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
3004 See the C<use> function, of which C<no> is the opposite.
3007 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3011 Interprets EXPR as an octal string and returns the corresponding
3012 value. (If EXPR happens to start off with C<0x>, interprets it as a
3013 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3014 binary string. Leading whitespace is ignored in all three cases.)
3015 The following will handle decimal, binary, octal, and hex in the standard
3018 $val = oct($val) if $val =~ /^0/;
3020 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3021 in octal), use sprintf() or printf():
3023 $perms = (stat("filename"))[2] & 07777;
3024 $oct_perms = sprintf "%lo", $perms;
3026 The oct() function is commonly used when a string such as C<644> needs
3027 to be converted into a file mode, for example. (Although perl will
3028 automatically convert strings into numbers as needed, this automatic
3029 conversion assumes base 10.)
3031 =item open FILEHANDLE,EXPR
3032 X<open> X<pipe> X<file, open> X<fopen>
3034 =item open FILEHANDLE,MODE,EXPR
3036 =item open FILEHANDLE,MODE,EXPR,LIST
3038 =item open FILEHANDLE,MODE,REFERENCE
3040 =item open FILEHANDLE
3042 Opens the file whose filename is given by EXPR, and associates it with
3045 (The following is a comprehensive reference to open(): for a gentler
3046 introduction you may consider L<perlopentut>.)
3048 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3049 the variable is assigned a reference to a new anonymous filehandle,
3050 otherwise if FILEHANDLE is an expression, its value is used as the name of
3051 the real filehandle wanted. (This is considered a symbolic reference, so
3052 C<use strict 'refs'> should I<not> be in effect.)
3054 If EXPR is omitted, the scalar variable of the same name as the
3055 FILEHANDLE contains the filename. (Note that lexical variables--those
3056 declared with C<my>--will not work for this purpose; so if you're
3057 using C<my>, specify EXPR in your call to open.)
3059 If three or more arguments are specified then the mode of opening and
3060 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3061 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3062 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3063 the file is opened for appending, again being created if necessary.
3065 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3066 indicate that you want both read and write access to the file; thus
3067 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3068 '+>' >> mode would clobber the file first. You can't usually use
3069 either read-write mode for updating textfiles, since they have
3070 variable length records. See the B<-i> switch in L<perlrun> for a
3071 better approach. The file is created with permissions of C<0666>
3072 modified by the process' C<umask> value.
3074 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3075 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3077 In the 2-arguments (and 1-argument) form of the call the mode and
3078 filename should be concatenated (in this order), possibly separated by
3079 spaces. It is possible to omit the mode in these forms if the mode is
3082 If the filename begins with C<'|'>, the filename is interpreted as a
3083 command to which output is to be piped, and if the filename ends with a
3084 C<'|'>, the filename is interpreted as a command which pipes output to
3085 us. See L<perlipc/"Using open() for IPC">
3086 for more examples of this. (You are not allowed to C<open> to a command
3087 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3088 and L<perlipc/"Bidirectional Communication with Another Process">
3091 For three or more arguments if MODE is C<'|-'>, the filename is
3092 interpreted as a command to which output is to be piped, and if MODE
3093 is C<'-|'>, the filename is interpreted as a command which pipes
3094 output to us. In the 2-arguments (and 1-argument) form one should
3095 replace dash (C<'-'>) with the command.
3096 See L<perlipc/"Using open() for IPC"> for more examples of this.
3097 (You are not allowed to C<open> to a command that pipes both in I<and>
3098 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3099 L<perlipc/"Bidirectional Communication"> for alternatives.)
3101 In the three-or-more argument form of pipe opens, if LIST is specified
3102 (extra arguments after the command name) then LIST becomes arguments
3103 to the command invoked if the platform supports it. The meaning of
3104 C<open> with more than three arguments for non-pipe modes is not yet
3105 specified. Experimental "layers" may give extra LIST arguments
3108 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3109 and opening C<< '>-' >> opens STDOUT.
3111 You may use the three-argument form of open to specify IO "layers"
3112 (sometimes also referred to as "disciplines") to be applied to the handle
3113 that affect how the input and output are processed (see L<open> and
3114 L<PerlIO> for more details). For example
3116 open(FH, "<:encoding(UTF-8)", "file")
3118 will open the UTF-8 encoded file containing Unicode characters,
3119 see L<perluniintro>. Note that if layers are specified in the
3120 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3121 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3123 Open returns nonzero upon success, the undefined value otherwise. If
3124 the C<open> involved a pipe, the return value happens to be the pid of
3127 If you're running Perl on a system that distinguishes between text
3128 files and binary files, then you should check out L</binmode> for tips
3129 for dealing with this. The key distinction between systems that need
3130 C<binmode> and those that don't is their text file formats. Systems
3131 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3132 character, and which encode that character in C as C<"\n">, do not
3133 need C<binmode>. The rest need it.
3135 When opening a file, it's usually a bad idea to continue normal execution
3136 if the request failed, so C<open> is frequently used in connection with
3137 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3138 where you want to make a nicely formatted error message (but there are
3139 modules that can help with that problem)) you should always check
3140 the return value from opening a file. The infrequent exception is when
3141 working with an unopened filehandle is actually what you want to do.
3143 As a special case the 3-arg form with a read/write mode and the third
3144 argument being C<undef>:
3146 open(TMP, "+>", undef) or die ...
3148 opens a filehandle to an anonymous temporary file. Also using "+<"
3149 works for symmetry, but you really should consider writing something
3150 to the temporary file first. You will need to seek() to do the
3153 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3154 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3155 "in memory" files held in Perl scalars via:
3157 open($fh, '>', \$variable) || ..
3159 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3160 file, you have to close it first:
3163 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3168 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3169 while (<ARTICLE>) {...
3171 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3172 # if the open fails, output is discarded
3174 open(DBASE, '+<', 'dbase.mine') # open for update
3175 or die "Can't open 'dbase.mine' for update: $!";
3177 open(DBASE, '+<dbase.mine') # ditto
3178 or die "Can't open 'dbase.mine' for update: $!";
3180 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3181 or die "Can't start caesar: $!";
3183 open(ARTICLE, "caesar <$article |") # ditto
3184 or die "Can't start caesar: $!";
3186 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3187 or die "Can't start sort: $!";
3190 open(MEMORY,'>', \$var)
3191 or die "Can't open memory file: $!";
3192 print MEMORY "foo!\n"; # output will end up in $var
3194 # process argument list of files along with any includes
3196 foreach $file (@ARGV) {
3197 process($file, 'fh00');
3201 my($filename, $input) = @_;
3202 $input++; # this is a string increment
3203 unless (open($input, $filename)) {
3204 print STDERR "Can't open $filename: $!\n";
3209 while (<$input>) { # note use of indirection
3210 if (/^#include "(.*)"/) {
3211 process($1, $input);
3218 See L<perliol> for detailed info on PerlIO.
3220 You may also, in the Bourne shell tradition, specify an EXPR beginning
3221 with C<< '>&' >>, in which case the rest of the string is interpreted
3222 as the name of a filehandle (or file descriptor, if numeric) to be
3223 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3224 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3225 The mode you specify should match the mode of the original filehandle.
3226 (Duping a filehandle does not take into account any existing contents
3227 of IO buffers.) If you use the 3-arg form then you can pass either a
3228 number, the name of a filehandle or the normal "reference to a glob".
3230 Here is a script that saves, redirects, and restores C<STDOUT> and
3231 C<STDERR> using various methods:
3234 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3235 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3237 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3238 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3240 select STDERR; $| = 1; # make unbuffered
3241 select STDOUT; $| = 1; # make unbuffered
3243 print STDOUT "stdout 1\n"; # this works for
3244 print STDERR "stderr 1\n"; # subprocesses too
3246 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3247 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3249 print STDOUT "stdout 2\n";
3250 print STDERR "stderr 2\n";
3252 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3253 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3254 that file descriptor (and not call L<dup(2)>); this is more
3255 parsimonious of file descriptors. For example:
3257 # open for input, reusing the fileno of $fd
3258 open(FILEHANDLE, "<&=$fd")
3262 open(FILEHANDLE, "<&=", $fd)
3266 # open for append, using the fileno of OLDFH
3267 open(FH, ">>&=", OLDFH)
3271 open(FH, ">>&=OLDFH")
3273 Being parsimonious on filehandles is also useful (besides being
3274 parsimonious) for example when something is dependent on file
3275 descriptors, like for example locking using flock(). If you do just
3276 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3277 descriptor as B, and therefore flock(A) will not flock(B), and vice
3278 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3279 the same file descriptor.
3281 Note that if you are using Perls older than 5.8.0, Perl will be using
3282 the standard C libraries' fdopen() to implement the "=" functionality.
3283 On many UNIX systems fdopen() fails when file descriptors exceed a
3284 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3285 most often the default.
3287 You can see whether Perl has been compiled with PerlIO or not by
3288 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3289 is C<define>, you have PerlIO, otherwise you don't.
3291 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3292 with 2-arguments (or 1-argument) form of open(), then
3293 there is an implicit fork done, and the return value of open is the pid
3294 of the child within the parent process, and C<0> within the child
3295 process. (Use C<defined($pid)> to determine whether the open was successful.)
3296 The filehandle behaves normally for the parent, but i/o to that
3297 filehandle is piped from/to the STDOUT/STDIN of the child process.
3298 In the child process the filehandle isn't opened--i/o happens from/to
3299 the new STDOUT or STDIN. Typically this is used like the normal
3300 piped open when you want to exercise more control over just how the
3301 pipe command gets executed, such as when you are running setuid, and
3302 don't want to have to scan shell commands for metacharacters.
3303 The following triples are more or less equivalent:
3305 open(FOO, "|tr '[a-z]' '[A-Z]'");
3306 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3307 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3308 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3310 open(FOO, "cat -n '$file'|");
3311 open(FOO, '-|', "cat -n '$file'");
3312 open(FOO, '-|') || exec 'cat', '-n', $file;
3313 open(FOO, '-|', "cat", '-n', $file);
3315 The last example in each block shows the pipe as "list form", which is
3316 not yet supported on all platforms. A good rule of thumb is that if
3317 your platform has true C<fork()> (in other words, if your platform is
3318 UNIX) you can use the list form.
3320 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3322 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3323 output before any operation that may do a fork, but this may not be
3324 supported on some platforms (see L<perlport>). To be safe, you may need
3325 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3326 of C<IO::Handle> on any open handles.
3328 On systems that support a close-on-exec flag on files, the flag will
3329 be set for the newly opened file descriptor as determined by the value
3330 of $^F. See L<perlvar/$^F>.
3332 Closing any piped filehandle causes the parent process to wait for the
3333 child to finish, and returns the status value in C<$?> and
3334 C<${^CHILD_ERROR_NATIVE}>.
3336 The filename passed to 2-argument (or 1-argument) form of open() will
3337 have leading and trailing whitespace deleted, and the normal
3338 redirection characters honored. This property, known as "magic open",
3339 can often be used to good effect. A user could specify a filename of
3340 F<"rsh cat file |">, or you could change certain filenames as needed:
3342 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3343 open(FH, $filename) or die "Can't open $filename: $!";
3345 Use 3-argument form to open a file with arbitrary weird characters in it,
3347 open(FOO, '<', $file);
3349 otherwise it's necessary to protect any leading and trailing whitespace:
3351 $file =~ s#^(\s)#./$1#;
3352 open(FOO, "< $file\0");
3354 (this may not work on some bizarre filesystems). One should
3355 conscientiously choose between the I<magic> and 3-arguments form
3360 will allow the user to specify an argument of the form C<"rsh cat file |">,
3361 but will not work on a filename which happens to have a trailing space, while
3363 open IN, '<', $ARGV[0];
3365 will have exactly the opposite restrictions.
3367 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3368 should use the C<sysopen> function, which involves no such magic (but
3369 may use subtly different filemodes than Perl open(), which is mapped
3370 to C fopen()). This is
3371 another way to protect your filenames from interpretation. For example:
3374 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3375 or die "sysopen $path: $!";
3376 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3377 print HANDLE "stuff $$\n";
3379 print "File contains: ", <HANDLE>;
3381 Using the constructor from the C<IO::Handle> package (or one of its
3382 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3383 filehandles that have the scope of whatever variables hold references to
3384 them, and automatically close whenever and however you leave that scope:
3388 sub read_myfile_munged {
3390 my $handle = new IO::File;
3391 open($handle, "myfile") or die "myfile: $!";
3393 or return (); # Automatically closed here.
3394 mung $first or die "mung failed"; # Or here.
3395 return $first, <$handle> if $ALL; # Or here.
3399 See L</seek> for some details about mixing reading and writing.
3401 =item opendir DIRHANDLE,EXPR
3404 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3405 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3406 DIRHANDLE may be an expression whose value can be used as an indirect
3407 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3408 scalar variable (or array or hash element), the variable is assigned a
3409 reference to a new anonymous dirhandle.
3410 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3417 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3418 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3421 For the reverse, see L</chr>.
3422 See L<perlunicode> for more about Unicode.
3429 =item our EXPR : ATTRS
3431 =item our TYPE EXPR : ATTRS
3433 C<our> associates a simple name with a package variable in the current
3434 package for use within the current scope. When C<use strict 'vars'> is in
3435 effect, C<our> lets you use declared global variables without qualifying
3436 them with package names, within the lexical scope of the C<our> declaration.
3437 In this way C<our> differs from C<use vars>, which is package scoped.
3439 Unlike C<my>, which both allocates storage for a variable and associates
3440 a simple name with that storage for use within the current scope, C<our>
3441 associates a simple name with a package variable in the current package,
3442 for use within the current scope. In other words, C<our> has the same
3443 scoping rules as C<my>, but does not necessarily create a
3446 If more than one value is listed, the list must be placed
3452 An C<our> declaration declares a global variable that will be visible
3453 across its entire lexical scope, even across package boundaries. The
3454 package in which the variable is entered is determined at the point
3455 of the declaration, not at the point of use. This means the following
3459 our $bar; # declares $Foo::bar for rest of lexical scope
3463 print $bar; # prints 20, as it refers to $Foo::bar
3465 Multiple C<our> declarations with the same name in the same lexical
3466 scope are allowed if they are in different packages. If they happen
3467 to be in the same package, Perl will emit warnings if you have asked
3468 for them, just like multiple C<my> declarations. Unlike a second
3469 C<my> declaration, which will bind the name to a fresh variable, a
3470 second C<our> declaration in the same package, in the same scope, is
3475 our $bar; # declares $Foo::bar for rest of lexical scope
3479 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3480 print $bar; # prints 30
3482 our $bar; # emits warning but has no other effect
3483 print $bar; # still prints 30
3485 An C<our> declaration may also have a list of attributes associated
3488 The exact semantics and interface of TYPE and ATTRS are still
3489 evolving. TYPE is currently bound to the use of C<fields> pragma,
3490 and attributes are handled using the C<attributes> pragma, or starting
3491 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3492 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3493 L<attributes>, and L<Attribute::Handlers>.
3495 =item pack TEMPLATE,LIST
3498 Takes a LIST of values and converts it into a string using the rules
3499 given by the TEMPLATE. The resulting string is the concatenation of
3500 the converted values. Typically, each converted value looks
3501 like its machine-level representation. For example, on 32-bit machines
3502 an integer may be represented by a sequence of 4 bytes that will be
3503 converted to a sequence of 4 characters.
3505 The TEMPLATE is a sequence of characters that give the order and type
3506 of values, as follows:
3508 a A string with arbitrary binary data, will be null padded.
3509 A A text (ASCII) string, will be space padded.
3510 Z A null terminated (ASCIZ) string, will be null padded.
3512 b A bit string (ascending bit order inside each byte, like vec()).
3513 B A bit string (descending bit order inside each byte).
3514 h A hex string (low nybble first).
3515 H A hex string (high nybble first).
3517 c A signed char (8-bit) value.
3518 C An unsigned C char (octet) even under Unicode. Should normally not
3519 be used. See U and W instead.
3520 W An unsigned char value (can be greater than 255).
3522 s A signed short (16-bit) value.
3523 S An unsigned short value.
3525 l A signed long (32-bit) value.
