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, and
300 the argument may be parenthesized like any other unary operator. The
301 operator may be any of:
303 -r File is readable by effective uid/gid.
304 -w File is writable by effective uid/gid.
305 -x File is executable by effective uid/gid.
306 -o File is owned by effective uid.
308 -R File is readable by real uid/gid.
309 -W File is writable by real uid/gid.
310 -X File is executable by real uid/gid.
311 -O File is owned by real uid.
314 -z File has zero size (is empty).
315 -s File has nonzero size (returns size in bytes).
317 -f File is a plain file.
318 -d File is a directory.
319 -l File is a symbolic link.
320 -p File is a named pipe (FIFO), or Filehandle is a pipe.
322 -b File is a block special file.
323 -c File is a character special file.
324 -t Filehandle is opened to a tty.
326 -u File has setuid bit set.
327 -g File has setgid bit set.
328 -k File has sticky bit set.
330 -T File is an ASCII text file (heuristic guess).
331 -B File is a "binary" file (opposite of -T).
333 -M Script start time minus file modification time, in days.
334 -A Same for access time.
335 -C Same for inode change time (Unix, may differ for other platforms)
341 next unless -f $_; # ignore specials
345 The interpretation of the file permission operators C<-r>, C<-R>,
346 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
347 of the file and the uids and gids of the user. There may be other
348 reasons you can't actually read, write, or execute the file. Such
349 reasons may be for example network filesystem access controls, ACLs
350 (access control lists), read-only filesystems, and unrecognized
353 Also note that, for the superuser on the local filesystems, the C<-r>,
354 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
355 if any execute bit is set in the mode. Scripts run by the superuser
356 may thus need to do a stat() to determine the actual mode of the file,
357 or temporarily set their effective uid to something else.
359 If you are using ACLs, there is a pragma called C<filetest> that may
360 produce more accurate results than the bare stat() mode bits.
361 When under the C<use filetest 'access'> the above-mentioned filetests
362 will test whether the permission can (not) be granted using the
363 access() family of system calls. Also note that the C<-x> and C<-X> may
364 under this pragma return true even if there are no execute permission
365 bits set (nor any extra execute permission ACLs). This strangeness is
366 due to the underlying system calls' definitions. Read the
367 documentation for the C<filetest> pragma for more information.
369 Note that C<-s/a/b/> does not do a negated substitution. Saying
370 C<-exp($foo)> still works as expected, however--only single letters
371 following a minus are interpreted as file tests.
373 The C<-T> and C<-B> switches work as follows. The first block or so of the
374 file is examined for odd characters such as strange control codes or
375 characters with the high bit set. If too many strange characters (>30%)
376 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
377 containing null in the first block is considered a binary file. If C<-T>
378 or C<-B> is used on a filehandle, the current IO buffer is examined
379 rather than the first block. Both C<-T> and C<-B> return true on a null
380 file, or a file at EOF when testing a filehandle. Because you have to
381 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
382 against the file first, as in C<next unless -f $file && -T $file>.
384 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
385 the special filehandle consisting of a solitary underline, then the stat
386 structure of the previous file test (or stat operator) is used, saving
387 a system call. (This doesn't work with C<-t>, and you need to remember
388 that lstat() and C<-l> will leave values in the stat structure for the
389 symbolic link, not the real file.) (Also, if the stat buffer was filled by
390 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
393 print "Can do.\n" if -r $a || -w _ || -x _;
396 print "Readable\n" if -r _;
397 print "Writable\n" if -w _;
398 print "Executable\n" if -x _;
399 print "Setuid\n" if -u _;
400 print "Setgid\n" if -g _;
401 print "Sticky\n" if -k _;
402 print "Text\n" if -T _;
403 print "Binary\n" if -B _;
405 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
406 test operators, in a way that C<-f -w -x $file> is equivalent to
407 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
408 the return value of C<-f $file> as an argument to another filetest
409 operator, no special magic will happen.)
416 Returns the absolute value of its argument.
417 If VALUE is omitted, uses C<$_>.
419 =item accept NEWSOCKET,GENERICSOCKET
422 Accepts an incoming socket connect, just as the accept(2) system call
423 does. Returns the packed address if it succeeded, false otherwise.
424 See the example in L<perlipc/"Sockets: Client/Server Communication">.
426 On systems that support a close-on-exec flag on files, the flag will
427 be set for the newly opened file descriptor, as determined by the
428 value of $^F. See L<perlvar/$^F>.
437 Arranges to have a SIGALRM delivered to this process after the
438 specified number of wallclock seconds has elapsed. If SECONDS is not
439 specified, the value stored in C<$_> is used. (On some machines,
440 unfortunately, the elapsed time may be up to one second less or more
441 than you specified because of how seconds are counted, and process
442 scheduling may delay the delivery of the signal even further.)
444 Only one timer may be counting at once. Each call disables the
445 previous timer, and an argument of C<0> may be supplied to cancel the
446 previous timer without starting a new one. The returned value is the
447 amount of time remaining on the previous timer.
449 For delays of finer granularity than one second, you may use Perl's
450 four-argument version of select() leaving the first three arguments
451 undefined, or you might be able to use the C<syscall> interface to
452 access setitimer(2) if your system supports it. The Time::HiRes
453 module (from CPAN, and starting from Perl 5.8 part of the standard
454 distribution) may also prove useful.
456 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
457 (C<sleep> may be internally implemented in your system with C<alarm>)
459 If you want to use C<alarm> to time out a system call you need to use an
460 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
461 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
462 restart system calls on some systems. Using C<eval>/C<die> always works,
463 modulo the caveats given in L<perlipc/"Signals">.
466 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
468 $nread = sysread SOCKET, $buffer, $size;
472 die unless $@ eq "alarm\n"; # propagate unexpected errors
479 For more information see L<perlipc>.
482 X<atan2> X<arctangent> X<tan> X<tangent>
484 Returns the arctangent of Y/X in the range -PI to PI.
486 For the tangent operation, you may use the C<Math::Trig::tan>
487 function, or use the familiar relation:
489 sub tan { sin($_[0]) / cos($_[0]) }
491 Note that atan2(0, 0) is not well-defined.
493 =item bind SOCKET,NAME
496 Binds a network address to a socket, just as the bind system call
497 does. Returns true if it succeeded, false otherwise. NAME should be a
498 packed address of the appropriate type for the socket. See the examples in
499 L<perlipc/"Sockets: Client/Server Communication">.
501 =item binmode FILEHANDLE, LAYER
502 X<binmode> X<binary> X<text> X<DOS> X<Windows>
504 =item binmode FILEHANDLE
506 Arranges for FILEHANDLE to be read or written in "binary" or "text"
507 mode on systems where the run-time libraries distinguish between
508 binary and text files. If FILEHANDLE is an expression, the value is
509 taken as the name of the filehandle. Returns true on success,
510 otherwise it returns C<undef> and sets C<$!> (errno).
512 On some systems (in general, DOS and Windows-based systems) binmode()
513 is necessary when you're not working with a text file. For the sake
514 of portability it is a good idea to always use it when appropriate,
515 and to never use it when it isn't appropriate. Also, people can
516 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
518 In other words: regardless of platform, use binmode() on binary data,
519 like for example images.
521 If LAYER is present it is a single string, but may contain multiple
522 directives. The directives alter the behaviour of the file handle.
523 When LAYER is present using binmode on text file makes sense.
525 If LAYER is omitted or specified as C<:raw> the filehandle is made
526 suitable for passing binary data. This includes turning off possible CRLF
527 translation and marking it as bytes (as opposed to Unicode characters).
528 Note that, despite what may be implied in I<"Programming Perl"> (the
529 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
530 -- other layers which would affect the binary nature of the stream are
531 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
532 PERLIO environment variable.
534 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
535 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
536 establish default I/O layers. See L<open>.
538 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
539 in "Programming Perl, 3rd Edition". However, since the publishing of this
540 book, by many known as "Camel III", the consensus of the naming of this
541 functionality has moved from "discipline" to "layer". All documentation
542 of this version of Perl therefore refers to "layers" rather than to
543 "disciplines". Now back to the regularly scheduled documentation...>
545 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
546 C<:utf8> just marks the data as UTF-8 without further checking,
547 while C<:encoding(utf8)> checks the data for actually being valid
548 UTF-8. More details can be found in L<PerlIO::encoding>.
550 In general, binmode() should be called after open() but before any I/O
551 is done on the filehandle. Calling binmode() will normally flush any
552 pending buffered output data (and perhaps pending input data) on the
553 handle. An exception to this is the C<:encoding> layer that
554 changes the default character encoding of the handle, see L<open>.
555 The C<:encoding> layer sometimes needs to be called in
556 mid-stream, and it doesn't flush the stream. The C<:encoding>
557 also implicitly pushes on top of itself the C<:utf8> layer because
558 internally Perl will operate on UTF-8 encoded Unicode characters.
560 The operating system, device drivers, C libraries, and Perl run-time
561 system all work together to let the programmer treat a single
562 character (C<\n>) as the line terminator, irrespective of the external
563 representation. On many operating systems, the native text file
564 representation matches the internal representation, but on some
565 platforms the external representation of C<\n> is made up of more than
568 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
569 character to end each line in the external representation of text (even
570 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
571 on Unix and most VMS files). In other systems like OS/2, DOS and the
572 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
573 but what's stored in text files are the two characters C<\cM\cJ>. That
574 means that, if you don't use binmode() on these systems, C<\cM\cJ>
575 sequences on disk will be converted to C<\n> on input, and any C<\n> in
576 your program will be converted back to C<\cM\cJ> on output. This is what
577 you want for text files, but it can be disastrous for binary files.
579 Another consequence of using binmode() (on some systems) is that
580 special end-of-file markers will be seen as part of the data stream.
581 For systems from the Microsoft family this means that if your binary
582 data contains C<\cZ>, the I/O subsystem will regard it as the end of
583 the file, unless you use binmode().
585 binmode() is not only important for readline() and print() operations,
586 but also when using read(), seek(), sysread(), syswrite() and tell()
587 (see L<perlport> for more details). See the C<$/> and C<$\> variables
588 in L<perlvar> for how to manually set your input and output
589 line-termination sequences.
591 =item bless REF,CLASSNAME
596 This function tells the thingy referenced by REF that it is now an object
597 in the CLASSNAME package. If CLASSNAME is omitted, the current package
598 is used. Because a C<bless> is often the last thing in a constructor,
599 it returns the reference for convenience. Always use the two-argument
600 version if a derived class might inherit the function doing the blessing.
601 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
604 Consider always blessing objects in CLASSNAMEs that are mixed case.
605 Namespaces with all lowercase names are considered reserved for
606 Perl pragmata. Builtin types have all uppercase names. To prevent
607 confusion, you may wish to avoid such package names as well. Make sure
608 that CLASSNAME is a true value.
610 See L<perlmod/"Perl Modules">.
614 Break out of a C<given()> block.
616 This keyword is enabled by the "switch" feature: see L<feature>
617 for more information.
620 X<caller> X<call stack> X<stack> X<stack trace>
624 Returns the context of the current subroutine call. In scalar context,
625 returns the caller's package name if there is a caller, that is, if
626 we're in a subroutine or C<eval> or C<require>, and the undefined value
627 otherwise. In list context, returns
630 ($package, $filename, $line) = caller;
632 With EXPR, it returns some extra information that the debugger uses to
633 print a stack trace. The value of EXPR indicates how many call frames
634 to go back before the current one.
637 ($package, $filename, $line, $subroutine, $hasargs,
640 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
643 Here $subroutine may be C<(eval)> if the frame is not a subroutine
644 call, but an C<eval>. In such a case additional elements $evaltext and
645 C<$is_require> are set: C<$is_require> is true if the frame is created by a
646 C<require> or C<use> statement, $evaltext contains the text of the
647 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
648 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
649 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
650 frame.) $subroutine may also be C<(unknown)> if this particular
651 subroutine happens to have been deleted from the symbol table.
652 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
653 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
654 compiled with. The C<$hints> and C<$bitmask> values are subject to change
655 between versions of Perl, and are not meant for external use.
657 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
658 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
659 of this hash, as they are the actual values stored in the optree.
661 Furthermore, when called from within the DB package, caller returns more
662 detailed information: it sets the list variable C<@DB::args> to be the
663 arguments with which the subroutine was invoked.
665 Be aware that the optimizer might have optimized call frames away before
666 C<caller> had a chance to get the information. That means that C<caller(N)>
667 might not return information about the call frame you expect it do, for
668 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
669 previous time C<caller> was called.
676 =item chdir FILEHANDLE
678 =item chdir DIRHANDLE
682 Changes the working directory to EXPR, if possible. If EXPR is omitted,
683 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
684 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
685 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
686 neither is set, C<chdir> does nothing. It returns true upon success,
687 false otherwise. See the example under C<die>.
689 On systems that support fchdir, you might pass a file handle or
690 directory handle as argument. On systems that don't support fchdir,
691 passing handles produces a fatal error at run time.
694 X<chmod> X<permission> X<mode>
696 Changes the permissions of a list of files. The first element of the
697 list must be the numerical mode, which should probably be an octal
698 number, and which definitely should I<not> be a string of octal digits:
699 C<0644> is okay, C<'0644'> is not. Returns the number of files
700 successfully changed. See also L</oct>, if all you have is a string.
702 $cnt = chmod 0755, 'foo', 'bar';
703 chmod 0755, @executables;
704 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
706 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
707 $mode = 0644; chmod $mode, 'foo'; # this is best
709 On systems that support fchmod, you might pass file handles among the
710 files. On systems that don't support fchmod, passing file handles
711 produces a fatal error at run time. The file handles must be passed
712 as globs or references to be recognized. Barewords are considered
715 open(my $fh, "<", "foo");
716 my $perm = (stat $fh)[2] & 07777;
717 chmod($perm | 0600, $fh);
719 You can also import the symbolic C<S_I*> constants from the Fcntl
724 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
725 # This is identical to the chmod 0755 of the above example.
728 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
734 This safer version of L</chop> removes any trailing string
735 that corresponds to the current value of C<$/> (also known as
736 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
737 number of characters removed from all its arguments. It's often used to
738 remove the newline from the end of an input record when you're worried
739 that the final record may be missing its newline. When in paragraph
740 mode (C<$/ = "">), it removes all trailing newlines from the string.
741 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
742 a reference to an integer or the like, see L<perlvar>) chomp() won't
744 If VARIABLE is omitted, it chomps C<$_>. Example:
747 chomp; # avoid \n on last field
752 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
754 You can actually chomp anything that's an lvalue, including an assignment:
757 chomp($answer = <STDIN>);
759 If you chomp a list, each element is chomped, and the total number of
760 characters removed is returned.
762 Note that parentheses are necessary when you're chomping anything
763 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
764 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
765 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
766 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
776 Chops off the last character of a string and returns the character
777 chopped. It is much more efficient than C<s/.$//s> because it neither
778 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
779 If VARIABLE is a hash, it chops the hash's values, but not its keys.
781 You can actually chop anything that's an lvalue, including an assignment.
783 If you chop a list, each element is chopped. Only the value of the
784 last C<chop> is returned.
786 Note that C<chop> returns the last character. To return all but the last
787 character, use C<substr($string, 0, -1)>.
792 X<chown> X<owner> X<user> X<group>
794 Changes the owner (and group) of a list of files. The first two
795 elements of the list must be the I<numeric> uid and gid, in that
796 order. A value of -1 in either position is interpreted by most
797 systems to leave that value unchanged. Returns the number of files
798 successfully changed.
800 $cnt = chown $uid, $gid, 'foo', 'bar';
801 chown $uid, $gid, @filenames;
803 On systems that support fchown, you might pass file handles among the
804 files. On systems that don't support fchown, passing file handles
805 produces a fatal error at run time. The file handles must be passed
806 as globs or references to be recognized. Barewords are considered
809 Here's an example that looks up nonnumeric uids in the passwd file:
812 chomp($user = <STDIN>);
814 chomp($pattern = <STDIN>);
816 ($login,$pass,$uid,$gid) = getpwnam($user)
817 or die "$user not in passwd file";
819 @ary = glob($pattern); # expand filenames
820 chown $uid, $gid, @ary;
822 On most systems, you are not allowed to change the ownership of the
823 file unless you're the superuser, although you should be able to change
824 the group to any of your secondary groups. On insecure systems, these
825 restrictions may be relaxed, but this is not a portable assumption.
826 On POSIX systems, you can detect this condition this way:
828 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
829 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
832 X<chr> X<character> X<ASCII> X<Unicode>
836 Returns the character represented by that NUMBER in the character set.
837 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
838 chr(0x263a) is a Unicode smiley face.
840 Negative values give the Unicode replacement character (chr(0xfffd)),
841 except under the L<bytes> pragma, where low eight bits of the value
842 (truncated to an integer) are used.
844 If NUMBER is omitted, uses C<$_>.
846 For the reverse, use L</ord>.
848 Note that characters from 128 to 255 (inclusive) are by default
849 internally not encoded as UTF-8 for backward compatibility reasons.
851 See L<perlunicode> for more about Unicode.
853 =item chroot FILENAME
858 This function works like the system call by the same name: it makes the
859 named directory the new root directory for all further pathnames that
860 begin with a C</> by your process and all its children. (It doesn't
861 change your current working directory, which is unaffected.) For security
862 reasons, this call is restricted to the superuser. If FILENAME is
863 omitted, does a C<chroot> to C<$_>.
865 =item close FILEHANDLE
870 Closes the file or pipe associated with the file handle, flushes the IO
871 buffers, and closes the system file descriptor. Returns true if those
872 operations have succeeded and if no error was reported by any PerlIO
873 layer. Closes the currently selected filehandle if the argument is
876 You don't have to close FILEHANDLE if you are immediately going to do
877 another C<open> on it, because C<open> will close it for you. (See
878 C<open>.) However, an explicit C<close> on an input file resets the line
879 counter (C<$.>), while the implicit close done by C<open> does not.
881 If the file handle came from a piped open, C<close> will additionally
882 return false if one of the other system calls involved fails, or if the
883 program exits with non-zero status. (If the only problem was that the
884 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
885 also waits for the process executing on the pipe to complete, in case you
886 want to look at the output of the pipe afterwards, and
887 implicitly puts the exit status value of that command into C<$?> and
888 C<${^CHILD_ERROR_NATIVE}>.
890 Prematurely closing the read end of a pipe (i.e. before the process
891 writing to it at the other end has closed it) will result in a
892 SIGPIPE being delivered to the writer. If the other end can't
893 handle that, be sure to read all the data before closing the pipe.
897 open(OUTPUT, '|sort >foo') # pipe to sort
898 or die "Can't start sort: $!";
899 #... # print stuff to output
900 close OUTPUT # wait for sort to finish
901 or warn $! ? "Error closing sort pipe: $!"
902 : "Exit status $? from sort";
903 open(INPUT, 'foo') # get sort's results
904 or die "Can't open 'foo' for input: $!";
906 FILEHANDLE may be an expression whose value can be used as an indirect
907 filehandle, usually the real filehandle name.
909 =item closedir DIRHANDLE
912 Closes a directory opened by C<opendir> and returns the success of that
915 =item connect SOCKET,NAME
918 Attempts to connect to a remote socket, just as the connect system call
919 does. Returns true if it succeeded, false otherwise. NAME should be a
920 packed address of the appropriate type for the socket. See the examples in
921 L<perlipc/"Sockets: Client/Server Communication">.
928 C<continue> is actually a flow control statement rather than a function. If
929 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
930 C<foreach>), it is always executed just before the conditional is about to
931 be evaluated again, just like the third part of a C<for> loop in C. Thus
932 it can be used to increment a loop variable, even when the loop has been
933 continued via the C<next> statement (which is similar to the C C<continue>
936 C<last>, C<next>, or C<redo> may appear within a C<continue>
937 block. C<last> and C<redo> will behave as if they had been executed within
938 the main block. So will C<next>, but since it will execute a C<continue>
939 block, it may be more entertaining.
942 ### redo always comes here
945 ### next always comes here
947 # then back the top to re-check EXPR
949 ### last always comes here
951 Omitting the C<continue> section is semantically equivalent to using an
952 empty one, logically enough. In that case, C<next> goes directly back
953 to check the condition at the top of the loop.
955 If the "switch" feature is enabled, C<continue> is also a
956 function that will break out of the current C<when> or C<default>
957 block, and fall through to the next case. See L<feature> and
958 L<perlsyn/"Switch statements"> for more information.
962 X<cos> X<cosine> X<acos> X<arccosine>
966 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
967 takes cosine of C<$_>.
969 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
970 function, or use this relation:
972 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
974 =item crypt PLAINTEXT,SALT
975 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
976 X<decrypt> X<cryptography> X<passwd> X<encrypt>
978 Creates a digest string exactly like the crypt(3) function in the C
979 library (assuming that you actually have a version there that has not
980 been extirpated as a potential munitions).
982 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
983 into a short string, called a digest, which is returned. The same
984 PLAINTEXT and SALT will always return the same string, but there is no
985 (known) way to get the original PLAINTEXT from the hash. Small
986 changes in the PLAINTEXT or SALT will result in large changes in the
989 There is no decrypt function. This function isn't all that useful for
990 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
991 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
992 primarily used to check if two pieces of text are the same without
993 having to transmit or store the text itself. An example is checking
994 if a correct password is given. The digest of the password is stored,
995 not the password itself. The user types in a password that is
996 crypt()'d with the same salt as the stored digest. If the two digests
997 match the password is correct.
999 When verifying an existing digest string you should use the digest as
1000 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1001 to create the digest is visible as part of the digest. This ensures
1002 crypt() will hash the new string with the same salt as the digest.
1003 This allows your code to work with the standard L<crypt|/crypt> and
1004 with more exotic implementations. In other words, do not assume
1005 anything about the returned string itself, or how many bytes in the
1008 Traditionally the result is a string of 13 bytes: two first bytes of
1009 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1010 the first eight bytes of the digest string mattered, but alternative
1011 hashing schemes (like MD5), higher level security schemes (like C2),
1012 and implementations on non-UNIX platforms may produce different
1015 When choosing a new salt create a random two character string whose
1016 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1017 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1018 characters is just a recommendation; the characters allowed in
1019 the salt depend solely on your system's crypt library, and Perl can't
1020 restrict what salts C<crypt()> accepts.
1022 Here's an example that makes sure that whoever runs this program knows
1025 $pwd = (getpwuid($<))[1];
1027 system "stty -echo";
1029 chomp($word = <STDIN>);
1033 if (crypt($word, $pwd) ne $pwd) {
1039 Of course, typing in your own password to whoever asks you
1042 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1043 of data, not least of all because you can't get the information
1044 back. Look at the L<Digest> module for more robust algorithms.
1046 If using crypt() on a Unicode string (which I<potentially> has
1047 characters with codepoints above 255), Perl tries to make sense
1048 of the situation by trying to downgrade (a copy of the string)
1049 the string back to an eight-bit byte string before calling crypt()
1050 (on that copy). If that works, good. If not, crypt() dies with
1051 C<Wide character in crypt>.
1056 [This function has been largely superseded by the C<untie> function.]
1058 Breaks the binding between a DBM file and a hash.
1060 =item dbmopen HASH,DBNAME,MASK
1061 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1063 [This function has been largely superseded by the C<tie> function.]
1065 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1066 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1067 argument is I<not> a filehandle, even though it looks like one). DBNAME
1068 is the name of the database (without the F<.dir> or F<.pag> extension if
1069 any). If the database does not exist, it is created with protection
1070 specified by MASK (as modified by the C<umask>). If your system supports
1071 only the older DBM functions, you may perform only one C<dbmopen> in your
1072 program. In older versions of Perl, if your system had neither DBM nor
1073 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1076 If you don't have write access to the DBM file, you can only read hash
1077 variables, not set them. If you want to test whether you can write,
1078 either use file tests or try setting a dummy hash entry inside an C<eval>,
1079 which will trap the error.
1081 Note that functions such as C<keys> and C<values> may return huge lists
1082 when used on large DBM files. You may prefer to use the C<each>
1083 function to iterate over large DBM files. Example:
1085 # print out history file offsets
1086 dbmopen(%HIST,'/usr/lib/news/history',0666);
1087 while (($key,$val) = each %HIST) {
1088 print $key, ' = ', unpack('L',$val), "\n";
1092 See also L<AnyDBM_File> for a more general description of the pros and
1093 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1094 rich implementation.
1096 You can control which DBM library you use by loading that library
1097 before you call dbmopen():
1100 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1101 or die "Can't open netscape history file: $!";
1104 X<defined> X<undef> X<undefined>
1108 Returns a Boolean value telling whether EXPR has a value other than
1109 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1112 Many operations return C<undef> to indicate failure, end of file,
1113 system error, uninitialized variable, and other exceptional
1114 conditions. This function allows you to distinguish C<undef> from
1115 other values. (A simple Boolean test will not distinguish among
1116 C<undef>, zero, the empty string, and C<"0">, which are all equally
1117 false.) Note that since C<undef> is a valid scalar, its presence
1118 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1119 returns C<undef> when its argument is an empty array, I<or> when the
1120 element to return happens to be C<undef>.
1122 You may also use C<defined(&func)> to check whether subroutine C<&func>
1123 has ever been defined. The return value is unaffected by any forward
1124 declarations of C<&func>. Note that a subroutine which is not defined
1125 may still be callable: its package may have an C<AUTOLOAD> method that
1126 makes it spring into existence the first time that it is called -- see
1129 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1130 used to report whether memory for that aggregate has ever been
1131 allocated. This behavior may disappear in future versions of Perl.
1132 You should instead use a simple test for size:
1134 if (@an_array) { print "has array elements\n" }
1135 if (%a_hash) { print "has hash members\n" }
1137 When used on a hash element, it tells you whether the value is defined,
1138 not whether the key exists in the hash. Use L</exists> for the latter
1143 print if defined $switch{'D'};
1144 print "$val\n" while defined($val = pop(@ary));
1145 die "Can't readlink $sym: $!"
