4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides a scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, the scalar arguments will
18 be first, and the list argument will follow. (Note that there can ever
19 be only one such list argument.) For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for the elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use the parentheses, the simple (but occasionally
34 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
35 function, and precedence doesn't matter. Otherwise it's a list
36 operator or unary operator, and precedence does matter. And whitespace
37 between the function and left parenthesis doesn't count--so you need to
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
52 A few functions take no arguments at all, and therefore work as neither
53 unary nor list operators. These include such functions as C<time>
54 and C<endpwent>. For example, C<time+86_400> always means
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in a scalar context by
59 returning the undefined value, and in a list context by returning the
62 Remember the following important rule: There is B<no rule> that relates
63 the behavior of an expression in list context to its behavior in scalar
64 context, or vice versa. It might do two totally different things.
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in scalar context. Some operators return the
67 length of the list that would have been returned in list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
74 A named array in scalar context is quite different from what would at
75 first glance appear to be a list in scalar context. You can't get a list
76 like C<(1,2,3)> into being in scalar context, because the compiler knows
77 the context at compile time. It would generate the scalar comma operator
78 there, not the list construction version of the comma. That means it
79 was never a list to start with.
81 In general, functions in Perl that serve as wrappers for system calls
82 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule are C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 =head2 Perl Functions by Category
92 Here are Perl's functions (including things that look like
93 functions, like some keywords and named operators)
94 arranged by category. Some functions appear in more
99 =item Functions for SCALARs or strings
100 X<scalar> X<string> X<character>
102 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
103 C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
104 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
106 =item Regular expressions and pattern matching
107 X<regular expression> X<regex> X<regexp>
109 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
111 =item Numeric functions
112 X<numeric> X<number> X<trigonometric> X<trigonometry>
114 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115 C<sin>, C<sqrt>, C<srand>
117 =item Functions for real @ARRAYs
120 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
122 =item Functions for list data
125 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
127 =item Functions for real %HASHes
130 C<delete>, C<each>, C<exists>, C<keys>, C<values>
132 =item Input and output functions
133 X<I/O> X<input> X<output> X<dbm>
135 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
137 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
138 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
141 =item Functions for fixed length data or records
143 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
145 =item Functions for filehandles, files, or directories
146 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
148 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
149 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
150 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151 C<umask>, C<unlink>, C<utime>
153 =item Keywords related to the control flow of your Perl program
156 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
159 =item Keywords related to switch
161 C<break>, C<continue>, C<given>, C<when>, C<default>
163 (These are only available if you enable the "switch" feature.
164 See L<feature> and L<perlsyn/"Switch statements">.)
166 =item Keywords related to scoping
168 C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
171 (C<state> is only available if the "state" feature is enabled. See
174 =item Miscellaneous functions
176 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
177 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
179 =item Functions for processes and process groups
180 X<process> X<pid> X<process id>
182 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
183 C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
184 C<times>, C<wait>, C<waitpid>
186 =item Keywords related to perl modules
189 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
191 =item Keywords related to classes and object-orientedness
192 X<object> X<class> X<package>
194 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
197 =item Low-level socket functions
200 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
202 C<socket>, C<socketpair>
204 =item System V interprocess communication functions
205 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
207 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
210 =item Fetching user and group info
211 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
213 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215 C<getpwuid>, C<setgrent>, C<setpwent>
217 =item Fetching network info
218 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
220 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224 C<setnetent>, C<setprotoent>, C<setservent>
226 =item Time-related functions
229 C<gmtime>, C<localtime>, C<time>, C<times>
231 =item Functions new in perl5
234 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
236 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
237 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238 C<ucfirst>, C<untie>, C<use>, C<when>
240 * - C<sub> was a keyword in perl4, but in perl5 it is an
241 operator, which can be used in expressions.
243 =item Functions obsoleted in perl5
245 C<dbmclose>, C<dbmopen>
250 X<portability> X<Unix> X<portable>
252 Perl was born in Unix and can therefore access all common Unix
253 system calls. In non-Unix environments, the functionality of some
254 Unix system calls may not be available, or details of the available
255 functionality may differ slightly. The Perl functions affected
258 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
261 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
263 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
264 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
267 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
268 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
271 C<shmwrite>, C<socket>, C<socketpair>,
272 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273 C<times>, C<truncate>, C<umask>, C<unlink>,
274 C<utime>, C<wait>, C<waitpid>
276 For more information about the portability of these functions, see
277 L<perlport> and other available platform-specific documentation.
279 =head2 Alphabetical Listing of Perl Functions
284 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
285 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
293 A file test, where X is one of the letters listed below. This unary
294 operator takes one argument, either a filename, a filehandle, or a dirhandle,
295 and tests the associated file to see if something is true about it. If the
296 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
297 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
298 the undefined value if the file doesn't exist. Despite the funny
299 names, precedence is the same as any other named unary operator, and
300 the argument may be parenthesized like any other unary operator. The
301 operator may be any of:
303 -r File is readable by effective uid/gid.
304 -w File is writable by effective uid/gid.
305 -x File is executable by effective uid/gid.
306 -o File is owned by effective uid.
308 -R File is readable by real uid/gid.
309 -W File is writable by real uid/gid.
310 -X File is executable by real uid/gid.
311 -O File is owned by real uid.
314 -z File has zero size (is empty).
315 -s File has nonzero size (returns size in bytes).
317 -f File is a plain file.
318 -d File is a directory.
319 -l File is a symbolic link.
320 -p File is a named pipe (FIFO), or Filehandle is a pipe.
322 -b File is a block special file.
323 -c File is a character special file.
324 -t Filehandle is opened to a tty.
326 -u File has setuid bit set.
327 -g File has setgid bit set.
328 -k File has sticky bit set.
330 -T File is an ASCII text file (heuristic guess).
331 -B File is a "binary" file (opposite of -T).
333 -M Script start time minus file modification time, in days.
334 -A Same for access time.
335 -C Same for inode change time (Unix, may differ for other platforms)
341 next unless -f $_; # ignore specials
345 The interpretation of the file permission operators C<-r>, C<-R>,
346 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
347 of the file and the uids and gids of the user. There may be other
348 reasons you can't actually read, write, or execute the file. Such
349 reasons may be for example network filesystem access controls, ACLs
350 (access control lists), read-only filesystems, and unrecognized
353 Also note that, for the superuser on the local filesystems, the C<-r>,
354 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
355 if any execute bit is set in the mode. Scripts run by the superuser
356 may thus need to do a stat() to determine the actual mode of the file,
357 or temporarily set their effective uid to something else.
359 If you are using ACLs, there is a pragma called C<filetest> that may
360 produce more accurate results than the bare stat() mode bits.
361 When under the C<use filetest 'access'> the above-mentioned filetests
362 will test whether the permission can (not) be granted using the
363 access() family of system calls. Also note that the C<-x> and C<-X> may
364 under this pragma return true even if there are no execute permission
365 bits set (nor any extra execute permission ACLs). This strangeness is
366 due to the underlying system calls' definitions. Read the
367 documentation for the C<filetest> pragma for more information.
369 Note that C<-s/a/b/> does not do a negated substitution. Saying
370 C<-exp($foo)> still works as expected, however--only single letters
371 following a minus are interpreted as file tests.
373 The C<-T> and C<-B> switches work as follows. The first block or so of the
374 file is examined for odd characters such as strange control codes or
375 characters with the high bit set. If too many strange characters (>30%)
376 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
377 containing null in the first block is considered a binary file. If C<-T>
378 or C<-B> is used on a filehandle, the current IO buffer is examined
379 rather than the first block. Both C<-T> and C<-B> return true on a null
380 file, or a file at EOF when testing a filehandle. Because you have to
381 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
382 against the file first, as in C<next unless -f $file && -T $file>.
384 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
385 the special filehandle consisting of a solitary underline, then the stat
386 structure of the previous file test (or stat operator) is used, saving
387 a system call. (This doesn't work with C<-t>, and you need to remember
388 that lstat() and C<-l> will leave values in the stat structure for the
389 symbolic link, not the real file.) (Also, if the stat buffer was filled by
390 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
393 print "Can do.\n" if -r $a || -w _ || -x _;
396 print "Readable\n" if -r _;
397 print "Writable\n" if -w _;
398 print "Executable\n" if -x _;
399 print "Setuid\n" if -u _;
400 print "Setgid\n" if -g _;
401 print "Sticky\n" if -k _;
402 print "Text\n" if -T _;
403 print "Binary\n" if -B _;
405 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
406 test operators, in a way that C<-f -w -x $file> is equivalent to
407 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
408 the return value of C<-f $file> as an argument to another filetest
409 operator, no special magic will happen.)
416 Returns the absolute value of its argument.
417 If VALUE is omitted, uses C<$_>.
419 =item accept NEWSOCKET,GENERICSOCKET
422 Accepts an incoming socket connect, just as the accept(2) system call
423 does. Returns the packed address if it succeeded, false otherwise.
424 See the example in L<perlipc/"Sockets: Client/Server Communication">.
426 On systems that support a close-on-exec flag on files, the flag will
427 be set for the newly opened file descriptor, as determined by the
428 value of $^F. See L<perlvar/$^F>.
437 Arranges to have a SIGALRM delivered to this process after the
438 specified number of wallclock seconds has elapsed. If SECONDS is not
439 specified, the value stored in C<$_> is used. (On some machines,
440 unfortunately, the elapsed time may be up to one second less or more
441 than you specified because of how seconds are counted, and process
442 scheduling may delay the delivery of the signal even further.)
444 Only one timer may be counting at once. Each call disables the
445 previous timer, and an argument of C<0> may be supplied to cancel the
446 previous timer without starting a new one. The returned value is the
447 amount of time remaining on the previous timer.
449 For delays of finer granularity than one second, you may use Perl's
450 four-argument version of select() leaving the first three arguments
451 undefined, or you might be able to use the C<syscall> interface to
452 access setitimer(2) if your system supports it. The Time::HiRes
453 module (from CPAN, and starting from Perl 5.8 part of the standard
454 distribution) may also prove useful.
456 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
457 (C<sleep> may be internally implemented in your system with C<alarm>)
459 If you want to use C<alarm> to time out a system call you need to use an
460 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
461 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
462 restart system calls on some systems. Using C<eval>/C<die> always works,
463 modulo the caveats given in L<perlipc/"Signals">.
466 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
468 $nread = sysread SOCKET, $buffer, $size;
472 die unless $@ eq "alarm\n"; # propagate unexpected errors
479 For more information see L<perlipc>.
482 X<atan2> X<arctangent> X<tan> X<tangent>
484 Returns the arctangent of Y/X in the range -PI to PI.
486 For the tangent operation, you may use the C<Math::Trig::tan>
487 function, or use the familiar relation:
489 sub tan { sin($_[0]) / cos($_[0]) }
491 Note that atan2(0, 0) is not well-defined.
493 =item bind SOCKET,NAME
496 Binds a network address to a socket, just as the bind system call
497 does. Returns true if it succeeded, false otherwise. NAME should be a
498 packed address of the appropriate type for the socket. See the examples in
499 L<perlipc/"Sockets: Client/Server Communication">.
501 =item binmode FILEHANDLE, LAYER
502 X<binmode> X<binary> X<text> X<DOS> X<Windows>
504 =item binmode FILEHANDLE
506 Arranges for FILEHANDLE to be read or written in "binary" or "text"
507 mode on systems where the run-time libraries distinguish between
508 binary and text files. If FILEHANDLE is an expression, the value is
509 taken as the name of the filehandle. Returns true on success,
510 otherwise it returns C<undef> and sets C<$!> (errno).
512 On some systems (in general, DOS and Windows-based systems) binmode()
513 is necessary when you're not working with a text file. For the sake
514 of portability it is a good idea to always use it when appropriate,
515 and to never use it when it isn't appropriate. Also, people can
516 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
518 In other words: regardless of platform, use binmode() on binary data,
519 like for example images.
521 If LAYER is present it is a single string, but may contain multiple
522 directives. The directives alter the behaviour of the file handle.
523 When LAYER is present using binmode on text file makes sense.
525 If LAYER is omitted or specified as C<:raw> the filehandle is made
526 suitable for passing binary data. This includes turning off possible CRLF
527 translation and marking it as bytes (as opposed to Unicode characters).
528 Note that, despite what may be implied in I<"Programming Perl"> (the
529 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
530 -- other layers which would affect the binary nature of the stream are
531 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
532 PERLIO environment variable.
534 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
535 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
536 establish default I/O layers. See L<open>.
538 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
539 in "Programming Perl, 3rd Edition". However, since the publishing of this
540 book, by many known as "Camel III", the consensus of the naming of this
541 functionality has moved from "discipline" to "layer". All documentation
542 of this version of Perl therefore refers to "layers" rather than to
543 "disciplines". Now back to the regularly scheduled documentation...>
545 To mark FILEHANDLE as UTF-8, use C<:utf8>.
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<$_>.
868 =item close FILEHANDLE
873 Closes the file or pipe associated with the file handle, flushes the IO
874 buffers, and closes the system file descriptor. Returns true if those
875 operations have succeeded and if no error was reported by any PerlIO
876 layer. Closes the currently selected filehandle if the argument is
879 You don't have to close FILEHANDLE if you are immediately going to do
880 another C<open> on it, because C<open> will close it for you. (See
881 C<open>.) However, an explicit C<close> on an input file resets the line
882 counter (C<$.>), while the implicit close done by C<open> does not.
884 If the file handle came from a piped open, C<close> will additionally
885 return false if one of the other system calls involved fails, or if the
886 program exits with non-zero status. (If the only problem was that the
887 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
888 also waits for the process executing on the pipe to complete, in case you
889 want to look at the output of the pipe afterwards, and
890 implicitly puts the exit status value of that command into C<$?> and
891 C<${^CHILD_ERROR_NATIVE}>.
893 Prematurely closing the read end of a pipe (i.e. before the process
894 writing to it at the other end has closed it) will result in a
895 SIGPIPE being delivered to the writer. If the other end can't
896 handle that, be sure to read all the data before closing the pipe.
900 open(OUTPUT, '|sort >foo') # pipe to sort
901 or die "Can't start sort: $!";
902 #... # print stuff to output
903 close OUTPUT # wait for sort to finish
904 or warn $! ? "Error closing sort pipe: $!"
905 : "Exit status $? from sort";
906 open(INPUT, 'foo') # get sort's results
907 or die "Can't open 'foo' for input: $!";
909 FILEHANDLE may be an expression whose value can be used as an indirect
910 filehandle, usually the real filehandle name.
912 =item closedir DIRHANDLE
915 Closes a directory opened by C<opendir> and returns the success of that
918 =item connect SOCKET,NAME
921 Attempts to connect to a remote socket, just as the connect system call
922 does. Returns true if it succeeded, false otherwise. NAME should be a
923 packed address of the appropriate type for the socket. See the examples in
924 L<perlipc/"Sockets: Client/Server Communication">.
931 C<continue> is actually a flow control statement rather than a function. If
932 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
933 C<foreach>), it is always executed just before the conditional is about to
934 be evaluated again, just like the third part of a C<for> loop in C. Thus
935 it can be used to increment a loop variable, even when the loop has been
936 continued via the C<next> statement (which is similar to the C C<continue>
939 C<last>, C<next>, or C<redo> may appear within a C<continue>
940 block. C<last> and C<redo> will behave as if they had been executed within
941 the main block. So will C<next>, but since it will execute a C<continue>
942 block, it may be more entertaining.
945 ### redo always comes here
948 ### next always comes here
950 # then back the top to re-check EXPR
952 ### last always comes here
954 Omitting the C<continue> section is semantically equivalent to using an
955 empty one, logically enough. In that case, C<next> goes directly back
956 to check the condition at the top of the loop.
958 If the "switch" feature is enabled, C<continue> is also a
959 function that will break out of the current C<when> or C<default>
960 block, and fall through to the next case. See L<feature> and
961 L<perlsyn/"Switch statements"> for more information.
965 X<cos> X<cosine> X<acos> X<arccosine>
969 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
970 takes cosine of C<$_>.
972 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
973 function, or use this relation:
975 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
977 =item crypt PLAINTEXT,SALT
978 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
979 X<decrypt> X<cryptography> X<passwd> X<encrypt>
981 Creates a digest string exactly like the crypt(3) function in the C
982 library (assuming that you actually have a version there that has not
983 been extirpated as a potential munitions).
985 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
986 into a short string, called a digest, which is returned. The same
987 PLAINTEXT and SALT will always return the same string, but there is no
988 (known) way to get the original PLAINTEXT from the hash. Small
989 changes in the PLAINTEXT or SALT will result in large changes in the
992 There is no decrypt function. This function isn't all that useful for
993 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
994 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
995 primarily used to check if two pieces of text are the same without
996 having to transmit or store the text itself. An example is checking
997 if a correct password is given. The digest of the password is stored,
998 not the password itself. The user types in a password that is
999 crypt()'d with the same salt as the stored digest. If the two digests
1000 match the password is correct.
1002 When verifying an existing digest string you should use the digest as
1003 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1004 to create the digest is visible as part of the digest. This ensures
1005 crypt() will hash the new string with the same salt as the digest.
1006 This allows your code to work with the standard L<crypt|/crypt> and
1007 with more exotic implementations. In other words, do not assume
1008 anything about the returned string itself, or how many bytes in the
1011 Traditionally the result is a string of 13 bytes: two first bytes of
1012 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1013 the first eight bytes of the digest string mattered, but alternative
1014 hashing schemes (like MD5), higher level security schemes (like C2),
1015 and implementations on non-UNIX platforms may produce different
1018 When choosing a new salt create a random two character string whose
1019 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1020 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1021 characters is just a recommendation; the characters allowed in
1022 the salt depend solely on your system's crypt library, and Perl can't
1023 restrict what salts C<crypt()> accepts.
1025 Here's an example that makes sure that whoever runs this program knows
1028 $pwd = (getpwuid($<))[1];
1030 system "stty -echo";
1032 chomp($word = <STDIN>);
1036 if (crypt($word, $pwd) ne $pwd) {
1042 Of course, typing in your own password to whoever asks you
1045 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1046 of data, not least of all because you can't get the information
1047 back. Look at the L<Digest> module for more robust algorithms.
1049 If using crypt() on a Unicode string (which I<potentially> has
1050 characters with codepoints above 255), Perl tries to make sense
1051 of the situation by trying to downgrade (a copy of the string)
1052 the string back to an eight-bit byte string before calling crypt()
1053 (on that copy). If that works, good. If not, crypt() dies with
1054 C<Wide character in crypt>.
1059 [This function has been largely superseded by the C<untie> function.]
1061 Breaks the binding between a DBM file and a hash.
1063 =item dbmopen HASH,DBNAME,MASK
1064 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1066 [This function has been largely superseded by the C<tie> function.]
1068 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1069 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1070 argument is I<not> a filehandle, even though it looks like one). DBNAME
1071 is the name of the database (without the F<.dir> or F<.pag> extension if
1072 any). If the database does not exist, it is created with protection
1073 specified by MASK (as modified by the C<umask>). If your system supports
1074 only the older DBM functions, you may perform only one C<dbmopen> in your
1075 program. In older versions of Perl, if your system had neither DBM nor
1076 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1079 If you don't have write access to the DBM file, you can only read hash
1080 variables, not set them. If you want to test whether you can write,
1081 either use file tests or try setting a dummy hash entry inside an C<eval>,
1082 which will trap the error.
1084 Note that functions such as C<keys> and C<values> may return huge lists
1085 when used on large DBM files. You may prefer to use the C<each>
1086 function to iterate over large DBM files. Example:
1088 # print out history file offsets
1089 dbmopen(%HIST,'/usr/lib/news/history',0666);
1090 while (($key,$val) = each %HIST) {
1091 print $key, ' = ', unpack('L',$val), "\n";
1095 See also L<AnyDBM_File> for a more general description of the pros and
1096 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1097 rich implementation.
1099 You can control which DBM library you use by loading that library
1100 before you call dbmopen():
1103 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1104 or die "Can't open netscape history file: $!";
1107 X<defined> X<undef> X<undefined>
1111 Returns a Boolean value telling whether EXPR has a value other than
1112 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1115 Many operations return C<undef> to indicate failure, end of file,
1116 system error, uninitialized variable, and other exceptional
1117 conditions. This function allows you to distinguish C<undef> from
1118 other values. (A simple Boolean test will not distinguish among
1119 C<undef>, zero, the empty string, and C<"0">, which are all equally
1120 false.) Note that since C<undef> is a valid scalar, its presence
1121 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1122 returns C<undef> when its argument is an empty array, I<or> when the
1123 element to return happens to be C<undef>.
1125 You may also use C<defined(&func)> to check whether subroutine C<&func>
1126 has ever been defined. The return value is unaffected by any forward
1127 declarations of C<&func>. Note that a subroutine which is not defined
1128 may still be callable: its package may have an C<AUTOLOAD> method that
1129 makes it spring into existence the first time that it is called -- see
1132 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1133 used to report whether memory for that aggregate has ever been
1134 allocated. This behavior may disappear in future versions of Perl.
1135 You should instead use a simple test for size:
1137 if (@an_array) { print "has array elements\n" }
1138 if (%a_hash) { print "has hash members\n" }
1140 When used on a hash element, it tells you whether the value is defined,
1141 not whether the key exists in the hash. Use L</exists> for the latter
1146 print if defined $switch{'D'};
1147 print "$val\n" while defined($val = pop(@ary));
1148 die "Can't readlink $sym: $!"
1149 unless defined($value = readlink $sym);
1150 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1151 $debugging = 0 unless defined $debugging;
1153 Note: Many folks tend to overuse C<defined>, and then are surprised to
1154 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1155 defined values. For example, if you say
1159 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1160 matched "nothing". It didn't really fail to match anything. Rather, it
1161 matched something that happened to be zero characters long. This is all
1162 very above-board and honest. When a function returns an undefined value,
1163 it's an admission that it couldn't give you an honest answer. So you
1164 should use C<defined> only when you're questioning the integrity of what
1165 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1168 See also L</undef>, L</exists>, L</ref>.
1173 Given an expression that specifies a hash element, array element, hash slice,
1174 or array slice, deletes the specified element(s) from the hash or array.
1175 In the case of an array, if the array elements happen to be at the end,
1176 the size of the array will shrink to the highest element that tests
1177 true for exists() (or 0 if no such element exists).
1179 Returns a list with the same number of elements as the number of elements
1180 for which deletion was attempted. Each element of that list consists of
1181 either the value of the element deleted, or the undefined value. In scalar
1182 context, this means that you get the value of the last element deleted (or
1183 the undefined value if that element did not exist).
1185 %hash = (foo => 11, bar => 22, baz => 33);
1186 $scalar = delete $hash{foo}; # $scalar is 11
1187 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1188 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1190 Deleting from C<%ENV> modifies the environment. Deleting from
1191 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1192 from a C<tie>d hash or array may not necessarily return anything.
1194 Deleting an array element effectively returns that position of the array
1195 to its initial, uninitialized state. Subsequently testing for the same
1196 element with exists() will return false. Also, deleting array elements
1197 in the middle of an array will not shift the index of the elements
1198 after them down. Use splice() for that. See L</exists>.
1200 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1202 foreach $key (keys %HASH) {
1206 foreach $index (0 .. $#ARRAY) {
1207 delete $ARRAY[$index];
1212 delete @HASH{keys %HASH};
1214 delete @ARRAY[0 .. $#ARRAY];
1216 But both of these are slower than just assigning the empty list
1217 or undefining %HASH or @ARRAY:
1219 %HASH = (); # completely empty %HASH
1220 undef %HASH; # forget %HASH ever existed
1222 @ARRAY = (); # completely empty @ARRAY
1223 undef @ARRAY; # forget @ARRAY ever existed
1225 Note that the EXPR can be arbitrarily complicated as long as the final
1226 operation is a hash element, array element, hash slice, or array slice
1229 delete $ref->[$x][$y]{$key};
1230 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1232 delete $ref->[$x][$y][$index];
1233 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1236 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1238 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1239 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1240 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1241 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1242 an C<eval(),> the error message is stuffed into C<$@> and the
1243 C<eval> is terminated with the undefined value. This makes
1244 C<die> the way to raise an exception.
1246 Equivalent examples:
1248 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1249 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1251 If the last element of LIST does not end in a newline, the current
1252 script line number and input line number (if any) are also printed,
1253 and a newline is supplied. Note that the "input line number" (also
1254 known as "chunk") is subject to whatever notion of "line" happens to
1255 be currently in effect, and is also available as the special variable
1256 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1258 Hint: sometimes appending C<", stopped"> to your message will cause it
1259 to make better sense when the string C<"at foo line 123"> is appended.
1260 Suppose you are running script "canasta".
1262 die "/etc/games is no good";
1263 die "/etc/games is no good, stopped";
1265 produce, respectively
1267 /etc/games is no good at canasta line 123.
1268 /etc/games is no good, stopped at canasta line 123.
1270 See also exit(), warn(), and the Carp module.
1272 If LIST is empty and C<$@> already contains a value (typically from a
1273 previous eval) that value is reused after appending C<"\t...propagated">.
1274 This is useful for propagating exceptions:
1277 die unless $@ =~ /Expected exception/;
1279 If LIST is empty and C<$@> contains an object reference that has a
1280 C<PROPAGATE> method, that method will be called with additional file
1281 and line number parameters. The return value replaces the value in
1282 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1285 If C<$@> is empty then the string C<"Died"> is used.