3526 L An unsigned long value.
3528 q A signed quad (64-bit) value.
3529 Q An unsigned quad value.
3530 (Quads are available only if your system supports 64-bit
3531 integer values _and_ if Perl has been compiled to support those.
3532 Causes a fatal error otherwise.)
3534 i A signed integer value.
3535 I A unsigned integer value.
3536 (This 'integer' is _at_least_ 32 bits wide. Its exact
3537 size depends on what a local C compiler calls 'int'.)
3539 n An unsigned short (16-bit) in "network" (big-endian) order.
3540 N An unsigned long (32-bit) in "network" (big-endian) order.
3541 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3542 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3544 j A Perl internal signed integer value (IV).
3545 J A Perl internal unsigned integer value (UV).
3547 f A single-precision float in the native format.
3548 d A double-precision float in the native format.
3550 F A Perl internal floating point value (NV) in the native format
3551 D A long double-precision float in the native format.
3552 (Long doubles are available only if your system supports long
3553 double values _and_ if Perl has been compiled to support those.
3554 Causes a fatal error otherwise.)
3556 p A pointer to a null-terminated string.
3557 P A pointer to a structure (fixed-length string).
3559 u A uuencoded string.
3560 U A Unicode character number. Encodes to UTF-8 internally
3561 (or UTF-EBCDIC in EBCDIC platforms).
3563 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3564 details). Its bytes represent an unsigned integer in base 128,
3565 most significant digit first, with as few digits as possible. Bit
3566 eight (the high bit) is set on each byte except the last.
3570 @ Null fill or truncate to absolute position, counted from the
3571 start of the innermost ()-group.
3572 . Null fill or truncate to absolute position specified by value.
3573 ( Start of a ()-group.
3575 One or more of the modifiers below may optionally follow some letters in the
3576 TEMPLATE (the second column lists the letters for which the modifier is
3579 ! sSlLiI Forces native (short, long, int) sizes instead
3580 of fixed (16-/32-bit) sizes.
3582 xX Make x and X act as alignment commands.
3584 nNvV Treat integers as signed instead of unsigned.
3586 @. Specify position as byte offset in the internal
3587 representation of the packed string. Efficient but
3590 > sSiIlLqQ Force big-endian byte-order on the type.
3591 jJfFdDpP (The "big end" touches the construct.)
3593 < sSiIlLqQ Force little-endian byte-order on the type.
3594 jJfFdDpP (The "little end" touches the construct.)
3596 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3597 in which case they force a certain byte-order on all components of
3598 that group, including subgroups.
3600 The following rules apply:
3606 Each letter may optionally be followed by a number giving a repeat
3607 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3608 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3609 that many values from the LIST. A C<*> for the repeat count means to
3610 use however many items are left, except for C<@>, C<x>, C<X>, where it
3611 is equivalent to C<0>, for <.> where it means relative to string start
3612 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3613 A numeric repeat count may optionally be enclosed in brackets, as in
3614 C<pack 'C[80]', @arr>.
3616 One can replace the numeric repeat count by a template enclosed in brackets;
3617 then the packed length of this template in bytes is used as a count.
3618 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3619 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3620 If the template in brackets contains alignment commands (such as C<x![d]>),
3621 its packed length is calculated as if the start of the template has the maximal
3624 When used with C<Z>, C<*> results in the addition of a trailing null
3625 byte (so the packed result will be one longer than the byte C<length>
3628 When used with C<@>, the repeat count represents an offset from the start
3629 of the innermost () group.
3631 When used with C<.>, the repeat count is used to determine the starting
3632 position from where the value offset is calculated. If the repeat count
3633 is 0, it's relative to the current position. If the repeat count is C<*>,
3634 the offset is relative to the start of the packed string. And if its an
3635 integer C<n> the offset is relative to the start of the n-th innermost
3636 () group (or the start of the string if C<n> is bigger then the group
3639 The repeat count for C<u> is interpreted as the maximal number of bytes
3640 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3641 count should not be more than 65.
3645 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3646 string of length count, padding with nulls or spaces as necessary. When
3647 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3648 after the first null, and C<a> returns data verbatim.
3650 If the value-to-pack is too long, it is truncated. If too long and an
3651 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3652 by a null byte. Thus C<Z> always packs a trailing null (except when the
3657 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3658 Each character of the input field of pack() generates 1 bit of the result.
3659 Each result bit is based on the least-significant bit of the corresponding
3660 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3661 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3663 Starting from the beginning of the input string of pack(), each 8-tuple
3664 of characters is converted to 1 character of output. With format C<b>
3665 the first character of the 8-tuple determines the least-significant bit of a
3666 character, and with format C<B> it determines the most-significant bit of
3669 If the length of the input string is not exactly divisible by 8, the
3670 remainder is packed as if the input string were padded by null characters
3671 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3673 If the input string of pack() is longer than needed, extra characters are
3674 ignored. A C<*> for the repeat count of pack() means to use all the
3675 characters of the input field. On unpack()ing the bits are converted to a
3676 string of C<"0">s and C<"1">s.
3680 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3681 representable as hexadecimal digits, 0-9a-f) long.
3683 Each character of the input field of pack() generates 4 bits of the result.
3684 For non-alphabetical characters the result is based on the 4 least-significant
3685 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3686 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3687 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3688 is compatible with the usual hexadecimal digits, so that C<"a"> and
3689 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3690 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3692 Starting from the beginning of the input string of pack(), each pair
3693 of characters is converted to 1 character of output. With format C<h> the
3694 first character of the pair determines the least-significant nybble of the
3695 output character, and with format C<H> it determines the most-significant
3698 If the length of the input string is not even, it behaves as if padded
3699 by a null character at the end. Similarly, during unpack()ing the "extra"
3700 nybbles are ignored.
3702 If the input string of pack() is longer than needed, extra characters are
3704 A C<*> for the repeat count of pack() means to use all the characters of
3705 the input field. On unpack()ing the nybbles are converted to a string
3706 of hexadecimal digits.
3710 The C<p> type packs a pointer to a null-terminated string. You are
3711 responsible for ensuring the string is not a temporary value (which can
3712 potentially get deallocated before you get around to using the packed result).
3713 The C<P> type packs a pointer to a structure of the size indicated by the
3714 length. A NULL pointer is created if the corresponding value for C<p> or
3715 C<P> is C<undef>, similarly for unpack().
3717 If your system has a strange pointer size (i.e. a pointer is neither as
3718 big as an int nor as big as a long), it may not be possible to pack or
3719 unpack pointers in big- or little-endian byte order. Attempting to do
3720 so will result in a fatal error.
3724 The C</> template character allows packing and unpacking of a sequence of
3725 items where the packed structure contains a packed item count followed by
3726 the packed items themselves.
3728 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3729 I<length-item> describes how the length value is packed. The ones likely
3730 to be of most use are integer-packing ones like C<n> (for Java strings),
3731 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3733 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3734 the minimum of that and the number of available items is used as argument
3735 for the I<length-item>. If it has no repeat count or uses a '*', the number
3736 of available items is used.
3738 For C<unpack> an internal stack of integer arguments unpacked so far is
3739 used. You write C</>I<sequence-item> and the repeat count is obtained by
3740 popping off the last element from the stack. The I<sequence-item> must not
3741 have a repeat count.
3743 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3744 the I<length-item> is a string length, not a number of strings. If there is
3745 an explicit repeat count for pack, the packed string will be adjusted to that
3748 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3749 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3750 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3751 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3752 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3754 The I<length-item> is not returned explicitly from C<unpack>.
3756 Adding a count to the I<length-item> letter is unlikely to do anything
3757 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3758 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3759 which Perl does not regard as legal in numeric strings.
3763 The integer types C<s>, C<S>, C<l>, and C<L> may be
3764 followed by a C<!> modifier to signify native shorts or
3765 longs--as you can see from above for example a bare C<l> does mean
3766 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3767 may be larger. This is an issue mainly in 64-bit platforms. You can
3768 see whether using C<!> makes any difference by
3770 print length(pack("s")), " ", length(pack("s!")), "\n";
3771 print length(pack("l")), " ", length(pack("l!")), "\n";
3773 C<i!> and C<I!> also work but only because of completeness;
3774 they are identical to C<i> and C<I>.
3776 The actual sizes (in bytes) of native shorts, ints, longs, and long
3777 longs on the platform where Perl was built are also available via
3781 print $Config{shortsize}, "\n";
3782 print $Config{intsize}, "\n";
3783 print $Config{longsize}, "\n";
3784 print $Config{longlongsize}, "\n";
3786 (The C<$Config{longlongsize}> will be undefined if your system does
3787 not support long longs.)
3791 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3792 are inherently non-portable between processors and operating systems
3793 because they obey the native byteorder and endianness. For example a
3794 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3795 (arranged in and handled by the CPU registers) into bytes as
3797 0x12 0x34 0x56 0x78 # big-endian
3798 0x78 0x56 0x34 0x12 # little-endian
3800 Basically, the Intel and VAX CPUs are little-endian, while everybody
3801 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3802 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3803 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3806 The names `big-endian' and `little-endian' are comic references to
3807 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3808 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3809 the egg-eating habits of the Lilliputians.
3811 Some systems may have even weirder byte orders such as
3816 You can see your system's preference with
3818 print join(" ", map { sprintf "%#02x", $_ }
3819 unpack("W*",pack("L",0x12345678))), "\n";
3821 The byteorder on the platform where Perl was built is also available
3825 print $Config{byteorder}, "\n";
3827 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3828 and C<'87654321'> are big-endian.
3830 If you want portable packed integers you can either use the formats
3831 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3832 modifiers. These modifiers are only available as of perl 5.9.2.
3833 See also L<perlport>.
3837 All integer and floating point formats as well as C<p> and C<P> and
3838 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3839 to force big- or little- endian byte-order, respectively.
3840 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3841 signed integers, 64-bit integers and floating point values. However,
3842 there are some things to keep in mind.
3844 Exchanging signed integers between different platforms only works
3845 if all platforms store them in the same format. Most platforms store
3846 signed integers in two's complement, so usually this is not an issue.
3848 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3849 formats on big- or little-endian machines. Otherwise, attempting to
3850 do so will result in a fatal error.
3852 Forcing big- or little-endian byte-order on floating point values for
3853 data exchange can only work if all platforms are using the same
3854 binary representation (e.g. IEEE floating point format). Even if all
3855 platforms are using IEEE, there may be subtle differences. Being able
3856 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3857 but also very dangerous if you don't know exactly what you're doing.
3858 It is definitely not a general way to portably store floating point
3861 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3862 all types inside the group that accept the byte-order modifiers,
3863 including all subgroups. It will silently be ignored for all other
3864 types. You are not allowed to override the byte-order within a group
3865 that already has a byte-order modifier suffix.
3869 Real numbers (floats and doubles) are in the native machine format only;
3870 due to the multiplicity of floating formats around, and the lack of a
3871 standard "network" representation, no facility for interchange has been
3872 made. This means that packed floating point data written on one machine
3873 may not be readable on another - even if both use IEEE floating point
3874 arithmetic (as the endian-ness of the memory representation is not part
3875 of the IEEE spec). See also L<perlport>.
3877 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3878 modifiers to force big- or little-endian byte-order on floating point values.
3880 Note that Perl uses doubles (or long doubles, if configured) internally for
3881 all numeric calculation, and converting from double into float and thence back
3882 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3883 will not in general equal $foo).
3887 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3888 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3889 where the packed string is processed in its UTF-8-encoded Unicode form on
3890 a byte by byte basis. Character mode is the default unless the format string
3891 starts with an C<U>. You can switch mode at any moment with an explicit
3892 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3893 or until the end of the ()-group in which it was entered.
3897 You must yourself do any alignment or padding by inserting for example
3898 enough C<'x'>es while packing. There is no way to pack() and unpack()
3899 could know where the characters are going to or coming from. Therefore
3900 C<pack> (and C<unpack>) handle their output and input as flat
3901 sequences of characters.
3905 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3906 take a repeat count, both as postfix, and for unpack() also via the C</>
3907 template character. Within each repetition of a group, positioning with
3908 C<@> starts again at 0. Therefore, the result of
3910 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3912 is the string "\0a\0\0bc".
3916 C<x> and C<X> accept C<!> modifier. In this case they act as
3917 alignment commands: they jump forward/back to the closest position
3918 aligned at a multiple of C<count> characters. For example, to pack() or
3919 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3920 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3921 aligned on the double's size.
3923 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3924 both result in no-ops.
3928 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3929 will represent signed 16-/32-bit integers in big-/little-endian order.
3930 This is only portable if all platforms sharing the packed data use the
3931 same binary representation for signed integers (e.g. all platforms are
3932 using two's complement representation).
3936 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3937 White space may be used to separate pack codes from each other, but
3938 modifiers and a repeat count must follow immediately.
3942 If TEMPLATE requires more arguments to pack() than actually given, pack()
3943 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3944 to pack() than actually given, extra arguments are ignored.
3950 $foo = pack("WWWW",65,66,67,68);
3952 $foo = pack("W4",65,66,67,68);
3954 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3955 # same thing with Unicode circled letters.
3956 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3957 # same thing with Unicode circled letters. You don't get the UTF-8
3958 # bytes because the U at the start of the format caused a switch to
3959 # U0-mode, so the UTF-8 bytes get joined into characters
3960 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3961 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3962 # This is the UTF-8 encoding of the string in the previous example
3964 $foo = pack("ccxxcc",65,66,67,68);
3967 # note: the above examples featuring "W" and "c" are true
3968 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3969 # and UTF-8. In EBCDIC the first example would be
3970 # $foo = pack("WWWW",193,194,195,196);
3972 $foo = pack("s2",1,2);
3973 # "\1\0\2\0" on little-endian
3974 # "\0\1\0\2" on big-endian
3976 $foo = pack("a4","abcd","x","y","z");
3979 $foo = pack("aaaa","abcd","x","y","z");
3982 $foo = pack("a14","abcdefg");
3983 # "abcdefg\0\0\0\0\0\0\0"
3985 $foo = pack("i9pl", gmtime);
3986 # a real struct tm (on my system anyway)
3988 $utmp_template = "Z8 Z8 Z16 L";
3989 $utmp = pack($utmp_template, @utmp1);
3990 # a struct utmp (BSDish)
3992 @utmp2 = unpack($utmp_template, $utmp);
3993 # "@utmp1" eq "@utmp2"
3996 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3999 $foo = pack('sx2l', 12, 34);
4000 # short 12, two zero bytes padding, long 34
4001 $bar = pack('s@4l', 12, 34);
4002 # short 12, zero fill to position 4, long 34
4004 $baz = pack('s.l', 12, 4, 34);
4005 # short 12, zero fill to position 4, long 34
4007 $foo = pack('nN', 42, 4711);
4008 # pack big-endian 16- and 32-bit unsigned integers
4009 $foo = pack('S>L>', 42, 4711);
4011 $foo = pack('s<l<', -42, 4711);
4012 # pack little-endian 16- and 32-bit signed integers
4013 $foo = pack('(sl)<', -42, 4711);
4016 The same template may generally also be used in unpack().
4018 =item package NAMESPACE
4019 X<package> X<module> X<namespace>
4023 Declares the compilation unit as being in the given namespace. The scope
4024 of the package declaration is from the declaration itself through the end
4025 of the enclosing block, file, or eval (the same as the C<my> operator).
4026 All further unqualified dynamic identifiers will be in this namespace.
4027 A package statement affects only dynamic variables--including those
4028 you've used C<local> on--but I<not> lexical variables, which are created
4029 with C<my>. Typically it would be the first declaration in a file to
4030 be included by the C<require> or C<use> operator. You can switch into a
4031 package in more than one place; it merely influences which symbol table
4032 is used by the compiler for the rest of that block. You can refer to
4033 variables and filehandles in other packages by prefixing the identifier
4034 with the package name and a double colon: C<$Package::Variable>.
4035 If the package name is null, the C<main> package as assumed. That is,
4036 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4037 still seen in older code).
4039 If NAMESPACE is omitted, then there is no current package, and all
4040 identifiers must be fully qualified or lexicals. However, you are
4041 strongly advised not to make use of this feature. Its use can cause
4042 unexpected behaviour, even crashing some versions of Perl. It is
4043 deprecated, and will be removed from a future release.
