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, scalar arguments
18 come first and list argument follow, and there can only ever
19 be 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 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 literal 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 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. Whitespace
37 between the function and left parenthesis doesn't count, so sometimes
38 you need to be careful:
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 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 ("syscalls")
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 available only if you enable the C<"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 available only if the C<"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 Perl 4, but in Perl 5 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 $_; # ignore specials
352 The interpretation of the file permission operators C<-r>, C<-R>,
353 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
354 of the file and the uids and gids of the user. There may be other
355 reasons you can't actually read, write, or execute the file: for
356 example network filesystem access controls, ACLs (access control lists),
357 read-only filesystems, and unrecognized executable formats. Note
358 that the use of these six specific operators to verify if some operation
359 is possible is usually a mistake, because it may be open to race
362 Also note that, for the superuser on the local filesystems, the C<-r>,
363 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
364 if any execute bit is set in the mode. Scripts run by the superuser
365 may thus need to do a stat() to determine the actual mode of the file,
366 or temporarily set their effective uid to something else.
368 If you are using ACLs, there is a pragma called C<filetest> that may
369 produce more accurate results than the bare stat() mode bits.
370 When under the C<use filetest 'access'> the above-mentioned filetests
371 test whether the permission can (not) be granted using the
372 access(2) family of system calls. Also note that the C<-x> and C<-X> may
373 under this pragma return true even if there are no execute permission
374 bits set (nor any extra execute permission ACLs). This strangeness is
375 due to the underlying system calls' definitions. Note also that, due to
376 the implementation of C<use filetest 'access'>, the C<_> special
377 filehandle won't cache the results of the file tests when this pragma is
378 in effect. Read the documentation for the C<filetest> pragma for more
381 Note that C<-s/a/b/> does not do a negated substitution. Saying
382 C<-exp($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 a zero byte 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 an empty
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> 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 fancy 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 accept(2)
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<$_> is used. (On some machines,
452 unfortunately, the elapsed time may be up to one second less or more
453 than you specified because of how seconds are counted, and process
454 scheduling may delay the delivery of the signal even further.)
456 Only one timer may be counting at once. Each call disables the
457 previous timer, and an argument of C<0> may be supplied to cancel the
458 previous timer without starting a new one. The returned value is the
459 amount of time remaining on the previous timer.
461 For delays of finer granularity than one second, the Time::HiRes module
462 (from CPAN, and starting from Perl 5.8 part of the standard
463 distribution) provides ualarm(). You may also use Perl's four-argument
464 version of select() leaving the first three arguments undefined, or you
465 might be able to use the C<syscall> interface to access setitimer(2) if
466 your system supports it. See L<perlfaq8> for details.
468 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
469 C<sleep> may be internally implemented on your system with C<alarm>.
471 If you want to use C<alarm> to time out a system call you need to use an
472 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
473 fail with C<$!> 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($_[0]) }
503 The return value for C<atan2(0,0)> is implementation-defined; consult
504 your atan2(3) manpage for more information.
506 =item bind SOCKET,NAME
509 Binds a network address to a socket, just as bind(2)
510 does. Returns true if it succeeded, false otherwise. NAME should be a
511 packed address of the appropriate type for the socket. See the examples in
512 L<perlipc/"Sockets: Client/Server Communication">.
514 =item binmode FILEHANDLE, LAYER
515 X<binmode> X<binary> X<text> X<DOS> X<Windows>
517 =item binmode FILEHANDLE
519 Arranges for FILEHANDLE to be read or written in "binary" or "text"
520 mode on systems where the run-time libraries distinguish between
521 binary and text files. If FILEHANDLE is an expression, the value is
522 taken as the name of the filehandle. Returns true on success,
523 otherwise it returns C<undef> and sets C<$!> (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 filehandle.
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, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
543 Other layers that 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>, 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() normally flushes 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 operates on UTF8-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 important not only 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 C<"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 I<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<$hinthash> is a reference to a hash containing the value of C<%^H> when the
671 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
672 of this hash, as they are the actual values stored in the optree.
674 Furthermore, when called from within the DB package, caller returns more
675 detailed information: it sets the list variable C<@DB::args> to be the
676 arguments with which the subroutine was invoked.
678 Be aware that the optimizer might have optimized call frames away before
679 C<caller> had a chance to get the information. That means that C<caller(N)>
680 might not return information about the call frame you expect it to, for
681 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
682 previous time C<caller> was called.
689 =item chdir FILEHANDLE
691 =item chdir DIRHANDLE
695 Changes the working directory to EXPR, if possible. If EXPR is omitted,
696 changes to the directory specified by C<$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 on success,
700 false otherwise. See the example under C<die>.
702 On systems that support fchdir(2), you may pass a filehandle or
703 directory handle as argument. On systems that don't support fchdir(2),
704 passing handles raises an exception.
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, but 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(2), you may pass filehandles among the
723 files. On systems that don't support fchmod(2), passing filehandles raises
724 an exception. Filehandles must be passed as globs or glob references to be
725 recognized; barewords are considered filenames.
727 open(my $fh, "<", "foo");
728 my $perm = (stat $fh)[2] & 07777;
729 chmod($perm | 0600, $fh);
731 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
734 use Fcntl qw( :mode );
735 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
736 # Identical to the chmod 0755 of the example above.
739 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
745 This safer version of L</chop> removes any trailing string
746 that corresponds to the current value of C<$/> (also known as
747 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
748 number of characters removed from all its arguments. It's often used to
749 remove the newline from the end of an input record when you're worried
750 that the final record may be missing its newline. When in paragraph
751 mode (C<$/ = "">), it removes all trailing newlines from the string.
752 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
753 a reference to an integer or the like, see L<perlvar>) chomp() won't
755 If VARIABLE is omitted, it chomps C<$_>. Example:
758 chomp; # avoid \n on last field
763 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
765 You can actually chomp anything that's an lvalue, including an assignment:
768 chomp($answer = <STDIN>);
770 If you chomp a list, each element is chomped, and the total number of
771 characters removed is returned.
773 Note that parentheses are necessary when you're chomping anything
774 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
775 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
776 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
777 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
787 Chops off the last character of a string and returns the character
788 chopped. It is much more efficient than C<s/.$//s> because it neither
789 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
790 If VARIABLE is a hash, it chops the hash's values, but not its keys.
792 You can actually chop anything that's an lvalue, including an assignment.
794 If you chop a list, each element is chopped. Only the value of the
795 last C<chop> is returned.
797 Note that C<chop> returns the last character. To return all but the last
798 character, use C<substr($string, 0, -1)>.
803 X<chown> X<owner> X<user> X<group>
805 Changes the owner (and group) of a list of files. The first two
806 elements of the list must be the I<numeric> uid and gid, in that
807 order. A value of -1 in either position is interpreted by most
808 systems to leave that value unchanged. Returns the number of files
809 successfully changed.
811 $cnt = chown $uid, $gid, 'foo', 'bar';
812 chown $uid, $gid, @filenames;
814 On systems that support fchown(2), you may pass filehandles among the
815 files. On systems that don't support fchown(2), passing filehandles raises
816 an exception. Filehandles must be passed as globs or glob references to be
817 recognized; barewords are considered filenames.
819 Here's an example that looks up nonnumeric uids in the passwd file:
822 chomp($user = <STDIN>);
824 chomp($pattern = <STDIN>);
826 ($login,$pass,$uid,$gid) = getpwnam($user)
827 or die "$user not in passwd file";
829 @ary = glob($pattern); # expand filenames
830 chown $uid, $gid, @ary;
832 On most systems, you are not allowed to change the ownership of the
833 file unless you're the superuser, although you should be able to change
834 the group to any of your secondary groups. On insecure systems, these
835 restrictions may be relaxed, but this is not a portable assumption.
836 On POSIX systems, you can detect this condition this way:
838 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
839 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
842 X<chr> X<character> X<ASCII> X<Unicode>
846 Returns the character represented by that NUMBER in the character set.
847 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
848 chr(0x263a) is a Unicode smiley face.
850 Negative values give the Unicode replacement character (chr(0xfffd)),
851 except under the L<bytes> pragma, where the low eight bits of the value
852 (truncated to an integer) are used.
854 If NUMBER is omitted, uses C<$_>.
856 For the reverse, use L</ord>.
858 Note that characters from 128 to 255 (inclusive) are by default
859 internally not encoded as UTF-8 for backward compatibility reasons.
861 See L<perlunicode> for more about Unicode.
863 =item chroot FILENAME
868 This function works like the system call by the same name: it makes the
869 named directory the new root directory for all further pathnames that
870 begin with a C</> by your process and all its children. (It doesn't
871 change your current working directory, which is unaffected.) For security
872 reasons, this call is restricted to the superuser. If FILENAME is
873 omitted, does a C<chroot> to C<$_>.
875 =item close FILEHANDLE
880 Closes the file or pipe associated with the filehandle, flushes the IO
881 buffers, and closes the system file descriptor. Returns true if those
882 operations have succeeded and if no error was reported by any PerlIO
883 layer. Closes the currently selected filehandle if the argument is
886 You don't have to close FILEHANDLE if you are immediately going to do
887 another C<open> on it, because C<open> closes it for you. (See
888 C<open>.) However, an explicit C<close> on an input file resets the line
889 counter (C<$.>), while the implicit close done by C<open> does not.
891 If the filehandle came from a piped open, C<close> returns false if one of
892 the other syscalls involved fails or if its program exits with non-zero
893 status. If the only problem was that the program exited non-zero, C<$!>
894 will be set to C<0>. Closing a pipe also waits for the process executing
895 on the pipe to exit--in case you wish to look at the output of the pipe
896 afterwards--and implicitly puts the exit status value of that command into
897 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
899 Closing the read end of a pipe before the process writing to it at the
900 other end is done writing results in the writer receiving a SIGPIPE. If
901 the other end can't handle that, be sure to read all the data before
906 open(OUTPUT, '|sort >foo') # pipe to sort
907 or die "Can't start sort: $!";
908 #... # print stuff to output
909 close OUTPUT # wait for sort to finish
910 or warn $! ? "Error closing sort pipe: $!"
911 : "Exit status $? from sort";
912 open(INPUT, 'foo') # get sort's results
913 or die "Can't open 'foo' for input: $!";
915 FILEHANDLE may be an expression whose value can be used as an indirect
916 filehandle, usually the real filehandle name.
918 =item closedir DIRHANDLE
921 Closes a directory opened by C<opendir> and returns the success of that
924 =item connect SOCKET,NAME
927 Attempts to connect to a remote socket, just like connect(2).
928 Returns true if it succeeded, false otherwise. NAME should be a
929 packed address of the appropriate type for the socket. See the examples in
930 L<perlipc/"Sockets: Client/Server Communication">.
937 C<continue> is actually a flow control statement rather than a function. If
938 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
939 C<foreach>), it is always executed just before the conditional is about to
940 be evaluated again, just like the third part of a C<for> loop in C. Thus
941 it can be used to increment a loop variable, even when the loop has been
942 continued via the C<next> statement (which is similar to the C C<continue>
945 C<last>, C<next>, or C<redo> may appear within a C<continue>
946 block; C<last> and C<redo> behave as if they had been executed within
947 the main block. So will C<next>, but since it will execute a C<continue>
948 block, it may be more entertaining.
951 ### redo always comes here
954 ### next always comes here
956 # then back the top to re-check EXPR
958 ### last always comes here
960 Omitting the C<continue> section is equivalent to using an
961 empty one, logically enough, so C<next> goes directly back
962 to check the condition at the top of the loop.
964 If the C<"switch"> feature is enabled, C<continue> is also a
965 function that exits the current C<when> (or C<default>) block and
966 falls through to the next one. See L<feature> and
967 L<perlsyn/"Switch statements"> for more information.
971 X<cos> X<cosine> X<acos> X<arccosine>
975 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
976 takes cosine of C<$_>.
978 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
979 function, or use this relation:
981 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
983 =item crypt PLAINTEXT,SALT
984 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
985 X<decrypt> X<cryptography> X<passwd> X<encrypt>
987 Creates a digest string exactly like the crypt(3) function in the C
988 library (assuming that you actually have a version there that has not
989 been extirpated as a potential munition).
991 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
992 into a short string, called a digest, which is returned. The same
993 PLAINTEXT and SALT will always return the same string, but there is no
994 (known) way to get the original PLAINTEXT from the hash. Small
995 changes in the PLAINTEXT or SALT will result in large changes in the
998 There is no decrypt function. This function isn't all that useful for
999 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1000 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1001 primarily used to check if two pieces of text are the same without
1002 having to transmit or store the text itself. An example is checking
1003 if a correct password is given. The digest of the password is stored,
1004 not the password itself. The user types in a password that is
1005 crypt()'d with the same salt as the stored digest. If the two digests
1006 match the password is correct.
1008 When verifying an existing digest string you should use the digest as
1009 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1010 to create the digest is visible as part of the digest. This ensures
1011 crypt() will hash the new string with the same salt as the digest.
1012 This allows your code to work with the standard L<crypt|/crypt> and
1013 with more exotic implementations. In other words, do not assume
1014 anything about the returned string itself, or how many bytes in the
1017 Traditionally the result is a string of 13 bytes: two first bytes of
1018 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1019 the first eight bytes of PLAINTEXT mattered. But alternative
1020 hashing schemes (like MD5), higher level security schemes (like C2),
1021 and implementations on non-Unix platforms may produce different
1024 When choosing a new salt create a random two character string whose
1025 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1026 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1027 characters is just a recommendation; the characters allowed in
1028 the salt depend solely on your system's crypt library, and Perl can't
1029 restrict what salts C<crypt()> accepts.
1031 Here's an example that makes sure that whoever runs this program knows
1034 $pwd = (getpwuid($<))[1];
1036 system "stty -echo";
1038 chomp($word = <STDIN>);
1042 if (crypt($word, $pwd) ne $pwd) {
1048 Of course, typing in your own password to whoever asks you
1051 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1052 of data, not least of all because you can't get the information
1053 back. Look at the L<Digest> module for more robust algorithms.
1055 If using crypt() on a Unicode string (which I<potentially> has
1056 characters with codepoints above 255), Perl tries to make sense
1057 of the situation by trying to downgrade (a copy of the string)
1058 the string back to an eight-bit byte string before calling crypt()
1059 (on that copy). If that works, good. If not, crypt() dies with
1060 C<Wide character in crypt>.
1065 [This function has been largely superseded by the C<untie> function.]
1067 Breaks the binding between a DBM file and a hash.
1069 =item dbmopen HASH,DBNAME,MASK
1070 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1072 [This function has been largely superseded by the C<tie> function.]
1074 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1075 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1076 argument is I<not> a filehandle, even though it looks like one). DBNAME
1077 is the name of the database (without the F<.dir> or F<.pag> extension if
1078 any). If the database does not exist, it is created with protection
1079 specified by MASK (as modified by the C<umask>). If your system supports
1080 only the older DBM functions, you may make only one C<dbmopen> call in your
1081 program. In older versions of Perl, if your system had neither DBM nor
1082 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1085 If you don't have write access to the DBM file, you can only read hash
1086 variables, not set them. If you want to test whether you can write,
1087 either use file tests or try setting a dummy hash entry inside an C<eval>
1090 Note that functions such as C<keys> and C<values> may return huge lists
1091 when used on large DBM files. You may prefer to use the C<each>
1092 function to iterate over large DBM files. Example:
1094 # print out history file offsets
1095 dbmopen(%HIST,'/usr/lib/news/history',0666);
1096 while (($key,$val) = each %HIST) {
1097 print $key, ' = ', unpack('L',$val), "\n";
1101 See also L<AnyDBM_File> for a more general description of the pros and
1102 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1103 rich implementation.
1105 You can control which DBM library you use by loading that library
1106 before you call dbmopen():
1109 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1110 or die "Can't open netscape history file: $!";
1113 X<defined> X<undef> X<undefined>
1117 Returns a Boolean value telling whether EXPR has a value other than
1118 the undefined value C<undef>. If EXPR is not present, C<$_> is
1121 Many operations return C<undef> to indicate failure, end of file,
1122 system error, uninitialized variable, and other exceptional
1123 conditions. This function allows you to distinguish C<undef> from
1124 other values. (A simple Boolean test will not distinguish among
1125 C<undef>, zero, the empty string, and C<"0">, which are all equally
1126 false.) Note that since C<undef> is a valid scalar, its presence
1127 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1128 returns C<undef> when its argument is an empty array, I<or> when the
1129 element to return happens to be C<undef>.
1131 You may also use C<defined(&func)> to check whether subroutine C<&func>
1132 has ever been defined. The return value is unaffected by any forward
1133 declarations of C<&func>. A subroutine that is not defined
1134 may still be callable: its package may have an C<AUTOLOAD> method that
1135 makes it spring into existence the first time that it is called; see
1138 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1139 used to report whether memory for that aggregate has ever been
1140 allocated. This behavior may disappear in future versions of Perl.
1141 You should instead use a simple test for size:
1143 if (@an_array) { print "has array elements\n" }
1144 if (%a_hash) { print "has hash members\n" }
1146 When used on a hash element, it tells you whether the value is defined,
1147 not whether the key exists in the hash. Use L</exists> for the latter
1152 print if defined $switch{'D'};
1153 print "$val\n" while defined($val = pop(@ary));
1154 die "Can't readlink $sym: $!"
1155 unless defined($value = readlink $sym);
1156 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1157 $debugging = 0 unless defined $debugging;
1159 Note: Many folks tend to overuse C<defined>, and then are surprised to
1160 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1161 defined values. For example, if you say
1165 The pattern match succeeds and C<$1> is defined, although it
1166 matched "nothing". It didn't really fail to match anything. Rather, it
1167 matched something that happened to be zero characters long. This is all
1168 very above-board and honest. When a function returns an undefined value,
1169 it's an admission that it couldn't give you an honest answer. So you
1170 should use C<defined> only when questioning the integrity of what
1171 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1174 See also L</undef>, L</exists>, L</ref>.
1179 Given an expression that specifies an element or slice of a hash, C<delete>
1180 deletes the specified elements from that hash so that exists() on that element
1181 no longer returns true. Setting a hash element to the undefined value does
1182 not remove its key, but deleting it does; see L</exists>.
1184 It returns the value or values deleted in list context, or the last such
1185 element in scalar context. The return list's length always matches that of
1186 the argument list: deleting non-existent elements returns the undefined value
1187 in their corresponding positions.
1189 delete() may also be used on arrays and array slices, but its behavior is less
1190 straightforward. Although exists() will return false for deleted entries,
1191 deleting array elements never changes indices of existing values; use shift()
1192 or splice() for that. However, if all deleted elements fall at the end of an
1193 array, the array's size shrinks to the position of the highest element that
1194 still tests true for exists(), or to 0 if none do.
1196 B<Be aware> that calling delete on array values is deprecated and likely to
1197 be removed in a future version of Perl.
1199 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1200 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1201 or array may not necessarily return anything; it depends on the implementation
1202 of the C<tied> package's DELETE method, which may do whatever it pleases.
1204 The C<delete local EXPR> construct localizes the deletion to the current
1205 block at run time. Until the block exits, elements locally deleted
1206 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1207 of composite types">.
1209 %hash = (foo => 11, bar => 22, baz => 33);
1210 $scalar = delete $hash{foo}; # $scalar is 11
1211 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1212 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1214 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1216 foreach $key (keys %HASH) {
1220 foreach $index (0 .. $#ARRAY) {
1221 delete $ARRAY[$index];
1226 delete @HASH{keys %HASH};
1228 delete @ARRAY[0 .. $#ARRAY];
1230 But both are slower than assigning the empty list
1231 or undefining %HASH or @ARRAY, which is the customary
1232 way to empty out an aggregate:
1234 %HASH = (); # completely empty %HASH
1235 undef %HASH; # forget %HASH ever existed
1237 @ARRAY = (); # completely empty @ARRAY
1238 undef @ARRAY; # forget @ARRAY ever existed
1240 The EXPR can be arbitrarily complicated provided its
1241 final operation is an element or slice of an aggregate:
1243 delete $ref->[$x][$y]{$key};
1244 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1246 delete $ref->[$x][$y][$index];
1247 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1250 X<die> X<throw> X<exception> X<raise> X<$@> 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 You can also call C<die> with a reference argument, and if this is trapped
1302 within an C<eval>, C<$@> contains that reference. This permits more
1303 elaborate exception handling using objects that maintain arbitrary state
1304 about the exception. Such a scheme is sometimes preferable to matching
1305 particular string values of C<$@> with regular expressions. Because C<$@>
1306 is a global variable and C<eval> may be used within object implementations,
1307 be careful that analyzing the error object doesn't replace the reference in
1308 the global variable. It's easiest to make a local copy of the reference
1309 before any manipulations. Here's an example:
1311 use Scalar::Util "blessed";
1313 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1314 if (my $ev_err = $@) {
1315 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1316 # handle Some::Module::Exception
1319 # handle all other possible exceptions
1323 Because Perl stringifies uncaught exception messages before display,
1324 you'll probably want to overload stringification operations on
1325 exception objects. See L<overload> for details about that.
1327 You can arrange for a callback to be run just before the C<die>
1328 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1329 handler is called with the error text and can change the error
1330 message, if it sees fit, by calling C<die> again. See
1331 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1332 L<"eval BLOCK"> for some examples. Although this feature was
1333 to be run only right before your program was to exit, this is not
1334 currently so: the C<$SIG{__DIE__}> hook is currently called
1335 even inside eval()ed blocks/strings! If one wants the hook to do
1336 nothing in such situations, put
1340 as the first line of the handler (see L<perlvar/$^S>). Because
1341 this promotes strange action at a distance, this counterintuitive
1342 behavior may be fixed in a future release.
1347 Not really a function. Returns the value of the last command in the
1348 sequence of commands indicated by BLOCK. When modified by the C<while> or
1349 C<until> loop modifier, executes the BLOCK once before testing the loop
1350 condition. (On other statements the loop modifiers test the conditional
1353 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1354 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1355 See L<perlsyn> for alternative strategies.
1357 =item do SUBROUTINE(LIST)
1360 This form of subroutine call is deprecated. See L<perlsub>.
1365 Uses the value of EXPR as a filename and executes the contents of the
1366 file as a Perl script.
1374 except that it's more efficient and concise, keeps track of the current
1375 filename for error messages, searches the @INC directories, and updates
1376 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1377 variables. It also differs in that code evaluated with C<do FILENAME>
1378 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1379 same, however, in that it does reparse the file every time you call it,
1380 so you probably don't want to do this inside a loop.
1382 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1383 error. If C<do> can read the file but cannot compile it, it
1384 returns undef and sets an error message in C<$@>. If the file is
1385 successfully compiled, C<do> returns the value of the last expression
1388 Inclusion of library modules is better done with the
1389 C<use> and C<require> operators, which also do automatic error checking
1390 and raise an exception if there's a problem.
1392 You might like to use C<do> to read in a program configuration
1393 file. Manual error checking can be done this way:
1395 # read in config files: system first, then user
1396 for $file ("/share/prog/defaults.rc",
1397 "$ENV{HOME}/.someprogrc")
1399 unless ($return = do $file) {
1400 warn "couldn't parse $file: $@" if $@;
1401 warn "couldn't do $file: $!" unless defined $return;
1402 warn "couldn't run $file" unless $return;
1407 X<dump> X<core> X<undump>
1411 This function causes an immediate core dump. See also the B<-u>
1412 command-line switch in L<perlrun>, which does the same thing.
1413 Primarily this is so that you can use the B<undump> program (not
1414 supplied) to turn your core dump into an executable binary after
1415 having initialized all your variables at the beginning of the
1416 program. When the new binary is executed it will begin by executing
1417 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1418 Think of it as a goto with an intervening core dump and reincarnation.
1419 If C<LABEL> is omitted, restarts the program from the top.
1421 B<WARNING>: Any files opened at the time of the dump will I<not>
1422 be open any more when the program is reincarnated, with possible
1423 resulting confusion by Perl.
1425 This function is now largely obsolete, mostly because it's very hard to
1426 convert a core file into an executable. That's why you should now invoke
1427 it as C<CORE::dump()>, if you don't want to be warned against a possible
1431 X<each> X<hash, iterator>
1436 When called in list context, returns a 2-element list consisting of the key
1437 and value for the next element of a hash, or the index and value for the
1438 next element of an array, so that you can iterate over it. When called in
1439 scalar context, returns only the key (not the value) in a hash, or the index
1442 Hash entries are returned in an apparently random order. The actual random
1443 order is subject to change in future versions of Perl, but it is
1444 guaranteed to be in the same order as either the C<keys> or C<values>
1445 function would produce on the same (unmodified) hash. Since Perl
1446 5.8.2 the ordering can be different even between different runs of Perl
1447 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1449 After C<each> has returned all entries from the hash or array, the next
1450 call to C<each> returns the empty list in list context and C<undef> in
1451 scalar context. The next call following that one restarts iteration. Each
1452 hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1453 and C<values>. The iterator is implicitly reset when C<each> has reached
1454 the end as just described; it can be explicitly reset by calling C<keys> or
1455 C<values> on the hash or array. If you add or delete a hash's elements
1456 while iterating over it, entries may be skipped or duplicated--so don't do
1457 that. Exception: It is always safe to delete the item most recently
1458 returned by C<each()>, so the following code works properly:
1460 while (($key, $value) = each %hash) {
1462 delete $hash{$key}; # This is safe
1465 This prints out your environment like the printenv(1) program,
1466 but in a different order:
1468 while (($key,$value) = each %ENV) {
1469 print "$key=$value\n";
1472 See also C<keys>, C<values> and C<sort>.
1474 =item eof FILEHANDLE
1483 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1484 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1485 gives the real filehandle. (Note that this function actually
1486 reads a character and then C<ungetc>s it, so isn't useful in an
1487 interactive context.) Do not read from a terminal file (or call
1488 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1489 as terminals may lose the end-of-file condition if you do.
1491 An C<eof> without an argument uses the last file read. Using C<eof()>
1492 with empty parentheses is different. It refers to the pseudo file
1493 formed from the files listed on the command line and accessed via the
1494 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1495 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1496 used will cause C<@ARGV> to be examined to determine if input is
1497 available. Similarly, an C<eof()> after C<< <> >> has returned
1498 end-of-file will assume you are processing another C<@ARGV> list,
1499 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1500 see L<perlop/"I/O Operators">.