1146 unless defined($value = readlink $sym);
1147 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1148 $debugging = 0 unless defined $debugging;
1150 Note: Many folks tend to overuse C<defined>, and then are surprised to
1151 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1152 defined values. For example, if you say
1156 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1157 matched "nothing". It didn't really fail to match anything. Rather, it
1158 matched something that happened to be zero characters long. This is all
1159 very above-board and honest. When a function returns an undefined value,
1160 it's an admission that it couldn't give you an honest answer. So you
1161 should use C<defined> only when you're questioning the integrity of what
1162 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1165 See also L</undef>, L</exists>, L</ref>.
1170 Given an expression that specifies a hash element, array element, hash slice,
1171 or array slice, deletes the specified element(s) from the hash or array.
1172 In the case of an array, if the array elements happen to be at the end,
1173 the size of the array will shrink to the highest element that tests
1174 true for exists() (or 0 if no such element exists).
1176 Returns a list with the same number of elements as the number of elements
1177 for which deletion was attempted. Each element of that list consists of
1178 either the value of the element deleted, or the undefined value. In scalar
1179 context, this means that you get the value of the last element deleted (or
1180 the undefined value if that element did not exist).
1182 %hash = (foo => 11, bar => 22, baz => 33);
1183 $scalar = delete $hash{foo}; # $scalar is 11
1184 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1185 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1187 Deleting from C<%ENV> modifies the environment. Deleting from
1188 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1189 from a C<tie>d hash or array may not necessarily return anything.
1191 Deleting an array element effectively returns that position of the array
1192 to its initial, uninitialized state. Subsequently testing for the same
1193 element with exists() will return false. Also, deleting array elements
1194 in the middle of an array will not shift the index of the elements
1195 after them down. Use splice() for that. See L</exists>.
1197 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1199 foreach $key (keys %HASH) {
1203 foreach $index (0 .. $#ARRAY) {
1204 delete $ARRAY[$index];
1209 delete @HASH{keys %HASH};
1211 delete @ARRAY[0 .. $#ARRAY];
1213 But both of these are slower than just assigning the empty list
1214 or undefining %HASH or @ARRAY:
1216 %HASH = (); # completely empty %HASH
1217 undef %HASH; # forget %HASH ever existed
1219 @ARRAY = (); # completely empty @ARRAY
1220 undef @ARRAY; # forget @ARRAY ever existed
1222 Note that the EXPR can be arbitrarily complicated as long as the final
1223 operation is a hash element, array element, hash slice, or array slice
1226 delete $ref->[$x][$y]{$key};
1227 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1229 delete $ref->[$x][$y][$index];
1230 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1233 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1235 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1236 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1237 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1238 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1239 an C<eval(),> the error message is stuffed into C<$@> and the
1240 C<eval> is terminated with the undefined value. This makes
1241 C<die> the way to raise an exception.
1243 Equivalent examples:
1245 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1246 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1248 If the last element of LIST does not end in a newline, the current
1249 script line number and input line number (if any) are also printed,
1250 and a newline is supplied. Note that the "input line number" (also
1251 known as "chunk") is subject to whatever notion of "line" happens to
1252 be currently in effect, and is also available as the special variable
1253 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1255 Hint: sometimes appending C<", stopped"> to your message will cause it
1256 to make better sense when the string C<"at foo line 123"> is appended.
1257 Suppose you are running script "canasta".
1259 die "/etc/games is no good";
1260 die "/etc/games is no good, stopped";
1262 produce, respectively
1264 /etc/games is no good at canasta line 123.
1265 /etc/games is no good, stopped at canasta line 123.
1267 See also exit(), warn(), and the Carp module.
1269 If LIST is empty and C<$@> already contains a value (typically from a
1270 previous eval) that value is reused after appending C<"\t...propagated">.
1271 This is useful for propagating exceptions:
1274 die unless $@ =~ /Expected exception/;
1276 If LIST is empty and C<$@> contains an object reference that has a
1277 C<PROPAGATE> method, that method will be called with additional file
1278 and line number parameters. The return value replaces the value in
1279 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1282 If C<$@> is empty then the string C<"Died"> is used.
1284 die() can also be called with a reference argument. If this happens to be
1285 trapped within an eval(), $@ contains the reference. This behavior permits
1286 a more elaborate exception handling implementation using objects that
1287 maintain arbitrary state about the nature of the exception. Such a scheme
1288 is sometimes preferable to matching particular string values of $@ using
1289 regular expressions. Because $@ is a global variable, and eval() may be
1290 used within object implementations, care must be taken that analyzing the
1291 error object doesn't replace the reference in the global variable. The
1292 easiest solution is to make a local copy of the reference before doing
1293 other manipulations. Here's an example:
1295 use Scalar::Util 'blessed';
1297 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1298 if (my $ev_err = $@) {
1299 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1300 # handle Some::Module::Exception
1303 # handle all other possible exceptions
1307 Because perl will stringify uncaught exception messages before displaying
1308 them, you may want to overload stringification operations on such custom
1309 exception objects. See L<overload> for details about that.
1311 You can arrange for a callback to be run just before the C<die>
1312 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1313 handler will be called with the error text and can change the error
1314 message, if it sees fit, by calling C<die> again. See
1315 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1316 L<"eval BLOCK"> for some examples. Although this feature was
1317 to be run only right before your program was to exit, this is not
1318 currently the case--the C<$SIG{__DIE__}> hook is currently called
1319 even inside eval()ed blocks/strings! If one wants the hook to do
1320 nothing in such situations, put
1324 as the first line of the handler (see L<perlvar/$^S>). Because
1325 this promotes strange action at a distance, this counterintuitive
1326 behavior may be fixed in a future release.
1331 Not really a function. Returns the value of the last command in the
1332 sequence of commands indicated by BLOCK. When modified by the C<while> or
1333 C<until> loop modifier, executes the BLOCK once before testing the loop
1334 condition. (On other statements the loop modifiers test the conditional
1337 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1338 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1339 See L<perlsyn> for alternative strategies.
1341 =item do SUBROUTINE(LIST)
1344 This form of subroutine call is deprecated. See L<perlsub>.
1349 Uses the value of EXPR as a filename and executes the contents of the
1350 file as a Perl script.
1358 except that it's more efficient and concise, keeps track of the current
1359 filename for error messages, searches the @INC directories, and updates
1360 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1361 variables. It also differs in that code evaluated with C<do FILENAME>
1362 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1363 same, however, in that it does reparse the file every time you call it,
1364 so you probably don't want to do this inside a loop.
1366 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1367 error. If C<do> can read the file but cannot compile it, it
1368 returns undef and sets an error message in C<$@>. If the file is
1369 successfully compiled, C<do> returns the value of the last expression
1372 Note that inclusion of library modules is better done with the
1373 C<use> and C<require> operators, which also do automatic error checking
1374 and raise an exception if there's a problem.
1376 You might like to use C<do> to read in a program configuration
1377 file. Manual error checking can be done this way:
1379 # read in config files: system first, then user
1380 for $file ("/share/prog/defaults.rc",
1381 "$ENV{HOME}/.someprogrc")
1383 unless ($return = do $file) {
1384 warn "couldn't parse $file: $@" if $@;
1385 warn "couldn't do $file: $!" unless defined $return;
1386 warn "couldn't run $file" unless $return;
1391 X<dump> X<core> X<undump>
1395 This function causes an immediate core dump. See also the B<-u>
1396 command-line switch in L<perlrun>, which does the same thing.
1397 Primarily this is so that you can use the B<undump> program (not
1398 supplied) to turn your core dump into an executable binary after
1399 having initialized all your variables at the beginning of the
1400 program. When the new binary is executed it will begin by executing
1401 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1402 Think of it as a goto with an intervening core dump and reincarnation.
1403 If C<LABEL> is omitted, restarts the program from the top.
1405 B<WARNING>: Any files opened at the time of the dump will I<not>
1406 be open any more when the program is reincarnated, with possible
1407 resulting confusion on the part of Perl.
1409 This function is now largely obsolete, partly because it's very
1410 hard to convert a core file into an executable, and because the
1411 real compiler backends for generating portable bytecode and compilable
1412 C code have superseded it. That's why you should now invoke it as
1413 C<CORE::dump()>, if you don't want to be warned against a possible
1416 If you're looking to use L<dump> to speed up your program, consider
1417 generating bytecode or native C code as described in L<perlcc>. If
1418 you're just trying to accelerate a CGI script, consider using the
1419 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1420 You might also consider autoloading or selfloading, which at least
1421 make your program I<appear> to run faster.
1424 X<each> X<hash, iterator>
1426 When called in list context, returns a 2-element list consisting of the
1427 key and value for the next element of a hash, so that you can iterate over
1428 it. When called in scalar context, returns only the key for the next
1429 element in the hash.
1431 Entries are returned in an apparently random order. The actual random
1432 order is subject to change in future versions of perl, but it is
1433 guaranteed to be in the same order as either the C<keys> or C<values>
1434 function would produce on the same (unmodified) hash. Since Perl
1435 5.8.1 the ordering is different even between different runs of Perl
1436 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1438 When the hash is entirely read, a null array is returned in list context
1439 (which when assigned produces a false (C<0>) value), and C<undef> in
1440 scalar context. The next call to C<each> after that will start iterating
1441 again. There is a single iterator for each hash, shared by all C<each>,
1442 C<keys>, and C<values> function calls in the program; it can be reset by
1443 reading all the elements from the hash, or by evaluating C<keys HASH> or
1444 C<values HASH>. If you add or delete elements of a hash while you're
1445 iterating over it, you may get entries skipped or duplicated, so
1446 don't. Exception: It is always safe to delete the item most recently
1447 returned by C<each()>, which means that the following code will work:
1449 while (($key, $value) = each %hash) {
1451 delete $hash{$key}; # This is safe
1454 The following prints out your environment like the printenv(1) program,
1455 only in a different order:
1457 while (($key,$value) = each %ENV) {
1458 print "$key=$value\n";
1461 See also C<keys>, C<values> and C<sort>.
1463 =item eof FILEHANDLE
1472 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1473 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1474 gives the real filehandle. (Note that this function actually
1475 reads a character and then C<ungetc>s it, so isn't very useful in an
1476 interactive context.) Do not read from a terminal file (or call
1477 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1478 as terminals may lose the end-of-file condition if you do.
1480 An C<eof> without an argument uses the last file read. Using C<eof()>
1481 with empty parentheses is very different. It refers to the pseudo file
1482 formed from the files listed on the command line and accessed via the
1483 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1484 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1485 used will cause C<@ARGV> to be examined to determine if input is
1486 available. Similarly, an C<eof()> after C<< <> >> has returned
1487 end-of-file will assume you are processing another C<@ARGV> list,
1488 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1489 see L<perlop/"I/O Operators">.
1491 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1492 detect the end of each file, C<eof()> will only detect the end of the
1493 last file. Examples:
1495 # reset line numbering on each input file
1497 next if /^\s*#/; # skip comments
1500 close ARGV if eof; # Not eof()!
1503 # insert dashes just before last line of last file
1505 if (eof()) { # check for end of last file
1506 print "--------------\n";
1509 last if eof(); # needed if we're reading from a terminal
1512 Practical hint: you almost never need to use C<eof> in Perl, because the
1513 input operators typically return C<undef> when they run out of data, or if
1517 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1518 X<error, handling> X<exception, handling>
1524 In the first form, the return value of EXPR is parsed and executed as if it
1525 were a little Perl program. The value of the expression (which is itself
1526 determined within scalar context) is first parsed, and if there weren't any
1527 errors, executed in the lexical context of the current Perl program, so
1528 that any variable settings or subroutine and format definitions remain
1529 afterwards. Note that the value is parsed every time the C<eval> executes.
1530 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1531 delay parsing and subsequent execution of the text of EXPR until run time.
1533 In the second form, the code within the BLOCK is parsed only once--at the
1534 same time the code surrounding the C<eval> itself was parsed--and executed
1535 within the context of the current Perl program. This form is typically
1536 used to trap exceptions more efficiently than the first (see below), while
1537 also providing the benefit of checking the code within BLOCK at compile
1540 The final semicolon, if any, may be omitted from the value of EXPR or within
1543 In both forms, the value returned is the value of the last expression
1544 evaluated inside the mini-program; a return statement may be also used, just
1545 as with subroutines. The expression providing the return value is evaluated
1546 in void, scalar, or list context, depending on the context of the C<eval>
1547 itself. See L</wantarray> for more on how the evaluation context can be
1550 If there is a syntax error or runtime error, or a C<die> statement is
1551 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1552 error message. If there was no error, C<$@> is guaranteed to be a null
1553 string. Beware that using C<eval> neither silences perl from printing
1554 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1555 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1556 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1557 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1559 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1560 determining whether a particular feature (such as C<socket> or C<symlink>)
1561 is implemented. It is also Perl's exception trapping mechanism, where
1562 the die operator is used to raise exceptions.
1564 If the code to be executed doesn't vary, you may use the eval-BLOCK
1565 form to trap run-time errors without incurring the penalty of
1566 recompiling each time. The error, if any, is still returned in C<$@>.
1569 # make divide-by-zero nonfatal
1570 eval { $answer = $a / $b; }; warn $@ if $@;
1572 # same thing, but less efficient
1573 eval '$answer = $a / $b'; warn $@ if $@;
1575 # a compile-time error
1576 eval { $answer = }; # WRONG
1579 eval '$answer ='; # sets $@
1581 Using the C<eval{}> form as an exception trap in libraries does have some
1582 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1583 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1584 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1585 as shown in this example:
1587 # a very private exception trap for divide-by-zero
1588 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1591 This is especially significant, given that C<__DIE__> hooks can call
1592 C<die> again, which has the effect of changing their error messages:
1594 # __DIE__ hooks may modify error messages
1596 local $SIG{'__DIE__'} =
1597 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1598 eval { die "foo lives here" };
1599 print $@ if $@; # prints "bar lives here"
1602 Because this promotes action at a distance, this counterintuitive behavior
1603 may be fixed in a future release.
1605 With an C<eval>, you should be especially careful to remember what's
1606 being looked at when:
1612 eval { $x }; # CASE 4
1614 eval "\$$x++"; # CASE 5
1617 Cases 1 and 2 above behave identically: they run the code contained in
1618 the variable $x. (Although case 2 has misleading double quotes making
1619 the reader wonder what else might be happening (nothing is).) Cases 3
1620 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1621 does nothing but return the value of $x. (Case 4 is preferred for
1622 purely visual reasons, but it also has the advantage of compiling at
1623 compile-time instead of at run-time.) Case 5 is a place where
1624 normally you I<would> like to use double quotes, except that in this
1625 particular situation, you can just use symbolic references instead, as
1628 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1629 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1631 Note that as a very special case, an C<eval ''> executed within the C<DB>
1632 package doesn't see the usual surrounding lexical scope, but rather the
1633 scope of the first non-DB piece of code that called it. You don't normally
1634 need to worry about this unless you are writing a Perl debugger.
1639 =item exec PROGRAM LIST
1641 The C<exec> function executes a system command I<and never returns>--
1642 use C<system> instead of C<exec> if you want it to return. It fails and
1643 returns false only if the command does not exist I<and> it is executed
1644 directly instead of via your system's command shell (see below).
1646 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1647 warns you if there is a following statement which isn't C<die>, C<warn>,
1648 or C<exit> (if C<-w> is set - but you always do that). If you
1649 I<really> want to follow an C<exec> with some other statement, you
1650 can use one of these styles to avoid the warning:
1652 exec ('foo') or print STDERR "couldn't exec foo: $!";
1653 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1655 If there is more than one argument in LIST, or if LIST is an array
1656 with more than one value, calls execvp(3) with the arguments in LIST.
1657 If there is only one scalar argument or an array with one element in it,
1658 the argument is checked for shell metacharacters, and if there are any,
1659 the entire argument is passed to the system's command shell for parsing
1660 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1661 If there are no shell metacharacters in the argument, it is split into
1662 words and passed directly to C<execvp>, which is more efficient.
1665 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1666 exec "sort $outfile | uniq";
1668 If you don't really want to execute the first argument, but want to lie
1669 to the program you are executing about its own name, you can specify
1670 the program you actually want to run as an "indirect object" (without a
1671 comma) in front of the LIST. (This always forces interpretation of the
1672 LIST as a multivalued list, even if there is only a single scalar in
1675 $shell = '/bin/csh';
1676 exec $shell '-sh'; # pretend it's a login shell
1680 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1682 When the arguments get executed via the system shell, results will
1683 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1686 Using an indirect object with C<exec> or C<system> is also more
1687 secure. This usage (which also works fine with system()) forces
1688 interpretation of the arguments as a multivalued list, even if the
1689 list had just one argument. That way you're safe from the shell
1690 expanding wildcards or splitting up words with whitespace in them.
1692 @args = ( "echo surprise" );
1694 exec @args; # subject to shell escapes
1696 exec { $args[0] } @args; # safe even with one-arg list
1698 The first version, the one without the indirect object, ran the I<echo>
1699 program, passing it C<"surprise"> an argument. The second version
1700 didn't--it tried to run a program literally called I<"echo surprise">,
1701 didn't find it, and set C<$?> to a non-zero value indicating failure.
1703 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1704 output before the exec, but this may not be supported on some platforms
1705 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1706 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1707 open handles in order to avoid lost output.
1709 Note that C<exec> will not call your C<END> blocks, nor will it call
1710 any C<DESTROY> methods in your objects.
1713 X<exists> X<autovivification>
1715 Given an expression that specifies a hash element or array element,
1716 returns true if the specified element in the hash or array has ever
1717 been initialized, even if the corresponding value is undefined. The
1718 element is not autovivified if it doesn't exist.
1720 print "Exists\n" if exists $hash{$key};
1721 print "Defined\n" if defined $hash{$key};
1722 print "True\n" if $hash{$key};
1724 print "Exists\n" if exists $array[$index];
1725 print "Defined\n" if defined $array[$index];
1726 print "True\n" if $array[$index];
1728 A hash or array element can be true only if it's defined, and defined if
1729 it exists, but the reverse doesn't necessarily hold true.
1731 Given an expression that specifies the name of a subroutine,
1732 returns true if the specified subroutine has ever been declared, even
1733 if it is undefined. Mentioning a subroutine name for exists or defined
1734 does not count as declaring it. Note that a subroutine which does not
1735 exist may still be callable: its package may have an C<AUTOLOAD>
1736 method that makes it spring into existence the first time that it is
1737 called -- see L<perlsub>.
1739 print "Exists\n" if exists &subroutine;
1740 print "Defined\n" if defined &subroutine;
1742 Note that the EXPR can be arbitrarily complicated as long as the final
1743 operation is a hash or array key lookup or subroutine name:
1745 if (exists $ref->{A}->{B}->{$key}) { }
1746 if (exists $hash{A}{B}{$key}) { }
1748 if (exists $ref->{A}->{B}->[$ix]) { }
1749 if (exists $hash{A}{B}[$ix]) { }
1751 if (exists &{$ref->{A}{B}{$key}}) { }
1753 Although the deepest nested array or hash will not spring into existence
1754 just because its existence was tested, any intervening ones will.
1755 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1756 into existence due to the existence test for the $key element above.
1757 This happens anywhere the arrow operator is used, including even:
1760 if (exists $ref->{"Some key"}) { }
1761 print $ref; # prints HASH(0x80d3d5c)
1763 This surprising autovivification in what does not at first--or even
1764 second--glance appear to be an lvalue context may be fixed in a future
1767 Use of a subroutine call, rather than a subroutine name, as an argument
1768 to exists() is an error.
1771 exists &sub(); # Error
1774 X<exit> X<terminate> X<abort>
1778 Evaluates EXPR and exits immediately with that value. Example:
1781 exit 0 if $ans =~ /^[Xx]/;
1783 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1784 universally recognized values for EXPR are C<0> for success and C<1>
1785 for error; other values are subject to interpretation depending on the
1786 environment in which the Perl program is running. For example, exiting
1787 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1788 the mailer to return the item undelivered, but that's not true everywhere.
1790 Don't use C<exit> to abort a subroutine if there's any chance that
1791 someone might want to trap whatever error happened. Use C<die> instead,
1792 which can be trapped by an C<eval>.
1794 The exit() function does not always exit immediately. It calls any
1795 defined C<END> routines first, but these C<END> routines may not
1796 themselves abort the exit. Likewise any object destructors that need to
1797 be called are called before the real exit. If this is a problem, you
1798 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1799 See L<perlmod> for details.
1802 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1806 Returns I<e> (the natural logarithm base) to the power of EXPR.
1807 If EXPR is omitted, gives C<exp($_)>.
1809 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1812 Implements the fcntl(2) function. You'll probably have to say
1816 first to get the correct constant definitions. Argument processing and
1817 value return works just like C<ioctl> below.
1821 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1822 or die "can't fcntl F_GETFL: $!";
1824 You don't have to check for C<defined> on the return from C<fcntl>.
1825 Like C<ioctl>, it maps a C<0> return from the system call into
1826 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1827 in numeric context. It is also exempt from the normal B<-w> warnings
1828 on improper numeric conversions.
1830 Note that C<fcntl> will produce a fatal error if used on a machine that
1831 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1832 manpage to learn what functions are available on your system.
1834 Here's an example of setting a filehandle named C<REMOTE> to be
1835 non-blocking at the system level. You'll have to negotiate C<$|>
1836 on your own, though.
1838 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1840 $flags = fcntl(REMOTE, F_GETFL, 0)
1841 or die "Can't get flags for the socket: $!\n";
1843 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1844 or die "Can't set flags for the socket: $!\n";
1846 =item fileno FILEHANDLE
1849 Returns the file descriptor for a filehandle, or undefined if the
1850 filehandle is not open. This is mainly useful for constructing
1851 bitmaps for C<select> and low-level POSIX tty-handling operations.
1852 If FILEHANDLE is an expression, the value is taken as an indirect
1853 filehandle, generally its name.
1855 You can use this to find out whether two handles refer to the
1856 same underlying descriptor:
1858 if (fileno(THIS) == fileno(THAT)) {
1859 print "THIS and THAT are dups\n";
1862 (Filehandles connected to memory objects via new features of C<open> may
1863 return undefined even though they are open.)
1866 =item flock FILEHANDLE,OPERATION
1867 X<flock> X<lock> X<locking>
1869 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1870 for success, false on failure. Produces a fatal error if used on a
1871 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1872 C<flock> is Perl's portable file locking interface, although it locks
1873 only entire files, not records.
1875 Two potentially non-obvious but traditional C<flock> semantics are
1876 that it waits indefinitely until the lock is granted, and that its locks
1877 B<merely advisory>. Such discretionary locks are more flexible, but offer
1878 fewer guarantees. This means that programs that do not also use C<flock>
1879 may modify files locked with C<flock>. See L<perlport>,
1880 your port's specific documentation, or your system-specific local manpages
1881 for details. It's best to assume traditional behavior if you're writing
1882 portable programs. (But if you're not, you should as always feel perfectly
1883 free to write for your own system's idiosyncrasies (sometimes called
1884 "features"). Slavish adherence to portability concerns shouldn't get
1885 in the way of your getting your job done.)
1887 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1888 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1889 you can use the symbolic names if you import them from the Fcntl module,
1890 either individually, or as a group using the ':flock' tag. LOCK_SH
1891 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1892 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1893 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1894 waiting for the lock (check the return status to see if you got it).
1896 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1897 before locking or unlocking it.
1899 Note that the emulation built with lockf(3) doesn't provide shared
1900 locks, and it requires that FILEHANDLE be open with write intent. These
1901 are the semantics that lockf(3) implements. Most if not all systems
1902 implement lockf(3) in terms of fcntl(2) locking, though, so the
1903 differing semantics shouldn't bite too many people.
1905 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1906 be open with read intent to use LOCK_SH and requires that it be open
1907 with write intent to use LOCK_EX.
1909 Note also that some versions of C<flock> cannot lock things over the
1910 network; you would need to use the more system-specific C<fcntl> for
1911 that. If you like you can force Perl to ignore your system's flock(2)
1912 function, and so provide its own fcntl(2)-based emulation, by passing
1913 the switch C<-Ud_flock> to the F<Configure> program when you configure
1916 Here's a mailbox appender for BSD systems.
1918 use Fcntl ':flock'; # import LOCK_* constants
1921 flock(MBOX,LOCK_EX);
1922 # and, in case someone appended
1923 # while we were waiting...
1928 flock(MBOX,LOCK_UN);
1931 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1932 or die "Can't open mailbox: $!";
1935 print MBOX $msg,"\n\n";
1938 On systems that support a real flock(), locks are inherited across fork()
1939 calls, whereas those that must resort to the more capricious fcntl()
1940 function lose the locks, making it harder to write servers.
1942 See also L<DB_File> for other flock() examples.
1945 X<fork> X<child> X<parent>
1947 Does a fork(2) system call to create a new process running the
1948 same program at the same point. It returns the child pid to the
1949 parent process, C<0> to the child process, or C<undef> if the fork is
1950 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1951 are shared, while everything else is copied. On most systems supporting
1952 fork(), great care has gone into making it extremely efficient (for
1953 example, using copy-on-write technology on data pages), making it the
1954 dominant paradigm for multitasking over the last few decades.
1956 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1957 output before forking the child process, but this may not be supported
1958 on some platforms (see L<perlport>). To be safe, you may need to set
1959 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1960 C<IO::Handle> on any open handles in order to avoid duplicate output.
1962 If you C<fork> without ever waiting on your children, you will
1963 accumulate zombies. On some systems, you can avoid this by setting
1964 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1965 forking and reaping moribund children.
1967 Note that if your forked child inherits system file descriptors like
1968 STDIN and STDOUT that are actually connected by a pipe or socket, even
1969 if you exit, then the remote server (such as, say, a CGI script or a
1970 backgrounded job launched from a remote shell) won't think you're done.
1971 You should reopen those to F</dev/null> if it's any issue.
1976 Declare a picture format for use by the C<write> function. For
1980 Test: @<<<<<<<< @||||| @>>>>>
1981 $str, $%, '$' . int($num)
1985 $num = $cost/$quantity;
1989 See L<perlform> for many details and examples.
1991 =item formline PICTURE,LIST
1994 This is an internal function used by C<format>s, though you may call it,
1995 too. It formats (see L<perlform>) a list of values according to the
1996 contents of PICTURE, placing the output into the format output
1997 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1998 Eventually, when a C<write> is done, the contents of
1999 C<$^A> are written to some filehandle. You could also read C<$^A>
2000 and then set C<$^A> back to C<"">. Note that a format typically
2001 does one C<formline> per line of form, but the C<formline> function itself
2002 doesn't care how many newlines are embedded in the PICTURE. This means
2003 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2004 You may therefore need to use multiple formlines to implement a single
2005 record format, just like the format compiler.
