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 Extension modules can also hook into the Perl parser to define new
90 kinds of keyword-headed expression. These may look like functions, but
91 may also look completely different. The syntax following the keyword
92 is defined entirely by the extension. If you are an implementor, see
93 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94 a module, see the module's documentation for details of the syntax that
97 =head2 Perl Functions by Category
100 Here are Perl's functions (including things that look like
101 functions, like some keywords and named operators)
102 arranged by category. Some functions appear in more
107 =item Functions for SCALARs or strings
108 X<scalar> X<string> X<character>
110 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
111 C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
112 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
114 =item Regular expressions and pattern matching
115 X<regular expression> X<regex> X<regexp>
117 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
119 =item Numeric functions
120 X<numeric> X<number> X<trigonometric> X<trigonometry>
122 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123 C<sin>, C<sqrt>, C<srand>
125 =item Functions for real @ARRAYs
128 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
130 =item Functions for list data
133 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
135 =item Functions for real %HASHes
138 C<delete>, C<each>, C<exists>, C<keys>, C<values>
140 =item Input and output functions
141 X<I/O> X<input> X<output> X<dbm>
143 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
145 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
146 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
149 =item Functions for fixed length data or records
151 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
153 =item Functions for filehandles, files, or directories
154 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
156 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
157 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
158 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159 C<umask>, C<unlink>, C<utime>
161 =item Keywords related to the control flow of your Perl program
164 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
165 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
167 =item Keywords related to switch
169 C<break>, C<continue>, C<given>, C<when>, C<default>
171 (These are only available if you enable the "switch" feature.
172 See L<feature> and L<perlsyn/"Switch statements">.)
174 =item Keywords related to scoping
176 C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
179 (C<state> is only available if the "state" feature is enabled. See
182 =item Miscellaneous functions
184 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
185 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
187 =item Functions for processes and process groups
188 X<process> X<pid> X<process id>
190 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
191 C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
192 C<times>, C<wait>, C<waitpid>
194 =item Keywords related to perl modules
197 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
199 =item Keywords related to classes and object-orientation
200 X<object> X<class> X<package>
202 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
205 =item Low-level socket functions
208 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
209 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
210 C<socket>, C<socketpair>
212 =item System V interprocess communication functions
213 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
215 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
216 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
218 =item Fetching user and group info
219 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
221 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
222 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
223 C<getpwuid>, C<setgrent>, C<setpwent>
225 =item Fetching network info
226 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
228 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
229 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
230 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
231 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
232 C<setnetent>, C<setprotoent>, C<setservent>
234 =item Time-related functions
237 C<gmtime>, C<localtime>, C<time>, C<times>
239 =item Functions new in perl5
242 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
243 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
244 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
245 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
246 C<ucfirst>, C<untie>, C<use>, C<when>
248 * - C<sub> was a keyword in perl4, but in perl5 it is an
249 operator, which can be used in expressions.
251 =item Functions obsoleted in perl5
253 C<dbmclose>, C<dbmopen>
258 X<portability> X<Unix> X<portable>
260 Perl was born in Unix and can therefore access all common Unix
261 system calls. In non-Unix environments, the functionality of some
262 Unix system calls may not be available, or details of the available
263 functionality may differ slightly. The Perl functions affected
266 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
267 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
268 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
269 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
270 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
271 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
272 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
273 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
274 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
275 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
276 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
277 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
278 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
279 C<shmwrite>, C<socket>, C<socketpair>,
280 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
281 C<times>, C<truncate>, C<umask>, C<unlink>,
282 C<utime>, C<wait>, C<waitpid>
284 For more information about the portability of these functions, see
285 L<perlport> and other available platform-specific documentation.
287 =head2 Alphabetical Listing of Perl Functions
292 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>
293 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
301 A file test, where X is one of the letters listed below. This unary
302 operator takes one argument, either a filename, a filehandle, or a dirhandle,
303 and tests the associated file to see if something is true about it. If the
304 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
305 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
306 the undefined value if the file doesn't exist. Despite the funny
307 names, precedence is the same as any other named unary operator. The
308 operator may be any of:
310 -r File is readable by effective uid/gid.
311 -w File is writable by effective uid/gid.
312 -x File is executable by effective uid/gid.
313 -o File is owned by effective uid.
315 -R File is readable by real uid/gid.
316 -W File is writable by real uid/gid.
317 -X File is executable by real uid/gid.
318 -O File is owned by real uid.
321 -z File has zero size (is empty).
322 -s File has nonzero size (returns size in bytes).
324 -f File is a plain file.
325 -d File is a directory.
326 -l File is a symbolic link.
327 -p File is a named pipe (FIFO), or Filehandle is a pipe.
329 -b File is a block special file.
330 -c File is a character special file.
331 -t Filehandle is opened to a tty.
333 -u File has setuid bit set.
334 -g File has setgid bit set.
335 -k File has sticky bit set.
337 -T File is an ASCII text file (heuristic guess).
338 -B File is a "binary" file (opposite of -T).
340 -M Script start time minus file modification time, in days.
341 -A Same for access time.
342 -C Same for inode change time (Unix, may differ for other platforms)
348 next unless -f $_; # ignore specials
352 The interpretation of the file permission operators C<-r>, C<-R>,
353 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
354 of the file and the uids and gids of the user. There may be other
355 reasons you can't actually read, write, or execute the file: for
356 example network filesystem access controls, ACLs (access control lists),
357 read-only filesystems, and unrecognized executable formats. Note
358 that the use of these six specific operators to verify if some operation
359 is possible is usually a mistake, because it may be open to race
362 Also note that, for the superuser on the local filesystems, the C<-r>,
363 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
364 if any execute bit is set in the mode. Scripts run by the superuser
365 may thus need to do a stat() to determine the actual mode of the file,
366 or temporarily set their effective uid to something else.
368 If you are using ACLs, there is a pragma called C<filetest> that may
369 produce more accurate results than the bare stat() mode bits.
370 When under the C<use filetest 'access'> the above-mentioned filetests
371 will test whether the permission can (not) be granted using the
372 access() family of system calls. Also note that the C<-x> and C<-X> may
373 under this pragma return true even if there are no execute permission
374 bits set (nor any extra execute permission ACLs). This strangeness is
375 due to the underlying system calls' definitions. Note also that, due to
376 the implementation of C<use filetest 'access'>, the C<_> special
377 filehandle won't cache the results of the file tests when this pragma is
378 in effect. Read the documentation for the C<filetest> pragma for more
381 Note that C<-s/a/b/> does not do a negated substitution. Saying
382 C<-exp($foo)> still works as expected, however--only single letters
383 following a minus are interpreted as file tests.
385 The C<-T> and C<-B> switches work as follows. The first block or so of the
386 file is examined for odd characters such as strange control codes or
387 characters with the high bit set. If too many strange characters (>30%)
388 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
389 containing null in the first block is considered a binary file. If C<-T>
390 or C<-B> is used on a filehandle, the current IO buffer is examined
391 rather than the first block. Both C<-T> and C<-B> return true on a null
392 file, or a file at EOF when testing a filehandle. Because you have to
393 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
394 against the file first, as in C<next unless -f $file && -T $file>.
396 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
397 the special filehandle consisting of a solitary underline, then the stat
398 structure of the previous file test (or stat operator) is used, saving
399 a system call. (This doesn't work with C<-t>, and you need to remember
400 that lstat() and C<-l> will leave values in the stat structure for the
401 symbolic link, not the real file.) (Also, if the stat buffer was filled by
402 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
405 print "Can do.\n" if -r $a || -w _ || -x _;
408 print "Readable\n" if -r _;
409 print "Writable\n" if -w _;
410 print "Executable\n" if -x _;
411 print "Setuid\n" if -u _;
412 print "Setgid\n" if -g _;
413 print "Sticky\n" if -k _;
414 print "Text\n" if -T _;
415 print "Binary\n" if -B _;
417 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
418 test operators, in a way that C<-f -w -x $file> is equivalent to
419 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
420 the return value of C<-f $file> as an argument to another filetest
421 operator, no special magic will happen.)
428 Returns the absolute value of its argument.
429 If VALUE is omitted, uses C<$_>.
431 =item accept NEWSOCKET,GENERICSOCKET
434 Accepts an incoming socket connect, just as the accept(2) system call
435 does. Returns the packed address if it succeeded, false otherwise.
436 See the example in L<perlipc/"Sockets: Client/Server Communication">.
438 On systems that support a close-on-exec flag on files, the flag will
439 be set for the newly opened file descriptor, as determined by the
440 value of $^F. See L<perlvar/$^F>.
449 Arranges to have a SIGALRM delivered to this process after the
450 specified number of wallclock seconds has elapsed. If SECONDS is not
451 specified, the value stored in C<$_> is used. (On some machines,
452 unfortunately, the elapsed time may be up to one second less or more
453 than you specified because of how seconds are counted, and process
454 scheduling may delay the delivery of the signal even further.)
456 Only one timer may be counting at once. Each call disables the
457 previous timer, and an argument of C<0> may be supplied to cancel the
458 previous timer without starting a new one. The returned value is the
459 amount of time remaining on the previous timer.
461 For delays of finer granularity than one second, the Time::HiRes module
462 (from CPAN, and starting from Perl 5.8 part of the standard
463 distribution) provides ualarm(). You may also use Perl's four-argument
464 version of select() leaving the first three arguments undefined, or you
465 might be able to use the C<syscall> interface to access setitimer(2) if
466 your system supports it. See L<perlfaq8> for details.
468 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
469 (C<sleep> may be internally implemented in your system with C<alarm>)
471 If you want to use C<alarm> to time out a system call you need to use an
472 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
473 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
474 restart system calls on some systems. Using C<eval>/C<die> always works,
475 modulo the caveats given in L<perlipc/"Signals">.
478 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
480 $nread = sysread SOCKET, $buffer, $size;
484 die unless $@ eq "alarm\n"; # propagate unexpected errors
491 For more information see L<perlipc>.
494 X<atan2> X<arctangent> X<tan> X<tangent>
496 Returns the arctangent of Y/X in the range -PI to PI.
498 For the tangent operation, you may use the C<Math::Trig::tan>
499 function, or use the familiar relation:
501 sub tan { sin($_[0]) / cos($_[0]) }
503 The return value for C<atan2(0,0)> is implementation-defined; consult
504 your atan2(3) manpage for more information.
506 =item bind SOCKET,NAME
509 Binds a network address to a socket, just as the bind system call
510 does. Returns true if it succeeded, false otherwise. NAME should be a
511 packed address of the appropriate type for the socket. See the examples in
512 L<perlipc/"Sockets: Client/Server Communication">.
514 =item binmode FILEHANDLE, LAYER
515 X<binmode> X<binary> X<text> X<DOS> X<Windows>
517 =item binmode FILEHANDLE
519 Arranges for FILEHANDLE to be read or written in "binary" or "text"
520 mode on systems where the run-time libraries distinguish between
521 binary and text files. If FILEHANDLE is an expression, the value is
522 taken as the name of the filehandle. Returns true on success,
523 otherwise it returns C<undef> and sets C<$!> (errno).
525 On some systems (in general, DOS and Windows-based systems) binmode()
526 is necessary when you're not working with a text file. For the sake
527 of portability it is a good idea to always use it when appropriate,
528 and to never use it when it isn't appropriate. Also, people can
529 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
531 In other words: regardless of platform, use binmode() on binary data,
532 like for example images.
534 If LAYER is present it is a single string, but may contain multiple
535 directives. The directives alter the behaviour of the file handle.
536 When LAYER is present using binmode on a text file makes sense.
538 If LAYER is omitted or specified as C<:raw> the filehandle is made
539 suitable for passing binary data. This includes turning off possible CRLF
540 translation and marking it as bytes (as opposed to Unicode characters).
541 Note that, despite what may be implied in I<"Programming Perl"> (the
542 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
543 -- other layers which would affect the binary nature of the stream are
544 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
545 PERLIO environment variable.
547 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
548 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
549 establish default I/O layers. See L<open>.
551 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
552 in "Programming Perl, 3rd Edition". However, since the publishing of this
553 book, by many known as "Camel III", the consensus of the naming of this
554 functionality has moved from "discipline" to "layer". All documentation
555 of this version of Perl therefore refers to "layers" rather than to
556 "disciplines". Now back to the regularly scheduled documentation...>
558 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
559 C<:utf8> just marks the data as UTF-8 without further checking,
560 while C<:encoding(utf8)> checks the data for actually being valid
561 UTF-8. More details can be found in L<PerlIO::encoding>.
563 In general, binmode() should be called after open() but before any I/O
564 is done on the filehandle. Calling binmode() will normally flush any
565 pending buffered output data (and perhaps pending input data) on the
566 handle. An exception to this is the C<:encoding> layer that
567 changes the default character encoding of the handle, see L<open>.
568 The C<:encoding> layer sometimes needs to be called in
569 mid-stream, and it doesn't flush the stream. The C<:encoding>
570 also implicitly pushes on top of itself the C<:utf8> layer because
571 internally Perl will operate on UTF-8 encoded Unicode characters.
573 The operating system, device drivers, C libraries, and Perl run-time
574 system all work together to let the programmer treat a single
575 character (C<\n>) as the line terminator, irrespective of the external
576 representation. On many operating systems, the native text file
577 representation matches the internal representation, but on some
578 platforms the external representation of C<\n> is made up of more than
581 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
582 character to end each line in the external representation of text (even
583 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
584 on Unix and most VMS files). In other systems like OS/2, DOS and the
585 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
586 but what's stored in text files are the two characters C<\cM\cJ>. That
587 means that, if you don't use binmode() on these systems, C<\cM\cJ>
588 sequences on disk will be converted to C<\n> on input, and any C<\n> in
589 your program will be converted back to C<\cM\cJ> on output. This is what
590 you want for text files, but it can be disastrous for binary files.
592 Another consequence of using binmode() (on some systems) is that
593 special end-of-file markers will be seen as part of the data stream.
594 For systems from the Microsoft family this means that if your binary
595 data contains C<\cZ>, the I/O subsystem will regard it as the end of
596 the file, unless you use binmode().
598 binmode() is not only important for readline() and print() operations,
599 but also when using read(), seek(), sysread(), syswrite() and tell()
600 (see L<perlport> for more details). See the C<$/> and C<$\> variables
601 in L<perlvar> for how to manually set your input and output
602 line-termination sequences.
604 =item bless REF,CLASSNAME
609 This function tells the thingy referenced by REF that it is now an object
610 in the CLASSNAME package. If CLASSNAME is omitted, the current package
611 is used. Because a C<bless> is often the last thing in a constructor,
612 it returns the reference for convenience. Always use the two-argument
613 version if a derived class might inherit the function doing the blessing.
614 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
617 Consider always blessing objects in CLASSNAMEs that are mixed case.
618 Namespaces with all lowercase names are considered reserved for
619 Perl pragmata. Builtin types have all uppercase names. To prevent
620 confusion, you may wish to avoid such package names as well. Make sure
621 that CLASSNAME is a true value.
623 See L<perlmod/"Perl Modules">.
627 Break out of a C<given()> block.
629 This keyword is enabled by the "switch" feature: see L<feature>
630 for more information.
633 X<caller> X<call stack> X<stack> X<stack trace>
637 Returns the context of the current subroutine call. In scalar context,
638 returns the caller's package name if there is a caller, that is, if
639 we're in a subroutine or C<eval> or C<require>, and the undefined value
640 otherwise. In list context, returns
643 ($package, $filename, $line) = caller;
645 With EXPR, it returns some extra information that the debugger uses to
646 print a stack trace. The value of EXPR indicates how many call frames
647 to go back before the current one.
650 ($package, $filename, $line, $subroutine, $hasargs,
653 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
656 Here $subroutine may be C<(eval)> if the frame is not a subroutine
657 call, but an C<eval>. In such a case additional elements $evaltext and
658 C<$is_require> are set: C<$is_require> is true if the frame is created by a
659 C<require> or C<use> statement, $evaltext contains the text of the
660 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
661 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
662 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
663 frame.) $subroutine may also be C<(unknown)> if this particular
664 subroutine happens to have been deleted from the symbol table.
665 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
666 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
667 compiled with. The C<$hints> and C<$bitmask> values are subject to change
668 between versions of Perl, and are not meant for external use.
670 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
671 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
672 of this hash, as they are the actual values stored in the optree.
674 Furthermore, when called from within the DB package, caller returns more
675 detailed information: it sets the list variable C<@DB::args> to be the
676 arguments with which the subroutine was invoked.
678 Be aware that the optimizer might have optimized call frames away before
679 C<caller> had a chance to get the information. That means that C<caller(N)>
680 might not return information about the call frame you expect it do, for
681 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
682 previous time C<caller> was called.
689 =item chdir FILEHANDLE
691 =item chdir DIRHANDLE
695 Changes the working directory to EXPR, if possible. If EXPR is omitted,
696 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
697 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
698 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
699 neither is set, C<chdir> does nothing. It returns true upon success,
700 false otherwise. See the example under C<die>.
702 On systems that support fchdir, you might pass a file handle or
703 directory handle as argument. On systems that don't support fchdir,
704 passing handles produces a fatal error at run time.
707 X<chmod> X<permission> X<mode>
709 Changes the permissions of a list of files. The first element of the
710 list must be the numerical mode, which should probably be an octal
711 number, and which definitely should I<not> be a string of octal digits:
712 C<0644> is okay, C<'0644'> is not. Returns the number of files
713 successfully changed. See also L</oct>, if all you have is a string.
715 $cnt = chmod 0755, 'foo', 'bar';
716 chmod 0755, @executables;
717 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
719 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
720 $mode = 0644; chmod $mode, 'foo'; # this is best
722 On systems that support fchmod, you might pass file handles among the
723 files. On systems that don't support fchmod, passing file handles
724 produces a fatal error at run time. The file handles must be passed
725 as globs or references to be recognized. Barewords are considered
728 open(my $fh, "<", "foo");
729 my $perm = (stat $fh)[2] & 07777;
730 chmod($perm | 0600, $fh);
732 You can also import the symbolic C<S_I*> constants from the Fcntl
737 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
738 # This is identical to the chmod 0755 of the above example.
741 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
747 This safer version of L</chop> removes any trailing string
748 that corresponds to the current value of C<$/> (also known as
749 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
750 number of characters removed from all its arguments. It's often used to
751 remove the newline from the end of an input record when you're worried
752 that the final record may be missing its newline. When in paragraph
753 mode (C<$/ = "">), it removes all trailing newlines from the string.
754 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
755 a reference to an integer or the like, see L<perlvar>) chomp() won't
757 If VARIABLE is omitted, it chomps C<$_>. Example:
760 chomp; # avoid \n on last field
765 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
767 You can actually chomp anything that's an lvalue, including an assignment:
770 chomp($answer = <STDIN>);
772 If you chomp a list, each element is chomped, and the total number of
773 characters removed is returned.
775 Note that parentheses are necessary when you're chomping anything
776 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
777 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
778 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
779 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
789 Chops off the last character of a string and returns the character
790 chopped. It is much more efficient than C<s/.$//s> because it neither
791 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
792 If VARIABLE is a hash, it chops the hash's values, but not its keys.
794 You can actually chop anything that's an lvalue, including an assignment.
796 If you chop a list, each element is chopped. Only the value of the
797 last C<chop> is returned.
799 Note that C<chop> returns the last character. To return all but the last
800 character, use C<substr($string, 0, -1)>.
805 X<chown> X<owner> X<user> X<group>
807 Changes the owner (and group) of a list of files. The first two
808 elements of the list must be the I<numeric> uid and gid, in that
809 order. A value of -1 in either position is interpreted by most
810 systems to leave that value unchanged. Returns the number of files
811 successfully changed.
813 $cnt = chown $uid, $gid, 'foo', 'bar';
814 chown $uid, $gid, @filenames;
816 On systems that support fchown, you might pass file handles among the
817 files. On systems that don't support fchown, passing file handles
818 produces a fatal error at run time. The file handles must be passed
819 as globs or references to be recognized. Barewords are considered
822 Here's an example that looks up nonnumeric uids in the passwd file:
825 chomp($user = <STDIN>);
827 chomp($pattern = <STDIN>);
829 ($login,$pass,$uid,$gid) = getpwnam($user)
830 or die "$user not in passwd file";
832 @ary = glob($pattern); # expand filenames
833 chown $uid, $gid, @ary;
835 On most systems, you are not allowed to change the ownership of the
836 file unless you're the superuser, although you should be able to change
837 the group to any of your secondary groups. On insecure systems, these
838 restrictions may be relaxed, but this is not a portable assumption.
839 On POSIX systems, you can detect this condition this way:
841 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
842 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
845 X<chr> X<character> X<ASCII> X<Unicode>
849 Returns the character represented by that NUMBER in the character set.
850 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
851 chr(0x263a) is a Unicode smiley face.
853 Negative values give the Unicode replacement character (chr(0xfffd)),
854 except under the L<bytes> pragma, where low eight bits of the value
855 (truncated to an integer) are used.
857 If NUMBER is omitted, uses C<$_>.
859 For the reverse, use L</ord>.
861 Note that characters from 128 to 255 (inclusive) are by default
862 internally not encoded as UTF-8 for backward compatibility reasons.
864 See L<perlunicode> for more about Unicode.
866 =item chroot FILENAME
871 This function works like the system call by the same name: it makes the
872 named directory the new root directory for all further pathnames that
873 begin with a C</> by your process and all its children. (It doesn't
874 change your current working directory, which is unaffected.) For security
875 reasons, this call is restricted to the superuser. If FILENAME is
876 omitted, does a C<chroot> to C<$_>.
878 =item close FILEHANDLE
883 Closes the file or pipe associated with the file handle, flushes the IO
884 buffers, and closes the system file descriptor. Returns true if those
885 operations have succeeded and if no error was reported by any PerlIO
886 layer. Closes the currently selected filehandle if the argument is
889 You don't have to close FILEHANDLE if you are immediately going to do
890 another C<open> on it, because C<open> will close it for you. (See
891 C<open>.) However, an explicit C<close> on an input file resets the line
892 counter (C<$.>), while the implicit close done by C<open> does not.
894 If the file handle came from a piped open, C<close> will additionally
895 return false if one of the other system calls involved fails, or if the
896 program exits with non-zero status. (If the only problem was that the
897 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
898 also waits for the process executing on the pipe to complete, in case you
899 want to look at the output of the pipe afterwards, and
900 implicitly puts the exit status value of that command into C<$?> and
901 C<${^CHILD_ERROR_NATIVE}>.
903 Prematurely closing the read end of a pipe (i.e. before the process
904 writing to it at the other end has closed it) will result in a
905 SIGPIPE being delivered to the writer. If the other end can't
906 handle that, be sure to read all the data before closing the pipe.
910 open(OUTPUT, '|sort >foo') # pipe to sort
911 or die "Can't start sort: $!";
912 #... # print stuff to output
913 close OUTPUT # wait for sort to finish
914 or warn $! ? "Error closing sort pipe: $!"
915 : "Exit status $? from sort";
916 open(INPUT, 'foo') # get sort's results
917 or die "Can't open 'foo' for input: $!";
919 FILEHANDLE may be an expression whose value can be used as an indirect
920 filehandle, usually the real filehandle name.
922 =item closedir DIRHANDLE
925 Closes a directory opened by C<opendir> and returns the success of that
928 =item connect SOCKET,NAME
931 Attempts to connect to a remote socket, just as the connect system call
932 does. Returns true if it succeeded, false otherwise. NAME should be a
933 packed address of the appropriate type for the socket. See the examples in
934 L<perlipc/"Sockets: Client/Server Communication">.
941 C<continue> is actually a flow control statement rather than a function. If
942 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
943 C<foreach>), it is always executed just before the conditional is about to
944 be evaluated again, just like the third part of a C<for> loop in C. Thus
945 it can be used to increment a loop variable, even when the loop has been
946 continued via the C<next> statement (which is similar to the C C<continue>
949 C<last>, C<next>, or C<redo> may appear within a C<continue>
950 block. C<last> and C<redo> will behave as if they had been executed within
951 the main block. So will C<next>, but since it will execute a C<continue>
952 block, it may be more entertaining.
955 ### redo always comes here
958 ### next always comes here
960 # then back the top to re-check EXPR
962 ### last always comes here
964 Omitting the C<continue> section is semantically equivalent to using an
965 empty one, logically enough. In that case, C<next> goes directly back
966 to check the condition at the top of the loop.
968 If the "switch" feature is enabled, C<continue> is also a
969 function that will break out of the current C<when> or C<default>
970 block, and fall through to the next case. See L<feature> and
971 L<perlsyn/"Switch statements"> for more information.
975 X<cos> X<cosine> X<acos> X<arccosine>
979 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
980 takes cosine of C<$_>.
982 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
983 function, or use this relation:
985 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
987 =item crypt PLAINTEXT,SALT
988 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
989 X<decrypt> X<cryptography> X<passwd> X<encrypt>
991 Creates a digest string exactly like the crypt(3) function in the C
992 library (assuming that you actually have a version there that has not
993 been extirpated as a potential munition).
995 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
996 into a short string, called a digest, which is returned. The same
997 PLAINTEXT and SALT will always return the same string, but there is no
998 (known) way to get the original PLAINTEXT from the hash. Small
999 changes in the PLAINTEXT or SALT will result in large changes in the
1002 There is no decrypt function. This function isn't all that useful for
1003 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1004 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1005 primarily used to check if two pieces of text are the same without
1006 having to transmit or store the text itself. An example is checking
1007 if a correct password is given. The digest of the password is stored,
1008 not the password itself. The user types in a password that is
1009 crypt()'d with the same salt as the stored digest. If the two digests
1010 match the password is correct.
1012 When verifying an existing digest string you should use the digest as
1013 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1014 to create the digest is visible as part of the digest. This ensures
1015 crypt() will hash the new string with the same salt as the digest.
1016 This allows your code to work with the standard L<crypt|/crypt> and
1017 with more exotic implementations. In other words, do not assume
1018 anything about the returned string itself, or how many bytes in the
1021 Traditionally the result is a string of 13 bytes: two first bytes of
1022 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1023 the first eight bytes of PLAINTEXT mattered. But alternative
1024 hashing schemes (like MD5), higher level security schemes (like C2),
1025 and implementations on non-UNIX platforms may produce different
1028 When choosing a new salt create a random two character string whose
1029 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1030 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1031 characters is just a recommendation; the characters allowed in
1032 the salt depend solely on your system's crypt library, and Perl can't
1033 restrict what salts C<crypt()> accepts.
1035 Here's an example that makes sure that whoever runs this program knows
1038 $pwd = (getpwuid($<))[1];
1040 system "stty -echo";
1042 chomp($word = <STDIN>);
1046 if (crypt($word, $pwd) ne $pwd) {
1052 Of course, typing in your own password to whoever asks you
1055 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1056 of data, not least of all because you can't get the information
1057 back. Look at the L<Digest> module for more robust algorithms.
1059 If using crypt() on a Unicode string (which I<potentially> has
1060 characters with codepoints above 255), Perl tries to make sense
1061 of the situation by trying to downgrade (a copy of the string)
1062 the string back to an eight-bit byte string before calling crypt()
1063 (on that copy). If that works, good. If not, crypt() dies with
1064 C<Wide character in crypt>.
1069 [This function has been largely superseded by the C<untie> function.]
1071 Breaks the binding between a DBM file and a hash.
1073 =item dbmopen HASH,DBNAME,MASK
1074 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1076 [This function has been largely superseded by the C<tie> function.]
