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-orientedness
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 $_; # ignore specials
344 The interpretation of the file permission operators C<-r>, C<-R>,
345 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
346 of the file and the uids and gids of the user. There may be other
347 reasons you can't actually read, write, or execute the file. Such
348 reasons may be for example network filesystem access controls, ACLs
349 (access control lists), read-only filesystems, and unrecognized
352 Also note that, for the superuser on the local filesystems, the C<-r>,
353 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
354 if any execute bit is set in the mode. Scripts run by the superuser
355 may thus need to do a stat() to determine the actual mode of the file,
356 or temporarily set their effective uid to something else.
358 If you are using ACLs, there is a pragma called C<filetest> that may
359 produce more accurate results than the bare stat() mode bits.
360 When under the C<use filetest 'access'> the above-mentioned filetests
361 will test whether the permission can (not) be granted using the
362 access() family of system calls. Also note that the C<-x> and C<-X> may
363 under this pragma return true even if there are no execute permission
364 bits set (nor any extra execute permission ACLs). This strangeness is
365 due to the underlying system calls' definitions. Read the
366 documentation for the C<filetest> pragma for more information.
368 Note that C<-s/a/b/> does not do a negated substitution. Saying
369 C<-exp($foo)> still works as expected, however--only single letters
370 following a minus are interpreted as file tests.
372 The C<-T> and C<-B> switches work as follows. The first block or so of the
373 file is examined for odd characters such as strange control codes or
374 characters with the high bit set. If too many strange characters (>30%)
375 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
376 containing null in the first block is considered a binary file. If C<-T>
377 or C<-B> is used on a filehandle, the current IO buffer is examined
378 rather than the first block. Both C<-T> and C<-B> return true on a null
379 file, or a file at EOF when testing a filehandle. Because you have to
380 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
381 against the file first, as in C<next unless -f $file && -T $file>.
383 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
384 the special filehandle consisting of a solitary underline, then the stat
385 structure of the previous file test (or stat operator) is used, saving
386 a system call. (This doesn't work with C<-t>, and you need to remember
387 that lstat() and C<-l> will leave values in the stat structure for the
388 symbolic link, not the real file.) (Also, if the stat buffer was filled by
389 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
392 print "Can do.\n" if -r $a || -w _ || -x _;
395 print "Readable\n" if -r _;
396 print "Writable\n" if -w _;
397 print "Executable\n" if -x _;
398 print "Setuid\n" if -u _;
399 print "Setgid\n" if -g _;
400 print "Sticky\n" if -k _;
401 print "Text\n" if -T _;
402 print "Binary\n" if -B _;
404 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
405 test operators, in a way that C<-f -w -x $file> is equivalent to
406 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
407 the return value of C<-f $file> as an argument to another filetest
408 operator, no special magic will happen.)
415 Returns the absolute value of its argument.
416 If VALUE is omitted, uses C<$_>.
418 =item accept NEWSOCKET,GENERICSOCKET
421 Accepts an incoming socket connect, just as the accept(2) system call
422 does. Returns the packed address if it succeeded, false otherwise.
423 See the example in L<perlipc/"Sockets: Client/Server Communication">.
425 On systems that support a close-on-exec flag on files, the flag will
426 be set for the newly opened file descriptor, as determined by the
427 value of $^F. See L<perlvar/$^F>.
436 Arranges to have a SIGALRM delivered to this process after the
437 specified number of wallclock seconds has elapsed. If SECONDS is not
438 specified, the value stored in C<$_> is used. (On some machines,
439 unfortunately, the elapsed time may be up to one second less or more
440 than you specified because of how seconds are counted, and process
441 scheduling may delay the delivery of the signal even further.)
443 Only one timer may be counting at once. Each call disables the
444 previous timer, and an argument of C<0> may be supplied to cancel the
445 previous timer without starting a new one. The returned value is the
446 amount of time remaining on the previous timer.
448 For delays of finer granularity than one second, you may use Perl's
449 four-argument version of select() leaving the first three arguments
450 undefined, or you might be able to use the C<syscall> interface to
451 access setitimer(2) if your system supports it. The Time::HiRes
452 module (from CPAN, and starting from Perl 5.8 part of the standard
453 distribution) may also prove useful.
455 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
456 (C<sleep> may be internally implemented in your system with C<alarm>)
458 If you want to use C<alarm> to time out a system call you need to use an
459 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
460 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
461 restart system calls on some systems. Using C<eval>/C<die> always works,
462 modulo the caveats given in L<perlipc/"Signals">.
465 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
467 $nread = sysread SOCKET, $buffer, $size;
471 die unless $@ eq "alarm\n"; # propagate unexpected errors
478 For more information see L<perlipc>.
481 X<atan2> X<arctangent> X<tan> X<tangent>
483 Returns the arctangent of Y/X in the range -PI to PI.
485 For the tangent operation, you may use the C<Math::Trig::tan>
486 function, or use the familiar relation:
488 sub tan { sin($_[0]) / cos($_[0]) }
490 Note that atan2(0, 0) is not well-defined.
492 =item bind SOCKET,NAME
495 Binds a network address to a socket, just as the bind system call
496 does. Returns true if it succeeded, false otherwise. NAME should be a
497 packed address of the appropriate type for the socket. See the examples in
498 L<perlipc/"Sockets: Client/Server Communication">.
500 =item binmode FILEHANDLE, LAYER
501 X<binmode> X<binary> X<text> X<DOS> X<Windows>
503 =item binmode FILEHANDLE
505 Arranges for FILEHANDLE to be read or written in "binary" or "text"
506 mode on systems where the run-time libraries distinguish between
507 binary and text files. If FILEHANDLE is an expression, the value is
508 taken as the name of the filehandle. Returns true on success,
509 otherwise it returns C<undef> and sets C<$!> (errno).
511 On some systems (in general, DOS and Windows-based systems) binmode()
512 is necessary when you're not working with a text file. For the sake
513 of portability it is a good idea to always use it when appropriate,
514 and to never use it when it isn't appropriate. Also, people can
515 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
517 In other words: regardless of platform, use binmode() on binary data,
518 like for example images.
520 If LAYER is present it is a single string, but may contain multiple
521 directives. The directives alter the behaviour of the file handle.
522 When LAYER is present using binmode on text file makes sense.
524 If LAYER is omitted or specified as C<:raw> the filehandle is made
525 suitable for passing binary data. This includes turning off possible CRLF
526 translation and marking it as bytes (as opposed to Unicode characters).
527 Note that, despite what may be implied in I<"Programming Perl"> (the
528 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
529 -- other layers which would affect the binary nature of the stream are
530 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
531 PERLIO environment variable.
533 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
534 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
535 establish default I/O layers. See L<open>.
537 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
538 in "Programming Perl, 3rd Edition". However, since the publishing of this
539 book, by many known as "Camel III", the consensus of the naming of this
540 functionality has moved from "discipline" to "layer". All documentation
541 of this version of Perl therefore refers to "layers" rather than to
542 "disciplines". Now back to the regularly scheduled documentation...>
544 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
545 C<:utf8> just marks the data as UTF-8 without further checking,
546 while C<:encoding(utf8)> checks the data for actually being valid
547 UTF-8. More details can be found in L<PerlIO::encoding>.
549 In general, binmode() should be called after open() but before any I/O
550 is done on the filehandle. Calling binmode() will normally flush any
551 pending buffered output data (and perhaps pending input data) on the
552 handle. An exception to this is the C<:encoding> layer that
553 changes the default character encoding of the handle, see L<open>.
554 The C<:encoding> layer sometimes needs to be called in
555 mid-stream, and it doesn't flush the stream. The C<:encoding>
556 also implicitly pushes on top of itself the C<:utf8> layer because
557 internally Perl will operate on UTF-8 encoded Unicode characters.
559 The operating system, device drivers, C libraries, and Perl run-time
560 system all work together to let the programmer treat a single
561 character (C<\n>) as the line terminator, irrespective of the external
562 representation. On many operating systems, the native text file
563 representation matches the internal representation, but on some
564 platforms the external representation of C<\n> is made up of more than
567 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
568 character to end each line in the external representation of text (even
569 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
570 on Unix and most VMS files). In other systems like OS/2, DOS and the
571 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
572 but what's stored in text files are the two characters C<\cM\cJ>. That
573 means that, if you don't use binmode() on these systems, C<\cM\cJ>
574 sequences on disk will be converted to C<\n> on input, and any C<\n> in
575 your program will be converted back to C<\cM\cJ> on output. This is what
576 you want for text files, but it can be disastrous for binary files.
578 Another consequence of using binmode() (on some systems) is that
579 special end-of-file markers will be seen as part of the data stream.
580 For systems from the Microsoft family this means that if your binary
581 data contains C<\cZ>, the I/O subsystem will regard it as the end of
582 the file, unless you use binmode().
584 binmode() is not only important for readline() and print() operations,
585 but also when using read(), seek(), sysread(), syswrite() and tell()
586 (see L<perlport> for more details). See the C<$/> and C<$\> variables
587 in L<perlvar> for how to manually set your input and output
588 line-termination sequences.
590 =item bless REF,CLASSNAME
595 This function tells the thingy referenced by REF that it is now an object
596 in the CLASSNAME package. If CLASSNAME is omitted, the current package
597 is used. Because a C<bless> is often the last thing in a constructor,
598 it returns the reference for convenience. Always use the two-argument
599 version if a derived class might inherit the function doing the blessing.
600 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
603 Consider always blessing objects in CLASSNAMEs that are mixed case.
604 Namespaces with all lowercase names are considered reserved for
605 Perl pragmata. Builtin types have all uppercase names. To prevent
606 confusion, you may wish to avoid such package names as well. Make sure
607 that CLASSNAME is a true value.
609 See L<perlmod/"Perl Modules">.
613 Break out of a C<given()> block.
615 This keyword is enabled by the "switch" feature: see L<feature>
616 for more information.
619 X<caller> X<call stack> X<stack> X<stack trace>
623 Returns the context of the current subroutine call. In scalar context,
624 returns the caller's package name if there is a caller, that is, if
625 we're in a subroutine or C<eval> or C<require>, and the undefined value
626 otherwise. In list context, returns
629 ($package, $filename, $line) = caller;
631 With EXPR, it returns some extra information that the debugger uses to
632 print a stack trace. The value of EXPR indicates how many call frames
633 to go back before the current one.
636 ($package, $filename, $line, $subroutine, $hasargs,
639 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
642 Here $subroutine may be C<(eval)> if the frame is not a subroutine
643 call, but an C<eval>. In such a case additional elements $evaltext and
644 C<$is_require> are set: C<$is_require> is true if the frame is created by a
645 C<require> or C<use> statement, $evaltext contains the text of the
646 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
647 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
648 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
649 frame.) $subroutine may also be C<(unknown)> if this particular
650 subroutine happens to have been deleted from the symbol table.
651 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
652 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
653 compiled with. The C<$hints> and C<$bitmask> values are subject to change
654 between versions of Perl, and are not meant for external use.
656 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
657 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
658 of this hash, as they are the actual values stored in the optree.
660 Furthermore, when called from within the DB package, caller returns more
661 detailed information: it sets the list variable C<@DB::args> to be the
662 arguments with which the subroutine was invoked.
664 Be aware that the optimizer might have optimized call frames away before
665 C<caller> had a chance to get the information. That means that C<caller(N)>
666 might not return information about the call frame you expect it do, for
667 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
668 previous time C<caller> was called.
675 =item chdir FILEHANDLE
677 =item chdir DIRHANDLE
681 Changes the working directory to EXPR, if possible. If EXPR is omitted,
682 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
683 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
684 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
685 neither is set, C<chdir> does nothing. It returns true upon success,
686 false otherwise. See the example under C<die>.
688 On systems that support fchdir, you might pass a file handle or
689 directory handle as argument. On systems that don't support fchdir,
690 passing handles produces a fatal error at run time.
693 X<chmod> X<permission> X<mode>
695 Changes the permissions of a list of files. The first element of the
696 list must be the numerical mode, which should probably be an octal
697 number, and which definitely should I<not> be a string of octal digits:
698 C<0644> is okay, C<'0644'> is not. Returns the number of files
699 successfully changed. See also L</oct>, if all you have is a string.
701 $cnt = chmod 0755, 'foo', 'bar';
702 chmod 0755, @executables;
703 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
705 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
706 $mode = 0644; chmod $mode, 'foo'; # this is best
708 On systems that support fchmod, you might pass file handles among the
709 files. On systems that don't support fchmod, passing file handles
710 produces a fatal error at run time. The file handles must be passed
711 as globs or references to be recognized. Barewords are considered
714 open(my $fh, "<", "foo");
715 my $perm = (stat $fh)[2] & 07777;
716 chmod($perm | 0600, $fh);
718 You can also import the symbolic C<S_I*> constants from the Fcntl
723 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
724 # This is identical to the chmod 0755 of the above example.
727 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
733 This safer version of L</chop> removes any trailing string
734 that corresponds to the current value of C<$/> (also known as
735 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
736 number of characters removed from all its arguments. It's often used to
737 remove the newline from the end of an input record when you're worried
738 that the final record may be missing its newline. When in paragraph
739 mode (C<$/ = "">), it removes all trailing newlines from the string.
740 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
741 a reference to an integer or the like, see L<perlvar>) chomp() won't
743 If VARIABLE is omitted, it chomps C<$_>. Example:
746 chomp; # avoid \n on last field
751 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
753 You can actually chomp anything that's an lvalue, including an assignment:
756 chomp($answer = <STDIN>);
758 If you chomp a list, each element is chomped, and the total number of
759 characters removed is returned.
761 Note that parentheses are necessary when you're chomping anything
762 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
763 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
764 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
765 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
775 Chops off the last character of a string and returns the character
776 chopped. It is much more efficient than C<s/.$//s> because it neither
777 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
778 If VARIABLE is a hash, it chops the hash's values, but not its keys.
780 You can actually chop anything that's an lvalue, including an assignment.
782 If you chop a list, each element is chopped. Only the value of the
783 last C<chop> is returned.
785 Note that C<chop> returns the last character. To return all but the last
786 character, use C<substr($string, 0, -1)>.
791 X<chown> X<owner> X<user> X<group>
793 Changes the owner (and group) of a list of files. The first two
794 elements of the list must be the I<numeric> uid and gid, in that
795 order. A value of -1 in either position is interpreted by most
796 systems to leave that value unchanged. Returns the number of files
797 successfully changed.
799 $cnt = chown $uid, $gid, 'foo', 'bar';
800 chown $uid, $gid, @filenames;
802 On systems that support fchown, you might pass file handles among the
803 files. On systems that don't support fchown, passing file handles
804 produces a fatal error at run time. The file handles must be passed
805 as globs or references to be recognized. Barewords are considered
808 Here's an example that looks up nonnumeric uids in the passwd file:
811 chomp($user = <STDIN>);
813 chomp($pattern = <STDIN>);
815 ($login,$pass,$uid,$gid) = getpwnam($user)
816 or die "$user not in passwd file";
818 @ary = glob($pattern); # expand filenames
819 chown $uid, $gid, @ary;
821 On most systems, you are not allowed to change the ownership of the
822 file unless you're the superuser, although you should be able to change
823 the group to any of your secondary groups. On insecure systems, these
824 restrictions may be relaxed, but this is not a portable assumption.
825 On POSIX systems, you can detect this condition this way:
827 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
828 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
831 X<chr> X<character> X<ASCII> X<Unicode>
835 Returns the character represented by that NUMBER in the character set.
836 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
837 chr(0x263a) is a Unicode smiley face.
839 Negative values give the Unicode replacement character (chr(0xfffd)),
840 except under the L<bytes> pragma, where low eight bits of the value
841 (truncated to an integer) are used.
843 If NUMBER is omitted, uses C<$_>.
845 For the reverse, use L</ord>.
847 Note that characters from 128 to 255 (inclusive) are by default
848 internally not encoded as UTF-8 for backward compatibility reasons.
850 See L<perlunicode> for more about Unicode.
852 =item chroot FILENAME
857 This function works like the system call by the same name: it makes the
858 named directory the new root directory for all further pathnames that
859 begin with a C</> by your process and all its children. (It doesn't
860 change your current working directory, which is unaffected.) For security
861 reasons, this call is restricted to the superuser. If FILENAME is
862 omitted, does a C<chroot> to C<$_>.
864 =item close FILEHANDLE
869 Closes the file or pipe associated with the file handle, flushes the IO
870 buffers, and closes the system file descriptor. Returns true if those
871 operations have succeeded and if no error was reported by any PerlIO
872 layer. Closes the currently selected filehandle if the argument is
875 You don't have to close FILEHANDLE if you are immediately going to do
876 another C<open> on it, because C<open> will close it for you. (See
877 C<open>.) However, an explicit C<close> on an input file resets the line
878 counter (C<$.>), while the implicit close done by C<open> does not.
880 If the file handle came from a piped open, C<close> will additionally
881 return false if one of the other system calls involved fails, or if the
882 program exits with non-zero status. (If the only problem was that the
883 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
884 also waits for the process executing on the pipe to complete, in case you
885 want to look at the output of the pipe afterwards, and
886 implicitly puts the exit status value of that command into C<$?> and
887 C<${^CHILD_ERROR_NATIVE}>.
889 Prematurely closing the read end of a pipe (i.e. before the process
890 writing to it at the other end has closed it) will result in a
891 SIGPIPE being delivered to the writer. If the other end can't
892 handle that, be sure to read all the data before closing the pipe.
896 open(OUTPUT, '|sort >foo') # pipe to sort
897 or die "Can't start sort: $!";
898 #... # print stuff to output
899 close OUTPUT # wait for sort to finish
900 or warn $! ? "Error closing sort pipe: $!"
901 : "Exit status $? from sort";
902 open(INPUT, 'foo') # get sort's results
903 or die "Can't open 'foo' for input: $!";
905 FILEHANDLE may be an expression whose value can be used as an indirect
906 filehandle, usually the real filehandle name.
908 =item closedir DIRHANDLE
911 Closes a directory opened by C<opendir> and returns the success of that
914 =item connect SOCKET,NAME
917 Attempts to connect to a remote socket, just as the connect system call
918 does. Returns true if it succeeded, false otherwise. NAME should be a
919 packed address of the appropriate type for the socket. See the examples in
920 L<perlipc/"Sockets: Client/Server Communication">.
927 C<continue> is actually a flow control statement rather than a function. If
928 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
929 C<foreach>), it is always executed just before the conditional is about to
930 be evaluated again, just like the third part of a C<for> loop in C. Thus
931 it can be used to increment a loop variable, even when the loop has been
932 continued via the C<next> statement (which is similar to the C C<continue>
935 C<last>, C<next>, or C<redo> may appear within a C<continue>
936 block. C<last> and C<redo> will behave as if they had been executed within
937 the main block. So will C<next>, but since it will execute a C<continue>
938 block, it may be more entertaining.
941 ### redo always comes here
944 ### next always comes here
946 # then back the top to re-check EXPR
948 ### last always comes here
950 Omitting the C<continue> section is semantically equivalent to using an
951 empty one, logically enough. In that case, C<next> goes directly back
952 to check the condition at the top of the loop.
954 If the "switch" feature is enabled, C<continue> is also a
955 function that will break out of the current C<when> or C<default>
956 block, and fall through to the next case. See L<feature> and
957 L<perlsyn/"Switch statements"> for more information.
961 X<cos> X<cosine> X<acos> X<arccosine>
965 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
966 takes cosine of C<$_>.
968 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
969 function, or use this relation:
971 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
973 =item crypt PLAINTEXT,SALT
974 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
975 X<decrypt> X<cryptography> X<passwd> X<encrypt>
977 Creates a digest string exactly like the crypt(3) function in the C
978 library (assuming that you actually have a version there that has not
979 been extirpated as a potential munitions).
981 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
982 into a short string, called a digest, which is returned. The same
983 PLAINTEXT and SALT will always return the same string, but there is no
984 (known) way to get the original PLAINTEXT from the hash. Small
985 changes in the PLAINTEXT or SALT will result in large changes in the
988 There is no decrypt function. This function isn't all that useful for
989 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
990 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
991 primarily used to check if two pieces of text are the same without
992 having to transmit or store the text itself. An example is checking
993 if a correct password is given. The digest of the password is stored,
994 not the password itself. The user types in a password that is
995 crypt()'d with the same salt as the stored digest. If the two digests
996 match the password is correct.
998 When verifying an existing digest string you should use the digest as
999 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1000 to create the digest is visible as part of the digest. This ensures
1001 crypt() will hash the new string with the same salt as the digest.
1002 This allows your code to work with the standard L<crypt|/crypt> and
1003 with more exotic implementations. In other words, do not assume
1004 anything about the returned string itself, or how many bytes in the
1007 Traditionally the result is a string of 13 bytes: two first bytes of
1008 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1009 the first eight bytes of the digest string mattered, but alternative
1010 hashing schemes (like MD5), higher level security schemes (like C2),
1011 and implementations on non-UNIX platforms may produce different
1014 When choosing a new salt create a random two character string whose
1015 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1016 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1017 characters is just a recommendation; the characters allowed in
1018 the salt depend solely on your system's crypt library, and Perl can't
1019 restrict what salts C<crypt()> accepts.
1021 Here's an example that makes sure that whoever runs this program knows
1024 $pwd = (getpwuid($<))[1];
1026 system "stty -echo";
1028 chomp($word = <STDIN>);
1032 if (crypt($word, $pwd) ne $pwd) {
1038 Of course, typing in your own password to whoever asks you
1041 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1042 of data, not least of all because you can't get the information
1043 back. Look at the L<Digest> module for more robust algorithms.
1045 If using crypt() on a Unicode string (which I<potentially> has
1046 characters with codepoints above 255), Perl tries to make sense
1047 of the situation by trying to downgrade (a copy of the string)
1048 the string back to an eight-bit byte string before calling crypt()
1049 (on that copy). If that works, good. If not, crypt() dies with
1050 C<Wide character in crypt>.
1055 [This function has been largely superseded by the C<untie> function.]
1057 Breaks the binding between a DBM file and a hash.
1059 =item dbmopen HASH,DBNAME,MASK
1060 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1062 [This function has been largely superseded by the C<tie> function.]
1064 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1065 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1066 argument is I<not> a filehandle, even though it looks like one). DBNAME
1067 is the name of the database (without the F<.dir> or F<.pag> extension if
1068 any). If the database does not exist, it is created with protection
1069 specified by MASK (as modified by the C<umask>). If your system supports
1070 only the older DBM functions, you may perform only one C<dbmopen> in your
1071 program. In older versions of Perl, if your system had neither DBM nor
1072 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1075 If you don't have write access to the DBM file, you can only read hash
1076 variables, not set them. If you want to test whether you can write,
1077 either use file tests or try setting a dummy hash entry inside an C<eval>,
1078 which will trap the error.
1080 Note that functions such as C<keys> and C<values> may return huge lists
1081 when used on large DBM files. You may prefer to use the C<each>
1082 function to iterate over large DBM files. Example:
1084 # print out history file offsets
1085 dbmopen(%HIST,'/usr/lib/news/history',0666);
1086 while (($key,$val) = each %HIST) {
1087 print $key, ' = ', unpack('L',$val), "\n";
1091 See also L<AnyDBM_File> for a more general description of the pros and
1092 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1093 rich implementation.
1095 You can control which DBM library you use by loading that library
1096 before you call dbmopen():
1099 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1100 or die "Can't open netscape history file: $!";
1103 X<defined> X<undef> X<undefined>
1107 Returns a Boolean value telling whether EXPR has a value other than
1108 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1111 Many operations return C<undef> to indicate failure, end of file,
1112 system error, uninitialized variable, and other exceptional
1113 conditions. This function allows you to distinguish C<undef> from
1114 other values. (A simple Boolean test will not distinguish among
1115 C<undef>, zero, the empty string, and C<"0">, which are all equally
1116 false.) Note that since C<undef> is a valid scalar, its presence
1117 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1118 returns C<undef> when its argument is an empty array, I<or> when the
1119 element to return happens to be C<undef>.
1121 You may also use C<defined(&func)> to check whether subroutine C<&func>
1122 has ever been defined. The return value is unaffected by any forward
1123 declarations of C<&func>. Note that a subroutine which is not defined
1124 may still be callable: its package may have an C<AUTOLOAD> method that
1125 makes it spring into existence the first time that it is called -- see
1128 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1129 used to report whether memory for that aggregate has ever been
1130 allocated. This behavior may disappear in future versions of Perl.
1131 You should instead use a simple test for size:
1133 if (@an_array) { print "has array elements\n" }
1134 if (%a_hash) { print "has hash members\n" }
1136 When used on a hash element, it tells you whether the value is defined,
1137 not whether the key exists in the hash. Use L</exists> for the latter
1142 print if defined $switch{'D'};
1143 print "$val\n" while defined($val = pop(@ary));
1144 die "Can't readlink $sym: $!"
1145 unless defined($value = readlink $sym);
1146 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1147 $debugging = 0 unless defined $debugging;
1149 Note: Many folks tend to overuse C<defined>, and then are surprised to
1150 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1151 defined values. For example, if you say
1155 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1156 matched "nothing". It didn't really fail to match anything. Rather, it
1157 matched something that happened to be zero characters long. This is all
1158 very above-board and honest. When a function returns an undefined value,
1159 it's an admission that it couldn't give you an honest answer. So you
1160 should use C<defined> only when you're questioning the integrity of what
1161 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1164 See also L</undef>, L</exists>, L</ref>.
1169 Given an expression that specifies a hash element, array element, hash slice,
1170 or array slice, deletes the specified element(s) from the hash or array.
1171 In the case of an array, if the array elements happen to be at the end,
1172 the size of the array will shrink to the highest element that tests
1173 true for exists() (or 0 if no such element exists).
1175 Returns a list with the same number of elements as the number of elements
1176 for which deletion was attempted. Each element of that list consists of
1177 either the value of the element deleted, or the undefined value. In scalar
1178 context, this means that you get the value of the last element deleted (or
1179 the undefined value if that element did not exist).