2007 Be careful if you put double quotes around the picture, because an C<@>
2008 character may be taken to mean the beginning of an array name.
2009 C<formline> always returns true. See L<perlform> for other examples.
2011 =item getc FILEHANDLE
2012 X<getc> X<getchar> X<character> X<file, read>
2016 Returns the next character from the input file attached to FILEHANDLE,
2017 or the undefined value at end of file, or if there was an error (in
2018 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2019 STDIN. This is not particularly efficient. However, it cannot be
2020 used by itself to fetch single characters without waiting for the user
2021 to hit enter. For that, try something more like:
2024 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2027 system "stty", '-icanon', 'eol', "\001";
2033 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2036 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2040 Determination of whether $BSD_STYLE should be set
2041 is left as an exercise to the reader.
2043 The C<POSIX::getattr> function can do this more portably on
2044 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2045 module from your nearest CPAN site; details on CPAN can be found on
2049 X<getlogin> X<login>
2051 This implements the C library function of the same name, which on most
2052 systems returns the current login from F</etc/utmp>, if any. If null,
2055 $login = getlogin || getpwuid($<) || "Kilroy";
2057 Do not consider C<getlogin> for authentication: it is not as
2058 secure as C<getpwuid>.
2060 =item getpeername SOCKET
2061 X<getpeername> X<peer>
2063 Returns the packed sockaddr address of other end of the SOCKET connection.
2066 $hersockaddr = getpeername(SOCK);
2067 ($port, $iaddr) = sockaddr_in($hersockaddr);
2068 $herhostname = gethostbyaddr($iaddr, AF_INET);
2069 $herstraddr = inet_ntoa($iaddr);
2074 Returns the current process group for the specified PID. Use
2075 a PID of C<0> to get the current process group for the
2076 current process. Will raise an exception if used on a machine that
2077 doesn't implement getpgrp(2). If PID is omitted, returns process
2078 group of current process. Note that the POSIX version of C<getpgrp>
2079 does not accept a PID argument, so only C<PID==0> is truly portable.
2082 X<getppid> X<parent> X<pid>
2084 Returns the process id of the parent process.
2086 Note for Linux users: on Linux, the C functions C<getpid()> and
2087 C<getppid()> return different values from different threads. In order to
2088 be portable, this behavior is not reflected by the perl-level function
2089 C<getppid()>, that returns a consistent value across threads. If you want
2090 to call the underlying C<getppid()>, you may use the CPAN module
2093 =item getpriority WHICH,WHO
2094 X<getpriority> X<priority> X<nice>
2096 Returns the current priority for a process, a process group, or a user.
2097 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2098 machine that doesn't implement getpriority(2).
2101 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2102 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2103 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2104 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2105 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2106 X<endnetent> X<endprotoent> X<endservent>
2110 =item gethostbyname NAME
2112 =item getnetbyname NAME
2114 =item getprotobyname NAME
2120 =item getservbyname NAME,PROTO
2122 =item gethostbyaddr ADDR,ADDRTYPE
2124 =item getnetbyaddr ADDR,ADDRTYPE
2126 =item getprotobynumber NUMBER
2128 =item getservbyport PORT,PROTO
2146 =item sethostent STAYOPEN
2148 =item setnetent STAYOPEN
2150 =item setprotoent STAYOPEN
2152 =item setservent STAYOPEN
2166 These routines perform the same functions as their counterparts in the
2167 system library. In list context, the return values from the
2168 various get routines are as follows:
2170 ($name,$passwd,$uid,$gid,
2171 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2172 ($name,$passwd,$gid,$members) = getgr*
2173 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2174 ($name,$aliases,$addrtype,$net) = getnet*
2175 ($name,$aliases,$proto) = getproto*
2176 ($name,$aliases,$port,$proto) = getserv*
2178 (If the entry doesn't exist you get a null list.)
2180 The exact meaning of the $gcos field varies but it usually contains
2181 the real name of the user (as opposed to the login name) and other
2182 information pertaining to the user. Beware, however, that in many
2183 system users are able to change this information and therefore it
2184 cannot be trusted and therefore the $gcos is tainted (see
2185 L<perlsec>). The $passwd and $shell, user's encrypted password and
2186 login shell, are also tainted, because of the same reason.
2188 In scalar context, you get the name, unless the function was a
2189 lookup by name, in which case you get the other thing, whatever it is.
2190 (If the entry doesn't exist you get the undefined value.) For example:
2192 $uid = getpwnam($name);
2193 $name = getpwuid($num);
2195 $gid = getgrnam($name);
2196 $name = getgrgid($num);
2200 In I<getpw*()> the fields $quota, $comment, and $expire are special
2201 cases in the sense that in many systems they are unsupported. If the
2202 $quota is unsupported, it is an empty scalar. If it is supported, it
2203 usually encodes the disk quota. If the $comment field is unsupported,
2204 it is an empty scalar. If it is supported it usually encodes some
2205 administrative comment about the user. In some systems the $quota
2206 field may be $change or $age, fields that have to do with password
2207 aging. In some systems the $comment field may be $class. The $expire
2208 field, if present, encodes the expiration period of the account or the
2209 password. For the availability and the exact meaning of these fields
2210 in your system, please consult your getpwnam(3) documentation and your
2211 F<pwd.h> file. You can also find out from within Perl what your
2212 $quota and $comment fields mean and whether you have the $expire field
2213 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2214 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2215 files are only supported if your vendor has implemented them in the
2216 intuitive fashion that calling the regular C library routines gets the
2217 shadow versions if you're running under privilege or if there exists
2218 the shadow(3) functions as found in System V (this includes Solaris
2219 and Linux.) Those systems that implement a proprietary shadow password
2220 facility are unlikely to be supported.
2222 The $members value returned by I<getgr*()> is a space separated list of
2223 the login names of the members of the group.
2225 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2226 C, it will be returned to you via C<$?> if the function call fails. The
2227 C<@addrs> value returned by a successful call is a list of the raw
2228 addresses returned by the corresponding system library call. In the
2229 Internet domain, each address is four bytes long and you can unpack it
2230 by saying something like:
2232 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2234 The Socket library makes this slightly easier:
2237 $iaddr = inet_aton("127.1"); # or whatever address
2238 $name = gethostbyaddr($iaddr, AF_INET);
2240 # or going the other way
2241 $straddr = inet_ntoa($iaddr);
2243 If you get tired of remembering which element of the return list
2244 contains which return value, by-name interfaces are provided
2245 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2246 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2247 and C<User::grent>. These override the normal built-ins, supplying
2248 versions that return objects with the appropriate names
2249 for each field. For example:
2253 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2255 Even though it looks like they're the same method calls (uid),
2256 they aren't, because a C<File::stat> object is different from
2257 a C<User::pwent> object.
2259 =item getsockname SOCKET
2262 Returns the packed sockaddr address of this end of the SOCKET connection,
2263 in case you don't know the address because you have several different
2264 IPs that the connection might have come in on.
2267 $mysockaddr = getsockname(SOCK);
2268 ($port, $myaddr) = sockaddr_in($mysockaddr);
2269 printf "Connect to %s [%s]\n",
2270 scalar gethostbyaddr($myaddr, AF_INET),
2273 =item getsockopt SOCKET,LEVEL,OPTNAME
2276 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2277 Options may exist at multiple protocol levels depending on the socket
2278 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2279 C<Socket> module) will exist. To query options at another level the
2280 protocol number of the appropriate protocol controlling the option
2281 should be supplied. For example, to indicate that an option is to be
2282 interpreted by the TCP protocol, LEVEL should be set to the protocol
2283 number of TCP, which you can get using getprotobyname.
2285 The call returns a packed string representing the requested socket option,
2286 or C<undef> if there is an error (the error reason will be in $!). What
2287 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2288 your system documentation for details. A very common case however is that
2289 the option is an integer, in which case the result will be a packed
2290 integer which you can decode using unpack with the C<i> (or C<I>) format.
2292 An example testing if Nagle's algorithm is turned on on a socket:
2294 use Socket qw(:all);
2296 defined(my $tcp = getprotobyname("tcp"))
2297 or die "Could not determine the protocol number for tcp";
2298 # my $tcp = IPPROTO_TCP; # Alternative
2299 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2300 or die "Could not query TCP_NODELAY socket option: $!";
2301 my $nodelay = unpack("I", $packed);
2302 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2306 X<glob> X<wildcard> X<filename, expansion> X<expand>
2310 In list context, returns a (possibly empty) list of filename expansions on
2311 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2312 scalar context, glob iterates through such filename expansions, returning
2313 undef when the list is exhausted. This is the internal function
2314 implementing the C<< <*.c> >> operator, but you can use it directly. If
2315 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2316 more detail in L<perlop/"I/O Operators">.
2318 Beginning with v5.6.0, this operator is implemented using the standard
2319 C<File::Glob> extension. See L<File::Glob> for details.
2322 X<gmtime> X<UTC> X<Greenwich>
2326 Works just like L<localtime> but the returned values are
2327 localized for the standard Greenwich time zone.
2329 Note: when called in list context, $isdst, the last value
2330 returned by gmtime is always C<0>. There is no
2331 Daylight Saving Time in GMT.
2333 See L<perlport/gmtime> for portability concerns.
2336 X<goto> X<jump> X<jmp>
2342 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2343 execution there. It may not be used to go into any construct that
2344 requires initialization, such as a subroutine or a C<foreach> loop. It
2345 also can't be used to go into a construct that is optimized away,
2346 or to get out of a block or subroutine given to C<sort>.
2347 It can be used to go almost anywhere else within the dynamic scope,
2348 including out of subroutines, but it's usually better to use some other
2349 construct such as C<last> or C<die>. The author of Perl has never felt the
2350 need to use this form of C<goto> (in Perl, that is--C is another matter).
2351 (The difference being that C does not offer named loops combined with
2352 loop control. Perl does, and this replaces most structured uses of C<goto>
2353 in other languages.)
2355 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2356 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2357 necessarily recommended if you're optimizing for maintainability:
2359 goto ("FOO", "BAR", "GLARCH")[$i];
2361 The C<goto-&NAME> form is quite different from the other forms of
2362 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2363 doesn't have the stigma associated with other gotos. Instead, it
2364 exits the current subroutine (losing any changes set by local()) and
2365 immediately calls in its place the named subroutine using the current
2366 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2367 load another subroutine and then pretend that the other subroutine had
2368 been called in the first place (except that any modifications to C<@_>
2369 in the current subroutine are propagated to the other subroutine.)
2370 After the C<goto>, not even C<caller> will be able to tell that this
2371 routine was called first.
2373 NAME needn't be the name of a subroutine; it can be a scalar variable
2374 containing a code reference, or a block that evaluates to a code
2377 =item grep BLOCK LIST
2380 =item grep EXPR,LIST
2382 This is similar in spirit to, but not the same as, grep(1) and its
2383 relatives. In particular, it is not limited to using regular expressions.
2385 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2386 C<$_> to each element) and returns the list value consisting of those
2387 elements for which the expression evaluated to true. In scalar
2388 context, returns the number of times the expression was true.
2390 @foo = grep(!/^#/, @bar); # weed out comments
2394 @foo = grep {!/^#/} @bar; # weed out comments
2396 Note that C<$_> is an alias to the list value, so it can be used to
2397 modify the elements of the LIST. While this is useful and supported,
2398 it can cause bizarre results if the elements of LIST are not variables.
2399 Similarly, grep returns aliases into the original list, much as a for
2400 loop's index variable aliases the list elements. That is, modifying an
2401 element of a list returned by grep (for example, in a C<foreach>, C<map>
2402 or another C<grep>) actually modifies the element in the original list.
2403 This is usually something to be avoided when writing clear code.
2405 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2406 been declared with C<my $_>) then, in addition to being locally aliased to
2407 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2408 can't be seen from the outside, avoiding any potential side-effects.
2410 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2413 X<hex> X<hexadecimal>
2417 Interprets EXPR as a hex string and returns the corresponding value.
2418 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2419 L</oct>.) If EXPR is omitted, uses C<$_>.
2421 print hex '0xAf'; # prints '175'
2422 print hex 'aF'; # same
2424 Hex strings may only represent integers. Strings that would cause
2425 integer overflow trigger a warning. Leading whitespace is not stripped,
2426 unlike oct(). To present something as hex, look into L</printf>,
2427 L</sprintf>, or L</unpack>.
2432 There is no builtin C<import> function. It is just an ordinary
2433 method (subroutine) defined (or inherited) by modules that wish to export
2434 names to another module. The C<use> function calls the C<import> method
2435 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2437 =item index STR,SUBSTR,POSITION
2438 X<index> X<indexOf> X<InStr>
2440 =item index STR,SUBSTR
2442 The index function searches for one string within another, but without
2443 the wildcard-like behavior of a full regular-expression pattern match.
2444 It returns the position of the first occurrence of SUBSTR in STR at
2445 or after POSITION. If POSITION is omitted, starts searching from the
2446 beginning of the string. POSITION before the beginning of the string
2447 or after its end is treated as if it were the beginning or the end,
2448 respectively. POSITION and the return value are based at C<0> (or whatever
2449 you've set the C<$[> variable to--but don't do that). If the substring
2450 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2453 X<int> X<integer> X<truncate> X<trunc> X<floor>
2457 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2458 You should not use this function for rounding: one because it truncates
2459 towards C<0>, and two because machine representations of floating point
2460 numbers can sometimes produce counterintuitive results. For example,
2461 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2462 because it's really more like -268.99999999999994315658 instead. Usually,
2463 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2464 functions will serve you better than will int().
2466 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2469 Implements the ioctl(2) function. You'll probably first have to say
2471 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2473 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2474 exist or doesn't have the correct definitions you'll have to roll your
2475 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2476 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2477 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2478 written depending on the FUNCTION--a pointer to the string value of SCALAR
2479 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2480 has no string value but does have a numeric value, that value will be
2481 passed rather than a pointer to the string value. To guarantee this to be
2482 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2483 functions may be needed to manipulate the values of structures used by
2486 The return value of C<ioctl> (and C<fcntl>) is as follows:
2488 if OS returns: then Perl returns:
2490 0 string "0 but true"
2491 anything else that number
2493 Thus Perl returns true on success and false on failure, yet you can
2494 still easily determine the actual value returned by the operating
2497 $retval = ioctl(...) || -1;
2498 printf "System returned %d\n", $retval;
2500 The special string C<"0 but true"> is exempt from B<-w> complaints
2501 about improper numeric conversions.
2503 =item join EXPR,LIST
2506 Joins the separate strings of LIST into a single string with fields
2507 separated by the value of EXPR, and returns that new string. Example:
2509 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2511 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2512 first argument. Compare L</split>.
2517 Returns a list consisting of all the keys of the named hash.
2518 (In scalar context, returns the number of keys.)
2520 The keys are returned in an apparently random order. The actual
2521 random order is subject to change in future versions of perl, but it
2522 is guaranteed to be the same order as either the C<values> or C<each>
2523 function produces (given that the hash has not been modified). Since
2524 Perl 5.8.1 the ordering is different even between different runs of
2525 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2528 As a side effect, calling keys() resets the HASH's internal iterator
2529 (see L</each>). In particular, calling keys() in void context resets
2530 the iterator with no other overhead.
2532 Here is yet another way to print your environment:
2535 @values = values %ENV;
2537 print pop(@keys), '=', pop(@values), "\n";
2540 or how about sorted by key:
2542 foreach $key (sort(keys %ENV)) {
2543 print $key, '=', $ENV{$key}, "\n";
2546 The returned values are copies of the original keys in the hash, so
2547 modifying them will not affect the original hash. Compare L</values>.
2549 To sort a hash by value, you'll need to use a C<sort> function.
2550 Here's a descending numeric sort of a hash by its values:
2552 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2553 printf "%4d %s\n", $hash{$key}, $key;
2556 As an lvalue C<keys> allows you to increase the number of hash buckets
2557 allocated for the given hash. This can gain you a measure of efficiency if
2558 you know the hash is going to get big. (This is similar to pre-extending
2559 an array by assigning a larger number to $#array.) If you say
2563 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2564 in fact, since it rounds up to the next power of two. These
2565 buckets will be retained even if you do C<%hash = ()>, use C<undef
2566 %hash> if you want to free the storage while C<%hash> is still in scope.
2567 You can't shrink the number of buckets allocated for the hash using
2568 C<keys> in this way (but you needn't worry about doing this by accident,
2569 as trying has no effect).
2571 See also C<each>, C<values> and C<sort>.
2573 =item kill SIGNAL, LIST
2576 Sends a signal to a list of processes. Returns the number of
2577 processes successfully signaled (which is not necessarily the
2578 same as the number actually killed).
2580 $cnt = kill 1, $child1, $child2;
2583 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2584 system call will check whether it's possible to send a signal to it (that
2585 means, to be brief, that the process is owned by the same user, or we are
2586 the super-user). This is a useful way to check that a child process is
2587 alive (even if only as a zombie) and hasn't changed its UID. See
2588 L<perlport> for notes on the portability of this construct.
2590 Unlike in the shell, if SIGNAL is negative, it kills
2591 process groups instead of processes. (On System V, a negative I<PROCESS>
2592 number will also kill process groups, but that's not portable.) That
2593 means you usually want to use positive not negative signals. You may also
2594 use a signal name in quotes.
2596 See L<perlipc/"Signals"> for more details.
2603 The C<last> command is like the C<break> statement in C (as used in
2604 loops); it immediately exits the loop in question. If the LABEL is
2605 omitted, the command refers to the innermost enclosing loop. The
2606 C<continue> block, if any, is not executed:
2608 LINE: while (<STDIN>) {
2609 last LINE if /^$/; # exit when done with header
2613 C<last> cannot be used to exit a block which returns a value such as
2614 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2615 a grep() or map() operation.
2617 Note that a block by itself is semantically identical to a loop
2618 that executes once. Thus C<last> can be used to effect an early
2619 exit out of such a block.
2621 See also L</continue> for an illustration of how C<last>, C<next>, and
2629 Returns a lowercased version of EXPR. This is the internal function
2630 implementing the C<\L> escape in double-quoted strings. Respects
2631 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2632 and L<perlunicode> for more details about locale and Unicode support.
2634 If EXPR is omitted, uses C<$_>.
2637 X<lcfirst> X<lowercase>
2641 Returns the value of EXPR with the first character lowercased. This
2642 is the internal function implementing the C<\l> escape in
2643 double-quoted strings. Respects current LC_CTYPE locale if C<use
2644 locale> in force. See L<perllocale> and L<perlunicode> for more
2645 details about locale and Unicode support.
2647 If EXPR is omitted, uses C<$_>.
2654 Returns the length in I<characters> of the value of EXPR. If EXPR is
2655 omitted, returns length of C<$_>. Note that this cannot be used on
2656 an entire array or hash to find out how many elements these have.
2657 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2659 Note the I<characters>: if the EXPR is in Unicode, you will get the
2660 number of characters, not the number of bytes. To get the length
2661 of the internal string in bytes, use C<bytes::length(EXPR)>, see
2662 L<bytes>. Note that the internal encoding is variable, and the number
2663 of bytes usually meaningless. To get the number of bytes that the
2664 string would have when encoded as UTF-8, use
2665 C<length(Encoding::encode_utf8(EXPR))>.
2667 =item link OLDFILE,NEWFILE
2670 Creates a new filename linked to the old filename. Returns true for
2671 success, false otherwise.
2673 =item listen SOCKET,QUEUESIZE
2676 Does the same thing that the listen system call does. Returns true if
2677 it succeeded, false otherwise. See the example in
2678 L<perlipc/"Sockets: Client/Server Communication">.
2683 You really probably want to be using C<my> instead, because C<local> isn't
2684 what most people think of as "local". See
2685 L<perlsub/"Private Variables via my()"> for details.
2687 A local modifies the listed variables to be local to the enclosing
2688 block, file, or eval. If more than one value is listed, the list must
2689 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2690 for details, including issues with tied arrays and hashes.
2692 =item localtime EXPR
2693 X<localtime> X<ctime>
2697 Converts a time as returned by the time function to a 9-element list
2698 with the time analyzed for the local time zone. Typically used as
2702 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2705 All list elements are numeric, and come straight out of the C `struct
2706 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2707 of the specified time.
2709 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2710 the range C<0..11> with 0 indicating January and 11 indicating December.
2711 This makes it easy to get a month name from a list:
2713 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2714 print "$abbr[$mon] $mday";
2715 # $mon=9, $mday=18 gives "Oct 18"
2717 C<$year> is the number of years since 1900, not just the last two digits
2718 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2719 to get a complete 4-digit year is simply:
2723 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2724 to do that, would you?
2726 To get the last two digits of the year (e.g., '01' in 2001) do:
2728 $year = sprintf("%02d", $year % 100);
2730 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2731 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2732 (or C<0..365> in leap years.)
2734 C<$isdst> is true if the specified time occurs during Daylight Saving
2735 Time, false otherwise.
2737 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2739 In scalar context, C<localtime()> returns the ctime(3) value:
2741 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2743 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2744 instead of local time use the L</gmtime> builtin. See also the
2745 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2746 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2747 and mktime(3) functions.
2749 To get somewhat similar but locale dependent date strings, set up your
2750 locale environment variables appropriately (please see L<perllocale>) and
2753 use POSIX qw(strftime);
2754 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2755 # or for GMT formatted appropriately for your locale:
2756 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2758 Note that the C<%a> and C<%b>, the short forms of the day of the week
2759 and the month of the year, may not necessarily be three characters wide.
2761 See L<perlport/localtime> for portability concerns.
2763 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2764 by-name access mechanism to the gmtime() and localtime() functions,
2767 For a comprehensive date and time representation look at the
2768 L<DateTime> module on CPAN.
2773 This function places an advisory lock on a shared variable, or referenced
2774 object contained in I<THING> until the lock goes out of scope.
2776 lock() is a "weak keyword" : this means that if you've defined a function
2777 by this name (before any calls to it), that function will be called
2778 instead. (However, if you've said C<use threads>, lock() is always a
2779 keyword.) See L<threads>.
2782 X<log> X<logarithm> X<e> X<ln> X<base>
2786 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2787 returns log of C<$_>. To get the log of another base, use basic algebra:
2788 The base-N log of a number is equal to the natural log of that number
2789 divided by the natural log of N. For example:
2793 return log($n)/log(10);
2796 See also L</exp> for the inverse operation.
2803 Does the same thing as the C<stat> function (including setting the
2804 special C<_> filehandle) but stats a symbolic link instead of the file
2805 the symbolic link points to. If symbolic links are unimplemented on
2806 your system, a normal C<stat> is done. For much more detailed
2807 information, please see the documentation for C<stat>.
2809 If EXPR is omitted, stats C<$_>.
2813 The match operator. See L<perlop>.
2815 =item map BLOCK LIST
2820 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2821 C<$_> to each element) and returns the list value composed of the
2822 results of each such evaluation. In scalar context, returns the
2823 total number of elements so generated. Evaluates BLOCK or EXPR in
2824 list context, so each element of LIST may produce zero, one, or
2825 more elements in the returned value.
2827 @chars = map(chr, @nums);
2829 translates a list of numbers to the corresponding characters. And
2831 %hash = map { get_a_key_for($_) => $_ } @array;
2833 is just a funny way to write
2837 $hash{get_a_key_for($_)} = $_;
2840 Note that C<$_> is an alias to the list value, so it can be used to
2841 modify the elements of the LIST. While this is useful and supported,
2842 it can cause bizarre results if the elements of LIST are not variables.
2843 Using a regular C<foreach> loop for this purpose would be clearer in
2844 most cases. See also L</grep> for an array composed of those items of
2845 the original list for which the BLOCK or EXPR evaluates to true.
2847 If C<$_> is lexical in the scope where the C<map> appears (because it has
2848 been declared with C<my $_>), then, in addition to being locally aliased to
2849 the list elements, C<$_> keeps being lexical inside the block; that is, it
2850 can't be seen from the outside, avoiding any potential side-effects.
2852 C<{> starts both hash references and blocks, so C<map { ...> could be either
2853 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2854 ahead for the closing C<}> it has to take a guess at which its dealing with
2855 based what it finds just after the C<{>. Usually it gets it right, but if it
2856 doesn't it won't realize something is wrong until it gets to the C<}> and
2857 encounters the missing (or unexpected) comma. The syntax error will be
2858 reported close to the C<}> but you'll need to change something near the C<{>
2859 such as using a unary C<+> to give perl some help:
2861 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2862 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2863 %hash = map { ("\L$_", 1) } @array # this also works
2864 %hash = map { lc($_), 1 } @array # as does this.
2865 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2867 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2869 or to force an anon hash constructor use C<+{>:
2871 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2873 and you get list of anonymous hashes each with only 1 entry.
2875 =item mkdir FILENAME,MASK
2876 X<mkdir> X<md> X<directory, create>
2878 =item mkdir FILENAME
2882 Creates the directory specified by FILENAME, with permissions
2883 specified by MASK (as modified by C<umask>). If it succeeds it
2884 returns true, otherwise it returns false and sets C<$!> (errno).
2885 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2888 In general, it is better to create directories with permissive MASK,
2889 and let the user modify that with their C<umask>, than it is to supply
2890 a restrictive MASK and give the user no way to be more permissive.
2891 The exceptions to this rule are when the file or directory should be
2892 kept private (mail files, for instance). The perlfunc(1) entry on
2893 C<umask> discusses the choice of MASK in more detail.
2895 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2896 number of trailing slashes. Some operating and filesystems do not get
2897 this right, so Perl automatically removes all trailing slashes to keep
2900 In order to recursively create a directory structure look at
2901 the C<mkpath> function of the L<File::Path> module.
