4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides a scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, the scalar arguments will
18 be first, and the list argument will follow. (Note that there can ever
19 be only one such list argument.) For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for the elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use the parentheses, the simple (but occasionally
34 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
35 function, and precedence doesn't matter. Otherwise it's a list
36 operator or unary operator, and precedence does matter. And whitespace
37 between the function and left parenthesis doesn't count--so you need to
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
52 A few functions take no arguments at all, and therefore work as neither
53 unary nor list operators. These include such functions as C<time>
54 and C<endpwent>. For example, C<time+86_400> always means
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in a scalar context by
59 returning the undefined value, and in a list context by returning the
62 Remember the following important rule: There is B<no rule> that relates
63 the behavior of an expression in list context to its behavior in scalar
64 context, or vice versa. It might do two totally different things.
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in scalar context. Some operators return the
67 length of the list that would have been returned in list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
74 A named array in scalar context is quite different from what would at
75 first glance appear to be a list in scalar context. You can't get a list
76 like C<(1,2,3)> into being in scalar context, because the compiler knows
77 the context at compile time. It would generate the scalar comma operator
78 there, not the list construction version of the comma. That means it
79 was never a list to start with.
81 In general, functions in Perl that serve as wrappers for system calls
82 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule are C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 =head2 Perl Functions by Category
92 Here are Perl's functions (including things that look like
93 functions, like some keywords and named operators)
94 arranged by category. Some functions appear in more
99 =item Functions for SCALARs or strings
100 X<scalar> X<string> X<character>
102 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
103 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
104 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
106 =item Regular expressions and pattern matching
107 X<regular expression> X<regex> X<regexp>
109 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
111 =item Numeric functions
112 X<numeric> X<number> X<trigonometric> X<trigonometry>
114 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115 C<sin>, C<sqrt>, C<srand>
117 =item Functions for real @ARRAYs
120 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
122 =item Functions for list data
125 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
127 =item Functions for real %HASHes
130 C<delete>, C<each>, C<exists>, C<keys>, C<values>
132 =item Input and output functions
133 X<I/O> X<input> X<output> X<dbm>
135 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
137 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
138 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
141 =item Functions for fixed length data or records
143 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
145 =item Functions for filehandles, files, or directories
146 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
148 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
149 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
150 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151 C<umask>, C<unlink>, C<utime>
153 =item Keywords related to the control flow of your Perl program
156 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
159 =item Keywords related to switch
161 C<break>, C<continue>, C<given>, C<when>, C<default>
163 (These are only available if you enable the "switch" feature.
164 See L<feature> and L<perlsyn/"Switch statements">.)
166 =item Keywords related to scoping
168 C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
171 (C<state> is only available if the "state" feature is enabled. See
174 =item Miscellaneous functions
176 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
177 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
179 =item Functions for processes and process groups
180 X<process> X<pid> X<process id>
182 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
183 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
184 C<times>, C<wait>, C<waitpid>
186 =item Keywords related to perl modules
189 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
191 =item Keywords related to classes and object-orientedness
192 X<object> X<class> X<package>
194 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
197 =item Low-level socket functions
200 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
202 C<socket>, C<socketpair>
204 =item System V interprocess communication functions
205 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
207 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
210 =item Fetching user and group info
211 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
213 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215 C<getpwuid>, C<setgrent>, C<setpwent>
217 =item Fetching network info
218 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
220 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224 C<setnetent>, C<setprotoent>, C<setservent>
226 =item Time-related functions
229 C<gmtime>, C<localtime>, C<time>, C<times>
231 =item Functions new in perl5
234 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
236 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr>, C<qw>, C<qx>,
237 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238 C<ucfirst>, C<untie>, C<use>, C<when>
240 * - C<sub> was a keyword in perl4, but in perl5 it is an
241 operator, which can be used in expressions.
243 =item Functions obsoleted in perl5
245 C<dbmclose>, C<dbmopen>
250 X<portability> X<Unix> X<portable>
252 Perl was born in Unix and can therefore access all common Unix
253 system calls. In non-Unix environments, the functionality of some
254 Unix system calls may not be available, or details of the available
255 functionality may differ slightly. The Perl functions affected
258 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
261 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
263 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
264 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
267 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
268 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
271 C<shmwrite>, C<socket>, C<socketpair>,
272 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273 C<times>, C<truncate>, C<umask>, C<unlink>,
274 C<utime>, C<wait>, C<waitpid>
276 For more information about the portability of these functions, see
277 L<perlport> and other available platform-specific documentation.
279 =head2 Alphabetical Listing of Perl Functions
284 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
285 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
293 A file test, where X is one of the letters listed below. This unary
294 operator takes one argument, either a filename, a filehandle, or a dirhandle,
295 and tests the associated file to see if something is true about it. If the
296 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
297 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
298 the undefined value if the file doesn't exist. Despite the funny
299 names, precedence is the same as any other named unary operator, and
300 the argument may be parenthesized like any other unary operator. The
301 operator may be any of:
303 -r File is readable by effective uid/gid.
304 -w File is writable by effective uid/gid.
305 -x File is executable by effective uid/gid.
306 -o File is owned by effective uid.
308 -R File is readable by real uid/gid.
309 -W File is writable by real uid/gid.
310 -X File is executable by real uid/gid.
311 -O File is owned by real uid.
314 -z File has zero size (is empty).
315 -s File has nonzero size (returns size in bytes).
317 -f File is a plain file.
318 -d File is a directory.
319 -l File is a symbolic link.
320 -p File is a named pipe (FIFO), or Filehandle is a pipe.
322 -b File is a block special file.
323 -c File is a character special file.
324 -t Filehandle is opened to a tty.
326 -u File has setuid bit set.
327 -g File has setgid bit set.
328 -k File has sticky bit set.
330 -T File is an ASCII text file (heuristic guess).
331 -B File is a "binary" file (opposite of -T).
333 -M Script start time minus file modification time, in days.
334 -A Same for access time.
335 -C Same for inode change time (Unix, may differ for other platforms)
341 next unless -f $_; # ignore specials
345 The interpretation of the file permission operators C<-r>, C<-R>,
346 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
347 of the file and the uids and gids of the user. There may be other
348 reasons you can't actually read, write, or execute the file. Such
349 reasons may be for example network filesystem access controls, ACLs
350 (access control lists), read-only filesystems, and unrecognized
353 Also note that, for the superuser on the local filesystems, the C<-r>,
354 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
355 if any execute bit is set in the mode. Scripts run by the superuser
356 may thus need to do a stat() to determine the actual mode of the file,
357 or temporarily set their effective uid to something else.
359 If you are using ACLs, there is a pragma called C<filetest> that may
360 produce more accurate results than the bare stat() mode bits.
361 When under the C<use filetest 'access'> the above-mentioned filetests
362 will test whether the permission can (not) be granted using the
363 access() family of system calls. Also note that the C<-x> and C<-X> may
364 under this pragma return true even if there are no execute permission
365 bits set (nor any extra execute permission ACLs). This strangeness is
366 due to the underlying system calls' definitions. Read the
367 documentation for the C<filetest> pragma for more information.
369 Note that C<-s/a/b/> does not do a negated substitution. Saying
370 C<-exp($foo)> still works as expected, however--only single letters
371 following a minus are interpreted as file tests.
373 The C<-T> and C<-B> switches work as follows. The first block or so of the
374 file is examined for odd characters such as strange control codes or
375 characters with the high bit set. If too many strange characters (>30%)
376 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
377 containing null in the first block is considered a binary file. If C<-T>
378 or C<-B> is used on a filehandle, the current IO buffer is examined
379 rather than the first block. Both C<-T> and C<-B> return true on a null
380 file, or a file at EOF when testing a filehandle. Because you have to
381 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
382 against the file first, as in C<next unless -f $file && -T $file>.
384 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
385 the special filehandle consisting of a solitary underline, then the stat
386 structure of the previous file test (or stat operator) is used, saving
387 a system call. (This doesn't work with C<-t>, and you need to remember
388 that lstat() and C<-l> will leave values in the stat structure for the
389 symbolic link, not the real file.) (Also, if the stat buffer was filled by
390 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
393 print "Can do.\n" if -r $a || -w _ || -x _;
396 print "Readable\n" if -r _;
397 print "Writable\n" if -w _;
398 print "Executable\n" if -x _;
399 print "Setuid\n" if -u _;
400 print "Setgid\n" if -g _;
401 print "Sticky\n" if -k _;
402 print "Text\n" if -T _;
403 print "Binary\n" if -B _;
405 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
406 test operators, in a way that C<-f -w -x $file> is equivalent to
407 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
408 the return value of C<-f $file> as an argument to another filetest
409 operator, no special magic will happen.)
416 Returns the absolute value of its argument.
417 If VALUE is omitted, uses C<$_>.
419 =item accept NEWSOCKET,GENERICSOCKET
422 Accepts an incoming socket connect, just as the accept(2) system call
423 does. Returns the packed address if it succeeded, false otherwise.
424 See the example in L<perlipc/"Sockets: Client/Server Communication">.
426 On systems that support a close-on-exec flag on files, the flag will
427 be set for the newly opened file descriptor, as determined by the
428 value of $^F. See L<perlvar/$^F>.
437 Arranges to have a SIGALRM delivered to this process after the
438 specified number of wallclock seconds has elapsed. If SECONDS is not
439 specified, the value stored in C<$_> is used. (On some machines,
440 unfortunately, the elapsed time may be up to one second less or more
441 than you specified because of how seconds are counted, and process
442 scheduling may delay the delivery of the signal even further.)
444 Only one timer may be counting at once. Each call disables the
445 previous timer, and an argument of C<0> may be supplied to cancel the
446 previous timer without starting a new one. The returned value is the
447 amount of time remaining on the previous timer.
449 For delays of finer granularity than one second, you may use Perl's
450 four-argument version of select() leaving the first three arguments
451 undefined, or you might be able to use the C<syscall> interface to
452 access setitimer(2) if your system supports it. The Time::HiRes
453 module (from CPAN, and starting from Perl 5.8 part of the standard
454 distribution) may also prove useful.
456 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
457 (C<sleep> may be internally implemented in your system with C<alarm>)
459 If you want to use C<alarm> to time out a system call you need to use an
460 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
461 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
462 restart system calls on some systems. Using C<eval>/C<die> always works,
463 modulo the caveats given in L<perlipc/"Signals">.
466 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
468 $nread = sysread SOCKET, $buffer, $size;
472 die unless $@ eq "alarm\n"; # propagate unexpected errors
479 For more information see L<perlipc>.
482 X<atan2> X<arctangent> X<tan> X<tangent>
484 Returns the arctangent of Y/X in the range -PI to PI.
486 For the tangent operation, you may use the C<Math::Trig::tan>
487 function, or use the familiar relation:
489 sub tan { sin($_[0]) / cos($_[0]) }
491 Note that atan2(0, 0) is not well-defined.
493 =item bind SOCKET,NAME
496 Binds a network address to a socket, just as the bind system call
497 does. Returns true if it succeeded, false otherwise. NAME should be a
498 packed address of the appropriate type for the socket. See the examples in
499 L<perlipc/"Sockets: Client/Server Communication">.
501 =item binmode FILEHANDLE, LAYER
502 X<binmode> X<binary> X<text> X<DOS> X<Windows>
504 =item binmode FILEHANDLE
506 Arranges for FILEHANDLE to be read or written in "binary" or "text"
507 mode on systems where the run-time libraries distinguish between
508 binary and text files. If FILEHANDLE is an expression, the value is
509 taken as the name of the filehandle. Returns true on success,
510 otherwise it returns C<undef> and sets C<$!> (errno).
512 On some systems (in general, DOS and Windows-based systems) binmode()
513 is necessary when you're not working with a text file. For the sake
514 of portability it is a good idea to always use it when appropriate,
515 and to never use it when it isn't appropriate. Also, people can
516 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
518 In other words: regardless of platform, use binmode() on binary data,
519 like for example images.
521 If LAYER is present it is a single string, but may contain multiple
522 directives. The directives alter the behaviour of the file handle.
523 When LAYER is present using binmode on text file makes sense.
525 If LAYER is omitted or specified as C<:raw> the filehandle is made
526 suitable for passing binary data. This includes turning off possible CRLF
527 translation and marking it as bytes (as opposed to Unicode characters).
528 Note that, despite what may be implied in I<"Programming Perl"> (the
529 Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
530 -- other layers which would affect binary nature of the stream are
531 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
532 PERLIO environment variable.
534 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
535 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
536 establish default I/O layers. See L<open>.
538 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
539 in "Programming Perl, 3rd Edition". However, since the publishing of this
540 book, by many known as "Camel III", the consensus of the naming of this
541 functionality has moved from "discipline" to "layer". All documentation
542 of this version of Perl therefore refers to "layers" rather than to
543 "disciplines". Now back to the regularly scheduled documentation...>
545 To mark FILEHANDLE as UTF-8, use C<:utf8>.
547 In general, binmode() should be called after open() but before any I/O
548 is done on the filehandle. Calling binmode() will normally flush any
549 pending buffered output data (and perhaps pending input data) on the
550 handle. An exception to this is the C<:encoding> layer that
551 changes the default character encoding of the handle, see L<open>.
552 The C<:encoding> layer sometimes needs to be called in
553 mid-stream, and it doesn't flush the stream. The C<:encoding>
554 also implicitly pushes on top of itself the C<:utf8> layer because
555 internally Perl will operate on UTF-8 encoded Unicode characters.
557 The operating system, device drivers, C libraries, and Perl run-time
558 system all work together to let the programmer treat a single
559 character (C<\n>) as the line terminator, irrespective of the external
560 representation. On many operating systems, the native text file
561 representation matches the internal representation, but on some
562 platforms the external representation of C<\n> is made up of more than
565 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
566 character to end each line in the external representation of text (even
567 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
568 on Unix and most VMS files). In other systems like OS/2, DOS and the
569 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
570 but what's stored in text files are the two characters C<\cM\cJ>. That
571 means that, if you don't use binmode() on these systems, C<\cM\cJ>
572 sequences on disk will be converted to C<\n> on input, and any C<\n> in
573 your program will be converted back to C<\cM\cJ> on output. This is what
574 you want for text files, but it can be disastrous for binary files.
576 Another consequence of using binmode() (on some systems) is that
577 special end-of-file markers will be seen as part of the data stream.
578 For systems from the Microsoft family this means that if your binary
579 data contains C<\cZ>, the I/O subsystem will regard it as the end of
580 the file, unless you use binmode().
582 binmode() is not only important for readline() and print() operations,
583 but also when using read(), seek(), sysread(), syswrite() and tell()
584 (see L<perlport> for more details). See the C<$/> and C<$\> variables
585 in L<perlvar> for how to manually set your input and output
586 line-termination sequences.
588 =item bless REF,CLASSNAME
593 This function tells the thingy referenced by REF that it is now an object
594 in the CLASSNAME package. If CLASSNAME is omitted, the current package
595 is used. Because a C<bless> is often the last thing in a constructor,
596 it returns the reference for convenience. Always use the two-argument
597 version if a derived class might inherit the function doing the blessing.
598 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
601 Consider always blessing objects in CLASSNAMEs that are mixed case.
602 Namespaces with all lowercase names are considered reserved for
603 Perl pragmata. Builtin types have all uppercase names. To prevent
604 confusion, you may wish to avoid such package names as well. Make sure
605 that CLASSNAME is a true value.
607 See L<perlmod/"Perl Modules">.
611 Break out of a C<given()> block.
613 This keyword is enabled by the "switch" feature: see L<feature>
614 for more information.
617 X<caller> X<call stack> X<stack> X<stack trace>
621 Returns the context of the current subroutine call. In scalar context,
622 returns the caller's package name if there is a caller, that is, if
623 we're in a subroutine or C<eval> or C<require>, and the undefined value
624 otherwise. In list context, returns
627 ($package, $filename, $line) = caller;
629 With EXPR, it returns some extra information that the debugger uses to
630 print a stack trace. The value of EXPR indicates how many call frames
631 to go back before the current one.
634 ($package, $filename, $line, $subroutine, $hasargs,
637 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
640 Here $subroutine may be C<(eval)> if the frame is not a subroutine
641 call, but an C<eval>. In such a case additional elements $evaltext and
642 C<$is_require> are set: C<$is_require> is true if the frame is created by a
643 C<require> or C<use> statement, $evaltext contains the text of the
644 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
645 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
646 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
647 frame.) $subroutine may also be C<(unknown)> if this particular
648 subroutine happens to have been deleted from the symbol table.
649 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
650 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
651 compiled with. The C<$hints> and C<$bitmask> values are subject to change
652 between versions of Perl, and are not meant for external use.
654 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
655 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
656 of this hash, as they are the actual values stored in the optree.
658 Furthermore, when called from within the DB package, caller returns more
659 detailed information: it sets the list variable C<@DB::args> to be the
660 arguments with which the subroutine was invoked.
662 Be aware that the optimizer might have optimized call frames away before
663 C<caller> had a chance to get the information. That means that C<caller(N)>
664 might not return information about the call frame you expect it do, for
665 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
666 previous time C<caller> was called.
673 =item chdir FILEHANDLE
675 =item chdir DIRHANDLE
679 Changes the working directory to EXPR, if possible. If EXPR is omitted,
680 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
681 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
682 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
683 neither is set, C<chdir> does nothing. It returns true upon success,
684 false otherwise. See the example under C<die>.
686 On systems that support fchdir, you might pass a file handle or
687 directory handle as argument. On systems that don't support fchdir,
688 passing handles produces a fatal error at run time.
691 X<chmod> X<permission> X<mode>
693 Changes the permissions of a list of files. The first element of the
694 list must be the numerical mode, which should probably be an octal
695 number, and which definitely should I<not> be a string of octal digits:
696 C<0644> is okay, C<'0644'> is not. Returns the number of files
697 successfully changed. See also L</oct>, if all you have is a string.
699 $cnt = chmod 0755, 'foo', 'bar';
700 chmod 0755, @executables;
701 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
703 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
704 $mode = 0644; chmod $mode, 'foo'; # this is best
706 On systems that support fchmod, you might pass file handles among the
707 files. On systems that don't support fchmod, passing file handles
708 produces a fatal error at run time. The file handles must be passed
709 as globs or references to be recognized. Barewords are considered
712 open(my $fh, "<", "foo");
713 my $perm = (stat $fh)[2] & 07777;
714 chmod($perm | 0600, $fh);
716 You can also import the symbolic C<S_I*> constants from the Fcntl
721 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
722 # This is identical to the chmod 0755 of the above example.
725 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
731 This safer version of L</chop> removes any trailing string
732 that corresponds to the current value of C<$/> (also known as
733 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
734 number of characters removed from all its arguments. It's often used to
735 remove the newline from the end of an input record when you're worried
736 that the final record may be missing its newline. When in paragraph
737 mode (C<$/ = "">), it removes all trailing newlines from the string.
738 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
739 a reference to an integer or the like, see L<perlvar>) chomp() won't
741 If VARIABLE is omitted, it chomps C<$_>. Example:
744 chomp; # avoid \n on last field
749 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
751 You can actually chomp anything that's an lvalue, including an assignment:
754 chomp($answer = <STDIN>);
756 If you chomp a list, each element is chomped, and the total number of
757 characters removed is returned.
759 If the C<encoding> pragma is in scope then the lengths returned are
760 calculated from the length of C<$/> in Unicode characters, which is not
761 always the same as the length of C<$/> in the native encoding.
763 Note that parentheses are necessary when you're chomping anything
764 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
765 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
766 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
767 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
777 Chops off the last character of a string and returns the character
778 chopped. It is much more efficient than C<s/.$//s> because it neither
779 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
780 If VARIABLE is a hash, it chops the hash's values, but not its keys.
782 You can actually chop anything that's an lvalue, including an assignment.
784 If you chop a list, each element is chopped. Only the value of the
785 last C<chop> is returned.
787 Note that C<chop> returns the last character. To return all but the last
788 character, use C<substr($string, 0, -1)>.
793 X<chown> X<owner> X<user> X<group>
795 Changes the owner (and group) of a list of files. The first two
796 elements of the list must be the I<numeric> uid and gid, in that
797 order. A value of -1 in either position is interpreted by most
798 systems to leave that value unchanged. Returns the number of files
799 successfully changed.
801 $cnt = chown $uid, $gid, 'foo', 'bar';
802 chown $uid, $gid, @filenames;
804 On systems that support fchown, you might pass file handles among the
805 files. On systems that don't support fchown, passing file handles
806 produces a fatal error at run time. The file handles must be passed
807 as globs or references to be recognized. Barewords are considered
810 Here's an example that looks up nonnumeric uids in the passwd file:
813 chomp($user = <STDIN>);
815 chomp($pattern = <STDIN>);
817 ($login,$pass,$uid,$gid) = getpwnam($user)
818 or die "$user not in passwd file";
820 @ary = glob($pattern); # expand filenames
821 chown $uid, $gid, @ary;
823 On most systems, you are not allowed to change the ownership of the
824 file unless you're the superuser, although you should be able to change
825 the group to any of your secondary groups. On insecure systems, these
826 restrictions may be relaxed, but this is not a portable assumption.
827 On POSIX systems, you can detect this condition this way:
829 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
830 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
833 X<chr> X<character> X<ASCII> X<Unicode>
837 Returns the character represented by that NUMBER in the character set.
838 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
839 chr(0x263a) is a Unicode smiley face. Note that characters from 128
840 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
841 backward compatibility reasons (but see L<encoding>).
843 Negative values give the Unicode replacement character (chr(0xfffd)),
844 except under the L<bytes> pragma, where low eight bits of the value
845 (truncated to an integer) are used.
847 If NUMBER is omitted, uses C<$_>.
849 For the reverse, use L</ord>.
851 Note that under the C<bytes> pragma the NUMBER is masked to
854 See L<perlunicode> and L<encoding> for more about Unicode.
856 =item chroot FILENAME
861 This function works like the system call by the same name: it makes the
862 named directory the new root directory for all further pathnames that
863 begin with a C</> by your process and all its children. (It doesn't
864 change your current working directory, which is unaffected.) For security
865 reasons, this call is restricted to the superuser. If FILENAME is
866 omitted, does a C<chroot> to C<$_>.
868 =item close FILEHANDLE
873 Closes the file or pipe associated with the file handle, returning
874 true only if IO buffers are successfully flushed and closes the system
875 file descriptor. Closes the currently selected filehandle if the
878 You don't have to close FILEHANDLE if you are immediately going to do
879 another C<open> on it, because C<open> will close it for you. (See
880 C<open>.) However, an explicit C<close> on an input file resets the line
881 counter (C<$.>), while the implicit close done by C<open> does not.
883 If the file handle came from a piped open, C<close> will additionally
884 return false if one of the other system calls involved fails, or if the
885 program exits with non-zero status. (If the only problem was that the
886 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
887 also waits for the process executing on the pipe to complete, in case you
888 want to look at the output of the pipe afterwards, and
889 implicitly puts the exit status value of that command into C<$?> and
890 C<${^CHILD_ERROR_NATIVE}>.
892 Prematurely closing the read end of a pipe (i.e. before the process
893 writing to it at the other end has closed it) will result in a
894 SIGPIPE being delivered to the writer. If the other end can't
895 handle that, be sure to read all the data before closing the pipe.
899 open(OUTPUT, '|sort >foo') # pipe to sort
900 or die "Can't start sort: $!";
901 #... # print stuff to output
902 close OUTPUT # wait for sort to finish
903 or warn $! ? "Error closing sort pipe: $!"
904 : "Exit status $? from sort";
905 open(INPUT, 'foo') # get sort's results
906 or die "Can't open 'foo' for input: $!";
908 FILEHANDLE may be an expression whose value can be used as an indirect
909 filehandle, usually the real filehandle name.
911 =item closedir DIRHANDLE
914 Closes a directory opened by C<opendir> and returns the success of that
917 =item connect SOCKET,NAME
920 Attempts to connect to a remote socket, just as the connect system call
921 does. Returns true if it succeeded, false otherwise. NAME should be a
922 packed address of the appropriate type for the socket. See the examples in
923 L<perlipc/"Sockets: Client/Server Communication">.
930 C<continue> is actually a flow control statement rather than a function. If
931 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
932 C<foreach>), it is always executed just before the conditional is about to
933 be evaluated again, just like the third part of a C<for> loop in C. Thus
934 it can be used to increment a loop variable, even when the loop has been
935 continued via the C<next> statement (which is similar to the C C<continue>
938 C<last>, C<next>, or C<redo> may appear within a C<continue>
939 block. C<last> and C<redo> will behave as if they had been executed within
940 the main block. So will C<next>, but since it will execute a C<continue>
941 block, it may be more entertaining.
944 ### redo always comes here
947 ### next always comes here
949 # then back the top to re-check EXPR
951 ### last always comes here
953 Omitting the C<continue> section is semantically equivalent to using an
954 empty one, logically enough. In that case, C<next> goes directly back
955 to check the condition at the top of the loop.
957 If the "switch" feature is enabled, C<continue> is also a
958 function that will break out of the current C<when> or C<default>
959 block, and fall through to the next case. See L<feature> and
960 L<perlsyn/"Switch statements"> for more information.
964 X<cos> X<cosine> X<acos> X<arccosine>
968 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
969 takes cosine of C<$_>.
971 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
972 function, or use this relation:
974 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
976 =item crypt PLAINTEXT,SALT
977 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
978 X<decrypt> X<cryptography> X<passwd> X<encrypt>
980 Creates a digest string exactly like the crypt(3) function in the C
981 library (assuming that you actually have a version there that has not
982 been extirpated as a potential munitions).
984 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
985 into a short string, called a digest, which is returned. The same
986 PLAINTEXT and SALT will always return the same string, but there is no
987 (known) way to get the original PLAINTEXT from the hash. Small
988 changes in the PLAINTEXT or SALT will result in large changes in the
991 There is no decrypt function. This function isn't all that useful for
992 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
993 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
994 primarily used to check if two pieces of text are the same without
995 having to transmit or store the text itself. An example is checking
996 if a correct password is given. The digest of the password is stored,
997 not the password itself. The user types in a password that is
998 crypt()'d with the same salt as the stored digest. If the two digests
999 match the password is correct.
1001 When verifying an existing digest string you should use the digest as
1002 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1003 to create the digest is visible as part of the digest. This ensures
1004 crypt() will hash the new string with the same salt as the digest.
1005 This allows your code to work with the standard L<crypt|/crypt> and
1006 with more exotic implementations. In other words, do not assume
1007 anything about the returned string itself, or how many bytes in the
1010 Traditionally the result is a string of 13 bytes: two first bytes of
1011 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1012 the first eight bytes of the digest string mattered, but alternative
1013 hashing schemes (like MD5), higher level security schemes (like C2),
1014 and implementations on non-UNIX platforms may produce different
1017 When choosing a new salt create a random two character string whose
1018 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1019 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1020 characters is just a recommendation; the characters allowed in
1021 the salt depend solely on your system's crypt library, and Perl can't
1022 restrict what salts C<crypt()> accepts.
