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/STRING/>, C<qq/STRING/>, 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/STRING/>, 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/STRING/>, 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> the simply inverse of C<:crlf>
530 -- other layers which would affect 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>.
547 In general, binmode() should be called after open() but before any I/O
548 is done on the filehandle. Calling binmode() will normally flush any
549 pending buffered output data (and perhaps pending input data) on the
550 handle. An exception to this is the C<:encoding> layer that
551 changes the default character encoding of the handle, see L<open>.
552 The C<:encoding> layer sometimes needs to be called in
553 mid-stream, and it doesn't flush the stream. The C<:encoding>
554 also implicitly pushes on top of itself the C<:utf8> layer because
555 internally Perl will operate on UTF-8 encoded Unicode characters.
557 The operating system, device drivers, C libraries, and Perl run-time
558 system all work together to let the programmer treat a single
559 character (C<\n>) as the line terminator, irrespective of the external
560 representation. On many operating systems, the native text file
561 representation matches the internal representation, but on some
562 platforms the external representation of C<\n> is made up of more than
565 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
566 character to end each line in the external representation of text (even
567 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
568 on Unix and most VMS files). In other systems like OS/2, DOS and the
569 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
570 but what's stored in text files are the two characters C<\cM\cJ>. That
571 means that, if you don't use binmode() on these systems, C<\cM\cJ>
572 sequences on disk will be converted to C<\n> on input, and any C<\n> in
573 your program will be converted back to C<\cM\cJ> on output. This is what
574 you want for text files, but it can be disastrous for binary files.
576 Another consequence of using binmode() (on some systems) is that
577 special end-of-file markers will be seen as part of the data stream.
578 For systems from the Microsoft family this means that if your binary
579 data contains C<\cZ>, the I/O subsystem will regard it as the end of
580 the file, unless you use binmode().
582 binmode() is not only important for readline() and print() operations,
583 but also when using read(), seek(), sysread(), syswrite() and tell()
584 (see L<perlport> for more details). See the C<$/> and C<$\> variables
585 in L<perlvar> for how to manually set your input and output
586 line-termination sequences.
588 =item bless REF,CLASSNAME
593 This function tells the thingy referenced by REF that it is now an object
594 in the CLASSNAME package. If CLASSNAME is omitted, the current package
595 is used. Because a C<bless> is often the last thing in a constructor,
596 it returns the reference for convenience. Always use the two-argument
597 version if a derived class might inherit the function doing the blessing.
598 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
601 Consider always blessing objects in CLASSNAMEs that are mixed case.
602 Namespaces with all lowercase names are considered reserved for
603 Perl pragmata. Builtin types have all uppercase names. To prevent
604 confusion, you may wish to avoid such package names as well. Make sure
605 that CLASSNAME is a true value.
607 See L<perlmod/"Perl Modules">.
611 Break out of a C<given()> block.
613 This keyword is enabled by the "switch" feature: see L<feature>
614 for more information.
617 X<caller> X<call stack> X<stack> X<stack trace>
621 Returns the context of the current subroutine call. In scalar context,
622 returns the caller's package name if there is a caller, that is, if
623 we're in a subroutine or C<eval> or C<require>, and the undefined value
624 otherwise. In list context, returns
626 ($package, $filename, $line) = caller;
628 With EXPR, it returns some extra information that the debugger uses to
629 print a stack trace. The value of EXPR indicates how many call frames
630 to go back before the current one.
632 ($package, $filename, $line, $subroutine, $hasargs,
633 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
636 Here $subroutine may be C<(eval)> if the frame is not a subroutine
637 call, but an C<eval>. In such a case additional elements $evaltext and
638 C<$is_require> are set: C<$is_require> is true if the frame is created by a
639 C<require> or C<use> statement, $evaltext contains the text of the
640 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
641 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
642 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
643 frame.) $subroutine may also be C<(unknown)> if this particular
644 subroutine happens to have been deleted from the symbol table.
645 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
646 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
647 compiled with. The C<$hints> and C<$bitmask> values are subject to change
648 between versions of Perl, and are not meant for external use.
650 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
651 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
652 of this hash, as they are the actual values stored in the optree.
654 Furthermore, when called from within the DB package, caller returns more
655 detailed information: it sets the list variable C<@DB::args> to be the
656 arguments with which the subroutine was invoked.
658 Be aware that the optimizer might have optimized call frames away before
659 C<caller> had a chance to get the information. That means that C<caller(N)>
660 might not return information about the call frame you expect it do, for
661 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
662 previous time C<caller> was called.
669 =item chdir FILEHANDLE
671 =item chdir DIRHANDLE
675 Changes the working directory to EXPR, if possible. If EXPR is omitted,
676 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
677 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
678 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
679 neither is set, C<chdir> does nothing. It returns true upon success,
680 false otherwise. See the example under C<die>.
682 On systems that support fchdir, you might pass a file handle or
683 directory handle as argument. On systems that don't support fchdir,
684 passing handles produces a fatal error at run time.
687 X<chmod> X<permission> X<mode>
689 Changes the permissions of a list of files. The first element of the
690 list must be the numerical mode, which should probably be an octal
691 number, and which definitely should I<not> be a string of octal digits:
692 C<0644> is okay, C<'0644'> is not. Returns the number of files
693 successfully changed. See also L</oct>, if all you have is a string.
695 $cnt = chmod 0755, 'foo', 'bar';
696 chmod 0755, @executables;
697 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
699 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
700 $mode = 0644; chmod $mode, 'foo'; # this is best
702 On systems that support fchmod, you might pass file handles among the
703 files. On systems that don't support fchmod, passing file handles
704 produces a fatal error at run time. The file handles must be passed
705 as globs or references to be recognized. Barewords are considered
708 open(my $fh, "<", "foo");
709 my $perm = (stat $fh)[2] & 07777;
710 chmod($perm | 0600, $fh);
712 You can also import the symbolic C<S_I*> constants from the Fcntl
717 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
718 # This is identical to the chmod 0755 of the above example.
721 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
727 This safer version of L</chop> removes any trailing string
728 that corresponds to the current value of C<$/> (also known as
729 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
730 number of characters removed from all its arguments. It's often used to
731 remove the newline from the end of an input record when you're worried
732 that the final record may be missing its newline. When in paragraph
733 mode (C<$/ = "">), it removes all trailing newlines from the string.
734 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
735 a reference to an integer or the like, see L<perlvar>) chomp() won't
737 If VARIABLE is omitted, it chomps C<$_>. Example:
740 chomp; # avoid \n on last field
745 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
747 You can actually chomp anything that's an lvalue, including an assignment:
750 chomp($answer = <STDIN>);
752 If you chomp a list, each element is chomped, and the total number of
753 characters removed is returned.
755 If the C<encoding> pragma is in scope then the lengths returned are
756 calculated from the length of C<$/> in Unicode characters, which is not
757 always the same as the length of C<$/> in the native encoding.
759 Note that parentheses are necessary when you're chomping anything
760 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
761 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
762 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
763 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
773 Chops off the last character of a string and returns the character
774 chopped. It is much more efficient than C<s/.$//s> because it neither
775 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
776 If VARIABLE is a hash, it chops the hash's values, but not its keys.
778 You can actually chop anything that's an lvalue, including an assignment.
780 If you chop a list, each element is chopped. Only the value of the
781 last C<chop> is returned.
783 Note that C<chop> returns the last character. To return all but the last
784 character, use C<substr($string, 0, -1)>.
789 X<chown> X<owner> X<user> X<group>
791 Changes the owner (and group) of a list of files. The first two
792 elements of the list must be the I<numeric> uid and gid, in that
793 order. A value of -1 in either position is interpreted by most
794 systems to leave that value unchanged. Returns the number of files
795 successfully changed.
797 $cnt = chown $uid, $gid, 'foo', 'bar';
798 chown $uid, $gid, @filenames;
800 On systems that support fchown, you might pass file handles among the
801 files. On systems that don't support fchown, passing file handles
802 produces a fatal error at run time. The file handles must be passed
803 as globs or references to be recognized. Barewords are considered
806 Here's an example that looks up nonnumeric uids in the passwd file:
809 chomp($user = <STDIN>);
811 chomp($pattern = <STDIN>);
813 ($login,$pass,$uid,$gid) = getpwnam($user)
814 or die "$user not in passwd file";
816 @ary = glob($pattern); # expand filenames
817 chown $uid, $gid, @ary;
819 On most systems, you are not allowed to change the ownership of the
820 file unless you're the superuser, although you should be able to change
821 the group to any of your secondary groups. On insecure systems, these
822 restrictions may be relaxed, but this is not a portable assumption.
823 On POSIX systems, you can detect this condition this way:
825 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
826 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
829 X<chr> X<character> X<ASCII> X<Unicode>
833 Returns the character represented by that NUMBER in the character set.
834 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
835 chr(0x263a) is a Unicode smiley face. Note that characters from 128
836 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
837 backward compatibility reasons (but see L<encoding>).
839 Negative values give the Unicode replacement character (chr(0xfffd)),
840 except under the L<bytes> pragma, where low eight bits of the value
841 (truncated to an integer) are used.
843 If NUMBER is omitted, uses C<$_>.
845 For the reverse, use L</ord>.
847 Note that under the C<bytes> pragma the NUMBER is masked to
850 See L<perlunicode> and L<encoding> for more about Unicode.
852 =item chroot FILENAME
857 This function works like the system call by the same name: it makes the
858 named directory the new root directory for all further pathnames that
859 begin with a C</> by your process and all its children. (It doesn't
860 change your current working directory, which is unaffected.) For security
861 reasons, this call is restricted to the superuser. If FILENAME is
862 omitted, does a C<chroot> to C<$_>.
864 =item close FILEHANDLE
869 Closes the file or pipe associated with the file handle, returning
870 true only if IO buffers are successfully flushed and closes the system
871 file descriptor. Closes the currently selected filehandle if the
874 You don't have to close FILEHANDLE if you are immediately going to do
875 another C<open> on it, because C<open> will close it for you. (See
876 C<open>.) However, an explicit C<close> on an input file resets the line
877 counter (C<$.>), while the implicit close done by C<open> does not.
879 If the file handle came from a piped open, C<close> will additionally
880 return false if one of the other system calls involved fails, or if the
881 program exits with non-zero status. (If the only problem was that the
882 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
883 also waits for the process executing on the pipe to complete, in case you
884 want to look at the output of the pipe afterwards, and
885 implicitly puts the exit status value of that command into C<$?> and
886 C<${^CHILD_ERROR_NATIVE}>.
888 Prematurely closing the read end of a pipe (i.e. before the process
889 writing to it at the other end has closed it) will result in a
890 SIGPIPE being delivered to the writer. If the other end can't
891 handle that, be sure to read all the data before closing the pipe.
895 open(OUTPUT, '|sort >foo') # pipe to sort
896 or die "Can't start sort: $!";
897 #... # print stuff to output
898 close OUTPUT # wait for sort to finish
899 or warn $! ? "Error closing sort pipe: $!"
900 : "Exit status $? from sort";
901 open(INPUT, 'foo') # get sort's results
902 or die "Can't open 'foo' for input: $!";
904 FILEHANDLE may be an expression whose value can be used as an indirect
905 filehandle, usually the real filehandle name.
907 =item closedir DIRHANDLE
910 Closes a directory opened by C<opendir> and returns the success of that
913 =item connect SOCKET,NAME
916 Attempts to connect to a remote socket, just as the connect system call
917 does. Returns true if it succeeded, false otherwise. NAME should be a
918 packed address of the appropriate type for the socket. See the examples in
919 L<perlipc/"Sockets: Client/Server Communication">.
926 C<continue> is actually a flow control statement rather than a function. If
927 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
928 C<foreach>), it is always executed just before the conditional is about to
929 be evaluated again, just like the third part of a C<for> loop in C. Thus
930 it can be used to increment a loop variable, even when the loop has been
931 continued via the C<next> statement (which is similar to the C C<continue>
934 C<last>, C<next>, or C<redo> may appear within a C<continue>
935 block. C<last> and C<redo> will behave as if they had been executed within
936 the main block. So will C<next>, but since it will execute a C<continue>
937 block, it may be more entertaining.
940 ### redo always comes here
943 ### next always comes here
945 # then back the top to re-check EXPR
947 ### last always comes here
949 Omitting the C<continue> section is semantically equivalent to using an
950 empty one, logically enough. In that case, C<next> goes directly back
951 to check the condition at the top of the loop.
953 If the "switch" feature is enabled, C<continue> is also a
954 function that will break out of the current C<when> or C<default>
955 block, and fall through to the next case. See L<feature> and
956 L<perlsyn/"Switch statements"> for more information.
960 X<cos> X<cosine> X<acos> X<arccosine>
964 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
965 takes cosine of C<$_>.
967 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
968 function, or use this relation:
970 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
972 =item crypt PLAINTEXT,SALT
973 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
974 X<decrypt> X<cryptography> X<passwd> X<encrypt>
976 Creates a digest string exactly like the crypt(3) function in the C
977 library (assuming that you actually have a version there that has not
978 been extirpated as a potential munitions).
980 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
981 into a short string, called a digest, which is returned. The same
982 PLAINTEXT and SALT will always return the same string, but there is no
983 (known) way to get the original PLAINTEXT from the hash. Small
984 changes in the PLAINTEXT or SALT will result in large changes in the
987 There is no decrypt function. This function isn't all that useful for
988 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
989 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
990 primarily used to check if two pieces of text are the same without
991 having to transmit or store the text itself. An example is checking
992 if a correct password is given. The digest of the password is stored,
993 not the password itself. The user types in a password that is
994 crypt()'d with the same salt as the stored digest. If the two digests
995 match the password is correct.
997 When verifying an existing digest string you should use the digest as
998 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
999 to create the digest is visible as part of the digest. This ensures
1000 crypt() will hash the new string with the same salt as the digest.
1001 This allows your code to work with the standard L<crypt|/crypt> and
1002 with more exotic implementations. In other words, do not assume
1003 anything about the returned string itself, or how many bytes in the
1006 Traditionally the result is a string of 13 bytes: two first bytes of
1007 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1008 the first eight bytes of the digest string mattered, but alternative
1009 hashing schemes (like MD5), higher level security schemes (like C2),
1010 and implementations on non-UNIX platforms may produce different
1013 When choosing a new salt create a random two character string whose
1014 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1015 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1016 characters is just a recommendation; the characters allowed in
1017 the salt depend solely on your system's crypt library, and Perl can't
1018 restrict what salts C<crypt()> accepts.
1020 Here's an example that makes sure that whoever runs this program knows
1023 $pwd = (getpwuid($<))[1];
1025 system "stty -echo";
1027 chomp($word = <STDIN>);
1031 if (crypt($word, $pwd) ne $pwd) {
1037 Of course, typing in your own password to whoever asks you
1040 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1041 of data, not least of all because you can't get the information
1042 back. Look at the L<Digest> module for more robust algorithms.
1044 If using crypt() on a Unicode string (which I<potentially> has
1045 characters with codepoints above 255), Perl tries to make sense
1046 of the situation by trying to downgrade (a copy of the string)
1047 the string back to an eight-bit byte string before calling crypt()
1048 (on that copy). If that works, good. If not, crypt() dies with
1049 C<Wide character in crypt>.
1054 [This function has been largely superseded by the C<untie> function.]
1056 Breaks the binding between a DBM file and a hash.
1058 =item dbmopen HASH,DBNAME,MASK
1059 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1061 [This function has been largely superseded by the C<tie> function.]
1063 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1064 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1065 argument is I<not> a filehandle, even though it looks like one). DBNAME
1066 is the name of the database (without the F<.dir> or F<.pag> extension if
1067 any). If the database does not exist, it is created with protection
1068 specified by MASK (as modified by the C<umask>). If your system supports
1069 only the older DBM functions, you may perform only one C<dbmopen> in your
1070 program. In older versions of Perl, if your system had neither DBM nor
1071 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1074 If you don't have write access to the DBM file, you can only read hash
1075 variables, not set them. If you want to test whether you can write,
1076 either use file tests or try setting a dummy hash entry inside an C<eval>,
1077 which will trap the error.
1079 Note that functions such as C<keys> and C<values> may return huge lists
1080 when used on large DBM files. You may prefer to use the C<each>
1081 function to iterate over large DBM files. Example:
1083 # print out history file offsets
1084 dbmopen(%HIST,'/usr/lib/news/history',0666);
1085 while (($key,$val) = each %HIST) {
1086 print $key, ' = ', unpack('L',$val), "\n";
1090 See also L<AnyDBM_File> for a more general description of the pros and
1091 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1092 rich implementation.
1094 You can control which DBM library you use by loading that library
1095 before you call dbmopen():
1098 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1099 or die "Can't open netscape history file: $!";
1102 X<defined> X<undef> X<undefined>
1106 Returns a Boolean value telling whether EXPR has a value other than
1107 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1110 Many operations return C<undef> to indicate failure, end of file,
1111 system error, uninitialized variable, and other exceptional
1112 conditions. This function allows you to distinguish C<undef> from
1113 other values. (A simple Boolean test will not distinguish among
1114 C<undef>, zero, the empty string, and C<"0">, which are all equally
1115 false.) Note that since C<undef> is a valid scalar, its presence
1116 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1117 returns C<undef> when its argument is an empty array, I<or> when the
1118 element to return happens to be C<undef>.
1120 You may also use C<defined(&func)> to check whether subroutine C<&func>
1121 has ever been defined. The return value is unaffected by any forward
1122 declarations of C<&func>. Note that a subroutine which is not defined
1123 may still be callable: its package may have an C<AUTOLOAD> method that
1124 makes it spring into existence the first time that it is called -- see
1127 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1128 used to report whether memory for that aggregate has ever been
1129 allocated. This behavior may disappear in future versions of Perl.
1130 You should instead use a simple test for size:
1132 if (@an_array) { print "has array elements\n" }
1133 if (%a_hash) { print "has hash members\n" }
1135 When used on a hash element, it tells you whether the value is defined,
1136 not whether the key exists in the hash. Use L</exists> for the latter
1141 print if defined $switch{'D'};
1142 print "$val\n" while defined($val = pop(@ary));
1143 die "Can't readlink $sym: $!"
1144 unless defined($value = readlink $sym);
1145 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1146 $debugging = 0 unless defined $debugging;
1148 Note: Many folks tend to overuse C<defined>, and then are surprised to
1149 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1150 defined values. For example, if you say
1154 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1155 matched "nothing". It didn't really fail to match anything. Rather, it
1156 matched something that happened to be zero characters long. This is all
1157 very above-board and honest. When a function returns an undefined value,
1158 it's an admission that it couldn't give you an honest answer. So you
1159 should use C<defined> only when you're questioning the integrity of what
1160 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1163 See also L</undef>, L</exists>, L</ref>.
1168 Given an expression that specifies a hash element, array element, hash slice,
1169 or array slice, deletes the specified element(s) from the hash or array.
1170 In the case of an array, if the array elements happen to be at the end,
1171 the size of the array will shrink to the highest element that tests
1172 true for exists() (or 0 if no such element exists).
1174 Returns a list with the same number of elements as the number of elements
1175 for which deletion was attempted. Each element of that list consists of
1176 either the value of the element deleted, or the undefined value. In scalar
1177 context, this means that you get the value of the last element deleted (or
1178 the undefined value if that element did not exist).
1180 %hash = (foo => 11, bar => 22, baz => 33);
1181 $scalar = delete $hash{foo}; # $scalar is 11
1182 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1183 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1185 Deleting from C<%ENV> modifies the environment. Deleting from
1186 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1187 from a C<tie>d hash or array may not necessarily return anything.
1189 Deleting an array element effectively returns that position of the array
1190 to its initial, uninitialized state. Subsequently testing for the same
1191 element with exists() will return false. Also, deleting array elements
1192 in the middle of an array will not shift the index of the elements
1193 after them down. Use splice() for that. See L</exists>.
1195 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1197 foreach $key (keys %HASH) {
1201 foreach $index (0 .. $#ARRAY) {
1202 delete $ARRAY[$index];
1207 delete @HASH{keys %HASH};
1209 delete @ARRAY[0 .. $#ARRAY];
1211 But both of these are slower than just assigning the empty list
1212 or undefining %HASH or @ARRAY:
1214 %HASH = (); # completely empty %HASH
1215 undef %HASH; # forget %HASH ever existed
1217 @ARRAY = (); # completely empty @ARRAY
1218 undef @ARRAY; # forget @ARRAY ever existed
1220 Note that the EXPR can be arbitrarily complicated as long as the final
1221 operation is a hash element, array element, hash slice, or array slice
1224 delete $ref->[$x][$y]{$key};
1225 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1227 delete $ref->[$x][$y][$index];
1228 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1231 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1233 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1234 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1235 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1236 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1237 an C<eval(),> the error message is stuffed into C<$@> and the
1238 C<eval> is terminated with the undefined value. This makes
1239 C<die> the way to raise an exception.
1241 Equivalent examples:
1243 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1244 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1246 If the last element of LIST does not end in a newline, the current
1247 script line number and input line number (if any) are also printed,
1248 and a newline is supplied. Note that the "input line number" (also
1249 known as "chunk") is subject to whatever notion of "line" happens to
1250 be currently in effect, and is also available as the special variable
1251 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1253 Hint: sometimes appending C<", stopped"> to your message will cause it
1254 to make better sense when the string C<"at foo line 123"> is appended.
1255 Suppose you are running script "canasta".
1257 die "/etc/games is no good";
1258 die "/etc/games is no good, stopped";
1260 produce, respectively
1262 /etc/games is no good at canasta line 123.
1263 /etc/games is no good, stopped at canasta line 123.
1265 See also exit(), warn(), and the Carp module.
1267 If LIST is empty and C<$@> already contains a value (typically from a
1268 previous eval) that value is reused after appending C<"\t...propagated">.
1269 This is useful for propagating exceptions:
1272 die unless $@ =~ /Expected exception/;
1274 If LIST is empty and C<$@> contains an object reference that has a
1275 C<PROPAGATE> method, that method will be called with additional file
1276 and line number parameters. The return value replaces the value in
1277 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1280 If C<$@> is empty then the string C<"Died"> is used.
1282 die() can also be called with a reference argument. If this happens to be
1283 trapped within an eval(), $@ contains the reference. This behavior permits
1284 a more elaborate exception handling implementation using objects that
1285 maintain arbitrary state about the nature of the exception. Such a scheme
1286 is sometimes preferable to matching particular string values of $@ using
1287 regular expressions. Here's an example:
1289 use Scalar::Util 'blessed';
1291 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1293 if (blessed($@) && $@->isa("Some::Module::Exception")) {
1294 # handle Some::Module::Exception
1297 # handle all other possible exceptions
1301 Because perl will stringify uncaught exception messages before displaying
1302 them, you may want to overload stringification operations on such custom
1303 exception objects. See L<overload> for details about that.
1305 You can arrange for a callback to be run just before the C<die>
1306 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1307 handler will be called with the error text and can change the error
1308 message, if it sees fit, by calling C<die> again. See
1309 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1310 L<"eval BLOCK"> for some examples. Although this feature was
1311 to be run only right before your program was to exit, this is not
1312 currently the case--the C<$SIG{__DIE__}> hook is currently called
1313 even inside eval()ed blocks/strings! If one wants the hook to do
1314 nothing in such situations, put
1318 as the first line of the handler (see L<perlvar/$^S>). Because
1319 this promotes strange action at a distance, this counterintuitive
1320 behavior may be fixed in a future release.
1325 Not really a function. Returns the value of the last command in the
1326 sequence of commands indicated by BLOCK. When modified by the C<while> or
1327 C<until> loop modifier, executes the BLOCK once before testing the loop
1328 condition. (On other statements the loop modifiers test the conditional
1331 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1332 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1333 See L<perlsyn> for alternative strategies.
1335 =item do SUBROUTINE(LIST)
1338 This form of subroutine call is deprecated. See L<perlsub>.
1343 Uses the value of EXPR as a filename and executes the contents of the
1344 file as a Perl script.
1352 except that it's more efficient and concise, keeps track of the current
1353 filename for error messages, searches the @INC directories, and updates
1354 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1355 variables. It also differs in that code evaluated with C<do FILENAME>
1356 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1357 same, however, in that it does reparse the file every time you call it,
1358 so you probably don't want to do this inside a loop.
1360 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1361 error. If C<do> can read the file but cannot compile it, it
1362 returns undef and sets an error message in C<$@>. If the file is
1363 successfully compiled, C<do> returns the value of the last expression
1366 Note that inclusion of library modules is better done with the
1367 C<use> and C<require> operators, which also do automatic error checking
1368 and raise an exception if there's a problem.
