4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides a scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, the scalar arguments will
18 be first, and the list argument will follow. (Note that there can ever
19 be only one such list argument.) For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for the elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use the parentheses, the simple (but occasionally
34 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
35 function, and precedence doesn't matter. Otherwise it's a list
36 operator or unary operator, and precedence does matter. And whitespace
37 between the function and left parenthesis doesn't count--so you need to
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
52 A few functions take no arguments at all, and therefore work as neither
53 unary nor list operators. These include such functions as C<time>
54 and C<endpwent>. For example, C<time+86_400> always means
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in a scalar context by
59 returning the undefined value, and in a list context by returning the
62 Remember the following important rule: There is B<no rule> that relates
63 the behavior of an expression in list context to its behavior in scalar
64 context, or vice versa. It might do two totally different things.
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in scalar context. Some operators return the
67 length of the list that would have been returned in list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
74 A named array in scalar context is quite different from what would at
75 first glance appear to be a list in scalar context. You can't get a list
76 like C<(1,2,3)> into being in scalar context, because the compiler knows
77 the context at compile time. It would generate the scalar comma operator
78 there, not the list construction version of the comma. That means it
79 was never a list to start with.
81 In general, functions in Perl that serve as wrappers for system calls
82 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule are C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 =head2 Perl Functions by Category
92 Here are Perl's functions (including things that look like
93 functions, like some keywords and named operators)
94 arranged by category. Some functions appear in more
99 =item Functions for SCALARs or strings
100 X<scalar> X<string> X<character>
102 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
103 C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
104 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
106 =item Regular expressions and pattern matching
107 X<regular expression> X<regex> X<regexp>
109 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
111 =item Numeric functions
112 X<numeric> X<number> X<trigonometric> X<trigonometry>
114 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115 C<sin>, C<sqrt>, C<srand>
117 =item Functions for real @ARRAYs
120 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
122 =item Functions for list data
125 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
127 =item Functions for real %HASHes
130 C<delete>, C<each>, C<exists>, C<keys>, C<values>
132 =item Input and output functions
133 X<I/O> X<input> X<output> X<dbm>
135 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
137 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
138 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
141 =item Functions for fixed length data or records
143 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
145 =item Functions for filehandles, files, or directories
146 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
148 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
149 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
150 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151 C<umask>, C<unlink>, C<utime>
153 =item Keywords related to the control flow of your Perl program
156 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
159 =item Keywords related to switch
161 C<break>, C<continue>, C<given>, C<when>, C<default>
163 (These are only available if you enable the "switch" feature.
164 See L<feature> and L<perlsyn/"Switch statements">.)
166 =item Keywords related to scoping
168 C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
171 (C<state> is only available if the "state" feature is enabled. See
174 =item Miscellaneous functions
176 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
177 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
179 =item Functions for processes and process groups
180 X<process> X<pid> X<process id>
182 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
183 C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
184 C<times>, C<wait>, C<waitpid>
186 =item Keywords related to perl modules
189 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
191 =item Keywords related to classes and object-orientation
192 X<object> X<class> X<package>
194 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
197 =item Low-level socket functions
200 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
202 C<socket>, C<socketpair>
204 =item System V interprocess communication functions
205 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
207 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
210 =item Fetching user and group info
211 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
213 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215 C<getpwuid>, C<setgrent>, C<setpwent>
217 =item Fetching network info
218 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
220 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224 C<setnetent>, C<setprotoent>, C<setservent>
226 =item Time-related functions
229 C<gmtime>, C<localtime>, C<time>, C<times>
231 =item Functions new in perl5
234 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
236 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
237 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238 C<ucfirst>, C<untie>, C<use>, C<when>
240 * - C<sub> was a keyword in perl4, but in perl5 it is an
241 operator, which can be used in expressions.
243 =item Functions obsoleted in perl5
245 C<dbmclose>, C<dbmopen>
250 X<portability> X<Unix> X<portable>
252 Perl was born in Unix and can therefore access all common Unix
253 system calls. In non-Unix environments, the functionality of some
254 Unix system calls may not be available, or details of the available
255 functionality may differ slightly. The Perl functions affected
258 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
261 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
263 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
264 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
267 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
268 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
271 C<shmwrite>, C<socket>, C<socketpair>,
272 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273 C<times>, C<truncate>, C<umask>, C<unlink>,
274 C<utime>, C<wait>, C<waitpid>
276 For more information about the portability of these functions, see
277 L<perlport> and other available platform-specific documentation.
279 =head2 Alphabetical Listing of Perl Functions
284 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
285 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
293 A file test, where X is one of the letters listed below. This unary
294 operator takes one argument, either a filename, a filehandle, or a dirhandle,
295 and tests the associated file to see if something is true about it. If the
296 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
297 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
298 the undefined value if the file doesn't exist. Despite the funny
299 names, precedence is the same as any other named unary operator. The
300 operator may be any of:
302 -r File is readable by effective uid/gid.
303 -w File is writable by effective uid/gid.
304 -x File is executable by effective uid/gid.
305 -o File is owned by effective uid.
307 -R File is readable by real uid/gid.
308 -W File is writable by real uid/gid.
309 -X File is executable by real uid/gid.
310 -O File is owned by real uid.
313 -z File has zero size (is empty).
314 -s File has nonzero size (returns size in bytes).
316 -f File is a plain file.
317 -d File is a directory.
318 -l File is a symbolic link.
319 -p File is a named pipe (FIFO), or Filehandle is a pipe.
321 -b File is a block special file.
322 -c File is a character special file.
323 -t Filehandle is opened to a tty.
325 -u File has setuid bit set.
326 -g File has setgid bit set.
327 -k File has sticky bit set.
329 -T File is an ASCII text file (heuristic guess).
330 -B File is a "binary" file (opposite of -T).
332 -M Script start time minus file modification time, in days.
333 -A Same for access time.
334 -C Same for inode change time (Unix, may differ for other platforms)
340 next unless -f $_; # ignore specials
344 The interpretation of the file permission operators C<-r>, C<-R>,
345 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
346 of the file and the uids and gids of the user. There may be other
347 reasons you can't actually read, write, or execute the file: for
348 example network filesystem access controls, ACLs (access control lists),
349 read-only filesystems, and unrecognized executable formats. Note
350 that the use of these six specific operators to verify if some operation
351 is possible is usually a mistake, because it may be open to race
354 Also note that, for the superuser on the local filesystems, the C<-r>,
355 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
356 if any execute bit is set in the mode. Scripts run by the superuser
357 may thus need to do a stat() to determine the actual mode of the file,
358 or temporarily set their effective uid to something else.
360 If you are using ACLs, there is a pragma called C<filetest> that may
361 produce more accurate results than the bare stat() mode bits.
362 When under the C<use filetest 'access'> the above-mentioned filetests
363 will test whether the permission can (not) be granted using the
364 access() family of system calls. Also note that the C<-x> and C<-X> may
365 under this pragma return true even if there are no execute permission
366 bits set (nor any extra execute permission ACLs). This strangeness is
367 due to the underlying system calls' definitions. Note also that, due to
368 the implementation of C<use filetest 'access'>, the C<_> special
369 filehandle won't cache the results of the file tests when this pragma is
370 in effect. Read the documentation for the C<filetest> pragma for more
373 Note that C<-s/a/b/> does not do a negated substitution. Saying
374 C<-exp($foo)> still works as expected, however--only single letters
375 following a minus are interpreted as file tests.
377 The C<-T> and C<-B> switches work as follows. The first block or so of the
378 file is examined for odd characters such as strange control codes or
379 characters with the high bit set. If too many strange characters (>30%)
380 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
381 containing null in the first block is considered a binary file. If C<-T>
382 or C<-B> is used on a filehandle, the current IO buffer is examined
383 rather than the first block. Both C<-T> and C<-B> return true on a null
384 file, or a file at EOF when testing a filehandle. Because you have to
385 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
386 against the file first, as in C<next unless -f $file && -T $file>.
388 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
389 the special filehandle consisting of a solitary underline, then the stat
390 structure of the previous file test (or stat operator) is used, saving
391 a system call. (This doesn't work with C<-t>, and you need to remember
392 that lstat() and C<-l> will leave values in the stat structure for the
393 symbolic link, not the real file.) (Also, if the stat buffer was filled by
394 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
397 print "Can do.\n" if -r $a || -w _ || -x _;
400 print "Readable\n" if -r _;
401 print "Writable\n" if -w _;
402 print "Executable\n" if -x _;
403 print "Setuid\n" if -u _;
404 print "Setgid\n" if -g _;
405 print "Sticky\n" if -k _;
406 print "Text\n" if -T _;
407 print "Binary\n" if -B _;
409 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
410 test operators, in a way that C<-f -w -x $file> is equivalent to
411 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
412 the return value of C<-f $file> as an argument to another filetest
413 operator, no special magic will happen.)
420 Returns the absolute value of its argument.
421 If VALUE is omitted, uses C<$_>.
423 =item accept NEWSOCKET,GENERICSOCKET
426 Accepts an incoming socket connect, just as the accept(2) system call
427 does. Returns the packed address if it succeeded, false otherwise.
428 See the example in L<perlipc/"Sockets: Client/Server Communication">.
430 On systems that support a close-on-exec flag on files, the flag will
431 be set for the newly opened file descriptor, as determined by the
432 value of $^F. See L<perlvar/$^F>.
441 Arranges to have a SIGALRM delivered to this process after the
442 specified number of wallclock seconds has elapsed. If SECONDS is not
443 specified, the value stored in C<$_> is used. (On some machines,
444 unfortunately, the elapsed time may be up to one second less or more
445 than you specified because of how seconds are counted, and process
446 scheduling may delay the delivery of the signal even further.)
448 Only one timer may be counting at once. Each call disables the
449 previous timer, and an argument of C<0> may be supplied to cancel the
450 previous timer without starting a new one. The returned value is the
451 amount of time remaining on the previous timer.
453 For delays of finer granularity than one second, the Time::HiRes module
454 (from CPAN, and starting from Perl 5.8 part of the standard
455 distribution) provides ualarm(). You may also use Perl's four-argument
456 version of select() leaving the first three arguments undefined, or you
457 might be able to use the C<syscall> interface to access setitimer(2) if
458 your system supports it. See L<perlfaq8> for details.
460 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
461 (C<sleep> may be internally implemented in your system with C<alarm>)
463 If you want to use C<alarm> to time out a system call you need to use an
464 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
465 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
466 restart system calls on some systems. Using C<eval>/C<die> always works,
467 modulo the caveats given in L<perlipc/"Signals">.
470 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
472 $nread = sysread SOCKET, $buffer, $size;
476 die unless $@ eq "alarm\n"; # propagate unexpected errors
483 For more information see L<perlipc>.
486 X<atan2> X<arctangent> X<tan> X<tangent>
488 Returns the arctangent of Y/X in the range -PI to PI.
490 For the tangent operation, you may use the C<Math::Trig::tan>
491 function, or use the familiar relation:
493 sub tan { sin($_[0]) / cos($_[0]) }
495 Note that atan2(0, 0) is not well-defined.
497 =item bind SOCKET,NAME
500 Binds a network address to a socket, just as the bind system call
501 does. Returns true if it succeeded, false otherwise. NAME should be a
502 packed address of the appropriate type for the socket. See the examples in
503 L<perlipc/"Sockets: Client/Server Communication">.
505 =item binmode FILEHANDLE, LAYER
506 X<binmode> X<binary> X<text> X<DOS> X<Windows>
508 =item binmode FILEHANDLE
510 Arranges for FILEHANDLE to be read or written in "binary" or "text"
511 mode on systems where the run-time libraries distinguish between
512 binary and text files. If FILEHANDLE is an expression, the value is
513 taken as the name of the filehandle. Returns true on success,
514 otherwise it returns C<undef> and sets C<$!> (errno).
516 On some systems (in general, DOS and Windows-based systems) binmode()
517 is necessary when you're not working with a text file. For the sake
518 of portability it is a good idea to always use it when appropriate,
519 and to never use it when it isn't appropriate. Also, people can
520 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
522 In other words: regardless of platform, use binmode() on binary data,
523 like for example images.
525 If LAYER is present it is a single string, but may contain multiple
526 directives. The directives alter the behaviour of the file handle.
527 When LAYER is present using binmode on text file makes sense.
529 If LAYER is omitted or specified as C<:raw> the filehandle is made
530 suitable for passing binary data. This includes turning off possible CRLF
531 translation and marking it as bytes (as opposed to Unicode characters).
532 Note that, despite what may be implied in I<"Programming Perl"> (the
533 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
534 -- other layers which would affect the binary nature of the stream are
535 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
536 PERLIO environment variable.
538 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
539 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
540 establish default I/O layers. See L<open>.
542 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
543 in "Programming Perl, 3rd Edition". However, since the publishing of this
544 book, by many known as "Camel III", the consensus of the naming of this
545 functionality has moved from "discipline" to "layer". All documentation
546 of this version of Perl therefore refers to "layers" rather than to
547 "disciplines". Now back to the regularly scheduled documentation...>
549 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
550 C<:utf8> just marks the data as UTF-8 without further checking,
551 while C<:encoding(utf8)> checks the data for actually being valid
552 UTF-8. More details can be found in L<PerlIO::encoding>.
554 In general, binmode() should be called after open() but before any I/O
555 is done on the filehandle. Calling binmode() will normally flush any
556 pending buffered output data (and perhaps pending input data) on the
557 handle. An exception to this is the C<:encoding> layer that
558 changes the default character encoding of the handle, see L<open>.
559 The C<:encoding> layer sometimes needs to be called in
560 mid-stream, and it doesn't flush the stream. The C<:encoding>
561 also implicitly pushes on top of itself the C<:utf8> layer because
562 internally Perl will operate on UTF-8 encoded Unicode characters.
564 The operating system, device drivers, C libraries, and Perl run-time
565 system all work together to let the programmer treat a single
566 character (C<\n>) as the line terminator, irrespective of the external
567 representation. On many operating systems, the native text file
568 representation matches the internal representation, but on some
569 platforms the external representation of C<\n> is made up of more than
572 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
573 character to end each line in the external representation of text (even
574 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
575 on Unix and most VMS files). In other systems like OS/2, DOS and the
576 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
577 but what's stored in text files are the two characters C<\cM\cJ>. That
578 means that, if you don't use binmode() on these systems, C<\cM\cJ>
579 sequences on disk will be converted to C<\n> on input, and any C<\n> in
580 your program will be converted back to C<\cM\cJ> on output. This is what
581 you want for text files, but it can be disastrous for binary files.
583 Another consequence of using binmode() (on some systems) is that
584 special end-of-file markers will be seen as part of the data stream.
585 For systems from the Microsoft family this means that if your binary
586 data contains C<\cZ>, the I/O subsystem will regard it as the end of
587 the file, unless you use binmode().
589 binmode() is not only important for readline() and print() operations,
590 but also when using read(), seek(), sysread(), syswrite() and tell()
591 (see L<perlport> for more details). See the C<$/> and C<$\> variables
592 in L<perlvar> for how to manually set your input and output
593 line-termination sequences.
595 =item bless REF,CLASSNAME
600 This function tells the thingy referenced by REF that it is now an object
601 in the CLASSNAME package. If CLASSNAME is omitted, the current package
602 is used. Because a C<bless> is often the last thing in a constructor,
603 it returns the reference for convenience. Always use the two-argument
604 version if a derived class might inherit the function doing the blessing.
605 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
608 Consider always blessing objects in CLASSNAMEs that are mixed case.
609 Namespaces with all lowercase names are considered reserved for
610 Perl pragmata. Builtin types have all uppercase names. To prevent
611 confusion, you may wish to avoid such package names as well. Make sure
612 that CLASSNAME is a true value.
614 See L<perlmod/"Perl Modules">.
618 Break out of a C<given()> block.
620 This keyword is enabled by the "switch" feature: see L<feature>
621 for more information.
624 X<caller> X<call stack> X<stack> X<stack trace>
628 Returns the context of the current subroutine call. In scalar context,
629 returns the caller's package name if there is a caller, that is, if
630 we're in a subroutine or C<eval> or C<require>, and the undefined value
631 otherwise. In list context, returns
634 ($package, $filename, $line) = caller;
636 With EXPR, it returns some extra information that the debugger uses to
637 print a stack trace. The value of EXPR indicates how many call frames
638 to go back before the current one.
641 ($package, $filename, $line, $subroutine, $hasargs,
644 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
647 Here $subroutine may be C<(eval)> if the frame is not a subroutine
648 call, but an C<eval>. In such a case additional elements $evaltext and
649 C<$is_require> are set: C<$is_require> is true if the frame is created by a
650 C<require> or C<use> statement, $evaltext contains the text of the
651 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
652 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
653 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
654 frame.) $subroutine may also be C<(unknown)> if this particular
655 subroutine happens to have been deleted from the symbol table.
656 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
657 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
658 compiled with. The C<$hints> and C<$bitmask> values are subject to change
659 between versions of Perl, and are not meant for external use.
661 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
662 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
663 of this hash, as they are the actual values stored in the optree.
665 Furthermore, when called from within the DB package, caller returns more
666 detailed information: it sets the list variable C<@DB::args> to be the
667 arguments with which the subroutine was invoked.
669 Be aware that the optimizer might have optimized call frames away before
670 C<caller> had a chance to get the information. That means that C<caller(N)>
671 might not return information about the call frame you expect it do, for
672 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
673 previous time C<caller> was called.
680 =item chdir FILEHANDLE
682 =item chdir DIRHANDLE
686 Changes the working directory to EXPR, if possible. If EXPR is omitted,
687 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
688 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
689 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
690 neither is set, C<chdir> does nothing. It returns true upon success,
691 false otherwise. See the example under C<die>.
693 On systems that support fchdir, you might pass a file handle or
694 directory handle as argument. On systems that don't support fchdir,
695 passing handles produces a fatal error at run time.
698 X<chmod> X<permission> X<mode>
700 Changes the permissions of a list of files. The first element of the
701 list must be the numerical mode, which should probably be an octal
702 number, and which definitely should I<not> be a string of octal digits:
703 C<0644> is okay, C<'0644'> is not. Returns the number of files
704 successfully changed. See also L</oct>, if all you have is a string.
706 $cnt = chmod 0755, 'foo', 'bar';
707 chmod 0755, @executables;
708 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
710 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
711 $mode = 0644; chmod $mode, 'foo'; # this is best
713 On systems that support fchmod, you might pass file handles among the
714 files. On systems that don't support fchmod, passing file handles
715 produces a fatal error at run time. The file handles must be passed
716 as globs or references to be recognized. Barewords are considered
719 open(my $fh, "<", "foo");
720 my $perm = (stat $fh)[2] & 07777;
721 chmod($perm | 0600, $fh);
723 You can also import the symbolic C<S_I*> constants from the Fcntl
728 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
729 # This is identical to the chmod 0755 of the above example.
732 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
738 This safer version of L</chop> removes any trailing string
739 that corresponds to the current value of C<$/> (also known as
740 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
741 number of characters removed from all its arguments. It's often used to
742 remove the newline from the end of an input record when you're worried
743 that the final record may be missing its newline. When in paragraph
744 mode (C<$/ = "">), it removes all trailing newlines from the string.
745 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
746 a reference to an integer or the like, see L<perlvar>) chomp() won't
748 If VARIABLE is omitted, it chomps C<$_>. Example:
751 chomp; # avoid \n on last field
756 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
758 You can actually chomp anything that's an lvalue, including an assignment:
761 chomp($answer = <STDIN>);
763 If you chomp a list, each element is chomped, and the total number of
764 characters removed is returned.
766 Note that parentheses are necessary when you're chomping anything
767 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
768 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
769 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
770 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
780 Chops off the last character of a string and returns the character
781 chopped. It is much more efficient than C<s/.$//s> because it neither
782 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
783 If VARIABLE is a hash, it chops the hash's values, but not its keys.
785 You can actually chop anything that's an lvalue, including an assignment.
787 If you chop a list, each element is chopped. Only the value of the
788 last C<chop> is returned.
790 Note that C<chop> returns the last character. To return all but the last
791 character, use C<substr($string, 0, -1)>.
796 X<chown> X<owner> X<user> X<group>
798 Changes the owner (and group) of a list of files. The first two
799 elements of the list must be the I<numeric> uid and gid, in that
800 order. A value of -1 in either position is interpreted by most
801 systems to leave that value unchanged. Returns the number of files
802 successfully changed.
804 $cnt = chown $uid, $gid, 'foo', 'bar';
805 chown $uid, $gid, @filenames;
807 On systems that support fchown, you might pass file handles among the
808 files. On systems that don't support fchown, passing file handles
809 produces a fatal error at run time. The file handles must be passed
810 as globs or references to be recognized. Barewords are considered
813 Here's an example that looks up nonnumeric uids in the passwd file:
816 chomp($user = <STDIN>);
818 chomp($pattern = <STDIN>);
820 ($login,$pass,$uid,$gid) = getpwnam($user)
821 or die "$user not in passwd file";
823 @ary = glob($pattern); # expand filenames
824 chown $uid, $gid, @ary;
826 On most systems, you are not allowed to change the ownership of the
827 file unless you're the superuser, although you should be able to change
828 the group to any of your secondary groups. On insecure systems, these
829 restrictions may be relaxed, but this is not a portable assumption.
830 On POSIX systems, you can detect this condition this way:
832 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
833 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
836 X<chr> X<character> X<ASCII> X<Unicode>
840 Returns the character represented by that NUMBER in the character set.
841 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
842 chr(0x263a) is a Unicode smiley face.
844 Negative values give the Unicode replacement character (chr(0xfffd)),
845 except under the L<bytes> pragma, where low eight bits of the value
846 (truncated to an integer) are used.
848 If NUMBER is omitted, uses C<$_>.
850 For the reverse, use L</ord>.
852 Note that characters from 128 to 255 (inclusive) are by default
853 internally not encoded as UTF-8 for backward compatibility reasons.
855 See L<perlunicode> for more about Unicode.
857 =item chroot FILENAME
862 This function works like the system call by the same name: it makes the
863 named directory the new root directory for all further pathnames that
864 begin with a C</> by your process and all its children. (It doesn't
865 change your current working directory, which is unaffected.) For security
866 reasons, this call is restricted to the superuser. If FILENAME is
867 omitted, does a C<chroot> to C<$_>.
869 =item close FILEHANDLE
874 Closes the file or pipe associated with the file handle, flushes the IO
875 buffers, and closes the system file descriptor. Returns true if those
876 operations have succeeded and if no error was reported by any PerlIO
877 layer. Closes the currently selected filehandle if the argument is
880 You don't have to close FILEHANDLE if you are immediately going to do
881 another C<open> on it, because C<open> will close it for you. (See
882 C<open>.) However, an explicit C<close> on an input file resets the line
883 counter (C<$.>), while the implicit close done by C<open> does not.
885 If the file handle came from a piped open, C<close> will additionally
886 return false if one of the other system calls involved fails, or if the
887 program exits with non-zero status. (If the only problem was that the
888 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
889 also waits for the process executing on the pipe to complete, in case you
890 want to look at the output of the pipe afterwards, and
891 implicitly puts the exit status value of that command into C<$?> and
892 C<${^CHILD_ERROR_NATIVE}>.
894 Prematurely closing the read end of a pipe (i.e. before the process
895 writing to it at the other end has closed it) will result in a
896 SIGPIPE being delivered to the writer. If the other end can't
897 handle that, be sure to read all the data before closing the pipe.
901 open(OUTPUT, '|sort >foo') # pipe to sort
902 or die "Can't start sort: $!";
903 #... # print stuff to output
904 close OUTPUT # wait for sort to finish
905 or warn $! ? "Error closing sort pipe: $!"
906 : "Exit status $? from sort";
907 open(INPUT, 'foo') # get sort's results
908 or die "Can't open 'foo' for input: $!";
910 FILEHANDLE may be an expression whose value can be used as an indirect
911 filehandle, usually the real filehandle name.
913 =item closedir DIRHANDLE
916 Closes a directory opened by C<opendir> and returns the success of that
919 =item connect SOCKET,NAME
922 Attempts to connect to a remote socket, just as the connect system call
923 does. Returns true if it succeeded, false otherwise. NAME should be a
924 packed address of the appropriate type for the socket. See the examples in
925 L<perlipc/"Sockets: Client/Server Communication">.
932 C<continue> is actually a flow control statement rather than a function. If
933 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
934 C<foreach>), it is always executed just before the conditional is about to
935 be evaluated again, just like the third part of a C<for> loop in C. Thus
936 it can be used to increment a loop variable, even when the loop has been
937 continued via the C<next> statement (which is similar to the C C<continue>
940 C<last>, C<next>, or C<redo> may appear within a C<continue>
941 block. C<last> and C<redo> will behave as if they had been executed within
942 the main block. So will C<next>, but since it will execute a C<continue>
943 block, it may be more entertaining.
946 ### redo always comes here
949 ### next always comes here
951 # then back the top to re-check EXPR
953 ### last always comes here
955 Omitting the C<continue> section is semantically equivalent to using an
956 empty one, logically enough. In that case, C<next> goes directly back
957 to check the condition at the top of the loop.
959 If the "switch" feature is enabled, C<continue> is also a
960 function that will break out of the current C<when> or C<default>
961 block, and fall through to the next case. See L<feature> and
962 L<perlsyn/"Switch statements"> for more information.
966 X<cos> X<cosine> X<acos> X<arccosine>
970 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
971 takes cosine of C<$_>.
973 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
974 function, or use this relation:
976 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
978 =item crypt PLAINTEXT,SALT
979 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
980 X<decrypt> X<cryptography> X<passwd> X<encrypt>
982 Creates a digest string exactly like the crypt(3) function in the C
983 library (assuming that you actually have a version there that has not
984 been extirpated as a potential munitions).
986 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
987 into a short string, called a digest, which is returned. The same
988 PLAINTEXT and SALT will always return the same string, but there is no
989 (known) way to get the original PLAINTEXT from the hash. Small
990 changes in the PLAINTEXT or SALT will result in large changes in the
993 There is no decrypt function. This function isn't all that useful for
994 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
995 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
996 primarily used to check if two pieces of text are the same without
997 having to transmit or store the text itself. An example is checking
998 if a correct password is given. The digest of the password is stored,
999 not the password itself. The user types in a password that is
1000 crypt()'d with the same salt as the stored digest. If the two digests
1001 match the password is correct.
1003 When verifying an existing digest string you should use the digest as
1004 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1005 to create the digest is visible as part of the digest. This ensures
1006 crypt() will hash the new string with the same salt as the digest.
1007 This allows your code to work with the standard L<crypt|/crypt> and
1008 with more exotic implementations. In other words, do not assume
1009 anything about the returned string itself, or how many bytes in the
1012 Traditionally the result is a string of 13 bytes: two first bytes of
1013 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1014 the first eight bytes of the digest string mattered, but alternative
1015 hashing schemes (like MD5), higher level security schemes (like C2),
1016 and implementations on non-UNIX platforms may produce different
1019 When choosing a new salt create a random two character string whose
1020 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1021 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1022 characters is just a recommendation; the characters allowed in
1023 the salt depend solely on your system's crypt library, and Perl can't
1024 restrict what salts C<crypt()> accepts.