1181 %hash = (foo => 11, bar => 22, baz => 33);
1182 $scalar = delete $hash{foo}; # $scalar is 11
1183 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1184 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1186 Deleting from C<%ENV> modifies the environment. Deleting from
1187 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1188 from a C<tie>d hash or array may not necessarily return anything.
1190 Deleting an array element effectively returns that position of the array
1191 to its initial, uninitialized state. Subsequently testing for the same
1192 element with exists() will return false. Also, deleting array elements
1193 in the middle of an array will not shift the index of the elements
1194 after them down. Use splice() for that. See L</exists>.
1196 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1198 foreach $key (keys %HASH) {
1202 foreach $index (0 .. $#ARRAY) {
1203 delete $ARRAY[$index];
1208 delete @HASH{keys %HASH};
1210 delete @ARRAY[0 .. $#ARRAY];
1212 But both of these are slower than just assigning the empty list
1213 or undefining %HASH or @ARRAY:
1215 %HASH = (); # completely empty %HASH
1216 undef %HASH; # forget %HASH ever existed
1218 @ARRAY = (); # completely empty @ARRAY
1219 undef @ARRAY; # forget @ARRAY ever existed
1221 Note that the EXPR can be arbitrarily complicated as long as the final
1222 operation is a hash element, array element, hash slice, or array slice
1225 delete $ref->[$x][$y]{$key};
1226 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1228 delete $ref->[$x][$y][$index];
1229 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1232 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1234 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1235 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1236 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1237 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1238 an C<eval(),> the error message is stuffed into C<$@> and the
1239 C<eval> is terminated with the undefined value. This makes
1240 C<die> the way to raise an exception.
1242 Equivalent examples:
1244 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1245 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1247 If the last element of LIST does not end in a newline, the current
1248 script line number and input line number (if any) are also printed,
1249 and a newline is supplied. Note that the "input line number" (also
1250 known as "chunk") is subject to whatever notion of "line" happens to
1251 be currently in effect, and is also available as the special variable
1252 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1254 Hint: sometimes appending C<", stopped"> to your message will cause it
1255 to make better sense when the string C<"at foo line 123"> is appended.
1256 Suppose you are running script "canasta".
1258 die "/etc/games is no good";
1259 die "/etc/games is no good, stopped";
1261 produce, respectively
1263 /etc/games is no good at canasta line 123.
1264 /etc/games is no good, stopped at canasta line 123.
1266 See also exit(), warn(), and the Carp module.
1268 If LIST is empty and C<$@> already contains a value (typically from a
1269 previous eval) that value is reused after appending C<"\t...propagated">.
1270 This is useful for propagating exceptions:
1273 die unless $@ =~ /Expected exception/;
1275 If LIST is empty and C<$@> contains an object reference that has a
1276 C<PROPAGATE> method, that method will be called with additional file
1277 and line number parameters. The return value replaces the value in
1278 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1281 If C<$@> is empty then the string C<"Died"> is used.
1283 die() can also be called with a reference argument. If this happens to be
1284 trapped within an eval(), $@ contains the reference. This behavior permits
1285 a more elaborate exception handling implementation using objects that
1286 maintain arbitrary state about the nature of the exception. Such a scheme
1287 is sometimes preferable to matching particular string values of $@ using
1288 regular expressions. Because $@ is a global variable, and eval() may be
1289 used within object implementations, care must be taken that analyzing the
1290 error object doesn't replace the reference in the global variable. The
1291 easiest solution is to make a local copy of the reference before doing
1292 other manipulations. Here's an example:
1294 use Scalar::Util 'blessed';
1296 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1297 if (my $ev_err = $@) {
1298 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1299 # handle Some::Module::Exception
1302 # handle all other possible exceptions
1306 Because perl will stringify uncaught exception messages before displaying
1307 them, you may want to overload stringification operations on such custom
1308 exception objects. See L<overload> for details about that.
1310 You can arrange for a callback to be run just before the C<die>
1311 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1312 handler will be called with the error text and can change the error
1313 message, if it sees fit, by calling C<die> again. See
1314 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1315 L<"eval BLOCK"> for some examples. Although this feature was
1316 to be run only right before your program was to exit, this is not
1317 currently the case--the C<$SIG{__DIE__}> hook is currently called
1318 even inside eval()ed blocks/strings! If one wants the hook to do
1319 nothing in such situations, put
1323 as the first line of the handler (see L<perlvar/$^S>). Because
1324 this promotes strange action at a distance, this counterintuitive
1325 behavior may be fixed in a future release.
1330 Not really a function. Returns the value of the last command in the
1331 sequence of commands indicated by BLOCK. When modified by the C<while> or
1332 C<until> loop modifier, executes the BLOCK once before testing the loop
1333 condition. (On other statements the loop modifiers test the conditional
1336 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1337 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1338 See L<perlsyn> for alternative strategies.
1340 =item do SUBROUTINE(LIST)
1343 This form of subroutine call is deprecated. See L<perlsub>.
1348 Uses the value of EXPR as a filename and executes the contents of the
1349 file as a Perl script.
1357 except that it's more efficient and concise, keeps track of the current
1358 filename for error messages, searches the @INC directories, and updates
1359 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1360 variables. It also differs in that code evaluated with C<do FILENAME>
1361 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1362 same, however, in that it does reparse the file every time you call it,
1363 so you probably don't want to do this inside a loop.
1365 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1366 error. If C<do> can read the file but cannot compile it, it
1367 returns undef and sets an error message in C<$@>. If the file is
1368 successfully compiled, C<do> returns the value of the last expression
1371 Note that inclusion of library modules is better done with the
1372 C<use> and C<require> operators, which also do automatic error checking
1373 and raise an exception if there's a problem.
1375 You might like to use C<do> to read in a program configuration
1376 file. Manual error checking can be done this way:
1378 # read in config files: system first, then user
1379 for $file ("/share/prog/defaults.rc",
1380 "$ENV{HOME}/.someprogrc")
1382 unless ($return = do $file) {
1383 warn "couldn't parse $file: $@" if $@;
1384 warn "couldn't do $file: $!" unless defined $return;
1385 warn "couldn't run $file" unless $return;
1390 X<dump> X<core> X<undump>
1394 This function causes an immediate core dump. See also the B<-u>
1395 command-line switch in L<perlrun>, which does the same thing.
1396 Primarily this is so that you can use the B<undump> program (not
1397 supplied) to turn your core dump into an executable binary after
1398 having initialized all your variables at the beginning of the
1399 program. When the new binary is executed it will begin by executing
1400 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1401 Think of it as a goto with an intervening core dump and reincarnation.
1402 If C<LABEL> is omitted, restarts the program from the top.
1404 B<WARNING>: Any files opened at the time of the dump will I<not>
1405 be open any more when the program is reincarnated, with possible
1406 resulting confusion on the part of Perl.
1408 This function is now largely obsolete, mostly because it's very hard to
1409 convert a core file into an executable. That's why you should now invoke
1410 it as C<CORE::dump()>, if you don't want to be warned against a possible
1414 X<each> X<hash, iterator>
1416 When called in list context, returns a 2-element list consisting of the
1417 key and value for the next element of a hash, so that you can iterate over
1418 it. When called in scalar context, returns only the key for the next
1419 element in the hash.
1421 Entries are returned in an apparently random order. The actual random
1422 order is subject to change in future versions of perl, but it is
1423 guaranteed to be in the same order as either the C<keys> or C<values>
1424 function would produce on the same (unmodified) hash. Since Perl
1425 5.8.1 the ordering is different even between different runs of Perl
1426 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1428 When the hash is entirely read, a null array is returned in list context
1429 (which when assigned produces a false (C<0>) value), and C<undef> in
1430 scalar context. The next call to C<each> after that will start iterating
1431 again. There is a single iterator for each hash, shared by all C<each>,
1432 C<keys>, and C<values> function calls in the program; it can be reset by
1433 reading all the elements from the hash, or by evaluating C<keys HASH> or
1434 C<values HASH>. If you add or delete elements of a hash while you're
1435 iterating over it, you may get entries skipped or duplicated, so
1436 don't. Exception: It is always safe to delete the item most recently
1437 returned by C<each()>, which means that the following code will work:
1439 while (($key, $value) = each %hash) {
1441 delete $hash{$key}; # This is safe
1444 The following prints out your environment like the printenv(1) program,
1445 only in a different order:
1447 while (($key,$value) = each %ENV) {
1448 print "$key=$value\n";
1451 See also C<keys>, C<values> and C<sort>.
1453 =item eof FILEHANDLE
1462 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1463 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1464 gives the real filehandle. (Note that this function actually
1465 reads a character and then C<ungetc>s it, so isn't very useful in an
1466 interactive context.) Do not read from a terminal file (or call
1467 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1468 as terminals may lose the end-of-file condition if you do.
1470 An C<eof> without an argument uses the last file read. Using C<eof()>
1471 with empty parentheses is very different. It refers to the pseudo file
1472 formed from the files listed on the command line and accessed via the
1473 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1474 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1475 used will cause C<@ARGV> to be examined to determine if input is
1476 available. Similarly, an C<eof()> after C<< <> >> has returned
1477 end-of-file will assume you are processing another C<@ARGV> list,
1478 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1479 see L<perlop/"I/O Operators">.
1481 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1482 detect the end of each file, C<eof()> will only detect the end of the
1483 last file. Examples:
1485 # reset line numbering on each input file
1487 next if /^\s*#/; # skip comments
1490 close ARGV if eof; # Not eof()!
1493 # insert dashes just before last line of last file
1495 if (eof()) { # check for end of last file
1496 print "--------------\n";
1499 last if eof(); # needed if we're reading from a terminal
1502 Practical hint: you almost never need to use C<eof> in Perl, because the
1503 input operators typically return C<undef> when they run out of data, or if
1507 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1508 X<error, handling> X<exception, handling>
1514 In the first form, the return value of EXPR is parsed and executed as if it
1515 were a little Perl program. The value of the expression (which is itself
1516 determined within scalar context) is first parsed, and if there weren't any
1517 errors, executed in the lexical context of the current Perl program, so
1518 that any variable settings or subroutine and format definitions remain
1519 afterwards. Note that the value is parsed every time the C<eval> executes.
1520 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1521 delay parsing and subsequent execution of the text of EXPR until run time.
1523 In the second form, the code within the BLOCK is parsed only once--at the
1524 same time the code surrounding the C<eval> itself was parsed--and executed
1525 within the context of the current Perl program. This form is typically
1526 used to trap exceptions more efficiently than the first (see below), while
1527 also providing the benefit of checking the code within BLOCK at compile
1530 The final semicolon, if any, may be omitted from the value of EXPR or within
1533 In both forms, the value returned is the value of the last expression
1534 evaluated inside the mini-program; a return statement may be also used, just
1535 as with subroutines. The expression providing the return value is evaluated
1536 in void, scalar, or list context, depending on the context of the C<eval>
1537 itself. See L</wantarray> for more on how the evaluation context can be
1540 If there is a syntax error or runtime error, or a C<die> statement is
1541 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1542 error message. If there was no error, C<$@> is guaranteed to be a null
1543 string. Beware that using C<eval> neither silences perl from printing
1544 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1545 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1546 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1547 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1549 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1550 determining whether a particular feature (such as C<socket> or C<symlink>)
1551 is implemented. It is also Perl's exception trapping mechanism, where
1552 the die operator is used to raise exceptions.
1554 If the code to be executed doesn't vary, you may use the eval-BLOCK
1555 form to trap run-time errors without incurring the penalty of
1556 recompiling each time. The error, if any, is still returned in C<$@>.
1559 # make divide-by-zero nonfatal
1560 eval { $answer = $a / $b; }; warn $@ if $@;
1562 # same thing, but less efficient
1563 eval '$answer = $a / $b'; warn $@ if $@;
1565 # a compile-time error
1566 eval { $answer = }; # WRONG
1569 eval '$answer ='; # sets $@
1571 Using the C<eval{}> form as an exception trap in libraries does have some
1572 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1573 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1574 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1575 as shown in this example:
1577 # a very private exception trap for divide-by-zero
1578 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1581 This is especially significant, given that C<__DIE__> hooks can call
1582 C<die> again, which has the effect of changing their error messages:
1584 # __DIE__ hooks may modify error messages
1586 local $SIG{'__DIE__'} =
1587 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1588 eval { die "foo lives here" };
1589 print $@ if $@; # prints "bar lives here"
1592 Because this promotes action at a distance, this counterintuitive behavior
1593 may be fixed in a future release.
1595 With an C<eval>, you should be especially careful to remember what's
1596 being looked at when:
1602 eval { $x }; # CASE 4
1604 eval "\$$x++"; # CASE 5
1607 Cases 1 and 2 above behave identically: they run the code contained in
1608 the variable $x. (Although case 2 has misleading double quotes making
1609 the reader wonder what else might be happening (nothing is).) Cases 3
1610 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1611 does nothing but return the value of $x. (Case 4 is preferred for
1612 purely visual reasons, but it also has the advantage of compiling at
1613 compile-time instead of at run-time.) Case 5 is a place where
1614 normally you I<would> like to use double quotes, except that in this
1615 particular situation, you can just use symbolic references instead, as
1618 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1619 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1621 Note that as a very special case, an C<eval ''> executed within the C<DB>
1622 package doesn't see the usual surrounding lexical scope, but rather the
1623 scope of the first non-DB piece of code that called it. You don't normally
1624 need to worry about this unless you are writing a Perl debugger.
1629 =item exec PROGRAM LIST
1631 The C<exec> function executes a system command I<and never returns>--
1632 use C<system> instead of C<exec> if you want it to return. It fails and
1633 returns false only if the command does not exist I<and> it is executed
1634 directly instead of via your system's command shell (see below).
1636 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1637 warns you if there is a following statement which isn't C<die>, C<warn>,
1638 or C<exit> (if C<-w> is set - but you always do that). If you
1639 I<really> want to follow an C<exec> with some other statement, you
1640 can use one of these styles to avoid the warning:
1642 exec ('foo') or print STDERR "couldn't exec foo: $!";
1643 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1645 If there is more than one argument in LIST, or if LIST is an array
1646 with more than one value, calls execvp(3) with the arguments in LIST.
1647 If there is only one scalar argument or an array with one element in it,
1648 the argument is checked for shell metacharacters, and if there are any,
1649 the entire argument is passed to the system's command shell for parsing
1650 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1651 If there are no shell metacharacters in the argument, it is split into
1652 words and passed directly to C<execvp>, which is more efficient.
1655 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1656 exec "sort $outfile | uniq";
1658 If you don't really want to execute the first argument, but want to lie
1659 to the program you are executing about its own name, you can specify
1660 the program you actually want to run as an "indirect object" (without a
1661 comma) in front of the LIST. (This always forces interpretation of the
1662 LIST as a multivalued list, even if there is only a single scalar in
1665 $shell = '/bin/csh';
1666 exec $shell '-sh'; # pretend it's a login shell
1670 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1672 When the arguments get executed via the system shell, results will
1673 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1676 Using an indirect object with C<exec> or C<system> is also more
1677 secure. This usage (which also works fine with system()) forces
1678 interpretation of the arguments as a multivalued list, even if the
1679 list had just one argument. That way you're safe from the shell
1680 expanding wildcards or splitting up words with whitespace in them.
1682 @args = ( "echo surprise" );
1684 exec @args; # subject to shell escapes
1686 exec { $args[0] } @args; # safe even with one-arg list
1688 The first version, the one without the indirect object, ran the I<echo>
1689 program, passing it C<"surprise"> an argument. The second version
1690 didn't--it tried to run a program literally called I<"echo surprise">,
1691 didn't find it, and set C<$?> to a non-zero value indicating failure.
1693 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1694 output before the exec, but this may not be supported on some platforms
1695 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1696 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1697 open handles in order to avoid lost output.
1699 Note that C<exec> will not call your C<END> blocks, nor will it call
1700 any C<DESTROY> methods in your objects.
1703 X<exists> X<autovivification>
1705 Given an expression that specifies a hash element or array element,
1706 returns true if the specified element in the hash or array has ever
1707 been initialized, even if the corresponding value is undefined. The
1708 element is not autovivified if it doesn't exist.
1710 print "Exists\n" if exists $hash{$key};
1711 print "Defined\n" if defined $hash{$key};
1712 print "True\n" if $hash{$key};
1714 print "Exists\n" if exists $array[$index];
1715 print "Defined\n" if defined $array[$index];
1716 print "True\n" if $array[$index];
1718 A hash or array element can be true only if it's defined, and defined if
1719 it exists, but the reverse doesn't necessarily hold true.
1721 Given an expression that specifies the name of a subroutine,
1722 returns true if the specified subroutine has ever been declared, even
1723 if it is undefined. Mentioning a subroutine name for exists or defined
1724 does not count as declaring it. Note that a subroutine which does not
1725 exist may still be callable: its package may have an C<AUTOLOAD>
1726 method that makes it spring into existence the first time that it is
1727 called -- see L<perlsub>.
1729 print "Exists\n" if exists &subroutine;
1730 print "Defined\n" if defined &subroutine;
1732 Note that the EXPR can be arbitrarily complicated as long as the final
1733 operation is a hash or array key lookup or subroutine name:
1735 if (exists $ref->{A}->{B}->{$key}) { }
1736 if (exists $hash{A}{B}{$key}) { }
1738 if (exists $ref->{A}->{B}->[$ix]) { }
1739 if (exists $hash{A}{B}[$ix]) { }
1741 if (exists &{$ref->{A}{B}{$key}}) { }
1743 Although the deepest nested array or hash will not spring into existence
1744 just because its existence was tested, any intervening ones will.
1745 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1746 into existence due to the existence test for the $key element above.
1747 This happens anywhere the arrow operator is used, including even:
1750 if (exists $ref->{"Some key"}) { }
1751 print $ref; # prints HASH(0x80d3d5c)
1753 This surprising autovivification in what does not at first--or even
1754 second--glance appear to be an lvalue context may be fixed in a future
1757 Use of a subroutine call, rather than a subroutine name, as an argument
1758 to exists() is an error.
1761 exists &sub(); # Error
1764 X<exit> X<terminate> X<abort>
1768 Evaluates EXPR and exits immediately with that value. Example:
1771 exit 0 if $ans =~ /^[Xx]/;
1773 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1774 universally recognized values for EXPR are C<0> for success and C<1>
1775 for error; other values are subject to interpretation depending on the
1776 environment in which the Perl program is running. For example, exiting
1777 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1778 the mailer to return the item undelivered, but that's not true everywhere.
1780 Don't use C<exit> to abort a subroutine if there's any chance that
1781 someone might want to trap whatever error happened. Use C<die> instead,
1782 which can be trapped by an C<eval>.
1784 The exit() function does not always exit immediately. It calls any
1785 defined C<END> routines first, but these C<END> routines may not
1786 themselves abort the exit. Likewise any object destructors that need to
1787 be called are called before the real exit. If this is a problem, you
1788 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1789 See L<perlmod> for details.
1792 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1796 Returns I<e> (the natural logarithm base) to the power of EXPR.
1797 If EXPR is omitted, gives C<exp($_)>.
1799 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1802 Implements the fcntl(2) function. You'll probably have to say
1806 first to get the correct constant definitions. Argument processing and
1807 value return works just like C<ioctl> below.
1811 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1812 or die "can't fcntl F_GETFL: $!";
1814 You don't have to check for C<defined> on the return from C<fcntl>.
1815 Like C<ioctl>, it maps a C<0> return from the system call into
1816 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1817 in numeric context. It is also exempt from the normal B<-w> warnings
1818 on improper numeric conversions.
1820 Note that C<fcntl> will produce a fatal error if used on a machine that
1821 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1822 manpage to learn what functions are available on your system.
1824 Here's an example of setting a filehandle named C<REMOTE> to be
1825 non-blocking at the system level. You'll have to negotiate C<$|>
1826 on your own, though.
1828 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1830 $flags = fcntl(REMOTE, F_GETFL, 0)
1831 or die "Can't get flags for the socket: $!\n";
1833 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1834 or die "Can't set flags for the socket: $!\n";
1836 =item fileno FILEHANDLE
1839 Returns the file descriptor for a filehandle, or undefined if the
1840 filehandle is not open. This is mainly useful for constructing
1841 bitmaps for C<select> and low-level POSIX tty-handling operations.
1842 If FILEHANDLE is an expression, the value is taken as an indirect
1843 filehandle, generally its name.
1845 You can use this to find out whether two handles refer to the
1846 same underlying descriptor:
1848 if (fileno(THIS) == fileno(THAT)) {
1849 print "THIS and THAT are dups\n";
1852 (Filehandles connected to memory objects via new features of C<open> may
1853 return undefined even though they are open.)
1856 =item flock FILEHANDLE,OPERATION
1857 X<flock> X<lock> X<locking>
1859 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1860 for success, false on failure. Produces a fatal error if used on a
1861 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1862 C<flock> is Perl's portable file locking interface, although it locks
1863 only entire files, not records.
1865 Two potentially non-obvious but traditional C<flock> semantics are
1866 that it waits indefinitely until the lock is granted, and that its locks
1867 B<merely advisory>. Such discretionary locks are more flexible, but offer
1868 fewer guarantees. This means that programs that do not also use C<flock>
1869 may modify files locked with C<flock>. See L<perlport>,
1870 your port's specific documentation, or your system-specific local manpages
1871 for details. It's best to assume traditional behavior if you're writing
1872 portable programs. (But if you're not, you should as always feel perfectly
1873 free to write for your own system's idiosyncrasies (sometimes called
1874 "features"). Slavish adherence to portability concerns shouldn't get
1875 in the way of your getting your job done.)
1877 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1878 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1879 you can use the symbolic names if you import them from the Fcntl module,
1880 either individually, or as a group using the ':flock' tag. LOCK_SH
1881 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1882 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1883 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1884 waiting for the lock (check the return status to see if you got it).
1886 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1887 before locking or unlocking it.
1889 Note that the emulation built with lockf(3) doesn't provide shared
1890 locks, and it requires that FILEHANDLE be open with write intent. These
1891 are the semantics that lockf(3) implements. Most if not all systems
1892 implement lockf(3) in terms of fcntl(2) locking, though, so the
1893 differing semantics shouldn't bite too many people.
1895 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1896 be open with read intent to use LOCK_SH and requires that it be open
1897 with write intent to use LOCK_EX.
1899 Note also that some versions of C<flock> cannot lock things over the
1900 network; you would need to use the more system-specific C<fcntl> for
1901 that. If you like you can force Perl to ignore your system's flock(2)
1902 function, and so provide its own fcntl(2)-based emulation, by passing
1903 the switch C<-Ud_flock> to the F<Configure> program when you configure
1906 Here's a mailbox appender for BSD systems.
1908 use Fcntl ':flock'; # import LOCK_* constants
1911 flock(MBOX,LOCK_EX);
1912 # and, in case someone appended
1913 # while we were waiting...
1918 flock(MBOX,LOCK_UN);
1921 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1922 or die "Can't open mailbox: $!";
1925 print MBOX $msg,"\n\n";
1928 On systems that support a real flock(), locks are inherited across fork()
1929 calls, whereas those that must resort to the more capricious fcntl()
1930 function lose the locks, making it harder to write servers.
1932 See also L<DB_File> for other flock() examples.
1935 X<fork> X<child> X<parent>
1937 Does a fork(2) system call to create a new process running the
1938 same program at the same point. It returns the child pid to the
1939 parent process, C<0> to the child process, or C<undef> if the fork is
1940 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1941 are shared, while everything else is copied. On most systems supporting
1942 fork(), great care has gone into making it extremely efficient (for
1943 example, using copy-on-write technology on data pages), making it the
1944 dominant paradigm for multitasking over the last few decades.
1946 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1947 output before forking the child process, but this may not be supported
1948 on some platforms (see L<perlport>). To be safe, you may need to set
1949 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1950 C<IO::Handle> on any open handles in order to avoid duplicate output.
1952 If you C<fork> without ever waiting on your children, you will
1953 accumulate zombies. On some systems, you can avoid this by setting
1954 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1955 forking and reaping moribund children.
1957 Note that if your forked child inherits system file descriptors like
1958 STDIN and STDOUT that are actually connected by a pipe or socket, even
1959 if you exit, then the remote server (such as, say, a CGI script or a
1960 backgrounded job launched from a remote shell) won't think you're done.
1961 You should reopen those to F</dev/null> if it's any issue.
1966 Declare a picture format for use by the C<write> function. For
1970 Test: @<<<<<<<< @||||| @>>>>>
1971 $str, $%, '$' . int($num)
1975 $num = $cost/$quantity;
1979 See L<perlform> for many details and examples.
1981 =item formline PICTURE,LIST
1984 This is an internal function used by C<format>s, though you may call it,
1985 too. It formats (see L<perlform>) a list of values according to the
1986 contents of PICTURE, placing the output into the format output
1987 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1988 Eventually, when a C<write> is done, the contents of
1989 C<$^A> are written to some filehandle. You could also read C<$^A>
1990 and then set C<$^A> back to C<"">. Note that a format typically
1991 does one C<formline> per line of form, but the C<formline> function itself
1992 doesn't care how many newlines are embedded in the PICTURE. This means
1993 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1994 You may therefore need to use multiple formlines to implement a single
1995 record format, just like the format compiler.
1997 Be careful if you put double quotes around the picture, because an C<@>
1998 character may be taken to mean the beginning of an array name.
1999 C<formline> always returns true. See L<perlform> for other examples.
2001 =item getc FILEHANDLE
2002 X<getc> X<getchar> X<character> X<file, read>
2006 Returns the next character from the input file attached to FILEHANDLE,
2007 or the undefined value at end of file, or if there was an error (in
2008 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2009 STDIN. This is not particularly efficient. However, it cannot be
2010 used by itself to fetch single characters without waiting for the user
2011 to hit enter. For that, try something more like:
2014 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2017 system "stty", '-icanon', 'eol', "\001";
2023 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2026 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2030 Determination of whether $BSD_STYLE should be set
2031 is left as an exercise to the reader.
