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<$hinthash> is a reference to a hash containing the value of C<%^H> when the
671 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
672 of this hash, as they are the actual values stored in the optree.
674 Furthermore, when called from within the DB package, caller returns more
675 detailed information: it sets the list variable C<@DB::args> to be the
676 arguments with which the subroutine was invoked.
678 Be aware that the optimizer might have optimized call frames away before
679 C<caller> had a chance to get the information. That means that C<caller(N)>
680 might not return information about the call frame you expect it to, for
681 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
682 previous time C<caller> was called.
684 Also be aware that setting C<@DB::args> is I<best effort>, intended for
685 debugging or generating backtraces, and should not be relied upon. In
686 particular, as C<@_> contains aliases to the caller's arguments, Perl does
687 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
688 subroutine makes to C<@_> or its contents, not the original values at call
689 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
690 elements, so under certain cases its elements may have become freed and
691 reallocated for other variables or temporary values. Finally, a side effect
692 of the current implementation means that the effects of C<shift @_> can
693 I<normally> be undone (but not C<pop @_> or other splicing, and not if a
694 reference to C<@_> has been taken, and subject to the caveat about reallocated
695 elements), so C<@DB::args> is actually a hybrid of the current state and
696 initial state of C<@_>. Buyer beware.
703 =item chdir FILEHANDLE
705 =item chdir DIRHANDLE
709 Changes the working directory to EXPR, if possible. If EXPR is omitted,
710 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
711 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
712 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
713 neither is set, C<chdir> does nothing. It returns true on success,
714 false otherwise. See the example under C<die>.
716 On systems that support fchdir(2), you may pass a filehandle or
717 directory handle as argument. On systems that don't support fchdir(2),
718 passing handles raises an exception.
721 X<chmod> X<permission> X<mode>
723 Changes the permissions of a list of files. The first element of the
724 list must be the numerical mode, which should probably be an octal
725 number, and which definitely should I<not> be a string of octal digits:
726 C<0644> is okay, but C<"0644"> is not. Returns the number of files
727 successfully changed. See also L</oct>, if all you have is a string.
729 $cnt = chmod 0755, "foo", "bar";
730 chmod 0755, @executables;
731 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
733 $mode = "0644"; chmod oct($mode), "foo"; # this is better
734 $mode = 0644; chmod $mode, "foo"; # this is best
736 On systems that support fchmod(2), you may pass filehandles among the
737 files. On systems that don't support fchmod(2), passing filehandles raises
738 an exception. Filehandles must be passed as globs or glob references to be
739 recognized; barewords are considered filenames.
741 open(my $fh, "<", "foo");
742 my $perm = (stat $fh)[2] & 07777;
743 chmod($perm | 0600, $fh);
745 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
748 use Fcntl qw( :mode );
749 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
750 # Identical to the chmod 0755 of the example above.
753 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
759 This safer version of L</chop> removes any trailing string
760 that corresponds to the current value of C<$/> (also known as
761 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
762 number of characters removed from all its arguments. It's often used to
763 remove the newline from the end of an input record when you're worried
764 that the final record may be missing its newline. When in paragraph
765 mode (C<$/ = "">), it removes all trailing newlines from the string.
766 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
767 a reference to an integer or the like, see L<perlvar>) chomp() won't
769 If VARIABLE is omitted, it chomps C<$_>. Example:
772 chomp; # avoid \n on last field
777 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
779 You can actually chomp anything that's an lvalue, including an assignment:
782 chomp($answer = <STDIN>);
784 If you chomp a list, each element is chomped, and the total number of
785 characters removed is returned.
787 Note that parentheses are necessary when you're chomping anything
788 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
789 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
790 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
791 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
801 Chops off the last character of a string and returns the character
802 chopped. It is much more efficient than C<s/.$//s> because it neither
803 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
804 If VARIABLE is a hash, it chops the hash's values, but not its keys.
806 You can actually chop anything that's an lvalue, including an assignment.
808 If you chop a list, each element is chopped. Only the value of the
809 last C<chop> is returned.
811 Note that C<chop> returns the last character. To return all but the last
812 character, use C<substr($string, 0, -1)>.
817 X<chown> X<owner> X<user> X<group>
819 Changes the owner (and group) of a list of files. The first two
820 elements of the list must be the I<numeric> uid and gid, in that
821 order. A value of -1 in either position is interpreted by most
822 systems to leave that value unchanged. Returns the number of files
823 successfully changed.
825 $cnt = chown $uid, $gid, 'foo', 'bar';
826 chown $uid, $gid, @filenames;
828 On systems that support fchown(2), you may pass filehandles among the
829 files. On systems that don't support fchown(2), passing filehandles raises
830 an exception. Filehandles must be passed as globs or glob references to be
831 recognized; barewords are considered filenames.
833 Here's an example that looks up nonnumeric uids in the passwd file:
836 chomp($user = <STDIN>);
838 chomp($pattern = <STDIN>);
840 ($login,$pass,$uid,$gid) = getpwnam($user)
841 or die "$user not in passwd file";
843 @ary = glob($pattern); # expand filenames
844 chown $uid, $gid, @ary;
846 On most systems, you are not allowed to change the ownership of the
847 file unless you're the superuser, although you should be able to change
848 the group to any of your secondary groups. On insecure systems, these
849 restrictions may be relaxed, but this is not a portable assumption.
850 On POSIX systems, you can detect this condition this way:
852 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
853 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
856 X<chr> X<character> X<ASCII> X<Unicode>
860 Returns the character represented by that NUMBER in the character set.
861 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
862 chr(0x263a) is a Unicode smiley face.
864 Negative values give the Unicode replacement character (chr(0xfffd)),
865 except under the L<bytes> pragma, where the low eight bits of the value
866 (truncated to an integer) are used.
868 If NUMBER is omitted, uses C<$_>.
870 For the reverse, use L</ord>.
872 Note that characters from 128 to 255 (inclusive) are by default
873 internally not encoded as UTF-8 for backward compatibility reasons.
875 See L<perlunicode> for more about Unicode.
877 =item chroot FILENAME
882 This function works like the system call by the same name: it makes the
883 named directory the new root directory for all further pathnames that
884 begin with a C</> by your process and all its children. (It doesn't
885 change your current working directory, which is unaffected.) For security
886 reasons, this call is restricted to the superuser. If FILENAME is
887 omitted, does a C<chroot> to C<$_>.
889 =item close FILEHANDLE
894 Closes the file or pipe associated with the filehandle, flushes the IO
895 buffers, and closes the system file descriptor. Returns true if those
896 operations have succeeded and if no error was reported by any PerlIO
897 layer. Closes the currently selected filehandle if the argument is
900 You don't have to close FILEHANDLE if you are immediately going to do
901 another C<open> on it, because C<open> closes it for you. (See
902 C<open>.) However, an explicit C<close> on an input file resets the line
903 counter (C<$.>), while the implicit close done by C<open> does not.
905 If the filehandle came from a piped open, C<close> returns false if one of
906 the other syscalls involved fails or if its program exits with non-zero
907 status. If the only problem was that the program exited non-zero, C<$!>
908 will be set to C<0>. Closing a pipe also waits for the process executing
909 on the pipe to exit--in case you wish to look at the output of the pipe
910 afterwards--and implicitly puts the exit status value of that command into
911 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
913 Closing the read end of a pipe before the process writing to it at the
914 other end is done writing results in the writer receiving a SIGPIPE. If
915 the other end can't handle that, be sure to read all the data before
920 open(OUTPUT, '|sort >foo') # pipe to sort
921 or die "Can't start sort: $!";
922 #... # print stuff to output
923 close OUTPUT # wait for sort to finish
924 or warn $! ? "Error closing sort pipe: $!"
925 : "Exit status $? from sort";
926 open(INPUT, 'foo') # get sort's results
927 or die "Can't open 'foo' for input: $!";
929 FILEHANDLE may be an expression whose value can be used as an indirect
930 filehandle, usually the real filehandle name.
932 =item closedir DIRHANDLE
935 Closes a directory opened by C<opendir> and returns the success of that
938 =item connect SOCKET,NAME
941 Attempts to connect to a remote socket, just like connect(2).
942 Returns true if it succeeded, false otherwise. NAME should be a
943 packed address of the appropriate type for the socket. See the examples in
944 L<perlipc/"Sockets: Client/Server Communication">.
951 C<continue> is actually a flow control statement rather than a function. If
952 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
953 C<foreach>), it is always executed just before the conditional is about to
954 be evaluated again, just like the third part of a C<for> loop in C. Thus
955 it can be used to increment a loop variable, even when the loop has been
956 continued via the C<next> statement (which is similar to the C C<continue>
959 C<last>, C<next>, or C<redo> may appear within a C<continue>
960 block; C<last> and C<redo> behave as if they had been executed within
961 the main block. So will C<next>, but since it will execute a C<continue>
962 block, it may be more entertaining.
965 ### redo always comes here
968 ### next always comes here
970 # then back the top to re-check EXPR
972 ### last always comes here
974 Omitting the C<continue> section is equivalent to using an
975 empty one, logically enough, so C<next> goes directly back
976 to check the condition at the top of the loop.
978 If the C<"switch"> feature is enabled, C<continue> is also a
979 function that exits the current C<when> (or C<default>) block and
980 falls through to the next one. See L<feature> and
981 L<perlsyn/"Switch statements"> for more information.
985 X<cos> X<cosine> X<acos> X<arccosine>
989 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
990 takes cosine of C<$_>.
992 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
993 function, or use this relation:
995 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
997 =item crypt PLAINTEXT,SALT
998 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
999 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1001 Creates a digest string exactly like the crypt(3) function in the C
1002 library (assuming that you actually have a version there that has not
1003 been extirpated as a potential munition).
1005 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
1006 into a short string, called a digest, which is returned. The same
1007 PLAINTEXT and SALT will always return the same string, but there is no
1008 (known) way to get the original PLAINTEXT from the hash. Small
1009 changes in the PLAINTEXT or SALT will result in large changes in the
1012 There is no decrypt function. This function isn't all that useful for
1013 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1014 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1015 primarily used to check if two pieces of text are the same without
1016 having to transmit or store the text itself. An example is checking
1017 if a correct password is given. The digest of the password is stored,
1018 not the password itself. The user types in a password that is
1019 crypt()'d with the same salt as the stored digest. If the two digests
1020 match the password is correct.
1022 When verifying an existing digest string you should use the digest as
1023 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1024 to create the digest is visible as part of the digest. This ensures
1025 crypt() will hash the new string with the same salt as the digest.
1026 This allows your code to work with the standard L<crypt|/crypt> and
1027 with more exotic implementations. In other words, do not assume
1028 anything about the returned string itself, or how many bytes in the
1031 Traditionally the result is a string of 13 bytes: two first bytes of
1032 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1033 the first eight bytes of PLAINTEXT mattered. But alternative
1034 hashing schemes (like MD5), higher level security schemes (like C2),
1035 and implementations on non-Unix platforms may produce different
1038 When choosing a new salt create a random two character string whose
1039 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1040 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1041 characters is just a recommendation; the characters allowed in
1042 the salt depend solely on your system's crypt library, and Perl can't
1043 restrict what salts C<crypt()> accepts.
1045 Here's an example that makes sure that whoever runs this program knows
1048 $pwd = (getpwuid($<))[1];
1050 system "stty -echo";
1052 chomp($word = <STDIN>);
1056 if (crypt($word, $pwd) ne $pwd) {
1062 Of course, typing in your own password to whoever asks you
1065 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1066 of data, not least of all because you can't get the information
1067 back. Look at the L<Digest> module for more robust algorithms.
1069 If using crypt() on a Unicode string (which I<potentially> has
1070 characters with codepoints above 255), Perl tries to make sense
1071 of the situation by trying to downgrade (a copy of the string)
1072 the string back to an eight-bit byte string before calling crypt()
1073 (on that copy). If that works, good. If not, crypt() dies with
1074 C<Wide character in crypt>.
1079 [This function has been largely superseded by the C<untie> function.]
1081 Breaks the binding between a DBM file and a hash.
1083 =item dbmopen HASH,DBNAME,MASK
1084 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1086 [This function has been largely superseded by the C<tie> function.]
1088 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1089 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1090 argument is I<not> a filehandle, even though it looks like one). DBNAME
1091 is the name of the database (without the F<.dir> or F<.pag> extension if
1092 any). If the database does not exist, it is created with protection
1093 specified by MASK (as modified by the C<umask>). If your system supports
1094 only the older DBM functions, you may make only one C<dbmopen> call in your
1095 program. In older versions of Perl, if your system had neither DBM nor
1096 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1099 If you don't have write access to the DBM file, you can only read hash
1100 variables, not set them. If you want to test whether you can write,
1101 either use file tests or try setting a dummy hash entry inside an C<eval>
1104 Note that functions such as C<keys> and C<values> may return huge lists
1105 when used on large DBM files. You may prefer to use the C<each>
1106 function to iterate over large DBM files. Example:
1108 # print out history file offsets
1109 dbmopen(%HIST,'/usr/lib/news/history',0666);
1110 while (($key,$val) = each %HIST) {
1111 print $key, ' = ', unpack('L',$val), "\n";
1115 See also L<AnyDBM_File> for a more general description of the pros and
1116 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1117 rich implementation.
1119 You can control which DBM library you use by loading that library
1120 before you call dbmopen():
1123 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1124 or die "Can't open netscape history file: $!";
1127 X<defined> X<undef> X<undefined>
1131 Returns a Boolean value telling whether EXPR has a value other than
1132 the undefined value C<undef>. If EXPR is not present, C<$_> is
1135 Many operations return C<undef> to indicate failure, end of file,
1136 system error, uninitialized variable, and other exceptional
1137 conditions. This function allows you to distinguish C<undef> from
1138 other values. (A simple Boolean test will not distinguish among
1139 C<undef>, zero, the empty string, and C<"0">, which are all equally
1140 false.) Note that since C<undef> is a valid scalar, its presence
1141 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1142 returns C<undef> when its argument is an empty array, I<or> when the
1143 element to return happens to be C<undef>.
1145 You may also use C<defined(&func)> to check whether subroutine C<&func>
1146 has ever been defined. The return value is unaffected by any forward
1147 declarations of C<&func>. A subroutine that is not defined
1148 may still be callable: its package may have an C<AUTOLOAD> method that
1149 makes it spring into existence the first time that it is called; see
1152 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1153 used to report whether memory for that aggregate has ever been
1154 allocated. This behavior may disappear in future versions of Perl.
1155 You should instead use a simple test for size:
1157 if (@an_array) { print "has array elements\n" }
1158 if (%a_hash) { print "has hash members\n" }
1160 When used on a hash element, it tells you whether the value is defined,
1161 not whether the key exists in the hash. Use L</exists> for the latter
1166 print if defined $switch{'D'};
1167 print "$val\n" while defined($val = pop(@ary));
1168 die "Can't readlink $sym: $!"
1169 unless defined($value = readlink $sym);
1170 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1171 $debugging = 0 unless defined $debugging;
1173 Note: Many folks tend to overuse C<defined>, and then are surprised to
1174 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1175 defined values. For example, if you say
1179 The pattern match succeeds and C<$1> is defined, although it
1180 matched "nothing". It didn't really fail to match anything. Rather, it
1181 matched something that happened to be zero characters long. This is all
1182 very above-board and honest. When a function returns an undefined value,
1183 it's an admission that it couldn't give you an honest answer. So you
1184 should use C<defined> only when questioning the integrity of what
1185 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1188 See also L</undef>, L</exists>, L</ref>.
1193 Given an expression that specifies an element or slice of a hash, C<delete>
1194 deletes the specified elements from that hash so that exists() on that element
1195 no longer returns true. Setting a hash element to the undefined value does
1196 not remove its key, but deleting it does; see L</exists>.
1198 It returns the value or values deleted in list context, or the last such
1199 element in scalar context. The return list's length always matches that of
1200 the argument list: deleting non-existent elements returns the undefined value
1201 in their corresponding positions.
1203 delete() may also be used on arrays and array slices, but its behavior is less
1204 straightforward. Although exists() will return false for deleted entries,
1205 deleting array elements never changes indices of existing values; use shift()
1206 or splice() for that. However, if all deleted elements fall at the end of an
1207 array, the array's size shrinks to the position of the highest element that
1208 still tests true for exists(), or to 0 if none do.
1210 B<Be aware> that calling delete on array values is deprecated and likely to
1211 be removed in a future version of Perl.
1213 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1214 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1215 or array may not necessarily return anything; it depends on the implementation
1216 of the C<tied> package's DELETE method, which may do whatever it pleases.
1218 The C<delete local EXPR> construct localizes the deletion to the current
1219 block at run time. Until the block exits, elements locally deleted
1220 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1221 of composite types">.
1223 %hash = (foo => 11, bar => 22, baz => 33);
1224 $scalar = delete $hash{foo}; # $scalar is 11
1225 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1226 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1228 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1230 foreach $key (keys %HASH) {
1234 foreach $index (0 .. $#ARRAY) {
1235 delete $ARRAY[$index];
1240 delete @HASH{keys %HASH};
1242 delete @ARRAY[0 .. $#ARRAY];
1244 But both are slower than assigning the empty list
1245 or undefining %HASH or @ARRAY, which is the customary
1246 way to empty out an aggregate:
1248 %HASH = (); # completely empty %HASH
1249 undef %HASH; # forget %HASH ever existed
1251 @ARRAY = (); # completely empty @ARRAY
1252 undef @ARRAY; # forget @ARRAY ever existed
1254 The EXPR can be arbitrarily complicated provided its
1255 final operation is an element or slice of an aggregate:
1257 delete $ref->[$x][$y]{$key};
1258 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1260 delete $ref->[$x][$y][$index];
1261 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1264 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1266 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1267 into C<$@> and the C<eval> is terminated with the undefined value.
1268 If the exception is outside of all enclosing C<eval>s, then the uncaught
1269 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1270 need to exit the process with a specific exit code, see L<exit>.
1272 Equivalent examples:
1274 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1275 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1277 If the last element of LIST does not end in a newline, the current
1278 script line number and input line number (if any) are also printed,
1279 and a newline is supplied. Note that the "input line number" (also
1280 known as "chunk") is subject to whatever notion of "line" happens to
1281 be currently in effect, and is also available as the special variable
1282 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1284 Hint: sometimes appending C<", stopped"> to your message will cause it
1285 to make better sense when the string C<"at foo line 123"> is appended.
1286 Suppose you are running script "canasta".
1288 die "/etc/games is no good";
1289 die "/etc/games is no good, stopped";
1291 produce, respectively
1293 /etc/games is no good at canasta line 123.
1294 /etc/games is no good, stopped at canasta line 123.
1296 If the output is empty and C<$@> already contains a value (typically from a
1297 previous eval) that value is reused after appending C<"\t...propagated">.
1298 This is useful for propagating exceptions:
1301 die unless $@ =~ /Expected exception/;
1303 If the output is empty and C<$@> contains an object reference that has a
1304 C<PROPAGATE> method, that method will be called with additional file
1305 and line number parameters. The return value replaces the value in
1306 C<$@>. i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1309 If C<$@> is empty then the string C<"Died"> is used.
1311 If an uncaught exception results in interpreter exit, the exit code is
1312 determined from the values of C<$!> and C<$?> with this pseudocode:
1314 exit $! if $!; # errno
1315 exit $? >> 8 if $? >> 8; # child exit status
1316 exit 255; # last resort
1318 The intent is to squeeze as much possible information about the likely cause
1319 into the limited space of the system exit code. However, as C<$!> is the value
1320 of C's C<errno>, which can be set by any system call, this means that the value
1321 of the exit code used by C<die> can be non-predictable, so should not be relied
1322 upon, other than to be non-zero.
1324 You can also call C<die> with a reference argument, and if this is trapped
1325 within an C<eval>, C<$@> contains that reference. This permits more
1326 elaborate exception handling using objects that maintain arbitrary state
1327 about the exception. Such a scheme is sometimes preferable to matching
1328 particular string values of C<$@> with regular expressions. Because C<$@>
1329 is a global variable and C<eval> may be used within object implementations,
1330 be careful that analyzing the error object doesn't replace the reference in
1331 the global variable. It's easiest to make a local copy of the reference
1332 before any manipulations. Here's an example:
1334 use Scalar::Util "blessed";
1336 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1337 if (my $ev_err = $@) {
1338 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1339 # handle Some::Module::Exception
1342 # handle all other possible exceptions
1346 Because Perl stringifies uncaught exception messages before display,
1347 you'll probably want to overload stringification operations on
1348 exception objects. See L<overload> for details about that.
1350 You can arrange for a callback to be run just before the C<die>
1351 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1352 handler is called with the error text and can change the error
1353 message, if it sees fit, by calling C<die> again. See
1354 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1355 L<"eval BLOCK"> for some examples. Although this feature was
1356 to be run only right before your program was to exit, this is not
1357 currently so: the C<$SIG{__DIE__}> hook is currently called
1358 even inside eval()ed blocks/strings! If one wants the hook to do
1359 nothing in such situations, put
1363 as the first line of the handler (see L<perlvar/$^S>). Because
1364 this promotes strange action at a distance, this counterintuitive
1365 behavior may be fixed in a future release.
1367 See also exit(), warn(), and the Carp module.
1372 Not really a function. Returns the value of the last command in the
1373 sequence of commands indicated by BLOCK. When modified by the C<while> or
1374 C<until> loop modifier, executes the BLOCK once before testing the loop
1375 condition. (On other statements the loop modifiers test the conditional
1378 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1379 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1380 See L<perlsyn> for alternative strategies.
1382 =item do SUBROUTINE(LIST)
1385 This form of subroutine call is deprecated. See L<perlsub>.
1390 Uses the value of EXPR as a filename and executes the contents of the
1391 file as a Perl script.
1399 except that it's more efficient and concise, keeps track of the current
1400 filename for error messages, searches the @INC directories, and updates
1401 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1402 variables. It also differs in that code evaluated with C<do FILENAME>
1403 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1404 same, however, in that it does reparse the file every time you call it,
1405 so you probably don't want to do this inside a loop.
1407 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1408 error. If C<do> can read the file but cannot compile it, it
1409 returns undef and sets an error message in C<$@>. If the file is
1410 successfully compiled, C<do> returns the value of the last expression
1413 Inclusion of library modules is better done with the
1414 C<use> and C<require> operators, which also do automatic error checking
1415 and raise an exception if there's a problem.
1417 You might like to use C<do> to read in a program configuration
1418 file. Manual error checking can be done this way:
1420 # read in config files: system first, then user
1421 for $file ("/share/prog/defaults.rc",
1422 "$ENV{HOME}/.someprogrc")
1424 unless ($return = do $file) {
1425 warn "couldn't parse $file: $@" if $@;
1426 warn "couldn't do $file: $!" unless defined $return;
1427 warn "couldn't run $file" unless $return;
1432 X<dump> X<core> X<undump>
1436 This function causes an immediate core dump. See also the B<-u>
1437 command-line switch in L<perlrun>, which does the same thing.
1438 Primarily this is so that you can use the B<undump> program (not
1439 supplied) to turn your core dump into an executable binary after
1440 having initialized all your variables at the beginning of the
1441 program. When the new binary is executed it will begin by executing
1442 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1443 Think of it as a goto with an intervening core dump and reincarnation.
1444 If C<LABEL> is omitted, restarts the program from the top.
1446 B<WARNING>: Any files opened at the time of the dump will I<not>
1447 be open any more when the program is reincarnated, with possible
1448 resulting confusion by Perl.
1450 This function is now largely obsolete, mostly because it's very hard to
1451 convert a core file into an executable. That's why you should now invoke
1452 it as C<CORE::dump()>, if you don't want to be warned against a possible
1456 X<each> X<hash, iterator>
1461 When called in list context, returns a 2-element list consisting of the key
1462 and value for the next element of a hash, or the index and value for the
1463 next element of an array, so that you can iterate over it. When called in
1464 scalar context, returns only the key (not the value) in a hash, or the index
1467 Hash entries are returned in an apparently random order. The actual random
1468 order is subject to change in future versions of Perl, but it is
1469 guaranteed to be in the same order as either the C<keys> or C<values>
1470 function would produce on the same (unmodified) hash. Since Perl
1471 5.8.2 the ordering can be different even between different runs of Perl
1472 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1474 After C<each> has returned all entries from the hash or array, the next
1475 call to C<each> returns the empty list in list context and C<undef> in
1476 scalar context. The next call following that one restarts iteration. Each
1477 hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1478 and C<values>. The iterator is implicitly reset when C<each> has reached
1479 the end as just described; it can be explicitly reset by calling C<keys> or
1480 C<values> on the hash or array. If you add or delete a hash's elements
1481 while iterating over it, entries may be skipped or duplicated--so don't do
1482 that. Exception: It is always safe to delete the item most recently
1483 returned by C<each()>, so the following code works properly:
1485 while (($key, $value) = each %hash) {
1487 delete $hash{$key}; # This is safe
1490 This prints out your environment like the printenv(1) program,
1491 but in a different order:
1493 while (($key,$value) = each %ENV) {
1494 print "$key=$value\n";
1497 See also C<keys>, C<values> and C<sort>.
1499 =item eof FILEHANDLE
1508 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1509 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1510 gives the real filehandle. (Note that this function actually
1511 reads a character and then C<ungetc>s it, so isn't useful in an
1512 interactive context.) Do not read from a terminal file (or call
1513 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1514 as terminals may lose the end-of-file condition if you do.
1516 An C<eof> without an argument uses the last file read. Using C<eof()>
1517 with empty parentheses is different. It refers to the pseudo file
1518 formed from the files listed on the command line and accessed via the
1519 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1520 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1521 used will cause C<@ARGV> to be examined to determine if input is
1522 available. Similarly, an C<eof()> after C<< <> >> has returned
1523 end-of-file will assume you are processing another C<@ARGV> list,
1524 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1525 see L<perlop/"I/O Operators">.
1527 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1528 detect the end of each file, C<eof()> will detect the end of only the
1529 last file. Examples:
1531 # reset line numbering on each input file
1533 next if /^\s*#/; # skip comments
1536 close ARGV if eof; # Not eof()!
