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//>, C<qq//>, 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//>, 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<reset>, C<scalar>, C<state>, 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//>, 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-orientation
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<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
236 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
237 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238 C<ucfirst>, C<untie>, C<use>, C<when>
240 * - C<sub> was a keyword in perl4, but in perl5 it is an
241 operator, which can be used in expressions.
243 =item Functions obsoleted in perl5
245 C<dbmclose>, C<dbmopen>
250 X<portability> X<Unix> X<portable>
252 Perl was born in Unix and can therefore access all common Unix
253 system calls. In non-Unix environments, the functionality of some
254 Unix system calls may not be available, or details of the available
255 functionality may differ slightly. The Perl functions affected
258 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
261 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
263 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
264 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
267 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
268 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
271 C<shmwrite>, C<socket>, C<socketpair>,
272 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273 C<times>, C<truncate>, C<umask>, C<unlink>,
274 C<utime>, C<wait>, C<waitpid>
276 For more information about the portability of these functions, see
277 L<perlport> and other available platform-specific documentation.
279 =head2 Alphabetical Listing of Perl Functions
284 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>
285 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
293 A file test, where X is one of the letters listed below. This unary
294 operator takes one argument, either a filename, a filehandle, or a dirhandle,
295 and tests the associated file to see if something is true about it. If the
296 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
297 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
298 the undefined value if the file doesn't exist. Despite the funny
299 names, precedence is the same as any other named unary operator. The
300 operator may be any of:
302 -r File is readable by effective uid/gid.
303 -w File is writable by effective uid/gid.
304 -x File is executable by effective uid/gid.
305 -o File is owned by effective uid.
307 -R File is readable by real uid/gid.
308 -W File is writable by real uid/gid.
309 -X File is executable by real uid/gid.
310 -O File is owned by real uid.
313 -z File has zero size (is empty).
314 -s File has nonzero size (returns size in bytes).
316 -f File is a plain file.
317 -d File is a directory.
318 -l File is a symbolic link.
319 -p File is a named pipe (FIFO), or Filehandle is a pipe.
321 -b File is a block special file.
322 -c File is a character special file.
323 -t Filehandle is opened to a tty.
325 -u File has setuid bit set.
326 -g File has setgid bit set.
327 -k File has sticky bit set.
329 -T File is an ASCII text file (heuristic guess).
330 -B File is a "binary" file (opposite of -T).
332 -M Script start time minus file modification time, in days.
333 -A Same for access time.
334 -C Same for inode change time (Unix, may differ for other platforms)
340 next unless -f $_; # ignore specials
344 The interpretation of the file permission operators C<-r>, C<-R>,
345 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
346 of the file and the uids and gids of the user. There may be other
347 reasons you can't actually read, write, or execute the file: for
348 example network filesystem access controls, ACLs (access control lists),
349 read-only filesystems, and unrecognized executable formats. Note
350 that the use of these six specific operators to verify if some operation
351 is possible is usually a mistake, because it may be open to race
354 Also note that, for the superuser on the local filesystems, the C<-r>,
355 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
356 if any execute bit is set in the mode. Scripts run by the superuser
357 may thus need to do a stat() to determine the actual mode of the file,
358 or temporarily set their effective uid to something else.
360 If you are using ACLs, there is a pragma called C<filetest> that may
361 produce more accurate results than the bare stat() mode bits.
362 When under the C<use filetest 'access'> the above-mentioned filetests
363 will test whether the permission can (not) be granted using the
364 access() family of system calls. Also note that the C<-x> and C<-X> may
365 under this pragma return true even if there are no execute permission
366 bits set (nor any extra execute permission ACLs). This strangeness is
367 due to the underlying system calls' definitions. Note also that, due to
368 the implementation of C<use filetest 'access'>, the C<_> special
369 filehandle won't cache the results of the file tests when this pragma is
370 in effect. Read the documentation for the C<filetest> pragma for more
373 Note that C<-s/a/b/> does not do a negated substitution. Saying
374 C<-exp($foo)> still works as expected, however--only single letters
375 following a minus are interpreted as file tests.
377 The C<-T> and C<-B> switches work as follows. The first block or so of the
378 file is examined for odd characters such as strange control codes or
379 characters with the high bit set. If too many strange characters (>30%)
380 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
381 containing null in the first block is considered a binary file. If C<-T>
382 or C<-B> is used on a filehandle, the current IO buffer is examined
383 rather than the first block. Both C<-T> and C<-B> return true on a null
384 file, or a file at EOF when testing a filehandle. Because you have to
385 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
386 against the file first, as in C<next unless -f $file && -T $file>.
388 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
389 the special filehandle consisting of a solitary underline, then the stat
390 structure of the previous file test (or stat operator) is used, saving
391 a system call. (This doesn't work with C<-t>, and you need to remember
392 that lstat() and C<-l> will leave values in the stat structure for the
393 symbolic link, not the real file.) (Also, if the stat buffer was filled by
394 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
397 print "Can do.\n" if -r $a || -w _ || -x _;
400 print "Readable\n" if -r _;
401 print "Writable\n" if -w _;
402 print "Executable\n" if -x _;
403 print "Setuid\n" if -u _;
404 print "Setgid\n" if -g _;
405 print "Sticky\n" if -k _;
406 print "Text\n" if -T _;
407 print "Binary\n" if -B _;
409 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
410 test operators, in a way that C<-f -w -x $file> is equivalent to
411 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
412 the return value of C<-f $file> as an argument to another filetest
413 operator, no special magic will happen.)
420 Returns the absolute value of its argument.
421 If VALUE is omitted, uses C<$_>.
423 =item accept NEWSOCKET,GENERICSOCKET
426 Accepts an incoming socket connect, just as the accept(2) system call
427 does. Returns the packed address if it succeeded, false otherwise.
428 See the example in L<perlipc/"Sockets: Client/Server Communication">.
430 On systems that support a close-on-exec flag on files, the flag will
431 be set for the newly opened file descriptor, as determined by the
432 value of $^F. See L<perlvar/$^F>.
441 Arranges to have a SIGALRM delivered to this process after the
442 specified number of wallclock seconds has elapsed. If SECONDS is not
443 specified, the value stored in C<$_> is used. (On some machines,
444 unfortunately, the elapsed time may be up to one second less or more
445 than you specified because of how seconds are counted, and process
446 scheduling may delay the delivery of the signal even further.)
448 Only one timer may be counting at once. Each call disables the
449 previous timer, and an argument of C<0> may be supplied to cancel the
450 previous timer without starting a new one. The returned value is the
451 amount of time remaining on the previous timer.
453 For delays of finer granularity than one second, the Time::HiRes module
454 (from CPAN, and starting from Perl 5.8 part of the standard
455 distribution) provides ualarm(). You may also use Perl's four-argument
456 version of select() leaving the first three arguments undefined, or you
457 might be able to use the C<syscall> interface to access setitimer(2) if
458 your system supports it. See L<perlfaq8> for details.
460 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
461 (C<sleep> may be internally implemented in your system with C<alarm>)
463 If you want to use C<alarm> to time out a system call you need to use an
464 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
465 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
466 restart system calls on some systems. Using C<eval>/C<die> always works,
467 modulo the caveats given in L<perlipc/"Signals">.
470 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
472 $nread = sysread SOCKET, $buffer, $size;
476 die unless $@ eq "alarm\n"; # propagate unexpected errors
483 For more information see L<perlipc>.
486 X<atan2> X<arctangent> X<tan> X<tangent>
488 Returns the arctangent of Y/X in the range -PI to PI.
490 For the tangent operation, you may use the C<Math::Trig::tan>
491 function, or use the familiar relation:
493 sub tan { sin($_[0]) / cos($_[0]) }
495 The return value for C<atan2(0,0)> is implementation-defined; consult
496 your atan2(3) manpage for more information.
498 =item bind SOCKET,NAME
501 Binds a network address to a socket, just as the bind system call
502 does. Returns true if it succeeded, false otherwise. NAME should be a
503 packed address of the appropriate type for the socket. See the examples in
504 L<perlipc/"Sockets: Client/Server Communication">.
506 =item binmode FILEHANDLE, LAYER
507 X<binmode> X<binary> X<text> X<DOS> X<Windows>
509 =item binmode FILEHANDLE
511 Arranges for FILEHANDLE to be read or written in "binary" or "text"
512 mode on systems where the run-time libraries distinguish between
513 binary and text files. If FILEHANDLE is an expression, the value is
514 taken as the name of the filehandle. Returns true on success,
515 otherwise it returns C<undef> and sets C<$!> (errno).
517 On some systems (in general, DOS and Windows-based systems) binmode()
518 is necessary when you're not working with a text file. For the sake
519 of portability it is a good idea to always use it when appropriate,
520 and to never use it when it isn't appropriate. Also, people can
521 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
523 In other words: regardless of platform, use binmode() on binary data,
524 like for example images.
526 If LAYER is present it is a single string, but may contain multiple
527 directives. The directives alter the behaviour of the file handle.
528 When LAYER is present using binmode on text file makes sense.
530 If LAYER is omitted or specified as C<:raw> the filehandle is made
531 suitable for passing binary data. This includes turning off possible CRLF
532 translation and marking it as bytes (as opposed to Unicode characters).
533 Note that, despite what may be implied in I<"Programming Perl"> (the
534 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
535 -- other layers which would affect the binary nature of the stream are
536 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
537 PERLIO environment variable.
539 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
540 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
541 establish default I/O layers. See L<open>.
543 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
544 in "Programming Perl, 3rd Edition". However, since the publishing of this
545 book, by many known as "Camel III", the consensus of the naming of this
546 functionality has moved from "discipline" to "layer". All documentation
547 of this version of Perl therefore refers to "layers" rather than to
548 "disciplines". Now back to the regularly scheduled documentation...>
550 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
551 C<:utf8> just marks the data as UTF-8 without further checking,
552 while C<:encoding(utf8)> checks the data for actually being valid
553 UTF-8. More details can be found in L<PerlIO::encoding>.
555 In general, binmode() should be called after open() but before any I/O
556 is done on the filehandle. Calling binmode() will normally flush any
557 pending buffered output data (and perhaps pending input data) on the
558 handle. An exception to this is the C<:encoding> layer that
559 changes the default character encoding of the handle, see L<open>.
560 The C<:encoding> layer sometimes needs to be called in
561 mid-stream, and it doesn't flush the stream. The C<:encoding>
562 also implicitly pushes on top of itself the C<:utf8> layer because
563 internally Perl will operate on UTF-8 encoded Unicode characters.
565 The operating system, device drivers, C libraries, and Perl run-time
566 system all work together to let the programmer treat a single
567 character (C<\n>) as the line terminator, irrespective of the external
568 representation. On many operating systems, the native text file
569 representation matches the internal representation, but on some
570 platforms the external representation of C<\n> is made up of more than
573 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
574 character to end each line in the external representation of text (even
575 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
576 on Unix and most VMS files). In other systems like OS/2, DOS and the
577 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
578 but what's stored in text files are the two characters C<\cM\cJ>. That
579 means that, if you don't use binmode() on these systems, C<\cM\cJ>
580 sequences on disk will be converted to C<\n> on input, and any C<\n> in
581 your program will be converted back to C<\cM\cJ> on output. This is what
582 you want for text files, but it can be disastrous for binary files.
584 Another consequence of using binmode() (on some systems) is that
585 special end-of-file markers will be seen as part of the data stream.
586 For systems from the Microsoft family this means that if your binary
587 data contains C<\cZ>, the I/O subsystem will regard it as the end of
588 the file, unless you use binmode().
590 binmode() is not only important for readline() and print() operations,
591 but also when using read(), seek(), sysread(), syswrite() and tell()
592 (see L<perlport> for more details). See the C<$/> and C<$\> variables
593 in L<perlvar> for how to manually set your input and output
594 line-termination sequences.
596 =item bless REF,CLASSNAME
601 This function tells the thingy referenced by REF that it is now an object
602 in the CLASSNAME package. If CLASSNAME is omitted, the current package
603 is used. Because a C<bless> is often the last thing in a constructor,
604 it returns the reference for convenience. Always use the two-argument
605 version if a derived class might inherit the function doing the blessing.
606 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
609 Consider always blessing objects in CLASSNAMEs that are mixed case.
610 Namespaces with all lowercase names are considered reserved for
611 Perl pragmata. Builtin types have all uppercase names. To prevent
612 confusion, you may wish to avoid such package names as well. Make sure
613 that CLASSNAME is a true value.
615 See L<perlmod/"Perl Modules">.
619 Break out of a C<given()> block.
621 This keyword is enabled by the "switch" feature: see L<feature>
622 for more information.
625 X<caller> X<call stack> X<stack> X<stack trace>
629 Returns the context of the current subroutine call. In scalar context,
630 returns the caller's package name if there is a caller, that is, if
631 we're in a subroutine or C<eval> or C<require>, and the undefined value
632 otherwise. In list context, returns
635 ($package, $filename, $line) = caller;
637 With EXPR, it returns some extra information that the debugger uses to
638 print a stack trace. The value of EXPR indicates how many call frames
639 to go back before the current one.
642 ($package, $filename, $line, $subroutine, $hasargs,
645 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
648 Here $subroutine may be C<(eval)> if the frame is not a subroutine
649 call, but an C<eval>. In such a case additional elements $evaltext and
650 C<$is_require> are set: C<$is_require> is true if the frame is created by a
651 C<require> or C<use> statement, $evaltext contains the text of the
652 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
653 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
654 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
655 frame.) $subroutine may also be C<(unknown)> if this particular
656 subroutine happens to have been deleted from the symbol table.
657 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
658 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
659 compiled with. The C<$hints> and C<$bitmask> values are subject to change
660 between versions of Perl, and are not meant for external use.
662 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
663 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
664 of this hash, as they are the actual values stored in the optree.
666 Furthermore, when called from within the DB package, caller returns more
667 detailed information: it sets the list variable C<@DB::args> to be the
668 arguments with which the subroutine was invoked.
670 Be aware that the optimizer might have optimized call frames away before
671 C<caller> had a chance to get the information. That means that C<caller(N)>
672 might not return information about the call frame you expect it do, for
673 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
674 previous time C<caller> was called.
681 =item chdir FILEHANDLE
683 =item chdir DIRHANDLE
687 Changes the working directory to EXPR, if possible. If EXPR is omitted,
688 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
689 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
690 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
691 neither is set, C<chdir> does nothing. It returns true upon success,
692 false otherwise. See the example under C<die>.
694 On systems that support fchdir, you might pass a file handle or
695 directory handle as argument. On systems that don't support fchdir,
696 passing handles produces a fatal error at run time.
699 X<chmod> X<permission> X<mode>
701 Changes the permissions of a list of files. The first element of the
702 list must be the numerical mode, which should probably be an octal
703 number, and which definitely should I<not> be a string of octal digits:
704 C<0644> is okay, C<'0644'> is not. Returns the number of files
705 successfully changed. See also L</oct>, if all you have is a string.
707 $cnt = chmod 0755, 'foo', 'bar';
708 chmod 0755, @executables;
709 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
711 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
712 $mode = 0644; chmod $mode, 'foo'; # this is best
714 On systems that support fchmod, you might pass file handles among the
715 files. On systems that don't support fchmod, passing file handles
716 produces a fatal error at run time. The file handles must be passed
717 as globs or references to be recognized. Barewords are considered
720 open(my $fh, "<", "foo");
721 my $perm = (stat $fh)[2] & 07777;
722 chmod($perm | 0600, $fh);
724 You can also import the symbolic C<S_I*> constants from the Fcntl
729 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
730 # This is identical to the chmod 0755 of the above example.
733 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
739 This safer version of L</chop> removes any trailing string
740 that corresponds to the current value of C<$/> (also known as
741 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
742 number of characters removed from all its arguments. It's often used to
743 remove the newline from the end of an input record when you're worried
744 that the final record may be missing its newline. When in paragraph
745 mode (C<$/ = "">), it removes all trailing newlines from the string.
746 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
747 a reference to an integer or the like, see L<perlvar>) chomp() won't
749 If VARIABLE is omitted, it chomps C<$_>. Example:
752 chomp; # avoid \n on last field
757 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
759 You can actually chomp anything that's an lvalue, including an assignment:
762 chomp($answer = <STDIN>);
764 If you chomp a list, each element is chomped, and the total number of
765 characters removed is returned.
767 Note that parentheses are necessary when you're chomping anything
768 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
769 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
770 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
771 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
781 Chops off the last character of a string and returns the character
782 chopped. It is much more efficient than C<s/.$//s> because it neither
783 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
784 If VARIABLE is a hash, it chops the hash's values, but not its keys.
786 You can actually chop anything that's an lvalue, including an assignment.
788 If you chop a list, each element is chopped. Only the value of the
789 last C<chop> is returned.
791 Note that C<chop> returns the last character. To return all but the last
792 character, use C<substr($string, 0, -1)>.
797 X<chown> X<owner> X<user> X<group>
799 Changes the owner (and group) of a list of files. The first two
800 elements of the list must be the I<numeric> uid and gid, in that
801 order. A value of -1 in either position is interpreted by most
802 systems to leave that value unchanged. Returns the number of files
803 successfully changed.
805 $cnt = chown $uid, $gid, 'foo', 'bar';
806 chown $uid, $gid, @filenames;
808 On systems that support fchown, you might pass file handles among the
809 files. On systems that don't support fchown, passing file handles
810 produces a fatal error at run time. The file handles must be passed
811 as globs or references to be recognized. Barewords are considered
814 Here's an example that looks up nonnumeric uids in the passwd file:
817 chomp($user = <STDIN>);
819 chomp($pattern = <STDIN>);
821 ($login,$pass,$uid,$gid) = getpwnam($user)
822 or die "$user not in passwd file";
824 @ary = glob($pattern); # expand filenames
825 chown $uid, $gid, @ary;
827 On most systems, you are not allowed to change the ownership of the
828 file unless you're the superuser, although you should be able to change
829 the group to any of your secondary groups. On insecure systems, these
830 restrictions may be relaxed, but this is not a portable assumption.
831 On POSIX systems, you can detect this condition this way:
833 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
834 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
837 X<chr> X<character> X<ASCII> X<Unicode>
841 Returns the character represented by that NUMBER in the character set.
842 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
843 chr(0x263a) is a Unicode smiley face.
845 Negative values give the Unicode replacement character (chr(0xfffd)),
846 except under the L<bytes> pragma, where low eight bits of the value
847 (truncated to an integer) are used.
849 If NUMBER is omitted, uses C<$_>.
851 For the reverse, use L</ord>.
853 Note that characters from 128 to 255 (inclusive) are by default
854 internally not encoded as UTF-8 for backward compatibility reasons.
856 See L<perlunicode> for more about Unicode.
858 =item chroot FILENAME
863 This function works like the system call by the same name: it makes the
864 named directory the new root directory for all further pathnames that
865 begin with a C</> by your process and all its children. (It doesn't
866 change your current working directory, which is unaffected.) For security
867 reasons, this call is restricted to the superuser. If FILENAME is
868 omitted, does a C<chroot> to C<$_>.
870 =item close FILEHANDLE
875 Closes the file or pipe associated with the file handle, flushes the IO
876 buffers, and closes the system file descriptor. Returns true if those
877 operations have succeeded and if no error was reported by any PerlIO
878 layer. Closes the currently selected filehandle if the argument is
881 You don't have to close FILEHANDLE if you are immediately going to do
882 another C<open> on it, because C<open> will close it for you. (See
883 C<open>.) However, an explicit C<close> on an input file resets the line
884 counter (C<$.>), while the implicit close done by C<open> does not.
886 If the file handle came from a piped open, C<close> will additionally
887 return false if one of the other system calls involved fails, or if the
888 program exits with non-zero status. (If the only problem was that the
889 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
890 also waits for the process executing on the pipe to complete, in case you
891 want to look at the output of the pipe afterwards, and
892 implicitly puts the exit status value of that command into C<$?> and
893 C<${^CHILD_ERROR_NATIVE}>.
895 Prematurely closing the read end of a pipe (i.e. before the process
896 writing to it at the other end has closed it) will result in a
897 SIGPIPE being delivered to the writer. If the other end can't
898 handle that, be sure to read all the data before closing the pipe.
902 open(OUTPUT, '|sort >foo') # pipe to sort
903 or die "Can't start sort: $!";
904 #... # print stuff to output
905 close OUTPUT # wait for sort to finish
906 or warn $! ? "Error closing sort pipe: $!"
907 : "Exit status $? from sort";
908 open(INPUT, 'foo') # get sort's results
909 or die "Can't open 'foo' for input: $!";
911 FILEHANDLE may be an expression whose value can be used as an indirect
912 filehandle, usually the real filehandle name.
914 =item closedir DIRHANDLE
917 Closes a directory opened by C<opendir> and returns the success of that
920 =item connect SOCKET,NAME
923 Attempts to connect to a remote socket, just as the connect system call
924 does. Returns true if it succeeded, false otherwise. NAME should be a
925 packed address of the appropriate type for the socket. See the examples in
926 L<perlipc/"Sockets: Client/Server Communication">.
933 C<continue> is actually a flow control statement rather than a function. If
934 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
935 C<foreach>), it is always executed just before the conditional is about to
936 be evaluated again, just like the third part of a C<for> loop in C. Thus
937 it can be used to increment a loop variable, even when the loop has been
938 continued via the C<next> statement (which is similar to the C C<continue>
941 C<last>, C<next>, or C<redo> may appear within a C<continue>
942 block. C<last> and C<redo> will behave as if they had been executed within
943 the main block. So will C<next>, but since it will execute a C<continue>
944 block, it may be more entertaining.
947 ### redo always comes here
950 ### next always comes here
952 # then back the top to re-check EXPR
954 ### last always comes here
956 Omitting the C<continue> section is semantically equivalent to using an
957 empty one, logically enough. In that case, C<next> goes directly back
958 to check the condition at the top of the loop.
960 If the "switch" feature is enabled, C<continue> is also a
961 function that will break out of the current C<when> or C<default>
962 block, and fall through to the next case. See L<feature> and
963 L<perlsyn/"Switch statements"> for more information.
967 X<cos> X<cosine> X<acos> X<arccosine>
971 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
972 takes cosine of C<$_>.
974 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
975 function, or use this relation:
977 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
979 =item crypt PLAINTEXT,SALT
980 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
981 X<decrypt> X<cryptography> X<passwd> X<encrypt>
983 Creates a digest string exactly like the crypt(3) function in the C
984 library (assuming that you actually have a version there that has not
985 been extirpated as a potential munitions).
987 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
988 into a short string, called a digest, which is returned. The same
989 PLAINTEXT and SALT will always return the same string, but there is no
990 (known) way to get the original PLAINTEXT from the hash. Small
991 changes in the PLAINTEXT or SALT will result in large changes in the
994 There is no decrypt function. This function isn't all that useful for
995 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
996 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
997 primarily used to check if two pieces of text are the same without
998 having to transmit or store the text itself. An example is checking
999 if a correct password is given. The digest of the password is stored,
1000 not the password itself. The user types in a password that is
1001 crypt()'d with the same salt as the stored digest. If the two digests
1002 match the password is correct.
1004 When verifying an existing digest string you should use the digest as
1005 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1006 to create the digest is visible as part of the digest. This ensures
1007 crypt() will hash the new string with the same salt as the digest.
1008 This allows your code to work with the standard L<crypt|/crypt> and
1009 with more exotic implementations. In other words, do not assume
1010 anything about the returned string itself, or how many bytes in the
1013 Traditionally the result is a string of 13 bytes: two first bytes of
1014 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1015 the first eight bytes of the digest string mattered, but alternative
1016 hashing schemes (like MD5), higher level security schemes (like C2),
1017 and implementations on non-UNIX platforms may produce different
1020 When choosing a new salt create a random two character string whose
1021 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1022 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1023 characters is just a recommendation; the characters allowed in
1024 the salt depend solely on your system's crypt library, and Perl can't
1025 restrict what salts C<crypt()> accepts.
1027 Here's an example that makes sure that whoever runs this program knows
1030 $pwd = (getpwuid($<))[1];
1032 system "stty -echo";
1034 chomp($word = <STDIN>);
1038 if (crypt($word, $pwd) ne $pwd) {
1044 Of course, typing in your own password to whoever asks you
1047 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1048 of data, not least of all because you can't get the information
1049 back. Look at the L<Digest> module for more robust algorithms.
1051 If using crypt() on a Unicode string (which I<potentially> has
1052 characters with codepoints above 255), Perl tries to make sense
1053 of the situation by trying to downgrade (a copy of the string)
1054 the string back to an eight-bit byte string before calling crypt()
1055 (on that copy). If that works, good. If not, crypt() dies with
1056 C<Wide character in crypt>.
1061 [This function has been largely superseded by the C<untie> function.]
1063 Breaks the binding between a DBM file and a hash.
1065 =item dbmopen HASH,DBNAME,MASK
1066 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1068 [This function has been largely superseded by the C<tie> function.]
1070 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1071 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1072 argument is I<not> a filehandle, even though it looks like one). DBNAME
1073 is the name of the database (without the F<.dir> or F<.pag> extension if
1074 any). If the database does not exist, it is created with protection
1075 specified by MASK (as modified by the C<umask>). If your system supports
1076 only the older DBM functions, you may perform only one C<dbmopen> in your
1077 program. In older versions of Perl, if your system had neither DBM nor
1078 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1081 If you don't have write access to the DBM file, you can only read hash
1082 variables, not set them. If you want to test whether you can write,
1083 either use file tests or try setting a dummy hash entry inside an C<eval>,
1084 which will trap the error.
1086 Note that functions such as C<keys> and C<values> may return huge lists
1087 when used on large DBM files. You may prefer to use the C<each>
1088 function to iterate over large DBM files. Example:
1090 # print out history file offsets
1091 dbmopen(%HIST,'/usr/lib/news/history',0666);
1092 while (($key,$val) = each %HIST) {
1093 print $key, ' = ', unpack('L',$val), "\n";
1097 See also L<AnyDBM_File> for a more general description of the pros and
1098 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1099 rich implementation.
1101 You can control which DBM library you use by loading that library
1102 before you call dbmopen():
1105 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1106 or die "Can't open netscape history file: $!";
1109 X<defined> X<undef> X<undefined>
1113 Returns a Boolean value telling whether EXPR has a value other than
1114 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1117 Many operations return C<undef> to indicate failure, end of file,
1118 system error, uninitialized variable, and other exceptional
1119 conditions. This function allows you to distinguish C<undef> from
1120 other values. (A simple Boolean test will not distinguish among
1121 C<undef>, zero, the empty string, and C<"0">, which are all equally
1122 false.) Note that since C<undef> is a valid scalar, its presence
1123 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1124 returns C<undef> when its argument is an empty array, I<or> when the
1125 element to return happens to be C<undef>.
