4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides a scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, the scalar arguments will
18 be first, and the list argument will follow. (Note that there can ever
19 be only one such list argument.) For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for the elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use the parentheses, the simple (but occasionally
34 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
35 function, and precedence doesn't matter. Otherwise it's a list
36 operator or unary operator, and precedence does matter. And whitespace
37 between the function and left parenthesis doesn't count--so you need to
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
52 A few functions take no arguments at all, and therefore work as neither
53 unary nor list operators. These include such functions as C<time>
54 and C<endpwent>. For example, C<time+86_400> always means
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in a scalar context by
59 returning the undefined value, and in a list context by returning the
62 Remember the following important rule: There is B<no rule> that relates
63 the behavior of an expression in list context to its behavior in scalar
64 context, or vice versa. It might do two totally different things.
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in scalar context. Some operators return the
67 length of the list that would have been returned in list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
74 A named array in scalar context is quite different from what would at
75 first glance appear to be a list in scalar context. You can't get a list
76 like C<(1,2,3)> into being in scalar context, because the compiler knows
77 the context at compile time. It would generate the scalar comma operator
78 there, not the list construction version of the comma. That means it
79 was never a list to start with.
81 In general, functions in Perl that serve as wrappers for system calls
82 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule are C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 =head2 Perl Functions by Category
92 Here are Perl's functions (including things that look like
93 functions, like some keywords and named operators)
94 arranged by category. Some functions appear in more
99 =item Functions for SCALARs or strings
100 X<scalar> X<string> X<character>
102 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
103 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
104 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
106 =item Regular expressions and pattern matching
107 X<regular expression> X<regex> X<regexp>
109 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
111 =item Numeric functions
112 X<numeric> X<number> X<trigonometric> X<trigonometry>
114 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115 C<sin>, C<sqrt>, C<srand>
117 =item Functions for real @ARRAYs
120 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
122 =item Functions for list data
125 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
127 =item Functions for real %HASHes
130 C<delete>, C<each>, C<exists>, C<keys>, C<values>
132 =item Input and output functions
133 X<I/O> X<input> X<output> X<dbm>
135 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
137 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
138 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
141 =item Functions for fixed length data or records
143 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
145 =item Functions for filehandles, files, or directories
146 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
148 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
149 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
150 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151 C<umask>, C<unlink>, C<utime>
153 =item Keywords related to the control flow of your Perl program
156 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
159 =item Keywords related to switch
161 C<break>, C<continue>, C<given>, C<when>, C<default>
163 (These are only available if you enable the "switch" feature.
164 See L<feature> and L<perlsyn/"Switch statements">.)
166 =item Keywords related to scoping
168 C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
171 (C<state> is only available if the "state" feature is enabled. See
174 =item Miscellaneous functions
176 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
177 C<state>, C<reset>, C<scalar>, C<undef>, C<wantarray>
179 =item Functions for processes and process groups
180 X<process> X<pid> X<process id>
182 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
183 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
184 C<times>, C<wait>, C<waitpid>
186 =item Keywords related to perl modules
189 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
191 =item Keywords related to classes and object-orientedness
192 X<object> X<class> X<package>
194 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
197 =item Low-level socket functions
200 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
202 C<socket>, C<socketpair>
204 =item System V interprocess communication functions
205 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
207 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
210 =item Fetching user and group info
211 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
213 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215 C<getpwuid>, C<setgrent>, C<setpwent>
217 =item Fetching network info
218 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
220 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224 C<setnetent>, C<setprotoent>, C<setservent>
226 =item Time-related functions
229 C<gmtime>, C<localtime>, C<time>, C<times>
231 =item Functions new in perl5
234 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
235 C<import>, C<lc>, C<lcfirst>, C<lock>, C<map>, C<my>, C<no>, C<our>,
237 C<prototype>, C<qr>, C<qw>, C<qx>, C<readline>, C<readpipe>, C<ref>,
238 C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>,
239 C<break>, C<continue>, C<given>, C<when>, C<default>
241 * - C<sub> was a keyword in perl4, but in perl5 it is an
242 operator, which can be used in expressions.
244 =item Functions obsoleted in perl5
246 C<dbmclose>, C<dbmopen>
251 X<portability> X<Unix> X<portable>
253 Perl was born in Unix and can therefore access all common Unix
254 system calls. In non-Unix environments, the functionality of some
255 Unix system calls may not be available, or details of the available
256 functionality may differ slightly. The Perl functions affected
259 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
260 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
261 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
262 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
263 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
264 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
265 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
266 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
267 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
268 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
269 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
270 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
271 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
272 C<shmwrite>, C<socket>, C<socketpair>,
273 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
274 C<times>, C<truncate>, C<umask>, C<unlink>,
275 C<utime>, C<wait>, C<waitpid>
277 For more information about the portability of these functions, see
278 L<perlport> and other available platform-specific documentation.
280 =head2 Alphabetical Listing of Perl Functions
285 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>
286 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
294 A file test, where X is one of the letters listed below. This unary
295 operator takes one argument, either a filename, a filehandle, or a dirhandle,
296 and tests the associated file to see if something is true about it. If the
297 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
298 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
299 the undefined value if the file doesn't exist. Despite the funny
300 names, precedence is the same as any other named unary operator, and
301 the argument may be parenthesized like any other unary operator. The
302 operator may be any of:
304 -r File is readable by effective uid/gid.
305 -w File is writable by effective uid/gid.
306 -x File is executable by effective uid/gid.
307 -o File is owned by effective uid.
309 -R File is readable by real uid/gid.
310 -W File is writable by real uid/gid.
311 -X File is executable by real uid/gid.
312 -O File is owned by real uid.
315 -z File has zero size (is empty).
316 -s File has nonzero size (returns size in bytes).
318 -f File is a plain file.
319 -d File is a directory.
320 -l File is a symbolic link.
321 -p File is a named pipe (FIFO), or Filehandle is a pipe.
323 -b File is a block special file.
324 -c File is a character special file.
325 -t Filehandle is opened to a tty.
327 -u File has setuid bit set.
328 -g File has setgid bit set.
329 -k File has sticky bit set.
331 -T File is an ASCII text file (heuristic guess).
332 -B File is a "binary" file (opposite of -T).
334 -M Script start time minus file modification time, in days.
335 -A Same for access time.
336 -C Same for inode change time (Unix, may differ for other platforms)
342 next unless -f $_; # ignore specials
346 The interpretation of the file permission operators C<-r>, C<-R>,
347 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
348 of the file and the uids and gids of the user. There may be other
349 reasons you can't actually read, write, or execute the file. Such
350 reasons may be for example network filesystem access controls, ACLs
351 (access control lists), read-only filesystems, and unrecognized
354 Also note that, for the superuser on the local filesystems, the C<-r>,
355 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
356 if any execute bit is set in the mode. Scripts run by the superuser
357 may thus need to do a stat() to determine the actual mode of the file,
358 or temporarily set their effective uid to something else.
360 If you are using ACLs, there is a pragma called C<filetest> that may
361 produce more accurate results than the bare stat() mode bits.
362 When under the C<use filetest 'access'> the above-mentioned filetests
363 will test whether the permission can (not) be granted using the
364 access() family of system calls. Also note that the C<-x> and C<-X> may
365 under this pragma return true even if there are no execute permission
366 bits set (nor any extra execute permission ACLs). This strangeness is
367 due to the underlying system calls' definitions. Read the
368 documentation for the C<filetest> pragma for more information.
370 Note that C<-s/a/b/> does not do a negated substitution. Saying
371 C<-exp($foo)> still works as expected, however--only single letters
372 following a minus are interpreted as file tests.
374 The C<-T> and C<-B> switches work as follows. The first block or so of the
375 file is examined for odd characters such as strange control codes or
376 characters with the high bit set. If too many strange characters (>30%)
377 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
378 containing null in the first block is considered a binary file. If C<-T>
379 or C<-B> is used on a filehandle, the current IO buffer is examined
380 rather than the first block. Both C<-T> and C<-B> return true on a null
381 file, or a file at EOF when testing a filehandle. Because you have to
382 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
383 against the file first, as in C<next unless -f $file && -T $file>.
385 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
386 the special filehandle consisting of a solitary underline, then the stat
387 structure of the previous file test (or stat operator) is used, saving
388 a system call. (This doesn't work with C<-t>, and you need to remember
389 that lstat() and C<-l> will leave values in the stat structure for the
390 symbolic link, not the real file.) (Also, if the stat buffer was filled by
391 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
394 print "Can do.\n" if -r $a || -w _ || -x _;
397 print "Readable\n" if -r _;
398 print "Writable\n" if -w _;
399 print "Executable\n" if -x _;
400 print "Setuid\n" if -u _;
401 print "Setgid\n" if -g _;
402 print "Sticky\n" if -k _;
403 print "Text\n" if -T _;
404 print "Binary\n" if -B _;
406 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
407 test operators, in a way that C<-f -w -x $file> is equivalent to
408 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
409 the return value of C<-f $file> as an argument to another filetest
410 operator, no special magic will happen.)
417 Returns the absolute value of its argument.
418 If VALUE is omitted, uses C<$_>.
420 =item accept NEWSOCKET,GENERICSOCKET
423 Accepts an incoming socket connect, just as the accept(2) system call
424 does. Returns the packed address if it succeeded, false otherwise.
425 See the example in L<perlipc/"Sockets: Client/Server Communication">.
427 On systems that support a close-on-exec flag on files, the flag will
428 be set for the newly opened file descriptor, as determined by the
429 value of $^F. See L<perlvar/$^F>.
438 Arranges to have a SIGALRM delivered to this process after the
439 specified number of wallclock seconds has elapsed. If SECONDS is not
440 specified, the value stored in C<$_> is used. (On some machines,
441 unfortunately, the elapsed time may be up to one second less or more
442 than you specified because of how seconds are counted, and process
443 scheduling may delay the delivery of the signal even further.)
445 Only one timer may be counting at once. Each call disables the
446 previous timer, and an argument of C<0> may be supplied to cancel the
447 previous timer without starting a new one. The returned value is the
448 amount of time remaining on the previous timer.
450 For delays of finer granularity than one second, you may use Perl's
451 four-argument version of select() leaving the first three arguments
452 undefined, or you might be able to use the C<syscall> interface to
453 access setitimer(2) if your system supports it. The Time::HiRes
454 module (from CPAN, and starting from Perl 5.8 part of the standard
455 distribution) may also prove useful.
457 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
458 (C<sleep> may be internally implemented in your system with C<alarm>)
460 If you want to use C<alarm> to time out a system call you need to use an
461 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
462 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
463 restart system calls on some systems. Using C<eval>/C<die> always works,
464 modulo the caveats given in L<perlipc/"Signals">.
467 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
469 $nread = sysread SOCKET, $buffer, $size;
473 die unless $@ eq "alarm\n"; # propagate unexpected errors
480 For more information see L<perlipc>.
483 X<atan2> X<arctangent> X<tan> X<tangent>
485 Returns the arctangent of Y/X in the range -PI to PI.
487 For the tangent operation, you may use the C<Math::Trig::tan>
488 function, or use the familiar relation:
490 sub tan { sin($_[0]) / cos($_[0]) }
492 Note that atan2(0, 0) is not well-defined.
494 =item bind SOCKET,NAME
497 Binds a network address to a socket, just as the bind system call
498 does. Returns true if it succeeded, false otherwise. NAME should be a
499 packed address of the appropriate type for the socket. See the examples in
500 L<perlipc/"Sockets: Client/Server Communication">.
502 =item binmode FILEHANDLE, LAYER
503 X<binmode> X<binary> X<text> X<DOS> X<Windows>
505 =item binmode FILEHANDLE
507 Arranges for FILEHANDLE to be read or written in "binary" or "text"
508 mode on systems where the run-time libraries distinguish between
509 binary and text files. If FILEHANDLE is an expression, the value is
510 taken as the name of the filehandle. Returns true on success,
511 otherwise it returns C<undef> and sets C<$!> (errno).
513 On some systems (in general, DOS and Windows-based systems) binmode()
514 is necessary when you're not working with a text file. For the sake
515 of portability it is a good idea to always use it when appropriate,
516 and to never use it when it isn't appropriate. Also, people can
517 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
519 In other words: regardless of platform, use binmode() on binary data,
520 like for example images.
522 If LAYER is present it is a single string, but may contain multiple
523 directives. The directives alter the behaviour of the file handle.
524 When LAYER is present using binmode on text file makes sense.
526 If LAYER is omitted or specified as C<:raw> the filehandle is made
527 suitable for passing binary data. This includes turning off possible CRLF
528 translation and marking it as bytes (as opposed to Unicode characters).
529 Note that, despite what may be implied in I<"Programming Perl"> (the
530 Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
531 -- other layers which would affect binary nature of the stream are
532 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
533 PERLIO environment variable.
535 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
536 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
537 establish default I/O layers. See L<open>.
539 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
540 in "Programming Perl, 3rd Edition". However, since the publishing of this
541 book, by many known as "Camel III", the consensus of the naming of this
542 functionality has moved from "discipline" to "layer". All documentation
543 of this version of Perl therefore refers to "layers" rather than to
544 "disciplines". Now back to the regularly scheduled documentation...>
546 To mark FILEHANDLE as UTF-8, use C<:utf8>.
548 In general, binmode() should be called after open() but before any I/O
549 is done on the filehandle. Calling binmode() will normally flush any
550 pending buffered output data (and perhaps pending input data) on the
551 handle. An exception to this is the C<:encoding> layer that
552 changes the default character encoding of the handle, see L<open>.
553 The C<:encoding> layer sometimes needs to be called in
554 mid-stream, and it doesn't flush the stream. The C<:encoding>
555 also implicitly pushes on top of itself the C<:utf8> layer because
556 internally Perl will operate on UTF-8 encoded Unicode characters.
558 The operating system, device drivers, C libraries, and Perl run-time
559 system all work together to let the programmer treat a single
560 character (C<\n>) as the line terminator, irrespective of the external
561 representation. On many operating systems, the native text file
562 representation matches the internal representation, but on some
563 platforms the external representation of C<\n> is made up of more than
566 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
567 character to end each line in the external representation of text (even
568 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
569 on Unix and most VMS files). In other systems like OS/2, DOS and the
570 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
571 but what's stored in text files are the two characters C<\cM\cJ>. That
572 means that, if you don't use binmode() on these systems, C<\cM\cJ>
573 sequences on disk will be converted to C<\n> on input, and any C<\n> in
574 your program will be converted back to C<\cM\cJ> on output. This is what
575 you want for text files, but it can be disastrous for binary files.
577 Another consequence of using binmode() (on some systems) is that
578 special end-of-file markers will be seen as part of the data stream.
579 For systems from the Microsoft family this means that if your binary
580 data contains C<\cZ>, the I/O subsystem will regard it as the end of
581 the file, unless you use binmode().
583 binmode() is not only important for readline() and print() operations,
584 but also when using read(), seek(), sysread(), syswrite() and tell()
585 (see L<perlport> for more details). See the C<$/> and C<$\> variables
586 in L<perlvar> for how to manually set your input and output
587 line-termination sequences.
589 =item bless REF,CLASSNAME
594 This function tells the thingy referenced by REF that it is now an object
595 in the CLASSNAME package. If CLASSNAME is omitted, the current package
596 is used. Because a C<bless> is often the last thing in a constructor,
597 it returns the reference for convenience. Always use the two-argument
598 version if a derived class might inherit the function doing the blessing.
599 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
602 Consider always blessing objects in CLASSNAMEs that are mixed case.
603 Namespaces with all lowercase names are considered reserved for
604 Perl pragmata. Builtin types have all uppercase names. To prevent
605 confusion, you may wish to avoid such package names as well. Make sure
606 that CLASSNAME is a true value.
608 See L<perlmod/"Perl Modules">.
612 Break out of a C<given()> block.
614 This keyword is enabled by the "switch" feature: see L<feature>
615 for more information.
618 X<caller> X<call stack> X<stack> X<stack trace>
622 Returns the context of the current subroutine call. In scalar context,
623 returns the caller's package name if there is a caller, that is, if
624 we're in a subroutine or C<eval> or C<require>, and the undefined value
625 otherwise. In list context, returns
627 ($package, $filename, $line) = caller;
629 With EXPR, it returns some extra information that the debugger uses to
630 print a stack trace. The value of EXPR indicates how many call frames
631 to go back before the current one.
633 ($package, $filename, $line, $subroutine, $hasargs,
634 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
637 Here $subroutine may be C<(eval)> if the frame is not a subroutine
638 call, but an C<eval>. In such a case additional elements $evaltext and
639 C<$is_require> are set: C<$is_require> is true if the frame is created by a
640 C<require> or C<use> statement, $evaltext contains the text of the
641 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
642 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
643 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
644 frame.) $subroutine may also be C<(unknown)> if this particular
645 subroutine happens to have been deleted from the symbol table.
646 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
647 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
648 compiled with. The C<$hints> and C<$bitmask> values are subject to change
649 between versions of Perl, and are not meant for external use.
651 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
652 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
653 of this hash, as they are the actual values stored in the optree.
655 Furthermore, when called from within the DB package, caller returns more
656 detailed information: it sets the list variable C<@DB::args> to be the
657 arguments with which the subroutine was invoked.
659 Be aware that the optimizer might have optimized call frames away before
660 C<caller> had a chance to get the information. That means that C<caller(N)>
661 might not return information about the call frame you expect it do, for
662 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
663 previous time C<caller> was called.
669 =item chdir FILEHANDLE
671 =item chdir DIRHANDLE
675 Changes the working directory to EXPR, if possible. If EXPR is omitted,
676 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
677 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
678 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
679 neither is set, C<chdir> does nothing. It returns true upon success,
680 false otherwise. See the example under C<die>.
682 On systems that support fchdir, you might pass a file handle or
683 directory handle as argument. On systems that don't support fchdir,
684 passing handles produces a fatal error at run time.
687 X<chmod> X<permission> X<mode>
689 Changes the permissions of a list of files. The first element of the
690 list must be the numerical mode, which should probably be an octal
691 number, and which definitely should I<not> be a string of octal digits:
692 C<0644> is okay, C<'0644'> is not. Returns the number of files
693 successfully changed. See also L</oct>, if all you have is a string.
695 $cnt = chmod 0755, 'foo', 'bar';
696 chmod 0755, @executables;
697 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
699 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
700 $mode = 0644; chmod $mode, 'foo'; # this is best
702 On systems that support fchmod, you might pass file handles among the
703 files. On systems that don't support fchmod, passing file handles
704 produces a fatal error at run time. The file handles must be passed
705 as globs or references to be recognized. Barewords are considered
708 open(my $fh, "<", "foo");
709 my $perm = (stat $fh)[2] & 07777;
710 chmod($perm | 0600, $fh);
712 You can also import the symbolic C<S_I*> constants from the Fcntl
717 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
718 # This is identical to the chmod 0755 of the above example.
721 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
727 This safer version of L</chop> removes any trailing string
728 that corresponds to the current value of C<$/> (also known as
729 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
730 number of characters removed from all its arguments. It's often used to
731 remove the newline from the end of an input record when you're worried
732 that the final record may be missing its newline. When in paragraph
733 mode (C<$/ = "">), it removes all trailing newlines from the string.
734 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
735 a reference to an integer or the like, see L<perlvar>) chomp() won't
737 If VARIABLE is omitted, it chomps C<$_>. Example:
740 chomp; # avoid \n on last field
745 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
747 You can actually chomp anything that's an lvalue, including an assignment:
750 chomp($answer = <STDIN>);
752 If you chomp a list, each element is chomped, and the total number of
753 characters removed is returned.
755 If the C<encoding> pragma is in scope then the lengths returned are
756 calculated from the length of C<$/> in Unicode characters, which is not
757 always the same as the length of C<$/> in the native encoding.
759 Note that parentheses are necessary when you're chomping anything
760 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
761 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
762 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
763 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
773 Chops off the last character of a string and returns the character
774 chopped. It is much more efficient than C<s/.$//s> because it neither
775 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
776 If VARIABLE is a hash, it chops the hash's values, but not its keys.
778 You can actually chop anything that's an lvalue, including an assignment.
780 If you chop a list, each element is chopped. Only the value of the
781 last C<chop> is returned.
783 Note that C<chop> returns the last character. To return all but the last
784 character, use C<substr($string, 0, -1)>.
789 X<chown> X<owner> X<user> X<group>
791 Changes the owner (and group) of a list of files. The first two
792 elements of the list must be the I<numeric> uid and gid, in that
793 order. A value of -1 in either position is interpreted by most
794 systems to leave that value unchanged. Returns the number of files
795 successfully changed.
797 $cnt = chown $uid, $gid, 'foo', 'bar';
798 chown $uid, $gid, @filenames;
800 On systems that support fchown, you might pass file handles among the
801 files. On systems that don't support fchown, passing file handles
802 produces a fatal error at run time. The file handles must be passed
803 as globs or references to be recognized. Barewords are considered
806 Here's an example that looks up nonnumeric uids in the passwd file:
809 chomp($user = <STDIN>);
811 chomp($pattern = <STDIN>);
813 ($login,$pass,$uid,$gid) = getpwnam($user)
814 or die "$user not in passwd file";
816 @ary = glob($pattern); # expand filenames
817 chown $uid, $gid, @ary;
819 On most systems, you are not allowed to change the ownership of the
820 file unless you're the superuser, although you should be able to change
821 the group to any of your secondary groups. On insecure systems, these
822 restrictions may be relaxed, but this is not a portable assumption.
823 On POSIX systems, you can detect this condition this way:
825 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
826 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
829 X<chr> X<character> X<ASCII> X<Unicode>
833 Returns the character represented by that NUMBER in the character set.
834 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
835 chr(0x263a) is a Unicode smiley face. Note that characters from 128
836 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
837 backward compatibility reasons (but see L<encoding>).
839 Negative values give the Unicode replacement character (chr(0xfffd)),
840 except under the L<bytes> pragma, where low eight bits of the value
841 (truncated to an integer) are used.
843 If NUMBER is omitted, uses C<$_>.
845 For the reverse, use L</ord>.
847 Note that under the C<bytes> pragma the NUMBER is masked to
850 See L<perlunicode> and L<encoding> for more about Unicode.
852 =item chroot FILENAME
857 This function works like the system call by the same name: it makes the
858 named directory the new root directory for all further pathnames that
859 begin with a C</> by your process and all its children. (It doesn't
860 change your current working directory, which is unaffected.) For security
861 reasons, this call is restricted to the superuser. If FILENAME is
862 omitted, does a C<chroot> to C<$_>.
864 =item close FILEHANDLE
869 Closes the file or pipe associated with the file handle, returning
870 true only if IO buffers are successfully flushed and closes the system
871 file descriptor. Closes the currently selected filehandle if the
874 You don't have to close FILEHANDLE if you are immediately going to do
875 another C<open> on it, because C<open> will close it for you. (See
876 C<open>.) However, an explicit C<close> on an input file resets the line
877 counter (C<$.>), while the implicit close done by C<open> does not.
879 If the file handle came from a piped open, C<close> will additionally
880 return false if one of the other system calls involved fails, or if the
881 program exits with non-zero status. (If the only problem was that the
882 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
883 also waits for the process executing on the pipe to complete, in case you
884 want to look at the output of the pipe afterwards, and
885 implicitly puts the exit status value of that command into C<$?> and
886 C<${^CHILD_ERROR_NATIVE}>.
888 Prematurely closing the read end of a pipe (i.e. before the process
889 writing to it at the other end has closed it) will result in a
890 SIGPIPE being delivered to the writer. If the other end can't
891 handle that, be sure to read all the data before closing the pipe.
895 open(OUTPUT, '|sort >foo') # pipe to sort
896 or die "Can't start sort: $!";
897 #... # print stuff to output
898 close OUTPUT # wait for sort to finish
899 or warn $! ? "Error closing sort pipe: $!"
900 : "Exit status $? from sort";
901 open(INPUT, 'foo') # get sort's results
902 or die "Can't open 'foo' for input: $!";
904 FILEHANDLE may be an expression whose value can be used as an indirect
905 filehandle, usually the real filehandle name.
907 =item closedir DIRHANDLE
910 Closes a directory opened by C<opendir> and returns the success of that
913 =item connect SOCKET,NAME
916 Attempts to connect to a remote socket, just as the connect system call
917 does. Returns true if it succeeded, false otherwise. NAME should be a
918 packed address of the appropriate type for the socket. See the examples in
919 L<perlipc/"Sockets: Client/Server Communication">.
926 C<continue> is actually a flow control statement rather than a function. If
927 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
928 C<foreach>), it is always executed just before the conditional is about to
929 be evaluated again, just like the third part of a C<for> loop in C. Thus
930 it can be used to increment a loop variable, even when the loop has been
931 continued via the C<next> statement (which is similar to the C C<continue>
934 C<last>, C<next>, or C<redo> may appear within a C<continue>
935 block. C<last> and C<redo> will behave as if they had been executed within
936 the main block. So will C<next>, but since it will execute a C<continue>
937 block, it may be more entertaining.
940 ### redo always comes here
943 ### next always comes here
945 # then back the top to re-check EXPR
947 ### last always comes here
949 Omitting the C<continue> section is semantically equivalent to using an
950 empty one, logically enough. In that case, C<next> goes directly back
951 to check the condition at the top of the loop.
953 If the "switch" feature is enabled, C<continue> is also a
954 function that will break out of the current C<when> or C<default>
955 block, and fall through to the next case. See L<feature> and
956 L<perlsyn/"Switch statements"> for more information.
960 X<cos> X<cosine> X<acos> X<arccosine>
964 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
965 takes cosine of C<$_>.
967 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
968 function, or use this relation:
970 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
972 =item crypt PLAINTEXT,SALT
973 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
974 X<decrypt> X<cryptography> X<passwd>
976 Creates a digest string exactly like the crypt(3) function in the C
977 library (assuming that you actually have a version there that has not
978 been extirpated as a potential munitions).
980 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
981 into a short string, called a digest, which is returned. The same
982 PLAINTEXT and SALT will always return the same string, but there is no
983 (known) way to get the original PLAINTEXT from the hash. Small
984 changes in the PLAINTEXT or SALT will result in large changes in the
987 There is no decrypt function. This function isn't all that useful for
988 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
989 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
990 primarily used to check if two pieces of text are the same without
991 having to transmit or store the text itself. An example is checking
992 if a correct password is given. The digest of the password is stored,
993 not the password itself. The user types in a password that is
994 crypt()'d with the same salt as the stored digest. If the two digests
995 match the password is correct.