1502 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1503 detect the end of each file, C<eof()> will detect the end of only the
1504 last file. Examples:
1506 # reset line numbering on each input file
1508 next if /^\s*#/; # skip comments
1511 close ARGV if eof; # Not eof()!
1514 # insert dashes just before last line of last file
1516 if (eof()) { # check for end of last file
1517 print "--------------\n";
1520 last if eof(); # needed if we're reading from a terminal
1523 Practical hint: you almost never need to use C<eof> in Perl, because the
1524 input operators typically return C<undef> when they run out of data, or if
1528 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1529 X<error, handling> X<exception, handling>
1535 In the first form, the return value of EXPR is parsed and executed as if it
1536 were a little Perl program. The value of the expression (which is itself
1537 determined within scalar context) is first parsed, and if there weren't any
1538 errors, executed in the lexical context of the current Perl program, so
1539 that any variable settings or subroutine and format definitions remain
1540 afterwards. Note that the value is parsed every time the C<eval> executes.
1541 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1542 delay parsing and subsequent execution of the text of EXPR until run time.
1544 In the second form, the code within the BLOCK is parsed only once--at the
1545 same time the code surrounding the C<eval> itself was parsed--and executed
1546 within the context of the current Perl program. This form is typically
1547 used to trap exceptions more efficiently than the first (see below), while
1548 also providing the benefit of checking the code within BLOCK at compile
1551 The final semicolon, if any, may be omitted from the value of EXPR or within
1554 In both forms, the value returned is the value of the last expression
1555 evaluated inside the mini-program; a return statement may be also used, just
1556 as with subroutines. The expression providing the return value is evaluated
1557 in void, scalar, or list context, depending on the context of the C<eval>
1558 itself. See L</wantarray> for more on how the evaluation context can be
1561 If there is a syntax error or runtime error, or a C<die> statement is
1562 executed, C<eval> returns an undefined value in scalar context
1563 or an empty list in list context, and C<$@> is set to the
1564 error message. If there was no error, C<$@> is guaranteed to be the empty
1565 string. Beware that using C<eval> neither silences Perl from printing
1566 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1567 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1568 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1569 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1571 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1572 determining whether a particular feature (such as C<socket> or C<symlink>)
1573 is implemented. It is also Perl's exception trapping mechanism, where
1574 the die operator is used to raise exceptions.
1576 If you want to trap errors when loading an XS module, some problems with
1577 the binary interface (such as Perl version skew) may be fatal even with
1578 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1580 If the code to be executed doesn't vary, you may use the eval-BLOCK
1581 form to trap run-time errors without incurring the penalty of
1582 recompiling each time. The error, if any, is still returned in C<$@>.
1585 # make divide-by-zero nonfatal
1586 eval { $answer = $a / $b; }; warn $@ if $@;
1588 # same thing, but less efficient
1589 eval '$answer = $a / $b'; warn $@ if $@;
1591 # a compile-time error
1592 eval { $answer = }; # WRONG
1595 eval '$answer ='; # sets $@
1597 Using the C<eval{}> form as an exception trap in libraries does have some
1598 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1599 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1600 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1601 as this example shows:
1603 # a private exception trap for divide-by-zero
1604 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1607 This is especially significant, given that C<__DIE__> hooks can call
1608 C<die> again, which has the effect of changing their error messages:
1610 # __DIE__ hooks may modify error messages
1612 local $SIG{'__DIE__'} =
1613 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1614 eval { die "foo lives here" };
1615 print $@ if $@; # prints "bar lives here"
1618 Because this promotes action at a distance, this counterintuitive behavior
1619 may be fixed in a future release.
1621 With an C<eval>, you should be especially careful to remember what's
1622 being looked at when:
1628 eval { $x }; # CASE 4
1630 eval "\$$x++"; # CASE 5
1633 Cases 1 and 2 above behave identically: they run the code contained in
1634 the variable $x. (Although case 2 has misleading double quotes making
1635 the reader wonder what else might be happening (nothing is).) Cases 3
1636 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1637 does nothing but return the value of $x. (Case 4 is preferred for
1638 purely visual reasons, but it also has the advantage of compiling at
1639 compile-time instead of at run-time.) Case 5 is a place where
1640 normally you I<would> like to use double quotes, except that in this
1641 particular situation, you can just use symbolic references instead, as
1644 The assignment to C<$@> occurs before restoration of localised variables,
1645 which means a temporary is required if you want to mask some but not all
1648 # alter $@ on nefarious repugnancy only
1652 local $@; # protect existing $@
1653 eval { test_repugnancy() };
1654 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1655 $@ =~ /nefarious/ and $e = $@;
1657 die $e if defined $e
1660 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1661 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1663 An C<eval ''> executed within the C<DB> package doesn't see the usual
1664 surrounding lexical scope, but rather the scope of the first non-DB piece
1665 of code that called it. You don't normally need to worry about this unless
1666 you are writing a Perl debugger.
1671 =item exec PROGRAM LIST
1673 The C<exec> function executes a system command I<and never returns>;
1674 use C<system> instead of C<exec> if you want it to return. It fails and
1675 returns false only if the command does not exist I<and> it is executed
1676 directly instead of via your system's command shell (see below).
1678 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1679 warns you if there is a following statement that isn't C<die>, C<warn>,
1680 or C<exit> (if C<-w> is set--but you always do that, right?). If you
1681 I<really> want to follow an C<exec> with some other statement, you
1682 can use one of these styles to avoid the warning:
1684 exec ('foo') or print STDERR "couldn't exec foo: $!";
1685 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1687 If there is more than one argument in LIST, or if LIST is an array
1688 with more than one value, calls execvp(3) with the arguments in LIST.
1689 If there is only one scalar argument or an array with one element in it,
1690 the argument is checked for shell metacharacters, and if there are any,
1691 the entire argument is passed to the system's command shell for parsing
1692 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1693 If there are no shell metacharacters in the argument, it is split into
1694 words and passed directly to C<execvp>, which is more efficient.
1697 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1698 exec "sort $outfile | uniq";
1700 If you don't really want to execute the first argument, but want to lie
1701 to the program you are executing about its own name, you can specify
1702 the program you actually want to run as an "indirect object" (without a
1703 comma) in front of the LIST. (This always forces interpretation of the
1704 LIST as a multivalued list, even if there is only a single scalar in
1707 $shell = '/bin/csh';
1708 exec $shell '-sh'; # pretend it's a login shell
1712 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1714 When the arguments get executed via the system shell, results are
1715 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1718 Using an indirect object with C<exec> or C<system> is also more
1719 secure. This usage (which also works fine with system()) forces
1720 interpretation of the arguments as a multivalued list, even if the
1721 list had just one argument. That way you're safe from the shell
1722 expanding wildcards or splitting up words with whitespace in them.
1724 @args = ( "echo surprise" );
1726 exec @args; # subject to shell escapes
1728 exec { $args[0] } @args; # safe even with one-arg list
1730 The first version, the one without the indirect object, ran the I<echo>
1731 program, passing it C<"surprise"> an argument. The second version didn't;
1732 it tried to run a program named I<"echo surprise">, didn't find it, and set
1733 C<$?> to a non-zero value indicating failure.
1735 Beginning with v5.6.0, Perl attempts to flush all files opened for
1736 output before the exec, but this may not be supported on some platforms
1737 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1738 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1739 open handles to avoid lost output.
1741 Note that C<exec> will not call your C<END> blocks, nor will it invoke
1742 C<DESTROY> methods on your objects.
1745 X<exists> X<autovivification>
1747 Given an expression that specifies an element of a hash, returns true if the
1748 specified element in the hash has ever been initialized, even if the
1749 corresponding value is undefined.
1751 print "Exists\n" if exists $hash{$key};
1752 print "Defined\n" if defined $hash{$key};
1753 print "True\n" if $hash{$key};
1755 exists may also be called on array elements, but its behavior is much less
1756 obvious, and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1757 that calling exists on array values is deprecated and likely to be removed in
1758 a future version of Perl.
1760 print "Exists\n" if exists $array[$index];
1761 print "Defined\n" if defined $array[$index];
1762 print "True\n" if $array[$index];
1764 A hash or array element can be true only if it's defined, and defined if
1765 it exists, but the reverse doesn't necessarily hold true.
1767 Given an expression that specifies the name of a subroutine,
1768 returns true if the specified subroutine has ever been declared, even
1769 if it is undefined. Mentioning a subroutine name for exists or defined
1770 does not count as declaring it. Note that a subroutine that does not
1771 exist may still be callable: its package may have an C<AUTOLOAD>
1772 method that makes it spring into existence the first time that it is
1773 called; see L<perlsub>.
1775 print "Exists\n" if exists &subroutine;
1776 print "Defined\n" if defined &subroutine;
1778 Note that the EXPR can be arbitrarily complicated as long as the final
1779 operation is a hash or array key lookup or subroutine name:
1781 if (exists $ref->{A}->{B}->{$key}) { }
1782 if (exists $hash{A}{B}{$key}) { }
1784 if (exists $ref->{A}->{B}->[$ix]) { }
1785 if (exists $hash{A}{B}[$ix]) { }
1787 if (exists &{$ref->{A}{B}{$key}}) { }
1789 Although the mostly deeply nested array or hash will not spring into
1790 existence just because its existence was tested, any intervening ones will.
1791 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1792 into existence due to the existence test for the $key element above.
1793 This happens anywhere the arrow operator is used, including even here:
1796 if (exists $ref->{"Some key"}) { }
1797 print $ref; # prints HASH(0x80d3d5c)
1799 This surprising autovivification in what does not at first--or even
1800 second--glance appear to be an lvalue context may be fixed in a future
1803 Use of a subroutine call, rather than a subroutine name, as an argument
1804 to exists() is an error.
1807 exists &sub(); # Error
1810 X<exit> X<terminate> X<abort>
1814 Evaluates EXPR and exits immediately with that value. Example:
1817 exit 0 if $ans =~ /^[Xx]/;
1819 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1820 universally recognized values for EXPR are C<0> for success and C<1>
1821 for error; other values are subject to interpretation depending on the
1822 environment in which the Perl program is running. For example, exiting
1823 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1824 the mailer to return the item undelivered, but that's not true everywhere.
1826 Don't use C<exit> to abort a subroutine if there's any chance that
1827 someone might want to trap whatever error happened. Use C<die> instead,
1828 which can be trapped by an C<eval>.
1830 The exit() function does not always exit immediately. It calls any
1831 defined C<END> routines first, but these C<END> routines may not
1832 themselves abort the exit. Likewise any object destructors that need to
1833 be called are called before the real exit. If this is a problem, you
1834 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1835 See L<perlmod> for details.
1838 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1842 Returns I<e> (the natural logarithm base) to the power of EXPR.
1843 If EXPR is omitted, gives C<exp($_)>.
1845 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1848 Implements the fcntl(2) function. You'll probably have to say
1852 first to get the correct constant definitions. Argument processing and
1853 value returned work just like C<ioctl> below.
1857 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1858 or die "can't fcntl F_GETFL: $!";
1860 You don't have to check for C<defined> on the return from C<fcntl>.
1861 Like C<ioctl>, it maps a C<0> return from the system call into
1862 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1863 in numeric context. It is also exempt from the normal B<-w> warnings
1864 on improper numeric conversions.
1866 Note that C<fcntl> raises an exception if used on a machine that
1867 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1868 manpage to learn what functions are available on your system.
1870 Here's an example of setting a filehandle named C<REMOTE> to be
1871 non-blocking at the system level. You'll have to negotiate C<$|>
1872 on your own, though.
1874 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1876 $flags = fcntl(REMOTE, F_GETFL, 0)
1877 or die "Can't get flags for the socket: $!\n";
1879 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1880 or die "Can't set flags for the socket: $!\n";
1882 =item fileno FILEHANDLE
1885 Returns the file descriptor for a filehandle, or undefined if the
1886 filehandle is not open. This is mainly useful for constructing
1887 bitmaps for C<select> and low-level POSIX tty-handling operations.
1888 If FILEHANDLE is an expression, the value is taken as an indirect
1889 filehandle, generally its name.
1891 You can use this to find out whether two handles refer to the
1892 same underlying descriptor:
1894 if (fileno(THIS) == fileno(THAT)) {
1895 print "THIS and THAT are dups\n";
1898 (Filehandles connected to memory objects via new features of C<open> may
1899 return undefined even though they are open.)
1902 =item flock FILEHANDLE,OPERATION
1903 X<flock> X<lock> X<locking>
1905 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1906 for success, false on failure. Produces a fatal error if used on a
1907 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1908 C<flock> is Perl's portable file locking interface, although it locks
1909 entire files only, not records.
1911 Two potentially non-obvious but traditional C<flock> semantics are
1912 that it waits indefinitely until the lock is granted, and that its locks
1913 B<merely advisory>. Such discretionary locks are more flexible, but offer
1914 fewer guarantees. This means that programs that do not also use C<flock>
1915 may modify files locked with C<flock>. See L<perlport>,
1916 your port's specific documentation, or your system-specific local manpages
1917 for details. It's best to assume traditional behavior if you're writing
1918 portable programs. (But if you're not, you should as always feel perfectly
1919 free to write for your own system's idiosyncrasies (sometimes called
1920 "features"). Slavish adherence to portability concerns shouldn't get
1921 in the way of your getting your job done.)
1923 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1924 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1925 you can use the symbolic names if you import them from the Fcntl module,
1926 either individually, or as a group using the ':flock' tag. LOCK_SH
1927 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1928 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1929 LOCK_SH or LOCK_EX then C<flock> returns immediately rather than blocking
1930 waiting for the lock; check the return status to see if you got it.
1932 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1933 before locking or unlocking it.
1935 Note that the emulation built with lockf(3) doesn't provide shared
1936 locks, and it requires that FILEHANDLE be open with write intent. These
1937 are the semantics that lockf(3) implements. Most if not all systems
1938 implement lockf(3) in terms of fcntl(2) locking, though, so the
1939 differing semantics shouldn't bite too many people.
1941 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1942 be open with read intent to use LOCK_SH and requires that it be open
1943 with write intent to use LOCK_EX.
1945 Note also that some versions of C<flock> cannot lock things over the
1946 network; you would need to use the more system-specific C<fcntl> for
1947 that. If you like you can force Perl to ignore your system's flock(2)
1948 function, and so provide its own fcntl(2)-based emulation, by passing
1949 the switch C<-Ud_flock> to the F<Configure> program when you configure
1952 Here's a mailbox appender for BSD systems.
1954 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
1958 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
1960 # and, in case someone appended while we were waiting...
1961 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
1966 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
1969 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
1970 or die "Can't open mailbox: $!";
1973 print $mbox $msg,"\n\n";
1976 On systems that support a real flock(2), locks are inherited across fork()
1977 calls, whereas those that must resort to the more capricious fcntl(2)
1978 function lose their locks, making it seriously harder to write servers.
1980 See also L<DB_File> for other flock() examples.
1983 X<fork> X<child> X<parent>
1985 Does a fork(2) system call to create a new process running the
1986 same program at the same point. It returns the child pid to the
1987 parent process, C<0> to the child process, or C<undef> if the fork is
1988 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1989 are shared, while everything else is copied. On most systems supporting
1990 fork(), great care has gone into making it extremely efficient (for
1991 example, using copy-on-write technology on data pages), making it the
1992 dominant paradigm for multitasking over the last few decades.
1994 Beginning with v5.6.0, Perl attempts to flush all files opened for
1995 output before forking the child process, but this may not be supported
1996 on some platforms (see L<perlport>). To be safe, you may need to set
1997 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1998 C<IO::Handle> on any open handles to avoid duplicate output.
2000 If you C<fork> without ever waiting on your children, you will
2001 accumulate zombies. On some systems, you can avoid this by setting
2002 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2003 forking and reaping moribund children.
2005 Note that if your forked child inherits system file descriptors like
2006 STDIN and STDOUT that are actually connected by a pipe or socket, even
2007 if you exit, then the remote server (such as, say, a CGI script or a
2008 backgrounded job launched from a remote shell) won't think you're done.
2009 You should reopen those to F</dev/null> if it's any issue.
2014 Declare a picture format for use by the C<write> function. For
2018 Test: @<<<<<<<< @||||| @>>>>>
2019 $str, $%, '$' . int($num)
2023 $num = $cost/$quantity;
2027 See L<perlform> for many details and examples.
2029 =item formline PICTURE,LIST
2032 This is an internal function used by C<format>s, though you may call it,
2033 too. It formats (see L<perlform>) a list of values according to the
2034 contents of PICTURE, placing the output into the format output
2035 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2036 Eventually, when a C<write> is done, the contents of
2037 C<$^A> are written to some filehandle. You could also read C<$^A>
2038 and then set C<$^A> back to C<"">. Note that a format typically
2039 does one C<formline> per line of form, but the C<formline> function itself
2040 doesn't care how many newlines are embedded in the PICTURE. This means
2041 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2042 You may therefore need to use multiple formlines to implement a single
2043 record format, just like the C<format> compiler.
2045 Be careful if you put double quotes around the picture, because an C<@>
2046 character may be taken to mean the beginning of an array name.
2047 C<formline> always returns true. See L<perlform> for other examples.
2049 =item getc FILEHANDLE
2050 X<getc> X<getchar> X<character> X<file, read>
2054 Returns the next character from the input file attached to FILEHANDLE,
2055 or the undefined value at end of file or if there was an error (in
2056 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2057 STDIN. This is not particularly efficient. However, it cannot be
2058 used by itself to fetch single characters without waiting for the user
2059 to hit enter. For that, try something more like:
2062 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2065 system "stty", '-icanon', 'eol', "\001";
2071 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2074 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2078 Determination of whether $BSD_STYLE should be set
2079 is left as an exercise to the reader.
2081 The C<POSIX::getattr> function can do this more portably on
2082 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2083 module from your nearest CPAN site; details on CPAN can be found on
2087 X<getlogin> X<login>
2089 This implements the C library function of the same name, which on most
2090 systems returns the current login from F</etc/utmp>, if any. If it
2091 returns the empty string, use C<getpwuid>.
2093 $login = getlogin || getpwuid($<) || "Kilroy";
2095 Do not consider C<getlogin> for authentication: it is not as
2096 secure as C<getpwuid>.
2098 =item getpeername SOCKET
2099 X<getpeername> X<peer>
2101 Returns the packed sockaddr address of other end of the SOCKET connection.
2104 $hersockaddr = getpeername(SOCK);
2105 ($port, $iaddr) = sockaddr_in($hersockaddr);
2106 $herhostname = gethostbyaddr($iaddr, AF_INET);
2107 $herstraddr = inet_ntoa($iaddr);
2112 Returns the current process group for the specified PID. Use
2113 a PID of C<0> to get the current process group for the
2114 current process. Will raise an exception if used on a machine that
2115 doesn't implement getpgrp(2). If PID is omitted, returns process
2116 group of current process. Note that the POSIX version of C<getpgrp>
2117 does not accept a PID argument, so only C<PID==0> is truly portable.
2120 X<getppid> X<parent> X<pid>
2122 Returns the process id of the parent process.
2124 Note for Linux users: on Linux, the C functions C<getpid()> and
2125 C<getppid()> return different values from different threads. In order to
2126 be portable, this behavior is not reflected by the Perl-level function
2127 C<getppid()>, that returns a consistent value across threads. If you want
2128 to call the underlying C<getppid()>, you may use the CPAN module
2131 =item getpriority WHICH,WHO
2132 X<getpriority> X<priority> X<nice>
2134 Returns the current priority for a process, a process group, or a user.
2135 (See C<getpriority(2)>.) Will raise a fatal exception if used on a
2136 machine that doesn't implement getpriority(2).
2139 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2140 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2141 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2142 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2143 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2144 X<endnetent> X<endprotoent> X<endservent>
2148 =item gethostbyname NAME
2150 =item getnetbyname NAME
2152 =item getprotobyname NAME
2158 =item getservbyname NAME,PROTO
2160 =item gethostbyaddr ADDR,ADDRTYPE
2162 =item getnetbyaddr ADDR,ADDRTYPE
2164 =item getprotobynumber NUMBER
2166 =item getservbyport PORT,PROTO
2184 =item sethostent STAYOPEN
2186 =item setnetent STAYOPEN
2188 =item setprotoent STAYOPEN
2190 =item setservent STAYOPEN
2204 These routines are the same as their counterparts in the
2205 system C library. In list context, the return values from the
2206 various get routines are as follows:
2208 ($name,$passwd,$uid,$gid,
2209 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2210 ($name,$passwd,$gid,$members) = getgr*
2211 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2212 ($name,$aliases,$addrtype,$net) = getnet*
2213 ($name,$aliases,$proto) = getproto*
2214 ($name,$aliases,$port,$proto) = getserv*
2216 (If the entry doesn't exist you get an empty list.)
2218 The exact meaning of the $gcos field varies but it usually contains
2219 the real name of the user (as opposed to the login name) and other
2220 information pertaining to the user. Beware, however, that in many
2221 system users are able to change this information and therefore it
2222 cannot be trusted and therefore the $gcos is tainted (see
2223 L<perlsec>). The $passwd and $shell, user's encrypted password and
2224 login shell, are also tainted, because of the same reason.
2226 In scalar context, you get the name, unless the function was a
2227 lookup by name, in which case you get the other thing, whatever it is.
2228 (If the entry doesn't exist you get the undefined value.) For example:
2230 $uid = getpwnam($name);
2231 $name = getpwuid($num);
2233 $gid = getgrnam($name);
2234 $name = getgrgid($num);
2238 In I<getpw*()> the fields $quota, $comment, and $expire are special
2239 in that they are unsupported on many systems. If the
2240 $quota is unsupported, it is an empty scalar. If it is supported, it
2241 usually encodes the disk quota. If the $comment field is unsupported,
2242 it is an empty scalar. If it is supported it usually encodes some
2243 administrative comment about the user. In some systems the $quota
2244 field may be $change or $age, fields that have to do with password
2245 aging. In some systems the $comment field may be $class. The $expire
2246 field, if present, encodes the expiration period of the account or the
2247 password. For the availability and the exact meaning of these fields
2248 in your system, please consult your getpwnam(3) documentation and your
2249 F<pwd.h> file. You can also find out from within Perl what your
2250 $quota and $comment fields mean and whether you have the $expire field
2251 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2252 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2253 files are supported only if your vendor has implemented them in the
2254 intuitive fashion that calling the regular C library routines gets the
2255 shadow versions if you're running under privilege or if there exists
2256 the shadow(3) functions as found in System V (this includes Solaris
2257 and Linux.) Those systems that implement a proprietary shadow password
2258 facility are unlikely to be supported.
2260 The $members value returned by I<getgr*()> is a space separated list of
2261 the login names of the members of the group.
2263 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2264 C, it will be returned to you via C<$?> if the function call fails. The
2265 C<@addrs> value returned by a successful call is a list of raw
2266 addresses returned by the corresponding library call. In the
2267 Internet domain, each address is four bytes long; you can unpack it
2268 by saying something like:
2270 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2272 The Socket library makes this slightly easier:
2275 $iaddr = inet_aton("127.1"); # or whatever address
2276 $name = gethostbyaddr($iaddr, AF_INET);
2278 # or going the other way
2279 $straddr = inet_ntoa($iaddr);
2281 In the opposite way, to resolve a hostname to the IP address
2285 $packed_ip = gethostbyname("www.perl.org");
2286 if (defined $packed_ip) {
2287 $ip_address = inet_ntoa($packed_ip);
2290 Make sure <gethostbyname()> is called in SCALAR context and that
2291 its return value is checked for definedness.
2293 If you get tired of remembering which element of the return list
2294 contains which return value, by-name interfaces are provided
2295 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2296 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2297 and C<User::grent>. These override the normal built-ins, supplying
2298 versions that return objects with the appropriate names
2299 for each field. For example:
2303 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2305 Even though it looks like they're the same method calls (uid),
2306 they aren't, because a C<File::stat> object is different from
2307 a C<User::pwent> object.
2309 =item getsockname SOCKET
2312 Returns the packed sockaddr address of this end of the SOCKET connection,
2313 in case you don't know the address because you have several different
2314 IPs that the connection might have come in on.
2317 $mysockaddr = getsockname(SOCK);
2318 ($port, $myaddr) = sockaddr_in($mysockaddr);
2319 printf "Connect to %s [%s]\n",
2320 scalar gethostbyaddr($myaddr, AF_INET),
2323 =item getsockopt SOCKET,LEVEL,OPTNAME
2326 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2327 Options may exist at multiple protocol levels depending on the socket
2328 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2329 C<Socket> module) will exist. To query options at another level the
2330 protocol number of the appropriate protocol controlling the option
2331 should be supplied. For example, to indicate that an option is to be
2332 interpreted by the TCP protocol, LEVEL should be set to the protocol
2333 number of TCP, which you can get using C<getprotobyname>.
2335 The function returns a packed string representing the requested socket
2336 option, or C<undef> on error, with the reason for the error placed in
2337 C<$!>). Just what is in the packed string depends on LEVEL and OPTNAME;
2338 consult getsockopt(2) for details. A common case is that the option is an
2339 integer, in which case the result is a packed integer, which you can decode
2340 using C<unpack> with the C<i> (or C<I>) format.
2342 An example to test whether Nagle's algorithm is turned on on a socket:
2344 use Socket qw(:all);
2346 defined(my $tcp = getprotobyname("tcp"))
2347 or die "Could not determine the protocol number for tcp";
2348 # my $tcp = IPPROTO_TCP; # Alternative
2349 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2350 or die "getsockopt TCP_NODELAY: $!";
2351 my $nodelay = unpack("I", $packed);
2352 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2356 X<glob> X<wildcard> X<filename, expansion> X<expand>
2360 In list context, returns a (possibly empty) list of filename expansions on
2361 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2362 scalar context, glob iterates through such filename expansions, returning
2363 undef when the list is exhausted. This is the internal function
2364 implementing the C<< <*.c> >> operator, but you can use it directly. If
2365 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2366 more detail in L<perlop/"I/O Operators">.