1370 You might like to use C<do> to read in a program configuration
1371 file. Manual error checking can be done this way:
1373 # read in config files: system first, then user
1374 for $file ("/share/prog/defaults.rc",
1375 "$ENV{HOME}/.someprogrc")
1377 unless ($return = do $file) {
1378 warn "couldn't parse $file: $@" if $@;
1379 warn "couldn't do $file: $!" unless defined $return;
1380 warn "couldn't run $file" unless $return;
1385 X<dump> X<core> X<undump>
1389 This function causes an immediate core dump. See also the B<-u>
1390 command-line switch in L<perlrun>, which does the same thing.
1391 Primarily this is so that you can use the B<undump> program (not
1392 supplied) to turn your core dump into an executable binary after
1393 having initialized all your variables at the beginning of the
1394 program. When the new binary is executed it will begin by executing
1395 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1396 Think of it as a goto with an intervening core dump and reincarnation.
1397 If C<LABEL> is omitted, restarts the program from the top.
1399 B<WARNING>: Any files opened at the time of the dump will I<not>
1400 be open any more when the program is reincarnated, with possible
1401 resulting confusion on the part of Perl.
1403 This function is now largely obsolete, partly because it's very
1404 hard to convert a core file into an executable, and because the
1405 real compiler backends for generating portable bytecode and compilable
1406 C code have superseded it. That's why you should now invoke it as
1407 C<CORE::dump()>, if you don't want to be warned against a possible
1410 If you're looking to use L<dump> to speed up your program, consider
1411 generating bytecode or native C code as described in L<perlcc>. If
1412 you're just trying to accelerate a CGI script, consider using the
1413 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1414 You might also consider autoloading or selfloading, which at least
1415 make your program I<appear> to run faster.
1418 X<each> X<hash, iterator>
1420 When called in list context, returns a 2-element list consisting of the
1421 key and value for the next element of a hash, so that you can iterate over
1422 it. When called in scalar context, returns only the key for the next
1423 element in the hash.
1425 Entries are returned in an apparently random order. The actual random
1426 order is subject to change in future versions of perl, but it is
1427 guaranteed to be in the same order as either the C<keys> or C<values>
1428 function would produce on the same (unmodified) hash. Since Perl
1429 5.8.1 the ordering is different even between different runs of Perl
1430 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1432 When the hash is entirely read, a null array is returned in list context
1433 (which when assigned produces a false (C<0>) value), and C<undef> in
1434 scalar context. The next call to C<each> after that will start iterating
1435 again. There is a single iterator for each hash, shared by all C<each>,
1436 C<keys>, and C<values> function calls in the program; it can be reset by
1437 reading all the elements from the hash, or by evaluating C<keys HASH> or
1438 C<values HASH>. If you add or delete elements of a hash while you're
1439 iterating over it, you may get entries skipped or duplicated, so
1440 don't. Exception: It is always safe to delete the item most recently
1441 returned by C<each()>, which means that the following code will work:
1443 while (($key, $value) = each %hash) {
1445 delete $hash{$key}; # This is safe
1448 The following prints out your environment like the printenv(1) program,
1449 only in a different order:
1451 while (($key,$value) = each %ENV) {
1452 print "$key=$value\n";
1455 See also C<keys>, C<values> and C<sort>.
1457 =item eof FILEHANDLE
1466 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1467 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1468 gives the real filehandle. (Note that this function actually
1469 reads a character and then C<ungetc>s it, so isn't very useful in an
1470 interactive context.) Do not read from a terminal file (or call
1471 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1472 as terminals may lose the end-of-file condition if you do.
1474 An C<eof> without an argument uses the last file read. Using C<eof()>
1475 with empty parentheses is very different. It refers to the pseudo file
1476 formed from the files listed on the command line and accessed via the
1477 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1478 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1479 used will cause C<@ARGV> to be examined to determine if input is
1480 available. Similarly, an C<eof()> after C<< <> >> has returned
1481 end-of-file will assume you are processing another C<@ARGV> list,
1482 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1483 see L<perlop/"I/O Operators">.
1485 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1486 detect the end of each file, C<eof()> will only detect the end of the
1487 last file. Examples:
1489 # reset line numbering on each input file
1491 next if /^\s*#/; # skip comments
1494 close ARGV if eof; # Not eof()!
1497 # insert dashes just before last line of last file
1499 if (eof()) { # check for end of last file
1500 print "--------------\n";
1503 last if eof(); # needed if we're reading from a terminal
1506 Practical hint: you almost never need to use C<eof> in Perl, because the
1507 input operators typically return C<undef> when they run out of data, or if
1511 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1512 X<error, handling> X<exception, handling>
1518 In the first form, the return value of EXPR is parsed and executed as if it
1519 were a little Perl program. The value of the expression (which is itself
1520 determined within scalar context) is first parsed, and if there weren't any
1521 errors, executed in the lexical context of the current Perl program, so
1522 that any variable settings or subroutine and format definitions remain
1523 afterwards. Note that the value is parsed every time the C<eval> executes.
1524 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1525 delay parsing and subsequent execution of the text of EXPR until run time.
1527 In the second form, the code within the BLOCK is parsed only once--at the
1528 same time the code surrounding the C<eval> itself was parsed--and executed
1529 within the context of the current Perl program. This form is typically
1530 used to trap exceptions more efficiently than the first (see below), while
1531 also providing the benefit of checking the code within BLOCK at compile
1534 The final semicolon, if any, may be omitted from the value of EXPR or within
1537 In both forms, the value returned is the value of the last expression
1538 evaluated inside the mini-program; a return statement may be also used, just
1539 as with subroutines. The expression providing the return value is evaluated
1540 in void, scalar, or list context, depending on the context of the C<eval>
1541 itself. See L</wantarray> for more on how the evaluation context can be
1544 If there is a syntax error or runtime error, or a C<die> statement is
1545 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1546 error message. If there was no error, C<$@> is guaranteed to be a null
1547 string. Beware that using C<eval> neither silences perl from printing
1548 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1549 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1550 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1551 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1553 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1554 determining whether a particular feature (such as C<socket> or C<symlink>)
1555 is implemented. It is also Perl's exception trapping mechanism, where
1556 the die operator is used to raise exceptions.
1558 If the code to be executed doesn't vary, you may use the eval-BLOCK
1559 form to trap run-time errors without incurring the penalty of
1560 recompiling each time. The error, if any, is still returned in C<$@>.
1563 # make divide-by-zero nonfatal
1564 eval { $answer = $a / $b; }; warn $@ if $@;
1566 # same thing, but less efficient
1567 eval '$answer = $a / $b'; warn $@ if $@;
1569 # a compile-time error
1570 eval { $answer = }; # WRONG
1573 eval '$answer ='; # sets $@
1575 Using the C<eval{}> form as an exception trap in libraries does have some
1576 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1577 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1578 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1579 as shown in this example:
1581 # a very private exception trap for divide-by-zero
1582 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1585 This is especially significant, given that C<__DIE__> hooks can call
1586 C<die> again, which has the effect of changing their error messages:
1588 # __DIE__ hooks may modify error messages
1590 local $SIG{'__DIE__'} =
1591 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1592 eval { die "foo lives here" };
1593 print $@ if $@; # prints "bar lives here"
1596 Because this promotes action at a distance, this counterintuitive behavior
1597 may be fixed in a future release.
1599 With an C<eval>, you should be especially careful to remember what's
1600 being looked at when:
1606 eval { $x }; # CASE 4
1608 eval "\$$x++"; # CASE 5
1611 Cases 1 and 2 above behave identically: they run the code contained in
1612 the variable $x. (Although case 2 has misleading double quotes making
1613 the reader wonder what else might be happening (nothing is).) Cases 3
1614 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1615 does nothing but return the value of $x. (Case 4 is preferred for
1616 purely visual reasons, but it also has the advantage of compiling at
1617 compile-time instead of at run-time.) Case 5 is a place where
1618 normally you I<would> like to use double quotes, except that in this
1619 particular situation, you can just use symbolic references instead, as
1622 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1623 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1625 Note that as a very special case, an C<eval ''> executed within the C<DB>
1626 package doesn't see the usual surrounding lexical scope, but rather the
1627 scope of the first non-DB piece of code that called it. You don't normally
1628 need to worry about this unless you are writing a Perl debugger.
1633 =item exec PROGRAM LIST
1635 The C<exec> function executes a system command I<and never returns>--
1636 use C<system> instead of C<exec> if you want it to return. It fails and
1637 returns false only if the command does not exist I<and> it is executed
1638 directly instead of via your system's command shell (see below).
1640 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1641 warns you if there is a following statement which isn't C<die>, C<warn>,
1642 or C<exit> (if C<-w> is set - but you always do that). If you
1643 I<really> want to follow an C<exec> with some other statement, you
1644 can use one of these styles to avoid the warning:
1646 exec ('foo') or print STDERR "couldn't exec foo: $!";
1647 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1649 If there is more than one argument in LIST, or if LIST is an array
1650 with more than one value, calls execvp(3) with the arguments in LIST.
1651 If there is only one scalar argument or an array with one element in it,
1652 the argument is checked for shell metacharacters, and if there are any,
1653 the entire argument is passed to the system's command shell for parsing
1654 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1655 If there are no shell metacharacters in the argument, it is split into
1656 words and passed directly to C<execvp>, which is more efficient.
1659 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1660 exec "sort $outfile | uniq";
1662 If you don't really want to execute the first argument, but want to lie
1663 to the program you are executing about its own name, you can specify
1664 the program you actually want to run as an "indirect object" (without a
1665 comma) in front of the LIST. (This always forces interpretation of the
1666 LIST as a multivalued list, even if there is only a single scalar in
1669 $shell = '/bin/csh';
1670 exec $shell '-sh'; # pretend it's a login shell
1674 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1676 When the arguments get executed via the system shell, results will
1677 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1680 Using an indirect object with C<exec> or C<system> is also more
1681 secure. This usage (which also works fine with system()) forces
1682 interpretation of the arguments as a multivalued list, even if the
1683 list had just one argument. That way you're safe from the shell
1684 expanding wildcards or splitting up words with whitespace in them.
1686 @args = ( "echo surprise" );
1688 exec @args; # subject to shell escapes
1690 exec { $args[0] } @args; # safe even with one-arg list
1692 The first version, the one without the indirect object, ran the I<echo>
1693 program, passing it C<"surprise"> an argument. The second version
1694 didn't--it tried to run a program literally called I<"echo surprise">,
1695 didn't find it, and set C<$?> to a non-zero value indicating failure.
1697 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1698 output before the exec, but this may not be supported on some platforms
1699 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1700 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1701 open handles in order to avoid lost output.
1703 Note that C<exec> will not call your C<END> blocks, nor will it call
1704 any C<DESTROY> methods in your objects.
1707 X<exists> X<autovivification>
1709 Given an expression that specifies a hash element or array element,
1710 returns true if the specified element in the hash or array has ever
1711 been initialized, even if the corresponding value is undefined. The
1712 element is not autovivified if it doesn't exist.
1714 print "Exists\n" if exists $hash{$key};
1715 print "Defined\n" if defined $hash{$key};
1716 print "True\n" if $hash{$key};
1718 print "Exists\n" if exists $array[$index];
1719 print "Defined\n" if defined $array[$index];
1720 print "True\n" if $array[$index];
1722 A hash or array element can be true only if it's defined, and defined if
1723 it exists, but the reverse doesn't necessarily hold true.
1725 Given an expression that specifies the name of a subroutine,
1726 returns true if the specified subroutine has ever been declared, even
1727 if it is undefined. Mentioning a subroutine name for exists or defined
1728 does not count as declaring it. Note that a subroutine which does not
1729 exist may still be callable: its package may have an C<AUTOLOAD>
1730 method that makes it spring into existence the first time that it is
1731 called -- see L<perlsub>.
1733 print "Exists\n" if exists &subroutine;
1734 print "Defined\n" if defined &subroutine;
1736 Note that the EXPR can be arbitrarily complicated as long as the final
1737 operation is a hash or array key lookup or subroutine name:
1739 if (exists $ref->{A}->{B}->{$key}) { }
1740 if (exists $hash{A}{B}{$key}) { }
1742 if (exists $ref->{A}->{B}->[$ix]) { }
1743 if (exists $hash{A}{B}[$ix]) { }
1745 if (exists &{$ref->{A}{B}{$key}}) { }
1747 Although the deepest nested array or hash will not spring into existence
1748 just because its existence was tested, any intervening ones will.
1749 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1750 into existence due to the existence test for the $key element above.
1751 This happens anywhere the arrow operator is used, including even:
1754 if (exists $ref->{"Some key"}) { }
1755 print $ref; # prints HASH(0x80d3d5c)
1757 This surprising autovivification in what does not at first--or even
1758 second--glance appear to be an lvalue context may be fixed in a future
1761 Use of a subroutine call, rather than a subroutine name, as an argument
1762 to exists() is an error.
1765 exists &sub(); # Error
1768 X<exit> X<terminate> X<abort>
1772 Evaluates EXPR and exits immediately with that value. Example:
1775 exit 0 if $ans =~ /^[Xx]/;
1777 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1778 universally recognized values for EXPR are C<0> for success and C<1>
1779 for error; other values are subject to interpretation depending on the
1780 environment in which the Perl program is running. For example, exiting
1781 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1782 the mailer to return the item undelivered, but that's not true everywhere.
1784 Don't use C<exit> to abort a subroutine if there's any chance that
1785 someone might want to trap whatever error happened. Use C<die> instead,
1786 which can be trapped by an C<eval>.
1788 The exit() function does not always exit immediately. It calls any
1789 defined C<END> routines first, but these C<END> routines may not
1790 themselves abort the exit. Likewise any object destructors that need to
1791 be called are called before the real exit. If this is a problem, you
1792 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1793 See L<perlmod> for details.
1796 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1800 Returns I<e> (the natural logarithm base) to the power of EXPR.
1801 If EXPR is omitted, gives C<exp($_)>.
1803 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1806 Implements the fcntl(2) function. You'll probably have to say
1810 first to get the correct constant definitions. Argument processing and
1811 value return works just like C<ioctl> below.
1815 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1816 or die "can't fcntl F_GETFL: $!";
1818 You don't have to check for C<defined> on the return from C<fcntl>.
1819 Like C<ioctl>, it maps a C<0> return from the system call into
1820 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1821 in numeric context. It is also exempt from the normal B<-w> warnings
1822 on improper numeric conversions.
1824 Note that C<fcntl> will produce a fatal error if used on a machine that
1825 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1826 manpage to learn what functions are available on your system.
1828 Here's an example of setting a filehandle named C<REMOTE> to be
1829 non-blocking at the system level. You'll have to negotiate C<$|>
1830 on your own, though.
1832 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1834 $flags = fcntl(REMOTE, F_GETFL, 0)
1835 or die "Can't get flags for the socket: $!\n";
1837 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1838 or die "Can't set flags for the socket: $!\n";
1840 =item fileno FILEHANDLE
1843 Returns the file descriptor for a filehandle, or undefined if the
1844 filehandle is not open. This is mainly useful for constructing
1845 bitmaps for C<select> and low-level POSIX tty-handling operations.
1846 If FILEHANDLE is an expression, the value is taken as an indirect
1847 filehandle, generally its name.
1849 You can use this to find out whether two handles refer to the
1850 same underlying descriptor:
1852 if (fileno(THIS) == fileno(THAT)) {
1853 print "THIS and THAT are dups\n";
1856 (Filehandles connected to memory objects via new features of C<open> may
1857 return undefined even though they are open.)
1860 =item flock FILEHANDLE,OPERATION
1861 X<flock> X<lock> X<locking>
1863 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1864 for success, false on failure. Produces a fatal error if used on a
1865 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1866 C<flock> is Perl's portable file locking interface, although it locks
1867 only entire files, not records.
1869 Two potentially non-obvious but traditional C<flock> semantics are
1870 that it waits indefinitely until the lock is granted, and that its locks
1871 B<merely advisory>. Such discretionary locks are more flexible, but offer
1872 fewer guarantees. This means that programs that do not also use C<flock>
1873 may modify files locked with C<flock>. See L<perlport>,
1874 your port's specific documentation, or your system-specific local manpages
1875 for details. It's best to assume traditional behavior if you're writing
1876 portable programs. (But if you're not, you should as always feel perfectly
1877 free to write for your own system's idiosyncrasies (sometimes called
1878 "features"). Slavish adherence to portability concerns shouldn't get
1879 in the way of your getting your job done.)
1881 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1882 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1883 you can use the symbolic names if you import them from the Fcntl module,
1884 either individually, or as a group using the ':flock' tag. LOCK_SH
1885 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1886 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1887 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1888 waiting for the lock (check the return status to see if you got it).
1890 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1891 before locking or unlocking it.
1893 Note that the emulation built with lockf(3) doesn't provide shared
1894 locks, and it requires that FILEHANDLE be open with write intent. These
1895 are the semantics that lockf(3) implements. Most if not all systems
1896 implement lockf(3) in terms of fcntl(2) locking, though, so the
1897 differing semantics shouldn't bite too many people.
1899 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1900 be open with read intent to use LOCK_SH and requires that it be open
1901 with write intent to use LOCK_EX.
1903 Note also that some versions of C<flock> cannot lock things over the
1904 network; you would need to use the more system-specific C<fcntl> for
1905 that. If you like you can force Perl to ignore your system's flock(2)
1906 function, and so provide its own fcntl(2)-based emulation, by passing
1907 the switch C<-Ud_flock> to the F<Configure> program when you configure
1910 Here's a mailbox appender for BSD systems.
1912 use Fcntl ':flock'; # import LOCK_* constants
1915 flock(MBOX,LOCK_EX);
1916 # and, in case someone appended
1917 # while we were waiting...
1922 flock(MBOX,LOCK_UN);
1925 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1926 or die "Can't open mailbox: $!";
1929 print MBOX $msg,"\n\n";
1932 On systems that support a real flock(), locks are inherited across fork()
1933 calls, whereas those that must resort to the more capricious fcntl()
1934 function lose the locks, making it harder to write servers.
1936 See also L<DB_File> for other flock() examples.
1939 X<fork> X<child> X<parent>
1941 Does a fork(2) system call to create a new process running the
1942 same program at the same point. It returns the child pid to the
1943 parent process, C<0> to the child process, or C<undef> if the fork is
1944 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1945 are shared, while everything else is copied. On most systems supporting
1946 fork(), great care has gone into making it extremely efficient (for
1947 example, using copy-on-write technology on data pages), making it the
1948 dominant paradigm for multitasking over the last few decades.
1950 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1951 output before forking the child process, but this may not be supported
1952 on some platforms (see L<perlport>). To be safe, you may need to set
1953 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1954 C<IO::Handle> on any open handles in order to avoid duplicate output.
1956 If you C<fork> without ever waiting on your children, you will
1957 accumulate zombies. On some systems, you can avoid this by setting
1958 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1959 forking and reaping moribund children.
1961 Note that if your forked child inherits system file descriptors like
1962 STDIN and STDOUT that are actually connected by a pipe or socket, even
1963 if you exit, then the remote server (such as, say, a CGI script or a
1964 backgrounded job launched from a remote shell) won't think you're done.
1965 You should reopen those to F</dev/null> if it's any issue.
1970 Declare a picture format for use by the C<write> function. For
1974 Test: @<<<<<<<< @||||| @>>>>>
1975 $str, $%, '$' . int($num)
1979 $num = $cost/$quantity;
1983 See L<perlform> for many details and examples.
1985 =item formline PICTURE,LIST
1988 This is an internal function used by C<format>s, though you may call it,
1989 too. It formats (see L<perlform>) a list of values according to the
1990 contents of PICTURE, placing the output into the format output
1991 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1992 Eventually, when a C<write> is done, the contents of
1993 C<$^A> are written to some filehandle. You could also read C<$^A>
1994 and then set C<$^A> back to C<"">. Note that a format typically
1995 does one C<formline> per line of form, but the C<formline> function itself
1996 doesn't care how many newlines are embedded in the PICTURE. This means
1997 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1998 You may therefore need to use multiple formlines to implement a single
1999 record format, just like the format compiler.
2001 Be careful if you put double quotes around the picture, because an C<@>
2002 character may be taken to mean the beginning of an array name.
2003 C<formline> always returns true. See L<perlform> for other examples.
2005 =item getc FILEHANDLE
2006 X<getc> X<getchar> X<character> X<file, read>
2010 Returns the next character from the input file attached to FILEHANDLE,
2011 or the undefined value at end of file, or if there was an error (in
2012 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2013 STDIN. This is not particularly efficient. However, it cannot be
2014 used by itself to fetch single characters without waiting for the user
2015 to hit enter. For that, try something more like:
2018 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2021 system "stty", '-icanon', 'eol', "\001";
2027 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2030 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2034 Determination of whether $BSD_STYLE should be set
2035 is left as an exercise to the reader.
2037 The C<POSIX::getattr> function can do this more portably on
2038 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2039 module from your nearest CPAN site; details on CPAN can be found on
2043 X<getlogin> X<login>
2045 This implements the C library function of the same name, which on most
2046 systems returns the current login from F</etc/utmp>, if any. If null,
2049 $login = getlogin || getpwuid($<) || "Kilroy";
2051 Do not consider C<getlogin> for authentication: it is not as
2052 secure as C<getpwuid>.
2054 =item getpeername SOCKET
2055 X<getpeername> X<peer>
2057 Returns the packed sockaddr address of other end of the SOCKET connection.
2060 $hersockaddr = getpeername(SOCK);
2061 ($port, $iaddr) = sockaddr_in($hersockaddr);
2062 $herhostname = gethostbyaddr($iaddr, AF_INET);
2063 $herstraddr = inet_ntoa($iaddr);
2068 Returns the current process group for the specified PID. Use
2069 a PID of C<0> to get the current process group for the
2070 current process. Will raise an exception if used on a machine that
2071 doesn't implement getpgrp(2). If PID is omitted, returns process
2072 group of current process. Note that the POSIX version of C<getpgrp>
2073 does not accept a PID argument, so only C<PID==0> is truly portable.
2076 X<getppid> X<parent> X<pid>
2078 Returns the process id of the parent process.
2080 Note for Linux users: on Linux, the C functions C<getpid()> and
2081 C<getppid()> return different values from different threads. In order to
2082 be portable, this behavior is not reflected by the perl-level function
2083 C<getppid()>, that returns a consistent value across threads. If you want
2084 to call the underlying C<getppid()>, you may use the CPAN module
2087 =item getpriority WHICH,WHO
2088 X<getpriority> X<priority> X<nice>
2090 Returns the current priority for a process, a process group, or a user.
2091 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2092 machine that doesn't implement getpriority(2).
2095 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2096 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2097 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2098 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2099 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2100 X<endnetent> X<endprotoent> X<endservent>
2104 =item gethostbyname NAME
2106 =item getnetbyname NAME
2108 =item getprotobyname NAME
2114 =item getservbyname NAME,PROTO
2116 =item gethostbyaddr ADDR,ADDRTYPE
2118 =item getnetbyaddr ADDR,ADDRTYPE
2120 =item getprotobynumber NUMBER
2122 =item getservbyport PORT,PROTO
2140 =item sethostent STAYOPEN
2142 =item setnetent STAYOPEN
2144 =item setprotoent STAYOPEN
2146 =item setservent STAYOPEN
2160 These routines perform the same functions as their counterparts in the
2161 system library. In list context, the return values from the
2162 various get routines are as follows:
2164 ($name,$passwd,$uid,$gid,
2165 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2166 ($name,$passwd,$gid,$members) = getgr*
2167 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2168 ($name,$aliases,$addrtype,$net) = getnet*
2169 ($name,$aliases,$proto) = getproto*
2170 ($name,$aliases,$port,$proto) = getserv*
2172 (If the entry doesn't exist you get a null list.)
2174 The exact meaning of the $gcos field varies but it usually contains
2175 the real name of the user (as opposed to the login name) and other
2176 information pertaining to the user. Beware, however, that in many
2177 system users are able to change this information and therefore it
2178 cannot be trusted and therefore the $gcos is tainted (see
2179 L<perlsec>). The $passwd and $shell, user's encrypted password and
2180 login shell, are also tainted, because of the same reason.
2182 In scalar context, you get the name, unless the function was a
2183 lookup by name, in which case you get the other thing, whatever it is.
2184 (If the entry doesn't exist you get the undefined value.) For example:
2186 $uid = getpwnam($name);
2187 $name = getpwuid($num);
2189 $gid = getgrnam($name);
2190 $name = getgrgid($num);
2194 In I<getpw*()> the fields $quota, $comment, and $expire are special
2195 cases in the sense that in many systems they are unsupported. If the
2196 $quota is unsupported, it is an empty scalar. If it is supported, it
2197 usually encodes the disk quota. If the $comment field is unsupported,
2198 it is an empty scalar. If it is supported it usually encodes some
2199 administrative comment about the user. In some systems the $quota
2200 field may be $change or $age, fields that have to do with password
2201 aging. In some systems the $comment field may be $class. The $expire
2202 field, if present, encodes the expiration period of the account or the
2203 password. For the availability and the exact meaning of these fields
2204 in your system, please consult your getpwnam(3) documentation and your
2205 F<pwd.h> file. You can also find out from within Perl what your
2206 $quota and $comment fields mean and whether you have the $expire field
2207 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2208 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2209 files are only supported if your vendor has implemented them in the
2210 intuitive fashion that calling the regular C library routines gets the
2211 shadow versions if you're running under privilege or if there exists
2212 the shadow(3) functions as found in System V (this includes Solaris
2213 and Linux.) Those systems that implement a proprietary shadow password
2214 facility are unlikely to be supported.
