4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides a scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, the scalar arguments will
18 be first, and the list argument will follow. (Note that there can ever
19 be only one such list argument.) For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for the elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use the parentheses, the simple (but occasionally
34 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
35 function, and precedence doesn't matter. Otherwise it's a list
36 operator or unary operator, and precedence does matter. And whitespace
37 between the function and left parenthesis doesn't count--so you need to
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
52 A few functions take no arguments at all, and therefore work as neither
53 unary nor list operators. These include such functions as C<time>
54 and C<endpwent>. For example, C<time+86_400> always means
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in a scalar context by
59 returning the undefined value, and in a list context by returning the
62 Remember the following important rule: There is B<no rule> that relates
63 the behavior of an expression in list context to its behavior in scalar
64 context, or vice versa. It might do two totally different things.
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in scalar context. Some operators return the
67 length of the list that would have been returned in list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
74 A named array in scalar context is quite different from what would at
75 first glance appear to be a list in scalar context. You can't get a list
76 like C<(1,2,3)> into being in scalar context, because the compiler knows
77 the context at compile time. It would generate the scalar comma operator
78 there, not the list construction version of the comma. That means it
79 was never a list to start with.
81 In general, functions in Perl that serve as wrappers for system calls
82 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule are C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 =head2 Perl Functions by Category
92 Here are Perl's functions (including things that look like
93 functions, like some keywords and named operators)
94 arranged by category. Some functions appear in more
99 =item Functions for SCALARs or strings
100 X<scalar> X<string> X<character>
102 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
103 C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
104 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
106 =item Regular expressions and pattern matching
107 X<regular expression> X<regex> X<regexp>
109 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
111 =item Numeric functions
112 X<numeric> X<number> X<trigonometric> X<trigonometry>
114 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115 C<sin>, C<sqrt>, C<srand>
117 =item Functions for real @ARRAYs
120 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
122 =item Functions for list data
125 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
127 =item Functions for real %HASHes
130 C<delete>, C<each>, C<exists>, C<keys>, C<values>
132 =item Input and output functions
133 X<I/O> X<input> X<output> X<dbm>
135 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
137 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
138 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
141 =item Functions for fixed length data or records
143 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
145 =item Functions for filehandles, files, or directories
146 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
148 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
149 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
150 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151 C<umask>, C<unlink>, C<utime>
153 =item Keywords related to the control flow of your Perl program
156 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
159 =item Keywords related to switch
161 C<break>, C<continue>, C<given>, C<when>, C<default>
163 (These are only available if you enable the "switch" feature.
164 See L<feature> and L<perlsyn/"Switch statements">.)
166 =item Keywords related to scoping
168 C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
171 (C<state> is only available if the "state" feature is enabled. See
174 =item Miscellaneous functions
176 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
177 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
179 =item Functions for processes and process groups
180 X<process> X<pid> X<process id>
182 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
183 C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
184 C<times>, C<wait>, C<waitpid>
186 =item Keywords related to perl modules
189 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
191 =item Keywords related to classes and object-orientation
192 X<object> X<class> X<package>
194 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
197 =item Low-level socket functions
200 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
202 C<socket>, C<socketpair>
204 =item System V interprocess communication functions
205 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
207 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
210 =item Fetching user and group info
211 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
213 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215 C<getpwuid>, C<setgrent>, C<setpwent>
217 =item Fetching network info
218 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
220 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224 C<setnetent>, C<setprotoent>, C<setservent>
226 =item Time-related functions
229 C<gmtime>, C<localtime>, C<time>, C<times>
231 =item Functions new in perl5
234 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
236 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
237 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238 C<ucfirst>, C<untie>, C<use>, C<when>
240 * - C<sub> was a keyword in perl4, but in perl5 it is an
241 operator, which can be used in expressions.
243 =item Functions obsoleted in perl5
245 C<dbmclose>, C<dbmopen>
250 X<portability> X<Unix> X<portable>
252 Perl was born in Unix and can therefore access all common Unix
253 system calls. In non-Unix environments, the functionality of some
254 Unix system calls may not be available, or details of the available
255 functionality may differ slightly. The Perl functions affected
258 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
261 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
263 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
264 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
267 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
268 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
271 C<shmwrite>, C<socket>, C<socketpair>,
272 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273 C<times>, C<truncate>, C<umask>, C<unlink>,
274 C<utime>, C<wait>, C<waitpid>
276 For more information about the portability of these functions, see
277 L<perlport> and other available platform-specific documentation.
279 =head2 Alphabetical Listing of Perl Functions
284 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
285 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
293 A file test, where X is one of the letters listed below. This unary
294 operator takes one argument, either a filename, a filehandle, or a dirhandle,
295 and tests the associated file to see if something is true about it. If the
296 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
297 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
298 the undefined value if the file doesn't exist. Despite the funny
299 names, precedence is the same as any other named unary operator. The
300 operator may be any of:
302 -r File is readable by effective uid/gid.
303 -w File is writable by effective uid/gid.
304 -x File is executable by effective uid/gid.
305 -o File is owned by effective uid.
307 -R File is readable by real uid/gid.
308 -W File is writable by real uid/gid.
309 -X File is executable by real uid/gid.
310 -O File is owned by real uid.
313 -z File has zero size (is empty).
314 -s File has nonzero size (returns size in bytes).
316 -f File is a plain file.
317 -d File is a directory.
318 -l File is a symbolic link.
319 -p File is a named pipe (FIFO), or Filehandle is a pipe.
321 -b File is a block special file.
322 -c File is a character special file.
323 -t Filehandle is opened to a tty.
325 -u File has setuid bit set.
326 -g File has setgid bit set.
327 -k File has sticky bit set.
329 -T File is an ASCII text file (heuristic guess).
330 -B File is a "binary" file (opposite of -T).
332 -M Script start time minus file modification time, in days.
333 -A Same for access time.
334 -C Same for inode change time (Unix, may differ for other platforms)
340 next unless -f $_; # ignore specials
344 The interpretation of the file permission operators C<-r>, C<-R>,
345 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
346 of the file and the uids and gids of the user. There may be other
347 reasons you can't actually read, write, or execute the file: for
348 example network filesystem access controls, ACLs (access control lists),
349 read-only filesystems, and unrecognized executable formats. Note
350 that the use of these six specific operators to verify if some operation
351 is possible is usually a mistake, because it may be open to race
354 Also note that, for the superuser on the local filesystems, the C<-r>,
355 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
356 if any execute bit is set in the mode. Scripts run by the superuser
357 may thus need to do a stat() to determine the actual mode of the file,
358 or temporarily set their effective uid to something else.
360 If you are using ACLs, there is a pragma called C<filetest> that may
361 produce more accurate results than the bare stat() mode bits.
362 When under the C<use filetest 'access'> the above-mentioned filetests
363 will test whether the permission can (not) be granted using the
364 access() family of system calls. Also note that the C<-x> and C<-X> may
365 under this pragma return true even if there are no execute permission
366 bits set (nor any extra execute permission ACLs). This strangeness is
367 due to the underlying system calls' definitions. Note also that, due to
368 the implementation of C<use filetest 'access'>, the C<_> special
369 filehandle won't cache the results of the file tests when this pragma is
370 in effect. Read the documentation for the C<filetest> pragma for more
373 Note that C<-s/a/b/> does not do a negated substitution. Saying
374 C<-exp($foo)> still works as expected, however--only single letters
375 following a minus are interpreted as file tests.
377 The C<-T> and C<-B> switches work as follows. The first block or so of the
378 file is examined for odd characters such as strange control codes or
379 characters with the high bit set. If too many strange characters (>30%)
380 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
381 containing null in the first block is considered a binary file. If C<-T>
382 or C<-B> is used on a filehandle, the current IO buffer is examined
383 rather than the first block. Both C<-T> and C<-B> return true on a null
384 file, or a file at EOF when testing a filehandle. Because you have to
385 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
386 against the file first, as in C<next unless -f $file && -T $file>.
388 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
389 the special filehandle consisting of a solitary underline, then the stat
390 structure of the previous file test (or stat operator) is used, saving
391 a system call. (This doesn't work with C<-t>, and you need to remember
392 that lstat() and C<-l> will leave values in the stat structure for the
393 symbolic link, not the real file.) (Also, if the stat buffer was filled by
394 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
397 print "Can do.\n" if -r $a || -w _ || -x _;
400 print "Readable\n" if -r _;
401 print "Writable\n" if -w _;
402 print "Executable\n" if -x _;
403 print "Setuid\n" if -u _;
404 print "Setgid\n" if -g _;
405 print "Sticky\n" if -k _;
406 print "Text\n" if -T _;
407 print "Binary\n" if -B _;
409 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
410 test operators, in a way that C<-f -w -x $file> is equivalent to
411 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
412 the return value of C<-f $file> as an argument to another filetest
413 operator, no special magic will happen.)
420 Returns the absolute value of its argument.
421 If VALUE is omitted, uses C<$_>.
423 =item accept NEWSOCKET,GENERICSOCKET
426 Accepts an incoming socket connect, just as the accept(2) system call
427 does. Returns the packed address if it succeeded, false otherwise.
428 See the example in L<perlipc/"Sockets: Client/Server Communication">.
430 On systems that support a close-on-exec flag on files, the flag will
431 be set for the newly opened file descriptor, as determined by the
432 value of $^F. See L<perlvar/$^F>.
441 Arranges to have a SIGALRM delivered to this process after the
442 specified number of wallclock seconds has elapsed. If SECONDS is not
443 specified, the value stored in C<$_> is used. (On some machines,
444 unfortunately, the elapsed time may be up to one second less or more
445 than you specified because of how seconds are counted, and process
446 scheduling may delay the delivery of the signal even further.)
448 Only one timer may be counting at once. Each call disables the
449 previous timer, and an argument of C<0> may be supplied to cancel the
450 previous timer without starting a new one. The returned value is the
451 amount of time remaining on the previous timer.
453 For delays of finer granularity than one second, the Time::HiRes module
454 (from CPAN, and starting from Perl 5.8 part of the standard
455 distribution) provides ualarm(). You may also use Perl's four-argument
456 version of select() leaving the first three arguments undefined, or you
457 might be able to use the C<syscall> interface to access setitimer(2) if
458 your system supports it. See L<perlfaq8> for details.
460 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
461 (C<sleep> may be internally implemented in your system with C<alarm>)
463 If you want to use C<alarm> to time out a system call you need to use an
464 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
465 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
466 restart system calls on some systems. Using C<eval>/C<die> always works,
467 modulo the caveats given in L<perlipc/"Signals">.
470 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
472 $nread = sysread SOCKET, $buffer, $size;
476 die unless $@ eq "alarm\n"; # propagate unexpected errors
483 For more information see L<perlipc>.
486 X<atan2> X<arctangent> X<tan> X<tangent>
488 Returns the arctangent of Y/X in the range -PI to PI.
490 For the tangent operation, you may use the C<Math::Trig::tan>
491 function, or use the familiar relation:
493 sub tan { sin($_[0]) / cos($_[0]) }
495 The return value for C<atan2(0,0)> is implementation-defined; consult
496 your atan2(3) manpage for more information.
498 =item bind SOCKET,NAME
501 Binds a network address to a socket, just as the bind system call
502 does. Returns true if it succeeded, false otherwise. NAME should be a
503 packed address of the appropriate type for the socket. See the examples in
504 L<perlipc/"Sockets: Client/Server Communication">.
506 =item binmode FILEHANDLE, LAYER
507 X<binmode> X<binary> X<text> X<DOS> X<Windows>
509 =item binmode FILEHANDLE
511 Arranges for FILEHANDLE to be read or written in "binary" or "text"
512 mode on systems where the run-time libraries distinguish between
513 binary and text files. If FILEHANDLE is an expression, the value is
514 taken as the name of the filehandle. Returns true on success,
515 otherwise it returns C<undef> and sets C<$!> (errno).
517 On some systems (in general, DOS and Windows-based systems) binmode()
518 is necessary when you're not working with a text file. For the sake
519 of portability it is a good idea to always use it when appropriate,
520 and to never use it when it isn't appropriate. Also, people can
521 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
523 In other words: regardless of platform, use binmode() on binary data,
524 like for example images.
526 If LAYER is present it is a single string, but may contain multiple
527 directives. The directives alter the behaviour of the file handle.
528 When LAYER is present using binmode on a text file makes sense.
530 If LAYER is omitted or specified as C<:raw> the filehandle is made
531 suitable for passing binary data. This includes turning off possible CRLF
532 translation and marking it as bytes (as opposed to Unicode characters).
533 Note that, despite what may be implied in I<"Programming Perl"> (the
534 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
535 -- other layers which would affect the binary nature of the stream are
536 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
537 PERLIO environment variable.
539 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
540 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
541 establish default I/O layers. See L<open>.
543 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
544 in "Programming Perl, 3rd Edition". However, since the publishing of this
545 book, by many known as "Camel III", the consensus of the naming of this
546 functionality has moved from "discipline" to "layer". All documentation
547 of this version of Perl therefore refers to "layers" rather than to
548 "disciplines". Now back to the regularly scheduled documentation...>
550 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
551 C<:utf8> just marks the data as UTF-8 without further checking,
552 while C<:encoding(utf8)> checks the data for actually being valid
553 UTF-8. More details can be found in L<PerlIO::encoding>.
555 In general, binmode() should be called after open() but before any I/O
556 is done on the filehandle. Calling binmode() will normally flush any
557 pending buffered output data (and perhaps pending input data) on the
558 handle. An exception to this is the C<:encoding> layer that
559 changes the default character encoding of the handle, see L<open>.
560 The C<:encoding> layer sometimes needs to be called in
561 mid-stream, and it doesn't flush the stream. The C<:encoding>
562 also implicitly pushes on top of itself the C<:utf8> layer because
563 internally Perl will operate on UTF-8 encoded Unicode characters.
565 The operating system, device drivers, C libraries, and Perl run-time
566 system all work together to let the programmer treat a single
567 character (C<\n>) as the line terminator, irrespective of the external
568 representation. On many operating systems, the native text file
569 representation matches the internal representation, but on some
570 platforms the external representation of C<\n> is made up of more than
573 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
574 character to end each line in the external representation of text (even
575 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
576 on Unix and most VMS files). In other systems like OS/2, DOS and the
577 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
578 but what's stored in text files are the two characters C<\cM\cJ>. That
579 means that, if you don't use binmode() on these systems, C<\cM\cJ>
580 sequences on disk will be converted to C<\n> on input, and any C<\n> in
581 your program will be converted back to C<\cM\cJ> on output. This is what
582 you want for text files, but it can be disastrous for binary files.
584 Another consequence of using binmode() (on some systems) is that
585 special end-of-file markers will be seen as part of the data stream.
586 For systems from the Microsoft family this means that if your binary
587 data contains C<\cZ>, the I/O subsystem will regard it as the end of
588 the file, unless you use binmode().
590 binmode() is not only important for readline() and print() operations,
591 but also when using read(), seek(), sysread(), syswrite() and tell()
592 (see L<perlport> for more details). See the C<$/> and C<$\> variables
593 in L<perlvar> for how to manually set your input and output
594 line-termination sequences.
596 =item bless REF,CLASSNAME
601 This function tells the thingy referenced by REF that it is now an object
602 in the CLASSNAME package. If CLASSNAME is omitted, the current package
603 is used. Because a C<bless> is often the last thing in a constructor,
604 it returns the reference for convenience. Always use the two-argument
605 version if a derived class might inherit the function doing the blessing.
606 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
609 Consider always blessing objects in CLASSNAMEs that are mixed case.
610 Namespaces with all lowercase names are considered reserved for
611 Perl pragmata. Builtin types have all uppercase names. To prevent
612 confusion, you may wish to avoid such package names as well. Make sure
613 that CLASSNAME is a true value.
615 See L<perlmod/"Perl Modules">.
619 Break out of a C<given()> block.
621 This keyword is enabled by the "switch" feature: see L<feature>
622 for more information.
625 X<caller> X<call stack> X<stack> X<stack trace>
629 Returns the context of the current subroutine call. In scalar context,
630 returns the caller's package name if there is a caller, that is, if
631 we're in a subroutine or C<eval> or C<require>, and the undefined value
632 otherwise. In list context, returns
635 ($package, $filename, $line) = caller;
637 With EXPR, it returns some extra information that the debugger uses to
638 print a stack trace. The value of EXPR indicates how many call frames
639 to go back before the current one.
642 ($package, $filename, $line, $subroutine, $hasargs,
645 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
648 Here $subroutine may be C<(eval)> if the frame is not a subroutine
649 call, but an C<eval>. In such a case additional elements $evaltext and
650 C<$is_require> are set: C<$is_require> is true if the frame is created by a
651 C<require> or C<use> statement, $evaltext contains the text of the
652 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
653 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
654 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
655 frame.) $subroutine may also be C<(unknown)> if this particular
656 subroutine happens to have been deleted from the symbol table.
657 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
658 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
659 compiled with. The C<$hints> and C<$bitmask> values are subject to change
660 between versions of Perl, and are not meant for external use.
662 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
663 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
664 of this hash, as they are the actual values stored in the optree.
666 Furthermore, when called from within the DB package, caller returns more
667 detailed information: it sets the list variable C<@DB::args> to be the
668 arguments with which the subroutine was invoked.
670 Be aware that the optimizer might have optimized call frames away before
671 C<caller> had a chance to get the information. That means that C<caller(N)>
672 might not return information about the call frame you expect it do, for
673 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
674 previous time C<caller> was called.
681 =item chdir FILEHANDLE
683 =item chdir DIRHANDLE
687 Changes the working directory to EXPR, if possible. If EXPR is omitted,
688 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
689 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
690 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
691 neither is set, C<chdir> does nothing. It returns true upon success,
692 false otherwise. See the example under C<die>.
694 On systems that support fchdir, you might pass a file handle or
695 directory handle as argument. On systems that don't support fchdir,
696 passing handles produces a fatal error at run time.
699 X<chmod> X<permission> X<mode>
701 Changes the permissions of a list of files. The first element of the
702 list must be the numerical mode, which should probably be an octal
703 number, and which definitely should I<not> be a string of octal digits:
704 C<0644> is okay, C<'0644'> is not. Returns the number of files
705 successfully changed. See also L</oct>, if all you have is a string.
707 $cnt = chmod 0755, 'foo', 'bar';
708 chmod 0755, @executables;
709 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
711 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
712 $mode = 0644; chmod $mode, 'foo'; # this is best
714 On systems that support fchmod, you might pass file handles among the
715 files. On systems that don't support fchmod, passing file handles
716 produces a fatal error at run time. The file handles must be passed
717 as globs or references to be recognized. Barewords are considered
720 open(my $fh, "<", "foo");
721 my $perm = (stat $fh)[2] & 07777;
722 chmod($perm | 0600, $fh);
724 You can also import the symbolic C<S_I*> constants from the Fcntl
729 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
730 # This is identical to the chmod 0755 of the above example.
733 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
739 This safer version of L</chop> removes any trailing string
740 that corresponds to the current value of C<$/> (also known as
741 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
742 number of characters removed from all its arguments. It's often used to
743 remove the newline from the end of an input record when you're worried
744 that the final record may be missing its newline. When in paragraph
745 mode (C<$/ = "">), it removes all trailing newlines from the string.
746 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
747 a reference to an integer or the like, see L<perlvar>) chomp() won't
749 If VARIABLE is omitted, it chomps C<$_>. Example:
752 chomp; # avoid \n on last field
757 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
759 You can actually chomp anything that's an lvalue, including an assignment:
762 chomp($answer = <STDIN>);
764 If you chomp a list, each element is chomped, and the total number of
765 characters removed is returned.
767 Note that parentheses are necessary when you're chomping anything
768 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
769 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
770 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
771 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
781 Chops off the last character of a string and returns the character
782 chopped. It is much more efficient than C<s/.$//s> because it neither
783 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
784 If VARIABLE is a hash, it chops the hash's values, but not its keys.
786 You can actually chop anything that's an lvalue, including an assignment.
788 If you chop a list, each element is chopped. Only the value of the
789 last C<chop> is returned.
791 Note that C<chop> returns the last character. To return all but the last
792 character, use C<substr($string, 0, -1)>.
797 X<chown> X<owner> X<user> X<group>
799 Changes the owner (and group) of a list of files. The first two
800 elements of the list must be the I<numeric> uid and gid, in that
801 order. A value of -1 in either position is interpreted by most
802 systems to leave that value unchanged. Returns the number of files
803 successfully changed.
805 $cnt = chown $uid, $gid, 'foo', 'bar';
806 chown $uid, $gid, @filenames;
808 On systems that support fchown, you might pass file handles among the
809 files. On systems that don't support fchown, passing file handles
810 produces a fatal error at run time. The file handles must be passed
811 as globs or references to be recognized. Barewords are considered
814 Here's an example that looks up nonnumeric uids in the passwd file:
817 chomp($user = <STDIN>);
819 chomp($pattern = <STDIN>);
821 ($login,$pass,$uid,$gid) = getpwnam($user)
822 or die "$user not in passwd file";
824 @ary = glob($pattern); # expand filenames
825 chown $uid, $gid, @ary;
827 On most systems, you are not allowed to change the ownership of the
828 file unless you're the superuser, although you should be able to change
829 the group to any of your secondary groups. On insecure systems, these
830 restrictions may be relaxed, but this is not a portable assumption.
831 On POSIX systems, you can detect this condition this way:
833 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
834 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
837 X<chr> X<character> X<ASCII> X<Unicode>
841 Returns the character represented by that NUMBER in the character set.
842 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
843 chr(0x263a) is a Unicode smiley face.
845 Negative values give the Unicode replacement character (chr(0xfffd)),
846 except under the L<bytes> pragma, where low eight bits of the value
847 (truncated to an integer) are used.
849 If NUMBER is omitted, uses C<$_>.
851 For the reverse, use L</ord>.
853 Note that characters from 128 to 255 (inclusive) are by default
854 internally not encoded as UTF-8 for backward compatibility reasons.
856 See L<perlunicode> for more about Unicode.
858 =item chroot FILENAME
863 This function works like the system call by the same name: it makes the
864 named directory the new root directory for all further pathnames that
865 begin with a C</> by your process and all its children. (It doesn't
866 change your current working directory, which is unaffected.) For security
867 reasons, this call is restricted to the superuser. If FILENAME is
868 omitted, does a C<chroot> to C<$_>.
870 =item close FILEHANDLE
875 Closes the file or pipe associated with the file handle, flushes the IO
876 buffers, and closes the system file descriptor. Returns true if those
877 operations have succeeded and if no error was reported by any PerlIO
878 layer. Closes the currently selected filehandle if the argument is
881 You don't have to close FILEHANDLE if you are immediately going to do
882 another C<open> on it, because C<open> will close it for you. (See
883 C<open>.) However, an explicit C<close> on an input file resets the line
884 counter (C<$.>), while the implicit close done by C<open> does not.
886 If the file handle came from a piped open, C<close> will additionally
887 return false if one of the other system calls involved fails, or if the
888 program exits with non-zero status. (If the only problem was that the
889 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
890 also waits for the process executing on the pipe to complete, in case you
891 want to look at the output of the pipe afterwards, and
892 implicitly puts the exit status value of that command into C<$?> and
893 C<${^CHILD_ERROR_NATIVE}>.
895 Prematurely closing the read end of a pipe (i.e. before the process
896 writing to it at the other end has closed it) will result in a
897 SIGPIPE being delivered to the writer. If the other end can't
898 handle that, be sure to read all the data before closing the pipe.
902 open(OUTPUT, '|sort >foo') # pipe to sort
903 or die "Can't start sort: $!";
904 #... # print stuff to output
905 close OUTPUT # wait for sort to finish
906 or warn $! ? "Error closing sort pipe: $!"
907 : "Exit status $? from sort";
908 open(INPUT, 'foo') # get sort's results
909 or die "Can't open 'foo' for input: $!";
911 FILEHANDLE may be an expression whose value can be used as an indirect
912 filehandle, usually the real filehandle name.
914 =item closedir DIRHANDLE
917 Closes a directory opened by C<opendir> and returns the success of that
920 =item connect SOCKET,NAME
923 Attempts to connect to a remote socket, just as the connect system call
924 does. Returns true if it succeeded, false otherwise. NAME should be a
925 packed address of the appropriate type for the socket. See the examples in
926 L<perlipc/"Sockets: Client/Server Communication">.
933 C<continue> is actually a flow control statement rather than a function. If
934 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
935 C<foreach>), it is always executed just before the conditional is about to
936 be evaluated again, just like the third part of a C<for> loop in C. Thus
937 it can be used to increment a loop variable, even when the loop has been
938 continued via the C<next> statement (which is similar to the C C<continue>
941 C<last>, C<next>, or C<redo> may appear within a C<continue>
942 block. C<last> and C<redo> will behave as if they had been executed within
943 the main block. So will C<next>, but since it will execute a C<continue>
944 block, it may be more entertaining.
947 ### redo always comes here
950 ### next always comes here
952 # then back the top to re-check EXPR
954 ### last always comes here
956 Omitting the C<continue> section is semantically equivalent to using an
957 empty one, logically enough. In that case, C<next> goes directly back
958 to check the condition at the top of the loop.
960 If the "switch" feature is enabled, C<continue> is also a
961 function that will break out of the current C<when> or C<default>
962 block, and fall through to the next case. See L<feature> and
963 L<perlsyn/"Switch statements"> for more information.
967 X<cos> X<cosine> X<acos> X<arccosine>
971 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
972 takes cosine of C<$_>.
974 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
975 function, or use this relation:
977 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
979 =item crypt PLAINTEXT,SALT
980 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
981 X<decrypt> X<cryptography> X<passwd> X<encrypt>
983 Creates a digest string exactly like the crypt(3) function in the C
984 library (assuming that you actually have a version there that has not
985 been extirpated as a potential munition).
987 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
988 into a short string, called a digest, which is returned. The same
989 PLAINTEXT and SALT will always return the same string, but there is no
990 (known) way to get the original PLAINTEXT from the hash. Small
991 changes in the PLAINTEXT or SALT will result in large changes in the
994 There is no decrypt function. This function isn't all that useful for
995 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
996 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
997 primarily used to check if two pieces of text are the same without
998 having to transmit or store the text itself. An example is checking
999 if a correct password is given. The digest of the password is stored,
1000 not the password itself. The user types in a password that is
1001 crypt()'d with the same salt as the stored digest. If the two digests
1002 match the password is correct.
1004 When verifying an existing digest string you should use the digest as
1005 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1006 to create the digest is visible as part of the digest. This ensures
1007 crypt() will hash the new string with the same salt as the digest.
1008 This allows your code to work with the standard L<crypt|/crypt> and
1009 with more exotic implementations. In other words, do not assume
1010 anything about the returned string itself, or how many bytes in the
1013 Traditionally the result is a string of 13 bytes: two first bytes of
1014 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1015 the first eight bytes of PLAINTEXT mattered. But alternative
1016 hashing schemes (like MD5), higher level security schemes (like C2),
1017 and implementations on non-UNIX platforms may produce different
1020 When choosing a new salt create a random two character string whose
1021 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1022 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1023 characters is just a recommendation; the characters allowed in
1024 the salt depend solely on your system's crypt library, and Perl can't
1025 restrict what salts C<crypt()> accepts.