1127 You may also use C<defined(&func)> to check whether subroutine C<&func>
1128 has ever been defined. The return value is unaffected by any forward
1129 declarations of C<&func>. Note that a subroutine which is not defined
1130 may still be callable: its package may have an C<AUTOLOAD> method that
1131 makes it spring into existence the first time that it is called -- see
1134 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1135 used to report whether memory for that aggregate has ever been
1136 allocated. This behavior may disappear in future versions of Perl.
1137 You should instead use a simple test for size:
1139 if (@an_array) { print "has array elements\n" }
1140 if (%a_hash) { print "has hash members\n" }
1142 When used on a hash element, it tells you whether the value is defined,
1143 not whether the key exists in the hash. Use L</exists> for the latter
1148 print if defined $switch{'D'};
1149 print "$val\n" while defined($val = pop(@ary));
1150 die "Can't readlink $sym: $!"
1151 unless defined($value = readlink $sym);
1152 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1153 $debugging = 0 unless defined $debugging;
1155 Note: Many folks tend to overuse C<defined>, and then are surprised to
1156 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1157 defined values. For example, if you say
1161 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1162 matched "nothing". It didn't really fail to match anything. Rather, it
1163 matched something that happened to be zero characters long. This is all
1164 very above-board and honest. When a function returns an undefined value,
1165 it's an admission that it couldn't give you an honest answer. So you
1166 should use C<defined> only when you're questioning the integrity of what
1167 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1170 See also L</undef>, L</exists>, L</ref>.
1175 Given an expression that specifies a hash element, array element, hash slice,
1176 or array slice, deletes the specified element(s) from the hash or array.
1177 In the case of an array, if the array elements happen to be at the end,
1178 the size of the array will shrink to the highest element that tests
1179 true for exists() (or 0 if no such element exists).
1181 Returns a list with the same number of elements as the number of elements
1182 for which deletion was attempted. Each element of that list consists of
1183 either the value of the element deleted, or the undefined value. In scalar
1184 context, this means that you get the value of the last element deleted (or
1185 the undefined value if that element did not exist).
1187 %hash = (foo => 11, bar => 22, baz => 33);
1188 $scalar = delete $hash{foo}; # $scalar is 11
1189 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1190 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1192 Deleting from C<%ENV> modifies the environment. Deleting from
1193 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1194 from a C<tie>d hash or array may not necessarily return anything.
1196 Deleting an array element effectively returns that position of the array
1197 to its initial, uninitialized state. Subsequently testing for the same
1198 element with exists() will return false. Also, deleting array elements
1199 in the middle of an array will not shift the index of the elements
1200 after them down. Use splice() for that. See L</exists>.
1202 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1204 foreach $key (keys %HASH) {
1208 foreach $index (0 .. $#ARRAY) {
1209 delete $ARRAY[$index];
1214 delete @HASH{keys %HASH};
1216 delete @ARRAY[0 .. $#ARRAY];
1218 But both of these are slower than just assigning the empty list
1219 or undefining %HASH or @ARRAY:
1221 %HASH = (); # completely empty %HASH
1222 undef %HASH; # forget %HASH ever existed
1224 @ARRAY = (); # completely empty @ARRAY
1225 undef @ARRAY; # forget @ARRAY ever existed
1227 Note that the EXPR can be arbitrarily complicated as long as the final
1228 operation is a hash element, array element, hash slice, or array slice
1231 delete $ref->[$x][$y]{$key};
1232 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1234 delete $ref->[$x][$y][$index];
1235 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1238 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1240 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1241 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1242 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1243 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1244 an C<eval(),> the error message is stuffed into C<$@> and the
1245 C<eval> is terminated with the undefined value. This makes
1246 C<die> the way to raise an exception.
1248 Equivalent examples:
1250 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1251 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1253 If the last element of LIST does not end in a newline, the current
1254 script line number and input line number (if any) are also printed,
1255 and a newline is supplied. Note that the "input line number" (also
1256 known as "chunk") is subject to whatever notion of "line" happens to
1257 be currently in effect, and is also available as the special variable
1258 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1260 Hint: sometimes appending C<", stopped"> to your message will cause it
1261 to make better sense when the string C<"at foo line 123"> is appended.
1262 Suppose you are running script "canasta".
1264 die "/etc/games is no good";
1265 die "/etc/games is no good, stopped";
1267 produce, respectively
1269 /etc/games is no good at canasta line 123.
1270 /etc/games is no good, stopped at canasta line 123.
1272 See also exit(), warn(), and the Carp module.
1274 If LIST is empty and C<$@> already contains a value (typically from a
1275 previous eval) that value is reused after appending C<"\t...propagated">.
1276 This is useful for propagating exceptions:
1279 die unless $@ =~ /Expected exception/;
1281 If LIST is empty and C<$@> contains an object reference that has a
1282 C<PROPAGATE> method, that method will be called with additional file
1283 and line number parameters. The return value replaces the value in
1284 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1287 If C<$@> is empty then the string C<"Died"> is used.
1289 die() can also be called with a reference argument. If this happens to be
1290 trapped within an eval(), $@ contains the reference. This behavior permits
1291 a more elaborate exception handling implementation using objects that
1292 maintain arbitrary state about the nature of the exception. Such a scheme
1293 is sometimes preferable to matching particular string values of $@ using
1294 regular expressions. Because $@ is a global variable, and eval() may be
1295 used within object implementations, care must be taken that analyzing the
1296 error object doesn't replace the reference in the global variable. The
1297 easiest solution is to make a local copy of the reference before doing
1298 other manipulations. Here's an example:
1300 use Scalar::Util 'blessed';
1302 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1303 if (my $ev_err = $@) {
1304 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1305 # handle Some::Module::Exception
1308 # handle all other possible exceptions
1312 Because perl will stringify uncaught exception messages before displaying
1313 them, you may want to overload stringification operations on such custom
1314 exception objects. See L<overload> for details about that.
1316 You can arrange for a callback to be run just before the C<die>
1317 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1318 handler will be called with the error text and can change the error
1319 message, if it sees fit, by calling C<die> again. See
1320 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1321 L<"eval BLOCK"> for some examples. Although this feature was
1322 to be run only right before your program was to exit, this is not
1323 currently the case--the C<$SIG{__DIE__}> hook is currently called
1324 even inside eval()ed blocks/strings! If one wants the hook to do
1325 nothing in such situations, put
1329 as the first line of the handler (see L<perlvar/$^S>). Because
1330 this promotes strange action at a distance, this counterintuitive
1331 behavior may be fixed in a future release.
1336 Not really a function. Returns the value of the last command in the
1337 sequence of commands indicated by BLOCK. When modified by the C<while> or
1338 C<until> loop modifier, executes the BLOCK once before testing the loop
1339 condition. (On other statements the loop modifiers test the conditional
1342 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1343 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1344 See L<perlsyn> for alternative strategies.
1346 =item do SUBROUTINE(LIST)
1349 This form of subroutine call is deprecated. See L<perlsub>.
1354 Uses the value of EXPR as a filename and executes the contents of the
1355 file as a Perl script.
1363 except that it's more efficient and concise, keeps track of the current
1364 filename for error messages, searches the @INC directories, and updates
1365 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1366 variables. It also differs in that code evaluated with C<do FILENAME>
1367 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1368 same, however, in that it does reparse the file every time you call it,
1369 so you probably don't want to do this inside a loop.
1371 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1372 error. If C<do> can read the file but cannot compile it, it
1373 returns undef and sets an error message in C<$@>. If the file is
1374 successfully compiled, C<do> returns the value of the last expression
1377 Note that inclusion of library modules is better done with the
1378 C<use> and C<require> operators, which also do automatic error checking
1379 and raise an exception if there's a problem.
1381 You might like to use C<do> to read in a program configuration
1382 file. Manual error checking can be done this way:
1384 # read in config files: system first, then user
1385 for $file ("/share/prog/defaults.rc",
1386 "$ENV{HOME}/.someprogrc")
1388 unless ($return = do $file) {
1389 warn "couldn't parse $file: $@" if $@;
1390 warn "couldn't do $file: $!" unless defined $return;
1391 warn "couldn't run $file" unless $return;
1396 X<dump> X<core> X<undump>
1400 This function causes an immediate core dump. See also the B<-u>
1401 command-line switch in L<perlrun>, which does the same thing.
1402 Primarily this is so that you can use the B<undump> program (not
1403 supplied) to turn your core dump into an executable binary after
1404 having initialized all your variables at the beginning of the
1405 program. When the new binary is executed it will begin by executing
1406 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1407 Think of it as a goto with an intervening core dump and reincarnation.
1408 If C<LABEL> is omitted, restarts the program from the top.
1410 B<WARNING>: Any files opened at the time of the dump will I<not>
1411 be open any more when the program is reincarnated, with possible
1412 resulting confusion on the part of Perl.
1414 This function is now largely obsolete, mostly because it's very hard to
1415 convert a core file into an executable. That's why you should now invoke
1416 it as C<CORE::dump()>, if you don't want to be warned against a possible
1420 X<each> X<hash, iterator>
1425 When called in list context, returns a 2-element list consisting of the
1426 key and value for the next element of a hash, or the index and value for
1427 the next element of an array, so that you can iterate over it. When called
1428 in scalar context, returns only the key for the next element in the hash
1429 (or the index for an array).
1431 Hash entries are returned in an apparently random order. The actual random
1432 order is subject to change in future versions of perl, but it is
1433 guaranteed to be in the same order as either the C<keys> or C<values>
1434 function would produce on the same (unmodified) hash. Since Perl
1435 5.8.2 the ordering can be different even between different runs of Perl
1436 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1438 When the hash or array is entirely read, a null array is returned in list
1439 context (which when assigned produces a false (C<0>) value), and C<undef> in
1440 scalar context. The next call to C<each> after that will start iterating
1441 again. There is a single iterator for each hash or array, shared by all
1442 C<each>, C<keys>, and C<values> function calls in the program; it can be
1443 reset by reading all the elements from the hash or array, or by evaluating
1444 C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1445 or delete elements of a hash while you're
1446 iterating over it, you may get entries skipped or duplicated, so
1447 don't. Exception: It is always safe to delete the item most recently
1448 returned by C<each()>, which means that the following code will work:
1450 while (($key, $value) = each %hash) {
1452 delete $hash{$key}; # This is safe
1455 The following prints out your environment like the printenv(1) program,
1456 only in a different order:
1458 while (($key,$value) = each %ENV) {
1459 print "$key=$value\n";
1462 See also C<keys>, C<values> and C<sort>.
1464 =item eof FILEHANDLE
1473 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1474 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1475 gives the real filehandle. (Note that this function actually
1476 reads a character and then C<ungetc>s it, so isn't very useful in an
1477 interactive context.) Do not read from a terminal file (or call
1478 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1479 as terminals may lose the end-of-file condition if you do.
1481 An C<eof> without an argument uses the last file read. Using C<eof()>
1482 with empty parentheses is very different. It refers to the pseudo file
1483 formed from the files listed on the command line and accessed via the
1484 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1485 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1486 used will cause C<@ARGV> to be examined to determine if input is
1487 available. Similarly, an C<eof()> after C<< <> >> has returned
1488 end-of-file will assume you are processing another C<@ARGV> list,
1489 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1490 see L<perlop/"I/O Operators">.
1492 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1493 detect the end of each file, C<eof()> will only detect the end of the
1494 last file. Examples:
1496 # reset line numbering on each input file
1498 next if /^\s*#/; # skip comments
1501 close ARGV if eof; # Not eof()!
1504 # insert dashes just before last line of last file
1506 if (eof()) { # check for end of last file
1507 print "--------------\n";
1510 last if eof(); # needed if we're reading from a terminal
1513 Practical hint: you almost never need to use C<eof> in Perl, because the
1514 input operators typically return C<undef> when they run out of data, or if
1518 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1519 X<error, handling> X<exception, handling>
1525 In the first form, the return value of EXPR is parsed and executed as if it
1526 were a little Perl program. The value of the expression (which is itself
1527 determined within scalar context) is first parsed, and if there weren't any
1528 errors, executed in the lexical context of the current Perl program, so
1529 that any variable settings or subroutine and format definitions remain
1530 afterwards. Note that the value is parsed every time the C<eval> executes.
1531 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1532 delay parsing and subsequent execution of the text of EXPR until run time.
1534 In the second form, the code within the BLOCK is parsed only once--at the
1535 same time the code surrounding the C<eval> itself was parsed--and executed
1536 within the context of the current Perl program. This form is typically
1537 used to trap exceptions more efficiently than the first (see below), while
1538 also providing the benefit of checking the code within BLOCK at compile
1541 The final semicolon, if any, may be omitted from the value of EXPR or within
1544 In both forms, the value returned is the value of the last expression
1545 evaluated inside the mini-program; a return statement may be also used, just
1546 as with subroutines. The expression providing the return value is evaluated
1547 in void, scalar, or list context, depending on the context of the C<eval>
1548 itself. See L</wantarray> for more on how the evaluation context can be
1551 If there is a syntax error or runtime error, or a C<die> statement is
1552 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1553 error message. If there was no error, C<$@> is guaranteed to be a null
1554 string. Beware that using C<eval> neither silences perl from printing
1555 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1556 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1557 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1558 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1560 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1561 determining whether a particular feature (such as C<socket> or C<symlink>)
1562 is implemented. It is also Perl's exception trapping mechanism, where
1563 the die operator is used to raise exceptions.
1565 If you want to trap errors when loading an XS module, some problems with
1566 the binary interface (such as Perl version skew) may be fatal even with
1567 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1569 If the code to be executed doesn't vary, you may use the eval-BLOCK
1570 form to trap run-time errors without incurring the penalty of
1571 recompiling each time. The error, if any, is still returned in C<$@>.
1574 # make divide-by-zero nonfatal
1575 eval { $answer = $a / $b; }; warn $@ if $@;
1577 # same thing, but less efficient
1578 eval '$answer = $a / $b'; warn $@ if $@;
1580 # a compile-time error
1581 eval { $answer = }; # WRONG
1584 eval '$answer ='; # sets $@
1586 Using the C<eval{}> form as an exception trap in libraries does have some
1587 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1588 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1589 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1590 as shown in this example:
1592 # a very private exception trap for divide-by-zero
1593 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1596 This is especially significant, given that C<__DIE__> hooks can call
1597 C<die> again, which has the effect of changing their error messages:
1599 # __DIE__ hooks may modify error messages
1601 local $SIG{'__DIE__'} =
1602 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1603 eval { die "foo lives here" };
1604 print $@ if $@; # prints "bar lives here"
1607 Because this promotes action at a distance, this counterintuitive behavior
1608 may be fixed in a future release.
1610 With an C<eval>, you should be especially careful to remember what's
1611 being looked at when:
1617 eval { $x }; # CASE 4
1619 eval "\$$x++"; # CASE 5
1622 Cases 1 and 2 above behave identically: they run the code contained in
1623 the variable $x. (Although case 2 has misleading double quotes making
1624 the reader wonder what else might be happening (nothing is).) Cases 3
1625 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1626 does nothing but return the value of $x. (Case 4 is preferred for
1627 purely visual reasons, but it also has the advantage of compiling at
1628 compile-time instead of at run-time.) Case 5 is a place where
1629 normally you I<would> like to use double quotes, except that in this
1630 particular situation, you can just use symbolic references instead, as
1633 The assignment to C<$@> occurs before restoration of localised variables,
1634 which means a temporary is required if you want to mask some but not all
1637 # alter $@ on nefarious repugnancy only
1641 local $@; # protect existing $@
1642 eval { test_repugnancy() };
1643 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1644 $@ =~ /nefarious/ and $e = $@;
1646 die $e if defined $e
1649 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1650 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1652 Note that as a very special case, an C<eval ''> executed within the C<DB>
1653 package doesn't see the usual surrounding lexical scope, but rather the
1654 scope of the first non-DB piece of code that called it. You don't normally
1655 need to worry about this unless you are writing a Perl debugger.
1660 =item exec PROGRAM LIST
1662 The C<exec> function executes a system command I<and never returns>--
1663 use C<system> instead of C<exec> if you want it to return. It fails and
1664 returns false only if the command does not exist I<and> it is executed
1665 directly instead of via your system's command shell (see below).
1667 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1668 warns you if there is a following statement which isn't C<die>, C<warn>,
1669 or C<exit> (if C<-w> is set - but you always do that). If you
1670 I<really> want to follow an C<exec> with some other statement, you
1671 can use one of these styles to avoid the warning:
1673 exec ('foo') or print STDERR "couldn't exec foo: $!";
1674 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1676 If there is more than one argument in LIST, or if LIST is an array
1677 with more than one value, calls execvp(3) with the arguments in LIST.
1678 If there is only one scalar argument or an array with one element in it,
1679 the argument is checked for shell metacharacters, and if there are any,
1680 the entire argument is passed to the system's command shell for parsing
1681 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1682 If there are no shell metacharacters in the argument, it is split into
1683 words and passed directly to C<execvp>, which is more efficient.
1686 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1687 exec "sort $outfile | uniq";
1689 If you don't really want to execute the first argument, but want to lie
1690 to the program you are executing about its own name, you can specify
1691 the program you actually want to run as an "indirect object" (without a
1692 comma) in front of the LIST. (This always forces interpretation of the
1693 LIST as a multivalued list, even if there is only a single scalar in
1696 $shell = '/bin/csh';
1697 exec $shell '-sh'; # pretend it's a login shell
1701 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1703 When the arguments get executed via the system shell, results will
1704 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1707 Using an indirect object with C<exec> or C<system> is also more
1708 secure. This usage (which also works fine with system()) forces
1709 interpretation of the arguments as a multivalued list, even if the
1710 list had just one argument. That way you're safe from the shell
1711 expanding wildcards or splitting up words with whitespace in them.
1713 @args = ( "echo surprise" );
1715 exec @args; # subject to shell escapes
1717 exec { $args[0] } @args; # safe even with one-arg list
1719 The first version, the one without the indirect object, ran the I<echo>
1720 program, passing it C<"surprise"> an argument. The second version
1721 didn't--it tried to run a program literally called I<"echo surprise">,
1722 didn't find it, and set C<$?> to a non-zero value indicating failure.
1724 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1725 output before the exec, but this may not be supported on some platforms
1726 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1727 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1728 open handles in order to avoid lost output.
1730 Note that C<exec> will not call your C<END> blocks, nor will it call
1731 any C<DESTROY> methods in your objects.
1734 X<exists> X<autovivification>
1736 Given an expression that specifies a hash element or array element,
1737 returns true if the specified element in the hash or array has ever
1738 been initialized, even if the corresponding value is undefined. The
1739 element is not autovivified if it doesn't exist.
1741 print "Exists\n" if exists $hash{$key};
1742 print "Defined\n" if defined $hash{$key};
1743 print "True\n" if $hash{$key};
1745 print "Exists\n" if exists $array[$index];
1746 print "Defined\n" if defined $array[$index];
1747 print "True\n" if $array[$index];
1749 A hash or array element can be true only if it's defined, and defined if
1750 it exists, but the reverse doesn't necessarily hold true.
1752 Given an expression that specifies the name of a subroutine,
1753 returns true if the specified subroutine has ever been declared, even
1754 if it is undefined. Mentioning a subroutine name for exists or defined
1755 does not count as declaring it. Note that a subroutine which does not
1756 exist may still be callable: its package may have an C<AUTOLOAD>
1757 method that makes it spring into existence the first time that it is
1758 called -- see L<perlsub>.
1760 print "Exists\n" if exists &subroutine;
1761 print "Defined\n" if defined &subroutine;
1763 Note that the EXPR can be arbitrarily complicated as long as the final
1764 operation is a hash or array key lookup or subroutine name:
1766 if (exists $ref->{A}->{B}->{$key}) { }
1767 if (exists $hash{A}{B}{$key}) { }
1769 if (exists $ref->{A}->{B}->[$ix]) { }
1770 if (exists $hash{A}{B}[$ix]) { }
1772 if (exists &{$ref->{A}{B}{$key}}) { }
1774 Although the deepest nested array or hash will not spring into existence
1775 just because its existence was tested, any intervening ones will.
1776 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1777 into existence due to the existence test for the $key element above.
1778 This happens anywhere the arrow operator is used, including even:
1781 if (exists $ref->{"Some key"}) { }
1782 print $ref; # prints HASH(0x80d3d5c)
1784 This surprising autovivification in what does not at first--or even
1785 second--glance appear to be an lvalue context may be fixed in a future
1788 Use of a subroutine call, rather than a subroutine name, as an argument
1789 to exists() is an error.
1792 exists &sub(); # Error
1795 X<exit> X<terminate> X<abort>
1799 Evaluates EXPR and exits immediately with that value. Example:
1802 exit 0 if $ans =~ /^[Xx]/;
1804 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1805 universally recognized values for EXPR are C<0> for success and C<1>
1806 for error; other values are subject to interpretation depending on the
1807 environment in which the Perl program is running. For example, exiting
1808 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1809 the mailer to return the item undelivered, but that's not true everywhere.
1811 Don't use C<exit> to abort a subroutine if there's any chance that
1812 someone might want to trap whatever error happened. Use C<die> instead,
1813 which can be trapped by an C<eval>.
1815 The exit() function does not always exit immediately. It calls any
1816 defined C<END> routines first, but these C<END> routines may not
1817 themselves abort the exit. Likewise any object destructors that need to
1818 be called are called before the real exit. If this is a problem, you
1819 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1820 See L<perlmod> for details.
1823 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1827 Returns I<e> (the natural logarithm base) to the power of EXPR.
1828 If EXPR is omitted, gives C<exp($_)>.
1830 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1833 Implements the fcntl(2) function. You'll probably have to say
1837 first to get the correct constant definitions. Argument processing and
1838 value return works just like C<ioctl> below.
1842 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1843 or die "can't fcntl F_GETFL: $!";
1845 You don't have to check for C<defined> on the return from C<fcntl>.
1846 Like C<ioctl>, it maps a C<0> return from the system call into
1847 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1848 in numeric context. It is also exempt from the normal B<-w> warnings
1849 on improper numeric conversions.
1851 Note that C<fcntl> will produce a fatal error if used on a machine that
1852 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1853 manpage to learn what functions are available on your system.
1855 Here's an example of setting a filehandle named C<REMOTE> to be
1856 non-blocking at the system level. You'll have to negotiate C<$|>
1857 on your own, though.
1859 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1861 $flags = fcntl(REMOTE, F_GETFL, 0)
1862 or die "Can't get flags for the socket: $!\n";
1864 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1865 or die "Can't set flags for the socket: $!\n";
1867 =item fileno FILEHANDLE
1870 Returns the file descriptor for a filehandle, or undefined if the
1871 filehandle is not open. This is mainly useful for constructing
1872 bitmaps for C<select> and low-level POSIX tty-handling operations.
1873 If FILEHANDLE is an expression, the value is taken as an indirect
1874 filehandle, generally its name.
1876 You can use this to find out whether two handles refer to the
1877 same underlying descriptor:
1879 if (fileno(THIS) == fileno(THAT)) {
1880 print "THIS and THAT are dups\n";
1883 (Filehandles connected to memory objects via new features of C<open> may
1884 return undefined even though they are open.)
1887 =item flock FILEHANDLE,OPERATION
1888 X<flock> X<lock> X<locking>
1890 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1891 for success, false on failure. Produces a fatal error if used on a
1892 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1893 C<flock> is Perl's portable file locking interface, although it locks
1894 only entire files, not records.
1896 Two potentially non-obvious but traditional C<flock> semantics are
1897 that it waits indefinitely until the lock is granted, and that its locks
1898 B<merely advisory>. Such discretionary locks are more flexible, but offer
1899 fewer guarantees. This means that programs that do not also use C<flock>
1900 may modify files locked with C<flock>. See L<perlport>,
1901 your port's specific documentation, or your system-specific local manpages
1902 for details. It's best to assume traditional behavior if you're writing
1903 portable programs. (But if you're not, you should as always feel perfectly
1904 free to write for your own system's idiosyncrasies (sometimes called
1905 "features"). Slavish adherence to portability concerns shouldn't get
1906 in the way of your getting your job done.)
1908 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1909 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1910 you can use the symbolic names if you import them from the Fcntl module,
1911 either individually, or as a group using the ':flock' tag. LOCK_SH
1912 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1913 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1914 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1915 waiting for the lock (check the return status to see if you got it).
1917 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1918 before locking or unlocking it.
1920 Note that the emulation built with lockf(3) doesn't provide shared
1921 locks, and it requires that FILEHANDLE be open with write intent. These
1922 are the semantics that lockf(3) implements. Most if not all systems
1923 implement lockf(3) in terms of fcntl(2) locking, though, so the
1924 differing semantics shouldn't bite too many people.
1926 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1927 be open with read intent to use LOCK_SH and requires that it be open
1928 with write intent to use LOCK_EX.
1930 Note also that some versions of C<flock> cannot lock things over the
1931 network; you would need to use the more system-specific C<fcntl> for
1932 that. If you like you can force Perl to ignore your system's flock(2)
1933 function, and so provide its own fcntl(2)-based emulation, by passing
1934 the switch C<-Ud_flock> to the F<Configure> program when you configure
1937 Here's a mailbox appender for BSD systems.
1939 use Fcntl ':flock'; # import LOCK_* constants
1942 flock(MBOX,LOCK_EX);
1943 # and, in case someone appended
1944 # while we were waiting...
1949 flock(MBOX,LOCK_UN);
1952 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
1953 or die "Can't open mailbox: $!";
1956 print $mbox $msg,"\n\n";
1959 On systems that support a real flock(), locks are inherited across fork()
1960 calls, whereas those that must resort to the more capricious fcntl()
1961 function lose the locks, making it harder to write servers.
1963 See also L<DB_File> for other flock() examples.
1966 X<fork> X<child> X<parent>
1968 Does a fork(2) system call to create a new process running the
1969 same program at the same point. It returns the child pid to the
1970 parent process, C<0> to the child process, or C<undef> if the fork is
1971 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1972 are shared, while everything else is copied. On most systems supporting
1973 fork(), great care has gone into making it extremely efficient (for
1974 example, using copy-on-write technology on data pages), making it the
1975 dominant paradigm for multitasking over the last few decades.
1977 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1978 output before forking the child process, but this may not be supported
1979 on some platforms (see L<perlport>). To be safe, you may need to set
1980 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1981 C<IO::Handle> on any open handles in order to avoid duplicate output.