1026 Here's an example that makes sure that whoever runs this program knows
1029 $pwd = (getpwuid($<))[1];
1031 system "stty -echo";
1033 chomp($word = <STDIN>);
1037 if (crypt($word, $pwd) ne $pwd) {
1043 Of course, typing in your own password to whoever asks you
1046 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1047 of data, not least of all because you can't get the information
1048 back. Look at the L<Digest> module for more robust algorithms.
1050 If using crypt() on a Unicode string (which I<potentially> has
1051 characters with codepoints above 255), Perl tries to make sense
1052 of the situation by trying to downgrade (a copy of the string)
1053 the string back to an eight-bit byte string before calling crypt()
1054 (on that copy). If that works, good. If not, crypt() dies with
1055 C<Wide character in crypt>.
1060 [This function has been largely superseded by the C<untie> function.]
1062 Breaks the binding between a DBM file and a hash.
1064 =item dbmopen HASH,DBNAME,MASK
1065 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1067 [This function has been largely superseded by the C<tie> function.]
1069 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1070 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1071 argument is I<not> a filehandle, even though it looks like one). DBNAME
1072 is the name of the database (without the F<.dir> or F<.pag> extension if
1073 any). If the database does not exist, it is created with protection
1074 specified by MASK (as modified by the C<umask>). If your system supports
1075 only the older DBM functions, you may perform only one C<dbmopen> in your
1076 program. In older versions of Perl, if your system had neither DBM nor
1077 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1080 If you don't have write access to the DBM file, you can only read hash
1081 variables, not set them. If you want to test whether you can write,
1082 either use file tests or try setting a dummy hash entry inside an C<eval>,
1083 which will trap the error.
1085 Note that functions such as C<keys> and C<values> may return huge lists
1086 when used on large DBM files. You may prefer to use the C<each>
1087 function to iterate over large DBM files. Example:
1089 # print out history file offsets
1090 dbmopen(%HIST,'/usr/lib/news/history',0666);
1091 while (($key,$val) = each %HIST) {
1092 print $key, ' = ', unpack('L',$val), "\n";
1096 See also L<AnyDBM_File> for a more general description of the pros and
1097 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1098 rich implementation.
1100 You can control which DBM library you use by loading that library
1101 before you call dbmopen():
1104 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1105 or die "Can't open netscape history file: $!";
1108 X<defined> X<undef> X<undefined>
1112 Returns a Boolean value telling whether EXPR has a value other than
1113 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1116 Many operations return C<undef> to indicate failure, end of file,
1117 system error, uninitialized variable, and other exceptional
1118 conditions. This function allows you to distinguish C<undef> from
1119 other values. (A simple Boolean test will not distinguish among
1120 C<undef>, zero, the empty string, and C<"0">, which are all equally
1121 false.) Note that since C<undef> is a valid scalar, its presence
1122 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1123 returns C<undef> when its argument is an empty array, I<or> when the
1124 element to return happens to be C<undef>.
1126 You may also use C<defined(&func)> to check whether subroutine C<&func>
1127 has ever been defined. The return value is unaffected by any forward
1128 declarations of C<&func>. Note that a subroutine which is not defined
1129 may still be callable: its package may have an C<AUTOLOAD> method that
1130 makes it spring into existence the first time that it is called -- see
1133 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1134 used to report whether memory for that aggregate has ever been
1135 allocated. This behavior may disappear in future versions of Perl.
1136 You should instead use a simple test for size:
1138 if (@an_array) { print "has array elements\n" }
1139 if (%a_hash) { print "has hash members\n" }
1141 When used on a hash element, it tells you whether the value is defined,
1142 not whether the key exists in the hash. Use L</exists> for the latter
1147 print if defined $switch{'D'};
1148 print "$val\n" while defined($val = pop(@ary));
1149 die "Can't readlink $sym: $!"
1150 unless defined($value = readlink $sym);
1151 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1152 $debugging = 0 unless defined $debugging;
1154 Note: Many folks tend to overuse C<defined>, and then are surprised to
1155 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1156 defined values. For example, if you say
1160 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1161 matched "nothing". It didn't really fail to match anything. Rather, it
1162 matched something that happened to be zero characters long. This is all
1163 very above-board and honest. When a function returns an undefined value,
1164 it's an admission that it couldn't give you an honest answer. So you
1165 should use C<defined> only when you're questioning the integrity of what
1166 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1169 See also L</undef>, L</exists>, L</ref>.
1174 Given an expression that specifies a hash element, array element, hash slice,
1175 or array slice, deletes the specified element(s) from the hash or array.
1176 In the case of an array, if the array elements happen to be at the end,
1177 the size of the array will shrink to the highest element that tests
1178 true for exists() (or 0 if no such element exists).
1180 Returns a list with the same number of elements as the number of elements
1181 for which deletion was attempted. Each element of that list consists of
1182 either the value of the element deleted, or the undefined value. In scalar
1183 context, this means that you get the value of the last element deleted (or
1184 the undefined value if that element did not exist).
1186 %hash = (foo => 11, bar => 22, baz => 33);
1187 $scalar = delete $hash{foo}; # $scalar is 11
1188 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1189 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1191 Deleting from C<%ENV> modifies the environment. Deleting from
1192 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1193 from a C<tie>d hash or array may not necessarily return anything.
1195 Deleting an array element effectively returns that position of the array
1196 to its initial, uninitialized state. Subsequently testing for the same
1197 element with exists() will return false. Also, deleting array elements
1198 in the middle of an array will not shift the index of the elements
1199 after them down. Use splice() for that. See L</exists>.
1201 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1203 foreach $key (keys %HASH) {
1207 foreach $index (0 .. $#ARRAY) {
1208 delete $ARRAY[$index];
1213 delete @HASH{keys %HASH};
1215 delete @ARRAY[0 .. $#ARRAY];
1217 But both of these are slower than just assigning the empty list
1218 or undefining %HASH or @ARRAY:
1220 %HASH = (); # completely empty %HASH
1221 undef %HASH; # forget %HASH ever existed
1223 @ARRAY = (); # completely empty @ARRAY
1224 undef @ARRAY; # forget @ARRAY ever existed
1226 Note that the EXPR can be arbitrarily complicated as long as the final
1227 operation is a hash element, array element, hash slice, or array slice
1230 delete $ref->[$x][$y]{$key};
1231 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1233 delete $ref->[$x][$y][$index];
1234 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1237 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1239 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1240 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1241 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1242 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1243 an C<eval(),> the error message is stuffed into C<$@> and the
1244 C<eval> is terminated with the undefined value. This makes
1245 C<die> the way to raise an exception.
1247 Equivalent examples:
1249 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1250 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1252 If the last element of LIST does not end in a newline, the current
1253 script line number and input line number (if any) are also printed,
1254 and a newline is supplied. Note that the "input line number" (also
1255 known as "chunk") is subject to whatever notion of "line" happens to
1256 be currently in effect, and is also available as the special variable
1257 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1259 Hint: sometimes appending C<", stopped"> to your message will cause it
1260 to make better sense when the string C<"at foo line 123"> is appended.
1261 Suppose you are running script "canasta".
1263 die "/etc/games is no good";
1264 die "/etc/games is no good, stopped";
1266 produce, respectively
1268 /etc/games is no good at canasta line 123.
1269 /etc/games is no good, stopped at canasta line 123.
1271 See also exit(), warn(), and the Carp module.
1273 If LIST is empty and C<$@> already contains a value (typically from a
1274 previous eval) that value is reused after appending C<"\t...propagated">.
1275 This is useful for propagating exceptions:
1278 die unless $@ =~ /Expected exception/;
1280 If LIST is empty and C<$@> contains an object reference that has a
1281 C<PROPAGATE> method, that method will be called with additional file
1282 and line number parameters. The return value replaces the value in
1283 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1286 If C<$@> is empty then the string C<"Died"> is used.
1288 die() can also be called with a reference argument. If this happens to be
1289 trapped within an eval(), $@ contains the reference. This behavior permits
1290 a more elaborate exception handling implementation using objects that
1291 maintain arbitrary state about the nature of the exception. Such a scheme
1292 is sometimes preferable to matching particular string values of $@ using
1293 regular expressions. Because $@ is a global variable, and eval() may be
1294 used within object implementations, care must be taken that analyzing the
1295 error object doesn't replace the reference in the global variable. The
1296 easiest solution is to make a local copy of the reference before doing
1297 other manipulations. Here's an example:
1299 use Scalar::Util 'blessed';
1301 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1302 if (my $ev_err = $@) {
1303 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1304 # handle Some::Module::Exception
1307 # handle all other possible exceptions
1311 Because perl will stringify uncaught exception messages before displaying
1312 them, you may want to overload stringification operations on such custom
1313 exception objects. See L<overload> for details about that.
1315 You can arrange for a callback to be run just before the C<die>
1316 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1317 handler will be called with the error text and can change the error
1318 message, if it sees fit, by calling C<die> again. See
1319 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1320 L<"eval BLOCK"> for some examples. Although this feature was
1321 to be run only right before your program was to exit, this is not
1322 currently the case--the C<$SIG{__DIE__}> hook is currently called
1323 even inside eval()ed blocks/strings! If one wants the hook to do
1324 nothing in such situations, put
1328 as the first line of the handler (see L<perlvar/$^S>). Because
1329 this promotes strange action at a distance, this counterintuitive
1330 behavior may be fixed in a future release.
1335 Not really a function. Returns the value of the last command in the
1336 sequence of commands indicated by BLOCK. When modified by the C<while> or
1337 C<until> loop modifier, executes the BLOCK once before testing the loop
1338 condition. (On other statements the loop modifiers test the conditional
1341 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1342 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1343 See L<perlsyn> for alternative strategies.
1345 =item do SUBROUTINE(LIST)
1348 This form of subroutine call is deprecated. See L<perlsub>.
1353 Uses the value of EXPR as a filename and executes the contents of the
1354 file as a Perl script.
1362 except that it's more efficient and concise, keeps track of the current
1363 filename for error messages, searches the @INC directories, and updates
1364 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1365 variables. It also differs in that code evaluated with C<do FILENAME>
1366 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1367 same, however, in that it does reparse the file every time you call it,
1368 so you probably don't want to do this inside a loop.
1370 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1371 error. If C<do> can read the file but cannot compile it, it
1372 returns undef and sets an error message in C<$@>. If the file is
1373 successfully compiled, C<do> returns the value of the last expression
1376 Note that inclusion of library modules is better done with the
1377 C<use> and C<require> operators, which also do automatic error checking
1378 and raise an exception if there's a problem.
1380 You might like to use C<do> to read in a program configuration
1381 file. Manual error checking can be done this way:
1383 # read in config files: system first, then user
1384 for $file ("/share/prog/defaults.rc",
1385 "$ENV{HOME}/.someprogrc")
1387 unless ($return = do $file) {
1388 warn "couldn't parse $file: $@" if $@;
1389 warn "couldn't do $file: $!" unless defined $return;
1390 warn "couldn't run $file" unless $return;
1395 X<dump> X<core> X<undump>
1399 This function causes an immediate core dump. See also the B<-u>
1400 command-line switch in L<perlrun>, which does the same thing.
1401 Primarily this is so that you can use the B<undump> program (not
1402 supplied) to turn your core dump into an executable binary after
1403 having initialized all your variables at the beginning of the
1404 program. When the new binary is executed it will begin by executing
1405 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1406 Think of it as a goto with an intervening core dump and reincarnation.
1407 If C<LABEL> is omitted, restarts the program from the top.
1409 B<WARNING>: Any files opened at the time of the dump will I<not>
1410 be open any more when the program is reincarnated, with possible
1411 resulting confusion on the part of Perl.
1413 This function is now largely obsolete, mostly because it's very hard to
1414 convert a core file into an executable. That's why you should now invoke
1415 it as C<CORE::dump()>, if you don't want to be warned against a possible
1419 X<each> X<hash, iterator>
1424 When called in list context, returns a 2-element list consisting of the
1425 key and value for the next element of a hash, or the index and value for
1426 the next element of an array, so that you can iterate over it. When called
1427 in scalar context, returns only the key for the next element in the hash
1428 (or the index for an array).
1430 Hash entries are returned in an apparently random order. The actual random
1431 order is subject to change in future versions of perl, but it is
1432 guaranteed to be in the same order as either the C<keys> or C<values>
1433 function would produce on the same (unmodified) hash. Since Perl
1434 5.8.2 the ordering can be different even between different runs of Perl
1435 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1437 When the hash or array is entirely read, a null array is returned in list
1438 context (which when assigned produces a false (C<0>) value), and C<undef> in
1439 scalar context. The next call to C<each> after that will start iterating
1440 again. There is a single iterator for each hash or array, shared by all
1441 C<each>, C<keys>, and C<values> function calls in the program; it can be
1442 reset by reading all the elements from the hash or array, or by evaluating
1443 C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1444 or delete elements of a hash while you're
1445 iterating over it, you may get entries skipped or duplicated, so
1446 don't. Exception: It is always safe to delete the item most recently
1447 returned by C<each()>, which means that the following code will work:
1449 while (($key, $value) = each %hash) {
1451 delete $hash{$key}; # This is safe
1454 The following prints out your environment like the printenv(1) program,
1455 only in a different order:
1457 while (($key,$value) = each %ENV) {
1458 print "$key=$value\n";
1461 See also C<keys>, C<values> and C<sort>.
1463 =item eof FILEHANDLE
1472 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1473 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1474 gives the real filehandle. (Note that this function actually
1475 reads a character and then C<ungetc>s it, so isn't very useful in an
1476 interactive context.) Do not read from a terminal file (or call
1477 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1478 as terminals may lose the end-of-file condition if you do.
1480 An C<eof> without an argument uses the last file read. Using C<eof()>
1481 with empty parentheses is very different. It refers to the pseudo file
1482 formed from the files listed on the command line and accessed via the
1483 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1484 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1485 used will cause C<@ARGV> to be examined to determine if input is
1486 available. Similarly, an C<eof()> after C<< <> >> has returned
1487 end-of-file will assume you are processing another C<@ARGV> list,
1488 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1489 see L<perlop/"I/O Operators">.
1491 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1492 detect the end of each file, C<eof()> will only detect the end of the
1493 last file. Examples:
1495 # reset line numbering on each input file
1497 next if /^\s*#/; # skip comments
1500 close ARGV if eof; # Not eof()!
1503 # insert dashes just before last line of last file
1505 if (eof()) { # check for end of last file
1506 print "--------------\n";
1509 last if eof(); # needed if we're reading from a terminal
1512 Practical hint: you almost never need to use C<eof> in Perl, because the
1513 input operators typically return C<undef> when they run out of data, or if
1517 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1518 X<error, handling> X<exception, handling>
1524 In the first form, the return value of EXPR is parsed and executed as if it
1525 were a little Perl program. The value of the expression (which is itself
1526 determined within scalar context) is first parsed, and if there weren't any
1527 errors, executed in the lexical context of the current Perl program, so
1528 that any variable settings or subroutine and format definitions remain
1529 afterwards. Note that the value is parsed every time the C<eval> executes.
1530 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1531 delay parsing and subsequent execution of the text of EXPR until run time.
1533 In the second form, the code within the BLOCK is parsed only once--at the
1534 same time the code surrounding the C<eval> itself was parsed--and executed
1535 within the context of the current Perl program. This form is typically
1536 used to trap exceptions more efficiently than the first (see below), while
1537 also providing the benefit of checking the code within BLOCK at compile
1540 The final semicolon, if any, may be omitted from the value of EXPR or within
1543 In both forms, the value returned is the value of the last expression
1544 evaluated inside the mini-program; a return statement may be also used, just
1545 as with subroutines. The expression providing the return value is evaluated
1546 in void, scalar, or list context, depending on the context of the C<eval>
1547 itself. See L</wantarray> for more on how the evaluation context can be
1550 If there is a syntax error or runtime error, or a C<die> statement is
1551 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1552 error message. If there was no error, C<$@> is guaranteed to be a null
1553 string. Beware that using C<eval> neither silences perl from printing
1554 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1555 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1556 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1557 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1559 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1560 determining whether a particular feature (such as C<socket> or C<symlink>)
1561 is implemented. It is also Perl's exception trapping mechanism, where
1562 the die operator is used to raise exceptions.
1564 If the code to be executed doesn't vary, you may use the eval-BLOCK
1565 form to trap run-time errors without incurring the penalty of
1566 recompiling each time. The error, if any, is still returned in C<$@>.
1569 # make divide-by-zero nonfatal
1570 eval { $answer = $a / $b; }; warn $@ if $@;
1572 # same thing, but less efficient
1573 eval '$answer = $a / $b'; warn $@ if $@;
1575 # a compile-time error
1576 eval { $answer = }; # WRONG
1579 eval '$answer ='; # sets $@
1581 Using the C<eval{}> form as an exception trap in libraries does have some
1582 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1583 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1584 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1585 as shown in this example:
1587 # a very private exception trap for divide-by-zero
1588 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1591 This is especially significant, given that C<__DIE__> hooks can call
1592 C<die> again, which has the effect of changing their error messages:
1594 # __DIE__ hooks may modify error messages
1596 local $SIG{'__DIE__'} =
1597 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1598 eval { die "foo lives here" };
1599 print $@ if $@; # prints "bar lives here"
1602 Because this promotes action at a distance, this counterintuitive behavior
1603 may be fixed in a future release.
1605 With an C<eval>, you should be especially careful to remember what's
1606 being looked at when:
1612 eval { $x }; # CASE 4
1614 eval "\$$x++"; # CASE 5
1617 Cases 1 and 2 above behave identically: they run the code contained in
1618 the variable $x. (Although case 2 has misleading double quotes making
1619 the reader wonder what else might be happening (nothing is).) Cases 3
1620 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1621 does nothing but return the value of $x. (Case 4 is preferred for
1622 purely visual reasons, but it also has the advantage of compiling at
1623 compile-time instead of at run-time.) Case 5 is a place where
1624 normally you I<would> like to use double quotes, except that in this
1625 particular situation, you can just use symbolic references instead, as
1628 The assignment to C<$@> occurs before restoration of localised variables,
1629 which means a temporary is required if you want to mask some but not all
1632 # alter $@ on nefarious repugnancy only
1636 local $@; # protect existing $@
1637 eval { test_repugnancy() };
1638 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1639 $@ =~ /nefarious/ and $e = $@;
1641 die $e if defined $e
1644 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1645 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1647 Note that as a very special case, an C<eval ''> executed within the C<DB>
1648 package doesn't see the usual surrounding lexical scope, but rather the
1649 scope of the first non-DB piece of code that called it. You don't normally
1650 need to worry about this unless you are writing a Perl debugger.
1655 =item exec PROGRAM LIST
1657 The C<exec> function executes a system command I<and never returns>--
1658 use C<system> instead of C<exec> if you want it to return. It fails and
1659 returns false only if the command does not exist I<and> it is executed
1660 directly instead of via your system's command shell (see below).
1662 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1663 warns you if there is a following statement which isn't C<die>, C<warn>,
1664 or C<exit> (if C<-w> is set - but you always do that). If you
1665 I<really> want to follow an C<exec> with some other statement, you
1666 can use one of these styles to avoid the warning:
1668 exec ('foo') or print STDERR "couldn't exec foo: $!";
1669 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1671 If there is more than one argument in LIST, or if LIST is an array
1672 with more than one value, calls execvp(3) with the arguments in LIST.
1673 If there is only one scalar argument or an array with one element in it,
1674 the argument is checked for shell metacharacters, and if there are any,
1675 the entire argument is passed to the system's command shell for parsing
1676 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1677 If there are no shell metacharacters in the argument, it is split into
1678 words and passed directly to C<execvp>, which is more efficient.
1681 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1682 exec "sort $outfile | uniq";
1684 If you don't really want to execute the first argument, but want to lie
1685 to the program you are executing about its own name, you can specify
1686 the program you actually want to run as an "indirect object" (without a
1687 comma) in front of the LIST. (This always forces interpretation of the
1688 LIST as a multivalued list, even if there is only a single scalar in
1691 $shell = '/bin/csh';
1692 exec $shell '-sh'; # pretend it's a login shell
1696 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1698 When the arguments get executed via the system shell, results will
1699 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1702 Using an indirect object with C<exec> or C<system> is also more
1703 secure. This usage (which also works fine with system()) forces
1704 interpretation of the arguments as a multivalued list, even if the
1705 list had just one argument. That way you're safe from the shell
1706 expanding wildcards or splitting up words with whitespace in them.
1708 @args = ( "echo surprise" );
1710 exec @args; # subject to shell escapes
1712 exec { $args[0] } @args; # safe even with one-arg list
1714 The first version, the one without the indirect object, ran the I<echo>
1715 program, passing it C<"surprise"> an argument. The second version
1716 didn't--it tried to run a program literally called I<"echo surprise">,
1717 didn't find it, and set C<$?> to a non-zero value indicating failure.
1719 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1720 output before the exec, but this may not be supported on some platforms
1721 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1722 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1723 open handles in order to avoid lost output.
1725 Note that C<exec> will not call your C<END> blocks, nor will it call
1726 any C<DESTROY> methods in your objects.
1729 X<exists> X<autovivification>
1731 Given an expression that specifies a hash element or array element,
1732 returns true if the specified element in the hash or array has ever
1733 been initialized, even if the corresponding value is undefined. The
1734 element is not autovivified if it doesn't exist.
1736 print "Exists\n" if exists $hash{$key};
1737 print "Defined\n" if defined $hash{$key};
1738 print "True\n" if $hash{$key};
1740 print "Exists\n" if exists $array[$index];
1741 print "Defined\n" if defined $array[$index];
1742 print "True\n" if $array[$index];
1744 A hash or array element can be true only if it's defined, and defined if
1745 it exists, but the reverse doesn't necessarily hold true.
1747 Given an expression that specifies the name of a subroutine,
1748 returns true if the specified subroutine has ever been declared, even
1749 if it is undefined. Mentioning a subroutine name for exists or defined
1750 does not count as declaring it. Note that a subroutine which does not
1751 exist may still be callable: its package may have an C<AUTOLOAD>
1752 method that makes it spring into existence the first time that it is
1753 called -- see L<perlsub>.
1755 print "Exists\n" if exists &subroutine;
1756 print "Defined\n" if defined &subroutine;
1758 Note that the EXPR can be arbitrarily complicated as long as the final
1759 operation is a hash or array key lookup or subroutine name:
1761 if (exists $ref->{A}->{B}->{$key}) { }
1762 if (exists $hash{A}{B}{$key}) { }
1764 if (exists $ref->{A}->{B}->[$ix]) { }
1765 if (exists $hash{A}{B}[$ix]) { }
1767 if (exists &{$ref->{A}{B}{$key}}) { }
1769 Although the deepest nested array or hash will not spring into existence
1770 just because its existence was tested, any intervening ones will.
1771 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1772 into existence due to the existence test for the $key element above.
1773 This happens anywhere the arrow operator is used, including even:
1776 if (exists $ref->{"Some key"}) { }
1777 print $ref; # prints HASH(0x80d3d5c)
1779 This surprising autovivification in what does not at first--or even
1780 second--glance appear to be an lvalue context may be fixed in a future
1783 Use of a subroutine call, rather than a subroutine name, as an argument
1784 to exists() is an error.
1787 exists &sub(); # Error
1790 X<exit> X<terminate> X<abort>
1794 Evaluates EXPR and exits immediately with that value. Example:
1797 exit 0 if $ans =~ /^[Xx]/;
1799 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1800 universally recognized values for EXPR are C<0> for success and C<1>
1801 for error; other values are subject to interpretation depending on the
1802 environment in which the Perl program is running. For example, exiting
1803 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1804 the mailer to return the item undelivered, but that's not true everywhere.
1806 Don't use C<exit> to abort a subroutine if there's any chance that
1807 someone might want to trap whatever error happened. Use C<die> instead,
1808 which can be trapped by an C<eval>.
1810 The exit() function does not always exit immediately. It calls any
1811 defined C<END> routines first, but these C<END> routines may not
1812 themselves abort the exit. Likewise any object destructors that need to
1813 be called are called before the real exit. If this is a problem, you
1814 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1815 See L<perlmod> for details.
1818 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1822 Returns I<e> (the natural logarithm base) to the power of EXPR.
1823 If EXPR is omitted, gives C<exp($_)>.
1825 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1828 Implements the fcntl(2) function. You'll probably have to say
1832 first to get the correct constant definitions. Argument processing and
1833 value return works just like C<ioctl> below.
1837 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1838 or die "can't fcntl F_GETFL: $!";
1840 You don't have to check for C<defined> on the return from C<fcntl>.
1841 Like C<ioctl>, it maps a C<0> return from the system call into
1842 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1843 in numeric context. It is also exempt from the normal B<-w> warnings
1844 on improper numeric conversions.
1846 Note that C<fcntl> will produce a fatal error if used on a machine that
1847 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1848 manpage to learn what functions are available on your system.
1850 Here's an example of setting a filehandle named C<REMOTE> to be
1851 non-blocking at the system level. You'll have to negotiate C<$|>
1852 on your own, though.
1854 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1856 $flags = fcntl(REMOTE, F_GETFL, 0)
1857 or die "Can't get flags for the socket: $!\n";
1859 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1860 or die "Can't set flags for the socket: $!\n";
1862 =item fileno FILEHANDLE
1865 Returns the file descriptor for a filehandle, or undefined if the
1866 filehandle is not open. This is mainly useful for constructing
1867 bitmaps for C<select> and low-level POSIX tty-handling operations.
1868 If FILEHANDLE is an expression, the value is taken as an indirect
1869 filehandle, generally its name.
1871 You can use this to find out whether two handles refer to the
1872 same underlying descriptor:
1874 if (fileno(THIS) == fileno(THAT)) {
1875 print "THIS and THAT are dups\n";
1878 (Filehandles connected to memory objects via new features of C<open> may
1879 return undefined even though they are open.)
1882 =item flock FILEHANDLE,OPERATION
1883 X<flock> X<lock> X<locking>
1885 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1886 for success, false on failure. Produces a fatal error if used on a
1887 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1888 C<flock> is Perl's portable file locking interface, although it locks
1889 only entire files, not records.
1891 Two potentially non-obvious but traditional C<flock> semantics are
1892 that it waits indefinitely until the lock is granted, and that its locks
1893 B<merely advisory>. Such discretionary locks are more flexible, but offer
1894 fewer guarantees. This means that programs that do not also use C<flock>
1895 may modify files locked with C<flock>. See L<perlport>,
1896 your port's specific documentation, or your system-specific local manpages
1897 for details. It's best to assume traditional behavior if you're writing
1898 portable programs. (But if you're not, you should as always feel perfectly
1899 free to write for your own system's idiosyncrasies (sometimes called
1900 "features"). Slavish adherence to portability concerns shouldn't get
1901 in the way of your getting your job done.)
