined> on the return from C<fcntl>.
1680 Like C<ioctl>, it maps a C<0> return from the system call into
1681 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1682 in numeric context. It is also exempt from the normal B<-w> warnings
1683 on improper numeric conversions.
1685 Note that C<fcntl> will produce a fatal error if used on a machine that
1686 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1687 manpage to learn what functions are available on your system.
1689 Here's an example of setting a filehandle named C<REMOTE> to be
1690 non-blocking at the system level. You'll have to negotiate C<$|>
1691 on your own, though.
1693 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1695 $flags = fcntl(REMOTE, F_GETFL, 0)
1696 or die "Can't get flags for the socket: $!\n";
1698 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1699 or die "Can't set flags for the socket: $!\n";
1701 =item fileno FILEHANDLE
1703 Returns the file descriptor for a filehandle, or undefined if the
1704 filehandle is not open. This is mainly useful for constructing
1705 bitmaps for C<select> and low-level POSIX tty-handling operations.
1706 If FILEHANDLE is an expression, the value is taken as an indirect
1707 filehandle, generally its name.
1709 You can use this to find out whether two handles refer to the
1710 same underlying descriptor:
1712 if (fileno(THIS) == fileno(THAT)) {
1713 print "THIS and THAT are dups\n";
1716 (Filehandles connected to memory objects via new features of C<open> may
1717 return undefined even though they are open.)
1720 =item flock FILEHANDLE,OPERATION
1722 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1723 for success, false on failure. Produces a fatal error if used on a
1724 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1725 C<flock> is Perl's portable file locking interface, although it locks
1726 only entire files, not records.
1728 Two potentially non-obvious but traditional C<flock> semantics are
1729 that it waits indefinitely until the lock is granted, and that its locks
1730 B<merely advisory>. Such discretionary locks are more flexible, but offer
1731 fewer guarantees. This means that files locked with C<flock> may be
1732 modified by programs that do not also use C<flock>. See L<perlport>,
1733 your port's specific documentation, or your system-specific local manpages
1734 for details. It's best to assume traditional behavior if you're writing
1735 portable programs. (But if you're not, you should as always feel perfectly
1736 free to write for your own system's idiosyncrasies (sometimes called
1737 "features"). Slavish adherence to portability concerns shouldn't get
1738 in the way of your getting your job done.)
1740 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1741 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1742 you can use the symbolic names if you import them from the Fcntl module,
1743 either individually, or as a group using the ':flock' tag. LOCK_SH
1744 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1745 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1746 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1747 waiting for the lock (check the return status to see if you got it).
1749 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1750 before locking or unlocking it.
1752 Note that the emulation built with lockf(3) doesn't provide shared
1753 locks, and it requires that FILEHANDLE be open with write intent. These
1754 are the semantics that lockf(3) implements. Most if not all systems
1755 implement lockf(3) in terms of fcntl(2) locking, though, so the
1756 differing semantics shouldn't bite too many people.
1758 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1759 be open with read intent to use LOCK_SH and requires that it be open
1760 with write intent to use LOCK_EX.
1762 Note also that some versions of C<flock> cannot lock things over the
1763 network; you would need to use the more system-specific C<fcntl> for
1764 that. If you like you can force Perl to ignore your system's flock(2)
1765 function, and so provide its own fcntl(2)-based emulation, by passing
1766 the switch C<-Ud_flock> to the F<Configure> program when you configure
1769 Here's a mailbox appender for BSD systems.
1771 use Fcntl ':flock'; # import LOCK_* constants
1774 flock(MBOX,LOCK_EX);
1775 # and, in case someone appended
1776 # while we were waiting...
1781 flock(MBOX,LOCK_UN);
1784 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1785 or die "Can't open mailbox: $!";
1788 print MBOX $msg,"\n\n";
1791 On systems that support a real flock(), locks are inherited across fork()
1792 calls, whereas those that must resort to the more capricious fcntl()
1793 function lose the locks, making it harder to write servers.
1795 See also L<DB_File> for other flock() examples.
1799 Does a fork(2) system call to create a new process running the
1800 same program at the same point. It returns the child pid to the
1801 parent process, C<0> to the child process, or C<undef> if the fork is
1802 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1803 are shared, while everything else is copied. On most systems supporting
1804 fork(), great care has gone into making it extremely efficient (for
1805 example, using copy-on-write technology on data pages), making it the
1806 dominant paradigm for multitasking over the last few decades.
1808 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1809 output before forking the child process, but this may not be supported
1810 on some platforms (see L<perlport>). To be safe, you may need to set
1811 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1812 C<IO::Handle> on any open handles in order to avoid duplicate output.
1814 If you C<fork> without ever waiting on your children, you will
1815 accumulate zombies. On some systems, you can avoid this by setting
1816 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1817 forking and reaping moribund children.
1819 Note that if your forked child inherits system file descriptors like
1820 STDIN and STDOUT that are actually connected by a pipe or socket, even
1821 if you exit, then the remote server (such as, say, a CGI script or a
1822 backgrounded job launched from a remote shell) won't think you're done.
1823 You should reopen those to F</dev/null> if it's any issue.
1827 Declare a picture format for use by the C<write> function. For
1831 Test: @<<<<<<<< @||||| @>>>>>
1832 $str, $%, '$' . int($num)
1836 $num = $cost/$quantity;
1840 See L<perlform> for many details and examples.
1842 =item formline PICTURE,LIST
1844 This is an internal function used by C<format>s, though you may call it,
1845 too. It formats (see L<perlform>) a list of values according to the
1846 contents of PICTURE, placing the output into the format output
1847 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1848 Eventually, when a C<write> is done, the contents of
1849 C<$^A> are written to some filehandle, but you could also read C<$^A>
1850 yourself and then set C<$^A> back to C<"">. Note that a format typically
1851 does one C<formline> per line of form, but the C<formline> function itself
1852 doesn't care how many newlines are embedded in the PICTURE. This means
1853 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1854 You may therefore need to use multiple formlines to implement a single
1855 record format, just like the format compiler.
1857 Be careful if you put double quotes around the picture, because an C<@>
1858 character may be taken to mean the beginning of an array name.
1859 C<formline> always returns true. See L<perlform> for other examples.
1861 =item getc FILEHANDLE
1865 Returns the next character from the input file attached to FILEHANDLE,
1866 or the undefined value at end of file, or if there was an error (in
1867 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1868 STDIN. This is not particularly efficient. However, it cannot be
1869 used by itself to fetch single characters without waiting for the user
1870 to hit enter. For that, try something more like:
1873 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1876 system "stty", '-icanon', 'eol', "\001";
1882 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1885 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1889 Determination of whether $BSD_STYLE should be set
1890 is left as an exercise to the reader.
1892 The C<POSIX::getattr> function can do this more portably on
1893 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1894 module from your nearest CPAN site; details on CPAN can be found on
1899 Implements the C library function of the same name, which on most
1900 systems returns the current login from F</etc/utmp>, if any. If null,
1903 $login = getlogin || getpwuid($<) || "Kilroy";
1905 Do not consider C<getlogin> for authentication: it is not as
1906 secure as C<getpwuid>.
1908 =item getpeername SOCKET
1910 Returns the packed sockaddr address of other end of the SOCKET connection.
1913 $hersockaddr = getpeername(SOCK);
1914 ($port, $iaddr) = sockaddr_in($hersockaddr);
1915 $herhostname = gethostbyaddr($iaddr, AF_INET);
1916 $herstraddr = inet_ntoa($iaddr);
1920 Returns the current process group for the specified PID. Use
1921 a PID of C<0> to get the current process group for the
1922 current process. Will raise an exception if used on a machine that
1923 doesn't implement getpgrp(2). If PID is omitted, returns process
1924 group of current process. Note that the POSIX version of C<getpgrp>
1925 does not accept a PID argument, so only C<PID==0> is truly portable.
1929 Returns the process id of the parent process.
1931 Note for Linux users: on Linux, the C functions C<getpid()> and
1932 C<getppid()> return different values from different threads. In order to
1933 be portable, this behavior is not reflected by the perl-level function
1934 C<getppid()>, that returns a consistent value across threads. If you want
1935 to call the underlying C<getppid()>, you may use the CPAN module
1938 =item getpriority WHICH,WHO
1940 Returns the current priority for a process, a process group, or a user.
1941 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1942 machine that doesn't implement getpriority(2).
1948 =item gethostbyname NAME
1950 =item getnetbyname NAME
1952 =item getprotobyname NAME
1958 =item getservbyname NAME,PROTO
1960 =item gethostbyaddr ADDR,ADDRTYPE
1962 =item getnetbyaddr ADDR,ADDRTYPE
1964 =item getprotobynumber NUMBER
1966 =item getservbyport PORT,PROTO
1984 =item sethostent STAYOPEN
1986 =item setnetent STAYOPEN
1988 =item setprotoent STAYOPEN
1990 =item setservent STAYOPEN
2004 These routines perform the same functions as their counterparts in the
2005 system library. In list context, the return values from the
2006 various get routines are as follows:
2008 ($name,$passwd,$uid,$gid,
2009 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2010 ($name,$passwd,$gid,$members) = getgr*
2011 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2012 ($name,$aliases,$addrtype,$net) = getnet*
2013 ($name,$aliases,$proto) = getproto*
2014 ($name,$aliases,$port,$proto) = getserv*
2016 (If the entry doesn't exist you get a null list.)
2018 The exact meaning of the $gcos field varies but it usually contains
2019 the real name of the user (as opposed to the login name) and other
2020 information pertaining to the user. Beware, however, that in many
2021 system users are able to change this information and therefore it
2022 cannot be trusted and therefore the $gcos is tainted (see
2023 L<perlsec>). The $passwd and $shell, user's encrypted password and
2024 login shell, are also tainted, because of the same reason.
2026 In scalar context, you get the name, unless the function was a
2027 lookup by name, in which case you get the other thing, whatever it is.
2028 (If the entry doesn't exist you get the undefined value.) For example:
2030 $uid = getpwnam($name);
2031 $name = getpwuid($num);
2033 $gid = getgrnam($name);
2034 $name = getgrgid($num);
2038 In I<getpw*()> the fields $quota, $comment, and $expire are special
2039 cases in the sense that in many systems they are unsupported. If the
2040 $quota is unsupported, it is an empty scalar. If it is supported, it
2041 usually encodes the disk quota. If the $comment field is unsupported,
2042 it is an empty scalar. If it is supported it usually encodes some
2043 administrative comment about the user. In some systems the $quota
2044 field may be $change or $age, fields that have to do with password
2045 aging. In some systems the $comment field may be $class. The $expire
2046 field, if present, encodes the expiration period of the account or the
2047 password. For the availability and the exact meaning of these fields
2048 in your system, please consult your getpwnam(3) documentation and your
2049 F<pwd.h> file. You can also find out from within Perl what your
2050 $quota and $comment fields mean and whether you have the $expire field
2051 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2052 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2053 files are only supported if your vendor has implemented them in the
2054 intuitive fashion that calling the regular C library routines gets the
2055 shadow versions if you're running under privilege or if there exists
2056 the shadow(3) functions as found in System V ( this includes Solaris
2057 and Linux.) Those systems which implement a proprietary shadow password
2058 facility are unlikely to be supported.
2060 The $members value returned by I<getgr*()> is a space separated list of
2061 the login names of the members of the group.
2063 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2064 C, it will be returned to you via C<$?> if the function call fails. The
2065 C<@addrs> value returned by a successful call is a list of the raw
2066 addresses returned by the corresponding system library call. In the
2067 Internet domain, each address is four bytes long and you can unpack it
2068 by saying something like:
2070 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2072 The Socket library makes this slightly easier:
2075 $iaddr = inet_aton("127.1"); # or whatever address
2076 $name = gethostbyaddr($iaddr, AF_INET);
2078 # or going the other way
2079 $straddr = inet_ntoa($iaddr);
2081 If you get tired of remembering which element of the return list
2082 contains which return value, by-name interfaces are provided
2083 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2084 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2085 and C<User::grent>. These override the normal built-ins, supplying
2086 versions that return objects with the appropriate names
2087 for each field. For example:
2091 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2093 Even though it looks like they're the same method calls (uid),
2094 they aren't, because a C<File::stat> object is different from
2095 a C<User::pwent> object.
2097 =item getsockname SOCKET
2099 Returns the packed sockaddr address of this end of the SOCKET connection,
2100 in case you don't know the address because you have several different
2101 IPs that the connection might have come in on.
2104 $mysockaddr = getsockname(SOCK);
2105 ($port, $myaddr) = sockaddr_in($mysockaddr);
2106 printf "Connect to %s [%s]\n",
2107 scalar gethostbyaddr($myaddr, AF_INET),
2110 =item getsockopt SOCKET,LEVEL,OPTNAME
2112 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2113 Options may exist at multiple protocol levels depending on the socket
2114 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2115 C<Socket> module) will exist. To query options at another level the
2116 protocol number of the appropriate protocol controlling the option
2117 should be supplied. For example, to indicate that an option is to be
2118 interpreted by the TCP protocol, LEVEL should be set to the protocol
2119 number of TCP, which you can get using getprotobyname.
2121 The call returns a packed string representing the requested socket option,
2122 or C<undef> if there is an error (the error reason will be in $!). What
2123 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2124 your system documentation for details. A very common case however is that
2125 the option is an integer, in which case the result will be an packed
2126 integer which you can decode using unpack with the C<i> (or C<I>) format.
2128 An example testing if Nagle's algorithm is turned on on a socket:
2130 use Socket qw(:all);
2132 defined(my $tcp = getprotobyname("tcp"))
2133 or die "Could not determine the protocol number for tcp";
2134 # my $tcp = IPPROTO_TCP; # Alternative
2135 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2136 or die "Could not query TCP_NODELAY socket option: $!";
2137 my $nodelay = unpack("I", $packed);
2138 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2145 In list context, returns a (possibly empty) list of filename expansions on
2146 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2147 scalar context, glob iterates through such filename expansions, returning
2148 undef when the list is exhausted. This is the internal function
2149 implementing the C<< <*.c> >> operator, but you can use it directly. If
2150 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2151 more detail in L<perlop/"I/O Operators">.
2153 Beginning with v5.6.0, this operator is implemented using the standard
2154 C<File::Glob> extension. See L<File::Glob> for details.
2160 Converts a time as returned by the time function to an 8-element list
2161 with the time localized for the standard Greenwich time zone.
2162 Typically used as follows:
2165 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2168 All list elements are numeric, and come straight out of the C `struct
2169 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2170 specified time. $mday is the day of the month, and $mon is the month
2171 itself, in the range C<0..11> with 0 indicating January and 11
2172 indicating December. $year is the number of years since 1900. That
2173 is, $year is C<123> in year 2023. $wday is the day of the week, with
2174 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2175 the year, in the range C<0..364> (or C<0..365> in leap years.)
2177 Note that the $year element is I<not> simply the last two digits of
2178 the year. If you assume it is, then you create non-Y2K-compliant
2179 programs--and you wouldn't want to do that, would you?
2181 The proper way to get a complete 4-digit year is simply:
2185 And to get the last two digits of the year (e.g., '01' in 2001) do:
2187 $year = sprintf("%02d", $year % 100);
2189 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2191 In scalar context, C<gmtime()> returns the ctime(3) value:
2193 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2195 If you need local time instead of GMT use the L</localtime> builtin.
2196 See also the C<timegm> function provided by the C<Time::Local> module,
2197 and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
2199 This scalar value is B<not> locale dependent (see L<perllocale>), but is
2200 instead a Perl builtin. To get somewhat similar but locale dependent date
2201 strings, see the example in L</localtime>.
2203 See L<perlport/gmtime> for portability concerns.
2211 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2212 execution there. It may not be used to go into any construct that
2213 requires initialization, such as a subroutine or a C<foreach> loop. It
2214 also can't be used to go into a construct that is optimized away,
2215 or to get out of a block or subroutine given to C<sort>.
2216 It can be used to go almost anywhere else within the dynamic scope,
2217 including out of subroutines, but it's usually better to use some other
2218 construct such as C<last> or C<die>. The author of Perl has never felt the
2219 need to use this form of C<goto> (in Perl, that is--C is another matter).
2220 (The difference being that C does not offer named loops combined with
2221 loop control. Perl does, and this replaces most structured uses of C<goto>
2222 in other languages.)
2224 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2225 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2226 necessarily recommended if you're optimizing for maintainability:
2228 goto ("FOO", "BAR", "GLARCH")[$i];
2230 The C<goto-&NAME> form is quite different from the other forms of
2231 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2232 doesn't have the stigma associated with other gotos. Instead, it
2233 exits the current subroutine (losing any changes set by local()) and
2234 immediately calls in its place the named subroutine using the current
2235 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2236 load another subroutine and then pretend that the other subroutine had
2237 been called in the first place (except that any modifications to C<@_>
2238 in the current subroutine are propagated to the other subroutine.)
2239 After the C<goto>, not even C<caller> will be able to tell that this
2240 routine was called first.
2242 NAME needn't be the name of a subroutine; it can be a scalar variable
2243 containing a code reference, or a block which evaluates to a code
2246 =item grep BLOCK LIST
2248 =item grep EXPR,LIST
2250 This is similar in spirit to, but not the same as, grep(1) and its
2251 relatives. In particular, it is not limited to using regular expressions.
2253 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2254 C<$_> to each element) and returns the list value consisting of those
2255 elements for which the expression evaluated to true. In scalar
2256 context, returns the number of times the expression was true.
2258 @foo = grep(!/^#/, @bar); # weed out comments
2262 @foo = grep {!/^#/} @bar; # weed out comments
2264 Note that C<$_> is an alias to the list value, so it can be used to
2265 modify the elements of the LIST. While this is useful and supported,
2266 it can cause bizarre results if the elements of LIST are not variables.
2267 Similarly, grep returns aliases into the original list, much as a for
2268 loop's index variable aliases the list elements. That is, modifying an
2269 element of a list returned by grep (for example, in a C<foreach>, C<map>
2270 or another C<grep>) actually modifies the element in the original list.
2271 This is usually something to be avoided when writing clear code.
2273 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2274 been declared with C<my $_>) then, in addition the be locally aliased to
2275 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2276 can't be seen from the outside, avoiding any potential side-effects.
2278 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2284 Interprets EXPR as a hex string and returns the corresponding value.
2285 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2286 L</oct>.) If EXPR is omitted, uses C<$_>.
2288 print hex '0xAf'; # prints '175'
2289 print hex 'aF'; # same
2291 Hex strings may only represent integers. Strings that would cause
2292 integer overflow trigger a warning. Leading whitespace is not stripped,
2293 unlike oct(). To present something as hex, look into L</printf>,
2294 L</sprintf>, or L</unpack>.
2298 There is no builtin C<import> function. It is just an ordinary
2299 method (subroutine) defined (or inherited) by modules that wish to export
2300 names to another module. The C<use> function calls the C<import> method
2301 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2303 =item index STR,SUBSTR,POSITION
2305 =item index STR,SUBSTR
2307 The index function searches for one string within another, but without
2308 the wildcard-like behavior of a full regular-expression pattern match.
2309 It returns the position of the first occurrence of SUBSTR in STR at
2310 or after POSITION. If POSITION is omitted, starts searching from the
2311 beginning of the string. The return value is based at C<0> (or whatever
2312 you've set the C<$[> variable to--but don't do that). If the substring
2313 is not found, returns one less than the base, ordinarily C<-1>.
2319 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2320 You should not use this function for rounding: one because it truncates
2321 towards C<0>, and two because machine representations of floating point
2322 numbers can sometimes produce counterintuitive results. For example,
2323 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2324 because it's really more like -268.99999999999994315658 instead. Usually,
2325 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2326 functions will serve you better than will int().