1983 If you C<fork> without ever waiting on your children, you will
1984 accumulate zombies. On some systems, you can avoid this by setting
1985 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1986 forking and reaping moribund children.
1988 Note that if your forked child inherits system file descriptors like
1989 STDIN and STDOUT that are actually connected by a pipe or socket, even
1990 if you exit, then the remote server (such as, say, a CGI script or a
1991 backgrounded job launched from a remote shell) won't think you're done.
1992 You should reopen those to F</dev/null> if it's any issue.
1997 Declare a picture format for use by the C<write> function. For
2001 Test: @<<<<<<<< @||||| @>>>>>
2002 $str, $%, '$' . int($num)
2006 $num = $cost/$quantity;
2010 See L<perlform> for many details and examples.
2012 =item formline PICTURE,LIST
2015 This is an internal function used by C<format>s, though you may call it,
2016 too. It formats (see L<perlform>) a list of values according to the
2017 contents of PICTURE, placing the output into the format output
2018 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2019 Eventually, when a C<write> is done, the contents of
2020 C<$^A> are written to some filehandle. You could also read C<$^A>
2021 and then set C<$^A> back to C<"">. Note that a format typically
2022 does one C<formline> per line of form, but the C<formline> function itself
2023 doesn't care how many newlines are embedded in the PICTURE. This means
2024 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2025 You may therefore need to use multiple formlines to implement a single
2026 record format, just like the format compiler.
2028 Be careful if you put double quotes around the picture, because an C<@>
2029 character may be taken to mean the beginning of an array name.
2030 C<formline> always returns true. See L<perlform> for other examples.
2032 =item getc FILEHANDLE
2033 X<getc> X<getchar> X<character> X<file, read>
2037 Returns the next character from the input file attached to FILEHANDLE,
2038 or the undefined value at end of file, or if there was an error (in
2039 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2040 STDIN. This is not particularly efficient. However, it cannot be
2041 used by itself to fetch single characters without waiting for the user
2042 to hit enter. For that, try something more like:
2045 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2048 system "stty", '-icanon', 'eol', "\001";
2054 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2057 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2061 Determination of whether $BSD_STYLE should be set
2062 is left as an exercise to the reader.
2064 The C<POSIX::getattr> function can do this more portably on
2065 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2066 module from your nearest CPAN site; details on CPAN can be found on
2070 X<getlogin> X<login>
2072 This implements the C library function of the same name, which on most
2073 systems returns the current login from F</etc/utmp>, if any. If null,
2076 $login = getlogin || getpwuid($<) || "Kilroy";
2078 Do not consider C<getlogin> for authentication: it is not as
2079 secure as C<getpwuid>.
2081 =item getpeername SOCKET
2082 X<getpeername> X<peer>
2084 Returns the packed sockaddr address of other end of the SOCKET connection.
2087 $hersockaddr = getpeername(SOCK);
2088 ($port, $iaddr) = sockaddr_in($hersockaddr);
2089 $herhostname = gethostbyaddr($iaddr, AF_INET);
2090 $herstraddr = inet_ntoa($iaddr);
2095 Returns the current process group for the specified PID. Use
2096 a PID of C<0> to get the current process group for the
2097 current process. Will raise an exception if used on a machine that
2098 doesn't implement getpgrp(2). If PID is omitted, returns process
2099 group of current process. Note that the POSIX version of C<getpgrp>
2100 does not accept a PID argument, so only C<PID==0> is truly portable.
2103 X<getppid> X<parent> X<pid>
2105 Returns the process id of the parent process.
2107 Note for Linux users: on Linux, the C functions C<getpid()> and
2108 C<getppid()> return different values from different threads. In order to
2109 be portable, this behavior is not reflected by the perl-level function
2110 C<getppid()>, that returns a consistent value across threads. If you want
2111 to call the underlying C<getppid()>, you may use the CPAN module
2114 =item getpriority WHICH,WHO
2115 X<getpriority> X<priority> X<nice>
2117 Returns the current priority for a process, a process group, or a user.
2118 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2119 machine that doesn't implement getpriority(2).
2122 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2123 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2124 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2125 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2126 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2127 X<endnetent> X<endprotoent> X<endservent>
2131 =item gethostbyname NAME
2133 =item getnetbyname NAME
2135 =item getprotobyname NAME
2141 =item getservbyname NAME,PROTO
2143 =item gethostbyaddr ADDR,ADDRTYPE
2145 =item getnetbyaddr ADDR,ADDRTYPE
2147 =item getprotobynumber NUMBER
2149 =item getservbyport PORT,PROTO
2167 =item sethostent STAYOPEN
2169 =item setnetent STAYOPEN
2171 =item setprotoent STAYOPEN
2173 =item setservent STAYOPEN
2187 These routines perform the same functions as their counterparts in the
2188 system library. In list context, the return values from the
2189 various get routines are as follows:
2191 ($name,$passwd,$uid,$gid,
2192 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2193 ($name,$passwd,$gid,$members) = getgr*
2194 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2195 ($name,$aliases,$addrtype,$net) = getnet*
2196 ($name,$aliases,$proto) = getproto*
2197 ($name,$aliases,$port,$proto) = getserv*
2199 (If the entry doesn't exist you get a null list.)
2201 The exact meaning of the $gcos field varies but it usually contains
2202 the real name of the user (as opposed to the login name) and other
2203 information pertaining to the user. Beware, however, that in many
2204 system users are able to change this information and therefore it
2205 cannot be trusted and therefore the $gcos is tainted (see
2206 L<perlsec>). The $passwd and $shell, user's encrypted password and
2207 login shell, are also tainted, because of the same reason.
2209 In scalar context, you get the name, unless the function was a
2210 lookup by name, in which case you get the other thing, whatever it is.
2211 (If the entry doesn't exist you get the undefined value.) For example:
2213 $uid = getpwnam($name);
2214 $name = getpwuid($num);
2216 $gid = getgrnam($name);
2217 $name = getgrgid($num);
2221 In I<getpw*()> the fields $quota, $comment, and $expire are special
2222 cases in the sense that in many systems they are unsupported. If the
2223 $quota is unsupported, it is an empty scalar. If it is supported, it
2224 usually encodes the disk quota. If the $comment field is unsupported,
2225 it is an empty scalar. If it is supported it usually encodes some
2226 administrative comment about the user. In some systems the $quota
2227 field may be $change or $age, fields that have to do with password
2228 aging. In some systems the $comment field may be $class. The $expire
2229 field, if present, encodes the expiration period of the account or the
2230 password. For the availability and the exact meaning of these fields
2231 in your system, please consult your getpwnam(3) documentation and your
2232 F<pwd.h> file. You can also find out from within Perl what your
2233 $quota and $comment fields mean and whether you have the $expire field
2234 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2235 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2236 files are only supported if your vendor has implemented them in the
2237 intuitive fashion that calling the regular C library routines gets the
2238 shadow versions if you're running under privilege or if there exists
2239 the shadow(3) functions as found in System V (this includes Solaris
2240 and Linux.) Those systems that implement a proprietary shadow password
2241 facility are unlikely to be supported.
2243 The $members value returned by I<getgr*()> is a space separated list of
2244 the login names of the members of the group.
2246 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2247 C, it will be returned to you via C<$?> if the function call fails. The
2248 C<@addrs> value returned by a successful call is a list of the raw
2249 addresses returned by the corresponding system library call. In the
2250 Internet domain, each address is four bytes long and you can unpack it
2251 by saying something like:
2253 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2255 The Socket library makes this slightly easier:
2258 $iaddr = inet_aton("127.1"); # or whatever address
2259 $name = gethostbyaddr($iaddr, AF_INET);
2261 # or going the other way
2262 $straddr = inet_ntoa($iaddr);
2264 In the opposite way, to resolve a hostname to the IP address
2268 $packed_ip = gethostbyname("www.perl.org");
2269 if (defined $packed_ip) {
2270 $ip_address = inet_ntoa($packed_ip);
2273 Make sure <gethostbyname()> is called in SCALAR context and that
2274 its return value is checked for definedness.
2276 If you get tired of remembering which element of the return list
2277 contains which return value, by-name interfaces are provided
2278 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2279 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2280 and C<User::grent>. These override the normal built-ins, supplying
2281 versions that return objects with the appropriate names
2282 for each field. For example:
2286 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2288 Even though it looks like they're the same method calls (uid),
2289 they aren't, because a C<File::stat> object is different from
2290 a C<User::pwent> object.
2292 =item getsockname SOCKET
2295 Returns the packed sockaddr address of this end of the SOCKET connection,
2296 in case you don't know the address because you have several different
2297 IPs that the connection might have come in on.
2300 $mysockaddr = getsockname(SOCK);
2301 ($port, $myaddr) = sockaddr_in($mysockaddr);
2302 printf "Connect to %s [%s]\n",
2303 scalar gethostbyaddr($myaddr, AF_INET),
2306 =item getsockopt SOCKET,LEVEL,OPTNAME
2309 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2310 Options may exist at multiple protocol levels depending on the socket
2311 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2312 C<Socket> module) will exist. To query options at another level the
2313 protocol number of the appropriate protocol controlling the option
2314 should be supplied. For example, to indicate that an option is to be
2315 interpreted by the TCP protocol, LEVEL should be set to the protocol
2316 number of TCP, which you can get using getprotobyname.
2318 The call returns a packed string representing the requested socket option,
2319 or C<undef> if there is an error (the error reason will be in $!). What
2320 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2321 your system documentation for details. A very common case however is that
2322 the option is an integer, in which case the result will be a packed
2323 integer which you can decode using unpack with the C<i> (or C<I>) format.
2325 An example testing if Nagle's algorithm is turned on on a socket:
2327 use Socket qw(:all);
2329 defined(my $tcp = getprotobyname("tcp"))
2330 or die "Could not determine the protocol number for tcp";
2331 # my $tcp = IPPROTO_TCP; # Alternative
2332 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2333 or die "Could not query TCP_NODELAY socket option: $!";
2334 my $nodelay = unpack("I", $packed);
2335 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2339 X<glob> X<wildcard> X<filename, expansion> X<expand>
2343 In list context, returns a (possibly empty) list of filename expansions on
2344 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2345 scalar context, glob iterates through such filename expansions, returning
2346 undef when the list is exhausted. This is the internal function
2347 implementing the C<< <*.c> >> operator, but you can use it directly. If
2348 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2349 more detail in L<perlop/"I/O Operators">.
2351 Beginning with v5.6.0, this operator is implemented using the standard
2352 C<File::Glob> extension. See L<File::Glob> for details.
2355 X<gmtime> X<UTC> X<Greenwich>
2359 Works just like L<localtime> but the returned values are
2360 localized for the standard Greenwich time zone.
2362 Note: when called in list context, $isdst, the last value
2363 returned by gmtime is always C<0>. There is no
2364 Daylight Saving Time in GMT.
2366 See L<perlport/gmtime> for portability concerns.
2369 X<goto> X<jump> X<jmp>
2375 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2376 execution there. It may not be used to go into any construct that
2377 requires initialization, such as a subroutine or a C<foreach> loop. It
2378 also can't be used to go into a construct that is optimized away,
2379 or to get out of a block or subroutine given to C<sort>.
2380 It can be used to go almost anywhere else within the dynamic scope,
2381 including out of subroutines, but it's usually better to use some other
2382 construct such as C<last> or C<die>. The author of Perl has never felt the
2383 need to use this form of C<goto> (in Perl, that is--C is another matter).
2384 (The difference being that C does not offer named loops combined with
2385 loop control. Perl does, and this replaces most structured uses of C<goto>
2386 in other languages.)
2388 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2389 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2390 necessarily recommended if you're optimizing for maintainability:
2392 goto ("FOO", "BAR", "GLARCH")[$i];
2394 The C<goto-&NAME> form is quite different from the other forms of
2395 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2396 doesn't have the stigma associated with other gotos. Instead, it
2397 exits the current subroutine (losing any changes set by local()) and
2398 immediately calls in its place the named subroutine using the current
2399 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2400 load another subroutine and then pretend that the other subroutine had
2401 been called in the first place (except that any modifications to C<@_>
2402 in the current subroutine are propagated to the other subroutine.)
2403 After the C<goto>, not even C<caller> will be able to tell that this
2404 routine was called first.
2406 NAME needn't be the name of a subroutine; it can be a scalar variable
2407 containing a code reference, or a block that evaluates to a code
2410 =item grep BLOCK LIST
2413 =item grep EXPR,LIST
2415 This is similar in spirit to, but not the same as, grep(1) and its
2416 relatives. In particular, it is not limited to using regular expressions.
2418 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2419 C<$_> to each element) and returns the list value consisting of those
2420 elements for which the expression evaluated to true. In scalar
2421 context, returns the number of times the expression was true.
2423 @foo = grep(!/^#/, @bar); # weed out comments
2427 @foo = grep {!/^#/} @bar; # weed out comments
2429 Note that C<$_> is an alias to the list value, so it can be used to
2430 modify the elements of the LIST. While this is useful and supported,
2431 it can cause bizarre results if the elements of LIST are not variables.
2432 Similarly, grep returns aliases into the original list, much as a for
2433 loop's index variable aliases the list elements. That is, modifying an
2434 element of a list returned by grep (for example, in a C<foreach>, C<map>
2435 or another C<grep>) actually modifies the element in the original list.
2436 This is usually something to be avoided when writing clear code.
2438 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2439 been declared with C<my $_>) then, in addition to being locally aliased to
2440 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2441 can't be seen from the outside, avoiding any potential side-effects.
2443 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2446 X<hex> X<hexadecimal>
2450 Interprets EXPR as a hex string and returns the corresponding value.
2451 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2452 L</oct>.) If EXPR is omitted, uses C<$_>.
2454 print hex '0xAf'; # prints '175'
2455 print hex 'aF'; # same
2457 Hex strings may only represent integers. Strings that would cause
2458 integer overflow trigger a warning. Leading whitespace is not stripped,
2459 unlike oct(). To present something as hex, look into L</printf>,
2460 L</sprintf>, or L</unpack>.
2465 There is no builtin C<import> function. It is just an ordinary
2466 method (subroutine) defined (or inherited) by modules that wish to export
2467 names to another module. The C<use> function calls the C<import> method
2468 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2470 =item index STR,SUBSTR,POSITION
2471 X<index> X<indexOf> X<InStr>
2473 =item index STR,SUBSTR
2475 The index function searches for one string within another, but without
2476 the wildcard-like behavior of a full regular-expression pattern match.
2477 It returns the position of the first occurrence of SUBSTR in STR at
2478 or after POSITION. If POSITION is omitted, starts searching from the
2479 beginning of the string. POSITION before the beginning of the string
2480 or after its end is treated as if it were the beginning or the end,
2481 respectively. POSITION and the return value are based at C<0> (or whatever
2482 you've set the C<$[> variable to--but don't do that). If the substring
2483 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2486 X<int> X<integer> X<truncate> X<trunc> X<floor>
2490 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2491 You should not use this function for rounding: one because it truncates
2492 towards C<0>, and two because machine representations of floating point
2493 numbers can sometimes produce counterintuitive results. For example,
2494 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2495 because it's really more like -268.99999999999994315658 instead. Usually,
2496 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2497 functions will serve you better than will int().
2499 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2502 Implements the ioctl(2) function. You'll probably first have to say
2504 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2506 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2507 exist or doesn't have the correct definitions you'll have to roll your
2508 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2509 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2510 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2511 written depending on the FUNCTION--a pointer to the string value of SCALAR
2512 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2513 has no string value but does have a numeric value, that value will be
2514 passed rather than a pointer to the string value. To guarantee this to be
2515 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2516 functions may be needed to manipulate the values of structures used by
2519 The return value of C<ioctl> (and C<fcntl>) is as follows:
2521 if OS returns: then Perl returns:
2523 0 string "0 but true"
2524 anything else that number
2526 Thus Perl returns true on success and false on failure, yet you can
2527 still easily determine the actual value returned by the operating
2530 $retval = ioctl(...) || -1;
2531 printf "System returned %d\n", $retval;
2533 The special string C<"0 but true"> is exempt from B<-w> complaints
2534 about improper numeric conversions.
2536 =item join EXPR,LIST
2539 Joins the separate strings of LIST into a single string with fields
2540 separated by the value of EXPR, and returns that new string. Example:
2542 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2544 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2545 first argument. Compare L</split>.
2552 Returns a list consisting of all the keys of the named hash, or the indices
2553 of an array. (In scalar context, returns the number of keys or indices.)
2555 The keys of a hash are returned in an apparently random order. The actual
2556 random order is subject to change in future versions of perl, but it
2557 is guaranteed to be the same order as either the C<values> or C<each>
2558 function produces (given that the hash has not been modified). Since
2559 Perl 5.8.1 the ordering is different even between different runs of
2560 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2563 As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
2564 (see L</each>). In particular, calling keys() in void context resets
2565 the iterator with no other overhead.
2567 Here is yet another way to print your environment:
2570 @values = values %ENV;
2572 print pop(@keys), '=', pop(@values), "\n";
2575 or how about sorted by key:
2577 foreach $key (sort(keys %ENV)) {
2578 print $key, '=', $ENV{$key}, "\n";
2581 The returned values are copies of the original keys in the hash, so
2582 modifying them will not affect the original hash. Compare L</values>.
2584 To sort a hash by value, you'll need to use a C<sort> function.
2585 Here's a descending numeric sort of a hash by its values:
2587 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2588 printf "%4d %s\n", $hash{$key}, $key;
2591 As an lvalue C<keys> allows you to increase the number of hash buckets
2592 allocated for the given hash. This can gain you a measure of efficiency if
2593 you know the hash is going to get big. (This is similar to pre-extending
2594 an array by assigning a larger number to $#array.) If you say
2598 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2599 in fact, since it rounds up to the next power of two. These
2600 buckets will be retained even if you do C<%hash = ()>, use C<undef
2601 %hash> if you want to free the storage while C<%hash> is still in scope.
2602 You can't shrink the number of buckets allocated for the hash using
2603 C<keys> in this way (but you needn't worry about doing this by accident,
2604 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2607 See also C<each>, C<values> and C<sort>.
2609 =item kill SIGNAL, LIST
2612 Sends a signal to a list of processes. Returns the number of
2613 processes successfully signaled (which is not necessarily the
2614 same as the number actually killed).
2616 $cnt = kill 1, $child1, $child2;
2619 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2620 system call will check whether it's possible to send a signal to it (that
2621 means, to be brief, that the process is owned by the same user, or we are
2622 the super-user). This is a useful way to check that a child process is
2623 alive (even if only as a zombie) and hasn't changed its UID. See
2624 L<perlport> for notes on the portability of this construct.
2626 Unlike in the shell, if SIGNAL is negative, it kills
2627 process groups instead of processes. (On System V, a negative I<PROCESS>
2628 number will also kill process groups, but that's not portable.) That
2629 means you usually want to use positive not negative signals. You may also
2630 use a signal name in quotes.
2632 See L<perlipc/"Signals"> for more details.
2639 The C<last> command is like the C<break> statement in C (as used in
2640 loops); it immediately exits the loop in question. If the LABEL is
2641 omitted, the command refers to the innermost enclosing loop. The
2642 C<continue> block, if any, is not executed:
2644 LINE: while (<STDIN>) {
2645 last LINE if /^$/; # exit when done with header
2649 C<last> cannot be used to exit a block which returns a value such as
2650 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2651 a grep() or map() operation.
2653 Note that a block by itself is semantically identical to a loop
2654 that executes once. Thus C<last> can be used to effect an early
2655 exit out of such a block.
2657 See also L</continue> for an illustration of how C<last>, C<next>, and
2665 Returns a lowercased version of EXPR. This is the internal function
2666 implementing the C<\L> escape in double-quoted strings. Respects
2667 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2668 and L<perlunicode> for more details about locale and Unicode support.
2670 If EXPR is omitted, uses C<$_>.
2673 X<lcfirst> X<lowercase>
2677 Returns the value of EXPR with the first character lowercased. This
2678 is the internal function implementing the C<\l> escape in
2679 double-quoted strings. Respects current LC_CTYPE locale if C<use
2680 locale> in force. See L<perllocale> and L<perlunicode> for more
2681 details about locale and Unicode support.
2683 If EXPR is omitted, uses C<$_>.
2690 Returns the length in I<characters> of the value of EXPR. If EXPR is
2691 omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2692 Note that this cannot be used on an entire array or hash to find out how
2693 many elements these have. For that, use C<scalar @array> and C<scalar keys
2694 %hash> respectively.
2696 Note the I<characters>: if the EXPR is in Unicode, you will get the
2697 number of characters, not the number of bytes. To get the length
2698 of the internal string in bytes, use C<bytes::length(EXPR)>, see
2699 L<bytes>. Note that the internal encoding is variable, and the number
2700 of bytes usually meaningless. To get the number of bytes that the
2701 string would have when encoded as UTF-8, use
2702 C<length(Encoding::encode_utf8(EXPR))>.
2704 =item link OLDFILE,NEWFILE
2707 Creates a new filename linked to the old filename. Returns true for
2708 success, false otherwise.
2710 =item listen SOCKET,QUEUESIZE
2713 Does the same thing that the listen system call does. Returns true if
2714 it succeeded, false otherwise. See the example in
2715 L<perlipc/"Sockets: Client/Server Communication">.
2720 You really probably want to be using C<my> instead, because C<local> isn't
2721 what most people think of as "local". See
2722 L<perlsub/"Private Variables via my()"> for details.
2724 A local modifies the listed variables to be local to the enclosing
2725 block, file, or eval. If more than one value is listed, the list must
2726 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2727 for details, including issues with tied arrays and hashes.
2729 =item localtime EXPR
2730 X<localtime> X<ctime>
2734 Converts a time as returned by the time function to a 9-element list
2735 with the time analyzed for the local time zone. Typically used as
2739 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2742 All list elements are numeric, and come straight out of the C `struct
2743 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2744 of the specified time.
2746 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2747 the range C<0..11> with 0 indicating January and 11 indicating December.
2748 This makes it easy to get a month name from a list:
2750 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2751 print "$abbr[$mon] $mday";
2752 # $mon=9, $mday=18 gives "Oct 18"
2754 C<$year> is the number of years since 1900, not just the last two digits
2755 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2756 to get a complete 4-digit year is simply:
2760 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2761 to do that, would you?
2763 To get the last two digits of the year (e.g., '01' in 2001) do:
2765 $year = sprintf("%02d", $year % 100);
2767 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2768 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2769 (or C<0..365> in leap years.)
2771 C<$isdst> is true if the specified time occurs during Daylight Saving
2772 Time, false otherwise.
2774 If EXPR is omitted, C<localtime()> uses the current time (as returned
2777 In scalar context, C<localtime()> returns the ctime(3) value:
2779 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2781 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2782 instead of local time use the L</gmtime> builtin. See also the
2783 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2784 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2785 and mktime(3) functions.
2787 To get somewhat similar but locale dependent date strings, set up your
2788 locale environment variables appropriately (please see L<perllocale>) and
2791 use POSIX qw(strftime);
2792 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2793 # or for GMT formatted appropriately for your locale:
2794 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2796 Note that the C<%a> and C<%b>, the short forms of the day of the week
2797 and the month of the year, may not necessarily be three characters wide.
2799 See L<perlport/localtime> for portability concerns.
2801 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2802 by-name access mechanism to the gmtime() and localtime() functions,
2805 For a comprehensive date and time representation look at the
2806 L<DateTime> module on CPAN.
2811 This function places an advisory lock on a shared variable, or referenced
2812 object contained in I<THING> until the lock goes out of scope.
2814 lock() is a "weak keyword" : this means that if you've defined a function
2815 by this name (before any calls to it), that function will be called
2816 instead. (However, if you've said C<use threads>, lock() is always a
2817 keyword.) See L<threads>.
2820 X<log> X<logarithm> X<e> X<ln> X<base>
2824 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2825 returns log of C<$_>. To get the log of another base, use basic algebra:
2826 The base-N log of a number is equal to the natural log of that number
2827 divided by the natural log of N. For example:
2831 return log($n)/log(10);
2834 See also L</exp> for the inverse operation.
2841 Does the same thing as the C<stat> function (including setting the
2842 special C<_> filehandle) but stats a symbolic link instead of the file
2843 the symbolic link points to. If symbolic links are unimplemented on
2844 your system, a normal C<stat> is done. For much more detailed
2845 information, please see the documentation for C<stat>.
2847 If EXPR is omitted, stats C<$_>.
2851 The match operator. See L<perlop>.
2853 =item map BLOCK LIST
2858 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2859 C<$_> to each element) and returns the list value composed of the
2860 results of each such evaluation. In scalar context, returns the
2861 total number of elements so generated. Evaluates BLOCK or EXPR in
2862 list context, so each element of LIST may produce zero, one, or
2863 more elements in the returned value.
2865 @chars = map(chr, @nums);
2867 translates a list of numbers to the corresponding characters. And
2869 %hash = map { get_a_key_for($_) => $_ } @array;
2871 is just a funny way to write
2875 $hash{get_a_key_for($_)} = $_;
2878 Note that C<$_> is an alias to the list value, so it can be used to
2879 modify the elements of the LIST. While this is useful and supported,
2880 it can cause bizarre results if the elements of LIST are not variables.
2881 Using a regular C<foreach> loop for this purpose would be clearer in
2882 most cases. See also L</grep> for an array composed of those items of
2883 the original list for which the BLOCK or EXPR evaluates to true.
2885 If C<$_> is lexical in the scope where the C<map> appears (because it has
2886 been declared with C<my $_>), then, in addition to being locally aliased to
2887 the list elements, C<$_> keeps being lexical inside the block; that is, it
2888 can't be seen from the outside, avoiding any potential side-effects.
2890 C<{> starts both hash references and blocks, so C<map { ...> could be either
2891 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2892 ahead for the closing C<}> it has to take a guess at which its dealing with
2893 based what it finds just after the C<{>. Usually it gets it right, but if it
2894 doesn't it won't realize something is wrong until it gets to the C<}> and
2895 encounters the missing (or unexpected) comma. The syntax error will be
2896 reported close to the C<}> but you'll need to change something near the C<{>
2897 such as using a unary C<+> to give perl some help:
2899 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2900 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2901 %hash = map { ("\L$_", 1) } @array # this also works
2902 %hash = map { lc($_), 1 } @array # as does this.
2903 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2905 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2907 or to force an anon hash constructor use C<+{>:
2909 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2911 and you get list of anonymous hashes each with only 1 entry.
2913 =item mkdir FILENAME,MASK
2914 X<mkdir> X<md> X<directory, create>
2916 =item mkdir FILENAME
2920 Creates the directory specified by FILENAME, with permissions
2921 specified by MASK (as modified by C<umask>). If it succeeds it
2922 returns true, otherwise it returns false and sets C<$!> (errno).