4045 See L<perlmod/"Packages"> for more information about packages, modules,
4046 and classes. See L<perlsub> for other scoping issues.
4048 =item pipe READHANDLE,WRITEHANDLE
4051 Opens a pair of connected pipes like the corresponding system call.
4052 Note that if you set up a loop of piped processes, deadlock can occur
4053 unless you are very careful. In addition, note that Perl's pipes use
4054 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4055 after each command, depending on the application.
4057 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4058 for examples of such things.
4060 On systems that support a close-on-exec flag on files, the flag will be set
4061 for the newly opened file descriptors as determined by the value of $^F.
4069 Pops and returns the last value of the array, shortening the array by
4070 one element. Has an effect similar to
4074 If there are no elements in the array, returns the undefined value
4075 (although this may happen at other times as well). If ARRAY is
4076 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4077 array in subroutines, just like C<shift>.
4080 X<pos> X<match, position>
4084 Returns the offset of where the last C<m//g> search left off for the variable
4085 in question (C<$_> is used when the variable is not specified). Note that
4086 0 is a valid match offset. C<undef> indicates that the search position
4087 is reset (usually due to match failure, but can also be because no match has
4088 yet been performed on the scalar). C<pos> directly accesses the location used
4089 by the regexp engine to store the offset, so assigning to C<pos> will change
4090 that offset, and so will also influence the C<\G> zero-width assertion in
4091 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4092 the return from C<pos> won't change either in this case. See L<perlre> and
4095 =item print FILEHANDLE LIST
4102 Prints a string or a list of strings. Returns true if successful.
4103 FILEHANDLE may be a scalar variable name, in which case the variable
4104 contains the name of or a reference to the filehandle, thus introducing
4105 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4106 the next token is a term, it may be misinterpreted as an operator
4107 unless you interpose a C<+> or put parentheses around the arguments.)
4108 If FILEHANDLE is omitted, prints by default to standard output (or
4109 to the last selected output channel--see L</select>). If LIST is
4110 also omitted, prints C<$_> to the currently selected output channel.
4111 To set the default output channel to something other than STDOUT
4112 use the select operation. The current value of C<$,> (if any) is
4113 printed between each LIST item. The current value of C<$\> (if
4114 any) is printed after the entire LIST has been printed. Because
4115 print takes a LIST, anything in the LIST is evaluated in list
4116 context, and any subroutine that you call will have one or more of
4117 its expressions evaluated in list context. Also be careful not to
4118 follow the print keyword with a left parenthesis unless you want
4119 the corresponding right parenthesis to terminate the arguments to
4120 the print--interpose a C<+> or put parentheses around all the
4123 Note that if you're storing FILEHANDLEs in an array, or if you're using
4124 any other expression more complex than a scalar variable to retrieve it,
4125 you will have to use a block returning the filehandle value instead:
4127 print { $files[$i] } "stuff\n";
4128 print { $OK ? STDOUT : STDERR } "stuff\n";
4130 =item printf FILEHANDLE FORMAT, LIST
4133 =item printf FORMAT, LIST
4135 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4136 (the output record separator) is not appended. The first argument
4137 of the list will be interpreted as the C<printf> format. See C<sprintf>
4138 for an explanation of the format argument. If C<use locale> is in effect,
4139 and POSIX::setlocale() has been called, the character used for the decimal
4140 separator in formatted floating point numbers is affected by the LC_NUMERIC
4141 locale. See L<perllocale> and L<POSIX>.
4143 Don't fall into the trap of using a C<printf> when a simple
4144 C<print> would do. The C<print> is more efficient and less
4147 =item prototype FUNCTION
4150 Returns the prototype of a function as a string (or C<undef> if the
4151 function has no prototype). FUNCTION is a reference to, or the name of,
4152 the function whose prototype you want to retrieve.
4154 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4155 name for Perl builtin. If the builtin is not I<overridable> (such as
4156 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4157 C<system>) returns C<undef> because the builtin does not really behave
4158 like a Perl function. Otherwise, the string describing the equivalent
4159 prototype is returned.
4161 =item push ARRAY,LIST
4164 Treats ARRAY as a stack, and pushes the values of LIST
4165 onto the end of ARRAY. The length of ARRAY increases by the length of
4166 LIST. Has the same effect as
4169 $ARRAY[++$#ARRAY] = $value;
4172 but is more efficient. Returns the number of elements in the array following
4173 the completed C<push>.
4185 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4187 =item quotemeta EXPR
4188 X<quotemeta> X<metacharacter>
4192 Returns the value of EXPR with all non-"word"
4193 characters backslashed. (That is, all characters not matching
4194 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4195 returned string, regardless of any locale settings.)
4196 This is the internal function implementing
4197 the C<\Q> escape in double-quoted strings.
4199 If EXPR is omitted, uses C<$_>.
4206 Returns a random fractional number greater than or equal to C<0> and less
4207 than the value of EXPR. (EXPR should be positive.) If EXPR is
4208 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4209 also special-cased as C<1> - this has not been documented before perl 5.8.0
4210 and is subject to change in future versions of perl. Automatically calls
4211 C<srand> unless C<srand> has already been called. See also C<srand>.
4213 Apply C<int()> to the value returned by C<rand()> if you want random
4214 integers instead of random fractional numbers. For example,
4218 returns a random integer between C<0> and C<9>, inclusive.
4220 (Note: If your rand function consistently returns numbers that are too
4221 large or too small, then your version of Perl was probably compiled
4222 with the wrong number of RANDBITS.)
4224 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4225 X<read> X<file, read>
4227 =item read FILEHANDLE,SCALAR,LENGTH
4229 Attempts to read LENGTH I<characters> of data into variable SCALAR
4230 from the specified FILEHANDLE. Returns the number of characters
4231 actually read, C<0> at end of file, or undef if there was an error (in
4232 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4233 so that the last character actually read is the last character of the
4234 scalar after the read.
4236 An OFFSET may be specified to place the read data at some place in the
4237 string other than the beginning. A negative OFFSET specifies
4238 placement at that many characters counting backwards from the end of
4239 the string. A positive OFFSET greater than the length of SCALAR
4240 results in the string being padded to the required size with C<"\0">
4241 bytes before the result of the read is appended.
4243 The call is actually implemented in terms of either Perl's or system's
4244 fread() call. To get a true read(2) system call, see C<sysread>.
4246 Note the I<characters>: depending on the status of the filehandle,
4247 either (8-bit) bytes or characters are read. By default all
4248 filehandles operate on bytes, but for example if the filehandle has
4249 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4250 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4251 characters, not bytes. Similarly for the C<:encoding> pragma:
4252 in that case pretty much any characters can be read.
4254 =item readdir DIRHANDLE
4257 Returns the next directory entry for a directory opened by C<opendir>.
4258 If used in list context, returns all the rest of the entries in the
4259 directory. If there are no more entries, returns an undefined value in
4260 scalar context or a null list in list context.
4262 If you're planning to filetest the return values out of a C<readdir>, you'd
4263 better prepend the directory in question. Otherwise, because we didn't
4264 C<chdir> there, it would have been testing the wrong file.
4266 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4267 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4271 X<readline> X<gets> X<fgets>
4273 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4274 context, each call reads and returns the next line, until end-of-file is
4275 reached, whereupon the subsequent call returns undef. In list context,
4276 reads until end-of-file is reached and returns a list of lines. Note that
4277 the notion of "line" used here is however you may have defined it
4278 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4280 When C<$/> is set to C<undef>, when readline() is in scalar
4281 context (i.e. file slurp mode), and when an empty file is read, it
4282 returns C<''> the first time, followed by C<undef> subsequently.
4284 This is the internal function implementing the C<< <EXPR> >>
4285 operator, but you can use it directly. The C<< <EXPR> >>
4286 operator is discussed in more detail in L<perlop/"I/O Operators">.
4289 $line = readline(*STDIN); # same thing
4291 If readline encounters an operating system error, C<$!> will be set with the
4292 corresponding error message. It can be helpful to check C<$!> when you are
4293 reading from filehandles you don't trust, such as a tty or a socket. The
4294 following example uses the operator form of C<readline>, and takes the necessary
4295 steps to ensure that C<readline> was successful.
4299 unless (defined( $line = <> )) {
4311 Returns the value of a symbolic link, if symbolic links are
4312 implemented. If not, gives a fatal error. If there is some system
4313 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4314 omitted, uses C<$_>.
4319 EXPR is executed as a system command.
4320 The collected standard output of the command is returned.
4321 In scalar context, it comes back as a single (potentially
4322 multi-line) string. In list context, returns a list of lines
4323 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4324 This is the internal function implementing the C<qx/EXPR/>
4325 operator, but you can use it directly. The C<qx/EXPR/>
4326 operator is discussed in more detail in L<perlop/"I/O Operators">.
4328 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4331 Receives a message on a socket. Attempts to receive LENGTH characters
4332 of data into variable SCALAR from the specified SOCKET filehandle.
4333 SCALAR will be grown or shrunk to the length actually read. Takes the
4334 same flags as the system call of the same name. Returns the address
4335 of the sender if SOCKET's protocol supports this; returns an empty
4336 string otherwise. If there's an error, returns the undefined value.
4337 This call is actually implemented in terms of recvfrom(2) system call.
4338 See L<perlipc/"UDP: Message Passing"> for examples.
4340 Note the I<characters>: depending on the status of the socket, either
4341 (8-bit) bytes or characters are received. By default all sockets
4342 operate on bytes, but for example if the socket has been changed using
4343 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4344 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4345 characters, not bytes. Similarly for the C<:encoding> pragma:
4346 in that case pretty much any characters can be read.
4353 The C<redo> command restarts the loop block without evaluating the
4354 conditional again. The C<continue> block, if any, is not executed. If
4355 the LABEL is omitted, the command refers to the innermost enclosing
4356 loop. Programs that want to lie to themselves about what was just input
4357 normally use this command:
4359 # a simpleminded Pascal comment stripper
4360 # (warning: assumes no { or } in strings)
4361 LINE: while (<STDIN>) {
4362 while (s|({.*}.*){.*}|$1 |) {}
4367 if (/}/) { # end of comment?
4376 C<redo> cannot be used to retry a block which returns a value such as
4377 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4378 a grep() or map() operation.
4380 Note that a block by itself is semantically identical to a loop
4381 that executes once. Thus C<redo> inside such a block will effectively
4382 turn it into a looping construct.
4384 See also L</continue> for an illustration of how C<last>, C<next>, and
4392 Returns a non-empty string if EXPR is a reference, the empty
4393 string otherwise. If EXPR
4394 is not specified, C<$_> will be used. The value returned depends on the
4395 type of thing the reference is a reference to.
4396 Builtin types include:
4410 If the referenced object has been blessed into a package, then that package
4411 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4413 if (ref($r) eq "HASH") {
4414 print "r is a reference to a hash.\n";
4417 print "r is not a reference at all.\n";
4420 The return value C<LVALUE> indicates a reference to an lvalue that is not
4421 a variable. You get this from taking the reference of function calls like
4422 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4423 to a L<version string|perldata\"Version Strings">.
4425 The result C<Regexp> indicates that the argument is a regular expression
4426 resulting from C<qr//>.
4428 See also L<perlref>.
4430 =item rename OLDNAME,NEWNAME
4431 X<rename> X<move> X<mv> X<ren>
4433 Changes the name of a file; an existing file NEWNAME will be
4434 clobbered. Returns true for success, false otherwise.
4436 Behavior of this function varies wildly depending on your system
4437 implementation. For example, it will usually not work across file system
4438 boundaries, even though the system I<mv> command sometimes compensates
4439 for this. Other restrictions include whether it works on directories,
4440 open files, or pre-existing files. Check L<perlport> and either the
4441 rename(2) manpage or equivalent system documentation for details.
4443 For a platform independent C<move> function look at the L<File::Copy>
4446 =item require VERSION
4453 Demands a version of Perl specified by VERSION, or demands some semantics
4454 specified by EXPR or by C<$_> if EXPR is not supplied.
4456 VERSION may be either a numeric argument such as 5.006, which will be
4457 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4458 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4459 VERSION is greater than the version of the current Perl interpreter.
4460 Compare with L</use>, which can do a similar check at compile time.
4462 Specifying VERSION as a literal of the form v5.6.1 should generally be
4463 avoided, because it leads to misleading error messages under earlier
4464 versions of Perl that do not support this syntax. The equivalent numeric
4465 version should be used instead.
4467 require v5.6.1; # run time version check
4468 require 5.6.1; # ditto
4469 require 5.006_001; # ditto; preferred for backwards compatibility
4471 Otherwise, C<require> demands that a library file be included if it
4472 hasn't already been included. The file is included via the do-FILE
4473 mechanism, which is essentially just a variety of C<eval>. Has
4474 semantics similar to the following subroutine:
4477 my ($filename) = @_;
4478 if (exists $INC{$filename}) {
4479 return 1 if $INC{$filename};
4480 die "Compilation failed in require";
4482 my ($realfilename,$result);
4484 foreach $prefix (@INC) {
4485 $realfilename = "$prefix/$filename";
4486 if (-f $realfilename) {
4487 $INC{$filename} = $realfilename;
4488 $result = do $realfilename;
4492 die "Can't find $filename in \@INC";
4495 $INC{$filename} = undef;
4497 } elsif (!$result) {
4498 delete $INC{$filename};
4499 die "$filename did not return true value";
4505 Note that the file will not be included twice under the same specified
4508 The file must return true as the last statement to indicate
4509 successful execution of any initialization code, so it's customary to
4510 end such a file with C<1;> unless you're sure it'll return true
4511 otherwise. But it's better just to put the C<1;>, in case you add more
4514 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4515 replaces "F<::>" with "F</>" in the filename for you,
4516 to make it easy to load standard modules. This form of loading of
4517 modules does not risk altering your namespace.
4519 In other words, if you try this:
4521 require Foo::Bar; # a splendid bareword
4523 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4524 directories specified in the C<@INC> array.
4526 But if you try this:
4528 $class = 'Foo::Bar';
4529 require $class; # $class is not a bareword
4531 require "Foo::Bar"; # not a bareword because of the ""
4533 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4534 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4536 eval "require $class";
4538 Now that you understand how C<require> looks for files in the case of a
4539 bareword argument, there is a little extra functionality going on behind
4540 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4541 first look for a similar filename with a "F<.pmc>" extension. If this file
4542 is found, it will be loaded in place of any file ending in a "F<.pm>"
4545 You can also insert hooks into the import facility, by putting directly
4546 Perl code into the @INC array. There are three forms of hooks: subroutine
4547 references, array references and blessed objects.
4549 Subroutine references are the simplest case. When the inclusion system
4550 walks through @INC and encounters a subroutine, this subroutine gets
4551 called with two parameters, the first being a reference to itself, and the
4552 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4553 subroutine should return nothing, or a list of up to three values in the
4560 A filehandle, from which the file will be read.
4564 A reference to a subroutine. If there is no filehandle (previous item),
4565 then this subroutine is expected to generate one line of source code per
4566 call, writing the line into C<$_> and returning 1, then returning 0 at
4567 "end of file". If there is a filehandle, then the subroutine will be
4568 called to act a simple source filter, with the line as read in C<$_>.
4569 Again, return 1 for each valid line, and 0 after all lines have been
4574 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4575 reference to the subroutine itself is passed in as C<$_[0]>.
4579 If an empty list, C<undef>, or nothing that matches the first 3 values above
4580 is returned then C<require> will look at the remaining elements of @INC.
4581 Note that this file handle must be a real file handle (strictly a typeglob,
4582 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4583 ignored and return value processing will stop there.
4585 If the hook is an array reference, its first element must be a subroutine
4586 reference. This subroutine is called as above, but the first parameter is
4587 the array reference. This enables to pass indirectly some arguments to
4590 In other words, you can write:
4592 push @INC, \&my_sub;
4594 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4600 push @INC, [ \&my_sub, $x, $y, ... ];
4602 my ($arrayref, $filename) = @_;
4603 # Retrieve $x, $y, ...