2368 Note that C<glob> splits its arguments on whitespace and treats
2369 each segment as separate pattern. As such, C<glob("*.c *.h")>
2370 matches all files with a F<.c> or F<.h> extension. The expression
2371 C<glob(".* *")> matchs all files in the current working directory.
2373 If non-empty braces are the only wildcard characters used in the
2374 C<glob>, no filenames are matched, but potentially many strings
2375 are returned. For example, this produces nine strings, one for
2376 each pairing of fruits and colors:
2378 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2380 Beginning with v5.6.0, this operator is implemented using the standard
2381 C<File::Glob> extension. See L<File::Glob> for details, including
2382 C<bsd_glob> which does not treat whitespace as a pattern separator.
2385 X<gmtime> X<UTC> X<Greenwich>
2389 Works just like L<localtime> but the returned values are
2390 localized for the standard Greenwich time zone.
2392 Note: when called in list context, $isdst, the last value
2393 returned by gmtime is always C<0>. There is no
2394 Daylight Saving Time in GMT.
2396 See L<perlport/gmtime> for portability concerns.
2399 X<goto> X<jump> X<jmp>
2405 The C<goto-LABEL> form finds the statement labeled with LABEL and
2406 resumes execution there. It can't be used to get out of a block or
2407 subroutine given to C<sort>. It can be used to go almost anywhere
2408 else within the dynamic scope, including out of subroutines, but it's
2409 usually better to use some other construct such as C<last> or C<die>.
2410 The author of Perl has never felt the need to use this form of C<goto>
2411 (in Perl, that is; C is another matter). (The difference is that C
2412 does not offer named loops combined with loop control. Perl does, and
2413 this replaces most structured uses of C<goto> in other languages.)
2415 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2416 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2417 necessarily recommended if you're optimizing for maintainability:
2419 goto ("FOO", "BAR", "GLARCH")[$i];
2421 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2422 deprecated and will issue a warning. Even then, it may not be used to
2423 go into any construct that requires initialization, such as a
2424 subroutine or a C<foreach> loop. It also can't be used to go into a
2425 construct that is optimized away.
2427 The C<goto-&NAME> form is quite different from the other forms of
2428 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2429 doesn't have the stigma associated with other gotos. Instead, it
2430 exits the current subroutine (losing any changes set by local()) and
2431 immediately calls in its place the named subroutine using the current
2432 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2433 load another subroutine and then pretend that the other subroutine had
2434 been called in the first place (except that any modifications to C<@_>
2435 in the current subroutine are propagated to the other subroutine.)
2436 After the C<goto>, not even C<caller> will be able to tell that this
2437 routine was called first.
2439 NAME needn't be the name of a subroutine; it can be a scalar variable
2440 containing a code reference, or a block that evaluates to a code
2443 =item grep BLOCK LIST
2446 =item grep EXPR,LIST
2448 This is similar in spirit to, but not the same as, grep(1) and its
2449 relatives. In particular, it is not limited to using regular expressions.
2451 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2452 C<$_> to each element) and returns the list value consisting of those
2453 elements for which the expression evaluated to true. In scalar
2454 context, returns the number of times the expression was true.
2456 @foo = grep(!/^#/, @bar); # weed out comments
2460 @foo = grep {!/^#/} @bar; # weed out comments
2462 Note that C<$_> is an alias to the list value, so it can be used to
2463 modify the elements of the LIST. While this is useful and supported,
2464 it can cause bizarre results if the elements of LIST are not variables.
2465 Similarly, grep returns aliases into the original list, much as a for
2466 loop's index variable aliases the list elements. That is, modifying an
2467 element of a list returned by grep (for example, in a C<foreach>, C<map>
2468 or another C<grep>) actually modifies the element in the original list.
2469 This is usually something to be avoided when writing clear code.
2471 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2472 been declared with C<my $_>) then, in addition to being locally aliased to
2473 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2474 can't be seen from the outside, avoiding any potential side-effects.
2476 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2479 X<hex> X<hexadecimal>
2483 Interprets EXPR as a hex string and returns the corresponding value.
2484 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2485 L</oct>.) If EXPR is omitted, uses C<$_>.
2487 print hex '0xAf'; # prints '175'
2488 print hex 'aF'; # same
2490 Hex strings may only represent integers. Strings that would cause
2491 integer overflow trigger a warning. Leading whitespace is not stripped,
2492 unlike oct(). To present something as hex, look into L</printf>,
2493 L</sprintf>, or L</unpack>.
2498 There is no builtin C<import> function. It is just an ordinary
2499 method (subroutine) defined (or inherited) by modules that wish to export
2500 names to another module. The C<use> function calls the C<import> method
2501 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2503 =item index STR,SUBSTR,POSITION
2504 X<index> X<indexOf> X<InStr>
2506 =item index STR,SUBSTR
2508 The index function searches for one string within another, but without
2509 the wildcard-like behavior of a full regular-expression pattern match.
2510 It returns the position of the first occurrence of SUBSTR in STR at
2511 or after POSITION. If POSITION is omitted, starts searching from the
2512 beginning of the string. POSITION before the beginning of the string
2513 or after its end is treated as if it were the beginning or the end,
2514 respectively. POSITION and the return value are based at C<0> (or whatever
2515 you've set the C<$[> variable to--but don't do that). If the substring
2516 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2519 X<int> X<integer> X<truncate> X<trunc> X<floor>
2523 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2524 You should not use this function for rounding: one because it truncates
2525 towards C<0>, and two because machine representations of floating-point
2526 numbers can sometimes produce counterintuitive results. For example,
2527 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2528 because it's really more like -268.99999999999994315658 instead. Usually,
2529 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2530 functions will serve you better than will int().
2532 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2535 Implements the ioctl(2) function. You'll probably first have to say
2537 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2539 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2540 exist or doesn't have the correct definitions you'll have to roll your
2541 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2542 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2543 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2544 written depending on the FUNCTION; a C pointer to the string value of SCALAR
2545 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2546 has no string value but does have a numeric value, that value will be
2547 passed rather than a pointer to the string value. To guarantee this to be
2548 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2549 functions may be needed to manipulate the values of structures used by
2552 The return value of C<ioctl> (and C<fcntl>) is as follows:
2554 if OS returns: then Perl returns:
2556 0 string "0 but true"
2557 anything else that number
2559 Thus Perl returns true on success and false on failure, yet you can
2560 still easily determine the actual value returned by the operating
2563 $retval = ioctl(...) || -1;
2564 printf "System returned %d\n", $retval;
2566 The special string C<"0 but true"> is exempt from B<-w> complaints
2567 about improper numeric conversions.
2569 =item join EXPR,LIST
2572 Joins the separate strings of LIST into a single string with fields
2573 separated by the value of EXPR, and returns that new string. Example:
2575 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2577 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2578 first argument. Compare L</split>.
2585 Returns a list consisting of all the keys of the named hash, or the indices
2586 of an array. (In scalar context, returns the number of keys or indices.)
2588 The keys of a hash are returned in an apparently random order. The actual
2589 random order is subject to change in future versions of Perl, but it
2590 is guaranteed to be the same order as either the C<values> or C<each>
2591 function produces (given that the hash has not been modified). Since
2592 Perl 5.8.1 the ordering is different even between different runs of
2593 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2596 As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
2597 (see L</each>). In particular, calling keys() in void context resets
2598 the iterator with no other overhead.
2600 Here is yet another way to print your environment:
2603 @values = values %ENV;
2605 print pop(@keys), '=', pop(@values), "\n";
2608 or how about sorted by key:
2610 foreach $key (sort(keys %ENV)) {
2611 print $key, '=', $ENV{$key}, "\n";
2614 The returned values are copies of the original keys in the hash, so
2615 modifying them will not affect the original hash. Compare L</values>.
2617 To sort a hash by value, you'll need to use a C<sort> function.
2618 Here's a descending numeric sort of a hash by its values:
2620 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2621 printf "%4d %s\n", $hash{$key}, $key;
2624 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
2625 allocated for the given hash. This can gain you a measure of efficiency if
2626 you know the hash is going to get big. (This is similar to pre-extending
2627 an array by assigning a larger number to $#array.) If you say
2631 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2632 in fact, since it rounds up to the next power of two. These
2633 buckets will be retained even if you do C<%hash = ()>, use C<undef
2634 %hash> if you want to free the storage while C<%hash> is still in scope.
2635 You can't shrink the number of buckets allocated for the hash using
2636 C<keys> in this way (but you needn't worry about doing this by accident,
2637 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2640 See also C<each>, C<values> and C<sort>.
2642 =item kill SIGNAL, LIST
2645 Sends a signal to a list of processes. Returns the number of
2646 processes successfully signaled (which is not necessarily the
2647 same as the number actually killed).
2649 $cnt = kill 1, $child1, $child2;
2652 If SIGNAL is zero, no signal is sent to the process, but C<kill>
2653 checks whether it's I<possible> to send a signal to it (that
2654 means, to be brief, that the process is owned by the same user, or we are
2655 the super-user). This is useful to check that a child process is still
2656 alive (even if only as a zombie) and hasn't changed its UID. See
2657 L<perlport> for notes on the portability of this construct.
2659 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2660 of processes. That means you usually want to use positive not negative signals.
2661 You may also use a signal name in quotes.
2663 The behavior of kill when a I<PROCESS> number is zero or negative depends on
2664 the operating system. For example, on POSIX-conforming systems, zero will
2665 signal the current process group and -1 will signal all processes.
2667 See L<perlipc/"Signals"> for more details.
2674 The C<last> command is like the C<break> statement in C (as used in
2675 loops); it immediately exits the loop in question. If the LABEL is
2676 omitted, the command refers to the innermost enclosing loop. The
2677 C<continue> block, if any, is not executed:
2679 LINE: while (<STDIN>) {
2680 last LINE if /^$/; # exit when done with header
2684 C<last> cannot be used to exit a block that returns a value such as
2685 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2686 a grep() or map() operation.
2688 Note that a block by itself is semantically identical to a loop
2689 that executes once. Thus C<last> can be used to effect an early
2690 exit out of such a block.
2692 See also L</continue> for an illustration of how C<last>, C<next>, and
2700 Returns a lowercased version of EXPR. This is the internal function
2701 implementing the C<\L> escape in double-quoted strings. Respects
2702 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2703 and L<perlunicode> for more details about locale and Unicode support.
2705 If EXPR is omitted, uses C<$_>.
2708 X<lcfirst> X<lowercase>
2712 Returns the value of EXPR with the first character lowercased. This
2713 is the internal function implementing the C<\l> escape in
2714 double-quoted strings. Respects current LC_CTYPE locale if C<use
2715 locale> in force. See L<perllocale> and L<perlunicode> for more
2716 details about locale and Unicode support.
2718 If EXPR is omitted, uses C<$_>.
2725 Returns the length in I<characters> of the value of EXPR. If EXPR is
2726 omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2728 This function cannot be used on an entire array or hash to find out how
2729 many elements these have. For that, use C<scalar @array> and C<scalar keys
2730 %hash>, respectively.
2732 Like all Perl character operations, length() normally deals in logical
2733 characters, not physical bytes. For how many bytes a string encoded as
2734 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2735 to C<use Encode> first). See L<Encode> and L<perlunicode>.
2737 =item link OLDFILE,NEWFILE
2740 Creates a new filename linked to the old filename. Returns true for
2741 success, false otherwise.
2743 =item listen SOCKET,QUEUESIZE
2746 Does the same thing that the listen(2) system call does. Returns true if
2747 it succeeded, false otherwise. See the example in
2748 L<perlipc/"Sockets: Client/Server Communication">.
2753 You really probably want to be using C<my> instead, because C<local> isn't
2754 what most people think of as "local". See
2755 L<perlsub/"Private Variables via my()"> for details.
2757 A local modifies the listed variables to be local to the enclosing
2758 block, file, or eval. If more than one value is listed, the list must
2759 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2760 for details, including issues with tied arrays and hashes.
2762 The C<delete local EXPR> construct can also be used to localize the deletion
2763 of array/hash elements to the current block.
2764 See L<perlsub/"Localized deletion of elements of composite types">.
2766 =item localtime EXPR
2767 X<localtime> X<ctime>
2771 Converts a time as returned by the time function to a 9-element list
2772 with the time analyzed for the local time zone. Typically used as
2776 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2779 All list elements are numeric, and come straight out of the C `struct
2780 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2781 of the specified time.
2783 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2784 the range C<0..11> with 0 indicating January and 11 indicating December.
2785 This makes it easy to get a month name from a list:
2787 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2788 print "$abbr[$mon] $mday";
2789 # $mon=9, $mday=18 gives "Oct 18"
2791 C<$year> is the number of years since 1900, not just the last two digits
2792 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2793 to get a 4-digit year is simply:
2797 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2798 to do that, would you?
2800 To get the last two digits of the year (e.g., '01' in 2001) do:
2802 $year = sprintf("%02d", $year % 100);
2804 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2805 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2806 (or C<0..365> in leap years.)
2808 C<$isdst> is true if the specified time occurs during Daylight Saving
2809 Time, false otherwise.
2811 If EXPR is omitted, C<localtime()> uses the current time (as returned
2814 In scalar context, C<localtime()> returns the ctime(3) value:
2816 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2818 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2819 instead of local time use the L</gmtime> builtin. See also the
2820 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2821 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2822 and mktime(3) functions.
2824 To get somewhat similar but locale dependent date strings, set up your
2825 locale environment variables appropriately (please see L<perllocale>) and
2828 use POSIX qw(strftime);
2829 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2830 # or for GMT formatted appropriately for your locale:
2831 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2833 Note that the C<%a> and C<%b>, the short forms of the day of the week
2834 and the month of the year, may not necessarily be three characters wide.
2836 See L<perlport/localtime> for portability concerns.
2838 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2839 by-name access mechanism to the gmtime() and localtime() functions,
2842 For a comprehensive date and time representation look at the
2843 L<DateTime> module on CPAN.
2848 This function places an advisory lock on a shared variable, or referenced
2849 object contained in I<THING> until the lock goes out of scope.
2851 lock() is a "weak keyword" : this means that if you've defined a function
2852 by this name (before any calls to it), that function will be called
2853 instead. If you are not under C<use threads::shared> this does nothing.
2854 See L<threads::shared>.
2857 X<log> X<logarithm> X<e> X<ln> X<base>
2861 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2862 returns log of C<$_>. To get the log of another base, use basic algebra:
2863 The base-N log of a number is equal to the natural log of that number
2864 divided by the natural log of N. For example:
2868 return log($n)/log(10);
2871 See also L</exp> for the inverse operation.
2878 Does the same thing as the C<stat> function (including setting the
2879 special C<_> filehandle) but stats a symbolic link instead of the file
2880 the symbolic link points to. If symbolic links are unimplemented on
2881 your system, a normal C<stat> is done. For much more detailed
2882 information, please see the documentation for C<stat>.
2884 If EXPR is omitted, stats C<$_>.
2888 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
2890 =item map BLOCK LIST
2895 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2896 C<$_> to each element) and returns the list value composed of the
2897 results of each such evaluation. In scalar context, returns the
2898 total number of elements so generated. Evaluates BLOCK or EXPR in
2899 list context, so each element of LIST may produce zero, one, or
2900 more elements in the returned value.
2902 @chars = map(chr, @nums);
2904 translates a list of numbers to the corresponding characters. And
2906 %hash = map { get_a_key_for($_) => $_ } @array;
2908 is just a funny way to write
2912 $hash{get_a_key_for($_)} = $_;
2915 Note that C<$_> is an alias to the list value, so it can be used to
2916 modify the elements of the LIST. While this is useful and supported,
2917 it can cause bizarre results if the elements of LIST are not variables.
2918 Using a regular C<foreach> loop for this purpose would be clearer in
2919 most cases. See also L</grep> for an array composed of those items of
2920 the original list for which the BLOCK or EXPR evaluates to true.
2922 If C<$_> is lexical in the scope where the C<map> appears (because it has
2923 been declared with C<my $_>), then, in addition to being locally aliased to
2924 the list elements, C<$_> keeps being lexical inside the block; that is, it
2925 can't be seen from the outside, avoiding any potential side-effects.
2927 C<{> starts both hash references and blocks, so C<map { ...> could be either
2928 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
2929 ahead for the closing C<}> it has to take a guess at which it's dealing with
2930 based on what it finds just after the C<{>. Usually it gets it right, but if it
2931 doesn't it won't realize something is wrong until it gets to the C<}> and
2932 encounters the missing (or unexpected) comma. The syntax error will be
2933 reported close to the C<}>, but you'll need to change something near the C<{>
2934 such as using a unary C<+> to give Perl some help:
2936 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
2937 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
2938 %hash = map { ("\L$_" => 1) } @array # this also works
2939 %hash = map { lc($_) => 1 } @array # as does this.
2940 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
2942 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2944 or to force an anon hash constructor use C<+{>:
2946 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
2948 to get a list of anonymous hashes each with only one entry apiece.
2950 =item mkdir FILENAME,MASK
2951 X<mkdir> X<md> X<directory, create>
2953 =item mkdir FILENAME
2957 Creates the directory specified by FILENAME, with permissions
2958 specified by MASK (as modified by C<umask>). If it succeeds it
2959 returns true, otherwise it returns false and sets C<$!> (errno).
2960 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2963 In general, it is better to create directories with permissive MASK,
2964 and let the user modify that with their C<umask>, than it is to supply
2965 a restrictive MASK and give the user no way to be more permissive.
2966 The exceptions to this rule are when the file or directory should be
2967 kept private (mail files, for instance). The perlfunc(1) entry on
2968 C<umask> discusses the choice of MASK in more detail.
2970 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2971 number of trailing slashes. Some operating and filesystems do not get
2972 this right, so Perl automatically removes all trailing slashes to keep
2975 To recursively create a directory structure, look at
2976 the C<mkpath> function of the L<File::Path> module.
2978 =item msgctl ID,CMD,ARG
2981 Calls the System V IPC function msgctl(2). You'll probably have to say
2985 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2986 then ARG must be a variable that will hold the returned C<msqid_ds>
2987 structure. Returns like C<ioctl>: the undefined value for error,
2988 C<"0 but true"> for zero, or the actual return value otherwise. See also
2989 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2991 =item msgget KEY,FLAGS
2994 Calls the System V IPC function msgget(2). Returns the message queue
2995 id, or the undefined value if there is an error. See also
2996 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2998 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3001 Calls the System V IPC function msgrcv to receive a message from
3002 message queue ID into variable VAR with a maximum message size of
3003 SIZE. Note that when a message is received, the message type as a
3004 native long integer will be the first thing in VAR, followed by the
3005 actual message. This packing may be opened with C<unpack("l! a*")>.
3006 Taints the variable. Returns true if successful, or false if there is
3007 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
3008 C<IPC::SysV::Msg> documentation.
3010 =item msgsnd ID,MSG,FLAGS
3013 Calls the System V IPC function msgsnd to send the message MSG to the
3014 message queue ID. MSG must begin with the native long integer message
3015 type, and be followed by the length of the actual message, and finally
3016 the message itself. This kind of packing can be achieved with
3017 C<pack("l! a*", $type, $message)>. Returns true if successful,
3018 or false if there is an error. See also C<IPC::SysV>
3019 and C<IPC::SysV::Msg> documentation.
3026 =item my EXPR : ATTRS
3028 =item my TYPE EXPR : ATTRS
3030 A C<my> declares the listed variables to be local (lexically) to the
3031 enclosing block, file, or C<eval>. If more than one value is listed,
3032 the list must be placed in parentheses.
3034 The exact semantics and interface of TYPE and ATTRS are still
3035 evolving. TYPE is currently bound to the use of C<fields> pragma,
3036 and attributes are handled using the C<attributes> pragma, or starting
3037 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3038 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3039 L<attributes>, and L<Attribute::Handlers>.
3046 The C<next> command is like the C<continue> statement in C; it starts
3047 the next iteration of the loop:
3049 LINE: while (<STDIN>) {
3050 next LINE if /^#/; # discard comments
3054 Note that if there were a C<continue> block on the above, it would get
3055 executed even on discarded lines. If LABEL is omitted, the command
3056 refers to the innermost enclosing loop.
3058 C<next> cannot be used to exit a block which returns a value such as
3059 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3060 a grep() or map() operation.
3062 Note that a block by itself is semantically identical to a loop
3063 that executes once. Thus C<next> will exit such a block early.
3065 See also L</continue> for an illustration of how C<last>, C<next>, and
3068 =item no MODULE VERSION LIST
3072 =item no MODULE VERSION
3074 =item no MODULE LIST
3080 See the C<use> function, of which C<no> is the opposite.
3083 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3087 Interprets EXPR as an octal string and returns the corresponding
3088 value. (If EXPR happens to start off with C<0x>, interprets it as a
3089 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3090 binary string. Leading whitespace is ignored in all three cases.)
3091 The following will handle decimal, binary, octal, and hex in standard
3094 $val = oct($val) if $val =~ /^0/;
3096 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3097 in octal), use sprintf() or printf():
3099 $dec_perms = (stat("filename"))[2] & 07777;
3100 $oct_perm_str = sprintf "%o", $perms;
3102 The oct() function is commonly used when a string such as C<644> needs
3103 to be converted into a file mode, for example. Although Perl
3104 automatically converts strings into numbers as needed, this automatic
3105 conversion assumes base 10.
3107 Leading white space is ignored without warning, as too are any trailing
3108 non-digits, such as a decimal point (C<oct> only handles non-negative
3109 integers, not negative integers or floating point).
3111 =item open FILEHANDLE,EXPR
3112 X<open> X<pipe> X<file, open> X<fopen>
3114 =item open FILEHANDLE,MODE,EXPR
3116 =item open FILEHANDLE,MODE,EXPR,LIST
3118 =item open FILEHANDLE,MODE,REFERENCE
3120 =item open FILEHANDLE
3122 Opens the file whose filename is given by EXPR, and associates it with
3125 Simple examples to open a file for reading:
3127 open(my $fh, '<', "input.txt") or die $!;
3131 open(my $fh, '>', "output.txt") or die $!;
3133 (The following is a comprehensive reference to open(): for a gentler
3134 introduction you may consider L<perlopentut>.)
3136 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3137 the variable is assigned a reference to a new anonymous filehandle,
3138 otherwise if FILEHANDLE is an expression, its value is used as the name of
3139 the real filehandle wanted. (This is considered a symbolic reference, so
3140 C<use strict 'refs'> should I<not> be in effect.)
3142 If EXPR is omitted, the scalar variable of the same name as the
3143 FILEHANDLE contains the filename. (Note that lexical variables--those
3144 declared with C<my>--will not work for this purpose; so if you're
3145 using C<my>, specify EXPR in your call to open.)
3147 If three or more arguments are specified then the mode of opening and
3148 the filename are separate. If MODE is C<< '<' >> or nothing, the file
3149 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3150 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3151 the file is opened for appending, again being created if necessary.
3153 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3154 indicate that you want both read and write access to the file; thus
3155 C<< '+<' >> is almost always preferred for read/write updates--the
3156 C<< '+>' >> mode would clobber the file first. You can't usually use
3157 either read-write mode for updating textfiles, since they have
3158 variable length records. See the B<-i> switch in L<perlrun> for a
3159 better approach. The file is created with permissions of C<0666>
3160 modified by the process's C<umask> value.
3162 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3163 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3165 In the two-argument (and one-argument) form of the call, the mode and
3166 filename should be concatenated (in that order), possibly separated by
3167 spaces. You may omit the mode in these forms when that mode is
3170 If the filename begins with C<'|'>, the filename is interpreted as a
3171 command to which output is to be piped, and if the filename ends with a
3172 C<'|'>, the filename is interpreted as a command that pipes output to
3173 us. See L<perlipc/"Using open() for IPC">
3174 for more examples of this. (You are not allowed to C<open> to a command
3175 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3176 and L<perlipc/"Bidirectional Communication with Another Process">
3179 For three or more arguments if MODE is C<'|-'>, the filename is
3180 interpreted as a command to which output is to be piped, and if MODE
3181 is C<'-|'>, the filename is interpreted as a command that pipes
3182 output to us. In the two-argument (and one-argument) form, one should
3183 replace dash (C<'-'>) with the command.
3184 See L<perlipc/"Using open() for IPC"> for more examples of this.
3185 (You are not allowed to C<open> to a command that pipes both in I<and>
3186 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3187 L<perlipc/"Bidirectional Communication"> for alternatives.)
3189 In the form of pipe opens taking three or more arguments, if LIST is specified
3190 (extra arguments after the command name) then LIST becomes arguments
3191 to the command invoked if the platform supports it. The meaning of
3192 C<open> with more than three arguments for non-pipe modes is not yet
3193 defined, but experimental "layers" may give extra LIST arguments
3196 In the two-argument (and one-argument) form, opening C<< '<-' >>
3197 or C<'-'> opens STDIN and opening C<< '>-' >> opens STDOUT.
3199 You may use the three-argument form of open to specify I/O layers
3200 (sometimes referred to as "disciplines") to apply to the handle
3201 that affect how the input and output are processed (see L<open> and
3202 L<PerlIO> for more details). For example:
3204 open(my $fh, "<:encoding(UTF-8)", "filename")
3205 || die "can't open UTF-8 encoded filename: $!";
3207 opens the UTF-8 encoded file containing Unicode characters;
3208 see L<perluniintro>. Note that if layers are specified in the
3209 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3210 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3212 Open returns nonzero on success, the undefined value otherwise. If
3213 the C<open> involved a pipe, the return value happens to be the pid of
3216 If you're running Perl on a system that distinguishes between text
3217 files and binary files, then you should check out L</binmode> for tips
3218 for dealing with this. The key distinction between systems that need
3219 C<binmode> and those that don't is their text file formats. Systems
3220 like Unix, Mac OS, and Plan 9, that end lines with a single
3221 character and encode that character in C as C<"\n"> do not
3222 need C<binmode>. The rest need it.
3224 When opening a file, it's seldom a good idea to continue
3225 if the request failed, so C<open> is frequently used with
3226 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3227 where you want to format a suitable error message (but there are
3228 modules that can help with that problem)) always check
3229 the return value from opening a file.