2033 The C<POSIX::getattr> function can do this more portably on
2034 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2035 module from your nearest CPAN site; details on CPAN can be found on
2039 X<getlogin> X<login>
2041 This implements the C library function of the same name, which on most
2042 systems returns the current login from F</etc/utmp>, if any. If null,
2045 $login = getlogin || getpwuid($<) || "Kilroy";
2047 Do not consider C<getlogin> for authentication: it is not as
2048 secure as C<getpwuid>.
2050 =item getpeername SOCKET
2051 X<getpeername> X<peer>
2053 Returns the packed sockaddr address of other end of the SOCKET connection.
2056 $hersockaddr = getpeername(SOCK);
2057 ($port, $iaddr) = sockaddr_in($hersockaddr);
2058 $herhostname = gethostbyaddr($iaddr, AF_INET);
2059 $herstraddr = inet_ntoa($iaddr);
2064 Returns the current process group for the specified PID. Use
2065 a PID of C<0> to get the current process group for the
2066 current process. Will raise an exception if used on a machine that
2067 doesn't implement getpgrp(2). If PID is omitted, returns process
2068 group of current process. Note that the POSIX version of C<getpgrp>
2069 does not accept a PID argument, so only C<PID==0> is truly portable.
2072 X<getppid> X<parent> X<pid>
2074 Returns the process id of the parent process.
2076 Note for Linux users: on Linux, the C functions C<getpid()> and
2077 C<getppid()> return different values from different threads. In order to
2078 be portable, this behavior is not reflected by the perl-level function
2079 C<getppid()>, that returns a consistent value across threads. If you want
2080 to call the underlying C<getppid()>, you may use the CPAN module
2083 =item getpriority WHICH,WHO
2084 X<getpriority> X<priority> X<nice>
2086 Returns the current priority for a process, a process group, or a user.
2087 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2088 machine that doesn't implement getpriority(2).
2091 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2092 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2093 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2094 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2095 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2096 X<endnetent> X<endprotoent> X<endservent>
2100 =item gethostbyname NAME
2102 =item getnetbyname NAME
2104 =item getprotobyname NAME
2110 =item getservbyname NAME,PROTO
2112 =item gethostbyaddr ADDR,ADDRTYPE
2114 =item getnetbyaddr ADDR,ADDRTYPE
2116 =item getprotobynumber NUMBER
2118 =item getservbyport PORT,PROTO
2136 =item sethostent STAYOPEN
2138 =item setnetent STAYOPEN
2140 =item setprotoent STAYOPEN
2142 =item setservent STAYOPEN
2156 These routines perform the same functions as their counterparts in the
2157 system library. In list context, the return values from the
2158 various get routines are as follows:
2160 ($name,$passwd,$uid,$gid,
2161 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2162 ($name,$passwd,$gid,$members) = getgr*
2163 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2164 ($name,$aliases,$addrtype,$net) = getnet*
2165 ($name,$aliases,$proto) = getproto*
2166 ($name,$aliases,$port,$proto) = getserv*
2168 (If the entry doesn't exist you get a null list.)
2170 The exact meaning of the $gcos field varies but it usually contains
2171 the real name of the user (as opposed to the login name) and other
2172 information pertaining to the user. Beware, however, that in many
2173 system users are able to change this information and therefore it
2174 cannot be trusted and therefore the $gcos is tainted (see
2175 L<perlsec>). The $passwd and $shell, user's encrypted password and
2176 login shell, are also tainted, because of the same reason.
2178 In scalar context, you get the name, unless the function was a
2179 lookup by name, in which case you get the other thing, whatever it is.
2180 (If the entry doesn't exist you get the undefined value.) For example:
2182 $uid = getpwnam($name);
2183 $name = getpwuid($num);
2185 $gid = getgrnam($name);
2186 $name = getgrgid($num);
2190 In I<getpw*()> the fields $quota, $comment, and $expire are special
2191 cases in the sense that in many systems they are unsupported. If the
2192 $quota is unsupported, it is an empty scalar. If it is supported, it
2193 usually encodes the disk quota. If the $comment field is unsupported,
2194 it is an empty scalar. If it is supported it usually encodes some
2195 administrative comment about the user. In some systems the $quota
2196 field may be $change or $age, fields that have to do with password
2197 aging. In some systems the $comment field may be $class. The $expire
2198 field, if present, encodes the expiration period of the account or the
2199 password. For the availability and the exact meaning of these fields
2200 in your system, please consult your getpwnam(3) documentation and your
2201 F<pwd.h> file. You can also find out from within Perl what your
2202 $quota and $comment fields mean and whether you have the $expire field
2203 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2204 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2205 files are only supported if your vendor has implemented them in the
2206 intuitive fashion that calling the regular C library routines gets the
2207 shadow versions if you're running under privilege or if there exists
2208 the shadow(3) functions as found in System V (this includes Solaris
2209 and Linux.) Those systems that implement a proprietary shadow password
2210 facility are unlikely to be supported.
2212 The $members value returned by I<getgr*()> is a space separated list of
2213 the login names of the members of the group.
2215 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2216 C, it will be returned to you via C<$?> if the function call fails. The
2217 C<@addrs> value returned by a successful call is a list of the raw
2218 addresses returned by the corresponding system library call. In the
2219 Internet domain, each address is four bytes long and you can unpack it
2220 by saying something like:
2222 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2224 The Socket library makes this slightly easier:
2227 $iaddr = inet_aton("127.1"); # or whatever address
2228 $name = gethostbyaddr($iaddr, AF_INET);
2230 # or going the other way
2231 $straddr = inet_ntoa($iaddr);
2233 If you get tired of remembering which element of the return list
2234 contains which return value, by-name interfaces are provided
2235 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2236 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2237 and C<User::grent>. These override the normal built-ins, supplying
2238 versions that return objects with the appropriate names
2239 for each field. For example:
2243 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2245 Even though it looks like they're the same method calls (uid),
2246 they aren't, because a C<File::stat> object is different from
2247 a C<User::pwent> object.
2249 =item getsockname SOCKET
2252 Returns the packed sockaddr address of this end of the SOCKET connection,
2253 in case you don't know the address because you have several different
2254 IPs that the connection might have come in on.
2257 $mysockaddr = getsockname(SOCK);
2258 ($port, $myaddr) = sockaddr_in($mysockaddr);
2259 printf "Connect to %s [%s]\n",
2260 scalar gethostbyaddr($myaddr, AF_INET),
2263 =item getsockopt SOCKET,LEVEL,OPTNAME
2266 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2267 Options may exist at multiple protocol levels depending on the socket
2268 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2269 C<Socket> module) will exist. To query options at another level the
2270 protocol number of the appropriate protocol controlling the option
2271 should be supplied. For example, to indicate that an option is to be
2272 interpreted by the TCP protocol, LEVEL should be set to the protocol
2273 number of TCP, which you can get using getprotobyname.
2275 The call returns a packed string representing the requested socket option,
2276 or C<undef> if there is an error (the error reason will be in $!). What
2277 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2278 your system documentation for details. A very common case however is that
2279 the option is an integer, in which case the result will be a packed
2280 integer which you can decode using unpack with the C<i> (or C<I>) format.
2282 An example testing if Nagle's algorithm is turned on on a socket:
2284 use Socket qw(:all);
2286 defined(my $tcp = getprotobyname("tcp"))
2287 or die "Could not determine the protocol number for tcp";
2288 # my $tcp = IPPROTO_TCP; # Alternative
2289 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2290 or die "Could not query TCP_NODELAY socket option: $!";
2291 my $nodelay = unpack("I", $packed);
2292 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2296 X<glob> X<wildcard> X<filename, expansion> X<expand>
2300 In list context, returns a (possibly empty) list of filename expansions on
2301 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2302 scalar context, glob iterates through such filename expansions, returning
2303 undef when the list is exhausted. This is the internal function
2304 implementing the C<< <*.c> >> operator, but you can use it directly. If
2305 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2306 more detail in L<perlop/"I/O Operators">.
2308 Beginning with v5.6.0, this operator is implemented using the standard
2309 C<File::Glob> extension. See L<File::Glob> for details.
2312 X<gmtime> X<UTC> X<Greenwich>
2316 Works just like L<localtime> but the returned values are
2317 localized for the standard Greenwich time zone.
2319 Note: when called in list context, $isdst, the last value
2320 returned by gmtime is always C<0>. There is no
2321 Daylight Saving Time in GMT.
2323 See L<perlport/gmtime> for portability concerns.
2326 X<goto> X<jump> X<jmp>
2332 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2333 execution there. It may not be used to go into any construct that
2334 requires initialization, such as a subroutine or a C<foreach> loop. It
2335 also can't be used to go into a construct that is optimized away,
2336 or to get out of a block or subroutine given to C<sort>.
2337 It can be used to go almost anywhere else within the dynamic scope,
2338 including out of subroutines, but it's usually better to use some other
2339 construct such as C<last> or C<die>. The author of Perl has never felt the
2340 need to use this form of C<goto> (in Perl, that is--C is another matter).
2341 (The difference being that C does not offer named loops combined with
2342 loop control. Perl does, and this replaces most structured uses of C<goto>
2343 in other languages.)
2345 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2346 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2347 necessarily recommended if you're optimizing for maintainability:
2349 goto ("FOO", "BAR", "GLARCH")[$i];
2351 The C<goto-&NAME> form is quite different from the other forms of
2352 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2353 doesn't have the stigma associated with other gotos. Instead, it
2354 exits the current subroutine (losing any changes set by local()) and
2355 immediately calls in its place the named subroutine using the current
2356 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2357 load another subroutine and then pretend that the other subroutine had
2358 been called in the first place (except that any modifications to C<@_>
2359 in the current subroutine are propagated to the other subroutine.)
2360 After the C<goto>, not even C<caller> will be able to tell that this
2361 routine was called first.
2363 NAME needn't be the name of a subroutine; it can be a scalar variable
2364 containing a code reference, or a block that evaluates to a code
2367 =item grep BLOCK LIST
2370 =item grep EXPR,LIST
2372 This is similar in spirit to, but not the same as, grep(1) and its
2373 relatives. In particular, it is not limited to using regular expressions.
2375 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2376 C<$_> to each element) and returns the list value consisting of those
2377 elements for which the expression evaluated to true. In scalar
2378 context, returns the number of times the expression was true.
2380 @foo = grep(!/^#/, @bar); # weed out comments
2384 @foo = grep {!/^#/} @bar; # weed out comments
2386 Note that C<$_> is an alias to the list value, so it can be used to
2387 modify the elements of the LIST. While this is useful and supported,
2388 it can cause bizarre results if the elements of LIST are not variables.
2389 Similarly, grep returns aliases into the original list, much as a for
2390 loop's index variable aliases the list elements. That is, modifying an
2391 element of a list returned by grep (for example, in a C<foreach>, C<map>
2392 or another C<grep>) actually modifies the element in the original list.
2393 This is usually something to be avoided when writing clear code.
2395 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2396 been declared with C<my $_>) then, in addition to being locally aliased to
2397 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2398 can't be seen from the outside, avoiding any potential side-effects.
2400 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2403 X<hex> X<hexadecimal>
2407 Interprets EXPR as a hex string and returns the corresponding value.
2408 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2409 L</oct>.) If EXPR is omitted, uses C<$_>.
2411 print hex '0xAf'; # prints '175'
2412 print hex 'aF'; # same
2414 Hex strings may only represent integers. Strings that would cause
2415 integer overflow trigger a warning. Leading whitespace is not stripped,
2416 unlike oct(). To present something as hex, look into L</printf>,
2417 L</sprintf>, or L</unpack>.
2422 There is no builtin C<import> function. It is just an ordinary
2423 method (subroutine) defined (or inherited) by modules that wish to export
2424 names to another module. The C<use> function calls the C<import> method
2425 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2427 =item index STR,SUBSTR,POSITION
2428 X<index> X<indexOf> X<InStr>
2430 =item index STR,SUBSTR
2432 The index function searches for one string within another, but without
2433 the wildcard-like behavior of a full regular-expression pattern match.
2434 It returns the position of the first occurrence of SUBSTR in STR at
2435 or after POSITION. If POSITION is omitted, starts searching from the
2436 beginning of the string. POSITION before the beginning of the string
2437 or after its end is treated as if it were the beginning or the end,
2438 respectively. POSITION and the return value are based at C<0> (or whatever
2439 you've set the C<$[> variable to--but don't do that). If the substring
2440 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2443 X<int> X<integer> X<truncate> X<trunc> X<floor>
2447 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2448 You should not use this function for rounding: one because it truncates
2449 towards C<0>, and two because machine representations of floating point
2450 numbers can sometimes produce counterintuitive results. For example,
2451 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2452 because it's really more like -268.99999999999994315658 instead. Usually,
2453 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2454 functions will serve you better than will int().
2456 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2459 Implements the ioctl(2) function. You'll probably first have to say
2461 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2463 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2464 exist or doesn't have the correct definitions you'll have to roll your
2465 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2466 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2467 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2468 written depending on the FUNCTION--a pointer to the string value of SCALAR
2469 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2470 has no string value but does have a numeric value, that value will be
2471 passed rather than a pointer to the string value. To guarantee this to be
2472 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2473 functions may be needed to manipulate the values of structures used by
2476 The return value of C<ioctl> (and C<fcntl>) is as follows:
2478 if OS returns: then Perl returns:
2480 0 string "0 but true"
2481 anything else that number
2483 Thus Perl returns true on success and false on failure, yet you can
2484 still easily determine the actual value returned by the operating
2487 $retval = ioctl(...) || -1;
2488 printf "System returned %d\n", $retval;
2490 The special string C<"0 but true"> is exempt from B<-w> complaints
2491 about improper numeric conversions.
2493 =item join EXPR,LIST
2496 Joins the separate strings of LIST into a single string with fields
2497 separated by the value of EXPR, and returns that new string. Example:
2499 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2501 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2502 first argument. Compare L</split>.
2507 Returns a list consisting of all the keys of the named hash.
2508 (In scalar context, returns the number of keys.)
2510 The keys are returned in an apparently random order. The actual
2511 random order is subject to change in future versions of perl, but it
2512 is guaranteed to be the same order as either the C<values> or C<each>
2513 function produces (given that the hash has not been modified). Since
2514 Perl 5.8.1 the ordering is different even between different runs of
2515 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2518 As a side effect, calling keys() resets the HASH's internal iterator
2519 (see L</each>). In particular, calling keys() in void context resets
2520 the iterator with no other overhead.
2522 Here is yet another way to print your environment:
2525 @values = values %ENV;
2527 print pop(@keys), '=', pop(@values), "\n";
2530 or how about sorted by key:
2532 foreach $key (sort(keys %ENV)) {
2533 print $key, '=', $ENV{$key}, "\n";
2536 The returned values are copies of the original keys in the hash, so
2537 modifying them will not affect the original hash. Compare L</values>.
2539 To sort a hash by value, you'll need to use a C<sort> function.
2540 Here's a descending numeric sort of a hash by its values:
2542 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2543 printf "%4d %s\n", $hash{$key}, $key;
2546 As an lvalue C<keys> allows you to increase the number of hash buckets
2547 allocated for the given hash. This can gain you a measure of efficiency if
2548 you know the hash is going to get big. (This is similar to pre-extending
2549 an array by assigning a larger number to $#array.) If you say
2553 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2554 in fact, since it rounds up to the next power of two. These
2555 buckets will be retained even if you do C<%hash = ()>, use C<undef
2556 %hash> if you want to free the storage while C<%hash> is still in scope.
2557 You can't shrink the number of buckets allocated for the hash using
2558 C<keys> in this way (but you needn't worry about doing this by accident,
2559 as trying has no effect).
2561 See also C<each>, C<values> and C<sort>.
2563 =item kill SIGNAL, LIST
2566 Sends a signal to a list of processes. Returns the number of
2567 processes successfully signaled (which is not necessarily the
2568 same as the number actually killed).
2570 $cnt = kill 1, $child1, $child2;
2573 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2574 system call will check whether it's possible to send a signal to it (that
2575 means, to be brief, that the process is owned by the same user, or we are
2576 the super-user). This is a useful way to check that a child process is
2577 alive (even if only as a zombie) and hasn't changed its UID. See
2578 L<perlport> for notes on the portability of this construct.
2580 Unlike in the shell, if SIGNAL is negative, it kills
2581 process groups instead of processes. (On System V, a negative I<PROCESS>
2582 number will also kill process groups, but that's not portable.) That
2583 means you usually want to use positive not negative signals. You may also
2584 use a signal name in quotes.
2586 See L<perlipc/"Signals"> for more details.
2593 The C<last> command is like the C<break> statement in C (as used in
2594 loops); it immediately exits the loop in question. If the LABEL is
2595 omitted, the command refers to the innermost enclosing loop. The
2596 C<continue> block, if any, is not executed:
2598 LINE: while (<STDIN>) {
2599 last LINE if /^$/; # exit when done with header
2603 C<last> cannot be used to exit a block which returns a value such as
2604 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2605 a grep() or map() operation.
2607 Note that a block by itself is semantically identical to a loop
2608 that executes once. Thus C<last> can be used to effect an early
2609 exit out of such a block.
2611 See also L</continue> for an illustration of how C<last>, C<next>, and
2619 Returns a lowercased version of EXPR. This is the internal function
2620 implementing the C<\L> escape in double-quoted strings. Respects
2621 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2622 and L<perlunicode> for more details about locale and Unicode support.
2624 If EXPR is omitted, uses C<$_>.
2627 X<lcfirst> X<lowercase>
2631 Returns the value of EXPR with the first character lowercased. This
2632 is the internal function implementing the C<\l> escape in
2633 double-quoted strings. Respects current LC_CTYPE locale if C<use
2634 locale> in force. See L<perllocale> and L<perlunicode> for more
2635 details about locale and Unicode support.
2637 If EXPR is omitted, uses C<$_>.
2644 Returns the length in I<characters> of the value of EXPR. If EXPR is
2645 omitted, returns length of C<$_>. Note that this cannot be used on
2646 an entire array or hash to find out how many elements these have.
2647 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2649 Note the I<characters>: if the EXPR is in Unicode, you will get the
2650 number of characters, not the number of bytes. To get the length
2651 of the internal string in bytes, use C<bytes::length(EXPR)>, see
2652 L<bytes>. Note that the internal encoding is variable, and the number
2653 of bytes usually meaningless. To get the number of bytes that the
2654 string would have when encoded as UTF-8, use
2655 C<length(Encoding::encode_utf8(EXPR))>.
2657 =item link OLDFILE,NEWFILE
2660 Creates a new filename linked to the old filename. Returns true for
2661 success, false otherwise.
2663 =item listen SOCKET,QUEUESIZE
2666 Does the same thing that the listen system call does. Returns true if
2667 it succeeded, false otherwise. See the example in
2668 L<perlipc/"Sockets: Client/Server Communication">.
2673 You really probably want to be using C<my> instead, because C<local> isn't
2674 what most people think of as "local". See
2675 L<perlsub/"Private Variables via my()"> for details.
2677 A local modifies the listed variables to be local to the enclosing
2678 block, file, or eval. If more than one value is listed, the list must
2679 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2680 for details, including issues with tied arrays and hashes.
2682 =item localtime EXPR
2683 X<localtime> X<ctime>
2687 Converts a time as returned by the time function to a 9-element list
2688 with the time analyzed for the local time zone. Typically used as
2692 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2695 All list elements are numeric, and come straight out of the C `struct
2696 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2697 of the specified time.
2699 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2700 the range C<0..11> with 0 indicating January and 11 indicating December.
2701 This makes it easy to get a month name from a list:
2703 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2704 print "$abbr[$mon] $mday";
2705 # $mon=9, $mday=18 gives "Oct 18"
2707 C<$year> is the number of years since 1900, not just the last two digits
2708 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2709 to get a complete 4-digit year is simply:
2713 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2714 to do that, would you?
2716 To get the last two digits of the year (e.g., '01' in 2001) do:
2718 $year = sprintf("%02d", $year % 100);
2720 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2721 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2722 (or C<0..365> in leap years.)
2724 C<$isdst> is true if the specified time occurs during Daylight Saving
2725 Time, false otherwise.
2727 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2729 In scalar context, C<localtime()> returns the ctime(3) value:
2731 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2733 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2734 instead of local time use the L</gmtime> builtin. See also the
2735 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2736 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2737 and mktime(3) functions.
2739 To get somewhat similar but locale dependent date strings, set up your
2740 locale environment variables appropriately (please see L<perllocale>) and
2743 use POSIX qw(strftime);
2744 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2745 # or for GMT formatted appropriately for your locale:
2746 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2748 Note that the C<%a> and C<%b>, the short forms of the day of the week
2749 and the month of the year, may not necessarily be three characters wide.
2751 See L<perlport/localtime> for portability concerns.
2753 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2754 by-name access mechanism to the gmtime() and localtime() functions,
2757 For a comprehensive date and time representation look at the
2758 L<DateTime> module on CPAN.
2763 This function places an advisory lock on a shared variable, or referenced
2764 object contained in I<THING> until the lock goes out of scope.
2766 lock() is a "weak keyword" : this means that if you've defined a function
2767 by this name (before any calls to it), that function will be called
2768 instead. (However, if you've said C<use threads>, lock() is always a
2769 keyword.) See L<threads>.
2772 X<log> X<logarithm> X<e> X<ln> X<base>
2776 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2777 returns log of C<$_>. To get the log of another base, use basic algebra:
2778 The base-N log of a number is equal to the natural log of that number
2779 divided by the natural log of N. For example:
2783 return log($n)/log(10);
2786 See also L</exp> for the inverse operation.
2793 Does the same thing as the C<stat> function (including setting the
2794 special C<_> filehandle) but stats a symbolic link instead of the file
2795 the symbolic link points to. If symbolic links are unimplemented on
2796 your system, a normal C<stat> is done. For much more detailed
2797 information, please see the documentation for C<stat>.
2799 If EXPR is omitted, stats C<$_>.
2803 The match operator. See L<perlop>.
2805 =item map BLOCK LIST
2810 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2811 C<$_> to each element) and returns the list value composed of the
2812 results of each such evaluation. In scalar context, returns the
2813 total number of elements so generated. Evaluates BLOCK or EXPR in
2814 list context, so each element of LIST may produce zero, one, or
2815 more elements in the returned value.
2817 @chars = map(chr, @nums);
2819 translates a list of numbers to the corresponding characters. And
2821 %hash = map { get_a_key_for($_) => $_ } @array;
2823 is just a funny way to write
2827 $hash{get_a_key_for($_)} = $_;
2830 Note that C<$_> is an alias to the list value, so it can be used to
2831 modify the elements of the LIST. While this is useful and supported,
2832 it can cause bizarre results if the elements of LIST are not variables.
2833 Using a regular C<foreach> loop for this purpose would be clearer in
2834 most cases. See also L</grep> for an array composed of those items of
2835 the original list for which the BLOCK or EXPR evaluates to true.
2837 If C<$_> is lexical in the scope where the C<map> appears (because it has
2838 been declared with C<my $_>), then, in addition to being locally aliased to
2839 the list elements, C<$_> keeps being lexical inside the block; that is, it
2840 can't be seen from the outside, avoiding any potential side-effects.
2842 C<{> starts both hash references and blocks, so C<map { ...> could be either
2843 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2844 ahead for the closing C<}> it has to take a guess at which its dealing with
2845 based what it finds just after the C<{>. Usually it gets it right, but if it
2846 doesn't it won't realize something is wrong until it gets to the C<}> and
2847 encounters the missing (or unexpected) comma. The syntax error will be
2848 reported close to the C<}> but you'll need to change something near the C<{>
2849 such as using a unary C<+> to give perl some help:
2851 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2852 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2853 %hash = map { ("\L$_", 1) } @array # this also works
2854 %hash = map { lc($_), 1 } @array # as does this.
2855 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2857 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2859 or to force an anon hash constructor use C<+{>:
2861 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2863 and you get list of anonymous hashes each with only 1 entry.
2865 =item mkdir FILENAME,MASK
2866 X<mkdir> X<md> X<directory, create>
2868 =item mkdir FILENAME
2872 Creates the directory specified by FILENAME, with permissions
2873 specified by MASK (as modified by C<umask>). If it succeeds it
2874 returns true, otherwise it returns false and sets C<$!> (errno).
2875 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2878 In general, it is better to create directories with permissive MASK,
2879 and let the user modify that with their C<umask>, than it is to supply
2880 a restrictive MASK and give the user no way to be more permissive.
2881 The exceptions to this rule are when the file or directory should be
2882 kept private (mail files, for instance). The perlfunc(1) entry on
2883 C<umask> discusses the choice of MASK in more detail.
2885 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2886 number of trailing slashes. Some operating and filesystems do not get
2887 this right, so Perl automatically removes all trailing slashes to keep
2890 In order to recursively create a directory structure look at
2891 the C<mkpath> function of the L<File::Path> module.
2893 =item msgctl ID,CMD,ARG
2896 Calls the System V IPC function msgctl(2). You'll probably have to say
2900 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2901 then ARG must be a variable that will hold the returned C<msqid_ds>
2902 structure. Returns like C<ioctl>: the undefined value for error,
2903 C<"0 but true"> for zero, or the actual return value otherwise. See also
2904 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2906 =item msgget KEY,FLAGS
2909 Calls the System V IPC function msgget(2). Returns the message queue
2910 id, or the undefined value if there is an error. See also
2911 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2913 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2916 Calls the System V IPC function msgrcv to receive a message from
2917 message queue ID into variable VAR with a maximum message size of
2918 SIZE. Note that when a message is received, the message type as a
2919 native long integer will be the first thing in VAR, followed by the
2920 actual message. This packing may be opened with C<unpack("l! a*")>.
2921 Taints the variable. Returns true if successful, or false if there is
2922 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2923 C<IPC::SysV::Msg> documentation.
2925 =item msgsnd ID,MSG,FLAGS
2928 Calls the System V IPC function msgsnd to send the message MSG to the
2929 message queue ID. MSG must begin with the native long integer message
2930 type, and be followed by the length of the actual message, and finally
2931 the message itself. This kind of packing can be achieved with
2932 C<pack("l! a*", $type, $message)>. Returns true if successful,
2933 or false if there is an error. See also C<IPC::SysV>
2934 and C<IPC::SysV::Msg> documentation.
