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
627 ($package, $filename, $line) = caller;
629 With EXPR, it returns some extra information that the debugger uses to
630 print a stack trace. The value of EXPR indicates how many call frames
631 to go back before the current one.
634 ($package, $filename, $line, $subroutine, $hasargs,
637 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
640 Here $subroutine may be C<(eval)> if the frame is not a subroutine
641 call, but an C<eval>. In such a case additional elements $evaltext and
642 C<$is_require> are set: C<$is_require> is true if the frame is created by a
643 C<require> or C<use> statement, $evaltext contains the text of the
644 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
645 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
646 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
647 frame.) $subroutine may also be C<(unknown)> if this particular
648 subroutine happens to have been deleted from the symbol table.
649 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
650 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
651 compiled with. The C<$hints> and C<$bitmask> values are subject to change
652 between versions of Perl, and are not meant for external use.
654 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
655 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
656 of this hash, as they are the actual values stored in the optree.
658 Furthermore, when called from within the DB package, caller returns more
659 detailed information: it sets the list variable C<@DB::args> to be the
660 arguments with which the subroutine was invoked.
662 Be aware that the optimizer might have optimized call frames away before
663 C<caller> had a chance to get the information. That means that C<caller(N)>
664 might not return information about the call frame you expect it do, for
665 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
666 previous time C<caller> was called.
673 =item chdir FILEHANDLE
675 =item chdir DIRHANDLE
679 Changes the working directory to EXPR, if possible. If EXPR is omitted,
680 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
681 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
682 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
683 neither is set, C<chdir> does nothing. It returns true upon success,
684 false otherwise. See the example under C<die>.
686 On systems that support fchdir, you might pass a file handle or
687 directory handle as argument. On systems that don't support fchdir,
688 passing handles produces a fatal error at run time.
691 X<chmod> X<permission> X<mode>
693 Changes the permissions of a list of files. The first element of the
694 list must be the numerical mode, which should probably be an octal
695 number, and which definitely should I<not> be a string of octal digits:
696 C<0644> is okay, C<'0644'> is not. Returns the number of files
697 successfully changed. See also L</oct>, if all you have is a string.
699 $cnt = chmod 0755, 'foo', 'bar';
700 chmod 0755, @executables;
701 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
703 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
704 $mode = 0644; chmod $mode, 'foo'; # this is best
706 On systems that support fchmod, you might pass file handles among the
707 files. On systems that don't support fchmod, passing file handles
708 produces a fatal error at run time. The file handles must be passed
709 as globs or references to be recognized. Barewords are considered
712 open(my $fh, "<", "foo");
713 my $perm = (stat $fh)[2] & 07777;
714 chmod($perm | 0600, $fh);
716 You can also import the symbolic C<S_I*> constants from the Fcntl
721 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
722 # This is identical to the chmod 0755 of the above example.
725 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
731 This safer version of L</chop> removes any trailing string
732 that corresponds to the current value of C<$/> (also known as
733 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
734 number of characters removed from all its arguments. It's often used to
735 remove the newline from the end of an input record when you're worried
736 that the final record may be missing its newline. When in paragraph
737 mode (C<$/ = "">), it removes all trailing newlines from the string.
738 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
739 a reference to an integer or the like, see L<perlvar>) chomp() won't
741 If VARIABLE is omitted, it chomps C<$_>. Example:
744 chomp; # avoid \n on last field
749 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
751 You can actually chomp anything that's an lvalue, including an assignment:
754 chomp($answer = <STDIN>);
756 If you chomp a list, each element is chomped, and the total number of
757 characters removed is returned.
759 If the C<encoding> pragma is in scope then the lengths returned are
760 calculated from the length of C<$/> in Unicode characters, which is not
761 always the same as the length of C<$/> in the native encoding.
763 Note that parentheses are necessary when you're chomping anything
764 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
765 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
766 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
767 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
777 Chops off the last character of a string and returns the character
778 chopped. It is much more efficient than C<s/.$//s> because it neither
779 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
780 If VARIABLE is a hash, it chops the hash's values, but not its keys.
782 You can actually chop anything that's an lvalue, including an assignment.
784 If you chop a list, each element is chopped. Only the value of the
785 last C<chop> is returned.
787 Note that C<chop> returns the last character. To return all but the last
788 character, use C<substr($string, 0, -1)>.
793 X<chown> X<owner> X<user> X<group>
795 Changes the owner (and group) of a list of files. The first two
796 elements of the list must be the I<numeric> uid and gid, in that
797 order. A value of -1 in either position is interpreted by most
798 systems to leave that value unchanged. Returns the number of files
799 successfully changed.
801 $cnt = chown $uid, $gid, 'foo', 'bar';
802 chown $uid, $gid, @filenames;
804 On systems that support fchown, you might pass file handles among the
805 files. On systems that don't support fchown, passing file handles
806 produces a fatal error at run time. The file handles must be passed
807 as globs or references to be recognized. Barewords are considered
810 Here's an example that looks up nonnumeric uids in the passwd file:
813 chomp($user = <STDIN>);
815 chomp($pattern = <STDIN>);
817 ($login,$pass,$uid,$gid) = getpwnam($user)
818 or die "$user not in passwd file";
820 @ary = glob($pattern); # expand filenames
821 chown $uid, $gid, @ary;
823 On most systems, you are not allowed to change the ownership of the
824 file unless you're the superuser, although you should be able to change
825 the group to any of your secondary groups. On insecure systems, these
826 restrictions may be relaxed, but this is not a portable assumption.
827 On POSIX systems, you can detect this condition this way:
829 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
830 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
833 X<chr> X<character> X<ASCII> X<Unicode>
837 Returns the character represented by that NUMBER in the character set.
838 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
839 chr(0x263a) is a Unicode smiley face. Note that characters from 128
840 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
841 backward compatibility reasons (but see L<encoding>).
843 Negative values give the Unicode replacement character (chr(0xfffd)),
844 except under the L<bytes> pragma, where low eight bits of the value
845 (truncated to an integer) are used.
847 If NUMBER is omitted, uses C<$_>.
849 For the reverse, use L</ord>.
851 Note that under the C<bytes> pragma the NUMBER is masked to
854 See L<perlunicode> and L<encoding> for more about Unicode.
856 =item chroot FILENAME
861 This function works like the system call by the same name: it makes the
862 named directory the new root directory for all further pathnames that
863 begin with a C</> by your process and all its children. (It doesn't
864 change your current working directory, which is unaffected.) For security
865 reasons, this call is restricted to the superuser. If FILENAME is
866 omitted, does a C<chroot> to C<$_>.
868 =item close FILEHANDLE
873 Closes the file or pipe associated with the file handle, returning
874 true only if IO buffers are successfully flushed and closes the system
875 file descriptor. Closes the currently selected filehandle if the
878 You don't have to close FILEHANDLE if you are immediately going to do
879 another C<open> on it, because C<open> will close it for you. (See
880 C<open>.) However, an explicit C<close> on an input file resets the line
881 counter (C<$.>), while the implicit close done by C<open> does not.
883 If the file handle came from a piped open, C<close> will additionally
884 return false if one of the other system calls involved fails, or if the
885 program exits with non-zero status. (If the only problem was that the
886 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
887 also waits for the process executing on the pipe to complete, in case you
888 want to look at the output of the pipe afterwards, and
889 implicitly puts the exit status value of that command into C<$?> and
890 C<${^CHILD_ERROR_NATIVE}>.
892 Prematurely closing the read end of a pipe (i.e. before the process
893 writing to it at the other end has closed it) will result in a
894 SIGPIPE being delivered to the writer. If the other end can't
895 handle that, be sure to read all the data before closing the pipe.
899 open(OUTPUT, '|sort >foo') # pipe to sort
900 or die "Can't start sort: $!";
901 #... # print stuff to output
902 close OUTPUT # wait for sort to finish
903 or warn $! ? "Error closing sort pipe: $!"
904 : "Exit status $? from sort";
905 open(INPUT, 'foo') # get sort's results
906 or die "Can't open 'foo' for input: $!";
908 FILEHANDLE may be an expression whose value can be used as an indirect
909 filehandle, usually the real filehandle name.
911 =item closedir DIRHANDLE
914 Closes a directory opened by C<opendir> and returns the success of that
917 =item connect SOCKET,NAME
920 Attempts to connect to a remote socket, just as the connect system call
921 does. Returns true if it succeeded, false otherwise. NAME should be a
922 packed address of the appropriate type for the socket. See the examples in
923 L<perlipc/"Sockets: Client/Server Communication">.
930 C<continue> is actually a flow control statement rather than a function. If
931 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
932 C<foreach>), it is always executed just before the conditional is about to
933 be evaluated again, just like the third part of a C<for> loop in C. Thus
934 it can be used to increment a loop variable, even when the loop has been
935 continued via the C<next> statement (which is similar to the C C<continue>
938 C<last>, C<next>, or C<redo> may appear within a C<continue>
939 block. C<last> and C<redo> will behave as if they had been executed within
940 the main block. So will C<next>, but since it will execute a C<continue>
941 block, it may be more entertaining.
944 ### redo always comes here
947 ### next always comes here
949 # then back the top to re-check EXPR
951 ### last always comes here
953 Omitting the C<continue> section is semantically equivalent to using an
954 empty one, logically enough. In that case, C<next> goes directly back
955 to check the condition at the top of the loop.
957 If the "switch" feature is enabled, C<continue> is also a
958 function that will break out of the current C<when> or C<default>
959 block, and fall through to the next case. See L<feature> and
960 L<perlsyn/"Switch statements"> for more information.
964 X<cos> X<cosine> X<acos> X<arccosine>
968 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
969 takes cosine of C<$_>.
971 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
972 function, or use this relation:
974 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
976 =item crypt PLAINTEXT,SALT
977 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
978 X<decrypt> X<cryptography> X<passwd> X<encrypt>
980 Creates a digest string exactly like the crypt(3) function in the C
981 library (assuming that you actually have a version there that has not
982 been extirpated as a potential munitions).
984 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
985 into a short string, called a digest, which is returned. The same
986 PLAINTEXT and SALT will always return the same string, but there is no
987 (known) way to get the original PLAINTEXT from the hash. Small
988 changes in the PLAINTEXT or SALT will result in large changes in the
991 There is no decrypt function. This function isn't all that useful for
992 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
993 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
994 primarily used to check if two pieces of text are the same without
995 having to transmit or store the text itself. An example is checking
996 if a correct password is given. The digest of the password is stored,
997 not the password itself. The user types in a password that is
998 crypt()'d with the same salt as the stored digest. If the two digests
999 match the password is correct.
1001 When verifying an existing digest string you should use the digest as
1002 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1003 to create the digest is visible as part of the digest. This ensures
1004 crypt() will hash the new string with the same salt as the digest.
1005 This allows your code to work with the standard L<crypt|/crypt> and
1006 with more exotic implementations. In other words, do not assume
1007 anything about the returned string itself, or how many bytes in the
1010 Traditionally the result is a string of 13 bytes: two first bytes of
1011 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1012 the first eight bytes of the digest string mattered, but alternative
1013 hashing schemes (like MD5), higher level security schemes (like C2),
1014 and implementations on non-UNIX platforms may produce different
1017 When choosing a new salt create a random two character string whose
1018 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1019 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1020 characters is just a recommendation; the characters allowed in
1021 the salt depend solely on your system's crypt library, and Perl can't
1022 restrict what salts C<crypt()> accepts.
1024 Here's an example that makes sure that whoever runs this program knows
1027 $pwd = (getpwuid($<))[1];
1029 system "stty -echo";
1031 chomp($word = <STDIN>);
1035 if (crypt($word, $pwd) ne $pwd) {
1041 Of course, typing in your own password to whoever asks you
1044 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1045 of data, not least of all because you can't get the information
1046 back. Look at the L<Digest> module for more robust algorithms.
1048 If using crypt() on a Unicode string (which I<potentially> has
1049 characters with codepoints above 255), Perl tries to make sense
1050 of the situation by trying to downgrade (a copy of the string)
1051 the string back to an eight-bit byte string before calling crypt()
1052 (on that copy). If that works, good. If not, crypt() dies with
1053 C<Wide character in crypt>.
1058 [This function has been largely superseded by the C<untie> function.]
1060 Breaks the binding between a DBM file and a hash.
1062 =item dbmopen HASH,DBNAME,MASK
1063 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1065 [This function has been largely superseded by the C<tie> function.]
1067 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1068 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1069 argument is I<not> a filehandle, even though it looks like one). DBNAME
1070 is the name of the database (without the F<.dir> or F<.pag> extension if
1071 any). If the database does not exist, it is created with protection
1072 specified by MASK (as modified by the C<umask>). If your system supports
1073 only the older DBM functions, you may perform only one C<dbmopen> in your
1074 program. In older versions of Perl, if your system had neither DBM nor
1075 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1078 If you don't have write access to the DBM file, you can only read hash
1079 variables, not set them. If you want to test whether you can write,
1080 either use file tests or try setting a dummy hash entry inside an C<eval>,
1081 which will trap the error.
1083 Note that functions such as C<keys> and C<values> may return huge lists
1084 when used on large DBM files. You may prefer to use the C<each>
1085 function to iterate over large DBM files. Example:
1087 # print out history file offsets
1088 dbmopen(%HIST,'/usr/lib/news/history',0666);
1089 while (($key,$val) = each %HIST) {
1090 print $key, ' = ', unpack('L',$val), "\n";
1094 See also L<AnyDBM_File> for a more general description of the pros and
1095 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1096 rich implementation.
1098 You can control which DBM library you use by loading that library
1099 before you call dbmopen():
1102 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1103 or die "Can't open netscape history file: $!";
1106 X<defined> X<undef> X<undefined>
1110 Returns a Boolean value telling whether EXPR has a value other than
1111 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1114 Many operations return C<undef> to indicate failure, end of file,
1115 system error, uninitialized variable, and other exceptional
1116 conditions. This function allows you to distinguish C<undef> from
1117 other values. (A simple Boolean test will not distinguish among
1118 C<undef>, zero, the empty string, and C<"0">, which are all equally
1119 false.) Note that since C<undef> is a valid scalar, its presence
1120 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1121 returns C<undef> when its argument is an empty array, I<or> when the
1122 element to return happens to be C<undef>.
1124 You may also use C<defined(&func)> to check whether subroutine C<&func>
1125 has ever been defined. The return value is unaffected by any forward
1126 declarations of C<&func>. Note that a subroutine which is not defined
1127 may still be callable: its package may have an C<AUTOLOAD> method that
1128 makes it spring into existence the first time that it is called -- see
1131 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1132 used to report whether memory for that aggregate has ever been
1133 allocated. This behavior may disappear in future versions of Perl.
1134 You should instead use a simple test for size:
1136 if (@an_array) { print "has array elements\n" }
1137 if (%a_hash) { print "has hash members\n" }
1139 When used on a hash element, it tells you whether the value is defined,
1140 not whether the key exists in the hash. Use L</exists> for the latter
1145 print if defined $switch{'D'};
1146 print "$val\n" while defined($val = pop(@ary));
1147 die "Can't readlink $sym: $!"
1148 unless defined($value = readlink $sym);
1149 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1150 $debugging = 0 unless defined $debugging;
1152 Note: Many folks tend to overuse C<defined>, and then are surprised to
1153 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1154 defined values. For example, if you say
1158 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1159 matched "nothing". It didn't really fail to match anything. Rather, it
1160 matched something that happened to be zero characters long. This is all
1161 very above-board and honest. When a function returns an undefined value,
1162 it's an admission that it couldn't give you an honest answer. So you
1163 should use C<defined> only when you're questioning the integrity of what
1164 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1167 See also L</undef>, L</exists>, L</ref>.
1172 Given an expression that specifies a hash element, array element, hash slice,
1173 or array slice, deletes the specified element(s) from the hash or array.
1174 In the case of an array, if the array elements happen to be at the end,
1175 the size of the array will shrink to the highest element that tests
1176 true for exists() (or 0 if no such element exists).
1178 Returns a list with the same number of elements as the number of elements
1179 for which deletion was attempted. Each element of that list consists of
1180 either the value of the element deleted, or the undefined value. In scalar
1181 context, this means that you get the value of the last element deleted (or
1182 the undefined value if that element did not exist).
1184 %hash = (foo => 11, bar => 22, baz => 33);
1185 $scalar = delete $hash{foo}; # $scalar is 11
1186 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1187 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1189 Deleting from C<%ENV> modifies the environment. Deleting from
1190 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1191 from a C<tie>d hash or array may not necessarily return anything.
1193 Deleting an array element effectively returns that position of the array
1194 to its initial, uninitialized state. Subsequently testing for the same
1195 element with exists() will return false. Also, deleting array elements
1196 in the middle of an array will not shift the index of the elements
1197 after them down. Use splice() for that. See L</exists>.
1199 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1201 foreach $key (keys %HASH) {
1205 foreach $index (0 .. $#ARRAY) {
1206 delete $ARRAY[$index];
1211 delete @HASH{keys %HASH};
1213 delete @ARRAY[0 .. $#ARRAY];
1215 But both of these are slower than just assigning the empty list
1216 or undefining %HASH or @ARRAY:
1218 %HASH = (); # completely empty %HASH
1219 undef %HASH; # forget %HASH ever existed
1221 @ARRAY = (); # completely empty @ARRAY
1222 undef @ARRAY; # forget @ARRAY ever existed
1224 Note that the EXPR can be arbitrarily complicated as long as the final
1225 operation is a hash element, array element, hash slice, or array slice
1228 delete $ref->[$x][$y]{$key};
1229 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1231 delete $ref->[$x][$y][$index];
1232 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1235 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1237 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1238 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1239 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1240 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1241 an C<eval(),> the error message is stuffed into C<$@> and the
1242 C<eval> is terminated with the undefined value. This makes
1243 C<die> the way to raise an exception.
1245 Equivalent examples:
1247 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1248 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1250 If the last element of LIST does not end in a newline, the current
1251 script line number and input line number (if any) are also printed,
1252 and a newline is supplied. Note that the "input line number" (also
1253 known as "chunk") is subject to whatever notion of "line" happens to
1254 be currently in effect, and is also available as the special variable
1255 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1257 Hint: sometimes appending C<", stopped"> to your message will cause it
1258 to make better sense when the string C<"at foo line 123"> is appended.
1259 Suppose you are running script "canasta".
1261 die "/etc/games is no good";
1262 die "/etc/games is no good, stopped";
1264 produce, respectively
1266 /etc/games is no good at canasta line 123.
1267 /etc/games is no good, stopped at canasta line 123.
1269 See also exit(), warn(), and the Carp module.
1271 If LIST is empty and C<$@> already contains a value (typically from a
1272 previous eval) that value is reused after appending C<"\t...propagated">.
1273 This is useful for propagating exceptions:
1276 die unless $@ =~ /Expected exception/;
1278 If LIST is empty and C<$@> contains an object reference that has a
1279 C<PROPAGATE> method, that method will be called with additional file
1280 and line number parameters. The return value replaces the value in
1281 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1284 If C<$@> is empty then the string C<"Died"> is used.
1286 die() can also be called with a reference argument. If this happens to be
1287 trapped within an eval(), $@ contains the reference. This behavior permits
1288 a more elaborate exception handling implementation using objects that
1289 maintain arbitrary state about the nature of the exception. Such a scheme
1290 is sometimes preferable to matching particular string values of $@ using
1291 regular expressions. Here's an example:
1293 use Scalar::Util 'blessed';
1295 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1297 if (blessed($@) && $@->isa("Some::Module::Exception")) {
1298 # handle Some::Module::Exception
1301 # handle all other possible exceptions
1305 Because perl will stringify uncaught exception messages before displaying
1306 them, you may want to overload stringification operations on such custom
1307 exception objects. See L<overload> for details about that.
1309 You can arrange for a callback to be run just before the C<die>
1310 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1311 handler will be called with the error text and can change the error
1312 message, if it sees fit, by calling C<die> again. See
1313 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1314 L<"eval BLOCK"> for some examples. Although this feature was
1315 to be run only right before your program was to exit, this is not
1316 currently the case--the C<$SIG{__DIE__}> hook is currently called
1317 even inside eval()ed blocks/strings! If one wants the hook to do
1318 nothing in such situations, put
1322 as the first line of the handler (see L<perlvar/$^S>). Because
1323 this promotes strange action at a distance, this counterintuitive
1324 behavior may be fixed in a future release.
1329 Not really a function. Returns the value of the last command in the
1330 sequence of commands indicated by BLOCK. When modified by the C<while> or
1331 C<until> loop modifier, executes the BLOCK once before testing the loop
1332 condition. (On other statements the loop modifiers test the conditional
1335 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1336 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1337 See L<perlsyn> for alternative strategies.
1339 =item do SUBROUTINE(LIST)
1342 This form of subroutine call is deprecated. See L<perlsub>.
1347 Uses the value of EXPR as a filename and executes the contents of the
1348 file as a Perl script.
1356 except that it's more efficient and concise, keeps track of the current
1357 filename for error messages, searches the @INC directories, and updates
1358 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1359 variables. It also differs in that code evaluated with C<do FILENAME>
1360 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1361 same, however, in that it does reparse the file every time you call it,
1362 so you probably don't want to do this inside a loop.
1364 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1365 error. If C<do> can read the file but cannot compile it, it
1366 returns undef and sets an error message in C<$@>. If the file is
1367 successfully compiled, C<do> returns the value of the last expression
1370 Note that inclusion of library modules is better done with the
1371 C<use> and C<require> operators, which also do automatic error checking
1372 and raise an exception if there's a problem.
1374 You might like to use C<do> to read in a program configuration
1375 file. Manual error checking can be done this way:
1377 # read in config files: system first, then user
1378 for $file ("/share/prog/defaults.rc",
1379 "$ENV{HOME}/.someprogrc")
1381 unless ($return = do $file) {
1382 warn "couldn't parse $file: $@" if $@;
1383 warn "couldn't do $file: $!" unless defined $return;
1384 warn "couldn't run $file" unless $return;
1389 X<dump> X<core> X<undump>
1393 This function causes an immediate core dump. See also the B<-u>
1394 command-line switch in L<perlrun>, which does the same thing.
1395 Primarily this is so that you can use the B<undump> program (not
1396 supplied) to turn your core dump into an executable binary after
1397 having initialized all your variables at the beginning of the
1398 program. When the new binary is executed it will begin by executing
1399 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1400 Think of it as a goto with an intervening core dump and reincarnation.
1401 If C<LABEL> is omitted, restarts the program from the top.
1403 B<WARNING>: Any files opened at the time of the dump will I<not>
1404 be open any more when the program is reincarnated, with possible
1405 resulting confusion on the part of Perl.
1407 This function is now largely obsolete, partly because it's very
1408 hard to convert a core file into an executable, and because the
1409 real compiler backends for generating portable bytecode and compilable
1410 C code have superseded it. That's why you should now invoke it as
1411 C<CORE::dump()>, if you don't want to be warned against a possible
1414 If you're looking to use L<dump> to speed up your program, consider
1415 generating bytecode or native C code as described in L<perlcc>. If
1416 you're just trying to accelerate a CGI script, consider using the
1417 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1418 You might also consider autoloading or selfloading, which at least
1419 make your program I<appear> to run faster.
1422 X<each> X<hash, iterator>
1424 When called in list context, returns a 2-element list consisting of the
1425 key and value for the next element of a hash, so that you can iterate over
1426 it. When called in scalar context, returns only the key for the next
1427 element in the hash.
1429 Entries are returned in an apparently random order. The actual random
1430 order is subject to change in future versions of perl, but it is
1431 guaranteed to be in the same order as either the C<keys> or C<values>
1432 function would produce on the same (unmodified) hash. Since Perl
1433 5.8.1 the ordering is different even between different runs of Perl
1434 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1436 When the hash is entirely read, a null array is returned in list context
1437 (which when assigned produces a false (C<0>) value), and C<undef> in
1438 scalar context. The next call to C<each> after that will start iterating
1439 again. There is a single iterator for each hash, shared by all C<each>,
1440 C<keys>, and C<values> function calls in the program; it can be reset by
1441 reading all the elements from the hash, or by evaluating C<keys HASH> or
1442 C<values HASH>. If you add or delete elements of a hash while you're
1443 iterating over it, you may get entries skipped or duplicated, so
1444 don't. Exception: It is always safe to delete the item most recently
1445 returned by C<each()>, which means that the following code will work:
1447 while (($key, $value) = each %hash) {
1449 delete $hash{$key}; # This is safe
1452 The following prints out your environment like the printenv(1) program,
1453 only in a different order:
1455 while (($key,$value) = each %ENV) {
1456 print "$key=$value\n";
1459 See also C<keys>, C<values> and C<sort>.
1461 =item eof FILEHANDLE
1470 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1471 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1472 gives the real filehandle. (Note that this function actually
1473 reads a character and then C<ungetc>s it, so isn't very useful in an
1474 interactive context.) Do not read from a terminal file (or call
1475 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1476 as terminals may lose the end-of-file condition if you do.