1078 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1079 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1080 argument is I<not> a filehandle, even though it looks like one). DBNAME
1081 is the name of the database (without the F<.dir> or F<.pag> extension if
1082 any). If the database does not exist, it is created with protection
1083 specified by MASK (as modified by the C<umask>). If your system supports
1084 only the older DBM functions, you may perform only one C<dbmopen> in your
1085 program. In older versions of Perl, if your system had neither DBM nor
1086 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1089 If you don't have write access to the DBM file, you can only read hash
1090 variables, not set them. If you want to test whether you can write,
1091 either use file tests or try setting a dummy hash entry inside an C<eval>,
1092 which will trap the error.
1094 Note that functions such as C<keys> and C<values> may return huge lists
1095 when used on large DBM files. You may prefer to use the C<each>
1096 function to iterate over large DBM files. Example:
1098 # print out history file offsets
1099 dbmopen(%HIST,'/usr/lib/news/history',0666);
1100 while (($key,$val) = each %HIST) {
1101 print $key, ' = ', unpack('L',$val), "\n";
1105 See also L<AnyDBM_File> for a more general description of the pros and
1106 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1107 rich implementation.
1109 You can control which DBM library you use by loading that library
1110 before you call dbmopen():
1113 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1114 or die "Can't open netscape history file: $!";
1117 X<defined> X<undef> X<undefined>
1121 Returns a Boolean value telling whether EXPR has a value other than
1122 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1125 Many operations return C<undef> to indicate failure, end of file,
1126 system error, uninitialized variable, and other exceptional
1127 conditions. This function allows you to distinguish C<undef> from
1128 other values. (A simple Boolean test will not distinguish among
1129 C<undef>, zero, the empty string, and C<"0">, which are all equally
1130 false.) Note that since C<undef> is a valid scalar, its presence
1131 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1132 returns C<undef> when its argument is an empty array, I<or> when the
1133 element to return happens to be C<undef>.
1135 You may also use C<defined(&func)> to check whether subroutine C<&func>
1136 has ever been defined. The return value is unaffected by any forward
1137 declarations of C<&func>. Note that a subroutine which is not defined
1138 may still be callable: its package may have an C<AUTOLOAD> method that
1139 makes it spring into existence the first time that it is called -- see
1142 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1143 used to report whether memory for that aggregate has ever been
1144 allocated. This behavior may disappear in future versions of Perl.
1145 You should instead use a simple test for size:
1147 if (@an_array) { print "has array elements\n" }
1148 if (%a_hash) { print "has hash members\n" }
1150 When used on a hash element, it tells you whether the value is defined,
1151 not whether the key exists in the hash. Use L</exists> for the latter
1156 print if defined $switch{'D'};
1157 print "$val\n" while defined($val = pop(@ary));
1158 die "Can't readlink $sym: $!"
1159 unless defined($value = readlink $sym);
1160 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1161 $debugging = 0 unless defined $debugging;
1163 Note: Many folks tend to overuse C<defined>, and then are surprised to
1164 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1165 defined values. For example, if you say
1169 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1170 matched "nothing". It didn't really fail to match anything. Rather, it
1171 matched something that happened to be zero characters long. This is all
1172 very above-board and honest. When a function returns an undefined value,
1173 it's an admission that it couldn't give you an honest answer. So you
1174 should use C<defined> only when you're questioning the integrity of what
1175 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1178 See also L</undef>, L</exists>, L</ref>.
1183 Given an expression that specifies a hash element, array element, hash slice,
1184 or array slice, deletes the specified element(s) from the hash or array.
1185 In the case of an array, if the array elements happen to be at the end,
1186 the size of the array will shrink to the highest element that tests
1187 true for exists() (or 0 if no such element exists).
1189 Returns a list with the same number of elements as the number of elements
1190 for which deletion was attempted. Each element of that list consists of
1191 either the value of the element deleted, or the undefined value. In scalar
1192 context, this means that you get the value of the last element deleted (or
1193 the undefined value if that element did not exist).
1195 %hash = (foo => 11, bar => 22, baz => 33);
1196 $scalar = delete $hash{foo}; # $scalar is 11
1197 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1198 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1200 Deleting from C<%ENV> modifies the environment. Deleting from
1201 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1202 from a C<tie>d hash or array may not necessarily return anything.
1204 Deleting an array element effectively returns that position of the array
1205 to its initial, uninitialized state. Subsequently testing for the same
1206 element with exists() will return false. Also, deleting array elements
1207 in the middle of an array will not shift the index of the elements
1208 after them down. Use splice() for that. See L</exists>.
1210 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1212 foreach $key (keys %HASH) {
1216 foreach $index (0 .. $#ARRAY) {
1217 delete $ARRAY[$index];
1222 delete @HASH{keys %HASH};
1224 delete @ARRAY[0 .. $#ARRAY];
1226 But both of these are slower than just assigning the empty list
1227 or undefining %HASH or @ARRAY:
1229 %HASH = (); # completely empty %HASH
1230 undef %HASH; # forget %HASH ever existed
1232 @ARRAY = (); # completely empty @ARRAY
1233 undef @ARRAY; # forget @ARRAY ever existed
1235 Note that the EXPR can be arbitrarily complicated as long as the final
1236 operation is a hash element, array element, hash slice, or array slice
1239 delete $ref->[$x][$y]{$key};
1240 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1242 delete $ref->[$x][$y][$index];
1243 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1245 The C<delete local EXPR> construct can also be used to localize the deletion
1246 of array/hash elements to the current block.
1247 See L<perlsub/"Localized deletion of elements of composite types">.
1250 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1252 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1253 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1254 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1255 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1256 an C<eval(),> the error message is stuffed into C<$@> and the
1257 C<eval> is terminated with the undefined value. This makes
1258 C<die> the way to raise an exception.
1260 Equivalent examples:
1262 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1263 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1265 If the last element of LIST does not end in a newline, the current
1266 script line number and input line number (if any) are also printed,
1267 and a newline is supplied. Note that the "input line number" (also
1268 known as "chunk") is subject to whatever notion of "line" happens to
1269 be currently in effect, and is also available as the special variable
1270 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1272 Hint: sometimes appending C<", stopped"> to your message will cause it
1273 to make better sense when the string C<"at foo line 123"> is appended.
1274 Suppose you are running script "canasta".
1276 die "/etc/games is no good";
1277 die "/etc/games is no good, stopped";
1279 produce, respectively
1281 /etc/games is no good at canasta line 123.
1282 /etc/games is no good, stopped at canasta line 123.
1284 See also exit(), warn(), and the Carp module.
1286 If the output is empty and C<$@> already contains a value (typically from a
1287 previous eval) that value is reused after appending C<"\t...propagated">.
1288 This is useful for propagating exceptions:
1291 die unless $@ =~ /Expected exception/;
1293 If the output is empty and C<$@> contains an object reference that has a
1294 C<PROPAGATE> method, that method will be called with additional file
1295 and line number parameters. The return value replaces the value in
1296 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1299 If C<$@> is empty then the string C<"Died"> is used.
1301 die() can also be called with a reference argument. If this happens to be
1302 trapped within an eval(), $@ contains the reference. This behavior permits
1303 a more elaborate exception handling implementation using objects that
1304 maintain arbitrary state about the nature of the exception. Such a scheme
1305 is sometimes preferable to matching particular string values of $@ using
1306 regular expressions. Because $@ is a global variable, and eval() may be
1307 used within object implementations, care must be taken that analyzing the
1308 error object doesn't replace the reference in the global variable. The
1309 easiest solution is to make a local copy of the reference before doing
1310 other manipulations. Here's an example:
1312 use Scalar::Util 'blessed';
1314 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1315 if (my $ev_err = $@) {
1316 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1317 # handle Some::Module::Exception
1320 # handle all other possible exceptions
1324 Because perl will stringify uncaught exception messages before displaying
1325 them, you may want to overload stringification operations on such custom
1326 exception objects. See L<overload> for details about that.
1328 You can arrange for a callback to be run just before the C<die>
1329 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1330 handler will be called with the error text and can change the error
1331 message, if it sees fit, by calling C<die> again. See
1332 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1333 L<"eval BLOCK"> for some examples. Although this feature was
1334 to be run only right before your program was to exit, this is not
1335 currently the case--the C<$SIG{__DIE__}> hook is currently called
1336 even inside eval()ed blocks/strings! If one wants the hook to do
1337 nothing in such situations, put
1341 as the first line of the handler (see L<perlvar/$^S>). Because
1342 this promotes strange action at a distance, this counterintuitive
1343 behavior may be fixed in a future release.
1348 Not really a function. Returns the value of the last command in the
1349 sequence of commands indicated by BLOCK. When modified by the C<while> or
1350 C<until> loop modifier, executes the BLOCK once before testing the loop
1351 condition. (On other statements the loop modifiers test the conditional
1354 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1355 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1356 See L<perlsyn> for alternative strategies.
1358 =item do SUBROUTINE(LIST)
1361 This form of subroutine call is deprecated. See L<perlsub>.
1366 Uses the value of EXPR as a filename and executes the contents of the
1367 file as a Perl script.
1375 except that it's more efficient and concise, keeps track of the current
1376 filename for error messages, searches the @INC directories, and updates
1377 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1378 variables. It also differs in that code evaluated with C<do FILENAME>
1379 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1380 same, however, in that it does reparse the file every time you call it,
1381 so you probably don't want to do this inside a loop.
1383 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1384 error. If C<do> can read the file but cannot compile it, it
1385 returns undef and sets an error message in C<$@>. If the file is
1386 successfully compiled, C<do> returns the value of the last expression
1389 Note that inclusion of library modules is better done with the
1390 C<use> and C<require> operators, which also do automatic error checking
1391 and raise an exception if there's a problem.
1393 You might like to use C<do> to read in a program configuration
1394 file. Manual error checking can be done this way:
1396 # read in config files: system first, then user
1397 for $file ("/share/prog/defaults.rc",
1398 "$ENV{HOME}/.someprogrc")
1400 unless ($return = do $file) {
1401 warn "couldn't parse $file: $@" if $@;
1402 warn "couldn't do $file: $!" unless defined $return;
1403 warn "couldn't run $file" unless $return;
1408 X<dump> X<core> X<undump>
1412 This function causes an immediate core dump. See also the B<-u>
1413 command-line switch in L<perlrun>, which does the same thing.
1414 Primarily this is so that you can use the B<undump> program (not
1415 supplied) to turn your core dump into an executable binary after
1416 having initialized all your variables at the beginning of the
1417 program. When the new binary is executed it will begin by executing
1418 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1419 Think of it as a goto with an intervening core dump and reincarnation.
1420 If C<LABEL> is omitted, restarts the program from the top.
1422 B<WARNING>: Any files opened at the time of the dump will I<not>
1423 be open any more when the program is reincarnated, with possible
1424 resulting confusion on the part of Perl.
1426 This function is now largely obsolete, mostly because it's very hard to
1427 convert a core file into an executable. That's why you should now invoke
1428 it as C<CORE::dump()>, if you don't want to be warned against a possible
1432 X<each> X<hash, iterator>
1437 When called in list context, returns a 2-element list consisting of the
1438 key and value for the next element of a hash, or the index and value for
1439 the next element of an array, so that you can iterate over it. When called
1440 in scalar context, returns only the key for the next element in the hash
1441 (or the index for an array).
1443 Hash entries are returned in an apparently random order. The actual random
1444 order is subject to change in future versions of perl, but it is
1445 guaranteed to be in the same order as either the C<keys> or C<values>
1446 function would produce on the same (unmodified) hash. Since Perl
1447 5.8.2 the ordering can be different even between different runs of Perl
1448 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1450 When the hash or array is entirely read, a null array is returned in list
1451 context (which when assigned produces a false (C<0>) value), and C<undef> in
1452 scalar context. The next call to C<each> after that will start iterating
1453 again. There is a single iterator for each hash or array, shared by all
1454 C<each>, C<keys>, and C<values> function calls in the program; it can be
1455 reset by reading all the elements from the hash or array, or by evaluating
1456 C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1457 or delete elements of a hash while you're
1458 iterating over it, you may get entries skipped or duplicated, so
1459 don't. Exception: It is always safe to delete the item most recently
1460 returned by C<each()>, which means that the following code will work:
1462 while (($key, $value) = each %hash) {
1464 delete $hash{$key}; # This is safe
1467 The following prints out your environment like the printenv(1) program,
1468 only in a different order:
1470 while (($key,$value) = each %ENV) {
1471 print "$key=$value\n";
1474 See also C<keys>, C<values> and C<sort>.
1476 =item eof FILEHANDLE
1485 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1486 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1487 gives the real filehandle. (Note that this function actually
1488 reads a character and then C<ungetc>s it, so isn't very useful in an
1489 interactive context.) Do not read from a terminal file (or call
1490 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1491 as terminals may lose the end-of-file condition if you do.
1493 An C<eof> without an argument uses the last file read. Using C<eof()>
1494 with empty parentheses is very different. It refers to the pseudo file
1495 formed from the files listed on the command line and accessed via the
1496 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1497 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1498 used will cause C<@ARGV> to be examined to determine if input is
1499 available. Similarly, an C<eof()> after C<< <> >> has returned
1500 end-of-file will assume you are processing another C<@ARGV> list,
1501 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1502 see L<perlop/"I/O Operators">.
1504 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1505 detect the end of each file, C<eof()> will only detect the end of the
1506 last file. Examples:
1508 # reset line numbering on each input file
1510 next if /^\s*#/; # skip comments
1513 close ARGV if eof; # Not eof()!
1516 # insert dashes just before last line of last file
1518 if (eof()) { # check for end of last file
1519 print "--------------\n";
1522 last if eof(); # needed if we're reading from a terminal
1525 Practical hint: you almost never need to use C<eof> in Perl, because the
1526 input operators typically return C<undef> when they run out of data, or if
1530 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1531 X<error, handling> X<exception, handling>
1537 In the first form, the return value of EXPR is parsed and executed as if it
1538 were a little Perl program. The value of the expression (which is itself
1539 determined within scalar context) is first parsed, and if there weren't any
1540 errors, executed in the lexical context of the current Perl program, so
1541 that any variable settings or subroutine and format definitions remain
1542 afterwards. Note that the value is parsed every time the C<eval> executes.
1543 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1544 delay parsing and subsequent execution of the text of EXPR until run time.
1546 In the second form, the code within the BLOCK is parsed only once--at the
1547 same time the code surrounding the C<eval> itself was parsed--and executed
1548 within the context of the current Perl program. This form is typically
1549 used to trap exceptions more efficiently than the first (see below), while
1550 also providing the benefit of checking the code within BLOCK at compile
1553 The final semicolon, if any, may be omitted from the value of EXPR or within
1556 In both forms, the value returned is the value of the last expression
1557 evaluated inside the mini-program; a return statement may be also used, just
1558 as with subroutines. The expression providing the return value is evaluated
1559 in void, scalar, or list context, depending on the context of the C<eval>
1560 itself. See L</wantarray> for more on how the evaluation context can be
1563 If there is a syntax error or runtime error, or a C<die> statement is
1564 executed, C<eval> returns an undefined value in scalar context
1565 or an empty list in list context, and C<$@> is set to the
1566 error message. If there was no error, C<$@> is guaranteed to be a null
1567 string. Beware that using C<eval> neither silences perl from printing
1568 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1569 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1570 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1571 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1573 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1574 determining whether a particular feature (such as C<socket> or C<symlink>)
1575 is implemented. It is also Perl's exception trapping mechanism, where
1576 the die operator is used to raise exceptions.
1578 If you want to trap errors when loading an XS module, some problems with
1579 the binary interface (such as Perl version skew) may be fatal even with
1580 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1582 If the code to be executed doesn't vary, you may use the eval-BLOCK
1583 form to trap run-time errors without incurring the penalty of
1584 recompiling each time. The error, if any, is still returned in C<$@>.
1587 # make divide-by-zero nonfatal
1588 eval { $answer = $a / $b; }; warn $@ if $@;
1590 # same thing, but less efficient
1591 eval '$answer = $a / $b'; warn $@ if $@;
1593 # a compile-time error
1594 eval { $answer = }; # WRONG
1597 eval '$answer ='; # sets $@
1599 Using the C<eval{}> form as an exception trap in libraries does have some
1600 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1601 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1602 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1603 as shown in this example:
1605 # a very private exception trap for divide-by-zero
1606 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1609 This is especially significant, given that C<__DIE__> hooks can call
1610 C<die> again, which has the effect of changing their error messages:
1612 # __DIE__ hooks may modify error messages
1614 local $SIG{'__DIE__'} =
1615 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1616 eval { die "foo lives here" };
1617 print $@ if $@; # prints "bar lives here"
1620 Because this promotes action at a distance, this counterintuitive behavior
1621 may be fixed in a future release.
1623 With an C<eval>, you should be especially careful to remember what's
1624 being looked at when:
1630 eval { $x }; # CASE 4
1632 eval "\$$x++"; # CASE 5
1635 Cases 1 and 2 above behave identically: they run the code contained in
1636 the variable $x. (Although case 2 has misleading double quotes making
1637 the reader wonder what else might be happening (nothing is).) Cases 3
1638 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1639 does nothing but return the value of $x. (Case 4 is preferred for
1640 purely visual reasons, but it also has the advantage of compiling at
1641 compile-time instead of at run-time.) Case 5 is a place where
1642 normally you I<would> like to use double quotes, except that in this
1643 particular situation, you can just use symbolic references instead, as
1646 The assignment to C<$@> occurs before restoration of localised variables,
1647 which means a temporary is required if you want to mask some but not all
1650 # alter $@ on nefarious repugnancy only
1654 local $@; # protect existing $@
1655 eval { test_repugnancy() };
1656 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1657 $@ =~ /nefarious/ and $e = $@;
1659 die $e if defined $e
1662 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1663 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1665 Note that as a very special case, an C<eval ''> executed within the C<DB>
1666 package doesn't see the usual surrounding lexical scope, but rather the
1667 scope of the first non-DB piece of code that called it. You don't normally
1668 need to worry about this unless you are writing a Perl debugger.
1673 =item exec PROGRAM LIST
1675 The C<exec> function executes a system command I<and never returns>--
1676 use C<system> instead of C<exec> if you want it to return. It fails and
1677 returns false only if the command does not exist I<and> it is executed
1678 directly instead of via your system's command shell (see below).
1680 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1681 warns you if there is a following statement which isn't C<die>, C<warn>,
1682 or C<exit> (if C<-w> is set - but you always do that). If you
1683 I<really> want to follow an C<exec> with some other statement, you
1684 can use one of these styles to avoid the warning:
1686 exec ('foo') or print STDERR "couldn't exec foo: $!";
1687 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1689 If there is more than one argument in LIST, or if LIST is an array
1690 with more than one value, calls execvp(3) with the arguments in LIST.
1691 If there is only one scalar argument or an array with one element in it,
1692 the argument is checked for shell metacharacters, and if there are any,
1693 the entire argument is passed to the system's command shell for parsing
1694 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1695 If there are no shell metacharacters in the argument, it is split into
1696 words and passed directly to C<execvp>, which is more efficient.
1699 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1700 exec "sort $outfile | uniq";
1702 If you don't really want to execute the first argument, but want to lie
1703 to the program you are executing about its own name, you can specify
1704 the program you actually want to run as an "indirect object" (without a
1705 comma) in front of the LIST. (This always forces interpretation of the
1706 LIST as a multivalued list, even if there is only a single scalar in
1709 $shell = '/bin/csh';
1710 exec $shell '-sh'; # pretend it's a login shell
1714 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1716 When the arguments get executed via the system shell, results will
1717 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1720 Using an indirect object with C<exec> or C<system> is also more
1721 secure. This usage (which also works fine with system()) forces
1722 interpretation of the arguments as a multivalued list, even if the
1723 list had just one argument. That way you're safe from the shell
1724 expanding wildcards or splitting up words with whitespace in them.
1726 @args = ( "echo surprise" );
1728 exec @args; # subject to shell escapes
1730 exec { $args[0] } @args; # safe even with one-arg list
1732 The first version, the one without the indirect object, ran the I<echo>
1733 program, passing it C<"surprise"> an argument. The second version
1734 didn't--it tried to run a program literally called I<"echo surprise">,
1735 didn't find it, and set C<$?> to a non-zero value indicating failure.
1737 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1738 output before the exec, but this may not be supported on some platforms
1739 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1740 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1741 open handles in order to avoid lost output.
1743 Note that C<exec> will not call your C<END> blocks, nor will it call
1744 any C<DESTROY> methods in your objects.
1747 X<exists> X<autovivification>
1749 Given an expression that specifies a hash element or array element,
1750 returns true if the specified element in the hash or array has ever
1751 been initialized, even if the corresponding value is undefined.
1753 print "Exists\n" if exists $hash{$key};
1754 print "Defined\n" if defined $hash{$key};
1755 print "True\n" if $hash{$key};
1757 print "Exists\n" if exists $array[$index];
1758 print "Defined\n" if defined $array[$index];
1759 print "True\n" if $array[$index];
1761 A hash or array element can be true only if it's defined, and defined if
1762 it exists, but the reverse doesn't necessarily hold true.
1764 Given an expression that specifies the name of a subroutine,
1765 returns true if the specified subroutine has ever been declared, even
1766 if it is undefined. Mentioning a subroutine name for exists or defined
1767 does not count as declaring it. Note that a subroutine which does not
1768 exist may still be callable: its package may have an C<AUTOLOAD>
1769 method that makes it spring into existence the first time that it is
1770 called -- see L<perlsub>.
1772 print "Exists\n" if exists &subroutine;
1773 print "Defined\n" if defined &subroutine;
1775 Note that the EXPR can be arbitrarily complicated as long as the final
1776 operation is a hash or array key lookup or subroutine name:
1778 if (exists $ref->{A}->{B}->{$key}) { }
1779 if (exists $hash{A}{B}{$key}) { }
1781 if (exists $ref->{A}->{B}->[$ix]) { }
1782 if (exists $hash{A}{B}[$ix]) { }
1784 if (exists &{$ref->{A}{B}{$key}}) { }
1786 Although the deepest nested array or hash will not spring into existence
1787 just because its existence was tested, any intervening ones will.
1788 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1789 into existence due to the existence test for the $key element above.
1790 This happens anywhere the arrow operator is used, including even:
1793 if (exists $ref->{"Some key"}) { }
1794 print $ref; # prints HASH(0x80d3d5c)
1796 This surprising autovivification in what does not at first--or even
1797 second--glance appear to be an lvalue context may be fixed in a future
1800 Use of a subroutine call, rather than a subroutine name, as an argument
1801 to exists() is an error.
1804 exists &sub(); # Error
1807 X<exit> X<terminate> X<abort>
1811 Evaluates EXPR and exits immediately with that value. Example:
1814 exit 0 if $ans =~ /^[Xx]/;
1816 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1817 universally recognized values for EXPR are C<0> for success and C<1>
1818 for error; other values are subject to interpretation depending on the
1819 environment in which the Perl program is running. For example, exiting
1820 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1821 the mailer to return the item undelivered, but that's not true everywhere.
1823 Don't use C<exit> to abort a subroutine if there's any chance that
1824 someone might want to trap whatever error happened. Use C<die> instead,
1825 which can be trapped by an C<eval>.
1827 The exit() function does not always exit immediately. It calls any
1828 defined C<END> routines first, but these C<END> routines may not
1829 themselves abort the exit. Likewise any object destructors that need to
1830 be called are called before the real exit. If this is a problem, you
1831 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1832 See L<perlmod> for details.
1835 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1839 Returns I<e> (the natural logarithm base) to the power of EXPR.
1840 If EXPR is omitted, gives C<exp($_)>.
1842 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1845 Implements the fcntl(2) function. You'll probably have to say
1849 first to get the correct constant definitions. Argument processing and
1850 value return works just like C<ioctl> below.
1854 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1855 or die "can't fcntl F_GETFL: $!";
1857 You don't have to check for C<defined> on the return from C<fcntl>.
1858 Like C<ioctl>, it maps a C<0> return from the system call into
1859 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1860 in numeric context. It is also exempt from the normal B<-w> warnings
1861 on improper numeric conversions.
1863 Note that C<fcntl> will produce a fatal error if used on a machine that
1864 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1865 manpage to learn what functions are available on your system.
1867 Here's an example of setting a filehandle named C<REMOTE> to be
1868 non-blocking at the system level. You'll have to negotiate C<$|>
1869 on your own, though.
1871 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1873 $flags = fcntl(REMOTE, F_GETFL, 0)
1874 or die "Can't get flags for the socket: $!\n";
1876 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1877 or die "Can't set flags for the socket: $!\n";
1879 =item fileno FILEHANDLE
1882 Returns the file descriptor for a filehandle, or undefined if the
1883 filehandle is not open. This is mainly useful for constructing
1884 bitmaps for C<select> and low-level POSIX tty-handling operations.
1885 If FILEHANDLE is an expression, the value is taken as an indirect
1886 filehandle, generally its name.
1888 You can use this to find out whether two handles refer to the
1889 same underlying descriptor:
1891 if (fileno(THIS) == fileno(THAT)) {
1892 print "THIS and THAT are dups\n";
1895 (Filehandles connected to memory objects via new features of C<open> may
1896 return undefined even though they are open.)
1899 =item flock FILEHANDLE,OPERATION
1900 X<flock> X<lock> X<locking>
1902 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1903 for success, false on failure. Produces a fatal error if used on a
1904 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1905 C<flock> is Perl's portable file locking interface, although it locks
1906 only entire files, not records.
1908 Two potentially non-obvious but traditional C<flock> semantics are
1909 that it waits indefinitely until the lock is granted, and that its locks
1910 B<merely advisory>. Such discretionary locks are more flexible, but offer
1911 fewer guarantees. This means that programs that do not also use C<flock>
1912 may modify files locked with C<flock>. See L<perlport>,
1913 your port's specific documentation, or your system-specific local manpages
1914 for details. It's best to assume traditional behavior if you're writing
1915 portable programs. (But if you're not, you should as always feel perfectly
1916 free to write for your own system's idiosyncrasies (sometimes called
1917 "features"). Slavish adherence to portability concerns shouldn't get
1918 in the way of your getting your job done.)
1920 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1921 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1922 you can use the symbolic names if you import them from the Fcntl module,
1923 either individually, or as a group using the ':flock' tag. LOCK_SH
1924 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1925 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1926 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1927 waiting for the lock (check the return status to see if you got it).
1929 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1930 before locking or unlocking it.
1932 Note that the emulation built with lockf(3) doesn't provide shared
1933 locks, and it requires that FILEHANDLE be open with write intent. These
1934 are the semantics that lockf(3) implements. Most if not all systems
1935 implement lockf(3) in terms of fcntl(2) locking, though, so the
1936 differing semantics shouldn't bite too many people.
1938 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1939 be open with read intent to use LOCK_SH and requires that it be open
1940 with write intent to use LOCK_EX.
1942 Note also that some versions of C<flock> cannot lock things over the
1943 network; you would need to use the more system-specific C<fcntl> for
1944 that. If you like you can force Perl to ignore your system's flock(2)
1945 function, and so provide its own fcntl(2)-based emulation, by passing
1946 the switch C<-Ud_flock> to the F<Configure> program when you configure
1949 Here's a mailbox appender for BSD systems.
1951 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
1955 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
1957 # and, in case someone appended while we were waiting...