1539 # insert dashes just before last line of last file
1541 if (eof()) { # check for end of last file
1542 print "--------------\n";
1545 last if eof(); # needed if we're reading from a terminal
1548 Practical hint: you almost never need to use C<eof> in Perl, because the
1549 input operators typically return C<undef> when they run out of data, or if
1553 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1554 X<error, handling> X<exception, handling>
1560 In the first form, the return value of EXPR is parsed and executed as if it
1561 were a little Perl program. The value of the expression (which is itself
1562 determined within scalar context) is first parsed, and if there weren't any
1563 errors, executed in the lexical context of the current Perl program, so
1564 that any variable settings or subroutine and format definitions remain
1565 afterwards. Note that the value is parsed every time the C<eval> executes.
1566 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1567 delay parsing and subsequent execution of the text of EXPR until run time.
1569 In the second form, the code within the BLOCK is parsed only once--at the
1570 same time the code surrounding the C<eval> itself was parsed--and executed
1571 within the context of the current Perl program. This form is typically
1572 used to trap exceptions more efficiently than the first (see below), while
1573 also providing the benefit of checking the code within BLOCK at compile
1576 The final semicolon, if any, may be omitted from the value of EXPR or within
1579 In both forms, the value returned is the value of the last expression
1580 evaluated inside the mini-program; a return statement may be also used, just
1581 as with subroutines. The expression providing the return value is evaluated
1582 in void, scalar, or list context, depending on the context of the C<eval>
1583 itself. See L</wantarray> for more on how the evaluation context can be
1586 If there is a syntax error or runtime error, or a C<die> statement is
1587 executed, C<eval> returns an undefined value in scalar context
1588 or an empty list in list context, and C<$@> is set to the
1589 error message. If there was no error, C<$@> is guaranteed to be the empty
1590 string. Beware that using C<eval> neither silences Perl from printing
1591 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1592 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1593 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1594 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1596 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1597 determining whether a particular feature (such as C<socket> or C<symlink>)
1598 is implemented. It is also Perl's exception trapping mechanism, where
1599 the die operator is used to raise exceptions.
1601 If you want to trap errors when loading an XS module, some problems with
1602 the binary interface (such as Perl version skew) may be fatal even with
1603 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1605 If the code to be executed doesn't vary, you may use the eval-BLOCK
1606 form to trap run-time errors without incurring the penalty of
1607 recompiling each time. The error, if any, is still returned in C<$@>.
1610 # make divide-by-zero nonfatal
1611 eval { $answer = $a / $b; }; warn $@ if $@;
1613 # same thing, but less efficient
1614 eval '$answer = $a / $b'; warn $@ if $@;
1616 # a compile-time error
1617 eval { $answer = }; # WRONG
1620 eval '$answer ='; # sets $@
1622 Using the C<eval{}> form as an exception trap in libraries does have some
1623 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1624 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1625 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1626 as this example shows:
1628 # a private exception trap for divide-by-zero
1629 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1632 This is especially significant, given that C<__DIE__> hooks can call
1633 C<die> again, which has the effect of changing their error messages:
1635 # __DIE__ hooks may modify error messages
1637 local $SIG{'__DIE__'} =
1638 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1639 eval { die "foo lives here" };
1640 print $@ if $@; # prints "bar lives here"
1643 Because this promotes action at a distance, this counterintuitive behavior
1644 may be fixed in a future release.
1646 With an C<eval>, you should be especially careful to remember what's
1647 being looked at when:
1653 eval { $x }; # CASE 4
1655 eval "\$$x++"; # CASE 5
1658 Cases 1 and 2 above behave identically: they run the code contained in
1659 the variable $x. (Although case 2 has misleading double quotes making
1660 the reader wonder what else might be happening (nothing is).) Cases 3
1661 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1662 does nothing but return the value of $x. (Case 4 is preferred for
1663 purely visual reasons, but it also has the advantage of compiling at
1664 compile-time instead of at run-time.) Case 5 is a place where
1665 normally you I<would> like to use double quotes, except that in this
1666 particular situation, you can just use symbolic references instead, as
1669 The assignment to C<$@> occurs before restoration of localised variables,
1670 which means a temporary is required if you want to mask some but not all
1673 # alter $@ on nefarious repugnancy only
1677 local $@; # protect existing $@
1678 eval { test_repugnancy() };
1679 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1680 $@ =~ /nefarious/ and $e = $@;
1682 die $e if defined $e
1685 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1686 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1688 An C<eval ''> executed within the C<DB> package doesn't see the usual
1689 surrounding lexical scope, but rather the scope of the first non-DB piece
1690 of code that called it. You don't normally need to worry about this unless
1691 you are writing a Perl debugger.
1696 =item exec PROGRAM LIST
1698 The C<exec> function executes a system command I<and never returns>;
1699 use C<system> instead of C<exec> if you want it to return. It fails and
1700 returns false only if the command does not exist I<and> it is executed
1701 directly instead of via your system's command shell (see below).
1703 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1704 warns you if there is a following statement that isn't C<die>, C<warn>,
1705 or C<exit> (if C<-w> is set--but you always do that, right?). If you
1706 I<really> want to follow an C<exec> with some other statement, you
1707 can use one of these styles to avoid the warning:
1709 exec ('foo') or print STDERR "couldn't exec foo: $!";
1710 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1712 If there is more than one argument in LIST, or if LIST is an array
1713 with more than one value, calls execvp(3) with the arguments in LIST.
1714 If there is only one scalar argument or an array with one element in it,
1715 the argument is checked for shell metacharacters, and if there are any,
1716 the entire argument is passed to the system's command shell for parsing
1717 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1718 If there are no shell metacharacters in the argument, it is split into
1719 words and passed directly to C<execvp>, which is more efficient.
1722 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1723 exec "sort $outfile | uniq";
1725 If you don't really want to execute the first argument, but want to lie
1726 to the program you are executing about its own name, you can specify
1727 the program you actually want to run as an "indirect object" (without a
1728 comma) in front of the LIST. (This always forces interpretation of the
1729 LIST as a multivalued list, even if there is only a single scalar in
1732 $shell = '/bin/csh';
1733 exec $shell '-sh'; # pretend it's a login shell
1737 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1739 When the arguments get executed via the system shell, results are
1740 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1743 Using an indirect object with C<exec> or C<system> is also more
1744 secure. This usage (which also works fine with system()) forces
1745 interpretation of the arguments as a multivalued list, even if the
1746 list had just one argument. That way you're safe from the shell
1747 expanding wildcards or splitting up words with whitespace in them.
1749 @args = ( "echo surprise" );
1751 exec @args; # subject to shell escapes
1753 exec { $args[0] } @args; # safe even with one-arg list
1755 The first version, the one without the indirect object, ran the I<echo>
1756 program, passing it C<"surprise"> an argument. The second version didn't;
1757 it tried to run a program named I<"echo surprise">, didn't find it, and set
1758 C<$?> to a non-zero value indicating failure.
1760 Beginning with v5.6.0, Perl attempts to flush all files opened for
1761 output before the exec, but this may not be supported on some platforms
1762 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1763 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1764 open handles to avoid lost output.
1766 Note that C<exec> will not call your C<END> blocks, nor will it invoke
1767 C<DESTROY> methods on your objects.
1770 X<exists> X<autovivification>
1772 Given an expression that specifies an element of a hash, returns true if the
1773 specified element in the hash has ever been initialized, even if the
1774 corresponding value is undefined.
1776 print "Exists\n" if exists $hash{$key};
1777 print "Defined\n" if defined $hash{$key};
1778 print "True\n" if $hash{$key};
1780 exists may also be called on array elements, but its behavior is much less
1781 obvious, and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1782 that calling exists on array values is deprecated and likely to be removed in
1783 a future version of Perl.
1785 print "Exists\n" if exists $array[$index];
1786 print "Defined\n" if defined $array[$index];
1787 print "True\n" if $array[$index];
1789 A hash or array element can be true only if it's defined, and defined if
1790 it exists, but the reverse doesn't necessarily hold true.
1792 Given an expression that specifies the name of a subroutine,
1793 returns true if the specified subroutine has ever been declared, even
1794 if it is undefined. Mentioning a subroutine name for exists or defined
1795 does not count as declaring it. Note that a subroutine that does not
1796 exist may still be callable: its package may have an C<AUTOLOAD>
1797 method that makes it spring into existence the first time that it is
1798 called; see L<perlsub>.
1800 print "Exists\n" if exists &subroutine;
1801 print "Defined\n" if defined &subroutine;
1803 Note that the EXPR can be arbitrarily complicated as long as the final
1804 operation is a hash or array key lookup or subroutine name:
1806 if (exists $ref->{A}->{B}->{$key}) { }
1807 if (exists $hash{A}{B}{$key}) { }
1809 if (exists $ref->{A}->{B}->[$ix]) { }
1810 if (exists $hash{A}{B}[$ix]) { }
1812 if (exists &{$ref->{A}{B}{$key}}) { }
1814 Although the mostly deeply nested array or hash will not spring into
1815 existence just because its existence was tested, any intervening ones will.
1816 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1817 into existence due to the existence test for the $key element above.
1818 This happens anywhere the arrow operator is used, including even here:
1821 if (exists $ref->{"Some key"}) { }
1822 print $ref; # prints HASH(0x80d3d5c)
1824 This surprising autovivification in what does not at first--or even
1825 second--glance appear to be an lvalue context may be fixed in a future
1828 Use of a subroutine call, rather than a subroutine name, as an argument
1829 to exists() is an error.
1832 exists &sub(); # Error
1835 X<exit> X<terminate> X<abort>
1839 Evaluates EXPR and exits immediately with that value. Example:
1842 exit 0 if $ans =~ /^[Xx]/;
1844 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1845 universally recognized values for EXPR are C<0> for success and C<1>
1846 for error; other values are subject to interpretation depending on the
1847 environment in which the Perl program is running. For example, exiting
1848 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1849 the mailer to return the item undelivered, but that's not true everywhere.
1851 Don't use C<exit> to abort a subroutine if there's any chance that
1852 someone might want to trap whatever error happened. Use C<die> instead,
1853 which can be trapped by an C<eval>.
1855 The exit() function does not always exit immediately. It calls any
1856 defined C<END> routines first, but these C<END> routines may not
1857 themselves abort the exit. Likewise any object destructors that need to
1858 be called are called before the real exit. If this is a problem, you
1859 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1860 See L<perlmod> for details.
1863 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1867 Returns I<e> (the natural logarithm base) to the power of EXPR.
1868 If EXPR is omitted, gives C<exp($_)>.
1870 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1873 Implements the fcntl(2) function. You'll probably have to say
1877 first to get the correct constant definitions. Argument processing and
1878 value returned work just like C<ioctl> below.
1882 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1883 or die "can't fcntl F_GETFL: $!";
1885 You don't have to check for C<defined> on the return from C<fcntl>.
1886 Like C<ioctl>, it maps a C<0> return from the system call into
1887 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1888 in numeric context. It is also exempt from the normal B<-w> warnings
1889 on improper numeric conversions.
1891 Note that C<fcntl> raises an exception if used on a machine that
1892 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1893 manpage to learn what functions are available on your system.
1895 Here's an example of setting a filehandle named C<REMOTE> to be
1896 non-blocking at the system level. You'll have to negotiate C<$|>
1897 on your own, though.
1899 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1901 $flags = fcntl(REMOTE, F_GETFL, 0)
1902 or die "Can't get flags for the socket: $!\n";
1904 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1905 or die "Can't set flags for the socket: $!\n";
1907 =item fileno FILEHANDLE
1910 Returns the file descriptor for a filehandle, or undefined if the
1911 filehandle is not open. This is mainly useful for constructing
1912 bitmaps for C<select> and low-level POSIX tty-handling operations.
1913 If FILEHANDLE is an expression, the value is taken as an indirect
1914 filehandle, generally its name.
1916 You can use this to find out whether two handles refer to the
1917 same underlying descriptor:
1919 if (fileno(THIS) == fileno(THAT)) {
1920 print "THIS and THAT are dups\n";
1923 (Filehandles connected to memory objects via new features of C<open> may
1924 return undefined even though they are open.)
1927 =item flock FILEHANDLE,OPERATION
1928 X<flock> X<lock> X<locking>
1930 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1931 for success, false on failure. Produces a fatal error if used on a
1932 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1933 C<flock> is Perl's portable file locking interface, although it locks
1934 entire files only, not records.
1936 Two potentially non-obvious but traditional C<flock> semantics are
1937 that it waits indefinitely until the lock is granted, and that its locks
1938 B<merely advisory>. Such discretionary locks are more flexible, but offer
1939 fewer guarantees. This means that programs that do not also use C<flock>
1940 may modify files locked with C<flock>. See L<perlport>,
1941 your port's specific documentation, or your system-specific local manpages
1942 for details. It's best to assume traditional behavior if you're writing
1943 portable programs. (But if you're not, you should as always feel perfectly
1944 free to write for your own system's idiosyncrasies (sometimes called
1945 "features"). Slavish adherence to portability concerns shouldn't get
1946 in the way of your getting your job done.)
1948 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1949 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1950 you can use the symbolic names if you import them from the Fcntl module,
1951 either individually, or as a group using the ':flock' tag. LOCK_SH
1952 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1953 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1954 LOCK_SH or LOCK_EX then C<flock> returns immediately rather than blocking
1955 waiting for the lock; check the return status to see if you got it.
1957 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1958 before locking or unlocking it.
1960 Note that the emulation built with lockf(3) doesn't provide shared
1961 locks, and it requires that FILEHANDLE be open with write intent. These
1962 are the semantics that lockf(3) implements. Most if not all systems
1963 implement lockf(3) in terms of fcntl(2) locking, though, so the
1964 differing semantics shouldn't bite too many people.
1966 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1967 be open with read intent to use LOCK_SH and requires that it be open
1968 with write intent to use LOCK_EX.
1970 Note also that some versions of C<flock> cannot lock things over the
1971 network; you would need to use the more system-specific C<fcntl> for
1972 that. If you like you can force Perl to ignore your system's flock(2)
1973 function, and so provide its own fcntl(2)-based emulation, by passing
1974 the switch C<-Ud_flock> to the F<Configure> program when you configure
1977 Here's a mailbox appender for BSD systems.
1979 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
1983 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
1985 # and, in case someone appended while we were waiting...
1986 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
1991 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
1994 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
1995 or die "Can't open mailbox: $!";
1998 print $mbox $msg,"\n\n";
2001 On systems that support a real flock(2), locks are inherited across fork()
2002 calls, whereas those that must resort to the more capricious fcntl(2)
2003 function lose their locks, making it seriously harder to write servers.
2005 See also L<DB_File> for other flock() examples.
2008 X<fork> X<child> X<parent>
2010 Does a fork(2) system call to create a new process running the
2011 same program at the same point. It returns the child pid to the
2012 parent process, C<0> to the child process, or C<undef> if the fork is
2013 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2014 are shared, while everything else is copied. On most systems supporting
2015 fork(), great care has gone into making it extremely efficient (for
2016 example, using copy-on-write technology on data pages), making it the
2017 dominant paradigm for multitasking over the last few decades.
2019 Beginning with v5.6.0, Perl attempts to flush all files opened for
2020 output before forking the child process, but this may not be supported
2021 on some platforms (see L<perlport>). To be safe, you may need to set
2022 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2023 C<IO::Handle> on any open handles to avoid duplicate output.
2025 If you C<fork> without ever waiting on your children, you will
2026 accumulate zombies. On some systems, you can avoid this by setting
2027 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2028 forking and reaping moribund children.
2030 Note that if your forked child inherits system file descriptors like
2031 STDIN and STDOUT that are actually connected by a pipe or socket, even
2032 if you exit, then the remote server (such as, say, a CGI script or a
2033 backgrounded job launched from a remote shell) won't think you're done.
2034 You should reopen those to F</dev/null> if it's any issue.
2039 Declare a picture format for use by the C<write> function. For
2043 Test: @<<<<<<<< @||||| @>>>>>
2044 $str, $%, '$' . int($num)
2048 $num = $cost/$quantity;
2052 See L<perlform> for many details and examples.
2054 =item formline PICTURE,LIST
2057 This is an internal function used by C<format>s, though you may call it,
2058 too. It formats (see L<perlform>) a list of values according to the
2059 contents of PICTURE, placing the output into the format output
2060 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2061 Eventually, when a C<write> is done, the contents of
2062 C<$^A> are written to some filehandle. You could also read C<$^A>
2063 and then set C<$^A> back to C<"">. Note that a format typically
2064 does one C<formline> per line of form, but the C<formline> function itself
2065 doesn't care how many newlines are embedded in the PICTURE. This means
2066 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2067 You may therefore need to use multiple formlines to implement a single
2068 record format, just like the C<format> compiler.
2070 Be careful if you put double quotes around the picture, because an C<@>
2071 character may be taken to mean the beginning of an array name.
2072 C<formline> always returns true. See L<perlform> for other examples.
2074 =item getc FILEHANDLE
2075 X<getc> X<getchar> X<character> X<file, read>
2079 Returns the next character from the input file attached to FILEHANDLE,
2080 or the undefined value at end of file or if there was an error (in
2081 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2082 STDIN. This is not particularly efficient. However, it cannot be
2083 used by itself to fetch single characters without waiting for the user
2084 to hit enter. For that, try something more like:
2087 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2090 system "stty", '-icanon', 'eol', "\001";
2096 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2099 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2103 Determination of whether $BSD_STYLE should be set
2104 is left as an exercise to the reader.
2106 The C<POSIX::getattr> function can do this more portably on
2107 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2108 module from your nearest CPAN site; details on CPAN can be found on
2112 X<getlogin> X<login>
2114 This implements the C library function of the same name, which on most
2115 systems returns the current login from F</etc/utmp>, if any. If it
2116 returns the empty string, use C<getpwuid>.
2118 $login = getlogin || getpwuid($<) || "Kilroy";
2120 Do not consider C<getlogin> for authentication: it is not as
2121 secure as C<getpwuid>.
2123 =item getpeername SOCKET
2124 X<getpeername> X<peer>
2126 Returns the packed sockaddr address of other end of the SOCKET connection.
2129 $hersockaddr = getpeername(SOCK);
2130 ($port, $iaddr) = sockaddr_in($hersockaddr);
2131 $herhostname = gethostbyaddr($iaddr, AF_INET);
2132 $herstraddr = inet_ntoa($iaddr);
2137 Returns the current process group for the specified PID. Use
2138 a PID of C<0> to get the current process group for the
2139 current process. Will raise an exception if used on a machine that
2140 doesn't implement getpgrp(2). If PID is omitted, returns process
2141 group of current process. Note that the POSIX version of C<getpgrp>
2142 does not accept a PID argument, so only C<PID==0> is truly portable.
2145 X<getppid> X<parent> X<pid>
2147 Returns the process id of the parent process.
2149 Note for Linux users: on Linux, the C functions C<getpid()> and
2150 C<getppid()> return different values from different threads. In order to
2151 be portable, this behavior is not reflected by the Perl-level function
2152 C<getppid()>, that returns a consistent value across threads. If you want
2153 to call the underlying C<getppid()>, you may use the CPAN module
2156 =item getpriority WHICH,WHO
2157 X<getpriority> X<priority> X<nice>
2159 Returns the current priority for a process, a process group, or a user.
2160 (See C<getpriority(2)>.) Will raise a fatal exception if used on a
2161 machine that doesn't implement getpriority(2).
2164 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2165 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2166 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2167 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2168 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2169 X<endnetent> X<endprotoent> X<endservent>
2173 =item gethostbyname NAME
2175 =item getnetbyname NAME
2177 =item getprotobyname NAME
2183 =item getservbyname NAME,PROTO
2185 =item gethostbyaddr ADDR,ADDRTYPE
2187 =item getnetbyaddr ADDR,ADDRTYPE
2189 =item getprotobynumber NUMBER
2191 =item getservbyport PORT,PROTO
2209 =item sethostent STAYOPEN
2211 =item setnetent STAYOPEN
2213 =item setprotoent STAYOPEN
2215 =item setservent STAYOPEN
2229 These routines are the same as their counterparts in the
2230 system C library. In list context, the return values from the
2231 various get routines are as follows:
2233 ($name,$passwd,$uid,$gid,
2234 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2235 ($name,$passwd,$gid,$members) = getgr*
2236 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2237 ($name,$aliases,$addrtype,$net) = getnet*
2238 ($name,$aliases,$proto) = getproto*
2239 ($name,$aliases,$port,$proto) = getserv*
2241 (If the entry doesn't exist you get an empty list.)
2243 The exact meaning of the $gcos field varies but it usually contains
2244 the real name of the user (as opposed to the login name) and other
2245 information pertaining to the user. Beware, however, that in many
2246 system users are able to change this information and therefore it
2247 cannot be trusted and therefore the $gcos is tainted (see
2248 L<perlsec>). The $passwd and $shell, user's encrypted password and
2249 login shell, are also tainted, because of the same reason.
2251 In scalar context, you get the name, unless the function was a
2252 lookup by name, in which case you get the other thing, whatever it is.
2253 (If the entry doesn't exist you get the undefined value.) For example:
2255 $uid = getpwnam($name);
2256 $name = getpwuid($num);
2258 $gid = getgrnam($name);
2259 $name = getgrgid($num);
2263 In I<getpw*()> the fields $quota, $comment, and $expire are special
2264 in that they are unsupported on many systems. If the
2265 $quota is unsupported, it is an empty scalar. If it is supported, it
2266 usually encodes the disk quota. If the $comment field is unsupported,
2267 it is an empty scalar. If it is supported it usually encodes some
2268 administrative comment about the user. In some systems the $quota
2269 field may be $change or $age, fields that have to do with password
2270 aging. In some systems the $comment field may be $class. The $expire
2271 field, if present, encodes the expiration period of the account or the
2272 password. For the availability and the exact meaning of these fields
2273 in your system, please consult your getpwnam(3) documentation and your
2274 F<pwd.h> file. You can also find out from within Perl what your
2275 $quota and $comment fields mean and whether you have the $expire field
2276 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2277 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2278 files are supported only if your vendor has implemented them in the
2279 intuitive fashion that calling the regular C library routines gets the
2280 shadow versions if you're running under privilege or if there exists
2281 the shadow(3) functions as found in System V (this includes Solaris
2282 and Linux.) Those systems that implement a proprietary shadow password
2283 facility are unlikely to be supported.
2285 The $members value returned by I<getgr*()> is a space separated list of
2286 the login names of the members of the group.
2288 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2289 C, it will be returned to you via C<$?> if the function call fails. The
2290 C<@addrs> value returned by a successful call is a list of raw
2291 addresses returned by the corresponding library call. In the
2292 Internet domain, each address is four bytes long; you can unpack it
2293 by saying something like:
2295 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2297 The Socket library makes this slightly easier:
2300 $iaddr = inet_aton("127.1"); # or whatever address
2301 $name = gethostbyaddr($iaddr, AF_INET);
2303 # or going the other way
2304 $straddr = inet_ntoa($iaddr);
2306 In the opposite way, to resolve a hostname to the IP address
2310 $packed_ip = gethostbyname("www.perl.org");
2311 if (defined $packed_ip) {
2312 $ip_address = inet_ntoa($packed_ip);
2315 Make sure <gethostbyname()> is called in SCALAR context and that
2316 its return value is checked for definedness.
2318 If you get tired of remembering which element of the return list
2319 contains which return value, by-name interfaces are provided
2320 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2321 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2322 and C<User::grent>. These override the normal built-ins, supplying
2323 versions that return objects with the appropriate names
2324 for each field. For example:
2328 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2330 Even though it looks like they're the same method calls (uid),
2331 they aren't, because a C<File::stat> object is different from
2332 a C<User::pwent> object.
2334 =item getsockname SOCKET
2337 Returns the packed sockaddr address of this end of the SOCKET connection,
2338 in case you don't know the address because you have several different
2339 IPs that the connection might have come in on.
2342 $mysockaddr = getsockname(SOCK);
2343 ($port, $myaddr) = sockaddr_in($mysockaddr);
2344 printf "Connect to %s [%s]\n",
2345 scalar gethostbyaddr($myaddr, AF_INET),
2348 =item getsockopt SOCKET,LEVEL,OPTNAME
2351 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2352 Options may exist at multiple protocol levels depending on the socket
2353 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2354 C<Socket> module) will exist. To query options at another level the
2355 protocol number of the appropriate protocol controlling the option
2356 should be supplied. For example, to indicate that an option is to be
2357 interpreted by the TCP protocol, LEVEL should be set to the protocol
2358 number of TCP, which you can get using C<getprotobyname>.
2360 The function returns a packed string representing the requested socket
2361 option, or C<undef> on error, with the reason for the error placed in
2362 C<$!>). Just what is in the packed string depends on LEVEL and OPTNAME;
2363 consult getsockopt(2) for details. A common case is that the option is an
2364 integer, in which case the result is a packed integer, which you can decode
2365 using C<unpack> with the C<i> (or C<I>) format.
2367 An example to test whether Nagle's algorithm is turned on on a socket:
2369 use Socket qw(:all);
2371 defined(my $tcp = getprotobyname("tcp"))
2372 or die "Could not determine the protocol number for tcp";
2373 # my $tcp = IPPROTO_TCP; # Alternative
2374 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2375 or die "getsockopt TCP_NODELAY: $!";
2376 my $nodelay = unpack("I", $packed);
2377 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2381 X<glob> X<wildcard> X<filename, expansion> X<expand>
2385 In list context, returns a (possibly empty) list of filename expansions on
2386 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2387 scalar context, glob iterates through such filename expansions, returning
2388 undef when the list is exhausted. This is the internal function
2389 implementing the C<< <*.c> >> operator, but you can use it directly. If
2390 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2391 more detail in L<perlop/"I/O Operators">.
2393 Note that C<glob> splits its arguments on whitespace and treats
2394 each segment as separate pattern. As such, C<glob("*.c *.h")>
2395 matches all files with a F<.c> or F<.h> extension. The expression
2396 C<glob(".* *")> matchs all files in the current working directory.
2398 If non-empty braces are the only wildcard characters used in the
2399 C<glob>, no filenames are matched, but potentially many strings
2400 are returned. For example, this produces nine strings, one for
2401 each pairing of fruits and colors:
2403 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2405 Beginning with v5.6.0, this operator is implemented using the standard
2406 C<File::Glob> extension. See L<File::Glob> for details, including
2407 C<bsd_glob> which does not treat whitespace as a pattern separator.
2410 X<gmtime> X<UTC> X<Greenwich>
2414 Works just like L<localtime> but the returned values are
2415 localized for the standard Greenwich time zone.