2903 =item msgctl ID,CMD,ARG
2906 Calls the System V IPC function msgctl(2). You'll probably have to say
2910 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2911 then ARG must be a variable that will hold the returned C<msqid_ds>
2912 structure. Returns like C<ioctl>: the undefined value for error,
2913 C<"0 but true"> for zero, or the actual return value otherwise. See also
2914 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2916 =item msgget KEY,FLAGS
2919 Calls the System V IPC function msgget(2). Returns the message queue
2920 id, or the undefined value if there is an error. See also
2921 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2923 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2926 Calls the System V IPC function msgrcv to receive a message from
2927 message queue ID into variable VAR with a maximum message size of
2928 SIZE. Note that when a message is received, the message type as a
2929 native long integer will be the first thing in VAR, followed by the
2930 actual message. This packing may be opened with C<unpack("l! a*")>.
2931 Taints the variable. Returns true if successful, or false if there is
2932 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2933 C<IPC::SysV::Msg> documentation.
2935 =item msgsnd ID,MSG,FLAGS
2938 Calls the System V IPC function msgsnd to send the message MSG to the
2939 message queue ID. MSG must begin with the native long integer message
2940 type, and be followed by the length of the actual message, and finally
2941 the message itself. This kind of packing can be achieved with
2942 C<pack("l! a*", $type, $message)>. Returns true if successful,
2943 or false if there is an error. See also C<IPC::SysV>
2944 and C<IPC::SysV::Msg> documentation.
2951 =item my EXPR : ATTRS
2953 =item my TYPE EXPR : ATTRS
2955 A C<my> declares the listed variables to be local (lexically) to the
2956 enclosing block, file, or C<eval>. If more than one value is listed,
2957 the list must be placed in parentheses.
2959 The exact semantics and interface of TYPE and ATTRS are still
2960 evolving. TYPE is currently bound to the use of C<fields> pragma,
2961 and attributes are handled using the C<attributes> pragma, or starting
2962 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2963 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2964 L<attributes>, and L<Attribute::Handlers>.
2971 The C<next> command is like the C<continue> statement in C; it starts
2972 the next iteration of the loop:
2974 LINE: while (<STDIN>) {
2975 next LINE if /^#/; # discard comments
2979 Note that if there were a C<continue> block on the above, it would get
2980 executed even on discarded lines. If the LABEL is omitted, the command
2981 refers to the innermost enclosing loop.
2983 C<next> cannot be used to exit a block which returns a value such as
2984 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2985 a grep() or map() operation.
2987 Note that a block by itself is semantically identical to a loop
2988 that executes once. Thus C<next> will exit such a block early.
2990 See also L</continue> for an illustration of how C<last>, C<next>, and
2993 =item no Module VERSION LIST
2996 =item no Module VERSION
2998 =item no Module LIST
3002 See the C<use> function, of which C<no> is the opposite.
3005 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3009 Interprets EXPR as an octal string and returns the corresponding
3010 value. (If EXPR happens to start off with C<0x>, interprets it as a
3011 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3012 binary string. Leading whitespace is ignored in all three cases.)
3013 The following will handle decimal, binary, octal, and hex in the standard
3016 $val = oct($val) if $val =~ /^0/;
3018 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3019 in octal), use sprintf() or printf():
3021 $perms = (stat("filename"))[2] & 07777;
3022 $oct_perms = sprintf "%lo", $perms;
3024 The oct() function is commonly used when a string such as C<644> needs
3025 to be converted into a file mode, for example. (Although perl will
3026 automatically convert strings into numbers as needed, this automatic
3027 conversion assumes base 10.)
3029 =item open FILEHANDLE,EXPR
3030 X<open> X<pipe> X<file, open> X<fopen>
3032 =item open FILEHANDLE,MODE,EXPR
3034 =item open FILEHANDLE,MODE,EXPR,LIST
3036 =item open FILEHANDLE,MODE,REFERENCE
3038 =item open FILEHANDLE
3040 Opens the file whose filename is given by EXPR, and associates it with
3043 (The following is a comprehensive reference to open(): for a gentler
3044 introduction you may consider L<perlopentut>.)
3046 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3047 the variable is assigned a reference to a new anonymous filehandle,
3048 otherwise if FILEHANDLE is an expression, its value is used as the name of
3049 the real filehandle wanted. (This is considered a symbolic reference, so
3050 C<use strict 'refs'> should I<not> be in effect.)
3052 If EXPR is omitted, the scalar variable of the same name as the
3053 FILEHANDLE contains the filename. (Note that lexical variables--those
3054 declared with C<my>--will not work for this purpose; so if you're
3055 using C<my>, specify EXPR in your call to open.)
3057 If three or more arguments are specified then the mode of opening and
3058 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3059 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3060 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3061 the file is opened for appending, again being created if necessary.
3063 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3064 indicate that you want both read and write access to the file; thus
3065 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3066 '+>' >> mode would clobber the file first. You can't usually use
3067 either read-write mode for updating textfiles, since they have
3068 variable length records. See the B<-i> switch in L<perlrun> for a
3069 better approach. The file is created with permissions of C<0666>
3070 modified by the process' C<umask> value.
3072 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3073 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3075 In the 2-arguments (and 1-argument) form of the call the mode and
3076 filename should be concatenated (in this order), possibly separated by
3077 spaces. It is possible to omit the mode in these forms if the mode is
3080 If the filename begins with C<'|'>, the filename is interpreted as a
3081 command to which output is to be piped, and if the filename ends with a
3082 C<'|'>, the filename is interpreted as a command which pipes output to
3083 us. See L<perlipc/"Using open() for IPC">
3084 for more examples of this. (You are not allowed to C<open> to a command
3085 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3086 and L<perlipc/"Bidirectional Communication with Another Process">
3089 For three or more arguments if MODE is C<'|-'>, the filename is
3090 interpreted as a command to which output is to be piped, and if MODE
3091 is C<'-|'>, the filename is interpreted as a command which pipes
3092 output to us. In the 2-arguments (and 1-argument) form one should
3093 replace dash (C<'-'>) with the command.
3094 See L<perlipc/"Using open() for IPC"> for more examples of this.
3095 (You are not allowed to C<open> to a command that pipes both in I<and>
3096 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3097 L<perlipc/"Bidirectional Communication"> for alternatives.)
3099 In the three-or-more argument form of pipe opens, if LIST is specified
3100 (extra arguments after the command name) then LIST becomes arguments
3101 to the command invoked if the platform supports it. The meaning of
3102 C<open> with more than three arguments for non-pipe modes is not yet
3103 specified. Experimental "layers" may give extra LIST arguments
3106 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3107 and opening C<< '>-' >> opens STDOUT.
3109 You may use the three-argument form of open to specify IO "layers"
3110 (sometimes also referred to as "disciplines") to be applied to the handle
3111 that affect how the input and output are processed (see L<open> and
3112 L<PerlIO> for more details). For example
3114 open(FH, "<:encoding(UTF-8)", "file")
3116 will open the UTF-8 encoded file containing Unicode characters,
3117 see L<perluniintro>. Note that if layers are specified in the
3118 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3119 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3121 Open returns nonzero upon success, the undefined value otherwise. If
3122 the C<open> involved a pipe, the return value happens to be the pid of
3125 If you're running Perl on a system that distinguishes between text
3126 files and binary files, then you should check out L</binmode> for tips
3127 for dealing with this. The key distinction between systems that need
3128 C<binmode> and those that don't is their text file formats. Systems
3129 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3130 character, and which encode that character in C as C<"\n">, do not
3131 need C<binmode>. The rest need it.
3133 When opening a file, it's usually a bad idea to continue normal execution
3134 if the request failed, so C<open> is frequently used in connection with
3135 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3136 where you want to make a nicely formatted error message (but there are
3137 modules that can help with that problem)) you should always check
3138 the return value from opening a file. The infrequent exception is when
3139 working with an unopened filehandle is actually what you want to do.
3141 As a special case the 3-arg form with a read/write mode and the third
3142 argument being C<undef>:
3144 open(TMP, "+>", undef) or die ...
3146 opens a filehandle to an anonymous temporary file. Also using "+<"
3147 works for symmetry, but you really should consider writing something
3148 to the temporary file first. You will need to seek() to do the
3151 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3152 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3153 "in memory" files held in Perl scalars via:
3155 open($fh, '>', \$variable) || ..
3157 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3158 file, you have to close it first:
3161 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3166 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3167 while (<ARTICLE>) {...
3169 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3170 # if the open fails, output is discarded
3172 open(DBASE, '+<', 'dbase.mine') # open for update
3173 or die "Can't open 'dbase.mine' for update: $!";
3175 open(DBASE, '+<dbase.mine') # ditto
3176 or die "Can't open 'dbase.mine' for update: $!";
3178 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3179 or die "Can't start caesar: $!";
3181 open(ARTICLE, "caesar <$article |") # ditto
3182 or die "Can't start caesar: $!";
3184 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3185 or die "Can't start sort: $!";
3188 open(MEMORY,'>', \$var)
3189 or die "Can't open memory file: $!";
3190 print MEMORY "foo!\n"; # output will end up in $var
3192 # process argument list of files along with any includes
3194 foreach $file (@ARGV) {
3195 process($file, 'fh00');
3199 my($filename, $input) = @_;
3200 $input++; # this is a string increment
3201 unless (open($input, $filename)) {
3202 print STDERR "Can't open $filename: $!\n";
3207 while (<$input>) { # note use of indirection
3208 if (/^#include "(.*)"/) {
3209 process($1, $input);
3216 See L<perliol> for detailed info on PerlIO.
3218 You may also, in the Bourne shell tradition, specify an EXPR beginning
3219 with C<< '>&' >>, in which case the rest of the string is interpreted
3220 as the name of a filehandle (or file descriptor, if numeric) to be
3221 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3222 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3223 The mode you specify should match the mode of the original filehandle.
3224 (Duping a filehandle does not take into account any existing contents
3225 of IO buffers.) If you use the 3-arg form then you can pass either a
3226 number, the name of a filehandle or the normal "reference to a glob".
3228 Here is a script that saves, redirects, and restores C<STDOUT> and
3229 C<STDERR> using various methods:
3232 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3233 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3235 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3236 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3238 select STDERR; $| = 1; # make unbuffered
3239 select STDOUT; $| = 1; # make unbuffered
3241 print STDOUT "stdout 1\n"; # this works for
3242 print STDERR "stderr 1\n"; # subprocesses too
3244 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3245 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3247 print STDOUT "stdout 2\n";
3248 print STDERR "stderr 2\n";
3250 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3251 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3252 that file descriptor (and not call L<dup(2)>); this is more
3253 parsimonious of file descriptors. For example:
3255 # open for input, reusing the fileno of $fd
3256 open(FILEHANDLE, "<&=$fd")
3260 open(FILEHANDLE, "<&=", $fd)
3264 # open for append, using the fileno of OLDFH
3265 open(FH, ">>&=", OLDFH)
3269 open(FH, ">>&=OLDFH")
3271 Being parsimonious on filehandles is also useful (besides being
3272 parsimonious) for example when something is dependent on file
3273 descriptors, like for example locking using flock(). If you do just
3274 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3275 descriptor as B, and therefore flock(A) will not flock(B), and vice
3276 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3277 the same file descriptor.
3279 Note that if you are using Perls older than 5.8.0, Perl will be using
3280 the standard C libraries' fdopen() to implement the "=" functionality.
3281 On many UNIX systems fdopen() fails when file descriptors exceed a
3282 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3283 most often the default.
3285 You can see whether Perl has been compiled with PerlIO or not by
3286 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3287 is C<define>, you have PerlIO, otherwise you don't.
3289 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3290 with 2-arguments (or 1-argument) form of open(), then
3291 there is an implicit fork done, and the return value of open is the pid
3292 of the child within the parent process, and C<0> within the child
3293 process. (Use C<defined($pid)> to determine whether the open was successful.)
3294 The filehandle behaves normally for the parent, but i/o to that
3295 filehandle is piped from/to the STDOUT/STDIN of the child process.
3296 In the child process the filehandle isn't opened--i/o happens from/to
3297 the new STDOUT or STDIN. Typically this is used like the normal
3298 piped open when you want to exercise more control over just how the
3299 pipe command gets executed, such as when you are running setuid, and
3300 don't want to have to scan shell commands for metacharacters.
3301 The following triples are more or less equivalent:
3303 open(FOO, "|tr '[a-z]' '[A-Z]'");
3304 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3305 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3306 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3308 open(FOO, "cat -n '$file'|");
3309 open(FOO, '-|', "cat -n '$file'");
3310 open(FOO, '-|') || exec 'cat', '-n', $file;
3311 open(FOO, '-|', "cat", '-n', $file);
3313 The last example in each block shows the pipe as "list form", which is
3314 not yet supported on all platforms. A good rule of thumb is that if
3315 your platform has true C<fork()> (in other words, if your platform is
3316 UNIX) you can use the list form.
3318 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3320 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3321 output before any operation that may do a fork, but this may not be
3322 supported on some platforms (see L<perlport>). To be safe, you may need
3323 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3324 of C<IO::Handle> on any open handles.
3326 On systems that support a close-on-exec flag on files, the flag will
3327 be set for the newly opened file descriptor as determined by the value
3328 of $^F. See L<perlvar/$^F>.
3330 Closing any piped filehandle causes the parent process to wait for the
3331 child to finish, and returns the status value in C<$?> and
3332 C<${^CHILD_ERROR_NATIVE}>.
3334 The filename passed to 2-argument (or 1-argument) form of open() will
3335 have leading and trailing whitespace deleted, and the normal
3336 redirection characters honored. This property, known as "magic open",
3337 can often be used to good effect. A user could specify a filename of
3338 F<"rsh cat file |">, or you could change certain filenames as needed:
3340 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3341 open(FH, $filename) or die "Can't open $filename: $!";
3343 Use 3-argument form to open a file with arbitrary weird characters in it,
3345 open(FOO, '<', $file);
3347 otherwise it's necessary to protect any leading and trailing whitespace:
3349 $file =~ s#^(\s)#./$1#;
3350 open(FOO, "< $file\0");
3352 (this may not work on some bizarre filesystems). One should
3353 conscientiously choose between the I<magic> and 3-arguments form
3358 will allow the user to specify an argument of the form C<"rsh cat file |">,
3359 but will not work on a filename which happens to have a trailing space, while
3361 open IN, '<', $ARGV[0];
3363 will have exactly the opposite restrictions.
3365 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3366 should use the C<sysopen> function, which involves no such magic (but
3367 may use subtly different filemodes than Perl open(), which is mapped
3368 to C fopen()). This is
3369 another way to protect your filenames from interpretation. For example:
3372 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3373 or die "sysopen $path: $!";
3374 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3375 print HANDLE "stuff $$\n";
3377 print "File contains: ", <HANDLE>;
3379 Using the constructor from the C<IO::Handle> package (or one of its
3380 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3381 filehandles that have the scope of whatever variables hold references to
3382 them, and automatically close whenever and however you leave that scope:
3386 sub read_myfile_munged {
3388 my $handle = new IO::File;
3389 open($handle, "myfile") or die "myfile: $!";
3391 or return (); # Automatically closed here.
3392 mung $first or die "mung failed"; # Or here.
3393 return $first, <$handle> if $ALL; # Or here.
3397 See L</seek> for some details about mixing reading and writing.
3399 =item opendir DIRHANDLE,EXPR
3402 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3403 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3404 DIRHANDLE may be an expression whose value can be used as an indirect
3405 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3406 scalar variable (or array or hash element), the variable is assigned a
3407 reference to a new anonymous dirhandle.
3408 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3415 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3416 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3419 For the reverse, see L</chr>.
3420 See L<perlunicode> for more about Unicode.
3427 =item our EXPR : ATTRS
3429 =item our TYPE EXPR : ATTRS
3431 C<our> associates a simple name with a package variable in the current
3432 package for use within the current scope. When C<use strict 'vars'> is in
3433 effect, C<our> lets you use declared global variables without qualifying
3434 them with package names, within the lexical scope of the C<our> declaration.
3435 In this way C<our> differs from C<use vars>, which is package scoped.
3437 Unlike C<my>, which both allocates storage for a variable and associates
3438 a simple name with that storage for use within the current scope, C<our>
3439 associates a simple name with a package variable in the current package,
3440 for use within the current scope. In other words, C<our> has the same
3441 scoping rules as C<my>, but does not necessarily create a
3444 If more than one value is listed, the list must be placed
3450 An C<our> declaration declares a global variable that will be visible
3451 across its entire lexical scope, even across package boundaries. The
3452 package in which the variable is entered is determined at the point
3453 of the declaration, not at the point of use. This means the following
3457 our $bar; # declares $Foo::bar for rest of lexical scope
3461 print $bar; # prints 20, as it refers to $Foo::bar
3463 Multiple C<our> declarations with the same name in the same lexical
3464 scope are allowed if they are in different packages. If they happen
3465 to be in the same package, Perl will emit warnings if you have asked
3466 for them, just like multiple C<my> declarations. Unlike a second
3467 C<my> declaration, which will bind the name to a fresh variable, a
3468 second C<our> declaration in the same package, in the same scope, is
3473 our $bar; # declares $Foo::bar for rest of lexical scope
3477 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3478 print $bar; # prints 30
3480 our $bar; # emits warning but has no other effect
3481 print $bar; # still prints 30
3483 An C<our> declaration may also have a list of attributes associated
3486 The exact semantics and interface of TYPE and ATTRS are still
3487 evolving. TYPE is currently bound to the use of C<fields> pragma,
3488 and attributes are handled using the C<attributes> pragma, or starting
3489 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3490 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3491 L<attributes>, and L<Attribute::Handlers>.
3493 =item pack TEMPLATE,LIST
3496 Takes a LIST of values and converts it into a string using the rules
3497 given by the TEMPLATE. The resulting string is the concatenation of
3498 the converted values. Typically, each converted value looks
3499 like its machine-level representation. For example, on 32-bit machines
3500 an integer may be represented by a sequence of 4 bytes that will be
3501 converted to a sequence of 4 characters.
3503 The TEMPLATE is a sequence of characters that give the order and type
3504 of values, as follows:
3506 a A string with arbitrary binary data, will be null padded.
3507 A A text (ASCII) string, will be space padded.
3508 Z A null terminated (ASCIZ) string, will be null padded.
3510 b A bit string (ascending bit order inside each byte, like vec()).
3511 B A bit string (descending bit order inside each byte).
3512 h A hex string (low nybble first).
3513 H A hex string (high nybble first).
3515 c A signed char (8-bit) value.
3516 C An unsigned C char (octet) even under Unicode. Should normally not
3517 be used. See U and W instead.
3518 W An unsigned char value (can be greater than 255).
3520 s A signed short (16-bit) value.
3521 S An unsigned short value.
3523 l A signed long (32-bit) value.
3524 L An unsigned long value.
3526 q A signed quad (64-bit) value.
3527 Q An unsigned quad value.
3528 (Quads are available only if your system supports 64-bit
3529 integer values _and_ if Perl has been compiled to support those.
3530 Causes a fatal error otherwise.)
3532 i A signed integer value.
3533 I A unsigned integer value.
3534 (This 'integer' is _at_least_ 32 bits wide. Its exact
3535 size depends on what a local C compiler calls 'int'.)
3537 n An unsigned short (16-bit) in "network" (big-endian) order.
3538 N An unsigned long (32-bit) in "network" (big-endian) order.
3539 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3540 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3542 j A Perl internal signed integer value (IV).
3543 J A Perl internal unsigned integer value (UV).
3545 f A single-precision float in the native format.
3546 d A double-precision float in the native format.
3548 F A Perl internal floating point value (NV) in the native format
3549 D A long double-precision float in the native format.
3550 (Long doubles are available only if your system supports long
3551 double values _and_ if Perl has been compiled to support those.
3552 Causes a fatal error otherwise.)
3554 p A pointer to a null-terminated string.
3555 P A pointer to a structure (fixed-length string).
3557 u A uuencoded string.
3558 U A Unicode character number. Encodes to UTF-8 internally
3559 (or UTF-EBCDIC in EBCDIC platforms).
3561 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3562 details). Its bytes represent an unsigned integer in base 128,
3563 most significant digit first, with as few digits as possible. Bit
3564 eight (the high bit) is set on each byte except the last.
3568 @ Null fill or truncate to absolute position, counted from the
3569 start of the innermost ()-group.
3570 . Null fill or truncate to absolute position specified by value.
3571 ( Start of a ()-group.
3573 One or more of the modifiers below may optionally follow some letters in the
3574 TEMPLATE (the second column lists the letters for which the modifier is
3577 ! sSlLiI Forces native (short, long, int) sizes instead
3578 of fixed (16-/32-bit) sizes.
3580 xX Make x and X act as alignment commands.
3582 nNvV Treat integers as signed instead of unsigned.
3584 @. Specify position as byte offset in the internal
3585 representation of the packed string. Efficient but
3588 > sSiIlLqQ Force big-endian byte-order on the type.
3589 jJfFdDpP (The "big end" touches the construct.)
3591 < sSiIlLqQ Force little-endian byte-order on the type.
3592 jJfFdDpP (The "little end" touches the construct.)
3594 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3595 in which case they force a certain byte-order on all components of
3596 that group, including subgroups.
3598 The following rules apply:
3604 Each letter may optionally be followed by a number giving a repeat
3605 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3606 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3607 that many values from the LIST. A C<*> for the repeat count means to
3608 use however many items are left, except for C<@>, C<x>, C<X>, where it
3609 is equivalent to C<0>, for <.> where it means relative to string start
3610 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3611 A numeric repeat count may optionally be enclosed in brackets, as in
3612 C<pack 'C[80]', @arr>.
3614 One can replace the numeric repeat count by a template enclosed in brackets;
3615 then the packed length of this template in bytes is used as a count.
3616 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3617 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3618 If the template in brackets contains alignment commands (such as C<x![d]>),
3619 its packed length is calculated as if the start of the template has the maximal
3622 When used with C<Z>, C<*> results in the addition of a trailing null
3623 byte (so the packed result will be one longer than the byte C<length>
3626 When used with C<@>, the repeat count represents an offset from the start
3627 of the innermost () group.
3629 When used with C<.>, the repeat count is used to determine the starting
3630 position from where the value offset is calculated. If the repeat count
3631 is 0, it's relative to the current position. If the repeat count is C<*>,
3632 the offset is relative to the start of the packed string. And if its an
3633 integer C<n> the offset is relative to the start of the n-th innermost
3634 () group (or the start of the string if C<n> is bigger then the group
3637 The repeat count for C<u> is interpreted as the maximal number of bytes
3638 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3639 count should not be more than 65.
3643 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3644 string of length count, padding with nulls or spaces as necessary. When
3645 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3646 after the first null, and C<a> returns data verbatim.
3648 If the value-to-pack is too long, it is truncated. If too long and an
3649 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3650 by a null byte. Thus C<Z> always packs a trailing null (except when the
3655 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3656 Each character of the input field of pack() generates 1 bit of the result.
3657 Each result bit is based on the least-significant bit of the corresponding
3658 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3659 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3661 Starting from the beginning of the input string of pack(), each 8-tuple
3662 of characters is converted to 1 character of output. With format C<b>
3663 the first character of the 8-tuple determines the least-significant bit of a
3664 character, and with format C<B> it determines the most-significant bit of
3667 If the length of the input string is not exactly divisible by 8, the
3668 remainder is packed as if the input string were padded by null characters
3669 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3671 If the input string of pack() is longer than needed, extra characters are
3672 ignored. A C<*> for the repeat count of pack() means to use all the
3673 characters of the input field. On unpack()ing the bits are converted to a
3674 string of C<"0">s and C<"1">s.
3678 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3679 representable as hexadecimal digits, 0-9a-f) long.
3681 Each character of the input field of pack() generates 4 bits of the result.
3682 For non-alphabetical characters the result is based on the 4 least-significant
3683 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3684 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3685 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3686 is compatible with the usual hexadecimal digits, so that C<"a"> and
3687 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3688 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3690 Starting from the beginning of the input string of pack(), each pair
3691 of characters is converted to 1 character of output. With format C<h> the
3692 first character of the pair determines the least-significant nybble of the
3693 output character, and with format C<H> it determines the most-significant
3696 If the length of the input string is not even, it behaves as if padded
3697 by a null character at the end. Similarly, during unpack()ing the "extra"
3698 nybbles are ignored.
3700 If the input string of pack() is longer than needed, extra characters are
3702 A C<*> for the repeat count of pack() means to use all the characters of
3703 the input field. On unpack()ing the nybbles are converted to a string
3704 of hexadecimal digits.
3708 The C<p> type packs a pointer to a null-terminated string. You are
3709 responsible for ensuring the string is not a temporary value (which can
3710 potentially get deallocated before you get around to using the packed result).
3711 The C<P> type packs a pointer to a structure of the size indicated by the
3712 length. A NULL pointer is created if the corresponding value for C<p> or
3713 C<P> is C<undef>, similarly for unpack().
3715 If your system has a strange pointer size (i.e. a pointer is neither as
3716 big as an int nor as big as a long), it may not be possible to pack or
3717 unpack pointers in big- or little-endian byte order. Attempting to do
3718 so will result in a fatal error.
3722 The C</> template character allows packing and unpacking of a sequence of
3723 items where the packed structure contains a packed item count followed by
3724 the packed items themselves.
3726 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3727 I<length-item> describes how the length value is packed. The ones likely
3728 to be of most use are integer-packing ones like C<n> (for Java strings),
3729 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3731 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3732 the minimum of that and the number of available items is used as argument
3733 for the I<length-item>. If it has no repeat count or uses a '*', the number
3734 of available items is used.
3736 For C<unpack> an internal stack of integer arguments unpacked so far is
3737 used. You write C</>I<sequence-item> and the repeat count is obtained by
3738 popping off the last element from the stack. The I<sequence-item> must not
3739 have a repeat count.
3741 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3742 the I<length-item> is a string length, not a number of strings. If there is
3743 an explicit repeat count for pack, the packed string will be adjusted to that
3746 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3747 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3748 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3749 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3750 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3752 The I<length-item> is not returned explicitly from C<unpack>.
3754 Adding a count to the I<length-item> letter is unlikely to do anything
3755 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3756 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3757 which Perl does not regard as legal in numeric strings.