1958 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
1963 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
1966 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
1967 or die "Can't open mailbox: $!";
1970 print $mbox $msg,"\n\n";
1973 On systems that support a real flock(), locks are inherited across fork()
1974 calls, whereas those that must resort to the more capricious fcntl()
1975 function lose the locks, making it harder to write servers.
1977 See also L<DB_File> for other flock() examples.
1980 X<fork> X<child> X<parent>
1982 Does a fork(2) system call to create a new process running the
1983 same program at the same point. It returns the child pid to the
1984 parent process, C<0> to the child process, or C<undef> if the fork is
1985 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1986 are shared, while everything else is copied. On most systems supporting
1987 fork(), great care has gone into making it extremely efficient (for
1988 example, using copy-on-write technology on data pages), making it the
1989 dominant paradigm for multitasking over the last few decades.
1991 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1992 output before forking the child process, but this may not be supported
1993 on some platforms (see L<perlport>). To be safe, you may need to set
1994 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1995 C<IO::Handle> on any open handles in order to avoid duplicate output.
1997 If you C<fork> without ever waiting on your children, you will
1998 accumulate zombies. On some systems, you can avoid this by setting
1999 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2000 forking and reaping moribund children.
2002 Note that if your forked child inherits system file descriptors like
2003 STDIN and STDOUT that are actually connected by a pipe or socket, even
2004 if you exit, then the remote server (such as, say, a CGI script or a
2005 backgrounded job launched from a remote shell) won't think you're done.
2006 You should reopen those to F</dev/null> if it's any issue.
2011 Declare a picture format for use by the C<write> function. For
2015 Test: @<<<<<<<< @||||| @>>>>>
2016 $str, $%, '$' . int($num)
2020 $num = $cost/$quantity;
2024 See L<perlform> for many details and examples.
2026 =item formline PICTURE,LIST
2029 This is an internal function used by C<format>s, though you may call it,
2030 too. It formats (see L<perlform>) a list of values according to the
2031 contents of PICTURE, placing the output into the format output
2032 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2033 Eventually, when a C<write> is done, the contents of
2034 C<$^A> are written to some filehandle. You could also read C<$^A>
2035 and then set C<$^A> back to C<"">. Note that a format typically
2036 does one C<formline> per line of form, but the C<formline> function itself
2037 doesn't care how many newlines are embedded in the PICTURE. This means
2038 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2039 You may therefore need to use multiple formlines to implement a single
2040 record format, just like the format compiler.
2042 Be careful if you put double quotes around the picture, because an C<@>
2043 character may be taken to mean the beginning of an array name.
2044 C<formline> always returns true. See L<perlform> for other examples.
2046 =item getc FILEHANDLE
2047 X<getc> X<getchar> X<character> X<file, read>
2051 Returns the next character from the input file attached to FILEHANDLE,
2052 or the undefined value at end of file, or if there was an error (in
2053 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2054 STDIN. This is not particularly efficient. However, it cannot be
2055 used by itself to fetch single characters without waiting for the user
2056 to hit enter. For that, try something more like:
2059 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2062 system "stty", '-icanon', 'eol', "\001";
2068 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2071 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2075 Determination of whether $BSD_STYLE should be set
2076 is left as an exercise to the reader.
2078 The C<POSIX::getattr> function can do this more portably on
2079 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2080 module from your nearest CPAN site; details on CPAN can be found on
2084 X<getlogin> X<login>
2086 This implements the C library function of the same name, which on most
2087 systems returns the current login from F</etc/utmp>, if any. If null,
2090 $login = getlogin || getpwuid($<) || "Kilroy";
2092 Do not consider C<getlogin> for authentication: it is not as
2093 secure as C<getpwuid>.
2095 =item getpeername SOCKET
2096 X<getpeername> X<peer>
2098 Returns the packed sockaddr address of other end of the SOCKET connection.
2101 $hersockaddr = getpeername(SOCK);
2102 ($port, $iaddr) = sockaddr_in($hersockaddr);
2103 $herhostname = gethostbyaddr($iaddr, AF_INET);
2104 $herstraddr = inet_ntoa($iaddr);
2109 Returns the current process group for the specified PID. Use
2110 a PID of C<0> to get the current process group for the
2111 current process. Will raise an exception if used on a machine that
2112 doesn't implement getpgrp(2). If PID is omitted, returns process
2113 group of current process. Note that the POSIX version of C<getpgrp>
2114 does not accept a PID argument, so only C<PID==0> is truly portable.
2117 X<getppid> X<parent> X<pid>
2119 Returns the process id of the parent process.
2121 Note for Linux users: on Linux, the C functions C<getpid()> and
2122 C<getppid()> return different values from different threads. In order to
2123 be portable, this behavior is not reflected by the perl-level function
2124 C<getppid()>, that returns a consistent value across threads. If you want
2125 to call the underlying C<getppid()>, you may use the CPAN module
2128 =item getpriority WHICH,WHO
2129 X<getpriority> X<priority> X<nice>
2131 Returns the current priority for a process, a process group, or a user.
2132 (See C<getpriority(2)>.) Will raise a fatal exception if used on a
2133 machine that doesn't implement getpriority(2).
2136 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2137 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2138 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2139 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2140 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2141 X<endnetent> X<endprotoent> X<endservent>
2145 =item gethostbyname NAME
2147 =item getnetbyname NAME
2149 =item getprotobyname NAME
2155 =item getservbyname NAME,PROTO
2157 =item gethostbyaddr ADDR,ADDRTYPE
2159 =item getnetbyaddr ADDR,ADDRTYPE
2161 =item getprotobynumber NUMBER
2163 =item getservbyport PORT,PROTO
2181 =item sethostent STAYOPEN
2183 =item setnetent STAYOPEN
2185 =item setprotoent STAYOPEN
2187 =item setservent STAYOPEN
2201 These routines perform the same functions as their counterparts in the
2202 system library. In list context, the return values from the
2203 various get routines are as follows:
2205 ($name,$passwd,$uid,$gid,
2206 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2207 ($name,$passwd,$gid,$members) = getgr*
2208 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2209 ($name,$aliases,$addrtype,$net) = getnet*
2210 ($name,$aliases,$proto) = getproto*
2211 ($name,$aliases,$port,$proto) = getserv*
2213 (If the entry doesn't exist you get a null list.)
2215 The exact meaning of the $gcos field varies but it usually contains
2216 the real name of the user (as opposed to the login name) and other
2217 information pertaining to the user. Beware, however, that in many
2218 system users are able to change this information and therefore it
2219 cannot be trusted and therefore the $gcos is tainted (see
2220 L<perlsec>). The $passwd and $shell, user's encrypted password and
2221 login shell, are also tainted, because of the same reason.
2223 In scalar context, you get the name, unless the function was a
2224 lookup by name, in which case you get the other thing, whatever it is.
2225 (If the entry doesn't exist you get the undefined value.) For example:
2227 $uid = getpwnam($name);
2228 $name = getpwuid($num);
2230 $gid = getgrnam($name);
2231 $name = getgrgid($num);
2235 In I<getpw*()> the fields $quota, $comment, and $expire are special
2236 cases in the sense that in many systems they are unsupported. If the
2237 $quota is unsupported, it is an empty scalar. If it is supported, it
2238 usually encodes the disk quota. If the $comment field is unsupported,
2239 it is an empty scalar. If it is supported it usually encodes some
2240 administrative comment about the user. In some systems the $quota
2241 field may be $change or $age, fields that have to do with password
2242 aging. In some systems the $comment field may be $class. The $expire
2243 field, if present, encodes the expiration period of the account or the
2244 password. For the availability and the exact meaning of these fields
2245 in your system, please consult your getpwnam(3) documentation and your
2246 F<pwd.h> file. You can also find out from within Perl what your
2247 $quota and $comment fields mean and whether you have the $expire field
2248 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2249 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2250 files are only supported if your vendor has implemented them in the
2251 intuitive fashion that calling the regular C library routines gets the
2252 shadow versions if you're running under privilege or if there exists
2253 the shadow(3) functions as found in System V (this includes Solaris
2254 and Linux.) Those systems that implement a proprietary shadow password
2255 facility are unlikely to be supported.
2257 The $members value returned by I<getgr*()> is a space separated list of
2258 the login names of the members of the group.
2260 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2261 C, it will be returned to you via C<$?> if the function call fails. The
2262 C<@addrs> value returned by a successful call is a list of the raw
2263 addresses returned by the corresponding system library call. In the
2264 Internet domain, each address is four bytes long and you can unpack it
2265 by saying something like:
2267 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2269 The Socket library makes this slightly easier:
2272 $iaddr = inet_aton("127.1"); # or whatever address
2273 $name = gethostbyaddr($iaddr, AF_INET);
2275 # or going the other way
2276 $straddr = inet_ntoa($iaddr);
2278 In the opposite way, to resolve a hostname to the IP address
2282 $packed_ip = gethostbyname("www.perl.org");
2283 if (defined $packed_ip) {
2284 $ip_address = inet_ntoa($packed_ip);
2287 Make sure <gethostbyname()> is called in SCALAR context and that
2288 its return value is checked for definedness.
2290 If you get tired of remembering which element of the return list
2291 contains which return value, by-name interfaces are provided
2292 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2293 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2294 and C<User::grent>. These override the normal built-ins, supplying
2295 versions that return objects with the appropriate names
2296 for each field. For example:
2300 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2302 Even though it looks like they're the same method calls (uid),
2303 they aren't, because a C<File::stat> object is different from
2304 a C<User::pwent> object.
2306 =item getsockname SOCKET
2309 Returns the packed sockaddr address of this end of the SOCKET connection,
2310 in case you don't know the address because you have several different
2311 IPs that the connection might have come in on.
2314 $mysockaddr = getsockname(SOCK);
2315 ($port, $myaddr) = sockaddr_in($mysockaddr);
2316 printf "Connect to %s [%s]\n",
2317 scalar gethostbyaddr($myaddr, AF_INET),
2320 =item getsockopt SOCKET,LEVEL,OPTNAME
2323 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2324 Options may exist at multiple protocol levels depending on the socket
2325 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2326 C<Socket> module) will exist. To query options at another level the
2327 protocol number of the appropriate protocol controlling the option
2328 should be supplied. For example, to indicate that an option is to be
2329 interpreted by the TCP protocol, LEVEL should be set to the protocol
2330 number of TCP, which you can get using getprotobyname.
2332 The call returns a packed string representing the requested socket option,
2333 or C<undef> if there is an error (the error reason will be in $!). What
2334 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2335 your system documentation for details. A very common case however is that
2336 the option is an integer, in which case the result will be a packed
2337 integer which you can decode using unpack with the C<i> (or C<I>) format.
2339 An example testing if Nagle's algorithm is turned on on a socket:
2341 use Socket qw(:all);
2343 defined(my $tcp = getprotobyname("tcp"))
2344 or die "Could not determine the protocol number for tcp";
2345 # my $tcp = IPPROTO_TCP; # Alternative
2346 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2347 or die "Could not query TCP_NODELAY socket option: $!";
2348 my $nodelay = unpack("I", $packed);
2349 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2353 X<glob> X<wildcard> X<filename, expansion> X<expand>
2357 In list context, returns a (possibly empty) list of filename expansions on
2358 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2359 scalar context, glob iterates through such filename expansions, returning
2360 undef when the list is exhausted. This is the internal function
2361 implementing the C<< <*.c> >> operator, but you can use it directly. If
2362 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2363 more detail in L<perlop/"I/O Operators">.
2365 Note that C<glob> will split its arguments on whitespace, treating
2366 each segment as separate pattern. As such, C<glob('*.c *.h')> would
2367 match all files with a F<.c> or F<.h> extension. The expression
2368 C<glob('.* *')> would match all files in the current working directory.
2370 Beginning with v5.6.0, this operator is implemented using the standard
2371 C<File::Glob> extension. See L<File::Glob> for details, including
2372 C<bsd_glob> which does not treat whitespace as a pattern separator.
2375 X<gmtime> X<UTC> X<Greenwich>
2379 Works just like L<localtime> but the returned values are
2380 localized for the standard Greenwich time zone.
2382 Note: when called in list context, $isdst, the last value
2383 returned by gmtime is always C<0>. There is no
2384 Daylight Saving Time in GMT.
2386 See L<perlport/gmtime> for portability concerns.
2389 X<goto> X<jump> X<jmp>
2395 The C<goto-LABEL> form finds the statement labeled with LABEL and
2396 resumes execution there. It can't be used to get out of a block or
2397 subroutine given to C<sort>. It can be used to go almost anywhere
2398 else within the dynamic scope, including out of subroutines, but it's
2399 usually better to use some other construct such as C<last> or C<die>.
2400 The author of Perl has never felt the need to use this form of C<goto>
2401 (in Perl, that is--C is another matter). (The difference being that C
2402 does not offer named loops combined with loop control. Perl does, and
2403 this replaces most structured uses of C<goto> in other languages.)
2405 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2406 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2407 necessarily recommended if you're optimizing for maintainability:
2409 goto ("FOO", "BAR", "GLARCH")[$i];
2411 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2412 deprecated and will issue a warning. Even then, it may not be used to
2413 go into any construct that requires initialization, such as a
2414 subroutine or a C<foreach> loop. It also can't be used to go into a
2415 construct that is optimized away.
2417 The C<goto-&NAME> form is quite different from the other forms of
2418 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2419 doesn't have the stigma associated with other gotos. Instead, it
2420 exits the current subroutine (losing any changes set by local()) and
2421 immediately calls in its place the named subroutine using the current
2422 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2423 load another subroutine and then pretend that the other subroutine had
2424 been called in the first place (except that any modifications to C<@_>
2425 in the current subroutine are propagated to the other subroutine.)
2426 After the C<goto>, not even C<caller> will be able to tell that this
2427 routine was called first.
2429 NAME needn't be the name of a subroutine; it can be a scalar variable
2430 containing a code reference, or a block that evaluates to a code
2433 =item grep BLOCK LIST
2436 =item grep EXPR,LIST
2438 This is similar in spirit to, but not the same as, grep(1) and its
2439 relatives. In particular, it is not limited to using regular expressions.
2441 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2442 C<$_> to each element) and returns the list value consisting of those
2443 elements for which the expression evaluated to true. In scalar
2444 context, returns the number of times the expression was true.
2446 @foo = grep(!/^#/, @bar); # weed out comments
2450 @foo = grep {!/^#/} @bar; # weed out comments
2452 Note that C<$_> is an alias to the list value, so it can be used to
2453 modify the elements of the LIST. While this is useful and supported,
2454 it can cause bizarre results if the elements of LIST are not variables.
2455 Similarly, grep returns aliases into the original list, much as a for
2456 loop's index variable aliases the list elements. That is, modifying an
2457 element of a list returned by grep (for example, in a C<foreach>, C<map>
2458 or another C<grep>) actually modifies the element in the original list.
2459 This is usually something to be avoided when writing clear code.
2461 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2462 been declared with C<my $_>) then, in addition to being locally aliased to
2463 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2464 can't be seen from the outside, avoiding any potential side-effects.
2466 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2469 X<hex> X<hexadecimal>
2473 Interprets EXPR as a hex string and returns the corresponding value.
2474 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2475 L</oct>.) If EXPR is omitted, uses C<$_>.
2477 print hex '0xAf'; # prints '175'
2478 print hex 'aF'; # same
2480 Hex strings may only represent integers. Strings that would cause
2481 integer overflow trigger a warning. Leading whitespace is not stripped,
2482 unlike oct(). To present something as hex, look into L</printf>,
2483 L</sprintf>, or L</unpack>.
2488 There is no builtin C<import> function. It is just an ordinary
2489 method (subroutine) defined (or inherited) by modules that wish to export
2490 names to another module. The C<use> function calls the C<import> method
2491 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2493 =item index STR,SUBSTR,POSITION
2494 X<index> X<indexOf> X<InStr>
2496 =item index STR,SUBSTR
2498 The index function searches for one string within another, but without
2499 the wildcard-like behavior of a full regular-expression pattern match.
2500 It returns the position of the first occurrence of SUBSTR in STR at
2501 or after POSITION. If POSITION is omitted, starts searching from the
2502 beginning of the string. POSITION before the beginning of the string
2503 or after its end is treated as if it were the beginning or the end,
2504 respectively. POSITION and the return value are based at C<0> (or whatever
2505 you've set the C<$[> variable to--but don't do that). If the substring
2506 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2509 X<int> X<integer> X<truncate> X<trunc> X<floor>
2513 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2514 You should not use this function for rounding: one because it truncates
2515 towards C<0>, and two because machine representations of floating point
2516 numbers can sometimes produce counterintuitive results. For example,
2517 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2518 because it's really more like -268.99999999999994315658 instead. Usually,
2519 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2520 functions will serve you better than will int().
2522 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2525 Implements the ioctl(2) function. You'll probably first have to say
2527 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2529 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2530 exist or doesn't have the correct definitions you'll have to roll your
2531 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2532 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2533 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2534 written depending on the FUNCTION--a pointer to the string value of SCALAR
2535 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2536 has no string value but does have a numeric value, that value will be
2537 passed rather than a pointer to the string value. To guarantee this to be
2538 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2539 functions may be needed to manipulate the values of structures used by
2542 The return value of C<ioctl> (and C<fcntl>) is as follows:
2544 if OS returns: then Perl returns:
2546 0 string "0 but true"
2547 anything else that number
2549 Thus Perl returns true on success and false on failure, yet you can
2550 still easily determine the actual value returned by the operating
2553 $retval = ioctl(...) || -1;
2554 printf "System returned %d\n", $retval;
2556 The special string C<"0 but true"> is exempt from B<-w> complaints
2557 about improper numeric conversions.
2559 =item join EXPR,LIST
2562 Joins the separate strings of LIST into a single string with fields
2563 separated by the value of EXPR, and returns that new string. Example:
2565 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2567 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2568 first argument. Compare L</split>.
2575 Returns a list consisting of all the keys of the named hash, or the indices
2576 of an array. (In scalar context, returns the number of keys or indices.)
2578 The keys of a hash are returned in an apparently random order. The actual
2579 random order is subject to change in future versions of perl, but it
2580 is guaranteed to be the same order as either the C<values> or C<each>
2581 function produces (given that the hash has not been modified). Since
2582 Perl 5.8.1 the ordering is different even between different runs of
2583 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2586 As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
2587 (see L</each>). In particular, calling keys() in void context resets
2588 the iterator with no other overhead.
2590 Here is yet another way to print your environment:
2593 @values = values %ENV;
2595 print pop(@keys), '=', pop(@values), "\n";
2598 or how about sorted by key:
2600 foreach $key (sort(keys %ENV)) {
2601 print $key, '=', $ENV{$key}, "\n";
2604 The returned values are copies of the original keys in the hash, so
2605 modifying them will not affect the original hash. Compare L</values>.
2607 To sort a hash by value, you'll need to use a C<sort> function.
2608 Here's a descending numeric sort of a hash by its values:
2610 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2611 printf "%4d %s\n", $hash{$key}, $key;
2614 As an lvalue C<keys> allows you to increase the number of hash buckets
2615 allocated for the given hash. This can gain you a measure of efficiency if
2616 you know the hash is going to get big. (This is similar to pre-extending
2617 an array by assigning a larger number to $#array.) If you say
2621 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2622 in fact, since it rounds up to the next power of two. These
2623 buckets will be retained even if you do C<%hash = ()>, use C<undef
2624 %hash> if you want to free the storage while C<%hash> is still in scope.
2625 You can't shrink the number of buckets allocated for the hash using
2626 C<keys> in this way (but you needn't worry about doing this by accident,
2627 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2630 See also C<each>, C<values> and C<sort>.
2632 =item kill SIGNAL, LIST
2635 Sends a signal to a list of processes. Returns the number of
2636 processes successfully signaled (which is not necessarily the
2637 same as the number actually killed).
2639 $cnt = kill 1, $child1, $child2;
2642 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2643 system call will check whether it's possible to send a signal to it (that
2644 means, to be brief, that the process is owned by the same user, or we are
2645 the super-user). This is a useful way to check that a child process is
2646 alive (even if only as a zombie) and hasn't changed its UID. See
2647 L<perlport> for notes on the portability of this construct.
2649 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2650 of processes. That means you usually want to use positive not negative signals.
2651 You may also use a signal name in quotes.
2653 The behavior of kill when a I<PROCESS> number is zero or negative depends on
2654 the operating system. For example, on POSIX-conforming systems, zero will
2655 signal the current process group and -1 will signal all processes.
2657 See L<perlipc/"Signals"> for more details.
2664 The C<last> command is like the C<break> statement in C (as used in
2665 loops); it immediately exits the loop in question. If the LABEL is
2666 omitted, the command refers to the innermost enclosing loop. The
2667 C<continue> block, if any, is not executed:
2669 LINE: while (<STDIN>) {
2670 last LINE if /^$/; # exit when done with header
2674 C<last> cannot be used to exit a block which returns a value such as
2675 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2676 a grep() or map() operation.
2678 Note that a block by itself is semantically identical to a loop
2679 that executes once. Thus C<last> can be used to effect an early
2680 exit out of such a block.
2682 See also L</continue> for an illustration of how C<last>, C<next>, and
2690 Returns a lowercased version of EXPR. This is the internal function
2691 implementing the C<\L> escape in double-quoted strings. Respects
2692 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2693 and L<perlunicode> for more details about locale and Unicode support.
2695 If EXPR is omitted, uses C<$_>.
2698 X<lcfirst> X<lowercase>
2702 Returns the value of EXPR with the first character lowercased. This
2703 is the internal function implementing the C<\l> escape in
2704 double-quoted strings. Respects current LC_CTYPE locale if C<use
2705 locale> in force. See L<perllocale> and L<perlunicode> for more
2706 details about locale and Unicode support.
2708 If EXPR is omitted, uses C<$_>.
2715 Returns the length in I<characters> of the value of EXPR. If EXPR is
2716 omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2717 Note that this cannot be used on an entire array or hash to find out how
2718 many elements these have. For that, use C<scalar @array> and C<scalar keys
2719 %hash> respectively.
2721 Note the I<characters>: if the EXPR is in Unicode, you will get the
2722 number of characters, not the number of bytes. To get the length
2723 of the internal string in bytes, use C<bytes::length(EXPR)>, see
2724 L<bytes>. Note that the internal encoding is variable, and the number
2725 of bytes usually meaningless. To get the number of bytes that the
2726 string would have when encoded as UTF-8, use
2727 C<length(Encoding::encode_utf8(EXPR))>.
2729 =item link OLDFILE,NEWFILE
2732 Creates a new filename linked to the old filename. Returns true for
2733 success, false otherwise.
2735 =item listen SOCKET,QUEUESIZE
2738 Does the same thing that the listen system call does. Returns true if
2739 it succeeded, false otherwise. See the example in
2740 L<perlipc/"Sockets: Client/Server Communication">.
2745 You really probably want to be using C<my> instead, because C<local> isn't
2746 what most people think of as "local". See
2747 L<perlsub/"Private Variables via my()"> for details.
2749 A local modifies the listed variables to be local to the enclosing
2750 block, file, or eval. If more than one value is listed, the list must
2751 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2752 for details, including issues with tied arrays and hashes.
2754 The C<delete local EXPR> construct can also be used to localize the deletion
2755 of array/hash elements to the current block.
2756 See L<perlsub/"Localized deletion of elements of composite types">.
2758 =item localtime EXPR
2759 X<localtime> X<ctime>
2763 Converts a time as returned by the time function to a 9-element list
2764 with the time analyzed for the local time zone. Typically used as
2768 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2771 All list elements are numeric, and come straight out of the C `struct
2772 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2773 of the specified time.
2775 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2776 the range C<0..11> with 0 indicating January and 11 indicating December.
2777 This makes it easy to get a month name from a list:
2779 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2780 print "$abbr[$mon] $mday";
2781 # $mon=9, $mday=18 gives "Oct 18"
2783 C<$year> is the number of years since 1900, not just the last two digits
2784 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2785 to get a complete 4-digit year is simply:
2789 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2790 to do that, would you?
2792 To get the last two digits of the year (e.g., '01' in 2001) do:
2794 $year = sprintf("%02d", $year % 100);
2796 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2797 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2798 (or C<0..365> in leap years.)
2800 C<$isdst> is true if the specified time occurs during Daylight Saving
2801 Time, false otherwise.
2803 If EXPR is omitted, C<localtime()> uses the current time (as returned
2806 In scalar context, C<localtime()> returns the ctime(3) value:
2808 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2810 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2811 instead of local time use the L</gmtime> builtin. See also the
2812 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2813 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2814 and mktime(3) functions.
2816 To get somewhat similar but locale dependent date strings, set up your
2817 locale environment variables appropriately (please see L<perllocale>) and
2820 use POSIX qw(strftime);
2821 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2822 # or for GMT formatted appropriately for your locale:
2823 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2825 Note that the C<%a> and C<%b>, the short forms of the day of the week
2826 and the month of the year, may not necessarily be three characters wide.
2828 See L<perlport/localtime> for portability concerns.
2830 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2831 by-name access mechanism to the gmtime() and localtime() functions,
2834 For a comprehensive date and time representation look at the
2835 L<DateTime> module on CPAN.
2840 This function places an advisory lock on a shared variable, or referenced
2841 object contained in I<THING> until the lock goes out of scope.
2843 lock() is a "weak keyword" : this means that if you've defined a function
2844 by this name (before any calls to it), that function will be called
2845 instead. (However, if you've said C<use threads>, lock() is always a
2846 keyword.) See L<threads>.
2849 X<log> X<logarithm> X<e> X<ln> X<base>
2853 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2854 returns log of C<$_>. To get the log of another base, use basic algebra:
2855 The base-N log of a number is equal to the natural log of that number
2856 divided by the natural log of N. For example:
2860 return log($n)/log(10);
2863 See also L</exp> for the inverse operation.
2870 Does the same thing as the C<stat> function (including setting the
2871 special C<_> filehandle) but stats a symbolic link instead of the file
2872 the symbolic link points to. If symbolic links are unimplemented on
2873 your system, a normal C<stat> is done. For much more detailed
2874 information, please see the documentation for C<stat>.
2876 If EXPR is omitted, stats C<$_>.
2880 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
2882 =item map BLOCK LIST
2887 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2888 C<$_> to each element) and returns the list value composed of the
2889 results of each such evaluation. In scalar context, returns the
2890 total number of elements so generated. Evaluates BLOCK or EXPR in
2891 list context, so each element of LIST may produce zero, one, or
2892 more elements in the returned value.
2894 @chars = map(chr, @nums);
2896 translates a list of numbers to the corresponding characters. And
2898 %hash = map { get_a_key_for($_) => $_ } @array;
2900 is just a funny way to write
2904 $hash{get_a_key_for($_)} = $_;
2907 Note that C<$_> is an alias to the list value, so it can be used to
2908 modify the elements of the LIST. While this is useful and supported,
2909 it can cause bizarre results if the elements of LIST are not variables.
2910 Using a regular C<foreach> loop for this purpose would be clearer in
2911 most cases. See also L</grep> for an array composed of those items of
2912 the original list for which the BLOCK or EXPR evaluates to true.
2914 If C<$_> is lexical in the scope where the C<map> appears (because it has
2915 been declared with C<my $_>), then, in addition to being locally aliased to
2916 the list elements, C<$_> keeps being lexical inside the block; that is, it
2917 can't be seen from the outside, avoiding any potential side-effects.