1478 An C<eof> without an argument uses the last file read. Using C<eof()>
1479 with empty parentheses is very different. It refers to the pseudo file
1480 formed from the files listed on the command line and accessed via the
1481 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1482 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1483 used will cause C<@ARGV> to be examined to determine if input is
1484 available. Similarly, an C<eof()> after C<< <> >> has returned
1485 end-of-file will assume you are processing another C<@ARGV> list,
1486 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1487 see L<perlop/"I/O Operators">.
1489 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1490 detect the end of each file, C<eof()> will only detect the end of the
1491 last file. Examples:
1493 # reset line numbering on each input file
1495 next if /^\s*#/; # skip comments
1498 close ARGV if eof; # Not eof()!
1501 # insert dashes just before last line of last file
1503 if (eof()) { # check for end of last file
1504 print "--------------\n";
1507 last if eof(); # needed if we're reading from a terminal
1510 Practical hint: you almost never need to use C<eof> in Perl, because the
1511 input operators typically return C<undef> when they run out of data, or if
1515 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1516 X<error, handling> X<exception, handling>
1522 In the first form, the return value of EXPR is parsed and executed as if it
1523 were a little Perl program. The value of the expression (which is itself
1524 determined within scalar context) is first parsed, and if there weren't any
1525 errors, executed in the lexical context of the current Perl program, so
1526 that any variable settings or subroutine and format definitions remain
1527 afterwards. Note that the value is parsed every time the C<eval> executes.
1528 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1529 delay parsing and subsequent execution of the text of EXPR until run time.
1531 In the second form, the code within the BLOCK is parsed only once--at the
1532 same time the code surrounding the C<eval> itself was parsed--and executed
1533 within the context of the current Perl program. This form is typically
1534 used to trap exceptions more efficiently than the first (see below), while
1535 also providing the benefit of checking the code within BLOCK at compile
1538 The final semicolon, if any, may be omitted from the value of EXPR or within
1541 In both forms, the value returned is the value of the last expression
1542 evaluated inside the mini-program; a return statement may be also used, just
1543 as with subroutines. The expression providing the return value is evaluated
1544 in void, scalar, or list context, depending on the context of the C<eval>
1545 itself. See L</wantarray> for more on how the evaluation context can be
1548 If there is a syntax error or runtime error, or a C<die> statement is
1549 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1550 error message. If there was no error, C<$@> is guaranteed to be a null
1551 string. Beware that using C<eval> neither silences perl from printing
1552 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1553 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1554 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1555 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1557 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1558 determining whether a particular feature (such as C<socket> or C<symlink>)
1559 is implemented. It is also Perl's exception trapping mechanism, where
1560 the die operator is used to raise exceptions.
1562 If the code to be executed doesn't vary, you may use the eval-BLOCK
1563 form to trap run-time errors without incurring the penalty of
1564 recompiling each time. The error, if any, is still returned in C<$@>.
1567 # make divide-by-zero nonfatal
1568 eval { $answer = $a / $b; }; warn $@ if $@;
1570 # same thing, but less efficient
1571 eval '$answer = $a / $b'; warn $@ if $@;
1573 # a compile-time error
1574 eval { $answer = }; # WRONG
1577 eval '$answer ='; # sets $@
1579 Using the C<eval{}> form as an exception trap in libraries does have some
1580 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1581 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1582 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1583 as shown in this example:
1585 # a very private exception trap for divide-by-zero
1586 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1589 This is especially significant, given that C<__DIE__> hooks can call
1590 C<die> again, which has the effect of changing their error messages:
1592 # __DIE__ hooks may modify error messages
1594 local $SIG{'__DIE__'} =
1595 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1596 eval { die "foo lives here" };
1597 print $@ if $@; # prints "bar lives here"
1600 Because this promotes action at a distance, this counterintuitive behavior
1601 may be fixed in a future release.
1603 With an C<eval>, you should be especially careful to remember what's
1604 being looked at when:
1610 eval { $x }; # CASE 4
1612 eval "\$$x++"; # CASE 5
1615 Cases 1 and 2 above behave identically: they run the code contained in
1616 the variable $x. (Although case 2 has misleading double quotes making
1617 the reader wonder what else might be happening (nothing is).) Cases 3
1618 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1619 does nothing but return the value of $x. (Case 4 is preferred for
1620 purely visual reasons, but it also has the advantage of compiling at
1621 compile-time instead of at run-time.) Case 5 is a place where
1622 normally you I<would> like to use double quotes, except that in this
1623 particular situation, you can just use symbolic references instead, as
1626 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1627 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1629 Note that as a very special case, an C<eval ''> executed within the C<DB>
1630 package doesn't see the usual surrounding lexical scope, but rather the
1631 scope of the first non-DB piece of code that called it. You don't normally
1632 need to worry about this unless you are writing a Perl debugger.
1637 =item exec PROGRAM LIST
1639 The C<exec> function executes a system command I<and never returns>--
1640 use C<system> instead of C<exec> if you want it to return. It fails and
1641 returns false only if the command does not exist I<and> it is executed
1642 directly instead of via your system's command shell (see below).
1644 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1645 warns you if there is a following statement which isn't C<die>, C<warn>,
1646 or C<exit> (if C<-w> is set - but you always do that). If you
1647 I<really> want to follow an C<exec> with some other statement, you
1648 can use one of these styles to avoid the warning:
1650 exec ('foo') or print STDERR "couldn't exec foo: $!";
1651 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1653 If there is more than one argument in LIST, or if LIST is an array
1654 with more than one value, calls execvp(3) with the arguments in LIST.
1655 If there is only one scalar argument or an array with one element in it,
1656 the argument is checked for shell metacharacters, and if there are any,
1657 the entire argument is passed to the system's command shell for parsing
1658 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1659 If there are no shell metacharacters in the argument, it is split into
1660 words and passed directly to C<execvp>, which is more efficient.
1663 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1664 exec "sort $outfile | uniq";
1666 If you don't really want to execute the first argument, but want to lie
1667 to the program you are executing about its own name, you can specify
1668 the program you actually want to run as an "indirect object" (without a
1669 comma) in front of the LIST. (This always forces interpretation of the
1670 LIST as a multivalued list, even if there is only a single scalar in
1673 $shell = '/bin/csh';
1674 exec $shell '-sh'; # pretend it's a login shell
1678 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1680 When the arguments get executed via the system shell, results will
1681 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1684 Using an indirect object with C<exec> or C<system> is also more
1685 secure. This usage (which also works fine with system()) forces
1686 interpretation of the arguments as a multivalued list, even if the
1687 list had just one argument. That way you're safe from the shell
1688 expanding wildcards or splitting up words with whitespace in them.
1690 @args = ( "echo surprise" );
1692 exec @args; # subject to shell escapes
1694 exec { $args[0] } @args; # safe even with one-arg list
1696 The first version, the one without the indirect object, ran the I<echo>
1697 program, passing it C<"surprise"> an argument. The second version
1698 didn't--it tried to run a program literally called I<"echo surprise">,
1699 didn't find it, and set C<$?> to a non-zero value indicating failure.
1701 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1702 output before the exec, but this may not be supported on some platforms
1703 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1704 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1705 open handles in order to avoid lost output.
1707 Note that C<exec> will not call your C<END> blocks, nor will it call
1708 any C<DESTROY> methods in your objects.
1711 X<exists> X<autovivification>
1713 Given an expression that specifies a hash element or array element,
1714 returns true if the specified element in the hash or array has ever
1715 been initialized, even if the corresponding value is undefined. The
1716 element is not autovivified if it doesn't exist.
1718 print "Exists\n" if exists $hash{$key};
1719 print "Defined\n" if defined $hash{$key};
1720 print "True\n" if $hash{$key};
1722 print "Exists\n" if exists $array[$index];
1723 print "Defined\n" if defined $array[$index];
1724 print "True\n" if $array[$index];
1726 A hash or array element can be true only if it's defined, and defined if
1727 it exists, but the reverse doesn't necessarily hold true.
1729 Given an expression that specifies the name of a subroutine,
1730 returns true if the specified subroutine has ever been declared, even
1731 if it is undefined. Mentioning a subroutine name for exists or defined
1732 does not count as declaring it. Note that a subroutine which does not
1733 exist may still be callable: its package may have an C<AUTOLOAD>
1734 method that makes it spring into existence the first time that it is
1735 called -- see L<perlsub>.
1737 print "Exists\n" if exists &subroutine;
1738 print "Defined\n" if defined &subroutine;
1740 Note that the EXPR can be arbitrarily complicated as long as the final
1741 operation is a hash or array key lookup or subroutine name:
1743 if (exists $ref->{A}->{B}->{$key}) { }
1744 if (exists $hash{A}{B}{$key}) { }
1746 if (exists $ref->{A}->{B}->[$ix]) { }
1747 if (exists $hash{A}{B}[$ix]) { }
1749 if (exists &{$ref->{A}{B}{$key}}) { }
1751 Although the deepest nested array or hash will not spring into existence
1752 just because its existence was tested, any intervening ones will.
1753 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1754 into existence due to the existence test for the $key element above.
1755 This happens anywhere the arrow operator is used, including even:
1758 if (exists $ref->{"Some key"}) { }
1759 print $ref; # prints HASH(0x80d3d5c)
1761 This surprising autovivification in what does not at first--or even
1762 second--glance appear to be an lvalue context may be fixed in a future
1765 Use of a subroutine call, rather than a subroutine name, as an argument
1766 to exists() is an error.
1769 exists &sub(); # Error
1772 X<exit> X<terminate> X<abort>
1776 Evaluates EXPR and exits immediately with that value. Example:
1779 exit 0 if $ans =~ /^[Xx]/;
1781 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1782 universally recognized values for EXPR are C<0> for success and C<1>
1783 for error; other values are subject to interpretation depending on the
1784 environment in which the Perl program is running. For example, exiting
1785 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1786 the mailer to return the item undelivered, but that's not true everywhere.
1788 Don't use C<exit> to abort a subroutine if there's any chance that
1789 someone might want to trap whatever error happened. Use C<die> instead,
1790 which can be trapped by an C<eval>.
1792 The exit() function does not always exit immediately. It calls any
1793 defined C<END> routines first, but these C<END> routines may not
1794 themselves abort the exit. Likewise any object destructors that need to
1795 be called are called before the real exit. If this is a problem, you
1796 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1797 See L<perlmod> for details.
1800 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1804 Returns I<e> (the natural logarithm base) to the power of EXPR.
1805 If EXPR is omitted, gives C<exp($_)>.
1807 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1810 Implements the fcntl(2) function. You'll probably have to say
1814 first to get the correct constant definitions. Argument processing and
1815 value return works just like C<ioctl> below.
1819 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1820 or die "can't fcntl F_GETFL: $!";
1822 You don't have to check for C<defined> on the return from C<fcntl>.
1823 Like C<ioctl>, it maps a C<0> return from the system call into
1824 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1825 in numeric context. It is also exempt from the normal B<-w> warnings
1826 on improper numeric conversions.
1828 Note that C<fcntl> will produce a fatal error if used on a machine that
1829 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1830 manpage to learn what functions are available on your system.
1832 Here's an example of setting a filehandle named C<REMOTE> to be
1833 non-blocking at the system level. You'll have to negotiate C<$|>
1834 on your own, though.
1836 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1838 $flags = fcntl(REMOTE, F_GETFL, 0)
1839 or die "Can't get flags for the socket: $!\n";
1841 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1842 or die "Can't set flags for the socket: $!\n";
1844 =item fileno FILEHANDLE
1847 Returns the file descriptor for a filehandle, or undefined if the
1848 filehandle is not open. This is mainly useful for constructing
1849 bitmaps for C<select> and low-level POSIX tty-handling operations.
1850 If FILEHANDLE is an expression, the value is taken as an indirect
1851 filehandle, generally its name.
1853 You can use this to find out whether two handles refer to the
1854 same underlying descriptor:
1856 if (fileno(THIS) == fileno(THAT)) {
1857 print "THIS and THAT are dups\n";
1860 (Filehandles connected to memory objects via new features of C<open> may
1861 return undefined even though they are open.)
1864 =item flock FILEHANDLE,OPERATION
1865 X<flock> X<lock> X<locking>
1867 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1868 for success, false on failure. Produces a fatal error if used on a
1869 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1870 C<flock> is Perl's portable file locking interface, although it locks
1871 only entire files, not records.
1873 Two potentially non-obvious but traditional C<flock> semantics are
1874 that it waits indefinitely until the lock is granted, and that its locks
1875 B<merely advisory>. Such discretionary locks are more flexible, but offer
1876 fewer guarantees. This means that programs that do not also use C<flock>
1877 may modify files locked with C<flock>. See L<perlport>,
1878 your port's specific documentation, or your system-specific local manpages
1879 for details. It's best to assume traditional behavior if you're writing
1880 portable programs. (But if you're not, you should as always feel perfectly
1881 free to write for your own system's idiosyncrasies (sometimes called
1882 "features"). Slavish adherence to portability concerns shouldn't get
1883 in the way of your getting your job done.)
1885 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1886 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1887 you can use the symbolic names if you import them from the Fcntl module,
1888 either individually, or as a group using the ':flock' tag. LOCK_SH
1889 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1890 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1891 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1892 waiting for the lock (check the return status to see if you got it).
1894 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1895 before locking or unlocking it.
1897 Note that the emulation built with lockf(3) doesn't provide shared
1898 locks, and it requires that FILEHANDLE be open with write intent. These
1899 are the semantics that lockf(3) implements. Most if not all systems
1900 implement lockf(3) in terms of fcntl(2) locking, though, so the
1901 differing semantics shouldn't bite too many people.
1903 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1904 be open with read intent to use LOCK_SH and requires that it be open
1905 with write intent to use LOCK_EX.
1907 Note also that some versions of C<flock> cannot lock things over the
1908 network; you would need to use the more system-specific C<fcntl> for
1909 that. If you like you can force Perl to ignore your system's flock(2)
1910 function, and so provide its own fcntl(2)-based emulation, by passing
1911 the switch C<-Ud_flock> to the F<Configure> program when you configure
1914 Here's a mailbox appender for BSD systems.
1916 use Fcntl ':flock'; # import LOCK_* constants
1919 flock(MBOX,LOCK_EX);
1920 # and, in case someone appended
1921 # while we were waiting...
1926 flock(MBOX,LOCK_UN);
1929 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1930 or die "Can't open mailbox: $!";
1933 print MBOX $msg,"\n\n";
1936 On systems that support a real flock(), locks are inherited across fork()
1937 calls, whereas those that must resort to the more capricious fcntl()
1938 function lose the locks, making it harder to write servers.
1940 See also L<DB_File> for other flock() examples.
1943 X<fork> X<child> X<parent>
1945 Does a fork(2) system call to create a new process running the
1946 same program at the same point. It returns the child pid to the
1947 parent process, C<0> to the child process, or C<undef> if the fork is
1948 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1949 are shared, while everything else is copied. On most systems supporting
1950 fork(), great care has gone into making it extremely efficient (for
1951 example, using copy-on-write technology on data pages), making it the
1952 dominant paradigm for multitasking over the last few decades.
1954 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1955 output before forking the child process, but this may not be supported
1956 on some platforms (see L<perlport>). To be safe, you may need to set
1957 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1958 C<IO::Handle> on any open handles in order to avoid duplicate output.
1960 If you C<fork> without ever waiting on your children, you will
1961 accumulate zombies. On some systems, you can avoid this by setting
1962 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1963 forking and reaping moribund children.
1965 Note that if your forked child inherits system file descriptors like
1966 STDIN and STDOUT that are actually connected by a pipe or socket, even
1967 if you exit, then the remote server (such as, say, a CGI script or a
1968 backgrounded job launched from a remote shell) won't think you're done.
1969 You should reopen those to F</dev/null> if it's any issue.
1974 Declare a picture format for use by the C<write> function. For
1978 Test: @<<<<<<<< @||||| @>>>>>
1979 $str, $%, '$' . int($num)
1983 $num = $cost/$quantity;
1987 See L<perlform> for many details and examples.
1989 =item formline PICTURE,LIST
1992 This is an internal function used by C<format>s, though you may call it,
1993 too. It formats (see L<perlform>) a list of values according to the
1994 contents of PICTURE, placing the output into the format output
1995 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1996 Eventually, when a C<write> is done, the contents of
1997 C<$^A> are written to some filehandle. You could also read C<$^A>
1998 and then set C<$^A> back to C<"">. Note that a format typically
1999 does one C<formline> per line of form, but the C<formline> function itself
2000 doesn't care how many newlines are embedded in the PICTURE. This means
2001 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2002 You may therefore need to use multiple formlines to implement a single
2003 record format, just like the format compiler.
2005 Be careful if you put double quotes around the picture, because an C<@>
2006 character may be taken to mean the beginning of an array name.
2007 C<formline> always returns true. See L<perlform> for other examples.
2009 =item getc FILEHANDLE
2010 X<getc> X<getchar> X<character> X<file, read>
2014 Returns the next character from the input file attached to FILEHANDLE,
2015 or the undefined value at end of file, or if there was an error (in
2016 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2017 STDIN. This is not particularly efficient. However, it cannot be
2018 used by itself to fetch single characters without waiting for the user
2019 to hit enter. For that, try something more like:
2022 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2025 system "stty", '-icanon', 'eol', "\001";
2031 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2034 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2038 Determination of whether $BSD_STYLE should be set
2039 is left as an exercise to the reader.
2041 The C<POSIX::getattr> function can do this more portably on
2042 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2043 module from your nearest CPAN site; details on CPAN can be found on
2047 X<getlogin> X<login>
2049 This implements the C library function of the same name, which on most
2050 systems returns the current login from F</etc/utmp>, if any. If null,
2053 $login = getlogin || getpwuid($<) || "Kilroy";
2055 Do not consider C<getlogin> for authentication: it is not as
2056 secure as C<getpwuid>.
2058 =item getpeername SOCKET
2059 X<getpeername> X<peer>
2061 Returns the packed sockaddr address of other end of the SOCKET connection.
2064 $hersockaddr = getpeername(SOCK);
2065 ($port, $iaddr) = sockaddr_in($hersockaddr);
2066 $herhostname = gethostbyaddr($iaddr, AF_INET);
2067 $herstraddr = inet_ntoa($iaddr);
2072 Returns the current process group for the specified PID. Use
2073 a PID of C<0> to get the current process group for the
2074 current process. Will raise an exception if used on a machine that
2075 doesn't implement getpgrp(2). If PID is omitted, returns process
2076 group of current process. Note that the POSIX version of C<getpgrp>
2077 does not accept a PID argument, so only C<PID==0> is truly portable.
2080 X<getppid> X<parent> X<pid>
2082 Returns the process id of the parent process.
2084 Note for Linux users: on Linux, the C functions C<getpid()> and
2085 C<getppid()> return different values from different threads. In order to
2086 be portable, this behavior is not reflected by the perl-level function
2087 C<getppid()>, that returns a consistent value across threads. If you want
2088 to call the underlying C<getppid()>, you may use the CPAN module
2091 =item getpriority WHICH,WHO
2092 X<getpriority> X<priority> X<nice>
2094 Returns the current priority for a process, a process group, or a user.
2095 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2096 machine that doesn't implement getpriority(2).
2099 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2100 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2101 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2102 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2103 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2104 X<endnetent> X<endprotoent> X<endservent>
2108 =item gethostbyname NAME
2110 =item getnetbyname NAME
2112 =item getprotobyname NAME
2118 =item getservbyname NAME,PROTO
2120 =item gethostbyaddr ADDR,ADDRTYPE
2122 =item getnetbyaddr ADDR,ADDRTYPE
2124 =item getprotobynumber NUMBER
2126 =item getservbyport PORT,PROTO
2144 =item sethostent STAYOPEN
2146 =item setnetent STAYOPEN
2148 =item setprotoent STAYOPEN
2150 =item setservent STAYOPEN
2164 These routines perform the same functions as their counterparts in the
2165 system library. In list context, the return values from the
2166 various get routines are as follows:
2168 ($name,$passwd,$uid,$gid,
2169 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2170 ($name,$passwd,$gid,$members) = getgr*
2171 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2172 ($name,$aliases,$addrtype,$net) = getnet*
2173 ($name,$aliases,$proto) = getproto*
2174 ($name,$aliases,$port,$proto) = getserv*
2176 (If the entry doesn't exist you get a null list.)
2178 The exact meaning of the $gcos field varies but it usually contains
2179 the real name of the user (as opposed to the login name) and other
2180 information pertaining to the user. Beware, however, that in many
2181 system users are able to change this information and therefore it
2182 cannot be trusted and therefore the $gcos is tainted (see
2183 L<perlsec>). The $passwd and $shell, user's encrypted password and
2184 login shell, are also tainted, because of the same reason.
2186 In scalar context, you get the name, unless the function was a
2187 lookup by name, in which case you get the other thing, whatever it is.
2188 (If the entry doesn't exist you get the undefined value.) For example:
2190 $uid = getpwnam($name);
2191 $name = getpwuid($num);
2193 $gid = getgrnam($name);
2194 $name = getgrgid($num);
2198 In I<getpw*()> the fields $quota, $comment, and $expire are special
2199 cases in the sense that in many systems they are unsupported. If the
2200 $quota is unsupported, it is an empty scalar. If it is supported, it
2201 usually encodes the disk quota. If the $comment field is unsupported,
2202 it is an empty scalar. If it is supported it usually encodes some
2203 administrative comment about the user. In some systems the $quota
2204 field may be $change or $age, fields that have to do with password
2205 aging. In some systems the $comment field may be $class. The $expire
2206 field, if present, encodes the expiration period of the account or the
2207 password. For the availability and the exact meaning of these fields
2208 in your system, please consult your getpwnam(3) documentation and your
2209 F<pwd.h> file. You can also find out from within Perl what your
2210 $quota and $comment fields mean and whether you have the $expire field
2211 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2212 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2213 files are only supported if your vendor has implemented them in the
2214 intuitive fashion that calling the regular C library routines gets the
2215 shadow versions if you're running under privilege or if there exists
2216 the shadow(3) functions as found in System V (this includes Solaris
2217 and Linux.) Those systems that implement a proprietary shadow password
2218 facility are unlikely to be supported.
2220 The $members value returned by I<getgr*()> is a space separated list of
2221 the login names of the members of the group.
2223 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2224 C, it will be returned to you via C<$?> if the function call fails. The
2225 C<@addrs> value returned by a successful call is a list of the raw
2226 addresses returned by the corresponding system library call. In the
2227 Internet domain, each address is four bytes long and you can unpack it
2228 by saying something like:
2230 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2232 The Socket library makes this slightly easier:
2235 $iaddr = inet_aton("127.1"); # or whatever address
2236 $name = gethostbyaddr($iaddr, AF_INET);
2238 # or going the other way
2239 $straddr = inet_ntoa($iaddr);
2241 If you get tired of remembering which element of the return list
2242 contains which return value, by-name interfaces are provided
2243 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2244 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2245 and C<User::grent>. These override the normal built-ins, supplying
2246 versions that return objects with the appropriate names
2247 for each field. For example:
2251 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2253 Even though it looks like they're the same method calls (uid),
2254 they aren't, because a C<File::stat> object is different from
2255 a C<User::pwent> object.
2257 =item getsockname SOCKET
2260 Returns the packed sockaddr address of this end of the SOCKET connection,
2261 in case you don't know the address because you have several different
2262 IPs that the connection might have come in on.
2265 $mysockaddr = getsockname(SOCK);
2266 ($port, $myaddr) = sockaddr_in($mysockaddr);
2267 printf "Connect to %s [%s]\n",
2268 scalar gethostbyaddr($myaddr, AF_INET),
2271 =item getsockopt SOCKET,LEVEL,OPTNAME
2274 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2275 Options may exist at multiple protocol levels depending on the socket
2276 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2277 C<Socket> module) will exist. To query options at another level the
2278 protocol number of the appropriate protocol controlling the option
2279 should be supplied. For example, to indicate that an option is to be
2280 interpreted by the TCP protocol, LEVEL should be set to the protocol
2281 number of TCP, which you can get using getprotobyname.
2283 The call returns a packed string representing the requested socket option,
2284 or C<undef> if there is an error (the error reason will be in $!). What
2285 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2286 your system documentation for details. A very common case however is that
2287 the option is an integer, in which case the result will be a packed
2288 integer which you can decode using unpack with the C<i> (or C<I>) format.