997 When verifying an existing digest string you should use the digest as
998 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
999 to create the digest is visible as part of the digest. This ensures
1000 crypt() will hash the new string with the same salt as the digest.
1001 This allows your code to work with the standard L<crypt|/crypt> and
1002 with more exotic implementations. In other words, do not assume
1003 anything about the returned string itself, or how many bytes in the
1006 Traditionally the result is a string of 13 bytes: two first bytes of
1007 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1008 the first eight bytes of the digest string mattered, but alternative
1009 hashing schemes (like MD5), higher level security schemes (like C2),
1010 and implementations on non-UNIX platforms may produce different
1013 When choosing a new salt create a random two character string whose
1014 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1015 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1016 characters is just a recommendation; the characters allowed in
1017 the salt depend solely on your system's crypt library, and Perl can't
1018 restrict what salts C<crypt()> accepts.
1020 Here's an example that makes sure that whoever runs this program knows
1023 $pwd = (getpwuid($<))[1];
1025 system "stty -echo";
1027 chomp($word = <STDIN>);
1031 if (crypt($word, $pwd) ne $pwd) {
1037 Of course, typing in your own password to whoever asks you
1040 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1041 of data, not least of all because you can't get the information
1042 back. Look at the L<Digest> module for more robust algorithms.
1044 If using crypt() on a Unicode string (which I<potentially> has
1045 characters with codepoints above 255), Perl tries to make sense
1046 of the situation by trying to downgrade (a copy of the string)
1047 the string back to an eight-bit byte string before calling crypt()
1048 (on that copy). If that works, good. If not, crypt() dies with
1049 C<Wide character in crypt>.
1054 [This function has been largely superseded by the C<untie> function.]
1056 Breaks the binding between a DBM file and a hash.
1058 =item dbmopen HASH,DBNAME,MASK
1059 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1061 [This function has been largely superseded by the C<tie> function.]
1063 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1064 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1065 argument is I<not> a filehandle, even though it looks like one). DBNAME
1066 is the name of the database (without the F<.dir> or F<.pag> extension if
1067 any). If the database does not exist, it is created with protection
1068 specified by MASK (as modified by the C<umask>). If your system supports
1069 only the older DBM functions, you may perform only one C<dbmopen> in your
1070 program. In older versions of Perl, if your system had neither DBM nor
1071 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1074 If you don't have write access to the DBM file, you can only read hash
1075 variables, not set them. If you want to test whether you can write,
1076 either use file tests or try setting a dummy hash entry inside an C<eval>,
1077 which will trap the error.
1079 Note that functions such as C<keys> and C<values> may return huge lists
1080 when used on large DBM files. You may prefer to use the C<each>
1081 function to iterate over large DBM files. Example:
1083 # print out history file offsets
1084 dbmopen(%HIST,'/usr/lib/news/history',0666);
1085 while (($key,$val) = each %HIST) {
1086 print $key, ' = ', unpack('L',$val), "\n";
1090 See also L<AnyDBM_File> for a more general description of the pros and
1091 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1092 rich implementation.
1094 You can control which DBM library you use by loading that library
1095 before you call dbmopen():
1098 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1099 or die "Can't open netscape history file: $!";
1102 X<defined> X<undef> X<undefined>
1106 Returns a Boolean value telling whether EXPR has a value other than
1107 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1110 Many operations return C<undef> to indicate failure, end of file,
1111 system error, uninitialized variable, and other exceptional
1112 conditions. This function allows you to distinguish C<undef> from
1113 other values. (A simple Boolean test will not distinguish among
1114 C<undef>, zero, the empty string, and C<"0">, which are all equally
1115 false.) Note that since C<undef> is a valid scalar, its presence
1116 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1117 returns C<undef> when its argument is an empty array, I<or> when the
1118 element to return happens to be C<undef>.
1120 You may also use C<defined(&func)> to check whether subroutine C<&func>
1121 has ever been defined. The return value is unaffected by any forward
1122 declarations of C<&func>. Note that a subroutine which is not defined
1123 may still be callable: its package may have an C<AUTOLOAD> method that
1124 makes it spring into existence the first time that it is called -- see
1127 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1128 used to report whether memory for that aggregate has ever been
1129 allocated. This behavior may disappear in future versions of Perl.
1130 You should instead use a simple test for size:
1132 if (@an_array) { print "has array elements\n" }
1133 if (%a_hash) { print "has hash members\n" }
1135 When used on a hash element, it tells you whether the value is defined,
1136 not whether the key exists in the hash. Use L</exists> for the latter
1141 print if defined $switch{'D'};
1142 print "$val\n" while defined($val = pop(@ary));
1143 die "Can't readlink $sym: $!"
1144 unless defined($value = readlink $sym);
1145 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1146 $debugging = 0 unless defined $debugging;
1148 Note: Many folks tend to overuse C<defined>, and then are surprised to
1149 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1150 defined values. For example, if you say
1154 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1155 matched "nothing". It didn't really fail to match anything. Rather, it
1156 matched something that happened to be zero characters long. This is all
1157 very above-board and honest. When a function returns an undefined value,
1158 it's an admission that it couldn't give you an honest answer. So you
1159 should use C<defined> only when you're questioning the integrity of what
1160 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1163 See also L</undef>, L</exists>, L</ref>.
1168 Given an expression that specifies a hash element, array element, hash slice,
1169 or array slice, deletes the specified element(s) from the hash or array.
1170 In the case of an array, if the array elements happen to be at the end,
1171 the size of the array will shrink to the highest element that tests
1172 true for exists() (or 0 if no such element exists).
1174 Returns a list with the same number of elements as the number of elements
1175 for which deletion was attempted. Each element of that list consists of
1176 either the value of the element deleted, or the undefined value. In scalar
1177 context, this means that you get the value of the last element deleted (or
1178 the undefined value if that element did not exist).
1180 %hash = (foo => 11, bar => 22, baz => 33);
1181 $scalar = delete $hash{foo}; # $scalar is 11
1182 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1183 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1185 Deleting from C<%ENV> modifies the environment. Deleting from
1186 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1187 from a C<tie>d hash or array may not necessarily return anything.
1189 Deleting an array element effectively returns that position of the array
1190 to its initial, uninitialized state. Subsequently testing for the same
1191 element with exists() will return false. Also, deleting array elements
1192 in the middle of an array will not shift the index of the elements
1193 after them down. Use splice() for that. See L</exists>.
1195 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1197 foreach $key (keys %HASH) {
1201 foreach $index (0 .. $#ARRAY) {
1202 delete $ARRAY[$index];
1207 delete @HASH{keys %HASH};
1209 delete @ARRAY[0 .. $#ARRAY];
1211 But both of these are slower than just assigning the empty list
1212 or undefining %HASH or @ARRAY:
1214 %HASH = (); # completely empty %HASH
1215 undef %HASH; # forget %HASH ever existed
1217 @ARRAY = (); # completely empty @ARRAY
1218 undef @ARRAY; # forget @ARRAY ever existed
1220 Note that the EXPR can be arbitrarily complicated as long as the final
1221 operation is a hash element, array element, hash slice, or array slice
1224 delete $ref->[$x][$y]{$key};
1225 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1227 delete $ref->[$x][$y][$index];
1228 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1231 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1233 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1234 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1235 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1236 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1237 an C<eval(),> the error message is stuffed into C<$@> and the
1238 C<eval> is terminated with the undefined value. This makes
1239 C<die> the way to raise an exception.
1241 Equivalent examples:
1243 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1244 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1246 If the last element of LIST does not end in a newline, the current
1247 script line number and input line number (if any) are also printed,
1248 and a newline is supplied. Note that the "input line number" (also
1249 known as "chunk") is subject to whatever notion of "line" happens to
1250 be currently in effect, and is also available as the special variable
1251 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1253 Hint: sometimes appending C<", stopped"> to your message will cause it
1254 to make better sense when the string C<"at foo line 123"> is appended.
1255 Suppose you are running script "canasta".
1257 die "/etc/games is no good";
1258 die "/etc/games is no good, stopped";
1260 produce, respectively
1262 /etc/games is no good at canasta line 123.
1263 /etc/games is no good, stopped at canasta line 123.
1265 See also exit(), warn(), and the Carp module.
1267 If LIST is empty and C<$@> already contains a value (typically from a
1268 previous eval) that value is reused after appending C<"\t...propagated">.
1269 This is useful for propagating exceptions:
1272 die unless $@ =~ /Expected exception/;
1274 If LIST is empty and C<$@> contains an object reference that has a
1275 C<PROPAGATE> method, that method will be called with additional file
1276 and line number parameters. The return value replaces the value in
1277 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1280 If C<$@> is empty then the string C<"Died"> is used.
1282 die() can also be called with a reference argument. If this happens to be
1283 trapped within an eval(), $@ contains the reference. This behavior permits
1284 a more elaborate exception handling implementation using objects that
1285 maintain arbitrary state about the nature of the exception. Such a scheme
1286 is sometimes preferable to matching particular string values of $@ using
1287 regular expressions. Here's an example:
1289 use Scalar::Util 'blessed';
1291 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1293 if (blessed($@) && $@->isa("Some::Module::Exception")) {
1294 # handle Some::Module::Exception
1297 # handle all other possible exceptions
1301 Because perl will stringify uncaught exception messages before displaying
1302 them, you may want to overload stringification operations on such custom
1303 exception objects. See L<overload> for details about that.
1305 You can arrange for a callback to be run just before the C<die>
1306 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1307 handler will be called with the error text and can change the error
1308 message, if it sees fit, by calling C<die> again. See
1309 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1310 L<"eval BLOCK"> for some examples. Although this feature was
1311 to be run only right before your program was to exit, this is not
1312 currently the case--the C<$SIG{__DIE__}> hook is currently called
1313 even inside eval()ed blocks/strings! If one wants the hook to do
1314 nothing in such situations, put
1318 as the first line of the handler (see L<perlvar/$^S>). Because
1319 this promotes strange action at a distance, this counterintuitive
1320 behavior may be fixed in a future release.
1325 Not really a function. Returns the value of the last command in the
1326 sequence of commands indicated by BLOCK. When modified by the C<while> or
1327 C<until> loop modifier, executes the BLOCK once before testing the loop
1328 condition. (On other statements the loop modifiers test the conditional
1331 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1332 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1333 See L<perlsyn> for alternative strategies.
1335 =item do SUBROUTINE(LIST)
1338 This form of subroutine call is deprecated. See L<perlsub>.
1343 Uses the value of EXPR as a filename and executes the contents of the
1344 file as a Perl script.
1352 except that it's more efficient and concise, keeps track of the current
1353 filename for error messages, searches the @INC directories, and updates
1354 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1355 variables. It also differs in that code evaluated with C<do FILENAME>
1356 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1357 same, however, in that it does reparse the file every time you call it,
1358 so you probably don't want to do this inside a loop.
1360 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1361 error. If C<do> can read the file but cannot compile it, it
1362 returns undef and sets an error message in C<$@>. If the file is
1363 successfully compiled, C<do> returns the value of the last expression
1366 Note that inclusion of library modules is better done with the
1367 C<use> and C<require> operators, which also do automatic error checking
1368 and raise an exception if there's a problem.
1370 You might like to use C<do> to read in a program configuration
1371 file. Manual error checking can be done this way:
1373 # read in config files: system first, then user
1374 for $file ("/share/prog/defaults.rc",
1375 "$ENV{HOME}/.someprogrc")
1377 unless ($return = do $file) {
1378 warn "couldn't parse $file: $@" if $@;
1379 warn "couldn't do $file: $!" unless defined $return;
1380 warn "couldn't run $file" unless $return;
1385 X<dump> X<core> X<undump>
1389 This function causes an immediate core dump. See also the B<-u>
1390 command-line switch in L<perlrun>, which does the same thing.
1391 Primarily this is so that you can use the B<undump> program (not
1392 supplied) to turn your core dump into an executable binary after
1393 having initialized all your variables at the beginning of the
1394 program. When the new binary is executed it will begin by executing
1395 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1396 Think of it as a goto with an intervening core dump and reincarnation.
1397 If C<LABEL> is omitted, restarts the program from the top.
1399 B<WARNING>: Any files opened at the time of the dump will I<not>
1400 be open any more when the program is reincarnated, with possible
1401 resulting confusion on the part of Perl.
1403 This function is now largely obsolete, partly because it's very
1404 hard to convert a core file into an executable, and because the
1405 real compiler backends for generating portable bytecode and compilable
1406 C code have superseded it. That's why you should now invoke it as
1407 C<CORE::dump()>, if you don't want to be warned against a possible
1410 If you're looking to use L<dump> to speed up your program, consider
1411 generating bytecode or native C code as described in L<perlcc>. If
1412 you're just trying to accelerate a CGI script, consider using the
1413 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1414 You might also consider autoloading or selfloading, which at least
1415 make your program I<appear> to run faster.
1418 X<each> X<hash, iterator>
1420 When called in list context, returns a 2-element list consisting of the
1421 key and value for the next element of a hash, so that you can iterate over
1422 it. When called in scalar context, returns only the key for the next
1423 element in the hash.
1425 Entries are returned in an apparently random order. The actual random
1426 order is subject to change in future versions of perl, but it is
1427 guaranteed to be in the same order as either the C<keys> or C<values>
1428 function would produce on the same (unmodified) hash. Since Perl
1429 5.8.1 the ordering is different even between different runs of Perl
1430 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1432 When the hash is entirely read, a null array is returned in list context
1433 (which when assigned produces a false (C<0>) value), and C<undef> in
1434 scalar context. The next call to C<each> after that will start iterating
1435 again. There is a single iterator for each hash, shared by all C<each>,
1436 C<keys>, and C<values> function calls in the program; it can be reset by
1437 reading all the elements from the hash, or by evaluating C<keys HASH> or
1438 C<values HASH>. If you add or delete elements of a hash while you're
1439 iterating over it, you may get entries skipped or duplicated, so
1440 don't. Exception: It is always safe to delete the item most recently
1441 returned by C<each()>, which means that the following code will work:
1443 while (($key, $value) = each %hash) {
1445 delete $hash{$key}; # This is safe
1448 The following prints out your environment like the printenv(1) program,
1449 only in a different order:
1451 while (($key,$value) = each %ENV) {
1452 print "$key=$value\n";
1455 See also C<keys>, C<values> and C<sort>.
1457 =item eof FILEHANDLE
1466 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1467 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1468 gives the real filehandle. (Note that this function actually
1469 reads a character and then C<ungetc>s it, so isn't very useful in an
1470 interactive context.) Do not read from a terminal file (or call
1471 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1472 as terminals may lose the end-of-file condition if you do.
1474 An C<eof> without an argument uses the last file read. Using C<eof()>
1475 with empty parentheses is very different. It refers to the pseudo file
1476 formed from the files listed on the command line and accessed via the
1477 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1478 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1479 used will cause C<@ARGV> to be examined to determine if input is
1480 available. Similarly, an C<eof()> after C<< <> >> has returned
1481 end-of-file will assume you are processing another C<@ARGV> list,
1482 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1483 see L<perlop/"I/O Operators">.
1485 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1486 detect the end of each file, C<eof()> will only detect the end of the
1487 last file. Examples:
1489 # reset line numbering on each input file
1491 next if /^\s*#/; # skip comments
1494 close ARGV if eof; # Not eof()!
1497 # insert dashes just before last line of last file
1499 if (eof()) { # check for end of last file
1500 print "--------------\n";
1503 last if eof(); # needed if we're reading from a terminal
1506 Practical hint: you almost never need to use C<eof> in Perl, because the
1507 input operators typically return C<undef> when they run out of data, or if
1511 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1517 In the first form, the return value of EXPR is parsed and executed as if it
1518 were a little Perl program. The value of the expression (which is itself
1519 determined within scalar context) is first parsed, and if there weren't any
1520 errors, executed in the lexical context of the current Perl program, so
1521 that any variable settings or subroutine and format definitions remain
1522 afterwards. Note that the value is parsed every time the C<eval> executes.
1523 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1524 delay parsing and subsequent execution of the text of EXPR until run time.
1526 In the second form, the code within the BLOCK is parsed only once--at the
1527 same time the code surrounding the C<eval> itself was parsed--and executed
1528 within the context of the current Perl program. This form is typically
1529 used to trap exceptions more efficiently than the first (see below), while
1530 also providing the benefit of checking the code within BLOCK at compile
1533 The final semicolon, if any, may be omitted from the value of EXPR or within
1536 In both forms, the value returned is the value of the last expression
1537 evaluated inside the mini-program; a return statement may be also used, just
1538 as with subroutines. The expression providing the return value is evaluated
1539 in void, scalar, or list context, depending on the context of the C<eval>
1540 itself. See L</wantarray> for more on how the evaluation context can be
1543 If there is a syntax error or runtime error, or a C<die> statement is
1544 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1545 error message. If there was no error, C<$@> is guaranteed to be a null
1546 string. Beware that using C<eval> neither silences perl from printing
1547 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1548 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1549 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1550 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1552 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1553 determining whether a particular feature (such as C<socket> or C<symlink>)
1554 is implemented. It is also Perl's exception trapping mechanism, where
1555 the die operator is used to raise exceptions.
1557 If the code to be executed doesn't vary, you may use the eval-BLOCK
1558 form to trap run-time errors without incurring the penalty of
1559 recompiling each time. The error, if any, is still returned in C<$@>.
1562 # make divide-by-zero nonfatal
1563 eval { $answer = $a / $b; }; warn $@ if $@;
1565 # same thing, but less efficient
1566 eval '$answer = $a / $b'; warn $@ if $@;
1568 # a compile-time error
1569 eval { $answer = }; # WRONG
1572 eval '$answer ='; # sets $@
1574 Using the C<eval{}> form as an exception trap in libraries does have some
1575 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1576 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1577 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1578 as shown in this example:
1580 # a very private exception trap for divide-by-zero
1581 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1584 This is especially significant, given that C<__DIE__> hooks can call
1585 C<die> again, which has the effect of changing their error messages:
1587 # __DIE__ hooks may modify error messages
1589 local $SIG{'__DIE__'} =
1590 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1591 eval { die "foo lives here" };
1592 print $@ if $@; # prints "bar lives here"
1595 Because this promotes action at a distance, this counterintuitive behavior
1596 may be fixed in a future release.
1598 With an C<eval>, you should be especially careful to remember what's
1599 being looked at when:
1605 eval { $x }; # CASE 4
1607 eval "\$$x++"; # CASE 5
1610 Cases 1 and 2 above behave identically: they run the code contained in
1611 the variable $x. (Although case 2 has misleading double quotes making
1612 the reader wonder what else might be happening (nothing is).) Cases 3
1613 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1614 does nothing but return the value of $x. (Case 4 is preferred for
1615 purely visual reasons, but it also has the advantage of compiling at
1616 compile-time instead of at run-time.) Case 5 is a place where
1617 normally you I<would> like to use double quotes, except that in this
1618 particular situation, you can just use symbolic references instead, as
1621 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1622 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1624 Note that as a very special case, an C<eval ''> executed within the C<DB>
1625 package doesn't see the usual surrounding lexical scope, but rather the
1626 scope of the first non-DB piece of code that called it. You don't normally
1627 need to worry about this unless you are writing a Perl debugger.
1632 =item exec PROGRAM LIST
1634 The C<exec> function executes a system command I<and never returns>--
1635 use C<system> instead of C<exec> if you want it to return. It fails and
1636 returns false only if the command does not exist I<and> it is executed
1637 directly instead of via your system's command shell (see below).
1639 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1640 warns you if there is a following statement which isn't C<die>, C<warn>,
1641 or C<exit> (if C<-w> is set - but you always do that). If you
1642 I<really> want to follow an C<exec> with some other statement, you
1643 can use one of these styles to avoid the warning:
1645 exec ('foo') or print STDERR "couldn't exec foo: $!";
1646 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1648 If there is more than one argument in LIST, or if LIST is an array
1649 with more than one value, calls execvp(3) with the arguments in LIST.
1650 If there is only one scalar argument or an array with one element in it,
1651 the argument is checked for shell metacharacters, and if there are any,
1652 the entire argument is passed to the system's command shell for parsing
1653 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1654 If there are no shell metacharacters in the argument, it is split into
1655 words and passed directly to C<execvp>, which is more efficient.
1658 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1659 exec "sort $outfile | uniq";
1661 If you don't really want to execute the first argument, but want to lie
1662 to the program you are executing about its own name, you can specify
1663 the program you actually want to run as an "indirect object" (without a
1664 comma) in front of the LIST. (This always forces interpretation of the
1665 LIST as a multivalued list, even if there is only a single scalar in
1668 $shell = '/bin/csh';
1669 exec $shell '-sh'; # pretend it's a login shell
1673 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1675 When the arguments get executed via the system shell, results will
1676 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1679 Using an indirect object with C<exec> or C<system> is also more
1680 secure. This usage (which also works fine with system()) forces
1681 interpretation of the arguments as a multivalued list, even if the
1682 list had just one argument. That way you're safe from the shell
1683 expanding wildcards or splitting up words with whitespace in them.
1685 @args = ( "echo surprise" );
1687 exec @args; # subject to shell escapes
1689 exec { $args[0] } @args; # safe even with one-arg list
1691 The first version, the one without the indirect object, ran the I<echo>
1692 program, passing it C<"surprise"> an argument. The second version
1693 didn't--it tried to run a program literally called I<"echo surprise">,
1694 didn't find it, and set C<$?> to a non-zero value indicating failure.
1696 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1697 output before the exec, but this may not be supported on some platforms
1698 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1699 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1700 open handles in order to avoid lost output.
1702 Note that C<exec> will not call your C<END> blocks, nor will it call
1703 any C<DESTROY> methods in your objects.
1706 X<exists> X<autovivification>
1708 Given an expression that specifies a hash element or array element,
1709 returns true if the specified element in the hash or array has ever
1710 been initialized, even if the corresponding value is undefined. The
1711 element is not autovivified if it doesn't exist.
1713 print "Exists\n" if exists $hash{$key};
1714 print "Defined\n" if defined $hash{$key};
1715 print "True\n" if $hash{$key};
1717 print "Exists\n" if exists $array[$index];
1718 print "Defined\n" if defined $array[$index];
1719 print "True\n" if $array[$index];
1721 A hash or array element can be true only if it's defined, and defined if
1722 it exists, but the reverse doesn't necessarily hold true.
1724 Given an expression that specifies the name of a subroutine,
1725 returns true if the specified subroutine has ever been declared, even
1726 if it is undefined. Mentioning a subroutine name for exists or defined
1727 does not count as declaring it. Note that a subroutine which does not
1728 exist may still be callable: its package may have an C<AUTOLOAD>
1729 method that makes it spring into existence the first time that it is
1730 called -- see L<perlsub>.
1732 print "Exists\n" if exists &subroutine;
1733 print "Defined\n" if defined &subroutine;
1735 Note that the EXPR can be arbitrarily complicated as long as the final
1736 operation is a hash or array key lookup or subroutine name:
1738 if (exists $ref->{A}->{B}->{$key}) { }
1739 if (exists $hash{A}{B}{$key}) { }
1741 if (exists $ref->{A}->{B}->[$ix]) { }
1742 if (exists $hash{A}{B}[$ix]) { }
1744 if (exists &{$ref->{A}{B}{$key}}) { }
1746 Although the deepest nested array or hash will not spring into existence
1747 just because its existence was tested, any intervening ones will.
1748 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1749 into existence due to the existence test for the $key element above.
1750 This happens anywhere the arrow operator is used, including even:
1753 if (exists $ref->{"Some key"}) { }
1754 print $ref; # prints HASH(0x80d3d5c)
1756 This surprising autovivification in what does not at first--or even
1757 second--glance appear to be an lvalue context may be fixed in a future
1760 Use of a subroutine call, rather than a subroutine name, as an argument
1761 to exists() is an error.
1764 exists &sub(); # Error
1767 X<exit> X<terminate> X<abort>
1771 Evaluates EXPR and exits immediately with that value. Example:
1774 exit 0 if $ans =~ /^[Xx]/;
1776 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1777 universally recognized values for EXPR are C<0> for success and C<1>
1778 for error; other values are subject to interpretation depending on the
1779 environment in which the Perl program is running. For example, exiting
1780 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1781 the mailer to return the item undelivered, but that's not true everywhere.
1783 Don't use C<exit> to abort a subroutine if there's any chance that
1784 someone might want to trap whatever error happened. Use C<die> instead,
1785 which can be trapped by an C<eval>.
1787 The exit() function does not always exit immediately. It calls any
1788 defined C<END> routines first, but these C<END> routines may not
1789 themselves abort the exit. Likewise any object destructors that need to
1790 be called are called before the real exit. If this is a problem, you
1791 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1792 See L<perlmod> for details.
1795 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1799 Returns I<e> (the natural logarithm base) to the power of EXPR.
1800 If EXPR is omitted, gives C<exp($_)>.
1802 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1805 Implements the fcntl(2) function. You'll probably have to say
1809 first to get the correct constant definitions. Argument processing and
1810 value return works just like C<ioctl> below.
1814 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1815 or die "can't fcntl F_GETFL: $!";
1817 You don't have to check for C<defined> on the return from C<fcntl>.
1818 Like C<ioctl>, it maps a C<0> return from the system call into
1819 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1820 in numeric context. It is also exempt from the normal B<-w> warnings
1821 on improper numeric conversions.
1823 Note that C<fcntl> will produce a fatal error if used on a machine that
1824 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1825 manpage to learn what functions are available on your system.
1827 Here's an example of setting a filehandle named C<REMOTE> to be
1828 non-blocking at the system level. You'll have to negotiate C<$|>
1829 on your own, though.
1831 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1833 $flags = fcntl(REMOTE, F_GETFL, 0)
1834 or die "Can't get flags for the socket: $!\n";
1836 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1837 or die "Can't set flags for the socket: $!\n";
1839 =item fileno FILEHANDLE
1842 Returns the file descriptor for a filehandle, or undefined if the
1843 filehandle is not open. This is mainly useful for constructing
1844 bitmaps for C<select> and low-level POSIX tty-handling operations.
1845 If FILEHANDLE is an expression, the value is taken as an indirect
1846 filehandle, generally its name.
1848 You can use this to find out whether two handles refer to the
1849 same underlying descriptor:
1851 if (fileno(THIS) == fileno(THAT)) {
1852 print "THIS and THAT are dups\n";
1855 (Filehandles connected to memory objects via new features of C<open> may
1856 return undefined even though they are open.)