4604 my @parameters = @$arrayref[1..$#$arrayref];
4608 If the hook is an object, it must provide an INC method that will be
4609 called as above, the first parameter being the object itself. (Note that
4610 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4611 into package C<main>.) Here is a typical code layout:
4617 my ($self, $filename) = @_;
4621 # In the main program
4622 push @INC, new Foo(...);
4624 Note that these hooks are also permitted to set the %INC entry
4625 corresponding to the files they have loaded. See L<perlvar/%INC>.
4627 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4634 Generally used in a C<continue> block at the end of a loop to clear
4635 variables and reset C<??> searches so that they work again. The
4636 expression is interpreted as a list of single characters (hyphens
4637 allowed for ranges). All variables and arrays beginning with one of
4638 those letters are reset to their pristine state. If the expression is
4639 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4640 only variables or searches in the current package. Always returns
4643 reset 'X'; # reset all X variables
4644 reset 'a-z'; # reset lower case variables
4645 reset; # just reset ?one-time? searches
4647 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4648 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4649 variables--lexical variables are unaffected, but they clean themselves
4650 up on scope exit anyway, so you'll probably want to use them instead.
4658 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4659 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4660 context, depending on how the return value will be used, and the context
4661 may vary from one execution to the next (see C<wantarray>). If no EXPR
4662 is given, returns an empty list in list context, the undefined value in
4663 scalar context, and (of course) nothing at all in a void context.
4665 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4666 or do FILE will automatically return the value of the last expression
4670 X<reverse> X<rev> X<invert>
4672 In list context, returns a list value consisting of the elements
4673 of LIST in the opposite order. In scalar context, concatenates the
4674 elements of LIST and returns a string value with all characters
4675 in the opposite order.
4677 print reverse <>; # line tac, last line first
4679 undef $/; # for efficiency of <>
4680 print scalar reverse <>; # character tac, last line tsrif
4682 Used without arguments in scalar context, reverse() reverses C<$_>.
4684 This operator is also handy for inverting a hash, although there are some
4685 caveats. If a value is duplicated in the original hash, only one of those
4686 can be represented as a key in the inverted hash. Also, this has to
4687 unwind one hash and build a whole new one, which may take some time
4688 on a large hash, such as from a DBM file.
4690 %by_name = reverse %by_address; # Invert the hash
4692 =item rewinddir DIRHANDLE
4695 Sets the current position to the beginning of the directory for the
4696 C<readdir> routine on DIRHANDLE.
4698 =item rindex STR,SUBSTR,POSITION
4701 =item rindex STR,SUBSTR
4703 Works just like index() except that it returns the position of the I<last>
4704 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4705 last occurrence beginning at or before that position.
4707 =item rmdir FILENAME
4708 X<rmdir> X<rd> X<directory, remove>
4712 Deletes the directory specified by FILENAME if that directory is
4713 empty. If it succeeds it returns true, otherwise it returns false and
4714 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4716 To remove a directory tree recursively (C<rm -rf> on unix) look at
4717 the C<rmtree> function of the L<File::Path> module.
4721 The substitution operator. See L<perlop>.
4723 =item say FILEHANDLE LIST
4730 Just like C<print>, but implicitly appends a newline.
4731 C<say LIST> is simply an abbreviation for C<{ local $/ = "\n"; print
4734 This keyword is only available when the "say" feature is
4735 enabled: see L<feature>.
4738 X<scalar> X<context>
4740 Forces EXPR to be interpreted in scalar context and returns the value
4743 @counts = ( scalar @a, scalar @b, scalar @c );
4745 There is no equivalent operator to force an expression to
4746 be interpolated in list context because in practice, this is never
4747 needed. If you really wanted to do so, however, you could use
4748 the construction C<@{[ (some expression) ]}>, but usually a simple
4749 C<(some expression)> suffices.
4751 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4752 parenthesized list, this behaves as a scalar comma expression, evaluating
4753 all but the last element in void context and returning the final element
4754 evaluated in scalar context. This is seldom what you want.
4756 The following single statement:
4758 print uc(scalar(&foo,$bar)),$baz;
4760 is the moral equivalent of these two:
4763 print(uc($bar),$baz);
4765 See L<perlop> for more details on unary operators and the comma operator.
4767 =item seek FILEHANDLE,POSITION,WHENCE
4768 X<seek> X<fseek> X<filehandle, position>
4770 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4771 FILEHANDLE may be an expression whose value gives the name of the
4772 filehandle. The values for WHENCE are C<0> to set the new position
4773 I<in bytes> to POSITION, C<1> to set it to the current position plus
4774 POSITION, and C<2> to set it to EOF plus POSITION (typically
4775 negative). For WHENCE you may use the constants C<SEEK_SET>,
4776 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4777 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4780 Note the I<in bytes>: even if the filehandle has been set to
4781 operate on characters (for example by using the C<:utf8> open
4782 layer), tell() will return byte offsets, not character offsets
4783 (because implementing that would render seek() and tell() rather slow).
4785 If you want to position file for C<sysread> or C<syswrite>, don't use
4786 C<seek>--buffering makes its effect on the file's system position
4787 unpredictable and non-portable. Use C<sysseek> instead.
4789 Due to the rules and rigors of ANSI C, on some systems you have to do a
4790 seek whenever you switch between reading and writing. Amongst other
4791 things, this may have the effect of calling stdio's clearerr(3).
4792 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4796 This is also useful for applications emulating C<tail -f>. Once you hit
4797 EOF on your read, and then sleep for a while, you might have to stick in a
4798 seek() to reset things. The C<seek> doesn't change the current position,
4799 but it I<does> clear the end-of-file condition on the handle, so that the
4800 next C<< <FILE> >> makes Perl try again to read something. We hope.
4802 If that doesn't work (some IO implementations are particularly
4803 cantankerous), then you may need something more like this:
4806 for ($curpos = tell(FILE); $_ = <FILE>;
4807 $curpos = tell(FILE)) {
4808 # search for some stuff and put it into files
4810 sleep($for_a_while);
4811 seek(FILE, $curpos, 0);
4814 =item seekdir DIRHANDLE,POS
4817 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4818 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4819 about possible directory compaction as the corresponding system library
4822 =item select FILEHANDLE
4823 X<select> X<filehandle, default>
4827 Returns the currently selected filehandle. Sets the current default
4828 filehandle for output, if FILEHANDLE is supplied. This has two
4829 effects: first, a C<write> or a C<print> without a filehandle will
4830 default to this FILEHANDLE. Second, references to variables related to
4831 output will refer to this output channel. For example, if you have to
4832 set the top of form format for more than one output channel, you might
4840 FILEHANDLE may be an expression whose value gives the name of the
4841 actual filehandle. Thus:
4843 $oldfh = select(STDERR); $| = 1; select($oldfh);
4845 Some programmers may prefer to think of filehandles as objects with
4846 methods, preferring to write the last example as:
4849 STDERR->autoflush(1);
4851 =item select RBITS,WBITS,EBITS,TIMEOUT
4854 This calls the select(2) system call with the bit masks specified, which
4855 can be constructed using C<fileno> and C<vec>, along these lines:
4857 $rin = $win = $ein = '';
4858 vec($rin,fileno(STDIN),1) = 1;
4859 vec($win,fileno(STDOUT),1) = 1;
4862 If you want to select on many filehandles you might wish to write a
4866 my(@fhlist) = split(' ',$_[0]);
4869 vec($bits,fileno($_),1) = 1;
4873 $rin = fhbits('STDIN TTY SOCK');
4877 ($nfound,$timeleft) =
4878 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4880 or to block until something becomes ready just do this
4882 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4884 Most systems do not bother to return anything useful in $timeleft, so
4885 calling select() in scalar context just returns $nfound.
4887 Any of the bit masks can also be undef. The timeout, if specified, is
4888 in seconds, which may be fractional. Note: not all implementations are
4889 capable of returning the $timeleft. If not, they always return
4890 $timeleft equal to the supplied $timeout.
4892 You can effect a sleep of 250 milliseconds this way:
4894 select(undef, undef, undef, 0.25);
4896 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4897 is implementation-dependent. See also L<perlport> for notes on the
4898 portability of C<select>.
4900 On error, C<select> behaves like the select(2) system call : it returns
4903 Note: on some Unixes, the select(2) system call may report a socket file
4904 descriptor as "ready for reading", when actually no data is available,
4905 thus a subsequent read blocks. It can be avoided using always the
4906 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4909 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4910 or <FH>) with C<select>, except as permitted by POSIX, and even
4911 then only on POSIX systems. You have to use C<sysread> instead.
4913 =item semctl ID,SEMNUM,CMD,ARG
4916 Calls the System V IPC function C<semctl>. You'll probably have to say
4920 first to get the correct constant definitions. If CMD is IPC_STAT or
4921 GETALL, then ARG must be a variable that will hold the returned
4922 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4923 the undefined value for error, "C<0 but true>" for zero, or the actual
4924 return value otherwise. The ARG must consist of a vector of native
4925 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4926 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4929 =item semget KEY,NSEMS,FLAGS
4932 Calls the System V IPC function semget. Returns the semaphore id, or
4933 the undefined value if there is an error. See also
4934 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4937 =item semop KEY,OPSTRING
4940 Calls the System V IPC function semop to perform semaphore operations
4941 such as signalling and waiting. OPSTRING must be a packed array of
4942 semop structures. Each semop structure can be generated with
4943 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4944 implies the number of semaphore operations. Returns true if
4945 successful, or false if there is an error. As an example, the
4946 following code waits on semaphore $semnum of semaphore id $semid:
4948 $semop = pack("s!3", $semnum, -1, 0);
4949 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4951 To signal the semaphore, replace C<-1> with C<1>. See also
4952 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4955 =item send SOCKET,MSG,FLAGS,TO
4958 =item send SOCKET,MSG,FLAGS
4960 Sends a message on a socket. Attempts to send the scalar MSG to the
4961 SOCKET filehandle. Takes the same flags as the system call of the
4962 same name. On unconnected sockets you must specify a destination to
4963 send TO, in which case it does a C C<sendto>. Returns the number of
4964 characters sent, or the undefined value if there is an error. The C
4965 system call sendmsg(2) is currently unimplemented. See
4966 L<perlipc/"UDP: Message Passing"> for examples.
4968 Note the I<characters>: depending on the status of the socket, either
4969 (8-bit) bytes or characters are sent. By default all sockets operate
4970 on bytes, but for example if the socket has been changed using
4971 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4972 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4973 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4974 in that case pretty much any characters can be sent.
4976 =item setpgrp PID,PGRP
4979 Sets the current process group for the specified PID, C<0> for the current
4980 process. Will produce a fatal error if used on a machine that doesn't
4981 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4982 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4983 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4986 =item setpriority WHICH,WHO,PRIORITY
4987 X<setpriority> X<priority> X<nice> X<renice>
4989 Sets the current priority for a process, a process group, or a user.
4990 (See setpriority(2).) Will produce a fatal error if used on a machine
4991 that doesn't implement setpriority(2).
4993 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4996 Sets the socket option requested. Returns undefined if there is an
4997 error. Use integer constants provided by the C<Socket> module for
4998 LEVEL and OPNAME. Values for LEVEL can also be obtained from
4999 getprotobyname. OPTVAL might either be a packed string or an integer.
5000 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5002 An example disabling the Nagle's algorithm for a socket:
5004 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5005 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5012 Shifts the first value of the array off and returns it, shortening the
5013 array by 1 and moving everything down. If there are no elements in the
5014 array, returns the undefined value. If ARRAY is omitted, shifts the
5015 C<@_> array within the lexical scope of subroutines and formats, and the
5016 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5017 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5018 C<UNITCHECK {}> and C<END {}> constructs.
5020 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5021 same thing to the left end of an array that C<pop> and C<push> do to the
5024 =item shmctl ID,CMD,ARG
5027 Calls the System V IPC function shmctl. You'll probably have to say
5031 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5032 then ARG must be a variable that will hold the returned C<shmid_ds>
5033 structure. Returns like ioctl: the undefined value for error, "C<0> but
5034 true" for zero, or the actual return value otherwise.
5035 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5037 =item shmget KEY,SIZE,FLAGS
5040 Calls the System V IPC function shmget. Returns the shared memory
5041 segment id, or the undefined value if there is an error.
5042 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5044 =item shmread ID,VAR,POS,SIZE
5048 =item shmwrite ID,STRING,POS,SIZE
5050 Reads or writes the System V shared memory segment ID starting at
5051 position POS for size SIZE by attaching to it, copying in/out, and
5052 detaching from it. When reading, VAR must be a variable that will
5053 hold the data read. When writing, if STRING is too long, only SIZE
5054 bytes are used; if STRING is too short, nulls are written to fill out
5055 SIZE bytes. Return true if successful, or false if there is an error.
5056 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5057 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5059 =item shutdown SOCKET,HOW
5062 Shuts down a socket connection in the manner indicated by HOW, which
5063 has the same interpretation as in the system call of the same name.
5065 shutdown(SOCKET, 0); # I/we have stopped reading data
5066 shutdown(SOCKET, 1); # I/we have stopped writing data
5067 shutdown(SOCKET, 2); # I/we have stopped using this socket
5069 This is useful with sockets when you want to tell the other
5070 side you're done writing but not done reading, or vice versa.
5071 It's also a more insistent form of close because it also
5072 disables the file descriptor in any forked copies in other
5076 X<sin> X<sine> X<asin> X<arcsine>
5080 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5081 returns sine of C<$_>.
5083 For the inverse sine operation, you may use the C<Math::Trig::asin>
5084 function, or use this relation:
5086 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5093 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5094 May be interrupted if the process receives a signal such as C<SIGALRM>.
5095 Returns the number of seconds actually slept. You probably cannot
5096 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5099 On some older systems, it may sleep up to a full second less than what
5100 you requested, depending on how it counts seconds. Most modern systems
5101 always sleep the full amount. They may appear to sleep longer than that,
5102 however, because your process might not be scheduled right away in a
5103 busy multitasking system.
5105 For delays of finer granularity than one second, you may use Perl's
5106 C<syscall> interface to access setitimer(2) if your system supports
5107 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5108 and starting from Perl 5.8 part of the standard distribution) may also
5111 See also the POSIX module's C<pause> function.
5113 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5116 Opens a socket of the specified kind and attaches it to filehandle
5117 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5118 the system call of the same name. You should C<use Socket> first
5119 to get the proper definitions imported. See the examples in
5120 L<perlipc/"Sockets: Client/Server Communication">.
5122 On systems that support a close-on-exec flag on files, the flag will
5123 be set for the newly opened file descriptor, as determined by the
5124 value of $^F. See L<perlvar/$^F>.
5126 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5129 Creates an unnamed pair of sockets in the specified domain, of the
5130 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5131 for the system call of the same name. If unimplemented, yields a fatal
5132 error. Returns true if successful.
5134 On systems that support a close-on-exec flag on files, the flag will
5135 be set for the newly opened file descriptors, as determined by the value
5136 of $^F. See L<perlvar/$^F>.
5138 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5139 to C<pipe(Rdr, Wtr)> is essentially:
5142 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5143 shutdown(Rdr, 1); # no more writing for reader
5144 shutdown(Wtr, 0); # no more reading for writer
5146 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5147 emulate socketpair using IP sockets to localhost if your system implements
5148 sockets but not socketpair.
5150 =item sort SUBNAME LIST
5151 X<sort> X<qsort> X<quicksort> X<mergesort>
5153 =item sort BLOCK LIST
5157 In list context, this sorts the LIST and returns the sorted list value.
5158 In scalar context, the behaviour of C<sort()> is undefined.
5160 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5161 order. If SUBNAME is specified, it gives the name of a subroutine
5162 that returns an integer less than, equal to, or greater than C<0>,
5163 depending on how the elements of the list are to be ordered. (The C<<
5164 <=> >> and C<cmp> operators are extremely useful in such routines.)
5165 SUBNAME may be a scalar variable name (unsubscripted), in which case
5166 the value provides the name of (or a reference to) the actual
5167 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5168 an anonymous, in-line sort subroutine.