2216 The $members value returned by I<getgr*()> is a space separated list of
2217 the login names of the members of the group.
2219 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2220 C, it will be returned to you via C<$?> if the function call fails. The
2221 C<@addrs> value returned by a successful call is a list of the raw
2222 addresses returned by the corresponding system library call. In the
2223 Internet domain, each address is four bytes long and you can unpack it
2224 by saying something like:
2226 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2228 The Socket library makes this slightly easier:
2231 $iaddr = inet_aton("127.1"); # or whatever address
2232 $name = gethostbyaddr($iaddr, AF_INET);
2234 # or going the other way
2235 $straddr = inet_ntoa($iaddr);
2237 If you get tired of remembering which element of the return list
2238 contains which return value, by-name interfaces are provided
2239 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2240 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2241 and C<User::grent>. These override the normal built-ins, supplying
2242 versions that return objects with the appropriate names
2243 for each field. For example:
2247 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2249 Even though it looks like they're the same method calls (uid),
2250 they aren't, because a C<File::stat> object is different from
2251 a C<User::pwent> object.
2253 =item getsockname SOCKET
2256 Returns the packed sockaddr address of this end of the SOCKET connection,
2257 in case you don't know the address because you have several different
2258 IPs that the connection might have come in on.
2261 $mysockaddr = getsockname(SOCK);
2262 ($port, $myaddr) = sockaddr_in($mysockaddr);
2263 printf "Connect to %s [%s]\n",
2264 scalar gethostbyaddr($myaddr, AF_INET),
2267 =item getsockopt SOCKET,LEVEL,OPTNAME
2270 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2271 Options may exist at multiple protocol levels depending on the socket
2272 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2273 C<Socket> module) will exist. To query options at another level the
2274 protocol number of the appropriate protocol controlling the option
2275 should be supplied. For example, to indicate that an option is to be
2276 interpreted by the TCP protocol, LEVEL should be set to the protocol
2277 number of TCP, which you can get using getprotobyname.
2279 The call returns a packed string representing the requested socket option,
2280 or C<undef> if there is an error (the error reason will be in $!). What
2281 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2282 your system documentation for details. A very common case however is that
2283 the option is an integer, in which case the result will be a packed
2284 integer which you can decode using unpack with the C<i> (or C<I>) format.
2286 An example testing if Nagle's algorithm is turned on on a socket:
2288 use Socket qw(:all);
2290 defined(my $tcp = getprotobyname("tcp"))
2291 or die "Could not determine the protocol number for tcp";
2292 # my $tcp = IPPROTO_TCP; # Alternative
2293 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2294 or die "Could not query TCP_NODELAY socket option: $!";
2295 my $nodelay = unpack("I", $packed);
2296 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2300 X<glob> X<wildcard> X<filename, expansion> X<expand>
2304 In list context, returns a (possibly empty) list of filename expansions on
2305 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2306 scalar context, glob iterates through such filename expansions, returning
2307 undef when the list is exhausted. This is the internal function
2308 implementing the C<< <*.c> >> operator, but you can use it directly. If
2309 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2310 more detail in L<perlop/"I/O Operators">.
2312 Beginning with v5.6.0, this operator is implemented using the standard
2313 C<File::Glob> extension. See L<File::Glob> for details.
2316 X<gmtime> X<UTC> X<Greenwich>
2320 Works just like L<localtime> but the returned values are
2321 localized for the standard Greenwich time zone.
2323 Note: when called in list context, $isdst, the last value
2324 returned by gmtime is always C<0>. There is no
2325 Daylight Saving Time in GMT.
2327 See L<perlport/gmtime> for portability concerns.
2330 X<goto> X<jump> X<jmp>
2336 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2337 execution there. It may not be used to go into any construct that
2338 requires initialization, such as a subroutine or a C<foreach> loop. It
2339 also can't be used to go into a construct that is optimized away,
2340 or to get out of a block or subroutine given to C<sort>.
2341 It can be used to go almost anywhere else within the dynamic scope,
2342 including out of subroutines, but it's usually better to use some other
2343 construct such as C<last> or C<die>. The author of Perl has never felt the
2344 need to use this form of C<goto> (in Perl, that is--C is another matter).
2345 (The difference being that C does not offer named loops combined with
2346 loop control. Perl does, and this replaces most structured uses of C<goto>
2347 in other languages.)
2349 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2350 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2351 necessarily recommended if you're optimizing for maintainability:
2353 goto ("FOO", "BAR", "GLARCH")[$i];
2355 The C<goto-&NAME> form is quite different from the other forms of
2356 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2357 doesn't have the stigma associated with other gotos. Instead, it
2358 exits the current subroutine (losing any changes set by local()) and
2359 immediately calls in its place the named subroutine using the current
2360 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2361 load another subroutine and then pretend that the other subroutine had
2362 been called in the first place (except that any modifications to C<@_>
2363 in the current subroutine are propagated to the other subroutine.)
2364 After the C<goto>, not even C<caller> will be able to tell that this
2365 routine was called first.
2367 NAME needn't be the name of a subroutine; it can be a scalar variable
2368 containing a code reference, or a block that evaluates to a code
2371 =item grep BLOCK LIST
2374 =item grep EXPR,LIST
2376 This is similar in spirit to, but not the same as, grep(1) and its
2377 relatives. In particular, it is not limited to using regular expressions.
2379 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2380 C<$_> to each element) and returns the list value consisting of those
2381 elements for which the expression evaluated to true. In scalar
2382 context, returns the number of times the expression was true.
2384 @foo = grep(!/^#/, @bar); # weed out comments
2388 @foo = grep {!/^#/} @bar; # weed out comments
2390 Note that C<$_> is an alias to the list value, so it can be used to
2391 modify the elements of the LIST. While this is useful and supported,
2392 it can cause bizarre results if the elements of LIST are not variables.
2393 Similarly, grep returns aliases into the original list, much as a for
2394 loop's index variable aliases the list elements. That is, modifying an
2395 element of a list returned by grep (for example, in a C<foreach>, C<map>
2396 or another C<grep>) actually modifies the element in the original list.
2397 This is usually something to be avoided when writing clear code.
2399 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2400 been declared with C<my $_>) then, in addition to being locally aliased to
2401 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2402 can't be seen from the outside, avoiding any potential side-effects.
2404 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2407 X<hex> X<hexadecimal>
2411 Interprets EXPR as a hex string and returns the corresponding value.
2412 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2413 L</oct>.) If EXPR is omitted, uses C<$_>.
2415 print hex '0xAf'; # prints '175'
2416 print hex 'aF'; # same
2418 Hex strings may only represent integers. Strings that would cause
2419 integer overflow trigger a warning. Leading whitespace is not stripped,
2420 unlike oct(). To present something as hex, look into L</printf>,
2421 L</sprintf>, or L</unpack>.
2426 There is no builtin C<import> function. It is just an ordinary
2427 method (subroutine) defined (or inherited) by modules that wish to export
2428 names to another module. The C<use> function calls the C<import> method
2429 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2431 =item index STR,SUBSTR,POSITION
2432 X<index> X<indexOf> X<InStr>
2434 =item index STR,SUBSTR
2436 The index function searches for one string within another, but without
2437 the wildcard-like behavior of a full regular-expression pattern match.
2438 It returns the position of the first occurrence of SUBSTR in STR at
2439 or after POSITION. If POSITION is omitted, starts searching from the
2440 beginning of the string. POSITION before the beginning of the string
2441 or after its end is treated as if it were the beginning or the end,
2442 respectively. POSITION and the return value are based at C<0> (or whatever
2443 you've set the C<$[> variable to--but don't do that). If the substring
2444 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2447 X<int> X<integer> X<truncate> X<trunc> X<floor>
2451 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2452 You should not use this function for rounding: one because it truncates
2453 towards C<0>, and two because machine representations of floating point
2454 numbers can sometimes produce counterintuitive results. For example,
2455 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2456 because it's really more like -268.99999999999994315658 instead. Usually,
2457 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2458 functions will serve you better than will int().
2460 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2463 Implements the ioctl(2) function. You'll probably first have to say
2465 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2467 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2468 exist or doesn't have the correct definitions you'll have to roll your
2469 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2470 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2471 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2472 written depending on the FUNCTION--a pointer to the string value of SCALAR
2473 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2474 has no string value but does have a numeric value, that value will be
2475 passed rather than a pointer to the string value. To guarantee this to be
2476 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2477 functions may be needed to manipulate the values of structures used by
2480 The return value of C<ioctl> (and C<fcntl>) is as follows:
2482 if OS returns: then Perl returns:
2484 0 string "0 but true"
2485 anything else that number
2487 Thus Perl returns true on success and false on failure, yet you can
2488 still easily determine the actual value returned by the operating
2491 $retval = ioctl(...) || -1;
2492 printf "System returned %d\n", $retval;
2494 The special string C<"0 but true"> is exempt from B<-w> complaints
2495 about improper numeric conversions.
2497 =item join EXPR,LIST
2500 Joins the separate strings of LIST into a single string with fields
2501 separated by the value of EXPR, and returns that new string. Example:
2503 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2505 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2506 first argument. Compare L</split>.
2511 Returns a list consisting of all the keys of the named hash.
2512 (In scalar context, returns the number of keys.)
2514 The keys are returned in an apparently random order. The actual
2515 random order is subject to change in future versions of perl, but it
2516 is guaranteed to be the same order as either the C<values> or C<each>
2517 function produces (given that the hash has not been modified). Since
2518 Perl 5.8.1 the ordering is different even between different runs of
2519 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2522 As a side effect, calling keys() resets the HASH's internal iterator
2523 (see L</each>). In particular, calling keys() in void context resets
2524 the iterator with no other overhead.
2526 Here is yet another way to print your environment:
2529 @values = values %ENV;
2531 print pop(@keys), '=', pop(@values), "\n";
2534 or how about sorted by key:
2536 foreach $key (sort(keys %ENV)) {
2537 print $key, '=', $ENV{$key}, "\n";
2540 The returned values are copies of the original keys in the hash, so
2541 modifying them will not affect the original hash. Compare L</values>.
2543 To sort a hash by value, you'll need to use a C<sort> function.
2544 Here's a descending numeric sort of a hash by its values:
2546 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2547 printf "%4d %s\n", $hash{$key}, $key;
2550 As an lvalue C<keys> allows you to increase the number of hash buckets
2551 allocated for the given hash. This can gain you a measure of efficiency if
2552 you know the hash is going to get big. (This is similar to pre-extending
2553 an array by assigning a larger number to $#array.) If you say
2557 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2558 in fact, since it rounds up to the next power of two. These
2559 buckets will be retained even if you do C<%hash = ()>, use C<undef
2560 %hash> if you want to free the storage while C<%hash> is still in scope.
2561 You can't shrink the number of buckets allocated for the hash using
2562 C<keys> in this way (but you needn't worry about doing this by accident,
2563 as trying has no effect).
2565 See also C<each>, C<values> and C<sort>.
2567 =item kill SIGNAL, LIST
2570 Sends a signal to a list of processes. Returns the number of
2571 processes successfully signaled (which is not necessarily the
2572 same as the number actually killed).
2574 $cnt = kill 1, $child1, $child2;
2577 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2578 system call will check whether it's possible to send a signal to it (that
2579 means, to be brief, that the process is owned by the same user, or we are
2580 the super-user). This is a useful way to check that a child process is
2581 alive and hasn't changed its UID. See L<perlport> for notes on the
2582 portability of this construct.
2584 Unlike in the shell, if SIGNAL is negative, it kills
2585 process groups instead of processes. (On System V, a negative I<PROCESS>
2586 number will also kill process groups, but that's not portable.) That
2587 means you usually want to use positive not negative signals. You may also
2588 use a signal name in quotes.
2590 See L<perlipc/"Signals"> for more details.
2597 The C<last> command is like the C<break> statement in C (as used in
2598 loops); it immediately exits the loop in question. If the LABEL is
2599 omitted, the command refers to the innermost enclosing loop. The
2600 C<continue> block, if any, is not executed:
2602 LINE: while (<STDIN>) {
2603 last LINE if /^$/; # exit when done with header
2607 C<last> cannot be used to exit a block which returns a value such as
2608 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2609 a grep() or map() operation.
2611 Note that a block by itself is semantically identical to a loop
2612 that executes once. Thus C<last> can be used to effect an early
2613 exit out of such a block.
2615 See also L</continue> for an illustration of how C<last>, C<next>, and
2623 Returns a lowercased version of EXPR. This is the internal function
2624 implementing the C<\L> escape in double-quoted strings. Respects
2625 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2626 and L<perlunicode> for more details about locale and Unicode support.
2628 If EXPR is omitted, uses C<$_>.
2631 X<lcfirst> X<lowercase>
2635 Returns the value of EXPR with the first character lowercased. This
2636 is the internal function implementing the C<\l> escape in
2637 double-quoted strings. Respects current LC_CTYPE locale if C<use
2638 locale> in force. See L<perllocale> and L<perlunicode> for more
2639 details about locale and Unicode support.
2641 If EXPR is omitted, uses C<$_>.
2648 Returns the length in I<characters> of the value of EXPR. If EXPR is
2649 omitted, returns length of C<$_>. Note that this cannot be used on
2650 an entire array or hash to find out how many elements these have.
2651 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2653 Note the I<characters>: if the EXPR is in Unicode, you will get the
2654 number of characters, not the number of bytes. To get the length
2655 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2657 =item link OLDFILE,NEWFILE
2660 Creates a new filename linked to the old filename. Returns true for
2661 success, false otherwise.
2663 =item listen SOCKET,QUEUESIZE
2666 Does the same thing that the listen system call does. Returns true if
2667 it succeeded, false otherwise. See the example in
2668 L<perlipc/"Sockets: Client/Server Communication">.
2673 You really probably want to be using C<my> instead, because C<local> isn't
2674 what most people think of as "local". See
2675 L<perlsub/"Private Variables via my()"> for details.
2677 A local modifies the listed variables to be local to the enclosing
2678 block, file, or eval. If more than one value is listed, the list must
2679 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2680 for details, including issues with tied arrays and hashes.
2682 =item localtime EXPR
2683 X<localtime> X<ctime>
2687 Converts a time as returned by the time function to a 9-element list
2688 with the time analyzed for the local time zone. Typically used as
2692 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2695 All list elements are numeric, and come straight out of the C `struct
2696 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2697 of the specified time.
2699 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2700 the range C<0..11> with 0 indicating January and 11 indicating December.
2701 This makes it easy to get a month name from a list:
2703 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2704 print "$abbr[$mon] $mday";
2705 # $mon=9, $mday=18 gives "Oct 18"
2707 C<$year> is the number of years since 1900, not just the last two digits
2708 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2709 to get a complete 4-digit year is simply:
2713 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2714 to do that, would you?
2716 To get the last two digits of the year (e.g., '01' in 2001) do:
2718 $year = sprintf("%02d", $year % 100);
2720 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2721 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2722 (or C<0..365> in leap years.)
2724 C<$isdst> is true if the specified time occurs during Daylight Saving
2725 Time, false otherwise.
2727 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2729 In scalar context, C<localtime()> returns the ctime(3) value:
2731 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2733 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2734 instead of local time use the L</gmtime> builtin. See also the
2735 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2736 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2737 and mktime(3) functions.
2739 To get somewhat similar but locale dependent date strings, set up your
2740 locale environment variables appropriately (please see L<perllocale>) and
2743 use POSIX qw(strftime);
2744 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2745 # or for GMT formatted appropriately for your locale:
2746 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2748 Note that the C<%a> and C<%b>, the short forms of the day of the week
2749 and the month of the year, may not necessarily be three characters wide.
2751 See L<perlport/localtime> for portability concerns.
2753 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2754 by-name access mechanism to the gmtime() and localtime() functions,
2757 For a comprehensive date and time representation look at the
2758 L<DateTime> module on CPAN.
2763 This function places an advisory lock on a shared variable, or referenced
2764 object contained in I<THING> until the lock goes out of scope.
2766 lock() is a "weak keyword" : this means that if you've defined a function
2767 by this name (before any calls to it), that function will be called
2768 instead. (However, if you've said C<use threads>, lock() is always a
2769 keyword.) See L<threads>.
2772 X<log> X<logarithm> X<e> X<ln> X<base>
2776 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2777 returns log of C<$_>. To get the log of another base, use basic algebra:
2778 The base-N log of a number is equal to the natural log of that number
2779 divided by the natural log of N. For example:
2783 return log($n)/log(10);
2786 See also L</exp> for the inverse operation.
2793 Does the same thing as the C<stat> function (including setting the
2794 special C<_> filehandle) but stats a symbolic link instead of the file
2795 the symbolic link points to. If symbolic links are unimplemented on
2796 your system, a normal C<stat> is done. For much more detailed
2797 information, please see the documentation for C<stat>.
2799 If EXPR is omitted, stats C<$_>.
2803 The match operator. See L<perlop>.
2805 =item map BLOCK LIST
2810 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2811 C<$_> to each element) and returns the list value composed of the
2812 results of each such evaluation. In scalar context, returns the
2813 total number of elements so generated. Evaluates BLOCK or EXPR in
2814 list context, so each element of LIST may produce zero, one, or
2815 more elements in the returned value.
2817 @chars = map(chr, @nums);
2819 translates a list of numbers to the corresponding characters. And
2821 %hash = map { getkey($_) => $_ } @array;
2823 is just a funny way to write
2826 foreach $_ (@array) {
2827 $hash{getkey($_)} = $_;
2830 Note that C<$_> is an alias to the list value, so it can be used to
2831 modify the elements of the LIST. While this is useful and supported,
2832 it can cause bizarre results if the elements of LIST are not variables.
2833 Using a regular C<foreach> loop for this purpose would be clearer in
2834 most cases. See also L</grep> for an array composed of those items of
2835 the original list for which the BLOCK or EXPR evaluates to true.
2837 If C<$_> is lexical in the scope where the C<map> appears (because it has
2838 been declared with C<my $_>) then, in addition to being locally aliased to
2839 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2840 can't be seen from the outside, avoiding any potential side-effects.
2842 C<{> starts both hash references and blocks, so C<map { ...> could be either
2843 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2844 ahead for the closing C<}> it has to take a guess at which its dealing with
2845 based what it finds just after the C<{>. Usually it gets it right, but if it
2846 doesn't it won't realize something is wrong until it gets to the C<}> and
2847 encounters the missing (or unexpected) comma. The syntax error will be
2848 reported close to the C<}> but you'll need to change something near the C<{>
2849 such as using a unary C<+> to give perl some help:
2851 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2852 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2853 %hash = map { ("\L$_", 1) } @array # this also works
2854 %hash = map { lc($_), 1 } @array # as does this.
2855 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2857 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2859 or to force an anon hash constructor use C<+{>
2861 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2863 and you get list of anonymous hashes each with only 1 entry.
2865 =item mkdir FILENAME,MASK
2866 X<mkdir> X<md> X<directory, create>
2868 =item mkdir FILENAME
2872 Creates the directory specified by FILENAME, with permissions
2873 specified by MASK (as modified by C<umask>). If it succeeds it
2874 returns true, otherwise it returns false and sets C<$!> (errno).
2875 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2878 In general, it is better to create directories with permissive MASK,
2879 and let the user modify that with their C<umask>, than it is to supply
2880 a restrictive MASK and give the user no way to be more permissive.
2881 The exceptions to this rule are when the file or directory should be
2882 kept private (mail files, for instance). The perlfunc(1) entry on
2883 C<umask> discusses the choice of MASK in more detail.
2885 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2886 number of trailing slashes. Some operating and filesystems do not get
2887 this right, so Perl automatically removes all trailing slashes to keep
2890 In order to recursively create a directory structure look at
2891 the C<mkpath> function of the L<File::Path> module.
2893 =item msgctl ID,CMD,ARG
2896 Calls the System V IPC function msgctl(2). You'll probably have to say
2900 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2901 then ARG must be a variable that will hold the returned C<msqid_ds>
2902 structure. Returns like C<ioctl>: the undefined value for error,
2903 C<"0 but true"> for zero, or the actual return value otherwise. See also
2904 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2906 =item msgget KEY,FLAGS
2909 Calls the System V IPC function msgget(2). Returns the message queue
2910 id, or the undefined value if there is an error. See also
2911 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2913 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2916 Calls the System V IPC function msgrcv to receive a message from
2917 message queue ID into variable VAR with a maximum message size of
2918 SIZE. Note that when a message is received, the message type as a
2919 native long integer will be the first thing in VAR, followed by the
2920 actual message. This packing may be opened with C<unpack("l! a*")>.
2921 Taints the variable. Returns true if successful, or false if there is
2922 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2923 C<IPC::SysV::Msg> documentation.
2925 =item msgsnd ID,MSG,FLAGS
2928 Calls the System V IPC function msgsnd to send the message MSG to the
2929 message queue ID. MSG must begin with the native long integer message
2930 type, and be followed by the length of the actual message, and finally
2931 the message itself. This kind of packing can be achieved with
2932 C<pack("l! a*", $type, $message)>. Returns true if successful,
2933 or false if there is an error. See also C<IPC::SysV>
2934 and C<IPC::SysV::Msg> documentation.
2941 =item my EXPR : ATTRS
2943 =item my TYPE EXPR : ATTRS
2945 A C<my> declares the listed variables to be local (lexically) to the
2946 enclosing block, file, or C<eval>. If more than one value is listed,
2947 the list must be placed in parentheses.
2949 The exact semantics and interface of TYPE and ATTRS are still
2950 evolving. TYPE is currently bound to the use of C<fields> pragma,
2951 and attributes are handled using the C<attributes> pragma, or starting
2952 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2953 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2954 L<attributes>, and L<Attribute::Handlers>.
2961 The C<next> command is like the C<continue> statement in C; it starts
2962 the next iteration of the loop:
2964 LINE: while (<STDIN>) {
2965 next LINE if /^#/; # discard comments
2969 Note that if there were a C<continue> block on the above, it would get
2970 executed even on discarded lines. If the LABEL is omitted, the command
2971 refers to the innermost enclosing loop.
2973 C<next> cannot be used to exit a block which returns a value such as
2974 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2975 a grep() or map() operation.
2977 Note that a block by itself is semantically identical to a loop
2978 that executes once. Thus C<next> will exit such a block early.
2980 See also L</continue> for an illustration of how C<last>, C<next>, and
2983 =item no Module VERSION LIST
2986 =item no Module VERSION
2988 =item no Module LIST
2992 See the C<use> function, of which C<no> is the opposite.
2995 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
2999 Interprets EXPR as an octal string and returns the corresponding
3000 value. (If EXPR happens to start off with C<0x>, interprets it as a
3001 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3002 binary string. Leading whitespace is ignored in all three cases.)
3003 The following will handle decimal, binary, octal, and hex in the standard
3006 $val = oct($val) if $val =~ /^0/;
3008 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3009 in octal), use sprintf() or printf():
3011 $perms = (stat("filename"))[2] & 07777;
3012 $oct_perms = sprintf "%lo", $perms;
3014 The oct() function is commonly used when a string such as C<644> needs
3015 to be converted into a file mode, for example. (Although perl will
3016 automatically convert strings into numbers as needed, this automatic
3017 conversion assumes base 10.)
3019 =item open FILEHANDLE,EXPR
3020 X<open> X<pipe> X<file, open> X<fopen>
3022 =item open FILEHANDLE,MODE,EXPR
3024 =item open FILEHANDLE,MODE,EXPR,LIST
3026 =item open FILEHANDLE,MODE,REFERENCE
3028 =item open FILEHANDLE
3030 Opens the file whose filename is given by EXPR, and associates it with
3033 (The following is a comprehensive reference to open(): for a gentler
3034 introduction you may consider L<perlopentut>.)
3036 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3037 the variable is assigned a reference to a new anonymous filehandle,
3038 otherwise if FILEHANDLE is an expression, its value is used as the name of
3039 the real filehandle wanted. (This is considered a symbolic reference, so
3040 C<use strict 'refs'> should I<not> be in effect.)
3042 If EXPR is omitted, the scalar variable of the same name as the
3043 FILEHANDLE contains the filename. (Note that lexical variables--those
3044 declared with C<my>--will not work for this purpose; so if you're
3045 using C<my>, specify EXPR in your call to open.)
3047 If three or more arguments are specified then the mode of opening and
3048 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3049 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3050 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3051 the file is opened for appending, again being created if necessary.