2919 C<{> starts both hash references and blocks, so C<map { ...> could be either
2920 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2921 ahead for the closing C<}> it has to take a guess at which its dealing with
2922 based what it finds just after the C<{>. Usually it gets it right, but if it
2923 doesn't it won't realize something is wrong until it gets to the C<}> and
2924 encounters the missing (or unexpected) comma. The syntax error will be
2925 reported close to the C<}> but you'll need to change something near the C<{>
2926 such as using a unary C<+> to give perl some help:
2928 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2929 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2930 %hash = map { ("\L$_", 1) } @array # this also works
2931 %hash = map { lc($_), 1 } @array # as does this.
2932 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2934 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2936 or to force an anon hash constructor use C<+{>:
2938 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2940 and you get list of anonymous hashes each with only 1 entry.
2942 =item mkdir FILENAME,MASK
2943 X<mkdir> X<md> X<directory, create>
2945 =item mkdir FILENAME
2949 Creates the directory specified by FILENAME, with permissions
2950 specified by MASK (as modified by C<umask>). If it succeeds it
2951 returns true, otherwise it returns false and sets C<$!> (errno).
2952 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2955 In general, it is better to create directories with permissive MASK,
2956 and let the user modify that with their C<umask>, than it is to supply
2957 a restrictive MASK and give the user no way to be more permissive.
2958 The exceptions to this rule are when the file or directory should be
2959 kept private (mail files, for instance). The perlfunc(1) entry on
2960 C<umask> discusses the choice of MASK in more detail.
2962 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2963 number of trailing slashes. Some operating and filesystems do not get
2964 this right, so Perl automatically removes all trailing slashes to keep
2967 In order to recursively create a directory structure look at
2968 the C<mkpath> function of the L<File::Path> module.
2970 =item msgctl ID,CMD,ARG
2973 Calls the System V IPC function msgctl(2). You'll probably have to say
2977 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2978 then ARG must be a variable that will hold the returned C<msqid_ds>
2979 structure. Returns like C<ioctl>: the undefined value for error,
2980 C<"0 but true"> for zero, or the actual return value otherwise. See also
2981 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2983 =item msgget KEY,FLAGS
2986 Calls the System V IPC function msgget(2). Returns the message queue
2987 id, or the undefined value if there is an error. See also
2988 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2990 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2993 Calls the System V IPC function msgrcv to receive a message from
2994 message queue ID into variable VAR with a maximum message size of
2995 SIZE. Note that when a message is received, the message type as a
2996 native long integer will be the first thing in VAR, followed by the
2997 actual message. This packing may be opened with C<unpack("l! a*")>.
2998 Taints the variable. Returns true if successful, or false if there is
2999 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
3000 C<IPC::SysV::Msg> documentation.
3002 =item msgsnd ID,MSG,FLAGS
3005 Calls the System V IPC function msgsnd to send the message MSG to the
3006 message queue ID. MSG must begin with the native long integer message
3007 type, and be followed by the length of the actual message, and finally
3008 the message itself. This kind of packing can be achieved with
3009 C<pack("l! a*", $type, $message)>. Returns true if successful,
3010 or false if there is an error. See also C<IPC::SysV>
3011 and C<IPC::SysV::Msg> documentation.
3018 =item my EXPR : ATTRS
3020 =item my TYPE EXPR : ATTRS
3022 A C<my> declares the listed variables to be local (lexically) to the
3023 enclosing block, file, or C<eval>. If more than one value is listed,
3024 the list must be placed in parentheses.
3026 The exact semantics and interface of TYPE and ATTRS are still
3027 evolving. TYPE is currently bound to the use of C<fields> pragma,
3028 and attributes are handled using the C<attributes> pragma, or starting
3029 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3030 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3031 L<attributes>, and L<Attribute::Handlers>.
3038 The C<next> command is like the C<continue> statement in C; it starts
3039 the next iteration of the loop:
3041 LINE: while (<STDIN>) {
3042 next LINE if /^#/; # discard comments
3046 Note that if there were a C<continue> block on the above, it would get
3047 executed even on discarded lines. If the LABEL is omitted, the command
3048 refers to the innermost enclosing loop.
3050 C<next> cannot be used to exit a block which returns a value such as
3051 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3052 a grep() or map() operation.
3054 Note that a block by itself is semantically identical to a loop
3055 that executes once. Thus C<next> will exit such a block early.
3057 See also L</continue> for an illustration of how C<last>, C<next>, and
3060 =item no Module VERSION LIST
3063 =item no Module VERSION
3065 =item no Module LIST
3071 See the C<use> function, of which C<no> is the opposite.
3074 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3078 Interprets EXPR as an octal string and returns the corresponding
3079 value. (If EXPR happens to start off with C<0x>, interprets it as a
3080 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3081 binary string. Leading whitespace is ignored in all three cases.)
3082 The following will handle decimal, binary, octal, and hex in the standard
3085 $val = oct($val) if $val =~ /^0/;
3087 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3088 in octal), use sprintf() or printf():
3090 $perms = (stat("filename"))[2] & 07777;
3091 $oct_perms = sprintf "%lo", $perms;
3093 The oct() function is commonly used when a string such as C<644> needs
3094 to be converted into a file mode, for example. (Although perl will
3095 automatically convert strings into numbers as needed, this automatic
3096 conversion assumes base 10.)
3098 =item open FILEHANDLE,EXPR
3099 X<open> X<pipe> X<file, open> X<fopen>
3101 =item open FILEHANDLE,MODE,EXPR
3103 =item open FILEHANDLE,MODE,EXPR,LIST
3105 =item open FILEHANDLE,MODE,REFERENCE
3107 =item open FILEHANDLE
3109 Opens the file whose filename is given by EXPR, and associates it with
3112 Simple examples to open a file for reading:
3114 open(my $fh, '<', "input.txt") or die $!;
3118 open(my $fh, '>', "output.txt") or die $!;
3120 (The following is a comprehensive reference to open(): for a gentler
3121 introduction you may consider L<perlopentut>.)
3123 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3124 the variable is assigned a reference to a new anonymous filehandle,
3125 otherwise if FILEHANDLE is an expression, its value is used as the name of
3126 the real filehandle wanted. (This is considered a symbolic reference, so
3127 C<use strict 'refs'> should I<not> be in effect.)
3129 If EXPR is omitted, the scalar variable of the same name as the
3130 FILEHANDLE contains the filename. (Note that lexical variables--those
3131 declared with C<my>--will not work for this purpose; so if you're
3132 using C<my>, specify EXPR in your call to open.)
3134 If three or more arguments are specified then the mode of opening and
3135 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3136 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3137 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3138 the file is opened for appending, again being created if necessary.
3140 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3141 indicate that you want both read and write access to the file; thus
3142 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3143 '+>' >> mode would clobber the file first. You can't usually use
3144 either read-write mode for updating textfiles, since they have
3145 variable length records. See the B<-i> switch in L<perlrun> for a
3146 better approach. The file is created with permissions of C<0666>
3147 modified by the process' C<umask> value.
3149 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3150 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3152 In the 2-arguments (and 1-argument) form of the call the mode and
3153 filename should be concatenated (in this order), possibly separated by
3154 spaces. It is possible to omit the mode in these forms if the mode is
3157 If the filename begins with C<'|'>, the filename is interpreted as a
3158 command to which output is to be piped, and if the filename ends with a
3159 C<'|'>, the filename is interpreted as a command which pipes output to
3160 us. See L<perlipc/"Using open() for IPC">
3161 for more examples of this. (You are not allowed to C<open> to a command
3162 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3163 and L<perlipc/"Bidirectional Communication with Another Process">
3166 For three or more arguments if MODE is C<'|-'>, the filename is
3167 interpreted as a command to which output is to be piped, and if MODE
3168 is C<'-|'>, the filename is interpreted as a command which pipes
3169 output to us. In the 2-arguments (and 1-argument) form one should
3170 replace dash (C<'-'>) with the command.
3171 See L<perlipc/"Using open() for IPC"> for more examples of this.
3172 (You are not allowed to C<open> to a command that pipes both in I<and>
3173 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3174 L<perlipc/"Bidirectional Communication"> for alternatives.)
3176 In the three-or-more argument form of pipe opens, if LIST is specified
3177 (extra arguments after the command name) then LIST becomes arguments
3178 to the command invoked if the platform supports it. The meaning of
3179 C<open> with more than three arguments for non-pipe modes is not yet
3180 specified. Experimental "layers" may give extra LIST arguments
3183 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3184 and opening C<< '>-' >> opens STDOUT.
3186 You may use the three-argument form of open to specify IO "layers"
3187 (sometimes also referred to as "disciplines") to be applied to the handle
3188 that affect how the input and output are processed (see L<open> and
3189 L<PerlIO> for more details). For example
3191 open(my $fh, "<:encoding(UTF-8)", "file")
3193 will open the UTF-8 encoded file containing Unicode characters,
3194 see L<perluniintro>. Note that if layers are specified in the
3195 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3196 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3198 Open returns nonzero upon success, the undefined value otherwise. If
3199 the C<open> involved a pipe, the return value happens to be the pid of
3202 If you're running Perl on a system that distinguishes between text
3203 files and binary files, then you should check out L</binmode> for tips
3204 for dealing with this. The key distinction between systems that need
3205 C<binmode> and those that don't is their text file formats. Systems
3206 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3207 character, and which encode that character in C as C<"\n">, do not
3208 need C<binmode>. The rest need it.
3210 When opening a file, it's usually a bad idea to continue normal execution
3211 if the request failed, so C<open> is frequently used in connection with
3212 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3213 where you want to make a nicely formatted error message (but there are
3214 modules that can help with that problem)) you should always check
3215 the return value from opening a file. The infrequent exception is when
3216 working with an unopened filehandle is actually what you want to do.
3218 As a special case the 3-arg form with a read/write mode and the third
3219 argument being C<undef>:
3221 open(my $tmp, "+>", undef) or die ...
3223 opens a filehandle to an anonymous temporary file. Also using "+<"
3224 works for symmetry, but you really should consider writing something
3225 to the temporary file first. You will need to seek() to do the
3228 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3229 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3230 "in memory" files held in Perl scalars via:
3232 open($fh, '>', \$variable) || ..
3234 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3235 file, you have to close it first:
3238 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3243 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3244 while (<ARTICLE>) {...
3246 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3247 # if the open fails, output is discarded
3249 open(my $dbase, '+<', 'dbase.mine') # open for update
3250 or die "Can't open 'dbase.mine' for update: $!";
3252 open(my $dbase, '+<dbase.mine') # ditto
3253 or die "Can't open 'dbase.mine' for update: $!";
3255 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3256 or die "Can't start caesar: $!";
3258 open(ARTICLE, "caesar <$article |") # ditto
3259 or die "Can't start caesar: $!";
3261 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3262 or die "Can't start sort: $!";
3265 open(MEMORY,'>', \$var)
3266 or die "Can't open memory file: $!";
3267 print MEMORY "foo!\n"; # output will end up in $var
3269 # process argument list of files along with any includes
3271 foreach $file (@ARGV) {
3272 process($file, 'fh00');
3276 my($filename, $input) = @_;
3277 $input++; # this is a string increment
3278 unless (open($input, $filename)) {
3279 print STDERR "Can't open $filename: $!\n";
3284 while (<$input>) { # note use of indirection
3285 if (/^#include "(.*)"/) {
3286 process($1, $input);
3293 See L<perliol> for detailed info on PerlIO.
3295 You may also, in the Bourne shell tradition, specify an EXPR beginning
3296 with C<< '>&' >>, in which case the rest of the string is interpreted
3297 as the name of a filehandle (or file descriptor, if numeric) to be
3298 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
3299 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3300 The mode you specify should match the mode of the original filehandle.
3301 (Duping a filehandle does not take into account any existing contents
3302 of IO buffers.) If you use the 3-arg form then you can pass either a
3303 number, the name of a filehandle or the normal "reference to a glob".
3305 Here is a script that saves, redirects, and restores C<STDOUT> and
3306 C<STDERR> using various methods:
3309 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3310 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3312 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3313 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3315 select STDERR; $| = 1; # make unbuffered
3316 select STDOUT; $| = 1; # make unbuffered
3318 print STDOUT "stdout 1\n"; # this works for
3319 print STDERR "stderr 1\n"; # subprocesses too
3321 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3322 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3324 print STDOUT "stdout 2\n";
3325 print STDERR "stderr 2\n";
3327 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3328 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3329 that file descriptor (and not call C<dup(2)>); this is more
3330 parsimonious of file descriptors. For example:
3332 # open for input, reusing the fileno of $fd
3333 open(FILEHANDLE, "<&=$fd")
3337 open(FILEHANDLE, "<&=", $fd)
3341 # open for append, using the fileno of OLDFH
3342 open(FH, ">>&=", OLDFH)
3346 open(FH, ">>&=OLDFH")
3348 Being parsimonious on filehandles is also useful (besides being
3349 parsimonious) for example when something is dependent on file
3350 descriptors, like for example locking using flock(). If you do just
3351 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3352 descriptor as B, and therefore flock(A) will not flock(B), and vice
3353 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3354 the same file descriptor.
3356 Note that if you are using Perls older than 5.8.0, Perl will be using
3357 the standard C libraries' fdopen() to implement the "=" functionality.
3358 On many UNIX systems fdopen() fails when file descriptors exceed a
3359 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3360 most often the default.
3362 You can see whether Perl has been compiled with PerlIO or not by
3363 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3364 is C<define>, you have PerlIO, otherwise you don't.
3366 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3367 with 2-arguments (or 1-argument) form of open(), then
3368 there is an implicit fork done, and the return value of open is the pid
3369 of the child within the parent process, and C<0> within the child
3370 process. (Use C<defined($pid)> to determine whether the open was successful.)
3371 The filehandle behaves normally for the parent, but i/o to that
3372 filehandle is piped from/to the STDOUT/STDIN of the child process.
3373 In the child process the filehandle isn't opened--i/o happens from/to
3374 the new STDOUT or STDIN. Typically this is used like the normal
3375 piped open when you want to exercise more control over just how the
3376 pipe command gets executed, such as when you are running setuid, and
3377 don't want to have to scan shell commands for metacharacters.
3378 The following triples are more or less equivalent:
3380 open(FOO, "|tr '[a-z]' '[A-Z]'");
3381 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3382 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3383 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3385 open(FOO, "cat -n '$file'|");
3386 open(FOO, '-|', "cat -n '$file'");
3387 open(FOO, '-|') || exec 'cat', '-n', $file;
3388 open(FOO, '-|', "cat", '-n', $file);
3390 The last example in each block shows the pipe as "list form", which is
3391 not yet supported on all platforms. A good rule of thumb is that if
3392 your platform has true C<fork()> (in other words, if your platform is
3393 UNIX) you can use the list form.
3395 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3397 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3398 output before any operation that may do a fork, but this may not be
3399 supported on some platforms (see L<perlport>). To be safe, you may need
3400 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3401 of C<IO::Handle> on any open handles.
3403 On systems that support a close-on-exec flag on files, the flag will
3404 be set for the newly opened file descriptor as determined by the value
3405 of $^F. See L<perlvar/$^F>.
3407 Closing any piped filehandle causes the parent process to wait for the
3408 child to finish, and returns the status value in C<$?> and
3409 C<${^CHILD_ERROR_NATIVE}>.
3411 The filename passed to 2-argument (or 1-argument) form of open() will
3412 have leading and trailing whitespace deleted, and the normal
3413 redirection characters honored. This property, known as "magic open",
3414 can often be used to good effect. A user could specify a filename of
3415 F<"rsh cat file |">, or you could change certain filenames as needed:
3417 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3418 open(FH, $filename) or die "Can't open $filename: $!";
3420 Use 3-argument form to open a file with arbitrary weird characters in it,
3422 open(FOO, '<', $file);
3424 otherwise it's necessary to protect any leading and trailing whitespace:
3426 $file =~ s#^(\s)#./$1#;
3427 open(FOO, "< $file\0");
3429 (this may not work on some bizarre filesystems). One should
3430 conscientiously choose between the I<magic> and 3-arguments form
3435 will allow the user to specify an argument of the form C<"rsh cat file |">,
3436 but will not work on a filename which happens to have a trailing space, while
3438 open IN, '<', $ARGV[0];
3440 will have exactly the opposite restrictions.
3442 If you want a "real" C C<open> (see C<open(2)> on your system), then you
3443 should use the C<sysopen> function, which involves no such magic (but
3444 may use subtly different filemodes than Perl open(), which is mapped
3445 to C fopen()). This is
3446 another way to protect your filenames from interpretation. For example:
3449 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3450 or die "sysopen $path: $!";
3451 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3452 print HANDLE "stuff $$\n";
3454 print "File contains: ", <HANDLE>;
3456 Using the constructor from the C<IO::Handle> package (or one of its
3457 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3458 filehandles that have the scope of whatever variables hold references to
3459 them, and automatically close whenever and however you leave that scope:
3463 sub read_myfile_munged {
3465 my $handle = IO::File->new;
3466 open($handle, "myfile") or die "myfile: $!";
3468 or return (); # Automatically closed here.
3469 mung $first or die "mung failed"; # Or here.
3470 return $first, <$handle> if $ALL; # Or here.
3474 See L</seek> for some details about mixing reading and writing.
3476 =item opendir DIRHANDLE,EXPR
3479 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3480 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3481 DIRHANDLE may be an expression whose value can be used as an indirect
3482 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3483 scalar variable (or array or hash element), the variable is assigned a
3484 reference to a new anonymous dirhandle.
3485 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3487 See example at C<readdir>.
3494 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3495 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3498 For the reverse, see L</chr>.
3499 See L<perlunicode> for more about Unicode.
3506 =item our EXPR : ATTRS
3508 =item our TYPE EXPR : ATTRS
3510 C<our> associates a simple name with a package variable in the current
3511 package for use within the current scope. When C<use strict 'vars'> is in
3512 effect, C<our> lets you use declared global variables without qualifying
3513 them with package names, within the lexical scope of the C<our> declaration.
3514 In this way C<our> differs from C<use vars>, which is package scoped.
3516 Unlike C<my>, which both allocates storage for a variable and associates
3517 a simple name with that storage for use within the current scope, C<our>
3518 associates a simple name with a package variable in the current package,
3519 for use within the current scope. In other words, C<our> has the same
3520 scoping rules as C<my>, but does not necessarily create a
3523 If more than one value is listed, the list must be placed
3529 An C<our> declaration declares a global variable that will be visible
3530 across its entire lexical scope, even across package boundaries. The
3531 package in which the variable is entered is determined at the point
3532 of the declaration, not at the point of use. This means the following
3536 our $bar; # declares $Foo::bar for rest of lexical scope
3540 print $bar; # prints 20, as it refers to $Foo::bar
3542 Multiple C<our> declarations with the same name in the same lexical
3543 scope are allowed if they are in different packages. If they happen
3544 to be in the same package, Perl will emit warnings if you have asked
3545 for them, just like multiple C<my> declarations. Unlike a second
3546 C<my> declaration, which will bind the name to a fresh variable, a
3547 second C<our> declaration in the same package, in the same scope, is
3552 our $bar; # declares $Foo::bar for rest of lexical scope
3556 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3557 print $bar; # prints 30
3559 our $bar; # emits warning but has no other effect
3560 print $bar; # still prints 30
3562 An C<our> declaration may also have a list of attributes associated
3565 The exact semantics and interface of TYPE and ATTRS are still
3566 evolving. TYPE is currently bound to the use of C<fields> pragma,
3567 and attributes are handled using the C<attributes> pragma, or starting
3568 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3569 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3570 L<attributes>, and L<Attribute::Handlers>.
3572 =item pack TEMPLATE,LIST
3575 Takes a LIST of values and converts it into a string using the rules
3576 given by the TEMPLATE. The resulting string is the concatenation of
3577 the converted values. Typically, each converted value looks
3578 like its machine-level representation. For example, on 32-bit machines
3579 an integer may be represented by a sequence of 4 bytes that will be
3580 converted to a sequence of 4 characters.
3582 The TEMPLATE is a sequence of characters that give the order and type
3583 of values, as follows:
3585 a A string with arbitrary binary data, will be null padded.
3586 A A text (ASCII) string, will be space padded.
3587 Z A null terminated (ASCIZ) string, will be null padded.
3589 b A bit string (ascending bit order inside each byte, like vec()).
3590 B A bit string (descending bit order inside each byte).
3591 h A hex string (low nybble first).
3592 H A hex string (high nybble first).
3594 c A signed char (8-bit) value.
3595 C An unsigned char (octet) value.
3596 W An unsigned char value (can be greater than 255).
3598 s A signed short (16-bit) value.
3599 S An unsigned short value.
3601 l A signed long (32-bit) value.
3602 L An unsigned long value.
3604 q A signed quad (64-bit) value.
3605 Q An unsigned quad value.
3606 (Quads are available only if your system supports 64-bit
3607 integer values _and_ if Perl has been compiled to support those.
3608 Causes a fatal error otherwise.)
3610 i A signed integer value.
3611 I A unsigned integer value.
3612 (This 'integer' is _at_least_ 32 bits wide. Its exact
3613 size depends on what a local C compiler calls 'int'.)
3615 n An unsigned short (16-bit) in "network" (big-endian) order.
3616 N An unsigned long (32-bit) in "network" (big-endian) order.
3617 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3618 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3620 j A Perl internal signed integer value (IV).
3621 J A Perl internal unsigned integer value (UV).
3623 f A single-precision float in the native format.
3624 d A double-precision float in the native format.
3626 F A Perl internal floating point value (NV) in the native format
3627 D A long double-precision float in the native format.
3628 (Long doubles are available only if your system supports long
3629 double values _and_ if Perl has been compiled to support those.
3630 Causes a fatal error otherwise.)
3632 p A pointer to a null-terminated string.
3633 P A pointer to a structure (fixed-length string).
3635 u A uuencoded string.
3636 U A Unicode character number. Encodes to a character in character mode
3637 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3639 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3640 details). Its bytes represent an unsigned integer in base 128,
3641 most significant digit first, with as few digits as possible. Bit
3642 eight (the high bit) is set on each byte except the last.
3646 @ Null fill or truncate to absolute position, counted from the
3647 start of the innermost ()-group.
3648 . Null fill or truncate to absolute position specified by value.
3649 ( Start of a ()-group.
3651 One or more of the modifiers below may optionally follow some letters in the
3652 TEMPLATE (the second column lists the letters for which the modifier is
3655 ! sSlLiI Forces native (short, long, int) sizes instead
3656 of fixed (16-/32-bit) sizes.
3658 xX Make x and X act as alignment commands.
3660 nNvV Treat integers as signed instead of unsigned.
3662 @. Specify position as byte offset in the internal
3663 representation of the packed string. Efficient but
3666 > sSiIlLqQ Force big-endian byte-order on the type.
3667 jJfFdDpP (The "big end" touches the construct.)
3669 < sSiIlLqQ Force little-endian byte-order on the type.
3670 jJfFdDpP (The "little end" touches the construct.)
3672 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3673 in which case they force a certain byte-order on all components of
3674 that group, including subgroups.
3676 The following rules apply:
3682 Each letter may optionally be followed by a number giving a repeat
3683 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3684 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3685 that many values from the LIST. A C<*> for the repeat count means to
3686 use however many items are left, except for C<@>, C<x>, C<X>, where it
3687 is equivalent to C<0>, for <.> where it means relative to string start
3688 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3689 A numeric repeat count may optionally be enclosed in brackets, as in
3690 C<pack 'C[80]', @arr>.
3692 One can replace the numeric repeat count by a template enclosed in brackets;
3693 then the packed length of this template in bytes is used as a count.
3694 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3695 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3696 If the template in brackets contains alignment commands (such as C<x![d]>),
3697 its packed length is calculated as if the start of the template has the maximal
3700 When used with C<Z>, C<*> results in the addition of a trailing null
3701 byte (so the packed result will be one longer than the byte C<length>
3704 When used with C<@>, the repeat count represents an offset from the start
3705 of the innermost () group.
3707 When used with C<.>, the repeat count is used to determine the starting
3708 position from where the value offset is calculated. If the repeat count
3709 is 0, it's relative to the current position. If the repeat count is C<*>,
3710 the offset is relative to the start of the packed string. And if its an
3711 integer C<n> the offset is relative to the start of the n-th innermost
3712 () group (or the start of the string if C<n> is bigger then the group
3715 The repeat count for C<u> is interpreted as the maximal number of bytes
3716 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3717 count should not be more than 65.
3721 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3722 string of length count, padding with nulls or spaces as necessary. When
3723 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3724 after the first null, and C<a> returns data verbatim.
3726 If the value-to-pack is too long, it is truncated. If too long and an
3727 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3728 by a null byte. Thus C<Z> always packs a trailing null (except when the
3733 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3734 Each character of the input field of pack() generates 1 bit of the result.
3735 Each result bit is based on the least-significant bit of the corresponding
3736 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3737 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3739 Starting from the beginning of the input string of pack(), each 8-tuple
3740 of characters is converted to 1 character of output. With format C<b>
3741 the first character of the 8-tuple determines the least-significant bit of a
3742 character, and with format C<B> it determines the most-significant bit of
3745 If the length of the input string is not exactly divisible by 8, the
3746 remainder is packed as if the input string were padded by null characters
3747 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3749 If the input string of pack() is longer than needed, extra characters are
3750 ignored. A C<*> for the repeat count of pack() means to use all the
3751 characters of the input field. On unpack()ing the bits are converted to a
3752 string of C<"0">s and C<"1">s.
3756 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3757 representable as hexadecimal digits, 0-9a-f) long.
3759 Each character of the input field of pack() generates 4 bits of the result.
3760 For non-alphabetical characters the result is based on the 4 least-significant
3761 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3762 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3763 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3764 is compatible with the usual hexadecimal digits, so that C<"a"> and
3765 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3766 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3768 Starting from the beginning of the input string of pack(), each pair
3769 of characters is converted to 1 character of output. With format C<h> the
3770 first character of the pair determines the least-significant nybble of the
3771 output character, and with format C<H> it determines the most-significant
3774 If the length of the input string is not even, it behaves as if padded
3775 by a null character at the end. Similarly, during unpack()ing the "extra"
3776 nybbles are ignored.
3778 If the input string of pack() is longer than needed, extra characters are
3780 A C<*> for the repeat count of pack() means to use all the characters of
3781 the input field. On unpack()ing the nybbles are converted to a string
3782 of hexadecimal digits.
3786 The C<p> type packs a pointer to a null-terminated string. You are
3787 responsible for ensuring the string is not a temporary value (which can
3788 potentially get deallocated before you get around to using the packed result).
3789 The C<P> type packs a pointer to a structure of the size indicated by the
3790 length. A NULL pointer is created if the corresponding value for C<p> or
3791 C<P> is C<undef>, similarly for unpack().
3793 If your system has a strange pointer size (i.e. a pointer is neither as
3794 big as an int nor as big as a long), it may not be possible to pack or
3795 unpack pointers in big- or little-endian byte order. Attempting to do
3796 so will result in a fatal error.
3800 The C</> template character allows packing and unpacking of a sequence of
3801 items where the packed structure contains a packed item count followed by
3802 the packed items themselves.
3804 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3805 I<length-item> describes how the length value is packed. The ones likely
3806 to be of most use are integer-packing ones like C<n> (for Java strings),
3807 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3809 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3810 the minimum of that and the number of available items is used as argument
3811 for the I<length-item>. If it has no repeat count or uses a '*', the number
3812 of available items is used.