1027 Here's an example that makes sure that whoever runs this program knows
1030 $pwd = (getpwuid($<))[1];
1032 system "stty -echo";
1034 chomp($word = <STDIN>);
1038 if (crypt($word, $pwd) ne $pwd) {
1044 Of course, typing in your own password to whoever asks you
1047 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1048 of data, not least of all because you can't get the information
1049 back. Look at the L<Digest> module for more robust algorithms.
1051 If using crypt() on a Unicode string (which I<potentially> has
1052 characters with codepoints above 255), Perl tries to make sense
1053 of the situation by trying to downgrade (a copy of the string)
1054 the string back to an eight-bit byte string before calling crypt()
1055 (on that copy). If that works, good. If not, crypt() dies with
1056 C<Wide character in crypt>.
1061 [This function has been largely superseded by the C<untie> function.]
1063 Breaks the binding between a DBM file and a hash.
1065 =item dbmopen HASH,DBNAME,MASK
1066 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1068 [This function has been largely superseded by the C<tie> function.]
1070 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1071 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1072 argument is I<not> a filehandle, even though it looks like one). DBNAME
1073 is the name of the database (without the F<.dir> or F<.pag> extension if
1074 any). If the database does not exist, it is created with protection
1075 specified by MASK (as modified by the C<umask>). If your system supports
1076 only the older DBM functions, you may perform only one C<dbmopen> in your
1077 program. In older versions of Perl, if your system had neither DBM nor
1078 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1081 If you don't have write access to the DBM file, you can only read hash
1082 variables, not set them. If you want to test whether you can write,
1083 either use file tests or try setting a dummy hash entry inside an C<eval>,
1084 which will trap the error.
1086 Note that functions such as C<keys> and C<values> may return huge lists
1087 when used on large DBM files. You may prefer to use the C<each>
1088 function to iterate over large DBM files. Example:
1090 # print out history file offsets
1091 dbmopen(%HIST,'/usr/lib/news/history',0666);
1092 while (($key,$val) = each %HIST) {
1093 print $key, ' = ', unpack('L',$val), "\n";
1097 See also L<AnyDBM_File> for a more general description of the pros and
1098 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1099 rich implementation.
1101 You can control which DBM library you use by loading that library
1102 before you call dbmopen():
1105 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1106 or die "Can't open netscape history file: $!";
1109 X<defined> X<undef> X<undefined>
1113 Returns a Boolean value telling whether EXPR has a value other than
1114 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1117 Many operations return C<undef> to indicate failure, end of file,
1118 system error, uninitialized variable, and other exceptional
1119 conditions. This function allows you to distinguish C<undef> from
1120 other values. (A simple Boolean test will not distinguish among
1121 C<undef>, zero, the empty string, and C<"0">, which are all equally
1122 false.) Note that since C<undef> is a valid scalar, its presence
1123 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1124 returns C<undef> when its argument is an empty array, I<or> when the
1125 element to return happens to be C<undef>.
1127 You may also use C<defined(&func)> to check whether subroutine C<&func>
1128 has ever been defined. The return value is unaffected by any forward
1129 declarations of C<&func>. Note that a subroutine which is not defined
1130 may still be callable: its package may have an C<AUTOLOAD> method that
1131 makes it spring into existence the first time that it is called -- see
1134 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1135 used to report whether memory for that aggregate has ever been
1136 allocated. This behavior may disappear in future versions of Perl.
1137 You should instead use a simple test for size:
1139 if (@an_array) { print "has array elements\n" }
1140 if (%a_hash) { print "has hash members\n" }
1142 When used on a hash element, it tells you whether the value is defined,
1143 not whether the key exists in the hash. Use L</exists> for the latter
1148 print if defined $switch{'D'};
1149 print "$val\n" while defined($val = pop(@ary));
1150 die "Can't readlink $sym: $!"
1151 unless defined($value = readlink $sym);
1152 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1153 $debugging = 0 unless defined $debugging;
1155 Note: Many folks tend to overuse C<defined>, and then are surprised to
1156 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1157 defined values. For example, if you say
1161 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1162 matched "nothing". It didn't really fail to match anything. Rather, it
1163 matched something that happened to be zero characters long. This is all
1164 very above-board and honest. When a function returns an undefined value,
1165 it's an admission that it couldn't give you an honest answer. So you
1166 should use C<defined> only when you're questioning the integrity of what
1167 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1170 See also L</undef>, L</exists>, L</ref>.
1175 Given an expression that specifies a hash element, array element, hash slice,
1176 or array slice, deletes the specified element(s) from the hash or array.
1177 In the case of an array, if the array elements happen to be at the end,
1178 the size of the array will shrink to the highest element that tests
1179 true for exists() (or 0 if no such element exists).
1181 Returns a list with the same number of elements as the number of elements
1182 for which deletion was attempted. Each element of that list consists of
1183 either the value of the element deleted, or the undefined value. In scalar
1184 context, this means that you get the value of the last element deleted (or
1185 the undefined value if that element did not exist).
1187 %hash = (foo => 11, bar => 22, baz => 33);
1188 $scalar = delete $hash{foo}; # $scalar is 11
1189 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1190 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1192 Deleting from C<%ENV> modifies the environment. Deleting from
1193 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1194 from a C<tie>d hash or array may not necessarily return anything.
1196 Deleting an array element effectively returns that position of the array
1197 to its initial, uninitialized state. Subsequently testing for the same
1198 element with exists() will return false. Also, deleting array elements
1199 in the middle of an array will not shift the index of the elements
1200 after them down. Use splice() for that. See L</exists>.
1202 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1204 foreach $key (keys %HASH) {
1208 foreach $index (0 .. $#ARRAY) {
1209 delete $ARRAY[$index];
1214 delete @HASH{keys %HASH};
1216 delete @ARRAY[0 .. $#ARRAY];
1218 But both of these are slower than just assigning the empty list
1219 or undefining %HASH or @ARRAY:
1221 %HASH = (); # completely empty %HASH
1222 undef %HASH; # forget %HASH ever existed
1224 @ARRAY = (); # completely empty @ARRAY
1225 undef @ARRAY; # forget @ARRAY ever existed
1227 Note that the EXPR can be arbitrarily complicated as long as the final
1228 operation is a hash element, array element, hash slice, or array slice
1231 delete $ref->[$x][$y]{$key};
1232 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1234 delete $ref->[$x][$y][$index];
1235 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1237 The C<delete local EXPR> construct can also be used to localize the deletion
1238 of array/hash elements to the current block.
1239 See L<perlsub/"Localized deletion of elements of composite types">.
1242 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1244 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1245 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1246 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1247 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1248 an C<eval(),> the error message is stuffed into C<$@> and the
1249 C<eval> is terminated with the undefined value. This makes
1250 C<die> the way to raise an exception.
1252 Equivalent examples:
1254 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1255 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1257 If the last element of LIST does not end in a newline, the current
1258 script line number and input line number (if any) are also printed,
1259 and a newline is supplied. Note that the "input line number" (also
1260 known as "chunk") is subject to whatever notion of "line" happens to
1261 be currently in effect, and is also available as the special variable
1262 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1264 Hint: sometimes appending C<", stopped"> to your message will cause it
1265 to make better sense when the string C<"at foo line 123"> is appended.
1266 Suppose you are running script "canasta".
1268 die "/etc/games is no good";
1269 die "/etc/games is no good, stopped";
1271 produce, respectively
1273 /etc/games is no good at canasta line 123.
1274 /etc/games is no good, stopped at canasta line 123.
1276 See also exit(), warn(), and the Carp module.
1278 If the output is empty and C<$@> already contains a value (typically from a
1279 previous eval) that value is reused after appending C<"\t...propagated">.
1280 This is useful for propagating exceptions:
1283 die unless $@ =~ /Expected exception/;
1285 If the output is empty and C<$@> contains an object reference that has a
1286 C<PROPAGATE> method, that method will be called with additional file
1287 and line number parameters. The return value replaces the value in
1288 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1291 If C<$@> is empty then the string C<"Died"> is used.
1293 die() can also be called with a reference argument. If this happens to be
1294 trapped within an eval(), $@ contains the reference. This behavior permits
1295 a more elaborate exception handling implementation using objects that
1296 maintain arbitrary state about the nature of the exception. Such a scheme
1297 is sometimes preferable to matching particular string values of $@ using
1298 regular expressions. Because $@ is a global variable, and eval() may be
1299 used within object implementations, care must be taken that analyzing the
1300 error object doesn't replace the reference in the global variable. The
1301 easiest solution is to make a local copy of the reference before doing
1302 other manipulations. Here's an example:
1304 use Scalar::Util 'blessed';
1306 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1307 if (my $ev_err = $@) {
1308 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1309 # handle Some::Module::Exception
1312 # handle all other possible exceptions
1316 Because perl will stringify uncaught exception messages before displaying
1317 them, you may want to overload stringification operations on such custom
1318 exception objects. See L<overload> for details about that.
1320 You can arrange for a callback to be run just before the C<die>
1321 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1322 handler will be called with the error text and can change the error
1323 message, if it sees fit, by calling C<die> again. See
1324 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1325 L<"eval BLOCK"> for some examples. Although this feature was
1326 to be run only right before your program was to exit, this is not
1327 currently the case--the C<$SIG{__DIE__}> hook is currently called
1328 even inside eval()ed blocks/strings! If one wants the hook to do
1329 nothing in such situations, put
1333 as the first line of the handler (see L<perlvar/$^S>). Because
1334 this promotes strange action at a distance, this counterintuitive
1335 behavior may be fixed in a future release.
1340 Not really a function. Returns the value of the last command in the
1341 sequence of commands indicated by BLOCK. When modified by the C<while> or
1342 C<until> loop modifier, executes the BLOCK once before testing the loop
1343 condition. (On other statements the loop modifiers test the conditional
1346 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1347 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1348 See L<perlsyn> for alternative strategies.
1350 =item do SUBROUTINE(LIST)
1353 This form of subroutine call is deprecated. See L<perlsub>.
1358 Uses the value of EXPR as a filename and executes the contents of the
1359 file as a Perl script.
1367 except that it's more efficient and concise, keeps track of the current
1368 filename for error messages, searches the @INC directories, and updates
1369 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1370 variables. It also differs in that code evaluated with C<do FILENAME>
1371 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1372 same, however, in that it does reparse the file every time you call it,
1373 so you probably don't want to do this inside a loop.
1375 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1376 error. If C<do> can read the file but cannot compile it, it
1377 returns undef and sets an error message in C<$@>. If the file is
1378 successfully compiled, C<do> returns the value of the last expression
1381 Note that inclusion of library modules is better done with the
1382 C<use> and C<require> operators, which also do automatic error checking
1383 and raise an exception if there's a problem.
1385 You might like to use C<do> to read in a program configuration
1386 file. Manual error checking can be done this way:
1388 # read in config files: system first, then user
1389 for $file ("/share/prog/defaults.rc",
1390 "$ENV{HOME}/.someprogrc")
1392 unless ($return = do $file) {
1393 warn "couldn't parse $file: $@" if $@;
1394 warn "couldn't do $file: $!" unless defined $return;
1395 warn "couldn't run $file" unless $return;
1400 X<dump> X<core> X<undump>
1404 This function causes an immediate core dump. See also the B<-u>
1405 command-line switch in L<perlrun>, which does the same thing.
1406 Primarily this is so that you can use the B<undump> program (not
1407 supplied) to turn your core dump into an executable binary after
1408 having initialized all your variables at the beginning of the
1409 program. When the new binary is executed it will begin by executing
1410 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1411 Think of it as a goto with an intervening core dump and reincarnation.
1412 If C<LABEL> is omitted, restarts the program from the top.
1414 B<WARNING>: Any files opened at the time of the dump will I<not>
1415 be open any more when the program is reincarnated, with possible
1416 resulting confusion on the part of Perl.
1418 This function is now largely obsolete, mostly because it's very hard to
1419 convert a core file into an executable. That's why you should now invoke
1420 it as C<CORE::dump()>, if you don't want to be warned against a possible
1424 X<each> X<hash, iterator>
1429 When called in list context, returns a 2-element list consisting of the
1430 key and value for the next element of a hash, or the index and value for
1431 the next element of an array, so that you can iterate over it. When called
1432 in scalar context, returns only the key for the next element in the hash
1433 (or the index for an array).
1435 Hash entries are returned in an apparently random order. The actual random
1436 order is subject to change in future versions of perl, but it is
1437 guaranteed to be in the same order as either the C<keys> or C<values>
1438 function would produce on the same (unmodified) hash. Since Perl
1439 5.8.2 the ordering can be different even between different runs of Perl
1440 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1442 When the hash or array is entirely read, a null array is returned in list
1443 context (which when assigned produces a false (C<0>) value), and C<undef> in
1444 scalar context. The next call to C<each> after that will start iterating
1445 again. There is a single iterator for each hash or array, shared by all
1446 C<each>, C<keys>, and C<values> function calls in the program; it can be
1447 reset by reading all the elements from the hash or array, or by evaluating
1448 C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1449 or delete elements of a hash while you're
1450 iterating over it, you may get entries skipped or duplicated, so
1451 don't. Exception: It is always safe to delete the item most recently
1452 returned by C<each()>, which means that the following code will work:
1454 while (($key, $value) = each %hash) {
1456 delete $hash{$key}; # This is safe
1459 The following prints out your environment like the printenv(1) program,
1460 only in a different order:
1462 while (($key,$value) = each %ENV) {
1463 print "$key=$value\n";
1466 See also C<keys>, C<values> and C<sort>.
1468 =item eof FILEHANDLE
1477 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1478 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1479 gives the real filehandle. (Note that this function actually
1480 reads a character and then C<ungetc>s it, so isn't very useful in an
1481 interactive context.) Do not read from a terminal file (or call
1482 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1483 as terminals may lose the end-of-file condition if you do.
1485 An C<eof> without an argument uses the last file read. Using C<eof()>
1486 with empty parentheses is very different. It refers to the pseudo file
1487 formed from the files listed on the command line and accessed via the
1488 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1489 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1490 used will cause C<@ARGV> to be examined to determine if input is
1491 available. Similarly, an C<eof()> after C<< <> >> has returned
1492 end-of-file will assume you are processing another C<@ARGV> list,
1493 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1494 see L<perlop/"I/O Operators">.
1496 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1497 detect the end of each file, C<eof()> will only detect the end of the
1498 last file. Examples:
1500 # reset line numbering on each input file
1502 next if /^\s*#/; # skip comments
1505 close ARGV if eof; # Not eof()!
1508 # insert dashes just before last line of last file
1510 if (eof()) { # check for end of last file
1511 print "--------------\n";
1514 last if eof(); # needed if we're reading from a terminal
1517 Practical hint: you almost never need to use C<eof> in Perl, because the
1518 input operators typically return C<undef> when they run out of data, or if
1522 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1523 X<error, handling> X<exception, handling>
1529 In the first form, the return value of EXPR is parsed and executed as if it
1530 were a little Perl program. The value of the expression (which is itself
1531 determined within scalar context) is first parsed, and if there weren't any
1532 errors, executed in the lexical context of the current Perl program, so
1533 that any variable settings or subroutine and format definitions remain
1534 afterwards. Note that the value is parsed every time the C<eval> executes.
1535 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1536 delay parsing and subsequent execution of the text of EXPR until run time.
1538 In the second form, the code within the BLOCK is parsed only once--at the
1539 same time the code surrounding the C<eval> itself was parsed--and executed
1540 within the context of the current Perl program. This form is typically
1541 used to trap exceptions more efficiently than the first (see below), while
1542 also providing the benefit of checking the code within BLOCK at compile
1545 The final semicolon, if any, may be omitted from the value of EXPR or within
1548 In both forms, the value returned is the value of the last expression
1549 evaluated inside the mini-program; a return statement may be also used, just
1550 as with subroutines. The expression providing the return value is evaluated
1551 in void, scalar, or list context, depending on the context of the C<eval>
1552 itself. See L</wantarray> for more on how the evaluation context can be
1555 If there is a syntax error or runtime error, or a C<die> statement is
1556 executed, C<eval> returns an undefined value in scalar context
1557 or an empty list in list context, and C<$@> is set to the
1558 error message. If there was no error, C<$@> is guaranteed to be a null
1559 string. Beware that using C<eval> neither silences perl from printing
1560 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1561 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1562 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1563 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1565 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1566 determining whether a particular feature (such as C<socket> or C<symlink>)
1567 is implemented. It is also Perl's exception trapping mechanism, where
1568 the die operator is used to raise exceptions.
1570 If you want to trap errors when loading an XS module, some problems with
1571 the binary interface (such as Perl version skew) may be fatal even with
1572 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1574 If the code to be executed doesn't vary, you may use the eval-BLOCK
1575 form to trap run-time errors without incurring the penalty of
1576 recompiling each time. The error, if any, is still returned in C<$@>.
1579 # make divide-by-zero nonfatal
1580 eval { $answer = $a / $b; }; warn $@ if $@;
1582 # same thing, but less efficient
1583 eval '$answer = $a / $b'; warn $@ if $@;
1585 # a compile-time error
1586 eval { $answer = }; # WRONG
1589 eval '$answer ='; # sets $@
1591 Using the C<eval{}> form as an exception trap in libraries does have some
1592 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1593 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1594 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1595 as shown in this example:
1597 # a very private exception trap for divide-by-zero
1598 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1601 This is especially significant, given that C<__DIE__> hooks can call
1602 C<die> again, which has the effect of changing their error messages:
1604 # __DIE__ hooks may modify error messages
1606 local $SIG{'__DIE__'} =
1607 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1608 eval { die "foo lives here" };
1609 print $@ if $@; # prints "bar lives here"
1612 Because this promotes action at a distance, this counterintuitive behavior
1613 may be fixed in a future release.
1615 With an C<eval>, you should be especially careful to remember what's
1616 being looked at when:
1622 eval { $x }; # CASE 4
1624 eval "\$$x++"; # CASE 5
1627 Cases 1 and 2 above behave identically: they run the code contained in
1628 the variable $x. (Although case 2 has misleading double quotes making
1629 the reader wonder what else might be happening (nothing is).) Cases 3
1630 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1631 does nothing but return the value of $x. (Case 4 is preferred for
1632 purely visual reasons, but it also has the advantage of compiling at
1633 compile-time instead of at run-time.) Case 5 is a place where
1634 normally you I<would> like to use double quotes, except that in this
1635 particular situation, you can just use symbolic references instead, as
1638 The assignment to C<$@> occurs before restoration of localised variables,
1639 which means a temporary is required if you want to mask some but not all
1642 # alter $@ on nefarious repugnancy only
1646 local $@; # protect existing $@
1647 eval { test_repugnancy() };
1648 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1649 $@ =~ /nefarious/ and $e = $@;
1651 die $e if defined $e
1654 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1655 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1657 Note that as a very special case, an C<eval ''> executed within the C<DB>
1658 package doesn't see the usual surrounding lexical scope, but rather the
1659 scope of the first non-DB piece of code that called it. You don't normally
1660 need to worry about this unless you are writing a Perl debugger.
1665 =item exec PROGRAM LIST
1667 The C<exec> function executes a system command I<and never returns>--
1668 use C<system> instead of C<exec> if you want it to return. It fails and
1669 returns false only if the command does not exist I<and> it is executed
1670 directly instead of via your system's command shell (see below).
1672 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1673 warns you if there is a following statement which isn't C<die>, C<warn>,
1674 or C<exit> (if C<-w> is set - but you always do that). If you
1675 I<really> want to follow an C<exec> with some other statement, you
1676 can use one of these styles to avoid the warning:
1678 exec ('foo') or print STDERR "couldn't exec foo: $!";
1679 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1681 If there is more than one argument in LIST, or if LIST is an array
1682 with more than one value, calls execvp(3) with the arguments in LIST.
1683 If there is only one scalar argument or an array with one element in it,
1684 the argument is checked for shell metacharacters, and if there are any,
1685 the entire argument is passed to the system's command shell for parsing
1686 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1687 If there are no shell metacharacters in the argument, it is split into
1688 words and passed directly to C<execvp>, which is more efficient.
1691 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1692 exec "sort $outfile | uniq";
1694 If you don't really want to execute the first argument, but want to lie
1695 to the program you are executing about its own name, you can specify
1696 the program you actually want to run as an "indirect object" (without a
1697 comma) in front of the LIST. (This always forces interpretation of the
1698 LIST as a multivalued list, even if there is only a single scalar in
1701 $shell = '/bin/csh';
1702 exec $shell '-sh'; # pretend it's a login shell
1706 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1708 When the arguments get executed via the system shell, results will
1709 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1712 Using an indirect object with C<exec> or C<system> is also more
1713 secure. This usage (which also works fine with system()) forces
1714 interpretation of the arguments as a multivalued list, even if the
1715 list had just one argument. That way you're safe from the shell
1716 expanding wildcards or splitting up words with whitespace in them.
1718 @args = ( "echo surprise" );
1720 exec @args; # subject to shell escapes
1722 exec { $args[0] } @args; # safe even with one-arg list
1724 The first version, the one without the indirect object, ran the I<echo>
1725 program, passing it C<"surprise"> an argument. The second version
1726 didn't--it tried to run a program literally called I<"echo surprise">,
1727 didn't find it, and set C<$?> to a non-zero value indicating failure.
1729 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1730 output before the exec, but this may not be supported on some platforms
1731 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1732 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1733 open handles in order to avoid lost output.
1735 Note that C<exec> will not call your C<END> blocks, nor will it call
1736 any C<DESTROY> methods in your objects.
1739 X<exists> X<autovivification>
1741 Given an expression that specifies a hash element or array element,
1742 returns true if the specified element in the hash or array has ever
1743 been initialized, even if the corresponding value is undefined.
1745 print "Exists\n" if exists $hash{$key};
1746 print "Defined\n" if defined $hash{$key};
1747 print "True\n" if $hash{$key};
1749 print "Exists\n" if exists $array[$index];
1750 print "Defined\n" if defined $array[$index];
1751 print "True\n" if $array[$index];
1753 A hash or array element can be true only if it's defined, and defined if
1754 it exists, but the reverse doesn't necessarily hold true.
1756 Given an expression that specifies the name of a subroutine,
1757 returns true if the specified subroutine has ever been declared, even
1758 if it is undefined. Mentioning a subroutine name for exists or defined
1759 does not count as declaring it. Note that a subroutine which does not
1760 exist may still be callable: its package may have an C<AUTOLOAD>
1761 method that makes it spring into existence the first time that it is
1762 called -- see L<perlsub>.
1764 print "Exists\n" if exists &subroutine;
1765 print "Defined\n" if defined &subroutine;
1767 Note that the EXPR can be arbitrarily complicated as long as the final
1768 operation is a hash or array key lookup or subroutine name:
1770 if (exists $ref->{A}->{B}->{$key}) { }
1771 if (exists $hash{A}{B}{$key}) { }
1773 if (exists $ref->{A}->{B}->[$ix]) { }
1774 if (exists $hash{A}{B}[$ix]) { }
1776 if (exists &{$ref->{A}{B}{$key}}) { }
1778 Although the deepest nested array or hash will not spring into existence
1779 just because its existence was tested, any intervening ones will.
1780 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1781 into existence due to the existence test for the $key element above.
1782 This happens anywhere the arrow operator is used, including even:
1785 if (exists $ref->{"Some key"}) { }
1786 print $ref; # prints HASH(0x80d3d5c)
1788 This surprising autovivification in what does not at first--or even
1789 second--glance appear to be an lvalue context may be fixed in a future
1792 Use of a subroutine call, rather than a subroutine name, as an argument
1793 to exists() is an error.
1796 exists &sub(); # Error
1799 X<exit> X<terminate> X<abort>
1803 Evaluates EXPR and exits immediately with that value. Example:
1806 exit 0 if $ans =~ /^[Xx]/;
1808 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1809 universally recognized values for EXPR are C<0> for success and C<1>
1810 for error; other values are subject to interpretation depending on the
1811 environment in which the Perl program is running. For example, exiting
1812 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1813 the mailer to return the item undelivered, but that's not true everywhere.
1815 Don't use C<exit> to abort a subroutine if there's any chance that
1816 someone might want to trap whatever error happened. Use C<die> instead,
1817 which can be trapped by an C<eval>.
1819 The exit() function does not always exit immediately. It calls any
1820 defined C<END> routines first, but these C<END> routines may not
1821 themselves abort the exit. Likewise any object destructors that need to
1822 be called are called before the real exit. If this is a problem, you
1823 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1824 See L<perlmod> for details.
1827 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1831 Returns I<e> (the natural logarithm base) to the power of EXPR.
1832 If EXPR is omitted, gives C<exp($_)>.
1834 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1837 Implements the fcntl(2) function. You'll probably have to say
1841 first to get the correct constant definitions. Argument processing and
1842 value return works just like C<ioctl> below.
1846 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1847 or die "can't fcntl F_GETFL: $!";
1849 You don't have to check for C<defined> on the return from C<fcntl>.
1850 Like C<ioctl>, it maps a C<0> return from the system call into
1851 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1852 in numeric context. It is also exempt from the normal B<-w> warnings
1853 on improper numeric conversions.
1855 Note that C<fcntl> will produce a fatal error if used on a machine that
1856 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1857 manpage to learn what functions are available on your system.
1859 Here's an example of setting a filehandle named C<REMOTE> to be
1860 non-blocking at the system level. You'll have to negotiate C<$|>
1861 on your own, though.
1863 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1865 $flags = fcntl(REMOTE, F_GETFL, 0)
1866 or die "Can't get flags for the socket: $!\n";
1868 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1869 or die "Can't set flags for the socket: $!\n";
1871 =item fileno FILEHANDLE
1874 Returns the file descriptor for a filehandle, or undefined if the
1875 filehandle is not open. This is mainly useful for constructing
1876 bitmaps for C<select> and low-level POSIX tty-handling operations.
1877 If FILEHANDLE is an expression, the value is taken as an indirect
1878 filehandle, generally its name.
1880 You can use this to find out whether two handles refer to the
1881 same underlying descriptor:
1883 if (fileno(THIS) == fileno(THAT)) {
1884 print "THIS and THAT are dups\n";
1887 (Filehandles connected to memory objects via new features of C<open> may
1888 return undefined even though they are open.)
1891 =item flock FILEHANDLE,OPERATION
1892 X<flock> X<lock> X<locking>
1894 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1895 for success, false on failure. Produces a fatal error if used on a
1896 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1897 C<flock> is Perl's portable file locking interface, although it locks
1898 only entire files, not records.
1900 Two potentially non-obvious but traditional C<flock> semantics are
1901 that it waits indefinitely until the lock is granted, and that its locks
1902 B<merely advisory>. Such discretionary locks are more flexible, but offer
1903 fewer guarantees. This means that programs that do not also use C<flock>
1904 may modify files locked with C<flock>. See L<perlport>,
1905 your port's specific documentation, or your system-specific local manpages
1906 for details. It's best to assume traditional behavior if you're writing
1907 portable programs. (But if you're not, you should as always feel perfectly
1908 free to write for your own system's idiosyncrasies (sometimes called
1909 "features"). Slavish adherence to portability concerns shouldn't get
1910 in the way of your getting your job done.)
1912 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1913 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1914 you can use the symbolic names if you import them from the Fcntl module,
1915 either individually, or as a group using the ':flock' tag. LOCK_SH
1916 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1917 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1918 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1919 waiting for the lock (check the return status to see if you got it).