2941 =item my EXPR : ATTRS
2943 =item my TYPE EXPR : ATTRS
2945 A C<my> declares the listed variables to be local (lexically) to the
2946 enclosing block, file, or C<eval>. If more than one value is listed,
2947 the list must be placed in parentheses.
2949 The exact semantics and interface of TYPE and ATTRS are still
2950 evolving. TYPE is currently bound to the use of C<fields> pragma,
2951 and attributes are handled using the C<attributes> pragma, or starting
2952 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2953 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2954 L<attributes>, and L<Attribute::Handlers>.
2961 The C<next> command is like the C<continue> statement in C; it starts
2962 the next iteration of the loop:
2964 LINE: while (<STDIN>) {
2965 next LINE if /^#/; # discard comments
2969 Note that if there were a C<continue> block on the above, it would get
2970 executed even on discarded lines. If the LABEL is omitted, the command
2971 refers to the innermost enclosing loop.
2973 C<next> cannot be used to exit a block which returns a value such as
2974 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2975 a grep() or map() operation.
2977 Note that a block by itself is semantically identical to a loop
2978 that executes once. Thus C<next> will exit such a block early.
2980 See also L</continue> for an illustration of how C<last>, C<next>, and
2983 =item no Module VERSION LIST
2986 =item no Module VERSION
2988 =item no Module LIST
2994 See the C<use> function, of which C<no> is the opposite.
2997 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3001 Interprets EXPR as an octal string and returns the corresponding
3002 value. (If EXPR happens to start off with C<0x>, interprets it as a
3003 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3004 binary string. Leading whitespace is ignored in all three cases.)
3005 The following will handle decimal, binary, octal, and hex in the standard
3008 $val = oct($val) if $val =~ /^0/;
3010 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3011 in octal), use sprintf() or printf():
3013 $perms = (stat("filename"))[2] & 07777;
3014 $oct_perms = sprintf "%lo", $perms;
3016 The oct() function is commonly used when a string such as C<644> needs
3017 to be converted into a file mode, for example. (Although perl will
3018 automatically convert strings into numbers as needed, this automatic
3019 conversion assumes base 10.)
3021 =item open FILEHANDLE,EXPR
3022 X<open> X<pipe> X<file, open> X<fopen>
3024 =item open FILEHANDLE,MODE,EXPR
3026 =item open FILEHANDLE,MODE,EXPR,LIST
3028 =item open FILEHANDLE,MODE,REFERENCE
3030 =item open FILEHANDLE
3032 Opens the file whose filename is given by EXPR, and associates it with
3035 (The following is a comprehensive reference to open(): for a gentler
3036 introduction you may consider L<perlopentut>.)
3038 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3039 the variable is assigned a reference to a new anonymous filehandle,
3040 otherwise if FILEHANDLE is an expression, its value is used as the name of
3041 the real filehandle wanted. (This is considered a symbolic reference, so
3042 C<use strict 'refs'> should I<not> be in effect.)
3044 If EXPR is omitted, the scalar variable of the same name as the
3045 FILEHANDLE contains the filename. (Note that lexical variables--those
3046 declared with C<my>--will not work for this purpose; so if you're
3047 using C<my>, specify EXPR in your call to open.)
3049 If three or more arguments are specified then the mode of opening and
3050 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3051 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3052 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3053 the file is opened for appending, again being created if necessary.
3055 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3056 indicate that you want both read and write access to the file; thus
3057 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3058 '+>' >> mode would clobber the file first. You can't usually use
3059 either read-write mode for updating textfiles, since they have
3060 variable length records. See the B<-i> switch in L<perlrun> for a
3061 better approach. The file is created with permissions of C<0666>
3062 modified by the process' C<umask> value.
3064 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3065 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3067 In the 2-arguments (and 1-argument) form of the call the mode and
3068 filename should be concatenated (in this order), possibly separated by
3069 spaces. It is possible to omit the mode in these forms if the mode is
3072 If the filename begins with C<'|'>, the filename is interpreted as a
3073 command to which output is to be piped, and if the filename ends with a
3074 C<'|'>, the filename is interpreted as a command which pipes output to
3075 us. See L<perlipc/"Using open() for IPC">
3076 for more examples of this. (You are not allowed to C<open> to a command
3077 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3078 and L<perlipc/"Bidirectional Communication with Another Process">
3081 For three or more arguments if MODE is C<'|-'>, the filename is
3082 interpreted as a command to which output is to be piped, and if MODE
3083 is C<'-|'>, the filename is interpreted as a command which pipes
3084 output to us. In the 2-arguments (and 1-argument) form one should
3085 replace dash (C<'-'>) with the command.
3086 See L<perlipc/"Using open() for IPC"> for more examples of this.
3087 (You are not allowed to C<open> to a command that pipes both in I<and>
3088 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3089 L<perlipc/"Bidirectional Communication"> for alternatives.)
3091 In the three-or-more argument form of pipe opens, if LIST is specified
3092 (extra arguments after the command name) then LIST becomes arguments
3093 to the command invoked if the platform supports it. The meaning of
3094 C<open> with more than three arguments for non-pipe modes is not yet
3095 specified. Experimental "layers" may give extra LIST arguments
3098 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3099 and opening C<< '>-' >> opens STDOUT.
3101 You may use the three-argument form of open to specify IO "layers"
3102 (sometimes also referred to as "disciplines") to be applied to the handle
3103 that affect how the input and output are processed (see L<open> and
3104 L<PerlIO> for more details). For example
3106 open(FH, "<:encoding(UTF-8)", "file")
3108 will open the UTF-8 encoded file containing Unicode characters,
3109 see L<perluniintro>. Note that if layers are specified in the
3110 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3111 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3113 Open returns nonzero upon success, the undefined value otherwise. If
3114 the C<open> involved a pipe, the return value happens to be the pid of
3117 If you're running Perl on a system that distinguishes between text
3118 files and binary files, then you should check out L</binmode> for tips
3119 for dealing with this. The key distinction between systems that need
3120 C<binmode> and those that don't is their text file formats. Systems
3121 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3122 character, and which encode that character in C as C<"\n">, do not
3123 need C<binmode>. The rest need it.
3125 When opening a file, it's usually a bad idea to continue normal execution
3126 if the request failed, so C<open> is frequently used in connection with
3127 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3128 where you want to make a nicely formatted error message (but there are
3129 modules that can help with that problem)) you should always check
3130 the return value from opening a file. The infrequent exception is when
3131 working with an unopened filehandle is actually what you want to do.
3133 As a special case the 3-arg form with a read/write mode and the third
3134 argument being C<undef>:
3136 open(TMP, "+>", undef) or die ...
3138 opens a filehandle to an anonymous temporary file. Also using "+<"
3139 works for symmetry, but you really should consider writing something
3140 to the temporary file first. You will need to seek() to do the
3143 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3144 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3145 "in memory" files held in Perl scalars via:
3147 open($fh, '>', \$variable) || ..
3149 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3150 file, you have to close it first:
3153 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3158 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3159 while (<ARTICLE>) {...
3161 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3162 # if the open fails, output is discarded
3164 open(DBASE, '+<', 'dbase.mine') # open for update
3165 or die "Can't open 'dbase.mine' for update: $!";
3167 open(DBASE, '+<dbase.mine') # ditto
3168 or die "Can't open 'dbase.mine' for update: $!";
3170 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3171 or die "Can't start caesar: $!";
3173 open(ARTICLE, "caesar <$article |") # ditto
3174 or die "Can't start caesar: $!";
3176 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3177 or die "Can't start sort: $!";
3180 open(MEMORY,'>', \$var)
3181 or die "Can't open memory file: $!";
3182 print MEMORY "foo!\n"; # output will end up in $var
3184 # process argument list of files along with any includes
3186 foreach $file (@ARGV) {
3187 process($file, 'fh00');
3191 my($filename, $input) = @_;
3192 $input++; # this is a string increment
3193 unless (open($input, $filename)) {
3194 print STDERR "Can't open $filename: $!\n";
3199 while (<$input>) { # note use of indirection
3200 if (/^#include "(.*)"/) {
3201 process($1, $input);
3208 See L<perliol> for detailed info on PerlIO.
3210 You may also, in the Bourne shell tradition, specify an EXPR beginning
3211 with C<< '>&' >>, in which case the rest of the string is interpreted
3212 as the name of a filehandle (or file descriptor, if numeric) to be
3213 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3214 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3215 The mode you specify should match the mode of the original filehandle.
3216 (Duping a filehandle does not take into account any existing contents
3217 of IO buffers.) If you use the 3-arg form then you can pass either a
3218 number, the name of a filehandle or the normal "reference to a glob".
3220 Here is a script that saves, redirects, and restores C<STDOUT> and
3221 C<STDERR> using various methods:
3224 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3225 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3227 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3228 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3230 select STDERR; $| = 1; # make unbuffered
3231 select STDOUT; $| = 1; # make unbuffered
3233 print STDOUT "stdout 1\n"; # this works for
3234 print STDERR "stderr 1\n"; # subprocesses too
3236 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3237 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3239 print STDOUT "stdout 2\n";
3240 print STDERR "stderr 2\n";
3242 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3243 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3244 that file descriptor (and not call L<dup(2)>); this is more
3245 parsimonious of file descriptors. For example:
3247 # open for input, reusing the fileno of $fd
3248 open(FILEHANDLE, "<&=$fd")
3252 open(FILEHANDLE, "<&=", $fd)
3256 # open for append, using the fileno of OLDFH
3257 open(FH, ">>&=", OLDFH)
3261 open(FH, ">>&=OLDFH")
3263 Being parsimonious on filehandles is also useful (besides being
3264 parsimonious) for example when something is dependent on file
3265 descriptors, like for example locking using flock(). If you do just
3266 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3267 descriptor as B, and therefore flock(A) will not flock(B), and vice
3268 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3269 the same file descriptor.
3271 Note that if you are using Perls older than 5.8.0, Perl will be using
3272 the standard C libraries' fdopen() to implement the "=" functionality.
3273 On many UNIX systems fdopen() fails when file descriptors exceed a
3274 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3275 most often the default.
3277 You can see whether Perl has been compiled with PerlIO or not by
3278 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3279 is C<define>, you have PerlIO, otherwise you don't.
3281 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3282 with 2-arguments (or 1-argument) form of open(), then
3283 there is an implicit fork done, and the return value of open is the pid
3284 of the child within the parent process, and C<0> within the child
3285 process. (Use C<defined($pid)> to determine whether the open was successful.)
3286 The filehandle behaves normally for the parent, but i/o to that
3287 filehandle is piped from/to the STDOUT/STDIN of the child process.
3288 In the child process the filehandle isn't opened--i/o happens from/to
3289 the new STDOUT or STDIN. Typically this is used like the normal
3290 piped open when you want to exercise more control over just how the
3291 pipe command gets executed, such as when you are running setuid, and
3292 don't want to have to scan shell commands for metacharacters.
3293 The following triples are more or less equivalent:
3295 open(FOO, "|tr '[a-z]' '[A-Z]'");
3296 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3297 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3298 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3300 open(FOO, "cat -n '$file'|");
3301 open(FOO, '-|', "cat -n '$file'");
3302 open(FOO, '-|') || exec 'cat', '-n', $file;
3303 open(FOO, '-|', "cat", '-n', $file);
3305 The last example in each block shows the pipe as "list form", which is
3306 not yet supported on all platforms. A good rule of thumb is that if
3307 your platform has true C<fork()> (in other words, if your platform is
3308 UNIX) you can use the list form.
3310 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3312 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3313 output before any operation that may do a fork, but this may not be
3314 supported on some platforms (see L<perlport>). To be safe, you may need
3315 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3316 of C<IO::Handle> on any open handles.
3318 On systems that support a close-on-exec flag on files, the flag will
3319 be set for the newly opened file descriptor as determined by the value
3320 of $^F. See L<perlvar/$^F>.
3322 Closing any piped filehandle causes the parent process to wait for the
3323 child to finish, and returns the status value in C<$?> and
3324 C<${^CHILD_ERROR_NATIVE}>.
3326 The filename passed to 2-argument (or 1-argument) form of open() will
3327 have leading and trailing whitespace deleted, and the normal
3328 redirection characters honored. This property, known as "magic open",
3329 can often be used to good effect. A user could specify a filename of
3330 F<"rsh cat file |">, or you could change certain filenames as needed:
3332 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3333 open(FH, $filename) or die "Can't open $filename: $!";
3335 Use 3-argument form to open a file with arbitrary weird characters in it,
3337 open(FOO, '<', $file);
3339 otherwise it's necessary to protect any leading and trailing whitespace:
3341 $file =~ s#^(\s)#./$1#;
3342 open(FOO, "< $file\0");
3344 (this may not work on some bizarre filesystems). One should
3345 conscientiously choose between the I<magic> and 3-arguments form
3350 will allow the user to specify an argument of the form C<"rsh cat file |">,
3351 but will not work on a filename which happens to have a trailing space, while
3353 open IN, '<', $ARGV[0];
3355 will have exactly the opposite restrictions.
3357 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3358 should use the C<sysopen> function, which involves no such magic (but
3359 may use subtly different filemodes than Perl open(), which is mapped
3360 to C fopen()). This is
3361 another way to protect your filenames from interpretation. For example:
3364 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3365 or die "sysopen $path: $!";
3366 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3367 print HANDLE "stuff $$\n";
3369 print "File contains: ", <HANDLE>;
3371 Using the constructor from the C<IO::Handle> package (or one of its
3372 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3373 filehandles that have the scope of whatever variables hold references to
3374 them, and automatically close whenever and however you leave that scope:
3378 sub read_myfile_munged {
3380 my $handle = new IO::File;
3381 open($handle, "myfile") or die "myfile: $!";
3383 or return (); # Automatically closed here.
3384 mung $first or die "mung failed"; # Or here.
3385 return $first, <$handle> if $ALL; # Or here.
3389 See L</seek> for some details about mixing reading and writing.
3391 =item opendir DIRHANDLE,EXPR
3394 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3395 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3396 DIRHANDLE may be an expression whose value can be used as an indirect
3397 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3398 scalar variable (or array or hash element), the variable is assigned a
3399 reference to a new anonymous dirhandle.
3400 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3407 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3408 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3411 For the reverse, see L</chr>.
3412 See L<perlunicode> for more about Unicode.
3419 =item our EXPR : ATTRS
3421 =item our TYPE EXPR : ATTRS
3423 C<our> associates a simple name with a package variable in the current
3424 package for use within the current scope. When C<use strict 'vars'> is in
3425 effect, C<our> lets you use declared global variables without qualifying
3426 them with package names, within the lexical scope of the C<our> declaration.
3427 In this way C<our> differs from C<use vars>, which is package scoped.
3429 Unlike C<my>, which both allocates storage for a variable and associates
3430 a simple name with that storage for use within the current scope, C<our>
3431 associates a simple name with a package variable in the current package,
3432 for use within the current scope. In other words, C<our> has the same
3433 scoping rules as C<my>, but does not necessarily create a
3436 If more than one value is listed, the list must be placed
3442 An C<our> declaration declares a global variable that will be visible
3443 across its entire lexical scope, even across package boundaries. The
3444 package in which the variable is entered is determined at the point
3445 of the declaration, not at the point of use. This means the following
3449 our $bar; # declares $Foo::bar for rest of lexical scope
3453 print $bar; # prints 20, as it refers to $Foo::bar
3455 Multiple C<our> declarations with the same name in the same lexical
3456 scope are allowed if they are in different packages. If they happen
3457 to be in the same package, Perl will emit warnings if you have asked
3458 for them, just like multiple C<my> declarations. Unlike a second
3459 C<my> declaration, which will bind the name to a fresh variable, a
3460 second C<our> declaration in the same package, in the same scope, is
3465 our $bar; # declares $Foo::bar for rest of lexical scope
3469 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3470 print $bar; # prints 30
3472 our $bar; # emits warning but has no other effect
3473 print $bar; # still prints 30
3475 An C<our> declaration may also have a list of attributes associated
3478 The exact semantics and interface of TYPE and ATTRS are still
3479 evolving. TYPE is currently bound to the use of C<fields> pragma,
3480 and attributes are handled using the C<attributes> pragma, or starting
3481 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3482 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3483 L<attributes>, and L<Attribute::Handlers>.
3485 =item pack TEMPLATE,LIST
3488 Takes a LIST of values and converts it into a string using the rules
3489 given by the TEMPLATE. The resulting string is the concatenation of
3490 the converted values. Typically, each converted value looks
3491 like its machine-level representation. For example, on 32-bit machines
3492 an integer may be represented by a sequence of 4 bytes that will be
3493 converted to a sequence of 4 characters.
3495 The TEMPLATE is a sequence of characters that give the order and type
3496 of values, as follows:
3498 a A string with arbitrary binary data, will be null padded.
3499 A A text (ASCII) string, will be space padded.
3500 Z A null terminated (ASCIZ) string, will be null padded.
3502 b A bit string (ascending bit order inside each byte, like vec()).
3503 B A bit string (descending bit order inside each byte).
3504 h A hex string (low nybble first).
3505 H A hex string (high nybble first).
3507 c A signed char (8-bit) value.
3508 C An unsigned char (octet) value.
3509 W An unsigned char value (can be greater than 255).
3511 s A signed short (16-bit) value.
3512 S An unsigned short value.
3514 l A signed long (32-bit) value.
3515 L An unsigned long value.
3517 q A signed quad (64-bit) value.
3518 Q An unsigned quad value.
3519 (Quads are available only if your system supports 64-bit
3520 integer values _and_ if Perl has been compiled to support those.
3521 Causes a fatal error otherwise.)
3523 i A signed integer value.
3524 I A unsigned integer value.
3525 (This 'integer' is _at_least_ 32 bits wide. Its exact
3526 size depends on what a local C compiler calls 'int'.)
3528 n An unsigned short (16-bit) in "network" (big-endian) order.
3529 N An unsigned long (32-bit) in "network" (big-endian) order.
3530 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3531 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3533 j A Perl internal signed integer value (IV).
3534 J A Perl internal unsigned integer value (UV).
3536 f A single-precision float in the native format.
3537 d A double-precision float in the native format.
3539 F A Perl internal floating point value (NV) in the native format
3540 D A long double-precision float in the native format.
3541 (Long doubles are available only if your system supports long
3542 double values _and_ if Perl has been compiled to support those.
3543 Causes a fatal error otherwise.)
3545 p A pointer to a null-terminated string.
3546 P A pointer to a structure (fixed-length string).
3548 u A uuencoded string.
3549 U A Unicode character number. Encodes to a character in character mode
3550 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3552 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3553 details). Its bytes represent an unsigned integer in base 128,
3554 most significant digit first, with as few digits as possible. Bit
3555 eight (the high bit) is set on each byte except the last.
3559 @ Null fill or truncate to absolute position, counted from the
3560 start of the innermost ()-group.
3561 . Null fill or truncate to absolute position specified by value.
3562 ( Start of a ()-group.
3564 One or more of the modifiers below may optionally follow some letters in the
3565 TEMPLATE (the second column lists the letters for which the modifier is
3568 ! sSlLiI Forces native (short, long, int) sizes instead
3569 of fixed (16-/32-bit) sizes.
3571 xX Make x and X act as alignment commands.
3573 nNvV Treat integers as signed instead of unsigned.
3575 @. Specify position as byte offset in the internal
3576 representation of the packed string. Efficient but
3579 > sSiIlLqQ Force big-endian byte-order on the type.
3580 jJfFdDpP (The "big end" touches the construct.)
3582 < sSiIlLqQ Force little-endian byte-order on the type.
3583 jJfFdDpP (The "little end" touches the construct.)
3585 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3586 in which case they force a certain byte-order on all components of
3587 that group, including subgroups.
3589 The following rules apply:
3595 Each letter may optionally be followed by a number giving a repeat
3596 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3597 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3598 that many values from the LIST. A C<*> for the repeat count means to
3599 use however many items are left, except for C<@>, C<x>, C<X>, where it
3600 is equivalent to C<0>, for <.> where it means relative to string start
3601 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3602 A numeric repeat count may optionally be enclosed in brackets, as in
3603 C<pack 'C[80]', @arr>.
3605 One can replace the numeric repeat count by a template enclosed in brackets;
3606 then the packed length of this template in bytes is used as a count.
3607 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3608 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3609 If the template in brackets contains alignment commands (such as C<x![d]>),
3610 its packed length is calculated as if the start of the template has the maximal
3613 When used with C<Z>, C<*> results in the addition of a trailing null
3614 byte (so the packed result will be one longer than the byte C<length>
3617 When used with C<@>, the repeat count represents an offset from the start
3618 of the innermost () group.
3620 When used with C<.>, the repeat count is used to determine the starting
3621 position from where the value offset is calculated. If the repeat count
3622 is 0, it's relative to the current position. If the repeat count is C<*>,
3623 the offset is relative to the start of the packed string. And if its an
3624 integer C<n> the offset is relative to the start of the n-th innermost
3625 () group (or the start of the string if C<n> is bigger then the group
3628 The repeat count for C<u> is interpreted as the maximal number of bytes
3629 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3630 count should not be more than 65.
3634 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3635 string of length count, padding with nulls or spaces as necessary. When
3636 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3637 after the first null, and C<a> returns data verbatim.
3639 If the value-to-pack is too long, it is truncated. If too long and an
3640 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3641 by a null byte. Thus C<Z> always packs a trailing null (except when the
3646 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3647 Each character of the input field of pack() generates 1 bit of the result.
3648 Each result bit is based on the least-significant bit of the corresponding
3649 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3650 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3652 Starting from the beginning of the input string of pack(), each 8-tuple
3653 of characters is converted to 1 character of output. With format C<b>
3654 the first character of the 8-tuple determines the least-significant bit of a
3655 character, and with format C<B> it determines the most-significant bit of
3658 If the length of the input string is not exactly divisible by 8, the
3659 remainder is packed as if the input string were padded by null characters
3660 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3662 If the input string of pack() is longer than needed, extra characters are
3663 ignored. A C<*> for the repeat count of pack() means to use all the
3664 characters of the input field. On unpack()ing the bits are converted to a
3665 string of C<"0">s and C<"1">s.
3669 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3670 representable as hexadecimal digits, 0-9a-f) long.
3672 Each character of the input field of pack() generates 4 bits of the result.
3673 For non-alphabetical characters the result is based on the 4 least-significant
3674 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3675 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3676 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3677 is compatible with the usual hexadecimal digits, so that C<"a"> and
3678 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3679 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3681 Starting from the beginning of the input string of pack(), each pair
3682 of characters is converted to 1 character of output. With format C<h> the
3683 first character of the pair determines the least-significant nybble of the
3684 output character, and with format C<H> it determines the most-significant
3687 If the length of the input string is not even, it behaves as if padded
3688 by a null character at the end. Similarly, during unpack()ing the "extra"
3689 nybbles are ignored.
3691 If the input string of pack() is longer than needed, extra characters are
3693 A C<*> for the repeat count of pack() means to use all the characters of
3694 the input field. On unpack()ing the nybbles are converted to a string
3695 of hexadecimal digits.
3699 The C<p> type packs a pointer to a null-terminated string. You are
3700 responsible for ensuring the string is not a temporary value (which can
3701 potentially get deallocated before you get around to using the packed result).
3702 The C<P> type packs a pointer to a structure of the size indicated by the
3703 length. A NULL pointer is created if the corresponding value for C<p> or
3704 C<P> is C<undef>, similarly for unpack().
3706 If your system has a strange pointer size (i.e. a pointer is neither as
3707 big as an int nor as big as a long), it may not be possible to pack or
3708 unpack pointers in big- or little-endian byte order. Attempting to do
3709 so will result in a fatal error.
3713 The C</> template character allows packing and unpacking of a sequence of
3714 items where the packed structure contains a packed item count followed by
3715 the packed items themselves.
3717 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3718 I<length-item> describes how the length value is packed. The ones likely
3719 to be of most use are integer-packing ones like C<n> (for Java strings),
3720 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3722 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3723 the minimum of that and the number of available items is used as argument
3724 for the I<length-item>. If it has no repeat count or uses a '*', the number
3725 of available items is used.
3727 For C<unpack> an internal stack of integer arguments unpacked so far is
3728 used. You write C</>I<sequence-item> and the repeat count is obtained by
3729 popping off the last element from the stack. The I<sequence-item> must not
3730 have a repeat count.
3732 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3733 the I<length-item> is a string length, not a number of strings. If there is
3734 an explicit repeat count for pack, the packed string will be adjusted to that
3737 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3738 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3739 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3740 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3741 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3743 The I<length-item> is not returned explicitly from C<unpack>.
3745 Adding a count to the I<length-item> letter is unlikely to do anything
3746 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3747 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3748 which Perl does not regard as legal in numeric strings.
3752 The integer types C<s>, C<S>, C<l>, and C<L> may be
3753 followed by a C<!> modifier to signify native shorts or
3754 longs--as you can see from above for example a bare C<l> does mean
3755 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3756 may be larger. This is an issue mainly in 64-bit platforms. You can
3757 see whether using C<!> makes any difference by
3759 print length(pack("s")), " ", length(pack("s!")), "\n";
3760 print length(pack("l")), " ", length(pack("l!")), "\n";
3762 C<i!> and C<I!> also work but only because of completeness;
3763 they are identical to C<i> and C<I>.
3765 The actual sizes (in bytes) of native shorts, ints, longs, and long
3766 longs on the platform where Perl was built are also available via
3770 print $Config{shortsize}, "\n";
3771 print $Config{intsize}, "\n";
3772 print $Config{longsize}, "\n";
3773 print $Config{longlongsize}, "\n";
3775 (The C<$Config{longlongsize}> will be undefined if your system does
3776 not support long longs.)
