4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides a scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, the scalar arguments will
18 be first, and the list argument will follow. (Note that there can ever
19 be only one such list argument.) For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for the elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use the parentheses, the simple (but occasionally
34 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
35 function, and precedence doesn't matter. Otherwise it's a list
36 operator or unary operator, and precedence does matter. And whitespace
37 between the function and left parenthesis doesn't count--so you need to
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
52 A few functions take no arguments at all, and therefore work as neither
53 unary nor list operators. These include such functions as C<time>
54 and C<endpwent>. For example, C<time+86_400> always means
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in a scalar context by
59 returning the undefined value, and in a list context by returning the
62 Remember the following important rule: There is B<no rule> that relates
63 the behavior of an expression in list context to its behavior in scalar
64 context, or vice versa. It might do two totally different things.
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in scalar context. Some operators return the
67 length of the list that would have been returned in list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
74 A named array in scalar context is quite different from what would at
75 first glance appear to be a list in scalar context. You can't get a list
76 like C<(1,2,3)> into being in scalar context, because the compiler knows
77 the context at compile time. It would generate the scalar comma operator
78 there, not the list construction version of the comma. That means it
79 was never a list to start with.
81 In general, functions in Perl that serve as wrappers for system calls
82 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule are C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 =head2 Perl Functions by Category
92 Here are Perl's functions (including things that look like
93 functions, like some keywords and named operators)
94 arranged by category. Some functions appear in more
99 =item Functions for SCALARs or strings
100 X<scalar> X<string> X<character>
102 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
103 C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
104 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
106 =item Regular expressions and pattern matching
107 X<regular expression> X<regex> X<regexp>
109 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
111 =item Numeric functions
112 X<numeric> X<number> X<trigonometric> X<trigonometry>
114 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115 C<sin>, C<sqrt>, C<srand>
117 =item Functions for real @ARRAYs
120 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
122 =item Functions for list data
125 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
127 =item Functions for real %HASHes
130 C<delete>, C<each>, C<exists>, C<keys>, C<values>
132 =item Input and output functions
133 X<I/O> X<input> X<output> X<dbm>
135 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
137 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
138 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
141 =item Functions for fixed length data or records
143 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
145 =item Functions for filehandles, files, or directories
146 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
148 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
149 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
150 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151 C<umask>, C<unlink>, C<utime>
153 =item Keywords related to the control flow of your Perl program
156 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
159 =item Keywords related to switch
161 C<break>, C<continue>, C<given>, C<when>, C<default>
163 (These are only available if you enable the "switch" feature.
164 See L<feature> and L<perlsyn/"Switch statements">.)
166 =item Keywords related to scoping
168 C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
171 (C<state> is only available if the "state" feature is enabled. See
174 =item Miscellaneous functions
176 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
177 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
179 =item Functions for processes and process groups
180 X<process> X<pid> X<process id>
182 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
183 C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
184 C<times>, C<wait>, C<waitpid>
186 =item Keywords related to perl modules
189 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
191 =item Keywords related to classes and object-orientation
192 X<object> X<class> X<package>
194 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
197 =item Low-level socket functions
200 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
202 C<socket>, C<socketpair>
204 =item System V interprocess communication functions
205 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
207 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
210 =item Fetching user and group info
211 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
213 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215 C<getpwuid>, C<setgrent>, C<setpwent>
217 =item Fetching network info
218 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
220 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224 C<setnetent>, C<setprotoent>, C<setservent>
226 =item Time-related functions
229 C<gmtime>, C<localtime>, C<time>, C<times>
231 =item Functions new in perl5
234 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
236 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
237 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238 C<ucfirst>, C<untie>, C<use>, C<when>
240 * - C<sub> was a keyword in perl4, but in perl5 it is an
241 operator, which can be used in expressions.
243 =item Functions obsoleted in perl5
245 C<dbmclose>, C<dbmopen>
250 X<portability> X<Unix> X<portable>
252 Perl was born in Unix and can therefore access all common Unix
253 system calls. In non-Unix environments, the functionality of some
254 Unix system calls may not be available, or details of the available
255 functionality may differ slightly. The Perl functions affected
258 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
261 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
263 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
264 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
267 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
268 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
271 C<shmwrite>, C<socket>, C<socketpair>,
272 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273 C<times>, C<truncate>, C<umask>, C<unlink>,
274 C<utime>, C<wait>, C<waitpid>
276 For more information about the portability of these functions, see
277 L<perlport> and other available platform-specific documentation.
279 =head2 Alphabetical Listing of Perl Functions
284 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
285 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
293 A file test, where X is one of the letters listed below. This unary
294 operator takes one argument, either a filename, a filehandle, or a dirhandle,
295 and tests the associated file to see if something is true about it. If the
296 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
297 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
298 the undefined value if the file doesn't exist. Despite the funny
299 names, precedence is the same as any other named unary operator. The
300 operator may be any of:
302 -r File is readable by effective uid/gid.
303 -w File is writable by effective uid/gid.
304 -x File is executable by effective uid/gid.
305 -o File is owned by effective uid.
307 -R File is readable by real uid/gid.
308 -W File is writable by real uid/gid.
309 -X File is executable by real uid/gid.
310 -O File is owned by real uid.
313 -z File has zero size (is empty).
314 -s File has nonzero size (returns size in bytes).
316 -f File is a plain file.
317 -d File is a directory.
318 -l File is a symbolic link.
319 -p File is a named pipe (FIFO), or Filehandle is a pipe.
321 -b File is a block special file.
322 -c File is a character special file.
323 -t Filehandle is opened to a tty.
325 -u File has setuid bit set.
326 -g File has setgid bit set.
327 -k File has sticky bit set.
329 -T File is an ASCII text file (heuristic guess).
330 -B File is a "binary" file (opposite of -T).
332 -M Script start time minus file modification time, in days.
333 -A Same for access time.
334 -C Same for inode change time (Unix, may differ for other platforms)
340 next unless -f $_; # ignore specials
344 The interpretation of the file permission operators C<-r>, C<-R>,
345 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
346 of the file and the uids and gids of the user. There may be other
347 reasons you can't actually read, write, or execute the file: for
348 example network filesystem access controls, ACLs (access control lists),
349 read-only filesystems, and unrecognized executable formats. Note
350 that the use of these six specific operators to verify if some operation
351 is possible is usually a mistake, because it may be open to race
354 Also note that, for the superuser on the local filesystems, the C<-r>,
355 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
356 if any execute bit is set in the mode. Scripts run by the superuser
357 may thus need to do a stat() to determine the actual mode of the file,
358 or temporarily set their effective uid to something else.
360 If you are using ACLs, there is a pragma called C<filetest> that may
361 produce more accurate results than the bare stat() mode bits.
362 When under the C<use filetest 'access'> the above-mentioned filetests
363 will test whether the permission can (not) be granted using the
364 access() family of system calls. Also note that the C<-x> and C<-X> may
365 under this pragma return true even if there are no execute permission
366 bits set (nor any extra execute permission ACLs). This strangeness is
367 due to the underlying system calls' definitions. Note also that, due to
368 the implementation of C<use filetest 'access'>, the C<_> special
369 filehandle won't cache the results of the file tests when this pragma is
370 in effect. Read the documentation for the C<filetest> pragma for more
373 Note that C<-s/a/b/> does not do a negated substitution. Saying
374 C<-exp($foo)> still works as expected, however--only single letters
375 following a minus are interpreted as file tests.
377 The C<-T> and C<-B> switches work as follows. The first block or so of the
378 file is examined for odd characters such as strange control codes or
379 characters with the high bit set. If too many strange characters (>30%)
380 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
381 containing null in the first block is considered a binary file. If C<-T>
382 or C<-B> is used on a filehandle, the current IO buffer is examined
383 rather than the first block. Both C<-T> and C<-B> return true on a null
384 file, or a file at EOF when testing a filehandle. Because you have to
385 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
386 against the file first, as in C<next unless -f $file && -T $file>.
388 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
389 the special filehandle consisting of a solitary underline, then the stat
390 structure of the previous file test (or stat operator) is used, saving
391 a system call. (This doesn't work with C<-t>, and you need to remember
392 that lstat() and C<-l> will leave values in the stat structure for the
393 symbolic link, not the real file.) (Also, if the stat buffer was filled by
394 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
397 print "Can do.\n" if -r $a || -w _ || -x _;
400 print "Readable\n" if -r _;
401 print "Writable\n" if -w _;
402 print "Executable\n" if -x _;
403 print "Setuid\n" if -u _;
404 print "Setgid\n" if -g _;
405 print "Sticky\n" if -k _;
406 print "Text\n" if -T _;
407 print "Binary\n" if -B _;
409 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
410 test operators, in a way that C<-f -w -x $file> is equivalent to
411 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
412 the return value of C<-f $file> as an argument to another filetest
413 operator, no special magic will happen.)
420 Returns the absolute value of its argument.
421 If VALUE is omitted, uses C<$_>.
423 =item accept NEWSOCKET,GENERICSOCKET
426 Accepts an incoming socket connect, just as the accept(2) system call
427 does. Returns the packed address if it succeeded, false otherwise.
428 See the example in L<perlipc/"Sockets: Client/Server Communication">.
430 On systems that support a close-on-exec flag on files, the flag will
431 be set for the newly opened file descriptor, as determined by the
432 value of $^F. See L<perlvar/$^F>.
441 Arranges to have a SIGALRM delivered to this process after the
442 specified number of wallclock seconds has elapsed. If SECONDS is not
443 specified, the value stored in C<$_> is used. (On some machines,
444 unfortunately, the elapsed time may be up to one second less or more
445 than you specified because of how seconds are counted, and process
446 scheduling may delay the delivery of the signal even further.)
448 Only one timer may be counting at once. Each call disables the
449 previous timer, and an argument of C<0> may be supplied to cancel the
450 previous timer without starting a new one. The returned value is the
451 amount of time remaining on the previous timer.
453 For delays of finer granularity than one second, the Time::HiRes module
454 (from CPAN, and starting from Perl 5.8 part of the standard
455 distribution) provides ualarm(). You may also use Perl's four-argument
456 version of select() leaving the first three arguments undefined, or you
457 might be able to use the C<syscall> interface to access setitimer(2) if
458 your system supports it. See L<perlfaq8> for details.
460 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
461 (C<sleep> may be internally implemented in your system with C<alarm>)
463 If you want to use C<alarm> to time out a system call you need to use an
464 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
465 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
466 restart system calls on some systems. Using C<eval>/C<die> always works,
467 modulo the caveats given in L<perlipc/"Signals">.
470 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
472 $nread = sysread SOCKET, $buffer, $size;
476 die unless $@ eq "alarm\n"; # propagate unexpected errors
483 For more information see L<perlipc>.
486 X<atan2> X<arctangent> X<tan> X<tangent>
488 Returns the arctangent of Y/X in the range -PI to PI.
490 For the tangent operation, you may use the C<Math::Trig::tan>
491 function, or use the familiar relation:
493 sub tan { sin($_[0]) / cos($_[0]) }
495 The return value for C<atan2(0,0)> is implementation-defined; consult
496 your atan2(3) manpage for more information.
498 =item bind SOCKET,NAME
501 Binds a network address to a socket, just as the bind system call
502 does. Returns true if it succeeded, false otherwise. NAME should be a
503 packed address of the appropriate type for the socket. See the examples in
504 L<perlipc/"Sockets: Client/Server Communication">.
506 =item binmode FILEHANDLE, LAYER
507 X<binmode> X<binary> X<text> X<DOS> X<Windows>
509 =item binmode FILEHANDLE
511 Arranges for FILEHANDLE to be read or written in "binary" or "text"
512 mode on systems where the run-time libraries distinguish between
513 binary and text files. If FILEHANDLE is an expression, the value is
514 taken as the name of the filehandle. Returns true on success,
515 otherwise it returns C<undef> and sets C<$!> (errno).
517 On some systems (in general, DOS and Windows-based systems) binmode()
518 is necessary when you're not working with a text file. For the sake
519 of portability it is a good idea to always use it when appropriate,
520 and to never use it when it isn't appropriate. Also, people can
521 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
523 In other words: regardless of platform, use binmode() on binary data,
524 like for example images.
526 If LAYER is present it is a single string, but may contain multiple
527 directives. The directives alter the behaviour of the file handle.
528 When LAYER is present using binmode on text file makes sense.
530 If LAYER is omitted or specified as C<:raw> the filehandle is made
531 suitable for passing binary data. This includes turning off possible CRLF
532 translation and marking it as bytes (as opposed to Unicode characters).
533 Note that, despite what may be implied in I<"Programming Perl"> (the
534 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
535 -- other layers which would affect the binary nature of the stream are
536 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
537 PERLIO environment variable.
539 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
540 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
541 establish default I/O layers. See L<open>.
543 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
544 in "Programming Perl, 3rd Edition". However, since the publishing of this
545 book, by many known as "Camel III", the consensus of the naming of this
546 functionality has moved from "discipline" to "layer". All documentation
547 of this version of Perl therefore refers to "layers" rather than to
548 "disciplines". Now back to the regularly scheduled documentation...>
550 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
551 C<:utf8> just marks the data as UTF-8 without further checking,
552 while C<:encoding(utf8)> checks the data for actually being valid
553 UTF-8. More details can be found in L<PerlIO::encoding>.
555 In general, binmode() should be called after open() but before any I/O
556 is done on the filehandle. Calling binmode() will normally flush any
557 pending buffered output data (and perhaps pending input data) on the
558 handle. An exception to this is the C<:encoding> layer that
559 changes the default character encoding of the handle, see L<open>.
560 The C<:encoding> layer sometimes needs to be called in
561 mid-stream, and it doesn't flush the stream. The C<:encoding>
562 also implicitly pushes on top of itself the C<:utf8> layer because
563 internally Perl will operate on UTF-8 encoded Unicode characters.
565 The operating system, device drivers, C libraries, and Perl run-time
566 system all work together to let the programmer treat a single
567 character (C<\n>) as the line terminator, irrespective of the external
568 representation. On many operating systems, the native text file
569 representation matches the internal representation, but on some
570 platforms the external representation of C<\n> is made up of more than
573 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
574 character to end each line in the external representation of text (even
575 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
576 on Unix and most VMS files). In other systems like OS/2, DOS and the
577 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
578 but what's stored in text files are the two characters C<\cM\cJ>. That
579 means that, if you don't use binmode() on these systems, C<\cM\cJ>
580 sequences on disk will be converted to C<\n> on input, and any C<\n> in
581 your program will be converted back to C<\cM\cJ> on output. This is what
582 you want for text files, but it can be disastrous for binary files.
584 Another consequence of using binmode() (on some systems) is that
585 special end-of-file markers will be seen as part of the data stream.
586 For systems from the Microsoft family this means that if your binary
587 data contains C<\cZ>, the I/O subsystem will regard it as the end of
588 the file, unless you use binmode().
590 binmode() is not only important for readline() and print() operations,
591 but also when using read(), seek(), sysread(), syswrite() and tell()
592 (see L<perlport> for more details). See the C<$/> and C<$\> variables
593 in L<perlvar> for how to manually set your input and output
594 line-termination sequences.
596 =item bless REF,CLASSNAME
601 This function tells the thingy referenced by REF that it is now an object
602 in the CLASSNAME package. If CLASSNAME is omitted, the current package
603 is used. Because a C<bless> is often the last thing in a constructor,
604 it returns the reference for convenience. Always use the two-argument
605 version if a derived class might inherit the function doing the blessing.
606 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
609 Consider always blessing objects in CLASSNAMEs that are mixed case.
610 Namespaces with all lowercase names are considered reserved for
611 Perl pragmata. Builtin types have all uppercase names. To prevent
612 confusion, you may wish to avoid such package names as well. Make sure
613 that CLASSNAME is a true value.
615 See L<perlmod/"Perl Modules">.
619 Break out of a C<given()> block.
621 This keyword is enabled by the "switch" feature: see L<feature>
622 for more information.
625 X<caller> X<call stack> X<stack> X<stack trace>
629 Returns the context of the current subroutine call. In scalar context,
630 returns the caller's package name if there is a caller, that is, if
631 we're in a subroutine or C<eval> or C<require>, and the undefined value
632 otherwise. In list context, returns
635 ($package, $filename, $line) = caller;
637 With EXPR, it returns some extra information that the debugger uses to
638 print a stack trace. The value of EXPR indicates how many call frames
639 to go back before the current one.
642 ($package, $filename, $line, $subroutine, $hasargs,
645 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
648 Here $subroutine may be C<(eval)> if the frame is not a subroutine
649 call, but an C<eval>. In such a case additional elements $evaltext and
650 C<$is_require> are set: C<$is_require> is true if the frame is created by a
651 C<require> or C<use> statement, $evaltext contains the text of the
652 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
653 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
654 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
655 frame.) $subroutine may also be C<(unknown)> if this particular
656 subroutine happens to have been deleted from the symbol table.
657 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
658 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
659 compiled with. The C<$hints> and C<$bitmask> values are subject to change
660 between versions of Perl, and are not meant for external use.
662 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
663 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
664 of this hash, as they are the actual values stored in the optree.
666 Furthermore, when called from within the DB package, caller returns more
667 detailed information: it sets the list variable C<@DB::args> to be the
668 arguments with which the subroutine was invoked.
670 Be aware that the optimizer might have optimized call frames away before
671 C<caller> had a chance to get the information. That means that C<caller(N)>
672 might not return information about the call frame you expect it do, for
673 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
674 previous time C<caller> was called.
681 =item chdir FILEHANDLE
683 =item chdir DIRHANDLE
687 Changes the working directory to EXPR, if possible. If EXPR is omitted,
688 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
689 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
690 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
691 neither is set, C<chdir> does nothing. It returns true upon success,
692 false otherwise. See the example under C<die>.
694 On systems that support fchdir, you might pass a file handle or
695 directory handle as argument. On systems that don't support fchdir,
696 passing handles produces a fatal error at run time.
699 X<chmod> X<permission> X<mode>
701 Changes the permissions of a list of files. The first element of the
702 list must be the numerical mode, which should probably be an octal
703 number, and which definitely should I<not> be a string of octal digits:
704 C<0644> is okay, C<'0644'> is not. Returns the number of files
705 successfully changed. See also L</oct>, if all you have is a string.
707 $cnt = chmod 0755, 'foo', 'bar';
708 chmod 0755, @executables;
709 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
711 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
712 $mode = 0644; chmod $mode, 'foo'; # this is best
714 On systems that support fchmod, you might pass file handles among the
715 files. On systems that don't support fchmod, passing file handles
716 produces a fatal error at run time. The file handles must be passed
717 as globs or references to be recognized. Barewords are considered
720 open(my $fh, "<", "foo");
721 my $perm = (stat $fh)[2] & 07777;
722 chmod($perm | 0600, $fh);
724 You can also import the symbolic C<S_I*> constants from the Fcntl
729 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
730 # This is identical to the chmod 0755 of the above example.
733 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
739 This safer version of L</chop> removes any trailing string
740 that corresponds to the current value of C<$/> (also known as
741 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
742 number of characters removed from all its arguments. It's often used to
743 remove the newline from the end of an input record when you're worried
744 that the final record may be missing its newline. When in paragraph
745 mode (C<$/ = "">), it removes all trailing newlines from the string.
746 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
747 a reference to an integer or the like, see L<perlvar>) chomp() won't
749 If VARIABLE is omitted, it chomps C<$_>. Example:
752 chomp; # avoid \n on last field
757 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
759 You can actually chomp anything that's an lvalue, including an assignment:
762 chomp($answer = <STDIN>);
764 If you chomp a list, each element is chomped, and the total number of
765 characters removed is returned.
767 Note that parentheses are necessary when you're chomping anything
768 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
769 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
770 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
771 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
781 Chops off the last character of a string and returns the character
782 chopped. It is much more efficient than C<s/.$//s> because it neither
783 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
784 If VARIABLE is a hash, it chops the hash's values, but not its keys.
786 You can actually chop anything that's an lvalue, including an assignment.
788 If you chop a list, each element is chopped. Only the value of the
789 last C<chop> is returned.
791 Note that C<chop> returns the last character. To return all but the last
792 character, use C<substr($string, 0, -1)>.
797 X<chown> X<owner> X<user> X<group>
799 Changes the owner (and group) of a list of files. The first two
800 elements of the list must be the I<numeric> uid and gid, in that
801 order. A value of -1 in either position is interpreted by most
802 systems to leave that value unchanged. Returns the number of files
803 successfully changed.
805 $cnt = chown $uid, $gid, 'foo', 'bar';
806 chown $uid, $gid, @filenames;
808 On systems that support fchown, you might pass file handles among the
809 files. On systems that don't support fchown, passing file handles
810 produces a fatal error at run time. The file handles must be passed
811 as globs or references to be recognized. Barewords are considered
814 Here's an example that looks up nonnumeric uids in the passwd file:
817 chomp($user = <STDIN>);
819 chomp($pattern = <STDIN>);
821 ($login,$pass,$uid,$gid) = getpwnam($user)
822 or die "$user not in passwd file";
824 @ary = glob($pattern); # expand filenames
825 chown $uid, $gid, @ary;
827 On most systems, you are not allowed to change the ownership of the
828 file unless you're the superuser, although you should be able to change
829 the group to any of your secondary groups. On insecure systems, these
830 restrictions may be relaxed, but this is not a portable assumption.
831 On POSIX systems, you can detect this condition this way:
833 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
834 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
837 X<chr> X<character> X<ASCII> X<Unicode>
841 Returns the character represented by that NUMBER in the character set.
842 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
843 chr(0x263a) is a Unicode smiley face.
845 Negative values give the Unicode replacement character (chr(0xfffd)),
846 except under the L<bytes> pragma, where low eight bits of the value
847 (truncated to an integer) are used.
849 If NUMBER is omitted, uses C<$_>.
851 For the reverse, use L</ord>.
853 Note that characters from 128 to 255 (inclusive) are by default
854 internally not encoded as UTF-8 for backward compatibility reasons.
856 See L<perlunicode> for more about Unicode.
858 =item chroot FILENAME
863 This function works like the system call by the same name: it makes the
864 named directory the new root directory for all further pathnames that
865 begin with a C</> by your process and all its children. (It doesn't
866 change your current working directory, which is unaffected.) For security
867 reasons, this call is restricted to the superuser. If FILENAME is
868 omitted, does a C<chroot> to C<$_>.
870 =item close FILEHANDLE
875 Closes the file or pipe associated with the file handle, flushes the IO
876 buffers, and closes the system file descriptor. Returns true if those
877 operations have succeeded and if no error was reported by any PerlIO
878 layer. Closes the currently selected filehandle if the argument is
881 You don't have to close FILEHANDLE if you are immediately going to do
882 another C<open> on it, because C<open> will close it for you. (See
883 C<open>.) However, an explicit C<close> on an input file resets the line
884 counter (C<$.>), while the implicit close done by C<open> does not.
886 If the file handle came from a piped open, C<close> will additionally
887 return false if one of the other system calls involved fails, or if the
888 program exits with non-zero status. (If the only problem was that the
889 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
890 also waits for the process executing on the pipe to complete, in case you
891 want to look at the output of the pipe afterwards, and
892 implicitly puts the exit status value of that command into C<$?> and
893 C<${^CHILD_ERROR_NATIVE}>.
895 Prematurely closing the read end of a pipe (i.e. before the process
896 writing to it at the other end has closed it) will result in a
897 SIGPIPE being delivered to the writer. If the other end can't
898 handle that, be sure to read all the data before closing the pipe.
902 open(OUTPUT, '|sort >foo') # pipe to sort
903 or die "Can't start sort: $!";
904 #... # print stuff to output
905 close OUTPUT # wait for sort to finish
906 or warn $! ? "Error closing sort pipe: $!"
907 : "Exit status $? from sort";
908 open(INPUT, 'foo') # get sort's results
909 or die "Can't open 'foo' for input: $!";
911 FILEHANDLE may be an expression whose value can be used as an indirect
912 filehandle, usually the real filehandle name.
914 =item closedir DIRHANDLE
917 Closes a directory opened by C<opendir> and returns the success of that
920 =item connect SOCKET,NAME
923 Attempts to connect to a remote socket, just as the connect system call
924 does. Returns true if it succeeded, false otherwise. NAME should be a
925 packed address of the appropriate type for the socket. See the examples in
926 L<perlipc/"Sockets: Client/Server Communication">.
933 C<continue> is actually a flow control statement rather than a function. If
934 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
935 C<foreach>), it is always executed just before the conditional is about to
936 be evaluated again, just like the third part of a C<for> loop in C. Thus
937 it can be used to increment a loop variable, even when the loop has been
938 continued via the C<next> statement (which is similar to the C C<continue>
941 C<last>, C<next>, or C<redo> may appear within a C<continue>
942 block. C<last> and C<redo> will behave as if they had been executed within
943 the main block. So will C<next>, but since it will execute a C<continue>
944 block, it may be more entertaining.
947 ### redo always comes here
950 ### next always comes here
952 # then back the top to re-check EXPR
954 ### last always comes here
956 Omitting the C<continue> section is semantically equivalent to using an
957 empty one, logically enough. In that case, C<next> goes directly back
958 to check the condition at the top of the loop.
960 If the "switch" feature is enabled, C<continue> is also a
961 function that will break out of the current C<when> or C<default>
962 block, and fall through to the next case. See L<feature> and
963 L<perlsyn/"Switch statements"> for more information.
967 X<cos> X<cosine> X<acos> X<arccosine>
971 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
972 takes cosine of C<$_>.
974 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
975 function, or use this relation:
977 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
979 =item crypt PLAINTEXT,SALT
980 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
981 X<decrypt> X<cryptography> X<passwd> X<encrypt>
983 Creates a digest string exactly like the crypt(3) function in the C
984 library (assuming that you actually have a version there that has not
985 been extirpated as a potential munitions).
987 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
988 into a short string, called a digest, which is returned. The same
989 PLAINTEXT and SALT will always return the same string, but there is no
990 (known) way to get the original PLAINTEXT from the hash. Small
991 changes in the PLAINTEXT or SALT will result in large changes in the
994 There is no decrypt function. This function isn't all that useful for
995 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
996 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
997 primarily used to check if two pieces of text are the same without
998 having to transmit or store the text itself. An example is checking
999 if a correct password is given. The digest of the password is stored,
1000 not the password itself. The user types in a password that is
1001 crypt()'d with the same salt as the stored digest. If the two digests
1002 match the password is correct.
1004 When verifying an existing digest string you should use the digest as
1005 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1006 to create the digest is visible as part of the digest. This ensures
1007 crypt() will hash the new string with the same salt as the digest.
1008 This allows your code to work with the standard L<crypt|/crypt> and
1009 with more exotic implementations. In other words, do not assume
1010 anything about the returned string itself, or how many bytes in the
1013 Traditionally the result is a string of 13 bytes: two first bytes of
1014 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1015 the first eight bytes of the digest string mattered, but alternative
1016 hashing schemes (like MD5), higher level security schemes (like C2),
1017 and implementations on non-UNIX platforms may produce different
1020 When choosing a new salt create a random two character string whose
1021 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1022 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1023 characters is just a recommendation; the characters allowed in
1024 the salt depend solely on your system's crypt library, and Perl can't
1025 restrict what salts C<crypt()> accepts.
