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($_[0]) }
495 Note that atan2(0, 0) is not well-defined.
497 =item bind SOCKET,NAME
500 Binds a network address to a socket, just as the bind system call
501 does. Returns true if it succeeded, false otherwise. NAME should be a
502 packed address of the appropriate type for the socket. See the examples in
503 L<perlipc/"Sockets: Client/Server Communication">.
505 =item binmode FILEHANDLE, LAYER
506 X<binmode> X<binary> X<text> X<DOS> X<Windows>
508 =item binmode FILEHANDLE
510 Arranges for FILEHANDLE to be read or written in "binary" or "text"
511 mode on systems where the run-time libraries distinguish between
512 binary and text files. If FILEHANDLE is an expression, the value is
513 taken as the name of the filehandle. Returns true on success,
514 otherwise it returns C<undef> and sets C<$!> (errno).
516 On some systems (in general, DOS and Windows-based systems) binmode()
517 is necessary when you're not working with a text file. For the sake
518 of portability it is a good idea to always use it when appropriate,
519 and to never use it when it isn't appropriate. Also, people can
520 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
522 In other words: regardless of platform, use binmode() on binary data,
523 like for example images.
525 If LAYER is present it is a single string, but may contain multiple
526 directives. The directives alter the behaviour of the file handle.
527 When LAYER is present using binmode on text file makes sense.
529 If LAYER is omitted or specified as C<:raw> the filehandle is made
530 suitable for passing binary data. This includes turning off possible CRLF
531 translation and marking it as bytes (as opposed to Unicode characters).
532 Note that, despite what may be implied in I<"Programming Perl"> (the
533 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
534 -- other layers which would affect the binary nature of the stream are
535 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
536 PERLIO environment variable.
538 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
539 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
540 establish default I/O layers. See L<open>.
542 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
543 in "Programming Perl, 3rd Edition". However, since the publishing of this
544 book, by many known as "Camel III", the consensus of the naming of this
545 functionality has moved from "discipline" to "layer". All documentation
546 of this version of Perl therefore refers to "layers" rather than to
547 "disciplines". Now back to the regularly scheduled documentation...>
549 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
550 C<:utf8> just marks the data as UTF-8 without further checking,
551 while C<:encoding(utf8)> checks the data for actually being valid
552 UTF-8. More details can be found in L<PerlIO::encoding>.
554 In general, binmode() should be called after open() but before any I/O
555 is done on the filehandle. Calling binmode() will normally flush any
556 pending buffered output data (and perhaps pending input data) on the
557 handle. An exception to this is the C<:encoding> layer that
558 changes the default character encoding of the handle, see L<open>.
559 The C<:encoding> layer sometimes needs to be called in
560 mid-stream, and it doesn't flush the stream. The C<:encoding>
561 also implicitly pushes on top of itself the C<:utf8> layer because
562 internally Perl will operate on UTF-8 encoded Unicode characters.
564 The operating system, device drivers, C libraries, and Perl run-time
565 system all work together to let the programmer treat a single
566 character (C<\n>) as the line terminator, irrespective of the external
567 representation. On many operating systems, the native text file
568 representation matches the internal representation, but on some
569 platforms the external representation of C<\n> is made up of more than
572 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
573 character to end each line in the external representation of text (even
574 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
575 on Unix and most VMS files). In other systems like OS/2, DOS and the
576 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
577 but what's stored in text files are the two characters C<\cM\cJ>. That
578 means that, if you don't use binmode() on these systems, C<\cM\cJ>
579 sequences on disk will be converted to C<\n> on input, and any C<\n> in
580 your program will be converted back to C<\cM\cJ> on output. This is what
581 you want for text files, but it can be disastrous for binary files.
583 Another consequence of using binmode() (on some systems) is that
584 special end-of-file markers will be seen as part of the data stream.
585 For systems from the Microsoft family this means that if your binary
586 data contains C<\cZ>, the I/O subsystem will regard it as the end of
587 the file, unless you use binmode().
589 binmode() is not only important for readline() and print() operations,
590 but also when using read(), seek(), sysread(), syswrite() and tell()
591 (see L<perlport> for more details). See the C<$/> and C<$\> variables
592 in L<perlvar> for how to manually set your input and output
593 line-termination sequences.
595 =item bless REF,CLASSNAME
600 This function tells the thingy referenced by REF that it is now an object
601 in the CLASSNAME package. If CLASSNAME is omitted, the current package
602 is used. Because a C<bless> is often the last thing in a constructor,
603 it returns the reference for convenience. Always use the two-argument
604 version if a derived class might inherit the function doing the blessing.
605 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
608 Consider always blessing objects in CLASSNAMEs that are mixed case.
609 Namespaces with all lowercase names are considered reserved for
610 Perl pragmata. Builtin types have all uppercase names. To prevent
611 confusion, you may wish to avoid such package names as well. Make sure
612 that CLASSNAME is a true value.
614 See L<perlmod/"Perl Modules">.
618 Break out of a C<given()> block.
620 This keyword is enabled by the "switch" feature: see L<feature>
621 for more information.
624 X<caller> X<call stack> X<stack> X<stack trace>
628 Returns the context of the current subroutine call. In scalar context,
629 returns the caller's package name if there is a caller, that is, if
630 we're in a subroutine or C<eval> or C<require>, and the undefined value
631 otherwise. In list context, returns
634 ($package, $filename, $line) = caller;
636 With EXPR, it returns some extra information that the debugger uses to
637 print a stack trace. The value of EXPR indicates how many call frames
638 to go back before the current one.
641 ($package, $filename, $line, $subroutine, $hasargs,
644 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
647 Here $subroutine may be C<(eval)> if the frame is not a subroutine
648 call, but an C<eval>. In such a case additional elements $evaltext and
649 C<$is_require> are set: C<$is_require> is true if the frame is created by a
650 C<require> or C<use> statement, $evaltext contains the text of the
651 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
652 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
653 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
654 frame.) $subroutine may also be C<(unknown)> if this particular
655 subroutine happens to have been deleted from the symbol table.
656 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
657 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
658 compiled with. The C<$hints> and C<$bitmask> values are subject to change
659 between versions of Perl, and are not meant for external use.
661 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
662 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
663 of this hash, as they are the actual values stored in the optree.
665 Furthermore, when called from within the DB package, caller returns more
666 detailed information: it sets the list variable C<@DB::args> to be the
667 arguments with which the subroutine was invoked.
669 Be aware that the optimizer might have optimized call frames away before
670 C<caller> had a chance to get the information. That means that C<caller(N)>
671 might not return information about the call frame you expect it do, for
672 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
673 previous time C<caller> was called.
680 =item chdir FILEHANDLE
682 =item chdir DIRHANDLE
686 Changes the working directory to EXPR, if possible. If EXPR is omitted,
687 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
688 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
689 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
690 neither is set, C<chdir> does nothing. It returns true upon success,
691 false otherwise. See the example under C<die>.
693 On systems that support fchdir, you might pass a file handle or
694 directory handle as argument. On systems that don't support fchdir,
695 passing handles produces a fatal error at run time.
698 X<chmod> X<permission> X<mode>
700 Changes the permissions of a list of files. The first element of the
701 list must be the numerical mode, which should probably be an octal
702 number, and which definitely should I<not> be a string of octal digits:
703 C<0644> is okay, C<'0644'> is not. Returns the number of files
704 successfully changed. See also L</oct>, if all you have is a string.
706 $cnt = chmod 0755, 'foo', 'bar';
707 chmod 0755, @executables;
708 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
710 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
711 $mode = 0644; chmod $mode, 'foo'; # this is best
713 On systems that support fchmod, you might pass file handles among the
714 files. On systems that don't support fchmod, passing file handles
715 produces a fatal error at run time. The file handles must be passed
716 as globs or references to be recognized. Barewords are considered
719 open(my $fh, "<", "foo");
720 my $perm = (stat $fh)[2] & 07777;
721 chmod($perm | 0600, $fh);
723 You can also import the symbolic C<S_I*> constants from the Fcntl
728 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
729 # This is identical to the chmod 0755 of the above example.
732 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
738 This safer version of L</chop> removes any trailing string
739 that corresponds to the current value of C<$/> (also known as
740 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
741 number of characters removed from all its arguments. It's often used to
742 remove the newline from the end of an input record when you're worried
743 that the final record may be missing its newline. When in paragraph
744 mode (C<$/ = "">), it removes all trailing newlines from the string.
745 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
746 a reference to an integer or the like, see L<perlvar>) chomp() won't
748 If VARIABLE is omitted, it chomps C<$_>. Example:
751 chomp; # avoid \n on last field
756 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
758 You can actually chomp anything that's an lvalue, including an assignment:
761 chomp($answer = <STDIN>);
763 If you chomp a list, each element is chomped, and the total number of
764 characters removed is returned.
766 Note that parentheses are necessary when you're chomping anything
767 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
768 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
769 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
770 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
780 Chops off the last character of a string and returns the character
781 chopped. It is much more efficient than C<s/.$//s> because it neither
782 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
783 If VARIABLE is a hash, it chops the hash's values, but not its keys.
785 You can actually chop anything that's an lvalue, including an assignment.
787 If you chop a list, each element is chopped. Only the value of the
788 last C<chop> is returned.
790 Note that C<chop> returns the last character. To return all but the last
791 character, use C<substr($string, 0, -1)>.
796 X<chown> X<owner> X<user> X<group>
798 Changes the owner (and group) of a list of files. The first two
799 elements of the list must be the I<numeric> uid and gid, in that
800 order. A value of -1 in either position is interpreted by most
801 systems to leave that value unchanged. Returns the number of files
802 successfully changed.
804 $cnt = chown $uid, $gid, 'foo', 'bar';
805 chown $uid, $gid, @filenames;
807 On systems that support fchown, you might pass file handles among the
808 files. On systems that don't support fchown, passing file handles
809 produces a fatal error at run time. The file handles must be passed
810 as globs or references to be recognized. Barewords are considered
813 Here's an example that looks up nonnumeric uids in the passwd file:
816 chomp($user = <STDIN>);
818 chomp($pattern = <STDIN>);
820 ($login,$pass,$uid,$gid) = getpwnam($user)
821 or die "$user not in passwd file";
823 @ary = glob($pattern); # expand filenames
824 chown $uid, $gid, @ary;
826 On most systems, you are not allowed to change the ownership of the
827 file unless you're the superuser, although you should be able to change
828 the group to any of your secondary groups. On insecure systems, these
829 restrictions may be relaxed, but this is not a portable assumption.
830 On POSIX systems, you can detect this condition this way:
832 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
833 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
836 X<chr> X<character> X<ASCII> X<Unicode>
840 Returns the character represented by that NUMBER in the character set.
841 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
842 chr(0x263a) is a Unicode smiley face.
844 Negative values give the Unicode replacement character (chr(0xfffd)),
845 except under the L<bytes> pragma, where low eight bits of the value
846 (truncated to an integer) are used.
848 If NUMBER is omitted, uses C<$_>.
850 For the reverse, use L</ord>.
852 Note that characters from 128 to 255 (inclusive) are by default
853 internally not encoded as UTF-8 for backward compatibility reasons.
855 See L<perlunicode> for more about Unicode.
857 =item chroot FILENAME
862 This function works like the system call by the same name: it makes the
863 named directory the new root directory for all further pathnames that
864 begin with a C</> by your process and all its children. (It doesn't
865 change your current working directory, which is unaffected.) For security
866 reasons, this call is restricted to the superuser. If FILENAME is
867 omitted, does a C<chroot> to C<$_>.
869 =item close FILEHANDLE
874 Closes the file or pipe associated with the file handle, flushes the IO
875 buffers, and closes the system file descriptor. Returns true if those
876 operations have succeeded and if no error was reported by any PerlIO
877 layer. Closes the currently selected filehandle if the argument is
880 You don't have to close FILEHANDLE if you are immediately going to do
881 another C<open> on it, because C<open> will close it for you. (See
882 C<open>.) However, an explicit C<close> on an input file resets the line
883 counter (C<$.>), while the implicit close done by C<open> does not.
885 If the file handle came from a piped open, C<close> will additionally
886 return false if one of the other system calls involved fails, or if the
887 program exits with non-zero status. (If the only problem was that the
888 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
889 also waits for the process executing on the pipe to complete, in case you
890 want to look at the output of the pipe afterwards, and
891 implicitly puts the exit status value of that command into C<$?> and
892 C<${^CHILD_ERROR_NATIVE}>.
894 Prematurely closing the read end of a pipe (i.e. before the process
895 writing to it at the other end has closed it) will result in a
896 SIGPIPE being delivered to the writer. If the other end can't
897 handle that, be sure to read all the data before closing the pipe.
901 open(OUTPUT, '|sort >foo') # pipe to sort
902 or die "Can't start sort: $!";
903 #... # print stuff to output
904 close OUTPUT # wait for sort to finish
905 or warn $! ? "Error closing sort pipe: $!"
906 : "Exit status $? from sort";
907 open(INPUT, 'foo') # get sort's results
908 or die "Can't open 'foo' for input: $!";
910 FILEHANDLE may be an expression whose value can be used as an indirect
911 filehandle, usually the real filehandle name.
913 =item closedir DIRHANDLE
916 Closes a directory opened by C<opendir> and returns the success of that
919 =item connect SOCKET,NAME
922 Attempts to connect to a remote socket, just as the connect system call
923 does. Returns true if it succeeded, false otherwise. NAME should be a
924 packed address of the appropriate type for the socket. See the examples in
925 L<perlipc/"Sockets: Client/Server Communication">.
932 C<continue> is actually a flow control statement rather than a function. If
933 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
934 C<foreach>), it is always executed just before the conditional is about to
935 be evaluated again, just like the third part of a C<for> loop in C. Thus
936 it can be used to increment a loop variable, even when the loop has been
937 continued via the C<next> statement (which is similar to the C C<continue>
940 C<last>, C<next>, or C<redo> may appear within a C<continue>
941 block. C<last> and C<redo> will behave as if they had been executed within
942 the main block. So will C<next>, but since it will execute a C<continue>
943 block, it may be more entertaining.
946 ### redo always comes here
949 ### next always comes here
951 # then back the top to re-check EXPR
953 ### last always comes here
955 Omitting the C<continue> section is semantically equivalent to using an
956 empty one, logically enough. In that case, C<next> goes directly back
957 to check the condition at the top of the loop.
959 If the "switch" feature is enabled, C<continue> is also a
960 function that will break out of the current C<when> or C<default>
961 block, and fall through to the next case. See L<feature> and
962 L<perlsyn/"Switch statements"> for more information.
966 X<cos> X<cosine> X<acos> X<arccosine>
970 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
971 takes cosine of C<$_>.
973 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
974 function, or use this relation:
976 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
978 =item crypt PLAINTEXT,SALT
979 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
980 X<decrypt> X<cryptography> X<passwd> X<encrypt>
982 Creates a digest string exactly like the crypt(3) function in the C
983 library (assuming that you actually have a version there that has not
984 been extirpated as a potential munitions).
986 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
987 into a short string, called a digest, which is returned. The same
988 PLAINTEXT and SALT will always return the same string, but there is no
989 (known) way to get the original PLAINTEXT from the hash. Small
990 changes in the PLAINTEXT or SALT will result in large changes in the
993 There is no decrypt function. This function isn't all that useful for
994 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
995 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
996 primarily used to check if two pieces of text are the same without
997 having to transmit or store the text itself. An example is checking
998 if a correct password is given. The digest of the password is stored,
999 not the password itself. The user types in a password that is
1000 crypt()'d with the same salt as the stored digest. If the two digests
1001 match the password is correct.
1003 When verifying an existing digest string you should use the digest as
1004 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1005 to create the digest is visible as part of the digest. This ensures
1006 crypt() will hash the new string with the same salt as the digest.
1007 This allows your code to work with the standard L<crypt|/crypt> and
1008 with more exotic implementations. In other words, do not assume
1009 anything about the returned string itself, or how many bytes in the
1012 Traditionally the result is a string of 13 bytes: two first bytes of
1013 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1014 the first eight bytes of the digest string mattered, but alternative
1015 hashing schemes (like MD5), higher level security schemes (like C2),
1016 and implementations on non-UNIX platforms may produce different
1019 When choosing a new salt create a random two character string whose
1020 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1021 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1022 characters is just a recommendation; the characters allowed in
1023 the salt depend solely on your system's crypt library, and Perl can't
1024 restrict what salts C<crypt()> accepts.
1026 Here's an example that makes sure that whoever runs this program knows
1029 $pwd = (getpwuid($<))[1];
1031 system "stty -echo";
1033 chomp($word = <STDIN>);
1037 if (crypt($word, $pwd) ne $pwd) {
1043 Of course, typing in your own password to whoever asks you
1046 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1047 of data, not least of all because you can't get the information
1048 back. Look at the L<Digest> module for more robust algorithms.
1050 If using crypt() on a Unicode string (which I<potentially> has
1051 characters with codepoints above 255), Perl tries to make sense
1052 of the situation by trying to downgrade (a copy of the string)
1053 the string back to an eight-bit byte string before calling crypt()
1054 (on that copy). If that works, good. If not, crypt() dies with
1055 C<Wide character in crypt>.
1060 [This function has been largely superseded by the C<untie> function.]
1062 Breaks the binding between a DBM file and a hash.
1064 =item dbmopen HASH,DBNAME,MASK
1065 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1067 [This function has been largely superseded by the C<tie> function.]
1069 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1070 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1071 argument is I<not> a filehandle, even though it looks like one). DBNAME
1072 is the name of the database (without the F<.dir> or F<.pag> extension if
1073 any). If the database does not exist, it is created with protection
1074 specified by MASK (as modified by the C<umask>). If your system supports
1075 only the older DBM functions, you may perform only one C<dbmopen> in your
1076 program. In older versions of Perl, if your system had neither DBM nor
1077 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1080 If you don't have write access to the DBM file, you can only read hash
1081 variables, not set them. If you want to test whether you can write,
1082 either use file tests or try setting a dummy hash entry inside an C<eval>,
1083 which will trap the error.
1085 Note that functions such as C<keys> and C<values> may return huge lists
1086 when used on large DBM files. You may prefer to use the C<each>
1087 function to iterate over large DBM files. Example:
1089 # print out history file offsets
1090 dbmopen(%HIST,'/usr/lib/news/history',0666);
1091 while (($key,$val) = each %HIST) {
1092 print $key, ' = ', unpack('L',$val), "\n";
1096 See also L<AnyDBM_File> for a more general description of the pros and
1097 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1098 rich implementation.
1100 You can control which DBM library you use by loading that library
1101 before you call dbmopen():
1104 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1105 or die "Can't open netscape history file: $!";
1108 X<defined> X<undef> X<undefined>
1112 Returns a Boolean value telling whether EXPR has a value other than
1113 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1116 Many operations return C<undef> to indicate failure, end of file,
1117 system error, uninitialized variable, and other exceptional
1118 conditions. This function allows you to distinguish C<undef> from
1119 other values. (A simple Boolean test will not distinguish among
1120 C<undef>, zero, the empty string, and C<"0">, which are all equally
1121 false.) Note that since C<undef> is a valid scalar, its presence
1122 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1123 returns C<undef> when its argument is an empty array, I<or> when the
1124 element to return happens to be C<undef>.
1126 You may also use C<defined(&func)> to check whether subroutine C<&func>
1127 has ever been defined. The return value is unaffected by any forward
1128 declarations of C<&func>. Note that a subroutine which is not defined
1129 may still be callable: its package may have an C<AUTOLOAD> method that
1130 makes it spring into existence the first time that it is called -- see
1133 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1134 used to report whether memory for that aggregate has ever been
1135 allocated. This behavior may disappear in future versions of Perl.
1136 You should instead use a simple test for size:
1138 if (@an_array) { print "has array elements\n" }
1139 if (%a_hash) { print "has hash members\n" }
1141 When used on a hash element, it tells you whether the value is defined,
1142 not whether the key exists in the hash. Use L</exists> for the latter
1147 print if defined $switch{'D'};
1148 print "$val\n" while defined($val = pop(@ary));
1149 die "Can't readlink $sym: $!"
1150 unless defined($value = readlink $sym);
1151 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1152 $debugging = 0 unless defined $debugging;
1154 Note: Many folks tend to overuse C<defined>, and then are surprised to
1155 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1156 defined values. For example, if you say
1160 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1161 matched "nothing". It didn't really fail to match anything. Rather, it
1162 matched something that happened to be zero characters long. This is all
1163 very above-board and honest. When a function returns an undefined value,
1164 it's an admission that it couldn't give you an honest answer. So you
1165 should use C<defined> only when you're questioning the integrity of what
1166 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1169 See also L</undef>, L</exists>, L</ref>.
1174 Given an expression that specifies a hash element, array element, hash slice,
1175 or array slice, deletes the specified element(s) from the hash or array.
1176 In the case of an array, if the array elements happen to be at the end,
1177 the size of the array will shrink to the highest element that tests
1178 true for exists() (or 0 if no such element exists).
1180 Returns a list with the same number of elements as the number of elements
1181 for which deletion was attempted. Each element of that list consists of
1182 either the value of the element deleted, or the undefined value. In scalar
1183 context, this means that you get the value of the last element deleted (or
1184 the undefined value if that element did not exist).
1186 %hash = (foo => 11, bar => 22, baz => 33);
1187 $scalar = delete $hash{foo}; # $scalar is 11
1188 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1189 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1191 Deleting from C<%ENV> modifies the environment. Deleting from
1192 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1193 from a C<tie>d hash or array may not necessarily return anything.
1195 Deleting an array element effectively returns that position of the array
1196 to its initial, uninitialized state. Subsequently testing for the same
1197 element with exists() will return false. Also, deleting array elements
1198 in the middle of an array will not shift the index of the elements
1199 after them down. Use splice() for that. See L</exists>.
1201 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1203 foreach $key (keys %HASH) {
1207 foreach $index (0 .. $#ARRAY) {
1208 delete $ARRAY[$index];
1213 delete @HASH{keys %HASH};
1215 delete @ARRAY[0 .. $#ARRAY];
1217 But both of these are slower than just assigning the empty list
1218 or undefining %HASH or @ARRAY:
1220 %HASH = (); # completely empty %HASH
1221 undef %HASH; # forget %HASH ever existed
1223 @ARRAY = (); # completely empty @ARRAY
1224 undef @ARRAY; # forget @ARRAY ever existed
1226 Note that the EXPR can be arbitrarily complicated as long as the final
1227 operation is a hash element, array element, hash slice, or array slice
1230 delete $ref->[$x][$y]{$key};
1231 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1233 delete $ref->[$x][$y][$index];
1234 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1237 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1239 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1240 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1241 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1242 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1243 an C<eval(),> the error message is stuffed into C<$@> and the
1244 C<eval> is terminated with the undefined value. This makes
1245 C<die> the way to raise an exception.
1247 Equivalent examples:
1249 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1250 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1252 If the last element of LIST does not end in a newline, the current
1253 script line number and input line number (if any) are also printed,
1254 and a newline is supplied. Note that the "input line number" (also
1255 known as "chunk") is subject to whatever notion of "line" happens to
1256 be currently in effect, and is also available as the special variable
1257 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1259 Hint: sometimes appending C<", stopped"> to your message will cause it
1260 to make better sense when the string C<"at foo line 123"> is appended.
1261 Suppose you are running script "canasta".
1263 die "/etc/games is no good";
1264 die "/etc/games is no good, stopped";
1266 produce, respectively
1268 /etc/games is no good at canasta line 123.
1269 /etc/games is no good, stopped at canasta line 123.
1271 See also exit(), warn(), and the Carp module.
1273 If LIST is empty and C<$@> already contains a value (typically from a
1274 previous eval) that value is reused after appending C<"\t...propagated">.
1275 This is useful for propagating exceptions:
1278 die unless $@ =~ /Expected exception/;
1280 If LIST is empty and C<$@> contains an object reference that has a
1281 C<PROPAGATE> method, that method will be called with additional file
1282 and line number parameters. The return value replaces the value in
1283 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1286 If C<$@> is empty then the string C<"Died"> is used.
1288 die() can also be called with a reference argument. If this happens to be
1289 trapped within an eval(), $@ contains the reference. This behavior permits
1290 a more elaborate exception handling implementation using objects that
1291 maintain arbitrary state about the nature of the exception. Such a scheme
1292 is sometimes preferable to matching particular string values of $@ using
1293 regular expressions. Because $@ is a global variable, and eval() may be
1294 used within object implementations, care must be taken that analyzing the
1295 error object doesn't replace the reference in the global variable. The
1296 easiest solution is to make a local copy of the reference before doing
1297 other manipulations. Here's an example:
1299 use Scalar::Util 'blessed';
1301 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1302 if (my $ev_err = $@) {
1303 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1304 # handle Some::Module::Exception
1307 # handle all other possible exceptions
1311 Because perl will stringify uncaught exception messages before displaying
1312 them, you may want to overload stringification operations on such custom
1313 exception objects. See L<overload> for details about that.
1315 You can arrange for a callback to be run just before the C<die>
1316 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1317 handler will be called with the error text and can change the error
1318 message, if it sees fit, by calling C<die> again. See
1319 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1320 L<"eval BLOCK"> for some examples. Although this feature was
1321 to be run only right before your program was to exit, this is not
1322 currently the case--the C<$SIG{__DIE__}> hook is currently called
1323 even inside eval()ed blocks/strings! If one wants the hook to do
1324 nothing in such situations, put
1328 as the first line of the handler (see L<perlvar/$^S>). Because
1329 this promotes strange action at a distance, this counterintuitive
1330 behavior may be fixed in a future release.