3780 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3781 are inherently non-portable between processors and operating systems
3782 because they obey the native byteorder and endianness. For example a
3783 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3784 (arranged in and handled by the CPU registers) into bytes as
3786 0x12 0x34 0x56 0x78 # big-endian
3787 0x78 0x56 0x34 0x12 # little-endian
3789 Basically, the Intel and VAX CPUs are little-endian, while everybody
3790 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3791 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3792 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3795 The names `big-endian' and `little-endian' are comic references to
3796 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3797 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3798 the egg-eating habits of the Lilliputians.
3800 Some systems may have even weirder byte orders such as
3805 You can see your system's preference with
3807 print join(" ", map { sprintf "%#02x", $_ }
3808 unpack("W*",pack("L",0x12345678))), "\n";
3810 The byteorder on the platform where Perl was built is also available
3814 print $Config{byteorder}, "\n";
3816 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3817 and C<'87654321'> are big-endian.
3819 If you want portable packed integers you can either use the formats
3820 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3821 modifiers. These modifiers are only available as of perl 5.9.2.
3822 See also L<perlport>.
3826 All integer and floating point formats as well as C<p> and C<P> and
3827 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3828 to force big- or little- endian byte-order, respectively.
3829 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3830 signed integers, 64-bit integers and floating point values. However,
3831 there are some things to keep in mind.
3833 Exchanging signed integers between different platforms only works
3834 if all platforms store them in the same format. Most platforms store
3835 signed integers in two's complement, so usually this is not an issue.
3837 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3838 formats on big- or little-endian machines. Otherwise, attempting to
3839 do so will result in a fatal error.
3841 Forcing big- or little-endian byte-order on floating point values for
3842 data exchange can only work if all platforms are using the same
3843 binary representation (e.g. IEEE floating point format). Even if all
3844 platforms are using IEEE, there may be subtle differences. Being able
3845 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3846 but also very dangerous if you don't know exactly what you're doing.
3847 It is definitely not a general way to portably store floating point
3850 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3851 all types inside the group that accept the byte-order modifiers,
3852 including all subgroups. It will silently be ignored for all other
3853 types. You are not allowed to override the byte-order within a group
3854 that already has a byte-order modifier suffix.
3858 Real numbers (floats and doubles) are in the native machine format only;
3859 due to the multiplicity of floating formats around, and the lack of a
3860 standard "network" representation, no facility for interchange has been
3861 made. This means that packed floating point data written on one machine
3862 may not be readable on another - even if both use IEEE floating point
3863 arithmetic (as the endian-ness of the memory representation is not part
3864 of the IEEE spec). See also L<perlport>.
3866 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3867 modifiers to force big- or little-endian byte-order on floating point values.
3869 Note that Perl uses doubles (or long doubles, if configured) internally for
3870 all numeric calculation, and converting from double into float and thence back
3871 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3872 will not in general equal $foo).
3876 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3877 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3878 where the packed string is processed in its UTF-8-encoded Unicode form on
3879 a byte by byte basis. Character mode is the default unless the format string
3880 starts with an C<U>. You can switch mode at any moment with an explicit
3881 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3882 or until the end of the ()-group in which it was entered.
3886 You must yourself do any alignment or padding by inserting for example
3887 enough C<'x'>es while packing. There is no way to pack() and unpack()
3888 could know where the characters are going to or coming from. Therefore
3889 C<pack> (and C<unpack>) handle their output and input as flat
3890 sequences of characters.
3894 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3895 take a repeat count, both as postfix, and for unpack() also via the C</>
3896 template character. Within each repetition of a group, positioning with
3897 C<@> starts again at 0. Therefore, the result of
3899 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3901 is the string "\0a\0\0bc".
3905 C<x> and C<X> accept C<!> modifier. In this case they act as
3906 alignment commands: they jump forward/back to the closest position
3907 aligned at a multiple of C<count> characters. For example, to pack() or
3908 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3909 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3910 aligned on the double's size.
3912 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3913 both result in no-ops.
3917 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3918 will represent signed 16-/32-bit integers in big-/little-endian order.
3919 This is only portable if all platforms sharing the packed data use the
3920 same binary representation for signed integers (e.g. all platforms are
3921 using two's complement representation).
3925 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3926 White space may be used to separate pack codes from each other, but
3927 modifiers and a repeat count must follow immediately.
3931 If TEMPLATE requires more arguments to pack() than actually given, pack()
3932 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3933 to pack() than actually given, extra arguments are ignored.
3939 $foo = pack("WWWW",65,66,67,68);
3941 $foo = pack("W4",65,66,67,68);
3943 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3944 # same thing with Unicode circled letters.
3945 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3946 # same thing with Unicode circled letters. You don't get the UTF-8
3947 # bytes because the U at the start of the format caused a switch to
3948 # U0-mode, so the UTF-8 bytes get joined into characters
3949 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3950 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3951 # This is the UTF-8 encoding of the string in the previous example
3953 $foo = pack("ccxxcc",65,66,67,68);
3956 # note: the above examples featuring "W" and "c" are true
3957 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3958 # and UTF-8. In EBCDIC the first example would be
3959 # $foo = pack("WWWW",193,194,195,196);
3961 $foo = pack("s2",1,2);
3962 # "\1\0\2\0" on little-endian
3963 # "\0\1\0\2" on big-endian
3965 $foo = pack("a4","abcd","x","y","z");
3968 $foo = pack("aaaa","abcd","x","y","z");
3971 $foo = pack("a14","abcdefg");
3972 # "abcdefg\0\0\0\0\0\0\0"
3974 $foo = pack("i9pl", gmtime);
3975 # a real struct tm (on my system anyway)
3977 $utmp_template = "Z8 Z8 Z16 L";
3978 $utmp = pack($utmp_template, @utmp1);
3979 # a struct utmp (BSDish)
3981 @utmp2 = unpack($utmp_template, $utmp);
3982 # "@utmp1" eq "@utmp2"
3985 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3988 $foo = pack('sx2l', 12, 34);
3989 # short 12, two zero bytes padding, long 34
3990 $bar = pack('s@4l', 12, 34);
3991 # short 12, zero fill to position 4, long 34
3993 $baz = pack('s.l', 12, 4, 34);
3994 # short 12, zero fill to position 4, long 34
3996 $foo = pack('nN', 42, 4711);
3997 # pack big-endian 16- and 32-bit unsigned integers
3998 $foo = pack('S>L>', 42, 4711);
4000 $foo = pack('s<l<', -42, 4711);
4001 # pack little-endian 16- and 32-bit signed integers
4002 $foo = pack('(sl)<', -42, 4711);
4005 The same template may generally also be used in unpack().
4007 =item package NAMESPACE
4008 X<package> X<module> X<namespace>
4012 Declares the compilation unit as being in the given namespace. The scope
4013 of the package declaration is from the declaration itself through the end
4014 of the enclosing block, file, or eval (the same as the C<my> operator).
4015 All further unqualified dynamic identifiers will be in this namespace.
4016 A package statement affects only dynamic variables--including those
4017 you've used C<local> on--but I<not> lexical variables, which are created
4018 with C<my>. Typically it would be the first declaration in a file to
4019 be included by the C<require> or C<use> operator. You can switch into a
4020 package in more than one place; it merely influences which symbol table
4021 is used by the compiler for the rest of that block. You can refer to
4022 variables and filehandles in other packages by prefixing the identifier
4023 with the package name and a double colon: C<$Package::Variable>.
4024 If the package name is null, the C<main> package as assumed. That is,
4025 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4026 still seen in older code).
4028 If NAMESPACE is omitted, then there is no current package, and all
4029 identifiers must be fully qualified or lexicals. However, you are
4030 strongly advised not to make use of this feature. Its use can cause
4031 unexpected behaviour, even crashing some versions of Perl. It is
4032 deprecated, and will be removed from a future release.
4034 See L<perlmod/"Packages"> for more information about packages, modules,
4035 and classes. See L<perlsub> for other scoping issues.
4037 =item pipe READHANDLE,WRITEHANDLE
4040 Opens a pair of connected pipes like the corresponding system call.
4041 Note that if you set up a loop of piped processes, deadlock can occur
4042 unless you are very careful. In addition, note that Perl's pipes use
4043 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4044 after each command, depending on the application.
4046 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4047 for examples of such things.
4049 On systems that support a close-on-exec flag on files, the flag will be set
4050 for the newly opened file descriptors as determined by the value of $^F.
4058 Pops and returns the last value of the array, shortening the array by
4061 If there are no elements in the array, returns the undefined value
4062 (although this may happen at other times as well). If ARRAY is
4063 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4064 array in subroutines, just like C<shift>.
4067 X<pos> X<match, position>
4071 Returns the offset of where the last C<m//g> search left off for the variable
4072 in question (C<$_> is used when the variable is not specified). Note that
4073 0 is a valid match offset. C<undef> indicates that the search position
4074 is reset (usually due to match failure, but can also be because no match has
4075 yet been performed on the scalar). C<pos> directly accesses the location used
4076 by the regexp engine to store the offset, so assigning to C<pos> will change
4077 that offset, and so will also influence the C<\G> zero-width assertion in
4078 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4079 the return from C<pos> won't change either in this case. See L<perlre> and
4082 =item print FILEHANDLE LIST
4089 Prints a string or a list of strings. Returns true if successful.
4090 FILEHANDLE may be a scalar variable name, in which case the variable
4091 contains the name of or a reference to the filehandle, thus introducing
4092 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4093 the next token is a term, it may be misinterpreted as an operator
4094 unless you interpose a C<+> or put parentheses around the arguments.)
4095 If FILEHANDLE is omitted, prints by default to standard output (or
4096 to the last selected output channel--see L</select>). If LIST is
4097 also omitted, prints C<$_> to the currently selected output channel.
4098 To set the default output channel to something other than STDOUT
4099 use the select operation. The current value of C<$,> (if any) is
4100 printed between each LIST item. The current value of C<$\> (if
4101 any) is printed after the entire LIST has been printed. Because
4102 print takes a LIST, anything in the LIST is evaluated in list
4103 context, and any subroutine that you call will have one or more of
4104 its expressions evaluated in list context. Also be careful not to
4105 follow the print keyword with a left parenthesis unless you want
4106 the corresponding right parenthesis to terminate the arguments to
4107 the print--interpose a C<+> or put parentheses around all the
4110 Note that if you're storing FILEHANDLEs in an array, or if you're using
4111 any other expression more complex than a scalar variable to retrieve it,
4112 you will have to use a block returning the filehandle value instead:
4114 print { $files[$i] } "stuff\n";
4115 print { $OK ? STDOUT : STDERR } "stuff\n";
4117 =item printf FILEHANDLE FORMAT, LIST
4120 =item printf FORMAT, LIST
4122 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4123 (the output record separator) is not appended. The first argument
4124 of the list will be interpreted as the C<printf> format. See C<sprintf>
4125 for an explanation of the format argument. If C<use locale> is in effect,
4126 and POSIX::setlocale() has been called, the character used for the decimal
4127 separator in formatted floating point numbers is affected by the LC_NUMERIC
4128 locale. See L<perllocale> and L<POSIX>.
4130 Don't fall into the trap of using a C<printf> when a simple
4131 C<print> would do. The C<print> is more efficient and less
4134 =item prototype FUNCTION
4137 Returns the prototype of a function as a string (or C<undef> if the
4138 function has no prototype). FUNCTION is a reference to, or the name of,
4139 the function whose prototype you want to retrieve.
4141 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4142 name for Perl builtin. If the builtin is not I<overridable> (such as
4143 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4144 C<system>) returns C<undef> because the builtin does not really behave
4145 like a Perl function. Otherwise, the string describing the equivalent
4146 prototype is returned.
4148 =item push ARRAY,LIST
4151 Treats ARRAY as a stack, and pushes the values of LIST
4152 onto the end of ARRAY. The length of ARRAY increases by the length of
4153 LIST. Has the same effect as
4156 $ARRAY[++$#ARRAY] = $value;
4159 but is more efficient. Returns the number of elements in the array following
4160 the completed C<push>.
4172 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4174 =item quotemeta EXPR
4175 X<quotemeta> X<metacharacter>
4179 Returns the value of EXPR with all non-"word"
4180 characters backslashed. (That is, all characters not matching
4181 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4182 returned string, regardless of any locale settings.)
4183 This is the internal function implementing
4184 the C<\Q> escape in double-quoted strings.
4186 If EXPR is omitted, uses C<$_>.
4193 Returns a random fractional number greater than or equal to C<0> and less
4194 than the value of EXPR. (EXPR should be positive.) If EXPR is
4195 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4196 also special-cased as C<1> - this has not been documented before perl 5.8.0
4197 and is subject to change in future versions of perl. Automatically calls
4198 C<srand> unless C<srand> has already been called. See also C<srand>.
4200 Apply C<int()> to the value returned by C<rand()> if you want random
4201 integers instead of random fractional numbers. For example,
4205 returns a random integer between C<0> and C<9>, inclusive.
4207 (Note: If your rand function consistently returns numbers that are too
4208 large or too small, then your version of Perl was probably compiled
4209 with the wrong number of RANDBITS.)
4211 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4212 X<read> X<file, read>
4214 =item read FILEHANDLE,SCALAR,LENGTH
4216 Attempts to read LENGTH I<characters> of data into variable SCALAR
4217 from the specified FILEHANDLE. Returns the number of characters
4218 actually read, C<0> at end of file, or undef if there was an error (in
4219 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4220 so that the last character actually read is the last character of the
4221 scalar after the read.
4223 An OFFSET may be specified to place the read data at some place in the
4224 string other than the beginning. A negative OFFSET specifies
4225 placement at that many characters counting backwards from the end of
4226 the string. A positive OFFSET greater than the length of SCALAR
4227 results in the string being padded to the required size with C<"\0">
4228 bytes before the result of the read is appended.
4230 The call is actually implemented in terms of either Perl's or system's
4231 fread() call. To get a true read(2) system call, see C<sysread>.
4233 Note the I<characters>: depending on the status of the filehandle,
4234 either (8-bit) bytes or characters are read. By default all
4235 filehandles operate on bytes, but for example if the filehandle has
4236 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4237 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4238 characters, not bytes. Similarly for the C<:encoding> pragma:
4239 in that case pretty much any characters can be read.
4241 =item readdir DIRHANDLE
4244 Returns the next directory entry for a directory opened by C<opendir>.
4245 If used in list context, returns all the rest of the entries in the
4246 directory. If there are no more entries, returns an undefined value in
4247 scalar context or a null list in list context.
4249 If you're planning to filetest the return values out of a C<readdir>, you'd
4250 better prepend the directory in question. Otherwise, because we didn't
4251 C<chdir> there, it would have been testing the wrong file.
4253 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4254 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4260 X<readline> X<gets> X<fgets>
4262 Reads from the filehandle whose typeglob is contained in EXPR (or from
4263 *ARGV if EXPR is not provided). In scalar context, each call reads and
4264 returns the next line, until end-of-file is reached, whereupon the
4265 subsequent call returns undef. In list context, reads until end-of-file
4266 is reached and returns a list of lines. Note that the notion of "line"
4267 used here is however you may have defined it with C<$/> or
4268 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4270 When C<$/> is set to C<undef>, when readline() is in scalar
4271 context (i.e. file slurp mode), and when an empty file is read, it
4272 returns C<''> the first time, followed by C<undef> subsequently.
4274 This is the internal function implementing the C<< <EXPR> >>
4275 operator, but you can use it directly. The C<< <EXPR> >>
4276 operator is discussed in more detail in L<perlop/"I/O Operators">.
4279 $line = readline(*STDIN); # same thing
4281 If readline encounters an operating system error, C<$!> will be set with the
4282 corresponding error message. It can be helpful to check C<$!> when you are
4283 reading from filehandles you don't trust, such as a tty or a socket. The
4284 following example uses the operator form of C<readline>, and takes the necessary
4285 steps to ensure that C<readline> was successful.
4289 unless (defined( $line = <> )) {
4301 Returns the value of a symbolic link, if symbolic links are
4302 implemented. If not, gives a fatal error. If there is some system
4303 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4304 omitted, uses C<$_>.
4311 EXPR is executed as a system command.
4312 The collected standard output of the command is returned.
4313 In scalar context, it comes back as a single (potentially
4314 multi-line) string. In list context, returns a list of lines
4315 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4316 This is the internal function implementing the C<qx/EXPR/>
4317 operator, but you can use it directly. The C<qx/EXPR/>
4318 operator is discussed in more detail in L<perlop/"I/O Operators">.
4319 If EXPR is omitted, uses C<$_>.
4321 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4324 Receives a message on a socket. Attempts to receive LENGTH characters
4325 of data into variable SCALAR from the specified SOCKET filehandle.
4326 SCALAR will be grown or shrunk to the length actually read. Takes the
4327 same flags as the system call of the same name. Returns the address
4328 of the sender if SOCKET's protocol supports this; returns an empty
4329 string otherwise. If there's an error, returns the undefined value.
4330 This call is actually implemented in terms of recvfrom(2) system call.
4331 See L<perlipc/"UDP: Message Passing"> for examples.
4333 Note the I<characters>: depending on the status of the socket, either
4334 (8-bit) bytes or characters are received. By default all sockets
4335 operate on bytes, but for example if the socket has been changed using
4336 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4337 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4338 characters, not bytes. Similarly for the C<:encoding> pragma:
4339 in that case pretty much any characters can be read.
4346 The C<redo> command restarts the loop block without evaluating the
4347 conditional again. The C<continue> block, if any, is not executed. If
4348 the LABEL is omitted, the command refers to the innermost enclosing
4349 loop. Programs that want to lie to themselves about what was just input
4350 normally use this command:
4352 # a simpleminded Pascal comment stripper
4353 # (warning: assumes no { or } in strings)
4354 LINE: while (<STDIN>) {
4355 while (s|({.*}.*){.*}|$1 |) {}
4360 if (/}/) { # end of comment?
4369 C<redo> cannot be used to retry a block which returns a value such as
4370 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4371 a grep() or map() operation.
4373 Note that a block by itself is semantically identical to a loop
4374 that executes once. Thus C<redo> inside such a block will effectively
4375 turn it into a looping construct.
4377 See also L</continue> for an illustration of how C<last>, C<next>, and
4385 Returns a non-empty string if EXPR is a reference, the empty
4386 string otherwise. If EXPR
4387 is not specified, C<$_> will be used. The value returned depends on the
4388 type of thing the reference is a reference to.
4389 Builtin types include:
4403 If the referenced object has been blessed into a package, then that package
4404 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4406 if (ref($r) eq "HASH") {
4407 print "r is a reference to a hash.\n";
4410 print "r is not a reference at all.\n";
4413 The return value C<LVALUE> indicates a reference to an lvalue that is not
4414 a variable. You get this from taking the reference of function calls like
4415 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4416 to a L<version string|perldata/"Version Strings">.
4418 The result C<Regexp> indicates that the argument is a regular expression
4419 resulting from C<qr//>.
4421 See also L<perlref>.
4423 =item rename OLDNAME,NEWNAME
4424 X<rename> X<move> X<mv> X<ren>
4426 Changes the name of a file; an existing file NEWNAME will be
4427 clobbered. Returns true for success, false otherwise.
4429 Behavior of this function varies wildly depending on your system
4430 implementation. For example, it will usually not work across file system
4431 boundaries, even though the system I<mv> command sometimes compensates
4432 for this. Other restrictions include whether it works on directories,
4433 open files, or pre-existing files. Check L<perlport> and either the
4434 rename(2) manpage or equivalent system documentation for details.
4436 For a platform independent C<move> function look at the L<File::Copy>
4439 =item require VERSION
4446 Demands a version of Perl specified by VERSION, or demands some semantics
4447 specified by EXPR or by C<$_> if EXPR is not supplied.
4449 VERSION may be either a numeric argument such as 5.006, which will be
4450 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4451 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4452 VERSION is greater than the version of the current Perl interpreter.
4453 Compare with L</use>, which can do a similar check at compile time.
4455 Specifying VERSION as a literal of the form v5.6.1 should generally be
4456 avoided, because it leads to misleading error messages under earlier
4457 versions of Perl that do not support this syntax. The equivalent numeric
4458 version should be used instead.
4460 require v5.6.1; # run time version check
4461 require 5.6.1; # ditto
4462 require 5.006_001; # ditto; preferred for backwards compatibility
4464 Otherwise, C<require> demands that a library file be included if it
4465 hasn't already been included. The file is included via the do-FILE
4466 mechanism, which is essentially just a variety of C<eval> with the
4467 caveat that lexical variables in the invoking script will be invisible
4468 to the included code. Has semantics similar to the following subroutine:
4471 my ($filename) = @_;
4472 if (exists $INC{$filename}) {
4473 return 1 if $INC{$filename};
4474 die "Compilation failed in require";
4476 my ($realfilename,$result);
4478 foreach $prefix (@INC) {
4479 $realfilename = "$prefix/$filename";
4480 if (-f $realfilename) {
4481 $INC{$filename} = $realfilename;
4482 $result = do $realfilename;
4486 die "Can't find $filename in \@INC";
4489 $INC{$filename} = undef;
4491 } elsif (!$result) {
4492 delete $INC{$filename};
4493 die "$filename did not return true value";
4499 Note that the file will not be included twice under the same specified
4502 The file must return true as the last statement to indicate
4503 successful execution of any initialization code, so it's customary to
4504 end such a file with C<1;> unless you're sure it'll return true
4505 otherwise. But it's better just to put the C<1;>, in case you add more
4508 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4509 replaces "F<::>" with "F</>" in the filename for you,
4510 to make it easy to load standard modules. This form of loading of
4511 modules does not risk altering your namespace.
4513 In other words, if you try this:
4515 require Foo::Bar; # a splendid bareword
4517 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4518 directories specified in the C<@INC> array.
4520 But if you try this:
4522 $class = 'Foo::Bar';
4523 require $class; # $class is not a bareword
4525 require "Foo::Bar"; # not a bareword because of the ""
4527 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4528 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4530 eval "require $class";
4532 Now that you understand how C<require> looks for files in the case of a
4533 bareword argument, there is a little extra functionality going on behind
4534 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4535 first look for a similar filename with a "F<.pmc>" extension. If this file
4536 is found, it will be loaded in place of any file ending in a "F<.pm>"
4539 You can also insert hooks into the import facility, by putting directly
4540 Perl code into the @INC array. There are three forms of hooks: subroutine
4541 references, array references and blessed objects.
4543 Subroutine references are the simplest case. When the inclusion system
4544 walks through @INC and encounters a subroutine, this subroutine gets
4545 called with two parameters, the first being a reference to itself, and the
4546 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4547 subroutine should return nothing, or a list of up to three values in the
4554 A filehandle, from which the file will be read.
4558 A reference to a subroutine. If there is no filehandle (previous item),
4559 then this subroutine is expected to generate one line of source code per
4560 call, writing the line into C<$_> and returning 1, then returning 0 at
4561 "end of file". If there is a filehandle, then the subroutine will be
4562 called to act a simple source filter, with the line as read in C<$_>.
4563 Again, return 1 for each valid line, and 0 after all lines have been
4568 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4569 reference to the subroutine itself is passed in as C<$_[0]>.
4573 If an empty list, C<undef>, or nothing that matches the first 3 values above
4574 is returned then C<require> will look at the remaining elements of @INC.
4575 Note that this file handle must be a real file handle (strictly a typeglob,
4576 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4577 ignored and return value processing will stop there.
4579 If the hook is an array reference, its first element must be a subroutine
4580 reference. This subroutine is called as above, but the first parameter is
4581 the array reference. This enables to pass indirectly some arguments to
4584 In other words, you can write:
4586 push @INC, \&my_sub;
4588 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4594 push @INC, [ \&my_sub, $x, $y, ... ];
4596 my ($arrayref, $filename) = @_;
4597 # Retrieve $x, $y, ...
4598 my @parameters = @$arrayref[1..$#$arrayref];
4602 If the hook is an object, it must provide an INC method that will be
4603 called as above, the first parameter being the object itself. (Note that
4604 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4605 into package C<main>.) Here is a typical code layout:
4611 my ($self, $filename) = @_;
4615 # In the main program
4616 push @INC, new Foo(...);
4618 Note that these hooks are also permitted to set the %INC entry
4619 corresponding to the files they have loaded. See L<perlvar/%INC>.
4621 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4628 Generally used in a C<continue> block at the end of a loop to clear
4629 variables and reset C<??> searches so that they work again. The
4630 expression is interpreted as a list of single characters (hyphens
4631 allowed for ranges). All variables and arrays beginning with one of
4632 those letters are reset to their pristine state. If the expression is
4633 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4634 only variables or searches in the current package. Always returns
4637 reset 'X'; # reset all X variables
4638 reset 'a-z'; # reset lower case variables
4639 reset; # just reset ?one-time? searches
4641 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4642 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4643 variables--lexical variables are unaffected, but they clean themselves
4644 up on scope exit anyway, so you'll probably want to use them instead.
4652 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4653 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4654 context, depending on how the return value will be used, and the context
4655 may vary from one execution to the next (see C<wantarray>). If no EXPR
4656 is given, returns an empty list in list context, the undefined value in
4657 scalar context, and (of course) nothing at all in a void context.
4659 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4660 or do FILE will automatically return the value of the last expression
4664 X<reverse> X<rev> X<invert>
4666 In list context, returns a list value consisting of the elements
4667 of LIST in the opposite order. In scalar context, concatenates the
4668 elements of LIST and returns a string value with all characters
4669 in the opposite order.
4671 print reverse <>; # line tac, last line first
4673 undef $/; # for efficiency of <>
4674 print scalar reverse <>; # character tac, last line tsrif
4676 Used without arguments in scalar context, reverse() reverses C<$_>.
4678 This operator is also handy for inverting a hash, although there are some
4679 caveats. If a value is duplicated in the original hash, only one of those
4680 can be represented as a key in the inverted hash. Also, this has to
4681 unwind one hash and build a whole new one, which may take some time
4682 on a large hash, such as from a DBM file.
4684 %by_name = reverse %by_address; # Invert the hash
4686 =item rewinddir DIRHANDLE
4689 Sets the current position to the beginning of the directory for the
4690 C<readdir> routine on DIRHANDLE.
4692 =item rindex STR,SUBSTR,POSITION
4695 =item rindex STR,SUBSTR
4697 Works just like index() except that it returns the position of the I<last>
4698 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4699 last occurrence beginning at or before that position.