3814 For C<unpack> an internal stack of integer arguments unpacked so far is
3815 used. You write C</>I<sequence-item> and the repeat count is obtained by
3816 popping off the last element from the stack. The I<sequence-item> must not
3817 have a repeat count.
3819 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3820 the I<length-item> is a string length, not a number of strings. If there is
3821 an explicit repeat count for pack, the packed string will be adjusted to that
3824 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3825 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3826 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3827 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3828 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3830 The I<length-item> is not returned explicitly from C<unpack>.
3832 Adding a count to the I<length-item> letter is unlikely to do anything
3833 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3834 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3835 which Perl does not regard as legal in numeric strings.
3839 The integer types C<s>, C<S>, C<l>, and C<L> may be
3840 followed by a C<!> modifier to signify native shorts or
3841 longs--as you can see from above for example a bare C<l> does mean
3842 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3843 may be larger. This is an issue mainly in 64-bit platforms. You can
3844 see whether using C<!> makes any difference by
3846 print length(pack("s")), " ", length(pack("s!")), "\n";
3847 print length(pack("l")), " ", length(pack("l!")), "\n";
3849 C<i!> and C<I!> also work but only because of completeness;
3850 they are identical to C<i> and C<I>.
3852 The actual sizes (in bytes) of native shorts, ints, longs, and long
3853 longs on the platform where Perl was built are also available via
3857 print $Config{shortsize}, "\n";
3858 print $Config{intsize}, "\n";
3859 print $Config{longsize}, "\n";
3860 print $Config{longlongsize}, "\n";
3862 (The C<$Config{longlongsize}> will be undefined if your system does
3863 not support long longs.)
3867 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3868 are inherently non-portable between processors and operating systems
3869 because they obey the native byteorder and endianness. For example a
3870 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3871 (arranged in and handled by the CPU registers) into bytes as
3873 0x12 0x34 0x56 0x78 # big-endian
3874 0x78 0x56 0x34 0x12 # little-endian
3876 Basically, the Intel and VAX CPUs are little-endian, while everybody
3877 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3878 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3879 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3882 The names `big-endian' and `little-endian' are comic references to
3883 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3884 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3885 the egg-eating habits of the Lilliputians.
3887 Some systems may have even weirder byte orders such as
3892 You can see your system's preference with
3894 print join(" ", map { sprintf "%#02x", $_ }
3895 unpack("W*",pack("L",0x12345678))), "\n";
3897 The byteorder on the platform where Perl was built is also available
3901 print $Config{byteorder}, "\n";
3903 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3904 and C<'87654321'> are big-endian.
3906 If you want portable packed integers you can either use the formats
3907 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3908 modifiers. These modifiers are only available as of perl 5.9.2.
3909 See also L<perlport>.
3913 All integer and floating point formats as well as C<p> and C<P> and
3914 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3915 to force big- or little- endian byte-order, respectively.
3916 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3917 signed integers, 64-bit integers and floating point values. However,
3918 there are some things to keep in mind.
3920 Exchanging signed integers between different platforms only works
3921 if all platforms store them in the same format. Most platforms store
3922 signed integers in two's complement, so usually this is not an issue.
3924 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3925 formats on big- or little-endian machines. Otherwise, attempting to
3926 do so will result in a fatal error.
3928 Forcing big- or little-endian byte-order on floating point values for
3929 data exchange can only work if all platforms are using the same
3930 binary representation (e.g. IEEE floating point format). Even if all
3931 platforms are using IEEE, there may be subtle differences. Being able
3932 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3933 but also very dangerous if you don't know exactly what you're doing.
3934 It is definitely not a general way to portably store floating point
3937 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3938 all types inside the group that accept the byte-order modifiers,
3939 including all subgroups. It will silently be ignored for all other
3940 types. You are not allowed to override the byte-order within a group
3941 that already has a byte-order modifier suffix.
3945 Real numbers (floats and doubles) are in the native machine format only;
3946 due to the multiplicity of floating formats around, and the lack of a
3947 standard "network" representation, no facility for interchange has been
3948 made. This means that packed floating point data written on one machine
3949 may not be readable on another - even if both use IEEE floating point
3950 arithmetic (as the endian-ness of the memory representation is not part
3951 of the IEEE spec). See also L<perlport>.
3953 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3954 modifiers to force big- or little-endian byte-order on floating point values.
3956 Note that Perl uses doubles (or long doubles, if configured) internally for
3957 all numeric calculation, and converting from double into float and thence back
3958 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3959 will not in general equal $foo).
3963 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3964 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3965 where the packed string is processed in its UTF-8-encoded Unicode form on
3966 a byte by byte basis. Character mode is the default unless the format string
3967 starts with an C<U>. You can switch mode at any moment with an explicit
3968 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3969 or until the end of the ()-group in which it was entered.
3973 You must yourself do any alignment or padding by inserting for example
3974 enough C<'x'>es while packing. There is no way to pack() and unpack()
3975 could know where the characters are going to or coming from. Therefore
3976 C<pack> (and C<unpack>) handle their output and input as flat
3977 sequences of characters.
3981 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3982 take a repeat count, both as postfix, and for unpack() also via the C</>
3983 template character. Within each repetition of a group, positioning with
3984 C<@> starts again at 0. Therefore, the result of
3986 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3988 is the string "\0a\0\0bc".
3992 C<x> and C<X> accept C<!> modifier. In this case they act as
3993 alignment commands: they jump forward/back to the closest position
3994 aligned at a multiple of C<count> characters. For example, to pack() or
3995 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3996 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3997 aligned on the double's size.
3999 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
4000 both result in no-ops.
4004 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
4005 will represent signed 16-/32-bit integers in big-/little-endian order.
4006 This is only portable if all platforms sharing the packed data use the
4007 same binary representation for signed integers (e.g. all platforms are
4008 using two's complement representation).
4012 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
4013 White space may be used to separate pack codes from each other, but
4014 modifiers and a repeat count must follow immediately.
4018 If TEMPLATE requires more arguments to pack() than actually given, pack()
4019 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4020 to pack() than actually given, extra arguments are ignored.
4026 $foo = pack("WWWW",65,66,67,68);
4028 $foo = pack("W4",65,66,67,68);
4030 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4031 # same thing with Unicode circled letters.
4032 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4033 # same thing with Unicode circled letters. You don't get the UTF-8
4034 # bytes because the U at the start of the format caused a switch to
4035 # U0-mode, so the UTF-8 bytes get joined into characters
4036 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4037 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4038 # This is the UTF-8 encoding of the string in the previous example
4040 $foo = pack("ccxxcc",65,66,67,68);
4043 # note: the above examples featuring "W" and "c" are true
4044 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4045 # and UTF-8. In EBCDIC the first example would be
4046 # $foo = pack("WWWW",193,194,195,196);
4048 $foo = pack("s2",1,2);
4049 # "\1\0\2\0" on little-endian
4050 # "\0\1\0\2" on big-endian
4052 $foo = pack("a4","abcd","x","y","z");
4055 $foo = pack("aaaa","abcd","x","y","z");
4058 $foo = pack("a14","abcdefg");
4059 # "abcdefg\0\0\0\0\0\0\0"
4061 $foo = pack("i9pl", gmtime);
4062 # a real struct tm (on my system anyway)
4064 $utmp_template = "Z8 Z8 Z16 L";
4065 $utmp = pack($utmp_template, @utmp1);
4066 # a struct utmp (BSDish)
4068 @utmp2 = unpack($utmp_template, $utmp);
4069 # "@utmp1" eq "@utmp2"
4072 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4075 $foo = pack('sx2l', 12, 34);
4076 # short 12, two zero bytes padding, long 34
4077 $bar = pack('s@4l', 12, 34);
4078 # short 12, zero fill to position 4, long 34
4080 $baz = pack('s.l', 12, 4, 34);
4081 # short 12, zero fill to position 4, long 34
4083 $foo = pack('nN', 42, 4711);
4084 # pack big-endian 16- and 32-bit unsigned integers
4085 $foo = pack('S>L>', 42, 4711);
4087 $foo = pack('s<l<', -42, 4711);
4088 # pack little-endian 16- and 32-bit signed integers
4089 $foo = pack('(sl)<', -42, 4711);
4092 The same template may generally also be used in unpack().
4094 =item package NAMESPACE VERSION
4095 X<package> X<module> X<namespace> X<version>
4097 =item package NAMESPACE
4099 Declares the compilation unit as being in the given namespace. The scope
4100 of the package declaration is from the declaration itself through the end
4101 of the enclosing block, file, or eval (the same as the C<my> operator).
4102 All further unqualified dynamic identifiers will be in this namespace.
4103 A package statement affects only dynamic variables--including those
4104 you've used C<local> on--but I<not> lexical variables, which are created
4105 with C<my>. Typically it would be the first declaration in a file to
4106 be included by the C<require> or C<use> operator. You can switch into a
4107 package in more than one place; it merely influences which symbol table
4108 is used by the compiler for the rest of that block. You can refer to
4109 variables and filehandles in other packages by prefixing the identifier
4110 with the package name and a double colon: C<$Package::Variable>.
4111 If the package name is null, the C<main> package as assumed. That is,
4112 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4113 still seen in older code).
4115 If VERSION is provided, C<package> also sets the C<$VERSION> variable in the
4116 given namespace. VERSION must be be a numeric literal or v-string; it is
4117 parsed exactly the same way as a VERSION argument to C<use MODULE VERSION>.
4118 C<$VERSION> should only be set once per package.
4120 See L<perlmod/"Packages"> for more information about packages, modules,
4121 and classes. See L<perlsub> for other scoping issues.
4123 =item pipe READHANDLE,WRITEHANDLE
4126 Opens a pair of connected pipes like the corresponding system call.
4127 Note that if you set up a loop of piped processes, deadlock can occur
4128 unless you are very careful. In addition, note that Perl's pipes use
4129 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4130 after each command, depending on the application.
4132 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4133 for examples of such things.
4135 On systems that support a close-on-exec flag on files, the flag will be set
4136 for the newly opened file descriptors as determined by the value of $^F.
4144 Pops and returns the last value of the array, shortening the array by
4147 If there are no elements in the array, returns the undefined value
4148 (although this may happen at other times as well). If ARRAY is
4149 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4150 array in subroutines, just like C<shift>.
4153 X<pos> X<match, position>
4157 Returns the offset of where the last C<m//g> search left off for the variable
4158 in question (C<$_> is used when the variable is not specified). Note that
4159 0 is a valid match offset. C<undef> indicates that the search position
4160 is reset (usually due to match failure, but can also be because no match has
4161 yet been performed on the scalar). C<pos> directly accesses the location used
4162 by the regexp engine to store the offset, so assigning to C<pos> will change
4163 that offset, and so will also influence the C<\G> zero-width assertion in
4164 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4165 the return from C<pos> won't change either in this case. See L<perlre> and
4168 =item print FILEHANDLE LIST
4175 Prints a string or a list of strings. Returns true if successful.
4176 FILEHANDLE may be a scalar variable name, in which case the variable
4177 contains the name of or a reference to the filehandle, thus introducing
4178 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4179 the next token is a term, it may be misinterpreted as an operator
4180 unless you interpose a C<+> or put parentheses around the arguments.)
4181 If FILEHANDLE is omitted, prints by default to standard output (or
4182 to the last selected output channel--see L</select>). If LIST is
4183 also omitted, prints C<$_> to the currently selected output channel.
4184 To set the default output channel to something other than STDOUT
4185 use the select operation. The current value of C<$,> (if any) is
4186 printed between each LIST item. The current value of C<$\> (if
4187 any) is printed after the entire LIST has been printed. Because
4188 print takes a LIST, anything in the LIST is evaluated in list
4189 context, and any subroutine that you call will have one or more of
4190 its expressions evaluated in list context. Also be careful not to
4191 follow the print keyword with a left parenthesis unless you want
4192 the corresponding right parenthesis to terminate the arguments to
4193 the print--interpose a C<+> or put parentheses around all the
4196 Note that if you're storing FILEHANDLEs in an array, or if you're using
4197 any other expression more complex than a scalar variable to retrieve it,
4198 you will have to use a block returning the filehandle value instead:
4200 print { $files[$i] } "stuff\n";
4201 print { $OK ? STDOUT : STDERR } "stuff\n";
4203 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
4204 L<perlipc> for more on signal handling.
4206 =item printf FILEHANDLE FORMAT, LIST
4209 =item printf FORMAT, LIST
4211 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4212 (the output record separator) is not appended. The first argument
4213 of the list will be interpreted as the C<printf> format. See C<sprintf>
4214 for an explanation of the format argument. If C<use locale> is in effect,
4215 and POSIX::setlocale() has been called, the character used for the decimal
4216 separator in formatted floating point numbers is affected by the LC_NUMERIC
4217 locale. See L<perllocale> and L<POSIX>.
4219 Don't fall into the trap of using a C<printf> when a simple
4220 C<print> would do. The C<print> is more efficient and less
4223 =item prototype FUNCTION
4226 Returns the prototype of a function as a string (or C<undef> if the
4227 function has no prototype). FUNCTION is a reference to, or the name of,
4228 the function whose prototype you want to retrieve.
4230 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4231 name for Perl builtin. If the builtin is not I<overridable> (such as
4232 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4233 (such as C<system>), prototype() returns C<undef>, because the builtin
4234 does not really behave like a Perl function. Otherwise, the string
4235 describing the equivalent prototype is returned.
4237 =item push ARRAY,LIST
4240 Treats ARRAY as a stack, and pushes the values of LIST
4241 onto the end of ARRAY. The length of ARRAY increases by the length of
4242 LIST. Has the same effect as
4245 $ARRAY[++$#ARRAY] = $value;
4248 but is more efficient. Returns the number of elements in the array following
4249 the completed C<push>.
4259 Generalized quotes. See L<perlop/"Quote-Like Operators">.
4263 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
4265 =item quotemeta EXPR
4266 X<quotemeta> X<metacharacter>
4270 Returns the value of EXPR with all non-"word"
4271 characters backslashed. (That is, all characters not matching
4272 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4273 returned string, regardless of any locale settings.)
4274 This is the internal function implementing
4275 the C<\Q> escape in double-quoted strings.
4277 If EXPR is omitted, uses C<$_>.
4284 Returns a random fractional number greater than or equal to C<0> and less
4285 than the value of EXPR. (EXPR should be positive.) If EXPR is
4286 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4287 also special-cased as C<1> - this has not been documented before perl 5.8.0
4288 and is subject to change in future versions of perl. Automatically calls
4289 C<srand> unless C<srand> has already been called. See also C<srand>.
4291 Apply C<int()> to the value returned by C<rand()> if you want random
4292 integers instead of random fractional numbers. For example,
4296 returns a random integer between C<0> and C<9>, inclusive.
4298 (Note: If your rand function consistently returns numbers that are too
4299 large or too small, then your version of Perl was probably compiled
4300 with the wrong number of RANDBITS.)
4302 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4303 X<read> X<file, read>
4305 =item read FILEHANDLE,SCALAR,LENGTH
4307 Attempts to read LENGTH I<characters> of data into variable SCALAR
4308 from the specified FILEHANDLE. Returns the number of characters
4309 actually read, C<0> at end of file, or undef if there was an error (in
4310 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4311 so that the last character actually read is the last character of the
4312 scalar after the read.
4314 An OFFSET may be specified to place the read data at some place in the
4315 string other than the beginning. A negative OFFSET specifies
4316 placement at that many characters counting backwards from the end of
4317 the string. A positive OFFSET greater than the length of SCALAR
4318 results in the string being padded to the required size with C<"\0">
4319 bytes before the result of the read is appended.
4321 The call is actually implemented in terms of either Perl's or system's
4322 fread() call. To get a true read(2) system call, see C<sysread>.
4324 Note the I<characters>: depending on the status of the filehandle,
4325 either (8-bit) bytes or characters are read. By default all
4326 filehandles operate on bytes, but for example if the filehandle has
4327 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4328 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4329 characters, not bytes. Similarly for the C<:encoding> pragma:
4330 in that case pretty much any characters can be read.
4332 =item readdir DIRHANDLE
4335 Returns the next directory entry for a directory opened by C<opendir>.
4336 If used in list context, returns all the rest of the entries in the
4337 directory. If there are no more entries, returns an undefined value in
4338 scalar context or a null list in list context.
4340 If you're planning to filetest the return values out of a C<readdir>, you'd
4341 better prepend the directory in question. Otherwise, because we didn't
4342 C<chdir> there, it would have been testing the wrong file.
4344 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4345 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4348 As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop,
4349 which will set C<$_> on every iteration.
4351 opendir(my $dh, $some_dir) || die;
4352 while(readdir $dh) {
4353 print "$some_dir/$_\n";
4360 X<readline> X<gets> X<fgets>
4362 Reads from the filehandle whose typeglob is contained in EXPR (or from
4363 *ARGV if EXPR is not provided). In scalar context, each call reads and
4364 returns the next line, until end-of-file is reached, whereupon the
4365 subsequent call returns C<undef>. In list context, reads until end-of-file
4366 is reached and returns a list of lines. Note that the notion of "line"
4367 used here is however you may have defined it with C<$/> or
4368 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4370 When C<$/> is set to C<undef>, when C<readline> is in scalar
4371 context (i.e. file slurp mode), and when an empty file is read, it
4372 returns C<''> the first time, followed by C<undef> subsequently.
4374 This is the internal function implementing the C<< <EXPR> >>
4375 operator, but you can use it directly. The C<< <EXPR> >>
4376 operator is discussed in more detail in L<perlop/"I/O Operators">.
4379 $line = readline(*STDIN); # same thing
4381 If C<readline> encounters an operating system error, C<$!> will be set
4382 with the corresponding error message. It can be helpful to check
4383 C<$!> when you are reading from filehandles you don't trust, such as a
4384 tty or a socket. The following example uses the operator form of
4385 C<readline> and dies if the result is not defined.
4387 while ( ! eof($fh) ) {
4388 defined( $_ = <$fh> ) or die "readline failed: $!";
4392 Note that you have can't handle C<readline> errors that way with the
4393 C<ARGV> filehandle. In that case, you have to open each element of
4394 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
4396 foreach my $arg (@ARGV) {
4397 open(my $fh, $arg) or warn "Can't open $arg: $!";
4399 while ( ! eof($fh) ) {
4400 defined( $_ = <$fh> )
4401 or die "readline failed for $arg: $!";
4411 Returns the value of a symbolic link, if symbolic links are
4412 implemented. If not, gives a fatal error. If there is some system
4413 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4414 omitted, uses C<$_>.
4421 EXPR is executed as a system command.
4422 The collected standard output of the command is returned.
4423 In scalar context, it comes back as a single (potentially
4424 multi-line) string. In list context, returns a list of lines
4425 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4426 This is the internal function implementing the C<qx/EXPR/>
4427 operator, but you can use it directly. The C<qx/EXPR/>
4428 operator is discussed in more detail in L<perlop/"I/O Operators">.
4429 If EXPR is omitted, uses C<$_>.
4431 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4434 Receives a message on a socket. Attempts to receive LENGTH characters
4435 of data into variable SCALAR from the specified SOCKET filehandle.
4436 SCALAR will be grown or shrunk to the length actually read. Takes the
4437 same flags as the system call of the same name. Returns the address
4438 of the sender if SOCKET's protocol supports this; returns an empty
4439 string otherwise. If there's an error, returns the undefined value.
4440 This call is actually implemented in terms of recvfrom(2) system call.
4441 See L<perlipc/"UDP: Message Passing"> for examples.
4443 Note the I<characters>: depending on the status of the socket, either
4444 (8-bit) bytes or characters are received. By default all sockets
4445 operate on bytes, but for example if the socket has been changed using
4446 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4447 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4448 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4449 case pretty much any characters can be read.
4456 The C<redo> command restarts the loop block without evaluating the
4457 conditional again. The C<continue> block, if any, is not executed. If
4458 the LABEL is omitted, the command refers to the innermost enclosing
4459 loop. Programs that want to lie to themselves about what was just input
4460 normally use this command:
4462 # a simpleminded Pascal comment stripper
4463 # (warning: assumes no { or } in strings)
4464 LINE: while (<STDIN>) {
4465 while (s|({.*}.*){.*}|$1 |) {}
4470 if (/}/) { # end of comment?
4479 C<redo> cannot be used to retry a block which returns a value such as
4480 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4481 a grep() or map() operation.
4483 Note that a block by itself is semantically identical to a loop
4484 that executes once. Thus C<redo> inside such a block will effectively
4485 turn it into a looping construct.
4487 See also L</continue> for an illustration of how C<last>, C<next>, and
4495 Returns a non-empty string if EXPR is a reference, the empty
4496 string otherwise. If EXPR
4497 is not specified, C<$_> will be used. The value returned depends on the
4498 type of thing the reference is a reference to.
4499 Builtin types include:
4513 If the referenced object has been blessed into a package, then that package
4514 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4516 if (ref($r) eq "HASH") {
4517 print "r is a reference to a hash.\n";
4520 print "r is not a reference at all.\n";
4523 The return value C<LVALUE> indicates a reference to an lvalue that is not
4524 a variable. You get this from taking the reference of function calls like
4525 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4526 to a L<version string|perldata/"Version Strings">.
4528 The result C<Regexp> indicates that the argument is a regular expression
4529 resulting from C<qr//>.
4531 See also L<perlref>.
4533 =item rename OLDNAME,NEWNAME
4534 X<rename> X<move> X<mv> X<ren>
4536 Changes the name of a file; an existing file NEWNAME will be
4537 clobbered. Returns true for success, false otherwise.
4539 Behavior of this function varies wildly depending on your system
4540 implementation. For example, it will usually not work across file system
4541 boundaries, even though the system I<mv> command sometimes compensates
4542 for this. Other restrictions include whether it works on directories,
4543 open files, or pre-existing files. Check L<perlport> and either the
4544 rename(2) manpage or equivalent system documentation for details.
4546 For a platform independent C<move> function look at the L<File::Copy>
4549 =item require VERSION
4556 Demands a version of Perl specified by VERSION, or demands some semantics
4557 specified by EXPR or by C<$_> if EXPR is not supplied.
4559 VERSION may be either a numeric argument such as 5.006, which will be
4560 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4561 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4562 VERSION is greater than the version of the current Perl interpreter.
4563 Compare with L</use>, which can do a similar check at compile time.
4565 Specifying VERSION as a literal of the form v5.6.1 should generally be
4566 avoided, because it leads to misleading error messages under earlier
4567 versions of Perl that do not support this syntax. The equivalent numeric
4568 version should be used instead.
4570 require v5.6.1; # run time version check
4571 require 5.6.1; # ditto
4572 require 5.006_001; # ditto; preferred for backwards compatibility
4574 Otherwise, C<require> demands that a library file be included if it
4575 hasn't already been included. The file is included via the do-FILE
4576 mechanism, which is essentially just a variety of C<eval> with the
4577 caveat that lexical variables in the invoking script will be invisible
4578 to the included code. Has semantics similar to the following subroutine:
4581 my ($filename) = @_;
4582 if (exists $INC{$filename}) {
4583 return 1 if $INC{$filename};
4584 die "Compilation failed in require";
4586 my ($realfilename,$result);
4588 foreach $prefix (@INC) {
4589 $realfilename = "$prefix/$filename";
4590 if (-f $realfilename) {
4591 $INC{$filename} = $realfilename;
4592 $result = do $realfilename;
4596 die "Can't find $filename in \@INC";
4599 $INC{$filename} = undef;
4601 } elsif (!$result) {
4602 delete $INC{$filename};
4603 die "$filename did not return true value";
4609 Note that the file will not be included twice under the same specified
4612 The file must return true as the last statement to indicate
4613 successful execution of any initialization code, so it's customary to
4614 end such a file with C<1;> unless you're sure it'll return true
4615 otherwise. But it's better just to put the C<1;>, in case you add more
4618 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4619 replaces "F<::>" with "F</>" in the filename for you,
4620 to make it easy to load standard modules. This form of loading of
4621 modules does not risk altering your namespace.
4623 In other words, if you try this:
4625 require Foo::Bar; # a splendid bareword
4627 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4628 directories specified in the C<@INC> array.
4630 But if you try this:
4632 $class = 'Foo::Bar';
4633 require $class; # $class is not a bareword
4635 require "Foo::Bar"; # not a bareword because of the ""
4637 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4638 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4640 eval "require $class";
4642 Now that you understand how C<require> looks for files in the case of a
4643 bareword argument, there is a little extra functionality going on behind
4644 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4645 first look for a similar filename with a "F<.pmc>" extension. If this file
4646 is found, it will be loaded in place of any file ending in a "F<.pm>"
4649 You can also insert hooks into the import facility, by putting directly
4650 Perl code into the @INC array. There are three forms of hooks: subroutine
4651 references, array references and blessed objects.
4653 Subroutine references are the simplest case. When the inclusion system
4654 walks through @INC and encounters a subroutine, this subroutine gets
4655 called with two parameters, the first being a reference to itself, and the
4656 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4657 subroutine should return nothing, or a list of up to three values in the
4664 A filehandle, from which the file will be read.
4668 A reference to a subroutine. If there is no filehandle (previous item),
4669 then this subroutine is expected to generate one line of source code per
4670 call, writing the line into C<$_> and returning 1, then returning 0 at
4671 "end of file". If there is a filehandle, then the subroutine will be
4672 called to act as a simple source filter, with the line as read in C<$_>.
4673 Again, return 1 for each valid line, and 0 after all lines have been
4678 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4679 reference to the subroutine itself is passed in as C<$_[0]>.
4683 If an empty list, C<undef>, or nothing that matches the first 3 values above
4684 is returned then C<require> will look at the remaining elements of @INC.
4685 Note that this file handle must be a real file handle (strictly a typeglob,
4686 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4687 ignored and return value processing will stop there.
4689 If the hook is an array reference, its first element must be a subroutine
4690 reference. This subroutine is called as above, but the first parameter is
4691 the array reference. This enables to pass indirectly some arguments to
4694 In other words, you can write:
4696 push @INC, \&my_sub;
4698 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4704 push @INC, [ \&my_sub, $x, $y, ... ];
4706 my ($arrayref, $filename) = @_;
4707 # Retrieve $x, $y, ...
4708 my @parameters = @$arrayref[1..$#$arrayref];
4712 If the hook is an object, it must provide an INC method that will be
4713 called as above, the first parameter being the object itself. (Note that
4714 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4715 into package C<main>.) Here is a typical code layout:
4721 my ($self, $filename) = @_;
4725 # In the main program
4726 push @INC, Foo->new(...);
4728 Note that these hooks are also permitted to set the %INC entry
4729 corresponding to the files they have loaded. See L<perlvar/%INC>.
4731 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4738 Generally used in a C<continue> block at the end of a loop to clear
4739 variables and reset C<??> searches so that they work again. The
4740 expression is interpreted as a list of single characters (hyphens
4741 allowed for ranges). All variables and arrays beginning with one of
4742 those letters are reset to their pristine state. If the expression is
4743 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4744 only variables or searches in the current package. Always returns
4747 reset 'X'; # reset all X variables
4748 reset 'a-z'; # reset lower case variables
4749 reset; # just reset ?one-time? searches
4751 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4752 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4753 variables--lexical variables are unaffected, but they clean themselves
4754 up on scope exit anyway, so you'll probably want to use them instead.
4762 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4763 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4764 context, depending on how the return value will be used, and the context
4765 may vary from one execution to the next (see C<wantarray>). If no EXPR
4766 is given, returns an empty list in list context, the undefined value in
4767 scalar context, and (of course) nothing at all in a void context.