1335 Not really a function. Returns the value of the last command in the
1336 sequence of commands indicated by BLOCK. When modified by the C<while> or
1337 C<until> loop modifier, executes the BLOCK once before testing the loop
1338 condition. (On other statements the loop modifiers test the conditional
1341 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1342 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1343 See L<perlsyn> for alternative strategies.
1345 =item do SUBROUTINE(LIST)
1348 This form of subroutine call is deprecated. See L<perlsub>.
1353 Uses the value of EXPR as a filename and executes the contents of the
1354 file as a Perl script.
1362 except that it's more efficient and concise, keeps track of the current
1363 filename for error messages, searches the @INC directories, and updates
1364 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1365 variables. It also differs in that code evaluated with C<do FILENAME>
1366 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1367 same, however, in that it does reparse the file every time you call it,
1368 so you probably don't want to do this inside a loop.
1370 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1371 error. If C<do> can read the file but cannot compile it, it
1372 returns undef and sets an error message in C<$@>. If the file is
1373 successfully compiled, C<do> returns the value of the last expression
1376 Note that inclusion of library modules is better done with the
1377 C<use> and C<require> operators, which also do automatic error checking
1378 and raise an exception if there's a problem.
1380 You might like to use C<do> to read in a program configuration
1381 file. Manual error checking can be done this way:
1383 # read in config files: system first, then user
1384 for $file ("/share/prog/defaults.rc",
1385 "$ENV{HOME}/.someprogrc")
1387 unless ($return = do $file) {
1388 warn "couldn't parse $file: $@" if $@;
1389 warn "couldn't do $file: $!" unless defined $return;
1390 warn "couldn't run $file" unless $return;
1395 X<dump> X<core> X<undump>
1399 This function causes an immediate core dump. See also the B<-u>
1400 command-line switch in L<perlrun>, which does the same thing.
1401 Primarily this is so that you can use the B<undump> program (not
1402 supplied) to turn your core dump into an executable binary after
1403 having initialized all your variables at the beginning of the
1404 program. When the new binary is executed it will begin by executing
1405 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1406 Think of it as a goto with an intervening core dump and reincarnation.
1407 If C<LABEL> is omitted, restarts the program from the top.
1409 B<WARNING>: Any files opened at the time of the dump will I<not>
1410 be open any more when the program is reincarnated, with possible
1411 resulting confusion on the part of Perl.
1413 This function is now largely obsolete, mostly because it's very hard to
1414 convert a core file into an executable. That's why you should now invoke
1415 it as C<CORE::dump()>, if you don't want to be warned against a possible
1419 X<each> X<hash, iterator>
1424 When called in list context, returns a 2-element list consisting of the
1425 key and value for the next element of a hash, or the index and value for
1426 the next element of an array, so that you can iterate over it. When called
1427 in scalar context, returns only the key for the next element in the hash
1428 (or the index for an array).
1430 Hash entries are returned in an apparently random order. The actual random
1431 order is subject to change in future versions of perl, but it is
1432 guaranteed to be in the same order as either the C<keys> or C<values>
1433 function would produce on the same (unmodified) hash. Since Perl
1434 5.8.2 the ordering can be different even between different runs of Perl
1435 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1437 When the hash or array is entirely read, a null array is returned in list
1438 context (which when assigned produces a false (C<0>) value), and C<undef> in
1439 scalar context. The next call to C<each> after that will start iterating
1440 again. There is a single iterator for each hash or array, shared by all
1441 C<each>, C<keys>, and C<values> function calls in the program; it can be
1442 reset by reading all the elements from the hash or array, or by evaluating
1443 C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1444 or delete elements of a hash while you're
1445 iterating over it, you may get entries skipped or duplicated, so
1446 don't. Exception: It is always safe to delete the item most recently
1447 returned by C<each()>, which means that the following code will work:
1449 while (($key, $value) = each %hash) {
1451 delete $hash{$key}; # This is safe
1454 The following prints out your environment like the printenv(1) program,
1455 only in a different order:
1457 while (($key,$value) = each %ENV) {
1458 print "$key=$value\n";
1461 See also C<keys>, C<values> and C<sort>.
1463 =item eof FILEHANDLE
1472 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1473 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1474 gives the real filehandle. (Note that this function actually
1475 reads a character and then C<ungetc>s it, so isn't very useful in an
1476 interactive context.) Do not read from a terminal file (or call
1477 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1478 as terminals may lose the end-of-file condition if you do.
1480 An C<eof> without an argument uses the last file read. Using C<eof()>
1481 with empty parentheses is very different. It refers to the pseudo file
1482 formed from the files listed on the command line and accessed via the
1483 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1484 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1485 used will cause C<@ARGV> to be examined to determine if input is
1486 available. Similarly, an C<eof()> after C<< <> >> has returned
1487 end-of-file will assume you are processing another C<@ARGV> list,
1488 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1489 see L<perlop/"I/O Operators">.
1491 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1492 detect the end of each file, C<eof()> will only detect the end of the
1493 last file. Examples:
1495 # reset line numbering on each input file
1497 next if /^\s*#/; # skip comments
1500 close ARGV if eof; # Not eof()!
1503 # insert dashes just before last line of last file
1505 if (eof()) { # check for end of last file
1506 print "--------------\n";
1509 last if eof(); # needed if we're reading from a terminal
1512 Practical hint: you almost never need to use C<eof> in Perl, because the
1513 input operators typically return C<undef> when they run out of data, or if
1517 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1518 X<error, handling> X<exception, handling>
1524 In the first form, the return value of EXPR is parsed and executed as if it
1525 were a little Perl program. The value of the expression (which is itself
1526 determined within scalar context) is first parsed, and if there weren't any
1527 errors, executed in the lexical context of the current Perl program, so
1528 that any variable settings or subroutine and format definitions remain
1529 afterwards. Note that the value is parsed every time the C<eval> executes.
1530 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1531 delay parsing and subsequent execution of the text of EXPR until run time.
1533 In the second form, the code within the BLOCK is parsed only once--at the
1534 same time the code surrounding the C<eval> itself was parsed--and executed
1535 within the context of the current Perl program. This form is typically
1536 used to trap exceptions more efficiently than the first (see below), while
1537 also providing the benefit of checking the code within BLOCK at compile
1540 The final semicolon, if any, may be omitted from the value of EXPR or within
1543 In both forms, the value returned is the value of the last expression
1544 evaluated inside the mini-program; a return statement may be also used, just
1545 as with subroutines. The expression providing the return value is evaluated
1546 in void, scalar, or list context, depending on the context of the C<eval>
1547 itself. See L</wantarray> for more on how the evaluation context can be
1550 If there is a syntax error or runtime error, or a C<die> statement is
1551 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1552 error message. If there was no error, C<$@> is guaranteed to be a null
1553 string. Beware that using C<eval> neither silences perl from printing
1554 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1555 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1556 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1557 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1559 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1560 determining whether a particular feature (such as C<socket> or C<symlink>)
1561 is implemented. It is also Perl's exception trapping mechanism, where
1562 the die operator is used to raise exceptions.
1564 If the code to be executed doesn't vary, you may use the eval-BLOCK
1565 form to trap run-time errors without incurring the penalty of
1566 recompiling each time. The error, if any, is still returned in C<$@>.
1569 # make divide-by-zero nonfatal
1570 eval { $answer = $a / $b; }; warn $@ if $@;
1572 # same thing, but less efficient
1573 eval '$answer = $a / $b'; warn $@ if $@;
1575 # a compile-time error
1576 eval { $answer = }; # WRONG
1579 eval '$answer ='; # sets $@
1581 Using the C<eval{}> form as an exception trap in libraries does have some
1582 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1583 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1584 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1585 as shown in this example:
1587 # a very private exception trap for divide-by-zero
1588 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1591 This is especially significant, given that C<__DIE__> hooks can call
1592 C<die> again, which has the effect of changing their error messages:
1594 # __DIE__ hooks may modify error messages
1596 local $SIG{'__DIE__'} =
1597 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1598 eval { die "foo lives here" };
1599 print $@ if $@; # prints "bar lives here"
1602 Because this promotes action at a distance, this counterintuitive behavior
1603 may be fixed in a future release.
1605 With an C<eval>, you should be especially careful to remember what's
1606 being looked at when:
1612 eval { $x }; # CASE 4
1614 eval "\$$x++"; # CASE 5
1617 Cases 1 and 2 above behave identically: they run the code contained in
1618 the variable $x. (Although case 2 has misleading double quotes making
1619 the reader wonder what else might be happening (nothing is).) Cases 3
1620 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1621 does nothing but return the value of $x. (Case 4 is preferred for
1622 purely visual reasons, but it also has the advantage of compiling at
1623 compile-time instead of at run-time.) Case 5 is a place where
1624 normally you I<would> like to use double quotes, except that in this
1625 particular situation, you can just use symbolic references instead, as
1628 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1629 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1631 Note that as a very special case, an C<eval ''> executed within the C<DB>
1632 package doesn't see the usual surrounding lexical scope, but rather the
1633 scope of the first non-DB piece of code that called it. You don't normally
1634 need to worry about this unless you are writing a Perl debugger.
1639 =item exec PROGRAM LIST
1641 The C<exec> function executes a system command I<and never returns>--
1642 use C<system> instead of C<exec> if you want it to return. It fails and
1643 returns false only if the command does not exist I<and> it is executed
1644 directly instead of via your system's command shell (see below).
1646 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1647 warns you if there is a following statement which isn't C<die>, C<warn>,
1648 or C<exit> (if C<-w> is set - but you always do that). If you
1649 I<really> want to follow an C<exec> with some other statement, you
1650 can use one of these styles to avoid the warning:
1652 exec ('foo') or print STDERR "couldn't exec foo: $!";
1653 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1655 If there is more than one argument in LIST, or if LIST is an array
1656 with more than one value, calls execvp(3) with the arguments in LIST.
1657 If there is only one scalar argument or an array with one element in it,
1658 the argument is checked for shell metacharacters, and if there are any,
1659 the entire argument is passed to the system's command shell for parsing
1660 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1661 If there are no shell metacharacters in the argument, it is split into
1662 words and passed directly to C<execvp>, which is more efficient.
1665 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1666 exec "sort $outfile | uniq";
1668 If you don't really want to execute the first argument, but want to lie
1669 to the program you are executing about its own name, you can specify
1670 the program you actually want to run as an "indirect object" (without a
1671 comma) in front of the LIST. (This always forces interpretation of the
1672 LIST as a multivalued list, even if there is only a single scalar in
1675 $shell = '/bin/csh';
1676 exec $shell '-sh'; # pretend it's a login shell
1680 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1682 When the arguments get executed via the system shell, results will
1683 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1686 Using an indirect object with C<exec> or C<system> is also more
1687 secure. This usage (which also works fine with system()) forces
1688 interpretation of the arguments as a multivalued list, even if the
1689 list had just one argument. That way you're safe from the shell
1690 expanding wildcards or splitting up words with whitespace in them.
1692 @args = ( "echo surprise" );
1694 exec @args; # subject to shell escapes
1696 exec { $args[0] } @args; # safe even with one-arg list
1698 The first version, the one without the indirect object, ran the I<echo>
1699 program, passing it C<"surprise"> an argument. The second version
1700 didn't--it tried to run a program literally called I<"echo surprise">,
1701 didn't find it, and set C<$?> to a non-zero value indicating failure.
1703 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1704 output before the exec, but this may not be supported on some platforms
1705 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1706 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1707 open handles in order to avoid lost output.
1709 Note that C<exec> will not call your C<END> blocks, nor will it call
1710 any C<DESTROY> methods in your objects.
1713 X<exists> X<autovivification>
1715 Given an expression that specifies a hash element or array element,
1716 returns true if the specified element in the hash or array has ever
1717 been initialized, even if the corresponding value is undefined. The
1718 element is not autovivified if it doesn't exist.
1720 print "Exists\n" if exists $hash{$key};
1721 print "Defined\n" if defined $hash{$key};
1722 print "True\n" if $hash{$key};
1724 print "Exists\n" if exists $array[$index];
1725 print "Defined\n" if defined $array[$index];
1726 print "True\n" if $array[$index];
1728 A hash or array element can be true only if it's defined, and defined if
1729 it exists, but the reverse doesn't necessarily hold true.
1731 Given an expression that specifies the name of a subroutine,
1732 returns true if the specified subroutine has ever been declared, even
1733 if it is undefined. Mentioning a subroutine name for exists or defined
1734 does not count as declaring it. Note that a subroutine which does not
1735 exist may still be callable: its package may have an C<AUTOLOAD>
1736 method that makes it spring into existence the first time that it is
1737 called -- see L<perlsub>.
1739 print "Exists\n" if exists &subroutine;
1740 print "Defined\n" if defined &subroutine;
1742 Note that the EXPR can be arbitrarily complicated as long as the final
1743 operation is a hash or array key lookup or subroutine name:
1745 if (exists $ref->{A}->{B}->{$key}) { }
1746 if (exists $hash{A}{B}{$key}) { }
1748 if (exists $ref->{A}->{B}->[$ix]) { }
1749 if (exists $hash{A}{B}[$ix]) { }
1751 if (exists &{$ref->{A}{B}{$key}}) { }
1753 Although the deepest nested array or hash will not spring into existence
1754 just because its existence was tested, any intervening ones will.
1755 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1756 into existence due to the existence test for the $key element above.
1757 This happens anywhere the arrow operator is used, including even:
1760 if (exists $ref->{"Some key"}) { }
1761 print $ref; # prints HASH(0x80d3d5c)
1763 This surprising autovivification in what does not at first--or even
1764 second--glance appear to be an lvalue context may be fixed in a future
1767 Use of a subroutine call, rather than a subroutine name, as an argument
1768 to exists() is an error.
1771 exists &sub(); # Error
1774 X<exit> X<terminate> X<abort>
1778 Evaluates EXPR and exits immediately with that value. Example:
1781 exit 0 if $ans =~ /^[Xx]/;
1783 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1784 universally recognized values for EXPR are C<0> for success and C<1>
1785 for error; other values are subject to interpretation depending on the
1786 environment in which the Perl program is running. For example, exiting
1787 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1788 the mailer to return the item undelivered, but that's not true everywhere.
1790 Don't use C<exit> to abort a subroutine if there's any chance that
1791 someone might want to trap whatever error happened. Use C<die> instead,
1792 which can be trapped by an C<eval>.
1794 The exit() function does not always exit immediately. It calls any
1795 defined C<END> routines first, but these C<END> routines may not
1796 themselves abort the exit. Likewise any object destructors that need to
1797 be called are called before the real exit. If this is a problem, you
1798 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1799 See L<perlmod> for details.
1802 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1806 Returns I<e> (the natural logarithm base) to the power of EXPR.
1807 If EXPR is omitted, gives C<exp($_)>.
1809 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1812 Implements the fcntl(2) function. You'll probably have to say
1816 first to get the correct constant definitions. Argument processing and
1817 value return works just like C<ioctl> below.
1821 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1822 or die "can't fcntl F_GETFL: $!";
1824 You don't have to check for C<defined> on the return from C<fcntl>.
1825 Like C<ioctl>, it maps a C<0> return from the system call into
1826 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1827 in numeric context. It is also exempt from the normal B<-w> warnings
1828 on improper numeric conversions.
1830 Note that C<fcntl> will produce a fatal error if used on a machine that
1831 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1832 manpage to learn what functions are available on your system.
1834 Here's an example of setting a filehandle named C<REMOTE> to be
1835 non-blocking at the system level. You'll have to negotiate C<$|>
1836 on your own, though.
1838 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1840 $flags = fcntl(REMOTE, F_GETFL, 0)
1841 or die "Can't get flags for the socket: $!\n";
1843 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1844 or die "Can't set flags for the socket: $!\n";
1846 =item fileno FILEHANDLE
1849 Returns the file descriptor for a filehandle, or undefined if the
1850 filehandle is not open. This is mainly useful for constructing
1851 bitmaps for C<select> and low-level POSIX tty-handling operations.
1852 If FILEHANDLE is an expression, the value is taken as an indirect
1853 filehandle, generally its name.
1855 You can use this to find out whether two handles refer to the
1856 same underlying descriptor:
1858 if (fileno(THIS) == fileno(THAT)) {
1859 print "THIS and THAT are dups\n";
1862 (Filehandles connected to memory objects via new features of C<open> may
1863 return undefined even though they are open.)
1866 =item flock FILEHANDLE,OPERATION
1867 X<flock> X<lock> X<locking>
1869 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1870 for success, false on failure. Produces a fatal error if used on a
1871 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1872 C<flock> is Perl's portable file locking interface, although it locks
1873 only entire files, not records.
1875 Two potentially non-obvious but traditional C<flock> semantics are
1876 that it waits indefinitely until the lock is granted, and that its locks
1877 B<merely advisory>. Such discretionary locks are more flexible, but offer
1878 fewer guarantees. This means that programs that do not also use C<flock>
1879 may modify files locked with C<flock>. See L<perlport>,
1880 your port's specific documentation, or your system-specific local manpages
1881 for details. It's best to assume traditional behavior if you're writing
1882 portable programs. (But if you're not, you should as always feel perfectly
1883 free to write for your own system's idiosyncrasies (sometimes called
1884 "features"). Slavish adherence to portability concerns shouldn't get
1885 in the way of your getting your job done.)
1887 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1888 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1889 you can use the symbolic names if you import them from the Fcntl module,
1890 either individually, or as a group using the ':flock' tag. LOCK_SH
1891 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1892 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1893 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1894 waiting for the lock (check the return status to see if you got it).
1896 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1897 before locking or unlocking it.
1899 Note that the emulation built with lockf(3) doesn't provide shared
1900 locks, and it requires that FILEHANDLE be open with write intent. These
1901 are the semantics that lockf(3) implements. Most if not all systems
1902 implement lockf(3) in terms of fcntl(2) locking, though, so the
1903 differing semantics shouldn't bite too many people.
1905 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1906 be open with read intent to use LOCK_SH and requires that it be open
1907 with write intent to use LOCK_EX.
1909 Note also that some versions of C<flock> cannot lock things over the
1910 network; you would need to use the more system-specific C<fcntl> for
1911 that. If you like you can force Perl to ignore your system's flock(2)
1912 function, and so provide its own fcntl(2)-based emulation, by passing
1913 the switch C<-Ud_flock> to the F<Configure> program when you configure
1916 Here's a mailbox appender for BSD systems.
1918 use Fcntl ':flock'; # import LOCK_* constants
1921 flock(MBOX,LOCK_EX);
1922 # and, in case someone appended
1923 # while we were waiting...
1928 flock(MBOX,LOCK_UN);
1931 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
1932 or die "Can't open mailbox: $!";
1935 print $mbox $msg,"\n\n";
1938 On systems that support a real flock(), locks are inherited across fork()
1939 calls, whereas those that must resort to the more capricious fcntl()
1940 function lose the locks, making it harder to write servers.
1942 See also L<DB_File> for other flock() examples.
1945 X<fork> X<child> X<parent>
1947 Does a fork(2) system call to create a new process running the
1948 same program at the same point. It returns the child pid to the
1949 parent process, C<0> to the child process, or C<undef> if the fork is
1950 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1951 are shared, while everything else is copied. On most systems supporting
1952 fork(), great care has gone into making it extremely efficient (for
1953 example, using copy-on-write technology on data pages), making it the
1954 dominant paradigm for multitasking over the last few decades.
1956 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1957 output before forking the child process, but this may not be supported
1958 on some platforms (see L<perlport>). To be safe, you may need to set
1959 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1960 C<IO::Handle> on any open handles in order to avoid duplicate output.
1962 If you C<fork> without ever waiting on your children, you will
1963 accumulate zombies. On some systems, you can avoid this by setting
1964 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1965 forking and reaping moribund children.
1967 Note that if your forked child inherits system file descriptors like
1968 STDIN and STDOUT that are actually connected by a pipe or socket, even
1969 if you exit, then the remote server (such as, say, a CGI script or a
1970 backgrounded job launched from a remote shell) won't think you're done.
1971 You should reopen those to F</dev/null> if it's any issue.
1976 Declare a picture format for use by the C<write> function. For
1980 Test: @<<<<<<<< @||||| @>>>>>
1981 $str, $%, '$' . int($num)
1985 $num = $cost/$quantity;
1989 See L<perlform> for many details and examples.
1991 =item formline PICTURE,LIST
1994 This is an internal function used by C<format>s, though you may call it,
1995 too. It formats (see L<perlform>) a list of values according to the
1996 contents of PICTURE, placing the output into the format output
1997 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1998 Eventually, when a C<write> is done, the contents of
1999 C<$^A> are written to some filehandle. You could also read C<$^A>
2000 and then set C<$^A> back to C<"">. Note that a format typically
2001 does one C<formline> per line of form, but the C<formline> function itself
2002 doesn't care how many newlines are embedded in the PICTURE. This means
2003 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2004 You may therefore need to use multiple formlines to implement a single
2005 record format, just like the format compiler.
2007 Be careful if you put double quotes around the picture, because an C<@>
2008 character may be taken to mean the beginning of an array name.
2009 C<formline> always returns true. See L<perlform> for other examples.
2011 =item getc FILEHANDLE
2012 X<getc> X<getchar> X<character> X<file, read>
2016 Returns the next character from the input file attached to FILEHANDLE,
2017 or the undefined value at end of file, or if there was an error (in
2018 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2019 STDIN. This is not particularly efficient. However, it cannot be
2020 used by itself to fetch single characters without waiting for the user
2021 to hit enter. For that, try something more like:
2024 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2027 system "stty", '-icanon', 'eol', "\001";
2033 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2036 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2040 Determination of whether $BSD_STYLE should be set
2041 is left as an exercise to the reader.
2043 The C<POSIX::getattr> function can do this more portably on
2044 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2045 module from your nearest CPAN site; details on CPAN can be found on
2049 X<getlogin> X<login>
2051 This implements the C library function of the same name, which on most
2052 systems returns the current login from F</etc/utmp>, if any. If null,
2055 $login = getlogin || getpwuid($<) || "Kilroy";
2057 Do not consider C<getlogin> for authentication: it is not as
2058 secure as C<getpwuid>.
2060 =item getpeername SOCKET
2061 X<getpeername> X<peer>
2063 Returns the packed sockaddr address of other end of the SOCKET connection.
2066 $hersockaddr = getpeername(SOCK);
2067 ($port, $iaddr) = sockaddr_in($hersockaddr);
2068 $herhostname = gethostbyaddr($iaddr, AF_INET);
2069 $herstraddr = inet_ntoa($iaddr);
2074 Returns the current process group for the specified PID. Use
2075 a PID of C<0> to get the current process group for the
2076 current process. Will raise an exception if used on a machine that
2077 doesn't implement getpgrp(2). If PID is omitted, returns process
2078 group of current process. Note that the POSIX version of C<getpgrp>
2079 does not accept a PID argument, so only C<PID==0> is truly portable.
2082 X<getppid> X<parent> X<pid>
2084 Returns the process id of the parent process.
2086 Note for Linux users: on Linux, the C functions C<getpid()> and
2087 C<getppid()> return different values from different threads. In order to
2088 be portable, this behavior is not reflected by the perl-level function
2089 C<getppid()>, that returns a consistent value across threads. If you want
2090 to call the underlying C<getppid()>, you may use the CPAN module
2093 =item getpriority WHICH,WHO
2094 X<getpriority> X<priority> X<nice>
2096 Returns the current priority for a process, a process group, or a user.
2097 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2098 machine that doesn't implement getpriority(2).
2101 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2102 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2103 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2104 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2105 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2106 X<endnetent> X<endprotoent> X<endservent>
2110 =item gethostbyname NAME
2112 =item getnetbyname NAME
2114 =item getprotobyname NAME
2120 =item getservbyname NAME,PROTO
2122 =item gethostbyaddr ADDR,ADDRTYPE
2124 =item getnetbyaddr ADDR,ADDRTYPE
2126 =item getprotobynumber NUMBER
2128 =item getservbyport PORT,PROTO
2146 =item sethostent STAYOPEN
2148 =item setnetent STAYOPEN
2150 =item setprotoent STAYOPEN
2152 =item setservent STAYOPEN
2166 These routines perform the same functions as their counterparts in the
2167 system library. In list context, the return values from the
2168 various get routines are as follows:
2170 ($name,$passwd,$uid,$gid,
2171 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2172 ($name,$passwd,$gid,$members) = getgr*
2173 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2174 ($name,$aliases,$addrtype,$net) = getnet*
2175 ($name,$aliases,$proto) = getproto*
2176 ($name,$aliases,$port,$proto) = getserv*
2178 (If the entry doesn't exist you get a null list.)
2180 The exact meaning of the $gcos field varies but it usually contains
2181 the real name of the user (as opposed to the login name) and other
2182 information pertaining to the user. Beware, however, that in many
2183 system users are able to change this information and therefore it
2184 cannot be trusted and therefore the $gcos is tainted (see
2185 L<perlsec>). The $passwd and $shell, user's encrypted password and
2186 login shell, are also tainted, because of the same reason.
2188 In scalar context, you get the name, unless the function was a
2189 lookup by name, in which case you get the other thing, whatever it is.