1287 die() can also be called with a reference argument. If this happens to be
1288 trapped within an eval(), $@ contains the reference. This behavior permits
1289 a more elaborate exception handling implementation using objects that
1290 maintain arbitrary state about the nature of the exception. Such a scheme
1291 is sometimes preferable to matching particular string values of $@ using
1292 regular expressions. Because $@ is a global variable, and eval() may be
1293 used within object implementations, care must be taken that analyzing the
1294 error object doesn't replace the reference in the global variable. The
1295 easiest solution is to make a local copy of the reference before doing
1296 other manipulations. Here's an example:
1298 use Scalar::Util 'blessed';
1300 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1301 if (my $ev_err = $@) {
1302 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1303 # handle Some::Module::Exception
1306 # handle all other possible exceptions
1310 Because perl will stringify uncaught exception messages before displaying
1311 them, you may want to overload stringification operations on such custom
1312 exception objects. See L<overload> for details about that.
1314 You can arrange for a callback to be run just before the C<die>
1315 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1316 handler will be called with the error text and can change the error
1317 message, if it sees fit, by calling C<die> again. See
1318 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1319 L<"eval BLOCK"> for some examples. Although this feature was
1320 to be run only right before your program was to exit, this is not
1321 currently the case--the C<$SIG{__DIE__}> hook is currently called
1322 even inside eval()ed blocks/strings! If one wants the hook to do
1323 nothing in such situations, put
1327 as the first line of the handler (see L<perlvar/$^S>). Because
1328 this promotes strange action at a distance, this counterintuitive
1329 behavior may be fixed in a future release.
1334 Not really a function. Returns the value of the last command in the
1335 sequence of commands indicated by BLOCK. When modified by the C<while> or
1336 C<until> loop modifier, executes the BLOCK once before testing the loop
1337 condition. (On other statements the loop modifiers test the conditional
1340 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1341 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1342 See L<perlsyn> for alternative strategies.
1344 =item do SUBROUTINE(LIST)
1347 This form of subroutine call is deprecated. See L<perlsub>.
1352 Uses the value of EXPR as a filename and executes the contents of the
1353 file as a Perl script.
1361 except that it's more efficient and concise, keeps track of the current
1362 filename for error messages, searches the @INC directories, and updates
1363 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1364 variables. It also differs in that code evaluated with C<do FILENAME>
1365 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1366 same, however, in that it does reparse the file every time you call it,
1367 so you probably don't want to do this inside a loop.
1369 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1370 error. If C<do> can read the file but cannot compile it, it
1371 returns undef and sets an error message in C<$@>. If the file is
1372 successfully compiled, C<do> returns the value of the last expression
1375 Note that inclusion of library modules is better done with the
1376 C<use> and C<require> operators, which also do automatic error checking
1377 and raise an exception if there's a problem.
1379 You might like to use C<do> to read in a program configuration
1380 file. Manual error checking can be done this way:
1382 # read in config files: system first, then user
1383 for $file ("/share/prog/defaults.rc",
1384 "$ENV{HOME}/.someprogrc")
1386 unless ($return = do $file) {
1387 warn "couldn't parse $file: $@" if $@;
1388 warn "couldn't do $file: $!" unless defined $return;
1389 warn "couldn't run $file" unless $return;
1394 X<dump> X<core> X<undump>
1398 This function causes an immediate core dump. See also the B<-u>
1399 command-line switch in L<perlrun>, which does the same thing.
1400 Primarily this is so that you can use the B<undump> program (not
1401 supplied) to turn your core dump into an executable binary after
1402 having initialized all your variables at the beginning of the
1403 program. When the new binary is executed it will begin by executing
1404 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1405 Think of it as a goto with an intervening core dump and reincarnation.
1406 If C<LABEL> is omitted, restarts the program from the top.
1408 B<WARNING>: Any files opened at the time of the dump will I<not>
1409 be open any more when the program is reincarnated, with possible
1410 resulting confusion on the part of Perl.
1412 This function is now largely obsolete, partly because it's very
1413 hard to convert a core file into an executable, and because the
1414 real compiler backends for generating portable bytecode and compilable
1415 C code have superseded it. That's why you should now invoke it as
1416 C<CORE::dump()>, if you don't want to be warned against a possible
1419 If you're looking to use L<dump> to speed up your program, consider
1420 generating bytecode or native C code as described in L<perlcc>. If
1421 you're just trying to accelerate a CGI script, consider using the
1422 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1423 You might also consider autoloading or selfloading, which at least
1424 make your program I<appear> to run faster.
1427 X<each> X<hash, iterator>
1429 When called in list context, returns a 2-element list consisting of the
1430 key and value for the next element of a hash, so that you can iterate over
1431 it. When called in scalar context, returns only the key for the next
1432 element in the hash.
1434 Entries are returned in an apparently random order. The actual random
1435 order is subject to change in future versions of perl, but it is
1436 guaranteed to be in the same order as either the C<keys> or C<values>
1437 function would produce on the same (unmodified) hash. Since Perl
1438 5.8.1 the ordering is different even between different runs of Perl
1439 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1441 When the hash is entirely read, a null array is returned in list context
1442 (which when assigned produces a false (C<0>) value), and C<undef> in
1443 scalar context. The next call to C<each> after that will start iterating
1444 again. There is a single iterator for each hash, shared by all C<each>,
1445 C<keys>, and C<values> function calls in the program; it can be reset by
1446 reading all the elements from the hash, or by evaluating C<keys HASH> or
1447 C<values HASH>. If you add or delete elements of a hash while you're
1448 iterating over it, you may get entries skipped or duplicated, so
1449 don't. Exception: It is always safe to delete the item most recently
1450 returned by C<each()>, which means that the following code will work:
1452 while (($key, $value) = each %hash) {
1454 delete $hash{$key}; # This is safe
1457 The following prints out your environment like the printenv(1) program,
1458 only in a different order:
1460 while (($key,$value) = each %ENV) {
1461 print "$key=$value\n";
1464 See also C<keys>, C<values> and C<sort>.
1466 =item eof FILEHANDLE
1475 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1476 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1477 gives the real filehandle. (Note that this function actually
1478 reads a character and then C<ungetc>s it, so isn't very useful in an
1479 interactive context.) Do not read from a terminal file (or call
1480 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1481 as terminals may lose the end-of-file condition if you do.
1483 An C<eof> without an argument uses the last file read. Using C<eof()>
1484 with empty parentheses is very different. It refers to the pseudo file
1485 formed from the files listed on the command line and accessed via the
1486 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1487 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1488 used will cause C<@ARGV> to be examined to determine if input is
1489 available. Similarly, an C<eof()> after C<< <> >> has returned
1490 end-of-file will assume you are processing another C<@ARGV> list,
1491 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1492 see L<perlop/"I/O Operators">.
1494 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1495 detect the end of each file, C<eof()> will only detect the end of the
1496 last file. Examples:
1498 # reset line numbering on each input file
1500 next if /^\s*#/; # skip comments
1503 close ARGV if eof; # Not eof()!
1506 # insert dashes just before last line of last file
1508 if (eof()) { # check for end of last file
1509 print "--------------\n";
1512 last if eof(); # needed if we're reading from a terminal
1515 Practical hint: you almost never need to use C<eof> in Perl, because the
1516 input operators typically return C<undef> when they run out of data, or if
1520 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1521 X<error, handling> X<exception, handling>
1527 In the first form, the return value of EXPR is parsed and executed as if it
1528 were a little Perl program. The value of the expression (which is itself
1529 determined within scalar context) is first parsed, and if there weren't any
1530 errors, executed in the lexical context of the current Perl program, so
1531 that any variable settings or subroutine and format definitions remain
1532 afterwards. Note that the value is parsed every time the C<eval> executes.
1533 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1534 delay parsing and subsequent execution of the text of EXPR until run time.
1536 In the second form, the code within the BLOCK is parsed only once--at the
1537 same time the code surrounding the C<eval> itself was parsed--and executed
1538 within the context of the current Perl program. This form is typically
1539 used to trap exceptions more efficiently than the first (see below), while
1540 also providing the benefit of checking the code within BLOCK at compile
1543 The final semicolon, if any, may be omitted from the value of EXPR or within
1546 In both forms, the value returned is the value of the last expression
1547 evaluated inside the mini-program; a return statement may be also used, just
1548 as with subroutines. The expression providing the return value is evaluated
1549 in void, scalar, or list context, depending on the context of the C<eval>
1550 itself. See L</wantarray> for more on how the evaluation context can be
1553 If there is a syntax error or runtime error, or a C<die> statement is
1554 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1555 error message. If there was no error, C<$@> is guaranteed to be a null
1556 string. Beware that using C<eval> neither silences perl from printing
1557 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1558 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1559 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1560 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1562 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1563 determining whether a particular feature (such as C<socket> or C<symlink>)
1564 is implemented. It is also Perl's exception trapping mechanism, where
1565 the die operator is used to raise exceptions.
1567 If the code to be executed doesn't vary, you may use the eval-BLOCK
1568 form to trap run-time errors without incurring the penalty of
1569 recompiling each time. The error, if any, is still returned in C<$@>.
1572 # make divide-by-zero nonfatal
1573 eval { $answer = $a / $b; }; warn $@ if $@;
1575 # same thing, but less efficient
1576 eval '$answer = $a / $b'; warn $@ if $@;
1578 # a compile-time error
1579 eval { $answer = }; # WRONG
1582 eval '$answer ='; # sets $@
1584 Using the C<eval{}> form as an exception trap in libraries does have some
1585 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1586 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1587 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1588 as shown in this example:
1590 # a very private exception trap for divide-by-zero
1591 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1594 This is especially significant, given that C<__DIE__> hooks can call
1595 C<die> again, which has the effect of changing their error messages:
1597 # __DIE__ hooks may modify error messages
1599 local $SIG{'__DIE__'} =
1600 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1601 eval { die "foo lives here" };
1602 print $@ if $@; # prints "bar lives here"
1605 Because this promotes action at a distance, this counterintuitive behavior
1606 may be fixed in a future release.
1608 With an C<eval>, you should be especially careful to remember what's
1609 being looked at when:
1615 eval { $x }; # CASE 4
1617 eval "\$$x++"; # CASE 5
1620 Cases 1 and 2 above behave identically: they run the code contained in
1621 the variable $x. (Although case 2 has misleading double quotes making
1622 the reader wonder what else might be happening (nothing is).) Cases 3
1623 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1624 does nothing but return the value of $x. (Case 4 is preferred for
1625 purely visual reasons, but it also has the advantage of compiling at
1626 compile-time instead of at run-time.) Case 5 is a place where
1627 normally you I<would> like to use double quotes, except that in this
1628 particular situation, you can just use symbolic references instead, as
1631 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1632 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1634 Note that as a very special case, an C<eval ''> executed within the C<DB>
1635 package doesn't see the usual surrounding lexical scope, but rather the
1636 scope of the first non-DB piece of code that called it. You don't normally
1637 need to worry about this unless you are writing a Perl debugger.
1642 =item exec PROGRAM LIST
1644 The C<exec> function executes a system command I<and never returns>--
1645 use C<system> instead of C<exec> if you want it to return. It fails and
1646 returns false only if the command does not exist I<and> it is executed
1647 directly instead of via your system's command shell (see below).
1649 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1650 warns you if there is a following statement which isn't C<die>, C<warn>,
1651 or C<exit> (if C<-w> is set - but you always do that). If you
1652 I<really> want to follow an C<exec> with some other statement, you
1653 can use one of these styles to avoid the warning:
1655 exec ('foo') or print STDERR "couldn't exec foo: $!";
1656 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1658 If there is more than one argument in LIST, or if LIST is an array
1659 with more than one value, calls execvp(3) with the arguments in LIST.
1660 If there is only one scalar argument or an array with one element in it,
1661 the argument is checked for shell metacharacters, and if there are any,
1662 the entire argument is passed to the system's command shell for parsing
1663 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1664 If there are no shell metacharacters in the argument, it is split into
1665 words and passed directly to C<execvp>, which is more efficient.
1668 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1669 exec "sort $outfile | uniq";
1671 If you don't really want to execute the first argument, but want to lie
1672 to the program you are executing about its own name, you can specify
1673 the program you actually want to run as an "indirect object" (without a
1674 comma) in front of the LIST. (This always forces interpretation of the
1675 LIST as a multivalued list, even if there is only a single scalar in
1678 $shell = '/bin/csh';
1679 exec $shell '-sh'; # pretend it's a login shell
1683 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1685 When the arguments get executed via the system shell, results will
1686 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1689 Using an indirect object with C<exec> or C<system> is also more
1690 secure. This usage (which also works fine with system()) forces
1691 interpretation of the arguments as a multivalued list, even if the
1692 list had just one argument. That way you're safe from the shell
1693 expanding wildcards or splitting up words with whitespace in them.
1695 @args = ( "echo surprise" );
1697 exec @args; # subject to shell escapes
1699 exec { $args[0] } @args; # safe even with one-arg list
1701 The first version, the one without the indirect object, ran the I<echo>
1702 program, passing it C<"surprise"> an argument. The second version
1703 didn't--it tried to run a program literally called I<"echo surprise">,
1704 didn't find it, and set C<$?> to a non-zero value indicating failure.
1706 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1707 output before the exec, but this may not be supported on some platforms
1708 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1709 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1710 open handles in order to avoid lost output.
1712 Note that C<exec> will not call your C<END> blocks, nor will it call
1713 any C<DESTROY> methods in your objects.
1716 X<exists> X<autovivification>
1718 Given an expression that specifies a hash element or array element,
1719 returns true if the specified element in the hash or array has ever
1720 been initialized, even if the corresponding value is undefined. The
1721 element is not autovivified if it doesn't exist.
1723 print "Exists\n" if exists $hash{$key};
1724 print "Defined\n" if defined $hash{$key};
1725 print "True\n" if $hash{$key};
1727 print "Exists\n" if exists $array[$index];
1728 print "Defined\n" if defined $array[$index];
1729 print "True\n" if $array[$index];
1731 A hash or array element can be true only if it's defined, and defined if
1732 it exists, but the reverse doesn't necessarily hold true.
1734 Given an expression that specifies the name of a subroutine,
1735 returns true if the specified subroutine has ever been declared, even
1736 if it is undefined. Mentioning a subroutine name for exists or defined
1737 does not count as declaring it. Note that a subroutine which does not
1738 exist may still be callable: its package may have an C<AUTOLOAD>
1739 method that makes it spring into existence the first time that it is
1740 called -- see L<perlsub>.
1742 print "Exists\n" if exists &subroutine;
1743 print "Defined\n" if defined &subroutine;
1745 Note that the EXPR can be arbitrarily complicated as long as the final
1746 operation is a hash or array key lookup or subroutine name:
1748 if (exists $ref->{A}->{B}->{$key}) { }
1749 if (exists $hash{A}{B}{$key}) { }
1751 if (exists $ref->{A}->{B}->[$ix]) { }
1752 if (exists $hash{A}{B}[$ix]) { }
1754 if (exists &{$ref->{A}{B}{$key}}) { }
1756 Although the deepest nested array or hash will not spring into existence
1757 just because its existence was tested, any intervening ones will.
1758 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1759 into existence due to the existence test for the $key element above.
1760 This happens anywhere the arrow operator is used, including even:
1763 if (exists $ref->{"Some key"}) { }
1764 print $ref; # prints HASH(0x80d3d5c)
1766 This surprising autovivification in what does not at first--or even
1767 second--glance appear to be an lvalue context may be fixed in a future
1770 Use of a subroutine call, rather than a subroutine name, as an argument
1771 to exists() is an error.
1774 exists &sub(); # Error
1777 X<exit> X<terminate> X<abort>
1781 Evaluates EXPR and exits immediately with that value. Example:
1784 exit 0 if $ans =~ /^[Xx]/;
1786 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1787 universally recognized values for EXPR are C<0> for success and C<1>
1788 for error; other values are subject to interpretation depending on the
1789 environment in which the Perl program is running. For example, exiting
1790 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1791 the mailer to return the item undelivered, but that's not true everywhere.
1793 Don't use C<exit> to abort a subroutine if there's any chance that
1794 someone might want to trap whatever error happened. Use C<die> instead,
1795 which can be trapped by an C<eval>.
1797 The exit() function does not always exit immediately. It calls any
1798 defined C<END> routines first, but these C<END> routines may not
1799 themselves abort the exit. Likewise any object destructors that need to
1800 be called are called before the real exit. If this is a problem, you
1801 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1802 See L<perlmod> for details.
1805 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1809 Returns I<e> (the natural logarithm base) to the power of EXPR.
1810 If EXPR is omitted, gives C<exp($_)>.
1812 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1815 Implements the fcntl(2) function. You'll probably have to say
1819 first to get the correct constant definitions. Argument processing and
1820 value return works just like C<ioctl> below.
1824 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1825 or die "can't fcntl F_GETFL: $!";
1827 You don't have to check for C<defined> on the return from C<fcntl>.
1828 Like C<ioctl>, it maps a C<0> return from the system call into
1829 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1830 in numeric context. It is also exempt from the normal B<-w> warnings
1831 on improper numeric conversions.
1833 Note that C<fcntl> will produce a fatal error if used on a machine that
1834 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1835 manpage to learn what functions are available on your system.
1837 Here's an example of setting a filehandle named C<REMOTE> to be
1838 non-blocking at the system level. You'll have to negotiate C<$|>
1839 on your own, though.
1841 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1843 $flags = fcntl(REMOTE, F_GETFL, 0)
1844 or die "Can't get flags for the socket: $!\n";
1846 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1847 or die "Can't set flags for the socket: $!\n";
1849 =item fileno FILEHANDLE
1852 Returns the file descriptor for a filehandle, or undefined if the
1853 filehandle is not open. This is mainly useful for constructing
1854 bitmaps for C<select> and low-level POSIX tty-handling operations.
1855 If FILEHANDLE is an expression, the value is taken as an indirect
1856 filehandle, generally its name.
1858 You can use this to find out whether two handles refer to the
1859 same underlying descriptor:
1861 if (fileno(THIS) == fileno(THAT)) {
1862 print "THIS and THAT are dups\n";
1865 (Filehandles connected to memory objects via new features of C<open> may
1866 return undefined even though they are open.)
1869 =item flock FILEHANDLE,OPERATION
1870 X<flock> X<lock> X<locking>
1872 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1873 for success, false on failure. Produces a fatal error if used on a
1874 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1875 C<flock> is Perl's portable file locking interface, although it locks
1876 only entire files, not records.
1878 Two potentially non-obvious but traditional C<flock> semantics are
1879 that it waits indefinitely until the lock is granted, and that its locks
1880 B<merely advisory>. Such discretionary locks are more flexible, but offer
1881 fewer guarantees. This means that programs that do not also use C<flock>
1882 may modify files locked with C<flock>. See L<perlport>,
1883 your port's specific documentation, or your system-specific local manpages
1884 for details. It's best to assume traditional behavior if you're writing
1885 portable programs. (But if you're not, you should as always feel perfectly
1886 free to write for your own system's idiosyncrasies (sometimes called
1887 "features"). Slavish adherence to portability concerns shouldn't get
1888 in the way of your getting your job done.)
1890 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1891 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1892 you can use the symbolic names if you import them from the Fcntl module,
1893 either individually, or as a group using the ':flock' tag. LOCK_SH
1894 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1895 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1896 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1897 waiting for the lock (check the return status to see if you got it).
1899 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1900 before locking or unlocking it.
1902 Note that the emulation built with lockf(3) doesn't provide shared
1903 locks, and it requires that FILEHANDLE be open with write intent. These
1904 are the semantics that lockf(3) implements. Most if not all systems
1905 implement lockf(3) in terms of fcntl(2) locking, though, so the
1906 differing semantics shouldn't bite too many people.
1908 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1909 be open with read intent to use LOCK_SH and requires that it be open
1910 with write intent to use LOCK_EX.
1912 Note also that some versions of C<flock> cannot lock things over the
1913 network; you would need to use the more system-specific C<fcntl> for
1914 that. If you like you can force Perl to ignore your system's flock(2)
1915 function, and so provide its own fcntl(2)-based emulation, by passing
1916 the switch C<-Ud_flock> to the F<Configure> program when you configure
1919 Here's a mailbox appender for BSD systems.
1921 use Fcntl ':flock'; # import LOCK_* constants
1924 flock(MBOX,LOCK_EX);
1925 # and, in case someone appended
1926 # while we were waiting...
1931 flock(MBOX,LOCK_UN);
1934 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1935 or die "Can't open mailbox: $!";
1938 print MBOX $msg,"\n\n";
1941 On systems that support a real flock(), locks are inherited across fork()
1942 calls, whereas those that must resort to the more capricious fcntl()
1943 function lose the locks, making it harder to write servers.
1945 See also L<DB_File> for other flock() examples.
1948 X<fork> X<child> X<parent>
1950 Does a fork(2) system call to create a new process running the
1951 same program at the same point. It returns the child pid to the
1952 parent process, C<0> to the child process, or C<undef> if the fork is
1953 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1954 are shared, while everything else is copied. On most systems supporting
1955 fork(), great care has gone into making it extremely efficient (for
1956 example, using copy-on-write technology on data pages), making it the
1957 dominant paradigm for multitasking over the last few decades.
1959 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1960 output before forking the child process, but this may not be supported
1961 on some platforms (see L<perlport>). To be safe, you may need to set
1962 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1963 C<IO::Handle> on any open handles in order to avoid duplicate output.
1965 If you C<fork> without ever waiting on your children, you will
1966 accumulate zombies. On some systems, you can avoid this by setting
1967 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1968 forking and reaping moribund children.
1970 Note that if your forked child inherits system file descriptors like
1971 STDIN and STDOUT that are actually connected by a pipe or socket, even
1972 if you exit, then the remote server (such as, say, a CGI script or a
1973 backgrounded job launched from a remote shell) won't think you're done.
1974 You should reopen those to F</dev/null> if it's any issue.
1979 Declare a picture format for use by the C<write> function. For
1983 Test: @<<<<<<<< @||||| @>>>>>
1984 $str, $%, '$' . int($num)
1988 $num = $cost/$quantity;
1992 See L<perlform> for many details and examples.
1994 =item formline PICTURE,LIST
1997 This is an internal function used by C<format>s, though you may call it,
1998 too. It formats (see L<perlform>) a list of values according to the
1999 contents of PICTURE, placing the output into the format output
2000 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2001 Eventually, when a C<write> is done, the contents of
2002 C<$^A> are written to some filehandle. You could also read C<$^A>
2003 and then set C<$^A> back to C<"">. Note that a format typically
2004 does one C<formline> per line of form, but the C<formline> function itself
2005 doesn't care how many newlines are embedded in the PICTURE. This means
2006 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2007 You may therefore need to use multiple formlines to implement a single
2008 record format, just like the format compiler.
2010 Be careful if you put double quotes around the picture, because an C<@>
2011 character may be taken to mean the beginning of an array name.
2012 C<formline> always returns true. See L<perlform> for other examples.
2014 =item getc FILEHANDLE
2015 X<getc> X<getchar> X<character> X<file, read>
2019 Returns the next character from the input file attached to FILEHANDLE,
2020 or the undefined value at end of file, or if there was an error (in
2021 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2022 STDIN. This is not particularly efficient. However, it cannot be
2023 used by itself to fetch single characters without waiting for the user
2024 to hit enter. For that, try something more like:
2027 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2030 system "stty", '-icanon', 'eol', "\001";
2036 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2039 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2043 Determination of whether $BSD_STYLE should be set
2044 is left as an exercise to the reader.
2046 The C<POSIX::getattr> function can do this more portably on
2047 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2048 module from your nearest CPAN site; details on CPAN can be found on
2052 X<getlogin> X<login>
2054 This implements the C library function of the same name, which on most
2055 systems returns the current login from F</etc/utmp>, if any. If null,
2058 $login = getlogin || getpwuid($<) || "Kilroy";
2060 Do not consider C<getlogin> for authentication: it is not as
2061 secure as C<getpwuid>.
2063 =item getpeername SOCKET
2064 X<getpeername> X<peer>
2066 Returns the packed sockaddr address of other end of the SOCKET connection.
2069 $hersockaddr = getpeername(SOCK);
2070 ($port, $iaddr) = sockaddr_in($hersockaddr);
2071 $herhostname = gethostbyaddr($iaddr, AF_INET);
2072 $herstraddr = inet_ntoa($iaddr);
2077 Returns the current process group for the specified PID. Use
2078 a PID of C<0> to get the current process group for the
2079 current process. Will raise an exception if used on a machine that
2080 doesn't implement getpgrp(2). If PID is omitted, returns process
2081 group of current process. Note that the POSIX version of C<getpgrp>
2082 does not accept a PID argument, so only C<PID==0> is truly portable.
2085 X<getppid> X<parent> X<pid>
2087 Returns the process id of the parent process.
2089 Note for Linux users: on Linux, the C functions C<getpid()> and
2090 C<getppid()> return different values from different threads. In order to
2091 be portable, this behavior is not reflected by the perl-level function
2092 C<getppid()>, that returns a consistent value across threads. If you want
2093 to call the underlying C<getppid()>, you may use the CPAN module
2096 =item getpriority WHICH,WHO
2097 X<getpriority> X<priority> X<nice>
2099 Returns the current priority for a process, a process group, or a user.
2100 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2101 machine that doesn't implement getpriority(2).