5170 If the subroutine's prototype is C<($$)>, the elements to be compared
5171 are passed by reference in C<@_>, as for a normal subroutine. This is
5172 slower than unprototyped subroutines, where the elements to be
5173 compared are passed into the subroutine
5174 as the package global variables $a and $b (see example below). Note that
5175 in the latter case, it is usually counter-productive to declare $a and
5178 The values to be compared are always passed by reference and should not
5181 You also cannot exit out of the sort block or subroutine using any of the
5182 loop control operators described in L<perlsyn> or with C<goto>.
5184 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5185 current collation locale. See L<perllocale>.
5187 sort() returns aliases into the original list, much as a for loop's index
5188 variable aliases the list elements. That is, modifying an element of a
5189 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5190 actually modifies the element in the original list. This is usually
5191 something to be avoided when writing clear code.
5193 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5194 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5195 preserves the input order of elements that compare equal. Although
5196 quicksort's run time is O(NlogN) when averaged over all arrays of
5197 length N, the time can be O(N**2), I<quadratic> behavior, for some
5198 inputs.) In 5.7, the quicksort implementation was replaced with
5199 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5200 But benchmarks indicated that for some inputs, on some platforms,
5201 the original quicksort was faster. 5.8 has a sort pragma for
5202 limited control of the sort. Its rather blunt control of the
5203 underlying algorithm may not persist into future Perls, but the
5204 ability to characterize the input or output in implementation
5205 independent ways quite probably will. See L<sort>.
5210 @articles = sort @files;
5212 # same thing, but with explicit sort routine
5213 @articles = sort {$a cmp $b} @files;
5215 # now case-insensitively
5216 @articles = sort {uc($a) cmp uc($b)} @files;
5218 # same thing in reversed order
5219 @articles = sort {$b cmp $a} @files;
5221 # sort numerically ascending
5222 @articles = sort {$a <=> $b} @files;
5224 # sort numerically descending
5225 @articles = sort {$b <=> $a} @files;
5227 # this sorts the %age hash by value instead of key
5228 # using an in-line function
5229 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5231 # sort using explicit subroutine name
5233 $age{$a} <=> $age{$b}; # presuming numeric
5235 @sortedclass = sort byage @class;
5237 sub backwards { $b cmp $a }
5238 @harry = qw(dog cat x Cain Abel);
5239 @george = qw(gone chased yz Punished Axed);
5241 # prints AbelCaincatdogx
5242 print sort backwards @harry;
5243 # prints xdogcatCainAbel
5244 print sort @george, 'to', @harry;
5245 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5247 # inefficiently sort by descending numeric compare using
5248 # the first integer after the first = sign, or the
5249 # whole record case-insensitively otherwise
5252 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5257 # same thing, but much more efficiently;
5258 # we'll build auxiliary indices instead
5262 push @nums, /=(\d+)/;
5267 $nums[$b] <=> $nums[$a]
5269 $caps[$a] cmp $caps[$b]
5273 # same thing, but without any temps
5274 @new = map { $_->[0] }
5275 sort { $b->[1] <=> $a->[1]
5278 } map { [$_, /=(\d+)/, uc($_)] } @old;
5280 # using a prototype allows you to use any comparison subroutine
5281 # as a sort subroutine (including other package's subroutines)
5283 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5286 @new = sort other::backwards @old;
5288 # guarantee stability, regardless of algorithm
5290 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5292 # force use of mergesort (not portable outside Perl 5.8)
5293 use sort '_mergesort'; # note discouraging _
5294 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5296 If you're using strict, you I<must not> declare $a
5297 and $b as lexicals. They are package globals. That means
5298 if you're in the C<main> package and type
5300 @articles = sort {$b <=> $a} @files;
5302 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5303 but if you're in the C<FooPack> package, it's the same as typing
5305 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5307 The comparison function is required to behave. If it returns
5308 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5309 sometimes saying the opposite, for example) the results are not
5312 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5313 (not-a-number), and because C<sort> will trigger a fatal error unless the
5314 result of a comparison is defined, when sorting with a comparison function
5315 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5316 The following example takes advantage of the fact that C<NaN != NaN> to
5317 eliminate any C<NaN>s from the input.
5319 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5321 =item splice ARRAY,OFFSET,LENGTH,LIST
5324 =item splice ARRAY,OFFSET,LENGTH
5326 =item splice ARRAY,OFFSET
5330 Removes the elements designated by OFFSET and LENGTH from an array, and
5331 replaces them with the elements of LIST, if any. In list context,
5332 returns the elements removed from the array. In scalar context,
5333 returns the last element removed, or C<undef> if no elements are
5334 removed. The array grows or shrinks as necessary.
5335 If OFFSET is negative then it starts that far from the end of the array.
5336 If LENGTH is omitted, removes everything from OFFSET onward.
5337 If LENGTH is negative, removes the elements from OFFSET onward
5338 except for -LENGTH elements at the end of the array.
5339 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5340 past the end of the array, perl issues a warning, and splices at the
5343 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5345 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5346 pop(@a) splice(@a,-1)
5347 shift(@a) splice(@a,0,1)
5348 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5349 $a[$i] = $y splice(@a,$i,1,$y)
5351 Example, assuming array lengths are passed before arrays:
5353 sub aeq { # compare two list values
5354 my(@a) = splice(@_,0,shift);
5355 my(@b) = splice(@_,0,shift);
5356 return 0 unless @a == @b; # same len?
5358 return 0 if pop(@a) ne pop(@b);
5362 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5364 =item split /PATTERN/,EXPR,LIMIT
5367 =item split /PATTERN/,EXPR
5369 =item split /PATTERN/
5373 Splits the string EXPR into a list of strings and returns that list. By
5374 default, empty leading fields are preserved, and empty trailing ones are
5375 deleted. (If all fields are empty, they are considered to be trailing.)
5377 In scalar context, returns the number of fields found and splits into
5378 the C<@_> array. Use of split in scalar context is deprecated, however,
5379 because it clobbers your subroutine arguments.
5381 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5382 splits on whitespace (after skipping any leading whitespace). Anything
5383 matching PATTERN is taken to be a delimiter separating the fields. (Note
5384 that the delimiter may be longer than one character.)
5386 If LIMIT is specified and positive, it represents the maximum number
5387 of fields the EXPR will be split into, though the actual number of
5388 fields returned depends on the number of times PATTERN matches within
5389 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5390 stripped (which potential users of C<pop> would do well to remember).
5391 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5392 had been specified. Note that splitting an EXPR that evaluates to the
5393 empty string always returns the empty list, regardless of the LIMIT
5396 A pattern matching the null string (not to be confused with
5397 a null pattern C<//>, which is just one member of the set of patterns
5398 matching a null string) will split the value of EXPR into separate
5399 characters at each point it matches that way. For example:
5401 print join(':', split(/ */, 'hi there'));
5403 produces the output 'h:i:t:h:e:r:e'.
5405 As a special case for C<split>, using the empty pattern C<//> specifically
5406 matches only the null string, and is not be confused with the regular use
5407 of C<//> to mean "the last successful pattern match". So, for C<split>,
5410 print join(':', split(//, 'hi there'));
5412 produces the output 'h:i: :t:h:e:r:e'.
5414 Empty leading (or trailing) fields are produced when there are positive
5415 width matches at the beginning (or end) of the string; a zero-width match
5416 at the beginning (or end) of the string does not produce an empty field.
5419 print join(':', split(/(?=\w)/, 'hi there!'));
5421 produces the output 'h:i :t:h:e:r:e!'.
5423 The LIMIT parameter can be used to split a line partially
5425 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5427 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5428 a LIMIT one larger than the number of variables in the list, to avoid
5429 unnecessary work. For the list above LIMIT would have been 4 by
5430 default. In time critical applications it behooves you not to split
5431 into more fields than you really need.
5433 If the PATTERN contains parentheses, additional list elements are
5434 created from each matching substring in the delimiter.
5436 split(/([,-])/, "1-10,20", 3);
5438 produces the list value
5440 (1, '-', 10, ',', 20)
5442 If you had the entire header of a normal Unix email message in $header,
5443 you could split it up into fields and their values this way:
5445 $header =~ s/\n\s+/ /g; # fix continuation lines
5446 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5448 The pattern C</PATTERN/> may be replaced with an expression to specify
5449 patterns that vary at runtime. (To do runtime compilation only once,
5450 use C</$variable/o>.)
5452 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5453 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5454 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5455 will give you as many null initial fields as there are leading spaces.
5456 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5457 whitespace produces a null first field. A C<split> with no arguments
5458 really does a S<C<split(' ', $_)>> internally.
5460 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5465 open(PASSWD, '/etc/passwd');
5468 ($login, $passwd, $uid, $gid,
5469 $gcos, $home, $shell) = split(/:/);
5473 As with regular pattern matching, any capturing parentheses that are not
5474 matched in a C<split()> will be set to C<undef> when returned:
5476 @fields = split /(A)|B/, "1A2B3";
5477 # @fields is (1, 'A', 2, undef, 3)
5479 =item sprintf FORMAT, LIST
5482 Returns a string formatted by the usual C<printf> conventions of the C
5483 library function C<sprintf>. See below for more details
5484 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5485 the general principles.
5489 # Format number with up to 8 leading zeroes
5490 $result = sprintf("%08d", $number);
5492 # Round number to 3 digits after decimal point
5493 $rounded = sprintf("%.3f", $number);
5495 Perl does its own C<sprintf> formatting--it emulates the C
5496 function C<sprintf>, but it doesn't use it (except for floating-point
5497 numbers, and even then only the standard modifiers are allowed). As a
5498 result, any non-standard extensions in your local C<sprintf> are not
5499 available from Perl.
5501 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5502 pass it an array as your first argument. The array is given scalar context,
5503 and instead of using the 0th element of the array as the format, Perl will
5504 use the count of elements in the array as the format, which is almost never
5507 Perl's C<sprintf> permits the following universally-known conversions:
5510 %c a character with the given number
5512 %d a signed integer, in decimal
5513 %u an unsigned integer, in decimal
5514 %o an unsigned integer, in octal
5515 %x an unsigned integer, in hexadecimal
5516 %e a floating-point number, in scientific notation
5517 %f a floating-point number, in fixed decimal notation
5518 %g a floating-point number, in %e or %f notation
5520 In addition, Perl permits the following widely-supported conversions:
5522 %X like %x, but using upper-case letters
5523 %E like %e, but using an upper-case "E"
5524 %G like %g, but with an upper-case "E" (if applicable)
5525 %b an unsigned integer, in binary
5526 %B like %b, but using an upper-case "B" with the # flag
5527 %p a pointer (outputs the Perl value's address in hexadecimal)
5528 %n special: *stores* the number of characters output so far
5529 into the next variable in the parameter list
5531 Finally, for backward (and we do mean "backward") compatibility, Perl
5532 permits these unnecessary but widely-supported conversions:
5535 %D a synonym for %ld
5536 %U a synonym for %lu
5537 %O a synonym for %lo
5540 Note that the number of exponent digits in the scientific notation produced
5541 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5542 exponent less than 100 is system-dependent: it may be three or less
5543 (zero-padded as necessary). In other words, 1.23 times ten to the
5544 99th may be either "1.23e99" or "1.23e099".
5546 Between the C<%> and the format letter, you may specify a number of
5547 additional attributes controlling the interpretation of the format.
5548 In order, these are:
5552 =item format parameter index
5554 An explicit format parameter index, such as C<2$>. By default sprintf
5555 will format the next unused argument in the list, but this allows you
5556 to take the arguments out of order, e.g.:
5558 printf '%2$d %1$d', 12, 34; # prints "34 12"
5559 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5565 space prefix positive number with a space
5566 + prefix positive number with a plus sign
5567 - left-justify within the field
5568 0 use zeros, not spaces, to right-justify
5569 # ensure the leading "0" for any octal,
5570 prefix non-zero hexadecimal with "0x" or "0X",
5571 prefix non-zero binary with "0b" or "0B"
5575 printf '<% d>', 12; # prints "< 12>"
5576 printf '<%+d>', 12; # prints "<+12>"
5577 printf '<%6s>', 12; # prints "< 12>"
5578 printf '<%-6s>', 12; # prints "<12 >"
5579 printf '<%06s>', 12; # prints "<000012>"
5580 printf '<%#o>', 12; # prints "<014>"
5581 printf '<%#x>', 12; # prints "<0xc>"
5582 printf '<%#X>', 12; # prints "<0XC>"
5583 printf '<%#b>', 12; # prints "<0b1100>"
5584 printf '<%#B>', 12; # prints "<0B1100>"
5586 When a space and a plus sign are given as the flags at once,
5587 a plus sign is used to prefix a positive number.
5589 printf '<%+ d>', 12; # prints "<+12>"
5590 printf '<% +d>', 12; # prints "<+12>"
5592 When the # flag and a precision are given in the %o conversion,
5593 the precision is incremented if it's necessary for the leading "0".
5595 printf '<%#.5o>', 012; # prints "<00012>"
5596 printf '<%#.5o>', 012345; # prints "<012345>"
5597 printf '<%#.0o>', 0; # prints "<0>"
5601 This flag tells perl to interpret the supplied string as a vector of
5602 integers, one for each character in the string. Perl applies the format to
5603 each integer in turn, then joins the resulting strings with a separator (a
5604 dot C<.> by default). This can be useful for displaying ordinal values of
5605 characters in arbitrary strings:
5607 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5608 printf "version is v%vd\n", $^V; # Perl's version
5610 Put an asterisk C<*> before the C<v> to override the string to
5611 use to separate the numbers:
5613 printf "address is %*vX\n", ":", $addr; # IPv6 address
5614 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5616 You can also explicitly specify the argument number to use for
5617 the join string using e.g. C<*2$v>:
5619 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5621 =item (minimum) width
5623 Arguments are usually formatted to be only as wide as required to
5624 display the given value. You can override the width by putting
5625 a number here, or get the width from the next argument (with C<*>)
5626 or from a specified argument (with e.g. C<*2$>):
5628 printf '<%s>', "a"; # prints "<a>"
5629 printf '<%6s>', "a"; # prints "< a>"
5630 printf '<%*s>', 6, "a"; # prints "< a>"
5631 printf '<%*2$s>', "a", 6; # prints "< a>"
5632 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5634 If a field width obtained through C<*> is negative, it has the same
5635 effect as the C<-> flag: left-justification.
5637 =item precision, or maximum width
5640 You can specify a precision (for numeric conversions) or a maximum
5641 width (for string conversions) by specifying a C<.> followed by a number.
5642 For floating point formats, with the exception of 'g' and 'G', this specifies
5643 the number of decimal places to show (the default being 6), e.g.:
5645 # these examples are subject to system-specific variation
5646 printf '<%f>', 1; # prints "<1.000000>"
5647 printf '<%.1f>', 1; # prints "<1.0>"
5648 printf '<%.0f>', 1; # prints "<1>"
5649 printf '<%e>', 10; # prints "<1.000000e+01>"
5650 printf '<%.1e>', 10; # prints "<1.0e+01>"
5652 For 'g' and 'G', this specifies the maximum number of digits to show,
5653 including prior to the decimal point as well as after it, e.g.:
5655 # these examples are subject to system-specific variation
5656 printf '<%g>', 1; # prints "<1>"
5657 printf '<%.10g>', 1; # prints "<1>"
5658 printf '<%g>', 100; # prints "<100>"
5659 printf '<%.1g>', 100; # prints "<1e+02>"
5660 printf '<%.2g>', 100.01; # prints "<1e+02>"
5661 printf '<%.5g>', 100.01; # prints "<100.01>"
5662 printf '<%.4g>', 100.01; # prints "<100>"
5664 For integer conversions, specifying a precision implies that the
5665 output of the number itself should be zero-padded to this width,
5666 where the 0 flag is ignored:
5668 printf '<%.6d>', 1; # prints "<000001>"
5669 printf '<%+.6d>', 1; # prints "<+000001>"
5670 printf '<%-10.6d>', 1; # prints "<000001 >"
5671 printf '<%10.6d>', 1; # prints "< 000001>"
5672 printf '<%010.6d>', 1; # prints "< 000001>"
5673 printf '<%+10.6d>', 1; # prints "< +000001>"
5675 printf '<%.6x>', 1; # prints "<000001>"
5676 printf '<%#.6x>', 1; # prints "<0x000001>"
5677 printf '<%-10.6x>', 1; # prints "<000001 >"
5678 printf '<%10.6x>', 1; # prints "< 000001>"
5679 printf '<%010.6x>', 1; # prints "< 000001>"
5680 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5682 For string conversions, specifying a precision truncates the string
5683 to fit in the specified width:
5685 printf '<%.5s>', "truncated"; # prints "<trunc>"
5686 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5688 You can also get the precision from the next argument using C<.*>:
5690 printf '<%.6x>', 1; # prints "<000001>"
5691 printf '<%.*x>', 6, 1; # prints "<000001>"
5693 If a precision obtained through C<*> is negative, it has the same
5694 effect as no precision.