2190 (If the entry doesn't exist you get the undefined value.) For example:
2192 $uid = getpwnam($name);
2193 $name = getpwuid($num);
2195 $gid = getgrnam($name);
2196 $name = getgrgid($num);
2200 In I<getpw*()> the fields $quota, $comment, and $expire are special
2201 cases in the sense that in many systems they are unsupported. If the
2202 $quota is unsupported, it is an empty scalar. If it is supported, it
2203 usually encodes the disk quota. If the $comment field is unsupported,
2204 it is an empty scalar. If it is supported it usually encodes some
2205 administrative comment about the user. In some systems the $quota
2206 field may be $change or $age, fields that have to do with password
2207 aging. In some systems the $comment field may be $class. The $expire
2208 field, if present, encodes the expiration period of the account or the
2209 password. For the availability and the exact meaning of these fields
2210 in your system, please consult your getpwnam(3) documentation and your
2211 F<pwd.h> file. You can also find out from within Perl what your
2212 $quota and $comment fields mean and whether you have the $expire field
2213 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2214 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2215 files are only supported if your vendor has implemented them in the
2216 intuitive fashion that calling the regular C library routines gets the
2217 shadow versions if you're running under privilege or if there exists
2218 the shadow(3) functions as found in System V (this includes Solaris
2219 and Linux.) Those systems that implement a proprietary shadow password
2220 facility are unlikely to be supported.
2222 The $members value returned by I<getgr*()> is a space separated list of
2223 the login names of the members of the group.
2225 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2226 C, it will be returned to you via C<$?> if the function call fails. The
2227 C<@addrs> value returned by a successful call is a list of the raw
2228 addresses returned by the corresponding system library call. In the
2229 Internet domain, each address is four bytes long and you can unpack it
2230 by saying something like:
2232 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2234 The Socket library makes this slightly easier:
2237 $iaddr = inet_aton("127.1"); # or whatever address
2238 $name = gethostbyaddr($iaddr, AF_INET);
2240 # or going the other way
2241 $straddr = inet_ntoa($iaddr);
2243 In the opposite way, to resolve a hostname to the IP address
2247 $packed_ip = gethostbyname("www.perl.org");
2248 if (defined $packed_ip) {
2249 $ip_address = inet_ntoa($packed_ip);
2252 Make sure <gethostbyname()> is called in SCALAR context and that
2253 its return value is checked for definedness.
2255 If you get tired of remembering which element of the return list
2256 contains which return value, by-name interfaces are provided
2257 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2258 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2259 and C<User::grent>. These override the normal built-ins, supplying
2260 versions that return objects with the appropriate names
2261 for each field. For example:
2265 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2267 Even though it looks like they're the same method calls (uid),
2268 they aren't, because a C<File::stat> object is different from
2269 a C<User::pwent> object.
2271 =item getsockname SOCKET
2274 Returns the packed sockaddr address of this end of the SOCKET connection,
2275 in case you don't know the address because you have several different
2276 IPs that the connection might have come in on.
2279 $mysockaddr = getsockname(SOCK);
2280 ($port, $myaddr) = sockaddr_in($mysockaddr);
2281 printf "Connect to %s [%s]\n",
2282 scalar gethostbyaddr($myaddr, AF_INET),
2285 =item getsockopt SOCKET,LEVEL,OPTNAME
2288 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2289 Options may exist at multiple protocol levels depending on the socket
2290 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2291 C<Socket> module) will exist. To query options at another level the
2292 protocol number of the appropriate protocol controlling the option
2293 should be supplied. For example, to indicate that an option is to be
2294 interpreted by the TCP protocol, LEVEL should be set to the protocol
2295 number of TCP, which you can get using getprotobyname.
2297 The call returns a packed string representing the requested socket option,
2298 or C<undef> if there is an error (the error reason will be in $!). What
2299 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2300 your system documentation for details. A very common case however is that
2301 the option is an integer, in which case the result will be a packed
2302 integer which you can decode using unpack with the C<i> (or C<I>) format.
2304 An example testing if Nagle's algorithm is turned on on a socket:
2306 use Socket qw(:all);
2308 defined(my $tcp = getprotobyname("tcp"))
2309 or die "Could not determine the protocol number for tcp";
2310 # my $tcp = IPPROTO_TCP; # Alternative
2311 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2312 or die "Could not query TCP_NODELAY socket option: $!";
2313 my $nodelay = unpack("I", $packed);
2314 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2318 X<glob> X<wildcard> X<filename, expansion> X<expand>
2322 In list context, returns a (possibly empty) list of filename expansions on
2323 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2324 scalar context, glob iterates through such filename expansions, returning
2325 undef when the list is exhausted. This is the internal function
2326 implementing the C<< <*.c> >> operator, but you can use it directly. If
2327 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2328 more detail in L<perlop/"I/O Operators">.
2330 Beginning with v5.6.0, this operator is implemented using the standard
2331 C<File::Glob> extension. See L<File::Glob> for details.
2334 X<gmtime> X<UTC> X<Greenwich>
2338 Works just like L<localtime> but the returned values are
2339 localized for the standard Greenwich time zone.
2341 Note: when called in list context, $isdst, the last value
2342 returned by gmtime is always C<0>. There is no
2343 Daylight Saving Time in GMT.
2345 See L<perlport/gmtime> for portability concerns.
2348 X<goto> X<jump> X<jmp>
2354 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2355 execution there. It may not be used to go into any construct that
2356 requires initialization, such as a subroutine or a C<foreach> loop. It
2357 also can't be used to go into a construct that is optimized away,
2358 or to get out of a block or subroutine given to C<sort>.
2359 It can be used to go almost anywhere else within the dynamic scope,
2360 including out of subroutines, but it's usually better to use some other
2361 construct such as C<last> or C<die>. The author of Perl has never felt the
2362 need to use this form of C<goto> (in Perl, that is--C is another matter).
2363 (The difference being that C does not offer named loops combined with
2364 loop control. Perl does, and this replaces most structured uses of C<goto>
2365 in other languages.)
2367 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2368 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2369 necessarily recommended if you're optimizing for maintainability:
2371 goto ("FOO", "BAR", "GLARCH")[$i];
2373 The C<goto-&NAME> form is quite different from the other forms of
2374 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2375 doesn't have the stigma associated with other gotos. Instead, it
2376 exits the current subroutine (losing any changes set by local()) and
2377 immediately calls in its place the named subroutine using the current
2378 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2379 load another subroutine and then pretend that the other subroutine had
2380 been called in the first place (except that any modifications to C<@_>
2381 in the current subroutine are propagated to the other subroutine.)
2382 After the C<goto>, not even C<caller> will be able to tell that this
2383 routine was called first.
2385 NAME needn't be the name of a subroutine; it can be a scalar variable
2386 containing a code reference, or a block that evaluates to a code
2389 =item grep BLOCK LIST
2392 =item grep EXPR,LIST
2394 This is similar in spirit to, but not the same as, grep(1) and its
2395 relatives. In particular, it is not limited to using regular expressions.
2397 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2398 C<$_> to each element) and returns the list value consisting of those
2399 elements for which the expression evaluated to true. In scalar
2400 context, returns the number of times the expression was true.
2402 @foo = grep(!/^#/, @bar); # weed out comments
2406 @foo = grep {!/^#/} @bar; # weed out comments
2408 Note that C<$_> is an alias to the list value, so it can be used to
2409 modify the elements of the LIST. While this is useful and supported,
2410 it can cause bizarre results if the elements of LIST are not variables.
2411 Similarly, grep returns aliases into the original list, much as a for
2412 loop's index variable aliases the list elements. That is, modifying an
2413 element of a list returned by grep (for example, in a C<foreach>, C<map>
2414 or another C<grep>) actually modifies the element in the original list.
2415 This is usually something to be avoided when writing clear code.
2417 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2418 been declared with C<my $_>) then, in addition to being locally aliased to
2419 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2420 can't be seen from the outside, avoiding any potential side-effects.
2422 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2425 X<hex> X<hexadecimal>
2429 Interprets EXPR as a hex string and returns the corresponding value.
2430 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2431 L</oct>.) If EXPR is omitted, uses C<$_>.
2433 print hex '0xAf'; # prints '175'
2434 print hex 'aF'; # same
2436 Hex strings may only represent integers. Strings that would cause
2437 integer overflow trigger a warning. Leading whitespace is not stripped,
2438 unlike oct(). To present something as hex, look into L</printf>,
2439 L</sprintf>, or L</unpack>.
2444 There is no builtin C<import> function. It is just an ordinary
2445 method (subroutine) defined (or inherited) by modules that wish to export
2446 names to another module. The C<use> function calls the C<import> method
2447 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2449 =item index STR,SUBSTR,POSITION
2450 X<index> X<indexOf> X<InStr>
2452 =item index STR,SUBSTR
2454 The index function searches for one string within another, but without
2455 the wildcard-like behavior of a full regular-expression pattern match.
2456 It returns the position of the first occurrence of SUBSTR in STR at
2457 or after POSITION. If POSITION is omitted, starts searching from the
2458 beginning of the string. POSITION before the beginning of the string
2459 or after its end is treated as if it were the beginning or the end,
2460 respectively. POSITION and the return value are based at C<0> (or whatever
2461 you've set the C<$[> variable to--but don't do that). If the substring
2462 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2465 X<int> X<integer> X<truncate> X<trunc> X<floor>
2469 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2470 You should not use this function for rounding: one because it truncates
2471 towards C<0>, and two because machine representations of floating point
2472 numbers can sometimes produce counterintuitive results. For example,
2473 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2474 because it's really more like -268.99999999999994315658 instead. Usually,
2475 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2476 functions will serve you better than will int().
2478 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2481 Implements the ioctl(2) function. You'll probably first have to say
2483 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2485 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2486 exist or doesn't have the correct definitions you'll have to roll your
2487 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2488 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2489 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2490 written depending on the FUNCTION--a pointer to the string value of SCALAR
2491 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2492 has no string value but does have a numeric value, that value will be
2493 passed rather than a pointer to the string value. To guarantee this to be
2494 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2495 functions may be needed to manipulate the values of structures used by
2498 The return value of C<ioctl> (and C<fcntl>) is as follows:
2500 if OS returns: then Perl returns:
2502 0 string "0 but true"
2503 anything else that number
2505 Thus Perl returns true on success and false on failure, yet you can
2506 still easily determine the actual value returned by the operating
2509 $retval = ioctl(...) || -1;
2510 printf "System returned %d\n", $retval;
2512 The special string C<"0 but true"> is exempt from B<-w> complaints
2513 about improper numeric conversions.
2515 =item join EXPR,LIST
2518 Joins the separate strings of LIST into a single string with fields
2519 separated by the value of EXPR, and returns that new string. Example:
2521 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2523 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2524 first argument. Compare L</split>.
2531 Returns a list consisting of all the keys of the named hash, or the indices
2532 of an array. (In scalar context, returns the number of keys or indices.)
2534 The keys of a hash are returned in an apparently random order. The actual
2535 random order is subject to change in future versions of perl, but it
2536 is guaranteed to be the same order as either the C<values> or C<each>
2537 function produces (given that the hash has not been modified). Since
2538 Perl 5.8.1 the ordering is different even between different runs of
2539 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2542 As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
2543 (see L</each>). In particular, calling keys() in void context resets
2544 the iterator with no other overhead.
2546 Here is yet another way to print your environment:
2549 @values = values %ENV;
2551 print pop(@keys), '=', pop(@values), "\n";
2554 or how about sorted by key:
2556 foreach $key (sort(keys %ENV)) {
2557 print $key, '=', $ENV{$key}, "\n";
2560 The returned values are copies of the original keys in the hash, so
2561 modifying them will not affect the original hash. Compare L</values>.
2563 To sort a hash by value, you'll need to use a C<sort> function.
2564 Here's a descending numeric sort of a hash by its values:
2566 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2567 printf "%4d %s\n", $hash{$key}, $key;
2570 As an lvalue C<keys> allows you to increase the number of hash buckets
2571 allocated for the given hash. This can gain you a measure of efficiency if
2572 you know the hash is going to get big. (This is similar to pre-extending
2573 an array by assigning a larger number to $#array.) If you say
2577 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2578 in fact, since it rounds up to the next power of two. These
2579 buckets will be retained even if you do C<%hash = ()>, use C<undef
2580 %hash> if you want to free the storage while C<%hash> is still in scope.
2581 You can't shrink the number of buckets allocated for the hash using
2582 C<keys> in this way (but you needn't worry about doing this by accident,
2583 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2586 See also C<each>, C<values> and C<sort>.
2588 =item kill SIGNAL, LIST
2591 Sends a signal to a list of processes. Returns the number of
2592 processes successfully signaled (which is not necessarily the
2593 same as the number actually killed).
2595 $cnt = kill 1, $child1, $child2;
2598 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2599 system call will check whether it's possible to send a signal to it (that
2600 means, to be brief, that the process is owned by the same user, or we are
2601 the super-user). This is a useful way to check that a child process is
2602 alive (even if only as a zombie) and hasn't changed its UID. See
2603 L<perlport> for notes on the portability of this construct.
2605 Unlike in the shell, if SIGNAL is negative, it kills
2606 process groups instead of processes. (On System V, a negative I<PROCESS>
2607 number will also kill process groups, but that's not portable.) That
2608 means you usually want to use positive not negative signals. You may also
2609 use a signal name in quotes.
2611 See L<perlipc/"Signals"> for more details.
2618 The C<last> command is like the C<break> statement in C (as used in
2619 loops); it immediately exits the loop in question. If the LABEL is
2620 omitted, the command refers to the innermost enclosing loop. The
2621 C<continue> block, if any, is not executed:
2623 LINE: while (<STDIN>) {
2624 last LINE if /^$/; # exit when done with header
2628 C<last> cannot be used to exit a block which returns a value such as
2629 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2630 a grep() or map() operation.
2632 Note that a block by itself is semantically identical to a loop
2633 that executes once. Thus C<last> can be used to effect an early
2634 exit out of such a block.
2636 See also L</continue> for an illustration of how C<last>, C<next>, and
2644 Returns a lowercased version of EXPR. This is the internal function
2645 implementing the C<\L> escape in double-quoted strings. Respects
2646 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2647 and L<perlunicode> for more details about locale and Unicode support.
2649 If EXPR is omitted, uses C<$_>.
2652 X<lcfirst> X<lowercase>
2656 Returns the value of EXPR with the first character lowercased. This
2657 is the internal function implementing the C<\l> escape in
2658 double-quoted strings. Respects current LC_CTYPE locale if C<use
2659 locale> in force. See L<perllocale> and L<perlunicode> for more
2660 details about locale and Unicode support.
2662 If EXPR is omitted, uses C<$_>.
2669 Returns the length in I<characters> of the value of EXPR. If EXPR is
2670 omitted, returns length of C<$_>. Note that this cannot be used on
2671 an entire array or hash to find out how many elements these have.
2672 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2674 Note the I<characters>: if the EXPR is in Unicode, you will get the
2675 number of characters, not the number of bytes. To get the length
2676 of the internal string in bytes, use C<bytes::length(EXPR)>, see
2677 L<bytes>. Note that the internal encoding is variable, and the number
2678 of bytes usually meaningless. To get the number of bytes that the
2679 string would have when encoded as UTF-8, use
2680 C<length(Encoding::encode_utf8(EXPR))>.
2682 =item link OLDFILE,NEWFILE
2685 Creates a new filename linked to the old filename. Returns true for
2686 success, false otherwise.
2688 =item listen SOCKET,QUEUESIZE
2691 Does the same thing that the listen system call does. Returns true if
2692 it succeeded, false otherwise. See the example in
2693 L<perlipc/"Sockets: Client/Server Communication">.
2698 You really probably want to be using C<my> instead, because C<local> isn't
2699 what most people think of as "local". See
2700 L<perlsub/"Private Variables via my()"> for details.
2702 A local modifies the listed variables to be local to the enclosing
2703 block, file, or eval. If more than one value is listed, the list must
2704 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2705 for details, including issues with tied arrays and hashes.
2707 =item localtime EXPR
2708 X<localtime> X<ctime>
2712 Converts a time as returned by the time function to a 9-element list
2713 with the time analyzed for the local time zone. Typically used as
2717 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2720 All list elements are numeric, and come straight out of the C `struct
2721 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2722 of the specified time.
2724 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2725 the range C<0..11> with 0 indicating January and 11 indicating December.
2726 This makes it easy to get a month name from a list:
2728 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2729 print "$abbr[$mon] $mday";
2730 # $mon=9, $mday=18 gives "Oct 18"
2732 C<$year> is the number of years since 1900, not just the last two digits
2733 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2734 to get a complete 4-digit year is simply:
2738 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2739 to do that, would you?
2741 To get the last two digits of the year (e.g., '01' in 2001) do:
2743 $year = sprintf("%02d", $year % 100);
2745 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2746 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2747 (or C<0..365> in leap years.)
2749 C<$isdst> is true if the specified time occurs during Daylight Saving
2750 Time, false otherwise.
2752 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2754 In scalar context, C<localtime()> returns the ctime(3) value:
2756 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2758 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2759 instead of local time use the L</gmtime> builtin. See also the
2760 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2761 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2762 and mktime(3) functions.
2764 To get somewhat similar but locale dependent date strings, set up your
2765 locale environment variables appropriately (please see L<perllocale>) and
2768 use POSIX qw(strftime);
2769 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2770 # or for GMT formatted appropriately for your locale:
2771 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2773 Note that the C<%a> and C<%b>, the short forms of the day of the week
2774 and the month of the year, may not necessarily be three characters wide.
2776 See L<perlport/localtime> for portability concerns.
2778 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2779 by-name access mechanism to the gmtime() and localtime() functions,
2782 For a comprehensive date and time representation look at the
2783 L<DateTime> module on CPAN.
2788 This function places an advisory lock on a shared variable, or referenced
2789 object contained in I<THING> until the lock goes out of scope.
2791 lock() is a "weak keyword" : this means that if you've defined a function
2792 by this name (before any calls to it), that function will be called
2793 instead. (However, if you've said C<use threads>, lock() is always a
2794 keyword.) See L<threads>.
2797 X<log> X<logarithm> X<e> X<ln> X<base>
2801 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2802 returns log of C<$_>. To get the log of another base, use basic algebra:
2803 The base-N log of a number is equal to the natural log of that number
2804 divided by the natural log of N. For example:
2808 return log($n)/log(10);
2811 See also L</exp> for the inverse operation.
2818 Does the same thing as the C<stat> function (including setting the
2819 special C<_> filehandle) but stats a symbolic link instead of the file
2820 the symbolic link points to. If symbolic links are unimplemented on
2821 your system, a normal C<stat> is done. For much more detailed
2822 information, please see the documentation for C<stat>.
2824 If EXPR is omitted, stats C<$_>.
2828 The match operator. See L<perlop>.
2830 =item map BLOCK LIST
2835 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2836 C<$_> to each element) and returns the list value composed of the
2837 results of each such evaluation. In scalar context, returns the
2838 total number of elements so generated. Evaluates BLOCK or EXPR in
2839 list context, so each element of LIST may produce zero, one, or
2840 more elements in the returned value.
2842 @chars = map(chr, @nums);
2844 translates a list of numbers to the corresponding characters. And
2846 %hash = map { get_a_key_for($_) => $_ } @array;
2848 is just a funny way to write
2852 $hash{get_a_key_for($_)} = $_;
2855 Note that C<$_> is an alias to the list value, so it can be used to
2856 modify the elements of the LIST. While this is useful and supported,
2857 it can cause bizarre results if the elements of LIST are not variables.
2858 Using a regular C<foreach> loop for this purpose would be clearer in
2859 most cases. See also L</grep> for an array composed of those items of
2860 the original list for which the BLOCK or EXPR evaluates to true.
2862 If C<$_> is lexical in the scope where the C<map> appears (because it has
2863 been declared with C<my $_>), then, in addition to being locally aliased to
2864 the list elements, C<$_> keeps being lexical inside the block; that is, it
2865 can't be seen from the outside, avoiding any potential side-effects.
2867 C<{> starts both hash references and blocks, so C<map { ...> could be either
2868 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2869 ahead for the closing C<}> it has to take a guess at which its dealing with
2870 based what it finds just after the C<{>. Usually it gets it right, but if it
2871 doesn't it won't realize something is wrong until it gets to the C<}> and
2872 encounters the missing (or unexpected) comma. The syntax error will be
2873 reported close to the C<}> but you'll need to change something near the C<{>
2874 such as using a unary C<+> to give perl some help:
2876 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2877 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2878 %hash = map { ("\L$_", 1) } @array # this also works
2879 %hash = map { lc($_), 1 } @array # as does this.
2880 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2882 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2884 or to force an anon hash constructor use C<+{>:
2886 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2888 and you get list of anonymous hashes each with only 1 entry.
2890 =item mkdir FILENAME,MASK
2891 X<mkdir> X<md> X<directory, create>
2893 =item mkdir FILENAME
2897 Creates the directory specified by FILENAME, with permissions
2898 specified by MASK (as modified by C<umask>). If it succeeds it
2899 returns true, otherwise it returns false and sets C<$!> (errno).
2900 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2903 In general, it is better to create directories with permissive MASK,
2904 and let the user modify that with their C<umask>, than it is to supply
2905 a restrictive MASK and give the user no way to be more permissive.
2906 The exceptions to this rule are when the file or directory should be
2907 kept private (mail files, for instance). The perlfunc(1) entry on
2908 C<umask> discusses the choice of MASK in more detail.
2910 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2911 number of trailing slashes. Some operating and filesystems do not get
2912 this right, so Perl automatically removes all trailing slashes to keep
2915 In order to recursively create a directory structure look at
2916 the C<mkpath> function of the L<File::Path> module.
2918 =item msgctl ID,CMD,ARG
2921 Calls the System V IPC function msgctl(2). You'll probably have to say
2925 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2926 then ARG must be a variable that will hold the returned C<msqid_ds>
2927 structure. Returns like C<ioctl>: the undefined value for error,
2928 C<"0 but true"> for zero, or the actual return value otherwise. See also
2929 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2931 =item msgget KEY,FLAGS
2934 Calls the System V IPC function msgget(2). Returns the message queue
2935 id, or the undefined value if there is an error. See also
2936 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2938 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2941 Calls the System V IPC function msgrcv to receive a message from
2942 message queue ID into variable VAR with a maximum message size of
2943 SIZE. Note that when a message is received, the message type as a
2944 native long integer will be the first thing in VAR, followed by the
2945 actual message. This packing may be opened with C<unpack("l! a*")>.
2946 Taints the variable. Returns true if successful, or false if there is
2947 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2948 C<IPC::SysV::Msg> documentation.
2950 =item msgsnd ID,MSG,FLAGS
2953 Calls the System V IPC function msgsnd to send the message MSG to the
2954 message queue ID. MSG must begin with the native long integer message
2955 type, and be followed by the length of the actual message, and finally
2956 the message itself. This kind of packing can be achieved with
2957 C<pack("l! a*", $type, $message)>. Returns true if successful,
2958 or false if there is an error. See also C<IPC::SysV>
2959 and C<IPC::SysV::Msg> documentation.
2966 =item my EXPR : ATTRS
2968 =item my TYPE EXPR : ATTRS
2970 A C<my> declares the listed variables to be local (lexically) to the
2971 enclosing block, file, or C<eval>. If more than one value is listed,
2972 the list must be placed in parentheses.
2974 The exact semantics and interface of TYPE and ATTRS are still
2975 evolving. TYPE is currently bound to the use of C<fields> pragma,
2976 and attributes are handled using the C<attributes> pragma, or starting
2977 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2978 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2979 L<attributes>, and L<Attribute::Handlers>.
2986 The C<next> command is like the C<continue> statement in C; it starts
2987 the next iteration of the loop:
2989 LINE: while (<STDIN>) {
2990 next LINE if /^#/; # discard comments
2994 Note that if there were a C<continue> block on the above, it would get
2995 executed even on discarded lines. If the LABEL is omitted, the command
2996 refers to the innermost enclosing loop.
2998 C<next> cannot be used to exit a block which returns a value such as
2999 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3000 a grep() or map() operation.
3002 Note that a block by itself is semantically identical to a loop
3003 that executes once. Thus C<next> will exit such a block early.
3005 See also L</continue> for an illustration of how C<last>, C<next>, and
3008 =item no Module VERSION LIST
3011 =item no Module VERSION
3013 =item no Module LIST
3019 See the C<use> function, of which C<no> is the opposite.
3022 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3026 Interprets EXPR as an octal string and returns the corresponding
3027 value. (If EXPR happens to start off with C<0x>, interprets it as a
3028 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3029 binary string. Leading whitespace is ignored in all three cases.)
3030 The following will handle decimal, binary, octal, and hex in the standard
3033 $val = oct($val) if $val =~ /^0/;
3035 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3036 in octal), use sprintf() or printf():
3038 $perms = (stat("filename"))[2] & 07777;
3039 $oct_perms = sprintf "%lo", $perms;
3041 The oct() function is commonly used when a string such as C<644> needs
3042 to be converted into a file mode, for example. (Although perl will
3043 automatically convert strings into numbers as needed, this automatic
3044 conversion assumes base 10.)
3046 =item open FILEHANDLE,EXPR
3047 X<open> X<pipe> X<file, open> X<fopen>
3049 =item open FILEHANDLE,MODE,EXPR
3051 =item open FILEHANDLE,MODE,EXPR,LIST
3053 =item open FILEHANDLE,MODE,REFERENCE
3055 =item open FILEHANDLE
3057 Opens the file whose filename is given by EXPR, and associates it with
3060 Simple examples to open a file for reading:
3062 open(my $fh, '<', "input.txt") or die $!;
3066 open(my $fh, '>', "output.txt") or die $!;
3068 (The following is a comprehensive reference to open(): for a gentler
3069 introduction you may consider L<perlopentut>.)