2104 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2105 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2106 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2107 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2108 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2109 X<endnetent> X<endprotoent> X<endservent>
2113 =item gethostbyname NAME
2115 =item getnetbyname NAME
2117 =item getprotobyname NAME
2123 =item getservbyname NAME,PROTO
2125 =item gethostbyaddr ADDR,ADDRTYPE
2127 =item getnetbyaddr ADDR,ADDRTYPE
2129 =item getprotobynumber NUMBER
2131 =item getservbyport PORT,PROTO
2149 =item sethostent STAYOPEN
2151 =item setnetent STAYOPEN
2153 =item setprotoent STAYOPEN
2155 =item setservent STAYOPEN
2169 These routines perform the same functions as their counterparts in the
2170 system library. In list context, the return values from the
2171 various get routines are as follows:
2173 ($name,$passwd,$uid,$gid,
2174 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2175 ($name,$passwd,$gid,$members) = getgr*
2176 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2177 ($name,$aliases,$addrtype,$net) = getnet*
2178 ($name,$aliases,$proto) = getproto*
2179 ($name,$aliases,$port,$proto) = getserv*
2181 (If the entry doesn't exist you get a null list.)
2183 The exact meaning of the $gcos field varies but it usually contains
2184 the real name of the user (as opposed to the login name) and other
2185 information pertaining to the user. Beware, however, that in many
2186 system users are able to change this information and therefore it
2187 cannot be trusted and therefore the $gcos is tainted (see
2188 L<perlsec>). The $passwd and $shell, user's encrypted password and
2189 login shell, are also tainted, because of the same reason.
2191 In scalar context, you get the name, unless the function was a
2192 lookup by name, in which case you get the other thing, whatever it is.
2193 (If the entry doesn't exist you get the undefined value.) For example:
2195 $uid = getpwnam($name);
2196 $name = getpwuid($num);
2198 $gid = getgrnam($name);
2199 $name = getgrgid($num);
2203 In I<getpw*()> the fields $quota, $comment, and $expire are special
2204 cases in the sense that in many systems they are unsupported. If the
2205 $quota is unsupported, it is an empty scalar. If it is supported, it
2206 usually encodes the disk quota. If the $comment field is unsupported,
2207 it is an empty scalar. If it is supported it usually encodes some
2208 administrative comment about the user. In some systems the $quota
2209 field may be $change or $age, fields that have to do with password
2210 aging. In some systems the $comment field may be $class. The $expire
2211 field, if present, encodes the expiration period of the account or the
2212 password. For the availability and the exact meaning of these fields
2213 in your system, please consult your getpwnam(3) documentation and your
2214 F<pwd.h> file. You can also find out from within Perl what your
2215 $quota and $comment fields mean and whether you have the $expire field
2216 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2217 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2218 files are only supported if your vendor has implemented them in the
2219 intuitive fashion that calling the regular C library routines gets the
2220 shadow versions if you're running under privilege or if there exists
2221 the shadow(3) functions as found in System V (this includes Solaris
2222 and Linux.) Those systems that implement a proprietary shadow password
2223 facility are unlikely to be supported.
2225 The $members value returned by I<getgr*()> is a space separated list of
2226 the login names of the members of the group.
2228 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2229 C, it will be returned to you via C<$?> if the function call fails. The
2230 C<@addrs> value returned by a successful call is a list of the raw
2231 addresses returned by the corresponding system library call. In the
2232 Internet domain, each address is four bytes long and you can unpack it
2233 by saying something like:
2235 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2237 The Socket library makes this slightly easier:
2240 $iaddr = inet_aton("127.1"); # or whatever address
2241 $name = gethostbyaddr($iaddr, AF_INET);
2243 # or going the other way
2244 $straddr = inet_ntoa($iaddr);
2246 If you get tired of remembering which element of the return list
2247 contains which return value, by-name interfaces are provided
2248 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2249 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2250 and C<User::grent>. These override the normal built-ins, supplying
2251 versions that return objects with the appropriate names
2252 for each field. For example:
2256 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2258 Even though it looks like they're the same method calls (uid),
2259 they aren't, because a C<File::stat> object is different from
2260 a C<User::pwent> object.
2262 =item getsockname SOCKET
2265 Returns the packed sockaddr address of this end of the SOCKET connection,
2266 in case you don't know the address because you have several different
2267 IPs that the connection might have come in on.
2270 $mysockaddr = getsockname(SOCK);
2271 ($port, $myaddr) = sockaddr_in($mysockaddr);
2272 printf "Connect to %s [%s]\n",
2273 scalar gethostbyaddr($myaddr, AF_INET),
2276 =item getsockopt SOCKET,LEVEL,OPTNAME
2279 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2280 Options may exist at multiple protocol levels depending on the socket
2281 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2282 C<Socket> module) will exist. To query options at another level the
2283 protocol number of the appropriate protocol controlling the option
2284 should be supplied. For example, to indicate that an option is to be
2285 interpreted by the TCP protocol, LEVEL should be set to the protocol
2286 number of TCP, which you can get using getprotobyname.
2288 The call returns a packed string representing the requested socket option,
2289 or C<undef> if there is an error (the error reason will be in $!). What
2290 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2291 your system documentation for details. A very common case however is that
2292 the option is an integer, in which case the result will be a packed
2293 integer which you can decode using unpack with the C<i> (or C<I>) format.
2295 An example testing if Nagle's algorithm is turned on on a socket:
2297 use Socket qw(:all);
2299 defined(my $tcp = getprotobyname("tcp"))
2300 or die "Could not determine the protocol number for tcp";
2301 # my $tcp = IPPROTO_TCP; # Alternative
2302 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2303 or die "Could not query TCP_NODELAY socket option: $!";
2304 my $nodelay = unpack("I", $packed);
2305 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2309 X<glob> X<wildcard> X<filename, expansion> X<expand>
2313 In list context, returns a (possibly empty) list of filename expansions on
2314 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2315 scalar context, glob iterates through such filename expansions, returning
2316 undef when the list is exhausted. This is the internal function
2317 implementing the C<< <*.c> >> operator, but you can use it directly. If
2318 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2319 more detail in L<perlop/"I/O Operators">.
2321 Beginning with v5.6.0, this operator is implemented using the standard
2322 C<File::Glob> extension. See L<File::Glob> for details.
2325 X<gmtime> X<UTC> X<Greenwich>
2329 Works just like L<localtime> but the returned values are
2330 localized for the standard Greenwich time zone.
2332 Note: when called in list context, $isdst, the last value
2333 returned by gmtime is always C<0>. There is no
2334 Daylight Saving Time in GMT.
2336 See L<perlport/gmtime> for portability concerns.
2339 X<goto> X<jump> X<jmp>
2345 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2346 execution there. It may not be used to go into any construct that
2347 requires initialization, such as a subroutine or a C<foreach> loop. It
2348 also can't be used to go into a construct that is optimized away,
2349 or to get out of a block or subroutine given to C<sort>.
2350 It can be used to go almost anywhere else within the dynamic scope,
2351 including out of subroutines, but it's usually better to use some other
2352 construct such as C<last> or C<die>. The author of Perl has never felt the
2353 need to use this form of C<goto> (in Perl, that is--C is another matter).
2354 (The difference being that C does not offer named loops combined with
2355 loop control. Perl does, and this replaces most structured uses of C<goto>
2356 in other languages.)
2358 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2359 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2360 necessarily recommended if you're optimizing for maintainability:
2362 goto ("FOO", "BAR", "GLARCH")[$i];
2364 The C<goto-&NAME> form is quite different from the other forms of
2365 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2366 doesn't have the stigma associated with other gotos. Instead, it
2367 exits the current subroutine (losing any changes set by local()) and
2368 immediately calls in its place the named subroutine using the current
2369 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2370 load another subroutine and then pretend that the other subroutine had
2371 been called in the first place (except that any modifications to C<@_>
2372 in the current subroutine are propagated to the other subroutine.)
2373 After the C<goto>, not even C<caller> will be able to tell that this
2374 routine was called first.
2376 NAME needn't be the name of a subroutine; it can be a scalar variable
2377 containing a code reference, or a block that evaluates to a code
2380 =item grep BLOCK LIST
2383 =item grep EXPR,LIST
2385 This is similar in spirit to, but not the same as, grep(1) and its
2386 relatives. In particular, it is not limited to using regular expressions.
2388 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2389 C<$_> to each element) and returns the list value consisting of those
2390 elements for which the expression evaluated to true. In scalar
2391 context, returns the number of times the expression was true.
2393 @foo = grep(!/^#/, @bar); # weed out comments
2397 @foo = grep {!/^#/} @bar; # weed out comments
2399 Note that C<$_> is an alias to the list value, so it can be used to
2400 modify the elements of the LIST. While this is useful and supported,
2401 it can cause bizarre results if the elements of LIST are not variables.
2402 Similarly, grep returns aliases into the original list, much as a for
2403 loop's index variable aliases the list elements. That is, modifying an
2404 element of a list returned by grep (for example, in a C<foreach>, C<map>
2405 or another C<grep>) actually modifies the element in the original list.
2406 This is usually something to be avoided when writing clear code.
2408 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2409 been declared with C<my $_>) then, in addition to being locally aliased to
2410 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2411 can't be seen from the outside, avoiding any potential side-effects.
2413 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2416 X<hex> X<hexadecimal>
2420 Interprets EXPR as a hex string and returns the corresponding value.
2421 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2422 L</oct>.) If EXPR is omitted, uses C<$_>.
2424 print hex '0xAf'; # prints '175'
2425 print hex 'aF'; # same
2427 Hex strings may only represent integers. Strings that would cause
2428 integer overflow trigger a warning. Leading whitespace is not stripped,
2429 unlike oct(). To present something as hex, look into L</printf>,
2430 L</sprintf>, or L</unpack>.
2435 There is no builtin C<import> function. It is just an ordinary
2436 method (subroutine) defined (or inherited) by modules that wish to export
2437 names to another module. The C<use> function calls the C<import> method
2438 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2440 =item index STR,SUBSTR,POSITION
2441 X<index> X<indexOf> X<InStr>
2443 =item index STR,SUBSTR
2445 The index function searches for one string within another, but without
2446 the wildcard-like behavior of a full regular-expression pattern match.
2447 It returns the position of the first occurrence of SUBSTR in STR at
2448 or after POSITION. If POSITION is omitted, starts searching from the
2449 beginning of the string. POSITION before the beginning of the string
2450 or after its end is treated as if it were the beginning or the end,
2451 respectively. POSITION and the return value are based at C<0> (or whatever
2452 you've set the C<$[> variable to--but don't do that). If the substring
2453 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2456 X<int> X<integer> X<truncate> X<trunc> X<floor>
2460 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2461 You should not use this function for rounding: one because it truncates
2462 towards C<0>, and two because machine representations of floating point
2463 numbers can sometimes produce counterintuitive results. For example,
2464 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2465 because it's really more like -268.99999999999994315658 instead. Usually,
2466 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2467 functions will serve you better than will int().
2469 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2472 Implements the ioctl(2) function. You'll probably first have to say
2474 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2476 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2477 exist or doesn't have the correct definitions you'll have to roll your
2478 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2479 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2480 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2481 written depending on the FUNCTION--a pointer to the string value of SCALAR
2482 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2483 has no string value but does have a numeric value, that value will be
2484 passed rather than a pointer to the string value. To guarantee this to be
2485 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2486 functions may be needed to manipulate the values of structures used by
2489 The return value of C<ioctl> (and C<fcntl>) is as follows:
2491 if OS returns: then Perl returns:
2493 0 string "0 but true"
2494 anything else that number
2496 Thus Perl returns true on success and false on failure, yet you can
2497 still easily determine the actual value returned by the operating
2500 $retval = ioctl(...) || -1;
2501 printf "System returned %d\n", $retval;
2503 The special string C<"0 but true"> is exempt from B<-w> complaints
2504 about improper numeric conversions.
2506 =item join EXPR,LIST
2509 Joins the separate strings of LIST into a single string with fields
2510 separated by the value of EXPR, and returns that new string. Example:
2512 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2514 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2515 first argument. Compare L</split>.
2520 Returns a list consisting of all the keys of the named hash.
2521 (In scalar context, returns the number of keys.)
2523 The keys are returned in an apparently random order. The actual
2524 random order is subject to change in future versions of perl, but it
2525 is guaranteed to be the same order as either the C<values> or C<each>
2526 function produces (given that the hash has not been modified). Since
2527 Perl 5.8.1 the ordering is different even between different runs of
2528 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2531 As a side effect, calling keys() resets the HASH's internal iterator
2532 (see L</each>). In particular, calling keys() in void context resets
2533 the iterator with no other overhead.
2535 Here is yet another way to print your environment:
2538 @values = values %ENV;
2540 print pop(@keys), '=', pop(@values), "\n";
2543 or how about sorted by key:
2545 foreach $key (sort(keys %ENV)) {
2546 print $key, '=', $ENV{$key}, "\n";
2549 The returned values are copies of the original keys in the hash, so
2550 modifying them will not affect the original hash. Compare L</values>.
2552 To sort a hash by value, you'll need to use a C<sort> function.
2553 Here's a descending numeric sort of a hash by its values:
2555 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2556 printf "%4d %s\n", $hash{$key}, $key;
2559 As an lvalue C<keys> allows you to increase the number of hash buckets
2560 allocated for the given hash. This can gain you a measure of efficiency if
2561 you know the hash is going to get big. (This is similar to pre-extending
2562 an array by assigning a larger number to $#array.) If you say
2566 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2567 in fact, since it rounds up to the next power of two. These
2568 buckets will be retained even if you do C<%hash = ()>, use C<undef
2569 %hash> if you want to free the storage while C<%hash> is still in scope.
2570 You can't shrink the number of buckets allocated for the hash using
2571 C<keys> in this way (but you needn't worry about doing this by accident,
2572 as trying has no effect).
2574 See also C<each>, C<values> and C<sort>.
2576 =item kill SIGNAL, LIST
2579 Sends a signal to a list of processes. Returns the number of
2580 processes successfully signaled (which is not necessarily the
2581 same as the number actually killed).
2583 $cnt = kill 1, $child1, $child2;
2586 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2587 system call will check whether it's possible to send a signal to it (that
2588 means, to be brief, that the process is owned by the same user, or we are
2589 the super-user). This is a useful way to check that a child process is
2590 alive (even if only as a zombie) and hasn't changed its UID. See
2591 L<perlport> for notes on the portability of this construct.
2593 Unlike in the shell, if SIGNAL is negative, it kills
2594 process groups instead of processes. (On System V, a negative I<PROCESS>
2595 number will also kill process groups, but that's not portable.) That
2596 means you usually want to use positive not negative signals. You may also
2597 use a signal name in quotes.
2599 See L<perlipc/"Signals"> for more details.
2606 The C<last> command is like the C<break> statement in C (as used in
2607 loops); it immediately exits the loop in question. If the LABEL is
2608 omitted, the command refers to the innermost enclosing loop. The
2609 C<continue> block, if any, is not executed:
2611 LINE: while (<STDIN>) {
2612 last LINE if /^$/; # exit when done with header
2616 C<last> cannot be used to exit a block which returns a value such as
2617 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2618 a grep() or map() operation.
2620 Note that a block by itself is semantically identical to a loop
2621 that executes once. Thus C<last> can be used to effect an early
2622 exit out of such a block.
2624 See also L</continue> for an illustration of how C<last>, C<next>, and
2632 Returns a lowercased version of EXPR. This is the internal function
2633 implementing the C<\L> escape in double-quoted strings. Respects
2634 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2635 and L<perlunicode> for more details about locale and Unicode support.
2637 If EXPR is omitted, uses C<$_>.
2640 X<lcfirst> X<lowercase>
2644 Returns the value of EXPR with the first character lowercased. This
2645 is the internal function implementing the C<\l> escape in
2646 double-quoted strings. Respects current LC_CTYPE locale if C<use
2647 locale> in force. See L<perllocale> and L<perlunicode> for more
2648 details about locale and Unicode support.
2650 If EXPR is omitted, uses C<$_>.
2657 Returns the length in I<characters> of the value of EXPR. If EXPR is
2658 omitted, returns length of C<$_>. Note that this cannot be used on
2659 an entire array or hash to find out how many elements these have.
2660 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2662 Note the I<characters>: if the EXPR is in Unicode, you will get the
2663 number of characters, not the number of bytes. To get the length
2664 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2666 =item link OLDFILE,NEWFILE
2669 Creates a new filename linked to the old filename. Returns true for
2670 success, false otherwise.
2672 =item listen SOCKET,QUEUESIZE
2675 Does the same thing that the listen system call does. Returns true if
2676 it succeeded, false otherwise. See the example in
2677 L<perlipc/"Sockets: Client/Server Communication">.
2682 You really probably want to be using C<my> instead, because C<local> isn't
2683 what most people think of as "local". See
2684 L<perlsub/"Private Variables via my()"> for details.
2686 A local modifies the listed variables to be local to the enclosing
2687 block, file, or eval. If more than one value is listed, the list must
2688 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2689 for details, including issues with tied arrays and hashes.
2691 =item localtime EXPR
2692 X<localtime> X<ctime>
2696 Converts a time as returned by the time function to a 9-element list
2697 with the time analyzed for the local time zone. Typically used as
2701 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2704 All list elements are numeric, and come straight out of the C `struct
2705 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2706 of the specified time.
2708 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2709 the range C<0..11> with 0 indicating January and 11 indicating December.
2710 This makes it easy to get a month name from a list:
2712 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2713 print "$abbr[$mon] $mday";
2714 # $mon=9, $mday=18 gives "Oct 18"
2716 C<$year> is the number of years since 1900, not just the last two digits
2717 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2718 to get a complete 4-digit year is simply:
2722 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2723 to do that, would you?
2725 To get the last two digits of the year (e.g., '01' in 2001) do:
2727 $year = sprintf("%02d", $year % 100);
2729 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2730 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2731 (or C<0..365> in leap years.)
2733 C<$isdst> is true if the specified time occurs during Daylight Saving
2734 Time, false otherwise.
2736 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2738 In scalar context, C<localtime()> returns the ctime(3) value:
2740 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2742 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2743 instead of local time use the L</gmtime> builtin. See also the
2744 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2745 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2746 and mktime(3) functions.
2748 To get somewhat similar but locale dependent date strings, set up your
2749 locale environment variables appropriately (please see L<perllocale>) and
2752 use POSIX qw(strftime);
2753 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2754 # or for GMT formatted appropriately for your locale:
2755 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2757 Note that the C<%a> and C<%b>, the short forms of the day of the week
2758 and the month of the year, may not necessarily be three characters wide.
2760 See L<perlport/localtime> for portability concerns.
2762 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2763 by-name access mechanism to the gmtime() and localtime() functions,
2766 For a comprehensive date and time representation look at the
2767 L<DateTime> module on CPAN.
2772 This function places an advisory lock on a shared variable, or referenced
2773 object contained in I<THING> until the lock goes out of scope.
2775 lock() is a "weak keyword" : this means that if you've defined a function
2776 by this name (before any calls to it), that function will be called
2777 instead. (However, if you've said C<use threads>, lock() is always a
2778 keyword.) See L<threads>.
2781 X<log> X<logarithm> X<e> X<ln> X<base>
2785 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2786 returns log of C<$_>. To get the log of another base, use basic algebra:
2787 The base-N log of a number is equal to the natural log of that number
2788 divided by the natural log of N. For example:
2792 return log($n)/log(10);
2795 See also L</exp> for the inverse operation.
2802 Does the same thing as the C<stat> function (including setting the
2803 special C<_> filehandle) but stats a symbolic link instead of the file
2804 the symbolic link points to. If symbolic links are unimplemented on
2805 your system, a normal C<stat> is done. For much more detailed
2806 information, please see the documentation for C<stat>.
2808 If EXPR is omitted, stats C<$_>.
2812 The match operator. See L<perlop>.
2814 =item map BLOCK LIST
2819 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2820 C<$_> to each element) and returns the list value composed of the
2821 results of each such evaluation. In scalar context, returns the
2822 total number of elements so generated. Evaluates BLOCK or EXPR in
2823 list context, so each element of LIST may produce zero, one, or
2824 more elements in the returned value.
2826 @chars = map(chr, @nums);
2828 translates a list of numbers to the corresponding characters. And
2830 %hash = map { get_a_key_for($_) => $_ } @array;
2832 is just a funny way to write
2836 $hash{get_a_key_for($_)} = $_;
2839 Note that C<$_> is an alias to the list value, so it can be used to
2840 modify the elements of the LIST. While this is useful and supported,
2841 it can cause bizarre results if the elements of LIST are not variables.
2842 Using a regular C<foreach> loop for this purpose would be clearer in
2843 most cases. See also L</grep> for an array composed of those items of
2844 the original list for which the BLOCK or EXPR evaluates to true.
2846 If C<$_> is lexical in the scope where the C<map> appears (because it has
2847 been declared with C<my $_>), then, in addition to being locally aliased to
2848 the list elements, C<$_> keeps being lexical inside the block; that is, it
2849 can't be seen from the outside, avoiding any potential side-effects.
2851 C<{> starts both hash references and blocks, so C<map { ...> could be either
2852 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2853 ahead for the closing C<}> it has to take a guess at which its dealing with
2854 based what it finds just after the C<{>. Usually it gets it right, but if it
2855 doesn't it won't realize something is wrong until it gets to the C<}> and
2856 encounters the missing (or unexpected) comma. The syntax error will be
2857 reported close to the C<}> but you'll need to change something near the C<{>
2858 such as using a unary C<+> to give perl some help:
2860 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2861 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2862 %hash = map { ("\L$_", 1) } @array # this also works
2863 %hash = map { lc($_), 1 } @array # as does this.
2864 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2866 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2868 or to force an anon hash constructor use C<+{>:
2870 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2872 and you get list of anonymous hashes each with only 1 entry.
2874 =item mkdir FILENAME,MASK
2875 X<mkdir> X<md> X<directory, create>
2877 =item mkdir FILENAME
2881 Creates the directory specified by FILENAME, with permissions
2882 specified by MASK (as modified by C<umask>). If it succeeds it
2883 returns true, otherwise it returns false and sets C<$!> (errno).
2884 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2887 In general, it is better to create directories with permissive MASK,
2888 and let the user modify that with their C<umask>, than it is to supply
2889 a restrictive MASK and give the user no way to be more permissive.
2890 The exceptions to this rule are when the file or directory should be
2891 kept private (mail files, for instance). The perlfunc(1) entry on
2892 C<umask> discusses the choice of MASK in more detail.
2894 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2895 number of trailing slashes. Some operating and filesystems do not get
2896 this right, so Perl automatically removes all trailing slashes to keep
2899 In order to recursively create a directory structure look at
2900 the C<mkpath> function of the L<File::Path> module.
2902 =item msgctl ID,CMD,ARG
2905 Calls the System V IPC function msgctl(2). You'll probably have to say
2909 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2910 then ARG must be a variable that will hold the returned C<msqid_ds>
2911 structure. Returns like C<ioctl>: the undefined value for error,
2912 C<"0 but true"> for zero, or the actual return value otherwise. See also
2913 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2915 =item msgget KEY,FLAGS
2918 Calls the System V IPC function msgget(2). Returns the message queue
2919 id, or the undefined value if there is an error. See also
2920 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2922 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2925 Calls the System V IPC function msgrcv to receive a message from
2926 message queue ID into variable VAR with a maximum message size of
2927 SIZE. Note that when a message is received, the message type as a
2928 native long integer will be the first thing in VAR, followed by the
2929 actual message. This packing may be opened with C<unpack("l! a*")>.
2930 Taints the variable. Returns true if successful, or false if there is
2931 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2932 C<IPC::SysV::Msg> documentation.
2934 =item msgsnd ID,MSG,FLAGS
2937 Calls the System V IPC function msgsnd to send the message MSG to the
2938 message queue ID. MSG must begin with the native long integer message
2939 type, and be followed by the length of the actual message, and finally
2940 the message itself. This kind of packing can be achieved with
2941 C<pack("l! a*", $type, $message)>. Returns true if successful,
2942 or false if there is an error. See also C<IPC::SysV>
2943 and C<IPC::SysV::Msg> documentation.
2950 =item my EXPR : ATTRS
2952 =item my TYPE EXPR : ATTRS
2954 A C<my> declares the listed variables to be local (lexically) to the
2955 enclosing block, file, or C<eval>. If more than one value is listed,
2956 the list must be placed in parentheses.
2958 The exact semantics and interface of TYPE and ATTRS are still
2959 evolving. TYPE is currently bound to the use of C<fields> pragma,
2960 and attributes are handled using the C<attributes> pragma, or starting
2961 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2962 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2963 L<attributes>, and L<Attribute::Handlers>.
2970 The C<next> command is like the C<continue> statement in C; it starts
2971 the next iteration of the loop:
2973 LINE: while (<STDIN>) {
2974 next LINE if /^#/; # discard comments
2978 Note that if there were a C<continue> block on the above, it would get
2979 executed even on discarded lines. If the LABEL is omitted, the command
2980 refers to the innermost enclosing loop.
2982 C<next> cannot be used to exit a block which returns a value such as
2983 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2984 a grep() or map() operation.
2986 Note that a block by itself is semantically identical to a loop
2987 that executes once. Thus C<next> will exit such a block early.
2989 See also L</continue> for an illustration of how C<last>, C<next>, and
2992 =item no Module VERSION LIST
2995 =item no Module VERSION
2997 =item no Module LIST
3001 See the C<use> function, of which C<no> is the opposite.
3004 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3008 Interprets EXPR as an octal string and returns the corresponding
3009 value. (If EXPR happens to start off with C<0x>, interprets it as a
3010 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3011 binary string. Leading whitespace is ignored in all three cases.)