1859 =item flock FILEHANDLE,OPERATION
1860 X<flock> X<lock> X<locking>
1862 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1863 for success, false on failure. Produces a fatal error if used on a
1864 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1865 C<flock> is Perl's portable file locking interface, although it locks
1866 only entire files, not records.
1868 Two potentially non-obvious but traditional C<flock> semantics are
1869 that it waits indefinitely until the lock is granted, and that its locks
1870 B<merely advisory>. Such discretionary locks are more flexible, but offer
1871 fewer guarantees. This means that programs that do not also use C<flock>
1872 may modify files locked with C<flock>. See L<perlport>,
1873 your port's specific documentation, or your system-specific local manpages
1874 for details. It's best to assume traditional behavior if you're writing
1875 portable programs. (But if you're not, you should as always feel perfectly
1876 free to write for your own system's idiosyncrasies (sometimes called
1877 "features"). Slavish adherence to portability concerns shouldn't get
1878 in the way of your getting your job done.)
1880 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1881 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1882 you can use the symbolic names if you import them from the Fcntl module,
1883 either individually, or as a group using the ':flock' tag. LOCK_SH
1884 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1885 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1886 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1887 waiting for the lock (check the return status to see if you got it).
1889 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1890 before locking or unlocking it.
1892 Note that the emulation built with lockf(3) doesn't provide shared
1893 locks, and it requires that FILEHANDLE be open with write intent. These
1894 are the semantics that lockf(3) implements. Most if not all systems
1895 implement lockf(3) in terms of fcntl(2) locking, though, so the
1896 differing semantics shouldn't bite too many people.
1898 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1899 be open with read intent to use LOCK_SH and requires that it be open
1900 with write intent to use LOCK_EX.
1902 Note also that some versions of C<flock> cannot lock things over the
1903 network; you would need to use the more system-specific C<fcntl> for
1904 that. If you like you can force Perl to ignore your system's flock(2)
1905 function, and so provide its own fcntl(2)-based emulation, by passing
1906 the switch C<-Ud_flock> to the F<Configure> program when you configure
1909 Here's a mailbox appender for BSD systems.
1911 use Fcntl ':flock'; # import LOCK_* constants
1914 flock(MBOX,LOCK_EX);
1915 # and, in case someone appended
1916 # while we were waiting...
1921 flock(MBOX,LOCK_UN);
1924 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1925 or die "Can't open mailbox: $!";
1928 print MBOX $msg,"\n\n";
1931 On systems that support a real flock(), locks are inherited across fork()
1932 calls, whereas those that must resort to the more capricious fcntl()
1933 function lose the locks, making it harder to write servers.
1935 See also L<DB_File> for other flock() examples.
1938 X<fork> X<child> X<parent>
1940 Does a fork(2) system call to create a new process running the
1941 same program at the same point. It returns the child pid to the
1942 parent process, C<0> to the child process, or C<undef> if the fork is
1943 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1944 are shared, while everything else is copied. On most systems supporting
1945 fork(), great care has gone into making it extremely efficient (for
1946 example, using copy-on-write technology on data pages), making it the
1947 dominant paradigm for multitasking over the last few decades.
1949 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1950 output before forking the child process, but this may not be supported
1951 on some platforms (see L<perlport>). To be safe, you may need to set
1952 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1953 C<IO::Handle> on any open handles in order to avoid duplicate output.
1955 If you C<fork> without ever waiting on your children, you will
1956 accumulate zombies. On some systems, you can avoid this by setting
1957 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1958 forking and reaping moribund children.
1960 Note that if your forked child inherits system file descriptors like
1961 STDIN and STDOUT that are actually connected by a pipe or socket, even
1962 if you exit, then the remote server (such as, say, a CGI script or a
1963 backgrounded job launched from a remote shell) won't think you're done.
1964 You should reopen those to F</dev/null> if it's any issue.
1969 Declare a picture format for use by the C<write> function. For
1973 Test: @<<<<<<<< @||||| @>>>>>
1974 $str, $%, '$' . int($num)
1978 $num = $cost/$quantity;
1982 See L<perlform> for many details and examples.
1984 =item formline PICTURE,LIST
1987 This is an internal function used by C<format>s, though you may call it,
1988 too. It formats (see L<perlform>) a list of values according to the
1989 contents of PICTURE, placing the output into the format output
1990 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1991 Eventually, when a C<write> is done, the contents of
1992 C<$^A> are written to some filehandle. You could also read C<$^A>
1993 and then set C<$^A> back to C<"">. Note that a format typically
1994 does one C<formline> per line of form, but the C<formline> function itself
1995 doesn't care how many newlines are embedded in the PICTURE. This means
1996 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1997 You may therefore need to use multiple formlines to implement a single
1998 record format, just like the format compiler.
2000 Be careful if you put double quotes around the picture, because an C<@>
2001 character may be taken to mean the beginning of an array name.
2002 C<formline> always returns true. See L<perlform> for other examples.
2004 =item getc FILEHANDLE
2009 Returns the next character from the input file attached to FILEHANDLE,
2010 or the undefined value at end of file, or if there was an error (in
2011 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2012 STDIN. This is not particularly efficient. However, it cannot be
2013 used by itself to fetch single characters without waiting for the user
2014 to hit enter. For that, try something more like:
2017 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2020 system "stty", '-icanon', 'eol', "\001";
2026 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2029 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2033 Determination of whether $BSD_STYLE should be set
2034 is left as an exercise to the reader.
2036 The C<POSIX::getattr> function can do this more portably on
2037 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2038 module from your nearest CPAN site; details on CPAN can be found on
2042 X<getlogin> X<login>
2044 This implements the C library function of the same name, which on most
2045 systems returns the current login from F</etc/utmp>, if any. If null,
2048 $login = getlogin || getpwuid($<) || "Kilroy";
2050 Do not consider C<getlogin> for authentication: it is not as
2051 secure as C<getpwuid>.
2053 =item getpeername SOCKET
2054 X<getpeername> X<peer>
2056 Returns the packed sockaddr address of other end of the SOCKET connection.
2059 $hersockaddr = getpeername(SOCK);
2060 ($port, $iaddr) = sockaddr_in($hersockaddr);
2061 $herhostname = gethostbyaddr($iaddr, AF_INET);
2062 $herstraddr = inet_ntoa($iaddr);
2067 Returns the current process group for the specified PID. Use
2068 a PID of C<0> to get the current process group for the
2069 current process. Will raise an exception if used on a machine that
2070 doesn't implement getpgrp(2). If PID is omitted, returns process
2071 group of current process. Note that the POSIX version of C<getpgrp>
2072 does not accept a PID argument, so only C<PID==0> is truly portable.
2075 X<getppid> X<parent> X<pid>
2077 Returns the process id of the parent process.
2079 Note for Linux users: on Linux, the C functions C<getpid()> and
2080 C<getppid()> return different values from different threads. In order to
2081 be portable, this behavior is not reflected by the perl-level function
2082 C<getppid()>, that returns a consistent value across threads. If you want
2083 to call the underlying C<getppid()>, you may use the CPAN module
2086 =item getpriority WHICH,WHO
2087 X<getpriority> X<priority> X<nice>
2089 Returns the current priority for a process, a process group, or a user.
2090 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2091 machine that doesn't implement getpriority(2).
2094 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2095 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2096 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2097 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2098 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2099 X<endnetent> X<endprotoent> X<endservent>
2103 =item gethostbyname NAME
2105 =item getnetbyname NAME
2107 =item getprotobyname NAME
2113 =item getservbyname NAME,PROTO
2115 =item gethostbyaddr ADDR,ADDRTYPE
2117 =item getnetbyaddr ADDR,ADDRTYPE
2119 =item getprotobynumber NUMBER
2121 =item getservbyport PORT,PROTO
2139 =item sethostent STAYOPEN
2141 =item setnetent STAYOPEN
2143 =item setprotoent STAYOPEN
2145 =item setservent STAYOPEN
2159 These routines perform the same functions as their counterparts in the
2160 system library. In list context, the return values from the
2161 various get routines are as follows:
2163 ($name,$passwd,$uid,$gid,
2164 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2165 ($name,$passwd,$gid,$members) = getgr*
2166 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2167 ($name,$aliases,$addrtype,$net) = getnet*
2168 ($name,$aliases,$proto) = getproto*
2169 ($name,$aliases,$port,$proto) = getserv*
2171 (If the entry doesn't exist you get a null list.)
2173 The exact meaning of the $gcos field varies but it usually contains
2174 the real name of the user (as opposed to the login name) and other
2175 information pertaining to the user. Beware, however, that in many
2176 system users are able to change this information and therefore it
2177 cannot be trusted and therefore the $gcos is tainted (see
2178 L<perlsec>). The $passwd and $shell, user's encrypted password and
2179 login shell, are also tainted, because of the same reason.
2181 In scalar context, you get the name, unless the function was a
2182 lookup by name, in which case you get the other thing, whatever it is.
2183 (If the entry doesn't exist you get the undefined value.) For example:
2185 $uid = getpwnam($name);
2186 $name = getpwuid($num);
2188 $gid = getgrnam($name);
2189 $name = getgrgid($num);
2193 In I<getpw*()> the fields $quota, $comment, and $expire are special
2194 cases in the sense that in many systems they are unsupported. If the
2195 $quota is unsupported, it is an empty scalar. If it is supported, it
2196 usually encodes the disk quota. If the $comment field is unsupported,
2197 it is an empty scalar. If it is supported it usually encodes some
2198 administrative comment about the user. In some systems the $quota
2199 field may be $change or $age, fields that have to do with password
2200 aging. In some systems the $comment field may be $class. The $expire
2201 field, if present, encodes the expiration period of the account or the
2202 password. For the availability and the exact meaning of these fields
2203 in your system, please consult your getpwnam(3) documentation and your
2204 F<pwd.h> file. You can also find out from within Perl what your
2205 $quota and $comment fields mean and whether you have the $expire field
2206 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2207 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2208 files are only supported if your vendor has implemented them in the
2209 intuitive fashion that calling the regular C library routines gets the
2210 shadow versions if you're running under privilege or if there exists
2211 the shadow(3) functions as found in System V (this includes Solaris
2212 and Linux.) Those systems that implement a proprietary shadow password
2213 facility are unlikely to be supported.
2215 The $members value returned by I<getgr*()> is a space separated list of
2216 the login names of the members of the group.
2218 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2219 C, it will be returned to you via C<$?> if the function call fails. The
2220 C<@addrs> value returned by a successful call is a list of the raw
2221 addresses returned by the corresponding system library call. In the
2222 Internet domain, each address is four bytes long and you can unpack it
2223 by saying something like:
2225 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2227 The Socket library makes this slightly easier:
2230 $iaddr = inet_aton("127.1"); # or whatever address
2231 $name = gethostbyaddr($iaddr, AF_INET);
2233 # or going the other way
2234 $straddr = inet_ntoa($iaddr);
2236 If you get tired of remembering which element of the return list
2237 contains which return value, by-name interfaces are provided
2238 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2239 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2240 and C<User::grent>. These override the normal built-ins, supplying
2241 versions that return objects with the appropriate names
2242 for each field. For example:
2246 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2248 Even though it looks like they're the same method calls (uid),
2249 they aren't, because a C<File::stat> object is different from
2250 a C<User::pwent> object.
2252 =item getsockname SOCKET
2255 Returns the packed sockaddr address of this end of the SOCKET connection,
2256 in case you don't know the address because you have several different
2257 IPs that the connection might have come in on.
2260 $mysockaddr = getsockname(SOCK);
2261 ($port, $myaddr) = sockaddr_in($mysockaddr);
2262 printf "Connect to %s [%s]\n",
2263 scalar gethostbyaddr($myaddr, AF_INET),
2266 =item getsockopt SOCKET,LEVEL,OPTNAME
2269 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2270 Options may exist at multiple protocol levels depending on the socket
2271 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2272 C<Socket> module) will exist. To query options at another level the
2273 protocol number of the appropriate protocol controlling the option
2274 should be supplied. For example, to indicate that an option is to be
2275 interpreted by the TCP protocol, LEVEL should be set to the protocol
2276 number of TCP, which you can get using getprotobyname.
2278 The call returns a packed string representing the requested socket option,
2279 or C<undef> if there is an error (the error reason will be in $!). What
2280 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2281 your system documentation for details. A very common case however is that
2282 the option is an integer, in which case the result will be a packed
2283 integer which you can decode using unpack with the C<i> (or C<I>) format.
2285 An example testing if Nagle's algorithm is turned on on a socket:
2287 use Socket qw(:all);
2289 defined(my $tcp = getprotobyname("tcp"))
2290 or die "Could not determine the protocol number for tcp";
2291 # my $tcp = IPPROTO_TCP; # Alternative
2292 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2293 or die "Could not query TCP_NODELAY socket option: $!";
2294 my $nodelay = unpack("I", $packed);
2295 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2299 X<glob> X<wildcard> X<filename, expansion> X<expand>
2303 In list context, returns a (possibly empty) list of filename expansions on
2304 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2305 scalar context, glob iterates through such filename expansions, returning
2306 undef when the list is exhausted. This is the internal function
2307 implementing the C<< <*.c> >> operator, but you can use it directly. If
2308 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2309 more detail in L<perlop/"I/O Operators">.
2311 Beginning with v5.6.0, this operator is implemented using the standard
2312 C<File::Glob> extension. See L<File::Glob> for details.
2315 X<gmtime> X<UTC> X<Greenwich>
2319 Converts a time as returned by the time function to an 9-element list
2320 with the time localized for the standard Greenwich time zone.
2321 Typically used as follows:
2324 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2327 All list elements are numeric, and come straight out of the C `struct
2328 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2329 specified time. $mday is the day of the month, and $mon is the month
2330 itself, in the range C<0..11> with 0 indicating January and 11
2331 indicating December. $year is the number of years since 1900. That
2332 is, $year is C<123> in year 2023. $wday is the day of the week, with
2333 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2334 the year, in the range C<0..364> (or C<0..365> in leap years). $isdst
2337 Note that the $year element is I<not> simply the last two digits of
2338 the year. If you assume it is then you create non-Y2K-compliant
2339 programs--and you wouldn't want to do that, would you?
2341 The proper way to get a complete 4-digit year is simply:
2345 And to get the last two digits of the year (e.g., '01' in 2001) do:
2347 $year = sprintf("%02d", $year % 100);
2349 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2351 In scalar context, C<gmtime()> returns the ctime(3) value:
2353 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2355 If you need local time instead of GMT use the L</localtime> builtin.
2356 See also the C<timegm> function provided by the C<Time::Local> module,
2357 and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
2359 This scalar value is B<not> locale dependent (see L<perllocale>), but is
2360 instead a Perl builtin. To get somewhat similar but locale dependent date
2361 strings, see the example in L</localtime>.
2363 See L<perlport/gmtime> for portability concerns.
2366 X<goto> X<jump> X<jmp>
2372 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2373 execution there. It may not be used to go into any construct that
2374 requires initialization, such as a subroutine or a C<foreach> loop. It
2375 also can't be used to go into a construct that is optimized away,
2376 or to get out of a block or subroutine given to C<sort>.
2377 It can be used to go almost anywhere else within the dynamic scope,
2378 including out of subroutines, but it's usually better to use some other
2379 construct such as C<last> or C<die>. The author of Perl has never felt the
2380 need to use this form of C<goto> (in Perl, that is--C is another matter).
2381 (The difference being that C does not offer named loops combined with
2382 loop control. Perl does, and this replaces most structured uses of C<goto>
2383 in other languages.)
2385 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2386 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2387 necessarily recommended if you're optimizing for maintainability:
2389 goto ("FOO", "BAR", "GLARCH")[$i];
2391 The C<goto-&NAME> form is quite different from the other forms of
2392 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2393 doesn't have the stigma associated with other gotos. Instead, it
2394 exits the current subroutine (losing any changes set by local()) and
2395 immediately calls in its place the named subroutine using the current
2396 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2397 load another subroutine and then pretend that the other subroutine had
2398 been called in the first place (except that any modifications to C<@_>
2399 in the current subroutine are propagated to the other subroutine.)
2400 After the C<goto>, not even C<caller> will be able to tell that this
2401 routine was called first.
2403 NAME needn't be the name of a subroutine; it can be a scalar variable
2404 containing a code reference, or a block that evaluates to a code
2407 =item grep BLOCK LIST
2410 =item grep EXPR,LIST
2412 This is similar in spirit to, but not the same as, grep(1) and its
2413 relatives. In particular, it is not limited to using regular expressions.
2415 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2416 C<$_> to each element) and returns the list value consisting of those
2417 elements for which the expression evaluated to true. In scalar
2418 context, returns the number of times the expression was true.
2420 @foo = grep(!/^#/, @bar); # weed out comments
2424 @foo = grep {!/^#/} @bar; # weed out comments
2426 Note that C<$_> is an alias to the list value, so it can be used to
2427 modify the elements of the LIST. While this is useful and supported,
2428 it can cause bizarre results if the elements of LIST are not variables.
2429 Similarly, grep returns aliases into the original list, much as a for
2430 loop's index variable aliases the list elements. That is, modifying an
2431 element of a list returned by grep (for example, in a C<foreach>, C<map>
2432 or another C<grep>) actually modifies the element in the original list.
2433 This is usually something to be avoided when writing clear code.
2435 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2436 been declared with C<my $_>) then, in addition to being locally aliased to
2437 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2438 can't be seen from the outside, avoiding any potential side-effects.
2440 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2443 X<hex> X<hexadecimal>
2447 Interprets EXPR as a hex string and returns the corresponding value.
2448 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2449 L</oct>.) If EXPR is omitted, uses C<$_>.
2451 print hex '0xAf'; # prints '175'
2452 print hex 'aF'; # same
2454 Hex strings may only represent integers. Strings that would cause
2455 integer overflow trigger a warning. Leading whitespace is not stripped,
2456 unlike oct(). To present something as hex, look into L</printf>,
2457 L</sprintf>, or L</unpack>.
2462 There is no builtin C<import> function. It is just an ordinary
2463 method (subroutine) defined (or inherited) by modules that wish to export
2464 names to another module. The C<use> function calls the C<import> method
2465 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2467 =item index STR,SUBSTR,POSITION
2468 X<index> X<indexOf> X<InStr>
2470 =item index STR,SUBSTR
2472 The index function searches for one string within another, but without
2473 the wildcard-like behavior of a full regular-expression pattern match.
2474 It returns the position of the first occurrence of SUBSTR in STR at
2475 or after POSITION. If POSITION is omitted, starts searching from the
2476 beginning of the string. POSITION before the beginning of the string
2477 or after its end is treated as if it were the beginning or the end,
2478 respectively. POSITION and the return value are based at C<0> (or whatever
2479 you've set the C<$[> variable to--but don't do that). If the substring
2480 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2483 X<int> X<integer> X<truncate> X<trunc>
2487 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2488 You should not use this function for rounding: one because it truncates
2489 towards C<0>, and two because machine representations of floating point
2490 numbers can sometimes produce counterintuitive results. For example,
2491 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2492 because it's really more like -268.99999999999994315658 instead. Usually,
2493 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2494 functions will serve you better than will int().
2496 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2499 Implements the ioctl(2) function. You'll probably first have to say
2501 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2503 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2504 exist or doesn't have the correct definitions you'll have to roll your
2505 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2506 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2507 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2508 written depending on the FUNCTION--a pointer to the string value of SCALAR
2509 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2510 has no string value but does have a numeric value, that value will be
2511 passed rather than a pointer to the string value. To guarantee this to be
2512 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2513 functions may be needed to manipulate the values of structures used by
2516 The return value of C<ioctl> (and C<fcntl>) is as follows:
2518 if OS returns: then Perl returns:
2520 0 string "0 but true"
2521 anything else that number
2523 Thus Perl returns true on success and false on failure, yet you can
2524 still easily determine the actual value returned by the operating
2527 $retval = ioctl(...) || -1;
2528 printf "System returned %d\n", $retval;
2530 The special string C<"0 but true"> is exempt from B<-w> complaints
2531 about improper numeric conversions.
2533 =item join EXPR,LIST
2536 Joins the separate strings of LIST into a single string with fields
2537 separated by the value of EXPR, and returns that new string. Example:
2539 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2541 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2542 first argument. Compare L</split>.
2547 Returns a list consisting of all the keys of the named hash.
2548 (In scalar context, returns the number of keys.)
2550 The keys are returned in an apparently random order. The actual
2551 random order is subject to change in future versions of perl, but it
2552 is guaranteed to be the same order as either the C<values> or C<each>
2553 function produces (given that the hash has not been modified). Since
2554 Perl 5.8.1 the ordering is different even between different runs of
2555 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2558 As a side effect, calling keys() resets the HASH's internal iterator
2559 (see L</each>). In particular, calling keys() in void context resets
2560 the iterator with no other overhead.
2562 Here is yet another way to print your environment:
2565 @values = values %ENV;
2567 print pop(@keys), '=', pop(@values), "\n";
2570 or how about sorted by key:
2572 foreach $key (sort(keys %ENV)) {
2573 print $key, '=', $ENV{$key}, "\n";
2576 The returned values are copies of the original keys in the hash, so
2577 modifying them will not affect the original hash. Compare L</values>.
2579 To sort a hash by value, you'll need to use a C<sort> function.
2580 Here's a descending numeric sort of a hash by its values:
2582 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2583 printf "%4d %s\n", $hash{$key}, $key;
2586 As an lvalue C<keys> allows you to increase the number of hash buckets
2587 allocated for the given hash. This can gain you a measure of efficiency if
2588 you know the hash is going to get big. (This is similar to pre-extending
2589 an array by assigning a larger number to $#array.) If you say
2593 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2594 in fact, since it rounds up to the next power of two. These
2595 buckets will be retained even if you do C<%hash = ()>, use C<undef
2596 %hash> if you want to free the storage while C<%hash> is still in scope.
2597 You can't shrink the number of buckets allocated for the hash using
2598 C<keys> in this way (but you needn't worry about doing this by accident,
2599 as trying has no effect).
2601 See also C<each>, C<values> and C<sort>.
2603 =item kill SIGNAL, LIST
2606 Sends a signal to a list of processes. Returns the number of
2607 processes successfully signaled (which is not necessarily the
2608 same as the number actually killed).
2610 $cnt = kill 1, $child1, $child2;
2613 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2614 system call will check whether it's possible to send a signal to it (that
2615 means, to be brief, that the process is owned by the same user, or we are
2616 the super-user). This is a useful way to check that a child process is
2617 alive and hasn't changed its UID. See L<perlport> for notes on the
2618 portability of this construct.
2620 Unlike in the shell, if SIGNAL is negative, it kills
2621 process groups instead of processes. (On System V, a negative I<PROCESS>
2622 number will also kill process groups, but that's not portable.) That
2623 means you usually want to use positive not negative signals. You may also
2624 use a signal name in quotes.
2626 See L<perlipc/"Signals"> for more details.
2633 The C<last> command is like the C<break> statement in C (as used in
2634 loops); it immediately exits the loop in question. If the LABEL is
2635 omitted, the command refers to the innermost enclosing loop. The
2636 C<continue> block, if any, is not executed:
2638 LINE: while (<STDIN>) {
2639 last LINE if /^$/; # exit when done with header
2643 C<last> cannot be used to exit a block which returns a value such as
2644 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2645 a grep() or map() operation.
2647 Note that a block by itself is semantically identical to a loop
2648 that executes once. Thus C<last> can be used to effect an early
2649 exit out of such a block.
2651 See also L</continue> for an illustration of how C<last>, C<next>, and
2659 Returns a lowercased version of EXPR. This is the internal function
2660 implementing the C<\L> escape in double-quoted strings. Respects
2661 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2662 and L<perlunicode> for more details about locale and Unicode support.
2664 If EXPR is omitted, uses C<$_>.
2667 X<lcfirst> X<lowercase>
2671 Returns the value of EXPR with the first character lowercased. This
2672 is the internal function implementing the C<\l> escape in
2673 double-quoted strings. Respects current LC_CTYPE locale if C<use
2674 locale> in force. See L<perllocale> and L<perlunicode> for more
2675 details about locale and Unicode support.
2677 If EXPR is omitted, uses C<$_>.
2684 Returns the length in I<characters> of the value of EXPR. If EXPR is
2685 omitted, returns length of C<$_>. Note that this cannot be used on
2686 an entire array or hash to find out how many elements these have.
2687 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2689 Note the I<characters>: if the EXPR is in Unicode, you will get the
2690 number of characters, not the number of bytes. To get the length
2691 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2693 =item link OLDFILE,NEWFILE
2696 Creates a new filename linked to the old filename. Returns true for
2697 success, false otherwise.
2699 =item listen SOCKET,QUEUESIZE
2702 Does the same thing that the listen system call does. Returns true if
2703 it succeeded, false otherwise. See the example in
2704 L<perlipc/"Sockets: Client/Server Communication">.
2709 You really probably want to be using C<my> instead, because C<local> isn't
2710 what most people think of as "local". See
2711 L<perlsub/"Private Variables via my()"> for details.
2713 A local modifies the listed variables to be local to the enclosing
2714 block, file, or eval. If more than one value is listed, the list must
2715 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2716 for details, including issues with tied arrays and hashes.
2718 =item localtime EXPR
2723 Converts a time as returned by the time function to a 9-element list
2724 with the time analyzed for the local time zone. Typically used as
2728 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2731 All list elements are numeric, and come straight out of the C `struct
2732 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2733 of the specified time.
2735 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2736 the range C<0..11> with 0 indicating January and 11 indicating December.
2737 This makes it easy to get a month name from a list:
2739 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2740 print "$abbr[$mon] $mday";
2741 # $mon=9, $mday=18 gives "Oct 18"
2743 C<$year> is the number of years since 1900, not just the last two digits
2744 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2745 to get a complete 4-digit year is simply:
2749 To get the last two digits of the year (e.g., '01' in 2001) do:
2751 $year = sprintf("%02d", $year % 100);
2753 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2754 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2755 (or C<0..365> in leap years.)
2757 C<$isdst> is true if the specified time occurs during Daylight Saving
2758 Time, false otherwise.
2760 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2762 In scalar context, C<localtime()> returns the ctime(3) value:
2764 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2766 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2767 instead of local time use the L</gmtime> builtin. See also the
2768 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2769 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2770 and mktime(3) functions.