2417 Note: when called in list context, $isdst, the last value
2418 returned by gmtime is always C<0>. There is no
2419 Daylight Saving Time in GMT.
2421 See L<perlport/gmtime> for portability concerns.
2424 X<goto> X<jump> X<jmp>
2430 The C<goto-LABEL> form finds the statement labeled with LABEL and
2431 resumes execution there. It can't be used to get out of a block or
2432 subroutine given to C<sort>. It can be used to go almost anywhere
2433 else within the dynamic scope, including out of subroutines, but it's
2434 usually better to use some other construct such as C<last> or C<die>.
2435 The author of Perl has never felt the need to use this form of C<goto>
2436 (in Perl, that is; C is another matter). (The difference is that C
2437 does not offer named loops combined with loop control. Perl does, and
2438 this replaces most structured uses of C<goto> in other languages.)
2440 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2441 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2442 necessarily recommended if you're optimizing for maintainability:
2444 goto ("FOO", "BAR", "GLARCH")[$i];
2446 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2447 deprecated and will issue a warning. Even then, it may not be used to
2448 go into any construct that requires initialization, such as a
2449 subroutine or a C<foreach> loop. It also can't be used to go into a
2450 construct that is optimized away.
2452 The C<goto-&NAME> form is quite different from the other forms of
2453 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2454 doesn't have the stigma associated with other gotos. Instead, it
2455 exits the current subroutine (losing any changes set by local()) and
2456 immediately calls in its place the named subroutine using the current
2457 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2458 load another subroutine and then pretend that the other subroutine had
2459 been called in the first place (except that any modifications to C<@_>
2460 in the current subroutine are propagated to the other subroutine.)
2461 After the C<goto>, not even C<caller> will be able to tell that this
2462 routine was called first.
2464 NAME needn't be the name of a subroutine; it can be a scalar variable
2465 containing a code reference, or a block that evaluates to a code
2468 =item grep BLOCK LIST
2471 =item grep EXPR,LIST
2473 This is similar in spirit to, but not the same as, grep(1) and its
2474 relatives. In particular, it is not limited to using regular expressions.
2476 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2477 C<$_> to each element) and returns the list value consisting of those
2478 elements for which the expression evaluated to true. In scalar
2479 context, returns the number of times the expression was true.
2481 @foo = grep(!/^#/, @bar); # weed out comments
2485 @foo = grep {!/^#/} @bar; # weed out comments
2487 Note that C<$_> is an alias to the list value, so it can be used to
2488 modify the elements of the LIST. While this is useful and supported,
2489 it can cause bizarre results if the elements of LIST are not variables.
2490 Similarly, grep returns aliases into the original list, much as a for
2491 loop's index variable aliases the list elements. That is, modifying an
2492 element of a list returned by grep (for example, in a C<foreach>, C<map>
2493 or another C<grep>) actually modifies the element in the original list.
2494 This is usually something to be avoided when writing clear code.
2496 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2497 been declared with C<my $_>) then, in addition to being locally aliased to
2498 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2499 can't be seen from the outside, avoiding any potential side-effects.
2501 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2504 X<hex> X<hexadecimal>
2508 Interprets EXPR as a hex string and returns the corresponding value.
2509 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2510 L</oct>.) If EXPR is omitted, uses C<$_>.
2512 print hex '0xAf'; # prints '175'
2513 print hex 'aF'; # same
2515 Hex strings may only represent integers. Strings that would cause
2516 integer overflow trigger a warning. Leading whitespace is not stripped,
2517 unlike oct(). To present something as hex, look into L</printf>,
2518 L</sprintf>, or L</unpack>.
2523 There is no builtin C<import> function. It is just an ordinary
2524 method (subroutine) defined (or inherited) by modules that wish to export
2525 names to another module. The C<use> function calls the C<import> method
2526 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2528 =item index STR,SUBSTR,POSITION
2529 X<index> X<indexOf> X<InStr>
2531 =item index STR,SUBSTR
2533 The index function searches for one string within another, but without
2534 the wildcard-like behavior of a full regular-expression pattern match.
2535 It returns the position of the first occurrence of SUBSTR in STR at
2536 or after POSITION. If POSITION is omitted, starts searching from the
2537 beginning of the string. POSITION before the beginning of the string
2538 or after its end is treated as if it were the beginning or the end,
2539 respectively. POSITION and the return value are based at C<0> (or whatever
2540 you've set the C<$[> variable to--but don't do that). If the substring
2541 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2544 X<int> X<integer> X<truncate> X<trunc> X<floor>
2548 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2549 You should not use this function for rounding: one because it truncates
2550 towards C<0>, and two because machine representations of floating-point
2551 numbers can sometimes produce counterintuitive results. For example,
2552 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2553 because it's really more like -268.99999999999994315658 instead. Usually,
2554 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2555 functions will serve you better than will int().
2557 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2560 Implements the ioctl(2) function. You'll probably first have to say
2562 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2564 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2565 exist or doesn't have the correct definitions you'll have to roll your
2566 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2567 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2568 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2569 written depending on the FUNCTION; a C pointer to the string value of SCALAR
2570 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2571 has no string value but does have a numeric value, that value will be
2572 passed rather than a pointer to the string value. To guarantee this to be
2573 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2574 functions may be needed to manipulate the values of structures used by
2577 The return value of C<ioctl> (and C<fcntl>) is as follows:
2579 if OS returns: then Perl returns:
2581 0 string "0 but true"
2582 anything else that number
2584 Thus Perl returns true on success and false on failure, yet you can
2585 still easily determine the actual value returned by the operating
2588 $retval = ioctl(...) || -1;
2589 printf "System returned %d\n", $retval;
2591 The special string C<"0 but true"> is exempt from B<-w> complaints
2592 about improper numeric conversions.
2594 =item join EXPR,LIST
2597 Joins the separate strings of LIST into a single string with fields
2598 separated by the value of EXPR, and returns that new string. Example:
2600 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2602 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2603 first argument. Compare L</split>.
2610 Returns a list consisting of all the keys of the named hash, or the indices
2611 of an array. (In scalar context, returns the number of keys or indices.)
2613 The keys of a hash are returned in an apparently random order. The actual
2614 random order is subject to change in future versions of Perl, but it
2615 is guaranteed to be the same order as either the C<values> or C<each>
2616 function produces (given that the hash has not been modified). Since
2617 Perl 5.8.1 the ordering is different even between different runs of
2618 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2621 As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
2622 (see L</each>). In particular, calling keys() in void context resets
2623 the iterator with no other overhead.
2625 Here is yet another way to print your environment:
2628 @values = values %ENV;
2630 print pop(@keys), '=', pop(@values), "\n";
2633 or how about sorted by key:
2635 foreach $key (sort(keys %ENV)) {
2636 print $key, '=', $ENV{$key}, "\n";
2639 The returned values are copies of the original keys in the hash, so
2640 modifying them will not affect the original hash. Compare L</values>.
2642 To sort a hash by value, you'll need to use a C<sort> function.
2643 Here's a descending numeric sort of a hash by its values:
2645 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2646 printf "%4d %s\n", $hash{$key}, $key;
2649 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
2650 allocated for the given hash. This can gain you a measure of efficiency if
2651 you know the hash is going to get big. (This is similar to pre-extending
2652 an array by assigning a larger number to $#array.) If you say
2656 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2657 in fact, since it rounds up to the next power of two. These
2658 buckets will be retained even if you do C<%hash = ()>, use C<undef
2659 %hash> if you want to free the storage while C<%hash> is still in scope.
2660 You can't shrink the number of buckets allocated for the hash using
2661 C<keys> in this way (but you needn't worry about doing this by accident,
2662 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2665 See also C<each>, C<values> and C<sort>.
2667 =item kill SIGNAL, LIST
2670 Sends a signal to a list of processes. Returns the number of
2671 processes successfully signaled (which is not necessarily the
2672 same as the number actually killed).
2674 $cnt = kill 1, $child1, $child2;
2677 If SIGNAL is zero, no signal is sent to the process, but C<kill>
2678 checks whether it's I<possible> to send a signal to it (that
2679 means, to be brief, that the process is owned by the same user, or we are
2680 the super-user). This is useful to check that a child process is still
2681 alive (even if only as a zombie) and hasn't changed its UID. See
2682 L<perlport> for notes on the portability of this construct.
2684 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2685 of processes. That means you usually want to use positive not negative signals.
2686 You may also use a signal name in quotes.
2688 The behavior of kill when a I<PROCESS> number is zero or negative depends on
2689 the operating system. For example, on POSIX-conforming systems, zero will
2690 signal the current process group and -1 will signal all processes.
2692 See L<perlipc/"Signals"> for more details.
2699 The C<last> command is like the C<break> statement in C (as used in
2700 loops); it immediately exits the loop in question. If the LABEL is
2701 omitted, the command refers to the innermost enclosing loop. The
2702 C<continue> block, if any, is not executed:
2704 LINE: while (<STDIN>) {
2705 last LINE if /^$/; # exit when done with header
2709 C<last> cannot be used to exit a block that returns a value such as
2710 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2711 a grep() or map() operation.
2713 Note that a block by itself is semantically identical to a loop
2714 that executes once. Thus C<last> can be used to effect an early
2715 exit out of such a block.
2717 See also L</continue> for an illustration of how C<last>, C<next>, and
2725 Returns a lowercased version of EXPR. This is the internal function
2726 implementing the C<\L> escape in double-quoted strings.
2728 If EXPR is omitted, uses C<$_>.
2730 What gets returned depends on several factors:
2734 =item If C<use bytes> is in effect:
2738 =item On EBCDIC platforms
2740 The results are what the C language system call C<tolower()> returns.
2742 =item On ASCII platforms
2744 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2749 =item Otherwise, If EXPR has the UTF8 flag set
2751 If the current package has a subroutine named C<ToLower>, it will be used to
2752 change the case (See L<perlunicode/User-Defined Case Mappings>.)
2753 Otherwise Unicode semantics are used for the case change.
2755 =item Otherwise, if C<use locale> is in effect
2757 Respects current LC_CTYPE locale. See L<perllocale>.
2759 =item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2761 Unicode semantics are used for the case change. Any subroutine named
2762 C<ToLower> will not be used.
2768 =item On EBCDIC platforms
2770 The results are what the C language system call C<tolower()> returns.
2772 =item On ASCII platforms
2774 ASCII semantics are used for the case change. The lowercase of any character
2775 outside the ASCII range is the character itself.
2782 X<lcfirst> X<lowercase>
2786 Returns the value of EXPR with the first character lowercased. This
2787 is the internal function implementing the C<\l> escape in
2788 double-quoted strings.
2790 If EXPR is omitted, uses C<$_>.
2792 This function behaves the same way under various pragma, such as in a locale,
2800 Returns the length in I<characters> of the value of EXPR. If EXPR is
2801 omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2803 This function cannot be used on an entire array or hash to find out how
2804 many elements these have. For that, use C<scalar @array> and C<scalar keys
2805 %hash>, respectively.
2807 Like all Perl character operations, length() normally deals in logical
2808 characters, not physical bytes. For how many bytes a string encoded as
2809 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2810 to C<use Encode> first). See L<Encode> and L<perlunicode>.
2812 =item link OLDFILE,NEWFILE
2815 Creates a new filename linked to the old filename. Returns true for
2816 success, false otherwise.
2818 =item listen SOCKET,QUEUESIZE
2821 Does the same thing that the listen(2) system call does. Returns true if
2822 it succeeded, false otherwise. See the example in
2823 L<perlipc/"Sockets: Client/Server Communication">.
2828 You really probably want to be using C<my> instead, because C<local> isn't
2829 what most people think of as "local". See
2830 L<perlsub/"Private Variables via my()"> for details.
2832 A local modifies the listed variables to be local to the enclosing
2833 block, file, or eval. If more than one value is listed, the list must
2834 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2835 for details, including issues with tied arrays and hashes.
2837 The C<delete local EXPR> construct can also be used to localize the deletion
2838 of array/hash elements to the current block.
2839 See L<perlsub/"Localized deletion of elements of composite types">.
2841 =item localtime EXPR
2842 X<localtime> X<ctime>
2846 Converts a time as returned by the time function to a 9-element list
2847 with the time analyzed for the local time zone. Typically used as
2851 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2854 All list elements are numeric, and come straight out of the C `struct
2855 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2856 of the specified time.
2858 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2859 the range C<0..11> with 0 indicating January and 11 indicating December.
2860 This makes it easy to get a month name from a list:
2862 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2863 print "$abbr[$mon] $mday";
2864 # $mon=9, $mday=18 gives "Oct 18"
2866 C<$year> is the number of years since 1900, not just the last two digits
2867 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2868 to get a 4-digit year is simply:
2872 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2873 to do that, would you?
2875 To get the last two digits of the year (e.g., '01' in 2001) do:
2877 $year = sprintf("%02d", $year % 100);
2879 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2880 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2881 (or C<0..365> in leap years.)
2883 C<$isdst> is true if the specified time occurs during Daylight Saving
2884 Time, false otherwise.
2886 If EXPR is omitted, C<localtime()> uses the current time (as returned
2889 In scalar context, C<localtime()> returns the ctime(3) value:
2891 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2893 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2894 instead of local time use the L</gmtime> builtin. See also the
2895 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2896 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2897 and mktime(3) functions.
2899 To get somewhat similar but locale dependent date strings, set up your
2900 locale environment variables appropriately (please see L<perllocale>) and
2903 use POSIX qw(strftime);
2904 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2905 # or for GMT formatted appropriately for your locale:
2906 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2908 Note that the C<%a> and C<%b>, the short forms of the day of the week
2909 and the month of the year, may not necessarily be three characters wide.
2911 See L<perlport/localtime> for portability concerns.
2913 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2914 by-name access mechanism to the gmtime() and localtime() functions,
2917 For a comprehensive date and time representation look at the
2918 L<DateTime> module on CPAN.
2923 This function places an advisory lock on a shared variable, or referenced
2924 object contained in I<THING> until the lock goes out of scope.
2926 lock() is a "weak keyword" : this means that if you've defined a function
2927 by this name (before any calls to it), that function will be called
2928 instead. If you are not under C<use threads::shared> this does nothing.
2929 See L<threads::shared>.
2932 X<log> X<logarithm> X<e> X<ln> X<base>
2936 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2937 returns log of C<$_>. To get the log of another base, use basic algebra:
2938 The base-N log of a number is equal to the natural log of that number
2939 divided by the natural log of N. For example:
2943 return log($n)/log(10);
2946 See also L</exp> for the inverse operation.
2953 Does the same thing as the C<stat> function (including setting the
2954 special C<_> filehandle) but stats a symbolic link instead of the file
2955 the symbolic link points to. If symbolic links are unimplemented on
2956 your system, a normal C<stat> is done. For much more detailed
2957 information, please see the documentation for C<stat>.
2959 If EXPR is omitted, stats C<$_>.
2963 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
2965 =item map BLOCK LIST
2970 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2971 C<$_> to each element) and returns the list value composed of the
2972 results of each such evaluation. In scalar context, returns the
2973 total number of elements so generated. Evaluates BLOCK or EXPR in
2974 list context, so each element of LIST may produce zero, one, or
2975 more elements in the returned value.
2977 @chars = map(chr, @nums);
2979 translates a list of numbers to the corresponding characters. And
2981 %hash = map { get_a_key_for($_) => $_ } @array;
2983 is just a funny way to write
2987 $hash{get_a_key_for($_)} = $_;
2990 Note that C<$_> is an alias to the list value, so it can be used to
2991 modify the elements of the LIST. While this is useful and supported,
2992 it can cause bizarre results if the elements of LIST are not variables.
2993 Using a regular C<foreach> loop for this purpose would be clearer in
2994 most cases. See also L</grep> for an array composed of those items of
2995 the original list for which the BLOCK or EXPR evaluates to true.
2997 If C<$_> is lexical in the scope where the C<map> appears (because it has
2998 been declared with C<my $_>), then, in addition to being locally aliased to
2999 the list elements, C<$_> keeps being lexical inside the block; that is, it
3000 can't be seen from the outside, avoiding any potential side-effects.
3002 C<{> starts both hash references and blocks, so C<map { ...> could be either
3003 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3004 ahead for the closing C<}> it has to take a guess at which it's dealing with
3005 based on what it finds just after the C<{>. Usually it gets it right, but if it
3006 doesn't it won't realize something is wrong until it gets to the C<}> and
3007 encounters the missing (or unexpected) comma. The syntax error will be
3008 reported close to the C<}>, but you'll need to change something near the C<{>
3009 such as using a unary C<+> to give Perl some help:
3011 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3012 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3013 %hash = map { ("\L$_" => 1) } @array # this also works
3014 %hash = map { lc($_) => 1 } @array # as does this.
3015 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3017 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3019 or to force an anon hash constructor use C<+{>:
3021 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
3023 to get a list of anonymous hashes each with only one entry apiece.
3025 =item mkdir FILENAME,MASK
3026 X<mkdir> X<md> X<directory, create>
3028 =item mkdir FILENAME
3032 Creates the directory specified by FILENAME, with permissions
3033 specified by MASK (as modified by C<umask>). If it succeeds it
3034 returns true, otherwise it returns false and sets C<$!> (errno).
3035 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
3038 In general, it is better to create directories with permissive MASK,
3039 and let the user modify that with their C<umask>, than it is to supply
3040 a restrictive MASK and give the user no way to be more permissive.
3041 The exceptions to this rule are when the file or directory should be
3042 kept private (mail files, for instance). The perlfunc(1) entry on
3043 C<umask> discusses the choice of MASK in more detail.
3045 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3046 number of trailing slashes. Some operating and filesystems do not get
3047 this right, so Perl automatically removes all trailing slashes to keep
3050 To recursively create a directory structure, look at
3051 the C<mkpath> function of the L<File::Path> module.
3053 =item msgctl ID,CMD,ARG
3056 Calls the System V IPC function msgctl(2). You'll probably have to say
3060 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3061 then ARG must be a variable that will hold the returned C<msqid_ds>
3062 structure. Returns like C<ioctl>: the undefined value for error,
3063 C<"0 but true"> for zero, or the actual return value otherwise. See also
3064 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
3066 =item msgget KEY,FLAGS
3069 Calls the System V IPC function msgget(2). Returns the message queue
3070 id, or the undefined value if there is an error. See also
3071 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
3073 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3076 Calls the System V IPC function msgrcv to receive a message from
3077 message queue ID into variable VAR with a maximum message size of
3078 SIZE. Note that when a message is received, the message type as a
3079 native long integer will be the first thing in VAR, followed by the
3080 actual message. This packing may be opened with C<unpack("l! a*")>.
3081 Taints the variable. Returns true if successful, or false if there is
3082 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
3083 C<IPC::SysV::Msg> documentation.
3085 =item msgsnd ID,MSG,FLAGS
3088 Calls the System V IPC function msgsnd to send the message MSG to the
3089 message queue ID. MSG must begin with the native long integer message
3090 type, and be followed by the length of the actual message, and finally
3091 the message itself. This kind of packing can be achieved with
3092 C<pack("l! a*", $type, $message)>. Returns true if successful,
3093 or false if there is an error. See also C<IPC::SysV>
3094 and C<IPC::SysV::Msg> documentation.
3101 =item my EXPR : ATTRS
3103 =item my TYPE EXPR : ATTRS
3105 A C<my> declares the listed variables to be local (lexically) to the
3106 enclosing block, file, or C<eval>. If more than one value is listed,
3107 the list must be placed in parentheses.
3109 The exact semantics and interface of TYPE and ATTRS are still
3110 evolving. TYPE is currently bound to the use of C<fields> pragma,
3111 and attributes are handled using the C<attributes> pragma, or starting
3112 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3113 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3114 L<attributes>, and L<Attribute::Handlers>.
3121 The C<next> command is like the C<continue> statement in C; it starts
3122 the next iteration of the loop:
3124 LINE: while (<STDIN>) {
3125 next LINE if /^#/; # discard comments
3129 Note that if there were a C<continue> block on the above, it would get
3130 executed even on discarded lines. If LABEL is omitted, the command
3131 refers to the innermost enclosing loop.
3133 C<next> cannot be used to exit a block which returns a value such as
3134 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3135 a grep() or map() operation.
3137 Note that a block by itself is semantically identical to a loop
3138 that executes once. Thus C<next> will exit such a block early.
3140 See also L</continue> for an illustration of how C<last>, C<next>, and
3143 =item no MODULE VERSION LIST
3147 =item no MODULE VERSION
3149 =item no MODULE LIST
3155 See the C<use> function, of which C<no> is the opposite.
3158 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3162 Interprets EXPR as an octal string and returns the corresponding
3163 value. (If EXPR happens to start off with C<0x>, interprets it as a
3164 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3165 binary string. Leading whitespace is ignored in all three cases.)
3166 The following will handle decimal, binary, octal, and hex in standard
3169 $val = oct($val) if $val =~ /^0/;
3171 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3172 in octal), use sprintf() or printf():
3174 $dec_perms = (stat("filename"))[2] & 07777;
3175 $oct_perm_str = sprintf "%o", $perms;
3177 The oct() function is commonly used when a string such as C<644> needs
3178 to be converted into a file mode, for example. Although Perl
3179 automatically converts strings into numbers as needed, this automatic
3180 conversion assumes base 10.
3182 Leading white space is ignored without warning, as too are any trailing
3183 non-digits, such as a decimal point (C<oct> only handles non-negative
3184 integers, not negative integers or floating point).
3186 =item open FILEHANDLE,EXPR
3187 X<open> X<pipe> X<file, open> X<fopen>
3189 =item open FILEHANDLE,MODE,EXPR
3191 =item open FILEHANDLE,MODE,EXPR,LIST
3193 =item open FILEHANDLE,MODE,REFERENCE
3195 =item open FILEHANDLE
3197 Opens the file whose filename is given by EXPR, and associates it with
3200 Simple examples to open a file for reading:
3202 open(my $fh, '<', "input.txt") or die $!;
3206 open(my $fh, '>', "output.txt") or die $!;
3208 (The following is a comprehensive reference to open(): for a gentler
3209 introduction you may consider L<perlopentut>.)
3211 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3212 the variable is assigned a reference to a new anonymous filehandle,
3213 otherwise if FILEHANDLE is an expression, its value is used as the name of
3214 the real filehandle wanted. (This is considered a symbolic reference, so
3215 C<use strict 'refs'> should I<not> be in effect.)
3217 If EXPR is omitted, the scalar variable of the same name as the
3218 FILEHANDLE contains the filename. (Note that lexical variables--those
3219 declared with C<my>--will not work for this purpose; so if you're
3220 using C<my>, specify EXPR in your call to open.)
3222 If three or more arguments are specified then the mode of opening and
3223 the filename are separate. If MODE is C<< '<' >> or nothing, the file
3224 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3225 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3226 the file is opened for appending, again being created if necessary.
3228 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3229 indicate that you want both read and write access to the file; thus
3230 C<< '+<' >> is almost always preferred for read/write updates--the
3231 C<< '+>' >> mode would clobber the file first. You can't usually use
3232 either read-write mode for updating textfiles, since they have
3233 variable length records. See the B<-i> switch in L<perlrun> for a
3234 better approach. The file is created with permissions of C<0666>
3235 modified by the process's C<umask> value.
3237 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3238 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3240 In the two-argument (and one-argument) form of the call, the mode and
3241 filename should be concatenated (in that order), possibly separated by
3242 spaces. You may omit the mode in these forms when that mode is
3245 If the filename begins with C<'|'>, the filename is interpreted as a
3246 command to which output is to be piped, and if the filename ends with a
3247 C<'|'>, the filename is interpreted as a command that pipes output to
3248 us. See L<perlipc/"Using open() for IPC">
3249 for more examples of this. (You are not allowed to C<open> to a command
3250 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3251 and L<perlipc/"Bidirectional Communication with Another Process">
3254 For three or more arguments if MODE is C<'|-'>, the filename is
3255 interpreted as a command to which output is to be piped, and if MODE
3256 is C<'-|'>, the filename is interpreted as a command that pipes
3257 output to us. In the two-argument (and one-argument) form, one should
3258 replace dash (C<'-'>) with the command.
3259 See L<perlipc/"Using open() for IPC"> for more examples of this.
3260 (You are not allowed to C<open> to a command that pipes both in I<and>
3261 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3262 L<perlipc/"Bidirectional Communication"> for alternatives.)
3264 In the form of pipe opens taking three or more arguments, if LIST is specified
3265 (extra arguments after the command name) then LIST becomes arguments
3266 to the command invoked if the platform supports it. The meaning of
3267 C<open> with more than three arguments for non-pipe modes is not yet
3268 defined, but experimental "layers" may give extra LIST arguments
3271 In the two-argument (and one-argument) form, opening C<< '<-' >>
3272 or C<'-'> opens STDIN and opening C<< '>-' >> opens STDOUT.
3274 You may use the three-argument form of open to specify I/O layers
3275 (sometimes referred to as "disciplines") to apply to the handle
3276 that affect how the input and output are processed (see L<open> and
3277 L<PerlIO> for more details). For example:
3279 open(my $fh, "<:encoding(UTF-8)", "filename")
3280 || die "can't open UTF-8 encoded filename: $!";
3282 opens the UTF-8 encoded file containing Unicode characters;
3283 see L<perluniintro>. Note that if layers are specified in the
3284 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3285 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3287 Open returns nonzero on success, the undefined value otherwise. If
3288 the C<open> involved a pipe, the return value happens to be the pid of
3291 If you're running Perl on a system that distinguishes between text
3292 files and binary files, then you should check out L</binmode> for tips
3293 for dealing with this. The key distinction between systems that need
3294 C<binmode> and those that don't is their text file formats. Systems
3295 like Unix, Mac OS, and Plan 9, that end lines with a single
3296 character and encode that character in C as C<"\n"> do not
3297 need C<binmode>. The rest need it.
3299 When opening a file, it's seldom a good idea to continue
3300 if the request failed, so C<open> is frequently used with
3301 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3302 where you want to format a suitable error message (but there are
3303 modules that can help with that problem)) always check
3304 the return value from opening a file.
3306 As a special case the 3-arg form with a read/write mode and the third
3307 argument being C<undef>:
3309 open(my $tmp, "+>", undef) or die ...