1027 Here's an example that makes sure that whoever runs this program knows
1030 $pwd = (getpwuid($<))[1];
1032 system "stty -echo";
1034 chomp($word = <STDIN>);
1038 if (crypt($word, $pwd) ne $pwd) {
1044 Of course, typing in your own password to whoever asks you
1047 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1048 of data, not least of all because you can't get the information
1049 back. Look at the L<Digest> module for more robust algorithms.
1051 If using crypt() on a Unicode string (which I<potentially> has
1052 characters with codepoints above 255), Perl tries to make sense
1053 of the situation by trying to downgrade (a copy of the string)
1054 the string back to an eight-bit byte string before calling crypt()
1055 (on that copy). If that works, good. If not, crypt() dies with
1056 C<Wide character in crypt>.
1061 [This function has been largely superseded by the C<untie> function.]
1063 Breaks the binding between a DBM file and a hash.
1065 =item dbmopen HASH,DBNAME,MASK
1066 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1068 [This function has been largely superseded by the C<tie> function.]
1070 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1071 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1072 argument is I<not> a filehandle, even though it looks like one). DBNAME
1073 is the name of the database (without the F<.dir> or F<.pag> extension if
1074 any). If the database does not exist, it is created with protection
1075 specified by MASK (as modified by the C<umask>). If your system supports
1076 only the older DBM functions, you may perform only one C<dbmopen> in your
1077 program. In older versions of Perl, if your system had neither DBM nor
1078 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1081 If you don't have write access to the DBM file, you can only read hash
1082 variables, not set them. If you want to test whether you can write,
1083 either use file tests or try setting a dummy hash entry inside an C<eval>,
1084 which will trap the error.
1086 Note that functions such as C<keys> and C<values> may return huge lists
1087 when used on large DBM files. You may prefer to use the C<each>
1088 function to iterate over large DBM files. Example:
1090 # print out history file offsets
1091 dbmopen(%HIST,'/usr/lib/news/history',0666);
1092 while (($key,$val) = each %HIST) {
1093 print $key, ' = ', unpack('L',$val), "\n";
1097 See also L<AnyDBM_File> for a more general description of the pros and
1098 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1099 rich implementation.
1101 You can control which DBM library you use by loading that library
1102 before you call dbmopen():
1105 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1106 or die "Can't open netscape history file: $!";
1109 X<defined> X<undef> X<undefined>
1113 Returns a Boolean value telling whether EXPR has a value other than
1114 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1117 Many operations return C<undef> to indicate failure, end of file,
1118 system error, uninitialized variable, and other exceptional
1119 conditions. This function allows you to distinguish C<undef> from
1120 other values. (A simple Boolean test will not distinguish among
1121 C<undef>, zero, the empty string, and C<"0">, which are all equally
1122 false.) Note that since C<undef> is a valid scalar, its presence
1123 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1124 returns C<undef> when its argument is an empty array, I<or> when the
1125 element to return happens to be C<undef>.
1127 You may also use C<defined(&func)> to check whether subroutine C<&func>
1128 has ever been defined. The return value is unaffected by any forward
1129 declarations of C<&func>. Note that a subroutine which is not defined
1130 may still be callable: its package may have an C<AUTOLOAD> method that
1131 makes it spring into existence the first time that it is called -- see
1134 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1135 used to report whether memory for that aggregate has ever been
1136 allocated. This behavior may disappear in future versions of Perl.
1137 You should instead use a simple test for size:
1139 if (@an_array) { print "has array elements\n" }
1140 if (%a_hash) { print "has hash members\n" }
1142 When used on a hash element, it tells you whether the value is defined,
1143 not whether the key exists in the hash. Use L</exists> for the latter
1148 print if defined $switch{'D'};
1149 print "$val\n" while defined($val = pop(@ary));
1150 die "Can't readlink $sym: $!"
1151 unless defined($value = readlink $sym);
1152 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1153 $debugging = 0 unless defined $debugging;
1155 Note: Many folks tend to overuse C<defined>, and then are surprised to
1156 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1157 defined values. For example, if you say
1161 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1162 matched "nothing". It didn't really fail to match anything. Rather, it
1163 matched something that happened to be zero characters long. This is all
1164 very above-board and honest. When a function returns an undefined value,
1165 it's an admission that it couldn't give you an honest answer. So you
1166 should use C<defined> only when you're questioning the integrity of what
1167 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1170 See also L</undef>, L</exists>, L</ref>.
1175 Given an expression that specifies a hash element, array element, hash slice,
1176 or array slice, deletes the specified element(s) from the hash or array.
1177 In the case of an array, if the array elements happen to be at the end,
1178 the size of the array will shrink to the highest element that tests
1179 true for exists() (or 0 if no such element exists).
1181 Returns a list with the same number of elements as the number of elements
1182 for which deletion was attempted. Each element of that list consists of
1183 either the value of the element deleted, or the undefined value. In scalar
1184 context, this means that you get the value of the last element deleted (or
1185 the undefined value if that element did not exist).
1187 %hash = (foo => 11, bar => 22, baz => 33);
1188 $scalar = delete $hash{foo}; # $scalar is 11
1189 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1190 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1192 Deleting from C<%ENV> modifies the environment. Deleting from
1193 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1194 from a C<tie>d hash or array may not necessarily return anything.
1196 Deleting an array element effectively returns that position of the array
1197 to its initial, uninitialized state. Subsequently testing for the same
1198 element with exists() will return false. Also, deleting array elements
1199 in the middle of an array will not shift the index of the elements
1200 after them down. Use splice() for that. See L</exists>.
1202 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1204 foreach $key (keys %HASH) {
1208 foreach $index (0 .. $#ARRAY) {
1209 delete $ARRAY[$index];
1214 delete @HASH{keys %HASH};
1216 delete @ARRAY[0 .. $#ARRAY];
1218 But both of these are slower than just assigning the empty list
1219 or undefining %HASH or @ARRAY:
1221 %HASH = (); # completely empty %HASH
1222 undef %HASH; # forget %HASH ever existed
1224 @ARRAY = (); # completely empty @ARRAY
1225 undef @ARRAY; # forget @ARRAY ever existed
1227 Note that the EXPR can be arbitrarily complicated as long as the final
1228 operation is a hash element, array element, hash slice, or array slice
1231 delete $ref->[$x][$y]{$key};
1232 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1234 delete $ref->[$x][$y][$index];
1235 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1238 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1240 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1241 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1242 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1243 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1244 an C<eval(),> the error message is stuffed into C<$@> and the
1245 C<eval> is terminated with the undefined value. This makes
1246 C<die> the way to raise an exception.
1248 Equivalent examples:
1250 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1251 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1253 If the last element of LIST does not end in a newline, the current
1254 script line number and input line number (if any) are also printed,
1255 and a newline is supplied. Note that the "input line number" (also
1256 known as "chunk") is subject to whatever notion of "line" happens to
1257 be currently in effect, and is also available as the special variable
1258 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1260 Hint: sometimes appending C<", stopped"> to your message will cause it
1261 to make better sense when the string C<"at foo line 123"> is appended.
1262 Suppose you are running script "canasta".
1264 die "/etc/games is no good";
1265 die "/etc/games is no good, stopped";
1267 produce, respectively
1269 /etc/games is no good at canasta line 123.
1270 /etc/games is no good, stopped at canasta line 123.
1272 See also exit(), warn(), and the Carp module.
1274 If LIST is empty and C<$@> already contains a value (typically from a
1275 previous eval) that value is reused after appending C<"\t...propagated">.
1276 This is useful for propagating exceptions:
1279 die unless $@ =~ /Expected exception/;
1281 If LIST is empty and C<$@> contains an object reference that has a
1282 C<PROPAGATE> method, that method will be called with additional file
1283 and line number parameters. The return value replaces the value in
1284 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1287 If C<$@> is empty then the string C<"Died"> is used.
1289 die() can also be called with a reference argument. If this happens to be
1290 trapped within an eval(), $@ contains the reference. This behavior permits
1291 a more elaborate exception handling implementation using objects that
1292 maintain arbitrary state about the nature of the exception. Such a scheme
1293 is sometimes preferable to matching particular string values of $@ using
1294 regular expressions. Because $@ is a global variable, and eval() may be
1295 used within object implementations, care must be taken that analyzing the
1296 error object doesn't replace the reference in the global variable. The
1297 easiest solution is to make a local copy of the reference before doing
1298 other manipulations. Here's an example:
1300 use Scalar::Util 'blessed';
1302 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1303 if (my $ev_err = $@) {
1304 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1305 # handle Some::Module::Exception
1308 # handle all other possible exceptions
1312 Because perl will stringify uncaught exception messages before displaying
1313 them, you may want to overload stringification operations on such custom
1314 exception objects. See L<overload> for details about that.
1316 You can arrange for a callback to be run just before the C<die>
1317 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1318 handler will be called with the error text and can change the error
1319 message, if it sees fit, by calling C<die> again. See
1320 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1321 L<"eval BLOCK"> for some examples. Although this feature was
1322 to be run only right before your program was to exit, this is not
1323 currently the case--the C<$SIG{__DIE__}> hook is currently called
1324 even inside eval()ed blocks/strings! If one wants the hook to do
1325 nothing in such situations, put
1329 as the first line of the handler (see L<perlvar/$^S>). Because
1330 this promotes strange action at a distance, this counterintuitive
1331 behavior may be fixed in a future release.
1336 Not really a function. Returns the value of the last command in the
1337 sequence of commands indicated by BLOCK. When modified by the C<while> or
1338 C<until> loop modifier, executes the BLOCK once before testing the loop
1339 condition. (On other statements the loop modifiers test the conditional
1342 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1343 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1344 See L<perlsyn> for alternative strategies.
1346 =item do SUBROUTINE(LIST)
1349 This form of subroutine call is deprecated. See L<perlsub>.
1354 Uses the value of EXPR as a filename and executes the contents of the
1355 file as a Perl script.
1363 except that it's more efficient and concise, keeps track of the current
1364 filename for error messages, searches the @INC directories, and updates
1365 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1366 variables. It also differs in that code evaluated with C<do FILENAME>
1367 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1368 same, however, in that it does reparse the file every time you call it,
1369 so you probably don't want to do this inside a loop.
1371 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1372 error. If C<do> can read the file but cannot compile it, it
1373 returns undef and sets an error message in C<$@>. If the file is
1374 successfully compiled, C<do> returns the value of the last expression
1377 Note that inclusion of library modules is better done with the
1378 C<use> and C<require> operators, which also do automatic error checking
1379 and raise an exception if there's a problem.
1381 You might like to use C<do> to read in a program configuration
1382 file. Manual error checking can be done this way:
1384 # read in config files: system first, then user
1385 for $file ("/share/prog/defaults.rc",
1386 "$ENV{HOME}/.someprogrc")
1388 unless ($return = do $file) {
1389 warn "couldn't parse $file: $@" if $@;
1390 warn "couldn't do $file: $!" unless defined $return;
1391 warn "couldn't run $file" unless $return;
1396 X<dump> X<core> X<undump>
1400 This function causes an immediate core dump. See also the B<-u>
1401 command-line switch in L<perlrun>, which does the same thing.
1402 Primarily this is so that you can use the B<undump> program (not
1403 supplied) to turn your core dump into an executable binary after
1404 having initialized all your variables at the beginning of the
1405 program. When the new binary is executed it will begin by executing
1406 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1407 Think of it as a goto with an intervening core dump and reincarnation.
1408 If C<LABEL> is omitted, restarts the program from the top.
1410 B<WARNING>: Any files opened at the time of the dump will I<not>
1411 be open any more when the program is reincarnated, with possible
1412 resulting confusion on the part of Perl.
1414 This function is now largely obsolete, mostly because it's very hard to
1415 convert a core file into an executable. That's why you should now invoke
1416 it as C<CORE::dump()>, if you don't want to be warned against a possible
1420 X<each> X<hash, iterator>
1425 When called in list context, returns a 2-element list consisting of the
1426 key and value for the next element of a hash, or the index and value for
1427 the next element of an array, so that you can iterate over it. When called
1428 in scalar context, returns only the key for the next element in the hash
1429 (or the index for an array).
1431 Hash entries are returned in an apparently random order. The actual random
1432 order is subject to change in future versions of perl, but it is
1433 guaranteed to be in the same order as either the C<keys> or C<values>
1434 function would produce on the same (unmodified) hash. Since Perl
1435 5.8.2 the ordering can be different even between different runs of Perl
1436 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1438 When the hash or array is entirely read, a null array is returned in list
1439 context (which when assigned produces a false (C<0>) value), and C<undef> in
1440 scalar context. The next call to C<each> after that will start iterating
1441 again. There is a single iterator for each hash or array, shared by all
1442 C<each>, C<keys>, and C<values> function calls in the program; it can be
1443 reset by reading all the elements from the hash or array, or by evaluating
1444 C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1445 or delete elements of a hash while you're
1446 iterating over it, you may get entries skipped or duplicated, so
1447 don't. Exception: It is always safe to delete the item most recently
1448 returned by C<each()>, which means that the following code will work:
1450 while (($key, $value) = each %hash) {
1452 delete $hash{$key}; # This is safe
1455 The following prints out your environment like the printenv(1) program,
1456 only in a different order:
1458 while (($key,$value) = each %ENV) {
1459 print "$key=$value\n";
1462 See also C<keys>, C<values> and C<sort>.
1464 =item eof FILEHANDLE
1473 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1474 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1475 gives the real filehandle. (Note that this function actually
1476 reads a character and then C<ungetc>s it, so isn't very useful in an
1477 interactive context.) Do not read from a terminal file (or call
1478 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1479 as terminals may lose the end-of-file condition if you do.
1481 An C<eof> without an argument uses the last file read. Using C<eof()>
1482 with empty parentheses is very different. It refers to the pseudo file
1483 formed from the files listed on the command line and accessed via the
1484 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1485 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1486 used will cause C<@ARGV> to be examined to determine if input is
1487 available. Similarly, an C<eof()> after C<< <> >> has returned
1488 end-of-file will assume you are processing another C<@ARGV> list,
1489 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1490 see L<perlop/"I/O Operators">.
1492 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1493 detect the end of each file, C<eof()> will only detect the end of the
1494 last file. Examples:
1496 # reset line numbering on each input file
1498 next if /^\s*#/; # skip comments
1501 close ARGV if eof; # Not eof()!
1504 # insert dashes just before last line of last file
1506 if (eof()) { # check for end of last file
1507 print "--------------\n";
1510 last if eof(); # needed if we're reading from a terminal
1513 Practical hint: you almost never need to use C<eof> in Perl, because the
1514 input operators typically return C<undef> when they run out of data, or if
1518 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1519 X<error, handling> X<exception, handling>
1525 In the first form, the return value of EXPR is parsed and executed as if it
1526 were a little Perl program. The value of the expression (which is itself
1527 determined within scalar context) is first parsed, and if there weren't any
1528 errors, executed in the lexical context of the current Perl program, so
1529 that any variable settings or subroutine and format definitions remain
1530 afterwards. Note that the value is parsed every time the C<eval> executes.
1531 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1532 delay parsing and subsequent execution of the text of EXPR until run time.
1534 In the second form, the code within the BLOCK is parsed only once--at the
1535 same time the code surrounding the C<eval> itself was parsed--and executed
1536 within the context of the current Perl program. This form is typically
1537 used to trap exceptions more efficiently than the first (see below), while
1538 also providing the benefit of checking the code within BLOCK at compile
1541 The final semicolon, if any, may be omitted from the value of EXPR or within
1544 In both forms, the value returned is the value of the last expression
1545 evaluated inside the mini-program; a return statement may be also used, just
1546 as with subroutines. The expression providing the return value is evaluated
1547 in void, scalar, or list context, depending on the context of the C<eval>
1548 itself. See L</wantarray> for more on how the evaluation context can be
1551 If there is a syntax error or runtime error, or a C<die> statement is
1552 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1553 error message. If there was no error, C<$@> is guaranteed to be a null
1554 string. Beware that using C<eval> neither silences perl from printing
1555 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1556 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1557 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1558 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1560 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1561 determining whether a particular feature (such as C<socket> or C<symlink>)
1562 is implemented. It is also Perl's exception trapping mechanism, where
1563 the die operator is used to raise exceptions.
1565 If the code to be executed doesn't vary, you may use the eval-BLOCK
1566 form to trap run-time errors without incurring the penalty of
1567 recompiling each time. The error, if any, is still returned in C<$@>.
1570 # make divide-by-zero nonfatal
1571 eval { $answer = $a / $b; }; warn $@ if $@;
1573 # same thing, but less efficient
1574 eval '$answer = $a / $b'; warn $@ if $@;
1576 # a compile-time error
1577 eval { $answer = }; # WRONG
1580 eval '$answer ='; # sets $@
1582 Using the C<eval{}> form as an exception trap in libraries does have some
1583 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1584 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1585 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1586 as shown in this example:
1588 # a very private exception trap for divide-by-zero
1589 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1592 This is especially significant, given that C<__DIE__> hooks can call
1593 C<die> again, which has the effect of changing their error messages:
1595 # __DIE__ hooks may modify error messages
1597 local $SIG{'__DIE__'} =
1598 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1599 eval { die "foo lives here" };
1600 print $@ if $@; # prints "bar lives here"
1603 Because this promotes action at a distance, this counterintuitive behavior
1604 may be fixed in a future release.
1606 With an C<eval>, you should be especially careful to remember what's
1607 being looked at when:
1613 eval { $x }; # CASE 4
1615 eval "\$$x++"; # CASE 5
1618 Cases 1 and 2 above behave identically: they run the code contained in
1619 the variable $x. (Although case 2 has misleading double quotes making
1620 the reader wonder what else might be happening (nothing is).) Cases 3
1621 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1622 does nothing but return the value of $x. (Case 4 is preferred for
1623 purely visual reasons, but it also has the advantage of compiling at
1624 compile-time instead of at run-time.) Case 5 is a place where
1625 normally you I<would> like to use double quotes, except that in this
1626 particular situation, you can just use symbolic references instead, as
1629 The assignment to C<$@> occurs before restoration of localised variables,
1630 which means a temporary is required if you want to mask some but not all
1633 # alter $@ on nefarious repugnancy only
1637 local $@; # protect existing $@
1638 eval { test_repugnancy() };
1639 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1640 $@ =~ /nefarious/ and $e = $@;
1642 die $e if defined $e
1645 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1646 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1648 Note that as a very special case, an C<eval ''> executed within the C<DB>
1649 package doesn't see the usual surrounding lexical scope, but rather the
1650 scope of the first non-DB piece of code that called it. You don't normally
1651 need to worry about this unless you are writing a Perl debugger.
1656 =item exec PROGRAM LIST
1658 The C<exec> function executes a system command I<and never returns>--
1659 use C<system> instead of C<exec> if you want it to return. It fails and
1660 returns false only if the command does not exist I<and> it is executed
1661 directly instead of via your system's command shell (see below).
1663 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1664 warns you if there is a following statement which isn't C<die>, C<warn>,
1665 or C<exit> (if C<-w> is set - but you always do that). If you
1666 I<really> want to follow an C<exec> with some other statement, you
1667 can use one of these styles to avoid the warning:
1669 exec ('foo') or print STDERR "couldn't exec foo: $!";
1670 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1672 If there is more than one argument in LIST, or if LIST is an array
1673 with more than one value, calls execvp(3) with the arguments in LIST.
1674 If there is only one scalar argument or an array with one element in it,
1675 the argument is checked for shell metacharacters, and if there are any,
1676 the entire argument is passed to the system's command shell for parsing
1677 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1678 If there are no shell metacharacters in the argument, it is split into
1679 words and passed directly to C<execvp>, which is more efficient.
1682 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1683 exec "sort $outfile | uniq";
1685 If you don't really want to execute the first argument, but want to lie
1686 to the program you are executing about its own name, you can specify
1687 the program you actually want to run as an "indirect object" (without a
1688 comma) in front of the LIST. (This always forces interpretation of the
1689 LIST as a multivalued list, even if there is only a single scalar in
1692 $shell = '/bin/csh';
1693 exec $shell '-sh'; # pretend it's a login shell
1697 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1699 When the arguments get executed via the system shell, results will
1700 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1703 Using an indirect object with C<exec> or C<system> is also more
1704 secure. This usage (which also works fine with system()) forces
1705 interpretation of the arguments as a multivalued list, even if the
1706 list had just one argument. That way you're safe from the shell
1707 expanding wildcards or splitting up words with whitespace in them.
1709 @args = ( "echo surprise" );
1711 exec @args; # subject to shell escapes
1713 exec { $args[0] } @args; # safe even with one-arg list
1715 The first version, the one without the indirect object, ran the I<echo>
1716 program, passing it C<"surprise"> an argument. The second version
1717 didn't--it tried to run a program literally called I<"echo surprise">,
1718 didn't find it, and set C<$?> to a non-zero value indicating failure.
1720 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1721 output before the exec, but this may not be supported on some platforms
1722 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1723 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1724 open handles in order to avoid lost output.
1726 Note that C<exec> will not call your C<END> blocks, nor will it call
1727 any C<DESTROY> methods in your objects.
1730 X<exists> X<autovivification>
1732 Given an expression that specifies a hash element or array element,
1733 returns true if the specified element in the hash or array has ever
1734 been initialized, even if the corresponding value is undefined. The
1735 element is not autovivified if it doesn't exist.
1737 print "Exists\n" if exists $hash{$key};
1738 print "Defined\n" if defined $hash{$key};
1739 print "True\n" if $hash{$key};
1741 print "Exists\n" if exists $array[$index];
1742 print "Defined\n" if defined $array[$index];
1743 print "True\n" if $array[$index];
1745 A hash or array element can be true only if it's defined, and defined if
1746 it exists, but the reverse doesn't necessarily hold true.
1748 Given an expression that specifies the name of a subroutine,
1749 returns true if the specified subroutine has ever been declared, even
1750 if it is undefined. Mentioning a subroutine name for exists or defined
1751 does not count as declaring it. Note that a subroutine which does not
1752 exist may still be callable: its package may have an C<AUTOLOAD>
1753 method that makes it spring into existence the first time that it is
1754 called -- see L<perlsub>.
1756 print "Exists\n" if exists &subroutine;
1757 print "Defined\n" if defined &subroutine;
1759 Note that the EXPR can be arbitrarily complicated as long as the final
1760 operation is a hash or array key lookup or subroutine name:
1762 if (exists $ref->{A}->{B}->{$key}) { }
1763 if (exists $hash{A}{B}{$key}) { }
1765 if (exists $ref->{A}->{B}->[$ix]) { }
1766 if (exists $hash{A}{B}[$ix]) { }
1768 if (exists &{$ref->{A}{B}{$key}}) { }
1770 Although the deepest nested array or hash will not spring into existence
1771 just because its existence was tested, any intervening ones will.
1772 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1773 into existence due to the existence test for the $key element above.
1774 This happens anywhere the arrow operator is used, including even:
1777 if (exists $ref->{"Some key"}) { }
1778 print $ref; # prints HASH(0x80d3d5c)
1780 This surprising autovivification in what does not at first--or even
1781 second--glance appear to be an lvalue context may be fixed in a future
1784 Use of a subroutine call, rather than a subroutine name, as an argument
1785 to exists() is an error.
1788 exists &sub(); # Error
1791 X<exit> X<terminate> X<abort>
1795 Evaluates EXPR and exits immediately with that value. Example:
1798 exit 0 if $ans =~ /^[Xx]/;
1800 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1801 universally recognized values for EXPR are C<0> for success and C<1>
1802 for error; other values are subject to interpretation depending on the
1803 environment in which the Perl program is running. For example, exiting
1804 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1805 the mailer to return the item undelivered, but that's not true everywhere.
1807 Don't use C<exit> to abort a subroutine if there's any chance that
1808 someone might want to trap whatever error happened. Use C<die> instead,
1809 which can be trapped by an C<eval>.
1811 The exit() function does not always exit immediately. It calls any
1812 defined C<END> routines first, but these C<END> routines may not
1813 themselves abort the exit. Likewise any object destructors that need to
1814 be called are called before the real exit. If this is a problem, you
1815 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1816 See L<perlmod> for details.
1819 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1823 Returns I<e> (the natural logarithm base) to the power of EXPR.
1824 If EXPR is omitted, gives C<exp($_)>.
1826 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1829 Implements the fcntl(2) function. You'll probably have to say
1833 first to get the correct constant definitions. Argument processing and
1834 value return works just like C<ioctl> below.
1838 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1839 or die "can't fcntl F_GETFL: $!";
1841 You don't have to check for C<defined> on the return from C<fcntl>.
1842 Like C<ioctl>, it maps a C<0> return from the system call into
1843 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1844 in numeric context. It is also exempt from the normal B<-w> warnings
1845 on improper numeric conversions.
1847 Note that C<fcntl> will produce a fatal error if used on a machine that
1848 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1849 manpage to learn what functions are available on your system.
1851 Here's an example of setting a filehandle named C<REMOTE> to be
1852 non-blocking at the system level. You'll have to negotiate C<$|>
1853 on your own, though.
1855 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1857 $flags = fcntl(REMOTE, F_GETFL, 0)
1858 or die "Can't get flags for the socket: $!\n";
1860 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1861 or die "Can't set flags for the socket: $!\n";
1863 =item fileno FILEHANDLE
1866 Returns the file descriptor for a filehandle, or undefined if the
1867 filehandle is not open. This is mainly useful for constructing
1868 bitmaps for C<select> and low-level POSIX tty-handling operations.
1869 If FILEHANDLE is an expression, the value is taken as an indirect
1870 filehandle, generally its name.
1872 You can use this to find out whether two handles refer to the
1873 same underlying descriptor:
1875 if (fileno(THIS) == fileno(THAT)) {
1876 print "THIS and THAT are dups\n";
1879 (Filehandles connected to memory objects via new features of C<open> may
1880 return undefined even though they are open.)
1883 =item flock FILEHANDLE,OPERATION
1884 X<flock> X<lock> X<locking>
1886 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1887 for success, false on failure. Produces a fatal error if used on a
1888 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1889 C<flock> is Perl's portable file locking interface, although it locks
1890 only entire files, not records.
1892 Two potentially non-obvious but traditional C<flock> semantics are
1893 that it waits indefinitely until the lock is granted, and that its locks
1894 B<merely advisory>. Such discretionary locks are more flexible, but offer
1895 fewer guarantees. This means that programs that do not also use C<flock>
1896 may modify files locked with C<flock>. See L<perlport>,
1897 your port's specific documentation, or your system-specific local manpages
1898 for details. It's best to assume traditional behavior if you're writing
1899 portable programs. (But if you're not, you should as always feel perfectly
1900 free to write for your own system's idiosyncrasies (sometimes called
1901 "features"). Slavish adherence to portability concerns shouldn't get
1902 in the way of your getting your job done.)