1903 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1904 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1905 you can use the symbolic names if you import them from the Fcntl module,
1906 either individually, or as a group using the ':flock' tag. LOCK_SH
1907 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1908 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1909 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1910 waiting for the lock (check the return status to see if you got it).
1912 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1913 before locking or unlocking it.
1915 Note that the emulation built with lockf(3) doesn't provide shared
1916 locks, and it requires that FILEHANDLE be open with write intent. These
1917 are the semantics that lockf(3) implements. Most if not all systems
1918 implement lockf(3) in terms of fcntl(2) locking, though, so the
1919 differing semantics shouldn't bite too many people.
1921 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1922 be open with read intent to use LOCK_SH and requires that it be open
1923 with write intent to use LOCK_EX.
1925 Note also that some versions of C<flock> cannot lock things over the
1926 network; you would need to use the more system-specific C<fcntl> for
1927 that. If you like you can force Perl to ignore your system's flock(2)
1928 function, and so provide its own fcntl(2)-based emulation, by passing
1929 the switch C<-Ud_flock> to the F<Configure> program when you configure
1932 Here's a mailbox appender for BSD systems.
1934 use Fcntl ':flock'; # import LOCK_* constants
1937 flock(MBOX,LOCK_EX);
1938 # and, in case someone appended
1939 # while we were waiting...
1944 flock(MBOX,LOCK_UN);
1947 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
1948 or die "Can't open mailbox: $!";
1951 print $mbox $msg,"\n\n";
1954 On systems that support a real flock(), locks are inherited across fork()
1955 calls, whereas those that must resort to the more capricious fcntl()
1956 function lose the locks, making it harder to write servers.
1958 See also L<DB_File> for other flock() examples.
1961 X<fork> X<child> X<parent>
1963 Does a fork(2) system call to create a new process running the
1964 same program at the same point. It returns the child pid to the
1965 parent process, C<0> to the child process, or C<undef> if the fork is
1966 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1967 are shared, while everything else is copied. On most systems supporting
1968 fork(), great care has gone into making it extremely efficient (for
1969 example, using copy-on-write technology on data pages), making it the
1970 dominant paradigm for multitasking over the last few decades.
1972 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1973 output before forking the child process, but this may not be supported
1974 on some platforms (see L<perlport>). To be safe, you may need to set
1975 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1976 C<IO::Handle> on any open handles in order to avoid duplicate output.
1978 If you C<fork> without ever waiting on your children, you will
1979 accumulate zombies. On some systems, you can avoid this by setting
1980 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1981 forking and reaping moribund children.
1983 Note that if your forked child inherits system file descriptors like
1984 STDIN and STDOUT that are actually connected by a pipe or socket, even
1985 if you exit, then the remote server (such as, say, a CGI script or a
1986 backgrounded job launched from a remote shell) won't think you're done.
1987 You should reopen those to F</dev/null> if it's any issue.
1992 Declare a picture format for use by the C<write> function. For
1996 Test: @<<<<<<<< @||||| @>>>>>
1997 $str, $%, '$' . int($num)
2001 $num = $cost/$quantity;
2005 See L<perlform> for many details and examples.
2007 =item formline PICTURE,LIST
2010 This is an internal function used by C<format>s, though you may call it,
2011 too. It formats (see L<perlform>) a list of values according to the
2012 contents of PICTURE, placing the output into the format output
2013 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2014 Eventually, when a C<write> is done, the contents of
2015 C<$^A> are written to some filehandle. You could also read C<$^A>
2016 and then set C<$^A> back to C<"">. Note that a format typically
2017 does one C<formline> per line of form, but the C<formline> function itself
2018 doesn't care how many newlines are embedded in the PICTURE. This means
2019 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2020 You may therefore need to use multiple formlines to implement a single
2021 record format, just like the format compiler.
2023 Be careful if you put double quotes around the picture, because an C<@>
2024 character may be taken to mean the beginning of an array name.
2025 C<formline> always returns true. See L<perlform> for other examples.
2027 =item getc FILEHANDLE
2028 X<getc> X<getchar> X<character> X<file, read>
2032 Returns the next character from the input file attached to FILEHANDLE,
2033 or the undefined value at end of file, or if there was an error (in
2034 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2035 STDIN. This is not particularly efficient. However, it cannot be
2036 used by itself to fetch single characters without waiting for the user
2037 to hit enter. For that, try something more like:
2040 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2043 system "stty", '-icanon', 'eol', "\001";
2049 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2052 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2056 Determination of whether $BSD_STYLE should be set
2057 is left as an exercise to the reader.
2059 The C<POSIX::getattr> function can do this more portably on
2060 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2061 module from your nearest CPAN site; details on CPAN can be found on
2065 X<getlogin> X<login>
2067 This implements the C library function of the same name, which on most
2068 systems returns the current login from F</etc/utmp>, if any. If null,
2071 $login = getlogin || getpwuid($<) || "Kilroy";
2073 Do not consider C<getlogin> for authentication: it is not as
2074 secure as C<getpwuid>.
2076 =item getpeername SOCKET
2077 X<getpeername> X<peer>
2079 Returns the packed sockaddr address of other end of the SOCKET connection.
2082 $hersockaddr = getpeername(SOCK);
2083 ($port, $iaddr) = sockaddr_in($hersockaddr);
2084 $herhostname = gethostbyaddr($iaddr, AF_INET);
2085 $herstraddr = inet_ntoa($iaddr);
2090 Returns the current process group for the specified PID. Use
2091 a PID of C<0> to get the current process group for the
2092 current process. Will raise an exception if used on a machine that
2093 doesn't implement getpgrp(2). If PID is omitted, returns process
2094 group of current process. Note that the POSIX version of C<getpgrp>
2095 does not accept a PID argument, so only C<PID==0> is truly portable.
2098 X<getppid> X<parent> X<pid>
2100 Returns the process id of the parent process.
2102 Note for Linux users: on Linux, the C functions C<getpid()> and
2103 C<getppid()> return different values from different threads. In order to
2104 be portable, this behavior is not reflected by the perl-level function
2105 C<getppid()>, that returns a consistent value across threads. If you want
2106 to call the underlying C<getppid()>, you may use the CPAN module
2109 =item getpriority WHICH,WHO
2110 X<getpriority> X<priority> X<nice>
2112 Returns the current priority for a process, a process group, or a user.
2113 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2114 machine that doesn't implement getpriority(2).
2117 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2118 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2119 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2120 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2121 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2122 X<endnetent> X<endprotoent> X<endservent>
2126 =item gethostbyname NAME
2128 =item getnetbyname NAME
2130 =item getprotobyname NAME
2136 =item getservbyname NAME,PROTO
2138 =item gethostbyaddr ADDR,ADDRTYPE
2140 =item getnetbyaddr ADDR,ADDRTYPE
2142 =item getprotobynumber NUMBER
2144 =item getservbyport PORT,PROTO
2162 =item sethostent STAYOPEN
2164 =item setnetent STAYOPEN
2166 =item setprotoent STAYOPEN
2168 =item setservent STAYOPEN
2182 These routines perform the same functions as their counterparts in the
2183 system library. In list context, the return values from the
2184 various get routines are as follows:
2186 ($name,$passwd,$uid,$gid,
2187 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2188 ($name,$passwd,$gid,$members) = getgr*
2189 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2190 ($name,$aliases,$addrtype,$net) = getnet*
2191 ($name,$aliases,$proto) = getproto*
2192 ($name,$aliases,$port,$proto) = getserv*
2194 (If the entry doesn't exist you get a null list.)
2196 The exact meaning of the $gcos field varies but it usually contains
2197 the real name of the user (as opposed to the login name) and other
2198 information pertaining to the user. Beware, however, that in many
2199 system users are able to change this information and therefore it
2200 cannot be trusted and therefore the $gcos is tainted (see
2201 L<perlsec>). The $passwd and $shell, user's encrypted password and
2202 login shell, are also tainted, because of the same reason.
2204 In scalar context, you get the name, unless the function was a
2205 lookup by name, in which case you get the other thing, whatever it is.
2206 (If the entry doesn't exist you get the undefined value.) For example:
2208 $uid = getpwnam($name);
2209 $name = getpwuid($num);
2211 $gid = getgrnam($name);
2212 $name = getgrgid($num);
2216 In I<getpw*()> the fields $quota, $comment, and $expire are special
2217 cases in the sense that in many systems they are unsupported. If the
2218 $quota is unsupported, it is an empty scalar. If it is supported, it
2219 usually encodes the disk quota. If the $comment field is unsupported,
2220 it is an empty scalar. If it is supported it usually encodes some
2221 administrative comment about the user. In some systems the $quota
2222 field may be $change or $age, fields that have to do with password
2223 aging. In some systems the $comment field may be $class. The $expire
2224 field, if present, encodes the expiration period of the account or the
2225 password. For the availability and the exact meaning of these fields
2226 in your system, please consult your getpwnam(3) documentation and your
2227 F<pwd.h> file. You can also find out from within Perl what your
2228 $quota and $comment fields mean and whether you have the $expire field
2229 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2230 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2231 files are only supported if your vendor has implemented them in the
2232 intuitive fashion that calling the regular C library routines gets the
2233 shadow versions if you're running under privilege or if there exists
2234 the shadow(3) functions as found in System V (this includes Solaris
2235 and Linux.) Those systems that implement a proprietary shadow password
2236 facility are unlikely to be supported.
2238 The $members value returned by I<getgr*()> is a space separated list of
2239 the login names of the members of the group.
2241 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2242 C, it will be returned to you via C<$?> if the function call fails. The
2243 C<@addrs> value returned by a successful call is a list of the raw
2244 addresses returned by the corresponding system library call. In the
2245 Internet domain, each address is four bytes long and you can unpack it
2246 by saying something like:
2248 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2250 The Socket library makes this slightly easier:
2253 $iaddr = inet_aton("127.1"); # or whatever address
2254 $name = gethostbyaddr($iaddr, AF_INET);
2256 # or going the other way
2257 $straddr = inet_ntoa($iaddr);
2259 In the opposite way, to resolve a hostname to the IP address
2263 $packed_ip = gethostbyname("www.perl.org");
2264 if (defined $packed_ip) {
2265 $ip_address = inet_ntoa($packed_ip);
2268 Make sure <gethostbyname()> is called in SCALAR context and that
2269 its return value is checked for definedness.
2271 If you get tired of remembering which element of the return list
2272 contains which return value, by-name interfaces are provided
2273 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2274 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2275 and C<User::grent>. These override the normal built-ins, supplying
2276 versions that return objects with the appropriate names
2277 for each field. For example:
2281 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2283 Even though it looks like they're the same method calls (uid),
2284 they aren't, because a C<File::stat> object is different from
2285 a C<User::pwent> object.
2287 =item getsockname SOCKET
2290 Returns the packed sockaddr address of this end of the SOCKET connection,
2291 in case you don't know the address because you have several different
2292 IPs that the connection might have come in on.
2295 $mysockaddr = getsockname(SOCK);
2296 ($port, $myaddr) = sockaddr_in($mysockaddr);
2297 printf "Connect to %s [%s]\n",
2298 scalar gethostbyaddr($myaddr, AF_INET),
2301 =item getsockopt SOCKET,LEVEL,OPTNAME
2304 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2305 Options may exist at multiple protocol levels depending on the socket
2306 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2307 C<Socket> module) will exist. To query options at another level the
2308 protocol number of the appropriate protocol controlling the option
2309 should be supplied. For example, to indicate that an option is to be
2310 interpreted by the TCP protocol, LEVEL should be set to the protocol
2311 number of TCP, which you can get using getprotobyname.
2313 The call returns a packed string representing the requested socket option,
2314 or C<undef> if there is an error (the error reason will be in $!). What
2315 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2316 your system documentation for details. A very common case however is that
2317 the option is an integer, in which case the result will be a packed
2318 integer which you can decode using unpack with the C<i> (or C<I>) format.
2320 An example testing if Nagle's algorithm is turned on on a socket:
2322 use Socket qw(:all);
2324 defined(my $tcp = getprotobyname("tcp"))
2325 or die "Could not determine the protocol number for tcp";
2326 # my $tcp = IPPROTO_TCP; # Alternative
2327 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2328 or die "Could not query TCP_NODELAY socket option: $!";
2329 my $nodelay = unpack("I", $packed);
2330 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2334 X<glob> X<wildcard> X<filename, expansion> X<expand>
2338 In list context, returns a (possibly empty) list of filename expansions on
2339 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2340 scalar context, glob iterates through such filename expansions, returning
2341 undef when the list is exhausted. This is the internal function
2342 implementing the C<< <*.c> >> operator, but you can use it directly. If
2343 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2344 more detail in L<perlop/"I/O Operators">.
2346 Beginning with v5.6.0, this operator is implemented using the standard
2347 C<File::Glob> extension. See L<File::Glob> for details.
2350 X<gmtime> X<UTC> X<Greenwich>
2354 Works just like L<localtime> but the returned values are
2355 localized for the standard Greenwich time zone.
2357 Note: when called in list context, $isdst, the last value
2358 returned by gmtime is always C<0>. There is no
2359 Daylight Saving Time in GMT.
2361 See L<perlport/gmtime> for portability concerns.
2364 X<goto> X<jump> X<jmp>
2370 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2371 execution there. It may not be used to go into any construct that
2372 requires initialization, such as a subroutine or a C<foreach> loop. It
2373 also can't be used to go into a construct that is optimized away,
2374 or to get out of a block or subroutine given to C<sort>.
2375 It can be used to go almost anywhere else within the dynamic scope,
2376 including out of subroutines, but it's usually better to use some other
2377 construct such as C<last> or C<die>. The author of Perl has never felt the
2378 need to use this form of C<goto> (in Perl, that is--C is another matter).
2379 (The difference being that C does not offer named loops combined with
2380 loop control. Perl does, and this replaces most structured uses of C<goto>
2381 in other languages.)
2383 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2384 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2385 necessarily recommended if you're optimizing for maintainability:
2387 goto ("FOO", "BAR", "GLARCH")[$i];
2389 The C<goto-&NAME> form is quite different from the other forms of
2390 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2391 doesn't have the stigma associated with other gotos. Instead, it
2392 exits the current subroutine (losing any changes set by local()) and
2393 immediately calls in its place the named subroutine using the current
2394 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2395 load another subroutine and then pretend that the other subroutine had
2396 been called in the first place (except that any modifications to C<@_>
2397 in the current subroutine are propagated to the other subroutine.)
2398 After the C<goto>, not even C<caller> will be able to tell that this
2399 routine was called first.
2401 NAME needn't be the name of a subroutine; it can be a scalar variable
2402 containing a code reference, or a block that evaluates to a code
2405 =item grep BLOCK LIST
2408 =item grep EXPR,LIST
2410 This is similar in spirit to, but not the same as, grep(1) and its
2411 relatives. In particular, it is not limited to using regular expressions.
2413 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2414 C<$_> to each element) and returns the list value consisting of those
2415 elements for which the expression evaluated to true. In scalar
2416 context, returns the number of times the expression was true.
2418 @foo = grep(!/^#/, @bar); # weed out comments
2422 @foo = grep {!/^#/} @bar; # weed out comments
2424 Note that C<$_> is an alias to the list value, so it can be used to
2425 modify the elements of the LIST. While this is useful and supported,
2426 it can cause bizarre results if the elements of LIST are not variables.
2427 Similarly, grep returns aliases into the original list, much as a for
2428 loop's index variable aliases the list elements. That is, modifying an
2429 element of a list returned by grep (for example, in a C<foreach>, C<map>
2430 or another C<grep>) actually modifies the element in the original list.
2431 This is usually something to be avoided when writing clear code.
2433 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2434 been declared with C<my $_>) then, in addition to being locally aliased to
2435 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2436 can't be seen from the outside, avoiding any potential side-effects.
2438 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2441 X<hex> X<hexadecimal>
2445 Interprets EXPR as a hex string and returns the corresponding value.
2446 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2447 L</oct>.) If EXPR is omitted, uses C<$_>.
2449 print hex '0xAf'; # prints '175'
2450 print hex 'aF'; # same
2452 Hex strings may only represent integers. Strings that would cause
2453 integer overflow trigger a warning. Leading whitespace is not stripped,
2454 unlike oct(). To present something as hex, look into L</printf>,
2455 L</sprintf>, or L</unpack>.
2460 There is no builtin C<import> function. It is just an ordinary
2461 method (subroutine) defined (or inherited) by modules that wish to export
2462 names to another module. The C<use> function calls the C<import> method
2463 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2465 =item index STR,SUBSTR,POSITION
2466 X<index> X<indexOf> X<InStr>
2468 =item index STR,SUBSTR
2470 The index function searches for one string within another, but without
2471 the wildcard-like behavior of a full regular-expression pattern match.
2472 It returns the position of the first occurrence of SUBSTR in STR at
2473 or after POSITION. If POSITION is omitted, starts searching from the
2474 beginning of the string. POSITION before the beginning of the string
2475 or after its end is treated as if it were the beginning or the end,
2476 respectively. POSITION and the return value are based at C<0> (or whatever
2477 you've set the C<$[> variable to--but don't do that). If the substring
2478 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2481 X<int> X<integer> X<truncate> X<trunc> X<floor>
2485 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2486 You should not use this function for rounding: one because it truncates
2487 towards C<0>, and two because machine representations of floating point
2488 numbers can sometimes produce counterintuitive results. For example,
2489 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2490 because it's really more like -268.99999999999994315658 instead. Usually,
2491 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2492 functions will serve you better than will int().
2494 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2497 Implements the ioctl(2) function. You'll probably first have to say
2499 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2501 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2502 exist or doesn't have the correct definitions you'll have to roll your
2503 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2504 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2505 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2506 written depending on the FUNCTION--a pointer to the string value of SCALAR
2507 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2508 has no string value but does have a numeric value, that value will be
2509 passed rather than a pointer to the string value. To guarantee this to be
2510 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2511 functions may be needed to manipulate the values of structures used by
2514 The return value of C<ioctl> (and C<fcntl>) is as follows:
2516 if OS returns: then Perl returns:
2518 0 string "0 but true"
2519 anything else that number
2521 Thus Perl returns true on success and false on failure, yet you can
2522 still easily determine the actual value returned by the operating
2525 $retval = ioctl(...) || -1;
2526 printf "System returned %d\n", $retval;
2528 The special string C<"0 but true"> is exempt from B<-w> complaints
2529 about improper numeric conversions.
2531 =item join EXPR,LIST
2534 Joins the separate strings of LIST into a single string with fields
2535 separated by the value of EXPR, and returns that new string. Example:
2537 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2539 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2540 first argument. Compare L</split>.
2547 Returns a list consisting of all the keys of the named hash, or the indices
2548 of an array. (In scalar context, returns the number of keys or indices.)
2550 The keys of a hash are returned in an apparently random order. The actual
2551 random order is subject to change in future versions of perl, but it
2552 is guaranteed to be the same order as either the C<values> or C<each>
2553 function produces (given that the hash has not been modified). Since
2554 Perl 5.8.1 the ordering is different even between different runs of
2555 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2558 As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
2559 (see L</each>). In particular, calling keys() in void context resets
2560 the iterator with no other overhead.
2562 Here is yet another way to print your environment:
2565 @values = values %ENV;
2567 print pop(@keys), '=', pop(@values), "\n";
2570 or how about sorted by key:
2572 foreach $key (sort(keys %ENV)) {
2573 print $key, '=', $ENV{$key}, "\n";
2576 The returned values are copies of the original keys in the hash, so
2577 modifying them will not affect the original hash. Compare L</values>.
2579 To sort a hash by value, you'll need to use a C<sort> function.
2580 Here's a descending numeric sort of a hash by its values:
2582 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2583 printf "%4d %s\n", $hash{$key}, $key;
2586 As an lvalue C<keys> allows you to increase the number of hash buckets
2587 allocated for the given hash. This can gain you a measure of efficiency if
2588 you know the hash is going to get big. (This is similar to pre-extending
2589 an array by assigning a larger number to $#array.) If you say
2593 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2594 in fact, since it rounds up to the next power of two. These
2595 buckets will be retained even if you do C<%hash = ()>, use C<undef
2596 %hash> if you want to free the storage while C<%hash> is still in scope.
2597 You can't shrink the number of buckets allocated for the hash using
2598 C<keys> in this way (but you needn't worry about doing this by accident,
2599 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2602 See also C<each>, C<values> and C<sort>.
2604 =item kill SIGNAL, LIST
2607 Sends a signal to a list of processes. Returns the number of
2608 processes successfully signaled (which is not necessarily the
2609 same as the number actually killed).
2611 $cnt = kill 1, $child1, $child2;
2614 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2615 system call will check whether it's possible to send a signal to it (that
2616 means, to be brief, that the process is owned by the same user, or we are
2617 the super-user). This is a useful way to check that a child process is
2618 alive (even if only as a zombie) and hasn't changed its UID. See
2619 L<perlport> for notes on the portability of this construct.
2621 Unlike in the shell, if SIGNAL is negative, it kills
2622 process groups instead of processes. (On System V, a negative I<PROCESS>
2623 number will also kill process groups, but that's not portable.) That
2624 means you usually want to use positive not negative signals. You may also
2625 use a signal name in quotes.
2627 See L<perlipc/"Signals"> for more details.
2634 The C<last> command is like the C<break> statement in C (as used in
2635 loops); it immediately exits the loop in question. If the LABEL is
2636 omitted, the command refers to the innermost enclosing loop. The
2637 C<continue> block, if any, is not executed:
2639 LINE: while (<STDIN>) {
2640 last LINE if /^$/; # exit when done with header
2644 C<last> cannot be used to exit a block which returns a value such as
2645 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2646 a grep() or map() operation.
2648 Note that a block by itself is semantically identical to a loop
2649 that executes once. Thus C<last> can be used to effect an early
2650 exit out of such a block.
2652 See also L</continue> for an illustration of how C<last>, C<next>, and
2660 Returns a lowercased version of EXPR. This is the internal function
2661 implementing the C<\L> escape in double-quoted strings. Respects
2662 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2663 and L<perlunicode> for more details about locale and Unicode support.
2665 If EXPR is omitted, uses C<$_>.
2668 X<lcfirst> X<lowercase>
2672 Returns the value of EXPR with the first character lowercased. This
2673 is the internal function implementing the C<\l> escape in
2674 double-quoted strings. Respects current LC_CTYPE locale if C<use
2675 locale> in force. See L<perllocale> and L<perlunicode> for more
2676 details about locale and Unicode support.
2678 If EXPR is omitted, uses C<$_>.
2685 Returns the length in I<characters> of the value of EXPR. If EXPR is
2686 omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2687 Note that this cannot be used on an entire array or hash to find out how
2688 many elements these have. For that, use C<scalar @array> and C<scalar keys
2689 %hash> respectively.
2691 Note the I<characters>: if the EXPR is in Unicode, you will get the
2692 number of characters, not the number of bytes. To get the length
2693 of the internal string in bytes, use C<bytes::length(EXPR)>, see
2694 L<bytes>. Note that the internal encoding is variable, and the number
2695 of bytes usually meaningless. To get the number of bytes that the
2696 string would have when encoded as UTF-8, use
2697 C<length(Encoding::encode_utf8(EXPR))>.
2699 =item link OLDFILE,NEWFILE
2702 Creates a new filename linked to the old filename. Returns true for
2703 success, false otherwise.
2705 =item listen SOCKET,QUEUESIZE
2708 Does the same thing that the listen system call does. Returns true if
2709 it succeeded, false otherwise. See the example in
2710 L<perlipc/"Sockets: Client/Server Communication">.
2715 You really probably want to be using C<my> instead, because C<local> isn't
2716 what most people think of as "local". See
2717 L<perlsub/"Private Variables via my()"> for details.
2719 A local modifies the listed variables to be local to the enclosing
2720 block, file, or eval. If more than one value is listed, the list must
2721 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2722 for details, including issues with tied arrays and hashes.
2724 =item localtime EXPR
2725 X<localtime> X<ctime>
2729 Converts a time as returned by the time function to a 9-element list
2730 with the time analyzed for the local time zone. Typically used as
2734 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2737 All list elements are numeric, and come straight out of the C `struct
2738 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2739 of the specified time.
2741 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2742 the range C<0..11> with 0 indicating January and 11 indicating December.
2743 This makes it easy to get a month name from a list:
2745 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2746 print "$abbr[$mon] $mday";
2747 # $mon=9, $mday=18 gives "Oct 18"
2749 C<$year> is the number of years since 1900, not just the last two digits
2750 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2751 to get a complete 4-digit year is simply:
2755 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2756 to do that, would you?
2758 To get the last two digits of the year (e.g., '01' in 2001) do:
2760 $year = sprintf("%02d", $year % 100);
2762 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2763 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2764 (or C<0..365> in leap years.)
2766 C<$isdst> is true if the specified time occurs during Daylight Saving
2767 Time, false otherwise.
2769 If EXPR is omitted, C<localtime()> uses the current time (as returned
2772 In scalar context, C<localtime()> returns the ctime(3) value:
2774 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2776 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2777 instead of local time use the L</gmtime> builtin. See also the
2778 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2779 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2780 and mktime(3) functions.
2782 To get somewhat similar but locale dependent date strings, set up your
2783 locale environment variables appropriately (please see L<perllocale>) and
2786 use POSIX qw(strftime);
2787 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2788 # or for GMT formatted appropriately for your locale:
2789 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2791 Note that the C<%a> and C<%b>, the short forms of the day of the week
2792 and the month of the year, may not necessarily be three characters wide.
2794 See L<perlport/localtime> for portability concerns.
2796 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2797 by-name access mechanism to the gmtime() and localtime() functions,
2800 For a comprehensive date and time representation look at the
2801 L<DateTime> module on CPAN.
2806 This function places an advisory lock on a shared variable, or referenced
2807 object contained in I<THING> until the lock goes out of scope.
2809 lock() is a "weak keyword" : this means that if you've defined a function
2810 by this name (before any calls to it), that function will be called
2811 instead. (However, if you've said C<use threads>, lock() is always a
2812 keyword.) See L<threads>.
2815 X<log> X<logarithm> X<e> X<ln> X<base>
2819 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2820 returns log of C<$_>. To get the log of another base, use basic algebra:
2821 The base-N log of a number is equal to the natural log of that number
2822 divided by the natural log of N. For example:
2826 return log($n)/log(10);
2829 See also L</exp> for the inverse operation.
2836 Does the same thing as the C<stat> function (including setting the
2837 special C<_> filehandle) but stats a symbolic link instead of the file
2838 the symbolic link points to. If symbolic links are unimplemented on
2839 your system, a normal C<stat> is done. For much more detailed
2840 information, please see the documentation for C<stat>.