2923 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2926 In general, it is better to create directories with permissive MASK,
2927 and let the user modify that with their C<umask>, than it is to supply
2928 a restrictive MASK and give the user no way to be more permissive.
2929 The exceptions to this rule are when the file or directory should be
2930 kept private (mail files, for instance). The perlfunc(1) entry on
2931 C<umask> discusses the choice of MASK in more detail.
2933 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2934 number of trailing slashes. Some operating and filesystems do not get
2935 this right, so Perl automatically removes all trailing slashes to keep
2938 In order to recursively create a directory structure look at
2939 the C<mkpath> function of the L<File::Path> module.
2941 =item msgctl ID,CMD,ARG
2944 Calls the System V IPC function msgctl(2). You'll probably have to say
2948 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2949 then ARG must be a variable that will hold the returned C<msqid_ds>
2950 structure. Returns like C<ioctl>: the undefined value for error,
2951 C<"0 but true"> for zero, or the actual return value otherwise. See also
2952 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2954 =item msgget KEY,FLAGS
2957 Calls the System V IPC function msgget(2). Returns the message queue
2958 id, or the undefined value if there is an error. See also
2959 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2961 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2964 Calls the System V IPC function msgrcv to receive a message from
2965 message queue ID into variable VAR with a maximum message size of
2966 SIZE. Note that when a message is received, the message type as a
2967 native long integer will be the first thing in VAR, followed by the
2968 actual message. This packing may be opened with C<unpack("l! a*")>.
2969 Taints the variable. Returns true if successful, or false if there is
2970 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2971 C<IPC::SysV::Msg> documentation.
2973 =item msgsnd ID,MSG,FLAGS
2976 Calls the System V IPC function msgsnd to send the message MSG to the
2977 message queue ID. MSG must begin with the native long integer message
2978 type, and be followed by the length of the actual message, and finally
2979 the message itself. This kind of packing can be achieved with
2980 C<pack("l! a*", $type, $message)>. Returns true if successful,
2981 or false if there is an error. See also C<IPC::SysV>
2982 and C<IPC::SysV::Msg> documentation.
2989 =item my EXPR : ATTRS
2991 =item my TYPE EXPR : ATTRS
2993 A C<my> declares the listed variables to be local (lexically) to the
2994 enclosing block, file, or C<eval>. If more than one value is listed,
2995 the list must be placed in parentheses.
2997 The exact semantics and interface of TYPE and ATTRS are still
2998 evolving. TYPE is currently bound to the use of C<fields> pragma,
2999 and attributes are handled using the C<attributes> pragma, or starting
3000 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3001 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3002 L<attributes>, and L<Attribute::Handlers>.
3009 The C<next> command is like the C<continue> statement in C; it starts
3010 the next iteration of the loop:
3012 LINE: while (<STDIN>) {
3013 next LINE if /^#/; # discard comments
3017 Note that if there were a C<continue> block on the above, it would get
3018 executed even on discarded lines. If the LABEL is omitted, the command
3019 refers to the innermost enclosing loop.
3021 C<next> cannot be used to exit a block which returns a value such as
3022 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3023 a grep() or map() operation.
3025 Note that a block by itself is semantically identical to a loop
3026 that executes once. Thus C<next> will exit such a block early.
3028 See also L</continue> for an illustration of how C<last>, C<next>, and
3031 =item no Module VERSION LIST
3034 =item no Module VERSION
3036 =item no Module LIST
3042 See the C<use> function, of which C<no> is the opposite.
3045 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3049 Interprets EXPR as an octal string and returns the corresponding
3050 value. (If EXPR happens to start off with C<0x>, interprets it as a
3051 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3052 binary string. Leading whitespace is ignored in all three cases.)
3053 The following will handle decimal, binary, octal, and hex in the standard
3056 $val = oct($val) if $val =~ /^0/;
3058 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3059 in octal), use sprintf() or printf():
3061 $perms = (stat("filename"))[2] & 07777;
3062 $oct_perms = sprintf "%lo", $perms;
3064 The oct() function is commonly used when a string such as C<644> needs
3065 to be converted into a file mode, for example. (Although perl will
3066 automatically convert strings into numbers as needed, this automatic
3067 conversion assumes base 10.)
3069 =item open FILEHANDLE,EXPR
3070 X<open> X<pipe> X<file, open> X<fopen>
3072 =item open FILEHANDLE,MODE,EXPR
3074 =item open FILEHANDLE,MODE,EXPR,LIST
3076 =item open FILEHANDLE,MODE,REFERENCE
3078 =item open FILEHANDLE
3080 Opens the file whose filename is given by EXPR, and associates it with
3083 Simple examples to open a file for reading:
3085 open(my $fh, '<', "input.txt") or die $!;
3089 open(my $fh, '>', "output.txt") or die $!;
3091 (The following is a comprehensive reference to open(): for a gentler
3092 introduction you may consider L<perlopentut>.)
3094 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3095 the variable is assigned a reference to a new anonymous filehandle,
3096 otherwise if FILEHANDLE is an expression, its value is used as the name of
3097 the real filehandle wanted. (This is considered a symbolic reference, so
3098 C<use strict 'refs'> should I<not> be in effect.)
3100 If EXPR is omitted, the scalar variable of the same name as the
3101 FILEHANDLE contains the filename. (Note that lexical variables--those
3102 declared with C<my>--will not work for this purpose; so if you're
3103 using C<my>, specify EXPR in your call to open.)
3105 If three or more arguments are specified then the mode of opening and
3106 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3107 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3108 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3109 the file is opened for appending, again being created if necessary.
3111 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3112 indicate that you want both read and write access to the file; thus
3113 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3114 '+>' >> mode would clobber the file first. You can't usually use
3115 either read-write mode for updating textfiles, since they have
3116 variable length records. See the B<-i> switch in L<perlrun> for a
3117 better approach. The file is created with permissions of C<0666>
3118 modified by the process' C<umask> value.
3120 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3121 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3123 In the 2-arguments (and 1-argument) form of the call the mode and
3124 filename should be concatenated (in this order), possibly separated by
3125 spaces. It is possible to omit the mode in these forms if the mode is
3128 If the filename begins with C<'|'>, the filename is interpreted as a
3129 command to which output is to be piped, and if the filename ends with a
3130 C<'|'>, the filename is interpreted as a command which pipes output to
3131 us. See L<perlipc/"Using open() for IPC">
3132 for more examples of this. (You are not allowed to C<open> to a command
3133 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3134 and L<perlipc/"Bidirectional Communication with Another Process">
3137 For three or more arguments if MODE is C<'|-'>, the filename is
3138 interpreted as a command to which output is to be piped, and if MODE
3139 is C<'-|'>, the filename is interpreted as a command which pipes
3140 output to us. In the 2-arguments (and 1-argument) form one should
3141 replace dash (C<'-'>) with the command.
3142 See L<perlipc/"Using open() for IPC"> for more examples of this.
3143 (You are not allowed to C<open> to a command that pipes both in I<and>
3144 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3145 L<perlipc/"Bidirectional Communication"> for alternatives.)
3147 In the three-or-more argument form of pipe opens, if LIST is specified
3148 (extra arguments after the command name) then LIST becomes arguments
3149 to the command invoked if the platform supports it. The meaning of
3150 C<open> with more than three arguments for non-pipe modes is not yet
3151 specified. Experimental "layers" may give extra LIST arguments
3154 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3155 and opening C<< '>-' >> opens STDOUT.
3157 You may use the three-argument form of open to specify IO "layers"
3158 (sometimes also referred to as "disciplines") to be applied to the handle
3159 that affect how the input and output are processed (see L<open> and
3160 L<PerlIO> for more details). For example
3162 open(my $fh, "<:encoding(UTF-8)", "file")
3164 will open the UTF-8 encoded file containing Unicode characters,
3165 see L<perluniintro>. Note that if layers are specified in the
3166 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3167 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3169 Open returns nonzero upon success, the undefined value otherwise. If
3170 the C<open> involved a pipe, the return value happens to be the pid of
3173 If you're running Perl on a system that distinguishes between text
3174 files and binary files, then you should check out L</binmode> for tips
3175 for dealing with this. The key distinction between systems that need
3176 C<binmode> and those that don't is their text file formats. Systems
3177 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3178 character, and which encode that character in C as C<"\n">, do not
3179 need C<binmode>. The rest need it.
3181 When opening a file, it's usually a bad idea to continue normal execution
3182 if the request failed, so C<open> is frequently used in connection with
3183 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3184 where you want to make a nicely formatted error message (but there are
3185 modules that can help with that problem)) you should always check
3186 the return value from opening a file. The infrequent exception is when
3187 working with an unopened filehandle is actually what you want to do.
3189 As a special case the 3-arg form with a read/write mode and the third
3190 argument being C<undef>:
3192 open(my $tmp, "+>", undef) or die ...
3194 opens a filehandle to an anonymous temporary file. Also using "+<"
3195 works for symmetry, but you really should consider writing something
3196 to the temporary file first. You will need to seek() to do the
3199 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3200 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3201 "in memory" files held in Perl scalars via:
3203 open($fh, '>', \$variable) || ..
3205 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3206 file, you have to close it first:
3209 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3214 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3215 while (<ARTICLE>) {...
3217 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3218 # if the open fails, output is discarded
3220 open(my $dbase, '+<', 'dbase.mine') # open for update
3221 or die "Can't open 'dbase.mine' for update: $!";
3223 open(my $dbase, '+<dbase.mine') # ditto
3224 or die "Can't open 'dbase.mine' for update: $!";
3226 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3227 or die "Can't start caesar: $!";
3229 open(ARTICLE, "caesar <$article |") # ditto
3230 or die "Can't start caesar: $!";
3232 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3233 or die "Can't start sort: $!";
3236 open(MEMORY,'>', \$var)
3237 or die "Can't open memory file: $!";
3238 print MEMORY "foo!\n"; # output will end up in $var
3240 # process argument list of files along with any includes
3242 foreach $file (@ARGV) {
3243 process($file, 'fh00');
3247 my($filename, $input) = @_;
3248 $input++; # this is a string increment
3249 unless (open($input, $filename)) {
3250 print STDERR "Can't open $filename: $!\n";
3255 while (<$input>) { # note use of indirection
3256 if (/^#include "(.*)"/) {
3257 process($1, $input);
3264 See L<perliol> for detailed info on PerlIO.
3266 You may also, in the Bourne shell tradition, specify an EXPR beginning
3267 with C<< '>&' >>, in which case the rest of the string is interpreted
3268 as the name of a filehandle (or file descriptor, if numeric) to be
3269 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3270 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3271 The mode you specify should match the mode of the original filehandle.
3272 (Duping a filehandle does not take into account any existing contents
3273 of IO buffers.) If you use the 3-arg form then you can pass either a
3274 number, the name of a filehandle or the normal "reference to a glob".
3276 Here is a script that saves, redirects, and restores C<STDOUT> and
3277 C<STDERR> using various methods:
3280 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3281 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3283 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3284 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3286 select STDERR; $| = 1; # make unbuffered
3287 select STDOUT; $| = 1; # make unbuffered
3289 print STDOUT "stdout 1\n"; # this works for
3290 print STDERR "stderr 1\n"; # subprocesses too
3292 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3293 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3295 print STDOUT "stdout 2\n";
3296 print STDERR "stderr 2\n";
3298 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3299 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3300 that file descriptor (and not call L<dup(2)>); this is more
3301 parsimonious of file descriptors. For example:
3303 # open for input, reusing the fileno of $fd
3304 open(FILEHANDLE, "<&=$fd")
3308 open(FILEHANDLE, "<&=", $fd)
3312 # open for append, using the fileno of OLDFH
3313 open(FH, ">>&=", OLDFH)
3317 open(FH, ">>&=OLDFH")
3319 Being parsimonious on filehandles is also useful (besides being
3320 parsimonious) for example when something is dependent on file
3321 descriptors, like for example locking using flock(). If you do just
3322 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3323 descriptor as B, and therefore flock(A) will not flock(B), and vice
3324 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3325 the same file descriptor.
3327 Note that if you are using Perls older than 5.8.0, Perl will be using
3328 the standard C libraries' fdopen() to implement the "=" functionality.
3329 On many UNIX systems fdopen() fails when file descriptors exceed a
3330 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3331 most often the default.
3333 You can see whether Perl has been compiled with PerlIO or not by
3334 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3335 is C<define>, you have PerlIO, otherwise you don't.
3337 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3338 with 2-arguments (or 1-argument) form of open(), then
3339 there is an implicit fork done, and the return value of open is the pid
3340 of the child within the parent process, and C<0> within the child
3341 process. (Use C<defined($pid)> to determine whether the open was successful.)
3342 The filehandle behaves normally for the parent, but i/o to that
3343 filehandle is piped from/to the STDOUT/STDIN of the child process.
3344 In the child process the filehandle isn't opened--i/o happens from/to
3345 the new STDOUT or STDIN. Typically this is used like the normal
3346 piped open when you want to exercise more control over just how the
3347 pipe command gets executed, such as when you are running setuid, and
3348 don't want to have to scan shell commands for metacharacters.
3349 The following triples are more or less equivalent:
3351 open(FOO, "|tr '[a-z]' '[A-Z]'");
3352 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3353 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3354 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3356 open(FOO, "cat -n '$file'|");
3357 open(FOO, '-|', "cat -n '$file'");
3358 open(FOO, '-|') || exec 'cat', '-n', $file;
3359 open(FOO, '-|', "cat", '-n', $file);
3361 The last example in each block shows the pipe as "list form", which is
3362 not yet supported on all platforms. A good rule of thumb is that if
3363 your platform has true C<fork()> (in other words, if your platform is
3364 UNIX) you can use the list form.
3366 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3368 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3369 output before any operation that may do a fork, but this may not be
3370 supported on some platforms (see L<perlport>). To be safe, you may need
3371 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3372 of C<IO::Handle> on any open handles.
3374 On systems that support a close-on-exec flag on files, the flag will
3375 be set for the newly opened file descriptor as determined by the value
3376 of $^F. See L<perlvar/$^F>.
3378 Closing any piped filehandle causes the parent process to wait for the
3379 child to finish, and returns the status value in C<$?> and
3380 C<${^CHILD_ERROR_NATIVE}>.
3382 The filename passed to 2-argument (or 1-argument) form of open() will
3383 have leading and trailing whitespace deleted, and the normal
3384 redirection characters honored. This property, known as "magic open",
3385 can often be used to good effect. A user could specify a filename of
3386 F<"rsh cat file |">, or you could change certain filenames as needed:
3388 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3389 open(FH, $filename) or die "Can't open $filename: $!";
3391 Use 3-argument form to open a file with arbitrary weird characters in it,
3393 open(FOO, '<', $file);
3395 otherwise it's necessary to protect any leading and trailing whitespace:
3397 $file =~ s#^(\s)#./$1#;
3398 open(FOO, "< $file\0");
3400 (this may not work on some bizarre filesystems). One should
3401 conscientiously choose between the I<magic> and 3-arguments form
3406 will allow the user to specify an argument of the form C<"rsh cat file |">,
3407 but will not work on a filename which happens to have a trailing space, while
3409 open IN, '<', $ARGV[0];
3411 will have exactly the opposite restrictions.
3413 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3414 should use the C<sysopen> function, which involves no such magic (but
3415 may use subtly different filemodes than Perl open(), which is mapped
3416 to C fopen()). This is
3417 another way to protect your filenames from interpretation. For example:
3420 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3421 or die "sysopen $path: $!";
3422 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3423 print HANDLE "stuff $$\n";
3425 print "File contains: ", <HANDLE>;
3427 Using the constructor from the C<IO::Handle> package (or one of its
3428 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3429 filehandles that have the scope of whatever variables hold references to
3430 them, and automatically close whenever and however you leave that scope:
3434 sub read_myfile_munged {
3436 my $handle = IO::File->new;
3437 open($handle, "myfile") or die "myfile: $!";
3439 or return (); # Automatically closed here.
3440 mung $first or die "mung failed"; # Or here.
3441 return $first, <$handle> if $ALL; # Or here.
3445 See L</seek> for some details about mixing reading and writing.
3447 =item opendir DIRHANDLE,EXPR
3450 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3451 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3452 DIRHANDLE may be an expression whose value can be used as an indirect
3453 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3454 scalar variable (or array or hash element), the variable is assigned a
3455 reference to a new anonymous dirhandle.
3456 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3458 See example at C<readdir>.
3465 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3466 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3469 For the reverse, see L</chr>.
3470 See L<perlunicode> for more about Unicode.
3477 =item our EXPR : ATTRS
3479 =item our TYPE EXPR : ATTRS
3481 C<our> associates a simple name with a package variable in the current
3482 package for use within the current scope. When C<use strict 'vars'> is in
3483 effect, C<our> lets you use declared global variables without qualifying
3484 them with package names, within the lexical scope of the C<our> declaration.
3485 In this way C<our> differs from C<use vars>, which is package scoped.
3487 Unlike C<my>, which both allocates storage for a variable and associates
3488 a simple name with that storage for use within the current scope, C<our>
3489 associates a simple name with a package variable in the current package,
3490 for use within the current scope. In other words, C<our> has the same
3491 scoping rules as C<my>, but does not necessarily create a
3494 If more than one value is listed, the list must be placed
3500 An C<our> declaration declares a global variable that will be visible
3501 across its entire lexical scope, even across package boundaries. The
3502 package in which the variable is entered is determined at the point
3503 of the declaration, not at the point of use. This means the following
3507 our $bar; # declares $Foo::bar for rest of lexical scope
3511 print $bar; # prints 20, as it refers to $Foo::bar
3513 Multiple C<our> declarations with the same name in the same lexical
3514 scope are allowed if they are in different packages. If they happen
3515 to be in the same package, Perl will emit warnings if you have asked
3516 for them, just like multiple C<my> declarations. Unlike a second
3517 C<my> declaration, which will bind the name to a fresh variable, a
3518 second C<our> declaration in the same package, in the same scope, is
3523 our $bar; # declares $Foo::bar for rest of lexical scope
3527 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3528 print $bar; # prints 30
3530 our $bar; # emits warning but has no other effect
3531 print $bar; # still prints 30
3533 An C<our> declaration may also have a list of attributes associated
3536 The exact semantics and interface of TYPE and ATTRS are still
3537 evolving. TYPE is currently bound to the use of C<fields> pragma,
3538 and attributes are handled using the C<attributes> pragma, or starting
3539 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3540 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3541 L<attributes>, and L<Attribute::Handlers>.
3543 =item pack TEMPLATE,LIST
3546 Takes a LIST of values and converts it into a string using the rules
3547 given by the TEMPLATE. The resulting string is the concatenation of
3548 the converted values. Typically, each converted value looks
3549 like its machine-level representation. For example, on 32-bit machines
3550 an integer may be represented by a sequence of 4 bytes that will be
3551 converted to a sequence of 4 characters.
3553 The TEMPLATE is a sequence of characters that give the order and type
3554 of values, as follows:
3556 a A string with arbitrary binary data, will be null padded.
3557 A A text (ASCII) string, will be space padded.
3558 Z A null terminated (ASCIZ) string, will be null padded.
3560 b A bit string (ascending bit order inside each byte, like vec()).
3561 B A bit string (descending bit order inside each byte).
3562 h A hex string (low nybble first).
3563 H A hex string (high nybble first).
3565 c A signed char (8-bit) value.
3566 C An unsigned char (octet) value.
3567 W An unsigned char value (can be greater than 255).
3569 s A signed short (16-bit) value.
3570 S An unsigned short value.
3572 l A signed long (32-bit) value.
3573 L An unsigned long value.
3575 q A signed quad (64-bit) value.
3576 Q An unsigned quad value.
3577 (Quads are available only if your system supports 64-bit
3578 integer values _and_ if Perl has been compiled to support those.
3579 Causes a fatal error otherwise.)
3581 i A signed integer value.
3582 I A unsigned integer value.
3583 (This 'integer' is _at_least_ 32 bits wide. Its exact
3584 size depends on what a local C compiler calls 'int'.)
3586 n An unsigned short (16-bit) in "network" (big-endian) order.
3587 N An unsigned long (32-bit) in "network" (big-endian) order.
3588 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3589 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3591 j A Perl internal signed integer value (IV).
3592 J A Perl internal unsigned integer value (UV).
3594 f A single-precision float in the native format.
3595 d A double-precision float in the native format.
3597 F A Perl internal floating point value (NV) in the native format
3598 D A long double-precision float in the native format.
3599 (Long doubles are available only if your system supports long
3600 double values _and_ if Perl has been compiled to support those.
3601 Causes a fatal error otherwise.)
3603 p A pointer to a null-terminated string.
3604 P A pointer to a structure (fixed-length string).
3606 u A uuencoded string.
3607 U A Unicode character number. Encodes to a character in character mode
3608 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3610 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3611 details). Its bytes represent an unsigned integer in base 128,
3612 most significant digit first, with as few digits as possible. Bit
3613 eight (the high bit) is set on each byte except the last.
3617 @ Null fill or truncate to absolute position, counted from the
3618 start of the innermost ()-group.
3619 . Null fill or truncate to absolute position specified by value.
3620 ( Start of a ()-group.
3622 One or more of the modifiers below may optionally follow some letters in the
3623 TEMPLATE (the second column lists the letters for which the modifier is
3626 ! sSlLiI Forces native (short, long, int) sizes instead
3627 of fixed (16-/32-bit) sizes.
3629 xX Make x and X act as alignment commands.
3631 nNvV Treat integers as signed instead of unsigned.
3633 @. Specify position as byte offset in the internal
3634 representation of the packed string. Efficient but
3637 > sSiIlLqQ Force big-endian byte-order on the type.
3638 jJfFdDpP (The "big end" touches the construct.)
3640 < sSiIlLqQ Force little-endian byte-order on the type.
3641 jJfFdDpP (The "little end" touches the construct.)
3643 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3644 in which case they force a certain byte-order on all components of
3645 that group, including subgroups.
3647 The following rules apply:
3653 Each letter may optionally be followed by a number giving a repeat
3654 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3655 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3656 that many values from the LIST. A C<*> for the repeat count means to
3657 use however many items are left, except for C<@>, C<x>, C<X>, where it
3658 is equivalent to C<0>, for <.> where it means relative to string start
3659 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3660 A numeric repeat count may optionally be enclosed in brackets, as in
3661 C<pack 'C[80]', @arr>.
3663 One can replace the numeric repeat count by a template enclosed in brackets;
3664 then the packed length of this template in bytes is used as a count.
3665 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3666 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3667 If the template in brackets contains alignment commands (such as C<x![d]>),
3668 its packed length is calculated as if the start of the template has the maximal
3671 When used with C<Z>, C<*> results in the addition of a trailing null
3672 byte (so the packed result will be one longer than the byte C<length>
3675 When used with C<@>, the repeat count represents an offset from the start
3676 of the innermost () group.
3678 When used with C<.>, the repeat count is used to determine the starting
3679 position from where the value offset is calculated. If the repeat count
3680 is 0, it's relative to the current position. If the repeat count is C<*>,
3681 the offset is relative to the start of the packed string. And if its an
3682 integer C<n> the offset is relative to the start of the n-th innermost
3683 () group (or the start of the string if C<n> is bigger then the group
3686 The repeat count for C<u> is interpreted as the maximal number of bytes
3687 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3688 count should not be more than 65.
3692 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3693 string of length count, padding with nulls or spaces as necessary. When
3694 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3695 after the first null, and C<a> returns data verbatim.
3697 If the value-to-pack is too long, it is truncated. If too long and an
3698 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3699 by a null byte. Thus C<Z> always packs a trailing null (except when the
3704 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3705 Each character of the input field of pack() generates 1 bit of the result.
3706 Each result bit is based on the least-significant bit of the corresponding
3707 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3708 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3710 Starting from the beginning of the input string of pack(), each 8-tuple
3711 of characters is converted to 1 character of output. With format C<b>
3712 the first character of the 8-tuple determines the least-significant bit of a
3713 character, and with format C<B> it determines the most-significant bit of
3716 If the length of the input string is not exactly divisible by 8, the
3717 remainder is packed as if the input string were padded by null characters
3718 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3720 If the input string of pack() is longer than needed, extra characters are
3721 ignored. A C<*> for the repeat count of pack() means to use all the
3722 characters of the input field. On unpack()ing the bits are converted to a
3723 string of C<"0">s and C<"1">s.
3727 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3728 representable as hexadecimal digits, 0-9a-f) long.
3730 Each character of the input field of pack() generates 4 bits of the result.
3731 For non-alphabetical characters the result is based on the 4 least-significant
3732 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3733 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3734 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3735 is compatible with the usual hexadecimal digits, so that C<"a"> and
3736 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3737 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3739 Starting from the beginning of the input string of pack(), each pair
3740 of characters is converted to 1 character of output. With format C<h> the
3741 first character of the pair determines the least-significant nybble of the
3742 output character, and with format C<H> it determines the most-significant
3745 If the length of the input string is not even, it behaves as if padded
3746 by a null character at the end. Similarly, during unpack()ing the "extra"
3747 nybbles are ignored.
3749 If the input string of pack() is longer than needed, extra characters are
3751 A C<*> for the repeat count of pack() means to use all the characters of
3752 the input field. On unpack()ing the nybbles are converted to a string
3753 of hexadecimal digits.
3757 The C<p> type packs a pointer to a null-terminated string. You are
3758 responsible for ensuring the string is not a temporary value (which can
3759 potentially get deallocated before you get around to using the packed result).
3760 The C<P> type packs a pointer to a structure of the size indicated by the
3761 length. A NULL pointer is created if the corresponding value for C<p> or
3762 C<P> is C<undef>, similarly for unpack().
3764 If your system has a strange pointer size (i.e. a pointer is neither as
3765 big as an int nor as big as a long), it may not be possible to pack or
3766 unpack pointers in big- or little-endian byte order. Attempting to do
3767 so will result in a fatal error.
3771 The C</> template character allows packing and unpacking of a sequence of
3772 items where the packed structure contains a packed item count followed by
3773 the packed items themselves.
3775 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3776 I<length-item> describes how the length value is packed. The ones likely
3777 to be of most use are integer-packing ones like C<n> (for Java strings),
3778 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3780 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3781 the minimum of that and the number of available items is used as argument
3782 for the I<length-item>. If it has no repeat count or uses a '*', the number
3783 of available items is used.
3785 For C<unpack> an internal stack of integer arguments unpacked so far is
3786 used. You write C</>I<sequence-item> and the repeat count is obtained by
3787 popping off the last element from the stack. The I<sequence-item> must not
3788 have a repeat count.