4701 =item rmdir FILENAME
4702 X<rmdir> X<rd> X<directory, remove>
4706 Deletes the directory specified by FILENAME if that directory is
4707 empty. If it succeeds it returns true, otherwise it returns false and
4708 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4710 To remove a directory tree recursively (C<rm -rf> on unix) look at
4711 the C<rmtree> function of the L<File::Path> module.
4715 The substitution operator. See L<perlop>.
4717 =item say FILEHANDLE LIST
4724 Just like C<print>, but implicitly appends a newline.
4725 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
4728 This keyword is only available when the "say" feature is
4729 enabled: see L<feature>.
4732 X<scalar> X<context>
4734 Forces EXPR to be interpreted in scalar context and returns the value
4737 @counts = ( scalar @a, scalar @b, scalar @c );
4739 There is no equivalent operator to force an expression to
4740 be interpolated in list context because in practice, this is never
4741 needed. If you really wanted to do so, however, you could use
4742 the construction C<@{[ (some expression) ]}>, but usually a simple
4743 C<(some expression)> suffices.
4745 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4746 parenthesized list, this behaves as a scalar comma expression, evaluating
4747 all but the last element in void context and returning the final element
4748 evaluated in scalar context. This is seldom what you want.
4750 The following single statement:
4752 print uc(scalar(&foo,$bar)),$baz;
4754 is the moral equivalent of these two:
4757 print(uc($bar),$baz);
4759 See L<perlop> for more details on unary operators and the comma operator.
4761 =item seek FILEHANDLE,POSITION,WHENCE
4762 X<seek> X<fseek> X<filehandle, position>
4764 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4765 FILEHANDLE may be an expression whose value gives the name of the
4766 filehandle. The values for WHENCE are C<0> to set the new position
4767 I<in bytes> to POSITION, C<1> to set it to the current position plus
4768 POSITION, and C<2> to set it to EOF plus POSITION (typically
4769 negative). For WHENCE you may use the constants C<SEEK_SET>,
4770 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4771 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4774 Note the I<in bytes>: even if the filehandle has been set to
4775 operate on characters (for example by using the C<:utf8> open
4776 layer), tell() will return byte offsets, not character offsets
4777 (because implementing that would render seek() and tell() rather slow).
4779 If you want to position file for C<sysread> or C<syswrite>, don't use
4780 C<seek>--buffering makes its effect on the file's system position
4781 unpredictable and non-portable. Use C<sysseek> instead.
4783 Due to the rules and rigors of ANSI C, on some systems you have to do a
4784 seek whenever you switch between reading and writing. Amongst other
4785 things, this may have the effect of calling stdio's clearerr(3).
4786 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4790 This is also useful for applications emulating C<tail -f>. Once you hit
4791 EOF on your read, and then sleep for a while, you might have to stick in a
4792 seek() to reset things. The C<seek> doesn't change the current position,
4793 but it I<does> clear the end-of-file condition on the handle, so that the
4794 next C<< <FILE> >> makes Perl try again to read something. We hope.
4796 If that doesn't work (some IO implementations are particularly
4797 cantankerous), then you may need something more like this:
4800 for ($curpos = tell(FILE); $_ = <FILE>;
4801 $curpos = tell(FILE)) {
4802 # search for some stuff and put it into files
4804 sleep($for_a_while);
4805 seek(FILE, $curpos, 0);
4808 =item seekdir DIRHANDLE,POS
4811 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4812 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4813 about possible directory compaction as the corresponding system library
4816 =item select FILEHANDLE
4817 X<select> X<filehandle, default>
4821 Returns the currently selected filehandle. If FILEHANDLE is supplied,
4822 sets the new current default filehandle for output. This has two
4823 effects: first, a C<write> or a C<print> without a filehandle will
4824 default to this FILEHANDLE. Second, references to variables related to
4825 output will refer to this output channel. For example, if you have to
4826 set the top of form format for more than one output channel, you might
4834 FILEHANDLE may be an expression whose value gives the name of the
4835 actual filehandle. Thus:
4837 $oldfh = select(STDERR); $| = 1; select($oldfh);
4839 Some programmers may prefer to think of filehandles as objects with
4840 methods, preferring to write the last example as:
4843 STDERR->autoflush(1);
4845 =item select RBITS,WBITS,EBITS,TIMEOUT
4848 This calls the select(2) system call with the bit masks specified, which
4849 can be constructed using C<fileno> and C<vec>, along these lines:
4851 $rin = $win = $ein = '';
4852 vec($rin,fileno(STDIN),1) = 1;
4853 vec($win,fileno(STDOUT),1) = 1;
4856 If you want to select on many filehandles you might wish to write a
4860 my(@fhlist) = split(' ',$_[0]);
4863 vec($bits,fileno($_),1) = 1;
4867 $rin = fhbits('STDIN TTY SOCK');
4871 ($nfound,$timeleft) =
4872 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4874 or to block until something becomes ready just do this
4876 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4878 Most systems do not bother to return anything useful in $timeleft, so
4879 calling select() in scalar context just returns $nfound.
4881 Any of the bit masks can also be undef. The timeout, if specified, is
4882 in seconds, which may be fractional. Note: not all implementations are
4883 capable of returning the $timeleft. If not, they always return
4884 $timeleft equal to the supplied $timeout.
4886 You can effect a sleep of 250 milliseconds this way:
4888 select(undef, undef, undef, 0.25);
4890 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4891 is implementation-dependent. See also L<perlport> for notes on the
4892 portability of C<select>.
4894 On error, C<select> behaves like the select(2) system call : it returns
4897 Note: on some Unixes, the select(2) system call may report a socket file
4898 descriptor as "ready for reading", when actually no data is available,
4899 thus a subsequent read blocks. It can be avoided using always the
4900 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4903 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4904 or <FH>) with C<select>, except as permitted by POSIX, and even
4905 then only on POSIX systems. You have to use C<sysread> instead.
4907 =item semctl ID,SEMNUM,CMD,ARG
4910 Calls the System V IPC function C<semctl>. You'll probably have to say
4914 first to get the correct constant definitions. If CMD is IPC_STAT or
4915 GETALL, then ARG must be a variable that will hold the returned
4916 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4917 the undefined value for error, "C<0 but true>" for zero, or the actual
4918 return value otherwise. The ARG must consist of a vector of native
4919 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4920 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4923 =item semget KEY,NSEMS,FLAGS
4926 Calls the System V IPC function semget. Returns the semaphore id, or
4927 the undefined value if there is an error. See also
4928 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4931 =item semop KEY,OPSTRING
4934 Calls the System V IPC function semop to perform semaphore operations
4935 such as signalling and waiting. OPSTRING must be a packed array of
4936 semop structures. Each semop structure can be generated with
4937 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4938 implies the number of semaphore operations. Returns true if
4939 successful, or false if there is an error. As an example, the
4940 following code waits on semaphore $semnum of semaphore id $semid:
4942 $semop = pack("s!3", $semnum, -1, 0);
4943 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4945 To signal the semaphore, replace C<-1> with C<1>. See also
4946 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4949 =item send SOCKET,MSG,FLAGS,TO
4952 =item send SOCKET,MSG,FLAGS
4954 Sends a message on a socket. Attempts to send the scalar MSG to the
4955 SOCKET filehandle. Takes the same flags as the system call of the
4956 same name. On unconnected sockets you must specify a destination to
4957 send TO, in which case it does a C C<sendto>. Returns the number of
4958 characters sent, or the undefined value if there is an error. The C
4959 system call sendmsg(2) is currently unimplemented. See
4960 L<perlipc/"UDP: Message Passing"> for examples.
4962 Note the I<characters>: depending on the status of the socket, either
4963 (8-bit) bytes or characters are sent. By default all sockets operate
4964 on bytes, but for example if the socket has been changed using
4965 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4966 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4967 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4968 in that case pretty much any characters can be sent.
4970 =item setpgrp PID,PGRP
4973 Sets the current process group for the specified PID, C<0> for the current
4974 process. Will produce a fatal error if used on a machine that doesn't
4975 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4976 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4977 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4980 =item setpriority WHICH,WHO,PRIORITY
4981 X<setpriority> X<priority> X<nice> X<renice>
4983 Sets the current priority for a process, a process group, or a user.
4984 (See setpriority(2).) Will produce a fatal error if used on a machine
4985 that doesn't implement setpriority(2).
4987 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4990 Sets the socket option requested. Returns undefined if there is an
4991 error. Use integer constants provided by the C<Socket> module for
4992 LEVEL and OPNAME. Values for LEVEL can also be obtained from
4993 getprotobyname. OPTVAL might either be a packed string or an integer.
4994 An integer OPTVAL is shorthand for pack("i", OPTVAL).
4996 An example disabling the Nagle's algorithm for a socket:
4998 use Socket qw(IPPROTO_TCP TCP_NODELAY);
4999 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5006 Shifts the first value of the array off and returns it, shortening the
5007 array by 1 and moving everything down. If there are no elements in the
5008 array, returns the undefined value. If ARRAY is omitted, shifts the
5009 C<@_> array within the lexical scope of subroutines and formats, and the
5010 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5011 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5012 C<UNITCHECK {}> and C<END {}> constructs.
5014 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5015 same thing to the left end of an array that C<pop> and C<push> do to the
5018 =item shmctl ID,CMD,ARG
5021 Calls the System V IPC function shmctl. You'll probably have to say
5025 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5026 then ARG must be a variable that will hold the returned C<shmid_ds>
5027 structure. Returns like ioctl: the undefined value for error, "C<0> but
5028 true" for zero, or the actual return value otherwise.
5029 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5031 =item shmget KEY,SIZE,FLAGS
5034 Calls the System V IPC function shmget. Returns the shared memory
5035 segment id, or the undefined value if there is an error.
5036 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5038 =item shmread ID,VAR,POS,SIZE
5042 =item shmwrite ID,STRING,POS,SIZE
5044 Reads or writes the System V shared memory segment ID starting at
5045 position POS for size SIZE by attaching to it, copying in/out, and
5046 detaching from it. When reading, VAR must be a variable that will
5047 hold the data read. When writing, if STRING is too long, only SIZE
5048 bytes are used; if STRING is too short, nulls are written to fill out
5049 SIZE bytes. Return true if successful, or false if there is an error.
5050 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5051 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5053 =item shutdown SOCKET,HOW
5056 Shuts down a socket connection in the manner indicated by HOW, which
5057 has the same interpretation as in the system call of the same name.
5059 shutdown(SOCKET, 0); # I/we have stopped reading data
5060 shutdown(SOCKET, 1); # I/we have stopped writing data
5061 shutdown(SOCKET, 2); # I/we have stopped using this socket
5063 This is useful with sockets when you want to tell the other
5064 side you're done writing but not done reading, or vice versa.
5065 It's also a more insistent form of close because it also
5066 disables the file descriptor in any forked copies in other
5070 X<sin> X<sine> X<asin> X<arcsine>
5074 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5075 returns sine of C<$_>.
5077 For the inverse sine operation, you may use the C<Math::Trig::asin>
5078 function, or use this relation:
5080 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5087 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5088 May be interrupted if the process receives a signal such as C<SIGALRM>.
5089 Returns the number of seconds actually slept. You probably cannot
5090 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5093 On some older systems, it may sleep up to a full second less than what
5094 you requested, depending on how it counts seconds. Most modern systems
5095 always sleep the full amount. They may appear to sleep longer than that,
5096 however, because your process might not be scheduled right away in a
5097 busy multitasking system.
5099 For delays of finer granularity than one second, you may use Perl's
5100 C<syscall> interface to access setitimer(2) if your system supports
5101 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5102 and starting from Perl 5.8 part of the standard distribution) may also
5105 See also the POSIX module's C<pause> function.
5107 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5110 Opens a socket of the specified kind and attaches it to filehandle
5111 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5112 the system call of the same name. You should C<use Socket> first
5113 to get the proper definitions imported. See the examples in
5114 L<perlipc/"Sockets: Client/Server Communication">.
5116 On systems that support a close-on-exec flag on files, the flag will
5117 be set for the newly opened file descriptor, as determined by the
5118 value of $^F. See L<perlvar/$^F>.
5120 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5123 Creates an unnamed pair of sockets in the specified domain, of the
5124 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5125 for the system call of the same name. If unimplemented, yields a fatal
5126 error. Returns true if successful.
5128 On systems that support a close-on-exec flag on files, the flag will
5129 be set for the newly opened file descriptors, as determined by the value
5130 of $^F. See L<perlvar/$^F>.
5132 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5133 to C<pipe(Rdr, Wtr)> is essentially:
5136 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5137 shutdown(Rdr, 1); # no more writing for reader
5138 shutdown(Wtr, 0); # no more reading for writer
5140 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5141 emulate socketpair using IP sockets to localhost if your system implements
5142 sockets but not socketpair.
5144 =item sort SUBNAME LIST
5145 X<sort> X<qsort> X<quicksort> X<mergesort>
5147 =item sort BLOCK LIST
5151 In list context, this sorts the LIST and returns the sorted list value.
5152 In scalar context, the behaviour of C<sort()> is undefined.
5154 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5155 order. If SUBNAME is specified, it gives the name of a subroutine
5156 that returns an integer less than, equal to, or greater than C<0>,
5157 depending on how the elements of the list are to be ordered. (The C<<
5158 <=> >> and C<cmp> operators are extremely useful in such routines.)
5159 SUBNAME may be a scalar variable name (unsubscripted), in which case
5160 the value provides the name of (or a reference to) the actual
5161 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5162 an anonymous, in-line sort subroutine.
5164 If the subroutine's prototype is C<($$)>, the elements to be compared
5165 are passed by reference in C<@_>, as for a normal subroutine. This is
5166 slower than unprototyped subroutines, where the elements to be
5167 compared are passed into the subroutine
5168 as the package global variables $a and $b (see example below). Note that
5169 in the latter case, it is usually counter-productive to declare $a and
5172 The values to be compared are always passed by reference and should not
5175 You also cannot exit out of the sort block or subroutine using any of the
5176 loop control operators described in L<perlsyn> or with C<goto>.
5178 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5179 current collation locale. See L<perllocale>.
5181 sort() returns aliases into the original list, much as a for loop's index
5182 variable aliases the list elements. That is, modifying an element of a
5183 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5184 actually modifies the element in the original list. This is usually
5185 something to be avoided when writing clear code.
5187 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5188 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5189 preserves the input order of elements that compare equal. Although
5190 quicksort's run time is O(NlogN) when averaged over all arrays of
5191 length N, the time can be O(N**2), I<quadratic> behavior, for some
5192 inputs.) In 5.7, the quicksort implementation was replaced with
5193 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5194 But benchmarks indicated that for some inputs, on some platforms,
5195 the original quicksort was faster. 5.8 has a sort pragma for
5196 limited control of the sort. Its rather blunt control of the
5197 underlying algorithm may not persist into future Perls, but the
5198 ability to characterize the input or output in implementation
5199 independent ways quite probably will. See L<sort>.
5204 @articles = sort @files;
5206 # same thing, but with explicit sort routine
5207 @articles = sort {$a cmp $b} @files;
5209 # now case-insensitively
5210 @articles = sort {uc($a) cmp uc($b)} @files;
5212 # same thing in reversed order
5213 @articles = sort {$b cmp $a} @files;
5215 # sort numerically ascending
5216 @articles = sort {$a <=> $b} @files;
5218 # sort numerically descending
5219 @articles = sort {$b <=> $a} @files;
5221 # this sorts the %age hash by value instead of key
5222 # using an in-line function
5223 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5225 # sort using explicit subroutine name
5227 $age{$a} <=> $age{$b}; # presuming numeric
5229 @sortedclass = sort byage @class;
5231 sub backwards { $b cmp $a }
5232 @harry = qw(dog cat x Cain Abel);
5233 @george = qw(gone chased yz Punished Axed);
5235 # prints AbelCaincatdogx
5236 print sort backwards @harry;
5237 # prints xdogcatCainAbel
5238 print sort @george, 'to', @harry;
5239 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5241 # inefficiently sort by descending numeric compare using
5242 # the first integer after the first = sign, or the
5243 # whole record case-insensitively otherwise
5246 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5251 # same thing, but much more efficiently;
5252 # we'll build auxiliary indices instead
5256 push @nums, /=(\d+)/;
5261 $nums[$b] <=> $nums[$a]
5263 $caps[$a] cmp $caps[$b]
5267 # same thing, but without any temps
5268 @new = map { $_->[0] }
5269 sort { $b->[1] <=> $a->[1]
5272 } map { [$_, /=(\d+)/, uc($_)] } @old;
5274 # using a prototype allows you to use any comparison subroutine
5275 # as a sort subroutine (including other package's subroutines)
5277 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5280 @new = sort other::backwards @old;
5282 # guarantee stability, regardless of algorithm
5284 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5286 # force use of mergesort (not portable outside Perl 5.8)
5287 use sort '_mergesort'; # note discouraging _
5288 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5290 If you're using strict, you I<must not> declare $a
5291 and $b as lexicals. They are package globals. That means
5292 if you're in the C<main> package and type
5294 @articles = sort {$b <=> $a} @files;
5296 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5297 but if you're in the C<FooPack> package, it's the same as typing
5299 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5301 The comparison function is required to behave. If it returns
5302 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5303 sometimes saying the opposite, for example) the results are not
5306 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5307 (not-a-number), and because C<sort> will trigger a fatal error unless the
5308 result of a comparison is defined, when sorting with a comparison function
5309 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5310 The following example takes advantage of the fact that C<NaN != NaN> to
5311 eliminate any C<NaN>s from the input.
5313 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5315 =item splice ARRAY,OFFSET,LENGTH,LIST
5318 =item splice ARRAY,OFFSET,LENGTH
5320 =item splice ARRAY,OFFSET
5324 Removes the elements designated by OFFSET and LENGTH from an array, and
5325 replaces them with the elements of LIST, if any. In list context,
5326 returns the elements removed from the array. In scalar context,
5327 returns the last element removed, or C<undef> if no elements are
5328 removed. The array grows or shrinks as necessary.
5329 If OFFSET is negative then it starts that far from the end of the array.
5330 If LENGTH is omitted, removes everything from OFFSET onward.
5331 If LENGTH is negative, removes the elements from OFFSET onward
5332 except for -LENGTH elements at the end of the array.
5333 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5334 past the end of the array, perl issues a warning, and splices at the
5337 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5339 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5340 pop(@a) splice(@a,-1)
5341 shift(@a) splice(@a,0,1)
5342 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5343 $a[$i] = $y splice(@a,$i,1,$y)
5345 Example, assuming array lengths are passed before arrays:
5347 sub aeq { # compare two list values
5348 my(@a) = splice(@_,0,shift);
5349 my(@b) = splice(@_,0,shift);
5350 return 0 unless @a == @b; # same len?
5352 return 0 if pop(@a) ne pop(@b);
5356 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5358 =item split /PATTERN/,EXPR,LIMIT
5361 =item split /PATTERN/,EXPR
5363 =item split /PATTERN/
5367 Splits the string EXPR into a list of strings and returns that list. By
5368 default, empty leading fields are preserved, and empty trailing ones are
5369 deleted. (If all fields are empty, they are considered to be trailing.)
5371 In scalar context, returns the number of fields found and splits into
5372 the C<@_> array. Use of split in scalar context is deprecated, however,
5373 because it clobbers your subroutine arguments.
5375 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5376 splits on whitespace (after skipping any leading whitespace). Anything
5377 matching PATTERN is taken to be a delimiter separating the fields. (Note
5378 that the delimiter may be longer than one character.)
5380 If LIMIT is specified and positive, it represents the maximum number
5381 of fields the EXPR will be split into, though the actual number of
5382 fields returned depends on the number of times PATTERN matches within
5383 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5384 stripped (which potential users of C<pop> would do well to remember).
5385 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5386 had been specified. Note that splitting an EXPR that evaluates to the
5387 empty string always returns the empty list, regardless of the LIMIT
5390 A pattern matching the null string (not to be confused with
5391 a null pattern C<//>, which is just one member of the set of patterns
5392 matching a null string) will split the value of EXPR into separate
5393 characters at each point it matches that way. For example:
5395 print join(':', split(/ */, 'hi there'));
5397 produces the output 'h:i:t:h:e:r:e'.
5399 As a special case for C<split>, using the empty pattern C<//> specifically
5400 matches only the null string, and is not be confused with the regular use
5401 of C<//> to mean "the last successful pattern match". So, for C<split>,
5404 print join(':', split(//, 'hi there'));
5406 produces the output 'h:i: :t:h:e:r:e'.
5408 Empty leading (or trailing) fields are produced when there are positive
5409 width matches at the beginning (or end) of the string; a zero-width match
5410 at the beginning (or end) of the string does not produce an empty field.
5413 print join(':', split(/(?=\w)/, 'hi there!'));
5415 produces the output 'h:i :t:h:e:r:e!'.
5417 The LIMIT parameter can be used to split a line partially
5419 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5421 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5422 a LIMIT one larger than the number of variables in the list, to avoid
5423 unnecessary work. For the list above LIMIT would have been 4 by
5424 default. In time critical applications it behooves you not to split
5425 into more fields than you really need.
5427 If the PATTERN contains parentheses, additional list elements are
5428 created from each matching substring in the delimiter.
5430 split(/([,-])/, "1-10,20", 3);
5432 produces the list value
5434 (1, '-', 10, ',', 20)
5436 If you had the entire header of a normal Unix email message in $header,
5437 you could split it up into fields and their values this way:
5439 $header =~ s/\n\s+/ /g; # fix continuation lines
5440 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5442 The pattern C</PATTERN/> may be replaced with an expression to specify
5443 patterns that vary at runtime. (To do runtime compilation only once,
5444 use C</$variable/o>.)
5446 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5447 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5448 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5449 will give you as many null initial fields as there are leading spaces.
5450 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5451 whitespace produces a null first field. A C<split> with no arguments
5452 really does a S<C<split(' ', $_)>> internally.
5454 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5459 open(PASSWD, '/etc/passwd');
5462 ($login, $passwd, $uid, $gid,
5463 $gcos, $home, $shell) = split(/:/);
5467 As with regular pattern matching, any capturing parentheses that are not
5468 matched in a C<split()> will be set to C<undef> when returned:
5470 @fields = split /(A)|B/, "1A2B3";
5471 # @fields is (1, 'A', 2, undef, 3)
5473 =item sprintf FORMAT, LIST
5476 Returns a string formatted by the usual C<printf> conventions of the C
5477 library function C<sprintf>. See below for more details
5478 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5479 the general principles.
5483 # Format number with up to 8 leading zeroes
5484 $result = sprintf("%08d", $number);
5486 # Round number to 3 digits after decimal point
5487 $rounded = sprintf("%.3f", $number);
5489 Perl does its own C<sprintf> formatting--it emulates the C
5490 function C<sprintf>, but it doesn't use it (except for floating-point
5491 numbers, and even then only the standard modifiers are allowed). As a
5492 result, any non-standard extensions in your local C<sprintf> are not
5493 available from Perl.
5495 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5496 pass it an array as your first argument. The array is given scalar context,
5497 and instead of using the 0th element of the array as the format, Perl will
5498 use the count of elements in the array as the format, which is almost never
5501 Perl's C<sprintf> permits the following universally-known conversions:
5504 %c a character with the given number
5506 %d a signed integer, in decimal
5507 %u an unsigned integer, in decimal
5508 %o an unsigned integer, in octal
5509 %x an unsigned integer, in hexadecimal
5510 %e a floating-point number, in scientific notation
5511 %f a floating-point number, in fixed decimal notation
5512 %g a floating-point number, in %e or %f notation
5514 In addition, Perl permits the following widely-supported conversions:
5516 %X like %x, but using upper-case letters
5517 %E like %e, but using an upper-case "E"
5518 %G like %g, but with an upper-case "E" (if applicable)
5519 %b an unsigned integer, in binary
5520 %B like %b, but using an upper-case "B" with the # flag
5521 %p a pointer (outputs the Perl value's address in hexadecimal)
5522 %n special: *stores* the number of characters output so far
5523 into the next variable in the parameter list
5525 Finally, for backward (and we do mean "backward") compatibility, Perl
5526 permits these unnecessary but widely-supported conversions:
5529 %D a synonym for %ld
5530 %U a synonym for %lu
5531 %O a synonym for %lo
5534 Note that the number of exponent digits in the scientific notation produced
5535 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5536 exponent less than 100 is system-dependent: it may be three or less
5537 (zero-padded as necessary). In other words, 1.23 times ten to the
5538 99th may be either "1.23e99" or "1.23e099".
5540 Between the C<%> and the format letter, you may specify a number of
5541 additional attributes controlling the interpretation of the format.
5542 In order, these are:
5546 =item format parameter index
5548 An explicit format parameter index, such as C<2$>. By default sprintf
5549 will format the next unused argument in the list, but this allows you
5550 to take the arguments out of order, e.g.:
5552 printf '%2$d %1$d', 12, 34; # prints "34 12"
5553 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5559 space prefix positive number with a space
5560 + prefix positive number with a plus sign
5561 - left-justify within the field
5562 0 use zeros, not spaces, to right-justify
5563 # ensure the leading "0" for any octal,
5564 prefix non-zero hexadecimal with "0x" or "0X",
5565 prefix non-zero binary with "0b" or "0B"
5569 printf '<% d>', 12; # prints "< 12>"
5570 printf '<%+d>', 12; # prints "<+12>"
5571 printf '<%6s>', 12; # prints "< 12>"
5572 printf '<%-6s>', 12; # prints "<12 >"
5573 printf '<%06s>', 12; # prints "<000012>"
5574 printf '<%#o>', 12; # prints "<014>"
5575 printf '<%#x>', 12; # prints "<0xc>"
5576 printf '<%#X>', 12; # prints "<0XC>"
5577 printf '<%#b>', 12; # prints "<0b1100>"
5578 printf '<%#B>', 12; # prints "<0B1100>"
5580 When a space and a plus sign are given as the flags at once,
5581 a plus sign is used to prefix a positive number.