2290 An example testing if Nagle's algorithm is turned on on a socket:
2292 use Socket qw(:all);
2294 defined(my $tcp = getprotobyname("tcp"))
2295 or die "Could not determine the protocol number for tcp";
2296 # my $tcp = IPPROTO_TCP; # Alternative
2297 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2298 or die "Could not query TCP_NODELAY socket option: $!";
2299 my $nodelay = unpack("I", $packed);
2300 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2304 X<glob> X<wildcard> X<filename, expansion> X<expand>
2308 In list context, returns a (possibly empty) list of filename expansions on
2309 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2310 scalar context, glob iterates through such filename expansions, returning
2311 undef when the list is exhausted. This is the internal function
2312 implementing the C<< <*.c> >> operator, but you can use it directly. If
2313 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2314 more detail in L<perlop/"I/O Operators">.
2316 Beginning with v5.6.0, this operator is implemented using the standard
2317 C<File::Glob> extension. See L<File::Glob> for details.
2320 X<gmtime> X<UTC> X<Greenwich>
2324 Works just like L<localtime> but the returned values are
2325 localized for the standard Greenwich time zone.
2327 Note: when called in list context, $isdst, the last value
2328 returned by gmtime is always C<0>. There is no
2329 Daylight Saving Time in GMT.
2331 See L<perlport/gmtime> for portability concerns.
2334 X<goto> X<jump> X<jmp>
2340 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2341 execution there. It may not be used to go into any construct that
2342 requires initialization, such as a subroutine or a C<foreach> loop. It
2343 also can't be used to go into a construct that is optimized away,
2344 or to get out of a block or subroutine given to C<sort>.
2345 It can be used to go almost anywhere else within the dynamic scope,
2346 including out of subroutines, but it's usually better to use some other
2347 construct such as C<last> or C<die>. The author of Perl has never felt the
2348 need to use this form of C<goto> (in Perl, that is--C is another matter).
2349 (The difference being that C does not offer named loops combined with
2350 loop control. Perl does, and this replaces most structured uses of C<goto>
2351 in other languages.)
2353 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2354 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2355 necessarily recommended if you're optimizing for maintainability:
2357 goto ("FOO", "BAR", "GLARCH")[$i];
2359 The C<goto-&NAME> form is quite different from the other forms of
2360 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2361 doesn't have the stigma associated with other gotos. Instead, it
2362 exits the current subroutine (losing any changes set by local()) and
2363 immediately calls in its place the named subroutine using the current
2364 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2365 load another subroutine and then pretend that the other subroutine had
2366 been called in the first place (except that any modifications to C<@_>
2367 in the current subroutine are propagated to the other subroutine.)
2368 After the C<goto>, not even C<caller> will be able to tell that this
2369 routine was called first.
2371 NAME needn't be the name of a subroutine; it can be a scalar variable
2372 containing a code reference, or a block that evaluates to a code
2375 =item grep BLOCK LIST
2378 =item grep EXPR,LIST
2380 This is similar in spirit to, but not the same as, grep(1) and its
2381 relatives. In particular, it is not limited to using regular expressions.
2383 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2384 C<$_> to each element) and returns the list value consisting of those
2385 elements for which the expression evaluated to true. In scalar
2386 context, returns the number of times the expression was true.
2388 @foo = grep(!/^#/, @bar); # weed out comments
2392 @foo = grep {!/^#/} @bar; # weed out comments
2394 Note that C<$_> is an alias to the list value, so it can be used to
2395 modify the elements of the LIST. While this is useful and supported,
2396 it can cause bizarre results if the elements of LIST are not variables.
2397 Similarly, grep returns aliases into the original list, much as a for
2398 loop's index variable aliases the list elements. That is, modifying an
2399 element of a list returned by grep (for example, in a C<foreach>, C<map>
2400 or another C<grep>) actually modifies the element in the original list.
2401 This is usually something to be avoided when writing clear code.
2403 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2404 been declared with C<my $_>) then, in addition to being locally aliased to
2405 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2406 can't be seen from the outside, avoiding any potential side-effects.
2408 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2411 X<hex> X<hexadecimal>
2415 Interprets EXPR as a hex string and returns the corresponding value.
2416 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2417 L</oct>.) If EXPR is omitted, uses C<$_>.
2419 print hex '0xAf'; # prints '175'
2420 print hex 'aF'; # same
2422 Hex strings may only represent integers. Strings that would cause
2423 integer overflow trigger a warning. Leading whitespace is not stripped,
2424 unlike oct(). To present something as hex, look into L</printf>,
2425 L</sprintf>, or L</unpack>.
2430 There is no builtin C<import> function. It is just an ordinary
2431 method (subroutine) defined (or inherited) by modules that wish to export
2432 names to another module. The C<use> function calls the C<import> method
2433 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2435 =item index STR,SUBSTR,POSITION
2436 X<index> X<indexOf> X<InStr>
2438 =item index STR,SUBSTR
2440 The index function searches for one string within another, but without
2441 the wildcard-like behavior of a full regular-expression pattern match.
2442 It returns the position of the first occurrence of SUBSTR in STR at
2443 or after POSITION. If POSITION is omitted, starts searching from the
2444 beginning of the string. POSITION before the beginning of the string
2445 or after its end is treated as if it were the beginning or the end,
2446 respectively. POSITION and the return value are based at C<0> (or whatever
2447 you've set the C<$[> variable to--but don't do that). If the substring
2448 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2451 X<int> X<integer> X<truncate> X<trunc> X<floor>
2455 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2456 You should not use this function for rounding: one because it truncates
2457 towards C<0>, and two because machine representations of floating point
2458 numbers can sometimes produce counterintuitive results. For example,
2459 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2460 because it's really more like -268.99999999999994315658 instead. Usually,
2461 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2462 functions will serve you better than will int().
2464 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2467 Implements the ioctl(2) function. You'll probably first have to say
2469 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2471 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2472 exist or doesn't have the correct definitions you'll have to roll your
2473 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2474 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2475 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2476 written depending on the FUNCTION--a pointer to the string value of SCALAR
2477 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2478 has no string value but does have a numeric value, that value will be
2479 passed rather than a pointer to the string value. To guarantee this to be
2480 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2481 functions may be needed to manipulate the values of structures used by
2484 The return value of C<ioctl> (and C<fcntl>) is as follows:
2486 if OS returns: then Perl returns:
2488 0 string "0 but true"
2489 anything else that number
2491 Thus Perl returns true on success and false on failure, yet you can
2492 still easily determine the actual value returned by the operating
2495 $retval = ioctl(...) || -1;
2496 printf "System returned %d\n", $retval;
2498 The special string C<"0 but true"> is exempt from B<-w> complaints
2499 about improper numeric conversions.
2501 =item join EXPR,LIST
2504 Joins the separate strings of LIST into a single string with fields
2505 separated by the value of EXPR, and returns that new string. Example:
2507 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2509 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2510 first argument. Compare L</split>.
2515 Returns a list consisting of all the keys of the named hash.
2516 (In scalar context, returns the number of keys.)
2518 The keys are returned in an apparently random order. The actual
2519 random order is subject to change in future versions of perl, but it
2520 is guaranteed to be the same order as either the C<values> or C<each>
2521 function produces (given that the hash has not been modified). Since
2522 Perl 5.8.1 the ordering is different even between different runs of
2523 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2526 As a side effect, calling keys() resets the HASH's internal iterator
2527 (see L</each>). In particular, calling keys() in void context resets
2528 the iterator with no other overhead.
2530 Here is yet another way to print your environment:
2533 @values = values %ENV;
2535 print pop(@keys), '=', pop(@values), "\n";
2538 or how about sorted by key:
2540 foreach $key (sort(keys %ENV)) {
2541 print $key, '=', $ENV{$key}, "\n";
2544 The returned values are copies of the original keys in the hash, so
2545 modifying them will not affect the original hash. Compare L</values>.
2547 To sort a hash by value, you'll need to use a C<sort> function.
2548 Here's a descending numeric sort of a hash by its values:
2550 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2551 printf "%4d %s\n", $hash{$key}, $key;
2554 As an lvalue C<keys> allows you to increase the number of hash buckets
2555 allocated for the given hash. This can gain you a measure of efficiency if
2556 you know the hash is going to get big. (This is similar to pre-extending
2557 an array by assigning a larger number to $#array.) If you say
2561 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2562 in fact, since it rounds up to the next power of two. These
2563 buckets will be retained even if you do C<%hash = ()>, use C<undef
2564 %hash> if you want to free the storage while C<%hash> is still in scope.
2565 You can't shrink the number of buckets allocated for the hash using
2566 C<keys> in this way (but you needn't worry about doing this by accident,
2567 as trying has no effect).
2569 See also C<each>, C<values> and C<sort>.
2571 =item kill SIGNAL, LIST
2574 Sends a signal to a list of processes. Returns the number of
2575 processes successfully signaled (which is not necessarily the
2576 same as the number actually killed).
2578 $cnt = kill 1, $child1, $child2;
2581 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2582 system call will check whether it's possible to send a signal to it (that
2583 means, to be brief, that the process is owned by the same user, or we are
2584 the super-user). This is a useful way to check that a child process is
2585 alive (even if only as a zombie) and hasn't changed its UID. See
2586 L<perlport> for notes on the portability of this construct.
2588 Unlike in the shell, if SIGNAL is negative, it kills
2589 process groups instead of processes. (On System V, a negative I<PROCESS>
2590 number will also kill process groups, but that's not portable.) That
2591 means you usually want to use positive not negative signals. You may also
2592 use a signal name in quotes.
2594 See L<perlipc/"Signals"> for more details.
2601 The C<last> command is like the C<break> statement in C (as used in
2602 loops); it immediately exits the loop in question. If the LABEL is
2603 omitted, the command refers to the innermost enclosing loop. The
2604 C<continue> block, if any, is not executed:
2606 LINE: while (<STDIN>) {
2607 last LINE if /^$/; # exit when done with header
2611 C<last> cannot be used to exit a block which returns a value such as
2612 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2613 a grep() or map() operation.
2615 Note that a block by itself is semantically identical to a loop
2616 that executes once. Thus C<last> can be used to effect an early
2617 exit out of such a block.
2619 See also L</continue> for an illustration of how C<last>, C<next>, and
2627 Returns a lowercased version of EXPR. This is the internal function
2628 implementing the C<\L> escape in double-quoted strings. Respects
2629 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2630 and L<perlunicode> for more details about locale and Unicode support.
2632 If EXPR is omitted, uses C<$_>.
2635 X<lcfirst> X<lowercase>
2639 Returns the value of EXPR with the first character lowercased. This
2640 is the internal function implementing the C<\l> escape in
2641 double-quoted strings. Respects current LC_CTYPE locale if C<use
2642 locale> in force. See L<perllocale> and L<perlunicode> for more
2643 details about locale and Unicode support.
2645 If EXPR is omitted, uses C<$_>.
2652 Returns the length in I<characters> of the value of EXPR. If EXPR is
2653 omitted, returns length of C<$_>. Note that this cannot be used on
2654 an entire array or hash to find out how many elements these have.
2655 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2657 Note the I<characters>: if the EXPR is in Unicode, you will get the
2658 number of characters, not the number of bytes. To get the length
2659 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2661 =item link OLDFILE,NEWFILE
2664 Creates a new filename linked to the old filename. Returns true for
2665 success, false otherwise.
2667 =item listen SOCKET,QUEUESIZE
2670 Does the same thing that the listen system call does. Returns true if
2671 it succeeded, false otherwise. See the example in
2672 L<perlipc/"Sockets: Client/Server Communication">.
2677 You really probably want to be using C<my> instead, because C<local> isn't
2678 what most people think of as "local". See
2679 L<perlsub/"Private Variables via my()"> for details.
2681 A local modifies the listed variables to be local to the enclosing
2682 block, file, or eval. If more than one value is listed, the list must
2683 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2684 for details, including issues with tied arrays and hashes.
2686 =item localtime EXPR
2687 X<localtime> X<ctime>
2691 Converts a time as returned by the time function to a 9-element list
2692 with the time analyzed for the local time zone. Typically used as
2696 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2699 All list elements are numeric, and come straight out of the C `struct
2700 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2701 of the specified time.
2703 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2704 the range C<0..11> with 0 indicating January and 11 indicating December.
2705 This makes it easy to get a month name from a list:
2707 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2708 print "$abbr[$mon] $mday";
2709 # $mon=9, $mday=18 gives "Oct 18"
2711 C<$year> is the number of years since 1900, not just the last two digits
2712 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2713 to get a complete 4-digit year is simply:
2717 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2718 to do that, would you?
2720 To get the last two digits of the year (e.g., '01' in 2001) do:
2722 $year = sprintf("%02d", $year % 100);
2724 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2725 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2726 (or C<0..365> in leap years.)
2728 C<$isdst> is true if the specified time occurs during Daylight Saving
2729 Time, false otherwise.
2731 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2733 In scalar context, C<localtime()> returns the ctime(3) value:
2735 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2737 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2738 instead of local time use the L</gmtime> builtin. See also the
2739 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2740 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2741 and mktime(3) functions.
2743 To get somewhat similar but locale dependent date strings, set up your
2744 locale environment variables appropriately (please see L<perllocale>) and
2747 use POSIX qw(strftime);
2748 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2749 # or for GMT formatted appropriately for your locale:
2750 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2752 Note that the C<%a> and C<%b>, the short forms of the day of the week
2753 and the month of the year, may not necessarily be three characters wide.
2755 See L<perlport/localtime> for portability concerns.
2757 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2758 by-name access mechanism to the gmtime() and localtime() functions,
2761 For a comprehensive date and time representation look at the
2762 L<DateTime> module on CPAN.
2767 This function places an advisory lock on a shared variable, or referenced
2768 object contained in I<THING> until the lock goes out of scope.
2770 lock() is a "weak keyword" : this means that if you've defined a function
2771 by this name (before any calls to it), that function will be called
2772 instead. (However, if you've said C<use threads>, lock() is always a
2773 keyword.) See L<threads>.
2776 X<log> X<logarithm> X<e> X<ln> X<base>
2780 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2781 returns log of C<$_>. To get the log of another base, use basic algebra:
2782 The base-N log of a number is equal to the natural log of that number
2783 divided by the natural log of N. For example:
2787 return log($n)/log(10);
2790 See also L</exp> for the inverse operation.
2797 Does the same thing as the C<stat> function (including setting the
2798 special C<_> filehandle) but stats a symbolic link instead of the file
2799 the symbolic link points to. If symbolic links are unimplemented on
2800 your system, a normal C<stat> is done. For much more detailed
2801 information, please see the documentation for C<stat>.
2803 If EXPR is omitted, stats C<$_>.
2807 The match operator. See L<perlop>.
2809 =item map BLOCK LIST
2814 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2815 C<$_> to each element) and returns the list value composed of the
2816 results of each such evaluation. In scalar context, returns the
2817 total number of elements so generated. Evaluates BLOCK or EXPR in
2818 list context, so each element of LIST may produce zero, one, or
2819 more elements in the returned value.
2821 @chars = map(chr, @nums);
2823 translates a list of numbers to the corresponding characters. And
2825 %hash = map { getkey($_) => $_ } @array;
2827 is just a funny way to write
2830 foreach $_ (@array) {
2831 $hash{getkey($_)} = $_;
2834 Note that C<$_> is an alias to the list value, so it can be used to
2835 modify the elements of the LIST. While this is useful and supported,
2836 it can cause bizarre results if the elements of LIST are not variables.
2837 Using a regular C<foreach> loop for this purpose would be clearer in
2838 most cases. See also L</grep> for an array composed of those items of
2839 the original list for which the BLOCK or EXPR evaluates to true.
2841 If C<$_> is lexical in the scope where the C<map> appears (because it has
2842 been declared with C<my $_>) then, in addition to being locally aliased to
2843 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2844 can't be seen from the outside, avoiding any potential side-effects.
2846 C<{> starts both hash references and blocks, so C<map { ...> could be either
2847 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2848 ahead for the closing C<}> it has to take a guess at which its dealing with
2849 based what it finds just after the C<{>. Usually it gets it right, but if it
2850 doesn't it won't realize something is wrong until it gets to the C<}> and
2851 encounters the missing (or unexpected) comma. The syntax error will be
2852 reported close to the C<}> but you'll need to change something near the C<{>
2853 such as using a unary C<+> to give perl some help:
2855 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2856 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2857 %hash = map { ("\L$_", 1) } @array # this also works
2858 %hash = map { lc($_), 1 } @array # as does this.
2859 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2861 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2863 or to force an anon hash constructor use C<+{>
2865 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2867 and you get list of anonymous hashes each with only 1 entry.
2869 =item mkdir FILENAME,MASK
2870 X<mkdir> X<md> X<directory, create>
2872 =item mkdir FILENAME
2876 Creates the directory specified by FILENAME, with permissions
2877 specified by MASK (as modified by C<umask>). If it succeeds it
2878 returns true, otherwise it returns false and sets C<$!> (errno).
2879 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2882 In general, it is better to create directories with permissive MASK,
2883 and let the user modify that with their C<umask>, than it is to supply
2884 a restrictive MASK and give the user no way to be more permissive.
2885 The exceptions to this rule are when the file or directory should be
2886 kept private (mail files, for instance). The perlfunc(1) entry on
2887 C<umask> discusses the choice of MASK in more detail.
2889 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2890 number of trailing slashes. Some operating and filesystems do not get
2891 this right, so Perl automatically removes all trailing slashes to keep
2894 In order to recursively create a directory structure look at
2895 the C<mkpath> function of the L<File::Path> module.
2897 =item msgctl ID,CMD,ARG
2900 Calls the System V IPC function msgctl(2). You'll probably have to say
2904 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2905 then ARG must be a variable that will hold the returned C<msqid_ds>
2906 structure. Returns like C<ioctl>: the undefined value for error,
2907 C<"0 but true"> for zero, or the actual return value otherwise. See also
2908 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2910 =item msgget KEY,FLAGS
2913 Calls the System V IPC function msgget(2). Returns the message queue
2914 id, or the undefined value if there is an error. See also
2915 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2917 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2920 Calls the System V IPC function msgrcv to receive a message from
2921 message queue ID into variable VAR with a maximum message size of
2922 SIZE. Note that when a message is received, the message type as a
2923 native long integer will be the first thing in VAR, followed by the
2924 actual message. This packing may be opened with C<unpack("l! a*")>.
2925 Taints the variable. Returns true if successful, or false if there is
2926 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2927 C<IPC::SysV::Msg> documentation.
2929 =item msgsnd ID,MSG,FLAGS
2932 Calls the System V IPC function msgsnd to send the message MSG to the
2933 message queue ID. MSG must begin with the native long integer message
2934 type, and be followed by the length of the actual message, and finally
2935 the message itself. This kind of packing can be achieved with
2936 C<pack("l! a*", $type, $message)>. Returns true if successful,
2937 or false if there is an error. See also C<IPC::SysV>
2938 and C<IPC::SysV::Msg> documentation.
2945 =item my EXPR : ATTRS
2947 =item my TYPE EXPR : ATTRS
2949 A C<my> declares the listed variables to be local (lexically) to the
2950 enclosing block, file, or C<eval>. If more than one value is listed,
2951 the list must be placed in parentheses.
2953 The exact semantics and interface of TYPE and ATTRS are still
2954 evolving. TYPE is currently bound to the use of C<fields> pragma,
2955 and attributes are handled using the C<attributes> pragma, or starting
2956 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2957 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2958 L<attributes>, and L<Attribute::Handlers>.
2965 The C<next> command is like the C<continue> statement in C; it starts
2966 the next iteration of the loop:
2968 LINE: while (<STDIN>) {
2969 next LINE if /^#/; # discard comments
2973 Note that if there were a C<continue> block on the above, it would get
2974 executed even on discarded lines. If the LABEL is omitted, the command
2975 refers to the innermost enclosing loop.
2977 C<next> cannot be used to exit a block which returns a value such as
2978 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2979 a grep() or map() operation.
2981 Note that a block by itself is semantically identical to a loop
2982 that executes once. Thus C<next> will exit such a block early.
2984 See also L</continue> for an illustration of how C<last>, C<next>, and
2987 =item no Module VERSION LIST
2990 =item no Module VERSION
2992 =item no Module LIST
2996 See the C<use> function, of which C<no> is the opposite.
2999 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3003 Interprets EXPR as an octal string and returns the corresponding
3004 value. (If EXPR happens to start off with C<0x>, interprets it as a
3005 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3006 binary string. Leading whitespace is ignored in all three cases.)
3007 The following will handle decimal, binary, octal, and hex in the standard
3010 $val = oct($val) if $val =~ /^0/;
3012 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3013 in octal), use sprintf() or printf():
3015 $perms = (stat("filename"))[2] & 07777;
3016 $oct_perms = sprintf "%lo", $perms;
3018 The oct() function is commonly used when a string such as C<644> needs
3019 to be converted into a file mode, for example. (Although perl will
3020 automatically convert strings into numbers as needed, this automatic
3021 conversion assumes base 10.)
3023 =item open FILEHANDLE,EXPR
3024 X<open> X<pipe> X<file, open> X<fopen>
3026 =item open FILEHANDLE,MODE,EXPR
3028 =item open FILEHANDLE,MODE,EXPR,LIST
3030 =item open FILEHANDLE,MODE,REFERENCE
3032 =item open FILEHANDLE
3034 Opens the file whose filename is given by EXPR, and associates it with
3037 (The following is a comprehensive reference to open(): for a gentler
3038 introduction you may consider L<perlopentut>.)
3040 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3041 the variable is assigned a reference to a new anonymous filehandle,
3042 otherwise if FILEHANDLE is an expression, its value is used as the name of
3043 the real filehandle wanted. (This is considered a symbolic reference, so
3044 C<use strict 'refs'> should I<not> be in effect.)
3046 If EXPR is omitted, the scalar variable of the same name as the
3047 FILEHANDLE contains the filename. (Note that lexical variables--those
3048 declared with C<my>--will not work for this purpose; so if you're
3049 using C<my>, specify EXPR in your call to open.)
3051 If three or more arguments are specified then the mode of opening and
3052 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3053 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3054 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3055 the file is opened for appending, again being created if necessary.
3057 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3058 indicate that you want both read and write access to the file; thus
3059 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3060 '+>' >> mode would clobber the file first. You can't usually use
3061 either read-write mode for updating textfiles, since they have
3062 variable length records. See the B<-i> switch in L<perlrun> for a
3063 better approach. The file is created with permissions of C<0666>
3064 modified by the process' C<umask> value.
3066 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3067 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3069 In the 2-arguments (and 1-argument) form of the call the mode and
3070 filename should be concatenated (in this order), possibly separated by
3071 spaces. It is possible to omit the mode in these forms if the mode is
3074 If the filename begins with C<'|'>, the filename is interpreted as a
3075 command to which output is to be piped, and if the filename ends with a
3076 C<'|'>, the filename is interpreted as a command which pipes output to
3077 us. See L<perlipc/"Using open() for IPC">
3078 for more examples of this. (You are not allowed to C<open> to a command
3079 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3080 and L<perlipc/"Bidirectional Communication with Another Process">
3083 For three or more arguments if MODE is C<'|-'>, the filename is
3084 interpreted as a command to which output is to be piped, and if MODE
3085 is C<'-|'>, the filename is interpreted as a command which pipes
3086 output to us. In the 2-arguments (and 1-argument) form one should
3087 replace dash (C<'-'>) with the command.
3088 See L<perlipc/"Using open() for IPC"> for more examples of this.
3089 (You are not allowed to C<open> to a command that pipes both in I<and>
3090 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3091 L<perlipc/"Bidirectional Communication"> for alternatives.)
3093 In the three-or-more argument form of pipe opens, if LIST is specified
3094 (extra arguments after the command name) then LIST becomes arguments
3095 to the command invoked if the platform supports it. The meaning of
3096 C<open> with more than three arguments for non-pipe modes is not yet
3097 specified. Experimental "layers" may give extra LIST arguments
3100 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3101 and opening C<< '>-' >> opens STDOUT.
3103 You may use the three-argument form of open to specify IO "layers"
3104 (sometimes also referred to as "disciplines") to be applied to the handle
3105 that affect how the input and output are processed (see L<open> and
3106 L<PerlIO> for more details). For example
3108 open(FH, "<:utf8", "file")
3110 will open the UTF-8 encoded file containing Unicode characters,
3111 see L<perluniintro>. Note that if layers are specified in the
3112 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3113 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3115 Open returns nonzero upon success, the undefined value otherwise. If
3116 the C<open> involved a pipe, the return value happens to be the pid of
3119 If you're running Perl on a system that distinguishes between text
3120 files and binary files, then you should check out L</binmode> for tips
3121 for dealing with this. The key distinction between systems that need
3122 C<binmode> and those that don't is their text file formats. Systems
3123 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3124 character, and which encode that character in C as C<"\n">, do not
3125 need C<binmode>. The rest need it.