3761 The integer types C<s>, C<S>, C<l>, and C<L> may be
3762 followed by a C<!> modifier to signify native shorts or
3763 longs--as you can see from above for example a bare C<l> does mean
3764 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3765 may be larger. This is an issue mainly in 64-bit platforms. You can
3766 see whether using C<!> makes any difference by
3768 print length(pack("s")), " ", length(pack("s!")), "\n";
3769 print length(pack("l")), " ", length(pack("l!")), "\n";
3771 C<i!> and C<I!> also work but only because of completeness;
3772 they are identical to C<i> and C<I>.
3774 The actual sizes (in bytes) of native shorts, ints, longs, and long
3775 longs on the platform where Perl was built are also available via
3779 print $Config{shortsize}, "\n";
3780 print $Config{intsize}, "\n";
3781 print $Config{longsize}, "\n";
3782 print $Config{longlongsize}, "\n";
3784 (The C<$Config{longlongsize}> will be undefined if your system does
3785 not support long longs.)
3789 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3790 are inherently non-portable between processors and operating systems
3791 because they obey the native byteorder and endianness. For example a
3792 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3793 (arranged in and handled by the CPU registers) into bytes as
3795 0x12 0x34 0x56 0x78 # big-endian
3796 0x78 0x56 0x34 0x12 # little-endian
3798 Basically, the Intel and VAX CPUs are little-endian, while everybody
3799 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3800 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3801 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3804 The names `big-endian' and `little-endian' are comic references to
3805 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3806 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3807 the egg-eating habits of the Lilliputians.
3809 Some systems may have even weirder byte orders such as
3814 You can see your system's preference with
3816 print join(" ", map { sprintf "%#02x", $_ }
3817 unpack("W*",pack("L",0x12345678))), "\n";
3819 The byteorder on the platform where Perl was built is also available
3823 print $Config{byteorder}, "\n";
3825 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3826 and C<'87654321'> are big-endian.
3828 If you want portable packed integers you can either use the formats
3829 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3830 modifiers. These modifiers are only available as of perl 5.9.2.
3831 See also L<perlport>.
3835 All integer and floating point formats as well as C<p> and C<P> and
3836 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3837 to force big- or little- endian byte-order, respectively.
3838 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3839 signed integers, 64-bit integers and floating point values. However,
3840 there are some things to keep in mind.
3842 Exchanging signed integers between different platforms only works
3843 if all platforms store them in the same format. Most platforms store
3844 signed integers in two's complement, so usually this is not an issue.
3846 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3847 formats on big- or little-endian machines. Otherwise, attempting to
3848 do so will result in a fatal error.
3850 Forcing big- or little-endian byte-order on floating point values for
3851 data exchange can only work if all platforms are using the same
3852 binary representation (e.g. IEEE floating point format). Even if all
3853 platforms are using IEEE, there may be subtle differences. Being able
3854 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3855 but also very dangerous if you don't know exactly what you're doing.
3856 It is definitely not a general way to portably store floating point
3859 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3860 all types inside the group that accept the byte-order modifiers,
3861 including all subgroups. It will silently be ignored for all other
3862 types. You are not allowed to override the byte-order within a group
3863 that already has a byte-order modifier suffix.
3867 Real numbers (floats and doubles) are in the native machine format only;
3868 due to the multiplicity of floating formats around, and the lack of a
3869 standard "network" representation, no facility for interchange has been
3870 made. This means that packed floating point data written on one machine
3871 may not be readable on another - even if both use IEEE floating point
3872 arithmetic (as the endian-ness of the memory representation is not part
3873 of the IEEE spec). See also L<perlport>.
3875 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3876 modifiers to force big- or little-endian byte-order on floating point values.
3878 Note that Perl uses doubles (or long doubles, if configured) internally for
3879 all numeric calculation, and converting from double into float and thence back
3880 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3881 will not in general equal $foo).
3885 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3886 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3887 where the packed string is processed in its UTF-8-encoded Unicode form on
3888 a byte by byte basis. Character mode is the default unless the format string
3889 starts with an C<U>. You can switch mode at any moment with an explicit
3890 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3891 or until the end of the ()-group in which it was entered.
3895 You must yourself do any alignment or padding by inserting for example
3896 enough C<'x'>es while packing. There is no way to pack() and unpack()
3897 could know where the characters are going to or coming from. Therefore
3898 C<pack> (and C<unpack>) handle their output and input as flat
3899 sequences of characters.
3903 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3904 take a repeat count, both as postfix, and for unpack() also via the C</>
3905 template character. Within each repetition of a group, positioning with
3906 C<@> starts again at 0. Therefore, the result of
3908 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3910 is the string "\0a\0\0bc".
3914 C<x> and C<X> accept C<!> modifier. In this case they act as
3915 alignment commands: they jump forward/back to the closest position
3916 aligned at a multiple of C<count> characters. For example, to pack() or
3917 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3918 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3919 aligned on the double's size.
3921 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3922 both result in no-ops.
3926 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3927 will represent signed 16-/32-bit integers in big-/little-endian order.
3928 This is only portable if all platforms sharing the packed data use the
3929 same binary representation for signed integers (e.g. all platforms are
3930 using two's complement representation).
3934 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3935 White space may be used to separate pack codes from each other, but
3936 modifiers and a repeat count must follow immediately.
3940 If TEMPLATE requires more arguments to pack() than actually given, pack()
3941 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3942 to pack() than actually given, extra arguments are ignored.
3948 $foo = pack("WWWW",65,66,67,68);
3950 $foo = pack("W4",65,66,67,68);
3952 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3953 # same thing with Unicode circled letters.
3954 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3955 # same thing with Unicode circled letters. You don't get the UTF-8
3956 # bytes because the U at the start of the format caused a switch to
3957 # U0-mode, so the UTF-8 bytes get joined into characters
3958 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3959 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3960 # This is the UTF-8 encoding of the string in the previous example
3962 $foo = pack("ccxxcc",65,66,67,68);
3965 # note: the above examples featuring "W" and "c" are true
3966 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3967 # and UTF-8. In EBCDIC the first example would be
3968 # $foo = pack("WWWW",193,194,195,196);
3970 $foo = pack("s2",1,2);
3971 # "\1\0\2\0" on little-endian
3972 # "\0\1\0\2" on big-endian
3974 $foo = pack("a4","abcd","x","y","z");
3977 $foo = pack("aaaa","abcd","x","y","z");
3980 $foo = pack("a14","abcdefg");
3981 # "abcdefg\0\0\0\0\0\0\0"
3983 $foo = pack("i9pl", gmtime);
3984 # a real struct tm (on my system anyway)
3986 $utmp_template = "Z8 Z8 Z16 L";
3987 $utmp = pack($utmp_template, @utmp1);
3988 # a struct utmp (BSDish)
3990 @utmp2 = unpack($utmp_template, $utmp);
3991 # "@utmp1" eq "@utmp2"
3994 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3997 $foo = pack('sx2l', 12, 34);
3998 # short 12, two zero bytes padding, long 34
3999 $bar = pack('s@4l', 12, 34);
4000 # short 12, zero fill to position 4, long 34
4002 $baz = pack('s.l', 12, 4, 34);
4003 # short 12, zero fill to position 4, long 34
4005 $foo = pack('nN', 42, 4711);
4006 # pack big-endian 16- and 32-bit unsigned integers
4007 $foo = pack('S>L>', 42, 4711);
4009 $foo = pack('s<l<', -42, 4711);
4010 # pack little-endian 16- and 32-bit signed integers
4011 $foo = pack('(sl)<', -42, 4711);
4014 The same template may generally also be used in unpack().
4016 =item package NAMESPACE
4017 X<package> X<module> X<namespace>
4021 Declares the compilation unit as being in the given namespace. The scope
4022 of the package declaration is from the declaration itself through the end
4023 of the enclosing block, file, or eval (the same as the C<my> operator).
4024 All further unqualified dynamic identifiers will be in this namespace.
4025 A package statement affects only dynamic variables--including those
4026 you've used C<local> on--but I<not> lexical variables, which are created
4027 with C<my>. Typically it would be the first declaration in a file to
4028 be included by the C<require> or C<use> operator. You can switch into a
4029 package in more than one place; it merely influences which symbol table
4030 is used by the compiler for the rest of that block. You can refer to
4031 variables and filehandles in other packages by prefixing the identifier
4032 with the package name and a double colon: C<$Package::Variable>.
4033 If the package name is null, the C<main> package as assumed. That is,
4034 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4035 still seen in older code).
4037 If NAMESPACE is omitted, then there is no current package, and all
4038 identifiers must be fully qualified or lexicals. However, you are
4039 strongly advised not to make use of this feature. Its use can cause
4040 unexpected behaviour, even crashing some versions of Perl. It is
4041 deprecated, and will be removed from a future release.
4043 See L<perlmod/"Packages"> for more information about packages, modules,
4044 and classes. See L<perlsub> for other scoping issues.
4046 =item pipe READHANDLE,WRITEHANDLE
4049 Opens a pair of connected pipes like the corresponding system call.
4050 Note that if you set up a loop of piped processes, deadlock can occur
4051 unless you are very careful. In addition, note that Perl's pipes use
4052 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4053 after each command, depending on the application.
4055 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4056 for examples of such things.
4058 On systems that support a close-on-exec flag on files, the flag will be set
4059 for the newly opened file descriptors as determined by the value of $^F.
4067 Pops and returns the last value of the array, shortening the array by
4068 one element. Has an effect similar to
4072 If there are no elements in the array, returns the undefined value
4073 (although this may happen at other times as well). If ARRAY is
4074 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4075 array in subroutines, just like C<shift>.
4078 X<pos> X<match, position>
4082 Returns the offset of where the last C<m//g> search left off for the variable
4083 in question (C<$_> is used when the variable is not specified). Note that
4084 0 is a valid match offset. C<undef> indicates that the search position
4085 is reset (usually due to match failure, but can also be because no match has
4086 yet been performed on the scalar). C<pos> directly accesses the location used
4087 by the regexp engine to store the offset, so assigning to C<pos> will change
4088 that offset, and so will also influence the C<\G> zero-width assertion in
4089 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4090 the return from C<pos> won't change either in this case. See L<perlre> and
4093 =item print FILEHANDLE LIST
4100 Prints a string or a list of strings. Returns true if successful.
4101 FILEHANDLE may be a scalar variable name, in which case the variable
4102 contains the name of or a reference to the filehandle, thus introducing
4103 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4104 the next token is a term, it may be misinterpreted as an operator
4105 unless you interpose a C<+> or put parentheses around the arguments.)
4106 If FILEHANDLE is omitted, prints by default to standard output (or
4107 to the last selected output channel--see L</select>). If LIST is
4108 also omitted, prints C<$_> to the currently selected output channel.
4109 To set the default output channel to something other than STDOUT
4110 use the select operation. The current value of C<$,> (if any) is
4111 printed between each LIST item. The current value of C<$\> (if
4112 any) is printed after the entire LIST has been printed. Because
4113 print takes a LIST, anything in the LIST is evaluated in list
4114 context, and any subroutine that you call will have one or more of
4115 its expressions evaluated in list context. Also be careful not to
4116 follow the print keyword with a left parenthesis unless you want
4117 the corresponding right parenthesis to terminate the arguments to
4118 the print--interpose a C<+> or put parentheses around all the
4121 Note that if you're storing FILEHANDLEs in an array, or if you're using
4122 any other expression more complex than a scalar variable to retrieve it,
4123 you will have to use a block returning the filehandle value instead:
4125 print { $files[$i] } "stuff\n";
4126 print { $OK ? STDOUT : STDERR } "stuff\n";
4128 =item printf FILEHANDLE FORMAT, LIST
4131 =item printf FORMAT, LIST
4133 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4134 (the output record separator) is not appended. The first argument
4135 of the list will be interpreted as the C<printf> format. See C<sprintf>
4136 for an explanation of the format argument. If C<use locale> is in effect,
4137 and POSIX::setlocale() has been called, the character used for the decimal
4138 separator in formatted floating point numbers is affected by the LC_NUMERIC
4139 locale. See L<perllocale> and L<POSIX>.
4141 Don't fall into the trap of using a C<printf> when a simple
4142 C<print> would do. The C<print> is more efficient and less
4145 =item prototype FUNCTION
4148 Returns the prototype of a function as a string (or C<undef> if the
4149 function has no prototype). FUNCTION is a reference to, or the name of,
4150 the function whose prototype you want to retrieve.
4152 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4153 name for Perl builtin. If the builtin is not I<overridable> (such as
4154 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4155 C<system>) returns C<undef> because the builtin does not really behave
4156 like a Perl function. Otherwise, the string describing the equivalent
4157 prototype is returned.
4159 =item push ARRAY,LIST
4162 Treats ARRAY as a stack, and pushes the values of LIST
4163 onto the end of ARRAY. The length of ARRAY increases by the length of
4164 LIST. Has the same effect as
4167 $ARRAY[++$#ARRAY] = $value;
4170 but is more efficient. Returns the number of elements in the array following
4171 the completed C<push>.
4183 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4185 =item quotemeta EXPR
4186 X<quotemeta> X<metacharacter>
4190 Returns the value of EXPR with all non-"word"
4191 characters backslashed. (That is, all characters not matching
4192 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4193 returned string, regardless of any locale settings.)
4194 This is the internal function implementing
4195 the C<\Q> escape in double-quoted strings.
4197 If EXPR is omitted, uses C<$_>.
4204 Returns a random fractional number greater than or equal to C<0> and less
4205 than the value of EXPR. (EXPR should be positive.) If EXPR is
4206 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4207 also special-cased as C<1> - this has not been documented before perl 5.8.0
4208 and is subject to change in future versions of perl. Automatically calls
4209 C<srand> unless C<srand> has already been called. See also C<srand>.
4211 Apply C<int()> to the value returned by C<rand()> if you want random
4212 integers instead of random fractional numbers. For example,
4216 returns a random integer between C<0> and C<9>, inclusive.
4218 (Note: If your rand function consistently returns numbers that are too
4219 large or too small, then your version of Perl was probably compiled
4220 with the wrong number of RANDBITS.)
4222 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4223 X<read> X<file, read>
4225 =item read FILEHANDLE,SCALAR,LENGTH
4227 Attempts to read LENGTH I<characters> of data into variable SCALAR
4228 from the specified FILEHANDLE. Returns the number of characters
4229 actually read, C<0> at end of file, or undef if there was an error (in
4230 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4231 so that the last character actually read is the last character of the
4232 scalar after the read.
4234 An OFFSET may be specified to place the read data at some place in the
4235 string other than the beginning. A negative OFFSET specifies
4236 placement at that many characters counting backwards from the end of
4237 the string. A positive OFFSET greater than the length of SCALAR
4238 results in the string being padded to the required size with C<"\0">
4239 bytes before the result of the read is appended.
4241 The call is actually implemented in terms of either Perl's or system's
4242 fread() call. To get a true read(2) system call, see C<sysread>.
4244 Note the I<characters>: depending on the status of the filehandle,
4245 either (8-bit) bytes or characters are read. By default all
4246 filehandles operate on bytes, but for example if the filehandle has
4247 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4248 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4249 characters, not bytes. Similarly for the C<:encoding> pragma:
4250 in that case pretty much any characters can be read.
4252 =item readdir DIRHANDLE
4255 Returns the next directory entry for a directory opened by C<opendir>.
4256 If used in list context, returns all the rest of the entries in the
4257 directory. If there are no more entries, returns an undefined value in
4258 scalar context or a null list in list context.
4260 If you're planning to filetest the return values out of a C<readdir>, you'd
4261 better prepend the directory in question. Otherwise, because we didn't
4262 C<chdir> there, it would have been testing the wrong file.
4264 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4265 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4269 X<readline> X<gets> X<fgets>
4271 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4272 context, each call reads and returns the next line, until end-of-file is
4273 reached, whereupon the subsequent call returns undef. In list context,
4274 reads until end-of-file is reached and returns a list of lines. Note that
4275 the notion of "line" used here is however you may have defined it
4276 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4278 When C<$/> is set to C<undef>, when readline() is in scalar
4279 context (i.e. file slurp mode), and when an empty file is read, it
4280 returns C<''> the first time, followed by C<undef> subsequently.
4282 This is the internal function implementing the C<< <EXPR> >>
4283 operator, but you can use it directly. The C<< <EXPR> >>
4284 operator is discussed in more detail in L<perlop/"I/O Operators">.
4287 $line = readline(*STDIN); # same thing
4289 If readline encounters an operating system error, C<$!> will be set with the
4290 corresponding error message. It can be helpful to check C<$!> when you are
4291 reading from filehandles you don't trust, such as a tty or a socket. The
4292 following example uses the operator form of C<readline>, and takes the necessary
4293 steps to ensure that C<readline> was successful.
4297 unless (defined( $line = <> )) {
4309 Returns the value of a symbolic link, if symbolic links are
4310 implemented. If not, gives a fatal error. If there is some system
4311 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4312 omitted, uses C<$_>.
4317 EXPR is executed as a system command.
4318 The collected standard output of the command is returned.
4319 In scalar context, it comes back as a single (potentially
4320 multi-line) string. In list context, returns a list of lines
4321 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4322 This is the internal function implementing the C<qx/EXPR/>
4323 operator, but you can use it directly. The C<qx/EXPR/>
4324 operator is discussed in more detail in L<perlop/"I/O Operators">.
4326 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4329 Receives a message on a socket. Attempts to receive LENGTH characters
4330 of data into variable SCALAR from the specified SOCKET filehandle.
4331 SCALAR will be grown or shrunk to the length actually read. Takes the
4332 same flags as the system call of the same name. Returns the address
4333 of the sender if SOCKET's protocol supports this; returns an empty
4334 string otherwise. If there's an error, returns the undefined value.
4335 This call is actually implemented in terms of recvfrom(2) system call.
4336 See L<perlipc/"UDP: Message Passing"> for examples.
4338 Note the I<characters>: depending on the status of the socket, either
4339 (8-bit) bytes or characters are received. By default all sockets
4340 operate on bytes, but for example if the socket has been changed using
4341 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4342 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4343 characters, not bytes. Similarly for the C<:encoding> pragma:
4344 in that case pretty much any characters can be read.
4351 The C<redo> command restarts the loop block without evaluating the
4352 conditional again. The C<continue> block, if any, is not executed. If
4353 the LABEL is omitted, the command refers to the innermost enclosing
4354 loop. Programs that want to lie to themselves about what was just input
4355 normally use this command:
4357 # a simpleminded Pascal comment stripper
4358 # (warning: assumes no { or } in strings)
4359 LINE: while (<STDIN>) {
4360 while (s|({.*}.*){.*}|$1 |) {}
4365 if (/}/) { # end of comment?
4374 C<redo> cannot be used to retry a block which returns a value such as
4375 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4376 a grep() or map() operation.
4378 Note that a block by itself is semantically identical to a loop
4379 that executes once. Thus C<redo> inside such a block will effectively
4380 turn it into a looping construct.
4382 See also L</continue> for an illustration of how C<last>, C<next>, and
4390 Returns a non-empty string if EXPR is a reference, the empty
4391 string otherwise. If EXPR
4392 is not specified, C<$_> will be used. The value returned depends on the
4393 type of thing the reference is a reference to.
4394 Builtin types include:
4408 If the referenced object has been blessed into a package, then that package
4409 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4411 if (ref($r) eq "HASH") {
4412 print "r is a reference to a hash.\n";
4415 print "r is not a reference at all.\n";
4418 See also L<perlref>.
4420 =item rename OLDNAME,NEWNAME
4421 X<rename> X<move> X<mv> X<ren>
4423 Changes the name of a file; an existing file NEWNAME will be
4424 clobbered. Returns true for success, false otherwise.
4426 Behavior of this function varies wildly depending on your system
4427 implementation. For example, it will usually not work across file system
4428 boundaries, even though the system I<mv> command sometimes compensates
4429 for this. Other restrictions include whether it works on directories,
4430 open files, or pre-existing files. Check L<perlport> and either the
4431 rename(2) manpage or equivalent system documentation for details.
4433 For a platform independent C<move> function look at the L<File::Copy>
4436 =item require VERSION
4443 Demands a version of Perl specified by VERSION, or demands some semantics
4444 specified by EXPR or by C<$_> if EXPR is not supplied.
4446 VERSION may be either a numeric argument such as 5.006, which will be
4447 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4448 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4449 VERSION is greater than the version of the current Perl interpreter.
4450 Compare with L</use>, which can do a similar check at compile time.
4452 Specifying VERSION as a literal of the form v5.6.1 should generally be
4453 avoided, because it leads to misleading error messages under earlier
4454 versions of Perl that do not support this syntax. The equivalent numeric
4455 version should be used instead.
4457 require v5.6.1; # run time version check
4458 require 5.6.1; # ditto
4459 require 5.006_001; # ditto; preferred for backwards compatibility
4461 Otherwise, C<require> demands that a library file be included if it
4462 hasn't already been included. The file is included via the do-FILE
4463 mechanism, which is essentially just a variety of C<eval>. Has
4464 semantics similar to the following subroutine:
4467 my ($filename) = @_;
4468 if (exists $INC{$filename}) {
4469 return 1 if $INC{$filename};
4470 die "Compilation failed in require";
4472 my ($realfilename,$result);
4474 foreach $prefix (@INC) {
4475 $realfilename = "$prefix/$filename";
4476 if (-f $realfilename) {
4477 $INC{$filename} = $realfilename;
4478 $result = do $realfilename;
4482 die "Can't find $filename in \@INC";
4485 $INC{$filename} = undef;
4487 } elsif (!$result) {
4488 delete $INC{$filename};
4489 die "$filename did not return true value";
4495 Note that the file will not be included twice under the same specified
4498 The file must return true as the last statement to indicate
4499 successful execution of any initialization code, so it's customary to
4500 end such a file with C<1;> unless you're sure it'll return true
4501 otherwise. But it's better just to put the C<1;>, in case you add more
4504 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4505 replaces "F<::>" with "F</>" in the filename for you,
4506 to make it easy to load standard modules. This form of loading of
4507 modules does not risk altering your namespace.
4509 In other words, if you try this:
4511 require Foo::Bar; # a splendid bareword
4513 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4514 directories specified in the C<@INC> array.
4516 But if you try this:
4518 $class = 'Foo::Bar';
4519 require $class; # $class is not a bareword
4521 require "Foo::Bar"; # not a bareword because of the ""
4523 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4524 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4526 eval "require $class";
4528 Now that you understand how C<require> looks for files in the case of a
4529 bareword argument, there is a little extra functionality going on behind
4530 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4531 first look for a similar filename with a "F<.pmc>" extension. If this file
4532 is found, it will be loaded in place of any file ending in a "F<.pm>"
4535 You can also insert hooks into the import facility, by putting directly
4536 Perl code into the @INC array. There are three forms of hooks: subroutine
4537 references, array references and blessed objects.
4539 Subroutine references are the simplest case. When the inclusion system
4540 walks through @INC and encounters a subroutine, this subroutine gets
4541 called with two parameters, the first being a reference to itself, and the
4542 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4543 subroutine should return nothing, or a list of up to three values in the
4550 A filehandle, from which the file will be read.
4554 A reference to a subroutine. If there is no filehandle (previous item),
4555 then this subroutine is expected to generate one line of source code per
4556 call, writing the line into C<$_> and returning 1, then returning 0 at
4557 "end of file". If there is a filehandle, then the subroutine will be
4558 called to act a simple source filter, with the line as read in C<$_>.
4559 Again, return 1 for each valid line, and 0 after all lines have been
4564 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4565 reference to the subroutine itself is passed in as C<$_[0]>.
4569 If an empty list, C<undef>, or nothing that matches the first 3 values above
4570 is returned then C<require> will look at the remaining elements of @INC.
4571 Note that this file handle must be a real file handle (strictly a typeglob,
4572 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4573 ignored and return value processing will stop there.
4575 If the hook is an array reference, its first element must be a subroutine
4576 reference. This subroutine is called as above, but the first parameter is
4577 the array reference. This enables to pass indirectly some arguments to
4580 In other words, you can write:
4582 push @INC, \&my_sub;
4584 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4590 push @INC, [ \&my_sub, $x, $y, ... ];
4592 my ($arrayref, $filename) = @_;
4593 # Retrieve $x, $y, ...
4594 my @parameters = @$arrayref[1..$#$arrayref];
4598 If the hook is an object, it must provide an INC method that will be
4599 called as above, the first parameter being the object itself. (Note that
4600 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4601 into package C<main>.) Here is a typical code layout:
4607 my ($self, $filename) = @_;
4611 # In the main program
4612 push @INC, new Foo(...);
4614 Note that these hooks are also permitted to set the %INC entry
4615 corresponding to the files they have loaded. See L<perlvar/%INC>.
4617 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4624 Generally used in a C<continue> block at the end of a loop to clear
4625 variables and reset C<??> searches so that they work again. The
4626 expression is interpreted as a list of single characters (hyphens
4627 allowed for ranges). All variables and arrays beginning with one of
4628 those letters are reset to their pristine state. If the expression is
4629 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4630 only variables or searches in the current package. Always returns
4633 reset 'X'; # reset all X variables
4634 reset 'a-z'; # reset lower case variables
4635 reset; # just reset ?one-time? searches
4637 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4638 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4639 variables--lexical variables are unaffected, but they clean themselves
4640 up on scope exit anyway, so you'll probably want to use them instead.
4648 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4649 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4650 context, depending on how the return value will be used, and the context
4651 may vary from one execution to the next (see C<wantarray>). If no EXPR
4652 is given, returns an empty list in list context, the undefined value in
4653 scalar context, and (of course) nothing at all in a void context.
4655 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4656 or do FILE will automatically return the value of the last expression
4660 X<reverse> X<rev> X<invert>
4662 In list context, returns a list value consisting of the elements
4663 of LIST in the opposite order. In scalar context, concatenates the
4664 elements of LIST and returns a string value with all characters
4665 in the opposite order.
4667 print reverse <>; # line tac, last line first
4669 undef $/; # for efficiency of <>
4670 print scalar reverse <>; # character tac, last line tsrif
4672 Used without arguments in scalar context, reverse() reverses C<$_>.
4674 This operator is also handy for inverting a hash, although there are some
4675 caveats. If a value is duplicated in the original hash, only one of those
4676 can be represented as a key in the inverted hash. Also, this has to
4677 unwind one hash and build a whole new one, which may take some time
4678 on a large hash, such as from a DBM file.