3071 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3072 the variable is assigned a reference to a new anonymous filehandle,
3073 otherwise if FILEHANDLE is an expression, its value is used as the name of
3074 the real filehandle wanted. (This is considered a symbolic reference, so
3075 C<use strict 'refs'> should I<not> be in effect.)
3077 If EXPR is omitted, the scalar variable of the same name as the
3078 FILEHANDLE contains the filename. (Note that lexical variables--those
3079 declared with C<my>--will not work for this purpose; so if you're
3080 using C<my>, specify EXPR in your call to open.)
3082 If three or more arguments are specified then the mode of opening and
3083 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3084 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3085 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3086 the file is opened for appending, again being created if necessary.
3088 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3089 indicate that you want both read and write access to the file; thus
3090 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3091 '+>' >> mode would clobber the file first. You can't usually use
3092 either read-write mode for updating textfiles, since they have
3093 variable length records. See the B<-i> switch in L<perlrun> for a
3094 better approach. The file is created with permissions of C<0666>
3095 modified by the process' C<umask> value.
3097 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3098 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3100 In the 2-arguments (and 1-argument) form of the call the mode and
3101 filename should be concatenated (in this order), possibly separated by
3102 spaces. It is possible to omit the mode in these forms if the mode is
3105 If the filename begins with C<'|'>, the filename is interpreted as a
3106 command to which output is to be piped, and if the filename ends with a
3107 C<'|'>, the filename is interpreted as a command which pipes output to
3108 us. See L<perlipc/"Using open() for IPC">
3109 for more examples of this. (You are not allowed to C<open> to a command
3110 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3111 and L<perlipc/"Bidirectional Communication with Another Process">
3114 For three or more arguments if MODE is C<'|-'>, the filename is
3115 interpreted as a command to which output is to be piped, and if MODE
3116 is C<'-|'>, the filename is interpreted as a command which pipes
3117 output to us. In the 2-arguments (and 1-argument) form one should
3118 replace dash (C<'-'>) with the command.
3119 See L<perlipc/"Using open() for IPC"> for more examples of this.
3120 (You are not allowed to C<open> to a command that pipes both in I<and>
3121 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3122 L<perlipc/"Bidirectional Communication"> for alternatives.)
3124 In the three-or-more argument form of pipe opens, if LIST is specified
3125 (extra arguments after the command name) then LIST becomes arguments
3126 to the command invoked if the platform supports it. The meaning of
3127 C<open> with more than three arguments for non-pipe modes is not yet
3128 specified. Experimental "layers" may give extra LIST arguments
3131 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3132 and opening C<< '>-' >> opens STDOUT.
3134 You may use the three-argument form of open to specify IO "layers"
3135 (sometimes also referred to as "disciplines") to be applied to the handle
3136 that affect how the input and output are processed (see L<open> and
3137 L<PerlIO> for more details). For example
3139 open(my $fh, "<:encoding(UTF-8)", "file")
3141 will open the UTF-8 encoded file containing Unicode characters,
3142 see L<perluniintro>. Note that if layers are specified in the
3143 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3144 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3146 Open returns nonzero upon success, the undefined value otherwise. If
3147 the C<open> involved a pipe, the return value happens to be the pid of
3150 If you're running Perl on a system that distinguishes between text
3151 files and binary files, then you should check out L</binmode> for tips
3152 for dealing with this. The key distinction between systems that need
3153 C<binmode> and those that don't is their text file formats. Systems
3154 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3155 character, and which encode that character in C as C<"\n">, do not
3156 need C<binmode>. The rest need it.
3158 When opening a file, it's usually a bad idea to continue normal execution
3159 if the request failed, so C<open> is frequently used in connection with
3160 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3161 where you want to make a nicely formatted error message (but there are
3162 modules that can help with that problem)) you should always check
3163 the return value from opening a file. The infrequent exception is when
3164 working with an unopened filehandle is actually what you want to do.
3166 As a special case the 3-arg form with a read/write mode and the third
3167 argument being C<undef>:
3169 open(my $tmp, "+>", undef) or die ...
3171 opens a filehandle to an anonymous temporary file. Also using "+<"
3172 works for symmetry, but you really should consider writing something
3173 to the temporary file first. You will need to seek() to do the
3176 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3177 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3178 "in memory" files held in Perl scalars via:
3180 open($fh, '>', \$variable) || ..
3182 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3183 file, you have to close it first:
3186 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3191 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3192 while (<ARTICLE>) {...
3194 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3195 # if the open fails, output is discarded
3197 open(my $dbase, '+<', 'dbase.mine') # open for update
3198 or die "Can't open 'dbase.mine' for update: $!";
3200 open(my $dbase, '+<dbase.mine') # ditto
3201 or die "Can't open 'dbase.mine' for update: $!";
3203 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3204 or die "Can't start caesar: $!";
3206 open(ARTICLE, "caesar <$article |") # ditto
3207 or die "Can't start caesar: $!";
3209 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3210 or die "Can't start sort: $!";
3213 open(MEMORY,'>', \$var)
3214 or die "Can't open memory file: $!";
3215 print MEMORY "foo!\n"; # output will end up in $var
3217 # process argument list of files along with any includes
3219 foreach $file (@ARGV) {
3220 process($file, 'fh00');
3224 my($filename, $input) = @_;
3225 $input++; # this is a string increment
3226 unless (open($input, $filename)) {
3227 print STDERR "Can't open $filename: $!\n";
3232 while (<$input>) { # note use of indirection
3233 if (/^#include "(.*)"/) {
3234 process($1, $input);
3241 See L<perliol> for detailed info on PerlIO.
3243 You may also, in the Bourne shell tradition, specify an EXPR beginning
3244 with C<< '>&' >>, in which case the rest of the string is interpreted
3245 as the name of a filehandle (or file descriptor, if numeric) to be
3246 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3247 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3248 The mode you specify should match the mode of the original filehandle.
3249 (Duping a filehandle does not take into account any existing contents
3250 of IO buffers.) If you use the 3-arg form then you can pass either a
3251 number, the name of a filehandle or the normal "reference to a glob".
3253 Here is a script that saves, redirects, and restores C<STDOUT> and
3254 C<STDERR> using various methods:
3257 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3258 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3260 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3261 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3263 select STDERR; $| = 1; # make unbuffered
3264 select STDOUT; $| = 1; # make unbuffered
3266 print STDOUT "stdout 1\n"; # this works for
3267 print STDERR "stderr 1\n"; # subprocesses too
3269 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3270 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3272 print STDOUT "stdout 2\n";
3273 print STDERR "stderr 2\n";
3275 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3276 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3277 that file descriptor (and not call L<dup(2)>); this is more
3278 parsimonious of file descriptors. For example:
3280 # open for input, reusing the fileno of $fd
3281 open(FILEHANDLE, "<&=$fd")
3285 open(FILEHANDLE, "<&=", $fd)
3289 # open for append, using the fileno of OLDFH
3290 open(FH, ">>&=", OLDFH)
3294 open(FH, ">>&=OLDFH")
3296 Being parsimonious on filehandles is also useful (besides being
3297 parsimonious) for example when something is dependent on file
3298 descriptors, like for example locking using flock(). If you do just
3299 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3300 descriptor as B, and therefore flock(A) will not flock(B), and vice
3301 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3302 the same file descriptor.
3304 Note that if you are using Perls older than 5.8.0, Perl will be using
3305 the standard C libraries' fdopen() to implement the "=" functionality.
3306 On many UNIX systems fdopen() fails when file descriptors exceed a
3307 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3308 most often the default.
3310 You can see whether Perl has been compiled with PerlIO or not by
3311 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3312 is C<define>, you have PerlIO, otherwise you don't.
3314 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3315 with 2-arguments (or 1-argument) form of open(), then
3316 there is an implicit fork done, and the return value of open is the pid
3317 of the child within the parent process, and C<0> within the child
3318 process. (Use C<defined($pid)> to determine whether the open was successful.)
3319 The filehandle behaves normally for the parent, but i/o to that
3320 filehandle is piped from/to the STDOUT/STDIN of the child process.
3321 In the child process the filehandle isn't opened--i/o happens from/to
3322 the new STDOUT or STDIN. Typically this is used like the normal
3323 piped open when you want to exercise more control over just how the
3324 pipe command gets executed, such as when you are running setuid, and
3325 don't want to have to scan shell commands for metacharacters.
3326 The following triples are more or less equivalent:
3328 open(FOO, "|tr '[a-z]' '[A-Z]'");
3329 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3330 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3331 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3333 open(FOO, "cat -n '$file'|");
3334 open(FOO, '-|', "cat -n '$file'");
3335 open(FOO, '-|') || exec 'cat', '-n', $file;
3336 open(FOO, '-|', "cat", '-n', $file);
3338 The last example in each block shows the pipe as "list form", which is
3339 not yet supported on all platforms. A good rule of thumb is that if
3340 your platform has true C<fork()> (in other words, if your platform is
3341 UNIX) you can use the list form.
3343 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3345 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3346 output before any operation that may do a fork, but this may not be
3347 supported on some platforms (see L<perlport>). To be safe, you may need
3348 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3349 of C<IO::Handle> on any open handles.
3351 On systems that support a close-on-exec flag on files, the flag will
3352 be set for the newly opened file descriptor as determined by the value
3353 of $^F. See L<perlvar/$^F>.
3355 Closing any piped filehandle causes the parent process to wait for the
3356 child to finish, and returns the status value in C<$?> and
3357 C<${^CHILD_ERROR_NATIVE}>.
3359 The filename passed to 2-argument (or 1-argument) form of open() will
3360 have leading and trailing whitespace deleted, and the normal
3361 redirection characters honored. This property, known as "magic open",
3362 can often be used to good effect. A user could specify a filename of
3363 F<"rsh cat file |">, or you could change certain filenames as needed:
3365 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3366 open(FH, $filename) or die "Can't open $filename: $!";
3368 Use 3-argument form to open a file with arbitrary weird characters in it,
3370 open(FOO, '<', $file);
3372 otherwise it's necessary to protect any leading and trailing whitespace:
3374 $file =~ s#^(\s)#./$1#;
3375 open(FOO, "< $file\0");
3377 (this may not work on some bizarre filesystems). One should
3378 conscientiously choose between the I<magic> and 3-arguments form
3383 will allow the user to specify an argument of the form C<"rsh cat file |">,
3384 but will not work on a filename which happens to have a trailing space, while
3386 open IN, '<', $ARGV[0];
3388 will have exactly the opposite restrictions.
3390 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3391 should use the C<sysopen> function, which involves no such magic (but
3392 may use subtly different filemodes than Perl open(), which is mapped
3393 to C fopen()). This is
3394 another way to protect your filenames from interpretation. For example:
3397 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3398 or die "sysopen $path: $!";
3399 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3400 print HANDLE "stuff $$\n";
3402 print "File contains: ", <HANDLE>;
3404 Using the constructor from the C<IO::Handle> package (or one of its
3405 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3406 filehandles that have the scope of whatever variables hold references to
3407 them, and automatically close whenever and however you leave that scope:
3411 sub read_myfile_munged {
3413 my $handle = IO::File->new;
3414 open($handle, "myfile") or die "myfile: $!";
3416 or return (); # Automatically closed here.
3417 mung $first or die "mung failed"; # Or here.
3418 return $first, <$handle> if $ALL; # Or here.
3422 See L</seek> for some details about mixing reading and writing.
3424 =item opendir DIRHANDLE,EXPR
3427 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3428 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3429 DIRHANDLE may be an expression whose value can be used as an indirect
3430 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3431 scalar variable (or array or hash element), the variable is assigned a
3432 reference to a new anonymous dirhandle.
3433 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3435 See example at C<readdir>.
3442 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3443 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3446 For the reverse, see L</chr>.
3447 See L<perlunicode> for more about Unicode.
3454 =item our EXPR : ATTRS
3456 =item our TYPE EXPR : ATTRS
3458 C<our> associates a simple name with a package variable in the current
3459 package for use within the current scope. When C<use strict 'vars'> is in
3460 effect, C<our> lets you use declared global variables without qualifying
3461 them with package names, within the lexical scope of the C<our> declaration.
3462 In this way C<our> differs from C<use vars>, which is package scoped.
3464 Unlike C<my>, which both allocates storage for a variable and associates
3465 a simple name with that storage for use within the current scope, C<our>
3466 associates a simple name with a package variable in the current package,
3467 for use within the current scope. In other words, C<our> has the same
3468 scoping rules as C<my>, but does not necessarily create a
3471 If more than one value is listed, the list must be placed
3477 An C<our> declaration declares a global variable that will be visible
3478 across its entire lexical scope, even across package boundaries. The
3479 package in which the variable is entered is determined at the point
3480 of the declaration, not at the point of use. This means the following
3484 our $bar; # declares $Foo::bar for rest of lexical scope
3488 print $bar; # prints 20, as it refers to $Foo::bar
3490 Multiple C<our> declarations with the same name in the same lexical
3491 scope are allowed if they are in different packages. If they happen
3492 to be in the same package, Perl will emit warnings if you have asked
3493 for them, just like multiple C<my> declarations. Unlike a second
3494 C<my> declaration, which will bind the name to a fresh variable, a
3495 second C<our> declaration in the same package, in the same scope, is
3500 our $bar; # declares $Foo::bar for rest of lexical scope
3504 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3505 print $bar; # prints 30
3507 our $bar; # emits warning but has no other effect
3508 print $bar; # still prints 30
3510 An C<our> declaration may also have a list of attributes associated
3513 The exact semantics and interface of TYPE and ATTRS are still
3514 evolving. TYPE is currently bound to the use of C<fields> pragma,
3515 and attributes are handled using the C<attributes> pragma, or starting
3516 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3517 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3518 L<attributes>, and L<Attribute::Handlers>.
3520 =item pack TEMPLATE,LIST
3523 Takes a LIST of values and converts it into a string using the rules
3524 given by the TEMPLATE. The resulting string is the concatenation of
3525 the converted values. Typically, each converted value looks
3526 like its machine-level representation. For example, on 32-bit machines
3527 an integer may be represented by a sequence of 4 bytes that will be
3528 converted to a sequence of 4 characters.
3530 The TEMPLATE is a sequence of characters that give the order and type
3531 of values, as follows:
3533 a A string with arbitrary binary data, will be null padded.
3534 A A text (ASCII) string, will be space padded.
3535 Z A null terminated (ASCIZ) string, will be null padded.
3537 b A bit string (ascending bit order inside each byte, like vec()).
3538 B A bit string (descending bit order inside each byte).
3539 h A hex string (low nybble first).
3540 H A hex string (high nybble first).
3542 c A signed char (8-bit) value.
3543 C An unsigned char (octet) value.
3544 W An unsigned char value (can be greater than 255).
3546 s A signed short (16-bit) value.
3547 S An unsigned short value.
3549 l A signed long (32-bit) value.
3550 L An unsigned long value.
3552 q A signed quad (64-bit) value.
3553 Q An unsigned quad value.
3554 (Quads are available only if your system supports 64-bit
3555 integer values _and_ if Perl has been compiled to support those.
3556 Causes a fatal error otherwise.)
3558 i A signed integer value.
3559 I A unsigned integer value.
3560 (This 'integer' is _at_least_ 32 bits wide. Its exact
3561 size depends on what a local C compiler calls 'int'.)
3563 n An unsigned short (16-bit) in "network" (big-endian) order.
3564 N An unsigned long (32-bit) in "network" (big-endian) order.
3565 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3566 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3568 j A Perl internal signed integer value (IV).
3569 J A Perl internal unsigned integer value (UV).
3571 f A single-precision float in the native format.
3572 d A double-precision float in the native format.
3574 F A Perl internal floating point value (NV) in the native format
3575 D A long double-precision float in the native format.
3576 (Long doubles are available only if your system supports long
3577 double values _and_ if Perl has been compiled to support those.
3578 Causes a fatal error otherwise.)
3580 p A pointer to a null-terminated string.
3581 P A pointer to a structure (fixed-length string).
3583 u A uuencoded string.
3584 U A Unicode character number. Encodes to a character in character mode
3585 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3587 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3588 details). Its bytes represent an unsigned integer in base 128,
3589 most significant digit first, with as few digits as possible. Bit
3590 eight (the high bit) is set on each byte except the last.
3594 @ Null fill or truncate to absolute position, counted from the
3595 start of the innermost ()-group.
3596 . Null fill or truncate to absolute position specified by value.
3597 ( Start of a ()-group.
3599 One or more of the modifiers below may optionally follow some letters in the
3600 TEMPLATE (the second column lists the letters for which the modifier is
3603 ! sSlLiI Forces native (short, long, int) sizes instead
3604 of fixed (16-/32-bit) sizes.
3606 xX Make x and X act as alignment commands.
3608 nNvV Treat integers as signed instead of unsigned.
3610 @. Specify position as byte offset in the internal
3611 representation of the packed string. Efficient but
3614 > sSiIlLqQ Force big-endian byte-order on the type.
3615 jJfFdDpP (The "big end" touches the construct.)
3617 < sSiIlLqQ Force little-endian byte-order on the type.
3618 jJfFdDpP (The "little end" touches the construct.)
3620 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3621 in which case they force a certain byte-order on all components of
3622 that group, including subgroups.
3624 The following rules apply:
3630 Each letter may optionally be followed by a number giving a repeat
3631 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3632 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3633 that many values from the LIST. A C<*> for the repeat count means to
3634 use however many items are left, except for C<@>, C<x>, C<X>, where it
3635 is equivalent to C<0>, for <.> where it means relative to string start
3636 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3637 A numeric repeat count may optionally be enclosed in brackets, as in
3638 C<pack 'C[80]', @arr>.
3640 One can replace the numeric repeat count by a template enclosed in brackets;
3641 then the packed length of this template in bytes is used as a count.
3642 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3643 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3644 If the template in brackets contains alignment commands (such as C<x![d]>),
3645 its packed length is calculated as if the start of the template has the maximal
3648 When used with C<Z>, C<*> results in the addition of a trailing null
3649 byte (so the packed result will be one longer than the byte C<length>
3652 When used with C<@>, the repeat count represents an offset from the start
3653 of the innermost () group.
3655 When used with C<.>, the repeat count is used to determine the starting
3656 position from where the value offset is calculated. If the repeat count
3657 is 0, it's relative to the current position. If the repeat count is C<*>,
3658 the offset is relative to the start of the packed string. And if its an
3659 integer C<n> the offset is relative to the start of the n-th innermost
3660 () group (or the start of the string if C<n> is bigger then the group
3663 The repeat count for C<u> is interpreted as the maximal number of bytes
3664 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3665 count should not be more than 65.
3669 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3670 string of length count, padding with nulls or spaces as necessary. When
3671 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3672 after the first null, and C<a> returns data verbatim.
3674 If the value-to-pack is too long, it is truncated. If too long and an
3675 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3676 by a null byte. Thus C<Z> always packs a trailing null (except when the
3681 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3682 Each character of the input field of pack() generates 1 bit of the result.
3683 Each result bit is based on the least-significant bit of the corresponding
3684 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3685 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3687 Starting from the beginning of the input string of pack(), each 8-tuple
3688 of characters is converted to 1 character of output. With format C<b>
3689 the first character of the 8-tuple determines the least-significant bit of a
3690 character, and with format C<B> it determines the most-significant bit of
3693 If the length of the input string is not exactly divisible by 8, the
3694 remainder is packed as if the input string were padded by null characters
3695 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3697 If the input string of pack() is longer than needed, extra characters are
3698 ignored. A C<*> for the repeat count of pack() means to use all the
3699 characters of the input field. On unpack()ing the bits are converted to a
3700 string of C<"0">s and C<"1">s.
3704 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3705 representable as hexadecimal digits, 0-9a-f) long.
3707 Each character of the input field of pack() generates 4 bits of the result.
3708 For non-alphabetical characters the result is based on the 4 least-significant
3709 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3710 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3711 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3712 is compatible with the usual hexadecimal digits, so that C<"a"> and
3713 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3714 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3716 Starting from the beginning of the input string of pack(), each pair
3717 of characters is converted to 1 character of output. With format C<h> the
3718 first character of the pair determines the least-significant nybble of the
3719 output character, and with format C<H> it determines the most-significant
3722 If the length of the input string is not even, it behaves as if padded
3723 by a null character at the end. Similarly, during unpack()ing the "extra"
3724 nybbles are ignored.
3726 If the input string of pack() is longer than needed, extra characters are
3728 A C<*> for the repeat count of pack() means to use all the characters of
3729 the input field. On unpack()ing the nybbles are converted to a string
3730 of hexadecimal digits.
3734 The C<p> type packs a pointer to a null-terminated string. You are
3735 responsible for ensuring the string is not a temporary value (which can
3736 potentially get deallocated before you get around to using the packed result).
3737 The C<P> type packs a pointer to a structure of the size indicated by the
3738 length. A NULL pointer is created if the corresponding value for C<p> or
3739 C<P> is C<undef>, similarly for unpack().
3741 If your system has a strange pointer size (i.e. a pointer is neither as
3742 big as an int nor as big as a long), it may not be possible to pack or
3743 unpack pointers in big- or little-endian byte order. Attempting to do
3744 so will result in a fatal error.
3748 The C</> template character allows packing and unpacking of a sequence of
3749 items where the packed structure contains a packed item count followed by
3750 the packed items themselves.
3752 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3753 I<length-item> describes how the length value is packed. The ones likely
3754 to be of most use are integer-packing ones like C<n> (for Java strings),
3755 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3757 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3758 the minimum of that and the number of available items is used as argument
3759 for the I<length-item>. If it has no repeat count or uses a '*', the number
3760 of available items is used.
3762 For C<unpack> an internal stack of integer arguments unpacked so far is
3763 used. You write C</>I<sequence-item> and the repeat count is obtained by
3764 popping off the last element from the stack. The I<sequence-item> must not
3765 have a repeat count.
3767 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3768 the I<length-item> is a string length, not a number of strings. If there is
3769 an explicit repeat count for pack, the packed string will be adjusted to that
3772 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3773 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3774 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3775 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3776 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3778 The I<length-item> is not returned explicitly from C<unpack>.
3780 Adding a count to the I<length-item> letter is unlikely to do anything
3781 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3782 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3783 which Perl does not regard as legal in numeric strings.
3787 The integer types C<s>, C<S>, C<l>, and C<L> may be
3788 followed by a C<!> modifier to signify native shorts or
3789 longs--as you can see from above for example a bare C<l> does mean
3790 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3791 may be larger. This is an issue mainly in 64-bit platforms. You can
3792 see whether using C<!> makes any difference by
3794 print length(pack("s")), " ", length(pack("s!")), "\n";
3795 print length(pack("l")), " ", length(pack("l!")), "\n";
3797 C<i!> and C<I!> also work but only because of completeness;
3798 they are identical to C<i> and C<I>.
3800 The actual sizes (in bytes) of native shorts, ints, longs, and long
3801 longs on the platform where Perl was built are also available via
3805 print $Config{shortsize}, "\n";
3806 print $Config{intsize}, "\n";
3807 print $Config{longsize}, "\n";
3808 print $Config{longlongsize}, "\n";
3810 (The C<$Config{longlongsize}> will be undefined if your system does
3811 not support long longs.)
3815 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3816 are inherently non-portable between processors and operating systems
3817 because they obey the native byteorder and endianness. For example a
3818 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3819 (arranged in and handled by the CPU registers) into bytes as
3821 0x12 0x34 0x56 0x78 # big-endian
3822 0x78 0x56 0x34 0x12 # little-endian
3824 Basically, the Intel and VAX CPUs are little-endian, while everybody
3825 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3826 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3827 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3830 The names `big-endian' and `little-endian' are comic references to
3831 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3832 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3833 the egg-eating habits of the Lilliputians.
3835 Some systems may have even weirder byte orders such as
3840 You can see your system's preference with
3842 print join(" ", map { sprintf "%#02x", $_ }
3843 unpack("W*",pack("L",0x12345678))), "\n";
3845 The byteorder on the platform where Perl was built is also available
3849 print $Config{byteorder}, "\n";
3851 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3852 and C<'87654321'> are big-endian.
3854 If you want portable packed integers you can either use the formats
3855 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3856 modifiers. These modifiers are only available as of perl 5.9.2.
3857 See also L<perlport>.
3861 All integer and floating point formats as well as C<p> and C<P> and
3862 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3863 to force big- or little- endian byte-order, respectively.
3864 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3865 signed integers, 64-bit integers and floating point values. However,
3866 there are some things to keep in mind.
3868 Exchanging signed integers between different platforms only works
3869 if all platforms store them in the same format. Most platforms store
3870 signed integers in two's complement, so usually this is not an issue.
3872 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3873 formats on big- or little-endian machines. Otherwise, attempting to
3874 do so will result in a fatal error.
3876 Forcing big- or little-endian byte-order on floating point values for
3877 data exchange can only work if all platforms are using the same
3878 binary representation (e.g. IEEE floating point format). Even if all
3879 platforms are using IEEE, there may be subtle differences. Being able
3880 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3881 but also very dangerous if you don't know exactly what you're doing.
3882 It is definitely not a general way to portably store floating point
3885 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3886 all types inside the group that accept the byte-order modifiers,
3887 including all subgroups. It will silently be ignored for all other
3888 types. You are not allowed to override the byte-order within a group
3889 that already has a byte-order modifier suffix.
3893 Real numbers (floats and doubles) are in the native machine format only;
3894 due to the multiplicity of floating formats around, and the lack of a
3895 standard "network" representation, no facility for interchange has been
3896 made. This means that packed floating point data written on one machine
3897 may not be readable on another - even if both use IEEE floating point
3898 arithmetic (as the endian-ness of the memory representation is not part
3899 of the IEEE spec). See also L<perlport>.
3901 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3902 modifiers to force big- or little-endian byte-order on floating point values.