2328 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2330 Implements the ioctl(2) function. You'll probably first have to say
2332 require "sys/ioctl.ph"; # probably in $Config{archlib}/ioctl.ph
2334 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2335 exist or doesn't have the correct definitions you'll have to roll your
2336 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2337 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2338 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2339 written depending on the FUNCTION--a pointer to the string value of SCALAR
2340 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2341 has no string value but does have a numeric value, that value will be
2342 passed rather than a pointer to the string value. To guarantee this to be
2343 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2344 functions may be needed to manipulate the values of structures used by
2347 The return value of C<ioctl> (and C<fcntl>) is as follows:
2349 if OS returns: then Perl returns:
2351 0 string "0 but true"
2352 anything else that number
2354 Thus Perl returns true on success and false on failure, yet you can
2355 still easily determine the actual value returned by the operating
2358 $retval = ioctl(...) || -1;
2359 printf "System returned %d\n", $retval;
2361 The special string C<"0 but true"> is exempt from B<-w> complaints
2362 about improper numeric conversions.
2364 =item join EXPR,LIST
2366 Joins the separate strings of LIST into a single string with fields
2367 separated by the value of EXPR, and returns that new string. Example:
2369 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2371 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2372 first argument. Compare L</split>.
2376 Returns a list consisting of all the keys of the named hash.
2377 (In scalar context, returns the number of keys.)
2379 The keys are returned in an apparently random order. The actual
2380 random order is subject to change in future versions of perl, but it
2381 is guaranteed to be the same order as either the C<values> or C<each>
2382 function produces (given that the hash has not been modified). Since
2383 Perl 5.8.1 the ordering is different even between different runs of
2384 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2387 As a side effect, calling keys() resets the HASH's internal iterator,
2388 see L</each>. (In particular, calling keys() in void context resets
2389 the iterator with no other overhead.)
2391 Here is yet another way to print your environment:
2394 @values = values %ENV;
2396 print pop(@keys), '=', pop(@values), "\n";
2399 or how about sorted by key:
2401 foreach $key (sort(keys %ENV)) {
2402 print $key, '=', $ENV{$key}, "\n";
2405 The returned values are copies of the original keys in the hash, so
2406 modifying them will not affect the original hash. Compare L</values>.
2408 To sort a hash by value, you'll need to use a C<sort> function.
2409 Here's a descending numeric sort of a hash by its values:
2411 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2412 printf "%4d %s\n", $hash{$key}, $key;
2415 As an lvalue C<keys> allows you to increase the number of hash buckets
2416 allocated for the given hash. This can gain you a measure of efficiency if
2417 you know the hash is going to get big. (This is similar to pre-extending
2418 an array by assigning a larger number to $#array.) If you say
2422 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2423 in fact, since it rounds up to the next power of two. These
2424 buckets will be retained even if you do C<%hash = ()>, use C<undef
2425 %hash> if you want to free the storage while C<%hash> is still in scope.
2426 You can't shrink the number of buckets allocated for the hash using
2427 C<keys> in this way (but you needn't worry about doing this by accident,
2428 as trying has no effect).
2430 See also C<each>, C<values> and C<sort>.
2432 =item kill SIGNAL, LIST
2434 Sends a signal to a list of processes. Returns the number of
2435 processes successfully signaled (which is not necessarily the
2436 same as the number actually killed).
2438 $cnt = kill 1, $child1, $child2;
2441 If SIGNAL is zero, no signal is sent to the process. This is a
2442 useful way to check that a child process is alive and hasn't changed
2443 its UID. See L<perlport> for notes on the portability of this
2446 Unlike in the shell, if SIGNAL is negative, it kills
2447 process groups instead of processes. (On System V, a negative I<PROCESS>
2448 number will also kill process groups, but that's not portable.) That
2449 means you usually want to use positive not negative signals. You may also
2450 use a signal name in quotes.
2452 See L<perlipc/"Signals"> for more details.
2458 The C<last> command is like the C<break> statement in C (as used in
2459 loops); it immediately exits the loop in question. If the LABEL is
2460 omitted, the command refers to the innermost enclosing loop. The
2461 C<continue> block, if any, is not executed:
2463 LINE: while (<STDIN>) {
2464 last LINE if /^$/; # exit when done with header
2468 C<last> cannot be used to exit a block which returns a value such as
2469 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2470 a grep() or map() operation.
2472 Note that a block by itself is semantically identical to a loop
2473 that executes once. Thus C<last> can be used to effect an early
2474 exit out of such a block.
2476 See also L</continue> for an illustration of how C<last>, C<next>, and
2483 Returns a lowercased version of EXPR. This is the internal function
2484 implementing the C<\L> escape in double-quoted strings. Respects
2485 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2486 and L<perlunicode> for more details about locale and Unicode support.
2488 If EXPR is omitted, uses C<$_>.
2494 Returns the value of EXPR with the first character lowercased. This
2495 is the internal function implementing the C<\l> escape in
2496 double-quoted strings. Respects current LC_CTYPE locale if C<use
2497 locale> in force. See L<perllocale> and L<perlunicode> for more
2498 details about locale and Unicode support.
2500 If EXPR is omitted, uses C<$_>.
2506 Returns the length in I<characters> of the value of EXPR. If EXPR is
2507 omitted, returns length of C<$_>. Note that this cannot be used on
2508 an entire array or hash to find out how many elements these have.
2509 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2511 Note the I<characters>: if the EXPR is in Unicode, you will get the
2512 number of characters, not the number of bytes. To get the length
2513 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2515 =item link OLDFILE,NEWFILE
2517 Creates a new filename linked to the old filename. Returns true for
2518 success, false otherwise.
2520 =item listen SOCKET,QUEUESIZE
2522 Does the same thing that the listen system call does. Returns true if
2523 it succeeded, false otherwise. See the example in
2524 L<perlipc/"Sockets: Client/Server Communication">.
2528 You really probably want to be using C<my> instead, because C<local> isn't
2529 what most people think of as "local". See
2530 L<perlsub/"Private Variables via my()"> for details.
2532 A local modifies the listed variables to be local to the enclosing
2533 block, file, or eval. If more than one value is listed, the list must
2534 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2535 for details, including issues with tied arrays and hashes.
2537 =item localtime EXPR
2541 Converts a time as returned by the time function to a 9-element list
2542 with the time analyzed for the local time zone. Typically used as
2546 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2549 All list elements are numeric, and come straight out of the C `struct
2550 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2551 of the specified time.
2553 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2554 the range C<0..11> with 0 indicating January and 11 indicating December.
2555 This makes it easy to get a month name from a list:
2557 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2558 print "$abbr[$mon] $mday";
2559 # $mon=9, $mday=18 gives "Oct 18"
2561 C<$year> is the number of years since 1900, not just the last two digits
2562 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2563 to get a complete 4-digit year is simply:
2567 To get the last two digits of the year (e.g., '01' in 2001) do:
2569 $year = sprintf("%02d", $year % 100);
2571 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2572 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2573 (or C<0..365> in leap years.)
2575 C<$isdst> is true if the specified time occurs during Daylight Saving
2576 Time, false otherwise.
2578 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2580 In scalar context, C<localtime()> returns the ctime(3) value:
2582 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2584 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2585 instead of local time use the L</gmtime> builtin. See also the
2586 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2587 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2588 and mktime(3) functions.
2590 To get somewhat similar but locale dependent date strings, set up your
2591 locale environment variables appropriately (please see L<perllocale>) and
2594 use POSIX qw(strftime);
2595 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2596 # or for GMT formatted appropriately for your locale:
2597 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2599 Note that the C<%a> and C<%b>, the short forms of the day of the week
2600 and the month of the year, may not necessarily be three characters wide.
2602 See L<perlport/localtime> for portability concerns.
2606 This function places an advisory lock on a shared variable, or referenced
2607 object contained in I<THING> until the lock goes out of scope.
2609 lock() is a "weak keyword" : this means that if you've defined a function
2610 by this name (before any calls to it), that function will be called
2611 instead. (However, if you've said C<use threads>, lock() is always a
2612 keyword.) See L<threads>.
2618 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2619 returns log of C<$_>. To get the log of another base, use basic algebra:
2620 The base-N log of a number is equal to the natural log of that number
2621 divided by the natural log of N. For example:
2625 return log($n)/log(10);
2628 See also L</exp> for the inverse operation.
2634 Does the same thing as the C<stat> function (including setting the
2635 special C<_> filehandle) but stats a symbolic link instead of the file
2636 the symbolic link points to. If symbolic links are unimplemented on
2637 your system, a normal C<stat> is done. For much more detailed
2638 information, please see the documentation for C<stat>.
2640 If EXPR is omitted, stats C<$_>.
2644 The match operator. See L<perlop>.
2646 =item map BLOCK LIST
2650 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2651 C<$_> to each element) and returns the list value composed of the
2652 results of each such evaluation. In scalar context, returns the
2653 total number of elements so generated. Evaluates BLOCK or EXPR in
2654 list context, so each element of LIST may produce zero, one, or
2655 more elements in the returned value.
2657 @chars = map(chr, @nums);
2659 translates a list of numbers to the corresponding characters. And
2661 %hash = map { getkey($_) => $_ } @array;
2663 is just a funny way to write
2666 foreach $_ (@array) {
2667 $hash{getkey($_)} = $_;
2670 Note that C<$_> is an alias to the list value, so it can be used to
2671 modify the elements of the LIST. While this is useful and supported,
2672 it can cause bizarre results if the elements of LIST are not variables.
2673 Using a regular C<foreach> loop for this purpose would be clearer in
2674 most cases. See also L</grep> for an array composed of those items of
2675 the original list for which the BLOCK or EXPR evaluates to true.
2677 If C<$_> is lexical in the scope where the C<map> appears (because it has
2678 been declared with C<my $_>) then, in addition the be locally aliased to
2679 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2680 can't be seen from the outside, avoiding any potential side-effects.
2682 C<{> starts both hash references and blocks, so C<map { ...> could be either
2683 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2684 ahead for the closing C<}> it has to take a guess at which its dealing with
2685 based what it finds just after the C<{>. Usually it gets it right, but if it
2686 doesn't it won't realize something is wrong until it gets to the C<}> and
2687 encounters the missing (or unexpected) comma. The syntax error will be
2688 reported close to the C<}> but you'll need to change something near the C<{>
2689 such as using a unary C<+> to give perl some help:
2691 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2692 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2693 %hash = map { ("\L$_", 1) } @array # this also works
2694 %hash = map { lc($_), 1 } @array # as does this.
2695 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2697 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2699 or to force an anon hash constructor use C<+{>
2701 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2703 and you get list of anonymous hashes each with only 1 entry.
2705 =item mkdir FILENAME,MASK
2707 =item mkdir FILENAME
2711 Creates the directory specified by FILENAME, with permissions
2712 specified by MASK (as modified by C<umask>). If it succeeds it
2713 returns true, otherwise it returns false and sets C<$!> (errno).
2714 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2717 In general, it is better to create directories with permissive MASK,
2718 and let the user modify that with their C<umask>, than it is to supply
2719 a restrictive MASK and give the user no way to be more permissive.
2720 The exceptions to this rule are when the file or directory should be
2721 kept private (mail files, for instance). The perlfunc(1) entry on
2722 C<umask> discusses the choice of MASK in more detail.
2724 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2725 number of trailing slashes. Some operating and filesystems do not get
2726 this right, so Perl automatically removes all trailing slashes to keep
2729 =item msgctl ID,CMD,ARG
2731 Calls the System V IPC function msgctl(2). You'll probably have to say
2735 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2736 then ARG must be a variable which will hold the returned C<msqid_ds>
2737 structure. Returns like C<ioctl>: the undefined value for error,
2738 C<"0 but true"> for zero, or the actual return value otherwise. See also
2739 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2741 =item msgget KEY,FLAGS
2743 Calls the System V IPC function msgget(2). Returns the message queue
2744 id, or the undefined value if there is an error. See also
2745 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2747 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2749 Calls the System V IPC function msgrcv to receive a message from
2750 message queue ID into variable VAR with a maximum message size of
2751 SIZE. Note that when a message is received, the message type as a
2752 native long integer will be the first thing in VAR, followed by the
2753 actual message. This packing may be opened with C<unpack("l! a*")>.
2754 Taints the variable. Returns true if successful, or false if there is
2755 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2756 C<IPC::SysV::Msg> documentation.
2758 =item msgsnd ID,MSG,FLAGS
2760 Calls the System V IPC function msgsnd to send the message MSG to the
2761 message queue ID. MSG must begin with the native long integer message
2762 type, and be followed by the length of the actual message, and finally
2763 the message itself. This kind of packing can be achieved with
2764 C<pack("l! a*", $type, $message)>. Returns true if successful,
2765 or false if there is an error. See also C<IPC::SysV>
2766 and C<IPC::SysV::Msg> documentation.
2772 =item my EXPR : ATTRS
2774 =item my TYPE EXPR : ATTRS
2776 A C<my> declares the listed variables to be local (lexically) to the
2777 enclosing block, file, or C<eval>. If more than one value is listed,
2778 the list must be placed in parentheses.
2780 The exact semantics and interface of TYPE and ATTRS are still
2781 evolving. TYPE is currently bound to the use of C<fields> pragma,
2782 and attributes are handled using the C<attributes> pragma, or starting
2783 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2784 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2785 L<attributes>, and L<Attribute::Handlers>.
2791 The C<next> command is like the C<continue> statement in C; it starts
2792 the next iteration of the loop:
2794 LINE: while (<STDIN>) {
2795 next LINE if /^#/; # discard comments
2799 Note that if there were a C<continue> block on the above, it would get
2800 executed even on discarded lines. If the LABEL is omitted, the command
2801 refers to the innermost enclosing loop.
2803 C<next> cannot be used to exit a block which returns a value such as
2804 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2805 a grep() or map() operation.
2807 Note that a block by itself is semantically identical to a loop
2808 that executes once. Thus C<next> will exit such a block early.
2810 See also L</continue> for an illustration of how C<last>, C<next>, and
2813 =item no Module VERSION LIST
2815 =item no Module VERSION
2817 =item no Module LIST
2821 See the C<use> function, of which C<no> is the opposite.
2827 Interprets EXPR as an octal string and returns the corresponding
2828 value. (If EXPR happens to start off with C<0x>, interprets it as a
2829 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2830 binary string. Leading whitespace is ignored in all three cases.)
2831 The following will handle decimal, binary, octal, and hex in the standard
2834 $val = oct($val) if $val =~ /^0/;
2836 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2837 in octal), use sprintf() or printf():
2839 $perms = (stat("filename"))[2] & 07777;
2840 $oct_perms = sprintf "%lo", $perms;
2842 The oct() function is commonly used when a string such as C<644> needs
2843 to be converted into a file mode, for example. (Although perl will
2844 automatically convert strings into numbers as needed, this automatic
2845 conversion assumes base 10.)
2847 =item open FILEHANDLE,EXPR
2849 =item open FILEHANDLE,MODE,EXPR
2851 =item open FILEHANDLE,MODE,EXPR,LIST
2853 =item open FILEHANDLE,MODE,REFERENCE
2855 =item open FILEHANDLE
2857 Opens the file whose filename is given by EXPR, and associates it with
2860 (The following is a comprehensive reference to open(): for a gentler
2861 introduction you may consider L<perlopentut>.)
2863 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2864 the variable is assigned a reference to a new anonymous filehandle,
2865 otherwise if FILEHANDLE is an expression, its value is used as the name of
2866 the real filehandle wanted. (This is considered a symbolic reference, so
2867 C<use strict 'refs'> should I<not> be in effect.)
2869 If EXPR is omitted, the scalar variable of the same name as the
2870 FILEHANDLE contains the filename. (Note that lexical variables--those
2871 declared with C<my>--will not work for this purpose; so if you're
2872 using C<my>, specify EXPR in your call to open.)
2874 If three or more arguments are specified then the mode of opening and
2875 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2876 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2877 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2878 the file is opened for appending, again being created if necessary.
2880 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2881 indicate that you want both read and write access to the file; thus
2882 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2883 '+>' >> mode would clobber the file first. You can't usually use
2884 either read-write mode for updating textfiles, since they have
2885 variable length records. See the B<-i> switch in L<perlrun> for a
2886 better approach. The file is created with permissions of C<0666>
2887 modified by the process' C<umask> value.
2889 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2890 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2892 In the 2-arguments (and 1-argument) form of the call the mode and
2893 filename should be concatenated (in this order), possibly separated by
2894 spaces. It is possible to omit the mode in these forms if the mode is
2897 If the filename begins with C<'|'>, the filename is interpreted as a
2898 command to which output is to be piped, and if the filename ends with a
2899 C<'|'>, the filename is interpreted as a command which pipes output to
2900 us. See L<perlipc/"Using open() for IPC">
2901 for more examples of this. (You are not allowed to C<open> to a command
2902 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2903 and L<perlipc/"Bidirectional Communication with Another Process">
2906 For three or more arguments if MODE is C<'|-'>, the filename is
2907 interpreted as a command to which output is to be piped, and if MODE
2908 is C<'-|'>, the filename is interpreted as a command which pipes
2909 output to us. In the 2-arguments (and 1-argument) form one should
2910 replace dash (C<'-'>) with the command.
2911 See L<perlipc/"Using open() for IPC"> for more examples of this.
2912 (You are not allowed to C<open> to a command that pipes both in I<and>
2913 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2914 L<perlipc/"Bidirectional Communication"> for alternatives.)
2916 In the three-or-more argument form of pipe opens, if LIST is specified
2917 (extra arguments after the command name) then LIST becomes arguments
2918 to the command invoked if the platform supports it. The meaning of
2919 C<open> with more than three arguments for non-pipe modes is not yet
2920 specified. Experimental "layers" may give extra LIST arguments
2923 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2924 and opening C<< '>-' >> opens STDOUT.
2926 You may use the three-argument form of open to specify IO "layers"
2927 (sometimes also referred to as "disciplines") to be applied to the handle
2928 that affect how the input and output are processed (see L<open> and
2929 L<PerlIO> for more details). For example
2931 open(FH, "<:utf8", "file")
2933 will open the UTF-8 encoded file containing Unicode characters,
2934 see L<perluniintro>. (Note that if layers are specified in the
2935 three-arg form then default layers set by the C<open> pragma are
2938 Open returns nonzero upon success, the undefined value otherwise. If
2939 the C<open> involved a pipe, the return value happens to be the pid of
2942 If you're running Perl on a system that distinguishes between text
2943 files and binary files, then you should check out L</binmode> for tips
2944 for dealing with this. The key distinction between systems that need
2945 C<binmode> and those that don't is their text file formats. Systems
2946 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2947 character, and which encode that character in C as C<"\n">, do not
2948 need C<binmode>. The rest need it.
2950 When opening a file, it's usually a bad idea to continue normal execution
2951 if the request failed, so C<open> is frequently used in connection with
2952 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2953 where you want to make a nicely formatted error message (but there are
2954 modules that can help with that problem)) you should always check
2955 the return value from opening a file. The infrequent exception is when
2956 working with an unopened filehandle is actually what you want to do.
2958 As a special case the 3 arg form with a read/write mode and the third
2959 argument being C<undef>:
2961 open(TMP, "+>", undef) or die ...
2963 opens a filehandle to an anonymous temporary file. Also using "+<"
2964 works for symmetry, but you really should consider writing something
2965 to the temporary file first. You will need to seek() to do the
2968 Since v5.8.0, perl has built using PerlIO by default. Unless you've
2969 changed this (ie Configure -Uuseperlio), you can open file handles to
2970 "in memory" files held in Perl scalars via:
2972 open($fh, '>', \$variable) || ..