3127 When opening a file, it's usually a bad idea to continue normal execution
3128 if the request failed, so C<open> is frequently used in connection with
3129 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3130 where you want to make a nicely formatted error message (but there are
3131 modules that can help with that problem)) you should always check
3132 the return value from opening a file. The infrequent exception is when
3133 working with an unopened filehandle is actually what you want to do.
3135 As a special case the 3-arg form with a read/write mode and the third
3136 argument being C<undef>:
3138 open(TMP, "+>", undef) or die ...
3140 opens a filehandle to an anonymous temporary file. Also using "+<"
3141 works for symmetry, but you really should consider writing something
3142 to the temporary file first. You will need to seek() to do the
3145 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3146 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3147 "in memory" files held in Perl scalars via:
3149 open($fh, '>', \$variable) || ..
3151 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3152 file, you have to close it first:
3155 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3160 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3161 while (<ARTICLE>) {...
3163 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3164 # if the open fails, output is discarded
3166 open(DBASE, '+<', 'dbase.mine') # open for update
3167 or die "Can't open 'dbase.mine' for update: $!";
3169 open(DBASE, '+<dbase.mine') # ditto
3170 or die "Can't open 'dbase.mine' for update: $!";
3172 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3173 or die "Can't start caesar: $!";
3175 open(ARTICLE, "caesar <$article |") # ditto
3176 or die "Can't start caesar: $!";
3178 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3179 or die "Can't start sort: $!";
3182 open(MEMORY,'>', \$var)
3183 or die "Can't open memory file: $!";
3184 print MEMORY "foo!\n"; # output will end up in $var
3186 # process argument list of files along with any includes
3188 foreach $file (@ARGV) {
3189 process($file, 'fh00');
3193 my($filename, $input) = @_;
3194 $input++; # this is a string increment
3195 unless (open($input, $filename)) {
3196 print STDERR "Can't open $filename: $!\n";
3201 while (<$input>) { # note use of indirection
3202 if (/^#include "(.*)"/) {
3203 process($1, $input);
3210 See L<perliol> for detailed info on PerlIO.
3212 You may also, in the Bourne shell tradition, specify an EXPR beginning
3213 with C<< '>&' >>, in which case the rest of the string is interpreted
3214 as the name of a filehandle (or file descriptor, if numeric) to be
3215 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3216 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3217 The mode you specify should match the mode of the original filehandle.
3218 (Duping a filehandle does not take into account any existing contents
3219 of IO buffers.) If you use the 3-arg form then you can pass either a
3220 number, the name of a filehandle or the normal "reference to a glob".
3222 Here is a script that saves, redirects, and restores C<STDOUT> and
3223 C<STDERR> using various methods:
3226 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3227 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3229 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3230 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3232 select STDERR; $| = 1; # make unbuffered
3233 select STDOUT; $| = 1; # make unbuffered
3235 print STDOUT "stdout 1\n"; # this works for
3236 print STDERR "stderr 1\n"; # subprocesses too
3238 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3239 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3241 print STDOUT "stdout 2\n";
3242 print STDERR "stderr 2\n";
3244 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3245 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3246 that file descriptor (and not call L<dup(2)>); this is more
3247 parsimonious of file descriptors. For example:
3249 # open for input, reusing the fileno of $fd
3250 open(FILEHANDLE, "<&=$fd")
3254 open(FILEHANDLE, "<&=", $fd)
3258 # open for append, using the fileno of OLDFH
3259 open(FH, ">>&=", OLDFH)
3263 open(FH, ">>&=OLDFH")
3265 Being parsimonious on filehandles is also useful (besides being
3266 parsimonious) for example when something is dependent on file
3267 descriptors, like for example locking using flock(). If you do just
3268 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3269 descriptor as B, and therefore flock(A) will not flock(B), and vice
3270 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3271 the same file descriptor.
3273 Note that if you are using Perls older than 5.8.0, Perl will be using
3274 the standard C libraries' fdopen() to implement the "=" functionality.
3275 On many UNIX systems fdopen() fails when file descriptors exceed a
3276 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3277 most often the default.
3279 You can see whether Perl has been compiled with PerlIO or not by
3280 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3281 is C<define>, you have PerlIO, otherwise you don't.
3283 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3284 with 2-arguments (or 1-argument) form of open(), then
3285 there is an implicit fork done, and the return value of open is the pid
3286 of the child within the parent process, and C<0> within the child
3287 process. (Use C<defined($pid)> to determine whether the open was successful.)
3288 The filehandle behaves normally for the parent, but i/o to that
3289 filehandle is piped from/to the STDOUT/STDIN of the child process.
3290 In the child process the filehandle isn't opened--i/o happens from/to
3291 the new STDOUT or STDIN. Typically this is used like the normal
3292 piped open when you want to exercise more control over just how the
3293 pipe command gets executed, such as when you are running setuid, and
3294 don't want to have to scan shell commands for metacharacters.
3295 The following triples are more or less equivalent:
3297 open(FOO, "|tr '[a-z]' '[A-Z]'");
3298 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3299 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3300 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3302 open(FOO, "cat -n '$file'|");
3303 open(FOO, '-|', "cat -n '$file'");
3304 open(FOO, '-|') || exec 'cat', '-n', $file;
3305 open(FOO, '-|', "cat", '-n', $file);
3307 The last example in each block shows the pipe as "list form", which is
3308 not yet supported on all platforms. A good rule of thumb is that if
3309 your platform has true C<fork()> (in other words, if your platform is
3310 UNIX) you can use the list form.
3312 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3314 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3315 output before any operation that may do a fork, but this may not be
3316 supported on some platforms (see L<perlport>). To be safe, you may need
3317 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3318 of C<IO::Handle> on any open handles.
3320 On systems that support a close-on-exec flag on files, the flag will
3321 be set for the newly opened file descriptor as determined by the value
3322 of $^F. See L<perlvar/$^F>.
3324 Closing any piped filehandle causes the parent process to wait for the
3325 child to finish, and returns the status value in C<$?> and
3326 C<${^CHILD_ERROR_NATIVE}>.
3328 The filename passed to 2-argument (or 1-argument) form of open() will
3329 have leading and trailing whitespace deleted, and the normal
3330 redirection characters honored. This property, known as "magic open",
3331 can often be used to good effect. A user could specify a filename of
3332 F<"rsh cat file |">, or you could change certain filenames as needed:
3334 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3335 open(FH, $filename) or die "Can't open $filename: $!";
3337 Use 3-argument form to open a file with arbitrary weird characters in it,
3339 open(FOO, '<', $file);
3341 otherwise it's necessary to protect any leading and trailing whitespace:
3343 $file =~ s#^(\s)#./$1#;
3344 open(FOO, "< $file\0");
3346 (this may not work on some bizarre filesystems). One should
3347 conscientiously choose between the I<magic> and 3-arguments form
3352 will allow the user to specify an argument of the form C<"rsh cat file |">,
3353 but will not work on a filename which happens to have a trailing space, while
3355 open IN, '<', $ARGV[0];
3357 will have exactly the opposite restrictions.
3359 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3360 should use the C<sysopen> function, which involves no such magic (but
3361 may use subtly different filemodes than Perl open(), which is mapped
3362 to C fopen()). This is
3363 another way to protect your filenames from interpretation. For example:
3366 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3367 or die "sysopen $path: $!";
3368 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3369 print HANDLE "stuff $$\n";
3371 print "File contains: ", <HANDLE>;
3373 Using the constructor from the C<IO::Handle> package (or one of its
3374 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3375 filehandles that have the scope of whatever variables hold references to
3376 them, and automatically close whenever and however you leave that scope:
3380 sub read_myfile_munged {
3382 my $handle = new IO::File;
3383 open($handle, "myfile") or die "myfile: $!";
3385 or return (); # Automatically closed here.
3386 mung $first or die "mung failed"; # Or here.
3387 return $first, <$handle> if $ALL; # Or here.
3391 See L</seek> for some details about mixing reading and writing.
3393 =item opendir DIRHANDLE,EXPR
3396 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3397 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3398 DIRHANDLE may be an expression whose value can be used as an indirect
3399 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3400 scalar variable (or array or hash element), the variable is assigned a
3401 reference to a new anonymous dirhandle.
3402 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3409 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3410 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3413 For the reverse, see L</chr>.
3414 See L<perlunicode> and L<encoding> for more about Unicode.
3421 =item our EXPR : ATTRS
3423 =item our TYPE EXPR : ATTRS
3425 C<our> associates a simple name with a package variable in the current
3426 package for use within the current scope. When C<use strict 'vars'> is in
3427 effect, C<our> lets you use declared global variables without qualifying
3428 them with package names, within the lexical scope of the C<our> declaration.
3429 In this way C<our> differs from C<use vars>, which is package scoped.
3431 Unlike C<my>, which both allocates storage for a variable and associates
3432 a simple name with that storage for use within the current scope, C<our>
3433 associates a simple name with a package variable in the current package,
3434 for use within the current scope. In other words, C<our> has the same
3435 scoping rules as C<my>, but does not necessarily create a
3438 If more than one value is listed, the list must be placed
3444 An C<our> declaration declares a global variable that will be visible
3445 across its entire lexical scope, even across package boundaries. The
3446 package in which the variable is entered is determined at the point
3447 of the declaration, not at the point of use. This means the following
3451 our $bar; # declares $Foo::bar for rest of lexical scope
3455 print $bar; # prints 20, as it refers to $Foo::bar
3457 Multiple C<our> declarations with the same name in the same lexical
3458 scope are allowed if they are in different packages. If they happen
3459 to be in the same package, Perl will emit warnings if you have asked
3460 for them, just like multiple C<my> declarations. Unlike a second
3461 C<my> declaration, which will bind the name to a fresh variable, a
3462 second C<our> declaration in the same package, in the same scope, is
3467 our $bar; # declares $Foo::bar for rest of lexical scope
3471 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3472 print $bar; # prints 30
3474 our $bar; # emits warning but has no other effect
3475 print $bar; # still prints 30
3477 An C<our> declaration may also have a list of attributes associated
3480 The exact semantics and interface of TYPE and ATTRS are still
3481 evolving. TYPE is currently bound to the use of C<fields> pragma,
3482 and attributes are handled using the C<attributes> pragma, or starting
3483 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3484 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3485 L<attributes>, and L<Attribute::Handlers>.
3487 =item pack TEMPLATE,LIST
3490 Takes a LIST of values and converts it into a string using the rules
3491 given by the TEMPLATE. The resulting string is the concatenation of
3492 the converted values. Typically, each converted value looks
3493 like its machine-level representation. For example, on 32-bit machines
3494 an integer may be represented by a sequence of 4 bytes that will be
3495 converted to a sequence of 4 characters.
3497 The TEMPLATE is a sequence of characters that give the order and type
3498 of values, as follows:
3500 a A string with arbitrary binary data, will be null padded.
3501 A A text (ASCII) string, will be space padded.
3502 Z A null terminated (ASCIZ) string, will be null padded.
3504 b A bit string (ascending bit order inside each byte, like vec()).
3505 B A bit string (descending bit order inside each byte).
3506 h A hex string (low nybble first).
3507 H A hex string (high nybble first).
3509 c A signed char (8-bit) value.
3510 C An unsigned C char (octet) even under Unicode. Should normally not
3511 be used. See U and W instead.
3512 W An unsigned char value (can be greater than 255).
3514 s A signed short (16-bit) value.
3515 S An unsigned short value.
3517 l A signed long (32-bit) value.
3518 L An unsigned long value.
3520 q A signed quad (64-bit) value.
3521 Q An unsigned quad value.
3522 (Quads are available only if your system supports 64-bit
3523 integer values _and_ if Perl has been compiled to support those.
3524 Causes a fatal error otherwise.)
3526 i A signed integer value.
3527 I A unsigned integer value.
3528 (This 'integer' is _at_least_ 32 bits wide. Its exact
3529 size depends on what a local C compiler calls 'int'.)
3531 n An unsigned short (16-bit) in "network" (big-endian) order.
3532 N An unsigned long (32-bit) in "network" (big-endian) order.
3533 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3534 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3536 j A Perl internal signed integer value (IV).
3537 J A Perl internal unsigned integer value (UV).
3539 f A single-precision float in the native format.
3540 d A double-precision float in the native format.
3542 F A Perl internal floating point value (NV) in the native format
3543 D A long double-precision float in the native format.
3544 (Long doubles are available only if your system supports long
3545 double values _and_ if Perl has been compiled to support those.
3546 Causes a fatal error otherwise.)
3548 p A pointer to a null-terminated string.
3549 P A pointer to a structure (fixed-length string).
3551 u A uuencoded string.
3552 U A Unicode character number. Encodes to UTF-8 internally
3553 (or UTF-EBCDIC in EBCDIC platforms).
3555 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3556 details). Its bytes represent an unsigned integer in base 128,
3557 most significant digit first, with as few digits as possible. Bit
3558 eight (the high bit) is set on each byte except the last.
3562 @ Null fill or truncate to absolute position, counted from the
3563 start of the innermost ()-group.
3564 . Null fill or truncate to absolute position specified by value.
3565 ( Start of a ()-group.
3567 One or more of the modifiers below may optionally follow some letters in the
3568 TEMPLATE (the second column lists the letters for which the modifier is
3571 ! sSlLiI Forces native (short, long, int) sizes instead
3572 of fixed (16-/32-bit) sizes.
3574 xX Make x and X act as alignment commands.
3576 nNvV Treat integers as signed instead of unsigned.
3578 @. Specify position as byte offset in the internal
3579 representation of the packed string. Efficient but
3582 > sSiIlLqQ Force big-endian byte-order on the type.
3583 jJfFdDpP (The "big end" touches the construct.)
3585 < sSiIlLqQ Force little-endian byte-order on the type.
3586 jJfFdDpP (The "little end" touches the construct.)
3588 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3589 in which case they force a certain byte-order on all components of
3590 that group, including subgroups.
3592 The following rules apply:
3598 Each letter may optionally be followed by a number giving a repeat
3599 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3600 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3601 that many values from the LIST. A C<*> for the repeat count means to
3602 use however many items are left, except for C<@>, C<x>, C<X>, where it
3603 is equivalent to C<0>, for <.> where it means relative to string start
3604 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3605 A numeric repeat count may optionally be enclosed in brackets, as in
3606 C<pack 'C[80]', @arr>.
3608 One can replace the numeric repeat count by a template enclosed in brackets;
3609 then the packed length of this template in bytes is used as a count.
3610 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3611 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3612 If the template in brackets contains alignment commands (such as C<x![d]>),
3613 its packed length is calculated as if the start of the template has the maximal
3616 When used with C<Z>, C<*> results in the addition of a trailing null
3617 byte (so the packed result will be one longer than the byte C<length>
3620 When used with C<@>, the repeat count represents an offset from the start
3621 of the innermost () group.
3623 When used with C<.>, the repeat count is used to determine the starting
3624 position from where the value offset is calculated. If the repeat count
3625 is 0, it's relative to the current position. If the repeat count is C<*>,
3626 the offset is relative to the start of the packed string. And if its an
3627 integer C<n> the offset is relative to the start of the n-th innermost
3628 () group (or the start of the string if C<n> is bigger then the group
3631 The repeat count for C<u> is interpreted as the maximal number of bytes
3632 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3633 count should not be more than 65.
3637 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3638 string of length count, padding with nulls or spaces as necessary. When
3639 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3640 after the first null, and C<a> returns data verbatim.
3642 If the value-to-pack is too long, it is truncated. If too long and an
3643 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3644 by a null byte. Thus C<Z> always packs a trailing null (except when the
3649 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3650 Each character of the input field of pack() generates 1 bit of the result.
3651 Each result bit is based on the least-significant bit of the corresponding
3652 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3653 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3655 Starting from the beginning of the input string of pack(), each 8-tuple
3656 of characters is converted to 1 character of output. With format C<b>
3657 the first character of the 8-tuple determines the least-significant bit of a
3658 character, and with format C<B> it determines the most-significant bit of
3661 If the length of the input string is not exactly divisible by 8, the
3662 remainder is packed as if the input string were padded by null characters
3663 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3665 If the input string of pack() is longer than needed, extra characters are
3666 ignored. A C<*> for the repeat count of pack() means to use all the
3667 characters of the input field. On unpack()ing the bits are converted to a
3668 string of C<"0">s and C<"1">s.
3672 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3673 representable as hexadecimal digits, 0-9a-f) long.
3675 Each character of the input field of pack() generates 4 bits of the result.
3676 For non-alphabetical characters the result is based on the 4 least-significant
3677 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3678 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3679 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3680 is compatible with the usual hexadecimal digits, so that C<"a"> and
3681 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3682 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3684 Starting from the beginning of the input string of pack(), each pair
3685 of characters is converted to 1 character of output. With format C<h> the
3686 first character of the pair determines the least-significant nybble of the
3687 output character, and with format C<H> it determines the most-significant
3690 If the length of the input string is not even, it behaves as if padded
3691 by a null character at the end. Similarly, during unpack()ing the "extra"
3692 nybbles are ignored.
3694 If the input string of pack() is longer than needed, extra characters are
3696 A C<*> for the repeat count of pack() means to use all the characters of
3697 the input field. On unpack()ing the nybbles are converted to a string
3698 of hexadecimal digits.
3702 The C<p> type packs a pointer to a null-terminated string. You are
3703 responsible for ensuring the string is not a temporary value (which can
3704 potentially get deallocated before you get around to using the packed result).
3705 The C<P> type packs a pointer to a structure of the size indicated by the
3706 length. A NULL pointer is created if the corresponding value for C<p> or
3707 C<P> is C<undef>, similarly for unpack().
3709 If your system has a strange pointer size (i.e. a pointer is neither as
3710 big as an int nor as big as a long), it may not be possible to pack or
3711 unpack pointers in big- or little-endian byte order. Attempting to do
3712 so will result in a fatal error.
3716 The C</> template character allows packing and unpacking of a sequence of
3717 items where the packed structure contains a packed item count followed by
3718 the packed items themselves.
3720 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3721 I<length-item> describes how the length value is packed. The ones likely
3722 to be of most use are integer-packing ones like C<n> (for Java strings),
3723 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3725 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3726 the minimum of that and the number of available items is used as argument
3727 for the I<length-item>. If it has no repeat count or uses a '*', the number
3728 of available items is used.
3730 For C<unpack> an internal stack of integer arguments unpacked so far is
3731 used. You write C</>I<sequence-item> and the repeat count is obtained by
3732 popping off the last element from the stack. The I<sequence-item> must not
3733 have a repeat count.
3735 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3736 the I<length-item> is a string length, not a number of strings. If there is
3737 an explicit repeat count for pack, the packed string will be adjusted to that
3740 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3741 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3742 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3743 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3744 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3746 The I<length-item> is not returned explicitly from C<unpack>.
3748 Adding a count to the I<length-item> letter is unlikely to do anything
3749 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3750 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3751 which Perl does not regard as legal in numeric strings.
3755 The integer types C<s>, C<S>, C<l>, and C<L> may be
3756 followed by a C<!> modifier to signify native shorts or
3757 longs--as you can see from above for example a bare C<l> does mean
3758 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3759 may be larger. This is an issue mainly in 64-bit platforms. You can
3760 see whether using C<!> makes any difference by
3762 print length(pack("s")), " ", length(pack("s!")), "\n";
3763 print length(pack("l")), " ", length(pack("l!")), "\n";
3765 C<i!> and C<I!> also work but only because of completeness;
3766 they are identical to C<i> and C<I>.
3768 The actual sizes (in bytes) of native shorts, ints, longs, and long
3769 longs on the platform where Perl was built are also available via
3773 print $Config{shortsize}, "\n";
3774 print $Config{intsize}, "\n";
3775 print $Config{longsize}, "\n";
3776 print $Config{longlongsize}, "\n";
3778 (The C<$Config{longlongsize}> will be undefined if your system does
3779 not support long longs.)
3783 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3784 are inherently non-portable between processors and operating systems
3785 because they obey the native byteorder and endianness. For example a
3786 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3787 (arranged in and handled by the CPU registers) into bytes as
3789 0x12 0x34 0x56 0x78 # big-endian
3790 0x78 0x56 0x34 0x12 # little-endian
3792 Basically, the Intel and VAX CPUs are little-endian, while everybody
3793 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3794 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3795 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3798 The names `big-endian' and `little-endian' are comic references to
3799 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3800 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3801 the egg-eating habits of the Lilliputians.
3803 Some systems may have even weirder byte orders such as
3808 You can see your system's preference with
3810 print join(" ", map { sprintf "%#02x", $_ }
3811 unpack("W*",pack("L",0x12345678))), "\n";
3813 The byteorder on the platform where Perl was built is also available
3817 print $Config{byteorder}, "\n";
3819 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3820 and C<'87654321'> are big-endian.
3822 If you want portable packed integers you can either use the formats
3823 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3824 modifiers. These modifiers are only available as of perl 5.9.2.
3825 See also L<perlport>.
3829 All integer and floating point formats as well as C<p> and C<P> and
3830 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3831 to force big- or little- endian byte-order, respectively.
3832 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3833 signed integers, 64-bit integers and floating point values. However,
3834 there are some things to keep in mind.
3836 Exchanging signed integers between different platforms only works
3837 if all platforms store them in the same format. Most platforms store
3838 signed integers in two's complement, so usually this is not an issue.
3840 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3841 formats on big- or little-endian machines. Otherwise, attempting to
3842 do so will result in a fatal error.
3844 Forcing big- or little-endian byte-order on floating point values for
3845 data exchange can only work if all platforms are using the same
3846 binary representation (e.g. IEEE floating point format). Even if all
3847 platforms are using IEEE, there may be subtle differences. Being able
3848 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3849 but also very dangerous if you don't know exactly what you're doing.
3850 It is definitely not a general way to portably store floating point
3853 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3854 all types inside the group that accept the byte-order modifiers,
3855 including all subgroups. It will silently be ignored for all other
3856 types. You are not allowed to override the byte-order within a group
3857 that already has a byte-order modifier suffix.
3861 Real numbers (floats and doubles) are in the native machine format only;
3862 due to the multiplicity of floating formats around, and the lack of a
3863 standard "network" representation, no facility for interchange has been
3864 made. This means that packed floating point data written on one machine
3865 may not be readable on another - even if both use IEEE floating point
3866 arithmetic (as the endian-ness of the memory representation is not part
3867 of the IEEE spec). See also L<perlport>.
3869 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3870 modifiers to force big- or little-endian byte-order on floating point values.
3872 Note that Perl uses doubles (or long doubles, if configured) internally for
3873 all numeric calculation, and converting from double into float and thence back
3874 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3875 will not in general equal $foo).
3879 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3880 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3881 where the packed string is processed in its UTF-8-encoded Unicode form on
3882 a byte by byte basis. Character mode is the default unless the format string
3883 starts with an C<U>. You can switch mode at any moment with an explicit
3884 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3885 or until the end of the ()-group in which it was entered.
3889 You must yourself do any alignment or padding by inserting for example
3890 enough C<'x'>es while packing. There is no way to pack() and unpack()
3891 could know where the characters are going to or coming from. Therefore
3892 C<pack> (and C<unpack>) handle their output and input as flat
3893 sequences of characters.
3897 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3898 take a repeat count, both as postfix, and for unpack() also via the C</>
3899 template character. Within each repetition of a group, positioning with
3900 C<@> starts again at 0. Therefore, the result of
3902 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3904 is the string "\0a\0\0bc".
3908 C<x> and C<X> accept C<!> modifier. In this case they act as
3909 alignment commands: they jump forward/back to the closest position
3910 aligned at a multiple of C<count> characters. For example, to pack() or
3911 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3912 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3913 aligned on the double's size.
3915 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3916 both result in no-ops.
3920 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3921 will represent signed 16-/32-bit integers in big-/little-endian order.
3922 This is only portable if all platforms sharing the packed data use the
3923 same binary representation for signed integers (e.g. all platforms are
3924 using two's complement representation).
3928 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3929 White space may be used to separate pack codes from each other, but
3930 modifiers and a repeat count must follow immediately.
3934 If TEMPLATE requires more arguments to pack() than actually given, pack()
3935 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3936 to pack() than actually given, extra arguments are ignored.
3942 $foo = pack("WWWW",65,66,67,68);
3944 $foo = pack("W4",65,66,67,68);
3946 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3947 # same thing with Unicode circled letters.