4769 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4770 or do FILE will automatically return the value of the last expression
4774 X<reverse> X<rev> X<invert>
4776 In list context, returns a list value consisting of the elements
4777 of LIST in the opposite order. In scalar context, concatenates the
4778 elements of LIST and returns a string value with all characters
4779 in the opposite order.
4781 print join(", ", reverse "world", "Hello"); # Hello, world
4783 print scalar reverse "dlrow ,", "olleH"; # Hello, world
4785 Used without arguments in scalar context, reverse() reverses C<$_>.
4787 $_ = "dlrow ,olleH";
4788 print reverse; # No output, list context
4789 print scalar reverse; # Hello, world
4791 Note that reversing an array to itself (as in C<@a = reverse @a>) will
4792 preserve non-existent elements whenever possible, i.e. for non magical
4793 arrays or tied arrays with C<EXISTS> and C<DELETE> methods.
4795 This operator is also handy for inverting a hash, although there are some
4796 caveats. If a value is duplicated in the original hash, only one of those
4797 can be represented as a key in the inverted hash. Also, this has to
4798 unwind one hash and build a whole new one, which may take some time
4799 on a large hash, such as from a DBM file.
4801 %by_name = reverse %by_address; # Invert the hash
4803 =item rewinddir DIRHANDLE
4806 Sets the current position to the beginning of the directory for the
4807 C<readdir> routine on DIRHANDLE.
4809 =item rindex STR,SUBSTR,POSITION
4812 =item rindex STR,SUBSTR
4814 Works just like index() except that it returns the position of the I<last>
4815 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4816 last occurrence beginning at or before that position.
4818 =item rmdir FILENAME
4819 X<rmdir> X<rd> X<directory, remove>
4823 Deletes the directory specified by FILENAME if that directory is
4824 empty. If it succeeds it returns true, otherwise it returns false and
4825 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4827 To remove a directory tree recursively (C<rm -rf> on unix) look at
4828 the C<rmtree> function of the L<File::Path> module.
4832 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
4834 =item say FILEHANDLE LIST
4841 Just like C<print>, but implicitly appends a newline.
4842 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
4845 This keyword is only available when the "say" feature is
4846 enabled: see L<feature>.
4849 X<scalar> X<context>
4851 Forces EXPR to be interpreted in scalar context and returns the value
4854 @counts = ( scalar @a, scalar @b, scalar @c );
4856 There is no equivalent operator to force an expression to
4857 be interpolated in list context because in practice, this is never
4858 needed. If you really wanted to do so, however, you could use
4859 the construction C<@{[ (some expression) ]}>, but usually a simple
4860 C<(some expression)> suffices.
4862 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4863 parenthesized list, this behaves as a scalar comma expression, evaluating
4864 all but the last element in void context and returning the final element
4865 evaluated in scalar context. This is seldom what you want.
4867 The following single statement:
4869 print uc(scalar(&foo,$bar)),$baz;
4871 is the moral equivalent of these two:
4874 print(uc($bar),$baz);
4876 See L<perlop> for more details on unary operators and the comma operator.
4878 =item seek FILEHANDLE,POSITION,WHENCE
4879 X<seek> X<fseek> X<filehandle, position>
4881 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4882 FILEHANDLE may be an expression whose value gives the name of the
4883 filehandle. The values for WHENCE are C<0> to set the new position
4884 I<in bytes> to POSITION, C<1> to set it to the current position plus
4885 POSITION, and C<2> to set it to EOF plus POSITION (typically
4886 negative). For WHENCE you may use the constants C<SEEK_SET>,
4887 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4888 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4891 Note the I<in bytes>: even if the filehandle has been set to
4892 operate on characters (for example by using the C<:encoding(utf8)> open
4893 layer), tell() will return byte offsets, not character offsets
4894 (because implementing that would render seek() and tell() rather slow).
4896 If you want to position file for C<sysread> or C<syswrite>, don't use
4897 C<seek>--buffering makes its effect on the file's system position
4898 unpredictable and non-portable. Use C<sysseek> instead.
4900 Due to the rules and rigors of ANSI C, on some systems you have to do a
4901 seek whenever you switch between reading and writing. Amongst other
4902 things, this may have the effect of calling stdio's clearerr(3).
4903 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4907 This is also useful for applications emulating C<tail -f>. Once you hit
4908 EOF on your read, and then sleep for a while, you might have to stick in a
4909 seek() to reset things. The C<seek> doesn't change the current position,
4910 but it I<does> clear the end-of-file condition on the handle, so that the
4911 next C<< <FILE> >> makes Perl try again to read something. We hope.
4913 If that doesn't work (some IO implementations are particularly
4914 cantankerous), then you may need something more like this:
4917 for ($curpos = tell(FILE); $_ = <FILE>;
4918 $curpos = tell(FILE)) {
4919 # search for some stuff and put it into files
4921 sleep($for_a_while);
4922 seek(FILE, $curpos, 0);
4925 =item seekdir DIRHANDLE,POS
4928 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4929 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4930 about possible directory compaction as the corresponding system library
4933 =item select FILEHANDLE
4934 X<select> X<filehandle, default>
4938 Returns the currently selected filehandle. If FILEHANDLE is supplied,
4939 sets the new current default filehandle for output. This has two
4940 effects: first, a C<write> or a C<print> without a filehandle will
4941 default to this FILEHANDLE. Second, references to variables related to
4942 output will refer to this output channel. For example, if you have to
4943 set the top of form format for more than one output channel, you might
4951 FILEHANDLE may be an expression whose value gives the name of the
4952 actual filehandle. Thus:
4954 $oldfh = select(STDERR); $| = 1; select($oldfh);
4956 Some programmers may prefer to think of filehandles as objects with
4957 methods, preferring to write the last example as:
4960 STDERR->autoflush(1);
4962 =item select RBITS,WBITS,EBITS,TIMEOUT
4965 This calls the select(2) system call with the bit masks specified, which
4966 can be constructed using C<fileno> and C<vec>, along these lines:
4968 $rin = $win = $ein = '';
4969 vec($rin,fileno(STDIN),1) = 1;
4970 vec($win,fileno(STDOUT),1) = 1;
4973 If you want to select on many filehandles you might wish to write a
4977 my(@fhlist) = split(' ',$_[0]);
4980 vec($bits,fileno($_),1) = 1;
4984 $rin = fhbits('STDIN TTY SOCK');
4988 ($nfound,$timeleft) =
4989 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4991 or to block until something becomes ready just do this
4993 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4995 Most systems do not bother to return anything useful in $timeleft, so
4996 calling select() in scalar context just returns $nfound.
4998 Any of the bit masks can also be undef. The timeout, if specified, is
4999 in seconds, which may be fractional. Note: not all implementations are
5000 capable of returning the $timeleft. If not, they always return
5001 $timeleft equal to the supplied $timeout.
5003 You can effect a sleep of 250 milliseconds this way:
5005 select(undef, undef, undef, 0.25);
5007 Note that whether C<select> gets restarted after signals (say, SIGALRM)
5008 is implementation-dependent. See also L<perlport> for notes on the
5009 portability of C<select>.
5011 On error, C<select> behaves like the select(2) system call : it returns
5014 Note: on some Unixes, the select(2) system call may report a socket file
5015 descriptor as "ready for reading", when actually no data is available,
5016 thus a subsequent read blocks. It can be avoided using always the
5017 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
5020 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
5021 or <FH>) with C<select>, except as permitted by POSIX, and even
5022 then only on POSIX systems. You have to use C<sysread> instead.
5024 =item semctl ID,SEMNUM,CMD,ARG
5027 Calls the System V IPC function C<semctl>. You'll probably have to say
5031 first to get the correct constant definitions. If CMD is IPC_STAT or
5032 GETALL, then ARG must be a variable that will hold the returned
5033 semid_ds structure or semaphore value array. Returns like C<ioctl>:
5034 the undefined value for error, "C<0 but true>" for zero, or the actual
5035 return value otherwise. The ARG must consist of a vector of native
5036 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
5037 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
5040 =item semget KEY,NSEMS,FLAGS
5043 Calls the System V IPC function semget. Returns the semaphore id, or
5044 the undefined value if there is an error. See also
5045 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
5048 =item semop KEY,OPSTRING
5051 Calls the System V IPC function semop to perform semaphore operations
5052 such as signalling and waiting. OPSTRING must be a packed array of
5053 semop structures. Each semop structure can be generated with
5054 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
5055 implies the number of semaphore operations. Returns true if
5056 successful, or false if there is an error. As an example, the
5057 following code waits on semaphore $semnum of semaphore id $semid:
5059 $semop = pack("s!3", $semnum, -1, 0);
5060 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5062 To signal the semaphore, replace C<-1> with C<1>. See also
5063 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
5066 =item send SOCKET,MSG,FLAGS,TO
5069 =item send SOCKET,MSG,FLAGS
5071 Sends a message on a socket. Attempts to send the scalar MSG to the
5072 SOCKET filehandle. Takes the same flags as the system call of the
5073 same name. On unconnected sockets you must specify a destination to
5074 send TO, in which case it does a C C<sendto>. Returns the number of
5075 characters sent, or the undefined value if there is an error. The C
5076 system call sendmsg(2) is currently unimplemented. See
5077 L<perlipc/"UDP: Message Passing"> for examples.
5079 Note the I<characters>: depending on the status of the socket, either
5080 (8-bit) bytes or characters are sent. By default all sockets operate
5081 on bytes, but for example if the socket has been changed using
5082 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5083 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5084 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5085 pragma: in that case pretty much any characters can be sent.
5087 =item setpgrp PID,PGRP
5090 Sets the current process group for the specified PID, C<0> for the current
5091 process. Will produce a fatal error if used on a machine that doesn't
5092 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5093 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5094 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5097 =item setpriority WHICH,WHO,PRIORITY
5098 X<setpriority> X<priority> X<nice> X<renice>
5100 Sets the current priority for a process, a process group, or a user.
5101 (See setpriority(2).) Will produce a fatal error if used on a machine
5102 that doesn't implement setpriority(2).
5104 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5107 Sets the socket option requested. Returns undefined if there is an
5108 error. Use integer constants provided by the C<Socket> module for
5109 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5110 getprotobyname. OPTVAL might either be a packed string or an integer.
5111 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5113 An example disabling the Nagle's algorithm for a socket:
5115 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5116 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5123 Shifts the first value of the array off and returns it, shortening the
5124 array by 1 and moving everything down. If there are no elements in the
5125 array, returns the undefined value. If ARRAY is omitted, shifts the
5126 C<@_> array within the lexical scope of subroutines and formats, and the
5127 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5128 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5129 C<UNITCHECK {}> and C<END {}> constructs.
5131 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5132 same thing to the left end of an array that C<pop> and C<push> do to the
5135 =item shmctl ID,CMD,ARG
5138 Calls the System V IPC function shmctl. You'll probably have to say
5142 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5143 then ARG must be a variable that will hold the returned C<shmid_ds>
5144 structure. Returns like ioctl: the undefined value for error, "C<0> but
5145 true" for zero, or the actual return value otherwise.
5146 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5148 =item shmget KEY,SIZE,FLAGS
5151 Calls the System V IPC function shmget. Returns the shared memory
5152 segment id, or the undefined value if there is an error.
5153 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5155 =item shmread ID,VAR,POS,SIZE
5159 =item shmwrite ID,STRING,POS,SIZE
5161 Reads or writes the System V shared memory segment ID starting at
5162 position POS for size SIZE by attaching to it, copying in/out, and
5163 detaching from it. When reading, VAR must be a variable that will
5164 hold the data read. When writing, if STRING is too long, only SIZE
5165 bytes are used; if STRING is too short, nulls are written to fill out
5166 SIZE bytes. Return true if successful, or false if there is an error.
5167 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5168 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5170 =item shutdown SOCKET,HOW
5173 Shuts down a socket connection in the manner indicated by HOW, which
5174 has the same interpretation as in the system call of the same name.
5176 shutdown(SOCKET, 0); # I/we have stopped reading data
5177 shutdown(SOCKET, 1); # I/we have stopped writing data
5178 shutdown(SOCKET, 2); # I/we have stopped using this socket
5180 This is useful with sockets when you want to tell the other
5181 side you're done writing but not done reading, or vice versa.
5182 It's also a more insistent form of close because it also
5183 disables the file descriptor in any forked copies in other
5186 Returns C<1> for success. In the case of error, returns C<undef> if
5187 the first argument is not a valid filehandle, or returns C<0> and sets
5188 C<$!> for any other failure.
5191 X<sin> X<sine> X<asin> X<arcsine>
5195 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5196 returns sine of C<$_>.
5198 For the inverse sine operation, you may use the C<Math::Trig::asin>
5199 function, or use this relation:
5201 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5208 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5209 Returns the number of seconds actually slept.
5211 May be interrupted if the process receives a signal such as C<SIGALRM>.
5214 local $SIG{ALARM} = sub { die "Alarm!\n" };
5217 die $@ unless $@ eq "Alarm!\n";
5219 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
5220 is often implemented using C<alarm>.
5222 On some older systems, it may sleep up to a full second less than what
5223 you requested, depending on how it counts seconds. Most modern systems
5224 always sleep the full amount. They may appear to sleep longer than that,
5225 however, because your process might not be scheduled right away in a
5226 busy multitasking system.
5228 For delays of finer granularity than one second, the Time::HiRes module
5229 (from CPAN, and starting from Perl 5.8 part of the standard
5230 distribution) provides usleep(). You may also use Perl's four-argument
5231 version of select() leaving the first three arguments undefined, or you
5232 might be able to use the C<syscall> interface to access setitimer(2) if
5233 your system supports it. See L<perlfaq8> for details.
5235 See also the POSIX module's C<pause> function.
5237 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5240 Opens a socket of the specified kind and attaches it to filehandle
5241 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5242 the system call of the same name. You should C<use Socket> first
5243 to get the proper definitions imported. See the examples in
5244 L<perlipc/"Sockets: Client/Server Communication">.
5246 On systems that support a close-on-exec flag on files, the flag will
5247 be set for the newly opened file descriptor, as determined by the
5248 value of $^F. See L<perlvar/$^F>.
5250 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5253 Creates an unnamed pair of sockets in the specified domain, of the
5254 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5255 for the system call of the same name. If unimplemented, yields a fatal
5256 error. Returns true if successful.
5258 On systems that support a close-on-exec flag on files, the flag will
5259 be set for the newly opened file descriptors, as determined by the value
5260 of $^F. See L<perlvar/$^F>.
5262 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5263 to C<pipe(Rdr, Wtr)> is essentially:
5266 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5267 shutdown(Rdr, 1); # no more writing for reader
5268 shutdown(Wtr, 0); # no more reading for writer
5270 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5271 emulate socketpair using IP sockets to localhost if your system implements
5272 sockets but not socketpair.
5274 =item sort SUBNAME LIST
5275 X<sort> X<qsort> X<quicksort> X<mergesort>
5277 =item sort BLOCK LIST
5281 In list context, this sorts the LIST and returns the sorted list value.
5282 In scalar context, the behaviour of C<sort()> is undefined.
5284 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5285 order. If SUBNAME is specified, it gives the name of a subroutine
5286 that returns an integer less than, equal to, or greater than C<0>,
5287 depending on how the elements of the list are to be ordered. (The C<<
5288 <=> >> and C<cmp> operators are extremely useful in such routines.)
5289 SUBNAME may be a scalar variable name (unsubscripted), in which case
5290 the value provides the name of (or a reference to) the actual
5291 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5292 an anonymous, in-line sort subroutine.
5294 If the subroutine's prototype is C<($$)>, the elements to be compared
5295 are passed by reference in C<@_>, as for a normal subroutine. This is
5296 slower than unprototyped subroutines, where the elements to be
5297 compared are passed into the subroutine
5298 as the package global variables $a and $b (see example below). Note that
5299 in the latter case, it is usually counter-productive to declare $a and
5302 The values to be compared are always passed by reference and should not
5305 You also cannot exit out of the sort block or subroutine using any of the
5306 loop control operators described in L<perlsyn> or with C<goto>.
5308 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5309 current collation locale. See L<perllocale>.
5311 sort() returns aliases into the original list, much as a for loop's index
5312 variable aliases the list elements. That is, modifying an element of a
5313 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5314 actually modifies the element in the original list. This is usually
5315 something to be avoided when writing clear code.
5317 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5318 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5319 preserves the input order of elements that compare equal. Although
5320 quicksort's run time is O(NlogN) when averaged over all arrays of
5321 length N, the time can be O(N**2), I<quadratic> behavior, for some
5322 inputs.) In 5.7, the quicksort implementation was replaced with
5323 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5324 But benchmarks indicated that for some inputs, on some platforms,
5325 the original quicksort was faster. 5.8 has a sort pragma for
5326 limited control of the sort. Its rather blunt control of the
5327 underlying algorithm may not persist into future Perls, but the
5328 ability to characterize the input or output in implementation
5329 independent ways quite probably will. See L<the sort pragma|sort>.
5334 @articles = sort @files;
5336 # same thing, but with explicit sort routine
5337 @articles = sort {$a cmp $b} @files;
5339 # now case-insensitively
5340 @articles = sort {uc($a) cmp uc($b)} @files;
5342 # same thing in reversed order
5343 @articles = sort {$b cmp $a} @files;
5345 # sort numerically ascending
5346 @articles = sort {$a <=> $b} @files;
5348 # sort numerically descending
5349 @articles = sort {$b <=> $a} @files;
5351 # this sorts the %age hash by value instead of key
5352 # using an in-line function
5353 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5355 # sort using explicit subroutine name
5357 $age{$a} <=> $age{$b}; # presuming numeric
5359 @sortedclass = sort byage @class;
5361 sub backwards { $b cmp $a }
5362 @harry = qw(dog cat x Cain Abel);
5363 @george = qw(gone chased yz Punished Axed);
5365 # prints AbelCaincatdogx
5366 print sort backwards @harry;
5367 # prints xdogcatCainAbel
5368 print sort @george, 'to', @harry;
5369 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5371 # inefficiently sort by descending numeric compare using
5372 # the first integer after the first = sign, or the
5373 # whole record case-insensitively otherwise
5376 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5381 # same thing, but much more efficiently;
5382 # we'll build auxiliary indices instead
5384 my @nums = @caps = ();
5386 push @nums, ( /=(\d+)/ ? $1 : undef );
5390 my @new = @old[ sort {
5391 $nums[$b] <=> $nums[$a]
5393 $caps[$a] cmp $caps[$b]
5397 # same thing, but without any temps
5398 @new = map { $_->[0] }
5399 sort { $b->[1] <=> $a->[1]
5402 } map { [$_, /=(\d+)/, uc($_)] } @old;
5404 # using a prototype allows you to use any comparison subroutine
5405 # as a sort subroutine (including other package's subroutines)
5407 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5410 @new = sort other::backwards @old;
5412 # guarantee stability, regardless of algorithm
5414 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5416 # force use of mergesort (not portable outside Perl 5.8)
5417 use sort '_mergesort'; # note discouraging _
5418 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5420 Warning: syntactical care is required when sorting the list returned from
5421 a function. If you want to sort the list returned by the function call
5422 C<find_records(@key)>, you can use:
5424 @contact = sort { $a cmp $b } find_records @key;
5425 @contact = sort +find_records(@key);
5426 @contact = sort &find_records(@key);
5427 @contact = sort(find_records(@key));
5429 If instead you want to sort the array @key with the comparison routine
5430 C<find_records()> then you can use:
5432 @contact = sort { find_records() } @key;
5433 @contact = sort find_records(@key);
5434 @contact = sort(find_records @key);
5435 @contact = sort(find_records (@key));
5437 If you're using strict, you I<must not> declare $a
5438 and $b as lexicals. They are package globals. That means
5439 that if you're in the C<main> package and type
5441 @articles = sort {$b <=> $a} @files;
5443 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5444 but if you're in the C<FooPack> package, it's the same as typing
5446 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5448 The comparison function is required to behave. If it returns
5449 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5450 sometimes saying the opposite, for example) the results are not
5453 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5454 (not-a-number), and because C<sort> will trigger a fatal error unless the
5455 result of a comparison is defined, when sorting with a comparison function
5456 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5457 The following example takes advantage of the fact that C<NaN != NaN> to
5458 eliminate any C<NaN>s from the input.
5460 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5462 =item splice ARRAY,OFFSET,LENGTH,LIST
5465 =item splice ARRAY,OFFSET,LENGTH
5467 =item splice ARRAY,OFFSET
5471 Removes the elements designated by OFFSET and LENGTH from an array, and
5472 replaces them with the elements of LIST, if any. In list context,
5473 returns the elements removed from the array. In scalar context,
5474 returns the last element removed, or C<undef> if no elements are
5475 removed. The array grows or shrinks as necessary.
5476 If OFFSET is negative then it starts that far from the end of the array.
5477 If LENGTH is omitted, removes everything from OFFSET onward.
5478 If LENGTH is negative, removes the elements from OFFSET onward
5479 except for -LENGTH elements at the end of the array.
5480 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5481 past the end of the array, perl issues a warning, and splices at the
5484 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5486 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5487 pop(@a) splice(@a,-1)
5488 shift(@a) splice(@a,0,1)
5489 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5490 $a[$i] = $y splice(@a,$i,1,$y)
5492 Example, assuming array lengths are passed before arrays:
5494 sub aeq { # compare two list values
5495 my(@a) = splice(@_,0,shift);
5496 my(@b) = splice(@_,0,shift);
5497 return 0 unless @a == @b; # same len?
5499 return 0 if pop(@a) ne pop(@b);
5503 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5505 =item split /PATTERN/,EXPR,LIMIT
5508 =item split /PATTERN/,EXPR
5510 =item split /PATTERN/
5514 Splits the string EXPR into a list of strings and returns that list. By
5515 default, empty leading fields are preserved, and empty trailing ones are
5516 deleted. (If all fields are empty, they are considered to be trailing.)
5518 In scalar context, returns the number of fields found.
5520 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5521 splits on whitespace (after skipping any leading whitespace). Anything
5522 matching PATTERN is taken to be a delimiter separating the fields. (Note
5523 that the delimiter may be longer than one character.)
5525 If LIMIT is specified and positive, it represents the maximum number
5526 of fields the EXPR will be split into, though the actual number of
5527 fields returned depends on the number of times PATTERN matches within
5528 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5529 stripped (which potential users of C<pop> would do well to remember).
5530 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5531 had been specified. Note that splitting an EXPR that evaluates to the
5532 empty string always returns the empty list, regardless of the LIMIT
5535 A pattern matching the null string (not to be confused with
5536 a null pattern C<//>, which is just one member of the set of patterns
5537 matching a null string) will split the value of EXPR into separate
5538 characters at each point it matches that way. For example:
5540 print join(':', split(/ */, 'hi there')), "\n";
5542 produces the output 'h:i:t:h:e:r:e'.
5544 As a special case for C<split>, using the empty pattern C<//> specifically
5545 matches only the null string, and is not be confused with the regular use
5546 of C<//> to mean "the last successful pattern match". So, for C<split>,
5549 print join(':', split(//, 'hi there')), "\n";
5551 produces the output 'h:i: :t:h:e:r:e'.
5553 Empty leading fields are produced when there are positive-width matches at
5554 the beginning of the string; a zero-width match at the beginning of
5555 the string does not produce an empty field. For example:
5557 print join(':', split(/(?=\w)/, 'hi there!'));
5559 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
5560 hand, are produced when there is a match at the end of the string (and
5561 when LIMIT is given and is not 0), regardless of the length of the match.
5564 print join(':', split(//, 'hi there!', -1)), "\n";
5565 print join(':', split(/\W/, 'hi there!', -1)), "\n";
5567 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
5568 both with an empty trailing field.
5570 The LIMIT parameter can be used to split a line partially
5572 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5574 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5575 a LIMIT one larger than the number of variables in the list, to avoid
5576 unnecessary work. For the list above LIMIT would have been 4 by
5577 default. In time critical applications it behooves you not to split
5578 into more fields than you really need.
5580 If the PATTERN contains parentheses, additional list elements are
5581 created from each matching substring in the delimiter.
5583 split(/([,-])/, "1-10,20", 3);
5585 produces the list value
5587 (1, '-', 10, ',', 20)
5589 If you had the entire header of a normal Unix email message in $header,
5590 you could split it up into fields and their values this way:
5592 $header =~ s/\n(?=\s)//g; # fix continuation lines
5593 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5595 The pattern C</PATTERN/> may be replaced with an expression to specify
5596 patterns that vary at runtime. (To do runtime compilation only once,
5597 use C</$variable/o>.)
5599 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5600 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5601 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5602 will give you as many null initial fields as there are leading spaces.
5603 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5604 whitespace produces a null first field. A C<split> with no arguments
5605 really does a S<C<split(' ', $_)>> internally.
5607 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5612 open(PASSWD, '/etc/passwd');
5615 ($login, $passwd, $uid, $gid,
5616 $gcos, $home, $shell) = split(/:/);
5620 As with regular pattern matching, any capturing parentheses that are not
5621 matched in a C<split()> will be set to C<undef> when returned:
5623 @fields = split /(A)|B/, "1A2B3";
5624 # @fields is (1, 'A', 2, undef, 3)
5626 =item sprintf FORMAT, LIST
5629 Returns a string formatted by the usual C<printf> conventions of the C
5630 library function C<sprintf>. See below for more details
5631 and see C<sprintf(3)> or C<printf(3)> on your system for an explanation of
5632 the general principles.
5636 # Format number with up to 8 leading zeroes
5637 $result = sprintf("%08d", $number);
5639 # Round number to 3 digits after decimal point
5640 $rounded = sprintf("%.3f", $number);
5642 Perl does its own C<sprintf> formatting--it emulates the C
5643 function C<sprintf>, but it doesn't use it (except for floating-point
5644 numbers, and even then only the standard modifiers are allowed). As a
5645 result, any non-standard extensions in your local C<sprintf> are not
5646 available from Perl.
5648 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5649 pass it an array as your first argument. The array is given scalar context,
5650 and instead of using the 0th element of the array as the format, Perl will
5651 use the count of elements in the array as the format, which is almost never
5654 Perl's C<sprintf> permits the following universally-known conversions:
5657 %c a character with the given number
5659 %d a signed integer, in decimal
5660 %u an unsigned integer, in decimal
5661 %o an unsigned integer, in octal
5662 %x an unsigned integer, in hexadecimal
5663 %e a floating-point number, in scientific notation
5664 %f a floating-point number, in fixed decimal notation
5665 %g a floating-point number, in %e or %f notation
5667 In addition, Perl permits the following widely-supported conversions:
5669 %X like %x, but using upper-case letters
5670 %E like %e, but using an upper-case "E"
5671 %G like %g, but with an upper-case "E" (if applicable)
5672 %b an unsigned integer, in binary
5673 %B like %b, but using an upper-case "B" with the # flag
5674 %p a pointer (outputs the Perl value's address in hexadecimal)
5675 %n special: *stores* the number of characters output so far
5676 into the next variable in the parameter list
5678 Finally, for backward (and we do mean "backward") compatibility, Perl
5679 permits these unnecessary but widely-supported conversions:
5682 %D a synonym for %ld
5683 %U a synonym for %lu
5684 %O a synonym for %lo
5687 Note that the number of exponent digits in the scientific notation produced
5688 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5689 exponent less than 100 is system-dependent: it may be three or less
5690 (zero-padded as necessary). In other words, 1.23 times ten to the
5691 99th may be either "1.23e99" or "1.23e099".