3904 Note that Perl uses doubles (or long doubles, if configured) internally for
3905 all numeric calculation, and converting from double into float and thence back
3906 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3907 will not in general equal $foo).
3911 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3912 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3913 where the packed string is processed in its UTF-8-encoded Unicode form on
3914 a byte by byte basis. Character mode is the default unless the format string
3915 starts with an C<U>. You can switch mode at any moment with an explicit
3916 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3917 or until the end of the ()-group in which it was entered.
3921 You must yourself do any alignment or padding by inserting for example
3922 enough C<'x'>es while packing. There is no way to pack() and unpack()
3923 could know where the characters are going to or coming from. Therefore
3924 C<pack> (and C<unpack>) handle their output and input as flat
3925 sequences of characters.
3929 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3930 take a repeat count, both as postfix, and for unpack() also via the C</>
3931 template character. Within each repetition of a group, positioning with
3932 C<@> starts again at 0. Therefore, the result of
3934 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3936 is the string "\0a\0\0bc".
3940 C<x> and C<X> accept C<!> modifier. In this case they act as
3941 alignment commands: they jump forward/back to the closest position
3942 aligned at a multiple of C<count> characters. For example, to pack() or
3943 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3944 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3945 aligned on the double's size.
3947 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3948 both result in no-ops.
3952 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3953 will represent signed 16-/32-bit integers in big-/little-endian order.
3954 This is only portable if all platforms sharing the packed data use the
3955 same binary representation for signed integers (e.g. all platforms are
3956 using two's complement representation).
3960 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3961 White space may be used to separate pack codes from each other, but
3962 modifiers and a repeat count must follow immediately.
3966 If TEMPLATE requires more arguments to pack() than actually given, pack()
3967 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3968 to pack() than actually given, extra arguments are ignored.
3974 $foo = pack("WWWW",65,66,67,68);
3976 $foo = pack("W4",65,66,67,68);
3978 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3979 # same thing with Unicode circled letters.
3980 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3981 # same thing with Unicode circled letters. You don't get the UTF-8
3982 # bytes because the U at the start of the format caused a switch to
3983 # U0-mode, so the UTF-8 bytes get joined into characters
3984 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3985 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3986 # This is the UTF-8 encoding of the string in the previous example
3988 $foo = pack("ccxxcc",65,66,67,68);
3991 # note: the above examples featuring "W" and "c" are true
3992 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3993 # and UTF-8. In EBCDIC the first example would be
3994 # $foo = pack("WWWW",193,194,195,196);
3996 $foo = pack("s2",1,2);
3997 # "\1\0\2\0" on little-endian
3998 # "\0\1\0\2" on big-endian
4000 $foo = pack("a4","abcd","x","y","z");
4003 $foo = pack("aaaa","abcd","x","y","z");
4006 $foo = pack("a14","abcdefg");
4007 # "abcdefg\0\0\0\0\0\0\0"
4009 $foo = pack("i9pl", gmtime);
4010 # a real struct tm (on my system anyway)
4012 $utmp_template = "Z8 Z8 Z16 L";
4013 $utmp = pack($utmp_template, @utmp1);
4014 # a struct utmp (BSDish)
4016 @utmp2 = unpack($utmp_template, $utmp);
4017 # "@utmp1" eq "@utmp2"
4020 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4023 $foo = pack('sx2l', 12, 34);
4024 # short 12, two zero bytes padding, long 34
4025 $bar = pack('s@4l', 12, 34);
4026 # short 12, zero fill to position 4, long 34
4028 $baz = pack('s.l', 12, 4, 34);
4029 # short 12, zero fill to position 4, long 34
4031 $foo = pack('nN', 42, 4711);
4032 # pack big-endian 16- and 32-bit unsigned integers
4033 $foo = pack('S>L>', 42, 4711);
4035 $foo = pack('s<l<', -42, 4711);
4036 # pack little-endian 16- and 32-bit signed integers
4037 $foo = pack('(sl)<', -42, 4711);
4040 The same template may generally also be used in unpack().
4042 =item package NAMESPACE
4043 X<package> X<module> X<namespace>
4047 Declares the compilation unit as being in the given namespace. The scope
4048 of the package declaration is from the declaration itself through the end
4049 of the enclosing block, file, or eval (the same as the C<my> operator).
4050 All further unqualified dynamic identifiers will be in this namespace.
4051 A package statement affects only dynamic variables--including those
4052 you've used C<local> on--but I<not> lexical variables, which are created
4053 with C<my>. Typically it would be the first declaration in a file to
4054 be included by the C<require> or C<use> operator. You can switch into a
4055 package in more than one place; it merely influences which symbol table
4056 is used by the compiler for the rest of that block. You can refer to
4057 variables and filehandles in other packages by prefixing the identifier
4058 with the package name and a double colon: C<$Package::Variable>.
4059 If the package name is null, the C<main> package as assumed. That is,
4060 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4061 still seen in older code).
4063 If NAMESPACE is omitted, then there is no current package, and all
4064 identifiers must be fully qualified or lexicals. However, you are
4065 strongly advised not to make use of this feature. Its use can cause
4066 unexpected behaviour, even crashing some versions of Perl. It is
4067 deprecated, and will be removed from a future release.
4069 See L<perlmod/"Packages"> for more information about packages, modules,
4070 and classes. See L<perlsub> for other scoping issues.
4072 =item pipe READHANDLE,WRITEHANDLE
4075 Opens a pair of connected pipes like the corresponding system call.
4076 Note that if you set up a loop of piped processes, deadlock can occur
4077 unless you are very careful. In addition, note that Perl's pipes use
4078 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4079 after each command, depending on the application.
4081 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4082 for examples of such things.
4084 On systems that support a close-on-exec flag on files, the flag will be set
4085 for the newly opened file descriptors as determined by the value of $^F.
4093 Pops and returns the last value of the array, shortening the array by
4096 If there are no elements in the array, returns the undefined value
4097 (although this may happen at other times as well). If ARRAY is
4098 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4099 array in subroutines, just like C<shift>.
4102 X<pos> X<match, position>
4106 Returns the offset of where the last C<m//g> search left off for the variable
4107 in question (C<$_> is used when the variable is not specified). Note that
4108 0 is a valid match offset. C<undef> indicates that the search position
4109 is reset (usually due to match failure, but can also be because no match has
4110 yet been performed on the scalar). C<pos> directly accesses the location used
4111 by the regexp engine to store the offset, so assigning to C<pos> will change
4112 that offset, and so will also influence the C<\G> zero-width assertion in
4113 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4114 the return from C<pos> won't change either in this case. See L<perlre> and
4117 =item print FILEHANDLE LIST
4124 Prints a string or a list of strings. Returns true if successful.
4125 FILEHANDLE may be a scalar variable name, in which case the variable
4126 contains the name of or a reference to the filehandle, thus introducing
4127 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4128 the next token is a term, it may be misinterpreted as an operator
4129 unless you interpose a C<+> or put parentheses around the arguments.)
4130 If FILEHANDLE is omitted, prints by default to standard output (or
4131 to the last selected output channel--see L</select>). If LIST is
4132 also omitted, prints C<$_> to the currently selected output channel.
4133 To set the default output channel to something other than STDOUT
4134 use the select operation. The current value of C<$,> (if any) is
4135 printed between each LIST item. The current value of C<$\> (if
4136 any) is printed after the entire LIST has been printed. Because
4137 print takes a LIST, anything in the LIST is evaluated in list
4138 context, and any subroutine that you call will have one or more of
4139 its expressions evaluated in list context. Also be careful not to
4140 follow the print keyword with a left parenthesis unless you want
4141 the corresponding right parenthesis to terminate the arguments to
4142 the print--interpose a C<+> or put parentheses around all the
4145 Note that if you're storing FILEHANDLEs in an array, or if you're using
4146 any other expression more complex than a scalar variable to retrieve it,
4147 you will have to use a block returning the filehandle value instead:
4149 print { $files[$i] } "stuff\n";
4150 print { $OK ? STDOUT : STDERR } "stuff\n";
4152 =item printf FILEHANDLE FORMAT, LIST
4155 =item printf FORMAT, LIST
4157 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4158 (the output record separator) is not appended. The first argument
4159 of the list will be interpreted as the C<printf> format. See C<sprintf>
4160 for an explanation of the format argument. If C<use locale> is in effect,
4161 and POSIX::setlocale() has been called, the character used for the decimal
4162 separator in formatted floating point numbers is affected by the LC_NUMERIC
4163 locale. See L<perllocale> and L<POSIX>.
4165 Don't fall into the trap of using a C<printf> when a simple
4166 C<print> would do. The C<print> is more efficient and less
4169 =item prototype FUNCTION
4172 Returns the prototype of a function as a string (or C<undef> if the
4173 function has no prototype). FUNCTION is a reference to, or the name of,
4174 the function whose prototype you want to retrieve.
4176 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4177 name for Perl builtin. If the builtin is not I<overridable> (such as
4178 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4179 (such as C<system>), prototype() returns C<undef>, because the builtin
4180 does not really behave like a Perl function. Otherwise, the string
4181 describing the equivalent prototype is returned.
4183 =item push ARRAY,LIST
4186 Treats ARRAY as a stack, and pushes the values of LIST
4187 onto the end of ARRAY. The length of ARRAY increases by the length of
4188 LIST. Has the same effect as
4191 $ARRAY[++$#ARRAY] = $value;
4194 but is more efficient. Returns the number of elements in the array following
4195 the completed C<push>.
4207 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4209 =item quotemeta EXPR
4210 X<quotemeta> X<metacharacter>
4214 Returns the value of EXPR with all non-"word"
4215 characters backslashed. (That is, all characters not matching
4216 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4217 returned string, regardless of any locale settings.)
4218 This is the internal function implementing
4219 the C<\Q> escape in double-quoted strings.
4221 If EXPR is omitted, uses C<$_>.
4228 Returns a random fractional number greater than or equal to C<0> and less
4229 than the value of EXPR. (EXPR should be positive.) If EXPR is
4230 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4231 also special-cased as C<1> - this has not been documented before perl 5.8.0
4232 and is subject to change in future versions of perl. Automatically calls
4233 C<srand> unless C<srand> has already been called. See also C<srand>.
4235 Apply C<int()> to the value returned by C<rand()> if you want random
4236 integers instead of random fractional numbers. For example,
4240 returns a random integer between C<0> and C<9>, inclusive.
4242 (Note: If your rand function consistently returns numbers that are too
4243 large or too small, then your version of Perl was probably compiled
4244 with the wrong number of RANDBITS.)
4246 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4247 X<read> X<file, read>
4249 =item read FILEHANDLE,SCALAR,LENGTH
4251 Attempts to read LENGTH I<characters> of data into variable SCALAR
4252 from the specified FILEHANDLE. Returns the number of characters
4253 actually read, C<0> at end of file, or undef if there was an error (in
4254 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4255 so that the last character actually read is the last character of the
4256 scalar after the read.
4258 An OFFSET may be specified to place the read data at some place in the
4259 string other than the beginning. A negative OFFSET specifies
4260 placement at that many characters counting backwards from the end of
4261 the string. A positive OFFSET greater than the length of SCALAR
4262 results in the string being padded to the required size with C<"\0">
4263 bytes before the result of the read is appended.
4265 The call is actually implemented in terms of either Perl's or system's
4266 fread() call. To get a true read(2) system call, see C<sysread>.
4268 Note the I<characters>: depending on the status of the filehandle,
4269 either (8-bit) bytes or characters are read. By default all
4270 filehandles operate on bytes, but for example if the filehandle has
4271 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4272 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4273 characters, not bytes. Similarly for the C<:encoding> pragma:
4274 in that case pretty much any characters can be read.
4276 =item readdir DIRHANDLE
4279 Returns the next directory entry for a directory opened by C<opendir>.
4280 If used in list context, returns all the rest of the entries in the
4281 directory. If there are no more entries, returns an undefined value in
4282 scalar context or a null list in list context.
4284 If you're planning to filetest the return values out of a C<readdir>, you'd
4285 better prepend the directory in question. Otherwise, because we didn't
4286 C<chdir> there, it would have been testing the wrong file.
4288 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4289 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4295 X<readline> X<gets> X<fgets>
4297 Reads from the filehandle whose typeglob is contained in EXPR (or from
4298 *ARGV if EXPR is not provided). In scalar context, each call reads and
4299 returns the next line, until end-of-file is reached, whereupon the
4300 subsequent call returns undef. In list context, reads until end-of-file
4301 is reached and returns a list of lines. Note that the notion of "line"
4302 used here is however you may have defined it with C<$/> or
4303 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4305 When C<$/> is set to C<undef>, when readline() is in scalar
4306 context (i.e. file slurp mode), and when an empty file is read, it
4307 returns C<''> the first time, followed by C<undef> subsequently.
4309 This is the internal function implementing the C<< <EXPR> >>
4310 operator, but you can use it directly. The C<< <EXPR> >>
4311 operator is discussed in more detail in L<perlop/"I/O Operators">.
4314 $line = readline(*STDIN); # same thing
4316 If readline encounters an operating system error, C<$!> will be set with the
4317 corresponding error message. It can be helpful to check C<$!> when you are
4318 reading from filehandles you don't trust, such as a tty or a socket. The
4319 following example uses the operator form of C<readline>, and takes the necessary
4320 steps to ensure that C<readline> was successful.
4324 unless (defined( $line = <> )) {
4336 Returns the value of a symbolic link, if symbolic links are
4337 implemented. If not, gives a fatal error. If there is some system
4338 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4339 omitted, uses C<$_>.
4346 EXPR is executed as a system command.
4347 The collected standard output of the command is returned.
4348 In scalar context, it comes back as a single (potentially
4349 multi-line) string. In list context, returns a list of lines
4350 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4351 This is the internal function implementing the C<qx/EXPR/>
4352 operator, but you can use it directly. The C<qx/EXPR/>
4353 operator is discussed in more detail in L<perlop/"I/O Operators">.
4354 If EXPR is omitted, uses C<$_>.
4356 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4359 Receives a message on a socket. Attempts to receive LENGTH characters
4360 of data into variable SCALAR from the specified SOCKET filehandle.
4361 SCALAR will be grown or shrunk to the length actually read. Takes the
4362 same flags as the system call of the same name. Returns the address
4363 of the sender if SOCKET's protocol supports this; returns an empty
4364 string otherwise. If there's an error, returns the undefined value.
4365 This call is actually implemented in terms of recvfrom(2) system call.
4366 See L<perlipc/"UDP: Message Passing"> for examples.
4368 Note the I<characters>: depending on the status of the socket, either
4369 (8-bit) bytes or characters are received. By default all sockets
4370 operate on bytes, but for example if the socket has been changed using
4371 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4372 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4373 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4374 case pretty much any characters can be read.
4381 The C<redo> command restarts the loop block without evaluating the
4382 conditional again. The C<continue> block, if any, is not executed. If
4383 the LABEL is omitted, the command refers to the innermost enclosing
4384 loop. Programs that want to lie to themselves about what was just input
4385 normally use this command:
4387 # a simpleminded Pascal comment stripper
4388 # (warning: assumes no { or } in strings)
4389 LINE: while (<STDIN>) {
4390 while (s|({.*}.*){.*}|$1 |) {}
4395 if (/}/) { # end of comment?
4404 C<redo> cannot be used to retry a block which returns a value such as
4405 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4406 a grep() or map() operation.
4408 Note that a block by itself is semantically identical to a loop
4409 that executes once. Thus C<redo> inside such a block will effectively
4410 turn it into a looping construct.
4412 See also L</continue> for an illustration of how C<last>, C<next>, and
4420 Returns a non-empty string if EXPR is a reference, the empty
4421 string otherwise. If EXPR
4422 is not specified, C<$_> will be used. The value returned depends on the
4423 type of thing the reference is a reference to.
4424 Builtin types include:
4438 If the referenced object has been blessed into a package, then that package
4439 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4441 if (ref($r) eq "HASH") {
4442 print "r is a reference to a hash.\n";
4445 print "r is not a reference at all.\n";
4448 The return value C<LVALUE> indicates a reference to an lvalue that is not
4449 a variable. You get this from taking the reference of function calls like
4450 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4451 to a L<version string|perldata/"Version Strings">.
4453 The result C<Regexp> indicates that the argument is a regular expression
4454 resulting from C<qr//>.
4456 See also L<perlref>.
4458 =item rename OLDNAME,NEWNAME
4459 X<rename> X<move> X<mv> X<ren>
4461 Changes the name of a file; an existing file NEWNAME will be
4462 clobbered. Returns true for success, false otherwise.
4464 Behavior of this function varies wildly depending on your system
4465 implementation. For example, it will usually not work across file system
4466 boundaries, even though the system I<mv> command sometimes compensates
4467 for this. Other restrictions include whether it works on directories,
4468 open files, or pre-existing files. Check L<perlport> and either the
4469 rename(2) manpage or equivalent system documentation for details.
4471 For a platform independent C<move> function look at the L<File::Copy>
4474 =item require VERSION
4481 Demands a version of Perl specified by VERSION, or demands some semantics
4482 specified by EXPR or by C<$_> if EXPR is not supplied.
4484 VERSION may be either a numeric argument such as 5.006, which will be
4485 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4486 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4487 VERSION is greater than the version of the current Perl interpreter.
4488 Compare with L</use>, which can do a similar check at compile time.
4490 Specifying VERSION as a literal of the form v5.6.1 should generally be
4491 avoided, because it leads to misleading error messages under earlier
4492 versions of Perl that do not support this syntax. The equivalent numeric
4493 version should be used instead.
4495 require v5.6.1; # run time version check
4496 require 5.6.1; # ditto
4497 require 5.006_001; # ditto; preferred for backwards compatibility
4499 Otherwise, C<require> demands that a library file be included if it
4500 hasn't already been included. The file is included via the do-FILE
4501 mechanism, which is essentially just a variety of C<eval> with the
4502 caveat that lexical variables in the invoking script will be invisible
4503 to the included code. Has semantics similar to the following subroutine:
4506 my ($filename) = @_;
4507 if (exists $INC{$filename}) {
4508 return 1 if $INC{$filename};
4509 die "Compilation failed in require";
4511 my ($realfilename,$result);
4513 foreach $prefix (@INC) {
4514 $realfilename = "$prefix/$filename";
4515 if (-f $realfilename) {
4516 $INC{$filename} = $realfilename;
4517 $result = do $realfilename;
4521 die "Can't find $filename in \@INC";
4524 $INC{$filename} = undef;
4526 } elsif (!$result) {
4527 delete $INC{$filename};
4528 die "$filename did not return true value";
4534 Note that the file will not be included twice under the same specified
4537 The file must return true as the last statement to indicate
4538 successful execution of any initialization code, so it's customary to
4539 end such a file with C<1;> unless you're sure it'll return true
4540 otherwise. But it's better just to put the C<1;>, in case you add more
4543 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4544 replaces "F<::>" with "F</>" in the filename for you,
4545 to make it easy to load standard modules. This form of loading of
4546 modules does not risk altering your namespace.
4548 In other words, if you try this:
4550 require Foo::Bar; # a splendid bareword
4552 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4553 directories specified in the C<@INC> array.
4555 But if you try this:
4557 $class = 'Foo::Bar';
4558 require $class; # $class is not a bareword
4560 require "Foo::Bar"; # not a bareword because of the ""
4562 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4563 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4565 eval "require $class";
4567 Now that you understand how C<require> looks for files in the case of a
4568 bareword argument, there is a little extra functionality going on behind
4569 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4570 first look for a similar filename with a "F<.pmc>" extension. If this file
4571 is found, it will be loaded in place of any file ending in a "F<.pm>"
4574 You can also insert hooks into the import facility, by putting directly
4575 Perl code into the @INC array. There are three forms of hooks: subroutine
4576 references, array references and blessed objects.
4578 Subroutine references are the simplest case. When the inclusion system
4579 walks through @INC and encounters a subroutine, this subroutine gets
4580 called with two parameters, the first being a reference to itself, and the
4581 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4582 subroutine should return nothing, or a list of up to three values in the
4589 A filehandle, from which the file will be read.
4593 A reference to a subroutine. If there is no filehandle (previous item),
4594 then this subroutine is expected to generate one line of source code per
4595 call, writing the line into C<$_> and returning 1, then returning 0 at
4596 "end of file". If there is a filehandle, then the subroutine will be
4597 called to act a simple source filter, with the line as read in C<$_>.
4598 Again, return 1 for each valid line, and 0 after all lines have been
4603 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4604 reference to the subroutine itself is passed in as C<$_[0]>.
4608 If an empty list, C<undef>, or nothing that matches the first 3 values above
4609 is returned then C<require> will look at the remaining elements of @INC.
4610 Note that this file handle must be a real file handle (strictly a typeglob,
4611 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4612 ignored and return value processing will stop there.
4614 If the hook is an array reference, its first element must be a subroutine
4615 reference. This subroutine is called as above, but the first parameter is
4616 the array reference. This enables to pass indirectly some arguments to
4619 In other words, you can write:
4621 push @INC, \&my_sub;
4623 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4629 push @INC, [ \&my_sub, $x, $y, ... ];
4631 my ($arrayref, $filename) = @_;
4632 # Retrieve $x, $y, ...
4633 my @parameters = @$arrayref[1..$#$arrayref];
4637 If the hook is an object, it must provide an INC method that will be
4638 called as above, the first parameter being the object itself. (Note that
4639 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4640 into package C<main>.) Here is a typical code layout:
4646 my ($self, $filename) = @_;
4650 # In the main program
4651 push @INC, new Foo(...);
4653 Note that these hooks are also permitted to set the %INC entry
4654 corresponding to the files they have loaded. See L<perlvar/%INC>.
4656 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4663 Generally used in a C<continue> block at the end of a loop to clear
4664 variables and reset C<??> searches so that they work again. The
4665 expression is interpreted as a list of single characters (hyphens
4666 allowed for ranges). All variables and arrays beginning with one of
4667 those letters are reset to their pristine state. If the expression is
4668 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4669 only variables or searches in the current package. Always returns
4672 reset 'X'; # reset all X variables
4673 reset 'a-z'; # reset lower case variables
4674 reset; # just reset ?one-time? searches
4676 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4677 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4678 variables--lexical variables are unaffected, but they clean themselves
4679 up on scope exit anyway, so you'll probably want to use them instead.
4687 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4688 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4689 context, depending on how the return value will be used, and the context
4690 may vary from one execution to the next (see C<wantarray>). If no EXPR
4691 is given, returns an empty list in list context, the undefined value in
4692 scalar context, and (of course) nothing at all in a void context.
4694 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4695 or do FILE will automatically return the value of the last expression
4699 X<reverse> X<rev> X<invert>
4701 In list context, returns a list value consisting of the elements
4702 of LIST in the opposite order. In scalar context, concatenates the
4703 elements of LIST and returns a string value with all characters
4704 in the opposite order.
4706 print reverse <>; # line tac, last line first
4708 undef $/; # for efficiency of <>
4709 print scalar reverse <>; # character tac, last line tsrif
4711 Used without arguments in scalar context, reverse() reverses C<$_>.
4713 This operator is also handy for inverting a hash, although there are some
4714 caveats. If a value is duplicated in the original hash, only one of those
4715 can be represented as a key in the inverted hash. Also, this has to
4716 unwind one hash and build a whole new one, which may take some time
4717 on a large hash, such as from a DBM file.
4719 %by_name = reverse %by_address; # Invert the hash
4721 =item rewinddir DIRHANDLE
4724 Sets the current position to the beginning of the directory for the
4725 C<readdir> routine on DIRHANDLE.
4727 =item rindex STR,SUBSTR,POSITION
4730 =item rindex STR,SUBSTR
4732 Works just like index() except that it returns the position of the I<last>
4733 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4734 last occurrence beginning at or before that position.
4736 =item rmdir FILENAME
4737 X<rmdir> X<rd> X<directory, remove>
4741 Deletes the directory specified by FILENAME if that directory is
4742 empty. If it succeeds it returns true, otherwise it returns false and
4743 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4745 To remove a directory tree recursively (C<rm -rf> on unix) look at
4746 the C<rmtree> function of the L<File::Path> module.
4750 The substitution operator. See L<perlop>.
4752 =item say FILEHANDLE LIST
4759 Just like C<print>, but implicitly appends a newline.
4760 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
4763 This keyword is only available when the "say" feature is
4764 enabled: see L<feature>.
4767 X<scalar> X<context>
4769 Forces EXPR to be interpreted in scalar context and returns the value
4772 @counts = ( scalar @a, scalar @b, scalar @c );
4774 There is no equivalent operator to force an expression to
4775 be interpolated in list context because in practice, this is never
4776 needed. If you really wanted to do so, however, you could use
4777 the construction C<@{[ (some expression) ]}>, but usually a simple
4778 C<(some expression)> suffices.
4780 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4781 parenthesized list, this behaves as a scalar comma expression, evaluating
4782 all but the last element in void context and returning the final element
4783 evaluated in scalar context. This is seldom what you want.
4785 The following single statement:
4787 print uc(scalar(&foo,$bar)),$baz;
4789 is the moral equivalent of these two:
4792 print(uc($bar),$baz);
4794 See L<perlop> for more details on unary operators and the comma operator.
4796 =item seek FILEHANDLE,POSITION,WHENCE
4797 X<seek> X<fseek> X<filehandle, position>
4799 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4800 FILEHANDLE may be an expression whose value gives the name of the
4801 filehandle. The values for WHENCE are C<0> to set the new position
4802 I<in bytes> to POSITION, C<1> to set it to the current position plus
4803 POSITION, and C<2> to set it to EOF plus POSITION (typically
4804 negative). For WHENCE you may use the constants C<SEEK_SET>,
4805 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4806 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4809 Note the I<in bytes>: even if the filehandle has been set to
4810 operate on characters (for example by using the C<:encoding(utf8)> open
4811 layer), tell() will return byte offsets, not character offsets
4812 (because implementing that would render seek() and tell() rather slow).