1024 Here's an example that makes sure that whoever runs this program knows
1027 $pwd = (getpwuid($<))[1];
1029 system "stty -echo";
1031 chomp($word = <STDIN>);
1035 if (crypt($word, $pwd) ne $pwd) {
1041 Of course, typing in your own password to whoever asks you
1044 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1045 of data, not least of all because you can't get the information
1046 back. Look at the L<Digest> module for more robust algorithms.
1048 If using crypt() on a Unicode string (which I<potentially> has
1049 characters with codepoints above 255), Perl tries to make sense
1050 of the situation by trying to downgrade (a copy of the string)
1051 the string back to an eight-bit byte string before calling crypt()
1052 (on that copy). If that works, good. If not, crypt() dies with
1053 C<Wide character in crypt>.
1058 [This function has been largely superseded by the C<untie> function.]
1060 Breaks the binding between a DBM file and a hash.
1062 =item dbmopen HASH,DBNAME,MASK
1063 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1065 [This function has been largely superseded by the C<tie> function.]
1067 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1068 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1069 argument is I<not> a filehandle, even though it looks like one). DBNAME
1070 is the name of the database (without the F<.dir> or F<.pag> extension if
1071 any). If the database does not exist, it is created with protection
1072 specified by MASK (as modified by the C<umask>). If your system supports
1073 only the older DBM functions, you may perform only one C<dbmopen> in your
1074 program. In older versions of Perl, if your system had neither DBM nor
1075 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1078 If you don't have write access to the DBM file, you can only read hash
1079 variables, not set them. If you want to test whether you can write,
1080 either use file tests or try setting a dummy hash entry inside an C<eval>,
1081 which will trap the error.
1083 Note that functions such as C<keys> and C<values> may return huge lists
1084 when used on large DBM files. You may prefer to use the C<each>
1085 function to iterate over large DBM files. Example:
1087 # print out history file offsets
1088 dbmopen(%HIST,'/usr/lib/news/history',0666);
1089 while (($key,$val) = each %HIST) {
1090 print $key, ' = ', unpack('L',$val), "\n";
1094 See also L<AnyDBM_File> for a more general description of the pros and
1095 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1096 rich implementation.
1098 You can control which DBM library you use by loading that library
1099 before you call dbmopen():
1102 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1103 or die "Can't open netscape history file: $!";
1106 X<defined> X<undef> X<undefined>
1110 Returns a Boolean value telling whether EXPR has a value other than
1111 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1114 Many operations return C<undef> to indicate failure, end of file,
1115 system error, uninitialized variable, and other exceptional
1116 conditions. This function allows you to distinguish C<undef> from
1117 other values. (A simple Boolean test will not distinguish among
1118 C<undef>, zero, the empty string, and C<"0">, which are all equally
1119 false.) Note that since C<undef> is a valid scalar, its presence
1120 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1121 returns C<undef> when its argument is an empty array, I<or> when the
1122 element to return happens to be C<undef>.
1124 You may also use C<defined(&func)> to check whether subroutine C<&func>
1125 has ever been defined. The return value is unaffected by any forward
1126 declarations of C<&func>. Note that a subroutine which is not defined
1127 may still be callable: its package may have an C<AUTOLOAD> method that
1128 makes it spring into existence the first time that it is called -- see
1131 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1132 used to report whether memory for that aggregate has ever been
1133 allocated. This behavior may disappear in future versions of Perl.
1134 You should instead use a simple test for size:
1136 if (@an_array) { print "has array elements\n" }
1137 if (%a_hash) { print "has hash members\n" }
1139 When used on a hash element, it tells you whether the value is defined,
1140 not whether the key exists in the hash. Use L</exists> for the latter
1145 print if defined $switch{'D'};
1146 print "$val\n" while defined($val = pop(@ary));
1147 die "Can't readlink $sym: $!"
1148 unless defined($value = readlink $sym);
1149 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1150 $debugging = 0 unless defined $debugging;
1152 Note: Many folks tend to overuse C<defined>, and then are surprised to
1153 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1154 defined values. For example, if you say
1158 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1159 matched "nothing". It didn't really fail to match anything. Rather, it
1160 matched something that happened to be zero characters long. This is all
1161 very above-board and honest. When a function returns an undefined value,
1162 it's an admission that it couldn't give you an honest answer. So you
1163 should use C<defined> only when you're questioning the integrity of what
1164 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1167 See also L</undef>, L</exists>, L</ref>.
1172 Given an expression that specifies a hash element, array element, hash slice,
1173 or array slice, deletes the specified element(s) from the hash or array.
1174 In the case of an array, if the array elements happen to be at the end,
1175 the size of the array will shrink to the highest element that tests
1176 true for exists() (or 0 if no such element exists).
1178 Returns a list with the same number of elements as the number of elements
1179 for which deletion was attempted. Each element of that list consists of
1180 either the value of the element deleted, or the undefined value. In scalar
1181 context, this means that you get the value of the last element deleted (or
1182 the undefined value if that element did not exist).
1184 %hash = (foo => 11, bar => 22, baz => 33);
1185 $scalar = delete $hash{foo}; # $scalar is 11
1186 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1187 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1189 Deleting from C<%ENV> modifies the environment. Deleting from
1190 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1191 from a C<tie>d hash or array may not necessarily return anything.
1193 Deleting an array element effectively returns that position of the array
1194 to its initial, uninitialized state. Subsequently testing for the same
1195 element with exists() will return false. Also, deleting array elements
1196 in the middle of an array will not shift the index of the elements
1197 after them down. Use splice() for that. See L</exists>.
1199 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1201 foreach $key (keys %HASH) {
1205 foreach $index (0 .. $#ARRAY) {
1206 delete $ARRAY[$index];
1211 delete @HASH{keys %HASH};
1213 delete @ARRAY[0 .. $#ARRAY];
1215 But both of these are slower than just assigning the empty list
1216 or undefining %HASH or @ARRAY:
1218 %HASH = (); # completely empty %HASH
1219 undef %HASH; # forget %HASH ever existed
1221 @ARRAY = (); # completely empty @ARRAY
1222 undef @ARRAY; # forget @ARRAY ever existed
1224 Note that the EXPR can be arbitrarily complicated as long as the final
1225 operation is a hash element, array element, hash slice, or array slice
1228 delete $ref->[$x][$y]{$key};
1229 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1231 delete $ref->[$x][$y][$index];
1232 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1235 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1237 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1238 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1239 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1240 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1241 an C<eval(),> the error message is stuffed into C<$@> and the
1242 C<eval> is terminated with the undefined value. This makes
1243 C<die> the way to raise an exception.
1245 Equivalent examples:
1247 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1248 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1250 If the last element of LIST does not end in a newline, the current
1251 script line number and input line number (if any) are also printed,
1252 and a newline is supplied. Note that the "input line number" (also
1253 known as "chunk") is subject to whatever notion of "line" happens to
1254 be currently in effect, and is also available as the special variable
1255 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1257 Hint: sometimes appending C<", stopped"> to your message will cause it
1258 to make better sense when the string C<"at foo line 123"> is appended.
1259 Suppose you are running script "canasta".
1261 die "/etc/games is no good";
1262 die "/etc/games is no good, stopped";
1264 produce, respectively
1266 /etc/games is no good at canasta line 123.
1267 /etc/games is no good, stopped at canasta line 123.
1269 See also exit(), warn(), and the Carp module.
1271 If LIST is empty and C<$@> already contains a value (typically from a
1272 previous eval) that value is reused after appending C<"\t...propagated">.
1273 This is useful for propagating exceptions:
1276 die unless $@ =~ /Expected exception/;
1278 If LIST is empty and C<$@> contains an object reference that has a
1279 C<PROPAGATE> method, that method will be called with additional file
1280 and line number parameters. The return value replaces the value in
1281 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1284 If C<$@> is empty then the string C<"Died"> is used.
1286 die() can also be called with a reference argument. If this happens to be
1287 trapped within an eval(), $@ contains the reference. This behavior permits
1288 a more elaborate exception handling implementation using objects that
1289 maintain arbitrary state about the nature of the exception. Such a scheme
1290 is sometimes preferable to matching particular string values of $@ using
1291 regular expressions. Because $@ is a global variable, and eval() may be
1292 used within object implementations, care must be taken that analyzing the
1293 error object doesn't replace the reference in the global variable. The
1294 easiest solution is to make a local copy of the reference before doing
1295 other manipulations. Here's an example:
1297 use Scalar::Util 'blessed';
1299 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1300 if (my $ev_err = $@) {
1301 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1302 # handle Some::Module::Exception
1305 # handle all other possible exceptions
1309 Because perl will stringify uncaught exception messages before displaying
1310 them, you may want to overload stringification operations on such custom
1311 exception objects. See L<overload> for details about that.
1313 You can arrange for a callback to be run just before the C<die>
1314 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1315 handler will be called with the error text and can change the error
1316 message, if it sees fit, by calling C<die> again. See
1317 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1318 L<"eval BLOCK"> for some examples. Although this feature was
1319 to be run only right before your program was to exit, this is not
1320 currently the case--the C<$SIG{__DIE__}> hook is currently called
1321 even inside eval()ed blocks/strings! If one wants the hook to do
1322 nothing in such situations, put
1326 as the first line of the handler (see L<perlvar/$^S>). Because
1327 this promotes strange action at a distance, this counterintuitive
1328 behavior may be fixed in a future release.
1333 Not really a function. Returns the value of the last command in the
1334 sequence of commands indicated by BLOCK. When modified by the C<while> or
1335 C<until> loop modifier, executes the BLOCK once before testing the loop
1336 condition. (On other statements the loop modifiers test the conditional
1339 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1340 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1341 See L<perlsyn> for alternative strategies.
1343 =item do SUBROUTINE(LIST)
1346 This form of subroutine call is deprecated. See L<perlsub>.
1351 Uses the value of EXPR as a filename and executes the contents of the
1352 file as a Perl script.
1360 except that it's more efficient and concise, keeps track of the current
1361 filename for error messages, searches the @INC directories, and updates
1362 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1363 variables. It also differs in that code evaluated with C<do FILENAME>
1364 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1365 same, however, in that it does reparse the file every time you call it,
1366 so you probably don't want to do this inside a loop.
1368 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1369 error. If C<do> can read the file but cannot compile it, it
1370 returns undef and sets an error message in C<$@>. If the file is
1371 successfully compiled, C<do> returns the value of the last expression
1374 Note that inclusion of library modules is better done with the
1375 C<use> and C<require> operators, which also do automatic error checking
1376 and raise an exception if there's a problem.
1378 You might like to use C<do> to read in a program configuration
1379 file. Manual error checking can be done this way:
1381 # read in config files: system first, then user
1382 for $file ("/share/prog/defaults.rc",
1383 "$ENV{HOME}/.someprogrc")
1385 unless ($return = do $file) {
1386 warn "couldn't parse $file: $@" if $@;
1387 warn "couldn't do $file: $!" unless defined $return;
1388 warn "couldn't run $file" unless $return;
1393 X<dump> X<core> X<undump>
1397 This function causes an immediate core dump. See also the B<-u>
1398 command-line switch in L<perlrun>, which does the same thing.
1399 Primarily this is so that you can use the B<undump> program (not
1400 supplied) to turn your core dump into an executable binary after
1401 having initialized all your variables at the beginning of the
1402 program. When the new binary is executed it will begin by executing
1403 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1404 Think of it as a goto with an intervening core dump and reincarnation.
1405 If C<LABEL> is omitted, restarts the program from the top.
1407 B<WARNING>: Any files opened at the time of the dump will I<not>
1408 be open any more when the program is reincarnated, with possible
1409 resulting confusion on the part of Perl.
1411 This function is now largely obsolete, partly because it's very
1412 hard to convert a core file into an executable, and because the
1413 real compiler backends for generating portable bytecode and compilable
1414 C code have superseded it. That's why you should now invoke it as
1415 C<CORE::dump()>, if you don't want to be warned against a possible
1418 If you're looking to use L<dump> to speed up your program, consider
1419 generating bytecode or native C code as described in L<perlcc>. If
1420 you're just trying to accelerate a CGI script, consider using the
1421 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1422 You might also consider autoloading or selfloading, which at least
1423 make your program I<appear> to run faster.
1426 X<each> X<hash, iterator>
1428 When called in list context, returns a 2-element list consisting of the
1429 key and value for the next element of a hash, so that you can iterate over
1430 it. When called in scalar context, returns only the key for the next
1431 element in the hash.
1433 Entries are returned in an apparently random order. The actual random
1434 order is subject to change in future versions of perl, but it is
1435 guaranteed to be in the same order as either the C<keys> or C<values>
1436 function would produce on the same (unmodified) hash. Since Perl
1437 5.8.1 the ordering is different even between different runs of Perl
1438 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1440 When the hash is entirely read, a null array is returned in list context
1441 (which when assigned produces a false (C<0>) value), and C<undef> in
1442 scalar context. The next call to C<each> after that will start iterating
1443 again. There is a single iterator for each hash, shared by all C<each>,
1444 C<keys>, and C<values> function calls in the program; it can be reset by
1445 reading all the elements from the hash, or by evaluating C<keys HASH> or
1446 C<values HASH>. If you add or delete elements of a hash while you're
1447 iterating over it, you may get entries skipped or duplicated, so
1448 don't. Exception: It is always safe to delete the item most recently
1449 returned by C<each()>, which means that the following code will work:
1451 while (($key, $value) = each %hash) {
1453 delete $hash{$key}; # This is safe
1456 The following prints out your environment like the printenv(1) program,
1457 only in a different order:
1459 while (($key,$value) = each %ENV) {
1460 print "$key=$value\n";
1463 See also C<keys>, C<values> and C<sort>.
1465 =item eof FILEHANDLE
1474 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1475 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1476 gives the real filehandle. (Note that this function actually
1477 reads a character and then C<ungetc>s it, so isn't very useful in an
1478 interactive context.) Do not read from a terminal file (or call
1479 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1480 as terminals may lose the end-of-file condition if you do.
1482 An C<eof> without an argument uses the last file read. Using C<eof()>
1483 with empty parentheses is very different. It refers to the pseudo file
1484 formed from the files listed on the command line and accessed via the
1485 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1486 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1487 used will cause C<@ARGV> to be examined to determine if input is
1488 available. Similarly, an C<eof()> after C<< <> >> has returned
1489 end-of-file will assume you are processing another C<@ARGV> list,
1490 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1491 see L<perlop/"I/O Operators">.
1493 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1494 detect the end of each file, C<eof()> will only detect the end of the
1495 last file. Examples:
1497 # reset line numbering on each input file
1499 next if /^\s*#/; # skip comments
1502 close ARGV if eof; # Not eof()!
1505 # insert dashes just before last line of last file
1507 if (eof()) { # check for end of last file
1508 print "--------------\n";
1511 last if eof(); # needed if we're reading from a terminal
1514 Practical hint: you almost never need to use C<eof> in Perl, because the
1515 input operators typically return C<undef> when they run out of data, or if
1519 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1520 X<error, handling> X<exception, handling>
1526 In the first form, the return value of EXPR is parsed and executed as if it
1527 were a little Perl program. The value of the expression (which is itself
1528 determined within scalar context) is first parsed, and if there weren't any
1529 errors, executed in the lexical context of the current Perl program, so
1530 that any variable settings or subroutine and format definitions remain
1531 afterwards. Note that the value is parsed every time the C<eval> executes.
1532 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1533 delay parsing and subsequent execution of the text of EXPR until run time.
1535 In the second form, the code within the BLOCK is parsed only once--at the
1536 same time the code surrounding the C<eval> itself was parsed--and executed
1537 within the context of the current Perl program. This form is typically
1538 used to trap exceptions more efficiently than the first (see below), while
1539 also providing the benefit of checking the code within BLOCK at compile
1542 The final semicolon, if any, may be omitted from the value of EXPR or within
1545 In both forms, the value returned is the value of the last expression
1546 evaluated inside the mini-program; a return statement may be also used, just
1547 as with subroutines. The expression providing the return value is evaluated
1548 in void, scalar, or list context, depending on the context of the C<eval>
1549 itself. See L</wantarray> for more on how the evaluation context can be
1552 If there is a syntax error or runtime error, or a C<die> statement is
1553 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1554 error message. If there was no error, C<$@> is guaranteed to be a null
1555 string. Beware that using C<eval> neither silences perl from printing
1556 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1557 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1558 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1559 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1561 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1562 determining whether a particular feature (such as C<socket> or C<symlink>)
1563 is implemented. It is also Perl's exception trapping mechanism, where
1564 the die operator is used to raise exceptions.
1566 If the code to be executed doesn't vary, you may use the eval-BLOCK
1567 form to trap run-time errors without incurring the penalty of
1568 recompiling each time. The error, if any, is still returned in C<$@>.
1571 # make divide-by-zero nonfatal
1572 eval { $answer = $a / $b; }; warn $@ if $@;
1574 # same thing, but less efficient
1575 eval '$answer = $a / $b'; warn $@ if $@;
1577 # a compile-time error
1578 eval { $answer = }; # WRONG
1581 eval '$answer ='; # sets $@
1583 Using the C<eval{}> form as an exception trap in libraries does have some
1584 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1585 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1586 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1587 as shown in this example:
1589 # a very private exception trap for divide-by-zero
1590 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1593 This is especially significant, given that C<__DIE__> hooks can call
1594 C<die> again, which has the effect of changing their error messages:
1596 # __DIE__ hooks may modify error messages
1598 local $SIG{'__DIE__'} =
1599 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1600 eval { die "foo lives here" };
1601 print $@ if $@; # prints "bar lives here"
1604 Because this promotes action at a distance, this counterintuitive behavior
1605 may be fixed in a future release.
1607 With an C<eval>, you should be especially careful to remember what's
1608 being looked at when:
1614 eval { $x }; # CASE 4
1616 eval "\$$x++"; # CASE 5
1619 Cases 1 and 2 above behave identically: they run the code contained in
1620 the variable $x. (Although case 2 has misleading double quotes making
1621 the reader wonder what else might be happening (nothing is).) Cases 3
1622 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1623 does nothing but return the value of $x. (Case 4 is preferred for
1624 purely visual reasons, but it also has the advantage of compiling at
1625 compile-time instead of at run-time.) Case 5 is a place where
1626 normally you I<would> like to use double quotes, except that in this
1627 particular situation, you can just use symbolic references instead, as
1630 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1631 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1633 Note that as a very special case, an C<eval ''> executed within the C<DB>
1634 package doesn't see the usual surrounding lexical scope, but rather the
1635 scope of the first non-DB piece of code that called it. You don't normally
1636 need to worry about this unless you are writing a Perl debugger.
1641 =item exec PROGRAM LIST
1643 The C<exec> function executes a system command I<and never returns>--
1644 use C<system> instead of C<exec> if you want it to return. It fails and
1645 returns false only if the command does not exist I<and> it is executed
1646 directly instead of via your system's command shell (see below).
1648 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1649 warns you if there is a following statement which isn't C<die>, C<warn>,
1650 or C<exit> (if C<-w> is set - but you always do that). If you
1651 I<really> want to follow an C<exec> with some other statement, you
1652 can use one of these styles to avoid the warning:
1654 exec ('foo') or print STDERR "couldn't exec foo: $!";
1655 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1657 If there is more than one argument in LIST, or if LIST is an array
1658 with more than one value, calls execvp(3) with the arguments in LIST.
1659 If there is only one scalar argument or an array with one element in it,
1660 the argument is checked for shell metacharacters, and if there are any,
1661 the entire argument is passed to the system's command shell for parsing
1662 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1663 If there are no shell metacharacters in the argument, it is split into
1664 words and passed directly to C<execvp>, which is more efficient.
1667 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1668 exec "sort $outfile | uniq";
1670 If you don't really want to execute the first argument, but want to lie
1671 to the program you are executing about its own name, you can specify
1672 the program you actually want to run as an "indirect object" (without a
1673 comma) in front of the LIST. (This always forces interpretation of the
1674 LIST as a multivalued list, even if there is only a single scalar in
1677 $shell = '/bin/csh';
1678 exec $shell '-sh'; # pretend it's a login shell
1682 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1684 When the arguments get executed via the system shell, results will
1685 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1688 Using an indirect object with C<exec> or C<system> is also more
1689 secure. This usage (which also works fine with system()) forces
1690 interpretation of the arguments as a multivalued list, even if the
1691 list had just one argument. That way you're safe from the shell
1692 expanding wildcards or splitting up words with whitespace in them.
1694 @args = ( "echo surprise" );
1696 exec @args; # subject to shell escapes
1698 exec { $args[0] } @args; # safe even with one-arg list
1700 The first version, the one without the indirect object, ran the I<echo>
1701 program, passing it C<"surprise"> an argument. The second version
1702 didn't--it tried to run a program literally called I<"echo surprise">,
1703 didn't find it, and set C<$?> to a non-zero value indicating failure.
1705 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1706 output before the exec, but this may not be supported on some platforms
1707 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1708 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1709 open handles in order to avoid lost output.
1711 Note that C<exec> will not call your C<END> blocks, nor will it call
1712 any C<DESTROY> methods in your objects.
1715 X<exists> X<autovivification>
1717 Given an expression that specifies a hash element or array element,
1718 returns true if the specified element in the hash or array has ever
1719 been initialized, even if the corresponding value is undefined. The
1720 element is not autovivified if it doesn't exist.
1722 print "Exists\n" if exists $hash{$key};
1723 print "Defined\n" if defined $hash{$key};
1724 print "True\n" if $hash{$key};
1726 print "Exists\n" if exists $array[$index];
1727 print "Defined\n" if defined $array[$index];
1728 print "True\n" if $array[$index];
1730 A hash or array element can be true only if it's defined, and defined if
1731 it exists, but the reverse doesn't necessarily hold true.
1733 Given an expression that specifies the name of a subroutine,
1734 returns true if the specified subroutine has ever been declared, even
1735 if it is undefined. Mentioning a subroutine name for exists or defined
1736 does not count as declaring it. Note that a subroutine which does not
1737 exist may still be callable: its package may have an C<AUTOLOAD>
1738 method that makes it spring into existence the first time that it is
1739 called -- see L<perlsub>.
1741 print "Exists\n" if exists &subroutine;
1742 print "Defined\n" if defined &subroutine;
1744 Note that the EXPR can be arbitrarily complicated as long as the final
1745 operation is a hash or array key lookup or subroutine name:
1747 if (exists $ref->{A}->{B}->{$key}) { }
1748 if (exists $hash{A}{B}{$key}) { }
1750 if (exists $ref->{A}->{B}->[$ix]) { }
1751 if (exists $hash{A}{B}[$ix]) { }
1753 if (exists &{$ref->{A}{B}{$key}}) { }
1755 Although the deepest nested array or hash will not spring into existence
1756 just because its existence was tested, any intervening ones will.
1757 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1758 into existence due to the existence test for the $key element above.
1759 This happens anywhere the arrow operator is used, including even:
1762 if (exists $ref->{"Some key"}) { }
1763 print $ref; # prints HASH(0x80d3d5c)
1765 This surprising autovivification in what does not at first--or even
1766 second--glance appear to be an lvalue context may be fixed in a future
1769 Use of a subroutine call, rather than a subroutine name, as an argument
1770 to exists() is an error.
1773 exists &sub(); # Error
1776 X<exit> X<terminate> X<abort>
1780 Evaluates EXPR and exits immediately with that value. Example:
1783 exit 0 if $ans =~ /^[Xx]/;
1785 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1786 universally recognized values for EXPR are C<0> for success and C<1>
1787 for error; other values are subject to interpretation depending on the
1788 environment in which the Perl program is running. For example, exiting
1789 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1790 the mailer to return the item undelivered, but that's not true everywhere.
1792 Don't use C<exit> to abort a subroutine if there's any chance that
1793 someone might want to trap whatever error happened. Use C<die> instead,
1794 which can be trapped by an C<eval>.
1796 The exit() function does not always exit immediately. It calls any
1797 defined C<END> routines first, but these C<END> routines may not
1798 themselves abort the exit. Likewise any object destructors that need to
1799 be called are called before the real exit. If this is a problem, you
1800 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1801 See L<perlmod> for details.
1804 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1808 Returns I<e> (the natural logarithm base) to the power of EXPR.
1809 If EXPR is omitted, gives C<exp($_)>.
1811 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1814 Implements the fcntl(2) function. You'll probably have to say
1818 first to get the correct constant definitions. Argument processing and
1819 value return works just like C<ioctl> below.
1823 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1824 or die "can't fcntl F_GETFL: $!";
1826 You don't have to check for C<defined> on the return from C<fcntl>.
1827 Like C<ioctl>, it maps a C<0> return from the system call into
1828 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1829 in numeric context. It is also exempt from the normal B<-w> warnings
1830 on improper numeric conversions.
1832 Note that C<fcntl> will produce a fatal error if used on a machine that
1833 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1834 manpage to learn what functions are available on your system.
1836 Here's an example of setting a filehandle named C<REMOTE> to be
1837 non-blocking at the system level. You'll have to negotiate C<$|>
1838 on your own, though.
1840 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1842 $flags = fcntl(REMOTE, F_GETFL, 0)
1843 or die "Can't get flags for the socket: $!\n";
1845 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1846 or die "Can't set flags for the socket: $!\n";
1848 =item fileno FILEHANDLE
1851 Returns the file descriptor for a filehandle, or undefined if the
1852 filehandle is not open. This is mainly useful for constructing
1853 bitmaps for C<select> and low-level POSIX tty-handling operations.
1854 If FILEHANDLE is an expression, the value is taken as an indirect
1855 filehandle, generally its name.
1857 You can use this to find out whether two handles refer to the
1858 same underlying descriptor:
1860 if (fileno(THIS) == fileno(THAT)) {
1861 print "THIS and THAT are dups\n";
1864 (Filehandles connected to memory objects via new features of C<open> may
1865 return undefined even though they are open.)
1868 =item flock FILEHANDLE,OPERATION
1869 X<flock> X<lock> X<locking>
1871 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1872 for success, false on failure. Produces a fatal error if used on a
1873 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1874 C<flock> is Perl's portable file locking interface, although it locks
1875 only entire files, not records.
1877 Two potentially non-obvious but traditional C<flock> semantics are
1878 that it waits indefinitely until the lock is granted, and that its locks
1879 B<merely advisory>. Such discretionary locks are more flexible, but offer
1880 fewer guarantees. This means that programs that do not also use C<flock>
1881 may modify files locked with C<flock>. See L<perlport>,
1882 your port's specific documentation, or your system-specific local manpages
1883 for details. It's best to assume traditional behavior if you're writing
1884 portable programs. (But if you're not, you should as always feel perfectly
1885 free to write for your own system's idiosyncrasies (sometimes called
1886 "features"). Slavish adherence to portability concerns shouldn't get
1887 in the way of your getting your job done.)