3790 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3791 the I<length-item> is a string length, not a number of strings. If there is
3792 an explicit repeat count for pack, the packed string will be adjusted to that
3795 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3796 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3797 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3798 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3799 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3801 The I<length-item> is not returned explicitly from C<unpack>.
3803 Adding a count to the I<length-item> letter is unlikely to do anything
3804 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3805 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3806 which Perl does not regard as legal in numeric strings.
3810 The integer types C<s>, C<S>, C<l>, and C<L> may be
3811 followed by a C<!> modifier to signify native shorts or
3812 longs--as you can see from above for example a bare C<l> does mean
3813 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3814 may be larger. This is an issue mainly in 64-bit platforms. You can
3815 see whether using C<!> makes any difference by
3817 print length(pack("s")), " ", length(pack("s!")), "\n";
3818 print length(pack("l")), " ", length(pack("l!")), "\n";
3820 C<i!> and C<I!> also work but only because of completeness;
3821 they are identical to C<i> and C<I>.
3823 The actual sizes (in bytes) of native shorts, ints, longs, and long
3824 longs on the platform where Perl was built are also available via
3828 print $Config{shortsize}, "\n";
3829 print $Config{intsize}, "\n";
3830 print $Config{longsize}, "\n";
3831 print $Config{longlongsize}, "\n";
3833 (The C<$Config{longlongsize}> will be undefined if your system does
3834 not support long longs.)
3838 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3839 are inherently non-portable between processors and operating systems
3840 because they obey the native byteorder and endianness. For example a
3841 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3842 (arranged in and handled by the CPU registers) into bytes as
3844 0x12 0x34 0x56 0x78 # big-endian
3845 0x78 0x56 0x34 0x12 # little-endian
3847 Basically, the Intel and VAX CPUs are little-endian, while everybody
3848 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3849 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3850 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3853 The names `big-endian' and `little-endian' are comic references to
3854 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3855 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3856 the egg-eating habits of the Lilliputians.
3858 Some systems may have even weirder byte orders such as
3863 You can see your system's preference with
3865 print join(" ", map { sprintf "%#02x", $_ }
3866 unpack("W*",pack("L",0x12345678))), "\n";
3868 The byteorder on the platform where Perl was built is also available
3872 print $Config{byteorder}, "\n";
3874 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3875 and C<'87654321'> are big-endian.
3877 If you want portable packed integers you can either use the formats
3878 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3879 modifiers. These modifiers are only available as of perl 5.9.2.
3880 See also L<perlport>.
3884 All integer and floating point formats as well as C<p> and C<P> and
3885 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3886 to force big- or little- endian byte-order, respectively.
3887 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3888 signed integers, 64-bit integers and floating point values. However,
3889 there are some things to keep in mind.
3891 Exchanging signed integers between different platforms only works
3892 if all platforms store them in the same format. Most platforms store
3893 signed integers in two's complement, so usually this is not an issue.
3895 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3896 formats on big- or little-endian machines. Otherwise, attempting to
3897 do so will result in a fatal error.
3899 Forcing big- or little-endian byte-order on floating point values for
3900 data exchange can only work if all platforms are using the same
3901 binary representation (e.g. IEEE floating point format). Even if all
3902 platforms are using IEEE, there may be subtle differences. Being able
3903 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3904 but also very dangerous if you don't know exactly what you're doing.
3905 It is definitely not a general way to portably store floating point
3908 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3909 all types inside the group that accept the byte-order modifiers,
3910 including all subgroups. It will silently be ignored for all other
3911 types. You are not allowed to override the byte-order within a group
3912 that already has a byte-order modifier suffix.
3916 Real numbers (floats and doubles) are in the native machine format only;
3917 due to the multiplicity of floating formats around, and the lack of a
3918 standard "network" representation, no facility for interchange has been
3919 made. This means that packed floating point data written on one machine
3920 may not be readable on another - even if both use IEEE floating point
3921 arithmetic (as the endian-ness of the memory representation is not part
3922 of the IEEE spec). See also L<perlport>.
3924 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3925 modifiers to force big- or little-endian byte-order on floating point values.
3927 Note that Perl uses doubles (or long doubles, if configured) internally for
3928 all numeric calculation, and converting from double into float and thence back
3929 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3930 will not in general equal $foo).
3934 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3935 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3936 where the packed string is processed in its UTF-8-encoded Unicode form on
3937 a byte by byte basis. Character mode is the default unless the format string
3938 starts with an C<U>. You can switch mode at any moment with an explicit
3939 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3940 or until the end of the ()-group in which it was entered.
3944 You must yourself do any alignment or padding by inserting for example
3945 enough C<'x'>es while packing. There is no way to pack() and unpack()
3946 could know where the characters are going to or coming from. Therefore
3947 C<pack> (and C<unpack>) handle their output and input as flat
3948 sequences of characters.
3952 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3953 take a repeat count, both as postfix, and for unpack() also via the C</>
3954 template character. Within each repetition of a group, positioning with
3955 C<@> starts again at 0. Therefore, the result of
3957 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3959 is the string "\0a\0\0bc".
3963 C<x> and C<X> accept C<!> modifier. In this case they act as
3964 alignment commands: they jump forward/back to the closest position
3965 aligned at a multiple of C<count> characters. For example, to pack() or
3966 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3967 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3968 aligned on the double's size.
3970 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3971 both result in no-ops.
3975 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3976 will represent signed 16-/32-bit integers in big-/little-endian order.
3977 This is only portable if all platforms sharing the packed data use the
3978 same binary representation for signed integers (e.g. all platforms are
3979 using two's complement representation).
3983 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3984 White space may be used to separate pack codes from each other, but
3985 modifiers and a repeat count must follow immediately.
3989 If TEMPLATE requires more arguments to pack() than actually given, pack()
3990 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3991 to pack() than actually given, extra arguments are ignored.
3997 $foo = pack("WWWW",65,66,67,68);
3999 $foo = pack("W4",65,66,67,68);
4001 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4002 # same thing with Unicode circled letters.
4003 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4004 # same thing with Unicode circled letters. You don't get the UTF-8
4005 # bytes because the U at the start of the format caused a switch to
4006 # U0-mode, so the UTF-8 bytes get joined into characters
4007 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4008 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4009 # This is the UTF-8 encoding of the string in the previous example
4011 $foo = pack("ccxxcc",65,66,67,68);
4014 # note: the above examples featuring "W" and "c" are true
4015 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4016 # and UTF-8. In EBCDIC the first example would be
4017 # $foo = pack("WWWW",193,194,195,196);
4019 $foo = pack("s2",1,2);
4020 # "\1\0\2\0" on little-endian
4021 # "\0\1\0\2" on big-endian
4023 $foo = pack("a4","abcd","x","y","z");
4026 $foo = pack("aaaa","abcd","x","y","z");
4029 $foo = pack("a14","abcdefg");
4030 # "abcdefg\0\0\0\0\0\0\0"
4032 $foo = pack("i9pl", gmtime);
4033 # a real struct tm (on my system anyway)
4035 $utmp_template = "Z8 Z8 Z16 L";
4036 $utmp = pack($utmp_template, @utmp1);
4037 # a struct utmp (BSDish)
4039 @utmp2 = unpack($utmp_template, $utmp);
4040 # "@utmp1" eq "@utmp2"
4043 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4046 $foo = pack('sx2l', 12, 34);
4047 # short 12, two zero bytes padding, long 34
4048 $bar = pack('s@4l', 12, 34);
4049 # short 12, zero fill to position 4, long 34
4051 $baz = pack('s.l', 12, 4, 34);
4052 # short 12, zero fill to position 4, long 34
4054 $foo = pack('nN', 42, 4711);
4055 # pack big-endian 16- and 32-bit unsigned integers
4056 $foo = pack('S>L>', 42, 4711);
4058 $foo = pack('s<l<', -42, 4711);
4059 # pack little-endian 16- and 32-bit signed integers
4060 $foo = pack('(sl)<', -42, 4711);
4063 The same template may generally also be used in unpack().
4065 =item package NAMESPACE
4066 X<package> X<module> X<namespace>
4070 Declares the compilation unit as being in the given namespace. The scope
4071 of the package declaration is from the declaration itself through the end
4072 of the enclosing block, file, or eval (the same as the C<my> operator).
4073 All further unqualified dynamic identifiers will be in this namespace.
4074 A package statement affects only dynamic variables--including those
4075 you've used C<local> on--but I<not> lexical variables, which are created
4076 with C<my>. Typically it would be the first declaration in a file to
4077 be included by the C<require> or C<use> operator. You can switch into a
4078 package in more than one place; it merely influences which symbol table
4079 is used by the compiler for the rest of that block. You can refer to
4080 variables and filehandles in other packages by prefixing the identifier
4081 with the package name and a double colon: C<$Package::Variable>.
4082 If the package name is null, the C<main> package as assumed. That is,
4083 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4084 still seen in older code).
4086 See L<perlmod/"Packages"> for more information about packages, modules,
4087 and classes. See L<perlsub> for other scoping issues.
4089 =item pipe READHANDLE,WRITEHANDLE
4092 Opens a pair of connected pipes like the corresponding system call.
4093 Note that if you set up a loop of piped processes, deadlock can occur
4094 unless you are very careful. In addition, note that Perl's pipes use
4095 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4096 after each command, depending on the application.
4098 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4099 for examples of such things.
4101 On systems that support a close-on-exec flag on files, the flag will be set
4102 for the newly opened file descriptors as determined by the value of $^F.
4110 Pops and returns the last value of the array, shortening the array by
4113 If there are no elements in the array, returns the undefined value
4114 (although this may happen at other times as well). If ARRAY is
4115 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4116 array in subroutines, just like C<shift>.
4119 X<pos> X<match, position>
4123 Returns the offset of where the last C<m//g> search left off for the variable
4124 in question (C<$_> is used when the variable is not specified). Note that
4125 0 is a valid match offset. C<undef> indicates that the search position
4126 is reset (usually due to match failure, but can also be because no match has
4127 yet been performed on the scalar). C<pos> directly accesses the location used
4128 by the regexp engine to store the offset, so assigning to C<pos> will change
4129 that offset, and so will also influence the C<\G> zero-width assertion in
4130 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4131 the return from C<pos> won't change either in this case. See L<perlre> and
4134 =item print FILEHANDLE LIST
4141 Prints a string or a list of strings. Returns true if successful.
4142 FILEHANDLE may be a scalar variable name, in which case the variable
4143 contains the name of or a reference to the filehandle, thus introducing
4144 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4145 the next token is a term, it may be misinterpreted as an operator
4146 unless you interpose a C<+> or put parentheses around the arguments.)
4147 If FILEHANDLE is omitted, prints by default to standard output (or
4148 to the last selected output channel--see L</select>). If LIST is
4149 also omitted, prints C<$_> to the currently selected output channel.
4150 To set the default output channel to something other than STDOUT
4151 use the select operation. The current value of C<$,> (if any) is
4152 printed between each LIST item. The current value of C<$\> (if
4153 any) is printed after the entire LIST has been printed. Because
4154 print takes a LIST, anything in the LIST is evaluated in list
4155 context, and any subroutine that you call will have one or more of
4156 its expressions evaluated in list context. Also be careful not to
4157 follow the print keyword with a left parenthesis unless you want
4158 the corresponding right parenthesis to terminate the arguments to
4159 the print--interpose a C<+> or put parentheses around all the
4162 Note that if you're storing FILEHANDLEs in an array, or if you're using
4163 any other expression more complex than a scalar variable to retrieve it,
4164 you will have to use a block returning the filehandle value instead:
4166 print { $files[$i] } "stuff\n";
4167 print { $OK ? STDOUT : STDERR } "stuff\n";
4169 =item printf FILEHANDLE FORMAT, LIST
4172 =item printf FORMAT, LIST
4174 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4175 (the output record separator) is not appended. The first argument
4176 of the list will be interpreted as the C<printf> format. See C<sprintf>
4177 for an explanation of the format argument. If C<use locale> is in effect,
4178 and POSIX::setlocale() has been called, the character used for the decimal
4179 separator in formatted floating point numbers is affected by the LC_NUMERIC
4180 locale. See L<perllocale> and L<POSIX>.
4182 Don't fall into the trap of using a C<printf> when a simple
4183 C<print> would do. The C<print> is more efficient and less
4186 =item prototype FUNCTION
4189 Returns the prototype of a function as a string (or C<undef> if the
4190 function has no prototype). FUNCTION is a reference to, or the name of,
4191 the function whose prototype you want to retrieve.
4193 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4194 name for Perl builtin. If the builtin is not I<overridable> (such as
4195 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4196 (such as C<system>), prototype() returns C<undef>, because the builtin
4197 does not really behave like a Perl function. Otherwise, the string
4198 describing the equivalent prototype is returned.
4200 =item push ARRAY,LIST
4203 Treats ARRAY as a stack, and pushes the values of LIST
4204 onto the end of ARRAY. The length of ARRAY increases by the length of
4205 LIST. Has the same effect as
4208 $ARRAY[++$#ARRAY] = $value;
4211 but is more efficient. Returns the number of elements in the array following
4212 the completed C<push>.
4222 Generalized quotes. See L<perlop/"Quote-Like Operators">.
4226 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
4228 =item quotemeta EXPR
4229 X<quotemeta> X<metacharacter>
4233 Returns the value of EXPR with all non-"word"
4234 characters backslashed. (That is, all characters not matching
4235 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4236 returned string, regardless of any locale settings.)
4237 This is the internal function implementing
4238 the C<\Q> escape in double-quoted strings.
4240 If EXPR is omitted, uses C<$_>.
4247 Returns a random fractional number greater than or equal to C<0> and less
4248 than the value of EXPR. (EXPR should be positive.) If EXPR is
4249 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4250 also special-cased as C<1> - this has not been documented before perl 5.8.0
4251 and is subject to change in future versions of perl. Automatically calls
4252 C<srand> unless C<srand> has already been called. See also C<srand>.
4254 Apply C<int()> to the value returned by C<rand()> if you want random
4255 integers instead of random fractional numbers. For example,
4259 returns a random integer between C<0> and C<9>, inclusive.
4261 (Note: If your rand function consistently returns numbers that are too
4262 large or too small, then your version of Perl was probably compiled
4263 with the wrong number of RANDBITS.)
4265 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4266 X<read> X<file, read>
4268 =item read FILEHANDLE,SCALAR,LENGTH
4270 Attempts to read LENGTH I<characters> of data into variable SCALAR
4271 from the specified FILEHANDLE. Returns the number of characters
4272 actually read, C<0> at end of file, or undef if there was an error (in
4273 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4274 so that the last character actually read is the last character of the
4275 scalar after the read.
4277 An OFFSET may be specified to place the read data at some place in the
4278 string other than the beginning. A negative OFFSET specifies
4279 placement at that many characters counting backwards from the end of
4280 the string. A positive OFFSET greater than the length of SCALAR
4281 results in the string being padded to the required size with C<"\0">
4282 bytes before the result of the read is appended.
4284 The call is actually implemented in terms of either Perl's or system's
4285 fread() call. To get a true read(2) system call, see C<sysread>.
4287 Note the I<characters>: depending on the status of the filehandle,
4288 either (8-bit) bytes or characters are read. By default all
4289 filehandles operate on bytes, but for example if the filehandle has
4290 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4291 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4292 characters, not bytes. Similarly for the C<:encoding> pragma:
4293 in that case pretty much any characters can be read.
4295 =item readdir DIRHANDLE
4298 Returns the next directory entry for a directory opened by C<opendir>.
4299 If used in list context, returns all the rest of the entries in the
4300 directory. If there are no more entries, returns an undefined value in
4301 scalar context or a null list in list context.
4303 If you're planning to filetest the return values out of a C<readdir>, you'd
4304 better prepend the directory in question. Otherwise, because we didn't
4305 C<chdir> there, it would have been testing the wrong file.
4307 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4308 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4314 X<readline> X<gets> X<fgets>
4316 Reads from the filehandle whose typeglob is contained in EXPR (or from
4317 *ARGV if EXPR is not provided). In scalar context, each call reads and
4318 returns the next line, until end-of-file is reached, whereupon the
4319 subsequent call returns undef. In list context, reads until end-of-file
4320 is reached and returns a list of lines. Note that the notion of "line"
4321 used here is however you may have defined it with C<$/> or
4322 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4324 When C<$/> is set to C<undef>, when readline() is in scalar
4325 context (i.e. file slurp mode), and when an empty file is read, it
4326 returns C<''> the first time, followed by C<undef> subsequently.
4328 This is the internal function implementing the C<< <EXPR> >>
4329 operator, but you can use it directly. The C<< <EXPR> >>
4330 operator is discussed in more detail in L<perlop/"I/O Operators">.
4333 $line = readline(*STDIN); # same thing
4335 If readline encounters an operating system error, C<$!> will be set with the
4336 corresponding error message. It can be helpful to check C<$!> when you are
4337 reading from filehandles you don't trust, such as a tty or a socket. The
4338 following example uses the operator form of C<readline>, and takes the necessary
4339 steps to ensure that C<readline> was successful.
4343 unless (defined( $line = <> )) {
4355 Returns the value of a symbolic link, if symbolic links are
4356 implemented. If not, gives a fatal error. If there is some system
4357 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4358 omitted, uses C<$_>.
4365 EXPR is executed as a system command.
4366 The collected standard output of the command is returned.
4367 In scalar context, it comes back as a single (potentially
4368 multi-line) string. In list context, returns a list of lines
4369 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4370 This is the internal function implementing the C<qx/EXPR/>
4371 operator, but you can use it directly. The C<qx/EXPR/>
4372 operator is discussed in more detail in L<perlop/"I/O Operators">.
4373 If EXPR is omitted, uses C<$_>.
4375 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4378 Receives a message on a socket. Attempts to receive LENGTH characters
4379 of data into variable SCALAR from the specified SOCKET filehandle.
4380 SCALAR will be grown or shrunk to the length actually read. Takes the
4381 same flags as the system call of the same name. Returns the address
4382 of the sender if SOCKET's protocol supports this; returns an empty
4383 string otherwise. If there's an error, returns the undefined value.
4384 This call is actually implemented in terms of recvfrom(2) system call.
4385 See L<perlipc/"UDP: Message Passing"> for examples.
4387 Note the I<characters>: depending on the status of the socket, either
4388 (8-bit) bytes or characters are received. By default all sockets
4389 operate on bytes, but for example if the socket has been changed using
4390 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4391 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4392 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4393 case pretty much any characters can be read.
4400 The C<redo> command restarts the loop block without evaluating the
4401 conditional again. The C<continue> block, if any, is not executed. If
4402 the LABEL is omitted, the command refers to the innermost enclosing
4403 loop. Programs that want to lie to themselves about what was just input
4404 normally use this command:
4406 # a simpleminded Pascal comment stripper
4407 # (warning: assumes no { or } in strings)
4408 LINE: while (<STDIN>) {
4409 while (s|({.*}.*){.*}|$1 |) {}
4414 if (/}/) { # end of comment?
4423 C<redo> cannot be used to retry a block which returns a value such as
4424 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4425 a grep() or map() operation.
4427 Note that a block by itself is semantically identical to a loop
4428 that executes once. Thus C<redo> inside such a block will effectively
4429 turn it into a looping construct.
4431 See also L</continue> for an illustration of how C<last>, C<next>, and
4439 Returns a non-empty string if EXPR is a reference, the empty
4440 string otherwise. If EXPR
4441 is not specified, C<$_> will be used. The value returned depends on the
4442 type of thing the reference is a reference to.
4443 Builtin types include:
4457 If the referenced object has been blessed into a package, then that package
4458 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4460 if (ref($r) eq "HASH") {
4461 print "r is a reference to a hash.\n";
4464 print "r is not a reference at all.\n";
4467 The return value C<LVALUE> indicates a reference to an lvalue that is not
4468 a variable. You get this from taking the reference of function calls like
4469 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4470 to a L<version string|perldata/"Version Strings">.
4472 The result C<Regexp> indicates that the argument is a regular expression
4473 resulting from C<qr//>.
4475 See also L<perlref>.
4477 =item rename OLDNAME,NEWNAME
4478 X<rename> X<move> X<mv> X<ren>
4480 Changes the name of a file; an existing file NEWNAME will be
4481 clobbered. Returns true for success, false otherwise.
4483 Behavior of this function varies wildly depending on your system
4484 implementation. For example, it will usually not work across file system
4485 boundaries, even though the system I<mv> command sometimes compensates
4486 for this. Other restrictions include whether it works on directories,
4487 open files, or pre-existing files. Check L<perlport> and either the
4488 rename(2) manpage or equivalent system documentation for details.
4490 For a platform independent C<move> function look at the L<File::Copy>
4493 =item require VERSION
4500 Demands a version of Perl specified by VERSION, or demands some semantics
4501 specified by EXPR or by C<$_> if EXPR is not supplied.
4503 VERSION may be either a numeric argument such as 5.006, which will be
4504 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4505 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4506 VERSION is greater than the version of the current Perl interpreter.
4507 Compare with L</use>, which can do a similar check at compile time.
4509 Specifying VERSION as a literal of the form v5.6.1 should generally be
4510 avoided, because it leads to misleading error messages under earlier
4511 versions of Perl that do not support this syntax. The equivalent numeric
4512 version should be used instead.
4514 require v5.6.1; # run time version check
4515 require 5.6.1; # ditto
4516 require 5.006_001; # ditto; preferred for backwards compatibility
4518 Otherwise, C<require> demands that a library file be included if it
4519 hasn't already been included. The file is included via the do-FILE
4520 mechanism, which is essentially just a variety of C<eval> with the
4521 caveat that lexical variables in the invoking script will be invisible
4522 to the included code. Has semantics similar to the following subroutine:
4525 my ($filename) = @_;
4526 if (exists $INC{$filename}) {
4527 return 1 if $INC{$filename};
4528 die "Compilation failed in require";
4530 my ($realfilename,$result);
4532 foreach $prefix (@INC) {
4533 $realfilename = "$prefix/$filename";
4534 if (-f $realfilename) {
4535 $INC{$filename} = $realfilename;
4536 $result = do $realfilename;
4540 die "Can't find $filename in \@INC";
4543 $INC{$filename} = undef;
4545 } elsif (!$result) {
4546 delete $INC{$filename};
4547 die "$filename did not return true value";
4553 Note that the file will not be included twice under the same specified
4556 The file must return true as the last statement to indicate
4557 successful execution of any initialization code, so it's customary to
4558 end such a file with C<1;> unless you're sure it'll return true
4559 otherwise. But it's better just to put the C<1;>, in case you add more
4562 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4563 replaces "F<::>" with "F</>" in the filename for you,
4564 to make it easy to load standard modules. This form of loading of
4565 modules does not risk altering your namespace.
4567 In other words, if you try this:
4569 require Foo::Bar; # a splendid bareword
4571 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4572 directories specified in the C<@INC> array.
4574 But if you try this:
4576 $class = 'Foo::Bar';
4577 require $class; # $class is not a bareword
4579 require "Foo::Bar"; # not a bareword because of the ""
4581 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4582 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4584 eval "require $class";
4586 Now that you understand how C<require> looks for files in the case of a
4587 bareword argument, there is a little extra functionality going on behind
4588 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4589 first look for a similar filename with a "F<.pmc>" extension. If this file
4590 is found, it will be loaded in place of any file ending in a "F<.pm>"
4593 You can also insert hooks into the import facility, by putting directly
4594 Perl code into the @INC array. There are three forms of hooks: subroutine
4595 references, array references and blessed objects.
4597 Subroutine references are the simplest case. When the inclusion system
4598 walks through @INC and encounters a subroutine, this subroutine gets
4599 called with two parameters, the first being a reference to itself, and the
4600 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4601 subroutine should return nothing, or a list of up to three values in the
4608 A filehandle, from which the file will be read.
4612 A reference to a subroutine. If there is no filehandle (previous item),
4613 then this subroutine is expected to generate one line of source code per
4614 call, writing the line into C<$_> and returning 1, then returning 0 at
4615 "end of file". If there is a filehandle, then the subroutine will be
4616 called to act a simple source filter, with the line as read in C<$_>.
4617 Again, return 1 for each valid line, and 0 after all lines have been
4622 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4623 reference to the subroutine itself is passed in as C<$_[0]>.
4627 If an empty list, C<undef>, or nothing that matches the first 3 values above
4628 is returned then C<require> will look at the remaining elements of @INC.
4629 Note that this file handle must be a real file handle (strictly a typeglob,
4630 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4631 ignored and return value processing will stop there.
4633 If the hook is an array reference, its first element must be a subroutine
4634 reference. This subroutine is called as above, but the first parameter is
4635 the array reference. This enables to pass indirectly some arguments to
4638 In other words, you can write:
4640 push @INC, \&my_sub;
4642 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4648 push @INC, [ \&my_sub, $x, $y, ... ];
4650 my ($arrayref, $filename) = @_;
4651 # Retrieve $x, $y, ...
4652 my @parameters = @$arrayref[1..$#$arrayref];
4656 If the hook is an object, it must provide an INC method that will be
4657 called as above, the first parameter being the object itself. (Note that
4658 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4659 into package C<main>.) Here is a typical code layout:
4665 my ($self, $filename) = @_;
4669 # In the main program
4670 push @INC, new Foo(...);
4672 Note that these hooks are also permitted to set the %INC entry
4673 corresponding to the files they have loaded. See L<perlvar/%INC>.
4675 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4682 Generally used in a C<continue> block at the end of a loop to clear
4683 variables and reset C<??> searches so that they work again. The
4684 expression is interpreted as a list of single characters (hyphens
4685 allowed for ranges). All variables and arrays beginning with one of
4686 those letters are reset to their pristine state. If the expression is
4687 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4688 only variables or searches in the current package. Always returns
4691 reset 'X'; # reset all X variables
4692 reset 'a-z'; # reset lower case variables
4693 reset; # just reset ?one-time? searches
4695 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4696 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4697 variables--lexical variables are unaffected, but they clean themselves
4698 up on scope exit anyway, so you'll probably want to use them instead.
4706 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4707 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4708 context, depending on how the return value will be used, and the context
4709 may vary from one execution to the next (see C<wantarray>). If no EXPR
4710 is given, returns an empty list in list context, the undefined value in
4711 scalar context, and (of course) nothing at all in a void context.
4713 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4714 or do FILE will automatically return the value of the last expression
4718 X<reverse> X<rev> X<invert>
4720 In list context, returns a list value consisting of the elements
4721 of LIST in the opposite order. In scalar context, concatenates the
4722 elements of LIST and returns a string value with all characters
4723 in the opposite order.