4680 %by_name = reverse %by_address; # Invert the hash
4682 =item rewinddir DIRHANDLE
4685 Sets the current position to the beginning of the directory for the
4686 C<readdir> routine on DIRHANDLE.
4688 =item rindex STR,SUBSTR,POSITION
4691 =item rindex STR,SUBSTR
4693 Works just like index() except that it returns the position of the I<last>
4694 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4695 last occurrence beginning at or before that position.
4697 =item rmdir FILENAME
4698 X<rmdir> X<rd> X<directory, remove>
4702 Deletes the directory specified by FILENAME if that directory is
4703 empty. If it succeeds it returns true, otherwise it returns false and
4704 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4706 To remove a directory tree recursively (C<rm -rf> on unix) look at
4707 the C<rmtree> function of the L<File::Path> module.
4711 The substitution operator. See L<perlop>.
4713 =item say FILEHANDLE LIST
4720 Just like C<print>, but implicitly appends a newline.
4721 C<say LIST> is simply an abbreviation for C<{ local $/ = "\n"; print
4724 This keyword is only available when the "say" feature is
4725 enabled: see L<feature>.
4728 X<scalar> X<context>
4730 Forces EXPR to be interpreted in scalar context and returns the value
4733 @counts = ( scalar @a, scalar @b, scalar @c );
4735 There is no equivalent operator to force an expression to
4736 be interpolated in list context because in practice, this is never
4737 needed. If you really wanted to do so, however, you could use
4738 the construction C<@{[ (some expression) ]}>, but usually a simple
4739 C<(some expression)> suffices.
4741 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4742 parenthesized list, this behaves as a scalar comma expression, evaluating
4743 all but the last element in void context and returning the final element
4744 evaluated in scalar context. This is seldom what you want.
4746 The following single statement:
4748 print uc(scalar(&foo,$bar)),$baz;
4750 is the moral equivalent of these two:
4753 print(uc($bar),$baz);
4755 See L<perlop> for more details on unary operators and the comma operator.
4757 =item seek FILEHANDLE,POSITION,WHENCE
4758 X<seek> X<fseek> X<filehandle, position>
4760 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4761 FILEHANDLE may be an expression whose value gives the name of the
4762 filehandle. The values for WHENCE are C<0> to set the new position
4763 I<in bytes> to POSITION, C<1> to set it to the current position plus
4764 POSITION, and C<2> to set it to EOF plus POSITION (typically
4765 negative). For WHENCE you may use the constants C<SEEK_SET>,
4766 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4767 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4770 Note the I<in bytes>: even if the filehandle has been set to
4771 operate on characters (for example by using the C<:utf8> open
4772 layer), tell() will return byte offsets, not character offsets
4773 (because implementing that would render seek() and tell() rather slow).
4775 If you want to position file for C<sysread> or C<syswrite>, don't use
4776 C<seek>--buffering makes its effect on the file's system position
4777 unpredictable and non-portable. Use C<sysseek> instead.
4779 Due to the rules and rigors of ANSI C, on some systems you have to do a
4780 seek whenever you switch between reading and writing. Amongst other
4781 things, this may have the effect of calling stdio's clearerr(3).
4782 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4786 This is also useful for applications emulating C<tail -f>. Once you hit
4787 EOF on your read, and then sleep for a while, you might have to stick in a
4788 seek() to reset things. The C<seek> doesn't change the current position,
4789 but it I<does> clear the end-of-file condition on the handle, so that the
4790 next C<< <FILE> >> makes Perl try again to read something. We hope.
4792 If that doesn't work (some IO implementations are particularly
4793 cantankerous), then you may need something more like this:
4796 for ($curpos = tell(FILE); $_ = <FILE>;
4797 $curpos = tell(FILE)) {
4798 # search for some stuff and put it into files
4800 sleep($for_a_while);
4801 seek(FILE, $curpos, 0);
4804 =item seekdir DIRHANDLE,POS
4807 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4808 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4809 about possible directory compaction as the corresponding system library
4812 =item select FILEHANDLE
4813 X<select> X<filehandle, default>
4817 Returns the currently selected filehandle. Sets the current default
4818 filehandle for output, if FILEHANDLE is supplied. This has two
4819 effects: first, a C<write> or a C<print> without a filehandle will
4820 default to this FILEHANDLE. Second, references to variables related to
4821 output will refer to this output channel. For example, if you have to
4822 set the top of form format for more than one output channel, you might
4830 FILEHANDLE may be an expression whose value gives the name of the
4831 actual filehandle. Thus:
4833 $oldfh = select(STDERR); $| = 1; select($oldfh);
4835 Some programmers may prefer to think of filehandles as objects with
4836 methods, preferring to write the last example as:
4839 STDERR->autoflush(1);
4841 =item select RBITS,WBITS,EBITS,TIMEOUT
4844 This calls the select(2) system call with the bit masks specified, which
4845 can be constructed using C<fileno> and C<vec>, along these lines:
4847 $rin = $win = $ein = '';
4848 vec($rin,fileno(STDIN),1) = 1;
4849 vec($win,fileno(STDOUT),1) = 1;
4852 If you want to select on many filehandles you might wish to write a
4856 my(@fhlist) = split(' ',$_[0]);
4859 vec($bits,fileno($_),1) = 1;
4863 $rin = fhbits('STDIN TTY SOCK');
4867 ($nfound,$timeleft) =
4868 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4870 or to block until something becomes ready just do this
4872 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4874 Most systems do not bother to return anything useful in $timeleft, so
4875 calling select() in scalar context just returns $nfound.
4877 Any of the bit masks can also be undef. The timeout, if specified, is
4878 in seconds, which may be fractional. Note: not all implementations are
4879 capable of returning the $timeleft. If not, they always return
4880 $timeleft equal to the supplied $timeout.
4882 You can effect a sleep of 250 milliseconds this way:
4884 select(undef, undef, undef, 0.25);
4886 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4887 is implementation-dependent. See also L<perlport> for notes on the
4888 portability of C<select>.
4890 On error, C<select> behaves like the select(2) system call : it returns
4893 Note: on some Unixes, the select(2) system call may report a socket file
4894 descriptor as "ready for reading", when actually no data is available,
4895 thus a subsequent read blocks. It can be avoided using always the
4896 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4899 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4900 or <FH>) with C<select>, except as permitted by POSIX, and even
4901 then only on POSIX systems. You have to use C<sysread> instead.
4903 =item semctl ID,SEMNUM,CMD,ARG
4906 Calls the System V IPC function C<semctl>. You'll probably have to say
4910 first to get the correct constant definitions. If CMD is IPC_STAT or
4911 GETALL, then ARG must be a variable that will hold the returned
4912 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4913 the undefined value for error, "C<0 but true>" for zero, or the actual
4914 return value otherwise. The ARG must consist of a vector of native
4915 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4916 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4919 =item semget KEY,NSEMS,FLAGS
4922 Calls the System V IPC function semget. Returns the semaphore id, or
4923 the undefined value if there is an error. See also
4924 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4927 =item semop KEY,OPSTRING
4930 Calls the System V IPC function semop to perform semaphore operations
4931 such as signalling and waiting. OPSTRING must be a packed array of
4932 semop structures. Each semop structure can be generated with
4933 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4934 implies the number of semaphore operations. Returns true if
4935 successful, or false if there is an error. As an example, the
4936 following code waits on semaphore $semnum of semaphore id $semid:
4938 $semop = pack("s!3", $semnum, -1, 0);
4939 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4941 To signal the semaphore, replace C<-1> with C<1>. See also
4942 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4945 =item send SOCKET,MSG,FLAGS,TO
4948 =item send SOCKET,MSG,FLAGS
4950 Sends a message on a socket. Attempts to send the scalar MSG to the
4951 SOCKET filehandle. Takes the same flags as the system call of the
4952 same name. On unconnected sockets you must specify a destination to
4953 send TO, in which case it does a C C<sendto>. Returns the number of
4954 characters sent, or the undefined value if there is an error. The C
4955 system call sendmsg(2) is currently unimplemented. See
4956 L<perlipc/"UDP: Message Passing"> for examples.
4958 Note the I<characters>: depending on the status of the socket, either
4959 (8-bit) bytes or characters are sent. By default all sockets operate
4960 on bytes, but for example if the socket has been changed using
4961 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4962 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4963 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4964 in that case pretty much any characters can be sent.
4966 =item setpgrp PID,PGRP
4969 Sets the current process group for the specified PID, C<0> for the current
4970 process. Will produce a fatal error if used on a machine that doesn't
4971 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4972 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4973 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4976 =item setpriority WHICH,WHO,PRIORITY
4977 X<setpriority> X<priority> X<nice> X<renice>
4979 Sets the current priority for a process, a process group, or a user.
4980 (See setpriority(2).) Will produce a fatal error if used on a machine
4981 that doesn't implement setpriority(2).
4983 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4986 Sets the socket option requested. Returns undefined if there is an
4987 error. Use integer constants provided by the C<Socket> module for
4988 LEVEL and OPNAME. Values for LEVEL can also be obtained from
4989 getprotobyname. OPTVAL might either be a packed string or an integer.
4990 An integer OPTVAL is shorthand for pack("i", OPTVAL).
4992 An example disabling the Nagle's algorithm for a socket:
4994 use Socket qw(IPPROTO_TCP TCP_NODELAY);
4995 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5002 Shifts the first value of the array off and returns it, shortening the
5003 array by 1 and moving everything down. If there are no elements in the
5004 array, returns the undefined value. If ARRAY is omitted, shifts the
5005 C<@_> array within the lexical scope of subroutines and formats, and the
5006 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5007 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5008 C<UNITCHECK {}> and C<END {}> constructs.
5010 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5011 same thing to the left end of an array that C<pop> and C<push> do to the
5014 =item shmctl ID,CMD,ARG
5017 Calls the System V IPC function shmctl. You'll probably have to say
5021 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5022 then ARG must be a variable that will hold the returned C<shmid_ds>
5023 structure. Returns like ioctl: the undefined value for error, "C<0> but
5024 true" for zero, or the actual return value otherwise.
5025 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5027 =item shmget KEY,SIZE,FLAGS
5030 Calls the System V IPC function shmget. Returns the shared memory
5031 segment id, or the undefined value if there is an error.
5032 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5034 =item shmread ID,VAR,POS,SIZE
5038 =item shmwrite ID,STRING,POS,SIZE
5040 Reads or writes the System V shared memory segment ID starting at
5041 position POS for size SIZE by attaching to it, copying in/out, and
5042 detaching from it. When reading, VAR must be a variable that will
5043 hold the data read. When writing, if STRING is too long, only SIZE
5044 bytes are used; if STRING is too short, nulls are written to fill out
5045 SIZE bytes. Return true if successful, or false if there is an error.
5046 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5047 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5049 =item shutdown SOCKET,HOW
5052 Shuts down a socket connection in the manner indicated by HOW, which
5053 has the same interpretation as in the system call of the same name.
5055 shutdown(SOCKET, 0); # I/we have stopped reading data
5056 shutdown(SOCKET, 1); # I/we have stopped writing data
5057 shutdown(SOCKET, 2); # I/we have stopped using this socket
5059 This is useful with sockets when you want to tell the other
5060 side you're done writing but not done reading, or vice versa.
5061 It's also a more insistent form of close because it also
5062 disables the file descriptor in any forked copies in other
5066 X<sin> X<sine> X<asin> X<arcsine>
5070 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5071 returns sine of C<$_>.
5073 For the inverse sine operation, you may use the C<Math::Trig::asin>
5074 function, or use this relation:
5076 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5083 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5084 May be interrupted if the process receives a signal such as C<SIGALRM>.
5085 Returns the number of seconds actually slept. You probably cannot
5086 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5089 On some older systems, it may sleep up to a full second less than what
5090 you requested, depending on how it counts seconds. Most modern systems
5091 always sleep the full amount. They may appear to sleep longer than that,
5092 however, because your process might not be scheduled right away in a
5093 busy multitasking system.
5095 For delays of finer granularity than one second, you may use Perl's
5096 C<syscall> interface to access setitimer(2) if your system supports
5097 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5098 and starting from Perl 5.8 part of the standard distribution) may also
5101 See also the POSIX module's C<pause> function.
5103 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5106 Opens a socket of the specified kind and attaches it to filehandle
5107 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5108 the system call of the same name. You should C<use Socket> first
5109 to get the proper definitions imported. See the examples in
5110 L<perlipc/"Sockets: Client/Server Communication">.
5112 On systems that support a close-on-exec flag on files, the flag will
5113 be set for the newly opened file descriptor, as determined by the
5114 value of $^F. See L<perlvar/$^F>.
5116 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5119 Creates an unnamed pair of sockets in the specified domain, of the
5120 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5121 for the system call of the same name. If unimplemented, yields a fatal
5122 error. Returns true if successful.
5124 On systems that support a close-on-exec flag on files, the flag will
5125 be set for the newly opened file descriptors, as determined by the value
5126 of $^F. See L<perlvar/$^F>.
5128 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5129 to C<pipe(Rdr, Wtr)> is essentially:
5132 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5133 shutdown(Rdr, 1); # no more writing for reader
5134 shutdown(Wtr, 0); # no more reading for writer
5136 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5137 emulate socketpair using IP sockets to localhost if your system implements
5138 sockets but not socketpair.
5140 =item sort SUBNAME LIST
5141 X<sort> X<qsort> X<quicksort> X<mergesort>
5143 =item sort BLOCK LIST
5147 In list context, this sorts the LIST and returns the sorted list value.
5148 In scalar context, the behaviour of C<sort()> is undefined.
5150 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5151 order. If SUBNAME is specified, it gives the name of a subroutine
5152 that returns an integer less than, equal to, or greater than C<0>,
5153 depending on how the elements of the list are to be ordered. (The C<<
5154 <=> >> and C<cmp> operators are extremely useful in such routines.)
5155 SUBNAME may be a scalar variable name (unsubscripted), in which case
5156 the value provides the name of (or a reference to) the actual
5157 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5158 an anonymous, in-line sort subroutine.
5160 If the subroutine's prototype is C<($$)>, the elements to be compared
5161 are passed by reference in C<@_>, as for a normal subroutine. This is
5162 slower than unprototyped subroutines, where the elements to be
5163 compared are passed into the subroutine
5164 as the package global variables $a and $b (see example below). Note that
5165 in the latter case, it is usually counter-productive to declare $a and
5168 The values to be compared are always passed by reference and should not
5171 You also cannot exit out of the sort block or subroutine using any of the
5172 loop control operators described in L<perlsyn> or with C<goto>.
5174 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5175 current collation locale. See L<perllocale>.
5177 sort() returns aliases into the original list, much as a for loop's index
5178 variable aliases the list elements. That is, modifying an element of a
5179 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5180 actually modifies the element in the original list. This is usually
5181 something to be avoided when writing clear code.
5183 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5184 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5185 preserves the input order of elements that compare equal. Although
5186 quicksort's run time is O(NlogN) when averaged over all arrays of
5187 length N, the time can be O(N**2), I<quadratic> behavior, for some
5188 inputs.) In 5.7, the quicksort implementation was replaced with
5189 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5190 But benchmarks indicated that for some inputs, on some platforms,
5191 the original quicksort was faster. 5.8 has a sort pragma for
5192 limited control of the sort. Its rather blunt control of the
5193 underlying algorithm may not persist into future Perls, but the
5194 ability to characterize the input or output in implementation
5195 independent ways quite probably will. See L<sort>.
5200 @articles = sort @files;
5202 # same thing, but with explicit sort routine
5203 @articles = sort {$a cmp $b} @files;
5205 # now case-insensitively
5206 @articles = sort {uc($a) cmp uc($b)} @files;
5208 # same thing in reversed order
5209 @articles = sort {$b cmp $a} @files;
5211 # sort numerically ascending
5212 @articles = sort {$a <=> $b} @files;
5214 # sort numerically descending
5215 @articles = sort {$b <=> $a} @files;
5217 # this sorts the %age hash by value instead of key
5218 # using an in-line function
5219 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5221 # sort using explicit subroutine name
5223 $age{$a} <=> $age{$b}; # presuming numeric
5225 @sortedclass = sort byage @class;
5227 sub backwards { $b cmp $a }
5228 @harry = qw(dog cat x Cain Abel);
5229 @george = qw(gone chased yz Punished Axed);
5231 # prints AbelCaincatdogx
5232 print sort backwards @harry;
5233 # prints xdogcatCainAbel
5234 print sort @george, 'to', @harry;
5235 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5237 # inefficiently sort by descending numeric compare using
5238 # the first integer after the first = sign, or the
5239 # whole record case-insensitively otherwise
5242 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5247 # same thing, but much more efficiently;
5248 # we'll build auxiliary indices instead
5252 push @nums, /=(\d+)/;
5257 $nums[$b] <=> $nums[$a]
5259 $caps[$a] cmp $caps[$b]
5263 # same thing, but without any temps
5264 @new = map { $_->[0] }
5265 sort { $b->[1] <=> $a->[1]
5268 } map { [$_, /=(\d+)/, uc($_)] } @old;
5270 # using a prototype allows you to use any comparison subroutine
5271 # as a sort subroutine (including other package's subroutines)
5273 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5276 @new = sort other::backwards @old;
5278 # guarantee stability, regardless of algorithm
5280 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5282 # force use of mergesort (not portable outside Perl 5.8)
5283 use sort '_mergesort'; # note discouraging _
5284 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5286 If you're using strict, you I<must not> declare $a
5287 and $b as lexicals. They are package globals. That means
5288 if you're in the C<main> package and type
5290 @articles = sort {$b <=> $a} @files;
5292 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5293 but if you're in the C<FooPack> package, it's the same as typing
5295 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5297 The comparison function is required to behave. If it returns
5298 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5299 sometimes saying the opposite, for example) the results are not
5302 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5303 (not-a-number), and because C<sort> will trigger a fatal error unless the
5304 result of a comparison is defined, when sorting with a comparison function
5305 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5306 The following example takes advantage of the fact that C<NaN != NaN> to
5307 eliminate any C<NaN>s from the input.
5309 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5311 =item splice ARRAY,OFFSET,LENGTH,LIST
5314 =item splice ARRAY,OFFSET,LENGTH
5316 =item splice ARRAY,OFFSET
5320 Removes the elements designated by OFFSET and LENGTH from an array, and
5321 replaces them with the elements of LIST, if any. In list context,
5322 returns the elements removed from the array. In scalar context,
5323 returns the last element removed, or C<undef> if no elements are
5324 removed. The array grows or shrinks as necessary.
5325 If OFFSET is negative then it starts that far from the end of the array.
5326 If LENGTH is omitted, removes everything from OFFSET onward.
5327 If LENGTH is negative, removes the elements from OFFSET onward
5328 except for -LENGTH elements at the end of the array.
5329 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5330 past the end of the array, perl issues a warning, and splices at the
5333 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5335 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5336 pop(@a) splice(@a,-1)
5337 shift(@a) splice(@a,0,1)
5338 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5339 $a[$i] = $y splice(@a,$i,1,$y)
5341 Example, assuming array lengths are passed before arrays:
5343 sub aeq { # compare two list values
5344 my(@a) = splice(@_,0,shift);
5345 my(@b) = splice(@_,0,shift);
5346 return 0 unless @a == @b; # same len?
5348 return 0 if pop(@a) ne pop(@b);
5352 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5354 =item split /PATTERN/,EXPR,LIMIT
5357 =item split /PATTERN/,EXPR
5359 =item split /PATTERN/
5363 Splits the string EXPR into a list of strings and returns that list. By
5364 default, empty leading fields are preserved, and empty trailing ones are
5365 deleted. (If all fields are empty, they are considered to be trailing.)
5367 In scalar context, returns the number of fields found and splits into
5368 the C<@_> array. Use of split in scalar context is deprecated, however,
5369 because it clobbers your subroutine arguments.
5371 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5372 splits on whitespace (after skipping any leading whitespace). Anything
5373 matching PATTERN is taken to be a delimiter separating the fields. (Note
5374 that the delimiter may be longer than one character.)
5376 If LIMIT is specified and positive, it represents the maximum number
5377 of fields the EXPR will be split into, though the actual number of
5378 fields returned depends on the number of times PATTERN matches within
5379 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5380 stripped (which potential users of C<pop> would do well to remember).
5381 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5382 had been specified. Note that splitting an EXPR that evaluates to the
5383 empty string always returns the empty list, regardless of the LIMIT
5386 A pattern matching the null string (not to be confused with
5387 a null pattern C<//>, which is just one member of the set of patterns
5388 matching a null string) will split the value of EXPR into separate
5389 characters at each point it matches that way. For example:
5391 print join(':', split(/ */, 'hi there'));
5393 produces the output 'h:i:t:h:e:r:e'.
5395 As a special case for C<split>, using the empty pattern C<//> specifically
5396 matches only the null string, and is not be confused with the regular use
5397 of C<//> to mean "the last successful pattern match". So, for C<split>,
5400 print join(':', split(//, 'hi there'));
5402 produces the output 'h:i: :t:h:e:r:e'.
5404 Empty leading (or trailing) fields are produced when there are positive
5405 width matches at the beginning (or end) of the string; a zero-width match
5406 at the beginning (or end) of the string does not produce an empty field.
5409 print join(':', split(/(?=\w)/, 'hi there!'));
5411 produces the output 'h:i :t:h:e:r:e!'.
5413 The LIMIT parameter can be used to split a line partially
5415 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5417 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5418 a LIMIT one larger than the number of variables in the list, to avoid
5419 unnecessary work. For the list above LIMIT would have been 4 by
5420 default. In time critical applications it behooves you not to split
5421 into more fields than you really need.
5423 If the PATTERN contains parentheses, additional list elements are
5424 created from each matching substring in the delimiter.
5426 split(/([,-])/, "1-10,20", 3);
5428 produces the list value
5430 (1, '-', 10, ',', 20)
5432 If you had the entire header of a normal Unix email message in $header,
5433 you could split it up into fields and their values this way:
5435 $header =~ s/\n\s+/ /g; # fix continuation lines
5436 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5438 The pattern C</PATTERN/> may be replaced with an expression to specify
5439 patterns that vary at runtime. (To do runtime compilation only once,
5440 use C</$variable/o>.)
5442 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5443 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5444 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5445 will give you as many null initial fields as there are leading spaces.
5446 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5447 whitespace produces a null first field. A C<split> with no arguments
5448 really does a S<C<split(' ', $_)>> internally.
5450 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5455 open(PASSWD, '/etc/passwd');
5458 ($login, $passwd, $uid, $gid,
5459 $gcos, $home, $shell) = split(/:/);
5463 As with regular pattern matching, any capturing parentheses that are not
5464 matched in a C<split()> will be set to C<undef> when returned:
5466 @fields = split /(A)|B/, "1A2B3";
5467 # @fields is (1, 'A', 2, undef, 3)
5469 =item sprintf FORMAT, LIST
5472 Returns a string formatted by the usual C<printf> conventions of the C
5473 library function C<sprintf>. See below for more details
5474 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5475 the general principles.
5479 # Format number with up to 8 leading zeroes
5480 $result = sprintf("%08d", $number);
5482 # Round number to 3 digits after decimal point
5483 $rounded = sprintf("%.3f", $number);
5485 Perl does its own C<sprintf> formatting--it emulates the C
5486 function C<sprintf>, but it doesn't use it (except for floating-point
5487 numbers, and even then only the standard modifiers are allowed). As a
5488 result, any non-standard extensions in your local C<sprintf> are not
5489 available from Perl.
5491 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5492 pass it an array as your first argument. The array is given scalar context,
5493 and instead of using the 0th element of the array as the format, Perl will
5494 use the count of elements in the array as the format, which is almost never
5497 Perl's C<sprintf> permits the following universally-known conversions:
5500 %c a character with the given number
5502 %d a signed integer, in decimal
5503 %u an unsigned integer, in decimal
5504 %o an unsigned integer, in octal
5505 %x an unsigned integer, in hexadecimal
5506 %e a floating-point number, in scientific notation
5507 %f a floating-point number, in fixed decimal notation
5508 %g a floating-point number, in %e or %f notation
5510 In addition, Perl permits the following widely-supported conversions:
5512 %X like %x, but using upper-case letters
5513 %E like %e, but using an upper-case "E"
5514 %G like %g, but with an upper-case "E" (if applicable)
5515 %b an unsigned integer, in binary
5516 %B like %b, but using an upper-case "B" with the # flag
5517 %p a pointer (outputs the Perl value's address in hexadecimal)
5518 %n special: *stores* the number of characters output so far
5519 into the next variable in the parameter list
5521 Finally, for backward (and we do mean "backward") compatibility, Perl
5522 permits these unnecessary but widely-supported conversions:
5525 %D a synonym for %ld
5526 %U a synonym for %lu
5527 %O a synonym for %lo
5530 Note that the number of exponent digits in the scientific notation produced
5531 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5532 exponent less than 100 is system-dependent: it may be three or less
5533 (zero-padded as necessary). In other words, 1.23 times ten to the
5534 99th may be either "1.23e99" or "1.23e099".
5536 Between the C<%> and the format letter, you may specify a number of
5537 additional attributes controlling the interpretation of the format.
5538 In order, these are:
5542 =item format parameter index
5544 An explicit format parameter index, such as C<2$>. By default sprintf
5545 will format the next unused argument in the list, but this allows you
5546 to take the arguments out of order, e.g.:
5548 printf '%2$d %1$d', 12, 34; # prints "34 12"
5549 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5555 space prefix positive number with a space
5556 + prefix positive number with a plus sign
5557 - left-justify within the field
5558 0 use zeros, not spaces, to right-justify
5559 # ensure the leading "0" for any octal,
5560 prefix non-zero hexadecimal with "0x" or "0X",
5561 prefix non-zero binary with "0b" or "0B"
5565 printf '<% d>', 12; # prints "< 12>"
5566 printf '<%+d>', 12; # prints "<+12>"
5567 printf '<%6s>', 12; # prints "< 12>"
5568 printf '<%-6s>', 12; # prints "<12 >"
5569 printf '<%06s>', 12; # prints "<000012>"
5570 printf '<%#o>', 12; # prints "<014>"
5571 printf '<%#x>', 12; # prints "<0xc>"
5572 printf '<%#X>', 12; # prints "<0XC>"
5573 printf '<%#b>', 12; # prints "<0b1100>"
5574 printf '<%#B>', 12; # prints "<0B1100>"
5576 When a space and a plus sign are given as the flags at once,
5577 a plus sign is used to prefix a positive number.