1921 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1922 before locking or unlocking it.
1924 Note that the emulation built with lockf(3) doesn't provide shared
1925 locks, and it requires that FILEHANDLE be open with write intent. These
1926 are the semantics that lockf(3) implements. Most if not all systems
1927 implement lockf(3) in terms of fcntl(2) locking, though, so the
1928 differing semantics shouldn't bite too many people.
1930 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1931 be open with read intent to use LOCK_SH and requires that it be open
1932 with write intent to use LOCK_EX.
1934 Note also that some versions of C<flock> cannot lock things over the
1935 network; you would need to use the more system-specific C<fcntl> for
1936 that. If you like you can force Perl to ignore your system's flock(2)
1937 function, and so provide its own fcntl(2)-based emulation, by passing
1938 the switch C<-Ud_flock> to the F<Configure> program when you configure
1941 Here's a mailbox appender for BSD systems.
1943 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
1947 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
1949 # and, in case someone appended while we were waiting...
1950 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
1955 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
1958 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
1959 or die "Can't open mailbox: $!";
1962 print $mbox $msg,"\n\n";
1965 On systems that support a real flock(), locks are inherited across fork()
1966 calls, whereas those that must resort to the more capricious fcntl()
1967 function lose the locks, making it harder to write servers.
1969 See also L<DB_File> for other flock() examples.
1972 X<fork> X<child> X<parent>
1974 Does a fork(2) system call to create a new process running the
1975 same program at the same point. It returns the child pid to the
1976 parent process, C<0> to the child process, or C<undef> if the fork is
1977 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1978 are shared, while everything else is copied. On most systems supporting
1979 fork(), great care has gone into making it extremely efficient (for
1980 example, using copy-on-write technology on data pages), making it the
1981 dominant paradigm for multitasking over the last few decades.
1983 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1984 output before forking the child process, but this may not be supported
1985 on some platforms (see L<perlport>). To be safe, you may need to set
1986 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1987 C<IO::Handle> on any open handles in order to avoid duplicate output.
1989 If you C<fork> without ever waiting on your children, you will
1990 accumulate zombies. On some systems, you can avoid this by setting
1991 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1992 forking and reaping moribund children.
1994 Note that if your forked child inherits system file descriptors like
1995 STDIN and STDOUT that are actually connected by a pipe or socket, even
1996 if you exit, then the remote server (such as, say, a CGI script or a
1997 backgrounded job launched from a remote shell) won't think you're done.
1998 You should reopen those to F</dev/null> if it's any issue.
2003 Declare a picture format for use by the C<write> function. For
2007 Test: @<<<<<<<< @||||| @>>>>>
2008 $str, $%, '$' . int($num)
2012 $num = $cost/$quantity;
2016 See L<perlform> for many details and examples.
2018 =item formline PICTURE,LIST
2021 This is an internal function used by C<format>s, though you may call it,
2022 too. It formats (see L<perlform>) a list of values according to the
2023 contents of PICTURE, placing the output into the format output
2024 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2025 Eventually, when a C<write> is done, the contents of
2026 C<$^A> are written to some filehandle. You could also read C<$^A>
2027 and then set C<$^A> back to C<"">. Note that a format typically
2028 does one C<formline> per line of form, but the C<formline> function itself
2029 doesn't care how many newlines are embedded in the PICTURE. This means
2030 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2031 You may therefore need to use multiple formlines to implement a single
2032 record format, just like the format compiler.
2034 Be careful if you put double quotes around the picture, because an C<@>
2035 character may be taken to mean the beginning of an array name.
2036 C<formline> always returns true. See L<perlform> for other examples.
2038 =item getc FILEHANDLE
2039 X<getc> X<getchar> X<character> X<file, read>
2043 Returns the next character from the input file attached to FILEHANDLE,
2044 or the undefined value at end of file, or if there was an error (in
2045 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2046 STDIN. This is not particularly efficient. However, it cannot be
2047 used by itself to fetch single characters without waiting for the user
2048 to hit enter. For that, try something more like:
2051 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2054 system "stty", '-icanon', 'eol', "\001";
2060 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2063 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2067 Determination of whether $BSD_STYLE should be set
2068 is left as an exercise to the reader.
2070 The C<POSIX::getattr> function can do this more portably on
2071 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2072 module from your nearest CPAN site; details on CPAN can be found on
2076 X<getlogin> X<login>
2078 This implements the C library function of the same name, which on most
2079 systems returns the current login from F</etc/utmp>, if any. If null,
2082 $login = getlogin || getpwuid($<) || "Kilroy";
2084 Do not consider C<getlogin> for authentication: it is not as
2085 secure as C<getpwuid>.
2087 =item getpeername SOCKET
2088 X<getpeername> X<peer>
2090 Returns the packed sockaddr address of other end of the SOCKET connection.
2093 $hersockaddr = getpeername(SOCK);
2094 ($port, $iaddr) = sockaddr_in($hersockaddr);
2095 $herhostname = gethostbyaddr($iaddr, AF_INET);
2096 $herstraddr = inet_ntoa($iaddr);
2101 Returns the current process group for the specified PID. Use
2102 a PID of C<0> to get the current process group for the
2103 current process. Will raise an exception if used on a machine that
2104 doesn't implement getpgrp(2). If PID is omitted, returns process
2105 group of current process. Note that the POSIX version of C<getpgrp>
2106 does not accept a PID argument, so only C<PID==0> is truly portable.
2109 X<getppid> X<parent> X<pid>
2111 Returns the process id of the parent process.
2113 Note for Linux users: on Linux, the C functions C<getpid()> and
2114 C<getppid()> return different values from different threads. In order to
2115 be portable, this behavior is not reflected by the perl-level function
2116 C<getppid()>, that returns a consistent value across threads. If you want
2117 to call the underlying C<getppid()>, you may use the CPAN module
2120 =item getpriority WHICH,WHO
2121 X<getpriority> X<priority> X<nice>
2123 Returns the current priority for a process, a process group, or a user.
2124 (See C<getpriority(2)>.) Will raise a fatal exception if used on a
2125 machine that doesn't implement getpriority(2).
2128 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2129 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2130 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2131 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2132 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2133 X<endnetent> X<endprotoent> X<endservent>
2137 =item gethostbyname NAME
2139 =item getnetbyname NAME
2141 =item getprotobyname NAME
2147 =item getservbyname NAME,PROTO
2149 =item gethostbyaddr ADDR,ADDRTYPE
2151 =item getnetbyaddr ADDR,ADDRTYPE
2153 =item getprotobynumber NUMBER
2155 =item getservbyport PORT,PROTO
2173 =item sethostent STAYOPEN
2175 =item setnetent STAYOPEN
2177 =item setprotoent STAYOPEN
2179 =item setservent STAYOPEN
2193 These routines perform the same functions as their counterparts in the
2194 system library. In list context, the return values from the
2195 various get routines are as follows:
2197 ($name,$passwd,$uid,$gid,
2198 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2199 ($name,$passwd,$gid,$members) = getgr*
2200 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2201 ($name,$aliases,$addrtype,$net) = getnet*
2202 ($name,$aliases,$proto) = getproto*
2203 ($name,$aliases,$port,$proto) = getserv*
2205 (If the entry doesn't exist you get a null list.)
2207 The exact meaning of the $gcos field varies but it usually contains
2208 the real name of the user (as opposed to the login name) and other
2209 information pertaining to the user. Beware, however, that in many
2210 system users are able to change this information and therefore it
2211 cannot be trusted and therefore the $gcos is tainted (see
2212 L<perlsec>). The $passwd and $shell, user's encrypted password and
2213 login shell, are also tainted, because of the same reason.
2215 In scalar context, you get the name, unless the function was a
2216 lookup by name, in which case you get the other thing, whatever it is.
2217 (If the entry doesn't exist you get the undefined value.) For example:
2219 $uid = getpwnam($name);
2220 $name = getpwuid($num);
2222 $gid = getgrnam($name);
2223 $name = getgrgid($num);
2227 In I<getpw*()> the fields $quota, $comment, and $expire are special
2228 cases in the sense that in many systems they are unsupported. If the
2229 $quota is unsupported, it is an empty scalar. If it is supported, it
2230 usually encodes the disk quota. If the $comment field is unsupported,
2231 it is an empty scalar. If it is supported it usually encodes some
2232 administrative comment about the user. In some systems the $quota
2233 field may be $change or $age, fields that have to do with password
2234 aging. In some systems the $comment field may be $class. The $expire
2235 field, if present, encodes the expiration period of the account or the
2236 password. For the availability and the exact meaning of these fields
2237 in your system, please consult your getpwnam(3) documentation and your
2238 F<pwd.h> file. You can also find out from within Perl what your
2239 $quota and $comment fields mean and whether you have the $expire field
2240 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2241 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2242 files are only supported if your vendor has implemented them in the
2243 intuitive fashion that calling the regular C library routines gets the
2244 shadow versions if you're running under privilege or if there exists
2245 the shadow(3) functions as found in System V (this includes Solaris
2246 and Linux.) Those systems that implement a proprietary shadow password
2247 facility are unlikely to be supported.
2249 The $members value returned by I<getgr*()> is a space separated list of
2250 the login names of the members of the group.
2252 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2253 C, it will be returned to you via C<$?> if the function call fails. The
2254 C<@addrs> value returned by a successful call is a list of the raw
2255 addresses returned by the corresponding system library call. In the
2256 Internet domain, each address is four bytes long and you can unpack it
2257 by saying something like:
2259 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2261 The Socket library makes this slightly easier:
2264 $iaddr = inet_aton("127.1"); # or whatever address
2265 $name = gethostbyaddr($iaddr, AF_INET);
2267 # or going the other way
2268 $straddr = inet_ntoa($iaddr);
2270 In the opposite way, to resolve a hostname to the IP address
2274 $packed_ip = gethostbyname("www.perl.org");
2275 if (defined $packed_ip) {
2276 $ip_address = inet_ntoa($packed_ip);
2279 Make sure <gethostbyname()> is called in SCALAR context and that
2280 its return value is checked for definedness.
2282 If you get tired of remembering which element of the return list
2283 contains which return value, by-name interfaces are provided
2284 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2285 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2286 and C<User::grent>. These override the normal built-ins, supplying
2287 versions that return objects with the appropriate names
2288 for each field. For example:
2292 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2294 Even though it looks like they're the same method calls (uid),
2295 they aren't, because a C<File::stat> object is different from
2296 a C<User::pwent> object.
2298 =item getsockname SOCKET
2301 Returns the packed sockaddr address of this end of the SOCKET connection,
2302 in case you don't know the address because you have several different
2303 IPs that the connection might have come in on.
2306 $mysockaddr = getsockname(SOCK);
2307 ($port, $myaddr) = sockaddr_in($mysockaddr);
2308 printf "Connect to %s [%s]\n",
2309 scalar gethostbyaddr($myaddr, AF_INET),
2312 =item getsockopt SOCKET,LEVEL,OPTNAME
2315 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2316 Options may exist at multiple protocol levels depending on the socket
2317 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2318 C<Socket> module) will exist. To query options at another level the
2319 protocol number of the appropriate protocol controlling the option
2320 should be supplied. For example, to indicate that an option is to be
2321 interpreted by the TCP protocol, LEVEL should be set to the protocol
2322 number of TCP, which you can get using getprotobyname.
2324 The call returns a packed string representing the requested socket option,
2325 or C<undef> if there is an error (the error reason will be in $!). What
2326 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2327 your system documentation for details. A very common case however is that
2328 the option is an integer, in which case the result will be a packed
2329 integer which you can decode using unpack with the C<i> (or C<I>) format.
2331 An example testing if Nagle's algorithm is turned on on a socket:
2333 use Socket qw(:all);
2335 defined(my $tcp = getprotobyname("tcp"))
2336 or die "Could not determine the protocol number for tcp";
2337 # my $tcp = IPPROTO_TCP; # Alternative
2338 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2339 or die "Could not query TCP_NODELAY socket option: $!";
2340 my $nodelay = unpack("I", $packed);
2341 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2345 X<glob> X<wildcard> X<filename, expansion> X<expand>
2349 In list context, returns a (possibly empty) list of filename expansions on
2350 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2351 scalar context, glob iterates through such filename expansions, returning
2352 undef when the list is exhausted. This is the internal function
2353 implementing the C<< <*.c> >> operator, but you can use it directly. If
2354 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2355 more detail in L<perlop/"I/O Operators">.
2357 Note that C<glob> will split its arguments on whitespace, treating
2358 each segment as separate pattern. As such, C<glob('*.c *.h')> would
2359 match all files with a F<.c> or F<.h> extension. The expression
2360 C<glob('.* *')> would match all files in the current working directory.
2362 Beginning with v5.6.0, this operator is implemented using the standard
2363 C<File::Glob> extension. See L<File::Glob> for details, including
2364 C<bsd_glob> which does not treat whitespace as a pattern separator.
2367 X<gmtime> X<UTC> X<Greenwich>
2371 Works just like L<localtime> but the returned values are
2372 localized for the standard Greenwich time zone.
2374 Note: when called in list context, $isdst, the last value
2375 returned by gmtime is always C<0>. There is no
2376 Daylight Saving Time in GMT.
2378 See L<perlport/gmtime> for portability concerns.
2381 X<goto> X<jump> X<jmp>
2387 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2388 execution there. It may not be used to go into any construct that
2389 requires initialization, such as a subroutine or a C<foreach> loop. It
2390 also can't be used to go into a construct that is optimized away,
2391 or to get out of a block or subroutine given to C<sort>.
2392 It can be used to go almost anywhere else within the dynamic scope,
2393 including out of subroutines, but it's usually better to use some other
2394 construct such as C<last> or C<die>. The author of Perl has never felt the
2395 need to use this form of C<goto> (in Perl, that is--C is another matter).
2396 (The difference being that C does not offer named loops combined with
2397 loop control. Perl does, and this replaces most structured uses of C<goto>
2398 in other languages.)
2400 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2401 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2402 necessarily recommended if you're optimizing for maintainability:
2404 goto ("FOO", "BAR", "GLARCH")[$i];
2406 The C<goto-&NAME> form is quite different from the other forms of
2407 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2408 doesn't have the stigma associated with other gotos. Instead, it
2409 exits the current subroutine (losing any changes set by local()) and
2410 immediately calls in its place the named subroutine using the current
2411 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2412 load another subroutine and then pretend that the other subroutine had
2413 been called in the first place (except that any modifications to C<@_>
2414 in the current subroutine are propagated to the other subroutine.)
2415 After the C<goto>, not even C<caller> will be able to tell that this
2416 routine was called first.
2418 NAME needn't be the name of a subroutine; it can be a scalar variable
2419 containing a code reference, or a block that evaluates to a code
2422 =item grep BLOCK LIST
2425 =item grep EXPR,LIST
2427 This is similar in spirit to, but not the same as, grep(1) and its
2428 relatives. In particular, it is not limited to using regular expressions.
2430 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2431 C<$_> to each element) and returns the list value consisting of those
2432 elements for which the expression evaluated to true. In scalar
2433 context, returns the number of times the expression was true.
2435 @foo = grep(!/^#/, @bar); # weed out comments
2439 @foo = grep {!/^#/} @bar; # weed out comments
2441 Note that C<$_> is an alias to the list value, so it can be used to
2442 modify the elements of the LIST. While this is useful and supported,
2443 it can cause bizarre results if the elements of LIST are not variables.
2444 Similarly, grep returns aliases into the original list, much as a for
2445 loop's index variable aliases the list elements. That is, modifying an
2446 element of a list returned by grep (for example, in a C<foreach>, C<map>
2447 or another C<grep>) actually modifies the element in the original list.
2448 This is usually something to be avoided when writing clear code.
2450 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2451 been declared with C<my $_>) then, in addition to being locally aliased to
2452 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2453 can't be seen from the outside, avoiding any potential side-effects.
2455 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2458 X<hex> X<hexadecimal>
2462 Interprets EXPR as a hex string and returns the corresponding value.
2463 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2464 L</oct>.) If EXPR is omitted, uses C<$_>.
2466 print hex '0xAf'; # prints '175'
2467 print hex 'aF'; # same
2469 Hex strings may only represent integers. Strings that would cause
2470 integer overflow trigger a warning. Leading whitespace is not stripped,
2471 unlike oct(). To present something as hex, look into L</printf>,
2472 L</sprintf>, or L</unpack>.
2477 There is no builtin C<import> function. It is just an ordinary
2478 method (subroutine) defined (or inherited) by modules that wish to export
2479 names to another module. The C<use> function calls the C<import> method
2480 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2482 =item index STR,SUBSTR,POSITION
2483 X<index> X<indexOf> X<InStr>
2485 =item index STR,SUBSTR
2487 The index function searches for one string within another, but without
2488 the wildcard-like behavior of a full regular-expression pattern match.
2489 It returns the position of the first occurrence of SUBSTR in STR at
2490 or after POSITION. If POSITION is omitted, starts searching from the
2491 beginning of the string. POSITION before the beginning of the string
2492 or after its end is treated as if it were the beginning or the end,
2493 respectively. POSITION and the return value are based at C<0> (or whatever
2494 you've set the C<$[> variable to--but don't do that). If the substring
2495 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2498 X<int> X<integer> X<truncate> X<trunc> X<floor>
2502 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2503 You should not use this function for rounding: one because it truncates
2504 towards C<0>, and two because machine representations of floating point
2505 numbers can sometimes produce counterintuitive results. For example,
2506 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2507 because it's really more like -268.99999999999994315658 instead. Usually,
2508 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2509 functions will serve you better than will int().
2511 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2514 Implements the ioctl(2) function. You'll probably first have to say
2516 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2518 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2519 exist or doesn't have the correct definitions you'll have to roll your
2520 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2521 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2522 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2523 written depending on the FUNCTION--a pointer to the string value of SCALAR
2524 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2525 has no string value but does have a numeric value, that value will be
2526 passed rather than a pointer to the string value. To guarantee this to be
2527 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2528 functions may be needed to manipulate the values of structures used by
2531 The return value of C<ioctl> (and C<fcntl>) is as follows:
2533 if OS returns: then Perl returns:
2535 0 string "0 but true"
2536 anything else that number
2538 Thus Perl returns true on success and false on failure, yet you can
2539 still easily determine the actual value returned by the operating
2542 $retval = ioctl(...) || -1;
2543 printf "System returned %d\n", $retval;
2545 The special string C<"0 but true"> is exempt from B<-w> complaints
2546 about improper numeric conversions.
2548 =item join EXPR,LIST
2551 Joins the separate strings of LIST into a single string with fields
2552 separated by the value of EXPR, and returns that new string. Example:
2554 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2556 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2557 first argument. Compare L</split>.
2564 Returns a list consisting of all the keys of the named hash, or the indices
2565 of an array. (In scalar context, returns the number of keys or indices.)
2567 The keys of a hash are returned in an apparently random order. The actual
2568 random order is subject to change in future versions of perl, but it
2569 is guaranteed to be the same order as either the C<values> or C<each>
2570 function produces (given that the hash has not been modified). Since
2571 Perl 5.8.1 the ordering is different even between different runs of
2572 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2575 As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
2576 (see L</each>). In particular, calling keys() in void context resets
2577 the iterator with no other overhead.
2579 Here is yet another way to print your environment:
2582 @values = values %ENV;
2584 print pop(@keys), '=', pop(@values), "\n";
2587 or how about sorted by key:
2589 foreach $key (sort(keys %ENV)) {
2590 print $key, '=', $ENV{$key}, "\n";
2593 The returned values are copies of the original keys in the hash, so
2594 modifying them will not affect the original hash. Compare L</values>.
2596 To sort a hash by value, you'll need to use a C<sort> function.
2597 Here's a descending numeric sort of a hash by its values:
2599 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2600 printf "%4d %s\n", $hash{$key}, $key;
2603 As an lvalue C<keys> allows you to increase the number of hash buckets
2604 allocated for the given hash. This can gain you a measure of efficiency if
2605 you know the hash is going to get big. (This is similar to pre-extending
2606 an array by assigning a larger number to $#array.) If you say
2610 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2611 in fact, since it rounds up to the next power of two. These
2612 buckets will be retained even if you do C<%hash = ()>, use C<undef
2613 %hash> if you want to free the storage while C<%hash> is still in scope.
2614 You can't shrink the number of buckets allocated for the hash using
2615 C<keys> in this way (but you needn't worry about doing this by accident,
2616 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2619 See also C<each>, C<values> and C<sort>.
2621 =item kill SIGNAL, LIST
2624 Sends a signal to a list of processes. Returns the number of
2625 processes successfully signaled (which is not necessarily the
2626 same as the number actually killed).
2628 $cnt = kill 1, $child1, $child2;
2631 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2632 system call will check whether it's possible to send a signal to it (that
2633 means, to be brief, that the process is owned by the same user, or we are
2634 the super-user). This is a useful way to check that a child process is
2635 alive (even if only as a zombie) and hasn't changed its UID. See
2636 L<perlport> for notes on the portability of this construct.
2638 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2639 of processes. That means you usually want to use positive not negative signals.
2640 You may also use a signal name in quotes.
2642 The behavior of kill when a I<PROCESS> number is zero or negative depends on
2643 the operating system. For example, on POSIX-conforming systems, zero will
2644 signal the current process group and -1 will signal all processes.
2646 See L<perlipc/"Signals"> for more details.
2653 The C<last> command is like the C<break> statement in C (as used in
2654 loops); it immediately exits the loop in question. If the LABEL is
2655 omitted, the command refers to the innermost enclosing loop. The
2656 C<continue> block, if any, is not executed:
2658 LINE: while (<STDIN>) {
2659 last LINE if /^$/; # exit when done with header
2663 C<last> cannot be used to exit a block which returns a value such as
2664 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2665 a grep() or map() operation.
2667 Note that a block by itself is semantically identical to a loop
2668 that executes once. Thus C<last> can be used to effect an early
2669 exit out of such a block.
2671 See also L</continue> for an illustration of how C<last>, C<next>, and
2679 Returns a lowercased version of EXPR. This is the internal function
2680 implementing the C<\L> escape in double-quoted strings. Respects
2681 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2682 and L<perlunicode> for more details about locale and Unicode support.
2684 If EXPR is omitted, uses C<$_>.
2687 X<lcfirst> X<lowercase>
2691 Returns the value of EXPR with the first character lowercased. This
2692 is the internal function implementing the C<\l> escape in
2693 double-quoted strings. Respects current LC_CTYPE locale if C<use
2694 locale> in force. See L<perllocale> and L<perlunicode> for more
2695 details about locale and Unicode support.
2697 If EXPR is omitted, uses C<$_>.
2704 Returns the length in I<characters> of the value of EXPR. If EXPR is
2705 omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2706 Note that this cannot be used on an entire array or hash to find out how
2707 many elements these have. For that, use C<scalar @array> and C<scalar keys
2708 %hash> respectively.
2710 Note the I<characters>: if the EXPR is in Unicode, you will get the
2711 number of characters, not the number of bytes. To get the length
2712 of the internal string in bytes, use C<bytes::length(EXPR)>, see
2713 L<bytes>. Note that the internal encoding is variable, and the number
2714 of bytes usually meaningless. To get the number of bytes that the
2715 string would have when encoded as UTF-8, use
2716 C<length(Encoding::encode_utf8(EXPR))>.
2718 =item link OLDFILE,NEWFILE
2721 Creates a new filename linked to the old filename. Returns true for
2722 success, false otherwise.
2724 =item listen SOCKET,QUEUESIZE
2727 Does the same thing that the listen system call does. Returns true if
2728 it succeeded, false otherwise. See the example in
2729 L<perlipc/"Sockets: Client/Server Communication">.
2734 You really probably want to be using C<my> instead, because C<local> isn't
2735 what most people think of as "local". See
2736 L<perlsub/"Private Variables via my()"> for details.
2738 A local modifies the listed variables to be local to the enclosing
2739 block, file, or eval. If more than one value is listed, the list must
2740 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2741 for details, including issues with tied arrays and hashes.
2743 The C<delete local EXPR> construct can also be used to localize the deletion
2744 of array/hash elements to the current block.
2745 See L<perlsub/"Localized deletion of elements of composite types">.
2747 =item localtime EXPR
2748 X<localtime> X<ctime>
2752 Converts a time as returned by the time function to a 9-element list
2753 with the time analyzed for the local time zone. Typically used as
2757 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2760 All list elements are numeric, and come straight out of the C `struct
2761 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2762 of the specified time.
2764 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2765 the range C<0..11> with 0 indicating January and 11 indicating December.
2766 This makes it easy to get a month name from a list:
2768 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2769 print "$abbr[$mon] $mday";
2770 # $mon=9, $mday=18 gives "Oct 18"
2772 C<$year> is the number of years since 1900, not just the last two digits
2773 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2774 to get a complete 4-digit year is simply:
2778 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2779 to do that, would you?
2781 To get the last two digits of the year (e.g., '01' in 2001) do:
2783 $year = sprintf("%02d", $year % 100);
2785 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2786 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2787 (or C<0..365> in leap years.)
2789 C<$isdst> is true if the specified time occurs during Daylight Saving
2790 Time, false otherwise.
2792 If EXPR is omitted, C<localtime()> uses the current time (as returned
2795 In scalar context, C<localtime()> returns the ctime(3) value:
2797 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2799 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2800 instead of local time use the L</gmtime> builtin. See also the
2801 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2802 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2803 and mktime(3) functions.
2805 To get somewhat similar but locale dependent date strings, set up your
2806 locale environment variables appropriately (please see L<perllocale>) and
2809 use POSIX qw(strftime);
2810 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2811 # or for GMT formatted appropriately for your locale:
2812 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2814 Note that the C<%a> and C<%b>, the short forms of the day of the week
2815 and the month of the year, may not necessarily be three characters wide.
2817 See L<perlport/localtime> for portability concerns.
2819 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2820 by-name access mechanism to the gmtime() and localtime() functions,
2823 For a comprehensive date and time representation look at the
2824 L<DateTime> module on CPAN.
2829 This function places an advisory lock on a shared variable, or referenced
2830 object contained in I<THING> until the lock goes out of scope.
2832 lock() is a "weak keyword" : this means that if you've defined a function
2833 by this name (before any calls to it), that function will be called
2834 instead. (However, if you've said C<use threads>, lock() is always a
2835 keyword.) See L<threads>.
2838 X<log> X<logarithm> X<e> X<ln> X<base>
2842 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2843 returns log of C<$_>. To get the log of another base, use basic algebra:
2844 The base-N log of a number is equal to the natural log of that number
2845 divided by the natural log of N. For example:
2849 return log($n)/log(10);
2852 See also L</exp> for the inverse operation.
2859 Does the same thing as the C<stat> function (including setting the
2860 special C<_> filehandle) but stats a symbolic link instead of the file
2861 the symbolic link points to. If symbolic links are unimplemented on
2862 your system, a normal C<stat> is done. For much more detailed
2863 information, please see the documentation for C<stat>.
2865 If EXPR is omitted, stats C<$_>.
2869 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
2871 =item map BLOCK LIST
2876 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2877 C<$_> to each element) and returns the list value composed of the
2878 results of each such evaluation. In scalar context, returns the
2879 total number of elements so generated. Evaluates BLOCK or EXPR in
2880 list context, so each element of LIST may produce zero, one, or
2881 more elements in the returned value.