3231 As a special case the 3-arg form with a read/write mode and the third
3232 argument being C<undef>:
3234 open(my $tmp, "+>", undef) or die ...
3236 opens a filehandle to an anonymous temporary file. Also using "+<"
3237 works for symmetry, but you really should consider writing something
3238 to the temporary file first. You will need to seek() to do the
3241 Since v5.8.0, Perl has built using PerlIO by default. Unless you've
3242 changed this (i.e., Configure -Uuseperlio), you can open filehandles
3243 directly to Perl scalars via:
3245 open($fh, '>', \$variable) || ..
3247 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3250 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3255 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3256 while (<ARTICLE>) {...
3258 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3259 # if the open fails, output is discarded
3261 open(my $dbase, '+<', 'dbase.mine') # open for update
3262 or die "Can't open 'dbase.mine' for update: $!";
3264 open(my $dbase, '+<dbase.mine') # ditto
3265 or die "Can't open 'dbase.mine' for update: $!";
3267 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3268 or die "Can't start caesar: $!";
3270 open(ARTICLE, "caesar <$article |") # ditto
3271 or die "Can't start caesar: $!";
3273 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3274 or die "Can't start sort: $!";
3277 open(MEMORY,'>', \$var)
3278 or die "Can't open memory file: $!";
3279 print MEMORY "foo!\n"; # output will appear in $var
3281 # process argument list of files along with any includes
3283 foreach $file (@ARGV) {
3284 process($file, 'fh00');
3288 my($filename, $input) = @_;
3289 $input++; # this is a string increment
3290 unless (open($input, $filename)) {
3291 print STDERR "Can't open $filename: $!\n";
3296 while (<$input>) { # note use of indirection
3297 if (/^#include "(.*)"/) {
3298 process($1, $input);
3305 See L<perliol> for detailed info on PerlIO.
3307 You may also, in the Bourne shell tradition, specify an EXPR beginning
3308 with C<< '>&' >>, in which case the rest of the string is interpreted
3309 as the name of a filehandle (or file descriptor, if numeric) to be
3310 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
3311 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3312 The mode you specify should match the mode of the original filehandle.
3313 (Duping a filehandle does not take into account any existing contents
3314 of IO buffers.) If you use the 3-arg form then you can pass either a
3315 number, the name of a filehandle or the normal "reference to a glob".
3317 Here is a script that saves, redirects, and restores C<STDOUT> and
3318 C<STDERR> using various methods:
3321 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3322 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3324 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3325 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3327 select STDERR; $| = 1; # make unbuffered
3328 select STDOUT; $| = 1; # make unbuffered
3330 print STDOUT "stdout 1\n"; # this works for
3331 print STDERR "stderr 1\n"; # subprocesses too
3333 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3334 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3336 print STDOUT "stdout 2\n";
3337 print STDERR "stderr 2\n";
3339 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3340 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3341 that file descriptor (and not call C<dup(2)>); this is more
3342 parsimonious of file descriptors. For example:
3344 # open for input, reusing the fileno of $fd
3345 open(FILEHANDLE, "<&=$fd")
3349 open(FILEHANDLE, "<&=", $fd)
3353 # open for append, using the fileno of OLDFH
3354 open(FH, ">>&=", OLDFH)
3358 open(FH, ">>&=OLDFH")
3360 Being parsimonious on filehandles is also useful (besides being
3361 parsimonious) for example when something is dependent on file
3362 descriptors, like for example locking using flock(). If you do just
3363 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3364 descriptor as B, and therefore flock(A) will not flock(B), and vice
3365 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3366 the same file descriptor.
3368 Note that if you are using Perls older than 5.8.0, Perl will be using
3369 the standard C libraries' fdopen() to implement the "=" functionality.
3370 On many Unix systems fdopen() fails when file descriptors exceed a
3371 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3372 most often the default.
3374 You can see whether Perl has been compiled with PerlIO or not by
3375 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3376 is C<define>, you have PerlIO, otherwise you don't.
3378 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3379 with 2-arguments (or 1-argument) form of open(), then
3380 there is an implicit fork done, and the return value of open is the pid
3381 of the child within the parent process, and C<0> within the child
3382 process. (Use C<defined($pid)> to determine whether the open was successful.)
3383 The filehandle behaves normally for the parent, but I/O to that
3384 filehandle is piped from/to the STDOUT/STDIN of the child process.
3385 In the child process, the filehandle isn't opened--I/O happens from/to
3386 the new STDOUT/STDIN. Typically this is used like the normal
3387 piped open when you want to exercise more control over just how the
3388 pipe command gets executed, such as when running setuid and
3389 you don't want to have to scan shell commands for metacharacters.
3391 The following triples are more or less equivalent:
3393 open(FOO, "|tr '[a-z]' '[A-Z]'");
3394 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3395 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3396 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3398 open(FOO, "cat -n '$file'|");
3399 open(FOO, '-|', "cat -n '$file'");
3400 open(FOO, '-|') || exec 'cat', '-n', $file;
3401 open(FOO, '-|', "cat", '-n', $file);
3403 The last example in each block shows the pipe as "list form", which is
3404 not yet supported on all platforms. A good rule of thumb is that if
3405 your platform has true C<fork()> (in other words, if your platform is
3406 Unix) you can use the list form.
3408 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3410 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3411 output before any operation that may do a fork, but this may not be
3412 supported on some platforms (see L<perlport>). To be safe, you may need
3413 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3414 of C<IO::Handle> on any open handles.
3416 On systems that support a close-on-exec flag on files, the flag will
3417 be set for the newly opened file descriptor as determined by the value
3418 of $^F. See L<perlvar/$^F>.
3420 Closing any piped filehandle causes the parent process to wait for the
3421 child to finish, and returns the status value in C<$?> and
3422 C<${^CHILD_ERROR_NATIVE}>.
3424 The filename passed to 2-argument (or 1-argument) form of open() will
3425 have leading and trailing whitespace deleted, and the normal
3426 redirection characters honored. This property, known as "magic open",
3427 can often be used to good effect. A user could specify a filename of
3428 F<"rsh cat file |">, or you could change certain filenames as needed:
3430 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3431 open(FH, $filename) or die "Can't open $filename: $!";
3433 Use 3-argument form to open a file with arbitrary weird characters in it,
3435 open(FOO, '<', $file);
3437 otherwise it's necessary to protect any leading and trailing whitespace:
3439 $file =~ s#^(\s)#./$1#;
3440 open(FOO, "< $file\0");
3442 (this may not work on some bizarre filesystems). One should
3443 conscientiously choose between the I<magic> and 3-arguments form
3448 will allow the user to specify an argument of the form C<"rsh cat file |">,
3449 but will not work on a filename that happens to have a trailing space, while
3451 open IN, '<', $ARGV[0];
3453 will have exactly the opposite restrictions.
3455 If you want a "real" C C<open> (see C<open(2)> on your system), then you
3456 should use the C<sysopen> function, which involves no such magic (but
3457 may use subtly different filemodes than Perl open(), which is mapped
3458 to C fopen()). This is
3459 another way to protect your filenames from interpretation. For example:
3462 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3463 or die "sysopen $path: $!";
3464 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3465 print HANDLE "stuff $$\n";
3467 print "File contains: ", <HANDLE>;
3469 Using the constructor from the C<IO::Handle> package (or one of its
3470 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3471 filehandles that have the scope of whatever variables hold references to
3472 them, and automatically close whenever and however you leave that scope:
3476 sub read_myfile_munged {
3478 my $handle = IO::File->new;
3479 open($handle, "myfile") or die "myfile: $!";
3481 or return (); # Automatically closed here.
3482 mung $first or die "mung failed"; # Or here.
3483 return $first, <$handle> if $ALL; # Or here.
3487 See L</seek> for some details about mixing reading and writing.
3489 =item opendir DIRHANDLE,EXPR
3492 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3493 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3494 DIRHANDLE may be an expression whose value can be used as an indirect
3495 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3496 scalar variable (or array or hash element), the variable is assigned a
3497 reference to a new anonymous dirhandle.
3498 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3500 See example at C<readdir>.
3507 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3508 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3511 For the reverse, see L</chr>.
3512 See L<perlunicode> for more about Unicode.
3519 =item our EXPR : ATTRS
3521 =item our TYPE EXPR : ATTRS
3523 C<our> associates a simple name with a package variable in the current
3524 package for use within the current scope. When C<use strict 'vars'> is in
3525 effect, C<our> lets you use declared global variables without qualifying
3526 them with package names, within the lexical scope of the C<our> declaration.
3527 In this way C<our> differs from C<use vars>, which is package scoped.
3529 Unlike C<my>, which both allocates storage for a variable and associates
3530 a simple name with that storage for use within the current scope, C<our>
3531 associates a simple name with a package variable in the current package,
3532 for use within the current scope. In other words, C<our> has the same
3533 scoping rules as C<my>, but does not necessarily create a
3536 If more than one value is listed, the list must be placed
3542 An C<our> declaration declares a global variable that will be visible
3543 across its entire lexical scope, even across package boundaries. The
3544 package in which the variable is entered is determined at the point
3545 of the declaration, not at the point of use. This means the following
3549 our $bar; # declares $Foo::bar for rest of lexical scope
3553 print $bar; # prints 20, as it refers to $Foo::bar
3555 Multiple C<our> declarations with the same name in the same lexical
3556 scope are allowed if they are in different packages. If they happen
3557 to be in the same package, Perl will emit warnings if you have asked
3558 for them, just like multiple C<my> declarations. Unlike a second
3559 C<my> declaration, which will bind the name to a fresh variable, a
3560 second C<our> declaration in the same package, in the same scope, is
3565 our $bar; # declares $Foo::bar for rest of lexical scope
3569 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3570 print $bar; # prints 30
3572 our $bar; # emits warning but has no other effect
3573 print $bar; # still prints 30
3575 An C<our> declaration may also have a list of attributes associated
3578 The exact semantics and interface of TYPE and ATTRS are still
3579 evolving. TYPE is currently bound to the use of C<fields> pragma,
3580 and attributes are handled using the C<attributes> pragma, or starting
3581 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3582 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3583 L<attributes>, and L<Attribute::Handlers>.
3585 =item pack TEMPLATE,LIST
3588 Takes a LIST of values and converts it into a string using the rules
3589 given by the TEMPLATE. The resulting string is the concatenation of
3590 the converted values. Typically, each converted value looks
3591 like its machine-level representation. For example, on 32-bit machines
3592 an integer may be represented by a sequence of 4 bytes, which will in
3593 Perl be presented as a string that's 4 characters long.
3595 The TEMPLATE is a sequence of characters that give the order and type
3596 of values, as follows:
3598 a A string with arbitrary binary data, will be null padded.
3599 A A text (ASCII) string, will be space padded.
3600 Z A null-terminated (ASCIZ) string, will be null padded.
3602 b A bit string (ascending bit order inside each byte, like vec()).
3603 B A bit string (descending bit order inside each byte).
3604 h A hex string (low nybble first).
3605 H A hex string (high nybble first).
3607 c A signed char (8-bit) value.
3608 C An unsigned char (octet) value.
3609 W An unsigned char value (can be greater than 255).
3611 s A signed short (16-bit) value.
3612 S An unsigned short value.
3614 l A signed long (32-bit) value.
3615 L An unsigned long value.
3617 q A signed quad (64-bit) value.
3618 Q An unsigned quad value.
3619 (Quads are available only if your system supports 64-bit
3620 integer values _and_ if Perl has been compiled to support those.
3621 Raises an exception otherwise.)
3623 i A signed integer value.
3624 I A unsigned integer value.
3625 (This 'integer' is _at_least_ 32 bits wide. Its exact
3626 size depends on what a local C compiler calls 'int'.)
3628 n An unsigned short (16-bit) in "network" (big-endian) order.
3629 N An unsigned long (32-bit) in "network" (big-endian) order.
3630 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3631 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3633 j A Perl internal signed integer value (IV).
3634 J A Perl internal unsigned integer value (UV).
3636 f A single-precision float in native format.
3637 d A double-precision float in native format.
3639 F A Perl internal floating-point value (NV) in native format
3640 D A float of long-double precision in native format.
3641 (Long doubles are available only if your system supports long
3642 double values _and_ if Perl has been compiled to support those.
3643 Raises an exception otherwise.)
3645 p A pointer to a null-terminated string.
3646 P A pointer to a structure (fixed-length string).
3648 u A uuencoded string.
3649 U A Unicode character number. Encodes to a character in character mode
3650 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3652 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3653 details). Its bytes represent an unsigned integer in base 128,
3654 most significant digit first, with as few digits as possible. Bit
3655 eight (the high bit) is set on each byte except the last.
3657 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
3659 @ Null-fill or truncate to absolute position, counted from the
3660 start of the innermost ()-group.
3661 . Null-fill or truncate to absolute position specified by the value.
3662 ( Start of a ()-group.
3664 One or more modifiers below may optionally follow certain letters in the
3665 TEMPLATE (the second column lists letters for which the modifier is valid):
3667 ! sSlLiI Forces native (short, long, int) sizes instead
3668 of fixed (16-/32-bit) sizes.
3670 xX Make x and X act as alignment commands.
3672 nNvV Treat integers as signed instead of unsigned.
3674 @. Specify position as byte offset in the internal
3675 representation of the packed string. Efficient but
3678 > sSiIlLqQ Force big-endian byte-order on the type.
3679 jJfFdDpP (The "big end" touches the construct.)
3681 < sSiIlLqQ Force little-endian byte-order on the type.
3682 jJfFdDpP (The "little end" touches the construct.)
3684 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
3685 to force a particular byte-order on all components in that group,
3686 including all its subgroups.
3688 The following rules apply:
3694 Each letter may optionally be followed by a number indicating the repeat
3695 count. A numeric repeat count may optionally be enclosed in brackets, as
3696 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
3697 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
3698 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
3699 something else, dscribed below. Supplying a C<*> for the repeat count
3700 instead of a number means to use however many items are left, except for:
3706 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
3710 <.>, where it means relative to the start of the string.
3714 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
3718 One can replace a numeric repeat count with a template letter enclosed in
3719 brackets to use the packed byte length of the bracketed template for the
3722 For example, the template C<x[L]> skips as many bytes as in a packed long,
3723 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
3724 variable-expanded) unpacks. If the template in brackets contains alignment
3725 commands (such as C<x![d]>), its packed length is calculated as if the
3726 start of the template had the maximal possible alignment.
3728 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
3729 trailing null byte, so the resulting string is always one byte longer than
3730 the byte length of the item itself.
3732 When used with C<@>, the repeat count represents an offset from the start
3733 of the innermost C<()> group.
3735 When used with C<.>, the repeat count determines the starting position to
3736 calculate the value offset as follows:
3742 If the repeat count is C<0>, it's relative to the current position.
3746 If the repeat count is C<*>, the offset is relative to the start of the
3751 And if it's an integer I<n>, the offset is relative to the start of the
3752 I<n>th innermost C<()> group, or to the start of the string if I<n> is
3753 bigger then the group level.
3757 The repeat count for C<u> is interpreted as the maximal number of bytes
3758 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3759 count should not be more than 65.
3763 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3764 string of length count, padding with nulls or spaces as needed. When
3765 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3766 after the first null, and C<a> returns data without any sort of trimming.
3768 If the value to pack is too long, the result is truncated. If it's too
3769 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
3770 followed by a null byte. Thus C<Z> always packs a trailing null, except
3771 for when the count is 0.
3775 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
3776 Each such format generates 1 bit of the result.
3778 Each result bit is based on the least-significant bit of the corresponding
3779 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3780 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
3782 Starting from the beginning of the input string, each 8-tuple
3783 of characters is converted to 1 character of output. With format C<b>,
3784 the first character of the 8-tuple determines the least-significant bit of a
3785 character; with format C<B>, it determines the most-significant bit of
3788 If the length of the input string is not evenly divisible by 8, the
3789 remainder is packed as if the input string were padded by null characters
3790 at the end. Similarly during unpacking, "extra" bits are ignored.
3792 If the input string is longer than needed, remaining characters are ignored.
3794 A C<*> for the repeat count uses all characters of the input field.
3795 On unpacking, bits are converted to a string of C<"0">s and C<"1">s.
3799 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
3800 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
3802 For each such format, pack() generates 4 bits of the result.
3803 With non-alphabetical characters, the result is based on the 4 least-significant
3804 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3805 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3806 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F">, the result
3807 is compatible with the usual hexadecimal digits, so that C<"a"> and
3808 C<"A"> both generate the nybble C<0xa==10>. Do not use any characters
3809 but these with this format.
3811 Starting from the beginning of the template to pack(), each pair
3812 of characters is converted to 1 character of output. With format C<h>, the
3813 first character of the pair determines the least-significant nybble of the
3814 output character; with format C<H>, it determines the most-significant
3817 If the length of the input string is not even, it behaves as if padded by
3818 a null character at the end. Similarly, "extra" nybbles are ignored during
3821 If the input string is longer than needed, extra characters are ignored.
3823 A C<*> for the repeat count uses all characters of the input field. For
3824 unpack(), nybbles are converted to a string of hexadecimal digits.
3828 The C<p> format packs a pointer to a null-terminated string. You are
3829 responsible for ensuring that the string is not a temporary value, as that
3830 could potentially get deallocated before you got around to using the packed
3831 result. The C<P> format packs a pointer to a structure of the size indicated
3832 by the length. A null pointer is created if the corresponding value for
3833 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
3834 unpacks into C<undef>.
3836 If your system has a strange pointer size--meaning a pointer is neither as
3837 big as an int nor as big as a long--it may not be possible to pack or
3838 unpack pointers in big- or little-endian byte order. Attempting to do
3839 so raises an exception.
3843 The C</> template character allows packing and unpacking of a sequence of
3844 items where the packed structure contains a packed item count followed by
3845 the packed items themselves. This is useful when the structure you're
3846 unpacking has encoded the sizes or repeat counts for some of its fields
3847 within the structure itself as separate fields.
3849 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
3850 I<length-item> describes how the length value is packed. Formats likely
3851 to be of most use are integer-packing ones like C<n> for Java strings,
3852 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
3854 For C<pack>, I<sequence-item> may have a repeat count, in which case
3855 the minimum of that and the number of available items is used as the argument
3856 for I<length-item>. If it has no repeat count or uses a '*', the number
3857 of available items is used.
3859 For C<unpack>, an internal stack of integer arguments unpacked so far is
3860 used. You write C</>I<sequence-item> and the repeat count is obtained by
3861 popping off the last element from the stack. The I<sequence-item> must not
3862 have a repeat count.
3864 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
3865 the I<length-item> is the string length, not the number of strings. With
3866 an explicit repeat count for pack, the packed string is adjusted to that
3867 length. For example:
3869 unpack("W/a", "\04Gurusamy") gives ("Guru")
3870 unpack("a3/A A*", "007 Bond J ") gives (" Bond", "J")
3871 unpack("a3 x2 /A A*", "007: Bond, J.") gives ("Bond, J", ".")
3873 pack("n/a* w/a","hello,","world") gives "\000\006hello,\005world"
3874 pack("a/W2", ord("a") .. ord("z")) gives "2ab"
3876 The I<length-item> is not returned explicitly from C<unpack>.
3878 Supplying a count to the I<length-item> format letter is only useful with
3879 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
3880 introduce C<"\000"> characters, which Perl does not regard as legal in
3885 The integer types C<s>, C<S>, C<l>, and C<L> may be
3886 followed by a C<!> modifier to specify native shorts or
3887 longs. As shown in the example above, a bare C<l> means
3888 exactly 32 bits, although the native C<long> as seen by the local C compiler
3889 may be larger. This is mainly an issue on 64-bit platforms. You can
3890 see whether using C<!> makes any difference this way:
3892 printf "format s is %d, s! is %d\n",
3893 length pack("s"), length pack("s!");
3895 printf "format l is %d, l! is %d\n",
3896 length pack("l"), length pack("l!");
3899 C<i!> and C<I!> are also allowed, but only for completeness' sake:
3900 they are identical to C<i> and C<I>.
3902 The actual sizes (in bytes) of native shorts, ints, longs, and long
3903 longs on the platform where Perl was built are also available from
3906 $ perl -V:{short,int,long{,long}}size
3912 or programmatically via the C<Config> module:
3915 print $Config{shortsize}, "\n";
3916 print $Config{intsize}, "\n";
3917 print $Config{longsize}, "\n";
3918 print $Config{longlongsize}, "\n";
3920 C<$Config{longlongsize}> is undefined on systems without
3925 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
3926 inherently non-portable between processors and operating systems because
3927 they obey native byteorder and endianness. For example, a 4-byte integer
3928 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
3929 handled by the CPU registers) into bytes as
3931 0x12 0x34 0x56 0x78 # big-endian
3932 0x78 0x56 0x34 0x12 # little-endian
3934 Basically, Intel and VAX CPUs are little-endian, while everybody else,
3935 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
3936 big-endian. Alpha and MIPS can be either: Digital/Compaq used/uses them in
3937 little-endian mode, but SGI/Cray uses them in big-endian mode.
3939 The names I<big-endian> and I<little-endian> are comic references to the
3940 egg-eating habits of the little-endian Lilliputians and the big-endian
3941 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
3942 This entered computer lingo via the paper "On Holy Wars and a Plea for
3943 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
3945 Some systems may have even weirder byte orders such as
3950 You can determine your system endianness with this incantation:
3952 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
3954 The byteorder on the platform where Perl was built is also available
3958 print "$Config{byteorder}\n";
3960 or from the command line:
3964 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
3965 and C<"87654321"> are big-endian.
3967 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
3968 and C<V> or else use the C<< > >> and C<< < >> modifiers described
3969 immediately below. See also L<perlport>.
3973 Starting with Perl 5.9.2, integer and floating-point formats, along with
3974 the C<p> and C<P> formats and C<()> groups, may all be followed by the
3975 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
3976 or little-endian byte-order. These modifiers are especially useful
3977 given how C<n>, C<N>, C<v> and C<V> don't cover signed integers,
3978 64-bit integers, or floating-point values.
3980 Here are some concerns to keep in mind when using endianness modifier:
3986 Exchanging signed integers between different platforms works only
3987 when all platforms store them in the same format. Most platforms store
3988 signed integers in two's-complement notation, so usually this is not an issue.
3992 The C<< > >> or C<< < >> modifiers can only be used on floating-point
3993 formats on big- or little-endian machines. Otherwise, attempting to
3994 use them raises an exception.
3998 Forcing big- or little-endian byte-order on floating-point values for
3999 data exchange can work only if all platforms use the same
4000 binary representation such as IEEE floating-point. Even if all
4001 platforms are using IEEE, there may still be subtle differences. Being able
4002 to use C<< > >> or C<< < >> on floating-point values can be useful,
4003 but also dangerous if you don't know exactly what you're doing.
4004 It is not a general way to portably store floating-point values.
4008 When using C<< > >> or C<< < >> on a C<()> group, this affects
4009 all types inside the group that accept byte-order modifiers,
4010 including all subgroups. It is silently ignored for all other
4011 types. You are not allowed to override the byte-order within a group
4012 that already has a byte-order modifier suffix.
4018 Real numbers (floats and doubles) are in native machine format only.
4019 Due to the multiplicity of floating-point formats and the lack of a
4020 standard "network" representation for them, no facility for interchange has been
4021 made. This means that packed floating-point data written on one machine
4022 may not be readable on another, even if both use IEEE floating-point
4023 arithmetic (because the endianness of the memory representation is not part
4024 of the IEEE spec). See also L<perlport>.
4026 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4027 modifiers to force big- or little-endian byte-order on floating-point values.
4029 Because Perl uses doubles (or long doubles, if configured) internally for
4030 all numeric calculation, converting from double into float and thence
4031 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4032 will not in general equal $foo.
4036 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4037 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4038 where the packed string is processed in its UTF-8-encoded Unicode form on
4039 a byte-by-byte basis. Character mode is the default unless the format string
4040 starts with C<U>. You can always switch mode mid-format with an explicit
4041 C<C0> or C<U0> in the format. This mode remains in effect until the next
4042 mode change, or until the end of the C<()> group it (directly) applies to.
4046 You must yourself do any alignment or padding by inserting, for example,
4047 enough C<"x">es while packing. There is no way for pack() and unpack()
4048 to know where characters are going to or coming from, so they
4049 handle their output and input as flat sequences of characters.
4053 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4054 take a repeat count either as postfix, or for unpack(), also via the C</>
4055 template character. Within each repetition of a group, positioning with
4056 C<@> starts over at 0. Therefore, the result of
4058 pack("@1A((@2A)@3A)", qw[X Y Z])
4060 is the string C<"\0X\0\0YZ">.
4064 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4065 jump forward or back to the closest position aligned at a multiple of C<count>
4066 characters. For example, to pack() or unpack() a C structure like
4069 char c; /* one signed, 8-bit character */
4074 one may need to use the template C<c x![d] d c[2]>. This assumes that
4075 doubles must be aligned to the size of double.
4077 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4082 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4083 represent signed 16-/32-bit integers in big-/little-endian order.
4084 This is portable only when all platforms sharing packed data use the
4085 same binary representation for signed integers; for example, when all
4086 platforms use two's-complement representation.
4090 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4091 White space can separate pack codes from each other, but modifiers and
4092 repeat counts must follow immediately. Breaking complex templates into
4093 individual line-by-line components, suitably annotated, can do as much to
4094 improve legibility and maintainability of pack/unpack formats as C</x> can
4095 for complicated pattern matches.
4099 If TEMPLATE requires more arguments that pack() is given, pack()
4100 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4101 than given, extra arguments are ignored.
4107 $foo = pack("WWWW",65,66,67,68);
4109 $foo = pack("W4",65,66,67,68);
4111 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4112 # same thing with Unicode circled letters.