3948 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3949 # same thing with Unicode circled letters. You don't get the UTF-8
3950 # bytes because the U at the start of the format caused a switch to
3951 # U0-mode, so the UTF-8 bytes get joined into characters
3952 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3953 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3954 # This is the UTF-8 encoding of the string in the previous example
3956 $foo = pack("ccxxcc",65,66,67,68);
3959 # note: the above examples featuring "W" and "c" are true
3960 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3961 # and UTF-8. In EBCDIC the first example would be
3962 # $foo = pack("WWWW",193,194,195,196);
3964 $foo = pack("s2",1,2);
3965 # "\1\0\2\0" on little-endian
3966 # "\0\1\0\2" on big-endian
3968 $foo = pack("a4","abcd","x","y","z");
3971 $foo = pack("aaaa","abcd","x","y","z");
3974 $foo = pack("a14","abcdefg");
3975 # "abcdefg\0\0\0\0\0\0\0"
3977 $foo = pack("i9pl", gmtime);
3978 # a real struct tm (on my system anyway)
3980 $utmp_template = "Z8 Z8 Z16 L";
3981 $utmp = pack($utmp_template, @utmp1);
3982 # a struct utmp (BSDish)
3984 @utmp2 = unpack($utmp_template, $utmp);
3985 # "@utmp1" eq "@utmp2"
3988 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3991 $foo = pack('sx2l', 12, 34);
3992 # short 12, two zero bytes padding, long 34
3993 $bar = pack('s@4l', 12, 34);
3994 # short 12, zero fill to position 4, long 34
3996 $baz = pack('s.l', 12, 4, 34);
3997 # short 12, zero fill to position 4, long 34
3999 $foo = pack('nN', 42, 4711);
4000 # pack big-endian 16- and 32-bit unsigned integers
4001 $foo = pack('S>L>', 42, 4711);
4003 $foo = pack('s<l<', -42, 4711);
4004 # pack little-endian 16- and 32-bit signed integers
4005 $foo = pack('(sl)<', -42, 4711);
4008 The same template may generally also be used in unpack().
4010 =item package NAMESPACE
4011 X<package> X<module> X<namespace>
4015 Declares the compilation unit as being in the given namespace. The scope
4016 of the package declaration is from the declaration itself through the end
4017 of the enclosing block, file, or eval (the same as the C<my> operator).
4018 All further unqualified dynamic identifiers will be in this namespace.
4019 A package statement affects only dynamic variables--including those
4020 you've used C<local> on--but I<not> lexical variables, which are created
4021 with C<my>. Typically it would be the first declaration in a file to
4022 be included by the C<require> or C<use> operator. You can switch into a
4023 package in more than one place; it merely influences which symbol table
4024 is used by the compiler for the rest of that block. You can refer to
4025 variables and filehandles in other packages by prefixing the identifier
4026 with the package name and a double colon: C<$Package::Variable>.
4027 If the package name is null, the C<main> package as assumed. That is,
4028 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4029 still seen in older code).
4031 If NAMESPACE is omitted, then there is no current package, and all
4032 identifiers must be fully qualified or lexicals. However, you are
4033 strongly advised not to make use of this feature. Its use can cause
4034 unexpected behaviour, even crashing some versions of Perl. It is
4035 deprecated, and will be removed from a future release.
4037 See L<perlmod/"Packages"> for more information about packages, modules,
4038 and classes. See L<perlsub> for other scoping issues.
4040 =item pipe READHANDLE,WRITEHANDLE
4043 Opens a pair of connected pipes like the corresponding system call.
4044 Note that if you set up a loop of piped processes, deadlock can occur
4045 unless you are very careful. In addition, note that Perl's pipes use
4046 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4047 after each command, depending on the application.
4049 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4050 for examples of such things.
4052 On systems that support a close-on-exec flag on files, the flag will be set
4053 for the newly opened file descriptors as determined by the value of $^F.
4061 Pops and returns the last value of the array, shortening the array by
4062 one element. Has an effect similar to
4066 If there are no elements in the array, returns the undefined value
4067 (although this may happen at other times as well). If ARRAY is
4068 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4069 array in subroutines, just like C<shift>.
4072 X<pos> X<match, position>
4076 Returns the offset of where the last C<m//g> search left off for the variable
4077 in question (C<$_> is used when the variable is not specified). Note that
4078 0 is a valid match offset. C<undef> indicates that the search position
4079 is reset (usually due to match failure, but can also be because no match has
4080 yet been performed on the scalar). C<pos> directly accesses the location used
4081 by the regexp engine to store the offset, so assigning to C<pos> will change
4082 that offset, and so will also influence the C<\G> zero-width assertion in
4083 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4084 the return from C<pos> won't change either in this case. See L<perlre> and
4087 =item print FILEHANDLE LIST
4094 Prints a string or a list of strings. Returns true if successful.
4095 FILEHANDLE may be a scalar variable name, in which case the variable
4096 contains the name of or a reference to the filehandle, thus introducing
4097 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4098 the next token is a term, it may be misinterpreted as an operator
4099 unless you interpose a C<+> or put parentheses around the arguments.)
4100 If FILEHANDLE is omitted, prints by default to standard output (or
4101 to the last selected output channel--see L</select>). If LIST is
4102 also omitted, prints C<$_> to the currently selected output channel.
4103 To set the default output channel to something other than STDOUT
4104 use the select operation. The current value of C<$,> (if any) is
4105 printed between each LIST item. The current value of C<$\> (if
4106 any) is printed after the entire LIST has been printed. Because
4107 print takes a LIST, anything in the LIST is evaluated in list
4108 context, and any subroutine that you call will have one or more of
4109 its expressions evaluated in list context. Also be careful not to
4110 follow the print keyword with a left parenthesis unless you want
4111 the corresponding right parenthesis to terminate the arguments to
4112 the print--interpose a C<+> or put parentheses around all the
4115 Note that if you're storing FILEHANDLEs in an array, or if you're using
4116 any other expression more complex than a scalar variable to retrieve it,
4117 you will have to use a block returning the filehandle value instead:
4119 print { $files[$i] } "stuff\n";
4120 print { $OK ? STDOUT : STDERR } "stuff\n";
4122 =item printf FILEHANDLE FORMAT, LIST
4125 =item printf FORMAT, LIST
4127 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4128 (the output record separator) is not appended. The first argument
4129 of the list will be interpreted as the C<printf> format. See C<sprintf>
4130 for an explanation of the format argument. If C<use locale> is in effect,
4131 and POSIX::setlocale() has been called, the character used for the decimal
4132 separator in formatted floating point numbers is affected by the LC_NUMERIC
4133 locale. See L<perllocale> and L<POSIX>.
4135 Don't fall into the trap of using a C<printf> when a simple
4136 C<print> would do. The C<print> is more efficient and less
4139 =item prototype FUNCTION
4142 Returns the prototype of a function as a string (or C<undef> if the
4143 function has no prototype). FUNCTION is a reference to, or the name of,
4144 the function whose prototype you want to retrieve.
4146 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4147 name for Perl builtin. If the builtin is not I<overridable> (such as
4148 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4149 C<system>) returns C<undef> because the builtin does not really behave
4150 like a Perl function. Otherwise, the string describing the equivalent
4151 prototype is returned.
4153 =item push ARRAY,LIST
4156 Treats ARRAY as a stack, and pushes the values of LIST
4157 onto the end of ARRAY. The length of ARRAY increases by the length of
4158 LIST. Has the same effect as
4161 $ARRAY[++$#ARRAY] = $value;
4164 but is more efficient. Returns the number of elements in the array following
4165 the completed C<push>.
4177 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4179 =item quotemeta EXPR
4180 X<quotemeta> X<metacharacter>
4184 Returns the value of EXPR with all non-"word"
4185 characters backslashed. (That is, all characters not matching
4186 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4187 returned string, regardless of any locale settings.)
4188 This is the internal function implementing
4189 the C<\Q> escape in double-quoted strings.
4191 If EXPR is omitted, uses C<$_>.
4198 Returns a random fractional number greater than or equal to C<0> and less
4199 than the value of EXPR. (EXPR should be positive.) If EXPR is
4200 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4201 also special-cased as C<1> - this has not been documented before perl 5.8.0
4202 and is subject to change in future versions of perl. Automatically calls
4203 C<srand> unless C<srand> has already been called. See also C<srand>.
4205 Apply C<int()> to the value returned by C<rand()> if you want random
4206 integers instead of random fractional numbers. For example,
4210 returns a random integer between C<0> and C<9>, inclusive.
4212 (Note: If your rand function consistently returns numbers that are too
4213 large or too small, then your version of Perl was probably compiled
4214 with the wrong number of RANDBITS.)
4216 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4217 X<read> X<file, read>
4219 =item read FILEHANDLE,SCALAR,LENGTH
4221 Attempts to read LENGTH I<characters> of data into variable SCALAR
4222 from the specified FILEHANDLE. Returns the number of characters
4223 actually read, C<0> at end of file, or undef if there was an error (in
4224 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4225 so that the last character actually read is the last character of the
4226 scalar after the read.
4228 An OFFSET may be specified to place the read data at some place in the
4229 string other than the beginning. A negative OFFSET specifies
4230 placement at that many characters counting backwards from the end of
4231 the string. A positive OFFSET greater than the length of SCALAR
4232 results in the string being padded to the required size with C<"\0">
4233 bytes before the result of the read is appended.
4235 The call is actually implemented in terms of either Perl's or system's
4236 fread() call. To get a true read(2) system call, see C<sysread>.
4238 Note the I<characters>: depending on the status of the filehandle,
4239 either (8-bit) bytes or characters are read. By default all
4240 filehandles operate on bytes, but for example if the filehandle has
4241 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4242 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4243 characters, not bytes. Similarly for the C<:encoding> pragma:
4244 in that case pretty much any characters can be read.
4246 =item readdir DIRHANDLE
4249 Returns the next directory entry for a directory opened by C<opendir>.
4250 If used in list context, returns all the rest of the entries in the
4251 directory. If there are no more entries, returns an undefined value in
4252 scalar context or a null list in list context.
4254 If you're planning to filetest the return values out of a C<readdir>, you'd
4255 better prepend the directory in question. Otherwise, because we didn't
4256 C<chdir> there, it would have been testing the wrong file.
4258 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4259 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4263 X<readline> X<gets> X<fgets>
4265 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4266 context, each call reads and returns the next line, until end-of-file is
4267 reached, whereupon the subsequent call returns undef. In list context,
4268 reads until end-of-file is reached and returns a list of lines. Note that
4269 the notion of "line" used here is however you may have defined it
4270 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4272 When C<$/> is set to C<undef>, when readline() is in scalar
4273 context (i.e. file slurp mode), and when an empty file is read, it
4274 returns C<''> the first time, followed by C<undef> subsequently.
4276 This is the internal function implementing the C<< <EXPR> >>
4277 operator, but you can use it directly. The C<< <EXPR> >>
4278 operator is discussed in more detail in L<perlop/"I/O Operators">.
4281 $line = readline(*STDIN); # same thing
4283 If readline encounters an operating system error, C<$!> will be set with the
4284 corresponding error message. It can be helpful to check C<$!> when you are
4285 reading from filehandles you don't trust, such as a tty or a socket. The
4286 following example uses the operator form of C<readline>, and takes the necessary
4287 steps to ensure that C<readline> was successful.
4291 unless (defined( $line = <> )) {
4303 Returns the value of a symbolic link, if symbolic links are
4304 implemented. If not, gives a fatal error. If there is some system
4305 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4306 omitted, uses C<$_>.
4311 EXPR is executed as a system command.
4312 The collected standard output of the command is returned.
4313 In scalar context, it comes back as a single (potentially
4314 multi-line) string. In list context, returns a list of lines
4315 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4316 This is the internal function implementing the C<qx/EXPR/>
4317 operator, but you can use it directly. The C<qx/EXPR/>
4318 operator is discussed in more detail in L<perlop/"I/O Operators">.
4320 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4323 Receives a message on a socket. Attempts to receive LENGTH characters
4324 of data into variable SCALAR from the specified SOCKET filehandle.
4325 SCALAR will be grown or shrunk to the length actually read. Takes the
4326 same flags as the system call of the same name. Returns the address
4327 of the sender if SOCKET's protocol supports this; returns an empty
4328 string otherwise. If there's an error, returns the undefined value.
4329 This call is actually implemented in terms of recvfrom(2) system call.
4330 See L<perlipc/"UDP: Message Passing"> for examples.
4332 Note the I<characters>: depending on the status of the socket, either
4333 (8-bit) bytes or characters are received. By default all sockets
4334 operate on bytes, but for example if the socket has been changed using
4335 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4336 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4337 characters, not bytes. Similarly for the C<:encoding> pragma:
4338 in that case pretty much any characters can be read.
4345 The C<redo> command restarts the loop block without evaluating the
4346 conditional again. The C<continue> block, if any, is not executed. If
4347 the LABEL is omitted, the command refers to the innermost enclosing
4348 loop. Programs that want to lie to themselves about what was just input
4349 normally use this command:
4351 # a simpleminded Pascal comment stripper
4352 # (warning: assumes no { or } in strings)
4353 LINE: while (<STDIN>) {
4354 while (s|({.*}.*){.*}|$1 |) {}
4359 if (/}/) { # end of comment?
4368 C<redo> cannot be used to retry a block which returns a value such as
4369 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4370 a grep() or map() operation.
4372 Note that a block by itself is semantically identical to a loop
4373 that executes once. Thus C<redo> inside such a block will effectively
4374 turn it into a looping construct.
4376 See also L</continue> for an illustration of how C<last>, C<next>, and
4384 Returns a non-empty string if EXPR is a reference, the empty
4385 string otherwise. If EXPR
4386 is not specified, C<$_> will be used. The value returned depends on the
4387 type of thing the reference is a reference to.
4388 Builtin types include:
4402 If the referenced object has been blessed into a package, then that package
4403 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4405 if (ref($r) eq "HASH") {
4406 print "r is a reference to a hash.\n";
4409 print "r is not a reference at all.\n";
4412 See also L<perlref>.
4414 =item rename OLDNAME,NEWNAME
4415 X<rename> X<move> X<mv> X<ren>
4417 Changes the name of a file; an existing file NEWNAME will be
4418 clobbered. Returns true for success, false otherwise.
4420 Behavior of this function varies wildly depending on your system
4421 implementation. For example, it will usually not work across file system
4422 boundaries, even though the system I<mv> command sometimes compensates
4423 for this. Other restrictions include whether it works on directories,
4424 open files, or pre-existing files. Check L<perlport> and either the
4425 rename(2) manpage or equivalent system documentation for details.
4427 For a platform independent C<move> function look at the L<File::Copy>
4430 =item require VERSION
4437 Demands a version of Perl specified by VERSION, or demands some semantics
4438 specified by EXPR or by C<$_> if EXPR is not supplied.
4440 VERSION may be either a numeric argument such as 5.006, which will be
4441 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4442 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4443 VERSION is greater than the version of the current Perl interpreter.
4444 Compare with L</use>, which can do a similar check at compile time.
4446 Specifying VERSION as a literal of the form v5.6.1 should generally be
4447 avoided, because it leads to misleading error messages under earlier
4448 versions of Perl that do not support this syntax. The equivalent numeric
4449 version should be used instead.
4451 require v5.6.1; # run time version check
4452 require 5.6.1; # ditto
4453 require 5.006_001; # ditto; preferred for backwards compatibility
4455 Otherwise, C<require> demands that a library file be included if it
4456 hasn't already been included. The file is included via the do-FILE
4457 mechanism, which is essentially just a variety of C<eval>. Has
4458 semantics similar to the following subroutine:
4461 my ($filename) = @_;
4462 if (exists $INC{$filename}) {
4463 return 1 if $INC{$filename};
4464 die "Compilation failed in require";
4466 my ($realfilename,$result);
4468 foreach $prefix (@INC) {
4469 $realfilename = "$prefix/$filename";
4470 if (-f $realfilename) {
4471 $INC{$filename} = $realfilename;
4472 $result = do $realfilename;
4476 die "Can't find $filename in \@INC";
4479 $INC{$filename} = undef;
4481 } elsif (!$result) {
4482 delete $INC{$filename};
4483 die "$filename did not return true value";
4489 Note that the file will not be included twice under the same specified
4492 The file must return true as the last statement to indicate
4493 successful execution of any initialization code, so it's customary to
4494 end such a file with C<1;> unless you're sure it'll return true
4495 otherwise. But it's better just to put the C<1;>, in case you add more
4498 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4499 replaces "F<::>" with "F</>" in the filename for you,
4500 to make it easy to load standard modules. This form of loading of
4501 modules does not risk altering your namespace.
4503 In other words, if you try this:
4505 require Foo::Bar; # a splendid bareword
4507 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4508 directories specified in the C<@INC> array.
4510 But if you try this:
4512 $class = 'Foo::Bar';
4513 require $class; # $class is not a bareword
4515 require "Foo::Bar"; # not a bareword because of the ""
4517 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4518 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4520 eval "require $class";
4522 Now that you understand how C<require> looks for files in the case of a
4523 bareword argument, there is a little extra functionality going on behind
4524 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4525 first look for a similar filename with a "F<.pmc>" extension. If this file
4526 is found, it will be loaded in place of any file ending in a "F<.pm>"
4529 You can also insert hooks into the import facility, by putting directly
4530 Perl code into the @INC array. There are three forms of hooks: subroutine
4531 references, array references and blessed objects.
4533 Subroutine references are the simplest case. When the inclusion system
4534 walks through @INC and encounters a subroutine, this subroutine gets
4535 called with two parameters, the first being a reference to itself, and the
4536 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4537 subroutine should return nothing, or a list of up to 4 values in the
4544 A reference to a scalar, containing any initial source code to prepend to
4545 the file or generator output.
4550 A filehandle, from which the file will be read.
4554 A reference to a subroutine. If there is no filehandle (previous item),
4555 then this subroutine is expected to generate one line of source code per
4556 call, writing the line into C<$_> and returning 1, then returning 0 at
4557 "end of file". If there is a filehandle, then the subroutine will be
4558 called to act a simple source filter, with the line as read in C<$_>.
4559 Again, return 1 for each valid line, and 0 after all lines have been
4564 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4565 reference to the subroutine itself is passed in as C<$_[0]>.
4569 If an empty list, C<undef>, or nothing that matches the first 3 values above
4570 is returned then C<require> will look at the remaining elements of @INC.
4571 Note that this file handle must be a real file handle (strictly a typeglob,
4572 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4573 ignored and return value processing will stop there.
4575 If the hook is an array reference, its first element must be a subroutine
4576 reference. This subroutine is called as above, but the first parameter is
4577 the array reference. This enables to pass indirectly some arguments to
4580 In other words, you can write:
4582 push @INC, \&my_sub;
4584 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4590 push @INC, [ \&my_sub, $x, $y, ... ];
4592 my ($arrayref, $filename) = @_;
4593 # Retrieve $x, $y, ...
4594 my @parameters = @$arrayref[1..$#$arrayref];
4598 If the hook is an object, it must provide an INC method that will be
4599 called as above, the first parameter being the object itself. (Note that
4600 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4601 into package C<main>.) Here is a typical code layout:
4607 my ($self, $filename) = @_;
4611 # In the main program
4612 push @INC, new Foo(...);
4614 Note that these hooks are also permitted to set the %INC entry
4615 corresponding to the files they have loaded. See L<perlvar/%INC>.
4617 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4624 Generally used in a C<continue> block at the end of a loop to clear
4625 variables and reset C<??> searches so that they work again. The
4626 expression is interpreted as a list of single characters (hyphens
4627 allowed for ranges). All variables and arrays beginning with one of
4628 those letters are reset to their pristine state. If the expression is
4629 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4630 only variables or searches in the current package. Always returns
4633 reset 'X'; # reset all X variables
4634 reset 'a-z'; # reset lower case variables
4635 reset; # just reset ?one-time? searches
4637 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4638 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4639 variables--lexical variables are unaffected, but they clean themselves
4640 up on scope exit anyway, so you'll probably want to use them instead.
4648 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4649 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4650 context, depending on how the return value will be used, and the context
4651 may vary from one execution to the next (see C<wantarray>). If no EXPR
4652 is given, returns an empty list in list context, the undefined value in
4653 scalar context, and (of course) nothing at all in a void context.
4655 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4656 or do FILE will automatically return the value of the last expression
4660 X<reverse> X<rev> X<invert>
4662 In list context, returns a list value consisting of the elements
4663 of LIST in the opposite order. In scalar context, concatenates the
4664 elements of LIST and returns a string value with all characters
4665 in the opposite order.
4667 print reverse <>; # line tac, last line first
4669 undef $/; # for efficiency of <>
4670 print scalar reverse <>; # character tac, last line tsrif
4672 Used without arguments in scalar context, reverse() reverses C<$_>.
4674 This operator is also handy for inverting a hash, although there are some
4675 caveats. If a value is duplicated in the original hash, only one of those
4676 can be represented as a key in the inverted hash. Also, this has to
4677 unwind one hash and build a whole new one, which may take some time
4678 on a large hash, such as from a DBM file.
4680 %by_name = reverse %by_address; # Invert the hash
4682 =item rewinddir DIRHANDLE
4685 Sets the current position to the beginning of the directory for the
4686 C<readdir> routine on DIRHANDLE.
4688 =item rindex STR,SUBSTR,POSITION
4691 =item rindex STR,SUBSTR
4693 Works just like index() except that it returns the position of the I<last>
4694 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4695 last occurrence beginning at or before that position.
4697 =item rmdir FILENAME
4698 X<rmdir> X<rd> X<directory, remove>
4702 Deletes the directory specified by FILENAME if that directory is
4703 empty. If it succeeds it returns true, otherwise it returns false and
4704 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4706 To remove a directory tree recursively (C<rm -rf> on unix) look at
4707 the C<rmtree> function of the L<File::Path> module.
4711 The substitution operator. See L<perlop>.
4713 =item say FILEHANDLE LIST
4720 Just like C<print>, but implicitly appends a newline.
4721 C<say LIST> is simply an abbreviation for C<print LIST, "\n">,
4722 and C<say()> works just like C<print($_, "\n")>.
4724 That means that a call to say() appends any output record separator
4725 I<after> the added newline.
4727 This keyword is only available when the "say" feature is
4728 enabled: see L<feature>.
4731 X<scalar> X<context>
4733 Forces EXPR to be interpreted in scalar context and returns the value
4736 @counts = ( scalar @a, scalar @b, scalar @c );
4738 There is no equivalent operator to force an expression to
4739 be interpolated in list context because in practice, this is never
4740 needed. If you really wanted to do so, however, you could use
4741 the construction C<@{[ (some expression) ]}>, but usually a simple
4742 C<(some expression)> suffices.
4744 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4745 parenthesized list, this behaves as a scalar comma expression, evaluating
4746 all but the last element in void context and returning the final element
4747 evaluated in scalar context. This is seldom what you want.
4749 The following single statement:
4751 print uc(scalar(&foo,$bar)),$baz;
4753 is the moral equivalent of these two:
4756 print(uc($bar),$baz);
4758 See L<perlop> for more details on unary operators and the comma operator.
4760 =item seek FILEHANDLE,POSITION,WHENCE
4761 X<seek> X<fseek> X<filehandle, position>
4763 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4764 FILEHANDLE may be an expression whose value gives the name of the
4765 filehandle. The values for WHENCE are C<0> to set the new position
4766 I<in bytes> to POSITION, C<1> to set it to the current position plus
4767 POSITION, and C<2> to set it to EOF plus POSITION (typically
4768 negative). For WHENCE you may use the constants C<SEEK_SET>,
4769 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4770 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4773 Note the I<in bytes>: even if the filehandle has been set to
4774 operate on characters (for example by using the C<:utf8> open
4775 layer), tell() will return byte offsets, not character offsets
4776 (because implementing that would render seek() and tell() rather slow).
4778 If you want to position file for C<sysread> or C<syswrite>, don't use
4779 C<seek>--buffering makes its effect on the file's system position
4780 unpredictable and non-portable. Use C<sysseek> instead.
4782 Due to the rules and rigors of ANSI C, on some systems you have to do a
4783 seek whenever you switch between reading and writing. Amongst other
4784 things, this may have the effect of calling stdio's clearerr(3).
4785 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4789 This is also useful for applications emulating C<tail -f>. Once you hit
4790 EOF on your read, and then sleep for a while, you might have to stick in a
4791 seek() to reset things. The C<seek> doesn't change the current position,
4792 but it I<does> clear the end-of-file condition on the handle, so that the
4793 next C<< <FILE> >> makes Perl try again to read something. We hope.