2842 If EXPR is omitted, stats C<$_>.
2846 The match operator. See L<perlop>.
2848 =item map BLOCK LIST
2853 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2854 C<$_> to each element) and returns the list value composed of the
2855 results of each such evaluation. In scalar context, returns the
2856 total number of elements so generated. Evaluates BLOCK or EXPR in
2857 list context, so each element of LIST may produce zero, one, or
2858 more elements in the returned value.
2860 @chars = map(chr, @nums);
2862 translates a list of numbers to the corresponding characters. And
2864 %hash = map { get_a_key_for($_) => $_ } @array;
2866 is just a funny way to write
2870 $hash{get_a_key_for($_)} = $_;
2873 Note that C<$_> is an alias to the list value, so it can be used to
2874 modify the elements of the LIST. While this is useful and supported,
2875 it can cause bizarre results if the elements of LIST are not variables.
2876 Using a regular C<foreach> loop for this purpose would be clearer in
2877 most cases. See also L</grep> for an array composed of those items of
2878 the original list for which the BLOCK or EXPR evaluates to true.
2880 If C<$_> is lexical in the scope where the C<map> appears (because it has
2881 been declared with C<my $_>), then, in addition to being locally aliased to
2882 the list elements, C<$_> keeps being lexical inside the block; that is, it
2883 can't be seen from the outside, avoiding any potential side-effects.
2885 C<{> starts both hash references and blocks, so C<map { ...> could be either
2886 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2887 ahead for the closing C<}> it has to take a guess at which its dealing with
2888 based what it finds just after the C<{>. Usually it gets it right, but if it
2889 doesn't it won't realize something is wrong until it gets to the C<}> and
2890 encounters the missing (or unexpected) comma. The syntax error will be
2891 reported close to the C<}> but you'll need to change something near the C<{>
2892 such as using a unary C<+> to give perl some help:
2894 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2895 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2896 %hash = map { ("\L$_", 1) } @array # this also works
2897 %hash = map { lc($_), 1 } @array # as does this.
2898 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2900 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2902 or to force an anon hash constructor use C<+{>:
2904 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2906 and you get list of anonymous hashes each with only 1 entry.
2908 =item mkdir FILENAME,MASK
2909 X<mkdir> X<md> X<directory, create>
2911 =item mkdir FILENAME
2915 Creates the directory specified by FILENAME, with permissions
2916 specified by MASK (as modified by C<umask>). If it succeeds it
2917 returns true, otherwise it returns false and sets C<$!> (errno).
2918 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2921 In general, it is better to create directories with permissive MASK,
2922 and let the user modify that with their C<umask>, than it is to supply
2923 a restrictive MASK and give the user no way to be more permissive.
2924 The exceptions to this rule are when the file or directory should be
2925 kept private (mail files, for instance). The perlfunc(1) entry on
2926 C<umask> discusses the choice of MASK in more detail.
2928 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2929 number of trailing slashes. Some operating and filesystems do not get
2930 this right, so Perl automatically removes all trailing slashes to keep
2933 In order to recursively create a directory structure look at
2934 the C<mkpath> function of the L<File::Path> module.
2936 =item msgctl ID,CMD,ARG
2939 Calls the System V IPC function msgctl(2). You'll probably have to say
2943 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2944 then ARG must be a variable that will hold the returned C<msqid_ds>
2945 structure. Returns like C<ioctl>: the undefined value for error,
2946 C<"0 but true"> for zero, or the actual return value otherwise. See also
2947 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2949 =item msgget KEY,FLAGS
2952 Calls the System V IPC function msgget(2). Returns the message queue
2953 id, or the undefined value if there is an error. See also
2954 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2956 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2959 Calls the System V IPC function msgrcv to receive a message from
2960 message queue ID into variable VAR with a maximum message size of
2961 SIZE. Note that when a message is received, the message type as a
2962 native long integer will be the first thing in VAR, followed by the
2963 actual message. This packing may be opened with C<unpack("l! a*")>.
2964 Taints the variable. Returns true if successful, or false if there is
2965 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2966 C<IPC::SysV::Msg> documentation.
2968 =item msgsnd ID,MSG,FLAGS
2971 Calls the System V IPC function msgsnd to send the message MSG to the
2972 message queue ID. MSG must begin with the native long integer message
2973 type, and be followed by the length of the actual message, and finally
2974 the message itself. This kind of packing can be achieved with
2975 C<pack("l! a*", $type, $message)>. Returns true if successful,
2976 or false if there is an error. See also C<IPC::SysV>
2977 and C<IPC::SysV::Msg> documentation.
2984 =item my EXPR : ATTRS
2986 =item my TYPE EXPR : ATTRS
2988 A C<my> declares the listed variables to be local (lexically) to the
2989 enclosing block, file, or C<eval>. If more than one value is listed,
2990 the list must be placed in parentheses.
2992 The exact semantics and interface of TYPE and ATTRS are still
2993 evolving. TYPE is currently bound to the use of C<fields> pragma,
2994 and attributes are handled using the C<attributes> pragma, or starting
2995 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2996 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2997 L<attributes>, and L<Attribute::Handlers>.
3004 The C<next> command is like the C<continue> statement in C; it starts
3005 the next iteration of the loop:
3007 LINE: while (<STDIN>) {
3008 next LINE if /^#/; # discard comments
3012 Note that if there were a C<continue> block on the above, it would get
3013 executed even on discarded lines. If the LABEL is omitted, the command
3014 refers to the innermost enclosing loop.
3016 C<next> cannot be used to exit a block which returns a value such as
3017 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3018 a grep() or map() operation.
3020 Note that a block by itself is semantically identical to a loop
3021 that executes once. Thus C<next> will exit such a block early.
3023 See also L</continue> for an illustration of how C<last>, C<next>, and
3026 =item no Module VERSION LIST
3029 =item no Module VERSION
3031 =item no Module LIST
3037 See the C<use> function, of which C<no> is the opposite.
3040 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3044 Interprets EXPR as an octal string and returns the corresponding
3045 value. (If EXPR happens to start off with C<0x>, interprets it as a
3046 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3047 binary string. Leading whitespace is ignored in all three cases.)
3048 The following will handle decimal, binary, octal, and hex in the standard
3051 $val = oct($val) if $val =~ /^0/;
3053 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3054 in octal), use sprintf() or printf():
3056 $perms = (stat("filename"))[2] & 07777;
3057 $oct_perms = sprintf "%lo", $perms;
3059 The oct() function is commonly used when a string such as C<644> needs
3060 to be converted into a file mode, for example. (Although perl will
3061 automatically convert strings into numbers as needed, this automatic
3062 conversion assumes base 10.)
3064 =item open FILEHANDLE,EXPR
3065 X<open> X<pipe> X<file, open> X<fopen>
3067 =item open FILEHANDLE,MODE,EXPR
3069 =item open FILEHANDLE,MODE,EXPR,LIST
3071 =item open FILEHANDLE,MODE,REFERENCE
3073 =item open FILEHANDLE
3075 Opens the file whose filename is given by EXPR, and associates it with
3078 Simple examples to open a file for reading:
3080 open(my $fh, '<', "input.txt") or die $!;
3084 open(my $fh, '>', "output.txt") or die $!;
3086 (The following is a comprehensive reference to open(): for a gentler
3087 introduction you may consider L<perlopentut>.)
3089 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3090 the variable is assigned a reference to a new anonymous filehandle,
3091 otherwise if FILEHANDLE is an expression, its value is used as the name of
3092 the real filehandle wanted. (This is considered a symbolic reference, so
3093 C<use strict 'refs'> should I<not> be in effect.)
3095 If EXPR is omitted, the scalar variable of the same name as the
3096 FILEHANDLE contains the filename. (Note that lexical variables--those
3097 declared with C<my>--will not work for this purpose; so if you're
3098 using C<my>, specify EXPR in your call to open.)
3100 If three or more arguments are specified then the mode of opening and
3101 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3102 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3103 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3104 the file is opened for appending, again being created if necessary.
3106 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3107 indicate that you want both read and write access to the file; thus
3108 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3109 '+>' >> mode would clobber the file first. You can't usually use
3110 either read-write mode for updating textfiles, since they have
3111 variable length records. See the B<-i> switch in L<perlrun> for a
3112 better approach. The file is created with permissions of C<0666>
3113 modified by the process' C<umask> value.
3115 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3116 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3118 In the 2-arguments (and 1-argument) form of the call the mode and
3119 filename should be concatenated (in this order), possibly separated by
3120 spaces. It is possible to omit the mode in these forms if the mode is
3123 If the filename begins with C<'|'>, the filename is interpreted as a
3124 command to which output is to be piped, and if the filename ends with a
3125 C<'|'>, the filename is interpreted as a command which pipes output to
3126 us. See L<perlipc/"Using open() for IPC">
3127 for more examples of this. (You are not allowed to C<open> to a command
3128 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3129 and L<perlipc/"Bidirectional Communication with Another Process">
3132 For three or more arguments if MODE is C<'|-'>, the filename is
3133 interpreted as a command to which output is to be piped, and if MODE
3134 is C<'-|'>, the filename is interpreted as a command which pipes
3135 output to us. In the 2-arguments (and 1-argument) form one should
3136 replace dash (C<'-'>) with the command.
3137 See L<perlipc/"Using open() for IPC"> for more examples of this.
3138 (You are not allowed to C<open> to a command that pipes both in I<and>
3139 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3140 L<perlipc/"Bidirectional Communication"> for alternatives.)
3142 In the three-or-more argument form of pipe opens, if LIST is specified
3143 (extra arguments after the command name) then LIST becomes arguments
3144 to the command invoked if the platform supports it. The meaning of
3145 C<open> with more than three arguments for non-pipe modes is not yet
3146 specified. Experimental "layers" may give extra LIST arguments
3149 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3150 and opening C<< '>-' >> opens STDOUT.
3152 You may use the three-argument form of open to specify IO "layers"
3153 (sometimes also referred to as "disciplines") to be applied to the handle
3154 that affect how the input and output are processed (see L<open> and
3155 L<PerlIO> for more details). For example
3157 open(my $fh, "<:encoding(UTF-8)", "file")
3159 will open the UTF-8 encoded file containing Unicode characters,
3160 see L<perluniintro>. Note that if layers are specified in the
3161 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3162 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3164 Open returns nonzero upon success, the undefined value otherwise. If
3165 the C<open> involved a pipe, the return value happens to be the pid of
3168 If you're running Perl on a system that distinguishes between text
3169 files and binary files, then you should check out L</binmode> for tips
3170 for dealing with this. The key distinction between systems that need
3171 C<binmode> and those that don't is their text file formats. Systems
3172 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3173 character, and which encode that character in C as C<"\n">, do not
3174 need C<binmode>. The rest need it.
3176 When opening a file, it's usually a bad idea to continue normal execution
3177 if the request failed, so C<open> is frequently used in connection with
3178 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3179 where you want to make a nicely formatted error message (but there are
3180 modules that can help with that problem)) you should always check
3181 the return value from opening a file. The infrequent exception is when
3182 working with an unopened filehandle is actually what you want to do.
3184 As a special case the 3-arg form with a read/write mode and the third
3185 argument being C<undef>:
3187 open(my $tmp, "+>", undef) or die ...
3189 opens a filehandle to an anonymous temporary file. Also using "+<"
3190 works for symmetry, but you really should consider writing something
3191 to the temporary file first. You will need to seek() to do the
3194 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3195 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3196 "in memory" files held in Perl scalars via:
3198 open($fh, '>', \$variable) || ..
3200 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3201 file, you have to close it first:
3204 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3209 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3210 while (<ARTICLE>) {...
3212 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3213 # if the open fails, output is discarded
3215 open(my $dbase, '+<', 'dbase.mine') # open for update
3216 or die "Can't open 'dbase.mine' for update: $!";
3218 open(my $dbase, '+<dbase.mine') # ditto
3219 or die "Can't open 'dbase.mine' for update: $!";
3221 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3222 or die "Can't start caesar: $!";
3224 open(ARTICLE, "caesar <$article |") # ditto
3225 or die "Can't start caesar: $!";
3227 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3228 or die "Can't start sort: $!";
3231 open(MEMORY,'>', \$var)
3232 or die "Can't open memory file: $!";
3233 print MEMORY "foo!\n"; # output will end up in $var
3235 # process argument list of files along with any includes
3237 foreach $file (@ARGV) {
3238 process($file, 'fh00');
3242 my($filename, $input) = @_;
3243 $input++; # this is a string increment
3244 unless (open($input, $filename)) {
3245 print STDERR "Can't open $filename: $!\n";
3250 while (<$input>) { # note use of indirection
3251 if (/^#include "(.*)"/) {
3252 process($1, $input);
3259 See L<perliol> for detailed info on PerlIO.
3261 You may also, in the Bourne shell tradition, specify an EXPR beginning
3262 with C<< '>&' >>, in which case the rest of the string is interpreted
3263 as the name of a filehandle (or file descriptor, if numeric) to be
3264 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3265 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3266 The mode you specify should match the mode of the original filehandle.
3267 (Duping a filehandle does not take into account any existing contents
3268 of IO buffers.) If you use the 3-arg form then you can pass either a
3269 number, the name of a filehandle or the normal "reference to a glob".
3271 Here is a script that saves, redirects, and restores C<STDOUT> and
3272 C<STDERR> using various methods:
3275 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3276 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3278 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3279 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3281 select STDERR; $| = 1; # make unbuffered
3282 select STDOUT; $| = 1; # make unbuffered
3284 print STDOUT "stdout 1\n"; # this works for
3285 print STDERR "stderr 1\n"; # subprocesses too
3287 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3288 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3290 print STDOUT "stdout 2\n";
3291 print STDERR "stderr 2\n";
3293 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3294 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3295 that file descriptor (and not call L<dup(2)>); this is more
3296 parsimonious of file descriptors. For example:
3298 # open for input, reusing the fileno of $fd
3299 open(FILEHANDLE, "<&=$fd")
3303 open(FILEHANDLE, "<&=", $fd)
3307 # open for append, using the fileno of OLDFH
3308 open(FH, ">>&=", OLDFH)
3312 open(FH, ">>&=OLDFH")
3314 Being parsimonious on filehandles is also useful (besides being
3315 parsimonious) for example when something is dependent on file
3316 descriptors, like for example locking using flock(). If you do just
3317 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3318 descriptor as B, and therefore flock(A) will not flock(B), and vice
3319 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3320 the same file descriptor.
3322 Note that if you are using Perls older than 5.8.0, Perl will be using
3323 the standard C libraries' fdopen() to implement the "=" functionality.
3324 On many UNIX systems fdopen() fails when file descriptors exceed a
3325 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3326 most often the default.
3328 You can see whether Perl has been compiled with PerlIO or not by
3329 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3330 is C<define>, you have PerlIO, otherwise you don't.
3332 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3333 with 2-arguments (or 1-argument) form of open(), then
3334 there is an implicit fork done, and the return value of open is the pid
3335 of the child within the parent process, and C<0> within the child
3336 process. (Use C<defined($pid)> to determine whether the open was successful.)
3337 The filehandle behaves normally for the parent, but i/o to that
3338 filehandle is piped from/to the STDOUT/STDIN of the child process.
3339 In the child process the filehandle isn't opened--i/o happens from/to
3340 the new STDOUT or STDIN. Typically this is used like the normal
3341 piped open when you want to exercise more control over just how the
3342 pipe command gets executed, such as when you are running setuid, and
3343 don't want to have to scan shell commands for metacharacters.
3344 The following triples are more or less equivalent:
3346 open(FOO, "|tr '[a-z]' '[A-Z]'");
3347 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3348 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3349 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3351 open(FOO, "cat -n '$file'|");
3352 open(FOO, '-|', "cat -n '$file'");
3353 open(FOO, '-|') || exec 'cat', '-n', $file;
3354 open(FOO, '-|', "cat", '-n', $file);
3356 The last example in each block shows the pipe as "list form", which is
3357 not yet supported on all platforms. A good rule of thumb is that if
3358 your platform has true C<fork()> (in other words, if your platform is
3359 UNIX) you can use the list form.
3361 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3363 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3364 output before any operation that may do a fork, but this may not be
3365 supported on some platforms (see L<perlport>). To be safe, you may need
3366 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3367 of C<IO::Handle> on any open handles.
3369 On systems that support a close-on-exec flag on files, the flag will
3370 be set for the newly opened file descriptor as determined by the value
3371 of $^F. See L<perlvar/$^F>.
3373 Closing any piped filehandle causes the parent process to wait for the
3374 child to finish, and returns the status value in C<$?> and
3375 C<${^CHILD_ERROR_NATIVE}>.
3377 The filename passed to 2-argument (or 1-argument) form of open() will
3378 have leading and trailing whitespace deleted, and the normal
3379 redirection characters honored. This property, known as "magic open",
3380 can often be used to good effect. A user could specify a filename of
3381 F<"rsh cat file |">, or you could change certain filenames as needed:
3383 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3384 open(FH, $filename) or die "Can't open $filename: $!";
3386 Use 3-argument form to open a file with arbitrary weird characters in it,
3388 open(FOO, '<', $file);
3390 otherwise it's necessary to protect any leading and trailing whitespace:
3392 $file =~ s#^(\s)#./$1#;
3393 open(FOO, "< $file\0");
3395 (this may not work on some bizarre filesystems). One should
3396 conscientiously choose between the I<magic> and 3-arguments form
3401 will allow the user to specify an argument of the form C<"rsh cat file |">,
3402 but will not work on a filename which happens to have a trailing space, while
3404 open IN, '<', $ARGV[0];
3406 will have exactly the opposite restrictions.
3408 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3409 should use the C<sysopen> function, which involves no such magic (but
3410 may use subtly different filemodes than Perl open(), which is mapped
3411 to C fopen()). This is
3412 another way to protect your filenames from interpretation. For example:
3415 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3416 or die "sysopen $path: $!";
3417 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3418 print HANDLE "stuff $$\n";
3420 print "File contains: ", <HANDLE>;
3422 Using the constructor from the C<IO::Handle> package (or one of its
3423 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3424 filehandles that have the scope of whatever variables hold references to
3425 them, and automatically close whenever and however you leave that scope:
3429 sub read_myfile_munged {
3431 my $handle = IO::File->new;
3432 open($handle, "myfile") or die "myfile: $!";
3434 or return (); # Automatically closed here.
3435 mung $first or die "mung failed"; # Or here.
3436 return $first, <$handle> if $ALL; # Or here.
3440 See L</seek> for some details about mixing reading and writing.
3442 =item opendir DIRHANDLE,EXPR
3445 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3446 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3447 DIRHANDLE may be an expression whose value can be used as an indirect
3448 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3449 scalar variable (or array or hash element), the variable is assigned a
3450 reference to a new anonymous dirhandle.
3451 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3453 See example at C<readdir>.
3460 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3461 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3464 For the reverse, see L</chr>.
3465 See L<perlunicode> for more about Unicode.
3472 =item our EXPR : ATTRS
3474 =item our TYPE EXPR : ATTRS
3476 C<our> associates a simple name with a package variable in the current
3477 package for use within the current scope. When C<use strict 'vars'> is in
3478 effect, C<our> lets you use declared global variables without qualifying
3479 them with package names, within the lexical scope of the C<our> declaration.
3480 In this way C<our> differs from C<use vars>, which is package scoped.
3482 Unlike C<my>, which both allocates storage for a variable and associates
3483 a simple name with that storage for use within the current scope, C<our>
3484 associates a simple name with a package variable in the current package,
3485 for use within the current scope. In other words, C<our> has the same
3486 scoping rules as C<my>, but does not necessarily create a
3489 If more than one value is listed, the list must be placed
3495 An C<our> declaration declares a global variable that will be visible
3496 across its entire lexical scope, even across package boundaries. The
3497 package in which the variable is entered is determined at the point
3498 of the declaration, not at the point of use. This means the following
3502 our $bar; # declares $Foo::bar for rest of lexical scope
3506 print $bar; # prints 20, as it refers to $Foo::bar
3508 Multiple C<our> declarations with the same name in the same lexical
3509 scope are allowed if they are in different packages. If they happen
3510 to be in the same package, Perl will emit warnings if you have asked
3511 for them, just like multiple C<my> declarations. Unlike a second
3512 C<my> declaration, which will bind the name to a fresh variable, a
3513 second C<our> declaration in the same package, in the same scope, is
3518 our $bar; # declares $Foo::bar for rest of lexical scope
3522 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3523 print $bar; # prints 30
3525 our $bar; # emits warning but has no other effect
3526 print $bar; # still prints 30
3528 An C<our> declaration may also have a list of attributes associated
3531 The exact semantics and interface of TYPE and ATTRS are still
3532 evolving. TYPE is currently bound to the use of C<fields> pragma,
3533 and attributes are handled using the C<attributes> pragma, or starting
3534 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3535 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3536 L<attributes>, and L<Attribute::Handlers>.
3538 =item pack TEMPLATE,LIST
3541 Takes a LIST of values and converts it into a string using the rules
3542 given by the TEMPLATE. The resulting string is the concatenation of
3543 the converted values. Typically, each converted value looks
3544 like its machine-level representation. For example, on 32-bit machines
3545 an integer may be represented by a sequence of 4 bytes that will be
3546 converted to a sequence of 4 characters.
3548 The TEMPLATE is a sequence of characters that give the order and type
3549 of values, as follows:
3551 a A string with arbitrary binary data, will be null padded.
3552 A A text (ASCII) string, will be space padded.
3553 Z A null terminated (ASCIZ) string, will be null padded.
3555 b A bit string (ascending bit order inside each byte, like vec()).
3556 B A bit string (descending bit order inside each byte).
3557 h A hex string (low nybble first).
3558 H A hex string (high nybble first).
3560 c A signed char (8-bit) value.
3561 C An unsigned char (octet) value.
3562 W An unsigned char value (can be greater than 255).
3564 s A signed short (16-bit) value.
3565 S An unsigned short value.
3567 l A signed long (32-bit) value.
3568 L An unsigned long value.
3570 q A signed quad (64-bit) value.
3571 Q An unsigned quad value.
3572 (Quads are available only if your system supports 64-bit
3573 integer values _and_ if Perl has been compiled to support those.
3574 Causes a fatal error otherwise.)
3576 i A signed integer value.
3577 I A unsigned integer value.
3578 (This 'integer' is _at_least_ 32 bits wide. Its exact
3579 size depends on what a local C compiler calls 'int'.)
3581 n An unsigned short (16-bit) in "network" (big-endian) order.
3582 N An unsigned long (32-bit) in "network" (big-endian) order.
3583 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3584 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3586 j A Perl internal signed integer value (IV).
3587 J A Perl internal unsigned integer value (UV).
3589 f A single-precision float in the native format.
3590 d A double-precision float in the native format.
3592 F A Perl internal floating point value (NV) in the native format
3593 D A long double-precision float in the native format.
3594 (Long doubles are available only if your system supports long
3595 double values _and_ if Perl has been compiled to support those.
3596 Causes a fatal error otherwise.)
3598 p A pointer to a null-terminated string.
3599 P A pointer to a structure (fixed-length string).
3601 u A uuencoded string.
3602 U A Unicode character number. Encodes to a character in character mode
3603 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3605 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3606 details). Its bytes represent an unsigned integer in base 128,
3607 most significant digit first, with as few digits as possible. Bit
3608 eight (the high bit) is set on each byte except the last.
3612 @ Null fill or truncate to absolute position, counted from the
3613 start of the innermost ()-group.
3614 . Null fill or truncate to absolute position specified by value.
3615 ( Start of a ()-group.
3617 One or more of the modifiers below may optionally follow some letters in the
3618 TEMPLATE (the second column lists the letters for which the modifier is
3621 ! sSlLiI Forces native (short, long, int) sizes instead
3622 of fixed (16-/32-bit) sizes.
3624 xX Make x and X act as alignment commands.
3626 nNvV Treat integers as signed instead of unsigned.
3628 @. Specify position as byte offset in the internal
3629 representation of the packed string. Efficient but
3632 > sSiIlLqQ Force big-endian byte-order on the type.
3633 jJfFdDpP (The "big end" touches the construct.)
3635 < sSiIlLqQ Force little-endian byte-order on the type.
3636 jJfFdDpP (The "little end" touches the construct.)
3638 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3639 in which case they force a certain byte-order on all components of
3640 that group, including subgroups.
3642 The following rules apply:
3648 Each letter may optionally be followed by a number giving a repeat
3649 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3650 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3651 that many values from the LIST. A C<*> for the repeat count means to
3652 use however many items are left, except for C<@>, C<x>, C<X>, where it
3653 is equivalent to C<0>, for <.> where it means relative to string start
3654 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3655 A numeric repeat count may optionally be enclosed in brackets, as in
3656 C<pack 'C[80]', @arr>.
3658 One can replace the numeric repeat count by a template enclosed in brackets;
3659 then the packed length of this template in bytes is used as a count.
3660 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3661 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3662 If the template in brackets contains alignment commands (such as C<x![d]>),
3663 its packed length is calculated as if the start of the template has the maximal
3666 When used with C<Z>, C<*> results in the addition of a trailing null
3667 byte (so the packed result will be one longer than the byte C<length>
3670 When used with C<@>, the repeat count represents an offset from the start
3671 of the innermost () group.
3673 When used with C<.>, the repeat count is used to determine the starting
3674 position from where the value offset is calculated. If the repeat count
3675 is 0, it's relative to the current position. If the repeat count is C<*>,
3676 the offset is relative to the start of the packed string. And if its an
3677 integer C<n> the offset is relative to the start of the n-th innermost
3678 () group (or the start of the string if C<n> is bigger then the group
3681 The repeat count for C<u> is interpreted as the maximal number of bytes
3682 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3683 count should not be more than 65.
3687 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3688 string of length count, padding with nulls or spaces as necessary. When
3689 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3690 after the first null, and C<a> returns data verbatim.
3692 If the value-to-pack is too long, it is truncated. If too long and an
3693 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3694 by a null byte. Thus C<Z> always packs a trailing null (except when the
3699 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3700 Each character of the input field of pack() generates 1 bit of the result.
3701 Each result bit is based on the least-significant bit of the corresponding
3702 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3703 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3705 Starting from the beginning of the input string of pack(), each 8-tuple
3706 of characters is converted to 1 character of output. With format C<b>
3707 the first character of the 8-tuple determines the least-significant bit of a
3708 character, and with format C<B> it determines the most-significant bit of
3711 If the length of the input string is not exactly divisible by 8, the
3712 remainder is packed as if the input string were padded by null characters
3713 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3715 If the input string of pack() is longer than needed, extra characters are
3716 ignored. A C<*> for the repeat count of pack() means to use all the
3717 characters of the input field. On unpack()ing the bits are converted to a
3718 string of C<"0">s and C<"1">s.
3722 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3723 representable as hexadecimal digits, 0-9a-f) long.