1904 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1905 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1906 you can use the symbolic names if you import them from the Fcntl module,
1907 either individually, or as a group using the ':flock' tag. LOCK_SH
1908 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1909 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1910 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1911 waiting for the lock (check the return status to see if you got it).
1913 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1914 before locking or unlocking it.
1916 Note that the emulation built with lockf(3) doesn't provide shared
1917 locks, and it requires that FILEHANDLE be open with write intent. These
1918 are the semantics that lockf(3) implements. Most if not all systems
1919 implement lockf(3) in terms of fcntl(2) locking, though, so the
1920 differing semantics shouldn't bite too many people.
1922 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1923 be open with read intent to use LOCK_SH and requires that it be open
1924 with write intent to use LOCK_EX.
1926 Note also that some versions of C<flock> cannot lock things over the
1927 network; you would need to use the more system-specific C<fcntl> for
1928 that. If you like you can force Perl to ignore your system's flock(2)
1929 function, and so provide its own fcntl(2)-based emulation, by passing
1930 the switch C<-Ud_flock> to the F<Configure> program when you configure
1933 Here's a mailbox appender for BSD systems.
1935 use Fcntl ':flock'; # import LOCK_* constants
1938 flock(MBOX,LOCK_EX);
1939 # and, in case someone appended
1940 # while we were waiting...
1945 flock(MBOX,LOCK_UN);
1948 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
1949 or die "Can't open mailbox: $!";
1952 print $mbox $msg,"\n\n";
1955 On systems that support a real flock(), locks are inherited across fork()
1956 calls, whereas those that must resort to the more capricious fcntl()
1957 function lose the locks, making it harder to write servers.
1959 See also L<DB_File> for other flock() examples.
1962 X<fork> X<child> X<parent>
1964 Does a fork(2) system call to create a new process running the
1965 same program at the same point. It returns the child pid to the
1966 parent process, C<0> to the child process, or C<undef> if the fork is
1967 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1968 are shared, while everything else is copied. On most systems supporting
1969 fork(), great care has gone into making it extremely efficient (for
1970 example, using copy-on-write technology on data pages), making it the
1971 dominant paradigm for multitasking over the last few decades.
1973 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1974 output before forking the child process, but this may not be supported
1975 on some platforms (see L<perlport>). To be safe, you may need to set
1976 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1977 C<IO::Handle> on any open handles in order to avoid duplicate output.
1979 If you C<fork> without ever waiting on your children, you will
1980 accumulate zombies. On some systems, you can avoid this by setting
1981 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1982 forking and reaping moribund children.
1984 Note that if your forked child inherits system file descriptors like
1985 STDIN and STDOUT that are actually connected by a pipe or socket, even
1986 if you exit, then the remote server (such as, say, a CGI script or a
1987 backgrounded job launched from a remote shell) won't think you're done.
1988 You should reopen those to F</dev/null> if it's any issue.
1993 Declare a picture format for use by the C<write> function. For
1997 Test: @<<<<<<<< @||||| @>>>>>
1998 $str, $%, '$' . int($num)
2002 $num = $cost/$quantity;
2006 See L<perlform> for many details and examples.
2008 =item formline PICTURE,LIST
2011 This is an internal function used by C<format>s, though you may call it,
2012 too. It formats (see L<perlform>) a list of values according to the
2013 contents of PICTURE, placing the output into the format output
2014 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2015 Eventually, when a C<write> is done, the contents of
2016 C<$^A> are written to some filehandle. You could also read C<$^A>
2017 and then set C<$^A> back to C<"">. Note that a format typically
2018 does one C<formline> per line of form, but the C<formline> function itself
2019 doesn't care how many newlines are embedded in the PICTURE. This means
2020 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2021 You may therefore need to use multiple formlines to implement a single
2022 record format, just like the format compiler.
2024 Be careful if you put double quotes around the picture, because an C<@>
2025 character may be taken to mean the beginning of an array name.
2026 C<formline> always returns true. See L<perlform> for other examples.
2028 =item getc FILEHANDLE
2029 X<getc> X<getchar> X<character> X<file, read>
2033 Returns the next character from the input file attached to FILEHANDLE,
2034 or the undefined value at end of file, or if there was an error (in
2035 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2036 STDIN. This is not particularly efficient. However, it cannot be
2037 used by itself to fetch single characters without waiting for the user
2038 to hit enter. For that, try something more like:
2041 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2044 system "stty", '-icanon', 'eol', "\001";
2050 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2053 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2057 Determination of whether $BSD_STYLE should be set
2058 is left as an exercise to the reader.
2060 The C<POSIX::getattr> function can do this more portably on
2061 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2062 module from your nearest CPAN site; details on CPAN can be found on
2066 X<getlogin> X<login>
2068 This implements the C library function of the same name, which on most
2069 systems returns the current login from F</etc/utmp>, if any. If null,
2072 $login = getlogin || getpwuid($<) || "Kilroy";
2074 Do not consider C<getlogin> for authentication: it is not as
2075 secure as C<getpwuid>.
2077 =item getpeername SOCKET
2078 X<getpeername> X<peer>
2080 Returns the packed sockaddr address of other end of the SOCKET connection.
2083 $hersockaddr = getpeername(SOCK);
2084 ($port, $iaddr) = sockaddr_in($hersockaddr);
2085 $herhostname = gethostbyaddr($iaddr, AF_INET);
2086 $herstraddr = inet_ntoa($iaddr);
2091 Returns the current process group for the specified PID. Use
2092 a PID of C<0> to get the current process group for the
2093 current process. Will raise an exception if used on a machine that
2094 doesn't implement getpgrp(2). If PID is omitted, returns process
2095 group of current process. Note that the POSIX version of C<getpgrp>
2096 does not accept a PID argument, so only C<PID==0> is truly portable.
2099 X<getppid> X<parent> X<pid>
2101 Returns the process id of the parent process.
2103 Note for Linux users: on Linux, the C functions C<getpid()> and
2104 C<getppid()> return different values from different threads. In order to
2105 be portable, this behavior is not reflected by the perl-level function
2106 C<getppid()>, that returns a consistent value across threads. If you want
2107 to call the underlying C<getppid()>, you may use the CPAN module
2110 =item getpriority WHICH,WHO
2111 X<getpriority> X<priority> X<nice>
2113 Returns the current priority for a process, a process group, or a user.
2114 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2115 machine that doesn't implement getpriority(2).
2118 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2119 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2120 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2121 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2122 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2123 X<endnetent> X<endprotoent> X<endservent>
2127 =item gethostbyname NAME
2129 =item getnetbyname NAME
2131 =item getprotobyname NAME
2137 =item getservbyname NAME,PROTO
2139 =item gethostbyaddr ADDR,ADDRTYPE
2141 =item getnetbyaddr ADDR,ADDRTYPE
2143 =item getprotobynumber NUMBER
2145 =item getservbyport PORT,PROTO
2163 =item sethostent STAYOPEN
2165 =item setnetent STAYOPEN
2167 =item setprotoent STAYOPEN
2169 =item setservent STAYOPEN
2183 These routines perform the same functions as their counterparts in the
2184 system library. In list context, the return values from the
2185 various get routines are as follows:
2187 ($name,$passwd,$uid,$gid,
2188 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2189 ($name,$passwd,$gid,$members) = getgr*
2190 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2191 ($name,$aliases,$addrtype,$net) = getnet*
2192 ($name,$aliases,$proto) = getproto*
2193 ($name,$aliases,$port,$proto) = getserv*
2195 (If the entry doesn't exist you get a null list.)
2197 The exact meaning of the $gcos field varies but it usually contains
2198 the real name of the user (as opposed to the login name) and other
2199 information pertaining to the user. Beware, however, that in many
2200 system users are able to change this information and therefore it
2201 cannot be trusted and therefore the $gcos is tainted (see
2202 L<perlsec>). The $passwd and $shell, user's encrypted password and
2203 login shell, are also tainted, because of the same reason.
2205 In scalar context, you get the name, unless the function was a
2206 lookup by name, in which case you get the other thing, whatever it is.
2207 (If the entry doesn't exist you get the undefined value.) For example:
2209 $uid = getpwnam($name);
2210 $name = getpwuid($num);
2212 $gid = getgrnam($name);
2213 $name = getgrgid($num);
2217 In I<getpw*()> the fields $quota, $comment, and $expire are special
2218 cases in the sense that in many systems they are unsupported. If the
2219 $quota is unsupported, it is an empty scalar. If it is supported, it
2220 usually encodes the disk quota. If the $comment field is unsupported,
2221 it is an empty scalar. If it is supported it usually encodes some
2222 administrative comment about the user. In some systems the $quota
2223 field may be $change or $age, fields that have to do with password
2224 aging. In some systems the $comment field may be $class. The $expire
2225 field, if present, encodes the expiration period of the account or the
2226 password. For the availability and the exact meaning of these fields
2227 in your system, please consult your getpwnam(3) documentation and your
2228 F<pwd.h> file. You can also find out from within Perl what your
2229 $quota and $comment fields mean and whether you have the $expire field
2230 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2231 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2232 files are only supported if your vendor has implemented them in the
2233 intuitive fashion that calling the regular C library routines gets the
2234 shadow versions if you're running under privilege or if there exists
2235 the shadow(3) functions as found in System V (this includes Solaris
2236 and Linux.) Those systems that implement a proprietary shadow password
2237 facility are unlikely to be supported.
2239 The $members value returned by I<getgr*()> is a space separated list of
2240 the login names of the members of the group.
2242 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2243 C, it will be returned to you via C<$?> if the function call fails. The
2244 C<@addrs> value returned by a successful call is a list of the raw
2245 addresses returned by the corresponding system library call. In the
2246 Internet domain, each address is four bytes long and you can unpack it
2247 by saying something like:
2249 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2251 The Socket library makes this slightly easier:
2254 $iaddr = inet_aton("127.1"); # or whatever address
2255 $name = gethostbyaddr($iaddr, AF_INET);
2257 # or going the other way
2258 $straddr = inet_ntoa($iaddr);
2260 In the opposite way, to resolve a hostname to the IP address
2264 $packed_ip = gethostbyname("www.perl.org");
2265 if (defined $packed_ip) {
2266 $ip_address = inet_ntoa($packed_ip);
2269 Make sure <gethostbyname()> is called in SCALAR context and that
2270 its return value is checked for definedness.
2272 If you get tired of remembering which element of the return list
2273 contains which return value, by-name interfaces are provided
2274 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2275 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2276 and C<User::grent>. These override the normal built-ins, supplying
2277 versions that return objects with the appropriate names
2278 for each field. For example:
2282 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2284 Even though it looks like they're the same method calls (uid),
2285 they aren't, because a C<File::stat> object is different from
2286 a C<User::pwent> object.
2288 =item getsockname SOCKET
2291 Returns the packed sockaddr address of this end of the SOCKET connection,
2292 in case you don't know the address because you have several different
2293 IPs that the connection might have come in on.
2296 $mysockaddr = getsockname(SOCK);
2297 ($port, $myaddr) = sockaddr_in($mysockaddr);
2298 printf "Connect to %s [%s]\n",
2299 scalar gethostbyaddr($myaddr, AF_INET),
2302 =item getsockopt SOCKET,LEVEL,OPTNAME
2305 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2306 Options may exist at multiple protocol levels depending on the socket
2307 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2308 C<Socket> module) will exist. To query options at another level the
2309 protocol number of the appropriate protocol controlling the option
2310 should be supplied. For example, to indicate that an option is to be
2311 interpreted by the TCP protocol, LEVEL should be set to the protocol
2312 number of TCP, which you can get using getprotobyname.
2314 The call returns a packed string representing the requested socket option,
2315 or C<undef> if there is an error (the error reason will be in $!). What
2316 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2317 your system documentation for details. A very common case however is that
2318 the option is an integer, in which case the result will be a packed
2319 integer which you can decode using unpack with the C<i> (or C<I>) format.
2321 An example testing if Nagle's algorithm is turned on on a socket:
2323 use Socket qw(:all);
2325 defined(my $tcp = getprotobyname("tcp"))
2326 or die "Could not determine the protocol number for tcp";
2327 # my $tcp = IPPROTO_TCP; # Alternative
2328 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2329 or die "Could not query TCP_NODELAY socket option: $!";
2330 my $nodelay = unpack("I", $packed);
2331 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2335 X<glob> X<wildcard> X<filename, expansion> X<expand>
2339 In list context, returns a (possibly empty) list of filename expansions on
2340 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2341 scalar context, glob iterates through such filename expansions, returning
2342 undef when the list is exhausted. This is the internal function
2343 implementing the C<< <*.c> >> operator, but you can use it directly. If
2344 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2345 more detail in L<perlop/"I/O Operators">.
2347 Beginning with v5.6.0, this operator is implemented using the standard
2348 C<File::Glob> extension. See L<File::Glob> for details.
2351 X<gmtime> X<UTC> X<Greenwich>
2355 Works just like L<localtime> but the returned values are
2356 localized for the standard Greenwich time zone.
2358 Note: when called in list context, $isdst, the last value
2359 returned by gmtime is always C<0>. There is no
2360 Daylight Saving Time in GMT.
2362 See L<perlport/gmtime> for portability concerns.
2365 X<goto> X<jump> X<jmp>
2371 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2372 execution there. It may not be used to go into any construct that
2373 requires initialization, such as a subroutine or a C<foreach> loop. It
2374 also can't be used to go into a construct that is optimized away,
2375 or to get out of a block or subroutine given to C<sort>.
2376 It can be used to go almost anywhere else within the dynamic scope,
2377 including out of subroutines, but it's usually better to use some other
2378 construct such as C<last> or C<die>. The author of Perl has never felt the
2379 need to use this form of C<goto> (in Perl, that is--C is another matter).
2380 (The difference being that C does not offer named loops combined with
2381 loop control. Perl does, and this replaces most structured uses of C<goto>
2382 in other languages.)
2384 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2385 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2386 necessarily recommended if you're optimizing for maintainability:
2388 goto ("FOO", "BAR", "GLARCH")[$i];
2390 The C<goto-&NAME> form is quite different from the other forms of
2391 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2392 doesn't have the stigma associated with other gotos. Instead, it
2393 exits the current subroutine (losing any changes set by local()) and
2394 immediately calls in its place the named subroutine using the current
2395 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2396 load another subroutine and then pretend that the other subroutine had
2397 been called in the first place (except that any modifications to C<@_>
2398 in the current subroutine are propagated to the other subroutine.)
2399 After the C<goto>, not even C<caller> will be able to tell that this
2400 routine was called first.
2402 NAME needn't be the name of a subroutine; it can be a scalar variable
2403 containing a code reference, or a block that evaluates to a code
2406 =item grep BLOCK LIST
2409 =item grep EXPR,LIST
2411 This is similar in spirit to, but not the same as, grep(1) and its
2412 relatives. In particular, it is not limited to using regular expressions.
2414 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2415 C<$_> to each element) and returns the list value consisting of those
2416 elements for which the expression evaluated to true. In scalar
2417 context, returns the number of times the expression was true.
2419 @foo = grep(!/^#/, @bar); # weed out comments
2423 @foo = grep {!/^#/} @bar; # weed out comments
2425 Note that C<$_> is an alias to the list value, so it can be used to
2426 modify the elements of the LIST. While this is useful and supported,
2427 it can cause bizarre results if the elements of LIST are not variables.
2428 Similarly, grep returns aliases into the original list, much as a for
2429 loop's index variable aliases the list elements. That is, modifying an
2430 element of a list returned by grep (for example, in a C<foreach>, C<map>
2431 or another C<grep>) actually modifies the element in the original list.
2432 This is usually something to be avoided when writing clear code.
2434 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2435 been declared with C<my $_>) then, in addition to being locally aliased to
2436 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2437 can't be seen from the outside, avoiding any potential side-effects.
2439 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2442 X<hex> X<hexadecimal>
2446 Interprets EXPR as a hex string and returns the corresponding value.
2447 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2448 L</oct>.) If EXPR is omitted, uses C<$_>.
2450 print hex '0xAf'; # prints '175'
2451 print hex 'aF'; # same
2453 Hex strings may only represent integers. Strings that would cause
2454 integer overflow trigger a warning. Leading whitespace is not stripped,
2455 unlike oct(). To present something as hex, look into L</printf>,
2456 L</sprintf>, or L</unpack>.
2461 There is no builtin C<import> function. It is just an ordinary
2462 method (subroutine) defined (or inherited) by modules that wish to export
2463 names to another module. The C<use> function calls the C<import> method
2464 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2466 =item index STR,SUBSTR,POSITION
2467 X<index> X<indexOf> X<InStr>
2469 =item index STR,SUBSTR
2471 The index function searches for one string within another, but without
2472 the wildcard-like behavior of a full regular-expression pattern match.
2473 It returns the position of the first occurrence of SUBSTR in STR at
2474 or after POSITION. If POSITION is omitted, starts searching from the
2475 beginning of the string. POSITION before the beginning of the string
2476 or after its end is treated as if it were the beginning or the end,
2477 respectively. POSITION and the return value are based at C<0> (or whatever
2478 you've set the C<$[> variable to--but don't do that). If the substring
2479 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2482 X<int> X<integer> X<truncate> X<trunc> X<floor>
2486 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2487 You should not use this function for rounding: one because it truncates
2488 towards C<0>, and two because machine representations of floating point
2489 numbers can sometimes produce counterintuitive results. For example,
2490 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2491 because it's really more like -268.99999999999994315658 instead. Usually,
2492 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2493 functions will serve you better than will int().
2495 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2498 Implements the ioctl(2) function. You'll probably first have to say
2500 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2502 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2503 exist or doesn't have the correct definitions you'll have to roll your
2504 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2505 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2506 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2507 written depending on the FUNCTION--a pointer to the string value of SCALAR
2508 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2509 has no string value but does have a numeric value, that value will be
2510 passed rather than a pointer to the string value. To guarantee this to be
2511 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2512 functions may be needed to manipulate the values of structures used by
2515 The return value of C<ioctl> (and C<fcntl>) is as follows:
2517 if OS returns: then Perl returns:
2519 0 string "0 but true"
2520 anything else that number
2522 Thus Perl returns true on success and false on failure, yet you can
2523 still easily determine the actual value returned by the operating
2526 $retval = ioctl(...) || -1;
2527 printf "System returned %d\n", $retval;
2529 The special string C<"0 but true"> is exempt from B<-w> complaints
2530 about improper numeric conversions.
2532 =item join EXPR,LIST
2535 Joins the separate strings of LIST into a single string with fields
2536 separated by the value of EXPR, and returns that new string. Example:
2538 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2540 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2541 first argument. Compare L</split>.
2548 Returns a list consisting of all the keys of the named hash, or the indices
2549 of an array. (In scalar context, returns the number of keys or indices.)
2551 The keys of a hash are returned in an apparently random order. The actual
2552 random order is subject to change in future versions of perl, but it
2553 is guaranteed to be the same order as either the C<values> or C<each>
2554 function produces (given that the hash has not been modified). Since
2555 Perl 5.8.1 the ordering is different even between different runs of
2556 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2559 As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
2560 (see L</each>). In particular, calling keys() in void context resets
2561 the iterator with no other overhead.
2563 Here is yet another way to print your environment:
2566 @values = values %ENV;
2568 print pop(@keys), '=', pop(@values), "\n";
2571 or how about sorted by key:
2573 foreach $key (sort(keys %ENV)) {
2574 print $key, '=', $ENV{$key}, "\n";
2577 The returned values are copies of the original keys in the hash, so
2578 modifying them will not affect the original hash. Compare L</values>.
2580 To sort a hash by value, you'll need to use a C<sort> function.
2581 Here's a descending numeric sort of a hash by its values:
2583 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2584 printf "%4d %s\n", $hash{$key}, $key;
2587 As an lvalue C<keys> allows you to increase the number of hash buckets
2588 allocated for the given hash. This can gain you a measure of efficiency if
2589 you know the hash is going to get big. (This is similar to pre-extending
2590 an array by assigning a larger number to $#array.) If you say
2594 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2595 in fact, since it rounds up to the next power of two. These
2596 buckets will be retained even if you do C<%hash = ()>, use C<undef
2597 %hash> if you want to free the storage while C<%hash> is still in scope.
2598 You can't shrink the number of buckets allocated for the hash using
2599 C<keys> in this way (but you needn't worry about doing this by accident,
2600 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2603 See also C<each>, C<values> and C<sort>.
2605 =item kill SIGNAL, LIST
2608 Sends a signal to a list of processes. Returns the number of
2609 processes successfully signaled (which is not necessarily the
2610 same as the number actually killed).
2612 $cnt = kill 1, $child1, $child2;
2615 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2616 system call will check whether it's possible to send a signal to it (that
2617 means, to be brief, that the process is owned by the same user, or we are
2618 the super-user). This is a useful way to check that a child process is
2619 alive (even if only as a zombie) and hasn't changed its UID. See
2620 L<perlport> for notes on the portability of this construct.
2622 Unlike in the shell, if SIGNAL is negative, it kills
2623 process groups instead of processes. (On System V, a negative I<PROCESS>
2624 number will also kill process groups, but that's not portable.) That
2625 means you usually want to use positive not negative signals. You may also
2626 use a signal name in quotes.
2628 See L<perlipc/"Signals"> for more details.
2635 The C<last> command is like the C<break> statement in C (as used in
2636 loops); it immediately exits the loop in question. If the LABEL is
2637 omitted, the command refers to the innermost enclosing loop. The
2638 C<continue> block, if any, is not executed:
2640 LINE: while (<STDIN>) {
2641 last LINE if /^$/; # exit when done with header
2645 C<last> cannot be used to exit a block which returns a value such as
2646 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2647 a grep() or map() operation.
2649 Note that a block by itself is semantically identical to a loop
2650 that executes once. Thus C<last> can be used to effect an early
2651 exit out of such a block.
2653 See also L</continue> for an illustration of how C<last>, C<next>, and
2661 Returns a lowercased version of EXPR. This is the internal function
2662 implementing the C<\L> escape in double-quoted strings. Respects
2663 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2664 and L<perlunicode> for more details about locale and Unicode support.
2666 If EXPR is omitted, uses C<$_>.
2669 X<lcfirst> X<lowercase>
2673 Returns the value of EXPR with the first character lowercased. This
2674 is the internal function implementing the C<\l> escape in
2675 double-quoted strings. Respects current LC_CTYPE locale if C<use
2676 locale> in force. See L<perllocale> and L<perlunicode> for more
2677 details about locale and Unicode support.
2679 If EXPR is omitted, uses C<$_>.
2686 Returns the length in I<characters> of the value of EXPR. If EXPR is
2687 omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2688 Note that this cannot be used on an entire array or hash to find out how
2689 many elements these have. For that, use C<scalar @array> and C<scalar keys
2690 %hash> respectively.
2692 Note the I<characters>: if the EXPR is in Unicode, you will get the
2693 number of characters, not the number of bytes. To get the length
2694 of the internal string in bytes, use C<bytes::length(EXPR)>, see
2695 L<bytes>. Note that the internal encoding is variable, and the number
2696 of bytes usually meaningless. To get the number of bytes that the
2697 string would have when encoded as UTF-8, use
2698 C<length(Encoding::encode_utf8(EXPR))>.
2700 =item link OLDFILE,NEWFILE
2703 Creates a new filename linked to the old filename. Returns true for
2704 success, false otherwise.
2706 =item listen SOCKET,QUEUESIZE
2709 Does the same thing that the listen system call does. Returns true if
2710 it succeeded, false otherwise. See the example in
2711 L<perlipc/"Sockets: Client/Server Communication">.
2716 You really probably want to be using C<my> instead, because C<local> isn't
2717 what most people think of as "local". See
2718 L<perlsub/"Private Variables via my()"> for details.
2720 A local modifies the listed variables to be local to the enclosing
2721 block, file, or eval. If more than one value is listed, the list must
2722 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2723 for details, including issues with tied arrays and hashes.
2725 =item localtime EXPR
2726 X<localtime> X<ctime>
2730 Converts a time as returned by the time function to a 9-element list
2731 with the time analyzed for the local time zone. Typically used as
2735 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2738 All list elements are numeric, and come straight out of the C `struct
2739 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2740 of the specified time.
2742 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2743 the range C<0..11> with 0 indicating January and 11 indicating December.
2744 This makes it easy to get a month name from a list:
2746 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2747 print "$abbr[$mon] $mday";
2748 # $mon=9, $mday=18 gives "Oct 18"
2750 C<$year> is the number of years since 1900, not just the last two digits
2751 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2752 to get a complete 4-digit year is simply:
2756 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2757 to do that, would you?
2759 To get the last two digits of the year (e.g., '01' in 2001) do:
2761 $year = sprintf("%02d", $year % 100);
2763 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2764 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2765 (or C<0..365> in leap years.)
2767 C<$isdst> is true if the specified time occurs during Daylight Saving
2768 Time, false otherwise.
2770 If EXPR is omitted, C<localtime()> uses the current time (as returned
2773 In scalar context, C<localtime()> returns the ctime(3) value:
2775 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2777 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2778 instead of local time use the L</gmtime> builtin. See also the
2779 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2780 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2781 and mktime(3) functions.
2783 To get somewhat similar but locale dependent date strings, set up your
2784 locale environment variables appropriately (please see L<perllocale>) and
2787 use POSIX qw(strftime);
2788 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2789 # or for GMT formatted appropriately for your locale:
2790 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2792 Note that the C<%a> and C<%b>, the short forms of the day of the week
2793 and the month of the year, may not necessarily be three characters wide.
2795 See L<perlport/localtime> for portability concerns.
2797 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2798 by-name access mechanism to the gmtime() and localtime() functions,
2801 For a comprehensive date and time representation look at the
2802 L<DateTime> module on CPAN.
2807 This function places an advisory lock on a shared variable, or referenced
2808 object contained in I<THING> until the lock goes out of scope.
2810 lock() is a "weak keyword" : this means that if you've defined a function
2811 by this name (before any calls to it), that function will be called
2812 instead. (However, if you've said C<use threads>, lock() is always a
2813 keyword.) See L<threads>.
2816 X<log> X<logarithm> X<e> X<ln> X<base>
2820 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2821 returns log of C<$_>. To get the log of another base, use basic algebra:
2822 The base-N log of a number is equal to the natural log of that number
2823 divided by the natural log of N. For example:
2827 return log($n)/log(10);
2830 See also L</exp> for the inverse operation.
2837 Does the same thing as the C<stat> function (including setting the
2838 special C<_> filehandle) but stats a symbolic link instead of the file
2839 the symbolic link points to. If symbolic links are unimplemented on
2840 your system, a normal C<stat> is done. For much more detailed
2841 information, please see the documentation for C<stat>.