4795 If that doesn't work (some IO implementations are particularly
4796 cantankerous), then you may need something more like this:
4799 for ($curpos = tell(FILE); $_ = <FILE>;
4800 $curpos = tell(FILE)) {
4801 # search for some stuff and put it into files
4803 sleep($for_a_while);
4804 seek(FILE, $curpos, 0);
4807 =item seekdir DIRHANDLE,POS
4810 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4811 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4812 about possible directory compaction as the corresponding system library
4815 =item select FILEHANDLE
4816 X<select> X<filehandle, default>
4820 Returns the currently selected filehandle. Sets the current default
4821 filehandle for output, if FILEHANDLE is supplied. This has two
4822 effects: first, a C<write> or a C<print> without a filehandle will
4823 default to this FILEHANDLE. Second, references to variables related to
4824 output will refer to this output channel. For example, if you have to
4825 set the top of form format for more than one output channel, you might
4833 FILEHANDLE may be an expression whose value gives the name of the
4834 actual filehandle. Thus:
4836 $oldfh = select(STDERR); $| = 1; select($oldfh);
4838 Some programmers may prefer to think of filehandles as objects with
4839 methods, preferring to write the last example as:
4842 STDERR->autoflush(1);
4844 =item select RBITS,WBITS,EBITS,TIMEOUT
4847 This calls the select(2) system call with the bit masks specified, which
4848 can be constructed using C<fileno> and C<vec>, along these lines:
4850 $rin = $win = $ein = '';
4851 vec($rin,fileno(STDIN),1) = 1;
4852 vec($win,fileno(STDOUT),1) = 1;
4855 If you want to select on many filehandles you might wish to write a
4859 my(@fhlist) = split(' ',$_[0]);
4862 vec($bits,fileno($_),1) = 1;
4866 $rin = fhbits('STDIN TTY SOCK');
4870 ($nfound,$timeleft) =
4871 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4873 or to block until something becomes ready just do this
4875 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4877 Most systems do not bother to return anything useful in $timeleft, so
4878 calling select() in scalar context just returns $nfound.
4880 Any of the bit masks can also be undef. The timeout, if specified, is
4881 in seconds, which may be fractional. Note: not all implementations are
4882 capable of returning the $timeleft. If not, they always return
4883 $timeleft equal to the supplied $timeout.
4885 You can effect a sleep of 250 milliseconds this way:
4887 select(undef, undef, undef, 0.25);
4889 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4890 is implementation-dependent. See also L<perlport> for notes on the
4891 portability of C<select>.
4893 On error, C<select> behaves like the select(2) system call : it returns
4896 Note: on some Unixes, the select(2) system call may report a socket file
4897 descriptor as "ready for reading", when actually no data is available,
4898 thus a subsequent read blocks. It can be avoided using always the
4899 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4902 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4903 or <FH>) with C<select>, except as permitted by POSIX, and even
4904 then only on POSIX systems. You have to use C<sysread> instead.
4906 =item semctl ID,SEMNUM,CMD,ARG
4909 Calls the System V IPC function C<semctl>. You'll probably have to say
4913 first to get the correct constant definitions. If CMD is IPC_STAT or
4914 GETALL, then ARG must be a variable that will hold the returned
4915 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4916 the undefined value for error, "C<0 but true>" for zero, or the actual
4917 return value otherwise. The ARG must consist of a vector of native
4918 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4919 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4922 =item semget KEY,NSEMS,FLAGS
4925 Calls the System V IPC function semget. Returns the semaphore id, or
4926 the undefined value if there is an error. See also
4927 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4930 =item semop KEY,OPSTRING
4933 Calls the System V IPC function semop to perform semaphore operations
4934 such as signalling and waiting. OPSTRING must be a packed array of
4935 semop structures. Each semop structure can be generated with
4936 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4937 implies the number of semaphore operations. Returns true if
4938 successful, or false if there is an error. As an example, the
4939 following code waits on semaphore $semnum of semaphore id $semid:
4941 $semop = pack("s!3", $semnum, -1, 0);
4942 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4944 To signal the semaphore, replace C<-1> with C<1>. See also
4945 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4948 =item send SOCKET,MSG,FLAGS,TO
4951 =item send SOCKET,MSG,FLAGS
4953 Sends a message on a socket. Attempts to send the scalar MSG to the
4954 SOCKET filehandle. Takes the same flags as the system call of the
4955 same name. On unconnected sockets you must specify a destination to
4956 send TO, in which case it does a C C<sendto>. Returns the number of
4957 characters sent, or the undefined value if there is an error. The C
4958 system call sendmsg(2) is currently unimplemented. See
4959 L<perlipc/"UDP: Message Passing"> for examples.
4961 Note the I<characters>: depending on the status of the socket, either
4962 (8-bit) bytes or characters are sent. By default all sockets operate
4963 on bytes, but for example if the socket has been changed using
4964 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4965 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4966 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4967 in that case pretty much any characters can be sent.
4969 =item setpgrp PID,PGRP
4972 Sets the current process group for the specified PID, C<0> for the current
4973 process. Will produce a fatal error if used on a machine that doesn't
4974 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4975 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4976 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4979 =item setpriority WHICH,WHO,PRIORITY
4980 X<setpriority> X<priority> X<nice> X<renice>
4982 Sets the current priority for a process, a process group, or a user.
4983 (See setpriority(2).) Will produce a fatal error if used on a machine
4984 that doesn't implement setpriority(2).
4986 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4989 Sets the socket option requested. Returns undefined if there is an
4990 error. Use integer constants provided by the C<Socket> module for
4991 LEVEL and OPNAME. Values for LEVEL can also be obtained from
4992 getprotobyname. OPTVAL might either be a packed string or an integer.
4993 An integer OPTVAL is shorthand for pack("i", OPTVAL).
4995 An example disabling the Nagle's algorithm for a socket:
4997 use Socket qw(IPPROTO_TCP TCP_NODELAY);
4998 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5005 Shifts the first value of the array off and returns it, shortening the
5006 array by 1 and moving everything down. If there are no elements in the
5007 array, returns the undefined value. If ARRAY is omitted, shifts the
5008 C<@_> array within the lexical scope of subroutines and formats, and the
5009 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5010 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5011 C<UNITCHECK {}> and C<END {}> constructs.
5013 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5014 same thing to the left end of an array that C<pop> and C<push> do to the
5017 =item shmctl ID,CMD,ARG
5020 Calls the System V IPC function shmctl. You'll probably have to say
5024 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5025 then ARG must be a variable that will hold the returned C<shmid_ds>
5026 structure. Returns like ioctl: the undefined value for error, "C<0> but
5027 true" for zero, or the actual return value otherwise.
5028 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5030 =item shmget KEY,SIZE,FLAGS
5033 Calls the System V IPC function shmget. Returns the shared memory
5034 segment id, or the undefined value if there is an error.
5035 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5037 =item shmread ID,VAR,POS,SIZE
5041 =item shmwrite ID,STRING,POS,SIZE
5043 Reads or writes the System V shared memory segment ID starting at
5044 position POS for size SIZE by attaching to it, copying in/out, and
5045 detaching from it. When reading, VAR must be a variable that will
5046 hold the data read. When writing, if STRING is too long, only SIZE
5047 bytes are used; if STRING is too short, nulls are written to fill out
5048 SIZE bytes. Return true if successful, or false if there is an error.
5049 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5050 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5052 =item shutdown SOCKET,HOW
5055 Shuts down a socket connection in the manner indicated by HOW, which
5056 has the same interpretation as in the system call of the same name.
5058 shutdown(SOCKET, 0); # I/we have stopped reading data
5059 shutdown(SOCKET, 1); # I/we have stopped writing data
5060 shutdown(SOCKET, 2); # I/we have stopped using this socket
5062 This is useful with sockets when you want to tell the other
5063 side you're done writing but not done reading, or vice versa.
5064 It's also a more insistent form of close because it also
5065 disables the file descriptor in any forked copies in other
5069 X<sin> X<sine> X<asin> X<arcsine>
5073 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5074 returns sine of C<$_>.
5076 For the inverse sine operation, you may use the C<Math::Trig::asin>
5077 function, or use this relation:
5079 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5086 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5087 May be interrupted if the process receives a signal such as C<SIGALRM>.
5088 Returns the number of seconds actually slept. You probably cannot
5089 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5092 On some older systems, it may sleep up to a full second less than what
5093 you requested, depending on how it counts seconds. Most modern systems
5094 always sleep the full amount. They may appear to sleep longer than that,
5095 however, because your process might not be scheduled right away in a
5096 busy multitasking system.
5098 For delays of finer granularity than one second, you may use Perl's
5099 C<syscall> interface to access setitimer(2) if your system supports
5100 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5101 and starting from Perl 5.8 part of the standard distribution) may also
5104 See also the POSIX module's C<pause> function.
5106 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5109 Opens a socket of the specified kind and attaches it to filehandle
5110 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5111 the system call of the same name. You should C<use Socket> first
5112 to get the proper definitions imported. See the examples in
5113 L<perlipc/"Sockets: Client/Server Communication">.
5115 On systems that support a close-on-exec flag on files, the flag will
5116 be set for the newly opened file descriptor, as determined by the
5117 value of $^F. See L<perlvar/$^F>.
5119 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5122 Creates an unnamed pair of sockets in the specified domain, of the
5123 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5124 for the system call of the same name. If unimplemented, yields a fatal
5125 error. Returns true if successful.
5127 On systems that support a close-on-exec flag on files, the flag will
5128 be set for the newly opened file descriptors, as determined by the value
5129 of $^F. See L<perlvar/$^F>.
5131 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5132 to C<pipe(Rdr, Wtr)> is essentially:
5135 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5136 shutdown(Rdr, 1); # no more writing for reader
5137 shutdown(Wtr, 0); # no more reading for writer
5139 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5140 emulate socketpair using IP sockets to localhost if your system implements
5141 sockets but not socketpair.
5143 =item sort SUBNAME LIST
5144 X<sort> X<qsort> X<quicksort> X<mergesort>
5146 =item sort BLOCK LIST
5150 In list context, this sorts the LIST and returns the sorted list value.
5151 In scalar context, the behaviour of C<sort()> is undefined.
5153 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5154 order. If SUBNAME is specified, it gives the name of a subroutine
5155 that returns an integer less than, equal to, or greater than C<0>,
5156 depending on how the elements of the list are to be ordered. (The C<<
5157 <=> >> and C<cmp> operators are extremely useful in such routines.)
5158 SUBNAME may be a scalar variable name (unsubscripted), in which case
5159 the value provides the name of (or a reference to) the actual
5160 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5161 an anonymous, in-line sort subroutine.
5163 If the subroutine's prototype is C<($$)>, the elements to be compared
5164 are passed by reference in C<@_>, as for a normal subroutine. This is
5165 slower than unprototyped subroutines, where the elements to be
5166 compared are passed into the subroutine
5167 as the package global variables $a and $b (see example below). Note that
5168 in the latter case, it is usually counter-productive to declare $a and
5171 The values to be compared are always passed by reference and should not
5174 You also cannot exit out of the sort block or subroutine using any of the
5175 loop control operators described in L<perlsyn> or with C<goto>.
5177 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5178 current collation locale. See L<perllocale>.
5180 sort() returns aliases into the original list, much as a for loop's index
5181 variable aliases the list elements. That is, modifying an element of a
5182 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5183 actually modifies the element in the original list. This is usually
5184 something to be avoided when writing clear code.
5186 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5187 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5188 preserves the input order of elements that compare equal. Although
5189 quicksort's run time is O(NlogN) when averaged over all arrays of
5190 length N, the time can be O(N**2), I<quadratic> behavior, for some
5191 inputs.) In 5.7, the quicksort implementation was replaced with
5192 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5193 But benchmarks indicated that for some inputs, on some platforms,
5194 the original quicksort was faster. 5.8 has a sort pragma for
5195 limited control of the sort. Its rather blunt control of the
5196 underlying algorithm may not persist into future Perls, but the
5197 ability to characterize the input or output in implementation
5198 independent ways quite probably will. See L<sort>.
5203 @articles = sort @files;
5205 # same thing, but with explicit sort routine
5206 @articles = sort {$a cmp $b} @files;
5208 # now case-insensitively
5209 @articles = sort {uc($a) cmp uc($b)} @files;
5211 # same thing in reversed order
5212 @articles = sort {$b cmp $a} @files;
5214 # sort numerically ascending
5215 @articles = sort {$a <=> $b} @files;
5217 # sort numerically descending
5218 @articles = sort {$b <=> $a} @files;
5220 # this sorts the %age hash by value instead of key
5221 # using an in-line function
5222 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5224 # sort using explicit subroutine name
5226 $age{$a} <=> $age{$b}; # presuming numeric
5228 @sortedclass = sort byage @class;
5230 sub backwards { $b cmp $a }
5231 @harry = qw(dog cat x Cain Abel);
5232 @george = qw(gone chased yz Punished Axed);
5234 # prints AbelCaincatdogx
5235 print sort backwards @harry;
5236 # prints xdogcatCainAbel
5237 print sort @george, 'to', @harry;
5238 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5240 # inefficiently sort by descending numeric compare using
5241 # the first integer after the first = sign, or the
5242 # whole record case-insensitively otherwise
5245 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5250 # same thing, but much more efficiently;
5251 # we'll build auxiliary indices instead
5255 push @nums, /=(\d+)/;
5260 $nums[$b] <=> $nums[$a]
5262 $caps[$a] cmp $caps[$b]
5266 # same thing, but without any temps
5267 @new = map { $_->[0] }
5268 sort { $b->[1] <=> $a->[1]
5271 } map { [$_, /=(\d+)/, uc($_)] } @old;
5273 # using a prototype allows you to use any comparison subroutine
5274 # as a sort subroutine (including other package's subroutines)
5276 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5279 @new = sort other::backwards @old;
5281 # guarantee stability, regardless of algorithm
5283 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5285 # force use of mergesort (not portable outside Perl 5.8)
5286 use sort '_mergesort'; # note discouraging _
5287 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5289 If you're using strict, you I<must not> declare $a
5290 and $b as lexicals. They are package globals. That means
5291 if you're in the C<main> package and type
5293 @articles = sort {$b <=> $a} @files;
5295 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5296 but if you're in the C<FooPack> package, it's the same as typing
5298 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5300 The comparison function is required to behave. If it returns
5301 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5302 sometimes saying the opposite, for example) the results are not
5305 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5306 (not-a-number), and because C<sort> will trigger a fatal error unless the
5307 result of a comparison is defined, when sorting with a comparison function
5308 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5309 The following example takes advantage of the fact that C<NaN != NaN> to
5310 eliminate any C<NaN>s from the input.
5312 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5314 =item splice ARRAY,OFFSET,LENGTH,LIST
5317 =item splice ARRAY,OFFSET,LENGTH
5319 =item splice ARRAY,OFFSET
5323 Removes the elements designated by OFFSET and LENGTH from an array, and
5324 replaces them with the elements of LIST, if any. In list context,
5325 returns the elements removed from the array. In scalar context,
5326 returns the last element removed, or C<undef> if no elements are
5327 removed. The array grows or shrinks as necessary.
5328 If OFFSET is negative then it starts that far from the end of the array.
5329 If LENGTH is omitted, removes everything from OFFSET onward.
5330 If LENGTH is negative, removes the elements from OFFSET onward
5331 except for -LENGTH elements at the end of the array.
5332 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5333 past the end of the array, perl issues a warning, and splices at the
5336 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5338 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5339 pop(@a) splice(@a,-1)
5340 shift(@a) splice(@a,0,1)
5341 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5342 $a[$i] = $y splice(@a,$i,1,$y)
5344 Example, assuming array lengths are passed before arrays:
5346 sub aeq { # compare two list values
5347 my(@a) = splice(@_,0,shift);
5348 my(@b) = splice(@_,0,shift);
5349 return 0 unless @a == @b; # same len?
5351 return 0 if pop(@a) ne pop(@b);
5355 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5357 =item split /PATTERN/,EXPR,LIMIT
5360 =item split /PATTERN/,EXPR
5362 =item split /PATTERN/
5366 Splits the string EXPR into a list of strings and returns that list. By
5367 default, empty leading fields are preserved, and empty trailing ones are
5368 deleted. (If all fields are empty, they are considered to be trailing.)
5370 In scalar context, returns the number of fields found and splits into
5371 the C<@_> array. Use of split in scalar context is deprecated, however,
5372 because it clobbers your subroutine arguments.
5374 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5375 splits on whitespace (after skipping any leading whitespace). Anything
5376 matching PATTERN is taken to be a delimiter separating the fields. (Note
5377 that the delimiter may be longer than one character.)
5379 If LIMIT is specified and positive, it represents the maximum number
5380 of fields the EXPR will be split into, though the actual number of
5381 fields returned depends on the number of times PATTERN matches within
5382 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5383 stripped (which potential users of C<pop> would do well to remember).
5384 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5385 had been specified. Note that splitting an EXPR that evaluates to the
5386 empty string always returns the empty list, regardless of the LIMIT
5389 A pattern matching the null string (not to be confused with
5390 a null pattern C<//>, which is just one member of the set of patterns
5391 matching a null string) will split the value of EXPR into separate
5392 characters at each point it matches that way. For example:
5394 print join(':', split(/ */, 'hi there'));
5396 produces the output 'h:i:t:h:e:r:e'.
5398 As a special case for C<split>, using the empty pattern C<//> specifically
5399 matches only the null string, and is not be confused with the regular use
5400 of C<//> to mean "the last successful pattern match". So, for C<split>,
5403 print join(':', split(//, 'hi there'));
5405 produces the output 'h:i: :t:h:e:r:e'.
5407 Empty leading (or trailing) fields are produced when there are positive
5408 width matches at the beginning (or end) of the string; a zero-width match
5409 at the beginning (or end) of the string does not produce an empty field.
5412 print join(':', split(/(?=\w)/, 'hi there!'));
5414 produces the output 'h:i :t:h:e:r:e!'.
5416 The LIMIT parameter can be used to split a line partially
5418 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5420 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5421 a LIMIT one larger than the number of variables in the list, to avoid
5422 unnecessary work. For the list above LIMIT would have been 4 by
5423 default. In time critical applications it behooves you not to split
5424 into more fields than you really need.
5426 If the PATTERN contains parentheses, additional list elements are
5427 created from each matching substring in the delimiter.
5429 split(/([,-])/, "1-10,20", 3);
5431 produces the list value
5433 (1, '-', 10, ',', 20)
5435 If you had the entire header of a normal Unix email message in $header,
5436 you could split it up into fields and their values this way:
5438 $header =~ s/\n\s+/ /g; # fix continuation lines
5439 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5441 The pattern C</PATTERN/> may be replaced with an expression to specify
5442 patterns that vary at runtime. (To do runtime compilation only once,
5443 use C</$variable/o>.)
5445 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5446 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5447 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5448 will give you as many null initial fields as there are leading spaces.
5449 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5450 whitespace produces a null first field. A C<split> with no arguments
5451 really does a S<C<split(' ', $_)>> internally.
5453 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5458 open(PASSWD, '/etc/passwd');
5461 ($login, $passwd, $uid, $gid,
5462 $gcos, $home, $shell) = split(/:/);
5466 As with regular pattern matching, any capturing parentheses that are not
5467 matched in a C<split()> will be set to C<undef> when returned:
5469 @fields = split /(A)|B/, "1A2B3";
5470 # @fields is (1, 'A', 2, undef, 3)
5472 =item sprintf FORMAT, LIST
5475 Returns a string formatted by the usual C<printf> conventions of the C
5476 library function C<sprintf>. See below for more details
5477 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5478 the general principles.
5482 # Format number with up to 8 leading zeroes
5483 $result = sprintf("%08d", $number);
5485 # Round number to 3 digits after decimal point
5486 $rounded = sprintf("%.3f", $number);
5488 Perl does its own C<sprintf> formatting--it emulates the C
5489 function C<sprintf>, but it doesn't use it (except for floating-point
5490 numbers, and even then only the standard modifiers are allowed). As a
5491 result, any non-standard extensions in your local C<sprintf> are not
5492 available from Perl.
5494 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5495 pass it an array as your first argument. The array is given scalar context,
5496 and instead of using the 0th element of the array as the format, Perl will
5497 use the count of elements in the array as the format, which is almost never
5500 Perl's C<sprintf> permits the following universally-known conversions:
5503 %c a character with the given number
5505 %d a signed integer, in decimal
5506 %u an unsigned integer, in decimal
5507 %o an unsigned integer, in octal
5508 %x an unsigned integer, in hexadecimal
5509 %e a floating-point number, in scientific notation
5510 %f a floating-point number, in fixed decimal notation
5511 %g a floating-point number, in %e or %f notation
5513 In addition, Perl permits the following widely-supported conversions:
5515 %X like %x, but using upper-case letters
5516 %E like %e, but using an upper-case "E"
5517 %G like %g, but with an upper-case "E" (if applicable)
5518 %b an unsigned integer, in binary
5519 %B like %x, but using an upper-case "B" with the # flag
5520 %p a pointer (outputs the Perl value's address in hexadecimal)
5521 %n special: *stores* the number of characters output so far
5522 into the next variable in the parameter list
5524 Finally, for backward (and we do mean "backward") compatibility, Perl
5525 permits these unnecessary but widely-supported conversions:
5528 %D a synonym for %ld
5529 %U a synonym for %lu
5530 %O a synonym for %lo
5533 Note that the number of exponent digits in the scientific notation produced
5534 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5535 exponent less than 100 is system-dependent: it may be three or less
5536 (zero-padded as necessary). In other words, 1.23 times ten to the
5537 99th may be either "1.23e99" or "1.23e099".
5539 Between the C<%> and the format letter, you may specify a number of
5540 additional attributes controlling the interpretation of the format.
5541 In order, these are:
5545 =item format parameter index
5547 An explicit format parameter index, such as C<2$>. By default sprintf
5548 will format the next unused argument in the list, but this allows you
5549 to take the arguments out of order, e.g.:
5551 printf '%2$d %1$d', 12, 34; # prints "34 12"
5552 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5557 space prefix positive number with a space
5558 + prefix positive number with a plus sign
5559 - left-justify within the field
5560 0 use zeros, not spaces, to right-justify
5561 # prefix non-zero octal with "0", non-zero hex with "0x"
5562 or "0X", non-zero binary with "0b" or "OB"
5566 printf '<% d>', 12; # prints "< 12>"
5567 printf '<%+d>', 12; # prints "<+12>"
5568 printf '<%6s>', 12; # prints "< 12>"
5569 printf '<%-6s>', 12; # prints "<12 >"
5570 printf '<%06s>', 12; # prints "<000012>"
5571 printf '<%#x>', 12; # prints "<0xc>"
5573 When a space and a plus sign are given as the flags at once,
5574 a plus sign is used to prefix a positive number.
5576 printf '<%+ d>', 12; # prints "<+12>"
5577 printf '<% +d>', 12; # prints "<+12>"
5581 This flag tells perl to interpret the supplied string as a vector of
5582 integers, one for each character in the string. Perl applies the format to
5583 each integer in turn, then joins the resulting strings with a separator (a
5584 dot C<.> by default). This can be useful for displaying ordinal values of
5585 characters in arbitrary strings:
5587 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5588 printf "version is v%vd\n", $^V; # Perl's version
5590 Put an asterisk C<*> before the C<v> to override the string to
5591 use to separate the numbers:
5593 printf "address is %*vX\n", ":", $addr; # IPv6 address
5594 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5596 You can also explicitly specify the argument number to use for
5597 the join string using e.g. C<*2$v>:
5599 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5601 =item (minimum) width
5603 Arguments are usually formatted to be only as wide as required to
5604 display the given value. You can override the width by putting
5605 a number here, or get the width from the next argument (with C<*>)
5606 or from a specified argument (with e.g. C<*2$>):
5608 printf '<%s>', "a"; # prints "<a>"
5609 printf '<%6s>', "a"; # prints "< a>"
5610 printf '<%*s>', 6, "a"; # prints "< a>"
5611 printf '<%*2$s>', "a", 6; # prints "< a>"
5612 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5614 If a field width obtained through C<*> is negative, it has the same
5615 effect as the C<-> flag: left-justification.
5617 =item precision, or maximum width
5620 You can specify a precision (for numeric conversions) or a maximum
5621 width (for string conversions) by specifying a C<.> followed by a number.