5696 printf '<%.*s>', 7, "string"; # prints "<string>"
5697 printf '<%.*s>', 3, "string"; # prints "<str>"
5698 printf '<%.*s>', 0, "string"; # prints "<>"
5699 printf '<%.*s>', -1, "string"; # prints "<string>"
5701 printf '<%.*d>', 1, 0; # prints "<0>"
5702 printf '<%.*d>', 0, 0; # prints "<>"
5703 printf '<%.*d>', -1, 0; # prints "<0>"
5705 You cannot currently get the precision from a specified number,
5706 but it is intended that this will be possible in the future using
5709 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5713 For numeric conversions, you can specify the size to interpret the
5714 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5715 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5716 whatever the default integer size is on your platform (usually 32 or 64
5717 bits), but you can override this to use instead one of the standard C types,
5718 as supported by the compiler used to build Perl:
5720 l interpret integer as C type "long" or "unsigned long"
5721 h interpret integer as C type "short" or "unsigned short"
5722 q, L or ll interpret integer as C type "long long", "unsigned long long".
5723 or "quads" (typically 64-bit integers)
5725 The last will produce errors if Perl does not understand "quads" in your
5726 installation. (This requires that either the platform natively supports quads
5727 or Perl was specifically compiled to support quads.) You can find out
5728 whether your Perl supports quads via L<Config>:
5731 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5734 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5735 to be the default floating point size on your platform (double or long double),
5736 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5737 platform supports them. You can find out whether your Perl supports long
5738 doubles via L<Config>:
5741 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5743 You can find out whether Perl considers 'long double' to be the default
5744 floating point size to use on your platform via L<Config>:
5747 ($Config{uselongdouble} eq 'define') &&
5748 print "long doubles by default\n";
5750 It can also be the case that long doubles and doubles are the same thing:
5753 ($Config{doublesize} == $Config{longdblsize}) &&
5754 print "doubles are long doubles\n";
5756 The size specifier C<V> has no effect for Perl code, but it is supported
5757 for compatibility with XS code; it means 'use the standard size for
5758 a Perl integer (or floating-point number)', which is already the
5759 default for Perl code.
5761 =item order of arguments
5763 Normally, sprintf takes the next unused argument as the value to
5764 format for each format specification. If the format specification
5765 uses C<*> to require additional arguments, these are consumed from
5766 the argument list in the order in which they appear in the format
5767 specification I<before> the value to format. Where an argument is
5768 specified using an explicit index, this does not affect the normal
5769 order for the arguments (even when the explicitly specified index
5770 would have been the next argument in any case).
5774 printf '<%*.*s>', $a, $b, $c;
5776 would use C<$a> for the width, C<$b> for the precision and C<$c>
5777 as the value to format, while:
5779 print '<%*1$.*s>', $a, $b;
5781 would use C<$a> for the width and the precision, and C<$b> as the
5784 Here are some more examples - beware that when using an explicit
5785 index, the C<$> may need to be escaped:
5787 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5788 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5789 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5790 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5794 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5795 the character used for the decimal separator in formatted floating
5796 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5800 X<sqrt> X<root> X<square root>
5804 Return the square root of EXPR. If EXPR is omitted, returns square
5805 root of C<$_>. Only works on non-negative operands, unless you've
5806 loaded the standard Math::Complex module.
5809 print sqrt(-2); # prints 1.4142135623731i
5812 X<srand> X<seed> X<randseed>
5816 Sets the random number seed for the C<rand> operator.
5818 The point of the function is to "seed" the C<rand> function so that
5819 C<rand> can produce a different sequence each time you run your
5822 If srand() is not called explicitly, it is called implicitly at the
5823 first use of the C<rand> operator. However, this was not the case in
5824 versions of Perl before 5.004, so if your script will run under older
5825 Perl versions, it should call C<srand>.
5827 Most programs won't even call srand() at all, except those that
5828 need a cryptographically-strong starting point rather than the
5829 generally acceptable default, which is based on time of day,
5830 process ID, and memory allocation, or the F</dev/urandom> device,
5833 You can call srand($seed) with the same $seed to reproduce the
5834 I<same> sequence from rand(), but this is usually reserved for
5835 generating predictable results for testing or debugging.
5836 Otherwise, don't call srand() more than once in your program.
5838 Do B<not> call srand() (i.e. without an argument) more than once in
5839 a script. The internal state of the random number generator should
5840 contain more entropy than can be provided by any seed, so calling
5841 srand() again actually I<loses> randomness.
5843 Most implementations of C<srand> take an integer and will silently
5844 truncate decimal numbers. This means C<srand(42)> will usually
5845 produce the same results as C<srand(42.1)>. To be safe, always pass
5846 C<srand> an integer.
5848 In versions of Perl prior to 5.004 the default seed was just the
5849 current C<time>. This isn't a particularly good seed, so many old
5850 programs supply their own seed value (often C<time ^ $$> or C<time ^
5851 ($$ + ($$ << 15))>), but that isn't necessary any more.
5853 For cryptographic purposes, however, you need something much more random
5854 than the default seed. Checksumming the compressed output of one or more
5855 rapidly changing operating system status programs is the usual method. For
5858 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5860 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5863 Frequently called programs (like CGI scripts) that simply use
5867 for a seed can fall prey to the mathematical property that
5871 one-third of the time. So don't do that.
5873 =item stat FILEHANDLE
5874 X<stat> X<file, status> X<ctime>
5878 =item stat DIRHANDLE
5882 Returns a 13-element list giving the status info for a file, either
5883 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5884 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5887 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5888 $atime,$mtime,$ctime,$blksize,$blocks)
5891 Not all fields are supported on all filesystem types. Here are the
5892 meanings of the fields:
5894 0 dev device number of filesystem
5896 2 mode file mode (type and permissions)
5897 3 nlink number of (hard) links to the file
5898 4 uid numeric user ID of file's owner
5899 5 gid numeric group ID of file's owner
5900 6 rdev the device identifier (special files only)
5901 7 size total size of file, in bytes
5902 8 atime last access time in seconds since the epoch
5903 9 mtime last modify time in seconds since the epoch
5904 10 ctime inode change time in seconds since the epoch (*)
5905 11 blksize preferred block size for file system I/O
5906 12 blocks actual number of blocks allocated
5908 (The epoch was at 00:00 January 1, 1970 GMT.)
5910 (*) Not all fields are supported on all filesystem types. Notably, the
5911 ctime field is non-portable. In particular, you cannot expect it to be a
5912 "creation time", see L<perlport/"Files and Filesystems"> for details.
5914 If C<stat> is passed the special filehandle consisting of an underline, no
5915 stat is done, but the current contents of the stat structure from the
5916 last C<stat>, C<lstat>, or filetest are returned. Example:
5918 if (-x $file && (($d) = stat(_)) && $d < 0) {
5919 print "$file is executable NFS file\n";
5922 (This works on machines only for which the device number is negative
5925 Because the mode contains both the file type and its permissions, you
5926 should mask off the file type portion and (s)printf using a C<"%o">
5927 if you want to see the real permissions.
5929 $mode = (stat($filename))[2];
5930 printf "Permissions are %04o\n", $mode & 07777;
5932 In scalar context, C<stat> returns a boolean value indicating success
5933 or failure, and, if successful, sets the information associated with
5934 the special filehandle C<_>.
5936 The L<File::stat> module provides a convenient, by-name access mechanism:
5939 $sb = stat($filename);
5940 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5941 $filename, $sb->size, $sb->mode & 07777,
5942 scalar localtime $sb->mtime;
5944 You can import symbolic mode constants (C<S_IF*>) and functions
5945 (C<S_IS*>) from the Fcntl module:
5949 $mode = (stat($filename))[2];
5951 $user_rwx = ($mode & S_IRWXU) >> 6;
5952 $group_read = ($mode & S_IRGRP) >> 3;
5953 $other_execute = $mode & S_IXOTH;
5955 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5957 $is_setuid = $mode & S_ISUID;
5958 $is_directory = S_ISDIR($mode);
5960 You could write the last two using the C<-u> and C<-d> operators.
5961 The commonly available C<S_IF*> constants are
5963 # Permissions: read, write, execute, for user, group, others.
5965 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5966 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5967 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5969 # Setuid/Setgid/Stickiness/SaveText.
5970 # Note that the exact meaning of these is system dependent.
5972 S_ISUID S_ISGID S_ISVTX S_ISTXT
5974 # File types. Not necessarily all are available on your system.
5976 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5978 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5980 S_IREAD S_IWRITE S_IEXEC
5982 and the C<S_IF*> functions are
5984 S_IMODE($mode) the part of $mode containing the permission bits
5985 and the setuid/setgid/sticky bits
5987 S_IFMT($mode) the part of $mode containing the file type
5988 which can be bit-anded with e.g. S_IFREG
5989 or with the following functions
5991 # The operators -f, -d, -l, -b, -c, -p, and -S.
5993 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5994 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5996 # No direct -X operator counterpart, but for the first one
5997 # the -g operator is often equivalent. The ENFMT stands for
5998 # record flocking enforcement, a platform-dependent feature.
6000 S_ISENFMT($mode) S_ISWHT($mode)
6002 See your native chmod(2) and stat(2) documentation for more details
6003 about the C<S_*> constants. To get status info for a symbolic link
6004 instead of the target file behind the link, use the C<lstat> function.
6009 =item state TYPE EXPR
6011 =item state EXPR : ATTRS
6013 =item state TYPE EXPR : ATTRS
6015 C<state> declares a lexically scoped variable, just like C<my> does.
6016 However, those variables will be initialized only once, contrary to
6017 lexical variables that are reinitialized each time their enclosing block
6020 C<state> variables are only enabled when the C<feature 'state'> pragma is
6021 in effect. See L<feature>.
6028 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6029 doing many pattern matches on the string before it is next modified.
6030 This may or may not save time, depending on the nature and number of
6031 patterns you are searching on, and on the distribution of character
6032 frequencies in the string to be searched--you probably want to compare
6033 run times with and without it to see which runs faster. Those loops
6034 that scan for many short constant strings (including the constant
6035 parts of more complex patterns) will benefit most. You may have only
6036 one C<study> active at a time--if you study a different scalar the first
6037 is "unstudied". (The way C<study> works is this: a linked list of every
6038 character in the string to be searched is made, so we know, for
6039 example, where all the C<'k'> characters are. From each search string,
6040 the rarest character is selected, based on some static frequency tables
6041 constructed from some C programs and English text. Only those places
6042 that contain this "rarest" character are examined.)
6044 For example, here is a loop that inserts index producing entries
6045 before any line containing a certain pattern:
6049 print ".IX foo\n" if /\bfoo\b/;
6050 print ".IX bar\n" if /\bbar\b/;
6051 print ".IX blurfl\n" if /\bblurfl\b/;
6056 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6057 will be looked at, because C<f> is rarer than C<o>. In general, this is
6058 a big win except in pathological cases. The only question is whether
6059 it saves you more time than it took to build the linked list in the
6062 Note that if you have to look for strings that you don't know till
6063 runtime, you can build an entire loop as a string and C<eval> that to
6064 avoid recompiling all your patterns all the time. Together with
6065 undefining C<$/> to input entire files as one record, this can be very
6066 fast, often faster than specialized programs like fgrep(1). The following
6067 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6068 out the names of those files that contain a match:
6070 $search = 'while (<>) { study;';
6071 foreach $word (@words) {
6072 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6077 eval $search; # this screams
6078 $/ = "\n"; # put back to normal input delimiter
6079 foreach $file (sort keys(%seen)) {
6083 =item sub NAME BLOCK
6086 =item sub NAME (PROTO) BLOCK
6088 =item sub NAME : ATTRS BLOCK
6090 =item sub NAME (PROTO) : ATTRS BLOCK
6092 This is subroutine definition, not a real function I<per se>.
6093 Without a BLOCK it's just a forward declaration. Without a NAME,
6094 it's an anonymous function declaration, and does actually return
6095 a value: the CODE ref of the closure you just created.
6097 See L<perlsub> and L<perlref> for details about subroutines and
6098 references, and L<attributes> and L<Attribute::Handlers> for more
6099 information about attributes.
6101 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6102 X<substr> X<substring> X<mid> X<left> X<right>
6104 =item substr EXPR,OFFSET,LENGTH
6106 =item substr EXPR,OFFSET
6108 Extracts a substring out of EXPR and returns it. First character is at
6109 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6110 If OFFSET is negative (or more precisely, less than C<$[>), starts
6111 that far from the end of the string. If LENGTH is omitted, returns
6112 everything to the end of the string. If LENGTH is negative, leaves that
6113 many characters off the end of the string.
6115 my $s = "The black cat climbed the green tree";
6116 my $color = substr $s, 4, 5; # black
6117 my $middle = substr $s, 4, -11; # black cat climbed the
6118 my $end = substr $s, 14; # climbed the green tree
6119 my $tail = substr $s, -4; # tree
6120 my $z = substr $s, -4, 2; # tr
6122 You can use the substr() function as an lvalue, in which case EXPR
6123 must itself be an lvalue. If you assign something shorter than LENGTH,
6124 the string will shrink, and if you assign something longer than LENGTH,
6125 the string will grow to accommodate it. To keep the string the same
6126 length you may need to pad or chop your value using C<sprintf>.
6128 If OFFSET and LENGTH specify a substring that is partly outside the
6129 string, only the part within the string is returned. If the substring
6130 is beyond either end of the string, substr() returns the undefined
6131 value and produces a warning. When used as an lvalue, specifying a
6132 substring that is entirely outside the string is a fatal error.
6133 Here's an example showing the behavior for boundary cases:
6136 substr($name, 4) = 'dy'; # $name is now 'freddy'
6137 my $null = substr $name, 6, 2; # returns '' (no warning)
6138 my $oops = substr $name, 7; # returns undef, with warning
6139 substr($name, 7) = 'gap'; # fatal error
6141 An alternative to using substr() as an lvalue is to specify the
6142 replacement string as the 4th argument. This allows you to replace
6143 parts of the EXPR and return what was there before in one operation,
6144 just as you can with splice().
6146 my $s = "The black cat climbed the green tree";
6147 my $z = substr $s, 14, 7, "jumped from"; # climbed
6148 # $s is now "The black cat jumped from the green tree"
6150 Note that the lvalue returned by the 3-arg version of substr() acts as
6151 a 'magic bullet'; each time it is assigned to, it remembers which part
6152 of the original string is being modified; for example:
6155 for (substr($x,1,2)) {
6156 $_ = 'a'; print $x,"\n"; # prints 1a4
6157 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6159 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6162 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6165 =item symlink OLDFILE,NEWFILE
6166 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6168 Creates a new filename symbolically linked to the old filename.
6169 Returns C<1> for success, C<0> otherwise. On systems that don't support
6170 symbolic links, produces a fatal error at run time. To check for that,
6173 $symlink_exists = eval { symlink("",""); 1 };
6175 =item syscall NUMBER, LIST
6176 X<syscall> X<system call>
6178 Calls the system call specified as the first element of the list,
6179 passing the remaining elements as arguments to the system call. If
6180 unimplemented, produces a fatal error. The arguments are interpreted
6181 as follows: if a given argument is numeric, the argument is passed as
6182 an int. If not, the pointer to the string value is passed. You are
6183 responsible to make sure a string is pre-extended long enough to
6184 receive any result that might be written into a string. You can't use a
6185 string literal (or other read-only string) as an argument to C<syscall>
6186 because Perl has to assume that any string pointer might be written
6188 integer arguments are not literals and have never been interpreted in a
6189 numeric context, you may need to add C<0> to them to force them to look
6190 like numbers. This emulates the C<syswrite> function (or vice versa):
6192 require 'syscall.ph'; # may need to run h2ph
6194 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6196 Note that Perl supports passing of up to only 14 arguments to your system call,
6197 which in practice should usually suffice.