5579 printf '<%+ d>', 12; # prints "<+12>"
5580 printf '<% +d>', 12; # prints "<+12>"
5582 When the # flag and a precision are given in the %o conversion,
5583 the precision is incremented if it's necessary for the leading "0".
5585 printf '<%#.5o>', 012; # prints "<00012>"
5586 printf '<%#.5o>', 012345; # prints "<012345>"
5587 printf '<%#.0o>', 0; # prints "<0>"
5591 This flag tells perl to interpret the supplied string as a vector of
5592 integers, one for each character in the string. Perl applies the format to
5593 each integer in turn, then joins the resulting strings with a separator (a
5594 dot C<.> by default). This can be useful for displaying ordinal values of
5595 characters in arbitrary strings:
5597 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5598 printf "version is v%vd\n", $^V; # Perl's version
5600 Put an asterisk C<*> before the C<v> to override the string to
5601 use to separate the numbers:
5603 printf "address is %*vX\n", ":", $addr; # IPv6 address
5604 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5606 You can also explicitly specify the argument number to use for
5607 the join string using e.g. C<*2$v>:
5609 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5611 =item (minimum) width
5613 Arguments are usually formatted to be only as wide as required to
5614 display the given value. You can override the width by putting
5615 a number here, or get the width from the next argument (with C<*>)
5616 or from a specified argument (with e.g. C<*2$>):
5618 printf '<%s>', "a"; # prints "<a>"
5619 printf '<%6s>', "a"; # prints "< a>"
5620 printf '<%*s>', 6, "a"; # prints "< a>"
5621 printf '<%*2$s>', "a", 6; # prints "< a>"
5622 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5624 If a field width obtained through C<*> is negative, it has the same
5625 effect as the C<-> flag: left-justification.
5627 =item precision, or maximum width
5630 You can specify a precision (for numeric conversions) or a maximum
5631 width (for string conversions) by specifying a C<.> followed by a number.
5632 For floating point formats, with the exception of 'g' and 'G', this specifies
5633 the number of decimal places to show (the default being 6), e.g.:
5635 # these examples are subject to system-specific variation
5636 printf '<%f>', 1; # prints "<1.000000>"
5637 printf '<%.1f>', 1; # prints "<1.0>"
5638 printf '<%.0f>', 1; # prints "<1>"
5639 printf '<%e>', 10; # prints "<1.000000e+01>"
5640 printf '<%.1e>', 10; # prints "<1.0e+01>"
5642 For 'g' and 'G', this specifies the maximum number of digits to show,
5643 including prior to the decimal point as well as after it, e.g.:
5645 # these examples are subject to system-specific variation
5646 printf '<%g>', 1; # prints "<1>"
5647 printf '<%.10g>', 1; # prints "<1>"
5648 printf '<%g>', 100; # prints "<100>"
5649 printf '<%.1g>', 100; # prints "<1e+02>"
5650 printf '<%.2g>', 100.01; # prints "<1e+02>"
5651 printf '<%.5g>', 100.01; # prints "<100.01>"
5652 printf '<%.4g>', 100.01; # prints "<100>"
5654 For integer conversions, specifying a precision implies that the
5655 output of the number itself should be zero-padded to this width,
5656 where the 0 flag is ignored:
5658 printf '<%.6d>', 1; # prints "<000001>"
5659 printf '<%+.6d>', 1; # prints "<+000001>"
5660 printf '<%-10.6d>', 1; # prints "<000001 >"
5661 printf '<%10.6d>', 1; # prints "< 000001>"
5662 printf '<%010.6d>', 1; # prints "< 000001>"
5663 printf '<%+10.6d>', 1; # prints "< +000001>"
5665 printf '<%.6x>', 1; # prints "<000001>"
5666 printf '<%#.6x>', 1; # prints "<0x000001>"
5667 printf '<%-10.6x>', 1; # prints "<000001 >"
5668 printf '<%10.6x>', 1; # prints "< 000001>"
5669 printf '<%010.6x>', 1; # prints "< 000001>"
5670 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5672 For string conversions, specifying a precision truncates the string
5673 to fit in the specified width:
5675 printf '<%.5s>', "truncated"; # prints "<trunc>"
5676 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5678 You can also get the precision from the next argument using C<.*>:
5680 printf '<%.6x>', 1; # prints "<000001>"
5681 printf '<%.*x>', 6, 1; # prints "<000001>"
5683 If a precision obtained through C<*> is negative, it has the same
5684 effect as no precision.
5686 printf '<%.*s>', 7, "string"; # prints "<string>"
5687 printf '<%.*s>', 3, "string"; # prints "<str>"
5688 printf '<%.*s>', 0, "string"; # prints "<>"
5689 printf '<%.*s>', -1, "string"; # prints "<string>"
5691 printf '<%.*d>', 1, 0; # prints "<0>"
5692 printf '<%.*d>', 0, 0; # prints "<>"
5693 printf '<%.*d>', -1, 0; # prints "<0>"
5695 You cannot currently get the precision from a specified number,
5696 but it is intended that this will be possible in the future using
5699 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5703 For numeric conversions, you can specify the size to interpret the
5704 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5705 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5706 whatever the default integer size is on your platform (usually 32 or 64
5707 bits), but you can override this to use instead one of the standard C types,
5708 as supported by the compiler used to build Perl:
5710 l interpret integer as C type "long" or "unsigned long"
5711 h interpret integer as C type "short" or "unsigned short"
5712 q, L or ll interpret integer as C type "long long", "unsigned long long".
5713 or "quads" (typically 64-bit integers)
5715 The last will produce errors if Perl does not understand "quads" in your
5716 installation. (This requires that either the platform natively supports quads
5717 or Perl was specifically compiled to support quads.) You can find out
5718 whether your Perl supports quads via L<Config>:
5721 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5724 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5725 to be the default floating point size on your platform (double or long double),
5726 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5727 platform supports them. You can find out whether your Perl supports long
5728 doubles via L<Config>:
5731 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5733 You can find out whether Perl considers 'long double' to be the default
5734 floating point size to use on your platform via L<Config>:
5737 ($Config{uselongdouble} eq 'define') &&
5738 print "long doubles by default\n";
5740 It can also be the case that long doubles and doubles are the same thing:
5743 ($Config{doublesize} == $Config{longdblsize}) &&
5744 print "doubles are long doubles\n";
5746 The size specifier C<V> has no effect for Perl code, but it is supported
5747 for compatibility with XS code; it means 'use the standard size for
5748 a Perl integer (or floating-point number)', which is already the
5749 default for Perl code.
5751 =item order of arguments
5753 Normally, sprintf takes the next unused argument as the value to
5754 format for each format specification. If the format specification
5755 uses C<*> to require additional arguments, these are consumed from
5756 the argument list in the order in which they appear in the format
5757 specification I<before> the value to format. Where an argument is
5758 specified using an explicit index, this does not affect the normal
5759 order for the arguments (even when the explicitly specified index
5760 would have been the next argument in any case).
5764 printf '<%*.*s>', $a, $b, $c;
5766 would use C<$a> for the width, C<$b> for the precision and C<$c>
5767 as the value to format, while:
5769 print '<%*1$.*s>', $a, $b;
5771 would use C<$a> for the width and the precision, and C<$b> as the
5774 Here are some more examples - beware that when using an explicit
5775 index, the C<$> may need to be escaped:
5777 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5778 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5779 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5780 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5784 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5785 the character used for the decimal separator in formatted floating
5786 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5790 X<sqrt> X<root> X<square root>
5794 Return the square root of EXPR. If EXPR is omitted, returns square
5795 root of C<$_>. Only works on non-negative operands, unless you've
5796 loaded the standard Math::Complex module.
5799 print sqrt(-2); # prints 1.4142135623731i
5802 X<srand> X<seed> X<randseed>
5806 Sets the random number seed for the C<rand> operator.
5808 The point of the function is to "seed" the C<rand> function so that
5809 C<rand> can produce a different sequence each time you run your
5812 If srand() is not called explicitly, it is called implicitly at the
5813 first use of the C<rand> operator. However, this was not the case in
5814 versions of Perl before 5.004, so if your script will run under older
5815 Perl versions, it should call C<srand>.
5817 Most programs won't even call srand() at all, except those that
5818 need a cryptographically-strong starting point rather than the
5819 generally acceptable default, which is based on time of day,
5820 process ID, and memory allocation, or the F</dev/urandom> device,
5823 You can call srand($seed) with the same $seed to reproduce the
5824 I<same> sequence from rand(), but this is usually reserved for
5825 generating predictable results for testing or debugging.
5826 Otherwise, don't call srand() more than once in your program.
5828 Do B<not> call srand() (i.e. without an argument) more than once in
5829 a script. The internal state of the random number generator should
5830 contain more entropy than can be provided by any seed, so calling
5831 srand() again actually I<loses> randomness.
5833 Most implementations of C<srand> take an integer and will silently
5834 truncate decimal numbers. This means C<srand(42)> will usually
5835 produce the same results as C<srand(42.1)>. To be safe, always pass
5836 C<srand> an integer.
5838 In versions of Perl prior to 5.004 the default seed was just the
5839 current C<time>. This isn't a particularly good seed, so many old
5840 programs supply their own seed value (often C<time ^ $$> or C<time ^
5841 ($$ + ($$ << 15))>), but that isn't necessary any more.
5843 For cryptographic purposes, however, you need something much more random
5844 than the default seed. Checksumming the compressed output of one or more
5845 rapidly changing operating system status programs is the usual method. For
5848 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5850 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5853 Frequently called programs (like CGI scripts) that simply use
5857 for a seed can fall prey to the mathematical property that
5861 one-third of the time. So don't do that.
5863 =item stat FILEHANDLE
5864 X<stat> X<file, status> X<ctime>
5868 =item stat DIRHANDLE
5872 Returns a 13-element list giving the status info for a file, either
5873 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5874 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5877 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5878 $atime,$mtime,$ctime,$blksize,$blocks)
5881 Not all fields are supported on all filesystem types. Here are the
5882 meanings of the fields:
5884 0 dev device number of filesystem
5886 2 mode file mode (type and permissions)
5887 3 nlink number of (hard) links to the file
5888 4 uid numeric user ID of file's owner
5889 5 gid numeric group ID of file's owner
5890 6 rdev the device identifier (special files only)
5891 7 size total size of file, in bytes
5892 8 atime last access time in seconds since the epoch
5893 9 mtime last modify time in seconds since the epoch
5894 10 ctime inode change time in seconds since the epoch (*)
5895 11 blksize preferred block size for file system I/O
5896 12 blocks actual number of blocks allocated
5898 (The epoch was at 00:00 January 1, 1970 GMT.)
5900 (*) Not all fields are supported on all filesystem types. Notably, the
5901 ctime field is non-portable. In particular, you cannot expect it to be a
5902 "creation time", see L<perlport/"Files and Filesystems"> for details.
5904 If C<stat> is passed the special filehandle consisting of an underline, no
5905 stat is done, but the current contents of the stat structure from the
5906 last C<stat>, C<lstat>, or filetest are returned. Example:
5908 if (-x $file && (($d) = stat(_)) && $d < 0) {
5909 print "$file is executable NFS file\n";
5912 (This works on machines only for which the device number is negative
5915 Because the mode contains both the file type and its permissions, you
5916 should mask off the file type portion and (s)printf using a C<"%o">
5917 if you want to see the real permissions.
5919 $mode = (stat($filename))[2];
5920 printf "Permissions are %04o\n", $mode & 07777;
5922 In scalar context, C<stat> returns a boolean value indicating success
5923 or failure, and, if successful, sets the information associated with
5924 the special filehandle C<_>.
5926 The L<File::stat> module provides a convenient, by-name access mechanism:
5929 $sb = stat($filename);
5930 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5931 $filename, $sb->size, $sb->mode & 07777,
5932 scalar localtime $sb->mtime;
5934 You can import symbolic mode constants (C<S_IF*>) and functions
5935 (C<S_IS*>) from the Fcntl module:
5939 $mode = (stat($filename))[2];
5941 $user_rwx = ($mode & S_IRWXU) >> 6;
5942 $group_read = ($mode & S_IRGRP) >> 3;
5943 $other_execute = $mode & S_IXOTH;
5945 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5947 $is_setuid = $mode & S_ISUID;
5948 $is_directory = S_ISDIR($mode);
5950 You could write the last two using the C<-u> and C<-d> operators.
5951 The commonly available C<S_IF*> constants are
5953 # Permissions: read, write, execute, for user, group, others.
5955 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5956 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5957 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5959 # Setuid/Setgid/Stickiness/SaveText.
5960 # Note that the exact meaning of these is system dependent.
5962 S_ISUID S_ISGID S_ISVTX S_ISTXT
5964 # File types. Not necessarily all are available on your system.
5966 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5968 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5970 S_IREAD S_IWRITE S_IEXEC
5972 and the C<S_IF*> functions are
5974 S_IMODE($mode) the part of $mode containing the permission bits
5975 and the setuid/setgid/sticky bits
5977 S_IFMT($mode) the part of $mode containing the file type
5978 which can be bit-anded with e.g. S_IFREG
5979 or with the following functions
5981 # The operators -f, -d, -l, -b, -c, -p, and -S.
5983 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5984 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5986 # No direct -X operator counterpart, but for the first one
5987 # the -g operator is often equivalent. The ENFMT stands for
5988 # record flocking enforcement, a platform-dependent feature.
5990 S_ISENFMT($mode) S_ISWHT($mode)
5992 See your native chmod(2) and stat(2) documentation for more details
5993 about the C<S_*> constants. To get status info for a symbolic link
5994 instead of the target file behind the link, use the C<lstat> function.
5999 =item state TYPE EXPR
6001 =item state EXPR : ATTRS
6003 =item state TYPE EXPR : ATTRS
6005 C<state> declares a lexically scoped variable, just like C<my> does.
6006 However, those variables will be initialized only once, contrary to
6007 lexical variables that are reinitialized each time their enclosing block
6010 C<state> variables are only enabled when the C<feature 'state'> pragma is
6011 in effect. See L<feature>.
6018 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6019 doing many pattern matches on the string before it is next modified.
6020 This may or may not save time, depending on the nature and number of
6021 patterns you are searching on, and on the distribution of character
6022 frequencies in the string to be searched--you probably want to compare
6023 run times with and without it to see which runs faster. Those loops
6024 that scan for many short constant strings (including the constant
6025 parts of more complex patterns) will benefit most. You may have only
6026 one C<study> active at a time--if you study a different scalar the first
6027 is "unstudied". (The way C<study> works is this: a linked list of every
6028 character in the string to be searched is made, so we know, for
6029 example, where all the C<'k'> characters are. From each search string,
6030 the rarest character is selected, based on some static frequency tables
6031 constructed from some C programs and English text. Only those places
6032 that contain this "rarest" character are examined.)
6034 For example, here is a loop that inserts index producing entries
6035 before any line containing a certain pattern:
6039 print ".IX foo\n" if /\bfoo\b/;
6040 print ".IX bar\n" if /\bbar\b/;
6041 print ".IX blurfl\n" if /\bblurfl\b/;
6046 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6047 will be looked at, because C<f> is rarer than C<o>. In general, this is
6048 a big win except in pathological cases. The only question is whether
6049 it saves you more time than it took to build the linked list in the
6052 Note that if you have to look for strings that you don't know till
6053 runtime, you can build an entire loop as a string and C<eval> that to
6054 avoid recompiling all your patterns all the time. Together with
6055 undefining C<$/> to input entire files as one record, this can be very
6056 fast, often faster than specialized programs like fgrep(1). The following
6057 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6058 out the names of those files that contain a match:
6060 $search = 'while (<>) { study;';
6061 foreach $word (@words) {
6062 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6067 eval $search; # this screams
6068 $/ = "\n"; # put back to normal input delimiter
6069 foreach $file (sort keys(%seen)) {
6073 =item sub NAME BLOCK
6076 =item sub NAME (PROTO) BLOCK
6078 =item sub NAME : ATTRS BLOCK
6080 =item sub NAME (PROTO) : ATTRS BLOCK
6082 This is subroutine definition, not a real function I<per se>.
6083 Without a BLOCK it's just a forward declaration. Without a NAME,
6084 it's an anonymous function declaration, and does actually return
6085 a value: the CODE ref of the closure you just created.
6087 See L<perlsub> and L<perlref> for details about subroutines and
6088 references, and L<attributes> and L<Attribute::Handlers> for more
6089 information about attributes.
6091 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6092 X<substr> X<substring> X<mid> X<left> X<right>
6094 =item substr EXPR,OFFSET,LENGTH
6096 =item substr EXPR,OFFSET
6098 Extracts a substring out of EXPR and returns it. First character is at
6099 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6100 If OFFSET is negative (or more precisely, less than C<$[>), starts
6101 that far from the end of the string. If LENGTH is omitted, returns
6102 everything to the end of the string. If LENGTH is negative, leaves that
6103 many characters off the end of the string.
6105 my $s = "The black cat climbed the green tree";
6106 my $color = substr $s, 4, 5; # black
6107 my $middle = substr $s, 4, -11; # black cat climbed the
6108 my $end = substr $s, 14; # climbed the green tree
6109 my $tail = substr $s, -4; # tree
6110 my $z = substr $s, -4, 2; # tr
6112 You can use the substr() function as an lvalue, in which case EXPR
6113 must itself be an lvalue. If you assign something shorter than LENGTH,
6114 the string will shrink, and if you assign something longer than LENGTH,
6115 the string will grow to accommodate it. To keep the string the same
6116 length you may need to pad or chop your value using C<sprintf>.
6118 If OFFSET and LENGTH specify a substring that is partly outside the
6119 string, only the part within the string is returned. If the substring
6120 is beyond either end of the string, substr() returns the undefined
6121 value and produces a warning. When used as an lvalue, specifying a
6122 substring that is entirely outside the string is a fatal error.
6123 Here's an example showing the behavior for boundary cases:
6126 substr($name, 4) = 'dy'; # $name is now 'freddy'
6127 my $null = substr $name, 6, 2; # returns '' (no warning)
6128 my $oops = substr $name, 7; # returns undef, with warning
6129 substr($name, 7) = 'gap'; # fatal error
6131 An alternative to using substr() as an lvalue is to specify the
6132 replacement string as the 4th argument. This allows you to replace
6133 parts of the EXPR and return what was there before in one operation,
6134 just as you can with splice().
6136 my $s = "The black cat climbed the green tree";
6137 my $z = substr $s, 14, 7, "jumped from"; # climbed
6138 # $s is now "The black cat jumped from the green tree"
6140 Note that the lvalue returned by the 3-arg version of substr() acts as
6141 a 'magic bullet'; each time it is assigned to, it remembers which part
6142 of the original string is being modified; for example:
6145 for (substr($x,1,2)) {
6146 $_ = 'a'; print $x,"\n"; # prints 1a4
6147 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6149 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6152 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6155 =item symlink OLDFILE,NEWFILE
6156 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6158 Creates a new filename symbolically linked to the old filename.
6159 Returns C<1> for success, C<0> otherwise. On systems that don't support
6160 symbolic links, produces a fatal error at run time. To check for that,
6163 $symlink_exists = eval { symlink("",""); 1 };
6165 =item syscall NUMBER, LIST
6166 X<syscall> X<system call>
6168 Calls the system call specified as the first element of the list,
6169 passing the remaining elements as arguments to the system call. If
6170 unimplemented, produces a fatal error. The arguments are interpreted
6171 as follows: if a given argument is numeric, the argument is passed as
6172 an int. If not, the pointer to the string value is passed. You are
6173 responsible to make sure a string is pre-extended long enough to
6174 receive any result that might be written into a string. You can't use a
6175 string literal (or other read-only string) as an argument to C<syscall>
6176 because Perl has to assume that any string pointer might be written
6178 integer arguments are not literals and have never been interpreted in a
6179 numeric context, you may need to add C<0> to them to force them to look
6180 like numbers. This emulates the C<syswrite> function (or vice versa):
6182 require 'syscall.ph'; # may need to run h2ph
6184 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6186 Note that Perl supports passing of up to only 14 arguments to your system call,
6187 which in practice should usually suffice.
6189 Syscall returns whatever value returned by the system call it calls.
6190 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6191 Note that some system calls can legitimately return C<-1>. The proper
6192 way to handle such calls is to assign C<$!=0;> before the call and
6193 check the value of C<$!> if syscall returns C<-1>.
6195 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6196 number of the read end of the pipe it creates. There is no way
6197 to retrieve the file number of the other end. You can avoid this
6198 problem by using C<pipe> instead.
6200 =item sysopen FILEHANDLE,FILENAME,MODE
6203 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6205 Opens the file whose filename is given by FILENAME, and associates it
6206 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6207 the name of the real filehandle wanted. This function calls the
6208 underlying operating system's C<open> function with the parameters
6209 FILENAME, MODE, PERMS.
6211 The possible values and flag bits of the MODE parameter are
6212 system-dependent; they are available via the standard module C<Fcntl>.
6213 See the documentation of your operating system's C<open> to see which
6214 values and flag bits are available. You may combine several flags
6215 using the C<|>-operator.
6217 Some of the most common values are C<O_RDONLY> for opening the file in
6218 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6219 and C<O_RDWR> for opening the file in read-write mode.
6220 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6222 For historical reasons, some values work on almost every system
6223 supported by perl: zero means read-only, one means write-only, and two
6224 means read/write. We know that these values do I<not> work under
6225 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6226 use them in new code.
6228 If the file named by FILENAME does not exist and the C<open> call creates
6229 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6230 PERMS specifies the permissions of the newly created file. If you omit
6231 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6232 These permission values need to be in octal, and are modified by your
6233 process's current C<umask>.
6236 In many systems the C<O_EXCL> flag is available for opening files in
6237 exclusive mode. This is B<not> locking: exclusiveness means here that
6238 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6239 on network filesystems, and has no effect unless the C<O_CREAT> flag
6240 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6241 being opened if it is a symbolic link. It does not protect against
6242 symbolic links in the file's path.
6245 Sometimes you may want to truncate an already-existing file. This
6246 can be done using the C<O_TRUNC> flag. The behavior of
6247 C<O_TRUNC> with C<O_RDONLY> is undefined.
6250 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6251 that takes away the user's option to have a more permissive umask.
6252 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6255 Note that C<sysopen> depends on the fdopen() C library function.
6256 On many UNIX systems, fdopen() is known to fail when file descriptors
6257 exceed a certain value, typically 255. If you need more file
6258 descriptors than that, consider rebuilding Perl to use the C<sfio>
6259 library, or perhaps using the POSIX::open() function.
6261 See L<perlopentut> for a kinder, gentler explanation of opening files.
6263 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6266 =item sysread FILEHANDLE,SCALAR,LENGTH
6268 Attempts to read LENGTH bytes of data into variable SCALAR from the
6269 specified FILEHANDLE, using the system call read(2). It bypasses
6270 buffered IO, so mixing this with other kinds of reads, C<print>,
6271 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6272 perlio or stdio layers usually buffers data. Returns the number of
6273 bytes actually read, C<0> at end of file, or undef if there was an
6274 error (in the latter case C<$!> is also set). SCALAR will be grown or
6275 shrunk so that the last byte actually read is the last byte of the
6276 scalar after the read.
6278 An OFFSET may be specified to place the read data at some place in the
6279 string other than the beginning. A negative OFFSET specifies
6280 placement at that many characters counting backwards from the end of
6281 the string. A positive OFFSET greater than the length of SCALAR
6282 results in the string being padded to the required size with C<"\0">
6283 bytes before the result of the read is appended.
6285 There is no syseof() function, which is ok, since eof() doesn't work
6286 very well on device files (like ttys) anyway. Use sysread() and check
6287 for a return value for 0 to decide whether you're done.
6289 Note that if the filehandle has been marked as C<:utf8> Unicode
6290 characters are read instead of bytes (the LENGTH, OFFSET, and the
6291 return value of sysread() are in Unicode characters).
6292 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6293 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6295 =item sysseek FILEHANDLE,POSITION,WHENCE
6298 Sets FILEHANDLE's system position in bytes using the system call
6299 lseek(2). FILEHANDLE may be an expression whose value gives the name
6300 of the filehandle. The values for WHENCE are C<0> to set the new
6301 position to POSITION, C<1> to set the it to the current position plus
6302 POSITION, and C<2> to set it to EOF plus POSITION (typically
6305 Note the I<in bytes>: even if the filehandle has been set to operate
6306 on characters (for example by using the C<:utf8> I/O layer), tell()
6307 will return byte offsets, not character offsets (because implementing
6308 that would render sysseek() very slow).
6310 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6311 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6312 C<seek>, C<tell>, or C<eof> may cause confusion.
6314 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6315 and C<SEEK_END> (start of the file, current position, end of the file)
6316 from the Fcntl module. Use of the constants is also more portable
6317 than relying on 0, 1, and 2. For example to define a "systell" function:
6319 use Fcntl 'SEEK_CUR';
6320 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6322 Returns the new position, or the undefined value on failure. A position
6323 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6324 true on success and false on failure, yet you can still easily determine
6330 =item system PROGRAM LIST
6332 Does exactly the same thing as C<exec LIST>, except that a fork is
6333 done first, and the parent process waits for the child process to
6334 complete. Note that argument processing varies depending on the
6335 number of arguments. If there is more than one argument in LIST,
6336 or if LIST is an array with more than one value, starts the program
6337 given by the first element of the list with arguments given by the
6338 rest of the list. If there is only one scalar argument, the argument
6339 is checked for shell metacharacters, and if there are any, the
6340 entire argument is passed to the system's command shell for parsing
6341 (this is C</bin/sh -c> on Unix platforms, but varies on other
6342 platforms). If there are no shell metacharacters in the argument,
6343 it is split into words and passed directly to C<execvp>, which is
6346 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6347 output before any operation that may do a fork, but this may not be
6348 supported on some platforms (see L<perlport>). To be safe, you may need
6349 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6350 of C<IO::Handle> on any open handles.