2772 To get somewhat similar but locale dependent date strings, set up your
2773 locale environment variables appropriately (please see L<perllocale>) and
2776 use POSIX qw(strftime);
2777 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2778 # or for GMT formatted appropriately for your locale:
2779 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2781 Note that the C<%a> and C<%b>, the short forms of the day of the week
2782 and the month of the year, may not necessarily be three characters wide.
2784 See L<perlport/localtime> for portability concerns.
2789 This function places an advisory lock on a shared variable, or referenced
2790 object contained in I<THING> until the lock goes out of scope.
2792 lock() is a "weak keyword" : this means that if you've defined a function
2793 by this name (before any calls to it), that function will be called
2794 instead. (However, if you've said C<use threads>, lock() is always a
2795 keyword.) See L<threads>.
2798 X<log> X<logarithm> X<e> X<ln> X<base>
2802 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2803 returns log of C<$_>. To get the log of another base, use basic algebra:
2804 The base-N log of a number is equal to the natural log of that number
2805 divided by the natural log of N. For example:
2809 return log($n)/log(10);
2812 See also L</exp> for the inverse operation.
2819 Does the same thing as the C<stat> function (including setting the
2820 special C<_> filehandle) but stats a symbolic link instead of the file
2821 the symbolic link points to. If symbolic links are unimplemented on
2822 your system, a normal C<stat> is done. For much more detailed
2823 information, please see the documentation for C<stat>.
2825 If EXPR is omitted, stats C<$_>.
2829 The match operator. See L<perlop>.
2831 =item map BLOCK LIST
2836 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2837 C<$_> to each element) and returns the list value composed of the
2838 results of each such evaluation. In scalar context, returns the
2839 total number of elements so generated. Evaluates BLOCK or EXPR in
2840 list context, so each element of LIST may produce zero, one, or
2841 more elements in the returned value.
2843 @chars = map(chr, @nums);
2845 translates a list of numbers to the corresponding characters. And
2847 %hash = map { getkey($_) => $_ } @array;
2849 is just a funny way to write
2852 foreach $_ (@array) {
2853 $hash{getkey($_)} = $_;
2856 Note that C<$_> is an alias to the list value, so it can be used to
2857 modify the elements of the LIST. While this is useful and supported,
2858 it can cause bizarre results if the elements of LIST are not variables.
2859 Using a regular C<foreach> loop for this purpose would be clearer in
2860 most cases. See also L</grep> for an array composed of those items of
2861 the original list for which the BLOCK or EXPR evaluates to true.
2863 If C<$_> is lexical in the scope where the C<map> appears (because it has
2864 been declared with C<my $_>) then, in addition to being locally aliased to
2865 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2866 can't be seen from the outside, avoiding any potential side-effects.
2868 C<{> starts both hash references and blocks, so C<map { ...> could be either
2869 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2870 ahead for the closing C<}> it has to take a guess at which its dealing with
2871 based what it finds just after the C<{>. Usually it gets it right, but if it
2872 doesn't it won't realize something is wrong until it gets to the C<}> and
2873 encounters the missing (or unexpected) comma. The syntax error will be
2874 reported close to the C<}> but you'll need to change something near the C<{>
2875 such as using a unary C<+> to give perl some help:
2877 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2878 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2879 %hash = map { ("\L$_", 1) } @array # this also works
2880 %hash = map { lc($_), 1 } @array # as does this.
2881 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2883 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2885 or to force an anon hash constructor use C<+{>
2887 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2889 and you get list of anonymous hashes each with only 1 entry.
2891 =item mkdir FILENAME,MASK
2892 X<mkdir> X<md> X<directory, create>
2894 =item mkdir FILENAME
2898 Creates the directory specified by FILENAME, with permissions
2899 specified by MASK (as modified by C<umask>). If it succeeds it
2900 returns true, otherwise it returns false and sets C<$!> (errno).
2901 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2904 In general, it is better to create directories with permissive MASK,
2905 and let the user modify that with their C<umask>, than it is to supply
2906 a restrictive MASK and give the user no way to be more permissive.
2907 The exceptions to this rule are when the file or directory should be
2908 kept private (mail files, for instance). The perlfunc(1) entry on
2909 C<umask> discusses the choice of MASK in more detail.
2911 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2912 number of trailing slashes. Some operating and filesystems do not get
2913 this right, so Perl automatically removes all trailing slashes to keep
2916 =item msgctl ID,CMD,ARG
2919 Calls the System V IPC function msgctl(2). You'll probably have to say
2923 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2924 then ARG must be a variable that will hold the returned C<msqid_ds>
2925 structure. Returns like C<ioctl>: the undefined value for error,
2926 C<"0 but true"> for zero, or the actual return value otherwise. See also
2927 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2929 =item msgget KEY,FLAGS
2932 Calls the System V IPC function msgget(2). Returns the message queue
2933 id, or the undefined value if there is an error. See also
2934 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2936 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2939 Calls the System V IPC function msgrcv to receive a message from
2940 message queue ID into variable VAR with a maximum message size of
2941 SIZE. Note that when a message is received, the message type as a
2942 native long integer will be the first thing in VAR, followed by the
2943 actual message. This packing may be opened with C<unpack("l! a*")>.
2944 Taints the variable. Returns true if successful, or false if there is
2945 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2946 C<IPC::SysV::Msg> documentation.
2948 =item msgsnd ID,MSG,FLAGS
2951 Calls the System V IPC function msgsnd to send the message MSG to the
2952 message queue ID. MSG must begin with the native long integer message
2953 type, and be followed by the length of the actual message, and finally
2954 the message itself. This kind of packing can be achieved with
2955 C<pack("l! a*", $type, $message)>. Returns true if successful,
2956 or false if there is an error. See also C<IPC::SysV>
2957 and C<IPC::SysV::Msg> documentation.
2964 =item my EXPR : ATTRS
2966 =item my TYPE EXPR : ATTRS
2968 A C<my> declares the listed variables to be local (lexically) to the
2969 enclosing block, file, or C<eval>. If more than one value is listed,
2970 the list must be placed in parentheses.
2972 The exact semantics and interface of TYPE and ATTRS are still
2973 evolving. TYPE is currently bound to the use of C<fields> pragma,
2974 and attributes are handled using the C<attributes> pragma, or starting
2975 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2976 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2977 L<attributes>, and L<Attribute::Handlers>.
2984 The C<next> command is like the C<continue> statement in C; it starts
2985 the next iteration of the loop:
2987 LINE: while (<STDIN>) {
2988 next LINE if /^#/; # discard comments
2992 Note that if there were a C<continue> block on the above, it would get
2993 executed even on discarded lines. If the LABEL is omitted, the command
2994 refers to the innermost enclosing loop.
2996 C<next> cannot be used to exit a block which returns a value such as
2997 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2998 a grep() or map() operation.
3000 Note that a block by itself is semantically identical to a loop
3001 that executes once. Thus C<next> will exit such a block early.
3003 See also L</continue> for an illustration of how C<last>, C<next>, and
3006 =item no Module VERSION LIST
3009 =item no Module VERSION
3011 =item no Module LIST
3015 See the C<use> function, of which C<no> is the opposite.
3018 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3022 Interprets EXPR as an octal string and returns the corresponding
3023 value. (If EXPR happens to start off with C<0x>, interprets it as a
3024 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3025 binary string. Leading whitespace is ignored in all three cases.)
3026 The following will handle decimal, binary, octal, and hex in the standard
3029 $val = oct($val) if $val =~ /^0/;
3031 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3032 in octal), use sprintf() or printf():
3034 $perms = (stat("filename"))[2] & 07777;
3035 $oct_perms = sprintf "%lo", $perms;
3037 The oct() function is commonly used when a string such as C<644> needs
3038 to be converted into a file mode, for example. (Although perl will
3039 automatically convert strings into numbers as needed, this automatic
3040 conversion assumes base 10.)
3042 =item open FILEHANDLE,EXPR
3043 X<open> X<pipe> X<file, open> X<fopen>
3045 =item open FILEHANDLE,MODE,EXPR
3047 =item open FILEHANDLE,MODE,EXPR,LIST
3049 =item open FILEHANDLE,MODE,REFERENCE
3051 =item open FILEHANDLE
3053 Opens the file whose filename is given by EXPR, and associates it with
3056 (The following is a comprehensive reference to open(): for a gentler
3057 introduction you may consider L<perlopentut>.)
3059 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3060 the variable is assigned a reference to a new anonymous filehandle,
3061 otherwise if FILEHANDLE is an expression, its value is used as the name of
3062 the real filehandle wanted. (This is considered a symbolic reference, so
3063 C<use strict 'refs'> should I<not> be in effect.)
3065 If EXPR is omitted, the scalar variable of the same name as the
3066 FILEHANDLE contains the filename. (Note that lexical variables--those
3067 declared with C<my>--will not work for this purpose; so if you're
3068 using C<my>, specify EXPR in your call to open.)
3070 If three or more arguments are specified then the mode of opening and
3071 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3072 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3073 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3074 the file is opened for appending, again being created if necessary.
3076 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3077 indicate that you want both read and write access to the file; thus
3078 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3079 '+>' >> mode would clobber the file first. You can't usually use
3080 either read-write mode for updating textfiles, since they have
3081 variable length records. See the B<-i> switch in L<perlrun> for a
3082 better approach. The file is created with permissions of C<0666>
3083 modified by the process' C<umask> value.
3085 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3086 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3088 In the 2-arguments (and 1-argument) form of the call the mode and
3089 filename should be concatenated (in this order), possibly separated by
3090 spaces. It is possible to omit the mode in these forms if the mode is
3093 If the filename begins with C<'|'>, the filename is interpreted as a
3094 command to which output is to be piped, and if the filename ends with a
3095 C<'|'>, the filename is interpreted as a command which pipes output to
3096 us. See L<perlipc/"Using open() for IPC">
3097 for more examples of this. (You are not allowed to C<open> to a command
3098 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3099 and L<perlipc/"Bidirectional Communication with Another Process">
3102 For three or more arguments if MODE is C<'|-'>, the filename is
3103 interpreted as a command to which output is to be piped, and if MODE
3104 is C<'-|'>, the filename is interpreted as a command which pipes
3105 output to us. In the 2-arguments (and 1-argument) form one should
3106 replace dash (C<'-'>) with the command.
3107 See L<perlipc/"Using open() for IPC"> for more examples of this.
3108 (You are not allowed to C<open> to a command that pipes both in I<and>
3109 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3110 L<perlipc/"Bidirectional Communication"> for alternatives.)
3112 In the three-or-more argument form of pipe opens, if LIST is specified
3113 (extra arguments after the command name) then LIST becomes arguments
3114 to the command invoked if the platform supports it. The meaning of
3115 C<open> with more than three arguments for non-pipe modes is not yet
3116 specified. Experimental "layers" may give extra LIST arguments
3119 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3120 and opening C<< '>-' >> opens STDOUT.
3122 You may use the three-argument form of open to specify IO "layers"
3123 (sometimes also referred to as "disciplines") to be applied to the handle
3124 that affect how the input and output are processed (see L<open> and
3125 L<PerlIO> for more details). For example
3127 open(FH, "<:utf8", "file")
3129 will open the UTF-8 encoded file containing Unicode characters,
3130 see L<perluniintro>. Note that if layers are specified in the
3131 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3132 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3134 Open returns nonzero upon success, the undefined value otherwise. If
3135 the C<open> involved a pipe, the return value happens to be the pid of
3138 If you're running Perl on a system that distinguishes between text
3139 files and binary files, then you should check out L</binmode> for tips
3140 for dealing with this. The key distinction between systems that need
3141 C<binmode> and those that don't is their text file formats. Systems
3142 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3143 character, and which encode that character in C as C<"\n">, do not
3144 need C<binmode>. The rest need it.
3146 When opening a file, it's usually a bad idea to continue normal execution
3147 if the request failed, so C<open> is frequently used in connection with
3148 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3149 where you want to make a nicely formatted error message (but there are
3150 modules that can help with that problem)) you should always check
3151 the return value from opening a file. The infrequent exception is when
3152 working with an unopened filehandle is actually what you want to do.
3154 As a special case the 3-arg form with a read/write mode and the third
3155 argument being C<undef>:
3157 open(TMP, "+>", undef) or die ...
3159 opens a filehandle to an anonymous temporary file. Also using "+<"
3160 works for symmetry, but you really should consider writing something
3161 to the temporary file first. You will need to seek() to do the
3164 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3165 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3166 "in memory" files held in Perl scalars via:
3168 open($fh, '>', \$variable) || ..
3170 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3171 file, you have to close it first:
3174 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3179 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3180 while (<ARTICLE>) {...
3182 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3183 # if the open fails, output is discarded
3185 open(DBASE, '+<', 'dbase.mine') # open for update
3186 or die "Can't open 'dbase.mine' for update: $!";
3188 open(DBASE, '+<dbase.mine') # ditto
3189 or die "Can't open 'dbase.mine' for update: $!";
3191 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3192 or die "Can't start caesar: $!";
3194 open(ARTICLE, "caesar <$article |") # ditto
3195 or die "Can't start caesar: $!";
3197 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3198 or die "Can't start sort: $!";
3201 open(MEMORY,'>', \$var)
3202 or die "Can't open memory file: $!";
3203 print MEMORY "foo!\n"; # output will end up in $var
3205 # process argument list of files along with any includes
3207 foreach $file (@ARGV) {
3208 process($file, 'fh00');
3212 my($filename, $input) = @_;
3213 $input++; # this is a string increment
3214 unless (open($input, $filename)) {
3215 print STDERR "Can't open $filename: $!\n";
3220 while (<$input>) { # note use of indirection
3221 if (/^#include "(.*)"/) {
3222 process($1, $input);
3229 See L<perliol> for detailed info on PerlIO.
3231 You may also, in the Bourne shell tradition, specify an EXPR beginning
3232 with C<< '>&' >>, in which case the rest of the string is interpreted
3233 as the name of a filehandle (or file descriptor, if numeric) to be
3234 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3235 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3236 The mode you specify should match the mode of the original filehandle.
3237 (Duping a filehandle does not take into account any existing contents
3238 of IO buffers.) If you use the 3-arg form then you can pass either a
3239 number, the name of a filehandle or the normal "reference to a glob".
3241 Here is a script that saves, redirects, and restores C<STDOUT> and
3242 C<STDERR> using various methods:
3245 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3246 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3248 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3249 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3251 select STDERR; $| = 1; # make unbuffered
3252 select STDOUT; $| = 1; # make unbuffered
3254 print STDOUT "stdout 1\n"; # this works for
3255 print STDERR "stderr 1\n"; # subprocesses too
3257 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3258 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3260 print STDOUT "stdout 2\n";
3261 print STDERR "stderr 2\n";
3263 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3264 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3265 that file descriptor (and not call L<dup(2)>); this is more
3266 parsimonious of file descriptors. For example:
3268 # open for input, reusing the fileno of $fd
3269 open(FILEHANDLE, "<&=$fd")
3273 open(FILEHANDLE, "<&=", $fd)
3277 # open for append, using the fileno of OLDFH
3278 open(FH, ">>&=", OLDFH)
3282 open(FH, ">>&=OLDFH")
3284 Being parsimonious on filehandles is also useful (besides being
3285 parsimonious) for example when something is dependent on file
3286 descriptors, like for example locking using flock(). If you do just
3287 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3288 descriptor as B, and therefore flock(A) will not flock(B), and vice
3289 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3290 the same file descriptor.
3292 Note that if you are using Perls older than 5.8.0, Perl will be using
3293 the standard C libraries' fdopen() to implement the "=" functionality.
3294 On many UNIX systems fdopen() fails when file descriptors exceed a
3295 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3296 most often the default.
3298 You can see whether Perl has been compiled with PerlIO or not by
3299 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3300 is C<define>, you have PerlIO, otherwise you don't.
3302 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3303 with 2-arguments (or 1-argument) form of open(), then
3304 there is an implicit fork done, and the return value of open is the pid
3305 of the child within the parent process, and C<0> within the child
3306 process. (Use C<defined($pid)> to determine whether the open was successful.)
3307 The filehandle behaves normally for the parent, but i/o to that
3308 filehandle is piped from/to the STDOUT/STDIN of the child process.
3309 In the child process the filehandle isn't opened--i/o happens from/to
3310 the new STDOUT or STDIN. Typically this is used like the normal
3311 piped open when you want to exercise more control over just how the
3312 pipe command gets executed, such as when you are running setuid, and
3313 don't want to have to scan shell commands for metacharacters.
3314 The following triples are more or less equivalent:
3316 open(FOO, "|tr '[a-z]' '[A-Z]'");
3317 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3318 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3319 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3321 open(FOO, "cat -n '$file'|");
3322 open(FOO, '-|', "cat -n '$file'");
3323 open(FOO, '-|') || exec 'cat', '-n', $file;
3324 open(FOO, '-|', "cat", '-n', $file);
3326 The last example in each block shows the pipe as "list form", which is
3327 not yet supported on all platforms. A good rule of thumb is that if
3328 your platform has true C<fork()> (in other words, if your platform is
3329 UNIX) you can use the list form.
3331 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3333 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3334 output before any operation that may do a fork, but this may not be
3335 supported on some platforms (see L<perlport>). To be safe, you may need
3336 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3337 of C<IO::Handle> on any open handles.
3339 On systems that support a close-on-exec flag on files, the flag will
3340 be set for the newly opened file descriptor as determined by the value
3341 of $^F. See L<perlvar/$^F>.
3343 Closing any piped filehandle causes the parent process to wait for the
3344 child to finish, and returns the status value in C<$?> and
3345 C<${^CHILD_ERROR_NATIVE}>.
3347 The filename passed to 2-argument (or 1-argument) form of open() will
3348 have leading and trailing whitespace deleted, and the normal
3349 redirection characters honored. This property, known as "magic open",
3350 can often be used to good effect. A user could specify a filename of
3351 F<"rsh cat file |">, or you could change certain filenames as needed:
3353 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3354 open(FH, $filename) or die "Can't open $filename: $!";
3356 Use 3-argument form to open a file with arbitrary weird characters in it,
3358 open(FOO, '<', $file);
3360 otherwise it's necessary to protect any leading and trailing whitespace:
3362 $file =~ s#^(\s)#./$1#;
3363 open(FOO, "< $file\0");
3365 (this may not work on some bizarre filesystems). One should
3366 conscientiously choose between the I<magic> and 3-arguments form
3371 will allow the user to specify an argument of the form C<"rsh cat file |">,
3372 but will not work on a filename which happens to have a trailing space, while
3374 open IN, '<', $ARGV[0];
3376 will have exactly the opposite restrictions.
3378 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3379 should use the C<sysopen> function, which involves no such magic (but
3380 may use subtly different filemodes than Perl open(), which is mapped
3381 to C fopen()). This is
3382 another way to protect your filenames from interpretation. For example:
3385 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3386 or die "sysopen $path: $!";
3387 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3388 print HANDLE "stuff $$\n";
3390 print "File contains: ", <HANDLE>;
3392 Using the constructor from the C<IO::Handle> package (or one of its
3393 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3394 filehandles that have the scope of whatever variables hold references to
3395 them, and automatically close whenever and however you leave that scope:
3399 sub read_myfile_munged {
3401 my $handle = new IO::File;
3402 open($handle, "myfile") or die "myfile: $!";
3404 or return (); # Automatically closed here.
3405 mung $first or die "mung failed"; # Or here.
3406 return $first, <$handle> if $ALL; # Or here.
3410 See L</seek> for some details about mixing reading and writing.
3412 =item opendir DIRHANDLE,EXPR
3415 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3416 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3417 DIRHANDLE may be an expression whose value can be used as an indirect
3418 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3419 scalar variable (or array or hash element), the variable is assigned a
3420 reference to a new anonymous dirhandle.
3421 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3428 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3429 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3432 For the reverse, see L</chr>.
3433 See L<perlunicode> and L<encoding> for more about Unicode.
3440 =item our EXPR : ATTRS
3442 =item our TYPE EXPR : ATTRS
3444 C<our> associates a simple name with a package variable in the current
3445 package for use within the current scope. When C<use strict 'vars'> is in
3446 effect, C<our> lets you use declared global variables without qualifying
3447 them with package names, within the lexical scope of the C<our> declaration.
3448 In this way C<our> differs from C<use vars>, which is package scoped.
3450 Unlike C<my>, which both allocates storage for a variable and associates
3451 a simple name with that storage for use within the current scope, C<our>
3452 associates a simple name with a package variable in the current package,
3453 for use within the current scope. In other words, C<our> has the same
3454 scoping rules as C<my>, but does not necessarily create a
3457 If more than one value is listed, the list must be placed
3463 An C<our> declaration declares a global variable that will be visible
3464 across its entire lexical scope, even across package boundaries. The
3465 package in which the variable is entered is determined at the point
3466 of the declaration, not at the point of use. This means the following
3470 our $bar; # declares $Foo::bar for rest of lexical scope
3474 print $bar; # prints 20, as it refers to $Foo::bar
3476 Multiple C<our> declarations with the same name in the same lexical
3477 scope are allowed if they are in different packages. If they happen
3478 to be in the same package, Perl will emit warnings if you have asked
3479 for them, just like multiple C<my> declarations. Unlike a second
3480 C<my> declaration, which will bind the name to a fresh variable, a
3481 second C<our> declaration in the same package, in the same scope, is
3486 our $bar; # declares $Foo::bar for rest of lexical scope
3490 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3491 print $bar; # prints 30
3493 our $bar; # emits warning but has no other effect
3494 print $bar; # still prints 30
3496 An C<our> declaration may also have a list of attributes associated
3499 The exact semantics and interface of TYPE and ATTRS are still
3500 evolving. TYPE is currently bound to the use of C<fields> pragma,
3501 and attributes are handled using the C<attributes> pragma, or starting
3502 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3503 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3504 L<attributes>, and L<Attribute::Handlers>.
3506 =item pack TEMPLATE,LIST
3509 Takes a LIST of values and converts it into a string using the rules
3510 given by the TEMPLATE. The resulting string is the concatenation of
3511 the converted values. Typically, each converted value looks
3512 like its machine-level representation. For example, on 32-bit machines
3513 an integer may be represented by a sequence of 4 bytes that will be
3514 converted to a sequence of 4 characters.
3516 The TEMPLATE is a sequence of characters that give the order and type
3517 of values, as follows:
3519 a A string with arbitrary binary data, will be null padded.
3520 A A text (ASCII) string, will be space padded.
3521 Z A null terminated (ASCIZ) string, will be null padded.
3523 b A bit string (ascending bit order inside each byte, like vec()).
3524 B A bit string (descending bit order inside each byte).
3525 h A hex string (low nybble first).
3526 H A hex string (high nybble first).
3528 c A signed char (8-bit) value.
3529 C An unsigned C char (octet) even under Unicode. Should normally not
3530 be used. See U and W instead.
3531 W An unsigned char value (can be greater than 255).
3533 s A signed short (16-bit) value.
3534 S An unsigned short value.
3536 l A signed long (32-bit) value.
3537 L An unsigned long value.
3539 q A signed quad (64-bit) value.
3540 Q An unsigned quad value.
3541 (Quads are available only if your system supports 64-bit
3542 integer values _and_ if Perl has been compiled to support those.
3543 Causes a fatal error otherwise.)
3545 i A signed integer value.
3546 I A unsigned integer value.
3547 (This 'integer' is _at_least_ 32 bits wide. Its exact
3548 size depends on what a local C compiler calls 'int'.)
3550 n An unsigned short (16-bit) in "network" (big-endian) order.
3551 N An unsigned long (32-bit) in "network" (big-endian) order.
3552 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3553 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3555 j A Perl internal signed integer value (IV).
3556 J A Perl internal unsigned integer value (UV).
3558 f A single-precision float in the native format.
3559 d A double-precision float in the native format.
3561 F A Perl internal floating point value (NV) in the native format
3562 D A long double-precision float in the native format.
3563 (Long doubles are available only if your system supports long
3564 double values _and_ if Perl has been compiled to support those.
3565 Causes a fatal error otherwise.)
3567 p A pointer to a null-terminated string.
3568 P A pointer to a structure (fixed-length string).
3570 u A uuencoded string.
3571 U A Unicode character number. Encodes to UTF-8 internally
3572 (or UTF-EBCDIC in EBCDIC platforms).
3574 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3575 details). Its bytes represent an unsigned integer in base 128,
3576 most significant digit first, with as few digits as possible. Bit
3577 eight (the high bit) is set on each byte except the last.
3581 @ Null fill or truncate to absolute position, counted from the
3582 start of the innermost ()-group.
3583 . Null fill or truncate to absolute position specified by value.
3584 ( Start of a ()-group.
3586 One or more of the modifiers below may optionally follow some letters in the
3587 TEMPLATE (the second column lists the letters for which the modifier is
3590 ! sSlLiI Forces native (short, long, int) sizes instead
3591 of fixed (16-/32-bit) sizes.
3593 xX Make x and X act as alignment commands.
3595 nNvV Treat integers as signed instead of unsigned.
3597 @. Specify position as byte offset in the internal
3598 representation of the packed string. Efficient but
3601 > sSiIlLqQ Force big-endian byte-order on the type.
3602 jJfFdDpP (The "big end" touches the construct.)
3604 < sSiIlLqQ Force little-endian byte-order on the type.
3605 jJfFdDpP (The "little end" touches the construct.)
3607 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3608 in which case they force a certain byte-order on all components of
3609 that group, including subgroups.
3611 The following rules apply:
3617 Each letter may optionally be followed by a number giving a repeat
3618 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3619 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3620 that many values from the LIST. A C<*> for the repeat count means to
3621 use however many items are left, except for C<@>, C<x>, C<X>, where it
3622 is equivalent to C<0>, for <.> where it means relative to string start
3623 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3624 A numeric repeat count may optionally be enclosed in brackets, as in
3625 C<pack 'C[80]', @arr>.
3627 One can replace the numeric repeat count by a template enclosed in brackets;
3628 then the packed length of this template in bytes is used as a count.
3629 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3630 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3631 If the template in brackets contains alignment commands (such as C<x![d]>),
3632 its packed length is calculated as if the start of the template has the maximal
3635 When used with C<Z>, C<*> results in the addition of a trailing null
3636 byte (so the packed result will be one longer than the byte C<length>
3639 When used with C<@>, the repeat count represents an offset from the start
3640 of the innermost () group.