3311 opens a filehandle to an anonymous temporary file. Also using "+<"
3312 works for symmetry, but you really should consider writing something
3313 to the temporary file first. You will need to seek() to do the
3316 Since v5.8.0, Perl has built using PerlIO by default. Unless you've
3317 changed this (i.e., Configure -Uuseperlio), you can open filehandles
3318 directly to Perl scalars via:
3320 open($fh, '>', \$variable) || ..
3322 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3325 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3330 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3331 while (<ARTICLE>) {...
3333 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3334 # if the open fails, output is discarded
3336 open(my $dbase, '+<', 'dbase.mine') # open for update
3337 or die "Can't open 'dbase.mine' for update: $!";
3339 open(my $dbase, '+<dbase.mine') # ditto
3340 or die "Can't open 'dbase.mine' for update: $!";
3342 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3343 or die "Can't start caesar: $!";
3345 open(ARTICLE, "caesar <$article |") # ditto
3346 or die "Can't start caesar: $!";
3348 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3349 or die "Can't start sort: $!";
3352 open(MEMORY,'>', \$var)
3353 or die "Can't open memory file: $!";
3354 print MEMORY "foo!\n"; # output will appear in $var
3356 # process argument list of files along with any includes
3358 foreach $file (@ARGV) {
3359 process($file, 'fh00');
3363 my($filename, $input) = @_;
3364 $input++; # this is a string increment
3365 unless (open($input, $filename)) {
3366 print STDERR "Can't open $filename: $!\n";
3371 while (<$input>) { # note use of indirection
3372 if (/^#include "(.*)"/) {
3373 process($1, $input);
3380 See L<perliol> for detailed info on PerlIO.
3382 You may also, in the Bourne shell tradition, specify an EXPR beginning
3383 with C<< '>&' >>, in which case the rest of the string is interpreted
3384 as the name of a filehandle (or file descriptor, if numeric) to be
3385 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
3386 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3387 The mode you specify should match the mode of the original filehandle.
3388 (Duping a filehandle does not take into account any existing contents
3389 of IO buffers.) If you use the 3-arg form then you can pass either a
3390 number, the name of a filehandle or the normal "reference to a glob".
3392 Here is a script that saves, redirects, and restores C<STDOUT> and
3393 C<STDERR> using various methods:
3396 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3397 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3399 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3400 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3402 select STDERR; $| = 1; # make unbuffered
3403 select STDOUT; $| = 1; # make unbuffered
3405 print STDOUT "stdout 1\n"; # this works for
3406 print STDERR "stderr 1\n"; # subprocesses too
3408 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3409 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3411 print STDOUT "stdout 2\n";
3412 print STDERR "stderr 2\n";
3414 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3415 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3416 that file descriptor (and not call C<dup(2)>); this is more
3417 parsimonious of file descriptors. For example:
3419 # open for input, reusing the fileno of $fd
3420 open(FILEHANDLE, "<&=$fd")
3424 open(FILEHANDLE, "<&=", $fd)
3428 # open for append, using the fileno of OLDFH
3429 open(FH, ">>&=", OLDFH)
3433 open(FH, ">>&=OLDFH")
3435 Being parsimonious on filehandles is also useful (besides being
3436 parsimonious) for example when something is dependent on file
3437 descriptors, like for example locking using flock(). If you do just
3438 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3439 descriptor as B, and therefore flock(A) will not flock(B), and vice
3440 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3441 the same file descriptor.
3443 Note that if you are using Perls older than 5.8.0, Perl will be using
3444 the standard C libraries' fdopen() to implement the "=" functionality.
3445 On many Unix systems fdopen() fails when file descriptors exceed a
3446 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3447 most often the default.
3449 You can see whether Perl has been compiled with PerlIO or not by
3450 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3451 is C<define>, you have PerlIO, otherwise you don't.
3453 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3454 with 2-arguments (or 1-argument) form of open(), then
3455 there is an implicit fork done, and the return value of open is the pid
3456 of the child within the parent process, and C<0> within the child
3457 process. (Use C<defined($pid)> to determine whether the open was successful.)
3458 The filehandle behaves normally for the parent, but I/O to that
3459 filehandle is piped from/to the STDOUT/STDIN of the child process.
3460 In the child process, the filehandle isn't opened--I/O happens from/to
3461 the new STDOUT/STDIN. Typically this is used like the normal
3462 piped open when you want to exercise more control over just how the
3463 pipe command gets executed, such as when running setuid and
3464 you don't want to have to scan shell commands for metacharacters.
3466 The following triples are more or less equivalent:
3468 open(FOO, "|tr '[a-z]' '[A-Z]'");
3469 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3470 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3471 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3473 open(FOO, "cat -n '$file'|");
3474 open(FOO, '-|', "cat -n '$file'");
3475 open(FOO, '-|') || exec 'cat', '-n', $file;
3476 open(FOO, '-|', "cat", '-n', $file);
3478 The last example in each block shows the pipe as "list form", which is
3479 not yet supported on all platforms. A good rule of thumb is that if
3480 your platform has true C<fork()> (in other words, if your platform is
3481 Unix) you can use the list form.
3483 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3485 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3486 output before any operation that may do a fork, but this may not be
3487 supported on some platforms (see L<perlport>). To be safe, you may need
3488 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3489 of C<IO::Handle> on any open handles.
3491 On systems that support a close-on-exec flag on files, the flag will
3492 be set for the newly opened file descriptor as determined by the value
3493 of $^F. See L<perlvar/$^F>.
3495 Closing any piped filehandle causes the parent process to wait for the
3496 child to finish, and returns the status value in C<$?> and
3497 C<${^CHILD_ERROR_NATIVE}>.
3499 The filename passed to 2-argument (or 1-argument) form of open() will
3500 have leading and trailing whitespace deleted, and the normal
3501 redirection characters honored. This property, known as "magic open",
3502 can often be used to good effect. A user could specify a filename of
3503 F<"rsh cat file |">, or you could change certain filenames as needed:
3505 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3506 open(FH, $filename) or die "Can't open $filename: $!";
3508 Use 3-argument form to open a file with arbitrary weird characters in it,
3510 open(FOO, '<', $file);
3512 otherwise it's necessary to protect any leading and trailing whitespace:
3514 $file =~ s#^(\s)#./$1#;
3515 open(FOO, "< $file\0");
3517 (this may not work on some bizarre filesystems). One should
3518 conscientiously choose between the I<magic> and 3-arguments form
3523 will allow the user to specify an argument of the form C<"rsh cat file |">,
3524 but will not work on a filename that happens to have a trailing space, while
3526 open IN, '<', $ARGV[0];
3528 will have exactly the opposite restrictions.
3530 If you want a "real" C C<open> (see C<open(2)> on your system), then you
3531 should use the C<sysopen> function, which involves no such magic (but
3532 may use subtly different filemodes than Perl open(), which is mapped
3533 to C fopen()). This is
3534 another way to protect your filenames from interpretation. For example:
3537 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3538 or die "sysopen $path: $!";
3539 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3540 print HANDLE "stuff $$\n";
3542 print "File contains: ", <HANDLE>;
3544 Using the constructor from the C<IO::Handle> package (or one of its
3545 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3546 filehandles that have the scope of whatever variables hold references to
3547 them, and automatically close whenever and however you leave that scope:
3551 sub read_myfile_munged {
3553 my $handle = IO::File->new;
3554 open($handle, "myfile") or die "myfile: $!";
3556 or return (); # Automatically closed here.
3557 mung $first or die "mung failed"; # Or here.
3558 return $first, <$handle> if $ALL; # Or here.
3562 See L</seek> for some details about mixing reading and writing.
3564 =item opendir DIRHANDLE,EXPR
3567 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3568 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3569 DIRHANDLE may be an expression whose value can be used as an indirect
3570 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3571 scalar variable (or array or hash element), the variable is assigned a
3572 reference to a new anonymous dirhandle.
3573 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3575 See example at C<readdir>.
3582 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3583 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3586 For the reverse, see L</chr>.
3587 See L<perlunicode> for more about Unicode.
3594 =item our EXPR : ATTRS
3596 =item our TYPE EXPR : ATTRS
3598 C<our> associates a simple name with a package variable in the current
3599 package for use within the current scope. When C<use strict 'vars'> is in
3600 effect, C<our> lets you use declared global variables without qualifying
3601 them with package names, within the lexical scope of the C<our> declaration.
3602 In this way C<our> differs from C<use vars>, which is package scoped.
3604 Unlike C<my>, which both allocates storage for a variable and associates
3605 a simple name with that storage for use within the current scope, C<our>
3606 associates a simple name with a package variable in the current package,
3607 for use within the current scope. In other words, C<our> has the same
3608 scoping rules as C<my>, but does not necessarily create a
3611 If more than one value is listed, the list must be placed
3617 An C<our> declaration declares a global variable that will be visible
3618 across its entire lexical scope, even across package boundaries. The
3619 package in which the variable is entered is determined at the point
3620 of the declaration, not at the point of use. This means the following
3624 our $bar; # declares $Foo::bar for rest of lexical scope
3628 print $bar; # prints 20, as it refers to $Foo::bar
3630 Multiple C<our> declarations with the same name in the same lexical
3631 scope are allowed if they are in different packages. If they happen
3632 to be in the same package, Perl will emit warnings if you have asked
3633 for them, just like multiple C<my> declarations. Unlike a second
3634 C<my> declaration, which will bind the name to a fresh variable, a
3635 second C<our> declaration in the same package, in the same scope, is
3640 our $bar; # declares $Foo::bar for rest of lexical scope
3644 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3645 print $bar; # prints 30
3647 our $bar; # emits warning but has no other effect
3648 print $bar; # still prints 30
3650 An C<our> declaration may also have a list of attributes associated
3653 The exact semantics and interface of TYPE and ATTRS are still
3654 evolving. TYPE is currently bound to the use of C<fields> pragma,
3655 and attributes are handled using the C<attributes> pragma, or starting
3656 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3657 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3658 L<attributes>, and L<Attribute::Handlers>.
3660 =item pack TEMPLATE,LIST
3663 Takes a LIST of values and converts it into a string using the rules
3664 given by the TEMPLATE. The resulting string is the concatenation of
3665 the converted values. Typically, each converted value looks
3666 like its machine-level representation. For example, on 32-bit machines
3667 an integer may be represented by a sequence of 4 bytes, which will in
3668 Perl be presented as a string that's 4 characters long.
3670 See L<perlpacktut> for an introduction to this function.
3672 The TEMPLATE is a sequence of characters that give the order and type
3673 of values, as follows:
3675 a A string with arbitrary binary data, will be null padded.
3676 A A text (ASCII) string, will be space padded.
3677 Z A null-terminated (ASCIZ) string, will be null padded.
3679 b A bit string (ascending bit order inside each byte, like vec()).
3680 B A bit string (descending bit order inside each byte).
3681 h A hex string (low nybble first).
3682 H A hex string (high nybble first).
3684 c A signed char (8-bit) value.
3685 C An unsigned char (octet) value.
3686 W An unsigned char value (can be greater than 255).
3688 s A signed short (16-bit) value.
3689 S An unsigned short value.
3691 l A signed long (32-bit) value.
3692 L An unsigned long value.
3694 q A signed quad (64-bit) value.
3695 Q An unsigned quad value.
3696 (Quads are available only if your system supports 64-bit
3697 integer values _and_ if Perl has been compiled to support those.
3698 Raises an exception otherwise.)
3700 i A signed integer value.
3701 I A unsigned integer value.
3702 (This 'integer' is _at_least_ 32 bits wide. Its exact
3703 size depends on what a local C compiler calls 'int'.)
3705 n An unsigned short (16-bit) in "network" (big-endian) order.
3706 N An unsigned long (32-bit) in "network" (big-endian) order.
3707 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3708 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3710 j A Perl internal signed integer value (IV).
3711 J A Perl internal unsigned integer value (UV).
3713 f A single-precision float in native format.
3714 d A double-precision float in native format.
3716 F A Perl internal floating-point value (NV) in native format
3717 D A float of long-double precision in native format.
3718 (Long doubles are available only if your system supports long
3719 double values _and_ if Perl has been compiled to support those.
3720 Raises an exception otherwise.)
3722 p A pointer to a null-terminated string.
3723 P A pointer to a structure (fixed-length string).
3725 u A uuencoded string.
3726 U A Unicode character number. Encodes to a character in character mode
3727 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3729 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3730 details). Its bytes represent an unsigned integer in base 128,
3731 most significant digit first, with as few digits as possible. Bit
3732 eight (the high bit) is set on each byte except the last.
3734 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
3736 @ Null-fill or truncate to absolute position, counted from the
3737 start of the innermost ()-group.
3738 . Null-fill or truncate to absolute position specified by the value.
3739 ( Start of a ()-group.
3741 One or more modifiers below may optionally follow certain letters in the
3742 TEMPLATE (the second column lists letters for which the modifier is valid):
3744 ! sSlLiI Forces native (short, long, int) sizes instead
3745 of fixed (16-/32-bit) sizes.
3747 xX Make x and X act as alignment commands.
3749 nNvV Treat integers as signed instead of unsigned.
3751 @. Specify position as byte offset in the internal
3752 representation of the packed string. Efficient but
3755 > sSiIlLqQ Force big-endian byte-order on the type.
3756 jJfFdDpP (The "big end" touches the construct.)
3758 < sSiIlLqQ Force little-endian byte-order on the type.
3759 jJfFdDpP (The "little end" touches the construct.)
3761 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
3762 to force a particular byte-order on all components in that group,
3763 including all its subgroups.
3765 The following rules apply:
3771 Each letter may optionally be followed by a number indicating the repeat
3772 count. A numeric repeat count may optionally be enclosed in brackets, as
3773 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
3774 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
3775 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
3776 something else, dscribed below. Supplying a C<*> for the repeat count
3777 instead of a number means to use however many items are left, except for:
3783 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
3787 <.>, where it means relative to the start of the string.
3791 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
3795 One can replace a numeric repeat count with a template letter enclosed in
3796 brackets to use the packed byte length of the bracketed template for the
3799 For example, the template C<x[L]> skips as many bytes as in a packed long,
3800 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
3801 variable-expanded) unpacks. If the template in brackets contains alignment
3802 commands (such as C<x![d]>), its packed length is calculated as if the
3803 start of the template had the maximal possible alignment.
3805 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
3806 trailing null byte, so the resulting string is always one byte longer than
3807 the byte length of the item itself.
3809 When used with C<@>, the repeat count represents an offset from the start
3810 of the innermost C<()> group.
3812 When used with C<.>, the repeat count determines the starting position to
3813 calculate the value offset as follows:
3819 If the repeat count is C<0>, it's relative to the current position.
3823 If the repeat count is C<*>, the offset is relative to the start of the
3828 And if it's an integer I<n>, the offset is relative to the start of the
3829 I<n>th innermost C<()> group, or to the start of the string if I<n> is
3830 bigger then the group level.
3834 The repeat count for C<u> is interpreted as the maximal number of bytes
3835 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3836 count should not be more than 65.
3840 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3841 string of length count, padding with nulls or spaces as needed. When
3842 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3843 after the first null, and C<a> returns data without any sort of trimming.
3845 If the value to pack is too long, the result is truncated. If it's too
3846 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
3847 followed by a null byte. Thus C<Z> always packs a trailing null, except
3848 for when the count is 0.
3852 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
3853 Each such format generates 1 bit of the result.
3855 Each result bit is based on the least-significant bit of the corresponding
3856 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3857 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
3859 Starting from the beginning of the input string, each 8-tuple
3860 of characters is converted to 1 character of output. With format C<b>,
3861 the first character of the 8-tuple determines the least-significant bit of a
3862 character; with format C<B>, it determines the most-significant bit of
3865 If the length of the input string is not evenly divisible by 8, the
3866 remainder is packed as if the input string were padded by null characters
3867 at the end. Similarly during unpacking, "extra" bits are ignored.
3869 If the input string is longer than needed, remaining characters are ignored.
3871 A C<*> for the repeat count uses all characters of the input field.
3872 On unpacking, bits are converted to a string of C<"0">s and C<"1">s.
3876 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
3877 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
3879 For each such format, pack() generates 4 bits of the result.
3880 With non-alphabetical characters, the result is based on the 4 least-significant
3881 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3882 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3883 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F">, the result
3884 is compatible with the usual hexadecimal digits, so that C<"a"> and
3885 C<"A"> both generate the nybble C<0xa==10>. Do not use any characters
3886 but these with this format.
3888 Starting from the beginning of the template to pack(), each pair
3889 of characters is converted to 1 character of output. With format C<h>, the
3890 first character of the pair determines the least-significant nybble of the
3891 output character; with format C<H>, it determines the most-significant
3894 If the length of the input string is not even, it behaves as if padded by
3895 a null character at the end. Similarly, "extra" nybbles are ignored during
3898 If the input string is longer than needed, extra characters are ignored.
3900 A C<*> for the repeat count uses all characters of the input field. For
3901 unpack(), nybbles are converted to a string of hexadecimal digits.
3905 The C<p> format packs a pointer to a null-terminated string. You are
3906 responsible for ensuring that the string is not a temporary value, as that
3907 could potentially get deallocated before you got around to using the packed
3908 result. The C<P> format packs a pointer to a structure of the size indicated
3909 by the length. A null pointer is created if the corresponding value for
3910 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
3911 unpacks into C<undef>.
3913 If your system has a strange pointer size--meaning a pointer is neither as
3914 big as an int nor as big as a long--it may not be possible to pack or
3915 unpack pointers in big- or little-endian byte order. Attempting to do
3916 so raises an exception.
3920 The C</> template character allows packing and unpacking of a sequence of
3921 items where the packed structure contains a packed item count followed by
3922 the packed items themselves. This is useful when the structure you're
3923 unpacking has encoded the sizes or repeat counts for some of its fields
3924 within the structure itself as separate fields.
3926 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
3927 I<length-item> describes how the length value is packed. Formats likely
3928 to be of most use are integer-packing ones like C<n> for Java strings,
3929 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
3931 For C<pack>, I<sequence-item> may have a repeat count, in which case
3932 the minimum of that and the number of available items is used as the argument
3933 for I<length-item>. If it has no repeat count or uses a '*', the number
3934 of available items is used.
3936 For C<unpack>, an internal stack of integer arguments unpacked so far is
3937 used. You write C</>I<sequence-item> and the repeat count is obtained by
3938 popping off the last element from the stack. The I<sequence-item> must not
3939 have a repeat count.
3941 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
3942 the I<length-item> is the string length, not the number of strings. With
3943 an explicit repeat count for pack, the packed string is adjusted to that
3944 length. For example:
3946 unpack("W/a", "\04Gurusamy") gives ("Guru")
3947 unpack("a3/A A*", "007 Bond J ") gives (" Bond", "J")
3948 unpack("a3 x2 /A A*", "007: Bond, J.") gives ("Bond, J", ".")
3950 pack("n/a* w/a","hello,","world") gives "\000\006hello,\005world"
3951 pack("a/W2", ord("a") .. ord("z")) gives "2ab"
3953 The I<length-item> is not returned explicitly from C<unpack>.
3955 Supplying a count to the I<length-item> format letter is only useful with
3956 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
3957 introduce C<"\000"> characters, which Perl does not regard as legal in
3962 The integer types C<s>, C<S>, C<l>, and C<L> may be
3963 followed by a C<!> modifier to specify native shorts or
3964 longs. As shown in the example above, a bare C<l> means
3965 exactly 32 bits, although the native C<long> as seen by the local C compiler
3966 may be larger. This is mainly an issue on 64-bit platforms. You can
3967 see whether using C<!> makes any difference this way:
3969 printf "format s is %d, s! is %d\n",
3970 length pack("s"), length pack("s!");
3972 printf "format l is %d, l! is %d\n",
3973 length pack("l"), length pack("l!");
3976 C<i!> and C<I!> are also allowed, but only for completeness' sake:
3977 they are identical to C<i> and C<I>.
3979 The actual sizes (in bytes) of native shorts, ints, longs, and long
3980 longs on the platform where Perl was built are also available from
3983 $ perl -V:{short,int,long{,long}}size
3989 or programmatically via the C<Config> module:
3992 print $Config{shortsize}, "\n";
3993 print $Config{intsize}, "\n";
3994 print $Config{longsize}, "\n";
3995 print $Config{longlongsize}, "\n";
3997 C<$Config{longlongsize}> is undefined on systems without
4002 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4003 inherently non-portable between processors and operating systems because
4004 they obey native byteorder and endianness. For example, a 4-byte integer
4005 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4006 handled by the CPU registers) into bytes as
4008 0x12 0x34 0x56 0x78 # big-endian
4009 0x78 0x56 0x34 0x12 # little-endian
4011 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4012 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4013 big-endian. Alpha and MIPS can be either: Digital/Compaq used/uses them in
4014 little-endian mode, but SGI/Cray uses them in big-endian mode.
4016 The names I<big-endian> and I<little-endian> are comic references to the
4017 egg-eating habits of the little-endian Lilliputians and the big-endian
4018 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4019 This entered computer lingo via the paper "On Holy Wars and a Plea for
4020 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4022 Some systems may have even weirder byte orders such as
4027 You can determine your system endianness with this incantation:
4029 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4031 The byteorder on the platform where Perl was built is also available
4035 print "$Config{byteorder}\n";
4037 or from the command line:
4041 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4042 and C<"87654321"> are big-endian.
4044 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4045 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4046 immediately below. See also L<perlport>.
4050 Starting with Perl 5.9.2, integer and floating-point formats, along with
4051 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4052 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4053 or little-endian byte-order. These modifiers are especially useful
4054 given how C<n>, C<N>, C<v> and C<V> don't cover signed integers,
4055 64-bit integers, or floating-point values.
4057 Here are some concerns to keep in mind when using endianness modifier:
4063 Exchanging signed integers between different platforms works only
4064 when all platforms store them in the same format. Most platforms store
4065 signed integers in two's-complement notation, so usually this is not an issue.
4069 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4070 formats on big- or little-endian machines. Otherwise, attempting to
4071 use them raises an exception.
4075 Forcing big- or little-endian byte-order on floating-point values for
4076 data exchange can work only if all platforms use the same
4077 binary representation such as IEEE floating-point. Even if all
4078 platforms are using IEEE, there may still be subtle differences. Being able
4079 to use C<< > >> or C<< < >> on floating-point values can be useful,
4080 but also dangerous if you don't know exactly what you're doing.
4081 It is not a general way to portably store floating-point values.
4085 When using C<< > >> or C<< < >> on a C<()> group, this affects
4086 all types inside the group that accept byte-order modifiers,
4087 including all subgroups. It is silently ignored for all other
4088 types. You are not allowed to override the byte-order within a group
4089 that already has a byte-order modifier suffix.
4095 Real numbers (floats and doubles) are in native machine format only.
4096 Due to the multiplicity of floating-point formats and the lack of a
4097 standard "network" representation for them, no facility for interchange has been
4098 made. This means that packed floating-point data written on one machine
4099 may not be readable on another, even if both use IEEE floating-point
4100 arithmetic (because the endianness of the memory representation is not part
4101 of the IEEE spec). See also L<perlport>.
4103 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4104 modifiers to force big- or little-endian byte-order on floating-point values.
4106 Because Perl uses doubles (or long doubles, if configured) internally for
4107 all numeric calculation, converting from double into float and thence
4108 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4109 will not in general equal $foo.
4113 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4114 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4115 where the packed string is processed in its UTF-8-encoded Unicode form on
4116 a byte-by-byte basis. Character mode is the default unless the format string
4117 starts with C<U>. You can always switch mode mid-format with an explicit
4118 C<C0> or C<U0> in the format. This mode remains in effect until the next
4119 mode change, or until the end of the C<()> group it (directly) applies to.
4123 You must yourself do any alignment or padding by inserting, for example,
4124 enough C<"x">es while packing. There is no way for pack() and unpack()
4125 to know where characters are going to or coming from, so they
4126 handle their output and input as flat sequences of characters.
4130 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4131 take a repeat count either as postfix, or for unpack(), also via the C</>
4132 template character. Within each repetition of a group, positioning with
4133 C<@> starts over at 0. Therefore, the result of
4135 pack("@1A((@2A)@3A)", qw[X Y Z])
4137 is the string C<"\0X\0\0YZ">.
4141 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4142 jump forward or back to the closest position aligned at a multiple of C<count>
4143 characters. For example, to pack() or unpack() a C structure like
4146 char c; /* one signed, 8-bit character */
4151 one may need to use the template C<c x![d] d c[2]>. This assumes that
4152 doubles must be aligned to the size of double.
4154 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4159 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4160 represent signed 16-/32-bit integers in big-/little-endian order.
4161 This is portable only when all platforms sharing packed data use the
4162 same binary representation for signed integers; for example, when all
4163 platforms use two's-complement representation.
4167 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4168 White space can separate pack codes from each other, but modifiers and
4169 repeat counts must follow immediately. Breaking complex templates into
4170 individual line-by-line components, suitably annotated, can do as much to
4171 improve legibility and maintainability of pack/unpack formats as C</x> can
4172 for complicated pattern matches.
4176 If TEMPLATE requires more arguments that pack() is given, pack()
4177 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4178 than given, extra arguments are ignored.
4184 $foo = pack("WWWW",65,66,67,68);
4186 $foo = pack("W4",65,66,67,68);
4188 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4189 # same thing with Unicode circled letters.