4725 print join(", ", reverse "world", "Hello"); # Hello, world
4727 print scalar reverse "dlrow ,", "olleH"; # Hello, world
4729 Used without arguments in scalar context, reverse() reverses C<$_>.
4731 $_ = "dlrow ,olleH";
4732 print reverse; # No output, list context
4733 print scalar reverse; # Hello, world
4735 This operator is also handy for inverting a hash, although there are some
4736 caveats. If a value is duplicated in the original hash, only one of those
4737 can be represented as a key in the inverted hash. Also, this has to
4738 unwind one hash and build a whole new one, which may take some time
4739 on a large hash, such as from a DBM file.
4741 %by_name = reverse %by_address; # Invert the hash
4743 =item rewinddir DIRHANDLE
4746 Sets the current position to the beginning of the directory for the
4747 C<readdir> routine on DIRHANDLE.
4749 =item rindex STR,SUBSTR,POSITION
4752 =item rindex STR,SUBSTR
4754 Works just like index() except that it returns the position of the I<last>
4755 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4756 last occurrence beginning at or before that position.
4758 =item rmdir FILENAME
4759 X<rmdir> X<rd> X<directory, remove>
4763 Deletes the directory specified by FILENAME if that directory is
4764 empty. If it succeeds it returns true, otherwise it returns false and
4765 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4767 To remove a directory tree recursively (C<rm -rf> on unix) look at
4768 the C<rmtree> function of the L<File::Path> module.
4772 The substitution operator. See L<perlop>.
4774 =item say FILEHANDLE LIST
4781 Just like C<print>, but implicitly appends a newline.
4782 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
4785 This keyword is only available when the "say" feature is
4786 enabled: see L<feature>.
4789 X<scalar> X<context>
4791 Forces EXPR to be interpreted in scalar context and returns the value
4794 @counts = ( scalar @a, scalar @b, scalar @c );
4796 There is no equivalent operator to force an expression to
4797 be interpolated in list context because in practice, this is never
4798 needed. If you really wanted to do so, however, you could use
4799 the construction C<@{[ (some expression) ]}>, but usually a simple
4800 C<(some expression)> suffices.
4802 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4803 parenthesized list, this behaves as a scalar comma expression, evaluating
4804 all but the last element in void context and returning the final element
4805 evaluated in scalar context. This is seldom what you want.
4807 The following single statement:
4809 print uc(scalar(&foo,$bar)),$baz;
4811 is the moral equivalent of these two:
4814 print(uc($bar),$baz);
4816 See L<perlop> for more details on unary operators and the comma operator.
4818 =item seek FILEHANDLE,POSITION,WHENCE
4819 X<seek> X<fseek> X<filehandle, position>
4821 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4822 FILEHANDLE may be an expression whose value gives the name of the
4823 filehandle. The values for WHENCE are C<0> to set the new position
4824 I<in bytes> to POSITION, C<1> to set it to the current position plus
4825 POSITION, and C<2> to set it to EOF plus POSITION (typically
4826 negative). For WHENCE you may use the constants C<SEEK_SET>,
4827 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4828 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4831 Note the I<in bytes>: even if the filehandle has been set to
4832 operate on characters (for example by using the C<:encoding(utf8)> open
4833 layer), tell() will return byte offsets, not character offsets
4834 (because implementing that would render seek() and tell() rather slow).
4836 If you want to position file for C<sysread> or C<syswrite>, don't use
4837 C<seek>--buffering makes its effect on the file's system position
4838 unpredictable and non-portable. Use C<sysseek> instead.
4840 Due to the rules and rigors of ANSI C, on some systems you have to do a
4841 seek whenever you switch between reading and writing. Amongst other
4842 things, this may have the effect of calling stdio's clearerr(3).
4843 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4847 This is also useful for applications emulating C<tail -f>. Once you hit
4848 EOF on your read, and then sleep for a while, you might have to stick in a
4849 seek() to reset things. The C<seek> doesn't change the current position,
4850 but it I<does> clear the end-of-file condition on the handle, so that the
4851 next C<< <FILE> >> makes Perl try again to read something. We hope.
4853 If that doesn't work (some IO implementations are particularly
4854 cantankerous), then you may need something more like this:
4857 for ($curpos = tell(FILE); $_ = <FILE>;
4858 $curpos = tell(FILE)) {
4859 # search for some stuff and put it into files
4861 sleep($for_a_while);
4862 seek(FILE, $curpos, 0);
4865 =item seekdir DIRHANDLE,POS
4868 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4869 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4870 about possible directory compaction as the corresponding system library
4873 =item select FILEHANDLE
4874 X<select> X<filehandle, default>
4878 Returns the currently selected filehandle. If FILEHANDLE is supplied,
4879 sets the new current default filehandle for output. This has two
4880 effects: first, a C<write> or a C<print> without a filehandle will
4881 default to this FILEHANDLE. Second, references to variables related to
4882 output will refer to this output channel. For example, if you have to
4883 set the top of form format for more than one output channel, you might
4891 FILEHANDLE may be an expression whose value gives the name of the
4892 actual filehandle. Thus:
4894 $oldfh = select(STDERR); $| = 1; select($oldfh);
4896 Some programmers may prefer to think of filehandles as objects with
4897 methods, preferring to write the last example as:
4900 STDERR->autoflush(1);
4902 =item select RBITS,WBITS,EBITS,TIMEOUT
4905 This calls the select(2) system call with the bit masks specified, which
4906 can be constructed using C<fileno> and C<vec>, along these lines:
4908 $rin = $win = $ein = '';
4909 vec($rin,fileno(STDIN),1) = 1;
4910 vec($win,fileno(STDOUT),1) = 1;
4913 If you want to select on many filehandles you might wish to write a
4917 my(@fhlist) = split(' ',$_[0]);
4920 vec($bits,fileno($_),1) = 1;
4924 $rin = fhbits('STDIN TTY SOCK');
4928 ($nfound,$timeleft) =
4929 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4931 or to block until something becomes ready just do this
4933 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4935 Most systems do not bother to return anything useful in $timeleft, so
4936 calling select() in scalar context just returns $nfound.
4938 Any of the bit masks can also be undef. The timeout, if specified, is
4939 in seconds, which may be fractional. Note: not all implementations are
4940 capable of returning the $timeleft. If not, they always return
4941 $timeleft equal to the supplied $timeout.
4943 You can effect a sleep of 250 milliseconds this way:
4945 select(undef, undef, undef, 0.25);
4947 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4948 is implementation-dependent. See also L<perlport> for notes on the
4949 portability of C<select>.
4951 On error, C<select> behaves like the select(2) system call : it returns
4954 Note: on some Unixes, the select(2) system call may report a socket file
4955 descriptor as "ready for reading", when actually no data is available,
4956 thus a subsequent read blocks. It can be avoided using always the
4957 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4960 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4961 or <FH>) with C<select>, except as permitted by POSIX, and even
4962 then only on POSIX systems. You have to use C<sysread> instead.
4964 =item semctl ID,SEMNUM,CMD,ARG
4967 Calls the System V IPC function C<semctl>. You'll probably have to say
4971 first to get the correct constant definitions. If CMD is IPC_STAT or
4972 GETALL, then ARG must be a variable that will hold the returned
4973 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4974 the undefined value for error, "C<0 but true>" for zero, or the actual
4975 return value otherwise. The ARG must consist of a vector of native
4976 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4977 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4980 =item semget KEY,NSEMS,FLAGS
4983 Calls the System V IPC function semget. Returns the semaphore id, or
4984 the undefined value if there is an error. See also
4985 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4988 =item semop KEY,OPSTRING
4991 Calls the System V IPC function semop to perform semaphore operations
4992 such as signalling and waiting. OPSTRING must be a packed array of
4993 semop structures. Each semop structure can be generated with
4994 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4995 implies the number of semaphore operations. Returns true if
4996 successful, or false if there is an error. As an example, the
4997 following code waits on semaphore $semnum of semaphore id $semid:
4999 $semop = pack("s!3", $semnum, -1, 0);
5000 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5002 To signal the semaphore, replace C<-1> with C<1>. See also
5003 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
5006 =item send SOCKET,MSG,FLAGS,TO
5009 =item send SOCKET,MSG,FLAGS
5011 Sends a message on a socket. Attempts to send the scalar MSG to the
5012 SOCKET filehandle. Takes the same flags as the system call of the
5013 same name. On unconnected sockets you must specify a destination to
5014 send TO, in which case it does a C C<sendto>. Returns the number of
5015 characters sent, or the undefined value if there is an error. The C
5016 system call sendmsg(2) is currently unimplemented. See
5017 L<perlipc/"UDP: Message Passing"> for examples.
5019 Note the I<characters>: depending on the status of the socket, either
5020 (8-bit) bytes or characters are sent. By default all sockets operate
5021 on bytes, but for example if the socket has been changed using
5022 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5023 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5024 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5025 pragma: in that case pretty much any characters can be sent.
5027 =item setpgrp PID,PGRP
5030 Sets the current process group for the specified PID, C<0> for the current
5031 process. Will produce a fatal error if used on a machine that doesn't
5032 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5033 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5034 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5037 =item setpriority WHICH,WHO,PRIORITY
5038 X<setpriority> X<priority> X<nice> X<renice>
5040 Sets the current priority for a process, a process group, or a user.
5041 (See setpriority(2).) Will produce a fatal error if used on a machine
5042 that doesn't implement setpriority(2).
5044 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5047 Sets the socket option requested. Returns undefined if there is an
5048 error. Use integer constants provided by the C<Socket> module for
5049 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5050 getprotobyname. OPTVAL might either be a packed string or an integer.
5051 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5053 An example disabling the Nagle's algorithm for a socket:
5055 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5056 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5063 Shifts the first value of the array off and returns it, shortening the
5064 array by 1 and moving everything down. If there are no elements in the
5065 array, returns the undefined value. If ARRAY is omitted, shifts the
5066 C<@_> array within the lexical scope of subroutines and formats, and the
5067 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5068 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5069 C<UNITCHECK {}> and C<END {}> constructs.
5071 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5072 same thing to the left end of an array that C<pop> and C<push> do to the
5075 =item shmctl ID,CMD,ARG
5078 Calls the System V IPC function shmctl. You'll probably have to say
5082 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5083 then ARG must be a variable that will hold the returned C<shmid_ds>
5084 structure. Returns like ioctl: the undefined value for error, "C<0> but
5085 true" for zero, or the actual return value otherwise.
5086 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5088 =item shmget KEY,SIZE,FLAGS
5091 Calls the System V IPC function shmget. Returns the shared memory
5092 segment id, or the undefined value if there is an error.
5093 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5095 =item shmread ID,VAR,POS,SIZE
5099 =item shmwrite ID,STRING,POS,SIZE
5101 Reads or writes the System V shared memory segment ID starting at
5102 position POS for size SIZE by attaching to it, copying in/out, and
5103 detaching from it. When reading, VAR must be a variable that will
5104 hold the data read. When writing, if STRING is too long, only SIZE
5105 bytes are used; if STRING is too short, nulls are written to fill out
5106 SIZE bytes. Return true if successful, or false if there is an error.
5107 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5108 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5110 =item shutdown SOCKET,HOW
5113 Shuts down a socket connection in the manner indicated by HOW, which
5114 has the same interpretation as in the system call of the same name.
5116 shutdown(SOCKET, 0); # I/we have stopped reading data
5117 shutdown(SOCKET, 1); # I/we have stopped writing data
5118 shutdown(SOCKET, 2); # I/we have stopped using this socket
5120 This is useful with sockets when you want to tell the other
5121 side you're done writing but not done reading, or vice versa.
5122 It's also a more insistent form of close because it also
5123 disables the file descriptor in any forked copies in other
5126 Returns C<1> for success. In the case of error, returns C<undef> if
5127 the first argument is not a valid filehandle, or returns C<0> and sets
5128 C<$!> for any other failure.
5131 X<sin> X<sine> X<asin> X<arcsine>
5135 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5136 returns sine of C<$_>.
5138 For the inverse sine operation, you may use the C<Math::Trig::asin>
5139 function, or use this relation:
5141 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5148 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5149 May be interrupted if the process receives a signal such as C<SIGALRM>.
5150 Returns the number of seconds actually slept. You probably cannot
5151 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5154 On some older systems, it may sleep up to a full second less than what
5155 you requested, depending on how it counts seconds. Most modern systems
5156 always sleep the full amount. They may appear to sleep longer than that,
5157 however, because your process might not be scheduled right away in a
5158 busy multitasking system.
5160 For delays of finer granularity than one second, the Time::HiRes module
5161 (from CPAN, and starting from Perl 5.8 part of the standard
5162 distribution) provides usleep(). You may also use Perl's four-argument
5163 version of select() leaving the first three arguments undefined, or you
5164 might be able to use the C<syscall> interface to access setitimer(2) if
5165 your system supports it. See L<perlfaq8> for details.
5167 See also the POSIX module's C<pause> function.
5169 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5172 Opens a socket of the specified kind and attaches it to filehandle
5173 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5174 the system call of the same name. You should C<use Socket> first
5175 to get the proper definitions imported. See the examples in
5176 L<perlipc/"Sockets: Client/Server Communication">.
5178 On systems that support a close-on-exec flag on files, the flag will
5179 be set for the newly opened file descriptor, as determined by the
5180 value of $^F. See L<perlvar/$^F>.
5182 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5185 Creates an unnamed pair of sockets in the specified domain, of the
5186 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5187 for the system call of the same name. If unimplemented, yields a fatal
5188 error. Returns true if successful.
5190 On systems that support a close-on-exec flag on files, the flag will
5191 be set for the newly opened file descriptors, as determined by the value
5192 of $^F. See L<perlvar/$^F>.
5194 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5195 to C<pipe(Rdr, Wtr)> is essentially:
5198 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5199 shutdown(Rdr, 1); # no more writing for reader
5200 shutdown(Wtr, 0); # no more reading for writer
5202 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5203 emulate socketpair using IP sockets to localhost if your system implements
5204 sockets but not socketpair.
5206 =item sort SUBNAME LIST
5207 X<sort> X<qsort> X<quicksort> X<mergesort>
5209 =item sort BLOCK LIST
5213 In list context, this sorts the LIST and returns the sorted list value.
5214 In scalar context, the behaviour of C<sort()> is undefined.
5216 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5217 order. If SUBNAME is specified, it gives the name of a subroutine
5218 that returns an integer less than, equal to, or greater than C<0>,
5219 depending on how the elements of the list are to be ordered. (The C<<
5220 <=> >> and C<cmp> operators are extremely useful in such routines.)
5221 SUBNAME may be a scalar variable name (unsubscripted), in which case
5222 the value provides the name of (or a reference to) the actual
5223 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5224 an anonymous, in-line sort subroutine.
5226 If the subroutine's prototype is C<($$)>, the elements to be compared
5227 are passed by reference in C<@_>, as for a normal subroutine. This is
5228 slower than unprototyped subroutines, where the elements to be
5229 compared are passed into the subroutine
5230 as the package global variables $a and $b (see example below). Note that
5231 in the latter case, it is usually counter-productive to declare $a and
5234 The values to be compared are always passed by reference and should not
5237 You also cannot exit out of the sort block or subroutine using any of the
5238 loop control operators described in L<perlsyn> or with C<goto>.
5240 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5241 current collation locale. See L<perllocale>.
5243 sort() returns aliases into the original list, much as a for loop's index
5244 variable aliases the list elements. That is, modifying an element of a
5245 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5246 actually modifies the element in the original list. This is usually
5247 something to be avoided when writing clear code.
5249 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5250 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5251 preserves the input order of elements that compare equal. Although
5252 quicksort's run time is O(NlogN) when averaged over all arrays of
5253 length N, the time can be O(N**2), I<quadratic> behavior, for some
5254 inputs.) In 5.7, the quicksort implementation was replaced with
5255 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5256 But benchmarks indicated that for some inputs, on some platforms,
5257 the original quicksort was faster. 5.8 has a sort pragma for
5258 limited control of the sort. Its rather blunt control of the
5259 underlying algorithm may not persist into future Perls, but the
5260 ability to characterize the input or output in implementation
5261 independent ways quite probably will. See L<sort>.
5266 @articles = sort @files;
5268 # same thing, but with explicit sort routine
5269 @articles = sort {$a cmp $b} @files;
5271 # now case-insensitively
5272 @articles = sort {uc($a) cmp uc($b)} @files;
5274 # same thing in reversed order
5275 @articles = sort {$b cmp $a} @files;
5277 # sort numerically ascending
5278 @articles = sort {$a <=> $b} @files;
5280 # sort numerically descending
5281 @articles = sort {$b <=> $a} @files;
5283 # this sorts the %age hash by value instead of key
5284 # using an in-line function
5285 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5287 # sort using explicit subroutine name
5289 $age{$a} <=> $age{$b}; # presuming numeric
5291 @sortedclass = sort byage @class;
5293 sub backwards { $b cmp $a }
5294 @harry = qw(dog cat x Cain Abel);
5295 @george = qw(gone chased yz Punished Axed);
5297 # prints AbelCaincatdogx
5298 print sort backwards @harry;
5299 # prints xdogcatCainAbel
5300 print sort @george, 'to', @harry;
5301 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5303 # inefficiently sort by descending numeric compare using
5304 # the first integer after the first = sign, or the
5305 # whole record case-insensitively otherwise
5308 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5313 # same thing, but much more efficiently;
5314 # we'll build auxiliary indices instead
5318 push @nums, /=(\d+)/;
5323 $nums[$b] <=> $nums[$a]
5325 $caps[$a] cmp $caps[$b]
5329 # same thing, but without any temps
5330 @new = map { $_->[0] }
5331 sort { $b->[1] <=> $a->[1]
5334 } map { [$_, /=(\d+)/, uc($_)] } @old;
5336 # using a prototype allows you to use any comparison subroutine
5337 # as a sort subroutine (including other package's subroutines)
5339 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5342 @new = sort other::backwards @old;
5344 # guarantee stability, regardless of algorithm
5346 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5348 # force use of mergesort (not portable outside Perl 5.8)
5349 use sort '_mergesort'; # note discouraging _
5350 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5352 If you're using strict, you I<must not> declare $a
5353 and $b as lexicals. They are package globals. That means
5354 if you're in the C<main> package and type
5356 @articles = sort {$b <=> $a} @files;
5358 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5359 but if you're in the C<FooPack> package, it's the same as typing
5361 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5363 The comparison function is required to behave. If it returns
5364 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5365 sometimes saying the opposite, for example) the results are not
5368 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5369 (not-a-number), and because C<sort> will trigger a fatal error unless the
5370 result of a comparison is defined, when sorting with a comparison function
5371 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5372 The following example takes advantage of the fact that C<NaN != NaN> to
5373 eliminate any C<NaN>s from the input.
5375 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5377 =item splice ARRAY,OFFSET,LENGTH,LIST
5380 =item splice ARRAY,OFFSET,LENGTH
5382 =item splice ARRAY,OFFSET
5386 Removes the elements designated by OFFSET and LENGTH from an array, and
5387 replaces them with the elements of LIST, if any. In list context,
5388 returns the elements removed from the array. In scalar context,
5389 returns the last element removed, or C<undef> if no elements are
5390 removed. The array grows or shrinks as necessary.
5391 If OFFSET is negative then it starts that far from the end of the array.
5392 If LENGTH is omitted, removes everything from OFFSET onward.
5393 If LENGTH is negative, removes the elements from OFFSET onward
5394 except for -LENGTH elements at the end of the array.
5395 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5396 past the end of the array, perl issues a warning, and splices at the
5399 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5401 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5402 pop(@a) splice(@a,-1)
5403 shift(@a) splice(@a,0,1)
5404 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5405 $a[$i] = $y splice(@a,$i,1,$y)
5407 Example, assuming array lengths are passed before arrays:
5409 sub aeq { # compare two list values
5410 my(@a) = splice(@_,0,shift);
5411 my(@b) = splice(@_,0,shift);
5412 return 0 unless @a == @b; # same len?
5414 return 0 if pop(@a) ne pop(@b);
5418 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5420 =item split /PATTERN/,EXPR,LIMIT
5423 =item split /PATTERN/,EXPR
5425 =item split /PATTERN/
5429 Splits the string EXPR into a list of strings and returns that list. By
5430 default, empty leading fields are preserved, and empty trailing ones are
5431 deleted. (If all fields are empty, they are considered to be trailing.)
5433 In scalar context, returns the number of fields found and splits into
5434 the C<@_> array. Use of split in scalar context is deprecated, however,
5435 because it clobbers your subroutine arguments.
5437 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5438 splits on whitespace (after skipping any leading whitespace). Anything
5439 matching PATTERN is taken to be a delimiter separating the fields. (Note
5440 that the delimiter may be longer than one character.)
5442 If LIMIT is specified and positive, it represents the maximum number
5443 of fields the EXPR will be split into, though the actual number of
5444 fields returned depends on the number of times PATTERN matches within
5445 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5446 stripped (which potential users of C<pop> would do well to remember).
5447 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5448 had been specified. Note that splitting an EXPR that evaluates to the
5449 empty string always returns the empty list, regardless of the LIMIT
5452 A pattern matching the null string (not to be confused with
5453 a null pattern C<//>, which is just one member of the set of patterns
5454 matching a null string) will split the value of EXPR into separate
5455 characters at each point it matches that way. For example:
5457 print join(':', split(/ */, 'hi there')), "\n";
5459 produces the output 'h:i:t:h:e:r:e'.
5461 As a special case for C<split>, using the empty pattern C<//> specifically
5462 matches only the null string, and is not be confused with the regular use
5463 of C<//> to mean "the last successful pattern match". So, for C<split>,
5466 print join(':', split(//, 'hi there')), "\n";
5468 produces the output 'h:i: :t:h:e:r:e'.
5470 Empty leading fields are produced when there are positive-width matches at
5471 the beginning of the string; a zero-width match at the beginning of
5472 the string does not produce an empty field. For example:
5474 print join(':', split(/(?=\w)/, 'hi there!'));
5476 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
5477 hand, are produced when there is a match at the end of the string (and
5478 when LIMIT is given and is not 0), regardless of the length of the match.
5481 print join(':', split(//, 'hi there!', -1)), "\n";
5482 print join(':', split(/\W/, 'hi there!', -1)), "\n";
5484 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
5485 both with an empty trailing field.
5487 The LIMIT parameter can be used to split a line partially
5489 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5491 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5492 a LIMIT one larger than the number of variables in the list, to avoid
5493 unnecessary work. For the list above LIMIT would have been 4 by
5494 default. In time critical applications it behooves you not to split
5495 into more fields than you really need.
5497 If the PATTERN contains parentheses, additional list elements are
5498 created from each matching substring in the delimiter.
5500 split(/([,-])/, "1-10,20", 3);
5502 produces the list value
5504 (1, '-', 10, ',', 20)
5506 If you had the entire header of a normal Unix email message in $header,
5507 you could split it up into fields and their values this way:
5509 $header =~ s/\n\s+/ /g; # fix continuation lines
5510 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5512 The pattern C</PATTERN/> may be replaced with an expression to specify
5513 patterns that vary at runtime. (To do runtime compilation only once,
5514 use C</$variable/o>.)
5516 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5517 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5518 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5519 will give you as many null initial fields as there are leading spaces.
5520 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5521 whitespace produces a null first field. A C<split> with no arguments
5522 really does a S<C<split(' ', $_)>> internally.
5524 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5529 open(PASSWD, '/etc/passwd');
5532 ($login, $passwd, $uid, $gid,
5533 $gcos, $home, $shell) = split(/:/);
5537 As with regular pattern matching, any capturing parentheses that are not
5538 matched in a C<split()> will be set to C<undef> when returned:
5540 @fields = split /(A)|B/, "1A2B3";
5541 # @fields is (1, 'A', 2, undef, 3)
5543 =item sprintf FORMAT, LIST
5546 Returns a string formatted by the usual C<printf> conventions of the C
5547 library function C<sprintf>. See below for more details
5548 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5549 the general principles.
5553 # Format number with up to 8 leading zeroes
5554 $result = sprintf("%08d", $number);
5556 # Round number to 3 digits after decimal point
5557 $rounded = sprintf("%.3f", $number);
5559 Perl does its own C<sprintf> formatting--it emulates the C
5560 function C<sprintf>, but it doesn't use it (except for floating-point
5561 numbers, and even then only the standard modifiers are allowed). As a
5562 result, any non-standard extensions in your local C<sprintf> are not
5563 available from Perl.
5565 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5566 pass it an array as your first argument. The array is given scalar context,
5567 and instead of using the 0th element of the array as the format, Perl will
5568 use the count of elements in the array as the format, which is almost never
5571 Perl's C<sprintf> permits the following universally-known conversions:
5574 %c a character with the given number
5576 %d a signed integer, in decimal
5577 %u an unsigned integer, in decimal
5578 %o an unsigned integer, in octal
5579 %x an unsigned integer, in hexadecimal
5580 %e a floating-point number, in scientific notation
5581 %f a floating-point number, in fixed decimal notation
5582 %g a floating-point number, in %e or %f notation
5584 In addition, Perl permits the following widely-supported conversions:
5586 %X like %x, but using upper-case letters
5587 %E like %e, but using an upper-case "E"
5588 %G like %g, but with an upper-case "E" (if applicable)
5589 %b an unsigned integer, in binary
5590 %B like %b, but using an upper-case "B" with the # flag
5591 %p a pointer (outputs the Perl value's address in hexadecimal)
5592 %n special: *stores* the number of characters output so far
5593 into the next variable in the parameter list
5595 Finally, for backward (and we do mean "backward") compatibility, Perl
5596 permits these unnecessary but widely-supported conversions:
5599 %D a synonym for %ld
5600 %U a synonym for %lu
5601 %O a synonym for %lo
5604 Note that the number of exponent digits in the scientific notation produced
5605 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5606 exponent less than 100 is system-dependent: it may be three or less
5607 (zero-padded as necessary). In other words, 1.23 times ten to the
5608 99th may be either "1.23e99" or "1.23e099".
5610 Between the C<%> and the format letter, you may specify a number of
5611 additional attributes controlling the interpretation of the format.