2974 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2975 file, you have to close it first:
2978 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2983 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2984 while (<ARTICLE>) {...
2986 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2987 # if the open fails, output is discarded
2989 open(DBASE, '+<', 'dbase.mine') # open for update
2990 or die "Can't open 'dbase.mine' for update: $!";
2992 open(DBASE, '+<dbase.mine') # ditto
2993 or die "Can't open 'dbase.mine' for update: $!";
2995 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2996 or die "Can't start caesar: $!";
2998 open(ARTICLE, "caesar <$article |") # ditto
2999 or die "Can't start caesar: $!";
3001 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3002 or die "Can't start sort: $!";
3005 open(MEMORY,'>', \$var)
3006 or die "Can't open memory file: $!";
3007 print MEMORY "foo!\n"; # output will end up in $var
3009 # process argument list of files along with any includes
3011 foreach $file (@ARGV) {
3012 process($file, 'fh00');
3016 my($filename, $input) = @_;
3017 $input++; # this is a string increment
3018 unless (open($input, $filename)) {
3019 print STDERR "Can't open $filename: $!\n";
3024 while (<$input>) { # note use of indirection
3025 if (/^#include "(.*)"/) {
3026 process($1, $input);
3033 See L<perliol> for detailed info on PerlIO.
3035 You may also, in the Bourne shell tradition, specify an EXPR beginning
3036 with C<< '>&' >>, in which case the rest of the string is interpreted
3037 as the name of a filehandle (or file descriptor, if numeric) to be
3038 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3039 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3040 The mode you specify should match the mode of the original filehandle.
3041 (Duping a filehandle does not take into account any existing contents
3042 of IO buffers.) If you use the 3 arg form then you can pass either a
3043 number, the name of a filehandle or the normal "reference to a glob".
3045 Here is a script that saves, redirects, and restores C<STDOUT> and
3046 C<STDERR> using various methods:
3049 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3050 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3052 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3053 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3055 select STDERR; $| = 1; # make unbuffered
3056 select STDOUT; $| = 1; # make unbuffered
3058 print STDOUT "stdout 1\n"; # this works for
3059 print STDERR "stderr 1\n"; # subprocesses too
3061 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3062 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3064 print STDOUT "stdout 2\n";
3065 print STDERR "stderr 2\n";
3067 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3068 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3069 that file descriptor (and not call L<dup(2)>); this is more
3070 parsimonious of file descriptors. For example:
3072 # open for input, reusing the fileno of $fd
3073 open(FILEHANDLE, "<&=$fd")
3077 open(FILEHANDLE, "<&=", $fd)
3081 # open for append, using the fileno of OLDFH
3082 open(FH, ">>&=", OLDFH)
3086 open(FH, ">>&=OLDFH")
3088 Being parsimonious on filehandles is also useful (besides being
3089 parsimonious) for example when something is dependent on file
3090 descriptors, like for example locking using flock(). If you do just
3091 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3092 descriptor as B, and therefore flock(A) will not flock(B), and vice
3093 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3094 the same file descriptor.
3096 Note that if you are using Perls older than 5.8.0, Perl will be using
3097 the standard C libraries' fdopen() to implement the "=" functionality.
3098 On many UNIX systems fdopen() fails when file descriptors exceed a
3099 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3100 most often the default.
3102 You can see whether Perl has been compiled with PerlIO or not by
3103 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3104 is C<define>, you have PerlIO, otherwise you don't.
3106 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3107 with 2-arguments (or 1-argument) form of open(), then
3108 there is an implicit fork done, and the return value of open is the pid
3109 of the child within the parent process, and C<0> within the child
3110 process. (Use C<defined($pid)> to determine whether the open was successful.)
3111 The filehandle behaves normally for the parent, but i/o to that
3112 filehandle is piped from/to the STDOUT/STDIN of the child process.
3113 In the child process the filehandle isn't opened--i/o happens from/to
3114 the new STDOUT or STDIN. Typically this is used like the normal
3115 piped open when you want to exercise more control over just how the
3116 pipe command gets executed, such as when you are running setuid, and
3117 don't want to have to scan shell commands for metacharacters.
3118 The following triples are more or less equivalent:
3120 open(FOO, "|tr '[a-z]' '[A-Z]'");
3121 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3122 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3123 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3125 open(FOO, "cat -n '$file'|");
3126 open(FOO, '-|', "cat -n '$file'");
3127 open(FOO, '-|') || exec 'cat', '-n', $file;
3128 open(FOO, '-|', "cat", '-n', $file);
3130 The last example in each block shows the pipe as "list form", which is
3131 not yet supported on all platforms. A good rule of thumb is that if
3132 your platform has true C<fork()> (in other words, if your platform is
3133 UNIX) you can use the list form.
3135 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3137 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3138 output before any operation that may do a fork, but this may not be
3139 supported on some platforms (see L<perlport>). To be safe, you may need
3140 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3141 of C<IO::Handle> on any open handles.
3143 On systems that support a close-on-exec flag on files, the flag will
3144 be set for the newly opened file descriptor as determined by the value
3145 of $^F. See L<perlvar/$^F>.
3147 Closing any piped filehandle causes the parent process to wait for the
3148 child to finish, and returns the status value in C<$?> and
3149 C<${^CHILD_ERROR_NATIVE}>.
3151 The filename passed to 2-argument (or 1-argument) form of open() will
3152 have leading and trailing whitespace deleted, and the normal
3153 redirection characters honored. This property, known as "magic open",
3154 can often be used to good effect. A user could specify a filename of
3155 F<"rsh cat file |">, or you could change certain filenames as needed:
3157 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3158 open(FH, $filename) or die "Can't open $filename: $!";
3160 Use 3-argument form to open a file with arbitrary weird characters in it,
3162 open(FOO, '<', $file);
3164 otherwise it's necessary to protect any leading and trailing whitespace:
3166 $file =~ s#^(\s)#./$1#;
3167 open(FOO, "< $file\0");
3169 (this may not work on some bizarre filesystems). One should
3170 conscientiously choose between the I<magic> and 3-arguments form
3175 will allow the user to specify an argument of the form C<"rsh cat file |">,
3176 but will not work on a filename which happens to have a trailing space, while
3178 open IN, '<', $ARGV[0];
3180 will have exactly the opposite restrictions.
3182 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3183 should use the C<sysopen> function, which involves no such magic (but
3184 may use subtly different filemodes than Perl open(), which is mapped
3185 to C fopen()). This is
3186 another way to protect your filenames from interpretation. For example:
3189 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3190 or die "sysopen $path: $!";
3191 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3192 print HANDLE "stuff $$\n";
3194 print "File contains: ", <HANDLE>;
3196 Using the constructor from the C<IO::Handle> package (or one of its
3197 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3198 filehandles that have the scope of whatever variables hold references to
3199 them, and automatically close whenever and however you leave that scope:
3203 sub read_myfile_munged {
3205 my $handle = new IO::File;
3206 open($handle, "myfile") or die "myfile: $!";
3208 or return (); # Automatically closed here.
3209 mung $first or die "mung failed"; # Or here.
3210 return $first, <$handle> if $ALL; # Or here.
3214 See L</seek> for some details about mixing reading and writing.
3216 =item opendir DIRHANDLE,EXPR
3218 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3219 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3220 DIRHANDLE may be an expression whose value can be used as an indirect
3221 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3222 scalar variable (or array or hash element), the variable is assigned a
3223 reference to a new anonymous dirhandle.
3224 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3230 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3231 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3234 For the reverse, see L</chr>.
3235 See L<perlunicode> and L<encoding> for more about Unicode.
3241 =item our EXPR : ATTRS
3243 =item our TYPE EXPR : ATTRS
3245 An C<our> declares the listed variables to be valid globals within
3246 the enclosing block, file, or C<eval>. That is, it has the same
3247 scoping rules as a "my" declaration, but does not create a local
3248 variable. If more than one value is listed, the list must be placed
3249 in parentheses. The C<our> declaration has no semantic effect unless
3250 "use strict vars" is in effect, in which case it lets you use the
3251 declared global variable without qualifying it with a package name.
3252 (But only within the lexical scope of the C<our> declaration. In this
3253 it differs from "use vars", which is package scoped.)
3255 An C<our> declaration declares a global variable that will be visible
3256 across its entire lexical scope, even across package boundaries. The
3257 package in which the variable is entered is determined at the point
3258 of the declaration, not at the point of use. This means the following
3262 our $bar; # declares $Foo::bar for rest of lexical scope
3266 print $bar; # prints 20
3268 Multiple C<our> declarations in the same lexical scope are allowed
3269 if they are in different packages. If they happened to be in the same
3270 package, Perl will emit warnings if you have asked for them.
3274 our $bar; # declares $Foo::bar for rest of lexical scope
3278 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3279 print $bar; # prints 30
3281 our $bar; # emits warning
3283 An C<our> declaration may also have a list of attributes associated
3286 The exact semantics and interface of TYPE and ATTRS are still
3287 evolving. TYPE is currently bound to the use of C<fields> pragma,
3288 and attributes are handled using the C<attributes> pragma, or starting
3289 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3290 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3291 L<attributes>, and L<Attribute::Handlers>.
3293 The only currently recognized C<our()> attribute is C<unique> which
3294 indicates that a single copy of the global is to be used by all
3295 interpreters should the program happen to be running in a
3296 multi-interpreter environment. (The default behaviour would be for
3297 each interpreter to have its own copy of the global.) Examples:
3299 our @EXPORT : unique = qw(foo);
3300 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3301 our $VERSION : unique = "1.00";
3303 Note that this attribute also has the effect of making the global
3304 readonly when the first new interpreter is cloned (for example,
3305 when the first new thread is created).
3307 Multi-interpreter environments can come to being either through the
3308 fork() emulation on Windows platforms, or by embedding perl in a
3309 multi-threaded application. The C<unique> attribute does nothing in
3310 all other environments.
3312 Warning: the current implementation of this attribute operates on the
3313 typeglob associated with the variable; this means that C<our $x : unique>
3314 also has the effect of C<our @x : unique; our %x : unique>. This may be
3317 =item pack TEMPLATE,LIST
3319 Takes a LIST of values and converts it into a string using the rules
3320 given by the TEMPLATE. The resulting string is the concatenation of
3321 the converted values. Typically, each converted value looks
3322 like its machine-level representation. For example, on 32-bit machines
3323 an integer may be represented by a sequence of 4 bytes which will be
3324 converted to a sequence of 4 characters.
3326 The TEMPLATE is a sequence of characters that give the order and type
3327 of values, as follows:
3329 a A string with arbitrary binary data, will be null padded.
3330 A A text (ASCII) string, will be space padded.
3331 Z A null terminated (ASCIZ) string, will be null padded.
3333 b A bit string (ascending bit order inside each byte, like vec()).
3334 B A bit string (descending bit order inside each byte).
3335 h A hex string (low nybble first).
3336 H A hex string (high nybble first).
3338 c A signed char (8-bit) value.
3339 C An unsigned C char (octet) even under Unicode. Should normally not
3340 be used. See U and W instead.
3341 W An unsigned char value (can be greater than 255).
3343 s A signed short (16-bit) value.
3344 S An unsigned short value.
3346 l A signed long (32-bit) value.
3347 L An unsigned long value.
3349 q A signed quad (64-bit) value.
3350 Q An unsigned quad value.
3351 (Quads are available only if your system supports 64-bit
3352 integer values _and_ if Perl has been compiled to support those.
3353 Causes a fatal error otherwise.)
3355 i A signed integer value.
3356 I A unsigned integer value.
3357 (This 'integer' is _at_least_ 32 bits wide. Its exact
3358 size depends on what a local C compiler calls 'int'.)
3360 n An unsigned short (16-bit) in "network" (big-endian) order.
3361 N An unsigned long (32-bit) in "network" (big-endian) order.
3362 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3363 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3365 j A Perl internal signed integer value (IV).
3366 J A Perl internal unsigned integer value (UV).
3368 f A single-precision float in the native format.
3369 d A double-precision float in the native format.
3371 F A Perl internal floating point value (NV) in the native format
3372 D A long double-precision float in the native format.
3373 (Long doubles are available only if your system supports long
3374 double values _and_ if Perl has been compiled to support those.
3375 Causes a fatal error otherwise.)
3377 p A pointer to a null-terminated string.
3378 P A pointer to a structure (fixed-length string).
3380 u A uuencoded string.
3381 U A Unicode character number. Encodes to UTF-8 internally
3382 (or UTF-EBCDIC in EBCDIC platforms).
3384 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3385 details). Its bytes represent an unsigned integer in base 128,
3386 most significant digit first, with as few digits as possible. Bit
3387 eight (the high bit) is set on each byte except the last.
3391 @ Null fill or truncate to absolute position, counted from the
3392 start of the innermost ()-group.
3393 . Null fill or truncate to absolute position specified by value.
3394 ( Start of a ()-group.
3396 Some letters in the TEMPLATE may optionally be followed by one or
3397 more of these modifiers (the second column lists the letters for
3398 which the modifier is valid):
3400 ! sSlLiI Forces native (short, long, int) sizes instead
3401 of fixed (16-/32-bit) sizes.
3403 xX Make x and X act as alignment commands.
3405 nNvV Treat integers as signed instead of unsigned.
3407 @. Specify position as byte offset in the internal
3408 representation of the packed string. Efficient but
3411 > sSiIlLqQ Force big-endian byte-order on the type.
3412 jJfFdDpP (The "big end" touches the construct.)
3414 < sSiIlLqQ Force little-endian byte-order on the type.
3415 jJfFdDpP (The "little end" touches the construct.)
3417 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3418 in which case they force a certain byte-order on all components of
3419 that group, including subgroups.
3421 The following rules apply:
3427 Each letter may optionally be followed by a number giving a repeat
3428 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3429 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3430 that many values from the LIST. A C<*> for the repeat count means to
3431 use however many items are left, except for C<@>, C<x>, C<X>, where it
3432 is equivalent to C<0>, for <.> where it means relative to string start
3433 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3434 A numeric repeat count may optionally be enclosed in brackets, as in
3435 C<pack 'C[80]', @arr>.
3437 One can replace the numeric repeat count by a template enclosed in brackets;
3438 then the packed length of this template in bytes is used as a count.
3439 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3440 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3441 If the template in brackets contains alignment commands (such as C<x![d]>),
3442 its packed length is calculated as if the start of the template has the maximal
3445 When used with C<Z>, C<*> results in the addition of a trailing null
3446 byte (so the packed result will be one longer than the byte C<length>
3449 When used with C<@>, the repeat count represents an offset from the start
3450 of the innermost () group.
3452 When used with C<.>, the repeat count is used to determine the starting
3453 position from where the value offset is calculated. If the repeat count
3454 is 0, it's relative to the current position. If the repeat count is C<*>,
3455 the offset is relative to the start of the packed string. And if its an
3456 integer C<n> the offset is relative to the start of the n-th innermost
3457 () group (or the start of the string if C<n> is bigger then the group
3460 The repeat count for C<u> is interpreted as the maximal number of bytes
3461 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3462 count should not be more than 65.
3466 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3467 string of length count, padding with nulls or spaces as necessary. When
3468 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3469 after the first null, and C<a> returns data verbatim.
3471 If the value-to-pack is too long, it is truncated. If too long and an
3472 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3473 by a null byte. Thus C<Z> always packs a trailing null (except when the
3478 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3479 Each character of the input field of pack() generates 1 bit of the result.
3480 Each result bit is based on the least-significant bit of the corresponding
3481 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3482 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3484 Starting from the beginning of the input string of pack(), each 8-tuple
3485 of characters is converted to 1 character of output. With format C<b>
3486 the first character of the 8-tuple determines the least-significant bit of a
3487 character, and with format C<B> it determines the most-significant bit of
3490 If the length of the input string is not exactly divisible by 8, the
3491 remainder is packed as if the input string were padded by null characters
3492 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3494 If the input string of pack() is longer than needed, extra characters are
3495 ignored. A C<*> for the repeat count of pack() means to use all the
3496 characters of the input field. On unpack()ing the bits are converted to a
3497 string of C<"0">s and C<"1">s.
3501 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3502 representable as hexadecimal digits, 0-9a-f) long.
3504 Each character of the input field of pack() generates 4 bits of the result.
3505 For non-alphabetical characters the result is based on the 4 least-significant
3506 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3507 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3508 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3509 is compatible with the usual hexadecimal digits, so that C<"a"> and
3510 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3511 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3513 Starting from the beginning of the input string of pack(), each pair
3514 of characters is converted to 1 character of output. With format C<h> the
3515 first character of the pair determines the least-significant nybble of the
3516 output character, and with format C<H> it determines the most-significant
3519 If the length of the input string is not even, it behaves as if padded
3520 by a null character at the end. Similarly, during unpack()ing the "extra"
3521 nybbles are ignored.
3523 If the input string of pack() is longer than needed, extra characters are
3525 A C<*> for the repeat count of pack() means to use all the characters of
3526 the input field. On unpack()ing the nybbles are converted to a string
3527 of hexadecimal digits.
3531 The C<p> type packs a pointer to a null-terminated string. You are
3532 responsible for ensuring the string is not a temporary value (which can
3533 potentially get deallocated before you get around to using the packed result).
3534 The C<P> type packs a pointer to a structure of the size indicated by the
3535 length. A NULL pointer is created if the corresponding value for C<p> or
3536 C<P> is C<undef>, similarly for unpack().
3538 If your system has a strange pointer size (i.e. a pointer is neither as
3539 big as an int nor as big as a long), it may not be possible to pack or
3540 unpack pointers in big- or little-endian byte order. Attempting to do
3541 so will result in a fatal error.
3545 The C</> template character allows packing and unpacking of a sequence of
3546 items where the packed structure contains a packed item count followed by
3547 the packed items themselves.
3548 You write I<length-item>C</>I<sequence-item>.
3550 The I<length-item> can be any C<pack> template letter, and describes
3551 how the length value is packed. The ones likely to be of most use are
3552 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3553 SNMP) and C<N> (for Sun XDR).
3555 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3556 the minimum of that and the number of available items is used as argument
3557 for the I<length-item>. If it has no repeat count or uses a '*', the number
3558 of available items is used. For C<unpack> the repeat count is always obtained
3559 by decoding the packed item count, and the I<sequence-item> must not have a
3562 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3563 the I<length-item> is a string length, not a number of strings. If there is
3564 an explicit repeat count for pack, the packed string will be adjusted to that
3567 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3568 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond', 'J')
3569 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3570 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3572 The I<length-item> is not returned explicitly from C<unpack>.
3574 Adding a count to the I<length-item> letter is unlikely to do anything
3575 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3576 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3577 which Perl does not regard as legal in numeric strings.
3581 The integer types C<s>, C<S>, C<l>, and C<L> may be
3582 followed by a C<!> modifier to signify native shorts or
3583 longs--as you can see from above for example a bare C<l> does mean
3584 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3585 may be larger. This is an issue mainly in 64-bit platforms. You can
3586 see whether using C<!> makes any difference by
3588 print length(pack("s")), " ", length(pack("s!")), "\n";
3589 print length(pack("l")), " ", length(pack("l!")), "\n";
3591 C<i!> and C<I!> also work but only because of completeness;
3592 they are identical to C<i> and C<I>.