4190 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4191 # same thing with Unicode circled letters. You don't get the UTF-8
4192 # bytes because the U at the start of the format caused a switch to
4193 # U0-mode, so the UTF-8 bytes get joined into characters
4194 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4195 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4196 # This is the UTF-8 encoding of the string in the previous example
4198 $foo = pack("ccxxcc",65,66,67,68);
4201 # NOTE: The examples above featuring "W" and "c" are true
4202 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4203 # and UTF-8. On EBCDIC systems, the first example would be
4204 # $foo = pack("WWWW",193,194,195,196);
4206 $foo = pack("s2",1,2);
4207 # "\1\0\2\0" on little-endian
4208 # "\0\1\0\2" on big-endian
4210 $foo = pack("a4","abcd","x","y","z");
4213 $foo = pack("aaaa","abcd","x","y","z");
4216 $foo = pack("a14","abcdefg");
4217 # "abcdefg\0\0\0\0\0\0\0"
4219 $foo = pack("i9pl", gmtime);
4220 # a real struct tm (on my system anyway)
4222 $utmp_template = "Z8 Z8 Z16 L";
4223 $utmp = pack($utmp_template, @utmp1);
4224 # a struct utmp (BSDish)
4226 @utmp2 = unpack($utmp_template, $utmp);
4227 # "@utmp1" eq "@utmp2"
4230 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4233 $foo = pack('sx2l', 12, 34);
4234 # short 12, two zero bytes padding, long 34
4235 $bar = pack('s@4l', 12, 34);
4236 # short 12, zero fill to position 4, long 34
4238 $baz = pack('s.l', 12, 4, 34);
4239 # short 12, zero fill to position 4, long 34
4241 $foo = pack('nN', 42, 4711);
4242 # pack big-endian 16- and 32-bit unsigned integers
4243 $foo = pack('S>L>', 42, 4711);
4245 $foo = pack('s<l<', -42, 4711);
4246 # pack little-endian 16- and 32-bit signed integers
4247 $foo = pack('(sl)<', -42, 4711);
4250 The same template may generally also be used in unpack().
4252 =item package NAMESPACE VERSION
4253 X<package> X<module> X<namespace> X<version>
4255 =item package NAMESPACE
4257 =item package NAMESPACE VERSION BLOCK
4258 X<package> X<module> X<namespace> X<version>
4260 =item package NAMESPACE BLOCK
4262 Declares the BLOCK, or the rest of the compilation unit, as being in
4263 the given namespace. The scope of the package declaration is either the
4264 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
4265 itself through the end of the enclosing block, file, or eval (the same
4266 as the C<my> operator). All unqualified dynamic identifiers in this
4267 scope will be in the given namespace, except where overridden by another
4268 C<package> declaration.
4270 A package statement affects dynamic variables only, including those
4271 you've used C<local> on, but I<not> lexical variables, which are created
4272 with C<my> (or C<our> (or C<state>)). Typically it would be the first
4273 declaration in a file included by C<require> or C<use>. You can switch into a
4274 package in more than one place, since this only determines which default
4275 symbol table the compiler uses for the rest of that block. You can refer to
4276 identifiers in other packages than the current one by prefixing the identifier
4277 with the package name and a double colon, as in C<$SomePack::var>
4278 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
4279 package as assumed. That is, C<$::sail> is equivalent to
4280 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
4281 code, mostly from Perl 4).
4283 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
4284 namespace to a L<version> object with the VERSION provided. VERSION must be a
4285 "strict" style version number as defined by the L<version> module: a positive
4286 decimal number (integer or decimal-fraction) without exponentiation or else a
4287 dotted-decimal v-string with a leading 'v' character and at least three
4288 components. You should set C<$VERSION> only once per package.
4290 See L<perlmod/"Packages"> for more information about packages, modules,
4291 and classes. See L<perlsub> for other scoping issues.
4293 =item pipe READHANDLE,WRITEHANDLE
4296 Opens a pair of connected pipes like the corresponding system call.
4297 Note that if you set up a loop of piped processes, deadlock can occur
4298 unless you are very careful. In addition, note that Perl's pipes use
4299 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4300 after each command, depending on the application.
4302 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4303 for examples of such things.
4305 On systems that support a close-on-exec flag on files, that flag is set
4306 on all newly opened file descriptors whose C<fileno>s are I<higher> than
4307 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
4314 Pops and returns the last value of the array, shortening the array by
4317 Returns the undefined value if the array is empty, although this may also
4318 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
4319 main program, but the C<@_> array in subroutines, just like C<shift>.
4322 X<pos> X<match, position>
4326 Returns the offset of where the last C<m//g> search left off for the variable
4327 in question (C<$_> is used when the variable is not specified). Note that
4328 0 is a valid match offset. C<undef> indicates that the search position
4329 is reset (usually due to match failure, but can also be because no match has
4330 yet been run on the scalar). C<pos> directly accesses the location used
4331 by the regexp engine to store the offset, so assigning to C<pos> will change
4332 that offset, and so will also influence the C<\G> zero-width assertion in
4333 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4334 the return from C<pos> won't change either in this case. See L<perlre> and
4337 =item print FILEHANDLE LIST
4344 Prints a string or a list of strings. Returns true if successful.
4345 FILEHANDLE may be a scalar variable containing
4346 the name of or a reference to the filehandle, thus introducing
4347 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4348 the next token is a term, it may be misinterpreted as an operator
4349 unless you interpose a C<+> or put parentheses around the arguments.)
4350 If FILEHANDLE is omitted, prints to standard output by default, or
4351 to the last selected output channel; see L</select>. If LIST is
4352 also omitted, prints C<$_> to the currently selected output handle.
4353 To set the default output handle to something other than STDOUT
4354 use the select operation. The current value of C<$,> (if any) is
4355 printed between each LIST item. The current value of C<$\> (if
4356 any) is printed after the entire LIST has been printed. Because
4357 print takes a LIST, anything in the LIST is evaluated in list
4358 context, and any subroutine that you call will have one or more of
4359 its expressions evaluated in list context. Also be careful not to
4360 follow the print keyword with a left parenthesis unless you want
4361 the corresponding right parenthesis to terminate the arguments to
4362 the print; put parentheses around all the arguments
4363 (or interpose a C<+>, but that doesn't look as good).
4365 Note that if you're storing FILEHANDLEs in an array, or if you're using
4366 any other expression more complex than a scalar variable to retrieve it,
4367 you will have to use a block returning the filehandle value instead:
4369 print { $files[$i] } "stuff\n";
4370 print { $OK ? STDOUT : STDERR } "stuff\n";
4372 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
4373 L<perlipc> for more on signal handling.
4375 =item printf FILEHANDLE FORMAT, LIST
4378 =item printf FORMAT, LIST
4380 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4381 (the output record separator) is not appended. The first argument
4382 of the list will be interpreted as the C<printf> format. See C<sprintf>
4383 for an explanation of the format argument. If C<use locale> is in effect,
4384 and POSIX::setlocale() has been called, the character used for the decimal
4385 separator in formatted floating-point numbers is affected by the LC_NUMERIC
4386 locale. See L<perllocale> and L<POSIX>.
4388 Don't fall into the trap of using a C<printf> when a simple
4389 C<print> would do. The C<print> is more efficient and less
4392 =item prototype FUNCTION
4395 Returns the prototype of a function as a string (or C<undef> if the
4396 function has no prototype). FUNCTION is a reference to, or the name of,
4397 the function whose prototype you want to retrieve.
4399 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4400 name for a Perl builtin. If the builtin is not I<overridable> (such as
4401 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4402 (such as C<system>), prototype() returns C<undef>, because the builtin
4403 does not really behave like a Perl function. Otherwise, the string
4404 describing the equivalent prototype is returned.
4406 =item push ARRAY,LIST
4409 Treats ARRAY as a stack, and pushes the values of LIST
4410 onto the end of ARRAY. The length of ARRAY increases by the length of
4411 LIST. Has the same effect as
4414 $ARRAY[++$#ARRAY] = $value;
4417 but is more efficient. Returns the number of elements in the array following
4418 the completed C<push>.
4428 Generalized quotes. See L<perlop/"Quote-Like Operators">.
4432 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
4434 =item quotemeta EXPR
4435 X<quotemeta> X<metacharacter>
4439 Returns the value of EXPR with all non-"word"
4440 characters backslashed. (That is, all characters not matching
4441 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4442 returned string, regardless of any locale settings.)
4443 This is the internal function implementing
4444 the C<\Q> escape in double-quoted strings.
4446 If EXPR is omitted, uses C<$_>.
4448 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
4449 regular expressions, because by default an interpolated variable will be
4450 considered a mini-regular expression. For example:
4452 my $sentence = 'The quick brown fox jumped over the lazy dog';
4453 my $substring = 'quick.*?fox';
4454 $sentence =~ s{$substring}{big bad wolf};
4456 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
4460 my $sentence = 'The quick brown fox jumped over the lazy dog';
4461 my $substring = 'quick.*?fox';
4462 $sentence =~ s{\Q$substring\E}{big bad wolf};
4466 my $sentence = 'The quick brown fox jumped over the lazy dog';
4467 my $substring = 'quick.*?fox';
4468 my $quoted_substring = quotemeta($substring);
4469 $sentence =~ s{$quoted_substring}{big bad wolf};
4471 Will both leave the sentence as is. Normally, when accepting string input from
4472 the user, quotemeta() or C<\Q> must be used.
4479 Returns a random fractional number greater than or equal to C<0> and less
4480 than the value of EXPR. (EXPR should be positive.) If EXPR is
4481 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4482 also special-cased as C<1> (this was undocumented before Perl 5.8.0
4483 and is subject to change in future versions of Perl). Automatically calls
4484 C<srand> unless C<srand> has already been called. See also C<srand>.
4486 Apply C<int()> to the value returned by C<rand()> if you want random
4487 integers instead of random fractional numbers. For example,
4491 returns a random integer between C<0> and C<9>, inclusive.
4493 (Note: If your rand function consistently returns numbers that are too
4494 large or too small, then your version of Perl was probably compiled
4495 with the wrong number of RANDBITS.)
4497 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4498 X<read> X<file, read>
4500 =item read FILEHANDLE,SCALAR,LENGTH
4502 Attempts to read LENGTH I<characters> of data into variable SCALAR
4503 from the specified FILEHANDLE. Returns the number of characters
4504 actually read, C<0> at end of file, or undef if there was an error (in
4505 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4506 so that the last character actually read is the last character of the
4507 scalar after the read.
4509 An OFFSET may be specified to place the read data at some place in the
4510 string other than the beginning. A negative OFFSET specifies
4511 placement at that many characters counting backwards from the end of
4512 the string. A positive OFFSET greater than the length of SCALAR
4513 results in the string being padded to the required size with C<"\0">
4514 bytes before the result of the read is appended.
4516 The call is implemented in terms of either Perl's or your system's native
4517 fread(3) library function. To get a true read(2) system call, see C<sysread>.
4519 Note the I<characters>: depending on the status of the filehandle,
4520 either (8-bit) bytes or characters are read. By default all
4521 filehandles operate on bytes, but for example if the filehandle has
4522 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4523 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4524 characters, not bytes. Similarly for the C<:encoding> pragma:
4525 in that case pretty much any characters can be read.
4527 =item readdir DIRHANDLE
4530 Returns the next directory entry for a directory opened by C<opendir>.
4531 If used in list context, returns all the rest of the entries in the
4532 directory. If there are no more entries, returns the undefined value in
4533 scalar context and the empty list in list context.
4535 If you're planning to filetest the return values out of a C<readdir>, you'd
4536 better prepend the directory in question. Otherwise, because we didn't
4537 C<chdir> there, it would have been testing the wrong file.
4539 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4540 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4543 As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop,
4544 which will set C<$_> on every iteration.
4546 opendir(my $dh, $some_dir) || die;
4547 while(readdir $dh) {
4548 print "$some_dir/$_\n";
4555 X<readline> X<gets> X<fgets>
4557 Reads from the filehandle whose typeglob is contained in EXPR (or from
4558 *ARGV if EXPR is not provided). In scalar context, each call reads and
4559 returns the next line until end-of-file is reached, whereupon the
4560 subsequent call returns C<undef>. In list context, reads until end-of-file
4561 is reached and returns a list of lines. Note that the notion of "line"
4562 used here is whatever you may have defined with C<$/> or
4563 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4565 When C<$/> is set to C<undef>, when C<readline> is in scalar
4566 context (i.e., file slurp mode), and when an empty file is read, it
4567 returns C<''> the first time, followed by C<undef> subsequently.
4569 This is the internal function implementing the C<< <EXPR> >>
4570 operator, but you can use it directly. The C<< <EXPR> >>
4571 operator is discussed in more detail in L<perlop/"I/O Operators">.
4574 $line = readline(*STDIN); # same thing
4576 If C<readline> encounters an operating system error, C<$!> will be set
4577 with the corresponding error message. It can be helpful to check
4578 C<$!> when you are reading from filehandles you don't trust, such as a
4579 tty or a socket. The following example uses the operator form of
4580 C<readline> and dies if the result is not defined.
4582 while ( ! eof($fh) ) {
4583 defined( $_ = <$fh> ) or die "readline failed: $!";
4587 Note that you have can't handle C<readline> errors that way with the
4588 C<ARGV> filehandle. In that case, you have to open each element of
4589 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
4591 foreach my $arg (@ARGV) {
4592 open(my $fh, $arg) or warn "Can't open $arg: $!";
4594 while ( ! eof($fh) ) {
4595 defined( $_ = <$fh> )
4596 or die "readline failed for $arg: $!";
4606 Returns the value of a symbolic link, if symbolic links are
4607 implemented. If not, raises an exception. If there is a system
4608 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4609 omitted, uses C<$_>.
4616 EXPR is executed as a system command.
4617 The collected standard output of the command is returned.
4618 In scalar context, it comes back as a single (potentially
4619 multi-line) string. In list context, returns a list of lines
4620 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4621 This is the internal function implementing the C<qx/EXPR/>
4622 operator, but you can use it directly. The C<qx/EXPR/>
4623 operator is discussed in more detail in L<perlop/"I/O Operators">.
4624 If EXPR is omitted, uses C<$_>.
4626 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4629 Receives a message on a socket. Attempts to receive LENGTH characters
4630 of data into variable SCALAR from the specified SOCKET filehandle.
4631 SCALAR will be grown or shrunk to the length actually read. Takes the
4632 same flags as the system call of the same name. Returns the address
4633 of the sender if SOCKET's protocol supports this; returns an empty
4634 string otherwise. If there's an error, returns the undefined value.
4635 This call is actually implemented in terms of recvfrom(2) system call.
4636 See L<perlipc/"UDP: Message Passing"> for examples.
4638 Note the I<characters>: depending on the status of the socket, either
4639 (8-bit) bytes or characters are received. By default all sockets
4640 operate on bytes, but for example if the socket has been changed using
4641 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4642 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4643 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4644 case pretty much any characters can be read.
4651 The C<redo> command restarts the loop block without evaluating the
4652 conditional again. The C<continue> block, if any, is not executed. If
4653 the LABEL is omitted, the command refers to the innermost enclosing
4654 loop. Programs that want to lie to themselves about what was just input
4655 normally use this command:
4657 # a simpleminded Pascal comment stripper
4658 # (warning: assumes no { or } in strings)
4659 LINE: while (<STDIN>) {
4660 while (s|({.*}.*){.*}|$1 |) {}
4665 if (/}/) { # end of comment?
4674 C<redo> cannot be used to retry a block that returns a value such as
4675 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4676 a grep() or map() operation.
4678 Note that a block by itself is semantically identical to a loop
4679 that executes once. Thus C<redo> inside such a block will effectively
4680 turn it into a looping construct.
4682 See also L</continue> for an illustration of how C<last>, C<next>, and
4690 Returns a non-empty string if EXPR is a reference, the empty
4691 string otherwise. If EXPR
4692 is not specified, C<$_> will be used. The value returned depends on the
4693 type of thing the reference is a reference to.
4694 Builtin types include:
4708 If the referenced object has been blessed into a package, then that package
4709 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4711 if (ref($r) eq "HASH") {
4712 print "r is a reference to a hash.\n";
4715 print "r is not a reference at all.\n";
4718 The return value C<LVALUE> indicates a reference to an lvalue that is not
4719 a variable. You get this from taking the reference of function calls like
4720 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4721 to a L<version string|perldata/"Version Strings">.
4723 The result C<Regexp> indicates that the argument is a regular expression
4724 resulting from C<qr//>.
4726 See also L<perlref>.
4728 =item rename OLDNAME,NEWNAME
4729 X<rename> X<move> X<mv> X<ren>
4731 Changes the name of a file; an existing file NEWNAME will be
4732 clobbered. Returns true for success, false otherwise.
4734 Behavior of this function varies wildly depending on your system
4735 implementation. For example, it will usually not work across file system
4736 boundaries, even though the system I<mv> command sometimes compensates
4737 for this. Other restrictions include whether it works on directories,
4738 open files, or pre-existing files. Check L<perlport> and either the
4739 rename(2) manpage or equivalent system documentation for details.
4741 For a platform independent C<move> function look at the L<File::Copy>
4744 =item require VERSION
4751 Demands a version of Perl specified by VERSION, or demands some semantics
4752 specified by EXPR or by C<$_> if EXPR is not supplied.
4754 VERSION may be either a numeric argument such as 5.006, which will be
4755 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4756 to C<$^V> (aka $PERL_VERSION). An exception is raised if
4757 VERSION is greater than the version of the current Perl interpreter.
4758 Compare with L</use>, which can do a similar check at compile time.
4760 Specifying VERSION as a literal of the form v5.6.1 should generally be
4761 avoided, because it leads to misleading error messages under earlier
4762 versions of Perl that do not support this syntax. The equivalent numeric
4763 version should be used instead.
4765 require v5.6.1; # run time version check
4766 require 5.6.1; # ditto
4767 require 5.006_001; # ditto; preferred for backwards compatibility
4769 Otherwise, C<require> demands that a library file be included if it
4770 hasn't already been included. The file is included via the do-FILE
4771 mechanism, which is essentially just a variety of C<eval> with the
4772 caveat that lexical variables in the invoking script will be invisible
4773 to the included code. Has semantics similar to the following subroutine:
4776 my ($filename) = @_;
4777 if (exists $INC{$filename}) {
4778 return 1 if $INC{$filename};
4779 die "Compilation failed in require";
4781 my ($realfilename,$result);
4783 foreach $prefix (@INC) {
4784 $realfilename = "$prefix/$filename";
4785 if (-f $realfilename) {
4786 $INC{$filename} = $realfilename;
4787 $result = do $realfilename;
4791 die "Can't find $filename in \@INC";
4794 $INC{$filename} = undef;
4796 } elsif (!$result) {
4797 delete $INC{$filename};
4798 die "$filename did not return true value";
4804 Note that the file will not be included twice under the same specified
4807 The file must return true as the last statement to indicate
4808 successful execution of any initialization code, so it's customary to
4809 end such a file with C<1;> unless you're sure it'll return true
4810 otherwise. But it's better just to put the C<1;>, in case you add more
4813 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4814 replaces "F<::>" with "F</>" in the filename for you,
4815 to make it easy to load standard modules. This form of loading of
4816 modules does not risk altering your namespace.
4818 In other words, if you try this:
4820 require Foo::Bar; # a splendid bareword
4822 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4823 directories specified in the C<@INC> array.
4825 But if you try this:
4827 $class = 'Foo::Bar';
4828 require $class; # $class is not a bareword
4830 require "Foo::Bar"; # not a bareword because of the ""
4832 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4833 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4835 eval "require $class";
4837 Now that you understand how C<require> looks for files with a
4838 bareword argument, there is a little extra functionality going on behind
4839 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4840 first look for a similar filename with a "F<.pmc>" extension. If this file
4841 is found, it will be loaded in place of any file ending in a "F<.pm>"
4844 You can also insert hooks into the import facility, by putting Perl code
4845 directly into the @INC array. There are three forms of hooks: subroutine
4846 references, array references and blessed objects.
4848 Subroutine references are the simplest case. When the inclusion system
4849 walks through @INC and encounters a subroutine, this subroutine gets
4850 called with two parameters, the first a reference to itself, and the
4851 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
4852 subroutine should return either nothing or else a list of up to three
4853 values in the following order:
4859 A filehandle, from which the file will be read.
4863 A reference to a subroutine. If there is no filehandle (previous item),
4864 then this subroutine is expected to generate one line of source code per
4865 call, writing the line into C<$_> and returning 1, then returning 0 at
4866 end of file. If there is a filehandle, then the subroutine will be
4867 called to act as a simple source filter, with the line as read in C<$_>.
4868 Again, return 1 for each valid line, and 0 after all lines have been
4873 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4874 reference to the subroutine itself is passed in as C<$_[0]>.
4878 If an empty list, C<undef>, or nothing that matches the first 3 values above
4879 is returned, then C<require> looks at the remaining elements of @INC.
4880 Note that this filehandle must be a real filehandle (strictly a typeglob
4881 or reference to a typeglob, blessed or unblessed); tied filehandles will be
4882 ignored and return value processing will stop there.
4884 If the hook is an array reference, its first element must be a subroutine
4885 reference. This subroutine is called as above, but the first parameter is
4886 the array reference. This lets you indirectly pass arguments to
4889 In other words, you can write:
4891 push @INC, \&my_sub;
4893 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4899 push @INC, [ \&my_sub, $x, $y, ... ];
4901 my ($arrayref, $filename) = @_;
4902 # Retrieve $x, $y, ...
4903 my @parameters = @$arrayref[1..$#$arrayref];
4907 If the hook is an object, it must provide an INC method that will be
4908 called as above, the first parameter being the object itself. (Note that
4909 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4910 into package C<main>.) Here is a typical code layout:
4916 my ($self, $filename) = @_;
4920 # In the main program
4921 push @INC, Foo->new(...);
4923 These hooks are also permitted to set the %INC entry
4924 corresponding to the files they have loaded. See L<perlvar/%INC>.
4926 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4933 Generally used in a C<continue> block at the end of a loop to clear
4934 variables and reset C<??> searches so that they work again. The
4935 expression is interpreted as a list of single characters (hyphens
4936 allowed for ranges). All variables and arrays beginning with one of
4937 those letters are reset to their pristine state. If the expression is
4938 omitted, one-match searches (C<?pattern?>) are reset to match again.
4939 Only resets variables or searches in the current package. Always returns
4942 reset 'X'; # reset all X variables
4943 reset 'a-z'; # reset lower case variables
4944 reset; # just reset ?one-time? searches
4946 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4947 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4948 variables; lexical variables are unaffected, but they clean themselves
4949 up on scope exit anyway, so you'll probably want to use them instead.
4957 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4958 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4959 context, depending on how the return value will be used, and the context
4960 may vary from one execution to the next (see C<wantarray>). If no EXPR
4961 is given, returns an empty list in list context, the undefined value in
4962 scalar context, and (of course) nothing at all in void context.
4964 (In the absence of an explicit C<return>, a subroutine, eval,
4965 or do FILE automatically returns the value of the last expression
4969 X<reverse> X<rev> X<invert>
4971 In list context, returns a list value consisting of the elements
4972 of LIST in the opposite order. In scalar context, concatenates the
4973 elements of LIST and returns a string value with all characters
4974 in the opposite order.
4976 print join(", ", reverse "world", "Hello"); # Hello, world
4978 print scalar reverse "dlrow ,", "olleH"; # Hello, world
4980 Used without arguments in scalar context, reverse() reverses C<$_>.
4982 $_ = "dlrow ,olleH";
4983 print reverse; # No output, list context
4984 print scalar reverse; # Hello, world
4986 Note that reversing an array to itself (as in C<@a = reverse @a>) will
4987 preserve non-existent elements whenever possible, i.e., for non magical
4988 arrays or tied arrays with C<EXISTS> and C<DELETE> methods.
4990 This operator is also handy for inverting a hash, although there are some
4991 caveats. If a value is duplicated in the original hash, only one of those
4992 can be represented as a key in the inverted hash. Also, this has to
4993 unwind one hash and build a whole new one, which may take some time
4994 on a large hash, such as from a DBM file.
4996 %by_name = reverse %by_address; # Invert the hash
4998 =item rewinddir DIRHANDLE
5001 Sets the current position to the beginning of the directory for the
5002 C<readdir> routine on DIRHANDLE.
5004 =item rindex STR,SUBSTR,POSITION
5007 =item rindex STR,SUBSTR
5009 Works just like index() except that it returns the position of the I<last>
5010 occurrence of SUBSTR in STR. If POSITION is specified, returns the
5011 last occurrence beginning at or before that position.
5013 =item rmdir FILENAME
5014 X<rmdir> X<rd> X<directory, remove>
5018 Deletes the directory specified by FILENAME if that directory is
5019 empty. If it succeeds it returns true, otherwise it returns false and
5020 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
5022 To remove a directory tree recursively (C<rm -rf> on Unix) look at
5023 the C<rmtree> function of the L<File::Path> module.
5027 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
5029 =item say FILEHANDLE LIST
5036 Just like C<print>, but implicitly appends a newline.
5037 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
5040 This keyword is available only when the "say" feature is
5041 enabled: see L<feature>.
5044 X<scalar> X<context>
5046 Forces EXPR to be interpreted in scalar context and returns the value
5049 @counts = ( scalar @a, scalar @b, scalar @c );
5051 There is no equivalent operator to force an expression to
5052 be interpolated in list context because in practice, this is never
5053 needed. If you really wanted to do so, however, you could use
5054 the construction C<@{[ (some expression) ]}>, but usually a simple
5055 C<(some expression)> suffices.
5057 Because C<scalar> is a unary operator, if you accidentally use for EXPR a
5058 parenthesized list, this behaves as a scalar comma expression, evaluating
5059 all but the last element in void context and returning the final element
5060 evaluated in scalar context. This is seldom what you want.
5062 The following single statement:
5064 print uc(scalar(&foo,$bar)),$baz;
5066 is the moral equivalent of these two:
5069 print(uc($bar),$baz);
5071 See L<perlop> for more details on unary operators and the comma operator.
5073 =item seek FILEHANDLE,POSITION,WHENCE
5074 X<seek> X<fseek> X<filehandle, position>
5076 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
5077 FILEHANDLE may be an expression whose value gives the name of the
5078 filehandle. The values for WHENCE are C<0> to set the new position
5079 I<in bytes> to POSITION, C<1> to set it to the current position plus
5080 POSITION, and C<2> to set it to EOF plus POSITION (typically
5081 negative). For WHENCE you may use the constants C<SEEK_SET>,
5082 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
5083 of the file) from the Fcntl module. Returns C<1> on success, C<0>
5086 Note the I<in bytes>: even if the filehandle has been set to
5087 operate on characters (for example by using the C<:encoding(utf8)> open
5088 layer), tell() will return byte offsets, not character offsets
5089 (because implementing that would render seek() and tell() rather slow).
5091 If you want to position the file for C<sysread> or C<syswrite>, don't use
5092 C<seek>, because buffering makes its effect on the file's read-write position
5093 unpredictable and non-portable. Use C<sysseek> instead.
5095 Due to the rules and rigors of ANSI C, on some systems you have to do a
5096 seek whenever you switch between reading and writing. Amongst other
5097 things, this may have the effect of calling stdio's clearerr(3).