1889 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1890 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1891 you can use the symbolic names if you import them from the Fcntl module,
1892 either individually, or as a group using the ':flock' tag. LOCK_SH
1893 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1894 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1895 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1896 waiting for the lock (check the return status to see if you got it).
1898 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1899 before locking or unlocking it.
1901 Note that the emulation built with lockf(3) doesn't provide shared
1902 locks, and it requires that FILEHANDLE be open with write intent. These
1903 are the semantics that lockf(3) implements. Most if not all systems
1904 implement lockf(3) in terms of fcntl(2) locking, though, so the
1905 differing semantics shouldn't bite too many people.
1907 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1908 be open with read intent to use LOCK_SH and requires that it be open
1909 with write intent to use LOCK_EX.
1911 Note also that some versions of C<flock> cannot lock things over the
1912 network; you would need to use the more system-specific C<fcntl> for
1913 that. If you like you can force Perl to ignore your system's flock(2)
1914 function, and so provide its own fcntl(2)-based emulation, by passing
1915 the switch C<-Ud_flock> to the F<Configure> program when you configure
1918 Here's a mailbox appender for BSD systems.
1920 use Fcntl ':flock'; # import LOCK_* constants
1923 flock(MBOX,LOCK_EX);
1924 # and, in case someone appended
1925 # while we were waiting...
1930 flock(MBOX,LOCK_UN);
1933 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1934 or die "Can't open mailbox: $!";
1937 print MBOX $msg,"\n\n";
1940 On systems that support a real flock(), locks are inherited across fork()
1941 calls, whereas those that must resort to the more capricious fcntl()
1942 function lose the locks, making it harder to write servers.
1944 See also L<DB_File> for other flock() examples.
1947 X<fork> X<child> X<parent>
1949 Does a fork(2) system call to create a new process running the
1950 same program at the same point. It returns the child pid to the
1951 parent process, C<0> to the child process, or C<undef> if the fork is
1952 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1953 are shared, while everything else is copied. On most systems supporting
1954 fork(), great care has gone into making it extremely efficient (for
1955 example, using copy-on-write technology on data pages), making it the
1956 dominant paradigm for multitasking over the last few decades.
1958 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1959 output before forking the child process, but this may not be supported
1960 on some platforms (see L<perlport>). To be safe, you may need to set
1961 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1962 C<IO::Handle> on any open handles in order to avoid duplicate output.
1964 If you C<fork> without ever waiting on your children, you will
1965 accumulate zombies. On some systems, you can avoid this by setting
1966 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1967 forking and reaping moribund children.
1969 Note that if your forked child inherits system file descriptors like
1970 STDIN and STDOUT that are actually connected by a pipe or socket, even
1971 if you exit, then the remote server (such as, say, a CGI script or a
1972 backgrounded job launched from a remote shell) won't think you're done.
1973 You should reopen those to F</dev/null> if it's any issue.
1978 Declare a picture format for use by the C<write> function. For
1982 Test: @<<<<<<<< @||||| @>>>>>
1983 $str, $%, '$' . int($num)
1987 $num = $cost/$quantity;
1991 See L<perlform> for many details and examples.
1993 =item formline PICTURE,LIST
1996 This is an internal function used by C<format>s, though you may call it,
1997 too. It formats (see L<perlform>) a list of values according to the
1998 contents of PICTURE, placing the output into the format output
1999 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2000 Eventually, when a C<write> is done, the contents of
2001 C<$^A> are written to some filehandle. You could also read C<$^A>
2002 and then set C<$^A> back to C<"">. Note that a format typically
2003 does one C<formline> per line of form, but the C<formline> function itself
2004 doesn't care how many newlines are embedded in the PICTURE. This means
2005 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
2006 You may therefore need to use multiple formlines to implement a single
2007 record format, just like the format compiler.
2009 Be careful if you put double quotes around the picture, because an C<@>
2010 character may be taken to mean the beginning of an array name.
2011 C<formline> always returns true. See L<perlform> for other examples.
2013 =item getc FILEHANDLE
2014 X<getc> X<getchar> X<character> X<file, read>
2018 Returns the next character from the input file attached to FILEHANDLE,
2019 or the undefined value at end of file, or if there was an error (in
2020 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2021 STDIN. This is not particularly efficient. However, it cannot be
2022 used by itself to fetch single characters without waiting for the user
2023 to hit enter. For that, try something more like:
2026 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2029 system "stty", '-icanon', 'eol', "\001";
2035 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2038 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2042 Determination of whether $BSD_STYLE should be set
2043 is left as an exercise to the reader.
2045 The C<POSIX::getattr> function can do this more portably on
2046 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2047 module from your nearest CPAN site; details on CPAN can be found on
2051 X<getlogin> X<login>
2053 This implements the C library function of the same name, which on most
2054 systems returns the current login from F</etc/utmp>, if any. If null,
2057 $login = getlogin || getpwuid($<) || "Kilroy";
2059 Do not consider C<getlogin> for authentication: it is not as
2060 secure as C<getpwuid>.
2062 =item getpeername SOCKET
2063 X<getpeername> X<peer>
2065 Returns the packed sockaddr address of other end of the SOCKET connection.
2068 $hersockaddr = getpeername(SOCK);
2069 ($port, $iaddr) = sockaddr_in($hersockaddr);
2070 $herhostname = gethostbyaddr($iaddr, AF_INET);
2071 $herstraddr = inet_ntoa($iaddr);
2076 Returns the current process group for the specified PID. Use
2077 a PID of C<0> to get the current process group for the
2078 current process. Will raise an exception if used on a machine that
2079 doesn't implement getpgrp(2). If PID is omitted, returns process
2080 group of current process. Note that the POSIX version of C<getpgrp>
2081 does not accept a PID argument, so only C<PID==0> is truly portable.
2084 X<getppid> X<parent> X<pid>
2086 Returns the process id of the parent process.
2088 Note for Linux users: on Linux, the C functions C<getpid()> and
2089 C<getppid()> return different values from different threads. In order to
2090 be portable, this behavior is not reflected by the perl-level function
2091 C<getppid()>, that returns a consistent value across threads. If you want
2092 to call the underlying C<getppid()>, you may use the CPAN module
2095 =item getpriority WHICH,WHO
2096 X<getpriority> X<priority> X<nice>
2098 Returns the current priority for a process, a process group, or a user.
2099 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2100 machine that doesn't implement getpriority(2).
2103 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2104 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2105 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2106 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2107 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2108 X<endnetent> X<endprotoent> X<endservent>
2112 =item gethostbyname NAME
2114 =item getnetbyname NAME
2116 =item getprotobyname NAME
2122 =item getservbyname NAME,PROTO
2124 =item gethostbyaddr ADDR,ADDRTYPE
2126 =item getnetbyaddr ADDR,ADDRTYPE
2128 =item getprotobynumber NUMBER
2130 =item getservbyport PORT,PROTO
2148 =item sethostent STAYOPEN
2150 =item setnetent STAYOPEN
2152 =item setprotoent STAYOPEN
2154 =item setservent STAYOPEN
2168 These routines perform the same functions as their counterparts in the
2169 system library. In list context, the return values from the
2170 various get routines are as follows:
2172 ($name,$passwd,$uid,$gid,
2173 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2174 ($name,$passwd,$gid,$members) = getgr*
2175 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2176 ($name,$aliases,$addrtype,$net) = getnet*
2177 ($name,$aliases,$proto) = getproto*
2178 ($name,$aliases,$port,$proto) = getserv*
2180 (If the entry doesn't exist you get a null list.)
2182 The exact meaning of the $gcos field varies but it usually contains
2183 the real name of the user (as opposed to the login name) and other
2184 information pertaining to the user. Beware, however, that in many
2185 system users are able to change this information and therefore it
2186 cannot be trusted and therefore the $gcos is tainted (see
2187 L<perlsec>). The $passwd and $shell, user's encrypted password and
2188 login shell, are also tainted, because of the same reason.
2190 In scalar context, you get the name, unless the function was a
2191 lookup by name, in which case you get the other thing, whatever it is.
2192 (If the entry doesn't exist you get the undefined value.) For example:
2194 $uid = getpwnam($name);
2195 $name = getpwuid($num);
2197 $gid = getgrnam($name);
2198 $name = getgrgid($num);
2202 In I<getpw*()> the fields $quota, $comment, and $expire are special
2203 cases in the sense that in many systems they are unsupported. If the
2204 $quota is unsupported, it is an empty scalar. If it is supported, it
2205 usually encodes the disk quota. If the $comment field is unsupported,
2206 it is an empty scalar. If it is supported it usually encodes some
2207 administrative comment about the user. In some systems the $quota
2208 field may be $change or $age, fields that have to do with password
2209 aging. In some systems the $comment field may be $class. The $expire
2210 field, if present, encodes the expiration period of the account or the
2211 password. For the availability and the exact meaning of these fields
2212 in your system, please consult your getpwnam(3) documentation and your
2213 F<pwd.h> file. You can also find out from within Perl what your
2214 $quota and $comment fields mean and whether you have the $expire field
2215 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2216 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2217 files are only supported if your vendor has implemented them in the
2218 intuitive fashion that calling the regular C library routines gets the
2219 shadow versions if you're running under privilege or if there exists
2220 the shadow(3) functions as found in System V (this includes Solaris
2221 and Linux.) Those systems that implement a proprietary shadow password
2222 facility are unlikely to be supported.
2224 The $members value returned by I<getgr*()> is a space separated list of
2225 the login names of the members of the group.
2227 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2228 C, it will be returned to you via C<$?> if the function call fails. The
2229 C<@addrs> value returned by a successful call is a list of the raw
2230 addresses returned by the corresponding system library call. In the
2231 Internet domain, each address is four bytes long and you can unpack it
2232 by saying something like:
2234 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2236 The Socket library makes this slightly easier:
2239 $iaddr = inet_aton("127.1"); # or whatever address
2240 $name = gethostbyaddr($iaddr, AF_INET);
2242 # or going the other way
2243 $straddr = inet_ntoa($iaddr);
2245 If you get tired of remembering which element of the return list
2246 contains which return value, by-name interfaces are provided
2247 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2248 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2249 and C<User::grent>. These override the normal built-ins, supplying
2250 versions that return objects with the appropriate names
2251 for each field. For example:
2255 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2257 Even though it looks like they're the same method calls (uid),
2258 they aren't, because a C<File::stat> object is different from
2259 a C<User::pwent> object.
2261 =item getsockname SOCKET
2264 Returns the packed sockaddr address of this end of the SOCKET connection,
2265 in case you don't know the address because you have several different
2266 IPs that the connection might have come in on.
2269 $mysockaddr = getsockname(SOCK);
2270 ($port, $myaddr) = sockaddr_in($mysockaddr);
2271 printf "Connect to %s [%s]\n",
2272 scalar gethostbyaddr($myaddr, AF_INET),
2275 =item getsockopt SOCKET,LEVEL,OPTNAME
2278 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2279 Options may exist at multiple protocol levels depending on the socket
2280 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2281 C<Socket> module) will exist. To query options at another level the
2282 protocol number of the appropriate protocol controlling the option
2283 should be supplied. For example, to indicate that an option is to be
2284 interpreted by the TCP protocol, LEVEL should be set to the protocol
2285 number of TCP, which you can get using getprotobyname.
2287 The call returns a packed string representing the requested socket option,
2288 or C<undef> if there is an error (the error reason will be in $!). What
2289 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2290 your system documentation for details. A very common case however is that
2291 the option is an integer, in which case the result will be a packed
2292 integer which you can decode using unpack with the C<i> (or C<I>) format.
2294 An example testing if Nagle's algorithm is turned on on a socket:
2296 use Socket qw(:all);
2298 defined(my $tcp = getprotobyname("tcp"))
2299 or die "Could not determine the protocol number for tcp";
2300 # my $tcp = IPPROTO_TCP; # Alternative
2301 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2302 or die "Could not query TCP_NODELAY socket option: $!";
2303 my $nodelay = unpack("I", $packed);
2304 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2308 X<glob> X<wildcard> X<filename, expansion> X<expand>
2312 In list context, returns a (possibly empty) list of filename expansions on
2313 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2314 scalar context, glob iterates through such filename expansions, returning
2315 undef when the list is exhausted. This is the internal function
2316 implementing the C<< <*.c> >> operator, but you can use it directly. If
2317 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2318 more detail in L<perlop/"I/O Operators">.
2320 Beginning with v5.6.0, this operator is implemented using the standard
2321 C<File::Glob> extension. See L<File::Glob> for details.
2324 X<gmtime> X<UTC> X<Greenwich>
2328 Works just like L<localtime> but the returned values are
2329 localized for the standard Greenwich time zone.
2331 Note: when called in list context, $isdst, the last value
2332 returned by gmtime is always C<0>. There is no
2333 Daylight Saving Time in GMT.
2335 See L<perlport/gmtime> for portability concerns.
2338 X<goto> X<jump> X<jmp>
2344 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2345 execution there. It may not be used to go into any construct that
2346 requires initialization, such as a subroutine or a C<foreach> loop. It
2347 also can't be used to go into a construct that is optimized away,
2348 or to get out of a block or subroutine given to C<sort>.
2349 It can be used to go almost anywhere else within the dynamic scope,
2350 including out of subroutines, but it's usually better to use some other
2351 construct such as C<last> or C<die>. The author of Perl has never felt the
2352 need to use this form of C<goto> (in Perl, that is--C is another matter).
2353 (The difference being that C does not offer named loops combined with
2354 loop control. Perl does, and this replaces most structured uses of C<goto>
2355 in other languages.)
2357 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2358 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2359 necessarily recommended if you're optimizing for maintainability:
2361 goto ("FOO", "BAR", "GLARCH")[$i];
2363 The C<goto-&NAME> form is quite different from the other forms of
2364 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2365 doesn't have the stigma associated with other gotos. Instead, it
2366 exits the current subroutine (losing any changes set by local()) and
2367 immediately calls in its place the named subroutine using the current
2368 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2369 load another subroutine and then pretend that the other subroutine had
2370 been called in the first place (except that any modifications to C<@_>
2371 in the current subroutine are propagated to the other subroutine.)
2372 After the C<goto>, not even C<caller> will be able to tell that this
2373 routine was called first.
2375 NAME needn't be the name of a subroutine; it can be a scalar variable
2376 containing a code reference, or a block that evaluates to a code
2379 =item grep BLOCK LIST
2382 =item grep EXPR,LIST
2384 This is similar in spirit to, but not the same as, grep(1) and its
2385 relatives. In particular, it is not limited to using regular expressions.
2387 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2388 C<$_> to each element) and returns the list value consisting of those
2389 elements for which the expression evaluated to true. In scalar
2390 context, returns the number of times the expression was true.
2392 @foo = grep(!/^#/, @bar); # weed out comments
2396 @foo = grep {!/^#/} @bar; # weed out comments
2398 Note that C<$_> is an alias to the list value, so it can be used to
2399 modify the elements of the LIST. While this is useful and supported,
2400 it can cause bizarre results if the elements of LIST are not variables.
2401 Similarly, grep returns aliases into the original list, much as a for
2402 loop's index variable aliases the list elements. That is, modifying an
2403 element of a list returned by grep (for example, in a C<foreach>, C<map>
2404 or another C<grep>) actually modifies the element in the original list.
2405 This is usually something to be avoided when writing clear code.
2407 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2408 been declared with C<my $_>) then, in addition to being locally aliased to
2409 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2410 can't be seen from the outside, avoiding any potential side-effects.
2412 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2415 X<hex> X<hexadecimal>
2419 Interprets EXPR as a hex string and returns the corresponding value.
2420 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2421 L</oct>.) If EXPR is omitted, uses C<$_>.
2423 print hex '0xAf'; # prints '175'
2424 print hex 'aF'; # same
2426 Hex strings may only represent integers. Strings that would cause
2427 integer overflow trigger a warning. Leading whitespace is not stripped,
2428 unlike oct(). To present something as hex, look into L</printf>,
2429 L</sprintf>, or L</unpack>.
2434 There is no builtin C<import> function. It is just an ordinary
2435 method (subroutine) defined (or inherited) by modules that wish to export
2436 names to another module. The C<use> function calls the C<import> method
2437 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2439 =item index STR,SUBSTR,POSITION
2440 X<index> X<indexOf> X<InStr>
2442 =item index STR,SUBSTR
2444 The index function searches for one string within another, but without
2445 the wildcard-like behavior of a full regular-expression pattern match.
2446 It returns the position of the first occurrence of SUBSTR in STR at
2447 or after POSITION. If POSITION is omitted, starts searching from the
2448 beginning of the string. POSITION before the beginning of the string
2449 or after its end is treated as if it were the beginning or the end,
2450 respectively. POSITION and the return value are based at C<0> (or whatever
2451 you've set the C<$[> variable to--but don't do that). If the substring
2452 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2455 X<int> X<integer> X<truncate> X<trunc> X<floor>
2459 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2460 You should not use this function for rounding: one because it truncates
2461 towards C<0>, and two because machine representations of floating point
2462 numbers can sometimes produce counterintuitive results. For example,
2463 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2464 because it's really more like -268.99999999999994315658 instead. Usually,
2465 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2466 functions will serve you better than will int().
2468 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2471 Implements the ioctl(2) function. You'll probably first have to say
2473 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2475 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2476 exist or doesn't have the correct definitions you'll have to roll your
2477 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2478 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2479 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2480 written depending on the FUNCTION--a pointer to the string value of SCALAR
2481 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2482 has no string value but does have a numeric value, that value will be
2483 passed rather than a pointer to the string value. To guarantee this to be
2484 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2485 functions may be needed to manipulate the values of structures used by
2488 The return value of C<ioctl> (and C<fcntl>) is as follows:
2490 if OS returns: then Perl returns:
2492 0 string "0 but true"
2493 anything else that number
2495 Thus Perl returns true on success and false on failure, yet you can
2496 still easily determine the actual value returned by the operating
2499 $retval = ioctl(...) || -1;
2500 printf "System returned %d\n", $retval;
2502 The special string C<"0 but true"> is exempt from B<-w> complaints
2503 about improper numeric conversions.
2505 =item join EXPR,LIST
2508 Joins the separate strings of LIST into a single string with fields
2509 separated by the value of EXPR, and returns that new string. Example:
2511 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2513 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2514 first argument. Compare L</split>.
2519 Returns a list consisting of all the keys of the named hash.
2520 (In scalar context, returns the number of keys.)
2522 The keys are returned in an apparently random order. The actual
2523 random order is subject to change in future versions of perl, but it
2524 is guaranteed to be the same order as either the C<values> or C<each>
2525 function produces (given that the hash has not been modified). Since
2526 Perl 5.8.1 the ordering is different even between different runs of
2527 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2530 As a side effect, calling keys() resets the HASH's internal iterator
2531 (see L</each>). In particular, calling keys() in void context resets
2532 the iterator with no other overhead.
2534 Here is yet another way to print your environment:
2537 @values = values %ENV;
2539 print pop(@keys), '=', pop(@values), "\n";
2542 or how about sorted by key:
2544 foreach $key (sort(keys %ENV)) {
2545 print $key, '=', $ENV{$key}, "\n";
2548 The returned values are copies of the original keys in the hash, so
2549 modifying them will not affect the original hash. Compare L</values>.
2551 To sort a hash by value, you'll need to use a C<sort> function.
2552 Here's a descending numeric sort of a hash by its values:
2554 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2555 printf "%4d %s\n", $hash{$key}, $key;
2558 As an lvalue C<keys> allows you to increase the number of hash buckets
2559 allocated for the given hash. This can gain you a measure of efficiency if
2560 you know the hash is going to get big. (This is similar to pre-extending
2561 an array by assigning a larger number to $#array.) If you say
2565 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2566 in fact, since it rounds up to the next power of two. These
2567 buckets will be retained even if you do C<%hash = ()>, use C<undef
2568 %hash> if you want to free the storage while C<%hash> is still in scope.
2569 You can't shrink the number of buckets allocated for the hash using
2570 C<keys> in this way (but you needn't worry about doing this by accident,
2571 as trying has no effect).
2573 See also C<each>, C<values> and C<sort>.
2575 =item kill SIGNAL, LIST
2578 Sends a signal to a list of processes. Returns the number of
2579 processes successfully signaled (which is not necessarily the
2580 same as the number actually killed).
2582 $cnt = kill 1, $child1, $child2;
2585 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2586 system call will check whether it's possible to send a signal to it (that
2587 means, to be brief, that the process is owned by the same user, or we are
2588 the super-user). This is a useful way to check that a child process is
2589 alive (even if only as a zombie) and hasn't changed its UID. See
2590 L<perlport> for notes on the portability of this construct.
2592 Unlike in the shell, if SIGNAL is negative, it kills
2593 process groups instead of processes. (On System V, a negative I<PROCESS>
2594 number will also kill process groups, but that's not portable.) That
2595 means you usually want to use positive not negative signals. You may also
2596 use a signal name in quotes.
2598 See L<perlipc/"Signals"> for more details.
2605 The C<last> command is like the C<break> statement in C (as used in
2606 loops); it immediately exits the loop in question. If the LABEL is
2607 omitted, the command refers to the innermost enclosing loop. The
2608 C<continue> block, if any, is not executed:
2610 LINE: while (<STDIN>) {
2611 last LINE if /^$/; # exit when done with header
2615 C<last> cannot be used to exit a block which returns a value such as
2616 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2617 a grep() or map() operation.
2619 Note that a block by itself is semantically identical to a loop
2620 that executes once. Thus C<last> can be used to effect an early
2621 exit out of such a block.
2623 See also L</continue> for an illustration of how C<last>, C<next>, and
2631 Returns a lowercased version of EXPR. This is the internal function
2632 implementing the C<\L> escape in double-quoted strings. Respects
2633 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2634 and L<perlunicode> for more details about locale and Unicode support.
2636 If EXPR is omitted, uses C<$_>.
2639 X<lcfirst> X<lowercase>
2643 Returns the value of EXPR with the first character lowercased. This
2644 is the internal function implementing the C<\l> escape in
2645 double-quoted strings. Respects current LC_CTYPE locale if C<use
2646 locale> in force. See L<perllocale> and L<perlunicode> for more
2647 details about locale and Unicode support.
2649 If EXPR is omitted, uses C<$_>.
2656 Returns the length in I<characters> of the value of EXPR. If EXPR is
2657 omitted, returns length of C<$_>. Note that this cannot be used on
2658 an entire array or hash to find out how many elements these have.
2659 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2661 Note the I<characters>: if the EXPR is in Unicode, you will get the
2662 number of characters, not the number of bytes. To get the length
2663 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2665 =item link OLDFILE,NEWFILE
2668 Creates a new filename linked to the old filename. Returns true for
2669 success, false otherwise.
2671 =item listen SOCKET,QUEUESIZE
2674 Does the same thing that the listen system call does. Returns true if
2675 it succeeded, false otherwise. See the example in
2676 L<perlipc/"Sockets: Client/Server Communication">.
2681 You really probably want to be using C<my> instead, because C<local> isn't
2682 what most people think of as "local". See
2683 L<perlsub/"Private Variables via my()"> for details.
2685 A local modifies the listed variables to be local to the enclosing
2686 block, file, or eval. If more than one value is listed, the list must
2687 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2688 for details, including issues with tied arrays and hashes.
2690 =item localtime EXPR
2691 X<localtime> X<ctime>
2695 Converts a time as returned by the time function to a 9-element list
2696 with the time analyzed for the local time zone. Typically used as
2700 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2703 All list elements are numeric, and come straight out of the C `struct
2704 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2705 of the specified time.
2707 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2708 the range C<0..11> with 0 indicating January and 11 indicating December.
2709 This makes it easy to get a month name from a list:
2711 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2712 print "$abbr[$mon] $mday";
2713 # $mon=9, $mday=18 gives "Oct 18"
2715 C<$year> is the number of years since 1900, not just the last two digits
2716 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2717 to get a complete 4-digit year is simply:
2721 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2722 to do that, would you?
2724 To get the last two digits of the year (e.g., '01' in 2001) do:
2726 $year = sprintf("%02d", $year % 100);
2728 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2729 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2730 (or C<0..365> in leap years.)
2732 C<$isdst> is true if the specified time occurs during Daylight Saving
2733 Time, false otherwise.
2735 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2737 In scalar context, C<localtime()> returns the ctime(3) value:
2739 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2741 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2742 instead of local time use the L</gmtime> builtin. See also the
2743 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2744 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2745 and mktime(3) functions.
2747 To get somewhat similar but locale dependent date strings, set up your
2748 locale environment variables appropriately (please see L<perllocale>) and
2751 use POSIX qw(strftime);
2752 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2753 # or for GMT formatted appropriately for your locale:
2754 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2756 Note that the C<%a> and C<%b>, the short forms of the day of the week
2757 and the month of the year, may not necessarily be three characters wide.
2759 See L<perlport/localtime> for portability concerns.
2761 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2762 by-name access mechanism to the gmtime() and localtime() functions,
2765 For a comprehensive date and time representation look at the
2766 L<DateTime> module on CPAN.
2771 This function places an advisory lock on a shared variable, or referenced
2772 object contained in I<THING> until the lock goes out of scope.
2774 lock() is a "weak keyword" : this means that if you've defined a function
2775 by this name (before any calls to it), that function will be called
2776 instead. (However, if you've said C<use threads>, lock() is always a
2777 keyword.) See L<threads>.
2780 X<log> X<logarithm> X<e> X<ln> X<base>
2784 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2785 returns log of C<$_>. To get the log of another base, use basic algebra:
2786 The base-N log of a number is equal to the natural log of that number
2787 divided by the natural log of N. For example:
2791 return log($n)/log(10);
2794 See also L</exp> for the inverse operation.
2801 Does the same thing as the C<stat> function (including setting the
2802 special C<_> filehandle) but stats a symbolic link instead of the file
2803 the symbolic link points to. If symbolic links are unimplemented on
2804 your system, a normal C<stat> is done. For much more detailed
2805 information, please see the documentation for C<stat>.
2807 If EXPR is omitted, stats C<$_>.
2811 The match operator. See L<perlop>.
2813 =item map BLOCK LIST
2818 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2819 C<$_> to each element) and returns the list value composed of the
2820 results of each such evaluation. In scalar context, returns the
2821 total number of elements so generated. Evaluates BLOCK or EXPR in
2822 list context, so each element of LIST may produce zero, one, or
2823 more elements in the returned value.