3594 The actual sizes (in bytes) of native shorts, ints, longs, and long
3595 longs on the platform where Perl was built are also available via
3599 print $Config{shortsize}, "\n";
3600 print $Config{intsize}, "\n";
3601 print $Config{longsize}, "\n";
3602 print $Config{longlongsize}, "\n";
3604 (The C<$Config{longlongsize}> will be undefined if your system does
3605 not support long longs.)
3609 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3610 are inherently non-portable between processors and operating systems
3611 because they obey the native byteorder and endianness. For example a
3612 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3613 (arranged in and handled by the CPU registers) into bytes as
3615 0x12 0x34 0x56 0x78 # big-endian
3616 0x78 0x56 0x34 0x12 # little-endian
3618 Basically, the Intel and VAX CPUs are little-endian, while everybody
3619 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3620 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3621 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3624 The names `big-endian' and `little-endian' are comic references to
3625 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3626 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3627 the egg-eating habits of the Lilliputians.
3629 Some systems may have even weirder byte orders such as
3634 You can see your system's preference with
3636 print join(" ", map { sprintf "%#02x", $_ }
3637 unpack("W*",pack("L",0x12345678))), "\n";
3639 The byteorder on the platform where Perl was built is also available
3643 print $Config{byteorder}, "\n";
3645 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3646 and C<'87654321'> are big-endian.
3648 If you want portable packed integers you can either use the formats
3649 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3650 modifiers. These modifiers are only available as of perl 5.9.2.
3651 See also L<perlport>.
3655 All integer and floating point formats as well as C<p> and C<P> and
3656 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3657 to force big- or little- endian byte-order, respectively.
3658 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3659 signed integers, 64-bit integers and floating point values. However,
3660 there are some things to keep in mind.
3662 Exchanging signed integers between different platforms only works
3663 if all platforms store them in the same format. Most platforms store
3664 signed integers in two's complement, so usually this is not an issue.
3666 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3667 formats on big- or little-endian machines. Otherwise, attempting to
3668 do so will result in a fatal error.
3670 Forcing big- or little-endian byte-order on floating point values for
3671 data exchange can only work if all platforms are using the same
3672 binary representation (e.g. IEEE floating point format). Even if all
3673 platforms are using IEEE, there may be subtle differences. Being able
3674 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3675 but also very dangerous if you don't know exactly what you're doing.
3676 It is definetely not a general way to portably store floating point
3679 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3680 all types inside the group that accept the byte-order modifiers,
3681 including all subgroups. It will silently be ignored for all other
3682 types. You are not allowed to override the byte-order within a group
3683 that already has a byte-order modifier suffix.
3687 Real numbers (floats and doubles) are in the native machine format only;
3688 due to the multiplicity of floating formats around, and the lack of a
3689 standard "network" representation, no facility for interchange has been
3690 made. This means that packed floating point data written on one machine
3691 may not be readable on another - even if both use IEEE floating point
3692 arithmetic (as the endian-ness of the memory representation is not part
3693 of the IEEE spec). See also L<perlport>.
3695 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3696 modifiers to force big- or little-endian byte-order on floating point values.
3698 Note that Perl uses doubles (or long doubles, if configured) internally for
3699 all numeric calculation, and converting from double into float and thence back
3700 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3701 will not in general equal $foo).
3705 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3706 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3707 where the packed string is processed in its UTF-8-encoded Unicode form on
3708 a byte by byte basis. Character mode is the default unless the format string
3709 starts with an C<U>. You can switch mode at any moment with an explicit
3710 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3711 or until the end of the ()-group in which it was entered.
3715 You must yourself do any alignment or padding by inserting for example
3716 enough C<'x'>es while packing. There is no way to pack() and unpack()
3717 could know where the characters are going to or coming from. Therefore
3718 C<pack> (and C<unpack>) handle their output and input as flat
3719 sequences of characters.
3723 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3724 take a repeat count, both as postfix, and for unpack() also via the C</>
3725 template character. Within each repetition of a group, positioning with
3726 C<@> starts again at 0. Therefore, the result of
3728 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3730 is the string "\0a\0\0bc".
3734 C<x> and C<X> accept C<!> modifier. In this case they act as
3735 alignment commands: they jump forward/back to the closest position
3736 aligned at a multiple of C<count> characters. For example, to pack() or
3737 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3738 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3739 aligned on the double's size.
3741 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3742 both result in no-ops.
3746 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3747 will represent signed 16-/32-bit integers in big-/little-endian order.
3748 This is only portable if all platforms sharing the packed data use the
3749 same binary representation for signed integers (e.g. all platforms are
3750 using two's complement representation).
3754 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3755 White space may be used to separate pack codes from each other, but
3756 modifiers and a repeat count must follow immediately.
3760 If TEMPLATE requires more arguments to pack() than actually given, pack()
3761 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3762 to pack() than actually given, extra arguments are ignored.
3768 $foo = pack("WWWW",65,66,67,68);
3770 $foo = pack("W4",65,66,67,68);
3772 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3773 # same thing with Unicode circled letters.
3774 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3775 # same thing with Unicode circled letters. You don't get the UTF-8
3776 # bytes because the U at the start of the format caused a switch to
3777 # U0-mode, so the UTF-8 bytes get joined into characters
3778 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3779 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3780 # This is the UTF-8 encoding of the string in the previous example
3782 $foo = pack("ccxxcc",65,66,67,68);
3785 # note: the above examples featuring "W" and "c" are true
3786 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3787 # and UTF-8. In EBCDIC the first example would be
3788 # $foo = pack("WWWW",193,194,195,196);
3790 $foo = pack("s2",1,2);
3791 # "\1\0\2\0" on little-endian
3792 # "\0\1\0\2" on big-endian
3794 $foo = pack("a4","abcd","x","y","z");
3797 $foo = pack("aaaa","abcd","x","y","z");
3800 $foo = pack("a14","abcdefg");
3801 # "abcdefg\0\0\0\0\0\0\0"
3803 $foo = pack("i9pl", gmtime);
3804 # a real struct tm (on my system anyway)
3806 $utmp_template = "Z8 Z8 Z16 L";
3807 $utmp = pack($utmp_template, @utmp1);
3808 # a struct utmp (BSDish)
3810 @utmp2 = unpack($utmp_template, $utmp);
3811 # "@utmp1" eq "@utmp2"
3814 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3817 $foo = pack('sx2l', 12, 34);
3818 # short 12, two zero bytes padding, long 34
3819 $bar = pack('s@4l', 12, 34);
3820 # short 12, zero fill to position 4, long 34
3822 $baz = pack('s.l', 12, 4, 34);
3823 # short 12, zero fill to position 4, long 34
3825 $foo = pack('nN', 42, 4711);
3826 # pack big-endian 16- and 32-bit unsigned integers
3827 $foo = pack('S>L>', 42, 4711);
3829 $foo = pack('s<l<', -42, 4711);
3830 # pack little-endian 16- and 32-bit signed integers
3831 $foo = pack('(sl)<', -42, 4711);
3834 The same template may generally also be used in unpack().
3836 =item package NAMESPACE
3840 Declares the compilation unit as being in the given namespace. The scope
3841 of the package declaration is from the declaration itself through the end
3842 of the enclosing block, file, or eval (the same as the C<my> operator).
3843 All further unqualified dynamic identifiers will be in this namespace.
3844 A package statement affects only dynamic variables--including those
3845 you've used C<local> on--but I<not> lexical variables, which are created
3846 with C<my>. Typically it would be the first declaration in a file to
3847 be included by the C<require> or C<use> operator. You can switch into a
3848 package in more than one place; it merely influences which symbol table
3849 is used by the compiler for the rest of that block. You can refer to
3850 variables and filehandles in other packages by prefixing the identifier
3851 with the package name and a double colon: C<$Package::Variable>.
3852 If the package name is null, the C<main> package as assumed. That is,
3853 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3854 still seen in older code).
3856 If NAMESPACE is omitted, then there is no current package, and all
3857 identifiers must be fully qualified or lexicals. However, you are
3858 strongly advised not to make use of this feature. Its use can cause
3859 unexpected behaviour, even crashing some versions of Perl. It is
3860 deprecated, and will be removed from a future release.
3862 See L<perlmod/"Packages"> for more information about packages, modules,
3863 and classes. See L<perlsub> for other scoping issues.
3865 =item pipe READHANDLE,WRITEHANDLE
3867 Opens a pair of connected pipes like the corresponding system call.
3868 Note that if you set up a loop of piped processes, deadlock can occur
3869 unless you are very careful. In addition, note that Perl's pipes use
3870 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3871 after each command, depending on the application.
3873 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3874 for examples of such things.
3876 On systems that support a close-on-exec flag on files, the flag will be set
3877 for the newly opened file descriptors as determined by the value of $^F.
3884 Pops and returns the last value of the array, shortening the array by
3885 one element. Has an effect similar to
3889 If there are no elements in the array, returns the undefined value
3890 (although this may happen at other times as well). If ARRAY is
3891 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3892 array in subroutines, just like C<shift>.
3898 Returns the offset of where the last C<m//g> search left off for the variable
3899 in question (C<$_> is used when the variable is not specified). Note that
3900 0 is a valid match offset, while C<undef> indicates that the search position
3901 is reset (usually due to match failure, but can also be because no match has
3902 yet been performed on the scalar). C<pos> directly accesses the location used
3903 by the regexp engine to store the offset, so assigning to C<pos> will change
3904 that offset, and so will also influence the C<\G> zero-width assertion in
3905 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
3906 the return from C<pos> won't change either in this case. See L<perlre> and
3909 =item print FILEHANDLE LIST
3915 Prints a string or a list of strings. Returns true if successful.
3916 FILEHANDLE may be a scalar variable name, in which case the variable
3917 contains the name of or a reference to the filehandle, thus introducing
3918 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3919 the next token is a term, it may be misinterpreted as an operator
3920 unless you interpose a C<+> or put parentheses around the arguments.)
3921 If FILEHANDLE is omitted, prints by default to standard output (or
3922 to the last selected output channel--see L</select>). If LIST is
3923 also omitted, prints C<$_> to the currently selected output channel.
3924 To set the default output channel to something other than STDOUT
3925 use the select operation. The current value of C<$,> (if any) is
3926 printed between each LIST item. The current value of C<$\> (if
3927 any) is printed after the entire LIST has been printed. Because
3928 print takes a LIST, anything in the LIST is evaluated in list
3929 context, and any subroutine that you call will have one or more of
3930 its expressions evaluated in list context. Also be careful not to
3931 follow the print keyword with a left parenthesis unless you want
3932 the corresponding right parenthesis to terminate the arguments to
3933 the print--interpose a C<+> or put parentheses around all the
3936 Note that if you're storing FILEHANDLEs in an array, or if you're using
3937 any other expression more complex than a scalar variable to retrieve it,
3938 you will have to use a block returning the filehandle value instead:
3940 print { $files[$i] } "stuff\n";
3941 print { $OK ? STDOUT : STDERR } "stuff\n";
3943 =item printf FILEHANDLE FORMAT, LIST
3945 =item printf FORMAT, LIST
3947 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3948 (the output record separator) is not appended. The first argument
3949 of the list will be interpreted as the C<printf> format. See C<sprintf>
3950 for an explanation of the format argument. If C<use locale> is in effect,
3951 the character used for the decimal point in formatted real numbers is
3952 affected by the LC_NUMERIC locale. See L<perllocale>.
3954 Don't fall into the trap of using a C<printf> when a simple
3955 C<print> would do. The C<print> is more efficient and less
3958 =item prototype FUNCTION
3960 Returns the prototype of a function as a string (or C<undef> if the
3961 function has no prototype). FUNCTION is a reference to, or the name of,
3962 the function whose prototype you want to retrieve.
3964 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3965 name for Perl builtin. If the builtin is not I<overridable> (such as
3966 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3967 C<system>) returns C<undef> because the builtin does not really behave
3968 like a Perl function. Otherwise, the string describing the equivalent
3969 prototype is returned.
3971 =item push ARRAY,LIST
3973 Treats ARRAY as a stack, and pushes the values of LIST
3974 onto the end of ARRAY. The length of ARRAY increases by the length of
3975 LIST. Has the same effect as
3978 $ARRAY[++$#ARRAY] = $value;
3981 but is more efficient. Returns the new number of elements in the array.
3993 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3995 =item quotemeta EXPR
3999 Returns the value of EXPR with all non-"word"
4000 characters backslashed. (That is, all characters not matching
4001 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4002 returned string, regardless of any locale settings.)
4003 This is the internal function implementing
4004 the C<\Q> escape in double-quoted strings.
4006 If EXPR is omitted, uses C<$_>.
4012 Returns a random fractional number greater than or equal to C<0> and less
4013 than the value of EXPR. (EXPR should be positive.) If EXPR is
4014 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4015 also special-cased as C<1> - this has not been documented before perl 5.8.0
4016 and is subject to change in future versions of perl. Automatically calls
4017 C<srand> unless C<srand> has already been called. See also C<srand>.
4019 Apply C<int()> to the value returned by C<rand()> if you want random
4020 integers instead of random fractional numbers. For example,
4024 returns a random integer between C<0> and C<9>, inclusive.
4026 (Note: If your rand function consistently returns numbers that are too
4027 large or too small, then your version of Perl was probably compiled
4028 with the wrong number of RANDBITS.)
4030 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4032 =item read FILEHANDLE,SCALAR,LENGTH
4034 Attempts to read LENGTH I<characters> of data into variable SCALAR
4035 from the specified FILEHANDLE. Returns the number of characters
4036 actually read, C<0> at end of file, or undef if there was an error (in
4037 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4038 so that the last character actually read is the last character of the
4039 scalar after the read.
4041 An OFFSET may be specified to place the read data at some place in the
4042 string other than the beginning. A negative OFFSET specifies
4043 placement at that many characters counting backwards from the end of
4044 the string. A positive OFFSET greater than the length of SCALAR
4045 results in the string being padded to the required size with C<"\0">
4046 bytes before the result of the read is appended.
4048 The call is actually implemented in terms of either Perl's or system's
4049 fread() call. To get a true read(2) system call, see C<sysread>.
4051 Note the I<characters>: depending on the status of the filehandle,
4052 either (8-bit) bytes or characters are read. By default all
4053 filehandles operate on bytes, but for example if the filehandle has
4054 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4055 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4056 characters, not bytes. Similarly for the C<:encoding> pragma:
4057 in that case pretty much any characters can be read.
4059 =item readdir DIRHANDLE
4061 Returns the next directory entry for a directory opened by C<opendir>.
4062 If used in list context, returns all the rest of the entries in the
4063 directory. If there are no more entries, returns an undefined value in
4064 scalar context or a null list in list context.
4066 If you're planning to filetest the return values out of a C<readdir>, you'd
4067 better prepend the directory in question. Otherwise, because we didn't
4068 C<chdir> there, it would have been testing the wrong file.
4070 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4071 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4076 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4077 context, each call reads and returns the next line, until end-of-file is
4078 reached, whereupon the subsequent call returns undef. In list context,
4079 reads until end-of-file is reached and returns a list of lines. Note that
4080 the notion of "line" used here is however you may have defined it
4081 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4083 When C<$/> is set to C<undef>, when readline() is in scalar
4084 context (i.e. file slurp mode), and when an empty file is read, it
4085 returns C<''> the first time, followed by C<undef> subsequently.
4087 This is the internal function implementing the C<< <EXPR> >>
4088 operator, but you can use it directly. The C<< <EXPR> >>
4089 operator is discussed in more detail in L<perlop/"I/O Operators">.
4092 $line = readline(*STDIN); # same thing
4094 If readline encounters an operating system error, C<$!> will be set with the
4095 corresponding error message. It can be helpful to check C<$!> when you are
4096 reading from filehandles you don't trust, such as a tty or a socket. The
4097 following example uses the operator form of C<readline>, and takes the necessary
4098 steps to ensure that C<readline> was successful.
4102 unless (defined( $line = <> )) {
4113 Returns the value of a symbolic link, if symbolic links are
4114 implemented. If not, gives a fatal error. If there is some system
4115 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4116 omitted, uses C<$_>.
4120 EXPR is executed as a system command.
4121 The collected standard output of the command is returned.
4122 In scalar context, it comes back as a single (potentially
4123 multi-line) string. In list context, returns a list of lines
4124 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4125 This is the internal function implementing the C<qx/EXPR/>
4126 operator, but you can use it directly. The C<qx/EXPR/>
4127 operator is discussed in more detail in L<perlop/"I/O Operators">.
4129 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4131 Receives a message on a socket. Attempts to receive LENGTH characters
4132 of data into variable SCALAR from the specified SOCKET filehandle.
4133 SCALAR will be grown or shrunk to the length actually read. Takes the
4134 same flags as the system call of the same name. Returns the address
4135 of the sender if SOCKET's protocol supports this; returns an empty
4136 string otherwise. If there's an error, returns the undefined value.
4137 This call is actually implemented in terms of recvfrom(2) system call.
4138 See L<perlipc/"UDP: Message Passing"> for examples.
4140 Note the I<characters>: depending on the status of the socket, either
4141 (8-bit) bytes or characters are received. By default all sockets
4142 operate on bytes, but for example if the socket has been changed using
4143 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4144 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4145 characters, not bytes. Similarly for the C<:encoding> pragma:
4146 in that case pretty much any characters can be read.
4152 The C<redo> command restarts the loop block without evaluating the
4153 conditional again. The C<continue> block, if any, is not executed. If
4154 the LABEL is omitted, the command refers to the innermost enclosing
4155 loop. This command is normally used by programs that want to lie to
4156 themselves about what was just input:
4158 # a simpleminded Pascal comment stripper
4159 # (warning: assumes no { or } in strings)
4160 LINE: while (<STDIN>) {
4161 while (s|({.*}.*){.*}|$1 |) {}
4166 if (/}/) { # end of comment?
4175 C<redo> cannot be used to retry a block which returns a value such as
4176 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4177 a grep() or map() operation.
4179 Note that a block by itself is semantically identical to a loop
4180 that executes once. Thus C<redo> inside such a block will effectively
4181 turn it into a looping construct.
4183 See also L</continue> for an illustration of how C<last>, C<next>, and
4190 Returns a non-empty string if EXPR is a reference, the empty
4191 string otherwise. If EXPR
4192 is not specified, C<$_> will be used. The value returned depends on the
4193 type of thing the reference is a reference to.
4194 Builtin types include:
4204 If the referenced object has been blessed into a package, then that package
4205 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4207 if (ref($r) eq "HASH") {
4208 print "r is a reference to a hash.\n";
4211 print "r is not a reference at all.\n";
4214 See also L<perlref>.
4216 =item rename OLDNAME,NEWNAME
4218 Changes the name of a file; an existing file NEWNAME will be
4219 clobbered. Returns true for success, false otherwise.
4221 Behavior of this function varies wildly depending on your system
4222 implementation. For example, it will usually not work across file system
4223 boundaries, even though the system I<mv> command sometimes compensates
4224 for this. Other restrictions include whether it works on directories,
4225 open files, or pre-existing files. Check L<perlport> and either the
4226 rename(2) manpage or equivalent system documentation for details.
4228 =item require VERSION
4234 Demands a version of Perl specified by VERSION, or demands some semantics
4235 specified by EXPR or by C<$_> if EXPR is not supplied.
4237 VERSION may be either a numeric argument such as 5.006, which will be
4238 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4239 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4240 VERSION is greater than the version of the current Perl interpreter.
4241 Compare with L</use>, which can do a similar check at compile time.
4243 Specifying VERSION as a literal of the form v5.6.1 should generally be
4244 avoided, because it leads to misleading error messages under earlier
4245 versions of Perl which do not support this syntax. The equivalent numeric
4246 version should be used instead.