5583 printf '<%+ d>', 12; # prints "<+12>"
5584 printf '<% +d>', 12; # prints "<+12>"
5586 When the # flag and a precision are given in the %o conversion,
5587 the precision is incremented if it's necessary for the leading "0".
5589 printf '<%#.5o>', 012; # prints "<00012>"
5590 printf '<%#.5o>', 012345; # prints "<012345>"
5591 printf '<%#.0o>', 0; # prints "<0>"
5595 This flag tells perl to interpret the supplied string as a vector of
5596 integers, one for each character in the string. Perl applies the format to
5597 each integer in turn, then joins the resulting strings with a separator (a
5598 dot C<.> by default). This can be useful for displaying ordinal values of
5599 characters in arbitrary strings:
5601 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5602 printf "version is v%vd\n", $^V; # Perl's version
5604 Put an asterisk C<*> before the C<v> to override the string to
5605 use to separate the numbers:
5607 printf "address is %*vX\n", ":", $addr; # IPv6 address
5608 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5610 You can also explicitly specify the argument number to use for
5611 the join string using e.g. C<*2$v>:
5613 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5615 =item (minimum) width
5617 Arguments are usually formatted to be only as wide as required to
5618 display the given value. You can override the width by putting
5619 a number here, or get the width from the next argument (with C<*>)
5620 or from a specified argument (with e.g. C<*2$>):
5622 printf '<%s>', "a"; # prints "<a>"
5623 printf '<%6s>', "a"; # prints "< a>"
5624 printf '<%*s>', 6, "a"; # prints "< a>"
5625 printf '<%*2$s>', "a", 6; # prints "< a>"
5626 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5628 If a field width obtained through C<*> is negative, it has the same
5629 effect as the C<-> flag: left-justification.
5631 =item precision, or maximum width
5634 You can specify a precision (for numeric conversions) or a maximum
5635 width (for string conversions) by specifying a C<.> followed by a number.
5636 For floating point formats, with the exception of 'g' and 'G', this specifies
5637 the number of decimal places to show (the default being 6), e.g.:
5639 # these examples are subject to system-specific variation
5640 printf '<%f>', 1; # prints "<1.000000>"
5641 printf '<%.1f>', 1; # prints "<1.0>"
5642 printf '<%.0f>', 1; # prints "<1>"
5643 printf '<%e>', 10; # prints "<1.000000e+01>"
5644 printf '<%.1e>', 10; # prints "<1.0e+01>"
5646 For 'g' and 'G', this specifies the maximum number of digits to show,
5647 including prior to the decimal point as well as after it, e.g.:
5649 # these examples are subject to system-specific variation
5650 printf '<%g>', 1; # prints "<1>"
5651 printf '<%.10g>', 1; # prints "<1>"
5652 printf '<%g>', 100; # prints "<100>"
5653 printf '<%.1g>', 100; # prints "<1e+02>"
5654 printf '<%.2g>', 100.01; # prints "<1e+02>"
5655 printf '<%.5g>', 100.01; # prints "<100.01>"
5656 printf '<%.4g>', 100.01; # prints "<100>"
5658 For integer conversions, specifying a precision implies that the
5659 output of the number itself should be zero-padded to this width,
5660 where the 0 flag is ignored:
5662 printf '<%.6d>', 1; # prints "<000001>"
5663 printf '<%+.6d>', 1; # prints "<+000001>"
5664 printf '<%-10.6d>', 1; # prints "<000001 >"
5665 printf '<%10.6d>', 1; # prints "< 000001>"
5666 printf '<%010.6d>', 1; # prints "< 000001>"
5667 printf '<%+10.6d>', 1; # prints "< +000001>"
5669 printf '<%.6x>', 1; # prints "<000001>"
5670 printf '<%#.6x>', 1; # prints "<0x000001>"
5671 printf '<%-10.6x>', 1; # prints "<000001 >"
5672 printf '<%10.6x>', 1; # prints "< 000001>"
5673 printf '<%010.6x>', 1; # prints "< 000001>"
5674 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5676 For string conversions, specifying a precision truncates the string
5677 to fit in the specified width:
5679 printf '<%.5s>', "truncated"; # prints "<trunc>"
5680 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5682 You can also get the precision from the next argument using C<.*>:
5684 printf '<%.6x>', 1; # prints "<000001>"
5685 printf '<%.*x>', 6, 1; # prints "<000001>"
5687 If a precision obtained through C<*> is negative, it has the same
5688 effect as no precision.
5690 printf '<%.*s>', 7, "string"; # prints "<string>"
5691 printf '<%.*s>', 3, "string"; # prints "<str>"
5692 printf '<%.*s>', 0, "string"; # prints "<>"
5693 printf '<%.*s>', -1, "string"; # prints "<string>"
5695 printf '<%.*d>', 1, 0; # prints "<0>"
5696 printf '<%.*d>', 0, 0; # prints "<>"
5697 printf '<%.*d>', -1, 0; # prints "<0>"
5699 You cannot currently get the precision from a specified number,
5700 but it is intended that this will be possible in the future using
5703 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5707 For numeric conversions, you can specify the size to interpret the
5708 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5709 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5710 whatever the default integer size is on your platform (usually 32 or 64
5711 bits), but you can override this to use instead one of the standard C types,
5712 as supported by the compiler used to build Perl:
5714 l interpret integer as C type "long" or "unsigned long"
5715 h interpret integer as C type "short" or "unsigned short"
5716 q, L or ll interpret integer as C type "long long", "unsigned long long".
5717 or "quads" (typically 64-bit integers)
5719 The last will produce errors if Perl does not understand "quads" in your
5720 installation. (This requires that either the platform natively supports quads
5721 or Perl was specifically compiled to support quads.) You can find out
5722 whether your Perl supports quads via L<Config>:
5725 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5728 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5729 to be the default floating point size on your platform (double or long double),
5730 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5731 platform supports them. You can find out whether your Perl supports long
5732 doubles via L<Config>:
5735 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5737 You can find out whether Perl considers 'long double' to be the default
5738 floating point size to use on your platform via L<Config>:
5741 ($Config{uselongdouble} eq 'define') &&
5742 print "long doubles by default\n";
5744 It can also be the case that long doubles and doubles are the same thing:
5747 ($Config{doublesize} == $Config{longdblsize}) &&
5748 print "doubles are long doubles\n";
5750 The size specifier C<V> has no effect for Perl code, but it is supported
5751 for compatibility with XS code; it means 'use the standard size for
5752 a Perl integer (or floating-point number)', which is already the
5753 default for Perl code.
5755 =item order of arguments
5757 Normally, sprintf takes the next unused argument as the value to
5758 format for each format specification. If the format specification
5759 uses C<*> to require additional arguments, these are consumed from
5760 the argument list in the order in which they appear in the format
5761 specification I<before> the value to format. Where an argument is
5762 specified using an explicit index, this does not affect the normal
5763 order for the arguments (even when the explicitly specified index
5764 would have been the next argument in any case).
5768 printf '<%*.*s>', $a, $b, $c;
5770 would use C<$a> for the width, C<$b> for the precision and C<$c>
5771 as the value to format, while:
5773 print '<%*1$.*s>', $a, $b;
5775 would use C<$a> for the width and the precision, and C<$b> as the
5778 Here are some more examples - beware that when using an explicit
5779 index, the C<$> may need to be escaped:
5781 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5782 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5783 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5784 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5788 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5789 the character used for the decimal separator in formatted floating
5790 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5794 X<sqrt> X<root> X<square root>
5798 Return the square root of EXPR. If EXPR is omitted, returns square
5799 root of C<$_>. Only works on non-negative operands, unless you've
5800 loaded the standard Math::Complex module.
5803 print sqrt(-2); # prints 1.4142135623731i
5806 X<srand> X<seed> X<randseed>
5810 Sets the random number seed for the C<rand> operator.
5812 The point of the function is to "seed" the C<rand> function so that
5813 C<rand> can produce a different sequence each time you run your
5816 If srand() is not called explicitly, it is called implicitly at the
5817 first use of the C<rand> operator. However, this was not the case in
5818 versions of Perl before 5.004, so if your script will run under older
5819 Perl versions, it should call C<srand>.
5821 Most programs won't even call srand() at all, except those that
5822 need a cryptographically-strong starting point rather than the
5823 generally acceptable default, which is based on time of day,
5824 process ID, and memory allocation, or the F</dev/urandom> device,
5827 You can call srand($seed) with the same $seed to reproduce the
5828 I<same> sequence from rand(), but this is usually reserved for
5829 generating predictable results for testing or debugging.
5830 Otherwise, don't call srand() more than once in your program.
5832 Do B<not> call srand() (i.e. without an argument) more than once in
5833 a script. The internal state of the random number generator should
5834 contain more entropy than can be provided by any seed, so calling
5835 srand() again actually I<loses> randomness.
5837 Most implementations of C<srand> take an integer and will silently
5838 truncate decimal numbers. This means C<srand(42)> will usually
5839 produce the same results as C<srand(42.1)>. To be safe, always pass
5840 C<srand> an integer.
5842 In versions of Perl prior to 5.004 the default seed was just the
5843 current C<time>. This isn't a particularly good seed, so many old
5844 programs supply their own seed value (often C<time ^ $$> or C<time ^
5845 ($$ + ($$ << 15))>), but that isn't necessary any more.
5847 For cryptographic purposes, however, you need something much more random
5848 than the default seed. Checksumming the compressed output of one or more
5849 rapidly changing operating system status programs is the usual method. For
5852 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5854 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5857 Frequently called programs (like CGI scripts) that simply use
5861 for a seed can fall prey to the mathematical property that
5865 one-third of the time. So don't do that.
5867 =item stat FILEHANDLE
5868 X<stat> X<file, status> X<ctime>
5872 =item stat DIRHANDLE
5876 Returns a 13-element list giving the status info for a file, either
5877 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5878 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5881 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5882 $atime,$mtime,$ctime,$blksize,$blocks)
5885 Not all fields are supported on all filesystem types. Here are the
5886 meanings of the fields:
5888 0 dev device number of filesystem
5890 2 mode file mode (type and permissions)
5891 3 nlink number of (hard) links to the file
5892 4 uid numeric user ID of file's owner
5893 5 gid numeric group ID of file's owner
5894 6 rdev the device identifier (special files only)
5895 7 size total size of file, in bytes
5896 8 atime last access time in seconds since the epoch
5897 9 mtime last modify time in seconds since the epoch
5898 10 ctime inode change time in seconds since the epoch (*)
5899 11 blksize preferred block size for file system I/O
5900 12 blocks actual number of blocks allocated
5902 (The epoch was at 00:00 January 1, 1970 GMT.)
5904 (*) Not all fields are supported on all filesystem types. Notably, the
5905 ctime field is non-portable. In particular, you cannot expect it to be a
5906 "creation time", see L<perlport/"Files and Filesystems"> for details.
5908 If C<stat> is passed the special filehandle consisting of an underline, no
5909 stat is done, but the current contents of the stat structure from the
5910 last C<stat>, C<lstat>, or filetest are returned. Example:
5912 if (-x $file && (($d) = stat(_)) && $d < 0) {
5913 print "$file is executable NFS file\n";
5916 (This works on machines only for which the device number is negative
5919 Because the mode contains both the file type and its permissions, you
5920 should mask off the file type portion and (s)printf using a C<"%o">
5921 if you want to see the real permissions.
5923 $mode = (stat($filename))[2];
5924 printf "Permissions are %04o\n", $mode & 07777;
5926 In scalar context, C<stat> returns a boolean value indicating success
5927 or failure, and, if successful, sets the information associated with
5928 the special filehandle C<_>.
5930 The L<File::stat> module provides a convenient, by-name access mechanism:
5933 $sb = stat($filename);
5934 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5935 $filename, $sb->size, $sb->mode & 07777,
5936 scalar localtime $sb->mtime;
5938 You can import symbolic mode constants (C<S_IF*>) and functions
5939 (C<S_IS*>) from the Fcntl module:
5943 $mode = (stat($filename))[2];
5945 $user_rwx = ($mode & S_IRWXU) >> 6;
5946 $group_read = ($mode & S_IRGRP) >> 3;
5947 $other_execute = $mode & S_IXOTH;
5949 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5951 $is_setuid = $mode & S_ISUID;
5952 $is_directory = S_ISDIR($mode);
5954 You could write the last two using the C<-u> and C<-d> operators.
5955 The commonly available C<S_IF*> constants are
5957 # Permissions: read, write, execute, for user, group, others.
5959 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5960 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5961 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5963 # Setuid/Setgid/Stickiness/SaveText.
5964 # Note that the exact meaning of these is system dependent.
5966 S_ISUID S_ISGID S_ISVTX S_ISTXT
5968 # File types. Not necessarily all are available on your system.
5970 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5972 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5974 S_IREAD S_IWRITE S_IEXEC
5976 and the C<S_IF*> functions are
5978 S_IMODE($mode) the part of $mode containing the permission bits
5979 and the setuid/setgid/sticky bits
5981 S_IFMT($mode) the part of $mode containing the file type
5982 which can be bit-anded with e.g. S_IFREG
5983 or with the following functions
5985 # The operators -f, -d, -l, -b, -c, -p, and -S.
5987 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5988 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5990 # No direct -X operator counterpart, but for the first one
5991 # the -g operator is often equivalent. The ENFMT stands for
5992 # record flocking enforcement, a platform-dependent feature.
5994 S_ISENFMT($mode) S_ISWHT($mode)
5996 See your native chmod(2) and stat(2) documentation for more details
5997 about the C<S_*> constants. To get status info for a symbolic link
5998 instead of the target file behind the link, use the C<lstat> function.
6003 =item state TYPE EXPR
6005 =item state EXPR : ATTRS
6007 =item state TYPE EXPR : ATTRS
6009 C<state> declares a lexically scoped variable, just like C<my> does.
6010 However, those variables will never be reinitialized, contrary to
6011 lexical variables that are reinitialized each time their enclosing block
6014 C<state> variables are only enabled when the C<feature 'state'> pragma is
6015 in effect. See L<feature>.
6022 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6023 doing many pattern matches on the string before it is next modified.
6024 This may or may not save time, depending on the nature and number of
6025 patterns you are searching on, and on the distribution of character
6026 frequencies in the string to be searched--you probably want to compare
6027 run times with and without it to see which runs faster. Those loops
6028 that scan for many short constant strings (including the constant
6029 parts of more complex patterns) will benefit most. You may have only
6030 one C<study> active at a time--if you study a different scalar the first
6031 is "unstudied". (The way C<study> works is this: a linked list of every
6032 character in the string to be searched is made, so we know, for
6033 example, where all the C<'k'> characters are. From each search string,
6034 the rarest character is selected, based on some static frequency tables
6035 constructed from some C programs and English text. Only those places
6036 that contain this "rarest" character are examined.)
6038 For example, here is a loop that inserts index producing entries
6039 before any line containing a certain pattern:
6043 print ".IX foo\n" if /\bfoo\b/;
6044 print ".IX bar\n" if /\bbar\b/;
6045 print ".IX blurfl\n" if /\bblurfl\b/;
6050 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6051 will be looked at, because C<f> is rarer than C<o>. In general, this is
6052 a big win except in pathological cases. The only question is whether
6053 it saves you more time than it took to build the linked list in the
6056 Note that if you have to look for strings that you don't know till
6057 runtime, you can build an entire loop as a string and C<eval> that to
6058 avoid recompiling all your patterns all the time. Together with
6059 undefining C<$/> to input entire files as one record, this can be very
6060 fast, often faster than specialized programs like fgrep(1). The following
6061 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6062 out the names of those files that contain a match:
6064 $search = 'while (<>) { study;';
6065 foreach $word (@words) {
6066 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6071 eval $search; # this screams
6072 $/ = "\n"; # put back to normal input delimiter
6073 foreach $file (sort keys(%seen)) {
6077 =item sub NAME BLOCK
6080 =item sub NAME (PROTO) BLOCK
6082 =item sub NAME : ATTRS BLOCK
6084 =item sub NAME (PROTO) : ATTRS BLOCK
6086 This is subroutine definition, not a real function I<per se>.
6087 Without a BLOCK it's just a forward declaration. Without a NAME,
6088 it's an anonymous function declaration, and does actually return
6089 a value: the CODE ref of the closure you just created.
6091 See L<perlsub> and L<perlref> for details about subroutines and
6092 references, and L<attributes> and L<Attribute::Handlers> for more
6093 information about attributes.
6095 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6096 X<substr> X<substring> X<mid> X<left> X<right>
6098 =item substr EXPR,OFFSET,LENGTH
6100 =item substr EXPR,OFFSET
6102 Extracts a substring out of EXPR and returns it. First character is at
6103 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6104 If OFFSET is negative (or more precisely, less than C<$[>), starts
6105 that far from the end of the string. If LENGTH is omitted, returns
6106 everything to the end of the string. If LENGTH is negative, leaves that
6107 many characters off the end of the string.
6109 my $s = "The black cat climbed the green tree";
6110 my $color = substr $s, 4, 5; # black
6111 my $middle = substr $s, 4, -11; # black cat climbed the
6112 my $end = substr $s, 14; # climbed the green tree
6113 my $tail = substr $s, -4; # tree
6114 my $z = substr $s, -4, 2; # tr
6116 You can use the substr() function as an lvalue, in which case EXPR
6117 must itself be an lvalue. If you assign something shorter than LENGTH,
6118 the string will shrink, and if you assign something longer than LENGTH,
6119 the string will grow to accommodate it. To keep the string the same
6120 length you may need to pad or chop your value using C<sprintf>.
6122 If OFFSET and LENGTH specify a substring that is partly outside the
6123 string, only the part within the string is returned. If the substring
6124 is beyond either end of the string, substr() returns the undefined
6125 value and produces a warning. When used as an lvalue, specifying a
6126 substring that is entirely outside the string is a fatal error.
6127 Here's an example showing the behavior for boundary cases:
6130 substr($name, 4) = 'dy'; # $name is now 'freddy'
6131 my $null = substr $name, 6, 2; # returns '' (no warning)
6132 my $oops = substr $name, 7; # returns undef, with warning
6133 substr($name, 7) = 'gap'; # fatal error
6135 An alternative to using substr() as an lvalue is to specify the
6136 replacement string as the 4th argument. This allows you to replace
6137 parts of the EXPR and return what was there before in one operation,
6138 just as you can with splice().
6140 my $s = "The black cat climbed the green tree";
6141 my $z = substr $s, 14, 7, "jumped from"; # climbed
6142 # $s is now "The black cat jumped from the green tree"
6144 Note that the lvalue returned by the 3-arg version of substr() acts as
6145 a 'magic bullet'; each time it is assigned to, it remembers which part
6146 of the original string is being modified; for example:
6149 for (substr($x,1,2)) {
6150 $_ = 'a'; print $x,"\n"; # prints 1a4
6151 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6153 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6156 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6159 =item symlink OLDFILE,NEWFILE
6160 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6162 Creates a new filename symbolically linked to the old filename.
6163 Returns C<1> for success, C<0> otherwise. On systems that don't support
6164 symbolic links, produces a fatal error at run time. To check for that,
6167 $symlink_exists = eval { symlink("",""); 1 };
6169 =item syscall NUMBER, LIST
6170 X<syscall> X<system call>
6172 Calls the system call specified as the first element of the list,
6173 passing the remaining elements as arguments to the system call. If
6174 unimplemented, produces a fatal error. The arguments are interpreted
6175 as follows: if a given argument is numeric, the argument is passed as
6176 an int. If not, the pointer to the string value is passed. You are
6177 responsible to make sure a string is pre-extended long enough to
6178 receive any result that might be written into a string. You can't use a
6179 string literal (or other read-only string) as an argument to C<syscall>
6180 because Perl has to assume that any string pointer might be written
6182 integer arguments are not literals and have never been interpreted in a
6183 numeric context, you may need to add C<0> to them to force them to look
6184 like numbers. This emulates the C<syswrite> function (or vice versa):
6186 require 'syscall.ph'; # may need to run h2ph
6188 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6190 Note that Perl supports passing of up to only 14 arguments to your system call,
6191 which in practice should usually suffice.
6193 Syscall returns whatever value returned by the system call it calls.
6194 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6195 Note that some system calls can legitimately return C<-1>. The proper
6196 way to handle such calls is to assign C<$!=0;> before the call and
6197 check the value of C<$!> if syscall returns C<-1>.
6199 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6200 number of the read end of the pipe it creates. There is no way
6201 to retrieve the file number of the other end. You can avoid this
6202 problem by using C<pipe> instead.
6204 =item sysopen FILEHANDLE,FILENAME,MODE
6207 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6209 Opens the file whose filename is given by FILENAME, and associates it
6210 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6211 the name of the real filehandle wanted. This function calls the
6212 underlying operating system's C<open> function with the parameters
6213 FILENAME, MODE, PERMS.
6215 The possible values and flag bits of the MODE parameter are
6216 system-dependent; they are available via the standard module C<Fcntl>.
6217 See the documentation of your operating system's C<open> to see which
6218 values and flag bits are available. You may combine several flags
6219 using the C<|>-operator.
6221 Some of the most common values are C<O_RDONLY> for opening the file in
6222 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6223 and C<O_RDWR> for opening the file in read-write mode.
6224 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6226 For historical reasons, some values work on almost every system
6227 supported by perl: zero means read-only, one means write-only, and two
6228 means read/write. We know that these values do I<not> work under
6229 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6230 use them in new code.
6232 If the file named by FILENAME does not exist and the C<open> call creates
6233 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6234 PERMS specifies the permissions of the newly created file. If you omit
6235 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6236 These permission values need to be in octal, and are modified by your
6237 process's current C<umask>.
6240 In many systems the C<O_EXCL> flag is available for opening files in
6241 exclusive mode. This is B<not> locking: exclusiveness means here that
6242 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6243 on network filesystems, and has no effect unless the C<O_CREAT> flag
6244 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6245 being opened if it is a symbolic link. It does not protect against
6246 symbolic links in the file's path.
6249 Sometimes you may want to truncate an already-existing file. This
6250 can be done using the C<O_TRUNC> flag. The behavior of
6251 C<O_TRUNC> with C<O_RDONLY> is undefined.
6254 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6255 that takes away the user's option to have a more permissive umask.
6256 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6259 Note that C<sysopen> depends on the fdopen() C library function.
6260 On many UNIX systems, fdopen() is known to fail when file descriptors
6261 exceed a certain value, typically 255. If you need more file
6262 descriptors than that, consider rebuilding Perl to use the C<sfio>
6263 library, or perhaps using the POSIX::open() function.
6265 See L<perlopentut> for a kinder, gentler explanation of opening files.
6267 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6270 =item sysread FILEHANDLE,SCALAR,LENGTH
6272 Attempts to read LENGTH bytes of data into variable SCALAR from the
6273 specified FILEHANDLE, using the system call read(2). It bypasses
6274 buffered IO, so mixing this with other kinds of reads, C<print>,
6275 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6276 perlio or stdio layers usually buffers data. Returns the number of
6277 bytes actually read, C<0> at end of file, or undef if there was an
6278 error (in the latter case C<$!> is also set). SCALAR will be grown or
6279 shrunk so that the last byte actually read is the last byte of the
6280 scalar after the read.
6282 An OFFSET may be specified to place the read data at some place in the
6283 string other than the beginning. A negative OFFSET specifies
6284 placement at that many characters counting backwards from the end of
6285 the string. A positive OFFSET greater than the length of SCALAR
6286 results in the string being padded to the required size with C<"\0">
6287 bytes before the result of the read is appended.
6289 There is no syseof() function, which is ok, since eof() doesn't work
6290 very well on device files (like ttys) anyway. Use sysread() and check
6291 for a return value for 0 to decide whether you're done.
6293 Note that if the filehandle has been marked as C<:utf8> Unicode
6294 characters are read instead of bytes (the LENGTH, OFFSET, and the
6295 return value of sysread() are in Unicode characters).
6296 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6297 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6299 =item sysseek FILEHANDLE,POSITION,WHENCE
6302 Sets FILEHANDLE's system position in bytes using the system call
6303 lseek(2). FILEHANDLE may be an expression whose value gives the name
6304 of the filehandle. The values for WHENCE are C<0> to set the new
6305 position to POSITION, C<1> to set the it to the current position plus
6306 POSITION, and C<2> to set it to EOF plus POSITION (typically
6309 Note the I<in bytes>: even if the filehandle has been set to operate
6310 on characters (for example by using the C<:utf8> I/O layer), tell()
6311 will return byte offsets, not character offsets (because implementing
6312 that would render sysseek() very slow).
6314 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6315 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6316 C<seek>, C<tell>, or C<eof> may cause confusion.
6318 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6319 and C<SEEK_END> (start of the file, current position, end of the file)
6320 from the Fcntl module. Use of the constants is also more portable
6321 than relying on 0, 1, and 2. For example to define a "systell" function:
6323 use Fcntl 'SEEK_CUR';
6324 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6326 Returns the new position, or the undefined value on failure. A position
6327 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6328 true on success and false on failure, yet you can still easily determine
6334 =item system PROGRAM LIST
6336 Does exactly the same thing as C<exec LIST>, except that a fork is
6337 done first, and the parent process waits for the child process to
6338 complete. Note that argument processing varies depending on the
6339 number of arguments. If there is more than one argument in LIST,
6340 or if LIST is an array with more than one value, starts the program
6341 given by the first element of the list with arguments given by the
6342 rest of the list. If there is only one scalar argument, the argument
6343 is checked for shell metacharacters, and if there are any, the
6344 entire argument is passed to the system's command shell for parsing
6345 (this is C</bin/sh -c> on Unix platforms, but varies on other
6346 platforms). If there are no shell metacharacters in the argument,
6347 it is split into words and passed directly to C<execvp>, which is
6350 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6351 output before any operation that may do a fork, but this may not be
6352 supported on some platforms (see L<perlport>). To be safe, you may need
6353 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6354 of C<IO::Handle> on any open handles.