3725 Each character of the input field of pack() generates 4 bits of the result.
3726 For non-alphabetical characters the result is based on the 4 least-significant
3727 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3728 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3729 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3730 is compatible with the usual hexadecimal digits, so that C<"a"> and
3731 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3732 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3734 Starting from the beginning of the input string of pack(), each pair
3735 of characters is converted to 1 character of output. With format C<h> the
3736 first character of the pair determines the least-significant nybble of the
3737 output character, and with format C<H> it determines the most-significant
3740 If the length of the input string is not even, it behaves as if padded
3741 by a null character at the end. Similarly, during unpack()ing the "extra"
3742 nybbles are ignored.
3744 If the input string of pack() is longer than needed, extra characters are
3746 A C<*> for the repeat count of pack() means to use all the characters of
3747 the input field. On unpack()ing the nybbles are converted to a string
3748 of hexadecimal digits.
3752 The C<p> type packs a pointer to a null-terminated string. You are
3753 responsible for ensuring the string is not a temporary value (which can
3754 potentially get deallocated before you get around to using the packed result).
3755 The C<P> type packs a pointer to a structure of the size indicated by the
3756 length. A NULL pointer is created if the corresponding value for C<p> or
3757 C<P> is C<undef>, similarly for unpack().
3759 If your system has a strange pointer size (i.e. a pointer is neither as
3760 big as an int nor as big as a long), it may not be possible to pack or
3761 unpack pointers in big- or little-endian byte order. Attempting to do
3762 so will result in a fatal error.
3766 The C</> template character allows packing and unpacking of a sequence of
3767 items where the packed structure contains a packed item count followed by
3768 the packed items themselves.
3770 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3771 I<length-item> describes how the length value is packed. The ones likely
3772 to be of most use are integer-packing ones like C<n> (for Java strings),
3773 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3775 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3776 the minimum of that and the number of available items is used as argument
3777 for the I<length-item>. If it has no repeat count or uses a '*', the number
3778 of available items is used.
3780 For C<unpack> an internal stack of integer arguments unpacked so far is
3781 used. You write C</>I<sequence-item> and the repeat count is obtained by
3782 popping off the last element from the stack. The I<sequence-item> must not
3783 have a repeat count.
3785 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3786 the I<length-item> is a string length, not a number of strings. If there is
3787 an explicit repeat count for pack, the packed string will be adjusted to that
3790 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3791 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3792 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3793 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3794 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3796 The I<length-item> is not returned explicitly from C<unpack>.
3798 Adding a count to the I<length-item> letter is unlikely to do anything
3799 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3800 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3801 which Perl does not regard as legal in numeric strings.
3805 The integer types C<s>, C<S>, C<l>, and C<L> may be
3806 followed by a C<!> modifier to signify native shorts or
3807 longs--as you can see from above for example a bare C<l> does mean
3808 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3809 may be larger. This is an issue mainly in 64-bit platforms. You can
3810 see whether using C<!> makes any difference by
3812 print length(pack("s")), " ", length(pack("s!")), "\n";
3813 print length(pack("l")), " ", length(pack("l!")), "\n";
3815 C<i!> and C<I!> also work but only because of completeness;
3816 they are identical to C<i> and C<I>.
3818 The actual sizes (in bytes) of native shorts, ints, longs, and long
3819 longs on the platform where Perl was built are also available via
3823 print $Config{shortsize}, "\n";
3824 print $Config{intsize}, "\n";
3825 print $Config{longsize}, "\n";
3826 print $Config{longlongsize}, "\n";
3828 (The C<$Config{longlongsize}> will be undefined if your system does
3829 not support long longs.)
3833 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3834 are inherently non-portable between processors and operating systems
3835 because they obey the native byteorder and endianness. For example a
3836 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3837 (arranged in and handled by the CPU registers) into bytes as
3839 0x12 0x34 0x56 0x78 # big-endian
3840 0x78 0x56 0x34 0x12 # little-endian
3842 Basically, the Intel and VAX CPUs are little-endian, while everybody
3843 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3844 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3845 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3848 The names `big-endian' and `little-endian' are comic references to
3849 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3850 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3851 the egg-eating habits of the Lilliputians.
3853 Some systems may have even weirder byte orders such as
3858 You can see your system's preference with
3860 print join(" ", map { sprintf "%#02x", $_ }
3861 unpack("W*",pack("L",0x12345678))), "\n";
3863 The byteorder on the platform where Perl was built is also available
3867 print $Config{byteorder}, "\n";
3869 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3870 and C<'87654321'> are big-endian.
3872 If you want portable packed integers you can either use the formats
3873 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3874 modifiers. These modifiers are only available as of perl 5.9.2.
3875 See also L<perlport>.
3879 All integer and floating point formats as well as C<p> and C<P> and
3880 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3881 to force big- or little- endian byte-order, respectively.
3882 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3883 signed integers, 64-bit integers and floating point values. However,
3884 there are some things to keep in mind.
3886 Exchanging signed integers between different platforms only works
3887 if all platforms store them in the same format. Most platforms store
3888 signed integers in two's complement, so usually this is not an issue.
3890 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3891 formats on big- or little-endian machines. Otherwise, attempting to
3892 do so will result in a fatal error.
3894 Forcing big- or little-endian byte-order on floating point values for
3895 data exchange can only work if all platforms are using the same
3896 binary representation (e.g. IEEE floating point format). Even if all
3897 platforms are using IEEE, there may be subtle differences. Being able
3898 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3899 but also very dangerous if you don't know exactly what you're doing.
3900 It is definitely not a general way to portably store floating point
3903 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3904 all types inside the group that accept the byte-order modifiers,
3905 including all subgroups. It will silently be ignored for all other
3906 types. You are not allowed to override the byte-order within a group
3907 that already has a byte-order modifier suffix.
3911 Real numbers (floats and doubles) are in the native machine format only;
3912 due to the multiplicity of floating formats around, and the lack of a
3913 standard "network" representation, no facility for interchange has been
3914 made. This means that packed floating point data written on one machine
3915 may not be readable on another - even if both use IEEE floating point
3916 arithmetic (as the endian-ness of the memory representation is not part
3917 of the IEEE spec). See also L<perlport>.
3919 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3920 modifiers to force big- or little-endian byte-order on floating point values.
3922 Note that Perl uses doubles (or long doubles, if configured) internally for
3923 all numeric calculation, and converting from double into float and thence back
3924 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3925 will not in general equal $foo).
3929 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3930 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3931 where the packed string is processed in its UTF-8-encoded Unicode form on
3932 a byte by byte basis. Character mode is the default unless the format string
3933 starts with an C<U>. You can switch mode at any moment with an explicit
3934 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3935 or until the end of the ()-group in which it was entered.
3939 You must yourself do any alignment or padding by inserting for example
3940 enough C<'x'>es while packing. There is no way to pack() and unpack()
3941 could know where the characters are going to or coming from. Therefore
3942 C<pack> (and C<unpack>) handle their output and input as flat
3943 sequences of characters.
3947 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3948 take a repeat count, both as postfix, and for unpack() also via the C</>
3949 template character. Within each repetition of a group, positioning with
3950 C<@> starts again at 0. Therefore, the result of
3952 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3954 is the string "\0a\0\0bc".
3958 C<x> and C<X> accept C<!> modifier. In this case they act as
3959 alignment commands: they jump forward/back to the closest position
3960 aligned at a multiple of C<count> characters. For example, to pack() or
3961 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3962 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3963 aligned on the double's size.
3965 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3966 both result in no-ops.
3970 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3971 will represent signed 16-/32-bit integers in big-/little-endian order.
3972 This is only portable if all platforms sharing the packed data use the
3973 same binary representation for signed integers (e.g. all platforms are
3974 using two's complement representation).
3978 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3979 White space may be used to separate pack codes from each other, but
3980 modifiers and a repeat count must follow immediately.
3984 If TEMPLATE requires more arguments to pack() than actually given, pack()
3985 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3986 to pack() than actually given, extra arguments are ignored.
3992 $foo = pack("WWWW",65,66,67,68);
3994 $foo = pack("W4",65,66,67,68);
3996 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3997 # same thing with Unicode circled letters.
3998 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3999 # same thing with Unicode circled letters. You don't get the UTF-8
4000 # bytes because the U at the start of the format caused a switch to
4001 # U0-mode, so the UTF-8 bytes get joined into characters
4002 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4003 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4004 # This is the UTF-8 encoding of the string in the previous example
4006 $foo = pack("ccxxcc",65,66,67,68);
4009 # note: the above examples featuring "W" and "c" are true
4010 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4011 # and UTF-8. In EBCDIC the first example would be
4012 # $foo = pack("WWWW",193,194,195,196);
4014 $foo = pack("s2",1,2);
4015 # "\1\0\2\0" on little-endian
4016 # "\0\1\0\2" on big-endian
4018 $foo = pack("a4","abcd","x","y","z");
4021 $foo = pack("aaaa","abcd","x","y","z");
4024 $foo = pack("a14","abcdefg");
4025 # "abcdefg\0\0\0\0\0\0\0"
4027 $foo = pack("i9pl", gmtime);
4028 # a real struct tm (on my system anyway)
4030 $utmp_template = "Z8 Z8 Z16 L";
4031 $utmp = pack($utmp_template, @utmp1);
4032 # a struct utmp (BSDish)
4034 @utmp2 = unpack($utmp_template, $utmp);
4035 # "@utmp1" eq "@utmp2"
4038 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4041 $foo = pack('sx2l', 12, 34);
4042 # short 12, two zero bytes padding, long 34
4043 $bar = pack('s@4l', 12, 34);
4044 # short 12, zero fill to position 4, long 34
4046 $baz = pack('s.l', 12, 4, 34);
4047 # short 12, zero fill to position 4, long 34
4049 $foo = pack('nN', 42, 4711);
4050 # pack big-endian 16- and 32-bit unsigned integers
4051 $foo = pack('S>L>', 42, 4711);
4053 $foo = pack('s<l<', -42, 4711);
4054 # pack little-endian 16- and 32-bit signed integers
4055 $foo = pack('(sl)<', -42, 4711);
4058 The same template may generally also be used in unpack().
4060 =item package NAMESPACE
4061 X<package> X<module> X<namespace>
4065 Declares the compilation unit as being in the given namespace. The scope
4066 of the package declaration is from the declaration itself through the end
4067 of the enclosing block, file, or eval (the same as the C<my> operator).
4068 All further unqualified dynamic identifiers will be in this namespace.
4069 A package statement affects only dynamic variables--including those
4070 you've used C<local> on--but I<not> lexical variables, which are created
4071 with C<my>. Typically it would be the first declaration in a file to
4072 be included by the C<require> or C<use> operator. You can switch into a
4073 package in more than one place; it merely influences which symbol table
4074 is used by the compiler for the rest of that block. You can refer to
4075 variables and filehandles in other packages by prefixing the identifier
4076 with the package name and a double colon: C<$Package::Variable>.
4077 If the package name is null, the C<main> package as assumed. That is,
4078 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4079 still seen in older code).
4081 See L<perlmod/"Packages"> for more information about packages, modules,
4082 and classes. See L<perlsub> for other scoping issues.
4084 =item pipe READHANDLE,WRITEHANDLE
4087 Opens a pair of connected pipes like the corresponding system call.
4088 Note that if you set up a loop of piped processes, deadlock can occur
4089 unless you are very careful. In addition, note that Perl's pipes use
4090 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4091 after each command, depending on the application.
4093 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4094 for examples of such things.
4096 On systems that support a close-on-exec flag on files, the flag will be set
4097 for the newly opened file descriptors as determined by the value of $^F.
4105 Pops and returns the last value of the array, shortening the array by
4108 If there are no elements in the array, returns the undefined value
4109 (although this may happen at other times as well). If ARRAY is
4110 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4111 array in subroutines, just like C<shift>.
4114 X<pos> X<match, position>
4118 Returns the offset of where the last C<m//g> search left off for the variable
4119 in question (C<$_> is used when the variable is not specified). Note that
4120 0 is a valid match offset. C<undef> indicates that the search position
4121 is reset (usually due to match failure, but can also be because no match has
4122 yet been performed on the scalar). C<pos> directly accesses the location used
4123 by the regexp engine to store the offset, so assigning to C<pos> will change
4124 that offset, and so will also influence the C<\G> zero-width assertion in
4125 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4126 the return from C<pos> won't change either in this case. See L<perlre> and
4129 =item print FILEHANDLE LIST
4136 Prints a string or a list of strings. Returns true if successful.
4137 FILEHANDLE may be a scalar variable name, in which case the variable
4138 contains the name of or a reference to the filehandle, thus introducing
4139 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4140 the next token is a term, it may be misinterpreted as an operator
4141 unless you interpose a C<+> or put parentheses around the arguments.)
4142 If FILEHANDLE is omitted, prints by default to standard output (or
4143 to the last selected output channel--see L</select>). If LIST is
4144 also omitted, prints C<$_> to the currently selected output channel.
4145 To set the default output channel to something other than STDOUT
4146 use the select operation. The current value of C<$,> (if any) is
4147 printed between each LIST item. The current value of C<$\> (if
4148 any) is printed after the entire LIST has been printed. Because
4149 print takes a LIST, anything in the LIST is evaluated in list
4150 context, and any subroutine that you call will have one or more of
4151 its expressions evaluated in list context. Also be careful not to
4152 follow the print keyword with a left parenthesis unless you want
4153 the corresponding right parenthesis to terminate the arguments to
4154 the print--interpose a C<+> or put parentheses around all the
4157 Note that if you're storing FILEHANDLEs in an array, or if you're using
4158 any other expression more complex than a scalar variable to retrieve it,
4159 you will have to use a block returning the filehandle value instead:
4161 print { $files[$i] } "stuff\n";
4162 print { $OK ? STDOUT : STDERR } "stuff\n";
4164 =item printf FILEHANDLE FORMAT, LIST
4167 =item printf FORMAT, LIST
4169 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4170 (the output record separator) is not appended. The first argument
4171 of the list will be interpreted as the C<printf> format. See C<sprintf>
4172 for an explanation of the format argument. If C<use locale> is in effect,
4173 and POSIX::setlocale() has been called, the character used for the decimal
4174 separator in formatted floating point numbers is affected by the LC_NUMERIC
4175 locale. See L<perllocale> and L<POSIX>.
4177 Don't fall into the trap of using a C<printf> when a simple
4178 C<print> would do. The C<print> is more efficient and less
4181 =item prototype FUNCTION
4184 Returns the prototype of a function as a string (or C<undef> if the
4185 function has no prototype). FUNCTION is a reference to, or the name of,
4186 the function whose prototype you want to retrieve.
4188 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4189 name for Perl builtin. If the builtin is not I<overridable> (such as
4190 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4191 (such as C<system>), prototype() returns C<undef>, because the builtin
4192 does not really behave like a Perl function. Otherwise, the string
4193 describing the equivalent prototype is returned.
4195 =item push ARRAY,LIST
4198 Treats ARRAY as a stack, and pushes the values of LIST
4199 onto the end of ARRAY. The length of ARRAY increases by the length of
4200 LIST. Has the same effect as
4203 $ARRAY[++$#ARRAY] = $value;
4206 but is more efficient. Returns the number of elements in the array following
4207 the completed C<push>.
4219 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4221 =item quotemeta EXPR
4222 X<quotemeta> X<metacharacter>
4226 Returns the value of EXPR with all non-"word"
4227 characters backslashed. (That is, all characters not matching
4228 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4229 returned string, regardless of any locale settings.)
4230 This is the internal function implementing
4231 the C<\Q> escape in double-quoted strings.
4233 If EXPR is omitted, uses C<$_>.
4240 Returns a random fractional number greater than or equal to C<0> and less
4241 than the value of EXPR. (EXPR should be positive.) If EXPR is
4242 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4243 also special-cased as C<1> - this has not been documented before perl 5.8.0
4244 and is subject to change in future versions of perl. Automatically calls
4245 C<srand> unless C<srand> has already been called. See also C<srand>.
4247 Apply C<int()> to the value returned by C<rand()> if you want random
4248 integers instead of random fractional numbers. For example,
4252 returns a random integer between C<0> and C<9>, inclusive.
4254 (Note: If your rand function consistently returns numbers that are too
4255 large or too small, then your version of Perl was probably compiled
4256 with the wrong number of RANDBITS.)
4258 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4259 X<read> X<file, read>
4261 =item read FILEHANDLE,SCALAR,LENGTH
4263 Attempts to read LENGTH I<characters> of data into variable SCALAR
4264 from the specified FILEHANDLE. Returns the number of characters
4265 actually read, C<0> at end of file, or undef if there was an error (in
4266 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4267 so that the last character actually read is the last character of the
4268 scalar after the read.
4270 An OFFSET may be specified to place the read data at some place in the
4271 string other than the beginning. A negative OFFSET specifies
4272 placement at that many characters counting backwards from the end of
4273 the string. A positive OFFSET greater than the length of SCALAR
4274 results in the string being padded to the required size with C<"\0">
4275 bytes before the result of the read is appended.
4277 The call is actually implemented in terms of either Perl's or system's
4278 fread() call. To get a true read(2) system call, see C<sysread>.
4280 Note the I<characters>: depending on the status of the filehandle,
4281 either (8-bit) bytes or characters are read. By default all
4282 filehandles operate on bytes, but for example if the filehandle has
4283 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4284 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4285 characters, not bytes. Similarly for the C<:encoding> pragma:
4286 in that case pretty much any characters can be read.
4288 =item readdir DIRHANDLE
4291 Returns the next directory entry for a directory opened by C<opendir>.
4292 If used in list context, returns all the rest of the entries in the
4293 directory. If there are no more entries, returns an undefined value in
4294 scalar context or a null list in list context.
4296 If you're planning to filetest the return values out of a C<readdir>, you'd
4297 better prepend the directory in question. Otherwise, because we didn't
4298 C<chdir> there, it would have been testing the wrong file.
4300 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4301 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4307 X<readline> X<gets> X<fgets>
4309 Reads from the filehandle whose typeglob is contained in EXPR (or from
4310 *ARGV if EXPR is not provided). In scalar context, each call reads and
4311 returns the next line, until end-of-file is reached, whereupon the
4312 subsequent call returns undef. In list context, reads until end-of-file
4313 is reached and returns a list of lines. Note that the notion of "line"
4314 used here is however you may have defined it with C<$/> or
4315 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4317 When C<$/> is set to C<undef>, when readline() is in scalar
4318 context (i.e. file slurp mode), and when an empty file is read, it
4319 returns C<''> the first time, followed by C<undef> subsequently.
4321 This is the internal function implementing the C<< <EXPR> >>
4322 operator, but you can use it directly. The C<< <EXPR> >>
4323 operator is discussed in more detail in L<perlop/"I/O Operators">.
4326 $line = readline(*STDIN); # same thing
4328 If readline encounters an operating system error, C<$!> will be set with the
4329 corresponding error message. It can be helpful to check C<$!> when you are
4330 reading from filehandles you don't trust, such as a tty or a socket. The
4331 following example uses the operator form of C<readline>, and takes the necessary
4332 steps to ensure that C<readline> was successful.
4336 unless (defined( $line = <> )) {
4348 Returns the value of a symbolic link, if symbolic links are
4349 implemented. If not, gives a fatal error. If there is some system
4350 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4351 omitted, uses C<$_>.
4358 EXPR is executed as a system command.
4359 The collected standard output of the command is returned.
4360 In scalar context, it comes back as a single (potentially
4361 multi-line) string. In list context, returns a list of lines
4362 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4363 This is the internal function implementing the C<qx/EXPR/>
4364 operator, but you can use it directly. The C<qx/EXPR/>
4365 operator is discussed in more detail in L<perlop/"I/O Operators">.
4366 If EXPR is omitted, uses C<$_>.
4368 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4371 Receives a message on a socket. Attempts to receive LENGTH characters
4372 of data into variable SCALAR from the specified SOCKET filehandle.
4373 SCALAR will be grown or shrunk to the length actually read. Takes the
4374 same flags as the system call of the same name. Returns the address
4375 of the sender if SOCKET's protocol supports this; returns an empty
4376 string otherwise. If there's an error, returns the undefined value.
4377 This call is actually implemented in terms of recvfrom(2) system call.
4378 See L<perlipc/"UDP: Message Passing"> for examples.
4380 Note the I<characters>: depending on the status of the socket, either
4381 (8-bit) bytes or characters are received. By default all sockets
4382 operate on bytes, but for example if the socket has been changed using
4383 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4384 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4385 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4386 case pretty much any characters can be read.
4393 The C<redo> command restarts the loop block without evaluating the
4394 conditional again. The C<continue> block, if any, is not executed. If
4395 the LABEL is omitted, the command refers to the innermost enclosing
4396 loop. Programs that want to lie to themselves about what was just input
4397 normally use this command:
4399 # a simpleminded Pascal comment stripper
4400 # (warning: assumes no { or } in strings)
4401 LINE: while (<STDIN>) {
4402 while (s|({.*}.*){.*}|$1 |) {}
4407 if (/}/) { # end of comment?
4416 C<redo> cannot be used to retry a block which returns a value such as
4417 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4418 a grep() or map() operation.
4420 Note that a block by itself is semantically identical to a loop
4421 that executes once. Thus C<redo> inside such a block will effectively
4422 turn it into a looping construct.
4424 See also L</continue> for an illustration of how C<last>, C<next>, and
4432 Returns a non-empty string if EXPR is a reference, the empty
4433 string otherwise. If EXPR
4434 is not specified, C<$_> will be used. The value returned depends on the
4435 type of thing the reference is a reference to.
4436 Builtin types include:
4450 If the referenced object has been blessed into a package, then that package
4451 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4453 if (ref($r) eq "HASH") {
4454 print "r is a reference to a hash.\n";
4457 print "r is not a reference at all.\n";
4460 The return value C<LVALUE> indicates a reference to an lvalue that is not
4461 a variable. You get this from taking the reference of function calls like
4462 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4463 to a L<version string|perldata/"Version Strings">.
4465 The result C<Regexp> indicates that the argument is a regular expression
4466 resulting from C<qr//>.
4468 See also L<perlref>.
4470 =item rename OLDNAME,NEWNAME
4471 X<rename> X<move> X<mv> X<ren>
4473 Changes the name of a file; an existing file NEWNAME will be
4474 clobbered. Returns true for success, false otherwise.
4476 Behavior of this function varies wildly depending on your system
4477 implementation. For example, it will usually not work across file system
4478 boundaries, even though the system I<mv> command sometimes compensates
4479 for this. Other restrictions include whether it works on directories,
4480 open files, or pre-existing files. Check L<perlport> and either the
4481 rename(2) manpage or equivalent system documentation for details.
4483 For a platform independent C<move> function look at the L<File::Copy>
4486 =item require VERSION
4493 Demands a version of Perl specified by VERSION, or demands some semantics
4494 specified by EXPR or by C<$_> if EXPR is not supplied.
4496 VERSION may be either a numeric argument such as 5.006, which will be
4497 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4498 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4499 VERSION is greater than the version of the current Perl interpreter.
4500 Compare with L</use>, which can do a similar check at compile time.
4502 Specifying VERSION as a literal of the form v5.6.1 should generally be
4503 avoided, because it leads to misleading error messages under earlier
4504 versions of Perl that do not support this syntax. The equivalent numeric
4505 version should be used instead.
4507 require v5.6.1; # run time version check
4508 require 5.6.1; # ditto
4509 require 5.006_001; # ditto; preferred for backwards compatibility
4511 Otherwise, C<require> demands that a library file be included if it
4512 hasn't already been included. The file is included via the do-FILE
4513 mechanism, which is essentially just a variety of C<eval> with the
4514 caveat that lexical variables in the invoking script will be invisible
4515 to the included code. Has semantics similar to the following subroutine:
4518 my ($filename) = @_;
4519 if (exists $INC{$filename}) {
4520 return 1 if $INC{$filename};
4521 die "Compilation failed in require";
4523 my ($realfilename,$result);
4525 foreach $prefix (@INC) {
4526 $realfilename = "$prefix/$filename";
4527 if (-f $realfilename) {
4528 $INC{$filename} = $realfilename;
4529 $result = do $realfilename;
4533 die "Can't find $filename in \@INC";
4536 $INC{$filename} = undef;
4538 } elsif (!$result) {
4539 delete $INC{$filename};
4540 die "$filename did not return true value";
4546 Note that the file will not be included twice under the same specified
4549 The file must return true as the last statement to indicate
4550 successful execution of any initialization code, so it's customary to
4551 end such a file with C<1;> unless you're sure it'll return true
4552 otherwise. But it's better just to put the C<1;>, in case you add more
4555 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4556 replaces "F<::>" with "F</>" in the filename for you,
4557 to make it easy to load standard modules. This form of loading of
4558 modules does not risk altering your namespace.
4560 In other words, if you try this:
4562 require Foo::Bar; # a splendid bareword
4564 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4565 directories specified in the C<@INC> array.
4567 But if you try this:
4569 $class = 'Foo::Bar';
4570 require $class; # $class is not a bareword
4572 require "Foo::Bar"; # not a bareword because of the ""
4574 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4575 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4577 eval "require $class";
4579 Now that you understand how C<require> looks for files in the case of a
4580 bareword argument, there is a little extra functionality going on behind
4581 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4582 first look for a similar filename with a "F<.pmc>" extension. If this file
4583 is found, it will be loaded in place of any file ending in a "F<.pm>"
4586 You can also insert hooks into the import facility, by putting directly
4587 Perl code into the @INC array. There are three forms of hooks: subroutine
4588 references, array references and blessed objects.
4590 Subroutine references are the simplest case. When the inclusion system
4591 walks through @INC and encounters a subroutine, this subroutine gets
4592 called with two parameters, the first being a reference to itself, and the
4593 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4594 subroutine should return nothing, or a list of up to three values in the
4601 A filehandle, from which the file will be read.
4605 A reference to a subroutine. If there is no filehandle (previous item),
4606 then this subroutine is expected to generate one line of source code per
4607 call, writing the line into C<$_> and returning 1, then returning 0 at
4608 "end of file". If there is a filehandle, then the subroutine will be
4609 called to act a simple source filter, with the line as read in C<$_>.
4610 Again, return 1 for each valid line, and 0 after all lines have been
4615 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4616 reference to the subroutine itself is passed in as C<$_[0]>.
4620 If an empty list, C<undef>, or nothing that matches the first 3 values above
4621 is returned then C<require> will look at the remaining elements of @INC.
4622 Note that this file handle must be a real file handle (strictly a typeglob,
4623 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4624 ignored and return value processing will stop there.
4626 If the hook is an array reference, its first element must be a subroutine
4627 reference. This subroutine is called as above, but the first parameter is
4628 the array reference. This enables to pass indirectly some arguments to
4631 In other words, you can write:
4633 push @INC, \&my_sub;
4635 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4641 push @INC, [ \&my_sub, $x, $y, ... ];
4643 my ($arrayref, $filename) = @_;
4644 # Retrieve $x, $y, ...
4645 my @parameters = @$arrayref[1..$#$arrayref];
4649 If the hook is an object, it must provide an INC method that will be
4650 called as above, the first parameter being the object itself. (Note that
4651 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4652 into package C<main>.) Here is a typical code layout:
4658 my ($self, $filename) = @_;
4662 # In the main program
4663 push @INC, new Foo(...);
4665 Note that these hooks are also permitted to set the %INC entry
4666 corresponding to the files they have loaded. See L<perlvar/%INC>.