3053 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3054 indicate that you want both read and write access to the file; thus
3055 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3056 '+>' >> mode would clobber the file first. You can't usually use
3057 either read-write mode for updating textfiles, since they have
3058 variable length records. See the B<-i> switch in L<perlrun> for a
3059 better approach. The file is created with permissions of C<0666>
3060 modified by the process' C<umask> value.
3062 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3063 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3065 In the 2-arguments (and 1-argument) form of the call the mode and
3066 filename should be concatenated (in this order), possibly separated by
3067 spaces. It is possible to omit the mode in these forms if the mode is
3070 If the filename begins with C<'|'>, the filename is interpreted as a
3071 command to which output is to be piped, and if the filename ends with a
3072 C<'|'>, the filename is interpreted as a command which pipes output to
3073 us. See L<perlipc/"Using open() for IPC">
3074 for more examples of this. (You are not allowed to C<open> to a command
3075 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3076 and L<perlipc/"Bidirectional Communication with Another Process">
3079 For three or more arguments if MODE is C<'|-'>, the filename is
3080 interpreted as a command to which output is to be piped, and if MODE
3081 is C<'-|'>, the filename is interpreted as a command which pipes
3082 output to us. In the 2-arguments (and 1-argument) form one should
3083 replace dash (C<'-'>) with the command.
3084 See L<perlipc/"Using open() for IPC"> for more examples of this.
3085 (You are not allowed to C<open> to a command that pipes both in I<and>
3086 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3087 L<perlipc/"Bidirectional Communication"> for alternatives.)
3089 In the three-or-more argument form of pipe opens, if LIST is specified
3090 (extra arguments after the command name) then LIST becomes arguments
3091 to the command invoked if the platform supports it. The meaning of
3092 C<open> with more than three arguments for non-pipe modes is not yet
3093 specified. Experimental "layers" may give extra LIST arguments
3096 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3097 and opening C<< '>-' >> opens STDOUT.
3099 You may use the three-argument form of open to specify IO "layers"
3100 (sometimes also referred to as "disciplines") to be applied to the handle
3101 that affect how the input and output are processed (see L<open> and
3102 L<PerlIO> for more details). For example
3104 open(FH, "<:utf8", "file")
3106 will open the UTF-8 encoded file containing Unicode characters,
3107 see L<perluniintro>. Note that if layers are specified in the
3108 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3109 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3111 Open returns nonzero upon success, the undefined value otherwise. If
3112 the C<open> involved a pipe, the return value happens to be the pid of
3115 If you're running Perl on a system that distinguishes between text
3116 files and binary files, then you should check out L</binmode> for tips
3117 for dealing with this. The key distinction between systems that need
3118 C<binmode> and those that don't is their text file formats. Systems
3119 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3120 character, and which encode that character in C as C<"\n">, do not
3121 need C<binmode>. The rest need it.
3123 When opening a file, it's usually a bad idea to continue normal execution
3124 if the request failed, so C<open> is frequently used in connection with
3125 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3126 where you want to make a nicely formatted error message (but there are
3127 modules that can help with that problem)) you should always check
3128 the return value from opening a file. The infrequent exception is when
3129 working with an unopened filehandle is actually what you want to do.
3131 As a special case the 3-arg form with a read/write mode and the third
3132 argument being C<undef>:
3134 open(TMP, "+>", undef) or die ...
3136 opens a filehandle to an anonymous temporary file. Also using "+<"
3137 works for symmetry, but you really should consider writing something
3138 to the temporary file first. You will need to seek() to do the
3141 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3142 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3143 "in memory" files held in Perl scalars via:
3145 open($fh, '>', \$variable) || ..
3147 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3148 file, you have to close it first:
3151 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3156 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3157 while (<ARTICLE>) {...
3159 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3160 # if the open fails, output is discarded
3162 open(DBASE, '+<', 'dbase.mine') # open for update
3163 or die "Can't open 'dbase.mine' for update: $!";
3165 open(DBASE, '+<dbase.mine') # ditto
3166 or die "Can't open 'dbase.mine' for update: $!";
3168 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3169 or die "Can't start caesar: $!";
3171 open(ARTICLE, "caesar <$article |") # ditto
3172 or die "Can't start caesar: $!";
3174 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3175 or die "Can't start sort: $!";
3178 open(MEMORY,'>', \$var)
3179 or die "Can't open memory file: $!";
3180 print MEMORY "foo!\n"; # output will end up in $var
3182 # process argument list of files along with any includes
3184 foreach $file (@ARGV) {
3185 process($file, 'fh00');
3189 my($filename, $input) = @_;
3190 $input++; # this is a string increment
3191 unless (open($input, $filename)) {
3192 print STDERR "Can't open $filename: $!\n";
3197 while (<$input>) { # note use of indirection
3198 if (/^#include "(.*)"/) {
3199 process($1, $input);
3206 See L<perliol> for detailed info on PerlIO.
3208 You may also, in the Bourne shell tradition, specify an EXPR beginning
3209 with C<< '>&' >>, in which case the rest of the string is interpreted
3210 as the name of a filehandle (or file descriptor, if numeric) to be
3211 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3212 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3213 The mode you specify should match the mode of the original filehandle.
3214 (Duping a filehandle does not take into account any existing contents
3215 of IO buffers.) If you use the 3-arg form then you can pass either a
3216 number, the name of a filehandle or the normal "reference to a glob".
3218 Here is a script that saves, redirects, and restores C<STDOUT> and
3219 C<STDERR> using various methods:
3222 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3223 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3225 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3226 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3228 select STDERR; $| = 1; # make unbuffered
3229 select STDOUT; $| = 1; # make unbuffered
3231 print STDOUT "stdout 1\n"; # this works for
3232 print STDERR "stderr 1\n"; # subprocesses too
3234 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3235 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3237 print STDOUT "stdout 2\n";
3238 print STDERR "stderr 2\n";
3240 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3241 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3242 that file descriptor (and not call L<dup(2)>); this is more
3243 parsimonious of file descriptors. For example:
3245 # open for input, reusing the fileno of $fd
3246 open(FILEHANDLE, "<&=$fd")
3250 open(FILEHANDLE, "<&=", $fd)
3254 # open for append, using the fileno of OLDFH
3255 open(FH, ">>&=", OLDFH)
3259 open(FH, ">>&=OLDFH")
3261 Being parsimonious on filehandles is also useful (besides being
3262 parsimonious) for example when something is dependent on file
3263 descriptors, like for example locking using flock(). If you do just
3264 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3265 descriptor as B, and therefore flock(A) will not flock(B), and vice
3266 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3267 the same file descriptor.
3269 Note that if you are using Perls older than 5.8.0, Perl will be using
3270 the standard C libraries' fdopen() to implement the "=" functionality.
3271 On many UNIX systems fdopen() fails when file descriptors exceed a
3272 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3273 most often the default.
3275 You can see whether Perl has been compiled with PerlIO or not by
3276 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3277 is C<define>, you have PerlIO, otherwise you don't.
3279 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3280 with 2-arguments (or 1-argument) form of open(), then
3281 there is an implicit fork done, and the return value of open is the pid
3282 of the child within the parent process, and C<0> within the child
3283 process. (Use C<defined($pid)> to determine whether the open was successful.)
3284 The filehandle behaves normally for the parent, but i/o to that
3285 filehandle is piped from/to the STDOUT/STDIN of the child process.
3286 In the child process the filehandle isn't opened--i/o happens from/to
3287 the new STDOUT or STDIN. Typically this is used like the normal
3288 piped open when you want to exercise more control over just how the
3289 pipe command gets executed, such as when you are running setuid, and
3290 don't want to have to scan shell commands for metacharacters.
3291 The following triples are more or less equivalent:
3293 open(FOO, "|tr '[a-z]' '[A-Z]'");
3294 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3295 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3296 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3298 open(FOO, "cat -n '$file'|");
3299 open(FOO, '-|', "cat -n '$file'");
3300 open(FOO, '-|') || exec 'cat', '-n', $file;
3301 open(FOO, '-|', "cat", '-n', $file);
3303 The last example in each block shows the pipe as "list form", which is
3304 not yet supported on all platforms. A good rule of thumb is that if
3305 your platform has true C<fork()> (in other words, if your platform is
3306 UNIX) you can use the list form.
3308 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3310 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3311 output before any operation that may do a fork, but this may not be
3312 supported on some platforms (see L<perlport>). To be safe, you may need
3313 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3314 of C<IO::Handle> on any open handles.
3316 On systems that support a close-on-exec flag on files, the flag will
3317 be set for the newly opened file descriptor as determined by the value
3318 of $^F. See L<perlvar/$^F>.
3320 Closing any piped filehandle causes the parent process to wait for the
3321 child to finish, and returns the status value in C<$?> and
3322 C<${^CHILD_ERROR_NATIVE}>.
3324 The filename passed to 2-argument (or 1-argument) form of open() will
3325 have leading and trailing whitespace deleted, and the normal
3326 redirection characters honored. This property, known as "magic open",
3327 can often be used to good effect. A user could specify a filename of
3328 F<"rsh cat file |">, or you could change certain filenames as needed:
3330 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3331 open(FH, $filename) or die "Can't open $filename: $!";
3333 Use 3-argument form to open a file with arbitrary weird characters in it,
3335 open(FOO, '<', $file);
3337 otherwise it's necessary to protect any leading and trailing whitespace:
3339 $file =~ s#^(\s)#./$1#;
3340 open(FOO, "< $file\0");
3342 (this may not work on some bizarre filesystems). One should
3343 conscientiously choose between the I<magic> and 3-arguments form
3348 will allow the user to specify an argument of the form C<"rsh cat file |">,
3349 but will not work on a filename which happens to have a trailing space, while
3351 open IN, '<', $ARGV[0];
3353 will have exactly the opposite restrictions.
3355 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3356 should use the C<sysopen> function, which involves no such magic (but
3357 may use subtly different filemodes than Perl open(), which is mapped
3358 to C fopen()). This is
3359 another way to protect your filenames from interpretation. For example:
3362 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3363 or die "sysopen $path: $!";
3364 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3365 print HANDLE "stuff $$\n";
3367 print "File contains: ", <HANDLE>;
3369 Using the constructor from the C<IO::Handle> package (or one of its
3370 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3371 filehandles that have the scope of whatever variables hold references to
3372 them, and automatically close whenever and however you leave that scope:
3376 sub read_myfile_munged {
3378 my $handle = new IO::File;
3379 open($handle, "myfile") or die "myfile: $!";
3381 or return (); # Automatically closed here.
3382 mung $first or die "mung failed"; # Or here.
3383 return $first, <$handle> if $ALL; # Or here.
3387 See L</seek> for some details about mixing reading and writing.
3389 =item opendir DIRHANDLE,EXPR
3392 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3393 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3394 DIRHANDLE may be an expression whose value can be used as an indirect
3395 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3396 scalar variable (or array or hash element), the variable is assigned a
3397 reference to a new anonymous dirhandle.
3398 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3405 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3406 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3409 For the reverse, see L</chr>.
3410 See L<perlunicode> and L<encoding> for more about Unicode.
3417 =item our EXPR : ATTRS
3419 =item our TYPE EXPR : ATTRS
3421 C<our> associates a simple name with a package variable in the current
3422 package for use within the current scope. When C<use strict 'vars'> is in
3423 effect, C<our> lets you use declared global variables without qualifying
3424 them with package names, within the lexical scope of the C<our> declaration.
3425 In this way C<our> differs from C<use vars>, which is package scoped.
3427 Unlike C<my>, which both allocates storage for a variable and associates
3428 a simple name with that storage for use within the current scope, C<our>
3429 associates a simple name with a package variable in the current package,
3430 for use within the current scope. In other words, C<our> has the same
3431 scoping rules as C<my>, but does not necessarily create a
3434 If more than one value is listed, the list must be placed
3440 An C<our> declaration declares a global variable that will be visible
3441 across its entire lexical scope, even across package boundaries. The
3442 package in which the variable is entered is determined at the point
3443 of the declaration, not at the point of use. This means the following
3447 our $bar; # declares $Foo::bar for rest of lexical scope
3451 print $bar; # prints 20, as it refers to $Foo::bar
3453 Multiple C<our> declarations with the same name in the same lexical
3454 scope are allowed if they are in different packages. If they happen
3455 to be in the same package, Perl will emit warnings if you have asked
3456 for them, just like multiple C<my> declarations. Unlike a second
3457 C<my> declaration, which will bind the name to a fresh variable, a
3458 second C<our> declaration in the same package, in the same scope, is
3463 our $bar; # declares $Foo::bar for rest of lexical scope
3467 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3468 print $bar; # prints 30
3470 our $bar; # emits warning but has no other effect
3471 print $bar; # still prints 30
3473 An C<our> declaration may also have a list of attributes associated
3476 The exact semantics and interface of TYPE and ATTRS are still
3477 evolving. TYPE is currently bound to the use of C<fields> pragma,
3478 and attributes are handled using the C<attributes> pragma, or starting
3479 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3480 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3481 L<attributes>, and L<Attribute::Handlers>.
3483 =item pack TEMPLATE,LIST
3486 Takes a LIST of values and converts it into a string using the rules
3487 given by the TEMPLATE. The resulting string is the concatenation of
3488 the converted values. Typically, each converted value looks
3489 like its machine-level representation. For example, on 32-bit machines
3490 an integer may be represented by a sequence of 4 bytes that will be
3491 converted to a sequence of 4 characters.
3493 The TEMPLATE is a sequence of characters that give the order and type
3494 of values, as follows:
3496 a A string with arbitrary binary data, will be null padded.
3497 A A text (ASCII) string, will be space padded.
3498 Z A null terminated (ASCIZ) string, will be null padded.
3500 b A bit string (ascending bit order inside each byte, like vec()).
3501 B A bit string (descending bit order inside each byte).
3502 h A hex string (low nybble first).
3503 H A hex string (high nybble first).
3505 c A signed char (8-bit) value.
3506 C An unsigned C char (octet) even under Unicode. Should normally not
3507 be used. See U and W instead.
3508 W An unsigned char value (can be greater than 255).
3510 s A signed short (16-bit) value.
3511 S An unsigned short value.
3513 l A signed long (32-bit) value.
3514 L An unsigned long value.
3516 q A signed quad (64-bit) value.
3517 Q An unsigned quad value.
3518 (Quads are available only if your system supports 64-bit
3519 integer values _and_ if Perl has been compiled to support those.
3520 Causes a fatal error otherwise.)
3522 i A signed integer value.
3523 I A unsigned integer value.
3524 (This 'integer' is _at_least_ 32 bits wide. Its exact
3525 size depends on what a local C compiler calls 'int'.)
3527 n An unsigned short (16-bit) in "network" (big-endian) order.
3528 N An unsigned long (32-bit) in "network" (big-endian) order.
3529 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3530 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3532 j A Perl internal signed integer value (IV).
3533 J A Perl internal unsigned integer value (UV).
3535 f A single-precision float in the native format.
3536 d A double-precision float in the native format.
3538 F A Perl internal floating point value (NV) in the native format
3539 D A long double-precision float in the native format.
3540 (Long doubles are available only if your system supports long
3541 double values _and_ if Perl has been compiled to support those.
3542 Causes a fatal error otherwise.)
3544 p A pointer to a null-terminated string.
3545 P A pointer to a structure (fixed-length string).
3547 u A uuencoded string.
3548 U A Unicode character number. Encodes to UTF-8 internally
3549 (or UTF-EBCDIC in EBCDIC platforms).
3551 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3552 details). Its bytes represent an unsigned integer in base 128,
3553 most significant digit first, with as few digits as possible. Bit
3554 eight (the high bit) is set on each byte except the last.
3558 @ Null fill or truncate to absolute position, counted from the
3559 start of the innermost ()-group.
3560 . Null fill or truncate to absolute position specified by value.
3561 ( Start of a ()-group.
3563 One or more of the modifiers below may optionally follow some letters in the
3564 TEMPLATE (the second column lists the letters for which the modifier is
3567 ! sSlLiI Forces native (short, long, int) sizes instead
3568 of fixed (16-/32-bit) sizes.
3570 xX Make x and X act as alignment commands.
3572 nNvV Treat integers as signed instead of unsigned.
3574 @. Specify position as byte offset in the internal
3575 representation of the packed string. Efficient but
3578 > sSiIlLqQ Force big-endian byte-order on the type.
3579 jJfFdDpP (The "big end" touches the construct.)
3581 < sSiIlLqQ Force little-endian byte-order on the type.
3582 jJfFdDpP (The "little end" touches the construct.)
3584 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3585 in which case they force a certain byte-order on all components of
3586 that group, including subgroups.
3588 The following rules apply:
3594 Each letter may optionally be followed by a number giving a repeat
3595 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3596 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3597 that many values from the LIST. A C<*> for the repeat count means to
3598 use however many items are left, except for C<@>, C<x>, C<X>, where it
3599 is equivalent to C<0>, for <.> where it means relative to string start
3600 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3601 A numeric repeat count may optionally be enclosed in brackets, as in
3602 C<pack 'C[80]', @arr>.
3604 One can replace the numeric repeat count by a template enclosed in brackets;
3605 then the packed length of this template in bytes is used as a count.
3606 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3607 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3608 If the template in brackets contains alignment commands (such as C<x![d]>),
3609 its packed length is calculated as if the start of the template has the maximal
3612 When used with C<Z>, C<*> results in the addition of a trailing null
3613 byte (so the packed result will be one longer than the byte C<length>
3616 When used with C<@>, the repeat count represents an offset from the start
3617 of the innermost () group.
3619 When used with C<.>, the repeat count is used to determine the starting
3620 position from where the value offset is calculated. If the repeat count
3621 is 0, it's relative to the current position. If the repeat count is C<*>,
3622 the offset is relative to the start of the packed string. And if its an
3623 integer C<n> the offset is relative to the start of the n-th innermost
3624 () group (or the start of the string if C<n> is bigger then the group
3627 The repeat count for C<u> is interpreted as the maximal number of bytes
3628 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3629 count should not be more than 65.
3633 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3634 string of length count, padding with nulls or spaces as necessary. When
3635 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3636 after the first null, and C<a> returns data verbatim.
3638 If the value-to-pack is too long, it is truncated. If too long and an
3639 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3640 by a null byte. Thus C<Z> always packs a trailing null (except when the
3645 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3646 Each character of the input field of pack() generates 1 bit of the result.
3647 Each result bit is based on the least-significant bit of the corresponding
3648 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3649 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3651 Starting from the beginning of the input string of pack(), each 8-tuple
3652 of characters is converted to 1 character of output. With format C<b>
3653 the first character of the 8-tuple determines the least-significant bit of a
3654 character, and with format C<B> it determines the most-significant bit of
3657 If the length of the input string is not exactly divisible by 8, the
3658 remainder is packed as if the input string were padded by null characters
3659 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3661 If the input string of pack() is longer than needed, extra characters are
3662 ignored. A C<*> for the repeat count of pack() means to use all the
3663 characters of the input field. On unpack()ing the bits are converted to a
3664 string of C<"0">s and C<"1">s.
3668 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3669 representable as hexadecimal digits, 0-9a-f) long.
3671 Each character of the input field of pack() generates 4 bits of the result.
3672 For non-alphabetical characters the result is based on the 4 least-significant
3673 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3674 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3675 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3676 is compatible with the usual hexadecimal digits, so that C<"a"> and
3677 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3678 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3680 Starting from the beginning of the input string of pack(), each pair
3681 of characters is converted to 1 character of output. With format C<h> the
3682 first character of the pair determines the least-significant nybble of the
3683 output character, and with format C<H> it determines the most-significant
3686 If the length of the input string is not even, it behaves as if padded
3687 by a null character at the end. Similarly, during unpack()ing the "extra"
3688 nybbles are ignored.
3690 If the input string of pack() is longer than needed, extra characters are
3692 A C<*> for the repeat count of pack() means to use all the characters of
3693 the input field. On unpack()ing the nybbles are converted to a string
3694 of hexadecimal digits.
3698 The C<p> type packs a pointer to a null-terminated string. You are
3699 responsible for ensuring the string is not a temporary value (which can
3700 potentially get deallocated before you get around to using the packed result).
3701 The C<P> type packs a pointer to a structure of the size indicated by the
3702 length. A NULL pointer is created if the corresponding value for C<p> or
3703 C<P> is C<undef>, similarly for unpack().
3705 If your system has a strange pointer size (i.e. a pointer is neither as
3706 big as an int nor as big as a long), it may not be possible to pack or
3707 unpack pointers in big- or little-endian byte order. Attempting to do
3708 so will result in a fatal error.
3712 The C</> template character allows packing and unpacking of a sequence of
3713 items where the packed structure contains a packed item count followed by
3714 the packed items themselves.
3716 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3717 I<length-item> describes how the length value is packed. The ones likely
3718 to be of most use are integer-packing ones like C<n> (for Java strings),
3719 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3721 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3722 the minimum of that and the number of available items is used as argument
3723 for the I<length-item>. If it has no repeat count or uses a '*', the number
3724 of available items is used.
3726 For C<unpack> an internal stack of integer arguments unpacked so far is
3727 used. You write C</>I<sequence-item> and the repeat count is obtained by
3728 popping off the last element from the stack. The I<sequence-item> must not
3729 have a repeat count.
3731 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3732 the I<length-item> is a string length, not a number of strings. If there is
3733 an explicit repeat count for pack, the packed string will be adjusted to that
3736 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3737 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3738 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3739 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3740 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3742 The I<length-item> is not returned explicitly from C<unpack>.
3744 Adding a count to the I<length-item> letter is unlikely to do anything
3745 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3746 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3747 which Perl does not regard as legal in numeric strings.
3751 The integer types C<s>, C<S>, C<l>, and C<L> may be
3752 followed by a C<!> modifier to signify native shorts or
3753 longs--as you can see from above for example a bare C<l> does mean
3754 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3755 may be larger. This is an issue mainly in 64-bit platforms. You can
3756 see whether using C<!> makes any difference by
3758 print length(pack("s")), " ", length(pack("s!")), "\n";
3759 print length(pack("l")), " ", length(pack("l!")), "\n";
3761 C<i!> and C<I!> also work but only because of completeness;
3762 they are identical to C<i> and C<I>.
3764 The actual sizes (in bytes) of native shorts, ints, longs, and long
3765 longs on the platform where Perl was built are also available via
3769 print $Config{shortsize}, "\n";
3770 print $Config{intsize}, "\n";
3771 print $Config{longsize}, "\n";
3772 print $Config{longlongsize}, "\n";
3774 (The C<$Config{longlongsize}> will be undefined if your system does
3775 not support long longs.)
3779 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3780 are inherently non-portable between processors and operating systems
3781 because they obey the native byteorder and endianness. For example a
3782 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3783 (arranged in and handled by the CPU registers) into bytes as
3785 0x12 0x34 0x56 0x78 # big-endian
3786 0x78 0x56 0x34 0x12 # little-endian
3788 Basically, the Intel and VAX CPUs are little-endian, while everybody
3789 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3790 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3791 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3794 The names `big-endian' and `little-endian' are comic references to
3795 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3796 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3797 the egg-eating habits of the Lilliputians.
3799 Some systems may have even weirder byte orders such as
3804 You can see your system's preference with
3806 print join(" ", map { sprintf "%#02x", $_ }
3807 unpack("W*",pack("L",0x12345678))), "\n";
3809 The byteorder on the platform where Perl was built is also available
3813 print $Config{byteorder}, "\n";
3815 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3816 and C<'87654321'> are big-endian.
3818 If you want portable packed integers you can either use the formats
3819 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3820 modifiers. These modifiers are only available as of perl 5.9.2.
3821 See also L<perlport>.
3825 All integer and floating point formats as well as C<p> and C<P> and
3826 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3827 to force big- or little- endian byte-order, respectively.
3828 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3829 signed integers, 64-bit integers and floating point values. However,
3830 there are some things to keep in mind.
3832 Exchanging signed integers between different platforms only works
3833 if all platforms store them in the same format. Most platforms store
3834 signed integers in two's complement, so usually this is not an issue.
3836 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3837 formats on big- or little-endian machines. Otherwise, attempting to
3838 do so will result in a fatal error.
3840 Forcing big- or little-endian byte-order on floating point values for
3841 data exchange can only work if all platforms are using the same
3842 binary representation (e.g. IEEE floating point format). Even if all
3843 platforms are using IEEE, there may be subtle differences. Being able
3844 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3845 but also very dangerous if you don't know exactly what you're doing.
3846 It is definitely not a general way to portably store floating point
3849 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3850 all types inside the group that accept the byte-order modifiers,
3851 including all subgroups. It will silently be ignored for all other
3852 types. You are not allowed to override the byte-order within a group
3853 that already has a byte-order modifier suffix.
3857 Real numbers (floats and doubles) are in the native machine format only;
3858 due to the multiplicity of floating formats around, and the lack of a
3859 standard "network" representation, no facility for interchange has been
3860 made. This means that packed floating point data written on one machine
3861 may not be readable on another - even if both use IEEE floating point
3862 arithmetic (as the endian-ness of the memory representation is not part
3863 of the IEEE spec). See also L<perlport>.