2843 If EXPR is omitted, stats C<$_>.
2847 The match operator. See L<perlop>.
2849 =item map BLOCK LIST
2854 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2855 C<$_> to each element) and returns the list value composed of the
2856 results of each such evaluation. In scalar context, returns the
2857 total number of elements so generated. Evaluates BLOCK or EXPR in
2858 list context, so each element of LIST may produce zero, one, or
2859 more elements in the returned value.
2861 @chars = map(chr, @nums);
2863 translates a list of numbers to the corresponding characters. And
2865 %hash = map { get_a_key_for($_) => $_ } @array;
2867 is just a funny way to write
2871 $hash{get_a_key_for($_)} = $_;
2874 Note that C<$_> is an alias to the list value, so it can be used to
2875 modify the elements of the LIST. While this is useful and supported,
2876 it can cause bizarre results if the elements of LIST are not variables.
2877 Using a regular C<foreach> loop for this purpose would be clearer in
2878 most cases. See also L</grep> for an array composed of those items of
2879 the original list for which the BLOCK or EXPR evaluates to true.
2881 If C<$_> is lexical in the scope where the C<map> appears (because it has
2882 been declared with C<my $_>), then, in addition to being locally aliased to
2883 the list elements, C<$_> keeps being lexical inside the block; that is, it
2884 can't be seen from the outside, avoiding any potential side-effects.
2886 C<{> starts both hash references and blocks, so C<map { ...> could be either
2887 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2888 ahead for the closing C<}> it has to take a guess at which its dealing with
2889 based what it finds just after the C<{>. Usually it gets it right, but if it
2890 doesn't it won't realize something is wrong until it gets to the C<}> and
2891 encounters the missing (or unexpected) comma. The syntax error will be
2892 reported close to the C<}> but you'll need to change something near the C<{>
2893 such as using a unary C<+> to give perl some help:
2895 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2896 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2897 %hash = map { ("\L$_", 1) } @array # this also works
2898 %hash = map { lc($_), 1 } @array # as does this.
2899 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2901 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2903 or to force an anon hash constructor use C<+{>:
2905 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2907 and you get list of anonymous hashes each with only 1 entry.
2909 =item mkdir FILENAME,MASK
2910 X<mkdir> X<md> X<directory, create>
2912 =item mkdir FILENAME
2916 Creates the directory specified by FILENAME, with permissions
2917 specified by MASK (as modified by C<umask>). If it succeeds it
2918 returns true, otherwise it returns false and sets C<$!> (errno).
2919 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2922 In general, it is better to create directories with permissive MASK,
2923 and let the user modify that with their C<umask>, than it is to supply
2924 a restrictive MASK and give the user no way to be more permissive.
2925 The exceptions to this rule are when the file or directory should be
2926 kept private (mail files, for instance). The perlfunc(1) entry on
2927 C<umask> discusses the choice of MASK in more detail.
2929 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2930 number of trailing slashes. Some operating and filesystems do not get
2931 this right, so Perl automatically removes all trailing slashes to keep
2934 In order to recursively create a directory structure look at
2935 the C<mkpath> function of the L<File::Path> module.
2937 =item msgctl ID,CMD,ARG
2940 Calls the System V IPC function msgctl(2). You'll probably have to say
2944 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2945 then ARG must be a variable that will hold the returned C<msqid_ds>
2946 structure. Returns like C<ioctl>: the undefined value for error,
2947 C<"0 but true"> for zero, or the actual return value otherwise. See also
2948 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2950 =item msgget KEY,FLAGS
2953 Calls the System V IPC function msgget(2). Returns the message queue
2954 id, or the undefined value if there is an error. See also
2955 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2957 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2960 Calls the System V IPC function msgrcv to receive a message from
2961 message queue ID into variable VAR with a maximum message size of
2962 SIZE. Note that when a message is received, the message type as a
2963 native long integer will be the first thing in VAR, followed by the
2964 actual message. This packing may be opened with C<unpack("l! a*")>.
2965 Taints the variable. Returns true if successful, or false if there is
2966 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2967 C<IPC::SysV::Msg> documentation.
2969 =item msgsnd ID,MSG,FLAGS
2972 Calls the System V IPC function msgsnd to send the message MSG to the
2973 message queue ID. MSG must begin with the native long integer message
2974 type, and be followed by the length of the actual message, and finally
2975 the message itself. This kind of packing can be achieved with
2976 C<pack("l! a*", $type, $message)>. Returns true if successful,
2977 or false if there is an error. See also C<IPC::SysV>
2978 and C<IPC::SysV::Msg> documentation.
2985 =item my EXPR : ATTRS
2987 =item my TYPE EXPR : ATTRS
2989 A C<my> declares the listed variables to be local (lexically) to the
2990 enclosing block, file, or C<eval>. If more than one value is listed,
2991 the list must be placed in parentheses.
2993 The exact semantics and interface of TYPE and ATTRS are still
2994 evolving. TYPE is currently bound to the use of C<fields> pragma,
2995 and attributes are handled using the C<attributes> pragma, or starting
2996 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2997 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2998 L<attributes>, and L<Attribute::Handlers>.
3005 The C<next> command is like the C<continue> statement in C; it starts
3006 the next iteration of the loop:
3008 LINE: while (<STDIN>) {
3009 next LINE if /^#/; # discard comments
3013 Note that if there were a C<continue> block on the above, it would get
3014 executed even on discarded lines. If the LABEL is omitted, the command
3015 refers to the innermost enclosing loop.
3017 C<next> cannot be used to exit a block which returns a value such as
3018 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3019 a grep() or map() operation.
3021 Note that a block by itself is semantically identical to a loop
3022 that executes once. Thus C<next> will exit such a block early.
3024 See also L</continue> for an illustration of how C<last>, C<next>, and
3027 =item no Module VERSION LIST
3030 =item no Module VERSION
3032 =item no Module LIST
3038 See the C<use> function, of which C<no> is the opposite.
3041 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3045 Interprets EXPR as an octal string and returns the corresponding
3046 value. (If EXPR happens to start off with C<0x>, interprets it as a
3047 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3048 binary string. Leading whitespace is ignored in all three cases.)
3049 The following will handle decimal, binary, octal, and hex in the standard
3052 $val = oct($val) if $val =~ /^0/;
3054 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3055 in octal), use sprintf() or printf():
3057 $perms = (stat("filename"))[2] & 07777;
3058 $oct_perms = sprintf "%lo", $perms;
3060 The oct() function is commonly used when a string such as C<644> needs
3061 to be converted into a file mode, for example. (Although perl will
3062 automatically convert strings into numbers as needed, this automatic
3063 conversion assumes base 10.)
3065 =item open FILEHANDLE,EXPR
3066 X<open> X<pipe> X<file, open> X<fopen>
3068 =item open FILEHANDLE,MODE,EXPR
3070 =item open FILEHANDLE,MODE,EXPR,LIST
3072 =item open FILEHANDLE,MODE,REFERENCE
3074 =item open FILEHANDLE
3076 Opens the file whose filename is given by EXPR, and associates it with
3079 Simple examples to open a file for reading:
3081 open(my $fh, '<', "input.txt") or die $!;
3085 open(my $fh, '>', "output.txt") or die $!;
3087 (The following is a comprehensive reference to open(): for a gentler
3088 introduction you may consider L<perlopentut>.)
3090 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3091 the variable is assigned a reference to a new anonymous filehandle,
3092 otherwise if FILEHANDLE is an expression, its value is used as the name of
3093 the real filehandle wanted. (This is considered a symbolic reference, so
3094 C<use strict 'refs'> should I<not> be in effect.)
3096 If EXPR is omitted, the scalar variable of the same name as the
3097 FILEHANDLE contains the filename. (Note that lexical variables--those
3098 declared with C<my>--will not work for this purpose; so if you're
3099 using C<my>, specify EXPR in your call to open.)
3101 If three or more arguments are specified then the mode of opening and
3102 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3103 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3104 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3105 the file is opened for appending, again being created if necessary.
3107 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3108 indicate that you want both read and write access to the file; thus
3109 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3110 '+>' >> mode would clobber the file first. You can't usually use
3111 either read-write mode for updating textfiles, since they have
3112 variable length records. See the B<-i> switch in L<perlrun> for a
3113 better approach. The file is created with permissions of C<0666>
3114 modified by the process' C<umask> value.
3116 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3117 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3119 In the 2-arguments (and 1-argument) form of the call the mode and
3120 filename should be concatenated (in this order), possibly separated by
3121 spaces. It is possible to omit the mode in these forms if the mode is
3124 If the filename begins with C<'|'>, the filename is interpreted as a
3125 command to which output is to be piped, and if the filename ends with a
3126 C<'|'>, the filename is interpreted as a command which pipes output to
3127 us. See L<perlipc/"Using open() for IPC">
3128 for more examples of this. (You are not allowed to C<open> to a command
3129 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3130 and L<perlipc/"Bidirectional Communication with Another Process">
3133 For three or more arguments if MODE is C<'|-'>, the filename is
3134 interpreted as a command to which output is to be piped, and if MODE
3135 is C<'-|'>, the filename is interpreted as a command which pipes
3136 output to us. In the 2-arguments (and 1-argument) form one should
3137 replace dash (C<'-'>) with the command.
3138 See L<perlipc/"Using open() for IPC"> for more examples of this.
3139 (You are not allowed to C<open> to a command that pipes both in I<and>
3140 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3141 L<perlipc/"Bidirectional Communication"> for alternatives.)
3143 In the three-or-more argument form of pipe opens, if LIST is specified
3144 (extra arguments after the command name) then LIST becomes arguments
3145 to the command invoked if the platform supports it. The meaning of
3146 C<open> with more than three arguments for non-pipe modes is not yet
3147 specified. Experimental "layers" may give extra LIST arguments
3150 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3151 and opening C<< '>-' >> opens STDOUT.
3153 You may use the three-argument form of open to specify IO "layers"
3154 (sometimes also referred to as "disciplines") to be applied to the handle
3155 that affect how the input and output are processed (see L<open> and
3156 L<PerlIO> for more details). For example
3158 open(my $fh, "<:encoding(UTF-8)", "file")
3160 will open the UTF-8 encoded file containing Unicode characters,
3161 see L<perluniintro>. Note that if layers are specified in the
3162 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3163 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3165 Open returns nonzero upon success, the undefined value otherwise. If
3166 the C<open> involved a pipe, the return value happens to be the pid of
3169 If you're running Perl on a system that distinguishes between text
3170 files and binary files, then you should check out L</binmode> for tips
3171 for dealing with this. The key distinction between systems that need
3172 C<binmode> and those that don't is their text file formats. Systems
3173 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3174 character, and which encode that character in C as C<"\n">, do not
3175 need C<binmode>. The rest need it.
3177 When opening a file, it's usually a bad idea to continue normal execution
3178 if the request failed, so C<open> is frequently used in connection with
3179 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3180 where you want to make a nicely formatted error message (but there are
3181 modules that can help with that problem)) you should always check
3182 the return value from opening a file. The infrequent exception is when
3183 working with an unopened filehandle is actually what you want to do.
3185 As a special case the 3-arg form with a read/write mode and the third
3186 argument being C<undef>:
3188 open(my $tmp, "+>", undef) or die ...
3190 opens a filehandle to an anonymous temporary file. Also using "+<"
3191 works for symmetry, but you really should consider writing something
3192 to the temporary file first. You will need to seek() to do the
3195 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3196 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3197 "in memory" files held in Perl scalars via:
3199 open($fh, '>', \$variable) || ..
3201 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3202 file, you have to close it first:
3205 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3210 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3211 while (<ARTICLE>) {...
3213 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3214 # if the open fails, output is discarded
3216 open(my $dbase, '+<', 'dbase.mine') # open for update
3217 or die "Can't open 'dbase.mine' for update: $!";
3219 open(my $dbase, '+<dbase.mine') # ditto
3220 or die "Can't open 'dbase.mine' for update: $!";
3222 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3223 or die "Can't start caesar: $!";
3225 open(ARTICLE, "caesar <$article |") # ditto
3226 or die "Can't start caesar: $!";
3228 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3229 or die "Can't start sort: $!";
3232 open(MEMORY,'>', \$var)
3233 or die "Can't open memory file: $!";
3234 print MEMORY "foo!\n"; # output will end up in $var
3236 # process argument list of files along with any includes
3238 foreach $file (@ARGV) {
3239 process($file, 'fh00');
3243 my($filename, $input) = @_;
3244 $input++; # this is a string increment
3245 unless (open($input, $filename)) {
3246 print STDERR "Can't open $filename: $!\n";
3251 while (<$input>) { # note use of indirection
3252 if (/^#include "(.*)"/) {
3253 process($1, $input);
3260 See L<perliol> for detailed info on PerlIO.
3262 You may also, in the Bourne shell tradition, specify an EXPR beginning
3263 with C<< '>&' >>, in which case the rest of the string is interpreted
3264 as the name of a filehandle (or file descriptor, if numeric) to be
3265 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3266 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3267 The mode you specify should match the mode of the original filehandle.
3268 (Duping a filehandle does not take into account any existing contents
3269 of IO buffers.) If you use the 3-arg form then you can pass either a
3270 number, the name of a filehandle or the normal "reference to a glob".
3272 Here is a script that saves, redirects, and restores C<STDOUT> and
3273 C<STDERR> using various methods:
3276 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3277 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3279 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3280 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3282 select STDERR; $| = 1; # make unbuffered
3283 select STDOUT; $| = 1; # make unbuffered
3285 print STDOUT "stdout 1\n"; # this works for
3286 print STDERR "stderr 1\n"; # subprocesses too
3288 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3289 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3291 print STDOUT "stdout 2\n";
3292 print STDERR "stderr 2\n";
3294 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3295 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3296 that file descriptor (and not call L<dup(2)>); this is more
3297 parsimonious of file descriptors. For example:
3299 # open for input, reusing the fileno of $fd
3300 open(FILEHANDLE, "<&=$fd")
3304 open(FILEHANDLE, "<&=", $fd)
3308 # open for append, using the fileno of OLDFH
3309 open(FH, ">>&=", OLDFH)
3313 open(FH, ">>&=OLDFH")
3315 Being parsimonious on filehandles is also useful (besides being
3316 parsimonious) for example when something is dependent on file
3317 descriptors, like for example locking using flock(). If you do just
3318 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3319 descriptor as B, and therefore flock(A) will not flock(B), and vice
3320 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3321 the same file descriptor.
3323 Note that if you are using Perls older than 5.8.0, Perl will be using
3324 the standard C libraries' fdopen() to implement the "=" functionality.
3325 On many UNIX systems fdopen() fails when file descriptors exceed a
3326 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3327 most often the default.
3329 You can see whether Perl has been compiled with PerlIO or not by
3330 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3331 is C<define>, you have PerlIO, otherwise you don't.
3333 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3334 with 2-arguments (or 1-argument) form of open(), then
3335 there is an implicit fork done, and the return value of open is the pid
3336 of the child within the parent process, and C<0> within the child
3337 process. (Use C<defined($pid)> to determine whether the open was successful.)
3338 The filehandle behaves normally for the parent, but i/o to that
3339 filehandle is piped from/to the STDOUT/STDIN of the child process.
3340 In the child process the filehandle isn't opened--i/o happens from/to
3341 the new STDOUT or STDIN. Typically this is used like the normal
3342 piped open when you want to exercise more control over just how the
3343 pipe command gets executed, such as when you are running setuid, and
3344 don't want to have to scan shell commands for metacharacters.
3345 The following triples are more or less equivalent:
3347 open(FOO, "|tr '[a-z]' '[A-Z]'");
3348 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3349 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3350 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3352 open(FOO, "cat -n '$file'|");
3353 open(FOO, '-|', "cat -n '$file'");
3354 open(FOO, '-|') || exec 'cat', '-n', $file;
3355 open(FOO, '-|', "cat", '-n', $file);
3357 The last example in each block shows the pipe as "list form", which is
3358 not yet supported on all platforms. A good rule of thumb is that if
3359 your platform has true C<fork()> (in other words, if your platform is
3360 UNIX) you can use the list form.
3362 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3364 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3365 output before any operation that may do a fork, but this may not be
3366 supported on some platforms (see L<perlport>). To be safe, you may need
3367 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3368 of C<IO::Handle> on any open handles.
3370 On systems that support a close-on-exec flag on files, the flag will
3371 be set for the newly opened file descriptor as determined by the value
3372 of $^F. See L<perlvar/$^F>.
3374 Closing any piped filehandle causes the parent process to wait for the
3375 child to finish, and returns the status value in C<$?> and
3376 C<${^CHILD_ERROR_NATIVE}>.
3378 The filename passed to 2-argument (or 1-argument) form of open() will
3379 have leading and trailing whitespace deleted, and the normal
3380 redirection characters honored. This property, known as "magic open",
3381 can often be used to good effect. A user could specify a filename of
3382 F<"rsh cat file |">, or you could change certain filenames as needed:
3384 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3385 open(FH, $filename) or die "Can't open $filename: $!";
3387 Use 3-argument form to open a file with arbitrary weird characters in it,
3389 open(FOO, '<', $file);
3391 otherwise it's necessary to protect any leading and trailing whitespace:
3393 $file =~ s#^(\s)#./$1#;
3394 open(FOO, "< $file\0");
3396 (this may not work on some bizarre filesystems). One should
3397 conscientiously choose between the I<magic> and 3-arguments form
3402 will allow the user to specify an argument of the form C<"rsh cat file |">,
3403 but will not work on a filename which happens to have a trailing space, while
3405 open IN, '<', $ARGV[0];
3407 will have exactly the opposite restrictions.
3409 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3410 should use the C<sysopen> function, which involves no such magic (but
3411 may use subtly different filemodes than Perl open(), which is mapped
3412 to C fopen()). This is
3413 another way to protect your filenames from interpretation. For example:
3416 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3417 or die "sysopen $path: $!";
3418 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3419 print HANDLE "stuff $$\n";
3421 print "File contains: ", <HANDLE>;
3423 Using the constructor from the C<IO::Handle> package (or one of its
3424 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3425 filehandles that have the scope of whatever variables hold references to
3426 them, and automatically close whenever and however you leave that scope:
3430 sub read_myfile_munged {
3432 my $handle = IO::File->new;
3433 open($handle, "myfile") or die "myfile: $!";
3435 or return (); # Automatically closed here.
3436 mung $first or die "mung failed"; # Or here.
3437 return $first, <$handle> if $ALL; # Or here.
3441 See L</seek> for some details about mixing reading and writing.
3443 =item opendir DIRHANDLE,EXPR
3446 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3447 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3448 DIRHANDLE may be an expression whose value can be used as an indirect
3449 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3450 scalar variable (or array or hash element), the variable is assigned a
3451 reference to a new anonymous dirhandle.
3452 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3454 See example at C<readdir>.
3461 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3462 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3465 For the reverse, see L</chr>.
3466 See L<perlunicode> for more about Unicode.
3473 =item our EXPR : ATTRS
3475 =item our TYPE EXPR : ATTRS
3477 C<our> associates a simple name with a package variable in the current
3478 package for use within the current scope. When C<use strict 'vars'> is in
3479 effect, C<our> lets you use declared global variables without qualifying
3480 them with package names, within the lexical scope of the C<our> declaration.
3481 In this way C<our> differs from C<use vars>, which is package scoped.
3483 Unlike C<my>, which both allocates storage for a variable and associates
3484 a simple name with that storage for use within the current scope, C<our>
3485 associates a simple name with a package variable in the current package,
3486 for use within the current scope. In other words, C<our> has the same
3487 scoping rules as C<my>, but does not necessarily create a
3490 If more than one value is listed, the list must be placed
3496 An C<our> declaration declares a global variable that will be visible
3497 across its entire lexical scope, even across package boundaries. The
3498 package in which the variable is entered is determined at the point
3499 of the declaration, not at the point of use. This means the following
3503 our $bar; # declares $Foo::bar for rest of lexical scope
3507 print $bar; # prints 20, as it refers to $Foo::bar
3509 Multiple C<our> declarations with the same name in the same lexical
3510 scope are allowed if they are in different packages. If they happen
3511 to be in the same package, Perl will emit warnings if you have asked
3512 for them, just like multiple C<my> declarations. Unlike a second
3513 C<my> declaration, which will bind the name to a fresh variable, a
3514 second C<our> declaration in the same package, in the same scope, is
3519 our $bar; # declares $Foo::bar for rest of lexical scope
3523 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3524 print $bar; # prints 30
3526 our $bar; # emits warning but has no other effect
3527 print $bar; # still prints 30
3529 An C<our> declaration may also have a list of attributes associated
3532 The exact semantics and interface of TYPE and ATTRS are still
3533 evolving. TYPE is currently bound to the use of C<fields> pragma,
3534 and attributes are handled using the C<attributes> pragma, or starting
3535 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3536 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3537 L<attributes>, and L<Attribute::Handlers>.
3539 =item pack TEMPLATE,LIST
3542 Takes a LIST of values and converts it into a string using the rules
3543 given by the TEMPLATE. The resulting string is the concatenation of
3544 the converted values. Typically, each converted value looks
3545 like its machine-level representation. For example, on 32-bit machines
3546 an integer may be represented by a sequence of 4 bytes that will be
3547 converted to a sequence of 4 characters.
3549 The TEMPLATE is a sequence of characters that give the order and type
3550 of values, as follows:
3552 a A string with arbitrary binary data, will be null padded.
3553 A A text (ASCII) string, will be space padded.
3554 Z A null terminated (ASCIZ) string, will be null padded.
3556 b A bit string (ascending bit order inside each byte, like vec()).
3557 B A bit string (descending bit order inside each byte).
3558 h A hex string (low nybble first).
3559 H A hex string (high nybble first).
3561 c A signed char (8-bit) value.
3562 C An unsigned char (octet) value.
3563 W An unsigned char value (can be greater than 255).
3565 s A signed short (16-bit) value.
3566 S An unsigned short value.
3568 l A signed long (32-bit) value.
3569 L An unsigned long value.
3571 q A signed quad (64-bit) value.
3572 Q An unsigned quad value.
3573 (Quads are available only if your system supports 64-bit
3574 integer values _and_ if Perl has been compiled to support those.
3575 Causes a fatal error otherwise.)
3577 i A signed integer value.
3578 I A unsigned integer value.
3579 (This 'integer' is _at_least_ 32 bits wide. Its exact
3580 size depends on what a local C compiler calls 'int'.)
3582 n An unsigned short (16-bit) in "network" (big-endian) order.
3583 N An unsigned long (32-bit) in "network" (big-endian) order.
3584 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3585 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3587 j A Perl internal signed integer value (IV).
3588 J A Perl internal unsigned integer value (UV).
3590 f A single-precision float in the native format.
3591 d A double-precision float in the native format.
3593 F A Perl internal floating point value (NV) in the native format
3594 D A long double-precision float in the native format.
3595 (Long doubles are available only if your system supports long
3596 double values _and_ if Perl has been compiled to support those.
3597 Causes a fatal error otherwise.)
3599 p A pointer to a null-terminated string.
3600 P A pointer to a structure (fixed-length string).
3602 u A uuencoded string.
3603 U A Unicode character number. Encodes to a character in character mode
3604 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3606 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3607 details). Its bytes represent an unsigned integer in base 128,
3608 most significant digit first, with as few digits as possible. Bit
3609 eight (the high bit) is set on each byte except the last.
3613 @ Null fill or truncate to absolute position, counted from the
3614 start of the innermost ()-group.
3615 . Null fill or truncate to absolute position specified by value.
3616 ( Start of a ()-group.
3618 One or more of the modifiers below may optionally follow some letters in the
3619 TEMPLATE (the second column lists the letters for which the modifier is
3622 ! sSlLiI Forces native (short, long, int) sizes instead
3623 of fixed (16-/32-bit) sizes.
3625 xX Make x and X act as alignment commands.
3627 nNvV Treat integers as signed instead of unsigned.
3629 @. Specify position as byte offset in the internal
3630 representation of the packed string. Efficient but
3633 > sSiIlLqQ Force big-endian byte-order on the type.
3634 jJfFdDpP (The "big end" touches the construct.)
3636 < sSiIlLqQ Force little-endian byte-order on the type.
3637 jJfFdDpP (The "little end" touches the construct.)
3639 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3640 in which case they force a certain byte-order on all components of
3641 that group, including subgroups.
3643 The following rules apply:
3649 Each letter may optionally be followed by a number giving a repeat
3650 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3651 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3652 that many values from the LIST. A C<*> for the repeat count means to
3653 use however many items are left, except for C<@>, C<x>, C<X>, where it
3654 is equivalent to C<0>, for <.> where it means relative to string start
3655 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3656 A numeric repeat count may optionally be enclosed in brackets, as in
3657 C<pack 'C[80]', @arr>.
3659 One can replace the numeric repeat count by a template enclosed in brackets;
3660 then the packed length of this template in bytes is used as a count.
3661 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3662 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3663 If the template in brackets contains alignment commands (such as C<x![d]>),
3664 its packed length is calculated as if the start of the template has the maximal
3667 When used with C<Z>, C<*> results in the addition of a trailing null
3668 byte (so the packed result will be one longer than the byte C<length>
3671 When used with C<@>, the repeat count represents an offset from the start
3672 of the innermost () group.
3674 When used with C<.>, the repeat count is used to determine the starting
3675 position from where the value offset is calculated. If the repeat count
3676 is 0, it's relative to the current position. If the repeat count is C<*>,
3677 the offset is relative to the start of the packed string. And if its an
3678 integer C<n> the offset is relative to the start of the n-th innermost
3679 () group (or the start of the string if C<n> is bigger then the group
3682 The repeat count for C<u> is interpreted as the maximal number of bytes
3683 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3684 count should not be more than 65.
3688 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3689 string of length count, padding with nulls or spaces as necessary. When
3690 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3691 after the first null, and C<a> returns data verbatim.
3693 If the value-to-pack is too long, it is truncated. If too long and an
3694 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3695 by a null byte. Thus C<Z> always packs a trailing null (except when the
3700 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3701 Each character of the input field of pack() generates 1 bit of the result.
3702 Each result bit is based on the least-significant bit of the corresponding
3703 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3704 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3706 Starting from the beginning of the input string of pack(), each 8-tuple
3707 of characters is converted to 1 character of output. With format C<b>
3708 the first character of the 8-tuple determines the least-significant bit of a
3709 character, and with format C<B> it determines the most-significant bit of
3712 If the length of the input string is not exactly divisible by 8, the
3713 remainder is packed as if the input string were padded by null characters
3714 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3716 If the input string of pack() is longer than needed, extra characters are
3717 ignored. A C<*> for the repeat count of pack() means to use all the
3718 characters of the input field. On unpack()ing the bits are converted to a
3719 string of C<"0">s and C<"1">s.
3723 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3724 representable as hexadecimal digits, 0-9a-f) long.
3726 Each character of the input field of pack() generates 4 bits of the result.
3727 For non-alphabetical characters the result is based on the 4 least-significant
3728 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3729 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3730 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3731 is compatible with the usual hexadecimal digits, so that C<"a"> and
3732 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3733 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3735 Starting from the beginning of the input string of pack(), each pair
3736 of characters is converted to 1 character of output. With format C<h> the
3737 first character of the pair determines the least-significant nybble of the
3738 output character, and with format C<H> it determines the most-significant
3741 If the length of the input string is not even, it behaves as if padded
3742 by a null character at the end. Similarly, during unpack()ing the "extra"
3743 nybbles are ignored.