6199 Syscall returns whatever value returned by the system call it calls.
6200 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6201 Note that some system calls can legitimately return C<-1>. The proper
6202 way to handle such calls is to assign C<$!=0;> before the call and
6203 check the value of C<$!> if syscall returns C<-1>.
6205 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6206 number of the read end of the pipe it creates. There is no way
6207 to retrieve the file number of the other end. You can avoid this
6208 problem by using C<pipe> instead.
6210 =item sysopen FILEHANDLE,FILENAME,MODE
6213 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6215 Opens the file whose filename is given by FILENAME, and associates it
6216 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6217 the name of the real filehandle wanted. This function calls the
6218 underlying operating system's C<open> function with the parameters
6219 FILENAME, MODE, PERMS.
6221 The possible values and flag bits of the MODE parameter are
6222 system-dependent; they are available via the standard module C<Fcntl>.
6223 See the documentation of your operating system's C<open> to see which
6224 values and flag bits are available. You may combine several flags
6225 using the C<|>-operator.
6227 Some of the most common values are C<O_RDONLY> for opening the file in
6228 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6229 and C<O_RDWR> for opening the file in read-write mode.
6230 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6232 For historical reasons, some values work on almost every system
6233 supported by perl: zero means read-only, one means write-only, and two
6234 means read/write. We know that these values do I<not> work under
6235 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6236 use them in new code.
6238 If the file named by FILENAME does not exist and the C<open> call creates
6239 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6240 PERMS specifies the permissions of the newly created file. If you omit
6241 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6242 These permission values need to be in octal, and are modified by your
6243 process's current C<umask>.
6246 In many systems the C<O_EXCL> flag is available for opening files in
6247 exclusive mode. This is B<not> locking: exclusiveness means here that
6248 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6249 on network filesystems, and has no effect unless the C<O_CREAT> flag
6250 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6251 being opened if it is a symbolic link. It does not protect against
6252 symbolic links in the file's path.
6255 Sometimes you may want to truncate an already-existing file. This
6256 can be done using the C<O_TRUNC> flag. The behavior of
6257 C<O_TRUNC> with C<O_RDONLY> is undefined.
6260 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6261 that takes away the user's option to have a more permissive umask.
6262 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6265 Note that C<sysopen> depends on the fdopen() C library function.
6266 On many UNIX systems, fdopen() is known to fail when file descriptors
6267 exceed a certain value, typically 255. If you need more file
6268 descriptors than that, consider rebuilding Perl to use the C<sfio>
6269 library, or perhaps using the POSIX::open() function.
6271 See L<perlopentut> for a kinder, gentler explanation of opening files.
6273 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6276 =item sysread FILEHANDLE,SCALAR,LENGTH
6278 Attempts to read LENGTH bytes of data into variable SCALAR from the
6279 specified FILEHANDLE, using the system call read(2). It bypasses
6280 buffered IO, so mixing this with other kinds of reads, C<print>,
6281 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6282 perlio or stdio layers usually buffers data. Returns the number of
6283 bytes actually read, C<0> at end of file, or undef if there was an
6284 error (in the latter case C<$!> is also set). SCALAR will be grown or
6285 shrunk so that the last byte actually read is the last byte of the
6286 scalar after the read.
6288 An OFFSET may be specified to place the read data at some place in the
6289 string other than the beginning. A negative OFFSET specifies
6290 placement at that many characters counting backwards from the end of
6291 the string. A positive OFFSET greater than the length of SCALAR
6292 results in the string being padded to the required size with C<"\0">
6293 bytes before the result of the read is appended.
6295 There is no syseof() function, which is ok, since eof() doesn't work
6296 very well on device files (like ttys) anyway. Use sysread() and check
6297 for a return value for 0 to decide whether you're done.
6299 Note that if the filehandle has been marked as C<:utf8> Unicode
6300 characters are read instead of bytes (the LENGTH, OFFSET, and the
6301 return value of sysread() are in Unicode characters).
6302 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6303 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6305 =item sysseek FILEHANDLE,POSITION,WHENCE
6308 Sets FILEHANDLE's system position in bytes using the system call
6309 lseek(2). FILEHANDLE may be an expression whose value gives the name
6310 of the filehandle. The values for WHENCE are C<0> to set the new
6311 position to POSITION, C<1> to set the it to the current position plus
6312 POSITION, and C<2> to set it to EOF plus POSITION (typically
6315 Note the I<in bytes>: even if the filehandle has been set to operate
6316 on characters (for example by using the C<:utf8> I/O layer), tell()
6317 will return byte offsets, not character offsets (because implementing
6318 that would render sysseek() very slow).
6320 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6321 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6322 C<seek>, C<tell>, or C<eof> may cause confusion.
6324 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6325 and C<SEEK_END> (start of the file, current position, end of the file)
6326 from the Fcntl module. Use of the constants is also more portable
6327 than relying on 0, 1, and 2. For example to define a "systell" function:
6329 use Fcntl 'SEEK_CUR';
6330 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6332 Returns the new position, or the undefined value on failure. A position
6333 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6334 true on success and false on failure, yet you can still easily determine
6340 =item system PROGRAM LIST
6342 Does exactly the same thing as C<exec LIST>, except that a fork is
6343 done first, and the parent process waits for the child process to
6344 complete. Note that argument processing varies depending on the
6345 number of arguments. If there is more than one argument in LIST,
6346 or if LIST is an array with more than one value, starts the program
6347 given by the first element of the list with arguments given by the
6348 rest of the list. If there is only one scalar argument, the argument
6349 is checked for shell metacharacters, and if there are any, the
6350 entire argument is passed to the system's command shell for parsing
6351 (this is C</bin/sh -c> on Unix platforms, but varies on other
6352 platforms). If there are no shell metacharacters in the argument,
6353 it is split into words and passed directly to C<execvp>, which is
6356 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6357 output before any operation that may do a fork, but this may not be
6358 supported on some platforms (see L<perlport>). To be safe, you may need
6359 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6360 of C<IO::Handle> on any open handles.
6362 The return value is the exit status of the program as returned by the
6363 C<wait> call. To get the actual exit value, shift right by eight (see
6364 below). See also L</exec>. This is I<not> what you want to use to capture
6365 the output from a command, for that you should use merely backticks or
6366 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6367 indicates a failure to start the program or an error of the wait(2) system
6368 call (inspect $! for the reason).
6370 Like C<exec>, C<system> allows you to lie to a program about its name if
6371 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6373 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6374 C<system>, if you expect your program to terminate on receipt of these
6375 signals you will need to arrange to do so yourself based on the return
6378 @args = ("command", "arg1", "arg2");
6380 or die "system @args failed: $?"
6382 You can check all the failure possibilities by inspecting
6386 print "failed to execute: $!\n";
6389 printf "child died with signal %d, %s coredump\n",
6390 ($? & 127), ($? & 128) ? 'with' : 'without';
6393 printf "child exited with value %d\n", $? >> 8;
6396 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6397 with the W*() calls of the POSIX extension.
6399 When the arguments get executed via the system shell, results
6400 and return codes will be subject to its quirks and capabilities.
6401 See L<perlop/"`STRING`"> and L</exec> for details.
6403 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6406 =item syswrite FILEHANDLE,SCALAR,LENGTH
6408 =item syswrite FILEHANDLE,SCALAR
6410 Attempts to write LENGTH bytes of data from variable SCALAR to the
6411 specified FILEHANDLE, using the system call write(2). If LENGTH is
6412 not specified, writes whole SCALAR. It bypasses buffered IO, so
6413 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6414 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6415 stdio layers usually buffers data. Returns the number of bytes
6416 actually written, or C<undef> if there was an error (in this case the
6417 errno variable C<$!> is also set). If the LENGTH is greater than the
6418 available data in the SCALAR after the OFFSET, only as much data as is
6419 available will be written.
6421 An OFFSET may be specified to write the data from some part of the
6422 string other than the beginning. A negative OFFSET specifies writing
6423 that many characters counting backwards from the end of the string.
6424 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6426 Note that if the filehandle has been marked as C<:utf8>, Unicode
6427 characters are written instead of bytes (the LENGTH, OFFSET, and the
6428 return value of syswrite() are in UTF-8 encoded Unicode characters).
6429 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6430 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6432 =item tell FILEHANDLE
6437 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6438 error. FILEHANDLE may be an expression whose value gives the name of
6439 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6442 Note the I<in bytes>: even if the filehandle has been set to
6443 operate on characters (for example by using the C<:utf8> open
6444 layer), tell() will return byte offsets, not character offsets
6445 (because that would render seek() and tell() rather slow).
6447 The return value of tell() for the standard streams like the STDIN
6448 depends on the operating system: it may return -1 or something else.
6449 tell() on pipes, fifos, and sockets usually returns -1.
6451 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6453 Do not use tell() (or other buffered I/O operations) on a file handle
6454 that has been manipulated by sysread(), syswrite() or sysseek().
6455 Those functions ignore the buffering, while tell() does not.
6457 =item telldir DIRHANDLE
6460 Returns the current position of the C<readdir> routines on DIRHANDLE.
6461 Value may be given to C<seekdir> to access a particular location in a
6462 directory. C<telldir> has the same caveats about possible directory
6463 compaction as the corresponding system library routine.
6465 =item tie VARIABLE,CLASSNAME,LIST
6468 This function binds a variable to a package class that will provide the
6469 implementation for the variable. VARIABLE is the name of the variable
6470 to be enchanted. CLASSNAME is the name of a class implementing objects
6471 of correct type. Any additional arguments are passed to the C<new>
6472 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6473 or C<TIEHASH>). Typically these are arguments such as might be passed
6474 to the C<dbm_open()> function of C. The object returned by the C<new>
6475 method is also returned by the C<tie> function, which would be useful
6476 if you want to access other methods in CLASSNAME.
6478 Note that functions such as C<keys> and C<values> may return huge lists
6479 when used on large objects, like DBM files. You may prefer to use the
6480 C<each> function to iterate over such. Example:
6482 # print out history file offsets
6484 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6485 while (($key,$val) = each %HIST) {
6486 print $key, ' = ', unpack('L',$val), "\n";
6490 A class implementing a hash should have the following methods:
6492 TIEHASH classname, LIST
6494 STORE this, key, value
6499 NEXTKEY this, lastkey
6504 A class implementing an ordinary array should have the following methods:
6506 TIEARRAY classname, LIST
6508 STORE this, key, value
6510 STORESIZE this, count
6516 SPLICE this, offset, length, LIST
6521 A class implementing a file handle should have the following methods:
6523 TIEHANDLE classname, LIST
6524 READ this, scalar, length, offset
6527 WRITE this, scalar, length, offset
6529 PRINTF this, format, LIST
6533 SEEK this, position, whence
6535 OPEN this, mode, LIST
6540 A class implementing a scalar should have the following methods:
6542 TIESCALAR classname, LIST
6548 Not all methods indicated above need be implemented. See L<perltie>,
6549 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6551 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6552 for you--you need to do that explicitly yourself. See L<DB_File>
6553 or the F<Config> module for interesting C<tie> implementations.
6555 For further details see L<perltie>, L<"tied VARIABLE">.
6560 Returns a reference to the object underlying VARIABLE (the same value
6561 that was originally returned by the C<tie> call that bound the variable
6562 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6568 Returns the number of non-leap seconds since whatever time the system
6569 considers to be the epoch, suitable for feeding to C<gmtime> and
6570 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6571 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6572 1904 in the current local time zone for its epoch.
6574 For measuring time in better granularity than one second,
6575 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6576 Perl 5.8 part of the standard distribution), or if you have
6577 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6578 See L<perlfaq8> for details.
6580 For date and time processing look at the many related modules on CPAN.
6581 For a comprehensive date and time representation look at the
6587 Returns a four-element list giving the user and system times, in
6588 seconds, for this process and the children of this process.
6590 ($user,$system,$cuser,$csystem) = times;
6592 In scalar context, C<times> returns C<$user>.
6594 Note that times for children are included only after they terminate.
6598 The transliteration operator. Same as C<y///>. See L<perlop>.
6600 =item truncate FILEHANDLE,LENGTH
6603 =item truncate EXPR,LENGTH
6605 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6606 specified length. Produces a fatal error if truncate isn't implemented
6607 on your system. Returns true if successful, the undefined value
6610 The behavior is undefined if LENGTH is greater than the length of the
6613 The position in the file of FILEHANDLE is left unchanged. You may want to
6614 call L<seek> before writing to the file.
6617 X<uc> X<uppercase> X<toupper>
6621 Returns an uppercased version of EXPR. This is the internal function
6622 implementing the C<\U> escape in double-quoted strings. Respects
6623 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6624 and L<perlunicode> for more details about locale and Unicode support.
6625 It does not attempt to do titlecase mapping on initial letters. See
6626 C<ucfirst> for that.
6628 If EXPR is omitted, uses C<$_>.
6631 X<ucfirst> X<uppercase>
6635 Returns the value of EXPR with the first character in uppercase
6636 (titlecase in Unicode). This is the internal function implementing
6637 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6638 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6639 for more details about locale and Unicode support.
6641 If EXPR is omitted, uses C<$_>.
6648 Sets the umask for the process to EXPR and returns the previous value.
6649 If EXPR is omitted, merely returns the current umask.
6651 The Unix permission C<rwxr-x---> is represented as three sets of three
6652 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6653 and isn't one of the digits). The C<umask> value is such a number
6654 representing disabled permissions bits. The permission (or "mode")
6655 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6656 even if you tell C<sysopen> to create a file with permissions C<0777>,
6657 if your umask is C<0022> then the file will actually be created with
6658 permissions C<0755>. If your C<umask> were C<0027> (group can't
6659 write; others can't read, write, or execute), then passing
6660 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6663 Here's some advice: supply a creation mode of C<0666> for regular
6664 files (in C<sysopen>) and one of C<0777> for directories (in
6665 C<mkdir>) and executable files. This gives users the freedom of
6666 choice: if they want protected files, they might choose process umasks
6667 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6668 Programs should rarely if ever make policy decisions better left to
6669 the user. The exception to this is when writing files that should be
6670 kept private: mail files, web browser cookies, I<.rhosts> files, and
6673 If umask(2) is not implemented on your system and you are trying to
6674 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6675 fatal error at run time. If umask(2) is not implemented and you are
6676 not trying to restrict access for yourself, returns C<undef>.
6678 Remember that a umask is a number, usually given in octal; it is I<not> a
6679 string of octal digits. See also L</oct>, if all you have is a string.
6682 X<undef> X<undefine>
6686 Undefines the value of EXPR, which must be an lvalue. Use only on a
6687 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6688 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6689 will probably not do what you expect on most predefined variables or
6690 DBM list values, so don't do that; see L<delete>.) Always returns the
6691 undefined value. You can omit the EXPR, in which case nothing is
6692 undefined, but you still get an undefined value that you could, for
6693 instance, return from a subroutine, assign to a variable or pass as a
6694 parameter. Examples:
6697 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6701 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6702 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6703 select undef, undef, undef, 0.25;
6704 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6706 Note that this is a unary operator, not a list operator.
6709 X<unlink> X<delete> X<remove> X<rm> X<del>
6713 Deletes a list of files. Returns the number of files successfully
6716 $cnt = unlink 'a', 'b', 'c';
6720 Note: C<unlink> will not attempt to delete directories unless you are superuser
6721 and the B<-U> flag is supplied to Perl. Even if these conditions are
6722 met, be warned that unlinking a directory can inflict damage on your
6723 filesystem. Finally, using C<unlink> on directories is not supported on
6724 many operating systems. Use C<rmdir> instead.
6726 If LIST is omitted, uses C<$_>.
6728 =item unpack TEMPLATE,EXPR
6731 =item unpack TEMPLATE
6733 C<unpack> does the reverse of C<pack>: it takes a string
6734 and expands it out into a list of values.
6735 (In scalar context, it returns merely the first value produced.)