3865 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3866 modifiers to force big- or little-endian byte-order on floating point values.
3868 Note that Perl uses doubles (or long doubles, if configured) internally for
3869 all numeric calculation, and converting from double into float and thence back
3870 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3871 will not in general equal $foo).
3875 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3876 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3877 where the packed string is processed in its UTF-8-encoded Unicode form on
3878 a byte by byte basis. Character mode is the default unless the format string
3879 starts with an C<U>. You can switch mode at any moment with an explicit
3880 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3881 or until the end of the ()-group in which it was entered.
3885 You must yourself do any alignment or padding by inserting for example
3886 enough C<'x'>es while packing. There is no way to pack() and unpack()
3887 could know where the characters are going to or coming from. Therefore
3888 C<pack> (and C<unpack>) handle their output and input as flat
3889 sequences of characters.
3893 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3894 take a repeat count, both as postfix, and for unpack() also via the C</>
3895 template character. Within each repetition of a group, positioning with
3896 C<@> starts again at 0. Therefore, the result of
3898 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3900 is the string "\0a\0\0bc".
3904 C<x> and C<X> accept C<!> modifier. In this case they act as
3905 alignment commands: they jump forward/back to the closest position
3906 aligned at a multiple of C<count> characters. For example, to pack() or
3907 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3908 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3909 aligned on the double's size.
3911 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3912 both result in no-ops.
3916 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3917 will represent signed 16-/32-bit integers in big-/little-endian order.
3918 This is only portable if all platforms sharing the packed data use the
3919 same binary representation for signed integers (e.g. all platforms are
3920 using two's complement representation).
3924 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3925 White space may be used to separate pack codes from each other, but
3926 modifiers and a repeat count must follow immediately.
3930 If TEMPLATE requires more arguments to pack() than actually given, pack()
3931 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3932 to pack() than actually given, extra arguments are ignored.
3938 $foo = pack("WWWW",65,66,67,68);
3940 $foo = pack("W4",65,66,67,68);
3942 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3943 # same thing with Unicode circled letters.
3944 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3945 # same thing with Unicode circled letters. You don't get the UTF-8
3946 # bytes because the U at the start of the format caused a switch to
3947 # U0-mode, so the UTF-8 bytes get joined into characters
3948 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3949 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3950 # This is the UTF-8 encoding of the string in the previous example
3952 $foo = pack("ccxxcc",65,66,67,68);
3955 # note: the above examples featuring "W" and "c" are true
3956 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3957 # and UTF-8. In EBCDIC the first example would be
3958 # $foo = pack("WWWW",193,194,195,196);
3960 $foo = pack("s2",1,2);
3961 # "\1\0\2\0" on little-endian
3962 # "\0\1\0\2" on big-endian
3964 $foo = pack("a4","abcd","x","y","z");
3967 $foo = pack("aaaa","abcd","x","y","z");
3970 $foo = pack("a14","abcdefg");
3971 # "abcdefg\0\0\0\0\0\0\0"
3973 $foo = pack("i9pl", gmtime);
3974 # a real struct tm (on my system anyway)
3976 $utmp_template = "Z8 Z8 Z16 L";
3977 $utmp = pack($utmp_template, @utmp1);
3978 # a struct utmp (BSDish)
3980 @utmp2 = unpack($utmp_template, $utmp);
3981 # "@utmp1" eq "@utmp2"
3984 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3987 $foo = pack('sx2l', 12, 34);
3988 # short 12, two zero bytes padding, long 34
3989 $bar = pack('s@4l', 12, 34);
3990 # short 12, zero fill to position 4, long 34
3992 $baz = pack('s.l', 12, 4, 34);
3993 # short 12, zero fill to position 4, long 34
3995 $foo = pack('nN', 42, 4711);
3996 # pack big-endian 16- and 32-bit unsigned integers
3997 $foo = pack('S>L>', 42, 4711);
3999 $foo = pack('s<l<', -42, 4711);
4000 # pack little-endian 16- and 32-bit signed integers
4001 $foo = pack('(sl)<', -42, 4711);
4004 The same template may generally also be used in unpack().
4006 =item package NAMESPACE
4007 X<package> X<module> X<namespace>
4011 Declares the compilation unit as being in the given namespace. The scope
4012 of the package declaration is from the declaration itself through the end
4013 of the enclosing block, file, or eval (the same as the C<my> operator).
4014 All further unqualified dynamic identifiers will be in this namespace.
4015 A package statement affects only dynamic variables--including those
4016 you've used C<local> on--but I<not> lexical variables, which are created
4017 with C<my>. Typically it would be the first declaration in a file to
4018 be included by the C<require> or C<use> operator. You can switch into a
4019 package in more than one place; it merely influences which symbol table
4020 is used by the compiler for the rest of that block. You can refer to
4021 variables and filehandles in other packages by prefixing the identifier
4022 with the package name and a double colon: C<$Package::Variable>.
4023 If the package name is null, the C<main> package as assumed. That is,
4024 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4025 still seen in older code).
4027 If NAMESPACE is omitted, then there is no current package, and all
4028 identifiers must be fully qualified or lexicals. However, you are
4029 strongly advised not to make use of this feature. Its use can cause
4030 unexpected behaviour, even crashing some versions of Perl. It is
4031 deprecated, and will be removed from a future release.
4033 See L<perlmod/"Packages"> for more information about packages, modules,
4034 and classes. See L<perlsub> for other scoping issues.
4036 =item pipe READHANDLE,WRITEHANDLE
4039 Opens a pair of connected pipes like the corresponding system call.
4040 Note that if you set up a loop of piped processes, deadlock can occur
4041 unless you are very careful. In addition, note that Perl's pipes use
4042 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4043 after each command, depending on the application.
4045 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4046 for examples of such things.
4048 On systems that support a close-on-exec flag on files, the flag will be set
4049 for the newly opened file descriptors as determined by the value of $^F.
4057 Pops and returns the last value of the array, shortening the array by
4058 one element. Has an effect similar to
4062 If there are no elements in the array, returns the undefined value
4063 (although this may happen at other times as well). If ARRAY is
4064 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4065 array in subroutines, just like C<shift>.
4068 X<pos> X<match, position>
4072 Returns the offset of where the last C<m//g> search left off for the variable
4073 in question (C<$_> is used when the variable is not specified). Note that
4074 0 is a valid match offset. C<undef> indicates that the search position
4075 is reset (usually due to match failure, but can also be because no match has
4076 yet been performed on the scalar). C<pos> directly accesses the location used
4077 by the regexp engine to store the offset, so assigning to C<pos> will change
4078 that offset, and so will also influence the C<\G> zero-width assertion in
4079 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4080 the return from C<pos> won't change either in this case. See L<perlre> and
4083 =item print FILEHANDLE LIST
4090 Prints a string or a list of strings. Returns true if successful.
4091 FILEHANDLE may be a scalar variable name, in which case the variable
4092 contains the name of or a reference to the filehandle, thus introducing
4093 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4094 the next token is a term, it may be misinterpreted as an operator
4095 unless you interpose a C<+> or put parentheses around the arguments.)
4096 If FILEHANDLE is omitted, prints by default to standard output (or
4097 to the last selected output channel--see L</select>). If LIST is
4098 also omitted, prints C<$_> to the currently selected output channel.
4099 To set the default output channel to something other than STDOUT
4100 use the select operation. The current value of C<$,> (if any) is
4101 printed between each LIST item. The current value of C<$\> (if
4102 any) is printed after the entire LIST has been printed. Because
4103 print takes a LIST, anything in the LIST is evaluated in list
4104 context, and any subroutine that you call will have one or more of
4105 its expressions evaluated in list context. Also be careful not to
4106 follow the print keyword with a left parenthesis unless you want
4107 the corresponding right parenthesis to terminate the arguments to
4108 the print--interpose a C<+> or put parentheses around all the
4111 Note that if you're storing FILEHANDLEs in an array, or if you're using
4112 any other expression more complex than a scalar variable to retrieve it,
4113 you will have to use a block returning the filehandle value instead:
4115 print { $files[$i] } "stuff\n";
4116 print { $OK ? STDOUT : STDERR } "stuff\n";
4118 =item printf FILEHANDLE FORMAT, LIST
4121 =item printf FORMAT, LIST
4123 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4124 (the output record separator) is not appended. The first argument
4125 of the list will be interpreted as the C<printf> format. See C<sprintf>
4126 for an explanation of the format argument. If C<use locale> is in effect,
4127 the character used for the decimal point in formatted real numbers is
4128 affected by the LC_NUMERIC locale. See L<perllocale>.
4130 Don't fall into the trap of using a C<printf> when a simple
4131 C<print> would do. The C<print> is more efficient and less
4134 =item prototype FUNCTION
4137 Returns the prototype of a function as a string (or C<undef> if the
4138 function has no prototype). FUNCTION is a reference to, or the name of,
4139 the function whose prototype you want to retrieve.
4141 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4142 name for Perl builtin. If the builtin is not I<overridable> (such as
4143 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4144 C<system>) returns C<undef> because the builtin does not really behave
4145 like a Perl function. Otherwise, the string describing the equivalent
4146 prototype is returned.
4148 =item push ARRAY,LIST
4151 Treats ARRAY as a stack, and pushes the values of LIST
4152 onto the end of ARRAY. The length of ARRAY increases by the length of
4153 LIST. Has the same effect as
4156 $ARRAY[++$#ARRAY] = $value;
4159 but is more efficient. Returns the number of elements in the array following
4160 the completed C<push>.
4172 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4174 =item quotemeta EXPR
4175 X<quotemeta> X<metacharacter>
4179 Returns the value of EXPR with all non-"word"
4180 characters backslashed. (That is, all characters not matching
4181 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4182 returned string, regardless of any locale settings.)
4183 This is the internal function implementing
4184 the C<\Q> escape in double-quoted strings.
4186 If EXPR is omitted, uses C<$_>.
4193 Returns a random fractional number greater than or equal to C<0> and less
4194 than the value of EXPR. (EXPR should be positive.) If EXPR is
4195 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4196 also special-cased as C<1> - this has not been documented before perl 5.8.0
4197 and is subject to change in future versions of perl. Automatically calls
4198 C<srand> unless C<srand> has already been called. See also C<srand>.
4200 Apply C<int()> to the value returned by C<rand()> if you want random
4201 integers instead of random fractional numbers. For example,
4205 returns a random integer between C<0> and C<9>, inclusive.
4207 (Note: If your rand function consistently returns numbers that are too
4208 large or too small, then your version of Perl was probably compiled
4209 with the wrong number of RANDBITS.)
4211 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4212 X<read> X<file, read>
4214 =item read FILEHANDLE,SCALAR,LENGTH
4216 Attempts to read LENGTH I<characters> of data into variable SCALAR
4217 from the specified FILEHANDLE. Returns the number of characters
4218 actually read, C<0> at end of file, or undef if there was an error (in
4219 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4220 so that the last character actually read is the last character of the
4221 scalar after the read.
4223 An OFFSET may be specified to place the read data at some place in the
4224 string other than the beginning. A negative OFFSET specifies
4225 placement at that many characters counting backwards from the end of
4226 the string. A positive OFFSET greater than the length of SCALAR
4227 results in the string being padded to the required size with C<"\0">
4228 bytes before the result of the read is appended.
4230 The call is actually implemented in terms of either Perl's or system's
4231 fread() call. To get a true read(2) system call, see C<sysread>.
4233 Note the I<characters>: depending on the status of the filehandle,
4234 either (8-bit) bytes or characters are read. By default all
4235 filehandles operate on bytes, but for example if the filehandle has
4236 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4237 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4238 characters, not bytes. Similarly for the C<:encoding> pragma:
4239 in that case pretty much any characters can be read.
4241 =item readdir DIRHANDLE
4244 Returns the next directory entry for a directory opened by C<opendir>.
4245 If used in list context, returns all the rest of the entries in the
4246 directory. If there are no more entries, returns an undefined value in
4247 scalar context or a null list in list context.
4249 If you're planning to filetest the return values out of a C<readdir>, you'd
4250 better prepend the directory in question. Otherwise, because we didn't
4251 C<chdir> there, it would have been testing the wrong file.
4253 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4254 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4258 X<readline> X<gets> X<fgets>
4260 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4261 context, each call reads and returns the next line, until end-of-file is
4262 reached, whereupon the subsequent call returns undef. In list context,
4263 reads until end-of-file is reached and returns a list of lines. Note that
4264 the notion of "line" used here is however you may have defined it
4265 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4267 When C<$/> is set to C<undef>, when readline() is in scalar
4268 context (i.e. file slurp mode), and when an empty file is read, it
4269 returns C<''> the first time, followed by C<undef> subsequently.
4271 This is the internal function implementing the C<< <EXPR> >>
4272 operator, but you can use it directly. The C<< <EXPR> >>
4273 operator is discussed in more detail in L<perlop/"I/O Operators">.
4276 $line = readline(*STDIN); # same thing
4278 If readline encounters an operating system error, C<$!> will be set with the
4279 corresponding error message. It can be helpful to check C<$!> when you are
4280 reading from filehandles you don't trust, such as a tty or a socket. The
4281 following example uses the operator form of C<readline>, and takes the necessary
4282 steps to ensure that C<readline> was successful.
4286 unless (defined( $line = <> )) {
4298 Returns the value of a symbolic link, if symbolic links are
4299 implemented. If not, gives a fatal error. If there is some system
4300 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4301 omitted, uses C<$_>.
4306 EXPR is executed as a system command.
4307 The collected standard output of the command is returned.
4308 In scalar context, it comes back as a single (potentially
4309 multi-line) string. In list context, returns a list of lines
4310 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4311 This is the internal function implementing the C<qx/EXPR/>
4312 operator, but you can use it directly. The C<qx/EXPR/>
4313 operator is discussed in more detail in L<perlop/"I/O Operators">.
4315 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4318 Receives a message on a socket. Attempts to receive LENGTH characters
4319 of data into variable SCALAR from the specified SOCKET filehandle.
4320 SCALAR will be grown or shrunk to the length actually read. Takes the
4321 same flags as the system call of the same name. Returns the address
4322 of the sender if SOCKET's protocol supports this; returns an empty
4323 string otherwise. If there's an error, returns the undefined value.
4324 This call is actually implemented in terms of recvfrom(2) system call.
4325 See L<perlipc/"UDP: Message Passing"> for examples.
4327 Note the I<characters>: depending on the status of the socket, either
4328 (8-bit) bytes or characters are received. By default all sockets
4329 operate on bytes, but for example if the socket has been changed using
4330 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4331 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4332 characters, not bytes. Similarly for the C<:encoding> pragma:
4333 in that case pretty much any characters can be read.
4340 The C<redo> command restarts the loop block without evaluating the
4341 conditional again. The C<continue> block, if any, is not executed. If
4342 the LABEL is omitted, the command refers to the innermost enclosing
4343 loop. Programs that want to lie to themselves about what was just input
4344 normally use this command:
4346 # a simpleminded Pascal comment stripper
4347 # (warning: assumes no { or } in strings)
4348 LINE: while (<STDIN>) {
4349 while (s|({.*}.*){.*}|$1 |) {}
4354 if (/}/) { # end of comment?
4363 C<redo> cannot be used to retry a block which returns a value such as
4364 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4365 a grep() or map() operation.
4367 Note that a block by itself is semantically identical to a loop
4368 that executes once. Thus C<redo> inside such a block will effectively
4369 turn it into a looping construct.
4371 See also L</continue> for an illustration of how C<last>, C<next>, and
4379 Returns a non-empty string if EXPR is a reference, the empty
4380 string otherwise. If EXPR
4381 is not specified, C<$_> will be used. The value returned depends on the
4382 type of thing the reference is a reference to.
4383 Builtin types include:
4397 If the referenced object has been blessed into a package, then that package
4398 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4400 if (ref($r) eq "HASH") {
4401 print "r is a reference to a hash.\n";
4404 print "r is not a reference at all.\n";
4407 See also L<perlref>.
4409 =item rename OLDNAME,NEWNAME
4410 X<rename> X<move> X<mv> X<ren>
4412 Changes the name of a file; an existing file NEWNAME will be
4413 clobbered. Returns true for success, false otherwise.
4415 Behavior of this function varies wildly depending on your system
4416 implementation. For example, it will usually not work across file system
4417 boundaries, even though the system I<mv> command sometimes compensates
4418 for this. Other restrictions include whether it works on directories,
4419 open files, or pre-existing files. Check L<perlport> and either the
4420 rename(2) manpage or equivalent system documentation for details.
4422 For a platform independent C<move> function look at the L<File::Copy>
4425 =item require VERSION
4432 Demands a version of Perl specified by VERSION, or demands some semantics
4433 specified by EXPR or by C<$_> if EXPR is not supplied.
4435 VERSION may be either a numeric argument such as 5.006, which will be
4436 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4437 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4438 VERSION is greater than the version of the current Perl interpreter.
4439 Compare with L</use>, which can do a similar check at compile time.
4441 Specifying VERSION as a literal of the form v5.6.1 should generally be
4442 avoided, because it leads to misleading error messages under earlier
4443 versions of Perl that do not support this syntax. The equivalent numeric
4444 version should be used instead.
4446 require v5.6.1; # run time version check
4447 require 5.6.1; # ditto
4448 require 5.006_001; # ditto; preferred for backwards compatibility
4450 Otherwise, C<require> demands that a library file be included if it
4451 hasn't already been included. The file is included via the do-FILE
4452 mechanism, which is essentially just a variety of C<eval>. Has
4453 semantics similar to the following subroutine:
4456 my ($filename) = @_;
4457 if (exists $INC{$filename}) {
4458 return 1 if $INC{$filename};
4459 die "Compilation failed in require";
4461 my ($realfilename,$result);
4463 foreach $prefix (@INC) {
4464 $realfilename = "$prefix/$filename";
4465 if (-f $realfilename) {
4466 $INC{$filename} = $realfilename;
4467 $result = do $realfilename;
4471 die "Can't find $filename in \@INC";
4474 $INC{$filename} = undef;
4476 } elsif (!$result) {
4477 delete $INC{$filename};
4478 die "$filename did not return true value";
4484 Note that the file will not be included twice under the same specified
4487 The file must return true as the last statement to indicate
4488 successful execution of any initialization code, so it's customary to
4489 end such a file with C<1;> unless you're sure it'll return true
4490 otherwise. But it's better just to put the C<1;>, in case you add more
4493 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4494 replaces "F<::>" with "F</>" in the filename for you,
4495 to make it easy to load standard modules. This form of loading of
4496 modules does not risk altering your namespace.
4498 In other words, if you try this:
4500 require Foo::Bar; # a splendid bareword
4502 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4503 directories specified in the C<@INC> array.
4505 But if you try this:
4507 $class = 'Foo::Bar';
4508 require $class; # $class is not a bareword
4510 require "Foo::Bar"; # not a bareword because of the ""
4512 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4513 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4515 eval "require $class";
4517 Now that you understand how C<require> looks for files in the case of a
4518 bareword argument, there is a little extra functionality going on behind
4519 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4520 first look for a similar filename with a "F<.pmc>" extension. If this file
4521 is found, it will be loaded in place of any file ending in a "F<.pm>"
4524 You can also insert hooks into the import facility, by putting directly
4525 Perl code into the @INC array. There are three forms of hooks: subroutine
4526 references, array references and blessed objects.
4528 Subroutine references are the simplest case. When the inclusion system
4529 walks through @INC and encounters a subroutine, this subroutine gets
4530 called with two parameters, the first being a reference to itself, and the
4531 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4532 subroutine should return nothing, or a list of up to 4 values in the
4539 A reference to a scalar, containing any initial source code to prepend to
4540 the file or generator output.
4545 A filehandle, from which the file will be read.
4549 A reference to a subroutine. If there is no filehandle (previous item),
4550 then this subroutine is expected to generate one line of source code per
4551 call, writing the line into C<$_> and returning 1, then returning 0 at
4552 "end of file". If there is a filehandle, then the subroutine will be
4553 called to act a simple source filter, with the line as read in C<$_>.
4554 Again, return 1 for each valid line, and 0 after all lines have been
4559 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4560 reference to the subroutine itself is passed in as C<$_[0]>.
4564 If an empty list, C<undef>, or nothing that matches the first 3 values above
4565 is returned then C<require> will look at the remaining elements of @INC.
4566 Note that this file handle must be a real file handle (strictly a typeglob,
4567 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4568 ignored and return value processing will stop there.
4570 If the hook is an array reference, its first element must be a subroutine
4571 reference. This subroutine is called as above, but the first parameter is
4572 the array reference. This enables to pass indirectly some arguments to
4575 In other words, you can write:
4577 push @INC, \&my_sub;
4579 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4585 push @INC, [ \&my_sub, $x, $y, ... ];
4587 my ($arrayref, $filename) = @_;
4588 # Retrieve $x, $y, ...
4589 my @parameters = @$arrayref[1..$#$arrayref];
4593 If the hook is an object, it must provide an INC method that will be
4594 called as above, the first parameter being the object itself. (Note that
4595 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4596 into package C<main>.) Here is a typical code layout:
4602 my ($self, $filename) = @_;
4606 # In the main program
4607 push @INC, new Foo(...);
4609 Note that these hooks are also permitted to set the %INC entry
4610 corresponding to the files they have loaded. See L<perlvar/%INC>.
4612 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4619 Generally used in a C<continue> block at the end of a loop to clear
4620 variables and reset C<??> searches so that they work again. The
4621 expression is interpreted as a list of single characters (hyphens
4622 allowed for ranges). All variables and arrays beginning with one of
4623 those letters are reset to their pristine state. If the expression is
4624 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4625 only variables or searches in the current package. Always returns
4628 reset 'X'; # reset all X variables
4629 reset 'a-z'; # reset lower case variables
4630 reset; # just reset ?one-time? searches
4632 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4633 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4634 variables--lexical variables are unaffected, but they clean themselves
4635 up on scope exit anyway, so you'll probably want to use them instead.
4643 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4644 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4645 context, depending on how the return value will be used, and the context
4646 may vary from one execution to the next (see C<wantarray>). If no EXPR
4647 is given, returns an empty list in list context, the undefined value in
4648 scalar context, and (of course) nothing at all in a void context.
4650 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4651 or do FILE will automatically return the value of the last expression
4655 X<reverse> X<rev> X<invert>
4657 In list context, returns a list value consisting of the elements
4658 of LIST in the opposite order. In scalar context, concatenates the
4659 elements of LIST and returns a string value with all characters
4660 in the opposite order.
4662 print reverse <>; # line tac, last line first
4664 undef $/; # for efficiency of <>
4665 print scalar reverse <>; # character tac, last line tsrif
4667 Used without arguments in scalar context, reverse() reverses C<$_>.
4669 This operator is also handy for inverting a hash, although there are some
4670 caveats. If a value is duplicated in the original hash, only one of those
4671 can be represented as a key in the inverted hash. Also, this has to
4672 unwind one hash and build a whole new one, which may take some time
4673 on a large hash, such as from a DBM file.
4675 %by_name = reverse %by_address; # Invert the hash
4677 =item rewinddir DIRHANDLE
4680 Sets the current position to the beginning of the directory for the
4681 C<readdir> routine on DIRHANDLE.
4683 =item rindex STR,SUBSTR,POSITION
4686 =item rindex STR,SUBSTR
4688 Works just like index() except that it returns the position of the I<last>
4689 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4690 last occurrence beginning at or before that position.
4692 =item rmdir FILENAME
4693 X<rmdir> X<rd> X<directory, remove>
4697 Deletes the directory specified by FILENAME if that directory is
4698 empty. If it succeeds it returns true, otherwise it returns false and
4699 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4701 To remove a directory tree recursively (C<rm -rf> on unix) look at
4702 the C<rmtree> function of the L<File::Path> module.
4706 The substitution operator. See L<perlop>.
4708 =item say FILEHANDLE LIST
4715 Just like C<print>, but implicitly appends a newline.
4716 C<say LIST> is simply an abbreviation for C<print LIST, "\n">,
4717 and C<say()> works just like C<print($_, "\n")>.
4719 That means that a call to say() appends any output record separator
4720 I<after> the added newline.
4722 This keyword is only available when the "say" feature is
4723 enabled: see L<feature>.
4726 X<scalar> X<context>
4728 Forces EXPR to be interpreted in scalar context and returns the value
4731 @counts = ( scalar @a, scalar @b, scalar @c );
4733 There is no equivalent operator to force an expression to
4734 be interpolated in list context because in practice, this is never
4735 needed. If you really wanted to do so, however, you could use
4736 the construction C<@{[ (some expression) ]}>, but usually a simple
4737 C<(some expression)> suffices.
4739 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4740 parenthesized list, this behaves as a scalar comma expression, evaluating
4741 all but the last element in void context and returning the final element
4742 evaluated in scalar context. This is seldom what you want.