3745 If the input string of pack() is longer than needed, extra characters are
3747 A C<*> for the repeat count of pack() means to use all the characters of
3748 the input field. On unpack()ing the nybbles are converted to a string
3749 of hexadecimal digits.
3753 The C<p> type packs a pointer to a null-terminated string. You are
3754 responsible for ensuring the string is not a temporary value (which can
3755 potentially get deallocated before you get around to using the packed result).
3756 The C<P> type packs a pointer to a structure of the size indicated by the
3757 length. A NULL pointer is created if the corresponding value for C<p> or
3758 C<P> is C<undef>, similarly for unpack().
3760 If your system has a strange pointer size (i.e. a pointer is neither as
3761 big as an int nor as big as a long), it may not be possible to pack or
3762 unpack pointers in big- or little-endian byte order. Attempting to do
3763 so will result in a fatal error.
3767 The C</> template character allows packing and unpacking of a sequence of
3768 items where the packed structure contains a packed item count followed by
3769 the packed items themselves.
3771 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3772 I<length-item> describes how the length value is packed. The ones likely
3773 to be of most use are integer-packing ones like C<n> (for Java strings),
3774 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3776 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3777 the minimum of that and the number of available items is used as argument
3778 for the I<length-item>. If it has no repeat count or uses a '*', the number
3779 of available items is used.
3781 For C<unpack> an internal stack of integer arguments unpacked so far is
3782 used. You write C</>I<sequence-item> and the repeat count is obtained by
3783 popping off the last element from the stack. The I<sequence-item> must not
3784 have a repeat count.
3786 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3787 the I<length-item> is a string length, not a number of strings. If there is
3788 an explicit repeat count for pack, the packed string will be adjusted to that
3791 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3792 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3793 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3794 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3795 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3797 The I<length-item> is not returned explicitly from C<unpack>.
3799 Adding a count to the I<length-item> letter is unlikely to do anything
3800 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3801 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3802 which Perl does not regard as legal in numeric strings.
3806 The integer types C<s>, C<S>, C<l>, and C<L> may be
3807 followed by a C<!> modifier to signify native shorts or
3808 longs--as you can see from above for example a bare C<l> does mean
3809 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3810 may be larger. This is an issue mainly in 64-bit platforms. You can
3811 see whether using C<!> makes any difference by
3813 print length(pack("s")), " ", length(pack("s!")), "\n";
3814 print length(pack("l")), " ", length(pack("l!")), "\n";
3816 C<i!> and C<I!> also work but only because of completeness;
3817 they are identical to C<i> and C<I>.
3819 The actual sizes (in bytes) of native shorts, ints, longs, and long
3820 longs on the platform where Perl was built are also available via
3824 print $Config{shortsize}, "\n";
3825 print $Config{intsize}, "\n";
3826 print $Config{longsize}, "\n";
3827 print $Config{longlongsize}, "\n";
3829 (The C<$Config{longlongsize}> will be undefined if your system does
3830 not support long longs.)
3834 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3835 are inherently non-portable between processors and operating systems
3836 because they obey the native byteorder and endianness. For example a
3837 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3838 (arranged in and handled by the CPU registers) into bytes as
3840 0x12 0x34 0x56 0x78 # big-endian
3841 0x78 0x56 0x34 0x12 # little-endian
3843 Basically, the Intel and VAX CPUs are little-endian, while everybody
3844 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3845 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3846 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3849 The names `big-endian' and `little-endian' are comic references to
3850 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3851 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3852 the egg-eating habits of the Lilliputians.
3854 Some systems may have even weirder byte orders such as
3859 You can see your system's preference with
3861 print join(" ", map { sprintf "%#02x", $_ }
3862 unpack("W*",pack("L",0x12345678))), "\n";
3864 The byteorder on the platform where Perl was built is also available
3868 print $Config{byteorder}, "\n";
3870 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3871 and C<'87654321'> are big-endian.
3873 If you want portable packed integers you can either use the formats
3874 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3875 modifiers. These modifiers are only available as of perl 5.9.2.
3876 See also L<perlport>.
3880 All integer and floating point formats as well as C<p> and C<P> and
3881 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3882 to force big- or little- endian byte-order, respectively.
3883 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3884 signed integers, 64-bit integers and floating point values. However,
3885 there are some things to keep in mind.
3887 Exchanging signed integers between different platforms only works
3888 if all platforms store them in the same format. Most platforms store
3889 signed integers in two's complement, so usually this is not an issue.
3891 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3892 formats on big- or little-endian machines. Otherwise, attempting to
3893 do so will result in a fatal error.
3895 Forcing big- or little-endian byte-order on floating point values for
3896 data exchange can only work if all platforms are using the same
3897 binary representation (e.g. IEEE floating point format). Even if all
3898 platforms are using IEEE, there may be subtle differences. Being able
3899 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3900 but also very dangerous if you don't know exactly what you're doing.
3901 It is definitely not a general way to portably store floating point
3904 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3905 all types inside the group that accept the byte-order modifiers,
3906 including all subgroups. It will silently be ignored for all other
3907 types. You are not allowed to override the byte-order within a group
3908 that already has a byte-order modifier suffix.
3912 Real numbers (floats and doubles) are in the native machine format only;
3913 due to the multiplicity of floating formats around, and the lack of a
3914 standard "network" representation, no facility for interchange has been
3915 made. This means that packed floating point data written on one machine
3916 may not be readable on another - even if both use IEEE floating point
3917 arithmetic (as the endian-ness of the memory representation is not part
3918 of the IEEE spec). See also L<perlport>.
3920 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3921 modifiers to force big- or little-endian byte-order on floating point values.
3923 Note that Perl uses doubles (or long doubles, if configured) internally for
3924 all numeric calculation, and converting from double into float and thence back
3925 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3926 will not in general equal $foo).
3930 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3931 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3932 where the packed string is processed in its UTF-8-encoded Unicode form on
3933 a byte by byte basis. Character mode is the default unless the format string
3934 starts with an C<U>. You can switch mode at any moment with an explicit
3935 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3936 or until the end of the ()-group in which it was entered.
3940 You must yourself do any alignment or padding by inserting for example
3941 enough C<'x'>es while packing. There is no way to pack() and unpack()
3942 could know where the characters are going to or coming from. Therefore
3943 C<pack> (and C<unpack>) handle their output and input as flat
3944 sequences of characters.
3948 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3949 take a repeat count, both as postfix, and for unpack() also via the C</>
3950 template character. Within each repetition of a group, positioning with
3951 C<@> starts again at 0. Therefore, the result of
3953 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3955 is the string "\0a\0\0bc".
3959 C<x> and C<X> accept C<!> modifier. In this case they act as
3960 alignment commands: they jump forward/back to the closest position
3961 aligned at a multiple of C<count> characters. For example, to pack() or
3962 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3963 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3964 aligned on the double's size.
3966 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3967 both result in no-ops.
3971 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3972 will represent signed 16-/32-bit integers in big-/little-endian order.
3973 This is only portable if all platforms sharing the packed data use the
3974 same binary representation for signed integers (e.g. all platforms are
3975 using two's complement representation).
3979 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3980 White space may be used to separate pack codes from each other, but
3981 modifiers and a repeat count must follow immediately.
3985 If TEMPLATE requires more arguments to pack() than actually given, pack()
3986 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3987 to pack() than actually given, extra arguments are ignored.
3993 $foo = pack("WWWW",65,66,67,68);
3995 $foo = pack("W4",65,66,67,68);
3997 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3998 # same thing with Unicode circled letters.
3999 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4000 # same thing with Unicode circled letters. You don't get the UTF-8
4001 # bytes because the U at the start of the format caused a switch to
4002 # U0-mode, so the UTF-8 bytes get joined into characters
4003 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4004 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4005 # This is the UTF-8 encoding of the string in the previous example
4007 $foo = pack("ccxxcc",65,66,67,68);
4010 # note: the above examples featuring "W" and "c" are true
4011 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4012 # and UTF-8. In EBCDIC the first example would be
4013 # $foo = pack("WWWW",193,194,195,196);
4015 $foo = pack("s2",1,2);
4016 # "\1\0\2\0" on little-endian
4017 # "\0\1\0\2" on big-endian
4019 $foo = pack("a4","abcd","x","y","z");
4022 $foo = pack("aaaa","abcd","x","y","z");
4025 $foo = pack("a14","abcdefg");
4026 # "abcdefg\0\0\0\0\0\0\0"
4028 $foo = pack("i9pl", gmtime);
4029 # a real struct tm (on my system anyway)
4031 $utmp_template = "Z8 Z8 Z16 L";
4032 $utmp = pack($utmp_template, @utmp1);
4033 # a struct utmp (BSDish)
4035 @utmp2 = unpack($utmp_template, $utmp);
4036 # "@utmp1" eq "@utmp2"
4039 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4042 $foo = pack('sx2l', 12, 34);
4043 # short 12, two zero bytes padding, long 34
4044 $bar = pack('s@4l', 12, 34);
4045 # short 12, zero fill to position 4, long 34
4047 $baz = pack('s.l', 12, 4, 34);
4048 # short 12, zero fill to position 4, long 34
4050 $foo = pack('nN', 42, 4711);
4051 # pack big-endian 16- and 32-bit unsigned integers
4052 $foo = pack('S>L>', 42, 4711);
4054 $foo = pack('s<l<', -42, 4711);
4055 # pack little-endian 16- and 32-bit signed integers
4056 $foo = pack('(sl)<', -42, 4711);
4059 The same template may generally also be used in unpack().
4061 =item package NAMESPACE
4062 X<package> X<module> X<namespace>
4066 Declares the compilation unit as being in the given namespace. The scope
4067 of the package declaration is from the declaration itself through the end
4068 of the enclosing block, file, or eval (the same as the C<my> operator).
4069 All further unqualified dynamic identifiers will be in this namespace.
4070 A package statement affects only dynamic variables--including those
4071 you've used C<local> on--but I<not> lexical variables, which are created
4072 with C<my>. Typically it would be the first declaration in a file to
4073 be included by the C<require> or C<use> operator. You can switch into a
4074 package in more than one place; it merely influences which symbol table
4075 is used by the compiler for the rest of that block. You can refer to
4076 variables and filehandles in other packages by prefixing the identifier
4077 with the package name and a double colon: C<$Package::Variable>.
4078 If the package name is null, the C<main> package as assumed. That is,
4079 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4080 still seen in older code).
4082 See L<perlmod/"Packages"> for more information about packages, modules,
4083 and classes. See L<perlsub> for other scoping issues.
4085 =item pipe READHANDLE,WRITEHANDLE
4088 Opens a pair of connected pipes like the corresponding system call.
4089 Note that if you set up a loop of piped processes, deadlock can occur
4090 unless you are very careful. In addition, note that Perl's pipes use
4091 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4092 after each command, depending on the application.
4094 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4095 for examples of such things.
4097 On systems that support a close-on-exec flag on files, the flag will be set
4098 for the newly opened file descriptors as determined by the value of $^F.
4106 Pops and returns the last value of the array, shortening the array by
4109 If there are no elements in the array, returns the undefined value
4110 (although this may happen at other times as well). If ARRAY is
4111 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4112 array in subroutines, just like C<shift>.
4115 X<pos> X<match, position>
4119 Returns the offset of where the last C<m//g> search left off for the variable
4120 in question (C<$_> is used when the variable is not specified). Note that
4121 0 is a valid match offset. C<undef> indicates that the search position
4122 is reset (usually due to match failure, but can also be because no match has
4123 yet been performed on the scalar). C<pos> directly accesses the location used
4124 by the regexp engine to store the offset, so assigning to C<pos> will change
4125 that offset, and so will also influence the C<\G> zero-width assertion in
4126 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4127 the return from C<pos> won't change either in this case. See L<perlre> and
4130 =item print FILEHANDLE LIST
4137 Prints a string or a list of strings. Returns true if successful.
4138 FILEHANDLE may be a scalar variable name, in which case the variable
4139 contains the name of or a reference to the filehandle, thus introducing
4140 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4141 the next token is a term, it may be misinterpreted as an operator
4142 unless you interpose a C<+> or put parentheses around the arguments.)
4143 If FILEHANDLE is omitted, prints by default to standard output (or
4144 to the last selected output channel--see L</select>). If LIST is
4145 also omitted, prints C<$_> to the currently selected output channel.
4146 To set the default output channel to something other than STDOUT
4147 use the select operation. The current value of C<$,> (if any) is
4148 printed between each LIST item. The current value of C<$\> (if
4149 any) is printed after the entire LIST has been printed. Because
4150 print takes a LIST, anything in the LIST is evaluated in list
4151 context, and any subroutine that you call will have one or more of
4152 its expressions evaluated in list context. Also be careful not to
4153 follow the print keyword with a left parenthesis unless you want
4154 the corresponding right parenthesis to terminate the arguments to
4155 the print--interpose a C<+> or put parentheses around all the
4158 Note that if you're storing FILEHANDLEs in an array, or if you're using
4159 any other expression more complex than a scalar variable to retrieve it,
4160 you will have to use a block returning the filehandle value instead:
4162 print { $files[$i] } "stuff\n";
4163 print { $OK ? STDOUT : STDERR } "stuff\n";
4165 =item printf FILEHANDLE FORMAT, LIST
4168 =item printf FORMAT, LIST
4170 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4171 (the output record separator) is not appended. The first argument
4172 of the list will be interpreted as the C<printf> format. See C<sprintf>
4173 for an explanation of the format argument. If C<use locale> is in effect,
4174 and POSIX::setlocale() has been called, the character used for the decimal
4175 separator in formatted floating point numbers is affected by the LC_NUMERIC
4176 locale. See L<perllocale> and L<POSIX>.
4178 Don't fall into the trap of using a C<printf> when a simple
4179 C<print> would do. The C<print> is more efficient and less
4182 =item prototype FUNCTION
4185 Returns the prototype of a function as a string (or C<undef> if the
4186 function has no prototype). FUNCTION is a reference to, or the name of,
4187 the function whose prototype you want to retrieve.
4189 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4190 name for Perl builtin. If the builtin is not I<overridable> (such as
4191 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4192 (such as C<system>), prototype() returns C<undef>, because the builtin
4193 does not really behave like a Perl function. Otherwise, the string
4194 describing the equivalent prototype is returned.
4196 =item push ARRAY,LIST
4199 Treats ARRAY as a stack, and pushes the values of LIST
4200 onto the end of ARRAY. The length of ARRAY increases by the length of
4201 LIST. Has the same effect as
4204 $ARRAY[++$#ARRAY] = $value;
4207 but is more efficient. Returns the number of elements in the array following
4208 the completed C<push>.
4220 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4222 =item quotemeta EXPR
4223 X<quotemeta> X<metacharacter>
4227 Returns the value of EXPR with all non-"word"
4228 characters backslashed. (That is, all characters not matching
4229 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4230 returned string, regardless of any locale settings.)
4231 This is the internal function implementing
4232 the C<\Q> escape in double-quoted strings.
4234 If EXPR is omitted, uses C<$_>.
4241 Returns a random fractional number greater than or equal to C<0> and less
4242 than the value of EXPR. (EXPR should be positive.) If EXPR is
4243 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4244 also special-cased as C<1> - this has not been documented before perl 5.8.0
4245 and is subject to change in future versions of perl. Automatically calls
4246 C<srand> unless C<srand> has already been called. See also C<srand>.
4248 Apply C<int()> to the value returned by C<rand()> if you want random
4249 integers instead of random fractional numbers. For example,
4253 returns a random integer between C<0> and C<9>, inclusive.
4255 (Note: If your rand function consistently returns numbers that are too
4256 large or too small, then your version of Perl was probably compiled
4257 with the wrong number of RANDBITS.)
4259 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4260 X<read> X<file, read>
4262 =item read FILEHANDLE,SCALAR,LENGTH
4264 Attempts to read LENGTH I<characters> of data into variable SCALAR
4265 from the specified FILEHANDLE. Returns the number of characters
4266 actually read, C<0> at end of file, or undef if there was an error (in
4267 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4268 so that the last character actually read is the last character of the
4269 scalar after the read.
4271 An OFFSET may be specified to place the read data at some place in the
4272 string other than the beginning. A negative OFFSET specifies
4273 placement at that many characters counting backwards from the end of
4274 the string. A positive OFFSET greater than the length of SCALAR
4275 results in the string being padded to the required size with C<"\0">
4276 bytes before the result of the read is appended.
4278 The call is actually implemented in terms of either Perl's or system's
4279 fread() call. To get a true read(2) system call, see C<sysread>.
4281 Note the I<characters>: depending on the status of the filehandle,
4282 either (8-bit) bytes or characters are read. By default all
4283 filehandles operate on bytes, but for example if the filehandle has
4284 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4285 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4286 characters, not bytes. Similarly for the C<:encoding> pragma:
4287 in that case pretty much any characters can be read.
4289 =item readdir DIRHANDLE
4292 Returns the next directory entry for a directory opened by C<opendir>.
4293 If used in list context, returns all the rest of the entries in the
4294 directory. If there are no more entries, returns an undefined value in
4295 scalar context or a null list in list context.
4297 If you're planning to filetest the return values out of a C<readdir>, you'd
4298 better prepend the directory in question. Otherwise, because we didn't
4299 C<chdir> there, it would have been testing the wrong file.
4301 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4302 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4308 X<readline> X<gets> X<fgets>
4310 Reads from the filehandle whose typeglob is contained in EXPR (or from
4311 *ARGV if EXPR is not provided). In scalar context, each call reads and
4312 returns the next line, until end-of-file is reached, whereupon the
4313 subsequent call returns undef. In list context, reads until end-of-file
4314 is reached and returns a list of lines. Note that the notion of "line"
4315 used here is however you may have defined it with C<$/> or
4316 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4318 When C<$/> is set to C<undef>, when readline() is in scalar
4319 context (i.e. file slurp mode), and when an empty file is read, it
4320 returns C<''> the first time, followed by C<undef> subsequently.
4322 This is the internal function implementing the C<< <EXPR> >>
4323 operator, but you can use it directly. The C<< <EXPR> >>
4324 operator is discussed in more detail in L<perlop/"I/O Operators">.
4327 $line = readline(*STDIN); # same thing
4329 If readline encounters an operating system error, C<$!> will be set with the
4330 corresponding error message. It can be helpful to check C<$!> when you are
4331 reading from filehandles you don't trust, such as a tty or a socket. The
4332 following example uses the operator form of C<readline>, and takes the necessary
4333 steps to ensure that C<readline> was successful.
4337 unless (defined( $line = <> )) {
4349 Returns the value of a symbolic link, if symbolic links are
4350 implemented. If not, gives a fatal error. If there is some system
4351 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4352 omitted, uses C<$_>.
4359 EXPR is executed as a system command.
4360 The collected standard output of the command is returned.
4361 In scalar context, it comes back as a single (potentially
4362 multi-line) string. In list context, returns a list of lines
4363 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4364 This is the internal function implementing the C<qx/EXPR/>
4365 operator, but you can use it directly. The C<qx/EXPR/>
4366 operator is discussed in more detail in L<perlop/"I/O Operators">.
4367 If EXPR is omitted, uses C<$_>.
4369 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4372 Receives a message on a socket. Attempts to receive LENGTH characters
4373 of data into variable SCALAR from the specified SOCKET filehandle.
4374 SCALAR will be grown or shrunk to the length actually read. Takes the
4375 same flags as the system call of the same name. Returns the address
4376 of the sender if SOCKET's protocol supports this; returns an empty
4377 string otherwise. If there's an error, returns the undefined value.
4378 This call is actually implemented in terms of recvfrom(2) system call.
4379 See L<perlipc/"UDP: Message Passing"> for examples.
4381 Note the I<characters>: depending on the status of the socket, either
4382 (8-bit) bytes or characters are received. By default all sockets
4383 operate on bytes, but for example if the socket has been changed using
4384 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4385 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4386 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4387 case pretty much any characters can be read.
4394 The C<redo> command restarts the loop block without evaluating the
4395 conditional again. The C<continue> block, if any, is not executed. If
4396 the LABEL is omitted, the command refers to the innermost enclosing
4397 loop. Programs that want to lie to themselves about what was just input
4398 normally use this command:
4400 # a simpleminded Pascal comment stripper
4401 # (warning: assumes no { or } in strings)
4402 LINE: while (<STDIN>) {
4403 while (s|({.*}.*){.*}|$1 |) {}
4408 if (/}/) { # end of comment?
4417 C<redo> cannot be used to retry a block which returns a value such as
4418 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4419 a grep() or map() operation.
4421 Note that a block by itself is semantically identical to a loop
4422 that executes once. Thus C<redo> inside such a block will effectively
4423 turn it into a looping construct.
4425 See also L</continue> for an illustration of how C<last>, C<next>, and
4433 Returns a non-empty string if EXPR is a reference, the empty
4434 string otherwise. If EXPR
4435 is not specified, C<$_> will be used. The value returned depends on the
4436 type of thing the reference is a reference to.
4437 Builtin types include:
4451 If the referenced object has been blessed into a package, then that package
4452 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4454 if (ref($r) eq "HASH") {
4455 print "r is a reference to a hash.\n";
4458 print "r is not a reference at all.\n";
4461 The return value C<LVALUE> indicates a reference to an lvalue that is not
4462 a variable. You get this from taking the reference of function calls like
4463 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4464 to a L<version string|perldata/"Version Strings">.
4466 The result C<Regexp> indicates that the argument is a regular expression
4467 resulting from C<qr//>.
4469 See also L<perlref>.
4471 =item rename OLDNAME,NEWNAME
4472 X<rename> X<move> X<mv> X<ren>
4474 Changes the name of a file; an existing file NEWNAME will be
4475 clobbered. Returns true for success, false otherwise.
4477 Behavior of this function varies wildly depending on your system
4478 implementation. For example, it will usually not work across file system
4479 boundaries, even though the system I<mv> command sometimes compensates
4480 for this. Other restrictions include whether it works on directories,
4481 open files, or pre-existing files. Check L<perlport> and either the
4482 rename(2) manpage or equivalent system documentation for details.
4484 For a platform independent C<move> function look at the L<File::Copy>
4487 =item require VERSION
4494 Demands a version of Perl specified by VERSION, or demands some semantics
4495 specified by EXPR or by C<$_> if EXPR is not supplied.
4497 VERSION may be either a numeric argument such as 5.006, which will be
4498 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4499 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4500 VERSION is greater than the version of the current Perl interpreter.
4501 Compare with L</use>, which can do a similar check at compile time.
4503 Specifying VERSION as a literal of the form v5.6.1 should generally be
4504 avoided, because it leads to misleading error messages under earlier
4505 versions of Perl that do not support this syntax. The equivalent numeric
4506 version should be used instead.
4508 require v5.6.1; # run time version check
4509 require 5.6.1; # ditto
4510 require 5.006_001; # ditto; preferred for backwards compatibility
4512 Otherwise, C<require> demands that a library file be included if it
4513 hasn't already been included. The file is included via the do-FILE
4514 mechanism, which is essentially just a variety of C<eval> with the
4515 caveat that lexical variables in the invoking script will be invisible
4516 to the included code. Has semantics similar to the following subroutine:
4519 my ($filename) = @_;
4520 if (exists $INC{$filename}) {
4521 return 1 if $INC{$filename};
4522 die "Compilation failed in require";
4524 my ($realfilename,$result);
4526 foreach $prefix (@INC) {
4527 $realfilename = "$prefix/$filename";
4528 if (-f $realfilename) {
4529 $INC{$filename} = $realfilename;
4530 $result = do $realfilename;
4534 die "Can't find $filename in \@INC";
4537 $INC{$filename} = undef;
4539 } elsif (!$result) {
4540 delete $INC{$filename};
4541 die "$filename did not return true value";
4547 Note that the file will not be included twice under the same specified
4550 The file must return true as the last statement to indicate
4551 successful execution of any initialization code, so it's customary to
4552 end such a file with C<1;> unless you're sure it'll return true
4553 otherwise. But it's better just to put the C<1;>, in case you add more
4556 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4557 replaces "F<::>" with "F</>" in the filename for you,
4558 to make it easy to load standard modules. This form of loading of
4559 modules does not risk altering your namespace.
4561 In other words, if you try this:
4563 require Foo::Bar; # a splendid bareword
4565 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4566 directories specified in the C<@INC> array.
4568 But if you try this:
4570 $class = 'Foo::Bar';
4571 require $class; # $class is not a bareword
4573 require "Foo::Bar"; # not a bareword because of the ""
4575 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4576 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4578 eval "require $class";
4580 Now that you understand how C<require> looks for files in the case of a
4581 bareword argument, there is a little extra functionality going on behind
4582 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4583 first look for a similar filename with a "F<.pmc>" extension. If this file
4584 is found, it will be loaded in place of any file ending in a "F<.pm>"
4587 You can also insert hooks into the import facility, by putting directly
4588 Perl code into the @INC array. There are three forms of hooks: subroutine
4589 references, array references and blessed objects.
4591 Subroutine references are the simplest case. When the inclusion system
4592 walks through @INC and encounters a subroutine, this subroutine gets
4593 called with two parameters, the first being a reference to itself, and the
4594 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4595 subroutine should return nothing, or a list of up to three values in the
4602 A filehandle, from which the file will be read.
4606 A reference to a subroutine. If there is no filehandle (previous item),
4607 then this subroutine is expected to generate one line of source code per
4608 call, writing the line into C<$_> and returning 1, then returning 0 at
4609 "end of file". If there is a filehandle, then the subroutine will be
4610 called to act a simple source filter, with the line as read in C<$_>.
4611 Again, return 1 for each valid line, and 0 after all lines have been
4616 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4617 reference to the subroutine itself is passed in as C<$_[0]>.
4621 If an empty list, C<undef>, or nothing that matches the first 3 values above
4622 is returned then C<require> will look at the remaining elements of @INC.
4623 Note that this file handle must be a real file handle (strictly a typeglob,
4624 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4625 ignored and return value processing will stop there.
4627 If the hook is an array reference, its first element must be a subroutine
4628 reference. This subroutine is called as above, but the first parameter is
4629 the array reference. This enables to pass indirectly some arguments to
4632 In other words, you can write:
4634 push @INC, \&my_sub;
4636 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4642 push @INC, [ \&my_sub, $x, $y, ... ];
4644 my ($arrayref, $filename) = @_;
4645 # Retrieve $x, $y, ...
4646 my @parameters = @$arrayref[1..$#$arrayref];
4650 If the hook is an object, it must provide an INC method that will be
4651 called as above, the first parameter being the object itself. (Note that
4652 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4653 into package C<main>.) Here is a typical code layout:
4659 my ($self, $filename) = @_;
4663 # In the main program
4664 push @INC, new Foo(...);
4666 Note that these hooks are also permitted to set the %INC entry
4667 corresponding to the files they have loaded. See L<perlvar/%INC>.