5622 For floating point formats, with the exception of 'g' and 'G', this specifies
5623 the number of decimal places to show (the default being 6), e.g.:
5625 # these examples are subject to system-specific variation
5626 printf '<%f>', 1; # prints "<1.000000>"
5627 printf '<%.1f>', 1; # prints "<1.0>"
5628 printf '<%.0f>', 1; # prints "<1>"
5629 printf '<%e>', 10; # prints "<1.000000e+01>"
5630 printf '<%.1e>', 10; # prints "<1.0e+01>"
5632 For 'g' and 'G', this specifies the maximum number of digits to show,
5633 including prior to the decimal point as well as after it, e.g.:
5635 # these examples are subject to system-specific variation
5636 printf '<%g>', 1; # prints "<1>"
5637 printf '<%.10g>', 1; # prints "<1>"
5638 printf '<%g>', 100; # prints "<100>"
5639 printf '<%.1g>', 100; # prints "<1e+02>"
5640 printf '<%.2g>', 100.01; # prints "<1e+02>"
5641 printf '<%.5g>', 100.01; # prints "<100.01>"
5642 printf '<%.4g>', 100.01; # prints "<100>"
5644 For integer conversions, specifying a precision implies that the
5645 output of the number itself should be zero-padded to this width,
5646 where the 0 flag is ignored:
5648 printf '<%.6d>', 1; # prints "<000001>"
5649 printf '<%+.6d>', 1; # prints "<+000001>"
5650 printf '<%-10.6d>', 1; # prints "<000001 >"
5651 printf '<%10.6d>', 1; # prints "< 000001>"
5652 printf '<%010.6d>', 1; # prints "< 000001>"
5653 printf '<%+10.6d>', 1; # prints "< +000001>"
5655 printf '<%.6x>', 1; # prints "<000001>"
5656 printf '<%#.6x>', 1; # prints "<0x000001>"
5657 printf '<%-10.6x>', 1; # prints "<000001 >"
5658 printf '<%10.6x>', 1; # prints "< 000001>"
5659 printf '<%010.6x>', 1; # prints "< 000001>"
5660 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5662 For string conversions, specifying a precision truncates the string
5663 to fit in the specified width:
5665 printf '<%.5s>', "truncated"; # prints "<trunc>"
5666 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5668 You can also get the precision from the next argument using C<.*>:
5670 printf '<%.6x>', 1; # prints "<000001>"
5671 printf '<%.*x>', 6, 1; # prints "<000001>"
5673 If a precision obtained through C<*> is negative, it has the same
5674 effect as no precision.
5676 printf '<%.*s>', 7, "string"; # prints "<string>"
5677 printf '<%.*s>', 3, "string"; # prints "<str>"
5678 printf '<%.*s>', 0, "string"; # prints "<>"
5679 printf '<%.*s>', -1, "string"; # prints "<string>"
5681 printf '<%.*d>', 1, 0; # prints "<0>"
5682 printf '<%.*d>', 0, 0; # prints "<>"
5683 printf '<%.*d>', -1, 0; # prints "<0>"
5685 You cannot currently get the precision from a specified number,
5686 but it is intended that this will be possible in the future using
5689 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5693 For numeric conversions, you can specify the size to interpret the
5694 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5695 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5696 whatever the default integer size is on your platform (usually 32 or 64
5697 bits), but you can override this to use instead one of the standard C types,
5698 as supported by the compiler used to build Perl:
5700 l interpret integer as C type "long" or "unsigned long"
5701 h interpret integer as C type "short" or "unsigned short"
5702 q, L or ll interpret integer as C type "long long", "unsigned long long".
5703 or "quads" (typically 64-bit integers)
5705 The last will produce errors if Perl does not understand "quads" in your
5706 installation. (This requires that either the platform natively supports quads
5707 or Perl was specifically compiled to support quads.) You can find out
5708 whether your Perl supports quads via L<Config>:
5711 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5714 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5715 to be the default floating point size on your platform (double or long double),
5716 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5717 platform supports them. You can find out whether your Perl supports long
5718 doubles via L<Config>:
5721 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5723 You can find out whether Perl considers 'long double' to be the default
5724 floating point size to use on your platform via L<Config>:
5727 ($Config{uselongdouble} eq 'define') &&
5728 print "long doubles by default\n";
5730 It can also be the case that long doubles and doubles are the same thing:
5733 ($Config{doublesize} == $Config{longdblsize}) &&
5734 print "doubles are long doubles\n";
5736 The size specifier C<V> has no effect for Perl code, but it is supported
5737 for compatibility with XS code; it means 'use the standard size for
5738 a Perl integer (or floating-point number)', which is already the
5739 default for Perl code.
5741 =item order of arguments
5743 Normally, sprintf takes the next unused argument as the value to
5744 format for each format specification. If the format specification
5745 uses C<*> to require additional arguments, these are consumed from
5746 the argument list in the order in which they appear in the format
5747 specification I<before> the value to format. Where an argument is
5748 specified using an explicit index, this does not affect the normal
5749 order for the arguments (even when the explicitly specified index
5750 would have been the next argument in any case).
5754 printf '<%*.*s>', $a, $b, $c;
5756 would use C<$a> for the width, C<$b> for the precision and C<$c>
5757 as the value to format, while:
5759 print '<%*1$.*s>', $a, $b;
5761 would use C<$a> for the width and the precision, and C<$b> as the
5764 Here are some more examples - beware that when using an explicit
5765 index, the C<$> may need to be escaped:
5767 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5768 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5769 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5770 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5774 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5775 the character used for the decimal separator in formatted floating
5776 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5780 X<sqrt> X<root> X<square root>
5784 Return the square root of EXPR. If EXPR is omitted, returns square
5785 root of C<$_>. Only works on non-negative operands, unless you've
5786 loaded the standard Math::Complex module.
5789 print sqrt(-2); # prints 1.4142135623731i
5792 X<srand> X<seed> X<randseed>
5796 Sets the random number seed for the C<rand> operator.
5798 The point of the function is to "seed" the C<rand> function so that
5799 C<rand> can produce a different sequence each time you run your
5802 If srand() is not called explicitly, it is called implicitly at the
5803 first use of the C<rand> operator. However, this was not the case in
5804 versions of Perl before 5.004, so if your script will run under older
5805 Perl versions, it should call C<srand>.
5807 Most programs won't even call srand() at all, except those that
5808 need a cryptographically-strong starting point rather than the
5809 generally acceptable default, which is based on time of day,
5810 process ID, and memory allocation, or the F</dev/urandom> device,
5813 You can call srand($seed) with the same $seed to reproduce the
5814 I<same> sequence from rand(), but this is usually reserved for
5815 generating predictable results for testing or debugging.
5816 Otherwise, don't call srand() more than once in your program.
5818 Do B<not> call srand() (i.e. without an argument) more than once in
5819 a script. The internal state of the random number generator should
5820 contain more entropy than can be provided by any seed, so calling
5821 srand() again actually I<loses> randomness.
5823 Most implementations of C<srand> take an integer and will silently
5824 truncate decimal numbers. This means C<srand(42)> will usually
5825 produce the same results as C<srand(42.1)>. To be safe, always pass
5826 C<srand> an integer.
5828 In versions of Perl prior to 5.004 the default seed was just the
5829 current C<time>. This isn't a particularly good seed, so many old
5830 programs supply their own seed value (often C<time ^ $$> or C<time ^
5831 ($$ + ($$ << 15))>), but that isn't necessary any more.
5833 For cryptographic purposes, however, you need something much more random
5834 than the default seed. Checksumming the compressed output of one or more
5835 rapidly changing operating system status programs is the usual method. For
5838 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5840 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5843 Frequently called programs (like CGI scripts) that simply use
5847 for a seed can fall prey to the mathematical property that
5851 one-third of the time. So don't do that.
5853 =item stat FILEHANDLE
5854 X<stat> X<file, status> X<ctime>
5858 =item stat DIRHANDLE
5862 Returns a 13-element list giving the status info for a file, either
5863 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5864 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5867 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5868 $atime,$mtime,$ctime,$blksize,$blocks)
5871 Not all fields are supported on all filesystem types. Here are the
5872 meanings of the fields:
5874 0 dev device number of filesystem
5876 2 mode file mode (type and permissions)
5877 3 nlink number of (hard) links to the file
5878 4 uid numeric user ID of file's owner
5879 5 gid numeric group ID of file's owner
5880 6 rdev the device identifier (special files only)
5881 7 size total size of file, in bytes
5882 8 atime last access time in seconds since the epoch
5883 9 mtime last modify time in seconds since the epoch
5884 10 ctime inode change time in seconds since the epoch (*)
5885 11 blksize preferred block size for file system I/O
5886 12 blocks actual number of blocks allocated
5888 (The epoch was at 00:00 January 1, 1970 GMT.)
5890 (*) Not all fields are supported on all filesystem types. Notably, the
5891 ctime field is non-portable. In particular, you cannot expect it to be a
5892 "creation time", see L<perlport/"Files and Filesystems"> for details.
5894 If C<stat> is passed the special filehandle consisting of an underline, no
5895 stat is done, but the current contents of the stat structure from the
5896 last C<stat>, C<lstat>, or filetest are returned. Example:
5898 if (-x $file && (($d) = stat(_)) && $d < 0) {
5899 print "$file is executable NFS file\n";
5902 (This works on machines only for which the device number is negative
5905 Because the mode contains both the file type and its permissions, you
5906 should mask off the file type portion and (s)printf using a C<"%o">
5907 if you want to see the real permissions.
5909 $mode = (stat($filename))[2];
5910 printf "Permissions are %04o\n", $mode & 07777;
5912 In scalar context, C<stat> returns a boolean value indicating success
5913 or failure, and, if successful, sets the information associated with
5914 the special filehandle C<_>.
5916 The L<File::stat> module provides a convenient, by-name access mechanism:
5919 $sb = stat($filename);
5920 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5921 $filename, $sb->size, $sb->mode & 07777,
5922 scalar localtime $sb->mtime;
5924 You can import symbolic mode constants (C<S_IF*>) and functions
5925 (C<S_IS*>) from the Fcntl module:
5929 $mode = (stat($filename))[2];
5931 $user_rwx = ($mode & S_IRWXU) >> 6;
5932 $group_read = ($mode & S_IRGRP) >> 3;
5933 $other_execute = $mode & S_IXOTH;
5935 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5937 $is_setuid = $mode & S_ISUID;
5938 $is_directory = S_ISDIR($mode);
5940 You could write the last two using the C<-u> and C<-d> operators.
5941 The commonly available C<S_IF*> constants are
5943 # Permissions: read, write, execute, for user, group, others.
5945 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5946 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5947 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5949 # Setuid/Setgid/Stickiness/SaveText.
5950 # Note that the exact meaning of these is system dependent.
5952 S_ISUID S_ISGID S_ISVTX S_ISTXT
5954 # File types. Not necessarily all are available on your system.
5956 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5958 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5960 S_IREAD S_IWRITE S_IEXEC
5962 and the C<S_IF*> functions are
5964 S_IMODE($mode) the part of $mode containing the permission bits
5965 and the setuid/setgid/sticky bits
5967 S_IFMT($mode) the part of $mode containing the file type
5968 which can be bit-anded with e.g. S_IFREG
5969 or with the following functions
5971 # The operators -f, -d, -l, -b, -c, -p, and -S.
5973 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5974 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5976 # No direct -X operator counterpart, but for the first one
5977 # the -g operator is often equivalent. The ENFMT stands for
5978 # record flocking enforcement, a platform-dependent feature.
5980 S_ISENFMT($mode) S_ISWHT($mode)
5982 See your native chmod(2) and stat(2) documentation for more details
5983 about the C<S_*> constants. To get status info for a symbolic link
5984 instead of the target file behind the link, use the C<lstat> function.
5989 =item state TYPE EXPR
5991 =item state EXPR : ATTRS
5993 =item state TYPE EXPR : ATTRS
5995 C<state> declares a lexically scoped variable, just like C<my> does.
5996 However, those variables will be initialized only once, contrary to
5997 lexical variables that are reinitialized each time their enclosing block
6000 C<state> variables are only enabled when the C<feature 'state'> pragma is
6001 in effect. See L<feature>.
6008 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6009 doing many pattern matches on the string before it is next modified.
6010 This may or may not save time, depending on the nature and number of
6011 patterns you are searching on, and on the distribution of character
6012 frequencies in the string to be searched--you probably want to compare
6013 run times with and without it to see which runs faster. Those loops
6014 that scan for many short constant strings (including the constant
6015 parts of more complex patterns) will benefit most. You may have only
6016 one C<study> active at a time--if you study a different scalar the first
6017 is "unstudied". (The way C<study> works is this: a linked list of every
6018 character in the string to be searched is made, so we know, for
6019 example, where all the C<'k'> characters are. From each search string,
6020 the rarest character is selected, based on some static frequency tables
6021 constructed from some C programs and English text. Only those places
6022 that contain this "rarest" character are examined.)
6024 For example, here is a loop that inserts index producing entries
6025 before any line containing a certain pattern:
6029 print ".IX foo\n" if /\bfoo\b/;
6030 print ".IX bar\n" if /\bbar\b/;
6031 print ".IX blurfl\n" if /\bblurfl\b/;
6036 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6037 will be looked at, because C<f> is rarer than C<o>. In general, this is
6038 a big win except in pathological cases. The only question is whether
6039 it saves you more time than it took to build the linked list in the
6042 Note that if you have to look for strings that you don't know till
6043 runtime, you can build an entire loop as a string and C<eval> that to
6044 avoid recompiling all your patterns all the time. Together with
6045 undefining C<$/> to input entire files as one record, this can be very
6046 fast, often faster than specialized programs like fgrep(1). The following
6047 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6048 out the names of those files that contain a match:
6050 $search = 'while (<>) { study;';
6051 foreach $word (@words) {
6052 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6057 eval $search; # this screams
6058 $/ = "\n"; # put back to normal input delimiter
6059 foreach $file (sort keys(%seen)) {
6063 =item sub NAME BLOCK
6066 =item sub NAME (PROTO) BLOCK
6068 =item sub NAME : ATTRS BLOCK
6070 =item sub NAME (PROTO) : ATTRS BLOCK
6072 This is subroutine definition, not a real function I<per se>.
6073 Without a BLOCK it's just a forward declaration. Without a NAME,
6074 it's an anonymous function declaration, and does actually return
6075 a value: the CODE ref of the closure you just created.
6077 See L<perlsub> and L<perlref> for details about subroutines and
6078 references, and L<attributes> and L<Attribute::Handlers> for more
6079 information about attributes.
6081 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6082 X<substr> X<substring> X<mid> X<left> X<right>
6084 =item substr EXPR,OFFSET,LENGTH
6086 =item substr EXPR,OFFSET
6088 Extracts a substring out of EXPR and returns it. First character is at
6089 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6090 If OFFSET is negative (or more precisely, less than C<$[>), starts
6091 that far from the end of the string. If LENGTH is omitted, returns
6092 everything to the end of the string. If LENGTH is negative, leaves that
6093 many characters off the end of the string.
6095 my $s = "The black cat climbed the green tree";
6096 my $color = substr $s, 4, 5; # black
6097 my $middle = substr $s, 4, -11; # black cat climbed the
6098 my $end = substr $s, 14; # climbed the green tree
6099 my $tail = substr $s, -4; # tree
6100 my $z = substr $s, -4, 2; # tr
6102 You can use the substr() function as an lvalue, in which case EXPR
6103 must itself be an lvalue. If you assign something shorter than LENGTH,
6104 the string will shrink, and if you assign something longer than LENGTH,
6105 the string will grow to accommodate it. To keep the string the same
6106 length you may need to pad or chop your value using C<sprintf>.
6108 If OFFSET and LENGTH specify a substring that is partly outside the
6109 string, only the part within the string is returned. If the substring
6110 is beyond either end of the string, substr() returns the undefined
6111 value and produces a warning. When used as an lvalue, specifying a
6112 substring that is entirely outside the string is a fatal error.
6113 Here's an example showing the behavior for boundary cases:
6116 substr($name, 4) = 'dy'; # $name is now 'freddy'
6117 my $null = substr $name, 6, 2; # returns '' (no warning)
6118 my $oops = substr $name, 7; # returns undef, with warning
6119 substr($name, 7) = 'gap'; # fatal error
6121 An alternative to using substr() as an lvalue is to specify the
6122 replacement string as the 4th argument. This allows you to replace
6123 parts of the EXPR and return what was there before in one operation,
6124 just as you can with splice().
6126 my $s = "The black cat climbed the green tree";
6127 my $z = substr $s, 14, 7, "jumped from"; # climbed
6128 # $s is now "The black cat jumped from the green tree"
6130 Note that the lvalue returned by the 3-arg version of substr() acts as
6131 a 'magic bullet'; each time it is assigned to, it remembers which part
6132 of the original string is being modified; for example:
6135 for (substr($x,1,2)) {
6136 $_ = 'a'; print $x,"\n"; # prints 1a4
6137 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6139 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6142 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6145 =item symlink OLDFILE,NEWFILE
6146 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6148 Creates a new filename symbolically linked to the old filename.
6149 Returns C<1> for success, C<0> otherwise. On systems that don't support
6150 symbolic links, produces a fatal error at run time. To check for that,
6153 $symlink_exists = eval { symlink("",""); 1 };
6155 =item syscall NUMBER, LIST
6156 X<syscall> X<system call>
6158 Calls the system call specified as the first element of the list,
6159 passing the remaining elements as arguments to the system call. If
6160 unimplemented, produces a fatal error. The arguments are interpreted
6161 as follows: if a given argument is numeric, the argument is passed as
6162 an int. If not, the pointer to the string value is passed. You are
6163 responsible to make sure a string is pre-extended long enough to
6164 receive any result that might be written into a string. You can't use a
6165 string literal (or other read-only string) as an argument to C<syscall>
6166 because Perl has to assume that any string pointer might be written
6168 integer arguments are not literals and have never been interpreted in a
6169 numeric context, you may need to add C<0> to them to force them to look
6170 like numbers. This emulates the C<syswrite> function (or vice versa):
6172 require 'syscall.ph'; # may need to run h2ph
6174 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6176 Note that Perl supports passing of up to only 14 arguments to your system call,
6177 which in practice should usually suffice.
6179 Syscall returns whatever value returned by the system call it calls.
6180 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6181 Note that some system calls can legitimately return C<-1>. The proper
6182 way to handle such calls is to assign C<$!=0;> before the call and
6183 check the value of C<$!> if syscall returns C<-1>.
6185 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6186 number of the read end of the pipe it creates. There is no way
6187 to retrieve the file number of the other end. You can avoid this
6188 problem by using C<pipe> instead.
6190 =item sysopen FILEHANDLE,FILENAME,MODE
6193 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6195 Opens the file whose filename is given by FILENAME, and associates it
6196 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6197 the name of the real filehandle wanted. This function calls the
6198 underlying operating system's C<open> function with the parameters
6199 FILENAME, MODE, PERMS.
6201 The possible values and flag bits of the MODE parameter are
6202 system-dependent; they are available via the standard module C<Fcntl>.
6203 See the documentation of your operating system's C<open> to see which
6204 values and flag bits are available. You may combine several flags
6205 using the C<|>-operator.
6207 Some of the most common values are C<O_RDONLY> for opening the file in
6208 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6209 and C<O_RDWR> for opening the file in read-write mode.
6210 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6212 For historical reasons, some values work on almost every system
6213 supported by perl: zero means read-only, one means write-only, and two
6214 means read/write. We know that these values do I<not> work under
6215 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6216 use them in new code.
6218 If the file named by FILENAME does not exist and the C<open> call creates
6219 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6220 PERMS specifies the permissions of the newly created file. If you omit
6221 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6222 These permission values need to be in octal, and are modified by your
6223 process's current C<umask>.
6226 In many systems the C<O_EXCL> flag is available for opening files in
6227 exclusive mode. This is B<not> locking: exclusiveness means here that
6228 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6229 on network filesystems, and has no effect unless the C<O_CREAT> flag
6230 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6231 being opened if it is a symbolic link. It does not protect against
6232 symbolic links in the file's path.
6235 Sometimes you may want to truncate an already-existing file. This
6236 can be done using the C<O_TRUNC> flag. The behavior of
6237 C<O_TRUNC> with C<O_RDONLY> is undefined.
6240 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6241 that takes away the user's option to have a more permissive umask.
6242 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6245 Note that C<sysopen> depends on the fdopen() C library function.
6246 On many UNIX systems, fdopen() is known to fail when file descriptors
6247 exceed a certain value, typically 255. If you need more file
6248 descriptors than that, consider rebuilding Perl to use the C<sfio>
6249 library, or perhaps using the POSIX::open() function.
6251 See L<perlopentut> for a kinder, gentler explanation of opening files.
6253 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6256 =item sysread FILEHANDLE,SCALAR,LENGTH
6258 Attempts to read LENGTH bytes of data into variable SCALAR from the
6259 specified FILEHANDLE, using the system call read(2). It bypasses
6260 buffered IO, so mixing this with other kinds of reads, C<print>,
6261 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6262 perlio or stdio layers usually buffers data. Returns the number of
6263 bytes actually read, C<0> at end of file, or undef if there was an
6264 error (in the latter case C<$!> is also set). SCALAR will be grown or
6265 shrunk so that the last byte actually read is the last byte of the
6266 scalar after the read.
6268 An OFFSET may be specified to place the read data at some place in the
6269 string other than the beginning. A negative OFFSET specifies
6270 placement at that many characters counting backwards from the end of
6271 the string. A positive OFFSET greater than the length of SCALAR
6272 results in the string being padded to the required size with C<"\0">
6273 bytes before the result of the read is appended.
6275 There is no syseof() function, which is ok, since eof() doesn't work
6276 very well on device files (like ttys) anyway. Use sysread() and check
6277 for a return value for 0 to decide whether you're done.
6279 Note that if the filehandle has been marked as C<:utf8> Unicode
6280 characters are read instead of bytes (the LENGTH, OFFSET, and the
6281 return value of sysread() are in Unicode characters).
6282 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6283 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6285 =item sysseek FILEHANDLE,POSITION,WHENCE
6288 Sets FILEHANDLE's system position in bytes using the system call
6289 lseek(2). FILEHANDLE may be an expression whose value gives the name
6290 of the filehandle. The values for WHENCE are C<0> to set the new
6291 position to POSITION, C<1> to set the it to the current position plus
6292 POSITION, and C<2> to set it to EOF plus POSITION (typically
6295 Note the I<in bytes>: even if the filehandle has been set to operate
6296 on characters (for example by using the C<:utf8> I/O layer), tell()
6297 will return byte offsets, not character offsets (because implementing
6298 that would render sysseek() very slow).
6300 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6301 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6302 C<seek>, C<tell>, or C<eof> may cause confusion.
6304 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6305 and C<SEEK_END> (start of the file, current position, end of the file)
6306 from the Fcntl module. Use of the constants is also more portable
6307 than relying on 0, 1, and 2. For example to define a "systell" function:
6309 use Fcntl 'SEEK_CUR';
6310 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6312 Returns the new position, or the undefined value on failure. A position
6313 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6314 true on success and false on failure, yet you can still easily determine
6320 =item system PROGRAM LIST
6322 Does exactly the same thing as C<exec LIST>, except that a fork is
6323 done first, and the parent process waits for the child process to
6324 complete. Note that argument processing varies depending on the
6325 number of arguments. If there is more than one argument in LIST,
6326 or if LIST is an array with more than one value, starts the program
6327 given by the first element of the list with arguments given by the
6328 rest of the list. If there is only one scalar argument, the argument
6329 is checked for shell metacharacters, and if there are any, the
6330 entire argument is passed to the system's command shell for parsing
6331 (this is C</bin/sh -c> on Unix platforms, but varies on other
6332 platforms). If there are no shell metacharacters in the argument,
6333 it is split into words and passed directly to C<execvp>, which is
6336 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6337 output before any operation that may do a fork, but this may not be
6338 supported on some platforms (see L<perlport>). To be safe, you may need
6339 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6340 of C<IO::Handle> on any open handles.
6342 The return value is the exit status of the program as returned by the
6343 C<wait> call. To get the actual exit value, shift right by eight (see
6344 below). See also L</exec>. This is I<not> what you want to use to capture
6345 the output from a command, for that you should use merely backticks or
6346 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6347 indicates a failure to start the program or an error of the wait(2) system
6348 call (inspect $! for the reason).
6350 Like C<exec>, C<system> allows you to lie to a program about its name if
6351 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6353 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6354 C<system>, if you expect your program to terminate on receipt of these
6355 signals you will need to arrange to do so yourself based on the return
6358 @args = ("command", "arg1", "arg2");
6360 or die "system @args failed: $?"
6362 You can check all the failure possibilities by inspecting
6366 print "failed to execute: $!\n";
6369 printf "child died with signal %d, %s coredump\n",
6370 ($? & 127), ($? & 128) ? 'with' : 'without';
6373 printf "child exited with value %d\n", $? >> 8;
6376 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6377 with the W*() calls of the POSIX extension.
6379 When the arguments get executed via the system shell, results
6380 and return codes will be subject to its quirks and capabilities.