3012 The following will handle decimal, binary, octal, and hex in the standard
3015 $val = oct($val) if $val =~ /^0/;
3017 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3018 in octal), use sprintf() or printf():
3020 $perms = (stat("filename"))[2] & 07777;
3021 $oct_perms = sprintf "%lo", $perms;
3023 The oct() function is commonly used when a string such as C<644> needs
3024 to be converted into a file mode, for example. (Although perl will
3025 automatically convert strings into numbers as needed, this automatic
3026 conversion assumes base 10.)
3028 =item open FILEHANDLE,EXPR
3029 X<open> X<pipe> X<file, open> X<fopen>
3031 =item open FILEHANDLE,MODE,EXPR
3033 =item open FILEHANDLE,MODE,EXPR,LIST
3035 =item open FILEHANDLE,MODE,REFERENCE
3037 =item open FILEHANDLE
3039 Opens the file whose filename is given by EXPR, and associates it with
3042 (The following is a comprehensive reference to open(): for a gentler
3043 introduction you may consider L<perlopentut>.)
3045 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3046 the variable is assigned a reference to a new anonymous filehandle,
3047 otherwise if FILEHANDLE is an expression, its value is used as the name of
3048 the real filehandle wanted. (This is considered a symbolic reference, so
3049 C<use strict 'refs'> should I<not> be in effect.)
3051 If EXPR is omitted, the scalar variable of the same name as the
3052 FILEHANDLE contains the filename. (Note that lexical variables--those
3053 declared with C<my>--will not work for this purpose; so if you're
3054 using C<my>, specify EXPR in your call to open.)
3056 If three or more arguments are specified then the mode of opening and
3057 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3058 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3059 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3060 the file is opened for appending, again being created if necessary.
3062 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3063 indicate that you want both read and write access to the file; thus
3064 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3065 '+>' >> mode would clobber the file first. You can't usually use
3066 either read-write mode for updating textfiles, since they have
3067 variable length records. See the B<-i> switch in L<perlrun> for a
3068 better approach. The file is created with permissions of C<0666>
3069 modified by the process' C<umask> value.
3071 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3072 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3074 In the 2-arguments (and 1-argument) form of the call the mode and
3075 filename should be concatenated (in this order), possibly separated by
3076 spaces. It is possible to omit the mode in these forms if the mode is
3079 If the filename begins with C<'|'>, the filename is interpreted as a
3080 command to which output is to be piped, and if the filename ends with a
3081 C<'|'>, the filename is interpreted as a command which pipes output to
3082 us. See L<perlipc/"Using open() for IPC">
3083 for more examples of this. (You are not allowed to C<open> to a command
3084 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3085 and L<perlipc/"Bidirectional Communication with Another Process">
3088 For three or more arguments if MODE is C<'|-'>, the filename is
3089 interpreted as a command to which output is to be piped, and if MODE
3090 is C<'-|'>, the filename is interpreted as a command which pipes
3091 output to us. In the 2-arguments (and 1-argument) form one should
3092 replace dash (C<'-'>) with the command.
3093 See L<perlipc/"Using open() for IPC"> for more examples of this.
3094 (You are not allowed to C<open> to a command that pipes both in I<and>
3095 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3096 L<perlipc/"Bidirectional Communication"> for alternatives.)
3098 In the three-or-more argument form of pipe opens, if LIST is specified
3099 (extra arguments after the command name) then LIST becomes arguments
3100 to the command invoked if the platform supports it. The meaning of
3101 C<open> with more than three arguments for non-pipe modes is not yet
3102 specified. Experimental "layers" may give extra LIST arguments
3105 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3106 and opening C<< '>-' >> opens STDOUT.
3108 You may use the three-argument form of open to specify IO "layers"
3109 (sometimes also referred to as "disciplines") to be applied to the handle
3110 that affect how the input and output are processed (see L<open> and
3111 L<PerlIO> for more details). For example
3113 open(FH, "<:utf8", "file")
3115 will open the UTF-8 encoded file containing Unicode characters,
3116 see L<perluniintro>. Note that if layers are specified in the
3117 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3118 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3120 Open returns nonzero upon success, the undefined value otherwise. If
3121 the C<open> involved a pipe, the return value happens to be the pid of
3124 If you're running Perl on a system that distinguishes between text
3125 files and binary files, then you should check out L</binmode> for tips
3126 for dealing with this. The key distinction between systems that need
3127 C<binmode> and those that don't is their text file formats. Systems
3128 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3129 character, and which encode that character in C as C<"\n">, do not
3130 need C<binmode>. The rest need it.
3132 When opening a file, it's usually a bad idea to continue normal execution
3133 if the request failed, so C<open> is frequently used in connection with
3134 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3135 where you want to make a nicely formatted error message (but there are
3136 modules that can help with that problem)) you should always check
3137 the return value from opening a file. The infrequent exception is when
3138 working with an unopened filehandle is actually what you want to do.
3140 As a special case the 3-arg form with a read/write mode and the third
3141 argument being C<undef>:
3143 open(TMP, "+>", undef) or die ...
3145 opens a filehandle to an anonymous temporary file. Also using "+<"
3146 works for symmetry, but you really should consider writing something
3147 to the temporary file first. You will need to seek() to do the
3150 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3151 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3152 "in memory" files held in Perl scalars via:
3154 open($fh, '>', \$variable) || ..
3156 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3157 file, you have to close it first:
3160 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3165 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3166 while (<ARTICLE>) {...
3168 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3169 # if the open fails, output is discarded
3171 open(DBASE, '+<', 'dbase.mine') # open for update
3172 or die "Can't open 'dbase.mine' for update: $!";
3174 open(DBASE, '+<dbase.mine') # ditto
3175 or die "Can't open 'dbase.mine' for update: $!";
3177 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3178 or die "Can't start caesar: $!";
3180 open(ARTICLE, "caesar <$article |") # ditto
3181 or die "Can't start caesar: $!";
3183 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3184 or die "Can't start sort: $!";
3187 open(MEMORY,'>', \$var)
3188 or die "Can't open memory file: $!";
3189 print MEMORY "foo!\n"; # output will end up in $var
3191 # process argument list of files along with any includes
3193 foreach $file (@ARGV) {
3194 process($file, 'fh00');
3198 my($filename, $input) = @_;
3199 $input++; # this is a string increment
3200 unless (open($input, $filename)) {
3201 print STDERR "Can't open $filename: $!\n";
3206 while (<$input>) { # note use of indirection
3207 if (/^#include "(.*)"/) {
3208 process($1, $input);
3215 See L<perliol> for detailed info on PerlIO.
3217 You may also, in the Bourne shell tradition, specify an EXPR beginning
3218 with C<< '>&' >>, in which case the rest of the string is interpreted
3219 as the name of a filehandle (or file descriptor, if numeric) to be
3220 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3221 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3222 The mode you specify should match the mode of the original filehandle.
3223 (Duping a filehandle does not take into account any existing contents
3224 of IO buffers.) If you use the 3-arg form then you can pass either a
3225 number, the name of a filehandle or the normal "reference to a glob".
3227 Here is a script that saves, redirects, and restores C<STDOUT> and
3228 C<STDERR> using various methods:
3231 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3232 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3234 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3235 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3237 select STDERR; $| = 1; # make unbuffered
3238 select STDOUT; $| = 1; # make unbuffered
3240 print STDOUT "stdout 1\n"; # this works for
3241 print STDERR "stderr 1\n"; # subprocesses too
3243 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3244 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3246 print STDOUT "stdout 2\n";
3247 print STDERR "stderr 2\n";
3249 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3250 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3251 that file descriptor (and not call L<dup(2)>); this is more
3252 parsimonious of file descriptors. For example:
3254 # open for input, reusing the fileno of $fd
3255 open(FILEHANDLE, "<&=$fd")
3259 open(FILEHANDLE, "<&=", $fd)
3263 # open for append, using the fileno of OLDFH
3264 open(FH, ">>&=", OLDFH)
3268 open(FH, ">>&=OLDFH")
3270 Being parsimonious on filehandles is also useful (besides being
3271 parsimonious) for example when something is dependent on file
3272 descriptors, like for example locking using flock(). If you do just
3273 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3274 descriptor as B, and therefore flock(A) will not flock(B), and vice
3275 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3276 the same file descriptor.
3278 Note that if you are using Perls older than 5.8.0, Perl will be using
3279 the standard C libraries' fdopen() to implement the "=" functionality.
3280 On many UNIX systems fdopen() fails when file descriptors exceed a
3281 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3282 most often the default.
3284 You can see whether Perl has been compiled with PerlIO or not by
3285 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3286 is C<define>, you have PerlIO, otherwise you don't.
3288 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3289 with 2-arguments (or 1-argument) form of open(), then
3290 there is an implicit fork done, and the return value of open is the pid
3291 of the child within the parent process, and C<0> within the child
3292 process. (Use C<defined($pid)> to determine whether the open was successful.)
3293 The filehandle behaves normally for the parent, but i/o to that
3294 filehandle is piped from/to the STDOUT/STDIN of the child process.
3295 In the child process the filehandle isn't opened--i/o happens from/to
3296 the new STDOUT or STDIN. Typically this is used like the normal
3297 piped open when you want to exercise more control over just how the
3298 pipe command gets executed, such as when you are running setuid, and
3299 don't want to have to scan shell commands for metacharacters.
3300 The following triples are more or less equivalent:
3302 open(FOO, "|tr '[a-z]' '[A-Z]'");
3303 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3304 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3305 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3307 open(FOO, "cat -n '$file'|");
3308 open(FOO, '-|', "cat -n '$file'");
3309 open(FOO, '-|') || exec 'cat', '-n', $file;
3310 open(FOO, '-|', "cat", '-n', $file);
3312 The last example in each block shows the pipe as "list form", which is
3313 not yet supported on all platforms. A good rule of thumb is that if
3314 your platform has true C<fork()> (in other words, if your platform is
3315 UNIX) you can use the list form.
3317 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3319 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3320 output before any operation that may do a fork, but this may not be
3321 supported on some platforms (see L<perlport>). To be safe, you may need
3322 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3323 of C<IO::Handle> on any open handles.
3325 On systems that support a close-on-exec flag on files, the flag will
3326 be set for the newly opened file descriptor as determined by the value
3327 of $^F. See L<perlvar/$^F>.
3329 Closing any piped filehandle causes the parent process to wait for the
3330 child to finish, and returns the status value in C<$?> and
3331 C<${^CHILD_ERROR_NATIVE}>.
3333 The filename passed to 2-argument (or 1-argument) form of open() will
3334 have leading and trailing whitespace deleted, and the normal
3335 redirection characters honored. This property, known as "magic open",
3336 can often be used to good effect. A user could specify a filename of
3337 F<"rsh cat file |">, or you could change certain filenames as needed:
3339 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3340 open(FH, $filename) or die "Can't open $filename: $!";
3342 Use 3-argument form to open a file with arbitrary weird characters in it,
3344 open(FOO, '<', $file);
3346 otherwise it's necessary to protect any leading and trailing whitespace:
3348 $file =~ s#^(\s)#./$1#;
3349 open(FOO, "< $file\0");
3351 (this may not work on some bizarre filesystems). One should
3352 conscientiously choose between the I<magic> and 3-arguments form
3357 will allow the user to specify an argument of the form C<"rsh cat file |">,
3358 but will not work on a filename which happens to have a trailing space, while
3360 open IN, '<', $ARGV[0];
3362 will have exactly the opposite restrictions.
3364 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3365 should use the C<sysopen> function, which involves no such magic (but
3366 may use subtly different filemodes than Perl open(), which is mapped
3367 to C fopen()). This is
3368 another way to protect your filenames from interpretation. For example:
3371 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3372 or die "sysopen $path: $!";
3373 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3374 print HANDLE "stuff $$\n";
3376 print "File contains: ", <HANDLE>;
3378 Using the constructor from the C<IO::Handle> package (or one of its
3379 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3380 filehandles that have the scope of whatever variables hold references to
3381 them, and automatically close whenever and however you leave that scope:
3385 sub read_myfile_munged {
3387 my $handle = new IO::File;
3388 open($handle, "myfile") or die "myfile: $!";
3390 or return (); # Automatically closed here.
3391 mung $first or die "mung failed"; # Or here.
3392 return $first, <$handle> if $ALL; # Or here.
3396 See L</seek> for some details about mixing reading and writing.
3398 =item opendir DIRHANDLE,EXPR
3401 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3402 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3403 DIRHANDLE may be an expression whose value can be used as an indirect
3404 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3405 scalar variable (or array or hash element), the variable is assigned a
3406 reference to a new anonymous dirhandle.
3407 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3414 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3415 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3418 For the reverse, see L</chr>.
3419 See L<perlunicode> and L<encoding> for more about Unicode.
3426 =item our EXPR : ATTRS
3428 =item our TYPE EXPR : ATTRS
3430 C<our> associates a simple name with a package variable in the current
3431 package for use within the current scope. When C<use strict 'vars'> is in
3432 effect, C<our> lets you use declared global variables without qualifying
3433 them with package names, within the lexical scope of the C<our> declaration.
3434 In this way C<our> differs from C<use vars>, which is package scoped.
3436 Unlike C<my>, which both allocates storage for a variable and associates
3437 a simple name with that storage for use within the current scope, C<our>
3438 associates a simple name with a package variable in the current package,
3439 for use within the current scope. In other words, C<our> has the same
3440 scoping rules as C<my>, but does not necessarily create a
3443 If more than one value is listed, the list must be placed
3449 An C<our> declaration declares a global variable that will be visible
3450 across its entire lexical scope, even across package boundaries. The
3451 package in which the variable is entered is determined at the point
3452 of the declaration, not at the point of use. This means the following
3456 our $bar; # declares $Foo::bar for rest of lexical scope
3460 print $bar; # prints 20, as it refers to $Foo::bar
3462 Multiple C<our> declarations with the same name in the same lexical
3463 scope are allowed if they are in different packages. If they happen
3464 to be in the same package, Perl will emit warnings if you have asked
3465 for them, just like multiple C<my> declarations. Unlike a second
3466 C<my> declaration, which will bind the name to a fresh variable, a
3467 second C<our> declaration in the same package, in the same scope, is
3472 our $bar; # declares $Foo::bar for rest of lexical scope
3476 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3477 print $bar; # prints 30
3479 our $bar; # emits warning but has no other effect
3480 print $bar; # still prints 30
3482 An C<our> declaration may also have a list of attributes associated
3485 The exact semantics and interface of TYPE and ATTRS are still
3486 evolving. TYPE is currently bound to the use of C<fields> pragma,
3487 and attributes are handled using the C<attributes> pragma, or starting
3488 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3489 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3490 L<attributes>, and L<Attribute::Handlers>.
3492 =item pack TEMPLATE,LIST
3495 Takes a LIST of values and converts it into a string using the rules
3496 given by the TEMPLATE. The resulting string is the concatenation of
3497 the converted values. Typically, each converted value looks
3498 like its machine-level representation. For example, on 32-bit machines
3499 an integer may be represented by a sequence of 4 bytes that will be
3500 converted to a sequence of 4 characters.
3502 The TEMPLATE is a sequence of characters that give the order and type
3503 of values, as follows:
3505 a A string with arbitrary binary data, will be null padded.
3506 A A text (ASCII) string, will be space padded.
3507 Z A null terminated (ASCIZ) string, will be null padded.
3509 b A bit string (ascending bit order inside each byte, like vec()).
3510 B A bit string (descending bit order inside each byte).
3511 h A hex string (low nybble first).
3512 H A hex string (high nybble first).
3514 c A signed char (8-bit) value.
3515 C An unsigned C char (octet) even under Unicode. Should normally not
3516 be used. See U and W instead.
3517 W An unsigned char value (can be greater than 255).
3519 s A signed short (16-bit) value.
3520 S An unsigned short value.
3522 l A signed long (32-bit) value.
3523 L An unsigned long value.
3525 q A signed quad (64-bit) value.
3526 Q An unsigned quad value.
3527 (Quads are available only if your system supports 64-bit
3528 integer values _and_ if Perl has been compiled to support those.
3529 Causes a fatal error otherwise.)
3531 i A signed integer value.
3532 I A unsigned integer value.
3533 (This 'integer' is _at_least_ 32 bits wide. Its exact
3534 size depends on what a local C compiler calls 'int'.)
3536 n An unsigned short (16-bit) in "network" (big-endian) order.
3537 N An unsigned long (32-bit) in "network" (big-endian) order.
3538 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3539 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3541 j A Perl internal signed integer value (IV).
3542 J A Perl internal unsigned integer value (UV).
3544 f A single-precision float in the native format.
3545 d A double-precision float in the native format.
3547 F A Perl internal floating point value (NV) in the native format
3548 D A long double-precision float in the native format.
3549 (Long doubles are available only if your system supports long
3550 double values _and_ if Perl has been compiled to support those.
3551 Causes a fatal error otherwise.)
3553 p A pointer to a null-terminated string.
3554 P A pointer to a structure (fixed-length string).
3556 u A uuencoded string.
3557 U A Unicode character number. Encodes to UTF-8 internally
3558 (or UTF-EBCDIC in EBCDIC platforms).
3560 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3561 details). Its bytes represent an unsigned integer in base 128,
3562 most significant digit first, with as few digits as possible. Bit
3563 eight (the high bit) is set on each byte except the last.
3567 @ Null fill or truncate to absolute position, counted from the
3568 start of the innermost ()-group.
3569 . Null fill or truncate to absolute position specified by value.
3570 ( Start of a ()-group.
3572 One or more of the modifiers below may optionally follow some letters in the
3573 TEMPLATE (the second column lists the letters for which the modifier is
3576 ! sSlLiI Forces native (short, long, int) sizes instead
3577 of fixed (16-/32-bit) sizes.
3579 xX Make x and X act as alignment commands.
3581 nNvV Treat integers as signed instead of unsigned.
3583 @. Specify position as byte offset in the internal
3584 representation of the packed string. Efficient but
3587 > sSiIlLqQ Force big-endian byte-order on the type.
3588 jJfFdDpP (The "big end" touches the construct.)
3590 < sSiIlLqQ Force little-endian byte-order on the type.
3591 jJfFdDpP (The "little end" touches the construct.)
3593 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3594 in which case they force a certain byte-order on all components of
3595 that group, including subgroups.
3597 The following rules apply:
3603 Each letter may optionally be followed by a number giving a repeat
3604 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3605 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3606 that many values from the LIST. A C<*> for the repeat count means to
3607 use however many items are left, except for C<@>, C<x>, C<X>, where it
3608 is equivalent to C<0>, for <.> where it means relative to string start
3609 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3610 A numeric repeat count may optionally be enclosed in brackets, as in
3611 C<pack 'C[80]', @arr>.
3613 One can replace the numeric repeat count by a template enclosed in brackets;
3614 then the packed length of this template in bytes is used as a count.
3615 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3616 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3617 If the template in brackets contains alignment commands (such as C<x![d]>),
3618 its packed length is calculated as if the start of the template has the maximal
3621 When used with C<Z>, C<*> results in the addition of a trailing null
3622 byte (so the packed result will be one longer than the byte C<length>
3625 When used with C<@>, the repeat count represents an offset from the start
3626 of the innermost () group.
3628 When used with C<.>, the repeat count is used to determine the starting
3629 position from where the value offset is calculated. If the repeat count
3630 is 0, it's relative to the current position. If the repeat count is C<*>,
3631 the offset is relative to the start of the packed string. And if its an
3632 integer C<n> the offset is relative to the start of the n-th innermost
3633 () group (or the start of the string if C<n> is bigger then the group
3636 The repeat count for C<u> is interpreted as the maximal number of bytes
3637 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3638 count should not be more than 65.
3642 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3643 string of length count, padding with nulls or spaces as necessary. When
3644 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3645 after the first null, and C<a> returns data verbatim.
3647 If the value-to-pack is too long, it is truncated. If too long and an
3648 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3649 by a null byte. Thus C<Z> always packs a trailing null (except when the
3654 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3655 Each character of the input field of pack() generates 1 bit of the result.
3656 Each result bit is based on the least-significant bit of the corresponding
3657 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3658 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3660 Starting from the beginning of the input string of pack(), each 8-tuple
3661 of characters is converted to 1 character of output. With format C<b>
3662 the first character of the 8-tuple determines the least-significant bit of a
3663 character, and with format C<B> it determines the most-significant bit of
3666 If the length of the input string is not exactly divisible by 8, the
3667 remainder is packed as if the input string were padded by null characters
3668 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3670 If the input string of pack() is longer than needed, extra characters are
3671 ignored. A C<*> for the repeat count of pack() means to use all the
3672 characters of the input field. On unpack()ing the bits are converted to a
3673 string of C<"0">s and C<"1">s.
3677 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3678 representable as hexadecimal digits, 0-9a-f) long.
3680 Each character of the input field of pack() generates 4 bits of the result.
3681 For non-alphabetical characters the result is based on the 4 least-significant
3682 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3683 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3684 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3685 is compatible with the usual hexadecimal digits, so that C<"a"> and
3686 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3687 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3689 Starting from the beginning of the input string of pack(), each pair
3690 of characters is converted to 1 character of output. With format C<h> the
3691 first character of the pair determines the least-significant nybble of the
3692 output character, and with format C<H> it determines the most-significant
3695 If the length of the input string is not even, it behaves as if padded
3696 by a null character at the end. Similarly, during unpack()ing the "extra"
3697 nybbles are ignored.
3699 If the input string of pack() is longer than needed, extra characters are
3701 A C<*> for the repeat count of pack() means to use all the characters of
3702 the input field. On unpack()ing the nybbles are converted to a string
3703 of hexadecimal digits.
3707 The C<p> type packs a pointer to a null-terminated string. You are
3708 responsible for ensuring the string is not a temporary value (which can
3709 potentially get deallocated before you get around to using the packed result).
3710 The C<P> type packs a pointer to a structure of the size indicated by the
3711 length. A NULL pointer is created if the corresponding value for C<p> or
3712 C<P> is C<undef>, similarly for unpack().
3714 If your system has a strange pointer size (i.e. a pointer is neither as
3715 big as an int nor as big as a long), it may not be possible to pack or
3716 unpack pointers in big- or little-endian byte order. Attempting to do
3717 so will result in a fatal error.
3721 The C</> template character allows packing and unpacking of a sequence of
3722 items where the packed structure contains a packed item count followed by
3723 the packed items themselves.
3725 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3726 I<length-item> describes how the length value is packed. The ones likely
3727 to be of most use are integer-packing ones like C<n> (for Java strings),
3728 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3730 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3731 the minimum of that and the number of available items is used as argument
3732 for the I<length-item>. If it has no repeat count or uses a '*', the number
3733 of available items is used.
3735 For C<unpack> an internal stack of integer arguments unpacked so far is
3736 used. You write C</>I<sequence-item> and the repeat count is obtained by
3737 popping off the last element from the stack. The I<sequence-item> must not
3738 have a repeat count.
3740 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3741 the I<length-item> is a string length, not a number of strings. If there is
3742 an explicit repeat count for pack, the packed string will be adjusted to that
3745 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3746 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3747 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3748 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3749 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3751 The I<length-item> is not returned explicitly from C<unpack>.
3753 Adding a count to the I<length-item> letter is unlikely to do anything
3754 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3755 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3756 which Perl does not regard as legal in numeric strings.
3760 The integer types C<s>, C<S>, C<l>, and C<L> may be
3761 followed by a C<!> modifier to signify native shorts or
3762 longs--as you can see from above for example a bare C<l> does mean
3763 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3764 may be larger. This is an issue mainly in 64-bit platforms. You can
3765 see whether using C<!> makes any difference by
3767 print length(pack("s")), " ", length(pack("s!")), "\n";
3768 print length(pack("l")), " ", length(pack("l!")), "\n";
3770 C<i!> and C<I!> also work but only because of completeness;
3771 they are identical to C<i> and C<I>.
3773 The actual sizes (in bytes) of native shorts, ints, longs, and long
3774 longs on the platform where Perl was built are also available via
3778 print $Config{shortsize}, "\n";
3779 print $Config{intsize}, "\n";
3780 print $Config{longsize}, "\n";
3781 print $Config{longlongsize}, "\n";
3783 (The C<$Config{longlongsize}> will be undefined if your system does
3784 not support long longs.)
3788 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3789 are inherently non-portable between processors and operating systems
3790 because they obey the native byteorder and endianness. For example a
3791 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3792 (arranged in and handled by the CPU registers) into bytes as
3794 0x12 0x34 0x56 0x78 # big-endian
3795 0x78 0x56 0x34 0x12 # little-endian
3797 Basically, the Intel and VAX CPUs are little-endian, while everybody
3798 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3799 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3800 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3803 The names `big-endian' and `little-endian' are comic references to
3804 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3805 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3806 the egg-eating habits of the Lilliputians.
3808 Some systems may have even weirder byte orders such as
3813 You can see your system's preference with
3815 print join(" ", map { sprintf "%#02x", $_ }
3816 unpack("W*",pack("L",0x12345678))), "\n";
3818 The byteorder on the platform where Perl was built is also available
3822 print $Config{byteorder}, "\n";
3824 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3825 and C<'87654321'> are big-endian.
3827 If you want portable packed integers you can either use the formats
3828 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3829 modifiers. These modifiers are only available as of perl 5.9.2.
3830 See also L<perlport>.
3834 All integer and floating point formats as well as C<p> and C<P> and
3835 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3836 to force big- or little- endian byte-order, respectively.
3837 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3838 signed integers, 64-bit integers and floating point values. However,
3839 there are some things to keep in mind.
3841 Exchanging signed integers between different platforms only works
3842 if all platforms store them in the same format. Most platforms store
3843 signed integers in two's complement, so usually this is not an issue.
3845 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3846 formats on big- or little-endian machines. Otherwise, attempting to
3847 do so will result in a fatal error.