3642 When used with C<.>, the repeat count is used to determine the starting
3643 position from where the value offset is calculated. If the repeat count
3644 is 0, it's relative to the current position. If the repeat count is C<*>,
3645 the offset is relative to the start of the packed string. And if its an
3646 integer C<n> the offset is relative to the start of the n-th innermost
3647 () group (or the start of the string if C<n> is bigger then the group
3650 The repeat count for C<u> is interpreted as the maximal number of bytes
3651 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3652 count should not be more than 65.
3656 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3657 string of length count, padding with nulls or spaces as necessary. When
3658 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3659 after the first null, and C<a> returns data verbatim.
3661 If the value-to-pack is too long, it is truncated. If too long and an
3662 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3663 by a null byte. Thus C<Z> always packs a trailing null (except when the
3668 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3669 Each character of the input field of pack() generates 1 bit of the result.
3670 Each result bit is based on the least-significant bit of the corresponding
3671 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3672 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3674 Starting from the beginning of the input string of pack(), each 8-tuple
3675 of characters is converted to 1 character of output. With format C<b>
3676 the first character of the 8-tuple determines the least-significant bit of a
3677 character, and with format C<B> it determines the most-significant bit of
3680 If the length of the input string is not exactly divisible by 8, the
3681 remainder is packed as if the input string were padded by null characters
3682 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3684 If the input string of pack() is longer than needed, extra characters are
3685 ignored. A C<*> for the repeat count of pack() means to use all the
3686 characters of the input field. On unpack()ing the bits are converted to a
3687 string of C<"0">s and C<"1">s.
3691 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3692 representable as hexadecimal digits, 0-9a-f) long.
3694 Each character of the input field of pack() generates 4 bits of the result.
3695 For non-alphabetical characters the result is based on the 4 least-significant
3696 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3697 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3698 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3699 is compatible with the usual hexadecimal digits, so that C<"a"> and
3700 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3701 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3703 Starting from the beginning of the input string of pack(), each pair
3704 of characters is converted to 1 character of output. With format C<h> the
3705 first character of the pair determines the least-significant nybble of the
3706 output character, and with format C<H> it determines the most-significant
3709 If the length of the input string is not even, it behaves as if padded
3710 by a null character at the end. Similarly, during unpack()ing the "extra"
3711 nybbles are ignored.
3713 If the input string of pack() is longer than needed, extra characters are
3715 A C<*> for the repeat count of pack() means to use all the characters of
3716 the input field. On unpack()ing the nybbles are converted to a string
3717 of hexadecimal digits.
3721 The C<p> type packs a pointer to a null-terminated string. You are
3722 responsible for ensuring the string is not a temporary value (which can
3723 potentially get deallocated before you get around to using the packed result).
3724 The C<P> type packs a pointer to a structure of the size indicated by the
3725 length. A NULL pointer is created if the corresponding value for C<p> or
3726 C<P> is C<undef>, similarly for unpack().
3728 If your system has a strange pointer size (i.e. a pointer is neither as
3729 big as an int nor as big as a long), it may not be possible to pack or
3730 unpack pointers in big- or little-endian byte order. Attempting to do
3731 so will result in a fatal error.
3735 The C</> template character allows packing and unpacking of a sequence of
3736 items where the packed structure contains a packed item count followed by
3737 the packed items themselves.
3739 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3740 I<length-item> describes how the length value is packed. The ones likely
3741 to be of most use are integer-packing ones like C<n> (for Java strings),
3742 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3744 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3745 the minimum of that and the number of available items is used as argument
3746 for the I<length-item>. If it has no repeat count or uses a '*', the number
3747 of available items is used.
3749 For C<unpack> an internal stack of integer arguments unpacked so far is
3750 used. You write C</>I<sequence-item> and the repeat count is obtained by
3751 popping off the last element from the stack. The I<sequence-item> must not
3752 have a repeat count.
3754 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3755 the I<length-item> is a string length, not a number of strings. If there is
3756 an explicit repeat count for pack, the packed string will be adjusted to that
3759 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3760 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3761 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3762 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3763 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3765 The I<length-item> is not returned explicitly from C<unpack>.
3767 Adding a count to the I<length-item> letter is unlikely to do anything
3768 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3769 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3770 which Perl does not regard as legal in numeric strings.
3774 The integer types C<s>, C<S>, C<l>, and C<L> may be
3775 followed by a C<!> modifier to signify native shorts or
3776 longs--as you can see from above for example a bare C<l> does mean
3777 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3778 may be larger. This is an issue mainly in 64-bit platforms. You can
3779 see whether using C<!> makes any difference by
3781 print length(pack("s")), " ", length(pack("s!")), "\n";
3782 print length(pack("l")), " ", length(pack("l!")), "\n";
3784 C<i!> and C<I!> also work but only because of completeness;
3785 they are identical to C<i> and C<I>.
3787 The actual sizes (in bytes) of native shorts, ints, longs, and long
3788 longs on the platform where Perl was built are also available via
3792 print $Config{shortsize}, "\n";
3793 print $Config{intsize}, "\n";
3794 print $Config{longsize}, "\n";
3795 print $Config{longlongsize}, "\n";
3797 (The C<$Config{longlongsize}> will be undefined if your system does
3798 not support long longs.)
3802 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3803 are inherently non-portable between processors and operating systems
3804 because they obey the native byteorder and endianness. For example a
3805 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3806 (arranged in and handled by the CPU registers) into bytes as
3808 0x12 0x34 0x56 0x78 # big-endian
3809 0x78 0x56 0x34 0x12 # little-endian
3811 Basically, the Intel and VAX CPUs are little-endian, while everybody
3812 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3813 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3814 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3817 The names `big-endian' and `little-endian' are comic references to
3818 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3819 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3820 the egg-eating habits of the Lilliputians.
3822 Some systems may have even weirder byte orders such as
3827 You can see your system's preference with
3829 print join(" ", map { sprintf "%#02x", $_ }
3830 unpack("W*",pack("L",0x12345678))), "\n";
3832 The byteorder on the platform where Perl was built is also available
3836 print $Config{byteorder}, "\n";
3838 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3839 and C<'87654321'> are big-endian.
3841 If you want portable packed integers you can either use the formats
3842 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3843 modifiers. These modifiers are only available as of perl 5.9.2.
3844 See also L<perlport>.
3848 All integer and floating point formats as well as C<p> and C<P> and
3849 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3850 to force big- or little- endian byte-order, respectively.
3851 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3852 signed integers, 64-bit integers and floating point values. However,
3853 there are some things to keep in mind.
3855 Exchanging signed integers between different platforms only works
3856 if all platforms store them in the same format. Most platforms store
3857 signed integers in two's complement, so usually this is not an issue.
3859 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3860 formats on big- or little-endian machines. Otherwise, attempting to
3861 do so will result in a fatal error.
3863 Forcing big- or little-endian byte-order on floating point values for
3864 data exchange can only work if all platforms are using the same
3865 binary representation (e.g. IEEE floating point format). Even if all
3866 platforms are using IEEE, there may be subtle differences. Being able
3867 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3868 but also very dangerous if you don't know exactly what you're doing.
3869 It is definitely not a general way to portably store floating point
3872 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3873 all types inside the group that accept the byte-order modifiers,
3874 including all subgroups. It will silently be ignored for all other
3875 types. You are not allowed to override the byte-order within a group
3876 that already has a byte-order modifier suffix.
3880 Real numbers (floats and doubles) are in the native machine format only;
3881 due to the multiplicity of floating formats around, and the lack of a
3882 standard "network" representation, no facility for interchange has been
3883 made. This means that packed floating point data written on one machine
3884 may not be readable on another - even if both use IEEE floating point
3885 arithmetic (as the endian-ness of the memory representation is not part
3886 of the IEEE spec). See also L<perlport>.
3888 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3889 modifiers to force big- or little-endian byte-order on floating point values.
3891 Note that Perl uses doubles (or long doubles, if configured) internally for
3892 all numeric calculation, and converting from double into float and thence back
3893 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3894 will not in general equal $foo).
3898 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3899 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3900 where the packed string is processed in its UTF-8-encoded Unicode form on
3901 a byte by byte basis. Character mode is the default unless the format string
3902 starts with an C<U>. You can switch mode at any moment with an explicit
3903 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3904 or until the end of the ()-group in which it was entered.
3908 You must yourself do any alignment or padding by inserting for example
3909 enough C<'x'>es while packing. There is no way to pack() and unpack()
3910 could know where the characters are going to or coming from. Therefore
3911 C<pack> (and C<unpack>) handle their output and input as flat
3912 sequences of characters.
3916 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3917 take a repeat count, both as postfix, and for unpack() also via the C</>
3918 template character. Within each repetition of a group, positioning with
3919 C<@> starts again at 0. Therefore, the result of
3921 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3923 is the string "\0a\0\0bc".
3927 C<x> and C<X> accept C<!> modifier. In this case they act as
3928 alignment commands: they jump forward/back to the closest position
3929 aligned at a multiple of C<count> characters. For example, to pack() or
3930 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3931 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3932 aligned on the double's size.
3934 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3935 both result in no-ops.
3939 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3940 will represent signed 16-/32-bit integers in big-/little-endian order.
3941 This is only portable if all platforms sharing the packed data use the
3942 same binary representation for signed integers (e.g. all platforms are
3943 using two's complement representation).
3947 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3948 White space may be used to separate pack codes from each other, but
3949 modifiers and a repeat count must follow immediately.
3953 If TEMPLATE requires more arguments to pack() than actually given, pack()
3954 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3955 to pack() than actually given, extra arguments are ignored.
3961 $foo = pack("WWWW",65,66,67,68);
3963 $foo = pack("W4",65,66,67,68);
3965 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3966 # same thing with Unicode circled letters.
3967 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3968 # same thing with Unicode circled letters. You don't get the UTF-8
3969 # bytes because the U at the start of the format caused a switch to
3970 # U0-mode, so the UTF-8 bytes get joined into characters
3971 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3972 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3973 # This is the UTF-8 encoding of the string in the previous example
3975 $foo = pack("ccxxcc",65,66,67,68);
3978 # note: the above examples featuring "W" and "c" are true
3979 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3980 # and UTF-8. In EBCDIC the first example would be
3981 # $foo = pack("WWWW",193,194,195,196);
3983 $foo = pack("s2",1,2);
3984 # "\1\0\2\0" on little-endian
3985 # "\0\1\0\2" on big-endian
3987 $foo = pack("a4","abcd","x","y","z");
3990 $foo = pack("aaaa","abcd","x","y","z");
3993 $foo = pack("a14","abcdefg");
3994 # "abcdefg\0\0\0\0\0\0\0"
3996 $foo = pack("i9pl", gmtime);
3997 # a real struct tm (on my system anyway)
3999 $utmp_template = "Z8 Z8 Z16 L";
4000 $utmp = pack($utmp_template, @utmp1);
4001 # a struct utmp (BSDish)
4003 @utmp2 = unpack($utmp_template, $utmp);
4004 # "@utmp1" eq "@utmp2"
4007 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4010 $foo = pack('sx2l', 12, 34);
4011 # short 12, two zero bytes padding, long 34
4012 $bar = pack('s@4l', 12, 34);
4013 # short 12, zero fill to position 4, long 34
4015 $baz = pack('s.l', 12, 4, 34);
4016 # short 12, zero fill to position 4, long 34
4018 $foo = pack('nN', 42, 4711);
4019 # pack big-endian 16- and 32-bit unsigned integers
4020 $foo = pack('S>L>', 42, 4711);
4022 $foo = pack('s<l<', -42, 4711);
4023 # pack little-endian 16- and 32-bit signed integers
4024 $foo = pack('(sl)<', -42, 4711);
4027 The same template may generally also be used in unpack().
4029 =item package NAMESPACE
4030 X<package> X<module> X<namespace>
4034 Declares the compilation unit as being in the given namespace. The scope
4035 of the package declaration is from the declaration itself through the end
4036 of the enclosing block, file, or eval (the same as the C<my> operator).
4037 All further unqualified dynamic identifiers will be in this namespace.
4038 A package statement affects only dynamic variables--including those
4039 you've used C<local> on--but I<not> lexical variables, which are created
4040 with C<my>. Typically it would be the first declaration in a file to
4041 be included by the C<require> or C<use> operator. You can switch into a
4042 package in more than one place; it merely influences which symbol table
4043 is used by the compiler for the rest of that block. You can refer to
4044 variables and filehandles in other packages by prefixing the identifier
4045 with the package name and a double colon: C<$Package::Variable>.
4046 If the package name is null, the C<main> package as assumed. That is,
4047 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4048 still seen in older code).
4050 If NAMESPACE is omitted, then there is no current package, and all
4051 identifiers must be fully qualified or lexicals. However, you are
4052 strongly advised not to make use of this feature. Its use can cause
4053 unexpected behaviour, even crashing some versions of Perl. It is
4054 deprecated, and will be removed from a future release.
4056 See L<perlmod/"Packages"> for more information about packages, modules,
4057 and classes. See L<perlsub> for other scoping issues.
4059 =item pipe READHANDLE,WRITEHANDLE
4062 Opens a pair of connected pipes like the corresponding system call.
4063 Note that if you set up a loop of piped processes, deadlock can occur
4064 unless you are very careful. In addition, note that Perl's pipes use
4065 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4066 after each command, depending on the application.
4068 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4069 for examples of such things.
4071 On systems that support a close-on-exec flag on files, the flag will be set
4072 for the newly opened file descriptors as determined by the value of $^F.
4080 Pops and returns the last value of the array, shortening the array by
4081 one element. Has an effect similar to
4085 If there are no elements in the array, returns the undefined value
4086 (although this may happen at other times as well). If ARRAY is
4087 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4088 array in subroutines, just like C<shift>.
4091 X<pos> X<match, position>
4095 Returns the offset of where the last C<m//g> search left off for the variable
4096 in question (C<$_> is used when the variable is not specified). Note that
4097 0 is a valid match offset. C<undef> indicates that the search position
4098 is reset (usually due to match failure, but can also be because no match has
4099 yet been performed on the scalar). C<pos> directly accesses the location used
4100 by the regexp engine to store the offset, so assigning to C<pos> will change
4101 that offset, and so will also influence the C<\G> zero-width assertion in
4102 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4103 the return from C<pos> won't change either in this case. See L<perlre> and
4106 =item print FILEHANDLE LIST
4113 Prints a string or a list of strings. Returns true if successful.
4114 FILEHANDLE may be a scalar variable name, in which case the variable
4115 contains the name of or a reference to the filehandle, thus introducing
4116 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4117 the next token is a term, it may be misinterpreted as an operator
4118 unless you interpose a C<+> or put parentheses around the arguments.)
4119 If FILEHANDLE is omitted, prints by default to standard output (or
4120 to the last selected output channel--see L</select>). If LIST is
4121 also omitted, prints C<$_> to the currently selected output channel.
4122 To set the default output channel to something other than STDOUT
4123 use the select operation. The current value of C<$,> (if any) is
4124 printed between each LIST item. The current value of C<$\> (if
4125 any) is printed after the entire LIST has been printed. Because
4126 print takes a LIST, anything in the LIST is evaluated in list
4127 context, and any subroutine that you call will have one or more of
4128 its expressions evaluated in list context. Also be careful not to
4129 follow the print keyword with a left parenthesis unless you want
4130 the corresponding right parenthesis to terminate the arguments to
4131 the print--interpose a C<+> or put parentheses around all the
4134 Note that if you're storing FILEHANDLEs in an array, or if you're using
4135 any other expression more complex than a scalar variable to retrieve it,
4136 you will have to use a block returning the filehandle value instead:
4138 print { $files[$i] } "stuff\n";
4139 print { $OK ? STDOUT : STDERR } "stuff\n";
4141 =item printf FILEHANDLE FORMAT, LIST
4144 =item printf FORMAT, LIST
4146 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4147 (the output record separator) is not appended. The first argument
4148 of the list will be interpreted as the C<printf> format. See C<sprintf>
4149 for an explanation of the format argument. If C<use locale> is in effect,
4150 the character used for the decimal point in formatted real numbers is
4151 affected by the LC_NUMERIC locale. See L<perllocale>.
4153 Don't fall into the trap of using a C<printf> when a simple
4154 C<print> would do. The C<print> is more efficient and less
4157 =item prototype FUNCTION
4160 Returns the prototype of a function as a string (or C<undef> if the
4161 function has no prototype). FUNCTION is a reference to, or the name of,
4162 the function whose prototype you want to retrieve.
4164 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4165 name for Perl builtin. If the builtin is not I<overridable> (such as
4166 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4167 C<system>) returns C<undef> because the builtin does not really behave
4168 like a Perl function. Otherwise, the string describing the equivalent
4169 prototype is returned.
4171 =item push ARRAY,LIST
4174 Treats ARRAY as a stack, and pushes the values of LIST
4175 onto the end of ARRAY. The length of ARRAY increases by the length of
4176 LIST. Has the same effect as
4179 $ARRAY[++$#ARRAY] = $value;
4182 but is more efficient. Returns the number of elements in the array following
4183 the completed C<push>.
4195 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4197 =item quotemeta EXPR
4198 X<quotemeta> X<metacharacter>
4202 Returns the value of EXPR with all non-"word"
4203 characters backslashed. (That is, all characters not matching
4204 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4205 returned string, regardless of any locale settings.)
4206 This is the internal function implementing
4207 the C<\Q> escape in double-quoted strings.
4209 If EXPR is omitted, uses C<$_>.
4216 Returns a random fractional number greater than or equal to C<0> and less
4217 than the value of EXPR. (EXPR should be positive.) If EXPR is
4218 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4219 also special-cased as C<1> - this has not been documented before perl 5.8.0
4220 and is subject to change in future versions of perl. Automatically calls
4221 C<srand> unless C<srand> has already been called. See also C<srand>.
4223 Apply C<int()> to the value returned by C<rand()> if you want random
4224 integers instead of random fractional numbers. For example,
4228 returns a random integer between C<0> and C<9>, inclusive.
4230 (Note: If your rand function consistently returns numbers that are too
4231 large or too small, then your version of Perl was probably compiled
4232 with the wrong number of RANDBITS.)
4234 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4237 =item read FILEHANDLE,SCALAR,LENGTH
4239 Attempts to read LENGTH I<characters> of data into variable SCALAR
4240 from the specified FILEHANDLE. Returns the number of characters
4241 actually read, C<0> at end of file, or undef if there was an error (in
4242 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4243 so that the last character actually read is the last character of the
4244 scalar after the read.
4246 An OFFSET may be specified to place the read data at some place in the
4247 string other than the beginning. A negative OFFSET specifies
4248 placement at that many characters counting backwards from the end of
4249 the string. A positive OFFSET greater than the length of SCALAR
4250 results in the string being padded to the required size with C<"\0">
4251 bytes before the result of the read is appended.
4253 The call is actually implemented in terms of either Perl's or system's
4254 fread() call. To get a true read(2) system call, see C<sysread>.
4256 Note the I<characters>: depending on the status of the filehandle,
4257 either (8-bit) bytes or characters are read. By default all
4258 filehandles operate on bytes, but for example if the filehandle has
4259 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4260 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4261 characters, not bytes. Similarly for the C<:encoding> pragma:
4262 in that case pretty much any characters can be read.
4264 =item readdir DIRHANDLE
4267 Returns the next directory entry for a directory opened by C<opendir>.
4268 If used in list context, returns all the rest of the entries in the
4269 directory. If there are no more entries, returns an undefined value in
4270 scalar context or a null list in list context.
4272 If you're planning to filetest the return values out of a C<readdir>, you'd
4273 better prepend the directory in question. Otherwise, because we didn't
4274 C<chdir> there, it would have been testing the wrong file.
4276 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4277 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4281 X<readline> X<gets> X<fgets>
4283 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4284 context, each call reads and returns the next line, until end-of-file is
4285 reached, whereupon the subsequent call returns undef. In list context,
4286 reads until end-of-file is reached and returns a list of lines. Note that
4287 the notion of "line" used here is however you may have defined it
4288 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4290 When C<$/> is set to C<undef>, when readline() is in scalar
4291 context (i.e. file slurp mode), and when an empty file is read, it
4292 returns C<''> the first time, followed by C<undef> subsequently.
4294 This is the internal function implementing the C<< <EXPR> >>
4295 operator, but you can use it directly. The C<< <EXPR> >>
4296 operator is discussed in more detail in L<perlop/"I/O Operators">.
4299 $line = readline(*STDIN); # same thing
4301 If readline encounters an operating system error, C<$!> will be set with the
4302 corresponding error message. It can be helpful to check C<$!> when you are
4303 reading from filehandles you don't trust, such as a tty or a socket. The
4304 following example uses the operator form of C<readline>, and takes the necessary
4305 steps to ensure that C<readline> was successful.
4309 unless (defined( $line = <> )) {
4321 Returns the value of a symbolic link, if symbolic links are
4322 implemented. If not, gives a fatal error. If there is some system
4323 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4324 omitted, uses C<$_>.
4329 EXPR is executed as a system command.
4330 The collected standard output of the command is returned.
4331 In scalar context, it comes back as a single (potentially
4332 multi-line) string. In list context, returns a list of lines
4333 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4334 This is the internal function implementing the C<qx/EXPR/>
4335 operator, but you can use it directly. The C<qx/EXPR/>
4336 operator is discussed in more detail in L<perlop/"I/O Operators">.
4338 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4341 Receives a message on a socket. Attempts to receive LENGTH characters
4342 of data into variable SCALAR from the specified SOCKET filehandle.
4343 SCALAR will be grown or shrunk to the length actually read. Takes the
4344 same flags as the system call of the same name. Returns the address
4345 of the sender if SOCKET's protocol supports this; returns an empty
4346 string otherwise. If there's an error, returns the undefined value.
4347 This call is actually implemented in terms of recvfrom(2) system call.
4348 See L<perlipc/"UDP: Message Passing"> for examples.
4350 Note the I<characters>: depending on the status of the socket, either
4351 (8-bit) bytes or characters are received. By default all sockets
4352 operate on bytes, but for example if the socket has been changed using
4353 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4354 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4355 characters, not bytes. Similarly for the C<:encoding> pragma:
4356 in that case pretty much any characters can be read.
4363 The C<redo> command restarts the loop block without evaluating the
4364 conditional again. The C<continue> block, if any, is not executed. If
4365 the LABEL is omitted, the command refers to the innermost enclosing
4366 loop. Programs that want to lie to themselves about what was just input
4367 normally use this command:
4369 # a simpleminded Pascal comment stripper
4370 # (warning: assumes no { or } in strings)
4371 LINE: while (<STDIN>) {
4372 while (s|({.*}.*){.*}|$1 |) {}
4377 if (/}/) { # end of comment?
4386 C<redo> cannot be used to retry a block which returns a value such as
4387 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4388 a grep() or map() operation.
4390 Note that a block by itself is semantically identical to a loop
4391 that executes once. Thus C<redo> inside such a block will effectively
4392 turn it into a looping construct.
4394 See also L</continue> for an illustration of how C<last>, C<next>, and
4402 Returns a non-empty string if EXPR is a reference, the empty
4403 string otherwise. If EXPR
4404 is not specified, C<$_> will be used. The value returned depends on the
4405 type of thing the reference is a reference to.
4406 Builtin types include:
4420 If the referenced object has been blessed into a package, then that package
4421 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4423 if (ref($r) eq "HASH") {
4424 print "r is a reference to a hash.\n";
4427 print "r is not a reference at all.\n";
4430 See also L<perlref>.
4432 =item rename OLDNAME,NEWNAME
4433 X<rename> X<move> X<mv> X<ren>
4435 Changes the name of a file; an existing file NEWNAME will be
4436 clobbered. Returns true for success, false otherwise.
4438 Behavior of this function varies wildly depending on your system
4439 implementation. For example, it will usually not work across file system
4440 boundaries, even though the system I<mv> command sometimes compensates
4441 for this. Other restrictions include whether it works on directories,
4442 open files, or pre-existing files. Check L<perlport> and either the
4443 rename(2) manpage or equivalent system documentation for details.
4445 =item require VERSION
4452 Demands a version of Perl specified by VERSION, or demands some semantics
4453 specified by EXPR or by C<$_> if EXPR is not supplied.
4455 VERSION may be either a numeric argument such as 5.006, which will be
4456 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4457 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4458 VERSION is greater than the version of the current Perl interpreter.
4459 Compare with L</use>, which can do a similar check at compile time.
4461 Specifying VERSION as a literal of the form v5.6.1 should generally be
4462 avoided, because it leads to misleading error messages under earlier
4463 versions of Perl that do not support this syntax. The equivalent numeric
4464 version should be used instead.
4466 require v5.6.1; # run time version check
4467 require 5.6.1; # ditto
4468 require 5.006_001; # ditto; preferred for backwards compatibility
4470 Otherwise, C<require> demands that a library file be included if it
4471 hasn't already been included. The file is included via the do-FILE
4472 mechanism, which is essentially just a variety of C<eval>. Has
4473 semantics similar to the following subroutine:
4476 my ($filename) = @_;
4477 if (exists $INC{$filename}) {
4478 return 1 if $INC{$filename};
4479 die "Compilation failed in require";
4481 my ($realfilename,$result);
4483 foreach $prefix (@INC) {
4484 $realfilename = "$prefix/$filename";
4485 if (-f $realfilename) {
4486 $INC{$filename} = $realfilename;
4487 $result = do $realfilename;
4491 die "Can't find $filename in \@INC";
4494 $INC{$filename} = undef;
4496 } elsif (!$result) {
4497 delete $INC{$filename};
4498 die "$filename did not return true value";
4504 Note that the file will not be included twice under the same specified
4507 The file must return true as the last statement to indicate
4508 successful execution of any initialization code, so it's customary to
4509 end such a file with C<1;> unless you're sure it'll return true
4510 otherwise. But it's better just to put the C<1;>, in case you add more
4513 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4514 replaces "F<::>" with "F</>" in the filename for you,
4515 to make it easy to load standard modules. This form of loading of
4516 modules does not risk altering your namespace.
4518 In other words, if you try this:
4520 require Foo::Bar; # a splendid bareword
4522 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4523 directories specified in the C<@INC> array.
4525 But if you try this:
4527 $class = 'Foo::Bar';
4528 require $class; # $class is not a bareword
4530 require "Foo::Bar"; # not a bareword because of the ""
4532 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4533 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4535 eval "require $class";
4537 Now that you understand how C<require> looks for files in the case of a
4538 bareword argument, there is a little extra functionality going on behind
4539 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4540 first look for a similar filename with a "F<.pmc>" extension. If this file
4541 is found, it will be loaded in place of any file ending in a "F<.pm>"
4544 You can also insert hooks into the import facility, by putting directly
4545 Perl code into the @INC array. There are three forms of hooks: subroutine
4546 references, array references and blessed objects.