4744 The following single statement:
4746 print uc(scalar(&foo,$bar)),$baz;
4748 is the moral equivalent of these two:
4751 print(uc($bar),$baz);
4753 See L<perlop> for more details on unary operators and the comma operator.
4755 =item seek FILEHANDLE,POSITION,WHENCE
4756 X<seek> X<fseek> X<filehandle, position>
4758 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4759 FILEHANDLE may be an expression whose value gives the name of the
4760 filehandle. The values for WHENCE are C<0> to set the new position
4761 I<in bytes> to POSITION, C<1> to set it to the current position plus
4762 POSITION, and C<2> to set it to EOF plus POSITION (typically
4763 negative). For WHENCE you may use the constants C<SEEK_SET>,
4764 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4765 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4768 Note the I<in bytes>: even if the filehandle has been set to
4769 operate on characters (for example by using the C<:utf8> open
4770 layer), tell() will return byte offsets, not character offsets
4771 (because implementing that would render seek() and tell() rather slow).
4773 If you want to position file for C<sysread> or C<syswrite>, don't use
4774 C<seek>--buffering makes its effect on the file's system position
4775 unpredictable and non-portable. Use C<sysseek> instead.
4777 Due to the rules and rigors of ANSI C, on some systems you have to do a
4778 seek whenever you switch between reading and writing. Amongst other
4779 things, this may have the effect of calling stdio's clearerr(3).
4780 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4784 This is also useful for applications emulating C<tail -f>. Once you hit
4785 EOF on your read, and then sleep for a while, you might have to stick in a
4786 seek() to reset things. The C<seek> doesn't change the current position,
4787 but it I<does> clear the end-of-file condition on the handle, so that the
4788 next C<< <FILE> >> makes Perl try again to read something. We hope.
4790 If that doesn't work (some IO implementations are particularly
4791 cantankerous), then you may need something more like this:
4794 for ($curpos = tell(FILE); $_ = <FILE>;
4795 $curpos = tell(FILE)) {
4796 # search for some stuff and put it into files
4798 sleep($for_a_while);
4799 seek(FILE, $curpos, 0);
4802 =item seekdir DIRHANDLE,POS
4805 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4806 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4807 about possible directory compaction as the corresponding system library
4810 =item select FILEHANDLE
4811 X<select> X<filehandle, default>
4815 Returns the currently selected filehandle. Sets the current default
4816 filehandle for output, if FILEHANDLE is supplied. This has two
4817 effects: first, a C<write> or a C<print> without a filehandle will
4818 default to this FILEHANDLE. Second, references to variables related to
4819 output will refer to this output channel. For example, if you have to
4820 set the top of form format for more than one output channel, you might
4828 FILEHANDLE may be an expression whose value gives the name of the
4829 actual filehandle. Thus:
4831 $oldfh = select(STDERR); $| = 1; select($oldfh);
4833 Some programmers may prefer to think of filehandles as objects with
4834 methods, preferring to write the last example as:
4837 STDERR->autoflush(1);
4839 =item select RBITS,WBITS,EBITS,TIMEOUT
4842 This calls the select(2) system call with the bit masks specified, which
4843 can be constructed using C<fileno> and C<vec>, along these lines:
4845 $rin = $win = $ein = '';
4846 vec($rin,fileno(STDIN),1) = 1;
4847 vec($win,fileno(STDOUT),1) = 1;
4850 If you want to select on many filehandles you might wish to write a
4854 my(@fhlist) = split(' ',$_[0]);
4857 vec($bits,fileno($_),1) = 1;
4861 $rin = fhbits('STDIN TTY SOCK');
4865 ($nfound,$timeleft) =
4866 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4868 or to block until something becomes ready just do this
4870 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4872 Most systems do not bother to return anything useful in $timeleft, so
4873 calling select() in scalar context just returns $nfound.
4875 Any of the bit masks can also be undef. The timeout, if specified, is
4876 in seconds, which may be fractional. Note: not all implementations are
4877 capable of returning the $timeleft. If not, they always return
4878 $timeleft equal to the supplied $timeout.
4880 You can effect a sleep of 250 milliseconds this way:
4882 select(undef, undef, undef, 0.25);
4884 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4885 is implementation-dependent. See also L<perlport> for notes on the
4886 portability of C<select>.
4888 On error, C<select> behaves like the select(2) system call : it returns
4891 Note: on some Unixes, the select(2) system call may report a socket file
4892 descriptor as "ready for reading", when actually no data is available,
4893 thus a subsequent read blocks. It can be avoided using always the
4894 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4897 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4898 or <FH>) with C<select>, except as permitted by POSIX, and even
4899 then only on POSIX systems. You have to use C<sysread> instead.
4901 =item semctl ID,SEMNUM,CMD,ARG
4904 Calls the System V IPC function C<semctl>. You'll probably have to say
4908 first to get the correct constant definitions. If CMD is IPC_STAT or
4909 GETALL, then ARG must be a variable that will hold the returned
4910 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4911 the undefined value for error, "C<0 but true>" for zero, or the actual
4912 return value otherwise. The ARG must consist of a vector of native
4913 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4914 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4917 =item semget KEY,NSEMS,FLAGS
4920 Calls the System V IPC function semget. Returns the semaphore id, or
4921 the undefined value if there is an error. See also
4922 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4925 =item semop KEY,OPSTRING
4928 Calls the System V IPC function semop to perform semaphore operations
4929 such as signalling and waiting. OPSTRING must be a packed array of
4930 semop structures. Each semop structure can be generated with
4931 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4932 implies the number of semaphore operations. Returns true if
4933 successful, or false if there is an error. As an example, the
4934 following code waits on semaphore $semnum of semaphore id $semid:
4936 $semop = pack("s!3", $semnum, -1, 0);
4937 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4939 To signal the semaphore, replace C<-1> with C<1>. See also
4940 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4943 =item send SOCKET,MSG,FLAGS,TO
4946 =item send SOCKET,MSG,FLAGS
4948 Sends a message on a socket. Attempts to send the scalar MSG to the
4949 SOCKET filehandle. Takes the same flags as the system call of the
4950 same name. On unconnected sockets you must specify a destination to
4951 send TO, in which case it does a C C<sendto>. Returns the number of
4952 characters sent, or the undefined value if there is an error. The C
4953 system call sendmsg(2) is currently unimplemented. See
4954 L<perlipc/"UDP: Message Passing"> for examples.
4956 Note the I<characters>: depending on the status of the socket, either
4957 (8-bit) bytes or characters are sent. By default all sockets operate
4958 on bytes, but for example if the socket has been changed using
4959 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4960 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4961 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4962 in that case pretty much any characters can be sent.
4964 =item setpgrp PID,PGRP
4967 Sets the current process group for the specified PID, C<0> for the current
4968 process. Will produce a fatal error if used on a machine that doesn't
4969 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4970 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4971 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4974 =item setpriority WHICH,WHO,PRIORITY
4975 X<setpriority> X<priority> X<nice> X<renice>
4977 Sets the current priority for a process, a process group, or a user.
4978 (See setpriority(2).) Will produce a fatal error if used on a machine
4979 that doesn't implement setpriority(2).
4981 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4984 Sets the socket option requested. Returns undefined if there is an
4985 error. Use integer constants provided by the C<Socket> module for
4986 LEVEL and OPNAME. Values for LEVEL can also be obtained from
4987 getprotobyname. OPTVAL might either be a packed string or an integer.
4988 An integer OPTVAL is shorthand for pack("i", OPTVAL).
4990 An example disabling the Nagle's algorithm for a socket:
4992 use Socket qw(IPPROTO_TCP TCP_NODELAY);
4993 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5000 Shifts the first value of the array off and returns it, shortening the
5001 array by 1 and moving everything down. If there are no elements in the
5002 array, returns the undefined value. If ARRAY is omitted, shifts the
5003 C<@_> array within the lexical scope of subroutines and formats, and the
5004 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5005 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>
5006 and C<END {}> constructs.
5008 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5009 same thing to the left end of an array that C<pop> and C<push> do to the
5012 =item shmctl ID,CMD,ARG
5015 Calls the System V IPC function shmctl. You'll probably have to say
5019 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5020 then ARG must be a variable that will hold the returned C<shmid_ds>
5021 structure. Returns like ioctl: the undefined value for error, "C<0> but
5022 true" for zero, or the actual return value otherwise.
5023 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5025 =item shmget KEY,SIZE,FLAGS
5028 Calls the System V IPC function shmget. Returns the shared memory
5029 segment id, or the undefined value if there is an error.
5030 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5032 =item shmread ID,VAR,POS,SIZE
5036 =item shmwrite ID,STRING,POS,SIZE
5038 Reads or writes the System V shared memory segment ID starting at
5039 position POS for size SIZE by attaching to it, copying in/out, and
5040 detaching from it. When reading, VAR must be a variable that will
5041 hold the data read. When writing, if STRING is too long, only SIZE
5042 bytes are used; if STRING is too short, nulls are written to fill out
5043 SIZE bytes. Return true if successful, or false if there is an error.
5044 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5045 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5047 =item shutdown SOCKET,HOW
5050 Shuts down a socket connection in the manner indicated by HOW, which
5051 has the same interpretation as in the system call of the same name.
5053 shutdown(SOCKET, 0); # I/we have stopped reading data
5054 shutdown(SOCKET, 1); # I/we have stopped writing data
5055 shutdown(SOCKET, 2); # I/we have stopped using this socket
5057 This is useful with sockets when you want to tell the other
5058 side you're done writing but not done reading, or vice versa.
5059 It's also a more insistent form of close because it also
5060 disables the file descriptor in any forked copies in other
5064 X<sin> X<sine> X<asin> X<arcsine>
5068 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5069 returns sine of C<$_>.
5071 For the inverse sine operation, you may use the C<Math::Trig::asin>
5072 function, or use this relation:
5074 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5081 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5082 May be interrupted if the process receives a signal such as C<SIGALRM>.
5083 Returns the number of seconds actually slept. You probably cannot
5084 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5087 On some older systems, it may sleep up to a full second less than what
5088 you requested, depending on how it counts seconds. Most modern systems
5089 always sleep the full amount. They may appear to sleep longer than that,
5090 however, because your process might not be scheduled right away in a
5091 busy multitasking system.
5093 For delays of finer granularity than one second, you may use Perl's
5094 C<syscall> interface to access setitimer(2) if your system supports
5095 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5096 and starting from Perl 5.8 part of the standard distribution) may also
5099 See also the POSIX module's C<pause> function.
5101 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5104 Opens a socket of the specified kind and attaches it to filehandle
5105 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5106 the system call of the same name. You should C<use Socket> first
5107 to get the proper definitions imported. See the examples in
5108 L<perlipc/"Sockets: Client/Server Communication">.
5110 On systems that support a close-on-exec flag on files, the flag will
5111 be set for the newly opened file descriptor, as determined by the
5112 value of $^F. See L<perlvar/$^F>.
5114 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5117 Creates an unnamed pair of sockets in the specified domain, of the
5118 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5119 for the system call of the same name. If unimplemented, yields a fatal
5120 error. Returns true if successful.
5122 On systems that support a close-on-exec flag on files, the flag will
5123 be set for the newly opened file descriptors, as determined by the value
5124 of $^F. See L<perlvar/$^F>.
5126 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5127 to C<pipe(Rdr, Wtr)> is essentially:
5130 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5131 shutdown(Rdr, 1); # no more writing for reader
5132 shutdown(Wtr, 0); # no more reading for writer
5134 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5135 emulate socketpair using IP sockets to localhost if your system implements
5136 sockets but not socketpair.
5138 =item sort SUBNAME LIST
5139 X<sort> X<qsort> X<quicksort> X<mergesort>
5141 =item sort BLOCK LIST
5145 In list context, this sorts the LIST and returns the sorted list value.
5146 In scalar context, the behaviour of C<sort()> is undefined.
5148 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5149 order. If SUBNAME is specified, it gives the name of a subroutine
5150 that returns an integer less than, equal to, or greater than C<0>,
5151 depending on how the elements of the list are to be ordered. (The C<<
5152 <=> >> and C<cmp> operators are extremely useful in such routines.)
5153 SUBNAME may be a scalar variable name (unsubscripted), in which case
5154 the value provides the name of (or a reference to) the actual
5155 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5156 an anonymous, in-line sort subroutine.
5158 If the subroutine's prototype is C<($$)>, the elements to be compared
5159 are passed by reference in C<@_>, as for a normal subroutine. This is
5160 slower than unprototyped subroutines, where the elements to be
5161 compared are passed into the subroutine
5162 as the package global variables $a and $b (see example below). Note that
5163 in the latter case, it is usually counter-productive to declare $a and
5166 The values to be compared are always passed by reference and should not
5169 You also cannot exit out of the sort block or subroutine using any of the
5170 loop control operators described in L<perlsyn> or with C<goto>.
5172 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5173 current collation locale. See L<perllocale>.
5175 sort() returns aliases into the original list, much as a for loop's index
5176 variable aliases the list elements. That is, modifying an element of a
5177 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5178 actually modifies the element in the original list. This is usually
5179 something to be avoided when writing clear code.
5181 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5182 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5183 preserves the input order of elements that compare equal. Although
5184 quicksort's run time is O(NlogN) when averaged over all arrays of
5185 length N, the time can be O(N**2), I<quadratic> behavior, for some
5186 inputs.) In 5.7, the quicksort implementation was replaced with
5187 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5188 But benchmarks indicated that for some inputs, on some platforms,
5189 the original quicksort was faster. 5.8 has a sort pragma for
5190 limited control of the sort. Its rather blunt control of the
5191 underlying algorithm may not persist into future Perls, but the
5192 ability to characterize the input or output in implementation
5193 independent ways quite probably will. See L<sort>.
5198 @articles = sort @files;
5200 # same thing, but with explicit sort routine
5201 @articles = sort {$a cmp $b} @files;
5203 # now case-insensitively
5204 @articles = sort {uc($a) cmp uc($b)} @files;
5206 # same thing in reversed order
5207 @articles = sort {$b cmp $a} @files;
5209 # sort numerically ascending
5210 @articles = sort {$a <=> $b} @files;
5212 # sort numerically descending
5213 @articles = sort {$b <=> $a} @files;
5215 # this sorts the %age hash by value instead of key
5216 # using an in-line function
5217 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5219 # sort using explicit subroutine name
5221 $age{$a} <=> $age{$b}; # presuming numeric
5223 @sortedclass = sort byage @class;
5225 sub backwards { $b cmp $a }
5226 @harry = qw(dog cat x Cain Abel);
5227 @george = qw(gone chased yz Punished Axed);
5229 # prints AbelCaincatdogx
5230 print sort backwards @harry;
5231 # prints xdogcatCainAbel
5232 print sort @george, 'to', @harry;
5233 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5235 # inefficiently sort by descending numeric compare using
5236 # the first integer after the first = sign, or the
5237 # whole record case-insensitively otherwise
5240 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5245 # same thing, but much more efficiently;
5246 # we'll build auxiliary indices instead
5250 push @nums, /=(\d+)/;
5255 $nums[$b] <=> $nums[$a]
5257 $caps[$a] cmp $caps[$b]
5261 # same thing, but without any temps
5262 @new = map { $_->[0] }
5263 sort { $b->[1] <=> $a->[1]
5266 } map { [$_, /=(\d+)/, uc($_)] } @old;
5268 # using a prototype allows you to use any comparison subroutine
5269 # as a sort subroutine (including other package's subroutines)
5271 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5274 @new = sort other::backwards @old;
5276 # guarantee stability, regardless of algorithm
5278 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5280 # force use of mergesort (not portable outside Perl 5.8)
5281 use sort '_mergesort'; # note discouraging _
5282 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5284 If you're using strict, you I<must not> declare $a
5285 and $b as lexicals. They are package globals. That means
5286 if you're in the C<main> package and type
5288 @articles = sort {$b <=> $a} @files;
5290 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5291 but if you're in the C<FooPack> package, it's the same as typing
5293 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5295 The comparison function is required to behave. If it returns
5296 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5297 sometimes saying the opposite, for example) the results are not
5300 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5301 (not-a-number), and because C<sort> will trigger a fatal error unless the
5302 result of a comparison is defined, when sorting with a comparison function
5303 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5304 The following example takes advantage of the fact that C<NaN != NaN> to
5305 eliminate any C<NaN>s from the input.
5307 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5309 =item splice ARRAY,OFFSET,LENGTH,LIST
5312 =item splice ARRAY,OFFSET,LENGTH
5314 =item splice ARRAY,OFFSET
5318 Removes the elements designated by OFFSET and LENGTH from an array, and
5319 replaces them with the elements of LIST, if any. In list context,
5320 returns the elements removed from the array. In scalar context,
5321 returns the last element removed, or C<undef> if no elements are
5322 removed. The array grows or shrinks as necessary.
5323 If OFFSET is negative then it starts that far from the end of the array.
5324 If LENGTH is omitted, removes everything from OFFSET onward.
5325 If LENGTH is negative, removes the elements from OFFSET onward
5326 except for -LENGTH elements at the end of the array.
5327 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5328 past the end of the array, perl issues a warning, and splices at the
5331 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5333 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5334 pop(@a) splice(@a,-1)
5335 shift(@a) splice(@a,0,1)
5336 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5337 $a[$i] = $y splice(@a,$i,1,$y)
5339 Example, assuming array lengths are passed before arrays:
5341 sub aeq { # compare two list values
5342 my(@a) = splice(@_,0,shift);
5343 my(@b) = splice(@_,0,shift);
5344 return 0 unless @a == @b; # same len?
5346 return 0 if pop(@a) ne pop(@b);
5350 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5352 =item split /PATTERN/,EXPR,LIMIT
5355 =item split /PATTERN/,EXPR
5357 =item split /PATTERN/
5361 Splits the string EXPR into a list of strings and returns that list. By
5362 default, empty leading fields are preserved, and empty trailing ones are
5363 deleted. (If all fields are empty, they are considered to be trailing.)
5365 In scalar context, returns the number of fields found and splits into
5366 the C<@_> array. Use of split in scalar context is deprecated, however,
5367 because it clobbers your subroutine arguments.
5369 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5370 splits on whitespace (after skipping any leading whitespace). Anything
5371 matching PATTERN is taken to be a delimiter separating the fields. (Note
5372 that the delimiter may be longer than one character.)
5374 If LIMIT is specified and positive, it represents the maximum number
5375 of fields the EXPR will be split into, though the actual number of
5376 fields returned depends on the number of times PATTERN matches within
5377 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5378 stripped (which potential users of C<pop> would do well to remember).
5379 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5380 had been specified. Note that splitting an EXPR that evaluates to the
5381 empty string always returns the empty list, regardless of the LIMIT
5384 A pattern matching the null string (not to be confused with
5385 a null pattern C<//>, which is just one member of the set of patterns
5386 matching a null string) will split the value of EXPR into separate
5387 characters at each point it matches that way. For example:
5389 print join(':', split(/ */, 'hi there'));
5391 produces the output 'h:i:t:h:e:r:e'.
5393 As a special case for C<split>, using the empty pattern C<//> specifically
5394 matches only the null string, and is not be confused with the regular use
5395 of C<//> to mean "the last successful pattern match". So, for C<split>,
5398 print join(':', split(//, 'hi there'));
5400 produces the output 'h:i: :t:h:e:r:e'.
5402 Empty leading (or trailing) fields are produced when there are positive
5403 width matches at the beginning (or end) of the string; a zero-width match
5404 at the beginning (or end) of the string does not produce an empty field.
5407 print join(':', split(/(?=\w)/, 'hi there!'));
5409 produces the output 'h:i :t:h:e:r:e!'.
5411 The LIMIT parameter can be used to split a line partially
5413 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5415 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5416 a LIMIT one larger than the number of variables in the list, to avoid
5417 unnecessary work. For the list above LIMIT would have been 4 by
5418 default. In time critical applications it behooves you not to split
5419 into more fields than you really need.
5421 If the PATTERN contains parentheses, additional list elements are
5422 created from each matching substring in the delimiter.
5424 split(/([,-])/, "1-10,20", 3);
5426 produces the list value
5428 (1, '-', 10, ',', 20)
5430 If you had the entire header of a normal Unix email message in $header,
5431 you could split it up into fields and their values this way:
5433 $header =~ s/\n\s+/ /g; # fix continuation lines
5434 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5436 The pattern C</PATTERN/> may be replaced with an expression to specify
5437 patterns that vary at runtime. (To do runtime compilation only once,
5438 use C</$variable/o>.)
5440 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5441 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5442 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5443 will give you as many null initial fields as there are leading spaces.
5444 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5445 whitespace produces a null first field. A C<split> with no arguments
5446 really does a S<C<split(' ', $_)>> internally.
5448 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5453 open(PASSWD, '/etc/passwd');
5456 ($login, $passwd, $uid, $gid,
5457 $gcos, $home, $shell) = split(/:/);
5461 As with regular pattern matching, any capturing parentheses that are not
5462 matched in a C<split()> will be set to C<undef> when returned:
5464 @fields = split /(A)|B/, "1A2B3";
5465 # @fields is (1, 'A', 2, undef, 3)
5467 =item sprintf FORMAT, LIST
5470 Returns a string formatted by the usual C<printf> conventions of the C
5471 library function C<sprintf>. See below for more details
5472 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5473 the general principles.
5477 # Format number with up to 8 leading zeroes
5478 $result = sprintf("%08d", $number);
5480 # Round number to 3 digits after decimal point
5481 $rounded = sprintf("%.3f", $number);
5483 Perl does its own C<sprintf> formatting--it emulates the C
5484 function C<sprintf>, but it doesn't use it (except for floating-point
5485 numbers, and even then only the standard modifiers are allowed). As a
5486 result, any non-standard extensions in your local C<sprintf> are not
5487 available from Perl.
5489 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5490 pass it an array as your first argument. The array is given scalar context,
5491 and instead of using the 0th element of the array as the format, Perl will
5492 use the count of elements in the array as the format, which is almost never
5495 Perl's C<sprintf> permits the following universally-known conversions:
5498 %c a character with the given number
5500 %d a signed integer, in decimal
5501 %u an unsigned integer, in decimal
5502 %o an unsigned integer, in octal
5503 %x an unsigned integer, in hexadecimal
5504 %e a floating-point number, in scientific notation
5505 %f a floating-point number, in fixed decimal notation
5506 %g a floating-point number, in %e or %f notation
5508 In addition, Perl permits the following widely-supported conversions:
5510 %X like %x, but using upper-case letters
5511 %E like %e, but using an upper-case "E"
5512 %G like %g, but with an upper-case "E" (if applicable)
5513 %b an unsigned integer, in binary
5514 %p a pointer (outputs the Perl value's address in hexadecimal)
5515 %n special: *stores* the number of characters output so far
5516 into the next variable in the parameter list
5518 Finally, for backward (and we do mean "backward") compatibility, Perl
5519 permits these unnecessary but widely-supported conversions:
5522 %D a synonym for %ld
5523 %U a synonym for %lu
5524 %O a synonym for %lo
5527 Note that the number of exponent digits in the scientific notation produced
5528 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5529 exponent less than 100 is system-dependent: it may be three or less
5530 (zero-padded as necessary). In other words, 1.23 times ten to the
5531 99th may be either "1.23e99" or "1.23e099".
5533 Between the C<%> and the format letter, you may specify a number of
5534 additional attributes controlling the interpretation of the format.
5535 In order, these are:
5539 =item format parameter index
5541 An explicit format parameter index, such as C<2$>. By default sprintf
5542 will format the next unused argument in the list, but this allows you
5543 to take the arguments out of order, e.g.:
5545 printf '%2$d %1$d', 12, 34; # prints "34 12"
5546 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5551 space prefix positive number with a space
5552 + prefix positive number with a plus sign
5553 - left-justify within the field
5554 0 use zeros, not spaces, to right-justify
5555 # prefix non-zero octal with "0", non-zero hex with "0x",
5556 non-zero binary with "0b"
5560 printf '<% d>', 12; # prints "< 12>"
5561 printf '<%+d>', 12; # prints "<+12>"
5562 printf '<%6s>', 12; # prints "< 12>"
5563 printf '<%-6s>', 12; # prints "<12 >"
5564 printf '<%06s>', 12; # prints "<000012>"
5565 printf '<%#x>', 12; # prints "<0xc>"
5569 This flag tells perl to interpret the supplied string as a vector of
5570 integers, one for each character in the string. Perl applies the format to
5571 each integer in turn, then joins the resulting strings with a separator (a
5572 dot C<.> by default). This can be useful for displaying ordinal values of
5573 characters in arbitrary strings:
5575 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5576 printf "version is v%vd\n", $^V; # Perl's version
5578 Put an asterisk C<*> before the C<v> to override the string to
5579 use to separate the numbers:
5581 printf "address is %*vX\n", ":", $addr; # IPv6 address
5582 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5584 You can also explicitly specify the argument number to use for
5585 the join string using e.g. C<*2$v>:
5587 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5589 =item (minimum) width
5591 Arguments are usually formatted to be only as wide as required to
5592 display the given value. You can override the width by putting
5593 a number here, or get the width from the next argument (with C<*>)
5594 or from a specified argument (with e.g. C<*2$>):
5596 printf '<%s>', "a"; # prints "<a>"
5597 printf '<%6s>', "a"; # prints "< a>"
5598 printf '<%*s>', 6, "a"; # prints "< a>"
5599 printf '<%*2$s>', "a", 6; # prints "< a>"
5600 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5602 If a field width obtained through C<*> is negative, it has the same
5603 effect as the C<-> flag: left-justification.