4814 If you want to position file for C<sysread> or C<syswrite>, don't use
4815 C<seek>--buffering makes its effect on the file's system position
4816 unpredictable and non-portable. Use C<sysseek> instead.
4818 Due to the rules and rigors of ANSI C, on some systems you have to do a
4819 seek whenever you switch between reading and writing. Amongst other
4820 things, this may have the effect of calling stdio's clearerr(3).
4821 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4825 This is also useful for applications emulating C<tail -f>. Once you hit
4826 EOF on your read, and then sleep for a while, you might have to stick in a
4827 seek() to reset things. The C<seek> doesn't change the current position,
4828 but it I<does> clear the end-of-file condition on the handle, so that the
4829 next C<< <FILE> >> makes Perl try again to read something. We hope.
4831 If that doesn't work (some IO implementations are particularly
4832 cantankerous), then you may need something more like this:
4835 for ($curpos = tell(FILE); $_ = <FILE>;
4836 $curpos = tell(FILE)) {
4837 # search for some stuff and put it into files
4839 sleep($for_a_while);
4840 seek(FILE, $curpos, 0);
4843 =item seekdir DIRHANDLE,POS
4846 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4847 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4848 about possible directory compaction as the corresponding system library
4851 =item select FILEHANDLE
4852 X<select> X<filehandle, default>
4856 Returns the currently selected filehandle. If FILEHANDLE is supplied,
4857 sets the new current default filehandle for output. This has two
4858 effects: first, a C<write> or a C<print> without a filehandle will
4859 default to this FILEHANDLE. Second, references to variables related to
4860 output will refer to this output channel. For example, if you have to
4861 set the top of form format for more than one output channel, you might
4869 FILEHANDLE may be an expression whose value gives the name of the
4870 actual filehandle. Thus:
4872 $oldfh = select(STDERR); $| = 1; select($oldfh);
4874 Some programmers may prefer to think of filehandles as objects with
4875 methods, preferring to write the last example as:
4878 STDERR->autoflush(1);
4880 =item select RBITS,WBITS,EBITS,TIMEOUT
4883 This calls the select(2) system call with the bit masks specified, which
4884 can be constructed using C<fileno> and C<vec>, along these lines:
4886 $rin = $win = $ein = '';
4887 vec($rin,fileno(STDIN),1) = 1;
4888 vec($win,fileno(STDOUT),1) = 1;
4891 If you want to select on many filehandles you might wish to write a
4895 my(@fhlist) = split(' ',$_[0]);
4898 vec($bits,fileno($_),1) = 1;
4902 $rin = fhbits('STDIN TTY SOCK');
4906 ($nfound,$timeleft) =
4907 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4909 or to block until something becomes ready just do this
4911 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4913 Most systems do not bother to return anything useful in $timeleft, so
4914 calling select() in scalar context just returns $nfound.
4916 Any of the bit masks can also be undef. The timeout, if specified, is
4917 in seconds, which may be fractional. Note: not all implementations are
4918 capable of returning the $timeleft. If not, they always return
4919 $timeleft equal to the supplied $timeout.
4921 You can effect a sleep of 250 milliseconds this way:
4923 select(undef, undef, undef, 0.25);
4925 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4926 is implementation-dependent. See also L<perlport> for notes on the
4927 portability of C<select>.
4929 On error, C<select> behaves like the select(2) system call : it returns
4932 Note: on some Unixes, the select(2) system call may report a socket file
4933 descriptor as "ready for reading", when actually no data is available,
4934 thus a subsequent read blocks. It can be avoided using always the
4935 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4938 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4939 or <FH>) with C<select>, except as permitted by POSIX, and even
4940 then only on POSIX systems. You have to use C<sysread> instead.
4942 =item semctl ID,SEMNUM,CMD,ARG
4945 Calls the System V IPC function C<semctl>. You'll probably have to say
4949 first to get the correct constant definitions. If CMD is IPC_STAT or
4950 GETALL, then ARG must be a variable that will hold the returned
4951 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4952 the undefined value for error, "C<0 but true>" for zero, or the actual
4953 return value otherwise. The ARG must consist of a vector of native
4954 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4955 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4958 =item semget KEY,NSEMS,FLAGS
4961 Calls the System V IPC function semget. Returns the semaphore id, or
4962 the undefined value if there is an error. See also
4963 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4966 =item semop KEY,OPSTRING
4969 Calls the System V IPC function semop to perform semaphore operations
4970 such as signalling and waiting. OPSTRING must be a packed array of
4971 semop structures. Each semop structure can be generated with
4972 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4973 implies the number of semaphore operations. Returns true if
4974 successful, or false if there is an error. As an example, the
4975 following code waits on semaphore $semnum of semaphore id $semid:
4977 $semop = pack("s!3", $semnum, -1, 0);
4978 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4980 To signal the semaphore, replace C<-1> with C<1>. See also
4981 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4984 =item send SOCKET,MSG,FLAGS,TO
4987 =item send SOCKET,MSG,FLAGS
4989 Sends a message on a socket. Attempts to send the scalar MSG to the
4990 SOCKET filehandle. Takes the same flags as the system call of the
4991 same name. On unconnected sockets you must specify a destination to
4992 send TO, in which case it does a C C<sendto>. Returns the number of
4993 characters sent, or the undefined value if there is an error. The C
4994 system call sendmsg(2) is currently unimplemented. See
4995 L<perlipc/"UDP: Message Passing"> for examples.
4997 Note the I<characters>: depending on the status of the socket, either
4998 (8-bit) bytes or characters are sent. By default all sockets operate
4999 on bytes, but for example if the socket has been changed using
5000 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5001 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5002 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5003 pragma: in that case pretty much any characters can be sent.
5005 =item setpgrp PID,PGRP
5008 Sets the current process group for the specified PID, C<0> for the current
5009 process. Will produce a fatal error if used on a machine that doesn't
5010 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5011 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5012 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5015 =item setpriority WHICH,WHO,PRIORITY
5016 X<setpriority> X<priority> X<nice> X<renice>
5018 Sets the current priority for a process, a process group, or a user.
5019 (See setpriority(2).) Will produce a fatal error if used on a machine
5020 that doesn't implement setpriority(2).
5022 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5025 Sets the socket option requested. Returns undefined if there is an
5026 error. Use integer constants provided by the C<Socket> module for
5027 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5028 getprotobyname. OPTVAL might either be a packed string or an integer.
5029 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5031 An example disabling the Nagle's algorithm for a socket:
5033 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5034 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5041 Shifts the first value of the array off and returns it, shortening the
5042 array by 1 and moving everything down. If there are no elements in the
5043 array, returns the undefined value. If ARRAY is omitted, shifts the
5044 C<@_> array within the lexical scope of subroutines and formats, and the
5045 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5046 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5047 C<UNITCHECK {}> and C<END {}> constructs.
5049 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5050 same thing to the left end of an array that C<pop> and C<push> do to the
5053 =item shmctl ID,CMD,ARG
5056 Calls the System V IPC function shmctl. You'll probably have to say
5060 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5061 then ARG must be a variable that will hold the returned C<shmid_ds>
5062 structure. Returns like ioctl: the undefined value for error, "C<0> but
5063 true" for zero, or the actual return value otherwise.
5064 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5066 =item shmget KEY,SIZE,FLAGS
5069 Calls the System V IPC function shmget. Returns the shared memory
5070 segment id, or the undefined value if there is an error.
5071 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5073 =item shmread ID,VAR,POS,SIZE
5077 =item shmwrite ID,STRING,POS,SIZE
5079 Reads or writes the System V shared memory segment ID starting at
5080 position POS for size SIZE by attaching to it, copying in/out, and
5081 detaching from it. When reading, VAR must be a variable that will
5082 hold the data read. When writing, if STRING is too long, only SIZE
5083 bytes are used; if STRING is too short, nulls are written to fill out
5084 SIZE bytes. Return true if successful, or false if there is an error.
5085 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5086 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5088 =item shutdown SOCKET,HOW
5091 Shuts down a socket connection in the manner indicated by HOW, which
5092 has the same interpretation as in the system call of the same name.
5094 shutdown(SOCKET, 0); # I/we have stopped reading data
5095 shutdown(SOCKET, 1); # I/we have stopped writing data
5096 shutdown(SOCKET, 2); # I/we have stopped using this socket
5098 This is useful with sockets when you want to tell the other
5099 side you're done writing but not done reading, or vice versa.
5100 It's also a more insistent form of close because it also
5101 disables the file descriptor in any forked copies in other
5105 X<sin> X<sine> X<asin> X<arcsine>
5109 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5110 returns sine of C<$_>.
5112 For the inverse sine operation, you may use the C<Math::Trig::asin>
5113 function, or use this relation:
5115 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5122 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5123 May be interrupted if the process receives a signal such as C<SIGALRM>.
5124 Returns the number of seconds actually slept. You probably cannot
5125 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5128 On some older systems, it may sleep up to a full second less than what
5129 you requested, depending on how it counts seconds. Most modern systems
5130 always sleep the full amount. They may appear to sleep longer than that,
5131 however, because your process might not be scheduled right away in a
5132 busy multitasking system.
5134 For delays of finer granularity than one second, the Time::HiRes module
5135 (from CPAN, and starting from Perl 5.8 part of the standard
5136 distribution) provides usleep(). You may also use Perl's four-argument
5137 version of select() leaving the first three arguments undefined, or you
5138 might be able to use the C<syscall> interface to access setitimer(2) if
5139 your system supports it. See L<perlfaq8> for details.
5141 See also the POSIX module's C<pause> function.
5143 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5146 Opens a socket of the specified kind and attaches it to filehandle
5147 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5148 the system call of the same name. You should C<use Socket> first
5149 to get the proper definitions imported. See the examples in
5150 L<perlipc/"Sockets: Client/Server Communication">.
5152 On systems that support a close-on-exec flag on files, the flag will
5153 be set for the newly opened file descriptor, as determined by the
5154 value of $^F. See L<perlvar/$^F>.
5156 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5159 Creates an unnamed pair of sockets in the specified domain, of the
5160 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5161 for the system call of the same name. If unimplemented, yields a fatal
5162 error. Returns true if successful.
5164 On systems that support a close-on-exec flag on files, the flag will
5165 be set for the newly opened file descriptors, as determined by the value
5166 of $^F. See L<perlvar/$^F>.
5168 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5169 to C<pipe(Rdr, Wtr)> is essentially:
5172 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5173 shutdown(Rdr, 1); # no more writing for reader
5174 shutdown(Wtr, 0); # no more reading for writer
5176 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5177 emulate socketpair using IP sockets to localhost if your system implements
5178 sockets but not socketpair.
5180 =item sort SUBNAME LIST
5181 X<sort> X<qsort> X<quicksort> X<mergesort>
5183 =item sort BLOCK LIST
5187 In list context, this sorts the LIST and returns the sorted list value.
5188 In scalar context, the behaviour of C<sort()> is undefined.
5190 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5191 order. If SUBNAME is specified, it gives the name of a subroutine
5192 that returns an integer less than, equal to, or greater than C<0>,
5193 depending on how the elements of the list are to be ordered. (The C<<
5194 <=> >> and C<cmp> operators are extremely useful in such routines.)
5195 SUBNAME may be a scalar variable name (unsubscripted), in which case
5196 the value provides the name of (or a reference to) the actual
5197 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5198 an anonymous, in-line sort subroutine.
5200 If the subroutine's prototype is C<($$)>, the elements to be compared
5201 are passed by reference in C<@_>, as for a normal subroutine. This is
5202 slower than unprototyped subroutines, where the elements to be
5203 compared are passed into the subroutine
5204 as the package global variables $a and $b (see example below). Note that
5205 in the latter case, it is usually counter-productive to declare $a and
5208 The values to be compared are always passed by reference and should not
5211 You also cannot exit out of the sort block or subroutine using any of the
5212 loop control operators described in L<perlsyn> or with C<goto>.
5214 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5215 current collation locale. See L<perllocale>.
5217 sort() returns aliases into the original list, much as a for loop's index
5218 variable aliases the list elements. That is, modifying an element of a
5219 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5220 actually modifies the element in the original list. This is usually
5221 something to be avoided when writing clear code.
5223 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5224 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5225 preserves the input order of elements that compare equal. Although
5226 quicksort's run time is O(NlogN) when averaged over all arrays of
5227 length N, the time can be O(N**2), I<quadratic> behavior, for some
5228 inputs.) In 5.7, the quicksort implementation was replaced with
5229 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5230 But benchmarks indicated that for some inputs, on some platforms,
5231 the original quicksort was faster. 5.8 has a sort pragma for
5232 limited control of the sort. Its rather blunt control of the
5233 underlying algorithm may not persist into future Perls, but the
5234 ability to characterize the input or output in implementation
5235 independent ways quite probably will. See L<sort>.
5240 @articles = sort @files;
5242 # same thing, but with explicit sort routine
5243 @articles = sort {$a cmp $b} @files;
5245 # now case-insensitively
5246 @articles = sort {uc($a) cmp uc($b)} @files;
5248 # same thing in reversed order
5249 @articles = sort {$b cmp $a} @files;
5251 # sort numerically ascending
5252 @articles = sort {$a <=> $b} @files;
5254 # sort numerically descending
5255 @articles = sort {$b <=> $a} @files;
5257 # this sorts the %age hash by value instead of key
5258 # using an in-line function
5259 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5261 # sort using explicit subroutine name
5263 $age{$a} <=> $age{$b}; # presuming numeric
5265 @sortedclass = sort byage @class;
5267 sub backwards { $b cmp $a }
5268 @harry = qw(dog cat x Cain Abel);
5269 @george = qw(gone chased yz Punished Axed);
5271 # prints AbelCaincatdogx
5272 print sort backwards @harry;
5273 # prints xdogcatCainAbel
5274 print sort @george, 'to', @harry;
5275 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5277 # inefficiently sort by descending numeric compare using
5278 # the first integer after the first = sign, or the
5279 # whole record case-insensitively otherwise
5282 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5287 # same thing, but much more efficiently;
5288 # we'll build auxiliary indices instead
5292 push @nums, /=(\d+)/;
5297 $nums[$b] <=> $nums[$a]
5299 $caps[$a] cmp $caps[$b]
5303 # same thing, but without any temps
5304 @new = map { $_->[0] }
5305 sort { $b->[1] <=> $a->[1]
5308 } map { [$_, /=(\d+)/, uc($_)] } @old;
5310 # using a prototype allows you to use any comparison subroutine
5311 # as a sort subroutine (including other package's subroutines)
5313 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5316 @new = sort other::backwards @old;
5318 # guarantee stability, regardless of algorithm
5320 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5322 # force use of mergesort (not portable outside Perl 5.8)
5323 use sort '_mergesort'; # note discouraging _
5324 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5326 If you're using strict, you I<must not> declare $a
5327 and $b as lexicals. They are package globals. That means
5328 if you're in the C<main> package and type
5330 @articles = sort {$b <=> $a} @files;
5332 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5333 but if you're in the C<FooPack> package, it's the same as typing
5335 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5337 The comparison function is required to behave. If it returns
5338 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5339 sometimes saying the opposite, for example) the results are not
5342 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5343 (not-a-number), and because C<sort> will trigger a fatal error unless the
5344 result of a comparison is defined, when sorting with a comparison function
5345 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5346 The following example takes advantage of the fact that C<NaN != NaN> to
5347 eliminate any C<NaN>s from the input.
5349 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5351 =item splice ARRAY,OFFSET,LENGTH,LIST
5354 =item splice ARRAY,OFFSET,LENGTH
5356 =item splice ARRAY,OFFSET
5360 Removes the elements designated by OFFSET and LENGTH from an array, and
5361 replaces them with the elements of LIST, if any. In list context,
5362 returns the elements removed from the array. In scalar context,
5363 returns the last element removed, or C<undef> if no elements are
5364 removed. The array grows or shrinks as necessary.
5365 If OFFSET is negative then it starts that far from the end of the array.
5366 If LENGTH is omitted, removes everything from OFFSET onward.
5367 If LENGTH is negative, removes the elements from OFFSET onward
5368 except for -LENGTH elements at the end of the array.
5369 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5370 past the end of the array, perl issues a warning, and splices at the
5373 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5375 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5376 pop(@a) splice(@a,-1)
5377 shift(@a) splice(@a,0,1)
5378 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5379 $a[$i] = $y splice(@a,$i,1,$y)
5381 Example, assuming array lengths are passed before arrays:
5383 sub aeq { # compare two list values
5384 my(@a) = splice(@_,0,shift);
5385 my(@b) = splice(@_,0,shift);
5386 return 0 unless @a == @b; # same len?
5388 return 0 if pop(@a) ne pop(@b);
5392 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5394 =item split /PATTERN/,EXPR,LIMIT
5397 =item split /PATTERN/,EXPR
5399 =item split /PATTERN/
5403 Splits the string EXPR into a list of strings and returns that list. By
5404 default, empty leading fields are preserved, and empty trailing ones are
5405 deleted. (If all fields are empty, they are considered to be trailing.)
5407 In scalar context, returns the number of fields found and splits into
5408 the C<@_> array. Use of split in scalar context is deprecated, however,
5409 because it clobbers your subroutine arguments.
5411 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5412 splits on whitespace (after skipping any leading whitespace). Anything
5413 matching PATTERN is taken to be a delimiter separating the fields. (Note
5414 that the delimiter may be longer than one character.)
5416 If LIMIT is specified and positive, it represents the maximum number
5417 of fields the EXPR will be split into, though the actual number of
5418 fields returned depends on the number of times PATTERN matches within
5419 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5420 stripped (which potential users of C<pop> would do well to remember).
5421 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5422 had been specified. Note that splitting an EXPR that evaluates to the
5423 empty string always returns the empty list, regardless of the LIMIT
5426 A pattern matching the null string (not to be confused with
5427 a null pattern C<//>, which is just one member of the set of patterns
5428 matching a null string) will split the value of EXPR into separate
5429 characters at each point it matches that way. For example:
5431 print join(':', split(/ */, 'hi there'));
5433 produces the output 'h:i:t:h:e:r:e'.
5435 As a special case for C<split>, using the empty pattern C<//> specifically
5436 matches only the null string, and is not be confused with the regular use
5437 of C<//> to mean "the last successful pattern match". So, for C<split>,
5440 print join(':', split(//, 'hi there'));
5442 produces the output 'h:i: :t:h:e:r:e'.
5444 Empty leading fields are produced when there are positive-width matches at
5445 the beginning of the string; a zero-width match at the beginning of
5446 the string does not produce an empty field. For example:
5448 print join(':', split(/(?=\w)/, 'hi there!'));
5450 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
5451 hand, are produced when there is a match at the end of the string (and
5452 when LIMIT is given and is not 0), regardless of the length of the match.
5455 print join(':', split(//, 'hi there!', -1));
5456 print join(':', split(/\W/, 'hi there!', -1));
5458 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
5459 both with an empty trailing field.
5461 The LIMIT parameter can be used to split a line partially
5463 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5465 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5466 a LIMIT one larger than the number of variables in the list, to avoid
5467 unnecessary work. For the list above LIMIT would have been 4 by
5468 default. In time critical applications it behooves you not to split
5469 into more fields than you really need.
5471 If the PATTERN contains parentheses, additional list elements are
5472 created from each matching substring in the delimiter.
5474 split(/([,-])/, "1-10,20", 3);
5476 produces the list value
5478 (1, '-', 10, ',', 20)
5480 If you had the entire header of a normal Unix email message in $header,
5481 you could split it up into fields and their values this way:
5483 $header =~ s/\n\s+/ /g; # fix continuation lines
5484 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5486 The pattern C</PATTERN/> may be replaced with an expression to specify
5487 patterns that vary at runtime. (To do runtime compilation only once,
5488 use C</$variable/o>.)
5490 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5491 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5492 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5493 will give you as many null initial fields as there are leading spaces.
5494 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5495 whitespace produces a null first field. A C<split> with no arguments
5496 really does a S<C<split(' ', $_)>> internally.
5498 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5503 open(PASSWD, '/etc/passwd');
5506 ($login, $passwd, $uid, $gid,
5507 $gcos, $home, $shell) = split(/:/);
5511 As with regular pattern matching, any capturing parentheses that are not
5512 matched in a C<split()> will be set to C<undef> when returned:
5514 @fields = split /(A)|B/, "1A2B3";
5515 # @fields is (1, 'A', 2, undef, 3)
5517 =item sprintf FORMAT, LIST
5520 Returns a string formatted by the usual C<printf> conventions of the C
5521 library function C<sprintf>. See below for more details
5522 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5523 the general principles.
5527 # Format number with up to 8 leading zeroes
5528 $result = sprintf("%08d", $number);
5530 # Round number to 3 digits after decimal point
5531 $rounded = sprintf("%.3f", $number);
5533 Perl does its own C<sprintf> formatting--it emulates the C
5534 function C<sprintf>, but it doesn't use it (except for floating-point
5535 numbers, and even then only the standard modifiers are allowed). As a
5536 result, any non-standard extensions in your local C<sprintf> are not
5537 available from Perl.
5539 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5540 pass it an array as your first argument. The array is given scalar context,
5541 and instead of using the 0th element of the array as the format, Perl will
5542 use the count of elements in the array as the format, which is almost never
5545 Perl's C<sprintf> permits the following universally-known conversions:
5548 %c a character with the given number
5550 %d a signed integer, in decimal
5551 %u an unsigned integer, in decimal
5552 %o an unsigned integer, in octal
5553 %x an unsigned integer, in hexadecimal
5554 %e a floating-point number, in scientific notation
5555 %f a floating-point number, in fixed decimal notation
5556 %g a floating-point number, in %e or %f notation
5558 In addition, Perl permits the following widely-supported conversions:
5560 %X like %x, but using upper-case letters
5561 %E like %e, but using an upper-case "E"
5562 %G like %g, but with an upper-case "E" (if applicable)
5563 %b an unsigned integer, in binary
5564 %B like %b, but using an upper-case "B" with the # flag
5565 %p a pointer (outputs the Perl value's address in hexadecimal)
5566 %n special: *stores* the number of characters output so far
5567 into the next variable in the parameter list
5569 Finally, for backward (and we do mean "backward") compatibility, Perl
5570 permits these unnecessary but widely-supported conversions:
5573 %D a synonym for %ld
5574 %U a synonym for %lu
5575 %O a synonym for %lo
5578 Note that the number of exponent digits in the scientific notation produced
5579 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5580 exponent less than 100 is system-dependent: it may be three or less
5581 (zero-padded as necessary). In other words, 1.23 times ten to the
5582 99th may be either "1.23e99" or "1.23e099".
5584 Between the C<%> and the format letter, you may specify a number of
5585 additional attributes controlling the interpretation of the format.
5586 In order, these are:
5590 =item format parameter index
5592 An explicit format parameter index, such as C<2$>. By default sprintf
5593 will format the next unused argument in the list, but this allows you
5594 to take the arguments out of order, e.g.:
5596 printf '%2$d %1$d', 12, 34; # prints "34 12"
5597 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5603 space prefix positive number with a space
5604 + prefix positive number with a plus sign
5605 - left-justify within the field
5606 0 use zeros, not spaces, to right-justify
5607 # ensure the leading "0" for any octal,
5608 prefix non-zero hexadecimal with "0x" or "0X",
5609 prefix non-zero binary with "0b" or "0B"
5613 printf '<% d>', 12; # prints "< 12>"
5614 printf '<%+d>', 12; # prints "<+12>"
5615 printf '<%6s>', 12; # prints "< 12>"
5616 printf '<%-6s>', 12; # prints "<12 >"
5617 printf '<%06s>', 12; # prints "<000012>"
5618 printf '<%#o>', 12; # prints "<014>"
5619 printf '<%#x>', 12; # prints "<0xc>"
5620 printf '<%#X>', 12; # prints "<0XC>"
5621 printf '<%#b>', 12; # prints "<0b1100>"
5622 printf '<%#B>', 12; # prints "<0B1100>"
5624 When a space and a plus sign are given as the flags at once,
5625 a plus sign is used to prefix a positive number.
5627 printf '<%+ d>', 12; # prints "<+12>"
5628 printf '<% +d>', 12; # prints "<+12>"
5630 When the # flag and a precision are given in the %o conversion,
5631 the precision is incremented if it's necessary for the leading "0".
5633 printf '<%#.5o>', 012; # prints "<00012>"
5634 printf '<%#.5o>', 012345; # prints "<012345>"
5635 printf '<%#.0o>', 0; # prints "<0>"
5639 This flag tells perl to interpret the supplied string as a vector of
5640 integers, one for each character in the string. Perl applies the format to
5641 each integer in turn, then joins the resulting strings with a separator (a
5642 dot C<.> by default). This can be useful for displaying ordinal values of
5643 characters in arbitrary strings:
5645 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5646 printf "version is v%vd\n", $^V; # Perl's version
5648 Put an asterisk C<*> before the C<v> to override the string to
5649 use to separate the numbers:
5651 printf "address is %*vX\n", ":", $addr; # IPv6 address
5652 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5654 You can also explicitly specify the argument number to use for
5655 the join string using e.g. C<*2$v>:
5657 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5659 =item (minimum) width
5661 Arguments are usually formatted to be only as wide as required to
5662 display the given value. You can override the width by putting
5663 a number here, or get the width from the next argument (with C<*>)
5664 or from a specified argument (with e.g. C<*2$>):
5666 printf '<%s>', "a"; # prints "<a>"
5667 printf '<%6s>', "a"; # prints "< a>"
5668 printf '<%*s>', 6, "a"; # prints "< a>"
5669 printf '<%*2$s>', "a", 6; # prints "< a>"
5670 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5672 If a field width obtained through C<*> is negative, it has the same
5673 effect as the C<-> flag: left-justification.