2883 @chars = map(chr, @nums);
2885 translates a list of numbers to the corresponding characters. And
2887 %hash = map { get_a_key_for($_) => $_ } @array;
2889 is just a funny way to write
2893 $hash{get_a_key_for($_)} = $_;
2896 Note that C<$_> is an alias to the list value, so it can be used to
2897 modify the elements of the LIST. While this is useful and supported,
2898 it can cause bizarre results if the elements of LIST are not variables.
2899 Using a regular C<foreach> loop for this purpose would be clearer in
2900 most cases. See also L</grep> for an array composed of those items of
2901 the original list for which the BLOCK or EXPR evaluates to true.
2903 If C<$_> is lexical in the scope where the C<map> appears (because it has
2904 been declared with C<my $_>), then, in addition to being locally aliased to
2905 the list elements, C<$_> keeps being lexical inside the block; that is, it
2906 can't be seen from the outside, avoiding any potential side-effects.
2908 C<{> starts both hash references and blocks, so C<map { ...> could be either
2909 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2910 ahead for the closing C<}> it has to take a guess at which its dealing with
2911 based what it finds just after the C<{>. Usually it gets it right, but if it
2912 doesn't it won't realize something is wrong until it gets to the C<}> and
2913 encounters the missing (or unexpected) comma. The syntax error will be
2914 reported close to the C<}> but you'll need to change something near the C<{>
2915 such as using a unary C<+> to give perl some help:
2917 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2918 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2919 %hash = map { ("\L$_", 1) } @array # this also works
2920 %hash = map { lc($_), 1 } @array # as does this.
2921 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2923 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2925 or to force an anon hash constructor use C<+{>:
2927 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2929 and you get list of anonymous hashes each with only 1 entry.
2931 =item mkdir FILENAME,MASK
2932 X<mkdir> X<md> X<directory, create>
2934 =item mkdir FILENAME
2938 Creates the directory specified by FILENAME, with permissions
2939 specified by MASK (as modified by C<umask>). If it succeeds it
2940 returns true, otherwise it returns false and sets C<$!> (errno).
2941 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2944 In general, it is better to create directories with permissive MASK,
2945 and let the user modify that with their C<umask>, than it is to supply
2946 a restrictive MASK and give the user no way to be more permissive.
2947 The exceptions to this rule are when the file or directory should be
2948 kept private (mail files, for instance). The perlfunc(1) entry on
2949 C<umask> discusses the choice of MASK in more detail.
2951 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2952 number of trailing slashes. Some operating and filesystems do not get
2953 this right, so Perl automatically removes all trailing slashes to keep
2956 In order to recursively create a directory structure look at
2957 the C<mkpath> function of the L<File::Path> module.
2959 =item msgctl ID,CMD,ARG
2962 Calls the System V IPC function msgctl(2). You'll probably have to say
2966 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2967 then ARG must be a variable that will hold the returned C<msqid_ds>
2968 structure. Returns like C<ioctl>: the undefined value for error,
2969 C<"0 but true"> for zero, or the actual return value otherwise. See also
2970 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2972 =item msgget KEY,FLAGS
2975 Calls the System V IPC function msgget(2). Returns the message queue
2976 id, or the undefined value if there is an error. See also
2977 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2979 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2982 Calls the System V IPC function msgrcv to receive a message from
2983 message queue ID into variable VAR with a maximum message size of
2984 SIZE. Note that when a message is received, the message type as a
2985 native long integer will be the first thing in VAR, followed by the
2986 actual message. This packing may be opened with C<unpack("l! a*")>.
2987 Taints the variable. Returns true if successful, or false if there is
2988 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2989 C<IPC::SysV::Msg> documentation.
2991 =item msgsnd ID,MSG,FLAGS
2994 Calls the System V IPC function msgsnd to send the message MSG to the
2995 message queue ID. MSG must begin with the native long integer message
2996 type, and be followed by the length of the actual message, and finally
2997 the message itself. This kind of packing can be achieved with
2998 C<pack("l! a*", $type, $message)>. Returns true if successful,
2999 or false if there is an error. See also C<IPC::SysV>
3000 and C<IPC::SysV::Msg> documentation.
3007 =item my EXPR : ATTRS
3009 =item my TYPE EXPR : ATTRS
3011 A C<my> declares the listed variables to be local (lexically) to the
3012 enclosing block, file, or C<eval>. If more than one value is listed,
3013 the list must be placed in parentheses.
3015 The exact semantics and interface of TYPE and ATTRS are still
3016 evolving. TYPE is currently bound to the use of C<fields> pragma,
3017 and attributes are handled using the C<attributes> pragma, or starting
3018 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3019 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3020 L<attributes>, and L<Attribute::Handlers>.
3027 The C<next> command is like the C<continue> statement in C; it starts
3028 the next iteration of the loop:
3030 LINE: while (<STDIN>) {
3031 next LINE if /^#/; # discard comments
3035 Note that if there were a C<continue> block on the above, it would get
3036 executed even on discarded lines. If the LABEL is omitted, the command
3037 refers to the innermost enclosing loop.
3039 C<next> cannot be used to exit a block which returns a value such as
3040 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3041 a grep() or map() operation.
3043 Note that a block by itself is semantically identical to a loop
3044 that executes once. Thus C<next> will exit such a block early.
3046 See also L</continue> for an illustration of how C<last>, C<next>, and
3049 =item no Module VERSION LIST
3052 =item no Module VERSION
3054 =item no Module LIST
3060 See the C<use> function, of which C<no> is the opposite.
3063 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3067 Interprets EXPR as an octal string and returns the corresponding
3068 value. (If EXPR happens to start off with C<0x>, interprets it as a
3069 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3070 binary string. Leading whitespace is ignored in all three cases.)
3071 The following will handle decimal, binary, octal, and hex in the standard
3074 $val = oct($val) if $val =~ /^0/;
3076 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3077 in octal), use sprintf() or printf():
3079 $perms = (stat("filename"))[2] & 07777;
3080 $oct_perms = sprintf "%lo", $perms;
3082 The oct() function is commonly used when a string such as C<644> needs
3083 to be converted into a file mode, for example. (Although perl will
3084 automatically convert strings into numbers as needed, this automatic
3085 conversion assumes base 10.)
3087 =item open FILEHANDLE,EXPR
3088 X<open> X<pipe> X<file, open> X<fopen>
3090 =item open FILEHANDLE,MODE,EXPR
3092 =item open FILEHANDLE,MODE,EXPR,LIST
3094 =item open FILEHANDLE,MODE,REFERENCE
3096 =item open FILEHANDLE
3098 Opens the file whose filename is given by EXPR, and associates it with
3101 Simple examples to open a file for reading:
3103 open(my $fh, '<', "input.txt") or die $!;
3107 open(my $fh, '>', "output.txt") or die $!;
3109 (The following is a comprehensive reference to open(): for a gentler
3110 introduction you may consider L<perlopentut>.)
3112 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3113 the variable is assigned a reference to a new anonymous filehandle,
3114 otherwise if FILEHANDLE is an expression, its value is used as the name of
3115 the real filehandle wanted. (This is considered a symbolic reference, so
3116 C<use strict 'refs'> should I<not> be in effect.)
3118 If EXPR is omitted, the scalar variable of the same name as the
3119 FILEHANDLE contains the filename. (Note that lexical variables--those
3120 declared with C<my>--will not work for this purpose; so if you're
3121 using C<my>, specify EXPR in your call to open.)
3123 If three or more arguments are specified then the mode of opening and
3124 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3125 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3126 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3127 the file is opened for appending, again being created if necessary.
3129 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3130 indicate that you want both read and write access to the file; thus
3131 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3132 '+>' >> mode would clobber the file first. You can't usually use
3133 either read-write mode for updating textfiles, since they have
3134 variable length records. See the B<-i> switch in L<perlrun> for a
3135 better approach. The file is created with permissions of C<0666>
3136 modified by the process' C<umask> value.
3138 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3139 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3141 In the 2-arguments (and 1-argument) form of the call the mode and
3142 filename should be concatenated (in this order), possibly separated by
3143 spaces. It is possible to omit the mode in these forms if the mode is
3146 If the filename begins with C<'|'>, the filename is interpreted as a
3147 command to which output is to be piped, and if the filename ends with a
3148 C<'|'>, the filename is interpreted as a command which pipes output to
3149 us. See L<perlipc/"Using open() for IPC">
3150 for more examples of this. (You are not allowed to C<open> to a command
3151 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3152 and L<perlipc/"Bidirectional Communication with Another Process">
3155 For three or more arguments if MODE is C<'|-'>, the filename is
3156 interpreted as a command to which output is to be piped, and if MODE
3157 is C<'-|'>, the filename is interpreted as a command which pipes
3158 output to us. In the 2-arguments (and 1-argument) form one should
3159 replace dash (C<'-'>) with the command.
3160 See L<perlipc/"Using open() for IPC"> for more examples of this.
3161 (You are not allowed to C<open> to a command that pipes both in I<and>
3162 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3163 L<perlipc/"Bidirectional Communication"> for alternatives.)
3165 In the three-or-more argument form of pipe opens, if LIST is specified
3166 (extra arguments after the command name) then LIST becomes arguments
3167 to the command invoked if the platform supports it. The meaning of
3168 C<open> with more than three arguments for non-pipe modes is not yet
3169 specified. Experimental "layers" may give extra LIST arguments
3172 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3173 and opening C<< '>-' >> opens STDOUT.
3175 You may use the three-argument form of open to specify IO "layers"
3176 (sometimes also referred to as "disciplines") to be applied to the handle
3177 that affect how the input and output are processed (see L<open> and
3178 L<PerlIO> for more details). For example
3180 open(my $fh, "<:encoding(UTF-8)", "file")
3182 will open the UTF-8 encoded file containing Unicode characters,
3183 see L<perluniintro>. Note that if layers are specified in the
3184 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3185 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3187 Open returns nonzero upon success, the undefined value otherwise. If
3188 the C<open> involved a pipe, the return value happens to be the pid of
3191 If you're running Perl on a system that distinguishes between text
3192 files and binary files, then you should check out L</binmode> for tips
3193 for dealing with this. The key distinction between systems that need
3194 C<binmode> and those that don't is their text file formats. Systems
3195 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3196 character, and which encode that character in C as C<"\n">, do not
3197 need C<binmode>. The rest need it.
3199 When opening a file, it's usually a bad idea to continue normal execution
3200 if the request failed, so C<open> is frequently used in connection with
3201 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3202 where you want to make a nicely formatted error message (but there are
3203 modules that can help with that problem)) you should always check
3204 the return value from opening a file. The infrequent exception is when
3205 working with an unopened filehandle is actually what you want to do.
3207 As a special case the 3-arg form with a read/write mode and the third
3208 argument being C<undef>:
3210 open(my $tmp, "+>", undef) or die ...
3212 opens a filehandle to an anonymous temporary file. Also using "+<"
3213 works for symmetry, but you really should consider writing something
3214 to the temporary file first. You will need to seek() to do the
3217 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3218 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3219 "in memory" files held in Perl scalars via:
3221 open($fh, '>', \$variable) || ..
3223 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3224 file, you have to close it first:
3227 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3232 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3233 while (<ARTICLE>) {...
3235 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3236 # if the open fails, output is discarded
3238 open(my $dbase, '+<', 'dbase.mine') # open for update
3239 or die "Can't open 'dbase.mine' for update: $!";
3241 open(my $dbase, '+<dbase.mine') # ditto
3242 or die "Can't open 'dbase.mine' for update: $!";
3244 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3245 or die "Can't start caesar: $!";
3247 open(ARTICLE, "caesar <$article |") # ditto
3248 or die "Can't start caesar: $!";
3250 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3251 or die "Can't start sort: $!";
3254 open(MEMORY,'>', \$var)
3255 or die "Can't open memory file: $!";
3256 print MEMORY "foo!\n"; # output will end up in $var
3258 # process argument list of files along with any includes
3260 foreach $file (@ARGV) {
3261 process($file, 'fh00');
3265 my($filename, $input) = @_;
3266 $input++; # this is a string increment
3267 unless (open($input, $filename)) {
3268 print STDERR "Can't open $filename: $!\n";
3273 while (<$input>) { # note use of indirection
3274 if (/^#include "(.*)"/) {
3275 process($1, $input);
3282 See L<perliol> for detailed info on PerlIO.
3284 You may also, in the Bourne shell tradition, specify an EXPR beginning
3285 with C<< '>&' >>, in which case the rest of the string is interpreted
3286 as the name of a filehandle (or file descriptor, if numeric) to be
3287 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
3288 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3289 The mode you specify should match the mode of the original filehandle.
3290 (Duping a filehandle does not take into account any existing contents
3291 of IO buffers.) If you use the 3-arg form then you can pass either a
3292 number, the name of a filehandle or the normal "reference to a glob".
3294 Here is a script that saves, redirects, and restores C<STDOUT> and
3295 C<STDERR> using various methods:
3298 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3299 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3301 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3302 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3304 select STDERR; $| = 1; # make unbuffered
3305 select STDOUT; $| = 1; # make unbuffered
3307 print STDOUT "stdout 1\n"; # this works for
3308 print STDERR "stderr 1\n"; # subprocesses too
3310 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3311 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3313 print STDOUT "stdout 2\n";
3314 print STDERR "stderr 2\n";
3316 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3317 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3318 that file descriptor (and not call C<dup(2)>); this is more
3319 parsimonious of file descriptors. For example:
3321 # open for input, reusing the fileno of $fd
3322 open(FILEHANDLE, "<&=$fd")
3326 open(FILEHANDLE, "<&=", $fd)
3330 # open for append, using the fileno of OLDFH
3331 open(FH, ">>&=", OLDFH)
3335 open(FH, ">>&=OLDFH")
3337 Being parsimonious on filehandles is also useful (besides being
3338 parsimonious) for example when something is dependent on file
3339 descriptors, like for example locking using flock(). If you do just
3340 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3341 descriptor as B, and therefore flock(A) will not flock(B), and vice
3342 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3343 the same file descriptor.
3345 Note that if you are using Perls older than 5.8.0, Perl will be using
3346 the standard C libraries' fdopen() to implement the "=" functionality.
3347 On many UNIX systems fdopen() fails when file descriptors exceed a
3348 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3349 most often the default.
3351 You can see whether Perl has been compiled with PerlIO or not by
3352 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3353 is C<define>, you have PerlIO, otherwise you don't.
3355 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3356 with 2-arguments (or 1-argument) form of open(), then
3357 there is an implicit fork done, and the return value of open is the pid
3358 of the child within the parent process, and C<0> within the child
3359 process. (Use C<defined($pid)> to determine whether the open was successful.)
3360 The filehandle behaves normally for the parent, but i/o to that
3361 filehandle is piped from/to the STDOUT/STDIN of the child process.
3362 In the child process the filehandle isn't opened--i/o happens from/to
3363 the new STDOUT or STDIN. Typically this is used like the normal
3364 piped open when you want to exercise more control over just how the
3365 pipe command gets executed, such as when you are running setuid, and
3366 don't want to have to scan shell commands for metacharacters.
3367 The following triples are more or less equivalent:
3369 open(FOO, "|tr '[a-z]' '[A-Z]'");
3370 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3371 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3372 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3374 open(FOO, "cat -n '$file'|");
3375 open(FOO, '-|', "cat -n '$file'");
3376 open(FOO, '-|') || exec 'cat', '-n', $file;
3377 open(FOO, '-|', "cat", '-n', $file);
3379 The last example in each block shows the pipe as "list form", which is
3380 not yet supported on all platforms. A good rule of thumb is that if
3381 your platform has true C<fork()> (in other words, if your platform is
3382 UNIX) you can use the list form.
3384 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3386 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3387 output before any operation that may do a fork, but this may not be
3388 supported on some platforms (see L<perlport>). To be safe, you may need
3389 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3390 of C<IO::Handle> on any open handles.
3392 On systems that support a close-on-exec flag on files, the flag will
3393 be set for the newly opened file descriptor as determined by the value
3394 of $^F. See L<perlvar/$^F>.
3396 Closing any piped filehandle causes the parent process to wait for the
3397 child to finish, and returns the status value in C<$?> and
3398 C<${^CHILD_ERROR_NATIVE}>.
3400 The filename passed to 2-argument (or 1-argument) form of open() will
3401 have leading and trailing whitespace deleted, and the normal
3402 redirection characters honored. This property, known as "magic open",
3403 can often be used to good effect. A user could specify a filename of
3404 F<"rsh cat file |">, or you could change certain filenames as needed:
3406 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3407 open(FH, $filename) or die "Can't open $filename: $!";
3409 Use 3-argument form to open a file with arbitrary weird characters in it,
3411 open(FOO, '<', $file);
3413 otherwise it's necessary to protect any leading and trailing whitespace:
3415 $file =~ s#^(\s)#./$1#;
3416 open(FOO, "< $file\0");
3418 (this may not work on some bizarre filesystems). One should
3419 conscientiously choose between the I<magic> and 3-arguments form
3424 will allow the user to specify an argument of the form C<"rsh cat file |">,
3425 but will not work on a filename which happens to have a trailing space, while
3427 open IN, '<', $ARGV[0];
3429 will have exactly the opposite restrictions.
3431 If you want a "real" C C<open> (see C<open(2)> on your system), then you
3432 should use the C<sysopen> function, which involves no such magic (but
3433 may use subtly different filemodes than Perl open(), which is mapped
3434 to C fopen()). This is
3435 another way to protect your filenames from interpretation. For example:
3438 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3439 or die "sysopen $path: $!";
3440 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3441 print HANDLE "stuff $$\n";
3443 print "File contains: ", <HANDLE>;
3445 Using the constructor from the C<IO::Handle> package (or one of its
3446 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3447 filehandles that have the scope of whatever variables hold references to
3448 them, and automatically close whenever and however you leave that scope:
3452 sub read_myfile_munged {
3454 my $handle = IO::File->new;
3455 open($handle, "myfile") or die "myfile: $!";
3457 or return (); # Automatically closed here.
3458 mung $first or die "mung failed"; # Or here.
3459 return $first, <$handle> if $ALL; # Or here.
3463 See L</seek> for some details about mixing reading and writing.
3465 =item opendir DIRHANDLE,EXPR
3468 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3469 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3470 DIRHANDLE may be an expression whose value can be used as an indirect
3471 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3472 scalar variable (or array or hash element), the variable is assigned a
3473 reference to a new anonymous dirhandle.
3474 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3476 See example at C<readdir>.
3483 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3484 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3487 For the reverse, see L</chr>.
3488 See L<perlunicode> for more about Unicode.
3495 =item our EXPR : ATTRS
3497 =item our TYPE EXPR : ATTRS
3499 C<our> associates a simple name with a package variable in the current
3500 package for use within the current scope. When C<use strict 'vars'> is in
3501 effect, C<our> lets you use declared global variables without qualifying
3502 them with package names, within the lexical scope of the C<our> declaration.
3503 In this way C<our> differs from C<use vars>, which is package scoped.
3505 Unlike C<my>, which both allocates storage for a variable and associates
3506 a simple name with that storage for use within the current scope, C<our>
3507 associates a simple name with a package variable in the current package,
3508 for use within the current scope. In other words, C<our> has the same
3509 scoping rules as C<my>, but does not necessarily create a
3512 If more than one value is listed, the list must be placed
3518 An C<our> declaration declares a global variable that will be visible
3519 across its entire lexical scope, even across package boundaries. The
3520 package in which the variable is entered is determined at the point
3521 of the declaration, not at the point of use. This means the following
3525 our $bar; # declares $Foo::bar for rest of lexical scope
3529 print $bar; # prints 20, as it refers to $Foo::bar
3531 Multiple C<our> declarations with the same name in the same lexical
3532 scope are allowed if they are in different packages. If they happen
3533 to be in the same package, Perl will emit warnings if you have asked
3534 for them, just like multiple C<my> declarations. Unlike a second
3535 C<my> declaration, which will bind the name to a fresh variable, a
3536 second C<our> declaration in the same package, in the same scope, is
3541 our $bar; # declares $Foo::bar for rest of lexical scope
3545 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3546 print $bar; # prints 30
3548 our $bar; # emits warning but has no other effect
3549 print $bar; # still prints 30
3551 An C<our> declaration may also have a list of attributes associated
3554 The exact semantics and interface of TYPE and ATTRS are still
3555 evolving. TYPE is currently bound to the use of C<fields> pragma,
3556 and attributes are handled using the C<attributes> pragma, or starting
3557 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3558 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3559 L<attributes>, and L<Attribute::Handlers>.
3561 =item pack TEMPLATE,LIST
3564 Takes a LIST of values and converts it into a string using the rules
3565 given by the TEMPLATE. The resulting string is the concatenation of
3566 the converted values. Typically, each converted value looks
3567 like its machine-level representation. For example, on 32-bit machines
3568 an integer may be represented by a sequence of 4 bytes that will be
3569 converted to a sequence of 4 characters.
3571 The TEMPLATE is a sequence of characters that give the order and type
3572 of values, as follows:
3574 a A string with arbitrary binary data, will be null padded.
3575 A A text (ASCII) string, will be space padded.
3576 Z A null terminated (ASCIZ) string, will be null padded.
3578 b A bit string (ascending bit order inside each byte, like vec()).
3579 B A bit string (descending bit order inside each byte).
3580 h A hex string (low nybble first).
3581 H A hex string (high nybble first).
3583 c A signed char (8-bit) value.
3584 C An unsigned char (octet) value.
3585 W An unsigned char value (can be greater than 255).
3587 s A signed short (16-bit) value.
3588 S An unsigned short value.
3590 l A signed long (32-bit) value.
3591 L An unsigned long value.
3593 q A signed quad (64-bit) value.
3594 Q An unsigned quad value.
3595 (Quads are available only if your system supports 64-bit
3596 integer values _and_ if Perl has been compiled to support those.
3597 Causes a fatal error otherwise.)
3599 i A signed integer value.
3600 I A unsigned integer value.
3601 (This 'integer' is _at_least_ 32 bits wide. Its exact
3602 size depends on what a local C compiler calls 'int'.)
3604 n An unsigned short (16-bit) in "network" (big-endian) order.
3605 N An unsigned long (32-bit) in "network" (big-endian) order.
3606 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3607 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3609 j A Perl internal signed integer value (IV).
3610 J A Perl internal unsigned integer value (UV).
3612 f A single-precision float in the native format.
3613 d A double-precision float in the native format.
3615 F A Perl internal floating point value (NV) in the native format
3616 D A long double-precision float in the native format.
3617 (Long doubles are available only if your system supports long
3618 double values _and_ if Perl has been compiled to support those.
3619 Causes a fatal error otherwise.)
3621 p A pointer to a null-terminated string.
3622 P A pointer to a structure (fixed-length string).
3624 u A uuencoded string.
3625 U A Unicode character number. Encodes to a character in character mode
3626 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3628 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3629 details). Its bytes represent an unsigned integer in base 128,
3630 most significant digit first, with as few digits as possible. Bit
3631 eight (the high bit) is set on each byte except the last.
3635 @ Null fill or truncate to absolute position, counted from the
3636 start of the innermost ()-group.
3637 . Null fill or truncate to absolute position specified by value.
3638 ( Start of a ()-group.
3640 One or more of the modifiers below may optionally follow some letters in the
3641 TEMPLATE (the second column lists the letters for which the modifier is
3644 ! sSlLiI Forces native (short, long, int) sizes instead
3645 of fixed (16-/32-bit) sizes.
3647 xX Make x and X act as alignment commands.
3649 nNvV Treat integers as signed instead of unsigned.
3651 @. Specify position as byte offset in the internal
3652 representation of the packed string. Efficient but
3655 > sSiIlLqQ Force big-endian byte-order on the type.
3656 jJfFdDpP (The "big end" touches the construct.)
3658 < sSiIlLqQ Force little-endian byte-order on the type.
3659 jJfFdDpP (The "little end" touches the construct.)
3661 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3662 in which case they force a certain byte-order on all components of
3663 that group, including subgroups.
3665 The following rules apply:
3671 Each letter may optionally be followed by a number giving a repeat
3672 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3673 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3674 that many values from the LIST. A C<*> for the repeat count means to
3675 use however many items are left, except for C<@>, C<x>, C<X>, where it
3676 is equivalent to C<0>, for <.> where it means relative to string start
3677 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3678 A numeric repeat count may optionally be enclosed in brackets, as in
3679 C<pack 'C[80]', @arr>.
3681 One can replace the numeric repeat count by a template enclosed in brackets;
3682 then the packed length of this template in bytes is used as a count.
3683 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3684 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3685 If the template in brackets contains alignment commands (such as C<x![d]>),
3686 its packed length is calculated as if the start of the template has the maximal
3689 When used with C<Z>, C<*> results in the addition of a trailing null
3690 byte (so the packed result will be one longer than the byte C<length>
3693 When used with C<@>, the repeat count represents an offset from the start
3694 of the innermost () group.
3696 When used with C<.>, the repeat count is used to determine the starting
3697 position from where the value offset is calculated. If the repeat count
3698 is 0, it's relative to the current position. If the repeat count is C<*>,
3699 the offset is relative to the start of the packed string. And if its an
3700 integer C<n> the offset is relative to the start of the n-th innermost
3701 () group (or the start of the string if C<n> is bigger then the group
3704 The repeat count for C<u> is interpreted as the maximal number of bytes
3705 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3706 count should not be more than 65.
3710 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3711 string of length count, padding with nulls or spaces as necessary. When
3712 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3713 after the first null, and C<a> returns data verbatim.
3715 If the value-to-pack is too long, it is truncated. If too long and an
3716 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3717 by a null byte. Thus C<Z> always packs a trailing null (except when the
3722 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3723 Each character of the input field of pack() generates 1 bit of the result.
3724 Each result bit is based on the least-significant bit of the corresponding
3725 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3726 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3728 Starting from the beginning of the input string of pack(), each 8-tuple
3729 of characters is converted to 1 character of output. With format C<b>
3730 the first character of the 8-tuple determines the least-significant bit of a
3731 character, and with format C<B> it determines the most-significant bit of
3734 If the length of the input string is not exactly divisible by 8, the
3735 remainder is packed as if the input string were padded by null characters
3736 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3738 If the input string of pack() is longer than needed, extra characters are
3739 ignored. A C<*> for the repeat count of pack() means to use all the
3740 characters of the input field. On unpack()ing the bits are converted to a
3741 string of C<"0">s and C<"1">s.
3745 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3746 representable as hexadecimal digits, 0-9a-f) long.
3748 Each character of the input field of pack() generates 4 bits of the result.
3749 For non-alphabetical characters the result is based on the 4 least-significant
3750 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3751 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3752 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3753 is compatible with the usual hexadecimal digits, so that C<"a"> and
3754 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3755 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3757 Starting from the beginning of the input string of pack(), each pair
3758 of characters is converted to 1 character of output. With format C<h> the
3759 first character of the pair determines the least-significant nybble of the
3760 output character, and with format C<H> it determines the most-significant
3763 If the length of the input string is not even, it behaves as if padded
3764 by a null character at the end. Similarly, during unpack()ing the "extra"
3765 nybbles are ignored.
3767 If the input string of pack() is longer than needed, extra characters are
3769 A C<*> for the repeat count of pack() means to use all the characters of
3770 the input field. On unpack()ing the nybbles are converted to a string
3771 of hexadecimal digits.