4248 require v5.6.1; # run time version check
4249 require 5.6.1; # ditto
4250 require 5.006_001; # ditto; preferred for backwards compatibility
4252 Otherwise, demands that a library file be included if it hasn't already
4253 been included. The file is included via the do-FILE mechanism, which is
4254 essentially just a variety of C<eval>. Has semantics similar to the
4255 following subroutine:
4258 my ($filename) = @_;
4259 if (exists $INC{$filename}) {
4260 return 1 if $INC{$filename};
4261 die "Compilation failed in require";
4263 my ($realfilename,$result);
4265 foreach $prefix (@INC) {
4266 $realfilename = "$prefix/$filename";
4267 if (-f $realfilename) {
4268 $INC{$filename} = $realfilename;
4269 $result = do $realfilename;
4273 die "Can't find $filename in \@INC";
4276 $INC{$filename} = undef;
4278 } elsif (!$result) {
4279 delete $INC{$filename};
4280 die "$filename did not return true value";
4286 Note that the file will not be included twice under the same specified
4289 The file must return true as the last statement to indicate
4290 successful execution of any initialization code, so it's customary to
4291 end such a file with C<1;> unless you're sure it'll return true
4292 otherwise. But it's better just to put the C<1;>, in case you add more
4295 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4296 replaces "F<::>" with "F</>" in the filename for you,
4297 to make it easy to load standard modules. This form of loading of
4298 modules does not risk altering your namespace.
4300 In other words, if you try this:
4302 require Foo::Bar; # a splendid bareword
4304 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4305 directories specified in the C<@INC> array.
4307 But if you try this:
4309 $class = 'Foo::Bar';
4310 require $class; # $class is not a bareword
4312 require "Foo::Bar"; # not a bareword because of the ""
4314 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4315 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4317 eval "require $class";
4319 Now that you understand how C<require> looks for files in the case of
4320 a bareword argument, there is a little extra functionality going on
4321 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4322 it will first look for a filename with a "F<.pmc>" extension. A file
4323 with this extension is assumed to be Perl bytecode generated by
4324 L<B::Bytecode|B::Bytecode>. If this file is found, and its modification
4325 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4326 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4328 You can also insert hooks into the import facility, by putting directly
4329 Perl code into the @INC array. There are three forms of hooks: subroutine
4330 references, array references and blessed objects.
4332 Subroutine references are the simplest case. When the inclusion system
4333 walks through @INC and encounters a subroutine, this subroutine gets
4334 called with two parameters, the first being a reference to itself, and the
4335 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4336 subroutine should return C<undef> or a filehandle, from which the file to
4337 include will be read. If C<undef> is returned, C<require> will look at
4338 the remaining elements of @INC.
4340 If the hook is an array reference, its first element must be a subroutine
4341 reference. This subroutine is called as above, but the first parameter is
4342 the array reference. This enables to pass indirectly some arguments to
4345 In other words, you can write:
4347 push @INC, \&my_sub;
4349 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4355 push @INC, [ \&my_sub, $x, $y, ... ];
4357 my ($arrayref, $filename) = @_;
4358 # Retrieve $x, $y, ...
4359 my @parameters = @$arrayref[1..$#$arrayref];
4363 If the hook is an object, it must provide an INC method, that will be
4364 called as above, the first parameter being the object itself. (Note that
4365 you must fully qualify the sub's name, as it is always forced into package
4366 C<main>.) Here is a typical code layout:
4372 my ($self, $filename) = @_;
4376 # In the main program
4377 push @INC, new Foo(...);
4379 Note that these hooks are also permitted to set the %INC entry
4380 corresponding to the files they have loaded. See L<perlvar/%INC>.
4382 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4388 Generally used in a C<continue> block at the end of a loop to clear
4389 variables and reset C<??> searches so that they work again. The
4390 expression is interpreted as a list of single characters (hyphens
4391 allowed for ranges). All variables and arrays beginning with one of
4392 those letters are reset to their pristine state. If the expression is
4393 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4394 only variables or searches in the current package. Always returns
4397 reset 'X'; # reset all X variables
4398 reset 'a-z'; # reset lower case variables
4399 reset; # just reset ?one-time? searches
4401 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4402 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4403 variables--lexical variables are unaffected, but they clean themselves
4404 up on scope exit anyway, so you'll probably want to use them instead.
4411 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4412 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4413 context, depending on how the return value will be used, and the context
4414 may vary from one execution to the next (see C<wantarray>). If no EXPR
4415 is given, returns an empty list in list context, the undefined value in
4416 scalar context, and (of course) nothing at all in a void context.
4418 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4419 or do FILE will automatically return the value of the last expression
4424 In list context, returns a list value consisting of the elements
4425 of LIST in the opposite order. In scalar context, concatenates the
4426 elements of LIST and returns a string value with all characters
4427 in the opposite order.
4429 print reverse <>; # line tac, last line first
4431 undef $/; # for efficiency of <>
4432 print scalar reverse <>; # character tac, last line tsrif
4434 Used without arguments in scalar context, reverse() reverses C<$_>.
4436 This operator is also handy for inverting a hash, although there are some
4437 caveats. If a value is duplicated in the original hash, only one of those
4438 can be represented as a key in the inverted hash. Also, this has to
4439 unwind one hash and build a whole new one, which may take some time
4440 on a large hash, such as from a DBM file.
4442 %by_name = reverse %by_address; # Invert the hash
4444 =item rewinddir DIRHANDLE
4446 Sets the current position to the beginning of the directory for the
4447 C<readdir> routine on DIRHANDLE.
4449 =item rindex STR,SUBSTR,POSITION
4451 =item rindex STR,SUBSTR
4453 Works just like index() except that it returns the position of the LAST
4454 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4455 last occurrence at or before that position.
4457 =item rmdir FILENAME
4461 Deletes the directory specified by FILENAME if that directory is
4462 empty. If it succeeds it returns true, otherwise it returns false and
4463 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4467 The substitution operator. See L<perlop>.
4471 Forces EXPR to be interpreted in scalar context and returns the value
4474 @counts = ( scalar @a, scalar @b, scalar @c );
4476 There is no equivalent operator to force an expression to
4477 be interpolated in list context because in practice, this is never
4478 needed. If you really wanted to do so, however, you could use
4479 the construction C<@{[ (some expression) ]}>, but usually a simple
4480 C<(some expression)> suffices.
4482 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4483 parenthesized list, this behaves as a scalar comma expression, evaluating
4484 all but the last element in void context and returning the final element
4485 evaluated in scalar context. This is seldom what you want.
4487 The following single statement:
4489 print uc(scalar(&foo,$bar)),$baz;
4491 is the moral equivalent of these two:
4494 print(uc($bar),$baz);
4496 See L<perlop> for more details on unary operators and the comma operator.
4498 =item seek FILEHANDLE,POSITION,WHENCE
4500 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4501 FILEHANDLE may be an expression whose value gives the name of the
4502 filehandle. The values for WHENCE are C<0> to set the new position
4503 I<in bytes> to POSITION, C<1> to set it to the current position plus
4504 POSITION, and C<2> to set it to EOF plus POSITION (typically
4505 negative). For WHENCE you may use the constants C<SEEK_SET>,
4506 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4507 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4510 Note the I<in bytes>: even if the filehandle has been set to
4511 operate on characters (for example by using the C<:utf8> open
4512 layer), tell() will return byte offsets, not character offsets
4513 (because implementing that would render seek() and tell() rather slow).
4515 If you want to position file for C<sysread> or C<syswrite>, don't use
4516 C<seek>--buffering makes its effect on the file's system position
4517 unpredictable and non-portable. Use C<sysseek> instead.
4519 Due to the rules and rigors of ANSI C, on some systems you have to do a
4520 seek whenever you switch between reading and writing. Amongst other
4521 things, this may have the effect of calling stdio's clearerr(3).
4522 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4526 This is also useful for applications emulating C<tail -f>. Once you hit
4527 EOF on your read, and then sleep for a while, you might have to stick in a
4528 seek() to reset things. The C<seek> doesn't change the current position,
4529 but it I<does> clear the end-of-file condition on the handle, so that the
4530 next C<< <FILE> >> makes Perl try again to read something. We hope.
4532 If that doesn't work (some IO implementations are particularly
4533 cantankerous), then you may need something more like this:
4536 for ($curpos = tell(FILE); $_ = <FILE>;
4537 $curpos = tell(FILE)) {
4538 # search for some stuff and put it into files
4540 sleep($for_a_while);
4541 seek(FILE, $curpos, 0);
4544 =item seekdir DIRHANDLE,POS
4546 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4547 must be a value returned by C<telldir>. Has the same caveats about
4548 possible directory compaction as the corresponding system library
4551 =item select FILEHANDLE
4555 Returns the currently selected filehandle. Sets the current default
4556 filehandle for output, if FILEHANDLE is supplied. This has two
4557 effects: first, a C<write> or a C<print> without a filehandle will
4558 default to this FILEHANDLE. Second, references to variables related to
4559 output will refer to this output channel. For example, if you have to
4560 set the top of form format for more than one output channel, you might
4568 FILEHANDLE may be an expression whose value gives the name of the
4569 actual filehandle. Thus:
4571 $oldfh = select(STDERR); $| = 1; select($oldfh);
4573 Some programmers may prefer to think of filehandles as objects with
4574 methods, preferring to write the last example as:
4577 STDERR->autoflush(1);
4579 =item select RBITS,WBITS,EBITS,TIMEOUT
4581 This calls the select(2) system call with the bit masks specified, which
4582 can be constructed using C<fileno> and C<vec>, along these lines:
4584 $rin = $win = $ein = '';
4585 vec($rin,fileno(STDIN),1) = 1;
4586 vec($win,fileno(STDOUT),1) = 1;
4589 If you want to select on many filehandles you might wish to write a
4593 my(@fhlist) = split(' ',$_[0]);
4596 vec($bits,fileno($_),1) = 1;
4600 $rin = fhbits('STDIN TTY SOCK');
4604 ($nfound,$timeleft) =
4605 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4607 or to block until something becomes ready just do this
4609 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4611 Most systems do not bother to return anything useful in $timeleft, so
4612 calling select() in scalar context just returns $nfound.
4614 Any of the bit masks can also be undef. The timeout, if specified, is
4615 in seconds, which may be fractional. Note: not all implementations are
4616 capable of returning the $timeleft. If not, they always return
4617 $timeleft equal to the supplied $timeout.
4619 You can effect a sleep of 250 milliseconds this way:
4621 select(undef, undef, undef, 0.25);
4623 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4624 is implementation-dependent. See also L<perlport> for notes on the
4625 portability of C<select>.
4627 On error, C<select> returns C<undef> and sets C<$!>.
4629 Note: on some Unixes, the select(2) system call may report a socket file
4630 descriptor as "ready for reading", when actually no data is available,
4631 thus a subsequent read blocks. It can be avoided using always the
4632 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4635 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4636 or <FH>) with C<select>, except as permitted by POSIX, and even
4637 then only on POSIX systems. You have to use C<sysread> instead.
4639 =item semctl ID,SEMNUM,CMD,ARG
4641 Calls the System V IPC function C<semctl>. You'll probably have to say
4645 first to get the correct constant definitions. If CMD is IPC_STAT or
4646 GETALL, then ARG must be a variable which will hold the returned
4647 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4648 the undefined value for error, "C<0 but true>" for zero, or the actual
4649 return value otherwise. The ARG must consist of a vector of native
4650 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4651 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4654 =item semget KEY,NSEMS,FLAGS
4656 Calls the System V IPC function semget. Returns the semaphore id, or
4657 the undefined value if there is an error. See also
4658 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4661 =item semop KEY,OPSTRING
4663 Calls the System V IPC function semop to perform semaphore operations
4664 such as signalling and waiting. OPSTRING must be a packed array of
4665 semop structures. Each semop structure can be generated with
4666 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4667 operations is implied by the length of OPSTRING. Returns true if
4668 successful, or false if there is an error. As an example, the
4669 following code waits on semaphore $semnum of semaphore id $semid:
4671 $semop = pack("s!3", $semnum, -1, 0);
4672 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4674 To signal the semaphore, replace C<-1> with C<1>. See also
4675 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4678 =item send SOCKET,MSG,FLAGS,TO
4680 =item send SOCKET,MSG,FLAGS
4682 Sends a message on a socket. Attempts to send the scalar MSG to the
4683 SOCKET filehandle. Takes the same flags as the system call of the
4684 same name. On unconnected sockets you must specify a destination to
4685 send TO, in which case it does a C C<sendto>. Returns the number of
4686 characters sent, or the undefined value if there is an error. The C
4687 system call sendmsg(2) is currently unimplemented. See
4688 L<perlipc/"UDP: Message Passing"> for examples.
4690 Note the I<characters>: depending on the status of the socket, either
4691 (8-bit) bytes or characters are sent. By default all sockets operate
4692 on bytes, but for example if the socket has been changed using
4693 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4694 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4695 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4696 in that case pretty much any characters can be sent.
4698 =item setpgrp PID,PGRP
4700 Sets the current process group for the specified PID, C<0> for the current
4701 process. Will produce a fatal error if used on a machine that doesn't
4702 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4703 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4704 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4707 =item setpriority WHICH,WHO,PRIORITY
4709 Sets the current priority for a process, a process group, or a user.
4710 (See setpriority(2).) Will produce a fatal error if used on a machine
4711 that doesn't implement setpriority(2).
4713 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4715 Sets the socket option requested. Returns undefined if there is an
4716 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4723 Shifts the first value of the array off and returns it, shortening the
4724 array by 1 and moving everything down. If there are no elements in the
4725 array, returns the undefined value. If ARRAY is omitted, shifts the
4726 C<@_> array within the lexical scope of subroutines and formats, and the
4727 C<@ARGV> array at file scopes or within the lexical scopes established by
4728 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4731 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4732 same thing to the left end of an array that C<pop> and C<push> do to the
4735 =item shmctl ID,CMD,ARG
4737 Calls the System V IPC function shmctl. You'll probably have to say
4741 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4742 then ARG must be a variable which will hold the returned C<shmid_ds>
4743 structure. Returns like ioctl: the undefined value for error, "C<0> but
4744 true" for zero, or the actual return value otherwise.
4745 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4747 =item shmget KEY,SIZE,FLAGS
4749 Calls the System V IPC function shmget. Returns the shared memory
4750 segment id, or the undefined value if there is an error.
4751 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4753 =item shmread ID,VAR,POS,SIZE
4755 =item shmwrite ID,STRING,POS,SIZE
4757 Reads or writes the System V shared memory segment ID starting at
4758 position POS for size SIZE by attaching to it, copying in/out, and
4759 detaching from it. When reading, VAR must be a variable that will
4760 hold the data read. When writing, if STRING is too long, only SIZE
4761 bytes are used; if STRING is too short, nulls are written to fill out
4762 SIZE bytes. Return true if successful, or false if there is an error.
4763 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4764 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4766 =item shutdown SOCKET,HOW
4768 Shuts down a socket connection in the manner indicated by HOW, which
4769 has the same interpretation as in the system call of the same name.
4771 shutdown(SOCKET, 0); # I/we have stopped reading data
4772 shutdown(SOCKET, 1); # I/we have stopped writing data
4773 shutdown(SOCKET, 2); # I/we have stopped using this socket
4775 This is useful with sockets when you want to tell the other
4776 side you're done writing but not done reading, or vice versa.
4777 It's also a more insistent form of close because it also
4778 disables the file descriptor in any forked copies in other
4785 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4786 returns sine of C<$_>.
4788 For the inverse sine operation, you may use the C<Math::Trig::asin>
4789 function, or use this relation:
4791 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4797 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4798 May be interrupted if the process receives a signal such as C<SIGALRM>.
4799 Returns the number of seconds actually slept. You probably cannot
4800 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4803 On some older systems, it may sleep up to a full second less than what
4804 you requested, depending on how it counts seconds. Most modern systems
4805 always sleep the full amount. They may appear to sleep longer than that,
4806 however, because your process might not be scheduled right away in a
4807 busy multitasking system.
4809 For delays of finer granularity than one second, you may use Perl's
4810 C<syscall> interface to access setitimer(2) if your system supports
4811 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4812 and starting from Perl 5.8 part of the standard distribution) may also
4815 See also the POSIX module's C<pause> function.
4817 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4819 Opens a socket of the specified kind and attaches it to filehandle
4820 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4821 the system call of the same name. You should C<use Socket> first
4822 to get the proper definitions imported. See the examples in
4823 L<perlipc/"Sockets: Client/Server Communication">.
4825 On systems that support a close-on-exec flag on files, the flag will
4826 be set for the newly opened file descriptor, as determined by the
4827 value of $^F. See L<perlvar/$^F>.
4829 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4831 Creates an unnamed pair of sockets in the specified domain, of the
4832 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4833 for the system call of the same name. If unimplemented, yields a fatal
4834 error. Returns true if successful.
4836 On systems that support a close-on-exec flag on files, the flag will
4837 be set for the newly opened file descriptors, as determined by the value
4838 of $^F. See L<perlvar/$^F>.
4840 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4841 to C<pipe(Rdr, Wtr)> is essentially:
4844 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4845 shutdown(Rdr, 1); # no more writing for reader
4846 shutdown(Wtr, 0); # no more reading for writer
4848 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4849 emulate socketpair using IP sockets to localhost if your system implements
4850 sockets but not socketpair.
4852 =item sort SUBNAME LIST
4854 =item sort BLOCK LIST
4858 In list context, this sorts the LIST and returns the sorted list value.
4859 In scalar context, the behaviour of C<sort()> is undefined.
4861 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4862 order. If SUBNAME is specified, it gives the name of a subroutine
4863 that returns an integer less than, equal to, or greater than C<0>,
4864 depending on how the elements of the list are to be ordered. (The C<<
4865 <=> >> and C<cmp> operators are extremely useful in such routines.)
4866 SUBNAME may be a scalar variable name (unsubscripted), in which case
4867 the value provides the name of (or a reference to) the actual
4868 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4869 an anonymous, in-line sort subroutine.
4871 If the subroutine's prototype is C<($$)>, the elements to be compared
4872 are passed by reference in C<@_>, as for a normal subroutine. This is
4873 slower than unprototyped subroutines, where the elements to be
4874 compared are passed into the subroutine
4875 as the package global variables $a and $b (see example below). Note that
4876 in the latter case, it is usually counter-productive to declare $a and
4879 In either case, the subroutine may not be recursive. The values to be
4880 compared are always passed by reference, so don't modify them.
4882 You also cannot exit out of the sort block or subroutine using any of the
4883 loop control operators described in L<perlsyn> or with C<goto>.
4885 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4886 current collation locale. See L<perllocale>.
4888 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4889 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4890 preserves the input order of elements that compare equal. Although
4891 quicksort's run time is O(NlogN) when averaged over all arrays of
4892 length N, the time can be O(N**2), I<quadratic> behavior, for some
4893 inputs.) In 5.7, the quicksort implementation was replaced with
4894 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4895 But benchmarks indicated that for some inputs, on some platforms,
4896 the original quicksort was faster. 5.8 has a sort pragma for
4897 limited control of the sort. Its rather blunt control of the
4898 underlying algorithm may not persist into future perls, but the
4899 ability to characterize the input or output in implementation
4900 independent ways quite probably will. See L<sort>.