3849 Forcing big- or little-endian byte-order on floating point values for
3850 data exchange can only work if all platforms are using the same
3851 binary representation (e.g. IEEE floating point format). Even if all
3852 platforms are using IEEE, there may be subtle differences. Being able
3853 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3854 but also very dangerous if you don't know exactly what you're doing.
3855 It is definitely not a general way to portably store floating point
3858 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3859 all types inside the group that accept the byte-order modifiers,
3860 including all subgroups. It will silently be ignored for all other
3861 types. You are not allowed to override the byte-order within a group
3862 that already has a byte-order modifier suffix.
3866 Real numbers (floats and doubles) are in the native machine format only;
3867 due to the multiplicity of floating formats around, and the lack of a
3868 standard "network" representation, no facility for interchange has been
3869 made. This means that packed floating point data written on one machine
3870 may not be readable on another - even if both use IEEE floating point
3871 arithmetic (as the endian-ness of the memory representation is not part
3872 of the IEEE spec). See also L<perlport>.
3874 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3875 modifiers to force big- or little-endian byte-order on floating point values.
3877 Note that Perl uses doubles (or long doubles, if configured) internally for
3878 all numeric calculation, and converting from double into float and thence back
3879 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3880 will not in general equal $foo).
3884 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3885 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3886 where the packed string is processed in its UTF-8-encoded Unicode form on
3887 a byte by byte basis. Character mode is the default unless the format string
3888 starts with an C<U>. You can switch mode at any moment with an explicit
3889 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3890 or until the end of the ()-group in which it was entered.
3894 You must yourself do any alignment or padding by inserting for example
3895 enough C<'x'>es while packing. There is no way to pack() and unpack()
3896 could know where the characters are going to or coming from. Therefore
3897 C<pack> (and C<unpack>) handle their output and input as flat
3898 sequences of characters.
3902 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3903 take a repeat count, both as postfix, and for unpack() also via the C</>
3904 template character. Within each repetition of a group, positioning with
3905 C<@> starts again at 0. Therefore, the result of
3907 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3909 is the string "\0a\0\0bc".
3913 C<x> and C<X> accept C<!> modifier. In this case they act as
3914 alignment commands: they jump forward/back to the closest position
3915 aligned at a multiple of C<count> characters. For example, to pack() or
3916 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3917 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3918 aligned on the double's size.
3920 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3921 both result in no-ops.
3925 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3926 will represent signed 16-/32-bit integers in big-/little-endian order.
3927 This is only portable if all platforms sharing the packed data use the
3928 same binary representation for signed integers (e.g. all platforms are
3929 using two's complement representation).
3933 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3934 White space may be used to separate pack codes from each other, but
3935 modifiers and a repeat count must follow immediately.
3939 If TEMPLATE requires more arguments to pack() than actually given, pack()
3940 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3941 to pack() than actually given, extra arguments are ignored.
3947 $foo = pack("WWWW",65,66,67,68);
3949 $foo = pack("W4",65,66,67,68);
3951 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3952 # same thing with Unicode circled letters.
3953 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3954 # same thing with Unicode circled letters. You don't get the UTF-8
3955 # bytes because the U at the start of the format caused a switch to
3956 # U0-mode, so the UTF-8 bytes get joined into characters
3957 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3958 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3959 # This is the UTF-8 encoding of the string in the previous example
3961 $foo = pack("ccxxcc",65,66,67,68);
3964 # note: the above examples featuring "W" and "c" are true
3965 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3966 # and UTF-8. In EBCDIC the first example would be
3967 # $foo = pack("WWWW",193,194,195,196);
3969 $foo = pack("s2",1,2);
3970 # "\1\0\2\0" on little-endian
3971 # "\0\1\0\2" on big-endian
3973 $foo = pack("a4","abcd","x","y","z");
3976 $foo = pack("aaaa","abcd","x","y","z");
3979 $foo = pack("a14","abcdefg");
3980 # "abcdefg\0\0\0\0\0\0\0"
3982 $foo = pack("i9pl", gmtime);
3983 # a real struct tm (on my system anyway)
3985 $utmp_template = "Z8 Z8 Z16 L";
3986 $utmp = pack($utmp_template, @utmp1);
3987 # a struct utmp (BSDish)
3989 @utmp2 = unpack($utmp_template, $utmp);
3990 # "@utmp1" eq "@utmp2"
3993 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3996 $foo = pack('sx2l', 12, 34);
3997 # short 12, two zero bytes padding, long 34
3998 $bar = pack('s@4l', 12, 34);
3999 # short 12, zero fill to position 4, long 34
4001 $baz = pack('s.l', 12, 4, 34);
4002 # short 12, zero fill to position 4, long 34
4004 $foo = pack('nN', 42, 4711);
4005 # pack big-endian 16- and 32-bit unsigned integers
4006 $foo = pack('S>L>', 42, 4711);
4008 $foo = pack('s<l<', -42, 4711);
4009 # pack little-endian 16- and 32-bit signed integers
4010 $foo = pack('(sl)<', -42, 4711);
4013 The same template may generally also be used in unpack().
4015 =item package NAMESPACE
4016 X<package> X<module> X<namespace>
4020 Declares the compilation unit as being in the given namespace. The scope
4021 of the package declaration is from the declaration itself through the end
4022 of the enclosing block, file, or eval (the same as the C<my> operator).
4023 All further unqualified dynamic identifiers will be in this namespace.
4024 A package statement affects only dynamic variables--including those
4025 you've used C<local> on--but I<not> lexical variables, which are created
4026 with C<my>. Typically it would be the first declaration in a file to
4027 be included by the C<require> or C<use> operator. You can switch into a
4028 package in more than one place; it merely influences which symbol table
4029 is used by the compiler for the rest of that block. You can refer to
4030 variables and filehandles in other packages by prefixing the identifier
4031 with the package name and a double colon: C<$Package::Variable>.
4032 If the package name is null, the C<main> package as assumed. That is,
4033 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4034 still seen in older code).
4036 If NAMESPACE is omitted, then there is no current package, and all
4037 identifiers must be fully qualified or lexicals. However, you are
4038 strongly advised not to make use of this feature. Its use can cause
4039 unexpected behaviour, even crashing some versions of Perl. It is
4040 deprecated, and will be removed from a future release.
4042 See L<perlmod/"Packages"> for more information about packages, modules,
4043 and classes. See L<perlsub> for other scoping issues.
4045 =item pipe READHANDLE,WRITEHANDLE
4048 Opens a pair of connected pipes like the corresponding system call.
4049 Note that if you set up a loop of piped processes, deadlock can occur
4050 unless you are very careful. In addition, note that Perl's pipes use
4051 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4052 after each command, depending on the application.
4054 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4055 for examples of such things.
4057 On systems that support a close-on-exec flag on files, the flag will be set
4058 for the newly opened file descriptors as determined by the value of $^F.
4066 Pops and returns the last value of the array, shortening the array by
4067 one element. Has an effect similar to
4071 If there are no elements in the array, returns the undefined value
4072 (although this may happen at other times as well). If ARRAY is
4073 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4074 array in subroutines, just like C<shift>.
4077 X<pos> X<match, position>
4081 Returns the offset of where the last C<m//g> search left off for the variable
4082 in question (C<$_> is used when the variable is not specified). Note that
4083 0 is a valid match offset. C<undef> indicates that the search position
4084 is reset (usually due to match failure, but can also be because no match has
4085 yet been performed on the scalar). C<pos> directly accesses the location used
4086 by the regexp engine to store the offset, so assigning to C<pos> will change
4087 that offset, and so will also influence the C<\G> zero-width assertion in
4088 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4089 the return from C<pos> won't change either in this case. See L<perlre> and
4092 =item print FILEHANDLE LIST
4099 Prints a string or a list of strings. Returns true if successful.
4100 FILEHANDLE may be a scalar variable name, in which case the variable
4101 contains the name of or a reference to the filehandle, thus introducing
4102 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4103 the next token is a term, it may be misinterpreted as an operator
4104 unless you interpose a C<+> or put parentheses around the arguments.)
4105 If FILEHANDLE is omitted, prints by default to standard output (or
4106 to the last selected output channel--see L</select>). If LIST is
4107 also omitted, prints C<$_> to the currently selected output channel.
4108 To set the default output channel to something other than STDOUT
4109 use the select operation. The current value of C<$,> (if any) is
4110 printed between each LIST item. The current value of C<$\> (if
4111 any) is printed after the entire LIST has been printed. Because
4112 print takes a LIST, anything in the LIST is evaluated in list
4113 context, and any subroutine that you call will have one or more of
4114 its expressions evaluated in list context. Also be careful not to
4115 follow the print keyword with a left parenthesis unless you want
4116 the corresponding right parenthesis to terminate the arguments to
4117 the print--interpose a C<+> or put parentheses around all the
4120 Note that if you're storing FILEHANDLEs in an array, or if you're using
4121 any other expression more complex than a scalar variable to retrieve it,
4122 you will have to use a block returning the filehandle value instead:
4124 print { $files[$i] } "stuff\n";
4125 print { $OK ? STDOUT : STDERR } "stuff\n";
4127 =item printf FILEHANDLE FORMAT, LIST
4130 =item printf FORMAT, LIST
4132 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4133 (the output record separator) is not appended. The first argument
4134 of the list will be interpreted as the C<printf> format. See C<sprintf>
4135 for an explanation of the format argument. If C<use locale> is in effect,
4136 and POSIX::setlocale() has been called, the character used for the decimal
4137 separator in formatted floating point numbers is affected by the LC_NUMERIC
4138 locale. See L<perllocale> and L<POSIX>.
4140 Don't fall into the trap of using a C<printf> when a simple
4141 C<print> would do. The C<print> is more efficient and less
4144 =item prototype FUNCTION
4147 Returns the prototype of a function as a string (or C<undef> if the
4148 function has no prototype). FUNCTION is a reference to, or the name of,
4149 the function whose prototype you want to retrieve.
4151 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4152 name for Perl builtin. If the builtin is not I<overridable> (such as
4153 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4154 C<system>) returns C<undef> because the builtin does not really behave
4155 like a Perl function. Otherwise, the string describing the equivalent
4156 prototype is returned.
4158 =item push ARRAY,LIST
4161 Treats ARRAY as a stack, and pushes the values of LIST
4162 onto the end of ARRAY. The length of ARRAY increases by the length of
4163 LIST. Has the same effect as
4166 $ARRAY[++$#ARRAY] = $value;
4169 but is more efficient. Returns the number of elements in the array following
4170 the completed C<push>.
4182 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4184 =item quotemeta EXPR
4185 X<quotemeta> X<metacharacter>
4189 Returns the value of EXPR with all non-"word"
4190 characters backslashed. (That is, all characters not matching
4191 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4192 returned string, regardless of any locale settings.)
4193 This is the internal function implementing
4194 the C<\Q> escape in double-quoted strings.
4196 If EXPR is omitted, uses C<$_>.
4203 Returns a random fractional number greater than or equal to C<0> and less
4204 than the value of EXPR. (EXPR should be positive.) If EXPR is
4205 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4206 also special-cased as C<1> - this has not been documented before perl 5.8.0
4207 and is subject to change in future versions of perl. Automatically calls
4208 C<srand> unless C<srand> has already been called. See also C<srand>.
4210 Apply C<int()> to the value returned by C<rand()> if you want random
4211 integers instead of random fractional numbers. For example,
4215 returns a random integer between C<0> and C<9>, inclusive.
4217 (Note: If your rand function consistently returns numbers that are too
4218 large or too small, then your version of Perl was probably compiled
4219 with the wrong number of RANDBITS.)
4221 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4222 X<read> X<file, read>
4224 =item read FILEHANDLE,SCALAR,LENGTH
4226 Attempts to read LENGTH I<characters> of data into variable SCALAR
4227 from the specified FILEHANDLE. Returns the number of characters
4228 actually read, C<0> at end of file, or undef if there was an error (in
4229 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4230 so that the last character actually read is the last character of the
4231 scalar after the read.
4233 An OFFSET may be specified to place the read data at some place in the
4234 string other than the beginning. A negative OFFSET specifies
4235 placement at that many characters counting backwards from the end of
4236 the string. A positive OFFSET greater than the length of SCALAR
4237 results in the string being padded to the required size with C<"\0">
4238 bytes before the result of the read is appended.
4240 The call is actually implemented in terms of either Perl's or system's
4241 fread() call. To get a true read(2) system call, see C<sysread>.
4243 Note the I<characters>: depending on the status of the filehandle,
4244 either (8-bit) bytes or characters are read. By default all
4245 filehandles operate on bytes, but for example if the filehandle has
4246 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4247 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4248 characters, not bytes. Similarly for the C<:encoding> pragma:
4249 in that case pretty much any characters can be read.
4251 =item readdir DIRHANDLE
4254 Returns the next directory entry for a directory opened by C<opendir>.
4255 If used in list context, returns all the rest of the entries in the
4256 directory. If there are no more entries, returns an undefined value in
4257 scalar context or a null list in list context.
4259 If you're planning to filetest the return values out of a C<readdir>, you'd
4260 better prepend the directory in question. Otherwise, because we didn't
4261 C<chdir> there, it would have been testing the wrong file.
4263 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4264 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4268 X<readline> X<gets> X<fgets>
4270 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4271 context, each call reads and returns the next line, until end-of-file is
4272 reached, whereupon the subsequent call returns undef. In list context,
4273 reads until end-of-file is reached and returns a list of lines. Note that
4274 the notion of "line" used here is however you may have defined it
4275 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4277 When C<$/> is set to C<undef>, when readline() is in scalar
4278 context (i.e. file slurp mode), and when an empty file is read, it
4279 returns C<''> the first time, followed by C<undef> subsequently.
4281 This is the internal function implementing the C<< <EXPR> >>
4282 operator, but you can use it directly. The C<< <EXPR> >>
4283 operator is discussed in more detail in L<perlop/"I/O Operators">.
4286 $line = readline(*STDIN); # same thing
4288 If readline encounters an operating system error, C<$!> will be set with the
4289 corresponding error message. It can be helpful to check C<$!> when you are
4290 reading from filehandles you don't trust, such as a tty or a socket. The
4291 following example uses the operator form of C<readline>, and takes the necessary
4292 steps to ensure that C<readline> was successful.
4296 unless (defined( $line = <> )) {
4308 Returns the value of a symbolic link, if symbolic links are
4309 implemented. If not, gives a fatal error. If there is some system
4310 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4311 omitted, uses C<$_>.
4316 EXPR is executed as a system command.
4317 The collected standard output of the command is returned.
4318 In scalar context, it comes back as a single (potentially
4319 multi-line) string. In list context, returns a list of lines
4320 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4321 This is the internal function implementing the C<qx/EXPR/>
4322 operator, but you can use it directly. The C<qx/EXPR/>
4323 operator is discussed in more detail in L<perlop/"I/O Operators">.
4325 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4328 Receives a message on a socket. Attempts to receive LENGTH characters
4329 of data into variable SCALAR from the specified SOCKET filehandle.
4330 SCALAR will be grown or shrunk to the length actually read. Takes the
4331 same flags as the system call of the same name. Returns the address
4332 of the sender if SOCKET's protocol supports this; returns an empty
4333 string otherwise. If there's an error, returns the undefined value.
4334 This call is actually implemented in terms of recvfrom(2) system call.
4335 See L<perlipc/"UDP: Message Passing"> for examples.
4337 Note the I<characters>: depending on the status of the socket, either
4338 (8-bit) bytes or characters are received. By default all sockets
4339 operate on bytes, but for example if the socket has been changed using
4340 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4341 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4342 characters, not bytes. Similarly for the C<:encoding> pragma:
4343 in that case pretty much any characters can be read.
4350 The C<redo> command restarts the loop block without evaluating the
4351 conditional again. The C<continue> block, if any, is not executed. If
4352 the LABEL is omitted, the command refers to the innermost enclosing
4353 loop. Programs that want to lie to themselves about what was just input
4354 normally use this command:
4356 # a simpleminded Pascal comment stripper
4357 # (warning: assumes no { or } in strings)
4358 LINE: while (<STDIN>) {
4359 while (s|({.*}.*){.*}|$1 |) {}
4364 if (/}/) { # end of comment?
4373 C<redo> cannot be used to retry a block which returns a value such as
4374 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4375 a grep() or map() operation.
4377 Note that a block by itself is semantically identical to a loop
4378 that executes once. Thus C<redo> inside such a block will effectively
4379 turn it into a looping construct.
4381 See also L</continue> for an illustration of how C<last>, C<next>, and
4389 Returns a non-empty string if EXPR is a reference, the empty
4390 string otherwise. If EXPR
4391 is not specified, C<$_> will be used. The value returned depends on the
4392 type of thing the reference is a reference to.
4393 Builtin types include:
4407 If the referenced object has been blessed into a package, then that package
4408 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4410 if (ref($r) eq "HASH") {
4411 print "r is a reference to a hash.\n";
4414 print "r is not a reference at all.\n";
4417 See also L<perlref>.
4419 =item rename OLDNAME,NEWNAME
4420 X<rename> X<move> X<mv> X<ren>
4422 Changes the name of a file; an existing file NEWNAME will be
4423 clobbered. Returns true for success, false otherwise.
4425 Behavior of this function varies wildly depending on your system
4426 implementation. For example, it will usually not work across file system
4427 boundaries, even though the system I<mv> command sometimes compensates
4428 for this. Other restrictions include whether it works on directories,
4429 open files, or pre-existing files. Check L<perlport> and either the
4430 rename(2) manpage or equivalent system documentation for details.
4432 For a platform independent C<move> function look at the L<File::Copy>
4435 =item require VERSION
4442 Demands a version of Perl specified by VERSION, or demands some semantics
4443 specified by EXPR or by C<$_> if EXPR is not supplied.
4445 VERSION may be either a numeric argument such as 5.006, which will be
4446 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4447 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4448 VERSION is greater than the version of the current Perl interpreter.
4449 Compare with L</use>, which can do a similar check at compile time.
4451 Specifying VERSION as a literal of the form v5.6.1 should generally be
4452 avoided, because it leads to misleading error messages under earlier
4453 versions of Perl that do not support this syntax. The equivalent numeric
4454 version should be used instead.
4456 require v5.6.1; # run time version check
4457 require 5.6.1; # ditto
4458 require 5.006_001; # ditto; preferred for backwards compatibility
4460 Otherwise, C<require> demands that a library file be included if it
4461 hasn't already been included. The file is included via the do-FILE
4462 mechanism, which is essentially just a variety of C<eval>. Has
4463 semantics similar to the following subroutine:
4466 my ($filename) = @_;
4467 if (exists $INC{$filename}) {
4468 return 1 if $INC{$filename};
4469 die "Compilation failed in require";
4471 my ($realfilename,$result);
4473 foreach $prefix (@INC) {
4474 $realfilename = "$prefix/$filename";
4475 if (-f $realfilename) {
4476 $INC{$filename} = $realfilename;
4477 $result = do $realfilename;
4481 die "Can't find $filename in \@INC";
4484 $INC{$filename} = undef;
4486 } elsif (!$result) {
4487 delete $INC{$filename};
4488 die "$filename did not return true value";
4494 Note that the file will not be included twice under the same specified
4497 The file must return true as the last statement to indicate
4498 successful execution of any initialization code, so it's customary to
4499 end such a file with C<1;> unless you're sure it'll return true
4500 otherwise. But it's better just to put the C<1;>, in case you add more
4503 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4504 replaces "F<::>" with "F</>" in the filename for you,
4505 to make it easy to load standard modules. This form of loading of
4506 modules does not risk altering your namespace.
4508 In other words, if you try this:
4510 require Foo::Bar; # a splendid bareword
4512 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4513 directories specified in the C<@INC> array.
4515 But if you try this:
4517 $class = 'Foo::Bar';
4518 require $class; # $class is not a bareword
4520 require "Foo::Bar"; # not a bareword because of the ""
4522 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4523 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4525 eval "require $class";
4527 Now that you understand how C<require> looks for files in the case of a
4528 bareword argument, there is a little extra functionality going on behind
4529 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4530 first look for a similar filename with a "F<.pmc>" extension. If this file
4531 is found, it will be loaded in place of any file ending in a "F<.pm>"
4534 You can also insert hooks into the import facility, by putting directly
4535 Perl code into the @INC array. There are three forms of hooks: subroutine
4536 references, array references and blessed objects.
4538 Subroutine references are the simplest case. When the inclusion system
4539 walks through @INC and encounters a subroutine, this subroutine gets
4540 called with two parameters, the first being a reference to itself, and the
4541 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4542 subroutine should return nothing, or a list of up to three values in the
4549 A filehandle, from which the file will be read.
4553 A reference to a subroutine. If there is no filehandle (previous item),
4554 then this subroutine is expected to generate one line of source code per
4555 call, writing the line into C<$_> and returning 1, then returning 0 at
4556 "end of file". If there is a filehandle, then the subroutine will be
4557 called to act a simple source filter, with the line as read in C<$_>.
4558 Again, return 1 for each valid line, and 0 after all lines have been
4563 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4564 reference to the subroutine itself is passed in as C<$_[0]>.
4568 If an empty list, C<undef>, or nothing that matches the first 3 values above
4569 is returned then C<require> will look at the remaining elements of @INC.
4570 Note that this file handle must be a real file handle (strictly a typeglob,
4571 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4572 ignored and return value processing will stop there.
4574 If the hook is an array reference, its first element must be a subroutine
4575 reference. This subroutine is called as above, but the first parameter is
4576 the array reference. This enables to pass indirectly some arguments to
4579 In other words, you can write:
4581 push @INC, \&my_sub;
4583 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4589 push @INC, [ \&my_sub, $x, $y, ... ];
4591 my ($arrayref, $filename) = @_;
4592 # Retrieve $x, $y, ...
4593 my @parameters = @$arrayref[1..$#$arrayref];
4597 If the hook is an object, it must provide an INC method that will be
4598 called as above, the first parameter being the object itself. (Note that
4599 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4600 into package C<main>.) Here is a typical code layout:
4606 my ($self, $filename) = @_;
4610 # In the main program
4611 push @INC, new Foo(...);
4613 Note that these hooks are also permitted to set the %INC entry
4614 corresponding to the files they have loaded. See L<perlvar/%INC>.
4616 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4623 Generally used in a C<continue> block at the end of a loop to clear
4624 variables and reset C<??> searches so that they work again. The
4625 expression is interpreted as a list of single characters (hyphens
4626 allowed for ranges). All variables and arrays beginning with one of
4627 those letters are reset to their pristine state. If the expression is
4628 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4629 only variables or searches in the current package. Always returns
4632 reset 'X'; # reset all X variables
4633 reset 'a-z'; # reset lower case variables
4634 reset; # just reset ?one-time? searches
4636 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4637 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4638 variables--lexical variables are unaffected, but they clean themselves
4639 up on scope exit anyway, so you'll probably want to use them instead.
4647 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4648 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4649 context, depending on how the return value will be used, and the context
4650 may vary from one execution to the next (see C<wantarray>). If no EXPR
4651 is given, returns an empty list in list context, the undefined value in
4652 scalar context, and (of course) nothing at all in a void context.
4654 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4655 or do FILE will automatically return the value of the last expression
4659 X<reverse> X<rev> X<invert>
4661 In list context, returns a list value consisting of the elements
4662 of LIST in the opposite order. In scalar context, concatenates the
4663 elements of LIST and returns a string value with all characters
4664 in the opposite order.
4666 print reverse <>; # line tac, last line first
4668 undef $/; # for efficiency of <>
4669 print scalar reverse <>; # character tac, last line tsrif
4671 Used without arguments in scalar context, reverse() reverses C<$_>.
4673 This operator is also handy for inverting a hash, although there are some
4674 caveats. If a value is duplicated in the original hash, only one of those
4675 can be represented as a key in the inverted hash. Also, this has to
4676 unwind one hash and build a whole new one, which may take some time
4677 on a large hash, such as from a DBM file.
4679 %by_name = reverse %by_address; # Invert the hash
4681 =item rewinddir DIRHANDLE
4684 Sets the current position to the beginning of the directory for the
4685 C<readdir> routine on DIRHANDLE.
4687 =item rindex STR,SUBSTR,POSITION
4690 =item rindex STR,SUBSTR
4692 Works just like index() except that it returns the position of the I<last>
4693 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4694 last occurrence beginning at or before that position.
4696 =item rmdir FILENAME
4697 X<rmdir> X<rd> X<directory, remove>
4701 Deletes the directory specified by FILENAME if that directory is
4702 empty. If it succeeds it returns true, otherwise it returns false and
4703 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4705 To remove a directory tree recursively (C<rm -rf> on unix) look at
4706 the C<rmtree> function of the L<File::Path> module.
4710 The substitution operator. See L<perlop>.
4712 =item say FILEHANDLE LIST
4719 Just like C<print>, but implicitly appends a newline.
4720 C<say LIST> is simply an abbreviation for C<{ local $/ = "\n"; print
4723 This keyword is only available when the "say" feature is
4724 enabled: see L<feature>.
4727 X<scalar> X<context>
4729 Forces EXPR to be interpreted in scalar context and returns the value
4732 @counts = ( scalar @a, scalar @b, scalar @c );
4734 There is no equivalent operator to force an expression to
4735 be interpolated in list context because in practice, this is never
4736 needed. If you really wanted to do so, however, you could use
4737 the construction C<@{[ (some expression) ]}>, but usually a simple
4738 C<(some expression)> suffices.
4740 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4741 parenthesized list, this behaves as a scalar comma expression, evaluating
4742 all but the last element in void context and returning the final element
4743 evaluated in scalar context. This is seldom what you want.