4113 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4114 # same thing with Unicode circled letters. You don't get the UTF-8
4115 # bytes because the U at the start of the format caused a switch to
4116 # U0-mode, so the UTF-8 bytes get joined into characters
4117 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4118 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4119 # This is the UTF-8 encoding of the string in the previous example
4121 $foo = pack("ccxxcc",65,66,67,68);
4124 # NOTE: The examples above featuring "W" and "c" are true
4125 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4126 # and UTF-8. On EBCDIC systems, the first example would be
4127 # $foo = pack("WWWW",193,194,195,196);
4129 $foo = pack("s2",1,2);
4130 # "\1\0\2\0" on little-endian
4131 # "\0\1\0\2" on big-endian
4133 $foo = pack("a4","abcd","x","y","z");
4136 $foo = pack("aaaa","abcd","x","y","z");
4139 $foo = pack("a14","abcdefg");
4140 # "abcdefg\0\0\0\0\0\0\0"
4142 $foo = pack("i9pl", gmtime);
4143 # a real struct tm (on my system anyway)
4145 $utmp_template = "Z8 Z8 Z16 L";
4146 $utmp = pack($utmp_template, @utmp1);
4147 # a struct utmp (BSDish)
4149 @utmp2 = unpack($utmp_template, $utmp);
4150 # "@utmp1" eq "@utmp2"
4153 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4156 $foo = pack('sx2l', 12, 34);
4157 # short 12, two zero bytes padding, long 34
4158 $bar = pack('s@4l', 12, 34);
4159 # short 12, zero fill to position 4, long 34
4161 $baz = pack('s.l', 12, 4, 34);
4162 # short 12, zero fill to position 4, long 34
4164 $foo = pack('nN', 42, 4711);
4165 # pack big-endian 16- and 32-bit unsigned integers
4166 $foo = pack('S>L>', 42, 4711);
4168 $foo = pack('s<l<', -42, 4711);
4169 # pack little-endian 16- and 32-bit signed integers
4170 $foo = pack('(sl)<', -42, 4711);
4173 The same template may generally also be used in unpack().
4175 =item package NAMESPACE VERSION
4176 X<package> X<module> X<namespace> X<version>
4178 =item package NAMESPACE
4180 Declares the compilation unit as being in the given namespace. The scope
4181 of the package declaration is from the declaration itself through the end
4182 of the enclosing block, file, or eval (the same as the C<my> operator).
4183 All further unqualified dynamic identifiers will be in this namespace.
4184 A package statement affects dynamic variables only, including those
4185 you've used C<local> on, but I<not> lexical variables, which are created
4186 with C<my> (or C<our> (or C<state>)). Typically it would be the first
4187 declaration in a file included by C<require> or C<use>. You can switch into a
4188 package in more than one place, since this only determines which default
4189 symbol table the compiler uses for the rest of that block. You can refer to
4190 identifiers in other packages than the current one by prefixing the identifier
4191 with the package name and a double colon, as in C<$SomePack::var>
4192 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
4193 package as assumed. That is, C<$::sail> is equivalent to
4194 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
4195 code, mostly from Perl 4).
4197 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
4198 namespace to a L<version> object with the VERSION provided. VERSION must be a
4199 "strict" style version number as defined by the L<version> module: a positive
4200 decimal number (integer or decimal-fraction) without exponentiation or else a
4201 dotted-decimal v-string with a leading 'v' character and at least three
4202 components. You should set C<$VERSION> only once per package.
4204 See L<perlmod/"Packages"> for more information about packages, modules,
4205 and classes. See L<perlsub> for other scoping issues.
4207 =item pipe READHANDLE,WRITEHANDLE
4210 Opens a pair of connected pipes like the corresponding system call.
4211 Note that if you set up a loop of piped processes, deadlock can occur
4212 unless you are very careful. In addition, note that Perl's pipes use
4213 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4214 after each command, depending on the application.
4216 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4217 for examples of such things.
4219 On systems that support a close-on-exec flag on files, that flag is set
4220 on all newly opened file descriptors whose C<fileno>s are I<higher> than
4221 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
4228 Pops and returns the last value of the array, shortening the array by
4231 Returns the undefined value if the array is empty, although this may also
4232 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
4233 main program, but the C<@_> array in subroutines, just like C<shift>.
4236 X<pos> X<match, position>
4240 Returns the offset of where the last C<m//g> search left off for the variable
4241 in question (C<$_> is used when the variable is not specified). Note that
4242 0 is a valid match offset. C<undef> indicates that the search position
4243 is reset (usually due to match failure, but can also be because no match has
4244 yet been run on the scalar). C<pos> directly accesses the location used
4245 by the regexp engine to store the offset, so assigning to C<pos> will change
4246 that offset, and so will also influence the C<\G> zero-width assertion in
4247 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4248 the return from C<pos> won't change either in this case. See L<perlre> and
4251 =item print FILEHANDLE LIST
4258 Prints a string or a list of strings. Returns true if successful.
4259 FILEHANDLE may be a scalar variable containing
4260 the name of or a reference to the filehandle, thus introducing
4261 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4262 the next token is a term, it may be misinterpreted as an operator
4263 unless you interpose a C<+> or put parentheses around the arguments.)
4264 If FILEHANDLE is omitted, prints to standard output by default, or
4265 to the last selected output channel; see L</select>. If LIST is
4266 also omitted, prints C<$_> to the currently selected output handle.
4267 To set the default output handle to something other than STDOUT
4268 use the select operation. The current value of C<$,> (if any) is
4269 printed between each LIST item. The current value of C<$\> (if
4270 any) is printed after the entire LIST has been printed. Because
4271 print takes a LIST, anything in the LIST is evaluated in list
4272 context, and any subroutine that you call will have one or more of
4273 its expressions evaluated in list context. Also be careful not to
4274 follow the print keyword with a left parenthesis unless you want
4275 the corresponding right parenthesis to terminate the arguments to
4276 the print; put parentheses around all the arguments
4277 (or interpose a C<+>, but that doesn't look as good).
4279 Note that if you're storing FILEHANDLEs in an array, or if you're using
4280 any other expression more complex than a scalar variable to retrieve it,
4281 you will have to use a block returning the filehandle value instead:
4283 print { $files[$i] } "stuff\n";
4284 print { $OK ? STDOUT : STDERR } "stuff\n";
4286 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
4287 L<perlipc> for more on signal handling.
4289 =item printf FILEHANDLE FORMAT, LIST
4292 =item printf FORMAT, LIST
4294 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4295 (the output record separator) is not appended. The first argument
4296 of the list will be interpreted as the C<printf> format. See C<sprintf>
4297 for an explanation of the format argument. If C<use locale> is in effect,
4298 and POSIX::setlocale() has been called, the character used for the decimal
4299 separator in formatted floating-point numbers is affected by the LC_NUMERIC
4300 locale. See L<perllocale> and L<POSIX>.
4302 Don't fall into the trap of using a C<printf> when a simple
4303 C<print> would do. The C<print> is more efficient and less
4306 =item prototype FUNCTION
4309 Returns the prototype of a function as a string (or C<undef> if the
4310 function has no prototype). FUNCTION is a reference to, or the name of,
4311 the function whose prototype you want to retrieve.
4313 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4314 name for a Perl builtin. If the builtin is not I<overridable> (such as
4315 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4316 (such as C<system>), prototype() returns C<undef>, because the builtin
4317 does not really behave like a Perl function. Otherwise, the string
4318 describing the equivalent prototype is returned.
4320 =item push ARRAY,LIST
4323 Treats ARRAY as a stack, and pushes the values of LIST
4324 onto the end of ARRAY. The length of ARRAY increases by the length of
4325 LIST. Has the same effect as
4328 $ARRAY[++$#ARRAY] = $value;
4331 but is more efficient. Returns the number of elements in the array following
4332 the completed C<push>.
4342 Generalized quotes. See L<perlop/"Quote-Like Operators">.
4346 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
4348 =item quotemeta EXPR
4349 X<quotemeta> X<metacharacter>
4353 Returns the value of EXPR with all non-"word"
4354 characters backslashed. (That is, all characters not matching
4355 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4356 returned string, regardless of any locale settings.)
4357 This is the internal function implementing
4358 the C<\Q> escape in double-quoted strings.
4360 If EXPR is omitted, uses C<$_>.
4367 Returns a random fractional number greater than or equal to C<0> and less
4368 than the value of EXPR. (EXPR should be positive.) If EXPR is
4369 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4370 also special-cased as C<1> (this was undocumented before Perl 5.8.0
4371 and is subject to change in future versions of Perl). Automatically calls
4372 C<srand> unless C<srand> has already been called. See also C<srand>.
4374 Apply C<int()> to the value returned by C<rand()> if you want random
4375 integers instead of random fractional numbers. For example,
4379 returns a random integer between C<0> and C<9>, inclusive.
4381 (Note: If your rand function consistently returns numbers that are too
4382 large or too small, then your version of Perl was probably compiled
4383 with the wrong number of RANDBITS.)
4385 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4386 X<read> X<file, read>
4388 =item read FILEHANDLE,SCALAR,LENGTH
4390 Attempts to read LENGTH I<characters> of data into variable SCALAR
4391 from the specified FILEHANDLE. Returns the number of characters
4392 actually read, C<0> at end of file, or undef if there was an error (in
4393 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4394 so that the last character actually read is the last character of the
4395 scalar after the read.
4397 An OFFSET may be specified to place the read data at some place in the
4398 string other than the beginning. A negative OFFSET specifies
4399 placement at that many characters counting backwards from the end of
4400 the string. A positive OFFSET greater than the length of SCALAR
4401 results in the string being padded to the required size with C<"\0">
4402 bytes before the result of the read is appended.
4404 The call is implemented in terms of either Perl's or your system's native
4405 fread(3) library function. To get a true read(2) system call, see C<sysread>.
4407 Note the I<characters>: depending on the status of the filehandle,
4408 either (8-bit) bytes or characters are read. By default all
4409 filehandles operate on bytes, but for example if the filehandle has
4410 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4411 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4412 characters, not bytes. Similarly for the C<:encoding> pragma:
4413 in that case pretty much any characters can be read.
4415 =item readdir DIRHANDLE
4418 Returns the next directory entry for a directory opened by C<opendir>.
4419 If used in list context, returns all the rest of the entries in the
4420 directory. If there are no more entries, returns the undefined value in
4421 scalar context and the empty list in list context.
4423 If you're planning to filetest the return values out of a C<readdir>, you'd
4424 better prepend the directory in question. Otherwise, because we didn't
4425 C<chdir> there, it would have been testing the wrong file.
4427 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4428 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4431 As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop,
4432 which will set C<$_> on every iteration.
4434 opendir(my $dh, $some_dir) || die;
4435 while(readdir $dh) {
4436 print "$some_dir/$_\n";
4443 X<readline> X<gets> X<fgets>
4445 Reads from the filehandle whose typeglob is contained in EXPR (or from
4446 *ARGV if EXPR is not provided). In scalar context, each call reads and
4447 returns the next line until end-of-file is reached, whereupon the
4448 subsequent call returns C<undef>. In list context, reads until end-of-file
4449 is reached and returns a list of lines. Note that the notion of "line"
4450 used here is whatever you may have defined with C<$/> or
4451 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4453 When C<$/> is set to C<undef>, when C<readline> is in scalar
4454 context (i.e., file slurp mode), and when an empty file is read, it
4455 returns C<''> the first time, followed by C<undef> subsequently.
4457 This is the internal function implementing the C<< <EXPR> >>
4458 operator, but you can use it directly. The C<< <EXPR> >>
4459 operator is discussed in more detail in L<perlop/"I/O Operators">.
4462 $line = readline(*STDIN); # same thing
4464 If C<readline> encounters an operating system error, C<$!> will be set
4465 with the corresponding error message. It can be helpful to check
4466 C<$!> when you are reading from filehandles you don't trust, such as a
4467 tty or a socket. The following example uses the operator form of
4468 C<readline> and dies if the result is not defined.
4470 while ( ! eof($fh) ) {
4471 defined( $_ = <$fh> ) or die "readline failed: $!";
4475 Note that you have can't handle C<readline> errors that way with the
4476 C<ARGV> filehandle. In that case, you have to open each element of
4477 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
4479 foreach my $arg (@ARGV) {
4480 open(my $fh, $arg) or warn "Can't open $arg: $!";
4482 while ( ! eof($fh) ) {
4483 defined( $_ = <$fh> )
4484 or die "readline failed for $arg: $!";
4494 Returns the value of a symbolic link, if symbolic links are
4495 implemented. If not, raises an exception. If there is a system
4496 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4497 omitted, uses C<$_>.
4504 EXPR is executed as a system command.
4505 The collected standard output of the command is returned.
4506 In scalar context, it comes back as a single (potentially
4507 multi-line) string. In list context, returns a list of lines
4508 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4509 This is the internal function implementing the C<qx/EXPR/>
4510 operator, but you can use it directly. The C<qx/EXPR/>
4511 operator is discussed in more detail in L<perlop/"I/O Operators">.
4512 If EXPR is omitted, uses C<$_>.
4514 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4517 Receives a message on a socket. Attempts to receive LENGTH characters
4518 of data into variable SCALAR from the specified SOCKET filehandle.
4519 SCALAR will be grown or shrunk to the length actually read. Takes the
4520 same flags as the system call of the same name. Returns the address
4521 of the sender if SOCKET's protocol supports this; returns an empty
4522 string otherwise. If there's an error, returns the undefined value.
4523 This call is actually implemented in terms of recvfrom(2) system call.
4524 See L<perlipc/"UDP: Message Passing"> for examples.
4526 Note the I<characters>: depending on the status of the socket, either
4527 (8-bit) bytes or characters are received. By default all sockets
4528 operate on bytes, but for example if the socket has been changed using
4529 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4530 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4531 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4532 case pretty much any characters can be read.
4539 The C<redo> command restarts the loop block without evaluating the
4540 conditional again. The C<continue> block, if any, is not executed. If
4541 the LABEL is omitted, the command refers to the innermost enclosing
4542 loop. Programs that want to lie to themselves about what was just input
4543 normally use this command:
4545 # a simpleminded Pascal comment stripper
4546 # (warning: assumes no { or } in strings)
4547 LINE: while (<STDIN>) {
4548 while (s|({.*}.*){.*}|$1 |) {}
4553 if (/}/) { # end of comment?
4562 C<redo> cannot be used to retry a block that returns a value such as
4563 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4564 a grep() or map() operation.
4566 Note that a block by itself is semantically identical to a loop
4567 that executes once. Thus C<redo> inside such a block will effectively
4568 turn it into a looping construct.
4570 See also L</continue> for an illustration of how C<last>, C<next>, and
4578 Returns a non-empty string if EXPR is a reference, the empty
4579 string otherwise. If EXPR
4580 is not specified, C<$_> will be used. The value returned depends on the
4581 type of thing the reference is a reference to.
4582 Builtin types include:
4596 If the referenced object has been blessed into a package, then that package
4597 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4599 if (ref($r) eq "HASH") {
4600 print "r is a reference to a hash.\n";
4603 print "r is not a reference at all.\n";
4606 The return value C<LVALUE> indicates a reference to an lvalue that is not
4607 a variable. You get this from taking the reference of function calls like
4608 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4609 to a L<version string|perldata/"Version Strings">.
4611 The result C<Regexp> indicates that the argument is a regular expression
4612 resulting from C<qr//>.
4614 See also L<perlref>.
4616 =item rename OLDNAME,NEWNAME
4617 X<rename> X<move> X<mv> X<ren>
4619 Changes the name of a file; an existing file NEWNAME will be
4620 clobbered. Returns true for success, false otherwise.
4622 Behavior of this function varies wildly depending on your system
4623 implementation. For example, it will usually not work across file system
4624 boundaries, even though the system I<mv> command sometimes compensates
4625 for this. Other restrictions include whether it works on directories,
4626 open files, or pre-existing files. Check L<perlport> and either the
4627 rename(2) manpage or equivalent system documentation for details.
4629 For a platform independent C<move> function look at the L<File::Copy>
4632 =item require VERSION
4639 Demands a version of Perl specified by VERSION, or demands some semantics
4640 specified by EXPR or by C<$_> if EXPR is not supplied.
4642 VERSION may be either a numeric argument such as 5.006, which will be
4643 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4644 to C<$^V> (aka $PERL_VERSION). An exception is raised if
4645 VERSION is greater than the version of the current Perl interpreter.
4646 Compare with L</use>, which can do a similar check at compile time.
4648 Specifying VERSION as a literal of the form v5.6.1 should generally be
4649 avoided, because it leads to misleading error messages under earlier
4650 versions of Perl that do not support this syntax. The equivalent numeric
4651 version should be used instead.
4653 require v5.6.1; # run time version check
4654 require 5.6.1; # ditto
4655 require 5.006_001; # ditto; preferred for backwards compatibility
4657 Otherwise, C<require> demands that a library file be included if it
4658 hasn't already been included. The file is included via the do-FILE
4659 mechanism, which is essentially just a variety of C<eval> with the
4660 caveat that lexical variables in the invoking script will be invisible
4661 to the included code. Has semantics similar to the following subroutine:
4664 my ($filename) = @_;
4665 if (exists $INC{$filename}) {
4666 return 1 if $INC{$filename};
4667 die "Compilation failed in require";
4669 my ($realfilename,$result);
4671 foreach $prefix (@INC) {
4672 $realfilename = "$prefix/$filename";
4673 if (-f $realfilename) {
4674 $INC{$filename} = $realfilename;
4675 $result = do $realfilename;
4679 die "Can't find $filename in \@INC";
4682 $INC{$filename} = undef;
4684 } elsif (!$result) {
4685 delete $INC{$filename};
4686 die "$filename did not return true value";
4692 Note that the file will not be included twice under the same specified
4695 The file must return true as the last statement to indicate
4696 successful execution of any initialization code, so it's customary to
4697 end such a file with C<1;> unless you're sure it'll return true
4698 otherwise. But it's better just to put the C<1;>, in case you add more
4701 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4702 replaces "F<::>" with "F</>" in the filename for you,
4703 to make it easy to load standard modules. This form of loading of
4704 modules does not risk altering your namespace.
4706 In other words, if you try this:
4708 require Foo::Bar; # a splendid bareword
4710 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4711 directories specified in the C<@INC> array.
4713 But if you try this:
4715 $class = 'Foo::Bar';
4716 require $class; # $class is not a bareword
4718 require "Foo::Bar"; # not a bareword because of the ""
4720 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4721 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4723 eval "require $class";
4725 Now that you understand how C<require> looks for files with a
4726 bareword argument, there is a little extra functionality going on behind
4727 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4728 first look for a similar filename with a "F<.pmc>" extension. If this file
4729 is found, it will be loaded in place of any file ending in a "F<.pm>"
4732 You can also insert hooks into the import facility, by putting directly
4733 Perl code into the @INC array. There are three forms of hooks: subroutine
4734 references, array references and blessed objects.
4736 Subroutine references are the simplest case. When the inclusion system
4737 walks through @INC and encounters a subroutine, this subroutine gets
4738 called with two parameters, the first a reference to itself, and the
4739 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
4740 subroutine should return either nothing or else a list of up to three
4741 values in the following order:
4747 A filehandle, from which the file will be read.
4751 A reference to a subroutine. If there is no filehandle (previous item),
4752 then this subroutine is expected to generate one line of source code per
4753 call, writing the line into C<$_> and returning 1, then returning 0 at
4754 end of file. If there is a filehandle, then the subroutine will be
4755 called to act as a simple source filter, with the line as read in C<$_>.
4756 Again, return 1 for each valid line, and 0 after all lines have been
4761 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4762 reference to the subroutine itself is passed in as C<$_[0]>.
4766 If an empty list, C<undef>, or nothing that matches the first 3 values above
4767 is returned, then C<require> looks at the remaining elements of @INC.
4768 Note that this filehandle must be a real filehandle (strictly a typeglob
4769 or reference to a typeglob, blessed or unblessed); tied filehandles will be
4770 ignored and return value processing will stop there.
4772 If the hook is an array reference, its first element must be a subroutine
4773 reference. This subroutine is called as above, but the first parameter is
4774 the array reference. This lets you indirectly pass arguments to
4777 In other words, you can write:
4779 push @INC, \&my_sub;
4781 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4787 push @INC, [ \&my_sub, $x, $y, ... ];
4789 my ($arrayref, $filename) = @_;
4790 # Retrieve $x, $y, ...
4791 my @parameters = @$arrayref[1..$#$arrayref];
4795 If the hook is an object, it must provide an INC method that will be
4796 called as above, the first parameter being the object itself. (Note that
4797 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4798 into package C<main>.) Here is a typical code layout:
4804 my ($self, $filename) = @_;
4808 # In the main program
4809 push @INC, Foo->new(...);
4811 These hooks are also permitted to set the %INC entry
4812 corresponding to the files they have loaded. See L<perlvar/%INC>.
4814 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4821 Generally used in a C<continue> block at the end of a loop to clear
4822 variables and reset C<??> searches so that they work again. The
4823 expression is interpreted as a list of single characters (hyphens
4824 allowed for ranges). All variables and arrays beginning with one of
4825 those letters are reset to their pristine state. If the expression is
4826 omitted, one-match searches (C<?pattern?>) are reset to match again.
4827 Only resets variables or searches in the current package. Always returns
4830 reset 'X'; # reset all X variables
4831 reset 'a-z'; # reset lower case variables
4832 reset; # just reset ?one-time? searches
4834 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4835 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4836 variables; lexical variables are unaffected, but they clean themselves
4837 up on scope exit anyway, so you'll probably want to use them instead.
4845 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4846 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4847 context, depending on how the return value will be used, and the context
4848 may vary from one execution to the next (see C<wantarray>). If no EXPR
4849 is given, returns an empty list in list context, the undefined value in
4850 scalar context, and (of course) nothing at all in void context.
4852 (In the absence of an explicit C<return>, a subroutine, eval,
4853 or do FILE automatically returns the value of the last expression
4857 X<reverse> X<rev> X<invert>
4859 In list context, returns a list value consisting of the elements
4860 of LIST in the opposite order. In scalar context, concatenates the
4861 elements of LIST and returns a string value with all characters
4862 in the opposite order.
4864 print join(", ", reverse "world", "Hello"); # Hello, world
4866 print scalar reverse "dlrow ,", "olleH"; # Hello, world
4868 Used without arguments in scalar context, reverse() reverses C<$_>.
4870 $_ = "dlrow ,olleH";
4871 print reverse; # No output, list context
4872 print scalar reverse; # Hello, world
4874 Note that reversing an array to itself (as in C<@a = reverse @a>) will
4875 preserve non-existent elements whenever possible, i.e., for non magical
4876 arrays or tied arrays with C<EXISTS> and C<DELETE> methods.
4878 This operator is also handy for inverting a hash, although there are some
4879 caveats. If a value is duplicated in the original hash, only one of those
4880 can be represented as a key in the inverted hash. Also, this has to
4881 unwind one hash and build a whole new one, which may take some time
4882 on a large hash, such as from a DBM file.
4884 %by_name = reverse %by_address; # Invert the hash
4886 =item rewinddir DIRHANDLE
4889 Sets the current position to the beginning of the directory for the
4890 C<readdir> routine on DIRHANDLE.
4892 =item rindex STR,SUBSTR,POSITION
4895 =item rindex STR,SUBSTR
4897 Works just like index() except that it returns the position of the I<last>
4898 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4899 last occurrence beginning at or before that position.
4901 =item rmdir FILENAME
4902 X<rmdir> X<rd> X<directory, remove>
4906 Deletes the directory specified by FILENAME if that directory is
4907 empty. If it succeeds it returns true, otherwise it returns false and
4908 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4910 To remove a directory tree recursively (C<rm -rf> on Unix) look at
4911 the C<rmtree> function of the L<File::Path> module.
4915 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
4917 =item say FILEHANDLE LIST
4924 Just like C<print>, but implicitly appends a newline.
4925 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
4928 This keyword is available only when the "say" feature is
4929 enabled: see L<feature>.
4932 X<scalar> X<context>
4934 Forces EXPR to be interpreted in scalar context and returns the value
4937 @counts = ( scalar @a, scalar @b, scalar @c );
4939 There is no equivalent operator to force an expression to
4940 be interpolated in list context because in practice, this is never
4941 needed. If you really wanted to do so, however, you could use
4942 the construction C<@{[ (some expression) ]}>, but usually a simple
4943 C<(some expression)> suffices.
4945 Because C<scalar> is a unary operator, if you accidentally use for EXPR a
4946 parenthesized list, this behaves as a scalar comma expression, evaluating
4947 all but the last element in void context and returning the final element
4948 evaluated in scalar context. This is seldom what you want.
4950 The following single statement:
4952 print uc(scalar(&foo,$bar)),$baz;
4954 is the moral equivalent of these two:
4957 print(uc($bar),$baz);
4959 See L<perlop> for more details on unary operators and the comma operator.
4961 =item seek FILEHANDLE,POSITION,WHENCE
4962 X<seek> X<fseek> X<filehandle, position>
4964 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4965 FILEHANDLE may be an expression whose value gives the name of the
4966 filehandle. The values for WHENCE are C<0> to set the new position
4967 I<in bytes> to POSITION, C<1> to set it to the current position plus
4968 POSITION, and C<2> to set it to EOF plus POSITION (typically
4969 negative). For WHENCE you may use the constants C<SEEK_SET>,
4970 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4971 of the file) from the Fcntl module. Returns C<1> on success, C<0>
4974 Note the I<in bytes>: even if the filehandle has been set to
4975 operate on characters (for example by using the C<:encoding(utf8)> open
4976 layer), tell() will return byte offsets, not character offsets
4977 (because implementing that would render seek() and tell() rather slow).
4979 If you want to position the file for C<sysread> or C<syswrite>, don't use
4980 C<seek>, because buffering makes its effect on the file's read-write position
4981 unpredictable and non-portable. Use C<sysseek> instead.
4983 Due to the rules and rigors of ANSI C, on some systems you have to do a
4984 seek whenever you switch between reading and writing. Amongst other
4985 things, this may have the effect of calling stdio's clearerr(3).