5098 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
5102 This is also useful for applications emulating C<tail -f>. Once you hit
5103 EOF on your read and then sleep for a while, you (probably) have to stick in a
5104 dummy seek() to reset things. The C<seek> doesn't change the position,
5105 but it I<does> clear the end-of-file condition on the handle, so that the
5106 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
5108 If that doesn't work (some I/O implementations are particularly
5109 cantankerous), you might need something like this:
5112 for ($curpos = tell(FILE); $_ = <FILE>;
5113 $curpos = tell(FILE)) {
5114 # search for some stuff and put it into files
5116 sleep($for_a_while);
5117 seek(FILE, $curpos, 0);
5120 =item seekdir DIRHANDLE,POS
5123 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
5124 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
5125 about possible directory compaction as the corresponding system library
5128 =item select FILEHANDLE
5129 X<select> X<filehandle, default>
5133 Returns the currently selected filehandle. If FILEHANDLE is supplied,
5134 sets the new current default filehandle for output. This has two
5135 effects: first, a C<write> or a C<print> without a filehandle will
5136 default to this FILEHANDLE. Second, references to variables related to
5137 output will refer to this output channel. For example, if you have to
5138 set the top of form format for more than one output channel, you might
5146 FILEHANDLE may be an expression whose value gives the name of the
5147 actual filehandle. Thus:
5149 $oldfh = select(STDERR); $| = 1; select($oldfh);
5151 Some programmers may prefer to think of filehandles as objects with
5152 methods, preferring to write the last example as:
5155 STDERR->autoflush(1);
5157 =item select RBITS,WBITS,EBITS,TIMEOUT
5160 This calls the select(2) syscall with the bit masks specified, which
5161 can be constructed using C<fileno> and C<vec>, along these lines:
5163 $rin = $win = $ein = '';
5164 vec($rin,fileno(STDIN),1) = 1;
5165 vec($win,fileno(STDOUT),1) = 1;
5168 If you want to select on many filehandles, you may wish to write a
5169 subroutine like this:
5172 my(@fhlist) = split(' ',$_[0]);
5175 vec($bits,fileno($_),1) = 1;
5179 $rin = fhbits('STDIN TTY SOCK');
5183 ($nfound,$timeleft) =
5184 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
5186 or to block until something becomes ready just do this
5188 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
5190 Most systems do not bother to return anything useful in $timeleft, so
5191 calling select() in scalar context just returns $nfound.
5193 Any of the bit masks can also be undef. The timeout, if specified, is
5194 in seconds, which may be fractional. Note: not all implementations are
5195 capable of returning the $timeleft. If not, they always return
5196 $timeleft equal to the supplied $timeout.
5198 You can effect a sleep of 250 milliseconds this way:
5200 select(undef, undef, undef, 0.25);
5202 Note that whether C<select> gets restarted after signals (say, SIGALRM)
5203 is implementation-dependent. See also L<perlport> for notes on the
5204 portability of C<select>.
5206 On error, C<select> behaves like select(2): it returns
5209 On some Unixes, select(2) may report a socket file
5210 descriptor as "ready for reading" when no data is available, and
5211 thus a subsequent read blocks. This can be avoided if you always use
5212 O_NONBLOCK on the socket. See select(2) and fcntl(2) for further
5215 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
5216 or <FH>) with C<select>, except as permitted by POSIX, and even
5217 then only on POSIX systems. You have to use C<sysread> instead.
5219 =item semctl ID,SEMNUM,CMD,ARG
5222 Calls the System V IPC function semctl(2). You'll probably have to say
5226 first to get the correct constant definitions. If CMD is IPC_STAT or
5227 GETALL, then ARG must be a variable that will hold the returned
5228 semid_ds structure or semaphore value array. Returns like C<ioctl>:
5229 the undefined value for error, "C<0 but true>" for zero, or the actual
5230 return value otherwise. The ARG must consist of a vector of native
5231 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
5232 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
5235 =item semget KEY,NSEMS,FLAGS
5238 Calls the System V IPC function semget(2). Returns the semaphore id, or
5239 the undefined value if there is an error. See also
5240 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
5243 =item semop KEY,OPSTRING
5246 Calls the System V IPC function semop(2) for semaphore operations
5247 such as signalling and waiting. OPSTRING must be a packed array of
5248 semop structures. Each semop structure can be generated with
5249 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
5250 implies the number of semaphore operations. Returns true if
5251 successful, or false if there is an error. As an example, the
5252 following code waits on semaphore $semnum of semaphore id $semid:
5254 $semop = pack("s!3", $semnum, -1, 0);
5255 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5257 To signal the semaphore, replace C<-1> with C<1>. See also
5258 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
5261 =item send SOCKET,MSG,FLAGS,TO
5264 =item send SOCKET,MSG,FLAGS
5266 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
5267 filehandle. Takes the same flags as the system call of the same name. On
5268 unconnected sockets, you must specify a destination to I<send to>, in which
5269 case it does a sendto(2) syscall. Returns the number of characters sent,
5270 or the undefined value on error. The sendmsg(2) syscall is currently
5271 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
5273 Note the I<characters>: depending on the status of the socket, either
5274 (8-bit) bytes or characters are sent. By default all sockets operate
5275 on bytes, but for example if the socket has been changed using
5276 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5277 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5278 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5279 pragma: in that case pretty much any characters can be sent.
5281 =item setpgrp PID,PGRP
5284 Sets the current process group for the specified PID, C<0> for the current
5285 process. Raises an exception when used on a machine that doesn't
5286 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5287 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5288 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5291 =item setpriority WHICH,WHO,PRIORITY
5292 X<setpriority> X<priority> X<nice> X<renice>
5294 Sets the current priority for a process, a process group, or a user.
5295 (See setpriority(2).) Raises an exception when used on a machine
5296 that doesn't implement setpriority(2).
5298 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5301 Sets the socket option requested. Returns undefined if there is an
5302 error. Use integer constants provided by the C<Socket> module for
5303 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5304 getprotobyname. OPTVAL might either be a packed string or an integer.
5305 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5307 An example disabling Nagle's algorithm on a socket:
5309 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5310 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5317 Shifts the first value of the array off and returns it, shortening the
5318 array by 1 and moving everything down. If there are no elements in the
5319 array, returns the undefined value. If ARRAY is omitted, shifts the
5320 C<@_> array within the lexical scope of subroutines and formats, and the
5321 C<@ARGV> array outside a subroutine and also within the lexical scopes
5322 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5323 C<UNITCHECK {}> and C<END {}> constructs.
5325 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5326 same thing to the left end of an array that C<pop> and C<push> do to the
5329 =item shmctl ID,CMD,ARG
5332 Calls the System V IPC function shmctl. You'll probably have to say
5336 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5337 then ARG must be a variable that will hold the returned C<shmid_ds>
5338 structure. Returns like ioctl: the undefined value for error, "C<0> but
5339 true" for zero, or the actual return value otherwise.
5340 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5342 =item shmget KEY,SIZE,FLAGS
5345 Calls the System V IPC function shmget. Returns the shared memory
5346 segment id, or the undefined value if there is an error.
5347 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5349 =item shmread ID,VAR,POS,SIZE
5353 =item shmwrite ID,STRING,POS,SIZE
5355 Reads or writes the System V shared memory segment ID starting at
5356 position POS for size SIZE by attaching to it, copying in/out, and
5357 detaching from it. When reading, VAR must be a variable that will
5358 hold the data read. When writing, if STRING is too long, only SIZE
5359 bytes are used; if STRING is too short, nulls are written to fill out
5360 SIZE bytes. Return true if successful, or false if there is an error.
5361 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5362 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5364 =item shutdown SOCKET,HOW
5367 Shuts down a socket connection in the manner indicated by HOW, which
5368 has the same interpretation as in the syscall of the same name.
5370 shutdown(SOCKET, 0); # I/we have stopped reading data
5371 shutdown(SOCKET, 1); # I/we have stopped writing data
5372 shutdown(SOCKET, 2); # I/we have stopped using this socket
5374 This is useful with sockets when you want to tell the other
5375 side you're done writing but not done reading, or vice versa.
5376 It's also a more insistent form of close because it also
5377 disables the file descriptor in any forked copies in other
5380 Returns C<1> for success; on error, returns C<undef> if
5381 the first argument is not a valid filehandle, or returns C<0> and sets
5382 C<$!> for any other failure.
5385 X<sin> X<sine> X<asin> X<arcsine>
5389 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5390 returns sine of C<$_>.
5392 For the inverse sine operation, you may use the C<Math::Trig::asin>
5393 function, or use this relation:
5395 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5402 Causes the script to sleep for (integer) EXPR seconds, or forever if no
5403 argument is given. Returns the integer number of seconds actually slept.
5405 May be interrupted if the process receives a signal such as C<SIGALRM>.
5408 local $SIG{ALARM} = sub { die "Alarm!\n" };
5411 die $@ unless $@ eq "Alarm!\n";
5413 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
5414 is often implemented using C<alarm>.
5416 On some older systems, it may sleep up to a full second less than what
5417 you requested, depending on how it counts seconds. Most modern systems
5418 always sleep the full amount. They may appear to sleep longer than that,
5419 however, because your process might not be scheduled right away in a
5420 busy multitasking system.
5422 For delays of finer granularity than one second, the Time::HiRes module
5423 (from CPAN, and starting from Perl 5.8 part of the standard
5424 distribution) provides usleep(). You may also use Perl's four-argument
5425 version of select() leaving the first three arguments undefined, or you
5426 might be able to use the C<syscall> interface to access setitimer(2) if
5427 your system supports it. See L<perlfaq8> for details.
5429 See also the POSIX module's C<pause> function.
5431 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5434 Opens a socket of the specified kind and attaches it to filehandle
5435 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5436 the syscall of the same name. You should C<use Socket> first
5437 to get the proper definitions imported. See the examples in
5438 L<perlipc/"Sockets: Client/Server Communication">.
5440 On systems that support a close-on-exec flag on files, the flag will
5441 be set for the newly opened file descriptor, as determined by the
5442 value of $^F. See L<perlvar/$^F>.
5444 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5447 Creates an unnamed pair of sockets in the specified domain, of the
5448 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5449 for the syscall of the same name. If unimplemented, raises an exception.
5450 Returns true if successful.
5452 On systems that support a close-on-exec flag on files, the flag will
5453 be set for the newly opened file descriptors, as determined by the value
5454 of $^F. See L<perlvar/$^F>.
5456 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5457 to C<pipe(Rdr, Wtr)> is essentially:
5460 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5461 shutdown(Rdr, 1); # no more writing for reader
5462 shutdown(Wtr, 0); # no more reading for writer
5464 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5465 emulate socketpair using IP sockets to localhost if your system implements
5466 sockets but not socketpair.
5468 =item sort SUBNAME LIST
5469 X<sort> X<qsort> X<quicksort> X<mergesort>
5471 =item sort BLOCK LIST
5475 In list context, this sorts the LIST and returns the sorted list value.
5476 In scalar context, the behaviour of C<sort()> is undefined.
5478 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5479 order. If SUBNAME is specified, it gives the name of a subroutine
5480 that returns an integer less than, equal to, or greater than C<0>,
5481 depending on how the elements of the list are to be ordered. (The
5482 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
5483 SUBNAME may be a scalar variable name (unsubscripted), in which case
5484 the value provides the name of (or a reference to) the actual
5485 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5486 an anonymous, in-line sort subroutine.
5488 If the subroutine's prototype is C<($$)>, the elements to be compared
5489 are passed by reference in C<@_>, as for a normal subroutine. This is
5490 slower than unprototyped subroutines, where the elements to be
5491 compared are passed into the subroutine
5492 as the package global variables $a and $b (see example below). Note that
5493 in the latter case, it is usually counter-productive to declare $a and
5496 The values to be compared are always passed by reference and should not
5499 You also cannot exit out of the sort block or subroutine using any of the
5500 loop control operators described in L<perlsyn> or with C<goto>.
5502 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5503 current collation locale. See L<perllocale>.
5505 sort() returns aliases into the original list, much as a for loop's index
5506 variable aliases the list elements. That is, modifying an element of a
5507 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5508 actually modifies the element in the original list. This is usually
5509 something to be avoided when writing clear code.
5511 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5512 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5513 preserves the input order of elements that compare equal. Although
5514 quicksort's run time is O(NlogN) when averaged over all arrays of
5515 length N, the time can be O(N**2), I<quadratic> behavior, for some
5516 inputs.) In 5.7, the quicksort implementation was replaced with
5517 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5518 But benchmarks indicated that for some inputs, on some platforms,
5519 the original quicksort was faster. 5.8 has a sort pragma for
5520 limited control of the sort. Its rather blunt control of the
5521 underlying algorithm may not persist into future Perls, but the
5522 ability to characterize the input or output in implementation
5523 independent ways quite probably will. See L<the sort pragma|sort>.
5528 @articles = sort @files;
5530 # same thing, but with explicit sort routine
5531 @articles = sort {$a cmp $b} @files;
5533 # now case-insensitively
5534 @articles = sort {uc($a) cmp uc($b)} @files;
5536 # same thing in reversed order
5537 @articles = sort {$b cmp $a} @files;
5539 # sort numerically ascending
5540 @articles = sort {$a <=> $b} @files;
5542 # sort numerically descending
5543 @articles = sort {$b <=> $a} @files;
5545 # this sorts the %age hash by value instead of key
5546 # using an in-line function
5547 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5549 # sort using explicit subroutine name
5551 $age{$a} <=> $age{$b}; # presuming numeric
5553 @sortedclass = sort byage @class;
5555 sub backwards { $b cmp $a }
5556 @harry = qw(dog cat x Cain Abel);
5557 @george = qw(gone chased yz Punished Axed);
5559 # prints AbelCaincatdogx
5560 print sort backwards @harry;
5561 # prints xdogcatCainAbel
5562 print sort @george, 'to', @harry;
5563 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5565 # inefficiently sort by descending numeric compare using
5566 # the first integer after the first = sign, or the
5567 # whole record case-insensitively otherwise
5570 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5575 # same thing, but much more efficiently;
5576 # we'll build auxiliary indices instead
5578 my @nums = @caps = ();
5580 push @nums, ( /=(\d+)/ ? $1 : undef );
5584 my @new = @old[ sort {
5585 $nums[$b] <=> $nums[$a]
5587 $caps[$a] cmp $caps[$b]
5591 # same thing, but without any temps
5592 @new = map { $_->[0] }
5593 sort { $b->[1] <=> $a->[1]
5596 } map { [$_, /=(\d+)/, uc($_)] } @old;
5598 # using a prototype allows you to use any comparison subroutine
5599 # as a sort subroutine (including other package's subroutines)
5601 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5604 @new = sort other::backwards @old;
5606 # guarantee stability, regardless of algorithm
5608 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5610 # force use of mergesort (not portable outside Perl 5.8)
5611 use sort '_mergesort'; # note discouraging _
5612 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5614 Warning: syntactical care is required when sorting the list returned from
5615 a function. If you want to sort the list returned by the function call
5616 C<find_records(@key)>, you can use:
5618 @contact = sort { $a cmp $b } find_records @key;
5619 @contact = sort +find_records(@key);
5620 @contact = sort &find_records(@key);
5621 @contact = sort(find_records(@key));
5623 If instead you want to sort the array @key with the comparison routine
5624 C<find_records()> then you can use:
5626 @contact = sort { find_records() } @key;
5627 @contact = sort find_records(@key);
5628 @contact = sort(find_records @key);
5629 @contact = sort(find_records (@key));
5631 If you're using strict, you I<must not> declare $a
5632 and $b as lexicals. They are package globals. That means
5633 that if you're in the C<main> package and type
5635 @articles = sort {$b <=> $a} @files;
5637 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5638 but if you're in the C<FooPack> package, it's the same as typing
5640 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5642 The comparison function is required to behave. If it returns
5643 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5644 sometimes saying the opposite, for example) the results are not
5647 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5648 (not-a-number), and because C<sort> raises an exception unless the
5649 result of a comparison is defined, when sorting with a comparison function
5650 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5651 The following example takes advantage that C<NaN != NaN> to
5652 eliminate any C<NaN>s from the input list.
5654 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5656 =item splice ARRAY,OFFSET,LENGTH,LIST
5659 =item splice ARRAY,OFFSET,LENGTH
5661 =item splice ARRAY,OFFSET
5665 Removes the elements designated by OFFSET and LENGTH from an array, and
5666 replaces them with the elements of LIST, if any. In list context,
5667 returns the elements removed from the array. In scalar context,
5668 returns the last element removed, or C<undef> if no elements are
5669 removed. The array grows or shrinks as necessary.
5670 If OFFSET is negative then it starts that far from the end of the array.
5671 If LENGTH is omitted, removes everything from OFFSET onward.
5672 If LENGTH is negative, removes the elements from OFFSET onward
5673 except for -LENGTH elements at the end of the array.
5674 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5675 past the end of the array, Perl issues a warning, and splices at the
5678 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5680 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5681 pop(@a) splice(@a,-1)
5682 shift(@a) splice(@a,0,1)
5683 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5684 $a[$i] = $y splice(@a,$i,1,$y)
5686 Example, assuming array lengths are passed before arrays:
5688 sub aeq { # compare two list values
5689 my(@a) = splice(@_,0,shift);
5690 my(@b) = splice(@_,0,shift);
5691 return 0 unless @a == @b; # same len?
5693 return 0 if pop(@a) ne pop(@b);
5697 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5699 =item split /PATTERN/,EXPR,LIMIT
5702 =item split /PATTERN/,EXPR
5704 =item split /PATTERN/
5708 Splits the string EXPR into a list of strings and returns that list. By
5709 default, empty leading fields are preserved, and empty trailing ones are
5710 deleted. (If all fields are empty, they are considered to be trailing.)
5712 In scalar context, returns the number of fields found.
5714 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5715 splits on whitespace (after skipping any leading whitespace). Anything
5716 matching PATTERN is taken to be a delimiter separating the fields. (Note
5717 that the delimiter may be longer than one character.)
5719 If LIMIT is specified and positive, it represents the maximum number
5720 of fields the EXPR will be split into, though the actual number of
5721 fields returned depends on the number of times PATTERN matches within
5722 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5723 stripped (which potential users of C<pop> would do well to remember).
5724 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5725 had been specified. Note that splitting an EXPR that evaluates to the
5726 empty string always returns the empty list, regardless of the LIMIT
5729 A pattern matching the empty string (not to be confused with
5730 an empty pattern C<//>, which is just one member of the set of patterns
5731 matching the epmty string), splits EXPR into individual
5732 characters. For example:
5734 print join(':', split(/ */, 'hi there')), "\n";
5736 produces the output 'h:i:t:h:e:r:e'.
5738 As a special case for C<split>, the empty pattern C<//> specifically
5739 matches the empty string; this is not be confused with the normal use
5740 of an empty pattern to mean the last successful match. So to split
5741 a string into individual characters, the following:
5743 print join(':', split(//, 'hi there')), "\n";
5745 produces the output 'h:i: :t:h:e:r:e'.
5747 Empty leading fields are produced when there are positive-width matches at
5748 the beginning of the string; a zero-width match at the beginning of
5749 the string does not produce an empty field. For example:
5751 print join(':', split(/(?=\w)/, 'hi there!'));
5753 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
5754 hand, are produced when there is a match at the end of the string (and
5755 when LIMIT is given and is not 0), regardless of the length of the match.
5758 print join(':', split(//, 'hi there!', -1)), "\n";
5759 print join(':', split(/\W/, 'hi there!', -1)), "\n";
5761 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
5762 both with an empty trailing field.
5764 The LIMIT parameter can be used to split a line partially
5766 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5768 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5769 a LIMIT one larger than the number of variables in the list, to avoid
5770 unnecessary work. For the list above LIMIT would have been 4 by
5771 default. In time critical applications it behooves you not to split
5772 into more fields than you really need.
5774 If the PATTERN contains parentheses, additional list elements are
5775 created from each matching substring in the delimiter.
5777 split(/([,-])/, "1-10,20", 3);
5779 produces the list value
5781 (1, '-', 10, ',', 20)
5783 If you had the entire header of a normal Unix email message in $header,
5784 you could split it up into fields and their values this way:
5786 $header =~ s/\n(?=\s)//g; # fix continuation lines
5787 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5789 The pattern C</PATTERN/> may be replaced with an expression to specify
5790 patterns that vary at runtime. (To do runtime compilation only once,
5791 use C</$variable/o>.)
5793 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5794 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5795 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5796 will give you as many initial null fields (empty string) as there are leading spaces.
5797 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5798 whitespace produces a null first field. A C<split> with no arguments
5799 really does a S<C<split(' ', $_)>> internally.
5801 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5806 open(PASSWD, '/etc/passwd');
5809 ($login, $passwd, $uid, $gid,
5810 $gcos, $home, $shell) = split(/:/);
5814 As with regular pattern matching, any capturing parentheses that are not
5815 matched in a C<split()> will be set to C<undef> when returned:
5817 @fields = split /(A)|B/, "1A2B3";
5818 # @fields is (1, 'A', 2, undef, 3)
5820 =item sprintf FORMAT, LIST
5823 Returns a string formatted by the usual C<printf> conventions of the C
5824 library function C<sprintf>. See below for more details
5825 and see C<sprintf(3)> or C<printf(3)> on your system for an explanation of
5826 the general principles.
5830 # Format number with up to 8 leading zeroes
5831 $result = sprintf("%08d", $number);
5833 # Round number to 3 digits after decimal point
5834 $rounded = sprintf("%.3f", $number);
5836 Perl does its own C<sprintf> formatting: it emulates the C
5837 function sprintf(3), but doesn't use it except for floating-point
5838 numbers, and even then only standard modifiers are allowed.
5839 Non-standard extensions in your local sprintf(3) are
5840 therefore unavailable from Perl.
5842 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5843 pass it an array as your first argument. The array is given scalar context,
5844 and instead of using the 0th element of the array as the format, Perl will
5845 use the count of elements in the array as the format, which is almost never
5848 Perl's C<sprintf> permits the following universally-known conversions:
5851 %c a character with the given number
5853 %d a signed integer, in decimal
5854 %u an unsigned integer, in decimal
5855 %o an unsigned integer, in octal
5856 %x an unsigned integer, in hexadecimal
5857 %e a floating-point number, in scientific notation
5858 %f a floating-point number, in fixed decimal notation
5859 %g a floating-point number, in %e or %f notation
5861 In addition, Perl permits the following widely-supported conversions:
5863 %X like %x, but using upper-case letters
5864 %E like %e, but using an upper-case "E"
5865 %G like %g, but with an upper-case "E" (if applicable)
5866 %b an unsigned integer, in binary
5867 %B like %b, but using an upper-case "B" with the # flag
5868 %p a pointer (outputs the Perl value's address in hexadecimal)
5869 %n special: *stores* the number of characters output so far
5870 into the next variable in the parameter list
5872 Finally, for backward (and we do mean "backward") compatibility, Perl
5873 permits these unnecessary but widely-supported conversions:
5876 %D a synonym for %ld
5877 %U a synonym for %lu
5878 %O a synonym for %lo
5881 Note that the number of exponent digits in the scientific notation produced
5882 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5883 exponent less than 100 is system-dependent: it may be three or less
5884 (zero-padded as necessary). In other words, 1.23 times ten to the
5885 99th may be either "1.23e99" or "1.23e099".
5887 Between the C<%> and the format letter, you may specify several
5888 additional attributes controlling the interpretation of the format.
5889 In order, these are:
5893 =item format parameter index
5895 An explicit format parameter index, such as C<2$>. By default sprintf
5896 will format the next unused argument in the list, but this allows you
5897 to take the arguments out of order:
5899 printf '%2$d %1$d', 12, 34; # prints "34 12"
5900 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5906 space prefix non-negative number with a space
5907 + prefix non-negative number with a plus sign
5908 - left-justify within the field
5909 0 use zeros, not spaces, to right-justify
5910 # ensure the leading "0" for any octal,
5911 prefix non-zero hexadecimal with "0x" or "0X",
5912 prefix non-zero binary with "0b" or "0B"
5916 printf '<% d>', 12; # prints "< 12>"
5917 printf '<%+d>', 12; # prints "<+12>"
5918 printf '<%6s>', 12; # prints "< 12>"
5919 printf '<%-6s>', 12; # prints "<12 >"
5920 printf '<%06s>', 12; # prints "<000012>"
5921 printf '<%#o>', 12; # prints "<014>"
5922 printf '<%#x>', 12; # prints "<0xc>"
5923 printf '<%#X>', 12; # prints "<0XC>"
5924 printf '<%#b>', 12; # prints "<0b1100>"
5925 printf '<%#B>', 12; # prints "<0B1100>"
5927 When a space and a plus sign are given as the flags at once,
5928 a plus sign is used to prefix a positive number.
5930 printf '<%+ d>', 12; # prints "<+12>"
5931 printf '<% +d>', 12; # prints "<+12>"
5933 When the # flag and a precision are given in the %o conversion,
5934 the precision is incremented if it's necessary for the leading "0".
5936 printf '<%#.5o>', 012; # prints "<00012>"
5937 printf '<%#.5o>', 012345; # prints "<012345>"
5938 printf '<%#.0o>', 0; # prints "<0>"
5942 This flag tells Perl to interpret the supplied string as a vector of
5943 integers, one for each character in the string. Perl applies the format to
5944 each integer in turn, then joins the resulting strings with a separator (a
5945 dot C<.> by default). This can be useful for displaying ordinal values of
5946 characters in arbitrary strings:
5948 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5949 printf "version is v%vd\n", $^V; # Perl's version
5951 Put an asterisk C<*> before the C<v> to override the string to
5952 use to separate the numbers:
5954 printf "address is %*vX\n", ":", $addr; # IPv6 address
5955 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5957 You can also explicitly specify the argument number to use for
5958 the join string using something like C<*2$v>; for example:
5960 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5962 =item (minimum) width
5964 Arguments are usually formatted to be only as wide as required to
5965 display the given value. You can override the width by putting
5966 a number here, or get the width from the next argument (with C<*>)
5967 or from a specified argument (e.g., with C<*2$>):
5969 printf '<%s>', "a"; # prints "<a>"
5970 printf '<%6s>', "a"; # prints "< a>"
5971 printf '<%*s>', 6, "a"; # prints "< a>"
5972 printf '<%*2$s>', "a", 6; # prints "< a>"
5973 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5975 If a field width obtained through C<*> is negative, it has the same
5976 effect as the C<-> flag: left-justification.