4745 The following single statement:
4747 print uc(scalar(&foo,$bar)),$baz;
4749 is the moral equivalent of these two:
4752 print(uc($bar),$baz);
4754 See L<perlop> for more details on unary operators and the comma operator.
4756 =item seek FILEHANDLE,POSITION,WHENCE
4757 X<seek> X<fseek> X<filehandle, position>
4759 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4760 FILEHANDLE may be an expression whose value gives the name of the
4761 filehandle. The values for WHENCE are C<0> to set the new position
4762 I<in bytes> to POSITION, C<1> to set it to the current position plus
4763 POSITION, and C<2> to set it to EOF plus POSITION (typically
4764 negative). For WHENCE you may use the constants C<SEEK_SET>,
4765 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4766 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4769 Note the I<in bytes>: even if the filehandle has been set to
4770 operate on characters (for example by using the C<:utf8> open
4771 layer), tell() will return byte offsets, not character offsets
4772 (because implementing that would render seek() and tell() rather slow).
4774 If you want to position file for C<sysread> or C<syswrite>, don't use
4775 C<seek>--buffering makes its effect on the file's system position
4776 unpredictable and non-portable. Use C<sysseek> instead.
4778 Due to the rules and rigors of ANSI C, on some systems you have to do a
4779 seek whenever you switch between reading and writing. Amongst other
4780 things, this may have the effect of calling stdio's clearerr(3).
4781 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4785 This is also useful for applications emulating C<tail -f>. Once you hit
4786 EOF on your read, and then sleep for a while, you might have to stick in a
4787 seek() to reset things. The C<seek> doesn't change the current position,
4788 but it I<does> clear the end-of-file condition on the handle, so that the
4789 next C<< <FILE> >> makes Perl try again to read something. We hope.
4791 If that doesn't work (some IO implementations are particularly
4792 cantankerous), then you may need something more like this:
4795 for ($curpos = tell(FILE); $_ = <FILE>;
4796 $curpos = tell(FILE)) {
4797 # search for some stuff and put it into files
4799 sleep($for_a_while);
4800 seek(FILE, $curpos, 0);
4803 =item seekdir DIRHANDLE,POS
4806 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4807 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4808 about possible directory compaction as the corresponding system library
4811 =item select FILEHANDLE
4812 X<select> X<filehandle, default>
4816 Returns the currently selected filehandle. Sets the current default
4817 filehandle for output, if FILEHANDLE is supplied. This has two
4818 effects: first, a C<write> or a C<print> without a filehandle will
4819 default to this FILEHANDLE. Second, references to variables related to
4820 output will refer to this output channel. For example, if you have to
4821 set the top of form format for more than one output channel, you might
4829 FILEHANDLE may be an expression whose value gives the name of the
4830 actual filehandle. Thus:
4832 $oldfh = select(STDERR); $| = 1; select($oldfh);
4834 Some programmers may prefer to think of filehandles as objects with
4835 methods, preferring to write the last example as:
4838 STDERR->autoflush(1);
4840 =item select RBITS,WBITS,EBITS,TIMEOUT
4843 This calls the select(2) system call with the bit masks specified, which
4844 can be constructed using C<fileno> and C<vec>, along these lines:
4846 $rin = $win = $ein = '';
4847 vec($rin,fileno(STDIN),1) = 1;
4848 vec($win,fileno(STDOUT),1) = 1;
4851 If you want to select on many filehandles you might wish to write a
4855 my(@fhlist) = split(' ',$_[0]);
4858 vec($bits,fileno($_),1) = 1;
4862 $rin = fhbits('STDIN TTY SOCK');
4866 ($nfound,$timeleft) =
4867 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4869 or to block until something becomes ready just do this
4871 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4873 Most systems do not bother to return anything useful in $timeleft, so
4874 calling select() in scalar context just returns $nfound.
4876 Any of the bit masks can also be undef. The timeout, if specified, is
4877 in seconds, which may be fractional. Note: not all implementations are
4878 capable of returning the $timeleft. If not, they always return
4879 $timeleft equal to the supplied $timeout.
4881 You can effect a sleep of 250 milliseconds this way:
4883 select(undef, undef, undef, 0.25);
4885 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4886 is implementation-dependent. See also L<perlport> for notes on the
4887 portability of C<select>.
4889 On error, C<select> behaves like the select(2) system call : it returns
4892 Note: on some Unixes, the select(2) system call may report a socket file
4893 descriptor as "ready for reading", when actually no data is available,
4894 thus a subsequent read blocks. It can be avoided using always the
4895 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4898 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4899 or <FH>) with C<select>, except as permitted by POSIX, and even
4900 then only on POSIX systems. You have to use C<sysread> instead.
4902 =item semctl ID,SEMNUM,CMD,ARG
4905 Calls the System V IPC function C<semctl>. You'll probably have to say
4909 first to get the correct constant definitions. If CMD is IPC_STAT or
4910 GETALL, then ARG must be a variable that will hold the returned
4911 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4912 the undefined value for error, "C<0 but true>" for zero, or the actual
4913 return value otherwise. The ARG must consist of a vector of native
4914 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4915 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4918 =item semget KEY,NSEMS,FLAGS
4921 Calls the System V IPC function semget. Returns the semaphore id, or
4922 the undefined value if there is an error. See also
4923 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4926 =item semop KEY,OPSTRING
4929 Calls the System V IPC function semop to perform semaphore operations
4930 such as signalling and waiting. OPSTRING must be a packed array of
4931 semop structures. Each semop structure can be generated with
4932 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4933 implies the number of semaphore operations. Returns true if
4934 successful, or false if there is an error. As an example, the
4935 following code waits on semaphore $semnum of semaphore id $semid:
4937 $semop = pack("s!3", $semnum, -1, 0);
4938 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4940 To signal the semaphore, replace C<-1> with C<1>. See also
4941 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4944 =item send SOCKET,MSG,FLAGS,TO
4947 =item send SOCKET,MSG,FLAGS
4949 Sends a message on a socket. Attempts to send the scalar MSG to the
4950 SOCKET filehandle. Takes the same flags as the system call of the
4951 same name. On unconnected sockets you must specify a destination to
4952 send TO, in which case it does a C C<sendto>. Returns the number of
4953 characters sent, or the undefined value if there is an error. The C
4954 system call sendmsg(2) is currently unimplemented. See
4955 L<perlipc/"UDP: Message Passing"> for examples.
4957 Note the I<characters>: depending on the status of the socket, either
4958 (8-bit) bytes or characters are sent. By default all sockets operate
4959 on bytes, but for example if the socket has been changed using
4960 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4961 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4962 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4963 in that case pretty much any characters can be sent.
4965 =item setpgrp PID,PGRP
4968 Sets the current process group for the specified PID, C<0> for the current
4969 process. Will produce a fatal error if used on a machine that doesn't
4970 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4971 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4972 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4975 =item setpriority WHICH,WHO,PRIORITY
4976 X<setpriority> X<priority> X<nice> X<renice>
4978 Sets the current priority for a process, a process group, or a user.
4979 (See setpriority(2).) Will produce a fatal error if used on a machine
4980 that doesn't implement setpriority(2).
4982 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4985 Sets the socket option requested. Returns undefined if there is an
4986 error. Use integer constants provided by the C<Socket> module for
4987 LEVEL and OPNAME. Values for LEVEL can also be obtained from
4988 getprotobyname. OPTVAL might either be a packed string or an integer.
4989 An integer OPTVAL is shorthand for pack("i", OPTVAL).
4991 An example disabling the Nagle's algorithm for a socket:
4993 use Socket qw(IPPROTO_TCP TCP_NODELAY);
4994 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5001 Shifts the first value of the array off and returns it, shortening the
5002 array by 1 and moving everything down. If there are no elements in the
5003 array, returns the undefined value. If ARRAY is omitted, shifts the
5004 C<@_> array within the lexical scope of subroutines and formats, and the
5005 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5006 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5007 C<UNITCHECK {}> and C<END {}> constructs.
5009 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5010 same thing to the left end of an array that C<pop> and C<push> do to the
5013 =item shmctl ID,CMD,ARG
5016 Calls the System V IPC function shmctl. You'll probably have to say
5020 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5021 then ARG must be a variable that will hold the returned C<shmid_ds>
5022 structure. Returns like ioctl: the undefined value for error, "C<0> but
5023 true" for zero, or the actual return value otherwise.
5024 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5026 =item shmget KEY,SIZE,FLAGS
5029 Calls the System V IPC function shmget. Returns the shared memory
5030 segment id, or the undefined value if there is an error.
5031 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5033 =item shmread ID,VAR,POS,SIZE
5037 =item shmwrite ID,STRING,POS,SIZE
5039 Reads or writes the System V shared memory segment ID starting at
5040 position POS for size SIZE by attaching to it, copying in/out, and
5041 detaching from it. When reading, VAR must be a variable that will
5042 hold the data read. When writing, if STRING is too long, only SIZE
5043 bytes are used; if STRING is too short, nulls are written to fill out
5044 SIZE bytes. Return true if successful, or false if there is an error.
5045 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5046 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5048 =item shutdown SOCKET,HOW
5051 Shuts down a socket connection in the manner indicated by HOW, which
5052 has the same interpretation as in the system call of the same name.
5054 shutdown(SOCKET, 0); # I/we have stopped reading data
5055 shutdown(SOCKET, 1); # I/we have stopped writing data
5056 shutdown(SOCKET, 2); # I/we have stopped using this socket
5058 This is useful with sockets when you want to tell the other
5059 side you're done writing but not done reading, or vice versa.
5060 It's also a more insistent form of close because it also
5061 disables the file descriptor in any forked copies in other
5065 X<sin> X<sine> X<asin> X<arcsine>
5069 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5070 returns sine of C<$_>.
5072 For the inverse sine operation, you may use the C<Math::Trig::asin>
5073 function, or use this relation:
5075 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5082 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5083 May be interrupted if the process receives a signal such as C<SIGALRM>.
5084 Returns the number of seconds actually slept. You probably cannot
5085 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5088 On some older systems, it may sleep up to a full second less than what
5089 you requested, depending on how it counts seconds. Most modern systems
5090 always sleep the full amount. They may appear to sleep longer than that,
5091 however, because your process might not be scheduled right away in a
5092 busy multitasking system.
5094 For delays of finer granularity than one second, you may use Perl's
5095 C<syscall> interface to access setitimer(2) if your system supports
5096 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5097 and starting from Perl 5.8 part of the standard distribution) may also
5100 See also the POSIX module's C<pause> function.
5102 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5105 Opens a socket of the specified kind and attaches it to filehandle
5106 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5107 the system call of the same name. You should C<use Socket> first
5108 to get the proper definitions imported. See the examples in
5109 L<perlipc/"Sockets: Client/Server Communication">.
5111 On systems that support a close-on-exec flag on files, the flag will
5112 be set for the newly opened file descriptor, as determined by the
5113 value of $^F. See L<perlvar/$^F>.
5115 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5118 Creates an unnamed pair of sockets in the specified domain, of the
5119 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5120 for the system call of the same name. If unimplemented, yields a fatal
5121 error. Returns true if successful.
5123 On systems that support a close-on-exec flag on files, the flag will
5124 be set for the newly opened file descriptors, as determined by the value
5125 of $^F. See L<perlvar/$^F>.
5127 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5128 to C<pipe(Rdr, Wtr)> is essentially:
5131 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5132 shutdown(Rdr, 1); # no more writing for reader
5133 shutdown(Wtr, 0); # no more reading for writer
5135 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5136 emulate socketpair using IP sockets to localhost if your system implements
5137 sockets but not socketpair.
5139 =item sort SUBNAME LIST
5140 X<sort> X<qsort> X<quicksort> X<mergesort>
5142 =item sort BLOCK LIST
5146 In list context, this sorts the LIST and returns the sorted list value.
5147 In scalar context, the behaviour of C<sort()> is undefined.
5149 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5150 order. If SUBNAME is specified, it gives the name of a subroutine
5151 that returns an integer less than, equal to, or greater than C<0>,
5152 depending on how the elements of the list are to be ordered. (The C<<
5153 <=> >> and C<cmp> operators are extremely useful in such routines.)
5154 SUBNAME may be a scalar variable name (unsubscripted), in which case
5155 the value provides the name of (or a reference to) the actual
5156 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5157 an anonymous, in-line sort subroutine.
5159 If the subroutine's prototype is C<($$)>, the elements to be compared
5160 are passed by reference in C<@_>, as for a normal subroutine. This is
5161 slower than unprototyped subroutines, where the elements to be
5162 compared are passed into the subroutine
5163 as the package global variables $a and $b (see example below). Note that
5164 in the latter case, it is usually counter-productive to declare $a and
5167 The values to be compared are always passed by reference and should not
5170 You also cannot exit out of the sort block or subroutine using any of the
5171 loop control operators described in L<perlsyn> or with C<goto>.
5173 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5174 current collation locale. See L<perllocale>.
5176 sort() returns aliases into the original list, much as a for loop's index
5177 variable aliases the list elements. That is, modifying an element of a
5178 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5179 actually modifies the element in the original list. This is usually
5180 something to be avoided when writing clear code.
5182 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5183 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5184 preserves the input order of elements that compare equal. Although
5185 quicksort's run time is O(NlogN) when averaged over all arrays of
5186 length N, the time can be O(N**2), I<quadratic> behavior, for some
5187 inputs.) In 5.7, the quicksort implementation was replaced with
5188 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5189 But benchmarks indicated that for some inputs, on some platforms,
5190 the original quicksort was faster. 5.8 has a sort pragma for
5191 limited control of the sort. Its rather blunt control of the
5192 underlying algorithm may not persist into future Perls, but the
5193 ability to characterize the input or output in implementation
5194 independent ways quite probably will. See L<sort>.
5199 @articles = sort @files;
5201 # same thing, but with explicit sort routine
5202 @articles = sort {$a cmp $b} @files;
5204 # now case-insensitively
5205 @articles = sort {uc($a) cmp uc($b)} @files;
5207 # same thing in reversed order
5208 @articles = sort {$b cmp $a} @files;
5210 # sort numerically ascending
5211 @articles = sort {$a <=> $b} @files;
5213 # sort numerically descending
5214 @articles = sort {$b <=> $a} @files;
5216 # this sorts the %age hash by value instead of key
5217 # using an in-line function
5218 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5220 # sort using explicit subroutine name
5222 $age{$a} <=> $age{$b}; # presuming numeric
5224 @sortedclass = sort byage @class;
5226 sub backwards { $b cmp $a }
5227 @harry = qw(dog cat x Cain Abel);
5228 @george = qw(gone chased yz Punished Axed);
5230 # prints AbelCaincatdogx
5231 print sort backwards @harry;
5232 # prints xdogcatCainAbel
5233 print sort @george, 'to', @harry;
5234 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5236 # inefficiently sort by descending numeric compare using
5237 # the first integer after the first = sign, or the
5238 # whole record case-insensitively otherwise
5241 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5246 # same thing, but much more efficiently;
5247 # we'll build auxiliary indices instead
5251 push @nums, /=(\d+)/;
5256 $nums[$b] <=> $nums[$a]
5258 $caps[$a] cmp $caps[$b]
5262 # same thing, but without any temps
5263 @new = map { $_->[0] }
5264 sort { $b->[1] <=> $a->[1]
5267 } map { [$_, /=(\d+)/, uc($_)] } @old;
5269 # using a prototype allows you to use any comparison subroutine
5270 # as a sort subroutine (including other package's subroutines)
5272 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5275 @new = sort other::backwards @old;
5277 # guarantee stability, regardless of algorithm
5279 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5281 # force use of mergesort (not portable outside Perl 5.8)
5282 use sort '_mergesort'; # note discouraging _
5283 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5285 If you're using strict, you I<must not> declare $a
5286 and $b as lexicals. They are package globals. That means
5287 if you're in the C<main> package and type
5289 @articles = sort {$b <=> $a} @files;
5291 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5292 but if you're in the C<FooPack> package, it's the same as typing
5294 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5296 The comparison function is required to behave. If it returns
5297 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5298 sometimes saying the opposite, for example) the results are not
5301 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5302 (not-a-number), and because C<sort> will trigger a fatal error unless the
5303 result of a comparison is defined, when sorting with a comparison function
5304 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5305 The following example takes advantage of the fact that C<NaN != NaN> to
5306 eliminate any C<NaN>s from the input.
5308 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5310 =item splice ARRAY,OFFSET,LENGTH,LIST
5313 =item splice ARRAY,OFFSET,LENGTH
5315 =item splice ARRAY,OFFSET
5319 Removes the elements designated by OFFSET and LENGTH from an array, and
5320 replaces them with the elements of LIST, if any. In list context,
5321 returns the elements removed from the array. In scalar context,
5322 returns the last element removed, or C<undef> if no elements are
5323 removed. The array grows or shrinks as necessary.
5324 If OFFSET is negative then it starts that far from the end of the array.
5325 If LENGTH is omitted, removes everything from OFFSET onward.
5326 If LENGTH is negative, removes the elements from OFFSET onward
5327 except for -LENGTH elements at the end of the array.
5328 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5329 past the end of the array, perl issues a warning, and splices at the
5332 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5334 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5335 pop(@a) splice(@a,-1)
5336 shift(@a) splice(@a,0,1)
5337 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5338 $a[$i] = $y splice(@a,$i,1,$y)
5340 Example, assuming array lengths are passed before arrays:
5342 sub aeq { # compare two list values
5343 my(@a) = splice(@_,0,shift);
5344 my(@b) = splice(@_,0,shift);
5345 return 0 unless @a == @b; # same len?
5347 return 0 if pop(@a) ne pop(@b);
5351 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5353 =item split /PATTERN/,EXPR,LIMIT
5356 =item split /PATTERN/,EXPR
5358 =item split /PATTERN/
5362 Splits the string EXPR into a list of strings and returns that list. By
5363 default, empty leading fields are preserved, and empty trailing ones are
5364 deleted. (If all fields are empty, they are considered to be trailing.)
5366 In scalar context, returns the number of fields found and splits into
5367 the C<@_> array. Use of split in scalar context is deprecated, however,
5368 because it clobbers your subroutine arguments.
5370 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5371 splits on whitespace (after skipping any leading whitespace). Anything
5372 matching PATTERN is taken to be a delimiter separating the fields. (Note
5373 that the delimiter may be longer than one character.)
5375 If LIMIT is specified and positive, it represents the maximum number
5376 of fields the EXPR will be split into, though the actual number of
5377 fields returned depends on the number of times PATTERN matches within
5378 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5379 stripped (which potential users of C<pop> would do well to remember).
5380 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5381 had been specified. Note that splitting an EXPR that evaluates to the
5382 empty string always returns the empty list, regardless of the LIMIT
5385 A pattern matching the null string (not to be confused with
5386 a null pattern C<//>, which is just one member of the set of patterns
5387 matching a null string) will split the value of EXPR into separate
5388 characters at each point it matches that way. For example:
5390 print join(':', split(/ */, 'hi there'));
5392 produces the output 'h:i:t:h:e:r:e'.
5394 As a special case for C<split>, using the empty pattern C<//> specifically
5395 matches only the null string, and is not be confused with the regular use
5396 of C<//> to mean "the last successful pattern match". So, for C<split>,
5399 print join(':', split(//, 'hi there'));
5401 produces the output 'h:i: :t:h:e:r:e'.
5403 Empty leading (or trailing) fields are produced when there are positive
5404 width matches at the beginning (or end) of the string; a zero-width match
5405 at the beginning (or end) of the string does not produce an empty field.
5408 print join(':', split(/(?=\w)/, 'hi there!'));
5410 produces the output 'h:i :t:h:e:r:e!'.
5412 The LIMIT parameter can be used to split a line partially
5414 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5416 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5417 a LIMIT one larger than the number of variables in the list, to avoid
5418 unnecessary work. For the list above LIMIT would have been 4 by
5419 default. In time critical applications it behooves you not to split
5420 into more fields than you really need.
5422 If the PATTERN contains parentheses, additional list elements are
5423 created from each matching substring in the delimiter.
5425 split(/([,-])/, "1-10,20", 3);
5427 produces the list value
5429 (1, '-', 10, ',', 20)
5431 If you had the entire header of a normal Unix email message in $header,
5432 you could split it up into fields and their values this way:
5434 $header =~ s/\n\s+/ /g; # fix continuation lines
5435 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5437 The pattern C</PATTERN/> may be replaced with an expression to specify
5438 patterns that vary at runtime. (To do runtime compilation only once,
5439 use C</$variable/o>.)
5441 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5442 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5443 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5444 will give you as many null initial fields as there are leading spaces.
5445 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5446 whitespace produces a null first field. A C<split> with no arguments
5447 really does a S<C<split(' ', $_)>> internally.
5449 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5454 open(PASSWD, '/etc/passwd');
5457 ($login, $passwd, $uid, $gid,
5458 $gcos, $home, $shell) = split(/:/);
5462 As with regular pattern matching, any capturing parentheses that are not
5463 matched in a C<split()> will be set to C<undef> when returned:
5465 @fields = split /(A)|B/, "1A2B3";
5466 # @fields is (1, 'A', 2, undef, 3)
5468 =item sprintf FORMAT, LIST
5471 Returns a string formatted by the usual C<printf> conventions of the C
5472 library function C<sprintf>. See below for more details
5473 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5474 the general principles.
5478 # Format number with up to 8 leading zeroes
5479 $result = sprintf("%08d", $number);
5481 # Round number to 3 digits after decimal point
5482 $rounded = sprintf("%.3f", $number);
5484 Perl does its own C<sprintf> formatting--it emulates the C
5485 function C<sprintf>, but it doesn't use it (except for floating-point
5486 numbers, and even then only the standard modifiers are allowed). As a
5487 result, any non-standard extensions in your local C<sprintf> are not
5488 available from Perl.
5490 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5491 pass it an array as your first argument. The array is given scalar context,
5492 and instead of using the 0th element of the array as the format, Perl will
5493 use the count of elements in the array as the format, which is almost never
5496 Perl's C<sprintf> permits the following universally-known conversions:
5499 %c a character with the given number
5501 %d a signed integer, in decimal
5502 %u an unsigned integer, in decimal
5503 %o an unsigned integer, in octal
5504 %x an unsigned integer, in hexadecimal
5505 %e a floating-point number, in scientific notation
5506 %f a floating-point number, in fixed decimal notation
5507 %g a floating-point number, in %e or %f notation
5509 In addition, Perl permits the following widely-supported conversions:
5511 %X like %x, but using upper-case letters
5512 %E like %e, but using an upper-case "E"
5513 %G like %g, but with an upper-case "E" (if applicable)
5514 %b an unsigned integer, in binary
5515 %B like %b, but using an upper-case "B" with the # flag
5516 %p a pointer (outputs the Perl value's address in hexadecimal)
5517 %n special: *stores* the number of characters output so far
5518 into the next variable in the parameter list
5520 Finally, for backward (and we do mean "backward") compatibility, Perl
5521 permits these unnecessary but widely-supported conversions:
5524 %D a synonym for %ld
5525 %U a synonym for %lu
5526 %O a synonym for %lo
5529 Note that the number of exponent digits in the scientific notation produced
5530 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5531 exponent less than 100 is system-dependent: it may be three or less
5532 (zero-padded as necessary). In other words, 1.23 times ten to the
5533 99th may be either "1.23e99" or "1.23e099".
5535 Between the C<%> and the format letter, you may specify a number of
5536 additional attributes controlling the interpretation of the format.
5537 In order, these are:
5541 =item format parameter index
5543 An explicit format parameter index, such as C<2$>. By default sprintf
5544 will format the next unused argument in the list, but this allows you
5545 to take the arguments out of order, e.g.:
5547 printf '%2$d %1$d', 12, 34; # prints "34 12"
5548 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5554 space prefix positive number with a space
5555 + prefix positive number with a plus sign
5556 - left-justify within the field
5557 0 use zeros, not spaces, to right-justify
5558 # ensure the leading "0" for any octal,
5559 prefix non-zero hexadecimal with "0x" or "0X",
5560 prefix non-zero binary with "0b" or "0B"
5564 printf '<% d>', 12; # prints "< 12>"
5565 printf '<%+d>', 12; # prints "<+12>"
5566 printf '<%6s>', 12; # prints "< 12>"
5567 printf '<%-6s>', 12; # prints "<12 >"
5568 printf '<%06s>', 12; # prints "<000012>"
5569 printf '<%#o>', 12; # prints "<014>"
5570 printf '<%#x>', 12; # prints "<0xc>"
5571 printf '<%#X>', 12; # prints "<0XC>"
5572 printf '<%#b>', 12; # prints "<0b1100>"
5573 printf '<%#B>', 12; # prints "<0B1100>"
5575 When a space and a plus sign are given as the flags at once,
5576 a plus sign is used to prefix a positive number.
5578 printf '<%+ d>', 12; # prints "<+12>"
5579 printf '<% +d>', 12; # prints "<+12>"
5581 When the # flag and a precision are given in the %o conversion,
5582 the precision is incremented if it's necessary for the leading "0".