4986 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4990 This is also useful for applications emulating C<tail -f>. Once you hit
4991 EOF on your read and then sleep for a while, you (probably) have to stick in a
4992 dummy seek() to reset things. The C<seek> doesn't change the position,
4993 but it I<does> clear the end-of-file condition on the handle, so that the
4994 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
4996 If that doesn't work (some I/O implementations are particularly
4997 cantankerous), you might need something like this:
5000 for ($curpos = tell(FILE); $_ = <FILE>;
5001 $curpos = tell(FILE)) {
5002 # search for some stuff and put it into files
5004 sleep($for_a_while);
5005 seek(FILE, $curpos, 0);
5008 =item seekdir DIRHANDLE,POS
5011 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
5012 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
5013 about possible directory compaction as the corresponding system library
5016 =item select FILEHANDLE
5017 X<select> X<filehandle, default>
5021 Returns the currently selected filehandle. If FILEHANDLE is supplied,
5022 sets the new current default filehandle for output. This has two
5023 effects: first, a C<write> or a C<print> without a filehandle will
5024 default to this FILEHANDLE. Second, references to variables related to
5025 output will refer to this output channel. For example, if you have to
5026 set the top of form format for more than one output channel, you might
5034 FILEHANDLE may be an expression whose value gives the name of the
5035 actual filehandle. Thus:
5037 $oldfh = select(STDERR); $| = 1; select($oldfh);
5039 Some programmers may prefer to think of filehandles as objects with
5040 methods, preferring to write the last example as:
5043 STDERR->autoflush(1);
5045 =item select RBITS,WBITS,EBITS,TIMEOUT
5048 This calls the select(2) syscall with the bit masks specified, which
5049 can be constructed using C<fileno> and C<vec>, along these lines:
5051 $rin = $win = $ein = '';
5052 vec($rin,fileno(STDIN),1) = 1;
5053 vec($win,fileno(STDOUT),1) = 1;
5056 If you want to select on many filehandles, you may wish to write a
5057 subroutine like this:
5060 my(@fhlist) = split(' ',$_[0]);
5063 vec($bits,fileno($_),1) = 1;
5067 $rin = fhbits('STDIN TTY SOCK');
5071 ($nfound,$timeleft) =
5072 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
5074 or to block until something becomes ready just do this
5076 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
5078 Most systems do not bother to return anything useful in $timeleft, so
5079 calling select() in scalar context just returns $nfound.
5081 Any of the bit masks can also be undef. The timeout, if specified, is
5082 in seconds, which may be fractional. Note: not all implementations are
5083 capable of returning the $timeleft. If not, they always return
5084 $timeleft equal to the supplied $timeout.
5086 You can effect a sleep of 250 milliseconds this way:
5088 select(undef, undef, undef, 0.25);
5090 Note that whether C<select> gets restarted after signals (say, SIGALRM)
5091 is implementation-dependent. See also L<perlport> for notes on the
5092 portability of C<select>.
5094 On error, C<select> behaves like select(2): it returns
5097 On some Unixes, select(2) may report a socket file
5098 descriptor as "ready for reading" when no data is available, and
5099 thus a subsequent read blocks. This can be avoided if you always use
5100 O_NONBLOCK on the socket. See select(2) and fcntl(2) for further
5103 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
5104 or <FH>) with C<select>, except as permitted by POSIX, and even
5105 then only on POSIX systems. You have to use C<sysread> instead.
5107 =item semctl ID,SEMNUM,CMD,ARG
5110 Calls the System V IPC function semctl(2). You'll probably have to say
5114 first to get the correct constant definitions. If CMD is IPC_STAT or
5115 GETALL, then ARG must be a variable that will hold the returned
5116 semid_ds structure or semaphore value array. Returns like C<ioctl>:
5117 the undefined value for error, "C<0 but true>" for zero, or the actual
5118 return value otherwise. The ARG must consist of a vector of native
5119 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
5120 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
5123 =item semget KEY,NSEMS,FLAGS
5126 Calls the System V IPC function semget(2). Returns the semaphore id, or
5127 the undefined value if there is an error. See also
5128 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
5131 =item semop KEY,OPSTRING
5134 Calls the System V IPC function semop(2) for semaphore operations
5135 such as signalling and waiting. OPSTRING must be a packed array of
5136 semop structures. Each semop structure can be generated with
5137 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
5138 implies the number of semaphore operations. Returns true if
5139 successful, or false if there is an error. As an example, the
5140 following code waits on semaphore $semnum of semaphore id $semid:
5142 $semop = pack("s!3", $semnum, -1, 0);
5143 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5145 To signal the semaphore, replace C<-1> with C<1>. See also
5146 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
5149 =item send SOCKET,MSG,FLAGS,TO
5152 =item send SOCKET,MSG,FLAGS
5154 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
5155 filehandle. Takes the same flags as the system call of the same name. On
5156 unconnected sockets, you must specify a destination to I<send to>, in which
5157 case it does a sendto(2) syscall. Returns the number of characters sent,
5158 or the undefined value on error. The sendmsg(2) syscall is currently
5159 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
5161 Note the I<characters>: depending on the status of the socket, either
5162 (8-bit) bytes or characters are sent. By default all sockets operate
5163 on bytes, but for example if the socket has been changed using
5164 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5165 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5166 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5167 pragma: in that case pretty much any characters can be sent.
5169 =item setpgrp PID,PGRP
5172 Sets the current process group for the specified PID, C<0> for the current
5173 process. Raises an exception when used on a machine that doesn't
5174 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5175 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5176 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5179 =item setpriority WHICH,WHO,PRIORITY
5180 X<setpriority> X<priority> X<nice> X<renice>
5182 Sets the current priority for a process, a process group, or a user.
5183 (See setpriority(2).) Raises an exception when used on a machine
5184 that doesn't implement setpriority(2).
5186 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5189 Sets the socket option requested. Returns undefined if there is an
5190 error. Use integer constants provided by the C<Socket> module for
5191 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5192 getprotobyname. OPTVAL might either be a packed string or an integer.
5193 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5195 An example disabling Nagle's algorithm on a socket:
5197 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5198 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5205 Shifts the first value of the array off and returns it, shortening the
5206 array by 1 and moving everything down. If there are no elements in the
5207 array, returns the undefined value. If ARRAY is omitted, shifts the
5208 C<@_> array within the lexical scope of subroutines and formats, and the
5209 C<@ARGV> array outside a subroutine and also within the lexical scopes
5210 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5211 C<UNITCHECK {}> and C<END {}> constructs.
5213 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5214 same thing to the left end of an array that C<pop> and C<push> do to the
5217 =item shmctl ID,CMD,ARG
5220 Calls the System V IPC function shmctl. You'll probably have to say
5224 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5225 then ARG must be a variable that will hold the returned C<shmid_ds>
5226 structure. Returns like ioctl: the undefined value for error, "C<0> but
5227 true" for zero, or the actual return value otherwise.
5228 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5230 =item shmget KEY,SIZE,FLAGS
5233 Calls the System V IPC function shmget. Returns the shared memory
5234 segment id, or the undefined value if there is an error.
5235 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5237 =item shmread ID,VAR,POS,SIZE
5241 =item shmwrite ID,STRING,POS,SIZE
5243 Reads or writes the System V shared memory segment ID starting at
5244 position POS for size SIZE by attaching to it, copying in/out, and
5245 detaching from it. When reading, VAR must be a variable that will
5246 hold the data read. When writing, if STRING is too long, only SIZE
5247 bytes are used; if STRING is too short, nulls are written to fill out
5248 SIZE bytes. Return true if successful, or false if there is an error.
5249 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5250 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5252 =item shutdown SOCKET,HOW
5255 Shuts down a socket connection in the manner indicated by HOW, which
5256 has the same interpretation as in the syscall of the same name.
5258 shutdown(SOCKET, 0); # I/we have stopped reading data
5259 shutdown(SOCKET, 1); # I/we have stopped writing data
5260 shutdown(SOCKET, 2); # I/we have stopped using this socket
5262 This is useful with sockets when you want to tell the other
5263 side you're done writing but not done reading, or vice versa.
5264 It's also a more insistent form of close because it also
5265 disables the file descriptor in any forked copies in other
5268 Returns C<1> for success; on error, returns C<undef> if
5269 the first argument is not a valid filehandle, or returns C<0> and sets
5270 C<$!> for any other failure.
5273 X<sin> X<sine> X<asin> X<arcsine>
5277 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5278 returns sine of C<$_>.
5280 For the inverse sine operation, you may use the C<Math::Trig::asin>
5281 function, or use this relation:
5283 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5290 Causes the script to sleep for (integer) EXPR seconds, or forever if no
5291 argument is given. Returns the integer number of seconds actually slept.
5293 May be interrupted if the process receives a signal such as C<SIGALRM>.
5296 local $SIG{ALARM} = sub { die "Alarm!\n" };
5299 die $@ unless $@ eq "Alarm!\n";
5301 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
5302 is often implemented using C<alarm>.
5304 On some older systems, it may sleep up to a full second less than what
5305 you requested, depending on how it counts seconds. Most modern systems
5306 always sleep the full amount. They may appear to sleep longer than that,
5307 however, because your process might not be scheduled right away in a
5308 busy multitasking system.
5310 For delays of finer granularity than one second, the Time::HiRes module
5311 (from CPAN, and starting from Perl 5.8 part of the standard
5312 distribution) provides usleep(). You may also use Perl's four-argument
5313 version of select() leaving the first three arguments undefined, or you
5314 might be able to use the C<syscall> interface to access setitimer(2) if
5315 your system supports it. See L<perlfaq8> for details.
5317 See also the POSIX module's C<pause> function.
5319 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5322 Opens a socket of the specified kind and attaches it to filehandle
5323 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5324 the syscall of the same name. You should C<use Socket> first
5325 to get the proper definitions imported. See the examples in
5326 L<perlipc/"Sockets: Client/Server Communication">.
5328 On systems that support a close-on-exec flag on files, the flag will
5329 be set for the newly opened file descriptor, as determined by the
5330 value of $^F. See L<perlvar/$^F>.
5332 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5335 Creates an unnamed pair of sockets in the specified domain, of the
5336 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5337 for the syscall of the same name. If unimplemented, raises an exception.
5338 Returns true if successful.
5340 On systems that support a close-on-exec flag on files, the flag will
5341 be set for the newly opened file descriptors, as determined by the value
5342 of $^F. See L<perlvar/$^F>.
5344 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5345 to C<pipe(Rdr, Wtr)> is essentially:
5348 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5349 shutdown(Rdr, 1); # no more writing for reader
5350 shutdown(Wtr, 0); # no more reading for writer
5352 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5353 emulate socketpair using IP sockets to localhost if your system implements
5354 sockets but not socketpair.
5356 =item sort SUBNAME LIST
5357 X<sort> X<qsort> X<quicksort> X<mergesort>
5359 =item sort BLOCK LIST
5363 In list context, this sorts the LIST and returns the sorted list value.
5364 In scalar context, the behaviour of C<sort()> is undefined.
5366 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5367 order. If SUBNAME is specified, it gives the name of a subroutine
5368 that returns an integer less than, equal to, or greater than C<0>,
5369 depending on how the elements of the list are to be ordered. (The
5370 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
5371 SUBNAME may be a scalar variable name (unsubscripted), in which case
5372 the value provides the name of (or a reference to) the actual
5373 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5374 an anonymous, in-line sort subroutine.
5376 If the subroutine's prototype is C<($$)>, the elements to be compared
5377 are passed by reference in C<@_>, as for a normal subroutine. This is
5378 slower than unprototyped subroutines, where the elements to be
5379 compared are passed into the subroutine
5380 as the package global variables $a and $b (see example below). Note that
5381 in the latter case, it is usually counter-productive to declare $a and
5384 The values to be compared are always passed by reference and should not
5387 You also cannot exit out of the sort block or subroutine using any of the
5388 loop control operators described in L<perlsyn> or with C<goto>.
5390 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5391 current collation locale. See L<perllocale>.
5393 sort() returns aliases into the original list, much as a for loop's index
5394 variable aliases the list elements. That is, modifying an element of a
5395 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5396 actually modifies the element in the original list. This is usually
5397 something to be avoided when writing clear code.
5399 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5400 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5401 preserves the input order of elements that compare equal. Although
5402 quicksort's run time is O(NlogN) when averaged over all arrays of
5403 length N, the time can be O(N**2), I<quadratic> behavior, for some
5404 inputs.) In 5.7, the quicksort implementation was replaced with
5405 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5406 But benchmarks indicated that for some inputs, on some platforms,
5407 the original quicksort was faster. 5.8 has a sort pragma for
5408 limited control of the sort. Its rather blunt control of the
5409 underlying algorithm may not persist into future Perls, but the
5410 ability to characterize the input or output in implementation
5411 independent ways quite probably will. See L<the sort pragma|sort>.
5416 @articles = sort @files;
5418 # same thing, but with explicit sort routine
5419 @articles = sort {$a cmp $b} @files;
5421 # now case-insensitively
5422 @articles = sort {uc($a) cmp uc($b)} @files;
5424 # same thing in reversed order
5425 @articles = sort {$b cmp $a} @files;
5427 # sort numerically ascending
5428 @articles = sort {$a <=> $b} @files;
5430 # sort numerically descending
5431 @articles = sort {$b <=> $a} @files;
5433 # this sorts the %age hash by value instead of key
5434 # using an in-line function
5435 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5437 # sort using explicit subroutine name
5439 $age{$a} <=> $age{$b}; # presuming numeric
5441 @sortedclass = sort byage @class;
5443 sub backwards { $b cmp $a }
5444 @harry = qw(dog cat x Cain Abel);
5445 @george = qw(gone chased yz Punished Axed);
5447 # prints AbelCaincatdogx
5448 print sort backwards @harry;
5449 # prints xdogcatCainAbel
5450 print sort @george, 'to', @harry;
5451 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5453 # inefficiently sort by descending numeric compare using
5454 # the first integer after the first = sign, or the
5455 # whole record case-insensitively otherwise
5458 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5463 # same thing, but much more efficiently;
5464 # we'll build auxiliary indices instead
5466 my @nums = @caps = ();
5468 push @nums, ( /=(\d+)/ ? $1 : undef );
5472 my @new = @old[ sort {
5473 $nums[$b] <=> $nums[$a]
5475 $caps[$a] cmp $caps[$b]
5479 # same thing, but without any temps
5480 @new = map { $_->[0] }
5481 sort { $b->[1] <=> $a->[1]
5484 } map { [$_, /=(\d+)/, uc($_)] } @old;
5486 # using a prototype allows you to use any comparison subroutine
5487 # as a sort subroutine (including other package's subroutines)
5489 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5492 @new = sort other::backwards @old;
5494 # guarantee stability, regardless of algorithm
5496 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5498 # force use of mergesort (not portable outside Perl 5.8)
5499 use sort '_mergesort'; # note discouraging _
5500 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5502 Warning: syntactical care is required when sorting the list returned from
5503 a function. If you want to sort the list returned by the function call
5504 C<find_records(@key)>, you can use:
5506 @contact = sort { $a cmp $b } find_records @key;
5507 @contact = sort +find_records(@key);
5508 @contact = sort &find_records(@key);
5509 @contact = sort(find_records(@key));
5511 If instead you want to sort the array @key with the comparison routine
5512 C<find_records()> then you can use:
5514 @contact = sort { find_records() } @key;
5515 @contact = sort find_records(@key);
5516 @contact = sort(find_records @key);
5517 @contact = sort(find_records (@key));
5519 If you're using strict, you I<must not> declare $a
5520 and $b as lexicals. They are package globals. That means
5521 that if you're in the C<main> package and type
5523 @articles = sort {$b <=> $a} @files;
5525 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5526 but if you're in the C<FooPack> package, it's the same as typing
5528 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5530 The comparison function is required to behave. If it returns
5531 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5532 sometimes saying the opposite, for example) the results are not
5535 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5536 (not-a-number), and because C<sort> raises an exception unless the
5537 result of a comparison is defined, when sorting with a comparison function
5538 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5539 The following example takes advantage that C<NaN != NaN> to
5540 eliminate any C<NaN>s from the input list.
5542 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5544 =item splice ARRAY,OFFSET,LENGTH,LIST
5547 =item splice ARRAY,OFFSET,LENGTH
5549 =item splice ARRAY,OFFSET
5553 Removes the elements designated by OFFSET and LENGTH from an array, and
5554 replaces them with the elements of LIST, if any. In list context,
5555 returns the elements removed from the array. In scalar context,
5556 returns the last element removed, or C<undef> if no elements are
5557 removed. The array grows or shrinks as necessary.
5558 If OFFSET is negative then it starts that far from the end of the array.
5559 If LENGTH is omitted, removes everything from OFFSET onward.
5560 If LENGTH is negative, removes the elements from OFFSET onward
5561 except for -LENGTH elements at the end of the array.
5562 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5563 past the end of the array, Perl issues a warning, and splices at the
5566 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5568 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5569 pop(@a) splice(@a,-1)
5570 shift(@a) splice(@a,0,1)
5571 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5572 $a[$i] = $y splice(@a,$i,1,$y)
5574 Example, assuming array lengths are passed before arrays:
5576 sub aeq { # compare two list values
5577 my(@a) = splice(@_,0,shift);
5578 my(@b) = splice(@_,0,shift);
5579 return 0 unless @a == @b; # same len?
5581 return 0 if pop(@a) ne pop(@b);
5585 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5587 =item split /PATTERN/,EXPR,LIMIT
5590 =item split /PATTERN/,EXPR
5592 =item split /PATTERN/
5596 Splits the string EXPR into a list of strings and returns that list. By
5597 default, empty leading fields are preserved, and empty trailing ones are
5598 deleted. (If all fields are empty, they are considered to be trailing.)
5600 In scalar context, returns the number of fields found.
5602 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5603 splits on whitespace (after skipping any leading whitespace). Anything
5604 matching PATTERN is taken to be a delimiter separating the fields. (Note
5605 that the delimiter may be longer than one character.)
5607 If LIMIT is specified and positive, it represents the maximum number
5608 of fields the EXPR will be split into, though the actual number of
5609 fields returned depends on the number of times PATTERN matches within
5610 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5611 stripped (which potential users of C<pop> would do well to remember).
5612 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5613 had been specified. Note that splitting an EXPR that evaluates to the
5614 empty string always returns the empty list, regardless of the LIMIT
5617 A pattern matching the empty string (not to be confused with
5618 an empty pattern C<//>, which is just one member of the set of patterns
5619 matching the epmty string), splits EXPR into individual
5620 characters. For example:
5622 print join(':', split(/ */, 'hi there')), "\n";
5624 produces the output 'h:i:t:h:e:r:e'.
5626 As a special case for C<split>, the empty pattern C<//> specifically
5627 matches the empty string; this is not be confused with the normal use
5628 of an empty pattern to mean the last successful match. So to split
5629 a string into individual characters, the following:
5631 print join(':', split(//, 'hi there')), "\n";
5633 produces the output 'h:i: :t:h:e:r:e'.
5635 Empty leading fields are produced when there are positive-width matches at
5636 the beginning of the string; a zero-width match at the beginning of
5637 the string does not produce an empty field. For example:
5639 print join(':', split(/(?=\w)/, 'hi there!'));
5641 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
5642 hand, are produced when there is a match at the end of the string (and
5643 when LIMIT is given and is not 0), regardless of the length of the match.
5646 print join(':', split(//, 'hi there!', -1)), "\n";
5647 print join(':', split(/\W/, 'hi there!', -1)), "\n";
5649 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
5650 both with an empty trailing field.
5652 The LIMIT parameter can be used to split a line partially
5654 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5656 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5657 a LIMIT one larger than the number of variables in the list, to avoid
5658 unnecessary work. For the list above LIMIT would have been 4 by
5659 default. In time critical applications it behooves you not to split
5660 into more fields than you really need.
5662 If the PATTERN contains parentheses, additional list elements are
5663 created from each matching substring in the delimiter.
5665 split(/([,-])/, "1-10,20", 3);
5667 produces the list value
5669 (1, '-', 10, ',', 20)
5671 If you had the entire header of a normal Unix email message in $header,
5672 you could split it up into fields and their values this way:
5674 $header =~ s/\n(?=\s)//g; # fix continuation lines
5675 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5677 The pattern C</PATTERN/> may be replaced with an expression to specify
5678 patterns that vary at runtime. (To do runtime compilation only once,
5679 use C</$variable/o>.)
5681 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5682 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5683 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5684 will give you as many initial null fields (empty string) as there are leading spaces.
5685 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5686 whitespace produces a null first field. A C<split> with no arguments
5687 really does a S<C<split(' ', $_)>> internally.
5689 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5694 open(PASSWD, '/etc/passwd');
5697 ($login, $passwd, $uid, $gid,
5698 $gcos, $home, $shell) = split(/:/);
5702 As with regular pattern matching, any capturing parentheses that are not
5703 matched in a C<split()> will be set to C<undef> when returned:
5705 @fields = split /(A)|B/, "1A2B3";
5706 # @fields is (1, 'A', 2, undef, 3)
5708 =item sprintf FORMAT, LIST
5711 Returns a string formatted by the usual C<printf> conventions of the C
5712 library function C<sprintf>. See below for more details
5713 and see C<sprintf(3)> or C<printf(3)> on your system for an explanation of
5714 the general principles.
5718 # Format number with up to 8 leading zeroes
5719 $result = sprintf("%08d", $number);
5721 # Round number to 3 digits after decimal point
5722 $rounded = sprintf("%.3f", $number);
5724 Perl does its own C<sprintf> formatting: it emulates the C
5725 function sprintf(3), but doesn't use it except for floating-point
5726 numbers, and even then only standard modifiers are allowed.
5727 Non-standard extensions in your local sprintf(3) are
5728 therefore unavailable from Perl.
5730 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5731 pass it an array as your first argument. The array is given scalar context,
5732 and instead of using the 0th element of the array as the format, Perl will
5733 use the count of elements in the array as the format, which is almost never
5736 Perl's C<sprintf> permits the following universally-known conversions:
5739 %c a character with the given number
5741 %d a signed integer, in decimal
5742 %u an unsigned integer, in decimal
5743 %o an unsigned integer, in octal
5744 %x an unsigned integer, in hexadecimal
5745 %e a floating-point number, in scientific notation
5746 %f a floating-point number, in fixed decimal notation
5747 %g a floating-point number, in %e or %f notation
5749 In addition, Perl permits the following widely-supported conversions:
5751 %X like %x, but using upper-case letters
5752 %E like %e, but using an upper-case "E"
5753 %G like %g, but with an upper-case "E" (if applicable)
5754 %b an unsigned integer, in binary
5755 %B like %b, but using an upper-case "B" with the # flag
5756 %p a pointer (outputs the Perl value's address in hexadecimal)
5757 %n special: *stores* the number of characters output so far
5758 into the next variable in the parameter list
5760 Finally, for backward (and we do mean "backward") compatibility, Perl
5761 permits these unnecessary but widely-supported conversions:
5764 %D a synonym for %ld
5765 %U a synonym for %lu
5766 %O a synonym for %lo
5769 Note that the number of exponent digits in the scientific notation produced
5770 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5771 exponent less than 100 is system-dependent: it may be three or less
5772 (zero-padded as necessary). In other words, 1.23 times ten to the
5773 99th may be either "1.23e99" or "1.23e099".
5775 Between the C<%> and the format letter, you may specify several
5776 additional attributes controlling the interpretation of the format.
5777 In order, these are:
5781 =item format parameter index
5783 An explicit format parameter index, such as C<2$>. By default sprintf
5784 will format the next unused argument in the list, but this allows you
5785 to take the arguments out of order:
5787 printf '%2$d %1$d', 12, 34; # prints "34 12"
5788 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5794 space prefix non-negative number with a space
5795 + prefix non-negative number with a plus sign
5796 - left-justify within the field
5797 0 use zeros, not spaces, to right-justify
5798 # ensure the leading "0" for any octal,
5799 prefix non-zero hexadecimal with "0x" or "0X",
5800 prefix non-zero binary with "0b" or "0B"
5804 printf '<% d>', 12; # prints "< 12>"
5805 printf '<%+d>', 12; # prints "<+12>"
5806 printf '<%6s>', 12; # prints "< 12>"
5807 printf '<%-6s>', 12; # prints "<12 >"
5808 printf '<%06s>', 12; # prints "<000012>"
5809 printf '<%#o>', 12; # prints "<014>"
5810 printf '<%#x>', 12; # prints "<0xc>"
5811 printf '<%#X>', 12; # prints "<0XC>"
5812 printf '<%#b>', 12; # prints "<0b1100>"
5813 printf '<%#B>', 12; # prints "<0B1100>"
5815 When a space and a plus sign are given as the flags at once,
5816 a plus sign is used to prefix a positive number.
5818 printf '<%+ d>', 12; # prints "<+12>"
5819 printf '<% +d>', 12; # prints "<+12>"
5821 When the # flag and a precision are given in the %o conversion,
5822 the precision is incremented if it's necessary for the leading "0".
5824 printf '<%#.5o>', 012; # prints "<00012>"
5825 printf '<%#.5o>', 012345; # prints "<012345>"
5826 printf '<%#.0o>', 0; # prints "<0>"
5830 This flag tells Perl to interpret the supplied string as a vector of
5831 integers, one for each character in the string. Perl applies the format to
5832 each integer in turn, then joins the resulting strings with a separator (a
5833 dot C<.> by default). This can be useful for displaying ordinal values of
5834 characters in arbitrary strings:
5836 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5837 printf "version is v%vd\n", $^V; # Perl's version
5839 Put an asterisk C<*> before the C<v> to override the string to
5840 use to separate the numbers:
5842 printf "address is %*vX\n", ":", $addr; # IPv6 address
5843 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5845 You can also explicitly specify the argument number to use for
5846 the join string using something like C<*2$v>; for example:
5848 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5850 =item (minimum) width
5852 Arguments are usually formatted to be only as wide as required to
5853 display the given value. You can override the width by putting
5854 a number here, or get the width from the next argument (with C<*>)
5855 or from a specified argument (e.g., with C<*2$>):
5857 printf '<%s>', "a"; # prints "<a>"
5858 printf '<%6s>', "a"; # prints "< a>"
5859 printf '<%*s>', 6, "a"; # prints "< a>"
5860 printf '<%*2$s>', "a", 6; # prints "< a>"
5861 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5863 If a field width obtained through C<*> is negative, it has the same
5864 effect as the C<-> flag: left-justification.
5866 =item precision, or maximum width
5869 You can specify a precision (for numeric conversions) or a maximum
5870 width (for string conversions) by specifying a C<.> followed by a number.