3775 The C<p> type packs a pointer to a null-terminated string. You are
3776 responsible for ensuring the string is not a temporary value (which can
3777 potentially get deallocated before you get around to using the packed result).
3778 The C<P> type packs a pointer to a structure of the size indicated by the
3779 length. A NULL pointer is created if the corresponding value for C<p> or
3780 C<P> is C<undef>, similarly for unpack().
3782 If your system has a strange pointer size (i.e. a pointer is neither as
3783 big as an int nor as big as a long), it may not be possible to pack or
3784 unpack pointers in big- or little-endian byte order. Attempting to do
3785 so will result in a fatal error.
3789 The C</> template character allows packing and unpacking of a sequence of
3790 items where the packed structure contains a packed item count followed by
3791 the packed items themselves.
3793 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3794 I<length-item> describes how the length value is packed. The ones likely
3795 to be of most use are integer-packing ones like C<n> (for Java strings),
3796 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3798 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3799 the minimum of that and the number of available items is used as argument
3800 for the I<length-item>. If it has no repeat count or uses a '*', the number
3801 of available items is used.
3803 For C<unpack> an internal stack of integer arguments unpacked so far is
3804 used. You write C</>I<sequence-item> and the repeat count is obtained by
3805 popping off the last element from the stack. The I<sequence-item> must not
3806 have a repeat count.
3808 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3809 the I<length-item> is a string length, not a number of strings. If there is
3810 an explicit repeat count for pack, the packed string will be adjusted to that
3813 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3814 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3815 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3816 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3817 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3819 The I<length-item> is not returned explicitly from C<unpack>.
3821 Adding a count to the I<length-item> letter is unlikely to do anything
3822 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3823 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3824 which Perl does not regard as legal in numeric strings.
3828 The integer types C<s>, C<S>, C<l>, and C<L> may be
3829 followed by a C<!> modifier to signify native shorts or
3830 longs--as you can see from above for example a bare C<l> does mean
3831 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3832 may be larger. This is an issue mainly in 64-bit platforms. You can
3833 see whether using C<!> makes any difference by
3835 print length(pack("s")), " ", length(pack("s!")), "\n";
3836 print length(pack("l")), " ", length(pack("l!")), "\n";
3838 C<i!> and C<I!> also work but only because of completeness;
3839 they are identical to C<i> and C<I>.
3841 The actual sizes (in bytes) of native shorts, ints, longs, and long
3842 longs on the platform where Perl was built are also available via
3846 print $Config{shortsize}, "\n";
3847 print $Config{intsize}, "\n";
3848 print $Config{longsize}, "\n";
3849 print $Config{longlongsize}, "\n";
3851 (The C<$Config{longlongsize}> will be undefined if your system does
3852 not support long longs.)
3856 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3857 are inherently non-portable between processors and operating systems
3858 because they obey the native byteorder and endianness. For example a
3859 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3860 (arranged in and handled by the CPU registers) into bytes as
3862 0x12 0x34 0x56 0x78 # big-endian
3863 0x78 0x56 0x34 0x12 # little-endian
3865 Basically, the Intel and VAX CPUs are little-endian, while everybody
3866 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3867 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3868 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3871 The names `big-endian' and `little-endian' are comic references to
3872 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3873 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3874 the egg-eating habits of the Lilliputians.
3876 Some systems may have even weirder byte orders such as
3881 You can see your system's preference with
3883 print join(" ", map { sprintf "%#02x", $_ }
3884 unpack("W*",pack("L",0x12345678))), "\n";
3886 The byteorder on the platform where Perl was built is also available
3890 print $Config{byteorder}, "\n";
3892 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3893 and C<'87654321'> are big-endian.
3895 If you want portable packed integers you can either use the formats
3896 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3897 modifiers. These modifiers are only available as of perl 5.9.2.
3898 See also L<perlport>.
3902 All integer and floating point formats as well as C<p> and C<P> and
3903 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3904 to force big- or little- endian byte-order, respectively.
3905 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3906 signed integers, 64-bit integers and floating point values. However,
3907 there are some things to keep in mind.
3909 Exchanging signed integers between different platforms only works
3910 if all platforms store them in the same format. Most platforms store
3911 signed integers in two's complement, so usually this is not an issue.
3913 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3914 formats on big- or little-endian machines. Otherwise, attempting to
3915 do so will result in a fatal error.
3917 Forcing big- or little-endian byte-order on floating point values for
3918 data exchange can only work if all platforms are using the same
3919 binary representation (e.g. IEEE floating point format). Even if all
3920 platforms are using IEEE, there may be subtle differences. Being able
3921 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3922 but also very dangerous if you don't know exactly what you're doing.
3923 It is definitely not a general way to portably store floating point
3926 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3927 all types inside the group that accept the byte-order modifiers,
3928 including all subgroups. It will silently be ignored for all other
3929 types. You are not allowed to override the byte-order within a group
3930 that already has a byte-order modifier suffix.
3934 Real numbers (floats and doubles) are in the native machine format only;
3935 due to the multiplicity of floating formats around, and the lack of a
3936 standard "network" representation, no facility for interchange has been
3937 made. This means that packed floating point data written on one machine
3938 may not be readable on another - even if both use IEEE floating point
3939 arithmetic (as the endian-ness of the memory representation is not part
3940 of the IEEE spec). See also L<perlport>.
3942 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3943 modifiers to force big- or little-endian byte-order on floating point values.
3945 Note that Perl uses doubles (or long doubles, if configured) internally for
3946 all numeric calculation, and converting from double into float and thence back
3947 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3948 will not in general equal $foo).
3952 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3953 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3954 where the packed string is processed in its UTF-8-encoded Unicode form on
3955 a byte by byte basis. Character mode is the default unless the format string
3956 starts with an C<U>. You can switch mode at any moment with an explicit
3957 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3958 or until the end of the ()-group in which it was entered.
3962 You must yourself do any alignment or padding by inserting for example
3963 enough C<'x'>es while packing. There is no way to pack() and unpack()
3964 could know where the characters are going to or coming from. Therefore
3965 C<pack> (and C<unpack>) handle their output and input as flat
3966 sequences of characters.
3970 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3971 take a repeat count, both as postfix, and for unpack() also via the C</>
3972 template character. Within each repetition of a group, positioning with
3973 C<@> starts again at 0. Therefore, the result of
3975 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3977 is the string "\0a\0\0bc".
3981 C<x> and C<X> accept C<!> modifier. In this case they act as
3982 alignment commands: they jump forward/back to the closest position
3983 aligned at a multiple of C<count> characters. For example, to pack() or
3984 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3985 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3986 aligned on the double's size.
3988 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3989 both result in no-ops.
3993 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3994 will represent signed 16-/32-bit integers in big-/little-endian order.
3995 This is only portable if all platforms sharing the packed data use the
3996 same binary representation for signed integers (e.g. all platforms are
3997 using two's complement representation).
4001 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
4002 White space may be used to separate pack codes from each other, but
4003 modifiers and a repeat count must follow immediately.
4007 If TEMPLATE requires more arguments to pack() than actually given, pack()
4008 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4009 to pack() than actually given, extra arguments are ignored.
4015 $foo = pack("WWWW",65,66,67,68);
4017 $foo = pack("W4",65,66,67,68);
4019 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4020 # same thing with Unicode circled letters.
4021 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4022 # same thing with Unicode circled letters. You don't get the UTF-8
4023 # bytes because the U at the start of the format caused a switch to
4024 # U0-mode, so the UTF-8 bytes get joined into characters
4025 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4026 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4027 # This is the UTF-8 encoding of the string in the previous example
4029 $foo = pack("ccxxcc",65,66,67,68);
4032 # note: the above examples featuring "W" and "c" are true
4033 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4034 # and UTF-8. In EBCDIC the first example would be
4035 # $foo = pack("WWWW",193,194,195,196);
4037 $foo = pack("s2",1,2);
4038 # "\1\0\2\0" on little-endian
4039 # "\0\1\0\2" on big-endian
4041 $foo = pack("a4","abcd","x","y","z");
4044 $foo = pack("aaaa","abcd","x","y","z");
4047 $foo = pack("a14","abcdefg");
4048 # "abcdefg\0\0\0\0\0\0\0"
4050 $foo = pack("i9pl", gmtime);
4051 # a real struct tm (on my system anyway)
4053 $utmp_template = "Z8 Z8 Z16 L";
4054 $utmp = pack($utmp_template, @utmp1);
4055 # a struct utmp (BSDish)
4057 @utmp2 = unpack($utmp_template, $utmp);
4058 # "@utmp1" eq "@utmp2"
4061 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4064 $foo = pack('sx2l', 12, 34);
4065 # short 12, two zero bytes padding, long 34
4066 $bar = pack('s@4l', 12, 34);
4067 # short 12, zero fill to position 4, long 34
4069 $baz = pack('s.l', 12, 4, 34);
4070 # short 12, zero fill to position 4, long 34
4072 $foo = pack('nN', 42, 4711);
4073 # pack big-endian 16- and 32-bit unsigned integers
4074 $foo = pack('S>L>', 42, 4711);
4076 $foo = pack('s<l<', -42, 4711);
4077 # pack little-endian 16- and 32-bit signed integers
4078 $foo = pack('(sl)<', -42, 4711);
4081 The same template may generally also be used in unpack().
4083 =item package NAMESPACE
4084 X<package> X<module> X<namespace>
4086 Declares the compilation unit as being in the given namespace. The scope
4087 of the package declaration is from the declaration itself through the end
4088 of the enclosing block, file, or eval (the same as the C<my> operator).
4089 All further unqualified dynamic identifiers will be in this namespace.
4090 A package statement affects only dynamic variables--including those
4091 you've used C<local> on--but I<not> lexical variables, which are created
4092 with C<my>. Typically it would be the first declaration in a file to
4093 be included by the C<require> or C<use> operator. You can switch into a
4094 package in more than one place; it merely influences which symbol table
4095 is used by the compiler for the rest of that block. You can refer to
4096 variables and filehandles in other packages by prefixing the identifier
4097 with the package name and a double colon: C<$Package::Variable>.
4098 If the package name is null, the C<main> package as assumed. That is,
4099 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4100 still seen in older code).
4102 See L<perlmod/"Packages"> for more information about packages, modules,
4103 and classes. See L<perlsub> for other scoping issues.
4105 =item pipe READHANDLE,WRITEHANDLE
4108 Opens a pair of connected pipes like the corresponding system call.
4109 Note that if you set up a loop of piped processes, deadlock can occur
4110 unless you are very careful. In addition, note that Perl's pipes use
4111 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4112 after each command, depending on the application.
4114 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4115 for examples of such things.
4117 On systems that support a close-on-exec flag on files, the flag will be set
4118 for the newly opened file descriptors as determined by the value of $^F.
4126 Pops and returns the last value of the array, shortening the array by
4129 If there are no elements in the array, returns the undefined value
4130 (although this may happen at other times as well). If ARRAY is
4131 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4132 array in subroutines, just like C<shift>.
4135 X<pos> X<match, position>
4139 Returns the offset of where the last C<m//g> search left off for the variable
4140 in question (C<$_> is used when the variable is not specified). Note that
4141 0 is a valid match offset. C<undef> indicates that the search position
4142 is reset (usually due to match failure, but can also be because no match has
4143 yet been performed on the scalar). C<pos> directly accesses the location used
4144 by the regexp engine to store the offset, so assigning to C<pos> will change
4145 that offset, and so will also influence the C<\G> zero-width assertion in
4146 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4147 the return from C<pos> won't change either in this case. See L<perlre> and
4150 =item print FILEHANDLE LIST
4157 Prints a string or a list of strings. Returns true if successful.
4158 FILEHANDLE may be a scalar variable name, in which case the variable
4159 contains the name of or a reference to the filehandle, thus introducing
4160 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4161 the next token is a term, it may be misinterpreted as an operator
4162 unless you interpose a C<+> or put parentheses around the arguments.)
4163 If FILEHANDLE is omitted, prints by default to standard output (or
4164 to the last selected output channel--see L</select>). If LIST is
4165 also omitted, prints C<$_> to the currently selected output channel.
4166 To set the default output channel to something other than STDOUT
4167 use the select operation. The current value of C<$,> (if any) is
4168 printed between each LIST item. The current value of C<$\> (if
4169 any) is printed after the entire LIST has been printed. Because
4170 print takes a LIST, anything in the LIST is evaluated in list
4171 context, and any subroutine that you call will have one or more of
4172 its expressions evaluated in list context. Also be careful not to
4173 follow the print keyword with a left parenthesis unless you want
4174 the corresponding right parenthesis to terminate the arguments to
4175 the print--interpose a C<+> or put parentheses around all the
4178 Note that if you're storing FILEHANDLEs in an array, or if you're using
4179 any other expression more complex than a scalar variable to retrieve it,
4180 you will have to use a block returning the filehandle value instead:
4182 print { $files[$i] } "stuff\n";
4183 print { $OK ? STDOUT : STDERR } "stuff\n";
4185 =item printf FILEHANDLE FORMAT, LIST
4188 =item printf FORMAT, LIST
4190 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4191 (the output record separator) is not appended. The first argument
4192 of the list will be interpreted as the C<printf> format. See C<sprintf>
4193 for an explanation of the format argument. If C<use locale> is in effect,
4194 and POSIX::setlocale() has been called, the character used for the decimal
4195 separator in formatted floating point numbers is affected by the LC_NUMERIC
4196 locale. See L<perllocale> and L<POSIX>.
4198 Don't fall into the trap of using a C<printf> when a simple
4199 C<print> would do. The C<print> is more efficient and less
4202 =item prototype FUNCTION
4205 Returns the prototype of a function as a string (or C<undef> if the
4206 function has no prototype). FUNCTION is a reference to, or the name of,
4207 the function whose prototype you want to retrieve.
4209 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4210 name for Perl builtin. If the builtin is not I<overridable> (such as
4211 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4212 (such as C<system>), prototype() returns C<undef>, because the builtin
4213 does not really behave like a Perl function. Otherwise, the string
4214 describing the equivalent prototype is returned.
4216 =item push ARRAY,LIST
4219 Treats ARRAY as a stack, and pushes the values of LIST
4220 onto the end of ARRAY. The length of ARRAY increases by the length of
4221 LIST. Has the same effect as
4224 $ARRAY[++$#ARRAY] = $value;
4227 but is more efficient. Returns the number of elements in the array following
4228 the completed C<push>.
4238 Generalized quotes. See L<perlop/"Quote-Like Operators">.
4242 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
4244 =item quotemeta EXPR
4245 X<quotemeta> X<metacharacter>
4249 Returns the value of EXPR with all non-"word"
4250 characters backslashed. (That is, all characters not matching
4251 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4252 returned string, regardless of any locale settings.)
4253 This is the internal function implementing
4254 the C<\Q> escape in double-quoted strings.
4256 If EXPR is omitted, uses C<$_>.
4263 Returns a random fractional number greater than or equal to C<0> and less
4264 than the value of EXPR. (EXPR should be positive.) If EXPR is
4265 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4266 also special-cased as C<1> - this has not been documented before perl 5.8.0
4267 and is subject to change in future versions of perl. Automatically calls
4268 C<srand> unless C<srand> has already been called. See also C<srand>.
4270 Apply C<int()> to the value returned by C<rand()> if you want random
4271 integers instead of random fractional numbers. For example,
4275 returns a random integer between C<0> and C<9>, inclusive.
4277 (Note: If your rand function consistently returns numbers that are too
4278 large or too small, then your version of Perl was probably compiled
4279 with the wrong number of RANDBITS.)
4281 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4282 X<read> X<file, read>
4284 =item read FILEHANDLE,SCALAR,LENGTH
4286 Attempts to read LENGTH I<characters> of data into variable SCALAR
4287 from the specified FILEHANDLE. Returns the number of characters
4288 actually read, C<0> at end of file, or undef if there was an error (in
4289 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4290 so that the last character actually read is the last character of the
4291 scalar after the read.
4293 An OFFSET may be specified to place the read data at some place in the
4294 string other than the beginning. A negative OFFSET specifies
4295 placement at that many characters counting backwards from the end of
4296 the string. A positive OFFSET greater than the length of SCALAR
4297 results in the string being padded to the required size with C<"\0">
4298 bytes before the result of the read is appended.
4300 The call is actually implemented in terms of either Perl's or system's
4301 fread() call. To get a true read(2) system call, see C<sysread>.
4303 Note the I<characters>: depending on the status of the filehandle,
4304 either (8-bit) bytes or characters are read. By default all
4305 filehandles operate on bytes, but for example if the filehandle has
4306 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4307 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4308 characters, not bytes. Similarly for the C<:encoding> pragma:
4309 in that case pretty much any characters can be read.
4311 =item readdir DIRHANDLE
4314 Returns the next directory entry for a directory opened by C<opendir>.
4315 If used in list context, returns all the rest of the entries in the
4316 directory. If there are no more entries, returns an undefined value in
4317 scalar context or a null list in list context.
4319 If you're planning to filetest the return values out of a C<readdir>, you'd
4320 better prepend the directory in question. Otherwise, because we didn't
4321 C<chdir> there, it would have been testing the wrong file.
4323 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4324 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4330 X<readline> X<gets> X<fgets>
4332 Reads from the filehandle whose typeglob is contained in EXPR (or from
4333 *ARGV if EXPR is not provided). In scalar context, each call reads and
4334 returns the next line, until end-of-file is reached, whereupon the
4335 subsequent call returns C<undef>. In list context, reads until end-of-file
4336 is reached and returns a list of lines. Note that the notion of "line"
4337 used here is however you may have defined it with C<$/> or
4338 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4340 When C<$/> is set to C<undef>, when C<readline> is in scalar
4341 context (i.e. file slurp mode), and when an empty file is read, it
4342 returns C<''> the first time, followed by C<undef> subsequently.
4344 This is the internal function implementing the C<< <EXPR> >>
4345 operator, but you can use it directly. The C<< <EXPR> >>
4346 operator is discussed in more detail in L<perlop/"I/O Operators">.
4349 $line = readline(*STDIN); # same thing
4351 If C<readline> encounters an operating system error, C<$!> will be set
4352 with the corresponding error message. It can be helpful to check
4353 C<$!> when you are reading from filehandles you don't trust, such as a
4354 tty or a socket. The following example uses the operator form of
4355 C<readline> and dies if the result is not defined.
4357 while ( ! eof($fh) ) {
4358 defined( $_ = <$fh> ) or die "readline failed: $!";
4362 Note that you have can't handle C<readline> errors that way with the
4363 C<ARGV> filehandle. In that case, you have to open each element of
4364 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
4366 foreach my $arg (@ARGV) {
4367 open(my $fh, $arg) or warn "Can't open $arg: $!";
4369 while ( ! eof($fh) ) {
4370 defined( $_ = <$fh> )
4371 or die "readline failed for $arg: $!";
4381 Returns the value of a symbolic link, if symbolic links are
4382 implemented. If not, gives a fatal error. If there is some system
4383 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4384 omitted, uses C<$_>.
4391 EXPR is executed as a system command.
4392 The collected standard output of the command is returned.
4393 In scalar context, it comes back as a single (potentially
4394 multi-line) string. In list context, returns a list of lines
4395 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4396 This is the internal function implementing the C<qx/EXPR/>
4397 operator, but you can use it directly. The C<qx/EXPR/>
4398 operator is discussed in more detail in L<perlop/"I/O Operators">.
4399 If EXPR is omitted, uses C<$_>.
4401 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4404 Receives a message on a socket. Attempts to receive LENGTH characters
4405 of data into variable SCALAR from the specified SOCKET filehandle.
4406 SCALAR will be grown or shrunk to the length actually read. Takes the
4407 same flags as the system call of the same name. Returns the address
4408 of the sender if SOCKET's protocol supports this; returns an empty
4409 string otherwise. If there's an error, returns the undefined value.
4410 This call is actually implemented in terms of recvfrom(2) system call.
4411 See L<perlipc/"UDP: Message Passing"> for examples.
4413 Note the I<characters>: depending on the status of the socket, either
4414 (8-bit) bytes or characters are received. By default all sockets
4415 operate on bytes, but for example if the socket has been changed using
4416 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4417 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4418 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4419 case pretty much any characters can be read.
4426 The C<redo> command restarts the loop block without evaluating the
4427 conditional again. The C<continue> block, if any, is not executed. If
4428 the LABEL is omitted, the command refers to the innermost enclosing
4429 loop. Programs that want to lie to themselves about what was just input
4430 normally use this command:
4432 # a simpleminded Pascal comment stripper
4433 # (warning: assumes no { or } in strings)
4434 LINE: while (<STDIN>) {
4435 while (s|({.*}.*){.*}|$1 |) {}
4440 if (/}/) { # end of comment?
4449 C<redo> cannot be used to retry a block which returns a value such as
4450 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4451 a grep() or map() operation.
4453 Note that a block by itself is semantically identical to a loop
4454 that executes once. Thus C<redo> inside such a block will effectively
4455 turn it into a looping construct.
4457 See also L</continue> for an illustration of how C<last>, C<next>, and
4465 Returns a non-empty string if EXPR is a reference, the empty
4466 string otherwise. If EXPR
4467 is not specified, C<$_> will be used. The value returned depends on the
4468 type of thing the reference is a reference to.
4469 Builtin types include:
4483 If the referenced object has been blessed into a package, then that package
4484 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4486 if (ref($r) eq "HASH") {
4487 print "r is a reference to a hash.\n";
4490 print "r is not a reference at all.\n";
4493 The return value C<LVALUE> indicates a reference to an lvalue that is not
4494 a variable. You get this from taking the reference of function calls like
4495 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4496 to a L<version string|perldata/"Version Strings">.
4498 The result C<Regexp> indicates that the argument is a regular expression
4499 resulting from C<qr//>.
4501 See also L<perlref>.
4503 =item rename OLDNAME,NEWNAME
4504 X<rename> X<move> X<mv> X<ren>
4506 Changes the name of a file; an existing file NEWNAME will be
4507 clobbered. Returns true for success, false otherwise.
4509 Behavior of this function varies wildly depending on your system
4510 implementation. For example, it will usually not work across file system
4511 boundaries, even though the system I<mv> command sometimes compensates
4512 for this. Other restrictions include whether it works on directories,
4513 open files, or pre-existing files. Check L<perlport> and either the
4514 rename(2) manpage or equivalent system documentation for details.
4516 For a platform independent C<move> function look at the L<File::Copy>
4519 =item require VERSION
4526 Demands a version of Perl specified by VERSION, or demands some semantics
4527 specified by EXPR or by C<$_> if EXPR is not supplied.
4529 VERSION may be either a numeric argument such as 5.006, which will be
4530 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4531 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4532 VERSION is greater than the version of the current Perl interpreter.
4533 Compare with L</use>, which can do a similar check at compile time.
4535 Specifying VERSION as a literal of the form v5.6.1 should generally be
4536 avoided, because it leads to misleading error messages under earlier
4537 versions of Perl that do not support this syntax. The equivalent numeric
4538 version should be used instead.
4540 require v5.6.1; # run time version check
4541 require 5.6.1; # ditto
4542 require 5.006_001; # ditto; preferred for backwards compatibility
4544 Otherwise, C<require> demands that a library file be included if it
4545 hasn't already been included. The file is included via the do-FILE
4546 mechanism, which is essentially just a variety of C<eval> with the
4547 caveat that lexical variables in the invoking script will be invisible
4548 to the included code. Has semantics similar to the following subroutine:
4551 my ($filename) = @_;
4552 if (exists $INC{$filename}) {
4553 return 1 if $INC{$filename};
4554 die "Compilation failed in require";
4556 my ($realfilename,$result);
4558 foreach $prefix (@INC) {
4559 $realfilename = "$prefix/$filename";
4560 if (-f $realfilename) {
4561 $INC{$filename} = $realfilename;
4562 $result = do $realfilename;
4566 die "Can't find $filename in \@INC";
4569 $INC{$filename} = undef;
4571 } elsif (!$result) {
4572 delete $INC{$filename};
4573 die "$filename did not return true value";
4579 Note that the file will not be included twice under the same specified
4582 The file must return true as the last statement to indicate
4583 successful execution of any initialization code, so it's customary to
4584 end such a file with C<1;> unless you're sure it'll return true
4585 otherwise. But it's better just to put the C<1;>, in case you add more
4588 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4589 replaces "F<::>" with "F</>" in the filename for you,
4590 to make it easy to load standard modules. This form of loading of
4591 modules does not risk altering your namespace.
4593 In other words, if you try this:
4595 require Foo::Bar; # a splendid bareword
4597 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4598 directories specified in the C<@INC> array.
4600 But if you try this:
4602 $class = 'Foo::Bar';
4603 require $class; # $class is not a bareword
4605 require "Foo::Bar"; # not a bareword because of the ""
4607 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4608 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4610 eval "require $class";
4612 Now that you understand how C<require> looks for files in the case of a
4613 bareword argument, there is a little extra functionality going on behind
4614 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4615 first look for a similar filename with a "F<.pmc>" extension. If this file
4616 is found, it will be loaded in place of any file ending in a "F<.pm>"
4619 You can also insert hooks into the import facility, by putting directly
4620 Perl code into the @INC array. There are three forms of hooks: subroutine
4621 references, array references and blessed objects.
4623 Subroutine references are the simplest case. When the inclusion system
4624 walks through @INC and encounters a subroutine, this subroutine gets
4625 called with two parameters, the first being a reference to itself, and the
4626 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4627 subroutine should return nothing, or a list of up to three values in the
4634 A filehandle, from which the file will be read.
4638 A reference to a subroutine. If there is no filehandle (previous item),
4639 then this subroutine is expected to generate one line of source code per
4640 call, writing the line into C<$_> and returning 1, then returning 0 at
4641 "end of file". If there is a filehandle, then the subroutine will be
4642 called to act as a simple source filter, with the line as read in C<$_>.
4643 Again, return 1 for each valid line, and 0 after all lines have been
4648 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4649 reference to the subroutine itself is passed in as C<$_[0]>.
4653 If an empty list, C<undef>, or nothing that matches the first 3 values above
4654 is returned then C<require> will look at the remaining elements of @INC.
4655 Note that this file handle must be a real file handle (strictly a typeglob,
4656 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4657 ignored and return value processing will stop there.
4659 If the hook is an array reference, its first element must be a subroutine
4660 reference. This subroutine is called as above, but the first parameter is
4661 the array reference. This enables to pass indirectly some arguments to
4664 In other words, you can write:
4666 push @INC, \&my_sub;
4668 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4674 push @INC, [ \&my_sub, $x, $y, ... ];
4676 my ($arrayref, $filename) = @_;
4677 # Retrieve $x, $y, ...
4678 my @parameters = @$arrayref[1..$#$arrayref];
4682 If the hook is an object, it must provide an INC method that will be
4683 called as above, the first parameter being the object itself. (Note that
4684 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4685 into package C<main>.) Here is a typical code layout:
4691 my ($self, $filename) = @_;
4695 # In the main program
4696 push @INC, Foo->new(...);
4698 Note that these hooks are also permitted to set the %INC entry
4699 corresponding to the files they have loaded. See L<perlvar/%INC>.