2825 @chars = map(chr, @nums);
2827 translates a list of numbers to the corresponding characters. And
2829 %hash = map { getkey($_) => $_ } @array;
2831 is just a funny way to write
2834 foreach $_ (@array) {
2835 $hash{getkey($_)} = $_;
2838 Note that C<$_> is an alias to the list value, so it can be used to
2839 modify the elements of the LIST. While this is useful and supported,
2840 it can cause bizarre results if the elements of LIST are not variables.
2841 Using a regular C<foreach> loop for this purpose would be clearer in
2842 most cases. See also L</grep> for an array composed of those items of
2843 the original list for which the BLOCK or EXPR evaluates to true.
2845 If C<$_> is lexical in the scope where the C<map> appears (because it has
2846 been declared with C<my $_>) then, in addition to being locally aliased to
2847 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2848 can't be seen from the outside, avoiding any potential side-effects.
2850 C<{> starts both hash references and blocks, so C<map { ...> could be either
2851 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2852 ahead for the closing C<}> it has to take a guess at which its dealing with
2853 based what it finds just after the C<{>. Usually it gets it right, but if it
2854 doesn't it won't realize something is wrong until it gets to the C<}> and
2855 encounters the missing (or unexpected) comma. The syntax error will be
2856 reported close to the C<}> but you'll need to change something near the C<{>
2857 such as using a unary C<+> to give perl some help:
2859 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2860 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2861 %hash = map { ("\L$_", 1) } @array # this also works
2862 %hash = map { lc($_), 1 } @array # as does this.
2863 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2865 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2867 or to force an anon hash constructor use C<+{>
2869 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2871 and you get list of anonymous hashes each with only 1 entry.
2873 =item mkdir FILENAME,MASK
2874 X<mkdir> X<md> X<directory, create>
2876 =item mkdir FILENAME
2880 Creates the directory specified by FILENAME, with permissions
2881 specified by MASK (as modified by C<umask>). If it succeeds it
2882 returns true, otherwise it returns false and sets C<$!> (errno).
2883 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2886 In general, it is better to create directories with permissive MASK,
2887 and let the user modify that with their C<umask>, than it is to supply
2888 a restrictive MASK and give the user no way to be more permissive.
2889 The exceptions to this rule are when the file or directory should be
2890 kept private (mail files, for instance). The perlfunc(1) entry on
2891 C<umask> discusses the choice of MASK in more detail.
2893 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2894 number of trailing slashes. Some operating and filesystems do not get
2895 this right, so Perl automatically removes all trailing slashes to keep
2898 In order to recursively create a directory structure look at
2899 the C<mkpath> function of the L<File::Path> module.
2901 =item msgctl ID,CMD,ARG
2904 Calls the System V IPC function msgctl(2). You'll probably have to say
2908 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2909 then ARG must be a variable that will hold the returned C<msqid_ds>
2910 structure. Returns like C<ioctl>: the undefined value for error,
2911 C<"0 but true"> for zero, or the actual return value otherwise. See also
2912 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2914 =item msgget KEY,FLAGS
2917 Calls the System V IPC function msgget(2). Returns the message queue
2918 id, or the undefined value if there is an error. See also
2919 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2921 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2924 Calls the System V IPC function msgrcv to receive a message from
2925 message queue ID into variable VAR with a maximum message size of
2926 SIZE. Note that when a message is received, the message type as a
2927 native long integer will be the first thing in VAR, followed by the
2928 actual message. This packing may be opened with C<unpack("l! a*")>.
2929 Taints the variable. Returns true if successful, or false if there is
2930 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2931 C<IPC::SysV::Msg> documentation.
2933 =item msgsnd ID,MSG,FLAGS
2936 Calls the System V IPC function msgsnd to send the message MSG to the
2937 message queue ID. MSG must begin with the native long integer message
2938 type, and be followed by the length of the actual message, and finally
2939 the message itself. This kind of packing can be achieved with
2940 C<pack("l! a*", $type, $message)>. Returns true if successful,
2941 or false if there is an error. See also C<IPC::SysV>
2942 and C<IPC::SysV::Msg> documentation.
2949 =item my EXPR : ATTRS
2951 =item my TYPE EXPR : ATTRS
2953 A C<my> declares the listed variables to be local (lexically) to the
2954 enclosing block, file, or C<eval>. If more than one value is listed,
2955 the list must be placed in parentheses.
2957 The exact semantics and interface of TYPE and ATTRS are still
2958 evolving. TYPE is currently bound to the use of C<fields> pragma,
2959 and attributes are handled using the C<attributes> pragma, or starting
2960 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2961 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2962 L<attributes>, and L<Attribute::Handlers>.
2969 The C<next> command is like the C<continue> statement in C; it starts
2970 the next iteration of the loop:
2972 LINE: while (<STDIN>) {
2973 next LINE if /^#/; # discard comments
2977 Note that if there were a C<continue> block on the above, it would get
2978 executed even on discarded lines. If the LABEL is omitted, the command
2979 refers to the innermost enclosing loop.
2981 C<next> cannot be used to exit a block which returns a value such as
2982 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2983 a grep() or map() operation.
2985 Note that a block by itself is semantically identical to a loop
2986 that executes once. Thus C<next> will exit such a block early.
2988 See also L</continue> for an illustration of how C<last>, C<next>, and
2991 =item no Module VERSION LIST
2994 =item no Module VERSION
2996 =item no Module LIST
3000 See the C<use> function, of which C<no> is the opposite.
3003 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3007 Interprets EXPR as an octal string and returns the corresponding
3008 value. (If EXPR happens to start off with C<0x>, interprets it as a
3009 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3010 binary string. Leading whitespace is ignored in all three cases.)
3011 The following will handle decimal, binary, octal, and hex in the standard
3014 $val = oct($val) if $val =~ /^0/;
3016 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3017 in octal), use sprintf() or printf():
3019 $perms = (stat("filename"))[2] & 07777;
3020 $oct_perms = sprintf "%lo", $perms;
3022 The oct() function is commonly used when a string such as C<644> needs
3023 to be converted into a file mode, for example. (Although perl will
3024 automatically convert strings into numbers as needed, this automatic
3025 conversion assumes base 10.)
3027 =item open FILEHANDLE,EXPR
3028 X<open> X<pipe> X<file, open> X<fopen>
3030 =item open FILEHANDLE,MODE,EXPR
3032 =item open FILEHANDLE,MODE,EXPR,LIST
3034 =item open FILEHANDLE,MODE,REFERENCE
3036 =item open FILEHANDLE
3038 Opens the file whose filename is given by EXPR, and associates it with
3041 (The following is a comprehensive reference to open(): for a gentler
3042 introduction you may consider L<perlopentut>.)
3044 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3045 the variable is assigned a reference to a new anonymous filehandle,
3046 otherwise if FILEHANDLE is an expression, its value is used as the name of
3047 the real filehandle wanted. (This is considered a symbolic reference, so
3048 C<use strict 'refs'> should I<not> be in effect.)
3050 If EXPR is omitted, the scalar variable of the same name as the
3051 FILEHANDLE contains the filename. (Note that lexical variables--those
3052 declared with C<my>--will not work for this purpose; so if you're
3053 using C<my>, specify EXPR in your call to open.)
3055 If three or more arguments are specified then the mode of opening and
3056 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3057 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3058 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3059 the file is opened for appending, again being created if necessary.
3061 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3062 indicate that you want both read and write access to the file; thus
3063 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3064 '+>' >> mode would clobber the file first. You can't usually use
3065 either read-write mode for updating textfiles, since they have
3066 variable length records. See the B<-i> switch in L<perlrun> for a
3067 better approach. The file is created with permissions of C<0666>
3068 modified by the process' C<umask> value.
3070 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3071 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3073 In the 2-arguments (and 1-argument) form of the call the mode and
3074 filename should be concatenated (in this order), possibly separated by
3075 spaces. It is possible to omit the mode in these forms if the mode is
3078 If the filename begins with C<'|'>, the filename is interpreted as a
3079 command to which output is to be piped, and if the filename ends with a
3080 C<'|'>, the filename is interpreted as a command which pipes output to
3081 us. See L<perlipc/"Using open() for IPC">
3082 for more examples of this. (You are not allowed to C<open> to a command
3083 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3084 and L<perlipc/"Bidirectional Communication with Another Process">
3087 For three or more arguments if MODE is C<'|-'>, the filename is
3088 interpreted as a command to which output is to be piped, and if MODE
3089 is C<'-|'>, the filename is interpreted as a command which pipes
3090 output to us. In the 2-arguments (and 1-argument) form one should
3091 replace dash (C<'-'>) with the command.
3092 See L<perlipc/"Using open() for IPC"> for more examples of this.
3093 (You are not allowed to C<open> to a command that pipes both in I<and>
3094 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3095 L<perlipc/"Bidirectional Communication"> for alternatives.)
3097 In the three-or-more argument form of pipe opens, if LIST is specified
3098 (extra arguments after the command name) then LIST becomes arguments
3099 to the command invoked if the platform supports it. The meaning of
3100 C<open> with more than three arguments for non-pipe modes is not yet
3101 specified. Experimental "layers" may give extra LIST arguments
3104 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3105 and opening C<< '>-' >> opens STDOUT.
3107 You may use the three-argument form of open to specify IO "layers"
3108 (sometimes also referred to as "disciplines") to be applied to the handle
3109 that affect how the input and output are processed (see L<open> and
3110 L<PerlIO> for more details). For example
3112 open(FH, "<:utf8", "file")
3114 will open the UTF-8 encoded file containing Unicode characters,
3115 see L<perluniintro>. Note that if layers are specified in the
3116 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3117 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3119 Open returns nonzero upon success, the undefined value otherwise. If
3120 the C<open> involved a pipe, the return value happens to be the pid of
3123 If you're running Perl on a system that distinguishes between text
3124 files and binary files, then you should check out L</binmode> for tips
3125 for dealing with this. The key distinction between systems that need
3126 C<binmode> and those that don't is their text file formats. Systems
3127 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3128 character, and which encode that character in C as C<"\n">, do not
3129 need C<binmode>. The rest need it.
3131 When opening a file, it's usually a bad idea to continue normal execution
3132 if the request failed, so C<open> is frequently used in connection with
3133 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3134 where you want to make a nicely formatted error message (but there are
3135 modules that can help with that problem)) you should always check
3136 the return value from opening a file. The infrequent exception is when
3137 working with an unopened filehandle is actually what you want to do.
3139 As a special case the 3-arg form with a read/write mode and the third
3140 argument being C<undef>:
3142 open(TMP, "+>", undef) or die ...
3144 opens a filehandle to an anonymous temporary file. Also using "+<"
3145 works for symmetry, but you really should consider writing something
3146 to the temporary file first. You will need to seek() to do the
3149 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3150 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3151 "in memory" files held in Perl scalars via:
3153 open($fh, '>', \$variable) || ..
3155 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3156 file, you have to close it first:
3159 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3164 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3165 while (<ARTICLE>) {...
3167 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3168 # if the open fails, output is discarded
3170 open(DBASE, '+<', 'dbase.mine') # open for update
3171 or die "Can't open 'dbase.mine' for update: $!";
3173 open(DBASE, '+<dbase.mine') # ditto
3174 or die "Can't open 'dbase.mine' for update: $!";
3176 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3177 or die "Can't start caesar: $!";
3179 open(ARTICLE, "caesar <$article |") # ditto
3180 or die "Can't start caesar: $!";
3182 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3183 or die "Can't start sort: $!";
3186 open(MEMORY,'>', \$var)
3187 or die "Can't open memory file: $!";
3188 print MEMORY "foo!\n"; # output will end up in $var
3190 # process argument list of files along with any includes
3192 foreach $file (@ARGV) {
3193 process($file, 'fh00');
3197 my($filename, $input) = @_;
3198 $input++; # this is a string increment
3199 unless (open($input, $filename)) {
3200 print STDERR "Can't open $filename: $!\n";
3205 while (<$input>) { # note use of indirection
3206 if (/^#include "(.*)"/) {
3207 process($1, $input);
3214 See L<perliol> for detailed info on PerlIO.
3216 You may also, in the Bourne shell tradition, specify an EXPR beginning
3217 with C<< '>&' >>, in which case the rest of the string is interpreted
3218 as the name of a filehandle (or file descriptor, if numeric) to be
3219 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3220 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3221 The mode you specify should match the mode of the original filehandle.
3222 (Duping a filehandle does not take into account any existing contents
3223 of IO buffers.) If you use the 3-arg form then you can pass either a
3224 number, the name of a filehandle or the normal "reference to a glob".
3226 Here is a script that saves, redirects, and restores C<STDOUT> and
3227 C<STDERR> using various methods:
3230 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3231 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3233 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3234 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3236 select STDERR; $| = 1; # make unbuffered
3237 select STDOUT; $| = 1; # make unbuffered
3239 print STDOUT "stdout 1\n"; # this works for
3240 print STDERR "stderr 1\n"; # subprocesses too
3242 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3243 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3245 print STDOUT "stdout 2\n";
3246 print STDERR "stderr 2\n";
3248 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3249 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3250 that file descriptor (and not call L<dup(2)>); this is more
3251 parsimonious of file descriptors. For example:
3253 # open for input, reusing the fileno of $fd
3254 open(FILEHANDLE, "<&=$fd")
3258 open(FILEHANDLE, "<&=", $fd)
3262 # open for append, using the fileno of OLDFH
3263 open(FH, ">>&=", OLDFH)
3267 open(FH, ">>&=OLDFH")
3269 Being parsimonious on filehandles is also useful (besides being
3270 parsimonious) for example when something is dependent on file
3271 descriptors, like for example locking using flock(). If you do just
3272 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3273 descriptor as B, and therefore flock(A) will not flock(B), and vice
3274 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3275 the same file descriptor.
3277 Note that if you are using Perls older than 5.8.0, Perl will be using
3278 the standard C libraries' fdopen() to implement the "=" functionality.
3279 On many UNIX systems fdopen() fails when file descriptors exceed a
3280 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3281 most often the default.
3283 You can see whether Perl has been compiled with PerlIO or not by
3284 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3285 is C<define>, you have PerlIO, otherwise you don't.
3287 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3288 with 2-arguments (or 1-argument) form of open(), then
3289 there is an implicit fork done, and the return value of open is the pid
3290 of the child within the parent process, and C<0> within the child
3291 process. (Use C<defined($pid)> to determine whether the open was successful.)
3292 The filehandle behaves normally for the parent, but i/o to that
3293 filehandle is piped from/to the STDOUT/STDIN of the child process.
3294 In the child process the filehandle isn't opened--i/o happens from/to
3295 the new STDOUT or STDIN. Typically this is used like the normal
3296 piped open when you want to exercise more control over just how the
3297 pipe command gets executed, such as when you are running setuid, and
3298 don't want to have to scan shell commands for metacharacters.
3299 The following triples are more or less equivalent:
3301 open(FOO, "|tr '[a-z]' '[A-Z]'");
3302 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3303 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3304 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3306 open(FOO, "cat -n '$file'|");
3307 open(FOO, '-|', "cat -n '$file'");
3308 open(FOO, '-|') || exec 'cat', '-n', $file;
3309 open(FOO, '-|', "cat", '-n', $file);
3311 The last example in each block shows the pipe as "list form", which is
3312 not yet supported on all platforms. A good rule of thumb is that if
3313 your platform has true C<fork()> (in other words, if your platform is
3314 UNIX) you can use the list form.
3316 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3318 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3319 output before any operation that may do a fork, but this may not be
3320 supported on some platforms (see L<perlport>). To be safe, you may need
3321 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3322 of C<IO::Handle> on any open handles.
3324 On systems that support a close-on-exec flag on files, the flag will
3325 be set for the newly opened file descriptor as determined by the value
3326 of $^F. See L<perlvar/$^F>.
3328 Closing any piped filehandle causes the parent process to wait for the
3329 child to finish, and returns the status value in C<$?> and
3330 C<${^CHILD_ERROR_NATIVE}>.
3332 The filename passed to 2-argument (or 1-argument) form of open() will
3333 have leading and trailing whitespace deleted, and the normal
3334 redirection characters honored. This property, known as "magic open",
3335 can often be used to good effect. A user could specify a filename of
3336 F<"rsh cat file |">, or you could change certain filenames as needed:
3338 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3339 open(FH, $filename) or die "Can't open $filename: $!";
3341 Use 3-argument form to open a file with arbitrary weird characters in it,
3343 open(FOO, '<', $file);
3345 otherwise it's necessary to protect any leading and trailing whitespace:
3347 $file =~ s#^(\s)#./$1#;
3348 open(FOO, "< $file\0");
3350 (this may not work on some bizarre filesystems). One should
3351 conscientiously choose between the I<magic> and 3-arguments form
3356 will allow the user to specify an argument of the form C<"rsh cat file |">,
3357 but will not work on a filename which happens to have a trailing space, while
3359 open IN, '<', $ARGV[0];
3361 will have exactly the opposite restrictions.
3363 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3364 should use the C<sysopen> function, which involves no such magic (but
3365 may use subtly different filemodes than Perl open(), which is mapped
3366 to C fopen()). This is
3367 another way to protect your filenames from interpretation. For example:
3370 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3371 or die "sysopen $path: $!";
3372 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3373 print HANDLE "stuff $$\n";
3375 print "File contains: ", <HANDLE>;
3377 Using the constructor from the C<IO::Handle> package (or one of its
3378 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3379 filehandles that have the scope of whatever variables hold references to
3380 them, and automatically close whenever and however you leave that scope:
3384 sub read_myfile_munged {
3386 my $handle = new IO::File;
3387 open($handle, "myfile") or die "myfile: $!";
3389 or return (); # Automatically closed here.
3390 mung $first or die "mung failed"; # Or here.
3391 return $first, <$handle> if $ALL; # Or here.
3395 See L</seek> for some details about mixing reading and writing.
3397 =item opendir DIRHANDLE,EXPR
3400 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3401 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3402 DIRHANDLE may be an expression whose value can be used as an indirect
3403 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3404 scalar variable (or array or hash element), the variable is assigned a
3405 reference to a new anonymous dirhandle.
3406 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3413 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3414 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3417 For the reverse, see L</chr>.
3418 See L<perlunicode> and L<encoding> for more about Unicode.
3425 =item our EXPR : ATTRS
3427 =item our TYPE EXPR : ATTRS
3429 C<our> associates a simple name with a package variable in the current
3430 package for use within the current scope. When C<use strict 'vars'> is in
3431 effect, C<our> lets you use declared global variables without qualifying
3432 them with package names, within the lexical scope of the C<our> declaration.
3433 In this way C<our> differs from C<use vars>, which is package scoped.
3435 Unlike C<my>, which both allocates storage for a variable and associates
3436 a simple name with that storage for use within the current scope, C<our>
3437 associates a simple name with a package variable in the current package,
3438 for use within the current scope. In other words, C<our> has the same
3439 scoping rules as C<my>, but does not necessarily create a
3442 If more than one value is listed, the list must be placed
3448 An C<our> declaration declares a global variable that will be visible
3449 across its entire lexical scope, even across package boundaries. The
3450 package in which the variable is entered is determined at the point
3451 of the declaration, not at the point of use. This means the following
3455 our $bar; # declares $Foo::bar for rest of lexical scope
3459 print $bar; # prints 20, as it refers to $Foo::bar
3461 Multiple C<our> declarations with the same name in the same lexical
3462 scope are allowed if they are in different packages. If they happen
3463 to be in the same package, Perl will emit warnings if you have asked
3464 for them, just like multiple C<my> declarations. Unlike a second
3465 C<my> declaration, which will bind the name to a fresh variable, a
3466 second C<our> declaration in the same package, in the same scope, is
3471 our $bar; # declares $Foo::bar for rest of lexical scope
3475 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3476 print $bar; # prints 30
3478 our $bar; # emits warning but has no other effect
3479 print $bar; # still prints 30
3481 An C<our> declaration may also have a list of attributes associated
3484 The exact semantics and interface of TYPE and ATTRS are still
3485 evolving. TYPE is currently bound to the use of C<fields> pragma,
3486 and attributes are handled using the C<attributes> pragma, or starting
3487 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3488 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3489 L<attributes>, and L<Attribute::Handlers>.
3491 =item pack TEMPLATE,LIST
3494 Takes a LIST of values and converts it into a string using the rules
3495 given by the TEMPLATE. The resulting string is the concatenation of
3496 the converted values. Typically, each converted value looks
3497 like its machine-level representation. For example, on 32-bit machines
3498 an integer may be represented by a sequence of 4 bytes that will be
3499 converted to a sequence of 4 characters.
3501 The TEMPLATE is a sequence of characters that give the order and type
3502 of values, as follows:
3504 a A string with arbitrary binary data, will be null padded.
3505 A A text (ASCII) string, will be space padded.
3506 Z A null terminated (ASCIZ) string, will be null padded.
3508 b A bit string (ascending bit order inside each byte, like vec()).
3509 B A bit string (descending bit order inside each byte).
3510 h A hex string (low nybble first).
3511 H A hex string (high nybble first).
3513 c A signed char (8-bit) value.
3514 C An unsigned C char (octet) even under Unicode. Should normally not
3515 be used. See U and W instead.
3516 W An unsigned char value (can be greater than 255).
3518 s A signed short (16-bit) value.
3519 S An unsigned short value.
3521 l A signed long (32-bit) value.
3522 L An unsigned long value.
3524 q A signed quad (64-bit) value.
3525 Q An unsigned quad value.
3526 (Quads are available only if your system supports 64-bit
3527 integer values _and_ if Perl has been compiled to support those.
3528 Causes a fatal error otherwise.)
3530 i A signed integer value.
3531 I A unsigned integer value.
3532 (This 'integer' is _at_least_ 32 bits wide. Its exact
3533 size depends on what a local C compiler calls 'int'.)
3535 n An unsigned short (16-bit) in "network" (big-endian) order.
3536 N An unsigned long (32-bit) in "network" (big-endian) order.
3537 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3538 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3540 j A Perl internal signed integer value (IV).
3541 J A Perl internal unsigned integer value (UV).
3543 f A single-precision float in the native format.
3544 d A double-precision float in the native format.
3546 F A Perl internal floating point value (NV) in the native format
3547 D A long double-precision float in the native format.
3548 (Long doubles are available only if your system supports long
3549 double values _and_ if Perl has been compiled to support those.
3550 Causes a fatal error otherwise.)
3552 p A pointer to a null-terminated string.
3553 P A pointer to a structure (fixed-length string).
3555 u A uuencoded string.
3556 U A Unicode character number. Encodes to UTF-8 internally
3557 (or UTF-EBCDIC in EBCDIC platforms).
3559 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3560 details). Its bytes represent an unsigned integer in base 128,
3561 most significant digit first, with as few digits as possible. Bit
3562 eight (the high bit) is set on each byte except the last.
3566 @ Null fill or truncate to absolute position, counted from the
3567 start of the innermost ()-group.
3568 . Null fill or truncate to absolute position specified by value.
3569 ( Start of a ()-group.
3571 One or more of the modifiers below may optionally follow some letters in the
3572 TEMPLATE (the second column lists the letters for which the modifier is
3575 ! sSlLiI Forces native (short, long, int) sizes instead
3576 of fixed (16-/32-bit) sizes.
3578 xX Make x and X act as alignment commands.
3580 nNvV Treat integers as signed instead of unsigned.
3582 @. Specify position as byte offset in the internal
3583 representation of the packed string. Efficient but
3586 > sSiIlLqQ Force big-endian byte-order on the type.
3587 jJfFdDpP (The "big end" touches the construct.)
3589 < sSiIlLqQ Force little-endian byte-order on the type.
3590 jJfFdDpP (The "little end" touches the construct.)
3592 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3593 in which case they force a certain byte-order on all components of
3594 that group, including subgroups.
3596 The following rules apply:
3602 Each letter may optionally be followed by a number giving a repeat
3603 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3604 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3605 that many values from the LIST. A C<*> for the repeat count means to
3606 use however many items are left, except for C<@>, C<x>, C<X>, where it
3607 is equivalent to C<0>, for <.> where it means relative to string start
3608 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3609 A numeric repeat count may optionally be enclosed in brackets, as in
3610 C<pack 'C[80]', @arr>.
3612 One can replace the numeric repeat count by a template enclosed in brackets;
3613 then the packed length of this template in bytes is used as a count.
3614 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3615 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3616 If the template in brackets contains alignment commands (such as C<x![d]>),
3617 its packed length is calculated as if the start of the template has the maximal
3620 When used with C<Z>, C<*> results in the addition of a trailing null
3621 byte (so the packed result will be one longer than the byte C<length>
3624 When used with C<@>, the repeat count represents an offset from the start
3625 of the innermost () group.
3627 When used with C<.>, the repeat count is used to determine the starting
3628 position from where the value offset is calculated. If the repeat count
3629 is 0, it's relative to the current position. If the repeat count is C<*>,
3630 the offset is relative to the start of the packed string. And if its an
3631 integer C<n> the offset is relative to the start of the n-th innermost
3632 () group (or the start of the string if C<n> is bigger then the group
3635 The repeat count for C<u> is interpreted as the maximal number of bytes
3636 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3637 count should not be more than 65.
3641 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3642 string of length count, padding with nulls or spaces as necessary. When
3643 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3644 after the first null, and C<a> returns data verbatim.
3646 If the value-to-pack is too long, it is truncated. If too long and an
3647 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3648 by a null byte. Thus C<Z> always packs a trailing null (except when the
3653 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3654 Each character of the input field of pack() generates 1 bit of the result.
3655 Each result bit is based on the least-significant bit of the corresponding
3656 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3657 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3659 Starting from the beginning of the input string of pack(), each 8-tuple
3660 of characters is converted to 1 character of output. With format C<b>
3661 the first character of the 8-tuple determines the least-significant bit of a
3662 character, and with format C<B> it determines the most-significant bit of
3665 If the length of the input string is not exactly divisible by 8, the
3666 remainder is packed as if the input string were padded by null characters
3667 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3669 If the input string of pack() is longer than needed, extra characters are
3670 ignored. A C<*> for the repeat count of pack() means to use all the
3671 characters of the input field. On unpack()ing the bits are converted to a
3672 string of C<"0">s and C<"1">s.
3676 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3677 representable as hexadecimal digits, 0-9a-f) long.
3679 Each character of the input field of pack() generates 4 bits of the result.
3680 For non-alphabetical characters the result is based on the 4 least-significant
3681 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3682 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3683 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3684 is compatible with the usual hexadecimal digits, so that C<"a"> and
3685 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3686 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3688 Starting from the beginning of the input string of pack(), each pair
3689 of characters is converted to 1 character of output. With format C<h> the
3690 first character of the pair determines the least-significant nybble of the
3691 output character, and with format C<H> it determines the most-significant
3694 If the length of the input string is not even, it behaves as if padded
3695 by a null character at the end. Similarly, during unpack()ing the "extra"
3696 nybbles are ignored.