4669 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4676 Generally used in a C<continue> block at the end of a loop to clear
4677 variables and reset C<??> searches so that they work again. The
4678 expression is interpreted as a list of single characters (hyphens
4679 allowed for ranges). All variables and arrays beginning with one of
4680 those letters are reset to their pristine state. If the expression is
4681 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4682 only variables or searches in the current package. Always returns
4685 reset 'X'; # reset all X variables
4686 reset 'a-z'; # reset lower case variables
4687 reset; # just reset ?one-time? searches
4689 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4690 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4691 variables--lexical variables are unaffected, but they clean themselves
4692 up on scope exit anyway, so you'll probably want to use them instead.
4700 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4701 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4702 context, depending on how the return value will be used, and the context
4703 may vary from one execution to the next (see C<wantarray>). If no EXPR
4704 is given, returns an empty list in list context, the undefined value in
4705 scalar context, and (of course) nothing at all in a void context.
4707 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4708 or do FILE will automatically return the value of the last expression
4712 X<reverse> X<rev> X<invert>
4714 In list context, returns a list value consisting of the elements
4715 of LIST in the opposite order. In scalar context, concatenates the
4716 elements of LIST and returns a string value with all characters
4717 in the opposite order.
4719 print reverse <>; # line tac, last line first
4721 undef $/; # for efficiency of <>
4722 print scalar reverse <>; # character tac, last line tsrif
4724 Used without arguments in scalar context, reverse() reverses C<$_>.
4726 This operator is also handy for inverting a hash, although there are some
4727 caveats. If a value is duplicated in the original hash, only one of those
4728 can be represented as a key in the inverted hash. Also, this has to
4729 unwind one hash and build a whole new one, which may take some time
4730 on a large hash, such as from a DBM file.
4732 %by_name = reverse %by_address; # Invert the hash
4734 =item rewinddir DIRHANDLE
4737 Sets the current position to the beginning of the directory for the
4738 C<readdir> routine on DIRHANDLE.
4740 =item rindex STR,SUBSTR,POSITION
4743 =item rindex STR,SUBSTR
4745 Works just like index() except that it returns the position of the I<last>
4746 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4747 last occurrence beginning at or before that position.
4749 =item rmdir FILENAME
4750 X<rmdir> X<rd> X<directory, remove>
4754 Deletes the directory specified by FILENAME if that directory is
4755 empty. If it succeeds it returns true, otherwise it returns false and
4756 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4758 To remove a directory tree recursively (C<rm -rf> on unix) look at
4759 the C<rmtree> function of the L<File::Path> module.
4763 The substitution operator. See L<perlop>.
4765 =item say FILEHANDLE LIST
4772 Just like C<print>, but implicitly appends a newline.
4773 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
4776 This keyword is only available when the "say" feature is
4777 enabled: see L<feature>.
4780 X<scalar> X<context>
4782 Forces EXPR to be interpreted in scalar context and returns the value
4785 @counts = ( scalar @a, scalar @b, scalar @c );
4787 There is no equivalent operator to force an expression to
4788 be interpolated in list context because in practice, this is never
4789 needed. If you really wanted to do so, however, you could use
4790 the construction C<@{[ (some expression) ]}>, but usually a simple
4791 C<(some expression)> suffices.
4793 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4794 parenthesized list, this behaves as a scalar comma expression, evaluating
4795 all but the last element in void context and returning the final element
4796 evaluated in scalar context. This is seldom what you want.
4798 The following single statement:
4800 print uc(scalar(&foo,$bar)),$baz;
4802 is the moral equivalent of these two:
4805 print(uc($bar),$baz);
4807 See L<perlop> for more details on unary operators and the comma operator.
4809 =item seek FILEHANDLE,POSITION,WHENCE
4810 X<seek> X<fseek> X<filehandle, position>
4812 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4813 FILEHANDLE may be an expression whose value gives the name of the
4814 filehandle. The values for WHENCE are C<0> to set the new position
4815 I<in bytes> to POSITION, C<1> to set it to the current position plus
4816 POSITION, and C<2> to set it to EOF plus POSITION (typically
4817 negative). For WHENCE you may use the constants C<SEEK_SET>,
4818 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4819 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4822 Note the I<in bytes>: even if the filehandle has been set to
4823 operate on characters (for example by using the C<:encoding(utf8)> open
4824 layer), tell() will return byte offsets, not character offsets
4825 (because implementing that would render seek() and tell() rather slow).
4827 If you want to position file for C<sysread> or C<syswrite>, don't use
4828 C<seek>--buffering makes its effect on the file's system position
4829 unpredictable and non-portable. Use C<sysseek> instead.
4831 Due to the rules and rigors of ANSI C, on some systems you have to do a
4832 seek whenever you switch between reading and writing. Amongst other
4833 things, this may have the effect of calling stdio's clearerr(3).
4834 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4838 This is also useful for applications emulating C<tail -f>. Once you hit
4839 EOF on your read, and then sleep for a while, you might have to stick in a
4840 seek() to reset things. The C<seek> doesn't change the current position,
4841 but it I<does> clear the end-of-file condition on the handle, so that the
4842 next C<< <FILE> >> makes Perl try again to read something. We hope.
4844 If that doesn't work (some IO implementations are particularly
4845 cantankerous), then you may need something more like this:
4848 for ($curpos = tell(FILE); $_ = <FILE>;
4849 $curpos = tell(FILE)) {
4850 # search for some stuff and put it into files
4852 sleep($for_a_while);
4853 seek(FILE, $curpos, 0);
4856 =item seekdir DIRHANDLE,POS
4859 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4860 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4861 about possible directory compaction as the corresponding system library
4864 =item select FILEHANDLE
4865 X<select> X<filehandle, default>
4869 Returns the currently selected filehandle. If FILEHANDLE is supplied,
4870 sets the new current default filehandle for output. This has two
4871 effects: first, a C<write> or a C<print> without a filehandle will
4872 default to this FILEHANDLE. Second, references to variables related to
4873 output will refer to this output channel. For example, if you have to
4874 set the top of form format for more than one output channel, you might
4882 FILEHANDLE may be an expression whose value gives the name of the
4883 actual filehandle. Thus:
4885 $oldfh = select(STDERR); $| = 1; select($oldfh);
4887 Some programmers may prefer to think of filehandles as objects with
4888 methods, preferring to write the last example as:
4891 STDERR->autoflush(1);
4893 =item select RBITS,WBITS,EBITS,TIMEOUT
4896 This calls the select(2) system call with the bit masks specified, which
4897 can be constructed using C<fileno> and C<vec>, along these lines:
4899 $rin = $win = $ein = '';
4900 vec($rin,fileno(STDIN),1) = 1;
4901 vec($win,fileno(STDOUT),1) = 1;
4904 If you want to select on many filehandles you might wish to write a
4908 my(@fhlist) = split(' ',$_[0]);
4911 vec($bits,fileno($_),1) = 1;
4915 $rin = fhbits('STDIN TTY SOCK');
4919 ($nfound,$timeleft) =
4920 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4922 or to block until something becomes ready just do this
4924 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4926 Most systems do not bother to return anything useful in $timeleft, so
4927 calling select() in scalar context just returns $nfound.
4929 Any of the bit masks can also be undef. The timeout, if specified, is
4930 in seconds, which may be fractional. Note: not all implementations are
4931 capable of returning the $timeleft. If not, they always return
4932 $timeleft equal to the supplied $timeout.
4934 You can effect a sleep of 250 milliseconds this way:
4936 select(undef, undef, undef, 0.25);
4938 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4939 is implementation-dependent. See also L<perlport> for notes on the
4940 portability of C<select>.
4942 On error, C<select> behaves like the select(2) system call : it returns
4945 Note: on some Unixes, the select(2) system call may report a socket file
4946 descriptor as "ready for reading", when actually no data is available,
4947 thus a subsequent read blocks. It can be avoided using always the
4948 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4951 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4952 or <FH>) with C<select>, except as permitted by POSIX, and even
4953 then only on POSIX systems. You have to use C<sysread> instead.
4955 =item semctl ID,SEMNUM,CMD,ARG
4958 Calls the System V IPC function C<semctl>. You'll probably have to say
4962 first to get the correct constant definitions. If CMD is IPC_STAT or
4963 GETALL, then ARG must be a variable that will hold the returned
4964 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4965 the undefined value for error, "C<0 but true>" for zero, or the actual
4966 return value otherwise. The ARG must consist of a vector of native
4967 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4968 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4971 =item semget KEY,NSEMS,FLAGS
4974 Calls the System V IPC function semget. Returns the semaphore id, or
4975 the undefined value if there is an error. See also
4976 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4979 =item semop KEY,OPSTRING
4982 Calls the System V IPC function semop to perform semaphore operations
4983 such as signalling and waiting. OPSTRING must be a packed array of
4984 semop structures. Each semop structure can be generated with
4985 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4986 implies the number of semaphore operations. Returns true if
4987 successful, or false if there is an error. As an example, the
4988 following code waits on semaphore $semnum of semaphore id $semid:
4990 $semop = pack("s!3", $semnum, -1, 0);
4991 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4993 To signal the semaphore, replace C<-1> with C<1>. See also
4994 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4997 =item send SOCKET,MSG,FLAGS,TO
5000 =item send SOCKET,MSG,FLAGS
5002 Sends a message on a socket. Attempts to send the scalar MSG to the
5003 SOCKET filehandle. Takes the same flags as the system call of the
5004 same name. On unconnected sockets you must specify a destination to
5005 send TO, in which case it does a C C<sendto>. Returns the number of
5006 characters sent, or the undefined value if there is an error. The C
5007 system call sendmsg(2) is currently unimplemented. See
5008 L<perlipc/"UDP: Message Passing"> for examples.
5010 Note the I<characters>: depending on the status of the socket, either
5011 (8-bit) bytes or characters are sent. By default all sockets operate
5012 on bytes, but for example if the socket has been changed using
5013 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5014 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5015 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5016 pragma: in that case pretty much any characters can be sent.
5018 =item setpgrp PID,PGRP
5021 Sets the current process group for the specified PID, C<0> for the current
5022 process. Will produce a fatal error if used on a machine that doesn't
5023 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5024 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5025 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5028 =item setpriority WHICH,WHO,PRIORITY
5029 X<setpriority> X<priority> X<nice> X<renice>
5031 Sets the current priority for a process, a process group, or a user.
5032 (See setpriority(2).) Will produce a fatal error if used on a machine
5033 that doesn't implement setpriority(2).
5035 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5038 Sets the socket option requested. Returns undefined if there is an
5039 error. Use integer constants provided by the C<Socket> module for
5040 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5041 getprotobyname. OPTVAL might either be a packed string or an integer.
5042 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5044 An example disabling the Nagle's algorithm for a socket:
5046 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5047 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5054 Shifts the first value of the array off and returns it, shortening the
5055 array by 1 and moving everything down. If there are no elements in the
5056 array, returns the undefined value. If ARRAY is omitted, shifts the
5057 C<@_> array within the lexical scope of subroutines and formats, and the
5058 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5059 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5060 C<UNITCHECK {}> and C<END {}> constructs.
5062 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5063 same thing to the left end of an array that C<pop> and C<push> do to the
5066 =item shmctl ID,CMD,ARG
5069 Calls the System V IPC function shmctl. You'll probably have to say
5073 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5074 then ARG must be a variable that will hold the returned C<shmid_ds>
5075 structure. Returns like ioctl: the undefined value for error, "C<0> but
5076 true" for zero, or the actual return value otherwise.
5077 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5079 =item shmget KEY,SIZE,FLAGS
5082 Calls the System V IPC function shmget. Returns the shared memory
5083 segment id, or the undefined value if there is an error.
5084 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5086 =item shmread ID,VAR,POS,SIZE
5090 =item shmwrite ID,STRING,POS,SIZE
5092 Reads or writes the System V shared memory segment ID starting at
5093 position POS for size SIZE by attaching to it, copying in/out, and
5094 detaching from it. When reading, VAR must be a variable that will
5095 hold the data read. When writing, if STRING is too long, only SIZE
5096 bytes are used; if STRING is too short, nulls are written to fill out
5097 SIZE bytes. Return true if successful, or false if there is an error.
5098 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5099 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5101 =item shutdown SOCKET,HOW
5104 Shuts down a socket connection in the manner indicated by HOW, which
5105 has the same interpretation as in the system call of the same name.
5107 shutdown(SOCKET, 0); # I/we have stopped reading data
5108 shutdown(SOCKET, 1); # I/we have stopped writing data
5109 shutdown(SOCKET, 2); # I/we have stopped using this socket
5111 This is useful with sockets when you want to tell the other
5112 side you're done writing but not done reading, or vice versa.
5113 It's also a more insistent form of close because it also
5114 disables the file descriptor in any forked copies in other
5117 Returns C<1> for success. In the case of error, returns C<undef> if
5118 the first argument is not a valid filehandle, or returns C<0> and sets
5119 C<$!> for any other failure.
5122 X<sin> X<sine> X<asin> X<arcsine>
5126 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5127 returns sine of C<$_>.
5129 For the inverse sine operation, you may use the C<Math::Trig::asin>
5130 function, or use this relation:
5132 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5139 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5140 May be interrupted if the process receives a signal such as C<SIGALRM>.
5141 Returns the number of seconds actually slept. You probably cannot
5142 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5145 On some older systems, it may sleep up to a full second less than what
5146 you requested, depending on how it counts seconds. Most modern systems
5147 always sleep the full amount. They may appear to sleep longer than that,
5148 however, because your process might not be scheduled right away in a
5149 busy multitasking system.
5151 For delays of finer granularity than one second, the Time::HiRes module
5152 (from CPAN, and starting from Perl 5.8 part of the standard
5153 distribution) provides usleep(). You may also use Perl's four-argument
5154 version of select() leaving the first three arguments undefined, or you
5155 might be able to use the C<syscall> interface to access setitimer(2) if
5156 your system supports it. See L<perlfaq8> for details.
5158 See also the POSIX module's C<pause> function.
5160 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5163 Opens a socket of the specified kind and attaches it to filehandle
5164 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5165 the system call of the same name. You should C<use Socket> first
5166 to get the proper definitions imported. See the examples in
5167 L<perlipc/"Sockets: Client/Server Communication">.
5169 On systems that support a close-on-exec flag on files, the flag will
5170 be set for the newly opened file descriptor, as determined by the
5171 value of $^F. See L<perlvar/$^F>.
5173 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5176 Creates an unnamed pair of sockets in the specified domain, of the
5177 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5178 for the system call of the same name. If unimplemented, yields a fatal
5179 error. Returns true if successful.
5181 On systems that support a close-on-exec flag on files, the flag will
5182 be set for the newly opened file descriptors, as determined by the value
5183 of $^F. See L<perlvar/$^F>.
5185 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5186 to C<pipe(Rdr, Wtr)> is essentially:
5189 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5190 shutdown(Rdr, 1); # no more writing for reader
5191 shutdown(Wtr, 0); # no more reading for writer
5193 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5194 emulate socketpair using IP sockets to localhost if your system implements
5195 sockets but not socketpair.
5197 =item sort SUBNAME LIST
5198 X<sort> X<qsort> X<quicksort> X<mergesort>
5200 =item sort BLOCK LIST
5204 In list context, this sorts the LIST and returns the sorted list value.
5205 In scalar context, the behaviour of C<sort()> is undefined.
5207 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5208 order. If SUBNAME is specified, it gives the name of a subroutine
5209 that returns an integer less than, equal to, or greater than C<0>,
5210 depending on how the elements of the list are to be ordered. (The C<<
5211 <=> >> and C<cmp> operators are extremely useful in such routines.)
5212 SUBNAME may be a scalar variable name (unsubscripted), in which case
5213 the value provides the name of (or a reference to) the actual
5214 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5215 an anonymous, in-line sort subroutine.
5217 If the subroutine's prototype is C<($$)>, the elements to be compared
5218 are passed by reference in C<@_>, as for a normal subroutine. This is
5219 slower than unprototyped subroutines, where the elements to be
5220 compared are passed into the subroutine
5221 as the package global variables $a and $b (see example below). Note that
5222 in the latter case, it is usually counter-productive to declare $a and
5225 The values to be compared are always passed by reference and should not
5228 You also cannot exit out of the sort block or subroutine using any of the
5229 loop control operators described in L<perlsyn> or with C<goto>.
5231 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5232 current collation locale. See L<perllocale>.
5234 sort() returns aliases into the original list, much as a for loop's index
5235 variable aliases the list elements. That is, modifying an element of a
5236 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5237 actually modifies the element in the original list. This is usually
5238 something to be avoided when writing clear code.
5240 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5241 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5242 preserves the input order of elements that compare equal. Although
5243 quicksort's run time is O(NlogN) when averaged over all arrays of
5244 length N, the time can be O(N**2), I<quadratic> behavior, for some
5245 inputs.) In 5.7, the quicksort implementation was replaced with
5246 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5247 But benchmarks indicated that for some inputs, on some platforms,
5248 the original quicksort was faster. 5.8 has a sort pragma for
5249 limited control of the sort. Its rather blunt control of the
5250 underlying algorithm may not persist into future Perls, but the
5251 ability to characterize the input or output in implementation
5252 independent ways quite probably will. See L<sort>.
5257 @articles = sort @files;
5259 # same thing, but with explicit sort routine
5260 @articles = sort {$a cmp $b} @files;
5262 # now case-insensitively
5263 @articles = sort {uc($a) cmp uc($b)} @files;
5265 # same thing in reversed order
5266 @articles = sort {$b cmp $a} @files;
5268 # sort numerically ascending
5269 @articles = sort {$a <=> $b} @files;
5271 # sort numerically descending
5272 @articles = sort {$b <=> $a} @files;
5274 # this sorts the %age hash by value instead of key
5275 # using an in-line function
5276 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5278 # sort using explicit subroutine name
5280 $age{$a} <=> $age{$b}; # presuming numeric
5282 @sortedclass = sort byage @class;
5284 sub backwards { $b cmp $a }
5285 @harry = qw(dog cat x Cain Abel);
5286 @george = qw(gone chased yz Punished Axed);
5288 # prints AbelCaincatdogx
5289 print sort backwards @harry;
5290 # prints xdogcatCainAbel
5291 print sort @george, 'to', @harry;
5292 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5294 # inefficiently sort by descending numeric compare using
5295 # the first integer after the first = sign, or the
5296 # whole record case-insensitively otherwise
5299 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5304 # same thing, but much more efficiently;
5305 # we'll build auxiliary indices instead
5309 push @nums, /=(\d+)/;
5314 $nums[$b] <=> $nums[$a]
5316 $caps[$a] cmp $caps[$b]
5320 # same thing, but without any temps
5321 @new = map { $_->[0] }
5322 sort { $b->[1] <=> $a->[1]
5325 } map { [$_, /=(\d+)/, uc($_)] } @old;
5327 # using a prototype allows you to use any comparison subroutine
5328 # as a sort subroutine (including other package's subroutines)
5330 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5333 @new = sort other::backwards @old;
5335 # guarantee stability, regardless of algorithm
5337 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5339 # force use of mergesort (not portable outside Perl 5.8)
5340 use sort '_mergesort'; # note discouraging _
5341 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5343 If you're using strict, you I<must not> declare $a
5344 and $b as lexicals. They are package globals. That means
5345 if you're in the C<main> package and type
5347 @articles = sort {$b <=> $a} @files;
5349 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5350 but if you're in the C<FooPack> package, it's the same as typing
5352 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5354 The comparison function is required to behave. If it returns
5355 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5356 sometimes saying the opposite, for example) the results are not
5359 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5360 (not-a-number), and because C<sort> will trigger a fatal error unless the
5361 result of a comparison is defined, when sorting with a comparison function
5362 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5363 The following example takes advantage of the fact that C<NaN != NaN> to
5364 eliminate any C<NaN>s from the input.
5366 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5368 =item splice ARRAY,OFFSET,LENGTH,LIST
5371 =item splice ARRAY,OFFSET,LENGTH
5373 =item splice ARRAY,OFFSET
5377 Removes the elements designated by OFFSET and LENGTH from an array, and
5378 replaces them with the elements of LIST, if any. In list context,
5379 returns the elements removed from the array. In scalar context,
5380 returns the last element removed, or C<undef> if no elements are
5381 removed. The array grows or shrinks as necessary.
5382 If OFFSET is negative then it starts that far from the end of the array.
5383 If LENGTH is omitted, removes everything from OFFSET onward.
5384 If LENGTH is negative, removes the elements from OFFSET onward
5385 except for -LENGTH elements at the end of the array.
5386 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5387 past the end of the array, perl issues a warning, and splices at the
5390 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5392 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5393 pop(@a) splice(@a,-1)
5394 shift(@a) splice(@a,0,1)
5395 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5396 $a[$i] = $y splice(@a,$i,1,$y)
5398 Example, assuming array lengths are passed before arrays:
5400 sub aeq { # compare two list values
5401 my(@a) = splice(@_,0,shift);
5402 my(@b) = splice(@_,0,shift);
5403 return 0 unless @a == @b; # same len?
5405 return 0 if pop(@a) ne pop(@b);
5409 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5411 =item split /PATTERN/,EXPR,LIMIT
5414 =item split /PATTERN/,EXPR
5416 =item split /PATTERN/
5420 Splits the string EXPR into a list of strings and returns that list. By
5421 default, empty leading fields are preserved, and empty trailing ones are
5422 deleted. (If all fields are empty, they are considered to be trailing.)
5424 In scalar context, returns the number of fields found and splits into
5425 the C<@_> array. Use of split in scalar context is deprecated, however,
5426 because it clobbers your subroutine arguments.
5428 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5429 splits on whitespace (after skipping any leading whitespace). Anything
5430 matching PATTERN is taken to be a delimiter separating the fields. (Note
5431 that the delimiter may be longer than one character.)
5433 If LIMIT is specified and positive, it represents the maximum number
5434 of fields the EXPR will be split into, though the actual number of
5435 fields returned depends on the number of times PATTERN matches within
5436 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5437 stripped (which potential users of C<pop> would do well to remember).
5438 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5439 had been specified. Note that splitting an EXPR that evaluates to the
5440 empty string always returns the empty list, regardless of the LIMIT
5443 A pattern matching the null string (not to be confused with
5444 a null pattern C<//>, which is just one member of the set of patterns
5445 matching a null string) will split the value of EXPR into separate
5446 characters at each point it matches that way. For example:
5448 print join(':', split(/ */, 'hi there')), "\n";
5450 produces the output 'h:i:t:h:e:r:e'.
5452 As a special case for C<split>, using the empty pattern C<//> specifically
5453 matches only the null string, and is not be confused with the regular use
5454 of C<//> to mean "the last successful pattern match". So, for C<split>,
5457 print join(':', split(//, 'hi there')), "\n";
5459 produces the output 'h:i: :t:h:e:r:e'.
5461 Empty leading fields are produced when there are positive-width matches at
5462 the beginning of the string; a zero-width match at the beginning of
5463 the string does not produce an empty field. For example:
5465 print join(':', split(/(?=\w)/, 'hi there!'));
5467 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
5468 hand, are produced when there is a match at the end of the string (and
5469 when LIMIT is given and is not 0), regardless of the length of the match.
5472 print join(':', split(//, 'hi there!', -1)), "\n";
5473 print join(':', split(/\W/, 'hi there!', -1)), "\n";
5475 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
5476 both with an empty trailing field.
5478 The LIMIT parameter can be used to split a line partially
5480 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5482 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5483 a LIMIT one larger than the number of variables in the list, to avoid
5484 unnecessary work. For the list above LIMIT would have been 4 by
5485 default. In time critical applications it behooves you not to split
5486 into more fields than you really need.
5488 If the PATTERN contains parentheses, additional list elements are
5489 created from each matching substring in the delimiter.
5491 split(/([,-])/, "1-10,20", 3);
5493 produces the list value
5495 (1, '-', 10, ',', 20)
5497 If you had the entire header of a normal Unix email message in $header,
5498 you could split it up into fields and their values this way:
5500 $header =~ s/\n\s+/ /g; # fix continuation lines
5501 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5503 The pattern C</PATTERN/> may be replaced with an expression to specify
5504 patterns that vary at runtime. (To do runtime compilation only once,
5505 use C</$variable/o>.)
5507 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5508 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5509 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5510 will give you as many null initial fields as there are leading spaces.
5511 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5512 whitespace produces a null first field. A C<split> with no arguments
5513 really does a S<C<split(' ', $_)>> internally.
5515 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5520 open(PASSWD, '/etc/passwd');
5523 ($login, $passwd, $uid, $gid,
5524 $gcos, $home, $shell) = split(/:/);
5528 As with regular pattern matching, any capturing parentheses that are not
5529 matched in a C<split()> will be set to C<undef> when returned:
5531 @fields = split /(A)|B/, "1A2B3";
5532 # @fields is (1, 'A', 2, undef, 3)
5534 =item sprintf FORMAT, LIST
5537 Returns a string formatted by the usual C<printf> conventions of the C
5538 library function C<sprintf>. See below for more details
5539 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5540 the general principles.
5544 # Format number with up to 8 leading zeroes
5545 $result = sprintf("%08d", $number);
5547 # Round number to 3 digits after decimal point
5548 $rounded = sprintf("%.3f", $number);
5550 Perl does its own C<sprintf> formatting--it emulates the C
5551 function C<sprintf>, but it doesn't use it (except for floating-point
5552 numbers, and even then only the standard modifiers are allowed). As a
5553 result, any non-standard extensions in your local C<sprintf> are not
5554 available from Perl.
5556 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5557 pass it an array as your first argument. The array is given scalar context,
5558 and instead of using the 0th element of the array as the format, Perl will
5559 use the count of elements in the array as the format, which is almost never
5562 Perl's C<sprintf> permits the following universally-known conversions:
5565 %c a character with the given number
5567 %d a signed integer, in decimal
5568 %u an unsigned integer, in decimal
5569 %o an unsigned integer, in octal
5570 %x an unsigned integer, in hexadecimal
5571 %e a floating-point number, in scientific notation
5572 %f a floating-point number, in fixed decimal notation
5573 %g a floating-point number, in %e or %f notation
5575 In addition, Perl permits the following widely-supported conversions:
5577 %X like %x, but using upper-case letters
5578 %E like %e, but using an upper-case "E"
5579 %G like %g, but with an upper-case "E" (if applicable)
5580 %b an unsigned integer, in binary
5581 %B like %b, but using an upper-case "B" with the # flag
5582 %p a pointer (outputs the Perl value's address in hexadecimal)
5583 %n special: *stores* the number of characters output so far
5584 into the next variable in the parameter list
5586 Finally, for backward (and we do mean "backward") compatibility, Perl
5587 permits these unnecessary but widely-supported conversions:
5590 %D a synonym for %ld
5591 %U a synonym for %lu
5592 %O a synonym for %lo
5595 Note that the number of exponent digits in the scientific notation produced
5596 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5597 exponent less than 100 is system-dependent: it may be three or less
5598 (zero-padded as necessary). In other words, 1.23 times ten to the
5599 99th may be either "1.23e99" or "1.23e099".