4548 Subroutine references are the simplest case. When the inclusion system
4549 walks through @INC and encounters a subroutine, this subroutine gets
4550 called with two parameters, the first being a reference to itself, and the
4551 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4552 subroutine should return nothing, or a list of up to 4 values in the
4559 A reference to a scalar, containing any initial source code to prepend to
4560 the file or generator output.
4565 A filehandle, from which the file will be read.
4569 A reference to a subroutine. If there is no file handle, then this subroutine
4570 is expected to generate one line of source code per call, writing the line
4571 into C<$_> and returning 1, then returning 0 at "end of FILE" If there is a
4572 file handle then the subroutine will be called to act a simple source filter,
4573 with the line as read in C<$_>. Again, return 1 for each valid line, and 0
4574 after all lines have been returned.
4578 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4579 reference to the subroutine itself is passed in as C<$_[0]>.
4583 If an empty list, C<undef>, or nothing that matches the first 3 values above
4584 is returned then C<require> will look at the remaining elements of @INC.
4585 Note that this file handle must be a real file handle (strictly a typeglob,
4586 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4587 ignored and return value processing will stop there.
4589 If the hook is an array reference, its first element must be a subroutine
4590 reference. This subroutine is called as above, but the first parameter is
4591 the array reference. This enables to pass indirectly some arguments to
4594 In other words, you can write:
4596 push @INC, \&my_sub;
4598 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4604 push @INC, [ \&my_sub, $x, $y, ... ];
4606 my ($arrayref, $filename) = @_;
4607 # Retrieve $x, $y, ...
4608 my @parameters = @$arrayref[1..$#$arrayref];
4612 If the hook is an object, it must provide an INC method that will be
4613 called as above, the first parameter being the object itself. (Note that
4614 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4615 into package C<main>.) Here is a typical code layout:
4621 my ($self, $filename) = @_;
4625 # In the main program
4626 push @INC, new Foo(...);
4628 Note that these hooks are also permitted to set the %INC entry
4629 corresponding to the files they have loaded. See L<perlvar/%INC>.
4631 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4638 Generally used in a C<continue> block at the end of a loop to clear
4639 variables and reset C<??> searches so that they work again. The
4640 expression is interpreted as a list of single characters (hyphens
4641 allowed for ranges). All variables and arrays beginning with one of
4642 those letters are reset to their pristine state. If the expression is
4643 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4644 only variables or searches in the current package. Always returns
4647 reset 'X'; # reset all X variables
4648 reset 'a-z'; # reset lower case variables
4649 reset; # just reset ?one-time? searches
4651 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4652 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4653 variables--lexical variables are unaffected, but they clean themselves
4654 up on scope exit anyway, so you'll probably want to use them instead.
4662 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4663 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4664 context, depending on how the return value will be used, and the context
4665 may vary from one execution to the next (see C<wantarray>). If no EXPR
4666 is given, returns an empty list in list context, the undefined value in
4667 scalar context, and (of course) nothing at all in a void context.
4669 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4670 or do FILE will automatically return the value of the last expression
4674 X<reverse> X<rev> X<invert>
4676 In list context, returns a list value consisting of the elements
4677 of LIST in the opposite order. In scalar context, concatenates the
4678 elements of LIST and returns a string value with all characters
4679 in the opposite order.
4681 print reverse <>; # line tac, last line first
4683 undef $/; # for efficiency of <>
4684 print scalar reverse <>; # character tac, last line tsrif
4686 Used without arguments in scalar context, reverse() reverses C<$_>.
4688 This operator is also handy for inverting a hash, although there are some
4689 caveats. If a value is duplicated in the original hash, only one of those
4690 can be represented as a key in the inverted hash. Also, this has to
4691 unwind one hash and build a whole new one, which may take some time
4692 on a large hash, such as from a DBM file.
4694 %by_name = reverse %by_address; # Invert the hash
4696 =item rewinddir DIRHANDLE
4699 Sets the current position to the beginning of the directory for the
4700 C<readdir> routine on DIRHANDLE.
4702 =item rindex STR,SUBSTR,POSITION
4705 =item rindex STR,SUBSTR
4707 Works just like index() except that it returns the position of the I<last>
4708 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4709 last occurrence beginning at or before that position.
4711 =item rmdir FILENAME
4712 X<rmdir> X<rd> X<directory, remove>
4716 Deletes the directory specified by FILENAME if that directory is
4717 empty. If it succeeds it returns true, otherwise it returns false and
4718 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4722 The substitution operator. See L<perlop>.
4724 =item say FILEHANDLE LIST
4731 Just like C<print>, but implicitly appends a newline.
4732 C<say LIST> is simply an abbreviation for C<print LIST, "\n">,
4733 and C<say()> works just like C<print($_, "\n")>.
4735 That means that a call to say() appends any output record separator
4736 I<after> the added newline.
4738 This keyword is only available when the "say" feature is
4739 enabled: see L<feature>.
4742 X<scalar> X<context>
4744 Forces EXPR to be interpreted in scalar context and returns the value
4747 @counts = ( scalar @a, scalar @b, scalar @c );
4749 There is no equivalent operator to force an expression to
4750 be interpolated in list context because in practice, this is never
4751 needed. If you really wanted to do so, however, you could use
4752 the construction C<@{[ (some expression) ]}>, but usually a simple
4753 C<(some expression)> suffices.
4755 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4756 parenthesized list, this behaves as a scalar comma expression, evaluating
4757 all but the last element in void context and returning the final element
4758 evaluated in scalar context. This is seldom what you want.
4760 The following single statement:
4762 print uc(scalar(&foo,$bar)),$baz;
4764 is the moral equivalent of these two:
4767 print(uc($bar),$baz);
4769 See L<perlop> for more details on unary operators and the comma operator.
4771 =item seek FILEHANDLE,POSITION,WHENCE
4772 X<seek> X<fseek> X<filehandle, position>
4774 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4775 FILEHANDLE may be an expression whose value gives the name of the
4776 filehandle. The values for WHENCE are C<0> to set the new position
4777 I<in bytes> to POSITION, C<1> to set it to the current position plus
4778 POSITION, and C<2> to set it to EOF plus POSITION (typically
4779 negative). For WHENCE you may use the constants C<SEEK_SET>,
4780 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4781 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4784 Note the I<in bytes>: even if the filehandle has been set to
4785 operate on characters (for example by using the C<:utf8> open
4786 layer), tell() will return byte offsets, not character offsets
4787 (because implementing that would render seek() and tell() rather slow).
4789 If you want to position file for C<sysread> or C<syswrite>, don't use
4790 C<seek>--buffering makes its effect on the file's system position
4791 unpredictable and non-portable. Use C<sysseek> instead.
4793 Due to the rules and rigors of ANSI C, on some systems you have to do a
4794 seek whenever you switch between reading and writing. Amongst other
4795 things, this may have the effect of calling stdio's clearerr(3).
4796 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4800 This is also useful for applications emulating C<tail -f>. Once you hit
4801 EOF on your read, and then sleep for a while, you might have to stick in a
4802 seek() to reset things. The C<seek> doesn't change the current position,
4803 but it I<does> clear the end-of-file condition on the handle, so that the
4804 next C<< <FILE> >> makes Perl try again to read something. We hope.
4806 If that doesn't work (some IO implementations are particularly
4807 cantankerous), then you may need something more like this:
4810 for ($curpos = tell(FILE); $_ = <FILE>;
4811 $curpos = tell(FILE)) {
4812 # search for some stuff and put it into files
4814 sleep($for_a_while);
4815 seek(FILE, $curpos, 0);
4818 =item seekdir DIRHANDLE,POS
4821 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4822 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4823 about possible directory compaction as the corresponding system library
4826 =item select FILEHANDLE
4827 X<select> X<filehandle, default>
4831 Returns the currently selected filehandle. Sets the current default
4832 filehandle for output, if FILEHANDLE is supplied. This has two
4833 effects: first, a C<write> or a C<print> without a filehandle will
4834 default to this FILEHANDLE. Second, references to variables related to
4835 output will refer to this output channel. For example, if you have to
4836 set the top of form format for more than one output channel, you might
4844 FILEHANDLE may be an expression whose value gives the name of the
4845 actual filehandle. Thus:
4847 $oldfh = select(STDERR); $| = 1; select($oldfh);
4849 Some programmers may prefer to think of filehandles as objects with
4850 methods, preferring to write the last example as:
4853 STDERR->autoflush(1);
4855 =item select RBITS,WBITS,EBITS,TIMEOUT
4858 This calls the select(2) system call with the bit masks specified, which
4859 can be constructed using C<fileno> and C<vec>, along these lines:
4861 $rin = $win = $ein = '';
4862 vec($rin,fileno(STDIN),1) = 1;
4863 vec($win,fileno(STDOUT),1) = 1;
4866 If you want to select on many filehandles you might wish to write a
4870 my(@fhlist) = split(' ',$_[0]);
4873 vec($bits,fileno($_),1) = 1;
4877 $rin = fhbits('STDIN TTY SOCK');
4881 ($nfound,$timeleft) =
4882 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4884 or to block until something becomes ready just do this
4886 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4888 Most systems do not bother to return anything useful in $timeleft, so
4889 calling select() in scalar context just returns $nfound.
4891 Any of the bit masks can also be undef. The timeout, if specified, is
4892 in seconds, which may be fractional. Note: not all implementations are
4893 capable of returning the $timeleft. If not, they always return
4894 $timeleft equal to the supplied $timeout.
4896 You can effect a sleep of 250 milliseconds this way:
4898 select(undef, undef, undef, 0.25);
4900 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4901 is implementation-dependent. See also L<perlport> for notes on the
4902 portability of C<select>.
4904 On error, C<select> behaves like the select(2) system call : it returns
4907 Note: on some Unixes, the select(2) system call may report a socket file
4908 descriptor as "ready for reading", when actually no data is available,
4909 thus a subsequent read blocks. It can be avoided using always the
4910 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4913 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4914 or <FH>) with C<select>, except as permitted by POSIX, and even
4915 then only on POSIX systems. You have to use C<sysread> instead.
4917 =item semctl ID,SEMNUM,CMD,ARG
4920 Calls the System V IPC function C<semctl>. You'll probably have to say
4924 first to get the correct constant definitions. If CMD is IPC_STAT or
4925 GETALL, then ARG must be a variable that will hold the returned
4926 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4927 the undefined value for error, "C<0 but true>" for zero, or the actual
4928 return value otherwise. The ARG must consist of a vector of native
4929 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4930 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4933 =item semget KEY,NSEMS,FLAGS
4936 Calls the System V IPC function semget. Returns the semaphore id, or
4937 the undefined value if there is an error. See also
4938 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4941 =item semop KEY,OPSTRING
4944 Calls the System V IPC function semop to perform semaphore operations
4945 such as signalling and waiting. OPSTRING must be a packed array of
4946 semop structures. Each semop structure can be generated with
4947 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4948 implies the number of semaphore operations. Returns true if
4949 successful, or false if there is an error. As an example, the
4950 following code waits on semaphore $semnum of semaphore id $semid:
4952 $semop = pack("s!3", $semnum, -1, 0);
4953 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4955 To signal the semaphore, replace C<-1> with C<1>. See also
4956 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4959 =item send SOCKET,MSG,FLAGS,TO
4962 =item send SOCKET,MSG,FLAGS
4964 Sends a message on a socket. Attempts to send the scalar MSG to the
4965 SOCKET filehandle. Takes the same flags as the system call of the
4966 same name. On unconnected sockets you must specify a destination to
4967 send TO, in which case it does a C C<sendto>. Returns the number of
4968 characters sent, or the undefined value if there is an error. The C
4969 system call sendmsg(2) is currently unimplemented. See
4970 L<perlipc/"UDP: Message Passing"> for examples.
4972 Note the I<characters>: depending on the status of the socket, either
4973 (8-bit) bytes or characters are sent. By default all sockets operate
4974 on bytes, but for example if the socket has been changed using
4975 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4976 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4977 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4978 in that case pretty much any characters can be sent.
4980 =item setpgrp PID,PGRP
4983 Sets the current process group for the specified PID, C<0> for the current
4984 process. Will produce a fatal error if used on a machine that doesn't
4985 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4986 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4987 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4990 =item setpriority WHICH,WHO,PRIORITY
4991 X<setpriority> X<priority> X<nice> X<renice>
4993 Sets the current priority for a process, a process group, or a user.
4994 (See setpriority(2).) Will produce a fatal error if used on a machine
4995 that doesn't implement setpriority(2).
4997 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5000 Sets the socket option requested. Returns undefined if there is an
5001 error. Use integer constants provided by the C<Socket> module for
5002 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5003 getprotobyname. OPTVAL might either be a packed string or an integer.
5004 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5006 An example disabling the Nagle's algorithm for a socket:
5008 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5009 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5016 Shifts the first value of the array off and returns it, shortening the
5017 array by 1 and moving everything down. If there are no elements in the
5018 array, returns the undefined value. If ARRAY is omitted, shifts the
5019 C<@_> array within the lexical scope of subroutines and formats, and the
5020 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5021 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>
5022 and C<END {}> constructs.
5024 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5025 same thing to the left end of an array that C<pop> and C<push> do to the
5028 =item shmctl ID,CMD,ARG
5031 Calls the System V IPC function shmctl. You'll probably have to say
5035 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5036 then ARG must be a variable that will hold the returned C<shmid_ds>
5037 structure. Returns like ioctl: the undefined value for error, "C<0> but
5038 true" for zero, or the actual return value otherwise.
5039 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5041 =item shmget KEY,SIZE,FLAGS
5044 Calls the System V IPC function shmget. Returns the shared memory
5045 segment id, or the undefined value if there is an error.
5046 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5048 =item shmread ID,VAR,POS,SIZE
5052 =item shmwrite ID,STRING,POS,SIZE
5054 Reads or writes the System V shared memory segment ID starting at
5055 position POS for size SIZE by attaching to it, copying in/out, and
5056 detaching from it. When reading, VAR must be a variable that will
5057 hold the data read. When writing, if STRING is too long, only SIZE
5058 bytes are used; if STRING is too short, nulls are written to fill out
5059 SIZE bytes. Return true if successful, or false if there is an error.
5060 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5061 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5063 =item shutdown SOCKET,HOW
5066 Shuts down a socket connection in the manner indicated by HOW, which
5067 has the same interpretation as in the system call of the same name.
5069 shutdown(SOCKET, 0); # I/we have stopped reading data
5070 shutdown(SOCKET, 1); # I/we have stopped writing data
5071 shutdown(SOCKET, 2); # I/we have stopped using this socket
5073 This is useful with sockets when you want to tell the other
5074 side you're done writing but not done reading, or vice versa.
5075 It's also a more insistent form of close because it also
5076 disables the file descriptor in any forked copies in other
5080 X<sin> X<sine> X<asin> X<arcsine>
5084 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5085 returns sine of C<$_>.
5087 For the inverse sine operation, you may use the C<Math::Trig::asin>
5088 function, or use this relation:
5090 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5097 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5098 May be interrupted if the process receives a signal such as C<SIGALRM>.
5099 Returns the number of seconds actually slept. You probably cannot
5100 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5103 On some older systems, it may sleep up to a full second less than what
5104 you requested, depending on how it counts seconds. Most modern systems
5105 always sleep the full amount. They may appear to sleep longer than that,
5106 however, because your process might not be scheduled right away in a
5107 busy multitasking system.
5109 For delays of finer granularity than one second, you may use Perl's
5110 C<syscall> interface to access setitimer(2) if your system supports
5111 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5112 and starting from Perl 5.8 part of the standard distribution) may also
5115 See also the POSIX module's C<pause> function.
5117 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5120 Opens a socket of the specified kind and attaches it to filehandle
5121 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5122 the system call of the same name. You should C<use Socket> first
5123 to get the proper definitions imported. See the examples in
5124 L<perlipc/"Sockets: Client/Server Communication">.
5126 On systems that support a close-on-exec flag on files, the flag will
5127 be set for the newly opened file descriptor, as determined by the
5128 value of $^F. See L<perlvar/$^F>.
5130 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5133 Creates an unnamed pair of sockets in the specified domain, of the
5134 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5135 for the system call of the same name. If unimplemented, yields a fatal
5136 error. Returns true if successful.
5138 On systems that support a close-on-exec flag on files, the flag will
5139 be set for the newly opened file descriptors, as determined by the value
5140 of $^F. See L<perlvar/$^F>.
5142 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5143 to C<pipe(Rdr, Wtr)> is essentially:
5146 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5147 shutdown(Rdr, 1); # no more writing for reader
5148 shutdown(Wtr, 0); # no more reading for writer
5150 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5151 emulate socketpair using IP sockets to localhost if your system implements
5152 sockets but not socketpair.
5154 =item sort SUBNAME LIST
5155 X<sort> X<qsort> X<quicksort> X<mergesort>
5157 =item sort BLOCK LIST
5161 In list context, this sorts the LIST and returns the sorted list value.
5162 In scalar context, the behaviour of C<sort()> is undefined.
5164 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5165 order. If SUBNAME is specified, it gives the name of a subroutine
5166 that returns an integer less than, equal to, or greater than C<0>,
5167 depending on how the elements of the list are to be ordered. (The C<<
5168 <=> >> and C<cmp> operators are extremely useful in such routines.)
5169 SUBNAME may be a scalar variable name (unsubscripted), in which case
5170 the value provides the name of (or a reference to) the actual
5171 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5172 an anonymous, in-line sort subroutine.
5174 If the subroutine's prototype is C<($$)>, the elements to be compared
5175 are passed by reference in C<@_>, as for a normal subroutine. This is
5176 slower than unprototyped subroutines, where the elements to be
5177 compared are passed into the subroutine
5178 as the package global variables $a and $b (see example below). Note that
5179 in the latter case, it is usually counter-productive to declare $a and
5182 The values to be compared are always passed by reference and should not
5185 You also cannot exit out of the sort block or subroutine using any of the
5186 loop control operators described in L<perlsyn> or with C<goto>.
5188 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5189 current collation locale. See L<perllocale>.
5191 sort() returns aliases into the original list, much as a for loop's index
5192 variable aliases the list elements. That is, modifying an element of a
5193 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5194 actually modifies the element in the original list. This is usually
5195 something to be avoided when writing clear code.
5197 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5198 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5199 preserves the input order of elements that compare equal. Although
5200 quicksort's run time is O(NlogN) when averaged over all arrays of
5201 length N, the time can be O(N**2), I<quadratic> behavior, for some
5202 inputs.) In 5.7, the quicksort implementation was replaced with
5203 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5204 But benchmarks indicated that for some inputs, on some platforms,
5205 the original quicksort was faster. 5.8 has a sort pragma for
5206 limited control of the sort. Its rather blunt control of the
5207 underlying algorithm may not persist into future Perls, but the
5208 ability to characterize the input or output in implementation
5209 independent ways quite probably will. See L<sort>.
5214 @articles = sort @files;
5216 # same thing, but with explicit sort routine
5217 @articles = sort {$a cmp $b} @files;
5219 # now case-insensitively
5220 @articles = sort {uc($a) cmp uc($b)} @files;
5222 # same thing in reversed order
5223 @articles = sort {$b cmp $a} @files;
5225 # sort numerically ascending
5226 @articles = sort {$a <=> $b} @files;
5228 # sort numerically descending
5229 @articles = sort {$b <=> $a} @files;
5231 # this sorts the %age hash by value instead of key
5232 # using an in-line function
5233 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5235 # sort using explicit subroutine name
5237 $age{$a} <=> $age{$b}; # presuming numeric
5239 @sortedclass = sort byage @class;
5241 sub backwards { $b cmp $a }
5242 @harry = qw(dog cat x Cain Abel);
5243 @george = qw(gone chased yz Punished Axed);
5245 # prints AbelCaincatdogx
5246 print sort backwards @harry;
5247 # prints xdogcatCainAbel
5248 print sort @george, 'to', @harry;
5249 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5251 # inefficiently sort by descending numeric compare using
5252 # the first integer after the first = sign, or the
5253 # whole record case-insensitively otherwise
5256 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5261 # same thing, but much more efficiently;
5262 # we'll build auxiliary indices instead
5266 push @nums, /=(\d+)/;
5271 $nums[$b] <=> $nums[$a]
5273 $caps[$a] cmp $caps[$b]
5277 # same thing, but without any temps
5278 @new = map { $_->[0] }
5279 sort { $b->[1] <=> $a->[1]
5282 } map { [$_, /=(\d+)/, uc($_)] } @old;
5284 # using a prototype allows you to use any comparison subroutine
5285 # as a sort subroutine (including other package's subroutines)
5287 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5290 @new = sort other::backwards @old;
5292 # guarantee stability, regardless of algorithm
5294 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5296 # force use of mergesort (not portable outside Perl 5.8)
5297 use sort '_mergesort'; # note discouraging _
5298 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5300 If you're using strict, you I<must not> declare $a
5301 and $b as lexicals. They are package globals. That means
5302 if you're in the C<main> package and type
5304 @articles = sort {$b <=> $a} @files;
5306 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5307 but if you're in the C<FooPack> package, it's the same as typing
5309 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5311 The comparison function is required to behave. If it returns
5312 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5313 sometimes saying the opposite, for example) the results are not
5316 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5317 (not-a-number), and because C<sort> will trigger a fatal error unless the
5318 result of a comparison is defined, when sorting with a comparison function
5319 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5320 The following example takes advantage of the fact that C<NaN != NaN> to
5321 eliminate any C<NaN>s from the input.
5323 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5325 =item splice ARRAY,OFFSET,LENGTH,LIST
5328 =item splice ARRAY,OFFSET,LENGTH
5330 =item splice ARRAY,OFFSET
5334 Removes the elements designated by OFFSET and LENGTH from an array, and
5335 replaces them with the elements of LIST, if any. In list context,
5336 returns the elements removed from the array. In scalar context,
5337 returns the last element removed, or C<undef> if no elements are
5338 removed. The array grows or shrinks as necessary.
5339 If OFFSET is negative then it starts that far from the end of the array.
5340 If LENGTH is omitted, removes everything from OFFSET onward.
5341 If LENGTH is negative, removes the elements from OFFSET onward
5342 except for -LENGTH elements at the end of the array.
5343 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5344 past the end of the array, perl issues a warning, and splices at the
5347 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5349 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5350 pop(@a) splice(@a,-1)
5351 shift(@a) splice(@a,0,1)
5352 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5353 $a[$i] = $y splice(@a,$i,1,$y)
5355 Example, assuming array lengths are passed before arrays:
5357 sub aeq { # compare two list values
5358 my(@a) = splice(@_,0,shift);
5359 my(@b) = splice(@_,0,shift);
5360 return 0 unless @a == @b; # same len?
5362 return 0 if pop(@a) ne pop(@b);
5366 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5368 =item split /PATTERN/,EXPR,LIMIT
5371 =item split /PATTERN/,EXPR
5373 =item split /PATTERN/
5377 Splits the string EXPR into a list of strings and returns that list. By
5378 default, empty leading fields are preserved, and empty trailing ones are
5379 deleted. (If all fields are empty, they are considered to be trailing.)
5381 In scalar context, returns the number of fields found and splits into
5382 the C<@_> array. Use of split in scalar context is deprecated, however,
5383 because it clobbers your subroutine arguments.
5385 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5386 splits on whitespace (after skipping any leading whitespace). Anything
5387 matching PATTERN is taken to be a delimiter separating the fields. (Note
5388 that the delimiter may be longer than one character.)
5390 If LIMIT is specified and positive, it represents the maximum number
5391 of fields the EXPR will be split into, though the actual number of
5392 fields returned depends on the number of times PATTERN matches within
5393 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5394 stripped (which potential users of C<pop> would do well to remember).
5395 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5396 had been specified. Note that splitting an EXPR that evaluates to the
5397 empty string always returns the empty list, regardless of the LIMIT
5400 A pattern matching the null string (not to be confused with
5401 a null pattern C<//>, which is just one member of the set of patterns
5402 matching a null string) will split the value of EXPR into separate
5403 characters at each point it matches that way. For example:
5405 print join(':', split(/ */, 'hi there'));
5407 produces the output 'h:i:t:h:e:r:e'.
5409 As a special case for C<split>, using the empty pattern C<//> specifically
5410 matches only the null string, and is not be confused with the regular use
5411 of C<//> to mean "the last successful pattern match". So, for C<split>,
5414 print join(':', split(//, 'hi there'));
5416 produces the output 'h:i: :t:h:e:r:e'.
5418 Empty leading (or trailing) fields are produced when there are positive
5419 width matches at the beginning (or end) of the string; a zero-width match
5420 at the beginning (or end) of the string does not produce an empty field.
5423 print join(':', split(/(?=\w)/, 'hi there!'));
5425 produces the output 'h:i :t:h:e:r:e!'.
5427 The LIMIT parameter can be used to split a line partially
5429 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5431 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5432 a LIMIT one larger than the number of variables in the list, to avoid
5433 unnecessary work. For the list above LIMIT would have been 4 by
5434 default. In time critical applications it behooves you not to split
5435 into more fields than you really need.
5437 If the PATTERN contains parentheses, additional list elements are
5438 created from each matching substring in the delimiter.
5440 split(/([,-])/, "1-10,20", 3);
5442 produces the list value
5444 (1, '-', 10, ',', 20)
5446 If you had the entire header of a normal Unix email message in $header,
5447 you could split it up into fields and their values this way:
5449 $header =~ s/\n\s+/ /g; # fix continuation lines
5450 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5452 The pattern C</PATTERN/> may be replaced with an expression to specify
5453 patterns that vary at runtime. (To do runtime compilation only once,
5454 use C</$variable/o>.)
5456 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5457 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5458 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5459 will give you as many null initial fields as there are leading spaces.
5460 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5461 whitespace produces a null first field. A C<split> with no arguments
5462 really does a S<C<split(' ', $_)>> internally.
5464 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5469 open(PASSWD, '/etc/passwd');
5472 ($login, $passwd, $uid, $gid,
5473 $gcos, $home, $shell) = split(/:/);
5477 As with regular pattern matching, any capturing parentheses that are not
5478 matched in a C<split()> will be set to C<undef> when returned:
5480 @fields = split /(A)|B/, "1A2B3";
5481 # @fields is (1, 'A', 2, undef, 3)
5483 =item sprintf FORMAT, LIST
5486 Returns a string formatted by the usual C<printf> conventions of the C
5487 library function C<sprintf>. See below for more details
5488 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5489 the general principles.