5675 =item precision, or maximum width
5678 You can specify a precision (for numeric conversions) or a maximum
5679 width (for string conversions) by specifying a C<.> followed by a number.
5680 For floating point formats, with the exception of 'g' and 'G', this specifies
5681 the number of decimal places to show (the default being 6), e.g.:
5683 # these examples are subject to system-specific variation
5684 printf '<%f>', 1; # prints "<1.000000>"
5685 printf '<%.1f>', 1; # prints "<1.0>"
5686 printf '<%.0f>', 1; # prints "<1>"
5687 printf '<%e>', 10; # prints "<1.000000e+01>"
5688 printf '<%.1e>', 10; # prints "<1.0e+01>"
5690 For 'g' and 'G', this specifies the maximum number of digits to show,
5691 including prior to the decimal point as well as after it, e.g.:
5693 # these examples are subject to system-specific variation
5694 printf '<%g>', 1; # prints "<1>"
5695 printf '<%.10g>', 1; # prints "<1>"
5696 printf '<%g>', 100; # prints "<100>"
5697 printf '<%.1g>', 100; # prints "<1e+02>"
5698 printf '<%.2g>', 100.01; # prints "<1e+02>"
5699 printf '<%.5g>', 100.01; # prints "<100.01>"
5700 printf '<%.4g>', 100.01; # prints "<100>"
5702 For integer conversions, specifying a precision implies that the
5703 output of the number itself should be zero-padded to this width,
5704 where the 0 flag is ignored:
5706 printf '<%.6d>', 1; # prints "<000001>"
5707 printf '<%+.6d>', 1; # prints "<+000001>"
5708 printf '<%-10.6d>', 1; # prints "<000001 >"
5709 printf '<%10.6d>', 1; # prints "< 000001>"
5710 printf '<%010.6d>', 1; # prints "< 000001>"
5711 printf '<%+10.6d>', 1; # prints "< +000001>"
5713 printf '<%.6x>', 1; # prints "<000001>"
5714 printf '<%#.6x>', 1; # prints "<0x000001>"
5715 printf '<%-10.6x>', 1; # prints "<000001 >"
5716 printf '<%10.6x>', 1; # prints "< 000001>"
5717 printf '<%010.6x>', 1; # prints "< 000001>"
5718 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5720 For string conversions, specifying a precision truncates the string
5721 to fit in the specified width:
5723 printf '<%.5s>', "truncated"; # prints "<trunc>"
5724 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5726 You can also get the precision from the next argument using C<.*>:
5728 printf '<%.6x>', 1; # prints "<000001>"
5729 printf '<%.*x>', 6, 1; # prints "<000001>"
5731 If a precision obtained through C<*> is negative, it has the same
5732 effect as no precision.
5734 printf '<%.*s>', 7, "string"; # prints "<string>"
5735 printf '<%.*s>', 3, "string"; # prints "<str>"
5736 printf '<%.*s>', 0, "string"; # prints "<>"
5737 printf '<%.*s>', -1, "string"; # prints "<string>"
5739 printf '<%.*d>', 1, 0; # prints "<0>"
5740 printf '<%.*d>', 0, 0; # prints "<>"
5741 printf '<%.*d>', -1, 0; # prints "<0>"
5743 You cannot currently get the precision from a specified number,
5744 but it is intended that this will be possible in the future using
5747 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5751 For numeric conversions, you can specify the size to interpret the
5752 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5753 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5754 whatever the default integer size is on your platform (usually 32 or 64
5755 bits), but you can override this to use instead one of the standard C types,
5756 as supported by the compiler used to build Perl:
5758 l interpret integer as C type "long" or "unsigned long"
5759 h interpret integer as C type "short" or "unsigned short"
5760 q, L or ll interpret integer as C type "long long", "unsigned long long".
5761 or "quads" (typically 64-bit integers)
5763 The last will produce errors if Perl does not understand "quads" in your
5764 installation. (This requires that either the platform natively supports quads
5765 or Perl was specifically compiled to support quads.) You can find out
5766 whether your Perl supports quads via L<Config>:
5769 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5772 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5773 to be the default floating point size on your platform (double or long double),
5774 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5775 platform supports them. You can find out whether your Perl supports long
5776 doubles via L<Config>:
5779 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5781 You can find out whether Perl considers 'long double' to be the default
5782 floating point size to use on your platform via L<Config>:
5785 ($Config{uselongdouble} eq 'define') &&
5786 print "long doubles by default\n";
5788 It can also be the case that long doubles and doubles are the same thing:
5791 ($Config{doublesize} == $Config{longdblsize}) &&
5792 print "doubles are long doubles\n";
5794 The size specifier C<V> has no effect for Perl code, but it is supported
5795 for compatibility with XS code; it means 'use the standard size for
5796 a Perl integer (or floating-point number)', which is already the
5797 default for Perl code.
5799 =item order of arguments
5801 Normally, sprintf takes the next unused argument as the value to
5802 format for each format specification. If the format specification
5803 uses C<*> to require additional arguments, these are consumed from
5804 the argument list in the order in which they appear in the format
5805 specification I<before> the value to format. Where an argument is
5806 specified using an explicit index, this does not affect the normal
5807 order for the arguments (even when the explicitly specified index
5808 would have been the next argument in any case).
5812 printf '<%*.*s>', $a, $b, $c;
5814 would use C<$a> for the width, C<$b> for the precision and C<$c>
5815 as the value to format, while:
5817 printf '<%*1$.*s>', $a, $b;
5819 would use C<$a> for the width and the precision, and C<$b> as the
5822 Here are some more examples - beware that when using an explicit
5823 index, the C<$> may need to be escaped:
5825 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5826 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5827 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5828 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5832 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5833 the character used for the decimal separator in formatted floating
5834 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5838 X<sqrt> X<root> X<square root>
5842 Return the square root of EXPR. If EXPR is omitted, returns square
5843 root of C<$_>. Only works on non-negative operands, unless you've
5844 loaded the standard Math::Complex module.
5847 print sqrt(-2); # prints 1.4142135623731i
5850 X<srand> X<seed> X<randseed>
5854 Sets the random number seed for the C<rand> operator.
5856 The point of the function is to "seed" the C<rand> function so that
5857 C<rand> can produce a different sequence each time you run your
5860 If srand() is not called explicitly, it is called implicitly at the
5861 first use of the C<rand> operator. However, this was not the case in
5862 versions of Perl before 5.004, so if your script will run under older
5863 Perl versions, it should call C<srand>.
5865 Most programs won't even call srand() at all, except those that
5866 need a cryptographically-strong starting point rather than the
5867 generally acceptable default, which is based on time of day,
5868 process ID, and memory allocation, or the F</dev/urandom> device,
5871 You can call srand($seed) with the same $seed to reproduce the
5872 I<same> sequence from rand(), but this is usually reserved for
5873 generating predictable results for testing or debugging.
5874 Otherwise, don't call srand() more than once in your program.
5876 Do B<not> call srand() (i.e. without an argument) more than once in
5877 a script. The internal state of the random number generator should
5878 contain more entropy than can be provided by any seed, so calling
5879 srand() again actually I<loses> randomness.
5881 Most implementations of C<srand> take an integer and will silently
5882 truncate decimal numbers. This means C<srand(42)> will usually
5883 produce the same results as C<srand(42.1)>. To be safe, always pass
5884 C<srand> an integer.
5886 In versions of Perl prior to 5.004 the default seed was just the
5887 current C<time>. This isn't a particularly good seed, so many old
5888 programs supply their own seed value (often C<time ^ $$> or C<time ^
5889 ($$ + ($$ << 15))>), but that isn't necessary any more.
5891 For cryptographic purposes, however, you need something much more random
5892 than the default seed. Checksumming the compressed output of one or more
5893 rapidly changing operating system status programs is the usual method. For
5896 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
5898 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5901 Frequently called programs (like CGI scripts) that simply use
5905 for a seed can fall prey to the mathematical property that
5909 one-third of the time. So don't do that.
5911 =item stat FILEHANDLE
5912 X<stat> X<file, status> X<ctime>
5916 =item stat DIRHANDLE
5920 Returns a 13-element list giving the status info for a file, either
5921 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5922 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5925 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5926 $atime,$mtime,$ctime,$blksize,$blocks)
5929 Not all fields are supported on all filesystem types. Here are the
5930 meanings of the fields:
5932 0 dev device number of filesystem
5934 2 mode file mode (type and permissions)
5935 3 nlink number of (hard) links to the file
5936 4 uid numeric user ID of file's owner
5937 5 gid numeric group ID of file's owner
5938 6 rdev the device identifier (special files only)
5939 7 size total size of file, in bytes
5940 8 atime last access time in seconds since the epoch
5941 9 mtime last modify time in seconds since the epoch
5942 10 ctime inode change time in seconds since the epoch (*)
5943 11 blksize preferred block size for file system I/O
5944 12 blocks actual number of blocks allocated
5946 (The epoch was at 00:00 January 1, 1970 GMT.)
5948 (*) Not all fields are supported on all filesystem types. Notably, the
5949 ctime field is non-portable. In particular, you cannot expect it to be a
5950 "creation time", see L<perlport/"Files and Filesystems"> for details.
5952 If C<stat> is passed the special filehandle consisting of an underline, no
5953 stat is done, but the current contents of the stat structure from the
5954 last C<stat>, C<lstat>, or filetest are returned. Example:
5956 if (-x $file && (($d) = stat(_)) && $d < 0) {
5957 print "$file is executable NFS file\n";
5960 (This works on machines only for which the device number is negative
5963 Because the mode contains both the file type and its permissions, you
5964 should mask off the file type portion and (s)printf using a C<"%o">
5965 if you want to see the real permissions.
5967 $mode = (stat($filename))[2];
5968 printf "Permissions are %04o\n", $mode & 07777;
5970 In scalar context, C<stat> returns a boolean value indicating success
5971 or failure, and, if successful, sets the information associated with
5972 the special filehandle C<_>.
5974 The L<File::stat> module provides a convenient, by-name access mechanism:
5977 $sb = stat($filename);
5978 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5979 $filename, $sb->size, $sb->mode & 07777,
5980 scalar localtime $sb->mtime;
5982 You can import symbolic mode constants (C<S_IF*>) and functions
5983 (C<S_IS*>) from the Fcntl module:
5987 $mode = (stat($filename))[2];
5989 $user_rwx = ($mode & S_IRWXU) >> 6;
5990 $group_read = ($mode & S_IRGRP) >> 3;
5991 $other_execute = $mode & S_IXOTH;
5993 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5995 $is_setuid = $mode & S_ISUID;
5996 $is_directory = S_ISDIR($mode);
5998 You could write the last two using the C<-u> and C<-d> operators.
5999 The commonly available C<S_IF*> constants are
6001 # Permissions: read, write, execute, for user, group, others.
6003 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6004 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6005 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6007 # Setuid/Setgid/Stickiness/SaveText.
6008 # Note that the exact meaning of these is system dependent.
6010 S_ISUID S_ISGID S_ISVTX S_ISTXT
6012 # File types. Not necessarily all are available on your system.
6014 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6016 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
6018 S_IREAD S_IWRITE S_IEXEC
6020 and the C<S_IF*> functions are
6022 S_IMODE($mode) the part of $mode containing the permission bits
6023 and the setuid/setgid/sticky bits
6025 S_IFMT($mode) the part of $mode containing the file type
6026 which can be bit-anded with e.g. S_IFREG
6027 or with the following functions
6029 # The operators -f, -d, -l, -b, -c, -p, and -S.
6031 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6032 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6034 # No direct -X operator counterpart, but for the first one
6035 # the -g operator is often equivalent. The ENFMT stands for
6036 # record flocking enforcement, a platform-dependent feature.
6038 S_ISENFMT($mode) S_ISWHT($mode)
6040 See your native chmod(2) and stat(2) documentation for more details
6041 about the C<S_*> constants. To get status info for a symbolic link
6042 instead of the target file behind the link, use the C<lstat> function.
6047 =item state TYPE EXPR
6049 =item state EXPR : ATTRS
6051 =item state TYPE EXPR : ATTRS
6053 C<state> declares a lexically scoped variable, just like C<my> does.
6054 However, those variables will never be reinitialized, contrary to
6055 lexical variables that are reinitialized each time their enclosing block
6058 C<state> variables are only enabled when the C<feature 'state'> pragma is
6059 in effect. See L<feature>.
6066 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6067 doing many pattern matches on the string before it is next modified.
6068 This may or may not save time, depending on the nature and number of
6069 patterns you are searching on, and on the distribution of character
6070 frequencies in the string to be searched--you probably want to compare
6071 run times with and without it to see which runs faster. Those loops
6072 that scan for many short constant strings (including the constant
6073 parts of more complex patterns) will benefit most. You may have only
6074 one C<study> active at a time--if you study a different scalar the first
6075 is "unstudied". (The way C<study> works is this: a linked list of every
6076 character in the string to be searched is made, so we know, for
6077 example, where all the C<'k'> characters are. From each search string,
6078 the rarest character is selected, based on some static frequency tables
6079 constructed from some C programs and English text. Only those places
6080 that contain this "rarest" character are examined.)
6082 For example, here is a loop that inserts index producing entries
6083 before any line containing a certain pattern:
6087 print ".IX foo\n" if /\bfoo\b/;
6088 print ".IX bar\n" if /\bbar\b/;
6089 print ".IX blurfl\n" if /\bblurfl\b/;
6094 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6095 will be looked at, because C<f> is rarer than C<o>. In general, this is
6096 a big win except in pathological cases. The only question is whether
6097 it saves you more time than it took to build the linked list in the
6100 Note that if you have to look for strings that you don't know till
6101 runtime, you can build an entire loop as a string and C<eval> that to
6102 avoid recompiling all your patterns all the time. Together with
6103 undefining C<$/> to input entire files as one record, this can be very
6104 fast, often faster than specialized programs like fgrep(1). The following
6105 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6106 out the names of those files that contain a match:
6108 $search = 'while (<>) { study;';
6109 foreach $word (@words) {
6110 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6115 eval $search; # this screams
6116 $/ = "\n"; # put back to normal input delimiter
6117 foreach $file (sort keys(%seen)) {
6121 =item sub NAME BLOCK
6124 =item sub NAME (PROTO) BLOCK
6126 =item sub NAME : ATTRS BLOCK
6128 =item sub NAME (PROTO) : ATTRS BLOCK
6130 This is subroutine definition, not a real function I<per se>.
6131 Without a BLOCK it's just a forward declaration. Without a NAME,
6132 it's an anonymous function declaration, and does actually return
6133 a value: the CODE ref of the closure you just created.
6135 See L<perlsub> and L<perlref> for details about subroutines and
6136 references, and L<attributes> and L<Attribute::Handlers> for more
6137 information about attributes.
6139 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6140 X<substr> X<substring> X<mid> X<left> X<right>
6142 =item substr EXPR,OFFSET,LENGTH
6144 =item substr EXPR,OFFSET
6146 Extracts a substring out of EXPR and returns it. First character is at
6147 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6148 If OFFSET is negative (or more precisely, less than C<$[>), starts
6149 that far from the end of the string. If LENGTH is omitted, returns
6150 everything to the end of the string. If LENGTH is negative, leaves that
6151 many characters off the end of the string.
6153 my $s = "The black cat climbed the green tree";
6154 my $color = substr $s, 4, 5; # black
6155 my $middle = substr $s, 4, -11; # black cat climbed the
6156 my $end = substr $s, 14; # climbed the green tree
6157 my $tail = substr $s, -4; # tree
6158 my $z = substr $s, -4, 2; # tr
6160 You can use the substr() function as an lvalue, in which case EXPR
6161 must itself be an lvalue. If you assign something shorter than LENGTH,
6162 the string will shrink, and if you assign something longer than LENGTH,
6163 the string will grow to accommodate it. To keep the string the same
6164 length you may need to pad or chop your value using C<sprintf>.
6166 If OFFSET and LENGTH specify a substring that is partly outside the
6167 string, only the part within the string is returned. If the substring
6168 is beyond either end of the string, substr() returns the undefined
6169 value and produces a warning. When used as an lvalue, specifying a
6170 substring that is entirely outside the string is a fatal error.
6171 Here's an example showing the behavior for boundary cases:
6174 substr($name, 4) = 'dy'; # $name is now 'freddy'
6175 my $null = substr $name, 6, 2; # returns '' (no warning)
6176 my $oops = substr $name, 7; # returns undef, with warning
6177 substr($name, 7) = 'gap'; # fatal error
6179 An alternative to using substr() as an lvalue is to specify the
6180 replacement string as the 4th argument. This allows you to replace
6181 parts of the EXPR and return what was there before in one operation,
6182 just as you can with splice().
6184 my $s = "The black cat climbed the green tree";
6185 my $z = substr $s, 14, 7, "jumped from"; # climbed
6186 # $s is now "The black cat jumped from the green tree"
6188 Note that the lvalue returned by the 3-arg version of substr() acts as
6189 a 'magic bullet'; each time it is assigned to, it remembers which part
6190 of the original string is being modified; for example:
6193 for (substr($x,1,2)) {
6194 $_ = 'a'; print $x,"\n"; # prints 1a4
6195 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6197 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6200 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6203 =item symlink OLDFILE,NEWFILE
6204 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6206 Creates a new filename symbolically linked to the old filename.
6207 Returns C<1> for success, C<0> otherwise. On systems that don't support
6208 symbolic links, produces a fatal error at run time. To check for that,
6211 $symlink_exists = eval { symlink("",""); 1 };
6213 =item syscall NUMBER, LIST
6214 X<syscall> X<system call>
6216 Calls the system call specified as the first element of the list,
6217 passing the remaining elements as arguments to the system call. If
6218 unimplemented, produces a fatal error. The arguments are interpreted
6219 as follows: if a given argument is numeric, the argument is passed as
6220 an int. If not, the pointer to the string value is passed. You are
6221 responsible to make sure a string is pre-extended long enough to
6222 receive any result that might be written into a string. You can't use a
6223 string literal (or other read-only string) as an argument to C<syscall>
6224 because Perl has to assume that any string pointer might be written
6226 integer arguments are not literals and have never been interpreted in a
6227 numeric context, you may need to add C<0> to them to force them to look
6228 like numbers. This emulates the C<syswrite> function (or vice versa):
6230 require 'syscall.ph'; # may need to run h2ph
6232 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6234 Note that Perl supports passing of up to only 14 arguments to your system call,
6235 which in practice should usually suffice.
6237 Syscall returns whatever value returned by the system call it calls.
6238 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6239 Note that some system calls can legitimately return C<-1>. The proper
6240 way to handle such calls is to assign C<$!=0;> before the call and
6241 check the value of C<$!> if syscall returns C<-1>.
6243 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6244 number of the read end of the pipe it creates. There is no way
6245 to retrieve the file number of the other end. You can avoid this
6246 problem by using C<pipe> instead.
6248 =item sysopen FILEHANDLE,FILENAME,MODE
6251 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6253 Opens the file whose filename is given by FILENAME, and associates it
6254 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6255 the name of the real filehandle wanted. This function calls the
6256 underlying operating system's C<open> function with the parameters
6257 FILENAME, MODE, PERMS.
6259 The possible values and flag bits of the MODE parameter are
6260 system-dependent; they are available via the standard module C<Fcntl>.
6261 See the documentation of your operating system's C<open> to see which
6262 values and flag bits are available. You may combine several flags
6263 using the C<|>-operator.
6265 Some of the most common values are C<O_RDONLY> for opening the file in
6266 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6267 and C<O_RDWR> for opening the file in read-write mode.
6268 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6270 For historical reasons, some values work on almost every system
6271 supported by perl: zero means read-only, one means write-only, and two
6272 means read/write. We know that these values do I<not> work under
6273 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6274 use them in new code.
6276 If the file named by FILENAME does not exist and the C<open> call creates
6277 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6278 PERMS specifies the permissions of the newly created file. If you omit
6279 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6280 These permission values need to be in octal, and are modified by your
6281 process's current C<umask>.
6284 In many systems the C<O_EXCL> flag is available for opening files in
6285 exclusive mode. This is B<not> locking: exclusiveness means here that
6286 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6287 on network filesystems, and has no effect unless the C<O_CREAT> flag
6288 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6289 being opened if it is a symbolic link. It does not protect against
6290 symbolic links in the file's path.
6293 Sometimes you may want to truncate an already-existing file. This
6294 can be done using the C<O_TRUNC> flag. The behavior of
6295 C<O_TRUNC> with C<O_RDONLY> is undefined.
6298 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6299 that takes away the user's option to have a more permissive umask.
6300 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6303 Note that C<sysopen> depends on the fdopen() C library function.
6304 On many UNIX systems, fdopen() is known to fail when file descriptors
6305 exceed a certain value, typically 255. If you need more file
6306 descriptors than that, consider rebuilding Perl to use the C<sfio>
6307 library, or perhaps using the POSIX::open() function.
6309 See L<perlopentut> for a kinder, gentler explanation of opening files.
6311 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6314 =item sysread FILEHANDLE,SCALAR,LENGTH
6316 Attempts to read LENGTH bytes of data into variable SCALAR from the
6317 specified FILEHANDLE, using the system call read(2). It bypasses
6318 buffered IO, so mixing this with other kinds of reads, C<print>,
6319 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6320 perlio or stdio layers usually buffers data. Returns the number of
6321 bytes actually read, C<0> at end of file, or undef if there was an
6322 error (in the latter case C<$!> is also set). SCALAR will be grown or
6323 shrunk so that the last byte actually read is the last byte of the
6324 scalar after the read.
6326 An OFFSET may be specified to place the read data at some place in the
6327 string other than the beginning. A negative OFFSET specifies
6328 placement at that many characters counting backwards from the end of
6329 the string. A positive OFFSET greater than the length of SCALAR
6330 results in the string being padded to the required size with C<"\0">
6331 bytes before the result of the read is appended.
6333 There is no syseof() function, which is ok, since eof() doesn't work
6334 very well on device files (like ttys) anyway. Use sysread() and check
6335 for a return value for 0 to decide whether you're done.
6337 Note that if the filehandle has been marked as C<:utf8> Unicode
6338 characters are read instead of bytes (the LENGTH, OFFSET, and the
6339 return value of sysread() are in Unicode characters).
6340 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6341 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6343 =item sysseek FILEHANDLE,POSITION,WHENCE
6346 Sets FILEHANDLE's system position in bytes using the system call
6347 lseek(2). FILEHANDLE may be an expression whose value gives the name
6348 of the filehandle. The values for WHENCE are C<0> to set the new
6349 position to POSITION, C<1> to set the it to the current position plus
6350 POSITION, and C<2> to set it to EOF plus POSITION (typically
6353 Note the I<in bytes>: even if the filehandle has been set to operate
6354 on characters (for example by using the C<:encoding(utf8)> I/O layer),
6355 tell() will return byte offsets, not character offsets (because
6356 implementing that would render sysseek() very slow).
6358 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6359 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6360 C<seek>, C<tell>, or C<eof> may cause confusion.
6362 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6363 and C<SEEK_END> (start of the file, current position, end of the file)
6364 from the Fcntl module. Use of the constants is also more portable
6365 than relying on 0, 1, and 2. For example to define a "systell" function:
6367 use Fcntl 'SEEK_CUR';
6368 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6370 Returns the new position, or the undefined value on failure. A position
6371 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6372 true on success and false on failure, yet you can still easily determine
6378 =item system PROGRAM LIST
6380 Does exactly the same thing as C<exec LIST>, except that a fork is
6381 done first, and the parent process waits for the child process to
6382 complete. Note that argument processing varies depending on the
6383 number of arguments. If there is more than one argument in LIST,
6384 or if LIST is an array with more than one value, starts the program
6385 given by the first element of the list with arguments given by the
6386 rest of the list. If there is only one scalar argument, the argument
6387 is checked for shell metacharacters, and if there are any, the
6388 entire argument is passed to the system's command shell for parsing
6389 (this is C</bin/sh -c> on Unix platforms, but varies on other
6390 platforms). If there are no shell metacharacters in the argument,
6391 it is split into words and passed directly to C<execvp>, which is
6394 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6395 output before any operation that may do a fork, but this may not be
6396 supported on some platforms (see L<perlport>). To be safe, you may need
6397 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6398 of C<IO::Handle> on any open handles.
6400 The return value is the exit status of the program as returned by the
6401 C<wait> call. To get the actual exit value, shift right by eight (see
6402 below). See also L</exec>. This is I<not> what you want to use to capture
6403 the output from a command, for that you should use merely backticks or
6404 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6405 indicates a failure to start the program or an error of the wait(2) system
6406 call (inspect $! for the reason).
6408 Like C<exec>, C<system> allows you to lie to a program about its name if
6409 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6411 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6412 C<system>, if you expect your program to terminate on receipt of these
6413 signals you will need to arrange to do so yourself based on the return
6416 @args = ("command", "arg1", "arg2");
6418 or die "system @args failed: $?"