6737 If EXPR is omitted, unpacks the C<$_> string.
6739 The string is broken into chunks described by the TEMPLATE. Each chunk
6740 is converted separately to a value. Typically, either the string is a result
6741 of C<pack>, or the characters of the string represent a C structure of some
6744 The TEMPLATE has the same format as in the C<pack> function.
6745 Here's a subroutine that does substring:
6748 my($what,$where,$howmuch) = @_;
6749 unpack("x$where a$howmuch", $what);
6754 sub ordinal { unpack("W",$_[0]); } # same as ord()
6756 In addition to fields allowed in pack(), you may prefix a field with
6757 a %<number> to indicate that
6758 you want a <number>-bit checksum of the items instead of the items
6759 themselves. Default is a 16-bit checksum. Checksum is calculated by
6760 summing numeric values of expanded values (for string fields the sum of
6761 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6763 For example, the following
6764 computes the same number as the System V sum program:
6768 unpack("%32W*",<>) % 65535;
6771 The following efficiently counts the number of set bits in a bit vector:
6773 $setbits = unpack("%32b*", $selectmask);
6775 The C<p> and C<P> formats should be used with care. Since Perl
6776 has no way of checking whether the value passed to C<unpack()>
6777 corresponds to a valid memory location, passing a pointer value that's
6778 not known to be valid is likely to have disastrous consequences.
6780 If there are more pack codes or if the repeat count of a field or a group
6781 is larger than what the remainder of the input string allows, the result
6782 is not well defined: in some cases, the repeat count is decreased, or
6783 C<unpack()> will produce null strings or zeroes, or terminate with an
6784 error. If the input string is longer than one described by the TEMPLATE,
6785 the rest is ignored.
6787 See L</pack> for more examples and notes.
6789 =item untie VARIABLE
6792 Breaks the binding between a variable and a package. (See C<tie>.)
6793 Has no effect if the variable is not tied.
6795 =item unshift ARRAY,LIST
6798 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6799 depending on how you look at it. Prepends list to the front of the
6800 array, and returns the new number of elements in the array.
6802 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6804 Note the LIST is prepended whole, not one element at a time, so the
6805 prepended elements stay in the same order. Use C<reverse> to do the
6808 =item use Module VERSION LIST
6809 X<use> X<module> X<import>
6811 =item use Module VERSION
6813 =item use Module LIST
6819 Imports some semantics into the current package from the named module,
6820 generally by aliasing certain subroutine or variable names into your
6821 package. It is exactly equivalent to
6823 BEGIN { require Module; import Module LIST; }
6825 except that Module I<must> be a bareword.
6827 In the peculiar C<use VERSION> form, VERSION may be either a numeric
6828 argument such as 5.006, which will be compared to C<$]>, or a literal of
6829 the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A
6830 fatal error is produced if VERSION is greater than the version of the
6831 current Perl interpreter; Perl will not attempt to parse the rest of the
6832 file. Compare with L</require>, which can do a similar check at run time.
6833 Symmetrically, C<no VERSION> allows you to specify that you want a version
6834 of perl older than the specified one.
6836 Specifying VERSION as a literal of the form v5.6.1 should generally be
6837 avoided, because it leads to misleading error messages under earlier
6838 versions of Perl that do not support this syntax. The equivalent numeric
6839 version should be used instead.
6841 use v5.6.1; # compile time version check
6843 use 5.006_001; # ditto; preferred for backwards compatibility
6845 This is often useful if you need to check the current Perl version before
6846 C<use>ing library modules that have changed in incompatible ways from
6847 older versions of Perl. (We try not to do this more than we have to.)
6849 Also, if the specified perl version is greater than or equal to 5.9.5,
6850 C<use VERSION> will also load the C<feature> pragma and enable all
6851 features available in the requested version. See L<feature>.
6853 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6854 C<require> makes sure the module is loaded into memory if it hasn't been
6855 yet. The C<import> is not a builtin--it's just an ordinary static method
6856 call into the C<Module> package to tell the module to import the list of
6857 features back into the current package. The module can implement its
6858 C<import> method any way it likes, though most modules just choose to
6859 derive their C<import> method via inheritance from the C<Exporter> class that
6860 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6861 method can be found then the call is skipped, even if there is an AUTOLOAD
6864 If you do not want to call the package's C<import> method (for instance,
6865 to stop your namespace from being altered), explicitly supply the empty list:
6869 That is exactly equivalent to
6871 BEGIN { require Module }
6873 If the VERSION argument is present between Module and LIST, then the
6874 C<use> will call the VERSION method in class Module with the given
6875 version as an argument. The default VERSION method, inherited from
6876 the UNIVERSAL class, croaks if the given version is larger than the
6877 value of the variable C<$Module::VERSION>.
6879 Again, there is a distinction between omitting LIST (C<import> called
6880 with no arguments) and an explicit empty LIST C<()> (C<import> not
6881 called). Note that there is no comma after VERSION!
6883 Because this is a wide-open interface, pragmas (compiler directives)
6884 are also implemented this way. Currently implemented pragmas are:
6889 use sigtrap qw(SEGV BUS);
6890 use strict qw(subs vars refs);
6891 use subs qw(afunc blurfl);
6892 use warnings qw(all);
6893 use sort qw(stable _quicksort _mergesort);
6895 Some of these pseudo-modules import semantics into the current
6896 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6897 which import symbols into the current package (which are effective
6898 through the end of the file).
6900 There's a corresponding C<no> command that unimports meanings imported
6901 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6902 It behaves exactly as C<import> does with respect to VERSION, an
6903 omitted LIST, empty LIST, or no unimport method being found.
6909 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6910 for the C<-M> and C<-m> command-line options to perl that give C<use>
6911 functionality from the command-line.
6916 Changes the access and modification times on each file of a list of
6917 files. The first two elements of the list must be the NUMERICAL access
6918 and modification times, in that order. Returns the number of files
6919 successfully changed. The inode change time of each file is set
6920 to the current time. For example, this code has the same effect as the
6921 Unix touch(1) command when the files I<already exist> and belong to
6922 the user running the program:
6925 $atime = $mtime = time;
6926 utime $atime, $mtime, @ARGV;
6928 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6929 the utime(2) function in the C library will be called with a null second
6930 argument. On most systems, this will set the file's access and
6931 modification times to the current time (i.e. equivalent to the example
6932 above) and will even work on other users' files where you have write
6935 utime undef, undef, @ARGV;
6937 Under NFS this will use the time of the NFS server, not the time of
6938 the local machine. If there is a time synchronization problem, the
6939 NFS server and local machine will have different times. The Unix
6940 touch(1) command will in fact normally use this form instead of the
6941 one shown in the first example.
6943 Note that only passing one of the first two elements as C<undef> will
6944 be equivalent of passing it as 0 and will not have the same effect as
6945 described when they are both C<undef>. This case will also trigger an
6946 uninitialized warning.
6948 On systems that support futimes, you might pass file handles among the
6949 files. On systems that don't support futimes, passing file handles
6950 produces a fatal error at run time. The file handles must be passed
6951 as globs or references to be recognized. Barewords are considered
6957 Returns a list consisting of all the values of the named hash.
6958 (In a scalar context, returns the number of values.)
6960 The values are returned in an apparently random order. The actual
6961 random order is subject to change in future versions of perl, but it
6962 is guaranteed to be the same order as either the C<keys> or C<each>
6963 function would produce on the same (unmodified) hash. Since Perl
6964 5.8.1 the ordering is different even between different runs of Perl
6965 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6967 As a side effect, calling values() resets the HASH's internal iterator,
6968 see L</each>. (In particular, calling values() in void context resets
6969 the iterator with no other overhead.)
6971 Note that the values are not copied, which means modifying them will
6972 modify the contents of the hash:
6974 for (values %hash) { s/foo/bar/g } # modifies %hash values
6975 for (@hash{keys %hash}) { s/foo/bar/g } # same
6977 See also C<keys>, C<each>, and C<sort>.
6979 =item vec EXPR,OFFSET,BITS
6980 X<vec> X<bit> X<bit vector>
6982 Treats the string in EXPR as a bit vector made up of elements of
6983 width BITS, and returns the value of the element specified by OFFSET
6984 as an unsigned integer. BITS therefore specifies the number of bits
6985 that are reserved for each element in the bit vector. This must
6986 be a power of two from 1 to 32 (or 64, if your platform supports
6989 If BITS is 8, "elements" coincide with bytes of the input string.
6991 If BITS is 16 or more, bytes of the input string are grouped into chunks
6992 of size BITS/8, and each group is converted to a number as with
6993 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6994 for BITS==64). See L<"pack"> for details.
6996 If bits is 4 or less, the string is broken into bytes, then the bits
6997 of each byte are broken into 8/BITS groups. Bits of a byte are
6998 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6999 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7000 breaking the single input byte C<chr(0x36)> into two groups gives a list
7001 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7003 C<vec> may also be assigned to, in which case parentheses are needed
7004 to give the expression the correct precedence as in
7006 vec($image, $max_x * $x + $y, 8) = 3;
7008 If the selected element is outside the string, the value 0 is returned.
7009 If an element off the end of the string is written to, Perl will first
7010 extend the string with sufficiently many zero bytes. It is an error
7011 to try to write off the beginning of the string (i.e. negative OFFSET).
7013 If the string happens to be encoded as UTF-8 internally (and thus has
7014 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7015 internal byte string, not the conceptual character string, even if you
7016 only have characters with values less than 256.
7018 Strings created with C<vec> can also be manipulated with the logical
7019 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7020 vector operation is desired when both operands are strings.
7021 See L<perlop/"Bitwise String Operators">.
7023 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7024 The comments show the string after each step. Note that this code works
7025 in the same way on big-endian or little-endian machines.
7028 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7030 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7031 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7033 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7034 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7035 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7036 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7037 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7038 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7040 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7041 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7042 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7045 To transform a bit vector into a string or list of 0's and 1's, use these:
7047 $bits = unpack("b*", $vector);
7048 @bits = split(//, unpack("b*", $vector));
7050 If you know the exact length in bits, it can be used in place of the C<*>.
7052 Here is an example to illustrate how the bits actually fall in place:
7058 unpack("V",$_) 01234567890123456789012345678901
7059 ------------------------------------------------------------------
7064 for ($shift=0; $shift < $width; ++$shift) {
7065 for ($off=0; $off < 32/$width; ++$off) {
7066 $str = pack("B*", "0"x32);
7067 $bits = (1<<$shift);
7068 vec($str, $off, $width) = $bits;
7069 $res = unpack("b*",$str);
7070 $val = unpack("V", $str);
7077 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7078 $off, $width, $bits, $val, $res
7082 Regardless of the machine architecture on which it is run, the above
7083 example should print the following table:
7086 unpack("V",$_) 01234567890123456789012345678901
7087 ------------------------------------------------------------------
7088 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7089 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7090 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7091 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7092 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7093 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7094 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7095 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7096 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7097 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7098 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7099 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7100 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7101 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7102 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7103 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7104 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7105 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7106 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7107 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7108 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7109 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7110 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7111 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7112 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7113 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7114 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7115 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7116 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7117 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7118 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7119 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7120 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7121 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7122 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7123 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7124 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7125 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7126 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7127 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7128 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7129 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7130 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7131 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7132 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7133 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7134 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7135 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7136 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7137 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7138 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7139 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7140 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7141 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7142 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7143 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7144 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7145 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7146 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7147 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7148 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7149 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7150 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7151 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7152 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7153 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7154 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7155 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7156 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7157 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7158 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7159 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7160 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7161 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7162 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7163 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7164 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7165 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7166 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7167 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7168 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7169 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7170 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7171 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7172 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7173 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7174 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7175 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7176 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7177 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7178 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7179 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7180 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7181 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7182 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7183 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7184 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7185 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7186 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7187 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7188 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7189 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7190 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7191 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7192 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7193 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7194 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7195 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7196 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7197 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7198 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7199 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7200 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7201 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7202 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7203 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7204 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7205 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7206 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7207 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7208 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7209 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7210 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7211 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7212 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7213 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7214 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7215 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7220 Behaves like the wait(2) system call on your system: it waits for a child
7221 process to terminate and returns the pid of the deceased process, or
7222 C<-1> if there are no child processes. The status is returned in C<$?>
7223 and C<{^CHILD_ERROR_NATIVE}>.
7224 Note that a return value of C<-1> could mean that child processes are
7225 being automatically reaped, as described in L<perlipc>.
7227 =item waitpid PID,FLAGS
7230 Waits for a particular child process to terminate and returns the pid of
7231 the deceased process, or C<-1> if there is no such child process. On some
7232 systems, a value of 0 indicates that there are processes still running.
7233 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7235 use POSIX ":sys_wait_h";
7238 $kid = waitpid(-1, WNOHANG);
7241 then you can do a non-blocking wait for all pending zombie processes.
7242 Non-blocking wait is available on machines supporting either the
7243 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7244 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7245 system call by remembering the status values of processes that have
7246 exited but have not been harvested by the Perl script yet.)
7248 Note that on some systems, a return value of C<-1> could mean that child
7249 processes are being automatically reaped. See L<perlipc> for details,
7250 and for other examples.
7253 X<wantarray> X<context>
7255 Returns true if the context of the currently executing subroutine or
7256 C<eval> is looking for a list value. Returns false if the context is
7257 looking for a scalar. Returns the undefined value if the context is
7258 looking for no value (void context).
7260 return unless defined wantarray; # don't bother doing more
7261 my @a = complex_calculation();
7262 return wantarray ? @a : "@a";
7264 C<wantarray()>'s result is unspecified in the top level of a file,
7265 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7266 in a C<DESTROY> method.
7268 This function should have been named wantlist() instead.
7271 X<warn> X<warning> X<STDERR>
7273 Prints the value of LIST to STDERR. If the last element of LIST does
7274 not end in a newline, appends the same text as C<die> does.
7276 If LIST is empty and C<$@> already contains a value (typically from a
7277 previous eval) that value is used after appending C<"\t...caught">
7278 to C<$@>. This is useful for staying almost, but not entirely similar to
7281 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7283 No message is printed if there is a C<$SIG{__WARN__}> handler
7284 installed. It is the handler's responsibility to deal with the message
7285 as it sees fit (like, for instance, converting it into a C<die>). Most
7286 handlers must therefore make arrangements to actually display the
7287 warnings that they are not prepared to deal with, by calling C<warn>
7288 again in the handler. Note that this is quite safe and will not
7289 produce an endless loop, since C<__WARN__> hooks are not called from
7292 You will find this behavior is slightly different from that of
7293 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7294 instead call C<die> again to change it).
7296 Using a C<__WARN__> handler provides a powerful way to silence all
7297 warnings (even the so-called mandatory ones). An example:
7299 # wipe out *all* compile-time warnings
7300 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7302 my $foo = 20; # no warning about duplicate my $foo,
7303 # but hey, you asked for it!
7304 # no compile-time or run-time warnings before here
7307 # run-time warnings enabled after here
7308 warn "\$foo is alive and $foo!"; # does show up
7310 See L<perlvar> for details on setting C<%SIG> entries, and for more
7311 examples. See the Carp module for other kinds of warnings using its
7312 carp() and cluck() functions.
7314 =item write FILEHANDLE
7321 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7322 using the format associated with that file. By default the format for
7323 a file is the one having the same name as the filehandle, but the
7324 format for the current output channel (see the C<select> function) may be set
7325 explicitly by assigning the name of the format to the C<$~> variable.
7327 Top of form processing is handled automatically: if there is
7328 insufficient room on the current page for the formatted record, the
7329 page is advanced by writing a form feed, a special top-of-page format
7330 is used to format the new page header, and then the record is written.
7331 By default the top-of-page format is the name of the filehandle with
7332 "_TOP" appended, but it may be dynamically set to the format of your
7333 choice by assigning the name to the C<$^> variable while the filehandle is
7334 selected. The number of lines remaining on the current page is in
7335 variable C<$->, which can be set to C<0> to force a new page.
7337 If FILEHANDLE is unspecified, output goes to the current default output
7338 channel, which starts out as STDOUT but may be changed by the
7339 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7340 is evaluated and the resulting string is used to look up the name of
7341 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7343 Note that write is I<not> the opposite of C<read>. Unfortunately.
7347 The transliteration operator. Same as C<tr///>. See L<perlop>.