4701 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4708 Generally used in a C<continue> block at the end of a loop to clear
4709 variables and reset C<??> searches so that they work again. The
4710 expression is interpreted as a list of single characters (hyphens
4711 allowed for ranges). All variables and arrays beginning with one of
4712 those letters are reset to their pristine state. If the expression is
4713 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4714 only variables or searches in the current package. Always returns
4717 reset 'X'; # reset all X variables
4718 reset 'a-z'; # reset lower case variables
4719 reset; # just reset ?one-time? searches
4721 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4722 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4723 variables--lexical variables are unaffected, but they clean themselves
4724 up on scope exit anyway, so you'll probably want to use them instead.
4732 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4733 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4734 context, depending on how the return value will be used, and the context
4735 may vary from one execution to the next (see C<wantarray>). If no EXPR
4736 is given, returns an empty list in list context, the undefined value in
4737 scalar context, and (of course) nothing at all in a void context.
4739 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4740 or do FILE will automatically return the value of the last expression
4744 X<reverse> X<rev> X<invert>
4746 In list context, returns a list value consisting of the elements
4747 of LIST in the opposite order. In scalar context, concatenates the
4748 elements of LIST and returns a string value with all characters
4749 in the opposite order.
4751 print join(", ", reverse "world", "Hello"); # Hello, world
4753 print scalar reverse "dlrow ,", "olleH"; # Hello, world
4755 Used without arguments in scalar context, reverse() reverses C<$_>.
4757 $_ = "dlrow ,olleH";
4758 print reverse; # No output, list context
4759 print scalar reverse; # Hello, world
4761 This operator is also handy for inverting a hash, although there are some
4762 caveats. If a value is duplicated in the original hash, only one of those
4763 can be represented as a key in the inverted hash. Also, this has to
4764 unwind one hash and build a whole new one, which may take some time
4765 on a large hash, such as from a DBM file.
4767 %by_name = reverse %by_address; # Invert the hash
4769 =item rewinddir DIRHANDLE
4772 Sets the current position to the beginning of the directory for the
4773 C<readdir> routine on DIRHANDLE.
4775 =item rindex STR,SUBSTR,POSITION
4778 =item rindex STR,SUBSTR
4780 Works just like index() except that it returns the position of the I<last>
4781 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4782 last occurrence beginning at or before that position.
4784 =item rmdir FILENAME
4785 X<rmdir> X<rd> X<directory, remove>
4789 Deletes the directory specified by FILENAME if that directory is
4790 empty. If it succeeds it returns true, otherwise it returns false and
4791 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4793 To remove a directory tree recursively (C<rm -rf> on unix) look at
4794 the C<rmtree> function of the L<File::Path> module.
4798 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
4800 =item say FILEHANDLE LIST
4807 Just like C<print>, but implicitly appends a newline.
4808 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
4811 This keyword is only available when the "say" feature is
4812 enabled: see L<feature>.
4815 X<scalar> X<context>
4817 Forces EXPR to be interpreted in scalar context and returns the value
4820 @counts = ( scalar @a, scalar @b, scalar @c );
4822 There is no equivalent operator to force an expression to
4823 be interpolated in list context because in practice, this is never
4824 needed. If you really wanted to do so, however, you could use
4825 the construction C<@{[ (some expression) ]}>, but usually a simple
4826 C<(some expression)> suffices.
4828 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4829 parenthesized list, this behaves as a scalar comma expression, evaluating
4830 all but the last element in void context and returning the final element
4831 evaluated in scalar context. This is seldom what you want.
4833 The following single statement:
4835 print uc(scalar(&foo,$bar)),$baz;
4837 is the moral equivalent of these two:
4840 print(uc($bar),$baz);
4842 See L<perlop> for more details on unary operators and the comma operator.
4844 =item seek FILEHANDLE,POSITION,WHENCE
4845 X<seek> X<fseek> X<filehandle, position>
4847 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4848 FILEHANDLE may be an expression whose value gives the name of the
4849 filehandle. The values for WHENCE are C<0> to set the new position
4850 I<in bytes> to POSITION, C<1> to set it to the current position plus
4851 POSITION, and C<2> to set it to EOF plus POSITION (typically
4852 negative). For WHENCE you may use the constants C<SEEK_SET>,
4853 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4854 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4857 Note the I<in bytes>: even if the filehandle has been set to
4858 operate on characters (for example by using the C<:encoding(utf8)> open
4859 layer), tell() will return byte offsets, not character offsets
4860 (because implementing that would render seek() and tell() rather slow).
4862 If you want to position file for C<sysread> or C<syswrite>, don't use
4863 C<seek>--buffering makes its effect on the file's system position
4864 unpredictable and non-portable. Use C<sysseek> instead.
4866 Due to the rules and rigors of ANSI C, on some systems you have to do a
4867 seek whenever you switch between reading and writing. Amongst other
4868 things, this may have the effect of calling stdio's clearerr(3).
4869 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4873 This is also useful for applications emulating C<tail -f>. Once you hit
4874 EOF on your read, and then sleep for a while, you might have to stick in a
4875 seek() to reset things. The C<seek> doesn't change the current position,
4876 but it I<does> clear the end-of-file condition on the handle, so that the
4877 next C<< <FILE> >> makes Perl try again to read something. We hope.
4879 If that doesn't work (some IO implementations are particularly
4880 cantankerous), then you may need something more like this:
4883 for ($curpos = tell(FILE); $_ = <FILE>;
4884 $curpos = tell(FILE)) {
4885 # search for some stuff and put it into files
4887 sleep($for_a_while);
4888 seek(FILE, $curpos, 0);
4891 =item seekdir DIRHANDLE,POS
4894 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4895 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4896 about possible directory compaction as the corresponding system library
4899 =item select FILEHANDLE
4900 X<select> X<filehandle, default>
4904 Returns the currently selected filehandle. If FILEHANDLE is supplied,
4905 sets the new current default filehandle for output. This has two
4906 effects: first, a C<write> or a C<print> without a filehandle will
4907 default to this FILEHANDLE. Second, references to variables related to
4908 output will refer to this output channel. For example, if you have to
4909 set the top of form format for more than one output channel, you might
4917 FILEHANDLE may be an expression whose value gives the name of the
4918 actual filehandle. Thus:
4920 $oldfh = select(STDERR); $| = 1; select($oldfh);
4922 Some programmers may prefer to think of filehandles as objects with
4923 methods, preferring to write the last example as:
4926 STDERR->autoflush(1);
4928 =item select RBITS,WBITS,EBITS,TIMEOUT
4931 This calls the select(2) system call with the bit masks specified, which
4932 can be constructed using C<fileno> and C<vec>, along these lines:
4934 $rin = $win = $ein = '';
4935 vec($rin,fileno(STDIN),1) = 1;
4936 vec($win,fileno(STDOUT),1) = 1;
4939 If you want to select on many filehandles you might wish to write a
4943 my(@fhlist) = split(' ',$_[0]);
4946 vec($bits,fileno($_),1) = 1;
4950 $rin = fhbits('STDIN TTY SOCK');
4954 ($nfound,$timeleft) =
4955 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4957 or to block until something becomes ready just do this
4959 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4961 Most systems do not bother to return anything useful in $timeleft, so
4962 calling select() in scalar context just returns $nfound.
4964 Any of the bit masks can also be undef. The timeout, if specified, is
4965 in seconds, which may be fractional. Note: not all implementations are
4966 capable of returning the $timeleft. If not, they always return
4967 $timeleft equal to the supplied $timeout.
4969 You can effect a sleep of 250 milliseconds this way:
4971 select(undef, undef, undef, 0.25);
4973 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4974 is implementation-dependent. See also L<perlport> for notes on the
4975 portability of C<select>.
4977 On error, C<select> behaves like the select(2) system call : it returns
4980 Note: on some Unixes, the select(2) system call may report a socket file
4981 descriptor as "ready for reading", when actually no data is available,
4982 thus a subsequent read blocks. It can be avoided using always the
4983 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4986 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4987 or <FH>) with C<select>, except as permitted by POSIX, and even
4988 then only on POSIX systems. You have to use C<sysread> instead.
4990 =item semctl ID,SEMNUM,CMD,ARG
4993 Calls the System V IPC function C<semctl>. You'll probably have to say
4997 first to get the correct constant definitions. If CMD is IPC_STAT or
4998 GETALL, then ARG must be a variable that will hold the returned
4999 semid_ds structure or semaphore value array. Returns like C<ioctl>:
5000 the undefined value for error, "C<0 but true>" for zero, or the actual
5001 return value otherwise. The ARG must consist of a vector of native
5002 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
5003 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
5006 =item semget KEY,NSEMS,FLAGS
5009 Calls the System V IPC function semget. Returns the semaphore id, or
5010 the undefined value if there is an error. See also
5011 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
5014 =item semop KEY,OPSTRING
5017 Calls the System V IPC function semop to perform semaphore operations
5018 such as signalling and waiting. OPSTRING must be a packed array of
5019 semop structures. Each semop structure can be generated with
5020 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
5021 implies the number of semaphore operations. Returns true if
5022 successful, or false if there is an error. As an example, the
5023 following code waits on semaphore $semnum of semaphore id $semid:
5025 $semop = pack("s!3", $semnum, -1, 0);
5026 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5028 To signal the semaphore, replace C<-1> with C<1>. See also
5029 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
5032 =item send SOCKET,MSG,FLAGS,TO
5035 =item send SOCKET,MSG,FLAGS
5037 Sends a message on a socket. Attempts to send the scalar MSG to the
5038 SOCKET filehandle. Takes the same flags as the system call of the
5039 same name. On unconnected sockets you must specify a destination to
5040 send TO, in which case it does a C C<sendto>. Returns the number of
5041 characters sent, or the undefined value if there is an error. The C
5042 system call sendmsg(2) is currently unimplemented. See
5043 L<perlipc/"UDP: Message Passing"> for examples.
5045 Note the I<characters>: depending on the status of the socket, either
5046 (8-bit) bytes or characters are sent. By default all sockets operate
5047 on bytes, but for example if the socket has been changed using
5048 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5049 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5050 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5051 pragma: in that case pretty much any characters can be sent.
5053 =item setpgrp PID,PGRP
5056 Sets the current process group for the specified PID, C<0> for the current
5057 process. Will produce a fatal error if used on a machine that doesn't
5058 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5059 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5060 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5063 =item setpriority WHICH,WHO,PRIORITY
5064 X<setpriority> X<priority> X<nice> X<renice>
5066 Sets the current priority for a process, a process group, or a user.
5067 (See setpriority(2).) Will produce a fatal error if used on a machine
5068 that doesn't implement setpriority(2).
5070 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5073 Sets the socket option requested. Returns undefined if there is an
5074 error. Use integer constants provided by the C<Socket> module for
5075 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5076 getprotobyname. OPTVAL might either be a packed string or an integer.
5077 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5079 An example disabling the Nagle's algorithm for a socket:
5081 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5082 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5089 Shifts the first value of the array off and returns it, shortening the
5090 array by 1 and moving everything down. If there are no elements in the
5091 array, returns the undefined value. If ARRAY is omitted, shifts the
5092 C<@_> array within the lexical scope of subroutines and formats, and the
5093 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5094 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5095 C<UNITCHECK {}> and C<END {}> constructs.
5097 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5098 same thing to the left end of an array that C<pop> and C<push> do to the
5101 =item shmctl ID,CMD,ARG
5104 Calls the System V IPC function shmctl. You'll probably have to say
5108 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5109 then ARG must be a variable that will hold the returned C<shmid_ds>
5110 structure. Returns like ioctl: the undefined value for error, "C<0> but
5111 true" for zero, or the actual return value otherwise.
5112 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5114 =item shmget KEY,SIZE,FLAGS
5117 Calls the System V IPC function shmget. Returns the shared memory
5118 segment id, or the undefined value if there is an error.
5119 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5121 =item shmread ID,VAR,POS,SIZE
5125 =item shmwrite ID,STRING,POS,SIZE
5127 Reads or writes the System V shared memory segment ID starting at
5128 position POS for size SIZE by attaching to it, copying in/out, and
5129 detaching from it. When reading, VAR must be a variable that will
5130 hold the data read. When writing, if STRING is too long, only SIZE
5131 bytes are used; if STRING is too short, nulls are written to fill out
5132 SIZE bytes. Return true if successful, or false if there is an error.
5133 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5134 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5136 =item shutdown SOCKET,HOW
5139 Shuts down a socket connection in the manner indicated by HOW, which
5140 has the same interpretation as in the system call of the same name.
5142 shutdown(SOCKET, 0); # I/we have stopped reading data
5143 shutdown(SOCKET, 1); # I/we have stopped writing data
5144 shutdown(SOCKET, 2); # I/we have stopped using this socket
5146 This is useful with sockets when you want to tell the other
5147 side you're done writing but not done reading, or vice versa.
5148 It's also a more insistent form of close because it also
5149 disables the file descriptor in any forked copies in other
5152 Returns C<1> for success. In the case of error, returns C<undef> if
5153 the first argument is not a valid filehandle, or returns C<0> and sets
5154 C<$!> for any other failure.
5157 X<sin> X<sine> X<asin> X<arcsine>
5161 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5162 returns sine of C<$_>.
5164 For the inverse sine operation, you may use the C<Math::Trig::asin>
5165 function, or use this relation:
5167 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5174 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5175 Returns the number of seconds actually slept.
5177 May be interrupted if the process receives a signal such as C<SIGALRM>.
5180 local $SIG{ALARM} = sub { die "Alarm!\n" };
5183 die $@ unless $@ eq "Alarm!\n";
5185 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
5186 is often implemented using C<alarm>.
5188 On some older systems, it may sleep up to a full second less than what
5189 you requested, depending on how it counts seconds. Most modern systems
5190 always sleep the full amount. They may appear to sleep longer than that,
5191 however, because your process might not be scheduled right away in a
5192 busy multitasking system.
5194 For delays of finer granularity than one second, the Time::HiRes module
5195 (from CPAN, and starting from Perl 5.8 part of the standard
5196 distribution) provides usleep(). You may also use Perl's four-argument
5197 version of select() leaving the first three arguments undefined, or you
5198 might be able to use the C<syscall> interface to access setitimer(2) if
5199 your system supports it. See L<perlfaq8> for details.
5201 See also the POSIX module's C<pause> function.
5203 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5206 Opens a socket of the specified kind and attaches it to filehandle
5207 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5208 the system call of the same name. You should C<use Socket> first
5209 to get the proper definitions imported. See the examples in
5210 L<perlipc/"Sockets: Client/Server Communication">.
5212 On systems that support a close-on-exec flag on files, the flag will
5213 be set for the newly opened file descriptor, as determined by the
5214 value of $^F. See L<perlvar/$^F>.
5216 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5219 Creates an unnamed pair of sockets in the specified domain, of the
5220 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5221 for the system call of the same name. If unimplemented, yields a fatal
5222 error. Returns true if successful.
5224 On systems that support a close-on-exec flag on files, the flag will
5225 be set for the newly opened file descriptors, as determined by the value
5226 of $^F. See L<perlvar/$^F>.
5228 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5229 to C<pipe(Rdr, Wtr)> is essentially:
5232 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5233 shutdown(Rdr, 1); # no more writing for reader
5234 shutdown(Wtr, 0); # no more reading for writer
5236 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5237 emulate socketpair using IP sockets to localhost if your system implements
5238 sockets but not socketpair.
5240 =item sort SUBNAME LIST
5241 X<sort> X<qsort> X<quicksort> X<mergesort>
5243 =item sort BLOCK LIST
5247 In list context, this sorts the LIST and returns the sorted list value.
5248 In scalar context, the behaviour of C<sort()> is undefined.
5250 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5251 order. If SUBNAME is specified, it gives the name of a subroutine
5252 that returns an integer less than, equal to, or greater than C<0>,
5253 depending on how the elements of the list are to be ordered. (The C<<
5254 <=> >> and C<cmp> operators are extremely useful in such routines.)
5255 SUBNAME may be a scalar variable name (unsubscripted), in which case
5256 the value provides the name of (or a reference to) the actual
5257 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5258 an anonymous, in-line sort subroutine.
5260 If the subroutine's prototype is C<($$)>, the elements to be compared
5261 are passed by reference in C<@_>, as for a normal subroutine. This is
5262 slower than unprototyped subroutines, where the elements to be
5263 compared are passed into the subroutine
5264 as the package global variables $a and $b (see example below). Note that
5265 in the latter case, it is usually counter-productive to declare $a and
5268 The values to be compared are always passed by reference and should not
5271 You also cannot exit out of the sort block or subroutine using any of the
5272 loop control operators described in L<perlsyn> or with C<goto>.
5274 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5275 current collation locale. See L<perllocale>.
5277 sort() returns aliases into the original list, much as a for loop's index
5278 variable aliases the list elements. That is, modifying an element of a
5279 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5280 actually modifies the element in the original list. This is usually
5281 something to be avoided when writing clear code.
5283 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5284 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5285 preserves the input order of elements that compare equal. Although
5286 quicksort's run time is O(NlogN) when averaged over all arrays of
5287 length N, the time can be O(N**2), I<quadratic> behavior, for some
5288 inputs.) In 5.7, the quicksort implementation was replaced with
5289 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5290 But benchmarks indicated that for some inputs, on some platforms,
5291 the original quicksort was faster. 5.8 has a sort pragma for
5292 limited control of the sort. Its rather blunt control of the
5293 underlying algorithm may not persist into future Perls, but the
5294 ability to characterize the input or output in implementation
5295 independent ways quite probably will. See L<the sort pragma|sort>.
5300 @articles = sort @files;
5302 # same thing, but with explicit sort routine
5303 @articles = sort {$a cmp $b} @files;
5305 # now case-insensitively
5306 @articles = sort {uc($a) cmp uc($b)} @files;
5308 # same thing in reversed order
5309 @articles = sort {$b cmp $a} @files;
5311 # sort numerically ascending
5312 @articles = sort {$a <=> $b} @files;
5314 # sort numerically descending
5315 @articles = sort {$b <=> $a} @files;
5317 # this sorts the %age hash by value instead of key
5318 # using an in-line function
5319 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5321 # sort using explicit subroutine name
5323 $age{$a} <=> $age{$b}; # presuming numeric
5325 @sortedclass = sort byage @class;
5327 sub backwards { $b cmp $a }
5328 @harry = qw(dog cat x Cain Abel);
5329 @george = qw(gone chased yz Punished Axed);
5331 # prints AbelCaincatdogx
5332 print sort backwards @harry;
5333 # prints xdogcatCainAbel
5334 print sort @george, 'to', @harry;
5335 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5337 # inefficiently sort by descending numeric compare using
5338 # the first integer after the first = sign, or the
5339 # whole record case-insensitively otherwise
5342 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5347 # same thing, but much more efficiently;
5348 # we'll build auxiliary indices instead
5352 push @nums, /=(\d+)/;
5357 $nums[$b] <=> $nums[$a]
5359 $caps[$a] cmp $caps[$b]
5363 # same thing, but without any temps
5364 @new = map { $_->[0] }
5365 sort { $b->[1] <=> $a->[1]
5368 } map { [$_, /=(\d+)/, uc($_)] } @old;
5370 # using a prototype allows you to use any comparison subroutine
5371 # as a sort subroutine (including other package's subroutines)
5373 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5376 @new = sort other::backwards @old;
5378 # guarantee stability, regardless of algorithm
5380 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5382 # force use of mergesort (not portable outside Perl 5.8)
5383 use sort '_mergesort'; # note discouraging _
5384 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5386 Warning: syntactical care is required when sorting the list returned from
5387 a function. If you want to sort the list returned by the function call
5388 C<find_records(@key)>, you can use:
5390 @contact = sort { $a cmp $b } find_records @key;
5391 @contact = sort +find_records(@key);
5392 @contact = sort &find_records(@key);
5393 @contact = sort(find_records(@key));
5395 If instead you want to sort the array @key with the comparison routine
5396 C<find_records()> then you can use:
5398 @contact = sort { find_records() } @key;
5399 @contact = sort find_records(@key);
5400 @contact = sort(find_records @key);
5401 @contact = sort(find_records (@key));
5403 If you're using strict, you I<must not> declare $a
5404 and $b as lexicals. They are package globals. That means
5405 that if you're in the C<main> package and type
5407 @articles = sort {$b <=> $a} @files;
5409 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5410 but if you're in the C<FooPack> package, it's the same as typing
5412 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5414 The comparison function is required to behave. If it returns
5415 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5416 sometimes saying the opposite, for example) the results are not
5419 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5420 (not-a-number), and because C<sort> will trigger a fatal error unless the
5421 result of a comparison is defined, when sorting with a comparison function
5422 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5423 The following example takes advantage of the fact that C<NaN != NaN> to
5424 eliminate any C<NaN>s from the input.
5426 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5428 =item splice ARRAY,OFFSET,LENGTH,LIST
5431 =item splice ARRAY,OFFSET,LENGTH
5433 =item splice ARRAY,OFFSET
5437 Removes the elements designated by OFFSET and LENGTH from an array, and
5438 replaces them with the elements of LIST, if any. In list context,
5439 returns the elements removed from the array. In scalar context,
5440 returns the last element removed, or C<undef> if no elements are
5441 removed. The array grows or shrinks as necessary.
5442 If OFFSET is negative then it starts that far from the end of the array.
5443 If LENGTH is omitted, removes everything from OFFSET onward.
5444 If LENGTH is negative, removes the elements from OFFSET onward
5445 except for -LENGTH elements at the end of the array.
5446 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5447 past the end of the array, perl issues a warning, and splices at the
5450 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5452 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5453 pop(@a) splice(@a,-1)
5454 shift(@a) splice(@a,0,1)
5455 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5456 $a[$i] = $y splice(@a,$i,1,$y)
5458 Example, assuming array lengths are passed before arrays:
5460 sub aeq { # compare two list values
5461 my(@a) = splice(@_,0,shift);
5462 my(@b) = splice(@_,0,shift);
5463 return 0 unless @a == @b; # same len?
5465 return 0 if pop(@a) ne pop(@b);
5469 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5471 =item split /PATTERN/,EXPR,LIMIT
5474 =item split /PATTERN/,EXPR
5476 =item split /PATTERN/
5480 Splits the string EXPR into a list of strings and returns that list. By
5481 default, empty leading fields are preserved, and empty trailing ones are
5482 deleted. (If all fields are empty, they are considered to be trailing.)
5484 In scalar context, returns the number of fields found.
5486 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5487 splits on whitespace (after skipping any leading whitespace). Anything
5488 matching PATTERN is taken to be a delimiter separating the fields. (Note
5489 that the delimiter may be longer than one character.)
5491 If LIMIT is specified and positive, it represents the maximum number
5492 of fields the EXPR will be split into, though the actual number of
5493 fields returned depends on the number of times PATTERN matches within
5494 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5495 stripped (which potential users of C<pop> would do well to remember).
5496 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5497 had been specified. Note that splitting an EXPR that evaluates to the
5498 empty string always returns the empty list, regardless of the LIMIT
5501 A pattern matching the null string (not to be confused with
5502 a null pattern C<//>, which is just one member of the set of patterns
5503 matching a null string) will split the value of EXPR into separate
5504 characters at each point it matches that way. For example:
5506 print join(':', split(/ */, 'hi there')), "\n";
5508 produces the output 'h:i:t:h:e:r:e'.
5510 As a special case for C<split>, using the empty pattern C<//> specifically
5511 matches only the null string, and is not be confused with the regular use
5512 of C<//> to mean "the last successful pattern match". So, for C<split>,
5515 print join(':', split(//, 'hi there')), "\n";
5517 produces the output 'h:i: :t:h:e:r:e'.
5519 Empty leading fields are produced when there are positive-width matches at
5520 the beginning of the string; a zero-width match at the beginning of
5521 the string does not produce an empty field. For example:
5523 print join(':', split(/(?=\w)/, 'hi there!'));
5525 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
5526 hand, are produced when there is a match at the end of the string (and
5527 when LIMIT is given and is not 0), regardless of the length of the match.
5530 print join(':', split(//, 'hi there!', -1)), "\n";
5531 print join(':', split(/\W/, 'hi there!', -1)), "\n";
5533 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
5534 both with an empty trailing field.
5536 The LIMIT parameter can be used to split a line partially
5538 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5540 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5541 a LIMIT one larger than the number of variables in the list, to avoid
5542 unnecessary work. For the list above LIMIT would have been 4 by
5543 default. In time critical applications it behooves you not to split
5544 into more fields than you really need.
5546 If the PATTERN contains parentheses, additional list elements are
5547 created from each matching substring in the delimiter.
5549 split(/([,-])/, "1-10,20", 3);
5551 produces the list value
5553 (1, '-', 10, ',', 20)
5555 If you had the entire header of a normal Unix email message in $header,
5556 you could split it up into fields and their values this way:
5558 $header =~ s/\n(?=\s)//g; # fix continuation lines
5559 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5561 The pattern C</PATTERN/> may be replaced with an expression to specify
5562 patterns that vary at runtime. (To do runtime compilation only once,
5563 use C</$variable/o>.)
5565 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5566 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5567 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5568 will give you as many null initial fields as there are leading spaces.
5569 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5570 whitespace produces a null first field. A C<split> with no arguments
5571 really does a S<C<split(' ', $_)>> internally.
5573 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5578 open(PASSWD, '/etc/passwd');
5581 ($login, $passwd, $uid, $gid,
5582 $gcos, $home, $shell) = split(/:/);
5586 As with regular pattern matching, any capturing parentheses that are not
5587 matched in a C<split()> will be set to C<undef> when returned:
5589 @fields = split /(A)|B/, "1A2B3";
5590 # @fields is (1, 'A', 2, undef, 3)
5592 =item sprintf FORMAT, LIST
5595 Returns a string formatted by the usual C<printf> conventions of the C
5596 library function C<sprintf>. See below for more details
5597 and see C<sprintf(3)> or C<printf(3)> on your system for an explanation of
5598 the general principles.
5602 # Format number with up to 8 leading zeroes
5603 $result = sprintf("%08d", $number);
5605 # Round number to 3 digits after decimal point
5606 $rounded = sprintf("%.3f", $number);
5608 Perl does its own C<sprintf> formatting--it emulates the C
5609 function C<sprintf>, but it doesn't use it (except for floating-point
5610 numbers, and even then only the standard modifiers are allowed). As a
5611 result, any non-standard extensions in your local C<sprintf> are not
5612 available from Perl.
5614 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5615 pass it an array as your first argument. The array is given scalar context,
5616 and instead of using the 0th element of the array as the format, Perl will
5617 use the count of elements in the array as the format, which is almost never
5620 Perl's C<sprintf> permits the following universally-known conversions:
5623 %c a character with the given number
5625 %d a signed integer, in decimal
5626 %u an unsigned integer, in decimal
5627 %o an unsigned integer, in octal
5628 %x an unsigned integer, in hexadecimal
5629 %e a floating-point number, in scientific notation
5630 %f a floating-point number, in fixed decimal notation
5631 %g a floating-point number, in %e or %f notation
5633 In addition, Perl permits the following widely-supported conversions:
5635 %X like %x, but using upper-case letters
5636 %E like %e, but using an upper-case "E"
5637 %G like %g, but with an upper-case "E" (if applicable)
5638 %b an unsigned integer, in binary
5639 %B like %b, but using an upper-case "B" with the # flag
5640 %p a pointer (outputs the Perl value's address in hexadecimal)
5641 %n special: *stores* the number of characters output so far
5642 into the next variable in the parameter list
5644 Finally, for backward (and we do mean "backward") compatibility, Perl
5645 permits these unnecessary but widely-supported conversions:
5648 %D a synonym for %ld
5649 %U a synonym for %lu
5650 %O a synonym for %lo
5653 Note that the number of exponent digits in the scientific notation produced
5654 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5655 exponent less than 100 is system-dependent: it may be three or less
5656 (zero-padded as necessary). In other words, 1.23 times ten to the
5657 99th may be either "1.23e99" or "1.23e099".