5978 =item precision, or maximum width
5981 You can specify a precision (for numeric conversions) or a maximum
5982 width (for string conversions) by specifying a C<.> followed by a number.
5983 For floating-point formats except 'g' and 'G', this specifies
5984 how many places right of the decimal point to show (the default being 6).
5987 # these examples are subject to system-specific variation
5988 printf '<%f>', 1; # prints "<1.000000>"
5989 printf '<%.1f>', 1; # prints "<1.0>"
5990 printf '<%.0f>', 1; # prints "<1>"
5991 printf '<%e>', 10; # prints "<1.000000e+01>"
5992 printf '<%.1e>', 10; # prints "<1.0e+01>"
5994 For "g" and "G", this specifies the maximum number of digits to show,
5995 including thoe prior to the decimal point and those after it; for
5998 # These examples are subject to system-specific variation.
5999 printf '<%g>', 1; # prints "<1>"
6000 printf '<%.10g>', 1; # prints "<1>"
6001 printf '<%g>', 100; # prints "<100>"
6002 printf '<%.1g>', 100; # prints "<1e+02>"
6003 printf '<%.2g>', 100.01; # prints "<1e+02>"
6004 printf '<%.5g>', 100.01; # prints "<100.01>"
6005 printf '<%.4g>', 100.01; # prints "<100>"
6007 For integer conversions, specifying a precision implies that the
6008 output of the number itself should be zero-padded to this width,
6009 where the 0 flag is ignored:
6011 printf '<%.6d>', 1; # prints "<000001>"
6012 printf '<%+.6d>', 1; # prints "<+000001>"
6013 printf '<%-10.6d>', 1; # prints "<000001 >"
6014 printf '<%10.6d>', 1; # prints "< 000001>"
6015 printf '<%010.6d>', 1; # prints "< 000001>"
6016 printf '<%+10.6d>', 1; # prints "< +000001>"
6018 printf '<%.6x>', 1; # prints "<000001>"
6019 printf '<%#.6x>', 1; # prints "<0x000001>"
6020 printf '<%-10.6x>', 1; # prints "<000001 >"
6021 printf '<%10.6x>', 1; # prints "< 000001>"
6022 printf '<%010.6x>', 1; # prints "< 000001>"
6023 printf '<%#10.6x>', 1; # prints "< 0x000001>"
6025 For string conversions, specifying a precision truncates the string
6026 to fit the specified width:
6028 printf '<%.5s>', "truncated"; # prints "<trunc>"
6029 printf '<%10.5s>', "truncated"; # prints "< trunc>"
6031 You can also get the precision from the next argument using C<.*>:
6033 printf '<%.6x>', 1; # prints "<000001>"
6034 printf '<%.*x>', 6, 1; # prints "<000001>"
6036 If a precision obtained through C<*> is negative, it counts
6037 as having no precision at all.
6039 printf '<%.*s>', 7, "string"; # prints "<string>"
6040 printf '<%.*s>', 3, "string"; # prints "<str>"
6041 printf '<%.*s>', 0, "string"; # prints "<>"
6042 printf '<%.*s>', -1, "string"; # prints "<string>"
6044 printf '<%.*d>', 1, 0; # prints "<0>"
6045 printf '<%.*d>', 0, 0; # prints "<>"
6046 printf '<%.*d>', -1, 0; # prints "<0>"
6048 You cannot currently get the precision from a specified number,
6049 but it is intended that this will be possible in the future, for
6050 example using C<.*2$>:
6052 printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
6056 For numeric conversions, you can specify the size to interpret the
6057 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
6058 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
6059 whatever the default integer size is on your platform (usually 32 or 64
6060 bits), but you can override this to use instead one of the standard C types,
6061 as supported by the compiler used to build Perl:
6063 l interpret integer as C type "long" or "unsigned long"
6064 h interpret integer as C type "short" or "unsigned short"
6065 q, L or ll interpret integer as C type "long long", "unsigned long long".
6066 or "quads" (typically 64-bit integers)
6068 The last will raise an exception if Perl does not understand "quads" in your
6069 installation. (This requires either that the platform natively support quads,
6070 or that Perl were specifically compiled to support quads.) You can find out
6071 whether your Perl supports quads via L<Config>:
6074 if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
6075 print "Nice quads!\n";
6078 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
6079 to be the default floating-point size on your platform (double or long double),
6080 but you can force "long double" with C<q>, C<L>, or C<ll> if your
6081 platform supports them. You can find out whether your Perl supports long
6082 doubles via L<Config>:
6085 print "long doubles\n" if $Config{d_longdbl} eq "define";
6087 You can find out whether Perl considers "long double" to be the default
6088 floating-point size to use on your platform via L<Config>:
6091 if ($Config{uselongdouble} eq "define") {
6092 print "long doubles by default\n";
6095 It can also be that long doubles and doubles are the same thing:
6098 ($Config{doublesize} == $Config{longdblsize}) &&
6099 print "doubles are long doubles\n";
6101 The size specifier C<V> has no effect for Perl code, but is supported for
6102 compatibility with XS code. It means "use the standard size for a Perl
6103 integer or floating-point number", which is the default.
6105 =item order of arguments
6107 Normally, sprintf() takes the next unused argument as the value to
6108 format for each format specification. If the format specification
6109 uses C<*> to require additional arguments, these are consumed from
6110 the argument list in the order they appear in the format
6111 specification I<before> the value to format. Where an argument is
6112 specified by an explicit index, this does not affect the normal
6113 order for the arguments, even when the explicitly specified index
6114 would have been the next argument.
6118 printf "<%*.*s>", $a, $b, $c;
6120 uses C<$a> for the width, C<$b> for the precision, and C<$c>
6121 as the value to format; while:
6123 printf "<%*1$.*s>", $a, $b;
6125 would use C<$a> for the width and precision, and C<$b> as the
6128 Here are some more examples; be aware that when using an explicit
6129 index, the C<$> may need escaping:
6131 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
6132 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
6133 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
6134 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
6138 If C<use locale> is in effect and POSIX::setlocale() has been called,
6139 the character used for the decimal separator in formatted floating-point
6140 numbers is affected by the LC_NUMERIC locale. See L<perllocale>
6144 X<sqrt> X<root> X<square root>
6148 Return the positive square root of EXPR. If EXPR is omitted, uses
6149 C<$_>. Works only for non-negative operands unless you've
6150 loaded the C<Math::Complex> module.
6153 print sqrt(-4); # prints 2i
6156 X<srand> X<seed> X<randseed>
6160 Sets the random number seed for the C<rand> operator.
6162 The point of the function is to "seed" the C<rand> function so that
6163 C<rand> can produce a different sequence each time you run your
6166 If srand() is not called explicitly, it is called implicitly at the
6167 first use of the C<rand> operator. However, this was not true of
6168 versions of Perl before 5.004, so if your script will run under older
6169 Perl versions, it should call C<srand>.
6171 Most programs won't even call srand() at all, except those that
6172 need a cryptographically-strong starting point rather than the
6173 generally acceptable default, which is based on time of day,
6174 process ID, and memory allocation, or the F</dev/urandom> device
6177 You can call srand($seed) with the same $seed to reproduce the
6178 I<same> sequence from rand(), but this is usually reserved for
6179 generating predictable results for testing or debugging.
6180 Otherwise, don't call srand() more than once in your program.
6182 Do B<not> call srand() (i.e., without an argument) more than once in
6183 a script. The internal state of the random number generator should
6184 contain more entropy than can be provided by any seed, so calling
6185 srand() again actually I<loses> randomness.
6187 Most implementations of C<srand> take an integer and will silently
6188 truncate decimal numbers. This means C<srand(42)> will usually
6189 produce the same results as C<srand(42.1)>. To be safe, always pass
6190 C<srand> an integer.
6192 In versions of Perl prior to 5.004 the default seed was just the
6193 current C<time>. This isn't a particularly good seed, so many old
6194 programs supply their own seed value (often C<time ^ $$> or C<time ^
6195 ($$ + ($$ << 15))>), but that isn't necessary any more.
6197 For cryptographic purposes, however, you need something much more random
6198 than the default seed. Checksumming the compressed output of one or more
6199 rapidly changing operating system status programs is the usual method. For
6202 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
6204 If you're particularly concerned with this, search the CPAN for
6205 random number generator modules instead of rolling out your own.
6207 Frequently called programs (like CGI scripts) that simply use
6211 for a seed can fall prey to the mathematical property that
6215 one-third of the time. So don't do that.
6217 =item stat FILEHANDLE
6218 X<stat> X<file, status> X<ctime>
6222 =item stat DIRHANDLE
6226 Returns a 13-element list giving the status info for a file, either
6227 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
6228 omitted, it stats C<$_>. Returns the empty list if C<stat> fails. Typically
6231 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
6232 $atime,$mtime,$ctime,$blksize,$blocks)
6235 Not all fields are supported on all filesystem types. Here are the
6236 meanings of the fields:
6238 0 dev device number of filesystem
6240 2 mode file mode (type and permissions)
6241 3 nlink number of (hard) links to the file
6242 4 uid numeric user ID of file's owner
6243 5 gid numeric group ID of file's owner
6244 6 rdev the device identifier (special files only)
6245 7 size total size of file, in bytes
6246 8 atime last access time in seconds since the epoch
6247 9 mtime last modify time in seconds since the epoch
6248 10 ctime inode change time in seconds since the epoch (*)
6249 11 blksize preferred block size for file system I/O
6250 12 blocks actual number of blocks allocated
6252 (The epoch was at 00:00 January 1, 1970 GMT.)
6254 (*) Not all fields are supported on all filesystem types. Notably, the
6255 ctime field is non-portable. In particular, you cannot expect it to be a
6256 "creation time", see L<perlport/"Files and Filesystems"> for details.
6258 If C<stat> is passed the special filehandle consisting of an underline, no
6259 stat is done, but the current contents of the stat structure from the
6260 last C<stat>, C<lstat>, or filetest are returned. Example:
6262 if (-x $file && (($d) = stat(_)) && $d < 0) {
6263 print "$file is executable NFS file\n";
6266 (This works on machines only for which the device number is negative
6269 Because the mode contains both the file type and its permissions, you
6270 should mask off the file type portion and (s)printf using a C<"%o">
6271 if you want to see the real permissions.
6273 $mode = (stat($filename))[2];
6274 printf "Permissions are %04o\n", $mode & 07777;
6276 In scalar context, C<stat> returns a boolean value indicating success
6277 or failure, and, if successful, sets the information associated with
6278 the special filehandle C<_>.
6280 The L<File::stat> module provides a convenient, by-name access mechanism:
6283 $sb = stat($filename);
6284 printf "File is %s, size is %s, perm %04o, mtime %s\n",
6285 $filename, $sb->size, $sb->mode & 07777,
6286 scalar localtime $sb->mtime;
6288 You can import symbolic mode constants (C<S_IF*>) and functions
6289 (C<S_IS*>) from the Fcntl module:
6293 $mode = (stat($filename))[2];
6295 $user_rwx = ($mode & S_IRWXU) >> 6;
6296 $group_read = ($mode & S_IRGRP) >> 3;
6297 $other_execute = $mode & S_IXOTH;
6299 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6301 $is_setuid = $mode & S_ISUID;
6302 $is_directory = S_ISDIR($mode);
6304 You could write the last two using the C<-u> and C<-d> operators.
6305 Commonly available C<S_IF*> constants are:
6307 # Permissions: read, write, execute, for user, group, others.
6309 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6310 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6311 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6313 # Setuid/Setgid/Stickiness/SaveText.
6314 # Note that the exact meaning of these is system dependent.
6316 S_ISUID S_ISGID S_ISVTX S_ISTXT
6318 # File types. Not necessarily all are available on your system.
6320 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6322 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
6324 S_IREAD S_IWRITE S_IEXEC
6326 and the C<S_IF*> functions are
6328 S_IMODE($mode) the part of $mode containing the permission bits
6329 and the setuid/setgid/sticky bits
6331 S_IFMT($mode) the part of $mode containing the file type
6332 which can be bit-anded with (for example) S_IFREG
6333 or with the following functions
6335 # The operators -f, -d, -l, -b, -c, -p, and -S.
6337 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6338 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6340 # No direct -X operator counterpart, but for the first one
6341 # the -g operator is often equivalent. The ENFMT stands for
6342 # record flocking enforcement, a platform-dependent feature.
6344 S_ISENFMT($mode) S_ISWHT($mode)
6346 See your native chmod(2) and stat(2) documentation for more details
6347 about the C<S_*> constants. To get status info for a symbolic link
6348 instead of the target file behind the link, use the C<lstat> function.
6353 =item state TYPE EXPR
6355 =item state EXPR : ATTRS
6357 =item state TYPE EXPR : ATTRS
6359 C<state> declares a lexically scoped variable, just like C<my> does.
6360 However, those variables will never be reinitialized, contrary to
6361 lexical variables that are reinitialized each time their enclosing block
6364 C<state> variables are enabled only when the C<use feature "state"> pragma
6365 is in effect. See L<feature>.
6372 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6373 doing many pattern matches on the string before it is next modified.
6374 This may or may not save time, depending on the nature and number of
6375 patterns you are searching on, and on the distribution of character
6376 frequencies in the string to be searched; you probably want to compare
6377 run times with and without it to see which runs faster. Those loops
6378 that scan for many short constant strings (including the constant
6379 parts of more complex patterns) will benefit most. You may have only
6380 one C<study> active at a time: if you study a different scalar the first
6381 is "unstudied". (The way C<study> works is this: a linked list of every
6382 character in the string to be searched is made, so we know, for
6383 example, where all the C<'k'> characters are. From each search string,
6384 the rarest character is selected, based on some static frequency tables
6385 constructed from some C programs and English text. Only those places
6386 that contain this "rarest" character are examined.)
6388 For example, here is a loop that inserts index producing entries
6389 before any line containing a certain pattern:
6393 print ".IX foo\n" if /\bfoo\b/;
6394 print ".IX bar\n" if /\bbar\b/;
6395 print ".IX blurfl\n" if /\bblurfl\b/;
6400 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
6401 will be looked at, because C<f> is rarer than C<o>. In general, this is
6402 a big win except in pathological cases. The only question is whether
6403 it saves you more time than it took to build the linked list in the
6406 Note that if you have to look for strings that you don't know till
6407 runtime, you can build an entire loop as a string and C<eval> that to
6408 avoid recompiling all your patterns all the time. Together with
6409 undefining C<$/> to input entire files as one record, this can be quite
6410 fast, often faster than specialized programs like fgrep(1). The following
6411 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6412 out the names of those files that contain a match:
6414 $search = 'while (<>) { study;';
6415 foreach $word (@words) {
6416 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6421 eval $search; # this screams
6422 $/ = "\n"; # put back to normal input delimiter
6423 foreach $file (sort keys(%seen)) {
6427 =item sub NAME BLOCK
6430 =item sub NAME (PROTO) BLOCK
6432 =item sub NAME : ATTRS BLOCK
6434 =item sub NAME (PROTO) : ATTRS BLOCK
6436 This is subroutine definition, not a real function I<per se>.
6437 Without a BLOCK it's just a forward declaration. Without a NAME,
6438 it's an anonymous function declaration, and does actually return
6439 a value: the CODE ref of the closure you just created.
6441 See L<perlsub> and L<perlref> for details about subroutines and
6442 references, and L<attributes> and L<Attribute::Handlers> for more
6443 information about attributes.
6445 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6446 X<substr> X<substring> X<mid> X<left> X<right>
6448 =item substr EXPR,OFFSET,LENGTH
6450 =item substr EXPR,OFFSET
6452 Extracts a substring out of EXPR and returns it. First character is at
6453 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6454 If OFFSET is negative (or more precisely, less than C<$[>), starts
6455 that far from the end of the string. If LENGTH is omitted, returns
6456 everything to the end of the string. If LENGTH is negative, leaves that
6457 many characters off the end of the string.
6459 my $s = "The black cat climbed the green tree";
6460 my $color = substr $s, 4, 5; # black
6461 my $middle = substr $s, 4, -11; # black cat climbed the
6462 my $end = substr $s, 14; # climbed the green tree
6463 my $tail = substr $s, -4; # tree
6464 my $z = substr $s, -4, 2; # tr
6466 You can use the substr() function as an lvalue, in which case EXPR
6467 must itself be an lvalue. If you assign something shorter than LENGTH,
6468 the string will shrink, and if you assign something longer than LENGTH,
6469 the string will grow to accommodate it. To keep the string the same
6470 length, you may need to pad or chop your value using C<sprintf>.
6472 If OFFSET and LENGTH specify a substring that is partly outside the
6473 string, only the part within the string is returned. If the substring
6474 is beyond either end of the string, substr() returns the undefined
6475 value and produces a warning. When used as an lvalue, specifying a
6476 substring that is entirely outside the string raises an exception.
6477 Here's an example showing the behavior for boundary cases:
6480 substr($name, 4) = 'dy'; # $name is now 'freddy'
6481 my $null = substr $name, 6, 2; # returns "" (no warning)
6482 my $oops = substr $name, 7; # returns undef, with warning
6483 substr($name, 7) = 'gap'; # raises an exception
6485 An alternative to using substr() as an lvalue is to specify the
6486 replacement string as the 4th argument. This allows you to replace
6487 parts of the EXPR and return what was there before in one operation,
6488 just as you can with splice().
6490 my $s = "The black cat climbed the green tree";
6491 my $z = substr $s, 14, 7, "jumped from"; # climbed
6492 # $s is now "The black cat jumped from the green tree"
6494 Note that the lvalue returned by the 3-arg version of substr() acts as
6495 a 'magic bullet'; each time it is assigned to, it remembers which part
6496 of the original string is being modified; for example:
6499 for (substr($x,1,2)) {
6500 $_ = 'a'; print $x,"\n"; # prints 1a4
6501 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6503 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6506 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6509 =item symlink OLDFILE,NEWFILE
6510 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6512 Creates a new filename symbolically linked to the old filename.
6513 Returns C<1> for success, C<0> otherwise. On systems that don't support
6514 symbolic links, raises an exception. To check for that,
6517 $symlink_exists = eval { symlink("",""); 1 };
6519 =item syscall NUMBER, LIST
6520 X<syscall> X<system call>
6522 Calls the system call specified as the first element of the list,
6523 passing the remaining elements as arguments to the system call. If
6524 unimplemented, raises an exception. The arguments are interpreted
6525 as follows: if a given argument is numeric, the argument is passed as
6526 an int. If not, the pointer to the string value is passed. You are
6527 responsible to make sure a string is pre-extended long enough to
6528 receive any result that might be written into a string. You can't use a
6529 string literal (or other read-only string) as an argument to C<syscall>
6530 because Perl has to assume that any string pointer might be written
6532 integer arguments are not literals and have never been interpreted in a
6533 numeric context, you may need to add C<0> to them to force them to look
6534 like numbers. This emulates the C<syswrite> function (or vice versa):
6536 require 'syscall.ph'; # may need to run h2ph
6538 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6540 Note that Perl supports passing of up to only 14 arguments to your syscall,
6541 which in practice should (usually) suffice.
6543 Syscall returns whatever value returned by the system call it calls.
6544 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6545 Note that some system calls can legitimately return C<-1>. The proper
6546 way to handle such calls is to assign C<$!=0;> before the call and
6547 check the value of C<$!> if syscall returns C<-1>.
6549 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6550 number of the read end of the pipe it creates. There is no way
6551 to retrieve the file number of the other end. You can avoid this
6552 problem by using C<pipe> instead.
6554 =item sysopen FILEHANDLE,FILENAME,MODE
6557 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6559 Opens the file whose filename is given by FILENAME, and associates it
6560 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6561 the name of the real filehandle wanted. This function calls the
6562 underlying operating system's C<open> function with the parameters
6563 FILENAME, MODE, PERMS.
6565 The possible values and flag bits of the MODE parameter are
6566 system-dependent; they are available via the standard module C<Fcntl>.
6567 See the documentation of your operating system's C<open> to see which
6568 values and flag bits are available. You may combine several flags
6569 using the C<|>-operator.
6571 Some of the most common values are C<O_RDONLY> for opening the file in
6572 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6573 and C<O_RDWR> for opening the file in read-write mode.
6574 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6576 For historical reasons, some values work on almost every system
6577 supported by Perl: 0 means read-only, 1 means write-only, and 2
6578 means read/write. We know that these values do I<not> work under
6579 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6580 use them in new code.
6582 If the file named by FILENAME does not exist and the C<open> call creates
6583 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6584 PERMS specifies the permissions of the newly created file. If you omit
6585 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6586 These permission values need to be in octal, and are modified by your
6587 process's current C<umask>.
6590 In many systems the C<O_EXCL> flag is available for opening files in
6591 exclusive mode. This is B<not> locking: exclusiveness means here that
6592 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6593 on network filesystems, and has no effect unless the C<O_CREAT> flag
6594 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6595 being opened if it is a symbolic link. It does not protect against
6596 symbolic links in the file's path.
6599 Sometimes you may want to truncate an already-existing file. This
6600 can be done using the C<O_TRUNC> flag. The behavior of
6601 C<O_TRUNC> with C<O_RDONLY> is undefined.
6604 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6605 that takes away the user's option to have a more permissive umask.
6606 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6609 Note that C<sysopen> depends on the fdopen() C library function.
6610 On many Unix systems, fdopen() is known to fail when file descriptors
6611 exceed a certain value, typically 255. If you need more file
6612 descriptors than that, consider rebuilding Perl to use the C<sfio>
6613 library, or perhaps using the POSIX::open() function.
6615 See L<perlopentut> for a kinder, gentler explanation of opening files.
6617 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6620 =item sysread FILEHANDLE,SCALAR,LENGTH
6622 Attempts to read LENGTH bytes of data into variable SCALAR from the
6623 specified FILEHANDLE, using the read(2). It bypasses
6624 buffered IO, so mixing this with other kinds of reads, C<print>,
6625 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6626 perlio or stdio layers usually buffers data. Returns the number of
6627 bytes actually read, C<0> at end of file, or undef if there was an
6628 error (in the latter case C<$!> is also set). SCALAR will be grown or
6629 shrunk so that the last byte actually read is the last byte of the
6630 scalar after the read.
6632 An OFFSET may be specified to place the read data at some place in the
6633 string other than the beginning. A negative OFFSET specifies
6634 placement at that many characters counting backwards from the end of
6635 the string. A positive OFFSET greater than the length of SCALAR
6636 results in the string being padded to the required size with C<"\0">
6637 bytes before the result of the read is appended.
6639 There is no syseof() function, which is ok, since eof() doesn't work
6640 well on device files (like ttys) anyway. Use sysread() and check
6641 for a return value for 0 to decide whether you're done.
6643 Note that if the filehandle has been marked as C<:utf8> Unicode
6644 characters are read instead of bytes (the LENGTH, OFFSET, and the
6645 return value of sysread() are in Unicode characters).
6646 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6647 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6649 =item sysseek FILEHANDLE,POSITION,WHENCE
6652 Sets FILEHANDLE's system position in bytes using
6653 lseek(2). FILEHANDLE may be an expression whose value gives the name
6654 of the filehandle. The values for WHENCE are C<0> to set the new
6655 position to POSITION, C<1> to set the it to the current position plus
6656 POSITION, and C<2> to set it to EOF plus POSITION (typically
6659 Note the I<in bytes>: even if the filehandle has been set to operate
6660 on characters (for example by using the C<:encoding(utf8)> I/O layer),
6661 tell() will return byte offsets, not character offsets (because
6662 implementing that would render sysseek() unacceptably slow).
6664 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6665 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6666 C<seek>, C<tell>, or C<eof> may cause confusion.
6668 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6669 and C<SEEK_END> (start of the file, current position, end of the file)
6670 from the Fcntl module. Use of the constants is also more portable
6671 than relying on 0, 1, and 2. For example to define a "systell" function:
6673 use Fcntl 'SEEK_CUR';
6674 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6676 Returns the new position, or the undefined value on failure. A position
6677 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6678 true on success and false on failure, yet you can still easily determine
6684 =item system PROGRAM LIST
6686 Does exactly the same thing as C<exec LIST>, except that a fork is
6687 done first, and the parent process waits for the child process to
6688 exit. Note that argument processing varies depending on the
6689 number of arguments. If there is more than one argument in LIST,
6690 or if LIST is an array with more than one value, starts the program
6691 given by the first element of the list with arguments given by the
6692 rest of the list. If there is only one scalar argument, the argument
6693 is checked for shell metacharacters, and if there are any, the
6694 entire argument is passed to the system's command shell for parsing
6695 (this is C</bin/sh -c> on Unix platforms, but varies on other
6696 platforms). If there are no shell metacharacters in the argument,
6697 it is split into words and passed directly to C<execvp>, which is
6700 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6701 output before any operation that may do a fork, but this may not be
6702 supported on some platforms (see L<perlport>). To be safe, you may need
6703 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6704 of C<IO::Handle> on any open handles.
6706 The return value is the exit status of the program as returned by the
6707 C<wait> call. To get the actual exit value, shift right by eight (see
6708 below). See also L</exec>. This is I<not> what you want to use to capture
6709 the output from a command, for that you should use merely backticks or
6710 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6711 indicates a failure to start the program or an error of the wait(2) system
6712 call (inspect $! for the reason).
6714 If you'd like to make C<system> (and many other bits of Perl) die on error,
6715 have a look at the L<autodie> pragma.
6717 Like C<exec>, C<system> allows you to lie to a program about its name if
6718 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6720 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6721 C<system>, if you expect your program to terminate on receipt of these
6722 signals you will need to arrange to do so yourself based on the return
6725 @args = ("command", "arg1", "arg2");
6727 or die "system @args failed: $?"
6729 If you'd like to manually inspect C<system>'s failure, you can check all
6730 possible failure modes by inspecting C<$?> like this:
6733 print "failed to execute: $!\n";
6736 printf "child died with signal %d, %s coredump\n",
6737 ($? & 127), ($? & 128) ? 'with' : 'without';
6740 printf "child exited with value %d\n", $? >> 8;
6743 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
6744 with the C<W*()> calls from the POSIX module.
6746 When C<system>'s arguments are executed indirectly by the shell,
6747 results and return codes are subject to its quirks.