5612 In order, these are:
5616 =item format parameter index
5618 An explicit format parameter index, such as C<2$>. By default sprintf
5619 will format the next unused argument in the list, but this allows you
5620 to take the arguments out of order, e.g.:
5622 printf '%2$d %1$d', 12, 34; # prints "34 12"
5623 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5629 space prefix non-negative number with a space
5630 + prefix non-negative number with a plus sign
5631 - left-justify within the field
5632 0 use zeros, not spaces, to right-justify
5633 # ensure the leading "0" for any octal,
5634 prefix non-zero hexadecimal with "0x" or "0X",
5635 prefix non-zero binary with "0b" or "0B"
5639 printf '<% d>', 12; # prints "< 12>"
5640 printf '<%+d>', 12; # prints "<+12>"
5641 printf '<%6s>', 12; # prints "< 12>"
5642 printf '<%-6s>', 12; # prints "<12 >"
5643 printf '<%06s>', 12; # prints "<000012>"
5644 printf '<%#o>', 12; # prints "<014>"
5645 printf '<%#x>', 12; # prints "<0xc>"
5646 printf '<%#X>', 12; # prints "<0XC>"
5647 printf '<%#b>', 12; # prints "<0b1100>"
5648 printf '<%#B>', 12; # prints "<0B1100>"
5650 When a space and a plus sign are given as the flags at once,
5651 a plus sign is used to prefix a positive number.
5653 printf '<%+ d>', 12; # prints "<+12>"
5654 printf '<% +d>', 12; # prints "<+12>"
5656 When the # flag and a precision are given in the %o conversion,
5657 the precision is incremented if it's necessary for the leading "0".
5659 printf '<%#.5o>', 012; # prints "<00012>"
5660 printf '<%#.5o>', 012345; # prints "<012345>"
5661 printf '<%#.0o>', 0; # prints "<0>"
5665 This flag tells perl to interpret the supplied string as a vector of
5666 integers, one for each character in the string. Perl applies the format to
5667 each integer in turn, then joins the resulting strings with a separator (a
5668 dot C<.> by default). This can be useful for displaying ordinal values of
5669 characters in arbitrary strings:
5671 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5672 printf "version is v%vd\n", $^V; # Perl's version
5674 Put an asterisk C<*> before the C<v> to override the string to
5675 use to separate the numbers:
5677 printf "address is %*vX\n", ":", $addr; # IPv6 address
5678 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5680 You can also explicitly specify the argument number to use for
5681 the join string using e.g. C<*2$v>:
5683 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5685 =item (minimum) width
5687 Arguments are usually formatted to be only as wide as required to
5688 display the given value. You can override the width by putting
5689 a number here, or get the width from the next argument (with C<*>)
5690 or from a specified argument (with e.g. C<*2$>):
5692 printf '<%s>', "a"; # prints "<a>"
5693 printf '<%6s>', "a"; # prints "< a>"
5694 printf '<%*s>', 6, "a"; # prints "< a>"
5695 printf '<%*2$s>', "a", 6; # prints "< a>"
5696 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5698 If a field width obtained through C<*> is negative, it has the same
5699 effect as the C<-> flag: left-justification.
5701 =item precision, or maximum width
5704 You can specify a precision (for numeric conversions) or a maximum
5705 width (for string conversions) by specifying a C<.> followed by a number.
5706 For floating point formats, with the exception of 'g' and 'G', this specifies
5707 the number of decimal places to show (the default being 6), e.g.:
5709 # these examples are subject to system-specific variation
5710 printf '<%f>', 1; # prints "<1.000000>"
5711 printf '<%.1f>', 1; # prints "<1.0>"
5712 printf '<%.0f>', 1; # prints "<1>"
5713 printf '<%e>', 10; # prints "<1.000000e+01>"
5714 printf '<%.1e>', 10; # prints "<1.0e+01>"
5716 For 'g' and 'G', this specifies the maximum number of digits to show,
5717 including prior to the decimal point as well as after it, e.g.:
5719 # these examples are subject to system-specific variation
5720 printf '<%g>', 1; # prints "<1>"
5721 printf '<%.10g>', 1; # prints "<1>"
5722 printf '<%g>', 100; # prints "<100>"
5723 printf '<%.1g>', 100; # prints "<1e+02>"
5724 printf '<%.2g>', 100.01; # prints "<1e+02>"
5725 printf '<%.5g>', 100.01; # prints "<100.01>"
5726 printf '<%.4g>', 100.01; # prints "<100>"
5728 For integer conversions, specifying a precision implies that the
5729 output of the number itself should be zero-padded to this width,
5730 where the 0 flag is ignored:
5732 printf '<%.6d>', 1; # prints "<000001>"
5733 printf '<%+.6d>', 1; # prints "<+000001>"
5734 printf '<%-10.6d>', 1; # prints "<000001 >"
5735 printf '<%10.6d>', 1; # prints "< 000001>"
5736 printf '<%010.6d>', 1; # prints "< 000001>"
5737 printf '<%+10.6d>', 1; # prints "< +000001>"
5739 printf '<%.6x>', 1; # prints "<000001>"
5740 printf '<%#.6x>', 1; # prints "<0x000001>"
5741 printf '<%-10.6x>', 1; # prints "<000001 >"
5742 printf '<%10.6x>', 1; # prints "< 000001>"
5743 printf '<%010.6x>', 1; # prints "< 000001>"
5744 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5746 For string conversions, specifying a precision truncates the string
5747 to fit in the specified width:
5749 printf '<%.5s>', "truncated"; # prints "<trunc>"
5750 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5752 You can also get the precision from the next argument using C<.*>:
5754 printf '<%.6x>', 1; # prints "<000001>"
5755 printf '<%.*x>', 6, 1; # prints "<000001>"
5757 If a precision obtained through C<*> is negative, it has the same
5758 effect as no precision.
5760 printf '<%.*s>', 7, "string"; # prints "<string>"
5761 printf '<%.*s>', 3, "string"; # prints "<str>"
5762 printf '<%.*s>', 0, "string"; # prints "<>"
5763 printf '<%.*s>', -1, "string"; # prints "<string>"
5765 printf '<%.*d>', 1, 0; # prints "<0>"
5766 printf '<%.*d>', 0, 0; # prints "<>"
5767 printf '<%.*d>', -1, 0; # prints "<0>"
5769 You cannot currently get the precision from a specified number,
5770 but it is intended that this will be possible in the future using
5773 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5777 For numeric conversions, you can specify the size to interpret the
5778 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5779 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5780 whatever the default integer size is on your platform (usually 32 or 64
5781 bits), but you can override this to use instead one of the standard C types,
5782 as supported by the compiler used to build Perl:
5784 l interpret integer as C type "long" or "unsigned long"
5785 h interpret integer as C type "short" or "unsigned short"
5786 q, L or ll interpret integer as C type "long long", "unsigned long long".
5787 or "quads" (typically 64-bit integers)
5789 The last will produce errors if Perl does not understand "quads" in your
5790 installation. (This requires that either the platform natively supports quads
5791 or Perl was specifically compiled to support quads.) You can find out
5792 whether your Perl supports quads via L<Config>:
5795 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5798 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5799 to be the default floating point size on your platform (double or long double),
5800 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5801 platform supports them. You can find out whether your Perl supports long
5802 doubles via L<Config>:
5805 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5807 You can find out whether Perl considers 'long double' to be the default
5808 floating point size to use on your platform via L<Config>:
5811 ($Config{uselongdouble} eq 'define') &&
5812 print "long doubles by default\n";
5814 It can also be the case that long doubles and doubles are the same thing:
5817 ($Config{doublesize} == $Config{longdblsize}) &&
5818 print "doubles are long doubles\n";
5820 The size specifier C<V> has no effect for Perl code, but it is supported
5821 for compatibility with XS code; it means 'use the standard size for
5822 a Perl integer (or floating-point number)', which is already the
5823 default for Perl code.
5825 =item order of arguments
5827 Normally, sprintf takes the next unused argument as the value to
5828 format for each format specification. If the format specification
5829 uses C<*> to require additional arguments, these are consumed from
5830 the argument list in the order in which they appear in the format
5831 specification I<before> the value to format. Where an argument is
5832 specified using an explicit index, this does not affect the normal
5833 order for the arguments (even when the explicitly specified index
5834 would have been the next argument in any case).
5838 printf '<%*.*s>', $a, $b, $c;
5840 would use C<$a> for the width, C<$b> for the precision and C<$c>
5841 as the value to format, while:
5843 printf '<%*1$.*s>', $a, $b;
5845 would use C<$a> for the width and the precision, and C<$b> as the
5848 Here are some more examples - beware that when using an explicit
5849 index, the C<$> may need to be escaped:
5851 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5852 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5853 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5854 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5858 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5859 the character used for the decimal separator in formatted floating
5860 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5864 X<sqrt> X<root> X<square root>
5868 Return the square root of EXPR. If EXPR is omitted, returns square
5869 root of C<$_>. Only works on non-negative operands, unless you've
5870 loaded the standard Math::Complex module.
5873 print sqrt(-2); # prints 1.4142135623731i
5876 X<srand> X<seed> X<randseed>
5880 Sets the random number seed for the C<rand> operator.
5882 The point of the function is to "seed" the C<rand> function so that
5883 C<rand> can produce a different sequence each time you run your
5886 If srand() is not called explicitly, it is called implicitly at the
5887 first use of the C<rand> operator. However, this was not the case in
5888 versions of Perl before 5.004, so if your script will run under older
5889 Perl versions, it should call C<srand>.
5891 Most programs won't even call srand() at all, except those that
5892 need a cryptographically-strong starting point rather than the
5893 generally acceptable default, which is based on time of day,
5894 process ID, and memory allocation, or the F</dev/urandom> device,
5897 You can call srand($seed) with the same $seed to reproduce the
5898 I<same> sequence from rand(), but this is usually reserved for
5899 generating predictable results for testing or debugging.
5900 Otherwise, don't call srand() more than once in your program.
5902 Do B<not> call srand() (i.e. without an argument) more than once in
5903 a script. The internal state of the random number generator should
5904 contain more entropy than can be provided by any seed, so calling
5905 srand() again actually I<loses> randomness.
5907 Most implementations of C<srand> take an integer and will silently
5908 truncate decimal numbers. This means C<srand(42)> will usually
5909 produce the same results as C<srand(42.1)>. To be safe, always pass
5910 C<srand> an integer.
5912 In versions of Perl prior to 5.004 the default seed was just the
5913 current C<time>. This isn't a particularly good seed, so many old
5914 programs supply their own seed value (often C<time ^ $$> or C<time ^
5915 ($$ + ($$ << 15))>), but that isn't necessary any more.
5917 For cryptographic purposes, however, you need something much more random
5918 than the default seed. Checksumming the compressed output of one or more
5919 rapidly changing operating system status programs is the usual method. For
5922 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
5924 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5927 Frequently called programs (like CGI scripts) that simply use
5931 for a seed can fall prey to the mathematical property that
5935 one-third of the time. So don't do that.
5937 =item stat FILEHANDLE
5938 X<stat> X<file, status> X<ctime>
5942 =item stat DIRHANDLE
5946 Returns a 13-element list giving the status info for a file, either
5947 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5948 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5951 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5952 $atime,$mtime,$ctime,$blksize,$blocks)
5955 Not all fields are supported on all filesystem types. Here are the
5956 meanings of the fields:
5958 0 dev device number of filesystem
5960 2 mode file mode (type and permissions)
5961 3 nlink number of (hard) links to the file
5962 4 uid numeric user ID of file's owner
5963 5 gid numeric group ID of file's owner
5964 6 rdev the device identifier (special files only)
5965 7 size total size of file, in bytes
5966 8 atime last access time in seconds since the epoch
5967 9 mtime last modify time in seconds since the epoch
5968 10 ctime inode change time in seconds since the epoch (*)
5969 11 blksize preferred block size for file system I/O
5970 12 blocks actual number of blocks allocated
5972 (The epoch was at 00:00 January 1, 1970 GMT.)
5974 (*) Not all fields are supported on all filesystem types. Notably, the
5975 ctime field is non-portable. In particular, you cannot expect it to be a
5976 "creation time", see L<perlport/"Files and Filesystems"> for details.
5978 If C<stat> is passed the special filehandle consisting of an underline, no
5979 stat is done, but the current contents of the stat structure from the
5980 last C<stat>, C<lstat>, or filetest are returned. Example:
5982 if (-x $file && (($d) = stat(_)) && $d < 0) {
5983 print "$file is executable NFS file\n";
5986 (This works on machines only for which the device number is negative
5989 Because the mode contains both the file type and its permissions, you
5990 should mask off the file type portion and (s)printf using a C<"%o">
5991 if you want to see the real permissions.
5993 $mode = (stat($filename))[2];
5994 printf "Permissions are %04o\n", $mode & 07777;
5996 In scalar context, C<stat> returns a boolean value indicating success
5997 or failure, and, if successful, sets the information associated with
5998 the special filehandle C<_>.
6000 The L<File::stat> module provides a convenient, by-name access mechanism:
6003 $sb = stat($filename);
6004 printf "File is %s, size is %s, perm %04o, mtime %s\n",
6005 $filename, $sb->size, $sb->mode & 07777,
6006 scalar localtime $sb->mtime;
6008 You can import symbolic mode constants (C<S_IF*>) and functions
6009 (C<S_IS*>) from the Fcntl module:
6013 $mode = (stat($filename))[2];
6015 $user_rwx = ($mode & S_IRWXU) >> 6;
6016 $group_read = ($mode & S_IRGRP) >> 3;
6017 $other_execute = $mode & S_IXOTH;
6019 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6021 $is_setuid = $mode & S_ISUID;
6022 $is_directory = S_ISDIR($mode);
6024 You could write the last two using the C<-u> and C<-d> operators.
6025 The commonly available C<S_IF*> constants are
6027 # Permissions: read, write, execute, for user, group, others.
6029 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6030 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6031 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6033 # Setuid/Setgid/Stickiness/SaveText.
6034 # Note that the exact meaning of these is system dependent.
6036 S_ISUID S_ISGID S_ISVTX S_ISTXT
6038 # File types. Not necessarily all are available on your system.
6040 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6042 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
6044 S_IREAD S_IWRITE S_IEXEC
6046 and the C<S_IF*> functions are
6048 S_IMODE($mode) the part of $mode containing the permission bits
6049 and the setuid/setgid/sticky bits
6051 S_IFMT($mode) the part of $mode containing the file type
6052 which can be bit-anded with e.g. S_IFREG
6053 or with the following functions
6055 # The operators -f, -d, -l, -b, -c, -p, and -S.
6057 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6058 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6060 # No direct -X operator counterpart, but for the first one
6061 # the -g operator is often equivalent. The ENFMT stands for
6062 # record flocking enforcement, a platform-dependent feature.
6064 S_ISENFMT($mode) S_ISWHT($mode)
6066 See your native chmod(2) and stat(2) documentation for more details
6067 about the C<S_*> constants. To get status info for a symbolic link
6068 instead of the target file behind the link, use the C<lstat> function.
6073 =item state TYPE EXPR
6075 =item state EXPR : ATTRS
6077 =item state TYPE EXPR : ATTRS
6079 C<state> declares a lexically scoped variable, just like C<my> does.
6080 However, those variables will never be reinitialized, contrary to
6081 lexical variables that are reinitialized each time their enclosing block
6084 C<state> variables are only enabled when the C<feature 'state'> pragma is
6085 in effect. See L<feature>.
6092 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6093 doing many pattern matches on the string before it is next modified.
6094 This may or may not save time, depending on the nature and number of
6095 patterns you are searching on, and on the distribution of character
6096 frequencies in the string to be searched--you probably want to compare
6097 run times with and without it to see which runs faster. Those loops
6098 that scan for many short constant strings (including the constant
6099 parts of more complex patterns) will benefit most. You may have only
6100 one C<study> active at a time--if you study a different scalar the first
6101 is "unstudied". (The way C<study> works is this: a linked list of every
6102 character in the string to be searched is made, so we know, for
6103 example, where all the C<'k'> characters are. From each search string,
6104 the rarest character is selected, based on some static frequency tables
6105 constructed from some C programs and English text. Only those places
6106 that contain this "rarest" character are examined.)
6108 For example, here is a loop that inserts index producing entries
6109 before any line containing a certain pattern:
6113 print ".IX foo\n" if /\bfoo\b/;
6114 print ".IX bar\n" if /\bbar\b/;
6115 print ".IX blurfl\n" if /\bblurfl\b/;
6120 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6121 will be looked at, because C<f> is rarer than C<o>. In general, this is
6122 a big win except in pathological cases. The only question is whether
6123 it saves you more time than it took to build the linked list in the
6126 Note that if you have to look for strings that you don't know till
6127 runtime, you can build an entire loop as a string and C<eval> that to
6128 avoid recompiling all your patterns all the time. Together with
6129 undefining C<$/> to input entire files as one record, this can be very
6130 fast, often faster than specialized programs like fgrep(1). The following
6131 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6132 out the names of those files that contain a match:
6134 $search = 'while (<>) { study;';
6135 foreach $word (@words) {
6136 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6141 eval $search; # this screams
6142 $/ = "\n"; # put back to normal input delimiter
6143 foreach $file (sort keys(%seen)) {
6147 =item sub NAME BLOCK
6150 =item sub NAME (PROTO) BLOCK
6152 =item sub NAME : ATTRS BLOCK
6154 =item sub NAME (PROTO) : ATTRS BLOCK
6156 This is subroutine definition, not a real function I<per se>.
6157 Without a BLOCK it's just a forward declaration. Without a NAME,
6158 it's an anonymous function declaration, and does actually return
6159 a value: the CODE ref of the closure you just created.
6161 See L<perlsub> and L<perlref> for details about subroutines and
6162 references, and L<attributes> and L<Attribute::Handlers> for more
6163 information about attributes.
6165 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6166 X<substr> X<substring> X<mid> X<left> X<right>
6168 =item substr EXPR,OFFSET,LENGTH
6170 =item substr EXPR,OFFSET
6172 Extracts a substring out of EXPR and returns it. First character is at
6173 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6174 If OFFSET is negative (or more precisely, less than C<$[>), starts
6175 that far from the end of the string. If LENGTH is omitted, returns
6176 everything to the end of the string. If LENGTH is negative, leaves that
6177 many characters off the end of the string.
6179 my $s = "The black cat climbed the green tree";
6180 my $color = substr $s, 4, 5; # black
6181 my $middle = substr $s, 4, -11; # black cat climbed the
6182 my $end = substr $s, 14; # climbed the green tree
6183 my $tail = substr $s, -4; # tree
6184 my $z = substr $s, -4, 2; # tr
6186 You can use the substr() function as an lvalue, in which case EXPR
6187 must itself be an lvalue. If you assign something shorter than LENGTH,
6188 the string will shrink, and if you assign something longer than LENGTH,
6189 the string will grow to accommodate it. To keep the string the same
6190 length you may need to pad or chop your value using C<sprintf>.
6192 If OFFSET and LENGTH specify a substring that is partly outside the
6193 string, only the part within the string is returned. If the substring
6194 is beyond either end of the string, substr() returns the undefined
6195 value and produces a warning. When used as an lvalue, specifying a
6196 substring that is entirely outside the string is a fatal error.
6197 Here's an example showing the behavior for boundary cases:
6200 substr($name, 4) = 'dy'; # $name is now 'freddy'
6201 my $null = substr $name, 6, 2; # returns '' (no warning)
6202 my $oops = substr $name, 7; # returns undef, with warning
6203 substr($name, 7) = 'gap'; # fatal error
6205 An alternative to using substr() as an lvalue is to specify the
6206 replacement string as the 4th argument. This allows you to replace
6207 parts of the EXPR and return what was there before in one operation,
6208 just as you can with splice().
6210 my $s = "The black cat climbed the green tree";
6211 my $z = substr $s, 14, 7, "jumped from"; # climbed
6212 # $s is now "The black cat jumped from the green tree"
6214 Note that the lvalue returned by the 3-arg version of substr() acts as
6215 a 'magic bullet'; each time it is assigned to, it remembers which part
6216 of the original string is being modified; for example:
6219 for (substr($x,1,2)) {
6220 $_ = 'a'; print $x,"\n"; # prints 1a4
6221 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6223 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6226 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6229 =item symlink OLDFILE,NEWFILE
6230 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6232 Creates a new filename symbolically linked to the old filename.
6233 Returns C<1> for success, C<0> otherwise. On systems that don't support
6234 symbolic links, produces a fatal error at run time. To check for that,
6237 $symlink_exists = eval { symlink("",""); 1 };
6239 =item syscall NUMBER, LIST
6240 X<syscall> X<system call>
6242 Calls the system call specified as the first element of the list,
6243 passing the remaining elements as arguments to the system call. If
6244 unimplemented, produces a fatal error. The arguments are interpreted
6245 as follows: if a given argument is numeric, the argument is passed as
6246 an int. If not, the pointer to the string value is passed. You are
6247 responsible to make sure a string is pre-extended long enough to
6248 receive any result that might be written into a string. You can't use a
6249 string literal (or other read-only string) as an argument to C<syscall>
6250 because Perl has to assume that any string pointer might be written
6252 integer arguments are not literals and have never been interpreted in a
6253 numeric context, you may need to add C<0> to them to force them to look
6254 like numbers. This emulates the C<syswrite> function (or vice versa):
6256 require 'syscall.ph'; # may need to run h2ph
6258 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6260 Note that Perl supports passing of up to only 14 arguments to your system call,
6261 which in practice should usually suffice.
6263 Syscall returns whatever value returned by the system call it calls.
6264 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6265 Note that some system calls can legitimately return C<-1>. The proper
6266 way to handle such calls is to assign C<$!=0;> before the call and
6267 check the value of C<$!> if syscall returns C<-1>.
6269 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6270 number of the read end of the pipe it creates. There is no way
6271 to retrieve the file number of the other end. You can avoid this
6272 problem by using C<pipe> instead.
6274 =item sysopen FILEHANDLE,FILENAME,MODE
6277 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6279 Opens the file whose filename is given by FILENAME, and associates it
6280 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6281 the name of the real filehandle wanted. This function calls the
6282 underlying operating system's C<open> function with the parameters
6283 FILENAME, MODE, PERMS.
6285 The possible values and flag bits of the MODE parameter are
6286 system-dependent; they are available via the standard module C<Fcntl>.
6287 See the documentation of your operating system's C<open> to see which
6288 values and flag bits are available. You may combine several flags
6289 using the C<|>-operator.
6291 Some of the most common values are C<O_RDONLY> for opening the file in
6292 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6293 and C<O_RDWR> for opening the file in read-write mode.
6294 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6296 For historical reasons, some values work on almost every system
6297 supported by perl: zero means read-only, one means write-only, and two
6298 means read/write. We know that these values do I<not> work under
6299 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6300 use them in new code.
6302 If the file named by FILENAME does not exist and the C<open> call creates
6303 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6304 PERMS specifies the permissions of the newly created file. If you omit
6305 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6306 These permission values need to be in octal, and are modified by your
6307 process's current C<umask>.
6310 In many systems the C<O_EXCL> flag is available for opening files in
6311 exclusive mode. This is B<not> locking: exclusiveness means here that
6312 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6313 on network filesystems, and has no effect unless the C<O_CREAT> flag
6314 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6315 being opened if it is a symbolic link. It does not protect against
6316 symbolic links in the file's path.
6319 Sometimes you may want to truncate an already-existing file. This
6320 can be done using the C<O_TRUNC> flag. The behavior of
6321 C<O_TRUNC> with C<O_RDONLY> is undefined.
6324 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6325 that takes away the user's option to have a more permissive umask.
6326 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6329 Note that C<sysopen> depends on the fdopen() C library function.
6330 On many UNIX systems, fdopen() is known to fail when file descriptors
6331 exceed a certain value, typically 255. If you need more file
6332 descriptors than that, consider rebuilding Perl to use the C<sfio>
6333 library, or perhaps using the POSIX::open() function.
6335 See L<perlopentut> for a kinder, gentler explanation of opening files.
6337 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6340 =item sysread FILEHANDLE,SCALAR,LENGTH
6342 Attempts to read LENGTH bytes of data into variable SCALAR from the
6343 specified FILEHANDLE, using the system call read(2). It bypasses
6344 buffered IO, so mixing this with other kinds of reads, C<print>,
6345 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6346 perlio or stdio layers usually buffers data. Returns the number of
6347 bytes actually read, C<0> at end of file, or undef if there was an
6348 error (in the latter case C<$!> is also set). SCALAR will be grown or
6349 shrunk so that the last byte actually read is the last byte of the
6350 scalar after the read.
6352 An OFFSET may be specified to place the read data at some place in the
6353 string other than the beginning. A negative OFFSET specifies
6354 placement at that many characters counting backwards from the end of
6355 the string. A positive OFFSET greater than the length of SCALAR
6356 results in the string being padded to the required size with C<"\0">
6357 bytes before the result of the read is appended.
6359 There is no syseof() function, which is ok, since eof() doesn't work
6360 very well on device files (like ttys) anyway. Use sysread() and check
6361 for a return value for 0 to decide whether you're done.
6363 Note that if the filehandle has been marked as C<:utf8> Unicode
6364 characters are read instead of bytes (the LENGTH, OFFSET, and the
6365 return value of sysread() are in Unicode characters).
6366 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6367 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6369 =item sysseek FILEHANDLE,POSITION,WHENCE
6372 Sets FILEHANDLE's system position in bytes using the system call
6373 lseek(2). FILEHANDLE may be an expression whose value gives the name
6374 of the filehandle. The values for WHENCE are C<0> to set the new
6375 position to POSITION, C<1> to set the it to the current position plus
6376 POSITION, and C<2> to set it to EOF plus POSITION (typically
6379 Note the I<in bytes>: even if the filehandle has been set to operate
6380 on characters (for example by using the C<:encoding(utf8)> I/O layer),
6381 tell() will return byte offsets, not character offsets (because
6382 implementing that would render sysseek() very slow).
6384 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6385 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6386 C<seek>, C<tell>, or C<eof> may cause confusion.
6388 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6389 and C<SEEK_END> (start of the file, current position, end of the file)
6390 from the Fcntl module. Use of the constants is also more portable
6391 than relying on 0, 1, and 2. For example to define a "systell" function:
6393 use Fcntl 'SEEK_CUR';
6394 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6396 Returns the new position, or the undefined value on failure. A position
6397 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6398 true on success and false on failure, yet you can still easily determine
6404 =item system PROGRAM LIST
6406 Does exactly the same thing as C<exec LIST>, except that a fork is
6407 done first, and the parent process waits for the child process to
6408 complete. Note that argument processing varies depending on the
6409 number of arguments. If there is more than one argument in LIST,
6410 or if LIST is an array with more than one value, starts the program
6411 given by the first element of the list with arguments given by the
6412 rest of the list. If there is only one scalar argument, the argument
6413 is checked for shell metacharacters, and if there are any, the
6414 entire argument is passed to the system's command shell for parsing
6415 (this is C</bin/sh -c> on Unix platforms, but varies on other
6416 platforms). If there are no shell metacharacters in the argument,
6417 it is split into words and passed directly to C<execvp>, which is
6420 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6421 output before any operation that may do a fork, but this may not be
6422 supported on some platforms (see L<perlport>). To be safe, you may need
6423 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6424 of C<IO::Handle> on any open handles.