5659 Between the C<%> and the format letter, you may specify a number of
5660 additional attributes controlling the interpretation of the format.
5661 In order, these are:
5665 =item format parameter index
5667 An explicit format parameter index, such as C<2$>. By default sprintf
5668 will format the next unused argument in the list, but this allows you
5669 to take the arguments out of order, e.g.:
5671 printf '%2$d %1$d', 12, 34; # prints "34 12"
5672 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5678 space prefix non-negative number with a space
5679 + prefix non-negative number with a plus sign
5680 - left-justify within the field
5681 0 use zeros, not spaces, to right-justify
5682 # ensure the leading "0" for any octal,
5683 prefix non-zero hexadecimal with "0x" or "0X",
5684 prefix non-zero binary with "0b" or "0B"
5688 printf '<% d>', 12; # prints "< 12>"
5689 printf '<%+d>', 12; # prints "<+12>"
5690 printf '<%6s>', 12; # prints "< 12>"
5691 printf '<%-6s>', 12; # prints "<12 >"
5692 printf '<%06s>', 12; # prints "<000012>"
5693 printf '<%#o>', 12; # prints "<014>"
5694 printf '<%#x>', 12; # prints "<0xc>"
5695 printf '<%#X>', 12; # prints "<0XC>"
5696 printf '<%#b>', 12; # prints "<0b1100>"
5697 printf '<%#B>', 12; # prints "<0B1100>"
5699 When a space and a plus sign are given as the flags at once,
5700 a plus sign is used to prefix a positive number.
5702 printf '<%+ d>', 12; # prints "<+12>"
5703 printf '<% +d>', 12; # prints "<+12>"
5705 When the # flag and a precision are given in the %o conversion,
5706 the precision is incremented if it's necessary for the leading "0".
5708 printf '<%#.5o>', 012; # prints "<00012>"
5709 printf '<%#.5o>', 012345; # prints "<012345>"
5710 printf '<%#.0o>', 0; # prints "<0>"
5714 This flag tells perl to interpret the supplied string as a vector of
5715 integers, one for each character in the string. Perl applies the format to
5716 each integer in turn, then joins the resulting strings with a separator (a
5717 dot C<.> by default). This can be useful for displaying ordinal values of
5718 characters in arbitrary strings:
5720 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5721 printf "version is v%vd\n", $^V; # Perl's version
5723 Put an asterisk C<*> before the C<v> to override the string to
5724 use to separate the numbers:
5726 printf "address is %*vX\n", ":", $addr; # IPv6 address
5727 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5729 You can also explicitly specify the argument number to use for
5730 the join string using e.g. C<*2$v>:
5732 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5734 =item (minimum) width
5736 Arguments are usually formatted to be only as wide as required to
5737 display the given value. You can override the width by putting
5738 a number here, or get the width from the next argument (with C<*>)
5739 or from a specified argument (with e.g. C<*2$>):
5741 printf '<%s>', "a"; # prints "<a>"
5742 printf '<%6s>', "a"; # prints "< a>"
5743 printf '<%*s>', 6, "a"; # prints "< a>"
5744 printf '<%*2$s>', "a", 6; # prints "< a>"
5745 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5747 If a field width obtained through C<*> is negative, it has the same
5748 effect as the C<-> flag: left-justification.
5750 =item precision, or maximum width
5753 You can specify a precision (for numeric conversions) or a maximum
5754 width (for string conversions) by specifying a C<.> followed by a number.
5755 For floating point formats, with the exception of 'g' and 'G', this specifies
5756 the number of decimal places to show (the default being 6), e.g.:
5758 # these examples are subject to system-specific variation
5759 printf '<%f>', 1; # prints "<1.000000>"
5760 printf '<%.1f>', 1; # prints "<1.0>"
5761 printf '<%.0f>', 1; # prints "<1>"
5762 printf '<%e>', 10; # prints "<1.000000e+01>"
5763 printf '<%.1e>', 10; # prints "<1.0e+01>"
5765 For 'g' and 'G', this specifies the maximum number of digits to show,
5766 including prior to the decimal point as well as after it, e.g.:
5768 # these examples are subject to system-specific variation
5769 printf '<%g>', 1; # prints "<1>"
5770 printf '<%.10g>', 1; # prints "<1>"
5771 printf '<%g>', 100; # prints "<100>"
5772 printf '<%.1g>', 100; # prints "<1e+02>"
5773 printf '<%.2g>', 100.01; # prints "<1e+02>"
5774 printf '<%.5g>', 100.01; # prints "<100.01>"
5775 printf '<%.4g>', 100.01; # prints "<100>"
5777 For integer conversions, specifying a precision implies that the
5778 output of the number itself should be zero-padded to this width,
5779 where the 0 flag is ignored:
5781 printf '<%.6d>', 1; # prints "<000001>"
5782 printf '<%+.6d>', 1; # prints "<+000001>"
5783 printf '<%-10.6d>', 1; # prints "<000001 >"
5784 printf '<%10.6d>', 1; # prints "< 000001>"
5785 printf '<%010.6d>', 1; # prints "< 000001>"
5786 printf '<%+10.6d>', 1; # prints "< +000001>"
5788 printf '<%.6x>', 1; # prints "<000001>"
5789 printf '<%#.6x>', 1; # prints "<0x000001>"
5790 printf '<%-10.6x>', 1; # prints "<000001 >"
5791 printf '<%10.6x>', 1; # prints "< 000001>"
5792 printf '<%010.6x>', 1; # prints "< 000001>"
5793 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5795 For string conversions, specifying a precision truncates the string
5796 to fit in the specified width:
5798 printf '<%.5s>', "truncated"; # prints "<trunc>"
5799 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5801 You can also get the precision from the next argument using C<.*>:
5803 printf '<%.6x>', 1; # prints "<000001>"
5804 printf '<%.*x>', 6, 1; # prints "<000001>"
5806 If a precision obtained through C<*> is negative, it has the same
5807 effect as no precision.
5809 printf '<%.*s>', 7, "string"; # prints "<string>"
5810 printf '<%.*s>', 3, "string"; # prints "<str>"
5811 printf '<%.*s>', 0, "string"; # prints "<>"
5812 printf '<%.*s>', -1, "string"; # prints "<string>"
5814 printf '<%.*d>', 1, 0; # prints "<0>"
5815 printf '<%.*d>', 0, 0; # prints "<>"
5816 printf '<%.*d>', -1, 0; # prints "<0>"
5818 You cannot currently get the precision from a specified number,
5819 but it is intended that this will be possible in the future using
5822 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5826 For numeric conversions, you can specify the size to interpret the
5827 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5828 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5829 whatever the default integer size is on your platform (usually 32 or 64
5830 bits), but you can override this to use instead one of the standard C types,
5831 as supported by the compiler used to build Perl:
5833 l interpret integer as C type "long" or "unsigned long"
5834 h interpret integer as C type "short" or "unsigned short"
5835 q, L or ll interpret integer as C type "long long", "unsigned long long".
5836 or "quads" (typically 64-bit integers)
5838 The last will produce errors if Perl does not understand "quads" in your
5839 installation. (This requires that either the platform natively supports quads
5840 or Perl was specifically compiled to support quads.) You can find out
5841 whether your Perl supports quads via L<Config>:
5844 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5847 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5848 to be the default floating point size on your platform (double or long double),
5849 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5850 platform supports them. You can find out whether your Perl supports long
5851 doubles via L<Config>:
5854 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5856 You can find out whether Perl considers 'long double' to be the default
5857 floating point size to use on your platform via L<Config>:
5860 ($Config{uselongdouble} eq 'define') &&
5861 print "long doubles by default\n";
5863 It can also be the case that long doubles and doubles are the same thing:
5866 ($Config{doublesize} == $Config{longdblsize}) &&
5867 print "doubles are long doubles\n";
5869 The size specifier C<V> has no effect for Perl code, but it is supported
5870 for compatibility with XS code; it means 'use the standard size for
5871 a Perl integer (or floating-point number)', which is already the
5872 default for Perl code.
5874 =item order of arguments
5876 Normally, sprintf takes the next unused argument as the value to
5877 format for each format specification. If the format specification
5878 uses C<*> to require additional arguments, these are consumed from
5879 the argument list in the order in which they appear in the format
5880 specification I<before> the value to format. Where an argument is
5881 specified using an explicit index, this does not affect the normal
5882 order for the arguments (even when the explicitly specified index
5883 would have been the next argument in any case).
5887 printf '<%*.*s>', $a, $b, $c;
5889 would use C<$a> for the width, C<$b> for the precision and C<$c>
5890 as the value to format, while:
5892 printf '<%*1$.*s>', $a, $b;
5894 would use C<$a> for the width and the precision, and C<$b> as the
5897 Here are some more examples - beware that when using an explicit
5898 index, the C<$> may need to be escaped:
5900 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5901 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5902 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5903 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5907 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5908 the character used for the decimal separator in formatted floating
5909 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5913 X<sqrt> X<root> X<square root>
5917 Return the square root of EXPR. If EXPR is omitted, returns square
5918 root of C<$_>. Only works on non-negative operands, unless you've
5919 loaded the standard Math::Complex module.
5922 print sqrt(-2); # prints 1.4142135623731i
5925 X<srand> X<seed> X<randseed>
5929 Sets the random number seed for the C<rand> operator.
5931 The point of the function is to "seed" the C<rand> function so that
5932 C<rand> can produce a different sequence each time you run your
5935 If srand() is not called explicitly, it is called implicitly at the
5936 first use of the C<rand> operator. However, this was not the case in
5937 versions of Perl before 5.004, so if your script will run under older
5938 Perl versions, it should call C<srand>.
5940 Most programs won't even call srand() at all, except those that
5941 need a cryptographically-strong starting point rather than the
5942 generally acceptable default, which is based on time of day,
5943 process ID, and memory allocation, or the F</dev/urandom> device,
5946 You can call srand($seed) with the same $seed to reproduce the
5947 I<same> sequence from rand(), but this is usually reserved for
5948 generating predictable results for testing or debugging.
5949 Otherwise, don't call srand() more than once in your program.
5951 Do B<not> call srand() (i.e. without an argument) more than once in
5952 a script. The internal state of the random number generator should
5953 contain more entropy than can be provided by any seed, so calling
5954 srand() again actually I<loses> randomness.
5956 Most implementations of C<srand> take an integer and will silently
5957 truncate decimal numbers. This means C<srand(42)> will usually
5958 produce the same results as C<srand(42.1)>. To be safe, always pass
5959 C<srand> an integer.
5961 In versions of Perl prior to 5.004 the default seed was just the
5962 current C<time>. This isn't a particularly good seed, so many old
5963 programs supply their own seed value (often C<time ^ $$> or C<time ^
5964 ($$ + ($$ << 15))>), but that isn't necessary any more.
5966 For cryptographic purposes, however, you need something much more random
5967 than the default seed. Checksumming the compressed output of one or more
5968 rapidly changing operating system status programs is the usual method. For
5971 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
5973 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5976 Frequently called programs (like CGI scripts) that simply use
5980 for a seed can fall prey to the mathematical property that
5984 one-third of the time. So don't do that.
5986 =item stat FILEHANDLE
5987 X<stat> X<file, status> X<ctime>
5991 =item stat DIRHANDLE
5995 Returns a 13-element list giving the status info for a file, either
5996 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5997 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
6000 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
6001 $atime,$mtime,$ctime,$blksize,$blocks)
6004 Not all fields are supported on all filesystem types. Here are the
6005 meanings of the fields:
6007 0 dev device number of filesystem
6009 2 mode file mode (type and permissions)
6010 3 nlink number of (hard) links to the file
6011 4 uid numeric user ID of file's owner
6012 5 gid numeric group ID of file's owner
6013 6 rdev the device identifier (special files only)
6014 7 size total size of file, in bytes
6015 8 atime last access time in seconds since the epoch
6016 9 mtime last modify time in seconds since the epoch
6017 10 ctime inode change time in seconds since the epoch (*)
6018 11 blksize preferred block size for file system I/O
6019 12 blocks actual number of blocks allocated
6021 (The epoch was at 00:00 January 1, 1970 GMT.)
6023 (*) Not all fields are supported on all filesystem types. Notably, the
6024 ctime field is non-portable. In particular, you cannot expect it to be a
6025 "creation time", see L<perlport/"Files and Filesystems"> for details.
6027 If C<stat> is passed the special filehandle consisting of an underline, no
6028 stat is done, but the current contents of the stat structure from the
6029 last C<stat>, C<lstat>, or filetest are returned. Example:
6031 if (-x $file && (($d) = stat(_)) && $d < 0) {
6032 print "$file is executable NFS file\n";
6035 (This works on machines only for which the device number is negative
6038 Because the mode contains both the file type and its permissions, you
6039 should mask off the file type portion and (s)printf using a C<"%o">
6040 if you want to see the real permissions.
6042 $mode = (stat($filename))[2];
6043 printf "Permissions are %04o\n", $mode & 07777;
6045 In scalar context, C<stat> returns a boolean value indicating success
6046 or failure, and, if successful, sets the information associated with
6047 the special filehandle C<_>.
6049 The L<File::stat> module provides a convenient, by-name access mechanism:
6052 $sb = stat($filename);
6053 printf "File is %s, size is %s, perm %04o, mtime %s\n",
6054 $filename, $sb->size, $sb->mode & 07777,
6055 scalar localtime $sb->mtime;
6057 You can import symbolic mode constants (C<S_IF*>) and functions
6058 (C<S_IS*>) from the Fcntl module:
6062 $mode = (stat($filename))[2];
6064 $user_rwx = ($mode & S_IRWXU) >> 6;
6065 $group_read = ($mode & S_IRGRP) >> 3;
6066 $other_execute = $mode & S_IXOTH;
6068 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6070 $is_setuid = $mode & S_ISUID;
6071 $is_directory = S_ISDIR($mode);
6073 You could write the last two using the C<-u> and C<-d> operators.
6074 The commonly available C<S_IF*> constants are
6076 # Permissions: read, write, execute, for user, group, others.
6078 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6079 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6080 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6082 # Setuid/Setgid/Stickiness/SaveText.
6083 # Note that the exact meaning of these is system dependent.
6085 S_ISUID S_ISGID S_ISVTX S_ISTXT
6087 # File types. Not necessarily all are available on your system.
6089 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6091 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
6093 S_IREAD S_IWRITE S_IEXEC
6095 and the C<S_IF*> functions are
6097 S_IMODE($mode) the part of $mode containing the permission bits
6098 and the setuid/setgid/sticky bits
6100 S_IFMT($mode) the part of $mode containing the file type
6101 which can be bit-anded with e.g. S_IFREG
6102 or with the following functions
6104 # The operators -f, -d, -l, -b, -c, -p, and -S.
6106 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6107 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6109 # No direct -X operator counterpart, but for the first one
6110 # the -g operator is often equivalent. The ENFMT stands for
6111 # record flocking enforcement, a platform-dependent feature.
6113 S_ISENFMT($mode) S_ISWHT($mode)
6115 See your native chmod(2) and stat(2) documentation for more details
6116 about the C<S_*> constants. To get status info for a symbolic link
6117 instead of the target file behind the link, use the C<lstat> function.
6122 =item state TYPE EXPR
6124 =item state EXPR : ATTRS
6126 =item state TYPE EXPR : ATTRS
6128 C<state> declares a lexically scoped variable, just like C<my> does.
6129 However, those variables will never be reinitialized, contrary to
6130 lexical variables that are reinitialized each time their enclosing block
6133 C<state> variables are only enabled when the C<feature 'state'> pragma is
6134 in effect. See L<feature>.
6141 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6142 doing many pattern matches on the string before it is next modified.
6143 This may or may not save time, depending on the nature and number of
6144 patterns you are searching on, and on the distribution of character
6145 frequencies in the string to be searched--you probably want to compare
6146 run times with and without it to see which runs faster. Those loops
6147 that scan for many short constant strings (including the constant
6148 parts of more complex patterns) will benefit most. You may have only
6149 one C<study> active at a time--if you study a different scalar the first
6150 is "unstudied". (The way C<study> works is this: a linked list of every
6151 character in the string to be searched is made, so we know, for
6152 example, where all the C<'k'> characters are. From each search string,
6153 the rarest character is selected, based on some static frequency tables
6154 constructed from some C programs and English text. Only those places
6155 that contain this "rarest" character are examined.)
6157 For example, here is a loop that inserts index producing entries
6158 before any line containing a certain pattern:
6162 print ".IX foo\n" if /\bfoo\b/;
6163 print ".IX bar\n" if /\bbar\b/;
6164 print ".IX blurfl\n" if /\bblurfl\b/;
6169 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6170 will be looked at, because C<f> is rarer than C<o>. In general, this is
6171 a big win except in pathological cases. The only question is whether
6172 it saves you more time than it took to build the linked list in the
6175 Note that if you have to look for strings that you don't know till
6176 runtime, you can build an entire loop as a string and C<eval> that to
6177 avoid recompiling all your patterns all the time. Together with
6178 undefining C<$/> to input entire files as one record, this can be very
6179 fast, often faster than specialized programs like fgrep(1). The following
6180 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6181 out the names of those files that contain a match:
6183 $search = 'while (<>) { study;';
6184 foreach $word (@words) {
6185 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6190 eval $search; # this screams
6191 $/ = "\n"; # put back to normal input delimiter
6192 foreach $file (sort keys(%seen)) {
6196 =item sub NAME BLOCK
6199 =item sub NAME (PROTO) BLOCK
6201 =item sub NAME : ATTRS BLOCK
6203 =item sub NAME (PROTO) : ATTRS BLOCK
6205 This is subroutine definition, not a real function I<per se>.
6206 Without a BLOCK it's just a forward declaration. Without a NAME,
6207 it's an anonymous function declaration, and does actually return
6208 a value: the CODE ref of the closure you just created.
6210 See L<perlsub> and L<perlref> for details about subroutines and
6211 references, and L<attributes> and L<Attribute::Handlers> for more
6212 information about attributes.
6214 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6215 X<substr> X<substring> X<mid> X<left> X<right>
6217 =item substr EXPR,OFFSET,LENGTH
6219 =item substr EXPR,OFFSET
6221 Extracts a substring out of EXPR and returns it. First character is at
6222 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6223 If OFFSET is negative (or more precisely, less than C<$[>), starts
6224 that far from the end of the string. If LENGTH is omitted, returns
6225 everything to the end of the string. If LENGTH is negative, leaves that
6226 many characters off the end of the string.
6228 my $s = "The black cat climbed the green tree";
6229 my $color = substr $s, 4, 5; # black
6230 my $middle = substr $s, 4, -11; # black cat climbed the
6231 my $end = substr $s, 14; # climbed the green tree
6232 my $tail = substr $s, -4; # tree
6233 my $z = substr $s, -4, 2; # tr
6235 You can use the substr() function as an lvalue, in which case EXPR
6236 must itself be an lvalue. If you assign something shorter than LENGTH,
6237 the string will shrink, and if you assign something longer than LENGTH,
6238 the string will grow to accommodate it. To keep the string the same
6239 length you may need to pad or chop your value using C<sprintf>.
6241 If OFFSET and LENGTH specify a substring that is partly outside the
6242 string, only the part within the string is returned. If the substring
6243 is beyond either end of the string, substr() returns the undefined
6244 value and produces a warning. When used as an lvalue, specifying a
6245 substring that is entirely outside the string is a fatal error.
6246 Here's an example showing the behavior for boundary cases:
6249 substr($name, 4) = 'dy'; # $name is now 'freddy'
6250 my $null = substr $name, 6, 2; # returns '' (no warning)
6251 my $oops = substr $name, 7; # returns undef, with warning
6252 substr($name, 7) = 'gap'; # fatal error
6254 An alternative to using substr() as an lvalue is to specify the
6255 replacement string as the 4th argument. This allows you to replace
6256 parts of the EXPR and return what was there before in one operation,
6257 just as you can with splice().
6259 my $s = "The black cat climbed the green tree";
6260 my $z = substr $s, 14, 7, "jumped from"; # climbed
6261 # $s is now "The black cat jumped from the green tree"
6263 Note that the lvalue returned by the 3-arg version of substr() acts as
6264 a 'magic bullet'; each time it is assigned to, it remembers which part
6265 of the original string is being modified; for example:
6268 for (substr($x,1,2)) {
6269 $_ = 'a'; print $x,"\n"; # prints 1a4
6270 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6272 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6275 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6278 =item symlink OLDFILE,NEWFILE
6279 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6281 Creates a new filename symbolically linked to the old filename.
6282 Returns C<1> for success, C<0> otherwise. On systems that don't support
6283 symbolic links, produces a fatal error at run time. To check for that,
6286 $symlink_exists = eval { symlink("",""); 1 };
6288 =item syscall NUMBER, LIST
6289 X<syscall> X<system call>
6291 Calls the system call specified as the first element of the list,
6292 passing the remaining elements as arguments to the system call. If
6293 unimplemented, produces a fatal error. The arguments are interpreted
6294 as follows: if a given argument is numeric, the argument is passed as
6295 an int. If not, the pointer to the string value is passed. You are
6296 responsible to make sure a string is pre-extended long enough to
6297 receive any result that might be written into a string. You can't use a
6298 string literal (or other read-only string) as an argument to C<syscall>
6299 because Perl has to assume that any string pointer might be written
6301 integer arguments are not literals and have never been interpreted in a
6302 numeric context, you may need to add C<0> to them to force them to look
6303 like numbers. This emulates the C<syswrite> function (or vice versa):
6305 require 'syscall.ph'; # may need to run h2ph
6307 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6309 Note that Perl supports passing of up to only 14 arguments to your system call,
6310 which in practice should usually suffice.
6312 Syscall returns whatever value returned by the system call it calls.
6313 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6314 Note that some system calls can legitimately return C<-1>. The proper
6315 way to handle such calls is to assign C<$!=0;> before the call and
6316 check the value of C<$!> if syscall returns C<-1>.
6318 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6319 number of the read end of the pipe it creates. There is no way
6320 to retrieve the file number of the other end. You can avoid this
6321 problem by using C<pipe> instead.
6323 =item sysopen FILEHANDLE,FILENAME,MODE
6326 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6328 Opens the file whose filename is given by FILENAME, and associates it
6329 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6330 the name of the real filehandle wanted. This function calls the
6331 underlying operating system's C<open> function with the parameters
6332 FILENAME, MODE, PERMS.
6334 The possible values and flag bits of the MODE parameter are
6335 system-dependent; they are available via the standard module C<Fcntl>.
6336 See the documentation of your operating system's C<open> to see which
6337 values and flag bits are available. You may combine several flags
6338 using the C<|>-operator.
6340 Some of the most common values are C<O_RDONLY> for opening the file in
6341 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6342 and C<O_RDWR> for opening the file in read-write mode.
6343 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6345 For historical reasons, some values work on almost every system
6346 supported by perl: zero means read-only, one means write-only, and two
6347 means read/write. We know that these values do I<not> work under
6348 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6349 use them in new code.
6351 If the file named by FILENAME does not exist and the C<open> call creates
6352 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6353 PERMS specifies the permissions of the newly created file. If you omit
6354 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6355 These permission values need to be in octal, and are modified by your
6356 process's current C<umask>.
6359 In many systems the C<O_EXCL> flag is available for opening files in
6360 exclusive mode. This is B<not> locking: exclusiveness means here that
6361 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6362 on network filesystems, and has no effect unless the C<O_CREAT> flag
6363 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6364 being opened if it is a symbolic link. It does not protect against
6365 symbolic links in the file's path.
6368 Sometimes you may want to truncate an already-existing file. This
6369 can be done using the C<O_TRUNC> flag. The behavior of
6370 C<O_TRUNC> with C<O_RDONLY> is undefined.
6373 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6374 that takes away the user's option to have a more permissive umask.
6375 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6378 Note that C<sysopen> depends on the fdopen() C library function.
6379 On many UNIX systems, fdopen() is known to fail when file descriptors
6380 exceed a certain value, typically 255. If you need more file
6381 descriptors than that, consider rebuilding Perl to use the C<sfio>
6382 library, or perhaps using the POSIX::open() function.
6384 See L<perlopentut> for a kinder, gentler explanation of opening files.
6386 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6389 =item sysread FILEHANDLE,SCALAR,LENGTH
6391 Attempts to read LENGTH bytes of data into variable SCALAR from the
6392 specified FILEHANDLE, using the system call read(2). It bypasses
6393 buffered IO, so mixing this with other kinds of reads, C<print>,
6394 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6395 perlio or stdio layers usually buffers data. Returns the number of
6396 bytes actually read, C<0> at end of file, or undef if there was an
6397 error (in the latter case C<$!> is also set). SCALAR will be grown or
6398 shrunk so that the last byte actually read is the last byte of the
6399 scalar after the read.
6401 An OFFSET may be specified to place the read data at some place in the
6402 string other than the beginning. A negative OFFSET specifies
6403 placement at that many characters counting backwards from the end of
6404 the string. A positive OFFSET greater than the length of SCALAR
6405 results in the string being padded to the required size with C<"\0">
6406 bytes before the result of the read is appended.
6408 There is no syseof() function, which is ok, since eof() doesn't work
6409 very well on device files (like ttys) anyway. Use sysread() and check
6410 for a return value for 0 to decide whether you're done.
6412 Note that if the filehandle has been marked as C<:utf8> Unicode
6413 characters are read instead of bytes (the LENGTH, OFFSET, and the
6414 return value of sysread() are in Unicode characters).
6415 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6416 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6418 =item sysseek FILEHANDLE,POSITION,WHENCE
6421 Sets FILEHANDLE's system position in bytes using the system call
6422 lseek(2). FILEHANDLE may be an expression whose value gives the name
6423 of the filehandle. The values for WHENCE are C<0> to set the new
6424 position to POSITION, C<1> to set the it to the current position plus
6425 POSITION, and C<2> to set it to EOF plus POSITION (typically
6428 Note the I<in bytes>: even if the filehandle has been set to operate
6429 on characters (for example by using the C<:encoding(utf8)> I/O layer),
6430 tell() will return byte offsets, not character offsets (because
6431 implementing that would render sysseek() very slow).
6433 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6434 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6435 C<seek>, C<tell>, or C<eof> may cause confusion.
6437 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6438 and C<SEEK_END> (start of the file, current position, end of the file)
6439 from the Fcntl module. Use of the constants is also more portable
6440 than relying on 0, 1, and 2. For example to define a "systell" function:
6442 use Fcntl 'SEEK_CUR';
6443 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6445 Returns the new position, or the undefined value on failure. A position
6446 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6447 true on success and false on failure, yet you can still easily determine
6453 =item system PROGRAM LIST
6455 Does exactly the same thing as C<exec LIST>, except that a fork is
6456 done first, and the parent process waits for the child process to
6457 complete. Note that argument processing varies depending on the
6458 number of arguments. If there is more than one argument in LIST,
6459 or if LIST is an array with more than one value, starts the program
6460 given by the first element of the list with arguments given by the
6461 rest of the list. If there is only one scalar argument, the argument
6462 is checked for shell metacharacters, and if there are any, the
6463 entire argument is passed to the system's command shell for parsing
6464 (this is C</bin/sh -c> on Unix platforms, but varies on other
6465 platforms). If there are no shell metacharacters in the argument,
6466 it is split into words and passed directly to C<execvp>, which is
6469 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6470 output before any operation that may do a fork, but this may not be
6471 supported on some platforms (see L<perlport>). To be safe, you may need
6472 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6473 of C<IO::Handle> on any open handles.