4905 @articles = sort @files;
4907 # same thing, but with explicit sort routine
4908 @articles = sort {$a cmp $b} @files;
4910 # now case-insensitively
4911 @articles = sort {uc($a) cmp uc($b)} @files;
4913 # same thing in reversed order
4914 @articles = sort {$b cmp $a} @files;
4916 # sort numerically ascending
4917 @articles = sort {$a <=> $b} @files;
4919 # sort numerically descending
4920 @articles = sort {$b <=> $a} @files;
4922 # this sorts the %age hash by value instead of key
4923 # using an in-line function
4924 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4926 # sort using explicit subroutine name
4928 $age{$a} <=> $age{$b}; # presuming numeric
4930 @sortedclass = sort byage @class;
4932 sub backwards { $b cmp $a }
4933 @harry = qw(dog cat x Cain Abel);
4934 @george = qw(gone chased yz Punished Axed);
4936 # prints AbelCaincatdogx
4937 print sort backwards @harry;
4938 # prints xdogcatCainAbel
4939 print sort @george, 'to', @harry;
4940 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4942 # inefficiently sort by descending numeric compare using
4943 # the first integer after the first = sign, or the
4944 # whole record case-insensitively otherwise
4947 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4952 # same thing, but much more efficiently;
4953 # we'll build auxiliary indices instead
4957 push @nums, /=(\d+)/;
4962 $nums[$b] <=> $nums[$a]
4964 $caps[$a] cmp $caps[$b]
4968 # same thing, but without any temps
4969 @new = map { $_->[0] }
4970 sort { $b->[1] <=> $a->[1]
4973 } map { [$_, /=(\d+)/, uc($_)] } @old;
4975 # using a prototype allows you to use any comparison subroutine
4976 # as a sort subroutine (including other package's subroutines)
4978 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4981 @new = sort other::backwards @old;
4983 # guarantee stability, regardless of algorithm
4985 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4987 # force use of mergesort (not portable outside Perl 5.8)
4988 use sort '_mergesort'; # note discouraging _
4989 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4991 If you're using strict, you I<must not> declare $a
4992 and $b as lexicals. They are package globals. That means
4993 if you're in the C<main> package and type
4995 @articles = sort {$b <=> $a} @files;
4997 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4998 but if you're in the C<FooPack> package, it's the same as typing
5000 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5002 The comparison function is required to behave. If it returns
5003 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5004 sometimes saying the opposite, for example) the results are not
5007 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5008 (not-a-number), and because C<sort> will trigger a fatal error unless the
5009 result of a comparison is defined, when sorting with a comparison function
5010 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5011 The following example takes advantage of the fact that C<NaN != NaN> to
5012 eliminate any C<NaN>s from the input.
5014 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5016 =item splice ARRAY,OFFSET,LENGTH,LIST
5018 =item splice ARRAY,OFFSET,LENGTH
5020 =item splice ARRAY,OFFSET
5024 Removes the elements designated by OFFSET and LENGTH from an array, and
5025 replaces them with the elements of LIST, if any. In list context,
5026 returns the elements removed from the array. In scalar context,
5027 returns the last element removed, or C<undef> if no elements are
5028 removed. The array grows or shrinks as necessary.
5029 If OFFSET is negative then it starts that far from the end of the array.
5030 If LENGTH is omitted, removes everything from OFFSET onward.
5031 If LENGTH is negative, removes the elements from OFFSET onward
5032 except for -LENGTH elements at the end of the array.
5033 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5034 past the end of the array, perl issues a warning, and splices at the
5037 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5039 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5040 pop(@a) splice(@a,-1)
5041 shift(@a) splice(@a,0,1)
5042 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5043 $a[$i] = $y splice(@a,$i,1,$y)
5045 Example, assuming array lengths are passed before arrays:
5047 sub aeq { # compare two list values
5048 my(@a) = splice(@_,0,shift);
5049 my(@b) = splice(@_,0,shift);
5050 return 0 unless @a == @b; # same len?
5052 return 0 if pop(@a) ne pop(@b);
5056 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5058 =item split /PATTERN/,EXPR,LIMIT
5060 =item split /PATTERN/,EXPR
5062 =item split /PATTERN/
5066 Splits the string EXPR into a list of strings and returns that list. By
5067 default, empty leading fields are preserved, and empty trailing ones are
5068 deleted. (If all fields are empty, they are considered to be trailing.)
5070 In scalar context, returns the number of fields found and splits into
5071 the C<@_> array. Use of split in scalar context is deprecated, however,
5072 because it clobbers your subroutine arguments.
5074 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5075 splits on whitespace (after skipping any leading whitespace). Anything
5076 matching PATTERN is taken to be a delimiter separating the fields. (Note
5077 that the delimiter may be longer than one character.)
5079 If LIMIT is specified and positive, it represents the maximum number
5080 of fields the EXPR will be split into, though the actual number of
5081 fields returned depends on the number of times PATTERN matches within
5082 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5083 stripped (which potential users of C<pop> would do well to remember).
5084 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5085 had been specified. Note that splitting an EXPR that evaluates to the
5086 empty string always returns the empty list, regardless of the LIMIT
5089 A pattern matching the null string (not to be confused with
5090 a null pattern C<//>, which is just one member of the set of patterns
5091 matching a null string) will split the value of EXPR into separate
5092 characters at each point it matches that way. For example:
5094 print join(':', split(/ */, 'hi there'));
5096 produces the output 'h:i:t:h:e:r:e'.
5098 As a special case for C<split>, using the empty pattern C<//> specifically
5099 matches only the null string, and is not be confused with the regular use
5100 of C<//> to mean "the last successful pattern match". So, for C<split>,
5103 print join(':', split(//, 'hi there'));
5105 produces the output 'h:i: :t:h:e:r:e'.
5107 Empty leading (or trailing) fields are produced when there are positive
5108 width matches at the beginning (or end) of the string; a zero-width match
5109 at the beginning (or end) of the string does not produce an empty field.
5112 print join(':', split(/(?=\w)/, 'hi there!'));
5114 produces the output 'h:i :t:h:e:r:e!'.
5116 The LIMIT parameter can be used to split a line partially
5118 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5120 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5121 a LIMIT one larger than the number of variables in the list, to avoid
5122 unnecessary work. For the list above LIMIT would have been 4 by
5123 default. In time critical applications it behooves you not to split
5124 into more fields than you really need.
5126 If the PATTERN contains parentheses, additional list elements are
5127 created from each matching substring in the delimiter.
5129 split(/([,-])/, "1-10,20", 3);
5131 produces the list value
5133 (1, '-', 10, ',', 20)
5135 If you had the entire header of a normal Unix email message in $header,
5136 you could split it up into fields and their values this way:
5138 $header =~ s/\n\s+/ /g; # fix continuation lines
5139 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5141 The pattern C</PATTERN/> may be replaced with an expression to specify
5142 patterns that vary at runtime. (To do runtime compilation only once,
5143 use C</$variable/o>.)
5145 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5146 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5147 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5148 will give you as many null initial fields as there are leading spaces.
5149 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5150 whitespace produces a null first field. A C<split> with no arguments
5151 really does a S<C<split(' ', $_)>> internally.
5153 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5158 open(PASSWD, '/etc/passwd');
5161 ($login, $passwd, $uid, $gid,
5162 $gcos, $home, $shell) = split(/:/);
5166 As with regular pattern matching, any capturing parentheses that are not
5167 matched in a C<split()> will be set to C<undef> when returned:
5169 @fields = split /(A)|B/, "1A2B3";
5170 # @fields is (1, 'A', 2, undef, 3)
5172 =item sprintf FORMAT, LIST
5174 Returns a string formatted by the usual C<printf> conventions of the C
5175 library function C<sprintf>. See below for more details
5176 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5177 the general principles.
5181 # Format number with up to 8 leading zeroes
5182 $result = sprintf("%08d", $number);
5184 # Round number to 3 digits after decimal point
5185 $rounded = sprintf("%.3f", $number);
5187 Perl does its own C<sprintf> formatting--it emulates the C
5188 function C<sprintf>, but it doesn't use it (except for floating-point
5189 numbers, and even then only the standard modifiers are allowed). As a
5190 result, any non-standard extensions in your local C<sprintf> are not
5191 available from Perl.
5193 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5194 pass it an array as your first argument. The array is given scalar context,
5195 and instead of using the 0th element of the array as the format, Perl will
5196 use the count of elements in the array as the format, which is almost never
5199 Perl's C<sprintf> permits the following universally-known conversions:
5202 %c a character with the given number
5204 %d a signed integer, in decimal
5205 %u an unsigned integer, in decimal
5206 %o an unsigned integer, in octal
5207 %x an unsigned integer, in hexadecimal
5208 %e a floating-point number, in scientific notation
5209 %f a floating-point number, in fixed decimal notation
5210 %g a floating-point number, in %e or %f notation
5212 In addition, Perl permits the following widely-supported conversions:
5214 %X like %x, but using upper-case letters
5215 %E like %e, but using an upper-case "E"
5216 %G like %g, but with an upper-case "E" (if applicable)
5217 %b an unsigned integer, in binary
5218 %p a pointer (outputs the Perl value's address in hexadecimal)
5219 %n special: *stores* the number of characters output so far
5220 into the next variable in the parameter list
5222 Finally, for backward (and we do mean "backward") compatibility, Perl
5223 permits these unnecessary but widely-supported conversions:
5226 %D a synonym for %ld
5227 %U a synonym for %lu
5228 %O a synonym for %lo
5231 Note that the number of exponent digits in the scientific notation produced
5232 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5233 exponent less than 100 is system-dependent: it may be three or less
5234 (zero-padded as necessary). In other words, 1.23 times ten to the
5235 99th may be either "1.23e99" or "1.23e099".
5237 Between the C<%> and the format letter, you may specify a number of
5238 additional attributes controlling the interpretation of the format.
5239 In order, these are:
5243 =item format parameter index
5245 An explicit format parameter index, such as C<2$>. By default sprintf
5246 will format the next unused argument in the list, but this allows you
5247 to take the arguments out of order. Eg:
5249 printf '%2$d %1$d', 12, 34; # prints "34 12"
5250 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5255 space prefix positive number with a space
5256 + prefix positive number with a plus sign
5257 - left-justify within the field
5258 0 use zeros, not spaces, to right-justify
5259 # prefix non-zero octal with "0", non-zero hex with "0x",
5260 non-zero binary with "0b"
5264 printf '<% d>', 12; # prints "< 12>"
5265 printf '<%+d>', 12; # prints "<+12>"
5266 printf '<%6s>', 12; # prints "< 12>"
5267 printf '<%-6s>', 12; # prints "<12 >"
5268 printf '<%06s>', 12; # prints "<000012>"
5269 printf '<%#x>', 12; # prints "<0xc>"
5273 The vector flag C<v>, optionally specifying the join string to use.
5274 This flag tells perl to interpret the supplied string as a vector
5275 of integers, one for each character in the string, separated by
5276 a given string (a dot C<.> by default). This can be useful for
5277 displaying ordinal values of characters in arbitrary strings:
5279 printf "version is v%vd\n", $^V; # Perl's version
5281 Put an asterisk C<*> before the C<v> to override the string to
5282 use to separate the numbers:
5284 printf "address is %*vX\n", ":", $addr; # IPv6 address
5285 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5287 You can also explicitly specify the argument number to use for
5288 the join string using eg C<*2$v>:
5290 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5292 =item (minimum) width
5294 Arguments are usually formatted to be only as wide as required to
5295 display the given value. You can override the width by putting
5296 a number here, or get the width from the next argument (with C<*>)
5297 or from a specified argument (with eg C<*2$>):
5299 printf '<%s>', "a"; # prints "<a>"
5300 printf '<%6s>', "a"; # prints "< a>"
5301 printf '<%*s>', 6, "a"; # prints "< a>"
5302 printf '<%*2$s>', "a", 6; # prints "< a>"
5303 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5305 If a field width obtained through C<*> is negative, it has the same
5306 effect as the C<-> flag: left-justification.
5308 =item precision, or maximum width
5310 You can specify a precision (for numeric conversions) or a maximum
5311 width (for string conversions) by specifying a C<.> followed by a number.
5312 For floating point formats, with the exception of 'g' and 'G', this specifies
5313 the number of decimal places to show (the default being 6), eg:
5315 # these examples are subject to system-specific variation
5316 printf '<%f>', 1; # prints "<1.000000>"
5317 printf '<%.1f>', 1; # prints "<1.0>"
5318 printf '<%.0f>', 1; # prints "<1>"
5319 printf '<%e>', 10; # prints "<1.000000e+01>"
5320 printf '<%.1e>', 10; # prints "<1.0e+01>"
5322 For 'g' and 'G', this specifies the maximum number of digits to show,
5323 including prior to the decimal point as well as after it, eg:
5325 # these examples are subject to system-specific variation
5326 printf '<%g>', 1; # prints "<1>"
5327 printf '<%.10g>', 1; # prints "<1>"
5328 printf '<%g>', 100; # prints "<100>"
5329 printf '<%.1g>', 100; # prints "<1e+02>"
5330 printf '<%.2g>', 100.01; # prints "<1e+02>"
5331 printf '<%.5g>', 100.01; # prints "<100.01>"
5332 printf '<%.4g>', 100.01; # prints "<100>"
5334 For integer conversions, specifying a precision implies that the
5335 output of the number itself should be zero-padded to this width:
5337 printf '<%.6x>', 1; # prints "<000001>"
5338 printf '<%#.6x>', 1; # prints "<0x000001>"
5339 printf '<%-10.6x>', 1; # prints "<000001 >"
5341 For string conversions, specifying a precision truncates the string
5342 to fit in the specified width:
5344 printf '<%.5s>', "truncated"; # prints "<trunc>"
5345 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5347 You can also get the precision from the next argument using C<.*>:
5349 printf '<%.6x>', 1; # prints "<000001>"
5350 printf '<%.*x>', 6, 1; # prints "<000001>"
5352 You cannot currently get the precision from a specified number,
5353 but it is intended that this will be possible in the future using
5356 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5360 For numeric conversions, you can specify the size to interpret the
5361 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5362 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5363 whatever the default integer size is on your platform (usually 32 or 64
5364 bits), but you can override this to use instead one of the standard C types,
5365 as supported by the compiler used to build Perl:
5367 l interpret integer as C type "long" or "unsigned long"
5368 h interpret integer as C type "short" or "unsigned short"
5369 q, L or ll interpret integer as C type "long long", "unsigned long long".
5370 or "quads" (typically 64-bit integers)
5372 The last will produce errors if Perl does not understand "quads" in your
5373 installation. (This requires that either the platform natively supports quads
5374 or Perl was specifically compiled to support quads.) You can find out
5375 whether your Perl supports quads via L<Config>:
5378 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5381 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5382 to be the default floating point size on your platform (double or long double),
5383 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5384 platform supports them. You can find out whether your Perl supports long
5385 doubles via L<Config>:
5388 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5390 You can find out whether Perl considers 'long double' to be the default
5391 floating point size to use on your platform via L<Config>:
5394 ($Config{uselongdouble} eq 'define') &&
5395 print "long doubles by default\n";
5397 It can also be the case that long doubles and doubles are the same thing:
5400 ($Config{doublesize} == $Config{longdblsize}) &&
5401 print "doubles are long doubles\n";
5403 The size specifier C<V> has no effect for Perl code, but it is supported
5404 for compatibility with XS code; it means 'use the standard size for
5405 a Perl integer (or floating-point number)', which is already the
5406 default for Perl code.
5408 =item order of arguments
5410 Normally, sprintf takes the next unused argument as the value to
5411 format for each format specification. If the format specification
5412 uses C<*> to require additional arguments, these are consumed from
5413 the argument list in the order in which they appear in the format
5414 specification I<before> the value to format. Where an argument is
5415 specified using an explicit index, this does not affect the normal
5416 order for the arguments (even when the explicitly specified index
5417 would have been the next argument in any case).
5421 printf '<%*.*s>', $a, $b, $c;
5423 would use C<$a> for the width, C<$b> for the precision and C<$c>
5424 as the value to format, while:
5426 print '<%*1$.*s>', $a, $b;
5428 would use C<$a> for the width and the precision, and C<$b> as the
5431 Here are some more examples - beware that when using an explicit
5432 index, the C<$> may need to be escaped:
5434 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5435 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5436 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5437 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5441 If C<use locale> is in effect, the character used for the decimal
5442 point in formatted real numbers is affected by the LC_NUMERIC locale.
5449 Return the square root of EXPR. If EXPR is omitted, returns square
5450 root of C<$_>. Only works on non-negative operands, unless you've
5451 loaded the standard Math::Complex module.
5454 print sqrt(-2); # prints 1.4142135623731i
5460 Sets the random number seed for the C<rand> operator.
5462 The point of the function is to "seed" the C<rand> function so that
5463 C<rand> can produce a different sequence each time you run your
5466 If srand() is not called explicitly, it is called implicitly at the
5467 first use of the C<rand> operator. However, this was not the case in
5468 versions of Perl before 5.004, so if your script will run under older
5469 Perl versions, it should call C<srand>.
5471 Most programs won't even call srand() at all, except those that
5472 need a cryptographically-strong starting point rather than the
5473 generally acceptable default, which is based on time of day,
5474 process ID, and memory allocation, or the F</dev/urandom> device,
5477 You can call srand($seed) with the same $seed to reproduce the
5478 I<same> sequence from rand(), but this is usually reserved for
5479 generating predictable results for testing or debugging.
5480 Otherwise, don't call srand() more than once in your program.
5482 Do B<not> call srand() (i.e. without an argument) more than once in
5483 a script. The internal state of the random number generator should
5484 contain more entropy than can be provided by any seed, so calling
5485 srand() again actually I<loses> randomness.
5487 Most implementations of C<srand> take an integer and will silently
5488 truncate decimal numbers. This means C<srand(42)> will usually
5489 produce the same results as C<srand(42.1)>. To be safe, always pass
5490 C<srand> an integer.
5492 In versions of Perl prior to 5.004 the default seed was just the
5493 current C<time>. This isn't a particularly good seed, so many old
5494 programs supply their own seed value (often C<time ^ $$> or C<time ^
5495 ($$ + ($$ << 15))>), but that isn't necessary any more.
5497 Note that you need something much more random than the default seed for
5498 cryptographic purposes. Checksumming the compressed output of one or more
5499 rapidly changing operating system status programs is the usual method. For
5502 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5504 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5507 Frequently called programs (like CGI scripts) that simply use
5511 for a seed can fall prey to the mathematical property that
5515 one-third of the time. So don't do that.
5517 =item stat FILEHANDLE
5523 Returns a 13-element list giving the status info for a file, either
5524 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5525 it stats C<$_>. Returns a null list if the stat fails. Typically used
5528 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5529 $atime,$mtime,$ctime,$blksize,$blocks)
5532 Not all fields are supported on all filesystem types. Here are the
5533 meanings of the fields:
5535 0 dev device number of filesystem
5537 2 mode file mode (type and permissions)
5538 3 nlink number of (hard) links to the file
5539 4 uid numeric user ID of file's owner
5540 5 gid numeric group ID of file's owner
5541 6 rdev the device identifier (special files only)
5542 7 size total size of file, in bytes
5543 8 atime last access time in seconds since the epoch
5544 9 mtime last modify time in seconds since the epoch
5545 10 ctime inode change time in seconds since the epoch (*)
5546 11 blksize preferred block size for file system I/O
5547 12 blocks actual number of blocks allocated
5549 (The epoch was at 00:00 January 1, 1970 GMT.)
5551 (*) Not all fields are supported on all filesystem types. Notably, the
5552 ctime field is non-portable. In particular, you cannot expect it to be a
5553 "creation time", see L<perlport/"Files and Filesystems"> for details.