6426 The return value is the exit status of the program as returned by the
6427 C<wait> call. To get the actual exit value, shift right by eight (see
6428 below). See also L</exec>. This is I<not> what you want to use to capture
6429 the output from a command, for that you should use merely backticks or
6430 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6431 indicates a failure to start the program or an error of the wait(2) system
6432 call (inspect $! for the reason).
6434 Like C<exec>, C<system> allows you to lie to a program about its name if
6435 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6437 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6438 C<system>, if you expect your program to terminate on receipt of these
6439 signals you will need to arrange to do so yourself based on the return
6442 @args = ("command", "arg1", "arg2");
6444 or die "system @args failed: $?"
6446 You can check all the failure possibilities by inspecting
6450 print "failed to execute: $!\n";
6453 printf "child died with signal %d, %s coredump\n",
6454 ($? & 127), ($? & 128) ? 'with' : 'without';
6457 printf "child exited with value %d\n", $? >> 8;
6460 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6461 with the W*() calls of the POSIX extension.
6463 When the arguments get executed via the system shell, results
6464 and return codes will be subject to its quirks and capabilities.
6465 See L<perlop/"`STRING`"> and L</exec> for details.
6467 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6470 =item syswrite FILEHANDLE,SCALAR,LENGTH
6472 =item syswrite FILEHANDLE,SCALAR
6474 Attempts to write LENGTH bytes of data from variable SCALAR to the
6475 specified FILEHANDLE, using the system call write(2). If LENGTH is
6476 not specified, writes whole SCALAR. It bypasses buffered IO, so
6477 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6478 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6479 stdio layers usually buffers data. Returns the number of bytes
6480 actually written, or C<undef> if there was an error (in this case the
6481 errno variable C<$!> is also set). If the LENGTH is greater than the
6482 available data in the SCALAR after the OFFSET, only as much data as is
6483 available will be written.
6485 An OFFSET may be specified to write the data from some part of the
6486 string other than the beginning. A negative OFFSET specifies writing
6487 that many characters counting backwards from the end of the string.
6488 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6490 Note that if the filehandle has been marked as C<:utf8>, Unicode
6491 characters are written instead of bytes (the LENGTH, OFFSET, and the
6492 return value of syswrite() are in UTF-8 encoded Unicode characters).
6493 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6494 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6496 =item tell FILEHANDLE
6501 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6502 error. FILEHANDLE may be an expression whose value gives the name of
6503 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6506 Note the I<in bytes>: even if the filehandle has been set to
6507 operate on characters (for example by using the C<:encoding(utf8)> open
6508 layer), tell() will return byte offsets, not character offsets (because
6509 that would render seek() and tell() rather slow).
6511 The return value of tell() for the standard streams like the STDIN
6512 depends on the operating system: it may return -1 or something else.
6513 tell() on pipes, fifos, and sockets usually returns -1.
6515 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6517 Do not use tell() (or other buffered I/O operations) on a file handle
6518 that has been manipulated by sysread(), syswrite() or sysseek().
6519 Those functions ignore the buffering, while tell() does not.
6521 =item telldir DIRHANDLE
6524 Returns the current position of the C<readdir> routines on DIRHANDLE.
6525 Value may be given to C<seekdir> to access a particular location in a
6526 directory. C<telldir> has the same caveats about possible directory
6527 compaction as the corresponding system library routine.
6529 =item tie VARIABLE,CLASSNAME,LIST
6532 This function binds a variable to a package class that will provide the
6533 implementation for the variable. VARIABLE is the name of the variable
6534 to be enchanted. CLASSNAME is the name of a class implementing objects
6535 of correct type. Any additional arguments are passed to the C<new>
6536 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6537 or C<TIEHASH>). Typically these are arguments such as might be passed
6538 to the C<dbm_open()> function of C. The object returned by the C<new>
6539 method is also returned by the C<tie> function, which would be useful
6540 if you want to access other methods in CLASSNAME.
6542 Note that functions such as C<keys> and C<values> may return huge lists
6543 when used on large objects, like DBM files. You may prefer to use the
6544 C<each> function to iterate over such. Example:
6546 # print out history file offsets
6548 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6549 while (($key,$val) = each %HIST) {
6550 print $key, ' = ', unpack('L',$val), "\n";
6554 A class implementing a hash should have the following methods:
6556 TIEHASH classname, LIST
6558 STORE this, key, value
6563 NEXTKEY this, lastkey
6568 A class implementing an ordinary array should have the following methods:
6570 TIEARRAY classname, LIST
6572 STORE this, key, value
6574 STORESIZE this, count
6580 SPLICE this, offset, length, LIST
6585 A class implementing a file handle should have the following methods:
6587 TIEHANDLE classname, LIST
6588 READ this, scalar, length, offset
6591 WRITE this, scalar, length, offset
6593 PRINTF this, format, LIST
6597 SEEK this, position, whence
6599 OPEN this, mode, LIST
6604 A class implementing a scalar should have the following methods:
6606 TIESCALAR classname, LIST
6612 Not all methods indicated above need be implemented. See L<perltie>,
6613 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6615 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6616 for you--you need to do that explicitly yourself. See L<DB_File>
6617 or the F<Config> module for interesting C<tie> implementations.
6619 For further details see L<perltie>, L<"tied VARIABLE">.
6624 Returns a reference to the object underlying VARIABLE (the same value
6625 that was originally returned by the C<tie> call that bound the variable
6626 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6632 Returns the number of non-leap seconds since whatever time the system
6633 considers to be the epoch, suitable for feeding to C<gmtime> and
6634 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6635 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6636 1904 in the current local time zone for its epoch.
6638 For measuring time in better granularity than one second,
6639 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6640 Perl 5.8 part of the standard distribution), or if you have
6641 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6642 See L<perlfaq8> for details.
6644 For date and time processing look at the many related modules on CPAN.
6645 For a comprehensive date and time representation look at the
6651 Returns a four-element list giving the user and system times, in
6652 seconds, for this process and the children of this process.
6654 ($user,$system,$cuser,$csystem) = times;
6656 In scalar context, C<times> returns C<$user>.
6658 Note that times for children are included only after they terminate.
6662 The transliteration operator. Same as C<y///>. See L<perlop>.
6664 =item truncate FILEHANDLE,LENGTH
6667 =item truncate EXPR,LENGTH
6669 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6670 specified length. Produces a fatal error if truncate isn't implemented
6671 on your system. Returns true if successful, the undefined value
6674 The behavior is undefined if LENGTH is greater than the length of the
6677 The position in the file of FILEHANDLE is left unchanged. You may want to
6678 call L<seek> before writing to the file.
6681 X<uc> X<uppercase> X<toupper>
6685 Returns an uppercased version of EXPR. This is the internal function
6686 implementing the C<\U> escape in double-quoted strings. Respects
6687 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6688 and L<perlunicode> for more details about locale and Unicode support.
6689 It does not attempt to do titlecase mapping on initial letters. See
6690 C<ucfirst> for that.
6692 If EXPR is omitted, uses C<$_>.
6695 X<ucfirst> X<uppercase>
6699 Returns the value of EXPR with the first character in uppercase
6700 (titlecase in Unicode). This is the internal function implementing
6701 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6702 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6703 for more details about locale and Unicode support.
6705 If EXPR is omitted, uses C<$_>.
6712 Sets the umask for the process to EXPR and returns the previous value.
6713 If EXPR is omitted, merely returns the current umask.
6715 The Unix permission C<rwxr-x---> is represented as three sets of three
6716 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6717 and isn't one of the digits). The C<umask> value is such a number
6718 representing disabled permissions bits. The permission (or "mode")
6719 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6720 even if you tell C<sysopen> to create a file with permissions C<0777>,
6721 if your umask is C<0022> then the file will actually be created with
6722 permissions C<0755>. If your C<umask> were C<0027> (group can't
6723 write; others can't read, write, or execute), then passing
6724 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6727 Here's some advice: supply a creation mode of C<0666> for regular
6728 files (in C<sysopen>) and one of C<0777> for directories (in
6729 C<mkdir>) and executable files. This gives users the freedom of
6730 choice: if they want protected files, they might choose process umasks
6731 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6732 Programs should rarely if ever make policy decisions better left to
6733 the user. The exception to this is when writing files that should be
6734 kept private: mail files, web browser cookies, I<.rhosts> files, and
6737 If umask(2) is not implemented on your system and you are trying to
6738 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6739 fatal error at run time. If umask(2) is not implemented and you are
6740 not trying to restrict access for yourself, returns C<undef>.
6742 Remember that a umask is a number, usually given in octal; it is I<not> a
6743 string of octal digits. See also L</oct>, if all you have is a string.
6746 X<undef> X<undefine>
6750 Undefines the value of EXPR, which must be an lvalue. Use only on a
6751 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6752 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6753 will probably not do what you expect on most predefined variables or
6754 DBM list values, so don't do that; see L<delete>.) Always returns the
6755 undefined value. You can omit the EXPR, in which case nothing is
6756 undefined, but you still get an undefined value that you could, for
6757 instance, return from a subroutine, assign to a variable or pass as a
6758 parameter. Examples:
6761 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6765 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6766 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6767 select undef, undef, undef, 0.25;
6768 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6770 Note that this is a unary operator, not a list operator.
6773 X<unlink> X<delete> X<remove> X<rm> X<del>
6777 Deletes a list of files. Returns the number of files successfully
6780 $cnt = unlink 'a', 'b', 'c';
6784 Note: C<unlink> will not attempt to delete directories unless you are superuser
6785 and the B<-U> flag is supplied to Perl. Even if these conditions are
6786 met, be warned that unlinking a directory can inflict damage on your
6787 filesystem. Finally, using C<unlink> on directories is not supported on
6788 many operating systems. Use C<rmdir> instead.
6790 If LIST is omitted, uses C<$_>.
6792 =item unpack TEMPLATE,EXPR
6795 =item unpack TEMPLATE
6797 C<unpack> does the reverse of C<pack>: it takes a string
6798 and expands it out into a list of values.
6799 (In scalar context, it returns merely the first value produced.)
6801 If EXPR is omitted, unpacks the C<$_> string.
6803 The string is broken into chunks described by the TEMPLATE. Each chunk
6804 is converted separately to a value. Typically, either the string is a result
6805 of C<pack>, or the characters of the string represent a C structure of some
6808 The TEMPLATE has the same format as in the C<pack> function.
6809 Here's a subroutine that does substring:
6812 my($what,$where,$howmuch) = @_;
6813 unpack("x$where a$howmuch", $what);
6818 sub ordinal { unpack("W",$_[0]); } # same as ord()
6820 In addition to fields allowed in pack(), you may prefix a field with
6821 a %<number> to indicate that
6822 you want a <number>-bit checksum of the items instead of the items
6823 themselves. Default is a 16-bit checksum. Checksum is calculated by
6824 summing numeric values of expanded values (for string fields the sum of
6825 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6827 For example, the following
6828 computes the same number as the System V sum program:
6832 unpack("%32W*",<>) % 65535;
6835 The following efficiently counts the number of set bits in a bit vector:
6837 $setbits = unpack("%32b*", $selectmask);
6839 The C<p> and C<P> formats should be used with care. Since Perl
6840 has no way of checking whether the value passed to C<unpack()>
6841 corresponds to a valid memory location, passing a pointer value that's
6842 not known to be valid is likely to have disastrous consequences.
6844 If there are more pack codes or if the repeat count of a field or a group
6845 is larger than what the remainder of the input string allows, the result
6846 is not well defined: in some cases, the repeat count is decreased, or
6847 C<unpack()> will produce null strings or zeroes, or terminate with an
6848 error. If the input string is longer than one described by the TEMPLATE,
6849 the rest is ignored.
6851 See L</pack> for more examples and notes.
6853 =item untie VARIABLE
6856 Breaks the binding between a variable and a package. (See C<tie>.)
6857 Has no effect if the variable is not tied.
6859 =item unshift ARRAY,LIST
6862 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6863 depending on how you look at it. Prepends list to the front of the
6864 array, and returns the new number of elements in the array.
6866 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6868 Note the LIST is prepended whole, not one element at a time, so the
6869 prepended elements stay in the same order. Use C<reverse> to do the
6872 =item use Module VERSION LIST
6873 X<use> X<module> X<import>
6875 =item use Module VERSION
6877 =item use Module LIST
6883 Imports some semantics into the current package from the named module,
6884 generally by aliasing certain subroutine or variable names into your
6885 package. It is exactly equivalent to
6887 BEGIN { require Module; Module->import( LIST ); }
6889 except that Module I<must> be a bareword.
6891 In the peculiar C<use VERSION> form, VERSION may be either a numeric
6892 argument such as 5.006, which will be compared to C<$]>, or a literal of
6893 the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A
6894 fatal error is produced if VERSION is greater than the version of the
6895 current Perl interpreter; Perl will not attempt to parse the rest of the
6896 file. Compare with L</require>, which can do a similar check at run time.
6897 Symmetrically, C<no VERSION> allows you to specify that you want a version
6898 of perl older than the specified one.
6900 Specifying VERSION as a literal of the form v5.6.1 should generally be
6901 avoided, because it leads to misleading error messages under earlier
6902 versions of Perl (that is, prior to 5.6.0) that do not support this
6903 syntax. The equivalent numeric version should be used instead.
6905 use v5.6.1; # compile time version check
6907 use 5.006_001; # ditto; preferred for backwards compatibility
6909 This is often useful if you need to check the current Perl version before
6910 C<use>ing library modules that won't work with older versions of Perl.
6911 (We try not to do this more than we have to.)
6913 Also, if the specified perl version is greater than or equal to 5.9.5,
6914 C<use VERSION> will also load the C<feature> pragma and enable all
6915 features available in the requested version. See L<feature>.
6917 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6918 C<require> makes sure the module is loaded into memory if it hasn't been
6919 yet. The C<import> is not a builtin--it's just an ordinary static method
6920 call into the C<Module> package to tell the module to import the list of
6921 features back into the current package. The module can implement its
6922 C<import> method any way it likes, though most modules just choose to
6923 derive their C<import> method via inheritance from the C<Exporter> class that
6924 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6925 method can be found then the call is skipped, even if there is an AUTOLOAD
6928 If you do not want to call the package's C<import> method (for instance,
6929 to stop your namespace from being altered), explicitly supply the empty list:
6933 That is exactly equivalent to
6935 BEGIN { require Module }
6937 If the VERSION argument is present between Module and LIST, then the
6938 C<use> will call the VERSION method in class Module with the given
6939 version as an argument. The default VERSION method, inherited from
6940 the UNIVERSAL class, croaks if the given version is larger than the
6941 value of the variable C<$Module::VERSION>.
6943 Again, there is a distinction between omitting LIST (C<import> called
6944 with no arguments) and an explicit empty LIST C<()> (C<import> not
6945 called). Note that there is no comma after VERSION!
6947 Because this is a wide-open interface, pragmas (compiler directives)
6948 are also implemented this way. Currently implemented pragmas are:
6953 use sigtrap qw(SEGV BUS);
6954 use strict qw(subs vars refs);
6955 use subs qw(afunc blurfl);
6956 use warnings qw(all);
6957 use sort qw(stable _quicksort _mergesort);
6959 Some of these pseudo-modules import semantics into the current
6960 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6961 which import symbols into the current package (which are effective
6962 through the end of the file).
6964 There's a corresponding C<no> command that unimports meanings imported
6965 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6966 It behaves exactly as C<import> does with respect to VERSION, an
6967 omitted LIST, empty LIST, or no unimport method being found.
6973 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6974 for the C<-M> and C<-m> command-line options to perl that give C<use>
6975 functionality from the command-line.
6980 Changes the access and modification times on each file of a list of
6981 files. The first two elements of the list must be the NUMERICAL access
6982 and modification times, in that order. Returns the number of files
6983 successfully changed. The inode change time of each file is set
6984 to the current time. For example, this code has the same effect as the
6985 Unix touch(1) command when the files I<already exist> and belong to
6986 the user running the program:
6989 $atime = $mtime = time;
6990 utime $atime, $mtime, @ARGV;
6992 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6993 the utime(2) function in the C library will be called with a null second
6994 argument. On most systems, this will set the file's access and
6995 modification times to the current time (i.e. equivalent to the example
6996 above) and will even work on other users' files where you have write
6999 utime undef, undef, @ARGV;
7001 Under NFS this will use the time of the NFS server, not the time of
7002 the local machine. If there is a time synchronization problem, the
7003 NFS server and local machine will have different times. The Unix
7004 touch(1) command will in fact normally use this form instead of the
7005 one shown in the first example.
7007 Note that only passing one of the first two elements as C<undef> will
7008 be equivalent of passing it as 0 and will not have the same effect as
7009 described when they are both C<undef>. This case will also trigger an
7010 uninitialized warning.
7012 On systems that support futimes, you might pass file handles among the
7013 files. On systems that don't support futimes, passing file handles
7014 produces a fatal error at run time. The file handles must be passed
7015 as globs or references to be recognized. Barewords are considered
7023 Returns a list consisting of all the values of the named hash, or the values
7024 of an array. (In a scalar context, returns the number of values.)
7026 The values are returned in an apparently random order. The actual
7027 random order is subject to change in future versions of perl, but it
7028 is guaranteed to be the same order as either the C<keys> or C<each>
7029 function would produce on the same (unmodified) hash. Since Perl
7030 5.8.1 the ordering is different even between different runs of Perl
7031 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7033 As a side effect, calling values() resets the HASH or ARRAY's internal
7035 see L</each>. (In particular, calling values() in void context resets
7036 the iterator with no other overhead. Apart from resetting the iterator,
7037 C<values @array> in list context is no different to plain C<@array>.
7038 We recommend that you use void context C<keys @array> for this, but reasoned
7039 that it taking C<values @array> out would require more documentation than
7043 Note that the values are not copied, which means modifying them will
7044 modify the contents of the hash:
7046 for (values %hash) { s/foo/bar/g } # modifies %hash values
7047 for (@hash{keys %hash}) { s/foo/bar/g } # same
7049 See also C<keys>, C<each>, and C<sort>.
7051 =item vec EXPR,OFFSET,BITS
7052 X<vec> X<bit> X<bit vector>
7054 Treats the string in EXPR as a bit vector made up of elements of
7055 width BITS, and returns the value of the element specified by OFFSET
7056 as an unsigned integer. BITS therefore specifies the number of bits
7057 that are reserved for each element in the bit vector. This must
7058 be a power of two from 1 to 32 (or 64, if your platform supports
7061 If BITS is 8, "elements" coincide with bytes of the input string.
7063 If BITS is 16 or more, bytes of the input string are grouped into chunks
7064 of size BITS/8, and each group is converted to a number as with
7065 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7066 for BITS==64). See L<"pack"> for details.
7068 If bits is 4 or less, the string is broken into bytes, then the bits
7069 of each byte are broken into 8/BITS groups. Bits of a byte are
7070 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7071 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7072 breaking the single input byte C<chr(0x36)> into two groups gives a list
7073 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7075 C<vec> may also be assigned to, in which case parentheses are needed
7076 to give the expression the correct precedence as in
7078 vec($image, $max_x * $x + $y, 8) = 3;
7080 If the selected element is outside the string, the value 0 is returned.
7081 If an element off the end of the string is written to, Perl will first
7082 extend the string with sufficiently many zero bytes. It is an error
7083 to try to write off the beginning of the string (i.e. negative OFFSET).
7085 If the string happens to be encoded as UTF-8 internally (and thus has
7086 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7087 internal byte string, not the conceptual character string, even if you
7088 only have characters with values less than 256.
7090 Strings created with C<vec> can also be manipulated with the logical
7091 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7092 vector operation is desired when both operands are strings.
7093 See L<perlop/"Bitwise String Operators">.
7095 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7096 The comments show the string after each step. Note that this code works
7097 in the same way on big-endian or little-endian machines.
7100 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7102 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7103 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7105 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7106 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7107 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7108 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7109 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7110 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7112 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7113 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7114 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7117 To transform a bit vector into a string or list of 0's and 1's, use these:
7119 $bits = unpack("b*", $vector);
7120 @bits = split(//, unpack("b*", $vector));
7122 If you know the exact length in bits, it can be used in place of the C<*>.
7124 Here is an example to illustrate how the bits actually fall in place:
7130 unpack("V",$_) 01234567890123456789012345678901
7131 ------------------------------------------------------------------
7136 for ($shift=0; $shift < $width; ++$shift) {
7137 for ($off=0; $off < 32/$width; ++$off) {
7138 $str = pack("B*", "0"x32);
7139 $bits = (1<<$shift);
7140 vec($str, $off, $width) = $bits;
7141 $res = unpack("b*",$str);
7142 $val = unpack("V", $str);
7149 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7150 $off, $width, $bits, $val, $res
7154 Regardless of the machine architecture on which it is run, the above
7155 example should print the following table:
7158 unpack("V",$_) 01234567890123456789012345678901
7159 ------------------------------------------------------------------
7160 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7161 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7162 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7163 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7164 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7165 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7166 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7167 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7168 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7169 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7170 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7171 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7172 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7173 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7174 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7175 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7176 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7177 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7178 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7179 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7180 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7181 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7182 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7183 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7184 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7185 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7186 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7187 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7188 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7189 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7190 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7191 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7192 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7193 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7194 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7195 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7196 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7197 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7198 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7199 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7200 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7201 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7202 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7203 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7204 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7205 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7206 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7207 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7208 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7209 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7210 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7211 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7212 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7213 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7214 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7215 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7216 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7217 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7218 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7219 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7220 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7221 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7222 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7223 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7224 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7225 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7226 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7227 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7228 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7229 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7230 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7231 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7232 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7233 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7234 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7235 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7236 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7237 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7238 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7239 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7240 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7241 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7242 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7243 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7244 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7245 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7246 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7247 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7248 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7249 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7250 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7251 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7252 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7253 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7254 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7255 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7256 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7257 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7258 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7259 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7260 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7261 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7262 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7263 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7264 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7265 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7266 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7267 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7268 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7269 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7270 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7271 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7272 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7273 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7274 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7275 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7276 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7277 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7278 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7279 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7280 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7281 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7282 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7283 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7284 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7285 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7286 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7287 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7292 Behaves like the wait(2) system call on your system: it waits for a child
7293 process to terminate and returns the pid of the deceased process, or
7294 C<-1> if there are no child processes. The status is returned in C<$?>
7295 and C<{^CHILD_ERROR_NATIVE}>.
7296 Note that a return value of C<-1> could mean that child processes are
7297 being automatically reaped, as described in L<perlipc>.
7299 =item waitpid PID,FLAGS
7302 Waits for a particular child process to terminate and returns the pid of
7303 the deceased process, or C<-1> if there is no such child process. On some
7304 systems, a value of 0 indicates that there are processes still running.
7305 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7307 use POSIX ":sys_wait_h";
7310 $kid = waitpid(-1, WNOHANG);
7313 then you can do a non-blocking wait for all pending zombie processes.
7314 Non-blocking wait is available on machines supporting either the
7315 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7316 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7317 system call by remembering the status values of processes that have
7318 exited but have not been harvested by the Perl script yet.)
7320 Note that on some systems, a return value of C<-1> could mean that child
7321 processes are being automatically reaped. See L<perlipc> for details,
7322 and for other examples.
7325 X<wantarray> X<context>
7327 Returns true if the context of the currently executing subroutine or
7328 C<eval> is looking for a list value. Returns false if the context is
7329 looking for a scalar. Returns the undefined value if the context is
7330 looking for no value (void context).
7332 return unless defined wantarray; # don't bother doing more
7333 my @a = complex_calculation();
7334 return wantarray ? @a : "@a";
7336 C<wantarray()>'s result is unspecified in the top level of a file,
7337 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7338 in a C<DESTROY> method.
7340 This function should have been named wantlist() instead.
7343 X<warn> X<warning> X<STDERR>
7345 Prints the value of LIST to STDERR. If the last element of LIST does
7346 not end in a newline, it appends the same file/line number text as C<die>
7349 If LIST is empty and C<$@> already contains a value (typically from a
7350 previous eval) that value is used after appending C<"\t...caught">
7351 to C<$@>. This is useful for staying almost, but not entirely similar to
7354 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7356 No message is printed if there is a C<$SIG{__WARN__}> handler
7357 installed. It is the handler's responsibility to deal with the message
7358 as it sees fit (like, for instance, converting it into a C<die>). Most
7359 handlers must therefore make arrangements to actually display the
7360 warnings that they are not prepared to deal with, by calling C<warn>
7361 again in the handler. Note that this is quite safe and will not
7362 produce an endless loop, since C<__WARN__> hooks are not called from
7365 You will find this behavior is slightly different from that of
7366 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7367 instead call C<die> again to change it).
7369 Using a C<__WARN__> handler provides a powerful way to silence all
7370 warnings (even the so-called mandatory ones). An example:
7372 # wipe out *all* compile-time warnings
7373 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7375 my $foo = 20; # no warning about duplicate my $foo,
7376 # but hey, you asked for it!
7377 # no compile-time or run-time warnings before here
7380 # run-time warnings enabled after here
7381 warn "\$foo is alive and $foo!"; # does show up
7383 See L<perlvar> for details on setting C<%SIG> entries, and for more
7384 examples. See the Carp module for other kinds of warnings using its
7385 carp() and cluck() functions.
7387 =item write FILEHANDLE
7394 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7395 using the format associated with that file. By default the format for
7396 a file is the one having the same name as the filehandle, but the
7397 format for the current output channel (see the C<select> function) may be set
7398 explicitly by assigning the name of the format to the C<$~> variable.
7400 Top of form processing is handled automatically: if there is
7401 insufficient room on the current page for the formatted record, the
7402 page is advanced by writing a form feed, a special top-of-page format
7403 is used to format the new page header, and then the record is written.
7404 By default the top-of-page format is the name of the filehandle with
7405 "_TOP" appended, but it may be dynamically set to the format of your
7406 choice by assigning the name to the C<$^> variable while the filehandle is
7407 selected. The number of lines remaining on the current page is in
7408 variable C<$->, which can be set to C<0> to force a new page.
7410 If FILEHANDLE is unspecified, output goes to the current default output
7411 channel, which starts out as STDOUT but may be changed by the
7412 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7413 is evaluated and the resulting string is used to look up the name of
7414 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7416 Note that write is I<not> the opposite of C<read>. Unfortunately.
7420 The transliteration operator. Same as C<tr///>. See L<perlop>.