6352 The return value is the exit status of the program as returned by the
6353 C<wait> call. To get the actual exit value, shift right by eight (see
6354 below). See also L</exec>. This is I<not> what you want to use to capture
6355 the output from a command, for that you should use merely backticks or
6356 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6357 indicates a failure to start the program or an error of the wait(2) system
6358 call (inspect $! for the reason).
6360 Like C<exec>, C<system> allows you to lie to a program about its name if
6361 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6363 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6364 C<system>, if you expect your program to terminate on receipt of these
6365 signals you will need to arrange to do so yourself based on the return
6368 @args = ("command", "arg1", "arg2");
6370 or die "system @args failed: $?"
6372 You can check all the failure possibilities by inspecting
6376 print "failed to execute: $!\n";
6379 printf "child died with signal %d, %s coredump\n",
6380 ($? & 127), ($? & 128) ? 'with' : 'without';
6383 printf "child exited with value %d\n", $? >> 8;
6386 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6387 with the W*() calls of the POSIX extension.
6389 When the arguments get executed via the system shell, results
6390 and return codes will be subject to its quirks and capabilities.
6391 See L<perlop/"`STRING`"> and L</exec> for details.
6393 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6396 =item syswrite FILEHANDLE,SCALAR,LENGTH
6398 =item syswrite FILEHANDLE,SCALAR
6400 Attempts to write LENGTH bytes of data from variable SCALAR to the
6401 specified FILEHANDLE, using the system call write(2). If LENGTH is
6402 not specified, writes whole SCALAR. It bypasses buffered IO, so
6403 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6404 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6405 stdio layers usually buffers data. Returns the number of bytes
6406 actually written, or C<undef> if there was an error (in this case the
6407 errno variable C<$!> is also set). If the LENGTH is greater than the
6408 available data in the SCALAR after the OFFSET, only as much data as is
6409 available will be written.
6411 An OFFSET may be specified to write the data from some part of the
6412 string other than the beginning. A negative OFFSET specifies writing
6413 that many characters counting backwards from the end of the string.
6414 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6416 Note that if the filehandle has been marked as C<:utf8>, Unicode
6417 characters are written instead of bytes (the LENGTH, OFFSET, and the
6418 return value of syswrite() are in UTF-8 encoded Unicode characters).
6419 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6420 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6422 =item tell FILEHANDLE
6427 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6428 error. FILEHANDLE may be an expression whose value gives the name of
6429 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6432 Note the I<in bytes>: even if the filehandle has been set to
6433 operate on characters (for example by using the C<:utf8> open
6434 layer), tell() will return byte offsets, not character offsets
6435 (because that would render seek() and tell() rather slow).
6437 The return value of tell() for the standard streams like the STDIN
6438 depends on the operating system: it may return -1 or something else.
6439 tell() on pipes, fifos, and sockets usually returns -1.
6441 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6443 Do not use tell() (or other buffered I/O operations) on a file handle
6444 that has been manipulated by sysread(), syswrite() or sysseek().
6445 Those functions ignore the buffering, while tell() does not.
6447 =item telldir DIRHANDLE
6450 Returns the current position of the C<readdir> routines on DIRHANDLE.
6451 Value may be given to C<seekdir> to access a particular location in a
6452 directory. C<telldir> has the same caveats about possible directory
6453 compaction as the corresponding system library routine.
6455 =item tie VARIABLE,CLASSNAME,LIST
6458 This function binds a variable to a package class that will provide the
6459 implementation for the variable. VARIABLE is the name of the variable
6460 to be enchanted. CLASSNAME is the name of a class implementing objects
6461 of correct type. Any additional arguments are passed to the C<new>
6462 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6463 or C<TIEHASH>). Typically these are arguments such as might be passed
6464 to the C<dbm_open()> function of C. The object returned by the C<new>
6465 method is also returned by the C<tie> function, which would be useful
6466 if you want to access other methods in CLASSNAME.
6468 Note that functions such as C<keys> and C<values> may return huge lists
6469 when used on large objects, like DBM files. You may prefer to use the
6470 C<each> function to iterate over such. Example:
6472 # print out history file offsets
6474 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6475 while (($key,$val) = each %HIST) {
6476 print $key, ' = ', unpack('L',$val), "\n";
6480 A class implementing a hash should have the following methods:
6482 TIEHASH classname, LIST
6484 STORE this, key, value
6489 NEXTKEY this, lastkey
6494 A class implementing an ordinary array should have the following methods:
6496 TIEARRAY classname, LIST
6498 STORE this, key, value
6500 STORESIZE this, count
6506 SPLICE this, offset, length, LIST
6511 A class implementing a file handle should have the following methods:
6513 TIEHANDLE classname, LIST
6514 READ this, scalar, length, offset
6517 WRITE this, scalar, length, offset
6519 PRINTF this, format, LIST
6523 SEEK this, position, whence
6525 OPEN this, mode, LIST
6530 A class implementing a scalar should have the following methods:
6532 TIESCALAR classname, LIST
6538 Not all methods indicated above need be implemented. See L<perltie>,
6539 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6541 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6542 for you--you need to do that explicitly yourself. See L<DB_File>
6543 or the F<Config> module for interesting C<tie> implementations.
6545 For further details see L<perltie>, L<"tied VARIABLE">.
6550 Returns a reference to the object underlying VARIABLE (the same value
6551 that was originally returned by the C<tie> call that bound the variable
6552 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6558 Returns the number of non-leap seconds since whatever time the system
6559 considers to be the epoch, suitable for feeding to C<gmtime> and
6560 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6561 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6562 1904 in the current local time zone for its epoch.
6564 For measuring time in better granularity than one second,
6565 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6566 Perl 5.8 part of the standard distribution), or if you have
6567 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6568 See L<perlfaq8> for details.
6570 For date and time processing look at the many related modules on CPAN.
6571 For a comprehensive date and time representation look at the
6577 Returns a four-element list giving the user and system times, in
6578 seconds, for this process and the children of this process.
6580 ($user,$system,$cuser,$csystem) = times;
6582 In scalar context, C<times> returns C<$user>.
6584 Note that times for children are included only after they terminate.
6588 The transliteration operator. Same as C<y///>. See L<perlop>.
6590 =item truncate FILEHANDLE,LENGTH
6593 =item truncate EXPR,LENGTH
6595 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6596 specified length. Produces a fatal error if truncate isn't implemented
6597 on your system. Returns true if successful, the undefined value
6600 The behavior is undefined if LENGTH is greater than the length of the
6603 The position in the file of FILEHANDLE is left unchanged. You may want to
6604 call L<seek> before writing to the file.
6607 X<uc> X<uppercase> X<toupper>
6611 Returns an uppercased version of EXPR. This is the internal function
6612 implementing the C<\U> escape in double-quoted strings. Respects
6613 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6614 and L<perlunicode> for more details about locale and Unicode support.
6615 It does not attempt to do titlecase mapping on initial letters. See
6616 C<ucfirst> for that.
6618 If EXPR is omitted, uses C<$_>.
6621 X<ucfirst> X<uppercase>
6625 Returns the value of EXPR with the first character in uppercase
6626 (titlecase in Unicode). This is the internal function implementing
6627 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6628 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6629 for more details about locale and Unicode support.
6631 If EXPR is omitted, uses C<$_>.
6638 Sets the umask for the process to EXPR and returns the previous value.
6639 If EXPR is omitted, merely returns the current umask.
6641 The Unix permission C<rwxr-x---> is represented as three sets of three
6642 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6643 and isn't one of the digits). The C<umask> value is such a number
6644 representing disabled permissions bits. The permission (or "mode")
6645 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6646 even if you tell C<sysopen> to create a file with permissions C<0777>,
6647 if your umask is C<0022> then the file will actually be created with
6648 permissions C<0755>. If your C<umask> were C<0027> (group can't
6649 write; others can't read, write, or execute), then passing
6650 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6653 Here's some advice: supply a creation mode of C<0666> for regular
6654 files (in C<sysopen>) and one of C<0777> for directories (in
6655 C<mkdir>) and executable files. This gives users the freedom of
6656 choice: if they want protected files, they might choose process umasks
6657 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6658 Programs should rarely if ever make policy decisions better left to
6659 the user. The exception to this is when writing files that should be
6660 kept private: mail files, web browser cookies, I<.rhosts> files, and
6663 If umask(2) is not implemented on your system and you are trying to
6664 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6665 fatal error at run time. If umask(2) is not implemented and you are
6666 not trying to restrict access for yourself, returns C<undef>.
6668 Remember that a umask is a number, usually given in octal; it is I<not> a
6669 string of octal digits. See also L</oct>, if all you have is a string.
6672 X<undef> X<undefine>
6676 Undefines the value of EXPR, which must be an lvalue. Use only on a
6677 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6678 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6679 will probably not do what you expect on most predefined variables or
6680 DBM list values, so don't do that; see L<delete>.) Always returns the
6681 undefined value. You can omit the EXPR, in which case nothing is
6682 undefined, but you still get an undefined value that you could, for
6683 instance, return from a subroutine, assign to a variable or pass as a
6684 parameter. Examples:
6687 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6691 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6692 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6693 select undef, undef, undef, 0.25;
6694 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6696 Note that this is a unary operator, not a list operator.
6699 X<unlink> X<delete> X<remove> X<rm> X<del>
6703 Deletes a list of files. Returns the number of files successfully
6706 $cnt = unlink 'a', 'b', 'c';
6710 Note: C<unlink> will not attempt to delete directories unless you are superuser
6711 and the B<-U> flag is supplied to Perl. Even if these conditions are
6712 met, be warned that unlinking a directory can inflict damage on your
6713 filesystem. Finally, using C<unlink> on directories is not supported on
6714 many operating systems. Use C<rmdir> instead.
6716 If LIST is omitted, uses C<$_>.
6718 =item unpack TEMPLATE,EXPR
6721 =item unpack TEMPLATE
6723 C<unpack> does the reverse of C<pack>: it takes a string
6724 and expands it out into a list of values.
6725 (In scalar context, it returns merely the first value produced.)
6727 If EXPR is omitted, unpacks the C<$_> string.
6729 The string is broken into chunks described by the TEMPLATE. Each chunk
6730 is converted separately to a value. Typically, either the string is a result
6731 of C<pack>, or the characters of the string represent a C structure of some
6734 The TEMPLATE has the same format as in the C<pack> function.
6735 Here's a subroutine that does substring:
6738 my($what,$where,$howmuch) = @_;
6739 unpack("x$where a$howmuch", $what);
6744 sub ordinal { unpack("W",$_[0]); } # same as ord()
6746 In addition to fields allowed in pack(), you may prefix a field with
6747 a %<number> to indicate that
6748 you want a <number>-bit checksum of the items instead of the items
6749 themselves. Default is a 16-bit checksum. Checksum is calculated by
6750 summing numeric values of expanded values (for string fields the sum of
6751 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6753 For example, the following
6754 computes the same number as the System V sum program:
6758 unpack("%32W*",<>) % 65535;
6761 The following efficiently counts the number of set bits in a bit vector:
6763 $setbits = unpack("%32b*", $selectmask);
6765 The C<p> and C<P> formats should be used with care. Since Perl
6766 has no way of checking whether the value passed to C<unpack()>
6767 corresponds to a valid memory location, passing a pointer value that's
6768 not known to be valid is likely to have disastrous consequences.
6770 If there are more pack codes or if the repeat count of a field or a group
6771 is larger than what the remainder of the input string allows, the result
6772 is not well defined: in some cases, the repeat count is decreased, or
6773 C<unpack()> will produce null strings or zeroes, or terminate with an
6774 error. If the input string is longer than one described by the TEMPLATE,
6775 the rest is ignored.
6777 See L</pack> for more examples and notes.
6779 =item untie VARIABLE
6782 Breaks the binding between a variable and a package. (See C<tie>.)
6783 Has no effect if the variable is not tied.
6785 =item unshift ARRAY,LIST
6788 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6789 depending on how you look at it. Prepends list to the front of the
6790 array, and returns the new number of elements in the array.
6792 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6794 Note the LIST is prepended whole, not one element at a time, so the
6795 prepended elements stay in the same order. Use C<reverse> to do the
6798 =item use Module VERSION LIST
6799 X<use> X<module> X<import>
6801 =item use Module VERSION
6803 =item use Module LIST
6809 Imports some semantics into the current package from the named module,
6810 generally by aliasing certain subroutine or variable names into your
6811 package. It is exactly equivalent to
6813 BEGIN { require Module; import Module LIST; }
6815 except that Module I<must> be a bareword.
6817 VERSION may be either a numeric argument such as 5.006, which will be
6818 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6819 to C<$^V> (aka $PERL_VERSION). A fatal error is produced if VERSION is
6820 greater than the version of the current Perl interpreter; Perl will not
6821 attempt to parse the rest of the file. Compare with L</require>, which can
6822 do a similar check at run time.
6824 Specifying VERSION as a literal of the form v5.6.1 should generally be
6825 avoided, because it leads to misleading error messages under earlier
6826 versions of Perl that do not support this syntax. The equivalent numeric
6827 version should be used instead.
6829 use v5.6.1; # compile time version check
6831 use 5.006_001; # ditto; preferred for backwards compatibility
6833 This is often useful if you need to check the current Perl version before
6834 C<use>ing library modules that have changed in incompatible ways from
6835 older versions of Perl. (We try not to do this more than we have to.)
6837 If the specified perl version is greater than or equal to 5.9.5, C<use
6838 VERSION> will also load the C<feature> pragma and enable all features
6839 available in the requested version. See L<feature>.
6841 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6842 C<require> makes sure the module is loaded into memory if it hasn't been
6843 yet. The C<import> is not a builtin--it's just an ordinary static method
6844 call into the C<Module> package to tell the module to import the list of
6845 features back into the current package. The module can implement its
6846 C<import> method any way it likes, though most modules just choose to
6847 derive their C<import> method via inheritance from the C<Exporter> class that
6848 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6849 method can be found then the call is skipped, even if there is an AUTOLOAD
6852 If you do not want to call the package's C<import> method (for instance,
6853 to stop your namespace from being altered), explicitly supply the empty list:
6857 That is exactly equivalent to
6859 BEGIN { require Module }
6861 If the VERSION argument is present between Module and LIST, then the
6862 C<use> will call the VERSION method in class Module with the given
6863 version as an argument. The default VERSION method, inherited from
6864 the UNIVERSAL class, croaks if the given version is larger than the
6865 value of the variable C<$Module::VERSION>.
6867 Again, there is a distinction between omitting LIST (C<import> called
6868 with no arguments) and an explicit empty LIST C<()> (C<import> not
6869 called). Note that there is no comma after VERSION!
6871 Because this is a wide-open interface, pragmas (compiler directives)
6872 are also implemented this way. Currently implemented pragmas are:
6877 use sigtrap qw(SEGV BUS);
6878 use strict qw(subs vars refs);
6879 use subs qw(afunc blurfl);
6880 use warnings qw(all);
6881 use sort qw(stable _quicksort _mergesort);
6883 Some of these pseudo-modules import semantics into the current
6884 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6885 which import symbols into the current package (which are effective
6886 through the end of the file).
6888 There's a corresponding C<no> command that unimports meanings imported
6889 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6890 It behaves exactly as C<import> does with respect to VERSION, an
6891 omitted LIST, empty LIST, or no unimport method being found.
6897 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6898 for the C<-M> and C<-m> command-line options to perl that give C<use>
6899 functionality from the command-line.
6904 Changes the access and modification times on each file of a list of
6905 files. The first two elements of the list must be the NUMERICAL access
6906 and modification times, in that order. Returns the number of files
6907 successfully changed. The inode change time of each file is set
6908 to the current time. For example, this code has the same effect as the
6909 Unix touch(1) command when the files I<already exist> and belong to
6910 the user running the program:
6913 $atime = $mtime = time;
6914 utime $atime, $mtime, @ARGV;
6916 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6917 the utime(2) function in the C library will be called with a null second
6918 argument. On most systems, this will set the file's access and
6919 modification times to the current time (i.e. equivalent to the example
6920 above) and will even work on other users' files where you have write
6923 utime undef, undef, @ARGV;
6925 Under NFS this will use the time of the NFS server, not the time of
6926 the local machine. If there is a time synchronization problem, the
6927 NFS server and local machine will have different times. The Unix
6928 touch(1) command will in fact normally use this form instead of the
6929 one shown in the first example.
6931 Note that only passing one of the first two elements as C<undef> will
6932 be equivalent of passing it as 0 and will not have the same effect as
6933 described when they are both C<undef>. This case will also trigger an
6934 uninitialized warning.
6936 On systems that support futimes, you might pass file handles among the
6937 files. On systems that don't support futimes, passing file handles
6938 produces a fatal error at run time. The file handles must be passed
6939 as globs or references to be recognized. Barewords are considered
6945 Returns a list consisting of all the values of the named hash.
6946 (In a scalar context, returns the number of values.)
6948 The values are returned in an apparently random order. The actual
6949 random order is subject to change in future versions of perl, but it
6950 is guaranteed to be the same order as either the C<keys> or C<each>
6951 function would produce on the same (unmodified) hash. Since Perl
6952 5.8.1 the ordering is different even between different runs of Perl
6953 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6955 As a side effect, calling values() resets the HASH's internal iterator,
6956 see L</each>. (In particular, calling values() in void context resets
6957 the iterator with no other overhead.)
6959 Note that the values are not copied, which means modifying them will
6960 modify the contents of the hash:
6962 for (values %hash) { s/foo/bar/g } # modifies %hash values
6963 for (@hash{keys %hash}) { s/foo/bar/g } # same
6965 See also C<keys>, C<each>, and C<sort>.
6967 =item vec EXPR,OFFSET,BITS
6968 X<vec> X<bit> X<bit vector>
6970 Treats the string in EXPR as a bit vector made up of elements of
6971 width BITS, and returns the value of the element specified by OFFSET
6972 as an unsigned integer. BITS therefore specifies the number of bits
6973 that are reserved for each element in the bit vector. This must
6974 be a power of two from 1 to 32 (or 64, if your platform supports
6977 If BITS is 8, "elements" coincide with bytes of the input string.
6979 If BITS is 16 or more, bytes of the input string are grouped into chunks
6980 of size BITS/8, and each group is converted to a number as with
6981 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6982 for BITS==64). See L<"pack"> for details.
6984 If bits is 4 or less, the string is broken into bytes, then the bits
6985 of each byte are broken into 8/BITS groups. Bits of a byte are
6986 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6987 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6988 breaking the single input byte C<chr(0x36)> into two groups gives a list
6989 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6991 C<vec> may also be assigned to, in which case parentheses are needed
6992 to give the expression the correct precedence as in
6994 vec($image, $max_x * $x + $y, 8) = 3;
6996 If the selected element is outside the string, the value 0 is returned.
6997 If an element off the end of the string is written to, Perl will first
6998 extend the string with sufficiently many zero bytes. It is an error
6999 to try to write off the beginning of the string (i.e. negative OFFSET).
7001 If the string happens to be encoded as UTF-8 internally (and thus has
7002 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7003 internal byte string, not the conceptual character string, even if you
7004 only have characters with values less than 256.
7006 Strings created with C<vec> can also be manipulated with the logical
7007 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7008 vector operation is desired when both operands are strings.
7009 See L<perlop/"Bitwise String Operators">.
7011 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7012 The comments show the string after each step. Note that this code works
7013 in the same way on big-endian or little-endian machines.
7016 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7018 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7019 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7021 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7022 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7023 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7024 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7025 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7026 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7028 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7029 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7030 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7033 To transform a bit vector into a string or list of 0's and 1's, use these:
7035 $bits = unpack("b*", $vector);
7036 @bits = split(//, unpack("b*", $vector));
7038 If you know the exact length in bits, it can be used in place of the C<*>.
7040 Here is an example to illustrate how the bits actually fall in place:
7046 unpack("V",$_) 01234567890123456789012345678901
7047 ------------------------------------------------------------------
7052 for ($shift=0; $shift < $width; ++$shift) {
7053 for ($off=0; $off < 32/$width; ++$off) {
7054 $str = pack("B*", "0"x32);
7055 $bits = (1<<$shift);
7056 vec($str, $off, $width) = $bits;
7057 $res = unpack("b*",$str);
7058 $val = unpack("V", $str);
7065 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7066 $off, $width, $bits, $val, $res
7070 Regardless of the machine architecture on which it is run, the above
7071 example should print the following table:
7074 unpack("V",$_) 01234567890123456789012345678901
7075 ------------------------------------------------------------------
7076 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7077 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7078 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7079 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7080 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7081 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7082 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7083 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7084 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7085 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7086 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7087 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7088 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7089 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7090 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7091 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7092 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7093 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7094 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7095 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7096 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7097 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7098 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7099 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7100 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7101 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7102 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7103 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7104 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7105 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7106 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7107 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7108 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7109 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7110 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7111 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7112 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7113 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7114 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7115 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7116 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7117 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7118 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7119 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7120 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7121 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7122 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7123 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7124 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7125 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7126 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7127 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7128 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7129 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7130 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7131 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7132 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7133 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7134 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7135 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7136 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7137 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7138 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7139 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7140 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7141 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7142 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7143 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7144 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7145 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7146 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7147 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7148 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7149 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7150 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7151 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7152 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7153 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7154 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7155 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7156 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7157 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7158 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7159 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7160 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7161 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7162 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7163 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7164 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7165 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7166 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7167 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7168 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7169 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7170 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7171 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7172 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7173 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7174 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7175 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7176 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7177 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7178 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7179 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7180 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7181 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7182 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7183 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7184 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7185 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7186 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7187 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7188 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7189 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7190 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7191 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7192 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7193 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7194 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7195 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7196 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7197 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7198 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7199 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7200 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7201 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7202 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7203 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7208 Behaves like the wait(2) system call on your system: it waits for a child
7209 process to terminate and returns the pid of the deceased process, or
7210 C<-1> if there are no child processes. The status is returned in C<$?>
7211 and C<{^CHILD_ERROR_NATIVE}>.
7212 Note that a return value of C<-1> could mean that child processes are
7213 being automatically reaped, as described in L<perlipc>.
7215 =item waitpid PID,FLAGS
7218 Waits for a particular child process to terminate and returns the pid of
7219 the deceased process, or C<-1> if there is no such child process. On some
7220 systems, a value of 0 indicates that there are processes still running.
7221 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7223 use POSIX ":sys_wait_h";
7226 $kid = waitpid(-1, WNOHANG);
7229 then you can do a non-blocking wait for all pending zombie processes.
7230 Non-blocking wait is available on machines supporting either the
7231 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7232 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7233 system call by remembering the status values of processes that have
7234 exited but have not been harvested by the Perl script yet.)
7236 Note that on some systems, a return value of C<-1> could mean that child
7237 processes are being automatically reaped. See L<perlipc> for details,
7238 and for other examples.
7241 X<wantarray> X<context>
7243 Returns true if the context of the currently executing subroutine or
7244 C<eval> is looking for a list value. Returns false if the context is
7245 looking for a scalar. Returns the undefined value if the context is
7246 looking for no value (void context).
7248 return unless defined wantarray; # don't bother doing more
7249 my @a = complex_calculation();
7250 return wantarray ? @a : "@a";
7252 C<wantarray()>'s result is unspecified in the top level of a file,
7253 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7254 in a C<DESTROY> method.
7256 This function should have been named wantlist() instead.
7259 X<warn> X<warning> X<STDERR>
7261 Prints the value of LIST to STDERR. If the last element of LIST does
7262 not end in a newline, appends the same text as C<die> does.
7264 If LIST is empty and C<$@> already contains a value (typically from a
7265 previous eval) that value is used after appending C<"\t...caught">
7266 to C<$@>. This is useful for staying almost, but not entirely similar to
7269 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7271 No message is printed if there is a C<$SIG{__WARN__}> handler
7272 installed. It is the handler's responsibility to deal with the message
7273 as it sees fit (like, for instance, converting it into a C<die>). Most
7274 handlers must therefore make arrangements to actually display the
7275 warnings that they are not prepared to deal with, by calling C<warn>
7276 again in the handler. Note that this is quite safe and will not
7277 produce an endless loop, since C<__WARN__> hooks are not called from
7280 You will find this behavior is slightly different from that of
7281 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7282 instead call C<die> again to change it).
7284 Using a C<__WARN__> handler provides a powerful way to silence all
7285 warnings (even the so-called mandatory ones). An example:
7287 # wipe out *all* compile-time warnings
7288 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7290 my $foo = 20; # no warning about duplicate my $foo,
7291 # but hey, you asked for it!
7292 # no compile-time or run-time warnings before here
7295 # run-time warnings enabled after here
7296 warn "\$foo is alive and $foo!"; # does show up
7298 See L<perlvar> for details on setting C<%SIG> entries, and for more
7299 examples. See the Carp module for other kinds of warnings using its
7300 carp() and cluck() functions.
7302 =item write FILEHANDLE
7309 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7310 using the format associated with that file. By default the format for
7311 a file is the one having the same name as the filehandle, but the
7312 format for the current output channel (see the C<select> function) may be set
7313 explicitly by assigning the name of the format to the C<$~> variable.
7315 Top of form processing is handled automatically: if there is
7316 insufficient room on the current page for the formatted record, the
7317 page is advanced by writing a form feed, a special top-of-page format
7318 is used to format the new page header, and then the record is written.
7319 By default the top-of-page format is the name of the filehandle with
7320 "_TOP" appended, but it may be dynamically set to the format of your
7321 choice by assigning the name to the C<$^> variable while the filehandle is
7322 selected. The number of lines remaining on the current page is in
7323 variable C<$->, which can be set to C<0> to force a new page.
7325 If FILEHANDLE is unspecified, output goes to the current default output
7326 channel, which starts out as STDOUT but may be changed by the
7327 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7328 is evaluated and the resulting string is used to look up the name of
7329 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7331 Note that write is I<not> the opposite of C<read>. Unfortunately.
7335 The transliteration operator. Same as C<tr///>. See L<perlop>.