5693 Between the C<%> and the format letter, you may specify a number of
5694 additional attributes controlling the interpretation of the format.
5695 In order, these are:
5699 =item format parameter index
5701 An explicit format parameter index, such as C<2$>. By default sprintf
5702 will format the next unused argument in the list, but this allows you
5703 to take the arguments out of order, e.g.:
5705 printf '%2$d %1$d', 12, 34; # prints "34 12"
5706 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5712 space prefix non-negative number with a space
5713 + prefix non-negative number with a plus sign
5714 - left-justify within the field
5715 0 use zeros, not spaces, to right-justify
5716 # ensure the leading "0" for any octal,
5717 prefix non-zero hexadecimal with "0x" or "0X",
5718 prefix non-zero binary with "0b" or "0B"
5722 printf '<% d>', 12; # prints "< 12>"
5723 printf '<%+d>', 12; # prints "<+12>"
5724 printf '<%6s>', 12; # prints "< 12>"
5725 printf '<%-6s>', 12; # prints "<12 >"
5726 printf '<%06s>', 12; # prints "<000012>"
5727 printf '<%#o>', 12; # prints "<014>"
5728 printf '<%#x>', 12; # prints "<0xc>"
5729 printf '<%#X>', 12; # prints "<0XC>"
5730 printf '<%#b>', 12; # prints "<0b1100>"
5731 printf '<%#B>', 12; # prints "<0B1100>"
5733 When a space and a plus sign are given as the flags at once,
5734 a plus sign is used to prefix a positive number.
5736 printf '<%+ d>', 12; # prints "<+12>"
5737 printf '<% +d>', 12; # prints "<+12>"
5739 When the # flag and a precision are given in the %o conversion,
5740 the precision is incremented if it's necessary for the leading "0".
5742 printf '<%#.5o>', 012; # prints "<00012>"
5743 printf '<%#.5o>', 012345; # prints "<012345>"
5744 printf '<%#.0o>', 0; # prints "<0>"
5748 This flag tells perl to interpret the supplied string as a vector of
5749 integers, one for each character in the string. Perl applies the format to
5750 each integer in turn, then joins the resulting strings with a separator (a
5751 dot C<.> by default). This can be useful for displaying ordinal values of
5752 characters in arbitrary strings:
5754 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5755 printf "version is v%vd\n", $^V; # Perl's version
5757 Put an asterisk C<*> before the C<v> to override the string to
5758 use to separate the numbers:
5760 printf "address is %*vX\n", ":", $addr; # IPv6 address
5761 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5763 You can also explicitly specify the argument number to use for
5764 the join string using e.g. C<*2$v>:
5766 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5768 =item (minimum) width
5770 Arguments are usually formatted to be only as wide as required to
5771 display the given value. You can override the width by putting
5772 a number here, or get the width from the next argument (with C<*>)
5773 or from a specified argument (with e.g. C<*2$>):
5775 printf '<%s>', "a"; # prints "<a>"
5776 printf '<%6s>', "a"; # prints "< a>"
5777 printf '<%*s>', 6, "a"; # prints "< a>"
5778 printf '<%*2$s>', "a", 6; # prints "< a>"
5779 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5781 If a field width obtained through C<*> is negative, it has the same
5782 effect as the C<-> flag: left-justification.
5784 =item precision, or maximum width
5787 You can specify a precision (for numeric conversions) or a maximum
5788 width (for string conversions) by specifying a C<.> followed by a number.
5789 For floating point formats, with the exception of 'g' and 'G', this specifies
5790 the number of decimal places to show (the default being 6), e.g.:
5792 # these examples are subject to system-specific variation
5793 printf '<%f>', 1; # prints "<1.000000>"
5794 printf '<%.1f>', 1; # prints "<1.0>"
5795 printf '<%.0f>', 1; # prints "<1>"
5796 printf '<%e>', 10; # prints "<1.000000e+01>"
5797 printf '<%.1e>', 10; # prints "<1.0e+01>"
5799 For 'g' and 'G', this specifies the maximum number of digits to show,
5800 including prior to the decimal point as well as after it, e.g.:
5802 # these examples are subject to system-specific variation
5803 printf '<%g>', 1; # prints "<1>"
5804 printf '<%.10g>', 1; # prints "<1>"
5805 printf '<%g>', 100; # prints "<100>"
5806 printf '<%.1g>', 100; # prints "<1e+02>"
5807 printf '<%.2g>', 100.01; # prints "<1e+02>"
5808 printf '<%.5g>', 100.01; # prints "<100.01>"
5809 printf '<%.4g>', 100.01; # prints "<100>"
5811 For integer conversions, specifying a precision implies that the
5812 output of the number itself should be zero-padded to this width,
5813 where the 0 flag is ignored:
5815 printf '<%.6d>', 1; # prints "<000001>"
5816 printf '<%+.6d>', 1; # prints "<+000001>"
5817 printf '<%-10.6d>', 1; # prints "<000001 >"
5818 printf '<%10.6d>', 1; # prints "< 000001>"
5819 printf '<%010.6d>', 1; # prints "< 000001>"
5820 printf '<%+10.6d>', 1; # prints "< +000001>"
5822 printf '<%.6x>', 1; # prints "<000001>"
5823 printf '<%#.6x>', 1; # prints "<0x000001>"
5824 printf '<%-10.6x>', 1; # prints "<000001 >"
5825 printf '<%10.6x>', 1; # prints "< 000001>"
5826 printf '<%010.6x>', 1; # prints "< 000001>"
5827 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5829 For string conversions, specifying a precision truncates the string
5830 to fit in the specified width:
5832 printf '<%.5s>', "truncated"; # prints "<trunc>"
5833 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5835 You can also get the precision from the next argument using C<.*>:
5837 printf '<%.6x>', 1; # prints "<000001>"
5838 printf '<%.*x>', 6, 1; # prints "<000001>"
5840 If a precision obtained through C<*> is negative, it has the same
5841 effect as no precision.
5843 printf '<%.*s>', 7, "string"; # prints "<string>"
5844 printf '<%.*s>', 3, "string"; # prints "<str>"
5845 printf '<%.*s>', 0, "string"; # prints "<>"
5846 printf '<%.*s>', -1, "string"; # prints "<string>"
5848 printf '<%.*d>', 1, 0; # prints "<0>"
5849 printf '<%.*d>', 0, 0; # prints "<>"
5850 printf '<%.*d>', -1, 0; # prints "<0>"
5852 You cannot currently get the precision from a specified number,
5853 but it is intended that this will be possible in the future using
5856 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5860 For numeric conversions, you can specify the size to interpret the
5861 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5862 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5863 whatever the default integer size is on your platform (usually 32 or 64
5864 bits), but you can override this to use instead one of the standard C types,
5865 as supported by the compiler used to build Perl:
5867 l interpret integer as C type "long" or "unsigned long"
5868 h interpret integer as C type "short" or "unsigned short"
5869 q, L or ll interpret integer as C type "long long", "unsigned long long".
5870 or "quads" (typically 64-bit integers)
5872 The last will produce errors if Perl does not understand "quads" in your
5873 installation. (This requires that either the platform natively supports quads
5874 or Perl was specifically compiled to support quads.) You can find out
5875 whether your Perl supports quads via L<Config>:
5878 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5881 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5882 to be the default floating point size on your platform (double or long double),
5883 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5884 platform supports them. You can find out whether your Perl supports long
5885 doubles via L<Config>:
5888 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5890 You can find out whether Perl considers 'long double' to be the default
5891 floating point size to use on your platform via L<Config>:
5894 ($Config{uselongdouble} eq 'define') &&
5895 print "long doubles by default\n";
5897 It can also be the case that long doubles and doubles are the same thing:
5900 ($Config{doublesize} == $Config{longdblsize}) &&
5901 print "doubles are long doubles\n";
5903 The size specifier C<V> has no effect for Perl code, but it is supported
5904 for compatibility with XS code; it means 'use the standard size for
5905 a Perl integer (or floating-point number)', which is already the
5906 default for Perl code.
5908 =item order of arguments
5910 Normally, sprintf takes the next unused argument as the value to
5911 format for each format specification. If the format specification
5912 uses C<*> to require additional arguments, these are consumed from
5913 the argument list in the order in which they appear in the format
5914 specification I<before> the value to format. Where an argument is
5915 specified using an explicit index, this does not affect the normal
5916 order for the arguments (even when the explicitly specified index
5917 would have been the next argument in any case).
5921 printf '<%*.*s>', $a, $b, $c;
5923 would use C<$a> for the width, C<$b> for the precision and C<$c>
5924 as the value to format, while:
5926 printf '<%*1$.*s>', $a, $b;
5928 would use C<$a> for the width and the precision, and C<$b> as the
5931 Here are some more examples - beware that when using an explicit
5932 index, the C<$> may need to be escaped:
5934 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5935 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5936 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5937 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5941 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5942 the character used for the decimal separator in formatted floating
5943 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5947 X<sqrt> X<root> X<square root>
5951 Return the square root of EXPR. If EXPR is omitted, returns square
5952 root of C<$_>. Only works on non-negative operands, unless you've
5953 loaded the standard Math::Complex module.
5956 print sqrt(-2); # prints 1.4142135623731i
5959 X<srand> X<seed> X<randseed>
5963 Sets the random number seed for the C<rand> operator.
5965 The point of the function is to "seed" the C<rand> function so that
5966 C<rand> can produce a different sequence each time you run your
5969 If srand() is not called explicitly, it is called implicitly at the
5970 first use of the C<rand> operator. However, this was not the case in
5971 versions of Perl before 5.004, so if your script will run under older
5972 Perl versions, it should call C<srand>.
5974 Most programs won't even call srand() at all, except those that
5975 need a cryptographically-strong starting point rather than the
5976 generally acceptable default, which is based on time of day,
5977 process ID, and memory allocation, or the F</dev/urandom> device,
5980 You can call srand($seed) with the same $seed to reproduce the
5981 I<same> sequence from rand(), but this is usually reserved for
5982 generating predictable results for testing or debugging.
5983 Otherwise, don't call srand() more than once in your program.
5985 Do B<not> call srand() (i.e. without an argument) more than once in
5986 a script. The internal state of the random number generator should
5987 contain more entropy than can be provided by any seed, so calling
5988 srand() again actually I<loses> randomness.
5990 Most implementations of C<srand> take an integer and will silently
5991 truncate decimal numbers. This means C<srand(42)> will usually
5992 produce the same results as C<srand(42.1)>. To be safe, always pass
5993 C<srand> an integer.
5995 In versions of Perl prior to 5.004 the default seed was just the
5996 current C<time>. This isn't a particularly good seed, so many old
5997 programs supply their own seed value (often C<time ^ $$> or C<time ^
5998 ($$ + ($$ << 15))>), but that isn't necessary any more.
6000 For cryptographic purposes, however, you need something much more random
6001 than the default seed. Checksumming the compressed output of one or more
6002 rapidly changing operating system status programs is the usual method. For
6005 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
6007 If you're particularly concerned with this, see the C<Math::TrulyRandom>
6010 Frequently called programs (like CGI scripts) that simply use
6014 for a seed can fall prey to the mathematical property that
6018 one-third of the time. So don't do that.
6020 =item stat FILEHANDLE
6021 X<stat> X<file, status> X<ctime>
6025 =item stat DIRHANDLE
6029 Returns a 13-element list giving the status info for a file, either
6030 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
6031 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
6034 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
6035 $atime,$mtime,$ctime,$blksize,$blocks)
6038 Not all fields are supported on all filesystem types. Here are the
6039 meanings of the fields:
6041 0 dev device number of filesystem
6043 2 mode file mode (type and permissions)
6044 3 nlink number of (hard) links to the file
6045 4 uid numeric user ID of file's owner
6046 5 gid numeric group ID of file's owner
6047 6 rdev the device identifier (special files only)
6048 7 size total size of file, in bytes
6049 8 atime last access time in seconds since the epoch
6050 9 mtime last modify time in seconds since the epoch
6051 10 ctime inode change time in seconds since the epoch (*)
6052 11 blksize preferred block size for file system I/O
6053 12 blocks actual number of blocks allocated
6055 (The epoch was at 00:00 January 1, 1970 GMT.)
6057 (*) Not all fields are supported on all filesystem types. Notably, the
6058 ctime field is non-portable. In particular, you cannot expect it to be a
6059 "creation time", see L<perlport/"Files and Filesystems"> for details.
6061 If C<stat> is passed the special filehandle consisting of an underline, no
6062 stat is done, but the current contents of the stat structure from the
6063 last C<stat>, C<lstat>, or filetest are returned. Example:
6065 if (-x $file && (($d) = stat(_)) && $d < 0) {
6066 print "$file is executable NFS file\n";
6069 (This works on machines only for which the device number is negative
6072 Because the mode contains both the file type and its permissions, you
6073 should mask off the file type portion and (s)printf using a C<"%o">
6074 if you want to see the real permissions.
6076 $mode = (stat($filename))[2];
6077 printf "Permissions are %04o\n", $mode & 07777;
6079 In scalar context, C<stat> returns a boolean value indicating success
6080 or failure, and, if successful, sets the information associated with
6081 the special filehandle C<_>.
6083 The L<File::stat> module provides a convenient, by-name access mechanism:
6086 $sb = stat($filename);
6087 printf "File is %s, size is %s, perm %04o, mtime %s\n",
6088 $filename, $sb->size, $sb->mode & 07777,
6089 scalar localtime $sb->mtime;
6091 You can import symbolic mode constants (C<S_IF*>) and functions
6092 (C<S_IS*>) from the Fcntl module:
6096 $mode = (stat($filename))[2];
6098 $user_rwx = ($mode & S_IRWXU) >> 6;
6099 $group_read = ($mode & S_IRGRP) >> 3;
6100 $other_execute = $mode & S_IXOTH;
6102 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6104 $is_setuid = $mode & S_ISUID;
6105 $is_directory = S_ISDIR($mode);
6107 You could write the last two using the C<-u> and C<-d> operators.
6108 The commonly available C<S_IF*> constants are
6110 # Permissions: read, write, execute, for user, group, others.
6112 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6113 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6114 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6116 # Setuid/Setgid/Stickiness/SaveText.
6117 # Note that the exact meaning of these is system dependent.
6119 S_ISUID S_ISGID S_ISVTX S_ISTXT
6121 # File types. Not necessarily all are available on your system.
6123 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6125 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
6127 S_IREAD S_IWRITE S_IEXEC
6129 and the C<S_IF*> functions are
6131 S_IMODE($mode) the part of $mode containing the permission bits
6132 and the setuid/setgid/sticky bits
6134 S_IFMT($mode) the part of $mode containing the file type
6135 which can be bit-anded with e.g. S_IFREG
6136 or with the following functions
6138 # The operators -f, -d, -l, -b, -c, -p, and -S.
6140 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6141 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6143 # No direct -X operator counterpart, but for the first one
6144 # the -g operator is often equivalent. The ENFMT stands for
6145 # record flocking enforcement, a platform-dependent feature.
6147 S_ISENFMT($mode) S_ISWHT($mode)
6149 See your native chmod(2) and stat(2) documentation for more details
6150 about the C<S_*> constants. To get status info for a symbolic link
6151 instead of the target file behind the link, use the C<lstat> function.
6156 =item state TYPE EXPR
6158 =item state EXPR : ATTRS
6160 =item state TYPE EXPR : ATTRS
6162 C<state> declares a lexically scoped variable, just like C<my> does.
6163 However, those variables will never be reinitialized, contrary to
6164 lexical variables that are reinitialized each time their enclosing block
6167 C<state> variables are only enabled when the C<feature 'state'> pragma is
6168 in effect. See L<feature>.
6175 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6176 doing many pattern matches on the string before it is next modified.
6177 This may or may not save time, depending on the nature and number of
6178 patterns you are searching on, and on the distribution of character
6179 frequencies in the string to be searched--you probably want to compare
6180 run times with and without it to see which runs faster. Those loops
6181 that scan for many short constant strings (including the constant
6182 parts of more complex patterns) will benefit most. You may have only
6183 one C<study> active at a time--if you study a different scalar the first
6184 is "unstudied". (The way C<study> works is this: a linked list of every
6185 character in the string to be searched is made, so we know, for
6186 example, where all the C<'k'> characters are. From each search string,
6187 the rarest character is selected, based on some static frequency tables
6188 constructed from some C programs and English text. Only those places
6189 that contain this "rarest" character are examined.)
6191 For example, here is a loop that inserts index producing entries
6192 before any line containing a certain pattern:
6196 print ".IX foo\n" if /\bfoo\b/;
6197 print ".IX bar\n" if /\bbar\b/;
6198 print ".IX blurfl\n" if /\bblurfl\b/;
6203 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6204 will be looked at, because C<f> is rarer than C<o>. In general, this is
6205 a big win except in pathological cases. The only question is whether
6206 it saves you more time than it took to build the linked list in the
6209 Note that if you have to look for strings that you don't know till
6210 runtime, you can build an entire loop as a string and C<eval> that to
6211 avoid recompiling all your patterns all the time. Together with
6212 undefining C<$/> to input entire files as one record, this can be very
6213 fast, often faster than specialized programs like fgrep(1). The following
6214 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6215 out the names of those files that contain a match:
6217 $search = 'while (<>) { study;';
6218 foreach $word (@words) {
6219 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6224 eval $search; # this screams
6225 $/ = "\n"; # put back to normal input delimiter
6226 foreach $file (sort keys(%seen)) {
6230 =item sub NAME BLOCK
6233 =item sub NAME (PROTO) BLOCK
6235 =item sub NAME : ATTRS BLOCK
6237 =item sub NAME (PROTO) : ATTRS BLOCK
6239 This is subroutine definition, not a real function I<per se>.
6240 Without a BLOCK it's just a forward declaration. Without a NAME,
6241 it's an anonymous function declaration, and does actually return
6242 a value: the CODE ref of the closure you just created.
6244 See L<perlsub> and L<perlref> for details about subroutines and
6245 references, and L<attributes> and L<Attribute::Handlers> for more
6246 information about attributes.
6248 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6249 X<substr> X<substring> X<mid> X<left> X<right>
6251 =item substr EXPR,OFFSET,LENGTH
6253 =item substr EXPR,OFFSET
6255 Extracts a substring out of EXPR and returns it. First character is at
6256 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6257 If OFFSET is negative (or more precisely, less than C<$[>), starts
6258 that far from the end of the string. If LENGTH is omitted, returns
6259 everything to the end of the string. If LENGTH is negative, leaves that
6260 many characters off the end of the string.
6262 my $s = "The black cat climbed the green tree";
6263 my $color = substr $s, 4, 5; # black
6264 my $middle = substr $s, 4, -11; # black cat climbed the
6265 my $end = substr $s, 14; # climbed the green tree
6266 my $tail = substr $s, -4; # tree
6267 my $z = substr $s, -4, 2; # tr
6269 You can use the substr() function as an lvalue, in which case EXPR
6270 must itself be an lvalue. If you assign something shorter than LENGTH,
6271 the string will shrink, and if you assign something longer than LENGTH,
6272 the string will grow to accommodate it. To keep the string the same
6273 length you may need to pad or chop your value using C<sprintf>.
6275 If OFFSET and LENGTH specify a substring that is partly outside the
6276 string, only the part within the string is returned. If the substring
6277 is beyond either end of the string, substr() returns the undefined
6278 value and produces a warning. When used as an lvalue, specifying a
6279 substring that is entirely outside the string is a fatal error.
6280 Here's an example showing the behavior for boundary cases:
6283 substr($name, 4) = 'dy'; # $name is now 'freddy'
6284 my $null = substr $name, 6, 2; # returns '' (no warning)
6285 my $oops = substr $name, 7; # returns undef, with warning
6286 substr($name, 7) = 'gap'; # fatal error
6288 An alternative to using substr() as an lvalue is to specify the
6289 replacement string as the 4th argument. This allows you to replace
6290 parts of the EXPR and return what was there before in one operation,
6291 just as you can with splice().
6293 my $s = "The black cat climbed the green tree";
6294 my $z = substr $s, 14, 7, "jumped from"; # climbed
6295 # $s is now "The black cat jumped from the green tree"
6297 Note that the lvalue returned by the 3-arg version of substr() acts as
6298 a 'magic bullet'; each time it is assigned to, it remembers which part
6299 of the original string is being modified; for example:
6302 for (substr($x,1,2)) {
6303 $_ = 'a'; print $x,"\n"; # prints 1a4
6304 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6306 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6309 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6312 =item symlink OLDFILE,NEWFILE
6313 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6315 Creates a new filename symbolically linked to the old filename.
6316 Returns C<1> for success, C<0> otherwise. On systems that don't support
6317 symbolic links, produces a fatal error at run time. To check for that,
6320 $symlink_exists = eval { symlink("",""); 1 };
6322 =item syscall NUMBER, LIST
6323 X<syscall> X<system call>
6325 Calls the system call specified as the first element of the list,
6326 passing the remaining elements as arguments to the system call. If
6327 unimplemented, produces a fatal error. The arguments are interpreted
6328 as follows: if a given argument is numeric, the argument is passed as
6329 an int. If not, the pointer to the string value is passed. You are
6330 responsible to make sure a string is pre-extended long enough to
6331 receive any result that might be written into a string. You can't use a
6332 string literal (or other read-only string) as an argument to C<syscall>
6333 because Perl has to assume that any string pointer might be written
6335 integer arguments are not literals and have never been interpreted in a
6336 numeric context, you may need to add C<0> to them to force them to look
6337 like numbers. This emulates the C<syswrite> function (or vice versa):
6339 require 'syscall.ph'; # may need to run h2ph
6341 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6343 Note that Perl supports passing of up to only 14 arguments to your system call,
6344 which in practice should usually suffice.
6346 Syscall returns whatever value returned by the system call it calls.
6347 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6348 Note that some system calls can legitimately return C<-1>. The proper
6349 way to handle such calls is to assign C<$!=0;> before the call and
6350 check the value of C<$!> if syscall returns C<-1>.
6352 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6353 number of the read end of the pipe it creates. There is no way
6354 to retrieve the file number of the other end. You can avoid this
6355 problem by using C<pipe> instead.
6357 =item sysopen FILEHANDLE,FILENAME,MODE
6360 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6362 Opens the file whose filename is given by FILENAME, and associates it
6363 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6364 the name of the real filehandle wanted. This function calls the
6365 underlying operating system's C<open> function with the parameters
6366 FILENAME, MODE, PERMS.
6368 The possible values and flag bits of the MODE parameter are
6369 system-dependent; they are available via the standard module C<Fcntl>.
6370 See the documentation of your operating system's C<open> to see which
6371 values and flag bits are available. You may combine several flags
6372 using the C<|>-operator.
6374 Some of the most common values are C<O_RDONLY> for opening the file in
6375 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6376 and C<O_RDWR> for opening the file in read-write mode.
6377 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6379 For historical reasons, some values work on almost every system
6380 supported by perl: zero means read-only, one means write-only, and two
6381 means read/write. We know that these values do I<not> work under
6382 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6383 use them in new code.
6385 If the file named by FILENAME does not exist and the C<open> call creates
6386 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6387 PERMS specifies the permissions of the newly created file. If you omit
6388 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6389 These permission values need to be in octal, and are modified by your
6390 process's current C<umask>.
6393 In many systems the C<O_EXCL> flag is available for opening files in
6394 exclusive mode. This is B<not> locking: exclusiveness means here that
6395 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6396 on network filesystems, and has no effect unless the C<O_CREAT> flag
6397 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6398 being opened if it is a symbolic link. It does not protect against
6399 symbolic links in the file's path.
6402 Sometimes you may want to truncate an already-existing file. This
6403 can be done using the C<O_TRUNC> flag. The behavior of
6404 C<O_TRUNC> with C<O_RDONLY> is undefined.
6407 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6408 that takes away the user's option to have a more permissive umask.
6409 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6412 Note that C<sysopen> depends on the fdopen() C library function.
6413 On many UNIX systems, fdopen() is known to fail when file descriptors
6414 exceed a certain value, typically 255. If you need more file
6415 descriptors than that, consider rebuilding Perl to use the C<sfio>
6416 library, or perhaps using the POSIX::open() function.
6418 See L<perlopentut> for a kinder, gentler explanation of opening files.
6420 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6423 =item sysread FILEHANDLE,SCALAR,LENGTH
6425 Attempts to read LENGTH bytes of data into variable SCALAR from the
6426 specified FILEHANDLE, using the system call read(2). It bypasses
6427 buffered IO, so mixing this with other kinds of reads, C<print>,
6428 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6429 perlio or stdio layers usually buffers data. Returns the number of
6430 bytes actually read, C<0> at end of file, or undef if there was an
6431 error (in the latter case C<$!> is also set). SCALAR will be grown or
6432 shrunk so that the last byte actually read is the last byte of the
6433 scalar after the read.
6435 An OFFSET may be specified to place the read data at some place in the
6436 string other than the beginning. A negative OFFSET specifies
6437 placement at that many characters counting backwards from the end of
6438 the string. A positive OFFSET greater than the length of SCALAR
6439 results in the string being padded to the required size with C<"\0">
6440 bytes before the result of the read is appended.
6442 There is no syseof() function, which is ok, since eof() doesn't work
6443 very well on device files (like ttys) anyway. Use sysread() and check
6444 for a return value for 0 to decide whether you're done.
6446 Note that if the filehandle has been marked as C<:utf8> Unicode
6447 characters are read instead of bytes (the LENGTH, OFFSET, and the
6448 return value of sysread() are in Unicode characters).
6449 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6450 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6452 =item sysseek FILEHANDLE,POSITION,WHENCE
6455 Sets FILEHANDLE's system position in bytes using the system call
6456 lseek(2). FILEHANDLE may be an expression whose value gives the name
6457 of the filehandle. The values for WHENCE are C<0> to set the new
6458 position to POSITION, C<1> to set the it to the current position plus
6459 POSITION, and C<2> to set it to EOF plus POSITION (typically
6462 Note the I<in bytes>: even if the filehandle has been set to operate
6463 on characters (for example by using the C<:encoding(utf8)> I/O layer),
6464 tell() will return byte offsets, not character offsets (because
6465 implementing that would render sysseek() very slow).
6467 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6468 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6469 C<seek>, C<tell>, or C<eof> may cause confusion.
6471 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6472 and C<SEEK_END> (start of the file, current position, end of the file)
6473 from the Fcntl module. Use of the constants is also more portable
6474 than relying on 0, 1, and 2. For example to define a "systell" function:
6476 use Fcntl 'SEEK_CUR';
6477 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6479 Returns the new position, or the undefined value on failure. A position
6480 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6481 true on success and false on failure, yet you can still easily determine
6487 =item system PROGRAM LIST
6489 Does exactly the same thing as C<exec LIST>, except that a fork is
6490 done first, and the parent process waits for the child process to
6491 complete. Note that argument processing varies depending on the
6492 number of arguments. If there is more than one argument in LIST,
6493 or if LIST is an array with more than one value, starts the program
6494 given by the first element of the list with arguments given by the
6495 rest of the list. If there is only one scalar argument, the argument
6496 is checked for shell metacharacters, and if there are any, the
6497 entire argument is passed to the system's command shell for parsing
6498 (this is C</bin/sh -c> on Unix platforms, but varies on other
6499 platforms). If there are no shell metacharacters in the argument,
6500 it is split into words and passed directly to C<execvp>, which is
6503 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6504 output before any operation that may do a fork, but this may not be
6505 supported on some platforms (see L<perlport>). To be safe, you may need
6506 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6507 of C<IO::Handle> on any open handles.