6356 The return value is the exit status of the program as returned by the
6357 C<wait> call. To get the actual exit value, shift right by eight (see
6358 below). See also L</exec>. This is I<not> what you want to use to capture
6359 the output from a command, for that you should use merely backticks or
6360 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6361 indicates a failure to start the program or an error of the wait(2) system
6362 call (inspect $! for the reason).
6364 Like C<exec>, C<system> allows you to lie to a program about its name if
6365 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6367 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6368 C<system>, if you expect your program to terminate on receipt of these
6369 signals you will need to arrange to do so yourself based on the return
6372 @args = ("command", "arg1", "arg2");
6374 or die "system @args failed: $?"
6376 You can check all the failure possibilities by inspecting
6380 print "failed to execute: $!\n";
6383 printf "child died with signal %d, %s coredump\n",
6384 ($? & 127), ($? & 128) ? 'with' : 'without';
6387 printf "child exited with value %d\n", $? >> 8;
6390 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6391 with the W*() calls of the POSIX extension.
6393 When the arguments get executed via the system shell, results
6394 and return codes will be subject to its quirks and capabilities.
6395 See L<perlop/"`STRING`"> and L</exec> for details.
6397 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6400 =item syswrite FILEHANDLE,SCALAR,LENGTH
6402 =item syswrite FILEHANDLE,SCALAR
6404 Attempts to write LENGTH bytes of data from variable SCALAR to the
6405 specified FILEHANDLE, using the system call write(2). If LENGTH is
6406 not specified, writes whole SCALAR. It bypasses buffered IO, so
6407 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6408 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6409 stdio layers usually buffers data. Returns the number of bytes
6410 actually written, or C<undef> if there was an error (in this case the
6411 errno variable C<$!> is also set). If the LENGTH is greater than the
6412 available data in the SCALAR after the OFFSET, only as much data as is
6413 available will be written.
6415 An OFFSET may be specified to write the data from some part of the
6416 string other than the beginning. A negative OFFSET specifies writing
6417 that many characters counting backwards from the end of the string.
6418 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6420 Note that if the filehandle has been marked as C<:utf8>, Unicode
6421 characters are written instead of bytes (the LENGTH, OFFSET, and the
6422 return value of syswrite() are in UTF-8 encoded Unicode characters).
6423 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6424 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6426 =item tell FILEHANDLE
6431 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6432 error. FILEHANDLE may be an expression whose value gives the name of
6433 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6436 Note the I<in bytes>: even if the filehandle has been set to
6437 operate on characters (for example by using the C<:utf8> open
6438 layer), tell() will return byte offsets, not character offsets
6439 (because that would render seek() and tell() rather slow).
6441 The return value of tell() for the standard streams like the STDIN
6442 depends on the operating system: it may return -1 or something else.
6443 tell() on pipes, fifos, and sockets usually returns -1.
6445 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6447 Do not use tell() (or other buffered I/O operations) on a file handle
6448 that has been manipulated by sysread(), syswrite() or sysseek().
6449 Those functions ignore the buffering, while tell() does not.
6451 =item telldir DIRHANDLE
6454 Returns the current position of the C<readdir> routines on DIRHANDLE.
6455 Value may be given to C<seekdir> to access a particular location in a
6456 directory. C<telldir> has the same caveats about possible directory
6457 compaction as the corresponding system library routine.
6459 =item tie VARIABLE,CLASSNAME,LIST
6462 This function binds a variable to a package class that will provide the
6463 implementation for the variable. VARIABLE is the name of the variable
6464 to be enchanted. CLASSNAME is the name of a class implementing objects
6465 of correct type. Any additional arguments are passed to the C<new>
6466 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6467 or C<TIEHASH>). Typically these are arguments such as might be passed
6468 to the C<dbm_open()> function of C. The object returned by the C<new>
6469 method is also returned by the C<tie> function, which would be useful
6470 if you want to access other methods in CLASSNAME.
6472 Note that functions such as C<keys> and C<values> may return huge lists
6473 when used on large objects, like DBM files. You may prefer to use the
6474 C<each> function to iterate over such. Example:
6476 # print out history file offsets
6478 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6479 while (($key,$val) = each %HIST) {
6480 print $key, ' = ', unpack('L',$val), "\n";
6484 A class implementing a hash should have the following methods:
6486 TIEHASH classname, LIST
6488 STORE this, key, value
6493 NEXTKEY this, lastkey
6498 A class implementing an ordinary array should have the following methods:
6500 TIEARRAY classname, LIST
6502 STORE this, key, value
6504 STORESIZE this, count
6510 SPLICE this, offset, length, LIST
6515 A class implementing a file handle should have the following methods:
6517 TIEHANDLE classname, LIST
6518 READ this, scalar, length, offset
6521 WRITE this, scalar, length, offset
6523 PRINTF this, format, LIST
6527 SEEK this, position, whence
6529 OPEN this, mode, LIST
6534 A class implementing a scalar should have the following methods:
6536 TIESCALAR classname, LIST
6542 Not all methods indicated above need be implemented. See L<perltie>,
6543 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6545 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6546 for you--you need to do that explicitly yourself. See L<DB_File>
6547 or the F<Config> module for interesting C<tie> implementations.
6549 For further details see L<perltie>, L<"tied VARIABLE">.
6554 Returns a reference to the object underlying VARIABLE (the same value
6555 that was originally returned by the C<tie> call that bound the variable
6556 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6562 Returns the number of non-leap seconds since whatever time the system
6563 considers to be the epoch, suitable for feeding to C<gmtime> and
6564 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6565 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6566 1904 in the current local time zone for its epoch.
6568 For measuring time in better granularity than one second,
6569 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6570 Perl 5.8 part of the standard distribution), or if you have
6571 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6572 See L<perlfaq8> for details.
6574 For date and time processing look at the many related modules on CPAN.
6575 For a comprehensive date and time representation look at the
6581 Returns a four-element list giving the user and system times, in
6582 seconds, for this process and the children of this process.
6584 ($user,$system,$cuser,$csystem) = times;
6586 In scalar context, C<times> returns C<$user>.
6588 Note that times for children are included only after they terminate.
6592 The transliteration operator. Same as C<y///>. See L<perlop>.
6594 =item truncate FILEHANDLE,LENGTH
6597 =item truncate EXPR,LENGTH
6599 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6600 specified length. Produces a fatal error if truncate isn't implemented
6601 on your system. Returns true if successful, the undefined value
6604 The behavior is undefined if LENGTH is greater than the length of the
6607 The position in the file of FILEHANDLE is left unchanged. You may want to
6608 call L<seek> before writing to the file.
6611 X<uc> X<uppercase> X<toupper>
6615 Returns an uppercased version of EXPR. This is the internal function
6616 implementing the C<\U> escape in double-quoted strings. Respects
6617 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6618 and L<perlunicode> for more details about locale and Unicode support.
6619 It does not attempt to do titlecase mapping on initial letters. See
6620 C<ucfirst> for that.
6622 If EXPR is omitted, uses C<$_>.
6625 X<ucfirst> X<uppercase>
6629 Returns the value of EXPR with the first character in uppercase
6630 (titlecase in Unicode). This is the internal function implementing
6631 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6632 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6633 for more details about locale and Unicode support.
6635 If EXPR is omitted, uses C<$_>.
6642 Sets the umask for the process to EXPR and returns the previous value.
6643 If EXPR is omitted, merely returns the current umask.
6645 The Unix permission C<rwxr-x---> is represented as three sets of three
6646 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6647 and isn't one of the digits). The C<umask> value is such a number
6648 representing disabled permissions bits. The permission (or "mode")
6649 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6650 even if you tell C<sysopen> to create a file with permissions C<0777>,
6651 if your umask is C<0022> then the file will actually be created with
6652 permissions C<0755>. If your C<umask> were C<0027> (group can't
6653 write; others can't read, write, or execute), then passing
6654 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6657 Here's some advice: supply a creation mode of C<0666> for regular
6658 files (in C<sysopen>) and one of C<0777> for directories (in
6659 C<mkdir>) and executable files. This gives users the freedom of
6660 choice: if they want protected files, they might choose process umasks
6661 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6662 Programs should rarely if ever make policy decisions better left to
6663 the user. The exception to this is when writing files that should be
6664 kept private: mail files, web browser cookies, I<.rhosts> files, and
6667 If umask(2) is not implemented on your system and you are trying to
6668 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6669 fatal error at run time. If umask(2) is not implemented and you are
6670 not trying to restrict access for yourself, returns C<undef>.
6672 Remember that a umask is a number, usually given in octal; it is I<not> a
6673 string of octal digits. See also L</oct>, if all you have is a string.
6676 X<undef> X<undefine>
6680 Undefines the value of EXPR, which must be an lvalue. Use only on a
6681 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6682 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6683 will probably not do what you expect on most predefined variables or
6684 DBM list values, so don't do that; see L<delete>.) Always returns the
6685 undefined value. You can omit the EXPR, in which case nothing is
6686 undefined, but you still get an undefined value that you could, for
6687 instance, return from a subroutine, assign to a variable or pass as a
6688 parameter. Examples:
6691 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6695 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6696 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6697 select undef, undef, undef, 0.25;
6698 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6700 Note that this is a unary operator, not a list operator.
6703 X<unlink> X<delete> X<remove> X<rm> X<del>
6707 Deletes a list of files. Returns the number of files successfully
6710 $cnt = unlink 'a', 'b', 'c';
6714 Note: C<unlink> will not attempt to delete directories unless you are superuser
6715 and the B<-U> flag is supplied to Perl. Even if these conditions are
6716 met, be warned that unlinking a directory can inflict damage on your
6717 filesystem. Finally, using C<unlink> on directories is not supported on
6718 many operating systems. Use C<rmdir> instead.
6720 If LIST is omitted, uses C<$_>.
6722 =item unpack TEMPLATE,EXPR
6725 =item unpack TEMPLATE
6727 C<unpack> does the reverse of C<pack>: it takes a string
6728 and expands it out into a list of values.
6729 (In scalar context, it returns merely the first value produced.)
6731 If EXPR is omitted, unpacks the C<$_> string.
6733 The string is broken into chunks described by the TEMPLATE. Each chunk
6734 is converted separately to a value. Typically, either the string is a result
6735 of C<pack>, or the characters of the string represent a C structure of some
6738 The TEMPLATE has the same format as in the C<pack> function.
6739 Here's a subroutine that does substring:
6742 my($what,$where,$howmuch) = @_;
6743 unpack("x$where a$howmuch", $what);
6748 sub ordinal { unpack("W",$_[0]); } # same as ord()
6750 In addition to fields allowed in pack(), you may prefix a field with
6751 a %<number> to indicate that
6752 you want a <number>-bit checksum of the items instead of the items
6753 themselves. Default is a 16-bit checksum. Checksum is calculated by
6754 summing numeric values of expanded values (for string fields the sum of
6755 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6757 For example, the following
6758 computes the same number as the System V sum program:
6762 unpack("%32W*",<>) % 65535;
6765 The following efficiently counts the number of set bits in a bit vector:
6767 $setbits = unpack("%32b*", $selectmask);
6769 The C<p> and C<P> formats should be used with care. Since Perl
6770 has no way of checking whether the value passed to C<unpack()>
6771 corresponds to a valid memory location, passing a pointer value that's
6772 not known to be valid is likely to have disastrous consequences.
6774 If there are more pack codes or if the repeat count of a field or a group
6775 is larger than what the remainder of the input string allows, the result
6776 is not well defined: in some cases, the repeat count is decreased, or
6777 C<unpack()> will produce null strings or zeroes, or terminate with an
6778 error. If the input string is longer than one described by the TEMPLATE,
6779 the rest is ignored.
6781 See L</pack> for more examples and notes.
6783 =item untie VARIABLE
6786 Breaks the binding between a variable and a package. (See C<tie>.)
6787 Has no effect if the variable is not tied.
6789 =item unshift ARRAY,LIST
6792 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6793 depending on how you look at it. Prepends list to the front of the
6794 array, and returns the new number of elements in the array.
6796 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6798 Note the LIST is prepended whole, not one element at a time, so the
6799 prepended elements stay in the same order. Use C<reverse> to do the
6802 =item use Module VERSION LIST
6803 X<use> X<module> X<import>
6805 =item use Module VERSION
6807 =item use Module LIST
6813 Imports some semantics into the current package from the named module,
6814 generally by aliasing certain subroutine or variable names into your
6815 package. It is exactly equivalent to
6817 BEGIN { require Module; import Module LIST; }
6819 except that Module I<must> be a bareword.
6821 In the peculiar C<use VERSION> form, VERSION may be either a numeric
6822 argument such as 5.006, which will be compared to C<$]>, or a literal of
6823 the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A
6824 fatal error is produced if VERSION is greater than the version of the
6825 current Perl interpreter; Perl will not attempt to parse the rest of the
6826 file. Compare with L</require>, which can do a similar check at run time.
6827 Symmetrically, C<no VERSION> allows you to specify that you want a version
6828 of perl older than the specified one.
6830 Specifying VERSION as a literal of the form v5.6.1 should generally be
6831 avoided, because it leads to misleading error messages under earlier
6832 versions of Perl that do not support this syntax. The equivalent numeric
6833 version should be used instead.
6835 use v5.6.1; # compile time version check
6837 use 5.006_001; # ditto; preferred for backwards compatibility
6839 This is often useful if you need to check the current Perl version before
6840 C<use>ing library modules that have changed in incompatible ways from
6841 older versions of Perl. (We try not to do this more than we have to.)
6843 Also, if the specified perl version is greater than or equal to 5.9.5,
6844 C<use VERSION> will also load the C<feature> pragma and enable all
6845 features available in the requested version. See L<feature>.
6847 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6848 C<require> makes sure the module is loaded into memory if it hasn't been
6849 yet. The C<import> is not a builtin--it's just an ordinary static method
6850 call into the C<Module> package to tell the module to import the list of
6851 features back into the current package. The module can implement its
6852 C<import> method any way it likes, though most modules just choose to
6853 derive their C<import> method via inheritance from the C<Exporter> class that
6854 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6855 method can be found then the call is skipped, even if there is an AUTOLOAD
6858 If you do not want to call the package's C<import> method (for instance,
6859 to stop your namespace from being altered), explicitly supply the empty list:
6863 That is exactly equivalent to
6865 BEGIN { require Module }
6867 If the VERSION argument is present between Module and LIST, then the
6868 C<use> will call the VERSION method in class Module with the given
6869 version as an argument. The default VERSION method, inherited from
6870 the UNIVERSAL class, croaks if the given version is larger than the
6871 value of the variable C<$Module::VERSION>.
6873 Again, there is a distinction between omitting LIST (C<import> called
6874 with no arguments) and an explicit empty LIST C<()> (C<import> not
6875 called). Note that there is no comma after VERSION!
6877 Because this is a wide-open interface, pragmas (compiler directives)
6878 are also implemented this way. Currently implemented pragmas are:
6883 use sigtrap qw(SEGV BUS);
6884 use strict qw(subs vars refs);
6885 use subs qw(afunc blurfl);
6886 use warnings qw(all);
6887 use sort qw(stable _quicksort _mergesort);
6889 Some of these pseudo-modules import semantics into the current
6890 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6891 which import symbols into the current package (which are effective
6892 through the end of the file).
6894 There's a corresponding C<no> command that unimports meanings imported
6895 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6896 It behaves exactly as C<import> does with respect to VERSION, an
6897 omitted LIST, empty LIST, or no unimport method being found.
6903 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6904 for the C<-M> and C<-m> command-line options to perl that give C<use>
6905 functionality from the command-line.
6910 Changes the access and modification times on each file of a list of
6911 files. The first two elements of the list must be the NUMERICAL access
6912 and modification times, in that order. Returns the number of files
6913 successfully changed. The inode change time of each file is set
6914 to the current time. For example, this code has the same effect as the
6915 Unix touch(1) command when the files I<already exist> and belong to
6916 the user running the program:
6919 $atime = $mtime = time;
6920 utime $atime, $mtime, @ARGV;
6922 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6923 the utime(2) function in the C library will be called with a null second
6924 argument. On most systems, this will set the file's access and
6925 modification times to the current time (i.e. equivalent to the example
6926 above) and will even work on other users' files where you have write
6929 utime undef, undef, @ARGV;
6931 Under NFS this will use the time of the NFS server, not the time of
6932 the local machine. If there is a time synchronization problem, the
6933 NFS server and local machine will have different times. The Unix
6934 touch(1) command will in fact normally use this form instead of the
6935 one shown in the first example.
6937 Note that only passing one of the first two elements as C<undef> will
6938 be equivalent of passing it as 0 and will not have the same effect as
6939 described when they are both C<undef>. This case will also trigger an
6940 uninitialized warning.
6942 On systems that support futimes, you might pass file handles among the
6943 files. On systems that don't support futimes, passing file handles
6944 produces a fatal error at run time. The file handles must be passed
6945 as globs or references to be recognized. Barewords are considered
6951 Returns a list consisting of all the values of the named hash.
6952 (In a scalar context, returns the number of values.)
6954 The values are returned in an apparently random order. The actual
6955 random order is subject to change in future versions of perl, but it
6956 is guaranteed to be the same order as either the C<keys> or C<each>
6957 function would produce on the same (unmodified) hash. Since Perl
6958 5.8.1 the ordering is different even between different runs of Perl
6959 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6961 As a side effect, calling values() resets the HASH's internal iterator,
6962 see L</each>. (In particular, calling values() in void context resets
6963 the iterator with no other overhead.)
6965 Note that the values are not copied, which means modifying them will
6966 modify the contents of the hash:
6968 for (values %hash) { s/foo/bar/g } # modifies %hash values
6969 for (@hash{keys %hash}) { s/foo/bar/g } # same
6971 See also C<keys>, C<each>, and C<sort>.
6973 =item vec EXPR,OFFSET,BITS
6974 X<vec> X<bit> X<bit vector>
6976 Treats the string in EXPR as a bit vector made up of elements of
6977 width BITS, and returns the value of the element specified by OFFSET
6978 as an unsigned integer. BITS therefore specifies the number of bits
6979 that are reserved for each element in the bit vector. This must
6980 be a power of two from 1 to 32 (or 64, if your platform supports
6983 If BITS is 8, "elements" coincide with bytes of the input string.
6985 If BITS is 16 or more, bytes of the input string are grouped into chunks
6986 of size BITS/8, and each group is converted to a number as with
6987 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6988 for BITS==64). See L<"pack"> for details.
6990 If bits is 4 or less, the string is broken into bytes, then the bits
6991 of each byte are broken into 8/BITS groups. Bits of a byte are
6992 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6993 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6994 breaking the single input byte C<chr(0x36)> into two groups gives a list
6995 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6997 C<vec> may also be assigned to, in which case parentheses are needed
6998 to give the expression the correct precedence as in
7000 vec($image, $max_x * $x + $y, 8) = 3;
7002 If the selected element is outside the string, the value 0 is returned.
7003 If an element off the end of the string is written to, Perl will first
7004 extend the string with sufficiently many zero bytes. It is an error
7005 to try to write off the beginning of the string (i.e. negative OFFSET).
7007 If the string happens to be encoded as UTF-8 internally (and thus has
7008 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7009 internal byte string, not the conceptual character string, even if you
7010 only have characters with values less than 256.
7012 Strings created with C<vec> can also be manipulated with the logical
7013 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7014 vector operation is desired when both operands are strings.
7015 See L<perlop/"Bitwise String Operators">.
7017 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7018 The comments show the string after each step. Note that this code works
7019 in the same way on big-endian or little-endian machines.
7022 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7024 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7025 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7027 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7028 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7029 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7030 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7031 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7032 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7034 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7035 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7036 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7039 To transform a bit vector into a string or list of 0's and 1's, use these:
7041 $bits = unpack("b*", $vector);
7042 @bits = split(//, unpack("b*", $vector));
7044 If you know the exact length in bits, it can be used in place of the C<*>.
7046 Here is an example to illustrate how the bits actually fall in place:
7052 unpack("V",$_) 01234567890123456789012345678901
7053 ------------------------------------------------------------------
7058 for ($shift=0; $shift < $width; ++$shift) {
7059 for ($off=0; $off < 32/$width; ++$off) {
7060 $str = pack("B*", "0"x32);
7061 $bits = (1<<$shift);
7062 vec($str, $off, $width) = $bits;
7063 $res = unpack("b*",$str);
7064 $val = unpack("V", $str);
7071 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7072 $off, $width, $bits, $val, $res
7076 Regardless of the machine architecture on which it is run, the above
7077 example should print the following table:
7080 unpack("V",$_) 01234567890123456789012345678901
7081 ------------------------------------------------------------------
7082 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7083 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7084 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7085 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7086 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7087 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7088 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7089 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7090 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7091 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7092 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7093 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7094 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7095 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7096 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7097 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7098 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7099 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7100 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7101 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7102 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7103 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7104 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7105 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7106 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7107 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7108 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7109 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7110 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7111 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7112 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7113 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7114 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7115 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7116 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7117 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7118 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7119 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7120 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7121 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7122 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7123 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7124 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7125 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7126 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7127 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7128 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7129 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7130 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7131 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7132 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7133 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7134 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7135 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7136 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7137 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7138 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7139 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7140 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7141 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7142 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7143 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7144 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7145 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7146 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7147 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7148 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7149 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7150 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7151 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7152 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7153 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7154 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7155 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7156 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7157 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7158 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7159 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7160 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7161 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7162 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7163 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7164 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7165 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7166 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7167 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7168 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7169 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7170 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7171 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7172 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7173 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7174 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7175 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7176 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7177 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7178 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7179 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7180 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7181 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7182 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7183 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7184 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7185 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7186 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7187 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7188 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7189 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7190 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7191 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7192 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7193 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7194 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7195 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7196 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7197 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7198 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7199 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7200 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7201 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7202 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7203 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7204 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7205 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7206 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7207 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7208 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7209 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7214 Behaves like the wait(2) system call on your system: it waits for a child
7215 process to terminate and returns the pid of the deceased process, or
7216 C<-1> if there are no child processes. The status is returned in C<$?>
7217 and C<{^CHILD_ERROR_NATIVE}>.
7218 Note that a return value of C<-1> could mean that child processes are
7219 being automatically reaped, as described in L<perlipc>.
7221 =item waitpid PID,FLAGS
7224 Waits for a particular child process to terminate and returns the pid of
7225 the deceased process, or C<-1> if there is no such child process. On some
7226 systems, a value of 0 indicates that there are processes still running.
7227 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7229 use POSIX ":sys_wait_h";
7232 $kid = waitpid(-1, WNOHANG);
7235 then you can do a non-blocking wait for all pending zombie processes.
7236 Non-blocking wait is available on machines supporting either the
7237 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7238 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7239 system call by remembering the status values of processes that have
7240 exited but have not been harvested by the Perl script yet.)
7242 Note that on some systems, a return value of C<-1> could mean that child
7243 processes are being automatically reaped. See L<perlipc> for details,
7244 and for other examples.
7247 X<wantarray> X<context>
7249 Returns true if the context of the currently executing subroutine or
7250 C<eval> is looking for a list value. Returns false if the context is
7251 looking for a scalar. Returns the undefined value if the context is
7252 looking for no value (void context).
7254 return unless defined wantarray; # don't bother doing more
7255 my @a = complex_calculation();
7256 return wantarray ? @a : "@a";
7258 C<wantarray()>'s result is unspecified in the top level of a file,
7259 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7260 in a C<DESTROY> method.
7262 This function should have been named wantlist() instead.
7265 X<warn> X<warning> X<STDERR>
7267 Prints the value of LIST to STDERR. If the last element of LIST does
7268 not end in a newline, it appends the same file/line number text as C<die>
7271 If LIST is empty and C<$@> already contains a value (typically from a
7272 previous eval) that value is used after appending C<"\t...caught">
7273 to C<$@>. This is useful for staying almost, but not entirely similar to
7276 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7278 No message is printed if there is a C<$SIG{__WARN__}> handler
7279 installed. It is the handler's responsibility to deal with the message
7280 as it sees fit (like, for instance, converting it into a C<die>). Most
7281 handlers must therefore make arrangements to actually display the
7282 warnings that they are not prepared to deal with, by calling C<warn>
7283 again in the handler. Note that this is quite safe and will not
7284 produce an endless loop, since C<__WARN__> hooks are not called from
7287 You will find this behavior is slightly different from that of
7288 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7289 instead call C<die> again to change it).
7291 Using a C<__WARN__> handler provides a powerful way to silence all
7292 warnings (even the so-called mandatory ones). An example:
7294 # wipe out *all* compile-time warnings
7295 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7297 my $foo = 20; # no warning about duplicate my $foo,
7298 # but hey, you asked for it!
7299 # no compile-time or run-time warnings before here
7302 # run-time warnings enabled after here
7303 warn "\$foo is alive and $foo!"; # does show up
7305 See L<perlvar> for details on setting C<%SIG> entries, and for more
7306 examples. See the Carp module for other kinds of warnings using its
7307 carp() and cluck() functions.
7309 =item write FILEHANDLE
7316 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7317 using the format associated with that file. By default the format for
7318 a file is the one having the same name as the filehandle, but the
7319 format for the current output channel (see the C<select> function) may be set
7320 explicitly by assigning the name of the format to the C<$~> variable.
7322 Top of form processing is handled automatically: if there is
7323 insufficient room on the current page for the formatted record, the
7324 page is advanced by writing a form feed, a special top-of-page format
7325 is used to format the new page header, and then the record is written.
7326 By default the top-of-page format is the name of the filehandle with
7327 "_TOP" appended, but it may be dynamically set to the format of your
7328 choice by assigning the name to the C<$^> variable while the filehandle is
7329 selected. The number of lines remaining on the current page is in
7330 variable C<$->, which can be set to C<0> to force a new page.
7332 If FILEHANDLE is unspecified, output goes to the current default output
7333 channel, which starts out as STDOUT but may be changed by the
7334 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7335 is evaluated and the resulting string is used to look up the name of
7336 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7338 Note that write is I<not> the opposite of C<read>. Unfortunately.
7342 The transliteration operator. Same as C<tr///>. See L<perlop>.