6475 The return value is the exit status of the program as returned by the
6476 C<wait> call. To get the actual exit value, shift right by eight (see
6477 below). See also L</exec>. This is I<not> what you want to use to capture
6478 the output from a command, for that you should use merely backticks or
6479 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6480 indicates a failure to start the program or an error of the wait(2) system
6481 call (inspect $! for the reason).
6483 If you'd like to make C<system> (and many other bits of Perl) die on error,
6484 have a look at the L<autodie> pragma.
6486 Like C<exec>, C<system> allows you to lie to a program about its name if
6487 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6489 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6490 C<system>, if you expect your program to terminate on receipt of these
6491 signals you will need to arrange to do so yourself based on the return
6494 @args = ("command", "arg1", "arg2");
6496 or die "system @args failed: $?"
6498 If you'd like to manually inspect C<system>'s failure, you can check all
6499 possible failure modes by inspecting C<$?> like this:
6502 print "failed to execute: $!\n";
6505 printf "child died with signal %d, %s coredump\n",
6506 ($? & 127), ($? & 128) ? 'with' : 'without';
6509 printf "child exited with value %d\n", $? >> 8;
6512 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6513 with the W*() calls of the POSIX extension.
6515 When the arguments get executed via the system shell, results
6516 and return codes will be subject to its quirks and capabilities.
6517 See L<perlop/"`STRING`"> and L</exec> for details.
6519 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6522 =item syswrite FILEHANDLE,SCALAR,LENGTH
6524 =item syswrite FILEHANDLE,SCALAR
6526 Attempts to write LENGTH bytes of data from variable SCALAR to the
6527 specified FILEHANDLE, using the system call write(2). If LENGTH is
6528 not specified, writes whole SCALAR. It bypasses buffered IO, so
6529 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6530 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6531 stdio layers usually buffers data. Returns the number of bytes
6532 actually written, or C<undef> if there was an error (in this case the
6533 errno variable C<$!> is also set). If the LENGTH is greater than the
6534 available data in the SCALAR after the OFFSET, only as much data as is
6535 available will be written.
6537 An OFFSET may be specified to write the data from some part of the
6538 string other than the beginning. A negative OFFSET specifies writing
6539 that many characters counting backwards from the end of the string.
6540 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6542 Note that if the filehandle has been marked as C<:utf8>, Unicode
6543 characters are written instead of bytes (the LENGTH, OFFSET, and the
6544 return value of syswrite() are in UTF-8 encoded Unicode characters).
6545 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6546 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6548 =item tell FILEHANDLE
6553 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6554 error. FILEHANDLE may be an expression whose value gives the name of
6555 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6558 Note the I<in bytes>: even if the filehandle has been set to
6559 operate on characters (for example by using the C<:encoding(utf8)> open
6560 layer), tell() will return byte offsets, not character offsets (because
6561 that would render seek() and tell() rather slow).
6563 The return value of tell() for the standard streams like the STDIN
6564 depends on the operating system: it may return -1 or something else.
6565 tell() on pipes, fifos, and sockets usually returns -1.
6567 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6569 Do not use tell() (or other buffered I/O operations) on a file handle
6570 that has been manipulated by sysread(), syswrite() or sysseek().
6571 Those functions ignore the buffering, while tell() does not.
6573 =item telldir DIRHANDLE
6576 Returns the current position of the C<readdir> routines on DIRHANDLE.
6577 Value may be given to C<seekdir> to access a particular location in a
6578 directory. C<telldir> has the same caveats about possible directory
6579 compaction as the corresponding system library routine.
6581 =item tie VARIABLE,CLASSNAME,LIST
6584 This function binds a variable to a package class that will provide the
6585 implementation for the variable. VARIABLE is the name of the variable
6586 to be enchanted. CLASSNAME is the name of a class implementing objects
6587 of correct type. Any additional arguments are passed to the C<new>
6588 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6589 or C<TIEHASH>). Typically these are arguments such as might be passed
6590 to the C<dbm_open()> function of C. The object returned by the C<new>
6591 method is also returned by the C<tie> function, which would be useful
6592 if you want to access other methods in CLASSNAME.
6594 Note that functions such as C<keys> and C<values> may return huge lists
6595 when used on large objects, like DBM files. You may prefer to use the
6596 C<each> function to iterate over such. Example:
6598 # print out history file offsets
6600 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6601 while (($key,$val) = each %HIST) {
6602 print $key, ' = ', unpack('L',$val), "\n";
6606 A class implementing a hash should have the following methods:
6608 TIEHASH classname, LIST
6610 STORE this, key, value
6615 NEXTKEY this, lastkey
6620 A class implementing an ordinary array should have the following methods:
6622 TIEARRAY classname, LIST
6624 STORE this, key, value
6626 STORESIZE this, count
6632 SPLICE this, offset, length, LIST
6637 A class implementing a file handle should have the following methods:
6639 TIEHANDLE classname, LIST
6640 READ this, scalar, length, offset
6643 WRITE this, scalar, length, offset
6645 PRINTF this, format, LIST
6649 SEEK this, position, whence
6651 OPEN this, mode, LIST
6656 A class implementing a scalar should have the following methods:
6658 TIESCALAR classname, LIST
6664 Not all methods indicated above need be implemented. See L<perltie>,
6665 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6667 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6668 for you--you need to do that explicitly yourself. See L<DB_File>
6669 or the F<Config> module for interesting C<tie> implementations.
6671 For further details see L<perltie>, L<"tied VARIABLE">.
6676 Returns a reference to the object underlying VARIABLE (the same value
6677 that was originally returned by the C<tie> call that bound the variable
6678 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6684 Returns the number of non-leap seconds since whatever time the system
6685 considers to be the epoch, suitable for feeding to C<gmtime> and
6686 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6687 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6688 1904 in the current local time zone for its epoch.
6690 For measuring time in better granularity than one second,
6691 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6692 Perl 5.8 part of the standard distribution), or if you have
6693 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6694 See L<perlfaq8> for details.
6696 For date and time processing look at the many related modules on CPAN.
6697 For a comprehensive date and time representation look at the
6703 Returns a four-element list giving the user and system times, in
6704 seconds, for this process and the children of this process.
6706 ($user,$system,$cuser,$csystem) = times;
6708 In scalar context, C<times> returns C<$user>.
6710 Note that times for children are included only after they terminate.
6714 The transliteration operator. Same as C<y///>. See
6715 L<perlop/"Quote and Quote-like Operators">.
6717 =item truncate FILEHANDLE,LENGTH
6720 =item truncate EXPR,LENGTH
6722 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6723 specified length. Produces a fatal error if truncate isn't implemented
6724 on your system. Returns true if successful, the undefined value
6727 The behavior is undefined if LENGTH is greater than the length of the
6730 The position in the file of FILEHANDLE is left unchanged. You may want to
6731 call L<seek> before writing to the file.
6734 X<uc> X<uppercase> X<toupper>
6738 Returns an uppercased version of EXPR. This is the internal function
6739 implementing the C<\U> escape in double-quoted strings. Respects
6740 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6741 and L<perlunicode> for more details about locale and Unicode support.
6742 It does not attempt to do titlecase mapping on initial letters. See
6743 C<ucfirst> for that.
6745 If EXPR is omitted, uses C<$_>.
6748 X<ucfirst> X<uppercase>
6752 Returns the value of EXPR with the first character in uppercase
6753 (titlecase in Unicode). This is the internal function implementing
6754 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6755 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6756 for more details about locale and Unicode support.
6758 If EXPR is omitted, uses C<$_>.
6765 Sets the umask for the process to EXPR and returns the previous value.
6766 If EXPR is omitted, merely returns the current umask.
6768 The Unix permission C<rwxr-x---> is represented as three sets of three
6769 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6770 and isn't one of the digits). The C<umask> value is such a number
6771 representing disabled permissions bits. The permission (or "mode")
6772 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6773 even if you tell C<sysopen> to create a file with permissions C<0777>,
6774 if your umask is C<0022> then the file will actually be created with
6775 permissions C<0755>. If your C<umask> were C<0027> (group can't
6776 write; others can't read, write, or execute), then passing
6777 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6780 Here's some advice: supply a creation mode of C<0666> for regular
6781 files (in C<sysopen>) and one of C<0777> for directories (in
6782 C<mkdir>) and executable files. This gives users the freedom of
6783 choice: if they want protected files, they might choose process umasks
6784 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6785 Programs should rarely if ever make policy decisions better left to
6786 the user. The exception to this is when writing files that should be
6787 kept private: mail files, web browser cookies, I<.rhosts> files, and
6790 If umask(2) is not implemented on your system and you are trying to
6791 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6792 fatal error at run time. If umask(2) is not implemented and you are
6793 not trying to restrict access for yourself, returns C<undef>.
6795 Remember that a umask is a number, usually given in octal; it is I<not> a
6796 string of octal digits. See also L</oct>, if all you have is a string.
6799 X<undef> X<undefine>
6803 Undefines the value of EXPR, which must be an lvalue. Use only on a
6804 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6805 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6806 will probably not do what you expect on most predefined variables or
6807 DBM list values, so don't do that; see L<delete>.) Always returns the
6808 undefined value. You can omit the EXPR, in which case nothing is
6809 undefined, but you still get an undefined value that you could, for
6810 instance, return from a subroutine, assign to a variable or pass as a
6811 parameter. Examples:
6814 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6818 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6819 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6820 select undef, undef, undef, 0.25;
6821 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6823 Note that this is a unary operator, not a list operator.
6826 X<unlink> X<delete> X<remove> X<rm> X<del>
6830 Deletes a list of files. Returns the number of files successfully
6833 $cnt = unlink 'a', 'b', 'c';
6837 Note: C<unlink> will not attempt to delete directories unless you are superuser
6838 and the B<-U> flag is supplied to Perl. Even if these conditions are
6839 met, be warned that unlinking a directory can inflict damage on your
6840 filesystem. Finally, using C<unlink> on directories is not supported on
6841 many operating systems. Use C<rmdir> instead.
6843 If LIST is omitted, uses C<$_>.
6845 =item unpack TEMPLATE,EXPR
6848 =item unpack TEMPLATE
6850 C<unpack> does the reverse of C<pack>: it takes a string
6851 and expands it out into a list of values.
6852 (In scalar context, it returns merely the first value produced.)
6854 If EXPR is omitted, unpacks the C<$_> string.
6856 The string is broken into chunks described by the TEMPLATE. Each chunk
6857 is converted separately to a value. Typically, either the string is a result
6858 of C<pack>, or the characters of the string represent a C structure of some
6861 The TEMPLATE has the same format as in the C<pack> function.
6862 Here's a subroutine that does substring:
6865 my($what,$where,$howmuch) = @_;
6866 unpack("x$where a$howmuch", $what);
6871 sub ordinal { unpack("W",$_[0]); } # same as ord()
6873 In addition to fields allowed in pack(), you may prefix a field with
6874 a %<number> to indicate that
6875 you want a <number>-bit checksum of the items instead of the items
6876 themselves. Default is a 16-bit checksum. Checksum is calculated by
6877 summing numeric values of expanded values (for string fields the sum of
6878 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6880 For example, the following
6881 computes the same number as the System V sum program:
6885 unpack("%32W*",<>) % 65535;
6888 The following efficiently counts the number of set bits in a bit vector:
6890 $setbits = unpack("%32b*", $selectmask);
6892 The C<p> and C<P> formats should be used with care. Since Perl
6893 has no way of checking whether the value passed to C<unpack()>
6894 corresponds to a valid memory location, passing a pointer value that's
6895 not known to be valid is likely to have disastrous consequences.
6897 If there are more pack codes or if the repeat count of a field or a group
6898 is larger than what the remainder of the input string allows, the result
6899 is not well defined: in some cases, the repeat count is decreased, or
6900 C<unpack()> will produce null strings or zeroes, or terminate with an
6901 error. If the input string is longer than one described by the TEMPLATE,
6902 the rest is ignored.
6904 See L</pack> for more examples and notes.
6906 =item untie VARIABLE
6909 Breaks the binding between a variable and a package. (See C<tie>.)
6910 Has no effect if the variable is not tied.
6912 =item unshift ARRAY,LIST
6915 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6916 depending on how you look at it. Prepends list to the front of the
6917 array, and returns the new number of elements in the array.
6919 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6921 Note the LIST is prepended whole, not one element at a time, so the
6922 prepended elements stay in the same order. Use C<reverse> to do the
6925 =item use Module VERSION LIST
6926 X<use> X<module> X<import>
6928 =item use Module VERSION
6930 =item use Module LIST
6936 Imports some semantics into the current package from the named module,
6937 generally by aliasing certain subroutine or variable names into your
6938 package. It is exactly equivalent to
6940 BEGIN { require Module; Module->import( LIST ); }
6942 except that Module I<must> be a bareword.
6944 In the peculiar C<use VERSION> form, VERSION may be either a numeric
6945 argument such as 5.006, which will be compared to C<$]>, or a literal of
6946 the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A
6947 fatal error is produced if VERSION is greater than the version of the
6948 current Perl interpreter; Perl will not attempt to parse the rest of the
6949 file. Compare with L</require>, which can do a similar check at run time.
6950 Symmetrically, C<no VERSION> allows you to specify that you want a version
6951 of perl older than the specified one.
6953 Specifying VERSION as a literal of the form v5.6.1 should generally be
6954 avoided, because it leads to misleading error messages under earlier
6955 versions of Perl (that is, prior to 5.6.0) that do not support this
6956 syntax. The equivalent numeric version should be used instead.
6958 use v5.6.1; # compile time version check
6960 use 5.006_001; # ditto; preferred for backwards compatibility
6962 This is often useful if you need to check the current Perl version before
6963 C<use>ing library modules that won't work with older versions of Perl.
6964 (We try not to do this more than we have to.)
6966 Also, if the specified perl version is greater than or equal to 5.9.5,
6967 C<use VERSION> will also load the C<feature> pragma and enable all
6968 features available in the requested version. See L<feature>.
6969 Similarly, if the specified perl version is greater than or equal to
6970 5.11.0, strictures are enabled lexically as with C<use strict> (except
6971 that the F<strict.pm> file is not actually loaded).
6973 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6974 C<require> makes sure the module is loaded into memory if it hasn't been
6975 yet. The C<import> is not a builtin--it's just an ordinary static method
6976 call into the C<Module> package to tell the module to import the list of
6977 features back into the current package. The module can implement its
6978 C<import> method any way it likes, though most modules just choose to
6979 derive their C<import> method via inheritance from the C<Exporter> class that
6980 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6981 method can be found then the call is skipped, even if there is an AUTOLOAD
6984 If you do not want to call the package's C<import> method (for instance,
6985 to stop your namespace from being altered), explicitly supply the empty list:
6989 That is exactly equivalent to
6991 BEGIN { require Module }
6993 If the VERSION argument is present between Module and LIST, then the
6994 C<use> will call the VERSION method in class Module with the given
6995 version as an argument. The default VERSION method, inherited from
6996 the UNIVERSAL class, croaks if the given version is larger than the
6997 value of the variable C<$Module::VERSION>.
6999 Again, there is a distinction between omitting LIST (C<import> called
7000 with no arguments) and an explicit empty LIST C<()> (C<import> not
7001 called). Note that there is no comma after VERSION!
7003 Because this is a wide-open interface, pragmas (compiler directives)
7004 are also implemented this way. Currently implemented pragmas are:
7009 use sigtrap qw(SEGV BUS);
7010 use strict qw(subs vars refs);
7011 use subs qw(afunc blurfl);
7012 use warnings qw(all);
7013 use sort qw(stable _quicksort _mergesort);
7015 Some of these pseudo-modules import semantics into the current
7016 block scope (like C<strict> or C<integer>, unlike ordinary modules,
7017 which import symbols into the current package (which are effective
7018 through the end of the file).
7020 There's a corresponding C<no> command that unimports meanings imported
7021 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
7022 It behaves exactly as C<import> does with respect to VERSION, an
7023 omitted LIST, empty LIST, or no unimport method being found.
7029 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
7030 for the C<-M> and C<-m> command-line options to perl that give C<use>
7031 functionality from the command-line.
7036 Changes the access and modification times on each file of a list of
7037 files. The first two elements of the list must be the NUMERICAL access
7038 and modification times, in that order. Returns the number of files
7039 successfully changed. The inode change time of each file is set
7040 to the current time. For example, this code has the same effect as the
7041 Unix touch(1) command when the files I<already exist> and belong to
7042 the user running the program:
7045 $atime = $mtime = time;
7046 utime $atime, $mtime, @ARGV;
7048 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
7049 the utime(2) function in the C library will be called with a null second
7050 argument. On most systems, this will set the file's access and
7051 modification times to the current time (i.e. equivalent to the example
7052 above) and will even work on other users' files where you have write
7055 utime undef, undef, @ARGV;
7057 Under NFS this will use the time of the NFS server, not the time of
7058 the local machine. If there is a time synchronization problem, the
7059 NFS server and local machine will have different times. The Unix
7060 touch(1) command will in fact normally use this form instead of the
7061 one shown in the first example.
7063 Note that only passing one of the first two elements as C<undef> will
7064 be equivalent of passing it as 0 and will not have the same effect as
7065 described when they are both C<undef>. This case will also trigger an
7066 uninitialized warning.
7068 On systems that support futimes, you might pass file handles among the
7069 files. On systems that don't support futimes, passing file handles
7070 produces a fatal error at run time. The file handles must be passed
7071 as globs or references to be recognized. Barewords are considered
7079 Returns a list consisting of all the values of the named hash, or the values
7080 of an array. (In a scalar context, returns the number of values.)
7082 The values are returned in an apparently random order. The actual
7083 random order is subject to change in future versions of perl, but it
7084 is guaranteed to be the same order as either the C<keys> or C<each>
7085 function would produce on the same (unmodified) hash. Since Perl
7086 5.8.1 the ordering is different even between different runs of Perl
7087 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7089 As a side effect, calling values() resets the HASH or ARRAY's internal
7091 see L</each>. (In particular, calling values() in void context resets
7092 the iterator with no other overhead. Apart from resetting the iterator,
7093 C<values @array> in list context is no different to plain C<@array>.
7094 We recommend that you use void context C<keys @array> for this, but reasoned
7095 that it taking C<values @array> out would require more documentation than
7099 Note that the values are not copied, which means modifying them will
7100 modify the contents of the hash:
7102 for (values %hash) { s/foo/bar/g } # modifies %hash values
7103 for (@hash{keys %hash}) { s/foo/bar/g } # same
7105 See also C<keys>, C<each>, and C<sort>.
7107 =item vec EXPR,OFFSET,BITS
7108 X<vec> X<bit> X<bit vector>
7110 Treats the string in EXPR as a bit vector made up of elements of
7111 width BITS, and returns the value of the element specified by OFFSET
7112 as an unsigned integer. BITS therefore specifies the number of bits
7113 that are reserved for each element in the bit vector. This must
7114 be a power of two from 1 to 32 (or 64, if your platform supports
7117 If BITS is 8, "elements" coincide with bytes of the input string.
7119 If BITS is 16 or more, bytes of the input string are grouped into chunks
7120 of size BITS/8, and each group is converted to a number as with
7121 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7122 for BITS==64). See L<"pack"> for details.
7124 If bits is 4 or less, the string is broken into bytes, then the bits
7125 of each byte are broken into 8/BITS groups. Bits of a byte are
7126 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7127 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7128 breaking the single input byte C<chr(0x36)> into two groups gives a list
7129 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7131 C<vec> may also be assigned to, in which case parentheses are needed
7132 to give the expression the correct precedence as in
7134 vec($image, $max_x * $x + $y, 8) = 3;
7136 If the selected element is outside the string, the value 0 is returned.
7137 If an element off the end of the string is written to, Perl will first
7138 extend the string with sufficiently many zero bytes. It is an error
7139 to try to write off the beginning of the string (i.e. negative OFFSET).
7141 If the string happens to be encoded as UTF-8 internally (and thus has
7142 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7143 internal byte string, not the conceptual character string, even if you
7144 only have characters with values less than 256.
7146 Strings created with C<vec> can also be manipulated with the logical
7147 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7148 vector operation is desired when both operands are strings.
7149 See L<perlop/"Bitwise String Operators">.
7151 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7152 The comments show the string after each step. Note that this code works
7153 in the same way on big-endian or little-endian machines.
7156 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7158 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7159 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7161 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7162 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7163 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7164 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7165 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7166 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7168 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7169 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7170 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7173 To transform a bit vector into a string or list of 0's and 1's, use these:
7175 $bits = unpack("b*", $vector);
7176 @bits = split(//, unpack("b*", $vector));
7178 If you know the exact length in bits, it can be used in place of the C<*>.
7180 Here is an example to illustrate how the bits actually fall in place:
7186 unpack("V",$_) 01234567890123456789012345678901
7187 ------------------------------------------------------------------
7192 for ($shift=0; $shift < $width; ++$shift) {
7193 for ($off=0; $off < 32/$width; ++$off) {
7194 $str = pack("B*", "0"x32);
7195 $bits = (1<<$shift);
7196 vec($str, $off, $width) = $bits;
7197 $res = unpack("b*",$str);
7198 $val = unpack("V", $str);
7205 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7206 $off, $width, $bits, $val, $res
7210 Regardless of the machine architecture on which it is run, the above
7211 example should print the following table:
7214 unpack("V",$_) 01234567890123456789012345678901
7215 ------------------------------------------------------------------
7216 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7217 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7218 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7219 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7220 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7221 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7222 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7223 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7224 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7225 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7226 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7227 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7228 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7229 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7230 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7231 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7232 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7233 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7234 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7235 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7236 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7237 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7238 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7239 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7240 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7241 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7242 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7243 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7244 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7245 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7246 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7247 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7248 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7249 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7250 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7251 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7252 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7253 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7254 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7255 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7256 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7257 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7258 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7259 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7260 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7261 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7262 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7263 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7264 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7265 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7266 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7267 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7268 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7269 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7270 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7271 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7272 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7273 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7274 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7275 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7276 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7277 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7278 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7279 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7280 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7281 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7282 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7283 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7284 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7285 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7286 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7287 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7288 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7289 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7290 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7291 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7292 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7293 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7294 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7295 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7296 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7297 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7298 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7299 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7300 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7301 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7302 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7303 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7304 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7305 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7306 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7307 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7308 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7309 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7310 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7311 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7312 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7313 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7314 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7315 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7316 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7317 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7318 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7319 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7320 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7321 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7322 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7323 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7324 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7325 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7326 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7327 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7328 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7329 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7330 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7331 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7332 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7333 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7334 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7335 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7336 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7337 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7338 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7339 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7340 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7341 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7342 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7343 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7348 Behaves like the wait(2) system call on your system: it waits for a child
7349 process to terminate and returns the pid of the deceased process, or
7350 C<-1> if there are no child processes. The status is returned in C<$?>
7351 and C<${^CHILD_ERROR_NATIVE}>.
7352 Note that a return value of C<-1> could mean that child processes are
7353 being automatically reaped, as described in L<perlipc>.
7355 =item waitpid PID,FLAGS
7358 Waits for a particular child process to terminate and returns the pid of
7359 the deceased process, or C<-1> if there is no such child process. On some
7360 systems, a value of 0 indicates that there are processes still running.
7361 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
7363 use POSIX ":sys_wait_h";
7366 $kid = waitpid(-1, WNOHANG);
7369 then you can do a non-blocking wait for all pending zombie processes.
7370 Non-blocking wait is available on machines supporting either the
7371 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7372 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7373 system call by remembering the status values of processes that have
7374 exited but have not been harvested by the Perl script yet.)
7376 Note that on some systems, a return value of C<-1> could mean that child
7377 processes are being automatically reaped. See L<perlipc> for details,
7378 and for other examples.
7381 X<wantarray> X<context>
7383 Returns true if the context of the currently executing subroutine or
7384 C<eval> is looking for a list value. Returns false if the context is
7385 looking for a scalar. Returns the undefined value if the context is
7386 looking for no value (void context).
7388 return unless defined wantarray; # don't bother doing more
7389 my @a = complex_calculation();
7390 return wantarray ? @a : "@a";
7392 C<wantarray()>'s result is unspecified in the top level of a file,
7393 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7394 in a C<DESTROY> method.
7396 This function should have been named wantlist() instead.
7399 X<warn> X<warning> X<STDERR>
7401 Prints the value of LIST to STDERR. If the last element of LIST does
7402 not end in a newline, it appends the same file/line number text as C<die>
7405 If the output is empty and C<$@> already contains a value (typically from a
7406 previous eval) that value is used after appending C<"\t...caught">
7407 to C<$@>. This is useful for staying almost, but not entirely similar to
7410 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7412 No message is printed if there is a C<$SIG{__WARN__}> handler
7413 installed. It is the handler's responsibility to deal with the message
7414 as it sees fit (like, for instance, converting it into a C<die>). Most
7415 handlers must therefore make arrangements to actually display the
7416 warnings that they are not prepared to deal with, by calling C<warn>
7417 again in the handler. Note that this is quite safe and will not
7418 produce an endless loop, since C<__WARN__> hooks are not called from
7421 You will find this behavior is slightly different from that of
7422 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7423 instead call C<die> again to change it).
7425 Using a C<__WARN__> handler provides a powerful way to silence all
7426 warnings (even the so-called mandatory ones). An example:
7428 # wipe out *all* compile-time warnings
7429 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7431 my $foo = 20; # no warning about duplicate my $foo,
7432 # but hey, you asked for it!
7433 # no compile-time or run-time warnings before here
7436 # run-time warnings enabled after here
7437 warn "\$foo is alive and $foo!"; # does show up
7439 See L<perlvar> for details on setting C<%SIG> entries, and for more
7440 examples. See the Carp module for other kinds of warnings using its
7441 carp() and cluck() functions.
7443 =item write FILEHANDLE
7450 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7451 using the format associated with that file. By default the format for
7452 a file is the one having the same name as the filehandle, but the
7453 format for the current output channel (see the C<select> function) may be set
7454 explicitly by assigning the name of the format to the C<$~> variable.
7456 Top of form processing is handled automatically: if there is
7457 insufficient room on the current page for the formatted record, the
7458 page is advanced by writing a form feed, a special top-of-page format
7459 is used to format the new page header, and then the record is written.
7460 By default the top-of-page format is the name of the filehandle with
7461 "_TOP" appended, but it may be dynamically set to the format of your
7462 choice by assigning the name to the C<$^> variable while the filehandle is
7463 selected. The number of lines remaining on the current page is in
7464 variable C<$->, which can be set to C<0> to force a new page.
7466 If FILEHANDLE is unspecified, output goes to the current default output
7467 channel, which starts out as STDOUT but may be changed by the
7468 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7469 is evaluated and the resulting string is used to look up the name of
7470 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7472 Note that write is I<not> the opposite of C<read>. Unfortunately.
7476 The transliteration operator. Same as C<tr///>. See
7477 L<perlop/"Quote and Quote-like Operators">.