4668 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4675 Generally used in a C<continue> block at the end of a loop to clear
4676 variables and reset C<??> searches so that they work again. The
4677 expression is interpreted as a list of single characters (hyphens
4678 allowed for ranges). All variables and arrays beginning with one of
4679 those letters are reset to their pristine state. If the expression is
4680 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4681 only variables or searches in the current package. Always returns
4684 reset 'X'; # reset all X variables
4685 reset 'a-z'; # reset lower case variables
4686 reset; # just reset ?one-time? searches
4688 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4689 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4690 variables--lexical variables are unaffected, but they clean themselves
4691 up on scope exit anyway, so you'll probably want to use them instead.
4699 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4700 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4701 context, depending on how the return value will be used, and the context
4702 may vary from one execution to the next (see C<wantarray>). If no EXPR
4703 is given, returns an empty list in list context, the undefined value in
4704 scalar context, and (of course) nothing at all in a void context.
4706 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4707 or do FILE will automatically return the value of the last expression
4711 X<reverse> X<rev> X<invert>
4713 In list context, returns a list value consisting of the elements
4714 of LIST in the opposite order. In scalar context, concatenates the
4715 elements of LIST and returns a string value with all characters
4716 in the opposite order.
4718 print reverse <>; # line tac, last line first
4720 undef $/; # for efficiency of <>
4721 print scalar reverse <>; # character tac, last line tsrif
4723 Used without arguments in scalar context, reverse() reverses C<$_>.
4725 This operator is also handy for inverting a hash, although there are some
4726 caveats. If a value is duplicated in the original hash, only one of those
4727 can be represented as a key in the inverted hash. Also, this has to
4728 unwind one hash and build a whole new one, which may take some time
4729 on a large hash, such as from a DBM file.
4731 %by_name = reverse %by_address; # Invert the hash
4733 =item rewinddir DIRHANDLE
4736 Sets the current position to the beginning of the directory for the
4737 C<readdir> routine on DIRHANDLE.
4739 =item rindex STR,SUBSTR,POSITION
4742 =item rindex STR,SUBSTR
4744 Works just like index() except that it returns the position of the I<last>
4745 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4746 last occurrence beginning at or before that position.
4748 =item rmdir FILENAME
4749 X<rmdir> X<rd> X<directory, remove>
4753 Deletes the directory specified by FILENAME if that directory is
4754 empty. If it succeeds it returns true, otherwise it returns false and
4755 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4757 To remove a directory tree recursively (C<rm -rf> on unix) look at
4758 the C<rmtree> function of the L<File::Path> module.
4762 The substitution operator. See L<perlop>.
4764 =item say FILEHANDLE LIST
4771 Just like C<print>, but implicitly appends a newline.
4772 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
4775 This keyword is only available when the "say" feature is
4776 enabled: see L<feature>.
4779 X<scalar> X<context>
4781 Forces EXPR to be interpreted in scalar context and returns the value
4784 @counts = ( scalar @a, scalar @b, scalar @c );
4786 There is no equivalent operator to force an expression to
4787 be interpolated in list context because in practice, this is never
4788 needed. If you really wanted to do so, however, you could use
4789 the construction C<@{[ (some expression) ]}>, but usually a simple
4790 C<(some expression)> suffices.
4792 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4793 parenthesized list, this behaves as a scalar comma expression, evaluating
4794 all but the last element in void context and returning the final element
4795 evaluated in scalar context. This is seldom what you want.
4797 The following single statement:
4799 print uc(scalar(&foo,$bar)),$baz;
4801 is the moral equivalent of these two:
4804 print(uc($bar),$baz);
4806 See L<perlop> for more details on unary operators and the comma operator.
4808 =item seek FILEHANDLE,POSITION,WHENCE
4809 X<seek> X<fseek> X<filehandle, position>
4811 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4812 FILEHANDLE may be an expression whose value gives the name of the
4813 filehandle. The values for WHENCE are C<0> to set the new position
4814 I<in bytes> to POSITION, C<1> to set it to the current position plus
4815 POSITION, and C<2> to set it to EOF plus POSITION (typically
4816 negative). For WHENCE you may use the constants C<SEEK_SET>,
4817 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4818 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4821 Note the I<in bytes>: even if the filehandle has been set to
4822 operate on characters (for example by using the C<:encoding(utf8)> open
4823 layer), tell() will return byte offsets, not character offsets
4824 (because implementing that would render seek() and tell() rather slow).
4826 If you want to position file for C<sysread> or C<syswrite>, don't use
4827 C<seek>--buffering makes its effect on the file's system position
4828 unpredictable and non-portable. Use C<sysseek> instead.
4830 Due to the rules and rigors of ANSI C, on some systems you have to do a
4831 seek whenever you switch between reading and writing. Amongst other
4832 things, this may have the effect of calling stdio's clearerr(3).
4833 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4837 This is also useful for applications emulating C<tail -f>. Once you hit
4838 EOF on your read, and then sleep for a while, you might have to stick in a
4839 seek() to reset things. The C<seek> doesn't change the current position,
4840 but it I<does> clear the end-of-file condition on the handle, so that the
4841 next C<< <FILE> >> makes Perl try again to read something. We hope.
4843 If that doesn't work (some IO implementations are particularly
4844 cantankerous), then you may need something more like this:
4847 for ($curpos = tell(FILE); $_ = <FILE>;
4848 $curpos = tell(FILE)) {
4849 # search for some stuff and put it into files
4851 sleep($for_a_while);
4852 seek(FILE, $curpos, 0);
4855 =item seekdir DIRHANDLE,POS
4858 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4859 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4860 about possible directory compaction as the corresponding system library
4863 =item select FILEHANDLE
4864 X<select> X<filehandle, default>
4868 Returns the currently selected filehandle. If FILEHANDLE is supplied,
4869 sets the new current default filehandle for output. This has two
4870 effects: first, a C<write> or a C<print> without a filehandle will
4871 default to this FILEHANDLE. Second, references to variables related to
4872 output will refer to this output channel. For example, if you have to
4873 set the top of form format for more than one output channel, you might
4881 FILEHANDLE may be an expression whose value gives the name of the
4882 actual filehandle. Thus:
4884 $oldfh = select(STDERR); $| = 1; select($oldfh);
4886 Some programmers may prefer to think of filehandles as objects with
4887 methods, preferring to write the last example as:
4890 STDERR->autoflush(1);
4892 =item select RBITS,WBITS,EBITS,TIMEOUT
4895 This calls the select(2) system call with the bit masks specified, which
4896 can be constructed using C<fileno> and C<vec>, along these lines:
4898 $rin = $win = $ein = '';
4899 vec($rin,fileno(STDIN),1) = 1;
4900 vec($win,fileno(STDOUT),1) = 1;
4903 If you want to select on many filehandles you might wish to write a
4907 my(@fhlist) = split(' ',$_[0]);
4910 vec($bits,fileno($_),1) = 1;
4914 $rin = fhbits('STDIN TTY SOCK');
4918 ($nfound,$timeleft) =
4919 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4921 or to block until something becomes ready just do this
4923 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4925 Most systems do not bother to return anything useful in $timeleft, so
4926 calling select() in scalar context just returns $nfound.
4928 Any of the bit masks can also be undef. The timeout, if specified, is
4929 in seconds, which may be fractional. Note: not all implementations are
4930 capable of returning the $timeleft. If not, they always return
4931 $timeleft equal to the supplied $timeout.
4933 You can effect a sleep of 250 milliseconds this way:
4935 select(undef, undef, undef, 0.25);
4937 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4938 is implementation-dependent. See also L<perlport> for notes on the
4939 portability of C<select>.
4941 On error, C<select> behaves like the select(2) system call : it returns
4944 Note: on some Unixes, the select(2) system call may report a socket file
4945 descriptor as "ready for reading", when actually no data is available,
4946 thus a subsequent read blocks. It can be avoided using always the
4947 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4950 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4951 or <FH>) with C<select>, except as permitted by POSIX, and even
4952 then only on POSIX systems. You have to use C<sysread> instead.
4954 =item semctl ID,SEMNUM,CMD,ARG
4957 Calls the System V IPC function C<semctl>. You'll probably have to say
4961 first to get the correct constant definitions. If CMD is IPC_STAT or
4962 GETALL, then ARG must be a variable that will hold the returned
4963 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4964 the undefined value for error, "C<0 but true>" for zero, or the actual
4965 return value otherwise. The ARG must consist of a vector of native
4966 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4967 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4970 =item semget KEY,NSEMS,FLAGS
4973 Calls the System V IPC function semget. Returns the semaphore id, or
4974 the undefined value if there is an error. See also
4975 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4978 =item semop KEY,OPSTRING
4981 Calls the System V IPC function semop to perform semaphore operations
4982 such as signalling and waiting. OPSTRING must be a packed array of
4983 semop structures. Each semop structure can be generated with
4984 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4985 implies the number of semaphore operations. Returns true if
4986 successful, or false if there is an error. As an example, the
4987 following code waits on semaphore $semnum of semaphore id $semid:
4989 $semop = pack("s!3", $semnum, -1, 0);
4990 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4992 To signal the semaphore, replace C<-1> with C<1>. See also
4993 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4996 =item send SOCKET,MSG,FLAGS,TO
4999 =item send SOCKET,MSG,FLAGS
5001 Sends a message on a socket. Attempts to send the scalar MSG to the
5002 SOCKET filehandle. Takes the same flags as the system call of the
5003 same name. On unconnected sockets you must specify a destination to
5004 send TO, in which case it does a C C<sendto>. Returns the number of
5005 characters sent, or the undefined value if there is an error. The C
5006 system call sendmsg(2) is currently unimplemented. See
5007 L<perlipc/"UDP: Message Passing"> for examples.
5009 Note the I<characters>: depending on the status of the socket, either
5010 (8-bit) bytes or characters are sent. By default all sockets operate
5011 on bytes, but for example if the socket has been changed using
5012 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5013 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5014 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5015 pragma: in that case pretty much any characters can be sent.
5017 =item setpgrp PID,PGRP
5020 Sets the current process group for the specified PID, C<0> for the current
5021 process. Will produce a fatal error if used on a machine that doesn't
5022 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5023 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5024 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5027 =item setpriority WHICH,WHO,PRIORITY
5028 X<setpriority> X<priority> X<nice> X<renice>
5030 Sets the current priority for a process, a process group, or a user.
5031 (See setpriority(2).) Will produce a fatal error if used on a machine
5032 that doesn't implement setpriority(2).
5034 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5037 Sets the socket option requested. Returns undefined if there is an
5038 error. Use integer constants provided by the C<Socket> module for
5039 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5040 getprotobyname. OPTVAL might either be a packed string or an integer.
5041 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5043 An example disabling the Nagle's algorithm for a socket:
5045 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5046 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5053 Shifts the first value of the array off and returns it, shortening the
5054 array by 1 and moving everything down. If there are no elements in the
5055 array, returns the undefined value. If ARRAY is omitted, shifts the
5056 C<@_> array within the lexical scope of subroutines and formats, and the
5057 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5058 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5059 C<UNITCHECK {}> and C<END {}> constructs.
5061 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5062 same thing to the left end of an array that C<pop> and C<push> do to the
5065 =item shmctl ID,CMD,ARG
5068 Calls the System V IPC function shmctl. You'll probably have to say
5072 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5073 then ARG must be a variable that will hold the returned C<shmid_ds>
5074 structure. Returns like ioctl: the undefined value for error, "C<0> but
5075 true" for zero, or the actual return value otherwise.
5076 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5078 =item shmget KEY,SIZE,FLAGS
5081 Calls the System V IPC function shmget. Returns the shared memory
5082 segment id, or the undefined value if there is an error.
5083 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5085 =item shmread ID,VAR,POS,SIZE
5089 =item shmwrite ID,STRING,POS,SIZE
5091 Reads or writes the System V shared memory segment ID starting at
5092 position POS for size SIZE by attaching to it, copying in/out, and
5093 detaching from it. When reading, VAR must be a variable that will
5094 hold the data read. When writing, if STRING is too long, only SIZE
5095 bytes are used; if STRING is too short, nulls are written to fill out
5096 SIZE bytes. Return true if successful, or false if there is an error.
5097 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5098 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5100 =item shutdown SOCKET,HOW
5103 Shuts down a socket connection in the manner indicated by HOW, which
5104 has the same interpretation as in the system call of the same name.
5106 shutdown(SOCKET, 0); # I/we have stopped reading data
5107 shutdown(SOCKET, 1); # I/we have stopped writing data
5108 shutdown(SOCKET, 2); # I/we have stopped using this socket
5110 This is useful with sockets when you want to tell the other
5111 side you're done writing but not done reading, or vice versa.
5112 It's also a more insistent form of close because it also
5113 disables the file descriptor in any forked copies in other
5117 X<sin> X<sine> X<asin> X<arcsine>
5121 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5122 returns sine of C<$_>.
5124 For the inverse sine operation, you may use the C<Math::Trig::asin>
5125 function, or use this relation:
5127 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5134 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5135 May be interrupted if the process receives a signal such as C<SIGALRM>.
5136 Returns the number of seconds actually slept. You probably cannot
5137 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5140 On some older systems, it may sleep up to a full second less than what
5141 you requested, depending on how it counts seconds. Most modern systems
5142 always sleep the full amount. They may appear to sleep longer than that,
5143 however, because your process might not be scheduled right away in a
5144 busy multitasking system.
5146 For delays of finer granularity than one second, the Time::HiRes module
5147 (from CPAN, and starting from Perl 5.8 part of the standard
5148 distribution) provides usleep(). You may also use Perl's four-argument
5149 version of select() leaving the first three arguments undefined, or you
5150 might be able to use the C<syscall> interface to access setitimer(2) if
5151 your system supports it. See L<perlfaq8> for details.
5153 See also the POSIX module's C<pause> function.
5155 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5158 Opens a socket of the specified kind and attaches it to filehandle
5159 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5160 the system call of the same name. You should C<use Socket> first
5161 to get the proper definitions imported. See the examples in
5162 L<perlipc/"Sockets: Client/Server Communication">.
5164 On systems that support a close-on-exec flag on files, the flag will
5165 be set for the newly opened file descriptor, as determined by the
5166 value of $^F. See L<perlvar/$^F>.
5168 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5171 Creates an unnamed pair of sockets in the specified domain, of the
5172 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5173 for the system call of the same name. If unimplemented, yields a fatal
5174 error. Returns true if successful.
5176 On systems that support a close-on-exec flag on files, the flag will
5177 be set for the newly opened file descriptors, as determined by the value
5178 of $^F. See L<perlvar/$^F>.
5180 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5181 to C<pipe(Rdr, Wtr)> is essentially:
5184 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5185 shutdown(Rdr, 1); # no more writing for reader
5186 shutdown(Wtr, 0); # no more reading for writer
5188 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5189 emulate socketpair using IP sockets to localhost if your system implements
5190 sockets but not socketpair.
5192 =item sort SUBNAME LIST
5193 X<sort> X<qsort> X<quicksort> X<mergesort>
5195 =item sort BLOCK LIST
5199 In list context, this sorts the LIST and returns the sorted list value.
5200 In scalar context, the behaviour of C<sort()> is undefined.
5202 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5203 order. If SUBNAME is specified, it gives the name of a subroutine
5204 that returns an integer less than, equal to, or greater than C<0>,
5205 depending on how the elements of the list are to be ordered. (The C<<
5206 <=> >> and C<cmp> operators are extremely useful in such routines.)
5207 SUBNAME may be a scalar variable name (unsubscripted), in which case
5208 the value provides the name of (or a reference to) the actual
5209 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5210 an anonymous, in-line sort subroutine.
5212 If the subroutine's prototype is C<($$)>, the elements to be compared
5213 are passed by reference in C<@_>, as for a normal subroutine. This is
5214 slower than unprototyped subroutines, where the elements to be
5215 compared are passed into the subroutine
5216 as the package global variables $a and $b (see example below). Note that
5217 in the latter case, it is usually counter-productive to declare $a and
5220 The values to be compared are always passed by reference and should not
5223 You also cannot exit out of the sort block or subroutine using any of the
5224 loop control operators described in L<perlsyn> or with C<goto>.
5226 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5227 current collation locale. See L<perllocale>.
5229 sort() returns aliases into the original list, much as a for loop's index
5230 variable aliases the list elements. That is, modifying an element of a
5231 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5232 actually modifies the element in the original list. This is usually
5233 something to be avoided when writing clear code.
5235 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5236 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5237 preserves the input order of elements that compare equal. Although
5238 quicksort's run time is O(NlogN) when averaged over all arrays of
5239 length N, the time can be O(N**2), I<quadratic> behavior, for some
5240 inputs.) In 5.7, the quicksort implementation was replaced with
5241 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5242 But benchmarks indicated that for some inputs, on some platforms,
5243 the original quicksort was faster. 5.8 has a sort pragma for
5244 limited control of the sort. Its rather blunt control of the
5245 underlying algorithm may not persist into future Perls, but the
5246 ability to characterize the input or output in implementation
5247 independent ways quite probably will. See L<sort>.
5252 @articles = sort @files;
5254 # same thing, but with explicit sort routine
5255 @articles = sort {$a cmp $b} @files;
5257 # now case-insensitively
5258 @articles = sort {uc($a) cmp uc($b)} @files;
5260 # same thing in reversed order
5261 @articles = sort {$b cmp $a} @files;
5263 # sort numerically ascending
5264 @articles = sort {$a <=> $b} @files;
5266 # sort numerically descending
5267 @articles = sort {$b <=> $a} @files;
5269 # this sorts the %age hash by value instead of key
5270 # using an in-line function
5271 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5273 # sort using explicit subroutine name
5275 $age{$a} <=> $age{$b}; # presuming numeric
5277 @sortedclass = sort byage @class;
5279 sub backwards { $b cmp $a }
5280 @harry = qw(dog cat x Cain Abel);
5281 @george = qw(gone chased yz Punished Axed);
5283 # prints AbelCaincatdogx
5284 print sort backwards @harry;
5285 # prints xdogcatCainAbel
5286 print sort @george, 'to', @harry;
5287 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5289 # inefficiently sort by descending numeric compare using
5290 # the first integer after the first = sign, or the
5291 # whole record case-insensitively otherwise
5294 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5299 # same thing, but much more efficiently;
5300 # we'll build auxiliary indices instead
5304 push @nums, /=(\d+)/;
5309 $nums[$b] <=> $nums[$a]
5311 $caps[$a] cmp $caps[$b]
5315 # same thing, but without any temps
5316 @new = map { $_->[0] }
5317 sort { $b->[1] <=> $a->[1]
5320 } map { [$_, /=(\d+)/, uc($_)] } @old;
5322 # using a prototype allows you to use any comparison subroutine
5323 # as a sort subroutine (including other package's subroutines)
5325 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5328 @new = sort other::backwards @old;
5330 # guarantee stability, regardless of algorithm
5332 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5334 # force use of mergesort (not portable outside Perl 5.8)
5335 use sort '_mergesort'; # note discouraging _
5336 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5338 If you're using strict, you I<must not> declare $a
5339 and $b as lexicals. They are package globals. That means
5340 if you're in the C<main> package and type
5342 @articles = sort {$b <=> $a} @files;
5344 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5345 but if you're in the C<FooPack> package, it's the same as typing
5347 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5349 The comparison function is required to behave. If it returns
5350 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5351 sometimes saying the opposite, for example) the results are not
5354 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5355 (not-a-number), and because C<sort> will trigger a fatal error unless the
5356 result of a comparison is defined, when sorting with a comparison function
5357 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5358 The following example takes advantage of the fact that C<NaN != NaN> to
5359 eliminate any C<NaN>s from the input.
5361 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5363 =item splice ARRAY,OFFSET,LENGTH,LIST
5366 =item splice ARRAY,OFFSET,LENGTH
5368 =item splice ARRAY,OFFSET
5372 Removes the elements designated by OFFSET and LENGTH from an array, and
5373 replaces them with the elements of LIST, if any. In list context,
5374 returns the elements removed from the array. In scalar context,
5375 returns the last element removed, or C<undef> if no elements are
5376 removed. The array grows or shrinks as necessary.
5377 If OFFSET is negative then it starts that far from the end of the array.
5378 If LENGTH is omitted, removes everything from OFFSET onward.
5379 If LENGTH is negative, removes the elements from OFFSET onward
5380 except for -LENGTH elements at the end of the array.
5381 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5382 past the end of the array, perl issues a warning, and splices at the
5385 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5387 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5388 pop(@a) splice(@a,-1)
5389 shift(@a) splice(@a,0,1)
5390 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5391 $a[$i] = $y splice(@a,$i,1,$y)
5393 Example, assuming array lengths are passed before arrays:
5395 sub aeq { # compare two list values
5396 my(@a) = splice(@_,0,shift);
5397 my(@b) = splice(@_,0,shift);
5398 return 0 unless @a == @b; # same len?
5400 return 0 if pop(@a) ne pop(@b);
5404 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5406 =item split /PATTERN/,EXPR,LIMIT
5409 =item split /PATTERN/,EXPR
5411 =item split /PATTERN/
5415 Splits the string EXPR into a list of strings and returns that list. By
5416 default, empty leading fields are preserved, and empty trailing ones are
5417 deleted. (If all fields are empty, they are considered to be trailing.)
5419 In scalar context, returns the number of fields found and splits into
5420 the C<@_> array. Use of split in scalar context is deprecated, however,
5421 because it clobbers your subroutine arguments.
5423 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5424 splits on whitespace (after skipping any leading whitespace). Anything
5425 matching PATTERN is taken to be a delimiter separating the fields. (Note
5426 that the delimiter may be longer than one character.)
5428 If LIMIT is specified and positive, it represents the maximum number
5429 of fields the EXPR will be split into, though the actual number of
5430 fields returned depends on the number of times PATTERN matches within
5431 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5432 stripped (which potential users of C<pop> would do well to remember).
5433 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5434 had been specified. Note that splitting an EXPR that evaluates to the
5435 empty string always returns the empty list, regardless of the LIMIT
5438 A pattern matching the null string (not to be confused with
5439 a null pattern C<//>, which is just one member of the set of patterns
5440 matching a null string) will split the value of EXPR into separate
5441 characters at each point it matches that way. For example:
5443 print join(':', split(/ */, 'hi there'));
5445 produces the output 'h:i:t:h:e:r:e'.
5447 As a special case for C<split>, using the empty pattern C<//> specifically
5448 matches only the null string, and is not be confused with the regular use
5449 of C<//> to mean "the last successful pattern match". So, for C<split>,
5452 print join(':', split(//, 'hi there'));
5454 produces the output 'h:i: :t:h:e:r:e'.
5456 Empty leading fields are produced when there are positive-width matches at
5457 the beginning of the string; a zero-width match at the beginning of
5458 the string does not produce an empty field. For example:
5460 print join(':', split(/(?=\w)/, 'hi there!'));
5462 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
5463 hand, are produced when there is a match at the end of the string (and
5464 when LIMIT is given and is not 0), regardless of the length of the match.
5467 print join(':', split(//, 'hi there!', -1));
5468 print join(':', split(/\W/, 'hi there!', -1));
5470 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
5471 both with an empty trailing field.
5473 The LIMIT parameter can be used to split a line partially
5475 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5477 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5478 a LIMIT one larger than the number of variables in the list, to avoid
5479 unnecessary work. For the list above LIMIT would have been 4 by
5480 default. In time critical applications it behooves you not to split
5481 into more fields than you really need.
5483 If the PATTERN contains parentheses, additional list elements are
5484 created from each matching substring in the delimiter.
5486 split(/([,-])/, "1-10,20", 3);
5488 produces the list value
5490 (1, '-', 10, ',', 20)
5492 If you had the entire header of a normal Unix email message in $header,
5493 you could split it up into fields and their values this way:
5495 $header =~ s/\n\s+/ /g; # fix continuation lines
5496 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5498 The pattern C</PATTERN/> may be replaced with an expression to specify
5499 patterns that vary at runtime. (To do runtime compilation only once,
5500 use C</$variable/o>.)
5502 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5503 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5504 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5505 will give you as many null initial fields as there are leading spaces.
5506 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5507 whitespace produces a null first field. A C<split> with no arguments
5508 really does a S<C<split(' ', $_)>> internally.
5510 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5515 open(PASSWD, '/etc/passwd');
5518 ($login, $passwd, $uid, $gid,
5519 $gcos, $home, $shell) = split(/:/);
5523 As with regular pattern matching, any capturing parentheses that are not
5524 matched in a C<split()> will be set to C<undef> when returned:
5526 @fields = split /(A)|B/, "1A2B3";
5527 # @fields is (1, 'A', 2, undef, 3)
5529 =item sprintf FORMAT, LIST
5532 Returns a string formatted by the usual C<printf> conventions of the C
5533 library function C<sprintf>. See below for more details
5534 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5535 the general principles.
5539 # Format number with up to 8 leading zeroes
5540 $result = sprintf("%08d", $number);
5542 # Round number to 3 digits after decimal point
5543 $rounded = sprintf("%.3f", $number);
5545 Perl does its own C<sprintf> formatting--it emulates the C
5546 function C<sprintf>, but it doesn't use it (except for floating-point
5547 numbers, and even then only the standard modifiers are allowed). As a
5548 result, any non-standard extensions in your local C<sprintf> are not
5549 available from Perl.
5551 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5552 pass it an array as your first argument. The array is given scalar context,
5553 and instead of using the 0th element of the array as the format, Perl will
5554 use the count of elements in the array as the format, which is almost never
5557 Perl's C<sprintf> permits the following universally-known conversions:
5560 %c a character with the given number
5562 %d a signed integer, in decimal
5563 %u an unsigned integer, in decimal
5564 %o an unsigned integer, in octal
5565 %x an unsigned integer, in hexadecimal
5566 %e a floating-point number, in scientific notation
5567 %f a floating-point number, in fixed decimal notation
5568 %g a floating-point number, in %e or %f notation
5570 In addition, Perl permits the following widely-supported conversions:
5572 %X like %x, but using upper-case letters
5573 %E like %e, but using an upper-case "E"
5574 %G like %g, but with an upper-case "E" (if applicable)
5575 %b an unsigned integer, in binary
5576 %B like %b, but using an upper-case "B" with the # flag
5577 %p a pointer (outputs the Perl value's address in hexadecimal)
5578 %n special: *stores* the number of characters output so far
5579 into the next variable in the parameter list
5581 Finally, for backward (and we do mean "backward") compatibility, Perl
5582 permits these unnecessary but widely-supported conversions:
5585 %D a synonym for %ld
5586 %U a synonym for %lu
5587 %O a synonym for %lo
5590 Note that the number of exponent digits in the scientific notation produced
5591 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5592 exponent less than 100 is system-dependent: it may be three or less
5593 (zero-padded as necessary). In other words, 1.23 times ten to the
5594 99th may be either "1.23e99" or "1.23e099".