5605 =item precision, or maximum width
5608 You can specify a precision (for numeric conversions) or a maximum
5609 width (for string conversions) by specifying a C<.> followed by a number.
5610 For floating point formats, with the exception of 'g' and 'G', this specifies
5611 the number of decimal places to show (the default being 6), e.g.:
5613 # these examples are subject to system-specific variation
5614 printf '<%f>', 1; # prints "<1.000000>"
5615 printf '<%.1f>', 1; # prints "<1.0>"
5616 printf '<%.0f>', 1; # prints "<1>"
5617 printf '<%e>', 10; # prints "<1.000000e+01>"
5618 printf '<%.1e>', 10; # prints "<1.0e+01>"
5620 For 'g' and 'G', this specifies the maximum number of digits to show,
5621 including prior to the decimal point as well as after it, e.g.:
5623 # these examples are subject to system-specific variation
5624 printf '<%g>', 1; # prints "<1>"
5625 printf '<%.10g>', 1; # prints "<1>"
5626 printf '<%g>', 100; # prints "<100>"
5627 printf '<%.1g>', 100; # prints "<1e+02>"
5628 printf '<%.2g>', 100.01; # prints "<1e+02>"
5629 printf '<%.5g>', 100.01; # prints "<100.01>"
5630 printf '<%.4g>', 100.01; # prints "<100>"
5632 For integer conversions, specifying a precision implies that the
5633 output of the number itself should be zero-padded to this width:
5635 printf '<%.6x>', 1; # prints "<000001>"
5636 printf '<%#.6x>', 1; # prints "<0x000001>"
5637 printf '<%-10.6x>', 1; # prints "<000001 >"
5639 For string conversions, specifying a precision truncates the string
5640 to fit in the specified width:
5642 printf '<%.5s>', "truncated"; # prints "<trunc>"
5643 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5645 You can also get the precision from the next argument using C<.*>:
5647 printf '<%.6x>', 1; # prints "<000001>"
5648 printf '<%.*x>', 6, 1; # prints "<000001>"
5650 You cannot currently get the precision from a specified number,
5651 but it is intended that this will be possible in the future using
5654 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5658 For numeric conversions, you can specify the size to interpret the
5659 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5660 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5661 whatever the default integer size is on your platform (usually 32 or 64
5662 bits), but you can override this to use instead one of the standard C types,
5663 as supported by the compiler used to build Perl:
5665 l interpret integer as C type "long" or "unsigned long"
5666 h interpret integer as C type "short" or "unsigned short"
5667 q, L or ll interpret integer as C type "long long", "unsigned long long".
5668 or "quads" (typically 64-bit integers)
5670 The last will produce errors if Perl does not understand "quads" in your
5671 installation. (This requires that either the platform natively supports quads
5672 or Perl was specifically compiled to support quads.) You can find out
5673 whether your Perl supports quads via L<Config>:
5676 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5679 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5680 to be the default floating point size on your platform (double or long double),
5681 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5682 platform supports them. You can find out whether your Perl supports long
5683 doubles via L<Config>:
5686 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5688 You can find out whether Perl considers 'long double' to be the default
5689 floating point size to use on your platform via L<Config>:
5692 ($Config{uselongdouble} eq 'define') &&
5693 print "long doubles by default\n";
5695 It can also be the case that long doubles and doubles are the same thing:
5698 ($Config{doublesize} == $Config{longdblsize}) &&
5699 print "doubles are long doubles\n";
5701 The size specifier C<V> has no effect for Perl code, but it is supported
5702 for compatibility with XS code; it means 'use the standard size for
5703 a Perl integer (or floating-point number)', which is already the
5704 default for Perl code.
5706 =item order of arguments
5708 Normally, sprintf takes the next unused argument as the value to
5709 format for each format specification. If the format specification
5710 uses C<*> to require additional arguments, these are consumed from
5711 the argument list in the order in which they appear in the format
5712 specification I<before> the value to format. Where an argument is
5713 specified using an explicit index, this does not affect the normal
5714 order for the arguments (even when the explicitly specified index
5715 would have been the next argument in any case).
5719 printf '<%*.*s>', $a, $b, $c;
5721 would use C<$a> for the width, C<$b> for the precision and C<$c>
5722 as the value to format, while:
5724 print '<%*1$.*s>', $a, $b;
5726 would use C<$a> for the width and the precision, and C<$b> as the
5729 Here are some more examples - beware that when using an explicit
5730 index, the C<$> may need to be escaped:
5732 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5733 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5734 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5735 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5739 If C<use locale> is in effect, the character used for the decimal
5740 point in formatted real numbers is affected by the LC_NUMERIC locale.
5744 X<sqrt> X<root> X<square root>
5748 Return the square root of EXPR. If EXPR is omitted, returns square
5749 root of C<$_>. Only works on non-negative operands, unless you've
5750 loaded the standard Math::Complex module.
5753 print sqrt(-2); # prints 1.4142135623731i
5756 X<srand> X<seed> X<randseed>
5760 Sets the random number seed for the C<rand> operator.
5762 The point of the function is to "seed" the C<rand> function so that
5763 C<rand> can produce a different sequence each time you run your
5766 If srand() is not called explicitly, it is called implicitly at the
5767 first use of the C<rand> operator. However, this was not the case in
5768 versions of Perl before 5.004, so if your script will run under older
5769 Perl versions, it should call C<srand>.
5771 Most programs won't even call srand() at all, except those that
5772 need a cryptographically-strong starting point rather than the
5773 generally acceptable default, which is based on time of day,
5774 process ID, and memory allocation, or the F</dev/urandom> device,
5777 You can call srand($seed) with the same $seed to reproduce the
5778 I<same> sequence from rand(), but this is usually reserved for
5779 generating predictable results for testing or debugging.
5780 Otherwise, don't call srand() more than once in your program.
5782 Do B<not> call srand() (i.e. without an argument) more than once in
5783 a script. The internal state of the random number generator should
5784 contain more entropy than can be provided by any seed, so calling
5785 srand() again actually I<loses> randomness.
5787 Most implementations of C<srand> take an integer and will silently
5788 truncate decimal numbers. This means C<srand(42)> will usually
5789 produce the same results as C<srand(42.1)>. To be safe, always pass
5790 C<srand> an integer.
5792 In versions of Perl prior to 5.004 the default seed was just the
5793 current C<time>. This isn't a particularly good seed, so many old
5794 programs supply their own seed value (often C<time ^ $$> or C<time ^
5795 ($$ + ($$ << 15))>), but that isn't necessary any more.
5797 For cryptographic purposes, however, you need something much more random
5798 than the default seed. Checksumming the compressed output of one or more
5799 rapidly changing operating system status programs is the usual method. For
5802 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5804 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5807 Frequently called programs (like CGI scripts) that simply use
5811 for a seed can fall prey to the mathematical property that
5815 one-third of the time. So don't do that.
5817 =item stat FILEHANDLE
5818 X<stat> X<file, status> X<ctime>
5822 =item stat DIRHANDLE
5826 Returns a 13-element list giving the status info for a file, either
5827 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5828 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5831 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5832 $atime,$mtime,$ctime,$blksize,$blocks)
5835 Not all fields are supported on all filesystem types. Here are the
5836 meanings of the fields:
5838 0 dev device number of filesystem
5840 2 mode file mode (type and permissions)
5841 3 nlink number of (hard) links to the file
5842 4 uid numeric user ID of file's owner
5843 5 gid numeric group ID of file's owner
5844 6 rdev the device identifier (special files only)
5845 7 size total size of file, in bytes
5846 8 atime last access time in seconds since the epoch
5847 9 mtime last modify time in seconds since the epoch
5848 10 ctime inode change time in seconds since the epoch (*)
5849 11 blksize preferred block size for file system I/O
5850 12 blocks actual number of blocks allocated
5852 (The epoch was at 00:00 January 1, 1970 GMT.)
5854 (*) Not all fields are supported on all filesystem types. Notably, the
5855 ctime field is non-portable. In particular, you cannot expect it to be a
5856 "creation time", see L<perlport/"Files and Filesystems"> for details.
5858 If C<stat> is passed the special filehandle consisting of an underline, no
5859 stat is done, but the current contents of the stat structure from the
5860 last C<stat>, C<lstat>, or filetest are returned. Example:
5862 if (-x $file && (($d) = stat(_)) && $d < 0) {
5863 print "$file is executable NFS file\n";
5866 (This works on machines only for which the device number is negative
5869 Because the mode contains both the file type and its permissions, you
5870 should mask off the file type portion and (s)printf using a C<"%o">
5871 if you want to see the real permissions.
5873 $mode = (stat($filename))[2];
5874 printf "Permissions are %04o\n", $mode & 07777;
5876 In scalar context, C<stat> returns a boolean value indicating success
5877 or failure, and, if successful, sets the information associated with
5878 the special filehandle C<_>.
5880 The L<File::stat> module provides a convenient, by-name access mechanism:
5883 $sb = stat($filename);
5884 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5885 $filename, $sb->size, $sb->mode & 07777,
5886 scalar localtime $sb->mtime;
5888 You can import symbolic mode constants (C<S_IF*>) and functions
5889 (C<S_IS*>) from the Fcntl module:
5893 $mode = (stat($filename))[2];
5895 $user_rwx = ($mode & S_IRWXU) >> 6;
5896 $group_read = ($mode & S_IRGRP) >> 3;
5897 $other_execute = $mode & S_IXOTH;
5899 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5901 $is_setuid = $mode & S_ISUID;
5902 $is_setgid = S_ISDIR($mode);
5904 You could write the last two using the C<-u> and C<-d> operators.
5905 The commonly available C<S_IF*> constants are
5907 # Permissions: read, write, execute, for user, group, others.
5909 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5910 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5911 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5913 # Setuid/Setgid/Stickiness/SaveText.
5914 # Note that the exact meaning of these is system dependent.
5916 S_ISUID S_ISGID S_ISVTX S_ISTXT
5918 # File types. Not necessarily all are available on your system.
5920 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5922 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5924 S_IREAD S_IWRITE S_IEXEC
5926 and the C<S_IF*> functions are
5928 S_IMODE($mode) the part of $mode containing the permission bits
5929 and the setuid/setgid/sticky bits
5931 S_IFMT($mode) the part of $mode containing the file type
5932 which can be bit-anded with e.g. S_IFREG
5933 or with the following functions
5935 # The operators -f, -d, -l, -b, -c, -p, and -S.
5937 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5938 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5940 # No direct -X operator counterpart, but for the first one
5941 # the -g operator is often equivalent. The ENFMT stands for
5942 # record flocking enforcement, a platform-dependent feature.
5944 S_ISENFMT($mode) S_ISWHT($mode)
5946 See your native chmod(2) and stat(2) documentation for more details
5947 about the C<S_*> constants. To get status info for a symbolic link
5948 instead of the target file behind the link, use the C<lstat> function.
5953 =item state TYPE EXPR
5955 =item state EXPR : ATTRS
5957 =item state TYPE EXPR : ATTRS
5959 C<state> declares a lexically scoped variable, just like C<my> does.
5960 However, those variables will be initialized only once, contrary to
5961 lexical variables that are reinitialized each time their enclosing block
5964 C<state> variables are only enabled when the C<feature 'state'> pragma is
5965 in effect. See L<feature>.
5972 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5973 doing many pattern matches on the string before it is next modified.
5974 This may or may not save time, depending on the nature and number of
5975 patterns you are searching on, and on the distribution of character
5976 frequencies in the string to be searched--you probably want to compare
5977 run times with and without it to see which runs faster. Those loops
5978 that scan for many short constant strings (including the constant
5979 parts of more complex patterns) will benefit most. You may have only
5980 one C<study> active at a time--if you study a different scalar the first
5981 is "unstudied". (The way C<study> works is this: a linked list of every
5982 character in the string to be searched is made, so we know, for
5983 example, where all the C<'k'> characters are. From each search string,
5984 the rarest character is selected, based on some static frequency tables
5985 constructed from some C programs and English text. Only those places
5986 that contain this "rarest" character are examined.)
5988 For example, here is a loop that inserts index producing entries
5989 before any line containing a certain pattern:
5993 print ".IX foo\n" if /\bfoo\b/;
5994 print ".IX bar\n" if /\bbar\b/;
5995 print ".IX blurfl\n" if /\bblurfl\b/;
6000 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6001 will be looked at, because C<f> is rarer than C<o>. In general, this is
6002 a big win except in pathological cases. The only question is whether
6003 it saves you more time than it took to build the linked list in the
6006 Note that if you have to look for strings that you don't know till
6007 runtime, you can build an entire loop as a string and C<eval> that to
6008 avoid recompiling all your patterns all the time. Together with
6009 undefining C<$/> to input entire files as one record, this can be very
6010 fast, often faster than specialized programs like fgrep(1). The following
6011 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6012 out the names of those files that contain a match:
6014 $search = 'while (<>) { study;';
6015 foreach $word (@words) {
6016 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6021 eval $search; # this screams
6022 $/ = "\n"; # put back to normal input delimiter
6023 foreach $file (sort keys(%seen)) {
6027 =item sub NAME BLOCK
6030 =item sub NAME (PROTO) BLOCK
6032 =item sub NAME : ATTRS BLOCK
6034 =item sub NAME (PROTO) : ATTRS BLOCK
6036 This is subroutine definition, not a real function I<per se>.
6037 Without a BLOCK it's just a forward declaration. Without a NAME,
6038 it's an anonymous function declaration, and does actually return
6039 a value: the CODE ref of the closure you just created.
6041 See L<perlsub> and L<perlref> for details about subroutines and
6042 references, and L<attributes> and L<Attribute::Handlers> for more
6043 information about attributes.
6045 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6046 X<substr> X<substring> X<mid> X<left> X<right>
6048 =item substr EXPR,OFFSET,LENGTH
6050 =item substr EXPR,OFFSET
6052 Extracts a substring out of EXPR and returns it. First character is at
6053 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6054 If OFFSET is negative (or more precisely, less than C<$[>), starts
6055 that far from the end of the string. If LENGTH is omitted, returns
6056 everything to the end of the string. If LENGTH is negative, leaves that
6057 many characters off the end of the string.
6059 my $s = "The black cat climbed the green tree";
6060 my $color = substr $s, 4, 5; # black
6061 my $middle = substr $s, 4, -11; # black cat climbed the
6062 my $end = substr $s, 14; # climbed the green tree
6063 my $tail = substr $s, -4; # tree
6064 my $z = substr $s, -4, 2; # tr
6066 You can use the substr() function as an lvalue, in which case EXPR
6067 must itself be an lvalue. If you assign something shorter than LENGTH,
6068 the string will shrink, and if you assign something longer than LENGTH,
6069 the string will grow to accommodate it. To keep the string the same
6070 length you may need to pad or chop your value using C<sprintf>.
6072 If OFFSET and LENGTH specify a substring that is partly outside the
6073 string, only the part within the string is returned. If the substring
6074 is beyond either end of the string, substr() returns the undefined
6075 value and produces a warning. When used as an lvalue, specifying a
6076 substring that is entirely outside the string is a fatal error.
6077 Here's an example showing the behavior for boundary cases:
6080 substr($name, 4) = 'dy'; # $name is now 'freddy'
6081 my $null = substr $name, 6, 2; # returns '' (no warning)
6082 my $oops = substr $name, 7; # returns undef, with warning
6083 substr($name, 7) = 'gap'; # fatal error
6085 An alternative to using substr() as an lvalue is to specify the
6086 replacement string as the 4th argument. This allows you to replace
6087 parts of the EXPR and return what was there before in one operation,
6088 just as you can with splice().
6090 my $s = "The black cat climbed the green tree";
6091 my $z = substr $s, 14, 7, "jumped from"; # climbed
6092 # $s is now "The black cat jumped from the green tree"
6094 Note that the lvalue returned by the 3-arg version of substr() acts as
6095 a 'magic bullet'; each time it is assigned to, it remembers which part
6096 of the original string is being modified; for example:
6099 for (substr($x,1,2)) {
6100 $_ = 'a'; print $x,"\n"; # prints 1a4
6101 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6103 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6106 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6109 =item symlink OLDFILE,NEWFILE
6110 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6112 Creates a new filename symbolically linked to the old filename.
6113 Returns C<1> for success, C<0> otherwise. On systems that don't support
6114 symbolic links, produces a fatal error at run time. To check for that,
6117 $symlink_exists = eval { symlink("",""); 1 };
6119 =item syscall NUMBER, LIST
6120 X<syscall> X<system call>
6122 Calls the system call specified as the first element of the list,
6123 passing the remaining elements as arguments to the system call. If
6124 unimplemented, produces a fatal error. The arguments are interpreted
6125 as follows: if a given argument is numeric, the argument is passed as
6126 an int. If not, the pointer to the string value is passed. You are
6127 responsible to make sure a string is pre-extended long enough to
6128 receive any result that might be written into a string. You can't use a
6129 string literal (or other read-only string) as an argument to C<syscall>
6130 because Perl has to assume that any string pointer might be written
6132 integer arguments are not literals and have never been interpreted in a
6133 numeric context, you may need to add C<0> to them to force them to look
6134 like numbers. This emulates the C<syswrite> function (or vice versa):
6136 require 'syscall.ph'; # may need to run h2ph
6138 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6140 Note that Perl supports passing of up to only 14 arguments to your system call,
6141 which in practice should usually suffice.
6143 Syscall returns whatever value returned by the system call it calls.
6144 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6145 Note that some system calls can legitimately return C<-1>. The proper
6146 way to handle such calls is to assign C<$!=0;> before the call and
6147 check the value of C<$!> if syscall returns C<-1>.
6149 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6150 number of the read end of the pipe it creates. There is no way
6151 to retrieve the file number of the other end. You can avoid this
6152 problem by using C<pipe> instead.
6154 =item sysopen FILEHANDLE,FILENAME,MODE
6157 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6159 Opens the file whose filename is given by FILENAME, and associates it
6160 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6161 the name of the real filehandle wanted. This function calls the
6162 underlying operating system's C<open> function with the parameters
6163 FILENAME, MODE, PERMS.
6165 The possible values and flag bits of the MODE parameter are
6166 system-dependent; they are available via the standard module C<Fcntl>.
6167 See the documentation of your operating system's C<open> to see which
6168 values and flag bits are available. You may combine several flags
6169 using the C<|>-operator.
6171 Some of the most common values are C<O_RDONLY> for opening the file in
6172 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6173 and C<O_RDWR> for opening the file in read-write mode.
6174 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6176 For historical reasons, some values work on almost every system
6177 supported by perl: zero means read-only, one means write-only, and two
6178 means read/write. We know that these values do I<not> work under
6179 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6180 use them in new code.
6182 If the file named by FILENAME does not exist and the C<open> call creates
6183 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6184 PERMS specifies the permissions of the newly created file. If you omit
6185 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6186 These permission values need to be in octal, and are modified by your
6187 process's current C<umask>.
6190 In many systems the C<O_EXCL> flag is available for opening files in
6191 exclusive mode. This is B<not> locking: exclusiveness means here that
6192 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6193 on network filesystems, and has no effect unless the C<O_CREAT> flag
6194 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6195 being opened if it is a symbolic link. It does not protect against
6196 symbolic links in the file's path.
6199 Sometimes you may want to truncate an already-existing file. This
6200 can be done using the C<O_TRUNC> flag. The behavior of
6201 C<O_TRUNC> with C<O_RDONLY> is undefined.
6204 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6205 that takes away the user's option to have a more permissive umask.
6206 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6209 Note that C<sysopen> depends on the fdopen() C library function.
6210 On many UNIX systems, fdopen() is known to fail when file descriptors
6211 exceed a certain value, typically 255. If you need more file
6212 descriptors than that, consider rebuilding Perl to use the C<sfio>
6213 library, or perhaps using the POSIX::open() function.
6215 See L<perlopentut> for a kinder, gentler explanation of opening files.
6217 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6220 =item sysread FILEHANDLE,SCALAR,LENGTH
6222 Attempts to read LENGTH bytes of data into variable SCALAR from the
6223 specified FILEHANDLE, using the system call read(2). It bypasses
6224 buffered IO, so mixing this with other kinds of reads, C<print>,
6225 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6226 perlio or stdio layers usually buffers data. Returns the number of
6227 bytes actually read, C<0> at end of file, or undef if there was an
6228 error (in the latter case C<$!> is also set). SCALAR will be grown or
6229 shrunk so that the last byte actually read is the last byte of the
6230 scalar after the read.
6232 An OFFSET may be specified to place the read data at some place in the
6233 string other than the beginning. A negative OFFSET specifies
6234 placement at that many characters counting backwards from the end of
6235 the string. A positive OFFSET greater than the length of SCALAR
6236 results in the string being padded to the required size with C<"\0">
6237 bytes before the result of the read is appended.
6239 There is no syseof() function, which is ok, since eof() doesn't work
6240 very well on device files (like ttys) anyway. Use sysread() and check
6241 for a return value for 0 to decide whether you're done.
6243 Note that if the filehandle has been marked as C<:utf8> Unicode
6244 characters are read instead of bytes (the LENGTH, OFFSET, and the
6245 return value of sysread() are in Unicode characters).
6246 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6247 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6249 =item sysseek FILEHANDLE,POSITION,WHENCE
6252 Sets FILEHANDLE's system position in bytes using the system call
6253 lseek(2). FILEHANDLE may be an expression whose value gives the name
6254 of the filehandle. The values for WHENCE are C<0> to set the new
6255 position to POSITION, C<1> to set the it to the current position plus
6256 POSITION, and C<2> to set it to EOF plus POSITION (typically
6259 Note the I<in bytes>: even if the filehandle has been set to operate
6260 on characters (for example by using the C<:utf8> I/O layer), tell()
6261 will return byte offsets, not character offsets (because implementing
6262 that would render sysseek() very slow).
6264 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6265 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6266 C<seek>, C<tell>, or C<eof> may cause confusion.
6268 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6269 and C<SEEK_END> (start of the file, current position, end of the file)
6270 from the Fcntl module. Use of the constants is also more portable
6271 than relying on 0, 1, and 2. For example to define a "systell" function:
6273 use Fcntl 'SEEK_CUR';
6274 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6276 Returns the new position, or the undefined value on failure. A position
6277 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6278 true on success and false on failure, yet you can still easily determine
6284 =item system PROGRAM LIST
6286 Does exactly the same thing as C<exec LIST>, except that a fork is
6287 done first, and the parent process waits for the child process to
6288 complete. Note that argument processing varies depending on the
6289 number of arguments. If there is more than one argument in LIST,
6290 or if LIST is an array with more than one value, starts the program
6291 given by the first element of the list with arguments given by the
6292 rest of the list. If there is only one scalar argument, the argument
6293 is checked for shell metacharacters, and if there are any, the
6294 entire argument is passed to the system's command shell for parsing
6295 (this is C</bin/sh -c> on Unix platforms, but varies on other
6296 platforms). If there are no shell metacharacters in the argument,
6297 it is split into words and passed directly to C<execvp>, which is
6300 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6301 output before any operation that may do a fork, but this may not be
6302 supported on some platforms (see L<perlport>). To be safe, you may need
6303 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6304 of C<IO::Handle> on any open handles.
6306 The return value is the exit status of the program as returned by the
6307 C<wait> call. To get the actual exit value, shift right by eight (see
6308 below). See also L</exec>. This is I<not> what you want to use to capture
6309 the output from a command, for that you should use merely backticks or
6310 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6311 indicates a failure to start the program or an error of the wait(2) system
6312 call (inspect $! for the reason).
6314 Like C<exec>, C<system> allows you to lie to a program about its name if
6315 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6317 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6318 C<system>, if you expect your program to terminate on receipt of these
6319 signals you will need to arrange to do so yourself based on the return
6322 @args = ("command", "arg1", "arg2");
6324 or die "system @args failed: $?"
6326 You can check all the failure possibilities by inspecting
6330 print "failed to execute: $!\n";
6333 printf "child died with signal %d, %s coredump\n",
6334 ($? & 127), ($? & 128) ? 'with' : 'without';
6337 printf "child exited with value %d\n", $? >> 8;
6340 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6341 with the W*() calls of the POSIX extension.