3698 If the input string of pack() is longer than needed, extra characters are
3700 A C<*> for the repeat count of pack() means to use all the characters of
3701 the input field. On unpack()ing the nybbles are converted to a string
3702 of hexadecimal digits.
3706 The C<p> type packs a pointer to a null-terminated string. You are
3707 responsible for ensuring the string is not a temporary value (which can
3708 potentially get deallocated before you get around to using the packed result).
3709 The C<P> type packs a pointer to a structure of the size indicated by the
3710 length. A NULL pointer is created if the corresponding value for C<p> or
3711 C<P> is C<undef>, similarly for unpack().
3713 If your system has a strange pointer size (i.e. a pointer is neither as
3714 big as an int nor as big as a long), it may not be possible to pack or
3715 unpack pointers in big- or little-endian byte order. Attempting to do
3716 so will result in a fatal error.
3720 The C</> template character allows packing and unpacking of a sequence of
3721 items where the packed structure contains a packed item count followed by
3722 the packed items themselves.
3724 For C<pack> you write I<length-item>C</>I<sequence-item> and the
3725 I<length-item> describes how the length value is packed. The ones likely
3726 to be of most use are integer-packing ones like C<n> (for Java strings),
3727 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
3729 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3730 the minimum of that and the number of available items is used as argument
3731 for the I<length-item>. If it has no repeat count or uses a '*', the number
3732 of available items is used.
3734 For C<unpack> an internal stack of integer arguments unpacked so far is
3735 used. You write C</>I<sequence-item> and the repeat count is obtained by
3736 popping off the last element from the stack. The I<sequence-item> must not
3737 have a repeat count.
3739 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3740 the I<length-item> is a string length, not a number of strings. If there is
3741 an explicit repeat count for pack, the packed string will be adjusted to that
3744 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3745 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
3746 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
3747 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3748 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3750 The I<length-item> is not returned explicitly from C<unpack>.
3752 Adding a count to the I<length-item> letter is unlikely to do anything
3753 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3754 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3755 which Perl does not regard as legal in numeric strings.
3759 The integer types C<s>, C<S>, C<l>, and C<L> may be
3760 followed by a C<!> modifier to signify native shorts or
3761 longs--as you can see from above for example a bare C<l> does mean
3762 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3763 may be larger. This is an issue mainly in 64-bit platforms. You can
3764 see whether using C<!> makes any difference by
3766 print length(pack("s")), " ", length(pack("s!")), "\n";
3767 print length(pack("l")), " ", length(pack("l!")), "\n";
3769 C<i!> and C<I!> also work but only because of completeness;
3770 they are identical to C<i> and C<I>.
3772 The actual sizes (in bytes) of native shorts, ints, longs, and long
3773 longs on the platform where Perl was built are also available via
3777 print $Config{shortsize}, "\n";
3778 print $Config{intsize}, "\n";
3779 print $Config{longsize}, "\n";
3780 print $Config{longlongsize}, "\n";
3782 (The C<$Config{longlongsize}> will be undefined if your system does
3783 not support long longs.)
3787 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3788 are inherently non-portable between processors and operating systems
3789 because they obey the native byteorder and endianness. For example a
3790 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3791 (arranged in and handled by the CPU registers) into bytes as
3793 0x12 0x34 0x56 0x78 # big-endian
3794 0x78 0x56 0x34 0x12 # little-endian
3796 Basically, the Intel and VAX CPUs are little-endian, while everybody
3797 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3798 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3799 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3802 The names `big-endian' and `little-endian' are comic references to
3803 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3804 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3805 the egg-eating habits of the Lilliputians.
3807 Some systems may have even weirder byte orders such as
3812 You can see your system's preference with
3814 print join(" ", map { sprintf "%#02x", $_ }
3815 unpack("W*",pack("L",0x12345678))), "\n";
3817 The byteorder on the platform where Perl was built is also available
3821 print $Config{byteorder}, "\n";
3823 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3824 and C<'87654321'> are big-endian.
3826 If you want portable packed integers you can either use the formats
3827 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3828 modifiers. These modifiers are only available as of perl 5.9.2.
3829 See also L<perlport>.
3833 All integer and floating point formats as well as C<p> and C<P> and
3834 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3835 to force big- or little- endian byte-order, respectively.
3836 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3837 signed integers, 64-bit integers and floating point values. However,
3838 there are some things to keep in mind.
3840 Exchanging signed integers between different platforms only works
3841 if all platforms store them in the same format. Most platforms store
3842 signed integers in two's complement, so usually this is not an issue.
3844 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3845 formats on big- or little-endian machines. Otherwise, attempting to
3846 do so will result in a fatal error.
3848 Forcing big- or little-endian byte-order on floating point values for
3849 data exchange can only work if all platforms are using the same
3850 binary representation (e.g. IEEE floating point format). Even if all
3851 platforms are using IEEE, there may be subtle differences. Being able
3852 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3853 but also very dangerous if you don't know exactly what you're doing.
3854 It is definitely not a general way to portably store floating point
3857 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3858 all types inside the group that accept the byte-order modifiers,
3859 including all subgroups. It will silently be ignored for all other
3860 types. You are not allowed to override the byte-order within a group
3861 that already has a byte-order modifier suffix.
3865 Real numbers (floats and doubles) are in the native machine format only;
3866 due to the multiplicity of floating formats around, and the lack of a
3867 standard "network" representation, no facility for interchange has been
3868 made. This means that packed floating point data written on one machine
3869 may not be readable on another - even if both use IEEE floating point
3870 arithmetic (as the endian-ness of the memory representation is not part
3871 of the IEEE spec). See also L<perlport>.
3873 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3874 modifiers to force big- or little-endian byte-order on floating point values.
3876 Note that Perl uses doubles (or long doubles, if configured) internally for
3877 all numeric calculation, and converting from double into float and thence back
3878 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3879 will not in general equal $foo).
3883 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3884 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3885 where the packed string is processed in its UTF-8-encoded Unicode form on
3886 a byte by byte basis. Character mode is the default unless the format string
3887 starts with an C<U>. You can switch mode at any moment with an explicit
3888 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3889 or until the end of the ()-group in which it was entered.
3893 You must yourself do any alignment or padding by inserting for example
3894 enough C<'x'>es while packing. There is no way to pack() and unpack()
3895 could know where the characters are going to or coming from. Therefore
3896 C<pack> (and C<unpack>) handle their output and input as flat
3897 sequences of characters.
3901 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3902 take a repeat count, both as postfix, and for unpack() also via the C</>
3903 template character. Within each repetition of a group, positioning with
3904 C<@> starts again at 0. Therefore, the result of
3906 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3908 is the string "\0a\0\0bc".
3912 C<x> and C<X> accept C<!> modifier. In this case they act as
3913 alignment commands: they jump forward/back to the closest position
3914 aligned at a multiple of C<count> characters. For example, to pack() or
3915 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3916 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3917 aligned on the double's size.
3919 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3920 both result in no-ops.
3924 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3925 will represent signed 16-/32-bit integers in big-/little-endian order.
3926 This is only portable if all platforms sharing the packed data use the
3927 same binary representation for signed integers (e.g. all platforms are
3928 using two's complement representation).
3932 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3933 White space may be used to separate pack codes from each other, but
3934 modifiers and a repeat count must follow immediately.
3938 If TEMPLATE requires more arguments to pack() than actually given, pack()
3939 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3940 to pack() than actually given, extra arguments are ignored.
3946 $foo = pack("WWWW",65,66,67,68);
3948 $foo = pack("W4",65,66,67,68);
3950 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3951 # same thing with Unicode circled letters.
3952 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3953 # same thing with Unicode circled letters. You don't get the UTF-8
3954 # bytes because the U at the start of the format caused a switch to
3955 # U0-mode, so the UTF-8 bytes get joined into characters
3956 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3957 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3958 # This is the UTF-8 encoding of the string in the previous example
3960 $foo = pack("ccxxcc",65,66,67,68);
3963 # note: the above examples featuring "W" and "c" are true
3964 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3965 # and UTF-8. In EBCDIC the first example would be
3966 # $foo = pack("WWWW",193,194,195,196);
3968 $foo = pack("s2",1,2);
3969 # "\1\0\2\0" on little-endian
3970 # "\0\1\0\2" on big-endian
3972 $foo = pack("a4","abcd","x","y","z");
3975 $foo = pack("aaaa","abcd","x","y","z");
3978 $foo = pack("a14","abcdefg");
3979 # "abcdefg\0\0\0\0\0\0\0"
3981 $foo = pack("i9pl", gmtime);
3982 # a real struct tm (on my system anyway)
3984 $utmp_template = "Z8 Z8 Z16 L";
3985 $utmp = pack($utmp_template, @utmp1);
3986 # a struct utmp (BSDish)
3988 @utmp2 = unpack($utmp_template, $utmp);
3989 # "@utmp1" eq "@utmp2"
3992 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3995 $foo = pack('sx2l', 12, 34);
3996 # short 12, two zero bytes padding, long 34
3997 $bar = pack('s@4l', 12, 34);
3998 # short 12, zero fill to position 4, long 34
4000 $baz = pack('s.l', 12, 4, 34);
4001 # short 12, zero fill to position 4, long 34
4003 $foo = pack('nN', 42, 4711);
4004 # pack big-endian 16- and 32-bit unsigned integers
4005 $foo = pack('S>L>', 42, 4711);
4007 $foo = pack('s<l<', -42, 4711);
4008 # pack little-endian 16- and 32-bit signed integers
4009 $foo = pack('(sl)<', -42, 4711);
4012 The same template may generally also be used in unpack().
4014 =item package NAMESPACE
4015 X<package> X<module> X<namespace>
4019 Declares the compilation unit as being in the given namespace. The scope
4020 of the package declaration is from the declaration itself through the end
4021 of the enclosing block, file, or eval (the same as the C<my> operator).
4022 All further unqualified dynamic identifiers will be in this namespace.
4023 A package statement affects only dynamic variables--including those
4024 you've used C<local> on--but I<not> lexical variables, which are created
4025 with C<my>. Typically it would be the first declaration in a file to
4026 be included by the C<require> or C<use> operator. You can switch into a
4027 package in more than one place; it merely influences which symbol table
4028 is used by the compiler for the rest of that block. You can refer to
4029 variables and filehandles in other packages by prefixing the identifier
4030 with the package name and a double colon: C<$Package::Variable>.
4031 If the package name is null, the C<main> package as assumed. That is,
4032 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4033 still seen in older code).
4035 If NAMESPACE is omitted, then there is no current package, and all
4036 identifiers must be fully qualified or lexicals. However, you are
4037 strongly advised not to make use of this feature. Its use can cause
4038 unexpected behaviour, even crashing some versions of Perl. It is
4039 deprecated, and will be removed from a future release.
4041 See L<perlmod/"Packages"> for more information about packages, modules,
4042 and classes. See L<perlsub> for other scoping issues.
4044 =item pipe READHANDLE,WRITEHANDLE
4047 Opens a pair of connected pipes like the corresponding system call.
4048 Note that if you set up a loop of piped processes, deadlock can occur
4049 unless you are very careful. In addition, note that Perl's pipes use
4050 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4051 after each command, depending on the application.
4053 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4054 for examples of such things.
4056 On systems that support a close-on-exec flag on files, the flag will be set
4057 for the newly opened file descriptors as determined by the value of $^F.
4065 Pops and returns the last value of the array, shortening the array by
4066 one element. Has an effect similar to
4070 If there are no elements in the array, returns the undefined value
4071 (although this may happen at other times as well). If ARRAY is
4072 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4073 array in subroutines, just like C<shift>.
4076 X<pos> X<match, position>
4080 Returns the offset of where the last C<m//g> search left off for the variable
4081 in question (C<$_> is used when the variable is not specified). Note that
4082 0 is a valid match offset. C<undef> indicates that the search position
4083 is reset (usually due to match failure, but can also be because no match has
4084 yet been performed on the scalar). C<pos> directly accesses the location used
4085 by the regexp engine to store the offset, so assigning to C<pos> will change
4086 that offset, and so will also influence the C<\G> zero-width assertion in
4087 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4088 the return from C<pos> won't change either in this case. See L<perlre> and
4091 =item print FILEHANDLE LIST
4098 Prints a string or a list of strings. Returns true if successful.
4099 FILEHANDLE may be a scalar variable name, in which case the variable
4100 contains the name of or a reference to the filehandle, thus introducing
4101 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4102 the next token is a term, it may be misinterpreted as an operator
4103 unless you interpose a C<+> or put parentheses around the arguments.)
4104 If FILEHANDLE is omitted, prints by default to standard output (or
4105 to the last selected output channel--see L</select>). If LIST is
4106 also omitted, prints C<$_> to the currently selected output channel.
4107 To set the default output channel to something other than STDOUT
4108 use the select operation. The current value of C<$,> (if any) is
4109 printed between each LIST item. The current value of C<$\> (if
4110 any) is printed after the entire LIST has been printed. Because
4111 print takes a LIST, anything in the LIST is evaluated in list
4112 context, and any subroutine that you call will have one or more of
4113 its expressions evaluated in list context. Also be careful not to
4114 follow the print keyword with a left parenthesis unless you want
4115 the corresponding right parenthesis to terminate the arguments to
4116 the print--interpose a C<+> or put parentheses around all the
4119 Note that if you're storing FILEHANDLEs in an array, or if you're using
4120 any other expression more complex than a scalar variable to retrieve it,
4121 you will have to use a block returning the filehandle value instead:
4123 print { $files[$i] } "stuff\n";
4124 print { $OK ? STDOUT : STDERR } "stuff\n";
4126 =item printf FILEHANDLE FORMAT, LIST
4129 =item printf FORMAT, LIST
4131 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4132 (the output record separator) is not appended. The first argument
4133 of the list will be interpreted as the C<printf> format. See C<sprintf>
4134 for an explanation of the format argument. If C<use locale> is in effect,
4135 and POSIX::setlocale() has been called, the character used for the decimal
4136 separator in formatted floating point numbers is affected by the LC_NUMERIC
4137 locale. See L<perllocale> and L<POSIX>.
4139 Don't fall into the trap of using a C<printf> when a simple
4140 C<print> would do. The C<print> is more efficient and less
4143 =item prototype FUNCTION
4146 Returns the prototype of a function as a string (or C<undef> if the
4147 function has no prototype). FUNCTION is a reference to, or the name of,
4148 the function whose prototype you want to retrieve.
4150 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4151 name for Perl builtin. If the builtin is not I<overridable> (such as
4152 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4153 C<system>) returns C<undef> because the builtin does not really behave
4154 like a Perl function. Otherwise, the string describing the equivalent
4155 prototype is returned.
4157 =item push ARRAY,LIST
4160 Treats ARRAY as a stack, and pushes the values of LIST
4161 onto the end of ARRAY. The length of ARRAY increases by the length of
4162 LIST. Has the same effect as
4165 $ARRAY[++$#ARRAY] = $value;
4168 but is more efficient. Returns the number of elements in the array following
4169 the completed C<push>.
4181 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4183 =item quotemeta EXPR
4184 X<quotemeta> X<metacharacter>
4188 Returns the value of EXPR with all non-"word"
4189 characters backslashed. (That is, all characters not matching
4190 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4191 returned string, regardless of any locale settings.)
4192 This is the internal function implementing
4193 the C<\Q> escape in double-quoted strings.
4195 If EXPR is omitted, uses C<$_>.
4202 Returns a random fractional number greater than or equal to C<0> and less
4203 than the value of EXPR. (EXPR should be positive.) If EXPR is
4204 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4205 also special-cased as C<1> - this has not been documented before perl 5.8.0
4206 and is subject to change in future versions of perl. Automatically calls
4207 C<srand> unless C<srand> has already been called. See also C<srand>.
4209 Apply C<int()> to the value returned by C<rand()> if you want random
4210 integers instead of random fractional numbers. For example,
4214 returns a random integer between C<0> and C<9>, inclusive.
4216 (Note: If your rand function consistently returns numbers that are too
4217 large or too small, then your version of Perl was probably compiled
4218 with the wrong number of RANDBITS.)
4220 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4221 X<read> X<file, read>
4223 =item read FILEHANDLE,SCALAR,LENGTH
4225 Attempts to read LENGTH I<characters> of data into variable SCALAR
4226 from the specified FILEHANDLE. Returns the number of characters
4227 actually read, C<0> at end of file, or undef if there was an error (in
4228 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4229 so that the last character actually read is the last character of the
4230 scalar after the read.
4232 An OFFSET may be specified to place the read data at some place in the
4233 string other than the beginning. A negative OFFSET specifies
4234 placement at that many characters counting backwards from the end of
4235 the string. A positive OFFSET greater than the length of SCALAR
4236 results in the string being padded to the required size with C<"\0">
4237 bytes before the result of the read is appended.
4239 The call is actually implemented in terms of either Perl's or system's
4240 fread() call. To get a true read(2) system call, see C<sysread>.
4242 Note the I<characters>: depending on the status of the filehandle,
4243 either (8-bit) bytes or characters are read. By default all
4244 filehandles operate on bytes, but for example if the filehandle has
4245 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4246 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4247 characters, not bytes. Similarly for the C<:encoding> pragma:
4248 in that case pretty much any characters can be read.
4250 =item readdir DIRHANDLE
4253 Returns the next directory entry for a directory opened by C<opendir>.
4254 If used in list context, returns all the rest of the entries in the
4255 directory. If there are no more entries, returns an undefined value in
4256 scalar context or a null list in list context.
4258 If you're planning to filetest the return values out of a C<readdir>, you'd
4259 better prepend the directory in question. Otherwise, because we didn't
4260 C<chdir> there, it would have been testing the wrong file.
4262 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4263 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4267 X<readline> X<gets> X<fgets>
4269 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4270 context, each call reads and returns the next line, until end-of-file is
4271 reached, whereupon the subsequent call returns undef. In list context,
4272 reads until end-of-file is reached and returns a list of lines. Note that
4273 the notion of "line" used here is however you may have defined it
4274 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4276 When C<$/> is set to C<undef>, when readline() is in scalar
4277 context (i.e. file slurp mode), and when an empty file is read, it
4278 returns C<''> the first time, followed by C<undef> subsequently.
4280 This is the internal function implementing the C<< <EXPR> >>
4281 operator, but you can use it directly. The C<< <EXPR> >>
4282 operator is discussed in more detail in L<perlop/"I/O Operators">.
4285 $line = readline(*STDIN); # same thing
4287 If readline encounters an operating system error, C<$!> will be set with the
4288 corresponding error message. It can be helpful to check C<$!> when you are
4289 reading from filehandles you don't trust, such as a tty or a socket. The
4290 following example uses the operator form of C<readline>, and takes the necessary
4291 steps to ensure that C<readline> was successful.
4295 unless (defined( $line = <> )) {
4307 Returns the value of a symbolic link, if symbolic links are
4308 implemented. If not, gives a fatal error. If there is some system
4309 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4310 omitted, uses C<$_>.
4315 EXPR is executed as a system command.
4316 The collected standard output of the command is returned.
4317 In scalar context, it comes back as a single (potentially
4318 multi-line) string. In list context, returns a list of lines
4319 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4320 This is the internal function implementing the C<qx/EXPR/>
4321 operator, but you can use it directly. The C<qx/EXPR/>
4322 operator is discussed in more detail in L<perlop/"I/O Operators">.
4324 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4327 Receives a message on a socket. Attempts to receive LENGTH characters
4328 of data into variable SCALAR from the specified SOCKET filehandle.
4329 SCALAR will be grown or shrunk to the length actually read. Takes the
4330 same flags as the system call of the same name. Returns the address
4331 of the sender if SOCKET's protocol supports this; returns an empty
4332 string otherwise. If there's an error, returns the undefined value.
4333 This call is actually implemented in terms of recvfrom(2) system call.
4334 See L<perlipc/"UDP: Message Passing"> for examples.
4336 Note the I<characters>: depending on the status of the socket, either
4337 (8-bit) bytes or characters are received. By default all sockets
4338 operate on bytes, but for example if the socket has been changed using
4339 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4340 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4341 characters, not bytes. Similarly for the C<:encoding> pragma:
4342 in that case pretty much any characters can be read.
4349 The C<redo> command restarts the loop block without evaluating the
4350 conditional again. The C<continue> block, if any, is not executed. If
4351 the LABEL is omitted, the command refers to the innermost enclosing
4352 loop. Programs that want to lie to themselves about what was just input
4353 normally use this command:
4355 # a simpleminded Pascal comment stripper
4356 # (warning: assumes no { or } in strings)
4357 LINE: while (<STDIN>) {
4358 while (s|({.*}.*){.*}|$1 |) {}
4363 if (/}/) { # end of comment?
4372 C<redo> cannot be used to retry a block which returns a value such as
4373 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4374 a grep() or map() operation.
4376 Note that a block by itself is semantically identical to a loop
4377 that executes once. Thus C<redo> inside such a block will effectively
4378 turn it into a looping construct.
4380 See also L</continue> for an illustration of how C<last>, C<next>, and
4388 Returns a non-empty string if EXPR is a reference, the empty
4389 string otherwise. If EXPR
4390 is not specified, C<$_> will be used. The value returned depends on the
4391 type of thing the reference is a reference to.
4392 Builtin types include:
4406 If the referenced object has been blessed into a package, then that package
4407 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4409 if (ref($r) eq "HASH") {
4410 print "r is a reference to a hash.\n";
4413 print "r is not a reference at all.\n";
4416 See also L<perlref>.
4418 =item rename OLDNAME,NEWNAME
4419 X<rename> X<move> X<mv> X<ren>
4421 Changes the name of a file; an existing file NEWNAME will be
4422 clobbered. Returns true for success, false otherwise.
4424 Behavior of this function varies wildly depending on your system
4425 implementation. For example, it will usually not work across file system
4426 boundaries, even though the system I<mv> command sometimes compensates
4427 for this. Other restrictions include whether it works on directories,
4428 open files, or pre-existing files. Check L<perlport> and either the
4429 rename(2) manpage or equivalent system documentation for details.
4431 For a platform independent C<move> function look at the L<File::Copy>
4434 =item require VERSION
4441 Demands a version of Perl specified by VERSION, or demands some semantics
4442 specified by EXPR or by C<$_> if EXPR is not supplied.
4444 VERSION may be either a numeric argument such as 5.006, which will be
4445 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4446 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4447 VERSION is greater than the version of the current Perl interpreter.
4448 Compare with L</use>, which can do a similar check at compile time.
4450 Specifying VERSION as a literal of the form v5.6.1 should generally be
4451 avoided, because it leads to misleading error messages under earlier
4452 versions of Perl that do not support this syntax. The equivalent numeric
4453 version should be used instead.
4455 require v5.6.1; # run time version check
4456 require 5.6.1; # ditto
4457 require 5.006_001; # ditto; preferred for backwards compatibility
4459 Otherwise, C<require> demands that a library file be included if it
4460 hasn't already been included. The file is included via the do-FILE
4461 mechanism, which is essentially just a variety of C<eval>. Has
4462 semantics similar to the following subroutine:
4465 my ($filename) = @_;
4466 if (exists $INC{$filename}) {
4467 return 1 if $INC{$filename};
4468 die "Compilation failed in require";
4470 my ($realfilename,$result);
4472 foreach $prefix (@INC) {
4473 $realfilename = "$prefix/$filename";
4474 if (-f $realfilename) {
4475 $INC{$filename} = $realfilename;
4476 $result = do $realfilename;
4480 die "Can't find $filename in \@INC";
4483 $INC{$filename} = undef;
4485 } elsif (!$result) {
4486 delete $INC{$filename};
4487 die "$filename did not return true value";
4493 Note that the file will not be included twice under the same specified
4496 The file must return true as the last statement to indicate
4497 successful execution of any initialization code, so it's customary to
4498 end such a file with C<1;> unless you're sure it'll return true
4499 otherwise. But it's better just to put the C<1;>, in case you add more
4502 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4503 replaces "F<::>" with "F</>" in the filename for you,
4504 to make it easy to load standard modules. This form of loading of
4505 modules does not risk altering your namespace.
4507 In other words, if you try this:
4509 require Foo::Bar; # a splendid bareword
4511 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4512 directories specified in the C<@INC> array.
4514 But if you try this:
4516 $class = 'Foo::Bar';
4517 require $class; # $class is not a bareword
4519 require "Foo::Bar"; # not a bareword because of the ""
4521 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4522 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4524 eval "require $class";
4526 Now that you understand how C<require> looks for files in the case of a
4527 bareword argument, there is a little extra functionality going on behind
4528 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4529 first look for a similar filename with a "F<.pmc>" extension. If this file
4530 is found, it will be loaded in place of any file ending in a "F<.pm>"
4533 You can also insert hooks into the import facility, by putting directly
4534 Perl code into the @INC array. There are three forms of hooks: subroutine
4535 references, array references and blessed objects.
4537 Subroutine references are the simplest case. When the inclusion system
4538 walks through @INC and encounters a subroutine, this subroutine gets
4539 called with two parameters, the first being a reference to itself, and the
4540 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4541 subroutine should return nothing, or a list of up to three values in the
4548 A filehandle, from which the file will be read.
4552 A reference to a subroutine. If there is no filehandle (previous item),
4553 then this subroutine is expected to generate one line of source code per
4554 call, writing the line into C<$_> and returning 1, then returning 0 at
4555 "end of file". If there is a filehandle, then the subroutine will be
4556 called to act a simple source filter, with the line as read in C<$_>.
4557 Again, return 1 for each valid line, and 0 after all lines have been
4562 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4563 reference to the subroutine itself is passed in as C<$_[0]>.
4567 If an empty list, C<undef>, or nothing that matches the first 3 values above
4568 is returned then C<require> will look at the remaining elements of @INC.
4569 Note that this file handle must be a real file handle (strictly a typeglob,
4570 or reference to a typeglob, blessed or unblessed) - tied file handles will be
4571 ignored and return value processing will stop there.
4573 If the hook is an array reference, its first element must be a subroutine
4574 reference. This subroutine is called as above, but the first parameter is
4575 the array reference. This enables to pass indirectly some arguments to
4578 In other words, you can write:
4580 push @INC, \&my_sub;
4582 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4588 push @INC, [ \&my_sub, $x, $y, ... ];
4590 my ($arrayref, $filename) = @_;
4591 # Retrieve $x, $y, ...
4592 my @parameters = @$arrayref[1..$#$arrayref];
4596 If the hook is an object, it must provide an INC method that will be
4597 called as above, the first parameter being the object itself. (Note that
4598 you must fully qualify the sub's name, as unqualified C<INC> is always forced
4599 into package C<main>.) Here is a typical code layout:
4605 my ($self, $filename) = @_;
4609 # In the main program
4610 push @INC, new Foo(...);
4612 Note that these hooks are also permitted to set the %INC entry
4613 corresponding to the files they have loaded. See L<perlvar/%INC>.