5601 Between the C<%> and the format letter, you may specify a number of
5602 additional attributes controlling the interpretation of the format.
5603 In order, these are:
5607 =item format parameter index
5609 An explicit format parameter index, such as C<2$>. By default sprintf
5610 will format the next unused argument in the list, but this allows you
5611 to take the arguments out of order, e.g.:
5613 printf '%2$d %1$d', 12, 34; # prints "34 12"
5614 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5620 space prefix non-negative number with a space
5621 + prefix non-negative number with a plus sign
5622 - left-justify within the field
5623 0 use zeros, not spaces, to right-justify
5624 # ensure the leading "0" for any octal,
5625 prefix non-zero hexadecimal with "0x" or "0X",
5626 prefix non-zero binary with "0b" or "0B"
5630 printf '<% d>', 12; # prints "< 12>"
5631 printf '<%+d>', 12; # prints "<+12>"
5632 printf '<%6s>', 12; # prints "< 12>"
5633 printf '<%-6s>', 12; # prints "<12 >"
5634 printf '<%06s>', 12; # prints "<000012>"
5635 printf '<%#o>', 12; # prints "<014>"
5636 printf '<%#x>', 12; # prints "<0xc>"
5637 printf '<%#X>', 12; # prints "<0XC>"
5638 printf '<%#b>', 12; # prints "<0b1100>"
5639 printf '<%#B>', 12; # prints "<0B1100>"
5641 When a space and a plus sign are given as the flags at once,
5642 a plus sign is used to prefix a positive number.
5644 printf '<%+ d>', 12; # prints "<+12>"
5645 printf '<% +d>', 12; # prints "<+12>"
5647 When the # flag and a precision are given in the %o conversion,
5648 the precision is incremented if it's necessary for the leading "0".
5650 printf '<%#.5o>', 012; # prints "<00012>"
5651 printf '<%#.5o>', 012345; # prints "<012345>"
5652 printf '<%#.0o>', 0; # prints "<0>"
5656 This flag tells perl to interpret the supplied string as a vector of
5657 integers, one for each character in the string. Perl applies the format to
5658 each integer in turn, then joins the resulting strings with a separator (a
5659 dot C<.> by default). This can be useful for displaying ordinal values of
5660 characters in arbitrary strings:
5662 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5663 printf "version is v%vd\n", $^V; # Perl's version
5665 Put an asterisk C<*> before the C<v> to override the string to
5666 use to separate the numbers:
5668 printf "address is %*vX\n", ":", $addr; # IPv6 address
5669 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5671 You can also explicitly specify the argument number to use for
5672 the join string using e.g. C<*2$v>:
5674 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5676 =item (minimum) width
5678 Arguments are usually formatted to be only as wide as required to
5679 display the given value. You can override the width by putting
5680 a number here, or get the width from the next argument (with C<*>)
5681 or from a specified argument (with e.g. C<*2$>):
5683 printf '<%s>', "a"; # prints "<a>"
5684 printf '<%6s>', "a"; # prints "< a>"
5685 printf '<%*s>', 6, "a"; # prints "< a>"
5686 printf '<%*2$s>', "a", 6; # prints "< a>"
5687 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5689 If a field width obtained through C<*> is negative, it has the same
5690 effect as the C<-> flag: left-justification.
5692 =item precision, or maximum width
5695 You can specify a precision (for numeric conversions) or a maximum
5696 width (for string conversions) by specifying a C<.> followed by a number.
5697 For floating point formats, with the exception of 'g' and 'G', this specifies
5698 the number of decimal places to show (the default being 6), e.g.:
5700 # these examples are subject to system-specific variation
5701 printf '<%f>', 1; # prints "<1.000000>"
5702 printf '<%.1f>', 1; # prints "<1.0>"
5703 printf '<%.0f>', 1; # prints "<1>"
5704 printf '<%e>', 10; # prints "<1.000000e+01>"
5705 printf '<%.1e>', 10; # prints "<1.0e+01>"
5707 For 'g' and 'G', this specifies the maximum number of digits to show,
5708 including prior to the decimal point as well as after it, e.g.:
5710 # these examples are subject to system-specific variation
5711 printf '<%g>', 1; # prints "<1>"
5712 printf '<%.10g>', 1; # prints "<1>"
5713 printf '<%g>', 100; # prints "<100>"
5714 printf '<%.1g>', 100; # prints "<1e+02>"
5715 printf '<%.2g>', 100.01; # prints "<1e+02>"
5716 printf '<%.5g>', 100.01; # prints "<100.01>"
5717 printf '<%.4g>', 100.01; # prints "<100>"
5719 For integer conversions, specifying a precision implies that the
5720 output of the number itself should be zero-padded to this width,
5721 where the 0 flag is ignored:
5723 printf '<%.6d>', 1; # prints "<000001>"
5724 printf '<%+.6d>', 1; # prints "<+000001>"
5725 printf '<%-10.6d>', 1; # prints "<000001 >"
5726 printf '<%10.6d>', 1; # prints "< 000001>"
5727 printf '<%010.6d>', 1; # prints "< 000001>"
5728 printf '<%+10.6d>', 1; # prints "< +000001>"
5730 printf '<%.6x>', 1; # prints "<000001>"
5731 printf '<%#.6x>', 1; # prints "<0x000001>"
5732 printf '<%-10.6x>', 1; # prints "<000001 >"
5733 printf '<%10.6x>', 1; # prints "< 000001>"
5734 printf '<%010.6x>', 1; # prints "< 000001>"
5735 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5737 For string conversions, specifying a precision truncates the string
5738 to fit in the specified width:
5740 printf '<%.5s>', "truncated"; # prints "<trunc>"
5741 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5743 You can also get the precision from the next argument using C<.*>:
5745 printf '<%.6x>', 1; # prints "<000001>"
5746 printf '<%.*x>', 6, 1; # prints "<000001>"
5748 If a precision obtained through C<*> is negative, it has the same
5749 effect as no precision.
5751 printf '<%.*s>', 7, "string"; # prints "<string>"
5752 printf '<%.*s>', 3, "string"; # prints "<str>"
5753 printf '<%.*s>', 0, "string"; # prints "<>"
5754 printf '<%.*s>', -1, "string"; # prints "<string>"
5756 printf '<%.*d>', 1, 0; # prints "<0>"
5757 printf '<%.*d>', 0, 0; # prints "<>"
5758 printf '<%.*d>', -1, 0; # prints "<0>"
5760 You cannot currently get the precision from a specified number,
5761 but it is intended that this will be possible in the future using
5764 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5768 For numeric conversions, you can specify the size to interpret the
5769 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5770 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5771 whatever the default integer size is on your platform (usually 32 or 64
5772 bits), but you can override this to use instead one of the standard C types,
5773 as supported by the compiler used to build Perl:
5775 l interpret integer as C type "long" or "unsigned long"
5776 h interpret integer as C type "short" or "unsigned short"
5777 q, L or ll interpret integer as C type "long long", "unsigned long long".
5778 or "quads" (typically 64-bit integers)
5780 The last will produce errors if Perl does not understand "quads" in your
5781 installation. (This requires that either the platform natively supports quads
5782 or Perl was specifically compiled to support quads.) You can find out
5783 whether your Perl supports quads via L<Config>:
5786 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5789 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5790 to be the default floating point size on your platform (double or long double),
5791 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5792 platform supports them. You can find out whether your Perl supports long
5793 doubles via L<Config>:
5796 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5798 You can find out whether Perl considers 'long double' to be the default
5799 floating point size to use on your platform via L<Config>:
5802 ($Config{uselongdouble} eq 'define') &&
5803 print "long doubles by default\n";
5805 It can also be the case that long doubles and doubles are the same thing:
5808 ($Config{doublesize} == $Config{longdblsize}) &&
5809 print "doubles are long doubles\n";
5811 The size specifier C<V> has no effect for Perl code, but it is supported
5812 for compatibility with XS code; it means 'use the standard size for
5813 a Perl integer (or floating-point number)', which is already the
5814 default for Perl code.
5816 =item order of arguments
5818 Normally, sprintf takes the next unused argument as the value to
5819 format for each format specification. If the format specification
5820 uses C<*> to require additional arguments, these are consumed from
5821 the argument list in the order in which they appear in the format
5822 specification I<before> the value to format. Where an argument is
5823 specified using an explicit index, this does not affect the normal
5824 order for the arguments (even when the explicitly specified index
5825 would have been the next argument in any case).
5829 printf '<%*.*s>', $a, $b, $c;
5831 would use C<$a> for the width, C<$b> for the precision and C<$c>
5832 as the value to format, while:
5834 printf '<%*1$.*s>', $a, $b;
5836 would use C<$a> for the width and the precision, and C<$b> as the
5839 Here are some more examples - beware that when using an explicit
5840 index, the C<$> may need to be escaped:
5842 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5843 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5844 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5845 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5849 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5850 the character used for the decimal separator in formatted floating
5851 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5855 X<sqrt> X<root> X<square root>
5859 Return the square root of EXPR. If EXPR is omitted, returns square
5860 root of C<$_>. Only works on non-negative operands, unless you've
5861 loaded the standard Math::Complex module.
5864 print sqrt(-2); # prints 1.4142135623731i
5867 X<srand> X<seed> X<randseed>
5871 Sets the random number seed for the C<rand> operator.
5873 The point of the function is to "seed" the C<rand> function so that
5874 C<rand> can produce a different sequence each time you run your
5877 If srand() is not called explicitly, it is called implicitly at the
5878 first use of the C<rand> operator. However, this was not the case in
5879 versions of Perl before 5.004, so if your script will run under older
5880 Perl versions, it should call C<srand>.
5882 Most programs won't even call srand() at all, except those that
5883 need a cryptographically-strong starting point rather than the
5884 generally acceptable default, which is based on time of day,
5885 process ID, and memory allocation, or the F</dev/urandom> device,
5888 You can call srand($seed) with the same $seed to reproduce the
5889 I<same> sequence from rand(), but this is usually reserved for
5890 generating predictable results for testing or debugging.
5891 Otherwise, don't call srand() more than once in your program.
5893 Do B<not> call srand() (i.e. without an argument) more than once in
5894 a script. The internal state of the random number generator should
5895 contain more entropy than can be provided by any seed, so calling
5896 srand() again actually I<loses> randomness.
5898 Most implementations of C<srand> take an integer and will silently
5899 truncate decimal numbers. This means C<srand(42)> will usually
5900 produce the same results as C<srand(42.1)>. To be safe, always pass
5901 C<srand> an integer.
5903 In versions of Perl prior to 5.004 the default seed was just the
5904 current C<time>. This isn't a particularly good seed, so many old
5905 programs supply their own seed value (often C<time ^ $$> or C<time ^
5906 ($$ + ($$ << 15))>), but that isn't necessary any more.
5908 For cryptographic purposes, however, you need something much more random
5909 than the default seed. Checksumming the compressed output of one or more
5910 rapidly changing operating system status programs is the usual method. For
5913 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
5915 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5918 Frequently called programs (like CGI scripts) that simply use
5922 for a seed can fall prey to the mathematical property that
5926 one-third of the time. So don't do that.
5928 =item stat FILEHANDLE
5929 X<stat> X<file, status> X<ctime>
5933 =item stat DIRHANDLE
5937 Returns a 13-element list giving the status info for a file, either
5938 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5939 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5942 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5943 $atime,$mtime,$ctime,$blksize,$blocks)
5946 Not all fields are supported on all filesystem types. Here are the
5947 meanings of the fields:
5949 0 dev device number of filesystem
5951 2 mode file mode (type and permissions)
5952 3 nlink number of (hard) links to the file
5953 4 uid numeric user ID of file's owner
5954 5 gid numeric group ID of file's owner
5955 6 rdev the device identifier (special files only)
5956 7 size total size of file, in bytes
5957 8 atime last access time in seconds since the epoch
5958 9 mtime last modify time in seconds since the epoch
5959 10 ctime inode change time in seconds since the epoch (*)
5960 11 blksize preferred block size for file system I/O
5961 12 blocks actual number of blocks allocated
5963 (The epoch was at 00:00 January 1, 1970 GMT.)
5965 (*) Not all fields are supported on all filesystem types. Notably, the
5966 ctime field is non-portable. In particular, you cannot expect it to be a
5967 "creation time", see L<perlport/"Files and Filesystems"> for details.
5969 If C<stat> is passed the special filehandle consisting of an underline, no
5970 stat is done, but the current contents of the stat structure from the
5971 last C<stat>, C<lstat>, or filetest are returned. Example:
5973 if (-x $file && (($d) = stat(_)) && $d < 0) {
5974 print "$file is executable NFS file\n";
5977 (This works on machines only for which the device number is negative
5980 Because the mode contains both the file type and its permissions, you
5981 should mask off the file type portion and (s)printf using a C<"%o">
5982 if you want to see the real permissions.
5984 $mode = (stat($filename))[2];
5985 printf "Permissions are %04o\n", $mode & 07777;
5987 In scalar context, C<stat> returns a boolean value indicating success
5988 or failure, and, if successful, sets the information associated with
5989 the special filehandle C<_>.
5991 The L<File::stat> module provides a convenient, by-name access mechanism:
5994 $sb = stat($filename);
5995 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5996 $filename, $sb->size, $sb->mode & 07777,
5997 scalar localtime $sb->mtime;
5999 You can import symbolic mode constants (C<S_IF*>) and functions
6000 (C<S_IS*>) from the Fcntl module:
6004 $mode = (stat($filename))[2];
6006 $user_rwx = ($mode & S_IRWXU) >> 6;
6007 $group_read = ($mode & S_IRGRP) >> 3;
6008 $other_execute = $mode & S_IXOTH;
6010 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6012 $is_setuid = $mode & S_ISUID;
6013 $is_directory = S_ISDIR($mode);
6015 You could write the last two using the C<-u> and C<-d> operators.
6016 The commonly available C<S_IF*> constants are
6018 # Permissions: read, write, execute, for user, group, others.
6020 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6021 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6022 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6024 # Setuid/Setgid/Stickiness/SaveText.
6025 # Note that the exact meaning of these is system dependent.
6027 S_ISUID S_ISGID S_ISVTX S_ISTXT
6029 # File types. Not necessarily all are available on your system.
6031 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6033 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
6035 S_IREAD S_IWRITE S_IEXEC
6037 and the C<S_IF*> functions are
6039 S_IMODE($mode) the part of $mode containing the permission bits
6040 and the setuid/setgid/sticky bits
6042 S_IFMT($mode) the part of $mode containing the file type
6043 which can be bit-anded with e.g. S_IFREG
6044 or with the following functions
6046 # The operators -f, -d, -l, -b, -c, -p, and -S.
6048 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6049 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6051 # No direct -X operator counterpart, but for the first one
6052 # the -g operator is often equivalent. The ENFMT stands for
6053 # record flocking enforcement, a platform-dependent feature.
6055 S_ISENFMT($mode) S_ISWHT($mode)
6057 See your native chmod(2) and stat(2) documentation for more details
6058 about the C<S_*> constants. To get status info for a symbolic link
6059 instead of the target file behind the link, use the C<lstat> function.
6064 =item state TYPE EXPR
6066 =item state EXPR : ATTRS
6068 =item state TYPE EXPR : ATTRS
6070 C<state> declares a lexically scoped variable, just like C<my> does.
6071 However, those variables will never be reinitialized, contrary to
6072 lexical variables that are reinitialized each time their enclosing block
6075 C<state> variables are only enabled when the C<feature 'state'> pragma is
6076 in effect. See L<feature>.
6083 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6084 doing many pattern matches on the string before it is next modified.
6085 This may or may not save time, depending on the nature and number of
6086 patterns you are searching on, and on the distribution of character
6087 frequencies in the string to be searched--you probably want to compare
6088 run times with and without it to see which runs faster. Those loops
6089 that scan for many short constant strings (including the constant
6090 parts of more complex patterns) will benefit most. You may have only
6091 one C<study> active at a time--if you study a different scalar the first
6092 is "unstudied". (The way C<study> works is this: a linked list of every
6093 character in the string to be searched is made, so we know, for
6094 example, where all the C<'k'> characters are. From each search string,
6095 the rarest character is selected, based on some static frequency tables
6096 constructed from some C programs and English text. Only those places
6097 that contain this "rarest" character are examined.)
6099 For example, here is a loop that inserts index producing entries
6100 before any line containing a certain pattern:
6104 print ".IX foo\n" if /\bfoo\b/;
6105 print ".IX bar\n" if /\bbar\b/;
6106 print ".IX blurfl\n" if /\bblurfl\b/;
6111 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6112 will be looked at, because C<f> is rarer than C<o>. In general, this is
6113 a big win except in pathological cases. The only question is whether
6114 it saves you more time than it took to build the linked list in the
6117 Note that if you have to look for strings that you don't know till
6118 runtime, you can build an entire loop as a string and C<eval> that to
6119 avoid recompiling all your patterns all the time. Together with
6120 undefining C<$/> to input entire files as one record, this can be very
6121 fast, often faster than specialized programs like fgrep(1). The following
6122 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6123 out the names of those files that contain a match:
6125 $search = 'while (<>) { study;';
6126 foreach $word (@words) {
6127 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6132 eval $search; # this screams
6133 $/ = "\n"; # put back to normal input delimiter
6134 foreach $file (sort keys(%seen)) {
6138 =item sub NAME BLOCK
6141 =item sub NAME (PROTO) BLOCK
6143 =item sub NAME : ATTRS BLOCK
6145 =item sub NAME (PROTO) : ATTRS BLOCK
6147 This is subroutine definition, not a real function I<per se>.
6148 Without a BLOCK it's just a forward declaration. Without a NAME,
6149 it's an anonymous function declaration, and does actually return
6150 a value: the CODE ref of the closure you just created.
6152 See L<perlsub> and L<perlref> for details about subroutines and
6153 references, and L<attributes> and L<Attribute::Handlers> for more
6154 information about attributes.
6156 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6157 X<substr> X<substring> X<mid> X<left> X<right>
6159 =item substr EXPR,OFFSET,LENGTH
6161 =item substr EXPR,OFFSET
6163 Extracts a substring out of EXPR and returns it. First character is at
6164 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6165 If OFFSET is negative (or more precisely, less than C<$[>), starts
6166 that far from the end of the string. If LENGTH is omitted, returns
6167 everything to the end of the string. If LENGTH is negative, leaves that
6168 many characters off the end of the string.
6170 my $s = "The black cat climbed the green tree";
6171 my $color = substr $s, 4, 5; # black
6172 my $middle = substr $s, 4, -11; # black cat climbed the
6173 my $end = substr $s, 14; # climbed the green tree
6174 my $tail = substr $s, -4; # tree
6175 my $z = substr $s, -4, 2; # tr
6177 You can use the substr() function as an lvalue, in which case EXPR
6178 must itself be an lvalue. If you assign something shorter than LENGTH,
6179 the string will shrink, and if you assign something longer than LENGTH,
6180 the string will grow to accommodate it. To keep the string the same
6181 length you may need to pad or chop your value using C<sprintf>.
6183 If OFFSET and LENGTH specify a substring that is partly outside the
6184 string, only the part within the string is returned. If the substring
6185 is beyond either end of the string, substr() returns the undefined
6186 value and produces a warning. When used as an lvalue, specifying a
6187 substring that is entirely outside the string is a fatal error.
6188 Here's an example showing the behavior for boundary cases:
6191 substr($name, 4) = 'dy'; # $name is now 'freddy'
6192 my $null = substr $name, 6, 2; # returns '' (no warning)
6193 my $oops = substr $name, 7; # returns undef, with warning
6194 substr($name, 7) = 'gap'; # fatal error
6196 An alternative to using substr() as an lvalue is to specify the
6197 replacement string as the 4th argument. This allows you to replace
6198 parts of the EXPR and return what was there before in one operation,
6199 just as you can with splice().
6201 my $s = "The black cat climbed the green tree";
6202 my $z = substr $s, 14, 7, "jumped from"; # climbed
6203 # $s is now "The black cat jumped from the green tree"
6205 Note that the lvalue returned by the 3-arg version of substr() acts as
6206 a 'magic bullet'; each time it is assigned to, it remembers which part
6207 of the original string is being modified; for example:
6210 for (substr($x,1,2)) {
6211 $_ = 'a'; print $x,"\n"; # prints 1a4
6212 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6214 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6217 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6220 =item symlink OLDFILE,NEWFILE
6221 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6223 Creates a new filename symbolically linked to the old filename.
6224 Returns C<1> for success, C<0> otherwise. On systems that don't support
6225 symbolic links, produces a fatal error at run time. To check for that,
6228 $symlink_exists = eval { symlink("",""); 1 };
6230 =item syscall NUMBER, LIST
6231 X<syscall> X<system call>
6233 Calls the system call specified as the first element of the list,
6234 passing the remaining elements as arguments to the system call. If
6235 unimplemented, produces a fatal error. The arguments are interpreted
6236 as follows: if a given argument is numeric, the argument is passed as
6237 an int. If not, the pointer to the string value is passed. You are
6238 responsible to make sure a string is pre-extended long enough to
6239 receive any result that might be written into a string. You can't use a
6240 string literal (or other read-only string) as an argument to C<syscall>
6241 because Perl has to assume that any string pointer might be written
6243 integer arguments are not literals and have never been interpreted in a
6244 numeric context, you may need to add C<0> to them to force them to look
6245 like numbers. This emulates the C<syswrite> function (or vice versa):
6247 require 'syscall.ph'; # may need to run h2ph
6249 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6251 Note that Perl supports passing of up to only 14 arguments to your system call,
6252 which in practice should usually suffice.
6254 Syscall returns whatever value returned by the system call it calls.
6255 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6256 Note that some system calls can legitimately return C<-1>. The proper
6257 way to handle such calls is to assign C<$!=0;> before the call and
6258 check the value of C<$!> if syscall returns C<-1>.
6260 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6261 number of the read end of the pipe it creates. There is no way
6262 to retrieve the file number of the other end. You can avoid this
6263 problem by using C<pipe> instead.
6265 =item sysopen FILEHANDLE,FILENAME,MODE
6268 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6270 Opens the file whose filename is given by FILENAME, and associates it
6271 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6272 the name of the real filehandle wanted. This function calls the
6273 underlying operating system's C<open> function with the parameters
6274 FILENAME, MODE, PERMS.
6276 The possible values and flag bits of the MODE parameter are
6277 system-dependent; they are available via the standard module C<Fcntl>.
6278 See the documentation of your operating system's C<open> to see which
6279 values and flag bits are available. You may combine several flags
6280 using the C<|>-operator.
6282 Some of the most common values are C<O_RDONLY> for opening the file in
6283 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6284 and C<O_RDWR> for opening the file in read-write mode.
6285 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6287 For historical reasons, some values work on almost every system
6288 supported by perl: zero means read-only, one means write-only, and two
6289 means read/write. We know that these values do I<not> work under
6290 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6291 use them in new code.
6293 If the file named by FILENAME does not exist and the C<open> call creates
6294 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6295 PERMS specifies the permissions of the newly created file. If you omit
6296 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6297 These permission values need to be in octal, and are modified by your
6298 process's current C<umask>.
6301 In many systems the C<O_EXCL> flag is available for opening files in
6302 exclusive mode. This is B<not> locking: exclusiveness means here that
6303 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6304 on network filesystems, and has no effect unless the C<O_CREAT> flag
6305 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6306 being opened if it is a symbolic link. It does not protect against
6307 symbolic links in the file's path.
6310 Sometimes you may want to truncate an already-existing file. This
6311 can be done using the C<O_TRUNC> flag. The behavior of
6312 C<O_TRUNC> with C<O_RDONLY> is undefined.
6315 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6316 that takes away the user's option to have a more permissive umask.
6317 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6320 Note that C<sysopen> depends on the fdopen() C library function.
6321 On many UNIX systems, fdopen() is known to fail when file descriptors
6322 exceed a certain value, typically 255. If you need more file
6323 descriptors than that, consider rebuilding Perl to use the C<sfio>
6324 library, or perhaps using the POSIX::open() function.
6326 See L<perlopentut> for a kinder, gentler explanation of opening files.
6328 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6331 =item sysread FILEHANDLE,SCALAR,LENGTH
6333 Attempts to read LENGTH bytes of data into variable SCALAR from the
6334 specified FILEHANDLE, using the system call read(2). It bypasses
6335 buffered IO, so mixing this with other kinds of reads, C<print>,
6336 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6337 perlio or stdio layers usually buffers data. Returns the number of
6338 bytes actually read, C<0> at end of file, or undef if there was an
6339 error (in the latter case C<$!> is also set). SCALAR will be grown or
6340 shrunk so that the last byte actually read is the last byte of the
6341 scalar after the read.
6343 An OFFSET may be specified to place the read data at some place in the
6344 string other than the beginning. A negative OFFSET specifies
6345 placement at that many characters counting backwards from the end of
6346 the string. A positive OFFSET greater than the length of SCALAR
6347 results in the string being padded to the required size with C<"\0">
6348 bytes before the result of the read is appended.
6350 There is no syseof() function, which is ok, since eof() doesn't work
6351 very well on device files (like ttys) anyway. Use sysread() and check
6352 for a return value for 0 to decide whether you're done.
6354 Note that if the filehandle has been marked as C<:utf8> Unicode
6355 characters are read instead of bytes (the LENGTH, OFFSET, and the
6356 return value of sysread() are in Unicode characters).
6357 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6358 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6360 =item sysseek FILEHANDLE,POSITION,WHENCE
6363 Sets FILEHANDLE's system position in bytes using the system call
6364 lseek(2). FILEHANDLE may be an expression whose value gives the name
6365 of the filehandle. The values for WHENCE are C<0> to set the new
6366 position to POSITION, C<1> to set the it to the current position plus
6367 POSITION, and C<2> to set it to EOF plus POSITION (typically
6370 Note the I<in bytes>: even if the filehandle has been set to operate
6371 on characters (for example by using the C<:encoding(utf8)> I/O layer),
6372 tell() will return byte offsets, not character offsets (because
6373 implementing that would render sysseek() very slow).
6375 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6376 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6377 C<seek>, C<tell>, or C<eof> may cause confusion.
6379 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6380 and C<SEEK_END> (start of the file, current position, end of the file)
6381 from the Fcntl module. Use of the constants is also more portable
6382 than relying on 0, 1, and 2. For example to define a "systell" function:
6384 use Fcntl 'SEEK_CUR';
6385 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6387 Returns the new position, or the undefined value on failure. A position
6388 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6389 true on success and false on failure, yet you can still easily determine
6395 =item system PROGRAM LIST
6397 Does exactly the same thing as C<exec LIST>, except that a fork is
6398 done first, and the parent process waits for the child process to
6399 complete. Note that argument processing varies depending on the
6400 number of arguments. If there is more than one argument in LIST,
6401 or if LIST is an array with more than one value, starts the program
6402 given by the first element of the list with arguments given by the
6403 rest of the list. If there is only one scalar argument, the argument
6404 is checked for shell metacharacters, and if there are any, the
6405 entire argument is passed to the system's command shell for parsing
6406 (this is C</bin/sh -c> on Unix platforms, but varies on other
6407 platforms). If there are no shell metacharacters in the argument,
6408 it is split into words and passed directly to C<execvp>, which is
6411 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6412 output before any operation that may do a fork, but this may not be
6413 supported on some platforms (see L<perlport>). To be safe, you may need
6414 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6415 of C<IO::Handle> on any open handles.