5584 printf '<%#.5o>', 012; # prints "<00012>"
5585 printf '<%#.5o>', 012345; # prints "<012345>"
5586 printf '<%#.0o>', 0; # prints "<0>"
5590 This flag tells perl to interpret the supplied string as a vector of
5591 integers, one for each character in the string. Perl applies the format to
5592 each integer in turn, then joins the resulting strings with a separator (a
5593 dot C<.> by default). This can be useful for displaying ordinal values of
5594 characters in arbitrary strings:
5596 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5597 printf "version is v%vd\n", $^V; # Perl's version
5599 Put an asterisk C<*> before the C<v> to override the string to
5600 use to separate the numbers:
5602 printf "address is %*vX\n", ":", $addr; # IPv6 address
5603 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5605 You can also explicitly specify the argument number to use for
5606 the join string using e.g. C<*2$v>:
5608 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5610 =item (minimum) width
5612 Arguments are usually formatted to be only as wide as required to
5613 display the given value. You can override the width by putting
5614 a number here, or get the width from the next argument (with C<*>)
5615 or from a specified argument (with e.g. C<*2$>):
5617 printf '<%s>', "a"; # prints "<a>"
5618 printf '<%6s>', "a"; # prints "< a>"
5619 printf '<%*s>', 6, "a"; # prints "< a>"
5620 printf '<%*2$s>', "a", 6; # prints "< a>"
5621 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5623 If a field width obtained through C<*> is negative, it has the same
5624 effect as the C<-> flag: left-justification.
5626 =item precision, or maximum width
5629 You can specify a precision (for numeric conversions) or a maximum
5630 width (for string conversions) by specifying a C<.> followed by a number.
5631 For floating point formats, with the exception of 'g' and 'G', this specifies
5632 the number of decimal places to show (the default being 6), e.g.:
5634 # these examples are subject to system-specific variation
5635 printf '<%f>', 1; # prints "<1.000000>"
5636 printf '<%.1f>', 1; # prints "<1.0>"
5637 printf '<%.0f>', 1; # prints "<1>"
5638 printf '<%e>', 10; # prints "<1.000000e+01>"
5639 printf '<%.1e>', 10; # prints "<1.0e+01>"
5641 For 'g' and 'G', this specifies the maximum number of digits to show,
5642 including prior to the decimal point as well as after it, e.g.:
5644 # these examples are subject to system-specific variation
5645 printf '<%g>', 1; # prints "<1>"
5646 printf '<%.10g>', 1; # prints "<1>"
5647 printf '<%g>', 100; # prints "<100>"
5648 printf '<%.1g>', 100; # prints "<1e+02>"
5649 printf '<%.2g>', 100.01; # prints "<1e+02>"
5650 printf '<%.5g>', 100.01; # prints "<100.01>"
5651 printf '<%.4g>', 100.01; # prints "<100>"
5653 For integer conversions, specifying a precision implies that the
5654 output of the number itself should be zero-padded to this width,
5655 where the 0 flag is ignored:
5657 printf '<%.6d>', 1; # prints "<000001>"
5658 printf '<%+.6d>', 1; # prints "<+000001>"
5659 printf '<%-10.6d>', 1; # prints "<000001 >"
5660 printf '<%10.6d>', 1; # prints "< 000001>"
5661 printf '<%010.6d>', 1; # prints "< 000001>"
5662 printf '<%+10.6d>', 1; # prints "< +000001>"
5664 printf '<%.6x>', 1; # prints "<000001>"
5665 printf '<%#.6x>', 1; # prints "<0x000001>"
5666 printf '<%-10.6x>', 1; # prints "<000001 >"
5667 printf '<%10.6x>', 1; # prints "< 000001>"
5668 printf '<%010.6x>', 1; # prints "< 000001>"
5669 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5671 For string conversions, specifying a precision truncates the string
5672 to fit in the specified width:
5674 printf '<%.5s>', "truncated"; # prints "<trunc>"
5675 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5677 You can also get the precision from the next argument using C<.*>:
5679 printf '<%.6x>', 1; # prints "<000001>"
5680 printf '<%.*x>', 6, 1; # prints "<000001>"
5682 If a precision obtained through C<*> is negative, it has the same
5683 effect as no precision.
5685 printf '<%.*s>', 7, "string"; # prints "<string>"
5686 printf '<%.*s>', 3, "string"; # prints "<str>"
5687 printf '<%.*s>', 0, "string"; # prints "<>"
5688 printf '<%.*s>', -1, "string"; # prints "<string>"
5690 printf '<%.*d>', 1, 0; # prints "<0>"
5691 printf '<%.*d>', 0, 0; # prints "<>"
5692 printf '<%.*d>', -1, 0; # prints "<0>"
5694 You cannot currently get the precision from a specified number,
5695 but it is intended that this will be possible in the future using
5698 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5702 For numeric conversions, you can specify the size to interpret the
5703 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5704 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5705 whatever the default integer size is on your platform (usually 32 or 64
5706 bits), but you can override this to use instead one of the standard C types,
5707 as supported by the compiler used to build Perl:
5709 l interpret integer as C type "long" or "unsigned long"
5710 h interpret integer as C type "short" or "unsigned short"
5711 q, L or ll interpret integer as C type "long long", "unsigned long long".
5712 or "quads" (typically 64-bit integers)
5714 The last will produce errors if Perl does not understand "quads" in your
5715 installation. (This requires that either the platform natively supports quads
5716 or Perl was specifically compiled to support quads.) You can find out
5717 whether your Perl supports quads via L<Config>:
5720 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5723 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5724 to be the default floating point size on your platform (double or long double),
5725 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5726 platform supports them. You can find out whether your Perl supports long
5727 doubles via L<Config>:
5730 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5732 You can find out whether Perl considers 'long double' to be the default
5733 floating point size to use on your platform via L<Config>:
5736 ($Config{uselongdouble} eq 'define') &&
5737 print "long doubles by default\n";
5739 It can also be the case that long doubles and doubles are the same thing:
5742 ($Config{doublesize} == $Config{longdblsize}) &&
5743 print "doubles are long doubles\n";
5745 The size specifier C<V> has no effect for Perl code, but it is supported
5746 for compatibility with XS code; it means 'use the standard size for
5747 a Perl integer (or floating-point number)', which is already the
5748 default for Perl code.
5750 =item order of arguments
5752 Normally, sprintf takes the next unused argument as the value to
5753 format for each format specification. If the format specification
5754 uses C<*> to require additional arguments, these are consumed from
5755 the argument list in the order in which they appear in the format
5756 specification I<before> the value to format. Where an argument is
5757 specified using an explicit index, this does not affect the normal
5758 order for the arguments (even when the explicitly specified index
5759 would have been the next argument in any case).
5763 printf '<%*.*s>', $a, $b, $c;
5765 would use C<$a> for the width, C<$b> for the precision and C<$c>
5766 as the value to format, while:
5768 print '<%*1$.*s>', $a, $b;
5770 would use C<$a> for the width and the precision, and C<$b> as the
5773 Here are some more examples - beware that when using an explicit
5774 index, the C<$> may need to be escaped:
5776 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5777 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5778 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5779 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5783 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5784 the character used for the decimal separator in formatted floating
5785 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5789 X<sqrt> X<root> X<square root>
5793 Return the square root of EXPR. If EXPR is omitted, returns square
5794 root of C<$_>. Only works on non-negative operands, unless you've
5795 loaded the standard Math::Complex module.
5798 print sqrt(-2); # prints 1.4142135623731i
5801 X<srand> X<seed> X<randseed>
5805 Sets the random number seed for the C<rand> operator.
5807 The point of the function is to "seed" the C<rand> function so that
5808 C<rand> can produce a different sequence each time you run your
5811 If srand() is not called explicitly, it is called implicitly at the
5812 first use of the C<rand> operator. However, this was not the case in
5813 versions of Perl before 5.004, so if your script will run under older
5814 Perl versions, it should call C<srand>.
5816 Most programs won't even call srand() at all, except those that
5817 need a cryptographically-strong starting point rather than the
5818 generally acceptable default, which is based on time of day,
5819 process ID, and memory allocation, or the F</dev/urandom> device,
5822 You can call srand($seed) with the same $seed to reproduce the
5823 I<same> sequence from rand(), but this is usually reserved for
5824 generating predictable results for testing or debugging.
5825 Otherwise, don't call srand() more than once in your program.
5827 Do B<not> call srand() (i.e. without an argument) more than once in
5828 a script. The internal state of the random number generator should
5829 contain more entropy than can be provided by any seed, so calling
5830 srand() again actually I<loses> randomness.
5832 Most implementations of C<srand> take an integer and will silently
5833 truncate decimal numbers. This means C<srand(42)> will usually
5834 produce the same results as C<srand(42.1)>. To be safe, always pass
5835 C<srand> an integer.
5837 In versions of Perl prior to 5.004 the default seed was just the
5838 current C<time>. This isn't a particularly good seed, so many old
5839 programs supply their own seed value (often C<time ^ $$> or C<time ^
5840 ($$ + ($$ << 15))>), but that isn't necessary any more.
5842 For cryptographic purposes, however, you need something much more random
5843 than the default seed. Checksumming the compressed output of one or more
5844 rapidly changing operating system status programs is the usual method. For
5847 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5849 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5852 Frequently called programs (like CGI scripts) that simply use
5856 for a seed can fall prey to the mathematical property that
5860 one-third of the time. So don't do that.
5862 =item stat FILEHANDLE
5863 X<stat> X<file, status> X<ctime>
5867 =item stat DIRHANDLE
5871 Returns a 13-element list giving the status info for a file, either
5872 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5873 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5876 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5877 $atime,$mtime,$ctime,$blksize,$blocks)
5880 Not all fields are supported on all filesystem types. Here are the
5881 meanings of the fields:
5883 0 dev device number of filesystem
5885 2 mode file mode (type and permissions)
5886 3 nlink number of (hard) links to the file
5887 4 uid numeric user ID of file's owner
5888 5 gid numeric group ID of file's owner
5889 6 rdev the device identifier (special files only)
5890 7 size total size of file, in bytes
5891 8 atime last access time in seconds since the epoch
5892 9 mtime last modify time in seconds since the epoch
5893 10 ctime inode change time in seconds since the epoch (*)
5894 11 blksize preferred block size for file system I/O
5895 12 blocks actual number of blocks allocated
5897 (The epoch was at 00:00 January 1, 1970 GMT.)
5899 (*) Not all fields are supported on all filesystem types. Notably, the
5900 ctime field is non-portable. In particular, you cannot expect it to be a
5901 "creation time", see L<perlport/"Files and Filesystems"> for details.
5903 If C<stat> is passed the special filehandle consisting of an underline, no
5904 stat is done, but the current contents of the stat structure from the
5905 last C<stat>, C<lstat>, or filetest are returned. Example:
5907 if (-x $file && (($d) = stat(_)) && $d < 0) {
5908 print "$file is executable NFS file\n";
5911 (This works on machines only for which the device number is negative
5914 Because the mode contains both the file type and its permissions, you
5915 should mask off the file type portion and (s)printf using a C<"%o">
5916 if you want to see the real permissions.
5918 $mode = (stat($filename))[2];
5919 printf "Permissions are %04o\n", $mode & 07777;
5921 In scalar context, C<stat> returns a boolean value indicating success
5922 or failure, and, if successful, sets the information associated with
5923 the special filehandle C<_>.
5925 The L<File::stat> module provides a convenient, by-name access mechanism:
5928 $sb = stat($filename);
5929 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5930 $filename, $sb->size, $sb->mode & 07777,
5931 scalar localtime $sb->mtime;
5933 You can import symbolic mode constants (C<S_IF*>) and functions
5934 (C<S_IS*>) from the Fcntl module:
5938 $mode = (stat($filename))[2];
5940 $user_rwx = ($mode & S_IRWXU) >> 6;
5941 $group_read = ($mode & S_IRGRP) >> 3;
5942 $other_execute = $mode & S_IXOTH;
5944 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5946 $is_setuid = $mode & S_ISUID;
5947 $is_directory = S_ISDIR($mode);
5949 You could write the last two using the C<-u> and C<-d> operators.
5950 The commonly available C<S_IF*> constants are
5952 # Permissions: read, write, execute, for user, group, others.
5954 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5955 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5956 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5958 # Setuid/Setgid/Stickiness/SaveText.
5959 # Note that the exact meaning of these is system dependent.
5961 S_ISUID S_ISGID S_ISVTX S_ISTXT
5963 # File types. Not necessarily all are available on your system.
5965 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5967 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5969 S_IREAD S_IWRITE S_IEXEC
5971 and the C<S_IF*> functions are
5973 S_IMODE($mode) the part of $mode containing the permission bits
5974 and the setuid/setgid/sticky bits
5976 S_IFMT($mode) the part of $mode containing the file type
5977 which can be bit-anded with e.g. S_IFREG
5978 or with the following functions
5980 # The operators -f, -d, -l, -b, -c, -p, and -S.
5982 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5983 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5985 # No direct -X operator counterpart, but for the first one
5986 # the -g operator is often equivalent. The ENFMT stands for
5987 # record flocking enforcement, a platform-dependent feature.
5989 S_ISENFMT($mode) S_ISWHT($mode)
5991 See your native chmod(2) and stat(2) documentation for more details
5992 about the C<S_*> constants. To get status info for a symbolic link
5993 instead of the target file behind the link, use the C<lstat> function.
5998 =item state TYPE EXPR
6000 =item state EXPR : ATTRS
6002 =item state TYPE EXPR : ATTRS
6004 C<state> declares a lexically scoped variable, just like C<my> does.
6005 However, those variables will be initialized only once, contrary to
6006 lexical variables that are reinitialized each time their enclosing block
6009 C<state> variables are only enabled when the C<feature 'state'> pragma is
6010 in effect. See L<feature>.
6017 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6018 doing many pattern matches on the string before it is next modified.
6019 This may or may not save time, depending on the nature and number of
6020 patterns you are searching on, and on the distribution of character
6021 frequencies in the string to be searched--you probably want to compare
6022 run times with and without it to see which runs faster. Those loops
6023 that scan for many short constant strings (including the constant
6024 parts of more complex patterns) will benefit most. You may have only
6025 one C<study> active at a time--if you study a different scalar the first
6026 is "unstudied". (The way C<study> works is this: a linked list of every
6027 character in the string to be searched is made, so we know, for
6028 example, where all the C<'k'> characters are. From each search string,
6029 the rarest character is selected, based on some static frequency tables
6030 constructed from some C programs and English text. Only those places
6031 that contain this "rarest" character are examined.)
6033 For example, here is a loop that inserts index producing entries
6034 before any line containing a certain pattern:
6038 print ".IX foo\n" if /\bfoo\b/;
6039 print ".IX bar\n" if /\bbar\b/;
6040 print ".IX blurfl\n" if /\bblurfl\b/;
6045 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6046 will be looked at, because C<f> is rarer than C<o>. In general, this is
6047 a big win except in pathological cases. The only question is whether
6048 it saves you more time than it took to build the linked list in the
6051 Note that if you have to look for strings that you don't know till
6052 runtime, you can build an entire loop as a string and C<eval> that to
6053 avoid recompiling all your patterns all the time. Together with
6054 undefining C<$/> to input entire files as one record, this can be very
6055 fast, often faster than specialized programs like fgrep(1). The following
6056 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6057 out the names of those files that contain a match:
6059 $search = 'while (<>) { study;';
6060 foreach $word (@words) {
6061 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6066 eval $search; # this screams
6067 $/ = "\n"; # put back to normal input delimiter
6068 foreach $file (sort keys(%seen)) {
6072 =item sub NAME BLOCK
6075 =item sub NAME (PROTO) BLOCK
6077 =item sub NAME : ATTRS BLOCK
6079 =item sub NAME (PROTO) : ATTRS BLOCK
6081 This is subroutine definition, not a real function I<per se>.
6082 Without a BLOCK it's just a forward declaration. Without a NAME,
6083 it's an anonymous function declaration, and does actually return
6084 a value: the CODE ref of the closure you just created.
6086 See L<perlsub> and L<perlref> for details about subroutines and
6087 references, and L<attributes> and L<Attribute::Handlers> for more
6088 information about attributes.
6090 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6091 X<substr> X<substring> X<mid> X<left> X<right>
6093 =item substr EXPR,OFFSET,LENGTH
6095 =item substr EXPR,OFFSET
6097 Extracts a substring out of EXPR and returns it. First character is at
6098 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6099 If OFFSET is negative (or more precisely, less than C<$[>), starts
6100 that far from the end of the string. If LENGTH is omitted, returns
6101 everything to the end of the string. If LENGTH is negative, leaves that
6102 many characters off the end of the string.
6104 my $s = "The black cat climbed the green tree";
6105 my $color = substr $s, 4, 5; # black
6106 my $middle = substr $s, 4, -11; # black cat climbed the
6107 my $end = substr $s, 14; # climbed the green tree
6108 my $tail = substr $s, -4; # tree
6109 my $z = substr $s, -4, 2; # tr
6111 You can use the substr() function as an lvalue, in which case EXPR
6112 must itself be an lvalue. If you assign something shorter than LENGTH,
6113 the string will shrink, and if you assign something longer than LENGTH,
6114 the string will grow to accommodate it. To keep the string the same
6115 length you may need to pad or chop your value using C<sprintf>.
6117 If OFFSET and LENGTH specify a substring that is partly outside the
6118 string, only the part within the string is returned. If the substring
6119 is beyond either end of the string, substr() returns the undefined
6120 value and produces a warning. When used as an lvalue, specifying a
6121 substring that is entirely outside the string is a fatal error.
6122 Here's an example showing the behavior for boundary cases:
6125 substr($name, 4) = 'dy'; # $name is now 'freddy'
6126 my $null = substr $name, 6, 2; # returns '' (no warning)
6127 my $oops = substr $name, 7; # returns undef, with warning
6128 substr($name, 7) = 'gap'; # fatal error
6130 An alternative to using substr() as an lvalue is to specify the
6131 replacement string as the 4th argument. This allows you to replace
6132 parts of the EXPR and return what was there before in one operation,
6133 just as you can with splice().
6135 my $s = "The black cat climbed the green tree";
6136 my $z = substr $s, 14, 7, "jumped from"; # climbed
6137 # $s is now "The black cat jumped from the green tree"
6139 Note that the lvalue returned by the 3-arg version of substr() acts as
6140 a 'magic bullet'; each time it is assigned to, it remembers which part
6141 of the original string is being modified; for example:
6144 for (substr($x,1,2)) {
6145 $_ = 'a'; print $x,"\n"; # prints 1a4
6146 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6148 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6151 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6154 =item symlink OLDFILE,NEWFILE
6155 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6157 Creates a new filename symbolically linked to the old filename.
6158 Returns C<1> for success, C<0> otherwise. On systems that don't support
6159 symbolic links, produces a fatal error at run time. To check for that,
6162 $symlink_exists = eval { symlink("",""); 1 };
6164 =item syscall NUMBER, LIST
6165 X<syscall> X<system call>
6167 Calls the system call specified as the first element of the list,
6168 passing the remaining elements as arguments to the system call. If
6169 unimplemented, produces a fatal error. The arguments are interpreted
6170 as follows: if a given argument is numeric, the argument is passed as
6171 an int. If not, the pointer to the string value is passed. You are
6172 responsible to make sure a string is pre-extended long enough to
6173 receive any result that might be written into a string. You can't use a
6174 string literal (or other read-only string) as an argument to C<syscall>
6175 because Perl has to assume that any string pointer might be written
6177 integer arguments are not literals and have never been interpreted in a
6178 numeric context, you may need to add C<0> to them to force them to look
6179 like numbers. This emulates the C<syswrite> function (or vice versa):
6181 require 'syscall.ph'; # may need to run h2ph
6183 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6185 Note that Perl supports passing of up to only 14 arguments to your system call,
6186 which in practice should usually suffice.
6188 Syscall returns whatever value returned by the system call it calls.
6189 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6190 Note that some system calls can legitimately return C<-1>. The proper
6191 way to handle such calls is to assign C<$!=0;> before the call and
6192 check the value of C<$!> if syscall returns C<-1>.
6194 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6195 number of the read end of the pipe it creates. There is no way
6196 to retrieve the file number of the other end. You can avoid this
6197 problem by using C<pipe> instead.
6199 =item sysopen FILEHANDLE,FILENAME,MODE
6202 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6204 Opens the file whose filename is given by FILENAME, and associates it
6205 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6206 the name of the real filehandle wanted. This function calls the
6207 underlying operating system's C<open> function with the parameters
6208 FILENAME, MODE, PERMS.
6210 The possible values and flag bits of the MODE parameter are
6211 system-dependent; they are available via the standard module C<Fcntl>.
6212 See the documentation of your operating system's C<open> to see which
6213 values and flag bits are available. You may combine several flags
6214 using the C<|>-operator.
6216 Some of the most common values are C<O_RDONLY> for opening the file in
6217 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6218 and C<O_RDWR> for opening the file in read-write mode.
6219 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6221 For historical reasons, some values work on almost every system
6222 supported by perl: zero means read-only, one means write-only, and two
6223 means read/write. We know that these values do I<not> work under
6224 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6225 use them in new code.
6227 If the file named by FILENAME does not exist and the C<open> call creates
6228 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6229 PERMS specifies the permissions of the newly created file. If you omit
6230 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6231 These permission values need to be in octal, and are modified by your
6232 process's current C<umask>.
6235 In many systems the C<O_EXCL> flag is available for opening files in
6236 exclusive mode. This is B<not> locking: exclusiveness means here that
6237 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6238 on network filesystems, and has no effect unless the C<O_CREAT> flag
6239 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6240 being opened if it is a symbolic link. It does not protect against
6241 symbolic links in the file's path.
6244 Sometimes you may want to truncate an already-existing file. This
6245 can be done using the C<O_TRUNC> flag. The behavior of
6246 C<O_TRUNC> with C<O_RDONLY> is undefined.
6249 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6250 that takes away the user's option to have a more permissive umask.
6251 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6254 Note that C<sysopen> depends on the fdopen() C library function.
6255 On many UNIX systems, fdopen() is known to fail when file descriptors
6256 exceed a certain value, typically 255. If you need more file
6257 descriptors than that, consider rebuilding Perl to use the C<sfio>
6258 library, or perhaps using the POSIX::open() function.
6260 See L<perlopentut> for a kinder, gentler explanation of opening files.
6262 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6265 =item sysread FILEHANDLE,SCALAR,LENGTH
6267 Attempts to read LENGTH bytes of data into variable SCALAR from the
6268 specified FILEHANDLE, using the system call read(2). It bypasses
6269 buffered IO, so mixing this with other kinds of reads, C<print>,
6270 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6271 perlio or stdio layers usually buffers data. Returns the number of
6272 bytes actually read, C<0> at end of file, or undef if there was an
6273 error (in the latter case C<$!> is also set). SCALAR will be grown or
6274 shrunk so that the last byte actually read is the last byte of the
6275 scalar after the read.
6277 An OFFSET may be specified to place the read data at some place in the
6278 string other than the beginning. A negative OFFSET specifies
6279 placement at that many characters counting backwards from the end of
6280 the string. A positive OFFSET greater than the length of SCALAR
6281 results in the string being padded to the required size with C<"\0">
6282 bytes before the result of the read is appended.
6284 There is no syseof() function, which is ok, since eof() doesn't work
6285 very well on device files (like ttys) anyway. Use sysread() and check
6286 for a return value for 0 to decide whether you're done.
6288 Note that if the filehandle has been marked as C<:utf8> Unicode
6289 characters are read instead of bytes (the LENGTH, OFFSET, and the
6290 return value of sysread() are in Unicode characters).
6291 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6292 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6294 =item sysseek FILEHANDLE,POSITION,WHENCE
6297 Sets FILEHANDLE's system position in bytes using the system call
6298 lseek(2). FILEHANDLE may be an expression whose value gives the name
6299 of the filehandle. The values for WHENCE are C<0> to set the new
6300 position to POSITION, C<1> to set the it to the current position plus
6301 POSITION, and C<2> to set it to EOF plus POSITION (typically
6304 Note the I<in bytes>: even if the filehandle has been set to operate
6305 on characters (for example by using the C<:utf8> I/O layer), tell()
6306 will return byte offsets, not character offsets (because implementing
6307 that would render sysseek() very slow).
6309 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6310 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6311 C<seek>, C<tell>, or C<eof> may cause confusion.
6313 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6314 and C<SEEK_END> (start of the file, current position, end of the file)
6315 from the Fcntl module. Use of the constants is also more portable
6316 than relying on 0, 1, and 2. For example to define a "systell" function:
6318 use Fcntl 'SEEK_CUR';
6319 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6321 Returns the new position, or the undefined value on failure. A position
6322 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6323 true on success and false on failure, yet you can still easily determine
6329 =item system PROGRAM LIST
6331 Does exactly the same thing as C<exec LIST>, except that a fork is
6332 done first, and the parent process waits for the child process to
6333 complete. Note that argument processing varies depending on the
6334 number of arguments. If there is more than one argument in LIST,
6335 or if LIST is an array with more than one value, starts the program
6336 given by the first element of the list with arguments given by the
6337 rest of the list. If there is only one scalar argument, the argument
6338 is checked for shell metacharacters, and if there are any, the
6339 entire argument is passed to the system's command shell for parsing
6340 (this is C</bin/sh -c> on Unix platforms, but varies on other
6341 platforms). If there are no shell metacharacters in the argument,
6342 it is split into words and passed directly to C<execvp>, which is
6345 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6346 output before any operation that may do a fork, but this may not be
6347 supported on some platforms (see L<perlport>). To be safe, you may need
6348 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6349 of C<IO::Handle> on any open handles.
6351 The return value is the exit status of the program as returned by the
6352 C<wait> call. To get the actual exit value, shift right by eight (see
6353 below). See also L</exec>. This is I<not> what you want to use to capture
6354 the output from a command, for that you should use merely backticks or
6355 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6356 indicates a failure to start the program or an error of the wait(2) system
6357 call (inspect $! for the reason).