6381 See L<perlop/"`STRING`"> and L</exec> for details.
6383 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6386 =item syswrite FILEHANDLE,SCALAR,LENGTH
6388 =item syswrite FILEHANDLE,SCALAR
6390 Attempts to write LENGTH bytes of data from variable SCALAR to the
6391 specified FILEHANDLE, using the system call write(2). If LENGTH is
6392 not specified, writes whole SCALAR. It bypasses buffered IO, so
6393 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6394 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6395 stdio layers usually buffers data. Returns the number of bytes
6396 actually written, or C<undef> if there was an error (in this case the
6397 errno variable C<$!> is also set). If the LENGTH is greater than the
6398 available data in the SCALAR after the OFFSET, only as much data as is
6399 available will be written.
6401 An OFFSET may be specified to write the data from some part of the
6402 string other than the beginning. A negative OFFSET specifies writing
6403 that many characters counting backwards from the end of the string.
6404 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6406 Note that if the filehandle has been marked as C<:utf8>, Unicode
6407 characters are written instead of bytes (the LENGTH, OFFSET, and the
6408 return value of syswrite() are in UTF-8 encoded Unicode characters).
6409 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6410 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6412 =item tell FILEHANDLE
6417 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6418 error. FILEHANDLE may be an expression whose value gives the name of
6419 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6422 Note the I<in bytes>: even if the filehandle has been set to
6423 operate on characters (for example by using the C<:utf8> open
6424 layer), tell() will return byte offsets, not character offsets
6425 (because that would render seek() and tell() rather slow).
6427 The return value of tell() for the standard streams like the STDIN
6428 depends on the operating system: it may return -1 or something else.
6429 tell() on pipes, fifos, and sockets usually returns -1.
6431 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6433 Do not use tell() (or other buffered I/O operations) on a file handle
6434 that has been manipulated by sysread(), syswrite() or sysseek().
6435 Those functions ignore the buffering, while tell() does not.
6437 =item telldir DIRHANDLE
6440 Returns the current position of the C<readdir> routines on DIRHANDLE.
6441 Value may be given to C<seekdir> to access a particular location in a
6442 directory. C<telldir> has the same caveats about possible directory
6443 compaction as the corresponding system library routine.
6445 =item tie VARIABLE,CLASSNAME,LIST
6448 This function binds a variable to a package class that will provide the
6449 implementation for the variable. VARIABLE is the name of the variable
6450 to be enchanted. CLASSNAME is the name of a class implementing objects
6451 of correct type. Any additional arguments are passed to the C<new>
6452 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6453 or C<TIEHASH>). Typically these are arguments such as might be passed
6454 to the C<dbm_open()> function of C. The object returned by the C<new>
6455 method is also returned by the C<tie> function, which would be useful
6456 if you want to access other methods in CLASSNAME.
6458 Note that functions such as C<keys> and C<values> may return huge lists
6459 when used on large objects, like DBM files. You may prefer to use the
6460 C<each> function to iterate over such. Example:
6462 # print out history file offsets
6464 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6465 while (($key,$val) = each %HIST) {
6466 print $key, ' = ', unpack('L',$val), "\n";
6470 A class implementing a hash should have the following methods:
6472 TIEHASH classname, LIST
6474 STORE this, key, value
6479 NEXTKEY this, lastkey
6484 A class implementing an ordinary array should have the following methods:
6486 TIEARRAY classname, LIST
6488 STORE this, key, value
6490 STORESIZE this, count
6496 SPLICE this, offset, length, LIST
6501 A class implementing a file handle should have the following methods:
6503 TIEHANDLE classname, LIST
6504 READ this, scalar, length, offset
6507 WRITE this, scalar, length, offset
6509 PRINTF this, format, LIST
6513 SEEK this, position, whence
6515 OPEN this, mode, LIST
6520 A class implementing a scalar should have the following methods:
6522 TIESCALAR classname, LIST
6528 Not all methods indicated above need be implemented. See L<perltie>,
6529 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6531 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6532 for you--you need to do that explicitly yourself. See L<DB_File>
6533 or the F<Config> module for interesting C<tie> implementations.
6535 For further details see L<perltie>, L<"tied VARIABLE">.
6540 Returns a reference to the object underlying VARIABLE (the same value
6541 that was originally returned by the C<tie> call that bound the variable
6542 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6548 Returns the number of non-leap seconds since whatever time the system
6549 considers to be the epoch, suitable for feeding to C<gmtime> and
6550 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6551 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6552 1904 in the current local time zone for its epoch.
6554 For measuring time in better granularity than one second,
6555 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6556 Perl 5.8 part of the standard distribution), or if you have
6557 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6558 See L<perlfaq8> for details.
6560 For date and time processing look at the many related modules on CPAN.
6561 For a comprehensive date and time representation look at the
6567 Returns a four-element list giving the user and system times, in
6568 seconds, for this process and the children of this process.
6570 ($user,$system,$cuser,$csystem) = times;
6572 In scalar context, C<times> returns C<$user>.
6574 Note that times for children are included only after they terminate.
6578 The transliteration operator. Same as C<y///>. See L<perlop>.
6580 =item truncate FILEHANDLE,LENGTH
6583 =item truncate EXPR,LENGTH
6585 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6586 specified length. Produces a fatal error if truncate isn't implemented
6587 on your system. Returns true if successful, the undefined value
6590 The behavior is undefined if LENGTH is greater than the length of the
6593 The position in the file of FILEHANDLE is left unchanged. You may want to
6594 call L<seek> before writing to the file.
6597 X<uc> X<uppercase> X<toupper>
6601 Returns an uppercased version of EXPR. This is the internal function
6602 implementing the C<\U> escape in double-quoted strings. Respects
6603 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6604 and L<perlunicode> for more details about locale and Unicode support.
6605 It does not attempt to do titlecase mapping on initial letters. See
6606 C<ucfirst> for that.
6608 If EXPR is omitted, uses C<$_>.
6611 X<ucfirst> X<uppercase>
6615 Returns the value of EXPR with the first character in uppercase
6616 (titlecase in Unicode). This is the internal function implementing
6617 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6618 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6619 for more details about locale and Unicode support.
6621 If EXPR is omitted, uses C<$_>.
6628 Sets the umask for the process to EXPR and returns the previous value.
6629 If EXPR is omitted, merely returns the current umask.
6631 The Unix permission C<rwxr-x---> is represented as three sets of three
6632 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6633 and isn't one of the digits). The C<umask> value is such a number
6634 representing disabled permissions bits. The permission (or "mode")
6635 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6636 even if you tell C<sysopen> to create a file with permissions C<0777>,
6637 if your umask is C<0022> then the file will actually be created with
6638 permissions C<0755>. If your C<umask> were C<0027> (group can't
6639 write; others can't read, write, or execute), then passing
6640 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6643 Here's some advice: supply a creation mode of C<0666> for regular
6644 files (in C<sysopen>) and one of C<0777> for directories (in
6645 C<mkdir>) and executable files. This gives users the freedom of
6646 choice: if they want protected files, they might choose process umasks
6647 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6648 Programs should rarely if ever make policy decisions better left to
6649 the user. The exception to this is when writing files that should be
6650 kept private: mail files, web browser cookies, I<.rhosts> files, and
6653 If umask(2) is not implemented on your system and you are trying to
6654 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6655 fatal error at run time. If umask(2) is not implemented and you are
6656 not trying to restrict access for yourself, returns C<undef>.
6658 Remember that a umask is a number, usually given in octal; it is I<not> a
6659 string of octal digits. See also L</oct>, if all you have is a string.
6662 X<undef> X<undefine>
6666 Undefines the value of EXPR, which must be an lvalue. Use only on a
6667 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6668 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6669 will probably not do what you expect on most predefined variables or
6670 DBM list values, so don't do that; see L<delete>.) Always returns the
6671 undefined value. You can omit the EXPR, in which case nothing is
6672 undefined, but you still get an undefined value that you could, for
6673 instance, return from a subroutine, assign to a variable or pass as a
6674 parameter. Examples:
6677 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6681 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6682 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6683 select undef, undef, undef, 0.25;
6684 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6686 Note that this is a unary operator, not a list operator.
6689 X<unlink> X<delete> X<remove> X<rm> X<del>
6693 Deletes a list of files. Returns the number of files successfully
6696 $cnt = unlink 'a', 'b', 'c';
6700 Note: C<unlink> will not attempt to delete directories unless you are superuser
6701 and the B<-U> flag is supplied to Perl. Even if these conditions are
6702 met, be warned that unlinking a directory can inflict damage on your
6703 filesystem. Finally, using C<unlink> on directories is not supported on
6704 many operating systems. Use C<rmdir> instead.
6706 If LIST is omitted, uses C<$_>.
6708 =item unpack TEMPLATE,EXPR
6711 =item unpack TEMPLATE
6713 C<unpack> does the reverse of C<pack>: it takes a string
6714 and expands it out into a list of values.
6715 (In scalar context, it returns merely the first value produced.)
6717 If EXPR is omitted, unpacks the C<$_> string.
6719 The string is broken into chunks described by the TEMPLATE. Each chunk
6720 is converted separately to a value. Typically, either the string is a result
6721 of C<pack>, or the characters of the string represent a C structure of some
6724 The TEMPLATE has the same format as in the C<pack> function.
6725 Here's a subroutine that does substring:
6728 my($what,$where,$howmuch) = @_;
6729 unpack("x$where a$howmuch", $what);
6734 sub ordinal { unpack("W",$_[0]); } # same as ord()
6736 In addition to fields allowed in pack(), you may prefix a field with
6737 a %<number> to indicate that
6738 you want a <number>-bit checksum of the items instead of the items
6739 themselves. Default is a 16-bit checksum. Checksum is calculated by
6740 summing numeric values of expanded values (for string fields the sum of
6741 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6743 For example, the following
6744 computes the same number as the System V sum program:
6748 unpack("%32W*",<>) % 65535;
6751 The following efficiently counts the number of set bits in a bit vector:
6753 $setbits = unpack("%32b*", $selectmask);
6755 The C<p> and C<P> formats should be used with care. Since Perl
6756 has no way of checking whether the value passed to C<unpack()>
6757 corresponds to a valid memory location, passing a pointer value that's
6758 not known to be valid is likely to have disastrous consequences.
6760 If there are more pack codes or if the repeat count of a field or a group
6761 is larger than what the remainder of the input string allows, the result
6762 is not well defined: in some cases, the repeat count is decreased, or
6763 C<unpack()> will produce null strings or zeroes, or terminate with an
6764 error. If the input string is longer than one described by the TEMPLATE,
6765 the rest is ignored.
6767 See L</pack> for more examples and notes.
6769 =item untie VARIABLE
6772 Breaks the binding between a variable and a package. (See C<tie>.)
6773 Has no effect if the variable is not tied.
6775 =item unshift ARRAY,LIST
6778 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6779 depending on how you look at it. Prepends list to the front of the
6780 array, and returns the new number of elements in the array.
6782 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6784 Note the LIST is prepended whole, not one element at a time, so the
6785 prepended elements stay in the same order. Use C<reverse> to do the
6788 =item use Module VERSION LIST
6789 X<use> X<module> X<import>
6791 =item use Module VERSION
6793 =item use Module LIST
6799 Imports some semantics into the current package from the named module,
6800 generally by aliasing certain subroutine or variable names into your
6801 package. It is exactly equivalent to
6803 BEGIN { require Module; import Module LIST; }
6805 except that Module I<must> be a bareword.
6807 VERSION may be either a numeric argument such as 5.006, which will be
6808 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6809 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6810 greater than the version of the current Perl interpreter; Perl will not
6811 attempt to parse the rest of the file. Compare with L</require>, which can
6812 do a similar check at run time.
6814 Specifying VERSION as a literal of the form v5.6.1 should generally be
6815 avoided, because it leads to misleading error messages under earlier
6816 versions of Perl that do not support this syntax. The equivalent numeric
6817 version should be used instead.
6819 use v5.6.1; # compile time version check
6821 use 5.006_001; # ditto; preferred for backwards compatibility
6823 This is often useful if you need to check the current Perl version before
6824 C<use>ing library modules that have changed in incompatible ways from
6825 older versions of Perl. (We try not to do this more than we have to.)
6827 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6828 C<require> makes sure the module is loaded into memory if it hasn't been
6829 yet. The C<import> is not a builtin--it's just an ordinary static method
6830 call into the C<Module> package to tell the module to import the list of
6831 features back into the current package. The module can implement its
6832 C<import> method any way it likes, though most modules just choose to
6833 derive their C<import> method via inheritance from the C<Exporter> class that
6834 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6835 method can be found then the call is skipped, even if there is an AUTOLOAD
6838 If you do not want to call the package's C<import> method (for instance,
6839 to stop your namespace from being altered), explicitly supply the empty list:
6843 That is exactly equivalent to
6845 BEGIN { require Module }
6847 If the VERSION argument is present between Module and LIST, then the
6848 C<use> will call the VERSION method in class Module with the given
6849 version as an argument. The default VERSION method, inherited from
6850 the UNIVERSAL class, croaks if the given version is larger than the
6851 value of the variable C<$Module::VERSION>.
6853 Again, there is a distinction between omitting LIST (C<import> called
6854 with no arguments) and an explicit empty LIST C<()> (C<import> not
6855 called). Note that there is no comma after VERSION!
6857 Because this is a wide-open interface, pragmas (compiler directives)
6858 are also implemented this way. Currently implemented pragmas are:
6863 use sigtrap qw(SEGV BUS);
6864 use strict qw(subs vars refs);
6865 use subs qw(afunc blurfl);
6866 use warnings qw(all);
6867 use sort qw(stable _quicksort _mergesort);
6869 Some of these pseudo-modules import semantics into the current
6870 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6871 which import symbols into the current package (which are effective
6872 through the end of the file).
6874 There's a corresponding C<no> command that unimports meanings imported
6875 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6876 It behaves exactly as C<import> does with respect to VERSION, an
6877 omitted LIST, empty LIST, or no unimport method being found.
6883 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6884 for the C<-M> and C<-m> command-line options to perl that give C<use>
6885 functionality from the command-line.
6890 Changes the access and modification times on each file of a list of
6891 files. The first two elements of the list must be the NUMERICAL access
6892 and modification times, in that order. Returns the number of files
6893 successfully changed. The inode change time of each file is set
6894 to the current time. For example, this code has the same effect as the
6895 Unix touch(1) command when the files I<already exist> and belong to
6896 the user running the program:
6899 $atime = $mtime = time;
6900 utime $atime, $mtime, @ARGV;
6902 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6903 the utime(2) function in the C library will be called with a null second
6904 argument. On most systems, this will set the file's access and
6905 modification times to the current time (i.e. equivalent to the example
6906 above) and will even work on other users' files where you have write
6909 utime undef, undef, @ARGV;
6911 Under NFS this will use the time of the NFS server, not the time of
6912 the local machine. If there is a time synchronization problem, the
6913 NFS server and local machine will have different times. The Unix
6914 touch(1) command will in fact normally use this form instead of the
6915 one shown in the first example.
6917 Note that only passing one of the first two elements as C<undef> will
6918 be equivalent of passing it as 0 and will not have the same effect as
6919 described when they are both C<undef>. This case will also trigger an
6920 uninitialized warning.
6922 On systems that support futimes, you might pass file handles among the
6923 files. On systems that don't support futimes, passing file handles
6924 produces a fatal error at run time. The file handles must be passed
6925 as globs or references to be recognized. Barewords are considered
6931 Returns a list consisting of all the values of the named hash.
6932 (In a scalar context, returns the number of values.)
6934 The values are returned in an apparently random order. The actual
6935 random order is subject to change in future versions of perl, but it
6936 is guaranteed to be the same order as either the C<keys> or C<each>
6937 function would produce on the same (unmodified) hash. Since Perl
6938 5.8.1 the ordering is different even between different runs of Perl
6939 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6941 As a side effect, calling values() resets the HASH's internal iterator,
6942 see L</each>. (In particular, calling values() in void context resets
6943 the iterator with no other overhead.)
6945 Note that the values are not copied, which means modifying them will
6946 modify the contents of the hash:
6948 for (values %hash) { s/foo/bar/g } # modifies %hash values
6949 for (@hash{keys %hash}) { s/foo/bar/g } # same
6951 See also C<keys>, C<each>, and C<sort>.
6953 =item vec EXPR,OFFSET,BITS
6954 X<vec> X<bit> X<bit vector>
6956 Treats the string in EXPR as a bit vector made up of elements of
6957 width BITS, and returns the value of the element specified by OFFSET
6958 as an unsigned integer. BITS therefore specifies the number of bits
6959 that are reserved for each element in the bit vector. This must
6960 be a power of two from 1 to 32 (or 64, if your platform supports
6963 If BITS is 8, "elements" coincide with bytes of the input string.
6965 If BITS is 16 or more, bytes of the input string are grouped into chunks
6966 of size BITS/8, and each group is converted to a number as with
6967 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6968 for BITS==64). See L<"pack"> for details.
6970 If bits is 4 or less, the string is broken into bytes, then the bits
6971 of each byte are broken into 8/BITS groups. Bits of a byte are
6972 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6973 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6974 breaking the single input byte C<chr(0x36)> into two groups gives a list
6975 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6977 C<vec> may also be assigned to, in which case parentheses are needed
6978 to give the expression the correct precedence as in
6980 vec($image, $max_x * $x + $y, 8) = 3;
6982 If the selected element is outside the string, the value 0 is returned.
6983 If an element off the end of the string is written to, Perl will first
6984 extend the string with sufficiently many zero bytes. It is an error
6985 to try to write off the beginning of the string (i.e. negative OFFSET).
6987 The string should not contain any character with the value > 255 (which
6988 can only happen if you're using UTF-8 encoding). If it does, it will be
6989 treated as something that is not UTF-8 encoded. When the C<vec> was
6990 assigned to, other parts of your program will also no longer consider the
6991 string to be UTF-8 encoded. In other words, if you do have such characters
6992 in your string, vec() will operate on the actual byte string, and not the
6993 conceptual character string.
6995 Strings created with C<vec> can also be manipulated with the logical
6996 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6997 vector operation is desired when both operands are strings.
6998 See L<perlop/"Bitwise String Operators">.
7000 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7001 The comments show the string after each step. Note that this code works
7002 in the same way on big-endian or little-endian machines.
7005 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7007 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7008 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7010 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7011 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7012 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7013 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7014 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7015 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7017 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7018 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7019 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7022 To transform a bit vector into a string or list of 0's and 1's, use these:
7024 $bits = unpack("b*", $vector);
7025 @bits = split(//, unpack("b*", $vector));
7027 If you know the exact length in bits, it can be used in place of the C<*>.
7029 Here is an example to illustrate how the bits actually fall in place:
7035 unpack("V",$_) 01234567890123456789012345678901
7036 ------------------------------------------------------------------
7041 for ($shift=0; $shift < $width; ++$shift) {
7042 for ($off=0; $off < 32/$width; ++$off) {
7043 $str = pack("B*", "0"x32);
7044 $bits = (1<<$shift);
7045 vec($str, $off, $width) = $bits;
7046 $res = unpack("b*",$str);
7047 $val = unpack("V", $str);
7054 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7055 $off, $width, $bits, $val, $res
7059 Regardless of the machine architecture on which it is run, the above
7060 example should print the following table:
7063 unpack("V",$_) 01234567890123456789012345678901
7064 ------------------------------------------------------------------
7065 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7066 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7067 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7068 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7069 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7070 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7071 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7072 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7073 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7074 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7075 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7076 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7077 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7078 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7079 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7080 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7081 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7082 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7083 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7084 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7085 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7086 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7087 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7088 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7089 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7090 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7091 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7092 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7093 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7094 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7095 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7096 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7097 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7098 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7099 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7100 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7101 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7102 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7103 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7104 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7105 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7106 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7107 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7108 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7109 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7110 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7111 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7112 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7113 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7114 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7115 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7116 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7117 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7118 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7119 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7120 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7121 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7122 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7123 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7124 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7125 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7126 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7127 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7128 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7129 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7130 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7131 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7132 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7133 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7134 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7135 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7136 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7137 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7138 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7139 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7140 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7141 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7142 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7143 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7144 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7145 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7146 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7147 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7148 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7149 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7150 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7151 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7152 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7153 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7154 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7155 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7156 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7157 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7158 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7159 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7160 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7161 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7162 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7163 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7164 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7165 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7166 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7167 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7168 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7169 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7170 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7171 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7172 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7173 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7174 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7175 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7176 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7177 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7178 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7179 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7180 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7181 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7182 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7183 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7184 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7185 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7186 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7187 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7188 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7189 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7190 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7191 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7192 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7197 Behaves like the wait(2) system call on your system: it waits for a child
7198 process to terminate and returns the pid of the deceased process, or
7199 C<-1> if there are no child processes. The status is returned in C<$?>
7200 and C<{^CHILD_ERROR_NATIVE}>.
7201 Note that a return value of C<-1> could mean that child processes are
7202 being automatically reaped, as described in L<perlipc>.
7204 =item waitpid PID,FLAGS
7207 Waits for a particular child process to terminate and returns the pid of
7208 the deceased process, or C<-1> if there is no such child process. On some
7209 systems, a value of 0 indicates that there are processes still running.
7210 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7212 use POSIX ":sys_wait_h";
7215 $kid = waitpid(-1, WNOHANG);
7218 then you can do a non-blocking wait for all pending zombie processes.
7219 Non-blocking wait is available on machines supporting either the
7220 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7221 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7222 system call by remembering the status values of processes that have
7223 exited but have not been harvested by the Perl script yet.)
7225 Note that on some systems, a return value of C<-1> could mean that child
7226 processes are being automatically reaped. See L<perlipc> for details,
7227 and for other examples.
7230 X<wantarray> X<context>
7232 Returns true if the context of the currently executing subroutine or
7233 C<eval> is looking for a list value. Returns false if the context is
7234 looking for a scalar. Returns the undefined value if the context is
7235 looking for no value (void context).
7237 return unless defined wantarray; # don't bother doing more
7238 my @a = complex_calculation();
7239 return wantarray ? @a : "@a";
7241 C<wantarray()>'s result is unspecified in the top level of a file,
7242 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7243 in a C<DESTROY> method.
7245 This function should have been named wantlist() instead.
7248 X<warn> X<warning> X<STDERR>
7250 Produces a message on STDERR just like C<die>, but doesn't exit or throw
7253 If LIST is empty and C<$@> already contains a value (typically from a
7254 previous eval) that value is used after appending C<"\t...caught">
7255 to C<$@>. This is useful for staying almost, but not entirely similar to
7258 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7260 No message is printed if there is a C<$SIG{__WARN__}> handler
7261 installed. It is the handler's responsibility to deal with the message
7262 as it sees fit (like, for instance, converting it into a C<die>). Most
7263 handlers must therefore make arrangements to actually display the
7264 warnings that they are not prepared to deal with, by calling C<warn>
7265 again in the handler. Note that this is quite safe and will not
7266 produce an endless loop, since C<__WARN__> hooks are not called from
7269 You will find this behavior is slightly different from that of
7270 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7271 instead call C<die> again to change it).
7273 Using a C<__WARN__> handler provides a powerful way to silence all
7274 warnings (even the so-called mandatory ones). An example:
7276 # wipe out *all* compile-time warnings
7277 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7279 my $foo = 20; # no warning about duplicate my $foo,
7280 # but hey, you asked for it!
7281 # no compile-time or run-time warnings before here
7284 # run-time warnings enabled after here
7285 warn "\$foo is alive and $foo!"; # does show up
7287 See L<perlvar> for details on setting C<%SIG> entries, and for more
7288 examples. See the Carp module for other kinds of warnings using its
7289 carp() and cluck() functions.
7291 =item write FILEHANDLE
7298 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7299 using the format associated with that file. By default the format for
7300 a file is the one having the same name as the filehandle, but the
7301 format for the current output channel (see the C<select> function) may be set
7302 explicitly by assigning the name of the format to the C<$~> variable.
7304 Top of form processing is handled automatically: if there is
7305 insufficient room on the current page for the formatted record, the
7306 page is advanced by writing a form feed, a special top-of-page format
7307 is used to format the new page header, and then the record is written.
7308 By default the top-of-page format is the name of the filehandle with
7309 "_TOP" appended, but it may be dynamically set to the format of your
7310 choice by assigning the name to the C<$^> variable while the filehandle is
7311 selected. The number of lines remaining on the current page is in
7312 variable C<$->, which can be set to C<0> to force a new page.
7314 If FILEHANDLE is unspecified, output goes to the current default output
7315 channel, which starts out as STDOUT but may be changed by the
7316 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7317 is evaluated and the resulting string is used to look up the name of
7318 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7320 Note that write is I<not> the opposite of C<read>. Unfortunately.
7324 The transliteration operator. Same as C<tr///>. See L<perlop>.