5871 For floating-point formats except 'g' and 'G', this specifies
5872 how many places right of the decimal point to show (the default being 6).
5875 # these examples are subject to system-specific variation
5876 printf '<%f>', 1; # prints "<1.000000>"
5877 printf '<%.1f>', 1; # prints "<1.0>"
5878 printf '<%.0f>', 1; # prints "<1>"
5879 printf '<%e>', 10; # prints "<1.000000e+01>"
5880 printf '<%.1e>', 10; # prints "<1.0e+01>"
5882 For "g" and "G", this specifies the maximum number of digits to show,
5883 including thoe prior to the decimal point and those after it; for
5886 # These examples are subject to system-specific variation.
5887 printf '<%g>', 1; # prints "<1>"
5888 printf '<%.10g>', 1; # prints "<1>"
5889 printf '<%g>', 100; # prints "<100>"
5890 printf '<%.1g>', 100; # prints "<1e+02>"
5891 printf '<%.2g>', 100.01; # prints "<1e+02>"
5892 printf '<%.5g>', 100.01; # prints "<100.01>"
5893 printf '<%.4g>', 100.01; # prints "<100>"
5895 For integer conversions, specifying a precision implies that the
5896 output of the number itself should be zero-padded to this width,
5897 where the 0 flag is ignored:
5899 printf '<%.6d>', 1; # prints "<000001>"
5900 printf '<%+.6d>', 1; # prints "<+000001>"
5901 printf '<%-10.6d>', 1; # prints "<000001 >"
5902 printf '<%10.6d>', 1; # prints "< 000001>"
5903 printf '<%010.6d>', 1; # prints "< 000001>"
5904 printf '<%+10.6d>', 1; # prints "< +000001>"
5906 printf '<%.6x>', 1; # prints "<000001>"
5907 printf '<%#.6x>', 1; # prints "<0x000001>"
5908 printf '<%-10.6x>', 1; # prints "<000001 >"
5909 printf '<%10.6x>', 1; # prints "< 000001>"
5910 printf '<%010.6x>', 1; # prints "< 000001>"
5911 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5913 For string conversions, specifying a precision truncates the string
5914 to fit the specified width:
5916 printf '<%.5s>', "truncated"; # prints "<trunc>"
5917 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5919 You can also get the precision from the next argument using C<.*>:
5921 printf '<%.6x>', 1; # prints "<000001>"
5922 printf '<%.*x>', 6, 1; # prints "<000001>"
5924 If a precision obtained through C<*> is negative, it counts
5925 as having no precision at all.
5927 printf '<%.*s>', 7, "string"; # prints "<string>"
5928 printf '<%.*s>', 3, "string"; # prints "<str>"
5929 printf '<%.*s>', 0, "string"; # prints "<>"
5930 printf '<%.*s>', -1, "string"; # prints "<string>"
5932 printf '<%.*d>', 1, 0; # prints "<0>"
5933 printf '<%.*d>', 0, 0; # prints "<>"
5934 printf '<%.*d>', -1, 0; # prints "<0>"
5936 You cannot currently get the precision from a specified number,
5937 but it is intended that this will be possible in the future, for
5938 example using C<.*2$>:
5940 printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
5944 For numeric conversions, you can specify the size to interpret the
5945 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5946 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5947 whatever the default integer size is on your platform (usually 32 or 64
5948 bits), but you can override this to use instead one of the standard C types,
5949 as supported by the compiler used to build Perl:
5951 l interpret integer as C type "long" or "unsigned long"
5952 h interpret integer as C type "short" or "unsigned short"
5953 q, L or ll interpret integer as C type "long long", "unsigned long long".
5954 or "quads" (typically 64-bit integers)
5956 The last will raise an exception if Perl does not understand "quads" in your
5957 installation. (This requires either that the platform natively support quads,
5958 or that Perl were specifically compiled to support quads.) You can find out
5959 whether your Perl supports quads via L<Config>:
5962 if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
5963 print "Nice quads!\n";
5966 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
5967 to be the default floating-point size on your platform (double or long double),
5968 but you can force "long double" with C<q>, C<L>, or C<ll> if your
5969 platform supports them. You can find out whether your Perl supports long
5970 doubles via L<Config>:
5973 print "long doubles\n" if $Config{d_longdbl} eq "define";
5975 You can find out whether Perl considers "long double" to be the default
5976 floating-point size to use on your platform via L<Config>:
5979 if ($Config{uselongdouble} eq "define") {
5980 print "long doubles by default\n";
5983 It can also be that long doubles and doubles are the same thing:
5986 ($Config{doublesize} == $Config{longdblsize}) &&
5987 print "doubles are long doubles\n";
5989 The size specifier C<V> has no effect for Perl code, but is supported for
5990 compatibility with XS code. It means "use the standard size for a Perl
5991 integer or floating-point number", which is the default.
5993 =item order of arguments
5995 Normally, sprintf() takes the next unused argument as the value to
5996 format for each format specification. If the format specification
5997 uses C<*> to require additional arguments, these are consumed from
5998 the argument list in the order they appear in the format
5999 specification I<before> the value to format. Where an argument is
6000 specified by an explicit index, this does not affect the normal
6001 order for the arguments, even when the explicitly specified index
6002 would have been the next argument.
6006 printf "<%*.*s>", $a, $b, $c;
6008 uses C<$a> for the width, C<$b> for the precision, and C<$c>
6009 as the value to format; while:
6011 printf "<%*1$.*s>", $a, $b;
6013 would use C<$a> for the width and precision, and C<$b> as the
6016 Here are some more examples; be aware that when using an explicit
6017 index, the C<$> may need escaping:
6019 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
6020 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
6021 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
6022 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
6026 If C<use locale> is in effect and POSIX::setlocale() has been called,
6027 the character used for the decimal separator in formatted floating-point
6028 numbers is affected by the LC_NUMERIC locale. See L<perllocale>
6032 X<sqrt> X<root> X<square root>
6036 Return the positive square root of EXPR. If EXPR is omitted, uses
6037 C<$_>. Works only for non-negative operands unless you've
6038 loaded the C<Math::Complex> module.
6041 print sqrt(-4); # prints 2i
6044 X<srand> X<seed> X<randseed>
6048 Sets the random number seed for the C<rand> operator.
6050 The point of the function is to "seed" the C<rand> function so that
6051 C<rand> can produce a different sequence each time you run your
6054 If srand() is not called explicitly, it is called implicitly at the
6055 first use of the C<rand> operator. However, this was not true of
6056 versions of Perl before 5.004, so if your script will run under older
6057 Perl versions, it should call C<srand>.
6059 Most programs won't even call srand() at all, except those that
6060 need a cryptographically-strong starting point rather than the
6061 generally acceptable default, which is based on time of day,
6062 process ID, and memory allocation, or the F</dev/urandom> device
6065 You can call srand($seed) with the same $seed to reproduce the
6066 I<same> sequence from rand(), but this is usually reserved for
6067 generating predictable results for testing or debugging.
6068 Otherwise, don't call srand() more than once in your program.
6070 Do B<not> call srand() (i.e., without an argument) more than once in
6071 a script. The internal state of the random number generator should
6072 contain more entropy than can be provided by any seed, so calling
6073 srand() again actually I<loses> randomness.
6075 Most implementations of C<srand> take an integer and will silently
6076 truncate decimal numbers. This means C<srand(42)> will usually
6077 produce the same results as C<srand(42.1)>. To be safe, always pass
6078 C<srand> an integer.
6080 In versions of Perl prior to 5.004 the default seed was just the
6081 current C<time>. This isn't a particularly good seed, so many old
6082 programs supply their own seed value (often C<time ^ $$> or C<time ^
6083 ($$ + ($$ << 15))>), but that isn't necessary any more.
6085 For cryptographic purposes, however, you need something much more random
6086 than the default seed. Checksumming the compressed output of one or more
6087 rapidly changing operating system status programs is the usual method. For
6090 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
6092 If you're particularly concerned with this, see the C<Math::TrulyRandom>
6095 Frequently called programs (like CGI scripts) that simply use
6099 for a seed can fall prey to the mathematical property that
6103 one-third of the time. So don't do that.
6105 =item stat FILEHANDLE
6106 X<stat> X<file, status> X<ctime>
6110 =item stat DIRHANDLE
6114 Returns a 13-element list giving the status info for a file, either
6115 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
6116 omitted, it stats C<$_>. Returns the empty list if C<stat> fails. Typically
6119 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
6120 $atime,$mtime,$ctime,$blksize,$blocks)
6123 Not all fields are supported on all filesystem types. Here are the
6124 meanings of the fields:
6126 0 dev device number of filesystem
6128 2 mode file mode (type and permissions)
6129 3 nlink number of (hard) links to the file
6130 4 uid numeric user ID of file's owner
6131 5 gid numeric group ID of file's owner
6132 6 rdev the device identifier (special files only)
6133 7 size total size of file, in bytes
6134 8 atime last access time in seconds since the epoch
6135 9 mtime last modify time in seconds since the epoch
6136 10 ctime inode change time in seconds since the epoch (*)
6137 11 blksize preferred block size for file system I/O
6138 12 blocks actual number of blocks allocated
6140 (The epoch was at 00:00 January 1, 1970 GMT.)
6142 (*) Not all fields are supported on all filesystem types. Notably, the
6143 ctime field is non-portable. In particular, you cannot expect it to be a
6144 "creation time", see L<perlport/"Files and Filesystems"> for details.
6146 If C<stat> is passed the special filehandle consisting of an underline, no
6147 stat is done, but the current contents of the stat structure from the
6148 last C<stat>, C<lstat>, or filetest are returned. Example:
6150 if (-x $file && (($d) = stat(_)) && $d < 0) {
6151 print "$file is executable NFS file\n";
6154 (This works on machines only for which the device number is negative
6157 Because the mode contains both the file type and its permissions, you
6158 should mask off the file type portion and (s)printf using a C<"%o">
6159 if you want to see the real permissions.
6161 $mode = (stat($filename))[2];
6162 printf "Permissions are %04o\n", $mode & 07777;
6164 In scalar context, C<stat> returns a boolean value indicating success
6165 or failure, and, if successful, sets the information associated with
6166 the special filehandle C<_>.
6168 The L<File::stat> module provides a convenient, by-name access mechanism:
6171 $sb = stat($filename);
6172 printf "File is %s, size is %s, perm %04o, mtime %s\n",
6173 $filename, $sb->size, $sb->mode & 07777,
6174 scalar localtime $sb->mtime;
6176 You can import symbolic mode constants (C<S_IF*>) and functions
6177 (C<S_IS*>) from the Fcntl module:
6181 $mode = (stat($filename))[2];
6183 $user_rwx = ($mode & S_IRWXU) >> 6;
6184 $group_read = ($mode & S_IRGRP) >> 3;
6185 $other_execute = $mode & S_IXOTH;
6187 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6189 $is_setuid = $mode & S_ISUID;
6190 $is_directory = S_ISDIR($mode);
6192 You could write the last two using the C<-u> and C<-d> operators.
6193 Commonly available C<S_IF*> constants are:
6195 # Permissions: read, write, execute, for user, group, others.
6197 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6198 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6199 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6201 # Setuid/Setgid/Stickiness/SaveText.
6202 # Note that the exact meaning of these is system dependent.
6204 S_ISUID S_ISGID S_ISVTX S_ISTXT
6206 # File types. Not necessarily all are available on your system.
6208 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6210 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
6212 S_IREAD S_IWRITE S_IEXEC
6214 and the C<S_IF*> functions are
6216 S_IMODE($mode) the part of $mode containing the permission bits
6217 and the setuid/setgid/sticky bits
6219 S_IFMT($mode) the part of $mode containing the file type
6220 which can be bit-anded with (for example) S_IFREG
6221 or with the following functions
6223 # The operators -f, -d, -l, -b, -c, -p, and -S.
6225 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6226 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6228 # No direct -X operator counterpart, but for the first one
6229 # the -g operator is often equivalent. The ENFMT stands for
6230 # record flocking enforcement, a platform-dependent feature.
6232 S_ISENFMT($mode) S_ISWHT($mode)
6234 See your native chmod(2) and stat(2) documentation for more details
6235 about the C<S_*> constants. To get status info for a symbolic link
6236 instead of the target file behind the link, use the C<lstat> function.
6241 =item state TYPE EXPR
6243 =item state EXPR : ATTRS
6245 =item state TYPE EXPR : ATTRS
6247 C<state> declares a lexically scoped variable, just like C<my> does.
6248 However, those variables will never be reinitialized, contrary to
6249 lexical variables that are reinitialized each time their enclosing block
6252 C<state> variables are enabled only when the C<use feature "state"> pragma
6253 is in effect. See L<feature>.
6260 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6261 doing many pattern matches on the string before it is next modified.
6262 This may or may not save time, depending on the nature and number of
6263 patterns you are searching on, and on the distribution of character
6264 frequencies in the string to be searched; you probably want to compare
6265 run times with and without it to see which runs faster. Those loops
6266 that scan for many short constant strings (including the constant
6267 parts of more complex patterns) will benefit most. You may have only
6268 one C<study> active at a time: if you study a different scalar the first
6269 is "unstudied". (The way C<study> works is this: a linked list of every
6270 character in the string to be searched is made, so we know, for
6271 example, where all the C<'k'> characters are. From each search string,
6272 the rarest character is selected, based on some static frequency tables
6273 constructed from some C programs and English text. Only those places
6274 that contain this "rarest" character are examined.)
6276 For example, here is a loop that inserts index producing entries
6277 before any line containing a certain pattern:
6281 print ".IX foo\n" if /\bfoo\b/;
6282 print ".IX bar\n" if /\bbar\b/;
6283 print ".IX blurfl\n" if /\bblurfl\b/;
6288 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
6289 will be looked at, because C<f> is rarer than C<o>. In general, this is
6290 a big win except in pathological cases. The only question is whether
6291 it saves you more time than it took to build the linked list in the
6294 Note that if you have to look for strings that you don't know till
6295 runtime, you can build an entire loop as a string and C<eval> that to
6296 avoid recompiling all your patterns all the time. Together with
6297 undefining C<$/> to input entire files as one record, this can be quite
6298 fast, often faster than specialized programs like fgrep(1). The following
6299 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6300 out the names of those files that contain a match:
6302 $search = 'while (<>) { study;';
6303 foreach $word (@words) {
6304 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6309 eval $search; # this screams
6310 $/ = "\n"; # put back to normal input delimiter
6311 foreach $file (sort keys(%seen)) {
6315 =item sub NAME BLOCK
6318 =item sub NAME (PROTO) BLOCK
6320 =item sub NAME : ATTRS BLOCK
6322 =item sub NAME (PROTO) : ATTRS BLOCK
6324 This is subroutine definition, not a real function I<per se>.
6325 Without a BLOCK it's just a forward declaration. Without a NAME,
6326 it's an anonymous function declaration, and does actually return
6327 a value: the CODE ref of the closure you just created.
6329 See L<perlsub> and L<perlref> for details about subroutines and
6330 references, and L<attributes> and L<Attribute::Handlers> for more
6331 information about attributes.
6333 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6334 X<substr> X<substring> X<mid> X<left> X<right>
6336 =item substr EXPR,OFFSET,LENGTH
6338 =item substr EXPR,OFFSET
6340 Extracts a substring out of EXPR and returns it. First character is at
6341 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6342 If OFFSET is negative (or more precisely, less than C<$[>), starts
6343 that far from the end of the string. If LENGTH is omitted, returns
6344 everything to the end of the string. If LENGTH is negative, leaves that
6345 many characters off the end of the string.
6347 my $s = "The black cat climbed the green tree";
6348 my $color = substr $s, 4, 5; # black
6349 my $middle = substr $s, 4, -11; # black cat climbed the
6350 my $end = substr $s, 14; # climbed the green tree
6351 my $tail = substr $s, -4; # tree
6352 my $z = substr $s, -4, 2; # tr
6354 You can use the substr() function as an lvalue, in which case EXPR
6355 must itself be an lvalue. If you assign something shorter than LENGTH,
6356 the string will shrink, and if you assign something longer than LENGTH,
6357 the string will grow to accommodate it. To keep the string the same
6358 length, you may need to pad or chop your value using C<sprintf>.
6360 If OFFSET and LENGTH specify a substring that is partly outside the
6361 string, only the part within the string is returned. If the substring
6362 is beyond either end of the string, substr() returns the undefined
6363 value and produces a warning. When used as an lvalue, specifying a
6364 substring that is entirely outside the string raises an exception.
6365 Here's an example showing the behavior for boundary cases:
6368 substr($name, 4) = 'dy'; # $name is now 'freddy'
6369 my $null = substr $name, 6, 2; # returns "" (no warning)
6370 my $oops = substr $name, 7; # returns undef, with warning
6371 substr($name, 7) = 'gap'; # raises an exception
6373 An alternative to using substr() as an lvalue is to specify the
6374 replacement string as the 4th argument. This allows you to replace
6375 parts of the EXPR and return what was there before in one operation,
6376 just as you can with splice().
6378 my $s = "The black cat climbed the green tree";
6379 my $z = substr $s, 14, 7, "jumped from"; # climbed
6380 # $s is now "The black cat jumped from the green tree"
6382 Note that the lvalue returned by the 3-arg version of substr() acts as
6383 a 'magic bullet'; each time it is assigned to, it remembers which part
6384 of the original string is being modified; for example:
6387 for (substr($x,1,2)) {
6388 $_ = 'a'; print $x,"\n"; # prints 1a4
6389 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6391 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6394 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6397 =item symlink OLDFILE,NEWFILE
6398 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6400 Creates a new filename symbolically linked to the old filename.
6401 Returns C<1> for success, C<0> otherwise. On systems that don't support
6402 symbolic links, raises an exception. To check for that,
6405 $symlink_exists = eval { symlink("",""); 1 };
6407 =item syscall NUMBER, LIST
6408 X<syscall> X<system call>
6410 Calls the system call specified as the first element of the list,
6411 passing the remaining elements as arguments to the system call. If
6412 unimplemented, raises an exception. The arguments are interpreted
6413 as follows: if a given argument is numeric, the argument is passed as
6414 an int. If not, the pointer to the string value is passed. You are
6415 responsible to make sure a string is pre-extended long enough to
6416 receive any result that might be written into a string. You can't use a
6417 string literal (or other read-only string) as an argument to C<syscall>
6418 because Perl has to assume that any string pointer might be written
6420 integer arguments are not literals and have never been interpreted in a
6421 numeric context, you may need to add C<0> to them to force them to look
6422 like numbers. This emulates the C<syswrite> function (or vice versa):
6424 require 'syscall.ph'; # may need to run h2ph
6426 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6428 Note that Perl supports passing of up to only 14 arguments to your syscall,
6429 which in practice should (usually) suffice.
6431 Syscall returns whatever value returned by the system call it calls.
6432 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6433 Note that some system calls can legitimately return C<-1>. The proper
6434 way to handle such calls is to assign C<$!=0;> before the call and
6435 check the value of C<$!> if syscall returns C<-1>.
6437 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6438 number of the read end of the pipe it creates. There is no way
6439 to retrieve the file number of the other end. You can avoid this
6440 problem by using C<pipe> instead.
6442 =item sysopen FILEHANDLE,FILENAME,MODE
6445 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6447 Opens the file whose filename is given by FILENAME, and associates it
6448 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6449 the name of the real filehandle wanted. This function calls the
6450 underlying operating system's C<open> function with the parameters
6451 FILENAME, MODE, PERMS.
6453 The possible values and flag bits of the MODE parameter are
6454 system-dependent; they are available via the standard module C<Fcntl>.
6455 See the documentation of your operating system's C<open> to see which
6456 values and flag bits are available. You may combine several flags
6457 using the C<|>-operator.
6459 Some of the most common values are C<O_RDONLY> for opening the file in
6460 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6461 and C<O_RDWR> for opening the file in read-write mode.
6462 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6464 For historical reasons, some values work on almost every system
6465 supported by Perl: 0 means read-only, 1 means write-only, and 2
6466 means read/write. We know that these values do I<not> work under
6467 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6468 use them in new code.
6470 If the file named by FILENAME does not exist and the C<open> call creates
6471 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6472 PERMS specifies the permissions of the newly created file. If you omit
6473 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6474 These permission values need to be in octal, and are modified by your
6475 process's current C<umask>.
6478 In many systems the C<O_EXCL> flag is available for opening files in
6479 exclusive mode. This is B<not> locking: exclusiveness means here that
6480 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6481 on network filesystems, and has no effect unless the C<O_CREAT> flag
6482 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6483 being opened if it is a symbolic link. It does not protect against
6484 symbolic links in the file's path.
6487 Sometimes you may want to truncate an already-existing file. This
6488 can be done using the C<O_TRUNC> flag. The behavior of
6489 C<O_TRUNC> with C<O_RDONLY> is undefined.
6492 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6493 that takes away the user's option to have a more permissive umask.
6494 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6497 Note that C<sysopen> depends on the fdopen() C library function.
6498 On many Unix systems, fdopen() is known to fail when file descriptors
6499 exceed a certain value, typically 255. If you need more file
6500 descriptors than that, consider rebuilding Perl to use the C<sfio>
6501 library, or perhaps using the POSIX::open() function.
6503 See L<perlopentut> for a kinder, gentler explanation of opening files.
6505 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6508 =item sysread FILEHANDLE,SCALAR,LENGTH
6510 Attempts to read LENGTH bytes of data into variable SCALAR from the
6511 specified FILEHANDLE, using the read(2). It bypasses
6512 buffered IO, so mixing this with other kinds of reads, C<print>,
6513 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6514 perlio or stdio layers usually buffers data. Returns the number of
6515 bytes actually read, C<0> at end of file, or undef if there was an
6516 error (in the latter case C<$!> is also set). SCALAR will be grown or
6517 shrunk so that the last byte actually read is the last byte of the
6518 scalar after the read.
6520 An OFFSET may be specified to place the read data at some place in the
6521 string other than the beginning. A negative OFFSET specifies
6522 placement at that many characters counting backwards from the end of
6523 the string. A positive OFFSET greater than the length of SCALAR
6524 results in the string being padded to the required size with C<"\0">
6525 bytes before the result of the read is appended.
6527 There is no syseof() function, which is ok, since eof() doesn't work
6528 well on device files (like ttys) anyway. Use sysread() and check
6529 for a return value for 0 to decide whether you're done.
6531 Note that if the filehandle has been marked as C<:utf8> Unicode
6532 characters are read instead of bytes (the LENGTH, OFFSET, and the
6533 return value of sysread() are in Unicode characters).
6534 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6535 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6537 =item sysseek FILEHANDLE,POSITION,WHENCE
6540 Sets FILEHANDLE's system position in bytes using
6541 lseek(2). FILEHANDLE may be an expression whose value gives the name
6542 of the filehandle. The values for WHENCE are C<0> to set the new
6543 position to POSITION, C<1> to set the it to the current position plus
6544 POSITION, and C<2> to set it to EOF plus POSITION (typically
6547 Note the I<in bytes>: even if the filehandle has been set to operate
6548 on characters (for example by using the C<:encoding(utf8)> I/O layer),
6549 tell() will return byte offsets, not character offsets (because
6550 implementing that would render sysseek() unacceptably slow).
6552 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6553 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6554 C<seek>, C<tell>, or C<eof> may cause confusion.
6556 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6557 and C<SEEK_END> (start of the file, current position, end of the file)
6558 from the Fcntl module. Use of the constants is also more portable
6559 than relying on 0, 1, and 2. For example to define a "systell" function:
6561 use Fcntl 'SEEK_CUR';
6562 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6564 Returns the new position, or the undefined value on failure. A position
6565 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6566 true on success and false on failure, yet you can still easily determine
6572 =item system PROGRAM LIST
6574 Does exactly the same thing as C<exec LIST>, except that a fork is
6575 done first, and the parent process waits for the child process to
6576 exit. Note that argument processing varies depending on the
6577 number of arguments. If there is more than one argument in LIST,
6578 or if LIST is an array with more than one value, starts the program
6579 given by the first element of the list with arguments given by the
6580 rest of the list. If there is only one scalar argument, the argument
6581 is checked for shell metacharacters, and if there are any, the
6582 entire argument is passed to the system's command shell for parsing
6583 (this is C</bin/sh -c> on Unix platforms, but varies on other
6584 platforms). If there are no shell metacharacters in the argument,
6585 it is split into words and passed directly to C<execvp>, which is
6588 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6589 output before any operation that may do a fork, but this may not be
6590 supported on some platforms (see L<perlport>). To be safe, you may need
6591 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6592 of C<IO::Handle> on any open handles.
6594 The return value is the exit status of the program as returned by the
6595 C<wait> call. To get the actual exit value, shift right by eight (see
6596 below). See also L</exec>. This is I<not> what you want to use to capture
6597 the output from a command, for that you should use merely backticks or
6598 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6599 indicates a failure to start the program or an error of the wait(2) system
6600 call (inspect $! for the reason).
6602 If you'd like to make C<system> (and many other bits of Perl) die on error,
6603 have a look at the L<autodie> pragma.
6605 Like C<exec>, C<system> allows you to lie to a program about its name if
6606 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6608 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6609 C<system>, if you expect your program to terminate on receipt of these
6610 signals you will need to arrange to do so yourself based on the return
6613 @args = ("command", "arg1", "arg2");
6615 or die "system @args failed: $?"
6617 If you'd like to manually inspect C<system>'s failure, you can check all
6618 possible failure modes by inspecting C<$?> like this:
6621 print "failed to execute: $!\n";
6624 printf "child died with signal %d, %s coredump\n",
6625 ($? & 127), ($? & 128) ? 'with' : 'without';
6628 printf "child exited with value %d\n", $? >> 8;
6631 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
6632 with the C<W*()> calls from the POSIX module.
6634 When C<system>'s arguments are executed indirectly by the shell,
6635 results and return codes are subject to its quirks.