5596 Between the C<%> and the format letter, you may specify a number of
5597 additional attributes controlling the interpretation of the format.
5598 In order, these are:
5602 =item format parameter index
5604 An explicit format parameter index, such as C<2$>. By default sprintf
5605 will format the next unused argument in the list, but this allows you
5606 to take the arguments out of order, e.g.:
5608 printf '%2$d %1$d', 12, 34; # prints "34 12"
5609 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5615 space prefix non-negative number with a space
5616 + prefix non-negative number with a plus sign
5617 - left-justify within the field
5618 0 use zeros, not spaces, to right-justify
5619 # ensure the leading "0" for any octal,
5620 prefix non-zero hexadecimal with "0x" or "0X",
5621 prefix non-zero binary with "0b" or "0B"
5625 printf '<% d>', 12; # prints "< 12>"
5626 printf '<%+d>', 12; # prints "<+12>"
5627 printf '<%6s>', 12; # prints "< 12>"
5628 printf '<%-6s>', 12; # prints "<12 >"
5629 printf '<%06s>', 12; # prints "<000012>"
5630 printf '<%#o>', 12; # prints "<014>"
5631 printf '<%#x>', 12; # prints "<0xc>"
5632 printf '<%#X>', 12; # prints "<0XC>"
5633 printf '<%#b>', 12; # prints "<0b1100>"
5634 printf '<%#B>', 12; # prints "<0B1100>"
5636 When a space and a plus sign are given as the flags at once,
5637 a plus sign is used to prefix a positive number.
5639 printf '<%+ d>', 12; # prints "<+12>"
5640 printf '<% +d>', 12; # prints "<+12>"
5642 When the # flag and a precision are given in the %o conversion,
5643 the precision is incremented if it's necessary for the leading "0".
5645 printf '<%#.5o>', 012; # prints "<00012>"
5646 printf '<%#.5o>', 012345; # prints "<012345>"
5647 printf '<%#.0o>', 0; # prints "<0>"
5651 This flag tells perl to interpret the supplied string as a vector of
5652 integers, one for each character in the string. Perl applies the format to
5653 each integer in turn, then joins the resulting strings with a separator (a
5654 dot C<.> by default). This can be useful for displaying ordinal values of
5655 characters in arbitrary strings:
5657 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5658 printf "version is v%vd\n", $^V; # Perl's version
5660 Put an asterisk C<*> before the C<v> to override the string to
5661 use to separate the numbers:
5663 printf "address is %*vX\n", ":", $addr; # IPv6 address
5664 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5666 You can also explicitly specify the argument number to use for
5667 the join string using e.g. C<*2$v>:
5669 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5671 =item (minimum) width
5673 Arguments are usually formatted to be only as wide as required to
5674 display the given value. You can override the width by putting
5675 a number here, or get the width from the next argument (with C<*>)
5676 or from a specified argument (with e.g. C<*2$>):
5678 printf '<%s>', "a"; # prints "<a>"
5679 printf '<%6s>', "a"; # prints "< a>"
5680 printf '<%*s>', 6, "a"; # prints "< a>"
5681 printf '<%*2$s>', "a", 6; # prints "< a>"
5682 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5684 If a field width obtained through C<*> is negative, it has the same
5685 effect as the C<-> flag: left-justification.
5687 =item precision, or maximum width
5690 You can specify a precision (for numeric conversions) or a maximum
5691 width (for string conversions) by specifying a C<.> followed by a number.
5692 For floating point formats, with the exception of 'g' and 'G', this specifies
5693 the number of decimal places to show (the default being 6), e.g.:
5695 # these examples are subject to system-specific variation
5696 printf '<%f>', 1; # prints "<1.000000>"
5697 printf '<%.1f>', 1; # prints "<1.0>"
5698 printf '<%.0f>', 1; # prints "<1>"
5699 printf '<%e>', 10; # prints "<1.000000e+01>"
5700 printf '<%.1e>', 10; # prints "<1.0e+01>"
5702 For 'g' and 'G', this specifies the maximum number of digits to show,
5703 including prior to the decimal point as well as after it, e.g.:
5705 # these examples are subject to system-specific variation
5706 printf '<%g>', 1; # prints "<1>"
5707 printf '<%.10g>', 1; # prints "<1>"
5708 printf '<%g>', 100; # prints "<100>"
5709 printf '<%.1g>', 100; # prints "<1e+02>"
5710 printf '<%.2g>', 100.01; # prints "<1e+02>"
5711 printf '<%.5g>', 100.01; # prints "<100.01>"
5712 printf '<%.4g>', 100.01; # prints "<100>"
5714 For integer conversions, specifying a precision implies that the
5715 output of the number itself should be zero-padded to this width,
5716 where the 0 flag is ignored:
5718 printf '<%.6d>', 1; # prints "<000001>"
5719 printf '<%+.6d>', 1; # prints "<+000001>"
5720 printf '<%-10.6d>', 1; # prints "<000001 >"
5721 printf '<%10.6d>', 1; # prints "< 000001>"
5722 printf '<%010.6d>', 1; # prints "< 000001>"
5723 printf '<%+10.6d>', 1; # prints "< +000001>"
5725 printf '<%.6x>', 1; # prints "<000001>"
5726 printf '<%#.6x>', 1; # prints "<0x000001>"
5727 printf '<%-10.6x>', 1; # prints "<000001 >"
5728 printf '<%10.6x>', 1; # prints "< 000001>"
5729 printf '<%010.6x>', 1; # prints "< 000001>"
5730 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5732 For string conversions, specifying a precision truncates the string
5733 to fit in the specified width:
5735 printf '<%.5s>', "truncated"; # prints "<trunc>"
5736 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5738 You can also get the precision from the next argument using C<.*>:
5740 printf '<%.6x>', 1; # prints "<000001>"
5741 printf '<%.*x>', 6, 1; # prints "<000001>"
5743 If a precision obtained through C<*> is negative, it has the same
5744 effect as no precision.
5746 printf '<%.*s>', 7, "string"; # prints "<string>"
5747 printf '<%.*s>', 3, "string"; # prints "<str>"
5748 printf '<%.*s>', 0, "string"; # prints "<>"
5749 printf '<%.*s>', -1, "string"; # prints "<string>"
5751 printf '<%.*d>', 1, 0; # prints "<0>"
5752 printf '<%.*d>', 0, 0; # prints "<>"
5753 printf '<%.*d>', -1, 0; # prints "<0>"
5755 You cannot currently get the precision from a specified number,
5756 but it is intended that this will be possible in the future using
5759 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5763 For numeric conversions, you can specify the size to interpret the
5764 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5765 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5766 whatever the default integer size is on your platform (usually 32 or 64
5767 bits), but you can override this to use instead one of the standard C types,
5768 as supported by the compiler used to build Perl:
5770 l interpret integer as C type "long" or "unsigned long"
5771 h interpret integer as C type "short" or "unsigned short"
5772 q, L or ll interpret integer as C type "long long", "unsigned long long".
5773 or "quads" (typically 64-bit integers)
5775 The last will produce errors if Perl does not understand "quads" in your
5776 installation. (This requires that either the platform natively supports quads
5777 or Perl was specifically compiled to support quads.) You can find out
5778 whether your Perl supports quads via L<Config>:
5781 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5784 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5785 to be the default floating point size on your platform (double or long double),
5786 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5787 platform supports them. You can find out whether your Perl supports long
5788 doubles via L<Config>:
5791 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5793 You can find out whether Perl considers 'long double' to be the default
5794 floating point size to use on your platform via L<Config>:
5797 ($Config{uselongdouble} eq 'define') &&
5798 print "long doubles by default\n";
5800 It can also be the case that long doubles and doubles are the same thing:
5803 ($Config{doublesize} == $Config{longdblsize}) &&
5804 print "doubles are long doubles\n";
5806 The size specifier C<V> has no effect for Perl code, but it is supported
5807 for compatibility with XS code; it means 'use the standard size for
5808 a Perl integer (or floating-point number)', which is already the
5809 default for Perl code.
5811 =item order of arguments
5813 Normally, sprintf takes the next unused argument as the value to
5814 format for each format specification. If the format specification
5815 uses C<*> to require additional arguments, these are consumed from
5816 the argument list in the order in which they appear in the format
5817 specification I<before> the value to format. Where an argument is
5818 specified using an explicit index, this does not affect the normal
5819 order for the arguments (even when the explicitly specified index
5820 would have been the next argument in any case).
5824 printf '<%*.*s>', $a, $b, $c;
5826 would use C<$a> for the width, C<$b> for the precision and C<$c>
5827 as the value to format, while:
5829 printf '<%*1$.*s>', $a, $b;
5831 would use C<$a> for the width and the precision, and C<$b> as the
5834 Here are some more examples - beware that when using an explicit
5835 index, the C<$> may need to be escaped:
5837 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5838 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5839 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5840 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5844 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5845 the character used for the decimal separator in formatted floating
5846 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5850 X<sqrt> X<root> X<square root>
5854 Return the square root of EXPR. If EXPR is omitted, returns square
5855 root of C<$_>. Only works on non-negative operands, unless you've
5856 loaded the standard Math::Complex module.
5859 print sqrt(-2); # prints 1.4142135623731i
5862 X<srand> X<seed> X<randseed>
5866 Sets the random number seed for the C<rand> operator.
5868 The point of the function is to "seed" the C<rand> function so that
5869 C<rand> can produce a different sequence each time you run your
5872 If srand() is not called explicitly, it is called implicitly at the
5873 first use of the C<rand> operator. However, this was not the case in
5874 versions of Perl before 5.004, so if your script will run under older
5875 Perl versions, it should call C<srand>.
5877 Most programs won't even call srand() at all, except those that
5878 need a cryptographically-strong starting point rather than the
5879 generally acceptable default, which is based on time of day,
5880 process ID, and memory allocation, or the F</dev/urandom> device,
5883 You can call srand($seed) with the same $seed to reproduce the
5884 I<same> sequence from rand(), but this is usually reserved for
5885 generating predictable results for testing or debugging.
5886 Otherwise, don't call srand() more than once in your program.
5888 Do B<not> call srand() (i.e. without an argument) more than once in
5889 a script. The internal state of the random number generator should
5890 contain more entropy than can be provided by any seed, so calling
5891 srand() again actually I<loses> randomness.
5893 Most implementations of C<srand> take an integer and will silently
5894 truncate decimal numbers. This means C<srand(42)> will usually
5895 produce the same results as C<srand(42.1)>. To be safe, always pass
5896 C<srand> an integer.
5898 In versions of Perl prior to 5.004 the default seed was just the
5899 current C<time>. This isn't a particularly good seed, so many old
5900 programs supply their own seed value (often C<time ^ $$> or C<time ^
5901 ($$ + ($$ << 15))>), but that isn't necessary any more.
5903 For cryptographic purposes, however, you need something much more random
5904 than the default seed. Checksumming the compressed output of one or more
5905 rapidly changing operating system status programs is the usual method. For
5908 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
5910 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5913 Frequently called programs (like CGI scripts) that simply use
5917 for a seed can fall prey to the mathematical property that
5921 one-third of the time. So don't do that.
5923 =item stat FILEHANDLE
5924 X<stat> X<file, status> X<ctime>
5928 =item stat DIRHANDLE
5932 Returns a 13-element list giving the status info for a file, either
5933 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5934 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5937 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5938 $atime,$mtime,$ctime,$blksize,$blocks)
5941 Not all fields are supported on all filesystem types. Here are the
5942 meanings of the fields:
5944 0 dev device number of filesystem
5946 2 mode file mode (type and permissions)
5947 3 nlink number of (hard) links to the file
5948 4 uid numeric user ID of file's owner
5949 5 gid numeric group ID of file's owner
5950 6 rdev the device identifier (special files only)
5951 7 size total size of file, in bytes
5952 8 atime last access time in seconds since the epoch
5953 9 mtime last modify time in seconds since the epoch
5954 10 ctime inode change time in seconds since the epoch (*)
5955 11 blksize preferred block size for file system I/O
5956 12 blocks actual number of blocks allocated
5958 (The epoch was at 00:00 January 1, 1970 GMT.)
5960 (*) Not all fields are supported on all filesystem types. Notably, the
5961 ctime field is non-portable. In particular, you cannot expect it to be a
5962 "creation time", see L<perlport/"Files and Filesystems"> for details.
5964 If C<stat> is passed the special filehandle consisting of an underline, no
5965 stat is done, but the current contents of the stat structure from the
5966 last C<stat>, C<lstat>, or filetest are returned. Example:
5968 if (-x $file && (($d) = stat(_)) && $d < 0) {
5969 print "$file is executable NFS file\n";
5972 (This works on machines only for which the device number is negative
5975 Because the mode contains both the file type and its permissions, you
5976 should mask off the file type portion and (s)printf using a C<"%o">
5977 if you want to see the real permissions.
5979 $mode = (stat($filename))[2];
5980 printf "Permissions are %04o\n", $mode & 07777;
5982 In scalar context, C<stat> returns a boolean value indicating success
5983 or failure, and, if successful, sets the information associated with
5984 the special filehandle C<_>.
5986 The L<File::stat> module provides a convenient, by-name access mechanism:
5989 $sb = stat($filename);
5990 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5991 $filename, $sb->size, $sb->mode & 07777,
5992 scalar localtime $sb->mtime;
5994 You can import symbolic mode constants (C<S_IF*>) and functions
5995 (C<S_IS*>) from the Fcntl module:
5999 $mode = (stat($filename))[2];
6001 $user_rwx = ($mode & S_IRWXU) >> 6;
6002 $group_read = ($mode & S_IRGRP) >> 3;
6003 $other_execute = $mode & S_IXOTH;
6005 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6007 $is_setuid = $mode & S_ISUID;
6008 $is_directory = S_ISDIR($mode);
6010 You could write the last two using the C<-u> and C<-d> operators.
6011 The commonly available C<S_IF*> constants are
6013 # Permissions: read, write, execute, for user, group, others.
6015 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6016 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6017 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6019 # Setuid/Setgid/Stickiness/SaveText.
6020 # Note that the exact meaning of these is system dependent.
6022 S_ISUID S_ISGID S_ISVTX S_ISTXT
6024 # File types. Not necessarily all are available on your system.
6026 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6028 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
6030 S_IREAD S_IWRITE S_IEXEC
6032 and the C<S_IF*> functions are
6034 S_IMODE($mode) the part of $mode containing the permission bits
6035 and the setuid/setgid/sticky bits
6037 S_IFMT($mode) the part of $mode containing the file type
6038 which can be bit-anded with e.g. S_IFREG
6039 or with the following functions
6041 # The operators -f, -d, -l, -b, -c, -p, and -S.
6043 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6044 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6046 # No direct -X operator counterpart, but for the first one
6047 # the -g operator is often equivalent. The ENFMT stands for
6048 # record flocking enforcement, a platform-dependent feature.
6050 S_ISENFMT($mode) S_ISWHT($mode)
6052 See your native chmod(2) and stat(2) documentation for more details
6053 about the C<S_*> constants. To get status info for a symbolic link
6054 instead of the target file behind the link, use the C<lstat> function.
6059 =item state TYPE EXPR
6061 =item state EXPR : ATTRS
6063 =item state TYPE EXPR : ATTRS
6065 C<state> declares a lexically scoped variable, just like C<my> does.
6066 However, those variables will never be reinitialized, contrary to
6067 lexical variables that are reinitialized each time their enclosing block
6070 C<state> variables are only enabled when the C<feature 'state'> pragma is
6071 in effect. See L<feature>.
6078 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6079 doing many pattern matches on the string before it is next modified.
6080 This may or may not save time, depending on the nature and number of
6081 patterns you are searching on, and on the distribution of character
6082 frequencies in the string to be searched--you probably want to compare
6083 run times with and without it to see which runs faster. Those loops
6084 that scan for many short constant strings (including the constant
6085 parts of more complex patterns) will benefit most. You may have only
6086 one C<study> active at a time--if you study a different scalar the first
6087 is "unstudied". (The way C<study> works is this: a linked list of every
6088 character in the string to be searched is made, so we know, for
6089 example, where all the C<'k'> characters are. From each search string,
6090 the rarest character is selected, based on some static frequency tables
6091 constructed from some C programs and English text. Only those places
6092 that contain this "rarest" character are examined.)
6094 For example, here is a loop that inserts index producing entries
6095 before any line containing a certain pattern:
6099 print ".IX foo\n" if /\bfoo\b/;
6100 print ".IX bar\n" if /\bbar\b/;
6101 print ".IX blurfl\n" if /\bblurfl\b/;
6106 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6107 will be looked at, because C<f> is rarer than C<o>. In general, this is
6108 a big win except in pathological cases. The only question is whether
6109 it saves you more time than it took to build the linked list in the
6112 Note that if you have to look for strings that you don't know till
6113 runtime, you can build an entire loop as a string and C<eval> that to
6114 avoid recompiling all your patterns all the time. Together with
6115 undefining C<$/> to input entire files as one record, this can be very
6116 fast, often faster than specialized programs like fgrep(1). The following
6117 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6118 out the names of those files that contain a match:
6120 $search = 'while (<>) { study;';
6121 foreach $word (@words) {
6122 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6127 eval $search; # this screams
6128 $/ = "\n"; # put back to normal input delimiter
6129 foreach $file (sort keys(%seen)) {
6133 =item sub NAME BLOCK
6136 =item sub NAME (PROTO) BLOCK
6138 =item sub NAME : ATTRS BLOCK
6140 =item sub NAME (PROTO) : ATTRS BLOCK
6142 This is subroutine definition, not a real function I<per se>.
6143 Without a BLOCK it's just a forward declaration. Without a NAME,
6144 it's an anonymous function declaration, and does actually return
6145 a value: the CODE ref of the closure you just created.
6147 See L<perlsub> and L<perlref> for details about subroutines and
6148 references, and L<attributes> and L<Attribute::Handlers> for more
6149 information about attributes.
6151 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6152 X<substr> X<substring> X<mid> X<left> X<right>
6154 =item substr EXPR,OFFSET,LENGTH
6156 =item substr EXPR,OFFSET
6158 Extracts a substring out of EXPR and returns it. First character is at
6159 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6160 If OFFSET is negative (or more precisely, less than C<$[>), starts
6161 that far from the end of the string. If LENGTH is omitted, returns
6162 everything to the end of the string. If LENGTH is negative, leaves that
6163 many characters off the end of the string.
6165 my $s = "The black cat climbed the green tree";
6166 my $color = substr $s, 4, 5; # black
6167 my $middle = substr $s, 4, -11; # black cat climbed the
6168 my $end = substr $s, 14; # climbed the green tree
6169 my $tail = substr $s, -4; # tree
6170 my $z = substr $s, -4, 2; # tr
6172 You can use the substr() function as an lvalue, in which case EXPR
6173 must itself be an lvalue. If you assign something shorter than LENGTH,
6174 the string will shrink, and if you assign something longer than LENGTH,
6175 the string will grow to accommodate it. To keep the string the same
6176 length you may need to pad or chop your value using C<sprintf>.
6178 If OFFSET and LENGTH specify a substring that is partly outside the
6179 string, only the part within the string is returned. If the substring
6180 is beyond either end of the string, substr() returns the undefined
6181 value and produces a warning. When used as an lvalue, specifying a
6182 substring that is entirely outside the string is a fatal error.
6183 Here's an example showing the behavior for boundary cases:
6186 substr($name, 4) = 'dy'; # $name is now 'freddy'
6187 my $null = substr $name, 6, 2; # returns '' (no warning)
6188 my $oops = substr $name, 7; # returns undef, with warning
6189 substr($name, 7) = 'gap'; # fatal error
6191 An alternative to using substr() as an lvalue is to specify the
6192 replacement string as the 4th argument. This allows you to replace
6193 parts of the EXPR and return what was there before in one operation,
6194 just as you can with splice().
6196 my $s = "The black cat climbed the green tree";
6197 my $z = substr $s, 14, 7, "jumped from"; # climbed
6198 # $s is now "The black cat jumped from the green tree"
6200 Note that the lvalue returned by the 3-arg version of substr() acts as
6201 a 'magic bullet'; each time it is assigned to, it remembers which part
6202 of the original string is being modified; for example:
6205 for (substr($x,1,2)) {
6206 $_ = 'a'; print $x,"\n"; # prints 1a4
6207 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6209 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6212 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6215 =item symlink OLDFILE,NEWFILE
6216 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6218 Creates a new filename symbolically linked to the old filename.
6219 Returns C<1> for success, C<0> otherwise. On systems that don't support
6220 symbolic links, produces a fatal error at run time. To check for that,
6223 $symlink_exists = eval { symlink("",""); 1 };
6225 =item syscall NUMBER, LIST
6226 X<syscall> X<system call>
6228 Calls the system call specified as the first element of the list,
6229 passing the remaining elements as arguments to the system call. If
6230 unimplemented, produces a fatal error. The arguments are interpreted
6231 as follows: if a given argument is numeric, the argument is passed as
6232 an int. If not, the pointer to the string value is passed. You are
6233 responsible to make sure a string is pre-extended long enough to
6234 receive any result that might be written into a string. You can't use a
6235 string literal (or other read-only string) as an argument to C<syscall>
6236 because Perl has to assume that any string pointer might be written
6238 integer arguments are not literals and have never been interpreted in a
6239 numeric context, you may need to add C<0> to them to force them to look
6240 like numbers. This emulates the C<syswrite> function (or vice versa):
6242 require 'syscall.ph'; # may need to run h2ph
6244 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6246 Note that Perl supports passing of up to only 14 arguments to your system call,
6247 which in practice should usually suffice.
6249 Syscall returns whatever value returned by the system call it calls.
6250 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6251 Note that some system calls can legitimately return C<-1>. The proper
6252 way to handle such calls is to assign C<$!=0;> before the call and
6253 check the value of C<$!> if syscall returns C<-1>.
6255 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6256 number of the read end of the pipe it creates. There is no way
6257 to retrieve the file number of the other end. You can avoid this
6258 problem by using C<pipe> instead.
6260 =item sysopen FILEHANDLE,FILENAME,MODE
6263 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6265 Opens the file whose filename is given by FILENAME, and associates it
6266 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6267 the name of the real filehandle wanted. This function calls the
6268 underlying operating system's C<open> function with the parameters
6269 FILENAME, MODE, PERMS.
6271 The possible values and flag bits of the MODE parameter are
6272 system-dependent; they are available via the standard module C<Fcntl>.
6273 See the documentation of your operating system's C<open> to see which
6274 values and flag bits are available. You may combine several flags
6275 using the C<|>-operator.
6277 Some of the most common values are C<O_RDONLY> for opening the file in
6278 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6279 and C<O_RDWR> for opening the file in read-write mode.
6280 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6282 For historical reasons, some values work on almost every system
6283 supported by perl: zero means read-only, one means write-only, and two
6284 means read/write. We know that these values do I<not> work under
6285 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6286 use them in new code.
6288 If the file named by FILENAME does not exist and the C<open> call creates
6289 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6290 PERMS specifies the permissions of the newly created file. If you omit
6291 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6292 These permission values need to be in octal, and are modified by your
6293 process's current C<umask>.
6296 In many systems the C<O_EXCL> flag is available for opening files in
6297 exclusive mode. This is B<not> locking: exclusiveness means here that
6298 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6299 on network filesystems, and has no effect unless the C<O_CREAT> flag
6300 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6301 being opened if it is a symbolic link. It does not protect against
6302 symbolic links in the file's path.
6305 Sometimes you may want to truncate an already-existing file. This
6306 can be done using the C<O_TRUNC> flag. The behavior of
6307 C<O_TRUNC> with C<O_RDONLY> is undefined.
6310 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6311 that takes away the user's option to have a more permissive umask.
6312 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6315 Note that C<sysopen> depends on the fdopen() C library function.
6316 On many UNIX systems, fdopen() is known to fail when file descriptors
6317 exceed a certain value, typically 255. If you need more file
6318 descriptors than that, consider rebuilding Perl to use the C<sfio>
6319 library, or perhaps using the POSIX::open() function.
6321 See L<perlopentut> for a kinder, gentler explanation of opening files.
6323 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6326 =item sysread FILEHANDLE,SCALAR,LENGTH
6328 Attempts to read LENGTH bytes of data into variable SCALAR from the
6329 specified FILEHANDLE, using the system call read(2). It bypasses
6330 buffered IO, so mixing this with other kinds of reads, C<print>,
6331 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6332 perlio or stdio layers usually buffers data. Returns the number of
6333 bytes actually read, C<0> at end of file, or undef if there was an
6334 error (in the latter case C<$!> is also set). SCALAR will be grown or
6335 shrunk so that the last byte actually read is the last byte of the
6336 scalar after the read.
6338 An OFFSET may be specified to place the read data at some place in the
6339 string other than the beginning. A negative OFFSET specifies
6340 placement at that many characters counting backwards from the end of
6341 the string. A positive OFFSET greater than the length of SCALAR
6342 results in the string being padded to the required size with C<"\0">
6343 bytes before the result of the read is appended.
6345 There is no syseof() function, which is ok, since eof() doesn't work
6346 very well on device files (like ttys) anyway. Use sysread() and check
6347 for a return value for 0 to decide whether you're done.
6349 Note that if the filehandle has been marked as C<:utf8> Unicode
6350 characters are read instead of bytes (the LENGTH, OFFSET, and the
6351 return value of sysread() are in Unicode characters).
6352 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6353 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6355 =item sysseek FILEHANDLE,POSITION,WHENCE
6358 Sets FILEHANDLE's system position in bytes using the system call
6359 lseek(2). FILEHANDLE may be an expression whose value gives the name
6360 of the filehandle. The values for WHENCE are C<0> to set the new
6361 position to POSITION, C<1> to set the it to the current position plus
6362 POSITION, and C<2> to set it to EOF plus POSITION (typically
6365 Note the I<in bytes>: even if the filehandle has been set to operate
6366 on characters (for example by using the C<:encoding(utf8)> I/O layer),
6367 tell() will return byte offsets, not character offsets (because
6368 implementing that would render sysseek() very slow).
6370 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6371 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6372 C<seek>, C<tell>, or C<eof> may cause confusion.
6374 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6375 and C<SEEK_END> (start of the file, current position, end of the file)
6376 from the Fcntl module. Use of the constants is also more portable
6377 than relying on 0, 1, and 2. For example to define a "systell" function:
6379 use Fcntl 'SEEK_CUR';
6380 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6382 Returns the new position, or the undefined value on failure. A position
6383 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6384 true on success and false on failure, yet you can still easily determine
6390 =item system PROGRAM LIST
6392 Does exactly the same thing as C<exec LIST>, except that a fork is
6393 done first, and the parent process waits for the child process to
6394 complete. Note that argument processing varies depending on the
6395 number of arguments. If there is more than one argument in LIST,
6396 or if LIST is an array with more than one value, starts the program
6397 given by the first element of the list with arguments given by the
6398 rest of the list. If there is only one scalar argument, the argument
6399 is checked for shell metacharacters, and if there are any, the
6400 entire argument is passed to the system's command shell for parsing
6401 (this is C</bin/sh -c> on Unix platforms, but varies on other
6402 platforms). If there are no shell metacharacters in the argument,
6403 it is split into words and passed directly to C<execvp>, which is
6406 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6407 output before any operation that may do a fork, but this may not be
6408 supported on some platforms (see L<perlport>). To be safe, you may need
6409 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6410 of C<IO::Handle> on any open handles.