6509 The return value is the exit status of the program as returned by the
6510 C<wait> call. To get the actual exit value, shift right by eight (see
6511 below). See also L</exec>. This is I<not> what you want to use to capture
6512 the output from a command, for that you should use merely backticks or
6513 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6514 indicates a failure to start the program or an error of the wait(2) system
6515 call (inspect $! for the reason).
6517 If you'd like to make C<system> (and many other bits of Perl) die on error,
6518 have a look at the L<autodie> pragma.
6520 Like C<exec>, C<system> allows you to lie to a program about its name if
6521 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6523 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6524 C<system>, if you expect your program to terminate on receipt of these
6525 signals you will need to arrange to do so yourself based on the return
6528 @args = ("command", "arg1", "arg2");
6530 or die "system @args failed: $?"
6532 If you'd like to manually inspect C<system>'s failure, you can check all
6533 possible failure modes by inspecting C<$?> like this:
6536 print "failed to execute: $!\n";
6539 printf "child died with signal %d, %s coredump\n",
6540 ($? & 127), ($? & 128) ? 'with' : 'without';
6543 printf "child exited with value %d\n", $? >> 8;
6546 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6547 with the W*() calls of the POSIX extension.
6549 When the arguments get executed via the system shell, results
6550 and return codes will be subject to its quirks and capabilities.
6551 See L<perlop/"`STRING`"> and L</exec> for details.
6553 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6556 =item syswrite FILEHANDLE,SCALAR,LENGTH
6558 =item syswrite FILEHANDLE,SCALAR
6560 Attempts to write LENGTH bytes of data from variable SCALAR to the
6561 specified FILEHANDLE, using the system call write(2). If LENGTH is
6562 not specified, writes whole SCALAR. It bypasses buffered IO, so
6563 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6564 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6565 stdio layers usually buffers data. Returns the number of bytes
6566 actually written, or C<undef> if there was an error (in this case the
6567 errno variable C<$!> is also set). If the LENGTH is greater than the
6568 available data in the SCALAR after the OFFSET, only as much data as is
6569 available will be written.
6571 An OFFSET may be specified to write the data from some part of the
6572 string other than the beginning. A negative OFFSET specifies writing
6573 that many characters counting backwards from the end of the string.
6574 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6576 Note that if the filehandle has been marked as C<:utf8>, Unicode
6577 characters are written instead of bytes (the LENGTH, OFFSET, and the
6578 return value of syswrite() are in UTF-8 encoded Unicode characters).
6579 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6580 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6582 =item tell FILEHANDLE
6587 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6588 error. FILEHANDLE may be an expression whose value gives the name of
6589 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6592 Note the I<in bytes>: even if the filehandle has been set to
6593 operate on characters (for example by using the C<:encoding(utf8)> open
6594 layer), tell() will return byte offsets, not character offsets (because
6595 that would render seek() and tell() rather slow).
6597 The return value of tell() for the standard streams like the STDIN
6598 depends on the operating system: it may return -1 or something else.
6599 tell() on pipes, fifos, and sockets usually returns -1.
6601 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6603 Do not use tell() (or other buffered I/O operations) on a file handle
6604 that has been manipulated by sysread(), syswrite() or sysseek().
6605 Those functions ignore the buffering, while tell() does not.
6607 =item telldir DIRHANDLE
6610 Returns the current position of the C<readdir> routines on DIRHANDLE.
6611 Value may be given to C<seekdir> to access a particular location in a
6612 directory. C<telldir> has the same caveats about possible directory
6613 compaction as the corresponding system library routine.
6615 =item tie VARIABLE,CLASSNAME,LIST
6618 This function binds a variable to a package class that will provide the
6619 implementation for the variable. VARIABLE is the name of the variable
6620 to be enchanted. CLASSNAME is the name of a class implementing objects
6621 of correct type. Any additional arguments are passed to the C<new>
6622 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6623 or C<TIEHASH>). Typically these are arguments such as might be passed
6624 to the C<dbm_open()> function of C. The object returned by the C<new>
6625 method is also returned by the C<tie> function, which would be useful
6626 if you want to access other methods in CLASSNAME.
6628 Note that functions such as C<keys> and C<values> may return huge lists
6629 when used on large objects, like DBM files. You may prefer to use the
6630 C<each> function to iterate over such. Example:
6632 # print out history file offsets
6634 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6635 while (($key,$val) = each %HIST) {
6636 print $key, ' = ', unpack('L',$val), "\n";
6640 A class implementing a hash should have the following methods:
6642 TIEHASH classname, LIST
6644 STORE this, key, value
6649 NEXTKEY this, lastkey
6654 A class implementing an ordinary array should have the following methods:
6656 TIEARRAY classname, LIST
6658 STORE this, key, value
6660 STORESIZE this, count
6666 SPLICE this, offset, length, LIST
6671 A class implementing a file handle should have the following methods:
6673 TIEHANDLE classname, LIST
6674 READ this, scalar, length, offset
6677 WRITE this, scalar, length, offset
6679 PRINTF this, format, LIST
6683 SEEK this, position, whence
6685 OPEN this, mode, LIST
6690 A class implementing a scalar should have the following methods:
6692 TIESCALAR classname, LIST
6698 Not all methods indicated above need be implemented. See L<perltie>,
6699 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6701 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6702 for you--you need to do that explicitly yourself. See L<DB_File>
6703 or the F<Config> module for interesting C<tie> implementations.
6705 For further details see L<perltie>, L<"tied VARIABLE">.
6710 Returns a reference to the object underlying VARIABLE (the same value
6711 that was originally returned by the C<tie> call that bound the variable
6712 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6718 Returns the number of non-leap seconds since whatever time the system
6719 considers to be the epoch, suitable for feeding to C<gmtime> and
6720 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6721 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6722 1904 in the current local time zone for its epoch.
6724 For measuring time in better granularity than one second,
6725 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6726 Perl 5.8 part of the standard distribution), or if you have
6727 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6728 See L<perlfaq8> for details.
6730 For date and time processing look at the many related modules on CPAN.
6731 For a comprehensive date and time representation look at the
6737 Returns a four-element list giving the user and system times, in
6738 seconds, for this process and the children of this process.
6740 ($user,$system,$cuser,$csystem) = times;
6742 In scalar context, C<times> returns C<$user>.
6744 Note that times for children are included only after they terminate.
6748 The transliteration operator. Same as C<y///>. See
6749 L<perlop/"Quote and Quote-like Operators">.
6751 =item truncate FILEHANDLE,LENGTH
6754 =item truncate EXPR,LENGTH
6756 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6757 specified length. Produces a fatal error if truncate isn't implemented
6758 on your system. Returns true if successful, the undefined value
6761 The behavior is undefined if LENGTH is greater than the length of the
6764 The position in the file of FILEHANDLE is left unchanged. You may want to
6765 call L<seek> before writing to the file.
6768 X<uc> X<uppercase> X<toupper>
6772 Returns an uppercased version of EXPR. This is the internal function
6773 implementing the C<\U> escape in double-quoted strings. Respects
6774 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6775 and L<perlunicode> for more details about locale and Unicode support.
6776 It does not attempt to do titlecase mapping on initial letters. See
6777 C<ucfirst> for that.
6779 If EXPR is omitted, uses C<$_>.
6782 X<ucfirst> X<uppercase>
6786 Returns the value of EXPR with the first character in uppercase
6787 (titlecase in Unicode). This is the internal function implementing
6788 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6789 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6790 for more details about locale and Unicode support.
6792 If EXPR is omitted, uses C<$_>.
6799 Sets the umask for the process to EXPR and returns the previous value.
6800 If EXPR is omitted, merely returns the current umask.
6802 The Unix permission C<rwxr-x---> is represented as three sets of three
6803 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6804 and isn't one of the digits). The C<umask> value is such a number
6805 representing disabled permissions bits. The permission (or "mode")
6806 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6807 even if you tell C<sysopen> to create a file with permissions C<0777>,
6808 if your umask is C<0022> then the file will actually be created with
6809 permissions C<0755>. If your C<umask> were C<0027> (group can't
6810 write; others can't read, write, or execute), then passing
6811 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6814 Here's some advice: supply a creation mode of C<0666> for regular
6815 files (in C<sysopen>) and one of C<0777> for directories (in
6816 C<mkdir>) and executable files. This gives users the freedom of
6817 choice: if they want protected files, they might choose process umasks
6818 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6819 Programs should rarely if ever make policy decisions better left to
6820 the user. The exception to this is when writing files that should be
6821 kept private: mail files, web browser cookies, I<.rhosts> files, and
6824 If umask(2) is not implemented on your system and you are trying to
6825 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6826 fatal error at run time. If umask(2) is not implemented and you are
6827 not trying to restrict access for yourself, returns C<undef>.
6829 Remember that a umask is a number, usually given in octal; it is I<not> a
6830 string of octal digits. See also L</oct>, if all you have is a string.
6833 X<undef> X<undefine>
6837 Undefines the value of EXPR, which must be an lvalue. Use only on a
6838 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6839 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6840 will probably not do what you expect on most predefined variables or
6841 DBM list values, so don't do that; see L<delete>.) Always returns the
6842 undefined value. You can omit the EXPR, in which case nothing is
6843 undefined, but you still get an undefined value that you could, for
6844 instance, return from a subroutine, assign to a variable or pass as a
6845 parameter. Examples:
6848 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6852 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6853 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6854 select undef, undef, undef, 0.25;
6855 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6857 Note that this is a unary operator, not a list operator.
6860 X<unlink> X<delete> X<remove> X<rm> X<del>
6864 Deletes a list of files. On success, it returns the number of files
6865 it successfully deleted. On failure, it returns false and sets C<$!>
6868 my $unlinked = unlink 'a', 'b', 'c';
6870 unlink glob "*.bak";
6872 On error, C<unlink> will not tell you which files it could not remove.
6873 If you care about the files you could not remove, try them one
6876 foreach my $file ( @goners ) {
6877 unlink $file or warn "Could not unlink $file: $!";
6880 Note: C<unlink> will not attempt to delete directories unless you are
6881 superuser and the B<-U> flag is supplied to Perl. Even if these
6882 conditions are met, be warned that unlinking a directory can inflict
6883 damage on your filesystem. Finally, using C<unlink> on directories is
6884 not supported on many operating systems. Use C<rmdir> instead.
6886 If LIST is omitted, C<unlink> uses C<$_>.
6888 =item unpack TEMPLATE,EXPR
6891 =item unpack TEMPLATE
6893 C<unpack> does the reverse of C<pack>: it takes a string
6894 and expands it out into a list of values.
6895 (In scalar context, it returns merely the first value produced.)
6897 If EXPR is omitted, unpacks the C<$_> string.
6899 The string is broken into chunks described by the TEMPLATE. Each chunk
6900 is converted separately to a value. Typically, either the string is a result
6901 of C<pack>, or the characters of the string represent a C structure of some
6904 The TEMPLATE has the same format as in the C<pack> function.
6905 Here's a subroutine that does substring:
6908 my($what,$where,$howmuch) = @_;
6909 unpack("x$where a$howmuch", $what);
6914 sub ordinal { unpack("W",$_[0]); } # same as ord()
6916 In addition to fields allowed in pack(), you may prefix a field with
6917 a %<number> to indicate that
6918 you want a <number>-bit checksum of the items instead of the items
6919 themselves. Default is a 16-bit checksum. Checksum is calculated by
6920 summing numeric values of expanded values (for string fields the sum of
6921 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6923 For example, the following
6924 computes the same number as the System V sum program:
6928 unpack("%32W*",<>) % 65535;
6931 The following efficiently counts the number of set bits in a bit vector:
6933 $setbits = unpack("%32b*", $selectmask);
6935 The C<p> and C<P> formats should be used with care. Since Perl
6936 has no way of checking whether the value passed to C<unpack()>
6937 corresponds to a valid memory location, passing a pointer value that's
6938 not known to be valid is likely to have disastrous consequences.
6940 If there are more pack codes or if the repeat count of a field or a group
6941 is larger than what the remainder of the input string allows, the result
6942 is not well defined: in some cases, the repeat count is decreased, or
6943 C<unpack()> will produce null strings or zeroes, or terminate with an
6944 error. If the input string is longer than one described by the TEMPLATE,
6945 the rest is ignored.
6947 See L</pack> for more examples and notes.
6949 =item untie VARIABLE
6952 Breaks the binding between a variable and a package. (See C<tie>.)
6953 Has no effect if the variable is not tied.
6955 =item unshift ARRAY,LIST
6958 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6959 depending on how you look at it. Prepends list to the front of the
6960 array, and returns the new number of elements in the array.
6962 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6964 Note the LIST is prepended whole, not one element at a time, so the
6965 prepended elements stay in the same order. Use C<reverse> to do the
6968 =item use Module VERSION LIST
6969 X<use> X<module> X<import>
6971 =item use Module VERSION
6973 =item use Module LIST
6979 Imports some semantics into the current package from the named module,
6980 generally by aliasing certain subroutine or variable names into your
6981 package. It is exactly equivalent to
6983 BEGIN { require Module; Module->import( LIST ); }
6985 except that Module I<must> be a bareword.
6987 In the peculiar C<use VERSION> form, VERSION may be either a numeric
6988 argument such as 5.006, which will be compared to C<$]>, or a literal of
6989 the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A
6990 fatal error is produced if VERSION is greater than the version of the
6991 current Perl interpreter; Perl will not attempt to parse the rest of the
6992 file. Compare with L</require>, which can do a similar check at run time.
6993 Symmetrically, C<no VERSION> allows you to specify that you want a version
6994 of perl older than the specified one.
6996 Specifying VERSION as a literal of the form v5.6.1 should generally be
6997 avoided, because it leads to misleading error messages under earlier
6998 versions of Perl (that is, prior to 5.6.0) that do not support this
6999 syntax. The equivalent numeric version should be used instead.
7001 use v5.6.1; # compile time version check
7003 use 5.006_001; # ditto; preferred for backwards compatibility
7005 This is often useful if you need to check the current Perl version before
7006 C<use>ing library modules that won't work with older versions of Perl.
7007 (We try not to do this more than we have to.)
7009 Also, if the specified perl version is greater than or equal to 5.9.5,
7010 C<use VERSION> will also load the C<feature> pragma and enable all
7011 features available in the requested version. See L<feature>.
7012 Similarly, if the specified perl version is greater than or equal to
7013 5.11.0, strictures are enabled lexically as with C<use strict> (except
7014 that the F<strict.pm> file is not actually loaded).
7016 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
7017 C<require> makes sure the module is loaded into memory if it hasn't been
7018 yet. The C<import> is not a builtin--it's just an ordinary static method
7019 call into the C<Module> package to tell the module to import the list of
7020 features back into the current package. The module can implement its
7021 C<import> method any way it likes, though most modules just choose to
7022 derive their C<import> method via inheritance from the C<Exporter> class that
7023 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
7024 method can be found then the call is skipped, even if there is an AUTOLOAD
7027 If you do not want to call the package's C<import> method (for instance,
7028 to stop your namespace from being altered), explicitly supply the empty list:
7032 That is exactly equivalent to
7034 BEGIN { require Module }
7036 If the VERSION argument is present between Module and LIST, then the
7037 C<use> will call the VERSION method in class Module with the given
7038 version as an argument. The default VERSION method, inherited from
7039 the UNIVERSAL class, croaks if the given version is larger than the
7040 value of the variable C<$Module::VERSION>.
7042 Again, there is a distinction between omitting LIST (C<import> called
7043 with no arguments) and an explicit empty LIST C<()> (C<import> not
7044 called). Note that there is no comma after VERSION!
7046 Because this is a wide-open interface, pragmas (compiler directives)
7047 are also implemented this way. Currently implemented pragmas are:
7052 use sigtrap qw(SEGV BUS);
7053 use strict qw(subs vars refs);
7054 use subs qw(afunc blurfl);
7055 use warnings qw(all);
7056 use sort qw(stable _quicksort _mergesort);
7058 Some of these pseudo-modules import semantics into the current
7059 block scope (like C<strict> or C<integer>, unlike ordinary modules,
7060 which import symbols into the current package (which are effective
7061 through the end of the file).
7063 There's a corresponding C<no> command that unimports meanings imported
7064 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
7065 It behaves exactly as C<import> does with respect to VERSION, an
7066 omitted LIST, empty LIST, or no unimport method being found.
7072 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
7073 for the C<-M> and C<-m> command-line options to perl that give C<use>
7074 functionality from the command-line.
7079 Changes the access and modification times on each file of a list of
7080 files. The first two elements of the list must be the NUMERICAL access
7081 and modification times, in that order. Returns the number of files
7082 successfully changed. The inode change time of each file is set
7083 to the current time. For example, this code has the same effect as the
7084 Unix touch(1) command when the files I<already exist> and belong to
7085 the user running the program:
7088 $atime = $mtime = time;
7089 utime $atime, $mtime, @ARGV;
7091 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
7092 the utime(2) function in the C library will be called with a null second
7093 argument. On most systems, this will set the file's access and
7094 modification times to the current time (i.e. equivalent to the example
7095 above) and will even work on other users' files where you have write
7098 utime undef, undef, @ARGV;
7100 Under NFS this will use the time of the NFS server, not the time of
7101 the local machine. If there is a time synchronization problem, the
7102 NFS server and local machine will have different times. The Unix
7103 touch(1) command will in fact normally use this form instead of the
7104 one shown in the first example.
7106 Note that only passing one of the first two elements as C<undef> will
7107 be equivalent of passing it as 0 and will not have the same effect as
7108 described when they are both C<undef>. This case will also trigger an
7109 uninitialized warning.
7111 On systems that support futimes, you might pass file handles among the
7112 files. On systems that don't support futimes, passing file handles
7113 produces a fatal error at run time. The file handles must be passed
7114 as globs or references to be recognized. Barewords are considered
7122 Returns a list consisting of all the values of the named hash, or the values
7123 of an array. (In a scalar context, returns the number of values.)
7125 The values are returned in an apparently random order. The actual
7126 random order is subject to change in future versions of perl, but it
7127 is guaranteed to be the same order as either the C<keys> or C<each>
7128 function would produce on the same (unmodified) hash. Since Perl
7129 5.8.1 the ordering is different even between different runs of Perl
7130 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7132 As a side effect, calling values() resets the HASH or ARRAY's internal
7134 see L</each>. (In particular, calling values() in void context resets
7135 the iterator with no other overhead. Apart from resetting the iterator,
7136 C<values @array> in list context is no different to plain C<@array>.
7137 We recommend that you use void context C<keys @array> for this, but reasoned
7138 that it taking C<values @array> out would require more documentation than
7142 Note that the values are not copied, which means modifying them will
7143 modify the contents of the hash:
7145 for (values %hash) { s/foo/bar/g } # modifies %hash values
7146 for (@hash{keys %hash}) { s/foo/bar/g } # same
7148 See also C<keys>, C<each>, and C<sort>.
7150 =item vec EXPR,OFFSET,BITS
7151 X<vec> X<bit> X<bit vector>
7153 Treats the string in EXPR as a bit vector made up of elements of
7154 width BITS, and returns the value of the element specified by OFFSET
7155 as an unsigned integer. BITS therefore specifies the number of bits
7156 that are reserved for each element in the bit vector. This must
7157 be a power of two from 1 to 32 (or 64, if your platform supports
7160 If BITS is 8, "elements" coincide with bytes of the input string.
7162 If BITS is 16 or more, bytes of the input string are grouped into chunks
7163 of size BITS/8, and each group is converted to a number as with
7164 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7165 for BITS==64). See L<"pack"> for details.
7167 If bits is 4 or less, the string is broken into bytes, then the bits
7168 of each byte are broken into 8/BITS groups. Bits of a byte are
7169 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7170 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7171 breaking the single input byte C<chr(0x36)> into two groups gives a list
7172 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7174 C<vec> may also be assigned to, in which case parentheses are needed
7175 to give the expression the correct precedence as in
7177 vec($image, $max_x * $x + $y, 8) = 3;
7179 If the selected element is outside the string, the value 0 is returned.
7180 If an element off the end of the string is written to, Perl will first
7181 extend the string with sufficiently many zero bytes. It is an error
7182 to try to write off the beginning of the string (i.e. negative OFFSET).
7184 If the string happens to be encoded as UTF-8 internally (and thus has
7185 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7186 internal byte string, not the conceptual character string, even if you
7187 only have characters with values less than 256.
7189 Strings created with C<vec> can also be manipulated with the logical
7190 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7191 vector operation is desired when both operands are strings.
7192 See L<perlop/"Bitwise String Operators">.
7194 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7195 The comments show the string after each step. Note that this code works
7196 in the same way on big-endian or little-endian machines.
7199 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7201 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7202 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7204 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7205 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7206 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7207 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7208 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7209 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7211 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7212 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7213 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7216 To transform a bit vector into a string or list of 0's and 1's, use these:
7218 $bits = unpack("b*", $vector);
7219 @bits = split(//, unpack("b*", $vector));
7221 If you know the exact length in bits, it can be used in place of the C<*>.
7223 Here is an example to illustrate how the bits actually fall in place:
7229 unpack("V",$_) 01234567890123456789012345678901
7230 ------------------------------------------------------------------
7235 for ($shift=0; $shift < $width; ++$shift) {
7236 for ($off=0; $off < 32/$width; ++$off) {
7237 $str = pack("B*", "0"x32);
7238 $bits = (1<<$shift);
7239 vec($str, $off, $width) = $bits;
7240 $res = unpack("b*",$str);
7241 $val = unpack("V", $str);
7248 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7249 $off, $width, $bits, $val, $res
7253 Regardless of the machine architecture on which it is run, the above
7254 example should print the following table:
7257 unpack("V",$_) 01234567890123456789012345678901
7258 ------------------------------------------------------------------
7259 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7260 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7261 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7262 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7263 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7264 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7265 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7266 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7267 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7268 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7269 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7270 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7271 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7272 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7273 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7274 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7275 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7276 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7277 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7278 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7279 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7280 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7281 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7282 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7283 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7284 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7285 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7286 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7287 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7288 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7289 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7290 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7291 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7292 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7293 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7294 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7295 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7296 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7297 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7298 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7299 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7300 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7301 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7302 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7303 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7304 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7305 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7306 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7307 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7308 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7309 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7310 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7311 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7312 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7313 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7314 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7315 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7316 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7317 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7318 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7319 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7320 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7321 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7322 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7323 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7324 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7325 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7326 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7327 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7328 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7329 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7330 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7331 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7332 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7333 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7334 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7335 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7336 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7337 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7338 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7339 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7340 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7341 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7342 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7343 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7344 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7345 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7346 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7347 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7348 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7349 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7350 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7351 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7352 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7353 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7354 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7355 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7356 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7357 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7358 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7359 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7360 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7361 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7362 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7363 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7364 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7365 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7366 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7367 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7368 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7369 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7370 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7371 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7372 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7373 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7374 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7375 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7376 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7377 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7378 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7379 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7380 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7381 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7382 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7383 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7384 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7385 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7386 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7391 Behaves like the wait(2) system call on your system: it waits for a child
7392 process to terminate and returns the pid of the deceased process, or
7393 C<-1> if there are no child processes. The status is returned in C<$?>
7394 and C<${^CHILD_ERROR_NATIVE}>.
7395 Note that a return value of C<-1> could mean that child processes are
7396 being automatically reaped, as described in L<perlipc>.
7398 =item waitpid PID,FLAGS
7401 Waits for a particular child process to terminate and returns the pid of
7402 the deceased process, or C<-1> if there is no such child process. On some
7403 systems, a value of 0 indicates that there are processes still running.
7404 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
7406 use POSIX ":sys_wait_h";
7409 $kid = waitpid(-1, WNOHANG);
7412 then you can do a non-blocking wait for all pending zombie processes.
7413 Non-blocking wait is available on machines supporting either the
7414 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7415 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7416 system call by remembering the status values of processes that have
7417 exited but have not been harvested by the Perl script yet.)
7419 Note that on some systems, a return value of C<-1> could mean that child
7420 processes are being automatically reaped. See L<perlipc> for details,
7421 and for other examples.
7424 X<wantarray> X<context>
7426 Returns true if the context of the currently executing subroutine or
7427 C<eval> is looking for a list value. Returns false if the context is
7428 looking for a scalar. Returns the undefined value if the context is
7429 looking for no value (void context).
7431 return unless defined wantarray; # don't bother doing more
7432 my @a = complex_calculation();
7433 return wantarray ? @a : "@a";
7435 C<wantarray()>'s result is unspecified in the top level of a file,
7436 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7437 in a C<DESTROY> method.
7439 This function should have been named wantlist() instead.
7442 X<warn> X<warning> X<STDERR>
7444 Prints the value of LIST to STDERR. If the last element of LIST does
7445 not end in a newline, it appends the same file/line number text as C<die>
7448 If the output is empty and C<$@> already contains a value (typically from a
7449 previous eval) that value is used after appending C<"\t...caught">
7450 to C<$@>. This is useful for staying almost, but not entirely similar to
7453 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7455 No message is printed if there is a C<$SIG{__WARN__}> handler
7456 installed. It is the handler's responsibility to deal with the message
7457 as it sees fit (like, for instance, converting it into a C<die>). Most
7458 handlers must therefore make arrangements to actually display the
7459 warnings that they are not prepared to deal with, by calling C<warn>
7460 again in the handler. Note that this is quite safe and will not
7461 produce an endless loop, since C<__WARN__> hooks are not called from
7464 You will find this behavior is slightly different from that of
7465 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7466 instead call C<die> again to change it).
7468 Using a C<__WARN__> handler provides a powerful way to silence all
7469 warnings (even the so-called mandatory ones). An example:
7471 # wipe out *all* compile-time warnings
7472 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7474 my $foo = 20; # no warning about duplicate my $foo,
7475 # but hey, you asked for it!
7476 # no compile-time or run-time warnings before here
7479 # run-time warnings enabled after here
7480 warn "\$foo is alive and $foo!"; # does show up
7482 See L<perlvar> for details on setting C<%SIG> entries, and for more
7483 examples. See the Carp module for other kinds of warnings using its
7484 carp() and cluck() functions.
7486 =item write FILEHANDLE
7493 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7494 using the format associated with that file. By default the format for
7495 a file is the one having the same name as the filehandle, but the
7496 format for the current output channel (see the C<select> function) may be set
7497 explicitly by assigning the name of the format to the C<$~> variable.
7499 Top of form processing is handled automatically: if there is
7500 insufficient room on the current page for the formatted record, the
7501 page is advanced by writing a form feed, a special top-of-page format
7502 is used to format the new page header, and then the record is written.
7503 By default the top-of-page format is the name of the filehandle with
7504 "_TOP" appended, but it may be dynamically set to the format of your
7505 choice by assigning the name to the C<$^> variable while the filehandle is
7506 selected. The number of lines remaining on the current page is in
7507 variable C<$->, which can be set to C<0> to force a new page.
7509 If FILEHANDLE is unspecified, output goes to the current default output
7510 channel, which starts out as STDOUT but may be changed by the
7511 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7512 is evaluated and the resulting string is used to look up the name of
7513 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7515 Note that write is I<not> the opposite of C<read>. Unfortunately.
7519 The transliteration operator. Same as C<tr///>. See
7520 L<perlop/"Quote and Quote-like Operators">.