4615 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4622 Generally used in a C<continue> block at the end of a loop to clear
4623 variables and reset C<??> searches so that they work again. The
4624 expression is interpreted as a list of single characters (hyphens
4625 allowed for ranges). All variables and arrays beginning with one of
4626 those letters are reset to their pristine state. If the expression is
4627 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4628 only variables or searches in the current package. Always returns
4631 reset 'X'; # reset all X variables
4632 reset 'a-z'; # reset lower case variables
4633 reset; # just reset ?one-time? searches
4635 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4636 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4637 variables--lexical variables are unaffected, but they clean themselves
4638 up on scope exit anyway, so you'll probably want to use them instead.
4646 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4647 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4648 context, depending on how the return value will be used, and the context
4649 may vary from one execution to the next (see C<wantarray>). If no EXPR
4650 is given, returns an empty list in list context, the undefined value in
4651 scalar context, and (of course) nothing at all in a void context.
4653 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4654 or do FILE will automatically return the value of the last expression
4658 X<reverse> X<rev> X<invert>
4660 In list context, returns a list value consisting of the elements
4661 of LIST in the opposite order. In scalar context, concatenates the
4662 elements of LIST and returns a string value with all characters
4663 in the opposite order.
4665 print reverse <>; # line tac, last line first
4667 undef $/; # for efficiency of <>
4668 print scalar reverse <>; # character tac, last line tsrif
4670 Used without arguments in scalar context, reverse() reverses C<$_>.
4672 This operator is also handy for inverting a hash, although there are some
4673 caveats. If a value is duplicated in the original hash, only one of those
4674 can be represented as a key in the inverted hash. Also, this has to
4675 unwind one hash and build a whole new one, which may take some time
4676 on a large hash, such as from a DBM file.
4678 %by_name = reverse %by_address; # Invert the hash
4680 =item rewinddir DIRHANDLE
4683 Sets the current position to the beginning of the directory for the
4684 C<readdir> routine on DIRHANDLE.
4686 =item rindex STR,SUBSTR,POSITION
4689 =item rindex STR,SUBSTR
4691 Works just like index() except that it returns the position of the I<last>
4692 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4693 last occurrence beginning at or before that position.
4695 =item rmdir FILENAME
4696 X<rmdir> X<rd> X<directory, remove>
4700 Deletes the directory specified by FILENAME if that directory is
4701 empty. If it succeeds it returns true, otherwise it returns false and
4702 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4704 To remove a directory tree recursively (C<rm -rf> on unix) look at
4705 the C<rmtree> function of the L<File::Path> module.
4709 The substitution operator. See L<perlop>.
4711 =item say FILEHANDLE LIST
4718 Just like C<print>, but implicitly appends a newline.
4719 C<say LIST> is simply an abbreviation for C<{ local $/ = "\n"; print
4722 This keyword is only available when the "say" feature is
4723 enabled: see L<feature>.
4726 X<scalar> X<context>
4728 Forces EXPR to be interpreted in scalar context and returns the value
4731 @counts = ( scalar @a, scalar @b, scalar @c );
4733 There is no equivalent operator to force an expression to
4734 be interpolated in list context because in practice, this is never
4735 needed. If you really wanted to do so, however, you could use
4736 the construction C<@{[ (some expression) ]}>, but usually a simple
4737 C<(some expression)> suffices.
4739 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4740 parenthesized list, this behaves as a scalar comma expression, evaluating
4741 all but the last element in void context and returning the final element
4742 evaluated in scalar context. This is seldom what you want.
4744 The following single statement:
4746 print uc(scalar(&foo,$bar)),$baz;
4748 is the moral equivalent of these two:
4751 print(uc($bar),$baz);
4753 See L<perlop> for more details on unary operators and the comma operator.
4755 =item seek FILEHANDLE,POSITION,WHENCE
4756 X<seek> X<fseek> X<filehandle, position>
4758 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4759 FILEHANDLE may be an expression whose value gives the name of the
4760 filehandle. The values for WHENCE are C<0> to set the new position
4761 I<in bytes> to POSITION, C<1> to set it to the current position plus
4762 POSITION, and C<2> to set it to EOF plus POSITION (typically
4763 negative). For WHENCE you may use the constants C<SEEK_SET>,
4764 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4765 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4768 Note the I<in bytes>: even if the filehandle has been set to
4769 operate on characters (for example by using the C<:utf8> open
4770 layer), tell() will return byte offsets, not character offsets
4771 (because implementing that would render seek() and tell() rather slow).
4773 If you want to position file for C<sysread> or C<syswrite>, don't use
4774 C<seek>--buffering makes its effect on the file's system position
4775 unpredictable and non-portable. Use C<sysseek> instead.
4777 Due to the rules and rigors of ANSI C, on some systems you have to do a
4778 seek whenever you switch between reading and writing. Amongst other
4779 things, this may have the effect of calling stdio's clearerr(3).
4780 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4784 This is also useful for applications emulating C<tail -f>. Once you hit
4785 EOF on your read, and then sleep for a while, you might have to stick in a
4786 seek() to reset things. The C<seek> doesn't change the current position,
4787 but it I<does> clear the end-of-file condition on the handle, so that the
4788 next C<< <FILE> >> makes Perl try again to read something. We hope.
4790 If that doesn't work (some IO implementations are particularly
4791 cantankerous), then you may need something more like this:
4794 for ($curpos = tell(FILE); $_ = <FILE>;
4795 $curpos = tell(FILE)) {
4796 # search for some stuff and put it into files
4798 sleep($for_a_while);
4799 seek(FILE, $curpos, 0);
4802 =item seekdir DIRHANDLE,POS
4805 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4806 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4807 about possible directory compaction as the corresponding system library
4810 =item select FILEHANDLE
4811 X<select> X<filehandle, default>
4815 Returns the currently selected filehandle. Sets the current default
4816 filehandle for output, if FILEHANDLE is supplied. This has two
4817 effects: first, a C<write> or a C<print> without a filehandle will
4818 default to this FILEHANDLE. Second, references to variables related to
4819 output will refer to this output channel. For example, if you have to
4820 set the top of form format for more than one output channel, you might
4828 FILEHANDLE may be an expression whose value gives the name of the
4829 actual filehandle. Thus:
4831 $oldfh = select(STDERR); $| = 1; select($oldfh);
4833 Some programmers may prefer to think of filehandles as objects with
4834 methods, preferring to write the last example as:
4837 STDERR->autoflush(1);
4839 =item select RBITS,WBITS,EBITS,TIMEOUT
4842 This calls the select(2) system call with the bit masks specified, which
4843 can be constructed using C<fileno> and C<vec>, along these lines:
4845 $rin = $win = $ein = '';
4846 vec($rin,fileno(STDIN),1) = 1;
4847 vec($win,fileno(STDOUT),1) = 1;
4850 If you want to select on many filehandles you might wish to write a
4854 my(@fhlist) = split(' ',$_[0]);
4857 vec($bits,fileno($_),1) = 1;
4861 $rin = fhbits('STDIN TTY SOCK');
4865 ($nfound,$timeleft) =
4866 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4868 or to block until something becomes ready just do this
4870 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4872 Most systems do not bother to return anything useful in $timeleft, so
4873 calling select() in scalar context just returns $nfound.
4875 Any of the bit masks can also be undef. The timeout, if specified, is
4876 in seconds, which may be fractional. Note: not all implementations are
4877 capable of returning the $timeleft. If not, they always return
4878 $timeleft equal to the supplied $timeout.
4880 You can effect a sleep of 250 milliseconds this way:
4882 select(undef, undef, undef, 0.25);
4884 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4885 is implementation-dependent. See also L<perlport> for notes on the
4886 portability of C<select>.
4888 On error, C<select> behaves like the select(2) system call : it returns
4891 Note: on some Unixes, the select(2) system call may report a socket file
4892 descriptor as "ready for reading", when actually no data is available,
4893 thus a subsequent read blocks. It can be avoided using always the
4894 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4897 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4898 or <FH>) with C<select>, except as permitted by POSIX, and even
4899 then only on POSIX systems. You have to use C<sysread> instead.
4901 =item semctl ID,SEMNUM,CMD,ARG
4904 Calls the System V IPC function C<semctl>. You'll probably have to say
4908 first to get the correct constant definitions. If CMD is IPC_STAT or
4909 GETALL, then ARG must be a variable that will hold the returned
4910 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4911 the undefined value for error, "C<0 but true>" for zero, or the actual
4912 return value otherwise. The ARG must consist of a vector of native
4913 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4914 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4917 =item semget KEY,NSEMS,FLAGS
4920 Calls the System V IPC function semget. Returns the semaphore id, or
4921 the undefined value if there is an error. See also
4922 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4925 =item semop KEY,OPSTRING
4928 Calls the System V IPC function semop to perform semaphore operations
4929 such as signalling and waiting. OPSTRING must be a packed array of
4930 semop structures. Each semop structure can be generated with
4931 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4932 implies the number of semaphore operations. Returns true if
4933 successful, or false if there is an error. As an example, the
4934 following code waits on semaphore $semnum of semaphore id $semid:
4936 $semop = pack("s!3", $semnum, -1, 0);
4937 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4939 To signal the semaphore, replace C<-1> with C<1>. See also
4940 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4943 =item send SOCKET,MSG,FLAGS,TO
4946 =item send SOCKET,MSG,FLAGS
4948 Sends a message on a socket. Attempts to send the scalar MSG to the
4949 SOCKET filehandle. Takes the same flags as the system call of the
4950 same name. On unconnected sockets you must specify a destination to
4951 send TO, in which case it does a C C<sendto>. Returns the number of
4952 characters sent, or the undefined value if there is an error. The C
4953 system call sendmsg(2) is currently unimplemented. See
4954 L<perlipc/"UDP: Message Passing"> for examples.
4956 Note the I<characters>: depending on the status of the socket, either
4957 (8-bit) bytes or characters are sent. By default all sockets operate
4958 on bytes, but for example if the socket has been changed using
4959 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4960 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4961 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4962 in that case pretty much any characters can be sent.
4964 =item setpgrp PID,PGRP
4967 Sets the current process group for the specified PID, C<0> for the current
4968 process. Will produce a fatal error if used on a machine that doesn't
4969 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4970 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4971 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4974 =item setpriority WHICH,WHO,PRIORITY
4975 X<setpriority> X<priority> X<nice> X<renice>
4977 Sets the current priority for a process, a process group, or a user.
4978 (See setpriority(2).) Will produce a fatal error if used on a machine
4979 that doesn't implement setpriority(2).
4981 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4984 Sets the socket option requested. Returns undefined if there is an
4985 error. Use integer constants provided by the C<Socket> module for
4986 LEVEL and OPNAME. Values for LEVEL can also be obtained from
4987 getprotobyname. OPTVAL might either be a packed string or an integer.
4988 An integer OPTVAL is shorthand for pack("i", OPTVAL).
4990 An example disabling the Nagle's algorithm for a socket:
4992 use Socket qw(IPPROTO_TCP TCP_NODELAY);
4993 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5000 Shifts the first value of the array off and returns it, shortening the
5001 array by 1 and moving everything down. If there are no elements in the
5002 array, returns the undefined value. If ARRAY is omitted, shifts the
5003 C<@_> array within the lexical scope of subroutines and formats, and the
5004 C<@ARGV> array outside of a subroutine and also within the lexical scopes
5005 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5006 C<UNITCHECK {}> and C<END {}> constructs.
5008 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5009 same thing to the left end of an array that C<pop> and C<push> do to the
5012 =item shmctl ID,CMD,ARG
5015 Calls the System V IPC function shmctl. You'll probably have to say
5019 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5020 then ARG must be a variable that will hold the returned C<shmid_ds>
5021 structure. Returns like ioctl: the undefined value for error, "C<0> but
5022 true" for zero, or the actual return value otherwise.
5023 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5025 =item shmget KEY,SIZE,FLAGS
5028 Calls the System V IPC function shmget. Returns the shared memory
5029 segment id, or the undefined value if there is an error.
5030 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5032 =item shmread ID,VAR,POS,SIZE
5036 =item shmwrite ID,STRING,POS,SIZE
5038 Reads or writes the System V shared memory segment ID starting at
5039 position POS for size SIZE by attaching to it, copying in/out, and
5040 detaching from it. When reading, VAR must be a variable that will
5041 hold the data read. When writing, if STRING is too long, only SIZE
5042 bytes are used; if STRING is too short, nulls are written to fill out
5043 SIZE bytes. Return true if successful, or false if there is an error.
5044 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5045 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5047 =item shutdown SOCKET,HOW
5050 Shuts down a socket connection in the manner indicated by HOW, which
5051 has the same interpretation as in the system call of the same name.
5053 shutdown(SOCKET, 0); # I/we have stopped reading data
5054 shutdown(SOCKET, 1); # I/we have stopped writing data
5055 shutdown(SOCKET, 2); # I/we have stopped using this socket
5057 This is useful with sockets when you want to tell the other
5058 side you're done writing but not done reading, or vice versa.
5059 It's also a more insistent form of close because it also
5060 disables the file descriptor in any forked copies in other
5064 X<sin> X<sine> X<asin> X<arcsine>
5068 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5069 returns sine of C<$_>.
5071 For the inverse sine operation, you may use the C<Math::Trig::asin>
5072 function, or use this relation:
5074 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5081 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5082 May be interrupted if the process receives a signal such as C<SIGALRM>.
5083 Returns the number of seconds actually slept. You probably cannot
5084 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5087 On some older systems, it may sleep up to a full second less than what
5088 you requested, depending on how it counts seconds. Most modern systems
5089 always sleep the full amount. They may appear to sleep longer than that,
5090 however, because your process might not be scheduled right away in a
5091 busy multitasking system.
5093 For delays of finer granularity than one second, you may use Perl's
5094 C<syscall> interface to access setitimer(2) if your system supports
5095 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5096 and starting from Perl 5.8 part of the standard distribution) may also
5099 See also the POSIX module's C<pause> function.
5101 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5104 Opens a socket of the specified kind and attaches it to filehandle
5105 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5106 the system call of the same name. You should C<use Socket> first
5107 to get the proper definitions imported. See the examples in
5108 L<perlipc/"Sockets: Client/Server Communication">.
5110 On systems that support a close-on-exec flag on files, the flag will
5111 be set for the newly opened file descriptor, as determined by the
5112 value of $^F. See L<perlvar/$^F>.
5114 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5117 Creates an unnamed pair of sockets in the specified domain, of the
5118 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5119 for the system call of the same name. If unimplemented, yields a fatal
5120 error. Returns true if successful.
5122 On systems that support a close-on-exec flag on files, the flag will
5123 be set for the newly opened file descriptors, as determined by the value
5124 of $^F. See L<perlvar/$^F>.
5126 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5127 to C<pipe(Rdr, Wtr)> is essentially:
5130 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5131 shutdown(Rdr, 1); # no more writing for reader
5132 shutdown(Wtr, 0); # no more reading for writer
5134 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5135 emulate socketpair using IP sockets to localhost if your system implements
5136 sockets but not socketpair.
5138 =item sort SUBNAME LIST
5139 X<sort> X<qsort> X<quicksort> X<mergesort>
5141 =item sort BLOCK LIST
5145 In list context, this sorts the LIST and returns the sorted list value.
5146 In scalar context, the behaviour of C<sort()> is undefined.
5148 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5149 order. If SUBNAME is specified, it gives the name of a subroutine
5150 that returns an integer less than, equal to, or greater than C<0>,
5151 depending on how the elements of the list are to be ordered. (The C<<
5152 <=> >> and C<cmp> operators are extremely useful in such routines.)
5153 SUBNAME may be a scalar variable name (unsubscripted), in which case
5154 the value provides the name of (or a reference to) the actual
5155 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5156 an anonymous, in-line sort subroutine.
5158 If the subroutine's prototype is C<($$)>, the elements to be compared
5159 are passed by reference in C<@_>, as for a normal subroutine. This is
5160 slower than unprototyped subroutines, where the elements to be
5161 compared are passed into the subroutine
5162 as the package global variables $a and $b (see example below). Note that
5163 in the latter case, it is usually counter-productive to declare $a and
5166 The values to be compared are always passed by reference and should not
5169 You also cannot exit out of the sort block or subroutine using any of the
5170 loop control operators described in L<perlsyn> or with C<goto>.
5172 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5173 current collation locale. See L<perllocale>.
5175 sort() returns aliases into the original list, much as a for loop's index
5176 variable aliases the list elements. That is, modifying an element of a
5177 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5178 actually modifies the element in the original list. This is usually
5179 something to be avoided when writing clear code.
5181 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5182 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5183 preserves the input order of elements that compare equal. Although
5184 quicksort's run time is O(NlogN) when averaged over all arrays of
5185 length N, the time can be O(N**2), I<quadratic> behavior, for some
5186 inputs.) In 5.7, the quicksort implementation was replaced with
5187 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5188 But benchmarks indicated that for some inputs, on some platforms,
5189 the original quicksort was faster. 5.8 has a sort pragma for
5190 limited control of the sort. Its rather blunt control of the
5191 underlying algorithm may not persist into future Perls, but the
5192 ability to characterize the input or output in implementation
5193 independent ways quite probably will. See L<sort>.
5198 @articles = sort @files;
5200 # same thing, but with explicit sort routine
5201 @articles = sort {$a cmp $b} @files;
5203 # now case-insensitively
5204 @articles = sort {uc($a) cmp uc($b)} @files;
5206 # same thing in reversed order
5207 @articles = sort {$b cmp $a} @files;
5209 # sort numerically ascending
5210 @articles = sort {$a <=> $b} @files;
5212 # sort numerically descending
5213 @articles = sort {$b <=> $a} @files;
5215 # this sorts the %age hash by value instead of key
5216 # using an in-line function
5217 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5219 # sort using explicit subroutine name
5221 $age{$a} <=> $age{$b}; # presuming numeric
5223 @sortedclass = sort byage @class;
5225 sub backwards { $b cmp $a }
5226 @harry = qw(dog cat x Cain Abel);
5227 @george = qw(gone chased yz Punished Axed);
5229 # prints AbelCaincatdogx
5230 print sort backwards @harry;
5231 # prints xdogcatCainAbel
5232 print sort @george, 'to', @harry;
5233 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5235 # inefficiently sort by descending numeric compare using
5236 # the first integer after the first = sign, or the
5237 # whole record case-insensitively otherwise
5240 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5245 # same thing, but much more efficiently;
5246 # we'll build auxiliary indices instead
5250 push @nums, /=(\d+)/;
5255 $nums[$b] <=> $nums[$a]
5257 $caps[$a] cmp $caps[$b]
5261 # same thing, but without any temps
5262 @new = map { $_->[0] }
5263 sort { $b->[1] <=> $a->[1]
5266 } map { [$_, /=(\d+)/, uc($_)] } @old;
5268 # using a prototype allows you to use any comparison subroutine
5269 # as a sort subroutine (including other package's subroutines)
5271 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5274 @new = sort other::backwards @old;
5276 # guarantee stability, regardless of algorithm
5278 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5280 # force use of mergesort (not portable outside Perl 5.8)
5281 use sort '_mergesort'; # note discouraging _
5282 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5284 If you're using strict, you I<must not> declare $a
5285 and $b as lexicals. They are package globals. That means
5286 if you're in the C<main> package and type
5288 @articles = sort {$b <=> $a} @files;
5290 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5291 but if you're in the C<FooPack> package, it's the same as typing
5293 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5295 The comparison function is required to behave. If it returns
5296 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5297 sometimes saying the opposite, for example) the results are not
5300 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5301 (not-a-number), and because C<sort> will trigger a fatal error unless the
5302 result of a comparison is defined, when sorting with a comparison function
5303 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5304 The following example takes advantage of the fact that C<NaN != NaN> to
5305 eliminate any C<NaN>s from the input.
5307 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5309 =item splice ARRAY,OFFSET,LENGTH,LIST
5312 =item splice ARRAY,OFFSET,LENGTH
5314 =item splice ARRAY,OFFSET
5318 Removes the elements designated by OFFSET and LENGTH from an array, and
5319 replaces them with the elements of LIST, if any. In list context,
5320 returns the elements removed from the array. In scalar context,
5321 returns the last element removed, or C<undef> if no elements are
5322 removed. The array grows or shrinks as necessary.
5323 If OFFSET is negative then it starts that far from the end of the array.
5324 If LENGTH is omitted, removes everything from OFFSET onward.
5325 If LENGTH is negative, removes the elements from OFFSET onward
5326 except for -LENGTH elements at the end of the array.
5327 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5328 past the end of the array, perl issues a warning, and splices at the
5331 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5333 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5334 pop(@a) splice(@a,-1)
5335 shift(@a) splice(@a,0,1)
5336 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5337 $a[$i] = $y splice(@a,$i,1,$y)
5339 Example, assuming array lengths are passed before arrays:
5341 sub aeq { # compare two list values
5342 my(@a) = splice(@_,0,shift);
5343 my(@b) = splice(@_,0,shift);
5344 return 0 unless @a == @b; # same len?
5346 return 0 if pop(@a) ne pop(@b);
5350 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5352 =item split /PATTERN/,EXPR,LIMIT
5355 =item split /PATTERN/,EXPR
5357 =item split /PATTERN/
5361 Splits the string EXPR into a list of strings and returns that list. By
5362 default, empty leading fields are preserved, and empty trailing ones are
5363 deleted. (If all fields are empty, they are considered to be trailing.)
5365 In scalar context, returns the number of fields found and splits into
5366 the C<@_> array. Use of split in scalar context is deprecated, however,
5367 because it clobbers your subroutine arguments.
5369 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5370 splits on whitespace (after skipping any leading whitespace). Anything
5371 matching PATTERN is taken to be a delimiter separating the fields. (Note
5372 that the delimiter may be longer than one character.)
5374 If LIMIT is specified and positive, it represents the maximum number
5375 of fields the EXPR will be split into, though the actual number of
5376 fields returned depends on the number of times PATTERN matches within
5377 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5378 stripped (which potential users of C<pop> would do well to remember).
5379 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5380 had been specified. Note that splitting an EXPR that evaluates to the
5381 empty string always returns the empty list, regardless of the LIMIT
5384 A pattern matching the null string (not to be confused with
5385 a null pattern C<//>, which is just one member of the set of patterns
5386 matching a null string) will split the value of EXPR into separate
5387 characters at each point it matches that way. For example:
5389 print join(':', split(/ */, 'hi there'));
5391 produces the output 'h:i:t:h:e:r:e'.
5393 As a special case for C<split>, using the empty pattern C<//> specifically
5394 matches only the null string, and is not be confused with the regular use
5395 of C<//> to mean "the last successful pattern match". So, for C<split>,
5398 print join(':', split(//, 'hi there'));
5400 produces the output 'h:i: :t:h:e:r:e'.
5402 Empty leading (or trailing) fields are produced when there are positive
5403 width matches at the beginning (or end) of the string; a zero-width match
5404 at the beginning (or end) of the string does not produce an empty field.
5407 print join(':', split(/(?=\w)/, 'hi there!'));
5409 produces the output 'h:i :t:h:e:r:e!'.
5411 The LIMIT parameter can be used to split a line partially
5413 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5415 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5416 a LIMIT one larger than the number of variables in the list, to avoid
5417 unnecessary work. For the list above LIMIT would have been 4 by
5418 default. In time critical applications it behooves you not to split
5419 into more fields than you really need.
5421 If the PATTERN contains parentheses, additional list elements are
5422 created from each matching substring in the delimiter.
5424 split(/([,-])/, "1-10,20", 3);
5426 produces the list value
5428 (1, '-', 10, ',', 20)
5430 If you had the entire header of a normal Unix email message in $header,
5431 you could split it up into fields and their values this way:
5433 $header =~ s/\n\s+/ /g; # fix continuation lines
5434 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5436 The pattern C</PATTERN/> may be replaced with an expression to specify
5437 patterns that vary at runtime. (To do runtime compilation only once,
5438 use C</$variable/o>.)
5440 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5441 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5442 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5443 will give you as many null initial fields as there are leading spaces.
5444 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5445 whitespace produces a null first field. A C<split> with no arguments
5446 really does a S<C<split(' ', $_)>> internally.
5448 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5453 open(PASSWD, '/etc/passwd');
5456 ($login, $passwd, $uid, $gid,
5457 $gcos, $home, $shell) = split(/:/);
5461 As with regular pattern matching, any capturing parentheses that are not
5462 matched in a C<split()> will be set to C<undef> when returned:
5464 @fields = split /(A)|B/, "1A2B3";
5465 # @fields is (1, 'A', 2, undef, 3)
5467 =item sprintf FORMAT, LIST
5470 Returns a string formatted by the usual C<printf> conventions of the C
5471 library function C<sprintf>. See below for more details
5472 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5473 the general principles.
5477 # Format number with up to 8 leading zeroes
5478 $result = sprintf("%08d", $number);
5480 # Round number to 3 digits after decimal point
5481 $rounded = sprintf("%.3f", $number);
5483 Perl does its own C<sprintf> formatting--it emulates the C
5484 function C<sprintf>, but it doesn't use it (except for floating-point
5485 numbers, and even then only the standard modifiers are allowed). As a
5486 result, any non-standard extensions in your local C<sprintf> are not
5487 available from Perl.
5489 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5490 pass it an array as your first argument. The array is given scalar context,
5491 and instead of using the 0th element of the array as the format, Perl will
5492 use the count of elements in the array as the format, which is almost never
5495 Perl's C<sprintf> permits the following universally-known conversions:
5498 %c a character with the given number
5500 %d a signed integer, in decimal
5501 %u an unsigned integer, in decimal
5502 %o an unsigned integer, in octal
5503 %x an unsigned integer, in hexadecimal
5504 %e a floating-point number, in scientific notation
5505 %f a floating-point number, in fixed decimal notation
5506 %g a floating-point number, in %e or %f notation
5508 In addition, Perl permits the following widely-supported conversions:
5510 %X like %x, but using upper-case letters
5511 %E like %e, but using an upper-case "E"
5512 %G like %g, but with an upper-case "E" (if applicable)
5513 %b an unsigned integer, in binary
5514 %B like %b, but using an upper-case "B" with the # flag
5515 %p a pointer (outputs the Perl value's address in hexadecimal)
5516 %n special: *stores* the number of characters output so far
5517 into the next variable in the parameter list
5519 Finally, for backward (and we do mean "backward") compatibility, Perl
5520 permits these unnecessary but widely-supported conversions:
5523 %D a synonym for %ld
5524 %U a synonym for %lu
5525 %O a synonym for %lo
5528 Note that the number of exponent digits in the scientific notation produced
5529 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5530 exponent less than 100 is system-dependent: it may be three or less
5531 (zero-padded as necessary). In other words, 1.23 times ten to the
5532 99th may be either "1.23e99" or "1.23e099".