5555 If C<stat> is passed the special filehandle consisting of an underline, no
5556 stat is done, but the current contents of the stat structure from the
5557 last C<stat>, C<lstat>, or filetest are returned. Example:
5559 if (-x $file && (($d) = stat(_)) && $d < 0) {
5560 print "$file is executable NFS file\n";
5563 (This works on machines only for which the device number is negative
5566 Because the mode contains both the file type and its permissions, you
5567 should mask off the file type portion and (s)printf using a C<"%o">
5568 if you want to see the real permissions.
5570 $mode = (stat($filename))[2];
5571 printf "Permissions are %04o\n", $mode & 07777;
5573 In scalar context, C<stat> returns a boolean value indicating success
5574 or failure, and, if successful, sets the information associated with
5575 the special filehandle C<_>.
5577 The File::stat module provides a convenient, by-name access mechanism:
5580 $sb = stat($filename);
5581 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5582 $filename, $sb->size, $sb->mode & 07777,
5583 scalar localtime $sb->mtime;
5585 You can import symbolic mode constants (C<S_IF*>) and functions
5586 (C<S_IS*>) from the Fcntl module:
5590 $mode = (stat($filename))[2];
5592 $user_rwx = ($mode & S_IRWXU) >> 6;
5593 $group_read = ($mode & S_IRGRP) >> 3;
5594 $other_execute = $mode & S_IXOTH;
5596 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5598 $is_setuid = $mode & S_ISUID;
5599 $is_setgid = S_ISDIR($mode);
5601 You could write the last two using the C<-u> and C<-d> operators.
5602 The commonly available C<S_IF*> constants are
5604 # Permissions: read, write, execute, for user, group, others.
5606 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5607 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5608 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5610 # Setuid/Setgid/Stickiness/SaveText.
5611 # Note that the exact meaning of these is system dependent.
5613 S_ISUID S_ISGID S_ISVTX S_ISTXT
5615 # File types. Not necessarily all are available on your system.
5617 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5619 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5621 S_IREAD S_IWRITE S_IEXEC
5623 and the C<S_IF*> functions are
5625 S_IMODE($mode) the part of $mode containing the permission bits
5626 and the setuid/setgid/sticky bits
5628 S_IFMT($mode) the part of $mode containing the file type
5629 which can be bit-anded with e.g. S_IFREG
5630 or with the following functions
5632 # The operators -f, -d, -l, -b, -c, -p, and -S.
5634 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5635 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5637 # No direct -X operator counterpart, but for the first one
5638 # the -g operator is often equivalent. The ENFMT stands for
5639 # record flocking enforcement, a platform-dependent feature.
5641 S_ISENFMT($mode) S_ISWHT($mode)
5643 See your native chmod(2) and stat(2) documentation for more details
5644 about the C<S_*> constants. To get status info for a symbolic link
5645 instead of the target file behind the link, use the C<lstat> function.
5651 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5652 doing many pattern matches on the string before it is next modified.
5653 This may or may not save time, depending on the nature and number of
5654 patterns you are searching on, and on the distribution of character
5655 frequencies in the string to be searched--you probably want to compare
5656 run times with and without it to see which runs faster. Those loops
5657 which scan for many short constant strings (including the constant
5658 parts of more complex patterns) will benefit most. You may have only
5659 one C<study> active at a time--if you study a different scalar the first
5660 is "unstudied". (The way C<study> works is this: a linked list of every
5661 character in the string to be searched is made, so we know, for
5662 example, where all the C<'k'> characters are. From each search string,
5663 the rarest character is selected, based on some static frequency tables
5664 constructed from some C programs and English text. Only those places
5665 that contain this "rarest" character are examined.)
5667 For example, here is a loop that inserts index producing entries
5668 before any line containing a certain pattern:
5672 print ".IX foo\n" if /\bfoo\b/;
5673 print ".IX bar\n" if /\bbar\b/;
5674 print ".IX blurfl\n" if /\bblurfl\b/;
5679 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5680 will be looked at, because C<f> is rarer than C<o>. In general, this is
5681 a big win except in pathological cases. The only question is whether
5682 it saves you more time than it took to build the linked list in the
5685 Note that if you have to look for strings that you don't know till
5686 runtime, you can build an entire loop as a string and C<eval> that to
5687 avoid recompiling all your patterns all the time. Together with
5688 undefining C<$/> to input entire files as one record, this can be very
5689 fast, often faster than specialized programs like fgrep(1). The following
5690 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5691 out the names of those files that contain a match:
5693 $search = 'while (<>) { study;';
5694 foreach $word (@words) {
5695 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5700 eval $search; # this screams
5701 $/ = "\n"; # put back to normal input delimiter
5702 foreach $file (sort keys(%seen)) {
5706 =item sub NAME BLOCK
5708 =item sub NAME (PROTO) BLOCK
5710 =item sub NAME : ATTRS BLOCK
5712 =item sub NAME (PROTO) : ATTRS BLOCK
5714 This is subroutine definition, not a real function I<per se>.
5715 Without a BLOCK it's just a forward declaration. Without a NAME,
5716 it's an anonymous function declaration, and does actually return
5717 a value: the CODE ref of the closure you just created.
5719 See L<perlsub> and L<perlref> for details about subroutines and
5720 references, and L<attributes> and L<Attribute::Handlers> for more
5721 information about attributes.
5723 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5725 =item substr EXPR,OFFSET,LENGTH
5727 =item substr EXPR,OFFSET
5729 Extracts a substring out of EXPR and returns it. First character is at
5730 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5731 If OFFSET is negative (or more precisely, less than C<$[>), starts
5732 that far from the end of the string. If LENGTH is omitted, returns
5733 everything to the end of the string. If LENGTH is negative, leaves that
5734 many characters off the end of the string.
5736 You can use the substr() function as an lvalue, in which case EXPR
5737 must itself be an lvalue. If you assign something shorter than LENGTH,
5738 the string will shrink, and if you assign something longer than LENGTH,
5739 the string will grow to accommodate it. To keep the string the same
5740 length you may need to pad or chop your value using C<sprintf>.
5742 If OFFSET and LENGTH specify a substring that is partly outside the
5743 string, only the part within the string is returned. If the substring
5744 is beyond either end of the string, substr() returns the undefined
5745 value and produces a warning. When used as an lvalue, specifying a
5746 substring that is entirely outside the string is a fatal error.
5747 Here's an example showing the behavior for boundary cases:
5750 substr($name, 4) = 'dy'; # $name is now 'freddy'
5751 my $null = substr $name, 6, 2; # returns '' (no warning)
5752 my $oops = substr $name, 7; # returns undef, with warning
5753 substr($name, 7) = 'gap'; # fatal error
5755 An alternative to using substr() as an lvalue is to specify the
5756 replacement string as the 4th argument. This allows you to replace
5757 parts of the EXPR and return what was there before in one operation,
5758 just as you can with splice().
5760 Note that the lvalue returned by by the 3-arg version of substr() acts as
5761 a 'magic bullet'; each time it is assigned to, it remembers which part
5762 of the original string is being modified; for example:
5765 for (substr($x,1,2)) {
5766 $_ = 'a'; print $x,"\n"; # prints 1a4
5767 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
5769 $_ = 'pq'; print $x,"\n"; # prints 5pq9
5773 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
5776 =item symlink OLDFILE,NEWFILE
5778 Creates a new filename symbolically linked to the old filename.
5779 Returns C<1> for success, C<0> otherwise. On systems that don't support
5780 symbolic links, produces a fatal error at run time. To check for that,
5783 $symlink_exists = eval { symlink("",""); 1 };
5785 =item syscall NUMBER, LIST
5787 Calls the system call specified as the first element of the list,
5788 passing the remaining elements as arguments to the system call. If
5789 unimplemented, produces a fatal error. The arguments are interpreted
5790 as follows: if a given argument is numeric, the argument is passed as
5791 an int. If not, the pointer to the string value is passed. You are
5792 responsible to make sure a string is pre-extended long enough to
5793 receive any result that might be written into a string. You can't use a
5794 string literal (or other read-only string) as an argument to C<syscall>
5795 because Perl has to assume that any string pointer might be written
5797 integer arguments are not literals and have never been interpreted in a
5798 numeric context, you may need to add C<0> to them to force them to look
5799 like numbers. This emulates the C<syswrite> function (or vice versa):
5801 require 'syscall.ph'; # may need to run h2ph
5803 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5805 Note that Perl supports passing of up to only 14 arguments to your system call,
5806 which in practice should usually suffice.
5808 Syscall returns whatever value returned by the system call it calls.
5809 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5810 Note that some system calls can legitimately return C<-1>. The proper
5811 way to handle such calls is to assign C<$!=0;> before the call and
5812 check the value of C<$!> if syscall returns C<-1>.
5814 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5815 number of the read end of the pipe it creates. There is no way
5816 to retrieve the file number of the other end. You can avoid this
5817 problem by using C<pipe> instead.
5819 =item sysopen FILEHANDLE,FILENAME,MODE
5821 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5823 Opens the file whose filename is given by FILENAME, and associates it
5824 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5825 the name of the real filehandle wanted. This function calls the
5826 underlying operating system's C<open> function with the parameters
5827 FILENAME, MODE, PERMS.
5829 The possible values and flag bits of the MODE parameter are
5830 system-dependent; they are available via the standard module C<Fcntl>.
5831 See the documentation of your operating system's C<open> to see which
5832 values and flag bits are available. You may combine several flags
5833 using the C<|>-operator.
5835 Some of the most common values are C<O_RDONLY> for opening the file in
5836 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5837 and C<O_RDWR> for opening the file in read-write mode.
5839 For historical reasons, some values work on almost every system
5840 supported by perl: zero means read-only, one means write-only, and two
5841 means read/write. We know that these values do I<not> work under
5842 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5843 use them in new code.
5845 If the file named by FILENAME does not exist and the C<open> call creates
5846 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5847 PERMS specifies the permissions of the newly created file. If you omit
5848 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5849 These permission values need to be in octal, and are modified by your
5850 process's current C<umask>.
5852 In many systems the C<O_EXCL> flag is available for opening files in
5853 exclusive mode. This is B<not> locking: exclusiveness means here that
5854 if the file already exists, sysopen() fails. C<O_EXCL> may not work
5855 on network filesystems, and has no effect unless the C<O_CREAT> flag
5856 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
5857 being opened if it is a symbolic link. It does not protect against
5858 symbolic links in the file's path.
5860 Sometimes you may want to truncate an already-existing file. This
5861 can be done using the C<O_TRUNC> flag. The behavior of
5862 C<O_TRUNC> with C<O_RDONLY> is undefined.
5864 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5865 that takes away the user's option to have a more permissive umask.
5866 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5869 Note that C<sysopen> depends on the fdopen() C library function.
5870 On many UNIX systems, fdopen() is known to fail when file descriptors
5871 exceed a certain value, typically 255. If you need more file
5872 descriptors than that, consider rebuilding Perl to use the C<sfio>
5873 library, or perhaps using the POSIX::open() function.
5875 See L<perlopentut> for a kinder, gentler explanation of opening files.
5877 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5879 =item sysread FILEHANDLE,SCALAR,LENGTH
5881 Attempts to read LENGTH bytes of data into variable SCALAR from the
5882 specified FILEHANDLE, using the system call read(2). It bypasses
5883 buffered IO, so mixing this with other kinds of reads, C<print>,
5884 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5885 perlio or stdio layers usually buffers data. Returns the number of
5886 bytes actually read, C<0> at end of file, or undef if there was an
5887 error (in the latter case C<$!> is also set). SCALAR will be grown or
5888 shrunk so that the last byte actually read is the last byte of the
5889 scalar after the read.
5891 An OFFSET may be specified to place the read data at some place in the
5892 string other than the beginning. A negative OFFSET specifies
5893 placement at that many characters counting backwards from the end of
5894 the string. A positive OFFSET greater than the length of SCALAR
5895 results in the string being padded to the required size with C<"\0">
5896 bytes before the result of the read is appended.
5898 There is no syseof() function, which is ok, since eof() doesn't work
5899 very well on device files (like ttys) anyway. Use sysread() and check
5900 for a return value for 0 to decide whether you're done.
5902 Note that if the filehandle has been marked as C<:utf8> Unicode
5903 characters are read instead of bytes (the LENGTH, OFFSET, and the
5904 return value of sysread() are in Unicode characters).
5905 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5906 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5908 =item sysseek FILEHANDLE,POSITION,WHENCE
5910 Sets FILEHANDLE's system position in bytes using the system call
5911 lseek(2). FILEHANDLE may be an expression whose value gives the name
5912 of the filehandle. The values for WHENCE are C<0> to set the new
5913 position to POSITION, C<1> to set the it to the current position plus
5914 POSITION, and C<2> to set it to EOF plus POSITION (typically
5917 Note the I<in bytes>: even if the filehandle has been set to operate
5918 on characters (for example by using the C<:utf8> I/O layer), tell()
5919 will return byte offsets, not character offsets (because implementing
5920 that would render sysseek() very slow).
5922 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5923 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
5924 C<seek>, C<tell>, or C<eof> may cause confusion.
5926 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5927 and C<SEEK_END> (start of the file, current position, end of the file)
5928 from the Fcntl module. Use of the constants is also more portable
5929 than relying on 0, 1, and 2. For example to define a "systell" function:
5931 use Fcntl 'SEEK_CUR';
5932 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5934 Returns the new position, or the undefined value on failure. A position
5935 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5936 true on success and false on failure, yet you can still easily determine
5941 =item system PROGRAM LIST
5943 Does exactly the same thing as C<exec LIST>, except that a fork is
5944 done first, and the parent process waits for the child process to
5945 complete. Note that argument processing varies depending on the
5946 number of arguments. If there is more than one argument in LIST,
5947 or if LIST is an array with more than one value, starts the program
5948 given by the first element of the list with arguments given by the
5949 rest of the list. If there is only one scalar argument, the argument
5950 is checked for shell metacharacters, and if there are any, the
5951 entire argument is passed to the system's command shell for parsing
5952 (this is C</bin/sh -c> on Unix platforms, but varies on other
5953 platforms). If there are no shell metacharacters in the argument,
5954 it is split into words and passed directly to C<execvp>, which is
5957 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5958 output before any operation that may do a fork, but this may not be
5959 supported on some platforms (see L<perlport>). To be safe, you may need
5960 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5961 of C<IO::Handle> on any open handles.
5963 The return value is the exit status of the program as returned by the
5964 C<wait> call. To get the actual exit value shift right by eight (see below).
5965 See also L</exec>. This is I<not> what you want to use to capture
5966 the output from a command, for that you should use merely backticks or
5967 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5968 indicates a failure to start the program (inspect $! for the reason).
5970 Like C<exec>, C<system> allows you to lie to a program about its name if
5971 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5973 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
5974 C<system>, if you expect your program to terminate on receipt of these
5975 signals you will need to arrange to do so yourself based on the return
5978 @args = ("command", "arg1", "arg2");
5980 or die "system @args failed: $?"
5982 You can check all the failure possibilities by inspecting
5986 print "failed to execute: $!\n";
5989 printf "child died with signal %d, %s coredump\n",
5990 ($? & 127), ($? & 128) ? 'with' : 'without';
5993 printf "child exited with value %d\n", $? >> 8;
5996 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
5997 with the W*() calls of the POSIX extension.
5999 When the arguments get executed via the system shell, results
6000 and return codes will be subject to its quirks and capabilities.
6001 See L<perlop/"`STRING`"> and L</exec> for details.
6003 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6005 =item syswrite FILEHANDLE,SCALAR,LENGTH
6007 =item syswrite FILEHANDLE,SCALAR
6009 Attempts to write LENGTH bytes of data from variable SCALAR to the
6010 specified FILEHANDLE, using the system call write(2). If LENGTH is
6011 not specified, writes whole SCALAR. It bypasses buffered IO, so
6012 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6013 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6014 stdio layers usually buffers data. Returns the number of bytes
6015 actually written, or C<undef> if there was an error (in this case the
6016 errno variable C<$!> is also set). If the LENGTH is greater than the
6017 available data in the SCALAR after the OFFSET, only as much data as is
6018 available will be written.
6020 An OFFSET may be specified to write the data from some part of the
6021 string other than the beginning. A negative OFFSET specifies writing
6022 that many characters counting backwards from the end of the string.
6023 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6025 Note that if the filehandle has been marked as C<:utf8>, Unicode
6026 characters are written instead of bytes (the LENGTH, OFFSET, and the
6027 return value of syswrite() are in UTF-8 encoded Unicode characters).
6028 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6029 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6031 =item tell FILEHANDLE
6035 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6036 error. FILEHANDLE may be an expression whose value gives the name of
6037 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6040 Note the I<in bytes>: even if the filehandle has been set to
6041 operate on characters (for example by using the C<:utf8> open
6042 layer), tell() will return byte offsets, not character offsets
6043 (because that would render seek() and tell() rather slow).
6045 The return value of tell() for the standard streams like the STDIN
6046 depends on the operating system: it may return -1 or something else.
6047 tell() on pipes, fifos, and sockets usually returns -1.
6049 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6051 Do not use tell() (or other buffered I/O operations) on a file handle
6052 that has been manipulated by sysread(), syswrite() or sysseek().
6053 Those functions ignore the buffering, while tell() does not.
6055 =item telldir DIRHANDLE
6057 Returns the current position of the C<readdir> routines on DIRHANDLE.
6058 Value may be given to C<seekdir> to access a particular location in a
6059 directory. Has the same caveats about possible directory compaction as
6060 the corresponding system library routine.
6062 =item tie VARIABLE,CLASSNAME,LIST
6064 This function binds a variable to a package class that will provide the
6065 implementation for the variable. VARIABLE is the name of the variable
6066 to be enchanted. CLASSNAME is the name of a class implementing objects
6067 of correct type. Any additional arguments are passed to the C<new>
6068 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6069 or C<TIEHASH>). Typically these are arguments such as might be passed
6070 to the C<dbm_open()> function of C. The object returned by the C<new>
6071 method is also returned by the C<tie> function, which would be useful
6072 if you want to access other methods in CLASSNAME.
6074 Note that functions such as C<keys> and C<values> may return huge lists
6075 when used on large objects, like DBM files. You may prefer to use the
6076 C<each> function to iterate over such. Example:
6078 # print out history file offsets
6080 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6081 while (($key,$val) = each %HIST) {
6082 print $key, ' = ', unpack('L',$val), "\n";
6086 A class implementing a hash should have the following methods:
6088 TIEHASH classname, LIST
6090 STORE this, key, value
6095 NEXTKEY this, lastkey
6100 A class implementing an ordinary array should have the following methods:
6102 TIEARRAY classname, LIST
6104 STORE this, key, value
6106 STORESIZE this, count
6112 SPLICE this, offset, length, LIST
6117 A class implementing a file handle should have the following methods:
6119 TIEHANDLE classname, LIST
6120 READ this, scalar, length, offset
6123 WRITE this, scalar, length, offset
6125 PRINTF this, format, LIST
6129 SEEK this, position, whence
6131 OPEN this, mode, LIST
6136 A class implementing a scalar should have the following methods:
6138 TIESCALAR classname, LIST
6144 Not all methods indicated above need be implemented. See L<perltie>,
6145 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6147 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6148 for you--you need to do that explicitly yourself. See L<DB_File>
6149 or the F<Config> module for interesting C<tie> implementations.
6151 For further details see L<perltie>, L<"tied VARIABLE">.
6155 Returns a reference to the object underlying VARIABLE (the same value
6156 that was originally returned by the C<tie> call that bound the variable
6157 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6162 Returns the number of non-leap seconds since whatever time the system
6163 considers to be the epoch, suitable for feeding to C<gmtime> and
6164 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6165 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6166 1904 in the current local time zone for its epoch.