6412 The return value is the exit status of the program as returned by the
6413 C<wait> call. To get the actual exit value, shift right by eight (see
6414 below). See also L</exec>. This is I<not> what you want to use to capture
6415 the output from a command, for that you should use merely backticks or
6416 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6417 indicates a failure to start the program or an error of the wait(2) system
6418 call (inspect $! for the reason).
6420 Like C<exec>, C<system> allows you to lie to a program about its name if
6421 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6423 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6424 C<system>, if you expect your program to terminate on receipt of these
6425 signals you will need to arrange to do so yourself based on the return
6428 @args = ("command", "arg1", "arg2");
6430 or die "system @args failed: $?"
6432 You can check all the failure possibilities by inspecting
6436 print "failed to execute: $!\n";
6439 printf "child died with signal %d, %s coredump\n",
6440 ($? & 127), ($? & 128) ? 'with' : 'without';
6443 printf "child exited with value %d\n", $? >> 8;
6446 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6447 with the W*() calls of the POSIX extension.
6449 When the arguments get executed via the system shell, results
6450 and return codes will be subject to its quirks and capabilities.
6451 See L<perlop/"`STRING`"> and L</exec> for details.
6453 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6456 =item syswrite FILEHANDLE,SCALAR,LENGTH
6458 =item syswrite FILEHANDLE,SCALAR
6460 Attempts to write LENGTH bytes of data from variable SCALAR to the
6461 specified FILEHANDLE, using the system call write(2). If LENGTH is
6462 not specified, writes whole SCALAR. It bypasses buffered IO, so
6463 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6464 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6465 stdio layers usually buffers data. Returns the number of bytes
6466 actually written, or C<undef> if there was an error (in this case the
6467 errno variable C<$!> is also set). If the LENGTH is greater than the
6468 available data in the SCALAR after the OFFSET, only as much data as is
6469 available will be written.
6471 An OFFSET may be specified to write the data from some part of the
6472 string other than the beginning. A negative OFFSET specifies writing
6473 that many characters counting backwards from the end of the string.
6474 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6476 Note that if the filehandle has been marked as C<:utf8>, Unicode
6477 characters are written instead of bytes (the LENGTH, OFFSET, and the
6478 return value of syswrite() are in UTF-8 encoded Unicode characters).
6479 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6480 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6482 =item tell FILEHANDLE
6487 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6488 error. FILEHANDLE may be an expression whose value gives the name of
6489 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6492 Note the I<in bytes>: even if the filehandle has been set to
6493 operate on characters (for example by using the C<:encoding(utf8)> open
6494 layer), tell() will return byte offsets, not character offsets (because
6495 that would render seek() and tell() rather slow).
6497 The return value of tell() for the standard streams like the STDIN
6498 depends on the operating system: it may return -1 or something else.
6499 tell() on pipes, fifos, and sockets usually returns -1.
6501 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6503 Do not use tell() (or other buffered I/O operations) on a file handle
6504 that has been manipulated by sysread(), syswrite() or sysseek().
6505 Those functions ignore the buffering, while tell() does not.
6507 =item telldir DIRHANDLE
6510 Returns the current position of the C<readdir> routines on DIRHANDLE.
6511 Value may be given to C<seekdir> to access a particular location in a
6512 directory. C<telldir> has the same caveats about possible directory
6513 compaction as the corresponding system library routine.
6515 =item tie VARIABLE,CLASSNAME,LIST
6518 This function binds a variable to a package class that will provide the
6519 implementation for the variable. VARIABLE is the name of the variable
6520 to be enchanted. CLASSNAME is the name of a class implementing objects
6521 of correct type. Any additional arguments are passed to the C<new>
6522 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6523 or C<TIEHASH>). Typically these are arguments such as might be passed
6524 to the C<dbm_open()> function of C. The object returned by the C<new>
6525 method is also returned by the C<tie> function, which would be useful
6526 if you want to access other methods in CLASSNAME.
6528 Note that functions such as C<keys> and C<values> may return huge lists
6529 when used on large objects, like DBM files. You may prefer to use the
6530 C<each> function to iterate over such. Example:
6532 # print out history file offsets
6534 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6535 while (($key,$val) = each %HIST) {
6536 print $key, ' = ', unpack('L',$val), "\n";
6540 A class implementing a hash should have the following methods:
6542 TIEHASH classname, LIST
6544 STORE this, key, value
6549 NEXTKEY this, lastkey
6554 A class implementing an ordinary array should have the following methods:
6556 TIEARRAY classname, LIST
6558 STORE this, key, value
6560 STORESIZE this, count
6566 SPLICE this, offset, length, LIST
6571 A class implementing a file handle should have the following methods:
6573 TIEHANDLE classname, LIST
6574 READ this, scalar, length, offset
6577 WRITE this, scalar, length, offset
6579 PRINTF this, format, LIST
6583 SEEK this, position, whence
6585 OPEN this, mode, LIST
6590 A class implementing a scalar should have the following methods:
6592 TIESCALAR classname, LIST
6598 Not all methods indicated above need be implemented. See L<perltie>,
6599 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6601 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6602 for you--you need to do that explicitly yourself. See L<DB_File>
6603 or the F<Config> module for interesting C<tie> implementations.
6605 For further details see L<perltie>, L<"tied VARIABLE">.
6610 Returns a reference to the object underlying VARIABLE (the same value
6611 that was originally returned by the C<tie> call that bound the variable
6612 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6618 Returns the number of non-leap seconds since whatever time the system
6619 considers to be the epoch, suitable for feeding to C<gmtime> and
6620 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6621 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6622 1904 in the current local time zone for its epoch.
6624 For measuring time in better granularity than one second,
6625 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6626 Perl 5.8 part of the standard distribution), or if you have
6627 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6628 See L<perlfaq8> for details.
6630 For date and time processing look at the many related modules on CPAN.
6631 For a comprehensive date and time representation look at the
6637 Returns a four-element list giving the user and system times, in
6638 seconds, for this process and the children of this process.
6640 ($user,$system,$cuser,$csystem) = times;
6642 In scalar context, C<times> returns C<$user>.
6644 Note that times for children are included only after they terminate.
6648 The transliteration operator. Same as C<y///>. See L<perlop>.
6650 =item truncate FILEHANDLE,LENGTH
6653 =item truncate EXPR,LENGTH
6655 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6656 specified length. Produces a fatal error if truncate isn't implemented
6657 on your system. Returns true if successful, the undefined value
6660 The behavior is undefined if LENGTH is greater than the length of the
6663 The position in the file of FILEHANDLE is left unchanged. You may want to
6664 call L<seek> before writing to the file.
6667 X<uc> X<uppercase> X<toupper>
6671 Returns an uppercased version of EXPR. This is the internal function
6672 implementing the C<\U> escape in double-quoted strings. Respects
6673 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6674 and L<perlunicode> for more details about locale and Unicode support.
6675 It does not attempt to do titlecase mapping on initial letters. See
6676 C<ucfirst> for that.
6678 If EXPR is omitted, uses C<$_>.
6681 X<ucfirst> X<uppercase>
6685 Returns the value of EXPR with the first character in uppercase
6686 (titlecase in Unicode). This is the internal function implementing
6687 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6688 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6689 for more details about locale and Unicode support.
6691 If EXPR is omitted, uses C<$_>.
6698 Sets the umask for the process to EXPR and returns the previous value.
6699 If EXPR is omitted, merely returns the current umask.
6701 The Unix permission C<rwxr-x---> is represented as three sets of three
6702 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6703 and isn't one of the digits). The C<umask> value is such a number
6704 representing disabled permissions bits. The permission (or "mode")
6705 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6706 even if you tell C<sysopen> to create a file with permissions C<0777>,
6707 if your umask is C<0022> then the file will actually be created with
6708 permissions C<0755>. If your C<umask> were C<0027> (group can't
6709 write; others can't read, write, or execute), then passing
6710 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6713 Here's some advice: supply a creation mode of C<0666> for regular
6714 files (in C<sysopen>) and one of C<0777> for directories (in
6715 C<mkdir>) and executable files. This gives users the freedom of
6716 choice: if they want protected files, they might choose process umasks
6717 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6718 Programs should rarely if ever make policy decisions better left to
6719 the user. The exception to this is when writing files that should be
6720 kept private: mail files, web browser cookies, I<.rhosts> files, and
6723 If umask(2) is not implemented on your system and you are trying to
6724 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6725 fatal error at run time. If umask(2) is not implemented and you are
6726 not trying to restrict access for yourself, returns C<undef>.
6728 Remember that a umask is a number, usually given in octal; it is I<not> a
6729 string of octal digits. See also L</oct>, if all you have is a string.
6732 X<undef> X<undefine>
6736 Undefines the value of EXPR, which must be an lvalue. Use only on a
6737 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6738 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6739 will probably not do what you expect on most predefined variables or
6740 DBM list values, so don't do that; see L<delete>.) Always returns the
6741 undefined value. You can omit the EXPR, in which case nothing is
6742 undefined, but you still get an undefined value that you could, for
6743 instance, return from a subroutine, assign to a variable or pass as a
6744 parameter. Examples:
6747 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6751 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6752 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6753 select undef, undef, undef, 0.25;
6754 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6756 Note that this is a unary operator, not a list operator.
6759 X<unlink> X<delete> X<remove> X<rm> X<del>
6763 Deletes a list of files. Returns the number of files successfully
6766 $cnt = unlink 'a', 'b', 'c';
6770 Note: C<unlink> will not attempt to delete directories unless you are superuser
6771 and the B<-U> flag is supplied to Perl. Even if these conditions are
6772 met, be warned that unlinking a directory can inflict damage on your
6773 filesystem. Finally, using C<unlink> on directories is not supported on
6774 many operating systems. Use C<rmdir> instead.
6776 If LIST is omitted, uses C<$_>.
6778 =item unpack TEMPLATE,EXPR
6781 =item unpack TEMPLATE
6783 C<unpack> does the reverse of C<pack>: it takes a string
6784 and expands it out into a list of values.
6785 (In scalar context, it returns merely the first value produced.)
6787 If EXPR is omitted, unpacks the C<$_> string.
6789 The string is broken into chunks described by the TEMPLATE. Each chunk
6790 is converted separately to a value. Typically, either the string is a result
6791 of C<pack>, or the characters of the string represent a C structure of some
6794 The TEMPLATE has the same format as in the C<pack> function.
6795 Here's a subroutine that does substring:
6798 my($what,$where,$howmuch) = @_;
6799 unpack("x$where a$howmuch", $what);
6804 sub ordinal { unpack("W",$_[0]); } # same as ord()
6806 In addition to fields allowed in pack(), you may prefix a field with
6807 a %<number> to indicate that
6808 you want a <number>-bit checksum of the items instead of the items
6809 themselves. Default is a 16-bit checksum. Checksum is calculated by
6810 summing numeric values of expanded values (for string fields the sum of
6811 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6813 For example, the following
6814 computes the same number as the System V sum program:
6818 unpack("%32W*",<>) % 65535;
6821 The following efficiently counts the number of set bits in a bit vector:
6823 $setbits = unpack("%32b*", $selectmask);
6825 The C<p> and C<P> formats should be used with care. Since Perl
6826 has no way of checking whether the value passed to C<unpack()>
6827 corresponds to a valid memory location, passing a pointer value that's
6828 not known to be valid is likely to have disastrous consequences.
6830 If there are more pack codes or if the repeat count of a field or a group
6831 is larger than what the remainder of the input string allows, the result
6832 is not well defined: in some cases, the repeat count is decreased, or
6833 C<unpack()> will produce null strings or zeroes, or terminate with an
6834 error. If the input string is longer than one described by the TEMPLATE,
6835 the rest is ignored.
6837 See L</pack> for more examples and notes.
6839 =item untie VARIABLE
6842 Breaks the binding between a variable and a package. (See C<tie>.)
6843 Has no effect if the variable is not tied.
6845 =item unshift ARRAY,LIST
6848 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6849 depending on how you look at it. Prepends list to the front of the
6850 array, and returns the new number of elements in the array.
6852 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6854 Note the LIST is prepended whole, not one element at a time, so the
6855 prepended elements stay in the same order. Use C<reverse> to do the
6858 =item use Module VERSION LIST
6859 X<use> X<module> X<import>
6861 =item use Module VERSION
6863 =item use Module LIST
6869 Imports some semantics into the current package from the named module,
6870 generally by aliasing certain subroutine or variable names into your
6871 package. It is exactly equivalent to
6873 BEGIN { require Module; Module->import( LIST ); }
6875 except that Module I<must> be a bareword.
6877 In the peculiar C<use VERSION> form, VERSION may be either a numeric
6878 argument such as 5.006, which will be compared to C<$]>, or a literal of
6879 the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A
6880 fatal error is produced if VERSION is greater than the version of the
6881 current Perl interpreter; Perl will not attempt to parse the rest of the
6882 file. Compare with L</require>, which can do a similar check at run time.
6883 Symmetrically, C<no VERSION> allows you to specify that you want a version
6884 of perl older than the specified one.
6886 Specifying VERSION as a literal of the form v5.6.1 should generally be
6887 avoided, because it leads to misleading error messages under earlier
6888 versions of Perl (that is, prior to 5.6.0) that do not support this
6889 syntax. The equivalent numeric version should be used instead.
6891 use v5.6.1; # compile time version check
6893 use 5.006_001; # ditto; preferred for backwards compatibility
6895 This is often useful if you need to check the current Perl version before
6896 C<use>ing library modules that won't work with older versions of Perl.
6897 (We try not to do this more than we have to.)
6899 Also, if the specified perl version is greater than or equal to 5.9.5,
6900 C<use VERSION> will also load the C<feature> pragma and enable all
6901 features available in the requested version. See L<feature>.
6903 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6904 C<require> makes sure the module is loaded into memory if it hasn't been
6905 yet. The C<import> is not a builtin--it's just an ordinary static method
6906 call into the C<Module> package to tell the module to import the list of
6907 features back into the current package. The module can implement its
6908 C<import> method any way it likes, though most modules just choose to
6909 derive their C<import> method via inheritance from the C<Exporter> class that
6910 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6911 method can be found then the call is skipped, even if there is an AUTOLOAD
6914 If you do not want to call the package's C<import> method (for instance,
6915 to stop your namespace from being altered), explicitly supply the empty list:
6919 That is exactly equivalent to
6921 BEGIN { require Module }
6923 If the VERSION argument is present between Module and LIST, then the
6924 C<use> will call the VERSION method in class Module with the given
6925 version as an argument. The default VERSION method, inherited from
6926 the UNIVERSAL class, croaks if the given version is larger than the
6927 value of the variable C<$Module::VERSION>.
6929 Again, there is a distinction between omitting LIST (C<import> called
6930 with no arguments) and an explicit empty LIST C<()> (C<import> not
6931 called). Note that there is no comma after VERSION!
6933 Because this is a wide-open interface, pragmas (compiler directives)
6934 are also implemented this way. Currently implemented pragmas are:
6939 use sigtrap qw(SEGV BUS);
6940 use strict qw(subs vars refs);
6941 use subs qw(afunc blurfl);
6942 use warnings qw(all);
6943 use sort qw(stable _quicksort _mergesort);
6945 Some of these pseudo-modules import semantics into the current
6946 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6947 which import symbols into the current package (which are effective
6948 through the end of the file).
6950 There's a corresponding C<no> command that unimports meanings imported
6951 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6952 It behaves exactly as C<import> does with respect to VERSION, an
6953 omitted LIST, empty LIST, or no unimport method being found.
6959 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6960 for the C<-M> and C<-m> command-line options to perl that give C<use>
6961 functionality from the command-line.
6966 Changes the access and modification times on each file of a list of
6967 files. The first two elements of the list must be the NUMERICAL access
6968 and modification times, in that order. Returns the number of files
6969 successfully changed. The inode change time of each file is set
6970 to the current time. For example, this code has the same effect as the
6971 Unix touch(1) command when the files I<already exist> and belong to
6972 the user running the program:
6975 $atime = $mtime = time;
6976 utime $atime, $mtime, @ARGV;
6978 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6979 the utime(2) function in the C library will be called with a null second
6980 argument. On most systems, this will set the file's access and
6981 modification times to the current time (i.e. equivalent to the example
6982 above) and will even work on other users' files where you have write
6985 utime undef, undef, @ARGV;
6987 Under NFS this will use the time of the NFS server, not the time of
6988 the local machine. If there is a time synchronization problem, the
6989 NFS server and local machine will have different times. The Unix
6990 touch(1) command will in fact normally use this form instead of the
6991 one shown in the first example.
6993 Note that only passing one of the first two elements as C<undef> will
6994 be equivalent of passing it as 0 and will not have the same effect as
6995 described when they are both C<undef>. This case will also trigger an
6996 uninitialized warning.
6998 On systems that support futimes, you might pass file handles among the
6999 files. On systems that don't support futimes, passing file handles
7000 produces a fatal error at run time. The file handles must be passed
7001 as globs or references to be recognized. Barewords are considered
7009 Returns a list consisting of all the values of the named hash, or the values
7010 of an array. (In a scalar context, returns the number of values.)
7012 The values are returned in an apparently random order. The actual
7013 random order is subject to change in future versions of perl, but it
7014 is guaranteed to be the same order as either the C<keys> or C<each>
7015 function would produce on the same (unmodified) hash. Since Perl
7016 5.8.1 the ordering is different even between different runs of Perl
7017 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7019 As a side effect, calling values() resets the HASH or ARRAY's internal
7021 see L</each>. (In particular, calling values() in void context resets
7022 the iterator with no other overhead. Apart from resetting the iterator,
7023 C<values @array> in list context is no different to plain C<@array>.
7024 We recommend that you use void context C<keys @array> for this, but reasoned
7025 that it taking C<values @array> out would require more documentation than
7029 Note that the values are not copied, which means modifying them will
7030 modify the contents of the hash:
7032 for (values %hash) { s/foo/bar/g } # modifies %hash values
7033 for (@hash{keys %hash}) { s/foo/bar/g } # same
7035 See also C<keys>, C<each>, and C<sort>.
7037 =item vec EXPR,OFFSET,BITS
7038 X<vec> X<bit> X<bit vector>
7040 Treats the string in EXPR as a bit vector made up of elements of
7041 width BITS, and returns the value of the element specified by OFFSET
7042 as an unsigned integer. BITS therefore specifies the number of bits
7043 that are reserved for each element in the bit vector. This must
7044 be a power of two from 1 to 32 (or 64, if your platform supports
7047 If BITS is 8, "elements" coincide with bytes of the input string.
7049 If BITS is 16 or more, bytes of the input string are grouped into chunks
7050 of size BITS/8, and each group is converted to a number as with
7051 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7052 for BITS==64). See L<"pack"> for details.
7054 If bits is 4 or less, the string is broken into bytes, then the bits
7055 of each byte are broken into 8/BITS groups. Bits of a byte are
7056 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7057 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7058 breaking the single input byte C<chr(0x36)> into two groups gives a list
7059 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7061 C<vec> may also be assigned to, in which case parentheses are needed
7062 to give the expression the correct precedence as in
7064 vec($image, $max_x * $x + $y, 8) = 3;
7066 If the selected element is outside the string, the value 0 is returned.
7067 If an element off the end of the string is written to, Perl will first
7068 extend the string with sufficiently many zero bytes. It is an error
7069 to try to write off the beginning of the string (i.e. negative OFFSET).
7071 If the string happens to be encoded as UTF-8 internally (and thus has
7072 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7073 internal byte string, not the conceptual character string, even if you
7074 only have characters with values less than 256.
7076 Strings created with C<vec> can also be manipulated with the logical
7077 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7078 vector operation is desired when both operands are strings.
7079 See L<perlop/"Bitwise String Operators">.
7081 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7082 The comments show the string after each step. Note that this code works
7083 in the same way on big-endian or little-endian machines.
7086 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7088 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7089 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7091 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7092 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7093 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7094 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7095 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7096 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7098 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7099 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7100 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7103 To transform a bit vector into a string or list of 0's and 1's, use these:
7105 $bits = unpack("b*", $vector);
7106 @bits = split(//, unpack("b*", $vector));
7108 If you know the exact length in bits, it can be used in place of the C<*>.
7110 Here is an example to illustrate how the bits actually fall in place:
7116 unpack("V",$_) 01234567890123456789012345678901
7117 ------------------------------------------------------------------
7122 for ($shift=0; $shift < $width; ++$shift) {
7123 for ($off=0; $off < 32/$width; ++$off) {
7124 $str = pack("B*", "0"x32);
7125 $bits = (1<<$shift);
7126 vec($str, $off, $width) = $bits;
7127 $res = unpack("b*",$str);
7128 $val = unpack("V", $str);
7135 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7136 $off, $width, $bits, $val, $res
7140 Regardless of the machine architecture on which it is run, the above
7141 example should print the following table:
7144 unpack("V",$_) 01234567890123456789012345678901
7145 ------------------------------------------------------------------
7146 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7147 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7148 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7149 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7150 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7151 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7152 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7153 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7154 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7155 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7156 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7157 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7158 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7159 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7160 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7161 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7162 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7163 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7164 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7165 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7166 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7167 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7168 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7169 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7170 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7171 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7172 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7173 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7174 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7175 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7176 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7177 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7178 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7179 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7180 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7181 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7182 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7183 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7184 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7185 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7186 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7187 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7188 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7189 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7190 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7191 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7192 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7193 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7194 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7195 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7196 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7197 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7198 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7199 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7200 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7201 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7202 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7203 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7204 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7205 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7206 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7207 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7208 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7209 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7210 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7211 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7212 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7213 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7214 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7215 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7216 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7217 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7218 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7219 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7220 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7221 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7222 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7223 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7224 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7225 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7226 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7227 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7228 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7229 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7230 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7231 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7232 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7233 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7234 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7235 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7236 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7237 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7238 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7239 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7240 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7241 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7242 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7243 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7244 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7245 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7246 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7247 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7248 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7249 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7250 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7251 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7252 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7253 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7254 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7255 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7256 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7257 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7258 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7259 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7260 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7261 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7262 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7263 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7264 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7265 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7266 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7267 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7268 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7269 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7270 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7271 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7272 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7273 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7278 Behaves like the wait(2) system call on your system: it waits for a child
7279 process to terminate and returns the pid of the deceased process, or
7280 C<-1> if there are no child processes. The status is returned in C<$?>
7281 and C<{^CHILD_ERROR_NATIVE}>.
7282 Note that a return value of C<-1> could mean that child processes are
7283 being automatically reaped, as described in L<perlipc>.
7285 =item waitpid PID,FLAGS
7288 Waits for a particular child process to terminate and returns the pid of
7289 the deceased process, or C<-1> if there is no such child process. On some
7290 systems, a value of 0 indicates that there are processes still running.
7291 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7293 use POSIX ":sys_wait_h";
7296 $kid = waitpid(-1, WNOHANG);
7299 then you can do a non-blocking wait for all pending zombie processes.
7300 Non-blocking wait is available on machines supporting either the
7301 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7302 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7303 system call by remembering the status values of processes that have
7304 exited but have not been harvested by the Perl script yet.)
7306 Note that on some systems, a return value of C<-1> could mean that child
7307 processes are being automatically reaped. See L<perlipc> for details,
7308 and for other examples.
7311 X<wantarray> X<context>
7313 Returns true if the context of the currently executing subroutine or
7314 C<eval> is looking for a list value. Returns false if the context is
7315 looking for a scalar. Returns the undefined value if the context is
7316 looking for no value (void context).
7318 return unless defined wantarray; # don't bother doing more
7319 my @a = complex_calculation();
7320 return wantarray ? @a : "@a";
7322 C<wantarray()>'s result is unspecified in the top level of a file,
7323 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7324 in a C<DESTROY> method.
7326 This function should have been named wantlist() instead.
7329 X<warn> X<warning> X<STDERR>
7331 Prints the value of LIST to STDERR. If the last element of LIST does
7332 not end in a newline, it appends the same file/line number text as C<die>
7335 If LIST is empty and C<$@> already contains a value (typically from a
7336 previous eval) that value is used after appending C<"\t...caught">
7337 to C<$@>. This is useful for staying almost, but not entirely similar to
7340 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7342 No message is printed if there is a C<$SIG{__WARN__}> handler
7343 installed. It is the handler's responsibility to deal with the message
7344 as it sees fit (like, for instance, converting it into a C<die>). Most
7345 handlers must therefore make arrangements to actually display the
7346 warnings that they are not prepared to deal with, by calling C<warn>
7347 again in the handler. Note that this is quite safe and will not
7348 produce an endless loop, since C<__WARN__> hooks are not called from
7351 You will find this behavior is slightly different from that of
7352 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7353 instead call C<die> again to change it).
7355 Using a C<__WARN__> handler provides a powerful way to silence all
7356 warnings (even the so-called mandatory ones). An example:
7358 # wipe out *all* compile-time warnings
7359 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7361 my $foo = 20; # no warning about duplicate my $foo,
7362 # but hey, you asked for it!
7363 # no compile-time or run-time warnings before here
7366 # run-time warnings enabled after here
7367 warn "\$foo is alive and $foo!"; # does show up
7369 See L<perlvar> for details on setting C<%SIG> entries, and for more
7370 examples. See the Carp module for other kinds of warnings using its
7371 carp() and cluck() functions.
7373 =item write FILEHANDLE
7380 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7381 using the format associated with that file. By default the format for
7382 a file is the one having the same name as the filehandle, but the
7383 format for the current output channel (see the C<select> function) may be set
7384 explicitly by assigning the name of the format to the C<$~> variable.
7386 Top of form processing is handled automatically: if there is
7387 insufficient room on the current page for the formatted record, the
7388 page is advanced by writing a form feed, a special top-of-page format
7389 is used to format the new page header, and then the record is written.
7390 By default the top-of-page format is the name of the filehandle with
7391 "_TOP" appended, but it may be dynamically set to the format of your
7392 choice by assigning the name to the C<$^> variable while the filehandle is
7393 selected. The number of lines remaining on the current page is in
7394 variable C<$->, which can be set to C<0> to force a new page.
7396 If FILEHANDLE is unspecified, output goes to the current default output
7397 channel, which starts out as STDOUT but may be changed by the
7398 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7399 is evaluated and the resulting string is used to look up the name of
7400 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7402 Note that write is I<not> the opposite of C<read>. Unfortunately.
7406 The transliteration operator. Same as C<tr///>. See L<perlop>.