6343 When the arguments get executed via the system shell, results
6344 and return codes will be subject to its quirks and capabilities.
6345 See L<perlop/"`STRING`"> and L</exec> for details.
6347 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6350 =item syswrite FILEHANDLE,SCALAR,LENGTH
6352 =item syswrite FILEHANDLE,SCALAR
6354 Attempts to write LENGTH bytes of data from variable SCALAR to the
6355 specified FILEHANDLE, using the system call write(2). If LENGTH is
6356 not specified, writes whole SCALAR. It bypasses buffered IO, so
6357 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6358 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6359 stdio layers usually buffers data. Returns the number of bytes
6360 actually written, or C<undef> if there was an error (in this case the
6361 errno variable C<$!> is also set). If the LENGTH is greater than the
6362 available data in the SCALAR after the OFFSET, only as much data as is
6363 available will be written.
6365 An OFFSET may be specified to write the data from some part of the
6366 string other than the beginning. A negative OFFSET specifies writing
6367 that many characters counting backwards from the end of the string.
6368 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6370 Note that if the filehandle has been marked as C<:utf8>, Unicode
6371 characters are written instead of bytes (the LENGTH, OFFSET, and the
6372 return value of syswrite() are in UTF-8 encoded Unicode characters).
6373 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6374 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6376 =item tell FILEHANDLE
6381 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6382 error. FILEHANDLE may be an expression whose value gives the name of
6383 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6386 Note the I<in bytes>: even if the filehandle has been set to
6387 operate on characters (for example by using the C<:utf8> open
6388 layer), tell() will return byte offsets, not character offsets
6389 (because that would render seek() and tell() rather slow).
6391 The return value of tell() for the standard streams like the STDIN
6392 depends on the operating system: it may return -1 or something else.
6393 tell() on pipes, fifos, and sockets usually returns -1.
6395 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6397 Do not use tell() (or other buffered I/O operations) on a file handle
6398 that has been manipulated by sysread(), syswrite() or sysseek().
6399 Those functions ignore the buffering, while tell() does not.
6401 =item telldir DIRHANDLE
6404 Returns the current position of the C<readdir> routines on DIRHANDLE.
6405 Value may be given to C<seekdir> to access a particular location in a
6406 directory. C<telldir> has the same caveats about possible directory
6407 compaction as the corresponding system library routine.
6409 =item tie VARIABLE,CLASSNAME,LIST
6412 This function binds a variable to a package class that will provide the
6413 implementation for the variable. VARIABLE is the name of the variable
6414 to be enchanted. CLASSNAME is the name of a class implementing objects
6415 of correct type. Any additional arguments are passed to the C<new>
6416 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6417 or C<TIEHASH>). Typically these are arguments such as might be passed
6418 to the C<dbm_open()> function of C. The object returned by the C<new>
6419 method is also returned by the C<tie> function, which would be useful
6420 if you want to access other methods in CLASSNAME.
6422 Note that functions such as C<keys> and C<values> may return huge lists
6423 when used on large objects, like DBM files. You may prefer to use the
6424 C<each> function to iterate over such. Example:
6426 # print out history file offsets
6428 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6429 while (($key,$val) = each %HIST) {
6430 print $key, ' = ', unpack('L',$val), "\n";
6434 A class implementing a hash should have the following methods:
6436 TIEHASH classname, LIST
6438 STORE this, key, value
6443 NEXTKEY this, lastkey
6448 A class implementing an ordinary array should have the following methods:
6450 TIEARRAY classname, LIST
6452 STORE this, key, value
6454 STORESIZE this, count
6460 SPLICE this, offset, length, LIST
6465 A class implementing a file handle should have the following methods:
6467 TIEHANDLE classname, LIST
6468 READ this, scalar, length, offset
6471 WRITE this, scalar, length, offset
6473 PRINTF this, format, LIST
6477 SEEK this, position, whence
6479 OPEN this, mode, LIST
6484 A class implementing a scalar should have the following methods:
6486 TIESCALAR classname, LIST
6492 Not all methods indicated above need be implemented. See L<perltie>,
6493 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6495 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6496 for you--you need to do that explicitly yourself. See L<DB_File>
6497 or the F<Config> module for interesting C<tie> implementations.
6499 For further details see L<perltie>, L<"tied VARIABLE">.
6504 Returns a reference to the object underlying VARIABLE (the same value
6505 that was originally returned by the C<tie> call that bound the variable
6506 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6512 Returns the number of non-leap seconds since whatever time the system
6513 considers to be the epoch, suitable for feeding to C<gmtime> and
6514 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6515 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6516 1904 in the current local time zone for its epoch.
6518 For measuring time in better granularity than one second,
6519 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6520 Perl 5.8 part of the standard distribution), or if you have
6521 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6522 See L<perlfaq8> for details.
6524 For date and time processing look at the many related modules on CPAN.
6525 For a comprehensive date and time representation look at the
6531 Returns a four-element list giving the user and system times, in
6532 seconds, for this process and the children of this process.
6534 ($user,$system,$cuser,$csystem) = times;
6536 In scalar context, C<times> returns C<$user>.
6538 Note that times for children are included only after they terminate.
6542 The transliteration operator. Same as C<y///>. See L<perlop>.
6544 =item truncate FILEHANDLE,LENGTH
6547 =item truncate EXPR,LENGTH
6549 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6550 specified length. Produces a fatal error if truncate isn't implemented
6551 on your system. Returns true if successful, the undefined value
6554 The behavior is undefined if LENGTH is greater than the length of the
6557 The position in the file of FILEHANDLE is left unchanged. You may want to
6558 call L<seek> before writing to the file.
6561 X<uc> X<uppercase> X<toupper>
6565 Returns an uppercased version of EXPR. This is the internal function
6566 implementing the C<\U> escape in double-quoted strings. Respects
6567 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6568 and L<perlunicode> for more details about locale and Unicode support.
6569 It does not attempt to do titlecase mapping on initial letters. See
6570 C<ucfirst> for that.
6572 If EXPR is omitted, uses C<$_>.
6575 X<ucfirst> X<uppercase>
6579 Returns the value of EXPR with the first character in uppercase
6580 (titlecase in Unicode). This is the internal function implementing
6581 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6582 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6583 for more details about locale and Unicode support.
6585 If EXPR is omitted, uses C<$_>.
6592 Sets the umask for the process to EXPR and returns the previous value.
6593 If EXPR is omitted, merely returns the current umask.
6595 The Unix permission C<rwxr-x---> is represented as three sets of three
6596 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6597 and isn't one of the digits). The C<umask> value is such a number
6598 representing disabled permissions bits. The permission (or "mode")
6599 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6600 even if you tell C<sysopen> to create a file with permissions C<0777>,
6601 if your umask is C<0022> then the file will actually be created with
6602 permissions C<0755>. If your C<umask> were C<0027> (group can't
6603 write; others can't read, write, or execute), then passing
6604 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6607 Here's some advice: supply a creation mode of C<0666> for regular
6608 files (in C<sysopen>) and one of C<0777> for directories (in
6609 C<mkdir>) and executable files. This gives users the freedom of
6610 choice: if they want protected files, they might choose process umasks
6611 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6612 Programs should rarely if ever make policy decisions better left to
6613 the user. The exception to this is when writing files that should be
6614 kept private: mail files, web browser cookies, I<.rhosts> files, and
6617 If umask(2) is not implemented on your system and you are trying to
6618 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6619 fatal error at run time. If umask(2) is not implemented and you are
6620 not trying to restrict access for yourself, returns C<undef>.
6622 Remember that a umask is a number, usually given in octal; it is I<not> a
6623 string of octal digits. See also L</oct>, if all you have is a string.
6626 X<undef> X<undefine>
6630 Undefines the value of EXPR, which must be an lvalue. Use only on a
6631 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6632 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6633 will probably not do what you expect on most predefined variables or
6634 DBM list values, so don't do that; see L<delete>.) Always returns the
6635 undefined value. You can omit the EXPR, in which case nothing is
6636 undefined, but you still get an undefined value that you could, for
6637 instance, return from a subroutine, assign to a variable or pass as a
6638 parameter. Examples:
6641 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6645 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6646 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6647 select undef, undef, undef, 0.25;
6648 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6650 Note that this is a unary operator, not a list operator.
6653 X<unlink> X<delete> X<remove> X<rm> X<del>
6657 Deletes a list of files. Returns the number of files successfully
6660 $cnt = unlink 'a', 'b', 'c';
6664 Note: C<unlink> will not attempt to delete directories unless you are superuser
6665 and the B<-U> flag is supplied to Perl. Even if these conditions are
6666 met, be warned that unlinking a directory can inflict damage on your
6667 filesystem. Finally, using C<unlink> on directories is not supported on
6668 many operating systems. Use C<rmdir> instead.
6670 If LIST is omitted, uses C<$_>.
6672 =item unpack TEMPLATE,EXPR
6675 =item unpack TEMPLATE
6677 C<unpack> does the reverse of C<pack>: it takes a string
6678 and expands it out into a list of values.
6679 (In scalar context, it returns merely the first value produced.)
6681 If EXPR is omitted, unpacks the C<$_> string.
6683 The string is broken into chunks described by the TEMPLATE. Each chunk
6684 is converted separately to a value. Typically, either the string is a result
6685 of C<pack>, or the characters of the string represent a C structure of some
6688 The TEMPLATE has the same format as in the C<pack> function.
6689 Here's a subroutine that does substring:
6692 my($what,$where,$howmuch) = @_;
6693 unpack("x$where a$howmuch", $what);
6698 sub ordinal { unpack("W",$_[0]); } # same as ord()
6700 In addition to fields allowed in pack(), you may prefix a field with
6701 a %<number> to indicate that
6702 you want a <number>-bit checksum of the items instead of the items
6703 themselves. Default is a 16-bit checksum. Checksum is calculated by
6704 summing numeric values of expanded values (for string fields the sum of
6705 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6707 For example, the following
6708 computes the same number as the System V sum program:
6712 unpack("%32W*",<>) % 65535;
6715 The following efficiently counts the number of set bits in a bit vector:
6717 $setbits = unpack("%32b*", $selectmask);
6719 The C<p> and C<P> formats should be used with care. Since Perl
6720 has no way of checking whether the value passed to C<unpack()>
6721 corresponds to a valid memory location, passing a pointer value that's
6722 not known to be valid is likely to have disastrous consequences.
6724 If there are more pack codes or if the repeat count of a field or a group
6725 is larger than what the remainder of the input string allows, the result
6726 is not well defined: in some cases, the repeat count is decreased, or
6727 C<unpack()> will produce null strings or zeroes, or terminate with an
6728 error. If the input string is longer than one described by the TEMPLATE,
6729 the rest is ignored.
6731 See L</pack> for more examples and notes.
6733 =item untie VARIABLE
6736 Breaks the binding between a variable and a package. (See C<tie>.)
6737 Has no effect if the variable is not tied.
6739 =item unshift ARRAY,LIST
6742 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6743 depending on how you look at it. Prepends list to the front of the
6744 array, and returns the new number of elements in the array.
6746 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6748 Note the LIST is prepended whole, not one element at a time, so the
6749 prepended elements stay in the same order. Use C<reverse> to do the
6752 =item use Module VERSION LIST
6753 X<use> X<module> X<import>
6755 =item use Module VERSION
6757 =item use Module LIST
6763 Imports some semantics into the current package from the named module,
6764 generally by aliasing certain subroutine or variable names into your
6765 package. It is exactly equivalent to
6767 BEGIN { require Module; import Module LIST; }
6769 except that Module I<must> be a bareword.
6771 VERSION may be either a numeric argument such as 5.006, which will be
6772 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6773 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6774 greater than the version of the current Perl interpreter; Perl will not
6775 attempt to parse the rest of the file. Compare with L</require>, which can
6776 do a similar check at run time.
6778 Specifying VERSION as a literal of the form v5.6.1 should generally be
6779 avoided, because it leads to misleading error messages under earlier
6780 versions of Perl that do not support this syntax. The equivalent numeric
6781 version should be used instead.
6783 use v5.6.1; # compile time version check
6785 use 5.006_001; # ditto; preferred for backwards compatibility
6787 This is often useful if you need to check the current Perl version before
6788 C<use>ing library modules that have changed in incompatible ways from
6789 older versions of Perl. (We try not to do this more than we have to.)
6791 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6792 C<require> makes sure the module is loaded into memory if it hasn't been
6793 yet. The C<import> is not a builtin--it's just an ordinary static method
6794 call into the C<Module> package to tell the module to import the list of
6795 features back into the current package. The module can implement its
6796 C<import> method any way it likes, though most modules just choose to
6797 derive their C<import> method via inheritance from the C<Exporter> class that
6798 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6799 method can be found then the call is skipped, even if there is an AUTOLOAD
6802 If you do not want to call the package's C<import> method (for instance,
6803 to stop your namespace from being altered), explicitly supply the empty list:
6807 That is exactly equivalent to
6809 BEGIN { require Module }
6811 If the VERSION argument is present between Module and LIST, then the
6812 C<use> will call the VERSION method in class Module with the given
6813 version as an argument. The default VERSION method, inherited from
6814 the UNIVERSAL class, croaks if the given version is larger than the
6815 value of the variable C<$Module::VERSION>.
6817 Again, there is a distinction between omitting LIST (C<import> called
6818 with no arguments) and an explicit empty LIST C<()> (C<import> not
6819 called). Note that there is no comma after VERSION!
6821 Because this is a wide-open interface, pragmas (compiler directives)
6822 are also implemented this way. Currently implemented pragmas are:
6827 use sigtrap qw(SEGV BUS);
6828 use strict qw(subs vars refs);
6829 use subs qw(afunc blurfl);
6830 use warnings qw(all);
6831 use sort qw(stable _quicksort _mergesort);
6833 Some of these pseudo-modules import semantics into the current
6834 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6835 which import symbols into the current package (which are effective
6836 through the end of the file).
6838 There's a corresponding C<no> command that unimports meanings imported
6839 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6840 It behaves exactly as C<import> does with respect to VERSION, an
6841 omitted LIST, empty LIST, or no unimport method being found.
6847 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6848 for the C<-M> and C<-m> command-line options to perl that give C<use>
6849 functionality from the command-line.
6854 Changes the access and modification times on each file of a list of
6855 files. The first two elements of the list must be the NUMERICAL access
6856 and modification times, in that order. Returns the number of files
6857 successfully changed. The inode change time of each file is set
6858 to the current time. For example, this code has the same effect as the
6859 Unix touch(1) command when the files I<already exist> and belong to
6860 the user running the program:
6863 $atime = $mtime = time;
6864 utime $atime, $mtime, @ARGV;
6866 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6867 the utime(2) function in the C library will be called with a null second
6868 argument. On most systems, this will set the file's access and
6869 modification times to the current time (i.e. equivalent to the example
6870 above) and will even work on other users' files where you have write
6873 utime undef, undef, @ARGV;
6875 Under NFS this will use the time of the NFS server, not the time of
6876 the local machine. If there is a time synchronization problem, the
6877 NFS server and local machine will have different times. The Unix
6878 touch(1) command will in fact normally use this form instead of the
6879 one shown in the first example.
6881 Note that only passing one of the first two elements as C<undef> will
6882 be equivalent of passing it as 0 and will not have the same effect as
6883 described when they are both C<undef>. This case will also trigger an
6884 uninitialized warning.
6886 On systems that support futimes, you might pass file handles among the
6887 files. On systems that don't support futimes, passing file handles
6888 produces a fatal error at run time. The file handles must be passed
6889 as globs or references to be recognized. Barewords are considered
6895 Returns a list consisting of all the values of the named hash.
6896 (In a scalar context, returns the number of values.)
6898 The values are returned in an apparently random order. The actual
6899 random order is subject to change in future versions of perl, but it
6900 is guaranteed to be the same order as either the C<keys> or C<each>
6901 function would produce on the same (unmodified) hash. Since Perl
6902 5.8.1 the ordering is different even between different runs of Perl
6903 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6905 As a side effect, calling values() resets the HASH's internal iterator,
6906 see L</each>. (In particular, calling values() in void context resets
6907 the iterator with no other overhead.)
6909 Note that the values are not copied, which means modifying them will
6910 modify the contents of the hash:
6912 for (values %hash) { s/foo/bar/g } # modifies %hash values
6913 for (@hash{keys %hash}) { s/foo/bar/g } # same
6915 See also C<keys>, C<each>, and C<sort>.
6917 =item vec EXPR,OFFSET,BITS
6918 X<vec> X<bit> X<bit vector>
6920 Treats the string in EXPR as a bit vector made up of elements of
6921 width BITS, and returns the value of the element specified by OFFSET
6922 as an unsigned integer. BITS therefore specifies the number of bits
6923 that are reserved for each element in the bit vector. This must
6924 be a power of two from 1 to 32 (or 64, if your platform supports
6927 If BITS is 8, "elements" coincide with bytes of the input string.
6929 If BITS is 16 or more, bytes of the input string are grouped into chunks
6930 of size BITS/8, and each group is converted to a number as with
6931 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6932 for BITS==64). See L<"pack"> for details.
6934 If bits is 4 or less, the string is broken into bytes, then the bits
6935 of each byte are broken into 8/BITS groups. Bits of a byte are
6936 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6937 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6938 breaking the single input byte C<chr(0x36)> into two groups gives a list
6939 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6941 C<vec> may also be assigned to, in which case parentheses are needed
6942 to give the expression the correct precedence as in
6944 vec($image, $max_x * $x + $y, 8) = 3;
6946 If the selected element is outside the string, the value 0 is returned.
6947 If an element off the end of the string is written to, Perl will first
6948 extend the string with sufficiently many zero bytes. It is an error
6949 to try to write off the beginning of the string (i.e. negative OFFSET).
6951 The string should not contain any character with the value > 255 (which
6952 can only happen if you're using UTF-8 encoding). If it does, it will be
6953 treated as something that is not UTF-8 encoded. When the C<vec> was
6954 assigned to, other parts of your program will also no longer consider the
6955 string to be UTF-8 encoded. In other words, if you do have such characters
6956 in your string, vec() will operate on the actual byte string, and not the
6957 conceptual character string.
6959 Strings created with C<vec> can also be manipulated with the logical
6960 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6961 vector operation is desired when both operands are strings.
6962 See L<perlop/"Bitwise String Operators">.
6964 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6965 The comments show the string after each step. Note that this code works
6966 in the same way on big-endian or little-endian machines.
6969 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6971 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6972 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6974 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6975 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6976 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6977 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6978 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6979 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6981 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6982 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6983 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6986 To transform a bit vector into a string or list of 0's and 1's, use these:
6988 $bits = unpack("b*", $vector);
6989 @bits = split(//, unpack("b*", $vector));
6991 If you know the exact length in bits, it can be used in place of the C<*>.
6993 Here is an example to illustrate how the bits actually fall in place:
6999 unpack("V",$_) 01234567890123456789012345678901
7000 ------------------------------------------------------------------
7005 for ($shift=0; $shift < $width; ++$shift) {
7006 for ($off=0; $off < 32/$width; ++$off) {
7007 $str = pack("B*", "0"x32);
7008 $bits = (1<<$shift);
7009 vec($str, $off, $width) = $bits;
7010 $res = unpack("b*",$str);
7011 $val = unpack("V", $str);
7018 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7019 $off, $width, $bits, $val, $res
7023 Regardless of the machine architecture on which it is run, the above
7024 example should print the following table:
7027 unpack("V",$_) 01234567890123456789012345678901
7028 ------------------------------------------------------------------
7029 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7030 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7031 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7032 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7033 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7034 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7035 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7036 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7037 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7038 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7039 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7040 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7041 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7042 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7043 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7044 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7045 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7046 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7047 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7048 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7049 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7050 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7051 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7052 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7053 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7054 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7055 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7056 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7057 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7058 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7059 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7060 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7061 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7062 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7063 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7064 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7065 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7066 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7067 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7068 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7069 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7070 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7071 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7072 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7073 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7074 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7075 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7076 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7077 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7078 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7079 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7080 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7081 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7082 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7083 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7084 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7085 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7086 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7087 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7088 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7089 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7090 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7091 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7092 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7093 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7094 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7095 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7096 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7097 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7098 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7099 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7100 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7101 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7102 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7103 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7104 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7105 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7106 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7107 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7108 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7109 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7110 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7111 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7112 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7113 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7114 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7115 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7116 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7117 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7118 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7119 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7120 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7121 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7122 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7123 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7124 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7125 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7126 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7127 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7128 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7129 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7130 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7131 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7132 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7133 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7134 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7135 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7136 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7137 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7138 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7139 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7140 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7141 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7142 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7143 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7144 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7145 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7146 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7147 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7148 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7149 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7150 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7151 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7152 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7153 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7154 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7155 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7156 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7161 Behaves like the wait(2) system call on your system: it waits for a child
7162 process to terminate and returns the pid of the deceased process, or
7163 C<-1> if there are no child processes. The status is returned in C<$?>
7164 and C<{^CHILD_ERROR_NATIVE}>.
7165 Note that a return value of C<-1> could mean that child processes are
7166 being automatically reaped, as described in L<perlipc>.
7168 =item waitpid PID,FLAGS
7171 Waits for a particular child process to terminate and returns the pid of
7172 the deceased process, or C<-1> if there is no such child process. On some
7173 systems, a value of 0 indicates that there are processes still running.
7174 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7176 use POSIX ":sys_wait_h";
7179 $kid = waitpid(-1, WNOHANG);
7182 then you can do a non-blocking wait for all pending zombie processes.
7183 Non-blocking wait is available on machines supporting either the
7184 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7185 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7186 system call by remembering the status values of processes that have
7187 exited but have not been harvested by the Perl script yet.)
7189 Note that on some systems, a return value of C<-1> could mean that child
7190 processes are being automatically reaped. See L<perlipc> for details,
7191 and for other examples.
7194 X<wantarray> X<context>
7196 Returns true if the context of the currently executing subroutine or
7197 C<eval> is looking for a list value. Returns false if the context is
7198 looking for a scalar. Returns the undefined value if the context is
7199 looking for no value (void context).
7201 return unless defined wantarray; # don't bother doing more
7202 my @a = complex_calculation();
7203 return wantarray ? @a : "@a";
7205 C<wantarray()>'s result is unspecified in the top level of a file,
7206 in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
7209 This function should have been named wantlist() instead.
7212 X<warn> X<warning> X<STDERR>
7214 Produces a message on STDERR just like C<die>, but doesn't exit or throw
7217 If LIST is empty and C<$@> already contains a value (typically from a
7218 previous eval) that value is used after appending C<"\t...caught">
7219 to C<$@>. This is useful for staying almost, but not entirely similar to
7222 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7224 No message is printed if there is a C<$SIG{__WARN__}> handler
7225 installed. It is the handler's responsibility to deal with the message
7226 as it sees fit (like, for instance, converting it into a C<die>). Most
7227 handlers must therefore make arrangements to actually display the
7228 warnings that they are not prepared to deal with, by calling C<warn>
7229 again in the handler. Note that this is quite safe and will not
7230 produce an endless loop, since C<__WARN__> hooks are not called from
7233 You will find this behavior is slightly different from that of
7234 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7235 instead call C<die> again to change it).
7237 Using a C<__WARN__> handler provides a powerful way to silence all
7238 warnings (even the so-called mandatory ones). An example:
7240 # wipe out *all* compile-time warnings
7241 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7243 my $foo = 20; # no warning about duplicate my $foo,
7244 # but hey, you asked for it!
7245 # no compile-time or run-time warnings before here
7248 # run-time warnings enabled after here
7249 warn "\$foo is alive and $foo!"; # does show up
7251 See L<perlvar> for details on setting C<%SIG> entries, and for more
7252 examples. See the Carp module for other kinds of warnings using its
7253 carp() and cluck() functions.
7255 =item write FILEHANDLE
7262 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7263 using the format associated with that file. By default the format for
7264 a file is the one having the same name as the filehandle, but the
7265 format for the current output channel (see the C<select> function) may be set
7266 explicitly by assigning the name of the format to the C<$~> variable.
7268 Top of form processing is handled automatically: if there is
7269 insufficient room on the current page for the formatted record, the
7270 page is advanced by writing a form feed, a special top-of-page format
7271 is used to format the new page header, and then the record is written.
7272 By default the top-of-page format is the name of the filehandle with
7273 "_TOP" appended, but it may be dynamically set to the format of your
7274 choice by assigning the name to the C<$^> variable while the filehandle is
7275 selected. The number of lines remaining on the current page is in
7276 variable C<$->, which can be set to C<0> to force a new page.
7278 If FILEHANDLE is unspecified, output goes to the current default output
7279 channel, which starts out as STDOUT but may be changed by the
7280 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7281 is evaluated and the resulting string is used to look up the name of
7282 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7284 Note that write is I<not> the opposite of C<read>. Unfortunately.
7288 The transliteration operator. Same as C<tr///>. See L<perlop>.