6359 Like C<exec>, C<system> allows you to lie to a program about its name if
6360 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6362 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6363 C<system>, if you expect your program to terminate on receipt of these
6364 signals you will need to arrange to do so yourself based on the return
6367 @args = ("command", "arg1", "arg2");
6369 or die "system @args failed: $?"
6371 You can check all the failure possibilities by inspecting
6375 print "failed to execute: $!\n";
6378 printf "child died with signal %d, %s coredump\n",
6379 ($? & 127), ($? & 128) ? 'with' : 'without';
6382 printf "child exited with value %d\n", $? >> 8;
6385 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6386 with the W*() calls of the POSIX extension.
6388 When the arguments get executed via the system shell, results
6389 and return codes will be subject to its quirks and capabilities.
6390 See L<perlop/"`STRING`"> and L</exec> for details.
6392 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6395 =item syswrite FILEHANDLE,SCALAR,LENGTH
6397 =item syswrite FILEHANDLE,SCALAR
6399 Attempts to write LENGTH bytes of data from variable SCALAR to the
6400 specified FILEHANDLE, using the system call write(2). If LENGTH is
6401 not specified, writes whole SCALAR. It bypasses buffered IO, so
6402 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6403 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6404 stdio layers usually buffers data. Returns the number of bytes
6405 actually written, or C<undef> if there was an error (in this case the
6406 errno variable C<$!> is also set). If the LENGTH is greater than the
6407 available data in the SCALAR after the OFFSET, only as much data as is
6408 available will be written.
6410 An OFFSET may be specified to write the data from some part of the
6411 string other than the beginning. A negative OFFSET specifies writing
6412 that many characters counting backwards from the end of the string.
6413 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6415 Note that if the filehandle has been marked as C<:utf8>, Unicode
6416 characters are written instead of bytes (the LENGTH, OFFSET, and the
6417 return value of syswrite() are in UTF-8 encoded Unicode characters).
6418 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6419 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6421 =item tell FILEHANDLE
6426 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6427 error. FILEHANDLE may be an expression whose value gives the name of
6428 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6431 Note the I<in bytes>: even if the filehandle has been set to
6432 operate on characters (for example by using the C<:utf8> open
6433 layer), tell() will return byte offsets, not character offsets
6434 (because that would render seek() and tell() rather slow).
6436 The return value of tell() for the standard streams like the STDIN
6437 depends on the operating system: it may return -1 or something else.
6438 tell() on pipes, fifos, and sockets usually returns -1.
6440 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6442 Do not use tell() (or other buffered I/O operations) on a file handle
6443 that has been manipulated by sysread(), syswrite() or sysseek().
6444 Those functions ignore the buffering, while tell() does not.
6446 =item telldir DIRHANDLE
6449 Returns the current position of the C<readdir> routines on DIRHANDLE.
6450 Value may be given to C<seekdir> to access a particular location in a
6451 directory. C<telldir> has the same caveats about possible directory
6452 compaction as the corresponding system library routine.
6454 =item tie VARIABLE,CLASSNAME,LIST
6457 This function binds a variable to a package class that will provide the
6458 implementation for the variable. VARIABLE is the name of the variable
6459 to be enchanted. CLASSNAME is the name of a class implementing objects
6460 of correct type. Any additional arguments are passed to the C<new>
6461 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6462 or C<TIEHASH>). Typically these are arguments such as might be passed
6463 to the C<dbm_open()> function of C. The object returned by the C<new>
6464 method is also returned by the C<tie> function, which would be useful
6465 if you want to access other methods in CLASSNAME.
6467 Note that functions such as C<keys> and C<values> may return huge lists
6468 when used on large objects, like DBM files. You may prefer to use the
6469 C<each> function to iterate over such. Example:
6471 # print out history file offsets
6473 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6474 while (($key,$val) = each %HIST) {
6475 print $key, ' = ', unpack('L',$val), "\n";
6479 A class implementing a hash should have the following methods:
6481 TIEHASH classname, LIST
6483 STORE this, key, value
6488 NEXTKEY this, lastkey
6493 A class implementing an ordinary array should have the following methods:
6495 TIEARRAY classname, LIST
6497 STORE this, key, value
6499 STORESIZE this, count
6505 SPLICE this, offset, length, LIST
6510 A class implementing a file handle should have the following methods:
6512 TIEHANDLE classname, LIST
6513 READ this, scalar, length, offset
6516 WRITE this, scalar, length, offset
6518 PRINTF this, format, LIST
6522 SEEK this, position, whence
6524 OPEN this, mode, LIST
6529 A class implementing a scalar should have the following methods:
6531 TIESCALAR classname, LIST
6537 Not all methods indicated above need be implemented. See L<perltie>,
6538 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6540 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6541 for you--you need to do that explicitly yourself. See L<DB_File>
6542 or the F<Config> module for interesting C<tie> implementations.
6544 For further details see L<perltie>, L<"tied VARIABLE">.
6549 Returns a reference to the object underlying VARIABLE (the same value
6550 that was originally returned by the C<tie> call that bound the variable
6551 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6557 Returns the number of non-leap seconds since whatever time the system
6558 considers to be the epoch, suitable for feeding to C<gmtime> and
6559 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6560 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6561 1904 in the current local time zone for its epoch.
6563 For measuring time in better granularity than one second,
6564 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6565 Perl 5.8 part of the standard distribution), or if you have
6566 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6567 See L<perlfaq8> for details.
6569 For date and time processing look at the many related modules on CPAN.
6570 For a comprehensive date and time representation look at the
6576 Returns a four-element list giving the user and system times, in
6577 seconds, for this process and the children of this process.
6579 ($user,$system,$cuser,$csystem) = times;
6581 In scalar context, C<times> returns C<$user>.
6583 Note that times for children are included only after they terminate.
6587 The transliteration operator. Same as C<y///>. See L<perlop>.
6589 =item truncate FILEHANDLE,LENGTH
6592 =item truncate EXPR,LENGTH
6594 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6595 specified length. Produces a fatal error if truncate isn't implemented
6596 on your system. Returns true if successful, the undefined value
6599 The behavior is undefined if LENGTH is greater than the length of the
6602 The position in the file of FILEHANDLE is left unchanged. You may want to
6603 call L<seek> before writing to the file.
6606 X<uc> X<uppercase> X<toupper>
6610 Returns an uppercased version of EXPR. This is the internal function
6611 implementing the C<\U> escape in double-quoted strings. Respects
6612 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6613 and L<perlunicode> for more details about locale and Unicode support.
6614 It does not attempt to do titlecase mapping on initial letters. See
6615 C<ucfirst> for that.
6617 If EXPR is omitted, uses C<$_>.
6620 X<ucfirst> X<uppercase>
6624 Returns the value of EXPR with the first character in uppercase
6625 (titlecase in Unicode). This is the internal function implementing
6626 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6627 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6628 for more details about locale and Unicode support.
6630 If EXPR is omitted, uses C<$_>.
6637 Sets the umask for the process to EXPR and returns the previous value.
6638 If EXPR is omitted, merely returns the current umask.
6640 The Unix permission C<rwxr-x---> is represented as three sets of three
6641 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6642 and isn't one of the digits). The C<umask> value is such a number
6643 representing disabled permissions bits. The permission (or "mode")
6644 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6645 even if you tell C<sysopen> to create a file with permissions C<0777>,
6646 if your umask is C<0022> then the file will actually be created with
6647 permissions C<0755>. If your C<umask> were C<0027> (group can't
6648 write; others can't read, write, or execute), then passing
6649 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6652 Here's some advice: supply a creation mode of C<0666> for regular
6653 files (in C<sysopen>) and one of C<0777> for directories (in
6654 C<mkdir>) and executable files. This gives users the freedom of
6655 choice: if they want protected files, they might choose process umasks
6656 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6657 Programs should rarely if ever make policy decisions better left to
6658 the user. The exception to this is when writing files that should be
6659 kept private: mail files, web browser cookies, I<.rhosts> files, and
6662 If umask(2) is not implemented on your system and you are trying to
6663 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6664 fatal error at run time. If umask(2) is not implemented and you are
6665 not trying to restrict access for yourself, returns C<undef>.
6667 Remember that a umask is a number, usually given in octal; it is I<not> a
6668 string of octal digits. See also L</oct>, if all you have is a string.
6671 X<undef> X<undefine>
6675 Undefines the value of EXPR, which must be an lvalue. Use only on a
6676 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6677 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6678 will probably not do what you expect on most predefined variables or
6679 DBM list values, so don't do that; see L<delete>.) Always returns the
6680 undefined value. You can omit the EXPR, in which case nothing is
6681 undefined, but you still get an undefined value that you could, for
6682 instance, return from a subroutine, assign to a variable or pass as a
6683 parameter. Examples:
6686 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6690 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6691 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6692 select undef, undef, undef, 0.25;
6693 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6695 Note that this is a unary operator, not a list operator.
6698 X<unlink> X<delete> X<remove> X<rm> X<del>
6702 Deletes a list of files. Returns the number of files successfully
6705 $cnt = unlink 'a', 'b', 'c';
6709 Note: C<unlink> will not attempt to delete directories unless you are superuser
6710 and the B<-U> flag is supplied to Perl. Even if these conditions are
6711 met, be warned that unlinking a directory can inflict damage on your
6712 filesystem. Finally, using C<unlink> on directories is not supported on
6713 many operating systems. Use C<rmdir> instead.
6715 If LIST is omitted, uses C<$_>.
6717 =item unpack TEMPLATE,EXPR
6720 =item unpack TEMPLATE
6722 C<unpack> does the reverse of C<pack>: it takes a string
6723 and expands it out into a list of values.
6724 (In scalar context, it returns merely the first value produced.)
6726 If EXPR is omitted, unpacks the C<$_> string.
6728 The string is broken into chunks described by the TEMPLATE. Each chunk
6729 is converted separately to a value. Typically, either the string is a result
6730 of C<pack>, or the characters of the string represent a C structure of some
6733 The TEMPLATE has the same format as in the C<pack> function.
6734 Here's a subroutine that does substring:
6737 my($what,$where,$howmuch) = @_;
6738 unpack("x$where a$howmuch", $what);
6743 sub ordinal { unpack("W",$_[0]); } # same as ord()
6745 In addition to fields allowed in pack(), you may prefix a field with
6746 a %<number> to indicate that
6747 you want a <number>-bit checksum of the items instead of the items
6748 themselves. Default is a 16-bit checksum. Checksum is calculated by
6749 summing numeric values of expanded values (for string fields the sum of
6750 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6752 For example, the following
6753 computes the same number as the System V sum program:
6757 unpack("%32W*",<>) % 65535;
6760 The following efficiently counts the number of set bits in a bit vector:
6762 $setbits = unpack("%32b*", $selectmask);
6764 The C<p> and C<P> formats should be used with care. Since Perl
6765 has no way of checking whether the value passed to C<unpack()>
6766 corresponds to a valid memory location, passing a pointer value that's
6767 not known to be valid is likely to have disastrous consequences.
6769 If there are more pack codes or if the repeat count of a field or a group
6770 is larger than what the remainder of the input string allows, the result
6771 is not well defined: in some cases, the repeat count is decreased, or
6772 C<unpack()> will produce null strings or zeroes, or terminate with an
6773 error. If the input string is longer than one described by the TEMPLATE,
6774 the rest is ignored.
6776 See L</pack> for more examples and notes.
6778 =item untie VARIABLE
6781 Breaks the binding between a variable and a package. (See C<tie>.)
6782 Has no effect if the variable is not tied.
6784 =item unshift ARRAY,LIST
6787 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6788 depending on how you look at it. Prepends list to the front of the
6789 array, and returns the new number of elements in the array.
6791 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6793 Note the LIST is prepended whole, not one element at a time, so the
6794 prepended elements stay in the same order. Use C<reverse> to do the
6797 =item use Module VERSION LIST
6798 X<use> X<module> X<import>
6800 =item use Module VERSION
6802 =item use Module LIST
6808 Imports some semantics into the current package from the named module,
6809 generally by aliasing certain subroutine or variable names into your
6810 package. It is exactly equivalent to
6812 BEGIN { require Module; import Module LIST; }
6814 except that Module I<must> be a bareword.
6816 VERSION may be either a numeric argument such as 5.006, which will be
6817 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6818 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6819 greater than the version of the current Perl interpreter; Perl will not
6820 attempt to parse the rest of the file. Compare with L</require>, which can
6821 do a similar check at run time.
6823 Specifying VERSION as a literal of the form v5.6.1 should generally be
6824 avoided, because it leads to misleading error messages under earlier
6825 versions of Perl that do not support this syntax. The equivalent numeric
6826 version should be used instead.
6828 use v5.6.1; # compile time version check
6830 use 5.006_001; # ditto; preferred for backwards compatibility
6832 This is often useful if you need to check the current Perl version before
6833 C<use>ing library modules that have changed in incompatible ways from
6834 older versions of Perl. (We try not to do this more than we have to.)
6836 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6837 C<require> makes sure the module is loaded into memory if it hasn't been
6838 yet. The C<import> is not a builtin--it's just an ordinary static method
6839 call into the C<Module> package to tell the module to import the list of
6840 features back into the current package. The module can implement its
6841 C<import> method any way it likes, though most modules just choose to
6842 derive their C<import> method via inheritance from the C<Exporter> class that
6843 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6844 method can be found then the call is skipped, even if there is an AUTOLOAD
6847 If you do not want to call the package's C<import> method (for instance,
6848 to stop your namespace from being altered), explicitly supply the empty list:
6852 That is exactly equivalent to
6854 BEGIN { require Module }
6856 If the VERSION argument is present between Module and LIST, then the
6857 C<use> will call the VERSION method in class Module with the given
6858 version as an argument. The default VERSION method, inherited from
6859 the UNIVERSAL class, croaks if the given version is larger than the
6860 value of the variable C<$Module::VERSION>.
6862 Again, there is a distinction between omitting LIST (C<import> called
6863 with no arguments) and an explicit empty LIST C<()> (C<import> not
6864 called). Note that there is no comma after VERSION!
6866 Because this is a wide-open interface, pragmas (compiler directives)
6867 are also implemented this way. Currently implemented pragmas are:
6872 use sigtrap qw(SEGV BUS);
6873 use strict qw(subs vars refs);
6874 use subs qw(afunc blurfl);
6875 use warnings qw(all);
6876 use sort qw(stable _quicksort _mergesort);
6878 Some of these pseudo-modules import semantics into the current
6879 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6880 which import symbols into the current package (which are effective
6881 through the end of the file).
6883 There's a corresponding C<no> command that unimports meanings imported
6884 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6885 It behaves exactly as C<import> does with respect to VERSION, an
6886 omitted LIST, empty LIST, or no unimport method being found.
6892 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6893 for the C<-M> and C<-m> command-line options to perl that give C<use>
6894 functionality from the command-line.
6899 Changes the access and modification times on each file of a list of
6900 files. The first two elements of the list must be the NUMERICAL access
6901 and modification times, in that order. Returns the number of files
6902 successfully changed. The inode change time of each file is set
6903 to the current time. For example, this code has the same effect as the
6904 Unix touch(1) command when the files I<already exist> and belong to
6905 the user running the program:
6908 $atime = $mtime = time;
6909 utime $atime, $mtime, @ARGV;
6911 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6912 the utime(2) function in the C library will be called with a null second
6913 argument. On most systems, this will set the file's access and
6914 modification times to the current time (i.e. equivalent to the example
6915 above) and will even work on other users' files where you have write
6918 utime undef, undef, @ARGV;
6920 Under NFS this will use the time of the NFS server, not the time of
6921 the local machine. If there is a time synchronization problem, the
6922 NFS server and local machine will have different times. The Unix
6923 touch(1) command will in fact normally use this form instead of the
6924 one shown in the first example.
6926 Note that only passing one of the first two elements as C<undef> will
6927 be equivalent of passing it as 0 and will not have the same effect as
6928 described when they are both C<undef>. This case will also trigger an
6929 uninitialized warning.
6931 On systems that support futimes, you might pass file handles among the
6932 files. On systems that don't support futimes, passing file handles
6933 produces a fatal error at run time. The file handles must be passed
6934 as globs or references to be recognized. Barewords are considered
6940 Returns a list consisting of all the values of the named hash.
6941 (In a scalar context, returns the number of values.)
6943 The values are returned in an apparently random order. The actual
6944 random order is subject to change in future versions of perl, but it
6945 is guaranteed to be the same order as either the C<keys> or C<each>
6946 function would produce on the same (unmodified) hash. Since Perl
6947 5.8.1 the ordering is different even between different runs of Perl
6948 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6950 As a side effect, calling values() resets the HASH's internal iterator,
6951 see L</each>. (In particular, calling values() in void context resets
6952 the iterator with no other overhead.)
6954 Note that the values are not copied, which means modifying them will
6955 modify the contents of the hash:
6957 for (values %hash) { s/foo/bar/g } # modifies %hash values
6958 for (@hash{keys %hash}) { s/foo/bar/g } # same
6960 See also C<keys>, C<each>, and C<sort>.
6962 =item vec EXPR,OFFSET,BITS
6963 X<vec> X<bit> X<bit vector>
6965 Treats the string in EXPR as a bit vector made up of elements of
6966 width BITS, and returns the value of the element specified by OFFSET
6967 as an unsigned integer. BITS therefore specifies the number of bits
6968 that are reserved for each element in the bit vector. This must
6969 be a power of two from 1 to 32 (or 64, if your platform supports
6972 If BITS is 8, "elements" coincide with bytes of the input string.
6974 If BITS is 16 or more, bytes of the input string are grouped into chunks
6975 of size BITS/8, and each group is converted to a number as with
6976 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6977 for BITS==64). See L<"pack"> for details.
6979 If bits is 4 or less, the string is broken into bytes, then the bits
6980 of each byte are broken into 8/BITS groups. Bits of a byte are
6981 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6982 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6983 breaking the single input byte C<chr(0x36)> into two groups gives a list
6984 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6986 C<vec> may also be assigned to, in which case parentheses are needed
6987 to give the expression the correct precedence as in
6989 vec($image, $max_x * $x + $y, 8) = 3;
6991 If the selected element is outside the string, the value 0 is returned.
6992 If an element off the end of the string is written to, Perl will first
6993 extend the string with sufficiently many zero bytes. It is an error
6994 to try to write off the beginning of the string (i.e. negative OFFSET).
6996 The string should not contain any character with the value > 255 (which
6997 can only happen if you're using UTF-8 encoding). If it does, it will be
6998 treated as something that is not UTF-8 encoded. When the C<vec> was
6999 assigned to, other parts of your program will also no longer consider the
7000 string to be UTF-8 encoded. In other words, if you do have such characters
7001 in your string, vec() will operate on the actual byte string, and not the
7002 conceptual character string.
7004 Strings created with C<vec> can also be manipulated with the logical
7005 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7006 vector operation is desired when both operands are strings.
7007 See L<perlop/"Bitwise String Operators">.
7009 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7010 The comments show the string after each step. Note that this code works
7011 in the same way on big-endian or little-endian machines.
7014 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7016 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7017 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7019 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7020 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7021 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7022 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7023 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7024 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7026 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7027 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7028 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7031 To transform a bit vector into a string or list of 0's and 1's, use these:
7033 $bits = unpack("b*", $vector);
7034 @bits = split(//, unpack("b*", $vector));
7036 If you know the exact length in bits, it can be used in place of the C<*>.
7038 Here is an example to illustrate how the bits actually fall in place:
7044 unpack("V",$_) 01234567890123456789012345678901
7045 ------------------------------------------------------------------
7050 for ($shift=0; $shift < $width; ++$shift) {
7051 for ($off=0; $off < 32/$width; ++$off) {
7052 $str = pack("B*", "0"x32);
7053 $bits = (1<<$shift);
7054 vec($str, $off, $width) = $bits;
7055 $res = unpack("b*",$str);
7056 $val = unpack("V", $str);
7063 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7064 $off, $width, $bits, $val, $res
7068 Regardless of the machine architecture on which it is run, the above
7069 example should print the following table:
7072 unpack("V",$_) 01234567890123456789012345678901
7073 ------------------------------------------------------------------
7074 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7075 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7076 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7077 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7078 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7079 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7080 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7081 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7082 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7083 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7084 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7085 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7086 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7087 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7088 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7089 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7090 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7091 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7092 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7093 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7094 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7095 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7096 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7097 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7098 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7099 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7100 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7101 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7102 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7103 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7104 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7105 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7106 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7107 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7108 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7109 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7110 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7111 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7112 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7113 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7114 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7115 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7116 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7117 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7118 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7119 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7120 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7121 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7122 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7123 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7124 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7125 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7126 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7127 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7128 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7129 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7130 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7131 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7132 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7133 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7134 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7135 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7136 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7137 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7138 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7139 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7140 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7141 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7142 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7143 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7144 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7145 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7146 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7147 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7148 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7149 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7150 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7151 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7152 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7153 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7154 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7155 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7156 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7157 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7158 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7159 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7160 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7161 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7162 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7163 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7164 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7165 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7166 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7167 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7168 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7169 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7170 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7171 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7172 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7173 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7174 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7175 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7176 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7177 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7178 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7179 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7180 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7181 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7182 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7183 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7184 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7185 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7186 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7187 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7188 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7189 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7190 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7191 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7192 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7193 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7194 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7195 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7196 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7197 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7198 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7199 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7200 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7201 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7206 Behaves like the wait(2) system call on your system: it waits for a child
7207 process to terminate and returns the pid of the deceased process, or
7208 C<-1> if there are no child processes. The status is returned in C<$?>
7209 and C<{^CHILD_ERROR_NATIVE}>.
7210 Note that a return value of C<-1> could mean that child processes are
7211 being automatically reaped, as described in L<perlipc>.
7213 =item waitpid PID,FLAGS
7216 Waits for a particular child process to terminate and returns the pid of
7217 the deceased process, or C<-1> if there is no such child process. On some
7218 systems, a value of 0 indicates that there are processes still running.
7219 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7221 use POSIX ":sys_wait_h";
7224 $kid = waitpid(-1, WNOHANG);
7227 then you can do a non-blocking wait for all pending zombie processes.
7228 Non-blocking wait is available on machines supporting either the
7229 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7230 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7231 system call by remembering the status values of processes that have
7232 exited but have not been harvested by the Perl script yet.)
7234 Note that on some systems, a return value of C<-1> could mean that child
7235 processes are being automatically reaped. See L<perlipc> for details,
7236 and for other examples.
7239 X<wantarray> X<context>
7241 Returns true if the context of the currently executing subroutine or
7242 C<eval> is looking for a list value. Returns false if the context is
7243 looking for a scalar. Returns the undefined value if the context is
7244 looking for no value (void context).
7246 return unless defined wantarray; # don't bother doing more
7247 my @a = complex_calculation();
7248 return wantarray ? @a : "@a";
7250 C<wantarray()>'s result is unspecified in the top level of a file,
7251 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7252 in a C<DESTROY> method.
7254 This function should have been named wantlist() instead.
7257 X<warn> X<warning> X<STDERR>
7259 Prints the value of LIST to STDERR. If the last element of LIST does
7260 not end in a newline, appends the same text as C<die> does.
7262 If LIST is empty and C<$@> already contains a value (typically from a
7263 previous eval) that value is used after appending C<"\t...caught">
7264 to C<$@>. This is useful for staying almost, but not entirely similar to
7267 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7269 No message is printed if there is a C<$SIG{__WARN__}> handler
7270 installed. It is the handler's responsibility to deal with the message
7271 as it sees fit (like, for instance, converting it into a C<die>). Most
7272 handlers must therefore make arrangements to actually display the
7273 warnings that they are not prepared to deal with, by calling C<warn>
7274 again in the handler. Note that this is quite safe and will not
7275 produce an endless loop, since C<__WARN__> hooks are not called from
7278 You will find this behavior is slightly different from that of
7279 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7280 instead call C<die> again to change it).
7282 Using a C<__WARN__> handler provides a powerful way to silence all
7283 warnings (even the so-called mandatory ones). An example:
7285 # wipe out *all* compile-time warnings
7286 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7288 my $foo = 20; # no warning about duplicate my $foo,
7289 # but hey, you asked for it!
7290 # no compile-time or run-time warnings before here
7293 # run-time warnings enabled after here
7294 warn "\$foo is alive and $foo!"; # does show up
7296 See L<perlvar> for details on setting C<%SIG> entries, and for more
7297 examples. See the Carp module for other kinds of warnings using its
7298 carp() and cluck() functions.
7300 =item write FILEHANDLE
7307 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7308 using the format associated with that file. By default the format for
7309 a file is the one having the same name as the filehandle, but the
7310 format for the current output channel (see the C<select> function) may be set
7311 explicitly by assigning the name of the format to the C<$~> variable.
7313 Top of form processing is handled automatically: if there is
7314 insufficient room on the current page for the formatted record, the
7315 page is advanced by writing a form feed, a special top-of-page format
7316 is used to format the new page header, and then the record is written.
7317 By default the top-of-page format is the name of the filehandle with
7318 "_TOP" appended, but it may be dynamically set to the format of your
7319 choice by assigning the name to the C<$^> variable while the filehandle is
7320 selected. The number of lines remaining on the current page is in
7321 variable C<$->, which can be set to C<0> to force a new page.
7323 If FILEHANDLE is unspecified, output goes to the current default output
7324 channel, which starts out as STDOUT but may be changed by the
7325 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7326 is evaluated and the resulting string is used to look up the name of
7327 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7329 Note that write is I<not> the opposite of C<read>. Unfortunately.
7333 The transliteration operator. Same as C<tr///>. See L<perlop>.