5493 # Format number with up to 8 leading zeroes
5494 $result = sprintf("%08d", $number);
5496 # Round number to 3 digits after decimal point
5497 $rounded = sprintf("%.3f", $number);
5499 Perl does its own C<sprintf> formatting--it emulates the C
5500 function C<sprintf>, but it doesn't use it (except for floating-point
5501 numbers, and even then only the standard modifiers are allowed). As a
5502 result, any non-standard extensions in your local C<sprintf> are not
5503 available from Perl.
5505 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5506 pass it an array as your first argument. The array is given scalar context,
5507 and instead of using the 0th element of the array as the format, Perl will
5508 use the count of elements in the array as the format, which is almost never
5511 Perl's C<sprintf> permits the following universally-known conversions:
5514 %c a character with the given number
5516 %d a signed integer, in decimal
5517 %u an unsigned integer, in decimal
5518 %o an unsigned integer, in octal
5519 %x an unsigned integer, in hexadecimal
5520 %e a floating-point number, in scientific notation
5521 %f a floating-point number, in fixed decimal notation
5522 %g a floating-point number, in %e or %f notation
5524 In addition, Perl permits the following widely-supported conversions:
5526 %X like %x, but using upper-case letters
5527 %E like %e, but using an upper-case "E"
5528 %G like %g, but with an upper-case "E" (if applicable)
5529 %b an unsigned integer, in binary
5530 %p a pointer (outputs the Perl value's address in hexadecimal)
5531 %n special: *stores* the number of characters output so far
5532 into the next variable in the parameter list
5534 Finally, for backward (and we do mean "backward") compatibility, Perl
5535 permits these unnecessary but widely-supported conversions:
5538 %D a synonym for %ld
5539 %U a synonym for %lu
5540 %O a synonym for %lo
5543 Note that the number of exponent digits in the scientific notation produced
5544 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5545 exponent less than 100 is system-dependent: it may be three or less
5546 (zero-padded as necessary). In other words, 1.23 times ten to the
5547 99th may be either "1.23e99" or "1.23e099".
5549 Between the C<%> and the format letter, you may specify a number of
5550 additional attributes controlling the interpretation of the format.
5551 In order, these are:
5555 =item format parameter index
5557 An explicit format parameter index, such as C<2$>. By default sprintf
5558 will format the next unused argument in the list, but this allows you
5559 to take the arguments out of order, e.g.:
5561 printf '%2$d %1$d', 12, 34; # prints "34 12"
5562 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5567 space prefix positive number with a space
5568 + prefix positive number with a plus sign
5569 - left-justify within the field
5570 0 use zeros, not spaces, to right-justify
5571 # prefix non-zero octal with "0", non-zero hex with "0x",
5572 non-zero binary with "0b"
5576 printf '<% d>', 12; # prints "< 12>"
5577 printf '<%+d>', 12; # prints "<+12>"
5578 printf '<%6s>', 12; # prints "< 12>"
5579 printf '<%-6s>', 12; # prints "<12 >"
5580 printf '<%06s>', 12; # prints "<000012>"
5581 printf '<%#x>', 12; # prints "<0xc>"
5585 This flag tells perl to interpret the supplied string as a vector of
5586 integers, one for each character in the string. Perl applies the format to
5587 each integer in turn, then joins the resulting strings with a separator (a
5588 dot C<.> by default). This can be useful for displaying ordinal values of
5589 characters in arbitrary strings:
5591 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5592 printf "version is v%vd\n", $^V; # Perl's version
5594 Put an asterisk C<*> before the C<v> to override the string to
5595 use to separate the numbers:
5597 printf "address is %*vX\n", ":", $addr; # IPv6 address
5598 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5600 You can also explicitly specify the argument number to use for
5601 the join string using e.g. C<*2$v>:
5603 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5605 =item (minimum) width
5607 Arguments are usually formatted to be only as wide as required to
5608 display the given value. You can override the width by putting
5609 a number here, or get the width from the next argument (with C<*>)
5610 or from a specified argument (with e.g. C<*2$>):
5612 printf '<%s>', "a"; # prints "<a>"
5613 printf '<%6s>', "a"; # prints "< a>"
5614 printf '<%*s>', 6, "a"; # prints "< a>"
5615 printf '<%*2$s>', "a", 6; # prints "< a>"
5616 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5618 If a field width obtained through C<*> is negative, it has the same
5619 effect as the C<-> flag: left-justification.
5621 =item precision, or maximum width
5624 You can specify a precision (for numeric conversions) or a maximum
5625 width (for string conversions) by specifying a C<.> followed by a number.
5626 For floating point formats, with the exception of 'g' and 'G', this specifies
5627 the number of decimal places to show (the default being 6), e.g.:
5629 # these examples are subject to system-specific variation
5630 printf '<%f>', 1; # prints "<1.000000>"
5631 printf '<%.1f>', 1; # prints "<1.0>"
5632 printf '<%.0f>', 1; # prints "<1>"
5633 printf '<%e>', 10; # prints "<1.000000e+01>"
5634 printf '<%.1e>', 10; # prints "<1.0e+01>"
5636 For 'g' and 'G', this specifies the maximum number of digits to show,
5637 including prior to the decimal point as well as after it, e.g.:
5639 # these examples are subject to system-specific variation
5640 printf '<%g>', 1; # prints "<1>"
5641 printf '<%.10g>', 1; # prints "<1>"
5642 printf '<%g>', 100; # prints "<100>"
5643 printf '<%.1g>', 100; # prints "<1e+02>"
5644 printf '<%.2g>', 100.01; # prints "<1e+02>"
5645 printf '<%.5g>', 100.01; # prints "<100.01>"
5646 printf '<%.4g>', 100.01; # prints "<100>"
5648 For integer conversions, specifying a precision implies that the
5649 output of the number itself should be zero-padded to this width:
5651 printf '<%.6x>', 1; # prints "<000001>"
5652 printf '<%#.6x>', 1; # prints "<0x000001>"
5653 printf '<%-10.6x>', 1; # prints "<000001 >"
5655 For string conversions, specifying a precision truncates the string
5656 to fit in the specified width:
5658 printf '<%.5s>', "truncated"; # prints "<trunc>"
5659 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5661 You can also get the precision from the next argument using C<.*>:
5663 printf '<%.6x>', 1; # prints "<000001>"
5664 printf '<%.*x>', 6, 1; # prints "<000001>"
5666 You cannot currently get the precision from a specified number,
5667 but it is intended that this will be possible in the future using
5670 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5674 For numeric conversions, you can specify the size to interpret the
5675 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5676 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5677 whatever the default integer size is on your platform (usually 32 or 64
5678 bits), but you can override this to use instead one of the standard C types,
5679 as supported by the compiler used to build Perl:
5681 l interpret integer as C type "long" or "unsigned long"
5682 h interpret integer as C type "short" or "unsigned short"
5683 q, L or ll interpret integer as C type "long long", "unsigned long long".
5684 or "quads" (typically 64-bit integers)
5686 The last will produce errors if Perl does not understand "quads" in your
5687 installation. (This requires that either the platform natively supports quads
5688 or Perl was specifically compiled to support quads.) You can find out
5689 whether your Perl supports quads via L<Config>:
5692 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5695 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5696 to be the default floating point size on your platform (double or long double),
5697 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5698 platform supports them. You can find out whether your Perl supports long
5699 doubles via L<Config>:
5702 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5704 You can find out whether Perl considers 'long double' to be the default
5705 floating point size to use on your platform via L<Config>:
5708 ($Config{uselongdouble} eq 'define') &&
5709 print "long doubles by default\n";
5711 It can also be the case that long doubles and doubles are the same thing:
5714 ($Config{doublesize} == $Config{longdblsize}) &&
5715 print "doubles are long doubles\n";
5717 The size specifier C<V> has no effect for Perl code, but it is supported
5718 for compatibility with XS code; it means 'use the standard size for
5719 a Perl integer (or floating-point number)', which is already the
5720 default for Perl code.
5722 =item order of arguments
5724 Normally, sprintf takes the next unused argument as the value to
5725 format for each format specification. If the format specification
5726 uses C<*> to require additional arguments, these are consumed from
5727 the argument list in the order in which they appear in the format
5728 specification I<before> the value to format. Where an argument is
5729 specified using an explicit index, this does not affect the normal
5730 order for the arguments (even when the explicitly specified index
5731 would have been the next argument in any case).
5735 printf '<%*.*s>', $a, $b, $c;
5737 would use C<$a> for the width, C<$b> for the precision and C<$c>
5738 as the value to format, while:
5740 print '<%*1$.*s>', $a, $b;
5742 would use C<$a> for the width and the precision, and C<$b> as the
5745 Here are some more examples - beware that when using an explicit
5746 index, the C<$> may need to be escaped:
5748 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5749 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5750 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5751 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5755 If C<use locale> is in effect, the character used for the decimal
5756 point in formatted real numbers is affected by the LC_NUMERIC locale.
5760 X<sqrt> X<root> X<square root>
5764 Return the square root of EXPR. If EXPR is omitted, returns square
5765 root of C<$_>. Only works on non-negative operands, unless you've
5766 loaded the standard Math::Complex module.
5769 print sqrt(-2); # prints 1.4142135623731i
5772 X<srand> X<seed> X<randseed>
5776 Sets the random number seed for the C<rand> operator.
5778 The point of the function is to "seed" the C<rand> function so that
5779 C<rand> can produce a different sequence each time you run your
5782 If srand() is not called explicitly, it is called implicitly at the
5783 first use of the C<rand> operator. However, this was not the case in
5784 versions of Perl before 5.004, so if your script will run under older
5785 Perl versions, it should call C<srand>.
5787 Most programs won't even call srand() at all, except those that
5788 need a cryptographically-strong starting point rather than the
5789 generally acceptable default, which is based on time of day,
5790 process ID, and memory allocation, or the F</dev/urandom> device,
5793 You can call srand($seed) with the same $seed to reproduce the
5794 I<same> sequence from rand(), but this is usually reserved for
5795 generating predictable results for testing or debugging.
5796 Otherwise, don't call srand() more than once in your program.
5798 Do B<not> call srand() (i.e. without an argument) more than once in
5799 a script. The internal state of the random number generator should
5800 contain more entropy than can be provided by any seed, so calling
5801 srand() again actually I<loses> randomness.
5803 Most implementations of C<srand> take an integer and will silently
5804 truncate decimal numbers. This means C<srand(42)> will usually
5805 produce the same results as C<srand(42.1)>. To be safe, always pass
5806 C<srand> an integer.
5808 In versions of Perl prior to 5.004 the default seed was just the
5809 current C<time>. This isn't a particularly good seed, so many old
5810 programs supply their own seed value (often C<time ^ $$> or C<time ^
5811 ($$ + ($$ << 15))>), but that isn't necessary any more.
5813 For cryptographic purposes, however, you need something much more random
5814 than the default seed. Checksumming the compressed output of one or more
5815 rapidly changing operating system status programs is the usual method. For
5818 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5820 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5823 Frequently called programs (like CGI scripts) that simply use
5827 for a seed can fall prey to the mathematical property that
5831 one-third of the time. So don't do that.
5833 =item stat FILEHANDLE
5834 X<stat> X<file, status>
5838 =item stat DIRHANDLE
5842 Returns a 13-element list giving the status info for a file, either
5843 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5844 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5847 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5848 $atime,$mtime,$ctime,$blksize,$blocks)
5851 Not all fields are supported on all filesystem types. Here are the
5852 meanings of the fields:
5854 0 dev device number of filesystem
5856 2 mode file mode (type and permissions)
5857 3 nlink number of (hard) links to the file
5858 4 uid numeric user ID of file's owner
5859 5 gid numeric group ID of file's owner
5860 6 rdev the device identifier (special files only)
5861 7 size total size of file, in bytes
5862 8 atime last access time in seconds since the epoch
5863 9 mtime last modify time in seconds since the epoch
5864 10 ctime inode change time in seconds since the epoch (*)
5865 11 blksize preferred block size for file system I/O
5866 12 blocks actual number of blocks allocated
5868 (The epoch was at 00:00 January 1, 1970 GMT.)
5870 (*) Not all fields are supported on all filesystem types. Notably, the
5871 ctime field is non-portable. In particular, you cannot expect it to be a
5872 "creation time", see L<perlport/"Files and Filesystems"> for details.
5874 If C<stat> is passed the special filehandle consisting of an underline, no
5875 stat is done, but the current contents of the stat structure from the
5876 last C<stat>, C<lstat>, or filetest are returned. Example:
5878 if (-x $file && (($d) = stat(_)) && $d < 0) {
5879 print "$file is executable NFS file\n";
5882 (This works on machines only for which the device number is negative
5885 Because the mode contains both the file type and its permissions, you
5886 should mask off the file type portion and (s)printf using a C<"%o">
5887 if you want to see the real permissions.
5889 $mode = (stat($filename))[2];
5890 printf "Permissions are %04o\n", $mode & 07777;
5892 In scalar context, C<stat> returns a boolean value indicating success
5893 or failure, and, if successful, sets the information associated with
5894 the special filehandle C<_>.
5896 The File::stat module provides a convenient, by-name access mechanism:
5899 $sb = stat($filename);
5900 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5901 $filename, $sb->size, $sb->mode & 07777,
5902 scalar localtime $sb->mtime;
5904 You can import symbolic mode constants (C<S_IF*>) and functions
5905 (C<S_IS*>) from the Fcntl module:
5909 $mode = (stat($filename))[2];
5911 $user_rwx = ($mode & S_IRWXU) >> 6;
5912 $group_read = ($mode & S_IRGRP) >> 3;
5913 $other_execute = $mode & S_IXOTH;
5915 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5917 $is_setuid = $mode & S_ISUID;
5918 $is_setgid = S_ISDIR($mode);
5920 You could write the last two using the C<-u> and C<-d> operators.
5921 The commonly available C<S_IF*> constants are
5923 # Permissions: read, write, execute, for user, group, others.
5925 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5926 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5927 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5929 # Setuid/Setgid/Stickiness/SaveText.
5930 # Note that the exact meaning of these is system dependent.
5932 S_ISUID S_ISGID S_ISVTX S_ISTXT
5934 # File types. Not necessarily all are available on your system.
5936 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5938 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5940 S_IREAD S_IWRITE S_IEXEC
5942 and the C<S_IF*> functions are
5944 S_IMODE($mode) the part of $mode containing the permission bits
5945 and the setuid/setgid/sticky bits
5947 S_IFMT($mode) the part of $mode containing the file type
5948 which can be bit-anded with e.g. S_IFREG
5949 or with the following functions
5951 # The operators -f, -d, -l, -b, -c, -p, and -S.
5953 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5954 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5956 # No direct -X operator counterpart, but for the first one
5957 # the -g operator is often equivalent. The ENFMT stands for
5958 # record flocking enforcement, a platform-dependent feature.
5960 S_ISENFMT($mode) S_ISWHT($mode)
5962 See your native chmod(2) and stat(2) documentation for more details
5963 about the C<S_*> constants. To get status info for a symbolic link
5964 instead of the target file behind the link, use the C<lstat> function.
5969 =item state TYPE EXPR
5971 =item state EXPR : ATTRS
5973 =item state TYPE EXPR : ATTRS
5975 C<state> declares a lexically scoped variable, just like C<my> does.
5976 However, those variables will be initialized only once, contrary to
5977 lexical variables that are reinitialized each time their enclosing block
5980 C<state> variables are only enabled when the C<feature 'state'> pragma is
5981 in effect. See L<feature>.
5988 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5989 doing many pattern matches on the string before it is next modified.
5990 This may or may not save time, depending on the nature and number of
5991 patterns you are searching on, and on the distribution of character
5992 frequencies in the string to be searched--you probably want to compare
5993 run times with and without it to see which runs faster. Those loops
5994 that scan for many short constant strings (including the constant
5995 parts of more complex patterns) will benefit most. You may have only
5996 one C<study> active at a time--if you study a different scalar the first
5997 is "unstudied". (The way C<study> works is this: a linked list of every
5998 character in the string to be searched is made, so we know, for
5999 example, where all the C<'k'> characters are. From each search string,
6000 the rarest character is selected, based on some static frequency tables
6001 constructed from some C programs and English text. Only those places
6002 that contain this "rarest" character are examined.)
6004 For example, here is a loop that inserts index producing entries
6005 before any line containing a certain pattern:
6009 print ".IX foo\n" if /\bfoo\b/;
6010 print ".IX bar\n" if /\bbar\b/;
6011 print ".IX blurfl\n" if /\bblurfl\b/;
6016 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6017 will be looked at, because C<f> is rarer than C<o>. In general, this is
6018 a big win except in pathological cases. The only question is whether
6019 it saves you more time than it took to build the linked list in the
6022 Note that if you have to look for strings that you don't know till
6023 runtime, you can build an entire loop as a string and C<eval> that to
6024 avoid recompiling all your patterns all the time. Together with
6025 undefining C<$/> to input entire files as one record, this can be very
6026 fast, often faster than specialized programs like fgrep(1). The following
6027 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6028 out the names of those files that contain a match:
6030 $search = 'while (<>) { study;';
6031 foreach $word (@words) {
6032 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6037 eval $search; # this screams
6038 $/ = "\n"; # put back to normal input delimiter
6039 foreach $file (sort keys(%seen)) {
6043 =item sub NAME BLOCK
6046 =item sub NAME (PROTO) BLOCK
6048 =item sub NAME : ATTRS BLOCK
6050 =item sub NAME (PROTO) : ATTRS BLOCK
6052 This is subroutine definition, not a real function I<per se>.
6053 Without a BLOCK it's just a forward declaration. Without a NAME,
6054 it's an anonymous function declaration, and does actually return
6055 a value: the CODE ref of the closure you just created.
6057 See L<perlsub> and L<perlref> for details about subroutines and
6058 references, and L<attributes> and L<Attribute::Handlers> for more
6059 information about attributes.
6061 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6062 X<substr> X<substring> X<mid> X<left> X<right>
6064 =item substr EXPR,OFFSET,LENGTH
6066 =item substr EXPR,OFFSET
6068 Extracts a substring out of EXPR and returns it. First character is at
6069 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6070 If OFFSET is negative (or more precisely, less than C<$[>), starts
6071 that far from the end of the string. If LENGTH is omitted, returns
6072 everything to the end of the string. If LENGTH is negative, leaves that
6073 many characters off the end of the string.
6075 You can use the substr() function as an lvalue, in which case EXPR
6076 must itself be an lvalue. If you assign something shorter than LENGTH,
6077 the string will shrink, and if you assign something longer than LENGTH,
6078 the string will grow to accommodate it. To keep the string the same
6079 length you may need to pad or chop your value using C<sprintf>.
6081 If OFFSET and LENGTH specify a substring that is partly outside the
6082 string, only the part within the string is returned. If the substring
6083 is beyond either end of the string, substr() returns the undefined
6084 value and produces a warning. When used as an lvalue, specifying a
6085 substring that is entirely outside the string is a fatal error.
6086 Here's an example showing the behavior for boundary cases:
6089 substr($name, 4) = 'dy'; # $name is now 'freddy'
6090 my $null = substr $name, 6, 2; # returns '' (no warning)
6091 my $oops = substr $name, 7; # returns undef, with warning
6092 substr($name, 7) = 'gap'; # fatal error
6094 An alternative to using substr() as an lvalue is to specify the
6095 replacement string as the 4th argument. This allows you to replace
6096 parts of the EXPR and return what was there before in one operation,
6097 just as you can with splice().
6099 Note that the lvalue returned by the 3-arg version of substr() acts as
6100 a 'magic bullet'; each time it is assigned to, it remembers which part
6101 of the original string is being modified; for example:
6104 for (substr($x,1,2)) {
6105 $_ = 'a'; print $x,"\n"; # prints 1a4
6106 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6108 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6112 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6115 =item symlink OLDFILE,NEWFILE
6116 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6118 Creates a new filename symbolically linked to the old filename.
6119 Returns C<1> for success, C<0> otherwise. On systems that don't support
6120 symbolic links, produces a fatal error at run time. To check for that,
6123 $symlink_exists = eval { symlink("",""); 1 };
6125 =item syscall NUMBER, LIST
6126 X<syscall> X<system call>
6128 Calls the system call specified as the first element of the list,
6129 passing the remaining elements as arguments to the system call. If
6130 unimplemented, produces a fatal error. The arguments are interpreted
6131 as follows: if a given argument is numeric, the argument is passed as
6132 an int. If not, the pointer to the string value is passed. You are
6133 responsible to make sure a string is pre-extended long enough to
6134 receive any result that might be written into a string. You can't use a
6135 string literal (or other read-only string) as an argument to C<syscall>
6136 because Perl has to assume that any string pointer might be written
6138 integer arguments are not literals and have never been interpreted in a
6139 numeric context, you may need to add C<0> to them to force them to look
6140 like numbers. This emulates the C<syswrite> function (or vice versa):
6142 require 'syscall.ph'; # may need to run h2ph
6144 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6146 Note that Perl supports passing of up to only 14 arguments to your system call,
6147 which in practice should usually suffice.
6149 Syscall returns whatever value returned by the system call it calls.
6150 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6151 Note that some system calls can legitimately return C<-1>. The proper
6152 way to handle such calls is to assign C<$!=0;> before the call and
6153 check the value of C<$!> if syscall returns C<-1>.
6155 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6156 number of the read end of the pipe it creates. There is no way
6157 to retrieve the file number of the other end. You can avoid this
6158 problem by using C<pipe> instead.
6160 =item sysopen FILEHANDLE,FILENAME,MODE
6163 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6165 Opens the file whose filename is given by FILENAME, and associates it
6166 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6167 the name of the real filehandle wanted. This function calls the
6168 underlying operating system's C<open> function with the parameters
6169 FILENAME, MODE, PERMS.
6171 The possible values and flag bits of the MODE parameter are
6172 system-dependent; they are available via the standard module C<Fcntl>.
6173 See the documentation of your operating system's C<open> to see which
6174 values and flag bits are available. You may combine several flags
6175 using the C<|>-operator.
6177 Some of the most common values are C<O_RDONLY> for opening the file in
6178 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6179 and C<O_RDWR> for opening the file in read-write mode.
6180 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6182 For historical reasons, some values work on almost every system
6183 supported by perl: zero means read-only, one means write-only, and two
6184 means read/write. We know that these values do I<not> work under
6185 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6186 use them in new code.
6188 If the file named by FILENAME does not exist and the C<open> call creates
6189 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6190 PERMS specifies the permissions of the newly created file. If you omit
6191 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6192 These permission values need to be in octal, and are modified by your
6193 process's current C<umask>.
6196 In many systems the C<O_EXCL> flag is available for opening files in
6197 exclusive mode. This is B<not> locking: exclusiveness means here that
6198 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6199 on network filesystems, and has no effect unless the C<O_CREAT> flag
6200 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6201 being opened if it is a symbolic link. It does not protect against
6202 symbolic links in the file's path.
6205 Sometimes you may want to truncate an already-existing file. This
6206 can be done using the C<O_TRUNC> flag. The behavior of
6207 C<O_TRUNC> with C<O_RDONLY> is undefined.
6210 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6211 that takes away the user's option to have a more permissive umask.
6212 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6215 Note that C<sysopen> depends on the fdopen() C library function.
6216 On many UNIX systems, fdopen() is known to fail when file descriptors
6217 exceed a certain value, typically 255. If you need more file
6218 descriptors than that, consider rebuilding Perl to use the C<sfio>
6219 library, or perhaps using the POSIX::open() function.
6221 See L<perlopentut> for a kinder, gentler explanation of opening files.
6223 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6226 =item sysread FILEHANDLE,SCALAR,LENGTH
6228 Attempts to read LENGTH bytes of data into variable SCALAR from the
6229 specified FILEHANDLE, using the system call read(2). It bypasses
6230 buffered IO, so mixing this with other kinds of reads, C<print>,
6231 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6232 perlio or stdio layers usually buffers data. Returns the number of
6233 bytes actually read, C<0> at end of file, or undef if there was an
6234 error (in the latter case C<$!> is also set). SCALAR will be grown or
6235 shrunk so that the last byte actually read is the last byte of the
6236 scalar after the read.
6238 An OFFSET may be specified to place the read data at some place in the
6239 string other than the beginning. A negative OFFSET specifies
6240 placement at that many characters counting backwards from the end of
6241 the string. A positive OFFSET greater than the length of SCALAR
6242 results in the string being padded to the required size with C<"\0">
6243 bytes before the result of the read is appended.
6245 There is no syseof() function, which is ok, since eof() doesn't work
6246 very well on device files (like ttys) anyway. Use sysread() and check
6247 for a return value for 0 to decide whether you're done.
6249 Note that if the filehandle has been marked as C<:utf8> Unicode
6250 characters are read instead of bytes (the LENGTH, OFFSET, and the
6251 return value of sysread() are in Unicode characters).
6252 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6253 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6255 =item sysseek FILEHANDLE,POSITION,WHENCE
6258 Sets FILEHANDLE's system position in bytes using the system call
6259 lseek(2). FILEHANDLE may be an expression whose value gives the name
6260 of the filehandle. The values for WHENCE are C<0> to set the new
6261 position to POSITION, C<1> to set the it to the current position plus
6262 POSITION, and C<2> to set it to EOF plus POSITION (typically
6265 Note the I<in bytes>: even if the filehandle has been set to operate
6266 on characters (for example by using the C<:utf8> I/O layer), tell()
6267 will return byte offsets, not character offsets (because implementing
6268 that would render sysseek() very slow).
6270 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6271 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6272 C<seek>, C<tell>, or C<eof> may cause confusion.