6636 See L<perlop/"`STRING`"> and L</exec> for details.
6638 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6641 =item syswrite FILEHANDLE,SCALAR,LENGTH
6643 =item syswrite FILEHANDLE,SCALAR
6645 Attempts to write LENGTH bytes of data from variable SCALAR to the
6646 specified FILEHANDLE, using write(2). If LENGTH is
6647 not specified, writes whole SCALAR. It bypasses buffered IO, so
6648 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6649 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6650 stdio layers usually buffers data. Returns the number of bytes
6651 actually written, or C<undef> if there was an error (in this case the
6652 errno variable C<$!> is also set). If the LENGTH is greater than the
6653 data available in the SCALAR after the OFFSET, only as much data as is
6654 available will be written.
6656 An OFFSET may be specified to write the data from some part of the
6657 string other than the beginning. A negative OFFSET specifies writing
6658 that many characters counting backwards from the end of the string.
6659 If SCALAR is of length zero, you can only use an OFFSET of 0.
6661 B<Warning>: If the filehandle is marked C<:utf8>, Unicode characters
6662 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
6663 return value of syswrite() are in (UTF-8 encoded Unicode) characters.
6664 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6665 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6667 =item tell FILEHANDLE
6672 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6673 error. FILEHANDLE may be an expression whose value gives the name of
6674 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6677 Note the I<in bytes>: even if the filehandle has been set to
6678 operate on characters (for example by using the C<:encoding(utf8)> open
6679 layer), tell() will return byte offsets, not character offsets (because
6680 that would render seek() and tell() rather slow).
6682 The return value of tell() for the standard streams like the STDIN
6683 depends on the operating system: it may return -1 or something else.
6684 tell() on pipes, fifos, and sockets usually returns -1.
6686 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6688 Do not use tell() (or other buffered I/O operations) on a filehandle
6689 that has been manipulated by sysread(), syswrite() or sysseek().
6690 Those functions ignore the buffering, while tell() does not.
6692 =item telldir DIRHANDLE
6695 Returns the current position of the C<readdir> routines on DIRHANDLE.
6696 Value may be given to C<seekdir> to access a particular location in a
6697 directory. C<telldir> has the same caveats about possible directory
6698 compaction as the corresponding system library routine.
6700 =item tie VARIABLE,CLASSNAME,LIST
6703 This function binds a variable to a package class that will provide the
6704 implementation for the variable. VARIABLE is the name of the variable
6705 to be enchanted. CLASSNAME is the name of a class implementing objects
6706 of correct type. Any additional arguments are passed to the C<new>
6707 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6708 or C<TIEHASH>). Typically these are arguments such as might be passed
6709 to the C<dbm_open()> function of C. The object returned by the C<new>
6710 method is also returned by the C<tie> function, which would be useful
6711 if you want to access other methods in CLASSNAME.
6713 Note that functions such as C<keys> and C<values> may return huge lists
6714 when used on large objects, like DBM files. You may prefer to use the
6715 C<each> function to iterate over such. Example:
6717 # print out history file offsets
6719 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6720 while (($key,$val) = each %HIST) {
6721 print $key, ' = ', unpack('L',$val), "\n";
6725 A class implementing a hash should have the following methods:
6727 TIEHASH classname, LIST
6729 STORE this, key, value
6734 NEXTKEY this, lastkey
6739 A class implementing an ordinary array should have the following methods:
6741 TIEARRAY classname, LIST
6743 STORE this, key, value
6745 STORESIZE this, count
6751 SPLICE this, offset, length, LIST
6756 A class implementing a filehandle should have the following methods:
6758 TIEHANDLE classname, LIST
6759 READ this, scalar, length, offset
6762 WRITE this, scalar, length, offset
6764 PRINTF this, format, LIST
6768 SEEK this, position, whence
6770 OPEN this, mode, LIST
6775 A class implementing a scalar should have the following methods:
6777 TIESCALAR classname, LIST
6783 Not all methods indicated above need be implemented. See L<perltie>,
6784 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6786 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
6787 for you; you need to do that explicitly yourself. See L<DB_File>
6788 or the F<Config> module for interesting C<tie> implementations.
6790 For further details see L<perltie>, L<"tied VARIABLE">.
6795 Returns a reference to the object underlying VARIABLE (the same value
6796 that was originally returned by the C<tie> call that bound the variable
6797 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6803 Returns the number of non-leap seconds since whatever time the system
6804 considers to be the epoch, suitable for feeding to C<gmtime> and
6805 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6806 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6807 1904 in the current local time zone for its epoch.
6809 For measuring time in better granularity than one second,
6810 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6811 Perl 5.8 part of the standard distribution), or if you have
6812 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6813 See L<perlfaq8> for details.
6815 For date and time processing look at the many related modules on CPAN.
6816 For a comprehensive date and time representation look at the
6822 Returns a four-element list giving the user and system times, in
6823 seconds, for this process and the children of this process.
6825 ($user,$system,$cuser,$csystem) = times;
6827 In scalar context, C<times> returns C<$user>.
6829 Children's times are only included for terminated children.
6833 The transliteration operator. Same as C<y///>. See
6834 L<perlop/"Quote and Quote-like Operators">.
6836 =item truncate FILEHANDLE,LENGTH
6839 =item truncate EXPR,LENGTH
6841 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6842 specified length. Raises an exception if truncate isn't implemented
6843 on your system. Returns true if successful, the undefined value
6846 The behavior is undefined if LENGTH is greater than the length of the
6849 The position in the file of FILEHANDLE is left unchanged. You may want to
6850 call L<seek> before writing to the file.
6853 X<uc> X<uppercase> X<toupper>
6857 Returns an uppercased version of EXPR. This is the internal function
6858 implementing the C<\U> escape in double-quoted strings. Respects
6859 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6860 and L<perlunicode> for more details about locale and Unicode support.
6861 It does not attempt to do titlecase mapping on initial letters. See
6862 C<ucfirst> for that.
6864 If EXPR is omitted, uses C<$_>.
6867 X<ucfirst> X<uppercase>
6871 Returns the value of EXPR with the first character in uppercase
6872 (titlecase in Unicode). This is the internal function implementing
6873 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6874 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6875 for more details about locale and Unicode support.
6877 If EXPR is omitted, uses C<$_>.
6884 Sets the umask for the process to EXPR and returns the previous value.
6885 If EXPR is omitted, merely returns the current umask.
6887 The Unix permission C<rwxr-x---> is represented as three sets of three
6888 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6889 and isn't one of the digits). The C<umask> value is such a number
6890 representing disabled permissions bits. The permission (or "mode")
6891 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6892 even if you tell C<sysopen> to create a file with permissions C<0777>,
6893 if your umask is C<0022> then the file will actually be created with
6894 permissions C<0755>. If your C<umask> were C<0027> (group can't
6895 write; others can't read, write, or execute), then passing
6896 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6899 Here's some advice: supply a creation mode of C<0666> for regular
6900 files (in C<sysopen>) and one of C<0777> for directories (in
6901 C<mkdir>) and executable files. This gives users the freedom of
6902 choice: if they want protected files, they might choose process umasks
6903 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6904 Programs should rarely if ever make policy decisions better left to
6905 the user. The exception to this is when writing files that should be
6906 kept private: mail files, web browser cookies, I<.rhosts> files, and
6909 If umask(2) is not implemented on your system and you are trying to
6910 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
6911 raises an exception. If umask(2) is not implemented and you are
6912 not trying to restrict access for yourself, returns C<undef>.
6914 Remember that a umask is a number, usually given in octal; it is I<not> a
6915 string of octal digits. See also L</oct>, if all you have is a string.
6918 X<undef> X<undefine>
6922 Undefines the value of EXPR, which must be an lvalue. Use only on a
6923 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6924 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
6925 will probably not do what you expect on most predefined variables or
6926 DBM list values, so don't do that; see L<delete>. Always returns the
6927 undefined value. You can omit the EXPR, in which case nothing is
6928 undefined, but you still get an undefined value that you could, for
6929 instance, return from a subroutine, assign to a variable, or pass as a
6930 parameter. Examples:
6933 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6937 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6938 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6939 select undef, undef, undef, 0.25;
6940 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6942 Note that this is a unary operator, not a list operator.
6945 X<unlink> X<delete> X<remove> X<rm> X<del>
6949 Deletes a list of files. On success, it returns the number of files
6950 it successfully deleted. On failure, it returns false and sets C<$!>
6953 my $unlinked = unlink 'a', 'b', 'c';
6955 unlink glob "*.bak";
6957 On error, C<unlink> will not tell you which files it could not remove.
6958 If you want to know which files you could not remove, try them one
6961 foreach my $file ( @goners ) {
6962 unlink $file or warn "Could not unlink $file: $!";
6965 Note: C<unlink> will not attempt to delete directories unless you are
6966 superuser and the B<-U> flag is supplied to Perl. Even if these
6967 conditions are met, be warned that unlinking a directory can inflict
6968 damage on your filesystem. Finally, using C<unlink> on directories is
6969 not supported on many operating systems. Use C<rmdir> instead.
6971 If LIST is omitted, C<unlink> uses C<$_>.
6973 =item unpack TEMPLATE,EXPR
6976 =item unpack TEMPLATE
6978 C<unpack> does the reverse of C<pack>: it takes a string
6979 and expands it out into a list of values.
6980 (In scalar context, it returns merely the first value produced.)
6982 If EXPR is omitted, unpacks the C<$_> string.
6984 The string is broken into chunks described by the TEMPLATE. Each chunk
6985 is converted separately to a value. Typically, either the string is a result
6986 of C<pack>, or the characters of the string represent a C structure of some
6989 The TEMPLATE has the same format as in the C<pack> function.
6990 Here's a subroutine that does substring:
6993 my($what,$where,$howmuch) = @_;
6994 unpack("x$where a$howmuch", $what);
6999 sub ordinal { unpack("W",$_[0]); } # same as ord()
7001 In addition to fields allowed in pack(), you may prefix a field with
7002 a %<number> to indicate that
7003 you want a <number>-bit checksum of the items instead of the items
7004 themselves. Default is a 16-bit checksum. Checksum is calculated by
7005 summing numeric values of expanded values (for string fields the sum of
7006 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
7008 For example, the following
7009 computes the same number as the System V sum program:
7013 unpack("%32W*",<>) % 65535;
7016 The following efficiently counts the number of set bits in a bit vector:
7018 $setbits = unpack("%32b*", $selectmask);
7020 The C<p> and C<P> formats should be used with care. Since Perl
7021 has no way of checking whether the value passed to C<unpack()>
7022 corresponds to a valid memory location, passing a pointer value that's
7023 not known to be valid is likely to have disastrous consequences.
7025 If there are more pack codes or if the repeat count of a field or a group
7026 is larger than what the remainder of the input string allows, the result
7027 is not well defined: the repeat count may be decreased, or
7028 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
7029 If the input string is longer than one described by the TEMPLATE,
7030 the remainder of that input string is ignored.
7032 See L</pack> for more examples and notes.
7034 =item untie VARIABLE
7037 Breaks the binding between a variable and a package. (See C<tie>.)
7038 Has no effect if the variable is not tied.
7040 =item unshift ARRAY,LIST
7043 Does the opposite of a C<shift>. Or the opposite of a C<push>,
7044 depending on how you look at it. Prepends list to the front of the
7045 array, and returns the new number of elements in the array.
7047 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
7049 Note the LIST is prepended whole, not one element at a time, so the
7050 prepended elements stay in the same order. Use C<reverse> to do the
7053 =item use Module VERSION LIST
7054 X<use> X<module> X<import>
7056 =item use Module VERSION
7058 =item use Module LIST
7064 Imports some semantics into the current package from the named module,
7065 generally by aliasing certain subroutine or variable names into your
7066 package. It is exactly equivalent to
7068 BEGIN { require Module; Module->import( LIST ); }
7070 except that Module I<must> be a bareword.
7072 In the peculiar C<use VERSION> form, VERSION may be either a positive
7073 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
7074 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
7075 exception is raised if VERSION is greater than the version of the
7076 current Perl interpreter; Perl will not attempt to parse the rest of the
7077 file. Compare with L</require>, which can do a similar check at run time.
7078 Symmetrically, C<no VERSION> allows you to specify that you want a version
7079 of Perl older than the specified one.
7081 Specifying VERSION as a literal of the form v5.6.1 should generally be
7082 avoided, because it leads to misleading error messages under earlier
7083 versions of Perl (that is, prior to 5.6.0) that do not support this
7084 syntax. The equivalent numeric version should be used instead.
7086 use v5.6.1; # compile time version check
7088 use 5.006_001; # ditto; preferred for backwards compatibility
7090 This is often useful if you need to check the current Perl version before
7091 C<use>ing library modules that won't work with older versions of Perl.
7092 (We try not to do this more than we have to.)
7094 Also, if the specified Perl version is greater than or equal to 5.9.5,
7095 C<use VERSION> will also load the C<feature> pragma and enable all
7096 features available in the requested version. See L<feature>.
7097 Similarly, if the specified Perl version is greater than or equal to
7098 5.11.0, strictures are enabled lexically as with C<use strict> (except
7099 that the F<strict.pm> file is not actually loaded).
7101 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
7102 C<require> makes sure the module is loaded into memory if it hasn't been
7103 yet. The C<import> is not a builtin; it's just an ordinary static method
7104 call into the C<Module> package to tell the module to import the list of
7105 features back into the current package. The module can implement its
7106 C<import> method any way it likes, though most modules just choose to
7107 derive their C<import> method via inheritance from the C<Exporter> class that
7108 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
7109 method can be found then the call is skipped, even if there is an AUTOLOAD
7112 If you do not want to call the package's C<import> method (for instance,
7113 to stop your namespace from being altered), explicitly supply the empty list:
7117 That is exactly equivalent to
7119 BEGIN { require Module }
7121 If the VERSION argument is present between Module and LIST, then the
7122 C<use> will call the VERSION method in class Module with the given
7123 version as an argument. The default VERSION method, inherited from
7124 the UNIVERSAL class, croaks if the given version is larger than the
7125 value of the variable C<$Module::VERSION>.
7127 Again, there is a distinction between omitting LIST (C<import> called
7128 with no arguments) and an explicit empty LIST C<()> (C<import> not
7129 called). Note that there is no comma after VERSION!
7131 Because this is a wide-open interface, pragmas (compiler directives)
7132 are also implemented this way. Currently implemented pragmas are:
7137 use sigtrap qw(SEGV BUS);
7138 use strict qw(subs vars refs);
7139 use subs qw(afunc blurfl);
7140 use warnings qw(all);
7141 use sort qw(stable _quicksort _mergesort);
7143 Some of these pseudo-modules import semantics into the current
7144 block scope (like C<strict> or C<integer>, unlike ordinary modules,
7145 which import symbols into the current package (which are effective
7146 through the end of the file).
7148 Because C<use> takes effect at compile time, it doesn't respect the
7149 ordinary flow control of the code being compiled. In particular, putting
7150 a C<use> inside the false branch of a conditional doesn't prevent it
7151 from being processed. If a module or pragma only needs to be loaded
7152 conditionally, this can be done using the L<if> pragma:
7154 use if $] < 5.008, "utf8";
7155 use if WANT_WARNINGS, warnings => qw(all);
7157 There's a corresponding C<no> command that unimports meanings imported
7158 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
7159 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
7160 or no unimport method being found.
7166 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
7167 for the C<-M> and C<-m> command-line options to Perl that give C<use>
7168 functionality from the command-line.
7173 Changes the access and modification times on each file of a list of
7174 files. The first two elements of the list must be the NUMERICAL access
7175 and modification times, in that order. Returns the number of files
7176 successfully changed. The inode change time of each file is set
7177 to the current time. For example, this code has the same effect as the
7178 Unix touch(1) command when the files I<already exist> and belong to
7179 the user running the program:
7182 $atime = $mtime = time;
7183 utime $atime, $mtime, @ARGV;
7185 Since Perl 5.7.2, if the first two elements of the list are C<undef>,
7186 the utime(2) syscall from your C library is called with a null second
7187 argument. On most systems, this will set the file's access and
7188 modification times to the current time (i.e., equivalent to the example
7189 above) and will work even on files you don't own provided you have write
7193 utime(undef, undef, $file)
7194 || warn "couldn't touch $file: $!";
7197 Under NFS this will use the time of the NFS server, not the time of
7198 the local machine. If there is a time synchronization problem, the
7199 NFS server and local machine will have different times. The Unix
7200 touch(1) command will in fact normally use this form instead of the
7201 one shown in the first example.
7203 Passing only one of the first two elements as C<undef> is
7204 equivalent to passing a 0 and will not have the effect
7205 described when both are C<undef>. This also triggers an
7206 uninitialized warning.
7208 On systems that support futimes(2), you may pass filehandles among the
7209 files. On systems that don't support futimes(2), passing filehandles raises
7210 an exception. Filehandles must be passed as globs or glob references to be
7211 recognized; barewords are considered filenames.
7218 Returns a list consisting of all the values of the named hash, or the values
7219 of an array. (In a scalar context, returns the number of values.)
7221 The values are returned in an apparently random order. The actual
7222 random order is subject to change in future versions of Perl, but it
7223 is guaranteed to be the same order as either the C<keys> or C<each>
7224 function would produce on the same (unmodified) hash. Since Perl
7225 5.8.1 the ordering is different even between different runs of Perl
7226 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7228 As a side effect, calling values() resets the HASH or ARRAY's internal
7230 see L</each>. (In particular, calling values() in void context resets
7231 the iterator with no other overhead. Apart from resetting the iterator,
7232 C<values @array> in list context is the same as plain C<@array>.
7233 We recommend that you use void context C<keys @array> for this, but reasoned
7234 that it taking C<values @array> out would require more documentation than
7238 Note that the values are not copied, which means modifying them will
7239 modify the contents of the hash:
7241 for (values %hash) { s/foo/bar/g } # modifies %hash values
7242 for (@hash{keys %hash}) { s/foo/bar/g } # same
7244 See also C<keys>, C<each>, and C<sort>.
7246 =item vec EXPR,OFFSET,BITS
7247 X<vec> X<bit> X<bit vector>
7249 Treats the string in EXPR as a bit vector made up of elements of
7250 width BITS, and returns the value of the element specified by OFFSET
7251 as an unsigned integer. BITS therefore specifies the number of bits
7252 that are reserved for each element in the bit vector. This must
7253 be a power of two from 1 to 32 (or 64, if your platform supports
7256 If BITS is 8, "elements" coincide with bytes of the input string.
7258 If BITS is 16 or more, bytes of the input string are grouped into chunks
7259 of size BITS/8, and each group is converted to a number as with
7260 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7261 for BITS==64). See L<"pack"> for details.
7263 If bits is 4 or less, the string is broken into bytes, then the bits
7264 of each byte are broken into 8/BITS groups. Bits of a byte are
7265 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7266 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7267 breaking the single input byte C<chr(0x36)> into two groups gives a list
7268 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7270 C<vec> may also be assigned to, in which case parentheses are needed
7271 to give the expression the correct precedence as in
7273 vec($image, $max_x * $x + $y, 8) = 3;
7275 If the selected element is outside the string, the value 0 is returned.
7276 If an element off the end of the string is written to, Perl will first
7277 extend the string with sufficiently many zero bytes. It is an error
7278 to try to write off the beginning of the string (i.e., negative OFFSET).
7280 If the string happens to be encoded as UTF-8 internally (and thus has
7281 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7282 internal byte string, not the conceptual character string, even if you
7283 only have characters with values less than 256.
7285 Strings created with C<vec> can also be manipulated with the logical
7286 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7287 vector operation is desired when both operands are strings.
7288 See L<perlop/"Bitwise String Operators">.
7290 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7291 The comments show the string after each step. Note that this code works
7292 in the same way on big-endian or little-endian machines.
7295 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7297 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7298 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7300 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7301 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7302 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7303 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7304 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7305 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7307 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7308 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7309 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7312 To transform a bit vector into a string or list of 0's and 1's, use these:
7314 $bits = unpack("b*", $vector);
7315 @bits = split(//, unpack("b*", $vector));
7317 If you know the exact length in bits, it can be used in place of the C<*>.
7319 Here is an example to illustrate how the bits actually fall in place:
7325 unpack("V",$_) 01234567890123456789012345678901
7326 ------------------------------------------------------------------
7331 for ($shift=0; $shift < $width; ++$shift) {
7332 for ($off=0; $off < 32/$width; ++$off) {
7333 $str = pack("B*", "0"x32);
7334 $bits = (1<<$shift);
7335 vec($str, $off, $width) = $bits;
7336 $res = unpack("b*",$str);
7337 $val = unpack("V", $str);
7344 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7345 $off, $width, $bits, $val, $res
7349 Regardless of the machine architecture on which it runs, the
7350 example above should print the following table:
7353 unpack("V",$_) 01234567890123456789012345678901
7354 ------------------------------------------------------------------
7355 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7356 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7357 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7358 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7359 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7360 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7361 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7362 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7363 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7364 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7365 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7366 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7367 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7368 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7369 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7370 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7371 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7372 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7373 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7374 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7375 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7376 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7377 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7378 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7379 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7380 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7381 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7382 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7383 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7384 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7385 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7386 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7387 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7388 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7389 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7390 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7391 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7392 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7393 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7394 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7395 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7396 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7397 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7398 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7399 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7400 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7401 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7402 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7403 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7404 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7405 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7406 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7407 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7408 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7409 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7410 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7411 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7412 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7413 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7414 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7415 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7416 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7417 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7418 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7419 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7420 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7421 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7422 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7423 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7424 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7425 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7426 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7427 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7428 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7429 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7430 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7431 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7432 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7433 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7434 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7435 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7436 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7437 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7438 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7439 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7440 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7441 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7442 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7443 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7444 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7445 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7446 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7447 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7448 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7449 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7450 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7451 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7452 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7453 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7454 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7455 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7456 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7457 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7458 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7459 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7460 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7461 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7462 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7463 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7464 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7465 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7466 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7467 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7468 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7469 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7470 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7471 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7472 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7473 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7474 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7475 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7476 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7477 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7478 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7479 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7480 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7481 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7482 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7487 Behaves like wait(2) on your system: it waits for a child
7488 process to terminate and returns the pid of the deceased process, or
7489 C<-1> if there are no child processes. The status is returned in C<$?>
7490 and C<${^CHILD_ERROR_NATIVE}>.
7491 Note that a return value of C<-1> could mean that child processes are
7492 being automatically reaped, as described in L<perlipc>.
7494 =item waitpid PID,FLAGS
7497 Waits for a particular child process to terminate and returns the pid of
7498 the deceased process, or C<-1> if there is no such child process. On some
7499 systems, a value of 0 indicates that there are processes still running.
7500 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
7502 use POSIX ":sys_wait_h";
7505 $kid = waitpid(-1, WNOHANG);
7508 then you can do a non-blocking wait for all pending zombie processes.
7509 Non-blocking wait is available on machines supporting either the
7510 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
7511 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7512 system call by remembering the status values of processes that have
7513 exited but have not been harvested by the Perl script yet.)
7515 Note that on some systems, a return value of C<-1> could mean that child
7516 processes are being automatically reaped. See L<perlipc> for details,
7517 and for other examples.
7520 X<wantarray> X<context>
7522 Returns true if the context of the currently executing subroutine or
7523 C<eval> is looking for a list value. Returns false if the context is
7524 looking for a scalar. Returns the undefined value if the context is
7525 looking for no value (void context).
7527 return unless defined wantarray; # don't bother doing more
7528 my @a = complex_calculation();
7529 return wantarray ? @a : "@a";
7531 C<wantarray()>'s result is unspecified in the top level of a file,
7532 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7533 in a C<DESTROY> method.
7535 This function should have been named wantlist() instead.
7538 X<warn> X<warning> X<STDERR>
7540 Prints the value of LIST to STDERR. If the last element of LIST does
7541 not end in a newline, it appends the same file/line number text as C<die>
7544 If the output is empty and C<$@> already contains a value (typically from a
7545 previous eval) that value is used after appending C<"\t...caught">
7546 to C<$@>. This is useful for staying almost, but not entirely similar to
7549 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7551 No message is printed if there is a C<$SIG{__WARN__}> handler
7552 installed. It is the handler's responsibility to deal with the message
7553 as it sees fit (like, for instance, converting it into a C<die>). Most
7554 handlers must therefore arrange to actually display the
7555 warnings that they are not prepared to deal with, by calling C<warn>
7556 again in the handler. Note that this is quite safe and will not
7557 produce an endless loop, since C<__WARN__> hooks are not called from
7560 You will find this behavior is slightly different from that of
7561 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7562 instead call C<die> again to change it).
7564 Using a C<__WARN__> handler provides a powerful way to silence all
7565 warnings (even the so-called mandatory ones). An example:
7567 # wipe out *all* compile-time warnings
7568 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7570 my $foo = 20; # no warning about duplicate my $foo,
7571 # but hey, you asked for it!
7572 # no compile-time or run-time warnings before here
7575 # run-time warnings enabled after here
7576 warn "\$foo is alive and $foo!"; # does show up
7578 See L<perlvar> for details on setting C<%SIG> entries, and for more
7579 examples. See the Carp module for other kinds of warnings using its
7580 carp() and cluck() functions.
7582 =item write FILEHANDLE
7589 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7590 using the format associated with that file. By default the format for
7591 a file is the one having the same name as the filehandle, but the
7592 format for the current output channel (see the C<select> function) may be set
7593 explicitly by assigning the name of the format to the C<$~> variable.
7595 Top of form processing is handled automatically: if there is
7596 insufficient room on the current page for the formatted record, the
7597 page is advanced by writing a form feed, a special top-of-page format
7598 is used to format the new page header, and then the record is written.
7599 By default the top-of-page format is the name of the filehandle with
7600 "_TOP" appended, but it may be dynamically set to the format of your
7601 choice by assigning the name to the C<$^> variable while the filehandle is
7602 selected. The number of lines remaining on the current page is in
7603 variable C<$->, which can be set to C<0> to force a new page.
7605 If FILEHANDLE is unspecified, output goes to the current default output
7606 channel, which starts out as STDOUT but may be changed by the
7607 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7608 is evaluated and the resulting string is used to look up the name of
7609 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7611 Note that write is I<not> the opposite of C<read>. Unfortunately.
7615 The transliteration operator. Same as C<tr///>. See
7616 L<perlop/"Quote and Quote-like Operators">.