6748 See L<perlop/"`STRING`"> and L</exec> for details.
6750 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6753 =item syswrite FILEHANDLE,SCALAR,LENGTH
6755 =item syswrite FILEHANDLE,SCALAR
6757 Attempts to write LENGTH bytes of data from variable SCALAR to the
6758 specified FILEHANDLE, using write(2). If LENGTH is
6759 not specified, writes whole SCALAR. It bypasses buffered IO, so
6760 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6761 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6762 stdio layers usually buffers data. Returns the number of bytes
6763 actually written, or C<undef> if there was an error (in this case the
6764 errno variable C<$!> is also set). If the LENGTH is greater than the
6765 data available in the SCALAR after the OFFSET, only as much data as is
6766 available will be written.
6768 An OFFSET may be specified to write the data from some part of the
6769 string other than the beginning. A negative OFFSET specifies writing
6770 that many characters counting backwards from the end of the string.
6771 If SCALAR is of length zero, you can only use an OFFSET of 0.
6773 B<Warning>: If the filehandle is marked C<:utf8>, Unicode characters
6774 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
6775 return value of syswrite() are in (UTF-8 encoded Unicode) characters.
6776 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6777 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6779 =item tell FILEHANDLE
6784 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6785 error. FILEHANDLE may be an expression whose value gives the name of
6786 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6789 Note the I<in bytes>: even if the filehandle has been set to
6790 operate on characters (for example by using the C<:encoding(utf8)> open
6791 layer), tell() will return byte offsets, not character offsets (because
6792 that would render seek() and tell() rather slow).
6794 The return value of tell() for the standard streams like the STDIN
6795 depends on the operating system: it may return -1 or something else.
6796 tell() on pipes, fifos, and sockets usually returns -1.
6798 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6800 Do not use tell() (or other buffered I/O operations) on a filehandle
6801 that has been manipulated by sysread(), syswrite() or sysseek().
6802 Those functions ignore the buffering, while tell() does not.
6804 =item telldir DIRHANDLE
6807 Returns the current position of the C<readdir> routines on DIRHANDLE.
6808 Value may be given to C<seekdir> to access a particular location in a
6809 directory. C<telldir> has the same caveats about possible directory
6810 compaction as the corresponding system library routine.
6812 =item tie VARIABLE,CLASSNAME,LIST
6815 This function binds a variable to a package class that will provide the
6816 implementation for the variable. VARIABLE is the name of the variable
6817 to be enchanted. CLASSNAME is the name of a class implementing objects
6818 of correct type. Any additional arguments are passed to the C<new>
6819 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6820 or C<TIEHASH>). Typically these are arguments such as might be passed
6821 to the C<dbm_open()> function of C. The object returned by the C<new>
6822 method is also returned by the C<tie> function, which would be useful
6823 if you want to access other methods in CLASSNAME.
6825 Note that functions such as C<keys> and C<values> may return huge lists
6826 when used on large objects, like DBM files. You may prefer to use the
6827 C<each> function to iterate over such. Example:
6829 # print out history file offsets
6831 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6832 while (($key,$val) = each %HIST) {
6833 print $key, ' = ', unpack('L',$val), "\n";
6837 A class implementing a hash should have the following methods:
6839 TIEHASH classname, LIST
6841 STORE this, key, value
6846 NEXTKEY this, lastkey
6851 A class implementing an ordinary array should have the following methods:
6853 TIEARRAY classname, LIST
6855 STORE this, key, value
6857 STORESIZE this, count
6863 SPLICE this, offset, length, LIST
6868 A class implementing a filehandle should have the following methods:
6870 TIEHANDLE classname, LIST
6871 READ this, scalar, length, offset
6874 WRITE this, scalar, length, offset
6876 PRINTF this, format, LIST
6880 SEEK this, position, whence
6882 OPEN this, mode, LIST
6887 A class implementing a scalar should have the following methods:
6889 TIESCALAR classname, LIST
6895 Not all methods indicated above need be implemented. See L<perltie>,
6896 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6898 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
6899 for you; you need to do that explicitly yourself. See L<DB_File>
6900 or the F<Config> module for interesting C<tie> implementations.
6902 For further details see L<perltie>, L<"tied VARIABLE">.
6907 Returns a reference to the object underlying VARIABLE (the same value
6908 that was originally returned by the C<tie> call that bound the variable
6909 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6915 Returns the number of non-leap seconds since whatever time the system
6916 considers to be the epoch, suitable for feeding to C<gmtime> and
6917 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6918 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6919 1904 in the current local time zone for its epoch.
6921 For measuring time in better granularity than one second,
6922 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6923 Perl 5.8 part of the standard distribution), or if you have
6924 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6925 See L<perlfaq8> for details.
6927 For date and time processing look at the many related modules on CPAN.
6928 For a comprehensive date and time representation look at the
6934 Returns a four-element list giving the user and system times, in
6935 seconds, for this process and the children of this process.
6937 ($user,$system,$cuser,$csystem) = times;
6939 In scalar context, C<times> returns C<$user>.
6941 Children's times are only included for terminated children.
6945 The transliteration operator. Same as C<y///>. See
6946 L<perlop/"Quote and Quote-like Operators">.
6948 =item truncate FILEHANDLE,LENGTH
6951 =item truncate EXPR,LENGTH
6953 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6954 specified length. Raises an exception if truncate isn't implemented
6955 on your system. Returns true if successful, the undefined value
6958 The behavior is undefined if LENGTH is greater than the length of the
6961 The position in the file of FILEHANDLE is left unchanged. You may want to
6962 call L<seek> before writing to the file.
6965 X<uc> X<uppercase> X<toupper>
6969 Returns an uppercased version of EXPR. This is the internal function
6970 implementing the C<\U> escape in double-quoted strings.
6971 It does not attempt to do titlecase mapping on initial letters. See
6972 L</ucfirst> for that.
6974 If EXPR is omitted, uses C<$_>.
6976 This function behaves the same way under various pragma, such as in a locale,
6980 X<ucfirst> X<uppercase>
6984 Returns the value of EXPR with the first character in uppercase
6985 (titlecase in Unicode). This is the internal function implementing
6986 the C<\u> escape in double-quoted strings.
6988 If EXPR is omitted, uses C<$_>.
6990 This function behaves the same way under various pragma, such as in a locale,
6998 Sets the umask for the process to EXPR and returns the previous value.
6999 If EXPR is omitted, merely returns the current umask.
7001 The Unix permission C<rwxr-x---> is represented as three sets of three
7002 bits, or three octal digits: C<0750> (the leading 0 indicates octal
7003 and isn't one of the digits). The C<umask> value is such a number
7004 representing disabled permissions bits. The permission (or "mode")
7005 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
7006 even if you tell C<sysopen> to create a file with permissions C<0777>,
7007 if your umask is C<0022> then the file will actually be created with
7008 permissions C<0755>. If your C<umask> were C<0027> (group can't
7009 write; others can't read, write, or execute), then passing
7010 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
7013 Here's some advice: supply a creation mode of C<0666> for regular
7014 files (in C<sysopen>) and one of C<0777> for directories (in
7015 C<mkdir>) and executable files. This gives users the freedom of
7016 choice: if they want protected files, they might choose process umasks
7017 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
7018 Programs should rarely if ever make policy decisions better left to
7019 the user. The exception to this is when writing files that should be
7020 kept private: mail files, web browser cookies, I<.rhosts> files, and
7023 If umask(2) is not implemented on your system and you are trying to
7024 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
7025 raises an exception. If umask(2) is not implemented and you are
7026 not trying to restrict access for yourself, returns C<undef>.
7028 Remember that a umask is a number, usually given in octal; it is I<not> a
7029 string of octal digits. See also L</oct>, if all you have is a string.
7032 X<undef> X<undefine>
7036 Undefines the value of EXPR, which must be an lvalue. Use only on a
7037 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
7038 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
7039 will probably not do what you expect on most predefined variables or
7040 DBM list values, so don't do that; see L<delete>. Always returns the
7041 undefined value. You can omit the EXPR, in which case nothing is
7042 undefined, but you still get an undefined value that you could, for
7043 instance, return from a subroutine, assign to a variable, or pass as a
7044 parameter. Examples:
7047 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
7051 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
7052 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
7053 select undef, undef, undef, 0.25;
7054 ($a, $b, undef, $c) = &foo; # Ignore third value returned
7056 Note that this is a unary operator, not a list operator.
7059 X<unlink> X<delete> X<remove> X<rm> X<del>
7063 Deletes a list of files. On success, it returns the number of files
7064 it successfully deleted. On failure, it returns false and sets C<$!>
7067 my $unlinked = unlink 'a', 'b', 'c';
7069 unlink glob "*.bak";
7071 On error, C<unlink> will not tell you which files it could not remove.
7072 If you want to know which files you could not remove, try them one
7075 foreach my $file ( @goners ) {
7076 unlink $file or warn "Could not unlink $file: $!";
7079 Note: C<unlink> will not attempt to delete directories unless you are
7080 superuser and the B<-U> flag is supplied to Perl. Even if these
7081 conditions are met, be warned that unlinking a directory can inflict
7082 damage on your filesystem. Finally, using C<unlink> on directories is
7083 not supported on many operating systems. Use C<rmdir> instead.
7085 If LIST is omitted, C<unlink> uses C<$_>.
7087 =item unpack TEMPLATE,EXPR
7090 =item unpack TEMPLATE
7092 C<unpack> does the reverse of C<pack>: it takes a string
7093 and expands it out into a list of values.
7094 (In scalar context, it returns merely the first value produced.)
7096 If EXPR is omitted, unpacks the C<$_> string.
7097 See L<perlpacktut> for an introduction to this function.
7099 The string is broken into chunks described by the TEMPLATE. Each chunk
7100 is converted separately to a value. Typically, either the string is a result
7101 of C<pack>, or the characters of the string represent a C structure of some
7104 The TEMPLATE has the same format as in the C<pack> function.
7105 Here's a subroutine that does substring:
7108 my($what,$where,$howmuch) = @_;
7109 unpack("x$where a$howmuch", $what);
7114 sub ordinal { unpack("W",$_[0]); } # same as ord()
7116 In addition to fields allowed in pack(), you may prefix a field with
7117 a %<number> to indicate that
7118 you want a <number>-bit checksum of the items instead of the items
7119 themselves. Default is a 16-bit checksum. Checksum is calculated by
7120 summing numeric values of expanded values (for string fields the sum of
7121 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
7123 For example, the following
7124 computes the same number as the System V sum program:
7128 unpack("%32W*",<>) % 65535;
7131 The following efficiently counts the number of set bits in a bit vector:
7133 $setbits = unpack("%32b*", $selectmask);
7135 The C<p> and C<P> formats should be used with care. Since Perl
7136 has no way of checking whether the value passed to C<unpack()>
7137 corresponds to a valid memory location, passing a pointer value that's
7138 not known to be valid is likely to have disastrous consequences.
7140 If there are more pack codes or if the repeat count of a field or a group
7141 is larger than what the remainder of the input string allows, the result
7142 is not well defined: the repeat count may be decreased, or
7143 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
7144 If the input string is longer than one described by the TEMPLATE,
7145 the remainder of that input string is ignored.
7147 See L</pack> for more examples and notes.
7149 =item untie VARIABLE
7152 Breaks the binding between a variable and a package. (See C<tie>.)
7153 Has no effect if the variable is not tied.
7155 =item unshift ARRAY,LIST
7158 Does the opposite of a C<shift>. Or the opposite of a C<push>,
7159 depending on how you look at it. Prepends list to the front of the
7160 array, and returns the new number of elements in the array.
7162 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
7164 Note the LIST is prepended whole, not one element at a time, so the
7165 prepended elements stay in the same order. Use C<reverse> to do the
7168 =item use Module VERSION LIST
7169 X<use> X<module> X<import>
7171 =item use Module VERSION
7173 =item use Module LIST
7179 Imports some semantics into the current package from the named module,
7180 generally by aliasing certain subroutine or variable names into your
7181 package. It is exactly equivalent to
7183 BEGIN { require Module; Module->import( LIST ); }
7185 except that Module I<must> be a bareword.
7187 In the peculiar C<use VERSION> form, VERSION may be either a positive
7188 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
7189 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
7190 exception is raised if VERSION is greater than the version of the
7191 current Perl interpreter; Perl will not attempt to parse the rest of the
7192 file. Compare with L</require>, which can do a similar check at run time.
7193 Symmetrically, C<no VERSION> allows you to specify that you want a version
7194 of Perl older than the specified one.
7196 Specifying VERSION as a literal of the form v5.6.1 should generally be
7197 avoided, because it leads to misleading error messages under earlier
7198 versions of Perl (that is, prior to 5.6.0) that do not support this
7199 syntax. The equivalent numeric version should be used instead.
7201 use v5.6.1; # compile time version check
7203 use 5.006_001; # ditto; preferred for backwards compatibility
7205 This is often useful if you need to check the current Perl version before
7206 C<use>ing library modules that won't work with older versions of Perl.
7207 (We try not to do this more than we have to.)
7209 Also, if the specified Perl version is greater than or equal to 5.9.5,
7210 C<use VERSION> will also load the C<feature> pragma and enable all
7211 features available in the requested version. See L<feature>.
7212 Similarly, if the specified Perl version is greater than or equal to
7213 5.11.0, strictures are enabled lexically as with C<use strict> (except
7214 that the F<strict.pm> file is not actually loaded).
7216 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
7217 C<require> makes sure the module is loaded into memory if it hasn't been
7218 yet. The C<import> is not a builtin; it's just an ordinary static method
7219 call into the C<Module> package to tell the module to import the list of
7220 features back into the current package. The module can implement its
7221 C<import> method any way it likes, though most modules just choose to
7222 derive their C<import> method via inheritance from the C<Exporter> class that
7223 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
7224 method can be found then the call is skipped, even if there is an AUTOLOAD
7227 If you do not want to call the package's C<import> method (for instance,
7228 to stop your namespace from being altered), explicitly supply the empty list:
7232 That is exactly equivalent to
7234 BEGIN { require Module }
7236 If the VERSION argument is present between Module and LIST, then the
7237 C<use> will call the VERSION method in class Module with the given
7238 version as an argument. The default VERSION method, inherited from
7239 the UNIVERSAL class, croaks if the given version is larger than the
7240 value of the variable C<$Module::VERSION>.
7242 Again, there is a distinction between omitting LIST (C<import> called
7243 with no arguments) and an explicit empty LIST C<()> (C<import> not
7244 called). Note that there is no comma after VERSION!
7246 Because this is a wide-open interface, pragmas (compiler directives)
7247 are also implemented this way. Currently implemented pragmas are:
7252 use sigtrap qw(SEGV BUS);
7253 use strict qw(subs vars refs);
7254 use subs qw(afunc blurfl);
7255 use warnings qw(all);
7256 use sort qw(stable _quicksort _mergesort);
7258 Some of these pseudo-modules import semantics into the current
7259 block scope (like C<strict> or C<integer>, unlike ordinary modules,
7260 which import symbols into the current package (which are effective
7261 through the end of the file).
7263 Because C<use> takes effect at compile time, it doesn't respect the
7264 ordinary flow control of the code being compiled. In particular, putting
7265 a C<use> inside the false branch of a conditional doesn't prevent it
7266 from being processed. If a module or pragma only needs to be loaded
7267 conditionally, this can be done using the L<if> pragma:
7269 use if $] < 5.008, "utf8";
7270 use if WANT_WARNINGS, warnings => qw(all);
7272 There's a corresponding C<no> command that unimports meanings imported
7273 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
7274 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
7275 or no unimport method being found.
7281 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
7282 for the C<-M> and C<-m> command-line options to Perl that give C<use>
7283 functionality from the command-line.
7288 Changes the access and modification times on each file of a list of
7289 files. The first two elements of the list must be the NUMERICAL access
7290 and modification times, in that order. Returns the number of files
7291 successfully changed. The inode change time of each file is set
7292 to the current time. For example, this code has the same effect as the
7293 Unix touch(1) command when the files I<already exist> and belong to
7294 the user running the program:
7297 $atime = $mtime = time;
7298 utime $atime, $mtime, @ARGV;
7300 Since Perl 5.7.2, if the first two elements of the list are C<undef>,
7301 the utime(2) syscall from your C library is called with a null second
7302 argument. On most systems, this will set the file's access and
7303 modification times to the current time (i.e., equivalent to the example
7304 above) and will work even on files you don't own provided you have write
7308 utime(undef, undef, $file)
7309 || warn "couldn't touch $file: $!";
7312 Under NFS this will use the time of the NFS server, not the time of
7313 the local machine. If there is a time synchronization problem, the
7314 NFS server and local machine will have different times. The Unix
7315 touch(1) command will in fact normally use this form instead of the
7316 one shown in the first example.
7318 Passing only one of the first two elements as C<undef> is
7319 equivalent to passing a 0 and will not have the effect
7320 described when both are C<undef>. This also triggers an
7321 uninitialized warning.
7323 On systems that support futimes(2), you may pass filehandles among the
7324 files. On systems that don't support futimes(2), passing filehandles raises
7325 an exception. Filehandles must be passed as globs or glob references to be
7326 recognized; barewords are considered filenames.
7333 Returns a list consisting of all the values of the named hash, or the values
7334 of an array. (In a scalar context, returns the number of values.)
7336 The values are returned in an apparently random order. The actual
7337 random order is subject to change in future versions of Perl, but it
7338 is guaranteed to be the same order as either the C<keys> or C<each>
7339 function would produce on the same (unmodified) hash. Since Perl
7340 5.8.1 the ordering is different even between different runs of Perl
7341 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7343 As a side effect, calling values() resets the HASH or ARRAY's internal
7345 see L</each>. (In particular, calling values() in void context resets
7346 the iterator with no other overhead. Apart from resetting the iterator,
7347 C<values @array> in list context is the same as plain C<@array>.
7348 We recommend that you use void context C<keys @array> for this, but reasoned
7349 that it taking C<values @array> out would require more documentation than
7353 Note that the values are not copied, which means modifying them will
7354 modify the contents of the hash:
7356 for (values %hash) { s/foo/bar/g } # modifies %hash values
7357 for (@hash{keys %hash}) { s/foo/bar/g } # same
7359 See also C<keys>, C<each>, and C<sort>.
7361 =item vec EXPR,OFFSET,BITS
7362 X<vec> X<bit> X<bit vector>
7364 Treats the string in EXPR as a bit vector made up of elements of
7365 width BITS, and returns the value of the element specified by OFFSET
7366 as an unsigned integer. BITS therefore specifies the number of bits
7367 that are reserved for each element in the bit vector. This must
7368 be a power of two from 1 to 32 (or 64, if your platform supports
7371 If BITS is 8, "elements" coincide with bytes of the input string.
7373 If BITS is 16 or more, bytes of the input string are grouped into chunks
7374 of size BITS/8, and each group is converted to a number as with
7375 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7376 for BITS==64). See L<"pack"> for details.
7378 If bits is 4 or less, the string is broken into bytes, then the bits
7379 of each byte are broken into 8/BITS groups. Bits of a byte are
7380 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7381 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7382 breaking the single input byte C<chr(0x36)> into two groups gives a list
7383 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7385 C<vec> may also be assigned to, in which case parentheses are needed
7386 to give the expression the correct precedence as in
7388 vec($image, $max_x * $x + $y, 8) = 3;
7390 If the selected element is outside the string, the value 0 is returned.
7391 If an element off the end of the string is written to, Perl will first
7392 extend the string with sufficiently many zero bytes. It is an error
7393 to try to write off the beginning of the string (i.e., negative OFFSET).
7395 If the string happens to be encoded as UTF-8 internally (and thus has
7396 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7397 internal byte string, not the conceptual character string, even if you
7398 only have characters with values less than 256.
7400 Strings created with C<vec> can also be manipulated with the logical
7401 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7402 vector operation is desired when both operands are strings.
7403 See L<perlop/"Bitwise String Operators">.
7405 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7406 The comments show the string after each step. Note that this code works
7407 in the same way on big-endian or little-endian machines.
7410 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7412 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7413 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7415 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7416 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7417 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7418 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7419 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7420 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7422 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7423 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7424 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7427 To transform a bit vector into a string or list of 0's and 1's, use these:
7429 $bits = unpack("b*", $vector);
7430 @bits = split(//, unpack("b*", $vector));
7432 If you know the exact length in bits, it can be used in place of the C<*>.
7434 Here is an example to illustrate how the bits actually fall in place:
7440 unpack("V",$_) 01234567890123456789012345678901
7441 ------------------------------------------------------------------
7446 for ($shift=0; $shift < $width; ++$shift) {
7447 for ($off=0; $off < 32/$width; ++$off) {
7448 $str = pack("B*", "0"x32);
7449 $bits = (1<<$shift);
7450 vec($str, $off, $width) = $bits;
7451 $res = unpack("b*",$str);
7452 $val = unpack("V", $str);
7459 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7460 $off, $width, $bits, $val, $res
7464 Regardless of the machine architecture on which it runs, the
7465 example above should print the following table:
7468 unpack("V",$_) 01234567890123456789012345678901
7469 ------------------------------------------------------------------
7470 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7471 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7472 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7473 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7474 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7475 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7476 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7477 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7478 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7479 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7480 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7481 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7482 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7483 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7484 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7485 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7486 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7487 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7488 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7489 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7490 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7491 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7492 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7493 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7494 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7495 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7496 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7497 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7498 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7499 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7500 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7501 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7502 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7503 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7504 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7505 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7506 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7507 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7508 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7509 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7510 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7511 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7512 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7513 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7514 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7515 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7516 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7517 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7518 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7519 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7520 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7521 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7522 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7523 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7524 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7525 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7526 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7527 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7528 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7529 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7530 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7531 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7532 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7533 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7534 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7535 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7536 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7537 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7538 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7539 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7540 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7541 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7542 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7543 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7544 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7545 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7546 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7547 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7548 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7549 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7550 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7551 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7552 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7553 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7554 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7555 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7556 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7557 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7558 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7559 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7560 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7561 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7562 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7563 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7564 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7565 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7566 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7567 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7568 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7569 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7570 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7571 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7572 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7573 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7574 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7575 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7576 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7577 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7578 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7579 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7580 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7581 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7582 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7583 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7584 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7585 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7586 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7587 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7588 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7589 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7590 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7591 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7592 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7593 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7594 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7595 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7596 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7597 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7602 Behaves like wait(2) on your system: it waits for a child
7603 process to terminate and returns the pid of the deceased process, or
7604 C<-1> if there are no child processes. The status is returned in C<$?>
7605 and C<${^CHILD_ERROR_NATIVE}>.
7606 Note that a return value of C<-1> could mean that child processes are
7607 being automatically reaped, as described in L<perlipc>.
7609 =item waitpid PID,FLAGS
7612 Waits for a particular child process to terminate and returns the pid of
7613 the deceased process, or C<-1> if there is no such child process. On some
7614 systems, a value of 0 indicates that there are processes still running.
7615 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
7617 use POSIX ":sys_wait_h";
7620 $kid = waitpid(-1, WNOHANG);
7623 then you can do a non-blocking wait for all pending zombie processes.
7624 Non-blocking wait is available on machines supporting either the
7625 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
7626 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7627 system call by remembering the status values of processes that have
7628 exited but have not been harvested by the Perl script yet.)
7630 Note that on some systems, a return value of C<-1> could mean that child
7631 processes are being automatically reaped. See L<perlipc> for details,
7632 and for other examples.
7635 X<wantarray> X<context>
7637 Returns true if the context of the currently executing subroutine or
7638 C<eval> is looking for a list value. Returns false if the context is
7639 looking for a scalar. Returns the undefined value if the context is
7640 looking for no value (void context).
7642 return unless defined wantarray; # don't bother doing more
7643 my @a = complex_calculation();
7644 return wantarray ? @a : "@a";
7646 C<wantarray()>'s result is unspecified in the top level of a file,
7647 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7648 in a C<DESTROY> method.
7650 This function should have been named wantlist() instead.
7653 X<warn> X<warning> X<STDERR>
7655 Prints the value of LIST to STDERR. If the last element of LIST does
7656 not end in a newline, it appends the same file/line number text as C<die>
7659 If the output is empty and C<$@> already contains a value (typically from a
7660 previous eval) that value is used after appending C<"\t...caught">
7661 to C<$@>. This is useful for staying almost, but not entirely similar to
7664 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7666 No message is printed if there is a C<$SIG{__WARN__}> handler
7667 installed. It is the handler's responsibility to deal with the message
7668 as it sees fit (like, for instance, converting it into a C<die>). Most
7669 handlers must therefore arrange to actually display the
7670 warnings that they are not prepared to deal with, by calling C<warn>
7671 again in the handler. Note that this is quite safe and will not
7672 produce an endless loop, since C<__WARN__> hooks are not called from
7675 You will find this behavior is slightly different from that of
7676 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7677 instead call C<die> again to change it).
7679 Using a C<__WARN__> handler provides a powerful way to silence all
7680 warnings (even the so-called mandatory ones). An example:
7682 # wipe out *all* compile-time warnings
7683 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7685 my $foo = 20; # no warning about duplicate my $foo,
7686 # but hey, you asked for it!
7687 # no compile-time or run-time warnings before here
7690 # run-time warnings enabled after here
7691 warn "\$foo is alive and $foo!"; # does show up
7693 See L<perlvar> for details on setting C<%SIG> entries, and for more
7694 examples. See the Carp module for other kinds of warnings using its
7695 carp() and cluck() functions.
7697 =item write FILEHANDLE
7704 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7705 using the format associated with that file. By default the format for
7706 a file is the one having the same name as the filehandle, but the
7707 format for the current output channel (see the C<select> function) may be set
7708 explicitly by assigning the name of the format to the C<$~> variable.
7710 Top of form processing is handled automatically: if there is
7711 insufficient room on the current page for the formatted record, the
7712 page is advanced by writing a form feed, a special top-of-page format
7713 is used to format the new page header, and then the record is written.
7714 By default the top-of-page format is the name of the filehandle with
7715 "_TOP" appended, but it may be dynamically set to the format of your
7716 choice by assigning the name to the C<$^> variable while the filehandle is
7717 selected. The number of lines remaining on the current page is in
7718 variable C<$->, which can be set to C<0> to force a new page.
7720 If FILEHANDLE is unspecified, output goes to the current default output
7721 channel, which starts out as STDOUT but may be changed by the
7722 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7723 is evaluated and the resulting string is used to look up the name of
7724 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7726 Note that write is I<not> the opposite of C<read>. Unfortunately.
7730 The transliteration operator. Same as C<tr///>. See
7731 L<perlop/"Quote and Quote-like Operators">.