6274 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6275 and C<SEEK_END> (start of the file, current position, end of the file)
6276 from the Fcntl module. Use of the constants is also more portable
6277 than relying on 0, 1, and 2. For example to define a "systell" function:
6279 use Fcntl 'SEEK_CUR';
6280 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6282 Returns the new position, or the undefined value on failure. A position
6283 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6284 true on success and false on failure, yet you can still easily determine
6290 =item system PROGRAM LIST
6292 Does exactly the same thing as C<exec LIST>, except that a fork is
6293 done first, and the parent process waits for the child process to
6294 complete. Note that argument processing varies depending on the
6295 number of arguments. If there is more than one argument in LIST,
6296 or if LIST is an array with more than one value, starts the program
6297 given by the first element of the list with arguments given by the
6298 rest of the list. If there is only one scalar argument, the argument
6299 is checked for shell metacharacters, and if there are any, the
6300 entire argument is passed to the system's command shell for parsing
6301 (this is C</bin/sh -c> on Unix platforms, but varies on other
6302 platforms). If there are no shell metacharacters in the argument,
6303 it is split into words and passed directly to C<execvp>, which is
6306 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6307 output before any operation that may do a fork, but this may not be
6308 supported on some platforms (see L<perlport>). To be safe, you may need
6309 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6310 of C<IO::Handle> on any open handles.
6312 The return value is the exit status of the program as returned by the
6313 C<wait> call. To get the actual exit value, shift right by eight (see
6314 below). See also L</exec>. This is I<not> what you want to use to capture
6315 the output from a command, for that you should use merely backticks or
6316 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6317 indicates a failure to start the program or an error of the wait(2) system
6318 call (inspect $! for the reason).
6320 Like C<exec>, C<system> allows you to lie to a program about its name if
6321 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6323 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6324 C<system>, if you expect your program to terminate on receipt of these
6325 signals you will need to arrange to do so yourself based on the return
6328 @args = ("command", "arg1", "arg2");
6330 or die "system @args failed: $?"
6332 You can check all the failure possibilities by inspecting
6336 print "failed to execute: $!\n";
6339 printf "child died with signal %d, %s coredump\n",
6340 ($? & 127), ($? & 128) ? 'with' : 'without';
6343 printf "child exited with value %d\n", $? >> 8;
6346 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6347 with the W*() calls of the POSIX extension.
6349 When the arguments get executed via the system shell, results
6350 and return codes will be subject to its quirks and capabilities.
6351 See L<perlop/"`STRING`"> and L</exec> for details.
6353 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6356 =item syswrite FILEHANDLE,SCALAR,LENGTH
6358 =item syswrite FILEHANDLE,SCALAR
6360 Attempts to write LENGTH bytes of data from variable SCALAR to the
6361 specified FILEHANDLE, using the system call write(2). If LENGTH is
6362 not specified, writes whole SCALAR. It bypasses buffered IO, so
6363 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6364 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6365 stdio layers usually buffers data. Returns the number of bytes
6366 actually written, or C<undef> if there was an error (in this case the
6367 errno variable C<$!> is also set). If the LENGTH is greater than the
6368 available data in the SCALAR after the OFFSET, only as much data as is
6369 available will be written.
6371 An OFFSET may be specified to write the data from some part of the
6372 string other than the beginning. A negative OFFSET specifies writing
6373 that many characters counting backwards from the end of the string.
6374 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6376 Note that if the filehandle has been marked as C<:utf8>, Unicode
6377 characters are written instead of bytes (the LENGTH, OFFSET, and the
6378 return value of syswrite() are in UTF-8 encoded Unicode characters).
6379 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6380 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6382 =item tell FILEHANDLE
6387 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6388 error. FILEHANDLE may be an expression whose value gives the name of
6389 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6392 Note the I<in bytes>: even if the filehandle has been set to
6393 operate on characters (for example by using the C<:utf8> open
6394 layer), tell() will return byte offsets, not character offsets
6395 (because that would render seek() and tell() rather slow).
6397 The return value of tell() for the standard streams like the STDIN
6398 depends on the operating system: it may return -1 or something else.
6399 tell() on pipes, fifos, and sockets usually returns -1.
6401 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6403 Do not use tell() (or other buffered I/O operations) on a file handle
6404 that has been manipulated by sysread(), syswrite() or sysseek().
6405 Those functions ignore the buffering, while tell() does not.
6407 =item telldir DIRHANDLE
6410 Returns the current position of the C<readdir> routines on DIRHANDLE.
6411 Value may be given to C<seekdir> to access a particular location in a
6412 directory. C<telldir> has the same caveats about possible directory
6413 compaction as the corresponding system library routine.
6415 =item tie VARIABLE,CLASSNAME,LIST
6418 This function binds a variable to a package class that will provide the
6419 implementation for the variable. VARIABLE is the name of the variable
6420 to be enchanted. CLASSNAME is the name of a class implementing objects
6421 of correct type. Any additional arguments are passed to the C<new>
6422 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6423 or C<TIEHASH>). Typically these are arguments such as might be passed
6424 to the C<dbm_open()> function of C. The object returned by the C<new>
6425 method is also returned by the C<tie> function, which would be useful
6426 if you want to access other methods in CLASSNAME.
6428 Note that functions such as C<keys> and C<values> may return huge lists
6429 when used on large objects, like DBM files. You may prefer to use the
6430 C<each> function to iterate over such. Example:
6432 # print out history file offsets
6434 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6435 while (($key,$val) = each %HIST) {
6436 print $key, ' = ', unpack('L',$val), "\n";
6440 A class implementing a hash should have the following methods:
6442 TIEHASH classname, LIST
6444 STORE this, key, value
6449 NEXTKEY this, lastkey
6454 A class implementing an ordinary array should have the following methods:
6456 TIEARRAY classname, LIST
6458 STORE this, key, value
6460 STORESIZE this, count
6466 SPLICE this, offset, length, LIST
6471 A class implementing a file handle should have the following methods:
6473 TIEHANDLE classname, LIST
6474 READ this, scalar, length, offset
6477 WRITE this, scalar, length, offset
6479 PRINTF this, format, LIST
6483 SEEK this, position, whence
6485 OPEN this, mode, LIST
6490 A class implementing a scalar should have the following methods:
6492 TIESCALAR classname, LIST
6498 Not all methods indicated above need be implemented. See L<perltie>,
6499 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6501 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6502 for you--you need to do that explicitly yourself. See L<DB_File>
6503 or the F<Config> module for interesting C<tie> implementations.
6505 For further details see L<perltie>, L<"tied VARIABLE">.
6510 Returns a reference to the object underlying VARIABLE (the same value
6511 that was originally returned by the C<tie> call that bound the variable
6512 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6518 Returns the number of non-leap seconds since whatever time the system
6519 considers to be the epoch, suitable for feeding to C<gmtime> and
6520 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6521 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6522 1904 in the current local time zone for its epoch.
6524 For measuring time in better granularity than one second,
6525 you may use either the Time::HiRes module (from CPAN, and starting from
6526 Perl 5.8 part of the standard distribution), or if you have
6527 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6528 See L<perlfaq8> for details.
6533 Returns a four-element list giving the user and system times, in
6534 seconds, for this process and the children of this process.
6536 ($user,$system,$cuser,$csystem) = times;
6538 In scalar context, C<times> returns C<$user>.
6540 Note that times for children are included only after they terminate.
6544 The transliteration operator. Same as C<y///>. See L<perlop>.
6546 =item truncate FILEHANDLE,LENGTH
6549 =item truncate EXPR,LENGTH
6551 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6552 specified length. Produces a fatal error if truncate isn't implemented
6553 on your system. Returns true if successful, the undefined value
6556 The behavior is undefined if LENGTH is greater than the length of the
6559 The position in the file of FILEHANDLE is left unchanged. You may want to
6560 call L<seek> before writing to the file.
6563 X<uc> X<uppercase> X<toupper>
6567 Returns an uppercased version of EXPR. This is the internal function
6568 implementing the C<\U> escape in double-quoted strings. Respects
6569 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6570 and L<perlunicode> for more details about locale and Unicode support.
6571 It does not attempt to do titlecase mapping on initial letters. See
6572 C<ucfirst> for that.
6574 If EXPR is omitted, uses C<$_>.
6577 X<ucfirst> X<uppercase>
6581 Returns the value of EXPR with the first character in uppercase
6582 (titlecase in Unicode). This is the internal function implementing
6583 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6584 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6585 for more details about locale and Unicode support.
6587 If EXPR is omitted, uses C<$_>.
6594 Sets the umask for the process to EXPR and returns the previous value.
6595 If EXPR is omitted, merely returns the current umask.
6597 The Unix permission C<rwxr-x---> is represented as three sets of three
6598 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6599 and isn't one of the digits). The C<umask> value is such a number
6600 representing disabled permissions bits. The permission (or "mode")
6601 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6602 even if you tell C<sysopen> to create a file with permissions C<0777>,
6603 if your umask is C<0022> then the file will actually be created with
6604 permissions C<0755>. If your C<umask> were C<0027> (group can't
6605 write; others can't read, write, or execute), then passing
6606 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6609 Here's some advice: supply a creation mode of C<0666> for regular
6610 files (in C<sysopen>) and one of C<0777> for directories (in
6611 C<mkdir>) and executable files. This gives users the freedom of
6612 choice: if they want protected files, they might choose process umasks
6613 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6614 Programs should rarely if ever make policy decisions better left to
6615 the user. The exception to this is when writing files that should be
6616 kept private: mail files, web browser cookies, I<.rhosts> files, and
6619 If umask(2) is not implemented on your system and you are trying to
6620 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6621 fatal error at run time. If umask(2) is not implemented and you are
6622 not trying to restrict access for yourself, returns C<undef>.
6624 Remember that a umask is a number, usually given in octal; it is I<not> a
6625 string of octal digits. See also L</oct>, if all you have is a string.
6628 X<undef> X<undefine>
6632 Undefines the value of EXPR, which must be an lvalue. Use only on a
6633 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6634 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6635 will probably not do what you expect on most predefined variables or
6636 DBM list values, so don't do that; see L<delete>.) Always returns the
6637 undefined value. You can omit the EXPR, in which case nothing is
6638 undefined, but you still get an undefined value that you could, for
6639 instance, return from a subroutine, assign to a variable or pass as a
6640 parameter. Examples:
6643 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6647 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6648 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6649 select undef, undef, undef, 0.25;
6650 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6652 Note that this is a unary operator, not a list operator.
6655 X<unlink> X<delete> X<remove> X<rm>
6659 Deletes a list of files. Returns the number of files successfully
6662 $cnt = unlink 'a', 'b', 'c';
6666 Note: C<unlink> will not attempt to delete directories unless you are superuser
6667 and the B<-U> flag is supplied to Perl. Even if these conditions are
6668 met, be warned that unlinking a directory can inflict damage on your
6669 filesystem. Finally, using C<unlink> on directories is not supported on
6670 many operating systems. Use C<rmdir> instead.
6672 If LIST is omitted, uses C<$_>.
6674 =item unpack TEMPLATE,EXPR
6677 =item unpack TEMPLATE
6679 C<unpack> does the reverse of C<pack>: it takes a string
6680 and expands it out into a list of values.
6681 (In scalar context, it returns merely the first value produced.)
6683 If EXPR is omitted, unpacks the C<$_> string.
6685 The string is broken into chunks described by the TEMPLATE. Each chunk
6686 is converted separately to a value. Typically, either the string is a result
6687 of C<pack>, or the characters of the string represent a C structure of some
6690 The TEMPLATE has the same format as in the C<pack> function.
6691 Here's a subroutine that does substring:
6694 my($what,$where,$howmuch) = @_;
6695 unpack("x$where a$howmuch", $what);
6700 sub ordinal { unpack("W",$_[0]); } # same as ord()
6702 In addition to fields allowed in pack(), you may prefix a field with
6703 a %<number> to indicate that
6704 you want a <number>-bit checksum of the items instead of the items
6705 themselves. Default is a 16-bit checksum. Checksum is calculated by
6706 summing numeric values of expanded values (for string fields the sum of
6707 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6709 For example, the following
6710 computes the same number as the System V sum program:
6714 unpack("%32W*",<>) % 65535;
6717 The following efficiently counts the number of set bits in a bit vector:
6719 $setbits = unpack("%32b*", $selectmask);
6721 The C<p> and C<P> formats should be used with care. Since Perl
6722 has no way of checking whether the value passed to C<unpack()>
6723 corresponds to a valid memory location, passing a pointer value that's
6724 not known to be valid is likely to have disastrous consequences.
6726 If there are more pack codes or if the repeat count of a field or a group
6727 is larger than what the remainder of the input string allows, the result
6728 is not well defined: in some cases, the repeat count is decreased, or
6729 C<unpack()> will produce null strings or zeroes, or terminate with an
6730 error. If the input string is longer than one described by the TEMPLATE,
6731 the rest is ignored.
6733 See L</pack> for more examples and notes.
6735 =item untie VARIABLE
6738 Breaks the binding between a variable and a package. (See C<tie>.)
6739 Has no effect if the variable is not tied.
6741 =item unshift ARRAY,LIST
6744 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6745 depending on how you look at it. Prepends list to the front of the
6746 array, and returns the new number of elements in the array.
6748 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6750 Note the LIST is prepended whole, not one element at a time, so the
6751 prepended elements stay in the same order. Use C<reverse> to do the
6754 =item use Module VERSION LIST
6755 X<use> X<module> X<import>
6757 =item use Module VERSION
6759 =item use Module LIST
6765 Imports some semantics into the current package from the named module,
6766 generally by aliasing certain subroutine or variable names into your
6767 package. It is exactly equivalent to
6769 BEGIN { require Module; import Module LIST; }
6771 except that Module I<must> be a bareword.
6773 VERSION may be either a numeric argument such as 5.006, which will be
6774 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6775 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6776 greater than the version of the current Perl interpreter; Perl will not
6777 attempt to parse the rest of the file. Compare with L</require>, which can
6778 do a similar check at run time.
6780 Specifying VERSION as a literal of the form v5.6.1 should generally be
6781 avoided, because it leads to misleading error messages under earlier
6782 versions of Perl that do not support this syntax. The equivalent numeric
6783 version should be used instead.
6785 use v5.6.1; # compile time version check
6787 use 5.006_001; # ditto; preferred for backwards compatibility
6789 This is often useful if you need to check the current Perl version before
6790 C<use>ing library modules that have changed in incompatible ways from
6791 older versions of Perl. (We try not to do this more than we have to.)
6793 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6794 C<require> makes sure the module is loaded into memory if it hasn't been
6795 yet. The C<import> is not a builtin--it's just an ordinary static method
6796 call into the C<Module> package to tell the module to import the list of
6797 features back into the current package. The module can implement its
6798 C<import> method any way it likes, though most modules just choose to
6799 derive their C<import> method via inheritance from the C<Exporter> class that
6800 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6801 method can be found then the call is skipped, even if there is an AUTOLOAD
6804 If you do not want to call the package's C<import> method (for instance,
6805 to stop your namespace from being altered), explicitly supply the empty list:
6809 That is exactly equivalent to
6811 BEGIN { require Module }
6813 If the VERSION argument is present between Module and LIST, then the
6814 C<use> will call the VERSION method in class Module with the given
6815 version as an argument. The default VERSION method, inherited from
6816 the UNIVERSAL class, croaks if the given version is larger than the
6817 value of the variable C<$Module::VERSION>.
6819 Again, there is a distinction between omitting LIST (C<import> called
6820 with no arguments) and an explicit empty LIST C<()> (C<import> not
6821 called). Note that there is no comma after VERSION!
6823 Because this is a wide-open interface, pragmas (compiler directives)
6824 are also implemented this way. Currently implemented pragmas are:
6829 use sigtrap qw(SEGV BUS);
6830 use strict qw(subs vars refs);
6831 use subs qw(afunc blurfl);
6832 use warnings qw(all);
6833 use sort qw(stable _quicksort _mergesort);
6835 Some of these pseudo-modules import semantics into the current
6836 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6837 which import symbols into the current package (which are effective
6838 through the end of the file).
6840 There's a corresponding C<no> command that unimports meanings imported
6841 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6842 It behaves exactly as C<import> does with respect to VERSION, an
6843 omitted LIST, empty LIST, or no unimport method being found.
6849 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6850 for the C<-M> and C<-m> command-line options to perl that give C<use>
6851 functionality from the command-line.
6856 Changes the access and modification times on each file of a list of
6857 files. The first two elements of the list must be the NUMERICAL access
6858 and modification times, in that order. Returns the number of files
6859 successfully changed. The inode change time of each file is set
6860 to the current time. For example, this code has the same effect as the
6861 Unix touch(1) command when the files I<already exist> and belong to
6862 the user running the program:
6865 $atime = $mtime = time;
6866 utime $atime, $mtime, @ARGV;
6868 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6869 the utime(2) function in the C library will be called with a null second
6870 argument. On most systems, this will set the file's access and
6871 modification times to the current time (i.e. equivalent to the example
6872 above) and will even work on other users' files where you have write
6875 utime undef, undef, @ARGV;
6877 Under NFS this will use the time of the NFS server, not the time of
6878 the local machine. If there is a time synchronization problem, the
6879 NFS server and local machine will have different times. The Unix
6880 touch(1) command will in fact normally use this form instead of the
6881 one shown in the first example.
6883 Note that only passing one of the first two elements as C<undef> will
6884 be equivalent of passing it as 0 and will not have the same effect as
6885 described when they are both C<undef>. This case will also trigger an
6886 uninitialized warning.
6888 On systems that support futimes, you might pass file handles among the
6889 files. On systems that don't support futimes, passing file handles
6890 produces a fatal error at run time. The file handles must be passed
6891 as globs or references to be recognized. Barewords are considered
6897 Returns a list consisting of all the values of the named hash.
6898 (In a scalar context, returns the number of values.)
6900 The values are returned in an apparently random order. The actual
6901 random order is subject to change in future versions of perl, but it
6902 is guaranteed to be the same order as either the C<keys> or C<each>
6903 function would produce on the same (unmodified) hash. Since Perl
6904 5.8.1 the ordering is different even between different runs of Perl
6905 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6907 As a side effect, calling values() resets the HASH's internal iterator,
6908 see L</each>. (In particular, calling values() in void context resets
6909 the iterator with no other overhead.)
6911 Note that the values are not copied, which means modifying them will
6912 modify the contents of the hash:
6914 for (values %hash) { s/foo/bar/g } # modifies %hash values
6915 for (@hash{keys %hash}) { s/foo/bar/g } # same
6917 See also C<keys>, C<each>, and C<sort>.
6919 =item vec EXPR,OFFSET,BITS
6920 X<vec> X<bit> X<bit vector>
6922 Treats the string in EXPR as a bit vector made up of elements of
6923 width BITS, and returns the value of the element specified by OFFSET
6924 as an unsigned integer. BITS therefore specifies the number of bits
6925 that are reserved for each element in the bit vector. This must
6926 be a power of two from 1 to 32 (or 64, if your platform supports
6929 If BITS is 8, "elements" coincide with bytes of the input string.
6931 If BITS is 16 or more, bytes of the input string are grouped into chunks
6932 of size BITS/8, and each group is converted to a number as with
6933 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6934 for BITS==64). See L<"pack"> for details.
6936 If bits is 4 or less, the string is broken into bytes, then the bits
6937 of each byte are broken into 8/BITS groups. Bits of a byte are
6938 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6939 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6940 breaking the single input byte C<chr(0x36)> into two groups gives a list
6941 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6943 C<vec> may also be assigned to, in which case parentheses are needed
6944 to give the expression the correct precedence as in
6946 vec($image, $max_x * $x + $y, 8) = 3;
6948 If the selected element is outside the string, the value 0 is returned.
6949 If an element off the end of the string is written to, Perl will first
6950 extend the string with sufficiently many zero bytes. It is an error
6951 to try to write off the beginning of the string (i.e. negative OFFSET).
6953 The string should not contain any character with the value > 255 (which
6954 can only happen if you're using UTF-8 encoding). If it does, it will be
6955 treated as something that is not UTF-8 encoded. When the C<vec> was
6956 assigned to, other parts of your program will also no longer consider the
6957 string to be UTF-8 encoded. In other words, if you do have such characters
6958 in your string, vec() will operate on the actual byte string, and not the
6959 conceptual character string.
6961 Strings created with C<vec> can also be manipulated with the logical
6962 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6963 vector operation is desired when both operands are strings.
6964 See L<perlop/"Bitwise String Operators">.
6966 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6967 The comments show the string after each step. Note that this code works
6968 in the same way on big-endian or little-endian machines.
6971 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6973 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6974 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6976 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6977 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6978 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6979 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6980 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6981 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6983 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6984 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6985 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6988 To transform a bit vector into a string or list of 0's and 1's, use these:
6990 $bits = unpack("b*", $vector);
6991 @bits = split(//, unpack("b*", $vector));
6993 If you know the exact length in bits, it can be used in place of the C<*>.
6995 Here is an example to illustrate how the bits actually fall in place:
7001 unpack("V",$_) 01234567890123456789012345678901
7002 ------------------------------------------------------------------
7007 for ($shift=0; $shift < $width; ++$shift) {
7008 for ($off=0; $off < 32/$width; ++$off) {
7009 $str = pack("B*", "0"x32);
7010 $bits = (1<<$shift);
7011 vec($str, $off, $width) = $bits;
7012 $res = unpack("b*",$str);
7013 $val = unpack("V", $str);
7020 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7021 $off, $width, $bits, $val, $res
7025 Regardless of the machine architecture on which it is run, the above
7026 example should print the following table:
7029 unpack("V",$_) 01234567890123456789012345678901
7030 ------------------------------------------------------------------
7031 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7032 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7033 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7034 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7035 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7036 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7037 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7038 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7039 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7040 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7041 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7042 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7043 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7044 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7045 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7046 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7047 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7048 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7049 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7050 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7051 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7052 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7053 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7054 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7055 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7056 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7057 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7058 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7059 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7060 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7061 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7062 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7063 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7064 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7065 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7066 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7067 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7068 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7069 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7070 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7071 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7072 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7073 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7074 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7075 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7076 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7077 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7078 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7079 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7080 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7081 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7082 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7083 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7084 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7085 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7086 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7087 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7088 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7089 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7090 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7091 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7092 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7093 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7094 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7095 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7096 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7097 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7098 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7099 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7100 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7101 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7102 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7103 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7104 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7105 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7106 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7107 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7108 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7109 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7110 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7111 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7112 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7113 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7114 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7115 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7116 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7117 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7118 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7119 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7120 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7121 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7122 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7123 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7124 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7125 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7126 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7127 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7128 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7129 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7130 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7131 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7132 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7133 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7134 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7135 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7136 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7137 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7138 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7139 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7140 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7141 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7142 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7143 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7144 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7145 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7146 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7147 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7148 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7149 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7150 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7151 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7152 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7153 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7154 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7155 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7156 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7157 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7158 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7163 Behaves like the wait(2) system call on your system: it waits for a child
7164 process to terminate and returns the pid of the deceased process, or
7165 C<-1> if there are no child processes. The status is returned in C<$?>
7166 and C<{^CHILD_ERROR_NATIVE}>.
7167 Note that a return value of C<-1> could mean that child processes are
7168 being automatically reaped, as described in L<perlipc>.
7170 =item waitpid PID,FLAGS
7173 Waits for a particular child process to terminate and returns the pid of
7174 the deceased process, or C<-1> if there is no such child process. On some
7175 systems, a value of 0 indicates that there are processes still running.
7176 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7178 use POSIX ":sys_wait_h";
7181 $kid = waitpid(-1, WNOHANG);
7184 then you can do a non-blocking wait for all pending zombie processes.
7185 Non-blocking wait is available on machines supporting either the
7186 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7187 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7188 system call by remembering the status values of processes that have
7189 exited but have not been harvested by the Perl script yet.)
7191 Note that on some systems, a return value of C<-1> could mean that child
7192 processes are being automatically reaped. See L<perlipc> for details,
7193 and for other examples.
7196 X<wantarray> X<context>
7198 Returns true if the context of the currently executing subroutine or
7199 C<eval> is looking for a list value. Returns false if the context is
7200 looking for a scalar. Returns the undefined value if the context is
7201 looking for no value (void context).
7203 return unless defined wantarray; # don't bother doing more
7204 my @a = complex_calculation();
7205 return wantarray ? @a : "@a";
7207 C<wantarray()>'s result is unspecified in the top level of a file,
7208 in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
7211 This function should have been named wantlist() instead.
7214 X<warn> X<warning> X<STDERR>
7216 Produces a message on STDERR just like C<die>, but doesn't exit or throw
7219 If LIST is empty and C<$@> already contains a value (typically from a
7220 previous eval) that value is used after appending C<"\t...caught">
7221 to C<$@>. This is useful for staying almost, but not entirely similar to
7224 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7226 No message is printed if there is a C<$SIG{__WARN__}> handler
7227 installed. It is the handler's responsibility to deal with the message
7228 as it sees fit (like, for instance, converting it into a C<die>). Most
7229 handlers must therefore make arrangements to actually display the
7230 warnings that they are not prepared to deal with, by calling C<warn>
7231 again in the handler. Note that this is quite safe and will not
7232 produce an endless loop, since C<__WARN__> hooks are not called from
7235 You will find this behavior is slightly different from that of
7236 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7237 instead call C<die> again to change it).
7239 Using a C<__WARN__> handler provides a powerful way to silence all
7240 warnings (even the so-called mandatory ones). An example:
7242 # wipe out *all* compile-time warnings
7243 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7245 my $foo = 20; # no warning about duplicate my $foo,
7246 # but hey, you asked for it!
7247 # no compile-time or run-time warnings before here
7250 # run-time warnings enabled after here
7251 warn "\$foo is alive and $foo!"; # does show up
7253 See L<perlvar> for details on setting C<%SIG> entries, and for more
7254 examples. See the Carp module for other kinds of warnings using its
7255 carp() and cluck() functions.
7257 =item write FILEHANDLE
7264 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7265 using the format associated with that file. By default the format for
7266 a file is the one having the same name as the filehandle, but the
7267 format for the current output channel (see the C<select> function) may be set
7268 explicitly by assigning the name of the format to the C<$~> variable.
7270 Top of form processing is handled automatically: if there is
7271 insufficient room on the current page for the formatted record, the
7272 page is advanced by writing a form feed, a special top-of-page format
7273 is used to format the new page header, and then the record is written.
7274 By default the top-of-page format is the name of the filehandle with
7275 "_TOP" appended, but it may be dynamically set to the format of your
7276 choice by assigning the name to the C<$^> variable while the filehandle is
7277 selected. The number of lines remaining on the current page is in
7278 variable C<$->, which can be set to C<0> to force a new page.
7280 If FILEHANDLE is unspecified, output goes to the current default output
7281 channel, which starts out as STDOUT but may be changed by the
7282 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7283 is evaluated and the resulting string is used to look up the name of
7284 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7286 Note that write is I<not> the opposite of C<read>. Unfortunately.
7290 The transliteration operator. Same as C<tr///>. See L<perlop>.