6168 For measuring time in better granularity than one second,
6169 you may use either the Time::HiRes module (from CPAN, and starting from
6170 Perl 5.8 part of the standard distribution), or if you have
6171 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6172 See L<perlfaq8> for details.
6176 Returns a four-element list giving the user and system times, in
6177 seconds, for this process and the children of this process.
6179 ($user,$system,$cuser,$csystem) = times;
6181 In scalar context, C<times> returns C<$user>.
6185 The transliteration operator. Same as C<y///>. See L<perlop>.
6187 =item truncate FILEHANDLE,LENGTH
6189 =item truncate EXPR,LENGTH
6191 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6192 specified length. Produces a fatal error if truncate isn't implemented
6193 on your system. Returns true if successful, the undefined value
6196 The behavior is undefined if LENGTH is greater than the length of the
6203 Returns an uppercased version of EXPR. This is the internal function
6204 implementing the C<\U> escape in double-quoted strings. Respects
6205 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6206 and L<perlunicode> for more details about locale and Unicode support.
6207 It does not attempt to do titlecase mapping on initial letters. See
6208 C<ucfirst> for that.
6210 If EXPR is omitted, uses C<$_>.
6216 Returns the value of EXPR with the first character in uppercase
6217 (titlecase in Unicode). This is the internal function implementing
6218 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6219 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6220 for more details about locale and Unicode support.
6222 If EXPR is omitted, uses C<$_>.
6228 Sets the umask for the process to EXPR and returns the previous value.
6229 If EXPR is omitted, merely returns the current umask.
6231 The Unix permission C<rwxr-x---> is represented as three sets of three
6232 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6233 and isn't one of the digits). The C<umask> value is such a number
6234 representing disabled permissions bits. The permission (or "mode")
6235 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6236 even if you tell C<sysopen> to create a file with permissions C<0777>,
6237 if your umask is C<0022> then the file will actually be created with
6238 permissions C<0755>. If your C<umask> were C<0027> (group can't
6239 write; others can't read, write, or execute), then passing
6240 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6243 Here's some advice: supply a creation mode of C<0666> for regular
6244 files (in C<sysopen>) and one of C<0777> for directories (in
6245 C<mkdir>) and executable files. This gives users the freedom of
6246 choice: if they want protected files, they might choose process umasks
6247 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6248 Programs should rarely if ever make policy decisions better left to
6249 the user. The exception to this is when writing files that should be
6250 kept private: mail files, web browser cookies, I<.rhosts> files, and
6253 If umask(2) is not implemented on your system and you are trying to
6254 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6255 fatal error at run time. If umask(2) is not implemented and you are
6256 not trying to restrict access for yourself, returns C<undef>.
6258 Remember that a umask is a number, usually given in octal; it is I<not> a
6259 string of octal digits. See also L</oct>, if all you have is a string.
6265 Undefines the value of EXPR, which must be an lvalue. Use only on a
6266 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6267 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6268 will probably not do what you expect on most predefined variables or
6269 DBM list values, so don't do that; see L<delete>.) Always returns the
6270 undefined value. You can omit the EXPR, in which case nothing is
6271 undefined, but you still get an undefined value that you could, for
6272 instance, return from a subroutine, assign to a variable or pass as a
6273 parameter. Examples:
6276 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6280 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6281 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6282 select undef, undef, undef, 0.25;
6283 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6285 Note that this is a unary operator, not a list operator.
6291 Deletes a list of files. Returns the number of files successfully
6294 $cnt = unlink 'a', 'b', 'c';
6298 Note: C<unlink> will not delete directories unless you are superuser and
6299 the B<-U> flag is supplied to Perl. Even if these conditions are
6300 met, be warned that unlinking a directory can inflict damage on your
6301 filesystem. Use C<rmdir> instead.
6303 If LIST is omitted, uses C<$_>.
6305 =item unpack TEMPLATE,EXPR
6307 =item unpack TEMPLATE
6309 C<unpack> does the reverse of C<pack>: it takes a string
6310 and expands it out into a list of values.
6311 (In scalar context, it returns merely the first value produced.)
6313 If EXPR is omitted, unpacks the C<$_> string.
6315 The string is broken into chunks described by the TEMPLATE. Each chunk
6316 is converted separately to a value. Typically, either the string is a result
6317 of C<pack>, or the characters of the string represent a C structure of some
6320 The TEMPLATE has the same format as in the C<pack> function.
6321 Here's a subroutine that does substring:
6324 my($what,$where,$howmuch) = @_;
6325 unpack("x$where a$howmuch", $what);
6330 sub ordinal { unpack("W",$_[0]); } # same as ord()
6332 In addition to fields allowed in pack(), you may prefix a field with
6333 a %<number> to indicate that
6334 you want a <number>-bit checksum of the items instead of the items
6335 themselves. Default is a 16-bit checksum. Checksum is calculated by
6336 summing numeric values of expanded values (for string fields the sum of
6337 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6339 For example, the following
6340 computes the same number as the System V sum program:
6344 unpack("%32W*",<>) % 65535;
6347 The following efficiently counts the number of set bits in a bit vector:
6349 $setbits = unpack("%32b*", $selectmask);
6351 The C<p> and C<P> formats should be used with care. Since Perl
6352 has no way of checking whether the value passed to C<unpack()>
6353 corresponds to a valid memory location, passing a pointer value that's
6354 not known to be valid is likely to have disastrous consequences.
6356 If there are more pack codes or if the repeat count of a field or a group
6357 is larger than what the remainder of the input string allows, the result
6358 is not well defined: in some cases, the repeat count is decreased, or
6359 C<unpack()> will produce null strings or zeroes, or terminate with an
6360 error. If the input string is longer than one described by the TEMPLATE,
6361 the rest is ignored.
6363 See L</pack> for more examples and notes.
6365 =item untie VARIABLE
6367 Breaks the binding between a variable and a package. (See C<tie>.)
6368 Has no effect if the variable is not tied.
6370 =item unshift ARRAY,LIST
6372 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6373 depending on how you look at it. Prepends list to the front of the
6374 array, and returns the new number of elements in the array.
6376 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6378 Note the LIST is prepended whole, not one element at a time, so the
6379 prepended elements stay in the same order. Use C<reverse> to do the
6382 =item use Module VERSION LIST
6384 =item use Module VERSION
6386 =item use Module LIST
6392 Imports some semantics into the current package from the named module,
6393 generally by aliasing certain subroutine or variable names into your
6394 package. It is exactly equivalent to
6396 BEGIN { require Module; import Module LIST; }
6398 except that Module I<must> be a bareword.
6400 VERSION may be either a numeric argument such as 5.006, which will be
6401 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6402 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6403 greater than the version of the current Perl interpreter; Perl will not
6404 attempt to parse the rest of the file. Compare with L</require>, which can
6405 do a similar check at run time.
6407 Specifying VERSION as a literal of the form v5.6.1 should generally be
6408 avoided, because it leads to misleading error messages under earlier
6409 versions of Perl which do not support this syntax. The equivalent numeric
6410 version should be used instead.
6412 use v5.6.1; # compile time version check
6414 use 5.006_001; # ditto; preferred for backwards compatibility
6416 This is often useful if you need to check the current Perl version before
6417 C<use>ing library modules that have changed in incompatible ways from
6418 older versions of Perl. (We try not to do this more than we have to.)
6420 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6421 C<require> makes sure the module is loaded into memory if it hasn't been
6422 yet. The C<import> is not a builtin--it's just an ordinary static method
6423 call into the C<Module> package to tell the module to import the list of
6424 features back into the current package. The module can implement its
6425 C<import> method any way it likes, though most modules just choose to
6426 derive their C<import> method via inheritance from the C<Exporter> class that
6427 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6428 method can be found then the call is skipped, even if there is an AUTOLOAD
6431 If you do not want to call the package's C<import> method (for instance,
6432 to stop your namespace from being altered), explicitly supply the empty list:
6436 That is exactly equivalent to
6438 BEGIN { require Module }
6440 If the VERSION argument is present between Module and LIST, then the
6441 C<use> will call the VERSION method in class Module with the given
6442 version as an argument. The default VERSION method, inherited from
6443 the UNIVERSAL class, croaks if the given version is larger than the
6444 value of the variable C<$Module::VERSION>.
6446 Again, there is a distinction between omitting LIST (C<import> called
6447 with no arguments) and an explicit empty LIST C<()> (C<import> not
6448 called). Note that there is no comma after VERSION!
6450 Because this is a wide-open interface, pragmas (compiler directives)
6451 are also implemented this way. Currently implemented pragmas are:
6456 use sigtrap qw(SEGV BUS);
6457 use strict qw(subs vars refs);
6458 use subs qw(afunc blurfl);
6459 use warnings qw(all);
6460 use sort qw(stable _quicksort _mergesort);
6462 Some of these pseudo-modules import semantics into the current
6463 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6464 which import symbols into the current package (which are effective
6465 through the end of the file).
6467 There's a corresponding C<no> command that unimports meanings imported
6468 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6469 It behaves exactly as C<import> does with respect to VERSION, an
6470 omitted LIST, empty LIST, or no unimport method being found.
6476 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6477 for the C<-M> and C<-m> command-line options to perl that give C<use>
6478 functionality from the command-line.
6482 Changes the access and modification times on each file of a list of
6483 files. The first two elements of the list must be the NUMERICAL access
6484 and modification times, in that order. Returns the number of files
6485 successfully changed. The inode change time of each file is set
6486 to the current time. For example, this code has the same effect as the
6487 Unix touch(1) command when the files I<already exist> and belong to
6488 the user running the program:
6491 $atime = $mtime = time;
6492 utime $atime, $mtime, @ARGV;
6494 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6495 the utime(2) function in the C library will be called with a null second
6496 argument. On most systems, this will set the file's access and
6497 modification times to the current time (i.e. equivalent to the example
6498 above) and will even work on other users' files where you have write
6501 utime undef, undef, @ARGV;
6503 Under NFS this will use the time of the NFS server, not the time of
6504 the local machine. If there is a time synchronization problem, the
6505 NFS server and local machine will have different times. The Unix
6506 touch(1) command will in fact normally use this form instead of the
6507 one shown in the first example.
6509 Note that only passing one of the first two elements as C<undef> will
6510 be equivalent of passing it as 0 and will not have the same effect as
6511 described when they are both C<undef>. This case will also trigger an
6512 uninitialized warning.
6516 Returns a list consisting of all the values of the named hash.
6517 (In a scalar context, returns the number of values.)
6519 The values are returned in an apparently random order. The actual
6520 random order is subject to change in future versions of perl, but it
6521 is guaranteed to be the same order as either the C<keys> or C<each>
6522 function would produce on the same (unmodified) hash. Since Perl
6523 5.8.1 the ordering is different even between different runs of Perl
6524 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6526 As a side effect, calling values() resets the HASH's internal iterator,
6527 see L</each>. (In particular, calling values() in void context resets
6528 the iterator with no other overhead.)
6530 Note that the values are not copied, which means modifying them will
6531 modify the contents of the hash:
6533 for (values %hash) { s/foo/bar/g } # modifies %hash values
6534 for (@hash{keys %hash}) { s/foo/bar/g } # same
6536 See also C<keys>, C<each>, and C<sort>.
6538 =item vec EXPR,OFFSET,BITS
6540 Treats the string in EXPR as a bit vector made up of elements of
6541 width BITS, and returns the value of the element specified by OFFSET
6542 as an unsigned integer. BITS therefore specifies the number of bits
6543 that are reserved for each element in the bit vector. This must
6544 be a power of two from 1 to 32 (or 64, if your platform supports
6547 If BITS is 8, "elements" coincide with bytes of the input string.
6549 If BITS is 16 or more, bytes of the input string are grouped into chunks
6550 of size BITS/8, and each group is converted to a number as with
6551 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6552 for BITS==64). See L<"pack"> for details.
6554 If bits is 4 or less, the string is broken into bytes, then the bits
6555 of each byte are broken into 8/BITS groups. Bits of a byte are
6556 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6557 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6558 breaking the single input byte C<chr(0x36)> into two groups gives a list
6559 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6561 C<vec> may also be assigned to, in which case parentheses are needed
6562 to give the expression the correct precedence as in
6564 vec($image, $max_x * $x + $y, 8) = 3;
6566 If the selected element is outside the string, the value 0 is returned.
6567 If an element off the end of the string is written to, Perl will first
6568 extend the string with sufficiently many zero bytes. It is an error
6569 to try to write off the beginning of the string (i.e. negative OFFSET).
6571 The string should not contain any character with the value > 255 (which
6572 can only happen if you're using UTF-8 encoding). If it does, it will be
6573 treated as something which is not UTF-8 encoded. When the C<vec> was
6574 assigned to, other parts of your program will also no longer consider the
6575 string to be UTF-8 encoded. In other words, if you do have such characters
6576 in your string, vec() will operate on the actual byte string, and not the
6577 conceptual character string.
6579 Strings created with C<vec> can also be manipulated with the logical
6580 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6581 vector operation is desired when both operands are strings.
6582 See L<perlop/"Bitwise String Operators">.
6584 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6585 The comments show the string after each step. Note that this code works
6586 in the same way on big-endian or little-endian machines.
6589 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6591 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6592 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6594 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6595 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6596 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6597 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6598 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6599 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6601 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6602 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6603 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6606 To transform a bit vector into a string or list of 0's and 1's, use these:
6608 $bits = unpack("b*", $vector);
6609 @bits = split(//, unpack("b*", $vector));
6611 If you know the exact length in bits, it can be used in place of the C<*>.
6613 Here is an example to illustrate how the bits actually fall in place:
6619 unpack("V",$_) 01234567890123456789012345678901
6620 ------------------------------------------------------------------
6625 for ($shift=0; $shift < $width; ++$shift) {
6626 for ($off=0; $off < 32/$width; ++$off) {
6627 $str = pack("B*", "0"x32);
6628 $bits = (1<<$shift);
6629 vec($str, $off, $width) = $bits;
6630 $res = unpack("b*",$str);
6631 $val = unpack("V", $str);
6638 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6639 $off, $width, $bits, $val, $res
6643 Regardless of the machine architecture on which it is run, the above
6644 example should print the following table:
6647 unpack("V",$_) 01234567890123456789012345678901
6648 ------------------------------------------------------------------
6649 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6650 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6651 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6652 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6653 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6654 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6655 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6656 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6657 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6658 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6659 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6660 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6661 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6662 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6663 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6664 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6665 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6666 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6667 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6668 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6669 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6670 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6671 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6672 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6673 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6674 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6675 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6676 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6677 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6678 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6679 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6680 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6681 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6682 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6683 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6684 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6685 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6686 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6687 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6688 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6689 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6690 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6691 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6692 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6693 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6694 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6695 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6696 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6697 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6698 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6699 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6700 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6701 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6702 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6703 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6704 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6705 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6706 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6707 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6708 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6709 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6710 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6711 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6712 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6713 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6714 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6715 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6716 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6717 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6718 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6719 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6720 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6721 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6722 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6723 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6724 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6725 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6726 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6727 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6728 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6729 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6730 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6731 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6732 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6733 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6734 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6735 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6736 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6737 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6738 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6739 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6740 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6741 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6742 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6743 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6744 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6745 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6746 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6747 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6748 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6749 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6750 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6751 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6752 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6753 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6754 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6755 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6756 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6757 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6758 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6759 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6760 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6761 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6762 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6763 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6764 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6765 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6766 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6767 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6768 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6769 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6770 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6771 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6772 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6773 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6774 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6775 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6776 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6780 Behaves like the wait(2) system call on your system: it waits for a child
6781 process to terminate and returns the pid of the deceased process, or
6782 C<-1> if there are no child processes. The status is returned in C<$?>
6783 and C<{^CHILD_ERROR_NATIVE}>.
6784 Note that a return value of C<-1> could mean that child processes are
6785 being automatically reaped, as described in L<perlipc>.
6787 =item waitpid PID,FLAGS
6789 Waits for a particular child process to terminate and returns the pid of
6790 the deceased process, or C<-1> if there is no such child process. On some
6791 systems, a value of 0 indicates that there are processes still running.
6792 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
6794 use POSIX ":sys_wait_h";
6797 $kid = waitpid(-1, WNOHANG);
6800 then you can do a non-blocking wait for all pending zombie processes.
6801 Non-blocking wait is available on machines supporting either the
6802 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6803 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6804 system call by remembering the status values of processes that have
6805 exited but have not been harvested by the Perl script yet.)
6807 Note that on some systems, a return value of C<-1> could mean that child
6808 processes are being automatically reaped. See L<perlipc> for details,
6809 and for other examples.
6813 Returns true if the context of the currently executing subroutine or
6814 C<eval> is looking for a list value. Returns false if the context is
6815 looking for a scalar. Returns the undefined value if the context is
6816 looking for no value (void context).
6818 return unless defined wantarray; # don't bother doing more
6819 my @a = complex_calculation();
6820 return wantarray ? @a : "@a";
6822 C<wantarray()>'s result is unspecified in the top level of a file,
6823 in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
6826 This function should have been named wantlist() instead.
6830 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6833 If LIST is empty and C<$@> already contains a value (typically from a
6834 previous eval) that value is used after appending C<"\t...caught">
6835 to C<$@>. This is useful for staying almost, but not entirely similar to
6838 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6840 No message is printed if there is a C<$SIG{__WARN__}> handler
6841 installed. It is the handler's responsibility to deal with the message
6842 as it sees fit (like, for instance, converting it into a C<die>). Most
6843 handlers must therefore make arrangements to actually display the
6844 warnings that they are not prepared to deal with, by calling C<warn>
6845 again in the handler. Note that this is quite safe and will not
6846 produce an endless loop, since C<__WARN__> hooks are not called from
6849 You will find this behavior is slightly different from that of
6850 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6851 instead call C<die> again to change it).
6853 Using a C<__WARN__> handler provides a powerful way to silence all
6854 warnings (even the so-called mandatory ones). An example:
6856 # wipe out *all* compile-time warnings
6857 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6859 my $foo = 20; # no warning about duplicate my $foo,
6860 # but hey, you asked for it!
6861 # no compile-time or run-time warnings before here
6864 # run-time warnings enabled after here
6865 warn "\$foo is alive and $foo!"; # does show up
6867 See L<perlvar> for details on setting C<%SIG> entries, and for more
6868 examples. See the Carp module for other kinds of warnings using its
6869 carp() and cluck() functions.
6871 =item write FILEHANDLE
6877 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6878 using the format associated with that file. By default the format for
6879 a file is the one having the same name as the filehandle, but the
6880 format for the current output channel (see the C<select> function) may be set
6881 explicitly by assigning the name of the format to the C<$~> variable.
6883 Top of form processing is handled automatically: if there is
6884 insufficient room on the current page for the formatted record, the
6885 page is advanced by writing a form feed, a special top-of-page format
6886 is used to format the new page header, and then the record is written.
6887 By default the top-of-page format is the name of the filehandle with
6888 "_TOP" appended, but it may be dynamically set to the format of your
6889 choice by assigning the name to the C<$^> variable while the filehandle is
6890 selected. The number of lines remaining on the current page is in
6891 variable C<$->, which can be set to C<0> to force a new page.
6893 If FILEHANDLE is unspecified, output goes to the current default output
6894 channel, which starts out as STDOUT but may be changed by the
6895 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6896 is evaluated and the resulting string is used to look up the name of
6897 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6899 Note that write is I<not> the opposite of C<read>. Unfortunately.
6903 The transliteration operator. Same as C<tr///>. See L<perlop>.