5534 Between the C<%> and the format letter, you may specify a number of
5535 additional attributes controlling the interpretation of the format.
5536 In order, these are:
5540 =item format parameter index
5542 An explicit format parameter index, such as C<2$>. By default sprintf
5543 will format the next unused argument in the list, but this allows you
5544 to take the arguments out of order, e.g.:
5546 printf '%2$d %1$d', 12, 34; # prints "34 12"
5547 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5553 space prefix positive number with a space
5554 + prefix positive number with a plus sign
5555 - left-justify within the field
5556 0 use zeros, not spaces, to right-justify
5557 # ensure the leading "0" for any octal,
5558 prefix non-zero hexadecimal with "0x" or "0X",
5559 prefix non-zero binary with "0b" or "0B"
5563 printf '<% d>', 12; # prints "< 12>"
5564 printf '<%+d>', 12; # prints "<+12>"
5565 printf '<%6s>', 12; # prints "< 12>"
5566 printf '<%-6s>', 12; # prints "<12 >"
5567 printf '<%06s>', 12; # prints "<000012>"
5568 printf '<%#o>', 12; # prints "<014>"
5569 printf '<%#x>', 12; # prints "<0xc>"
5570 printf '<%#X>', 12; # prints "<0XC>"
5571 printf '<%#b>', 12; # prints "<0b1100>"
5572 printf '<%#B>', 12; # prints "<0B1100>"
5574 When a space and a plus sign are given as the flags at once,
5575 a plus sign is used to prefix a positive number.
5577 printf '<%+ d>', 12; # prints "<+12>"
5578 printf '<% +d>', 12; # prints "<+12>"
5580 When the # flag and a precision are given in the %o conversion,
5581 the precision is incremented if it's necessary for the leading "0".
5583 printf '<%#.5o>', 012; # prints "<00012>"
5584 printf '<%#.5o>', 012345; # prints "<012345>"
5585 printf '<%#.0o>', 0; # prints "<0>"
5589 This flag tells perl to interpret the supplied string as a vector of
5590 integers, one for each character in the string. Perl applies the format to
5591 each integer in turn, then joins the resulting strings with a separator (a
5592 dot C<.> by default). This can be useful for displaying ordinal values of
5593 characters in arbitrary strings:
5595 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5596 printf "version is v%vd\n", $^V; # Perl's version
5598 Put an asterisk C<*> before the C<v> to override the string to
5599 use to separate the numbers:
5601 printf "address is %*vX\n", ":", $addr; # IPv6 address
5602 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5604 You can also explicitly specify the argument number to use for
5605 the join string using e.g. C<*2$v>:
5607 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5609 =item (minimum) width
5611 Arguments are usually formatted to be only as wide as required to
5612 display the given value. You can override the width by putting
5613 a number here, or get the width from the next argument (with C<*>)
5614 or from a specified argument (with e.g. C<*2$>):
5616 printf '<%s>', "a"; # prints "<a>"
5617 printf '<%6s>', "a"; # prints "< a>"
5618 printf '<%*s>', 6, "a"; # prints "< a>"
5619 printf '<%*2$s>', "a", 6; # prints "< a>"
5620 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5622 If a field width obtained through C<*> is negative, it has the same
5623 effect as the C<-> flag: left-justification.
5625 =item precision, or maximum width
5628 You can specify a precision (for numeric conversions) or a maximum
5629 width (for string conversions) by specifying a C<.> followed by a number.
5630 For floating point formats, with the exception of 'g' and 'G', this specifies
5631 the number of decimal places to show (the default being 6), e.g.:
5633 # these examples are subject to system-specific variation
5634 printf '<%f>', 1; # prints "<1.000000>"
5635 printf '<%.1f>', 1; # prints "<1.0>"
5636 printf '<%.0f>', 1; # prints "<1>"
5637 printf '<%e>', 10; # prints "<1.000000e+01>"
5638 printf '<%.1e>', 10; # prints "<1.0e+01>"
5640 For 'g' and 'G', this specifies the maximum number of digits to show,
5641 including prior to the decimal point as well as after it, e.g.:
5643 # these examples are subject to system-specific variation
5644 printf '<%g>', 1; # prints "<1>"
5645 printf '<%.10g>', 1; # prints "<1>"
5646 printf '<%g>', 100; # prints "<100>"
5647 printf '<%.1g>', 100; # prints "<1e+02>"
5648 printf '<%.2g>', 100.01; # prints "<1e+02>"
5649 printf '<%.5g>', 100.01; # prints "<100.01>"
5650 printf '<%.4g>', 100.01; # prints "<100>"
5652 For integer conversions, specifying a precision implies that the
5653 output of the number itself should be zero-padded to this width,
5654 where the 0 flag is ignored:
5656 printf '<%.6d>', 1; # prints "<000001>"
5657 printf '<%+.6d>', 1; # prints "<+000001>"
5658 printf '<%-10.6d>', 1; # prints "<000001 >"
5659 printf '<%10.6d>', 1; # prints "< 000001>"
5660 printf '<%010.6d>', 1; # prints "< 000001>"
5661 printf '<%+10.6d>', 1; # prints "< +000001>"
5663 printf '<%.6x>', 1; # prints "<000001>"
5664 printf '<%#.6x>', 1; # prints "<0x000001>"
5665 printf '<%-10.6x>', 1; # prints "<000001 >"
5666 printf '<%10.6x>', 1; # prints "< 000001>"
5667 printf '<%010.6x>', 1; # prints "< 000001>"
5668 printf '<%#10.6x>', 1; # prints "< 0x000001>"
5670 For string conversions, specifying a precision truncates the string
5671 to fit in the specified width:
5673 printf '<%.5s>', "truncated"; # prints "<trunc>"
5674 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5676 You can also get the precision from the next argument using C<.*>:
5678 printf '<%.6x>', 1; # prints "<000001>"
5679 printf '<%.*x>', 6, 1; # prints "<000001>"
5681 If a precision obtained through C<*> is negative, it has the same
5682 effect as no precision.
5684 printf '<%.*s>', 7, "string"; # prints "<string>"
5685 printf '<%.*s>', 3, "string"; # prints "<str>"
5686 printf '<%.*s>', 0, "string"; # prints "<>"
5687 printf '<%.*s>', -1, "string"; # prints "<string>"
5689 printf '<%.*d>', 1, 0; # prints "<0>"
5690 printf '<%.*d>', 0, 0; # prints "<>"
5691 printf '<%.*d>', -1, 0; # prints "<0>"
5693 You cannot currently get the precision from a specified number,
5694 but it is intended that this will be possible in the future using
5697 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5701 For numeric conversions, you can specify the size to interpret the
5702 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5703 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5704 whatever the default integer size is on your platform (usually 32 or 64
5705 bits), but you can override this to use instead one of the standard C types,
5706 as supported by the compiler used to build Perl:
5708 l interpret integer as C type "long" or "unsigned long"
5709 h interpret integer as C type "short" or "unsigned short"
5710 q, L or ll interpret integer as C type "long long", "unsigned long long".
5711 or "quads" (typically 64-bit integers)
5713 The last will produce errors if Perl does not understand "quads" in your
5714 installation. (This requires that either the platform natively supports quads
5715 or Perl was specifically compiled to support quads.) You can find out
5716 whether your Perl supports quads via L<Config>:
5719 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5722 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5723 to be the default floating point size on your platform (double or long double),
5724 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5725 platform supports them. You can find out whether your Perl supports long
5726 doubles via L<Config>:
5729 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5731 You can find out whether Perl considers 'long double' to be the default
5732 floating point size to use on your platform via L<Config>:
5735 ($Config{uselongdouble} eq 'define') &&
5736 print "long doubles by default\n";
5738 It can also be the case that long doubles and doubles are the same thing:
5741 ($Config{doublesize} == $Config{longdblsize}) &&
5742 print "doubles are long doubles\n";
5744 The size specifier C<V> has no effect for Perl code, but it is supported
5745 for compatibility with XS code; it means 'use the standard size for
5746 a Perl integer (or floating-point number)', which is already the
5747 default for Perl code.
5749 =item order of arguments
5751 Normally, sprintf takes the next unused argument as the value to
5752 format for each format specification. If the format specification
5753 uses C<*> to require additional arguments, these are consumed from
5754 the argument list in the order in which they appear in the format
5755 specification I<before> the value to format. Where an argument is
5756 specified using an explicit index, this does not affect the normal
5757 order for the arguments (even when the explicitly specified index
5758 would have been the next argument in any case).
5762 printf '<%*.*s>', $a, $b, $c;
5764 would use C<$a> for the width, C<$b> for the precision and C<$c>
5765 as the value to format, while:
5767 print '<%*1$.*s>', $a, $b;
5769 would use C<$a> for the width and the precision, and C<$b> as the
5772 Here are some more examples - beware that when using an explicit
5773 index, the C<$> may need to be escaped:
5775 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5776 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5777 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5778 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5782 If C<use locale> is in effect, and POSIX::setlocale() has been called,
5783 the character used for the decimal separator in formatted floating
5784 point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
5788 X<sqrt> X<root> X<square root>
5792 Return the square root of EXPR. If EXPR is omitted, returns square
5793 root of C<$_>. Only works on non-negative operands, unless you've
5794 loaded the standard Math::Complex module.
5797 print sqrt(-2); # prints 1.4142135623731i
5800 X<srand> X<seed> X<randseed>
5804 Sets the random number seed for the C<rand> operator.
5806 The point of the function is to "seed" the C<rand> function so that
5807 C<rand> can produce a different sequence each time you run your
5810 If srand() is not called explicitly, it is called implicitly at the
5811 first use of the C<rand> operator. However, this was not the case in
5812 versions of Perl before 5.004, so if your script will run under older
5813 Perl versions, it should call C<srand>.
5815 Most programs won't even call srand() at all, except those that
5816 need a cryptographically-strong starting point rather than the
5817 generally acceptable default, which is based on time of day,
5818 process ID, and memory allocation, or the F</dev/urandom> device,
5821 You can call srand($seed) with the same $seed to reproduce the
5822 I<same> sequence from rand(), but this is usually reserved for
5823 generating predictable results for testing or debugging.
5824 Otherwise, don't call srand() more than once in your program.
5826 Do B<not> call srand() (i.e. without an argument) more than once in
5827 a script. The internal state of the random number generator should
5828 contain more entropy than can be provided by any seed, so calling
5829 srand() again actually I<loses> randomness.
5831 Most implementations of C<srand> take an integer and will silently
5832 truncate decimal numbers. This means C<srand(42)> will usually
5833 produce the same results as C<srand(42.1)>. To be safe, always pass
5834 C<srand> an integer.
5836 In versions of Perl prior to 5.004 the default seed was just the
5837 current C<time>. This isn't a particularly good seed, so many old
5838 programs supply their own seed value (often C<time ^ $$> or C<time ^
5839 ($$ + ($$ << 15))>), but that isn't necessary any more.
5841 For cryptographic purposes, however, you need something much more random
5842 than the default seed. Checksumming the compressed output of one or more
5843 rapidly changing operating system status programs is the usual method. For
5846 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5848 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5851 Frequently called programs (like CGI scripts) that simply use
5855 for a seed can fall prey to the mathematical property that
5859 one-third of the time. So don't do that.
5861 =item stat FILEHANDLE
5862 X<stat> X<file, status> X<ctime>
5866 =item stat DIRHANDLE
5870 Returns a 13-element list giving the status info for a file, either
5871 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
5872 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
5875 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5876 $atime,$mtime,$ctime,$blksize,$blocks)
5879 Not all fields are supported on all filesystem types. Here are the
5880 meanings of the fields:
5882 0 dev device number of filesystem
5884 2 mode file mode (type and permissions)
5885 3 nlink number of (hard) links to the file
5886 4 uid numeric user ID of file's owner
5887 5 gid numeric group ID of file's owner
5888 6 rdev the device identifier (special files only)
5889 7 size total size of file, in bytes
5890 8 atime last access time in seconds since the epoch
5891 9 mtime last modify time in seconds since the epoch
5892 10 ctime inode change time in seconds since the epoch (*)
5893 11 blksize preferred block size for file system I/O
5894 12 blocks actual number of blocks allocated
5896 (The epoch was at 00:00 January 1, 1970 GMT.)
5898 (*) Not all fields are supported on all filesystem types. Notably, the
5899 ctime field is non-portable. In particular, you cannot expect it to be a
5900 "creation time", see L<perlport/"Files and Filesystems"> for details.
5902 If C<stat> is passed the special filehandle consisting of an underline, no
5903 stat is done, but the current contents of the stat structure from the
5904 last C<stat>, C<lstat>, or filetest are returned. Example:
5906 if (-x $file && (($d) = stat(_)) && $d < 0) {
5907 print "$file is executable NFS file\n";
5910 (This works on machines only for which the device number is negative
5913 Because the mode contains both the file type and its permissions, you
5914 should mask off the file type portion and (s)printf using a C<"%o">
5915 if you want to see the real permissions.
5917 $mode = (stat($filename))[2];
5918 printf "Permissions are %04o\n", $mode & 07777;
5920 In scalar context, C<stat> returns a boolean value indicating success
5921 or failure, and, if successful, sets the information associated with
5922 the special filehandle C<_>.
5924 The L<File::stat> module provides a convenient, by-name access mechanism:
5927 $sb = stat($filename);
5928 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5929 $filename, $sb->size, $sb->mode & 07777,
5930 scalar localtime $sb->mtime;
5932 You can import symbolic mode constants (C<S_IF*>) and functions
5933 (C<S_IS*>) from the Fcntl module:
5937 $mode = (stat($filename))[2];
5939 $user_rwx = ($mode & S_IRWXU) >> 6;
5940 $group_read = ($mode & S_IRGRP) >> 3;
5941 $other_execute = $mode & S_IXOTH;
5943 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5945 $is_setuid = $mode & S_ISUID;
5946 $is_directory = S_ISDIR($mode);
5948 You could write the last two using the C<-u> and C<-d> operators.
5949 The commonly available C<S_IF*> constants are
5951 # Permissions: read, write, execute, for user, group, others.
5953 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5954 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5955 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5957 # Setuid/Setgid/Stickiness/SaveText.
5958 # Note that the exact meaning of these is system dependent.
5960 S_ISUID S_ISGID S_ISVTX S_ISTXT
5962 # File types. Not necessarily all are available on your system.
5964 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5966 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5968 S_IREAD S_IWRITE S_IEXEC
5970 and the C<S_IF*> functions are
5972 S_IMODE($mode) the part of $mode containing the permission bits
5973 and the setuid/setgid/sticky bits
5975 S_IFMT($mode) the part of $mode containing the file type
5976 which can be bit-anded with e.g. S_IFREG
5977 or with the following functions
5979 # The operators -f, -d, -l, -b, -c, -p, and -S.
5981 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5982 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5984 # No direct -X operator counterpart, but for the first one
5985 # the -g operator is often equivalent. The ENFMT stands for
5986 # record flocking enforcement, a platform-dependent feature.
5988 S_ISENFMT($mode) S_ISWHT($mode)
5990 See your native chmod(2) and stat(2) documentation for more details
5991 about the C<S_*> constants. To get status info for a symbolic link
5992 instead of the target file behind the link, use the C<lstat> function.
5997 =item state TYPE EXPR
5999 =item state EXPR : ATTRS
6001 =item state TYPE EXPR : ATTRS
6003 C<state> declares a lexically scoped variable, just like C<my> does.
6004 However, those variables will be initialized only once, contrary to
6005 lexical variables that are reinitialized each time their enclosing block
6008 C<state> variables are only enabled when the C<feature 'state'> pragma is
6009 in effect. See L<feature>.
6016 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6017 doing many pattern matches on the string before it is next modified.
6018 This may or may not save time, depending on the nature and number of
6019 patterns you are searching on, and on the distribution of character
6020 frequencies in the string to be searched--you probably want to compare
6021 run times with and without it to see which runs faster. Those loops
6022 that scan for many short constant strings (including the constant
6023 parts of more complex patterns) will benefit most. You may have only
6024 one C<study> active at a time--if you study a different scalar the first
6025 is "unstudied". (The way C<study> works is this: a linked list of every
6026 character in the string to be searched is made, so we know, for
6027 example, where all the C<'k'> characters are. From each search string,
6028 the rarest character is selected, based on some static frequency tables
6029 constructed from some C programs and English text. Only those places
6030 that contain this "rarest" character are examined.)
6032 For example, here is a loop that inserts index producing entries
6033 before any line containing a certain pattern:
6037 print ".IX foo\n" if /\bfoo\b/;
6038 print ".IX bar\n" if /\bbar\b/;
6039 print ".IX blurfl\n" if /\bblurfl\b/;
6044 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
6045 will be looked at, because C<f> is rarer than C<o>. In general, this is
6046 a big win except in pathological cases. The only question is whether
6047 it saves you more time than it took to build the linked list in the
6050 Note that if you have to look for strings that you don't know till
6051 runtime, you can build an entire loop as a string and C<eval> that to
6052 avoid recompiling all your patterns all the time. Together with
6053 undefining C<$/> to input entire files as one record, this can be very
6054 fast, often faster than specialized programs like fgrep(1). The following
6055 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6056 out the names of those files that contain a match:
6058 $search = 'while (<>) { study;';
6059 foreach $word (@words) {
6060 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6065 eval $search; # this screams
6066 $/ = "\n"; # put back to normal input delimiter
6067 foreach $file (sort keys(%seen)) {
6071 =item sub NAME BLOCK
6074 =item sub NAME (PROTO) BLOCK
6076 =item sub NAME : ATTRS BLOCK
6078 =item sub NAME (PROTO) : ATTRS BLOCK
6080 This is subroutine definition, not a real function I<per se>.
6081 Without a BLOCK it's just a forward declaration. Without a NAME,
6082 it's an anonymous function declaration, and does actually return
6083 a value: the CODE ref of the closure you just created.
6085 See L<perlsub> and L<perlref> for details about subroutines and
6086 references, and L<attributes> and L<Attribute::Handlers> for more
6087 information about attributes.
6089 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6090 X<substr> X<substring> X<mid> X<left> X<right>
6092 =item substr EXPR,OFFSET,LENGTH
6094 =item substr EXPR,OFFSET
6096 Extracts a substring out of EXPR and returns it. First character is at
6097 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6098 If OFFSET is negative (or more precisely, less than C<$[>), starts
6099 that far from the end of the string. If LENGTH is omitted, returns
6100 everything to the end of the string. If LENGTH is negative, leaves that
6101 many characters off the end of the string.
6103 my $s = "The black cat climbed the green tree";
6104 my $color = substr $s, 4, 5; # black
6105 my $middle = substr $s, 4, -11; # black cat climbed the
6106 my $end = substr $s, 14; # climbed the green tree
6107 my $tail = substr $s, -4; # tree
6108 my $z = substr $s, -4, 2; # tr
6110 You can use the substr() function as an lvalue, in which case EXPR
6111 must itself be an lvalue. If you assign something shorter than LENGTH,
6112 the string will shrink, and if you assign something longer than LENGTH,
6113 the string will grow to accommodate it. To keep the string the same
6114 length you may need to pad or chop your value using C<sprintf>.
6116 If OFFSET and LENGTH specify a substring that is partly outside the
6117 string, only the part within the string is returned. If the substring
6118 is beyond either end of the string, substr() returns the undefined
6119 value and produces a warning. When used as an lvalue, specifying a
6120 substring that is entirely outside the string is a fatal error.
6121 Here's an example showing the behavior for boundary cases:
6124 substr($name, 4) = 'dy'; # $name is now 'freddy'
6125 my $null = substr $name, 6, 2; # returns '' (no warning)
6126 my $oops = substr $name, 7; # returns undef, with warning
6127 substr($name, 7) = 'gap'; # fatal error
6129 An alternative to using substr() as an lvalue is to specify the
6130 replacement string as the 4th argument. This allows you to replace
6131 parts of the EXPR and return what was there before in one operation,
6132 just as you can with splice().
6134 my $s = "The black cat climbed the green tree";
6135 my $z = substr $s, 14, 7, "jumped from"; # climbed
6136 # $s is now "The black cat jumped from the green tree"
6138 Note that the lvalue returned by the 3-arg version of substr() acts as
6139 a 'magic bullet'; each time it is assigned to, it remembers which part
6140 of the original string is being modified; for example:
6143 for (substr($x,1,2)) {
6144 $_ = 'a'; print $x,"\n"; # prints 1a4
6145 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6147 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6150 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6153 =item symlink OLDFILE,NEWFILE
6154 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6156 Creates a new filename symbolically linked to the old filename.
6157 Returns C<1> for success, C<0> otherwise. On systems that don't support
6158 symbolic links, produces a fatal error at run time. To check for that,
6161 $symlink_exists = eval { symlink("",""); 1 };
6163 =item syscall NUMBER, LIST
6164 X<syscall> X<system call>
6166 Calls the system call specified as the first element of the list,
6167 passing the remaining elements as arguments to the system call. If
6168 unimplemented, produces a fatal error. The arguments are interpreted
6169 as follows: if a given argument is numeric, the argument is passed as
6170 an int. If not, the pointer to the string value is passed. You are
6171 responsible to make sure a string is pre-extended long enough to
6172 receive any result that might be written into a string. You can't use a
6173 string literal (or other read-only string) as an argument to C<syscall>
6174 because Perl has to assume that any string pointer might be written
6176 integer arguments are not literals and have never been interpreted in a
6177 numeric context, you may need to add C<0> to them to force them to look
6178 like numbers. This emulates the C<syswrite> function (or vice versa):
6180 require 'syscall.ph'; # may need to run h2ph
6182 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6184 Note that Perl supports passing of up to only 14 arguments to your system call,
6185 which in practice should usually suffice.
6187 Syscall returns whatever value returned by the system call it calls.
6188 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6189 Note that some system calls can legitimately return C<-1>. The proper
6190 way to handle such calls is to assign C<$!=0;> before the call and
6191 check the value of C<$!> if syscall returns C<-1>.
6193 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6194 number of the read end of the pipe it creates. There is no way
6195 to retrieve the file number of the other end. You can avoid this
6196 problem by using C<pipe> instead.
6198 =item sysopen FILEHANDLE,FILENAME,MODE
6201 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6203 Opens the file whose filename is given by FILENAME, and associates it
6204 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6205 the name of the real filehandle wanted. This function calls the
6206 underlying operating system's C<open> function with the parameters
6207 FILENAME, MODE, PERMS.
6209 The possible values and flag bits of the MODE parameter are
6210 system-dependent; they are available via the standard module C<Fcntl>.
6211 See the documentation of your operating system's C<open> to see which
6212 values and flag bits are available. You may combine several flags
6213 using the C<|>-operator.
6215 Some of the most common values are C<O_RDONLY> for opening the file in
6216 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6217 and C<O_RDWR> for opening the file in read-write mode.
6218 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6220 For historical reasons, some values work on almost every system
6221 supported by perl: zero means read-only, one means write-only, and two
6222 means read/write. We know that these values do I<not> work under
6223 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6224 use them in new code.
6226 If the file named by FILENAME does not exist and the C<open> call creates
6227 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6228 PERMS specifies the permissions of the newly created file. If you omit
6229 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6230 These permission values need to be in octal, and are modified by your
6231 process's current C<umask>.
6234 In many systems the C<O_EXCL> flag is available for opening files in
6235 exclusive mode. This is B<not> locking: exclusiveness means here that
6236 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6237 on network filesystems, and has no effect unless the C<O_CREAT> flag
6238 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6239 being opened if it is a symbolic link. It does not protect against
6240 symbolic links in the file's path.
6243 Sometimes you may want to truncate an already-existing file. This
6244 can be done using the C<O_TRUNC> flag. The behavior of
6245 C<O_TRUNC> with C<O_RDONLY> is undefined.
6248 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6249 that takes away the user's option to have a more permissive umask.
6250 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6253 Note that C<sysopen> depends on the fdopen() C library function.
6254 On many UNIX systems, fdopen() is known to fail when file descriptors
6255 exceed a certain value, typically 255. If you need more file
6256 descriptors than that, consider rebuilding Perl to use the C<sfio>
6257 library, or perhaps using the POSIX::open() function.
6259 See L<perlopentut> for a kinder, gentler explanation of opening files.
6261 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6264 =item sysread FILEHANDLE,SCALAR,LENGTH
6266 Attempts to read LENGTH bytes of data into variable SCALAR from the
6267 specified FILEHANDLE, using the system call read(2). It bypasses
6268 buffered IO, so mixing this with other kinds of reads, C<print>,
6269 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6270 perlio or stdio layers usually buffers data. Returns the number of
6271 bytes actually read, C<0> at end of file, or undef if there was an
6272 error (in the latter case C<$!> is also set). SCALAR will be grown or
6273 shrunk so that the last byte actually read is the last byte of the
6274 scalar after the read.
6276 An OFFSET may be specified to place the read data at some place in the
6277 string other than the beginning. A negative OFFSET specifies
6278 placement at that many characters counting backwards from the end of
6279 the string. A positive OFFSET greater than the length of SCALAR
6280 results in the string being padded to the required size with C<"\0">
6281 bytes before the result of the read is appended.
6283 There is no syseof() function, which is ok, since eof() doesn't work
6284 very well on device files (like ttys) anyway. Use sysread() and check
6285 for a return value for 0 to decide whether you're done.
6287 Note that if the filehandle has been marked as C<:utf8> Unicode
6288 characters are read instead of bytes (the LENGTH, OFFSET, and the
6289 return value of sysread() are in Unicode characters).
6290 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6291 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6293 =item sysseek FILEHANDLE,POSITION,WHENCE
6296 Sets FILEHANDLE's system position in bytes using the system call
6297 lseek(2). FILEHANDLE may be an expression whose value gives the name
6298 of the filehandle. The values for WHENCE are C<0> to set the new
6299 position to POSITION, C<1> to set the it to the current position plus
6300 POSITION, and C<2> to set it to EOF plus POSITION (typically
6303 Note the I<in bytes>: even if the filehandle has been set to operate
6304 on characters (for example by using the C<:utf8> I/O layer), tell()
6305 will return byte offsets, not character offsets (because implementing
6306 that would render sysseek() very slow).
6308 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6309 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6310 C<seek>, C<tell>, or C<eof> may cause confusion.