6420 You can check all the failure possibilities by inspecting
6424 print "failed to execute: $!\n";
6427 printf "child died with signal %d, %s coredump\n",
6428 ($? & 127), ($? & 128) ? 'with' : 'without';
6431 printf "child exited with value %d\n", $? >> 8;
6434 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6435 with the W*() calls of the POSIX extension.
6437 When the arguments get executed via the system shell, results
6438 and return codes will be subject to its quirks and capabilities.
6439 See L<perlop/"`STRING`"> and L</exec> for details.
6441 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6444 =item syswrite FILEHANDLE,SCALAR,LENGTH
6446 =item syswrite FILEHANDLE,SCALAR
6448 Attempts to write LENGTH bytes of data from variable SCALAR to the
6449 specified FILEHANDLE, using the system call write(2). If LENGTH is
6450 not specified, writes whole SCALAR. It bypasses buffered IO, so
6451 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6452 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6453 stdio layers usually buffers data. Returns the number of bytes
6454 actually written, or C<undef> if there was an error (in this case the
6455 errno variable C<$!> is also set). If the LENGTH is greater than the
6456 available data in the SCALAR after the OFFSET, only as much data as is
6457 available will be written.
6459 An OFFSET may be specified to write the data from some part of the
6460 string other than the beginning. A negative OFFSET specifies writing
6461 that many characters counting backwards from the end of the string.
6462 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6464 Note that if the filehandle has been marked as C<:utf8>, Unicode
6465 characters are written instead of bytes (the LENGTH, OFFSET, and the
6466 return value of syswrite() are in UTF-8 encoded Unicode characters).
6467 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6468 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6470 =item tell FILEHANDLE
6475 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6476 error. FILEHANDLE may be an expression whose value gives the name of
6477 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6480 Note the I<in bytes>: even if the filehandle has been set to
6481 operate on characters (for example by using the C<:encoding(utf8)> open
6482 layer), tell() will return byte offsets, not character offsets (because
6483 that would render seek() and tell() rather slow).
6485 The return value of tell() for the standard streams like the STDIN
6486 depends on the operating system: it may return -1 or something else.
6487 tell() on pipes, fifos, and sockets usually returns -1.
6489 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6491 Do not use tell() (or other buffered I/O operations) on a file handle
6492 that has been manipulated by sysread(), syswrite() or sysseek().
6493 Those functions ignore the buffering, while tell() does not.
6495 =item telldir DIRHANDLE
6498 Returns the current position of the C<readdir> routines on DIRHANDLE.
6499 Value may be given to C<seekdir> to access a particular location in a
6500 directory. C<telldir> has the same caveats about possible directory
6501 compaction as the corresponding system library routine.
6503 =item tie VARIABLE,CLASSNAME,LIST
6506 This function binds a variable to a package class that will provide the
6507 implementation for the variable. VARIABLE is the name of the variable
6508 to be enchanted. CLASSNAME is the name of a class implementing objects
6509 of correct type. Any additional arguments are passed to the C<new>
6510 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6511 or C<TIEHASH>). Typically these are arguments such as might be passed
6512 to the C<dbm_open()> function of C. The object returned by the C<new>
6513 method is also returned by the C<tie> function, which would be useful
6514 if you want to access other methods in CLASSNAME.
6516 Note that functions such as C<keys> and C<values> may return huge lists
6517 when used on large objects, like DBM files. You may prefer to use the
6518 C<each> function to iterate over such. Example:
6520 # print out history file offsets
6522 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6523 while (($key,$val) = each %HIST) {
6524 print $key, ' = ', unpack('L',$val), "\n";
6528 A class implementing a hash should have the following methods:
6530 TIEHASH classname, LIST
6532 STORE this, key, value
6537 NEXTKEY this, lastkey
6542 A class implementing an ordinary array should have the following methods:
6544 TIEARRAY classname, LIST
6546 STORE this, key, value
6548 STORESIZE this, count
6554 SPLICE this, offset, length, LIST
6559 A class implementing a file handle should have the following methods:
6561 TIEHANDLE classname, LIST
6562 READ this, scalar, length, offset
6565 WRITE this, scalar, length, offset
6567 PRINTF this, format, LIST
6571 SEEK this, position, whence
6573 OPEN this, mode, LIST
6578 A class implementing a scalar should have the following methods:
6580 TIESCALAR classname, LIST
6586 Not all methods indicated above need be implemented. See L<perltie>,
6587 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6589 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6590 for you--you need to do that explicitly yourself. See L<DB_File>
6591 or the F<Config> module for interesting C<tie> implementations.
6593 For further details see L<perltie>, L<"tied VARIABLE">.
6598 Returns a reference to the object underlying VARIABLE (the same value
6599 that was originally returned by the C<tie> call that bound the variable
6600 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6606 Returns the number of non-leap seconds since whatever time the system
6607 considers to be the epoch, suitable for feeding to C<gmtime> and
6608 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6609 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6610 1904 in the current local time zone for its epoch.
6612 For measuring time in better granularity than one second,
6613 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6614 Perl 5.8 part of the standard distribution), or if you have
6615 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6616 See L<perlfaq8> for details.
6618 For date and time processing look at the many related modules on CPAN.
6619 For a comprehensive date and time representation look at the
6625 Returns a four-element list giving the user and system times, in
6626 seconds, for this process and the children of this process.
6628 ($user,$system,$cuser,$csystem) = times;
6630 In scalar context, C<times> returns C<$user>.
6632 Note that times for children are included only after they terminate.
6636 The transliteration operator. Same as C<y///>. See L<perlop>.
6638 =item truncate FILEHANDLE,LENGTH
6641 =item truncate EXPR,LENGTH
6643 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6644 specified length. Produces a fatal error if truncate isn't implemented
6645 on your system. Returns true if successful, the undefined value
6648 The behavior is undefined if LENGTH is greater than the length of the
6651 The position in the file of FILEHANDLE is left unchanged. You may want to
6652 call L<seek> before writing to the file.
6655 X<uc> X<uppercase> X<toupper>
6659 Returns an uppercased version of EXPR. This is the internal function
6660 implementing the C<\U> escape in double-quoted strings. Respects
6661 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6662 and L<perlunicode> for more details about locale and Unicode support.
6663 It does not attempt to do titlecase mapping on initial letters. See
6664 C<ucfirst> for that.
6666 If EXPR is omitted, uses C<$_>.
6669 X<ucfirst> X<uppercase>
6673 Returns the value of EXPR with the first character in uppercase
6674 (titlecase in Unicode). This is the internal function implementing
6675 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6676 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6677 for more details about locale and Unicode support.
6679 If EXPR is omitted, uses C<$_>.
6686 Sets the umask for the process to EXPR and returns the previous value.
6687 If EXPR is omitted, merely returns the current umask.
6689 The Unix permission C<rwxr-x---> is represented as three sets of three
6690 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6691 and isn't one of the digits). The C<umask> value is such a number
6692 representing disabled permissions bits. The permission (or "mode")
6693 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6694 even if you tell C<sysopen> to create a file with permissions C<0777>,
6695 if your umask is C<0022> then the file will actually be created with
6696 permissions C<0755>. If your C<umask> were C<0027> (group can't
6697 write; others can't read, write, or execute), then passing
6698 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6701 Here's some advice: supply a creation mode of C<0666> for regular
6702 files (in C<sysopen>) and one of C<0777> for directories (in
6703 C<mkdir>) and executable files. This gives users the freedom of
6704 choice: if they want protected files, they might choose process umasks
6705 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6706 Programs should rarely if ever make policy decisions better left to
6707 the user. The exception to this is when writing files that should be
6708 kept private: mail files, web browser cookies, I<.rhosts> files, and
6711 If umask(2) is not implemented on your system and you are trying to
6712 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6713 fatal error at run time. If umask(2) is not implemented and you are
6714 not trying to restrict access for yourself, returns C<undef>.
6716 Remember that a umask is a number, usually given in octal; it is I<not> a
6717 string of octal digits. See also L</oct>, if all you have is a string.
6720 X<undef> X<undefine>
6724 Undefines the value of EXPR, which must be an lvalue. Use only on a
6725 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6726 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6727 will probably not do what you expect on most predefined variables or
6728 DBM list values, so don't do that; see L<delete>.) Always returns the
6729 undefined value. You can omit the EXPR, in which case nothing is
6730 undefined, but you still get an undefined value that you could, for
6731 instance, return from a subroutine, assign to a variable or pass as a
6732 parameter. Examples:
6735 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6739 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6740 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6741 select undef, undef, undef, 0.25;
6742 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6744 Note that this is a unary operator, not a list operator.
6747 X<unlink> X<delete> X<remove> X<rm> X<del>
6751 Deletes a list of files. Returns the number of files successfully
6754 $cnt = unlink 'a', 'b', 'c';
6758 Note: C<unlink> will not attempt to delete directories unless you are superuser
6759 and the B<-U> flag is supplied to Perl. Even if these conditions are
6760 met, be warned that unlinking a directory can inflict damage on your
6761 filesystem. Finally, using C<unlink> on directories is not supported on
6762 many operating systems. Use C<rmdir> instead.
6764 If LIST is omitted, uses C<$_>.
6766 =item unpack TEMPLATE,EXPR
6769 =item unpack TEMPLATE
6771 C<unpack> does the reverse of C<pack>: it takes a string
6772 and expands it out into a list of values.
6773 (In scalar context, it returns merely the first value produced.)
6775 If EXPR is omitted, unpacks the C<$_> string.
6777 The string is broken into chunks described by the TEMPLATE. Each chunk
6778 is converted separately to a value. Typically, either the string is a result
6779 of C<pack>, or the characters of the string represent a C structure of some
6782 The TEMPLATE has the same format as in the C<pack> function.
6783 Here's a subroutine that does substring:
6786 my($what,$where,$howmuch) = @_;
6787 unpack("x$where a$howmuch", $what);
6792 sub ordinal { unpack("W",$_[0]); } # same as ord()
6794 In addition to fields allowed in pack(), you may prefix a field with
6795 a %<number> to indicate that
6796 you want a <number>-bit checksum of the items instead of the items
6797 themselves. Default is a 16-bit checksum. Checksum is calculated by
6798 summing numeric values of expanded values (for string fields the sum of
6799 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6801 For example, the following
6802 computes the same number as the System V sum program:
6806 unpack("%32W*",<>) % 65535;
6809 The following efficiently counts the number of set bits in a bit vector:
6811 $setbits = unpack("%32b*", $selectmask);
6813 The C<p> and C<P> formats should be used with care. Since Perl
6814 has no way of checking whether the value passed to C<unpack()>
6815 corresponds to a valid memory location, passing a pointer value that's
6816 not known to be valid is likely to have disastrous consequences.
6818 If there are more pack codes or if the repeat count of a field or a group
6819 is larger than what the remainder of the input string allows, the result
6820 is not well defined: in some cases, the repeat count is decreased, or
6821 C<unpack()> will produce null strings or zeroes, or terminate with an
6822 error. If the input string is longer than one described by the TEMPLATE,
6823 the rest is ignored.
6825 See L</pack> for more examples and notes.
6827 =item untie VARIABLE
6830 Breaks the binding between a variable and a package. (See C<tie>.)
6831 Has no effect if the variable is not tied.
6833 =item unshift ARRAY,LIST
6836 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6837 depending on how you look at it. Prepends list to the front of the
6838 array, and returns the new number of elements in the array.
6840 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6842 Note the LIST is prepended whole, not one element at a time, so the
6843 prepended elements stay in the same order. Use C<reverse> to do the
6846 =item use Module VERSION LIST
6847 X<use> X<module> X<import>
6849 =item use Module VERSION
6851 =item use Module LIST
6857 Imports some semantics into the current package from the named module,
6858 generally by aliasing certain subroutine or variable names into your
6859 package. It is exactly equivalent to
6861 BEGIN { require Module; Module->import( LIST ); }
6863 except that Module I<must> be a bareword.
6865 In the peculiar C<use VERSION> form, VERSION may be either a numeric
6866 argument such as 5.006, which will be compared to C<$]>, or a literal of
6867 the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A
6868 fatal error is produced if VERSION is greater than the version of the
6869 current Perl interpreter; Perl will not attempt to parse the rest of the
6870 file. Compare with L</require>, which can do a similar check at run time.
6871 Symmetrically, C<no VERSION> allows you to specify that you want a version
6872 of perl older than the specified one.
6874 Specifying VERSION as a literal of the form v5.6.1 should generally be
6875 avoided, because it leads to misleading error messages under earlier
6876 versions of Perl (that is, prior to 5.6.0) that do not support this
6877 syntax. The equivalent numeric version should be used instead.
6879 use v5.6.1; # compile time version check
6881 use 5.006_001; # ditto; preferred for backwards compatibility
6883 This is often useful if you need to check the current Perl version before
6884 C<use>ing library modules that won't work with older versions of Perl.
6885 (We try not to do this more than we have to.)
6887 Also, if the specified perl version is greater than or equal to 5.9.5,
6888 C<use VERSION> will also load the C<feature> pragma and enable all
6889 features available in the requested version. See L<feature>.
6891 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6892 C<require> makes sure the module is loaded into memory if it hasn't been
6893 yet. The C<import> is not a builtin--it's just an ordinary static method
6894 call into the C<Module> package to tell the module to import the list of
6895 features back into the current package. The module can implement its
6896 C<import> method any way it likes, though most modules just choose to
6897 derive their C<import> method via inheritance from the C<Exporter> class that
6898 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6899 method can be found then the call is skipped, even if there is an AUTOLOAD
6902 If you do not want to call the package's C<import> method (for instance,
6903 to stop your namespace from being altered), explicitly supply the empty list:
6907 That is exactly equivalent to
6909 BEGIN { require Module }
6911 If the VERSION argument is present between Module and LIST, then the
6912 C<use> will call the VERSION method in class Module with the given
6913 version as an argument. The default VERSION method, inherited from
6914 the UNIVERSAL class, croaks if the given version is larger than the
6915 value of the variable C<$Module::VERSION>.
6917 Again, there is a distinction between omitting LIST (C<import> called
6918 with no arguments) and an explicit empty LIST C<()> (C<import> not
6919 called). Note that there is no comma after VERSION!
6921 Because this is a wide-open interface, pragmas (compiler directives)
6922 are also implemented this way. Currently implemented pragmas are:
6927 use sigtrap qw(SEGV BUS);
6928 use strict qw(subs vars refs);
6929 use subs qw(afunc blurfl);
6930 use warnings qw(all);
6931 use sort qw(stable _quicksort _mergesort);
6933 Some of these pseudo-modules import semantics into the current
6934 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6935 which import symbols into the current package (which are effective
6936 through the end of the file).
6938 There's a corresponding C<no> command that unimports meanings imported
6939 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6940 It behaves exactly as C<import> does with respect to VERSION, an
6941 omitted LIST, empty LIST, or no unimport method being found.
6947 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6948 for the C<-M> and C<-m> command-line options to perl that give C<use>
6949 functionality from the command-line.
6954 Changes the access and modification times on each file of a list of
6955 files. The first two elements of the list must be the NUMERICAL access
6956 and modification times, in that order. Returns the number of files
6957 successfully changed. The inode change time of each file is set
6958 to the current time. For example, this code has the same effect as the
6959 Unix touch(1) command when the files I<already exist> and belong to
6960 the user running the program:
6963 $atime = $mtime = time;
6964 utime $atime, $mtime, @ARGV;
6966 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6967 the utime(2) function in the C library will be called with a null second
6968 argument. On most systems, this will set the file's access and
6969 modification times to the current time (i.e. equivalent to the example
6970 above) and will even work on other users' files where you have write
6973 utime undef, undef, @ARGV;
6975 Under NFS this will use the time of the NFS server, not the time of
6976 the local machine. If there is a time synchronization problem, the
6977 NFS server and local machine will have different times. The Unix
6978 touch(1) command will in fact normally use this form instead of the
6979 one shown in the first example.
6981 Note that only passing one of the first two elements as C<undef> will
6982 be equivalent of passing it as 0 and will not have the same effect as
6983 described when they are both C<undef>. This case will also trigger an
6984 uninitialized warning.
6986 On systems that support futimes, you might pass file handles among the
6987 files. On systems that don't support futimes, passing file handles
6988 produces a fatal error at run time. The file handles must be passed
6989 as globs or references to be recognized. Barewords are considered
6997 Returns a list consisting of all the values of the named hash, or the values
6998 of an array. (In a scalar context, returns the number of values.)
7000 The values are returned in an apparently random order. The actual
7001 random order is subject to change in future versions of perl, but it
7002 is guaranteed to be the same order as either the C<keys> or C<each>
7003 function would produce on the same (unmodified) hash. Since Perl
7004 5.8.1 the ordering is different even between different runs of Perl
7005 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7007 As a side effect, calling values() resets the HASH or ARRAY's internal
7009 see L</each>. (In particular, calling values() in void context resets
7010 the iterator with no other overhead. Apart from resetting the iterator,
7011 C<values @array> in list context is no different to plain C<@array>.
7012 We recommend that you use void context C<keys @array> for this, but reasoned
7013 that it taking C<values @array> out would require more documentation than
7017 Note that the values are not copied, which means modifying them will
7018 modify the contents of the hash:
7020 for (values %hash) { s/foo/bar/g } # modifies %hash values
7021 for (@hash{keys %hash}) { s/foo/bar/g } # same
7023 See also C<keys>, C<each>, and C<sort>.
7025 =item vec EXPR,OFFSET,BITS
7026 X<vec> X<bit> X<bit vector>
7028 Treats the string in EXPR as a bit vector made up of elements of
7029 width BITS, and returns the value of the element specified by OFFSET
7030 as an unsigned integer. BITS therefore specifies the number of bits
7031 that are reserved for each element in the bit vector. This must
7032 be a power of two from 1 to 32 (or 64, if your platform supports
7035 If BITS is 8, "elements" coincide with bytes of the input string.
7037 If BITS is 16 or more, bytes of the input string are grouped into chunks
7038 of size BITS/8, and each group is converted to a number as with
7039 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7040 for BITS==64). See L<"pack"> for details.
7042 If bits is 4 or less, the string is broken into bytes, then the bits
7043 of each byte are broken into 8/BITS groups. Bits of a byte are
7044 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7045 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7046 breaking the single input byte C<chr(0x36)> into two groups gives a list
7047 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7049 C<vec> may also be assigned to, in which case parentheses are needed
7050 to give the expression the correct precedence as in
7052 vec($image, $max_x * $x + $y, 8) = 3;
7054 If the selected element is outside the string, the value 0 is returned.
7055 If an element off the end of the string is written to, Perl will first
7056 extend the string with sufficiently many zero bytes. It is an error
7057 to try to write off the beginning of the string (i.e. negative OFFSET).
7059 If the string happens to be encoded as UTF-8 internally (and thus has
7060 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7061 internal byte string, not the conceptual character string, even if you
7062 only have characters with values less than 256.
7064 Strings created with C<vec> can also be manipulated with the logical
7065 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7066 vector operation is desired when both operands are strings.
7067 See L<perlop/"Bitwise String Operators">.
7069 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7070 The comments show the string after each step. Note that this code works
7071 in the same way on big-endian or little-endian machines.
7074 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7076 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7077 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7079 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7080 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7081 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7082 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7083 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7084 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7086 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7087 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7088 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7091 To transform a bit vector into a string or list of 0's and 1's, use these:
7093 $bits = unpack("b*", $vector);
7094 @bits = split(//, unpack("b*", $vector));
7096 If you know the exact length in bits, it can be used in place of the C<*>.
7098 Here is an example to illustrate how the bits actually fall in place:
7104 unpack("V",$_) 01234567890123456789012345678901
7105 ------------------------------------------------------------------
7110 for ($shift=0; $shift < $width; ++$shift) {
7111 for ($off=0; $off < 32/$width; ++$off) {
7112 $str = pack("B*", "0"x32);
7113 $bits = (1<<$shift);
7114 vec($str, $off, $width) = $bits;
7115 $res = unpack("b*",$str);
7116 $val = unpack("V", $str);
7123 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7124 $off, $width, $bits, $val, $res
7128 Regardless of the machine architecture on which it is run, the above
7129 example should print the following table:
7132 unpack("V",$_) 01234567890123456789012345678901
7133 ------------------------------------------------------------------
7134 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7135 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7136 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7137 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7138 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7139 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7140 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7141 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7142 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7143 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7144 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7145 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7146 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7147 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7148 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7149 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7150 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7151 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7152 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7153 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7154 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7155 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7156 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7157 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7158 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7159 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7160 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7161 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7162 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7163 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7164 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7165 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7166 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7167 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7168 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7169 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7170 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7171 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7172 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7173 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7174 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7175 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7176 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7177 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7178 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7179 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7180 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7181 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7182 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7183 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7184 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7185 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7186 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7187 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7188 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7189 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7190 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7191 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7192 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7193 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7194 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7195 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7196 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7197 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7198 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7199 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7200 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7201 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7202 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7203 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7204 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7205 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7206 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7207 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7208 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7209 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7210 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7211 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7212 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7213 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7214 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7215 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7216 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7217 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7218 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7219 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7220 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7221 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7222 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7223 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7224 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7225 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7226 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7227 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7228 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7229 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7230 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7231 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7232 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7233 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7234 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7235 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7236 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7237 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7238 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7239 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7240 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7241 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7242 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7243 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7244 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7245 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7246 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7247 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7248 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7249 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7250 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7251 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7252 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7253 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7254 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7255 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7256 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7257 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7258 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7259 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7260 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7261 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7266 Behaves like the wait(2) system call on your system: it waits for a child
7267 process to terminate and returns the pid of the deceased process, or
7268 C<-1> if there are no child processes. The status is returned in C<$?>
7269 and C<{^CHILD_ERROR_NATIVE}>.
7270 Note that a return value of C<-1> could mean that child processes are
7271 being automatically reaped, as described in L<perlipc>.
7273 =item waitpid PID,FLAGS
7276 Waits for a particular child process to terminate and returns the pid of
7277 the deceased process, or C<-1> if there is no such child process. On some
7278 systems, a value of 0 indicates that there are processes still running.
7279 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7281 use POSIX ":sys_wait_h";
7284 $kid = waitpid(-1, WNOHANG);
7287 then you can do a non-blocking wait for all pending zombie processes.
7288 Non-blocking wait is available on machines supporting either the
7289 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7290 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7291 system call by remembering the status values of processes that have
7292 exited but have not been harvested by the Perl script yet.)
7294 Note that on some systems, a return value of C<-1> could mean that child
7295 processes are being automatically reaped. See L<perlipc> for details,
7296 and for other examples.
7299 X<wantarray> X<context>
7301 Returns true if the context of the currently executing subroutine or
7302 C<eval> is looking for a list value. Returns false if the context is
7303 looking for a scalar. Returns the undefined value if the context is
7304 looking for no value (void context).
7306 return unless defined wantarray; # don't bother doing more
7307 my @a = complex_calculation();
7308 return wantarray ? @a : "@a";
7310 C<wantarray()>'s result is unspecified in the top level of a file,
7311 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7312 in a C<DESTROY> method.
7314 This function should have been named wantlist() instead.
7317 X<warn> X<warning> X<STDERR>
7319 Prints the value of LIST to STDERR. If the last element of LIST does
7320 not end in a newline, it appends the same file/line number text as C<die>
7323 If LIST is empty and C<$@> already contains a value (typically from a
7324 previous eval) that value is used after appending C<"\t...caught">
7325 to C<$@>. This is useful for staying almost, but not entirely similar to
7328 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7330 No message is printed if there is a C<$SIG{__WARN__}> handler
7331 installed. It is the handler's responsibility to deal with the message
7332 as it sees fit (like, for instance, converting it into a C<die>). Most
7333 handlers must therefore make arrangements to actually display the
7334 warnings that they are not prepared to deal with, by calling C<warn>
7335 again in the handler. Note that this is quite safe and will not
7336 produce an endless loop, since C<__WARN__> hooks are not called from
7339 You will find this behavior is slightly different from that of
7340 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7341 instead call C<die> again to change it).
7343 Using a C<__WARN__> handler provides a powerful way to silence all
7344 warnings (even the so-called mandatory ones). An example:
7346 # wipe out *all* compile-time warnings
7347 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7349 my $foo = 20; # no warning about duplicate my $foo,
7350 # but hey, you asked for it!
7351 # no compile-time or run-time warnings before here
7354 # run-time warnings enabled after here
7355 warn "\$foo is alive and $foo!"; # does show up
7357 See L<perlvar> for details on setting C<%SIG> entries, and for more
7358 examples. See the Carp module for other kinds of warnings using its
7359 carp() and cluck() functions.
7361 =item write FILEHANDLE
7368 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7369 using the format associated with that file. By default the format for
7370 a file is the one having the same name as the filehandle, but the
7371 format for the current output channel (see the C<select> function) may be set
7372 explicitly by assigning the name of the format to the C<$~> variable.
7374 Top of form processing is handled automatically: if there is
7375 insufficient room on the current page for the formatted record, the
7376 page is advanced by writing a form feed, a special top-of-page format
7377 is used to format the new page header, and then the record is written.
7378 By default the top-of-page format is the name of the filehandle with
7379 "_TOP" appended, but it may be dynamically set to the format of your
7380 choice by assigning the name to the C<$^> variable while the filehandle is
7381 selected. The number of lines remaining on the current page is in
7382 variable C<$->, which can be set to C<0> to force a new page.
7384 If FILEHANDLE is unspecified, output goes to the current default output
7385 channel, which starts out as STDOUT but may be changed by the
7386 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7387 is evaluated and the resulting string is used to look up the name of
7388 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7390 Note that write is I<not> the opposite of C<read>. Unfortunately.
7394 The transliteration operator. Same as C<tr///>. See L<perlop>.