6417 The return value is the exit status of the program as returned by the
6418 C<wait> call. To get the actual exit value, shift right by eight (see
6419 below). See also L</exec>. This is I<not> what you want to use to capture
6420 the output from a command, for that you should use merely backticks or
6421 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6422 indicates a failure to start the program or an error of the wait(2) system
6423 call (inspect $! for the reason).
6425 Like C<exec>, C<system> allows you to lie to a program about its name if
6426 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6428 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6429 C<system>, if you expect your program to terminate on receipt of these
6430 signals you will need to arrange to do so yourself based on the return
6433 @args = ("command", "arg1", "arg2");
6435 or die "system @args failed: $?"
6437 You can check all the failure possibilities by inspecting
6441 print "failed to execute: $!\n";
6444 printf "child died with signal %d, %s coredump\n",
6445 ($? & 127), ($? & 128) ? 'with' : 'without';
6448 printf "child exited with value %d\n", $? >> 8;
6451 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6452 with the W*() calls of the POSIX extension.
6454 When the arguments get executed via the system shell, results
6455 and return codes will be subject to its quirks and capabilities.
6456 See L<perlop/"`STRING`"> and L</exec> for details.
6458 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6461 =item syswrite FILEHANDLE,SCALAR,LENGTH
6463 =item syswrite FILEHANDLE,SCALAR
6465 Attempts to write LENGTH bytes of data from variable SCALAR to the
6466 specified FILEHANDLE, using the system call write(2). If LENGTH is
6467 not specified, writes whole SCALAR. It bypasses buffered IO, so
6468 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6469 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6470 stdio layers usually buffers data. Returns the number of bytes
6471 actually written, or C<undef> if there was an error (in this case the
6472 errno variable C<$!> is also set). If the LENGTH is greater than the
6473 available data in the SCALAR after the OFFSET, only as much data as is
6474 available will be written.
6476 An OFFSET may be specified to write the data from some part of the
6477 string other than the beginning. A negative OFFSET specifies writing
6478 that many characters counting backwards from the end of the string.
6479 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6481 Note that if the filehandle has been marked as C<:utf8>, Unicode
6482 characters are written instead of bytes (the LENGTH, OFFSET, and the
6483 return value of syswrite() are in UTF-8 encoded Unicode characters).
6484 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6485 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6487 =item tell FILEHANDLE
6492 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6493 error. FILEHANDLE may be an expression whose value gives the name of
6494 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6497 Note the I<in bytes>: even if the filehandle has been set to
6498 operate on characters (for example by using the C<:encoding(utf8)> open
6499 layer), tell() will return byte offsets, not character offsets (because
6500 that would render seek() and tell() rather slow).
6502 The return value of tell() for the standard streams like the STDIN
6503 depends on the operating system: it may return -1 or something else.
6504 tell() on pipes, fifos, and sockets usually returns -1.
6506 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6508 Do not use tell() (or other buffered I/O operations) on a file handle
6509 that has been manipulated by sysread(), syswrite() or sysseek().
6510 Those functions ignore the buffering, while tell() does not.
6512 =item telldir DIRHANDLE
6515 Returns the current position of the C<readdir> routines on DIRHANDLE.
6516 Value may be given to C<seekdir> to access a particular location in a
6517 directory. C<telldir> has the same caveats about possible directory
6518 compaction as the corresponding system library routine.
6520 =item tie VARIABLE,CLASSNAME,LIST
6523 This function binds a variable to a package class that will provide the
6524 implementation for the variable. VARIABLE is the name of the variable
6525 to be enchanted. CLASSNAME is the name of a class implementing objects
6526 of correct type. Any additional arguments are passed to the C<new>
6527 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6528 or C<TIEHASH>). Typically these are arguments such as might be passed
6529 to the C<dbm_open()> function of C. The object returned by the C<new>
6530 method is also returned by the C<tie> function, which would be useful
6531 if you want to access other methods in CLASSNAME.
6533 Note that functions such as C<keys> and C<values> may return huge lists
6534 when used on large objects, like DBM files. You may prefer to use the
6535 C<each> function to iterate over such. Example:
6537 # print out history file offsets
6539 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6540 while (($key,$val) = each %HIST) {
6541 print $key, ' = ', unpack('L',$val), "\n";
6545 A class implementing a hash should have the following methods:
6547 TIEHASH classname, LIST
6549 STORE this, key, value
6554 NEXTKEY this, lastkey
6559 A class implementing an ordinary array should have the following methods:
6561 TIEARRAY classname, LIST
6563 STORE this, key, value
6565 STORESIZE this, count
6571 SPLICE this, offset, length, LIST
6576 A class implementing a file handle should have the following methods:
6578 TIEHANDLE classname, LIST
6579 READ this, scalar, length, offset
6582 WRITE this, scalar, length, offset
6584 PRINTF this, format, LIST
6588 SEEK this, position, whence
6590 OPEN this, mode, LIST
6595 A class implementing a scalar should have the following methods:
6597 TIESCALAR classname, LIST
6603 Not all methods indicated above need be implemented. See L<perltie>,
6604 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6606 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6607 for you--you need to do that explicitly yourself. See L<DB_File>
6608 or the F<Config> module for interesting C<tie> implementations.
6610 For further details see L<perltie>, L<"tied VARIABLE">.
6615 Returns a reference to the object underlying VARIABLE (the same value
6616 that was originally returned by the C<tie> call that bound the variable
6617 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6623 Returns the number of non-leap seconds since whatever time the system
6624 considers to be the epoch, suitable for feeding to C<gmtime> and
6625 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6626 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6627 1904 in the current local time zone for its epoch.
6629 For measuring time in better granularity than one second,
6630 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6631 Perl 5.8 part of the standard distribution), or if you have
6632 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6633 See L<perlfaq8> for details.
6635 For date and time processing look at the many related modules on CPAN.
6636 For a comprehensive date and time representation look at the
6642 Returns a four-element list giving the user and system times, in
6643 seconds, for this process and the children of this process.
6645 ($user,$system,$cuser,$csystem) = times;
6647 In scalar context, C<times> returns C<$user>.
6649 Note that times for children are included only after they terminate.
6653 The transliteration operator. Same as C<y///>. See L<perlop>.
6655 =item truncate FILEHANDLE,LENGTH
6658 =item truncate EXPR,LENGTH
6660 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6661 specified length. Produces a fatal error if truncate isn't implemented
6662 on your system. Returns true if successful, the undefined value
6665 The behavior is undefined if LENGTH is greater than the length of the
6668 The position in the file of FILEHANDLE is left unchanged. You may want to
6669 call L<seek> before writing to the file.
6672 X<uc> X<uppercase> X<toupper>
6676 Returns an uppercased version of EXPR. This is the internal function
6677 implementing the C<\U> escape in double-quoted strings. Respects
6678 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6679 and L<perlunicode> for more details about locale and Unicode support.
6680 It does not attempt to do titlecase mapping on initial letters. See
6681 C<ucfirst> for that.
6683 If EXPR is omitted, uses C<$_>.
6686 X<ucfirst> X<uppercase>
6690 Returns the value of EXPR with the first character in uppercase
6691 (titlecase in Unicode). This is the internal function implementing
6692 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6693 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6694 for more details about locale and Unicode support.
6696 If EXPR is omitted, uses C<$_>.
6703 Sets the umask for the process to EXPR and returns the previous value.
6704 If EXPR is omitted, merely returns the current umask.
6706 The Unix permission C<rwxr-x---> is represented as three sets of three
6707 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6708 and isn't one of the digits). The C<umask> value is such a number
6709 representing disabled permissions bits. The permission (or "mode")
6710 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6711 even if you tell C<sysopen> to create a file with permissions C<0777>,
6712 if your umask is C<0022> then the file will actually be created with
6713 permissions C<0755>. If your C<umask> were C<0027> (group can't
6714 write; others can't read, write, or execute), then passing
6715 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6718 Here's some advice: supply a creation mode of C<0666> for regular
6719 files (in C<sysopen>) and one of C<0777> for directories (in
6720 C<mkdir>) and executable files. This gives users the freedom of
6721 choice: if they want protected files, they might choose process umasks
6722 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6723 Programs should rarely if ever make policy decisions better left to
6724 the user. The exception to this is when writing files that should be
6725 kept private: mail files, web browser cookies, I<.rhosts> files, and
6728 If umask(2) is not implemented on your system and you are trying to
6729 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6730 fatal error at run time. If umask(2) is not implemented and you are
6731 not trying to restrict access for yourself, returns C<undef>.
6733 Remember that a umask is a number, usually given in octal; it is I<not> a
6734 string of octal digits. See also L</oct>, if all you have is a string.
6737 X<undef> X<undefine>
6741 Undefines the value of EXPR, which must be an lvalue. Use only on a
6742 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6743 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6744 will probably not do what you expect on most predefined variables or
6745 DBM list values, so don't do that; see L<delete>.) Always returns the
6746 undefined value. You can omit the EXPR, in which case nothing is
6747 undefined, but you still get an undefined value that you could, for
6748 instance, return from a subroutine, assign to a variable or pass as a
6749 parameter. Examples:
6752 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6756 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6757 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6758 select undef, undef, undef, 0.25;
6759 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6761 Note that this is a unary operator, not a list operator.
6764 X<unlink> X<delete> X<remove> X<rm> X<del>
6768 Deletes a list of files. Returns the number of files successfully
6771 $cnt = unlink 'a', 'b', 'c';
6775 Note: C<unlink> will not attempt to delete directories unless you are superuser
6776 and the B<-U> flag is supplied to Perl. Even if these conditions are
6777 met, be warned that unlinking a directory can inflict damage on your
6778 filesystem. Finally, using C<unlink> on directories is not supported on
6779 many operating systems. Use C<rmdir> instead.
6781 If LIST is omitted, uses C<$_>.
6783 =item unpack TEMPLATE,EXPR
6786 =item unpack TEMPLATE
6788 C<unpack> does the reverse of C<pack>: it takes a string
6789 and expands it out into a list of values.
6790 (In scalar context, it returns merely the first value produced.)
6792 If EXPR is omitted, unpacks the C<$_> string.
6794 The string is broken into chunks described by the TEMPLATE. Each chunk
6795 is converted separately to a value. Typically, either the string is a result
6796 of C<pack>, or the characters of the string represent a C structure of some
6799 The TEMPLATE has the same format as in the C<pack> function.
6800 Here's a subroutine that does substring:
6803 my($what,$where,$howmuch) = @_;
6804 unpack("x$where a$howmuch", $what);
6809 sub ordinal { unpack("W",$_[0]); } # same as ord()
6811 In addition to fields allowed in pack(), you may prefix a field with
6812 a %<number> to indicate that
6813 you want a <number>-bit checksum of the items instead of the items
6814 themselves. Default is a 16-bit checksum. Checksum is calculated by
6815 summing numeric values of expanded values (for string fields the sum of
6816 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6818 For example, the following
6819 computes the same number as the System V sum program:
6823 unpack("%32W*",<>) % 65535;
6826 The following efficiently counts the number of set bits in a bit vector:
6828 $setbits = unpack("%32b*", $selectmask);
6830 The C<p> and C<P> formats should be used with care. Since Perl
6831 has no way of checking whether the value passed to C<unpack()>
6832 corresponds to a valid memory location, passing a pointer value that's
6833 not known to be valid is likely to have disastrous consequences.
6835 If there are more pack codes or if the repeat count of a field or a group
6836 is larger than what the remainder of the input string allows, the result
6837 is not well defined: in some cases, the repeat count is decreased, or
6838 C<unpack()> will produce null strings or zeroes, or terminate with an
6839 error. If the input string is longer than one described by the TEMPLATE,
6840 the rest is ignored.
6842 See L</pack> for more examples and notes.
6844 =item untie VARIABLE
6847 Breaks the binding between a variable and a package. (See C<tie>.)
6848 Has no effect if the variable is not tied.
6850 =item unshift ARRAY,LIST
6853 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6854 depending on how you look at it. Prepends list to the front of the
6855 array, and returns the new number of elements in the array.
6857 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6859 Note the LIST is prepended whole, not one element at a time, so the
6860 prepended elements stay in the same order. Use C<reverse> to do the
6863 =item use Module VERSION LIST
6864 X<use> X<module> X<import>
6866 =item use Module VERSION
6868 =item use Module LIST
6874 Imports some semantics into the current package from the named module,
6875 generally by aliasing certain subroutine or variable names into your
6876 package. It is exactly equivalent to
6878 BEGIN { require Module; Module->import( LIST ); }
6880 except that Module I<must> be a bareword.
6882 In the peculiar C<use VERSION> form, VERSION may be either a numeric
6883 argument such as 5.006, which will be compared to C<$]>, or a literal of
6884 the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A
6885 fatal error is produced if VERSION is greater than the version of the
6886 current Perl interpreter; Perl will not attempt to parse the rest of the
6887 file. Compare with L</require>, which can do a similar check at run time.
6888 Symmetrically, C<no VERSION> allows you to specify that you want a version
6889 of perl older than the specified one.
6891 Specifying VERSION as a literal of the form v5.6.1 should generally be
6892 avoided, because it leads to misleading error messages under earlier
6893 versions of Perl (that is, prior to 5.6.0) that do not support this
6894 syntax. The equivalent numeric version should be used instead.
6896 use v5.6.1; # compile time version check
6898 use 5.006_001; # ditto; preferred for backwards compatibility
6900 This is often useful if you need to check the current Perl version before
6901 C<use>ing library modules that won't work with older versions of Perl.
6902 (We try not to do this more than we have to.)
6904 Also, if the specified perl version is greater than or equal to 5.9.5,
6905 C<use VERSION> will also load the C<feature> pragma and enable all
6906 features available in the requested version. See L<feature>.
6908 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6909 C<require> makes sure the module is loaded into memory if it hasn't been
6910 yet. The C<import> is not a builtin--it's just an ordinary static method
6911 call into the C<Module> package to tell the module to import the list of
6912 features back into the current package. The module can implement its
6913 C<import> method any way it likes, though most modules just choose to
6914 derive their C<import> method via inheritance from the C<Exporter> class that
6915 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6916 method can be found then the call is skipped, even if there is an AUTOLOAD
6919 If you do not want to call the package's C<import> method (for instance,
6920 to stop your namespace from being altered), explicitly supply the empty list:
6924 That is exactly equivalent to
6926 BEGIN { require Module }
6928 If the VERSION argument is present between Module and LIST, then the
6929 C<use> will call the VERSION method in class Module with the given
6930 version as an argument. The default VERSION method, inherited from
6931 the UNIVERSAL class, croaks if the given version is larger than the
6932 value of the variable C<$Module::VERSION>.
6934 Again, there is a distinction between omitting LIST (C<import> called
6935 with no arguments) and an explicit empty LIST C<()> (C<import> not
6936 called). Note that there is no comma after VERSION!
6938 Because this is a wide-open interface, pragmas (compiler directives)
6939 are also implemented this way. Currently implemented pragmas are:
6944 use sigtrap qw(SEGV BUS);
6945 use strict qw(subs vars refs);
6946 use subs qw(afunc blurfl);
6947 use warnings qw(all);
6948 use sort qw(stable _quicksort _mergesort);
6950 Some of these pseudo-modules import semantics into the current
6951 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6952 which import symbols into the current package (which are effective
6953 through the end of the file).
6955 There's a corresponding C<no> command that unimports meanings imported
6956 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6957 It behaves exactly as C<import> does with respect to VERSION, an
6958 omitted LIST, empty LIST, or no unimport method being found.
6964 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6965 for the C<-M> and C<-m> command-line options to perl that give C<use>
6966 functionality from the command-line.
6971 Changes the access and modification times on each file of a list of
6972 files. The first two elements of the list must be the NUMERICAL access
6973 and modification times, in that order. Returns the number of files
6974 successfully changed. The inode change time of each file is set
6975 to the current time. For example, this code has the same effect as the
6976 Unix touch(1) command when the files I<already exist> and belong to
6977 the user running the program:
6980 $atime = $mtime = time;
6981 utime $atime, $mtime, @ARGV;
6983 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6984 the utime(2) function in the C library will be called with a null second
6985 argument. On most systems, this will set the file's access and
6986 modification times to the current time (i.e. equivalent to the example
6987 above) and will even work on other users' files where you have write
6990 utime undef, undef, @ARGV;
6992 Under NFS this will use the time of the NFS server, not the time of
6993 the local machine. If there is a time synchronization problem, the
6994 NFS server and local machine will have different times. The Unix
6995 touch(1) command will in fact normally use this form instead of the
6996 one shown in the first example.
6998 Note that only passing one of the first two elements as C<undef> will
6999 be equivalent of passing it as 0 and will not have the same effect as
7000 described when they are both C<undef>. This case will also trigger an
7001 uninitialized warning.
7003 On systems that support futimes, you might pass file handles among the
7004 files. On systems that don't support futimes, passing file handles
7005 produces a fatal error at run time. The file handles must be passed
7006 as globs or references to be recognized. Barewords are considered
7014 Returns a list consisting of all the values of the named hash, or the values
7015 of an array. (In a scalar context, returns the number of values.)
7017 The values are returned in an apparently random order. The actual
7018 random order is subject to change in future versions of perl, but it
7019 is guaranteed to be the same order as either the C<keys> or C<each>
7020 function would produce on the same (unmodified) hash. Since Perl
7021 5.8.1 the ordering is different even between different runs of Perl
7022 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7024 As a side effect, calling values() resets the HASH or ARRAY's internal
7026 see L</each>. (In particular, calling values() in void context resets
7027 the iterator with no other overhead. Apart from resetting the iterator,
7028 C<values @array> in list context is no different to plain C<@array>.
7029 We recommend that you use void context C<keys @array> for this, but reasoned
7030 that it taking C<values @array> out would require more documentation than
7034 Note that the values are not copied, which means modifying them will
7035 modify the contents of the hash:
7037 for (values %hash) { s/foo/bar/g } # modifies %hash values
7038 for (@hash{keys %hash}) { s/foo/bar/g } # same
7040 See also C<keys>, C<each>, and C<sort>.
7042 =item vec EXPR,OFFSET,BITS
7043 X<vec> X<bit> X<bit vector>
7045 Treats the string in EXPR as a bit vector made up of elements of
7046 width BITS, and returns the value of the element specified by OFFSET
7047 as an unsigned integer. BITS therefore specifies the number of bits
7048 that are reserved for each element in the bit vector. This must
7049 be a power of two from 1 to 32 (or 64, if your platform supports
7052 If BITS is 8, "elements" coincide with bytes of the input string.
7054 If BITS is 16 or more, bytes of the input string are grouped into chunks
7055 of size BITS/8, and each group is converted to a number as with
7056 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7057 for BITS==64). See L<"pack"> for details.
7059 If bits is 4 or less, the string is broken into bytes, then the bits
7060 of each byte are broken into 8/BITS groups. Bits of a byte are
7061 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7062 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7063 breaking the single input byte C<chr(0x36)> into two groups gives a list
7064 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7066 C<vec> may also be assigned to, in which case parentheses are needed
7067 to give the expression the correct precedence as in
7069 vec($image, $max_x * $x + $y, 8) = 3;
7071 If the selected element is outside the string, the value 0 is returned.
7072 If an element off the end of the string is written to, Perl will first
7073 extend the string with sufficiently many zero bytes. It is an error
7074 to try to write off the beginning of the string (i.e. negative OFFSET).
7076 If the string happens to be encoded as UTF-8 internally (and thus has
7077 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7078 internal byte string, not the conceptual character string, even if you
7079 only have characters with values less than 256.
7081 Strings created with C<vec> can also be manipulated with the logical
7082 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7083 vector operation is desired when both operands are strings.
7084 See L<perlop/"Bitwise String Operators">.
7086 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7087 The comments show the string after each step. Note that this code works
7088 in the same way on big-endian or little-endian machines.
7091 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7093 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7094 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7096 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7097 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7098 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7099 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7100 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7101 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7103 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7104 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7105 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7108 To transform a bit vector into a string or list of 0's and 1's, use these:
7110 $bits = unpack("b*", $vector);
7111 @bits = split(//, unpack("b*", $vector));
7113 If you know the exact length in bits, it can be used in place of the C<*>.
7115 Here is an example to illustrate how the bits actually fall in place:
7121 unpack("V",$_) 01234567890123456789012345678901
7122 ------------------------------------------------------------------
7127 for ($shift=0; $shift < $width; ++$shift) {
7128 for ($off=0; $off < 32/$width; ++$off) {
7129 $str = pack("B*", "0"x32);
7130 $bits = (1<<$shift);
7131 vec($str, $off, $width) = $bits;
7132 $res = unpack("b*",$str);
7133 $val = unpack("V", $str);
7140 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7141 $off, $width, $bits, $val, $res
7145 Regardless of the machine architecture on which it is run, the above
7146 example should print the following table:
7149 unpack("V",$_) 01234567890123456789012345678901
7150 ------------------------------------------------------------------
7151 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7152 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7153 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7154 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7155 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7156 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7157 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7158 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7159 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7160 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7161 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7162 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7163 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7164 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7165 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7166 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7167 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7168 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7169 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7170 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7171 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7172 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7173 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7174 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7175 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7176 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7177 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7178 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7179 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7180 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7181 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7182 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7183 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7184 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7185 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7186 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7187 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7188 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7189 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7190 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7191 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7192 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7193 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7194 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7195 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7196 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7197 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7198 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7199 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7200 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7201 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7202 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7203 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7204 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7205 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7206 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7207 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7208 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7209 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7210 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7211 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7212 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7213 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7214 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7215 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7216 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7217 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7218 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7219 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7220 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7221 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7222 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7223 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7224 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7225 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7226 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7227 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7228 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7229 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7230 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7231 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7232 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7233 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7234 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7235 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7236 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7237 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7238 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7239 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7240 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7241 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7242 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7243 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7244 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7245 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7246 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7247 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7248 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7249 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7250 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7251 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7252 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7253 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7254 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7255 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7256 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7257 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7258 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7259 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7260 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7261 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7262 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7263 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7264 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7265 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7266 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7267 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7268 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7269 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7270 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7271 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7272 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7273 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7274 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7275 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7276 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7277 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7278 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7283 Behaves like the wait(2) system call on your system: it waits for a child
7284 process to terminate and returns the pid of the deceased process, or
7285 C<-1> if there are no child processes. The status is returned in C<$?>
7286 and C<{^CHILD_ERROR_NATIVE}>.
7287 Note that a return value of C<-1> could mean that child processes are
7288 being automatically reaped, as described in L<perlipc>.
7290 =item waitpid PID,FLAGS
7293 Waits for a particular child process to terminate and returns the pid of
7294 the deceased process, or C<-1> if there is no such child process. On some
7295 systems, a value of 0 indicates that there are processes still running.
7296 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7298 use POSIX ":sys_wait_h";
7301 $kid = waitpid(-1, WNOHANG);
7304 then you can do a non-blocking wait for all pending zombie processes.
7305 Non-blocking wait is available on machines supporting either the
7306 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7307 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7308 system call by remembering the status values of processes that have
7309 exited but have not been harvested by the Perl script yet.)
7311 Note that on some systems, a return value of C<-1> could mean that child
7312 processes are being automatically reaped. See L<perlipc> for details,
7313 and for other examples.
7316 X<wantarray> X<context>
7318 Returns true if the context of the currently executing subroutine or
7319 C<eval> is looking for a list value. Returns false if the context is
7320 looking for a scalar. Returns the undefined value if the context is
7321 looking for no value (void context).
7323 return unless defined wantarray; # don't bother doing more
7324 my @a = complex_calculation();
7325 return wantarray ? @a : "@a";
7327 C<wantarray()>'s result is unspecified in the top level of a file,
7328 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7329 in a C<DESTROY> method.
7331 This function should have been named wantlist() instead.
7334 X<warn> X<warning> X<STDERR>
7336 Prints the value of LIST to STDERR. If the last element of LIST does
7337 not end in a newline, it appends the same file/line number text as C<die>
7340 If LIST is empty and C<$@> already contains a value (typically from a
7341 previous eval) that value is used after appending C<"\t...caught">
7342 to C<$@>. This is useful for staying almost, but not entirely similar to
7345 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7347 No message is printed if there is a C<$SIG{__WARN__}> handler
7348 installed. It is the handler's responsibility to deal with the message
7349 as it sees fit (like, for instance, converting it into a C<die>). Most
7350 handlers must therefore make arrangements to actually display the
7351 warnings that they are not prepared to deal with, by calling C<warn>
7352 again in the handler. Note that this is quite safe and will not
7353 produce an endless loop, since C<__WARN__> hooks are not called from
7356 You will find this behavior is slightly different from that of
7357 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7358 instead call C<die> again to change it).
7360 Using a C<__WARN__> handler provides a powerful way to silence all
7361 warnings (even the so-called mandatory ones). An example:
7363 # wipe out *all* compile-time warnings
7364 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7366 my $foo = 20; # no warning about duplicate my $foo,
7367 # but hey, you asked for it!
7368 # no compile-time or run-time warnings before here
7371 # run-time warnings enabled after here
7372 warn "\$foo is alive and $foo!"; # does show up
7374 See L<perlvar> for details on setting C<%SIG> entries, and for more
7375 examples. See the Carp module for other kinds of warnings using its
7376 carp() and cluck() functions.
7378 =item write FILEHANDLE
7385 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7386 using the format associated with that file. By default the format for
7387 a file is the one having the same name as the filehandle, but the
7388 format for the current output channel (see the C<select> function) may be set
7389 explicitly by assigning the name of the format to the C<$~> variable.
7391 Top of form processing is handled automatically: if there is
7392 insufficient room on the current page for the formatted record, the
7393 page is advanced by writing a form feed, a special top-of-page format
7394 is used to format the new page header, and then the record is written.
7395 By default the top-of-page format is the name of the filehandle with
7396 "_TOP" appended, but it may be dynamically set to the format of your
7397 choice by assigning the name to the C<$^> variable while the filehandle is
7398 selected. The number of lines remaining on the current page is in
7399 variable C<$->, which can be set to C<0> to force a new page.
7401 If FILEHANDLE is unspecified, output goes to the current default output
7402 channel, which starts out as STDOUT but may be changed by the
7403 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7404 is evaluated and the resulting string is used to look up the name of
7405 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7407 Note that write is I<not> the opposite of C<read>. Unfortunately.
7411 The transliteration operator. Same as C<tr///>. See L<perlop>.