6312 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6313 and C<SEEK_END> (start of the file, current position, end of the file)
6314 from the Fcntl module. Use of the constants is also more portable
6315 than relying on 0, 1, and 2. For example to define a "systell" function:
6317 use Fcntl 'SEEK_CUR';
6318 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6320 Returns the new position, or the undefined value on failure. A position
6321 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6322 true on success and false on failure, yet you can still easily determine
6328 =item system PROGRAM LIST
6330 Does exactly the same thing as C<exec LIST>, except that a fork is
6331 done first, and the parent process waits for the child process to
6332 complete. Note that argument processing varies depending on the
6333 number of arguments. If there is more than one argument in LIST,
6334 or if LIST is an array with more than one value, starts the program
6335 given by the first element of the list with arguments given by the
6336 rest of the list. If there is only one scalar argument, the argument
6337 is checked for shell metacharacters, and if there are any, the
6338 entire argument is passed to the system's command shell for parsing
6339 (this is C</bin/sh -c> on Unix platforms, but varies on other
6340 platforms). If there are no shell metacharacters in the argument,
6341 it is split into words and passed directly to C<execvp>, which is
6344 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6345 output before any operation that may do a fork, but this may not be
6346 supported on some platforms (see L<perlport>). To be safe, you may need
6347 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6348 of C<IO::Handle> on any open handles.
6350 The return value is the exit status of the program as returned by the
6351 C<wait> call. To get the actual exit value, shift right by eight (see
6352 below). See also L</exec>. This is I<not> what you want to use to capture
6353 the output from a command, for that you should use merely backticks or
6354 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6355 indicates a failure to start the program or an error of the wait(2) system
6356 call (inspect $! for the reason).
6358 Like C<exec>, C<system> allows you to lie to a program about its name if
6359 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6361 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6362 C<system>, if you expect your program to terminate on receipt of these
6363 signals you will need to arrange to do so yourself based on the return
6366 @args = ("command", "arg1", "arg2");
6368 or die "system @args failed: $?"
6370 You can check all the failure possibilities by inspecting
6374 print "failed to execute: $!\n";
6377 printf "child died with signal %d, %s coredump\n",
6378 ($? & 127), ($? & 128) ? 'with' : 'without';
6381 printf "child exited with value %d\n", $? >> 8;
6384 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6385 with the W*() calls of the POSIX extension.
6387 When the arguments get executed via the system shell, results
6388 and return codes will be subject to its quirks and capabilities.
6389 See L<perlop/"`STRING`"> and L</exec> for details.
6391 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6394 =item syswrite FILEHANDLE,SCALAR,LENGTH
6396 =item syswrite FILEHANDLE,SCALAR
6398 Attempts to write LENGTH bytes of data from variable SCALAR to the
6399 specified FILEHANDLE, using the system call write(2). If LENGTH is
6400 not specified, writes whole SCALAR. It bypasses buffered IO, so
6401 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6402 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6403 stdio layers usually buffers data. Returns the number of bytes
6404 actually written, or C<undef> if there was an error (in this case the
6405 errno variable C<$!> is also set). If the LENGTH is greater than the
6406 available data in the SCALAR after the OFFSET, only as much data as is
6407 available will be written.
6409 An OFFSET may be specified to write the data from some part of the
6410 string other than the beginning. A negative OFFSET specifies writing
6411 that many characters counting backwards from the end of the string.
6412 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6414 Note that if the filehandle has been marked as C<:utf8>, Unicode
6415 characters are written instead of bytes (the LENGTH, OFFSET, and the
6416 return value of syswrite() are in UTF-8 encoded Unicode characters).
6417 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6418 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6420 =item tell FILEHANDLE
6425 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6426 error. FILEHANDLE may be an expression whose value gives the name of
6427 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6430 Note the I<in bytes>: even if the filehandle has been set to
6431 operate on characters (for example by using the C<:utf8> open
6432 layer), tell() will return byte offsets, not character offsets
6433 (because that would render seek() and tell() rather slow).
6435 The return value of tell() for the standard streams like the STDIN
6436 depends on the operating system: it may return -1 or something else.
6437 tell() on pipes, fifos, and sockets usually returns -1.
6439 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6441 Do not use tell() (or other buffered I/O operations) on a file handle
6442 that has been manipulated by sysread(), syswrite() or sysseek().
6443 Those functions ignore the buffering, while tell() does not.
6445 =item telldir DIRHANDLE
6448 Returns the current position of the C<readdir> routines on DIRHANDLE.
6449 Value may be given to C<seekdir> to access a particular location in a
6450 directory. C<telldir> has the same caveats about possible directory
6451 compaction as the corresponding system library routine.
6453 =item tie VARIABLE,CLASSNAME,LIST
6456 This function binds a variable to a package class that will provide the
6457 implementation for the variable. VARIABLE is the name of the variable
6458 to be enchanted. CLASSNAME is the name of a class implementing objects
6459 of correct type. Any additional arguments are passed to the C<new>
6460 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6461 or C<TIEHASH>). Typically these are arguments such as might be passed
6462 to the C<dbm_open()> function of C. The object returned by the C<new>
6463 method is also returned by the C<tie> function, which would be useful
6464 if you want to access other methods in CLASSNAME.
6466 Note that functions such as C<keys> and C<values> may return huge lists
6467 when used on large objects, like DBM files. You may prefer to use the
6468 C<each> function to iterate over such. Example:
6470 # print out history file offsets
6472 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6473 while (($key,$val) = each %HIST) {
6474 print $key, ' = ', unpack('L',$val), "\n";
6478 A class implementing a hash should have the following methods:
6480 TIEHASH classname, LIST
6482 STORE this, key, value
6487 NEXTKEY this, lastkey
6492 A class implementing an ordinary array should have the following methods:
6494 TIEARRAY classname, LIST
6496 STORE this, key, value
6498 STORESIZE this, count
6504 SPLICE this, offset, length, LIST
6509 A class implementing a file handle should have the following methods:
6511 TIEHANDLE classname, LIST
6512 READ this, scalar, length, offset
6515 WRITE this, scalar, length, offset
6517 PRINTF this, format, LIST
6521 SEEK this, position, whence
6523 OPEN this, mode, LIST
6528 A class implementing a scalar should have the following methods:
6530 TIESCALAR classname, LIST
6536 Not all methods indicated above need be implemented. See L<perltie>,
6537 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6539 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6540 for you--you need to do that explicitly yourself. See L<DB_File>
6541 or the F<Config> module for interesting C<tie> implementations.
6543 For further details see L<perltie>, L<"tied VARIABLE">.
6548 Returns a reference to the object underlying VARIABLE (the same value
6549 that was originally returned by the C<tie> call that bound the variable
6550 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6556 Returns the number of non-leap seconds since whatever time the system
6557 considers to be the epoch, suitable for feeding to C<gmtime> and
6558 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6559 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6560 1904 in the current local time zone for its epoch.
6562 For measuring time in better granularity than one second,
6563 you may use either the L<Time::HiRes> module (from CPAN, and starting from
6564 Perl 5.8 part of the standard distribution), or if you have
6565 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6566 See L<perlfaq8> for details.
6568 For date and time processing look at the many related modules on CPAN.
6569 For a comprehensive date and time representation look at the
6575 Returns a four-element list giving the user and system times, in
6576 seconds, for this process and the children of this process.
6578 ($user,$system,$cuser,$csystem) = times;
6580 In scalar context, C<times> returns C<$user>.
6582 Note that times for children are included only after they terminate.
6586 The transliteration operator. Same as C<y///>. See L<perlop>.
6588 =item truncate FILEHANDLE,LENGTH
6591 =item truncate EXPR,LENGTH
6593 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6594 specified length. Produces a fatal error if truncate isn't implemented
6595 on your system. Returns true if successful, the undefined value
6598 The behavior is undefined if LENGTH is greater than the length of the
6601 The position in the file of FILEHANDLE is left unchanged. You may want to
6602 call L<seek> before writing to the file.
6605 X<uc> X<uppercase> X<toupper>
6609 Returns an uppercased version of EXPR. This is the internal function
6610 implementing the C<\U> escape in double-quoted strings. Respects
6611 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6612 and L<perlunicode> for more details about locale and Unicode support.
6613 It does not attempt to do titlecase mapping on initial letters. See
6614 C<ucfirst> for that.
6616 If EXPR is omitted, uses C<$_>.
6619 X<ucfirst> X<uppercase>
6623 Returns the value of EXPR with the first character in uppercase
6624 (titlecase in Unicode). This is the internal function implementing
6625 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6626 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6627 for more details about locale and Unicode support.
6629 If EXPR is omitted, uses C<$_>.
6636 Sets the umask for the process to EXPR and returns the previous value.
6637 If EXPR is omitted, merely returns the current umask.
6639 The Unix permission C<rwxr-x---> is represented as three sets of three
6640 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6641 and isn't one of the digits). The C<umask> value is such a number
6642 representing disabled permissions bits. The permission (or "mode")
6643 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6644 even if you tell C<sysopen> to create a file with permissions C<0777>,
6645 if your umask is C<0022> then the file will actually be created with
6646 permissions C<0755>. If your C<umask> were C<0027> (group can't
6647 write; others can't read, write, or execute), then passing
6648 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6651 Here's some advice: supply a creation mode of C<0666> for regular
6652 files (in C<sysopen>) and one of C<0777> for directories (in
6653 C<mkdir>) and executable files. This gives users the freedom of
6654 choice: if they want protected files, they might choose process umasks
6655 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6656 Programs should rarely if ever make policy decisions better left to
6657 the user. The exception to this is when writing files that should be
6658 kept private: mail files, web browser cookies, I<.rhosts> files, and
6661 If umask(2) is not implemented on your system and you are trying to
6662 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6663 fatal error at run time. If umask(2) is not implemented and you are
6664 not trying to restrict access for yourself, returns C<undef>.
6666 Remember that a umask is a number, usually given in octal; it is I<not> a
6667 string of octal digits. See also L</oct>, if all you have is a string.
6670 X<undef> X<undefine>
6674 Undefines the value of EXPR, which must be an lvalue. Use only on a
6675 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6676 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6677 will probably not do what you expect on most predefined variables or
6678 DBM list values, so don't do that; see L<delete>.) Always returns the
6679 undefined value. You can omit the EXPR, in which case nothing is
6680 undefined, but you still get an undefined value that you could, for
6681 instance, return from a subroutine, assign to a variable or pass as a
6682 parameter. Examples:
6685 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6689 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6690 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6691 select undef, undef, undef, 0.25;
6692 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6694 Note that this is a unary operator, not a list operator.
6697 X<unlink> X<delete> X<remove> X<rm> X<del>
6701 Deletes a list of files. Returns the number of files successfully
6704 $cnt = unlink 'a', 'b', 'c';
6708 Note: C<unlink> will not attempt to delete directories unless you are superuser
6709 and the B<-U> flag is supplied to Perl. Even if these conditions are
6710 met, be warned that unlinking a directory can inflict damage on your
6711 filesystem. Finally, using C<unlink> on directories is not supported on
6712 many operating systems. Use C<rmdir> instead.
6714 If LIST is omitted, uses C<$_>.
6716 =item unpack TEMPLATE,EXPR
6719 =item unpack TEMPLATE
6721 C<unpack> does the reverse of C<pack>: it takes a string
6722 and expands it out into a list of values.
6723 (In scalar context, it returns merely the first value produced.)
6725 If EXPR is omitted, unpacks the C<$_> string.
6727 The string is broken into chunks described by the TEMPLATE. Each chunk
6728 is converted separately to a value. Typically, either the string is a result
6729 of C<pack>, or the characters of the string represent a C structure of some
6732 The TEMPLATE has the same format as in the C<pack> function.
6733 Here's a subroutine that does substring:
6736 my($what,$where,$howmuch) = @_;
6737 unpack("x$where a$howmuch", $what);
6742 sub ordinal { unpack("W",$_[0]); } # same as ord()
6744 In addition to fields allowed in pack(), you may prefix a field with
6745 a %<number> to indicate that
6746 you want a <number>-bit checksum of the items instead of the items
6747 themselves. Default is a 16-bit checksum. Checksum is calculated by
6748 summing numeric values of expanded values (for string fields the sum of
6749 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6751 For example, the following
6752 computes the same number as the System V sum program:
6756 unpack("%32W*",<>) % 65535;
6759 The following efficiently counts the number of set bits in a bit vector:
6761 $setbits = unpack("%32b*", $selectmask);
6763 The C<p> and C<P> formats should be used with care. Since Perl
6764 has no way of checking whether the value passed to C<unpack()>
6765 corresponds to a valid memory location, passing a pointer value that's
6766 not known to be valid is likely to have disastrous consequences.
6768 If there are more pack codes or if the repeat count of a field or a group
6769 is larger than what the remainder of the input string allows, the result
6770 is not well defined: in some cases, the repeat count is decreased, or
6771 C<unpack()> will produce null strings or zeroes, or terminate with an
6772 error. If the input string is longer than one described by the TEMPLATE,
6773 the rest is ignored.
6775 See L</pack> for more examples and notes.
6777 =item untie VARIABLE
6780 Breaks the binding between a variable and a package. (See C<tie>.)
6781 Has no effect if the variable is not tied.
6783 =item unshift ARRAY,LIST
6786 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6787 depending on how you look at it. Prepends list to the front of the
6788 array, and returns the new number of elements in the array.
6790 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6792 Note the LIST is prepended whole, not one element at a time, so the
6793 prepended elements stay in the same order. Use C<reverse> to do the
6796 =item use Module VERSION LIST
6797 X<use> X<module> X<import>
6799 =item use Module VERSION
6801 =item use Module LIST
6807 Imports some semantics into the current package from the named module,
6808 generally by aliasing certain subroutine or variable names into your
6809 package. It is exactly equivalent to
6811 BEGIN { require Module; import Module LIST; }
6813 except that Module I<must> be a bareword.
6815 VERSION may be either a numeric argument such as 5.006, which will be
6816 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6817 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6818 greater than the version of the current Perl interpreter; Perl will not
6819 attempt to parse the rest of the file. Compare with L</require>, which can
6820 do a similar check at run time.
6822 Specifying VERSION as a literal of the form v5.6.1 should generally be
6823 avoided, because it leads to misleading error messages under earlier
6824 versions of Perl that do not support this syntax. The equivalent numeric
6825 version should be used instead.
6827 use v5.6.1; # compile time version check
6829 use 5.006_001; # ditto; preferred for backwards compatibility
6831 This is often useful if you need to check the current Perl version before
6832 C<use>ing library modules that have changed in incompatible ways from
6833 older versions of Perl. (We try not to do this more than we have to.)
6835 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6836 C<require> makes sure the module is loaded into memory if it hasn't been
6837 yet. The C<import> is not a builtin--it's just an ordinary static method
6838 call into the C<Module> package to tell the module to import the list of
6839 features back into the current package. The module can implement its
6840 C<import> method any way it likes, though most modules just choose to
6841 derive their C<import> method via inheritance from the C<Exporter> class that
6842 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6843 method can be found then the call is skipped, even if there is an AUTOLOAD
6846 If you do not want to call the package's C<import> method (for instance,
6847 to stop your namespace from being altered), explicitly supply the empty list:
6851 That is exactly equivalent to
6853 BEGIN { require Module }
6855 If the VERSION argument is present between Module and LIST, then the
6856 C<use> will call the VERSION method in class Module with the given
6857 version as an argument. The default VERSION method, inherited from
6858 the UNIVERSAL class, croaks if the given version is larger than the
6859 value of the variable C<$Module::VERSION>.
6861 Again, there is a distinction between omitting LIST (C<import> called
6862 with no arguments) and an explicit empty LIST C<()> (C<import> not
6863 called). Note that there is no comma after VERSION!
6865 Because this is a wide-open interface, pragmas (compiler directives)
6866 are also implemented this way. Currently implemented pragmas are:
6871 use sigtrap qw(SEGV BUS);
6872 use strict qw(subs vars refs);
6873 use subs qw(afunc blurfl);
6874 use warnings qw(all);
6875 use sort qw(stable _quicksort _mergesort);
6877 Some of these pseudo-modules import semantics into the current
6878 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6879 which import symbols into the current package (which are effective
6880 through the end of the file).
6882 There's a corresponding C<no> command that unimports meanings imported
6883 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6884 It behaves exactly as C<import> does with respect to VERSION, an
6885 omitted LIST, empty LIST, or no unimport method being found.
6891 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6892 for the C<-M> and C<-m> command-line options to perl that give C<use>
6893 functionality from the command-line.
6898 Changes the access and modification times on each file of a list of
6899 files. The first two elements of the list must be the NUMERICAL access
6900 and modification times, in that order. Returns the number of files
6901 successfully changed. The inode change time of each file is set
6902 to the current time. For example, this code has the same effect as the
6903 Unix touch(1) command when the files I<already exist> and belong to
6904 the user running the program:
6907 $atime = $mtime = time;
6908 utime $atime, $mtime, @ARGV;
6910 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6911 the utime(2) function in the C library will be called with a null second
6912 argument. On most systems, this will set the file's access and
6913 modification times to the current time (i.e. equivalent to the example
6914 above) and will even work on other users' files where you have write
6917 utime undef, undef, @ARGV;
6919 Under NFS this will use the time of the NFS server, not the time of
6920 the local machine. If there is a time synchronization problem, the
6921 NFS server and local machine will have different times. The Unix
6922 touch(1) command will in fact normally use this form instead of the
6923 one shown in the first example.
6925 Note that only passing one of the first two elements as C<undef> will
6926 be equivalent of passing it as 0 and will not have the same effect as
6927 described when they are both C<undef>. This case will also trigger an
6928 uninitialized warning.
6930 On systems that support futimes, you might pass file handles among the
6931 files. On systems that don't support futimes, passing file handles
6932 produces a fatal error at run time. The file handles must be passed
6933 as globs or references to be recognized. Barewords are considered
6939 Returns a list consisting of all the values of the named hash.
6940 (In a scalar context, returns the number of values.)
6942 The values are returned in an apparently random order. The actual
6943 random order is subject to change in future versions of perl, but it
6944 is guaranteed to be the same order as either the C<keys> or C<each>
6945 function would produce on the same (unmodified) hash. Since Perl
6946 5.8.1 the ordering is different even between different runs of Perl
6947 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6949 As a side effect, calling values() resets the HASH's internal iterator,
6950 see L</each>. (In particular, calling values() in void context resets
6951 the iterator with no other overhead.)
6953 Note that the values are not copied, which means modifying them will
6954 modify the contents of the hash:
6956 for (values %hash) { s/foo/bar/g } # modifies %hash values
6957 for (@hash{keys %hash}) { s/foo/bar/g } # same
6959 See also C<keys>, C<each>, and C<sort>.
6961 =item vec EXPR,OFFSET,BITS
6962 X<vec> X<bit> X<bit vector>
6964 Treats the string in EXPR as a bit vector made up of elements of
6965 width BITS, and returns the value of the element specified by OFFSET
6966 as an unsigned integer. BITS therefore specifies the number of bits
6967 that are reserved for each element in the bit vector. This must
6968 be a power of two from 1 to 32 (or 64, if your platform supports
6971 If BITS is 8, "elements" coincide with bytes of the input string.
6973 If BITS is 16 or more, bytes of the input string are grouped into chunks
6974 of size BITS/8, and each group is converted to a number as with
6975 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6976 for BITS==64). See L<"pack"> for details.
6978 If bits is 4 or less, the string is broken into bytes, then the bits
6979 of each byte are broken into 8/BITS groups. Bits of a byte are
6980 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6981 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6982 breaking the single input byte C<chr(0x36)> into two groups gives a list
6983 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6985 C<vec> may also be assigned to, in which case parentheses are needed
6986 to give the expression the correct precedence as in
6988 vec($image, $max_x * $x + $y, 8) = 3;
6990 If the selected element is outside the string, the value 0 is returned.
6991 If an element off the end of the string is written to, Perl will first
6992 extend the string with sufficiently many zero bytes. It is an error
6993 to try to write off the beginning of the string (i.e. negative OFFSET).
6995 The string should not contain any character with the value > 255 (which
6996 can only happen if you're using UTF-8 encoding). If it does, it will be
6997 treated as something that is not UTF-8 encoded. When the C<vec> was
6998 assigned to, other parts of your program will also no longer consider the
6999 string to be UTF-8 encoded. In other words, if you do have such characters
7000 in your string, vec() will operate on the actual byte string, and not the
7001 conceptual character string.
7003 Strings created with C<vec> can also be manipulated with the logical
7004 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7005 vector operation is desired when both operands are strings.
7006 See L<perlop/"Bitwise String Operators">.
7008 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7009 The comments show the string after each step. Note that this code works
7010 in the same way on big-endian or little-endian machines.
7013 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7015 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7016 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7018 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7019 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7020 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7021 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7022 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7023 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7025 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7026 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7027 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7030 To transform a bit vector into a string or list of 0's and 1's, use these:
7032 $bits = unpack("b*", $vector);
7033 @bits = split(//, unpack("b*", $vector));
7035 If you know the exact length in bits, it can be used in place of the C<*>.
7037 Here is an example to illustrate how the bits actually fall in place:
7043 unpack("V",$_) 01234567890123456789012345678901
7044 ------------------------------------------------------------------
7049 for ($shift=0; $shift < $width; ++$shift) {
7050 for ($off=0; $off < 32/$width; ++$off) {
7051 $str = pack("B*", "0"x32);
7052 $bits = (1<<$shift);
7053 vec($str, $off, $width) = $bits;
7054 $res = unpack("b*",$str);
7055 $val = unpack("V", $str);
7062 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7063 $off, $width, $bits, $val, $res
7067 Regardless of the machine architecture on which it is run, the above
7068 example should print the following table:
7071 unpack("V",$_) 01234567890123456789012345678901
7072 ------------------------------------------------------------------
7073 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7074 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7075 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7076 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7077 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7078 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7079 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7080 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7081 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7082 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7083 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7084 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7085 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7086 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7087 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7088 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7089 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7090 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7091 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7092 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7093 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7094 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7095 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7096 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7097 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7098 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7099 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7100 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7101 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7102 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7103 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7104 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7105 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7106 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7107 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7108 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7109 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7110 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7111 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7112 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7113 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7114 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7115 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7116 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7117 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7118 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7119 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7120 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7121 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7122 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7123 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7124 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7125 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7126 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7127 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7128 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7129 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7130 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7131 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7132 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7133 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7134 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7135 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7136 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7137 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7138 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7139 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7140 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7141 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7142 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7143 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7144 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7145 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7146 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7147 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7148 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7149 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7150 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7151 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7152 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7153 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7154 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7155 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7156 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7157 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7158 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7159 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7160 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7161 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7162 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7163 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7164 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7165 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7166 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7167 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7168 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7169 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7170 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7171 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7172 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7173 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7174 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7175 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7176 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7177 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7178 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7179 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7180 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7181 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7182 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7183 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7184 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7185 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7186 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7187 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7188 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7189 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7190 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7191 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7192 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7193 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7194 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7195 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7196 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7197 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7198 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7199 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7200 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7205 Behaves like the wait(2) system call on your system: it waits for a child
7206 process to terminate and returns the pid of the deceased process, or
7207 C<-1> if there are no child processes. The status is returned in C<$?>
7208 and C<{^CHILD_ERROR_NATIVE}>.
7209 Note that a return value of C<-1> could mean that child processes are
7210 being automatically reaped, as described in L<perlipc>.
7212 =item waitpid PID,FLAGS
7215 Waits for a particular child process to terminate and returns the pid of
7216 the deceased process, or C<-1> if there is no such child process. On some
7217 systems, a value of 0 indicates that there are processes still running.
7218 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7220 use POSIX ":sys_wait_h";
7223 $kid = waitpid(-1, WNOHANG);
7226 then you can do a non-blocking wait for all pending zombie processes.
7227 Non-blocking wait is available on machines supporting either the
7228 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7229 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7230 system call by remembering the status values of processes that have
7231 exited but have not been harvested by the Perl script yet.)
7233 Note that on some systems, a return value of C<-1> could mean that child
7234 processes are being automatically reaped. See L<perlipc> for details,
7235 and for other examples.
7238 X<wantarray> X<context>
7240 Returns true if the context of the currently executing subroutine or
7241 C<eval> is looking for a list value. Returns false if the context is
7242 looking for a scalar. Returns the undefined value if the context is
7243 looking for no value (void context).
7245 return unless defined wantarray; # don't bother doing more
7246 my @a = complex_calculation();
7247 return wantarray ? @a : "@a";
7249 C<wantarray()>'s result is unspecified in the top level of a file,
7250 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7251 in a C<DESTROY> method.
7253 This function should have been named wantlist() instead.
7256 X<warn> X<warning> X<STDERR>
7258 Produces a message on STDERR just like C<die>, but doesn't exit or throw
7261 If LIST is empty and C<$@> already contains a value (typically from a
7262 previous eval) that value is used after appending C<"\t...caught">
7263 to C<$@>. This is useful for staying almost, but not entirely similar to
7266 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7268 No message is printed if there is a C<$SIG{__WARN__}> handler
7269 installed. It is the handler's responsibility to deal with the message
7270 as it sees fit (like, for instance, converting it into a C<die>). Most
7271 handlers must therefore make arrangements to actually display the
7272 warnings that they are not prepared to deal with, by calling C<warn>
7273 again in the handler. Note that this is quite safe and will not
7274 produce an endless loop, since C<__WARN__> hooks are not called from
7277 You will find this behavior is slightly different from that of
7278 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7279 instead call C<die> again to change it).
7281 Using a C<__WARN__> handler provides a powerful way to silence all
7282 warnings (even the so-called mandatory ones). An example:
7284 # wipe out *all* compile-time warnings
7285 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7287 my $foo = 20; # no warning about duplicate my $foo,
7288 # but hey, you asked for it!
7289 # no compile-time or run-time warnings before here
7292 # run-time warnings enabled after here
7293 warn "\$foo is alive and $foo!"; # does show up
7295 See L<perlvar> for details on setting C<%SIG> entries, and for more
7296 examples. See the Carp module for other kinds of warnings using its
7297 carp() and cluck() functions.
7299 =item write FILEHANDLE
7306 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7307 using the format associated with that file. By default the format for
7308 a file is the one having the same name as the filehandle, but the
7309 format for the current output channel (see the C<select> function) may be set
7310 explicitly by assigning the name of the format to the C<$~> variable.
7312 Top of form processing is handled automatically: if there is
7313 insufficient room on the current page for the formatted record, the
7314 page is advanced by writing a form feed, a special top-of-page format
7315 is used to format the new page header, and then the record is written.
7316 By default the top-of-page format is the name of the filehandle with
7317 "_TOP" appended, but it may be dynamically set to the format of your
7318 choice by assigning the name to the C<$^> variable while the filehandle is
7319 selected. The number of lines remaining on the current page is in
7320 variable C<$->, which can be set to C<0> to force a new page.
7322 If FILEHANDLE is unspecified, output goes to the current default output
7323 channel, which starts out as STDOUT but may be changed by the
7324 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7325 is evaluated and the resulting string is used to look up the name of
7326 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7328 Note that write is I<not> the opposite of C<read>. Unfortunately.
7332 The transliteration operator. Same as C<tr///>. See L<perlop>.